JPS6247895B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a novel method for crosslinking polyolefins. Crosslinked polyolefins such as crosslinked polyethylene are known as excellent materials with advanced physical properties that cannot be achieved with uncrosslinked polyolefins in terms of mechanical properties such as heat resistance and creep resistance.
It is used in a wide variety of applications. Many proposals have been made for crosslinking methods for obtaining such crosslinked polyolefins, and these can be broadly classified into physical crosslinking methods, chemical crosslinking methods, and methods that combine these methods. A physical crosslinking method is a method in which polyolefin is crosslinked by irradiating ionizing radiation such as an electron beam or gamma ray, or a polyolefin with a special composition is irradiated with ultraviolet rays, and a chemical crosslinking method is
This is a method in which an organic peroxide for generating radicals is kneaded in advance into polyolefin at a temperature below its decomposition temperature, which is then molded by extrusion or the like and then heated and crosslinked. Furthermore, the above-mentioned combination method involves mixing an organic peroxide into polyolefin in advance to form a molded product, irradiating it with ionizing radiation to slightly crosslink it, and then heat-treating it to form a molded product. This method accelerates the decomposition of oxides and completes chemical crosslinking. Although each of these crosslinking methods has its own characteristics, the reality is that they are not always satisfactory. First, in the case of chemical crosslinking methods, the temperature conditions during molding must be strictly controlled to avoid deterioration of workability due to crosslinking during molding, and this method originally Because it depends on decomposition, it is heated to a relatively high temperature during crosslinking, but this is difficult for molded products that require high dimensional accuracy due to heat shrinkage, thermal deformation, etc. Since there is a risk of deformation, and the crosslinking rate generally depends on the above-mentioned heating temperature, the heat transferability is greatly limited by the shape of the polyolefin used or the molded product, resulting in a limit to the crosslinking rate and its industrial value. In addition, the resin temperature may temporarily rise too high, resulting in a decrease in viscosity, and the gas produced by decomposition of the crosslinking agent may cause foaming, which can significantly reduce the physical properties. be. On the other hand, physical crosslinking methods generally require extremely large-scale equipment such as electron beam irradiation equipment in order to obtain stable crosslinked products, which causes high costs. There were concerns about various physical property deteriorations such as irradiation irregularities that tend to occur, deterioration of withstand voltage characteristics due to increased charge accumulation especially in the case of thick lines, and deterioration of elongation characteristics due to uneven distribution of crosslinking points. Instead of this radiation irradiation, there is a crosslinking method that uses high energy density light such as ultraviolet rays.
This crosslinking method using ultraviolet irradiation is originally a crosslinking method that utilizes a photopolymerization reaction, which is mainly used in the curing process of thermosetting polymers such as paints and insulating varnishes. This is an effective use of groups. Specifically, when a thermoplastic polymer is crosslinked by this method, a ketone ultraviolet absorber having a sensitizing action against ultraviolet rays, such as benzophenone, is kneaded in advance and the mixture is irradiated with ultraviolet rays. However, this method generally has drawbacks such as low crosslinking efficiency, difficulty in uniformizing crosslinking, and inability to obtain products with high crosslinking density. Furthermore, in this method using ultraviolet irradiation,
There is also a method of mixing a suitable crosslinking agent into polyolefin in advance to form a molded body, and irradiating it with ultraviolet rays at a temperature above the melting point of the polyethylene and below the crosslinking temperature, but in this case it is difficult to control the temperature. In any case, there are still unresolved problems with this ultraviolet irradiation method, such as the fact that the product to be crosslinked is held above its melting point temperature, which tends to cause thermal deformation and foaming. was. Finally, the method using a combination of the physical and chemical crosslinking methods described above has extremely large practical problems because the process is complicated and management is difficult. As a result of intensive studies to solve this problem, the inventors mixed an organic peroxide and an ultraviolet sensitizer into the polyolefin molded product in advance, and heated it to a temperature exceeding the degree of decomposition of the organic peroxide. They discovered that by irradiating the organic peroxide with ultraviolet rays of a specific wavelength, the organic peroxide was instantaneously decomposed and crosslinked, and the present invention was completed. That is, this invention allows an organic peroxide and an ultraviolet sensitizer to coexist in a polyolefin, and irradiates the polyolefin with ultraviolet rays with a wavelength of 300 nm or less at a temperature below the melting temperature of the polyolefin to decompose the organic peroxide and form a polyolefin. This is a method for crosslinking polyolefin, which is characterized by crosslinking. In the present invention, as described above, by irradiating ultraviolet rays of a specific wavelength in the presence of an organic peroxide and an ultraviolet sensitizer, the organic peroxide is rapidly decomposed in a relatively low temperature range, and the crosslinking reaction is rapid. By carrying out this process sufficiently, it is possible to reduce the deterioration of various properties such as the mechanical properties and electrical properties of conventional crosslinked molded products, and to solve the various problems mentioned above, such as minimizing the deterioration of crosslinking efficiency. This is thought to be the reason why it was possible to do so. In this invention, polyolefins include polyethylene and It refers to a copolymer containing ethylene as the main component, a mixture thereof, or a mixture of these polymers as the main component with other polybutadiene, polyisoprene, etc. These polyolefins may be mixed with other additives such as plasticizers, blowing agents, extenders, etc. as appropriate. Next, the organic peroxide used in this invention has a decomposition temperature (temperature at which the half-life is 10 hours) that is higher than the melting point of the polyolefin, preferably higher than 110°C. If the decomposition temperature is below this range, the kneading operation becomes difficult, which is not preferable. And the activation energy for its decomposition is about 25 ~
It is preferably about 40 Kcal/mol, preferably about 28 to 38 Kcal/mol. The organic peroxide of this invention has -0 in its molecule.
A compound having a -0- bond, preferably one effective in the crosslinking reaction of polyolefin resin, and more preferably a dialkyl and ketone peroxide whose common name for organic peroxides is classified based on the chemical structure. Among those selected from the group of substances, as mentioned above, those having a decomposition temperature higher than the melting point of the polyolefin resin. Typical examples include 1.1-
Bis(t-butylperoxy)3,5,5-trimethylcyclohexane, t-butylcumyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2 -5-dimethyl-2,5-di(t-butylperoxy)hexane-3 and the like. These organic peroxides are used in an amount of 10 parts by weight or less, preferably 0.1 to 5.0 parts by weight, based on 100 parts by weight of the polyolefin. Methods for incorporating this organic peroxide into polyolefin include kneading, dissolving the organic peroxide in a suitable solvent, and contacting and impregnating the solution, but industrially, heavy By attaching or impregnating the surface of the combined particles with an organic peroxide and supplying the resultant to the extruder, or by supplying the organic peroxide while the resin is in an unmolten or molten state in the extruder,
A method of kneading, dispersing, and extruding at a temperature below the decomposition temperature of the organic peroxide is preferred. Next, the ultraviolet sensitizer used in this invention includes benzoin ethers, α-acyl oxime esters, acetophenone derivatives such as chlorinated acetophenone, dialkoxyacetophenones, etc., or ketone/amine complexes such as benzophenone,
Benzyl ketals, such as benzyl dimethyl ketal. Such an ultraviolet sensitizer is added to the polyolefin by contact impregnation with its solution in the same way as the organic peroxide, or by kneading, and the amount added is 0.05 to 1% by weight, preferably 0.05 to 1% by weight based on the polyolefin.
It is 0.2-0.5% by weight. The method of the present invention is suitable for crosslinking molded objects whose thickness is as small as possible, such as films, sheets, etc.
Suitable for fibers, thin rods, etc. However, if crosslinking is sufficient to crosslink only the surface layer of the polyolefin molded product, there are no limitations on its thickness, shape, etc.
For example, it can be applied for purposes such as improving the appearance of the surface of foamed products. The ultraviolet light used in this invention has a wavelength of 300 nm or less,
In particular, it is preferable to irradiate with a mercury lamp mainly containing a bright line spectrum of 200 to 300 nm. The reason for this is that the spectrum with the highest intensity in this wavelength range is at a wavelength of 253.7 nm, and it is clear that this wavelength portion contributes most to the decomposition of the organic peroxide in this invention. It is. The irradiation time of ultraviolet rays is inversely proportional to the square of the distance between the object to be irradiated and the light source and the output of the light source. Practically 10
~300 seconds, preferably about 100 to 180 seconds is particularly preferred. Further, in the present invention, it is effective to temporarily heat the material to be crosslinked in order to enhance the crosslinking effect, and in this case, the heating temperature is set to be below the melting temperature of the polyolefin, preferably below its heat distortion temperature. In this invention, in order to efficiently carry out the crosslinking reaction described above, it is effective to mix an antioxidant with less crosslinking inhibition, and it is also effective to use a suitable reflector for the light source and to use a plurality of light sources. It is also preferable to use it to homogenize the irradiation. As is clear from the above description and the examples below, this invention is achieved by irradiating polyolefin with ultraviolet rays of a specific wavelength in the presence of an organic peroxide and an ultraviolet sensitizer, thereby reducing the organic peroxide content to a relatively low level. It decomposes rapidly in a temperature range, and crosslinking of the polyolefin is carried out efficiently and well.Therefore, there is little deterioration of various properties due to crosslinking, and the merits of practical application are significant. The effect of solving the problem is extremely large. The present invention will be specifically explained below with reference to Examples. Examples 1 to 4, Comparative Examples 1 to 4 Table 1 for low density polyethylene (Yukalon YF30, trade name) with melt index MI = 1.0
Dicumyl peroxide (organic peroxide) and benzophenone (ultraviolet sensitizer) were added at a ratio of 1,000,000 yen, and then uniformly dispersed and kneaded using a Brabender apparatus maintained at 130°C. The obtained compound,
It was molded into a sheet with a thickness of 0.3 mm using a hot press maintained at 135°C. A sheet piece 5 mm wide x 3 mm long was cut out from this sheet, placed on a water-cooled aluminum plate, and a low-pressure mercury lamp (manufactured by Toshiba, GL-15, output 15 W) was used to
From the distance of mm, as shown in Table 1, ultraviolet rays (wavelength 253.7n)
m) was irradiated. Note that N ₂ gas was flowed at 300 c.c./min in order to prevent the temperature of the irradiation atmosphere from rising and the generation of ozone. The gel fraction of the obtained crosslinked sheet was determined as the weight percentage of insoluble components after 10 hours of Soxhlet extraction with boiling xylene and is shown in the same table. As a comparative example, as shown in the same table, the same procedure as in the example was carried out except that it did not contain either dicumyl peroxide or benzophenone, and the results are shown in the same table.

