JPS6259642A - Pre-expanded particle of modified polyethylene resin and production thereof - Google Patents

Abstract

PURPOSE:To obtain pre-expanded particles which give expanded moldings having good rigidity and heat resistance, by expanding modified polyethylene resin particles obtd. by impregnating crosslinked high-density polyethylene resin particles with a styrene monomer and copolymerizing them. CONSTITUTION:High-density polyethylene resin particles are crosslinked to such an extent that the gel fraction is 10-40%. 100pts.wt. said crosslinked resin particles, 5-40pts.wt. styrene monomer and 0.01-2pts.wt. (per 100pts.wt. styrene monomer) radical polymn. initiator are suspended in an aq. medium, and the suspension wherein said resin particles are impregnated with said styrene monomer is heated to copolymerize them, thus obtaining modified polyethylene resin particles. The obtained resin particles is dispersed in an aq. medium in the presence of a volatile blowing agent and a dispersant in a pressure vessel and the dispersion is heated. While the pressure of the vessel is kept constant, the resin particles and the aq. medium are discharged from the vessel to a low-pressure zone at a temp. of not lower than the m.p. of the resin particle to lower than the m.p. +30 deg.C. The resulting pre-expanded particles has a relationship represented by the formula, wherein d is density (g/cm<3>) and n is cell population (number/1mm<2>).

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to novel modified polyethylene resin pre-expanded particles and a method for producing the same.

[Prior art]

Foamed molded products obtained using pre-expanded particles obtained from crosslinked polyolefin resins have other characteristics such as good oil resistance and resistance to repeated compression.

It has excellent impact resistance, and the molded product does not break due to impact, which is an excellent feature not found in other foam molded materials, and it is used as an important packaging material. However, when using crosslinked polyethylene resin as pre-expanded particles to obtain an in-mold foamed product, due to the low rigidity that is a characteristic of polyethylene resin, the pre-expanded particles and the foamed product after in-mold molding are The molded product tends to shrink, and special techniques are required to prevent it. Moreover, the obtained foamed molded product is soft, has low compressive elasticity, and has poor cushioning performance.

The disadvantage is that a larger amount of foamed molded product is required to package the same product compared to foamed polystyrene molded products and the like.

As a method to improve the rigidity of crosslinked polyethylene resin foam, a method has been proposed to use polyethylene resins with strong m properties, such as medium-density polyethylene and linear low-density polyethylene. In some cases, the polyethylene that can be used is limited because the more rigid polyethylene resin is used, the more difficult it is to crosslink and the more difficult it is to perform foam molding. However, a foam that can be used as a rigid structural material using crosslinked polyethylene has not yet been developed. On the other hand, a method of impregnating a polyethylene resin with a styrene monomer and copolymerizing it to increase rigidity is disclosed in Japanese Patent Publication No. 51-46138 and Japanese Patent Publication No. 54-44.
It is known from Publication No. 309, etc.

However, according to these known techniques, a foam molded product made from an impregnated copolymer with a low styrene content compared to a polyethylene resin, for example, a styrene content of 30 parts by weight or less per 100 parts by weight of the polyethylene resin, is produced using a blowing agent. For reasons such as poor retention and low rigidity, only pre-expanded particles with low expansion and shrinkage can be obtained, and foam molding thereof cannot be performed. Therefore, with these known techniques, a foam molded product that can be used for practical purposes cannot be molded unless the styrene component content of the styrene-impregnated copolymer is greater than 30 parts by weight per 100 parts by weight of polyethylene resin. However, there is a problem in that as the styrene component content increases, the properties of the polyethylene resin are impaired, and although the rigidity increases, the impact resistance strength decreases.

〔the purpose〕

The present invention provides a preliminary foam molded product that can withstand repeated compression, which is a feature of crosslinked polyethylene resin foam molded products, maintains the impact strength characteristics that do not break due to impact, and has improved rigidity and excellent heat resistance. The purpose is to provide expanded particles.

