EP0473712A1 - Adhesive compositions containing low molecular weight polyphenylene oxides - Google Patents

Abstract

The invention relates to self-adhesive and hot-melt adhesive compositions having improved tear adhesion defect (TEAD) temperatures comprising: (a) a block copolymer comprising at least two terminal blocks (A) of monoalkenyl arene polymer, as well as 'at least one intermediate block (B) diene with elastomeric conjugation, said block (A) comprising 8 to 55% by weight of said block copolymer; (b) about 50 to about 200 phr of a tacky resin compatible with the block (B); and (c) about 5 to about 50 phr of a low molecular weight polyphenylene oxide polymer, the molecular weight (Mvis) of said polyphenylene oxide polymer being between about 1,000 and about 5,000, the Tg being between about 100 ° and about 165 ° C, preferably between 140 and 163 ° C.

Description

ADHESIVE COMPOSITIONS CONTAINING LOW MOLECULAR WEIGHT POLYPHENEYLENE OXIDES

Thi s i s a conti nuation-i n-part of copendi ng U.S. Serial No. 355,532, fi l ed May 23, 1989, enti tled "Adhesive Composi tion Containing Low Mol ecular Weight Polyphenylene Oxides , " now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the use of low molecular weight polyphenylene oxides in adhesive blends comprising styrenic triblock copolymers such as polystyrene-polyisoprene-polystyrene (S-I-S) and polystyrene-polybutadiene-polystyrene (S-B-S) to provide increases in the shear adhesion failure temperatures (SAFT) of the correspondi ng pressure sensi ti ve, hot mel t pressure sensi ti ve or hot mel t

adhesives. The SAFT increases are obtained without significant hot melt formulation viscosity increases and with little impact on the pressure sensitive adhesives' tack or peel strength.

2. Description of the Prior Art

In U.S. Patent 3,660,531, there are disclosed polyblends containing: (A) greater than 50% of a thermoplastic resin matrix, said resin matrix consisting of polyphenylene oxide resin in combination with alkenyl aromatic resins; and (B) less than 50% of an elastomer selected from a group consisting of poly(butadiene), and random, block or graft copolymers of butadiene and styrene. The resulting blends exhibit unexpected thermoplastic properties

including improved melt processability and impact resistance without sacrificing the desirable heat distortion temperature and flexural modulus of unmodified polyphenylene oxide resin. The materials in this patent are thermoplastic resins and not adhesives, and the degree of polymerization (DP) of polyphenylene oxide is greater than about 100.

Commonly assigned U.S. Patents 3,835,200 and 3,994,856 di sclose respectively, polyphenyl ene ether and rubber styrene copolymer compositions containing rigid block copolymers of

conjugated dienes and vinyl aromati c compounds , and high impact rubber modified polystyrene compositions contai ning polyphenyl ene ether and vinyl aromatic block copolymers; however, the compositions are thermoplastics and the DP of the polyphenyl ene oxides is greater than 50.

Hot melt adhesive compositions are disclosed in Hansen, U.S. Patent 4,104,323. The adhesive composition is prepared by first melt blending a polyphenylene ether resin and a low molecular weight aromatic resin, and then blending the resulting mixture and a monoalkenyl arene/conjugated diene block copolymer, tackifying resin, and optional hydrocarbon processing oil. The molecular weight of the polyphenylene oxide in the polyphenylene oxide alloy is between 6,000 and 25,000. The glass transition temperature is between 170 and 205ºC. This melt blend avoids the use of solvents while also avoiding oxidative degradation of the block copolymer. The resulting polymer blend possesses a much higher service temperature when used as an adhesive.

An adhesive composition having improved high temperature properties is also disclosed in U.S. Patent 4,141,876. The

composition is prepared by melt blending a polyphenylene ether resin, a selectively hydrogenated arene/conjugated diene block copolymer, a tackifying resin, and optionally, a hydrocarbon processing oil. This patent is restricted to hydrogenated block copolymers which can withstand the extremely high blending temperatures required to disperse the polyphenylene oxide resins, (230ºC to 260ºC) and to polyphenylene oxide resins having a molecular weight (M_vis) between 6,000 and 25,000. The glass transition temperature of the resin is restricted to between 170° and 210ºC.

