GB2123223A - Split heat-shrinkable sleeve and process for producing the same - Google Patents

Abstract

A split heat-shrinkable sleeve comprising a heat-shrinkable plastic sheet 1 having an elongated porous member 2, 2% along each side edge, each porous member being made of a heat resistant material e.g. a metal screen or a glass fibre mesh or a heat resistant hard resin and having a hook 3 that engages the hook 3% on the other porous member. The invention also resides in a process for producing a split heat-shrinkable sleeve. <IMAGE>

Description

SPECIFICATION Split heat-shrinkable sleeve and process for producing the same The present invention relates to a split heatshrinkable sleeve used in protecting steel pipes, electric cables or joints thereof from corrosion, or making them waterproof or repairing the same.

Heat-shrinkable tubes are extensively used in protecting steel pipes, electric cables or joints thereof (hereunder these objects are collectively referred to as the article to be covered), making them waterproof or repairing the same. To provide complete corrosion protection or repair, a coat of adhesive or bonding agent is usually formed on the inner surface of the heat-shrinkable tube. The heatshrinkable tube is conventionally produced from a tube of a thermoplastic resin such as polyethylene crosslinked by iradiation with an electron beam. This resin tube is first heated to a temperature higher than its softening point to become expanded, then coo led in the expanded state. Before the tube is put to service, it is heated with a burner or a torch lamp to restore its diameter to the initial value.However, because of its shape, the heat-shrinkable tube must be slipped over the article to be covered before the connecting or repair work starts. This makes the handling of the tube quite awkward, and in some cases, the tube is entirely unsuitable for connecting or fixing purposes.

In the case where the heat-shrinkable tube is unsuitable, a split heat-shrinkable tube or sleeve made of a heat-shrinkable sheet having two side edges that can be joined together is used. The most important point to be considered in using this split heat-shrinkable sleeve is how to achieve a simple but strong and complete bonding of the two side edges of the heat-shrinkable sheet after it is wrapped around the article to be covered. Many methods have been proposed for joining the two side edges of the heat-shrinkable sheet, but none have proved completely satisfactory. One of the simplest method is to cause one side edge to overlap the other side edge through a layer of adhesive or bonding agent.

In orderto provide a complete joining, the overlapping portion may be covered with an adhesive tape.

But this method does not assure adequate bonding strength at elevated temperatures and often lets the overlapping side edges slip from each other when the sheet shrinks under heating. To prevent this, the overlapping portion must be held in position by hand or a suitable retainer, but then the efficiency of the joining operation is greatly reduced.

In an improved method, an elongated lip member is provided in an area forming a narrow band parallel to and slightly apart (retracted) from one of both side edges of the heat-shrinkable sheet, and the two side edges are caused to overlap with each other so as to make a heat-shrinkable tube. This method partially solves the problem with the first method (low bonding strength at high temperatures), but because the number of overlapping layers is three or four, the thermal conductivity is reduced to prevent heat from flowing to the lower layer and the resulting adhesion between each layer is not as great as initially expected.

Japanese Patent Publication No. 6191176 discloses a method using a split heat-shrinkable tube having constricted projections along the length of both side edges, which are joined by putting the projections together and retaining them with a metallic clamping device. The adhesive strength achieved by this method is satisfactory, but because of the low flexibility of the claiming device, the tube cannot be brought into intimate contact with the article to be coated if it has irregular surfaces, and an adequate amount of heat is not transferred to the lower part of the joint. As a further disadvantage, neither conventional method permits the use of a hot-melt adhesive for application to the inner wall of the sleeve because the adhesive is resistant to low temperatures, but not to elevated temperatures.

Therefore, the primary object of the present invention is to provide a split heat-shrinkable tube that can be produced without experiencing the disadvantages involved in the conventional methods of joining the two side edges of a split heatshrinkable sleeve and which achieves an increased adhesive strength in the joint. This object can be attained by a split heat-shrinkable sleeve comprising a heat-shrinkable plastic sheet having an elongated porous member along each side edge, said porous member being made of a heat resistant material and having a hook that engages the hook on the other porous member.

In the drawings, Figure 1 is a perspective view showing one embodiment of the split heat-shrinkable sleeve of the present invention; Figure 2 is a partial cutway view of the porous members on the sleeve of the present invention in the engaged state; Figure 3 is a cross section showing how the porous members are provided on a heat-shrinkable sheet; and Figure 4 is a cross section showing one side edge of the heat-shrinkable sleve with a porous member formed by the method illustrated in Figure 3.

