WO1991006609A1

WO1991006609A1 - Adhesive films

Info

Publication number: WO1991006609A1
Application number: PCT/GB1990/001664
Authority: WO
Inventors: Jozef Aleida Florent Smits; Guillaume Jozef Roox
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1989-11-01
Filing date: 1990-10-31
Publication date: 1991-05-16
Also published as: EP0498832A1; JPH05504982A; GB8924626D0

Abstract

An adhesive film having a protective backing layer and a thin layer of adhesive material comprising an ethylene derived polymer containing from 30 to 60 wt % of a monofunctional ethylenically unsaturated ester and having a melt index of form 0.1 to 10. The adhesive material may consist essentially of the ethylene derived polymers and does not contain viscosity reducing and/or tack improving additives.

Description

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ADHESIVE FILMS

Field of Invention

This invention relates to adhesive films for use particularly in temporarily protecting surfaces of vulnerable materials such as plastics, metals and ceramics in the course of manufacture, storage and transportation. Such films are referred to herein as TSP films.

Background of Invention

Ethylene-vinyl acetate copolymers are known which have a high vinyl acetate content made by solution polymerisation (see DE-3323531) . Such polymers have been used because of their elasto eric nature or because of their high compatability with tackifiers or other viscosity reducing components in curable hot melt adhesives (see CA 95-188246/72; DE-3342307) .

The polymerisation process is costly. Adhesive properties were only established in the presence of a tackifier. DD-92554 disclose high vinylacetate content, ethylene vinyl acetate copolymers polymerised by free radical high pressure polymerisation with from 0.005 to 1 % of a bifunctional ter onomer to give extrudable materials. Similar polymers are disclosed in DE 2122597, DE 2340743, DD 108757^"and DE 2403516; DD 142 806; DD 116 623 and JP 7327748 (Mitsubishi Petrochemical) . The elastomeric nature of the polymerisate was disclosed as the main attribute.

DD 114823 discloses the use of from 0.001 to 0.004 wt% of summetric bifunctional co onomer to provide more stable processing at high vinyl acetate contents. The level of bifunctional comonomer is such that Melt Index at 33 % VA is still high.

GB 1339511 discloses the use of from 0.05 to 10 % of asymmetric bifunctional co onomers to provide polymers of elastomeric character which are crosslinkable outside of the polymerisation reactor to provide heat and weather resistant materials. TSP films are known, sold by Novacel and other producers consisting of a polyethylene backing coated in a separate process step by solution or emulsion of an acrylic adhesive material. When applied to vulnerable materials the TSP films provide a smooth abrasion resistant covering so as to reduce amount of accidental damage in the handling and storage of such materials. Thickness of the polyethylene layer varies from 30 to 100 micron and that of the coating varies from 3 to 20 micron.

The production process involves the evaporation of the medium in which the acrylic adhesive is dissolved or dispersed which is costly and may be environmentally damaging.

At a Specialty Plastics conference on 1-4 December 1987 in Zurinch, Switzerland a temporary surface film was disclosed using an LDPE backing layer and a free-radical poly erid EVA of unknown VA content grafted with maleic anhydride. Adhesion is moderate; the grafting can lead to objectionable polymer degradation.

It is the object of the invention to provide a TSP film which combines economic manufacturing cost, good adhesion, low residuals upon tape removal after storage and other end-use characteristics. Summary of invention

The invention provides a multi-layer adhesive film having a protective backing layer and a thin layer of adhesive material applied in molten form comprising an ethylene derived polymer prepared by free radical high pressure polymerisation containing from 30 to 60 wt% of a monofunctional ethylenically unsaturated ester and having a melt index of from 0.1 to 10.

The adhesive layer can be extruded or coextruded with the protective backing layer. Good adhesive characteristic can be obtained without, if appropriate, use of a viscosity reducing^' agent or tack improving additive. No solvent needs to be used in film production. The adhesive layer may contain stabilisers, anti-oxidants etc so as to improve resistance to environmental degradation. The film may contain conventional pigments and colorants.

