LOW IONIC CONTENT CONDUCTIVE THERMOPLASTIC
MATERIALS
FIELD OF THE INVENTION
The present invention relates to conductive thermoplastic compositions that have a low content of ionic contaminants.
BACKGROUND OF THE INVENTION
Articles made from thermoplastic resins are commonly utilized in the material-handling devices, electronic devices and business equipment, for example chip carriers, and printer and copier components in contact with moving paper such as paper paths - and moving components themselves - such as ink-jet printer penholders. Electrostatic dissipation is an especially important issue within the electronic industry because of the inherently insulating nature of organic polymeric materials. Electrostatic dissipation (or discharge) is defined as a transfer of electrostatic charge between bodies at different potentials caused by direct contact or induced by an electrostatic field. As electronic devices become smaller and faster, their sensitivity to electrostatic dissipation (ESD) increases.
The US Department of Defense Handbook 263 (DOD-HDK-263) defines three categories of plastics for use in ESD protection: antistatic, static dissipating, and conductive. Characteristics of each type are listed in Table 1. Conductive fillers such as carbon fibers and carbon powder can be incorporated into polymeric materials to modify the electrical properties to achieve any of these three characteristics.
Table l Categories of Materials for ESD/EMI Protection
Electronic components such as disk cassettes are often required to be able to dissipate static electricity or to provide shielding from electromagnetic interference (EMI, so-called EMI shielding). Thermoplastic materials that have been filled with conductive additives are the materials of choice for such applications as they offer design flexibility and cost-effectiveness. Many
electronic, telecommunication and computer applications also require the ability to dissipate static electricity and/or to provide EMI shielding.
During the last ten years there has been an explosive growth in the development of electronic components for industrial, commercial and domestic environments. While many of these components can be fabricated from metal housings and conventional insulators, the use of engineered thermoplastic materials provides many advantages in terms of weight, cost and ease of manufacture.
However, when non-conductive thermoplastics are utilized in articles of manufacture that employ electronic components such as computing devices, microprocessors, circuit boards and the like, static electricity can build up affecting the performance of the electrical devices from capacitance, inductance and discharge effects.
Commercially available thermoplastic materials are almost uniformly insulating in character. In order to impart conductivity to such materials or to provide for EMI shielding conductive fillers are used. Conductive fillers span a wide range of materials ranging from metallic fillers in powder, flake, fiber and filament form and extending to carbon based conductive materials such as graphite, carbon powder, carbon fibers, carbon fibrils and the like. As used in this patent application a conductive filler is a filler that when dispersed in an insulating thermoplastic material lowers the bulk volumetric electrical resistivity of the filled material relative to the bulk volumetric electrical resistivity of the unfilled thermoplastic.
The chemical processes used to produce many thermoplastic materials such as polycarbonate, polystyrene, polyolefins, polyesters, polyethers and the like lead to the production of thermoplastic materials that are contaminated with varying levels of ionic species, for example Mg+2, Ti+4, Cr+3, Fe+s, and Al+3 and other cations; and F-, Ch, Br, SO4"2, N02", N03", and P04-3 and other anions. Many conductive fillers, such as conductive carbon black also contain
high levels of ions. Indeed, one such form of carbon, activated carbon, is used precisely for its ability to capture and retain contaminant cations and anions.
BRIEF STATEMENT OF THE INVENTION
The present invention provides for a process for reducing the concentration of ionic contaminants in thermoplastic materials comprising a conductive filler having an initial concentration of ionic contaminants comprising:
(a) providing a thermoplastic material in a molten state and
(b) contacting the thermoplastic material with
(i) water at a temperature sufficient to convert the water to steam or
(ii) steam
whereby the initial concentration of ionic contaminants is reduced to a final concentration of ionic contaminants wherein the final concentration of the ionic contaminants is below the initial concentration of the ionic contaminants in the thermoplastic material.
