GB2303374A - Surface coating composition - Google Patents

Description

2303374 Surface-coating composition and method of application.

This invention relates to a surface-coating composition for application to plastics or glass articles as a protective film.

Equipment or materials in current use are often made from, or comprise, relatively non-conductive plastics materials which may be subject to the build-up and periodic discharge of static electricity. The existance of a surface charge tends to encourage the deposition of dust particles, may interfere with the application of emulsion-type protective coatings to the surface and may cause damage in the case of sensitive electronic equipment. It is possible to reduce the build-up of static electricity by including a conductive, material such as a conductive powder, as an ingredient in the plastics material itself or by providing earthing wires or strips embedded in the plastics material. It would be desirable to provide a protective surface-coating composition which could be applied readily to a surface of articles or equipment, which is sufficiently hardwearing to survive normal use, which is preferably transparent or translucent so that the original surface finish of the plastics material is not obscured and which has good conducting properties.

The present invention provides a surface-coating composition, for application as a protective surface-coating to plastics or glass articles or equipment, the composition 1 comprising a synthetic mineral capable of imparting conductive properties to the composition, a binder for the mineral comprising an aqueous emulsion, suspension or dispersion of a polymer and a low temperature cross-linking agent (curing agent) for the polymer. The invention also provides a process for the application of the protective surface-coating to articles or equipment comprising applying a layer of the composition to a surface of the articles or equipment and maintaining the layer under cross-linking conditions to form a coating.

EP-A-0 374 937 describes a heat-sensitive recording material having a back-coating comprising a synthetic hectorite and a soap-free emulsiontype binder. The synthetic hectorite may be fluoride -containing or fluoride-free and may be gelforming or sol-forming. The purpose of the back-coating is to reduce blockages in the recording equipment during use and there is no conception of the use of the hectorite/binder composition as a protective coating. If a protective coating is required this is applied as a separate coating.

The synthetic mineral which is used in the composition of the present invention should preferably have a relatively highly charged siliceous anionic lattice indicated by a base exchange capacity of at least 80 m eq/100g. Particularly preferably the exchange capacity is at least 100 and possibly up to 170 m eq/100g. The synthetic mineral is preferably finely divided since this will enable a relatively homogeneous distribution in the composition. The synthetic mineral is preferably a clay mineral, for example, preferably, a synthetic hectorite such as, for example that available under the Trade Mark Laponite from Laporte Absorbents. It is further preferred that the synthetic clay mineral has content of hydroxyl groups of at least 2, particularly preferably at least 3, and possibly up to 4, per 8 atoms of silicon. These hydroxyl groups are available for reaction with the functional groups of some of the polymeric binders, for example with isocyanate groups of a polyurethane binder, and this combination is a preferred selection within the ambit of the present invention. Preferred grades of Laponite 2 synthetic hectorite are the fluoride-free RD and RDS grades which fulfill the preferences expressed above as to hydroxyl content.

The RDS grade of Laponite synthetic hectorite, and also the S, and XLS grades, contain 6.5% by weight of a phosphate deflocculant. According to this invention the synthetic hectorite advantageously contains, or is used in combination with, a larger quantity of flocculant than is usually used in conjunction with synthetic hectorite, namely at least 7.5%, particularly preferably at least 8.5%, by weight of the synthetic hectorite. Suitably the deflocculant is a polyphosphate or pyrophosphate deflocculant. The optimal range for retention in composition stability, is found to be 9.5% to 10.5% and the upper limit may be 15% or more. The increaseddeflocculant synthetic hectorite may suitably be Laponite JS. Fluoride-containing synthetic clays may be used in combination with the increased quantity of deflocculant.

z The polymeric binder is preferably, although not essentially, in the form of an emulsion latex. Many polymers cannot be emulsion polymerised but may be transferred from an organic medium into the form of an aqueous emulsion by emulsification of the medium followed by solvent stripping or phase inversion. The polymeric binder may be selected from suitable filmforming cross-linkable polymers such as polyesters, polyepoxides, vinyl polymers such as polypropylene, polybutadiene, polyisoprene, polyvinyl acetate or polyvinyl alcohol or, as indicated above, preferably from the polyurethanes or urethane copolymers. The polymeric binder may be helped to cure by using a catalyst if necessary although the last-mentioned polymers will normally cure without a catalyst and, when cross-linked, may provide particularly durable and transparent coatings.

