GB1562645A

GB1562645A - Reversibly expanding silicone rubber

Info

Publication number: GB1562645A
Application number: GB1278877A
Authority: GB
Original assignee: PRADER R
Current assignee: PRADER R
Priority date: 1976-11-29
Filing date: 1977-03-25
Publication date: 1980-03-12
Also published as: NL7614589A; JPS5616743B2; CA1092765A; FR2372370A1; JPS5369281A; JPS5654218B2; JPS5699650A; DE2704598A1

Description

(54) IMPROVEMENTS IN OR RELATING TO REVERSIBLY EXPANDING SILCONE RUBBER (71) We, VICTOR EDWARD ALTHOUSE, of 393 Traverso Avenue, Los Altos, California 94022, United States of America, and RANDOLPH DAVID PRADER, of 811 Mulberry Lane, Sunnyvale, California 94087, United States of America, both citizens of the United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates generally to ways and means of connecting an elastomeric article with a supporting surface. The present invention further relates to a method of expanding silicone rubber, to silicone rubber treated with a fluorocarbon, to a method of securing a silicone rubber enclosure element to a support structure and to a silicone rubber enclosure element. The present invention further relates to a silicone rubber article and method of securing a silicone rubber article, such as a length of tubing, to a support or supporting surface by first contacting the silicone rubber article with an improved expanding agent which effects expansion and thereafter causing shrinking of the article to substantially its original dimensions without adversely affecting the desirable properties of the silicone rubber.

Silicone rubber tubing and other shaped articles formed of a silicone rubber composition are widely used in thermal and electrical insulation systems and are particularly useful, because of their outstanding chemical and physical properties. In many applications it is desirable to have the silicone rubber article fit securely over and connect the adjacent ends of two tubular members. Commonly, this is achieved through the use of heat shrink technology after the tubular article is mechanically expanded to an oversized shape. In this oversized shape the article can be placed over the end of a tubular member or over the end of any like supporting structure, and, thereafter the silicone article is shrunk by heat treatment to substantially its original unexpanded dimensions. In so doing, the article will tend to conform securely to, and form a sealing engagement with, the supporting structure.

Heretofore, methods have been devised for using various swelling agents to effect dilating or swelling of tubes or rings formed of an elastomeric material, such as polyvinyl chloride elastomers. The expansion and contraction of the prior art elastomeric materials have been effected by the swelling agents being absorbed into the elastomeric material and extracting at least a portion of one or more of the organic components, such as a plasticizer, contained in the elastomeric material. When it is desirable that the elastomeric article be highly resistant to heat, however, the inclusion of a plasticizer or other extractable organic material in the elastomer is undesirable, because the residual organic material causes a significant reduction in the heat resistance of the elastomeric article.

According to a first aspect of the invention, there is provided a method of securing a silicone rubber enclosure element about a support structure, which method comprises contacting an enclosure element formed of a silicone rubber with a fluorocarbon until the inner dimension of the enclosure element has expanded in excess of the original internal dimension thereof, fitting the element in the expanded form over the support structure and thereafter causing the element to shrink and form a secure engagement by removal of fluorocarbon from the element.

The Fluorocarbon is preferably removed by evaporation, which advantageously may occure at ambient temperature and pressure.

According to a second aspect of the present invention, there is prdvided a silicone rubber enclosure element having an inner dimension thereof expanded in excess of the normal unexpanded dimension thereof and comprising an enclosure element formed essentially of silicone rubber impregnated with a fluorocarbon.

The present invention enables the provision of an improved method of securing an enclosure element(s) formed of a silicone rubber about a support structure which avoids mechanically stressing and heating the silicone rubber element(s) to effect expansion and contraction of the silicone rubber element(s).

The present invention further enables the provision of an improved method of fitting a silicone rubber element about a support structure which avoids having an extractable organic compound present in the silicone rubber element when formed.

The present invention further enables the provision of an improved method of fitting a silicone rubber element about a support structure which forms a secure connection between the silicone rubber element and the support structure more rapidly and safely than methods heretofore devised.