〔composition〕

According to the present invention, the gel fraction is 10 to 40 as the first invention.
Pre-foaming of modified polyethylene resin particles containing 5 to 40 parts by weight of styrene component per 100 parts by weight of polyethylene resin obtained by impregnating and copolymerizing % crosslinked high-density polyethylene resin particles with a styrene monomer. Consisting of particles,
Provided are pre-expanded modified polyethylene resin particles characterized by having a relationship expressed by the formula % between the density d (g/cm3) and the number of cells n (cells/1m), In addition, as a second invention, after crosslinking high density polyethylene resin particles to a gel fraction of 10 to 40%, the crosslinked resin particles 1
00 parts by weight, 5 to 40 parts by weight of a styrene monomer, and 0.01 to 2 parts by weight of a radical polymerization initiator per 100 parts by weight of the styrene monomer are suspended in an aqueous medium and heated in this state. The styrene monomer is then impregnated and polymerized into the crosslinked resin particles to obtain modified polyethylene resin particles, and then the resin particles are injected into an aqueous medium in the presence of a volatile blowing agent and a dispersant in a pressure-resistant container. After heating in this state to impregnate the resin particles with the volatile blowing agent, the resin particles are heated in this pressure container at a temperature not exceeding the melting point of the resin particles and 30°C higher than the melting point. while maintaining a constant pressure, releasing the resin particles from the container together with the aqueous medium into a low pressure zone,
Density d (g/aj) and number of bubbles n (pcs/Imrr
There is provided a method for producing pre-expanded particles of a modified polyethylene resin, which is characterized by obtaining pre-expanded particles having the relationship expressed by the formula 3 (%).

In the high density polyethylene resin particles used in the present invention, the high density polyethylene resin has a density of 0.9
A polyethylene resin having a weight of 50 to 0.970 g/cj is used. If the density is less than 0.950 g/cJ, the increase in stiffness of the foam obtained by foam molding the pre-expanded particles is not significant compared to that of prior art foams. In addition, it is preferable to use a high-density polyethylene resin used in the present invention having an MFR of less than 5 in terms of its crosslinking property and the rigidity of the resulting foam, and particularly preferable one is one having an MFR of less than 1. 6 The average particle diameter of the high-density polyethylene resin particles used in the present invention is preferably in the range of 0.3 mm to 2 mm; if it is smaller than 0.30111, pre-foaming becomes difficult; on the other hand, if it exceeds 21, This is not preferable because it increases the amount of fusion between particles during impregnation polymerization. The high-density polyethylene resin used in the present invention may be mixed with an appropriate amount of medium-density polyethylene or linear low-density polyethylene as long as the purpose of the present invention is not particularly impaired.

Examples of the styrenic monomer used in the present invention include styrene as well as nuclear substituted styrenes such as α-methylstyrene and para-methylstyrene.

To preferably produce the pre-expanded particles of the present invention, first,
The high-density polyethylene resin particles are crosslinked in advance to a gel fraction of 10 to 40%. In this case, crosslinking is carried out by a conventionally known method. For example, resin particles, water, an anti-fusing agent, and a crosslinking agent are mixed in an autoclave, the temperature is raised to the softening temperature of the resin while stirring, and the crosslinking agent is heated to the softening temperature of the resin. It can be obtained by impregnating a resin with the resin, and then raising and maintaining the temperature to the crosslinking temperature. Examples of the crosslinking agent include dicumyl peroxide, 1,1-bis(7-butylperoxy)3,3.5
-)-limethylcyclohexane, n-butyl-4,4
-bis(di-butylperoxy)valerate, di-butylcumyl peroxide, 2,5-dimethyl-2,5-
Organic peroxides such as di(butylperoxy)hexane are used. Moreover, divinylbenzene can also be used together with these crosslinking agents as a crosslinking aid. The amount of the crosslinking agent used is 0.05 parts by weight per 100 parts by weight of the resin.
The amount of divinylbenzene used is 5 parts by weight of O,OS per 100 parts by weight of the resin.

