GB2166978A - Preparation of shrink articles - Google Patents

Abstract

For the preparation of shrink articles from crosslinkable plastics materials which are coated on one or both sides with a crosslinkable hot melt adhesive, the material of the hot melt adhesive is only crosslinked during the shrink process carried out for purposes of assembly.

Description

SPECIFICATION Preparation of Shrink Articles This invention relates to the preparation of shrink tubing, shrink caps, sleeves and tapes, and other shrink articles from a crosslinkable plastics material coated on one or both sides with a crosslinkable hot melt adhesive.

It-has long been known that shrink articles, e.g.

caps or tubing, may be provided on the inside with a hot melt adhesive which in the shrunk-on state of, for example, a cap or a piece of tubing, provides improved adhesion to the substrate and also ensures that the gap between the shrunk article and the substrate is sealed in a moisture-impermeable manner. The hot melt adhesives preferentially used produce their adhesive property through thermoplastic flow and intensive wetting of the substrate. This adhesive property is lost by cross Winking, however, so that, for example, in coextruded two-layer tubing comprising an inner hot melt adhesive layer and an outer tubing wall of shrinkable material, there is a danger that during the crosslinking process the adhesive will also be crosslinked and hence lose its adhesiveness.On the other hand, it is possible, through the crosslinking or partial crosslinking of the adhesive, to improve its shear strength and thus improve the mechanical strength of the shrink bond. Attempts have been made to combine these seemingly incompatible effects (i.e. the adhesiveness and mechanical strength just mentioned) by using a hot melt adhesive having molecules which consist of functional groups and hydrocarbon sequences linking them, the latter being built up, prior to the crosslinking process required for the shrinkable material, from part-branched or multiple-branched (i.e. sidechain-bearing) molecular structural units.

However, this known approach (German Offenlegungschrift 3,111,064) results in considerable restriction of the choice of the hot melt adhesives usable for practical purposes.

Accordingly, it is an object of the present invention to enable any desired crosslinkable hot melt adhesive to give a secure bond between an inner surface of a shrink article and a surface of a substrate.

According to the invention, there is provided a process for the preparation of shrink tubing, a shrink cap, sleeve or tape, or another shrink article from a crosslinkable plastics material coated on one or both sides with a crosslinkable melt adhesive, wherein the material of the hot melt adhesive is only crosslinked during the shrink process carried out for purposes of assembly. This provision means that it is only during or at the end of the assembly process that the hot melt adhesive undergoes any abrupt change in viscosity, which increases shear strength; up to this point, the distension of the shrink article and the assembly is not hindered by an excessively viscous adhesive.In the process of the invention, on distending the shrink article so as to allow it to shrink back, the adhesive on the inner surface of the shaped article can, because of its good flow, slide quite freely and adapt itself to the relevant changes in shape of the shrink article. This avoids the danger of crack formation, warping or tearing of the adhesive layer off the inner surface of the shrink article. Because of the good flow, the adhesive can very easily creep into and wet all recesses and micro-roughnesses of the substrate, before crosslinking begins.

It is usually advantageous for the material of the hot melt adhesive to be chemically crosslinked by addition of one or more peroxides. In that case, so as to avoid crosslinking of the hot melt adhesive regardless of the temperatures envisaged for crosslinking of the shrink article, the crosslinking agent will only subsequently be introduced into the hot melt adhesive. This can advantageously be achieved by "entrainment diffusion", in which the article concerned (e.g. a stretched tape coated with an adhesive, or distended tubing provided with an adhesive on its inner surface, or an already distended cap with an adhesive layer provided on its inside) is passed through a solvent containing a peroxide, or is introduced into such a solvent.The peroxide which has diffused into the adhesive layer, since it requires heat in order to react, only reacts during the shrinking back to the original state, in the temperature range appropriate for such shrinking.

In such cases it is advantageous to match the peroxide decomposition temperature and the temperature required for the shrinking back. The adhesive, which first of all melts as the temperature is raised, undergoes an increase in viscosity when the crosslinking mechanism comes into effect, this increase leading to a substantial increase in the shear strength of the adhesive. In the process of the invention, this increased shear strength comes into effect just when it is needed to make the shrink bond secure, but it does not come into effect at an earlier stage at which (as in known techniques) a high shear strength of the adhesive would interfere with the assembling of the shrink article on the substrate.

For the purposes of the invention, any desired applicable materials can be used as the base for the hot melt adhesive, for example ethylene/vinyl acetate copolymers, acrylate polymers and also materials based on polyamides, copolyamides, thermoplastic rubbers or polyesters.

These adhesive materials can also be foamable, to give resilience in the bond between a shrink article and a substrate.

As crosslinking agents, the familiar peroxides, e.g.

dicumyl peroxide, cumyl tert.-butyl peroxide, or 2,5 -dimethyl -2,5 -di -(tert-butyl) - peroxyhexane, can be used, as also other freeradical initiators, e.g. 1,10 - decane - bis sulphanazide or p-quinonedioxime.

If, for example, shrinkable tapes are produced by a process according to the invention (these tapes being used, for example, in the form of bandages at connection points of pipes or pipelines, to ensure, through being wrapped round and shrunk at the connection points, that a tight and corrosionresistant encasing results), the procedure followed will be that the tape, after extrusion and coating with an adhesive, is crosslinked, e.g. by means of peroxides, by high energy radiation or by "moisture crosslinking" after grafting a silane on to the base molecule. The crosslinked tape is subsequently stretched to create the elastic "memory" forces, the non-crosslinked adhesive having easy flow during this stage, so that it can follow the stretching without difficulties.Thereafter the stretched and adhesive-coated tape is passed through a solvent containing a crosslinking agent, e.g. a peroxide, so that the crosslinking agent penetrates into the non-crosslinked adhesive layer. Only during subsequent assembly, when the tape thus produced is, for example, wound round a pipe connection and shrunk in the wound state on to the pipe by heating, does the viscosity of the adhesive rise after the crosslinking agent has come into effect, the shear strength thereby being increased. A secure, moisture-tight bond is thereby ensured.

