Patent Number: 044435657
Section: summary

The present invention relates to a resin composition containing metal foil fragments in a high concentration. In recent years, electric apparatus and devices equipped with an electromagnetic wave generating source, such as microwave ovens, televisions or micro-computors, have become popular. There are accordingly an increasing number of instances where such apparatus and devices give electromagnetic wave interferences against communication or measuring devices, or they receive electromagnetic wave interferences from outside. In order to avoid such interferences by electromagnetic waves, it is common to use a metal casing for the assembly of these apparatus or devices. However, the metal casing requires a number of process steps for its shaping or coating. Therefore, it has been desired to develop a thermoplastic resin material which can be readily shaped and which is highly effective to attenuate the electromagnetic waves to pass therethrough. Thermoplastic resins can easily be shaped. However, if a substantial amount of metal foil fragments such as aluminum or copper foil fragments which are effective for attenuation of the electromagnetic waves, is incorporated to provide an adequate shielding effect against the electromagnetic waves, it would be difficult to adequately knead the mixture by a single shaft extruder as the mixture tends to be hardly processable by the screw of the single shaft extruder. Further, it is difficult to adequately disperse the metal foil fragments in the resin by mere mechanical kneading, and the resin composition thereby obtained does not have adequate strength. Accordingly, it is an object of the present invention to provide such a resin composition which is highly effective to attenuate electromagnetic waves. Another object of the present invention is to overcome the abovementioned difficulties in the conventional technique and to provide a resin composition containing metal foil fragments in a high concentration and yet having good dispersibility and adequate strength. A further object of the present invention is to provide a process for preparing such a resin composition. The present invention provides a composition comprising 100 parts by weight of metal foil fragments, from 1 to 10 parts by weight of a first polymer covering the surfaces of the metal foil fragments and obtained by polymerizing at least one of monomers selected from the group consisting of acrylic acid, methacrylic acid, an alkylacrylate, an aminoalkylacrylate, an alkylmethacrylate and an aminoalkylmethacrylate in the presence of the metal foil fragments, and from 5 to 33 parts by weight of a second polymer obtained by polymerizing a mixture of an aromatic vinyl monomer and a monomer copolymerizable with the aromatic vinyl monomer. The present invention also provides a process for preparing a thermoplastic resin composition containing metal foil fragments, which comprises suspension-polymerizing from 1 to 10 parts by weight of at least one of monomers selected from the group consisting of acrylic acid, methacrylic acid, an alkylacrylate, an alkylmethacrylate, an aminoalkylacrylate and an aminoalkylmethacrylate in the presence of 100 parts by weight of metal foil fragments and then adding and suspension-polymerizing from 5 to 33 parts by weight, based on 100 parts by weight of the metal foil fragments, of a mixture of an aromatic vinyl monomer and a monomer copolymerizable with the aromatic vinyl monomer. Now, the present invention will be described in detail with reference to the preferred embodiments. The metal foil fragments used in the present invention are preferably made of a metal which is stable and does not fuse or evaporate at a temperature for shaping the thermoplastic resin (usually from 230.degree. to 260.degree. C.) and which has good electric conductivity, such as aluminum, zinc, copper, iron, tin, gold, silver or an alloy thereof. Further, in order to obtain greater effectiveness for attenuation of electromagnetic waves, it is preferred to use a combination of at least two different types of metal foil fragments. In such a case, a combination of metals belonging to different Groups in the Periodical Table is preferred since it is thereby possible to minimize the specific resistance of the resin composition thereby obtained. For instance, good results will be obtained by a combination of aluminum and zinc or aluminum and tin. In the case of the combination of aluminum and zinc foil fragments, the mixing ratio is preferably from 3 to 70% by weight of zinc foil fragments, the rests being aluminum foil fragments, more preferably from 5 to 20% by weight of zinc foil fragments, the rest being aluminum foil fragments. When the proportion of zinc foil fragments is less than 3% by weight or more than 70% by weight, no improvements in the reduction of the specific resistance and the attenuation effect against electromagnetic waves are observed as compared with the case where aluminum foil fragments or zinc foil fragments are used alone. The metal foil fragments preferably have a thickness of from 5 to 50 .mu.m and a quadrilateral shape of 0.5-5 mm.times.0.5-5 mm, more preferably 1-2 mm.times.1-2 mm. They may have any other shapes so long as they have a size similar to the above size. These metal foil fragments are covered on their surfaces with a polymer obtained by polymerizing at least one of monomers selected from the group consisting of acrylic acid, methacrylic acid, an alkylacrylate, an aminoalkylacrylate, an alkylmethacrylate and an aminoalkylmethacrylate. Such a polymer is suitable because it has good adhesion to the metal foil fragments. The amount of the polymer is from 1 to 10 parts by weight per 100 parts by weight of the metal foil fragments. If the amount is