The unsaturated ester may be vinyl acetate, methyl acrylate, ethylacrylate, butyl acrylate, methyl methacrylate alone or in combination. Preferred are vinyl acetate and methyl acrylate, the latter providing improved thermal stability. The ester content is preferably from 35 to 50 wt%. The polymer may be made of high molecular weight by solution polymerisation. Preferably however a high pressure free radical procedure is used so as to produce polymer more economically.

In high pressure FR polymerisation, higher monofunctional ester contents improve adhesion and flexibility important for intimate tape/substrate contact but their chain transfer activity can be counterbalanced advantageously by including higher amounts of the multifunctional monomer in polymerisation. If the ester content in combination with the multifunctional monomer is too low, the tape will not meet the high adhesion requirements for TSP tape.

The selection of a high level of a symmetric bifunctional material having equally reactive terminal double bends with very high levels of ester comonomers yields a material which is both extrudable in molten form and has high adhesion.

The high pressure polymerization may be effected in a conventional autoclave or tubular reactor. The reaction temperature is normally between 130 and 250°C and is preferably from 145 to 230°C. The higher the pressure the higher the molecular weight that can be produced but at the expense of energy consumption and subject to plant design limits. Thus, the preferred pressure range is from 1000 to 3000 kg/cm²/, more preferably from 1500 to 2500 kg/cm². The particular conditions are chosen having regard to the product required.

The cross-linking multifunctional monomer or modifier may be introduced into the reaction vessel to adjust the melt index of the product. The amount of modifier used therefore depends on the agent chosen and the final melt index sought, as well as the reaction conditions employed and vinyl acetate content since the latter acts as a chain transfer agent itself. Typically the amount of modifier will be from 0 to 25% of the reactor feed. Given the requirements specified herein for the melt index suitable agents and rate of addition can be established empirically.

There will be incorporation of the modifier into the EVA copolymer and it will constitute a third monomer in the reaction mixture.

The polymerization will usually be initiated by a free radical initiator such as a peroxide. Also conventional additives such as antioxidants and carriers for the initiator may be present according to conventional practice. The invention will now be described in more detail, though only by way of illustration, with reference to the accompany¬ ing drawing which is a schematic block diagram of an autoclave apparatus for preparing an EVA of the invention.

Ethylene is introduced at 45 kg/cm² into compressor 1 where it is compressed to about 150 kg/cm², then vinyl acetate and modifier are added and the combined feed is further compressed in compressor 2 to approximately 1550 kg/cm². The high pressure feed is then introduced into autoclave 3 fitted with stirrer 4. A free radical catalyst is introduced via line 5 and at separate points on the autoclave (not shown) . The autoclave 3 may be cooled or heated as appropriate to maintain the desired reaction temperature.

The formed polymer, together with unreacted material and impurities is taken via line 6 to a high pressure separator 7. Unreacted monomer separated off are recycled via heat exchanger 11 to compressor 2. The remainder of the output is fed to low pressure separator 8, from which the polymer is collected and fed to extruder 9 where it is formed into pellets. The unpolymerized material separated at 8 is fed via heat exchanger 12 to a small compressor 10. Impurities are separated off in a purge 13, and unreacted monomers are recycled to compressor 1.

Preferably the multifunctional monomer is doubly unsaturated and has -0- and/or C=0 moieties as part of a generally"!inear molecule to improve wetting and adhesion. Preferably termonomer is obtained by esterification from a glycol or other linear hydrocarbons having at least two alcohol groups. The termonomer is advantageously obtained from an acrylic acid or a homologue thereof.

Most preferably the termonomer is ethylene glycol dimethacrylate (EDMA) . Such termonomers can be very effective in increasing adhesion in combination with the ester derived polymer units but adhesion to non-polar substrates after cooling from coextrusion temperatures is low permitting a tape to be produced in roll form from merely two coextruded layers; the backing being preferably LDPE, LLDPE or HDPE.

The doubly unsaturated termonomer crosslinks pairs of polymer chains reducing the melt index and contributing to adhesion. Stable polymerisation condition can be set up permitting extended continuous high pressure polymerisation runs. Using such increased amounts of VA and EDMA an MI can be obtained of from 0.1 to 5, preferably from 0.2 TO 4 suitable for extrusion. Blow extrusion is performed suitably with an MI of from 0.5 to 1; cast extrusion at an MI of from 2-3.

