Fabio D. S. Larotonda,
Katia S. Matsui,
Sabrina S. Paes,
Joao Borges Laurindo
Department of Chemical and Food Engineering, Federal University of Santa Catarina,
Florianopolis, Brazil
Impregnation of Kraft Paper with Cassava-Starch Acetate - Analysis of the Tensile Strength,
Water Absorption and Water Vapor Permeability
Kraft paper is widely used in the manufacture of packing, but its hygroscopic properties and water
vapor barrier are not suitable for the storage of dehydrated products for long periods. The objective
of this work was to study the influence of the impregnation of this paper with cassava-starch acetate
(CSA, degree of substitution, DS, 1.41) and the consequent modification of tensile strength, water
absorption and water vapor permeability. SEM-micrographs were taken and experiments of water
absorption, tensile strength, water vapor permeability and water sorption isotherms were carried out
for original and CSA-impregnated Kraft paper. The results obtained showed that the impregnation
of the Kraft paper with CSA significantly decreases the water absorption and the water vapor
permeability. Water sorption isotherms showed great differences only at high relative humidity. In
relation to mechanical properties, the tensile strength of the Kraft paper increased with its
impregnation by CSA. Therefore, CSA impregnation was demonstrated to be a very interesting and
new alternative for the use of cassava starch to obtain waterproof paper.
Keywords: Kraft paper; Starch acetate; Impregnation; Waterproof
1 Introduction
Kraft paper is used classically as material for the manufacture of packaging which requires good
mechanical properties, such as industrial packaging for large volumes, due to the paper's high
mechanical resistance to tearing and to tensile forces. These properties are explained by the long
fibers used in the manufacture of Kraft paper and its basis weight of 30-150 g/m2 [1].
On the other hand, the hygroscopic properties and the water vapor barrier of Kraft paper are not
suitable for long-term storage of dehydrated products, whose shelf life would be reduced. For this
reason, we studied the impregnation of samples of commercial Kraft paper with CSA synthetized in
the laboratory. In this work, the term impregnation is used instead of coating, because this last term
is related to a outside layer covering the surface of a material. Impregnation promotes the input of a
solution in the pores of the material.
Starch is a natural polymer consisting of two types of molecules: amylose, formed by
predominantly linear glucose chains, and amylopectin, formed by branched glucose chains [2, 3].
Amongst the agricultural cultures that proCorrespondence: Joao Borges Laurindo, Department of Chemical and Food Engineering, Federal
University of Santa Catarina, BP 476 Florianopolis - SC, Brazil 88040-900. e-mail
joao@enq.ufsc.br.
duce starch, one of the most used in Brazil is the cassava culture, Brazil being the second largest
producer of this root worldwide, closely following Nigeria [4].
Many works approach the chemical modification of starch as an alternative to improve its
properties. The acetylation reaction is one of the most interesting. This reaction allows the
attainment of a thermoplastic and hydrophobic material [5, 6]. By acetylation part of the hydroxyl
groups of glucose monomers are converted into acetate ester groups, modifying the molecular
structure and consequently the properties and applications of starch [7, 8].
The degree of substitution (DS) of the acetylated starch indicates the average number of hydroxyl
groups (OH) substituted by acetyl groups per anhydroglucose unit of starch. The highest possible
DS is 3 because there are three OH groups available per anhydroglucose unit [9]. Narayan et al.
[10] have shown that starch acetates with DS between 1.2 and 1.7 have the most preferred balance
in mechanical properties, water resistance, processibility and rate of biodegradation.
Recently, there has been renewed interest in intermediate DS (1-2) and high DS (3) starch acetates
for applications such as biodegradable films, molded articles [11, 12], foams [13, 14] and coatings
for paper. In this last application, starch acetate was used to reduce water sensitivity of hydrophilic
materials, being included in foamed starch
trays and used to coat these trays, wheat gluten based films and paper sheets [15].
The objective of this work was the modification of the hy-groscopicity, water permeability and the
tensile strength of Kraft paper by means of impregnation with CSA obtained in the laboratory.
