AN ABSTRACT OF THE THESIS OF
Kai Gu for the degree of Master of Science in Wood Science presented on
May 21, 2010
Title: Evaluation of New Formaldehyde-free, Soy Flour-based Wood Adhesives for
Making Particleboard
Abstract approved:
_____________________________________________________________________
Kaichang Li
Formaldehyde-based adhesives such as urea-formaldehyde (UF), phenolformaldehyde (PF) are widely applied in wood-based composite industry. However,
these adhesives are all petrochemical-based and could not sustain in a long term due to
a limited reserve of oil and natural gas. Moreover, these adhesives emit carcinogenic
formaldehyde in the production or use of wood composite panels, thus reducing indoor
air quality and posing a health risk to human beings. Soy flour (SF) is an abundant,
readily available, renewable, and inexpensive material and is mainly used for food
application at present. Its potential for industrial applications has not been fully
realized. In this study, we evaluated two SF-based formaldehyde-free wood adhesives
for making M-2 Grade particleboard panels.
The first adhesive was composed of SF, polyethylenimine (PEI), maleic anhydride
(MA) and sodium hydroxide (NaOH). The weight ratio of SF/PEI/MA/NaOH was
7/1.0/0.32/0.1. The hot-press temperature, hot-press time, panel density and adhesive
usages were optimized in terms of enhancing the modulus of rupture (MOR), modulus
of elasticity (MOE) and internal bond (IB) of the particleboard panels. It was found
that the MOR, MOE and IB exceeded the minimum industrial requirements of M-2
particleboards under the following variables and conditions: hot-press temperature,
170 °C; hot-press time, 270 s; the adhesive usage of surface particles, 10 wt%; the
adhesive usage of the core particles, 8 wt%; and the targeted particleboard density,
0.80 g/cm3.
The second adhesive consisted of SF, a new curing agent (CA) and NaOH. Effects
of the weight ratio of adhesive components on strengths of the particleboard panels
were investigated. It was found that this adhesive resulted in the highest strengths at
the SF/NaOH/CA weight ratio of 9/0.3/1. Effects of hot-press temperature and hotpress time on strengths of the panels were also investigated. The MOR, MOE and IB
met the minimum industrial requirements of M-2 particleboard panels under the
following variables and conditions: hot-press temperature, 190 °C; hot-press time, 270
s; the adhesive usage of surface particles, 12 wt%; the adhesive usage of the core
particles, 10 wt%; and the targeted particleboard density, 0.80 g/cm3.
©Copyright by Kai Gu
May 21, 2010
All Rights Reserved
EVALUATION OF NEW FORMALDEHYDE-FREE,
SOY FLOUR-BASED WOOD ADHESIVES FOR MAKING PARTICLEBOARD
by
Kai Gu
A THESIS
Submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Presented May 21, 2010
Commencement June 2010
Master of Science thesis of Kai Gu presented on May 21, 2010.
APPROVED:
_____________________________________________________________________
Major Professor, representing Wood Science
_____________________________________________________________________
Head of the Department of Wood Science and Engineering
_____________________________________________________________________
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon
State University libraries. My signature below authorizes release of my thesis to any
reader upon request.
_____________________________________________________________________
Kai Gu, Author
ACKNOWLEDGEMENTS
An old Chinese idiom says: “The man who teaches me even for only one day
should be respected and treated as my father during my whole life”. At this moment, I
could only find this piece of sentence to express my sincere gratitude to Dr. Li for his
tutoring and help during my study period. His passion for academic pursuit and
intelligence in digging fountain from “dull and plain” knowledge mountain surface
first among other sparkling merits and definitely will guide me for my future life.
I also could not forget the happy time I spent with Milo Clauson who always
extends his hands when I was in need during my experiment and he really has a
golden heart for students. I confess: he is also really good at fixing broken stuffs.
Many former and current members in our group helped me a lot during last several
years and we are like a big family.
I also want to thank Dr. John A, Nairn, Dr. Willie E. Skip Rochefort and Dr HsiouLien Chen for being my committee members and spending their time reviewing my
thesis.
Here, I need acknowledge the financial support from USDA and Flakeboard for
granting wood furnish.
Last but not the least, I would like to thank my parents for their persistent support
and my fiancée Tang Min who always infuses me with love and lively passion for life.
TABLE OF CONTENTS
Page
CHAPTER 1 GENERAL INTRODUCTION ................................................................ 1
1.1 Definition and advantages of wood composites ................................................... 1
1.2 The production and application of wood composites ........................................... 2
1.2.1 Plywood ......................................................................................................... 2
1.2.2 Particleboard .................................................................................................. 2
1.2.2.1 Manufacture of particleboard .................................................................. 3
1.2.2.2 Adhesives used for making particleboard ............................................... 4
1.2.3 Oriented standboard (OSB) ........................................................................... 4
1.2.4 Medium-density fiberboard (MDF) ............................................................... 5
1.2.5 Composite lumber products ........................................................................... 5
1.3 Introduction of wood adhesives............................................................................ 6
1.3.1 Adhesives from petrochemicals ..................................................................... 7
1.3.1.1 Urea-formaldehyde (UF) resins .............................................................. 7
1.3.1.2 Melamine-formaldehyde resins (MF) ..................................................... 9
1.3.1.3 Phenol-formaldehyde resins (PF).......................................................... 10
1.3.1.4 Isocyanate adhesives ............................................................................. 11
1.3.1.5 Poly(vinyl acetate) (PVAc) adhesives .................................................. 13
1.3.1.6 Issues associated with synthetic wood adhesives.................................. 14
1.3.1.6.1 Dependence on petroleum and natural gas ..................................... 14
TABLE OF CONTENTS (Continued)
Page
1.3.1.6.2 Emission of carcinogenic formaldehyde ........................................ 14
1.3.2 Adhesives from natural resources ................................................................ 16
1.3.2.1 Animal adhesives .................................................................................. 16
1.3.2.2 Casein-based adhesives ......................................................................... 16
1.3.2.3 Blood-based adhesives .......................................................................... 17
1.3.2.4 Tannin-based adhesives ........................................................................ 17
1.3.2.5 Lignin-based adhesives ......................................................................... 18
1.3.2.6 Soy-based adhesives ............................................................................. 19
1.3.2.6.1 Production and composition of soybean......................................... 19
1.3.2.6.2 Soybean as a wood adhesive .......................................................... 21
1.3.2.6.3 Soy-based adhesives: their market and barriers ............................. 26
1.4 Reference ............................................................................................................ 28
CHAPTER 2 PREPARATION AND EVALUATION OF PARTICLEBOARD WITH
A SOY FLOUR-POLYETHYLENIMINE-MALEIC ANHYDRIDE ADHESIVE.... 35
2.1 Abstracts ............................................................................................................. 36
2.2 Keywords ............................................................................................................ 36
2.3 Introduction ........................................................................................................ 36
2.4 Materials and methods ........................................................................................ 38
2.4.1 Materials ...................................................................................................... 38
2.4.2 Preparation of SF-coated wood particles ..................................................... 38
TABLE OF CONTENTS (Continued)
Page
2.4.3 Preparation of PEI-MA-NaOH solution ...................................................... 39
2.4.4 Preparation of three-layer particleboard ...................................................... 39
2.4.5 Evaluation of mechanical properties of particleboard ................................. 40
2.4.5.1 Evaluation of mechanical properties of particleboard .......................... 40
2.4.5.2 Statistical analysis of data ..................................................................... 41
2.5 Results ................................................................................................................ 41
2.5.1 Effects of hot-press temperature on the mechanical properties of the
particleboard ......................................................................................................... 41
2.5.2 Effects of hot-press time on the mechanical properties of the particleboard
.............................................................................................................................. 43
2.5.3 Effects of core adhesive usage at different densities on the mechanical
properties of the particleboard .............................................................................. 45
2.5.4 Effects of surface adhesive usage on the mechanical properties of the
particleboard ......................................................................................................... 49
2.5.5 Effects of the moisture content of surface wood particles on the mechanical
properties of the particleboard .............................................................................. 51
2.5.6 Effects of adhesive composition on the mechanical properties of the
particleboard ......................................................................................................... 53
2.6 Discussions ......................................................................................................... 55
2.7 Conclusions ........................................................................................................ 59
2.8 Acknowledgements ............................................................................................ 59
2.9 References .......................................................................................................... 59
TABLE OF CONTENTS (Continued)
Page
CHAPTER 3 PREPARATION AND EVALUATION OF PARTICLEBOARD WITH
A SOY FLOUR-NEW CURING AGENT ADHESIVE ............................................. 61
3.1 Abstracts ............................................................................................................. 62
3.2 Key words ........................................................................................................... 62
3.3 Introduction ........................................................................................................ 62
3.4 Materials and methods ........................................................................................ 64
3.4.1 Materials ...................................................................................................... 64
3.4.2 Preparation of SF-coated wood particles ..................................................... 65
3.4.2.1 Wet method ........................................................................................... 65
3.4.2.2 Dry method ........................................................................................... 65
3.4.3 Preparation of CA-NaOH solution .............................................................. 66
3.4.4 Preparation of three-layer particleboard ...................................................... 66
3.4.5 Determination of mechanical properties of particleboard ........................... 67
3.4.5.1 Evaluation of mechanical properties of particleboard .......................... 67
3.4.5.2 Statistical analysis of data ..................................................................... 67
3.5 Results ................................................................................................................ 68
3.5.1 Effects of SF/CA weight ratio on the mechanical properties of the
particleboard ......................................................................................................... 68
3.5.2 Effects of NaOH usages on the mechanical properties of the particleboard 70
3.5.3 Effects of hot-press temperature on the mechanical properties of the
particleboard ......................................................................................................... 72
TABLE OF CONTENTS (Continued)
Page
3.5.4 Effects of hot-press time on the mechanical properties of the particleboard
.............................................................................................................................. 74
3.5.5 Effects of different preparation methods on the mechanical properties of the
particleboard ......................................................................................................... 76
3.6 Discussions ......................................................................................................... 78
3.7 Conclusions ........................................................................................................ 80
3.8 Acknowledgements ............................................................................................ 81
3.9 References .......................................................................................................... 81
CHAPTER 4 GENERAL CONCLUSIONS ................................................................ 83
BIBLIOGRAPHY ........................................................................................................ 84
LIST OF FIGURES
Figure
Page
1.1 Addition of urea and formaldehyde ......................................................................... 8
1.2 Condensation of hydroxymethylurea ....................................................................... 9
2.1 Effects of hot-press temperature on the MOR and MOE of particleboards ........... 42
2.2 Effect of hot-press time on the IB of particleboards. ............................................. 43
2.3 Effects of hot-press time on the MOR and MOE of particleboards. ...................... 44
2.4 Effect of hot-press time on the IB of particleboards .............................................. 45
2.5 Effect of the core adhesive usage on the MOR at a high density level and a low
density level. ................................................................................................................ 46
2.6 Effect of the core adhesive usage on the MOE at a high density level and a low
density level. ................................................................................................................ 48
2.7 Effect of the core adhesive usage on the MOE at a high density level and a low
density level. ................................................................................................................ 49
2.8 Effects of the surface adhesive usage on the MOR and MOE of particleboards ... 50
2.9 Effect of surface adhesive usage on the IB of particleboards ................................ 51
2.10 Effects of the moisture content of the surface wood particles on the MOR and
MOE of particleboards. ................................................................................................ 52
2.11 Effect of the moisture content of the surface wood particles on the IB of
particleboards. .............................................................................................................. 53
2.12 Effects of the adhesive composition on the MOR and MOE. .............................. 54
2.13 Effect of the adhesive composition on the IB ...................................................... 55
3.1 Effects of SF/curing agent ratio on the MOR and MOE of particleboards ............ 69
LIST OF FIGURES (Continued)
Figure
Page
3.2 Effect of SF/curing agent ratio on the IB of particleboards ................................... 70
3.3 Effects of NaOH / (SF + curing agent) ratio on the MOR and MOE of
particleboards. .............................................................................................................. 71
3.4 Effect of NaOH / (SF + curing agent) ratio on the IB of particleboards ................ 72
3.5 Effects of hot-press temperature on the MOR and MOE of particleboards ........... 73
3.6 Effect of hot-press temperature on the IB of particleboards .................................. 74
3.7 Effects of hot-press time on the MOR and MOE of particleboards ....................... 75
3.8 Effect of hot-press time on the IB of particleboards .............................................. 76
3.9 Effects of different preparation methods on the MOR and MOE of particleboards.
...................................................................................................................................... 77
3.10 Effect of different preparation methods on the IB of particleboards ................... 77
LIST OF TABLES
Table
Page
1.1 Source of raw materials for the production of major synthetic wood adhesives ... 14
1.2 Major components of soy proteins ......................................................................... 20
1.3 Composition of different soy protein products ...................................................... 21
1.4 Amino acid group chains involved in Chemical Modification .............................. 24
1
EVALUATION OF NEW
FORMALDEHYDE-FREE, SOY FLOUR-BASED ADHESIVES FOR MAKING
PARTICLEBOARD
CHAPTER 1 GENERAL INTRODUCTION
1.1 Definition and advantages of wood composites
Composites are defined as materials that are made up of distinct components while
attaining new properties out of these components. In this meaning, wood composites
are made of wood elements and other materials, such as adhesives. Wood composites
are classified in accordance with wood elements that are different in size and geometry.
