JPS5945689B2 - Crosslinking modification method for polyethylene resin - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for crosslinking and modifying polyethylene resin.

Conventionally, known crosslinking modification methods for polyethylene resin include a method of modifying crosslinking by irradiation with radiation, and a method of modifying crosslinking by heating after adding a chemical crosslinking agent such as an organic peroxide. .

To explain in a little more detail, as a method that uses radiation,
g) Method of cross-linking modification by irradiation with ionizing radiation, (mouth)
There is a method of cross-linking modification by irradiation with ultraviolet rays, and a method using organic peroxide is to incorporate organic peroxide into polyethylene resin and melt it at a temperature higher than the decomposition temperature of this peroxide. A method of molding, a method of kneading and molding at a temperature lower than the decomposition temperature of the medium organic peroxide, and then heating it above the decomposition temperature of the peroxide, and a method of irradiating the polyethylene resin containing the organic peroxide with radiation. A method is known in which the material is firstly crosslinked and then heated to a temperature higher than the decomposition temperature of the organic peroxide to achieve a high degree of crosslinking.

However, each of the above-mentioned crosslinking modification methods has the following drawbacks.

The crosslinking modification method (using ionizing radiation) has excellent crosslinking efficiency, but the degree of crosslinking differs between the surface layer and the inside of the molded product, resulting in a nonuniform crosslinked structure with many crosslinking points generated in the amorphous region. Moreover, since it uses ionizing radiation, its range of application is limited.

In the crosslinking modification method of irradiating ultraviolet light, which is a high-energy beam, simply irradiating polyethylene resin with ultraviolet rays does not provide enough crosslinking to modify the resin. UV irradiation is carried out in the presence of a crosslinking accelerator such as a photosensitizer, trichlorethylene, or divinylbenzene, but even with this method, it takes time to obtain a degree of crosslinking sufficient for modification, and Due to its low permeability into molded products, it has the disadvantage of resulting in a non-uniform crosslinked structure with different degrees of crosslinking between the surface layer and the interior of the molded product, as in the case of ionizing radiation.

On the other hand, in the ←→ crosslinking modification method using organic peroxide,
If a normal extruder is used, the fluidity of the polyethylene resin will deteriorate significantly as the degree of crosslinking increases, and therefore, the F method is inappropriate as a method for obtaining a product with a high degree of crosslinking.

The use of ultra-high pressure extruders may ameliorate this problem, but is not common. In the second crosslinking modification method using an organic peroxide, a molded product with a relatively higher degree of crosslinking can be obtained than in the case of ←→, but the molded product to be crosslinked may undergo thermal deformation during crosslinking.
However, if the organic peroxide is decomposed at a lower temperature to avoid thermal deformation, the decomposition time becomes longer and the time required for the crosslinking reaction also exceeds several hours.

Finally, in the crosslinking modification method (e) using an organic peroxide, the thermal deformation caused by heating can be improved because secondary crosslinking is performed by irradiating radiation in advance, but the crosslinking process is performed in two stages. The process becomes extremely complicated.

Moreover, primary crosslinking involves the above-mentioned problems because the crosslinking modification method is carried out using radiation. In order to solve the various drawbacks mentioned above, the inventors focused on a method of promoting crosslinking reaction using organic peroxides, and as a result of intensive research, they surprisingly found that ultraviolet rays alone could not crosslink polyethylene resin. However, we have discovered that the crosslinking reaction is significantly accelerated by irradiating organic peroxide-containing polyethylene resin with ultraviolet light at a temperature and time that is above the melting point and at a time that does not substantially crosslink with heat alone. This invention was arrived at by investigating this.

That is, the present invention includes adding an organic peroxide to a polyethylene resin, and then irradiating the polyethylene resin with ultraviolet rays at a temperature and time that are above the melting point of the resin and that are not substantially crosslinked by heat alone. This is a method for efficiently modifying polyethylene resin into a crosslinked product having a uniform crosslinked structure.

