JPS6137296B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an atmospheric pressure crosslinking method for polyolefin polymers mainly composed of polyethylene, and in particular, a method for crosslinking polyolefin polymers mainly composed of polyethylene using peroxides such as organic peroxides. It relates to a crosslinking method.

Cross-linked polyolefins are currently used in pipes, tubes,
It is widely used in sheets, films, electrical cable coatings, etc., and various methods for crosslinking polyolefins are also known.

However, when polyolefin is thermally crosslinked with an organic peroxide such as dicumyl peroxide, bubbles are generated in the polyolefin molded article. It is well known that this not only reduces commercial value but also causes problems in physical properties. for example,
Voids occurring in the wire coating cause dielectric breakdown.

Therefore, in chemical crosslinking of polyolefins, in order to suppress the generation of bubbles as described above, crosslinking is carried out using a composition containing an organic peroxide, for example, in a high-pressure heating medium or in a long die to which internal pressure can be applied.

All of these methods involve crosslinking under pressure, which results in poor workability and safety issues, and it is clear that they require a large investment in equipment.

The present invention was made to eliminate these drawbacks, and as a result of extensive research, we succeeded in obtaining a crosslinked product free of voids even under normal pressure.

That is, in the present invention, when thermally crosslinking a polyolefin polymer mainly composed of polyethylene, (1) the following bubble suppressants (a) di- or triaryloxy-S-triazines, (ro) are added to the polyolefin together with an organic peroxide. ) di- or triallyltrioxy-S-triazines, (c) polyallyl esters of polycarboxylic acids, (d) polyvinyl monomers
(e) Adding 0.1 to 5 phr of one or more compounds selected from monomers having two or more propargyl groups, (2) For crosslinking polyolefin mainly composed of polyethylene during thermal crosslinking. The gas concentration of the gas contained in the composition is 0.060cc/g.
It has been found that when degassing is performed as follows, a crosslinked product free of voids can be obtained even in thermal crosslinking at normal pressure.

In the case of the present invention, the reason why crosslinking can be achieved without producing bubbles even at normal pressure is that due to the action of the bubble suppressant added to the polyolefin mainly composed of polyethylene, the decomposition products of organic peroxides, i.e. , methane, acetophenone, cumyl alcohol, etc. may be chemically trapped, or the surface tension of the matrix may increase due to crosslinking promotion, which is a side effect of bubble suppressants. , it is thought that bubble formation is suppressed. Although the reason is not clear, it is certain that the use of a bubble suppressant will prevent the generation of bubbles.

However, by adding an organic peroxide and a bubble suppressant to polyolefin, and forming a sheet using a press molding machine, or coating electric wires using an extruder,
For example, when thermal crosslinking is performed in a silicone bath,
Although not many, large poids occur. This means that despite the addition of the bubble suppressant, a new cause of bubble generation is occurring. This is because air is forcibly dissolved in the polyolefin matrix mainly composed of polyethylene during extrusion molding or press molding due to the pressure, and the organic peroxide decomposes to produce decomposition products.
We have found that this is because when the matrices are heated to the crosslinking temperature, their solubility is exceeded and they immediately foam. By removing this cause, the bubble suppression effect is significantly improved, to the point where it is almost impossible to find any voids larger than 5〓.

The bubble suppressant used in the present invention is as follows. That is, (a) di- or triallyloxy-S-triazines, such as triallyl cyanurate,
2,4-diallyloxy-6-dodecylamino-
S-triazine, 2,4-diallyloxy-6-
Hexadecyloxy-S-triazine, 2,4-
Diaryloxy-6-heneicosaniloxy-S
- triazine, (b) di- or triallylisocyanuric acids, such as triallylisocyanurate,
2,4-diallyl-6-dodecylaminoisocyanurate, 2,4-diallyl-6-monopropyl isocyanurate, 2,4-diallyl-6
-heneicosanyl isocyanurate, (c) polyallyl esters of polycarboxylic acids, such as trimellitic acid triallyl ester, trimesic acid triallyl ester, pyromellitic acid triallyl ester, 6-carboxy-2,4- Diallyloxybenzene, diallyl phthalate, triallyl benzophenonetetracarboxylate, diallyl oxalate, diallyl succinate, diallyl adipate, (d) polyvinyl monomers, such as divinylbenzene, (e) two or more Monomers having a propargyl group, such as dipropargyl phthalate,
Examples include dipropargyl terephthalate and dipropargyl maleate.

The amount of these bubble suppressants added is 0.1 to 5 phr. Among these, compounds exhibiting a particularly preferable bubble suppressing effect are the compounds (a) and (c) above, and the compound (a) is particularly preferable. Compounds having three or more double bonds are preferable in terms of the bubble suppressing effect and crosslinking promoting effect, but in consideration of heat aging properties, compounds having two double bonds are more preferable. In some cases, it may be effective to use both in combination; for example, triallyl cyanurate and diethylene glycol diacrylate are used.

