Materials

FIELD OF THE INVENTION

This invention relates to an electrically conductive polyolefin composite comprising polypropylene or polyethylene or both and short carbon fiber as conductive filler.

BACKGROUND

In recent years, electric conductive thermoplastics emerged as potential replacements to the metals in various applications owing to their non-corrosive nature, high impact resistance, and cost effectiveness compared to metals. Among the thermoplastics, commodity polymers, for example polypropylene (PP) and polyethylene (PE) resins, are the obvious choices. This is due to their low cost, easy availability and excellent mechanical properties. Generally, these polymers are known as electrically insulating materials with conductivity values as low as 10 ^-7 - 10 ^-14 S cm- ¹ . However, their electrical conductivity can be tailored by reinforcing various conductive fillers to use in a wide range of applications such as electromagnetic shielding, automotive components, electronics manufacturing and ATEX (Atmospheres Explosibles) applications for rninimizing the risk of explosion.

A plastic material can be classified as conductive if it protects against electrostatic discharge (ESD; surface resistivity between 10 ⁵ -10 ¹² ohms/sq) or electromagnetic interference / radio frequency interference (EMI/RFI; surface resistivity of <10 ⁵ ohm/sq) according to the Electronic Industries Association (EIA) Standard 541.

Carbon fiber (CF) is versatile filler for various resin matrixes due to its low- density, excellent electrical conductivity and greater specific strength. A typical electrical resistivity of the carbon fiber is about 10 ^-2 -10 ^-4 Ωcm and may vary depending on the morphology, fiber size and preparation conditions. It is normally available in various forms such as prepregs, woven textiles, ravings, continuous and chopped fibers. Carbon fiber is widely used as light weight filler for advanced applications such as aerospace, aviation, automotive and to improve the electrical conductivity of the material. Fiber reinforced polymer composite parts can be fabricated by various techniques through extrusion filament, compression molding, puttrusion, and injection molding. Carbon reinforced polyolefin-based composites of the present invention can be molded in to any shape which having electrically conductive surfaces.

SUMMARY OF INVENTION

This invention relates to preparation of an electrically conductive polyolefin composite comprising polyolefin resin, maleic anhydride modified resin as compatibilizer, and carbon fiber as conductive filler. The process comprises the steps of mixing the conductive filler with the polymer in a twin-screw extruder thereby producing a polymer composite with enhanced electrical conductivity through better dispersion of the conductive fillers.

DETAILED DESCRIPTION

In one embodiment, polyacrylonitrile (PAN)-based carbon fibers used are a short (6 mm) and physically treated to open bundled fiber material that has a carbon content of 85 to 100 wt % and has density 1.83 gm/cm ³ , and at least partially has a graphite structure. Among these, PAN-based carbon fibers are preferred in terms of their very high strength. The carbon fibers are preferably used in the form of a bundle, and the number of single fibers contains from about 1000 to 480000 carbon filaments.

Various forms of carbon fiber can be included in the fiber-reinforced composite material including, but not limited to, continuous unidirectionally aligned fibers, woven, mat, and knitted fabrics. These can be used separately or in combination depending on applications. For making of a carbon fiber composite, unidirectionally aligned fibers, woven material is more preferable.

Generally, the carbon fibers to be used have been treated with suitable organic sizing agents to improve its adhesion characteristic. Similarly, it is preferable to preliminarily open the fiber bundle using physical or chemical treatment in order to increase their total surface area which improves the interfacial interaction between fiber and matrix. The interfacial adhesion of the carbon fibers with resin can be obtained by, for example, grafting of a monomer that contains an ethylenic double bond and a polar group. Nonexclusive examples of suitable polar groups include acid anhydrides and their derivatives.

The amount of the polyolefin that contains an emylenic double bond and a polar group in the same chain is not particularly limited, and is preferably I to 10 parts by weight for each 100 parts by weight of the main chain of polyolefin. An amount of smaller than 1 part by weight may result in insufficient adhesion to the carbon fibers, while an amount of larger man 20 parts by weight may adversely affect the physical properties of the composites.