According to the present invention, all the drawbacks of the various crosslinking modification methods mentioned above can be solved. That is, according to this invention, (a) the above (c), (d),
(b) The crosslinking reaction is significantly accelerated and the crosslinking efficiency is superior to that of the crosslinking modification method according to (e).
(c) The crosslinking reaction is carried out at a temperature higher than the melting point of the polyethylene resin, so the crosslinking points are crystalline and non-crystalline. Since it is produced without distinction of crystalline regions, a product having a uniform crosslinked structure can be obtained compared to the crosslinking modification methods (a) and (x); and (d) ultraviolet rays are irradiated at a temperature below the melting point of the polyethylene resin. Compared to the crosslinking modification method by irradiating ultraviolet light at a temperature above the melting point, the crosslinking modification method by irradiating ultraviolet rays at a temperature higher than the melting point has a high permeability to polyethylene resin.
Even if ultraviolet light in a long wavelength range of 00λ or more is used, the crosslinking reaction is promoted, so that uniform crosslinking modification of thick molded products is possible; (e) resin melting time (crosslinking reaction time) (f) Since the decomposition of the organic peroxide depends on the amount of ultraviolet irradiation, the degree of crosslinking can be easily controlled and molded products with a high degree of crosslinking can be obtained. The object of the present invention is to provide a method for crosslinking and modifying polyethylene resin, which is industrially extremely advantageous and achieves good results. The polyethylene resin used in this invention means polyethylene and a copolymer mainly composed of ethylene, and specific examples include low-density polyethylene, medium- and high-density polyethylene, ethylene/vinyl acetate copolymer, and ethylene/methacrylate copolymer. It means methyl acrylate copolymer, ethylene/ethyl acrylate copolymer, ethylene/propylene copolymer, etc., or a mixture thereof, and furthermore, it refers to a polymer containing the above polymer as a main component and containing other polymers such as polybutadiene, polyisoprene, etc. Also includes mixtures of.

Furthermore, components other than organic polymer substances, such as organic and inorganic substances such as plasticizers, blowing agents, and fillers, can also be added to these polyethylene resins. The organic peroxide used in this invention means a compound having a 10-0 bond in its molecule, and is preferably effective in the crosslinking reaction of polyethylene resin, and more preferably an organic peroxide. Among those selected from the group of dialkyl and ketone peroxides whose common names are classified based on their chemical structure, they have a decomposition temperature equal to or higher than the melting point of polyethylene resin.

Specific examples include 1-1-bis(t-butylperoxy)3,5,5-trimethylcyclohexane, t-butylcumyl peroxide, dicumyl peroxide, 2,
Representative examples include 5-dimethyl 2,5-di(t-butylperoxy)hexane and 2,5-dimethyl 2,5-di(t-butylperoxy)hexane-3. These organic peroxides are used in a proportion of 10 parts by weight or less based on 100 parts by weight of the polyethylene resin, preferably 0.5 parts by weight or less.
1 to 5.0 parts by weight are used. Methods for incorporating organic peroxide into polyethylene resin include kneading, dissolving organic peroxide in a suitable solvent, and contacting and impregnating the solution, but industrially,
The organic peroxide is attached to the surface of the polymer particles and fed to the extruder, or the organic peroxide is fed while the resin is unmolten or molten in the extruder, and the organic peroxide is decomposed. A method of cross-dispersion and extrusion at a temperature below the temperature is preferred. In this case, it can be shaped into a desired shape using an extrusion nozzle or the like. The crosslinked polyethylene resin obtained by the present invention is not limited to grains, plates, wires, films, molded products, or composite materials. The ultraviolet light used in this invention has a wavelength of 1000λ to 4
500λ ultraviolet rays, especially for 1850 to 4000A
UV rays are preferred.

As these ultraviolet ray sources, low pressure, high pressure, ultra-high pressure mercury lamps, xenon lamps, carbon arcs, etc. are used. The heating in this invention is carried out at a temperature and time that are higher than the melting point of the polyethylene resin, and which do not substantially cause crosslinking with heat alone, and vary depending on the polyethylene resin and organic peroxide used.

As an example of heating, a sheet of a composition containing 2 parts of dicumyl peroxide in 100 parts of high-pressure polyethylene (
Fig. 1 shows the heating conditions when the thickness is 2 mm). The shaded area in the figure indicates a temperature and time range that is higher than the melting point of the resin composition and that does not substantially cause crosslinking by heat alone. Note that the crosslinking time is the time from the time when the sample is placed in a hot air circulation heating oven set at the crosslinking temperature and the sample becomes molten. The atmospheric phase during ultraviolet irradiation is a gas phase,
It may be in any liquid phase, and specific heating methods include heating with a heater, using heat rays emitted from a silicone oil bath, an infrared lamp, and a high-pressure mercury lamp, and these can be used as appropriate. .

When performing crosslinking modification by irradiating ultraviolet rays in this invention, the crosslinkable molded article may be irradiated with ultraviolet rays while being continuously introduced into a heating zone, or irradiated with ultraviolet rays while being heated in batches. Methods and the like may be used, but are not particularly limited thereto. By irradiating the polyethylene resin with ultraviolet rays at a temperature equal to or higher than the melting point of the organic peroxide-containing polyethylene resin, a uniform crosslinked structure can be obtained.