Compared to the solid wood, wood composites can provide more uniform properties
because reduction of wood into small, relatively uniform particles, fibers, or flakes
averages out natural differences of wood. Wood composites with desired properties
can be manufactured by controlling the properties of the wood elements. Wood
composites can have superior resistance to fire, weathering and biological degradation
by adding different kinds of additives. Moreover, wood composites utilize the trees
more efficiently and turn a lot of production waste materials such as sawdust into
valuable products. Currently the most commonly used wood composites include
plywood, particleboard, oriented strandboard (OSB), medium density fiberboard
(MDF), and composite lumber products [1, 2].
2
1.2 The production and application of wood composites
1.2.1 Plywood
Plywood is a flat panel product made by bonding wood veneers together under heat
and pressure.
For overcoming the drawbacks of anisotropic properties of wood,
plywood is typically constructed with an odd number of layers with the grain direction
of adjacent layers being perpendicular to each other. There are two types of plywood:
structural softwood plywood and decorative hardwood plywood. Softwood plywood
is mainly used in structural applications such as roof, sub-floors, underlayment, wall
sheathing, and siding because softwood plywood is much stronger and stiffer than
hardwood plywood. Hardwood plywood is generally used for decorative purposes
such as cabinets, fixtures and furniture because hardwood could provide attractive
wood grains [2].
Phenol-formaldehyde (PF) resin is typically used for production of softwood
plywood that requires high strengths and high water resistance for structural uses,
while urea-formaldehyde (UF) resin is typically used for production of hardwood
plywood because of its low cost and light color [1-2]. Because of the high cost of
veneers the plywood market is expected to decline in the future.
1.2.2 Particleboard
Particleboard is manufactured by bonding homogeneous wood particles or waste
materials such as sawdust and shavings with adhesives to form a flat panel product
under heat and pressure. The most common particleboard has three layers including
3
two face layers and one core layer. The face layers consist of fine particles to get
smooth surfaces for overlaying, laminating, painting or veneering. The core layer
consists of coarse particles. Particleboard is less expensive to make than other wood
composites and can well utilize the waste woody materials. Another advantage of
particleboard is that it can be tailored to meet requirements of various applications.
However, particleboard is not as strong as solid wood and fiberboard. Particleboard is
mainly used for making furniture and kitchen cabinets [1-2].
1.2.2.1 Manufacture of particleboard
The first step of making particleboard is the preparation of wood particles.
Standard particleboard plants use combinations of chippers, hammermills, ring flakers,
ring mills and attrition mills to break the large pieces of wood into small particles.
Many plants buy raw woody materials in form of shavings, chips, mixed mill residues,
or sawdust. The specification for particle sizes is different for each particleboard plant,
but generally requires that the particles be slender for attaining high strength.
Fine
particles are used for face layers and coarse particles are used for the core layer. Once
particles are prepared, they are dried by means of rotary, disk or suspension drying.
Particle drying is a critical step in production of particleboard and the moisture content
of the particles leaving the dryer is usually in the range of 4% to 8%. After drying, the
particles are blended with an adhesive and additives. The adhesive-blended particles
typically have a final moisture content of near 10%. Then the adhesive-blended
particles are formed into a mat and pre-pressed to reduce mat thickness prior to hot
4
pressing. Hot-press temperature usually ranges from 140 °C to 165 °C, and pressure
in the range of 1.37 to 3.43 MPa for medium density particleboard [2].
1.2.2.2 Adhesives used for making particleboard
Most particleboard panels are used for interior applications and have lower
requirements in water resistance than those used for exterior applications. Therefore
urea-formaldehyde (UF) adhesive is most commonly used due to its low cost, short
curing time and light in color. The adhesive usage can range from 4 to 10% on the dry
weight of the particles. The adhesive usage of face layers is usually higher than that of
the core layer.
Other adhesives such as phenol-formaldehyde (PF) adhesive,
melamine-urea-formaldehyde (MUF) adhesive and isocyanate adhesives are also used
in some special cases [2].
1.2.3 Oriented strandboard (OSB)
OSB is a flat panel made with wood flakes (long and thin strip with a rectangular
shape) and adhesives under heat and pressure. OSB typically includes two face layers
and a core layer with longer flakes for face layers. The grain directions of wood flakes
are typically perpendicular between face and core layers. It costs less to produce OSB
than plywood, but OSB has similar mechanical properties to plywood. Therefore,
OSB has gradually replaced plywood in the market. OSB is mainly used in flooring,
wall and roof sheathing, furniture and industrial containers [2].
5
Two types of adhesives are currently used in the industry for making OSB.
Phenol-formaldehyde (PF) resin is used in the face layers and polyisocyanates (pMDI)
in the core layer.
1.2.4 Medium-density fiberboard (MDF)
MDF is a flat panel product made by bonding wood fibers with adhesives and
additives under heat and pressure. MDF has the density ranging from 640 to 800
g/cm3. MDF is usually produced with long fine wood fibers in a similar fashion to the
particleboard. When compared to particleboard and OSB, MDF is much stronger and
more dimensionally stable, but costs more as well [2].
MDF is widely used for furniture, flooring, interior door skins, moldings and
interior trim components. UF resin is the commonly used adhesive for making MDF.
1.2.5 Composite lumber products
Due to the decreasing availability of the high quality lumbers, a lot of lumber-like
composites (commonly called composite lumber) are now produced in a large quantity
every year. The commonly used composite lumber products include laminated veneer
lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL), and
oriented strand lumber (OSL).
LVL is produced by bonding many layers of veneers whose grains are parallel
under heat and pressure to form a board that is then cut into a lumber-like product.
The adhesives used for making LVL are PF resins and isocyanates. LVL is mainly
used for headers of garage doors, large windows and flanges in I-beams [3].
6
PSL is made from strands whose grains are parallel to the length of the final
product. The strands are prepared from veneers with the dimension of about ¾ inches
wide and at least 24 inches long. Exterior-type plywood adhesives such as PF are
applied onto the strands and a microwave type heating system is used to cure the
adhesives. After pressing the resulting board is cut into the small pieces with desired
dimensions.
The resulting PSL can replace the high-strength lumber or timber
materials [3].
LSL is very similar to PSL, but there are three major differences. LSL is made
from smaller strands with 12 inches long. The isocyanate resin is used to provide the
light-colored gluelines. A steam-injection press system is used to provide uniform
density of LSL. The major market of LSL is where clear lumbers are desired such as
wall studs, beams, columns and windows [1].
OSL is made with bonding nominal 12-inch-long strands with adhesives. In many
ways the production of OSL is similar to that of OSB except that the grains of strands
to making OSL are aligned in the same direction. For reducing production time a
steam-injection pressing is used. OSL is mainly used for making furniture [1].
1.3 Introduction of wood adhesives
Wood adhesives are essential components of wood composites. In the long history
of human civilization people gradually learned how to make adhesives for bonding
different materials together for their needs. As for wood adhesives, the history also
witnessed a development from primitiveness to high industrialization. In 2001, the
7
worldwide wood adhesive consumption was 13.3 million tons and total sale value
reached $6.1 billion [4]. Since 2001 the global wood composite output has increased
steadily which means an increasing consumption of wood adhesives. Wood adhesives
can be broadly classified as petrochemical-based and natural-material-based adhesives.
1.3.1 Adhesives from petrochemicals
1.3.1.1 Urea-formaldehyde (UF) resins
UF resins are one of the most important thermosetting adhesives in the world. UF
resins are prepared through the reaction of two simple chemicals: urea and
formaldehyde. The synthesis of UF resins takes two stages: addition (methylolation)
and condensation as shown in Figure 1.1 and Figure 1.2. In the first stage, urea is
hydroxymethylated by the addition of formaldehyde to amino groups of urea. A
mixture of mono-, di-, trihydroxymethylureas are generated. Tetrahydroxymethylurea
is almost undetectable [5]. The pH is generally controlled at 8-9. In the second stage,
the methylolated urea derivatives polymerize to provide the UF resins in the presence
of an acid catalyst, usually ammonium chloride. A wide range of conditions such as
molar ratio between urea and formaldehyde, reaction temperature, reaction time, and
pH can be modified to make UF resins for different end uses. Advantages of UF
resins include high reactivity, short hot-pressing time, clear glueline, cold tack ability,
aqueous system, no flammability and low price. However, UF resins have low water
and weather resistance because aminomethylene linkages are susceptible to hydrolysis
and therefore is not stable at high humidity and temperature. Another disadvantage of
8
UF resins is the emission of formaldehyde due to the unreacted formaldehyde and the
hydrolysis of methylene ether linkages (-CH2-O-) [6]. Extensive efforts have been
devoted to the reduction of formaldehyde emission. At present, UF resins are still
widely used for making interior plywood, particleboard and MDF.
Figure 1.1 Addition of urea and formaldehyde*
9
Figure 1.2 Condensation of hydroxymethylurea*
*Adopted from Encyclopedia of Materials: Science and Technology (A. H. Conner,
2001) [5].
1.3.1.2 Melamine-formaldehyde resins (MF)
MF resins are also important thermosetting adhesives in the wood composites
industry. The reactions between melamine and formaldehyde are very similar to those
between urea with formaldehyde, first the addition reaction and then the condensation
reaction.
However, the addition of formaldehyde to melamine occurs more easily
than that of formaldehyde to urea and the residual formaldehyde content in the MF
resins is much lower than that in the UF resins [7]. This is the reason why melamine
is often added into UF resins as a formaldehyde scavenger. The molar ratio of
formaldehyde to melamine is generally at 1.2 to 3.
The good stability of the
symmetrical triazine ring makes the MF adhesives very resistant to water once the
resin has been cured to the insoluble cross-linked state [8]. MF is mainly used for
10
making exterior or semi-exterior grade plywood and particleboard. MF is also used
for the impregnation of paper sheets in the production of overlays for the surface of
wood composites and laminates due to its hardness and transparency [8]. However,
melamine is much more expensive than urea. Melamine is often added to UF to make
melamine-urea-formaldehyde (MUF) adhesives. MUF is more water resistant than UF.
MUF is also used for exterior applications such as finger joints and edge-glued lumber.
1.3.1.3 Phenol-formaldehyde resins (PF)
PF resins are reaction products between phenol and formaldehyde. Depending on
the pH value of catalysts, PF resins could be classified as resol resins and novolac
resins.
As for resol resins, usually but not necessarily, a molar excess of formaldehyde is
used.
By using a basic catalyst, the formaldehyde attacks the ortho- and para-
positions of phenol to form methylol phenol. These methylolated phenol derivatives
are able to further react with each other or phenol to form methylene linkages (-CH2-)
or methylene ether bonds (-CH2-O-CH2-). The most important point in resol resins is
that, when an excess of formaldehyde is used, a sufficient number of methylol groups
remain reactive to complete the polymerization. As consequence, resol resins are
cured by elevating temperature and no additional formaldehyde is needed. Therefore
the resol resins are commonly referred as one-step resins. Although the resol resins
polymerize quickly at a high temperature, they also can continue the polymerization
reaction at ambient temperatures so they have limited shelf lives [9].
11
Novolac resins are made through the reactions between formaldehyde and phenol
in the presence of an acidic catalyst. Methylol phenol derivatives are first formed and
then further react with additional phenol to create a methylene bridge at either the
ortho- or the para-position of the phenolic aromatic ring. The reactions stop when the
formaldehyde reactant is exhausted, leaving some un-reacted phenol. Novolac resins
are thermoplastics. For use as wood adhesives, a cross-linking agent is usually added
to cure the novolac resins.
The most commonly used cross-linking agent is
hexamethylene tetramine (HEXA). Therefore, novolac resins are also referred as twostep resins [9].
PF resins are mainly used for exterior application for structural plywood, OSB and
LVL because they have good water and weather resistance. The reactions between
phenol and formaldehyde are not reversible; cured PF resins thus contribute little to
the formaldehyde emission. The formaldehyde emission is mainly from residual
formaldehyde in PF resins. PF resins have dark gluelines which are not desirable for
interior or decorative applications [10-11].
1.3.1.4 Isocyanate adhesives
Isocyanates are loosely referred to all compounds containing an isocyanate group (N=C=O). The isocyanate group has a very high reactivity with groups that contain
reactive hydrogen, such as amino and hydroxyl groups at room temperature. Those
compounds having two isocyanate groups are called diisocyanates. Today, the most
commonly used isocyanates are polymeric diphenylmethane diisocyanate (pMDI) in
12
the wood composites industry. PMDI is a mixture of monomeric diphenylmethane
diisocyanate and methylene-bridged polyaromatic polyisocyanates [12].
The isocyanate group first reacts with water to form an unstable carbamic acid that
easily gives off carbon dioxide to from an amine that can react with another isocyanate
group to form a biuret as illustrated below [13].
Isocyanate group reacts with water:
R-NCO + H2O
R-NH-C(=O)-OH
Carbamic acid gives off carbon dioxide:
R-NH-C(=O)-OH
R-NH2 + CO2
Amine reacts with another isocyanate:
R-NH2 + R-NCO
R-NH-C(=O)-NH-R
The whole curing process proceeds very rapidly and forms a complex cross-linking
network. Isocyanates may form covalent linkages with wood through urethane bonds
[12].
