WO1999035184A1

WO1999035184A1 - Polymer composition

Info

Publication number: WO1999035184A1
Application number: PCT/NL1998/000463
Authority: WO
Inventors: John Wilhelm Geus
Original assignee: Universiteit Utrecht; U-Cat B.V.
Priority date: 1998-01-08
Filing date: 1998-08-14
Publication date: 1999-07-15
Also published as: AU8753298A

Abstract

The invention relates to a polymer composition stabilized against discoloring and against the release of corrosive acid compounds, comprising at least one thermoplastic polymer having homogeneously dispersed therein an inorganic solid substance of alkaline reaction, consisting of elementary particles of a particle size of less than 0.5 νm, and to the use of such a solid substance in polymer compositions.

Description

Title: Polymer composition

This invention relates to the use of finely divided, solid, inorganic compounds of basic reaction, in polymers containing components of acid reaction, such as halogens. More particularly, this invention relates to the stabilization of polymers or synthetic resins against discoloring by the presence of such acid components, and against the occurrence of corrosion thereby caused, respectively. In addition, _ this invention relates to sheetlike inorganic materials serving as supports for polymerization catalysts.

Polymers in which the problems of corrosion and/or discoloring occur are inter alia polymers prepared by catalytic polymerization using Ziegler-Natta catalysts. Such catalysts usually consist of a chloride of titanium or vanadium, which is treated with an aluminum alkyl. It is technically cumbersome to remove these catalysts completely, or substantially completely, from the polymers. Hydrolysis of the chlorides leads to the formation of hydrogen chloride. In the processing of the polymers, hydrogen chloride is released from the polymer, which leads to corrosion of the processing equipment .

Another group of polymers in which the problems mentioned can occur is formed by the polymers based on vinyl chloride. In these polymers, too, (for instance through reaction with water) , hydrogen chloride may be released. The invention also covers polymers prepared with acid catalysts, such as polymers or synthetic resins prepared from epichlorohydrin, as well as polymers prepared using Friedel- Crafts catalysts. A last group is formed by polymers manufactured through suspension polymerization, such as polymers based on styrene, such as polystyrene, styrene-acrylonitrile polymers and acrylonitrile-butadiene-styrene (ABS) polymers. Here, sometimes use is made of alkaline catalysts, such as calcium phosphate, magnesium hydroxide or calcium carbonate. These catalysts are removed from the polymer by a treatment with sulfuric acid or hydrochloric acid. Generally, a slight amount of acid then remains behind in the polymer. Over the last decades, the use of polymers has increased enormously. In prolonged use of the polymers, the durability of polymers appears to be an increasingly important factor. Accordingly, there is a great need for compounds and methods for stabilizing such polymers.

According to the prior art, in Japanese laid-open patent application No. 3541/1958, it has been proposed to include in polymer a basic, solid compound, such as, for instance, an oxide or hydroxide of an alkaline earth metal. In Japanese laid-open patent application No. 3947/1974, it is proposed to disperse aluminum oxide or aluminum hydroxide in a polymer. In this publication, hydrotalcites are also mentioned. At present, in practice, especially hydrotalcites are used, to prevent the release of halogen hydrogen compounds .

The mineral hydrotalcite is a mixed magnesium-aluminum hydroxide/carbonate of the general formula

Mgι-._XA1_X (OH) ₂+χ.mH₂0. The compounds considered to fall under the hydrotalcites have a structure, the hydrotalcite structure, built up from layers of metal ions with hydroxyl groups on both sides; the layers are mutually bonded by hydrogen bridges. By the substitution of trivalent ions for bivalent ions, there is a positive charge of the layers, which is compensated by incorporation of anions between the hydroxyl groups of the layers. In the hydrotalcite, carbonate ions are incorporated. However, other anions can also be properly included in a hydrotalcite structure. Compounds having a hydrotalcite structure comprise a series of bivalent and trivalent metal ions. The preparation of hydrotalcites is discussed in US-A 3,539,306 and in GB-A 1 348 702. In the latter publication, the preparation of a whole series of hydrotalcites with different metal ions is described. The preparation of hydrotalcites with two different bivalent metal ions is mentioned in DE-C 2061 114.

