Process for crosslinking hydrolyzable copolymers

A process for crosslinking comprising the following steps: PA0 (a) admixing, in a processing zone, a copolymer containing at least one hydrolyzable silane moiety; a dihydrocarbyltin oxide; and PA1 (i) a carboxylic acid; PA1 (ii) a compound bearing at least one carboxylate moiety, which is capable of forming a carboxylic acid when subjected to heat and/or moisture; or PA1 (iii) mixtures thereof, with the proviso that: PA2 (A) in the event that component (ii) is present, the conditions in the processing zone are such that the carboxylate moiety forms a carboxylic acid; PA2 (B) the residence time of the mixture in the processing zone is sufficient to at least partially complete the processing of the copolymer, but of sufficient brevity to substantially avoid a reaction of the carboxylic acid with the dihydrocarbyltin oxide; and PA0 (b) passing the mixture from step (a) into a crosslinking zone under such reaction conditions that the carboxylic acid reacts with the dihydrocarbyltin oxide to form dihydrocarbyltin carboxylate, said crosslinking zone having a moisture content sufficient to crosslink the hydrolyzable copolymer in the presence of the dihydrocarbyltin carboxylate.

TECHNICAL FIELD 
This invention relates to a process for crosslinking hydrolyzable 
copolymers using a silanol condensation catalyst prepared in situ, and 
compositions therefor. 
BACKGROUND ART 
Scorch, i.e., premature crosslinking, in water curable resin systems is a 
widely recognized problem. It can result in numerous ways. The most 
difficult to control can be referred to as process scorch, which takes 
place in the presence of a silanol condensation catalyst. Process scorch 
is manifested just after the addition of the silanol condensation catalyst 
to the resin and, again, during the continuous processing of the water 
curable resin in, for example, an extruder. The former occurs with a rapid 
build up in viscosity and, in many cases, can be dramatic. The latter, 
generally, involves a more subtle build up of viscosity over time. In both 
cases, the extrusion is rendered difficult and ultimately results in an 
unacceptable extrudate and a shutdown of the operation. 
The art is constantly searching for processes and compositions which lead 
to a diminution or elimination of process scorch. 
DISCLOSURE OF THE INVENTION 
An object of this invention, therefore, is to provide a crosslinking 
process which solves the problem of process scorch and a composition 
useful in the process. 
Other objects and advantages will become apparent hereinafter. 
According to the present invention the above object is met by a process for 
crosslinking comprising the following steps: 
(a) admixing, in a processing zone, a copolymer containing at least one 
hydrolyzable silane moiety; a dihydrocarbyltin oxide; and 
(i) a carboxylic acid; 
(ii) a compound bearing at least one carboxylate moiety, which is capable 
of forming a carboxylic acid when subjected to heat and/or moisture; or 
(iii) mixtures thereof, with the proviso that: 
(A) in the event that component (ii) is present, the conditions in the 
processing zone are such that the carboxylate moiety forms a carboxylic 
acid; 
(B) the residence time of the mixture in the processing zone is sufficient 
to at least partially complete the processing of the copolymer, but of 
sufficient brevity to substantially avoid a reaction of the carboxylic 
acid with the dihydrocarbyltin oxide; and 
(b) passing the mixture from step (a) into a crosslinking zone under such 
reaction conditions that the carboxylic acid reacts with the 
dihydrocarbyltin oxide to form dihydrocarbyltin carboxylate, said 
crosslinking zone having a moisture content sufficient to crosslink the 
hydrolyzable copolymer in the presence of the dihydrocarbyltin carboxylate 
.

DETAILED DESCRIPTION 
The hydrolyzable resins useful in subject process derive their 
hydrolyzability from silane modification. They are commercially attractive 
because they can be simply and effectively cured with water. These 
copolymers are, therefore, susceptible to a broad range of processing 
conditions and are particularly useful in the preparation of extruded wire 
coatings, foams, pipe, and pond liners. 
