GB2177413A - Antistatic coating compositions - Google Patents

Abstract

Antistatic coating compositions for plastics substrates, more particularly but not exclusively for synthetic paper substrates, comprise an aqueous system containing a polymeric binder, a pigment and an antistatic agent. The binder contains carboxyl groups, and the binder: pigment dry weight ratio is in the range 15:100 to 50:100. The antistatic agent is a water soluble ionic compound (e.g. a sodium polyphosphate) present in a dry weight: pigment ratio of from 0.4:100 to 2.5:100. The composition also contains an agent which reacts with carboxyl groups of the binder on drying so as to insolubilise the binder (e.g. ammonium zirconium carbonate).

Description

SPECIFICATION Coating composition.

This invention relates to antistatic coating compositions for plastics substrates and more particularly but not exclusively for synthetic paper substrates.

It is well known to coat plastics substrates for the purpose of modifying the appearances or other characteristics of their surfaces and it is well established practice to coat plastics film and sheet materials with overall or partial coatings which may be laid down for example by coating or printing techniques.

An inherent problem with substrates based on plastics materials, particularly hydrophobic plastics materials is associated with the build-up of static charges which are difficult to dissipate and these charges lead to difficulties in subsequent handling of the plastics materials in further processing operations such as printing, for example. For plastics substrates which are coated this problem may be overcome or alleviated by the application of a coating having antistatic properties which may be inherent in the coating base material or induced by the incorporation of an antistatic additive such as an ionic compound.However, where the coated plastics substrate such as a coated synthetic paper is to be subsequently printed using the well known lithographic printing process, the addition of such antistatic agents to the coating composition has, so far, not produced a satisfactory solution since the antistatic agents tend to be leached out giving rise to a phenomenon known to the skilled addressee as "print mottle" due to non-uniform laying down of the ink, and which manifests itself in multiple-pass printing. Further, the presence of such antistatic agents in coatings can itself reduce the level of adhesion of lithographic ink to the coated substrate.

In the case of coated synthetic papers to be subsequently printed, the coating composition is advantageously water-based to enable the coating to be carried out on conventional paper coating equipment and using the paper industry's established techniques of production and control. However, coatings employed for plastics substrates in general require a higher latex binder: pigment ratio than is normal for conventional paper substrates (e.g. 22-35: 100 instead of 15: 100-dry weight ratio) in order to achieve a satisfactory bond between coating and substrate. The higher proportion of binder tends to introduce an unacceptably high degree of water sensitivity into the coating which can give rise to difficulties in printing of the coated products by the lithographic process where additional problems of adhesion of overlying webs, known as blocking, can also occur.

It is known that the water resistance of such coatings may be improved by the incorporation of melamine-formaldehyde or urea-formaldehyde resins. Such resins, while providing a matrix which rigidifies the coating and reduces its water sensitivity, give rise to an undesirable increase in surface resistivity of the coating and the problem of static build-up referred to above. While this problem may be overcome, as indicated above, by addition of certain static reducing compounds at the wet mix pre-cure stage for example, the aforementioned leaching out of such compounds and associated problems occur. Further, the very addition of such compounds can undesirably interfere with the curing of the melamine-formaldehyde or urea-formaldehyde resin.

In view of health concern over formaldehyde fumes, it has been proposed to replace the melamine-formadehyde or urea-formaldehyde resins in coating compositions for conventional paper and containing relatively low binder concentrations with a chemical means to insolubilize the binder. Such means include in particular zirconium comppounds soluble in the liquid coating medium where they may exist in the form of hydroxy bridged polymers which, depending on the desired effect, may be polycationic (e.g. zirconium oxychloride), neutral (e.g. zirconium acetate), or polyanionic (e.g. zirconium orthosulphate or ammonium zirconium carbonate). For latex systems normally used for paper coating, the polyanionic compounds are suitable and ammonium zirconium carbonate is particularly preferred for carboxyl group-containing polymer latices.The latter added in solution to the polymer latex causes cross-linking of such polymer latices on drying. Typically proposed binder: pigment dry ratios for such coatings are 12:100 with the relative proportion of ammonium zirconium carbonate in the west state being equivalent to 1 part of aqueous solution believed to be of 30% concentration. In addition the proposed coating may contain for example, 0.3 parts per 100 parts pigment of a sodium polyphosphate such as 'Calgon' sold by Albright & Wilson of London, employed as wetting agent or dispersing agent.