In Japanese patent 37487/1977, it is proposed to improve the (surface) properties of polymers by dispersing hydrotalcites or metal hydroxides in the polymer. Further details on the compounds to be dispersed are not formulated. In Japanese laid-open patent application No. 49258/1977, mention is made of stabilization of a polyolefin prepared according to a Ziegler-Natta polymerization by dispersion of at least 0.01% by weight, and preferably 0.1 to 1.0% by weight, of an inorganic solid substance of the general formula Mg_xAl_y (OH) ₂χ_+3y-._2z (A) _z. aH₂0, wherein M is Mg, Ca or Zn, A is C0₃ or HP0₄, and x, y, z, and a are positive numbers. In this case, too, no specific requirements are imposed on the hydrotalcite to be dispersed.

The surface of solid particles of basic reaction will generally react faster with halogen hydrogen than the interior of the solid particles. In view of this, it seems attractive to use solid particles that are as small as possible. In a conglomerate of small solid particles, however, pores of the same dimensions as the particles occur. Because the surfaces of virtually all substances of alkaline reaction are hydrophilic, water will condense in the pores and on the surface upon exposure to atmospheric air. ater- covered inorganic solid particles exhibit a slight interaction with polymers, which are generally hydrophobic. As a result, it is not quite possible to properly disperse the inorganic solid particles in the polymer; the small elementary inorganic particles will adhere more strongly to each other than to the polymer. Further, the water that is present in the conglomerate of small solid particles will evaporate upon a thermal treatment. This gives rise to bubbles in the polymer when shaping the polymer.

In view of these problems, small solid particles are not straightforwardly applicable for dispersion in polymers. Japanese patent specification No. 80447/1980 for that reason imposes requirements on the hydrotalcite to be dispersed. The size of the particles must be inversely proportionate to the BET surface area of the powder. Therefore the BET surface area is used to specify the mean particle size. Thus, in the above-mentioned Japanese patent specification No. 80447/1980, it is stated that the BET surface area should not be less than 30 m² per gram. Also the broadening of the X-ray diffraction maxima is used as a measure of the mean particle size. In the patent, it is mentioned that the dimension of the elementary crystallites in the <003> direction should be at least 60 nm. The elementary particles occur as porous conglomerates; obviously, the dimensions of these conglomerates are of great importance for the dispersibility. In this connection, the patent refers to second-order particle sizes. The patent mentions that these conglomerates may not be greater than 5 μm.

In EP-B 0 189 899 it is proposed to use hydrotalcites with two different bivalent metal ions. At least one of the metal ions is selected from Mg, Ca, Sr, and Ba, and at least one from the group of Zn, Cd, Pb, and Sn. At present, however, because of the health risks and the danger of environmental pollution, incorporating Cd and Pb compounds in particular, into polymers is considered undesirable.

The trivalent metal ion in the hydrotalcite structure is preferably aluminum. In this case, too, requirements are imposed on the dimensions of the elementary particles and on the (porous) conglomerates of the elementary particles. The BET surface area should not be more than 30 m² per gram and the size of the conglomerates is not more than about 5 μm, preferably not more than 2 μm and more preferably not more than 1 μm. EP-B 0 189 899 gives a very extensive list of additives that lead to a higher stability against discoloring and increase stability at higher temperatures. Mentioned, among others, are beta-diketones . In EP-B 0 256 872 polyvinyl chloride polymers are discussed, to which, for the purpose of stabilization, a hydrotalcite, magnesium oxide and a beta-diketone or a zinc salt of an organic acid has been added. The BET surface area of the hydrotalcite is less than about 50 m² per gram and in particular less than 20 m² per gram. The dimensions of the porous conglomerates are less than 5 μm and preferably less than 1 μm. The magnesium oxide is added to bind the water and the carbon dioxide that is released upon reaction of acid compounds with the hydrotalcite. The binding of water and carbon dioxide prevents the formation of bubbles in the polymer.

