hot melt adhesive

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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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