ELECTRICALLY CONDUCTIVE ARTICLE

Abstract: The invention relates to electrostatically controlled plastics articles, their manufacture and their use. An article according to the invention contains a conductive polymer, and the surface resistivity of its outer surface, expressed in OMEGA /square, is at minimum one decade higher than volume resistivity of its interior, expressed in OMEGA -cm. The conductive polymer used may be in particular polyaniline. The invention can be exploited in the manufacture from plastics, for example, of transportation containers or equipment casings in which electrostatic shielding is required.

Description: ELECTRICALLY CONDUCTIVE ARTICLE FIELD OF TECHNOLOGY The invention is associated with materials technology and relates to plastics articles protected from the generation of static electricity, their manufacture and their use. The invention can be exploited in manufacturing from plastics, for example, transportation containers or casings for equipment requiring control of static electricity. BACKGROUND The generation of static electricity is in many contexts detrimental or downright hazar¬ dous. In the handling of flammable or explosive substances, an electrostatic discharge may cause accidents. These are prevented by using antistatic containers and chambers. Electrostatic discharges may also damage sensitive electronic components both during their manufacture and during their use, and therefore antistatic casings, transportation equipment and manufacturing spaces are required. No static electricity is generated in well conductive metal articles. However, metal cannot be employed for all uses. In articles manufactured from plastics, the generation of static electricity is a special prob¬ lem. The generation of static electricity can be prevented by improving the conductivity of the surface of an article or the entire article. To improve the conductivity of the surface of an article, antistatic agents can be used. Their conductivity is in general based on ionic charge carriers. However, sufficient con¬ ductivity cannot always be achieved by using such agents. In addition, they tend to travel to the surface of the article and become emitted with time. They are also very sensitive to air humidity and lose their operating capacity in very dry conditions. To enhance electric conductivity it is possible to mix a conductive additive, such as carbon or metal, with plastics. To accomplish conductivity, quite a large amount of the additive is needed, which respectively deteriorates the properties of the blend. Plastics thus filled are not suitable for all uses. The fact that conductivity is difficult to adjust is a further problem. Also known are intrinsically conductive polymers. Their electrical conductivity is based on conjugated double bonds in the polymer chain, on ring structures, and on a doping , agent which is an electron donor or an electron acceptor. Known conductive polymers include, for example, polyacetylene, polyparaphenylene, polypyrrole, polythiophenes, polyphenyiene vinylene, and polyanilines. The greatest problem with conductive polymers has been their poor stability. Their poor melt-processibility has also been a problem. Conductive polymers can in principle be used together with other polymers. In this, however, there has been a problem in the poor compatibility of conductive polymers with the conventional polymers involved. Polymerization of a conductive polymer onto the surface of ordinary plastics has also been used, but this method is also unsatisfactory. In electrically conductive polyanilines the monomer is aniline or its derivative. In the polymer, the nitrogen atom is bound to the para-carbon atom of the benzene ring of the subsequent unit. Polyaniline may appear in a number of different forms, of which usually the so-called emeraldine form is used for conductive polymer applications. The doping agent used is usually a protonic acid, in particular sulfonic acid. A suitable acid can be used in excess to plasticize the doped polyaniline. To compensate for the acidity caused by the excess acid, a suitable metal compound can be used. Patent application publication EP 582919 A discloses one such method for the preparation of an electrically conductive polyaniline material. DESCRIPTION OF THE INVENTION General description An electrostatically controlled article according to Claim 1, containing a conductive polymer, has now been invented. Certain preferred embodiments of the invention are stated in the other claims. The most essential idea of the invention is that with one material the advantages of both well conductive as well as less conductive material can be achieved. This is a specific benefit while e.g. producing containers or packaging for the contents to be shielded against static electricity and electric fields. The novelty of the