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Terpene Phenol Resins - Tackifiers for the Next Generation of Adhesives
Erwin R. Ruckel, Manager of Research and Chief Scientist and Wayne K. Chu, Sr. Group Leader
- Rheology
Arizona Chemical Co., International Paper Corporate Research Center, Tuxedo, NY 10987
Abstract
Terpene phenol resins, prepared by the oligomerization of terpene and phenol feed
streams, can be molecularly engineered to have moderate to highly polar structures. As such
they can be tailored to be compatible with synthetic polymers such as polyurethanes or
biodegradable polyesters. By virtue of the hydrogen bonding between terpene phenol resins with
polar substrates, a reinforcement of the formulated composite is achieved, exemplified by an
increase in heat resistance of an adhesive bond.
This flexible, synthetic technology has been employed to prepare resins which can not
only function as tackifiers and adhesion promoters but also as ductility enhancers for engineering
thermoplastics and surface energy modifiers.
Introduction
The natural terpenes are very versatile materials which can be molecularly engineered to
provide resin structures having broad utility. A conventional carbocationic polymerization1 of
terpene streams provides polyterpene resins which have Hildebrand solubility parameters ()
ranging from 8.1 to ~8.4, which makes them ideal tackifiers for natural rubber, synthetic
polyisoprene and styrene-isoprene-styrene (SIS) type block copolymers. The incorporation of an
aromatic component into the terpene feed stream renders the resin somewhat more polar ( 8.6)
which permits compatibilization with polybutadiene, styrene-butadiene-styrene (SBS) or (SB)n
type block copolymers. Tackifying resins of highest polarity can be prepared by a Friedel Craftsoligomerization of terpene-phenol feedstreams2. The differentiation of these different classes of
terpene-based tackifying resins on a linear Hildebrand polarity scale is presented in Figure 1.
Resin Preparation
Terpene phenol resins are low molecular weight (MW) oligomers which are prepared by
a Friedel Crafts reaction. Protonation of the terpene, typically alpha-pinene, results in a
cationated species which reacts with the phenol. For steric reasons, the initial alkylation
(terpinylation) occurs at the para position of the phenol which is followed by subsequent limited
alkylation at one of the two ortho positions. Further chain extension beyond "dimer" occurs
when the pendant terpene groups (para and some ortho) are again cationized (see Figure 2).These protonated species can then react with another phenol monomer in a type of chain
extension.
Once alkylated, a phenol moiety becomes more susceptible to reaction at the phenolic
hydroxyl group; this gives rise to some O-alkylation by terpinyl moieties (see Figure 3). The
extent of O-alkylation can be controlled by the judicious choice of synthetic conditions and by
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the ratio of terpene to phenol in the feedstream. The net effect of O-alkylation and (C10H16)xplacements is to modulate the polarity of the terpene phenol resin downward to lower .
Resin Properties
The level of bound phenol in Arizona Chemical commercial terpene phenol resins,indicated in Figure 4, significantly affects the solubility profile map of the resin. The two-
dimensional solubility maps of a terpene phenol resin with 24 percent bound phenol (TP-24) and
a terpene phenol resin with 38 percent bound phenol (TP-38) are presented in Figure 5. The
dramatic influence of the higher level of phenol in TP-38 is seen by the extension of the polar
(right) side of the map.
To gain a visual perspective of the polarity relationship with other terpene-based resins, a
portion of the solubility maps of a resin prepared from an all-terpenic feedstream, and a resin
prepared from a terpene stream fortified with some styrene, are presented in Figure 6, along with
a TP-38.
A comparison of the most polar portions of the solubility maps illustrates the progressive
increase in polarity.
To complete the comparison of solubility profiles of the major classes of commercial
tackifying resins, those of two rosin esters are shown. Figure 7 presents the profiles of a
glycerine-based rosin ester and a pentaerythritol-based rosin ester.
The degree of hydrogen bonding that can be achieved by using a terpene phenol resin in
an adhesive construction can have important ramifications concerning the performance.