EP-B 0 301 509 mentions the use of dispersed hydrotalcites for preventing adherence of polymer foils. In this case, the hydrotalcite must be present as porous conglomerates which approximate the spherical shape as closely as possible. The size of these conglomerates is 1 to 8 μm, preferably 2 to 6 μm, and more preferably 3 to 6 μm.

Finally, in US-A 5,106,898, it is proposed to stabilize halogen containing polymers by using a melt of lubricant, such as hydrogenated castor oil, in which a hydrotalcite suspended in the polymer is dispersed.

EP-A 0 052 331 relates to dispersing a mixed hydroxide of magnesium and aluminum in polymers to avoid discoloring and corrosion in the processing. The BET surface area of the hydroxide should not be greater than 40 m² per gram and the mean size of the conglomerates should not exceed about 5 μm. Because hydrotalcites generally consist of smaller elementary particles, a hydrothermal treatment is employed. This increases the size of the elementary particles without loss of the structure through desorption of structurally bound water. However, such a hydrothermal treatment is costly. On the surface of these hydroxides, an alkali metal salt of a fatty acid is provided to improve the interaction with the polymer. Finally, a long series of conventional additives are mentioned, which can be added to the inorganic powder, such as beta-diketones .

It is a first object of the invention to provide a system for stabilization of polymers, more particularly of vinyl chloride polymers, in which the disadvantages of the prior art, as set out in detail above, do not occur or occur to a lesser extent.

It is a further object of the invention to provide a polymer which is stabilized against discoloring by acid compounds, and which does not give rise to corrosion of processing equipment.

Further objects and effects of the invention will appear from the following definition of the invention and discussion of the advantages thereof. The invention is based on the surprising insight that an excellent stabilization of polymers against discoloring and against the release of corrosive acid compounds can be achieved by homogeneous dispersion in the polymer of an inorganic, solid substance of alkaline reaction, consisting of elementary particles which are mutually not bonded or weakly bonded. These particles should possess a size of less than 0.5 μm.

The invention accordingly relates to a stabilized polymer composition comprising at least one thermoplastic polymer having homogeneously dispersed therein elementary particles of an inorganic solid substance of alkaline reaction, of a particle size of not more than 0.5 μm.

The solid substance consists of elementary particles of a size (diameter) of less than 0.5 μm, preferably less than 0.2 μm, and more preferably less than 0.1 μm. In this connection, 'homogeneous' is understood to mean that the number of inorganic particles per mm³ in the polymer differs by not more than a factor of three, and preferably by not more than a factor of two. The elementary particles should not be agglomerated to such an extent that units greater than 0.5 μm are formed. In the case where the particles do exhibit some agglomeration, the cohesive forces should be so slight that during dispersal in the polymer a break-up of the agglomerate occurs.

The amount of solid particles of alkaline reaction which is preferably included in the polymer varies within wide limits. This amount is especially determined by the extent of stabilization that is desired and the amounts, of acid components that may be released. In general, this amount will vary from 0.05 to 10% by weight, based on the weight of the polymer.

In a number of polymers, it may be of importance that the polymer is transparent. When admixing solid substances, this should be taken into account. If particles are used whose dimensions are less than the wavelength of the light, little scatter of the light occurs, so that transparency remains high. In that connection, it may therefore be useful to use sheetlike particles of a thickness less than the wavelength of the light, while the lateral dimensions can be greater. According to the invention, various types of polymers and polymer mixtures can be used in the polymer composition. Contemplated in view of the objective of the invention are especially polymers and polymer mixtures in which components can be formed which, owing to their acid character, can give rise to discoloring and/or corrosion. Examples include the types of polymer mentioned in the introduction, of which especially the vinyl chloride polymers, styrene polymers and the Ziegler-Natta polymers (polyethylene, polypropylene and variations thereof) are commercially important. As solid substance of alkaline reaction, alkaline metal oxides, carbonates, hydroxides or hydrotalcites can be used. It is surprising that with such small particles, whose BET surface area can be greater than 50 m² per gram, good results can be achieved, while the present state of the art indicates that the dimensions of the particles must not be selected to be too small. According to the prior art, hydrotalcites or hydroxides (which may or may not be of two different metal ions) are used with a BET surface area of less than 30 m² per gram.

