JP2002347108A - Method for manufacturing stretched crosslinked polyethylene sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easy manufacturing method for a polyethylene stretched sheet wherein so-called general purpose polyethylene as a material having a weight average molecular weight of 500,000 or less is super-stretch-formed 30 times or more. SOLUTION: A high density polyethylene having its weight average molecular weight of 500,000 or less, a polymerizable monomer, and a photopolymerization initiator are melted, kneaded and extruded into a sheet to be primarily stretched. After that, the sheet is irradiated with ultraviolet rays. Furthermore, the sheet is secondarily stretched to exceed the total stretching rate over 30 times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a stretched crosslinked polyethylene sheet using high-density polyethylene as a raw material.

[0002]

2. Description of the Related Art Generally, stretching a polyethylene molded article at a high magnification of 10 times or more is called "super-stretch molding". Conventionally, as described in, for example, Japanese Patent Publication No. 7-53423, a method of producing a stretched polyethylene sheet by ultra-stretch-molding an ultra-high-molecular-weight polyethylene molded article having a large molecular weight is known. However, a so-called general-purpose polyethylene molded article having a weight-average molecular weight of 500,000 or less has a low molecular weight, and therefore, it has been difficult to perform high-magnification stretch-molding, particularly, stretch-molding having a stretch ratio of 30 times or more. The creep performance is improved by cross-linking the sheet by irradiating ultraviolet rays, but the cross-linking state is different between the surface layer portion and the central portion depending on the transmission distance and absorption distance of ultraviolet rays. There was a problem in terms of difficulty.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to solve these problems and to provide a general-purpose polyethylene having a weight average molecular weight of 500,000 or less. It is an object of the present invention to provide a production method capable of performing super-stretch molding by a factor of 30 or more by using as a raw material.

[0004]

According to the present invention, a high-density polyethylene having a weight-average molecular weight, a polymerizable monomer, and a photopolymerization initiator are melt-kneaded under pressure, extruded into a sheet, and subjected to primary stretching. After the step, the sheet is irradiated with ultraviolet rays, and further subjected to secondary stretching, thereby stretching the sheet to a total stretching ratio of 30 times or more.

[0005] The present invention uses high density polyethylene. High-density polyethylene has high crystallinity and exhibits high rigidity after stretching. It is difficult for polypropylene to exhibit high rigidity after stretching. Further, low-density polyethylene cannot be used because of insufficient rigidity. The high-density polyethylene preferably has a density of 0.94 g / cm ³ or more.
If the density is less than 0.94 g / m ³ , it is difficult to expect an increase in strength and elastic modulus even after stretching, and the mechanical properties after stretching are inferior.

The present invention uses polyethylene having a weight average molecular weight of 500,000 or less. When the weight average molecular weight exceeds 500,000, it can be stretched at a high magnification and can exhibit high rigidity,
Extrusion molding and the like become difficult, and a plasticizer and the like are also required, which complicates the process.

The above polyethylene has a melt index (MI) of preferably from 0.1 to 20, more preferably from 1 to 10. When the MI is less than 0.1, a burden is imposed on a molding device such as an extruder, and when the MI is more than 10, stretching may be difficult. Further, high-density polyethylene may be used alone or in combination with another easily compatible resin.

As the polymerizable monomer used in the present invention, any polymerizable monomer which can be polymerized by a radical generated by a photopolymerization initiator can be used, but it has an effect of increasing the cohesive force by artificially increasing the molecular weight of polyethylene. The glass transition temperature after polymerization is 0 ° C. or higher, more preferably 10 ° C.
Those at 0 ° C. or higher are preferred.

The above-mentioned polymerizable monomer is preferably, in consideration of compatibility with polyethylene, specifically, triallyl isocyanurate, diallyl phthalate, isobornyl methacrylate and the like.

The amount of the polymerizable monomer is preferably 0.1 to 2 parts by weight based on 100 parts by weight of the polyethylene. If the amount of the polymerizable monomer is less than 0.1 part by weight, it is difficult to sufficiently obtain the modifying effect, and if the amount exceeds 2 parts by weight, no further effect can be expected.

It is preferable to add a polymer having an unsaturated group to the polyethylene, since the photocrosslinkability can be more efficiently improved. Examples of the polymer having an unsaturated group include 1,2-polybutadiene, which is commercially available under the trade name “RB-810” (manufactured by JSR Corporation).