invention is that the surface resistivity, expressed in Ω/square, of the article made of one material is at least a decade, preferably two decades higher than the volume resistivity of the article, expressed in Ω-cm. Thus the surface will dissipate a charge sufficiently slowly, controlling the electrostatic discharge. The interior layers of lower resistivity dissipate charges efficiently and provide shield against electric fields. The article is preferably produced with such a melt processing method and in such processing conditions that during production the material is exposed to shear rates equal to 100 1/s or higher. Brief description of the drawings Figure 1 shows a typical resistivity profile of a PP/PANI article. Figure 2 shows the conductivity profiles of the samples injection molded at melt tempera¬ ture 200 * C. Figure 3 shows the conductivity profiles of the samples injection molded at melt tempera¬ ture 230° C. Figure 4 shows the conductivity profile of the sample molded with 10 % injection speed of the maximum speed. Figure 5 shows he conductivity profile of the sample molded with maximum speed. Detailed description By an article is here meant generally any entire product, such as a sheet or a casing, or a component of a product, such as coating. An article may be manufactured from one blend containing a conductive polymer, or an article may be manufactured in which the surface layer and the interior layer are of different blends. In addition to a doped conductive polymer the material of the article may also contain a matrix material, such as a suitable plastic, and suitable additives. The surface resistivity of the outer surface is, for example, approx. IO 5 - IO 12 Ω/square, preferably approx. 10 6 -10 10 Ω/square, and most preferably approx. 10 6 -10° Ω/square. The volume resistivity of the interior is, for example approx. 10 "2 -10 8 Ωcm, preferably approx. 10 ~ -10 6 Ωcm, and most preferably approx. 10 2 -10 4 Ωcm. According to one embodiment, the article has a resistivity profile in which the resistivity first decreases from the surface towards the interior and thereafter, further towards the interior, increases to an approximately constant level. The conductive polymer is preferably a polyaniline (incl. polyaniline derivatives) which is very stable. The doping agent is in this case preferably a functional doping acid, whereby a good compatibility is also achieved with non-polar matrix plastics. The acid may be in particular an organic sulfonic, phosphoric or phosphonic acid. Preferably the acid is an aromatic sulfonic acid, in particular alkylbenzene sulfonic acid, especially dodecylbenzene sulfonic acid. The acid is preferably used in excess, whereby plasticity is increased. To compensate for the acidity in such a case, preferably a metal compound is additionally used, such as zinc oxide, in particular in the way described in publications EP 545729 A and EP 582919 A (these applications are incoφorated into the present specifi- cation by reference). A calcium compound, such as calcium carbonate, may also be added. The pH of the blend is, for example, approx. 3-8, typically approx. 4-7. Preferably the blend comprises a doped and plasticized melt-processible conductive polymer and a preferably melt-processible matrix plastic compatible therewith. They may be mixed together by using suitable processing machines, such as mixers and kneaders, by means of which the blend is exposed to shear forces. Preferably a screw mixer is used. The matrix plastic may contain a thermoplast, such as polyolefin, polyvinyl chloride, styrene-butadiene polymer, polyester, polyamide, acrylonitrile-butadiene-styrene polymer or polycarbonate, or a thermoset, such as phenol-formaldehyde polymer or melamine- formaldehyde polymer. Preferably a polyolefin, such as polypropylene or polyethylene, is used. The proportion of the doped and plasticized conductive polymer to the total blend is, for example, approx. 0.1-30 % by weight, preferably approx. 1-20 % by weight, and most preferably approx. 5-15 % by weight. The article can be manufactured especially with such a melt processing method in which the material is exposed to shear rates equal to 100 1/s or higher. The preferred melt processing method is injection molding which provides easy controlla¬ bility of shear forces needed to generate the desired conductivity profile. Controllability may be accomplished by changing raw materials, processing temperature, injection speed or other processing parameters having an effect on viscosity of the material or shear rate during processing. The viscosity ratio of doped and plasticized conductive polymer and matrix material as well as the volume ratio of those materials in blend are key parameters regarding to generation of the conductivity profile. The volume ratio is to a great extent determined according