SUMMARY OF THE INVENTION

A need exists in the practice of adhesive formulating to obtain adhesive compositions with higher service temperatures and manageable hot melt viscosities. The present invention describes the use of low molecular weight polyphenylene oxide resins (PPO) in hot melt or pressure sensitive adhesive compositions comprising:

(a) 100 phr of a block copolymer having at least two monoalkenyl arene polymer endblocks A and at least one elastomeric conjugated-diene midblock B, said blocks A comprising 8-551 by weight of the block copolymer. Illustrative of the blocks are styrenic block copolymers such as polystyrene-polybutadiene-polystyrene (S-B-S), polystyrene-polyisoprene-polystyrene (S-I-S), poly

(α-methylstyrene)-polyisoprene-poly (α-methyl styrene), or their selectively hydrogenated derivatives.

(b) about 50-200 phr (part per hundred rubber) of a tackifying resin compatible with the rubbery midblock of the block copolymers and

(c) about 5-50 phr of a low molecular weight PPO resin with glass transition temperature (T_g) between 100ºC and 165ºC,

preferably between 140° and 163ºC.

The tackifying resin, which is compatible with the elastomeric midblock of the triblock copolymer, is used to render the formulation tacky. Preferred tackifying resins are those derived from the copolymerization of diolefins and especially of C₅

diolefins such as piperylene with C₅ olefins such as

2-methyl-2-butene. These resins, such as ESCOREZ 1310LC, available commercially from Exxon Chemical, have ring and ball softening points between 80°C to 115ºC. Another useful tackifying resin, Zonatac 105 Lite, available from Arizona Chemicals, is prepared by the cationic polymerization of limonene and styrene. Other useful tackifying resins include those derived from rosin esters, terpenes, and terpene phenolic resins. Hydrogenated versions of the above are also useful.

Hydrocarbon extending oils (0-200 phr) can be employed in this application to modify the formulation viscosity and to increase the tackiness of the adhesive. The extending oils, referred to as paraffinic/naphthenic oils are fractions of refined petroleum products having less than 30% by weight aromatics and viscosities ranging from 100 to 500 SSU at 100ºF. Oils are commercially available such as Shellflex 371, a naphthenic oil manufactured by Shell.

The adhesive formulations are prepared by dissolving in a solvent such as toluene, and casting over a substrate such as mylar. Optionally, to apply the formulation as a hot melt, the components are melt blended in a Brabender mixer. The temperature for melt blending will depend upon the T_g of the PPO. This is a significant advantage of using PPO of lower T_g than that claimed in U.S.

Patents 4,104,323 and 4,141,876.

The invention discovery is that polyphenylene oxide copolymers, having low molecular weight and high glass transition temperatures, extend the temperature range of pressure sensitive and hot melt adhesive systems which contain styrenic triblock

copolymers. This is a consequence of their compatibilities with the polystyrene domains of the triblock copolymers used in these adhesive applications. Because these adhesive formulations are useful up to the glass transition temperature of the polystyrene domains, blending a high T_g PPO polymer into the polystyrene domains increases the T_g of the glassy domains and consequently increases the useful temperature range of the adhesive. The glass transition temperature range for the PPO resin, 100-165ºC, preferably 140-163ºC enables hot melt application of the adhesive formulation. Higher Tg PPO resins cannot be hot melt processed unless they are preblended with low molecular weight aromatic resins such as polystyrene as described in U.S. Patent 4,104,323. Furthermore, the lower Tg PPO's of this invention provide superior adhesive properties compared with the higher Tg PPO's.

According to the "Glossary of Terms Used in the Pressure Sensitive Tape Industry", a pressure sensitive adhesive is a material which is aggressively and permanently tacky, adheres without the need of more than finger pressure, exerts a strong holding force, and has sufficient cohesiveness and elasticity that it can be removed from substrates without leaving a residue. A hot melt adhesive, on the other hand, is a 100% nonvolatile thermoplastic material that is heated to a melt and applied to the substrate as a liquid. The hot melt bond forms after the liquid cools and solidifies. Some pressure sensitive adhesives, such as those based on block copolymers, are applied as hot melts, and are referred to as hot melt-pressure sensitive adhesives.

