Background of the Invention

The electrical insulation properties inherent in common thermoplastic materials pose serious problems for their users in certain appli- cations, primarily electronics, but also in explosive environments. Attempts to solve these problems with thermoplastic resins compounded with additives fall into the two broad categories of electrostatic control and electromagnetic interference or radio frequency interference (EMI/RFI) shielding. Electrostatic control compounds vary by their surface or volume resistivity. Anti-static compounds range from 10 9-1012 ohms per sq.; static dissipative compounds range from 10 5-109 ohms; and conductive compounds range from 10 2-105 ohm-cm. EMI/RFI shielding com- pounds typically range over 10 0-102 ohm-cm.

Anti-static and static dissipative compounds are typically produced with surface modifying addi¬ tives. These compounds suffer from moisture depen- dence, lack of permanence, tacky feel, and

contamination problems. Conductive compounds typical¬ ly employ conductive carbon powder additives such as Cabot XC-72 or Noury Ketjenblack. These powders embrittle their base resin and produce compounds which slough conductive carbon particles during use. The powder also makes all molded parts or articles black in color and inhibits material flow during molding.

A number of approaches are being taken in an attempt to solve the EMI/RFI shielding problem. One main approach is through the application of metallic surface coatings to the molded plastic articles. Such metallic surface coatings are applied by the use of vacuum deposition, metal foil linings, metal-filled coatings and other means. Each of these procedures suffers from one or more drawbacks with respect to costs, adhesion, scratch resistance, environmental resistance and difficulties in adequately protecting many of the different structural forms in which the molded plastic must be provided. In addition, attempts have been made to resolve the problem of EMI or RFI by formulation of composite plastic materials based upon the use of various fillers in thermoplastic matrices. For example, conductive fillers that have been employed for this purpose are carbon black, carbon fibers, silver coated glass beads and metallized glass fibers or carbon fibers. The fibers that have been employed in composite materials are

primarily subject to the disadvantage of being brittle such that they break up into shorter lengths in processing. Also, shorter length conductive fibers and particles require higher loadings in the plastic matrix leading to higher costs and embrittlement of the plastic matrix.

Examples of issued patents directed to past approaches to solving the problem of EMI or RFI include U.S. Pat. No. 4,155,896 which discloses the use of metallic fibers or glass fibers having a metallic coating to provide a conductive fiber for incorporation into an organic paint for conductivity. Aluminum coated glass fibers impregnated with resin and woven articles produced therefrom have been employed for EMI shielding as disclosed in U.S. Pat. No. 4,234,648. Thermoplastic compositions containing aluminum flakes, aluminum flakes with carbon fibers, carbon black, carbon fibers, carbon fibers with carbon black, metal coated glass fibers or steel fibers, for the formulation of conductive composite plastics have been disclosed in U.S. Pat. Nos. 4,404,125; 4,474,685; 4,486,490; 4,490,283; 4,500,595; 4,545,914 and 4,566,990. These patents are illustrative of the prior art attempts to make conductive thermoplastic composites or compositions having EMI/RFI shielding effectiveness upon molding into useful articles.

In spite of the significant effort to resolve the problem of EMI or RFI by coating molded plastics or formulating composite plastic pellets with conductive fillers for molding plastic parts, none of the products developed heretofore have proven com¬ pletely satisfactory. Summary of the Invention

In accordance with the present invention, a thermoplastic resin pellet is provided for forming useful articles having improved conductive or EMI/RFI shielding properties. The thermoplastic pellet is comprised of a thermoplastic matrix having incor- porated therein a multitude of flexible conductive fibers made from an organic polymer containing a conductive component such as a filler or coating. The thermoplastic pellets of this invention can be molded into a product having excellent conductivity or EMI/RFI shielding properties. Thus, the molded thermoplastic products of the present invention are very suitable for electrostatic control in material handling, packaging, and shipping containers, and EMI/RFI shielding purposes in a wide variety of end use products such as computers, transmitters, elec¬ tronic parts and the like. The flexible conductive polymeric fibers that are employed in the thermoplastic matrix of the pellet of this invention can be prepared according to

procedures that are known in the art. In fact, the use of electrically conductive carbon black, for instance, dispersed throughout fibers for antistatic textile applications is described in U.S. Pat. No. 2,845,962. U.S. Pats. Nos. 3,803,453; 3,823,035;

3,969,559; 4,045,949; 4,061,827; 4,207,376; 4,255,487; 4,303,733 and 4,388,370 are other examples of patents disclosing conductive organic polymeric fibers. However, it was not heretofore known to employ such a conductive fiber as a flexible conductive filler in a thermoplastic matrix in the form of elongated pellet or particle in order to overcome the deficiencies men¬ tioned in the background of this invention in the production of molded parts having conductivity or EMI/RFI shielding.