Therefore, the level of free phenolic hydroxyl functionality in the resin is a key parameter which
controls the interaction between resin and a polar polymer such as ethylene vinyl acetate (EVA).As expected, the concentration of the hydrogen bonded species is directly proportional to the
amount of free hydroxyl present in the resin. The ratio of free hydroxyl to terpinylated, i.e.,
ether, structure is presented in Figure 8:
TP-38, having the greatest level of available phenolic hydroxyl functionality for
hydrogen-bonding, has a predominantly, but not exclusively, alternating structure.
T-P- (T-P- )nT-P where n is 1 to 3
Key physical characteristics of Arizona Chemical commercial and developmental terpene
phenol resins are listed in the table labeled Figure 9. The glass transition temperatures wererecorded using a Seiko 220C DSC at 10o C/min scan. The number average MW are averages of
readings taken on a Knauer VPO.
Although the terpene phenol resins are brittle solids, their low MW permits prompt
melting at moderately elevated temperatures and rapid change in viscosity with temperature. The
viscosity dependence on temperature is illustrated in Figure 10.
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The free phenolic hydroxyl functionality of commercial terpene phenol resins, presented
in Figure 11, can be seen to correlate with the level of bound phenol.
The net effects of employing higher terpene:phenol feedstream ratios is a less polar resin
having less available free hydroxyl.
Arizona Chemical is currently focused on broadening its line of terpene phenol resins to
include midrange softening point products. To this extent, we have developed 100o C softening
point analogs to our commercial resins. Additional resins that are under development have
softening points from 50o C to 170o C, with a range of polarities. In order to control the
performance of terpene phenol resins there is need to regulate:
molecular weight, molecular weight distribution
percent hydroxyl
structure
terminal OH : interior OH ratio
The last three parameters govern the Hildebrand solubility parameter, , and the ability to
hydrogen-bond.
Utility/Performance
Terpene phenol resins display broad utility which includes:
rheological modifier for polar polymers
processing aid (ductility modifier) for engineering thermoplastics
adhesion promoter due to surface phenomena
antioxidant properties for thermal and oxidative stability
The inherent high polarity of terpene phenol resins permits compatibilization with the
following polymeric backbones:
polyurethanes (reactive hot melts)
repulpable adhesives
biodegradable adhesives based on polyhydroxy-butrate (PHV), polyhydroxy-valerate3 (PHB)
polylactide4 and starch-based hot melt adhesives
The patent literature is replete with references to the use of terpene phenol resins in
reactive hot melt adhesive constructions; some typical uses are the following:
structural assemblies
wood bonding
book binding
can labeling
exterior panels
vinyl clad windows
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In these formulations the terpene phenol resin dramatically increases heat resistance.
There is a growing interest in the use of terpene phenol resins in adhesives that are
designed to be environmentally compatible. By nature, these polymers which can be either
synthetic or natural, have backbone structures that are inherently very polar. This situationrequires a highly polar resin, which preferably has hydrogen-bonding capability, in order to be
compatible. These adhesive systems use raw materials derived from renewable natural resources
which can biodegrade at a faster rate than their synthetic counterparts.
An example of the dramatic effect that a resin has on adhesive performance, resulting
from hydrogen bonding, can be seen by comparison of the rheological and adhesivity
characteristics imparted by terpene phenol resins vis-a-vis rosin esters. This comparison is based
on waterborne formulations, but the results are similar using hot melt technology. Figure 12
shows a reference rheological trace of a butyl acrylate-based pressure sensitive adhesive tackified
with an Arizona Chemical rosin ester dispersion. Note the reduction in the storage modulus,
which measures the resistance to deformation, that results when 30 percent by weight of rosinester is incorporated into the formulation. A similar adhesive formulated with an experimental
terpene phenol resin shows a much lower reduction in the storage modulus which is a
consequence of the hydrogen-bonding phenomenon (Figure 13). The result of this rheological
difference can be seen in the adhesive data generated on these two formulations compared as
adhesive tape constructions (Figure 14). Most noteworthy is the dramatic improvement in shear
strength that results from the use of a terpene phenol resin.
Adhesive manufactures have compounded small-to-moderate amounts of terpene phenol
resin into EVA-based hot melt adhesives to enhance adhesion to polar substrates. It is believed
that an EVA/terpene phenol/wax hot melt adhesive, which is totally compatible in the fluid hot
melt, will partially phase separate on cooling. This phenomenon results in phenolic hydroxylgroups on the adhesive surface which can extend across the adhesive-substrate interface to
hydrogen-bond to polar functionality on the surface of the substrate (Figure 15).