Advantages of pMDI resins include their rapid curing and their ability to form
adhesive bonds in the presence of high moisture content.
Therefore, pMDI is
commonly used as a core resin for the production of OSB because the core has a lower
temperature than the faces during hot-pressing. PMDI can also bond wood to metal
plates such as hot-press platens very well. A releasing agent has to be used to coat the
hot-press platens if pMDI is used as a face resin in the production of OSB.
Isocyanates are volatile and hazardous. Special protective measures have to be taken
13
when pMDI is used for making OSB. Once pMDI is cured, the resulting OSB panels
are safe to use. PMDI is much more expensive than UF or PF resins [14].
1.3.1.5 Poly(vinyl acetate) (PVAc) adhesives
Until 1950s UF resins were widely used for bonding joints in the furniture industry.
However, they were gradually replaced by poly(vinyl acetate) (PVAc) adhesives
because of the long curing time of UF resins at ambient temperature. PVAc adhesives
are commonly known as „„white glue‟‟. They are manufactured via a conventional
radical polymerization technique in the presence of catalysts [15].
PVAc adhesives
are odorless, nonflammable emulsions. They set to coalesce to form a continuous
solid film after water evaporates. The whole process takes about 15 min at room
temperature and no hot-pressing is required. Therefore their application is very easy
and they do not damage tools of processing the PVAc-bonded products. However,
PVAc adhesives are thermoplastic and soften when heated, and their bond strength is
very dependent on the temperature used. They lose bonding strengths at over 70 °C
[15-18]. For improving their moisture and temperature resistance, PVAc adhesives
are incorporated with a crosslinkable monomer such as N-(hydroxymethyl)acrylamide
[19]. PVAc adhesives are very versatile, and they are supplied in different forms for
various applications such as veneering, edgebanding and jointing in furniture
production [15].
14
1.3.1.6 Issues associated with synthetic wood adhesives
1.3.1.6.1 Dependence on petroleum and natural gas
There are many concerns about synthetic wood adhesives although they are
predominantly used in the production of wood composites due to their superior
properties
For example, raw materials of these adhesives heavily rely on the
petroleum resources. The detailed raw material sources are listed in Table 1. 1.
Table 1.1 Source of raw materials for the production of major synthetic wood
adhesives*
*Adopted from Encyclopedia of materials (A. Conner, 2001) [5].
However, the total petroleum reserve in the world is limited and the price for the
crude oil and natural gas will continue to hike in a long run. Being very similar to
many industries that depend on petroleum and natural gas, wood composites industry
also needs to find a new way for future sustainable growth.
1.3.1.6.2 Emission of carcinogenic formaldehyde
Formaldehyde is a colorless, pungent-smelling gas which can causes watery eyes,
burning throat, nausea, and difficulty in breathing for humans at certain level. High
15
concentrations may trigger attacks in people with asthma [20]. In June of 2004 the
International Agency for Research on Cancer reclassified formaldehyde as a human
carcinogen [21]. Exposure to formaldehyde in indoor environments first gained public
attention in the early 1980s, in large part due to problems with mobile homes [22].
The most recent case involved in formaldehyde emission was that people living in
government-provided trailers after Hurrican Katrina reported adverse health effects,
including headaches, running noses, chronic respiratory problems and nose bleeds [23].
At present formaldehyde-based wood adhesives are predominantly used in interior
wood composites, such as cabinets, floorings, furniture, etc. Formaldehyde is the
main ingredient for making UF and PF resins. As for UF resins, formaldehyde is
emitted from the hydrolysis of unstable methylene-ether bonds in the presence of
water or at elevated temperatures.
As for PF resins, formaldehyde released is
generally from residual free formaldehyde in the adhesives [24]. Although a lot of
efforts have been devoted to solving this problem, the issue of formaldehyde emission
still persists especially for the UF resin application.
In 2007, the California Air Resources Board (CARB) passed a regulation for
reducing public exposure to formaldehyde. This regulation set new formaldehyde
emission limits from wood composites in two phases. The phase I implementation has
already been in force since January 1, 2009 [25]. Additionally, US Environment
Protection Agency (EPA) is also considering regulatory action on the national level to
protect against risks posed by formaldehyde emitted from wood composites [26-27].
16
1.3.2 Adhesives from natural resources
1.3.2.1 Animal adhesives
Animal glues are collagen derivatives that are made from animal hide, connective
tissues and bones. Animal glues are in essence protein-based adhesives [28]. The
general manufacturing procedure is as follows: animal hides are first soaked in water
and treated with lime. The hides are then rinsed to remove the lime, and a weak acid
solution is used to neutralize any residual lime. The hides are heated in water to
certain temperature to make the glue. The liquor glue is also dried and chopped into
pellets for storage [29].
The animal glue was once the most commonly used
woodworking glue for thousands of years until it was replaced by synthetic adhesives
in the 20th century because of their many undesirable properties such as poor moisture
resistance, susceptibility to biological degradation, and a relatively high price [30].
Animal adhesives are still used in high-quality furniture and critical applications such
as pianos.
1.3.2.2 Casein-based adhesives
Casein is prepared from milk. It precipitates when the milk is acidified to about pH
4.5 when its solubility is the minimum. The casein is held together by calcium ions
and hydrophobic interactions. It is readily dispersible in dilute alkaline and salt
solution such as sodium oxalate and sodium acetate and after water is removed the
residue forms as an adhesive [29]. The viscosity and consistence of a casein adhesive
can be altered by a number of protein denaturants such as sulfur compounds,
17
formaldehyde donors or complex metal salts [30-31]. Casein was perhaps the first
adhesive used for structural wood composites. Now casein is still used in bonding
door skins with linseed oil being added for improving the surface quality [31].
1.3.2.3 Blood-based adhesives
New technology was developed in 1910 to dry liquid blood without damaging its
protein while maintaining its solubility [32]. After that the blood could be collected,
transported and processed for industrial applications. Blood adhesives once played an
important role in World War I and II because there was an urgent need for water
resistant plywood in aircraft manufacturing. It was subsequently replaced by PF resins
in 1970s [33]. Blood-based adhesives are very heat sensitive, resulting in a very short
hot-pressing curing time [34]. At present, animal blood, in form of solid powder, is
used as an essential ingredient for foaming adhesives in the production of industrial
and structural plywood [32]. However, animal blood tends to degrade rapidly and has
a limited number of suppliers [35].
1.3.2.4 Tannin-based adhesives
There are two types of tannins: hydrolysable tannins and condensed tannins. The
hydrolysable tannins are composed of simple phenols and esters of a sugar with gallic
and digallic acids [36].
Its industrial uses are limited due to its lack of
macromolecular structure and limited worldwide production. The condensed tannins
account for more than 90% of the world production of commercial tannins and are
chemically and economical meaningful for making adhesives and resins [37]. Tannin-
18
formaldehyde adhesives are obtained by polycondensation of condensed tannins with
formaldehyde.
However, the purity of the condensed tannin extracts varies
considerably and most extracts have a large fraction of other substances which do not
react with formaldehyde. It also has been proved that it is very difficult and expensive
to refining these extracts so some fortifiers are added into the extracts to make
adhesives. Fortifiers include some common synthetic adhesives such as PF, UF and
MDI [38-42]. The tannin extracts generally have a high viscosity so a series of acid or
alkaline treatment are needed to tailor the viscosity of tannin-based adhesives for
making exterior-grade particleboard [41]. Tannin-based adhesives are rarely used in
North America wood composites industry due to a limited resource, but in southern
hemisphere they have been widely used.
1.3.2.5 Lignin-based adhesives
Lignin is one of the main components of wood and it is the second most abundant
natural polymer in the world. Lignin is mainly recovered as a waste product in pulp
mills. A lot of researches have been done to utilize this material, but worldwide largescale applications are still at a very low level. At present, most of spent liquor in pulp
mills is burned for recovering pulping chemicals and heat.
Although lignin is
composed of phenylpropane units, it is in fact a very complex polymer and does not
react like a simple phenol molecule. In the past a lot of condensation reactions were
investigated for crosslinking lignin, but these reactions were always associated with
high temperature and strong mineral acids which all had negative effects on the wood
19
particles [43-44]. Hydrogen peroxide was also used to replace strong mineral acids
through free radical condensation reactions, but use of peroxides was not favored in
wood composites plants [45]. Formaldehyde was also applied for crosslinking lignin
but a long curing time was unacceptable in real practice [46].
Other works
concentrated on partial substitution of phenol with lignin in making PF resins. It was
found that methylolated lignin could replace up to 30% PF in making plywood under
regular hot-press conditions without negative effects of the adhesive strength [47]. At
present, some modified lignin is used for the partial replacement of phenol.
1.3.2.6 Soy-based adhesives
1.3.2.6.1 Production and composition of soybean
Soybean, originated from eastern Asia, is one of the most important crops in the
USA. Soybean oil and soybean meal are two major products of soybean. At present,
soybean meal is mainly used as animal feed. In 2008, about 29.1 million metric tons
out of 38.4 million metric tons produced in the US was used to feed animals and
poultry. Only about 1.0 million metric tons was applied for human food and industrial
uses [48].
Soybean consists of about 40 wt% protein, 21 wt% oil, 34 wt% carbohydrate and
4.9 wt% ash [49]. Soybean oil is mainly composed of saturated and unsaturated
triglycerides. Soybean carbohydrates consist of the complex polysaccharides including
cellulose, hemicelluloses, and pectin. Soybean proteins are mainly storage proteins
that provide amino acids during seed germination and protein synthesis. The amino
20
acids of the storage protein are linked by amide bonds into polypeptide chains. The
polypeptide chains are associated and entangled into a three-dimensional complicated
structure by disulfide and hydrogen bonds [50]. Most soy proteins are globulins,
containing about 25% acidic amino acids (aspartic acid and glutamic acid), 20% basic
amino acids (lysine and arginine), and 20% hydrophobic amino acids (alanine, valine
and leucine). Aspartic acid and glutamic acid account for almost 30% of all amino
acids in soy protein [51].
Based on sedimentation coefficients obtained from
centrifugation [52], soy protein could be subdivided into 4 main components which
are listed in the Table 1.2.
Table 1.2 Major components of soy proteins*
Fraction Content (%)
Principal component
11S
52
Globulins
7S
35
Lipoxygenase, amylase, globulins
15S
5
Polymer
2S
8
Trypsin inhibitor, cytochrome
*Adopted from Functional properties of soybean proteins (Kinsella, 1979) [52]
Based on protein content, the soybean products are classified as: flour and grits
(40-55 wt% protein); protein concentrate (70 wt% protein); soybean protein isolate
(>90 wt% protein) [53]. The composition of all three soybean products is given in
Table 1.3.
21
Table 1.3 Composition of different soy protein products*
g/100 g
Soy
Soy protein concentrate
Soy protein isolate
product
flour
(SPC)
(SPI)
Protein (as is)
48
64
92
Fat (min)
0.3
0.3
0.5
Moisture (max)
10
10
<5
Fiber (crude)
3.0
4.5
<1
Ash
7
7
4
Carbohydrate
31-32
14-15
-
*Adopted from Functional properties of soybean proteins (Kinsella, 1979) [52]
1.3.2.6.2 Soybean as a wood adhesive
Soy-based adhesives were widely used in the production of wood composites from
the 1930s to the 1960s [54]. Those soy-based adhesives were inexpensive and easy to
make and handle. However, wood composite panels bonded with these soy-based
adhesives had relatively poor strength, poor water-resistance, and poor biological
stability, and require long press times for their production. These drawbacks led to
their replacements by petroleum-based adhesives.
As discussed previously,
petroleum-based adhesives have their own problems such as formaldehyde emission
and heavy dependence on petroleum and natural gas. With the gradual depletion of
the world oil reserve and ever-increasing concern about the environment and public
22
health, soy-based adhesives gained renewed interest in recent years. Currently there
are four approaches for developing soy-based adhesives.
The first approach is the addition of soy flour to the currently used synthetic
adhesives, so the usage of the synthetic adhesives is reduced. For example, soy flour
was added into PF or phenol-resorcinol-formaldehyde (PRF) adhesives to make hybrid
adhesives [55-62].
Unless the soy flour could replace synthetic resins to a large
portion while maintaining required mechanical properties of wood composites, the
wood composite industry is still heavily dependent upon petrochemicals.
The second approach is the use of soy flour as a foaming agent in PF resins for
making plywood. Animal blood protein is currently the main source of foaming
agents, but animal blood is prone to degradation and may cause some diseases if
inhaled by people. Research found that soy flour had good foaming properties and
adhesive strengths were at least equal to those of the control adhesive without any
foaming agentThe cost of the final adhesive was lower than the current adhesives
using blood-based foaming agents [35].
The third approach is that soybean protein is first denatured with different chemical
and enzymatic treatments and then used as wood adhesives. Denaturation is referred
to a process that changes the secondary, tertiary, or quaternary structures of a protein
molecule [63]. Improvements have been reported on the adhesive strength and water
resistance of small veneer laminates made in laboratories using soybean protein
samples that were denatured with alkali, protease, urea, guanidine hydrochloride, and
23
sodium dodecyl sulfate (SDS)[63-67]. However, none of these modified soybean
protein has been used in commercial production of wood composite panels such as
plywood. The mechanisms by which denaturing protein could improve the strength
and water-resistance of the resulting wood composites have been proposed as follows:
most soy proteins are globulins which mean that the soy protein has a very compact
structure.
