Process for the production of composite material based on a polymer matrix

0001The present invention relates to a process for the production of a composite material based on a polymer matrix. More particularly, the invention relates to a process for the preparation of a composite material based on a polymer matrix comprising, homogeneously dispersed therein, an organic and/or inorganic filler (charge).

0002As is known, in many applications there are used materials comprising a polymer matrix in which are dispersed organic and/or inorganic fillers which impart to the composite material particular properties such as colour or mechanical resistance.

0003These composite materials in general are produced by mixing the organic and/or inorganic filler in the form of a powder with the polymer likewise in the form of a powder, usually in the presence of a dispersing medium, for example oil. This type of process requires that the polymer be processable on special machines, and that, moreover, the materials used as fillers possess specific dispersability characteristics.

0004In the case of thermoplastic polymers, there are required both high temperatures as well as a high quantity of energy for the mixing of the filler with the polymer in the molten state.

0005It is also known to produce composite materials by using a process comprising mixing with an aqueous dispersion of the polymer (latex) the filler in the form of a powder, and by then coagulating the resulting dispersion and then separating the coagulated composite material. Even if this process of mixing requires small quantities of energy, the composite material thus obtained does not exhibit a satisfactory homogeneity as required in certain applications.

0006Moreover, the addition of the organic and/or inorganic filler to the latex may lead to the coagulation of the polymer before complete homogenization is achieved.

0007GB-A-2071112 discloses a method of producing filler-containing PTFE powder, in which aqueous dispersions of PTFE particles and filler particles are mixed and coagulated. A substance containing a polyvalent cation is added to the mixture to ensure that the filler particles have a positive charge. A water-immiscible organic liquid is required to coagulate the particles.

0008The present invention aims to provide a process for the preparation of a composite material based on a polymer matrix having homogeneously dispersed therein an organic and/or inorganic filler, which process overcomes or at least mitigates the above described disadvantages.

0009More particularly, the invention aims to provide a composite material based on a polymer matrix, comprising dispersed therein an organic and/or inorganic filler, having a high grade of uniformity, prepared by a quite simple process and with the use of only small quantities of energy.

0010It has now been surprisingly found that these aims may be achieved if the following conditions are satisfied: <ul id="ul0001" list-style="none"><li>a) the polymer matrix and the organic and/or inorganic filler be mixed together in the form of aqueous dispersions;</li><li>b) the particles of the dispersed systems have a high degree of dispersion and exhibit a comparable stability.</li></ul>

0011According to the present invention, such conditions are satisfied by a process for the production of a composite material based on a polymer matrix, comprising homogeneously dispersed therein an organic and/or inorganic filler, the said process comprising mixing together the single components in the form of aqueous dispersions in which the particles have a surface charge of the same sign and a zeta potential (in absolute value) of from 10 mV to 100 mV and the ratio (in absolute value) between the zeta potentials of the particles of each dispersion is from 0.2 to 5, and then by coagulating the mixed dispersion thus obtained.

0012It is preferred that the zeta potential (in absolute value) of the particles of each dispersion is from 15 to 70 mV and that the ratio (in absolute value) between the zeta potentials is from 0,5 to 2, in the process of the present invention.

0013In the preparation of the composite material, any polymer matrix can be used, but the matrix is preferably in the form of a latex, more preferably a matrix obtained by emulsion polymerization.

0014Examples of polymers that may be used in the process of the present invention are: fluorinated polymers and elastomers such as polytetrafluoroethylene; copolymers of vinylidene fluoride and hexafluoropropylene: terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene; terpolymers of vinylidene fluoride, tetrafluoroethylene and perfluoroalkyl-vinyl-ethers; copolymers of tetrafluoroethylene and perfluoroalkyl-vinyl-ethers; copolymers of tetrafluoroethylene and propylene; copolymers of vinylidene fluoride and chloro-trifluoro-ethylene; copolymers of vinylidene fluoride and hydropentafluoropropylene; terpolymers of vinylidene fluoride, hydropentafluoropropylene and tetrafluoroethylene, and the corresponding products having in the chain small quantities of trifluoro-bromo-ethylene; chlorinated (co)polymers such as polyvinylchloride and polyvinylidenechloride; polymers and copolymers such as polyacrylonitrile; polymethacrylates and polymethylmethacrylates; polybutadiene and polyisoprene; polycarbonates; acrylonitrile-butadiene-styrene polymers, known as ABS resins; vinyl (co)polymers such as polyvinylacetate, ethylene-vinylacetate (EVA) copolymers, and polyvinyl-alcohol (PVA): and in general any polymer and copolymer obtained by an emulsion polymerization process.

0015Examples of inorganic fillers that may be used in the preparation of the composite material are: pigments. graphite, bronze in powder form, ground glass in the form of spheres or fibres, molybdenum sulphide, silica, steel powder, metal oxides, carbonates, and silicates.

