CA1117470A

CA1117470A - Radiation cured adhesive composition

Info

Publication number: CA1117470A
Application number: CA000320604A
Authority: CA
Inventors: David R. Hansen; David J. St. Clair
Original assignee: Shell Canada Ltd
Current assignee: Shell Canada Ltd
Priority date: 1978-03-03
Filing date: 1979-01-31
Publication date: 1982-02-02
Also published as: DK152513B; DK152513C; JPS6239184B2; EP0004102A1; ES478208A1; US4133731A; AU521700B2; EP0004102B1; ATA157479A; AT366083B; JPS54132635A; AU4472879A; DE2964684D1; DK88079A

Abstract

ABSTRACT
RADIATION CURED ADHESIVE COMPOSITION
A radiation cured adhesive composition comprising a thermoplastic elastomeric block copolymer, a tackifying resin and a di-, tri- or tetra ester of acrylic or methacrylic acid and an at least dihydric aliphatic alcohol.

Description

1~1747~

RADIATION CURED ADHESIVE COMPOSITION

Thermoplastic elastomeric block copolymers have been proposed in the past to produce adhesive compositions, for example U.S.P. 3,239,478 discloses combinations of these block copolymers with tackifying resins and paraffinic extending oils. Limitations of these adhesive compositions are their relatively low service temperatures and their poor solvent resistance and for a number of applications it would be advantageous to have higher service and better solvent resistance temperatures. For example, these adhesives would be useful in paint shops if the masking tapes produced had good solvent resistance and a 105-120C
service temperature. They would also be useful as lamina-ting adhesives in food pouches if they could withstand boiling water temperatures and as furniture laminating adhesives if they could bear moderate loads at 120 C.
One means to improv~ the service temperature of block copolymer adhesive compositions i9 to chemically cure the adhesive with a phenol Pormaldehyde resin and heat as dis-- closed in U.S.P. 3,625,752. However, this procedure i9 undesirable because it is a very energy intensive process to heat the adhesive. Also, this process cannot be used for curing~pressure sensitive tapes because the plastic back-ing film would melt at the temperature required for curing.
A more energy efficient process is the radiation cure disclosed in U.S.P. 2,956,904. The compositions disclosed 1~1747~

therein, however, do not contain a supplemental crosslink promoting ingredient and therefore higher radiation doses are required.
In accordance with the present invention an adhesive composition with i.mproved service temperature and good solvent resistance is provided, which composition comprises:
(a) 100 parts by weight of a thermoplastic elas-tomeric block copolymer having at least two monoalkenyl-aromatic polymer end blocks A and at least one elastomeric con~ugated diene mid block B, said blocks A comprising 8-55~ by weight of the block copolymer;
(b) from 25 to 250 parts by weight of an adhesion-promoting resin compati.ble with block B; and (c) from 1 to 50 parts by weight of a di , tri-, or tetrafunctional ester of acrylic or methacrylic acid and at least dihydric aliphatic alcohol.
The composition having been cured by radiation.
The block copolymers employed in the present composi-tion are thermoplastic elastomers and have at least two monoalkenyl aromatic polymer end blocks A and at least one elastomeric conjugated diene polymer mid block B.
The number of blocks in the block copolymer is not of special importance and the macromolecular configuration may be linear, graft, radial or star depending upon the method by which the block copolymer is formed. Typical block copolymers of the most ~imple conflguration would have the structure polystyrene-polyisoprene-polystyrene and polystyrene-polybutadiene-polystyrene. A typical radial or star polymer would comprise one in which the diene block has three to four branches (radial) or fi.ve or more branches (star), the tip of each branch being connected to a polystyrene block. Other useful monoalkenyl-aromatic compounds from which the thermoplastic (non-elastomeric) b].ocks may be formed include alphamethyl 1~174'7~