The inorganic powders according to our invention, by contrast, have greater BET surface areas. Without limiting the import of the invention, the good action of inorganic powders for the -invention can be attributed to the fact that the elementary particles are not bound to each other and therefore do not include narrow pores. As a result, capillary condensation of water does not occur. Because the elementary particles are not intimately bonded to each other, it is very well possible to properly disperse the particles in the polymer.

For the preparation of mutually isolate, or just weakly interconnected, elementary particles of solid substance, use can be made of known methods .

A first eligible method for this purpose is the so-called flame hydrolysis. In that case, a suitable volatile chloride is introduced into a hydrogen-oxygen flame, whereby the chloride reacts with water. A suitable example is aluminum chloride, which reacts according to

2A1C1₃ + 3H₂0 => A1₂0₃ + 6HC1

The extremely finely divided aluminum oxide formed is separated with an electrostatic precipitator . With the inorganic powders according to the invention, it is of importance to prevent condensation of water vapor on the powder. This can be done by operating the precipitator at elevated temperature and cooling it after ending the production in an anhydrous gas stream.

It is also possible to decompose metallo-organic compounds in the gas phase or to convert them to metal oxide by oxidation. A long known method is one whereby volatile organometallic compounds, such as alcoholates, are allowed to decompose in a gas stream. In this case, too, condensation of water on the particles is to be avoided.

Another method for preparing finely divided inorganic powders according to the invention proceeds via so-called aerogels. In that case, the precipitation of the desired compound is carried out such that eventually a suspension of the compound in a non-aqueous liquid, such as ethyl alcohol, is obtained. This can be done by performing the precipitation in alcohol, for instance by hydrolysis with the stoichiometric amount of water of an alcoholate of the desired metal. It is also possible to perform the precipitation in aqueous medium and then to replace the water with alcohol. Under pressure, the suspension is now heated to a temperature above the critical point of the liquid, whereafter the supercritical phase is allowed to escape from the autoclave. Owing to the absence of a meniscus, the solid particles are not pressed onto each other, so that mutually isolate particles are obtained. In this method, water cannot be used, because under the hydrothermal conditions emerging upon heating of water to the critical temperature, the small solid particles will generally react to form (very) large crystals. Upon exposure to water vapor, the very thin structure of the material is rapidly lost. Exposure to water vapor must therefore be avoided, or a suitable compound must be priorly adsorbed onto the surface.

Another method for the preparation of inorganic powders starts from oxalates. At elevated temperature, metal oxalates first lose water of crystallization, whereupon decomposition into oxides or metals occurs. With base metals, such as magnesium or zinc, decomposition proceeds to oxides, whereby carbon monoxide is formed. In more noble metals, such as nickel or copper, the metal and carbon dioxide are formed. In the latter case, the decomposition will have to be performed in a stream of an oxygen-containing gas, to prepare the desired oxides. The oxalates decompose into extremely small particles, which exhibit a large BET surface area. The elementary particles, however, will exhibit some coherence. The conglomerates can be easily disintegrated by mechanical operations, such as grinding. Obviously, this will have to be done in a very dry atmosphere (air) to prevent hydration and strong conglomeration of the elementary particles. The oxalates can also be decomposed in an ascending gas stream. The particles will start to move and the conglomerates will disintegrate as a result of the mutual friction. In a part of the apparatus, the gas stream is set such that particles of the desired dimensions are entrained by the gas stream and subsequently separated.

According to a preferred embodiment of a method for preparing particles which can be utilized in the invention, oxalates of the desired metals, or an aqueous suspension of the oxalates, are charged to a fluidized bed of wear-resistant particles, which is maintained at a temperature above the decomposition temperature of the oxalate. Oxalate particles or oxide particles initially deposited on the wear-resistant particles will, owing to wear and by the gas stream, be transported out of the fluidized bed as extremely small particles. Subsequently, the particles are separated with a cyclone and/or an electrostatic precipitator.