Next, in the present invention, degassing refers to keeping the inside of the molding machine at reduced pressure during molding of the crosslinking composition to remove the main gas components contained in the composition and oxygen.
Nitrogen or volatile low-boiling liquid components generated during the kneading process are removed from the composition, and bubbling can be prevented by reducing the amount of gas contained as much as possible. However, the amount of gas contained in the crosslinking composition was determined by the following measuring method per 1 g of the composition.
It is desirable to remove the amount to 0.060 cc or less, preferably 0.055 cc or less, and even more preferably 0.045 cc or less. To measure the amount of gas here, make a sufficient vacuum in the container with the glass tube for introducing the gas, and heat the crosslinking composition to 80 to 100℃.
The gas in the composition is almost completely desorbed under vacuum while heating for 30 minutes, and this gas can be introduced into a constant volume system using a diffusion pump for measurement.
Note that the pressure inside the container can be read using, for example, a Macleod vacuum gauge. The amount of gas under standard conditions can be determined from the degree of vacuum (gas pressure) and its volume.

In addition, the method for removing gas components or low boiling point liquid components from the crosslinking composition in the present invention is as follows:
1) Gas is removed from the crosslinking composition by maintaining the inside of the molding machine at reduced pressure. In this vacuum molding method, the composition to be molded is desirably 100 mmHg or less.
It is maintained at 80 mmHg or less, more preferably 60 mmHg or less, and the higher the temperature is, the more desirable it is to promote the diffusion of gas components, etc. Usually, it is desirable that the temperature of the composition is at least 10°C higher than the softening temperature of the resin. . However, it is necessary to maintain the temperature below the decomposition temperature of the crosslinking agent and blowing agent.

Further, the heating time varies depending on the resin used, the gas components contained, the heating temperature, the thickness of the molded product, etc., but it is usually desirable to carry out the heating in the range of 2 to 30 minutes, particularly 5 to 20 minutes.

Examples of specific methods of vacuum molding include the following.

(1) Method of reducing the pressure inside the extruder: This can be done by opening a vacuum degassing hole in the extruder barrel and using a pump to reduce the pressure from the outside, or by sealing the hopper for material input. The inside of the extruder can also be kept at a reduced pressure by using a pump to reduce the pressure through the degassing hole attached to the hopper.

(2) Method of reducing the pressure inside the mold: When molding using a mold, attach a deaeration hole for vacuuming to the mold and use a pump to maintain the reduced pressure while molding.

In either case (1) or (2), care must be taken to ensure that molten resin does not leak out of the system through the vacuum degassing hole. This can be avoided by installing the hole so that the vacuum hole and the resin do not come into direct contact with each other, or by placing an object between the hole and the resin.

In the present invention, the pressure inside the molding machine is 100 mm.
Hg or less, 0.1mmHg or more, but preferably 80
mmHg or less, 0.1 mmHg or more, more preferably 60 mmHg or less, 0.1 mmHg or more. In order to remove trace amounts of oxygen that can harm the chemical crosslinking agent, the lower the pressure, the more preferable it is, but reducing the pressure below 0.1 mmHg is not preferable because additives such as the crosslinking agent will evaporate.

Another method for degassing is 2) heat treatment of the crosslinking composition. For example, heat treatment is performed on the molded product extruded from the extruder at 100 to 150°C.
The composition is introduced into a heating medium and heat-treated for 5 to 30 minutes to reduce the amount of gas contained in the composition to 0.060 cc/g or less. The heat medium is generally hot air, silicone, glycerin, or the like. Further, heating gas may be degassed using an intermittent heating method such as infrared rays. During this time, care must be taken not to decompose the crosslinking agent.

In the present invention, the polyolefin polymer mainly composed of polyethylene refers to polyethylene alone such as high-density polyethylene and low-density polyethylene, or to these polyethylenes, olefin polymers such as polypropylene and polybutene-1, ethylene-propylene copolymers, ethylene-propylene copolymers, etc. It refers to a mixture of olefin copolymers such as butene copolymer, ethylene-vinyl acetate copolymer, and ethylene-acrylic acid ester copolymer.

The crosslinking of the present invention can also be carried out, if necessary, by degassing and heating a polyolefin polymer mainly composed of polyethylene to which a known crosslinking agent and a bubble suppressant according to the present invention have been added. As the crosslinking agent, dicumyl peroxide, 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexane, 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexane - Organic peroxides such as 3,1,3-bis(tertiarybutylperoxy)isopropylbenzene, 1,10-decanebissulfonazide, m-
Examples include azide compounds such as phenylene diazide. The amount of crosslinking agent added is usually 0.01 to 3 phr.

The crosslinking of the present invention can of course be carried out under pressure, but it can also be carried out by heating under normal pressure. Heating can be done using infrared rays, hot air, nitrogen gas or
It is carried out at temperatures of 180°C to 300°C using silicone oil, glycerin bath, salt bath, and metal bath.

