RU2573517C2 - Method of making electric insulating composition - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: composition, wt % is obtained of high-pressure polyethylene - 59.6-79.6, filling material with polar surface - microcalcite - 20.0-40.0, calcium stearate - 0.3 and Richnox 1010 - 0.1. The filling material is pre-dried at 105°C up to absolutely dry state.

EFFECT: invention allows increase in homogeneity of produced composition due to hydrophobisation of the filling material surface.

2 tbl, 2 ex

Description

 The invention relates to the cable industry, and in particular to methods for producing an electrical insulating composition intended for insulation and cable sheaths and wires, characterized by reduced smoke emission during combustion.

A known method of applying to inorganic fillers of various hydrophobic products, for example, saturated or unsaturated with fatty acids or their salts, in particular oleic acid or stearic acid, or the corresponding oleates or stearates, or organosilanes or titanates [1. Sirotkina E.E., Mityushkin S.Yu., Borilo A.V. Polypropylene and talc-filled compositions based on it // Plastics. 1997. - No. 2. - S. 27-31].

The disadvantage of this method is the formation of a coating layer with an unsatisfactory appearance, which has a dull, rough surface. Pore formation is observed inside the fire retardant layer, which leads to a subsequent deterioration in the mechanical properties of this coating.

The closest is a method of preparing a fire-retardant composition, which consists in mixing a polymer base with an inorganic filler when heated at a given temperature and for a specified time in order to reduce the moisture contained in the fire-retardant filler, and then add a dehydrating agent to the mixture, which promotes the absorption of water. [RU 2237078 C2, IPC 7 С09К 21/02, C08J 3/20, C08L 23/08, НВВ 7/295 С09К 21/02, C08J 3/20, publ. 09/27/2004].

As the dehydrating agent, calcium oxide, calcium chloride, anhydrous alumina, zeolites, magnesium sulfate, magnesium oxide, barium oxide or mixtures thereof are used. This dehydrating agent can be added to the flame retardant composition during the mixing step (preparation of the mixture) or immediately before introduction into the extruder.

The dehydrating agent exerts its effect by absorbing the water present in the fire retardant filler, which is released during heating of the composition at the extrusion stage.

The adsorption mechanism is preferably of the irreversible type or the dehydrating agent can adsorb water reversibly, but with a low rate of moisture release at the extrusion temperature, in order to ensure

essentially, the absence of water in the vapor state during the extrusion step. This work prevents the formation of pores inside the fire retardant coating and / or the appearance of roughness on its surface. The amount of water released increases with increasing extrusion temperature, as a result of which the benefits of the presence of dehydrating agents become especially apparent when relatively high extrusion temperatures are used, typically above 180 ° C, preferably above 200 ° C. However, the known method has the following disadvantages:

1. The introduction of a dehydrating agent leads to additional costs (costs) of the reagents and may impair the physico-mechanical properties of the composition.

2. The complication of the method of obtaining the polymer composition by adding a dehydrating agent.

The objective of the invention is to provide a method for producing a homogeneous insulating composition, eliminating uneven mixing of the polymer and the filler due to the hydrophilic surface of the filler.

The problem is achieved in that in the proposed method for producing an electrical insulating composition, a polyolefin — high pressure polyethylene — is mixed with an inorganic filler. All components with a polar surface are pre-dried at a temperature of 105 ° C to a completely dry state.

Microcalcite, which in natural conditions has a polar surface, is used as a filler. The surface of polymeric substances (polyolefins) is apolar, therefore, in the process of mixing these components, the problem of uniform distribution of the powdered filler in the polymer matrix arises, which leads to a deterioration in the physicomechanical properties of the composition.

In addition to fillers, stabilizers are introduced into the polymer composition to prevent or slow down the thermal-oxidative and photo-oxidative degradation, which prevent the aging of polymeric materials and extend their service life. Richnox 1010 and calcium stearate were used as stabilizers in the following ratio of components, mass. %:

high pressure polyethylene 59.6-79.6 microcalcite 20,0-40,0 calcium stearate 0.3 richnox 1010 0.1

Richnox 1010 (Richnox 1010), (tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] methane). The molecular weight of 1178 g / mol. White powder with a density of 1015 g / cm ³ . The range of melting temperature is 110-125 ° C. Solubility in water <1 · 10 ^-4 g / l (at 20 ° C). It is soluble in acetone, ethyl acetate. It is used to stabilize polyolefins, such as polyethylene, polypropylene, polybutene, as well as for polyacetals, polyamides and polyurethanes.

Calcium stearate - (C ₁₇ H ₃₅ COO) ₂ Ca. White powder with a melting point of 175 ° C and a density of 1.035 g / cm ³ . It is soluble in benzene, toluene and other non-polar solvents. Not soluble in water.

Since the surface of richnox and calcium stearate is apolar, these powders mix well with the molten polyolefin.

