JP2009512774A - Process for producing long-fiber thermoplastic resin for conductive composite material and composite material formed thereby - Google Patents

Abstract

  The present invention relates to polymer articles comprising conductive fibers to provide an electrical electromagnetic interference (EMI) shield and methods for their manufacture. The present invention includes a method of forming a shielding material by impregnating a polymer material with conductive fibers via direct injection of the conductive fibers into an extrusion process. The present invention also includes products that are electromagnetically shielded and radio frequency by EMI shielding polymers and components formed of shielding polymers.

Description

Detailed Description of the Invention

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION The present invention relates to polymer products containing conductive fibers and methods for their manufacture to provide electrical electromagnetic interference (EMI) shielding. More specifically, the present invention relates to a method of forming a shield material by impregnating a polymer material with conductive fibers via direct injection of the conductive fibers into an extrusion process. The EMI shielded polymers of the present invention can be formed into a wide variety of products such as radio frequency and electromagnetic shielded plastic products.
BACKGROUND OF THE INVENTION As the use of electronic devices such as computers and other digital devices increases, there is an increasing concern about the dangers associated with electromagnetic radiation, particularly radar waves, microwaves and electromagnetic radiation (caused by electronic circuitry). As the electronics industry continues to grow rapidly, there is a need to create improved electromagnetic shielding materials that can be introduced into electronic products.
Many conductive materials have been developed to produce composite products, such as plastic products, due to electromagnetic shielding, static dissipation and other electrically enhanced properties. Plastic products formed from conductive materials are particularly advantageous compared to conventional metal materials because they are light, easily manufactured using injection molding techniques and low cost. Typically, these conductive materials are plastics and composites of conductive powder and chopped fiber.

Various techniques are used to introduce conductive powders and chopped fibers into composite products. FIG. 1 illustrates a commonly used conventional thermoplastic extrusion compounding technique. The thermoplastic resin 112 is supplied to the blender 110. The resin 112 is heated to the melting temperature and then fibers or powder (collectively referred to as 114) are fed to the blender 110 and mixed in the conductive powder or chopped fiber. The resin / broken fiber mixture is extruded at 118, cooled in a water bath 120, and then cut by a strand cutter 122 into pellets 124. The pellets 124 are then typically fed to the melt section of the injection molding machine (not shown).
The pellets in the method shown in FIG. 1 contain fibers that are broken due to the cutting action by the screws 116 and by the shear forces applied to melt the resin. The fibers are broken during the compounding process so that the resulting composite product contains only relatively short broken fibers. Shortened fibers impart reduced electromagnetic shielding properties to composite materials due to their reduced ability to conduct electricity and form conductive fiber networks in composite products. Alternatively, when mixing the conductive powder with the molten thermoplastic, typically a very large amount of conductive powder needs to be used. Such a large amount of powder may result in poor powder dispersion of the final product or reduced mechanical strength. Thus, composite products formed of ruptured fibers and powders require high loading or filler concentration, thereby reducing the mechanical strength of the formed composite product and material costs. Becomes higher.

Published US patent application US 2002/0108699 entitled “Method of Forming Conductive Fibers and Fiber Pellets”, which is hereby incorporated by reference in its entirety, includes the following steps (1)-( 8) Disclose electromagnetic shielding pellets formed by: (1) applying sizing to Ni coated fibers to produce fibers compatible with thermoplastic matrix material; (2) heat drying sizing (3) a step of coating the fiber with a thermoplastic matrix material; (4) a step of quenching the thermoplastic matrix material; (5) a step of drying the coated fiber; (6) cutting or pelletizing the coated fiber. (7) supplying pellets to an injection molding machine; and (8) melting the pellets and injection molding the parts.
Each step of the pelletized fiber approach creates material losses and inefficiencies, slows cycle times, and creates opportunities for defects. There are some additional drawbacks. By heating the thermoplastic polymer during the wire coating process and again during the injection molding process, the performance of the polymer is degraded. If the degradation is severe, the polymer can be decomposed and gases can be formed that result in voids and shields and subsequent loss of mechanical properties. Also, in order to achieve good mold filling, the polymer melt flow must be sufficient to satisfy partial ribs or other small features. Melt flow is achieved by the choice of thermoplastic material, temperature, residence time and screw shear. High shear by the screw results in a sufficiently high melt flow, but the conductive fibers are broken down to become smaller and shorter and reduce the fiber's ability to form a continuous network of fibers.

