GB2247681A - A photo-degradable plastics material - Google Patents

Abstract

A composition formable into a film or other shaped body having controllable photo-degradation characteristics, comprises a plastics material, a photo-stabilizer for the plastics material and a particulate mineral, the photo-stabilizer and the mineral, and the amounts thereof being chosen to give a controlled period of photo-stability to the plastics material. The composition may be formed into a plastic film, such as an agricultural mulching film. The plastics material may be a polyolefin, e.g. polyethylene, polypropylene and poly-1-butene, or polystyrene. The stabilizer may be selected from UV absorbers, quenchers and hindered amine light stabilizers, and is preferably one which decomposes in the presence of the particulate mineral. The mineral is preferably CaCO3 or a natural hydrous alumino-silicate, e.g. a kaolinitic clay or bentonite, which may be chemically activated. The composition may also be formed into packaging components, e.g. bags.

Description

A PLASTICS MATERIAL This invention relates to a degradable plastics material and is more particularly, but not exclusively, concerned with the provision of a plastics film which possesses a controlled period of photo-stability which is followed by rapid degradation.

The disposal of plastics is currently an important political and ecological issue. Whilst recycling is the most preferred "disposal" option, this depends on an efficient collection and separation system which can be impractical for a large sector of the industry.

This is particularly true for polyethylene film used for bags and as film in packaging applications, for garbage bags and as agricultural mulching film. The preferred disposal route for these materials is, therefore, degradation.

There is a need for a plastics material which is rapidly degradable, but in a controlled fashion and both bio- and photo- degradation are currently being investigated. Indeed, there are several proprietory compounds already on the market. Another area of interest is agricultural mulching film which should possess a specific lifetime to allow for protection of the ground until crops reach a size sufficient to give self-mulching, with the film then being required to break-down completely before harvesting.

Work has been conducted on the use of inorganic minerals in polyethylene film to confer on the film infra red absorption properties. For instance, British Patent Specification No. 1518016 describes a polyolefin composition containing an essentially dehydrated kaolinite. The polyolefin film produced from such a composition has improved properties in terms of transmission to infra red wavelength light.

According to the present invention there is provided a composition formable into a film or other shaped body having controllable photo-degradation characteristics, said composition comprising a plastics material, a photo-stabiliser for the plastics material and a particulate mineral, the stabiliser and the mineral and the amounts thereof being chosen to give a controlled period of photo-stability to the plastics material.

According to a second aspect of the present invention, there is provided a method of providing a plastics material with a predetermined period of photostability comprising the step of combining with the plastics material a photo-stabiliser therefor-and a particulate mineral, the stabiliser and the mineral and the amounts thereof chosen to give a controlled period of photo-stability.

According to a third aspect of the present invention there is provided the use of a particulate mineral in a photo-stabilised plastics material to provide a controlled period of photo-stability to said plastics material followed by a rapid degradation of the material.

By a "controlled period of photo-stability", we mean the length of time within which the plastics material retains at least 60% of its original tensile strength before a rapid decline sets in. The controlled period of stability may be any desired length of time, for instance from as little as a few days (e.g. from 5 to 10 days) up to over a hundred days, if required.

The composition of the present invention may be formed into many different types of shaped bodies; for instance, the composition may be formed into shaped articles such as packaging components, e.g. bags, and films such as agricultural mulching films.

All plastics materials are to some extent photodegradable; particularly important in the present invention, however, are the polyolefins. Other polyolefins which are useable in the present invention are homopolymers of olefinic hydrocarbons such as lowdensity and high-density polyethylene, polypropylene and poly-l-butene, as well as copolymers prepared by copolymerising, for example, ethylene with monomers such as vinyl acetate, propylene, carbon monoxide, 1butene and maleic anhydride. Another group of plastics materials which has been found to be useful in the present invention is the polystyrenes. The present invention has been found to be particularly suited to the controlled photo-stabilisation of polyethylene film, notably low density polyethylene (ldpe) and high density polyethylene (hpde).