Typically, commercial PPO's are derived from the

2,6-dimethyl phenol monomer. In accordance with this invention there is described the use of high T_g, PPO copolymers. One advantage of the use of the copolymers is the lower cost of the monomers such as o-cresol as compared with the more expensive 2,6-dimethyl phenol monomer thereby resulting in a lower cost PPO. Further, the use of comonomers yields the low molecular weight PPO resins which are best suited for these applications. The useful glass transition

temperature range for these PPO resins ranges from 100-165ºC, preferably between 140-163ºC. This range, less than that described in U.S. Patents 4,104,323 and 4,141,876, provides superior adhesive service temperature increases while allowing hot melt processibility below 200°C.

These low molecular weight polyphenylene oxides improve the high temperature performance of styrenic block copolymers in pressure sensitive adhesive systems. For example, a 7 parts per one hundred rubber (phr) loading of the PPO in a formulation provides about a 32ºF improvement in the shear adhesion failure temperature (SAFT) with little impact on the pressure sensitive adhesive's tack. DETAILED DESCRIPTION OF THE INVENTION

In the use of low molecular weight polyphenylene oxides to increase the service temperatures of block copolymer adhesive systems, the upper use temperature of these adhesives is limited to the softening temperature (T_g) of the polystyrene domains. In accordance with this invention, a high T_g PPO with good polystyrene thermodynamic compatibility increases the service temperature when blended into the adhesive formulation.

Block copolymers employed in the invention may have geometrical structures, however the invention does not depend on a particular structure, but rather upon the chemical constitution of each of the polymer blocks. Thus, the structures may be linear, radial, or branched so long as each copolymer has at least two polymer endblocks and at least one polymer midblock. Thus the invention contemplates (but is not limited to) the use of

configurations such as (A-B-A)_n where n varies from 1 to 20, and preferably from 1 to 3, most preferably 1. Methods for preparation of such polymers are well known in the art. Although the term triblock is used throughout it is to be understood that where applicable the radial and branched blocks are included.

The invention applies especially to the use of polymers having the configuration of the following typical species:

polystyrene-polybutadiene-polystyrene (SBS)

polystyrene-polyisoprene-polystyrene (SIS)

poly(alpha-methylstyrene)-polybutadiene-poly

(alpha-methyl styrene) (αMS-B-αMS)

poly(alpha-methylstyrene)-polyisoprene-poly

(alpha-methyl styrene) (αMS-I-αMS).

It is to be understood that both Blocks A and B may be either homopolymer or random copolymer blocks as long as each block predominates in at least one class of the monomers characterizing the blocks as defined. Thus, blocks A may comprise styrene/alpha- methylstyrene copolymer blocks or styrene/butadiene random copolymer blocks as long as the blocks individually predominate in monoalkenyl arenes. The term "monoalkenyl arene" includes styrene and its analogs and homologs including alpha-methyl styrene and

ring-substituted styrenes, particularly ring-methylated styrenes. The preferred monoalkenyl arenes are styrene and alpha-methyl styrene, and styrene is particularly preferred. The blocks B may comprise homopolymers of butadiene, isoprene, copolymers of butadiene and isoprene and copolymers of one of these two dienes with monoalkenyl arene as long as the blocks B predominate in conjugated diene units. The rubbery midblock of these polymers may be hydrogenated, but non-hydrogenated midblocks can also be used since excessively high blending temperatures are not generally required to prepare the blends of the present inventory. When the monomer employed is butadiene, it is preferred that between about 35 and about 55 mole percent of the condensed butadiene units in the butadiene polymer block, have a 1,2 configuration.

Polyphenylene oxides of the invention will have repeating units represented by the formula:

wherein the oxygen ether atom of one unit is connected to the benzene nucleus of the next adjoining unit, n is a positive integer of from 10 to about 40 thereby providing a MW range of about 1000-5000, and each Q is a monovalent substituent selected from the group consisting of hydrogen, halogen, hydrocarbon radicals, hydrocarbonoxy radicals, and halohydrocarbonoxy radicals having at least two carbon atoms between the halogen atom and phenyl nucleus. Especially preferred polyphenylene oxide resins for purposes of the present invention are those having alkyl substitutions in the two positions ortho to the oxygen ether atom - i.e. where each Q is alkyl, most preferably, having from 1 to 4 carbon atoms. The polyphenylene oxides employed in accordance with the invention, prepared from 2,6-xylenol and additional comonomers such as o-cresol, allow the cost of the PPO to be materially reduced.

