research shows the performance benefits of glycidyl esters.
Epoxies have been a leader in the adhesive and
structural-composite marketplaces for over 50 years. Over time, interest has
grown in using epoxy adhesives to replace rivets and to use epoxy composites to
cost-effectively replace heavier steel. Now more than ever, there is a focused
emphasis on finding alternative adhesives and construction composite materials
that offer even lower construction costs, better energy efficiency, and less
environmental pollution. Industries most likely to benefit include automotive,
high-performance sports equipment, industrial, marine and aerospace.
The epoxies used in these applications are typically one- or two-part systems
that offer desirable properties such as adhesion and/or structural strength.
The performance properties of epoxy resin formulations are further maximized by
the addition of modifiers. These modifiers may improve performance in adhesion,
toughness, glass-transition temperature (Tg
) or other
attributes formulators might need.
Epoxy modifiers come in a variety of forms and chemistries, each offering a
different set of benefits - and sometimes challenges. Most conventional
modifiers are higher-viscosity liquid or solid polymers that cause further
thickening to the uncured epoxy formulation, creating handling challenges.
These high formulation viscosities also limit the amount of low-cost fillers
that can be used. As a result, the market is showing increased interest in
lower-viscosity liquid modifiers that would allow for easier handling and the addition
of low-cost fillers without sacrificing performance.
Compatibility can influence the adhesion and/or toughening of the final
product. Therefore, it is important to consider compatibility in both the
pre-reaction mix and in the final cured epoxy system; some modifiers may be
compatible in the pre-reaction mix but may then phase separate upon curing. In
addition, others may be incompatible in the pre-reaction mix and stay
incompatible upon curing.
An example of a modifier that is compatible in the pre-reaction mix but
incompatible (phase separates) during cure is a carboxy-terminated butadiene
acrylonitrile copolymer (CTBN) that may or may not be adducted with epoxy
resin. CTBN polymers and adducts with bisphenol A epoxy resin serve as industry
benchmarks, as they are known for their unique morphology (see Figure 1) and
their ultimate performance. The size and shape of the spherical rubbery
inclusions that form during the epoxy matrix cure are controlled by the cure
A low-molecular-weight reactive diluent, such as an aliphatic polyglycidyl
ether or ester, is one modifier that is compatible in both the premix and the
cured formula. These materials do not phase separate at any time before or
after the epoxy matrix cures, resulting in a transparent, more ductile, or
elastomeric epoxy. However, they typically are not used to enhance adhesion or
Incompatible modifiers, however, do not significantly affect Tg
and do not enhance adhesion. On the other hand, they do provide toughening.
Examples of modifiers that are not compatible at any time before or after cure
are core shell particles or filled or hollow glass beads.
Epoxy formulations can be designed to maximize performance of certain attributes,
as desired in a specific final end-use application. However, this is usually
accomplished at the expense of other properties. With the exception of
incompatible modifiers, adding a modifier usually results in the loss of Tg
Proper formulation is important to achieve balance so that other properties of
the epoxies are not compromised in the process to improve a certain performance
This article examines the structure/property relationship of
compatible liquid modifiers. CVC Thermoset Specialties conducted a study to
show how variation in modifier molecular weight, backbone composition and
end-group functionality affects the modifier and formulation viscosity or
handling properties, as well as the Tg
, adhesive, and
toughness properties of the cured epoxy. As a result of this work, new modifier
chemistries have been developed and are documented further in the company’s
Three polymer backbones and two end group functions were used in the study. The
polymer backbones were dimer-based polyester, polybutadiene and
butadiene/acrylonitrile copolymer. The end group functions were bisphenol A epoxy
adducts and glycidyl esters.
All polymer backbones were carboxylic acid terminated. They were then further
reacted with either bisphenol A epoxy resin to form adduct esters or with
epichlorohydrin to form the glycidyl esters of the carboxylic acids. The
bisphenol A epoxy adducts were prepared by reaction of the carboxy-terminated
polyesters or butadiene-acrylonitrile copolymers (40 parts). The adducts were
prepared with an excess of bisphenol A epoxy resin (60 parts) using
triphenylphosphine as a catalyst so that the resulting product contained 40% of
the modifier. The glycidyl esters were synthesized by reaction of the
carboxy-terminated polyester, CTB or CTBN polymers with epichlorohydrin
followed by dehydrochlorination. The structures for the backbones and
end-functional groups are shown in Figure 2.