It is believed that the particulate mineral employed in the present invention progressively destroys the photo-stabiliser in the plastics composition thereby removing the means stabilising the plastics material. The plastics material retains its stability whilst the stabiliser is present but, on removal of a certain proportion of the stabiliser, there is a relatively sudden degradation of the plastics material. Accordingly, a period of stability can be built into a plastics material by controlling the relative quantities of the stabiliser and the particulate mineral. The precise quantities of the particulate mineral and the stabiliser necessary for a particular period of photo-stability can be determined empirically.Another factor which contributes to the degradation is polymer chain scission which occurs through interaction between the polymer chains and Lewis acid sites on the surface of the particulate mineral.

The particulate mineral is preferably a natural hydrous alumino-silicate material, more preferably a hydrous clay. Most preferably, the hydrous clay is a kaolinitic clay such as kaolin clay; it may also be a smectite clay such as bentonite. Other inorganic minerals are usable such as calcium carbonate. The particulate mineral may be chemically activated, for example, by treating it with an acid to increase the number of Lewis acid sites on its surface.

The stabiliser is one which decomposes in the presence of the particulate mineral and is particularly one which decomposes in the presence of a hydrous alumino-silicate material such as a kaolinitic clay.

The stabiliser may be an organic stabiliser, for example a W absorber such as a hydroxybenzophenone or hydroxyphenyl-benzotriazole or a "quencher", the best know of which are the organo-nickel quencher compounds such as those known by the names CYASORB UB-1084 and CHIMASSORB N705. These act not only as quenchers but also as decomposing agents of hydrogen peroxide and any scavengers. Another type of stabiliser is the hindered amine light stabiliser (HALS) which absorbs no light above 250nm and does not act either as a UV absorber or a quencher. The precise manner in which these photostabilisers operate is not known.

The stabiliser may, for example, be present in amounts up to 20%, preferably up to 10% by weight of the composition whilst the particulate mineral may typically be present in amounts up to 30%, preferably up to 15% by weight of the composition although it should be noted that the precise amounts and proportions are to be determined empirically so as to achieve the necessary degree of controlled degradation.

The particle size of the particulate mineral is important. The best results are achieved using a finely divided mineral. Preferably 60%, and more preferably 80%, of the particles of the mineral are smaller than 2 microns e.s.d.; most preferably, substantially all of the particles are smaller than 2 microns e.s.d.. Particularly suitable are the china clays marketed by ECC International Limited of Cornwall, United Kingdom known by the names of SUPREME and SPESWHITE.

The composition of the present invention may include other additives conventionally used in the manufacture of plastics film.

The invention will now be illustrated by the following Examples. In these Examples, dumb-bell shaped pieces for testing were cut from a film of thickness approximately SOum of the plastics material under consideration in accordance with the provisions of British Standard Specification BS 903: Part A2: 1971. The test pieces had the following dimensions: Overall length 75 mm Width of parallel-sided ends 12.5 + 1.0 mm Length of narrow parallel portion 25 + 1 mm Width of narrow parallel portion 4.0 + 1.0 mm Small radius 8.0 + 0.5 mm Large radius 12.5 + 1.0 mm The "small radius" is the radius of the curve outwards from the narrow parallel portion, and the "large radius" is the radius of the curve back towards the parallel-sided ends (see pages 6, 7 and 14 of BS 903: Part A2: 1971].In order to calculate the area of cross section of the narrow parallel portion of a test piece, the thickness was measured near the centre and at each end of the narrow parallel portion and the mean of the three measurements was determined.

The test pieces were conditioned for 3 hours at 20"C before testing, and the testing was carried out at a constant temperature of 20"C. The test pieces were stretched in an tensile testing machine, the ends of the dumb-bell shaped pieces being held, respectively, in a static grip and in a moving grip which was driven away from the static grip at a constant speed of 25 mm/min. The stretching was continued until the test piece broke, at which point the maximum tensile strength of the film in MPa, and the percentage increase in the length of the test piece at the breaking point was proceeded as the percentage elongation at break. Three test pieces were tested for each sample of film and the mean of the three readings for each of the tensile strength and the percentage elongation at break were recorded.

The samples of film which were subjected to ageing were placed in a QW Accelerated Weathering Tester operating on a 4 hour cycle incorporating ultraviolet radiation and condensation phases. The samples were mounted on aluminium backing plates (to increase the rate of degradation) and were removed after ageing for different lengths of time so that dumb-bell shaped test pieces could be cut from them for testing in the tensile testing machine. The ultraviolet radiation in the accelerated weathering tester was provided by UVA tubes emitting radiation in the wavelength range 315400 nm and the temperature during the radiation phase was maintained at 35"C, and during the condensation phase at 46"C.