Protein chains are held together by disulfide bonds, hydrogen bonds,
electrostatic attraction between oppositely charged groups, and hydrophobic
interactions [68]. The compact structure of protein, after denaturation, is unfolded and
some hydrophobic amino acids are exposed outward instead buried inside. Therefore,
the modified soy protein has higher hydrophobicity so as to enhance its water-resistant
properties [69]. The proposed mechanisms are flawed. The denaturing processes may
increase the hydrophobicity of the denatured protein.
However, the increased
hydrophobicity may slow down water penetration, but cannot make the adhesive more
water-resistant.
The fourth approach is the use of a cross-linking agent to cure the soybean protein.
Crosslinking is a process of chemically joining two or more molecules by a covalent
bond.
Soybean protein contains many different amino acids, and a number of
functional groups on the amino acid side chain are available for chemical modification
(Table 1.4). Among them some functional groups are more accessible than the others.
24
Table 1.4 Amino acid group chains involved in Chemical Modification*
Side chain
Chemical modification
Amino
Acylation, alkylation
Carboxyl
Esterification, amidation
Disulfide
Oxidation, reduction
Side chain
Chemical modification
Sulfhydryl
Oxidation, alkylation
Side chain
Chemical modification
Thioether
Oxidation, alkylation
Phenolic
Acylation, electrophilic substitution
Imidazole
Oxidation, alkylation
Indole
Oxidation, alkylation
*Adopted from Food protein properties and characterization (Shuryo Nakai, et al,
1996) [70].
Epoxy resins are good cross-linking agents for curing soy protein. The main
mechanism is that an epoxy functional group can react with amino, carboxyl or
hydroxyl groups in soy protein. Epoxy functional groups can be imparted onto the soy
protein before curing the adhesives [71-72], or epoxy-group containing materials, such
as epoxidized soybean oil is mixed with soy protein before cuing [73].
The
crosslinking reactions require high curing temperature and long curing time, which
make these epoxy-soy adhesives not suitable for making wood composite panels. In
25
addition, these adhesives are much more expensive than the currently used synthetic
adhesives.
Aldehydes can also react with amino, and hydroxyl groups of soybean protein.
They crosslink and denature soy protein, thus resulting in enhanced water-resistance,
increased pot life, improved assembly time tolerance, and increased water-holding
capacity [74]. Examples of compounds of this group include: tris-hydroxymethyl
nitromethane, dimethylol urea, aldehydic starch, glyceraldehyde, hexamethylene
tetramine, urea–formaldehyde, methylolated phenols [75].
However, linkages from
the reactions between aldehydes and soy protein are reversible under hydrothermal
conditions, so the final uses of these soy-based adhesives are still limited.
Inspired by the fact that some marine animals, such as mussels, can stick
themselves to the rock and other substances in seawater by secreting a glue called the
marine adhesives protein (MAP) [76], the researchers found that if the soy protein
isolate (SPI) was imparted with a 3, 4-dihydroxyphenyl group or a mercapto group
which were abundant in MAP the strength and water-resistance of wood composites
bonded with modified SPI could be greatly improved [77-78].
Wood composites bonded with SPI-PAE (a polyamidoamine-epichlorohydrin
adduct) adhesive had shear strengths comparable to or higher than those bonded with
commercial urea-formaldehyde resins. Wood composites bonded with this new
adhesive system had high water resistance and retained relatively high strength even
after they had undergone a boiling-water test [79]. This new soy-based adhesive has
26
been successfully used to replace urea-formaldehyde resins for the commercial
production of hardwood plywood since 2004.
Another formaldehyde-free soy-based adhesives consisting of soy flour,
polyethylenimine (PEI), maleic anhydride (MA) and sodium hydroxide has recently
been developed for making interior plywood [80]. At present, this adhesive is still too
expensive to be commercially viable because of the high price of PEI. However this
research represents a new way of developing soy-based adhesives.
1.3.2.6.3 Soy-based adhesives: their market and barriers
At present, there are three soy-based adhesives on the market [81]. The first soybased adhesive is soy-phenol-formaldehyde resins. In this adhesive, low cost soy
flour is converted into a soy hydrolyzate and then mixed with PF reins. The main
advantage of this adhesive system is that the inexpensive soy flour could replace up to
40% more expensive phenol without negative impacts on the properties of the final
wood composites. This adhesive could be used in softwood plywood, OSB and
engineered wood products. The second soy-based adhesive involves the use of soy
flour in a foamed PF glue for LVL. The soy flour is used to replace animal blood as a
foaming agent for PF resins. The benefits of this technology include a reduction of
adhesive usage and increase in user safety. The third soy-based adhesive system is
based on soy flour and PAE. This soy-based adhesive is easy to prepare, and can be
used under commonly used hot-press conditions in the industry.
adhesive is truly formaldehyde-free and environmentally friendly [82].
Moreover, this
27
These soy-based adhesives either reduce formaldehyde emissions from UF bonded
wood composites or reduce the cost of using PF in structural wood composites such as
OSB and softwood plywood. The continued success of soy-based adhesives depends
on relative prices of petrochemicals and soybean flour and regulations of
formaldehyde emission. The prices of petrochemicals for making UF and PF resins
have been volatile in recent years and are expected to increase in the future. The price
of the soy meal/soy flour has been relatively stable in the past twenty years and will
increase, but typically at a slower pace than petrochemicals. At some particular time
when the price of soybean flour is significantly higher than UF and PF resins, the soybased adhesives may lose their competitive edges. Regulations on formaldehyde
emission from wood composites also have significant impacts on the market share of
the soy-based adhesives.
28
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74. Lambuth, A.L., 1977. Soybean glues. In: Skeist, I. (Ed.), Handbook of
Adhesives, 2nd ed. Van Nostrand, New York, pp. 172–180.
34
75. Rakesh Kumar, Veena Choudhary, Saroj Mishra, I.K. Varma, Bo Mattiason,
Adhesives and plastics based on soy protein products, Industrial Crops and
Products 16 (2002) p. 155-172.
76. Waite, J.H., Nature‟s underwater adhesive specialist. International Journal of
Adhesion and Adhesives, 1987. 7: p.9-14.
77. Liu, Y. and K. Li, Modification of soy protein for wood adhesives using mussel
protein as a model: The influence of a mercapto group. Macromolecular Rapid
Communications, 2004. 25(21): p. 1835-1838.
78. Liu, Y. and K. Li, Chemical modification of soy protein for wood adhesives.
Macromolecular Rapid Communications, 2002. 23(13): p. 739-742.
79. Li, K., S. Peshkova, and X. Geng, Investigation of soy protein-Kymene adhesive
systems for wood composites. Journal of the American Oil Chemists' Society,
2004. 81(5): p. 487-491.
80. Jian Huang, K. Li, A new soy flour-based adhesive for making interior type II
plywood, Journal of the American Oil Chemists‟ Society, 85(2008). P. 63-70.
81. Leland Orr, Wood adhesives: a market opportunity study, United Soybean Board,
2007.
82. Market opportunity summary: soy-based adhesives, United Soybean Board,
2008.
35
CHAPTER 2 PREPARATION AND EVALUATION OF PARTICLEBOARD WITH
A SOY FLOUR-POLYETHYLENIMINE-MALEIC ANHYDRIDE ADHESIVE
36
2.1 Abstracts
A soy-based formaldehyde-free adhesive consisting of soy flour (SF),
polyethylenimine (PEI), maleic anhydride (MA) and NaOH was investigated for
making three-layer particleboard.
7/1.0/0.32/0.1.
The weight ratio of SF/PEI/MA/NaOH was
Hot-press temperature, hot-press time, particleboard density and
adhesive usage were optimized in terms of enhancing the modulus of rupture (MOR),
modulus of elasticity (MOE) and internal bond strength (IB) of the resulting
particleboard. The MOR, MOE and IB met the minimum industrial requirements of
M-2 particleboard under the following variables: hot-press temperature of 170 °C, hotpress time of 270 s, the adhesive usage of surface particles at 10 wt%, the adhesive
usage of the core particles at 8 wt%, and the targeted particleboard density of 0.80
g/cm3.
2.2 Keywords
Wood adhesive ∙ Particleboard ∙ Soy flour ∙ Polyethylenimine
2.3 Introduction
Particleboard is mainly composed of wood particles and an adhesive. The most
commonly used particleboard has three layers: two face layers and one core layer.
The face layers consist of fine particles, and the core layer is made of coarse particles.
Wood particles are first coated with an adhesive, and then formed into a mat that is
further hot-pressed to form a panel product [1]. At present, urea-formaldehyde (UF)
resin is the most commonly used adhesive for making particleboard. Formaldehyde is
37
considered to be carcinogenic to human [2].
Formaldehyde is emitted in the
production and use of particleboard bonded with UF resins. California Air Resource
Board has passed a tough regulation on limiting formaldehyde emission from wood
composite panel products sold and used in California. The phase II standard for
particleboard will take effect on January 1, 2011. Particleboard bonded with the
currently used UF resins may not be able to meet the phase II standard. Therefore,
there is an urgent need for development of a commercially viable formaldehyde-free
adhesive for making particleboard.
Soy flour is abundant, renewable, and inexpensive. The soy-based adhesive
was widely used for the commercial production of plywood from 1930s to 1960s and
then replaced by synthetic resins such as UF resins because of the low water-resistance
of plywood panels bonded with the soy-based adhesive [3]. There has been a renewed
interest in recent years for the development of water-resistant soy-based wood
adhesives. Through mimicking mussel adhesive protein, several formaldehyde-free
soy-based adhesives have been developed and one of the soy-based adhesive has been
used to replace UF resins for the commercial production of plywood since 2004 [4-6].
One of the newly developed formaldehyde-free adhesives consists of soy flour (SF),
polyethylenimine (PEI), and maleic anhydride (MA). This SF-PEI-MA adhesive was
successfully used for preparation of type II plywood [7].
This adhesive was
investigated in this study to see if it could be used for production of particleboard.
38
2.4 Materials and methods
2.4.1 Materials
SF (7% moisture content) was provided by Cargill Incorporated (Minneapolis, MN,
USA); a 50 wt% aqueous PEI solution (Mw = 750,000) and MA were purchased from
Sigma-Aldrich (Milwaukee, WI, USA). Douglas fir wood particles (surface and core
wood particles) were obtained from Flakeboard (Albany, OR).
2.4.2 Preparation of SF-coated wood particles
The following is a representative procedure for the preparation of SF-coated wood
particles. For coating surface wood particles, SF (175 g, dry weight) and water (696 g)
were sequentially added into a HOBART A-200 blender (Hobart, Topeka, KS) and
stirred for 10 min at room temperature to form 20% SF slurry. Surface wood particles
(1403 g, dry weight) were then added into the SF slurry in the HOBART blender and
the resulting mixture was stirred for 10 min. The resulting wet SF-coated wood
particles were put in a cloth bag, and dried in a rotary dryer (Speedqueen, Ripon, WI)
for 1 h. After drying, the SF-coated surface wood particles had the moisture content of
2%. SF-coated core wood particles (2% moisture content) was prepared by following
the same procedure as for the preparation of the SF-coated surface wood particles
using SF (245 g dry weight), water (898.9 g), and core wood particles (2456 g, dry
weight).
39
2.4.3 Preparation of PEI-MA-NaOH solution
The following was a representative procedure for the preparation of PEI-MANaOH solution. MA (19.2 g) and water (100 g) were added into a 400 mL beaker, and
stirred with a magnetic stirrer for 30 min. The resulting MA solution (50 g), the PEI
solution (50 g), and 1 N NaOH (65 g) were added into a KitchenAid mixer, and stirred
for 6 min. The resulting solution was then sprayed onto the oven-dried SF-coated
surface wood particles (1578 g, dry weight) in a rotary drum-blender. The resulting
adhesive-coated surface wood particles had the adhesive usage of 15 wt% (the
adhesive usage was defined as the dry weight of SF, PEI, MA and NaOH divided by
the dry weight of the wood particles) and had the moisture content of 9%. A mixture
of the PEI solution (70 g), the MA solution (69.2 g), and 1 N NaOH (90 g) was
sprayed onto the oven-dried SF-coated core wood particles (2701 g, dry weight) in the
rotary drum-blender.
The resulting adhesive-coated core wood particles had the
adhesive usage of 12% and had the moisture content of 9%. Adhesive-coated wood
particles with a different adhesive usage were prepared in accordance with the
previous described procedures while SF/PEI/MA/NaOH dry weight ratio was
maintained at 7/1/0.32/0.1.
2.4.4 Preparation of three-layer particleboard
The adhesive-coated surface wood particles (807 g, dry weight) were added onto a
forming box (60.96 cm × 60.96 cm) by hand to form a uniform layer. The adhesivecoated core wood particles (2748 g, dry weight) were evenly distributed on top of the
40
surface wood particles layer, followed by a uniform layer of the adhesive-coated
surface wood particles (807 g, dry weight). The resulting three-layered wood particle
mat was hand-pressed with a flat plywood panel (60.96 cm × 60.96 cm) and then hotpressed at 170 °C for 300 s. The target thickness was 17 mm. The hot-press time and
hot-press temperature were changed when their effects on particleboard properties
were investigated. Two particleboard panels were prepared for each experimental
variable.
