Radiation Curable Oligomers Combining
Superior Wear Properties with Enhanced
Chemical and Moisture Resistance
By William Schaeffer
Oaklands Corporate Center • 502 Thomas Jones Way • Exton, PA 19341 • 800-SARTOMER
www.sartomer.com
5120 07/04
This study will investigate the wear properties of an
epoxy acrylate and a urethane and compare its
performance to five new polyester acrylate oligomers.
These oligomers range in functionality from di to tetra,
thus offering a wide range of physical and performance
properties.
ABSTRACT
Protective coatings require excellent scratch and
abrasion qualities while being resistant to chemical
attack and moisture degradation. Acrylated urethane
oligomers can oft times provide the abrasion and
yellowing resistant properties necessary but may be
lacking in moisture resistance. Hardness properties are
relatively easy to obtain using highly crosslinked
materials that are usually based on hexafunctional
urethane acrylates. These oligomers while effective in
many applications, due to their high reactivity, flexibility
and resiliency are sometimes sacrificed. Epoxy acrylate
oligomers while being less expensive and more
chemical resistant than urethane oligomers may not have
the required wear properties. In this work the
performance of several novel acrylated polyester
oligomers will be explored, and the physical properties
compared and contrasted to both epoxy and urethane
acrylate oligomers and a competitive polyester acrylate
oligomer that is widely accepted in the industry using
industry standard test methods. The testing will
demonstrate that the wear properties and chemical
resistance can be enhanced over the standard epoxy
system using polyester acrylate oligomers offering a
cost effective alternative to urethane acrylate oligomers.
EXPERIMENTAL SECTION
Types of Oligomers Evaluated
Figure 1 illustrates the chemical structure of Bisphenol
A diepoxide reacted with acrylic acid to form a
Bisphenol A epoxy diacrylate. Acrylated epoxies are
fast curing and abrasion and chemical resistant but have
poor flexibility and will yellow when exposed to outdoor weathering conditions.
Figure 1
Epoxy Acrylate
O
O
O
O
O
O
HO
OH
• High gloss, high hardness
• Fast cure
• Chemical resistance
• High viscosity
• Yellowing
INTRODUCTION
Oligomers that impart a high degree of wear resistance
to a UV cured film find their way into many application
areas. The applications where abrasion resistance is
particularly critical are wood and plastic as these are
widely used in automotive, furniture and flooring
markets. These protective coatings must maintain their
appearance under adverse end use conditions.
Urethane acrylates crosslink into tough and flexible films
that are abrasion resistant and exhibit yellowing
resistant properties depending on the functionality and
the structure of the isocyanate and the polyol used in
the reaction. A typical urethane acrylate structure is
described in Figure 2.
Figure 2
Abrasion resistance is also one of the most complex
properties to attain, as it is both a surface and subsurface property. Thus abrasion resistance can involve
different mechanisms, which may be interrelated. In
addition, there is not a lone test method that relates
directly to actual product behavior. Understanding the
mechanisms of abrasion resistance is a challenge. It is
even more difficult to offer formulation concepts to
obtain abrasion resistant UV curable coatings.
Fortunately, the wide variety of products available to
the formulator makes this challenge somewhat easier.
Urethane Acrylate
Diisocyanate
O
R
O
Urethane Linkage
O
N
H
D
N
H
O
P
Polyol
Segment
2
O
O
O
N
H
D
N
H
O
R
Capping
Group
Aliphatic urethane acrylates that use polyester in the
backbone structure tend to be the most yellowing
resistant and are more flexible than the aromatic
urethanes. The aromatic urethanes usually are harder
and more chemical resistant and offer a good
compromise between an epoxy acrylate and an
aliphatic urethane in terms of wears resistance, adhesion
and flexibility. An even more effective cost/performance
benefit can be derived, by substituting a polyester
acrylate oligomer for the more expensive urethane
acrylate. Or blending the polyester with the urethane
to reduce the overall cost of the formulation while
enhancing wear and moisture resistance properties. In
this work we will investigate the performance
properties of each of these oligomer types along with
polyester acrylate oligomers in terms of abrasion
resistance.