In the present invention, the degree of crosslinking of the crosslinked resin particles is 10 to 40%, preferably 20 to 3%, expressed as a gel fraction.
Specified within the range of 5%. Gel fraction of crosslinked resin is 40%
If it exceeds this, the secondary foamability of the pre-expanded particles will deteriorate, and even if such pre-expanded particles are used for foam molding, it will not be possible to obtain a foam with one surface smooth and without voids. If the gel fraction of the crosslinked resin particles is less than 10%, the pre-expanded particles will have weak cells, and the foam obtained using such pre-expanded particles will be an open-cell foam, and will have closed-cell foams. It becomes difficult to obtain a foam.

Note that the gel fraction as used herein refers to the ratio of insoluble matter obtained after immersing crosslinked resin particles in boiling xylene for 8 hours, and is expressed by the following formula.

P (%) = -X100 P niger fraction (%) L: Weight of crosslinked resin particles M: Weight of insoluble matter Crosslinking of the high-density polyethylene resin particles.

This is carried out prior to the impregnation copolymerization of the styrene monomer into the resin particles, but in the case of high-density polyethylene resin, this is done at the decomposition temperature of the styrene monomer polymerization initiator (styrene polymerization temperature) and the high temperature. This is because the crosslinking temperatures of the density polyethylene resins are close to each other, so if crosslinking and polymerization are performed simultaneously, the gel fraction of the resulting copolymer particles will not be stable.

Next, the crosslinked resin particles obtained as described above are suspended in an aqueous medium together with a styrenic monomer and a radical polymerization initiator, and heated in this state to release the styrenic monomer. The crosslinked resin particles are impregnated and copolymerized to obtain modified polyethylene resin particles.

In this case, as the radical polymerization initiator, commonly used ones 5
For example, 1,1-bis(t-butylperoxy)-3,
Examples include 3,5-trimethylcyclohexane, di-butyl peroxide, dicumyl peroxide, L-butyl peroxybenzoate, and benzoyl peroxide. As described above, examples of the styrenic monomer include styrene and various nuclear substituted products thereof.

The proportion of styrene monomer used is 100% of crosslinked resin particles.
It is 5 to 40 parts by weight, preferably 20 to 40 parts by weight.

If the proportion of the styrene monomer used is less than the above range, the effect of improving the rigidity of the foam will be small, and if it exceeds the above range, it will be difficult to impregnate the crosslinked resin particles with the styrene monomer. The ratio of the radical polymerization initiator used is 100% of the styrene monomer. It is 0.01 to 2 parts by weight, preferably 0.1 to 1.5 parts by weight.

If it is less than the above range, the styrene monomer will not be completely polymerized, and if it exceeds the above range, the molecular weight of the polystyrene component copolymerized into the resin particles will be undesirably small.

In the present invention, the impregnation of the crosslinked resin particles with a radical polymerization initiator and a styrene monomer is usually 70 to 1
It is preferable to carry out the reaction at a temperature of 10° C. If the temperature is less than 70° C., it becomes difficult to impregnate the styrenic monomer.

If the temperature exceeds 110°C, a polymerization reaction starts before impregnation, resulting in fusion of the resin particles and adhesion of the styrene component to the particle surface, which is not preferable. The copolymerization reaction of the styrene monomer to the resin particles is usually preferably carried out at 85°C to 150°C.

Next, the modified resin particles obtained as described above are dispersed in an aqueous medium in the presence of a volatile blowing agent and a dispersant in a pressure-resistant container, and heated in this state to remove the volatile blowing agent. After impregnating the resin particles, the melting point of the resin particles or higher and 3
At a temperature not exceeding 0° C. higher, while maintaining the internal pressure of the pressure container at a constant pressure, the resin particles are discharged from the pressure container together with water into a low pressure zone to obtain pre-expanded resin particles.

In this case, volatile blowing agents include propane, butane,
Hydrocarbons and halogenated hydrocarbons exemplified by pentane, trichlorofluoromethane, dichlorofluoromethane, dichlorotetrafluoroethane, methyl chloride, ethyl chloride, methylene chloride, etc. are used. The amount of the volatile blowing agent added is 0.05 to 0.5 mol, preferably 0.1 to 0.35 mol, per 100 parts by weight of the resin particles.
If the amount added is less than 0.05 mol, only low foaming particles will be obtained, and if it exceeds 0.5 mol,
This is not preferable because the cells of the foamed particles become too fine or become open cells. Examples of the dispersant (anti-fusing agent) include aluminum oxide, titanium oxide, aluminum hydroxide, basic magnesium carbonate, basic zinc carbonate, and zinc carbonate. The amount of the dispersant added is 0.05 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the resin particles.

