GB2162191A - Alkene-vinyl acetate copolymer adhesives - Google Patents

Abstract

Pressure sensitive PVC adhesive compositions suitable for use with PVC sheets ranging from rigid tiles to flexible foils may be produced by using as an adhesive component a mixture of a segmented and a non-segmented monoalkene - vinyl acetate copolymer.

Description

SPECIFICATION Improvements in and relating to adhesives The present invention relates to improvements in and relating to adhesives and in particular to pressure sensitive adhesive compositions suitable for adhering PVC and to PVC sheets provided with a layer of such compositions.

PVC sheets ranging in thickness from tiles to films or foils are widely available for use as covering materials for floors, walls, shelves, books etc. Such sheets may be rigid as for example with floor tiles, or flexible as for example with foils for the use as wall coverings. Flexibility, where required, is achieved by the incorporation within. the PVC of plasticizers such as phthalates or chlorinated paraffins.

The PVC plasticizers however exhibit the tendency to migrate from the PVC sheet into the adhesive used to secure the sheet to a substrate. Indeed, where the adhesive composition itself contains components which are PVC plasticizers they tend to migrate into the PVC sheet.

Thus, where rigidity is required of the PVC sheet, it is not possible to use a plasticizer- containing adhesive as migration of the plasticizer will cause the sheet to soften. Conversely, with flexible PVC sheets it has generally been necessary to use adhesives which contain plasticizers in order to obtain a balance in the plasticizer migration to avoid the sheets becoming brittle and to lessen the reduction in adhesive strength that results from reactions between the adhesive component of the adhesive composition and the plasticizers migrating out of the PVC sheet.

It has also be found that with vinyl acetate co-polymer-based adhesives the weakening effect of PVC plasticizers such as phthalates on the adhesive is reduced where a segmented co-polymer, such as EL VAX 170, an ethylene-vinyl acetate (EVA) co-polymer available from E.l. Du Pont de Nemours and Company, is used.

By "segmented" it is meant that the polymer chain has regions or segments in which the occurrence of acetate side groups is relatively high and regions or segments in which their occurrence is relatively low. This may be contrasted with conventional or "non-segmented" vinyl acetate co-polymers in which the acetate side group distribution is random.

Segmented EVA's might thus be expected to be suitable for use as the adhesive component in pressure sensitive adhesives for flexible, i.e. plasticizer-containing, PVC sheets. However, we have found that, in practice, using segmented EVA, pressure sensitive adhesive compositions are obtained which possess adequate adhesive strengths only at undesirably high viscosities.

Relatively low viscosity is a desirable characteristic for pressure sensitive adhesives for a variety of reasons, some technical some economic. In general, the higher the viscosity, the higher must also be the application temperature: thin PVC films are heat deformable and so low adhesive application temperatures are thus technically desirable as well as economically desirable in view of the reduced energy expenditure. Furthermore, with high viscosity adhesive compositions, formulation must either be carried out at high temperatures or high shearing power mixing apparatus must be used.It is also generally preferred to minimise the heating of hot melt adhesive compositions as certain components, can be thermally degraded It is an object of the present invention to provide pressure sensitive PVC adhesives which have a combination of low viscosity, high cohesive strength, tolerance to PVC plasticizers, and tack levels suitable for use with PVC sheets ranging from rigid tiles to flexible foils.

It is a further object of the invention to provide pressure sensitive PVC adhesives which can be used for the adhesion of flexible and rigid PVC sheets.

We have found that pressure sensitive PVC adhesive compositions having the desired properties can be produced using as the adhesive component a mixture of segmented and non-segmented vinyl acetate co-polymers.

Thus, in one aspect, the invention provides a pressure sensitive PVC adhesive composition containing as an adhesive component a segmented and a non segmented monoalkene-vinyl acetate co-polymer, preferably a mixture of segmented and non-segmented EVA's.

In a preferred embodiment, the adhesive composition of the invention contains from 20 to 35%, especially preferably about 25 to 30% by weight of EVA's, both segmented and non-segmented. The weight ratio of the segmented to the non-segmented vinyl acetate co-polymers is conveniently from 1:1 to 2:1 and preferably is about 3:2.

Suitable non-segmented, i.e. conventional, EVA's for incorporation into the adhesive compositions of the invention include those having medium to high vinyl acetate contents of 28 to 40% and EVA's having vinyl acetate contents of from 30 to 40% are preferred. Mixtures of conventional EVA's can of course be used.

Commercially available conventional EVA's are characterised by both their vinyl acetate contents and their melt flow index (MFI). MFl's are effectively a measure of the molecular weight of the co-polymer and EVA's having MFl's within a wide range, eg 5-800, are suitable for use in the present invention.