The present invention further enables the provision of a silicone rubber element, such as silicone rubber tubing, in a form suitable for immediately fitting about a support structure without requiring supplemental stressing or heating equipment for expanding or shrinking the silicone element.

It has now been found that an enclosure element formed of silicone rubber can be effectively expanded and shrunk using an embodiment of a method in accordance with the present invention by treating a silicone rubber article in the form of a tubing or like shaped article with an expanding agent comprised essentially of a fluorocarbon compound and, after mounting the expanded silicone rubber article about a support structure, the outer dimensions of which are slightly larger than the original inner dimension of the unexpanded silicone rubber article, allowing the fluorocarbon compound to separate from the silicone rubber article at ambient temperature and pressure so that the expanded silicone rubber article returns to substantially its original dimensions and forms a secure engagement with the support structure.

The silicones or organo siloxane elastomeric materials which can be used in a method in accordance with the present invention are based on a silicon-oxygen structure to which various organic radicals are attached.. The three fundamental types of organosiloxanes (oils, rubber, and resins) may be considered as being built from three unit structures which have been called M, D and T units as follows:

M unit D unit T unit As will be evident, the M unit is monofunctional, the D unit is difunctional and the T unit is trifunctional. In the formation of silicone or organosiloxane polymers, the M units can be used only for dimers or as end units or chain stoppers. Linear polymers can be produced from the D units with M units as the terminating groups. Cross-linked polymers can be produced from the T units. Therefore, the three general types of organosiloxane materials referred to above can be considered as having the following arrangement of M, D and T units: Silicone oils: M(D)XM, where x is a small number Silicone rubber: M(D)xM, where x is a very large number Silicone resins: MDT combinations.

These silicone molecules can be tailored to achieve specific properties. For example, the R group in a dimethyl polysiloxane polymer can be replaced by or substituted with phenyl and/or vinyl or trifluoropropyl groups to achieve significant variations in properties.

Silicone rubbers which are suitable for use in the present invention include, but are not limited to methyl vinyl polysiloxane, phenyl vinyl methyl polysiloxane and trifluoropropylmethyl vinyl polysiloxane. The polysiloxane rubber compositions can be combined to provide special properties to the enclosure element.

The silicone elastomeric rubber material can be provided in the form of a tube, ring or other enclosed form and can be extruded in such forms by conventional apparatus and procedures. The silicone rubber elastomers can have, but are not required to have, minor amounts of other conventional additives admixed therewith, such as inert fillers, colouring agents, or vulcanising agents.

The outstanding properties which distinguish the silicone rubber materials from other elastomers is their great resistance to extremely high and low temperatures. Thus, a silicone rubber element can be exposed to temperatures up to about 200"C for from 2 to 5 years without losing its resilience and dielectric strength. The silicone rubber elastomers also exhibit excellent resistance to both ozone and corona. The volume resistivity ranges between from 10 to 14 to from 10 to 16 61.cam and the dielectric strength ranges between from 400 to 700 V.mm-1. When ignited, the silicones do not form objectionable acids, such as HCl, HCN, or SO2.

While silicone rubber compositions are generally resistant to solvents, such as alcohol and acetone and to many chlorinated hydrocarbons, it has now been discovered that very significant expansion and contraction of a silicone rubber article can be effected when the silicone rubber article formed of a silicone rubber, such as methyl vinyl polysiloxane rubber, trifluoropropylmethyl vinyl polysiloxane rubber or both, is treated with a fluorocarbon compound, such as trichloro-trifluoroethane, without normally requiring the presence of an organic plasticiser and without substantial loss of any of the desirable characteristics of the silicone rubber, such as prolonged stability at elevated temperatures, up to about 260"C and at temperatures as low as about 100"C, non-flammability, high degree of electrical stability, and good resistance to dielectric fatigue.