In the present invention, as described above, resin particles are dispersed in an aqueous medium and heated to impregnate the resin particles with a volatile foaming agent. This impregnation with a volatile blowing agent is preferably carried out at a temperature higher than the melting point of the resin particles, and by this impregnation, 0.03 to 0.4 parts by weight per 100 parts by weight of the resin particles.
The resulting resin particles contain 5 moles, preferably 0.05 to 0.35 moles of blowing agent. Next, as described above, the foaming agent-impregnated resin particles are foamed at a temperature in a temperature range that is higher than the melting point and does not exceed 30°C higher than the melting point, and at this foaming temperature, the internal pressure of the container is set to the required level. The resin particles are discharged together with an aqueous medium from the inside of the container to a low pressure zone outside the container while maintaining the pressure at . In this case, if a foaming temperature lower than the melting point is used, the expansion ratio of the pre-expanded particles will not increase, the cells of the expanded particles will become too fine, and the cell diameters will also become non-uniform, which is not preferable. On the other hand, as the foaming temperature,
It is not preferable to use a temperature higher than 30° C. above the melting point because the bubbles in the obtained pre-expanded particles become weak.

In the present invention, the melting point is determined by differential scanning calorimetry of polyethylene resin particles. Approximately 5mg for measurement
The crystal melting obtained when the sample was heated to 220°C at a heating rate of 10°C/min, the temperature was lowered to 50°C at a cooling rate of 10°C/min, and the temperature was measured again at 10°C/min. is the endothermic peak temperature of In the pre-foaming process in which resin particles impregnated with a foaming agent are discharged from a pressure container into a low pressure zone to foam the resin particles, the pressure inside the pressure container is 10 to 10.
0kg/CiG, preferably 20-50kg/cdG
A range of pressures is used.

In the present invention, pre-expanded particles are prepared by the above process with a density d (g/a+?) and a number n (cells/1mrrf) of bubbles.
) is characterized in that it generates a relationship expressed by the formula %formula%. When R is smaller than 3, the cell diameter is too large, and the foam obtained by foam molding the pre-expanded particles has poor surface smoothness.On the other hand, when R is larger than 50, If the cell diameter is too small, the secondary foaming power of the pre-expanded particles will be insufficient, resulting in a molded product with many voids or a high shrinkage rate, which is not preferable.

In order to produce pre-expanded particles having the relationship between the density and the number of cells expressed by the above relational expression, the expansion ratio of the pre-expanded particles obtained by the above method may be adjusted.

〔Example〕

Next, the present invention will be illustrated in more detail with reference to Examples.

Note that all parts and percentages shown below are based on weight.

Example 1 Polyethylene resin particles having the density, MFR (melt flow rate) and average particle diameter shown in Table 1 were crosslinked by a conventional method to obtain crosslinked resin particles A-F. The gel fraction of the crosslinked resin particles thus obtained is also shown in Table 1.

Table 1 Next, 100 parts of the crosslinked resin particles, a styrene monomer and a radical polymerization initiator in the proportions shown in Table 2 were added to an autoclave, along with 600 parts by weight of water and 0.12 parts by weight of sodium dodecylbenzenesulfonate. Part, tricalcium phosphate 4.
5 parts were blended and held at 90°C for 2 hours to impregnate the resin particles with the styrene monomer, and then held at 105°C for 4 hours to perform a copolymerization reaction to obtain modified resin particles.

Next, 100 parts of the resin particles and 3 parts of water were placed in an autoclave.
Fill the container with 0.00 parts, 0.3 parts of dispersant (fine granular aluminum oxide), and dichlorodifluoromethane in the amount shown in Table 2, maintain the contents at a predetermined foaming temperature, and then blow air into the container to 40 kg. Foaming treatment was performed by opening one end of the container while applying pressure to /ejG. The density and number of cells of the obtained pre-expanded particles are also shown in Table-2.