The curing or crosslinking of these silane modified copolymers is effected 
by exposing the copolymers to moisture. Without a silane condensation 
catalyst, however, moisture cure is exceedingly slow and, in the short 
time it takes to pass the resin through, e.g., an extruder, the amount of 
crosslinking achieved is negligible. 
It is understood that the term "copolymer" as used in this specification 
may include silane grafted olefin homopolymers and copolymers, and 
copolymers of one or more olefin monomers and an olefin silane monomer. 
The monomers on which the homopolymers and copolymers are based can be 
alpha-olefins or diolefins having 2 to 20 carbon atoms, particularly the 
lower alpha-olefins having 2 to 12 carbon atoms. Preferably, a major 
proportion, i.e., more than 50 percent by weight, of each copolymer is 
attributed to ethylene, propylene, or 1-butene. The silane monomer, which 
is either grafted or copolymerized, is unsaturated and has at least one 
hydrolyzable group. Various useful alkenyl alkoxy silanes are mentioned 
below. 
In addition to the alpha-olefin, diolefin, and silane monomers, the balance 
of the copolymer can be based on one or more various olefin monomers 
having 2 to 20 carbon atoms. Examples of useful monomers are the vinyl 
esters, alkyl methacrylates, and alkyl acrylates. Examples of these 
compounds are 1 hexene, 4-methyl-1 pentene, 1-undecene, ethyl acrylate, 
vinyl acetate, methyl methacrylate, 1,4 hexadiene, ethylidenenorbornene, 
dicyclopentadiene, butyl acrylate, and methyl acrylate. Silane modified 
terpolymers such as ethylene/propylene/ethylidene norbornene rubbers are 
of particular interest. 
Silane grafted copolymers can be prepared by the technique described below. 
In this copolymer, the portion attributed to the silane is present in an 
amount of about 0.1 percent to about 10 percent by weight based on the 
weight of the copolymer and is preferably incorporated into the copolymer 
in an amount of about 0.5 to about 4 percent by weight. The silane used to 
modify the copolymer can be, among others, a vinyl trialkoxy silane such 
as vinyl trimethoxy silane, vinyl triethoxy silane, or vinyl triisopropoxy 
silane. Generally speaking, any unsaturated monomeric organosilane having 
one or more hydrolyzable groups can be used. If slower water cure or 
better shelf stability is desired, vinyl triisobutyoxy silane, vinyl 
tributoxy silane, or vinyl tris-(2-ethyl hexoxy) silane can be used. 
A free radical generator or catalyst is used in the preparation of the 
silane grafted copolymer. Among the most useful free radical generators 
are dicumyl peroxide, lauryl peroxide, azobisisobutyronitrile, benzoyl 
peroxide, tertiary butyl perbenzoate, di(tertiary-butyl) peroxide, cumene 
hydroperoxide, 2,5-dimethyl-2,5-di (t-butylperoxy) hexene, 
2,5-dimethyl-2,5 di(t-butylperoxy)hexane, tertiary butyl hydroperoxide, 
and isopropyl percarbonate. The organic peroxides are preferred. About 
0.001 to about 5 percent by weight of free radical generator based on the 
weight of the polymer or copolymer is used, preferably about 0.001 to 
about 0.5 percent by weight. 