The process of the present invention further provides a process for reducing the concentration of ionic contaminants in thermoplastic materials having an initial concentration of ionic contaminants where the thermoplastic material is selected from the group consisting of end-capped polyacetals, poly (oxymethylene), polyformaldehyde, poly (trichloroacetaldehyde), poly (n- valeraldehyde), poly (acetaldehyde), poly (propionaldehyde), acrylic polymers, polyacrylonitrile, polyacrylamide, poly (acrylic acid), poly(methacrylic acid), poly(ethyl acrylate), poly(methyl methacrylate), fluorocarbon polymers, poly(tetrafluoroethylene), perfluorinated ethylene-propylene copolymers, ethylene-tetrafluoroethylene copolymers, poly(chlorotrifluoroethylene), ethylene-chlorotrifluoroethylene copolymers, poly(vinylidene fluoride), poly(vinyl fluoride), polyamides, poly(6-aminocaproic acid) or poly( epsilon-
caprolactam), poly(hexamethylene adipamide), poly(hexamethylene sebacamide), poly(n-amino-undecanoic acid), polyaramides, poly(imino-ι,3- phenyleneiminoisophthaloyl), poly(m-phenylene isophthalamide), parylenes, poly-p-xylylene, poly(chloro-p-xylylene), polyaryl ethers, poly(oxy-2,6- dimethyl-ι,4-phenylene), poly(p-phenylene oxide), polyaryl sulfones, poly(oxy-l,4-phenylenesulfonyl-ι,4-phenyleneoxy-ι,4-phenylene- isopropylidene-l,4-phenylene), poly(sulfonyl-ι,4-phenyleneoxyi,4- phenylenesulfonyl-4,4'-biphenylene), polycarbonates, such as poly(bisphenol- A), poly(carbonyldioxy-ι,4-phenyleneisopropylidene- 1,4-phenylene), polyesters, poly(ethylene terephthalate), poly(tetramethylene terephthalate), poly(cyclohexylene-ι,4-dimethylene terephthalate), poly(oxymethylene-i,4- cyclohexylenemethyleneoxyterephthaloyl), polyaryl sulfides, poly(p-phenylene sulfide), poly(thio-ι,4-phenylene), polyimides, poly(pvromellitimido-ι,4- phenylene), polyolefins, polyethylene, polypropylene, poly(ι-butene), poly(2- butene), poly(ι-pentene), poly(2-pentene), poly(3-methyl-ι-pentene), polyfø- methyl- 1 -pentene), ι,2-poly-ι,3-butadiene, ι,4-poly-ι,3-butadiene, polyisoprene, polychloroprene, polyacrylonitrile, poly(vinyl acetate), poly(vinylidene chloride), polystyrene, copolymers and terpolymers thereof.
The process of the present invention further provides a process of reducing the concentration of ionic contaminants in thermoplastic materials having an initial concentration of ionic contaminants wherein the thermoplastic is selected from the group consisting of polycarbonate, polyetherimide, polymers of acrylonitrile-butadiene-styrene, polymers of acrylonitrile-styrene-acrylic acid, polymers of acrylonitrile-styrene-methacrylic acid, polybutylene terephthalate, polyphenylene sulfide, polyphenylene ether, polyvinyl chloride, polystyrene and polyamide.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process for the removal of contaminant ionic species from composites comprising thermoplastics and some form of electrically conductive carbon wherein ' the composite has
increased electrical conductivity relative to the thermoplastic by itself. The present invention also provides for a composite composition that possesses lower levels of ionic contaminant species in the thermoplastic matrix of the composite. The present invention further provides for articles of manufacture made from the thermoplastic composites of the present invention wherein magnetically stored and encoded data housed in the article of manufacture remain stable for longer periods of time than in such an article manufactured from similar compositions but not subjected to the process of the present invention prior to their use in producing the article of manufacture.
When filled thermoplastic materials containing ionic contaminants are used to manufacture magnetic data storage devices such as tape cassettes, disc drives, integrated circuits and the like magnetic coupling can occur between the magnetic domains of the data storage portion of the article of manufacture and the contaminant ions causing loss of signal integrity and relaxation of the magnetic state utilized to encode data. The process of removing contaminant ions from thermoplastic composites does not affect the ordinary properties of the composite such as melt flow, impact strength, multi-axial impact, conductivity or resistivity however it does favorably affect the degree to which magnetic coupling and its associated relaxation occur with magnetically encoded states by reducing the extent to which this phenomenon is thought to occur between such states and the contaminant ions that normally have no deleterious effect on the composite.