2.4.5 Evaluation of mechanical properties of particleboard
2.4.5.1 Evaluation of mechanical properties of particleboard
Particleboard was cut into 7.62 cm × 46.99 cm rectangular specimens for the
measurement of modulus of rupture (MOR) and modulus of elasticity (MOE), and into
5.08 cm × 5.08 cm specimens for the measurement of internal bond strength (IB) in
accordance with ASTM D1037-99 (American Society for Testing and Materials, 1999)
using a MTS Sintech 1/G testing machine (MTS Systems Corp., Eden Prairie, MN).
The crosshead speeds were 9 mm/min for the measurement of MOR and MOE and 1.5
mm/min for the measurement of IB. For each particleboard panel, four specimens
were obtained for the measurement of MOR and MOE, and eight specimens for the
measurement of IB.
41
2.4.5.2 Statistical analysis of data
All data were analyzed with Welch modified two-sample t-test with a S-PLUS®
statistical software package (Edition version 8.0, Insightful Corp., Seattle, WA, USA).
All comparisons were based on 95% confidence interval.
2.5 Results
2.5.1 Effects of hot-press temperature on the mechanical properties of the
particleboard
Effects of hot-press temperature on the MOR and MOE of particleboard panels are
shown in Fig. 2.1. At all hot-press temperatures studied, the MOR exceeded the
minimum industrial requirement of 14.5 MPa (horizontal solid line) for M-2 Grade
particleboard. The average MOR value significantly increased by 23.7% when hotpress temperature was raised from 160 to 170 °C. In the range of 170 to 190 °C the
MOR did not significantly change. The MOR at 200 °C was statistically higher than
that at other hot-press temperatures except that at 180 °C. At all hot-press
temperatures studied, the MOE exceeded the minimum industrial requirement of 2.25
GPa (horizontal dashed line) for M-2 Grade particleboard (Fig. 2.1).
The MOE
significantly increased by 10.0% when hot-press temperature was raised from 160 to
170 °C. In the range of 170 to 190 °C the MOE did not significantly change.
However, the average MOE at 200 °C was significantly higher than that at all other
temperatures. Effect of the hot-press temperature on the IB of particleboard is shown
in Fig. 2.2 with the minimum industrial requirement (0.45 MPa) shown as a horizontal
42
dashed line. The IB exceeded the minimum industrial requirement at all hot-press
temperatures. The IB significantly increased by 14.6% when the temperature was
raised from 160 to 170 °C. The IB values in the 170 - 200 °C range were not
significantly different from each other.
With the MOR, MOE and IB all being
considered, 170 °C appeared to be the best hot-press temperature and was used for
subsequent investigations.
MOR (MPa)
MOE (GPa)
30
5
20
3
2
10
MOE (GPa)
MOR (MPa)
4
1
0
0
160
170
180
190
200
Hot-press temperature (C)
Fig. 2.1 Effects of hot-press temperature on the MOR and MOE of particleboard.
[surface adhesive usage (dry basis on wood particles), 15 wt%; core adhesive usage,
12 wt%; hot-press time, 300 s; the targeted panel density, 0.70 g/cm3. Data are the
means of eight replications, and the error bar represents one standard error of the mean]
43
1.0
IB (MPa)
0.8
0.6
0.4
0.2
0.0
160
170
180
190
200
Hot-press temperature (C)
Fig. 2.2 Effect of hot-press time on the IB of particleboard. (Processing parameters
and statistical details are the same as those in Fig. 2.1)
2.5.2 Effects of hot-press time on the mechanical properties of the particleboard
Effects of hot-press time on the MOR and MOE of particleboard are shown in Fig.
2.3. The MOR did not significantly change when the hot-press time was raised from
200 to 230 s. The MOR significantly increased when the hot-press time was raised
from 230 to 270 s. Further increase in the hot-press time from 270 to 300 s did not
significantly increase the MOR although the average MOR at 300 s was higher than
that at 270 s. The MOR markedly decreased when the hot-press time was increased
from 300 to 370 s. The MOR at all hot-press times tested met the minimum industrial
requirement (horizontal solid line, Fig. 2.3). The MOE remained the same when the
hot-press time was increased from 200 to 230 s. However, the MOE significantly
increased when the hot-press time was increased from 230 to 270 s. The MOE
remained the same when the hot-press times was increased from 270 to 300 s, and
44
then significantly decreased when the hot-press time was further increased from 300 to
370 s. The MOE at all hot-press temperature tested met the minimum industrial
requirement (horizontal dashed line, Fig. 2.3). The IB significantly increased when
the hot-press time was increased from 200 to 270 s (Fig. 2.4). The IB remained the
same when the hot-press time was in the range of 270 to 370 s. The IB at all hot-press
temperature tested met the minimum industrial requirement (Fig. 2.4). The 270 s was
used as the optimum hot-press time for subsequent evaluation.
MOR (MPa)
MOE (GPa)
5
20
4
15
3
10
2
5
1
0
MOE (GPa)
MOR (MPa)
25
0
200
230
270
300
370
Hot-press time (s)
Fig. 2.3 Effects of hot-press time on the MOR and MOE of particleboard. [surface
adhesive usage (dry basis on wood particles), 15 wt%; core adhesive usage, 12 wt%;
hot-press temperature, 170 °C; the targeted panel density, 0.70 g/cm3. Data are the
means of eight replications, and the error bar represents one standard error of the mean]
45
0.8
IB (MPa)
0.6
0.4
0.2
0.0
200
230
270
300
370
Hot-press time (s)
Fig. 2.4 Effect of hot-press time on the IB of particleboard. (Processing parameters
and statistical details are the same as those in Fig. 2.3)
2.5.3 Effects of core adhesive usage at different densities on the mechanical
properties of the particleboard
Effect of core adhesive usage on the MOR of particleboard panels made at two
different density levels is shown in Fig. 2.5. Particleboard under both density levels
still belonged to medium density particleboard. At the high density level (0.80 g/cm3),
the MOR remained the same when the core adhesive usage was reduced from 11 to 10
wt% and then significantly increased when the core adhesive usage was further
reduced from 10 to 9 wt% (Fig. 2.5). At the high density level, the MOR rapidly
decreased when the adhesive usage was reduced from 9 to 7 wt%. The MOR at all
adhesive usages tested exceeded the minimum industrial requirement (horizontal solid
line, Fig. 2.5). At the low density level (0.70 g/cm3), the MOR was statistically the
46
same when the adhesive usage was reduced from 11 to 8 wt%. The MOR at the core
adhesive usage of 9 wt% was significantly higher than that at 7 wt%, and the MOR at
8 wt% was statistically the same as that at 7 wt%. The MOR at the adhesive usage of
11 to 8 wt% met the minimum industrial requirement while the MOR at 7 wt% was
very close to meet the requirement (horizontal solid line, Fig. 2.5). At the same
adhesive usage, the MOR at the low density level was significantly lower than that at
the high density level.
Low density level
40
High density level
MOR (MPa)
30
20
10
0
11
10
9
8
7
Core adhesive usage (%)
Fig. 2.5 Effect of the core adhesive usage on the MOR at a high density level and a
low density level. [surface adhesive usage (dry basis on wood particles), 15 wt%; hotpress temperature, 170 °C; hot-press time, 270 s. Data are the means of eight
replications, and the error bar represents one standard error of the mean]
Effect of the core adhesive usage on the MOE of particleboard panels made at two
different density levels is shown in Fig. 2.6. At the high density level (0.80 g/cm3),
47
the MOE remained the same when the adhesive usage was reduced from 11 to 9 wt%
and then significantly decreased when the adhesive usage was further reduced from 9
to 7 wt%. All MOE values at the high density level exceeded the minimum industrial
requirement (horizontal solid line, Fig. 2.6). At the low density level (0.70 g/cm3), the
MOE significantly increased when the core adhesive usage was reduced from 11 to 10
wt% and then significantly decreased when the adhesive usage was reduced from 10 to
8 wt%. The MOE somehow significantly increased when the adhesive usage was
further reduced from 8 to 7 wt%.
All MOE at this density level exceeded the
minimum industrial requirement (horizontal solid line, Fig. 2.6). At each adhesive
usage tested, the MOE at the high density level was always higher than that at the low
density level (Fig. 2.6).
Low density level
6
High density level
MOE (GPa)
5
4
3
2
11
10
9
8
Core adhesive usage (%)
7
48
Fig. 2.6 Effect of the core adhesive usage on the MOE at a high density level and a
low density level. (Processing parameters and statistical details are the same as those
in Fig. 2.5)
Effect of the core adhesive usage on the IB of the particleboards made at two
different density levels is shown in Fig. 2.7. At the high density level (0.80 g/cm3),
the IB decreased significantly when the core adhesive usage was reduced from 11 to
10 wt%, but remained statistically the same when the adhesive usage was reduced
from 10 to 9 wt%. Further reduction of the adhesive usage from 9 to 8 wt% or from 8
to 7 wt% significantly decreased the IB. The IB at all adhesive usages except 7 wt%
met the minimum industrial requirement (horizontal solid line, Fig. 2.7). At the low
density level (0.70 g/cm3), the IB significantly decreased when the adhesive usage was
reduced from 11 to 8 wt% and then remained the same when the adhesive usage was
further reduced from 8 to 7 wt%. The IB met the minimum industrial requirement
when the adhesive usage was 10 wt% or higher (horizontal solid line, Fig. 2.7). At the
same adhesive usage, the IB at the high density level was significantly higher than that
at the low density level.
49
Low density level
1.0
High density level
IB (MPa)
0.8
0.6
0.4
0.2
0.0
11
10
9
8
7
Core adhesive usage (%)
Fig. 2.7 Effect of the core adhesive usage on the MOE at a high density level and a
low density level. (Processing parameters and statistical details are the same as those
in Fig. 2.5)
2.5.4 Effects of surface adhesive usage on the mechanical properties of the
particleboard
Effects of surface adhesive usage on the MOR and MOE of particleboard panels
are shown in Fig. 2.8. The MOR significantly decreased when the adhesive usage was
reduced from 15 to 13 wt% and then remained the same when the adhesive usage was
further reduced from 13 to 12 wt%. The MOR significantly decreased again when the
adhesive usage was further reduced from 12 to 11 wt%. The MOR at all surface
adhesive usages tested exceeded the minimum industrial requirement (horizontal solid
line, Fig. 2.8). The MOE did not significantly change when the surface adhesive
usage was reduced from 15 to 14 wt%, and then significantly decreased when the
adhesive usage was further reduced from 14 to 13 wt%. The MOE remained the same
50
when the adhesive usage was reduced from 13 to 12 wt% and then significantly
decreased when the adhesive usage was further reduced from 12 to 11 wt% (Fig. 2.8).
The MOE at all adhesive usages tested met the minimum industrial requirement
(horizontal dashed line, Fig. 2.8). The effect of the surface adhesive usage on the IB
of particleboard panels is shown in Fig. 2.9. The IB significantly decreased when the
adhesive usage was reduced from 15 to 13 wt%. The IB at 13 wt% was somehow
significantly lower than that at 12 wt%. The IB remained the same when the adhesive
usage was reduced from 12 to 11 wt%.
The IB met the minimum industrial
requirement (horizontal dashed line, Fig. 2.9) when the adhesive usage was at 14 wt%
or higher.
MOR (MPa)
30
5
MOE (GPa)
20
3
2
10
M OE (GPa)
MOR (MPa)
4
1
0
0
15
14
13
12
11
surface adhesive usage (%)
Fig. 2.8 Effects of the surface adhesive usage on the MOR and MOE of particleboard.
[core adhesive usage (dry basis on wood particles), 8 wt%; hot-press temperature
51
170 °C; hot-press time, 270 s; the targeted panel density, 0.80 g/cm3. Data are the
means of eight replications, and the error bar represents one standard error of the mean]
0.6
IB (MPa)
0.4
0.2
0.0
15
14
13
12
11
Surface adhesive usage (%)
Fig. 2.9 Effect of surface adhesive usage on the IB of particleboard. (Processing
parameters and statistical details are the same as those in Fig. 2.8)
2.5.5 Effects of the moisture content of surface wood particles on the mechanical
properties of the particleboard
Effects of the moisture content of the surface wood particles on the MOR and MOE
are showed in Fig. 2.10. The MOR significantly increased when the moisture content
was raised from 7.1 to 8.6 wt% and then remained the same when the moisture content
was further raised from 8.6 to 9.0 wt%. However, the MOR significantly increased
when the moisture content was raised from 9.0 to 9.4 wt%. The MOR at all moisture
contents tested met the minimum industrial requirement (horizontal solid line, Fig.