The cross-linking agent which is added to the present composition is selected to be effective in relation to the particular polymer being used at temperatures which will not 3 harm the surface being treated in the time required for cure. Preferably, the cross-linking agent is selected to be capable of curing the polymer at temperatures below 1500C, particularly preferably below 1400C. It is generally preferred that the cross-linking agent be capable of achieving cure at temperatures above 100C, particularly preferably above 500C. The curing duration is preferably below 2 hours or, for cure temperatures above 1000C, no more than about 1.5 hours. A suitable minimum curing duration may be 10 minutes. Preferred curing agents are the polyaziridines, metal-containing curing agents, for example zirconium compounds such as ammonium zirconium carbonate or metal-organic compounds such as the amino-functional product synthesised with a stable preneutralised complex and available as Chartwell B515-4.

Polymer latices or dispersions for use in this invention preferably have a polymer concentration of from 1% to 5% particularly preferably from 2% to 4% by weight of the surface coating composition. The concentration of synthetic mineral, particularly of the synthetic clay mineral, should preferably be at least 0.5% and up to 12% by weight of the composition. The upper limit is set by the nature of the mineral and the viscosity required in the composition to attain a sufficiently thick coating. If the mineral is a gelling grade of synthetic hectorite clay mineral, such as Laponite RD, the concentration may be up to about 2% whereas, if it is a sol-forming peptisercontaining grade such as Laponite RDS, the concentration may be up to about 12% by weight, or more, of the composition.

It is found that the surface-coating composition of the invention may be fast to gel physically where the quantity of polymeric dispersion is relatively high, for example 10% or more, by weight of the composition. This may give rise to application problems. To lengthen the period of stability for a given application it may be necessaty to reduce the concentration to no more than about 6%. It is found that if a quantity of a polar solvent such as propylene glycol is added, for example from 1% to 12% by weight of the composition the 4 stability period for a given concentration of dispersion is considerably increased. Where both a polar solvent and an increased quantity of deflocculant is used the period of stability may be at least 5 days. Such a composition is readily sprayable during this period.

The composition should preferably also contain an anionic or nonionic wetting agent. Examples of suitable wetting agents are the nonionic fluorosurfactants such as the 40% weight active product Zonyl FSN or the anionic polysiloxanes such as the polyether-modified product BYK 346. A suitable dispersant is a fully neutralised polycarboxylic acid such as Dispex N40. The quantity of the wetting agent or dispersant is preferably each at least 0.1% and, for example up to 5% by weight of the composition.

The substrate to be surface-treated acording to this invention may be any plastic, glass or other artefact surface which, due to itshigh resistivity, for example more than 1013 ohms/m2' may be subject to static accumulation. Such a surface may be of polyvinyl chloride, for example as a coating on a sheet or fabric or in the form of floor tiles or other formed articles, acrylic, styrene-acrylic, styrene acrylonitrile or acrylonitrile-butadiene-styrene copolymers (ABS) which may be in the form of sheets, polycarbonates or other plastics surfaces. The glass surface may be, for example, a television screen or computer monitor screen.

The substrate may be pre-treated to render it more amenable to coating by corona discharge in the case of PVC of glass, or by the use of a silane treatment. A layer of the composition according to the invention may be applied, for example by brushing, or by spraying particularly in the case of compositions containing a deflocculant and/or a polar organic solvent, preferably to a thickness of at least 20 micrometers and suitably up to 150 micrometers applied, in the case of coatings above 30 micrometers in thickness, in two or more layers with intervening drying. The coating may be dried by hot air drying or by other means and may be crosslinked, as appropriate, by the application of heat or by the use ionising radiation or by other means. For heat-induced cross-linking of the composition on PVC, textiles or glass a temperature of up to about 1500C may be suitable while for the cross-linking on ABS or acrylics the temperature is preferably maintained at not more than about 850C.

The conductivity of a surface coated with the composition may be judged by the decrease in the resistivity of the surface as measured, using a resistivity meter, in ohms per m2. Preferably the decrease is at least 103 but most preferably at least 104 ohms/m2.

The invention will now be illustrated by reference to the following Examples which are not intended to be limiting in any way on the scope of the invention.

The following are formulations of compositions according to the invention expressed in parts by weight.

Example 1 Whitcobond 223-8 polyurethane latex Zeneca CX-100 aziridine crosslinker Laponite RDS synthetic hectorite Zonyl FSN wetting agent Demineralised Water 10.0 0.2 7.5 0.5 81.8 The Laponite RDS is a sol-forming grade containing a gel retarding quantity of tetrasodium pyrophosphate. Laponite is a Trademark of Laporte Industries. Whitcobond is a Trademark of Baxenden Chemicals. Zonyl is a Trademark of Du Pont.

