IES80667B2 - Controlled degradation coatings and a method for their manufacture - Google Patents

Abstract

A coating for a biodegradable article comprises a hydratable coating or a chemically or physically discontinuous coating. Imperfections in the coating, which may comprise pores in the coating, enable water to penetrate to the underlying surface of the water soluble article. The nature and extent of the imperfections permits the rate of penetration of the coating by water to be controlled. Alternatively, the coating is a hydratable coating which swells and tears on hydration to expose the underlying article to moisture. By these means, the article can be protected from premature dissolution during its normal working life, but will degrade on disposal to waste, when the surface coating will degrade to permit water to reach and dissolve the underlying article. Typically, the body of the article comprises water soluble, biodegradable polyvinyl alcohol or polyvinylalcohol/polyvinylacetate copolymer. Chemical/physical discontinuity may be achieved by over-coating a surface already covered with a physically flawed coating with a hydratable coating.

Description

CONTROLLED DEGRADATION COATINGS AND A METHOD FOR THEIR MANUFACTURE The present invention relates to controlled degradation coatings and in particular to coatings for polyvinylalcohol and polyvinylalcohol/polyvinylacetate copolymers and to a method for their manufacture.

Polyvinyl alcohol (PVA) and polyvinylalcohol/polyvinylacetate co-polymers (PVA/PVAc) are known biodegradable materials. PVA and PVA/PVAc (of greater than 85% PVA) are soluble in warm and hot water. PVA/PVAc (of less than 85% PVA content) is readily soluble in water at room temperature. Post dissolution these materials will readily bio-degrade on exposure to bacteria present in the environment. These polymers and their part degradation products and degradation products are non-toxic and pose no risk of bio-accumulation or long term hazard to the environment.

PVA and PVA/PVAc are capable of being processed into a wide variety of useful articles by thermoplastic processes. Once an article has fulfilled its useful life, it is desirable that when it is discarded to waste (landfill or similar), the article will rapidly dissolve and subsequently biodegrade.

The application of PVA and PVA/PVAc copolymers in the manufacture of many types of articles is limited by the possibility of premature dissolution, which may occur should the article become damp during its working life. Also, the articles may absorb atmospheric moisture, rendering it softer and S 8 Ο 6 ό'/ -2mechanically weaker than designed. There are also problems of handling by humans in that when PVA/PVAc co-polymer materials absorb perspiration, they become sticky to touch.

W092/01556 describes the manufacture of coated PVA/PVAc copolymers by layering or surrounding PVA/PVAc (which is water soluble at 20°C) with a thin layer of PVA which is insoluble in water at 20°C. Thus an article may be manufactured which would be protected from dissolution until its waterproofing insoluble layer is breached. The copolymer and coating are io disclosed as being co-extruded, laminated or moulded, with or without an intervening filler layer.

Irish Patent No. S71912 discloses that a PVA/PVAc co-polymeric article may be waterproofed by coating the surface with a phenoxy based surface coating to render it resistant to premature dissolution. A portion of the article may be left uncoated to permit a route by which it may ultimately undergo total dissolution. Alternatively, the entire surface may be coated, in which case the article must be broken prior to disposal to expose the biodegradable copolymer.

The aforementioned processes may not fully protect a PVA or PVA/PVAc article during its working life and may be regarded as complex in application and potentially expensive. -3The present invention seeks to overcome the disadvantages of known coatings by providing a coating which permits controlled degradation of a coated article.

Accordingly, the present invention provides a water soluble article or material at least partially coated by a water resistant surface coating which is susceptible to breakdown or penetration by water so that the coating resists degradation on contact with or immersion in an aqueous medium under normal conditions which the article or material is likely to encounter during its useful working life, the coating being susceptible to eventual penetration and degradation by water on prolonged contact with or immersion of the article or material in an aqueous medium when the article or material is discarded to waste.

In one arrangement, the coating becomes hydrated on prolonged exposure to aqueous medium causing it to swell and tear, enabling water to contact the water soluble surface underlying the coating. In an alternative arrangement, the coating is formed with imperfections comprising microscopic discontinuities or pores which on prolonged exposure enable water to contact the surface underlying the coating.