to desired conductivity level. The matrix material, on the other hand, is selected according to requirements set for product properties. For the reasons above the conducti¬ vity profile is most easily controlled by adjusting processing parameters. The processing temperature is selected in such a way that the viscosity of the neat matrix material (Pas) in processing conditions is higher but not more than a decade higher than that of doped and plasticized conductive polymer. Shear forces during melt processing are affected by material viscosity as well as shear rates during processing. In injection molding process shear rate is essentially affected by speed of injection which can be defined as per cents of maximum speed of injection of the machine. For example a blend of a polypropylene block-copolymer and a doped and plasticized conductive polymer in which the viscosity of the neat polypropylene at the temperature range 190°C-230'C and 0,1 MPa shear stress is 200-300 Pas, is preferably injection molded at melt temperature under 230 °C, more preferably under 210 °C and by using a speed of injection which is preferably under 70 %, more preferably under 50 % and most preferably under 30 % of maximum speed of injection of the machine. Articles according to the invention may be manufactured from a plastic contaimng a conductive polymer. The advantages of plastics include good processibility, light weight, and in general also an economical price. No separate antistatic coating treatment is required in the manufacture of an article; the manufacture can be carried out in one step. Also, with conductive polymers a lower resistivity is accomplished than with coatings. Wearing of the surface will not increase resistivity, and in normal conditions the controlled electrostatic properties of the article will be retained for 5-10 years. If conductive polymers are used, resistivity is easier to adjust than if carbon or metal fillers are used, and a sufficiently low resistivity suitable for ESD and also EMI applica- tions is achieved reliably. If conductive polymers are used, the properties of the articles will also be better than if carbon or metal fillers are used, since the mechanical properties, such as impact strength, of the conductive polymer correspond better to the properties of the matrix plastic. Furthermore, soiling electrically conductive carbon or metal dust will not detach from a product according to the invention. The colorability of a product according to the invention is better than that of products obtained using carbon-filled materials. A further advantage of blends according to the invention over carbon-filled or metal-filled plastics is their good processibility. For example, the blend will not cause wear on the apparatus in the manner that metal-filled plastics do. The viscosity of the blend is low compared to carbon black or metal-filled plastics, and thus lower processing temperatures or higher processing speeds can be used. Due to enhanced flow properties it will also be possible to manufacture articles having a more complex structure. Owing to the good processibility, it is also possible to use various manufacturing processes, and, for example, it is possible to manufacture fiber or film, or to stretch the product. According to one embodiment of the invention, the entire article is manufactured from one plastics blend, which is melt-processed and is then allowed to solidify so that the desired resistivity profile is produced in the article. According to another embodiment, the article is manufactured from two different blends having different conductivities. Accor¬ ding to each embodiment of the invention, the article is preferably manufactured by injection molding. The plastics blend can also be used for manufacturing articles, for example, by casting in mold or by coating, in a suitable manner. Typical uses for the products according to the invention are packaging for the electronics industry, objects suited for the handling and transportation of chemicals or explosive products, transportation containers, casings, for example diskette casings, components, etc., for copying machines and suchlike, pieces intended for industrial painting, such as, for example, automobile buffers, etc. Articles according to the invention are especially suitable for uses in which shielding against static electricity or electromagnetic interferen¬ ce (EMI) is required. Protective casings for computers and other sensitive electronic equipment are also a significant use for products according to the invention, in particular for providing EMI shielding. Such uses may include protective boxes for printed circuits, casings for equipment, containers, and even spaces having the size of rooms. Example 1 In the experiments, an emeraldine base form of polyaniline was used, which was prepared according to