GB2168984A - A powdery filler for plastics - Google Patents

Abstract

A fire retarding filler for polyolefins with particle size less than 20 ???m is obtained by calcination of dolomites and/or dolomitic limestones and the subsequent pressure hydration and treatment with a hydrophobizing agent. The filler contains magnesium and calcium hydroxides as a mixture with calcium carbonate. Catalytical impurities from natural raw material are passivated by calcination and composite materials with this filler exhibit good mechanical properties and are nonflammable.

Description

SPECIFICATION A powdery filler for plastics This invention deals with a filler based on hydrated calcinated dolomite allowing production of polyolefins or other organic polymers with decreased flammability.

It is known that by addition of high concentrations of anorganic fillers to polyolefins it is possible to achieve an increasing modulus of elasticity together with preservation or even an increase of toughness using a filler with a suitable particle size distribution.

Presently, calcium carbonate belongs to the most important industrial fillers either in the form of microground limestone or'in the form of chemically precipitated calcium carbonate.

Composites of polyolefins with calcium carbonate achieve improved processing and mechanical properties, but even at high contents of calcium carbonate these composite materials remain flammable.

A recently developed filler acting aspire retarding agent is magnesium hydroxide. Its application makes possible to obtain self extinguishing or even inflammable polyolefins without generation of smoke and toxic products.

A disadvantage of magnesium hydroxide produced till nbw only by precipitation of magnesium salts (from seawater or solutions made by dissolving magnesium-containing minerals) is primarily its high price due to an expensive production method. Another disadvantage is seen in insufficient purity of the product.

The insufficient purity is the cause of a catalytic activity of fillers leading to a faster thermooxidative and photooxidative degradation of polyolefins. In the case of microground fillers, the catalytic activity is due to various compounds of transition metals (Cu, Mn, Fe, etc.) which are always present. In the case of chemically precipitated fillers these metals are removed by expensive procedures. Even the remaining traces are in the form of very reactive salts such as chlorides, nitrates, sulfites, sulfates, whose catalytical effects are extraordinarily high.

Another disadvantage of precipitated magnesium hydroxide is its tendency to form aggregates, thus making its mixing with plastics difficult. To achieve a good homogenization of composite material it is necessary to apply high shearing forces.

One of the methods employed to facilitate the homogenization of magnesium hydroxide with plastics is the use of a mixture of magnesium hydroxide with calcium carbonate with different ratios of components, according to Czechoslovak patent application PV 754-84. The microground calcium carbonate destroys aggregates of magnesium hydroxide microcrystals and then the mixture filler homogenizes easily with plastics. The elementary analysis of this mixture filler has a similar ratio of individual components as hydrated dolomites or dolomitic limestones with the difference that these hydrates made according to known methods contain only a small amount of carbonates.

Methods of calcinated dolomite hydration known until now are directed toward its use in magnesium cement, where the presence of Ca(OH)2 is necessary to form.the main component. Therefore magnesium oxide is mostly overcalcinated in production of dolomitic lime, because the calcination temperature is determined by processing conditions of calcium carbonate. Hydration of magnesium oxide calcinated under these conditions is difficult.

If a dolomite is calcinated at lower temperatures, e.g. 800"C, a half calcinated dolomite is produced, containing magnesium oxide together with undercomposed calcium carbonate. This is used e.g. for water treatment. According to Hungarian patent 75-13564 of 28.7.1977 the hydration of half-calcinated dolomite is facilitated by the addition of different activators (e.g. calcium chloride, triethanolamine, fatty alcohols). A favourable influence of different salts (e.g. calcium chloride) on acceleration of dolomitic lime hydration was described earlier by Kamiskas (Zh. neorg. khim, 18, 3144 (1973)).

Even if the described methods give dolomitic hydrate with a large surface and a small particle size, after drying the hydrate prepared in such a way, compact aggregates are formed which cannot be dispersed in polymer melts and which - used as fillers - influence negatively mechanical properties of organic polymers. Therefore it is necessary to achieve hydrophobization of the particles during hydration already by forming homogeneous protective coatings on the particles. A necessary condition for magnesium hydroxide which frees water due to heating to temperatures over 340"C to act as an effective fire retardant, is a perfect homogeneous dispergation of the filler particles.

The invention provides a powdery filler for plastics based on dolomite and/or dolomitic limestone containing magnesium and calcium hydroxides with calcium carbonate, its particle size being smaller than 20 lim, the surface of which has been treated treated with hydrophibizing substances.