Example 1 Granules of a low density polyethylene containing a conventional photostabiliser and a kaolin clay filler were mixed homogenously under low shear and then meltcompounded in a Baker-Perkins screw compounding extruder. The extrudate was subsequently formed into a plastics film of thickness SOum from which a control dumb-bell shaped test piece was cut before ageing.The filler in this Example was the kaolin clay produced by ECC International Limited under the trade name "SPESWHITE" which has a particle size distribution such that 0.5% by weight consisted of particles having an equivalent spherical diameter larger than lOum, 80% by weight consisted of particles having an equivalent spherical diameter smaller than 2pm and 60% by weight consisted of particles having an equivalent spherical diameter smaller than lum. The filled low density polyethylene composition comprised 90% by weight of low density polyethylene. A control film, without filler was also produced and tested.

The results obtained after weathering of the plastics film are shown in the graphs which form Figures 1 and 2. These illustrate the effect on tensile strength and elongation at break on weathering the polyethylene film. The line designated A in each graph relates to the filled, stabilised polyethylene of the invention and line B to an unfilled, but similarly stabilised polyethylene. From these graphs, the effects of the filler and W are shown. In the unfilled systems, there is progressive and virtually linear reduction in tensile strength and elongation in break measurement whilst, with the filled system, there are clear discontinuities. Visual assessment of the polyethylene materials showed that there was a qualitative difference in the nature of the cracking.

While all of the clays used gave cracks at the same time, the ultra-fine clays gave a network of very fine cracks which, when the experiment was terminated, caused the film to crumble completely. The crumbling of the film to give very small fragments is especially important when the film is used to mulch a crop such as strawberries, where the fruit is harvested by means of suction devices. If the film did not degrade into small fragments the orifices of the harvesting equipment would be frequently blocked.

Example 2 Plastic compositions were prepared comprising 90% by weight of low density polyethylene, 10% by weight of the same kaolin clay as was described in Example 1 above, and varying quantities of a hindered amine ultraviolet stabiliser of the type indicated by the formula:

In each case the plastics composition was prepared by hand mixing together 50 parts by weight of low density polyethylene pellets and 50 parts by weight of the kaolin clay, and compounding the mixture in a Baker Perkins MP2030 co-rotating, twin screw compounder heated to about 200 cm. A homogeneous polymer melt was obtained which was extruded through an annular die to form a tube which was blown into a film in a Betol 0250 film tower by means of a compressed air jet passing through the centre of the die. The film was guided from the film tower to a coiler station where it was reeled on to a core. The end product in each case was a low density polyethylene film containing 10% by weight of the kaolin clay filler and the desired quantity of stabiliser. The quantity of stabiliser was expressed as a percentage by weight based on the total weight of low density polyethylene and filler.

Samples of thickness about 5Opm were cut from each batch of film. Dumb-bell shaped test pieces were cut from one sample for measurement of the tensile strength and percentage elongation at break of the film in its unaged condition. The remaining samples were mounted on aluminium backing plates and aged in the QUV Accelerated Weathering Tester for varying lengths of times as described in Example 1 above. Dumb-bell shaped test pieces were cut from each of the aged samples for measurement of the tensile strength and percentage elongation at break of the aged film.

As a comparison, the experiment was repeated with a film prepared from a low density polyethylene composition containing no filler and no stabiliser.

The results of the tensile strength and percentage elongation at break are given in Tables I and II respectively which appear at the end of this description. In those Tables, "n.d." means not determined and "F" means failed.

The results for the tensile strength are also shown graphically in Figure 3. These results show that the length of time of exposure to ultraviolet radiation after which the film failed can be adjusted by incorporating different quantities of ultraviolet stabiliser in the composition. In was surprising that the composition containing 3% by weight of stabiliser failed before the composition containing 2% by weight of stabiliser, and no satisfactory explanation for this behaviour can be offered at present.