In the method of melt mixing, the order of addition is preferably such that a polyolefin resin, a treated carbon fiber, and a polyolefin that contains anhydride polar group in the same molecule are melt mixed to prepare a composite mixture. The device used for the melt mixing may be a twin screw or single screw extruder, a Banbury mixer, a heating press or the like. The heating temperature in melt mixing is preferably 160° C. to 250° C. in that the polyolefin resin is satisfactorily melted but is not decomposed.

The production method of the composite material of carbon fibers and a matrix resin is not particularly limited. Usually, an integral molding method is employed which includes the carbon fibers with the polyolefin in a molten state at a high temperature under pressure with use of a device (e.g., an extruder, an injection molding machine, pressing machine) followed by cooling and curing.

The polyolefin resin used in the present invention is not particularly limited, and various polyolefin resins can be used. Examples thereof include polyethylene, polypropylene, poly-l-butene, polyisobutylene, random copolymers or block copolymers of propylene with ethylene.

The form of the matrix resin to be used in production of the carbon fiber- reinforced composite material of the present invention is not particularly limited, and the matrix resin may be used in the form of pellets, plates, or powder. The amount of the resin in the fiber-reinforced composite material should be usually 70 to 90 wt %, preferably 80 to 90 wt % or higher.

EXAMPLES

The present invention is described based on the following examples which, however, are not intended to limit the scope of the present invention. The used materials and the measurement conditions of the properties are described below.

Carbon Fibers

Polyacrylonitrile (PAN) based chopped carbon fibers of 6 mm length (Trade name: Tenax® HT C493) and milled carbon fibers of 100 um and 60 um length (Trade name: Tenax®-A, HTM 100, Tenax®-A, HTM 60, bulk density 300 and 550 g/1 respectively) were supplied by Toho Tenax GmbH, Germany. The main characteristics of 6 mm length (Trade name: Tenax® HT C493) carbon fibers are listed in Table 1. These values are from Toho Tenax strand test method, based on JIS R7601.

Table 1: Properties of conductive carbon fiber

Debundling of carbon fiber:

A carbon fiber was placed in horizontal rotary ball mill pulverizer and to it steel balls of various diameters were placed. The materials to ball ratio was 1:5. This is rotated with various speed and time interval to obtain defibrillated carbon fiber. Similarly, in a heavy-duty laboratory grinder, a small amount of fiber was placed and grinded carefully to make it defibrillated. This fiber is debundled into many fibrils and enhances the surface area of carbon fiber.

Method of making carbon fiber, polyethylene conductive composite materials Example 1

The various amount of carbon fiber was dry blended to obtain their respective composites using an intermeshing, co-rotating twin screw extruder (Farrell FTX20, USA, screw dia 26 mm; 1/d ratio 35). The screw has both the dispersive and distributive mixing elements. The extruder was operating at a screw speed of 15-30 rpm and processing temperature is preferably 200°C or higher, and more preferably 230°C or higher with the maximum temperature ranging from 240°C - 260°C. The dry-blends were ted with 100-70 or higher parts of injection molding grade homopolyethylene (Hostalen ACP S331H: product of The National Industrialization Company ,(TASNEE) Saudi Arabia, Melt Flow Rate=2.1 gm/10 minute at 190°C/2.16 kg according to ISO 1133), 5-30 parts preferably 10-20 parts of Polyacrylonitrile (PAN) based chopped carbon fibers of 6 mm length (Tenax® HT C493, Germany), modified polyethylene, (Polybond ^® 3029: maleic anhydride modified high density polyethylene (PE-g-MA, lot OP2B18R000, melt index 4.0 g/10 min at 190 °C, 2.16 kg) from Chemtura™ USA) were melt blended.

The extrudate was cooled in a water bath, air-dried, and pelletized to obtain modified polyethylene resin. The pelletized modified polyethylene was injection molded in an Arburg plunger type injection molder (40 tons, Series SM 120, Asian Plastic Machinery Co., Double Toggle IM Machine) to obtain specimens of ASTM Type I (D638) in the temperature range of preferably 180°C or higher, and more preferably 200°C or higher with the maximum temperature ranging from 240°C - 260 °C.