The amount of the polymer having an unsaturated group is preferably 1 to 50 parts by weight based on 100 parts by weight of the polyethylene. If the amount is less than 1 part by weight, the modifying effect may not be sufficiently obtained, and if it exceeds 50 parts by weight, no further effect can be expected.

The polyethylene sheet of the present invention may optionally contain other additives in addition to the above. In particular, when the polymer having an unsaturated group is added, an antioxidant is added to improve the thermal decomposability, and an ultraviolet stabilizer and a heat stabilizer are added to improve the stability upon irradiation with ultraviolet light. It is preferable to do so.

As the above-mentioned antioxidant, t-butyldiphenylphosphite is preferably exemplified. Preferred examples of the heat stabilizer include diethylamine.

As the above-mentioned ultraviolet stabilizer, for example, bis (2,2,6-tetramethyl-4-bibezyl) sebaque-
Are preferred.

As the photopolymerization initiator used in the present invention, for example, benzophenone is thioxanthone, benzylmethylketal, α-aminoalkylphenone, 2-benzyl-2-dimethylamino-4′-monophorinbutyrophenone, Orthoisobenzophenone and the like can be mentioned.

In the present invention, a polymerizable monomer is efficiently crosslinked to polyethylene by using a photopolymerization initiator having a photosensitive wavelength range having light absorption peaks in the short wavelength range side and the long wavelength range side. It is preferable in that it can be performed.

Benzophenone is preferred as having an absorbance on the short wavelength side (220 to 280 nm). In addition, 2-benzyl-2-dimethylamino-4′-monoforinobutyrophenone is preferably used as one having an absorbance on the long wavelength side (280 to 360 nm). When the composition containing the high-density polyethylene, the polymerizable monomer, and the photopolymerization initiator is melt-kneaded and molded, the melting temperature is preferably 130 ° C. or more, more preferably 150 ° C.
２５０250 ° C. If the melting temperature is lower than 130 ° C., the melting of the high-density polyethylene may be incomplete, and a load is imposed on a molding machine such as an extruder. On the other hand, when the temperature exceeds 250 ° C., the polyethylene may be decomposed, discoloration or the like may occur, or the strength may be reduced due to a decrease in the molecular weight.

The means for forming the above polyethylene composition into a sheet is not particularly limited, but, for example, a usual roll forming method, calender forming method and the like can be adopted. The thickness of the polyethylene sheet (this is referred to as a sheet raw material) is preferably in the range of 2 to 5 mm. If it is less than 2 mm, the thickness of the sheet after stretching is too small, and if it is more than 5 mm, subsequent moldability may be difficult.

It is preferable that the raw sheet is subjected to a rolling treatment prior to the primary stretching. In the rolling process, the gap between a pair of rolling rolls rotating in opposite directions is made smaller than the thickness of the polyethylene sheet, and the polyethylene sheet is inserted between the rolling rolls to reduce the thickness of the polyethylene sheet and extend in the length direction. Let it. The temperature of the polyethylene sheet in this rolling step is about 70 to 125 ° C., preferably 90 to 120 ° C. If the temperature is lower than 70 ° C., uniform rolling becomes difficult. The sheet may melt out.

If the rolling ratio in the above-mentioned rolling step is too small, not only the effect of the rolling cannot be expected, but also the burden on the subsequent stretching step. Conversely, if the rolling ratio is excessively large, not only uniform rolling becomes difficult, but also the thickness of the polyethylene sheet after rolling becomes too small, and the polyethylene sheet may be cut during the subsequent primary stretching step. Therefore,
The rolling ratio is preferably about 2 to 10 times. The rolling ratio is represented by the following equation.

Rolling ratio = Thickness of raw sheet before rolling / Thickness of sheet after rolling

Further, in order to first stretch the stretched sheet, the stretched sheet may be strongly oriented in only one axial direction by being sandwiched between two pairs of pinch rolls having different rotation speeds and pulled while being heated.

When the raw sheet is rolled before the primary stretching, the product of the rolling ratio and the primary stretching ratio is defined as the primary stretching ratio, and the primary stretching ratio is preferably 10 times or more. More preferably, it is 20 to 30 times. When the primary stretching ratio is less than 10 times, it is difficult to finally stretch the film to 30 times or more. Conversely, if it exceeds 30 times, cracks are likely to occur and the whole is whitened, and stable stretching becomes difficult. In addition, the molding temperature of the polyethylene sheet during the above stretching is preferably 70 to 120 ° C. Outside this range, stretching breakage is likely to occur.