In another aspect of this invention, novel methods of making the thermoplastic pellets are disclosed using thermosetting or crosslinkable fiber compositions that are crosslinked during the formation of the pellet by extrusion. In this form of the invention, crosslinkable thermoplastic fibers con¬ taining conductive filler and crosslinking agent are coextruded with a surrounding thermoplastic matrix polymer during which the crosslinkable fibers are crosslinked to produce a strand containing flexible conductive fibers containing thermoset or crosslinked

polymer. The strands can then be formed into pellets for ultimately molding a conductive plastic article.

This invention offers a cost effective solution to the problems associated with existing conductive thermoplastic molding compounds or coated plastic articles now used to provide electrostatic control or EMI/RFI shielding. The method of making thermoplastic pellets of this invention containing the flexible organic conductive filaments involves drawing or extruding the organic polymer fiber through a molten resin bath to produce short or long fiber products wherein the fibers are contained in the matrix of plastic resin. The polymeric strands so produced can be cut to form pellets which may then be used to form the finished conductive part. Thus, the fiber length of the finished part is limited by the compound pellet which is cut from the strands produced by such extrusion processes. It has been found that flexible organic polymer fibers containing a conduc- tive filler or coating will pass through conventional thermoplastic processing equipment unbroken and without adversely affecting conductivity properties in the molded article. Thus, this invention overcomes problems associated with anti-static compounds and fiber breakage according to prior art techniques and the disadvantages associated therewith such as higher loadings leading to embrittlement of the plastic

matrix and higher costs which have rendered them commercially unacceptable.

An essential feature of the thermoplastic pellets of this invention is the employment of flexi- ble conductive organic polymer fibers (FCF) having a melt temperature significantly higher than the resin into which the conductive fibers are compounded. The conductive fibers may be of the thermoplastic or thermosetting nature as explained above and will be understood in view of this description. The con¬ ductivity of the molded articles produced from these FCF/thermoplastic pellets can be controlled via the amount of FCF in the compound, the conductivity of the FCF, or the length of the compound pellet.

Detailed Description

The composite pellets of this invention are thermoplastic so that they may be formed and molded products may be made therefrom. The flexible organic polymeric fiber component of the composite may be made from thermoplastic and thermosetting resins. The fiber component may be rendered conductive by blending coating or otherwise incorporating a conductive component in the fiber polymer. In its broadest sense, the composite pellet of this invention includes a flexible conductive organic polymer component in a matrix of thermoplastic resin. In such form, the fiber may be incorporated into the resin matrix by either of the above mentioned techniques, i.e., impregnation of fiber filaments in a molten resin bath or impregnating with a resinous suspension or emulsion and subsequently heat drying the resin around the filaments. While flexible conductive organic fibers have not been made in accordance with the principles of this invention, techniques such as those described in U.S. Pat. Nos. 2,877,501 or 3,042,570 may be adapted to make the pellets of this invention and in view of the description herein. These patents dis¬ closures are incorporated herein. As opposed to carbon fibers or other such fibers which suffer from breakage, the organic fibers of this invention are flexible and therefore receptive

to handling in the plastic matrix in the formation of pellets by blenders and extruders without breakage. After the pellets or elongated granules containing the flexible conductive fibers in the thermoplastic resin matrix are prepared, the resulting pellet composite may then be molded in accordance with known proce¬ dures. Homogenization of the pellets into uniformly molded product containing the conductive fibers will be achieved in the molding step. Conductive flexible organic polymeric fibers of this invention may be prepared in accordance with any one of a number of known techniques as set forth in the patents listed above, i.e., U.S. Pats. Nos. 3,803,453; 3,823,035; 3,969,559; 4,045,949; 4,061,827; 4,207,376; 4,255,487; 4,303,733 and 4,388,370. The disclosures of these patents are incorporated herein by reference as will be understood to those of ordi¬ nary skill in the art. The exact technique for forming the conductive fiber is not material to the broad claims of this invention. In general, it is desirable to have very thin fibers or filaments, consistent with the objective of obtaining conduc¬ tivity, EMI or RFI shielding properties in the resul¬ tant molded article. There is obviously a fiber diameter lower limit in order to achieve conductivity and processing stability. An advantage of the organic polymeric fiber composite of this invention is that

