JPS6032650B2 - Method for producing crosslinked polyolefin molded product - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a crosslinked polyolefin molded article with improved physical properties.

The physical properties of a molded body made of a crystalline polymer, such as a polyolefin molded body, are determined by the microstructure constituting the molded body.

In general, solids made of such thermoplastic polymers are rarely 100% crystalline, and most industrially produced molded products contain nearly half of the amorphous portion.

When attempting to utilize the phenomena mainly exhibited by the amorphous material, such as elasticity and chemical reactivity, for such a molded article, it is difficult to use a highly crystalline material.

On the other hand, when it is desired to obtain physical properties of the molded body that mainly depend on the crystalline portion, such as elastic modulus, creep resistance, and voltage resistance, a material with a large density and high crystallinity is advantageous. In addition to the concept of trying to bring out appropriate physical properties for a solid crystalline polymer material by varying the fractions of the two phases that make up the solid material, it is also possible to It is well known that there is a technique for crosslinking chains to improve heat resistance. However, when crosslinking is introduced into a highly crystalline polymer such as polyethylene, when it solidifies from a melt, crystallization is inhibited or the integrity of the crystal is damaged, resulting in a melting point that is proportional to the number of crosslinking points. As it descends, the density also decreases as an additional effect.

In this case, the above molecular chains are still partially cross-linked, so the physical properties at temperatures above the melting point are significantly improved, including various mechanical properties, whereas below the melting point The physical properties in the temperature range may be caused by the decrease in density, which may cause a decrease in other physical properties such as an increase in dielectric loss and a decrease in withstand voltage performance.

Furthermore, as a conventional method for increasing the degree of crystallinity of such a crystalline polymer material and improving its physical properties, a method of heat-treating the material is known.

However, if the material is a cross-linked polymer, even if it is heat-treated, the molecular chains are already fixed to each other by cross-linking, so there is almost no improvement in physical properties due to recrystallization. be. Furthermore, in the crystallization technology under high pressure that has been studied in recent years, crystal growth proceeds with almost 100% of the defects in the polymer structure removed. , the observed melting point is the melting point obtained by theoretical calculation (14
1.5qo). However, in this case, when the same crystallization technology under high pressure was applied to cross-linked polyethylene, the aforementioned cross-linking between molecular chains became an obstacle, and only a slight improvement in crystallinity was observed, which was not as good as the expected improvement in physical properties. It cannot be obtained. Here, the inventors fundamentally re-evaluated the various excellent physical properties of the crosslinked polyolefin mentioned above and the characteristics of the polymer microstructure obtained by extremely crystallizing it, and, as mentioned above, discovered both of these previously unknown properties. As a result of intensive research into the possibility of creating new materials with these properties, we recognized that the network structure of cross-linked polyolefins could be swollen by solvents, and this could lead to a high degree of crystallization due to a completely new mode of aggregation of molecular chains. He completed his invention.

That is, the present invention is a method for producing a crosslinked polyolefin molded article, which is characterized in that the crosslinked polyolefin molded article is swollen with a solvent, and then the solvent is removed by drying or the like.

In this invention, swelling of the material in a solvent as described above is possible. The occurrence of crystallization phenomena can be specifically observed by placing a crosslinked material in a solvent and raising the temperature, and by measuring the phase change of the material during the salting process.

FIG. 1 shows a differential thermal measurement system constructed in a test tube using thermocouples. Using two sheathed thermocouples A and B, one measures the temperature of the standard substance A' (for example, a block of tetrafluoroethylene) and the other measures the temperature of the measurement substance B' (crosslinked polyethylene), and the phase change of B' is measured. The solvent C to be used is contained in the test tube D, in which the temperature change accompanying the temperature change is recorded on a recorder as the temperature difference with A'. E is a heat medium that can be heated and cooled continuously at a constant rate by a temperature control device. Figure 2 shows the observation of the phase change pattern of a crosslinked sample (low-density PE) in xylene during the process of increasing the salt content of the solvent (xylene) and decreasing the temperature. Parts P, Q, and R represent endotherms observed when the sample swells and dissolves during the heating process, and S, T, U, and V represent heat generation due to crystallization that appears during the heating process. In P, the crosslinked polyethylene and the solvent are in a phase-separated state, and as the temperature rises, dissolution begins and endotherm occurs. At Q, the dissolution rate is maximum, and at R, dissolution is complete. At this point R, the crosslinked polyethylene is amorphous containing a solvent and exhibits a so-called swollen state (swollen gel). Crystallization of a swollen gel in a solvent is observed as an exothermic peak as the temperature of the solvent is lowered.