Oligomers and Compositions Tested
Table 1 describes the oligomers evaluated in this study.
The Bis A epoxy is the “control“ oligomer as it is widely
used in many applications and is well known for its
rapid cure and high abrasion and chemical resistance
qualities. These properties will be compared to a
urethane acrylate oligomer commonly used in
applications when wear resistance is required and
aliphatic in structure. A competitive polyester acrylate
oligomer will also be evaluated. The performance of
these materials will be contrasted and compared to
several unique and novel polyester oligomers recently
commercialized.
Table 1
Component
Chemical Description
Functionality
CN120
Bis A Epoxy
Competitive Oligomer
Polyester Acrylate
2
CN963E75
Aliphatic Urethane
2
CN2250
Polyester Acrylate
4
CN2251
Polyester Acrylate
3
CN2252
Polyester Acrylate
3
CN2253
Polyester Acrylate
4
CN2254
Polyester Acrylate
2
Monomers and Photoinitiator Used
Formulations based on urethane acrylate oligomers,
epoxy acrylate oligomer or polyester acrylate oligomers
were tested in a typical UV curable wood coating using
the monomeric diluent and photoinitiator (PI) package
described in Table II. These monomers were selected
for the following reasons: SR212, 1,3- butylene gylgol
diacrylate, is a low viscosity monomer offering good
2
stain resistance and solvency. SR508, dipropylene
glycol diacrylate, is an excellent low viscosity,
economical diluent that can replace hexane diol
diacrylate in most applications. And finally, SR454,
3-mole ethoxylated trimethylolpropane triacrylate, is
a low skin irritation, fast curing monomer for use in
free radical polymerization. The level of oligomer used
in each formulation was held constant at 50%. The
3
monomers listed above were made into a separate
blend comprised of 1/3 each. To the 50 parts of
oligomer were added 46 parts of the monomer blend.
The PI employed (KIP100F) is a mixture of an
oligomeric alpha hydroxy ketone and 2-hydroxy-2methylphenyl 1- propane. It is a highly reactive, nonyellowing PI for the polymerization of UV coatings.
The level of PI was held constant at 4.0 percent.
Table 4 offers a listing of the various end use tests
conducted. All the films were very similar in terms of
performance when examined from a standpoint of
MEK resistance, reverse impact, adherence and pencil hardness properties. This work will concentrate on
the differences observed in terms of wear resistance
and physical properties of the cured film.
Table 4
Table 2
Application & Physical
Property Testing Conducted
Monomeric Diluent and Photoinitiator
Package Employed
•
•
•
•
•
•
•
•
• SR508: Dipropylene Glycol Diacrylate (DPGDA)
• SR212: 1,3 Butylene Glycol Diacrylate (BGDA)
• SR454: 3Mole Ethoxylated Trimethylolpropane Triacrylate (3EO
TMPTA)
• KIP100F: Monomeric Alpha Hydroxy Ketone
• Final Composition as Tested:
50 parts oligomer
46 parts monomer blend
4 parts photoinitiator
• Gloss @ 60 Degree
Reflectance Angle
• Modulus
• Tensile
• % Elongation @
Break
These tests include CS17 and S33 Taber resistance.
The modulus, tensile, elongation at break , as well as,
the glass transition temperature of the cured films will
also be reported and in turn correlated to the wearthrough properties observed.
Process Conditions
The application and cure conditions outlined in Table
3 were used to prepare all of the test panels unless
otherwise indicated. The chromate pretreated aluminum
Q-panels were used so as to eliminate any surface
differences normally associated with wood or plastics
substrates that may effect the performance properties
of the coating, particularly adhesion. All coatings were
applied using a number 40 wire wound rod and the
cured samples were allow to equilibrate for 24 hours
in a constant temperature and humidity environment
(22°C/ 55% RH) before any tests were
conducted.Tests Conducted
Table 3
MEK Rub Resistance
610 Tape Adhesion
Reverse Impact
Pencil Hardness
S33 Taber Abrasion
CS17 Taber Abrasion
Reverse Impact
Glass TransitionTemp.