the polymeric fibers will have a minimal affect on the density, rheology, shrinkage and other physical properties of the matrix resin. Fiber length is important and the objective is to produce the longest fiber in the pellet and the finished part. Fiber length in the finished part then becomes limited by the compound pellet which is most preferably prepared by the above fiber extrusion processes in resin matrix and then cut into pellet form. Of course, the fibers could be fed into conventional compounding extruders. Fiber diameters in this form are on the order of about 0.0005" to 0.010". It is understood that by maxi¬ mizing the conductivity of the fiber and minimizing its diameter, cost effective conductivity and shield- ing composites can be produced. Usually a long fiber process produces pellets on the order of about 1/16 inch to 1 1/2 inches in length having a nominal cross-section of about 1/16 inch to about 1/4 inch. This long fiber process generally involves the use of continuous lengths of conductive organic polymer filaments which are passed through a bath containing the molten resin whereby such filaments become impreg¬ nated or coated with the desired quantity of resin. Once the continuous filaments are impregnated or coated they are continuously withdrawn from the bath, comingled either before or after passage through the heat source, and cooled to solidify the molten resin

around the conductive organic fibers followed by a substantially transverse severing operation to form the pellets or short pieces. Thus, in this form the flexible conductive organic fibers usually extend substantially parallel to each other and substantially parallel to the axis defined by the direction in which the materials are withdrawn from the bath. However, as is understood, rather than a bath of molten resin, the filaments may be impregnated with a resin sus- pension or emulsion and subsequently subjected to sufficient heat to dry and fuse the resin around the co ingled flexible organic fibers as described above. Typical extrusion compounding may also be employed.

The proportions by weight of the conductive fiber component in the final pellet can be varied over a range in order to achieve conductivity in the molded part. Generally from about 1 to about 20% by weight of the conductive fiber may be employed. The objec¬ tive is to obtain the desired electrostatic control or conductivity with a minimum amount and still achieve desired processing of resin along with other physical properties in the end product. Selection of the proportion will be dependent on the end application or the particular objective sought. For example, from about 1 to about 5% may be advantageous for electro¬ static dissipation in the end product whereas from about 1 to about 20% by weight may be necessary for

EMI or RFI shielding applications. Other conventional fillers, pigments and the like may be included in the pellets such as conventional glass fiber as an extender.

The thermoplastic resins suitable for use as the matrix resin in the composite pellet include polyolefins, particularly polypropylene and copolymers of ethylene and propylene; polystyrene, styrene- acrylonitrile polymers, polymers based on acrylo- nitrile-polybutadiene-styrene(ABS) ; nylons, particu¬ larly Nylon 6,6; polyphenyleneoxides or ethers; poly- phenylene oxide-polystyrene blends; polyphenylene sulfides; polyacetals; polysulfones; polycarbonates; polyurethanes; cellulose esters; polyesters such as polyethylene terephthalate; polymonochlorostyrene; acrylic polymers; polyvinyl chlorides; polyvinylidene chlorides; -copolymers of vinyl-chloride and vinylidene chloride, various thermoplastic elastomers such as those based on styrene and butadiene or ethylene or propylene; and blends of any of the foregoing resins. The flexible organic conductive filaments may be made from any of these thermoplastic resins just mentioned or from other thermoplastic resins or thermosetting resins. Of course, the flexible fibers in the pellet must have a melt temperature higher than the resin into which the fibers are compounded. In the case of a thermosetting composition, ethylenevinylacetate

copolymers may be employed, for instance, along with a crosslinking agent such as peroxide in the presence of a conductive filler such as carbon. Therefore, the filament or fiber of the composite may be either thermoplastic or thermosetting, again, depending upon the result intended. In either case, the objectives of this invention may be achieved by employing such a flexible organic polymeric conductive filament to overcome the deficiencies of the prior art thereby eliminating a breakage and maintaining the process- ability of the matrix resin to achieve the desired conductivity. Typical flexible organic polymeric filaments include Nylon 6,6 and polyethylenetere- phthalate, polyphenylenesulfides, teflon, and other high temperature polymeric fiber material. The requirement of the fiber is to have a sufficient melt temperature or processability higher than the matrix resin into which the flexible conductive fibers are blended so that the conductivity of the fiber may not be destroyed. Accordingly, in processing the com¬ posite material of the invention, the pellets are fed in a normal manner to a feed hopper of an injection molding machine. The pellets are processed through equipment in the usual manner at temperatures and conditions which render the resin molten for suitable injection into a mold whereby useful articles are made.

