NO760076L - - Google Patents

Description

Polyethylene molding compound for extrusion of

pipes and plates with low surface resistance.

It is known that in potentially explosive atmospheres only building materials with a surface resistance of less than 10 Q• ohms should be used

in order to safely avoid a static charge and the formation of friction. For this reason, plastics often cannot be used here, although they would be excellently suited due to their other properties.

It is true that numerous methods are known for antistatic finishing of plastics, a lowering of the surface resistance below 10 9 ohms, however, cannot be achieved with certainty over a longer period of time. Thus, surface-active compounds can be applied or incorporated on the surface or the surface can be changed, chemically, e.g. sulfonated (compare DOS 1',953,371). All these methods have the disadvantage that, depending on humidity and wear, the effect is completely or temporarily lost.

It is also known to reduce the surface resistance

of plastics by adding soot. In order to reach a value below 10 9 ■ ohms, in the meantime, with commercially available conductive soot, such as e.g. "Corax L", from Degussa AG required quantities of approx.

20% by weight.

Such high amounts, however, reduce the plastic's toughness, above all the impact resistance, so considerably that the mechanical properties are no longer sufficient for most applications.

It has now been found that by using a special carbon black already with very little carbon black addition, the desired reduction of the surface resistance below 10 q ohms is achieved, so that molding compounds are obtained which at the same time have a low surface resistance and good mechanical properties and are excellently suited for extrusion of pipes and plates, which can also be used where there is a risk of explosion, e.g. in mining.

The object of the invention is therefore a polyethylene molding compound for extruding pipes and plates with low surface resistance and consisting of 92.5 - 85, preferably 92 - 88% by weight polyethylene and 7.5 - 15, preferably 8-12% by weight carbon black, each once referred to the polyethylene-carbon black mixture, the mass being characterized in that the carbon black has a BET surface area of 300-150.0, preferably 600-1100 m /g, an oil absorption of 2.5 - 5.5", preferably 3.0 - 4, 0.ml/g, , a content of volatile components of 0.1 - 6.0, preferably 0.5~4% by weight, a pH value of 7 - 10, . preferably 8 - 9.5 Pg an ash content of 0.5 - 5.0, preferably 0.5 - 1.0 by weight as well as usual additives such as e.g. stabilizers, processing aids and lubricants. Such soot occurs as a by-product in known gasification processes for the production of carbon monoxide and hydrogen-containing gas mixtures of hydrocarbons, such as e.g. is discussed in "Advances in Petroleum Chemistry and Refining", Volume 10, Chapter 4, Pages 123-189 (Interscience Publishers, New York, 1965). This soot was, of course, already used together but with non-conductive fillers for the production of electrically conductive vulcanizates, (DOS 2,047,333), thereby explicitly referring to the fact that when this carbon black is used alone, many disadvantages arise as the workability is difficult and the mechanical properties are poor. From these statements, it could not be concluded that polyethylene mixtures with this carbon black without additional fillers are particularly suitable for extruding pipes and sheets with low surface resistance and good mechanical properties. As polyethylene for the mixture according to the invention, a linear polyethylene with a density of 0.94 - 0.96 g/cm-5 and a melt index MF I 190/5 according to DIN 53 735 of 0.2 - 2.0 is particularly suitable. preferably 0.3 - 1.6 g/10 minutes. Mixed polymers of ethylene with up to 8% by weight of propylene and/or butene-(1) are also suitable.

Suitable stabilizers are above all aromatic amines, e.g. octylated diphenylamine or derivatives of p-phenylenediamine, such as N-phenyl-N',-isopropyl-p-phenylenediamine or N,N'-di-2-naphthyl-p-phenylenediamine in amounts of 0.05 - 2.0, preferably 0 .1 - 1.0 wt/S, referred to the mixture.

As a lubricant or processing aids are preferably used metal stearates, e.g. calcium stearate or zinc stearate or glycerol esters of fatty acids with 12 to 24 C atoms, - e.g. glycerol stearate in amounts of 0.1 - 1.0% by weight, referred to the mixture.

By adding 7.5 8% by weight of this carbon black to polyethylene, the surface resistance in the molding compound is lowered below 10<9>ohm. Higher additions reduce the surface resistance more strongly and, however, also affect the mechanical properties more strongly, so that addition quantities above 15% by weight do not bring benefits. The best results are obtained with 8-10% by weight of soot, referred to the mixture. With these mixtures, the surface resistance is lowered to the desired value, while the mechanical properties are approximately not affected.

The porm masses can be produced according to usual methods, e.g. by premixing powders in slow-moving mixers with subsequent homogenization and granulation on twin-screw extruders. It has proven advantageous to firstly pre-mix the polyethylene with the stabilizers in a high-speed mixer and then to add the soot in a slow-speed mixer.

As soot under the influence of oxygen and heat has a network-forming effect on the polyethylene, the mechanical properties of the finished mixture can be improved by mixing and granulating under nitrogen.

