CA1101169A

CA1101169A - Conductive plastic with metalized glass fibers retained in partial clumps

Info

Publication number: CA1101169A
Application number: CA300,252A
Authority: CA
Inventors: Edward G. Crosby; Frederick C. Hornbeck
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-05-09
Filing date: 1978-03-31
Publication date: 1981-05-19
Also published as: JPS5419423B2; DE2819377A1; GB1590252A; SE425167B; JPS53138466A; FR2390463B1; IT1113175B; FR2390463A1; SE7804920L; IT7823046A0

Abstract

IMPROVED CONDUCTIVE PLASTIC WITH METALIZED
GLASS FIBERS RETAINED IN PARTIAL CLUMPS
ABSTRACT
Articles of a thermoplastic having an improved level of electromagnetic shielding are molded from plastic pellets that have a core of metalized glass fibers that are arranged to partially disperse through the molded article as individual fibers and partially to remain in clumps of generally aligned closely contacting fibers. The clumps have approximately the length of an individual fiber but they are substantially wider than an individual fiber. Improved conductivity is attributed to increased electrical bridging between fibers that is provided by the width of the clumps of fibers.

Description

Introduction United States Patent 4,194,114 issued March 25, 1980 ~1 to the assignee of the present application describes an improved 22 b#*~ique ~or producing m~lded th rmcplastic ~icles that ha~e levels 23 of electrical conductivity that are useful for many purposes.
24 The articles are molded from plastic pellets that contain a core of metalized glass fibers. The glass fibers are metalized 26 as they are drawn from a melt of glass and the metalized fibers 27 are formed into a roving. In the subsequent manufacture of the 28 plastic pellets, the roving and a thermoplastic are dxawn from 29 a plastic extruder with the ro~ing forming the core of an extrusion that is chopped into pellets. When an article is , .,, i ~

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1 molded from these pellets, the fibers of metalized glass are

2 distributed through the article and make it electrically

3 conductive.

4 As one example of an application for a conductive plastic, an article that has been molded from a plastic 6 that conducts only slightly can be yiven an electrical 7 charge and can then be painted by electrostatic painting 8 techniques. A more demanding requirement for conductive 9 plastics is presented by enclosures (covers) for electronic apparatus that requires electromagnetic shielding. The cover 11 isolates the apparatus from electromagnetic radiation that 12 could otherwise produce spurious signals in the circuits of 13 unshielded apparatus, and it similarly prevents the apparatus 14 within a shielded cover from transmitting signals to interfere with other nearby apparatus.
16 The shielding capability of a conductive plastic is 17 measured by first molding a sample plaque of the material to 18 be tested. The plaqu~ has standard dimensions and is mounted 19 in the window of a metal box that contains a radio transmitter.
The strength of the radio signal outside the box is measured 21 at various frequencies with the window open and with the window 22 covered by the test plaque. The ratio of the signal measured 23 when the window is open to the signal measured when the window 24 is closed is formed to express the attenuating effect of the test plaquP in decibels (db's).

