FIELD OF THE INVENTION

This invention relates to an electrically conductive polyolefin composite comprising carbon black conductive filler, and processes for preparing the same.

BACKGROUND

Polymer thermoplastics, owing to their high impact resistance, non-corrosive nature, being lighter and often more cost effective than metals, have of late, emerged as a potential replacement for metals in several applications. Among them, polypropylene (PP) and polyethylene (PE) are known for their excellent chemical resistance and mechanical properties. Polymers are electrically insulating materials with conductivity values as low as 10 ^"7 - 10 ^"!4 S cm ^'1 . However: increasing their electrical conductivity would allow them to be used in other applications such as in manufacturing of computer chips, EMI shielding, and dissipation of electrostatic discharge. According to the Electronic Industries Association (EIA) Standard 541, a plastic material would be classified as conductive one if it has the ability to protect against electrostatic discharge (BSD; surface resistivity between 10 0 ¹² ohms/sq) or electromagnetic interference / radio frequency interference (EMI/RFI; surface resistivity of <10 ⁵ ohm/sq).

Carbon black (CB), a particulate form of carbon is widely used in tires, elastomers (rubber), plastics, inks, and paint. In plastics, carbon black is used as a colorant, as a stabilizer to protect the polymer from UV radiation, as reinforcement, and to improve the electrical conductivity of the material. The typical electrical resistivity of carbon black is about 10 ^"2 -10 ^"3 Qcm and may vary depending on the morphology, particle size and preparation conditions. CBs having smaller aggregates, higher structures, and less volatiles are good for use as filler in a conductive polymer composite. A carbon black having a high electrical conductivity even at low filler loading levels (low percolation threshold) would be ideal to use. Carbon black particles with larger surface area and high degree of branching (allowing it to contact a larger amount of polymer) can impart good electrical conductivit in composites. SUMMARY OF INVENTION

This invention relates to an electrically conductive polyolefm composite and a process for preparing the same. The electrically conductive polyolefm composite comprises a polyolefm resin, maleic anhydride modified resin as a compatibilizer, and carbon black conductive filler. The process for preparing the composite comprises the steps of mixing the conductive filler with the poly mer in a twin-screw extruder thereby producing a polymer composite with enhanced electrical conductivity due to improved dispersion of the conductive fillers.

The present invention enables production of conductive carbon fiber filled polypropylene and polyethylene composite materials which have excellent electrical conductivity properties.

DETAILED DESCRIPTION

In one embodiment, conductive carbon black filled polyolefm composite material comprises a polyolefm selected from polyethylene (including HDPE), polypropylene and combinations thereof, as a matrix resin. Suitable conductive carbon blacks include, but are not limited to, granules such as EN SAC O® 250 G (granular form) which can be obtained from TIMCAL, Switzerland. This material has high structure and void volume that allows the formation and retention of conductive carbon networks at a very low filler loading. Some of the important features of this material are shown in Table 1.

Table 1: Properties of ENSACO® 250 G granules used in this study

The interfacial adhesion of the carbon black with the polyolefm resins can be obtained by grafting of a monomer that contains an etliylenic double bond and a polar group in the same molecule. For example, acid anhydrides and their derivatives can be used.

The amount of the polyolefm that contains an ethylenic double bond and a polar group in the same chain is not particularly limited, and is preferably 1 to 10 parts by weight for each 100 parts by weight of the main chain of polyolefm. An amount smaller than 1 part by weight may result in insufficient adhesion to the carbon fibers, while an amount of larger than 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 the polyolefm resin, conductive carbon black, and the polyolefm 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 such that the polyolefm resin is satisfactorily melted but is not decomposed.

The production method of the composite material of the carbon black and the matrix resin employs (but is not limited to) an integral mixing method which includes the carbon black with the polyolefm 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 polyolefm resin used in the present invention is not particularly limited, and various polyolefm resins can be used. Suitable examples include, but are not limited to, polypropylene, poly-l-butene, polyisobutylene, and random copolymers or block copolymers of propylene with ethylene.

The form of the matrix resin to be used in production of the conductive carbon black filled composite material of the present invention is not particularly limited. For example, the matrix resin may be used in the form of pellets, flakes, or powder.

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. Production Method for Carbon Black filled Polypropylene Composite Materials:

Various amounts of carbon black were dry blended with other ingredients 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 25-35 rpm and processing temperature is preferably 200°C - 260°C.

The dry-blends were fed with 60-100 parts of injection molding grade homopolypropylene ;

5 - 40 parts preferably 10- 30 parts or higher of EN SAC 250 G carbon black; modified polypropylene 0,5- 10 parts preferably 1-5 part (PRIEX 25097, maleic anhydride modified polypropylene;

and were melt blended.

The extrudate was cooled in a water bath, air-dried, and pelletized to obtain the modified polypropylene composite resins. ^' The pelletized modified polypropylene resins were compression molded into sheets for electrical resistivity measurements.

Examples 1-15

Conductive polypropylene composite materials were produced using the methods described above, but adjusting: (1) the amount of PRIEX, (2) the amount of carbon black; (3) extrusion temperature; and (4) screw speed 25 rpm. Resistivity measurements are shown in Table 2 below.