A segmented EVA co-polymer suitable for use in the adhesive compositions of the present invention is the Du Pont product ELVAX 170 referred to above. Other segmented EVA's, especially those having lower molecular weights than ELVAX 170 could also be used.

Besides incorporating segmented and non-segmented EVA's into the adhesive compositions of the invention, it may be desirable in certain cases to include a colouring agent, for example a whitener such as titanium dioxide. Such colouring agents are particularly readily dispersed within the adhesive if intro duced as solid dispersions in EVA. Thus for example small quantities, eg 0.1 to 4%, of a 1:1 dispersion of TiO2 in EVA (available under the trade name E/Va white M/B from ICI, Plastics Division of Welwyn Garden City) may be included in the adhesive composition.

The adhesive compositions of the invention generally also include a tackifying resin. This may comprise any suitable resin or resin mixture such as are conventional for PVC pressure sensitive adhesives.

In this respect suitable tackifying resins may include: rosins, natural or modified, e.g. rosin derivatives such as Pentalyn 344 available from Hercules Chemical Co. of London and Sylvatac available from Sylvachem Corporation of Jacksonville, Florida, U.S.A. oc-methyl styrene homopolymers or co-polymers, e.g.

Kristalex FR75 (an a-methyl styrene modified polymer) or Piccotex (a vinyl toluene-a-methyl styrene copolymer) both available from Hercules Chemical Co.; glycerol and pentaerythritol rosin esters; polyterpene resins; phenolic modified terpene resins; and aliphatic petroleum hydrocarbon resins. Where the adhesive composition of the invention is to be applied to transparent PVC sheets, a-methyl styrene polymers such as Kristalex are particularly preferred for use as the tackifying resins.

In the adhesive compositions of the invention it has in general been found preferable to use a mixture of a tackifying resin of medium to low softening point and a tackifying resin of medium to high softening point. preferably in a weight ratio of from 1:2 to 2:1 and especially preferably of about 7:5.

The tackifying resin components of the adhesive compositions of the invention conveniently constitute up to about 60% by weight of the composition and advantageously are present from 1.5 to 3.5 especially about 2.5, parts by weight from each part of the vinyl acetate co-polymer adhesive components. As is conventional in the art, high tackifying resin contents are used where the adhesive is to be applied to thick PVC sheets such as floor tiles.

The adhesive compositions of the invention may also contain an anti-oxidant. In this respect, anti-oxidants conventional to the art, such as butylated hydroxy toluene (BHT), are suitable although it is preferred to use pentaerythritol-tetrakis- 3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate (available from Ciba-Geigy UK Limited of Manchester under the trade name IRGANOX 1710) optionally in combination with BHT. The low viscosities of the adhesive compositions of the invention allow the compositions to be kept at elevated temperatures for relatively short periods during manufacture and also to be applied at relatively lovj temperatures.Thus the thermal degradation of the system is reduced and the quantities of anti-oxidant required within the compositions may be lower than with conventional PVC pressure sensitive adhesives. Generally, the anti-oxidant will be present in the compositions of the invention at up to 2%, preferably 0.1 to 1%, and especially preferably about 0.5%, by weight.

The viscosity and tack of the adhesive compositions of the invention may be adjusted to the desired levels by the inclusion within the compositions of viscosity and tack modifiers such as processing oils. In this respect, napthenic processing oils such as EDELEX OIL 45 and SHELLFLEX 371 (both available from Shell Chemicals UK Ltd of London) are particularly suitable. In general, the viscosity and tack modifiers will be present within the compositions of the invention at weight ratios of from 0.3:1 to 1:1 relative to the vinyl acetate co-polymer adhesive components. Particularly suitable viscosities for adhesive compositions according to the invention for use on PVC foils are from 20 to 35 poise at 1500C.

The adhesive compositions of the invention may also contain tack modifying waxes such as petroleum waxes (e.g. paraffin waxes), microcrystalline waxes, and synthetic waxes (e.g. Fischer Tropsch waxes).

Such tack modifying waxes may be present as up to about 5% by weight of the total adhesive composition.

The adhesive compositions of the invention represent an inprovement over conventional PVC pressure sensitive adhesives in several respects. In particular, the compositions of the invention exhibit improved and long term plasticizer tolerance even in the absence of a plasticizer from the adhesive formulation and improved balance between cohesive strength and viscosity.

With the improved PVC plasticizer tolerance, compositions according to the invention, unlike previous PVC pressure sensitive adhesives, can be used with both rigid and flexible sheets.