The expanding or swelling agent used for treating the silicone rubber element is selected from the group of fluorocarbon chemicals comprising the group of halogenated hydrocarbons (usually based on methane or ethane) containing one or more fluorine atoms. Such chemicals are commonly sold under the trade name FREON, a trademark of the E.I.

duPont de Nemours Company. The fluorocarbon used in the present invention is preferably liquid at the temperatures normally encountered when using the silicone rubber elements but should preferably not have a high boiling point, since the compound should preferably have a high rate of vaporisation and so should be characterised by a relatively high vapour pressure and a low heat of vaporisation. In addition the fluorocarbon should preferably be readily radially absorbed by the silicone rubber material. The fluorocarbon chemical, trichlorotrifluoroethane, satisfies all of the foregoing preferred physical and chemical requirements and, in addition has the desirable characteristis of exhibiting low toxicity with an OSHA threshold limit of 1000 p.p.m., has excellent resistance to both ozone and corona and is non-flammable. Trichlorotrifluoroethane is chemically non-reactive with most other materials and does not present a disposal problem. Among the group of fluorocarbons suitable for use in the present invention but are not limited thereto are trichloromonofluoromethane, trichlorotrifluoroethane and trichlorodifluoroethane.

In a preferred embodiment of a method in accordance with the present invention a silicone rubber enclosure element, such as a length of tubing, is preferably treated with the fluorocarbon dilating agent by immersing the silicone rubber enclosure element in a liquid fluorocarbon for a period sufficient to cause the silicone rubber enclosure element to expand approximately 50% in excess of its original inner diameter when the element is in the form of a ring or tube. The maximum swelling of the silicone rubber enclosure element when treated with the fluorocarbon liquid is essentially complete in about 2 to 4 hours. It is preferably to pretreat the silicone rubber enclosure element by sealably enclosing the element in a container along with sufficient fluorocarbon liquid to effect maximum expansion of the silicone rubber element and provide a slight excess of fluorocarbon vapour within the container so that the silicone rubber element will remain in the expanded form until the container is opened. The expanded silicone rubber element can be kept in the sealed container for an indefinite period. When opened. the silicone rubber element is ready for immediate use and, after mounting the expanded silicone rubber element about a supporting structure having an outer diameter slightly larger than the original inner diameter of the silicone rubber element. the silicone rubber element begins to shrink as the fluorocarbon chemical evaporates, forming a secure frictional engagement about the supporting structure when the silicone rubber element shrinks to its original dimensions.

The method of expanding and retracting a silicone rubber enclosure element, such as a ring or tubing has numerous applications and is particularly adapted for forming waterproof connections between electrical conductors or pipes, in the space and aircraft industry, and in the automotive industry.

The following Example further illustrates the present invention.

Example I In one useful embodiment of the present invention a tubing formed of methyl vinyl polysiloxane rubber sold under the trademark "Silastic", Medical Grade Elastic Tubing by Dow Corning and having as original dimensions an inner diameter (I.D.) of 0.435 inches, a wall thickness of 0.037 inches and a length of 3 inches was impregnated throughout by treating with 3.25 ml. of trichlorotrifluoroethane in a sealed container.

After 24 hours the inside diameter of the tubing increased to 0.6000 inches, and the length to 3.875 inches. On opening the sealed container and exposing the dilated tubing to atmospheric pressure for 105 minutes at 210C (70"F), the tubing recovered substantially to the original dimensions (actual recovered dimensions were 0.436 inches I.D., 3 inches length, 0.034 inches wall thickness).

Example II Slabs formed of methyl vinyl polysiloxane rubber sold under the trademark "Electrisil SE9095A-03" by General Electric and having cut ring dimensions of 1.0" outside diameter and 0.8" inside diameter with a thickness of 0.085" was impregnated throughout by treating with an excess of trichlorotrifluoroethane in a sealed container. After 24 hours the inside diameter of the ring increased to 1.3" outside diameter, 1.05" inside diameter, and 0.115" thickness. On opening the sealed container and exposing the dilated rings to atmospheric pressure for 105 minutes at 210C (70"F), the ring recovered to its approximate original dimensions (the actual recovered dimensions were 1.0" outside diameter, 0.8" inside diameter, and 0.085" thickness).