In other words, in crosslinking modification in a crystallized state below the melting point of polyethylene resin, the organic peroxide is localized in the amorphous region of the resin, so the dispersibility of the organic peroxide within the resin is poor. , the crosslinking reaction occurs selectively in the amorphous regions of the polyethylene resin, resulting in a heterogeneous crosslinked structure in which the crystalline regions remain uncrosslinked. In crosslinking modification, since there are no crystalline regions, the dispersibility of the organic peroxide in the resin is good, so the crosslinking reaction occurs uniformly within the polyethylene resin, resulting in a homogeneous state in which the crosslinking points are evenly distributed. A crosslinked structure will be obtained. Also,
In this way, by irradiating ultraviolet rays at a temperature equal to or higher than the melting point of the polyethylene resin and at a temperature and time such that heat alone does not substantially cause crosslinking, the crosslinking reaction of the polyethylene resin can be significantly accelerated.

This is because the decomposition of the organic peroxide occurs due to the synergistic effect of using thermal energy and ultraviolet energy at the same time. That is, the -0-0 bond of the undecomposed organic peroxide, which has been activated to an excited state by absorbing thermal energy, is easily and selectively dissociated by the action of ultraviolet energy, resulting in crosslinking of the polyethylene resin. In addition, the photosensitizing substance (for example, acetophenone in the case of dicumyl peroxide) that is produced as a by-product during the decomposition process of organic peroxide acts as a UV sensitizer, and together with the generated radicals, This is thought to be because it effectively acts on crosslinking of polyethylene resin. On the other hand, when polyethylene resin containing organic peroxide is irradiated with ionizing radiation (e.g. accelerated electron beam) instead of ultraviolet rays,
Since no accelerating effect of the organic peroxide on the crosslinking reaction is observed, the crosslinking accelerating effect in this invention is unique to ultraviolet energy.

The term "substantially non-crosslinked polyethylene resin" as used in the present invention means a polyethylene resin whose degree of crosslinking is substantially zero as measured by the following measuring method.

The degree of crosslinking in this invention is determined by the following formula by placing the polyethylene resin in a 200-mesh wire mesh basket, performing reflux extraction with boiling toluene for 5 hours, drying, and measuring the toluene insoluble content.

The melting point of polyethylene resin as used in this invention refers to the temperature when the crystalline portion of polyethylene resin is in a molten state. This refers to the peak temperature when measured.

The present invention will be described below with reference to Examples, but it goes without saying that the present invention is not limited to these Examples.

Example 1 High pressure polyethylene (density 0.9189/CC, MIl
．． 2.0 parts of dicumyl peroxide was mixed with 100 parts of O, melting point 103°C), kneaded at 130°C in a 20ml 1φ extruder, extruded into an uncrosslinked strand with a diameter of 2ml 1L (resin temperature 130°C), and immediately irradiated with ultraviolet rays. Device (diameter 25C
TL. A Toshiba high-pressure mercury lamp; type H-400 is housed in a cylindrical device made of a hard aluminum plate with a length of 30 CIIL.
Four Ps were placed evenly at a distance of about 7.5 degrees from the center of the device.

In order to prevent the high-pressure mercury lamp from overheating, an air cooling device was installed, and the heat inside the device was maintained at approximately 150° C. using heat radiation from the high-pressure mercury lamp. ), and while keeping the resin temperature at 145°C, 0.06, 0.3
, 1.0, 1.5, and 3.0 m/min, and irradiated with ultraviolet rays for 5.0, 1.0, 0.3, 0.2, and 0.1 minutes, respectively. The degree of crosslinking of the obtained crosslinked strands was measured and the results are shown in Table 1 and Figure 2. For comparison, the uncrosslinked strands prepared above were subjected to crosslinking modification by irradiation with ultraviolet rays at 20°C (the uncrosslinked strands were placed in a water tank made of a hard aluminum plate, and The sample was irradiated with the same amount of ultraviolet light as in the above example for 5 minutes from a distance of 7.5α from the sample.

) and crosslinking modification by heat alone at 145°C without UV irradiation (uncrosslinked strands were
It was placed in a hot air circulation oven heated to 5° C. and heated for 5 to 60 minutes. ), and the degree of crosslinking of the crosslinked strands was measured for each. The results are shown in Table 1 and Figure 2. As is clear from the above results, 20
When crosslinking modification was carried out by irradiating ultraviolet rays at 145°C for 5 minutes and by heat alone for 5 minutes at 145°C, no degree of crosslinking was obtained, whereas no degree of crosslinking was obtained.
In crosslinking modification by irradiating ultraviolet rays under heating conditions of 145°C, a high degree of crosslinking can be obtained even in a short time of 5 minutes or less, and the crosslinking efficiency is excellent.