less than 1 part by weight, no adequate coating effectiveness will be obtained. On the other hand, if the amount exceeds 10 parts by weight, there will be an excess amount of the polymer which does not attach to the surfaces of the metal foil fragments and which gives an adverse effect to the physical properties of the composition thereby obtained. The polymer to be used for coating the metal foil fragment may be a homopolymer of one of the above-mentioned monomers or copolymer of at least two such monomers. For instance, there may be used a homopolymer of methylmethacrylate or a copolymer of methylmethacrylate with from 10 to 30% by weight of acrylic acid. However, a copolymer of methylmethacrylate with from 30 to 70% by weight of an aminoalkylacrylate or an aminoalkylmethacrylate is particularly preferred since it is superior in the coatability on the metal foil fragments and the compatibility with other resins. In such a case, the aminoalkyl group in the aminoalkylalkylate or the aminoalkylmethacrylate is preferably a dimethylaminoethyl group ((CH.sub.3).sub.2 NCH.sub.2 CH.sub.2 --). However, other groups such as a diethylaminoethyl group and a dimethylaminopropyl group are also useful as the aminoalkyl group. As the alkyl group, a straight chain, branched chain or alicyclic alkyl group having from 1 to 10 carbon atoms is used. However, a methyl group or an ethyl group is preferred in view of the physical properties such as tensil strength of the polymer thereby obtained. The mixing ratio of the monomers in a mixture is not restricted to the above-mentioned range. However, if the mixing ratio is in the above-mentioned range, good results are obtainable in the coatability on the metal foil fragment surfaces, the dispersibility and the physical properties. The metal foil fragments may be coated with the above-mentioned polymer by dipping the metal foil fragments in a solution prepared by dissolving the polymer in a suitable organic solvent. However, it is preferred to suspension-polymerize the above-mentioned monomer or monomer mixture in the presence of the metal foil fragments, whereby the coating can readily be done. When a combination of two different types of metal foil fragments is used, it is preferred preliminarily mix the two types of the metal foil fragments and then to suspension-polymerize the monomer or monomer mixture in the presence of the metal foil fragment mixture. Then, from 5 to 33 parts by weight, based on 100 parts by weight of the metal foil fragments, of a polymer obtained by polymerizing a mixture of an aromatic vinyl monomer and a monomer copolymerizable with the aromatic vinyl monomer is incorporated so that total amount of the polymers, i.e. this polymer and the polymer covering the surfaces of the metal foil fragments, is brought to from 6 to 43 parts by weight per 100 parts by weight of the metal foil fragments. If the amount of the polymer obtained by the polymerization of the aromatic vinyl polymer and the monomer copolymerizable therewith, is less than 5 parts by weight, the bulk density of the composition thereby obtained tends to be small, whereby the composition tends to be hardly mixed with e.g. an ABS resin. On the other hand, if the amount exceeds 33 parts by weight, the concentration of the metal foil fragments tends to be insufficient when the composition is mixed with an ABS resin or other thermoplastic resin. As the aromatic vinyl monomer to be used in the present invention, there may be mentioned styrene, .alpha.-methylstyrene, vinyl toluene and a halogenated styrene. As the copolymerizable monomer, acrylonitrile, methacrylonitrile or an alkylester of acrylic acid or methacrylic acid is suitable. The concentration of the copolymerizable monomer in the monomer mixture is preferably from 20 to 50% by weight. If the concentration is outside this range, the composition thereby obtained tends to have poor compatibility with other resins. In the preparation of the composition of the present invention, it is preferred that the above-mentioned monomers are suspension-polymerized in the presence of the metal foil fragments. Namely, 100 parts by weight of the metal foil fragments and from 1 to 10 parts by weight of at least one of the monomers selected from the group consisting of acrylic acid, methacrylic acid, an alkylacrylate, an aminoalkylacrylate, an alkylmethacrylate and an aminoalkylmethacrylate are suspended in water and the suspension is suspension-polymerized in accordance with a usual method. As the polymerization initiator, a water soluble initiator such as a persulfate is preferably used to obtain a good coating on the foil surfaces. Upon completion of the polymerization, from 5 to 33 parts by weight of a mixture of the aromatic vinyl monomer and the monomer copolymerizable therewith is added and further suspension-polymerized. The composition thereby obtained is in the form of pellets in which some tens metal foils are laminated. The composition has a great bulk density and good compatibility with e.g. an ABS resin. The composition of the present invention can be used by itself. However, it may also be used as a mixture with other resin such as an ABS resin or an AS resin. In such a case, the mixing can be done by a single shaft extruder. The mixture thereby obtained has good physical properties and good electroconductivity. Especially when two types of the metal foil fragments are used in combination, particularly good reduction of the specific resistance and electromagnetic wave attenuation characteristics are obtainable. Further, the metal foils in the composition are hardly susceptible to peeling even when an outer force is applied. Now, the present invention will be described in further detail with reference to Examples, Comparative Examples and Application Examples.