Examples 2-14

Polyethylene composite material was produced in the same manner as in Example 1, except that the following variables were changed: (1) carbon fiber loading percentage, (2) processing temperature and (3) screw speed.

These specimens were tested for surface resistivity analysis of modified resins. Table 2 shows the results of surface resistivity of the composites for Examples 1-15.

Table 2: Surface resistivity of the conductive polyethylene composite

While not being limited by any particular mechanism, it is postulated that the higher the surface/volume resistivity, the lower the leakage current and the less conductive the material is.

Method of making carbon fiber polyethylene conductive composite materials Example 16

The various amounts of carbon fiber were dry blended to obtain their respective composites using an intermeshing, co-rotating twin screw extruder (Farrell FTX20, USA, screw dia 26 mm; 1/d ratio 35). The screw has both the dispersive and distributive mixing elements. The extruder was operating at a screw speed of 15-30 rpm and the processing temperature was preferably 200°C or higher, and more preferably 230°C or higher, with the maximum temperature ranging from 240°C - 260°C.

The dry-blends were fed with 100-70 or higher parts of injection and compression molding grade of homopolypropylene (PP H 4120 product of The National Industrialization Company, (TASNEE) Saudi Arabia, Melt Flow Rate=12 gm/10 minute at 230°C/2.16 kg according to ISO 1133), 5-30 parts, preferably 10-20 parts, of Polyacrylonitrile (PAN) based chopped carbon fibers of 6 mm length (Tenax ^® HTC493, Germany), modified polypropylene preferably 1-5 part (PRIEX 25097:, maleic anhydride modified polypropylene (PP-g-MA, Batch O-7101, 0.45% grafting, <50 ppm free; melt index 15 g/10 min at 190 °C, 2.16 kg, product of Addcomp, Holland) were melt blended.

The extrudate was cooled in a water bath, air-dried, and pelletized to obtain modified polypropylene resin. The pelletized modified polypropylene were injection molded in an Arburg plunger type injection molder (40 tons, Series SM 120, Asian Plastic Machinery Co., Double Toggle IM Machine) to obtain specimens of ASTM Type I (D638) in Ihe temperature range of preferably 200 °C or higher, and more preferably 220 °C or higher with the maximum temperature ranging from 240 - 260°C.

Examples 17-29

Polypropylene composite materials were produced in the same manner as in Example 16, except that the following variables were changed: (1) carbon fiber loading percentage, (2) processing temperature, and (3) screw speed.

These specimens were tested for surface resistivity analysis of modified resins. Table 3 shows the results of surface resistivity of the composites.

Comparative example 1:

Saleem et al. report -10 ⁴ Ohms/sq of the surface resistivity for the composition of 40wt % of carbon fiber polypropylene composites. See A Saleem, L Frormann, A Iqbal, Journal of Polymer Research 2007, 14, 121

Comparative example 2:

Fenegan et al. showed that 20 wt% of carbon fiber polypropylene composites exhibits ~10 ² Ohms/sq of the surface resistivity. See I.C.Finegan , G.G Tibbetts Journal of Material Research 2001, 16.

Comparative example 3:

Drubertski et al. reported on composites that were were initially dry mixed in the ratio of 20:80 (Carbon fiber: PP matrix) and then melt mixing by twin screw extruder, followed by the injection moulding. The volume resistivity of the comparative example reported as ~10 ¹ Ohms.cm. See M. Drubertski, A, Siegmann, M. Narkis, Journal of Material Science 2007, 42, 1

Comparative example 4:

A commercial grade of conductive polypropylene with carbon fiber can be found on the market. For example, RTP 199 X from RTP company which is 20wt% of carbon fiber in polypropylene shows ~10 ⁵ Ohm/sq of the surface resistivity in the technical datasheet. See RTP data sheet RTP 199 X (Benchmark material).

Surface resistivities of the comparative examples are added to Table 3. Table 3: Surface resistivity of the conductive polypropylene composites

While particular embodiments of the invention have been illustrated and described above, various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.