A typical technique for preparing a silane grafted polyethylene is as 
follows: the polyethylene used can be, for example, a low density 
polyethylene having a density of 0.90 and a melt index of 1.0. It can be 
made by the processes described in European Patent Applications 0 120 501 
and 0 120 503, both published on Oct. 3, 1984, wherein ethylene is 
polymerized together with an alpha olefin comonomer having 3 to 8 carbon 
atoms, or by other conventional techniques. In the present application, 
low density polyethylenes are considered to include copolymers of ethylene 
and a minor proportion of alpha olefin. 100 parts of ethylene copolymer, 
0.2 part of polymerized 1,3-dihydro-2,2,4-trimethylquinoline (an 
antioxidant), 0.1 part of dicumyl peroxide, and 4 parts of vinyl 
tris-(2-ethyl-hexoxy) silane are mixed in a laboratory Brabender mixer at 
a temperature in the range of about 80.degree. C. to about 115.degree. C., 
a temperature low enough to keep the dicumyl peroxide below its 
decomposition temperature. After mixing for five minutes, the temperature 
is raised to a temperature in the range of about 150.degree. C. to about 
220.degree. C. The batch is then mixed for 5 to 10 minutes during which 
grafting of the silane to the polyethylene occurs. The antioxidant is used 
as a radical trap to control the amount of crosslinking. This technique 
can be repeated, for example by using 3 parts of vinyltriisobutoxysilane; 
0.1 part dicumyl peroxide; and 0.1 part of the antioxidant, tetrakis 
[methylene(2-5 di tert-butyl-4-hydroxyhydrocinnamate] methane; initial 
mixing is in the range of 110.degree. C. to 120.degree. C.; grafting is 
for 5 minutes at 185.degree. C. 
A copolymer of ethylene and silane can be prepared by the process described 
in U.S. Pat. No. 3,225,018 issued on Dec. 21, 1965 or U.S. Pat. No. 
4,574,133 issued on Mar. 4, 1986. The portion of the copolymer attributed 
to the silane is in the range of about 0.5 to about 10 percent by weight 
based on the weight of the copolymer and is preferably in the range of 
about 0.5 to about 4 percent by weight. 
Various other processes for preparing silane grafted copolymers and 
numerous unsaturated silanes suitable for use in preparing these polymers 
and bearing hydrolyzable groups such as alkoxy, oxy aryl, oxyaliphatic, 
and halogen are mentioned in U.S. Pat. Nos. 3,075,948; 4,412,042; 
4,413,066; and 4,593,071. 
The dihydrocarbyltin oxide can have the formula: 
EQU R.sub.2 SnO 
wherein R is an alkyl or aryl radical and R can be alike or different. The 
R radical can have 1 to 20 carbon atoms and preferably has 1 to 8 carbon 
atoms. Examples of the R radical are methyl, ethyl, propyl, n-butyl, 
isobutyl, pentyl, hexyl, phenyl, and octyl. Examples of dihydrocarbyltin 
oxides useful in subject invention are dimethyltin oxide, dibutyltin 
oxide, dioctyltin oxide, diphenyltin oxide, methylphenyltin oxide, and 
dibenzyltin oxide. These organotin oxides are, generally, high melting, 
dispersible powdered solids, which show little or no activity in the 
crosslinking of hydrolyzable silane resins. 
The carboxylic acids can be aliphatic or aromatic carboxylic acids having 1 
to 30 carbon atoms, preferably 1 to 12 carbon atoms, and can be 
unsubstituted or substituted provided that the substituent is inert to the 
materials and conditions in the processing and crosslinking zones. 
Examples of suitable carboxylic acids are acetic, formic, propionic, 
butyric, octanoic, benzoic, salicylic, citric, maleic, oleic, isostearic, 
succinic, phthalic, stearic, and lauric. The carboxylate moieties of 
these, and other carboxylic acids, are examples of the carboxylate 
moieties which can be included in the compounds referred to below. The 
carboxylic acids react with the dihydrocarbyltin oxide at temperatures in 
the range of about 20.degree. C. to about 350.degree. C. and preferably 
about 50.degree. C. to about 150.degree. C. 
Compounds bearing carboxylate moieties which form carboxylic acid when 
subjected to heat and/or moisture can be exemplified by silane 
carboxylates, compounds or complexes formed by the combination of fillers 
and carboxylate moieties, organic acid anhydrides, and resins. 