It has been found that the use of finely divided hydrotalcites of a water content of less than 2% by weight has advantages.

According to a preferred embodiment of the invention, finely divided hydrotalcites of a water content of less than 2% by weight are used. The water content is calculated from the following stoichiometric composition of the hydrotalcite, the magnesium-aluminum hydrotalcite being used as an example, Mg₆Al₃ (C0₃) ι,₅ (OH) ι₈nH₂0. After drying at room temperature, n=6, while after drying at 180 to 220°C n=0 is measured. The water content of the hydrotalcite is calculated from the value of n. The hydrotalcite particles can be prepared by precipitating hydrotalcites by mixing a solution of a carbonate of an alkali metal with a solution of nitrates or chlorides of the bivalent and trivalent ions of which it is desired to prepare the hydrotalcite. Preferably, sodium carbonate is used as carbonate of an alkali metal, and magnesium nitrate and aluminum nitrate as bivalent and trivalent ions. An advantage of this combination is that such a hydrotalcite does not contain any heavy metal ions. After filtering off and thorough washing out, the hydrotalcites are dried at a temperature between 180 and 220°C. Surprisingly, it has been found that very thorough washing-out of the precipitate leads to a considerable improvement of the dispersibility of the hydrotalcite. It has been found that after washing out two or more times, the specific surface increases strongly, up to as much as twice the surface after filtering off, while the pore volume also increases appreciably. It has been found that a hydrotalcite, immediately after filtering off, has a surface area of 33 m²/g and a pore volume of 0.32 ml/g, while after washing out three times, these values are 71 and 0.57. It has further been found that such materials, optionally after rendering them hydrophobic, permit of particularly good dispersal in plastics. Rendering the materials hydrophobic can occur, for instance, in the manner as described hereinbelow.

A characteristic property of properly washed-out materials is that they exhibit only a small number of peaks in the X-ray diffraction pattern. The above-described material, washed out three times, exhibits, for instance, only nine peaks, viz. at the d-values: 7.577, 3.792, 2.592, 2.573, 2.550, 2.524, 2.313, 1.521 and 1.492. To (greatly) reduce the permeability of plastics to, for instance, water vapor, it is known to include sheetlike inorganic solid substances in the polymers. In addition to reducing permeability, inorganic materials consisting of very thin sheets have the property that admixture does not impair the transparency of plastics and hence the ability to provide the plastics with brilliant colors. It is therefore attractive to use, as filler, hydrotalcites in the form of thin sheets, the lateral dimensions of the sheets being much greater than the thickness of the sheets. According to the invention, for this purpose, sheetlike hydrotalcites are used whose BET surface area is of the same order of magnitude as that of the above-mentioned conventionally prepared hydrotalcites, viz. about 70 m² per gram. However, the elementary particles of the hydrotalcites are thin sheets of a thickness of less than 5 nm and lateral dimensions of 0.1 to 3 μm. Such sheetlike hydrotalcites are characterized by the so-called t-plot, which can be calculated from the physical adsorption of nitrogen. For the background of the measurement and the calculation of the t-plot, we refer to B.C. ippens, B.G. Linsen and J.H. de Boer, J. Catal . 3 (1964) 32. The slope of the t-plot, which is proportional to the exposed surface, of the hydrotalcite sheets according to this particular embodiment of the invention decreases strongly with greater thickness of the adsorbed layer of nitrogen molecules. This is caused by the fact that between the sheets only a limited number of layers of adsorbed nitrogen molecules can be received. If the adsorbed layer thickens, adsorption occurs only on the obviously smaller surface around conglomerates of sheets. Such relatively well crystallized hydrotalcites are prepared by precipitation from homogeneous solution. In doing so, the pH of an acid solution of magnesium ions and aluminum ions is homogeneously increased. Technically, this can be eminently done by adding urea to the solution and maintaining the solution at approximately 90°C. By hydrolysis of urea, ammonium ions and carbonate ions are then formed, so that the pH increases homogeneously and the hydrotalcite precipitates. Because the seed formation in precipitation from homogeneous solution is strongly suppressed, a limited number of seeds grow into relatively large sheets. According to a particular embodiment of the invention, such hydrotalcite sheets are used as support for olefin polymerization catalysts. In this case, for instance titanium trichloride or a suitable metallocene is applied to the hydrotalcite sheets. If the polymerization is then carried out with such a catalyst, the elementary sheets are very well distributed through the polymer upon completion of the polymerization reaction.

