CS256989B1 - Linear polystyrene based antistatic compound - Google Patents

Abstract

The composition of an antistatic mixture based on linear polyethylene is addressed, intended in particular for the production of containers for flammable liquids, pipes, foils and injection-molded products. This mixture contains, per 100 parts by weight of polyethylene with a density of 0.93 to 0.97 gcm"3, a flow index MPI 190 °C/49 N according to ČSN 640 861 in the range of 0.15 to 8 g/10 8 g/10 min, 5 to 16 parts by weight of carbon black with a specific BET surface area of 500 to 1,500 m^g-1, a pH of the aqueous extract of 4 to 7, produced by partial oxidation of petroleum and tar raw materials in the presence of an oxygen vapor mixture and 0.1 to 2 parts by weight of a stabilizer based on organic phosphites according to the invention. It may also contain lubricants and modifiers.

Description

The invention relates to an antistatic mixture based on linear polyethylene, which is intended for applications where static electricity charging of materials must not occur. It is particularly suitable for the production of containers for flammable liquids, pipes, films and injection molded products.

Due to its high electrical resistance, non-antistatic polyethylene easily becomes electrostatically charged. The electrical charge can cause a spark capable of igniting flammable substances or interfering with sensitive electrical equipment.

For this reason, untreated polyethylene cannot be used for the production of containers for the transport of flammable liquids, for the production of pipes intended for installation in explosive environments, etc.

In order for polyethylene to be used for these applications, its surface electrical resistance must be reduced to a value of less than 10 Ohm. Materials with this value of surface electrical resistance are antistatic in the sense of ČSN 33 2030 and therefore safe for use in flammable or explosive environments. It is known that a permanent reduction in the electrical resistance of plastics can be achieved by adding carbon black. It is also known that special carbon blacks with a large specific surface area are the most effective in reducing the electrical resistance of plastics, i.e. carbon blacks with a specific BET surface area of 500 to 1,200 m ³ g ¹ .

It is also known that the addition of carbon black to a thermoplastic polymer impairs its processability and negatively affects the mechanical properties of the material. The deterioration is proportional to the concentration of carbon black, and therefore it is advantageous to use special high-surface carbon blacks, which are effective even at relatively low concentrations.

The use of these carbon blacks brings with it problems consisting in the chemical activity of their surface. It was found that the thermal oxidation of polyethylene occurs in the presence of carbon black in a different way than the thermal oxidation of pure polyethylene. This fact follows from Table No. 1, which shows the induction periods of the thermal oxidation of polyethylene at 180 °C. It is clear from the table that the addition of carbon black to unstabilized polyethylene extends the induction period of thermal oxidation (mixture No. 3).

The table also shows that commonly phenolic type antioxidants, which effectively increase the thermo-oxidative stability of polyethylene without carbon black, completely fail in the presence of carbon black and even suppress the carbon black's own stabilizing effect (mixtures 4, 5 and 6).

For this reason, polyethylene blends containing carbon black stabilized with conventional antioxidants are difficult to process because the low thermo-oxidative stability of the material leads to crosslinking of the polyethylene, which causes poor surface quality during extrusion processing and deterioration of the mechanical properties of the products.

This problem can be solved by using special antioxidants capable of suppressing the oxidation and crosslinking of polyethylene during processing.

The antistatic mixture based on linear polyethylene according to the invention exhibits high thermo-oxidative stability and good processability. It is intended in particular for the production of containers for flammable liquids, pipes, foils and injection-molded products. It consists of 200 parts by weight of linear polyethylene with a density of 0.93 to 0.97 g cm ^-3 , a flow index MFI 190 °C/49 N according to ČSN 64 0861 in the range of 0.15 to 8 g/10 min, 5 to 16 parts by weight of carbon black with a specific 2-1 BET surface area of 500 to 1,500 mg , pH of the aqueous extract of 4 to 7, produced by partial oxidation of petroleum and tar raw materials in the presence of an oxygen vapor mixture and 0.1 to 2 parts by weight of cyclic phosphite based on pentaerythritol, or substituted triphenyl phosphite, while the mixture may contain up to 2 parts by weight of lubricants and up to 30 parts by weight of ethylenepropylenediene or ethylenepropylene rubber.

