Process for making radiation cured silicone rubber articles

A process for making radiation cured silicone rubber articles is disclosed wherein a hydroxyl-terminated polysiloxane having a molecular weight from about 50,000 to about 2,000,000, optionally modified by mixing with up to 85% of an end-stopped silicone rubber, is mixed with from about 10 to about 70 parts per hundred of rubber of a finely divided silica filler with a particle size in the reinforcing range and other inlet fillers as determined by desired final properties; the composition so prepared is formed into the desired shape at room temperature; the article so formed is precured to improve the mechanical properties of the material with which it is made by exposure to ammonia gas, ammonium hydroxide, or to the vapors or solutions of a volatile amine at room temperature; and the precured article is irradiated with high energy electrons or gamma radiation to effect a permanent cure of the material from which the article is formed.

BACKGROUND 
I. Field of Invention 
The present invention relates to processes for making radiation cured 
silicone rubber articles, and, in particular, to a process wherein 
articles formed of silicone rubber are treated substantially immediately 
after their formation to withstand without serious deformation the 
stresses involved in the mechanical handling necessary to convey such 
articles through irradiation apparatus. 
II. Summary of the Prior Art 
Unlike some of the organic rubbers, particularly the newer thermoplastic 
rubbers, silicone rubber must be crosslinked or vulcanized in order to 
have useful properties. The cross-linking process is usually referred to, 
in the case of silicone rubber as "cure" or "curing." Since the invention 
of silicone rubber, the cure has been done by incorporating free radical 
producing catalysts, typically organic peroxides or azo compounds, mixed 
with rubber and heating the composition to a high temperature, typically 
150.degree.-250.degree. C., for periods ranging from a few minutes to 
several hours. 
Virtually all commercial, heat-cured silicone rubbers today are cured with 
organic peroxides such as benzoyl peroxide, dichlorobenzoyl peroxide, 
tert-butyl perbenzoate or dicumyl peroxide. However, products cured by 
such curing agents all suffer from a disadvantage in that, after cure has 
been completed, the products contain chemical residues from the 
decomposition of the peroxide, and these residues tend to affect 
deleteriously properties such as heat-ageing, electrical resistivity and 
reversion resistance. The presence of these residues, moreover, limits the 
use of otherwise biologically inert silicone rubber products in medical 
applications, since the residues tend to be leached out of the rubber by 
body fluids, saline solutions and other liquid products used in medicine. 
There are other problems of peroxides curing catalysts. One is that they 
are very sensitive to catalyst "poisons" and many common rubber 
compounding ingredients, which otherwise would be included in silicone 
rubber to improve physical strength, flame retardancy, thermal ageing, and 
so on, cannot be used because they prevent or retard the peroxide cure. 
For example, most reinforcing carbon blacks, commonly used in organic 
rubbers, completely prevent peroxide cure of silicones. Certain peroxide 
residues, being volatile, cause porosity in the cured rubber and others 
tend to diffuse out of the compound forming unsightly oily films or 
crystals on the surface. 
More recently, liquid silicone rubbers have been introduced to the market, 
which cure by other mechanisms to solid, vulcanized rubber products. These 
liquid rubbers (which occasionally are pastes or semi-solids) typically 
cure at room temperature, or at relatively low temperatures, and are 
categorically referred to as "Room Temperature Vulcanizing" (RTV) silicone 
rubbers. There are several mechanisms for curing RTV rubbers. Some contain 
crosslinking agents which are activated by atmospheric moisture and others 
are cured by complex catalysts containing platinum salts. All of them 
yield chemical byproducts from the curing reaction, some of them 
relatively toxic chemicals such as organic amines, acetic acid or 
methanol. They all, therefore, suffer from the disadvantages cited for 
peroxide cure, above. 
It has long been known that these disadvantages can be avoided, and that 
cured silicone rubber parts free of deleterious catalyst residues can be 
obtained, by irradiation with high energy electrons or gamma rays, to 
effect the cure. For example, the curing of silicone rubber with electrons 
was disclosed and claimed in U.S. Pat. No. 2,763,609 to Lewis, et al. This 
curing process and the benefits resulting therefrom are discussed in 
detail in that patent, the disclosure of which is incorporated herein by 
reference. In spite of the significant advantages of the irradiation 
curing process, however, it has never been commercialized, to the best of 
our knowledge, because of a serious obstacle to production of commercial 
quantities of formed, irradiated silicone parts. The problem is that 
Silicone rubber, like other curable polymers, must usually be formed into 
the desired, final shape before cure because, after cure, it is 
crosslinked and, therefore, not formable by the usual processes such as 
molding, extrusion, or casting. 