It is known to use clay minerals in polymers. This is primarily done to reduce the permeability of the polymer to molecules such as water. In addition, uniformly admixed clay minerals can increase the mechanical strength of the polymer composition. So far, clay minerals have not been mentioned for the neutralization of hydrogen chloride. The provision of fillers for improving the above two properties, permeability and mechanical strength, is an additional objective of the present invention, in addition to the objective of incorporating hydrogen chloride released. It is known to expand the intermediate layer of natural clay minerals by reaction with quaternary ammonium ions with long hydrocarbon groups or with inorganic colloidal groups. Such materials are described in EP-A 0 747 322 and in WO-A 97/31057. With the clay minerals described in the two publications, special measures must be taken to allow the polymer to penetrate the intermediate layers, the space between the elementary clay sheets. It would be highly attractive if it were possible to start from clay sheets which are not stacked, allowing the water to be removed completely from the intermediate layers, without the clay structfure being lost.

Therefore, according to another preferred embodiment of the method according to the invention, synthetic clay minerals are used, prepared by reaction of bivalent metal ions with silicon dioxide at a pH of 5 to 9. The preparation of such clay minerals is described in detail in WO-A 9607613. Surprisingly, it has been found that the stacking of the elementary clay sheets in such synthetic clay minerals can be eminently controlled. Now, according to the present invention, the preparation of the clay minerals is carried out such that the elementary clay sheets are hardly stacked, if at all, and assume a so-called house-of-cards structure. If acid groups are desired on the surface of the clay sheets, the precipitation can be performed using urea or by addition of ammonium hydroxide. In that case, ammonium ions neutralize the excess of negative charge on the clay sheets. Upon thermal treatment at temperatures above approximately 400°C, the NH₄ ⁺ ions decompose to form NH₃, while protons remain behind on the surface. In general, however, the number of acid groups on the surface of the elementary clay sheets is relatively small. Upon contact with strong acids, the metal ions present in the octahedron layer react with the acid, so that neutralization occurs. When it is desired to raise the reaction rate of clay minerals, it is attractive to have alkali metal ions or alkali earth metal ions in the intermediate layer positions. Then a reaction to the corresponding chloride occurs, while acid positions remain behind.

With synthetic clay minerals as described in WO-A 9607613, the water can be completely removed from the intermediate layer by heating, without the accessible surface of the clay mineral becoming appreciably lower. In clay minerals with magnesium ions in the octahedron layer (hectorites) , the elementary clay sheets are very small and hardly any stacking occurs. These clay minerals can be heated to a temperature of 600°C without any appreciable loss of accessible surface. With zinc as octahedron ion, the elementary sheets are greater. Such clay minerals already exhibit a lower accessible surface upon heating at 500°C. Clay minerals with cobalt ions or nickel ions in the octahedron layer have an accessible surface and thermal stability in-between those of magnesium- and zinc-containing clay minerals. When incorporating different ions, for instance zinc and magnesium, into the octahedron layer, intermediary dimensions of the elementary clay sheets can be set . Synthetic clay minerals are hydrophilic, as are natural clay minerals. For the purpose of incorporation into polymers, it is advantageous to render the surface of the elementary sheets of the clay minerals hydrophobic. According to the invention, this occurs by treating the clay minerals with organic compounds capable of entering into a hydrogen bridge bond with the surface of the clay sheets. Such organic chemical compounds are known; preferably, according to the invention, compounds having one or more hydroxyl groups or primary amines are used. Other suitable compounds are mentioned, for instance, in EP-A 0 747 322. With clay sheets of slight lateral dimensions, it is further possible to make use of the hydroxyl groups bound to the ions located on the outside of the octahedron layer. These hydroxyl groups can be reacted, for instance, with suitable silanes, as described below for the other materials according to the present invention.