27 Conductive plastics are not conductive to the degree that 28 metal conductors are conductive. The metalized glass fibers 29 in the molded article averaye about a quarter of an inch in length and they provide conductivity to the Pxtent hat they 31 fortuitously touch or very nearly touch. Electromagnetic PO9-77-~15 -2-1 shielding may also be attributed to capacitive and inductive 2 coupliny between isvlated fibers.) In an ideal situation, 3 the fibers would have random positions and random orientations 4 so that electrical pathways would extend in three dimensions from each individual fiber. The fibers can be seen readily 6 in special test samples that are molded of clear plastic, 7 and on the surface of the samples the fibers appear to have 8 this random organization superimposed on the general fill 9 pattern that occurs in the surface of the plastic as it flows into the mold. However, when samples are disected 11 for further analysis it can be seen that within the body 12 of the sample the fibers line up parallel to each other in 13 the direction of the flow of the heated plastic into the mold.
14 The contact between fibers that lie parallel to each other is much reduced from the contact that would be expected from the 16 pattern of random orientation of the fibers that are visible ~
17 at the surface of a sample.
18 The random orientation of the fiber at the surface of a 19 plaque probably comes about because friction between the surace of the mold and the flowing plastic causes turbulence 21 in the plastic near the surface. The surface plastic hardens 22 firstl entrapping the randomly oriented fibers. Similarly, 23 when a test plaque mold is only partially filled, the leading 24 edge of the plastic partial plaque shows a random orientation of the fibers that is probably caused by turbulence ir~ this 26 region of the fIowing plastic mass. The inner portion of the 27 plaques remain hot and molten and produces less turbulence 28 and the fibers in this region are aligned in t~e general 29 direction of plastic flow during molding.
According to this inventio~ a plastic molding pellet has 31 a core of metalized glass fibers that are arranged to p~rtly 32 disperse as individual fibers 33 and to partially remain as a clump that has an appreciable 1 cross section as compared with the cross section of an 2 individual fiber. The fibers in a clump have numerous 3 points of contact and thus provide good electrical conductivity 4 throughou~ the clump. These fiber clumps also line up in the direction of flow of the plastic in a molded article, but 6 they have sufficient width in the direction across the 7 direction of flow to significantly raise the amount of 8 bridging that occurs between fibers in a molded article.
9 Fibers can be arranged in the core of a molding pellet in various ways that will cause the fibers to partially clump 11 in the molded article. For example, the core can be formed 12 of braided roving o metalized glass fibers. Alternatively, 13 a roving of twisted fibers provides a particularly advantageous 14 technique for producing pellets that cause clumps of fibers in the molded article. To produce a twisted roving, the 16 glass fibers are first formed into sub-bundles and several 17 sub-bundles are twisted about a central sub-bundle in a way that 18 resembles some wire rope. On the other hand, coiling of the 19 fibers, which occurs in some glass making processes, does not provide clumping, probably because the coils are 21 straightened out during pellet manufacture by the tension that 22 is applied to the roving of metalized glass in pellet manufacturing.
23 We attribute the clumping effect of twisted fibers to the 24 fact that the fibers within the twisted roving do not wet well to the plastic during the pellet manufacture. During a 26 subsequent injection molding operation with these pellets, the 27 previously unwetted fibers are not mixed as well with the plastic 28 and thus tend to remain in their original form as a sub-bundle 29 of the metalized glass roviny. Thus, we contemplate that other 1 techniques for isolating a sub-bundle of fibers from the 2 mixing action of the injection molding apparatus will 3 pxovide good clumping results.
4 Although the invention adds a step to the glass manu-facturing operation, it simplifies the subsequent handling 6 of the metalized glass roving by permitting the usual creeling 7 step to be eliminated.
8 The following examples will suggest other features and 9 advantages of the invention.
The Examples 11 Introduction 12 As described later, a number of test plaques were 13 molded at a uniform size of 3" x 6" x 1/4" thick. These 14 plaques are smaller than the plaques used for the tests that ar~ reported in our United States Patent 4,195,114 ~12" x 12" x V4") and 16 were tested in a smaller test chamber, and as a result the 17 shielding results are numerically lower than the results 18 reported in the related application. To show the comparison 19 between the clumped fibers of this specification and the non-clumped glass fibers of the related application, sample 21 1 was molded from pellets made from a roving of non-clumping, 22 metalized glass fibers. In all of the samples the pellets and 23 the molded product contained approximately 25 Wt% metalized 24 glass fibers~ The pellets were 3/4" in length and the compo-sitions of the glass and the metal coating were identical 26 in each example. The genexal conditions of the tests were 27 comparable, and we attribute the improved rPsults of the 28 pellets containing twisted or braided roving to the clumping 29 effect that these pellets produce in a molded test plaque.
The following table shows the shielding levels for 31 samples at frequencies from 12 tc 100 megahertz.
POg-77-~15 -5-1 Control Example 1 Example 2 Example 3 2 Frequency Sample 1 Sample 2 Sample 3 Sample 4Sa~ple 5 (megahertz) ~ 20 16 19 20-21 18-19 18-19 15-16 18-19 21~23 18 19-20 8 The Control SamPle - Sam~le 1 9 Three test plaques of structural roam were molded of polycar~onate contalning 25 -~,~7t~ metali~ed glass fibers in the 11 form of a straight rovlng, as described in United States Patent 4,195,114.
12 The shielding remained below 20db th ough 70 megahertz. The 13 shielding of the test sample rose to about 30db at 100 megahertz 14 The test was stopped at 100 megahertz. The conductivity of these test samples is suitable for many purposes when general 16 conductivity is desirable, and the shielding capability is 17 suitable ror many shielding applications. ~owever, it would 18 be desirable to have shielding levels of about 20db from 19 the 1/4" test samples.
Exam~le 1 -21 Metalized glass fibers containing a binder coating as 22 described in the related applicatio~ were formed into sub-bundles 23 contalning 100 fibers that were essentially straight without any 24 twisting. Eight sub-bundles ~ere woven into a braid having 800 fibers. The braid is the pattern co~only used for the 26 outer cylendrical conductor of coa~ial cables. The braid 27 had eight cross-overs per inch, but we contemplate that any 28 suitable number of cross overs per inch can be used~ T~o of 29 these braids were used to mold pellets of polycarbonate con-taining 1600 fibers to produce about 25 ~t~ of metalized 31 glass fibers. On the surface, these plaques display large 1 swirl patterns showing the path of the plastic flowing 2 into the mold. A plaque was buffed to improve the contrast 3 between the metalized fibers and the plastic, and at the surface 4 the fibers appeared to have a random 3-dimensional orientation that was generally independent of the swirl patterns.
6 Samples were subsequently fractured approximately 7 parallel to the direction of flow and across the direction 8 of flow to observe the distribution of the metalized glass 9 fibers. The sample contained both clumps of fibers and individual fibers. The clumps generally retained essentially 11 their initial length and individual fibers appeared in a 12 full range of lengths from dust particles to approximately 13 the length of the original pellets. The clumps appeared 14 to consist of generally parallel, densely packed, fibers and the braid pattern did not appear in the molded test samples. Both 16 the individual fibers and the clumps appeared to be aligned 17 in the direction of flow of the plastic through the mold. The 18 clumps appeared to be scattered at random with approximately 19 1/4" (about the length of a clump) separating nearby clumps.
Sample 2 21 In sample 2, two test plaques were molded of a mixture of 22 pellets having 50% pellets manufactured with two braids of 23 metalized glass fibers as described in this example and 50~
24 metalized glass fibers having a straight roving as described for the control sample. Thus, the test plaques had 12.5 Wt%
26 of metalized glass fibers in the form of the braid and a total 27 of 25 Wt% glass fibers. As the table shows, a useful improvement 28 in shielding was measured.