Also, since it is necessary to control the extent of polymerization to obtain PPO products of low molecular weights, the use of the comonomers and control of the amount of oxygen admitted to the reaction allows preparation of the low molecular weights necessary to the invention.

In general, the low molecular weight polyphenylene oxides are prepared using a cuprous chloride-pyridine catalyst system in chlorobenzene solution. Magnesium sulfate is used to remove moisture from the reactions. The products are isolated by precipitation with a 10% HCl/methanol solution, and are dissolved and reprecipitated to remove any residual traces of catalyst or diphenoquinone side products.

PPO yields and glass transition temperatures are controlled by varying the degree of polymerization. This is achieved by changing the reaction time and consequently the amount of oxygen. A longer reaction time permits the formation of higher molecular weight and high T_g products, which, when precipitated, afford higher recoveries.

Further control of the molecular weight is provided by the use of o-cresol or other comonomers, which give low degrees of polymerization with the present catalyst system.

Polyphenylene oxides of low molecular weight, useful in the invention, also can be prepared according to the Perec article in J. of Polymer Science (vol 25, p 2605) from 4 bromo-2,6-dimethylphenol as monomer (see Example 5).

EXAMPLE 1

Cuprous chloride (10g) and pyridine (50ml) are stirred for 30 minutes in 500 ml chlorobenzene. o-Cresol (48ml) and anhydrous magnesium sulfate (1.5g) are added and the reaction is stirred for 28 hours at room temperature. The insoluble portion of the reaction is filtered away and the resin is precipitated with a 10% HCl/methanol solution. The resin is isolated by filtration and washed with methanol. The yield is 15g (301) of a pale orange, brittle solid. The glass transition temperature of the solid is 103ºC and H-NMR of the solid shows a 1:1 ratio of aliphatic to aromatic protons.

EXAMPLE 2

Cuprous chloride (10g) and pyridine (50ml) are stirred for 30 minutes at room temperature in 500 ml chlorobenzene. o-Cresol (25g), 2,6-xylenol (25g), and anhydrous magnesium sulfate (1.5g) are added and the reaction is stirred for 28 hours. The insoluble portion of the reaction mixture is removed by filtration and the resin is isolated by precipitation with 10% HCl/methanol. The resin is isolated by filtration and redissolved in toluene and precipitated with methanol to remove any residual catalyst or dimeric side products. The yield is 38g (761) of a pale orange solid. The glass transition temperature is 105ºC and H-NMR analysis shows a 5:3 ratio of aliphatic to aromatic protons.

EXAMPLE 3

The procedure of Example 2 is followed except that 26.5g of 2,6-xylenol and 26.5 g of o-cresol are used. The reaction is stirred for 48 hours. A 70% yield of a pale orange solid is obtained. The glass transition temperature of the resin is 153ºC and H-NMR analysis shows a 9:5 ratio of aliphatic to aromatic protons.

EXAMPLE 4

The procedure of Example 1 is followed except that 50g of 2,6-xylenol is used instead of the o-cresol. Also, the resin is dissolved in toluene and precipitated with methanol. A 61% yield of a yellow solid is obtained. The glass transition temperature of the solid is 154°C and H-NMR analysis of the solid shows a 3:1 ratio of aliphatic to aromatic protons. EXAMPLE 5

4-Bromo-2,6-dimethylphenol (38g) is dissolved in 316 ml 6N NaOH. Ammonium hydrogen sulfate (5.08g) and 316 ml toluene are added, and the mixture is stirred for 23/4 hours, whereupon, it is quenched with dilute HCl. The toluene phase is separated and dried with magnesium sulfate, and the polymer is isolated by precipitation with methanol (5.5g). DSC and GPC analyses are performed with the following results: T_g=163°C, Mn=2600, Mw/Mn-1.53.

EXAMPLE 6

The procedure of Example 3 was followed except that 75g 2, 6-xylenol, 50 mol pyridine, 900 ml chlorobenzene, and 5g magnesium sulfate were used. The mixture was stirred for 72 hours. The polymer yield was 43.5g, and the glass transition temperature was 145ºC.

