It is desirable to provide films which are biodegradable. These films which may, for example, become contaminated with food and may be difficult to recycle by conventional means, may be composted so as to reduce their volume and to produce a useful product.

Commonly used biodegradable films comprise cellulose, especially in the form of regenerated cellulose or cellulose acetate. Such films have in general poor barrier properties to water unless the degree of substitution of the cellulose by acetate groups is high, or a layer of a barrier polymer is applied to the cellulose. In either event biodegradability suffers. This invention comprises biodegradable film comprising at least one layer (hereinafter referred to as the cellulose layer) comprising water swellable water insoluble cellulose or cellulose acetate and at least one layer comprising polyhydroxyalkanoate (PHA) . The PHA is suitably a polymer or copolymer of hydroxybutyric acid, for example a copolymer of (R)3- hydroxybutyric acid with (R)3-hydroxyvaleric acid. Residues of hydroxyvaleric acid may constitute 0 to 25% for example 3 to 20% of the total weight of the polymer. Suitable polymers are disclosed in European Patents 69497 and 52459.

In addition to providing a water vapour barrier, the PHA layer in general also enables the film to be heat sealed to similar sheets and to various other materials if it is a surface layer.

It is preferred that the cellulose layer should comprise regenerated cellulose or should comprise cellulose acetate with a degree of substitution of at most 2.5 and preferably at most 2.3, for example 0.2 to 2.2.

The cellulose layer may be for example 10 to 100 microns thick and the PHA layer 0.5 to 30, for example 1 to 10, microns thick.

Particular examples of the film are: a single cellulose layer and a single PHA layer; two cellulose

layers with a central PHA layer, or two PHA layers and a central cellulose layer.

The film may be produced from pre-formed cellulose film, for example as sold under the trade mark "Cellophane", by applying a layer of the PHA to it and preferably heating to cause the layer to adhere. The PHA may be applied as a solution, for example in a chlorinated solvent for example chloroform. More preferably it is applied as an aqueous suspension, followed by drying. The PHA is preferably to some extent amorphous; (percentage crystallinity typically 0 to 30 for solvent process material, 20-40 for virgin material) then the deposit easily adheres to other materials. If desired, a second cellulose layer may be applied and the result then preferably heated to leave a product in which the PHA layer is sandwiched between two cellulose layers. If the PHA layer is to be on an outer surface of the film and if the film is to be rolled up soon after formation it is preferred to crystallise the PHA to the extent of typically 40-60% by heating the film, thus rendering the PHA non adhesive.

The liquid solutions or dispersions of the PHA may be applied to the preformed cellulose film in controlled amounts by known means, for example by spreading with a Mayer bar or (preferably) by contacting the film with a gravure roll which is continuously wetted with the liquid, suitably by running a band of film over the top of a rotated gravure roll the bottom of which contacts a reservoir of the liquid. The drying of liquids may be carried out by hot gas heating, e.g. air heating or radiant heating. Solvents other than water are preferably recovered for re-use. It is preferred to use as little solvent or water as possible in order to facilitate drying and to minimise any necessary solvent recovery.

EXAMPLE Solvent Process

(A) A first latex (A) was produced as follows:

40 mL of a 5% (w/v) solution of polyhydroxybutyric acid (PHB) in CHC1 ₃ is placed in a 400 mL beaker. 200 mL

of 75 mM aqueous hexadecyl (cetyl) trimethylammonium bromide (CTAB) is layered carefully over the PHB solution. The contents of the beaker are then sonicated using a Dawe Soniprobe at maximum power (ca. 200 ) for 5 min using the standard (about Vi in. diameter) horn. To control CHC1 ₃ release into the air, the operation is carried out either in a fume cupboard or using an ice bath to control excess heating of the sample. The process is repeated until an adequate quantity of emulsion has been prepared. Once prepared the emulsion is stirred magnetically to prevent settling.

Removal of the solvent is then undertaken at the earliest opportunity as follows. 2.25 L batches of fresh emulsion are placed in 5L glass beakers, which are immersed in a 75°C recirculating water bath in a fume cupboard. Each beaker is fitted with an overhead mechanical stirrer. Foaming, which can be quite vigorous, is controllable by adjusting the stirring speed. Solvent removal is allowed to proceed for 8 hr; make-up water may be added as necessary. The final suspension should be odourless and somewhat less opaque than the original emulsion, and is stable indefinitely at room temperature. Precipitation of CTAB will gradually occur. This can be reversed by heating the suspension to 50°C.