Model Formula and Test Protocol
The model formulation for the study used liquid bisphenol A
epoxy resin, dicyandiamide as a curative and OmicureTM
U52M as an accelerator. Fumed silica was also added to aid in maintaining the
suspension prior to cure. Following is the formulation composition.
Bisphenol A liquid epoxy resin
15% modifier based on epoxy resin
6 phr dicyandiamide
3 phr Omicure U52M
2 phr fumed silica
Glass beads (250 micron) were added to maintain constant adhesive thickness in
the bond line. All formulation cures were conducted by heating the specimen to
125°C and holding for two hours.
Following are the tests and methods used in this study.
Viscosity: ASTM D 2393
of cured formulation: ASTM E 1356
Lap shear of cured formulation: ASTM D 1002
T-peel of cured formulation: ASTM D 1876
Fracture toughness (K1c) of cured formulation: ASTM D 5045
Formulation preparation included mixing all ingredients together at room
temperature using a Cowles mixer. The mixture was then applied to the substrate
or poured into a mold. Phosphate-treated, cold-rolled steel (1-by-4-inch) was
used as the substrate for all lap shear and T-peel testing. Phosphate-treated,
cold-rolled steel was chosen as the test substrate because this substrate is
known for providing the best cohesive bond for epoxy adhesives. The goal was to
have predominantly adhesive failure to ensure that the properties of the
adhesive were truly being measured.
Results and DiscussionBisphenol A Epoxy Adducted
Four bisphenol A epoxy adducted modifiers were prepared and tested in this
study. Two were based on carboxylic acid terminated dimer polyesters:
2101 and 2104. The other two adducts were
based on CTBN: HyproTM
1300X8 and 1300X13. Each
formulation was adjusted such that the modifier content was maintained at 15%
of the composition. All samples were cured as stated above.
The viscosities of these modifiers (see Table 1), ranging from about 100,000 to
450,000 cps at 25°C, are inherently higher than the viscosity of the bisphenol
A epoxy resin used in the formulation because of the adduction process. The
adduction process generates oligomers with the resin yielding higher molecular
weight species. The formulation viscosities (observed for Examples 2-5) are
also quite high compared to the unmodified Example 1 because of the adducts’
viscosities. The industry benchmark (Example 5) has the highest mix viscosity
of those in this study. As bisphenol A epoxy resin adducts are the most widely
used modifiers in the industry, this high viscosity has to be accommodated by
the use of more specialized and costly formulating process equipment, which
limits the amount of low-cost fillers used. Lower-viscosity modifiers are
always being requested by the industry.
of the cured modified formulations (Examples 2-5)
is about 15 to 25°C lower than the unmodified formulation. This is due to some
level of compatibility of the modifier in the epoxy matrix. Although these
adducts are higher in molecular weight, there is a compatibility factor that
comes into play because the bisphenol A epoxy moiety is now a part of the
molecule enhancing compatibility. Comparison of the Tg
of Examples 2-5 shows that Tg
increases with molecular
weight but does not appear to be correlated with the polymer backbone
The overall lap shear and T-peel
adhesive properties are far better than the standard, and quite similar, with
the exception of Example 2, which has a lower molecular weight and
glass-transition temperature than the other modifiers in this group. As both
polymer backbones are known to exhibit good adhesive properties, these findings
are not unexpected. Example 5 exhibits slightly higher adhesive strength,
probably because of higher polarity from the increased acrylonitrile content.
Fracture toughness (K1c
) properties within the set are
good compared to the unmodified formulation. The polyesters of Examples 2 and 3
are better than the CTBN adducts of Examples 4 and 5. The improved K1c
in Examples 2 and 3 are indicative of more ductility in the epoxy matrix as a
result of more compatibility and lower Tg
. Again, there
is a relationship showing that molecular weight has an affect on Tg
compatibility and toughness.