In summary, the yields, glass transition temperatures, and product compositions for the PPO products prepared for testing in adhesive formulations are as follows:

GLASS TRANSITION YIELD COMPOSITION

TEMPERATURE

Example 1 103ºC 30% o-Cresol

Example 2 105ºC 76% 3:4 Xylenol :o-Cresol

Example 3 153ºC 70% 1:1 Xylenol :o-Cresol

Example 4 154ºC 61% Xylenol

Example 5 163ºC 24% Xylenol

Example 6 145ºC 30% 1 :1 Xylenol :o-Cresol PPO product compositions were determined using H-NMR spectroscopy. A ratio of aliphatic protons (1.5-2.5 ppm) to aromatic protons (6-7.4 ppm) indicates the relative amounts of cresol and xylenol present in the resins. An entirely 2,6-xylenol product contains a 3:1 ratio of aliphatic to aromatic protons while an entirely o-cresol product contains a 1:1 ratio of aliphatic to aromatic protons. In the following embodiments, examples and comparisons, these materials were employed:

( 1 ) Kraton 1107; a styrene-isoprene-styrene block copolymer from

Shell having block molecular weights of about

13,000-160,000-13,000.

(2) ESC0REZ 1310LC; a C₅ olefin/diolefin tackifying resin from

Exxon Chemicals having a ring and ball softening point of

95ºC.

(3) Stereon 840A; a tapered styrene-butadiene-styrene block

copolymer from Firestone having Mn of 60,000 and 43 wt % styrene.

(4) Zonatac 105L; a limonene/styrene tackifying resin from

Arizona Chemicals having a ring and ball softening point of

105ºC.

(5) Shellflex 371; a naphthenic extending oil from Shell.

( 6) Irganox 1010; an antioxidant from CIBA-Geigy.

(7) Noryl; a PPO from General Electric having a Tg of 194ºC.

ADHESIVE TESTING WITH SIS FORMULATIONS

S-I-S formulations with E-1310LC as tackifier resin were prepared for testing as pressure sensitive adhesives. All PPO products were used at two different levels and the 90° quick stick, 180° peel, polyken tack, and shear adhesion failure temperatures were measured for each of the formulations. The formulations were cast from toluene onto mylar, and dried in an oven at 80ºC to give a .0015 in. coating.

The adhesive tests are those commonly employed by the pressure sensitive adhesive industry. In the shear adhesion failure temperature test, a l"xl" overlap of tape to a stainless steel substrate is made with a 4.5 pound roller. A 1 kg weight is hung from the tape and the assembly is placed in an oven. The temperature is increased at 40ºF/hour and the temperature at which the weight drops is recorded as the SAFT. In the polyken tack test, a steel probe contacts the adhesive tape with a specified force frr a 1 second dwell time. The force required to break the bond between the adhesive and the stainless steel probe is measured (g). The 180º peel test involves placing a length of tape on a stainless steel plate and laminating it with a 1-pound roller. The force (lb/in) required to peel the tape at a 180º angle on an Instron is recorded.

The results of the adhesive testing are summarized in Table 1.

It is apparent from Table 1 that the examples representative of the invention have superior SAFT than those of the comparative examples. This indicates that lower T_g PPO resins are also useful in increasing the service temperatures of adhesives. Finally, the tack and peel properties of these adhesives are not adversely affected by the PPO resins as indicated by maintenance of the peel and quick stick values with modest declines in polyken tack.

Figure 1 illustrates the adhesive performance findings for use of the PPO product of Example 3 with SIS formulations.

ADHESIVE TESTING WITH SBS FORMULATIONS

Stereon 840 (SBS) formulations with a Zonatac 105 Lite/ Shellflex 371 tackifying system were prepared for testing as hot melt adhesives. All four PPO products were used at different levels and the results of the 180° peel and shear adhesion failure temperatures are compiled in Table 2.

Figure 2 illustrates the superior adhesive performance findings for the use of the PPO product of this invention with SBS formulations.

The viscosity results indicate that the PPO products of the invention can be formulated into adhesive formulations for hot melts without significantly altering the viscosity profile.