The shielding effect can be measured by ASTM-D4935 that measures the far-field shielding effect or ASTM ES7-83 that measures the near-field shielding effect.
In US 2002/0108699, an electromagnetic shielding product having a shielding effect of 80 to 90 dB (far field) and less than 80 dB (near field) at a fiber load of 15% by mass and a frequency of 30 to 1500 MHz is manufactured.
An alternative dry blend method to form a shielded product requires that the cut conductive fibers be mixed directly with the resin during the injection molding operation. This typically results in very poor fiber dispersion and inconsistent electrical performance from part to part. US 2002/0108699 discloses that an electromagnetic shielding product having a shielding effect of 60 to 70 dB at a fiber load of 15% by mass and a frequency of 30 to 1500 MHz is produced by a dry blend method.
FIG. 1 shows an inline process according to US 2002/0108699, in which a fiber tow 103 is unwound from a package or spool 105 and drawn into an aqueous silane bath 106 so that a conductive layer is applied to the tow 103. Is done. Tow 103 is then drawn into aqueous silane bath 104 and oven 108. The tow 103 is then passed through a non-aqueous size bath 107. The tow 103 is then wound on a package (or spool) 113. The coated fibers are then pelletized and placed in an extruder of an injection molding machine.

Prior methods of forming EMI shield composite products are not completely satisfactory due to the short fiber length in finished parts that degrade the shield and require additional loading of the fiber. The average fiber length by wire coating, pelletizing and then injection molding the pellet was about 0.5 mm. Conductive fibers must be in contact with each other to form a continuous network and provide a shield, so 10-20% by weight carbon fiber or nickel-coated carbon fiber is necessary to make the EMI shield sufficient It is said. This high level of fiber loading increases the cost of the composite due to the high cost of the fibers and suppresses polymer flow in the mold. High fiber loading also significantly increases the modulus of the product but decreases impact resistance. EMI shielding resins are used in articles such as cell phones and laptop computers that are expected to resist the impact of drops from up to 1 m or higher without breaking. A thermoplastic resin filled in 10 to 20% by mass of the fiber becomes brittle and easily broken.
Another disadvantage of the previous method of forming an EMI shield composite product is poor fiber dispersion. Pelletized materials are difficult to process so that the fibers are well dispersed and do not form a sufficient network of conductive fibers in the composite material. Although good fiber dispersion is sometimes achieved, the pellets and fibers can be ground to an average fiber length of about 0.5 mm, resulting in a short fiber having a sufficient network of conductive fibers in the composite material. Do not form. In any case, it is necessary to overfill the composite material with conductive fibers to compensate for the inefficient network of conductive fibers in the composite material.
US Pat. No. 6,676,864 entitled “Resin and Fiber Compounding Method for Molding” (incorporated herein in its entirety by reference) manufactures a fiber reinforced resin and molds the resin. An apparatus and method for doing so are described. U.S. Pat. No. 6,676,864 describes a two-stage extruder that applies a shear force in the first step to melt the polymer and feeds a molten thermoplastic material to the mold in the second step. An injection molding apparatus is shown including: Reinforcing fibers, such as glass fibers, carbon graphite fibers or Kevlar fibers, are fed between the two stages in the extruder. The forming device of US Pat. No. 6,676,864 does not consider electromagnetic shielding.

SUMMARY OF THE INVENTION The present invention solves the problems associated with previous methods of forming EMI shielded polymers. Long fiber thermoplastic technology allows the conductive fibers to remain long enough to provide an EMI shield at low fiber loads. Due to the manufacturing method of long fiber thermoplastics to form EMI shield composite products, it also has high impact resistance, surface is more beautiful and extruded with low material cost and reduced waste and scrap And the injection molding process is improved.
The method for producing a long fiber thermoplastic resin and the composite material formed thereby for the conductive composite material of the present invention are simpler, more efficient and have improved properties than conventional methods. ing.

Detailed Description and Preferred Embodiments of the Invention A thermoplastic resin, preferably in the form of pellets, is provided from a resin supply 14 to a resin first extruder 12. The resin may be any of a variety of acceptable thermoplastic resins for the intended production purposes, such as polypropylene, nylon, polyurethane and polyester. The melting screw 16 rotates in the melting barrel 18 of the extruder 10. Although the shear force of the melt screw 16 may provide sufficient heating to melt and condition the polymer, the melt barrel 18 is equipped with an additional heat source as is known in the art. Also good.
The flow conditioning plate 20 can be used at the downstream end of the barrel 40 to regulate the flow of the resin 15 out of the extruder barrel 18 and into the coating die 22. Plate 20 typically restricts the flow of resin 15 due to diameter reduction or otherwise by restricting flow within barrel 18. The coating die 22 and device in contact with the resin 15 may include suitable heating elements to maintain the desired temperature of the resin. The pressure in the coating die can be monitored by a pressure transducer that provides a control signal to the drive motor 17 of the melt screw 16.
The tow of the shield fiber 26 is directly supplied by the fiber spool 24. The shield fiber may have a suitable composition, such as nickel, copper and / or a conductive material (coated on carbon, aramid, glass or other suitable support), or alternatively stainless steel, Copper or similar metal fibers may be used. The fiber is pulled through the spray nozzle 28 to the coating chamber 32 of the coating die 22. Shield fiber 26 is then homogeneously blended and coated with molten polymer material 15. The coated shield fiber 26 then exits the coating die 22 through the die orifice 36 of the variable insert 30. The diameter of the die orifice 36 can be adjusted by changing the insert 30 to control the ratio of shield fiber 26 to resin 15.

