WO1991004286A1

WO1991004286A1 - Improvements in or relating to the production of plastics material incorporating starch

Info

Publication number: WO1991004286A1
Application number: PCT/GB1990/001402
Authority: WO
Inventors: Gerald Joseph Louis Griffin
Original assignee: Epron Industries Limited
Priority date: 1989-09-12
Filing date: 1990-09-11
Publication date: 1991-04-04
Also published as: AU6404890A; EP0491799A1; GB8920626D0; CA2066437A1; IL95670A0; JPH05503541A

Abstract

A filler for plastics material comprises natural starch which, before admixture with synthetic resin, is treated in a solution of calcium hydroxide or other alkali-earth metal hydroxide or of a metal hydroxide coordination complex having an alkaline character and is subsequently washed and dried before mixing with the molten resin. In a method utilising the invention the resin comprises a polymer or copolymer which affords carboxylic acid groups. A strong bonding of the resin with the starch particles results which improves the strength of the filled plastics, as compared with the corresponding plastics having an untreated starch filler.

Description

Title: "Improvements in or relating to the production of plastics material incorporating starch"

THIS INVENTION relates to the production of plastics material incorporating starch.

The incorporation of natural starch as a particulate filler in thermoplastic formulations is known to afford various benefits such as enhanced degradability, which reduces the environmental problems presented by plastics products, such as packaging when no longer required, or such as the reduction of product costs, (natural starch is cheaper than synthetic polymers) and the conservation of non-renewable resources (i.e. the synthetic polymers for which the starch is substituted).

The particle sizes of natural starch grains, however, tend to be larger than the particle sizes that would be considered optimal for fillers destined for use in thermo¬ plastics compositions and, in consequence, the mechanical properties of plastics products incorporating starch as a filler will be less favourable than those of plastics products incorporating other fillers, for example will have lower strength, unless some action is taken to remedy the effect.

It has been demonstrated that treating the surface of the starch particles chemically in a way that makes them hydrophobic does regain a proportion of the lost strength but the cost of this treatment, usually involving reactive silicone derivatives, is high and largely negates the cost advantage obtained by using starch as a filler. It has also been shown possible to recover some of the strength loss by incorporating tough polymers as alloying constituents but this also is a costly addition.

The inventor has observed experimentally that it is possible to establish chemical reactions directly between matrix polymers and the starch particle surfaces in the presence of strong alkali such as sodium hydroxide provided that the polymer itself has reactive structures such as carboxylic acid groups in its molecules and it is reasonable to presume that this is because the mildly acidic nature of the hydrogen atoms in the hydroxy groups pendant from the starch molecules is encouraged by the strong alkali to react and form ester linkages with the carboxylic acid groups in the matrix polymers. The inventor has ascertained that this process is effective with crosslinkable polyester resins, as a special case, but it would not be practicable to apply this process to the common polymers used, for example, by the packaging industry because these common polymers are not normally acidic and because the use of strong alkalis such as sodium hydroxide is very troublesome in the treatment of natural starches because of the great tendency of these materials to gelatinise ,at high pH values. The inventor has now discovered that a useful improvement in the bonding between the surface of natural starch particles and a thermoplastics polymer matrix can be achieved by forming a chemical bridge between the starch hydroxy groups and an intermediate polymer phase by using polyvalent metal hydroxides, in particular the hydroxides of the alkaline earth metals which combine multiple valency with a degree of water solubility. The inventor has observed that natural starch particles can take up significant quantities of basic metal hydroxides from aqueous solution and . that the resultant particles, which are apparently unchanged physically, can interact with carboxylic acid groups in polymers with the metal atoms remaining in position. It is reasonable to presume on the basis of classical chemistry that in the first stage, in which the starch particles are treated in the metal hydroxide solution, the metal ions form an alkoxide linking with the starch hydroxy groups whilst in the second stage carboxylate salt molecules are formed linking the polymer molecules with the starch particles.