Concentration of the particles which result is carried out by centrifugation. On a small scale, this is done conveniently by two rounds of centrifugation, at 10,000 and 40,000g, 30 min each at 20°C. The pellet from each round is resuspended immediately in the desired volume of distilled water. On a larger scale, the suspension can be centrifuged in Beckman centrifuges using the JA.10 (6 x 400 mL) rotors operated at 10,000 rpm (20°C) . Under these conditions, partial collection of the particles can be effected by a 1 hr centrifugation; more complete recovery requires longer spin times (> 2hr) . Latex A was made up by resuspending particles from the centrifuge in water to produce as suspension containing 21% by weight of solids. The particles are typically 24% crystalline.

(B) A second latex, (B) comprising virgin particles, was made as follows:

Cells of Alcaligenes eutrophus were grown in batch culture in an aqueous medium on a mixture of glucose and propionic acid under phosphorus limitation to give a culture containing 160 g/1 of cells containing 70% of a 3-hydroxybutyrate (HB) /3-hydroxyvalerate (HV) copolymer with a molar HV content of 4% (the remainder of the polymer being HB) . The cell suspension, which had a dry weight of 160 g/1, was rapidly heated to 150°C by direct injection of steam at 20 bar in a continuous flow steam injector. The cell suspension flowrate was controlled to give a residence time of 80 seconds at 150°C. The cell suspension was then cooled to below 70°C in a continuous flow heat exchanger.

The resulting cell suspension, now at a cell dry weight of 120 g/1 due to dilution by condensed steam, was heated to 70°C and the pH raised to 8.0 by addition of NH ₄ OH solution and the suspension was agitated.

A solution of a proteolytic enzyme sold under the trade name L660 "Optimase", by Solvay, was then added to the cell suspension at a concentration of 0.5% w/w with respect to the cell dry weight. The pH of the suspension was maintained at 8.0 by additions of H ₄ OH solution during the 2 hours of the reaction.

At the completion of the reaction, a detergent sold under the trade name "Synperonic" All by Imperial Chemical Industries PLC was added to the suspension at a concentration of 1.0% w/w with respect to the cell dry weight. The pH of the suspension was maintained at 8.0 by addition of NH ₄ OH solution for 2 hours. The suspension was cooled to below 40°C. The suspended phase, now mainly PHA, was washed by diluting with water and centrifugation three times. The final washed polymer suspension (latex B) had a suspended solids content of 200 g/1. The purity of the individual polymer particles, approximately 1 micron in size and 30% crystalline was typically 96% pure HBHV. The typical composition of the impurities was peptidoglycan 1%; residual protein 2%; and

lipid impurities (methanol soluble 1%) . Film Coating

Regenerated cellulose film 25μ thick sold under the tradename "Cellophane" was clamped in the sample holder so that it was taut and ripple free. A Mayer bar was placed at the top of the film and the coating material was applied by a Pasteur Pipette to the film in front of the bar. The bar was then drawn down and over the film thus coating it with the latex. The coated film in the sample holder was then placed in an oven at 80°C for 30 seconds to evaporate off the water and form a continuous coating onto the film. The PHA layer was now typically 48% crystalline. Results

Water vapour transmission rate was measured on 3 replicates of film per sample. Measurements were made via the 'wax-cup' method and used temperate conditions i.e. 25°C/75% Relative Humidity. Results were expressed as g/m ² /day.

Coating Coat Weight g/m ² WVTR g/m ² /day

None - 735

Latex A 2.8 152,337,251 Av 210

Latex B 2.1 142,146,152 Av 147

Heat Sealability

2 Pieces of the film were held together with the coated surfaces touching and placed in between the jaws of the heat sealer. The jaws were brought down and the sample was heated for 10 s then allowed to cool for a further 10 s. The jaws were opened and the sample removed.

On inspection the cellulose film had been heat sealed successfully.

93TJL17s MS - 23 November 1994