The invention also provides a method of production of this powder' filler by calcination and hydration of dolomite and/or dolomitic limestone in which dolomites and/or dolomitic limestones are calcinated at temperatures of up to 11000C and then they are hydrated preferably by solutions of anionactive or nonionactive tensides.

The invention further provides an advantageous method of powdery filler production using amonium salts of fatty acids with 12 to 20 carbon atoms and also mixtures of fatty acids salts with esters of fatty acids with at least two free alcoholic groups as tensides.

Finally, the invention provides composite materials made from polyolefins and the powdery filler ac co.rding to this invention.

Dolomitic limestone, calcareous dolomites, dolomite and doiomitic magnesites are natural raw materials defined by content of magnesium carbonate.

The content is lowest in dolomitic limestones where the ratio of MgCO3 is at least 4.6 mass %, in calcareous dolomites 22.9 mass %, in dolomites 41.2 mass % and dolomitic magnesites up to 45.7 mass %.

The filler prepared according to this invention from these natural raw materials removes disadvantages described above, combining advantages ensuing from a possibility of simultaneous use of CaCO3 and Mg(OH)2. Production of the new filler is relatively simple and inexpensive because neither of the components is prepared by precipitation from solutions. The efficiency of the new filler as a nontoxic fire retardant opens broad possibilities for large-scale application of polyolefins in many fields such as building industry, transportation, electrical engineering and other industries.

The filler according to this invention originates from dolomite or from dolomitic magnesite and/or from dolomitic limestone, i.e. the starting raw material contains either the equimolecular ratio MgCO3:CaCO3 or an excess of MgCO3 or a minor excess of CaCO3. It is obvious that the composition of a starting raw material will have a decisive influence on the effectiveness of the filler as a fire retardant. The purity of a raw material influences the whiteness of both the filler and the composite material. The substantial advantage of the new filler (in comparison with mixed fillers of similar composition but produced by precipitation) is based on lowering the total catalytic activity without an expensive removal of the harmful impurities.

The calcination of the natural raw material can be performed at temperatures from 750 to 1100 C. At lower temperatures only the decomposition of MgCO3 into MgO and the partial transformation of the present CaCO3 to CaO occurs. At higher temperatures, MgO.CaO is formed. Besides these main chemical reactions transformation of active salts of transition metal compounds into inert oxides takes place during calcination. It means that the catalytic activity of natural dolomite is suppressed by the calcination.

The calcination of dolomite is carried out preferantially in rotating kilns. To preserve a high chemical purity, it is required to use a fuel with a minimum content of sulphur, for example natural gas. Calcinated and passivated dolomite is subjected to a successive hydration. Because hydration of MgO proceeds slower than hydration of CaO, it is advantageous to perform the hydration in pressure hydrators at temperatures from 150 to 220"C. As the crystal lattices of substances formed are different, spontaneous partition of material during hydration occurs accompanied by formation of very fine particle. By a suitable control of the hydration process it is possible either to obtain directly a product with a suitable particle size distribution for the given purpose or to perform just a classification of the product without grinding.

When extremely fine particles of filler are required, it is possible to grind the hydrated product prior to classification with substantially lower energy requirements in comparison with grinding of the original raw material.

The filler according to this invention is surface treated with a mixture of fatty acids, their salts, esters or partial esters of glycerol with fatty acids or with other anionactive or nonionogenic tensides. This treatment is done directly during the hydration process using solutions oremulsions of these compounds in water.

A reagent suitable for surface treatment must fulfill the following conditions: (1) It must be soluble in water (2) It must form hydrophobic layer on the filler surface (3) It must not decrease thermooxidative and photooxidative stability of filled polyolefins.

Ad 1: A treatment reagent must form a true or micellar solution in water. Referring to condition (2), it is necessary to consider the use of micellar solutions of substances which have hydrophobic chains in addition to a hydrophilic functional group. Hydrophilic functional groups can have an ionogenic or non ionogenic character. lonogenic functional groups are salts of carboxylic acids RCOO-X, where the cation X can be an ion of alkali metals (Nal, K+) or ammonium (NH4+).

Ammonium salts of organic acids are exceptionally suitable due to the fact that ammonium is released by chemisorption on the basic filler surface and then ammonia escapes during hydration. On the contrary, when metal salts are used, ions of these metals remain sorbed on the filler surface, this being undesirable.