It will be appreciated, however, that the invention is not to be regarded as bound to any particular theory as to the mechanism involved.

According to the invention, there is provided a method of forming a plastics material comprising a thermoplastic resin and a filler in the form of natural starch, wherein the starch, before admixture with the resin, is treated in an aqueous solution of an alkali-earth metal hydroxide, and is subsequently washed and dried before mixing with the molten thermoplastics resin, and wherein the resin comprises a polymer or co-polymer which affords carboxylic acid groups.

The necessary carboxylic acid groupings can be an integral part of the matrix polymer as introduced by copolymerisation, for example as in a copolymer of styrene and maleic anhydride, or more conveniently can be provided by making use of a minor addition of a carboxylic acid rich polymer such as polyethylene acrylic acid copolymer to an inert polymer typified by polyethylene. The poor adhesion that normally prevails between the highly polar untreated natural starch surface and the low polarity waxy surface of polyolefins is sharply improved by the creation of a layer of an acidic polymer such as polyethylene acrylic acid chemically bonded to the starch particle surface because the two polymers (polyolefins and acidic polymer) are mutually compatible.

Whilst the applicants first utilised, as the metal hydroxide, the obvious chemical candidate, calcium hydroxide, and obtained good results using calcium hydroxide, the chemistry has been confirmed by showing experimentally that exactly the same benefits can be achieved by using the dibasic hydroxides of the other alkaline earth metals from group two of the periodic classification, strontium hydroxide and barium hydroxide. It is likely that the same benefits can also be obtained by using any inorganic coordination complex with at least two hydroxy groups possessing alkaline character attached to the central metallic atom and which has a sufficient level of water solubility to enable the initial reaction with the starch hydroxy groups to reach equilibrium in a time period compatible with commercial manufacturing requirements.

Embodiments of the invention are described below, in the following examples.

Example I

A sufficient quantity of normal commercial maize starch was freed from surface metallic traces by a pre¬ liminary wash with dilute hydrochloric acid as a pre¬ cautionary measure to make sure that traces of monovalent sodium or potassium ions possibly remaining from its original manufacturing operations were removed. After careful washing to remove acid the starch was recovered by centrifugation and individual portions resuspended in solutions of calcium, strontium and barium hydroxides respectively. The concentrations and amounts of the hydroxide solution were controlled in such a way that excess metal ions could be detected in each case. The surplus metal hydroxide liquors were removed by centrifugation, the treated starches washed on the centrifuge with deionised water until no significant amounts of metal ions could be detected in the washings, and the treated starches were recovered by spray drying the starches recovered from the centrifuge as a slurry in further deionised water. One portion of starch was treated in exactly the same way but with the metallic hydroxide solution replaced by deionised water so as to provide a control sample. All of these starch samples were vacuum dried at 80°C until the moisture content was between 0.5 and 1$ by weight. The dry starches were then compounded as masterbatches at 50% weight con¬ centration in low density polyethylene of melt flow index 2 to which had been added 10% of a polyethylene acrylic acid copolymer sold by Exxon Chemicals Ltd. as "Escor 5100" having a melt flow index of 8 and an acid number of 70 mgs. of K0H per gramme. These masterbatches, made on a laboratory 2-roll mill, were granulated and blended with more low density polyethylene of melt flow index 2 at such proportion as to produce a final product containing 6% ww of starch and 1.2% of ethylene acrylic acid copolymer. The final product was a film of 50 micrometre thickness made by extrusion film blowing and using identical conditions of speed and temperature in each case. The films thus extruded were subjected to tensile testing following BS 2782 and particular note taken of the percentage elongation at break because this is a physical property known to be especially sensitive to reduction by the presence of poorly bonded particulate fillers. Figure 1 is a bar chart showing the elongation at break 'for the products incorporating respectively the starch treated by deionised water (identified as "Blank" in the bar chart) and the starch treated with calcium, strontium and barium hydroxide solutions. All tests were based on at least five samples.