llG116g 1 Sample 3 2 In sample 3, three test plaques were molded and tested 3 according to the procedure described for sample 2 except that 4 the pellet mixture had 75% pellets containing ~he braid of metalized glass and 25% pellets containing the straight un-6 twisted metalized glass fibers. Thus the molded plaques 7 had 18 Wt% metalized glass fibers ~n the form of a braided 8 roving and 7% metalized glass fibers in the form of the 9 straight roving. The plaques had a shielding level of more than 20db across the frequency spectrum of the test.
11 Example 2 12 Metalized glass fibers were formed in the way already 13 described into thirteen sub-bundles that each contained 14 100 metali ed glass fibers. One of these bundles formed a central member of a twisted roving. Six of the sub-bundles were 16 wound clockwise about the central sub-bundle and the remaining 17 six of the sub-bundles were wound counter clockwise about the 18 inner seven sub-bundles. Thus, the roving had a total of 19 1300 fibers. The twisted roving was used for manufacturing pellets having the roving as a core, in the way already 21 described. The pellets had about 25 Wt% metalized glass fibers.
22 The sub-bundles wexe twisted together on laboratory 23 apparatus o~ the type used for similarly twisting textile 24 fibers. The roving had about one quarter of a turn per inch.
Although this is only three quarters of a turn along the length 26 of a pellet, the fiber sub-hundles showed interlocking at a cut 27 end of a roving and did not readily come apartO Commercial 28 equipment for ~imilarly twisting textile fibers is available 29 and we contemplate that about eight turns per inch will be desirable. A minimum tension is required on metalized glass 31 fiber~ to prevent linting w~ich occurs when tension is too 32 low, probably because relati~e motion of the fibers of a 1 sub-bundle occurs under low tension and the fibers abrade 2 each other. A maximum tension is established by the ten~ion 3 at which the roving will break. It appears that within these 4 limits the tension on the sub~bundles is not critical except that the sub-bundles are given equal tension.
6 Sample 4 7 Three test plaques were molded of pellets manufactured 8 according to this example (without other pellets). Thus the 9 plaques had about 25 Wt% of metalized glass in the form of a twisted roving. The plaques wexe tested with the results shown 11 in the table. Although the test results are lower at some 12 frequencies than the samples of example 1, the results are 13 sufficiently better than the control test of sample 1 that 14 for most applications the twisted roving will be the preferred roving form for pellets for the improved shielding of this 16 invention.
17 Test plaques were fractured at approximately right angles 18 to the direction of flow and approximately parallel to the 19 flow and the fibers and clumps of fibers were observed. The pattern of clumps and the individual clumps were essentially 21 indistinguishable from samples molded with the braided roving.
22 The clumps appeared to correspond to the original sub-bundles 23 of about 100 fibers.
24 Subsequently, the plastic molding pellet~ of Example 2 were used for molding a multipart cover for a terminal device, 26 and the product tested satisfactorily.
27 Example 3 - Sa~ple 5 28 Three test plaques were molded from a mixture of pellets 29 having 75% pel~ets containing the twisted roving described in Example 2 and 25~ pellets containing the braided roving of ~L1~ 9 1 Example 1. The plaques were tested with results that are 2 generally comparable to the tests of sample 4. A test plaque 3 was then fractured and the brok~n edge was inspected with a 4 microscope. The plaque showed both individual fibers and clumps of fibers. There was no apparant difference among 6 the clumps that would show whether clumps arose from the braided 7 roving or from the twisted roving. These results appear to 8 support our belief that the twisted roving will be preferred for 9 most applications.