Stereon 840 (SBS) formulations with a Zonatac 105 Lite/She! Iflex 371 tackifying system were prepared for testing as hot melt adhesives and solvnet cast pressure sensitive adhesives. For the purpose of comparison of the PPO's of this invention with the PPO's of the prior art, the formulations were prepared with the PPO of Example 6 (Tg=145°C) and NORYL (Tg=194ºC). For the hot melt adhesive, viscosity, T-Peel , SAFT and PAFT were evaluated. For the solvent cast pressure sensitive adhesive, 180° peel and SAFT were evaluated.

The T-Peel test was performed according to the procedure of ASTM D01876-72, for both aluminum and polyethylene. The shear adhesion failure temperature (SAFT) was determined as descrieed above, except that a 500 g weight was used for a 1" x 1" overlap of Kraft paper bonded to Kraft paper. The peel adhesion failure temperature (PAFT) utilized the same geometry as the ASTM D-1876-72 T-Peel test except with a 1" x 1" overlap of Kraft paper bonded to Kraft paper. The PAFT evaluation was conducted in an oven with a 200 g weight attached. The reported temperature was the

temperature at which the bond failed when the oven was ramped at 40ºF/hour. The 180° peel test was as described above. The results of the adhesive testing are summarized in Table 4.

While the invention has focused on the use of certain particular low molecular weight PPO polymers having a T_g of about 100°C to about 165ºC to improve the upper temperature performances of styrene block copolymer adhesive systems, it is to be understood that a wide range of these polymers are suitable and that the compositions can be dictated by economic considerations. For example, the data show that cresol copolymers exhibited

performances comparable to the more expensive xylenol homopolymer. Therefore, many monomer combinations based on cresylic acids and phenol methylation products can be used without departing from the spirit and scope of the use of low molecular weight PPO polymers for high temperature applications in pressure sensitive and hot melt adhesive systems.

The most remarkable feature of the data presented in Table 4 is that the NORYL (tg-194ºC) could not be hot melt blended at temperatures up to 220ºC, in contrast to the low Tg (145ºC) PPO of example 6. Furthermore, the adhesive properties of the lower Tg PPO of Example 6 formulations are superior to properties achieved with the higher Tg NORYL resin formulations. These data demonstrate a clear advantage in processability and in adhesive performance for the low Tg PPO compositions of this invention versus the higher Tg PPO resins taught in the prior art.

Claims

CLAIMS :

1. An adhesive composition having improved shear adhesion failure temperatures (SAFT) comprising:

(a) 100 phr of a block copolymer having at least two monoalkenyl arene polymer endblocks A and at least one elastomeric conjugated-diene midblock B, said blocks A comprising 8-551 by weight of the block copolymer;

(b) about 50 to about 200 phr of a tackifying resin compatible with block B; and

(c) about 5 to about 50 phr of a low molecular weight polyphenylene oxide polymer, having a molecular weight (M_vis) between about 1000 and about 5,000, and a T_g of about 100º-165ºC.

2. The composition of claim 1 wherein the block polymer is a triblock.

3. The composition of claim 1, wherein the polyphenylene oxide polymer has a Tg of about 140º-163°C.

4. The composition of claim 2, wherein the triblock copolymer comprises styrene endblocks and a polyisoprene rubbery midblock.

5. The composition of claim 2, wherein the triblock copolymer comprises styrene endblocks and a butadiene rubbery midblock.

6. The composition of claim 1, wherein the block copolymer is a triblock copolymer having styrene-alpha-methyl styrene endblocks and a polyisoprene rubbery midblock.

7. The composition of claim 3, wherein the block copolymer is a triblock copolymer having styrene endblocks and a polyisoprene rubbery midblock.

8. The composition of claim 3, wherein the block copolymer is a triblock copolymer having styrene endblocks and a butadiene rubbery midblock.

9. The composition of claim 3, wherein the block copolymer is a triblock copolymer having styrene-alpha-methyl styrene endblocks and a polyisoprene rubbery midblock.

10. An adhesive tape comprising the adhesive composition of claim 1.

11. A substrate coated with the adhesive composition of claim 1.

12. The composition of claim 4, wherein the block copolymer is hydrogenated.

13. The composition of claim 7, wherein the block copolymer is hydrogenated.

14. The substrate of claim 12, wherein the substrate is metallic.

15. A hot melt adhesive comprising the composition of claim 1.

16. A pressure sensitive adhesive comprising the composition of claim 1.

17. A hot melt pressure sensitive adhesive comprising the composition of claim 1.