GB2277479A

GB2277479A - Multi-layer protection film

Info

Publication number: GB2277479A
Application number: GB9406765A
Authority: GB
Inventors: Kurt Kindinger; Heiner Oudehinken
Original assignee: Bischof and Klein SE and Co KG; Roehm GmbH Darmstadt
Current assignee: Bischof and Klein SE and Co KG; Roehm GmbH Darmstadt
Priority date: 1993-04-06
Filing date: 1994-04-06
Publication date: 1994-11-02
Anticipated expiration: 2014-04-06
Also published as: SE9400428D0; ITTO940249A0; ITTO940249A1; BE1008856A3; DE4311322A1; FR2703625B1; IT1273106B; SE515060C2; FR2703625A1; NL9400315A; NL195091C; SE9400428L; ES2092952B1; ES2092952A1; GB9406765D0; GB2277479B

Description

& 9 Multi-laver Protection Film 2277479 The invention relates to a

self-adhesive multilayer surface protection film based on polyolefins which may be used for protecting the surfaces of plastics sheets during storage and transportation, or during further processing or thermal shaping of such sheets.

From German Patent No. 27 18 510 it is known to provide the surfaces of thermoplastic plastics sheets of polymethylmethacrylate (PMMA) with a protective film after manufacture, this film being produced from low density (LD) polyethylene having a melting index between 1 and 4 g/10 min and being pretreated on the adhesive side by corona discharge in order to improve the adhesion of the film to the plastics sheet. At a surface temperature of 60-100C the adhesion of this protective film is however highly variable, with the result that it may become detached at the edges or even over the entire surface. At the other extreme, the film may be torn as it is pulled away from the sheets or mouldings. The wide variability in adhesion of such films depends on the surface temperature of the sheet during application of the film and additionally on the nature of the surface of the film, eg. microscopic variations in the surface in the case of blown film, as well as fluctuations in the intensity of pretreatment and tolerances in the polyethylene granules caused by the manufacturing process.

European Patent No. 357 487 describes a thermal forming process for PMMA sheets having a reduced viscosity of at least 1.5. At least one surface is protected by a polyethylene film which has been pretreated by corona discharge. The polyethylene has a density of less than 0.935 and a melting index of less than 1.0 g/10 min. The protective film has satisfactory 3 v adhesion (eg. 0.6 N/cm) to the PMMA sheets and is capable of being removed from the surface at any time without leaving any residue.

US Patent Nos. 5275866 and 5279883 disclose self adhesive surface protection films of polyethylene, one surface of which is subjected to corona pretreatment at between 40 and 44 mN/m, and which are subsequently provided with a coating of an adhesive based on polyacrylic acid esters or based on natural rubber. The self adhesive surface protection films are indeed characterised by uniform and more reproducible adhesion to sheets of plastics, but as a result of thermal stresses during storage or during thermal forming, slight traces of adhesive may be left on the edges when the protective film is torn off quickly and these traces are visible on transparent objects such as light shades.

In practice, it has been found that two-layer or multi-layered polyethylene protective films produced by coextrusion, having an adhesive layer of ethylene-vinyl acetate copolymer with a vinyl acetate content of about 18% by weight, react to temperature increases during storage and transportation and particularly during thermal forming by adhering much more strongly to the plastics sheets.

If the vinyl acetate content of the adhesive layer is lower, eg. 9% by weight, the adhesion at ambient temperature is insufficient for working of the sheets by cutting, milling or drilling, with the result that the film lifts in the areas adjoining the machined edges.

An increase in the degree of adhesion can be achieved by the known process of corona pretreatment, but here again the significant increase in adhesion as the temperature rises is a disadvantage. During thermal forming, in particular, the increase in adhesion as a result of the reduction in the film thickness in the deep-drawn areas of mouldings is detrimental in that the protective film tears easily when it is removed by the 1 1 end user.

The solutions to the problem presented by the prior art are only partly satisfactory. There is still a need, therefore, to provide improved surface protecting films which overcome the disadvantages described above, if possible whilst satisfying all the other requirements, and which may be pulled away from the plastics sheets after thermal stress, particularly thermal forming, with virtually the same or preferably slightly less peeling resistance than before the plastics sheets were subjected to stressing at ambient temperature.