styrene, tert-butyl sty-ene and other ring a]kylated styrenes as well as mixtures of the same. The conjugated d_ene monomer preferably has 4 to 5 carbon atoms, such as butadiene and isoprene. A much preferred conjugated diene is isoprene.
The average molecular weight~ Or each of the blocks may be varied as desired. The monoalkenyl-aromatic poly-mer blocks preferably have average molecular weights between about 5,000 and 125,000, more preferably between about 7,000 and about 50l000. The elastomeric conjugated diene po~ymer blocks prererablY have average molecular weights between about 15,000 and about 250,000, more pre-rerably between about 25,000 and about 150,000. The average molecular weights of the polystyrene end blocks are determined by gel permeation chromotography, whereas the polystyrene block content of the polymer is measured by infrared spectroscopy of the finished block polymer. The weight percentage of the thermoplastic monoalkenyl aromatic blocks in the finlshed block polymer should be between 8 and 55%, preferably between 10% and 30~ by weight.
The block copolymer by itself lacks the required adhesion. Therefore, it is necessary to add an adhesion promoting or tackirying resin that is compatible with the elastomeric conjugated diene block. A much preferred tackifying resin is a diene-olefin copolymer of piperylene and 2-methyl-2-butene having a softening point of about 95 C. This resin is available commercially under the r tradename Wingtack 95, and is prepared by the cationic polymerization Or 60% piperylene, 10S isoprene, 5S cyclo-pentadiene, 15% 2-methyl butene and about 10S dimer, as disclosed in U.S.P. 3,577,398. Other tackifying resins of the same general type may be employed in which the resinous copolymer comprises 20-80 weight percent of piperylene and 80 20 weight percent Or 2-methyl-2-butene.
The resins normally have softening points ~ring and ball) between about 80 C and about 115 C. Other adhesion promo-1117~7~

ting resins which may be employed include hydrogenated resins, esters of resins, polyterpenes, terpenephenol resins, and, poly-merized mixed olefins.
The amount of tackifying resin employed varies from 25 to 250, perferably between 50 to 150 parts by weight per 100 parts of block copolymer.
The adhesive compositions of the invention also may con-tain plasticizers such as rubber extending or compounding oils or liquid resins. These rubber compounding oils are well-known in the art and include both high saturates content and high aromatics con-tent oils. The amount of rubber compounding oil employed varies from 0 to 100, preferably from 10 to 60 parts by weight per 100 parts of block copolymer.
An essential component of the present invention is the acrylate or methacrylate ester coupling agent which promotes cross-linking of the block copolymer during exposure to the radiation.
The coupling agents employed herein are di-,tri-, and tetra-functional acrylic and methacylic esters of an at least dihydric aliphatic alcohol.
Preferred esters are at least tri-functional esters of an at least trihydric alcohol. Of this group the tetracrylic or tetramethacrylic esters of pentaerythritol and the triacrylic or trimethacrylic esters of trimethylolpropane are particularly pre-ferred.
Other suitable esters are diesters derived from di-, tri-, tetra or polyethylene glycol, 1,6 hexanediol, 1,4-butanediol, bisphenol A ethoxylated bisphenol A, and 1,3-(2,2-dimethyl)-propane-diol.

ill747~) The amount of coupling agent employed varies from 1 to 50, preferably from 2 to 25 parts by weight per 100 parts of block-copolymer.
The compositions of this invention may be modified with supplementary materials including pigments and fillers, as well as stabilizers and oxidation inhibitors.
The adhesive compositions of the present invention may be applied to the substrate from a solution of up to about 40% weight solids of the ingredients in a solvent such as toluene, the solvent being removed by evaporation prior to curing by exposure to the radiation. Alternatively, the ingredients may be mixed in a solvent, the mixture may be emulsified and the solvent evaporated, and the adhesive may be applied to the substrate as a 60-70% weight solids water-based emulsion, the water being removed by evaporation prior to curing. Adhesives of the present invention are especially suit-ed for preparation as hot melt adhesives.
The compositions of the present invention have been cured by exposure to high energy radiation such as electron beam radiat-ion or ultraviolet radiation, with electron beam being particularly preferred.
The electron beam radiation or high energy ionizing radiation which is employed to effect the cross-linking reaction can be obtained from any suitable source such as an atomic pile, a resonant transformer accelerator, a Van de Graaff electron acceler-ator, an electron accelerator, a betatron, a synchrotron, or a cyclotron. Radiation from these sources will produce ionizing radiation such as electrons, protons, neutrons, deuterons, gamma rays, X-rays, alpha particles, and beta part~cles.