Yet unlike most other polymers, silicone rubber is almost completely 
lacking in physical strength before cure. Uncured silicone rubber is 
extremely soft, easily deformed even by working with the bare hands, and 
it flows readily under low pressures, such as might be experienced when 
winding a silicone tube or sheet on a reel. Cure, whether by peroxide 
catalysts or by irradiation, improves the physical strength of silicone 
rubber dramatically. Tensile strength, for example, is increased 20-50 
times. 
Because of its low precure strength (referred to in the trade as "green 
strength"), it is impossible to convey extruded silicone tubing, or 
extruded silicone insulated wire, for example, from the extruder to the 
radiation vault, and to convey it under the electron beam repeatedly, 
without serious physical damage. If the silicone rubber is to be formed by 
molding, it is nearly impossible to remove it from the mold without 
damage, in order to irradiate it. Of course, it cannot be radiation-cured 
while still in the mold, at least by electron irradiation, because the 
electrons cannot penetrate the thick metal walls of a typical mold. This 
problem has prevented commercialization of the radiation cure process. 
SUMMARY OF THE PRESENT INVENTION 
It is, therefore, an object of this invention to provide a practical 
process for curing silicone rubber formed objects by the use of high 
energy radiation and, particularly, by the use of high energy electrons. 
It is a further object to cure silicone rubber without the use of heat 
energy, and without the use of chemical agents or catalysts which leave 
deleterious residues in the rubber. 
A still further object is to effect an apparent cure of silicone rubber 
formed objects, immediately after the forming process, so that they can be 
handled, stored and conveyed through irradiation equipment without 
physical damage. 
The present invention is based on the unexpected discovery that a compound 
of a silicone rubber at least 15% of which has, predominantly, silanol end 
groups, and a finely divided silica filler with a particle size in the 
reinforcing range, is strengthened markedly by a brief exposure, at room 
temperature, to ammonia gas, or ammonium hydroxide, or to the vapors or 
solutions of a volatile amine. Although it seems likely (although not 
proven) that this strengthening does not result from permanent, chemical 
crosslinking, but is, rather, an apparent and somewhat transient cure, the 
increase in mechanical properties obtained is more than adequate to enable 
the formed object, whether it be an extruded tube, wire insulation, thin 
sheet or molded object, to withstand the severe tensile, compressive and 
abrasive stresses involved in the mechanical handling necessary to convey 
it through irradiation apparatus. 
Crosslinking by irradiation with high energy electrons then introduces 
permanent, primary bond crosslinks. At least in the case of ammonia gas as 
the precure agent, the ammonia which diffuses into the compound to effect 
the precure seems to diffuse back out during, or soon after irradiation, 
and is not detectable by sensitive analytical techniques. Thus, 
crosslinking has been effected without leaving measurable chemical 
residues.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
In the preferred embodiment of the invention, a hydroxyl-terminated 
polysiloxane having a molecular weight from about 50,000 to about 
2,000,000, optionally modified by mixing with up to 85% of an end-stopped 
silicone rubber polymer, is mixed with from about 10 to about 70 parts per 
hundred of rubber of a fumed silica. Other inert fillers may be added to 
obtain desired property profiles. Suitable mixing equipment includes such 
high shear mixers as two-roll compounding mills or sigmablade doughmixers. 
The resulting compound is then formed to the desired final shape by an 
appropriate forming process. That is, tubes or rods are extruded through a 
die from a rubber extruder; wire insulation is extruded onto bare wire 
through a crosshead extruder; flat sheets are calendered or extruded 
through a slit die, and so on. Forming is normally carried out at or near 
room temperature. 
The fabricated rubber is then conveyed immediately through an atmosphere of 
ammonia or amine vapors, allowing a residence time in the vapors of from 
about 10 seconds to about 60 minutes, depending on the sample thickness 
and degree of cure required. The precuring step is also preferably carried 
out at or about room temperature. 
The fabricated rubber, which has now been strengthened greatly by the 
precuring process is then crosslinked by exposure to high energy 
radiation. High energy electrons are preferred as the radiation source, 
but gamma radiation may also be used. The radiation dose may be from about 
1 megarad to about 50 megarads with an optimum range from 5 megarads to 20 
megarads. 
