CS210555B1 - a method for increasing the stress resistance of linear polyethylene against corrosion - Google Patents

Abstract

Method for increasing the resistance of polyethylene to stress corrosion Ethylene homopolymers easily crack under stress in the environment of solutions of surfactants and solvents. According to the invention, increasing the resistance can be achieved by adding 5 to 60% by weight of modified inorganic fillers, e.g. kaolin or ground lime with an average particle size of 1 to 10 µm.

Description

The invention relates to a modification of the properties of a linear polyethylene.

It is known that linear polyethylene is one of the materials which easily breaks under tension in aqueous surfactant solutions and some organic solvents. This phenomenon is called stress corrosion and its removal would allow a wide range of polyethylene applications.

A known way to reduce the sensitivity of polyethylene to stress corrosion was the use of high molecular weight copolymers, for example, and butene-1. However, the workability of these copolymers is limited by their workability. First of all, such types of polyethylene are difficult to process by injection molding, which is the most important processing technology for the production of technical products, such as the automotive industry, in which gasoline and oil are commonly contacted. Furthermore, these copolymers are more expensive than linear polyethylene injection types.

The present invention is based on our finding that the addition of microdisperse fillers, such as calcium carbonate or kaolin, preferably having a particle size in the range of 1 to 10 µm, results in significant changes in the supermolecular structure of polyethylene which both cause changes in the mechanical properties of polyethylene. by significantly improving the stress-resistant corrosion resistance of polyethylene.

SUMMARY OF THE INVENTION The present invention provides a method for increasing the stress corrosion resistance of a linear polyethylene by modifying additives, characterized in that the modifying additive is micronized stearic acid-treated limestone or thick kaolin treated with an average particle size of 1 to 10 µm at 5 to 60% by weight polyethylene.

The relationships between the structure of polyethylene and its resistance to stress corrosion are not yet theoretically elucidated. A more likely explanation is the weakening of the structure of the amorphous intermellular regions by their swelling in the surfactant. It is known that a boundary transcrystalline structure is formed around the mineral filler, which differs considerably from the disordered semi-crystalline structure of the unfilled polymer. By studying nitric acid induced degradation kinetics of polyethylene loaded with modified kaolin, it was found that this boundary structure was less ordered than the original crystalline phase and more ordered than the original amorphous phase. From a certain filler concentration higher in the total volume of the composite material, this boundary structure of the matrix already prevails. On the other hand, at a very high filler concentration, an easily brittle fracture occurs under the action of the tenzoactive substance.

The stress corrosion resistance measurements are made according to ASTM O 2552 and using test specimens die-cut from a 1 mm pressed thickness.

The required voltage was set with an accuracy of 0.1 MPa, the measuring device used by us has twenty places.

210 SSS

The test specimens were tempered for 1 hour at 115 ° C prior to measurement to achieve equilibrium crystallization and then allowed to cool slowly to room temperature. The undiluted commercial detergent Jar 75 was used as a surfactant, the bath temperature set at (50.0-0.5) ° C and the tensile stress at (2.9-0.1) MPa.

The measurement results are processed in a logarithmic-probabilistic graph. The logarithmic axis is Time to failure for individual bodies in hours, on the probability axis the expression z ^ / (N + 1) x 100%, where z _i is the order of the body at rupture and N is the total number of bodies tested. On the resulting linear dependence ee subtracts a factor of * 50 'that is the probable time to failure of 50% of the test specimens. If no body bursts after 500 hours of exposure in the active bath, the experiment is terminated and the sample is marked as corrosion resistant under stress in that bath.

To quantitatively evaluate! influence of used mineral fillers, the stress corrosion resistance of several linear polyethylene samples was first evaluated. The table shows the results:

Characteristics

Flow index according to ČSN 640861 (h)

Copolymer 0.20 Homopolymer 1 0.34 Homopolymer 2 5.9 resistant

It can be seen from the table that both said homopolymers, of which type 1 is relatively high molecular weight, are not sufficiently resistant to stress corrosion.

Conversely, as will be shown in the examples below, with a wide concentration range of the added fillers, a satisfactory resistance is achieved in the polymer abrasion. The parts and prooents given in the examples are percent by weight.

Example 1

The powdered homopolymer 1 was mixed with an appropriate amount of milled limestone characterized by an average particle size of 4.5 µm, in which the proportion below 2 µm was 14% and the proportion above 10 µm was 1.6%, and which had been pre-treated with 0.5% stearic acid. The mixture was stabilized with 0.15% pentaerythritol tetrabis (3,5-di-tert-butyl-4-hydroxyphenyl) propionate. The powder mixture was metered into a twin wall mixer extruder and granulated, melt temperature 230 ° C. Plates of 1 mm thickness were prepared from the prepared granulate by pressing at 210 ° C and die-cut test bodies which were tempered before measurement

210 SB5 and cooled as described above.

The influence of micronized limestone concentration in composite material on the value is shown in the table:

Influence of micronized limestone on corrosion resistance under tension of homopolymer 1% CaCO ^ in mixture Ρ ^ θ (h)

0 86 10 680 20 May 500 30 500 40 500 50 880

It is apparent from the table that a very good stress corrosion resistance is achieved in the CaCO2 concentration range from 10 to 50%.

Example 2

The powdered homopolymer 2 was mixed with an appropriate amount of calcined kaolin with a 1% aminopropyl-triethyl etosysilane surface treatment, which was characterized by an average particle size of 2.3 µm with a particle size below 2 µm 37% and a particle size above 10 µm 0.2%. The mixture was stabilized with 0.1% 2,2-thiodiethyl-bis-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate. The preparation of the granulate and the preparation of the test specimens were carried out in the same manner as in Example 1.

The effect of the modified microdisperse kaolin concentration in the composite material on the stress corrosion resistance of * 50 is shown in the following table:

Influence of modified kaolin on corrosion resistance under tension of homopolymer 2

% kaolin in the mixture * 50 0 30 10 180 20 May 500 30 500 40 500

210 585

500

500

It is apparent from the table that in the range of 10 to 60% of the concentration, a high resistance to stress corrosion is achieved with a low-molecular-weight injection homopolymer.

Claims (1)

Method for increasing the stress corrosion resistance of a linear polyethylene by modifying additives, characterized in that the modifying agent is stearic acid-treated micronized limestone or silane-treated kaolin with an average number of from 1 to 10 µm in a concentration of 5 to 60% by weight based on polyethylene weight .