Description of Test Methods
MEK resistance is an indication of the degree of
cross-linking associated with a given film and was
tested as outlined in Appendix 8 in the Radiation Curing Test Methods reference. Testing was ended at 200
MEK double rub cycles. The higher the MEK resistance the better the cure.
Adhesion is a measure of the force required to remove
the coating from the substrate. Tests were conducted
in accordance with ASTM D3359 using 610 tape.
Process Conditions
Reverse impact is a measure of the flexibility of the
cured film applied to a substrate. It value is reported
as the force require to fracture the film, the greater the
force the better the flexibility. The force is reported in
inch/pounds.
• Substrate: Q- Panel, AL46 Chromate
Pretreated
• Film Thickness: 50 Microns (2.0 mils)
• Cure: 300 w/in. Hg Lamp 6 m/m (20 fpm)
• UV Dose: 700 mj/sq. in., IL 390 Light
Meter
• Panels Evaluated After 24 Hours
Pencil Hardness is determined using a calibrated set
of drawing leads that range from 6B, the softest, to
6H, the hardest. The first pencil that scratches the
4
surface is reported as the coating’s hardness. This is
specified as ASTM D3363.
Table 5
Hardness, Flexibility, Adhesion and Chemical
Resistance of the Oligomers Tested
Taber Abrasion relates to the ability of a coating to
resist abrasive wheels. All panels were tested in
accordance with ASTM 4060-84. Taber resistance
was tested in two ways. The first and less aggressive
method is conducted using an S33 wheel under a 500g
load and counting the number of rotations required to
wear-through 1.0 mil of film. The second is based on
weight loss in milligrams per 500 cycles using the CS17
wheel under a 1000 g. load.
• MEK Resistance: > 200 Double Rubs
• Pencil Hardness: > 6H
• Reverse Impact: < 2 in/lbs.
• Tape Adhesion: 80-100% Loss
Gloss is the property of the coating surface that causes
it to reflect light. A gloss meter is used to quantitatively
measure the light reflected from a surface. All panels
were measured as specified in ASTM D523 at a 60degree angle of reflectance.
RESULTS AND DISCUSSION
When evaluating and comparing the performance of
base oligomers, hardness, flexibility, adhesion and
chemical resistance are important. The results attained
are related in the following Table 5.
Glass transition temperature (Tg) is the temperature at which the coating changes from a hard abrasion resistant coating to a soft, rubbery material. Properties such as refractive index and tensile properties
change significantly at the glass transition temperature.
Usually, the higher the Tg the greater the coating hardness. The glass transition temperature was measured
using a differential scanning calorimeter in accordance
with ASTM D3248.
As the data indicates all of these coating are similar in
these performance categories, which is not surprising
when the high UV dose that each of the formulations
received is considered. In all cases the chemical
resistance is excellent, exceeding 200 MEK double
rubs while the pencil hardness is greater then 6H, also
indicative of a highly cross-linked film.
As expected with a high degree of conversion the
flexibility suffers. As reverse impact testing
demonstrates, only two pounds of force are required
to fracture the coating. Highly flexible coatings will,
for example resist from 40-80 pounds of force without
fracturing.
Tensile properties are important to the performance
of radiation cured coatings as they directly impact the
performance of the cured film. A load is applied to the
film using a tensile testing instrument to determine
properties such as modulus, elongation at break, and
strength. Tensile is the greatest stress a coating can
withstand prior to breaking. Modulus is a measure of
the stress required to elongate the film a given distance
and elongation at break is the distance a film will stretch
before breaking. These properties were tested in
accordance with ASTM D882.
As films are more highly cross-linked greater shrinkage of the film occurs resulting poor adherence. When
tape adhesion is tested, around 80% adhesion loss is
observed. However, adhesion and flexibility can oft
times be attained through changes in the monomer and
additive package, whereas wear-through resistance is
a property that is typically imparted by the oligomer.
Thus the second part of this study is dedicated to the
wear-through resistance of the neat oligomers and how
this relates to the tensile properties and Tg of the cure
film. The use of a reactive matting agent and its effect
on gloss and adhesion will also be reported.
