WO1989000098A2

WO1989000098A2 - Thermoplastic composite pellets, method of making them and conductive molded articles produced therefrom

Info

Publication number: WO1989000098A2
Application number: PCT/US1988/002044
Authority: WO
Inventors: Mark T. Banks
Original assignee: Banks Mark T
Priority date: 1987-07-06
Filing date: 1988-06-14
Publication date: 1989-01-12
Also published as: AU2307688A; KR890701302A; WO1989000098A3

Abstract

Thermoplastic pellets are provided having electric conductivity properties in molded articles produced therefrom. The pellets have a thermoplastic resin matrix having incorporated therein a multitude of flexible conductive fibers made from an organic polymer containing a conductive component. Upon processing the pellets under the action of heat into a molded article, conductivity is achieved without adversely affecting the fibers and their ability to impart conductivity to the molded article.

Description

THERMOPLASTIC COMPOSITE PELLETS,METHOD OF MAKING THEM AND
CONDUCTIVE MOLDED ARTICLES PRODUCED THEREFROM
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
i09-i012 ohms per sq.; static dissipative compounds
range from 105-109 ohms; and conductive compounds
range from 102-105 ohm-cm.

EMI/RFI shielding com
pounds typically range over i00-i02 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 typically 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 completely 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 incorporated 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, electronic 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 mentioned 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 containing 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 conductive 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 flexible 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 conductivity 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 disclosures 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 procedures. 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 ordinary 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 conductivity, EMI or RFI shielding properties in the resultant 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 maximizing the conductivity of the fiber and minimizing its diameter, cost effective conductivity and shielding 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 impregnated 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 suspension or emulsion and subsequently subjected to sufficient heat to dry and fuse the resin around the comingled 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 objective 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 electrostatic 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, styreneacrylonitrile polymers, polymers based on acrylonitrile-polybutadiene-styrene(ABS); nylons, particularly Nylon 6,6; polyphenyleneoxides or ethers; polyphenylene 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 processability of the matrix resin to achieve the desired conductivity. Typical flexible organic polymeric filaments include Nylon 6,6 and polyethyleneterephthalate, 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 composite 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 3000F 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 1
ONE FIVE
LDPE1 STRAND
STRANDS
Tensile Strength, PSI 1500 1700 1800
Elongation, % 500 38 22
Izod Impact Ft-Lbs/In - NB NB2
Unnotched Impact Ft-Lbs/In - NB NB
Specific Gravity 0.917 0.921 0.943 1/8" Mold Shrinkage In/In - 0.0263 0.0105
Surface Resistivity - 5.3x105 4.5x103
Ohms/Sq
% Fiber of SpG Calculation - 5 14
% Fiber by Pellet Diseotion - 5.9 15 1 Rexene PE-179 Published Values 2 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 dissipative and a conductive composite pellet of this invention 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.

TABLE II
A B C
Base Resin PE179 PE179 PE179
Strands of Fiberglass 2 2 2
Strands of F-911 Yarn 2 5 5
Pellet Length, Inches 1/4 1/4 3/8
Tensile Strength, PSI 1700 1800 1900.

Elongation, % 26.8 18.7 19.4
Flexural Strength, PSI 1300 1500 1400
Flexural Modulus,
PSI x 10
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 &commat; 264 PSI, OF 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 ethylenevinylacetate (EVA) fibers containing carbon and 1.5 wt. % peroxide cross-linking agent were extruded into a polypropylene matrix at a temperature of about 4004250F 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 approximately 4000F. Intact fibers in the extruded strands were produced and pelletts made therefrom were molded without fiber attritions.

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

Claims

CLAIMS:

1. A thermoplastic pellet providing electrical conductivity properties in a molded article comprising a thermoplastic resin pellet having a thermoplastic resin matrix and incorporated therein a multitude of flexible conductive fibers made from an organic polymer containing a-conductive component, said fibers having a melt temperature higher than said matrix resin and said pellets being processable under the action of heat into a conductive molded article without adversely affecting said flexible fibers and their ability to impart conductivity to the molded article.