[Table] According to the results in the table above, the gel fraction of the example product was 70.
While sufficient crosslinking was achieved at ~90%, the comparative examples were either extremely insufficiently crosslinked or not crosslinked at all. Example 5, Comparative Examples 5 to 8 A polyethylene sheet was obtained according to Example 1 with the composition shown in Table 2 below, and a crosslinked sheet was obtained by irradiation with ultraviolet light (Example 5). The gel fraction and mechanical elongation (%) of this sheet were measured and the results are shown in Table 2. For comparison, low density polyethylene (Yukalon YF30 mentioned above) with melt index MI = 1.0 was heated with 2% by weight DCP at 130°C using a Brabender apparatus.
After kneading, a sheet with a thickness of 0.5 mm is formed using a heat press, and then the surface temperature of the heat press is
The temperature was raised to 200°C (about 15 minutes), and the formed film was immediately cooled in cold water (about 23°C) to obtain a crosslinked film. The measured values of the gel fraction and mechanical elongation of this film are shown in Table 2 (Comparative Example 5). Furthermore, regarding a crosslinked product obtained by irradiating the DCP kneaded composition (containing 2% DCP) used in Comparative Example 5 above with electron beams at a total dose of 20 Mrad in air, and a crosslinked product obtained without kneading DCP. When the produced Gels were compared, almost no difference was observed (Comparative Examples 6 and 7). Next, in Comparative Example 8, the effect of benzophenone on electron beam crosslinking was investigated. In this case as well, the total electron beam dose is 20 Mrad.