The invention utilizes the good stabilizing effect of antioxidants based on organic phosphites, which is applicable in the case of using highly conductive carbon black produced by partial oxidation of petroleum and tar raw materials in the presence of an oxygen-vapor mixture, characterized by an acidic reaction of the aqueous extract due to the increased content of sulfur compounds.

From the examples given in Table 1 it is clear that when using this type of carbon black, it is possible to prepare mixtures highly resistant to thermal oxidation using antioxidants based on organic phosphites. The essence of the invention is further clarified by the fact that the high stabilizing efficiency of phosphites is linked to the specific chemical activity of carbon blacks produced by the above-mentioned process.

Table 2 compares the effects of stabilizers based on organic phosphites on

HDPE containing 10% of carbon black S 1 produced by this technology and 10% of carbon black S 2 produced by a different process. Although both types of carbon black are very similar (the specific surface area of carbon black particles measured by the BET method in both cases is around 900 m ² g ^-1 ), it is clear that good results can be achieved only with carbon black S 1 when using organic phosphites.

The claim that stabilizers based on organic phosphites suppress the crosslinking of HDPE containing carbon black S 1 and thus improve its processability is supported by Table 3. This table shows the results of measurements carried out on a capillary viscometer used to measure the melt flow index of polymers.

The HDPE sample with the appropriate additives was thermally exposed at 190 °C in the viscometer chamber for 0 to 120 min. After this time, the viscometer piston was loaded with a standard device and the flow time of a certain volume of melt through the capillary was measured. With increasing thermal exposure time, HDPE crosslinking occurs, which is manifested by an increase in melt viscosity and an increase in the time of outflow through the capillary.

It is clear that carbon black S 1, despite being able to suppress the thermooxidation of polyethylene to a certain extent due to the chemical activity of its surface (Table 1), significantly accelerates the crosslinking of polyethylene. The addition of an organic phosphite-based stabilizer prevents this crosslinking.

Carbon blacks suitable for preparing the mixture according to the invention are produced by partial oxidation of petroleum and tar raw materials in the presence of an oxygen vapor mixture. These carbon blacks have a specific BET surface area of 2-1 2-1

500 to 1,500 mg, preferably 800 to 1,200 mg, pH of the aqueous extract 4 to 7 and ash content maximum 1.5%. For stabilization, an antioxidant based on phosphites, preferably substituted triphenylphosphites, or a cyclic phosphite based on pentaerythritol, or a mixture of the above types of phosphites in a concentration of 0.1 to 2 parts by weight is used.

A mixture of glycerides of higher fatty acids, or calcium stearate, or zinc stearate, or paraffin, or polyethylene wax, or mixtures of some of the above lubricants in a concentration of 0 to 5 parts by weight may be used as a lubricant.

In case increased toughness or reduced stiffness is required, the mixture is modified with an additive of thermoplastic rubber. Commercially produced mixtures of EPDM rubber and polyethylene supplied in granular form, in an amount of 3 to 30 parts by weight, can be used advantageously.

Example 1 _3

100 parts by weight of ethylene-1-butene copolymer with a density of 0.953 gem and a flow index MFI 190 °C/49 N = 0.75 g/10 min produced by gas phase polymerization were mixed in a fluidized bed mixer with 0.25 parts by weight of trinonylphenylphosphite and 0.3 parts by weight of a mixture of palmitic and stearic acid monoglycerides (approx. 1:1) and 10 parts by weight of carbon black 2—1 with a specific BET surface area of 900 mg, pH of the aqueous extract 6, produced by partial oxidation of solid and tar raw materials in the presence of an oxygen vapor mixture. The mixture was granulated on a KO-kneader. This material was easily processable on conventional blow molding machines for the production of linear PE containers. The material properties are given in Table 4.

Example 2

A mixture according to Example 1 was prepared in a fluidized bed mixer, in which the carbon black content was increased to 12.5 parts by weight. A mixture consisting of 65% EPDM rubber and 35% PE was used to modify this mixture. The mixture containing carbon black and the mixture containing EPDM were dosed into a K0-kneader so that their weight representation in the resulting product was 10:2.2. The influence of the addition of EPDM modifiers on the properties of the material is evident from Table 4. This material was tested with good results in bottle blowing, extrusion of 0.3 mm thick foil on a single-screw extruder equipped with a slotted head and in extrusion of pipes with a diameter of 32 mm

Table 1

Mixture

C.