Silane carboxylates include those compounds having the formula: 
EQU R'--Si--(OCOR").sub.3 
wherein R' is hydrogen or an aliphatic or aromatic radical and R" is an 
aliphatic or aromatic radical. The aliphatic or aromatic radical can have 
1 to 30 carbon atoms and preferably has 1 to 12 carbon atoms. The radical 
can be alkyl, alkaryl, aryl, cycloaliphatic, or heterocyclic. Examples of 
suitable radicals are methyl, ethyl, propyl, n-butyl, isobutyl, pentyl, 
hexyl, octyl, lauryl, phenyl, and benzyl. The silicone carboxylates can be 
exemplified by methyl triacetoxy silane, butyl triacetoxy silane, vinyl 
triacetoxy silane, methyl tribenzoxy silane, butyl tribenzoxy silane, and 
vinyl tribenzoxy silane. 
The carboxylate moiety can also be attached to the surface of a 
conventional filler, by reaction to form a compound, e.g., Mg(OH).sub.2 
.multidot.OCOR", or a complex, e.g., Al(OH).sub.3 .multidot.HOCOR". In 
both cases, R" is as described above. Another example of a useful filler 
is Ca(OH).sub.2. 
The organic acid anhydrides can have 4 to 20 carbon atoms and preferably 4 
to 10 carbon atoms. Examples of suitable anhydrides are maleic anhydride, 
itaconic anhydride, crotonic anhydride, benzoic anhydride, and phthalic 
anhydride. 
Resins having carboxylate moieties are exemplified by ethylene/vinyl 
acetate copolymer, ethylene/ethyl acrylate copolymer, 
ethylene/methylacrylate/acrylic acid terpolymer, and ethylene/acrylic acid 
copolymer. 
The reaction of the dihydrocarbyltin oxide with the carboxylate moiety to 
place the tin oxide in an activated state, i.e., to provide a silanol 
condensation catalyst, and the subsequent crosslinking of a hydrolyzable 
silane modified polymer in its presence, can be illustrated as follows: 
##STR1## 
The processing zone can be a conventional extruder, e.g., a single screw 
type. A typical extruder has a hopper at its upstream end and a die at its 
downstream end. The hopper feeds into a barrel, which contains a screw. At 
the downstream end, between the end of the screw and the die, is a screen 
pack and a breaker plate. The screw portion of the extruder is considered 
to be divided up into three sections, the feed section, the compression 
section, and the metering section, and two zones, the back heat zone and 
the front heat zone, the sections and zones running from upstream to 
downstream. If it has more than one barrel, the barrels are connected in 
series. The length to diameter ratio of each barrel is in the range of 
about 16:1 to about 30:1. 
The processing zone can also be a conventional injection molding apparatus, 
or a rotomolding apparatus. 
In the event that the compound bearing the carboxylate moiety is present, 
the conditions in the processing zone are such that the carboxylate moiety 
forms a free carboxylic acid. As noted, this is accomplished through the 
application of heat and/or moisture. The temperature in the processing 
zone will generally be in the range of about 20.degree. C. to about 
350.degree. C. and is preferably in the range of about 50.degree. C. to 
about 150.degree. C. These are conventional extrusion and injection 
molding temperatures. In the case of ethylene/vinyl acetate copolymers, 
the heat will be sufficient to form the carboxylic acid. With respect to 
those compounds which need moisture to form the carboxylic acid, the 
formation is generally facilitated by the presence of heat in the ranges 
mentioned above. 
The residence time of the mixture in the processing zone is, for example, 
of sufficient length to complete all or part of the extrusion or injection 
molding. This time is in the range of about 20 to about 2000 seconds and 
is preferably about 60 to about 1000 seconds. In view of this brief 
residence time, the reaction of the carboxylic acid with the 
dihydrocarbyltin oxide is substantially avoided, i.e., kept to a minimum. 