The above clay materials according to the invention also lend themselves eminently for providing polymerization catalysts. After polymerization, the elementary sheets are then eminently distributed in the polymer and the properties of the polymer are optimally influenced.

If it is desired to combine the incorporation of clay materials into the polymers with a considerable capacity for incorporating hydrogen chloride, the following preferred embodiment of the material according to the invention is used. Positively charged colloidal particles or clusters of compounds of alkaline reaction are applied to the surface of the clay sheets. Owing to the negative charge of the clay sheets, application can be effected by exchange. Previously, in the patent application concerning the synthetic clay minerals, WO-A 9607613, a method has been described to prepare Alι₃ clusters with a high concentration and subsequently to apply them to the clay sheets. Surprisingly, it has been found that clay minerals with non-stacked sheets on which Alι₃ complexes have been provided by exchange are eminently suitable to improve the mechanical properties of polymers, to reduce the permeability to foreign molecules and to react with hydrogen chloride being released.

In a preferred embodiment of the invention, elementary particles are used whose surface is covered with an organic compound which prevents the adsorption of water vapor and which is compatible with the (often hydrophobic) polymers. Examples include a hydrogenated fatty acid triglyceride, a fatty acid, an alkali metal salt of a fatty acid, a silicone compound or the titanium equivalent of a silicone compound. The organic material is preferably applied to the surface of the elementary particles under such conditions that no water vapor can condense on the surface.

According to the invention, extremely finely divided inorganic powder can be included in a non-aqueous liquid and the mixture thus obtained can be treated mechanically, whereby conglomerates of elementary particles, if any are present, are broken down. For the mechanical treatment, known methods can be used, such as colloid mills. Ultrasound treatment can also be used to great advantage.

According to the invention, the extremely finely divided inorganic powder, preferably after it has been covered with a suitable compound which counteracts the interaction with water, is combined with other compounds, such as beta-diketones, which improve the stability of the polymer against discoloring.

The invention is further explained in and by the following example.

Preparation I

Preparation of finely divided aluminum oxide

The starting material is a commercial aluminum oxide powder marketed by Degussa. It is Alon-C, aluminum oxide with a surface area of 80 to 100 m² per gram, consisting of elementary particles of approximately 50 nm. The powder which has been exposed to the air is heated under vacuum at 90 tot 100°C for one hour to remove water, if any has been absorbed.

The thus pretreated powder is introduced at approximately 50 to 70°C into hardened soybean oil or sunflower oil. The weight ratio of the fat and the oxide is approximately three. The melting point of the hardened fat is 30 to 40°C, whereafter the thus obtained mixture is intensively agitated with an UltraTurrax. Finally, the resultant mixture is applied as particles of dimensions of 1 to 2 mm in molten form to a cooled moving aluminum plate on which the material solidifies. The thus obtained solid particles are finally removed from the aluminum plate.

Preparation II

Preparation of finely divided magnesium oxide

In this second example, the' starting material is magnesium nitrate, which is allowed to react with ammonium oxalate to form insoluble magnesium oxalate. After separation of the liquid and washing of the precipitate, the solid substance is dried at 120°C.

The magnesium oxalate is charged to a reactor through which a dried air stream is passed, and the water of crystallization is removed at about 150°C. Then the temperature is raised to about 250°C, whereby the dehydrated magnesium oxalate decomposes to carbon dioxide and magnesium oxide. After cooling in the dried air stream, the solid substance is included in molten hydrogenated fat. During transfer, contact with atmospheric air is avoided to prevent absorption of water by the magnesium oxide. The weight ratio of the fat and the oxide is approximately three. The melting point of the hardened fat is 30 to 40°C, whereafter the thus obtained mixture is intensively agitated with an UltraTurrax. Finally, the resultant mixture is applied as particles of dimensions of 1 to 2 mm in molten form to a cooled moving aluminum plate on which the material solidifies. The thus obtained solid particles are finally removed from the aluminum plate.