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~1747~) The curing reaction is conveniently effected at room temperature, but it can be conducted at lower or higher temperatures if so desired. Preferably, the curing is effected in an inert atmosphere to reduce oxidative degradation of the block copolymer.
The amount of radiation required depends primarily upon the type and concentration of acrylate or methacrylate ester employed and the level of curing desired. Suitable doses of electron beam radiation include 1 Mrad to 25 Mrads, preferably 2 Mrads to 10 Mrads. Suitable UV

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radiation doses are those received by a 0.04 mm thick adhesive passing under a medium pressure mercury lamp rated at 200 watts per 2.5 cm at line speeds of about 3 to about 24 m per minute, the preferred range being 7.5 to 12 m per minute.
When using ultraviolet radiation it is preferred to employ a photosensitizer in order to speed up the curing reaction. Useful photosensitizers are benzophenone, pro-piophenone, cyclopropyl phenyl ketone, acetophenone, 1.3.5-triacetyl benzene, benzaldehyde~ thioxanthane, anthraquinone, beta-naphthy3 phenyl ketone, beta-naph~
thaldehyde, beta-acetonaphthone, 2.3-pentanedlone, benzil, fluoronone, W rene, benzanthrone, and anthracene. While most of these are well-known photosensitizers, other photosensitizers would work equally well in the present inventi~n.
A preferred use of the present formulation is in the preparation Or pressure-sensitive adhesive tapes or in the manufacture of labels. The pressure-sensitive adhesive tape comprises a flexible backing sheet and a layer of the adhesive composition of the present invention coated on one major surface o~ the backing sheet. The backing sheet may be a plastic film, paper or any other suitable material and the tape may include variou~ other layers or coatings, such as primers, release coatings and the like, which are used in the manufacture of pressure-sen sitive adhesive tapes.
The invention is further lllustrated by means of the following Examples.
3 In these Examples the high temperature cohesive - strength of the irradiated adhesive compositions was measured by a Strippability Temperature Limit (STL) test.
STL is defined as the maximum temperature at which a tape can be rapidly peeled from a hot, stainless steel substrate without leaving an adhesive residue on the panel. The test is run by applying a 6 mm wide strip of 11174'7~) tape to a stainless steel panel whose temperature is controlled to give a temperature gradient of about 5.5 C
per cm between 37 and 230 C. Following a 30 second warm-up, the tape is stripped orf manually at high speed and at appro~imately a 90 angle. The temperature at which cohesivs failure occurs is recorded as the STL value.
The STL test measures whether an automot-ve masking tape can be removed c]eanly when it is stripped from a painted car as it leaves the paint baking ovens. Solvent resis-tance is determ'ned by immersing about 6 cm tape in tolu-ene for 24 hours at room temperature. Ir the adhesive d-'s-solves, solvent resistance ls poor. If lt only swells, solYent resistance is good.
In all examples, the adheslve composition was pre-! 15 pared in a toluene solution and applied as about a 0.04 mm dry adhesive layer to a 0.0?5 mm thermopla~tic film sub--strate. Eleotron beam irradiation was accomplished using a 450 mm wide ELECTROCURTAIN* PROCESSOR manufactured by Energy Sciences, Inc. Ultraviolet irradiation was accom-pllshed using a U~ Processor (QC 1202 N~A) supplied by Radiation Polymer Co. This unit has two medium pressure mercury lamps, each rated at 200 watts per 2.5 cm.
The adhesive compositions used in Examples 1-4 are shown in Table 1. The polystyrene-polyisoprene-polystyrene block copolymers tested were Shell's KRATON* 1107 Rubber, a linear (Styrene-isoprene)2 polymer, Phillips' Solprene*
421, a radial (Styrene isoprene)4 polymer and an experimen~
tal star (Styrene-isoprene)8 polymer. All three polymers had about the same mo1ecu1ar weights prior to coupling and 3 all three contain 15~w polystyrene blocks. The polystyrene-polybutadiene-po1ystyrene polymer tested was Shell's KRATO~*1102 Rubber, a linear (Styrene-butadiene)2 polymer Or 30~w polysty ene. The adheslon promoting tackifyine resins used wlth the po~ymers were Wingtack 95*
*trade mark D

1747~) -Resin, a diene-olefin resin, and Hercules' XPS 502 Resin, a modified terpene hydrocarbon res.n, respectively. The plasticizer used was SHELLFLEX~ 371, a low aromatic con-tent process oil. The stabilizer used was Zinc dibutyl-dithiocarbamate (ZDBC).