The body of the articles or material may comprise polyvinylalcohol polymer or polyvinyl-alcohol/polyvinylacetate copolymer. The rate of degradation of the coating is controlled by selection of the thickness and/or physical and/or chemical structure of the coating. The coating may include single or blended surface coating resins having slight hydrophilic character or hydrophobic -4character. The material used in manufacturing the coating preferably has low or no environmental toxicity.

The coating may include single or blended acrylic resin and one or more plasticisers, waxes and/or surfactants. It may further include a non-aqueous solvent selected from esters, ketones, alcohols, white spirit, chlorinated solvents or aromatics. Also present may be a pigment or dye.

The coating may be decorated by conventional printing methods, including screen, flexographic and gravure printing. Most preferably, the coating is thermoplastic and permits the coated article or material to be welded to itself or to another article or material . The components of the surface coating are conveniently selected to permit the article or material to be used in food contact applications.

The present invention also provides a coating composition suitable for use as a water resistant surface coating for a water soluble plastics article or material, the composition comprising single or blended resins having slight hydrophilic character or hydrophobic character, the resultant coating when applied to an article or material being resistant to degradation by water on contact with or immersion in an aqueous medium under normal conditions which the article or materials is likely to encounter during its useful working life and being susceptible to degradation on prolonged contact with or immersion in the aqueous medium when the article or material is discarded to waste, enabling water to contact and dissolve the article or material. -5The present invention relates to an improved method for the protection of water soluble PVA and PVA/PVAc copolymer articles or materials by surface coating with water resistant surface coatings, which would protect the articles, including materials, from moisture during their working life, yet degrade once discarded.

The surface coatings are typically manufactured from polymeric resin materials (e.g. acrylic resins). The resins will dissolve in one or more, or blends of non-aqueous solvents, typically esters, ketones, alcohols, white spirits, chlorinated solvents or aromatics. Non-toxic or low toxicity solvents are preferred.

The control of degradation characteristics of these surface coatings is achieved by chemical and/or physical weakness being incorporated into the coating by design, i.e. through choice of the type of quantities of resins and solvent components used.

For the purpose of this specification the following terms have the meaning defined below:20 Resins or blends of resins which are soluble in non-aqueous solvents yet contain polar moieties or moieties which may possess an affinity for water (e.g. acrylic polymers containing polar hydrophilic pendant groups, such as free carboxylic acids groups), are termed “slightly hydrophilic resins”.

Resins which contain no polar moieties are termed “hydrophobic resins”. -6Surface coatings with designed chemical susceptibility to degradation by moisture may be manufactured from slightly hydrophilic resins, or blends of slightly hydrophilic resins or hydrophobic resins.

Surface coatings with designed physical imperfections may be manufactured from slightly hydrophilic resins, or blends of slightly hydrophilic resins and hydrophobic resins, or hydrophobic resins, thereby rendering the coated article susceptible to degradation by moisture. The occurrence of physical imperfections can be regulated by agents which control film formation, e.g. solvent, plasticisers and solvent modifiers.

Surface coatings prepared from the above can be used fully to coat the PVA/PVAc article and will resist full immersion in water, and in some cases survive total immersion for several hours, thereby rendering PVA/PVAc products practical for many more applications. The limited nature of the water resistance will permit the plastic articles ultimately to dissolve on disposal and subsequently biodegrade.

The water resistant surface coating may be applied by any conventional means, including dipping, spraying, smearing, painting, printing, etc.

The present invention also relates to the manufacture of surface coating systems which permit control of the rate of breakdown of the surface coating by the blending of resins with differing degrees of hydrophobic and slight hydophilic character. The presence of the hydrophilic components in a largely hydrophobic surface coating provides zones of weakness, where -Ί moisture can cause breakdown of the coating permitting water to penetrate to the PVA or PVA/PVAc material beneath the coating layer. The greater the slight hydrophilic nature of the resin system the faster will be the rate of breakdown.

The present invention also relates to surface coated articles in which the thickness of the applied surface coating is controlled to enable the rate of penetration or breakdown of the surface coating to be controlled. The thinner the coating applied the faster would be the breakdown. The present invention also relates to surface coated articles having imperfections in the surface coating to enable the rate of penetration or breakdown of the surface coating to be controlled. The less perfect is the coating film formation the faster would be the breakdown. Such surface coatings may be made with blends of resins with differing degrees of hydrophobic and slightly hydrophilic character, or be made with single resins with hydrophobic character or made with single resins of slightly hydrophilic character.