These are S, T, U, and V. T is the crystallization start temperature, and the crystallization rate is maximum at U. At V, crystallization ends. These specific temperatures are all at a lower temperature than in the absence of solvent. The magnitude of this temperature drop also varies depending on the affinity between the polymer and the solvent and the concentration of the polymer placed in the solvent. Therefore, specifically swelling a crosslinked polymer is done by placing the material in a solvent maintained at a temperature above the melting point of the polymer in the solvent.

Furthermore, in the case of polyolefins, crystallization proceeds in a solvent by lowering the temperature, and the desired material can then be obtained by removing the solvent.

The polyolefin used in this invention is a homopolymer such as polyethylene, polypropylene, polybutene-1, poly4-methyl-pentene-1, polyhexene-1, or a copolymer consisting of two or more of these components. Alternatively, copolymers with other polymerizable monomers such as vinyl acetate and ethyl acrylate, and mixtures of two or more selected from these homopolymers, copolymers, and graft polymers are also included.

These polyolefin molded products are molded using commonly used extrusion molding machines, injection molding machines, etc., and include films, sheets, fibers, pipes, tubes, rods, etc.
It also includes various irregularly shaped products.

There are no particular limitations on the crosslinking method for the polyolefin molded article, and organic peroxides, azide compounds, polyfunctional monomers, etc.
Using a chemical crosslinking method, Y-ray, electron beam,
Any crosslinking method using radiation irradiation such as X-rays or a method of silane crosslinking in the presence of a peroxide, a silane compound, and a silanol condensation catalyst may be used.

The degree of crosslinking of the crosslinked polyolefin of the present invention is suitably at least 4% by weight, preferably at least 6% by weight in terms of gel percentage.

Below this lower limit, the shape of the molded article may be distorted during swelling, which is not preferable. The solvent used for swelling the crosslinked polyolefin molded article has a solubility parameter of 6.0 to 10.0 in consideration of compatibility with the polyolefin. Examples include hydrocarbon cyclic compounds such as benzene, xylene, toluene, decalin, and tetralin, and linear hydrocarbon compounds such as n-hebutane and n-decane. In practice, the treatment is carried out by immersing it in these solvents or leaving it in the solvent vapor, but the type of solvent, temperature, shape of the crosslinked polyolefin molded article,
The physical properties of the molded article can be changed in various ways depending on the dimensions, degree of crosslinking, processing time, etc.

Furthermore, by lightly subjecting the molded body to this treatment to improve the crystallization density only on the surface portion of the molded body, the wear resistance of the molded body can be improved.

Furthermore, if this treatment is carried out to an extreme, the entire molded product will become highly densified, and is particularly suitable for plastic insulating paper for OF cables that require strict characteristics such as voltage resistance and oil resistance. The crosslinked polyolefin molded article swollen in this manner becomes highly densified by cooling it in the presence of a solvent and then removing the solvent.

In this case, it is necessary to reduce the amount of remaining solvent as much as possible. This is because when a polyolefin coexists with a solvent, it lowers the heat resistance such as the softening temperature and melting temperature of the molded article, and also adversely affects other electrical properties. Specifically, the residual solvent needs to be at most 0.5% by weight, preferably at most 0.2% by weight, based on the molded article.