2 Materials and Methods
2.1 Kraft paper
The Kraft paper (75 g/m2) used in the experiments was purchased commercially in Florianopolis,
SC, Brazil.
2.2 Cassava-starch acetate (CSA)
The CSA used for the impregnation of the samples was synthetized in the laboratory, according to
US Patent n° 5,710,269 [8], with the replacement of catalytic agent MSA (methanesulfonic acid) by
concentrated sulfuric acid.
The cassava starch was provided by Amifar Ind. (Deo-dapolis/MS, Brazil), and contains 17%
amylose. In order to obtain CSA, cassava starch was mixed with acetic anhydride and glacial acetic
acid, and this mixture was heated to 40 °C under agitation. When the temperature of the mixture
reached 40 °C, the catalytic mixture (concentrated sulfuric acid glacial acetic acid) was added
carefully and gradually during the first 10 min of reaction. After that, the solution was heated up to
80 °C and kept at this temperature for 2 h. When the reaction was complete, cold water (5-6 °C)
was added to the solution to precipitate starch acetate. The precipitate was washed with distilled
water and dried in a ventilated oven for 12 h at 60 °C.
The synthetized CSA was evaluated in relation to the degree of substitution (DS) (or degree of
acetylation). The method utilized to evaluate the degree of substitution was based on Wurzburg
[16], who developed the following methodology: a sample of 1.00 g of the polymer is dissolved by
boiling it in 50 mL of an aqueous alcoholic solution (75%, v/v) and the polymer is saponified with
40 mL of a 0.5 N sodium hydroxide (NaOH) solution, then the excess of NaOH is back titrated with
0.5 N hydrochloric acid (HCI).
The degree of substitution (DS) is calculated from the degree of acetylation using Equation 2:
The degree of acetylation is calculated as follows:
where:
Vo = HCI in the absence of the polymer
Vp = HCI in the presence of the polymer
NHCl = HCI normality
The degree of substitution (DS) is calculated from the degree of acetylation using Equation 2:
The DS for the CSA obtained in the laboratory was 1.41 [17].
2.3 Preparation of samples
Samples with dimensions of 25x100 mm and 100x100 mm were taken from the commercial Kraft
paper using shears. The thicknesses of the samples were measured using a Digimatic digital
external micrometer (Mitutoyo Co., Kawasaki, Japan), with exactness of ±0.001 mm.
Measurements of thickness were made at four different points on each sample, and these were used
to calculate the average values. The solution used in Kraft paper impregnation was prepared from
the solubilization of starch acetate in chloroform at the ratio of 1:5. At this ratio a homogeneous
solution without precipitate was obtained, because CSA is soluble in chloroform. This solution was
placed in a container that in turn was placed in a dessica-tor, where the samples of Kraft paper were
immersed and kept for 10 min, with the aid of a screen and a weight to prevent the samples from
floating. Depending on the case, a vacuum of 80 kPa, produced by a vacuum pump (model TE-058Tecnal, Piracicaba, SP, Brazil), was used.
All the impregnated and non-impregnated samples were conditioned for seven days at 25 °C in
dessicators with a saturated solution of sodium chloride, producing an RH of 75%. The samples
were codified in accordance with Tab. 1.
2.4 Scanning electron microscopy (SEM)
Microscopy of the samples was carried out using a Philips XL-30 scanning electron microscope.
The samples were coated with a fine gold layer before the taking the micrographs. All samples were
examined using an accelerating voltage of 20 kV.
Tab. 1. Samples codification.
2.5 Water absorption
The method used to study the water absorption for the samples was based on the ABNT standard
method NBR NM ISO 535 [18], that is itself based on the Cobb method. This method consists of a
gravimetrical analysis, where the samples with known area are weighed before and after their
immersion in distilled water, for an exact time. The samples used in this experiment had dimensions
of 100x100 mm and the immersion time was 60 s.
2.6 Mechanical properties
To determine the mechanical properties of the Kraft paper, tensile strength tests were carried out in
accordance with theASTM standard method D828 [19], in a Universal Test Machine, model
DL2000 (EMIC, Sao Jose dos Pin-has, PR, Brazil) for mechanical tests of traction, using the load
cell R2797 (500 N). The samples used in this test had dimensions of 25x100 mm, with an average
thickness of 0.135 mm for PK samples, 0.170 mm for PKI samples, and 0.153 mm for PKIV
samples. The tests of traction for the samples of Kraft paper were carried out with and without
impregnation with starch acetate.