52
2.10). The MOE did not significantly change when the moisture content was raised
from 7.1 to 9.0 wt% and then significantly increased when the moisture content was
further raised from 9.0 to 9.4 wt%. All MOE at the moisture contents tested met the
minimum industrial requirement (horizontal solid line, Fig. 2.10). The effect of the
moisture content of the surface wood particles on the IB is showed in Fig. 2.11. The
IB significantly increased when the moisture content was increased from 7.1 to 8.6
wt% and then remained statistically the same when the moisture content was further
raised from 8.6 to 9.0 wt%. However, The IB reduced significantly when the moisture
content was further increased from 9.0 to 9.4 wt%.
The IB met the minimum
industrial requirement when the moisture content was in the range of 8.6 to 9.0 wt%.
MOR (MPa)
MOE (GPa)
30
5
20
3
2
10
MOE (GPa)
MOR (MPa)
4
1
0
0
7.1
8.6
9.0
9.4
Moisture content of the surface wood particles (%)
Fig. 2.10 Effects of the moisture content of the surface wood particles on the MOR
and MOE of particleboard. [surface adhesive usage (dry basis on wood particles), 10
wt%; core adhesive usage (dry basis on wood particles), 8 wt%; hot-press temperature
170 °C; hot-press time, 270 s; the targeted panel density, 0.80 g/cm3. Data are the
means of eight replications, and the error bar represents one standard error of the mean]
53
IB (MPa)
0.6
0.4
0.2
0.0
7.1
8.6
9.0
9.4
Moisture content of surface wood particles (%)
Fig. 2.11 Effect of the moisture content of the surface wood particles on the IB of
particleboard. (Processing parameters and statistical details are the same as those in
Fig. 2.10)
2.5.6 Effects of adhesive composition on the mechanical properties of the
particleboard
Effects of the adhesive composition on the MOR and MOE are showed in Fig. 2.12.
Adhesive I consisted of SF, PEI, MA and NaOH with the SF/PEI/MA/NaOH weight
ratio of 7/1.0/0.32/0.1. Adhesive II consisted of SF, PEI and MA with the SF/PEI/MA
weight ratio of 7/1.0/0.32. Adhesive III consisted of SF, PEI, succinic acid and NaOH
with the SF/PEI/succinic acid/NaOH weight ratio of 7/1.0/0.32/0.1. The MOR of the
adhesive I was significantly higher than that of the adhesive II and the adhesive III.
The MOR of the adhesive II and the adhesive III was statistically the same. The
average MOR of all adhesives met the minimum industrial requirement (horizontal
solid line, Fig. 2.12). The MOE of the adhesive I was significantly higher than that of
54
the adhesive II and the adhesive III. The adhesive II and the adhesive III had the same
MOE values. All MOE met the minimum industrial requirement (horizontal solid line,
Fig. 2.12). The effect of the adhesive composition on the IB is showed in Fig. 2.13.
The adhesive I and the adhesive III had the same IB. The IB of the adhesive II was
significantly lower than that of the adhesive I and the adhesive III. Moreover, the IB
of the adhesive I and the adhesive III just met the minimum industrial requirement
(horizontal dashed line, Fig. 2.13), whereas the IB of the adhesive II did not.
MOR (MPa)
25
5
3
10
2
5
1
0
0
II
iv
e
dh
es
A
A
dh
es
iv
e
iv
e
dh
es
A
II I
15
I
4
MOE (GPa)
MOR (MPa)
MOE (GPa)
20
Fig. 2.12 Effects of the adhesive composition on the MOR and MOE. (surface
adhesive usage, 10 wt%; core adhesive usage, 8 wt%; hot-press temperature 170 °C;
hot-press time 270 s; the targeted panel density, 0.80 g/cm3. Data are the means of
eight replications, and the error bar represents one standard error of the mean.
Adhesive I: SF/PEI/MA/NaOH; Adhesive II: SF/PEI/MA; Adhesive III:
SF/PEI/succinic acid/NaOH)
55
0.5
IB (MPa)
0.4
0.3
0.2
0.1
III
iv
e
dh
es
A
si
ve
dh
e
A
A
dh
es
iv
e
II
I
0.0
Fig. 2.13 Effect of the adhesive composition on the IB. (Processing parameters,
sample denotation and statistical details are the same as those in Fig. 2.12)
2.6 Discussions
The previous publication showed that SF/PEI/MA/NaOH at the weight ratio of
7/1.0/0.32/0.1 was an optimum recipe for making interior type II plywood [7]. We
assumed that this recipe was also optimum for making M-2 grade particleboards. The
function of the hot-pressing in particleboard production is to provide the necessary
heat and pressure for curing the adhesive, and consolidating the discrete particles into
a solid board. During the hot-pressing, the heat is transferred from surface to core so
the core temperature is generally lower than that of the surface temperature. Full
curing of an adhesive can be accomplished by either increasing hot-press temperature
at a fixed hot-press time or increasing hot-press time at a fixed hot-press temperature.
56
However, if the hot-press temperature was higher than that needed for full curing of
the adhesive at a fixed hot-press time or if the long hot-press time was longer than that
needed for full curing of the adhesive, mechanical properties of resultant particleboard
panels would either remain the same or decreased due to partial degradation of
adhesives and wood particles. Therefore, hot-press temperature and hot-press time
should be optimized. At a fixed hot-press time of 300 s, it appeared that the adhesive
was already fully cured at 170 °C and further increase in the hot-press temperature did
not significantly change the MOR, MOE and IB (Figs. 2.1 and 2.2). At a fixed hotpress temperature of 170 °C, it appeared that the adhesive was already fully cured at
the hot-press time of 270 s. The decreased MOR and MOE at the hot-press time of 370
s over 300 s implied that the hot-press time of 370 s was too long (Fig. 2.3).
The mechanical properties of the particleboard are highly dependent upon the panel
density. Increase in the density would increase the contact of the wood particles and
tightly consolidate the particle mat, thus increasing the strength. This explanation is
consistent with our results that at any given core adhesive usage, the particleboard
panels with the density of 0.80 g/cm3 always had higher MOR, MOE and IB than
those with the density of 0.70 g/cm3, respectively (Figs 2.5, 2.6, and 2.7).
In the preparation of particleboard panels, the adhesive was sprayed as fine droplets
on surfaces of wood particles. The adhesive coverage on wood particles had to be
sufficiently high to form good adhesive bonding. It was believed that not all surfaces
could be covered by the adhesive.
Therefore, the increase in the adhesive usage
57
would typically increase the strengths (MOR, MOE and IB) of the resulting
particleboard panels. The IB was particularly dependent upon the core adhesive usage
because failure of all test specimens occurred in the core layer during the IB test,
which explained why the IB almost decreased linearly when the core adhesive usage
was reduced from 11 wt% to 7 wt% at both high and low density levels (Fig. 2.7).
Stress first occurred on face layers of test specimens during the bending test.
Therefore, the MOR and MOE were more closely related to the strength of face layers
than that of the core layer unless the core layer was too weak to support the face layers
during the bending test. The MOR at both high and low density levels and MOE at
the high density level did not significantly decrease until when the core adhesive usage
was reduced from 9 wt% to 7 wt% (Figs 2.5 and 2.6). It is still poorly understood on
why the MOE at the low density level fluctuated when the core adhesive usage was in
the range of 11 wt% to 7 wt% (Fig. 2.6). That MOR and MOE gradually decreased
when the surface adhesive usage was reduced from 15 wt% to 11 wt% was consistent
with the explanation that the MOR and MOE were closely related to the surface
adhesive usage (Fig. 2.8).
The surface adhesive usage was not supposed to have great impact on the IB
because all test specimens failed at the core layer, which was not consistent with the
results shown in Fig. 2.9 where a reduction of the surface adhesive usage significantly
decreased the IB. One possible explanation was that reducing surface adhesive usage
also reduced the moisture content of the surface layer and thus reduced the heat
58
transfer from the surface layer to the core layer, which in return slowed down the cure
of the adhesive in the core layer. The results from Fig. 2.11 indeed indicated that the
IB increased significantly when the moisture content of surface layers was increased
from 7.1 to 8.6% (Fig. 2.11). The high moisture of face layers is not always helpful
for improving the IB though. During the hot-pressing, water was turned into stream,
resulting in the build-up of internal pressure. The stream would come out and disrupt
the adhesive bonds when the hot-press was open, thus reducing the IB, which
explained that the IB decreased when the moisture content of surface wood particles
was raised from 9.0 to 9.4%. Cell walls of wood particles were compressed during
hot-pressing and tended to bounce back when the hot-press was open.
phenomenon is known as springback in wood composite industry.
This
The internal
pressure from the stream build-up also contributes to the springback. The MOR, MOE
and IB are compromised results of adhesive strength and springback.
Effects of some adhesive components on the mechanical properties of the resulting
particleboard panels were also investigated. Results from Figs. 2.12 and 2.13 further
confirmed that NaOH was an essential component of this SF-PEI-MA adhesive. That
the adhesive I had higher MOR and MOE than the adhesive III appeared to indicate
that the double bond of MA played in important role in the adhesive, whereas that the
IB was the same for the adhesive I and the adhesive III suggested that the double bond
of MA did not play a significant role. Further investigation is warranted for a better
understanding of the exact roles of each component in the adhesive.
59
2.7 Conclusions
This study demonstrated that the formaldehyde-free, environmentally friendly SFPEI-MA-NaOH adhesive could be used for making M-2 grade particleboard panels.
The following variables and conditions were most desirable for making particleboard:
hot-press temperature, 170 °C; hot-press time, 270 s; the adhesive usage of surface
particles, 10 wt%; the adhesive usage of the core particles, 8 wt%; and the targeted
particleboard density, 0.80 g/cm3.
2.8 Acknowledgements
Wood particles were provided by Flakeboard (Albany, OR).
2.9 References
1
John A. Youngquist (1999) Wood handbook, Forest Product laboratory, MI, USA
2
Formaldehyde, International Agency for Research on Cancer classifies
formaldehyde as carcinogenic to humans, International Agency for Research on
Cancer Press release # 153 (2004).
3
Liu K (1997) Soybeans-chemistry, technology, and utilization. International
Thomson Publishing, New York
4
Li K, Peshkova S, Geng X (2004) Investigation of soy protein-Kymene adhesive
systems for wood composites. J Am Oil Chem Soc 81:487-491
5
Li K (2002) Formaldehyde-free lignocellulosic adhesives and composites made
from the adhesives. US Patent 7,252,735
6
Li K, Liu Y (2006) Modified protein adhesives and lignocellulosic composites
made from the adhesives. US Patent 7,060,798
60
7
Huang J, Li K (2008) New soy flour-based adhesive for making interior type II
plywood. J Am Oil Chem Soc, 85:63-70
8
Liu Y, Li K (2007) Development and characterization of adhesives from soy
protein for bonding wood. International Journal of Adhesion and Adhesives 27:
59-67
61
CHAPTER 3 PREPARATION AND EVALUATION OF PARTICLEBOARD
WITH A SOY FLOUR-NEW CURING AGENT ADHESIVE
62
3.1 Abstracts
Urea-formaldehyde (UF) resin is one of the most commonly used wood adhesives
for making interior particleboard. However, UF is petrochemical-based so it is not
sustainable in a long run. Moreover UF emits carcinogenic formaldehyde to cause
human health problem.
In this study we evaluated a new soy flour-based,
formaldehyde-free wood adhesive for making M-2 Grade particleboard. This adhesive
consisted of soy flour (SF)/sodium hydroxide (NaOH)/a new curing agent at a dry
weight ratio of 9/0.3/1.0. The modulus of rupture (MOR), modulus of elasticity
(MOE) and internal bond (IB) met the minimum industrial requirements of M-2
particleboards at the following variables: hot-press temperature at 190 °C, hot-press
time at 270 s, the adhesive usage of surface particles at 12 wt%, the adhesive usage of
the core particles at 10 wt%, and the target particleboard density of 0.80 g/cm3.
3.2 Key words
Wood adhesive ∙ Particleboard ∙ Soy flour ∙ Curing agent ∙ Crosslinking
3.3 Introduction
Particleboard is a flat panel product manufactured by bonding homogeneous wood
particles or wood waste materials such as sawdust, shavings with adhesives under the
heat and pressure. In general, particleboard has three layers including two face layers
and one core layer. The face layers consist of fine particles to get smooth surfaces for
laminating, overlaying, painting or veneering.
The core layer consists of coarse
materials. Particleboard is mainly used for making furniture and kitchen cabinets [1-
63
2]. Currently urea-formaldehyde (UF) resin is the most commonly used adhesives for
making interior particleboard. Urea and formaldehyde are two main raw materials for
making UF resins. However, formaldehyde is derived from natural gas and natural
gas is not sustainable in the long run because of the limited reserve of natural gas or
fossil oil [3]. Another negative impact of using UF as a wood adhesive is related to
the emission of formaldehyde. Formaldehyde is a colorless, pungent-smelling gas
which can have adverse effects on human health. In June of 2004 the International
Agency for Research on Cancer reclassified formaldehyde as a human carcinogen [4].
According to United State Environmental Protection Agency (EPA), the most
significant indoor sources of formaldehyde are wood composites made with UF resins
[5]. In response to this situation the California Air Resources Board (CARB) passed a
regulation for reducing public exposure to formaldehyde in 2007. This regulation
placed a new formaldehyde emission limit from wood composite panel products and
took effect in two phases. The phase I implementation has already been in force since
January 1, 2009 [6]. Additionally, EPA is also considering a regulatory action on the
national level to protect people against risks posed by formaldehyde emitted from
wood composite panels [7-8]. Thus, it is very crucial and also urgent to develop new
formaldehyde-free wood adhesives to replace UF resins.