We have now developed a multi-layered surface protection film which is largely able to achieve this object.

Viewed from one aspect, the invention provides a self adhesive multilayer protection film based on polyolefins comprising:

(a) a carrier layer having a layer thickness in the range of from 20 to 150 Am and comprising one or more layers of polyolefins; and (b) an adhesive layer having a layer thickness in the range of from 5 to 40 Am and comprising 99.5-80 wt. % of an ethylene vinyl ester copolymer and/or polyethylene or a mixture thereof; and from 0.5 to 20 wt. % of one or more adhesion-promoting resins.

The adhesive layer is conveniently of a thickness in the range from 5 to 20 Am and preferably comprises an ethylene vinyl ester copolymer having a vinyl ester content of 2 to 20 wt. %, more preferably of 7-10 wt. The adhesion-promoting resins are preferably present in an amount of from 2 to 20 wt. %.

The multi-layered surface protection films in accordance with the invention may conveniently be produced by Coextrusion using a flat or annular die (see Becker-Braun, Kunststoff-Handbuch, New Ed. Vol. 1, pg. 443-461, C. Hanser, 1990; H.F. Mark et al. Encyclopedia 1 1 of Polymer Science & Engineering 2nd Ed. Vol. 6, pg. 608-631, J. Wiley, 1986, Ullmanns Encyclopedia of Industrial Chemistry, 5th Ed. Vol. A 11, pg. 89-90, VCH, 1988).

of great practical importance is that the surface protection film, after being laminated onto plastics sheets at ambient temperature and at a higher residual heat after the production of the sheets by extrusion, has a degree of adhesion which is sufficient for practical applications and that this adhesion (measured as a peeling value in cN/cm) shows only a slight increase after thermal stressing caused by storage or transport conditions, and falls to lower peeling values after thermal forming. The protective film can therefore easily be detached, by the end user, from the sheets or mouldings without leaving any residue.

The polyolefins used as the basis for the film are preferably homo or copolymers of C,_, olefins. In a preferred embodiment of the invention the carrier layer comprises polyethylene eg. polyethylene having an MFI (according to ASTM D 1238) of 0.1-8 q/600s and a density of 0.91-0.94 kg/m3 (according to ASTM D 1505) (see Ullmanns Encyclopedia of Industrial Chemistry 5th Ed. Vol. A 21, p. 489-516, VCH 1992).

The ethylene-vinyl ester copolymers of the adhesive layer may be prepared by conventional methods (see Houben-Weyl, 4th Ed., Vol. E 20, 700-710, 1120, 1126, G. Thieme-Verlag, 1987, H.F. Mark et al., Encyclopedia of Polymer Science and Engineering, Vol. 17, 425-445, J. Wiley, 1989).

Vinyl ester comonomers suitable for use in the invention include, in particular, vinyl acetate, vinyl propionate, vinyl butyrate and the esters of branched aliphatic acids such as pivalic acid, 2-ethylhexyl acid and the R versalic acids. If the adhesive layer contains polyethylene, this is preferably a polyethylene having a density of 0.91- 0.94 kg/m3. As a guide to the A A polyethylene content of the adhesive layer, a figure of 10 to 60% by weight may be given.

Adhesion promoting resins suitable for use in the film according to the invention are known in the art as so-called tackifiers (additives for elastomers) in the manufacture of adhesives, particularly solventcontaining adhesives or hot melt adhesives or coating adhesives for self-adhesive strips. The addition of the tackifiers permanently increases the adhesive force. The tackifiers generally share the common feature of having a relatively low molecular mass, usually in the range from 200-2000 g/mol, with a wide molecular mass distribution and a glass transition temperature Tg above that of the elastomers and adequate compatibility with the elastomers; in practice their softening points are in the range from 50-150C. (See H.F. Mark et al. Encyclopedia of Polymer Science & Engineering, Vol. p. 509-510; Vol. 13, p. 347-348, J. Wiley; Salas, Handbook of Pressure Sensitive Adhesion Technology 2nd Ed., p. 3860, Van Nostrand Reinhold, 1989).