11 The pellets of the preceding examples were extruded from 12 polycarbonate but we contemplate that the invention will be 13 used with other plastics that are commonly used ~ith unmetalized 14 glass fibers for both foam and solid plastic molded articles.
A variety of techniques for giving the fibers clumping 16 characteristics will be apparent from the well developed 17 arts of twisting and braiding or otherwise uniting textile 18 fibers, fine metal wires, and the like. For example, outer 19 sub-bundles can be wrapped around an inner sub-bundle with many turns per inch as compared with the twisted sub-bundles 21 of Example 2. Similarly, the inner bundles can ~e tied together 22 with ~n outer net-like arrangement of a few fibers. ~owever, 23 it is an advantage of the invention that the roving can be made 24 ~y simple techniques such as ~wisting that should not add significantly to the manufacturing cost of the pellet.
26 From the preceding description of several sepcific 27 examples of the in~ention, tho~e skilled in the art will 28 recognize a variety of modification within the scope of the 29 claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A thermo plastic molding pellet for molding articles of plastic and metalized glass fibers by a process in which the fibers disperse through a molded article but tend to become aligned in the general direction of the flow of plastic into a mold and contact between adjacent fibers for electrical conductivity is thereby reduced, comprising, a core of metalized glass fibers and an outer body of a thermoplastic, wherein the metalized glass fibers in the core of a pellet are arranged to partially disperse in the molded article as individual fibers and partially to remain as clumps having a thickness that is appreciably greater than the thickness of an individual fiber for improving the fiber to fiber contact in the direction orthogonal to the general direction of plastic flow in a subsequently molded plastic article.

2. The plastic molding pellet of Claim 1 wherein said metalized glass fibers comprise an inner sub-bundle of fibers and an outer arrangement of fibers formed about said inner sub-bundle.

3. The plastic molding pellet of Claim 1 wherein said metalized glass fibers are arranged with an inner sub-bundle of fibers and an outer grouping of sub-bundles formed about said inner sub-bundle.
PO9-77-015 CLAIMS 1, 2 and 3

4. The plastic molding pellet of Claim 3 wherein said metalized glass fibers are arranged in a twist of sub-bundles of fibers around an inner sub-bundle.

5. The plastic molding pellet of Claim 4 wherein said metalized glass fibers are arranged in a twist of a first layer of sub-bundles in one direction about a central sub-bundle and a twist of a second layer of sub-bundles in the opposite direction about said first layer.

6. The plastic molding pellet of Claim 1 wherein said fibers are arranged in a braid of fiber sub-bundles.

7. The plastic molding pellet of Claim 1 wherein said fiber sub-bundles are formed into a cylindrical braid.

8. A thermoplastic molding pellet as in claim 1, 2 or 3 containing about 25 weight percent of the metallized glass fibers.

9. A thermoplastic molding pellet as in claim 4, 5 or 6 containing about 25 weight percent of the metallized glass fibers.