The improved balance between strength and viscosity gives rise to both technical and economic benefits. According to the invention adhesive compositions can be obtained having acceptable cohesive strengths but with lower viscosity than conventional PVC pressure sensitive adhesives of comparable strength. The reduction in viscosity permits application to be effected at lower temperatures, for example 110 to 140"C, and formulation to be effected using mixing apparatus of lower shearing power. The use of lower temperatures or lower power machines clearly results in lower operating costs. However, lower application temperatures are also desirable because of the liability of thin PVC foils to distortion at high temperatures. Furthermore, as mentioned above, the use of low temperatures results in reduced thermal degradation of temperature-sensitive components in the adhesive compositions with the result that smaller quantities of often relatively expensive anti-oxidant components need to be used.

The adhesive compositions of the invention are thus particularly suitable for application to PVC sheets and in another aspect the invention provides a PVC sheet bearing on at least one major surface a layer of a PVC pressure sensitive adhesive according to the invention.

The PVC sheets of the invention may range in thickness from thick tiles to thin foils and may be rigid or flexible. Sheet thicknesses are generally 0.1 to 0.5 mm for sheets in foil form and 1 to 5 mm for sheets in tile form.

It should be recognised that "PVC" is used herein in its conventional sense to denote vinyl chloride homo-or co-polymers which may optionally contain further components such as plasticizers, colouring agents, etc. Similarly, it will be recognised that thin flexible PVC sheets according to the invention need not be planar and indeed in general will be formed in rolls for convenience of packaging, storage and transport.

The adhesive compositions of the invention may be applied to the PVC sheet either directly or indirectly onto a substrate, for example a silicone release paper, which is then brought into contact with the PVC sheet. Adhesive coating weights of 15 to 50, preferably 20 to 25, gm2 generally give acceptable results.

The sheets of the invention advantageously also comprise a removabie cover sheet, e.g. a release paper, disposed over the adhesive layer.

The formulation of the adhesive compositions of the invention and their application to PVC sheets may be achieved using conventional procedures although, as mentioned above, lower shearing power apparatus and low application temperatures may be used.

The following Example is provided to illustrate the invention further without limiting the protection sought therefor: Example A PVC pressure sensitive adhesive was formulated from the following:

ELVAX 170 15% 25% EVA (segmented EVA co-polymer) *33-25 (EVA co-polymer) 10% SYLVATAC RX 25% 59.5% Tackifying Resin SYLVATAC 40 34.5% EDELEX OIL 45 15% 15% Processing oil

IRGANOX 1010 0.2% 0.5% Anti-oxidant RALOX BHT+ 0.3% 100% 100% *33-25 implies a 33% vinyl acetate content and a Metal Flow Index of 25.

+ available from Chem-Ox (DBR) Ltd of Esher, Surrey.

The adhesive composition of this Example has a viscosity at 150"C at 25 poise and, applied to PVC foil at a coating weight of 22g/m 2, exhibits a peel adhesion of 1400 g/cm.

Claims (15)

1. . A pressure sensitive PVC adhesive composition containing as an adhesive component a segmented and a non-segmented monoalkene-vinyl acetate copolymer.

2. .A composition as claimed in claim 1 wherein said non-segmented copolymer has a vinyl acetate content of from 30-40% by weight.

3. A composition as claimed in either of claims 1 and 2 comprising said segmented and said nonsegmented copolymers in a weight ratio of from 1:1 to 2:1.

4. A composition as claimed in any one of claims 1 to 3 comprising a segmented and a non-segmented ethylene-vinyl acetate copolymer.

5. A composition as claimed in claim 4 comprising 20 to 35% by weight ethylene-vinyl acetate copolymer.

6. A composition as claimed in any one of claims 1 to 5 further comprising at least one agent selected from: tackifying resins; antioxidants; viscosity and tack modifiers; and tack modifying waxes.

7. A composition as claimed in claim 6 containing a first and a second tackifying resin in a weight ratio of from 1:2 to 2:1, the softening point of the first tackifying resin being higher than that of the second.

8. A composition as claimed in either of claims 6 and 7 containing tackifying resin in a weight ratio of from 1.5:1 to 3.5:1 relative to the total weight of said vinyl acetate copolymers.

9. A composition as claimed in any one of claims 6 to 8 containing from 0.1 to 1% by weight antioxidant.

10. A composition as claimed in any one of the preceding claims free from PVC plasticizer.

11. An adhesive composition substantially as herein described and comprising a segmented and a non-segmented monoalkene-vinyl acetate copolymer.

12. A PVC sheet provided on at least one major surface with a layer of an adhesive composition as claimed in any one of claims 1 to 11.

13. A sheet as claimed in claim 12 wherein said layer of adhesive is further provided with a removable cover sheet.

14. A sheet as claimed in either one of claims 12 and 13 in the form of a tile of 1 to 5 mm thickness.

15. A sheet as claimed in either of claims 12 and 13 in the form of a foil of 0.1 to 0.5mm thickness.