We have now found that the relatively high binder: pigment ratio coatings required for plastics substrates such as synthetic papers for example, can have their binders similarly insolubilized and that dried coatings thereby produced by contain selected antistatic compounds in sufficient quantity to be effective in reducing the surface resistivity of the coating to a satisfactory level while not being readily leached out during lithographic printing or otherwise adversely affecting the lithographic printability of the coated plastics substrate.

According to the present invention there is provided a coating composition for a plastics substrate and comprising an aqueous system containing a polymeric binder, pigment and anti static agent characterised in that the binder contains carboxyl groups, that the binder: pigment dry weight ratio is in the range 15: 100 to 50:100, preferably 22:100 to 35:100, the antistatic agent comprises a water soluble ionic compound present in an amount corresponding to a dry weight: pigment ratio of from 0.4:100 to 2.5:100 and in that the composition also contains an insolubilizing agent which is capable of reacting with carboxyl groups of the binder on drying the composition to insolubilize (as hereinafter defined) the binder.

Also provided by the present invention is a method for producing a water resistant, antistatic, lithographically printable coating on a plastics substrate which method comprises preparing a coating composition according to this invention applying the composition to the surface of a plastics substrate and drying the coating at a temperature below 100"C thereby to insolubilize (as hereinafter defined) the binder component of the coating.

The polymeric binder employed in the composition of the present invention should be in aqueous solution or latex suspension, preferably the latter, and should contain carboxyl groups on the polymer chain of at least one polymeric constituent. Thus the binder may comprise a single polymer or a mixture of polymers subject to the foregoing requirement being met.

Accordingly, the binder may comprise starch or protein modified chemically or by physical addition of other polymeric species to provide the required carboxyl groups. Alternatively, the polymeric binder may comprise for example, a carboxylated styrene-butadiene copolymer or an acrylic polymer or copolymer, a vinyl acetate polymer or copolymer but preferably a carboxylate sytrene-acrylic copolymer. The carboxyl group-containing binder may contain other polymeric species provided they do not interfere undesirably with the properties of the coating. For example, a styrene-butadiene copolymer latex may be incorporated in an acrylic latex binder to modify the flexibility and toughness of a dried coating.

The binder content of the aqueous coating composition of this invention is chosen to suit individual requirements within the binder: pigment dry weight ratios specified. For example, a latex with relatively high polymer solids content may be selected where the coating is required to impart significant stiffness to the coated product and, conversely, lower solids content may be chosen. We have found that for relatively non-absorbent surfaces such as synthetic paper the binder content of the composition is advantageously in the range 15 to 30% by weight based on the binder plus aqueous phase and is preferably in the range 20 to 25%. Latices in a range of polymer contents are commercially available.

The pigment may be any dispersable solid but is preferably an inorganic filler or pigment. Such pigment may be, for example, a calcium carbonate, china clay, titanium dioxide or other such components as commonly used in coatings.

The antistatic agent employed in the composition of this invention should be a water soluble ionic compound. While the range of compounds extends from sodium chloride to a sodium polyphosphate, the latter is preferred where higher water resistance of the coating is required.

The proportion of antistatic agent incorporated in the coating composition should be such as to be in the dry weight: pigment ratio of 0.4:100 to 2.5:100.

Certain water soluble ionic compounds which may be employed in the compositions of this invention as antistatic agents may also act as dispersing agents. Thus, for example, a sodium polyphosphate may act both as dispersing agent and antistatic agent and, where this is so, a quantity higher in the range specified will usually be required. However, smaller amounts of such antistatic agents may be employed if their addition to the coating composition in its preparation is made, as is preferred, after the pigment has been dispersed in aqueous medium with the assistance of a specific dispersing agent such as an ammonium polyacrylate for example.

The insolubilizing agent for the polymeric binder should be selected to be water soluble and such as to be reactive with carboxyl groups of the polymeric binder on drying the coating composition of the invention at a temperature below 100"C to insolubilize the binder.