Polymer

Antioxidant type

Antioxidant concentration (h.d./ΙΟΟ h.d.PE)

Induction period of thermooxidation (h)

1 2 HDPE without additives HDPE phenolic AO 1 0.1 1 2 3 HDPE with 10 h.d. soot S 1 - - 5 4 HDPE with 10 h.d. soot S 1 phenolic AO 1 0.25 3.5 5 HDPE with 10 h.d. soot S 1 phenolic AO 2 0.25 2 6 HDPE with 20 h.d. soot S 1 phenolic AO 2 0.2 ¹ 2 .7 HDPE with 10 h.d. soot S 1 organic phosphite 0.25 12 P 1 8 HDPE '8 10 h.d. organic phosphite 0.5 27 soot S 1 P 1 9 HDPE with 10 h.d. organic phosphite soot S 1 P 2 0.25 6 10 HDPE with 10 h.d. soot S 1 organic phosphite P 2 0.5 20

Table 2

Type of antioxidant Antioxidant structure (h.d./100 h.d.PE) Thermo-oxidation induction period (h) soot S 1 soot S 2 organic phosphite P 1 0.25 12 3 organic phosphite P 1 0.5 27 3 organic phosphite P2 0.25 6 3 organic phosphite P 2 0.5 20 3

Table 3

Thermal exposure time θ (min) of the melt at 190 °C 0 30 60 120 3 Flow time 2 cm 12.7 14.2 17.8 HDPE without additives melts by capillary 40.5 51.5 87 to 145 106 to 189 HDPE with 10 h.d. (sec) soot 46 42.5 42 38.1 HDPE with 10.h.d. carbon black with 1 and 0.5 h.d. org. phosphite P 2

Table 4

mixture no. 1 mixture no. 2 notch toughness at - 40 °C (kJ/nr) 5.2 10.9 flexural modulus (MPa) 1,040 670 elongation at break (MPa) 20 60 tensile strength at break (MPa) 32 24 flow index 190 °C/212 N (g/10 min) 5.2 5.8 X ) surface electrical resistance (Ohm) 4.10 ^{* 2 * 4S} 4,3.10 ⁴

X) surface electrical resistance measured on foils cut from the walls of 1 liter bottles.

Mixture No. 1: HDPE 100 wt, d, phosphite Ρ 1 0.25 h.d. lubricant 0.3 h.d. soot S 1 10 h.d.

Mixture No. 2: HDPE 100 h.d.

phosphite Ρ 1 0.25 h.d. lubricant 0.3 h.d. carbon black S 1 12.5 h.d.

EPDM modifier 25 h.d.

Explanation of abbreviations used in tables 1 to 4

HDPE - copolymer of ethylene with 1-butene with a density of 0.953 gem and an MFI of 190 °C/49 N = 0.75 g/10 min

S 1 - highly conductive carbon black produced by partial oxidation of petroleum and tar raw materials in the presence of an oxygen-vapor mixture, BET specific surface area 900 m g — 1

S 2 - highly conductive carbon black Ketjenblack EC, specific surface area BET 940 m g

Ρ 1 - cyclic phosphite based on pentaerythritol, e.g. distearyl pentaerythritol diphosphite

P 2 - aryl tris(nonylphenyl)phosphite

AO 2 - sulfur-containing phenolic antioxidant

AO 2 - phenolic antioxidant

EPDM - ethylene propylene diene rubber

Claims (1)

SUBJECT OF THE INVENTION Linear polyethylene based antistatic blend for use in the manufacture of easily flammable containers, tubes, films and injection molded products, characterized in that it consists of 100 parts by weight of linear polyethylene with a density of 0.93 to 0.97 g, flow index MFI 190 ° C / 49 N according to CSN 640 861 in the range of 0.15 to 8 g / 10 min, 5 to 16 parts by weight 2 -1 parts of carbon black with specific surface area BET 500 mg, pH of water extract 4 to 7 produced by partial oxidation of petroleum and tar raw materials in the presence of an oxygen-vapor mixture and 0.1 to 2 parts by weight of cyclic phosphite based on pentaerythritol or substituted triphenyl phosphite, the mixture optionally further comprising up to 2 parts by weight of lubricants and up to 30 parts by weight of ethylene propylene diene or ethylene propylene rubber.