Where a surface coating is wholly made from a resin of hydrophobic character, the depth of surface covering of the PVA/PVAc article is controlled to permit the occurrence of microscopic flaws (or pores). This renders the material highly durable in moisture sensitive applications, yet permits a route for the ultimate breakdown when the article is discarded.

By the incorporation of one or more plasticisers into the surface coating, the coating may be imparted sufficient flexibility to offer good adhesion to compounded PVA/PVAc which is a relatively soft plastic. The presence of -8the plasticisers also serves to aid moisture resistance. Also the presence of a plasticiser aids the ultimate breakdown of the surface coating residue.

The surface coating plasticiser may be (a) any Cl to CIO mono-, di-or triester of adipic acid, azelaic acid, phthalic acid or trimellitic acid or (b) any mono-, di- or tri- Cl to CIO carboxylic acid esters of glycerol, propylene glycol, 1,3-propanediol or ethylene glycol, or (c) any blend of (a) and (b).. Materials which are non-toxic or of low toxicity, and with low migration properties are preferred.

The surface coating may include a wax material (eg Ceraflour waxes of BYK Cera) for added water resistance and improved anti-scratch properties. Also, if desired, surface active agents may be employed to assist wetting on application.

The surface coating may contain solvent modifiers to control the rate of solvent evaporation, thereby yielding improved coating film formation and by implication effect the occurrence of flaws (or pores) in the surface coating. Such modifiers include propylene glycol, 1,3-propanediol, ethylene glycol or any Cl to C4 mono- or di- ethers, or mono- or di- esters of the aforementioned.

The surface coating may permit decoration of a coated article by conventional means, e.g. Flexographic and Gravure printing inks. Also it may be coloured by pigments or dyes. The components of the surface coating may be chosen to permit varying degrees of gloss or matt finish. -9The surface coating may be thermoplastic in nature to permit heat sealing systems to be applied, such that coated articles or materials may be welded, e.g. to form bags from coated film materials.

The components of the surface coating may be chosen to permit a coated article to be used for food contact applications.

Hydrophobic resins suitable for use in the invention include - BMA copolymers (eg, NeoCryl B-842 of Zeneca Resins) - IBMA copolymers (eg, NeoCryl B-875 of Zeneca Resins) - Polyvinyl Chloride and its copolymer (eg Vinylite VMCH of Union Carbide) - Phenoxy resins (eg PKHH-MM of B&K Resins/Phenoxy Inc.) - Nitrocellulose (eg DHX 8/13 of ICI Explosives) - Cellulose Acetate resins - Ethyl Cellulose resins - Polyester resins - Polyurethane resins - Any IBMA, BMA, MMA, EMA, BA, Styrenic, maleic acid, butadiene, ethylene, propylene polymer or co- polymer of any of the aforementioned materials with an Acid value less than lmg KOH per gram.

Slightly Hydrophilic Resins useful in the invention include: -10- IBMA Copolymers (eg, NeoCryl 862 of Zeneca Resins) - Styrene/IBMA copolymers (eg, NeoCryl 880 of Zeneca Resins) - Carboxylated acrylic resins (eg; Surcol 836 or Surcol 860 of Allied Colloids) - Any IBMA, BMA, MMA, EMA, BA, Styrenic, maleic acid, butadiene, ethylene, propylene polymer or co-polymer of any of the aforementioned materials with an Acid value less than 100 or greater than or equal to lmg KOH per gram. io The above lists are not limiting and other suitable materials or combinations of materials will suggest themselves to the skilled person.

The present invention also relates to the preparation of PVA and PVA/PVAc surface coatings from acrylic based polymers. Acrylic based polymeric materials are preferred as they are potentially biodegradable and present a low risk to the environment.

The invention will now be described more particularly with reference to the following examples.

Preparation of Surface Coatings Solvents; Ethyl acetate (50 parts by weight (PBW)) and isopropanol (14 PBW) were combined. Resins; NeoCryl B-842 (0-30 PBW) and Surcol(R) 836 (30-0 PBW) were added with stirring. After complete dissolution additives; Dowinol (1.5 PBW) and plasticiser Bisoflex(R) DOA (4.5 PBW) -11 (total 100 PBW) were added and the varnish mixed until homogeneous. The compositions of five exemplary formulations A to E are shown in Table 1.

