CS212997B1 - Process for processing plastics mixtures - Google Patents

Abstract

The invention relates to the field of processing mixtures of plastics such as polyolefins with elastomers, olefin polymers and copolymers with electrically conductive carbon black depending on the size of the specific surface, to semiconducting products with a specific surface resistivity^* = 10^ohms or with a specific internal resistivity^ v = 10^ohm.m. The invention addresses the issue of the conductivity of the product depending on the amount and quality of carbon black and on the processing temperature. The essence of the invention lies in the fact that the temperature regime during the processing of these mixtures is set in all other sections, except for the first input zone, to the same temperature value, which is 40 to 80 °C higher than the usual processing temperature of the above mixtures, while the percentage content of carbon black in high-surface areas can be reduced to 15% by weight. The invention can be used in the production of semi-conductive plastic products for mining and quarrying purposes, in the production of plastic containers for transporting flammable materials, and also for products for environments where electrostatic charge should not arise.

Description

The subject of the invention is a method of processing plastic mixtures into semiconducting products. Plastic mixtures are understood to mean mixtures of polyolefins with elastomers and especially olefinic polymers and copolymers with electroconductive carbon black in a concentration of up to 15% by weight. Semiconducting products for electric current are determined with a specific surface resistance = 10^ ohms or with a specific internal resistance _v ⁼ 10^ ohms.m.

There are a number of patents in the patent literature describing semiconducting mixtures whose basic component is carbon black. They can be divided according to the quality of the effective conductive material (carbon black), the main characteristic of which is the size of the specific surface area determined by the Brunauer, Emmet, Teller method, hereinafter referred to as BET.

The first group includes patents such as DAS 2 500 933, which includes a polyethylene mixture with 7.5 to 15% conductive carbon black with a BET of 300 to 1 500 m/g for products used in mines, in the production of explosives, etc., or Japanese patent JA 057 802, which includes a semiconductive mixture with high mechanical strength, containing thermoplastic polyethylene, ethylene vinyl acetate copolymer, chlorinated polyethylene, polypropylene rubbers and conductive carbon black with a surface area of more than 500 m/g. By using conductive carbon black with a high specific surface area, it is possible to achieve semiconductivity of the mixture even with a reduced carbon black content; the mixtures have good elasticity and a specified internal resistance of 10 to 10 Ohm.cm. Other Japanese patents JA 068 152 and J5 1 132 484 are based on a similar principle.

The second group of patents that protect semiconducting mixtures can be documented from the description of the invention of the USSR AO 455 376 and AO 427 026, also DAS 2 521 654, where, among other additives, carbon black with a specific BET surface area of 50 to 200 m /g is used, the amount ranging from 20 to 50% by weight,

AND

The proposed solution minimizes the amount of conductive carbon black used, enables the achievement of satisfactory semiconducting properties in products made from a mixture of carbon black with a low specific

2 surface area (50 to 200 m /g) or with carbon black with a high specific surface area (BET 500 m /g to

000 m /g) while ensuring good mechanical properties, while the stability of the production process is not impaired.

The essence of the present invention is that the temperature regime during the processing of plastic mixtures into semiconducting products is set in all other sections, except for the first input zone, to the same value, which is higher by 30 to 80 °C compared to the usual processing temperature of the said mixtures, while the percentage content of carbon black with a high specific surface area can be reduced to 5 to 7% by weight, and for carbon black with a low specific surface area to 15% by weight. The further description and the examples provided serve to explain the invention in more detail, but do not limit the scope of the invention.

It was found that the processing of the polymer-carbon black mixture is influenced by several factors, but the decisive one was the influence of shear mechanical forces acting in the polymer melt on the carbon black particles (clusters). The claim was supported by experiments in which the same mixtures were processed by pressing, injection molding, extrusion and blowing under similar temperature conditions. While semiconducting properties of products (plates, foils) were achieved by pressing, other technologies went to the extreme of losing these properties. The explanation of the causes is as follows: in the polymer-carbon black mixture, conductive bridging of a given volume of polymer is achieved by agglomeration of individual carbon black particles into various formations (chains) and their mutual interconnection enables the transfer of electrons. When processing mixtures in screw extruders or injection molding machines, under normal processing conditions during processing

212 997 to strong mechanical stress of the polymer melt and carbon black particles, high shear stress degrades the carbon black structure, disrupts the contacts between the chains, which leads to a decrease or complete loss of semiconductivity of the products. Usually this problem is solved by increasing the content in the mixture to 20 to 50% by weight for low-surface carbon blacks, about 10% by weight for carbon blacks with a high specific surface area, which, in addition to increased economic demands, significantly affects the mechanical properties of the products.