The invention is explained in more detail in the Examples section which follows.

Examples Section In a first Example, a tape coated with a thermoplastic adhesive is sprayed, while travelling continuously, with a peroxide-containing solution consisting of an aromatic solvent, e.g. xylene, which contains 0.5 to 10%, preferably 3 to 5%, of dicumyl peroxide. At moderately elevated temperature the solvent diffuses into the adhesive layer and in doing so carries peroxide into it by "entrainment diffusion". The amount entrained is a function of the solution concentration and of the contact time (residence time), and the solution concentration and residence time of the tape under the spraying device can be varied according to the requirement of crosslinking agent.

The procedure of this first Example may be modified in that, instead of the spraying device employed in that procedure, use is made of a vessel filled with the peroxide-containing solution, through which the tape is passed continuously.

It will be understood that other shrink articles, e.g.

caps or tubing, can also be treated in the manner just described.

Also, solvents other than that just mentioned can be used. It is in fact a particular advantage of the process of the invention that a solvent can be chosen in accordance with the chemical nature of the adhesive into which the crosslinking agent is to be introduced. This means that the solvent can be so matched to the adhesive that it readily diffuses into the adhesive layer and achieves homogeneous distribution therein.

A further advantageous possible method of subsequently introducing the crosslinking agent is that of introducing (e.g.) a peroxide without a liquid vehicle directly into the adhesive layer, for example by roller application or doctor blade application.

As a modification of the method of introducing the crosslinking agent subsequently into the hot melt adhesive layer by entrainment diffusion, it can often be particularly advantageous to dissolve both the hot melt adhesive material and the peroxide in a solvent, and to apply the resulting solution to the surfaces of the shrink article which are to be coated.

Thus, a commercial polyamide adhesive can for example be dissolved together with dicumyl peroxide in toluene, and this solution applied to the surfaces of the shrink article which are to be coated.

This makes it possible to lower the application temperature of the hot melt adhesive well below 100"C. At such a low application temperature, there are no longer any difficulties in introducing the peroxides beforehand, i.e. during application, without danger of premature incipient crosslinking.

If a single application of a solution comprising the hot melt adhesive, the peroxide and the solvent does not suffice, the application procedure can be repeated until the coating thickness required for secure bonding of the shrink article to the substrate has in fact been reached.

In another procedure in accordance with the invention, a layer of hot melt adhesive free from peroxides is first applied to the surfaces of the shrink article which are to be coated, and thereafter at least a second layer of a solution of both peroxide(s) and hot melt adhesive is applied to this first layer of hot melt adhesive. In that case it is particularly advantageous if the second layer, or the further layers, of the dissolved hot melt adhesive also contains the amount of peroxide required for crosslinking, or at least incipient crosslinking, of the first layer. In this way, multiple application of the dissolved hot melt adhesive can be performed without increasing the danger of incipient crosslinking upon application of the hot melt adhesive.

In the case of the commonly used materials, the first hot melt adhesive layer can be applied in the hot state, i.e. at about 1 50'-1 80'C, on to the surfaces of the shrink article which are to be coated, and thereafter there can be applied a thin film of the same adhesive material, but in dissolved form.

It will be understood that the invention has been described above purely by way of example, and that various modifications of detail can be made within the ambit of the invention.

Claims (13)

1. Aprocess for the preparation of shrink tubing, a shrink cap, sleeve ortape, or anothershrink article from a crosslinkable plastics material coated on one or both sides with a crosslinkable melt adhesive, wherein the material of the hot melt adhesive is only crosslinked during the shrink process carried out for purposes of assembly.

2. A process according to claim 1, wherein the material of the hot melt adhesive is crosslinked chemically by addition of one or more peroxides.

3. A process according to claim 2, wherein the one or more peroxides are introduced into the hot melt adhesive layer by entrainment diffusion after crosslinking of the shrink article.

4. A process according to claim 1, 2 or 3, wherein an ethylene/vinyl acetate copolymer or acrylate polymer is used as a base for the hot melt adhesive.

5. A process according to claim 3, wherein an aromatic solvent which contains 0.5 to 10%, preferably 3 to 5%, of peroxide(s) is usedforthe entrainment diffusion of the peroxide(s).

6. A process according to claim 5, wherein the shrink article is sprayed with the peroxidecontaining solution.

7. A process according to claim 5, wherein the shrink article is stored in the peroxide-containing solution or is passed continuously through this solution.

8. A process according to claim 2, wherein the hot melt adhesive and the peroxide, both dissolved in a solvent, are applied on to the surface(s) of the shrink article to be coated.

9. A process according to claim 8, wherein a plurality of successive applications of the dissolved hot melt adhesive and peroxide are effected.

10. A process according to claim 2, wherein a layer of hot melt adhesive free from peroxide(s) is applied on to the surface(s) of the shrink article to be coated, and thereafter at least a second layer of a hot melt adhesive dissolved together with the peroxide is applied on to this first hot melt adhesive layer.

11. A process according to claim 10, wherein the second layer, comprising dissolved hot melt adhesive, also contains the amount of peroxide required for the crosslinking or at least incipient crosslinking of the first layer.

12. A process according to any of claims 1 to 11, wherein the introduction of the crosslinking agent into an already present hot melt adhesive layer, or the application of a solution containing the hot melt adhesive and peroxide on to the surface(s) to be coated, is effected with the shrinked article in the crosslinked and distended or stretched state.

13. A process according to claim 1, substantially as described in the foregoing Examples Section.