2.7 Water vapor permeability
The water vapor transmission rate (WVTR) was determined gravimetrically at 38 °C, based on
ASTM standard method E96 [20]. The samples (discs of paper, 90 mm in diameter) had been
previously conditioned at 90% relative humidity (RH) at 38 °C for 24 h. After this pre-conditioning,
the samples were placed in permeation cells with calcium chloride (CaCI2) and hermetically sealed
on the edges with a mixture of refined paraffin and microcrys-talline wax. These permeation cells
(capsules) were placed in a glass chamber of dimensions 400x400x250 mm, with a saturated
solution of barium chloride (BaCI2) to produce a relative humidity of 90% in its interior. A small
fan was installed inside the chamber, to promote circulation of internal air and to maintain a
homogeneous RH in this space. The chamber with the permeation cells was placed in an oven and
maintained at 38 °C.
The water vapor transference through the area of a sample (50 cm2) was measured by the weight
gain of the capsule in function of the time. The capsules were weighed at the beginning of the
experiment and every 2 h, for 12 h. Three samples of each group (PK, PKI and PKIV) were
analyzed in the experiments. The water vapor transmission rate (WVTR [g h-1 m-2]) was calculated
by Equation 3:
where Δm/Δt and are respectively the time mass variation of the capsule, in the steady state
condition, and the area of permeation (50 cm2). The term Δm/Δt was calculated by linear
regression of experimental mass transfer data, in the steady state. With the data of WVTR, the water
vapor permeabilities (WVP, [g mm h-1 m-2 mm Hg-1]) were calculated by Equation 4.
where e is the average thickness of the samples, Pvs is the pressure of saturation of the vapor at the
experimental temperature 38 °C, and Rh1 and RH2 are the relative humidities in the interior of the
chamber and in the interior of the capsule, respectively.
2.8 Water sorption isotherms
Water sorption isotherms were determined by the gravimetrical method. Samples with dimensions
of 30x30 mm were previously dried at 105 °C for 24 h. The samples were then placed in dessicators
with different RH, determined by the saturated saline solutions used. The experiment was carried
out at 35 °C. The samples were weighed periodically until they reached constant weight, after
which the samples' moistures were determined by the gravimetric method.
3 Results and Discussion
3.1 Scanning electron microscopy (SEM)
Representative micrographs of the original and the starch-acetate-impregnated samples of Kraft
paper are shown in Figs. 1a, 1b and 1c. The micrographs indicate that a starch acetate layer on the
surface of the Kraft paper was formed. Figs. 1 b and 1 c show that the starch acetate solution filled
the superficial pores shown in Fig. 1a. The change of the hygroscopic properties of the impregnated
Kraft paper is a consequence of this filling, which is hypothesized as taking place in the following
manner. The starch acetate film on the surface of the impregnated samples becomes fissured while
drying and the filling process can be promoted by the solvent's evaporation and consequent
substitution by air [21].
3.2 Water absorption
The results of the tests of water absorption for the different samples, as well as their thicknesses, are
presented in Tab. 2 and Fig. 2. The presented values are the averages of the values obtained in the
assays that were car-
Fig. 1. SEM-micrographs for the samples of Kraft paper (a) without impregnation, (b) impregnated
at atmospheric pressure and (c) impregnated under vacuum. In all cases, the used magnification was
100 x.
ried out in triplicate. The Kraft paper impregnated under vacuum absorbed 56.4% less water than
the non-impregnated Kraft paper and 11.4% less water than the impregnated Kraft paper under
atmospheric pressure.
It is interesting to observe that the average thickness of the impregnated paper at atmospheric
pressure (PKI) is higher than the average thickness of the paper impregnated under vacuum (PKIV).
This indicates that the superficial starch acetate film formed in the first case possesses a greater
thickness than the second. The lower water absorption of the vacuum impregnated paper suggests
that the vacuum promoted a greater penetration of the solution into the pores of the paper and
decreased its porosity.