Glycidyl ester modifiers were based on Priplast 2104, Hypro 2000X162 and three
butadiene/acrylonitrile copolymers: Hypro 1300X31, 1300X8, and 1300X13. These
materials, shown in Table 2 as Examples 6-9, have viscosities of ~40,000 to
~576,000 cps at 25°C. These viscosities are lower than viscosities seen for the
bisphenol A epoxy adducted materials in Table 1. The mix viscosities of the
formulated products also have a significantly lower viscosity than their epoxy
adduct counterparts. This is because they are simple glycidyl esters, not oligomeric.
Because the entire glycidyl ester molecule is the modifier, these modifiers are
used “as is” at 15% loading to achieve the desired modifier level in the epoxy
formulation. In contrast, because the modifiers comprise only 40% of the
bisphenol A epoxy adduct molecule, the epoxy adducts are used at much higher
levels in the formulation to achieve equivalent modifier loading.
A comparison of Tg
of Examples 6-10 in Table 2 with the
unmodified formulation (Example 1) shows that they are again lower, indicating
that there is some portion of the modifier still compatible with the epoxy
matrix. However, a comparison of Tg
with the previous
study (Table 1) shows that these glycidyl esters are less compatible and have
less of an affect on Tg
. Again, within the set of
Examples 6-10, the Tg
follows along with the concept
that higher molecular weight gives higher Tg
compatibility in the epoxy matrix.
The adhesive properties, as expressed by lap shear and T-peel, are improved
over the unmodified formulation in Example 1. The glycidyl
butadiene/acrylonitrile copolymers appear to be better than either the glycidyl
polyester or the glycidyl polybutadiene. A comparison of the glycidyl esters
(Table 2) with the bisphenol A adducts (Table 1) with a similar backbone
indicates that the glycidyl esters provide better adhesive properties and
at lower formulation viscosities.
The glycidyl ester modifiers also show good improvement in fracture toughness
compared to the unmodified formulation. The 3,000-molecular-weight glycidyl
ester shown in Example 6 exhibits the highest K1c
but the lowest Tg
. This, again, may be indicative of
ductility from some compatibility with the epoxy matrix. The glycidyl
polybutadiene ester and all the glycidyl butadiene/acrylonitrile copolymer
esters (Examples 7-10) generate similar K1c
Compact Tension Test Specimens: Thermoset Resin
Summary and Conclusion
The test results demonstrate that the modifiers examined in
this study, whether adducted or glycidated, provide significant advantages in
performance properties compared to the unmodified control formulation. Results
also showed that glycidyl ester modifiers provide significant advantages over
the bisphenol A epoxy adducts in their ability to reduce processing viscosity
without sacrificing adhesive or toughener performance.
Other conclusions of the study include the following.
- The glycidyl ester modifiers are lower in viscosity
and yield lower formulation mix viscosities than the industry
- Selection of the proper glycidyl ester will allow for higher Tg
and desired adhesive and toughener performance.
- The end group functionality appears to affect compatibility in the
cured epoxy matrix, which in turn affects Tg. Glycidyl
esters exhibit less compatibility than epoxy adducts and thus give higher Tg.
- The study shows that adhesive properties are affected by the polymer
backbone. Both polyester backbones and butadiene/acrylonitrile copolymer
backbones provide good adhesion.
- The best adhesion performance was achieved with glycidyl ester
modifiers prepared using a CTBN.
No individual modifier will yield optimal results in all applications. The
results of this study, however, demonstrate that there are newer alternatives
with the potential to deliver better performance results in applications in
which a low-viscosity modifier is needed. Glycidyl esters offer a number of
advantages over the industry benchmark and, therefore, should be considered as
the market continues to move toward products that are both energy- and
more information, contact Dr. Starner by phone at (856) 533-3021 or e-mail Bill.Starner@emeraldmaterials.com.
HyproTM, HyPoxTM, and
OmicureTM are trademarks of Emerald Performance
Materials companies. PriplastTM is a trademark of Croda
About the AuthorDr.
William Starner is technical director for CVC Thermoset Specialties, an Emerald
Performance Materials company. With more than 30 years in the industry, he has
experience in new molecule research, process development, manufacturing and
project commercialization. Starner is the creator and implementer of more than
100 products for the polyurethane and epoxy markets, and holds 26 patents and
numerous publications in these areas.