HEAT RECOVERABLE ARTICLE WITH STRAIN RELIEF

Abstract: A heat recoverable article made of a relatively stiff polymeric material, i.e. a material having a 2 % secant modulus of at least about 30,000 psi, and having at least three elongate outlet elements, or legs, is provided with a convoluted region having at least one convolute on at least one of the legs to provide strain relief. The article is particularly useful as a transition in a wire harness.

Description: HEAT RECOVERABLE ARTICLE WITH STRAIN RELIEF This invention relates to a heat recoverable article formed of a high modulus polymeric material and having intrinsic strain relief means, to a method of providing strain relief to a heat recoverable article formed of high modulus polymeric material and to a method of protecting a transition of a wire harness using said articl-e. Heat recoverable articles are used to protect various substrates, such as, cable splices, pipelines, wire har¬ nesses and the like. Such heat recoverable articles include tubing, boots, transitions, covers, tape, etc. Such heat recoverable articles are typically made from thermoplastic and/or elastomeric materials. Certain applications of heat shrinkable articles, such as, for example, wire and cable harnesses used in aircraft engines the article must be made of a material that is capable of withstanding temperatures of about 150-200°C and resistant, even at those temperatures, to chemicals such as fuel, de-icing solvents, hydraulic fluids, decontamination solvents and the like. Such materials include for example, vinylidene fluoride homo- and copolymers, such as Kynar, commercially available from Pennwalt, such as ethylene/ tetrafluoroethylene copolymers such as Tefzel commercially from Dupont, polyarylene ether ketones, and other high per¬ formance polymers. These materials, however, tend to be relatively stiff and have a high secant modulus. It has been found that heat-recoverable articles, such as boots, transitions, "T's," "Y's," and the like, made from these materials tend to split during installation of the harness, particularly where the outlet legs of the article and/or the adjacent wires must be bent to accommodate a tortuous path around engine components. This splitting is a result of excess strain at the ends of the outlet legs and/or at the crotch area of the article. A typical article of this type is the transition shown in Fig.l, where wire bundle 10 enters the transition 1 through a first elongate outlet, 2, and two smaller wire bundles 12 and 14 exit through outlet legs 3 and 4, respectively. We have now discovered that such articles can intrin¬ sically be provided with strain-relief if at least one of the outlet legs is provided with an intermediate convoluted region. The use of convoluted heat recoverable tubing to provide flexibility has been proposed in the art. For example, U.K. published patent application No. 2,125,237 discloses flexible convoluted tubing and European published patent application. No. 68,781 discloses a cable breakout article having convolutions which provide sufficient flexibility that one tubular outlet portion of the cable breakout can be bent away from the others- In order to achieve sufficient flexibility of the tubular outlet, it has been found that the outlets should be of a material having a relatively low modulus such as low density polyethylene which has a 2% Secant modulus of about 20,000. The UK patent application does not address the problem of providing strain relief to heat recoverable articles of relatively stiff polymeric material. One aspect of this invention comprises a heat reco¬ verable hollow article formed from a polymeric material having 2% Secant modulus of at least about 30,000 psi, said article having at least three integral elongate outlet ele¬ ments, each of said elements having an end region of substantially uniform cross section and at least one of said elements having an intermediate convoluted region which upon recovery of the article about a substrate remains con¬ voluted, said convoluted region providing intrinsic strain relief to the recovered article. A further aspect of this invention comprises a method of providing intrinsic strain relief to a hollow heat reco- verble article formed of a polymeric material having a 2% Secant modulus of at least about 30,000 psi, said article having at least three integral elongate outlet elements each having an end region of substantially uniform cross section, which comprises providing at least one of said elements with an intermediate convoluted region which upon recovery of the article remains convoluted. Another aspect of this invention comrpises a heat reco¬ verbale tubular article formed from a polymeric material having a 2% Secant modulus of at least 30,000 psi and having an intermediate convoluted region which upon recovery the article remains convoluted, said convoluted region providing intrinsic strain relief to the recovered article. Yet another aspect of this invention comprises a method of providing intrinsic strain relief to a heat recoverable tubular article having a 2% Secant modulus of at least 30,000 psi which comprises providing said article with an intermediate convoluted region which upon recovery of the article remains convoluted. Fig. 1 illustrates a typical prior art transition of a wire harness. Fig. 2 is a cross-sectional view of a transition in accordance with this invention. Fig. 3 shows a heat-recoverable tubular article 30 in accordance with this invention. The article has two end regions 31 and 32 and an intermediate convoluted region 33. The article of this invention is heat recoverable. A heat recoverable article is an article the dimensional configuration of which may be substantially to change when subjected to heat treatment. usually these articles recover, on heating, towards an original shape from which they have previously been deformed but the term "heat-recoverable", as used herein, also inclu¬ des an article which, on heating, adopts a new con¬ figuration, even if it has not been previously deformed. In their most common form, such articles comprise a heat-shrinkable sleeve made from a polymeric material exhi¬ biting the property of elastic or plastic memory as described, for example, in U.S. Patents 2,027,962; 3,086,242 and 3,957,372. As is made clear in, for example, U.S. Patent 2,027,962, the original dimensionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-stable form but, in other applications, a preformed dimensionally heat stable article is deformed to a dimensionally heat unstable form in a separate stage. One manner of producing a heat-recoverable article comprises molding the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric material, heating the article to a temperature above the crystalline melting point or, for amorphous materials the softening point, as the case may be, of the polymer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape. The article of this invention is made of a polymeric material having a 2% Secant modulus of at least about 30,000 psi, preferably at least about 50,000 psi and most pre¬ ferably at least about 75,000 psi. The 2% Secant modulus is generally below about 200,000 psi and preferably is below about 120,000 psi. The polymeric material preferably has a 2% Secant modulus in the range of about 50,000 psi to about 200,000 psi, preferably about 70,000 psi to about 150,000 psi. Preferred materials include vinylidene fluoride polymers, ethylene-tetrafluoroethylene copolymers, (poly aryl ether ketones) , and the like. A particularly preferred material is a thermoplastic copolymer of vinylidene fluoride and hexafluoropropylene. The wall thickness of the recovered article should be in the range of about 10 to about 200 mils, preferably about 20 to about 150 mils and most preferably about 40 to about 100 mils. Generally, strain relief in accordance with this invention is most advantageous for articles of very high modulus materials regardless of wall thickness or of relati¬ vely high modulus with relatively thick walls. Articles of relatively low modulus materials with relatively thick walls and articles of relatively high modulus materials with very thin generally walls do not require strain relief. As discussed above, heat recoverable articles having three or more elongate outlet elements, or legs, tend to split if made of high modulus polymeric material and sub¬ jected to strain, such as when used in a wire harness and either the recovered article or adjacent wires are bent to accommodate a tortuous path around adjacent structures. In accordance with this invention the article is provided with an intermediate convoluted region to provide strain relief. The convoluted region has at least one convolute and is generally formed as an integral part of the article. The convoluted region can be incorporated during molding or extrusion of the article. Fig. 2 shows a three-leg transition in accordance with this invention. In accordance with Fig. 2 transition 20 consists of elongate outlet elements 21, 22 and 23. Each of the legs is provided with an end region, 24, 25 and 27, respectively of substantially uniform cross-section to pro¬ vide a seal with the substrate, such as wires, if desired. In the embodiment illustrated in Fig. 2 , two of the outlet legs 22 and 23 are provided with an intermediate convoluted region, 27 and 28 respectively. The convolutions provide strain relief, transferring strain from the ends of the legs or crotch region, depending on where such strain is applied, and spreading that strain over the convoluted region so that it is not concentrated at one point. This enables the tran¬ sition or the adjacent wires to be bent as desired without resulting in, or at least decreasing the occurrence of, splitting of the article at the crotch area, 29, or the ends 25 and 26 of the legs, 22 and 23. The number of con¬ volutions provided and their size can be designed to provide the desired degree of strain relief to the article depending on its modulus and the ultimate end use. If desired, crotch area 28 can be designed so that its wall thickness is greater than the wall thickness of the legs. This further reduces the tendency of the crotch area to split. Fig. 3 shows a heat-recoverable tubular article 30 in accordance with this invention. The article has two end regions 31 and 32 and an intermediate convoluted region 33. Upon recovery the convoluted region provides strain relief which prevents splitting of the end regions when the substrate onto which it is recovered is bent at the region adjacent the article. While this invention has been described with reference to a three-legged transition, it is to be understood that the provision of strain relief in accordance with this invention can be advantageous for heat recoverable articles having any number of elongate outlets. Such articles include boots (2 outlet legs), tridents (4 outlet legs), T's (3 outlet legs) and the like.