Pipes made from the molding compounds according to the invention are above all suitable as transport pipes for flammable liquids or powders, which can cause static charging due to friction. -Of the plates, it can e.g. shafts, transport vessels or air shafts for quarries are produced.

Example 1.

Polyethylene of a density of 0.946 g/cm^ and a melt index MFI 190/5, measured according to DIN 53 735 of 0.3 g/10 minutes in powder form was premixed in a high-speed mixer with 0.4 wt% of N-phenyl-N -isopropyl-p-phenylenediamine and 0.2% by weight of glycerol stearate (mixture of 45% mono-, 40% di- and 15% tri-stearate). Then, in a slow-moving mixer, the quantities of a carbon black indicated in Table 1 were added, which had a BET surface area determined according to Brunauer, Emmet and Teller, J. Amer. chem. Soc. 60, (1938), page 309, of 950 m<2>/g, an oil absorption of

3.2 ml/g, a volatile matter content of 1.0 wt%, a pH value of 8 and an ash content of 0.7 wt%. The mixture was then homogenized on a twin screw extruder at a temperature of 180 to 250°C and granulated.' From the granulate, 1 resp. 4 mm thick plates and the specimen produced from them. To

for comparison, similar mixtures were prepared with a commercially available highly conductive carbon black "Corax" L, Degussa.AG with a BET surface area of 133 m /g, an oil absorption of 5.1 ml/g, a content of volatile components of 1% by weight 3 et ash content of 0.07% by weight and a pH value of 8. The test results obtained appear in table 1.

Of the mixture according to the invention with 8, 10 resp. 14% by weight of the special soot, it was possible to extrude tubes and plates with correspondingly good mechanical properties and the desired low surface resistance.

The surface resistance was measured according to DIN 53 482 with electrode device A at a measuring voltage of 1000 resp. 5 volts. With a resistance below 10 7 ohms, the voltage must be reduced to prevent breakdown.

Example 2.

92 parts of a polyethylene with a density of

0.947 g/cm<5>and a melt index MFI 190/5 of 0.5 g/10 min. in powder form, a slow-moving mixer was mixed with each time 8 parts of a carbon black, which had a BET surface area of 800 m /g, an oil absorption of 3.2 ml/g, a volatile matter content of 3.5 wt$, a pH value of 8 and an ash content of 0.7% by weight as well as 0.4% by weight of N,N'-di-2-naphthyl-p-phenylenediamine and the amounts of glycerol stearate indicated in Table 2 and granulated as indicated in Example 1 and prepared for test specimens. Before the usual granulation, each time half of the mixture was separated, granulated under nitrogen and similarly processed, into the test body. The results obtained appear in table 2.

Claims (13)

1. Polyethylene molding compound for extruding pipes and plates with low surface resistance and consisting of 92.5 - 85 wt% polyethylene and 7.5 - 15 wt# carbon black, each time'referred to the polyethylene-carbon black mixture, as well as in addition 0 .1 to 1.0% by weight of a stabilizer and 0.1 to 1.0% by weight of a lubricant resp. processing aid, each time referred to the total amount of polyethylene/carbon black, characterized in that the carbon black has a BET surface area of 300 - 1500 m /g, an oil absorption of 2.5~ 5.5 ml/g, a content of volatile components of 0.1 - 6.0% by weight, a pH value of 7 - 10 and an ash content of 0.5 - 5.0% by weight. 2. Polyethylene molding compound according to claim 1, characterized in that the carbon black has a BET surface area of 600 - 1100 m <2> /g. 3- Polyethylene molding compound according to claims 1 and 2, characterized in that the soot has an oil absorption ■ of 3.0 - 4.0 ml/g. 4. Polyethylene molding compound according to claims 1 to 3, characterized in that the soot has a content of volatile components of 0.5 - 4% by weight. 5. Polyethylene molding compound according to claims 1 to 4, characterized in that the soot has a pH value of 8 - 9.5. 6. Polyethylene molding compound according to requirements. 1 to 5-, characterized in that the soot has an ash content of 0.5" 1.0% by weight. 7. Polyethylene molding compound according to claims 1 to 6, characterized in that a polyethylene is used with a density of 0.94 - 0.96 g/cm <5> and a melt index MPI 190/5 of 0.2 - 2 g/ 10 minutes 8. Polyethylene molding compound according to claims 1 to 7, characterized in that a polyethylene of an MPI 190/5 of 0.3 - 1.6 g/10 min is used. 9. Polyethylene molding compound according to claims 1 to 8, characterized in that the mixture is stabilized with 0.1 - 1.0% by weight of aromatic amines. 10. Polyethylene molding compound according to claims 1 to 9, characterized in that the polyethylene mixture is stabilized with a derivative of p-phenylenediamine. 11. Polyethylene molding compound according to claims 1 to 10, characterized in that it contains metal stearate or glycerol esters of fatty acids with 12 - 24 G~ atoms as a processing aid. 12. Polyethylene molding compound according to claims 1 to 11, characterized in that it contains glycerin stearate as a processing aid. 13. Pipes made of polyethylene molding material corresponding to requirements !■ to 12.