From Figure 1 , it can be clearly seen that the low elongation quality caused in the blank by the presence of 6% of starch with no polyvalent metallic hydroxide treatment is greatly improved by the calcium hydroxide treatment and almost equal benefits are achieved by the use of strontium and barium hydroxides. The elongation at break value for the calcium hydroxide treated starch sample (358%) was effectively restored to equal the elongation at break value for the polyethylene film without additives of any kind (350%).

EXAMPLE II

Using maize starch treated to saturation with calcium hydroxide following the procedure as described in Example 1 above, a series of masterbatches were prepared all contain¬ ing 50% of the treated starch but progressively larger quantities of the ethylene acrylic acid copolymer "Escor EX 248" from 1% minimum to 25% maximum. When granulated and then diluted with further low density polyethylene of melt flow index 2 so as to ensure an identical starch content of 6%ww in the final film samples, the final mixes contained 0.00%, 0.12%, 0.60%, 1.20%, 1.80% and 3-00% of ethylene acrylic acid copolymer. Quantities of 50 micrometer thick extrusion blown film were produced from the six final mixes and samples of such film were tested for elongation at break. Taking the elongation at break values obtained by tensile testing of the film samples blown from these mixes and plotting these valves against the actual copolymer addition levels we can see, in Figure 2, that there is a clear optimum amount of acidic copolymer needed to produce the best value of elongation at break. Since we have advanced the explanation that the physical benefits observed arise from the simple chemical reaction of salt formation then it would be expected that from the stoichiometry of the reaction that best results would be obtained with equivalent amounts of acidic and basic components present and that a progressive deterioration in mechanical properties would result from progressive increases in the proportion of acid copolymer beyond this point. An excess of the acrylic acid copolymer could be expected to have a negative influence because of its low molecular weight.

Whilst, in the examples above, the starch content of the final product is 6% by weight, this value has been selected for convenience and the proportion of starch may be varied over a wide range depending upon the desired characteristics of the final product, or economic con¬ siderations. Thus, the starch content may be any value within the range providing 1 to 60% by volume. (At starch contents below 1% the advantages of using the starch filler become neglibible, whilst at starch contents above 60% by volume the strength of the product becomes reduced to an extent which is likely to be unacceptable for most purposes) .

It will also be appreciated that departures from the method described in the examples above are possible. Thus, for example, the acidic polymer or copolymer might, instead of being incorporated in the masterbatch, be introduced in blending the final product.

Claims

1. A method of forming a plastics material comprising a thermoplastic resin and a filler in the form of natural starch, wherein the starch, before admixture with the resin, is treated in a solution of an alkali-earth metal hydroxide in water or another ionising solvent, and is subsequently washed and dried before mixing with the molten thermo¬ plastics resin, and wherein the resin comprises a polymer or co-polymer which affords carboxylic acid groups.

2. A method of forming a plastics material comprising a thermoplastic resin and a filler in the form of natural starch, wherein the starch, before admixture with the resin, is treated in a solution, in water or another ionising solvent, of a metal hydroxide coordination complex having an alkaline character with at least two hydroxy groups attached to the metallic atom, and is subsequently washed and dried before mixing with the molten thermoplastics resin, and wherein the resin comprises a polymer or co-polymer which affords carboxylic acid groups.

3. A method according to claim 1 or claim 2 wherein the treated starch is first mixed with a carboxylic-acid-rich co-polymer and the resulting mixture is subsequently mixed with a polyolefine, the proportions of the constituents being such that the co-polymer forms a minor proportion of the resin content of the final product.

4. A method according to claim 3 wherein the co-polymer is an ethylene acrylic acid co-polymer and the polyolefine is polyethylene and wherein the final product comprises 6% by weight starch filler and 1.8% by weight of ethylene acrylic acid copolymer, with the remainder comprising polyethylene.

5. A plastics material made by the method of any preceding claim.