Tackifiers of the kind described, chemically speaking, fall into the same category as resins, terpene oligomers, cumarone indene resins, aliphatic petrochemical resins and modified phenol resins. As is known, terpene tackifiers are prepared by polymerisation, particularly cationic polymerisation of naturally occurring terpenes such as the pinenes, limonenes and the like.

The aliphatic petrochemical resins are generally obtained by oligomerisation of the C.- and Cg-petroleum fractions; hydrogenated fractions should also be mentioned in particular. Examples include hydrogenated hydrocarbon resins based on C9 fractions as copolymer with indene, a-methylstyrene and vinyl toluene.

Of special interest is a hydrogenated hydrocarbon resin based on polymerised cyclopentadiene which is subsequently hydrogenated.

1 t Suitable methods of preparing the surface protection films include, in particular, coextrusion of the starting materials which form the polyolefin carrier layer and the adhesive layer in a process known per se (see Kunststofftechnik, Coextruded Films and Sheets, VDI-Verlag, 1990).

Table 1 below summarises types of film in accordance with the invention and compares these to types of film according to the prior art (film types 4). All percentages are given by weight.

TABLE 1

Type of Composition film 1 Corona pretreatment LDPE 2 Adhesive film 9% vinyl acetate 3 Adhesive film 9% vinyl acetate + corona pretreatment 4 Adhesive film 18% vinyl acetate Adhesive film A) 8% vinyl acetate + 2% Hc resin 6 Adhesive film A) 8% vinyl acetate + 6% Hc resin 7 Adhesive film B) 9% vinyl acetate + 5% Hc resin 8 Adhesive film B) 9% vinyl acetate + 10% Hc resin 9 Adhesive film B) 9% vinyl acetate + 20% Hc resin (Hc = Hydrocarbon) The surface protection films according to the invention may be used in particular to protect sheets of acrylic glass and polycarbonate, particularly with a view to thermal forming thereof, during which the surface protection films may remain on the sheets of plastics. The acrylic glass sheets generally consist of standard commercial PMMA or copolymers of methylmethacrylate known per se which are used for this purpose, preferably with a reduced viscosity of at least 19- f 0 1.5. These may optionally be modified, eg. given a flame resistant or antistatic finish using known stabilisers or other additives. As is common practice, the sheets of acrylic glass may be tinted in a known manner (see Ullmanns Encyclopedia of Industrial Chemistry 5th Ed., Vol. A20, 459-507, VCH 1992). The sheets of acrylic glass may, for example, be made of socalled cast material, continuous cast material or extruded material (see H.F. Mark et al. Encyclopedia of Polymer Science & Engineering, 2nd Ed. Vol. 1, 276-278, J. Wiley, 1985).

The polycarbonate sheets are standard commercial products or correspond to the prior art (see KirkOthmer, 3rd Ed., Vol. 18, 480-494, J. Wiley, 1982). Generally, the sheet thickness is in the range from 0.5 to 12mm.

The adhesion desired for practical use of the protective film on plastics sheets, measured as peeling resistance on strips 50mm wide with a tear off angle of 180 and a tear off speed of 100 mm/min, should be 50180 cN/cm, in order to prevent the film from lifting at the edges or around the machined edges after the sheets provided with protective film have been processed by cutting, milling or drilling. For protection purely during transport, adhesion values of between 10-100 cNI cm are sufficient, but the degree of adhesion should not increase any further and should preferably fall after the material has been stored in the warm or shaped by thermal forming.