The present invention relates to the above-mentioned primary stretched raw sheet.
Is irradiated with ultraviolet rays. Polyethylene by UV irradiation
The polymerizable monomer contained in the polymer is polymerized and cross-linked,
The cohesion of the ethylene molecules is increased. The ultraviolet light is
High pressure mercury lamps are preferred because polymerization can be performed in a short time.
No. As the irradiation amount of the ultraviolet light, the peak illuminance 30 mW / c
m^TwoAbove, the light amount is 50 mJ / cm ^TwoHigh intensity and short time
Irradiation is preferred, if the irradiation time is too long, polymerizable
Separation from polyethylene occurs due to monomer polymerization.
There is a risk of rubbing.

In order to further stretch the polyethylene sheet obtained by polymerizing the polymerizable monomer to a total stretching ratio of 30 times or more in the same manner as in the above-described primary stretching, two pairs of polyethylene sheets having different rotation speeds are stretched. A method in which the polyethylene sheet is stretched in only one axial direction by pulling the polyethylene sheet in a heated state while sandwiching it between pinch rolls may be used. The total stretching ratio of the primary stretching and the secondary stretching is 30 times or more, preferably 40 times or more. The temperature of the polyethylene sheet for the second stretching is preferably 70 to 120 ° C. If the temperature is outside these temperature ranges, stretching may be interrupted. The stretching ratio is represented by the following formula.

Stretching ratio = Thickness of raw sheet / Thickness after sheet stretching

(Operation) According to claim 1, a polyethylene sheet obtained by melt-kneading and molding a high-density polyethylene having a weight-average molecular weight of 500,000 or less, a polymerizable monomer, and a photopolymerization initiator is primarily stretched 10 times or more. Then, since the polymerizable monomer in the polyethylene molded article is polymerized by irradiation with ultraviolet rays, the cohesive force of the polyethylene molecules is increased. Therefore, the total stretching ratio can be further increased to 30 times or more by the secondary stretching.

Further, by adding a polymer having an unsaturated group as a crosslinking efficiency improving component according to the second aspect, the gel fraction is improved, the crystal is less likely to be melted, and further stretchability is obtained.

According to the third aspect, by using two kinds of photopolymerizable initiators having different wavelength characteristics in combination, it is possible to efficiently produce a stretched polyethylene sheet having a high degree of crosslinking.

[0031]

(Example 1) High-density polyethylene (weight average molecular weight 3.3 × 10 ⁵ , MI = 1.0, melting point 135 ° C .; trade name “HY540”, manufactured by Nippon Polychem Co., Ltd.)
100 parts by weight, triallyl isocyanurate 1 part by weight,
And 0.5 parts by weight of benzophenone were melt-kneaded in a twin-screw kneading extruder (“PCM30”, manufactured by Ikegai Iron & Steel).

At this time, the polymerizable monomer and the polymerization initiator are poured into an extruder under pressure, melt-kneaded at 200 ° C. while mixing in high-density polyethylene, and extruded into a sheet. Calendar whose temperature is controlled to 90 ° C
The sheet was formed into a sheet having a width of 70 mm and a thickness of 2.5 mm by a molding machine, wound up, and then heated in a uniaxial direction with a heating roll at 120 ° C. (roll diameter: 6 inches; manufactured by Komatsu Ltd.). Rolled twice.

Next, using a hot air heating type two-stage stretching machine,
The first stage stretching ratio is 1.0 times, and the second stage stretching ratio is 1.
The primary stretching operation was performed three times, and the total stretching ratio was 2.3 times. The stretching temperature was 100 ° C. The stretching ratio of the rolling and the primary stretching is about 25 times, and the dimensions of the polyethylene sheet after the primary stretching are 35 mm in width and 20 mm in thickness.
The surface was smooth at 0 μm. Further, the polyethylene sheet was placed under a high pressure mercury lamp of 120 W / cm ² under
It was passed at a speed of 0 m / min to polymerize the polymerizable monomer inside.

