Polyisobutylene copolymers having reactive silyl grafts

Details Polyisobutylene copolymers having reactive silyl grafts Polyisobutylene copolymers having reactive silyl grafts

1999-03-02

2002-04-30

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S264000, C525S265000, C525S288000

Reexamination Certificate

active

06380316

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to polyisobutylene copolymers. More particularly, the invention relates to a copolymer of isobutylene and at least one conjugated diene which is grafted with an alkenyl-functional silane in the presence of a free-radical generator to provide a modified copolymer having reactive silyl functionality.
BACKGROUND OF THE INVENTION
Various organic polymers having moisture-sensitive silyl functionality are known in the art. Such systems may be prepared, for example, by hydrosilating a polymer having vinylic functionality along its main chain (or at terminals positions) with a reactive group-containing silane which also contains SiH functionality, the reaction being carried out in the presence of a catalyst such as platinum. When hydrocarbon polymers are so modified, they combine advantages inherent in moisture-curable silicone compositions with those of the hydrocarbon. That is, the modified polymer can be crosslinked via the reaction of hydrolyzable silyl groups at room temperature and the low gas and moisture permeability of the organic backbone is maintained. Therefore, such systems are eminently suitable for use as base polymers in sealants for gas barrier applications such as form-in-place gaskets, O-rings, rubber plugs/seals, medical and food container caps, and the like. However, the hydrosilation methods conventionally used to prepare such modified polymers and their precursors are expensive and complicated.
A different approach for preparing certain polymers having moisture-sensitive silyl functionality has been suggested by Scott in U.S. Pat. No. 3,646,155. This patent teaches that polyethylene, or a copolymer of ethylene with minor portions of propylene and/or butylene, can be reacted with a silane which bears both an aliphatically unsaturated group as well as a hydrolyzable group, the reaction taking place in the molten state and in the presence of a free-radical generating compound. The resulting modified polyethylene was crosslinked by exposing it to moisture, typically steam. Of course, such an ethylene polymer or copolymer generally has a high molecular weight and must be processed at temperatures above the melt point (typically above 140° C. according to Scott). Further, all of the examples of the Scott patent indicate that the melt index, which is inversely related to melt viscosity, actually decreases upon modification with the silane. Scott points out that his cured compositions generally exhibit properties similar to those of corresponding peroxide-cured systems. However, it is clear that the moisture-curable modified polymers taught by Scott must be processed/fabricated at high temperatures and are certainly not suited for the production of room-temperature vulcanizable (RTV) sealant compositions.
SUMMARY OF THE INVENTION
It has now been found that a copolymer of isobutylene with a conjugated diene can be reacted with a silane having both an alkenyl group and a silicon-bonded hydrolyzable group, the reaction being carried out in the presence of a free-radical generator, to provide a modified silyl-functional copolymer. Further, this silyl-functional copolymer, which can be crosslinked by exposure to moisture, exhibits a number average molecular weight (typically 2,000 to 50,000) which is considerably lower than that of the initial unmodified copolymer. In view of the relatively low gas and moisture permeability of the modified copolymer and its low viscosity at room temperature (i.e., can easily be dispensed from a caulking gun), it is an ideal base polymer for formulating sealants for gas barrier applications. Moreover, sealants prepared from such modified copolymers have similar properties to those based on the previously discussed expensive systems prepared by hydrosilation techniques.
The invention, therefore, relates to a method comprising: reacting
(A) 100 parts by weight of a copolymer of isobutylene with 0.5 to 15 mole percent of at least one C
4
to C
14
conjugated diene, said copolymer having a number average molecular weight of about 5,000 to 500,000 and
(B) 1 to 50 parts by weight of a silane having the formula
wherein R is an alkenyl group having 2 to 10 carbon atoms, R′ is independently selected from hydrocarbon or halogenated hydrocarbon groups which contain no aliphatic unsaturation, X is a silicon-bonded hydrolyzable group selected from alkoxy, acyloxy, ketoxime, amino, amido, aminoxy or alkenyloxy groups, and n is 2 or 3, said reaction taking place in the presence of
(C) 0.5 to 10 parts by weight of a free-radical generator,
whereby said silane is grafted onto said copolymer and the number average molecular weight of the latter is reduced, typically by fifty percent or more.
The invention further relates to a modified isobutylene copolymer prepared by the above method.
DETAILED DESCRIPTION OF THE INVENTION
The copolymer (A) is an interpolymer of 85 to 99.5 mole percent of isobutylene with 15 to 0.5 mole percent of C
4
to C
14
conjugated diene. Such copolymers are well known in the art and are also referred to as “butyl rubber.” Typical conjugated dienes may be illustrated by isoprene, butadiene, 2,3-dimethyl butadiene, piperylene, 2,5,-dimethylhexa-2,4-diene, cyclopentadiene, cyclohexadiene and methylcyclopentadiene. It is preferred that component (A) is a copolymer of isobutylene with isoprene, the latter monomer preferably being present at a level of 0.5 to 4 mole percent, most preferably at 2 to 3 mole percent. Typically, this copolymer has a number average molecular weight (M
n
) of about 5,000 to 500,000, preferably greater than 50,000, more preferably 100,000 to 300,000, before it is modified according to the instant method.
Silane (B) has the formula
wherein R is an alkenyl group having 2 to 10 carbon atoms which may be illustrated by vinyl, allyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl and decenyl. Either straight chain or branched groups may be used but the unsaturation must be a vinylic type (i.e., having a terminal unsaturated moiety). Preferred alkenyl groups are vinyl, allyl and hexenyl. R′ in formula (i) may be independently selected from hydrocarbon or halogenated hydrocarbon groups which contain no aliphatic unsaturation. These may be specifically exemplified by alkyl groups having 1 to 20 carbon atoms, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl and dodecyl; cycloalkyl groups, such as cyclohexyl and cycloheptyl; aryl groups having 6 to 12 carbon atoms, such as phenyl, tolyl and xylyl; aralkyl groups having 7 to 20 carbon atoms, such as benzyl and phenylethyl; halogenated alkyl groups having 1 to 20 carbon atoms, such as 3,3,3-trifluoropropyl and chloromethyl; and halogenated aryl groups having 6 to 12 carbon atoms, such as chlorobenzyl, chloronaphthyl. X in the above formula is a silicon-bonded hydrolyzable group selected from alkoxy, acyloxy, ketoxime, amino, amido, aminoxy or alkenyloxy groups, and preferably contains no more than 6 carbon atoms. Specific examples of such hydrolyzable groups include methoxy, ethoxy, propoxy, butoxy, acetoxy, propionoxy, —ON═C(Me)
2
, —ON═C(Et)(Me), —ON═C(Ph)
2
, —NH(Me), —NH(Et), and —NH(Ph)
2
, wherein Me, Et and Ph hereinafter denote methyl, ethyl and phenyl groups, respectively. Preferably, X is an alkoxy group having 1 to 4 carbons, most preferably a methoxy group. Finally, the subscript n in formula (i) is an integer having a value of 2 or 3. Highly preferred specific silanes are methylvinyldimethoxysilane, vinyltrimethoxy-silane and hexenyltrimethoxysilane
The type of free-radical generator (C) used in the method of the present invention is not specifically limited. This organic component generates free-radicals upon heating and may be selected from any of the known azo or diazo compounds, such as 2,2′-azobisisobutyronitrile and phenyl-azo-triphenylmethane. Preferably, the free-radical generator is selected from organic peroxides such as hydroperoxides, diacyl peroxides, ketone peroxides, peroxyesters, dialkyl peroxides, diaryl peroxides, aryl-alkyl peroxides peroxydi

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