Heat crosslinking is preferably carried out in a liquid having a specific gravity close to that of the polyolefin molded article, since deformation and eccentricity do not occur during heat crosslinking, and silicone baths, salt baths, etc. having a specific gravity of 2.5 or less are preferable. Therefore, when the bubble suppressant according to the present invention is used, since it has a crosslinking promoting effect, deformation eccentricity is further prevented, which is preferable.

The polyolefin used in the present invention contains 50% by weight.
The following other thermoplastic resins, rubber, calcium carbonate, talc, inorganic fillers such as glass fibers, known heat retardants, known antioxidants, pigments, etc. can also be added.

The present invention will be further described below with reference to Examples.

Example 1 Low density polyethylene (MI=1.0), dicumyl peroxide 1.5phr, triallyl cyanurate
0.7 phr was added, kneaded using oven rolls at 120°C, and formed into a sheet with a thickness of 3 mm using a steam press at 120°C. Immediately remove this formed press plate.
Degassing was performed in a hot air constant temperature bath at 120°C for 30 minutes.
The gas concentration of this press plate was 0.04 cc/g.
On the other hand, the gas content concentration of the press plate immediately after forming is
It was 0.10cc/g. These press plates are 50x50
When cross-linked by cutting into mm pieces and holding them in a salt bath at a temperature of 210℃ for 5 minutes, no air bubbles were observed in the ones that had been degassed, and those that had not been degassed immediately after molding had a diameter of Several 2 mm coarse voids were observed. In addition, the gel fraction of both was 90%. In Example 1, remarkable effects of the present invention were observed.

Example 2 1.5 phr of dicumyl peroxide and 0.9 phr of 2,4-diallyloxy-6-dodecylamino-S-triazine were added to low-density polyethylene (MI=1.0), and a 2 mm diameter extruder was added using a 40 mmφ extruder. It was molded into a rod shape. A lid was attached to the hopper of this extruder to keep it airtight, and a vacuum nozzle was attached to the hopper, and extrusion was performed while being pulled by a vacuum pump. The degree of vacuum when using this method is 15mm
It was Hg. The gas content concentration of this rod is 0.03
It was cc/g. This rod is continuously heated to 250℃.
When cross-linking was carried out by introducing the sample into a silicone oil bath at 0.degree. C., the cross-linking treatment could be carried out without generating voids. The cross section had a nearly perfect circular shape, and almost no deformation was observed.

Comparative Example In Example 2, polyethylene of exactly the same composition as in Example 2 except that it did not contain a bubble suppressant was extruded using the same hopper and the same method. The gas content concentration of this rod was 0.03 cc/g. When this rod was similarly crosslinked using the same apparatus, only a crosslinked rod having a large number of voids was obtained. Further, the cross section was an ellipse with a/b=1.4.

Even from this point of view, the remarkable effects of the present invention can be recognized in Example 2.

Example 3 Low density polyethylene (MI=2.0) with 1,3-
Add 1.0 phr of bis-(tertiarybutylperoxyisopropyl)benzene, 0.3 phr of triallyl isocyanurate, and 0.5 phr of diethylene glycol dimethacrylate, and vacuum in the same manner as in Example 2 using an ordinary extruder with a diameter of 40 mm. The rod was extruded while performing a pulling process. The extruded rod was continuously introduced into an infrared heating furnace and heated at a temperature of 120°C for 10 minutes, and immediately introduced into a salt bath at a temperature of 220°C for thermal crosslinking. This crosslinked rod contained no bubbles and had a nearly perfect circle in cross section. The gas content concentration of the rod immediately after leaving the infrared heating furnace was as low as 0.03 cc/g. Example 3
The remarkable effects of the present invention were also observed in the following cases.

Example 4 Using the same extruder with a hopper as used in Example 2, a twisted annealed copper body with an outer diameter of 3.0 mm was
1.2 phr of dicumyl peroxide and 2,4-diallyloxy-6 per 100 parts by weight of polyethylene
A crosslinking polyethylene composition uniformly kneaded at a ratio of 0.8 phr to -dodecylamino-S-triazine was extrusion coated to a coating thickness of 1.0 mm. This temperature
When cross-linked by immersing it in a glycerin bath heated to 210℃, no bubbles were generated in the coating layer.
We were able to manufacture a cross-linked polyethylene insulated cable with a degree of cross-linking of 90%. In addition, the same composition was extrusion coated and crosslinked without degassing, and it had many coarse voids. Also in Example 4, the remarkable effects of the present invention were observed.

Claims (1)

[Claims] 1. When thermally crosslinking a polyolefin polymer mainly composed of polyethylene, (1) the following bubble suppressant (a) di- or triaryloxy-S-triazines (b) di- or triaryloxy-S-triazines are added to the polymer together with an organic peroxide. or triallylisocyanuric acids (c) polyallyl esters of polycarboxylic acids (d) polyvinyl monomers (e) one or more compounds selected from monomers having two or more propargyl groups (2) a method for atmospheric pressure crosslinking of polyolefins, characterized by adding 0.1 to 5 phr of