For silicone rubber polymers to have useful mechanical properties they must 
be compounded with fillers of extremely small particle size, generally 
less than about 50 m.mu. in diameter and a high surface area, generally 
more than 150 M.sup.2 /g. This type of filler is known as a "reinforcing 
filler" as opposed to a filler of larger particle size and a smaller 
surface area which contributes little to the mechanical strength of a 
rubber, but rather serves primarily to reduce the cost of the compound. 
The reinforcing fillers used in silicone rubber are generally finely 
divided silicas, although carbon black fillers may also be used. The use 
of such "reinforcing fillers" is well known in the prior art and is 
described, for example, at pages 405-423 of "Reinforcement of Elastomers," 
edited by Gerard Kraus (Interscience Publishers, New York) in a chapter by 
John W. Sellers and Frank E. Toonder entitled, "Reinforcing Fine Particle 
Silicas and Silicates." 
For the operation of this invention silica fillers are essential. Fumed 
silicas are preferred. Fumed or pyrogenic silica comprises a form of 
silica described in U.S. Pat. No. 2,888,424. Such silicas are sold under 
the trade designation Cabosil, by the Cabot Corporation of Boston, 
Massachusetts. Another type of silica known as silica aerogel, not now 
sold commercially, but previously sold under the trade designation 
Santocel by the Monsanto Chemical Company, is also effective. Silica 
aerogels are described, for example, in U.S. Pat. Nos. 2,657,149 and 
2,093,454. 
Although fumed silicas or silica aerogels, or other equally finely divided 
silicas are essential to the process disclosed herein, they may be used in 
conjunction with other inert fillers or additives conventionally used to 
alter specific properties, reduce cost, or as pigments. Such fillers 
include clays, metal oxides, carbon black, and larger particle size 
silicas. Fillers may be process aids (such as silicone oil and a 
reinforcing filler), flame retardants, pigments, stabilizers, etc. It is 
well known that fillers may be surface coated or otherwise treated. 
In order to illustrate the improved properties attainable with this 
invention the following examples are presented. These examples are 
presented by way of disclosure only and should not be construed as being 
in any way limiting. In these examples SE-30 refers to a linear end-capped 
polydimethylsiloxane gum produced by General Electric Company; SE-33 
refers to a linear end-capped polydimethyl-vinylsiloxane gum produced by 
General Electric Company; SE-75 refers to a linear non-end-capped 
polydimethylsiloxane gum produced by General Electric Company; Cabosil 
MS-7 refers to a pyrogenic, high surface area silica produced by the Cabot 
Corporation; Hisil 233 refers to a medium surface area silica produced by 
PPG Industries, Inc.; Imsil A-10 refers to a low to medium surface area 
silica produced by Illinois Minerals Company; Santocel-C refers to a 
silica aerogel previously produced by the Monsanto Chemical Company; and 
washed P.D.M.S. refers to a polydimethylsiloxane having, predominantly, 
silanol end-groups. The latter polymers may be made by the KOH-catalyzed 
polymerization of octamethylcyclotetrosiloxane as disclosed in Warrick 
U.S. Pat. No. 2,541,137, issued Feb. 13, 1951 and subsequent removal of 
potassium ions by washing the polymer with water, or by other suitable 
techniques. Such polymers will hereinafter be referred to as 
"hydroxyl-terminated polydimethylsiloxanes." 
It will, of course, be understood by those skilled in the art that the 
reactive components are the silanol end-groups, and that the structure of 
the silicone chain may be varied considerably without departing from the 
scope of this invention. Thus, hydroxyl-terminated polysiloxanes, in which 
some of the methyl groups have been replaced by other aliphatic, aromatic, 
or cycloaliphatic radicals such as ethyl, vinyl, allyl, phenyl, etc. may 
also be employed. Methyl-phenyl type copolymers will probably require 
relatively higher dosage of irradiation. Similarly some of the methyl 
groups may be replaced by fluorinated, or other halogenated radicals. 