Next, the pre-expanded particles obtained above were heated with air at 1,0 k.
A particle internal pressure of g/ci G was applied, and a mold for molding (3
00 x 300 x 50 mm) and pressure 4.2
Foam molding was performed by heating with steam of kg/CiG.

The compression stiffness, shrinkage rate, surface condition, and fusion properties of the obtained molded bodies were evaluated, and the results are shown in Table 3.

The test methods and evaluation methods for each physical property of the molded article are as follows.

[Compression Katasa]

It was measured according to the method of JIS K 6767.

〔Shrinkage factor〕

It was molded in a mold of 300 x 300 x 501 m, and the shrinkage rate in the plane direction was measured.

〔Surface condition〕

Visually observe the surface condition of the molded product, and
■The number of places that are depressed by 2+am or more from the surface was investigated.

0... 2 places or less Δ... 3 to 5 places X... 6 places or more C Fusion adhesion] Judgment was made based on the fractured state of the fractured surface when the molded article was fractured by tension.

0... Interparticle fracture is less than 40% Δ... 〃 is less than 40 to 70% ×...
〃 is 70% or more Table 3 [Effect] As can be seen from the results shown in Table 3, the foam (1) obtained using the pre-expanded particles according to the present invention (Experiment No. 1 to 4) It can be seen that all of Samples (IV) to (IV) show good results in terms of both surface condition and fusion properties.

On the other hand, even if the pre-expanded particles were obtained using the same crosslinked resin particles A, the foams obtained using particles whose nX/dy3 value was outside the specified range of the present invention (Experiments N007 and 8) (■) and (■) cannot maintain excellent physical properties in both surface condition and fusion properties.

In addition, even if the R value is within the specified range of the present invention, the styrene component content is outside the specified range of the present invention (Experiment No.
The foam obtained from 9) ([) similarly did not show good results in both surface condition and fusion properties, and in this case,
It has extremely poor fusion properties and does not provide a practical foam.

Furthermore, even if the pre-expanded particles (experiment No. 5.6) with a gel fraction outside the specified range of the present invention have an R value within the specified range of the present invention, the foam (V) and ( ■)
Both have poor surface condition and fusion properties,
It is not suitable for the purpose of the present invention.

Claims (3)

[Claims] (1) Modification containing 5 to 40 parts by weight of a styrene component per 100 parts by weight of a polyethylene resin obtained by impregnating and polymerizing crosslinked high-density polyethylene resin particles with a gel fraction of 10 to 40% with a styrene monomer. It is made of pre-expanded polyethylene resin particles, and between its density d (g/cm^3) and the number of bubbles n (cells/1 mm^2), formula 3<n^1^
Pre-expanded modified polyethylene resin particles characterized by having the relationship expressed by /^2/d^1^/^3<50. (2) High-density polyethylene resin particles have a gel fraction of 1 in advance.
After crosslinking to 0 to 40%, this crosslinked resin particle 100
parts by weight, 5 to 40 parts by weight of a styrene monomer, and 0.01 to 2 parts by weight of a radical polymerization initiator per 100 parts by weight of the styrene monomer are suspended in an aqueous medium, and heated in this state. The crosslinked resin particles are impregnated and copolymerized with the styrene monomer to form modified polyethylene resin particles, and then the resin particles are injected into an aqueous medium in the presence of a volatile blowing agent and a dispersant in a pressure container. After heating in this state to impregnate the resin particles with the volatile foaming agent, the inside of the pressure container is heated at a temperature not exceeding the melting point of the resin particles and 30° C. higher than the melting point. While maintaining a constant pressure, resin particles are released from the container together with an aqueous medium into a low-pressure zone, and the equation 3 is calculated between the density d (g/cm^3) and the number of bubbles n (cells/1 mm^2). A method for producing pre-expanded particles of a modified polyethylene resin, the method comprising obtaining pre-expanded particles having the following relationship: <n^1^/^2/d^1^/^3<50. (3) The average particle diameter of the high-density polyethylene resin particles is 0.
．． 3. The method of claim 2, wherein the thickness is 3 to 2 mm.