Example III A trifluoropropyl methyl vinyl polysiloxane rubber strip having a length of 2.0" and width of 0.44" was impregnated throughout by treating with an excess of trichlorotrifluoroethane in a sealed container. After 24 hours the length of material increased to 2.25" and the width to 0.5". On opening the sealed container and exposing the dilated strips to atmospheric pressure for 105 minutes at 21"C (70"F), the strip recovered to its approximate original dimensions (the actual recovered dimensions were 2.0" long and 0.44" wide).

Similar results are obtained with other grades of methyl vinyl polysiloxane rubber materials and fluorocarbon expanding agents.

WHAT WE CLAIM IS: 1. A method of securing a silicone rubber enclosure element about a support structure, which method comprises contacting an enclosure element formed of a silicone rubber with a fluorocarbon until the inner dimension of the enclosure element has expanded in excess of the original internal dimension thereof, fitting the element in the expanded form over the support structure and thereafter causing the element to shrink and form a secure engagement by removal of fluorocarbon from the element.

2. A method according to Claim 1, wherein the fluorocarbon is removed by evaporation.

3. A method according to Claim 2, wherein the evaporation occurs at ambient temperature and pressure.

4. A method according to any one of Claims 1 to 3, wherein the silicone rubber comprises methyl vinyl polysiloxane, phenyl vinyl methyl polysiloxane or trifluoropropy- Imethyl vinyl polysiloxane or any combination of two or more thereof.

5. A method according to any one of Claims 1 to 3, wherein the silicone rubber is essentially methyl vinyl polysiloxane.

6. A method according to any one of Claims 1 to 5, wherein the fluorocarbon comprises trichlorotrifluoroethane or tetrachlorodifluoroethane or trichloromonofluoromethane or any combination of two or more thereof.

7. A method according to any one of Claims 1 to 5, wherein the fluorocarbon comprises trichlorotrifluoroethane.

8. A method according to any one of the preceding Claims, wherein the silicone rubber is essentially free of extractable organic compounds.

9. A silicone rubber enclosure element having an inner dimension thereof expanded in excess of the normal unexpanded dimension thereof and comprising an enclosure element formed essentially of silicone rubber impregnated with a fluorocarbon.

10. A silicone rubber enclosure element according to Claim 9, wherein the silicone rubber comprises methyl vinyl polysiloxane or phenyl vinyl methyl polysiloxane or trifluoropropylmethyl vinyl siloxane or any combination thereof.

11. A silicone rubber enclosure element according to Claim 9 or 10, wherein the enclosure element is impregnated with a fluorocarbon comprising trichlorotrifluoroethane, tetrachlorodifluoroethane, trichloromonofluoromethane or any combination of two or more thereof.

12. A silicone rubber enclosure element according to Claim 9 or 10, wherein the enclosure element is impregnated with trichlorotrifluoroethane.

13. A silicone rubber enclosure element according to Claim 9, wherein the enclosure element is formed of methyl vinyl polysiloxane and is impregnated with trichlorotrif- luoroethane.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

throughout by treating with 3.25 ml. of trichlorotrifluoroethane in a sealed container.

After 24 hours the inside diameter of the tubing increased to 0.6000 inches, and the length to 3.875 inches. On opening the sealed container and exposing the dilated tubing to atmospheric pressure for 105 minutes at 210C (70"F), the tubing recovered substantially to the original dimensions (actual recovered dimensions were 0.436 inches I.D., 3 inches length, 0.034 inches wall thickness).

Example II Slabs formed of methyl vinyl polysiloxane rubber sold under the trademark "Electrisil SE9095A-03" by General Electric and having cut ring dimensions of 1.0" outside diameter and 0.8" inside diameter with a thickness of 0.085" was impregnated throughout by treating with an excess of trichlorotrifluoroethane in a sealed container. After 24 hours the inside diameter of the ring increased to 1.3" outside diameter, 1.05" inside diameter, and 0.115" thickness. On opening the sealed container and exposing the dilated rings to atmospheric pressure for 105 minutes at 210C (70"F), the ring recovered to its approximate original dimensions (the actual recovered dimensions were 1.0" outside diameter, 0.8" inside diameter, and 0.085" thickness).