As used in this patent application, the term thermoplastic material covers all thermoplastic polymers that are derived from a monomeric species that contains at least one carbon atom. Thus the following list of thermoplastic materials is by way of illustration only: end-capped polyacetals, such as poly (oxymethylene) or polyformaldehyde, poly (trichloroacetaldehyde), poly (n- valeraldehyde), poly (acetaldehyde), poly (propionaldehyde), and the like; acrylic polymers, such as polyacrylamide, poly (acrylic acid), poly (methacrylic acid), poly (ethyl acrylate), poly (methyl methacrylate), and the like; fluorocarbon polymers, such as poly (tetrafluoroethylene), perfluorinated
ethylene-propylene copolymers, ethylene-tetrafluoroethylene copolymers, poly(chlorotrifluoroethylene), ethylene-chlorotrifluoroethylene copolymers, poly(vinylidene fluoride), poly(vinyl fluoride), and the like; polyamides, such as poly(6-aminocaproic acid) or poly( epsilon -caprolactam), poly(hexamethylene adipamide), poly(hexamethylene sebacamide), poly(n- amino-undecanoic acid), and the like; polyaramides, such as poly(imino-ι,3- phenyleneiminoisophthaloyl) or poly(m-phenylene isophthalamide), and the like; parylenes, such as poly-p-xylylene, poly(chloro-p-xylylene), and the like; polyaryl ethers, such as poly(oxy-2,6-dimethyl-ι,4-phenylene) or poly(p- phenylene oxide), and the like; polyaryl sulfones, such as poly(oxy-ι,4- phenylenesulfonyl-ι,4-phenyleneoxy-ι,4-phenylene-isopropylidene-l,4- phenylene), poly(sulfonyl-ι,4-phenyleneoxyι,4-phenylenesuIfonyl-4,4'- biphenylene), and the like; polycarbonates, such as poly(bisphenolA) or poly(carbonyldioxy-ι,4-phenyleneisopropylidene- 1,4-phenylene), and the like; polyesters, such as poly(ethylene terephthalate), poly(tetramethylene terephthalate), poly(cyclohexylene-ι,4-dimethylene terephthalate) or poly(oxymethylene-i,4-cyclohexylenemethyleneoxyterephthaloyl), and the like; polyaryl sulfides, such as poly(p-phenylene sulfide) or poly(thio-ι,4- phenylene), and the like; polyimides, such as poly(pyromellitimido-ι,4- phenylene), and the like; polyolefins, such as polyethylene, polypropylene, poly(ι-butene), poly(2-butene), poly(ι-pentene), poly(2-pentene), poly(3- methyl-i-pentene), poly(4-methyl- 1 -pentene), ι,2-poly-ι,3-butadiene, 1,4- poly- 1,3- butadiene, polyisoprene, polychloroprene, polyacrylonitrile, poly(vinyl acetate), poly(vinylidene chloride), polystyrene, and the like: copolymers of the foregoing, such as acrylonitrile-butadiene-styrene (ABS) copolymers, and the like. In certain embodiments, the polymeric fabric will be prepared from a polyolefin.
As broadly conceived the process of the present invention comprises contacting a molten thermoplastic comprising a conductive filler with steam to extract the contaminant ions from the thermoplastic. In the case of fillers that are porous materials such as activated carbon, carbon fibers and the like, contaminants in the filler may also be removed from the filler. The
application of a vacuum may assist in the steam stripping or removal of contaminant ions from the composite.
The process of the present invention achieves levels of ionic contamination in materials comprising thermoplastic polymers where the level of ionic contamination is below about 100 ppb (parts per billion) preferably below about 75 ppb more preferably below about 50 ppb and most preferably below about 25 ppb of ionic contaminants in the thermoplastic resin or thermoplastic resin composite.