Table 2: Surface resistivity of the conductive polypropy lene composites

Example

30 250 25 4.6 1.073 1.518 6.587E-01 4

Example

10 230 35 90.797 21.175 29.966 3.337E-02 5

Example

30 230 35 27.76 6.474 9.162 1.091E-01 6

Example

10 250 35 35.353 8.245 1 1 .668 8.571E-02 n

Example

30 250 35 3.476 0.81 1 1.147 8.717E-01 8

Example

3.18 240 30 5161.33 1203.668 1703.409 5.871E-04 9

Example

36.82 240 30 3.995 0.932 1.318 7.584E-01 10

Example

20 223 30 24.487 5.711 8.082 1.237E-01 11

Example

20 257 30 124.067 28.934 40.946 2.442E-02 12

Example

20 240 22 10.243 2.389 3.381 2.958E-01 13

Example

20 240 38 21.347 4.978 7.045 1.419E-01 14

Example

20 240 30 7.906 1.844 2.609 3.833E-01 15

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.

Comparative example 1 :

Commercial grades of the conductive polypropylene with carbon black can be found on the market. For example, PP-EL from Simona company which is polypropylene with carbon black based was measured and showed 2.38 x 10 ² Ohm/sq surface resistivity and a volume resistivity of 1.80 x 10 ¹ Ohm.cm.

Other comparative examples of carbon black and polypropylene matrix are reported in the literature and are summarized in the comparative examples below. One can see that the composites prepared using the inventive methods described herein are superior to those known in the art.

Comparative example 2:

Tchodakov group reported that the lowest volume resistivity achieved was ~10 ⁴ Ohm.cm for the 15 wt% loading of carbon black in polypropylene matrix. See R. Tchoudakov, O. Breuer, M. Narkis, A. Siegmann. Polym Networks Blends 1996,6, 1.

Comparative example 3:

Li et ai in 2.006 reported that 1.48 x 10 10 ² Ohm.cm. with the 25 wt% carbon black polypropylene composites. See Y. Li, S. Wang, Y. Zhang, Polymer & Polymer Composites 2006, 14, 377.

Comparative example 4:

Saleem reported -10 ⁷ Ohm.cm with the formulation of 15wt% carbon black in polypropylene matrix. See A Saleem, L Frormann, A Iqbal, Journal of Polymer Research 2007, 14, 121.

Comparative example 5:

In 2012, Wen et ai reported that volume resistivity was ~10 ³ Ohm.cm with the 8 wt% carbon black loading in polypropylene composites. See M. Wen, X Sun, L Su J Shen, J. Li S, Guo, Polymer 2012 53, 1602.

Comparative example 6:

Another comparative example reported by Moskalyuk et al. shows -10 ⁵ Ohm.cm with the high loading of carbon black (40wt%) in polypropylene matrix. See O. A. Moskalyuk , A. N. Aleshin, E. S. Tsobkallo, A. V. Krestimn, V. E. Yudin, Phys. Solid State 54, 2122. (2012). Production Method of Carbon Black filled Polyethylene Composite Materials:

Various amounts of carbon black were dry blended with other ingredients to obtain their respective composites using an interraeshing, co-rotating twin screw- extruder (FaiTell FTX20, USA, screw diameter 26 mm; 1/d ratio 35). The screw has both the dispersive and distributive mixing elements. The extmder was operating at a screw speed of 9-15 rpm and processing temperature is preferably 190° C - 260° C The dry-blends were fed with:

70-100 parts of injection and compression molding grade of homopolyethylene, 5 - 40 parts preferably 10- 30 parts or higher of ENSACO ^® 250 G carbon black;

modified polyethylene 0.5- 10 parts preferably 1-5 part (Polybond ^® 3029, maieic anhydride modified high density polyethylene:

and were melt blended.

The extrudate was cooled in a water bath, air-dried, and pelletized to obtain the modified polypropylene composite resins. The pelletized modified polypropylene resins were compression molded into sheets for electrical resistivity measurements.

Examples 16-30

Conductive polypropylene composite material produced using the above- described methods, but adjusting: ( 1) the amount of Polybond, (2) the amount of carbon black; (3) extrusion temperature; and (4) scre speed. The results are shown below in Table 3.

Table 3: Surface resistivity of the conductive polyethylene composites

Example

30 250 9 20, 175 4.705 6.658 1.502E-01 19

Example

10 230 15 7.18 1.674 2.370 4.220E-01 20

Example

30 230 15 1 1.21 1 2.615 3.700 2.703E-01 21

Example

10 250 15 55 ,085 12.846 18.180 5.501E-02 22

Example

30 250 15 767.7 179,035 253.366 3.947E-03 23

Example

10 240 12 7.127 1.662 2.352 4.251E-01 24

Example

30 240 12 139.9 32.626 46.172 2.166E-02 25

Example

20 230 12 187.95 43.832 62.030 1.612E-02 26

Example

20 250 12 1149.25 268.015 379.290 2.637E-03 27

Example

20 240 9 14.288 3.332 4.716 2.121E-01 28

Example

20 240 15 20.17 4.704 6.657 1.502E-01 29

Example

20 240 12 24 ,65 5.749 8.135 1.229E-01 30

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.