The crosslinking zone is usually a water bath through which the extruded or 
injection molded resin, i.e, the processed resin, is passed. The 
crosslinking zone can be operated at a temperature in the range of about 
20.degree. C. to about 200.degree. C. and is preferably operated at a 
temperature in the range of about 50.degree. C. to about 90.degree. C. The 
residence time in this zone can be in the range of about 0.01 to about 72 
hours and is preferably about 1 to about 24 hours. As an alternative to 
the water bath, the zone can be humidified, the relative humidity being at 
least about 50 percent. 
In the crosslinking zone, the carboxylic acid reacts with the 
dihydrocarbyltin oxide to provide the silanol condensation catalyst, which 
accelerates the moisture cure of the hydrolyzable resin. 
Conventional additives can be added to the mixture introduced into the 
processing zone. The amount of additive is usually in the range of about 
0.01 to about 50 percent based on the weight of the resin. Useful 
additives are antioxidants, ultraviolet absorbers, antistatic agents, 
pigments, dyes, fillers, slip agents, fire retardants, plasticizers, 
processing aids, lubricants, stabilizers, and smoke inhibitors. Blends of 
the hydrolyzable polymer and other polymers can be prepared in the 
processing zone provided that the resins to be blended with the 
hydrolyzable copolymer will not crosslink. Examples of these resins are 
low density polyethylene, high density polyethylene, polypropylene, linear 
low density polyethylene, and very low density polyethylene (equal to or 
less than 0.915 grams per cubic centimeter). 
The patents and applications mentioned in this specification are 
incorporated by reference herein. 
The invention is illustrated by the following examples: 
EXAMPLES 1 TO 4 
The addition copolymer of ethylene and vinyltrimethoxysilane is fluxed in a 
Brabender mixer at 130.degree. C. and, in examples 2, 3 and 4, different 
masterbatches ar added and dispersed during 30 seconds. After discharge, 
rheometer plaques are pressed, placed in a water bath at 70.degree. C., 
and rheometer readings are taken at various intervals. 
The masterbatch added in Example 2 is made up of 90 percent by weight 
ethylene/ethyl acrylate copolymer and 10 percent by weight dibutyltin 
dilaurate. 
The masterbatch added in Example 3 is made up of 92 percent by weight 
ethylene/ethyl acrylate copolymer and 8 percent by weight dibutyltin 
oxide. 
The masterbatch added in example 4 is made up of 88.9 percent by weight 
ethylene/ethyl acrylate copolymer; 8 percent by weight dibutyltin oxide; 
and 3.1 percent by weight maleic anhydride. 
The rheometer test procedure is described in U.S. Pat. No. 4,108,852 issued 
on Apr. 19, 1977. The rheometer reading is in pound-inches (lb.-in.). 
The rheometer readings for the various time intervals are as follows: 
______________________________________ 
Time Interval Example 
(hours in 70.degree. C. H.sub.2 O) 
1 2 3 4 
______________________________________ 
0 5 9 5 9 
1 -- 15 6 18 
4 -- 26 6 30 
24 -- 40 10 50 
48 7 51 16 55 
96 13 -- 23 -- 
336 25 -- 39 -- 
______________________________________ 
EXAMPLES 5 TO 9 
The addition copolymer of ethylene and vinyltrimethoxysilane is fluxed in a 
Brabender mixer at 130.degree. C., additives are added and mixed and 
dispersed during one minute. After discharge, rheometer plaques are 
pressed, placed in a water bath at 70.degree. C., and rheometer readings 
are taken at zero and 16 hours. Components, proportions, and results are 
as follows: 
______________________________________ 
Example 
5 6 7 8 9 
(parts by weight) 
______________________________________ 
Components 
copolymer 100 100 100 100 100 
dibutyltin oxide 
0.05 -- -- 0.05 0.05 
stearic acid -- 0.12 -- 0.12 -- 
ethyltriacetoxy silane 
-- -- 0.03 -- 0.03 
rheometer readinq 
0 hours 5 7 7 8 8 
16 hours 7 12 8 16 14 
______________________________________