DESCRIPTION OF PREFERRED EMBODIMENTS
In the process of manufacture of various thermoplastic polymers, the process of the present invention may be utilized by steam stripping the thermoplastic polymer while it is still molten either in a separate vessel that allows for the introduction of steam and the optional application of a vacuum to assist in removing the steam or in a process line that mimics the function of the extruder as hereinafter described. In these various embodiments the process of the invention is functionally described as a treatment of the thermoplastic material with steam whereby the final concentration of ionic contaminants is reduced relative to the initial concentration of ionic contaminants by the application of steam to the thermoplastic material. Additionally, it has been found that the application of a vacuum alone can produce a reduction in the level of contaminant ions.
The continuous extruder used for one embodiment of the process of the present invention can be conceived of as being partitioned into eight sequential sections, the screw design, connected as follows: 1) a first section, a feed section; followed by 2) a second section, a solids pumping and conveying section; followed by 3) a third section, a primary melting and mixing section; followed by 4) a fourth section, a first melt seal having a vent; followed by 5) a fifth section, a steam stripping section; followed by 6) a sixth section, a second melt seal having a vent; followed by 7) a seventh section, a melt pumping and conveying section;, followed by 8) an eighth section, a die. Molten
thermoplastic is sequentially processed through the sections of the extruder being steam stripped in section five between the first and second melt seals, finally exiting the extruder through the die at section eight.
Samples of conductive carbon black powder filled thermoplastics were prepared by compounding in a twin screw extruder. The screw design used for this purpose utilized a primary melting section to melt the polymer resin and mix the filler, in these particular cases, carbon black powder, and other additives as desired. Sequential to the primary melting section, i.e. downstream of the primary melting section, the screw design utilizes a steam stripping section that is a closed section achieved by creating two additional sections that are melt seals that are created for example by using left handed screw bushings or left handed kneading elements. Water was introduced (or injected) into the steam stripping section between the two melt seal sections. Due to the high heat the water is vaporized becoming steam (it should be noted that either water or steam may be injected, preferably steam produced from deionized water or deionized water itself). Narrow disc kneading elements, turbine kneading elements or other types of mixing elements can be used for distributive mixing in the steam stripping section. Distributive mixing allows even distribution of ingredients by continuous rearrangement and deformation of the polymer melt. The vapor in the steam stripping section is allowed to escape through the vent in the sequentially related, i.e. downstream section just after the second melt seal, this is also one of the places where if it is desirable to employ a vacuum that a vacuum may be applied. A vacuum may be optionally employed along with the steam injection to assist in removing steam from a steam stripping section. If it is desired, additional steam stripping sections may be added along with any necessary intervening melt seal sections.
EXPERIMENTAL TECHNIQUES
Material was compounded by the procedure described and the ion content of the compounded material was analyzed by the following procedure. A known
weight of the sample is dried in a clean centrifuge tube and then a known weight of water is added to the tube and the tube containing the water and the compounded sample is placed in an oven at 85 °C for one hour. After one hour the tube is removed from the oven and the content of leachable ions is analyzed using ion chromatography (IC) or inductively coupled plasma - atomic emission spectroscopy (ICP-AES).
Table 1 shows the leachable ion content achieved when a low ionic content polycarbonate resin, characterized in Table 2, is used and compounded with conductive carbon black. Despite a high level of contaminant ions in the carbon black used (Table 3) much lower levels of leachable ions are achieved in the compounded material (Table 1). Table 4 shows the results obtained when a higher ionic content polycarbonate resin is used and compounded with the carbon black. The results in Table 4 show the usefulness of the process of the present invention to lower the levels- of ionic contaminants in compounded composites comprising a thermoplastic and a filler. Table 5 shows the results of a similar study when the thermoplastic material is polystyrene.
Because the process of the present invention deals with reducing levels of contaminant ions to the parts per billion level, laboratory procedures and reagents consistent with the detection of low levels of species should be employed otherwise it will not be possible to detect the improvements sought.