It was experimentally found that maintaining the semiconducting properties of products even with a reduced percentage* of carbon black (low-surface carbon black approx. 15% by weight, high-surface carbon black 5 to 7% by weight) can be achieved by adjusting the technological regime, especially the temperature profile. By increasing temperatures, reducing the viscosity of the melt, and shear stress of the mixture, the possibility of disrupting the contact between the carbon black chains is limited. -The best results were achieved with the so-called uniform temperature profile setting, except for the first input zone, the same temperatures are set in the other zones of the cylinder and on the extrusion head (similarly to injection molding). In these cases, the semiconducting treatment is homogeneous, with the same quality of products in all places. The level of the set temperature profile takes into account the type of material, the melt index, and later on the data obtained from the flow curves and the technology used.

The advantages of the method are illustrated by the following examples:

The mixtures prepared in the usual way on a twin-roller (containing the basic polymer branched polyethylene with a melt index of 2g/10 min., low-surface acetylene carbon black (BET 50 to 70 m ^{* 2 *} /g) and high-surface thermal carbon black (BET 1,000 m /g) were processed after granulation by pressing, extrusion into flat and tubular films, injection molding under conditions common for the given type of polymer and under conditions found to be optimal in terms of electrical and mechanical properties of the products and in terms of process stability. The results of the experiments are given in examples 1, 2 and 3.

Example No. 1 - Pressed plates - electrical properties

Type of soot Concentration - % wt. Temperature Regime ( °C) Specific surface. resistance (Dhm)^ Specific internal resistance (Ohm.m)^ _in VP 5 160 5.1Ο ⁴ 7.10° VP 7 160 4.1Ο ⁴ 2.10° NP 15 160 3.1Ο ⁵ 8.10 ¹

O

VP - carbon black with high specific surface area (BET 1,000 m/g)

NP - carbon black with a low specific surface area (BET 50 to 70 m /g) (> _s (0hm) - measured according to the method of the Coal Research Institute Radvanice - St. test room No. 214 ý _v (0hm.m) - clamp method VUKI Bratislava

The results in Example 1 show that 5% by weight of high-surface area carbon black or 15% by weight of low-surface area carbon black can be used for a high-quality semiconducting treatment, the experimental temperature is sufficient.

Example No. 2 - Extrusion of flat foils - dependence of electrical properties on temperature regime (15% NP carbon black used), flat foil with a thickness of 0.1 mm is produced.

Temperature regime cylinder - head °C (> ‘.(Ohm) (> _in (0hm.em) Across Furthermore Across Along 130,160,180- 180, 180 1.4x10 ⁹ 1.3x10 ⁹ 1.4x10® 9.2xl0 ⁷ 130, 160,180- 200, 200 ^1.6x107 1.5x10 ⁷ 2.3xl0 ⁵ 1.9x10 ⁵ 150,200,200- 200, 200 6.6x10® 5.8x10® 1.4x10 ⁵ 9.8xl0 ⁴ 150,200,210- 230, 230 9.1x10 ⁵ 8.1x10 ⁵ 1.4x10 ^{4 1.3x104} 170,230,230- 230, 230 ^7.3x105 8.0xl0 ^{5 1.3x104} 1.1x10 ⁴ 170,230,230- 250, 250 2.6xl0 ^{5 2.25x105} 3.1x10 ^{3 2.6x103} 170,250,250- 250, 250 2.0xl0 ⁵ 2.Ox1O ^{5 2.5x103 2.5x103}

Example No. 2 summarizes the conditions for extrusion of a mixture of branched polyethylene with 15 wt. % of low-surface carbon black and the corresponding electrical properties of the produced foils. The strong influence of the temperature regime setting on the specific surface and internal resistance is shown. The table does not perfectly capture the homogeneity of the semiconducting treatment, the values given are average values. It was observed that with a uniform temperature profile setting, the semiconducting treatment is always homogeneous, while with an upward setting, irregularities occur, the specific surface and internal space change locally (Similar phenomena have been observed with other technologies.).