0016Examples of organic fillers that can be used are organic pigments, stabilizers, and any polymer or resin different from that which forms the polymer matrix.

0017According to the present invention, composite materials in which the filler (or charge) is homogeneously dispersed in the matrix, are obtained if the single components are mixed together starting from preformed dispersions in which the particles are completely wetted by the dispersing medium and have a surface charge of the same sign and comparable stabilities. By the term stability there is meant the capacity of the particles to remain in the dispersed state.

0018The stability of a dispersed system may be measured by the zeta potential, defined as the electric potential at the particle-solution plane of slippage. The zeta potential of particles in a dispersion may be determined by an electrokinetic means, such as electrophoresis, electro-osmosis, or potential/current flow, as described for example in "Colloid Science", vol. 1, by H.R. Kruyt, published by Elsevier Publishing Co., of 1952.

0019The adjusting of the zeta potential value of the components of each dispersion may be carried out by acting on the mechanisms which control the formation of the surface charge. A suitable method for varying the zeta potential is that of varying the pH of the dispersion or otherwise by adding surfactants, polyelectrolytes or ions specifically adsorbed, such as sodium laurylsulphate, ammonium perfluoro-octanoate, sodium polyacrylate, sodium hexametaphosphate, or sodium phosphate.

0020After the mixing of the dispersions of the single components, the resulting dispersion is coagulated. The coagulation phase is preferably carried out in a fast way in order to avoid the selective coagulation of the single components of the dispersion, with a consequential dishomogeneity of the coagulate.

0021Any technique for coagulating the dispersion may be used, such as vigorous stirring, the addition of electrolytes or the addition of a liquid in which the dispersions are not stable, or a combination of said techniques.

0022The preferred method in the process of the present invention is the addition of electrolytes such as sulphuric acid, magnesium nitrate, or aluminium sulphate, at a concentration at least sufficient to coagulate the dispersion, combined, if necessary, with a vigorous stirring.

0023The separation of the coagulate from the dispersing medium may be obtained by using conventional techniques such as floatation, filtration, centrifugation, decantation or a combination of said techniques.

0024The use of the various separation techniques depends on the polymeric matrix used. More particularly, when the polymer phase is polytetrafluoroethylene, a preferred method may be the floatation of the coagulate by using vigorous stirring, while for composite materials based on polyvinylchloride or on tetrafluoroethylene-vinylidene fluoride copolymer, it is advisable to effect the separation by filtration.

0025In order to avoid possible adverse effects due to the presence of electrolytes in the coagulate, after the separation there may be carried out one or more washings of the coagulate.

0026The composite material thus obtained according to the process of the invention, under examination by a microscope, is found to be substantially perfectly uniform.

0027The invention will be further described with reference to the following illustrative Examples. In the Examples, all parts and percentages are by weight, unless otherwise stated.

Example 1

0028A dispersion of a red pigment based on iron oxide (Bayer 110M) was prepared in distilled water, at a concentration of 10 g/l, by means of ultrasounds. The particles of dispersed pigment showed a zeta potential equal to -25 mV.

0029Thereupon, 20 ml of the dispersion were added, under mild stirring (50 r.p.m.) to a polytetrafluoroethylene latex (PTFE) containing 40 g of polymer; the thus obtained dispersion was then diluted with distilled water until there was obtained a concentration of the PTFE equal to 100 g/l. In the latex the PTFE particles showed a zeta potential of -30 mV, measured by microelectrophoresis.

0030To this mixed dispersion thus obtained there were added 10 ml of H₂SO₄ (30% by weight), while the stirring was rapidly increased to 700 r.p.m. The dispersion coagulated completely in a time less than 5 minutes. The stirring was maintained for another 10 minutes in order to float the coagulate.

0031The powder, separated from the liquid by filtration on a nylon net, was then dried for 12 hours at 140°C. A thin ribbon prepared with the pigmented product exhibited a perfectly uniform colour, mechanical and dielectric properties similar to those of a non-pigmented ribbon, and the absence of pigment coagulates under microscopic analysis.

Example 2

0032A pigment based on cadmium sulphoselenide (CP 2400, Ferro Co.. USA) was dispersed, at a concentration of 10 g/l, in an aqueous solution containing 1 g/l of sodium hexametaphosphate, by using a ball mill of the "Red Devil" type. The pigment particles had a zeta potential of -46 mV, measured by electro-osmosis.

003320 ml of the dispersion were then added, under mild stirring (50 r.p.m.), to a PTFE latex containing 40 g of polymer; the dispersion was thereupon diluted with distilled water to a concentration of 100 g/l. In the latex the PTFE particles showed a zeta potential of -30 mV, measured by electrophoresis.