Furthermore, as shown in Figure 2, crosslinking modification by irradiating ultraviolet rays at 145°C was carried out for 5 minutes, the time at which a substantial degree of crosslinking begins to be obtained in the case of crosslinking modification by heat alone. When the resin is aged, the degree of crosslinking of the resin has already reached saturation, and it can be seen that the crosslinking reaction is carried out more efficiently in a shorter time than in crosslinking modification using only heat. Example 2 High pressure polyethylene (density 0.9169/CCsM2O
Melting point: 101 C) 100 parts of dicumyl peroxide 0
．． 5 parts were mixed and kneaded and extruded at 130°C using a 2011φ extruder to obtain a diameter of 21! 1 of uncrosslinked strands were made.

The uncrosslinked strands were heated using a box-shaped ultraviolet irradiation device (width: 30 x 20 cm, length: 40 cm) made of a hard aluminum plate, and three Toshiba high-pressure mercury lamps, type H4OOP, installed. (The temperature was maintained at 110° C. using heat radiation from the high-pressure mercury lamp.), and ultraviolet rays were irradiated for 4 minutes at a distance of 15CT1L from the high-pressure mercury lamp. Table 1 shows the measurement results of the degree of crosslinking of the obtained crosslinked strands. In this way, even when the concentration of organic peroxide is 0.5 parts, a highly crosslinked product can be obtained in a short time.

Example 3 Ethylene/vinyl acetate copolymer (vinyl acetate content 15
wt%. MIO. 3, melting point 85°C), mixed with 1.0 part of dicumyl peroxide, kneaded and extruded the uncrosslinked strands under the same conditions as in Example 1, immediately introduced into the ultraviolet irradiation equipment, and mixed with 1.0 part of dicumyl peroxide. Ultraviolet rays were irradiated for 20 seconds under these conditions.

Table 1 shows the degree of crosslinking of the crosslinked strands obtained. For comparison, the uncrosslinked strands prepared above were subjected to crosslinking modification by irradiation with ultraviolet rays at 20°C under the same conditions as in the comparative example described in Example 1, and without irradiation with ultraviolet rays. Crosslinking modification by heat alone was carried out at 145° C. for 5 minutes each.

The degree of crosslinking is shown in Table 1. As is clear from these results, even when using an ethylene/vinyl acetate copolymer, crosslinking modification by irradiation with ultraviolet rays at 20°C and crosslinking modification by heat only at 145°C resulted in no degree of crosslinking. However, by crosslinking modification by irradiating ultraviolet rays under heating conditions of 145° C., a high degree of crosslinking can be obtained in a short time.

Example 4 The high-pressure polyethylene used in Example 1 and the organic peroxide shown in Table 1 were mixed in specified amounts and mixed in a 20 mmφ extruder for 11 minutes.
Knead at 0°C and extrude uncrosslinked strands with two diameters.
Immediately, the resin was introduced continuously into the ultraviolet irradiation equipment used in Example 1, and was heated for 0.3 m while maintaining the resin temperature at 140°C.
The sample was taken at a speed of 1/min and irradiated with ultraviolet rays for 1 minute.

Table 1 shows the measurement results of the degree of crosslinking of the obtained crosslinked strands. For comparison, the uncrosslinked strands prepared above were placed in a hot air circulation oven heated to 140° C., and crosslinked by heat alone for 1 minute. Table 1 shows the measurement results of the degree of crosslinking of the obtained crosslinked strands. Even when organic peroxides other than dicumyl peroxide are used, the crosslinking reaction is faster in crosslinking modification by irradiating ultraviolet light under heating conditions of 140°C than in crosslinking modification by heat alone. It is clear that this has been significantly promoted.

Example 5 The crosslinked strands obtained in Example 1 with a degree of crosslinking of 75% were tested using a scanning differential calorimeter (DSC-1 manufactured by Perkin Elma).
(type) to increase the melting peak temperature and heat of fusion at a heating rate of 8°C.
/min.