Example ?.~3 - Blowing of tabular foils - relationship between electrical and mechanical properties and temperature regime (or type and concentration of soot).

3 ] í concentration K) mass Temperature regime (°C) common designed according to the invention Surface resistance _s (ohm) Internal Resistance Vv (ohm.cm) Tensile strength ¹ /(MPa) PN Dextensibility (%) P N Impact resistance by falling weight (J/mm) Tear resistance - (kN/mm) P N i-; Resistance to further tearing (kN/mm) P N 5 140,160,180,200,200,200 >10 ⁹ >10 ⁹ 14.6 13.9 ±0.7 +1.9 529.6 474.8 +67.91115.5 7.9 102.7 70.7 +10.6+38.4 52.6 53.5 ±5.0 ±2.3 140,250,250,250,250 4x10® 4xl0 ² 13.3 10.3 +1.5 ±0.56 432 348 *137.2+250.9 7 140,160,180,200,200,200 Λ10 ⁹ ?10^ 12.5 15.2 +3.2 ±0.7 401.2 472.4 *135 ±83.6 8.9 93.8 91.6 ±3.4 75.8 46.4 53.8 ±7.2 +1.6 140,230,230,230 4x10^ 2x10° 12.86 11.6 ±0.3 ±1.07 318.8 170.4 *167.8 +34.8 7.28 85.5 77.4 19.5 ±8.5 32.7 48.3 +8.5 ±4.6 10 140,160,180,200,200,200 3xl0 ⁴ 1.3xlO ^c 16.2 16.5 ±2.3 19.0 226.8 227.6 + 64.2HÓ7 7.9 88.3 81.8 14.2 ±2.6 30.6 36.4 ±4.2121.9 — — — — 15 130,140,160,160,160,160 >10 ⁹ >10 ⁹ 13.8 13.8 10.3 10.7 142.8 47.6 + 62.7 ±13.9 8.57 L80,250,250,250,250,250 2x10® 3xio ² 14.67 11.8 a.i ti,o 154.4 111.2 + 24.7 +1.8 6.2 53.9 58.8 ±9.7 +9.7 25.1 20.5 ±2.3 +7.4 140,160,180,200,200,200 >10 ⁹ ;>10 ⁹ 17.8 19', 4 12.0 >2.2 521.8 800 + 74 1167 12.8 93.9 89.7 +io;3 ±18.4 67.8 53.4 ±1.4±7.8

tnémka: Temperature regime values 1-4 = extrusion cylinder*Machine, 5-6 extrusion head P * along, N = across

212,997

As shown in example δ. 3 β, with increasing carbon black content in the film, there is a clear decrease in ductility and resistance to further tearing. It is also evident that the proposed processing conditions allow a reduction in the carbon black content, e.g. from 10 to 5 to 7% by weight of high-surface carbon black, which contributes to both the improvement of mechanical properties (ductility, resistance to further tearing) and the economy of the process.

The method according to the invention can be used in the production of semiconducting plastic products for mining and quarrying purposes, in the production of plastic containers for transporting flammable materials, for products for environments where electrostatic charges should not be generated, in electrical engineering, vacuum technology, etc.

Claims (1)

SUBJECT OF THE INVENTION Process for processing a mixture of plastics such as branched and linear polyethylenes, polypropylenes, olefin copolymers and mixtures of polyolefins with each other, with elastomers, with electrically conductive carbon black into semiconducting products, characterized in that the temperature mode is set to the same in all other sections a temperature value which is 30 to 80 ° C higher than the usual processing temperature of said mixtures, the percentage of carbon black being reduced to 5 to 7% by weight for low-surface and 15% by weight to low-surface.