0034To the mixed dispersion were then rapidly added, under constant stirring, 20 ml of a Mg(NO₃)₂ 1 M solution, thereby causing bulk coagulation of the particles. The coagulate was then separated by floating, increasing the stirring to 700 r.p.m.

0035The powder had a uniform colour and was dried at 140°C for 12 hours. A ribbon prepared with the pigmented powder exhibited a uniform colour, dielectric and mechanical properties similar to those of a non-pigmented ribbon, and the absence of pigment coagulates under microscopic analysis.

Example 3

0036A sample of graphite (FOLIAC X6236) was dispersed at a concentration of 10 g/l, in an aqueous solution containing 1 g/l of sodium polyacrylate, by using a "Red Devil" ball mill. 60 ml of the dispersion were thereupon added to 120 ml of a PTFE latex containing 40 g of polymer. The zeta potential of the graphite particles, measured by electro-osmosis, was -15 mV, while the zeta potential of the PTFE, measured by electrophoresis, was -30 mV.

0037The dispersion thus obtained was then diluted, under mild stirring (50 r.p.m.), to a volume of 400 ml. Still under mild stirring, there were then added 20 ml of a Mg(NO₃)₂ 1 M solution, causing complete coagulation of the slurry. The thus coagulated polymer was then floated under stirring at about 700 r.p.m.

0038The obtained powder, dried at 140°C for 12 hours, was seen to be perfectly uniform under microscopic analysis. A thin ribbon prepared with the dried powder had a uniform colour.

Example 4

003940 ml of a slurry, obtained by dispersing 25 g of glass fibre in 100 ml of an aqueous solution containing 1 g/l of sodium hexametaphosphate, were added to 120 ml of a PTFE latex containing 40 g of the polymer. The zeta potential of the glass fibre, measured by electro-osmosis, was -51 mV, while the zeta potential of the PTFE, measured by electrophoresis, was -30 mV.

0040The obtained slurry, diluted to a volume of 400 ml, was then additioned with 20 ml of a Mg(NO₃)₂ 1 M solution, under a mild stirring (100 r.p.m.). The addition of the electrolyte caused complete coagulation of the dispersion. The coagulate was thereupon floated, under vigorous stirring (700 r.p.m.), for a period of 15 minutes.

0041The powder thus obtained, dried for 12 hours at 140°C, under microscopic analysis, was showed to be formed of glass fibres singularly dispersed in the polymer matrix.

Example 5

0042A PTFE latex was added, in an amount of 10% by weight, to a latex made of a Tecnoflon fluoroelastomer (Tecnoflon is a registered trade mark). The zeta potential of the PTFE latex was -30 mV, while the zeta potential of the fluoroelastomer was -50 mV, both measured by electrophoresis.

0043Then 200 ml of the mixed latex were slowly added to an equal volume of an aqueous solution containing 8 g/l of Al₂(SO₄)₃, maintained under vigorous stirring (500 r.p.m.). The slurry coagulated instantly.

0044The coagulate, after separation by filtration and drying for 12 hours at 60°C, appeared under microscopic analysis to be perfectly uniform.

Example 6

0045A PTFE latex, containing 40 g of polymer, was additioned with a dispersion of red pigment CP2400 (by Ferro Co., USA) and a TiO₂ dispersion. The total of added pigments was equal to 3.5% on the PTFE. The zeta potentials of the particles were: <ul id="ul0002" list-style="dash"><li>for the PTFE = -30 mV</li><li>for the red pigment = -46 mV</li><li>for the TiO₂ pigment = -40 mV.</li></ul>

0046There were prepared various dispersions, with ratios by weight of red pigment/TiO₂ pigment of: 10:1, 2.5:1, 1:1, 1:2.5, 1:5, 1:10 and 1:20.

0047The dispersions were coagulated as described in Example 2. The powders, dried for 12 hours at 140°C, were uniformly coloured. The colour of tablets, obtained by sintering of the powders at 400°C, ranged uniformly from red to pink as the red pigment/TiO₂ pigment ratio decreased.

Example 7

004866.7 ml of a polyvinylchloride latex (at a 300 g/l concentration) and 10 ml of a dispersion of a yellow F 897 pigment (Ferro Co.. USA) at a concentration of 10 g/l were mixed with an aqueous solution containing 0.1 g/l of sodium hexametaphosphate and 83.4 ml of distilled water. The zeta potentials of the particles, measured by electrophoresis, were: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">- for the polyvinyl chloride</entry><entry namest="col2" nameend="col2" align="right">-35 mV,</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">- for the pigment</entry><entry namest="col2" nameend="col2" align="right">-25 mV.</entry></row></tbody></tgroup></table></tables> Thereupon, under a mild stirring, there were admixed 20 ml of a 1 M solution of Mg(NO₃)₂, thereby causing complete coagulation of the slurry.

0049The coloured coagulate was then separated by filtration and dried for 48 hours at 50°C. The resulting powder was uniformly coloured.