Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2001-02-27
2003-07-22
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C264S614000, C264S641000, C106S472000
Reexamination Certificate
active
06596199
ABSTRACT:
FIELD OF THE INVENTION
This application relates to blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) resin that are reinforced with carbon fiber. The compositions have improved impact strength as well as excellent electrostatic dissipative characteristics.
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, notebook computer enclosures 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 insulative nature of 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 can be incorporated into polymeric materials to modify the electrical properties to achieve any of these three characteristics. In particular, carbon fibers facilitate dissipation of static charge and provide enhanced electromagnetic shielding. (See, for example, U.S. Pat. Nos. 4,559,164 and 5,004,561).
TABLE 1
Categories of Materials for ESD/EMI Protection
Material
Category
Material Description
Antistatic
Will not generate a charge.
Will not allow a charge to remain localized on part
surface.
Refers to a material′s ability to resist triboelectric charge
generation.
Static
Will not generate a charge.
Dissipating
Will not allow a charge to remain localized on part
surface.
Can safely bleed an electric charge to ground.
Surface resistivity between 10
5
and 10
9
Ohm/Sq.
Conductive
Will not generate a charge.
Will not allow a charge to remain localized on part
surface.
Can ground a charge quickly.
Will shield parts from electromagnetic fields.
Surface resistivity < 10
5
Ohm/Sq.
Carbon fiber suppliers treat fiber surfaces to tailor the fibers for specific resin(s). Typical surface treatments used include air oxidation, anodic oxidation and wet oxidation. (J. M. Clark and D. R. Secrist, “Effect of Fiber Surface Treatment on the Properties of Carbon Fiber Reinforced Nylon Composites,” Journal of Thermoplastic Composite Materials, Vol. 1, pp. 232-241 (1988); E. Fitzer and R. Weiss, “Effect of Surface Treatment and Sizing of C-Fibers on the Mechanical Properties of CFR Thermosetting and Thermoplastic Polymers,” Carbon, Vol. 25, No. 4, pp. 455-467, (1987)). Binder is applied to these surface treated fibers and fibers are chopped. The purpose of the binder is to hold fiber bundles together (during handling and feeding). The binder is supposed to dissolve in the polymer matrix at high temperatures experienced during compounding performed in extruders. Utilization of binders is described in U.S. Pat. Nos. 4,781,947; 5,298,576; 5,641,572 and 5,639,807.
While carbon fibers have been utilized to improve electrical properties in thermoplastic resins, such fibers also contribute significantly to the cost of the articles, and can be detrimental to the impact strength and processability of the thermoplastic resin when used for injection molding. This problem is particularly significant in blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene copolymer (ABS). Such blends offer outstanding balance of mechanical properties, heat resistance, flow and cost. Performance of PC-ABS blend-based materials can be tailored for a particular application by varying the ratio of PC and ABS components. PC-ABS blends are often formulated with carbon fibers to impart high modulus and static dissipative properties. Due to the ease of processing and high flow, carbon fiber-filled PC-ABS compounds can be used to fill thin wall sections and complex parts in injection molding processes. However, one of the drawbacks of carbon fiber-filled PC-ABS is its low impact strength. Improvement in impact strength of carbon fiber-filled PC-ABS compositions, without affecting the modulus, static dissipative characteristics and cost would therefore be very useful.
SUMMARY OF THE INVENTION
The present invention provides a carbon fiber-filled PC-ABS resin composition which has improved electrical properties at a given level of carbon fibers, and which does not suffer from as significant a decrease in impact strength as would result from the introduction of generic carbon fibers. In the composition of the invention, the carbon fibers are associated into bundles with a polyamide terpolymer binder. The bundles are dispersed within the PC-ABS blend. The compositions of the invention can be used for injection molding of articles for use as components in applications requiring static dissipation and/or EMI shielding. Such articles include, but are not limited to electronic devices, dust handling equipment and notebook computer enclosures.
DETAILED DESCRIPTION OF THE INVENTION
The composition of the invention comprises a polymer blend of polycarbonate and an acrylonitrile-butadiene-styrene copolymer, and carbon fibers associated into bundles with a polyamide terpolymer binder. The bundles are dispersed within the polymer blend.
The composition of the PC-ABS blend may be varied to achieve desired properties in the final material. In general, the blend will range in composition from 10 to 90% PC by weight, preferably about 45 to 75% by weight, with lower amounts of PC being used when it is desired to have a composition with good flow and larger amounts of PC being used to produce a more heat resistant product. The ABS component may be an acrylonitrile-butadiene-styrene terpolymer or a blend of styrene-butadiene rubber and styrene-acrylonitrile copolymer. As used herein, the term “acrylonitrile-butadiene-styrene copolymer” or “ABS” refers to either of these alternatives.” A specific PC-ABS blend which may be used is sold by General Electric Co. under the tradename CYCOLOY (for example CYCOLOY C6200).
Examples of carbon fibers which may be suitably employed in the composition of the invention include those sold under the following tradenames: FORTAFIL CA and FORTAFIL CM (Fortafil Fibers, Inc), ZOLTEK HT (Zoltek Corporation), TORAY (Toray Industries, Inc.), and GRAFIL (Mitsubishi). The fibers are associated into bundles with a polyamide terpolymer binder, such as the polyamide terpolymer binder sold by DuPont under the tradename ELVAMIDE. Such binder-treated fibers can be produced by a conventional carbon fiber manufacturing process. Continuous filament carbon fibers are produced by pyrolyzing, or decomposing by heating, carbon-containing fibers such as rayon, polyacrylonitrile and petroleum pitch. The carbon fibers retain the physical shape and surface texture of the precursor fibers from which they are made. After carbonization, the fibers are surface treated. Then, the binder is applied on the fiber surface, after which the fibers are chopped to produce chopped products.
In the binder application process, continuous fiber bundles are pulled in a wet bath to coat the fibers with a desired amount of binder. The binder-coated fiber bundles, which are called a “wet forming package” are then either dried to produce a “dried forming package” or passed directly to the chopping process. The amount of binder is suitably from 0.5 to 10% by weight of the fibers.
Chopped strands can be produced by either of two major processes. In the first process, dried-forming packages are used as the source. A number of strand ends are fed into a chopper, which chops them into the correct length, for example ⅛ inch to ½ inch (0.31 to 1.27 cm) in size. The product is then scr
General Electric Company
Kopec Mark
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