The split heat-shrinkable sleeve according to the present invention is hereunder described by reference to the accompanying drawings. Figure 1 is a perspective view showing one embodiment of the sleeve. The sleeve comprises a plastic sheet 1 that is shrinkable in the direction of its circumference upon application of heat and which has an elongated porous member 2 along one side edge and another elongated porous member 2' along the other side edge. The porous members 2 and 2' have engageable hooks 3 and 3', respectively, at one end. They are formed by bending the free end of each porous member inwardly or outwardly along both side edges of the heat-shrinkable sheet 1, as shown in Figure 1.However, it should be understood that the embodiment of Figure 1 is not the only case for the hooks, and in another embodiment, projections formed on one porous member may be pushed into holes made in the other porous member.

According to the present invention, a coat 4 of an adhesive or bonding agent (e.g. hot-melt adhesive, heat-hardening adhesive or grease) may be formed on the inner surface of the heat-shrinkable sheet 1 so that a firmer adhesion is provided between the heat-shrinkable sheet 1 and the article to be covered.

The porous member 2 or 2' is typically made of a metal screen (e.g. iron, copper, aluminum or stainless steel) or a sheet of one of punched metals that are perforated by punching. In practice, the porous member may be formed of a mesh of an inorganic material such as glass fibers, or a sheet of a heat-resistant hard resin.

A cross section of the hooks 3 and 3' in the enaged state is shown in Figure 2, wherein the numerals the same as those used in Figure 1 identify the same elements. One of the two porous members 2 and 2' (2' in Figure 2) extends far beyond the side edge of the heat-shrinkable sheet 1 on which said porous member is provided, whereas the other porous member (2 in Figure 2) is provided in a position retracted away from the other side edge of the sheet by a distance 5, which is large enough to cover the underside of the two porous members in the engaged state. In a preferred embodiment, the distance 5 is larger than the width of the two engaged porous members by 30 mm or more in order to provide a better corrosion protection and waterproofness.

Having the construction described above, the split heat-shrinkable sleeve of the present invention achieves the following advantages.

(1)The flame or hot air from a burner or torch lamp used in heating the sleeve can easily penetrate the porous member. With a metallic porous member, the underlying heat-shrinkable sheet (the area 5 in Figure 2) is given a sufficient amount of heat to achieve uniform and complete heat shrinkage of the sleeve and strong adhesion to the article to be covered.

(2)The porous member is flexible enough to permit the sieve to follow the contour of an article to be covered that has irregular surfaces.

(3) The sheet is heated after the hooks made of a heat resistant material are brought into engagement, so there is no possibility that they come out of the engagement or slip from each other during the heating.

The method of forming the two porous members on the heat-shrinkable sheet is hereunder described by reference to Figures 3 and 4. Figure 3 is a cross section showing how layers are arranged to provide a porous member on one side edge of the heatshrinkable sheet. An uncrosslinked plastic tape 7 containing a crosslinking agent is placed between one side of the elongated porous member 2' and the heat-shrinkable sheet 1, and between the other side of the porous member and a crosslinked plastic tape 6. The assembly is pressed with a hot press 8 in order to crosslink each plastic tape 7 with heat under pressure.

In this method, the plastic tapes 6 and 7 are desirably made of a material of the same type as the material of the heat-shrinkable sheet 1. By the term "same type" is meant that if the sheet 1 is made of polyethylene, the plastic tapes 6 and 7 may be made of any member of polyolefins including polyethylene. But if a particularly strong adhesion is necessary, the tapes are desirably made of polyethylene.

Suitable crosslinking agents are organic peroxides such as di-a-cumylperoxide, 2,5-dimethyl-2,5-di(t- butylperoxy)hexane, and 2,5-dimethyl-2,5-di(tbutylperoxy)-hexane-3. The amount of the crosslinking agents used varies with their type, and generally, they are used in an amount of from 1 to 5 wt%, but this value may be increased or decreased as required. In certain cases, crosslinking aids such as triallyl cyanurate and polyethylene glycol dimethacrylate may be added with advantage.

The heating conditions will be properly determined by the type of the crosslinking agent used or the temperature of its decomposition. If dicumyl peroxide is used as a crosslinking agent, heating may be effected at 200-250 C for a period of about 3 to 5 minutes.

After the split heat-shrinkable sleeve of the present invention is provided with porous members by the method described above, the sleeve may be cooled with air or water. Figure 4 is a cross section showing one side edge of the sleeve with a porous member, and the numerals the same as those used in Figure 3 identify the same elements.