the method disclosed in the publication Y. Cao, A. Andreatta, A.J. Heeger & P. Smith, Polymer 30 (1989), 2305, but by using sulfuric acid instead of hydrochloric acid in the polymerization. The doping agent used for polyaniline was a commercial dodecylbenzene sulfonic acid (DBSA), Sulfosoft. By mixing with a solidification screw at a temperature of 150 °C, a polyaniline/DBSA complex (PANI) was prepared which additionally contained zinc oxide and calcium carbonate. This complex was incorporated into polypropylene (PP) in the same apparatus at a temperature of 170 °C. The following compositions were prepared (ingredients in % by weight of the whole blend): l P2 P3 Polyaniline 1.77 1.11 1.77 DBSA 16.4 10.3 10.2 ZnO 1.56 0.98 1.06 CaC0 3 1.15 1.14 1.14 PP add 100 add 100 add 100 100x100x50 mm 3 boxes having a wall thickness of 2 mm were injection molded (200- 220 °C, residence time less than 3 min) from the blends. The boxes had hinged lids. For comparison, boxes were also made from two commercial polypropylene blends containing carbon black (PP/CB; Cl, C2). The boxes were otherwise similar in appearance, except that the PP/PANI boxes were green and the PP/CB cases were black. The PP/PANI boxes were approx. 8 % lighter in weight. The bending strength of the lids of the PP/PANI boxes was substantially better; the lids could be bent thousands of times without breaking, whereas the lids of the PP/CB boxes broke already after somewhat over ten bending times. Surface resistivity (R s ) was measured according to the standard DIN 53 482 by using 100 V potential and a ring electrode. The resistivity profile was measured using a 2-point probe moving along the direction of the depth of the article. For some samples the resistivities were also measured, by using a 4-point probe, from thin slices cut in the direction of the surface. The latter method is more elaborate, but it gives more accurate and more correct values, since the 4-point measurement eliminates the contact resistance. Figure 1 shows a typical resistivity profile of a PP/PANI article. The resistivity decreases from the surface towards the interior, but then again increases. The resistivity of a CB article is constant throughout the article. Figure 1 also shows the mean resistivities p c , and p R . p c has been calculated from the mean of the conductances and describes the resistivity encountered by an electromagnetic field while passing through the article. p R has been calculated from the mean of the resistances and describes the resistivity encountered by the current while striving to pass through the article . The lower the p c , the better the electrostatic properties. ESD protection properties were measured accordmg to the standard IEC 801-2. Either a loop (diameter 28 mm) or a dipole (length 50 mm) antenna was used inside the box for detecting disturbance. The antenna was connected to an oscilloscope for detecting the difference between maximum and minimum voltages. The boxes were tested by using 4kV direct and indirect charges. The direct charge was applied to the surface of the box. The indirect charge was applied to a point 10 cm from the box. The ESD measurement results are shown in Table 1. Table 1 Blend R E (Ω/D) Direct charge (V) p c (Ωcm) Indirect charge (V) Ring Dipole Ring Dipole Pl 2.7*10 β 3.7 5.4 6.5*10 s 1.9 2.6 P2 3.3*10 e 3.0 5.8 8.3*10 5 2.9 4.5 P3 2.3*10 8 1.2 2.2 2.6*10 s 3.8 6.8 Cl 1.1*10 4 4.2 8.6 7.7*10 5 2.3 3.5 C2 9.7*10 3 4.6 6.8 6.8*10 5 1.9 3.3 In the articles according to the invention, the ρ c is sufficiently low so that charge will not accumulate. It was observed in the measurements that a 1000 V charge discharged to 50 V in less than 2 s. However, a high R s prevents the charge from discharging by sparking. However, additionally the p R is sufficiently high (cf. Fig. 1); this prevents, for example, batteries from discharging through the box. Example 2 A blend was prepared containing polypropylene and PANI-complex prepared as described above in weight ratio 87,5/12,5 respectively. The blend was injection molded both at melt temperature 200 ° C and 230 °C. The conductivity profiles of the injection molded samples are presented in figures 2 and 3. It can be seen that surface layers ( 0-1000μm and 9000- 10 OOOμm) of the sample molded at higher melt temperature do not contain well conducti¬ ve layers unlike the surface layers of the sample molded at lower melt temperature. Example 3 The blend of example 2 was injection molded at melt temperature 20 l 'C both with 10 % injection speed of the maximum speed of the machine and with maximum injection speed. The conductivity profiles of the injection molded samples are presented in figures 4 and 5. It can be seen that the conductivity profile of the sample molded with lower injection speed is steeper than that of sample molded with higher injection speed.