The term "insolubilize" as used herein means to render the binder relatively unaffected by water. Suitable insolubilizing agents include in particular zirconium compounds soluble in the aqueous coating medium where they may exist in the form of hydroxy bridged polymers which as described earlier may bew polycationic neutral or polyanionic. For latex-based coating compositions the polyanionic compounds are suitable and ammonium zirconium carbonate is particularly preferred.

The amount of insolubilizing agent employed in the aueous coating composition will depend on the binder content and the degree to which it is to be insolubilized. However, the proportion of an ammonium zirconium carbonate insolubilizing agent incorporated in an acrylic latex may be in the region of 1 part 30% solution: 12 parts latex solids to adequately insolubilize the acrylic component on drying.

In operation of the process of the present invention it is preferred that, in preparation of the aqueous coating composition, at least a greater proportion of the pigment is uniformly dispersed in aqueous medium before the addition of the polymeric binder in solution or latex dispersion.

Such pigment dispersion is preferably assisted by using a dispersing agent and is also preferably conducted in alkaline aqueous medium, with a view to the final aqueous coating composition having a pH range of 7.2 to 10.5.

The pigment in the final aqueous coating composition preferably has a maximum particle size of less than 30 microns and will normally total at least 30% by weight based on the overall weight of the aqueous coating composition and particularly 35 to 50% but may be higher.

The amount of dispersing agent used may be for example in the dry weight: pigment ratio of from 0.5:100 to 10:100.

The aqueous coating composition may contain additional components commonly employed in the coating industry. For example, processing aids such as stearates especially calcium stearate may be incorporated to improve the behaviour of the coating composition when being applied to a substrate using coating machinery.

The aqueous coating composition of the present invention may be applied to the plastic substrate by any known applicable technique and where the substrate is in film or sheet form such as a synthetic paper, paper coating techniques will normally be employed such as roller coating with air-knife metering. However, print-coating may also be employed. The thickness of the wet coating may for example be in the range appropriate to give a dry coating weight of from 5 to 30 g/m2 preferably in the region of 10 g/m2.

Drying of the wet coating may be by any means whereby the temperature may be adequately controlled to keep the coated plastic substrate temperature below 100"C. In the case of coatings on synthetic papers for example, air drying temperatures in the region of 50"C may be advantageously employed to achieve adequately rapid drying while preserving a uniform dried coating.

It will be appreciated that the plastic substrate may be comprised of any plastics material.

However, particularly where the surface of such plastics material is strongly hydrophobic, modification of the surface by known chemical or corona discharge treatment may be desirable prior to coating to assist wetting by the coating composition during the coating operation and/or to assist in achieving a good bond between dried coating and substrate. Preferred plastics substrates are synthetic papers as described and claimed in British Patent Specification 1470372 incorporated herein by reference.

The following Example is given to illustrate but to in no way limit the scope of the present invention: Example Formulation made up as follows: Wet Dry Water 67.5 litres Ammonia (0.88) 2.7 litres Dispel A40 0.54 litres 0.27 litres Kronos AD 10.00 kg 10.00 kg SPS Clay 33.70 kg 33.70 kg Hydrocarb* 90 56.30 kg 56.30 kg Registered Trade Mark The above components were dispersed with high shear mixing for a minimum of 20 minutes; the following were then added with gentle agitation: Acronal S305D 58.00 kg 29.00 kg Nopcote C104 1.50 kg 0.75 kg AZC Solution 3.00 kg 0.90 kg Ammonia (0.88) 1.00 kg 20% Calgon Solution 2.50 kg 0.50 kg Registered Trade Mark The total mix was diluted with water to machine viscosity, filtered through 100 mesh screen and then applied to Polyart 2 base film using forward roll application and air knife metering to a coat weight of 10g/m2 dry on both faces. The coating was dried at 50"C for 10 minutes using a festoon tunnel.

The coated product was found to give rapid static charge dissipation and was satisfactorily printed by offset lithographic printing showing no significant leaching of antistatic agent from the coating and showed adequate water resistance of the coating.