Table 2 below shows typical values for the peeling resistance of various protective films according to the prior art described above (types 1-4) and according to the features of the protective film according to the invention (types 5-9), measured under laboratory conditions after the protective films have been applied to sheets of polymethylmethacrylate under the same contact pressure on strips measuring 50mm wide with a

8 - tear off angle of 180 and a tear off speed of 100 mm/ min:

TABLE 2

Adhesive values cN/cm Type of film After application After application at and storage 23C and storage 24 hours/23C 24 hours/80'C 1 lamination 4-10 4-10 80C 2 0.6 0.4 3 48 197 4 2.6 122 10 15 6 88 102 7 18 0.7 8 47 1.2 9 163 15 The use of the resins in conjunction with extrudable copolymers, preferably ethylene-vinyl ester copolymers, for coextruded surface protection films, produces the desired result that, with the surface protection films thus produced, the adhesive force is significantly increased but after a temperature increase in the sheets provided with the protective films, depending on the choice of adhesion promoting resins, the adhesion increase is only slight or may in some cases even fall significantly. This effect, caused by migration from the surface of the adhesive layer into the underlying layers of the adhesive layer or carrier layer, is of particular significance for the further processing of plastic sheets provided with protective film by thermal forming.

A- The invention is further described by way of the following non-limiting Examples:

EXAMPLES

Prenaration of the surface Drotection film The carrier layer is formed from an LD polyethylene film having the following characteristics:

Characteristics: MFI = 2.0 g/600s, thickness: 38 gm Density: 0.923 g/cm 3 An adhesive layer (12 gm thick) is applied to this film base by coextrusion with a flat die, this adhesive layer consisting of:

A) 94-98% ethylene-vinyl acetate copolymer having 7-10% vinyl acetate with the addition of 2-6% hydrogenated hydrocarbon resin, based on C-9 fraction, copolymer with indene, a-methylstyrene and vinyl toluene, or B) 80-95% ethylene-vinyl acetate copolymer having 7-10% vinyl acetate and with the addition of 5-20% hydrogenated hydrocarbon resin, based on cyclopentadiene, polymerised and hydrogenated.

I-B Preparation of the surface protection film by co-extrusion II Coatinq of the plastics sheets The surface protection films obtained according to the - 10 invention are laminated over the entire surface of the plastics sheets, with the exclusion of air, at temperatures (roller/semi- finished product) of 20'C to 800C with a laminating roller at a contact pressure of 1 to 4 bar.

III Thermal forminq of the coated plastics sheets The plastics sheets obtained as described above, especially PMMA sheets, can be thermally formed in a manner known per se. Heating is carried out in a circulating air cabinet or by IR radiation.

Results:

The protective films according to A) are characterised in that the addition of 2-6% of adhesion promoting resin, preferably the addition of 6% ARKON F-125, significantly increases the adhesion to plastics sheets, but after the temperature has increased the adhesion increases only slightly and reaches a level at which the film can readily be removed leaving no residues.

In the case of protective film B), the addition of 5-20% of adhesion promoting resin, preferably 20% of ESCOREZ 5320, also significantly increases the adhesion of the protective film to plastics sheets. The action of heat, eg. during thermal forming, may even cause a dramatic fall in the adhesion, as a result of which the protective film can easily be removed even from complicated mouldings such as sanitary items, despite a great reduction in the thickness of the film.

v 11 -

Claims

1. A self adhesive multi-layer protection film based on polyolefins comprising:

2. A film as claimed in claim 1, wherein the carrier layer comprises polyethylene.

3. A film as claimed in claim 1 or claim 2, wherein the ethylene vinyl ester copolymer has a vinyl ester content of 2-20 wt.%.

4. A film as claimed in any one of claims 1 to 3, wherein the ethylene vinyl ester copolymer is ethylene vinyl acetate.

5. A film as claimed in any one of claims 1 to 4, wherein the adhesive layer comprises polyethylene.

6. A film as claimed in claim 5, wherein the polyethylene is present in an amount of from 10 to 60 wt. %.

7. A process for preparing a film as claimed in any one of claims 1 to 6 which comprises co-extruding using a flat or annular die the carrier layer and the adhesive layer as defined in any one of claims 1 to 6.

8. Use of a film as claimed in any one of claims 1 to 6 for coating plastics surfaces.

9. Use of a film as claimed in any one of claims 1 to 6 for laminating sheets of polyacrylate plastics or polycarbonate plastics.

10. A self adhesive nulti-layer surface protection film substantially as herein described and with reference to any one of the Examples.

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