Claims: CLAIMS 1. An electrically conductive article, characterized in that it contains a conductive polymer and that the surface resistivity of its outer surface, expressed in Ω/square, is at minimum one decade, and preferably at minimum two decades, higher than volume resistivity of the interior, expressed in Ω-cm. 2. An article according to Claim 1, characterized in that the surface resistivity of the outer surface is approx. 10 5 -10 12 Ω/square, preferably approx. 10 6 -10 10 Ω/square, and most preferably approx. 10 6 -10 9 Ω/square, and the volume resistivity of the interior is approx. 10 2 -10 8 Ωcm, preferably approx. 10 2 -10 6 Ωcm, and most preferably approx. 10" 2 -10 4 Ωcm. 3. An article according to Claim 1 or 2, characterized in that the material contains a melt-processible conductive polymer component and preferably a melt-processible matrix plastic. 4. An article according to Claim 3, characterized in that the conductive polymer component is a mixture which contains a polyamline or a polyaniline derivative, rendered conductive by doping, preferably a polyaniline or a polyaniline derivative, doped with an acid. 5. An article according to Claim 4, characterized in that the mixture contains a polyani¬ line or a polyaniline derivative, doped with an acid, which acid is a functional doping acid, such as an organic, preferably aromatic, sulfonic acid, such as alkylbenzene sulfonic acid, and in particular dodecylbenzene sulfonic acid. 6. An article according to Claim 5, characterized in that the conductive polymer mixture contains a metal compound, such as zinc oxide. 7. An article according to any of Claims 3-6, characterized in that the matrix plastic contains a thermoplast, such as polyolefin, polyvinyl chloride, styrene-butadiene polymer, polyester, polyamide, acrylonitrile-butadiene-styrene polymer, or polycarbonate. 8. An article according to Claim 7, characterized in that the matrix plastic contains polyolefin, preferably polypropylene or polyethylene, most preferably polypropylene. 9. An article according to any of Claims 3-6, characterized in that the matrix plastic contains a thermoset, such as phenol-formaldehyde polymer or melamine-formaldehyde polymer. 10. An article according to any of Claims 1-9, characterized in that the entire article is manufactured from one material. 11. A method for the manufacture of an electrically conductive article, characterized in that an article which contains conductive polymer and the surface resistivity of the outer surface of which, expressed in Ω/square, is at minimum a decade higher than volume resistivity of its interior, expressed in Ω-cm, is manufactured preferably by a melt- processing method. 12. A method according to Claim 11, characterized in that the article is prepared with such a melt processing method that during processing the material is exposed to shear rates equal to 100 1/s or higher. 13. A method according to Claim 12, characterized in that the melt processing method is injection molding. 14. A method according to any of Claims 11-13, characterized in that by a melt proces- sing method is prepared an article which contains conductive polymer and melt processi- ble matrix polymer and that in the processing conditions the viscosity of the matrix polymer is higher than viscosity of conductive polymer, preferably higher than viscosity of conductive polymer but lower than ten times viscosity of conductive polymer. 15. A method according to Claims 13 or 14, characterized in that injection speed lower than 70 %, preferably lower than 50 % and most preferably lower than 30 % of maxi¬ mum speed of the machine is used. 16. A method for the prevention of electrostatic interferences caused by an article, characterized in that an article according to any of Claims 1-10 or an article manufactu¬ red according to Claim 11-15 is used. 17. A method for shielding a product from electrostatic interferences, characterized in that the product is placed inside an article according to any of Claims 1-10 or an article manufactured according to Claim 11-15.

Inventors: JUSSILA MATTI, KIRMANEN PAULI, SAVOLAINEN ESKO, VAEKIPARTA KIMMO, VIRTANEN ESA, JUSSILA, MATTI, KIRMANEN, PAULI, SAVOLAINEN, ESKO, VAEKIPARTA, KIMMO, VIRTANEN, ESA
Applicants: JUSSILA MATTI, KIRMANEN PAULI, NESTE OY, SAVOLAINEN ESKO, VAEKIPARTA KIMMO, VIRTANEN ESA, JUSSILA, MATTI, KIRMANEN, PAULI, SAVOLAINEN, ESKO, VAEKIPARTA, KIMMO, VIRTANEN, ESA
Application year: 1996
Main classification: C08L101
IPC codes: H01B1