These results indicate that the impregnation of the Kraft paper with starch acetate can be an
alternative for the reduction of the water-mate rial interaction.
3.3 Mechanical properties
The mechanical properties of the samples were analyzed by measuring tensile strength (σR) and
elongation at the rupture (εR). The experimental values of σR and εR are presented in Tab. 3 and Fig.
3.
Fig. 2. Water absorption of Kraft paper samples.
Tab. 2. Average values of water absorption for the samples of Kraft paper.
Tab. 3. Mechanical properties of original and impregnated Kraft paper determined experimentally
for samples of each type, i.e., PK, PKI and PKIV (the standard deviations are shown in brackets).
Fig. 3. Effect of impregnation with starch acetate on the mechanical properties of the Kraft paper.
Fig. 4. Curves of time/weight evolutions of the samples of Kraft paper (PK), Kraft paper
impregnated at atmospheric pressure (PKI) and Kraft paper impregnated under vacuum (PKIV).
These data show that the impregnation of the Kraft paper promoted significant modifications in its
mechanical properties. The impregnated samples supported higher tensions than the original ones.
The samples impregnated at atmospheric pressure and under vacuum supported tensions 3.46 times
and 1.62 times greater, respectively, than the tensions supported by the original Kraft paper. On the
other hand, elongation at rupture was greater for the samples of Kraft paper without impregnation.
3.4 Water vapor permeability
The time/mass evolution of the permeation cells was linear from the start of the experiments, as
presented in Fig.
4. The fact that a period of transient mass transfer was not observed can be attributed to the preconditioning of the samples. This linear behavior indicates that the permeation occurred in the
steady state. All linear regressions had presented correlation coefficients higher than 0.99.
The results obtained in the experiments of water vapor permeability for all samples are presented in
Tab. 4. Also given are the coefficients of correlation values (R2) of the linear regressions, the
angular coefficients (Δm/Δt ) of the curves, the average thicknesses of the samples, and the values
of WVTR and WVP for all samples, as well as the average values of these last properties.
Fig. 5. Comparison of the values of the water vapor permeabilities for PK, PKI and PKIV.
Fig. 6. Water sorption isotherms for the samples of Kraft paper (Δ-PK), impregnated Kraft paper at
atmospheric pressure (◊-PKI), and Kraft paper impregnated under vacuum (□-PKIV).
he WVTR of the Kraft paper impregnated under vacuum was 2.13 times lower than the WVTR of
the Kraft paper without impregnation and 1.16 times lower than the WVTR of the Kraft paper
impregnated at atmospheric pressure. As the samples had different thicknesses, it is more suitable to
compare their water vapor permeabilities. Thus, the WVP of the Kraft paper impregnated under
vacuum was almost two times lower, when compared to the WVP of the Kraft paper without
impregnation and 1.17 times lower than the WVP of the Kraft paper impregnated at atmospheric
pressure.
3.5 Water sorption isotherms
The water sorption isotherms determined at 35 °C are presented in Fig. 6. All the isotherms showed
a sigmoid shape (Type II isotherm). A slight reduction of the hygro-scopicity of impregnated Kraft
paper for RH lower than 85% can be observed. For higher RH, equilibrium moistures of
impregnated samples were strongly reduced. There were no significant differences between the
sorption curves of impregnated samples PKI and PKIV.
4 Conclusions
The results obtained in this work showed that significant reductions in water absorption and water
vapor permeability of Kraft paper might well be achieved by starch acetate impregnation. An
augmentation in the tensile strength was also observed, but it was not significant because many
cracks were present on the surfaces of the samples.
Therefore, the demonstrated starch acetate impregnation of Kraft paper is an interesting alternative
for the improvement of the hygroscopic properties and water vapor barrier of the Kraft paper.
Moreover, the use of starch acetate in the impregnation of hygroscopic materials reveals an
interesting alternative for the use of starch, adding value to this raw material and providing an
incentive for agricultural producers.
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(Received: October 23, 2002)
(Revised: February 22, 2003)
(Accepted: May 20, 2003)