The mixture of resin 15 and fibers 26 exits the coating die 22 and the shield fibers 26 can be cut by the blades 52 in the cutting chambers 50 in the housings 54, 56. The mixture of the resin 15 and the fiber 26 leaves the cutting chamber 50 and is conveyed to the extruder 60 through the orifice 58. The extruder 60 typically includes a barrel 62 that feeds a mixture of resin 15 and fibers 26 to an extrusion die 64. The supply screw 66 may rotate within the barrel 62 and optionally reciprocate along the axis 72 to supply a filling of molding material through the orifice 63 to the molding cavity 68 of the die 64. The supply screw 66 is driven by the power supply device 70.
The temperature within barrel 18, coating chamber 32, cutting chamber 50 and extruder 60 can be controlled by one or more heating elements and temperature probes controlled by a microprocessor (not shown). Suitable polymers include thermoplastic polymers such as acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene (PA) and other thermoplastic materials (appropriate mechanical, Having thermal and melt flow properties).
Other fibers include metal-coated glass and carbon fibers, including coatings of metals such as alumina, copper, nickel and lead. Metal plasma deposition, melt liquid phase growth and electrodeposition are preferred fiber coating methods.
The product molded in the molding cavity 68 includes a number of individual shield fibers 26 having a predetermined length such that each fiber is established by the action of the blade 52. The molded product contains approximately three times the length of the wire coating and pelletization process shown in FIG. The average length by wire coating, pelletization and pellet injection molding was about 0.5 mm. The expected average length of Ni-C fibers will be 1.5 mm or more.
The shielding properties of the molding material are determined by the number of fibers in contact to provide a conductive network. Long thermoplastic fibers should be able to reduce the number of fibers needed to achieve the same connectivity to 1/4 to 1/3 of the original amount.
Thus, rather than requiring a fiber load of 15-20% by weight, LFTP can reduce the required fiber to as low as 4-5% by weight of the composite material.
By reducing the amount of carbon fiber to approximately 5% load, the impact strength is highly improved. The loss of impact strength severely limits the number of conductive composite applications (eg, cell phone exteriors). An impact improvement of up to 50% or a further reduction of fiber content up to 5% by weight can be expected.
The 5% fiber loading also improves other problems with surface aesthetics, consumer product outer housings. If the fiber is on the surface, the appearance will be poor.
A fiber load of 5% can save fiber costs by 75%, which can cost over $ 30 / lb.

Examples The following examples are prophetic and all mechanical and shielding properties are estimated. In Example 1, acrylonitrile-butadiene-styrene (ABS) polymer is melted and 10 wt% nickel-coated carbon (NCC) fibers are added to the ABS. Cut the fiber to the appropriate length and extrude the ABS / fiber melt to form the composite part.
In Example 2, the ABS polymer is melted and 7.5 wt% NCC fibers are added to the ABS. Cut the fiber to the appropriate length and extrude the ABS / fiber melt to form the composite part.
In Example 3, the ABS polymer is melted and 5.0 wt% NCC fibers are added to the ABS. Cut the fiber to the appropriate length and extrude the ABS / fiber melt to form the composite part. The characteristics in the example are estimated below.

The invention of this application has been described above both generically and with respect to specific embodiments. While this invention has been described as being a preferred embodiment, a wide variety of alternatives known to those skilled in the art can be selected from within the general disclosure. Except as set forth in the following claims, the invention is not limited in any other way.
BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description of the invention Help explain the principle.

1 is a schematic diagram of a conventional wire coating method for compounding EMI shielded thermoplastic extrusion materials. 1 is a plan view, partially in section, of an extruder useful in carrying out the present invention. FIG.

Claims (8)

  A fiber reinforced polymer product with improved electromagnetic shielding properties, comprising a thermoplastic polymer and less than 10% by weight of conductive fibers, wherein the composite product has an electromagnetic shielding efficiency of at least 70 dB.   The fiber reinforced polymer product of claim 1, wherein the electromagnetic shielding efficiency is at least 90 dB.   The fiber reinforced polymer product according to claim 1, wherein the conductive fiber is present in an amount of less than 5% by weight.   The fiber-reinforced polymer product according to claim 1, wherein the thermoplastic polymer is selected from the group consisting of acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene (PA).   The fiber reinforced polymer product according to claim 1, wherein the deterioration of the properties of the thermoplastic polymer due to the melting process is substantially reduced. A method of manufacturing a composite material product comprising:
Melting the thermoplastic material,
Adding conductive fibers to the molten thermoplastic material;
Cutting the conductive fibers into a predetermined length and injection molding the thermoplastic material to form a composite product having an electromagnetic shielding efficiency of at least 70 dB;
A method comprising the steps of:   A digital device having an electromagnetic shield, wherein the shield comprises a thermoplastic polymer and less than 10% by weight of conductive fibers, and the electromagnetic shielding efficiency of the composite product is at least 70 dB.   The fiber reinforced polymer product according to claim 7, wherein the electromagnetic shielding efficiency is at least 90 dB.

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