The split heat-shrinkable sleeve of the present invention that is produced by the method described above has the following advantage. The resin of which the uncrosslinked tape containing a crosslinking agent is crosslinked in the holes in the porous member and provides a great resistance to shearing force. At the same time, the croslinked resin becomes chemically integral with the crosslinked tape and the heat-shrinkable tape, so the porous member can be firmly adhered to the shrinkable sheet without slippage.

An example of the process for producing the split heat-shrinkable sleeve of the present invention is hereunder described.

One side of a crosslinked heat-shrinkable polyethylene sheet having a shrinkage of 25% (thickness: 1.5 mm, width: 600 mm, length: 2100 mm) was given a coat of hot-melt adhesive in a thickness of about 1.0 mm. A polyethylene tape (thickness: 0.2 mm, width: 40mm) containing a heat-reactive crosslinking agent (dicumyl peroxide: 3 wt%) was put on both sides of a stainless steel screen (wire diameter: 0.5 mm, mesh size: 5 mm) that was 600 mm long and 90 mm wide. On top of the upper uncrosslinked polyethylene tape, a crosslinked polyethylene tape (thickness: 0.3 mm, width: 40 mm) was placed. The resulting tape assembly was placed along one side edge of the heatshrinkable sheet, and heated at 220 C under pressure so as to crosslink the polyethylene tapes sandwiching the stainless steel screen. By so doing, the heat-shrinkable sheet was provided along one of its side edges with an elongated porous member made of the stainless steel screen. The same procedure was repeated to provide a stainless screen in a narrow strip 230 mm apart from the other side edge of the heat-shrinkable sheet. The free end of the first stainless steel screen was bent downwardly by a length of 15 mm to form a hook, whereas the free end of the second stainless steel screen was bent upwardly by the same length.

The split heat-shrinkable sleeve thus prepared was slipped over the weld joint of a 600 A polyethylene lined pipe that had been heated at 1000C and caused to shink by the following procedure. First, the hooks on the two porous members were brought into engagement. Then, the sleeve was heated with two propane gas burners, one being applied to the right half of the sleeve and the other applied to the left half. It took 9 minutes and 45 seconds to shrink the sleeve by a sufficient degree to tightly fit around the pipe. During and afterthe heating operation, the two porous members remained in secure engagement and caused no wrinkles. After the shrinkage was completed, the adhesion of the two side edges of the sheet under the engaging porous members was checked: neither void nor water channel was detected, and the adhesive strength ranged from 5 to 15 kg/cm, and was as high as the strength of the other part of the sheet.

Claims (9)

1. A split heat-shrinkable sleeve comprising a heat-shrinkable plastic sheet having an elongated porous member along each side edge, said porous material being made of a heat resistant material and having a hook that engages the hook on the other porous material.

2. A split heat-shrinkable sleeve according to Claim 1 wherein one porous member extends beyond one side edge of the heat-shrinkable sheet by a distance well larger than the width of the two porous members in the engaged state.

3. A split heat-shrinkable sleeve according to Claim 1 wherein said heat resistant member is a metal.

4. A split heat-shrinkable sleeve according to Claim 2 or 3 wherein said porous member is in the form of a mesh, net or screen.

5. A split heat-shrinkable sleeve according to Claim 2 or 3 wherein said porous member is made of a sheet-like material that is perforated by punching.

6. A split heat-shrinkable sleeve according to Claim 1 or 2 wherein each of the two engageable hooks is formed by bending the free end of the porous member inwardly or outwardly along both side edges of the heat-shrinkable sheet.

7. A process for producing a split heat-shrinkable sleeve by providing, along the side edges of a heat-shrinkable plastic sheet, an elongated porous member made of a heat resistant material and having a hook that engages the hook on the other porous member, said process comprising placing an uncrosslinked plastic tape containing a cross-linking agent between one side of said elongated porous member and said heat-shrinkable plastic sheet and between the other side of said porous member and a crosslinked plastic tape positioned above said one side of the porous material, then heating said uncrosslinked plastic tapes under pressure until they become crosslinked so as to render them integral with said heat-shrinkable plastic sheet and the initially crosslinked plastic tape.

8. A process for producing a split heat-shrinkable sleeve substantially as herein before described with reference to the accompanying drawings.

9. A split heat-shrinkable sleeve substantially as hereinbefore described with reference to the accom 70 panying drawings.