Example III A trifluoropropyl methyl vinyl polysiloxane rubber strip having a length of 2.0" and width of 0.44" was impregnated throughout by treating with an excess of trichlorotrifluoroethane in a sealed container. After 24 hours the length of material increased to 2.25" and the width to 0.5". On opening the sealed container and exposing the dilated strips to atmospheric pressure for 105 minutes at 21"C (70"F), the strip recovered to its approximate original dimensions (the actual recovered dimensions were 2.0" long and 0.44" wide).

Similar results are obtained with other grades of methyl vinyl polysiloxane rubber materials and fluorocarbon expanding agents.

WHAT WE CLAIM IS: 1. A method of securing a silicone rubber enclosure element about a support structure, which method comprises contacting an enclosure element formed of a silicone rubber with a fluorocarbon until the inner dimension of the enclosure element has expanded in excess of the original internal dimension thereof, fitting the element in the expanded form over the support structure and thereafter causing the element to shrink and form a secure engagement by removal of fluorocarbon from the element.
2. A method according to Claim 1, wherein the fluorocarbon is removed by evaporation.
3. A method according to Claim 2, wherein the evaporation occurs at ambient temperature and pressure.
4. A method according to any one of Claims 1 to 3, wherein the silicone rubber comprises methyl vinyl polysiloxane, phenyl vinyl methyl polysiloxane or trifluoropropy- Imethyl vinyl polysiloxane or any combination of two or more thereof.
5. A method according to any one of Claims 1 to 3, wherein the silicone rubber is essentially methyl vinyl polysiloxane.
6. A method according to any one of Claims 1 to 5, wherein the fluorocarbon comprises trichlorotrifluoroethane or tetrachlorodifluoroethane or trichloromonofluoromethane or any combination of two or more thereof.
7. A method according to any one of Claims 1 to 5, wherein the fluorocarbon comprises trichlorotrifluoroethane.
8. A method according to any one of the preceding Claims, wherein the silicone rubber is essentially free of extractable organic compounds.
9. A silicone rubber enclosure element having an inner dimension thereof expanded in excess of the normal unexpanded dimension thereof and comprising an enclosure element formed essentially of silicone rubber impregnated with a fluorocarbon.
10. A silicone rubber enclosure element according to Claim 9, wherein the silicone rubber comprises methyl vinyl polysiloxane or phenyl vinyl methyl polysiloxane or trifluoropropylmethyl vinyl siloxane or any combination thereof.
11. A silicone rubber enclosure element according to Claim 9 or 10, wherein the enclosure element is impregnated with a fluorocarbon comprising trichlorotrifluoroethane, tetrachlorodifluoroethane, trichloromonofluoromethane or any combination of two or more thereof.
12. A silicone rubber enclosure element according to Claim 9 or 10, wherein the enclosure element is impregnated with trichlorotrifluoroethane.
13. A silicone rubber enclosure element according to Claim 9, wherein the enclosure element is formed of methyl vinyl polysiloxane and is impregnated with trichlorotrif- luoroethane.
14. A method of securing a silicone rubber enclosure element about a support

structure, in accordance with Claim 1 and substantially as described in foregoing Example I.
15. A method of securing a silicone rubber enclosure element about a support structure, in accordance with Claim 1 and substantially as described in foregoing Example II.
16. A method of securing a silicone rubber enclosure element about a support structure in accordance with Claim 1 and substantially as described in foregoing Example III.
17. A silicone rubber enclosure element in accordance with Claim 9 and substantially as described in foregoing Example I.
18. A silicone rubber enclosure element in accordance with Claim 9 and substantially as described in foregoing Example II.
19. A silicone rubber enclosure element in accordance with Claim 9 and substantially as described in foregoing Example III.
20. A silicone rubber enclosure element whenever fitted to a support structure by the method of any one of Claims 1 to 8 and 14 to 16 and/or with the silicone rubber enclosure element of any one of Claims 9 to 13 and 17 to 19.