Soybean is one of the most important crops grown in the US. Soybean oil and
soybean meal are two major soybean products. At present, a large portion of soybean
meal is mainly used for feeding poultry and other animals in the US. How to use this
64
abundant, inexpensive, renewable and environmentally friendly material for other
industrial purposes gains a lot of research interest. In fact soybean meals were used as
wood adhesives for making plywood in 1930s-1960s [9]. However, these soy-based
adhesives were replaced by synthetic adhesives because of plywood made with the
soy-based adhesives had poor water resistance. A number of new water-resistant soybased adhesives have been developed in recent years [10-15]. One of the adhesives,
consisting of soy flour and a petrochemical-based curing agent, has been
commercialized for the production of interior plywood since late 2004 [14]. A new
curing agent that can be derived from renewable glycerol and ammonia has recently
been developed. This soy flour-(curing agent) (SF-CA) adhesive is 100% based on
renewable materials. In this study, this SF-CA adhesive was evaluated to see if it has
potential of being used for production of particleboard.
3.4 Materials and methods
3.4.1 Materials
SF (8.5% moisture content) was provided by Cargill Incorporated (Minneapolis,
MN, USA); Douglas fir wood particles (surface and core wood particles) were
obtained from Flakeboard (Albany, OR). A 30 wt% aqueous new curing agent was
prepared and provided by Kaichang Li‟s laboratory at Oregon State University
(Corvallis, OR).
65
3.4.2 Preparation of SF-coated wood particles
3.4.2.1 Wet method
The following is a representative wet method for the preparation of SF-coated
wood particles. For coating surface wood particles, SF (181.3 g, dry weight) and water
(792.6 g) were sequentially added into a HOBART A-200 blender (Hobart, Topeka,
KS) and stirred for 10 min at room temperature to form 20% SF slurry. Surface wood
particles (1733 g, dry weight) were then added into the SF slurry in the Hobart blender
and the resulting mixture was stirred for 10 min. The resulting wet SF-coated wood
particles were put in a cloth bag, and dried in a rotary dryer (Speedqueen, Ripon, WI)
for1 h. After drying the SF-coated surface wood particles had the moisture content of
2%. SF-coated core wood particles (2% moisture content) was prepared by following
the same procedure for the preparation of the SF-coated surface wood particles using
SF (261.9 g, dry weight), water (1144.9 g), and core wood particles (2997.3 g, dry
weight). Most results in this study using wet method to prepare SF-coated wood
particles unless a dry method was mentioned.
3.4.2.2 Dry method
The following is dry method for the preparation of SF-coated wood particles. For
coating surface wood particles, SF (181.3 g, dry weight) and surface wood particles
(1733 g, dry weight) were poured in a rotary drum-blender and blended for 5 min to
mix SF with surface wood particles homogeneously. For coating core wood particles,
SF (261.9 g, dry weight) and core wood particles (2997.3 g, dry weight) were poured
66
in the rotary drum-blender and blended for 5 min to mix SF with core wood particles
homogeneously.
3.4.3 Preparation of CA-NaOH solution
The following was a representative procedure for the preparation of CA-NaOH
solution. NaOH (6.0 g) and water (110 g) were added into a 400 mL beaker, and
stirred with a magnetic stirrer for 30 min. The CA (67.2 g) was further added into the
beaker and stirred for another 5 min. The resulting solution was then sprayed onto the
wet or dry method-prepared SF-coated surface wood particles (1914.3 g, dry weight)
in a rotary drum-blender. The resulting adhesive-coated surface wood particles had
the adhesive usage of 12 wt% (the adhesive usage was defined as the dry weight of SF,
CA and NaOH divided by the dry weight of the wood particles) and had the moisture
content of 9%. A mixture of the CA solution (97 g), NaOH solution (8.7 g solid +
water 110 g) was sprayed onto the wet or dry method-prepared SF-coated core wood
particles (3259.2 g, dry weight) in the rotary drum-blender. The resulting adhesivecoated core wood particles had the adhesive usage of 10% and had the moisture
content of 7%. In this case, SF/CA//NaOH dry weight ratio was at 9/1.0/0.3.
3.4.4 Preparation of three-layer particleboard
The adhesive-coated surface wood particles (807 g, dry weight) were added onto a
forming box (60.96 cm × 60.96 cm) by hand to form a uniform layer. The adhesivecoated core wood particles (2748 g, dry weight) were evenly distributed on top of the
surface wood particle layer, followed by a uniform layer of the adhesive-coated
67
surface wood particles (807 g, dry weight). The resulting three-layered wood particle
mat was hand-pressed with a flat plywood panel (60.96 cm × 60.96 cm) and then hotpressed at 180 °C for 270 s. The target thickness was 17 mm. The target density was
0.80 g/cm3. The hot-press time and hot-press temperature were changed when their
effects on particleboard properties were investigated. Two particleboard panels were
prepared for each experimental variable.
3.4.5 Determination of mechanical properties of particleboard
3.4.5.1 Evaluation of mechanical properties of particleboard
Particleboard was cut into 7.62 cm × 46.99 cm rectangular specimens for the
measurement of modulus of rupture (MOR) and modulus of elasticity (MOE), and into
5.08 cm × 5.08 cm specimens for the measurement of internal bond strength (IB) in
accordance with ASTM D1037-99 (American Society for Testing and Materials, 1999)
using a MTS Sintech 1/G testing machine (MTS Systems Corp., Eden Prairie, MN).
The crosshead speeds were 9 mm/min for the measurement of MOR and MOE and 1.5
mm/min for the IB measurement. For each particleboard panel, four specimens were
obtained for the measurement of MOR and MOE, and eight specimens for the IB
measurement.
3.4.5.2 Statistical analysis of data
All data were analyzed with Welch modified two-sample t-test with a S-PLUS®
statistical software package (Edition version 8.0, Insightful Corp., Seattle, WA, USA).
All comparisons were based on 95% confidence interval.
68
3.5 Results
3.5.1 Effects of SF/CA weight ratio on the mechanical properties of the
particleboard
Effects of SF/CA weight ratio on the MOR and MOE are shown in Fig. 3.1. The
weight ratio was based on the dry weight of soy flour and dry solid weight of the CA.
The MOR significantly increased when the SF/CA weight ratio was reduced from 11/1
to 9/1. However, the MOR did further increase when the SF/CA weight ratio was
further reduced from 9/1 to 7/1. The MOR decreased when the SF/CA weight ratio
was reduced from 7/1 to 5/1 and then remained the same when the SF/CA weight ratio
was further reduced from 5/1 to 3/1. The MOR at the SF/CA weight ratios studied all
met the minimum industrial requirement of 14.5 MPa (horizontal solid line) for M-2
grade particleboard. The MOE increased with reducing the SF/CA ratio from 11/1 to
9/1, and remained the same with further reduction of the ratio from 9/1 to 7/1. The
MOE first decreased and then remained the same when the SF/CA ratio was reduced
from 7/1 to 5/1 and then to 3/1. The MOE at selected ratios all met the minimum
industrial requirement of 2.25 GPa (horizontal dashed line) for M-2 grade
particleboard.
Effect of SF/CA weight ratio on the IB is shown in Fig. 3.2. When the SF/curing
agent ratio was at 11/1 the average IB was the lowest and the IB was also significantly
lower than that at 9/1 or 5/1. The IB remained statistically the same when the SF/CA
69
weight ratio was in the range of 9/1 to 3/1. The IB at all ratios studied met the
minimum industrial requirement of 0.45 MPa shown as a horizontal dashed line.
MOR (MPa)
MOE (GPa)
25
4
3
15
2
10
MOE (GPa)
MOR (MPa)
20
1
5
0
0
11/1
9/1
7/1
5/1
3/1
SF/CA weight ratio
Fig. 3.1 Effects of SF/CA weight ratio on the MOR and MOE of particleboard.
[surface adhesive usage (dry basis on wood particles), 12 wt%; core adhesive usage
(dry basis on wood particles), 10 wt%; NaOH/CA (dry weight basis), 3 wt%; hot-press
temperature, 180 °C; hot-press time, 270 s; the targeted particleboard density, 0.80
g/cm3. Data are the means of eight replications, and the error bar represents one
standard error of the mean]
0.8
IB (MPa)
0.6
0.4
0.2
0.0
11/1
9/1
7/1
5/1
SF/CA weight ratio
3/1
70
Fig. 3.2 Effect of SF/CA weight ratio on the IB of particleboard.
parameters and statistical details are the same as those in Fig. 3.1)
(Processing
3.5.2 Effects of NaOH usages on the mechanical properties of the particleboard
Effects of NaOH usages on the MOR and MOE are shown in Fig. 3.3. The NaOH
usage was defined as a percentage of dry NaOH weight over/(dry weight of SF + dry
weight of CA). When no NaOH was added the average MOR was the lowest. The
MOR remained statistically the same when the NaOH usage was increased from 0
wt% to 1 wt% and then significantly increased when the NaOH usage was further
increased from 1 wt% to 3 wt%. However, the MOR significantly decreased when the
NaOH usage was increased from 3 to 5 wt%. The MOR remained statistically the
same when the NaOH usage was raised from 5 wt% to 7 wt%. The MOR at all NaOH
usages studied exceeded the minimum industrial requirement (horizontal solid line).
The MOE remained statistically the same when the NaOH usage was increased from 0
wt% to 1 wt% and then significantly increased when the NaOH usage was increased
from 1 wt% to 3 wt%. Further increasing the NaOH usage from 3 wt% to 5 wt%
decreased the MOE significantly and the MOE did not change significantly when the
NaOH usage was in the range of 5-7 wt%. The MOE at all NaOH usages studied
exceeded the minimum industrial requirement (horizontal dashed line).
Effect of NaOH usages on the IB is shown in Fig. 3.4. The IB did not significantly
change when the NaOH usage was raised from 0 wt% to 1 wt% and then significantly
increased when the NaOH usage was raised from 1 wt% to 3 wt%. However, the IB
71
significantly decreased when the NaOH usage was raised from 3 wt% to 5 wt% and
then remained the same when the NaOH usage was raised from 5 wt% to 7 wt%. the
IB all exceeded the minimum industrial requirement (horizontal dashed line) when the
NaOH usage was in the range of 1-7 wt%.
MOR (MPa)
MOE (GPa)
25
4
3
15
2
10
MOE (GPa)
MOR (MPa)
20
1
5
0
0
0%
1%
3%
5%
7%
NaOH usages
Fig. 3.3 Effects of NaOH usages on the MOR and MOE of particleboard. [surface
adhesive usage (dry basis on wood particles), 12 wt%; core adhesive usage (dry basis
on wood particles), 10 wt%; SF/CA weight ratio, 9/1; hot-press temperature, 180 °C;
hot-press time, 270 s; the targeted particleboard density, 0.80 g/cm3. Data are the
means of eight replications, and the error bar represents one standard error of the mean]
IB (MPa)
0.6
0.4
0.2
0.0
0%
1%
3%
NaOH usages
5%
7%
72
Fig. 3.4 Effect of NaOH usages on the IB of particleboard. (Processing parameters
and statistical details are the same as those in Fig. 3.3)
3.5.3 Effects of hot-press temperature on the mechanical properties of the
particleboard
Effects of hot-press temperature on the MOR and MOE are shown in Fig. 3.5. The
MOR did not change significantly when the temperature was in the range of 150160 °C. The MOR at each temperature was significantly higher than that at 160 °C
when the temperature was higher than 160 °C. In the range of 170-180 °C the MOR
did not change significantly. However, the MOR at 190 °C was significantly higher
than that at 170 or 180 °C. The MOR was the highest when the temperature was at
190 °C. However, the MOR decreased significantly when the temperature was further
raised from 190 to 200 °C.
All MOR met the minimum industrial requirement
(horizontal solid line) at all hot-press temperatures studied.
MOE remained statistically the same in the range of 150-160 °C. However, the
MOE at a temperature higher than 160 °C was significantly higher than that at 160 °C.
The MOE did not significantly change when the temperature was raised from 170 to
180 °C or from 180 to 190 °C. However, the MOE at 190 °C was significantly higher
than that at 170 °C. The MOE significantly decreased along with increasing the
temperature from 190 to 200 °C. MOE exceeded the minimum industrial requirement
(horizontal dashed line) at all hot-press temperatures studied.
73
Effect of hot-press temperature on the IB is shown in Fig. 3.6. When the hot-press
temperature was in the range of 150-160 °C, the IB remained statistically the same and
could not meet the minimum industrial requirement (horizontal dashed line). The IB
increased significantly when the temperature was raised from 160 to 170 °C and then
remained the same when the temperature was raised from 170 to 200 °C. When the
temperature was above 160 °C, all IB exceeded the minimum industrial requirement
(horizontal dashed line).
MOR (MPa)
MOE (GPa)
30
5
20
3
2
10
MOE (GPa)
MOR (MPa)
4
1
0
0
150
160
170
180
190
200
Hot press temperature (C)
Fig. 3.5 Effects of hot-press temperature on the MOR and MOE of particleboard.