Specific methods for removing the solvent include air drying, heating drying, vacuum drying, and replacing the solvent with a low boiling point solvent. In the present invention, other antioxidants, crosslinking auxiliaries, etc. may be added as appropriate to the extent that the properties thereof are not impaired.

As is clear from the above explanation and the Examples described later, in this invention, a molded product with improved physical properties can be obtained by a simple operation of swelling a crosslinked polyolefin molded product with a solvent and then removing the lag agent. It has the advantage of not requiring any special equipment, and has great industrial value.

The present invention will be specifically explained below with reference to Examples.

Example 1, Comparative Examples 1 and 2 High-density polyethylene (MII.0) was extruded using a T-die extruder at a die temperature of 220 pm to obtain a film with a thickness of 0.5 mm (Comparative Example 1 product).

This film was irradiated with Y-rays at 30 Mrad in a vacuum to obtain a crosslinked polyethylene film with a gel fraction of 70% (Comparative Example 2).

Next, this crosslinked polyethylene film was immersed in 12,000 g of xylene for 1 minute, left to cool, and then dried in vacuum (Example 1 product).

The characteristic values of each of these films were examined and the results are shown in Table 1.

Table 1 The crosslinked polyethylene film of Comparative Example 2 was heat-treated in a silicone oil bath at 120°C for 5 hours, but the density was 0.
It only increased to 956, and almost no improvement in characteristics was observed.

Example 2, Comparative Example 3 Low density polyethylene (MII.0, density 0.914ta'
cc) 10 parts by weight of dicumyl peroxide and 2 parts by weight of dicumyl peroxide are kneaded with a roll, then molded in a mold at 135 oo by using an electric press, and then the temperature is raised to 200°C and held for 10 minutes. A two-thickness crosslinked polyethylene sheet with a gel fraction of 8% by weight was obtained (Comparative Example 3).

This crosslinked polyethylene sheet was immersed in xylene at 100° C. for 10 minutes to swell, and then allowed to cool. The sheet was further washed to remove the xylene in acetone, and air-dried and vacuum-dried (Example 2 product).

The properties of these sheets were investigated and the results are shown in Table 2. Table 2 However, in the above table, *mark: Values in AC and DDB oil, and other values are in accordance with Table 1.

The cross-linked polyethylene sheet of Comparative Example 3 was heated at 300 atmospheric pressure and 250° C. without swelling in a piston-cylinder type high-pressure container using silicone oil as a pressure medium.
The density of the product, which was maintained at 100° C. and then allowed to fall under the same pressure for high-pressure crystallization, was 0.9102, and no improvement was observed in the softening temperature or other characteristic values.

Example 3, Comparative Example 4 Polypropylene (MII. rate of 0.895/cc)
A mixture of 100 parts by weight and 2 parts by weight of divinylbenzene was extruded into a film with a side thickness of 0.5 using a T-die extruder, and then cross-linked with a gel fraction of 65% by weight by irradiating Y-rays with regular AD. A polypropylene film was obtained (comparative example 4 products).

This film was soaked in 13,000 decalin for 20 minutes, taken out, allowed to stand, and further ultrasonically cleaned in acetone and air-dried and vacuum-dried (Example 3 product).

The properties of both films were investigated and the results are shown in Table 3.

Table 3 From the results of the above examples, especially Tables 1 to 3, it is clear that the molded product according to the present invention has a crosslinked structure and high density, so it has better heat resistance, mechanical properties, electrical properties, etc. than the comparative example products. It is clear that various properties have been significantly improved.

[Brief explanation of the drawing]

Figure 1 is a diagram of a differential thermal measurement system configured in a test tube using a thermocouple, and Figure 2 shows the phase change pattern of a crosslinked sample in xylene during the temperature rising and cooling process of the solvent (xylene). It is. Figure 2 Figure 1

Claims (1)

[Claims] 1. A crosslinked polyolefin molded product, which is characterized in that the crosslinked polyolefin molded product is swollen by placing it in a solvent kept at a temperature higher than the melting point of the polymer, and then the solvent is removed by drying or the like. Production method.