The controlled precure which is obtained by exposing hydroxyl-terminated 
polysiloxane rubber compounds to ammonia or amines is highly specific to 
the hydroxyl-terminated polymer as is clearly demonstrated by the data 
presented hereinbelow in Example I. Most commercial silicone rubber gums 
are linear polysiloxanes whose molecules are terminated with 
trimethylsilyl or other inert groups. When compounded with a silica 
filler, the resulting compounds are completely unaffected by ammonia or 
amines. On the other hand, a KOH-catalyzed polymer, as disclosed in the 
Warrick U.S. Pat. No. 2,541,137, made without trimethylsilyl endgroups, 
and still containing the potassium ions (i.e., not water washed) develops 
an apparent cure rapidly, when compounded with the silica filler, to such 
an extent that it is difficult to form by extrusion, molding, calendering, 
etc. 
However, it is possible to blend conventional silicone rubber polymers, 
endstopped with trimethylsilyl, or other monofunctional silyl groups, with 
the hydroxyl-terminated polysiloxane up to 85% of the mixture, and to 
obtain a useful degree of precure when exposed to ammonia or amine vapors. 
EXAMPLE I 
In the first series of experiments, various silicone polymers and fillers 
were mixed together at room temperature on a laboratory two-roll mill. The 
compounds were then stripped from the mill and pressed into 100 mil-thick 
sheets on a water-cooled laboratory press. The sheets were then exposed to 
an atmosphere of anhydrous ammonia for 10 minutes. These results are 
summarized in Table I. 
TABLE I 
__________________________________________________________________________ 
A B C D E F G H I J 
__________________________________________________________________________ 
SE-33 100 100 50 
SE-30 100 
SE-75 100 
100 
100 100 
50 
Washed PDMS 100 
Cabosil MS-7 
25 
25 25 
30 30 
Hisil 233 25 
25 
Imsil A-10 25 
Santocel-C 
Tensile (psi) Prior 
Nil 
Nil 
Nil 
Nil 
Nil 
Nil 
Nil 
Nil 
Nil 
to NH.sub.3 Exposure 
.dwnarw. 
.dwnarw. 
.dwnarw. 
.dwnarw. 
.dwnarw. 
Tensile (psi) After 
.dwnarw. 
.dwnarw. 
.dwnarw. 
.dwnarw. 
.dwnarw. 
300 
850 
220 
450 
NH.sub.3 Exposure 
% Elongation After 
.dwnarw. 
.dwnarw. 
.dwnarw. 
.dwnarw. 
.dwnarw. 
700 
750 
350 
900 
NH.sub.3 Exposure 
__________________________________________________________________________ 
EXAMPLE II 
In this example various amounts of pyrogenic silica were compounded with a 
non-end blocked polydimethylsiloxane gum (SE-75) and cured with anhydrous 
ammonia for 10 minutes. These data are summarized in Table II: 
TABLE II 
______________________________________ 
A B C D E 
______________________________________ 
SE-75 Non-end Capped 
100 100 100 100 100 
Polydimethylsiloxane 
Polymer (Parts) 
Cabosil MS-7 (phr) 
15 25 30 35 45 
Tensile After 10 
50 300 600 1100 1300 
Minutes' Exposure to NH.sub.3 
(psi) 
% Elongation After 10 
400 700 750 800 750 
Minutes' Exposure to NH.sub.3 
______________________________________ 
EXAMPLE III 
In this series of experiments, a formulation containing 100 parts of non 
end-capped polydimethylsiloxane gum and 40 parts of fumed silica were 
exposed to vapors of various organic amines for a period of 10 minutes. 
Tensile and elongation tests were then performed on these samples. The 
date are summarized in Table III. 
TABLE III 
______________________________________ 
TENSILE/ TENSILE/ 
ELON- ELON- 
GATION GATION 
PRIOR TO AFTER 
COMPOUND AMINE EXPOSURE EXPOSURE 
______________________________________ 
100 g. 
SE-75 Methyl- Nil 970-500 
40 g. fumed silica 
Amine 
100 g. 
SE-75 Dimethyl- Nil 1050/550 
40 g. fumed silica 
Amine 
100 g. 
SE-75 Trimethyl- 
Nil 1050/600 
40 g. fumed silica 
Amine 
100 g. 
SE-75 Allyl- Nil 900/600 
40 g. fumed silica 
Amine 
100 g. 
SE-75 Pyridine Nil 1050/650 
40 g. fumed silica 
______________________________________ 
As can be seen from this data, organic primary, secondary and tertiary 
amine vapors may be used to provide a pseudo-cure to the silicone rubber. 