Claims: What Is Claimed Is: 1. A heat recoverable hollow article formed from a poly¬ meric material having a 2% Secant modulus of at least about 30,000 psi, said article having at least three integral elongate outlet elements, each of said elements having an end region of substantially uniform cross sec¬ tion and at least one of said elements having an inter¬ mediate convoluted region having at least one convolute which upon recovery of the article about a substrate remains convoluted, said convoluted region providing intrinsic strain relief to the recovered article. 2. An article in accordance with Claim 1 wherein said poly¬ meric material has a 2% Secant modulus of at least about 50,000 psi, preferably at least 75,000 psi. 3. An article in accordance with Claim 1 wherein said poly¬ meric material has a 2% Secant modulus between about 50,000 and 200,000 psi, preferably between 75,000 and 120,000 psi. 4. An article in accordance with Claim 1 wherein the wall thickness of said at least one element is between 10 mils and about 200 mils when recovered. 5. An article in accordance with Claim 1 wherein at least two of said elements have an intermediate convoluted region. 6. An article in accordance with Claim 1 having three elongate outlet elements and wherein two of said ele¬ ments have an intermediate convoluted region. 7. An article in accordance with Claim 6 having a crotch area between said two elements wherein said crotch area has a wall thickness greater than the wall thickness of the end regions of said elements. 8. An article in accordance with Claim 1 wherein said poly¬ meric material is a vinylidene fluoride polymer. 9. A method of protecting a transition of a wire harness which comprises placing over the transition an article according to claim 1 and then heating the article to cause it to recover. 10. A method of providing intrinsic strain relief to a hollow heat recoverable article formed of a polymeric material having a 2% Secant modulus of at least about 30,000 psi, said article having at least three integral elongate outlet elements each having an end region of substantially uniform cross section, which comprises providing at least one of said elements with an inter¬ mediate convoluted region which upon recovery of the article remains convoluted. 11. A heat recoverable tubular article formed from a poly¬ meric material having a 2% Secant modulus of at least 30,000 psi and having an intermediate convoluted region having at least one convolute and which upon recovery the article remains convoluted, said convoluted region providing intrinsic strain relief to the recovered article. 12. A method of providing intrinsic strain relief to a heat recoverable tubular article having a 2% Secant modulus of at least 30,000 psi which comprises providing said article with an intermediate convoluted region which remains convoluted on recovery of the article.

Inventors: HANSEN ERLING, HANSEN, ERLING
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1989
Main classification: B29C61
IPC codes: B29C61, B29K105, H01R4, H02G3, H02G15