Example I

Incorporation into polyvinyl chloride

Amounts of 0.2, 0.3 and 0.5% by weight of the above- discussed mixtures of hardened fat and aluminum oxide or magnesium oxide are mixed with polyvinyl chloride granules in a Nauta mixer. After thorough mixing, the mixture is extruded to form rectangular sheets of a thickness of about 1 mm and dimensions of 3x20 cm.

To assess the stabilizing effect, these sheets are placed in an oven maintained at 150°C. The oven can contain 10 sheets. Then the sheets are pulled from the oven at a rate of 1 cm per hour. As a result, the front portion of the sheet is exposed to the high temperature for about one hour and the rear portion for about 20 hours. Stabilization by the powder is indicated by the length of the sheets that is discolored. As a blank, a sheet without stabilizing powder is also pulled from the oven.

Both with the magnesium oxide and with the aluminum oxide, the discoloring of the polyvinyl chloride is much less. It appears that about 2% by weight already yields a sufficient improvement of the stability of the polymer.

Claims

1. A polymer composition stabilized against discoloring and against the release of corrosive acid compounds, comprising at least one thermoplastic polymer having homogeneously dispersed therein an inorganic solid substance of alkaline reaction, consisting of elementary particles of a particle size of less than, 0.5 ╬╝m.

2. A polymer composition according to claim 1, wherein the particle size is less than 0.2 ╬╝m, and preferably less than 0.1 ╬╝m.

3. A polymer composition according to claim 1 or 2, wherein .the amount of said solid substance is between 0.05 and 10% by weight.

4. A polymer composition according to claims 1-3, wherein alkaline metal oxides, carbonates, hydroxides or hydrotalcites are used.

5. A polymer composition according to claims 1-4, wherein the solid substance is prepared by flame hydrolysis of suitable volatile metal chlorides.

6. A polymer composition according to claims 1-4, wherein the solid substance is obtained by decomposing volatile organometallic compounds in a gas stream which may or may not be oxidizing, followed by separation from the gas stream.

7. A polymer composition according to claim 6, wherein the solid substance is obtained by decomposing metal oxalates, which may or may not be mixed, in a gas stream which may or may not be oxidizing.

8. A polymer composition according to claim 7, wherein the solid substance is obtained by decomposition of the metal oxalate in a fluidized bed of wear-resistant particles.

9. A polymer composition according to claims 1-4, wherein the solid substance is obtained by preparation of an aerogel of the desired compounds.

10. A polymer composition according to claims 1-9, wherein the polymer is selected from the group consisting of vinyl chloride polymers, styrene polymers, polymers obtained through Ziegler-Natta catalysts, polymers obtained through Friedel-Crafts catalysts and mixtures of one or more of these polymers with one or more other polymers.

11. A polymer composition according to claims 1-10, wherein the surface of the extremely finely divided inorganic powder is covered with an organic compound which prevents the adsorption of water vapor and which is compatible with the polymer, preferably trialkyl chloro-silane, trialkoxyalkyl silane or a silicone elastomer.

12. A polymer composition according to claims 1-11, wherein further a supplementary stabilizing compound, such as beta-diketones, is used.

13. A polymer composition according to claims 1-12, wherein clay minerals are used, of which the elementary sheets are not stacked or stacked to a slight extent only.

14. A polymer composition according to claim 13, wherein the clay sheets are coated with particles of alkaline reaction, preferably colloidal particles or clusters of ions.

15. A polymer composition according to claim 14, wherein the clay sheets are coated with clusters of ions in the form of Al╧ç ions.

16. The use of an inorganic solid substance of alkaline reaction, consisting of elementary particles of a particle size of less than 0.5 ╬╝m in homogeneously dispersed form in a polymer composition for preventing discoloring of the polymer composition or corrosion of processing equipment through the release of acid compounds.

17. A method for preparing a solid substance of alkaline reaction, wherein a substantially anhydrous solid substance of alkaline reaction, substantially under exclusion of water, is introduced into a molten fat, to form granules of the solid substance in the fat.