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In Examples 1-4, adhesive compositions were lrradia-ted under a nitrogen blanket with e~ectron beam radiation at doses of 0, 1, 2, 5 and t0 megarads (Mrad). With ELECTROCURTAIN*equipment, radiation doses of 1, 2, 5 and 10 Mrad are equivalent to curlng line gpeed9 Or about 210, 105, 36 and 21 m per minute, respectively. Each of the formulations in Table 1 was tested at four levels (1, 3, 10 and 25 parts per 100 parts of block copolymer) Or eater coupling agent. In figure3 1-4 the vertical axis represents the radiation dose and on the horizontal axis the ester concentrations at 1, 3, 10 and 25 pbw are indicated.
Results are represented in terms Or contour diagrams Or STL on a grid Or radiation dose va. coupling agent concen-tration. An STL value of ~205C wa~ considered to indicate a satisractory degree of curing. Contour lines in Figures - 1_4 indicate combinations of radiation doae and coupling agent concentratlon which give an acceptable degree Or curing. To the left of these contour lines, there was little if any improvement in STL and solvent resistance was poor. To the right of these lines, STL was ~205 C
and the adhesive would not dissolve in toluene.
Example 1 The effect Or the choice of ester coupling agent was determined. Four esters were evaluated for their efrective-neas; two difunctional esters: HDODA and HDODM in formu-lations D and C respectively, and two trifunctional estera TMPTA and TMPTM in formulations B and A rea-pectively. Results in Figure 1 show that both acrylates cure the adhesive more readily than do the ~ethacrylatea.
That is, a lower radiation dose is required at a given ester concentration or a lower ester concentration is re-quired at a given radlation dooç.
Example 2 _ _ _ . _ The effect of a stabilizer was studied. Results in ~igure 2 show that the ~tabilizer zinc dibutyldithiocarba-*trade mark .

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mate included in formulation A makes curing more dif-ficult, formulation E, with no stabilizer cures easier.
Example 3 The effect of block polymer structure on the rate of cure was determined. Results in Figure 3 comparing Formula-tions A and F show that an adhesive based on either a poly-styrene-polyisoprene-polystyrene or a polystyrene-polybuta-diene-polystyrene block copolymer can be eleotron oured.
Results in Figure 4 comparing Formulations A, G and H
which are based on a linear, a radial and a star ooupled polymer, respectively, show that as the number of arms on the polymer used increases, the adhesive can be more readily cured.
Example 4 The effect Or including a plasticizer in the adhesive composition was investigated. Curing studies on Formulation I in Table 1 showed that its curing characteristics are the same as those of Formulation A shown in Figure 1. That is, the plasticizer did not appreciably 910w down the rate of cure.
Ex_ ple 5 The suitability of ultraviolet radiation for ini-tiation of curing was checked. The formulations and results are shown in Table 2. All three formulations, the com-parative composition (Formulation A) and two compositions of this invention (Formulations B and C), show poor STL
and dissol~e in toluene before exposure to UV radiation.
After exposure to UY radiation, the comparative sample shows some improvement in STL but its solvent resistance remains poor. Formulations B and C, however, show good STL and solvent resistance.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An adhesive composition comprising:
(a) 100 parts by weight of a thermoplastic elastomeric block copolymer having at least two monoalkenyl-aromatic polymer end blocks A and at least one elastomeric conjugated diene mid block B, said block A comprising 8-55% by weight of the block copolymer;
(b) from 25 to 250 parts by weight of a tackifying resin compatible with block B; and (c) from 1 to 50 parts by weight of a di-, tri-, or tetra functional ester of acrylic or methacrylic acid and an at least dihydric aliphatic alcohol, the composition having been cured by radiation.

2. A composition as claimed in claim 1 wherein the type of radiation cure is electron beam irradiation.

3. A composition as claimed in claim 1 wherein the type of radiation cure is ultraviolet irradiation.

4. A composition as claimed in any one of claims 1-3, where-in the radiation employed is between 2 and 25 Mrad.

5. A composition as claimed in any one of claims 1-3, where-in the block copolymer is a linear polystyrene-polyisoprene-poly-styrene block copolymer,

6. A composition as claimed in any one of claims 1-3, wherein the block copolymer is a radial polystyrene/polyisoprene block copolymer.

7, A composition as claimed in any one of claims 1-3, wherein the block copolymer is a star-shaped polystyrene/polyisoprene block copolymer.

8. A composition as claimed in claim 1, wherein the ester is an at least trifunctional ester of an at least trihydric alcohol.

9. A composition as claimed in claim 8, wherein the ester is the tetra-acrylic or tetra-methacrylic ester of pentaerythritol.

10. A composition as claimed in claim 8 wherein the ester is the tri-acrylic or tri-methacrylic ester of trimethylol propane.