'NeoCryl' is a registered Trademark of Zeneca Resins; 'Surcol' is a registered 5 Trademark of Allied Colloids; 'Bisoflex' is a registered Trademark of British Petroleum Co. and used under license by International Speciality Chemicals; 'Dowinol' is a registered Trademark of Dow Chemicals) TABLE 1 Example Resin Ratio NeoCryl B-842 Sucrol 836 A 30 0 B 25 5 C 20 10 D 1515 E 0 30 Coatings indicated Examples A to E were made as above. Samples of A to E were drawn down with a K-Bar to give a 4 micron wet coating on Polyethylene and PVA/PVAc film.

Coated Film Results: - Films coated with formulations A-D had good retention of strength on light exposure to moisture, whereas uncoated film control rapidly absorbed moisture and reverted to a semi-liquid paste. Formulations A-C had good adhesion to PVA/PVAc film on stretching and creasing with A and B having the best performance. Formulations D and E -12tended to have less flexibility and form microscopic cracks on creasing and stretching, which lead to mechanical weakness on immersion in water.

Examples A to E above were diluted 1:1 with ethyl acetate and a test 5 PVA/PVAc injection moulding was fully immersed into the resin and allowed to dry. The test moulding was a thimble shaped injection moulding of approximate dimensions; 43mm high, 16mm dia, wall thickness 1mm, base thickness 5mm and weighing approximately 3 gram. The thickness of the dry surface coatings was typically 3 to 5 microns, io The dried coated mouldings were immersed in 500 ml of pure water and the dissolution observed against an uncoated test moulding as control.

Coated Test Moulding Results: The characteristics shown by the article coated with the example A to E formulations, by comparison with an untreated control sample are shown in Table 2. -13TABLE2 Example Dissolution Properties of Test Mounting Control After 5 minutes immersion, the TM surface visibly gelled. 10 minutes TM was visibly swollen with dissolution evident. 1 hour the wall of the TM was part dissolved and distorted. 20 hours the wall of the TM has totally dissolved with the base remaining as a gelatinous plug of material. 36 hours complete dissolution of TM. A After 30 minutes some puckering of the surface coating near mechanical flaws. 1 hour slight increase size of pucker near surface flaws. 1 hour 30 minutes, some surface coating breakdown. 2 hours de-lamination. TM dissolves as control leaving a thin, friable residue of flexible clear film. B After 30 minutes some puckering of the surface coating. 1 hour some surface coating breakdown. 1 hour 30 minutes, surface coating breakdown and delamination. TM dissolves as control leaving a thin, friable residue of flexible clear film. C After 30 minutes some puckering of the surface coating. 1 hour slight opacity observed in surface coating, with slight coating breakdown. 1 hour 30 minutes, surface coating breakdown and de-lamination. TM dissolves as control leaving a thin, friable residue of slightly opaque film.

D After 5 minutes some puckering of the surface coating. 30 minutes slight opacity observed in surface coating with coating breakdown. 1 hour significant de-lamination. TM dissolves as control leaving a thin, very friable residue of opaque film. E After 5 minutes some puckering and breaking of the surface coating. 15 minutes surface coating breakdown and de-lamination. TM dissolves as control leaving dispersed particles of a thin friable residue of opaque film.

Microscopic Analysis Flat, circular test plaques were coated as indicated above. The dried surface coated plaques all appear clear and even to the naked eye. The surface coatings were then examined under a microscope, (an Olympus XA40 x 100 magnification) to reveal the coatings micro-structure. The results are shown in the accompanying Figures 1 to 6, which show respectively an untreated io control, Example A and Example B (Figures 1 to 3 respectively), Example C (Figures 4 and 5) and Example D (Figure 6).

The illustrations, Figures 1 to 6 represent a region 575 x 435 micron (10'6m).

The surface of the cohtrol (Figure 1) indicates a flat surface scored by fine straight lines. These details are attributed to fine linear abrasions on the mould of the test plaque. -15Example A (Figure 2) illustrates a smoother surface than the control, where the test plaque abrasions have been covered. There are numerous fine pores or depressions (1-7 micron in diameter) randomly distributed over the surface * coating at an approximate number of 2-300 per mm2.

Example B (Figure 3) illustrates a gently undulating surface, where the test plaque abrasions have been covered. There are numerous fine pores or depressions (1-15 micron in diameter) distributed in clusters over the surface, being typically situated in depressions in the undulating surface coating. The approximate number of pores is 3-400 per mm2.