Materials Description Dispex* - 50% Ammonium Polyacrylate solution in water available from Allied Colloids Limited, Bradford.

Kronos* AD - Anatase Titanium Dioxide powder, available from N.L. Chemicals Limited, Wilmslow.

SPS Clay - China Clay powder, available from EEC International, St. Austell.

Hydrocarb* - Calcium Carbonate powder, available from Croxton & Garry, Dorking.

Acronal* S305D - Believed to be a Carboxylated Styrene-Acrylic copolymer, available from BASF (UK), Cheadle Hulme.

Nopcote* C104 - 50% dispersion of calcium stearate in water, available from Diamond Shamrock, Leeds.

AZC Solution - 30% Ammonium Zirconium Carbonate solution, available from Magnesium Elektron Limited, Twickenham.

Calgon* - Sodium Polyphosphate, available from Albright & Wilson, London.

Polyart* 2 base film - A synthetic paper base film manufactured from high density polyethylene and available from BXL Plastics Limited.

*Registered Trade Mark.

Claims (19)

1. A coating composition for a plastics substrate and comprising an aqueous system containing polymeric binder, pigment and antistatic agent characterised in that the binder contains carboxyl groups, that the binder: pigment dry weight ratio is in the range 15:100 to 50: iso, preferably 22:100 to 35:100, the antistatic agent comprises a water soluble ionic compound present in an amount corresponding to a dry weight: pigment ratio of from 0.4: 100 to 2.5:100 and in that the composition also contains an insolubilizing agent which is capable of reacting with carboxyl groups of the binder on drying the composition to insolubilize (as hereinbefore defined) the binder.

2. A coating composition as claimed in claim 1 wherein the polymeric binder is present in latex suspension.

3. A coating composition as claimed in claim 2 wherein the binder content of the latex suspension is in the range 15 to 30%, preferably 20 to 25% by weight based on binder and aqueous phase.

4. A coating composition as claimed in any one of the previous claims wherein the binder is a carboxylated styrene-acrylic copolymer in latex suspension.

5. A coating composition as claimed in any one of the previous claims wherein the antistatic agent is a sodium polyphosphate.

6. A coating composition as claimed in any one of the previous claims wherein the insolubilizing agent is a polyanionic zirconium compound.

7. A coating composition as claimed in claim 6 wherein the insolubilizing agent is ammonium zirconium carbonate.

8. A coating composition as claimed in claim 7 wherein ammonium zirconium carbonate is present in an amount approximately corresponding to 1 part 30% solution: 12 parts latex solids.

9. A method for producing a water resistant, antistatic, lithographically printable coating on a plastics substrate which method comprises preparing a coating composition as claimed in claim 1, applying the composition to the surface of the plastics substrate and drying the coating at a temperature below 100 C thereby to insolubilize (as hereinbefore defined) the binder component of the coating.

10. A method as claimed in claim 9 wherein the preparation of the coating composition includes uniformly dispersing at least a greater proportion of the pigment in an aqueous medium before the addition of the polymeric binder in solution or latex dispersion.

11. A method as claimed in claim 10 wherein the pigment dispersion is assisted by use of a dispersing agent in an amount corresponding to a dry weight: pigment ratio of from 0.5:100 to 10:100.

12. A method as claimed in either claim 10 to claim 11 wherein the antistatic agent is added with the polymeric binder.

13. A method as claimed in any one of the previous claims wherein the plastics substrate is a film or sheet.

14. A method as claimed in claim 13 wherein the film or sheet is a synthetic paper.

15. A method as claimed in claim 14 wherein the synthetic paper is based on high density polyethylene.

16. A method as claimed in any one of claims 13, 14 and 15 wherein the coating composition is applied by roller coating with air knife metering or by print coating.

17. A method as claimed in claim 14 of claim 15 wherein the thickness of the coating composition is in the range appropriate to give a dry coating weight of from 5 to 30 g/m2 preferably in the region of 10 g/m2.

18. A method as claimed in any one of claims 9 to 17 wherein the drying of the coating is effected by air drying at a temperature in the region of 50"C.

19. A coated plastics substrate produced by the method claimed in any one of claims 9 to 18.