Title: "Degradable plastics material"

THIS INVENTION relates to degradable synthetic polymer com¬ positions and, more particularly, to improved degradable polymer compositions which are capable of withstanding at least short-term exposure to elevated temperatures without significant thermal degradation.

It is known to incorporate biologically sensitive particulate fillers, for example fillers of biological origin, in synthetic plastics formulations which are intended to be photo- or biodegradable. It has been observed that the inclusion of such biologically sensitive particulate fillers into synthetic polymer formulations can have an accelerating effect on the biological breakdown of those polymers, whether such breakdown is initiated by, for example, ultra-violet radiation or by prolonged exposure to moderate -temperature in biologically active environments. Thus, for example, our International Patent Application PCT/GB88/00386, published as W088/09354, discloses a degradable polymer composition which incorporates a normally stable chemically saturated polymer, a less stable chemically unsaturated compound in the form of a polymer or copolymer, an anti-oxidant active over a limited period and a directly biologically sensitive particulate filler

whereby the biological breakdown, in natural environments or in the specially biologically active surroundings of com¬ posting urban garbage, of the molecular fragments created by the onset of oxidative scission of the molecular chains of the polymer, is accelerated by the presence of the filler.

In the preferred embodiments disposed in PCT/GB88/00386, the particulate filler comprises natural starch, although natural protein, a natural cellulose product or powdered sugar are indicated as possible alter¬ natives.

Whilst the degradable plastics materials of PCT/GB88 /000386 can be compounded and, for example, extruded into sheets or films, at relatively low temperatures, for example around 150° C, in some applications for which the use of a degradable plastics would ideally, on environmental grounds, be used, it is necessary to raise the plastics material, briefly, to a temperature substantially above its melting or softening temperature, to form a molten mass of relatively low viscosity. An example of such an application is the process of extrusion coating of polyethylene onto paper, where it has been found necessary to raise the plastics material to a relatively high temperature, for example greater than 270°C, before application to the paper, in order to secure good adherence of the plastics material to the paper. Similar difficulties arise in the application of the biodegradation acceleration technique of PCT/GB88/ 00382 to polymers such as Nylon and polyethylene tereph- thalate, which have melting points in the region of 270°C and which therefore cannot be compounded with starch.

It has been found by the inventor that the use of natural starch as a filler for polymers limits the field of application of such filled polymers by virtue of the fact that the starch particles cannot withstand temperatures in excess of about 270°C because that is the temperature by

which all of the natural starches start to pyrolyse endo- thermically giving intense brown discolouration and evolving substantial volumes of odorous gases.

Similar limitations are inherent in the use of the other natural fillers proposed in PCT/GB88/00386. It is an object of the present invention to provide a biologically sensitive filler for biodegradable plastics materials which is more temperature resistant than the biologically sensi¬ tive fillers proposed hitherto.

According to the present invention, there is provided a biologically sensitive filler for incorporation in biode¬ gradable plastics materials, the filler comprising an alka¬ line earth, or Group III metal salt of a simple polybasic aliphatic acid or substituted aliphatic acid.

In preferred embodiments, the filler is selected from the group comprising calcium succinate, aluminium succinate, calcium glutamate and aluminium glutamate.

The literature of organic chemistry, for example the Beilstein compendium of properties of organic compounds, lists many properties such as melting points, refractive indices, etc. but does not list pyrolysis temperatures. Selecting candidate organic compounds for use as potential biodegradable fillers for plastics destined to be processed at high temperatures is, therefore, very difficult. Numerous trials with substances judged from considerations of their chemical character as likely fillers proved very disappointing when the target temperature was set at 300°C. It was, therefore, very surprising to discover that the calcium and aluminium salts of succinic acid could be held at this high temperature for several minutes without dis¬ colouring or melting and that they could be dispersed in polymer compounds at high temperatures without difficulty. Extending the search showed that the same metal salts of

glutamic acid behaved almost as well despite the presence of the more reactive amino group on this molecule. These calcium and aluminium salts have the advantage of being colourless, odourless, and non toxic as well as being rela¬ tively cheap and very easy to prepare. Both glutamic and succinic acids are naturally occurring materials and readily metabolised by micro-organisms as is necessary for them to function as degradation enhancers of synthetic plastics.

It is thought that many simple dibasic aliphatic acids and substituted aliphatic acids may, when stabilised by combination as salts with alkaline earth metals or group three metals, prove to be thermally stable at temperatures up to 300°C.

Embodiments of the invention are described in detail below.

In the accompanying drawings Figures 1 and 2 illus¬ trate plots, provided over respective testing periods, by a differential thermal analysis instrument, for a filler embodying the invention (Figure 1) and for potato starch.

In one embodiment of the invention, a heat-resistant biologically sensitive filler comprised the aluminium salt of succinic acid, which salt was prepared by adding potassium aluminium sulphate solution (potash alum) to a 1θ$ solution of succinic acid in hot water until no more precip¬ itate was formed. The white precipitate of aluminium succinate was recovered by vacuum filtration, washed on the filter, and oven dried at 90°C to constant weight. The fine white aluminium succinate powder was examined for thermal stability by exposing small samples in unsealed glass tubes to a temperature of 300°C maintained in a thermostatically controlled heating block. No change in the appearance of the sample over a period of several minutes indicated that it was thermally stable and this was confirmed by examining

the performance of further samples in a Du Pont differential thermal analysis instrument which had been calibrated using the melting point of pure metallic zinc as a reference. No thermal transitions could be detected in the behaviour of the aluminium succinate specimen up to a temperature of 360°C. Figures 1 and 2 are plots, from the above-noted instrument, respectively for aluminium succinate powder prepared as set out above and, for comparison, for purified potato starch. The pronounced negative excursion (endo- thermic) in Figure 2 at around 270°C is indicative of pyrolysis. By comparison, the plot of Figure 1 is substan¬ tially free from significant negative or positive excursions over the relevant temperature range.

A similar trial of the similarly prepared calcium salt of succinic acid gave essentially the same result apart from the loss of a small proportion of water at a temperature of about 120°C, presumed to be water of crystallisation. This water of crystallisation would normally have been removed in the preparation of a plastics filler by appropriate adjust¬ ment of the oven drying temperature.