[Table] Example 6, Comparative Example 9 Table 3 shows the gel fractions obtained by performing crosslinking by changing the irradiation time in the same manner as in the example except that the wavelength of the light source used in the following Table 3 was used. For comparison, the strongest emission line spectrum is used as a light source.
The same procedure was carried out except that crosslinking was carried out at 95° C. using a 365 nm high pressure mercury lamp, and the results are shown in the same table.

【table】

[Table] According to the above results, it is clear that cross-linking is sufficiently carried out in the example products, and that a longer irradiation time is effective against gel formation. I understand. At this time, if a type of light source with a different wavelength is used (for example, a high-pressure mercury lamp), the crosslinking effect cannot be improved no matter how long the irradiation time is. Examples 7-10 Ethylene vinyl acetate copolymer (EVA, Nippon Unicar Co., Ltd., melt index 30, density 0.94)
DQDJ-3868), as dicumyl peroxide (organic peroxide) and ultraviolet sensitizer.
Hexachlorobenzene was added, and the mixture was uniformly dispersed and kneaded using a Brabender apparatus kept at 130°C.
The obtained compound was molded into a sheet with a thickness of 0.3 mm using a hot press maintained at 135°C. Cut a sheet piece 5 mm wide x 30 mm long from this sheet, place it on an aluminum plate, and use a low-pressure mercury lamp (manufactured by Toshiba, GL-15, output 15 W) to expose it to ultraviolet light from a distance of 50 mm as shown in Table 4. A wavelength of 253.7 nm) was irradiated. Here, too, in order to prevent an increase in the temperature of the irradiation atmosphere and the generation of ozone, the aluminum plate was cooled with water using a corrugated pipe, and N ₂ gas was flowed at a rate of 8/min. The gel fraction of the obtained crosslinked sheet was determined as the weight percentage of insoluble components after 10 hours of Soxhlet extraction with boiling xylene and is shown in Table 4.

【table】

Claims (1)

[Claims] 1 A solid-phase molded product containing an organic peroxide and an ultraviolet sensitizer coexisting in a polyolefin is irradiated with ultraviolet rays with a wavelength of 300 nm or less at a temperature below the melting temperature of the polyolefin to decompose the organic peroxide and form a polyolefin. A method for crosslinking polyolefin, which comprises crosslinking.