5
Wear-through Resistance
There are numerous test methods used to characterize
the wear resistant properties of a coating or film. The
most reliable method is to test the coating under actual
end use conditions. Although this method is the most
reliable it is also the most time consuming and difficult
to replicate. Taber abrasion testing is very often used
to screen the performance of the coatings in a
laboratory situation. There are two Taber techniques
that are used. The first is usually called “Cycles to
Wear-through Method” and it simply entails counting
the number of cycles to wear through a mil section of
film under a 500-gram load. There are two grades of
abrasion wheels that are commonly used for this. The
most abrasive is designated S-42 while the one being
less abrasive is identified as S-33. This method is most
useful for rapid screening of materials.
However the Urethane Acrylate (#2UA) demonstrates
greatly enhanced performance over either of those
products.
Figure 3
S33 Taber Cycles/mil of Coating,
500g. Load
100
95
90
85
#1 EA
#3 IS
#2 UA
CN2252
CN2250
CN2251
CN2254
75
CN2253
80
Urethane acrylates are also used for abrasion resistant
coatings where improved yellowing resistance is
needed. The aliphatic urethane diacrylate
(CN963E75) commonly used for this application does
improve the wear resistance over the epoxy acrylate
showing failure at 93 cycles.
The second method is based on weight loss verses the
number of abrasion cycles using a CS-17 abrasion
wheel with a 1,000 gram loading. The test continues
to 2,000 cycles and the panels are weighed every 500
cycles with the weight loss reported. The lower the
weight loss the better the abrasion resistance. Both
techniques were used in this study.
The polyester acrylate oligomers on the other hand do
exhibit several cost/performance benefits, being
considerably less expensive then Urethane acrylates
and more yellowing resistant and far more wear resistant
then the epoxy acrylate oligomer. In this particular series
of tests the cycles to wear through ranged from 90 to
100 as compared to only 85 for the epoxy acrylate
oligomer.
Cycles to Wear-Through (S33)
It was soon determined that the S-42 media was too
abrasive to show differences between the coatings and
as a result all samples failed between 80 and 90 cycles.
All further testing was conducted using the less abrasive S33 wheel. Refer to Table 6. The Taber testing
was repeated using the S-33 wheel. As this wheel is
less abrasive it is a more discriminating method of determining differences in surface hardness. The “control” formulation that is based on the epoxy acrylate
(CN120) is often used for its abrasion resistant qualities. This formulation in fact has the poorest abrasion
resistance of all material tested, requiring only 83 cycles
to failure.
CS-17 Taber Results
Additional abrasion resistance testing was conducted
using the CS17 method that is based on weight loss
per 500 cycles out to a maximum of 2,000 cycles.
The wear-through properties of an epoxy acrylate
(CN120) were compared to a series of Polyethylene
acrylate oligomers along with a urethane acrylate
oligomer and a competitive polyester acrylate that has
gained wide acceptance in wear and abrasion resistant
applications. Refer to Table VIII for a listing of the
oligomers tested and details of the results attained.
The oligomer identified as #3 is a product that has
gained wide acceptance in the industry and is used in
many applications where abrasion resistance and wearthrough properties are required. As predicted this
oligomer does perform somewhat better when
compared to the Epoxy Acrylate oligomer (#1 EA).
The same performance trend that was observed during
the “Cycles to Wear-through” testing continues. The
6
epoxy acrylate has the poorest abrasion resistance with
160 milligrams of weight loss exhibited after 2,000
cycles. The difunctional aromatic urethane (#2 UA) is
somewhat better then the epoxy with the milligrams of
weight loss dropping to 120 after 2,000 cycles. In this
test the competitive oligomer is far worse then the
epoxy or the urethane with weight loss values rising to
180 mg. In contrast the new polyester acrylate
oligomers far outperform the “control” materials ranging
in weight loss from 90 to 95 mg. Again the “cost
effectiveness” is demonstrated with lower raw
materials cost than a urethane with better wear
properties than the epoxy acrylate.
Figure 4
CS17 Taber, 1,000 g Load
Mg Weight Loss/500 Cycles
#3 IS
#2 UA
#1 EA
CN2252
CN2250
CN2251
CN2254
500
1000
1500
2000
CN2253
200
180
160
140
120
100
80
60
40
20
0
As the data above relates, these oligomers are quite
strong having Modulus values ranging from a low of
90,000 for the difunctional CN2254 to a high of
200,000 form the tetra functional product CN2250.