Examples of nonionogenic functional groups are oligomers of ethylene oxide or ethylene glycol. To attain the necessary water solubility, it is necessary to have at least 10 units of ethylene oxide in the hydrophilic functional group.

Ad 2: In order to achieve a hydrophobic character of the filler surface, it is necessary to have a sufficient number of hydrophobic units in the hydrophobic group of the reagent. If this group is formed from hydrocarbons, it is necessary that the carbon number in the chain is at least 12. It means that following fatty acids obtained from natural plant or animal fats can be used either individually or in optional mixtures: lauric acid (C12H24O2), myristic acid (C14H2802), palmitic acid (C13H3202), stearic acid (C,8H36O3), arachidic acid (C20H4002), behenic acid (C22HO2), oleic acid (C18H32O3), erucic acid (C22H42O2).

The last two acids are examples of unsaturated acids whose double bonds may be used preferentially for creating chemical bonds between filler and polyolefins, thus decreasing creep of composite materials.

It is possible to use synthetic acids with a linear or branched chain containing at least 12 carbon atoms instead of natural fatty acids.

Ad 3: Transition metal salts (Fe, Mn, Cu, Cr, Ti) are catalysts of thermooxidative and photooxidative degradation of polyolefins. Even if we use a very pure raw material and a pure polylefin for production of composite materials, it is not possible to assure a complete absence of transition metals impurities as in the filler so as in the polymer where residues of polymerization catalysts are always present. It is known that only such compounds of these metals exhibit catalytical activity, when the transition metal is in lower valency state and in such a form that it could form active complex with air oxygen (See Zahradnickova A., Vesely K. : Chemicky prumysl 32, 533 (1982); Lunak S. Lederer P., Stopka F., Veprek-Siska J.: Coll. Czech. Chem.Commun. 46, 2455 (1981); Vesely K., Petruj J., zahradnickova A. : 28th IUPAC Microsymposium, Praha 8-11-7 1985). Fatty acids react with present transition metal oxides forming catalytically active salts which catalyse oxidation not only of polyolefins themselves but also of added phenolic antioxidants. This leads not only to a sharp decrease of composite materials stability but also to an unwanted colouration caused by phenolic antioxidants oxidation products. We have found out that the oxidative stability can be substantially improved by an addition of compounds forming stable complexes with transition metals. Therefore, it is advantageous to add - in addition of ammonium fatty acid salt, which acts as an emulsifier -, an ester of fatty acid with an polyhydric alcohol in which at least two OH groups remain free.Examples of suitable esters are: glycerol monostearate, glycerol monooleate, pentaerythritol monostearate, pentaerythritaol distearate, sorbital monostearate. These esters it is possible to emulgate easily in a water solution of an emulsifier e.g. an ammonium fatty acid salt.

The amount of water solution of emulsion of treatment reagents used for hydration of fully or partially calcinated dolomite can vary widely. It is suitable to use only a small excess (cca 50 molar %) of water of the steichiometric amount needed for hydration. In this case, the hydration proceeds under "dry" conditions, it means that the hydration heat of MgO (34.3 kJ/mol) or CaO (64.9 kJ/mol) is sufficient to evaporate the excess water and a powder with good flowing properties is obtained which it is not necessary to dry. Thus, the exact optimum amount of water used for hydration depends on the raw material composition, on its degree of calcination and on hydration process design.

The following examples illustrate more specifically the present invention. Parts and percents in the examples are given in mass units.

Example 1 Dolomite from Dolni Rozinka exhibiting the following composition was used MgO 18.8% CaO 30.2% Fe2O3 0.18% SiO2 1.1% AI2O3 0.31% Annealing mass loss 49.41% The dolomite was calcinated for 3 hours in a laboratory muffle oven at 1000 C. X ray diffraction analysis showed that the calcinated material contained MgO 34.1% CaO 40.4% CaCo3 24.5% Thus decarboxylation of MgCO3 was found to be complete while decarboxylation yieid of CaCo3 was 74%.

The calcinated material was hydrated in the laboratory autoclave at 200 "C for 6 hours in the presence of an excess of water containing 0.4 % of potassium stearate (calculated on the calcinated dolomite).

Filtration of the slurry through a screen with mesh diameter 40 ijm removed coarser particles from the slurry. Water from the filtrated slurry was evaporated and the powder dried 1 hour at 150 "C. A fine free flowing hydrophobic powder was obtained. The size of particles determined using an optical microscope was between 1 and 4 wm.