Example C (Figures 4 and 5) illustrates a gently undulating surface, where the test plaque abrasions have been covered. There are numerous round or ovate pores or depressions(30-100 micron in diameter) distributed evenly over the surface. The approximate number of pores being 34-45 per mm2.

Example D (Figure 6) illustrates an undulating surface where numerous ridges have formed, roughly running parallel and 2-300 micron apart. The test plaque abrasions have been covered. Though distributed evenly over the surface, the pores or depressions have increased in size (typically 100-150 micron in diameter) and are more irregular in shape, with some forming channels of up to 500 micron length. The approximate number of pores being 5-10 per mm2. In these larger pores, there are numerous smaller pores or depressions (1-20 micron in diameter). -16It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention, as defined in the appended claims.

Claims (5)

CLAIMS:

1. A water soluble article at least partially coated by a water resistant hydratable coating or a chemically and/or physically discontinuous surface coating which is susceptible to breakdown or penetration by water so that the coating resists degradation on contact with or immersion in an aqueous medium under normal conditions which the article is likely to encounter during its useful working life, the coating being susceptible to eventual penetration and degradation by water on prolonged contact with or immersion of the article in an aqueous medium when the article is discarded to waste, optionally in which the coating becomes hydrated on prolonged exposure to aqueous medium causing it to swell and tear, enabling water to contact the water soluble surface underlying the coating, the degradation of the coating being independent of the prevailing pH, or alternatively in which the coating is formed with imperfections comprising microscopic discontinuities or pores which on prolonged exposure enable water to contact the surface underlying the coating.

2. An article according to Claim 1, in which the body of the articles comprises polyvinylalcohol polymer or polyvinyl-alcohol/polyvinylacetate copolymer and in which the rate of degradation of the coating is controlled by selection of the thickness and/or physical and/or chemical structure of the coating, optionally in which the coating includes single or blended surface coating resins having slight hydrophilic character or hydrophobic character, optionally in which the coating includes material having low or no environmental toxicity, optionally in which the coating includes single or blended acrylic resin, * optionally in which the coating includes one or more plasticisers, -18optionally in which the coating includes one or more waxes and/or surfactants, ' optionally in which the coating includes a non-aqueous solvent selected from esters, 5 ketones, alcohols, white spirit, chlorinated solvents, and aliphatic or aromatic solvents, optionally in which the coating includes a pigment or dye, optionally in which the coating is decorated by conventional printing methods, including 10 screen, flexographic and gravure printing, optionally in which the coating is thermoplastic and permits the coated article or material to be welded, 15 optionally in which the components of the surface coating are selected to permit the article or material to be used in food contact applications, and optionally in which the coating is physically discontinuous and is coated over by a further aqueous or organic solvent based resin, or by a heat or adhesive bonded layer to 20 provide an additional dissolution resistant, modified surface for purposes of printing, adhesion, smoothness of finish or gloss, or further surface coating.

3. A method of preparing a moisture resistant water soluble article, comprising at least partially coating a water soluble article with a coating material which includes single or 25 blended surface coating resins having slight hydrophilic character or hydrophobic character, the coating being hydratable or being applied in such a way that it has chemical and/or physical discontinuities which render it susceptible to penetration by water so that * on exposure to moisture conditions which are not encountered by the article during its normal useful working life but which are encountered when the article is discarded to 30 waste, the coating degrades to permit water to contact and dissolve the article, -19optionally including adjusting the thickness and/or physical and/or chemical structure of the coating to control its rate of degradation on exposure to water.

4. A coating composition suitable for use as a water resistant hydratable coating or a 5 chemically and/or physically discontinuous surface coating for a water soluble article, the composition comprising single or blended resins having slight hydrophilic character or hydrophobic character, the resultant coating when applied to an article being resistant to degradation by water on contact with or immersion in an aqueous medium under normal conditions which the article is likely to encounter during its useful working life and being 10 susceptible to degradation on prolonged contact with or immersion in the aqueous medium when the article is discarded to waste, enabling water to contact and dissolve the article.

5. A water soluble article according to claim 1 or claim 2, or a method of preparing a moisture resistant water soluble article according to claim 3, or a coating composition 15 according to claim 4, substantially as herein described with reference to the accompanying drawings.