The main component with polar properties is a filler - microcalcite.

The amount of moisture adsorbed on the surface of the polar particles of the filler depends not only on the physical properties of the particles (particle size, wetting angle, adsorption properties, etc.), but also on the humidity of the environment. The higher the humidity, the greater the amount of moisture adsorbed on the polar surface, and thereby the greater the amount of water introduced into the polymer matrix.

The applicant proposes to dry the filler before mixing to a completely dry state, after which the surface of the filler becomes apolar. This was verified experimentally by the specific sedimentation volumes of the powder in liquids of different polarity. The humidity of the filler ranged from absolutely dry to maximum saturation at 100% air humidity. Samples of a powdered filler in an amount of 1 g are placed in test tubes and filled with polar (water) and apolar (octane) liquid, incubated for 1 day, after which the specific sedimentation volume of the precipitate is determined (Table 1).

The value of the volume occupied by the same sample in liquids of different polarity allows us to evaluate their wettability, based on the wetting coefficient proposed by us - K (Table 1):

K = V _n / V _an,

where V _p - specific sedimentation volume of the powder in the polar liquid, cm ³ / g;

V _ap - specific sedimentation volume of the powder in the apolar liquid, cm ³ / g

For values of K> 1, the surface of the studied material is hydrophobic, and for K <1, the surface is hydrophilic. The greater the value of K differs from unity, the

accordingly, the studied material is more hydrophobic or hydrophilic. If the specific sedimentation volumes are equal, the coefficient K = 1, which corresponds to a contact angle of 90 °.

Table 1 shows the specific sedimentation volumes of the filler in polar (water) and apolar (octane) media and the wetting coefficient of microcalcite K.

Table 2 shows examples of limit and transcendental values.

Analyzing the results, we can draw the following conclusions. A change in the specific sedimentation volume of microcalcite with a hydrophilic surface in liquids of different polarity made it possible to establish that the hydrophilicity of the surface depends on the hygroscopicity of the powder, i.e. the ability of a material to absorb moisture from the environment. It was found that the natural moisture content of microcalcite is 0.15%. And during the process of deep drying of the powder, the surface of the particles becomes hydrophobic. As shown in the table. 1, microcalcite having hygroscopic moisture on its surface has a hydrophilic surface. However, dried microcalcite is hydrophobic, since the specific sedimentation volume of the microcalcite precipitate in water exceeds the specific sedimentation volume of the precipitate in octane.

This suggests that hydrophobic interactions do not appear in completely dry material. However, powders with natural humidity or specially moistened to maximum hygroscopicity occupy different volumes in polar and apolar liquids. That is, water molecules adsorbed on the surface of a powdery material contribute to the manifestation of hydrophobic interactions.

Example 1. In a laboratory mixer of the Banbury type load 1.5 kg of a mixture of the following composition (% wt.):

high pressure polyethylene 10803-020  59.6 microcalcite  40 calcium stearate  0.3 richnox 1010  0.1

Microcalcite is taken with a natural moisture content of 0.15%. Mixing takes place at a temperature of 140 ° C for 8 minutes, then the resulting composition is extruded and the physicomechanical properties shown in Table 1 are determined. 2.

Example 2. A mixture is loaded into a Banbury-type laboratory mixer, as in Example 1. Microcalcite was previously dried at 105 ° C until completely dry.

condition. The components are mixed at 140 ° C for 4 minutes, then the resulting composition is granulated at the melt temperature and physical and mechanical tests are carried out (Table 2).

Examples of limit and transcendental values are given in table. 2, which shows that the mixing time of the composition with absolutely dry microcalcite is 3-4 minutes, while the use of moistened microcalcite increases the mixing time to 8-10 minutes.

The physicomechanical properties of the composition are also affected by the content of microcalcite from 20 to 40%.

At a 20% microcalcite content in the composition, the physicomechanical properties change insignificantly depending on the moisture content in the filler. However, in a composition with dried microcalcite, the physical and mechanical properties are slightly higher than in a polymer composition with a moistened filler.

With an increase in the filler content up to 40%, the physicomechanical indices for absolutely dry microcalcite are significantly improved compared to moistened filler: tensile strength from 11.3 to 13.0 MPa; elongation from 50 to 100%; Charpy impact strength without notch at -30 ° C from 43.4 to 58.8 kJ / m ² . In this case, the mixing time of the composition is reduced from 10 to 4 minutes.

Claims (1)

A method of obtaining an electrical insulating composition, comprising mixing a polyolefin — high pressure polyethylene, an inorganic filler with a polar surface — microcalcite, calcium stearate, richnox 1010 with preliminary drying of the filler at 105 ° C to an absolutely dry state in the following ratio of components, mass. %:
high pressure polyethylene 59.6-79.6 microcalcite 20,0-40,0 calcium stearate 0.3 richnox 1010 0.1