[surface adhesive usage (dry basis on wood particle), 12 wt%; core adhesive usage
(dry basis on wood particles), 10 wt%; SF/CA/NaOH, 9/1.0/0.3; hot-press time, 270 s;
the targeted particleboard density, 0.80 g/cm3. Data are the means of eight
replications, and the error bar represents one standard error of the mean]
74
IB (MPa)
0.6
0.4
0.2
0.0
150
160
170
180
190
200
Hot press temperature ( oC)
Fig. 3.6 Effect of hot-press temperature on the IB of particleboard. (Processing
parameters and statistical details are the same as those in Fig. 3.5)
3.5.4 Effects of hot-press time on the mechanical properties of the particleboard
Effects of hot-press time on the MOR and MOE are shown in Fig. 3.7. The MOR
decreased significantly when the hot-press time was raised from 200 to 240 s, and then
significantly increased when the hot-press time was further raised from 240 to 270 s.
The MOR significantly decreased along with increase in the time from 270 to 300 s
and then remained the same with the further increase in the time from 300 to 330 s.
All the MOR met the minimum industrial requirement (horizontal solid line) at the
hot-press times studied. The MOR was the highest at the hot-press time of 270 s. The
MOE had the same trend as the MOR. The MOE first decreased and then increased
when the hot-press time was raised from 200 to 240 s and then to 270 s. The MOE
decreased and then remained the same when the hot-press time was raised from 270 to
75
300 s and then to 330 s. The MOE was the highest at the hot-press time of 270 s. All
the MOE exceeded the minimum industrial requirement (horizontal dashed line).
Effect of the hot-press time on the IB is shown in Fig. 3.8. The IB at the hot-press
time of 200 s was significantly lower than that at any other hot-press time and was the
only one that did not meet the minimum industrial requirement (horizontal dashed
line). The IB increased significantly when the hot-press time was raised from 200 to
240 s. The IB remained the same in the temperature ranges of 240-270 s and 270-330
s. However, the IB at 300 s or 330 s was significantly higher than that at 240 s. The
IB at a hot-press time of over 240 s exceeded the minimum industrial requirement
(horizontal dashed line).
MOR (MPa)
MOE (GPa)
30
5
20
3
2
10
MOE (GPa)
MOR (MPa)
4
1
0
0
200
240
270
300
330
Hot-press time (s)
Fig. 3.7 Effects of hot-press time on the MOR and MOE of particleboard. [surface
adhesive usage (dry basis on wood particles), 12 wt%; core adhesive usage (dry basis
on wood particles), 10 wt%; SF/CA/NaOH, 9/1.0/0.3; hot-press temperature, 190 °C;
the targeted particleboard density, 0.80 g/cm3. Data are the means of eight
replications, and the error bar represents one standard error of the mean]
76
IB (MPa)
0.6
0.4
0.2
0.0
200
240
270
300
330
Hot-press time (s)
Fig. 3.8 Effect of hot-press time on the IB of particleboard. (Processing parameters
and statistical details are the same as those in Fig. 3.7)
3.5.5 Effects of different preparation methods on the mechanical properties of the
particleboard
Effects of two different methods for preparing particleboard on the MOR and MOE
are shown in Fig. 3.9. Wet method referred to the method described in section 3.4.2.1,
i.e., the SF was first prepared in the form of 20 wt% slurry and was then coated on the
wood particles. Dry method referred to the method described in the section 3.4.2.2,
i.e., an aqueous solution of CA, water and NaOH was directly sprayed onto a mixture
of SF and wood particles. The wet method was superior to the dry method in terms of
increasing the MOR and MOE (Fig. 3.9) although both MOR and MOE from either
method met the minimum industrial requirement (horizontal solid and dashed lines).
Effect of two different methods for preparing particleboard panels on the IB is shown
in Fig. 3.10. The IB from the wet method was significantly higher than that from the
77
dry method. The IB from the wet method met the minimum industrial requirement
(horizontal dashed line), whereas the IB from the dry method did not.
MOR (MPa)
MOE (GPa)
30
5
20
3
2
10
MOE (GPa)
MOR (MPa)
4
1
0
0
Wet method
Dry method
Fig. 3.9 Effects of two different methods for preparing particleboard on the MOR and
MOE of particleboards. [surface adhesive usage (dry basis on wood particle), 12 wt%;
core adhesive usage (dry basis on wood particles), 10 wt%; SF/CA/NaOH, 9/1.0/0.3;
hot-press temperature, 190 °C; hot-press time, 270 s; the targeted particleboard density,
0.80 g/cm3. Data are the means of eight replications, and the error bar represents one
standard error of the mean]
IB (MPa)
0.6
0.4
0.2
0.0
Wet method
Dry method
Fig. 3.10 Effect of two different methods for preparing particleboard on the IB of
particleboard. (Processing parameters and statistical details are the same as those in
Fig. 3.9)
78
3.6 Discussions
SF is mainly a mixture of soy protein and carbohydrates that contain a high amount
of polar functional groups such as hydroxyl, amino and carboxylic acid groups. These
poplar functional groups can form hydrogen bonds with wood substrates, which
explains why SF was widely used as a wood adhesive in 1930s to 1960s. However,
plywood panels bonded with SF alone are not water-resistant and not strong because
the hydrogen bonds can be easily disrupted by water. During the hot-pressing of
making particleboard, water inside the wood particle mat becomes stream that is
trapped inside the particleboard panel and will tend to come out when the hot-press is
open. The ventilation of the steam will disrupt the hydrogen bonding, thus reducing
the strength of the resulting particleboard. Under the hot-press conditions used in this
study, particleboard panels could not be made with SF alone as an adhesive in our
laboratory because all panels blew out, i.e., delaminated when the hot-press was open.
For improving the strength and water resistance of the resulting wood composite
panels, one of the most effective approaches is to use a curing agent (CA) that can
reacts with the polar functional groups in soy flour and crosslink soy flour and
potentially wood substrates, thus significantly increasing the bonding strengths
between the adhesive and wood and the strengths of adhesive networks. Therefore,
the amount of CA has to be sufficient for providing good crosslinking reactions, which
explains that the MOR, MOE and IB at 9/1 SF/CA weight ratio were much stronger
than those at 11/1 ratio (Fig. 3.1). The curing agent used in this study is low-
79
molecular weight polymers containing numerous reactive functional groups that are
able to react with the polar functional groups in soy flour. The low molecular weight
characteristic of the CA might lead to reduction of the strengths of adhesive networks,
which explains why the MOR and MOE decreased along with increasing the SF/CA
weight ratio from 9/1 to 3/1 (Fig. 3.1).
Most of reactive functional groups in the CA used in this study are expected to be
chlorohydrin (CH(OH)-CH2Cl). A base such as NaOH is known to be able to activate
the chlorohydrin group to form epoxy functional group and de-protonate the polar
functional groups, thus facilitating its reactions with polar functional groups in SF and
wood substrates.
This explains why addition of NaOH to the SF-CA adhesive
improved the MOR, MOE and IB. A strong base such as NaOH can also hydrolyze
the chloride group in the chlorohydrin, thus destroying the reactivity of the
chlorohydrin. Therefore excessive NaOH might lead to reduction of the particleboard
strengths, which is consistent with the results shown in Fig. 3.3.
All curing reactions of the adhesive occurred during the hot-pressing. At a
fixed hot-press time, the higher the hot-press temperature the higher the strengths,
which is consistent with the results shown in Figs. 3.5 and 2.6 that the MOR, MOE
and IB increased along with increasing the hot-press temperature. The adhesive would
be fully cured if the hot-press temperature was sufficiently high. Further increase in
the hot-press temperature would not further increase the strengths. As a matter of fact,
80
some degradations of wood and the adhesive might occur, thus reducing the strength if
the hot-press temperature was too high such as 200 °C in the Fig. 3.5.
At a fixed hot-press temperature, the longer the hot-press time the higher degree of
the curing reactions.
Results in Figs. 3.7 and 3.8 appear to suggest that 270 s was
required to fully cure the adhesive at the hot-press temperature of 180 °C.
Unlike the traditional aqueous UF resin which is directly sprayed on the surface of
the particles using regular air pump system, soy slurry with a high solid content is
highly viscous and is very difficult to spray. In this study, two methods (dry method
and wet method) were investigated for applying the SF-CA adhesive onto wood
particles.
According to the dry method, a CA solution was directly sprayed onto a
mixture of SF and wood particles. The adhesive-coated wood particles were then used
for making particleboard. According to the wet method, wood particles were first
coated with a dilute SF slurry and dried for reducing the moisture content of the SFcoated wood particles. A CA solution was then sprayed onto the SF-coated wood
particles. Results in Figs 3.9 and 3.10 suggested that the wet method was superior to
the dry method in terms of enhancing the MOR, MOE and IB. The dry method might
result in poor mixing between SF and CA, which led to low strengths of the resulting
particleboard.
3.7 Conclusions
In this study particleboard was successfully prepared and evaluated with a new soybased, formaldehyde-free wood adhesive. It was found that the best recipe of this
81
adhesive was composed of SF/curing agent/NaOH with a ratio of 9/1.0/0.3. The MOR,
MOE and IB met the minimum industrial requirements of M-2 particleboards at the
following variables: hot-press temperature at 190 °C, hot-press time at 270s, the
adhesive usage of surface particles at 12 wt%, the adhesive usage of the core particles
at 10 wt%, and the targeted particleboard density of 0.8 g/cm3.
3.8 Acknowledgements
We thank Flakeboard for providing wood particles.
3.9 References
1.
Haygreen, J.G. and J.L. Bowyer, Forest products and wood science. 3rd ed.
1996, Iowa State University Press: Ames, IA.
2.
John A. Youngquist, Wood handbook: wood as an engineering material. Chap. 10,
1999, Madison, WI U.S. Department of Agriculture, Forest Service, Forest
Products Laboratory.
3.
A. H. Conner, Wood: Adhesives, Encyclopedia of Materials: Science and
Technology, Elsevier Science Ltd, New York, 2001, p. 9583-9599.
4.
International Agency for Research on Cancer (2004) International Agency for
Research on Cancer classifies formaldehyde as carcinogenic to humans. Press
release #153.
5.
EPA website, http://www.epa.gov/iaq/formalde.html#Health Effects
6.
CEPA (2008) Composite wood ATCM. California Environmental Protection
Agency. http://www.arb.ca.gov/toxics/compwood/compwood.htm.
7.
EPA (2008) Formaldehyde emission from pressed wood products: Advance
notice of proposed rulemaking and notice of public meetings. EPA. US Federal
Register. Vol. 73 No. 233/Wednesday, December 3, 2008/Proposed Rules.
82
8.
EPA (2009) An introduction to indoor air quality- Formaldehyde. EPA.
http://www.epa.gov/iaq/fromaldehye.html.
9.
Liu K (1997) Soybeans-chemistry, technology, and utilization. International
Thomson Publishing, New York.
10. Liu Y, Li K (2002) Chemical modification of soy protein for woodadhesives.
Macromol Rapid Commun 23:739–742
11. Liu Y, Li K (2004) Modification of soy protein for wood adhesives using mussel
protein as a model: the influence of a mercapto group. Macromol Rapid Commun
25:1835–1838
12. Liu Y, Li K (2006) Preparation and characterization of demethylated lignin
polyethylenimine adhesives. J Adhes 82:593–605
13. Liu Y, Li K (2007) Development and characterization of adhesives from soy
protein for bonding wood. Int J Adhes Adhes 27:59–67
14. Li K, Peshkova S, Geng X (2004) Investigation of soy protein-Kymene-adhesive
systems for wood composites. J Am Oil Chem Soc 81:487–491
15. Huang J, Li K (2008) A new soy flour-based adhesive for making interior type II
plywood. J Am Oil Chem Soc 85:63–70
83
CHAPTER 4 GENERAL CONCLUSIONS
The adhesive consisting of soy flour (SF), polyethyleneimine (PEI) and maleic
anhydride (MA) and sodium hydroxide (NaOH) was evaluated for making
particleboard. The weight ratio of SF/PEI/MA/NaOH was 7/1.0/0.32/0.1. Hot-press
temperature, hot-press time, particleboard density and adhesive usage were optimized
in terms of enhancing the modulus of rupture (MOR), modulus of elasticity (MOE)
and internal bond strength (IB) of the resulting particleboard. The MOR, MOE and IB
met the minimum industrial requirements of M-2 particleboards at the following
variables: hot-press temperature at 170 °C, hot-press time at 270 s, the adhesive usage
of surface particles at 10 wt%, the adhesive usage of the core particles at 8 wt%, and
the targeted particleboard density of 0.80 g/cm3.
Another new adhesive consisting of SF, a novel curing agent (CA) and NaOH was
evaluated for making particleboard. The optimum SF/CA/NaOH weight ratio was
found to be 9/1.0/0.3 in terms of enhancing the MOR, MOE and IB of the resulting
particleboard panels. The MOR, MOE and IB could meet the minimum industrial
requirements of M-2 particleboard at the following variables: hot-press temperature at
190 °C, hot-press time at 270 s, the adhesive usage of surface particles at 12 wt%, the
adhesive usage of the core particles at 10 wt%, and the targeted particleboard density
of 0.80 g/cm3.
84
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