The following examples illustrate practice of the present invention and other variations thereof will be understood to a person of ordinary skill in this art. EXAMPLE 1

Employing a cross-head die compounding apparatus, a composite pellet containing a conductive Nylon 6 fiber in a low density polyethylene (LDPE) was made. The conductive Nylon 6 fiber was BASF F-911 which consisted of 90 filament, 21 denier conductive Nylon 6 fiber containing carbon. One strand of the conductive fiber was processed with the LDPE for comparison with five strands and for comparison with the base polymer. The pellets were made according to the long process technique referred to above in the detailed description by drawing long conductive nylon fibers through the molten LDPE at temperatures of about _. 300°F and cooling to form a strand which was then cut into pellets of about 1/16 to 1 1/2 inches in length and about 1/16 to 1/4 inch in cross-section. The results are reported in Table I.

TABLE I

LDPE ^J

STRANDS

Tensile Strength, PSI 1500

Elongation, % 500

Izod Impact Ft-Lbs/In

Unnotched Impact Ft-Lbs/In

Specific Gravity 0.917 u I

10 1/8" Mold Shrinkage In/In

Surface Resistivity Ohms/Sq

% Fiber of SpG Calculation

% Fiber by Pellet Disection

15 Rexene PE-179 Published Values

> 'Non-Breaking

The surface resistivity was measured with a fixture which clamps a 2 inch molded disc across two square metal electrodes (1/2 inch by 1/2 inch) 1/2 inch part. Approximately 440 psi of pressure is applied to get good contact and reproducible numbers. As may be ascertained, a molded disc prepared from the pellets provided surface resistivity on the order of 5,000 ohms per square and 500,000 ohms per square at a loading of about 15% and 5% by weight of fiber. Accordingly, such results establish a static dissipa¬ tive and a conductive composite pellet of this inven¬ tion which may be molded into useful conductive articles. EXAMPLE 2 For purposes of this Example, subsequent experiments were conducted in LDPE using glass fibers along with the conductive Nylon 6 fiber used in Example 1. Glass fibers were incorporated to give a means of calculating the amount of conductive fiber present using the measured yield values of the two types of fibers. Composite pellets were prepared in accordance with the procedure of Example 1 and the pellet sizes along with the data including surface resistivity for samples A, B and C are provided in Table II. "FCF" means flexible conductive fiber.

Base Resin

Strands of Fiberglass

Strands of F-911 Yarn

Pellet Length, Inches

Tensile Strength, PSI

Elongation, %

Flexural Strength, PS

Flexural Modulus,

PSI x 10* 6.92 5.7 5.59

Izod Impact, 1/4 Inch,

Ft-Lb/In 32 32 32

Unnotched Impact, 1/8 Inch, 64 64 64 Ft.-Lb/In

Deflection Temperature

Under Load @ 264 PSI, °F 128 126 124

Surface Resistivity,

Ohms/Square 80,000 4400 5000

Specific Gravity 0.96 0.98 0.97

Ash Content, Burn-Off, % 5.7 4.7 4.1

FCF, % by Calculation 9.8 20.1 17.6

EXAMPLE 3

The procedures of Examples 1-2 were repeated except single strands of cross-linkable ethylenevinyl- acetate (EVA) fibers containing carbon and 1.5 wt. % peroxide cross-linking agent were extruded into a polypropylene matrix at a temperature of about 400- 425 ^β F so that the strands were cross-linked by the heat of the molten polypropylene and pelletized. This example demonstrated that conductive thermosetting fibers may be made and formed in situ using the principles of this invention. EXAMPLE 4

In this example, a 3.5 inches co-rotating twin screw extruder was employed to compound pellets. Badische F109 carbon impregnated Nylon 6/6 fibers of about 1 1/2 inch in length and .010 inch diameter (584 denier) in an amount of about 4.1% by weight were fed with clear polypropylene pellets into the extruder. The barrel temperature on the twin screw was approxi- mately 400 ^β F. Intact fibers in the extruded strands were produced and pellets made therefrom were molded without fiber attrition!.

It will be understood with reference to the above detailed description and operating examples that other variations may be made without departing from the spirit of this invention.