In order to understand the reinforcement function of terpene phenol resins, the hydrogen
bonding interactions and compatibility of terpene phenol/EVA blends were investigated as
functions of the OH content of the terpene phenol resin. Detailed information about hydrogen-
bonding in the terpene phenol/EVA blends can be obtained from the IR absorption characteristics
of either the carbonyl group or the hydroxyl group. Figure 16 shows the FTIR spectra of blends
as a function of the OH content of the terpene phenol resin. Two conclusions can be drawn from
these spectra:
a) the neat EVA (33 percent vinyl acetate) shows a single carbonyl band at ~1,740
cm-1 but the terpene phenol/EVA blends have two carbonyl bands - one at 1,740 cm-1 resulting
from the "free" carbonyl group and the other at 1,710 cm-1 corresponding to the carbonyl group
hydrogen-bonded to the terpene phenol resin.
b) the greater the OH content in the terpene phenol resin, the more hydrogen bonds
between the terpene phenol resin and EVA, and the stronger the carbonyl band at 1,710 cm-1.
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The compatibility of terpene phenol resins was determined from the clarity of the blended
films. The three Arizona Chemical commercial terpene phenol resins were all highly compatible
with EVA and the presence of wax had no effect on compatibility. Due to the strong hydrogen-
bonding between terpene phenol and EVA, these components are compatible over a wide range
of OH and VA content.
Each hydrogen-bond between a terpene phenol resin and EVA will function as a physical
crosslink at room temperature, providing the hot melt adhesive with higher shear and cohesive
strength.
Thermal Stability
Terpene phenol resins are inherently thermally stable at 400o F in both inert and air
atmospheres (Figure 17). Under these conditions, chosen to simulate the highest temperature
that might be employed in commercial processing, no thermal "depolymerization" was observed
via gel permeation chromatography (GPC).
The molten stability of EVA-based hot melt adhesives tackified with NIREZ V-2040 is
outstanding and does not require an antioxidant to achieve this effect. Figure 18 shows no
surface skinning or gelling when the compounded adhesive is maintained for four days at 350 o F
in an air environment. This coupled with no viscosity increase indicates that terpene phenol
resins also possess antioxidant qualities.
Oxidative Stability
The close similarity of the terpene phenol resin structure to that of the hindered phenol
class of commercial antioxidants suggests that these resins also possess antioxidant properties.The expected antioxidant effect of terpene phenol resins was demonstrated by incorporating low
levels of resin into a very oxidation-sensitive SIS type of block copolymer and measuring the
extent of oxidation under accelerated aging conditions (70o C/15 days). Figure 19 shows that
KRATON D-1111 degrades extensively as measured by the appearance of the carbonyl band at
1,730 cm-1. Note the dramatic reduction in the extent of oxidation by the presence of as little as
10 weight percent terpene phenol resin. Since adhesive formulations typically contain 20 to 40
percent tackifying resin, those adhesives formulated with terpene phenol resins would have
inherent resistance to oxidation and, therefore, may not require the presence of a separate
antioxidant.
Summary
The unique structure of terpene phenol resins makes them ideal products for the
rheological modification of polar polymers. By virtue of hydrogen bonding interactions, these
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terpene phenol composites exhibit high internal strength. Consequently, formulations designed
for adhesivity characteristics have enhanced shear strength and shear adhesion failure
temperature (SAFT). Additionally, the hindered phenol type of structure of this class of resin
imparts significant oxidative stability to adhesive formulations.
Acknowledgments
The authors wish to thank their research colleagues X. Lu and R. P. Scharrer for their
assistance in conducting experiments and for their insight and guidance.
References
1. Iovine, Kauffman, Schoenberg and Puletti, U.S. Patent 5,252,646, National Starch and
Chemical.
2. Kauffman, Brady, Puletti and Raykovitz, U.S. Patent 5,169,889, National Starch and
Chemical.
3. Arlt and Ruckel, U.S. Patent 3,761,457, Arizona Chemical Co.4. Lahourcade and Bonneau, U.S. Patent 4,056,513, Les Derives Resiniques et Terpeniques,
Dax, France.
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