Example 2 Primal 358 acrylic latex Zeneca CX-100 aziridine crosslinker Laponite RDS synthetic hectorite Zonyl FSN wetting agent Demineralised Water 6 10.0 0.2 7.5 0.5 81.8 Primal is a trademark of Rohm and Haas Example 3 Beetle BT 338 melamine resin Chartwell B 515.4 crosslinker Ammonium chloride (25% by weight) Laponite RDS synthetic hectorite 10.0 0.3 1.0 7.5 Zonyl FSN wetting agent 0.5 Demineralised water 80.7 Beetle is a Trademark of BIP Speciality Resins.

The compositions described in Examples 1 to 3 were applied to some of following substrates and the water resistance and surface resistivity of the resulting films was assessed.

ABS Acrylic Styrene acrylic Each of these substrates was coated as follows. Two 30 um coatings were applied with intervening drying by means of a hot air drier. The final coating was dried by the same means and the composite coating was cured at a temperature of 850C to 87C for 20 minutes.

Polycarbonate The coatings were applied and dried as above and the composite coating was cured at a temperature of 730C to 750C for 20 minutes.

PVC-coated textile fabric PVC tile Glass Each of these substrates was coated as follows. Two 60um coatings were applied and dried as above. The curing was conducted at a temperature of 1300C for 60 minutes.

The coating/substrate combinations are identified below.

Composition Substrate Resistivity of Example No coated Ohms/m2 I PVC Tile 10 8 to 109 7 It ABS it Acryl i c it Styrene acrylic is Polycarbonate 2 PVC Fabric It 3 Glass 1017- 10 M All the coatings were resistant to repeated water-washings.

The formulations of further compositions according to tha invention are as follows. Example 4 Whitcobond 223-8 binder CX-100 crosslinker Laponite JS(10% tetrasodium pyrophosphate) Zonyl FSN wetting agent Dispex N40 dispersant Propylene glycol Demineralised water Example 5 Whitcobond 223-8 binder CX-100 crosslinker Laponite RDS(6.5% tetrasodium.

pyrophosphate) Zonyl FSN wetting agent Dispex N40 dispersant Propylene glycol Demineralised water 8 10.00 0.20 7.50 0.50 1.25 5.00 75.55 5.00 0.20 7.50 0.50 1.25 5.00 80.55

Claims (13)

Claims

1. A surface-coating composition, for application as a protective surfacecoating to plastics or glass articles or equipment, the composition comprising asynthetic mineral capable of imparting conductive properties to the composition, a binder for the mineral comprising an aqueous emulsion, suspension or dispersion of a polymer and a low temperature cross-linking agent for the polymer.

2. A composition as claimed in claim 1 wherein the synthetic mineral is a synthetic hectorite containing hydroxyl groups.

3. A composition as claimed in claim 1 or 2 wherein the synthetic hectorite is fluoride-free.

4. A composition as claimed in any preceding claim wherein the synthetic hectorite is used in combination with a polyphosphate or pyrophosphate in a quantity of from 1% to 10% by weight thereof.'

5. A composition as claimed in claim 4 wherein the quantity of polyphosphate of pyrophosphate is at least 7.5%.

6. A composition as claimed in any preceding claim wherein the synthetic hectorite is in from 0.5% to 12% by weight of the composition.

7. A composition as claimed in any preceding claim wherein the binder is an aqueous emulsion, suspension or dispersion of a urethane polymer or copolymer, an acrylic polymer or copolymer or a melamine resin.

8. A composition as claimed in any preceding claim wherein the low temperature cross-linking agent is capable of achieving cure within a period of from 10 minutes to 2 hours at a temperature in the range 50 to 140 degrees Centigrade.

9. A composition as claimed in any one of claims 1 to 7 containing a polar organic solvent.

10. A composition as claimed in any preceding claim wherein the low temperature cross-linking agent is a polyaziridine a zirconium compound or an organometallic compound.

9

11. A composition as claimed in any preceding claim also containing an anionic or nonionic wetting agent in from 0.1% to 5% by weight of the composition.

12. A composition as claimed in claim 1 comprising a synthetic hectorite, a deflocculant in at least 7.5% by weight of the synthetic hectorite, a polar organic solvent in at least 1% by weight of the composition, and a binder in at least 10 % by weight of the composition.

13. A composition as claimed in claim 12 wherein the deflocculant is present in at least 8.5% by weight of the synthetic hectorite.

14 A composition as claimed in any preceding claim and substantially as described herein with reference to any one of the Examples.

l" An article or equipment having a plastics or glass surface coated with a protective coating comprising a composition as claimed in any preceding claim in cured form.