Although there is a wide range associated these values
the glass transition temperature range is rather high and
narrow spanning 37 to 48C. This indicates a high degree
of toughness that is required to attain the necessary
abrasion and wear resistance properties.
Summary of Physical Properties
The following table offers a listing of the physical
properties of the new polyester acrylate oligomers
tested along with comparative data on the competitive
product. These were tested in a formulation consisting
of 50 parts oligomer and 15 parts each of DPGDA,
3EO TMPTA and BGDA along with 5 parts of a
photoinitiator. These oligomers were tested in this
fashion, as the “neat” materials were too brittle after
curing to handle without breaking.
Table 6
PHYSICAL PROPERTY DATA OF THE FORMULATED OLIGOMERS
Product
Designation
CN2250
CN2251
CN2252
CN2253
CN2254
Competitive Product
Modulus,
PSI
200,000
150,000
180,000
190,000
90,000
70,000
Tensile,
PSI
4,800
4,000
4,600
5,000
5,600
2,500
% Elongation
@ Break
3
3
2
3
9
17
7
Glass Transition
Temperature, Degrees C
48
37
40
45
40
29
Table 7
PHYSICAL PROPERTY DATA OF THE “NEAT” OLIGOMERS
Product
Designation
Product
Brookfield
Viscosity
@ 25C, cps
Brookfield
Viscosity
@ 60C, cps
APHA
Color
CN2250
120,000
1,800
50
0.5
10.15
1.53
CN2251
60,000
1,200
300
1.7
9.9
1.53
CN2252
140,000
2,000
100
1.0
9.93
1.54
CN2253
375,000
4,000
180
1.0
10.2
1.53
CN2254
350,000
6,000
280
1.6
10.0
1.53
25,000
1,500
600
2.5
10.05
1.49
Competitive
Product
Table 6 above relates the liquid properties of the
Polyester Acrylate Oligomers. Aside from viscosity,
which is very important to know when selecting an
oligomer for a given application there are two other
pertinent aspects that should be pointed out. The first
is color of the oligomer. If an oligomer has a high color
in the liquid phase it may impart a yellow or amber
appearance to the coated substrate especially if the
substrate is white . Note that one oligomer CN2250,
has a Gardner color of 0.5 and CN2252 and CN2253
are a Gardner 1.0 as compared to the competitive
product that is at 2.5.
Gardner
Density,
Color
Pounds per
Gallon
Refractive
Index
@ 25C
CONCLUSIONS/OBSERVATIONS
Through accepted industry standard laboratory testing
it has been shown that polyester back-boned
oligomers have superior wear and abrasion resistance
when compared to a typical epoxy acrylate oligomer
and they are a cost effective alternative to urethanes
when weathering or yellowing resistance is not of critical
importance. They may be blended with urethane
acrylates without compromising the physical properties
of the cured film. Or they may be blended with an
epoxy acrylate containing composition to upgrade the
performance properties of the final formulation without
severely impacting cost.
The refractive index (RI) of the oligomer can also be
important when the final applied coating requires high
gloss or reflectance. The higher the RI typically the
higher the gloss. Note that the RI for the new oligomers
is nominally 1.535 as compared 1.49 for the other
oligomer. When applied to a given substrate this
difference equates in a gloss difference that is 10-12
points higher as measured using a Gardner Glossmeter
@ a 60 degree angle of acceptance.
REFERENCES
1) Radiation Curing Test Methods, 1988
2) UV/EB Curing Primer, 1997
3) Chemical Resistance of Oligomeric Acrylates,
Bo Yang, 1998
4) Weather Resistant Oligomers, Bo Yang, 1996
Novel Urethane Acrylate Oligomers
The information in this bulletin is believed to be accurate but all recommendations are made without warranty, since the conditions of use are beyond SARTOMER Company’s control. The listed
properties are illustrative only, and not product specifications. SARTOMER Company disclaims any liability in connection with the use of the information, and does not warrant against infringement by
reason of the use of its products in combination with other material or in any process.
8