Example 1

Terpolymers were prepared in separate runs 1 to 5 an autoclave reactor at 165^βC and 52^βC feed gas temperature at a pressure of 2100 kg/cm² using t-butylperneodecanoate as initiator giving a product as shown in Table 1. Run 5 include 0.3 wt% of oleylpalmitamide anti-blocking agent. The materials produced were dry coated with Microthene (made by USI) .

Table 1

Run Mono- Bifunctional Total MI functional comonomer comonomer comonomer

1 2 3 4 5 6 7

Film preparation

The terpolymers prepared in runs 1 to 5 Example 1 were coextruded onto LDPE giving a thickness of 40 micron of LDPE and approximately 5 micron of the adhesive terpolymer layer.

The terpolymer displayed good adhesion onto glass, aluminum and steel and could be removed without leaving tape material. Runs 6, 7 are made using Levaprene (Registered Trade Mark) 450 and 500 respectively which are solution polymerised copolymers of ethylene and vinyl acetate containing 45 and 49 % of vinyl acetate respectively. The film properties are indicated in Table 2.

Table 2

Run 1 2 3 4 5 6 7 8

180 peel strength (N/cm) 0.29 0.11 0.14 0.28 0.24 0.24 0.24 0.76

loop tack (N/25mm) 0.46 0.31 0.41 0.49 0.51 0.37 0.21 0.17

Run 8 represents a comparative sample of an acrylate/solvent coated material from Novacel.

It can be seen that a superior looptack can be obtained with films produced by a simple coextrusion manufacturing technique. It can also be seen that runs 3, 4 and 5. containing at least 40% of the comonomer have even better looptack.

The material can be removed without leaving residue and can be made economically. In modifications of the invention a lower vinylacetate content EVA (i.e. a material having 28% VA) could be extruded between the backing layer and the above described adhesive layer to improve the bonding of these two layers.

Release layers such as silicon-containing layers may be used to facilitate unwinding of rolls of film. Other slip additives may be present in such a release layer.

The films may be used on new cars during production for example; to protect mirrors, skis, computer equipment or PMMA panels or as temporary sunscreens.

Claims

1. A multi-layer adhesive film having a protective backing layer and a thin layer of adhesive material applied in molten form comprising an ethylene derived polymer prepared by free radical high pressure polymerisation containing from 30 to 60 wt % of a monofunctional ethylenically unsaturated ester and having a melt^" index of from 0.1 to 10.

2. A film according to claim 1 in which the adhesive material consists essential of a thermoplastic ethylene derived polymer substantially free of viscosity reducing and/or tack improving additive.

3. A film according to claim 2 in which the ethylene derived polymer also contains from 1 to 15 wt % preferably from 3 to 12 wt% of a multifunctional ethylenically unsaturated monomer having at least two ethylenically unsaturated groups, preferably of equal reactivity.

4. A film according to claim 1 in which the monofunctional ester is vinyl acetate or methyl acrylate and/or is present in an amount of from 35 to 50 wt% in the polymer.

5. A film according to claim 3 in which the multifunctional monomer is used in an amount of from 3 to 10 wt % and/or has a boiling point of less than 300^*C at 1000 bar.

6. A film according to claim 3 in which the multifunctiona monomer contains -0- or C=0 groups and is linear and i preferably a multi-ester of a polyol and acrylic acid o alkyl acrylic acid such as EDMA or EBMA.

7. A film according to claim 1 in which the film has looptack greater than 0.4 N/25 mm; and a rolling bal tack less than 20 cm.

8. A film according to claim 3 in which the total comonome content of the monofunctional and multifunctional monomers is from at least 40 wt% and is preferably up t 50 wt%.

9. A -film according to any of the preceding claims in which the protective layer is of an olefin polymer, preferably non-polar such as LLDPE or LDPE; and/or in which an adhesion improving layer is provided between the backing layer and the layer of adhesive material; and/or in which a release layer is provided on the outside of the backing layer from improving release when rolled up.

10. A process for making a film according to claim 6 by coextruding, extrusion coating or by cast film extrusion.