The results are shown in Table 2. Furthermore, this cross-linked strand was cut to a length of about 3 m1 and made into granules, and the cross-linked granules and Freon 112 were placed in a pressure-resistant autoclave (with an internal volume of about 50 m).
．． 0d) and heated in a water method at 90°C for 1 hour to impregnate about 40% of Freon-12, and immediately add 50frL x 5
C! Place it in a mold of TL x 5 cir L and weigh 1.6 kg/d (
A pre-foamed product was prepared by heating for 15 seconds at a steam pressure of 100% (gauge pressure), and its apparent density and foaming state were examined. The results are shown in Table 2. In addition, the uncrosslinked strands produced in Example 1 were irradiated with ultraviolet rays for 6 hours in a 30°C water bath using the ultraviolet irradiation device used in the comparative example described in the section of Example 1, and the degree of crosslinking was 75. A cross-linked strand was created.

Next, the polyethylene used in Example 1 is 207! I
Diameter 171t7 with mφ extruder! After forming it into a L strand, it was irradiated with an electron beam of 20 megarads using an electron beam accelerator to produce a crosslinked strand with a degree of crosslinking of 75%. Next, the uncrosslinked strand produced in Example 1 was heated for 15 minutes in a hot air circulating oven heated to 180°C to perform crosslinking modification only by heat, producing a crosslinked strand with a degree of crosslinking of 75%. These crosslinked strands obtained and Example 1
The melting peak temperature and heat of fusion of the uncrosslinked strands prepared in Example 5 were measured using a scanning differential calorimeter in the same manner as in Example 5. Further, the uncrosslinked strands and the crosslinked strands were cut into 311 particles, pre-foamed bodies were prepared in the same manner as in Example 5, and the apparent density and foamed state were examined. The results are shown in Table 2. From these results,
Since the melting peak temperature and heat of fusion of a crosslinked strand obtained by irradiation with an electron beam are the same as those of an uncrosslinked strand, the crosslinking modification method by irradiation with an electron beam has the effect that the crosslinking point is amorphous. A large amount of crystalline regions are formed, forming a heterogeneous crosslinked structure in which the crystalline regions are preserved as they are.

Furthermore, since the melting peak temperature and heat of fusion of a crosslinked strand obtained by irradiating ultraviolet light at 30°C are the same as those of an uncrosslinked strand and a crosslinked strand obtained by irradiating an electron beam, polyethylene Because the organic peroxide is localized in the amorphous region of the resin, the crosslinking reaction occurs selectively in the amorphous region, and the crystalline region is thought to be maintained without being crosslinked. It has a heterogeneous crosslinked structure similar to the crosslinked strand obtained by irradiation with . On the other hand, the melting peak temperature and heat of fusion of the crosslinked strands obtained by crosslinking modification using only heat are significantly reduced compared to those of the uncrosslinked strands, indicating that the crystalline regions are also involved in the crosslinking point. It is a structure. Furthermore, the melting peak temperature and heat of fusion of crosslinked strands obtained by irradiation with ultraviolet rays at 145°C are greatly reduced compared to those of uncrosslinked strands, and that of crosslinked strands obtained by crosslinking modification using only heat. Since the melting peak temperature and the heat of fusion are the same, crosslinked strands obtained by irradiating with ultraviolet rays at 145°C have good dispersibility of organic peroxide, so that even the regions that originally have a crystalline structure can be It can be seen that a crosslinking reaction has occurred and a uniform crosslinked structure similar to the crosslinked strand obtained by crosslinking modification using only heat is obtained. moreover,
Comparing the apparent density and appearance of the foams made from these crosslinked strands, we found that the foams made from the crosslinked strands obtained by irradiation with ultraviolet light at 145°C and those obtained by crosslinking modification only by heat. The foams made from cross-linked strands showed the same apparent density and foaming state, and the foams made from cross-linked strands were obtained by irradiating with ultraviolet light at 30°C and those obtained by irradiating with electron beams. The foam made from the cross-linked strands obtained by this process has a high apparent density, is a non-uniform closed cell foam, and has a low expansion ratio. ,
It is clear that the strand has a uniform crosslinked structure similar to the crosslinked strand obtained by crosslinking modification using only heat. As is clear from the above examples, according to the present invention, the crosslinking reaction of polyethylene resin is significantly promoted and a crosslinked product having a uniform crosslinked structure can be obtained. It is applied to crosslinking of molded products such as stretched films and pipes, foamed sheets, and beads, and its industrial value is extremely great.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between crosslinking temperature and time in the present invention, and the shaded area in the figure shows the range of temperature and time that is higher than the melting point of the resin and does not substantially crosslink with heat alone. .

Claims (1)

[Claims] 1. Crosslinking of polyethylene resin, which involves adding an organic peroxide to polyethylene resin and then irradiating it with ultraviolet rays at a temperature and time that are above the melting point of the resin and that will not substantially crosslink with heat alone. Modification method.