Next, the polyethylene sheet was stretched to 1.4 times using a hot air heating type single-stage stretching machine, and the secondary stretching operation at a total stretching ratio of 35 times was completed. The secondary stretching temperature was 110 ° C. The dimensions of the stretched crosslinked polyethylene sheet obtained after the second stretching were 30 mm in width and 15 mm in thickness.
It was 0 μm. Incidentally, the final stretching ratio was the highest ratio at which the secondary stretching ratio operation could be performed stably. Here, “stable” was determined based on the fact that the stretching operation could be performed without causing stretching breakage for one hour.

Example 2 As a photopolymerization initiator having an absorbance on the long wavelength side, 2-benzyl-2-dimethylamino-4'-monoforinobutyrophenone (trade name "Irgacure-369", Cibaspeciality) Chemical company)
Was added in the same manner as in Example 1 except that benzophenone was used in combination with 0.5 part by weight. As the final stretch ratio,
It could be performed stably up to 37 times.

Example 3 As an unsaturated group-containing polymer, 1,2-polybutadiene (trade name "RB-81")
0, manufactured by JSR), antioxidant (t-butyldiphenylphosphite; trade name "Mark 260", manufactured by Asahi Denka Co., Ltd.), bis (2,2,6-tetramethyl-4-bibezil) A crosslinked polyethylene sheet having a thickness of 500 μm was prepared in the same manner as in Example 1 except that Sebacate (trade name “Tinupin 770”, manufactured by Ciba Specialty Chemicals) and diethylamine were added. As a result, stable stretching was possible up to 40 times as the final stretching magnification.

Example 4 As an unsaturated group-containing polymer, 1,2-polybutadiene (trade name "RB-81")
0, manufactured by JSR), antioxidant (t-butyldiphenylphosphite; trade name "Mark 260", manufactured by Asahi Denka Co., Ltd.), bis (2,2,6-tetramethyl-4-bibezil) A crosslinked polyethylene sheet having a thickness of 500 μm was prepared in the same manner as in Example 2 except that Sebacate (trade name “Tinupin 770”, manufactured by Ciba Specialty Chemicals) and diethylamine were added. As a result, it was possible to stably perform the final stretching magnification up to 43 times.

Comparative Example 1 A stretched sheet was obtained in the same manner as in Example 1 except that the polymerizable monomer and the photopolymerization initiator were fed into the extruder under normal pressure. As a result, stable stretching could only be performed up to 27 times as the final stretching magnification.

Comparative Example 2 A stretched sheet was obtained in the same manner as in Example 2, except that the sheet after the first stretching was not irradiated with ultraviolet rays. As a result, the final stretch ratio was 23
It could be performed only up to twice as stably.

[Evaluation] The degree of crosslinking in Examples 1 to 4 and Comparative Example was evaluated by dissolving xylene at 140 ° C. and determining the gel fraction obtained from the polymer residue after 24 hours. The drawing ratio of the finally obtained sheet is also shown.

[0041]

[Table 1]

[0042]

The stretch-crosslinked polyethylene sheet of the present invention has good extrudability of the raw sheet by using high-density polyethylene having a polymerization average molecular weight of 500,000 or less. Since the polymerizable monomer is uniformly dispersed in the polyethylene molecules by melt-kneading the high-density polyethylene under pressure, the sheet is first stretched and then subjected to photopolymerization and crosslinking, so that the cohesive force of the polyethylene molecules is increased. Is increased, whereby the secondary stretching can be performed after the primary stretching, and a stretched crosslinked polyethylene sheet having a total stretching ratio of 30 times or more can be easily obtained.

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Claims (3)

[Claims] 1. A high-density polyethylene having a weight-average molecular weight of 500,000 or less, a polymerizable monomer, and a photopolymerization initiator are melt-kneaded under pressure, extruded into a sheet, and subjected to a first stretching step. A method for producing a stretched crosslinked polyethylene sheet, comprising: irradiating a sheet with ultraviolet rays and then performing a second stretching to stretch the sheet to a total stretching ratio of 30 times or more. 2. The method for producing a stretch-crosslinked polyethylene sheet according to claim 1, wherein a polymer having an unsaturated bond is further added during the melt-kneading. 3. The process for producing a stretched crosslinked polyethylene sheet according to claim 1, wherein a photopolymerization initiator having a photosensitive region on the short wavelength side and a photosensitive region on the long wavelength side is used in combination.