EXAMPLE IV 
In this series of experiments, a formulation composed of 100 parts non-end 
capped polydimethylsiloxane gum and 40 parts of fumed silica were cured 
with anhydrous ammonia and then irradiated with high-energy electrons to 
specified doses. Tensile and elongation tests were performed on unaged 
specimens, specimens aged at 220.degree. C. for 50 hours, and specimens 
aged in boiling water (100.degree. C.) for 50 hours. These data are 
summarized in Table IV. 
TABLE IV 
______________________________________ 
DOSE (Mrads) 
0 2 5 10 
______________________________________ 
Initial Tensile 1200 1100 850 800 
Strength (psi) 
Initial Elongation (%) 
500 420 250 125 
Tensile After 50 Hours 
450 650 850 750 
at 220.degree. C. (psi) 
Elongation After 50 Hours 
350 325 200 50 
at 220.degree. C. (%) 
Tensile After 50 Hours 
&lt;100 650 850 700 
in Water at 100.degree. C. 
Elongation After 50 Hours 
&lt;50 450 250 100 
in Water at 100.degree. C. 
______________________________________ 
As can be seen from these data, a permanent cure is effected by irradiation 
with high-energy electrons. 
EXAMPLE V 
In this series of experiments, two formulations were cured with anhydrous 
ammonia and irradiated by high-energy electrons to the desired dose. 
Specimens were then subjected to compression at elevated temperatures and 
the % permanent set measured. These results are given in Table V. 
TABLE V 
______________________________________ 
COMPRESSION 
SET (%) 
DOSE (22 HOURS 
FORMULATION (MRADS) at 175.degree. C. 
______________________________________ 
100g SE-75 
40g Cabosil MS-7 2 54 
NH.sub.3 Cure for 10 Minutes 
5 38 
100g SE-75 
50g Cabosil MS-7 2 33 
NH.sub.3 Cure for 10 Minutes 
5 27 
______________________________________ 
EXAMPLE VI 
In this series of experiments, a compound composed of 100 parts non-end 
blocked polydimethylsiloxane gum, 40 parts fumed silica (Cabosil MS-7) 
treated with CF-11 73 (a silicone fluid manufactured by General Electric 
Company) in accordance with U.S. Pat. No. 2,939,009, 40 parts 
non-reinforcing silica (Imsil A-10) and 2 parts red iron oxide were 
blended on a two-roll mill and prehardened by exposure to NH.sub.3. The 
material was then permanently cross-linked by means of high energy 
electrons. This material, along with a conventional peroxide cured 
material (GE 9100-A) was then heat aged at 300.degree. C. for 100, 250 and 
350 hours. At the end of this time, physical properties were measured in 
order to determine the relative heat stability of these materials. These 
data are summarized in Table VI. 
TABLE VI 
__________________________________________________________________________ 
TENSILE/ELONGATION/DUROMETER 
DOSE 100 HRS. 
250 HRS. 
350 HRS. 
COMPOUND (MRADS) 
INITIAL 
AT 300.degree. C. 
AT 300.degree. C. 
AT 300.degree. C. 
__________________________________________________________________________ 
100 parts non-end 
blocked PDMS 
40 parts fumed silica 
40 parts non-reinforcing 
silica 
2 parts Fe.sub.2 O.sub.3 
10 930/160/68 
575/135/67 
475/100/77 
530/65/73 
100 parts non-end 
blocked PDMS 
40 parts fumed silica 
40 parts non-reinforcing 
silica 
2 parts Fe.sub.2 O.sub.3 
20 950/100/72 
470/75/70 
465/50/85 
515/50/80 
Peroxide cured 
polymethylvinylsiloxane 
(Peroxide 
1000/400/55 
545/100/75 
460/0/97 
700/0/96 
(GE 9100-A) cure) 
Peroxide cured 
polymethylvinylsiloxane 
(Peroxide 
1100/400/59 
690/75/85 
600/30/93 
550/0/95 
(GE SE-9058) 
cured) 
__________________________________________________________________________ 
As can be seen from these data, silicone crosslinked by high energy 
electrons appears to have superior heat stability (based on % elongation) 
as compared to peroxide crosslinked silicone. 
It will be understood that the embodiments, product, process, and practices 
described and portrayed herein have been presented by way of disclosure, 
rather than limitation, and that various modifications, substitutions, and 
combinations may be effected without departure from the spirit and scope 
of this invention in its broader aspects.