2. A pellet of claim 1 wherein the fibers are contained in an amount of about 1 to about 20% by weight of the pellet.

3. A pellet of claim 1 wherein the fibers are contained in an amount of about 1 to about 5% by weight of the pellet.

4. A pellet of claim 1 wherein the thermoplastic matrix resin is a member selected from the group consisting of polyolefins, polystyrene, styreneacrylonitrile polymers, acrylonitrile-polybutadienestyrene, nylon, polyphenylene sulfides, polyacetals, polysulfones, polycarbonates, polyurethanes, cellulose esters, polyester, acrylic polymers, polyvinyl chlorides, polyvinylidene chloride, polyphenylene oxides, polyphenylene ether, polyphenylene oxide-polystyrene blends and blends of any of the foregoing resins.

5. A pellet of claim 1 wherein the flexible conductive fiber comprises a thermoplastic or thermoset resin selected from the group consisting of polyolefins, polystyrene, styrene-acrylonitrile polymers, acrylonitrile-polybutadiene-styrene, nylon, polyphenylene sulfides, polyacetals, polysulfones, polycarbonates, polyurethanes, cellulose esters, polyester, acrylic polymers, polyvinyl chlorides, polyvinylidene chloride, polyphenylene oxides, polyphenylene ether, polyphenylene oxide-polystyrene blends and blends of any of the foregoing resins.

6. The pellet of claim 1 wherein the flexible fiber comprises a thermoplastic polymer containing a conductive filler or coating.

7. The pellet of claim 6 wherein said conductive filler is carbon powder.

8. The pellet of claim 1 wherein the flexible conductive fiber is conductive nylon and the thermoplastic resin matrix is a polyolefin.

9. Molded products characterized by conductivity derived from pellets of claim 1.

10. Molded products of claim 9 when formed by injection molding process.

11. Molded products of claim 9 characterized by their ability to be pigmented with color additives thereon imparting any one of various colors across the visible spectrum.

12. A pellet of claim 1 wherein the pellet has a length of from about 1/16 to about 1 1/2 itches and a nominal cross-section of about 1/16 to 1/4 inch.

13. A method of making thermoplastic pellets of claim 1 comprising passing a multitude of said flexible fibers through a molten thermoplastic resin to form a continuous strand of said fibers impregnated with said resin, cooling said continuous strand and cutting said continuous strand into smaller pieces to form said pellets.

14. The method of claim 14 wherein said fibers are made from a thermoplastic resin having a conductive filler or coating.

15. The method of claim 14 wherein said fibers are made from a thermosetting resin having a conductive filler or coating.

16. The method of claim 16 wherein said thermosetting resin is cross-linked during said passage through said molten resin.

17. A method of claim 14 wherein the thermoplastic matrix resin is a member selected from the group consisting of polyolefins, polystyrene, styrene-acrylonitrile polymers, acrylonitrilepolybutadiene-styrene, nylon, polyphenylene sulfides, polyacetals, polysulfones, polycarbonates, polyurethanes, cellulose esters, polyester, acrylic polymers, polyvinyl chlorides, polyvinylidene chloride, polyphenylene oxides, polyphenylene ether, polyphenylene oxide-polystyrene blends and blends of any of the foregoing resins.

18. A method of claim 14 wherein the flexible fiber comprises a thermoplastic or thermoset resin selected from the group consisting of polyolefins, polystyrene, styrene-acrylonitrile polymers, acrylonitrile-polybutadiene-styrene, nylon, polyphenylene sulfides, polyacetals, polysulfones, polycarbonates, polyurethanes, cellulose esters, polyester, acrylic polymers, polyvinyl chlorides, polyvinylidene chloride, polyphenylene oxides, polyphenylene ether, polyphenylene oxide-polystyrene blends and blends of any of the foregoing resins.