The DTA method showed the following composition of the dry hydrate Mg(OH)2 : 38.5% Ca(OH)2 : 41.7% CaCO3 : 19.8% Mixtures of polypropylene (Mosten 58.412) with fillers were kneaded in a laboratory kneader at 220 "C.

The resulting material was pressure moulded to obtain 4 mm thick sheets used either for flammability test (oxygen index method LOI) according to ASTM D 2863 or for evaluation of yield point (or,) and notched impact strength a (k) according to DIN 53 453.

The properties of polypropylene filled with modified dolomite hydrate (D) and microground limestone (V) are compared in the table: Sample PP D V LOI k k {0/o) ro/O) /Q/O) %vol (spa) (kJlm2) a 60 40 - 21.9 24.5 5.2 b 50 50 - 23.3 22.7 5.8 c 40 60 - 25.1 20.8 6.5 d 60 - 40 19.6 24.7 5.2 e 50 - 50 20.5 23.1 6.0 f 40 - 60 21.7 21.0 7.0 It is necessary to note that materials with oxygen index higher than 22.5% can be deciared as self-extinguishing materials. It is obvious that the new filler secures virtually identical properties of the filler polypropylene as the microground limestone does but in addition to it, a substantial flammability decrease is attained.

Example 2 Dolomite Varin exhibiting the following composition was used: MgO 21.19% CaO 31.19% SiO3+13O3 0.22% Foe203 0.044% The dolomite was calcinated for 3 hours in a laboratory muffle oven at 850 "C. X ray diffraction analysis showed that the calcinated material contained MgO 29.2% CaO 7.7% CaCO3 63.1% To the 100 g of calcinated material, 25 g of water solution containing 4 % ammonium atearate was added. After mixing, a wet lumpy product was obtained. It was placed into a mixed laboratory autoclave and heated for 2 hours on 120 C. After opening the autoclave, the dry free flowing hydrate was obtained. The DTA method showed following composition of the product.

MgO(OH)2 36.6% Ca(OH)2 8.8% CaCO3 54.6% In the air classifier, the fraction with particles larger than 10 SLm was separated and the finer fraction was used as a filler for the following mixtures with various polyolefins.They were prepared by kneading in a Plasticoder Brabender at 220 - 230 "C. The best pieces were pressure moulded at 230 C. The summary of prepared mixtures and their properties obtained is given in the table: TABLE: Properties of Composite Materials Polymer Filler Additive Fl31 N Fl49N LOI Flextural ak ak (%) (%) (% O2) Modulus (Gpa) (MPa) (kJ/m2) PP 40 Ca St 2 0.38 1.30 21.8 2.53 24.5 2.9 PP 40 GM 2 0.35 1.47 21.7 2.41 22.8 5.0 PP 60 GM 2 24.5 15.6 2.0 IPE 40 GM 2 0.05 0.36 22.0 18.1 18.8 rPE 40 GM 2 0.08 0.50 22.0 11.4 8.5 Notes: PP = Polypropylene Mosten 55212 - producer CHZ Litvinov IPE = Linear polyethylene Liten FB 29 - producer CHZ Litvinov rPE = Branched polyethylene Bralen RB 03-23 - producer Slovnaft Bratislava CaSt = Calcium stearate GM = glycerol monostearate

Claims (7)

1. A powdery filler for plastics based on dolomites and/or dolomitic limestones containing a mixture of magnesium and calcium hydroxides with calcium carbonate with particles smaller than 20 lim, the surface of which has' been treated with hydrophobizing substances.

2. A method of production of a powdery filler according to Claim 1 by calcination and hydration of dolomites and/or dolomitic limestones, in which dolomites and/or dolimitic limestones are calcinated at temperatures up to 1100 "C and then they are hydrated, preferably by solutions of anionactive or nonionagenic tensides.

3. A method of production of a powdery filler according to Claim 2 in which as tensides ammonium salts of fatty acids with 12 to 20 carbon atoms are used.

4. A method of production of a powder filler according to Claim 2 or 3 in which mixtures of fatty acid salts with esters of fatty acids with at least two free alcoholic groups are used.

5. Composite materials based on polyolefins and the powder filler according to Claim 1.

6. A powdery filler substantially as herein described.

7. A method of production of a powder filler substantially as herein described.