Example 3 Further plastics compositions were prepared by the method described in Example 2 above but containing granules of, respectively, high density polyethylene (HPDE), propylene/ethylene copolymer (PP (co)) and polystyrene (PS). Each composition was divided into two portions, there being incorporated into the first portion 10% by weight of the same kaolin clay as was described in Example 1 above, the second portion being left unfilled, No ultraviolet stabiliser was added to either composition. Each portion was formed into a film of thickness approximately 50pm as described in Example 2 above and samples of film were aged in the QW Accelerated Weathering Tester, samples being removed at intervals up to a total time of 700 hours to enable standard dumb-bell shaped test pieces to be cut and tested in the tensile testing machined.

The results are shown graphically in Figure 4 below.

It can be seen that, for each plastics composition, the film prepared from the portion containing the kaolin filler degrades more rapidly than the film prepared from the unfilled portion.

TABLE I Tensile Strength (MPa) No. of days aged 0 7 14 21 26 29 33 35 40 49 56 70 Comp.

Unfilled 24.09 18.15 20.26 19.52 16.92 14.90 15.58 12.51 15.46 17.23 n.d. 12.30 Filled 0% stab. 19.46 9.87 12.00 11.66 F -- -- -- -- -- -- - Filled 1% stab. 18.07 12.76 13.08 11.78 12.10 11.19 F -- -- -- -- -- - Filled 2% stab. 18.82 15.91 18.75 16.74 16.46 16.09 14.08 14.38 13.96 14.25 12.41 12.63 Filled 3% stab. 19.61 15.66 15.68 14.80 13.94 14.30 14.50 11.83 14.68 12.81 9.10 F TABLE II Percentage elongation at break No. of days aged 0 7 14 21 26 29 33 35 40 49 56 70 Comp.

Unfilled 455 365 338 324 173 137 92 112 72 127 n.d. 31 Filled 0% stab. 177 17 10 3 F -- -- -- -- -- -- - Filled 1% stab. 302 150 37 25 13 25 13 5 F -- -- -- -- - Filled 2% stab. 370 280 235 204 222 327 130 175 40 79 22 43 Filled 3% stab. 337 335 220 157 150 71 57 29 19 4 F

Claims (20)

1. A composition formable into a film or other shaped body having controllable photo-degradation characteristics, said composition comprising a plastics material, a photo-stabilizer for the plastics material and a particulate mineral, the photo-stabilizer and the mineral and the amounts thereof being chosen to give a controlled period of photo-stability to the plastics material.

2. A composition according to claim 1, wherein said plastics material is a polyolefin or a polystyrene.

3. A composition according to claim 1 or 2, wherein the plastics material is a low density polyethylene or a high density polyethylene.

4. A composition according to any one of claims 1 to 3, wherein the stabilizer is one which decomposes in the presence of the particulate mineral.

5. A composition according to any preceding claim, wherein the stabilizer is selected from UV absorbers, quenchers and hindered amine light stabilizers.

6. A composition according to any one of the preceding claims, wherein the stabilizer is present in an amount of up to 20% by weight of the composition.

7. A composition according to any one of the preceding claims, wherein the particulate mineral is a natural hydrous alumino-silicate material.

8. A composition according to claim 7, wherein the particulate mineral is a hydrous clay.

9. A composition according to claim 8, wherein the hydrous clay is a kaolinitic clay.

10. A composition according to claim 8, wherein the hydrous clay is a bentonite.

11. A composition according to any one of the preceding claims, wherein the particulate mineral is chemically activated.

12. A composition according to any one of the preceding claims, wherein the particulate mineral is present in an amount up to 30% by weight of the composition.

13. A composition according to any one of the preceding claims, wherein the particulate mineral is a finely divided mineral.

14. A composition according to claim 13, wherein at least 60% of the particles of the mineral are smaller than 2 microns equivalent spherical diameter.

15. A shaped body formed from a composition according to any one of the preceding claims.

16. A shaped body according to claim 15, which is a plastic film.

17. A mulching film formed from the composition as claimed in any one of claims 1 to 15.

18. A method of providing a plastics material with a predetermined period of photo-stability comprising the step of combining with the plastics material a photo-stabilizer therefor and a particulate mineral, the stabilizer and the mineral and the amounts thereof being chosen to give a controlled period of stability.

19. The use of a particulate mineral in a photostabilized plastics material to provide a controlled period of photo-stability to said plastics material followed by a rapid degradation of the material.

20. A composition formable into a film or other shaped body substantially as hereinbefore described, with reference to the accompanying drawings.