JPH0892407A - Crosslinked expanded polyethylene resin particle, expanded molding made therefrom, and bed core material made of the molding - Google Patents

Abstract

PURPOSE: To obtain expanded particles which can give expanded moldings having small shrinkage and well-balanced compressive strength and recovery properties by mixing two specified polyethylene resins with each other under specified conditions and expanding and crosslinking the resultant mixture. CONSTITUTION: These particles are obtained by expanding and crosslinking a mixture prepared by mixing 50-90wt.% polyethylene resin (A) having a density 0.920-0.935g/cm<3> with 10-50wt.% another polyethylene resin (B) having a density of 0.940-0.970g/cm<3> under such conditions that the following relationship holds: 0.935<DA.CA+DB.CB<0.945(CA+CB=1) (wherein DA is the density (g/cm<3> ) of resin A; DB is the density (g/cm<3> ) of resin B; DA is the weight ratio of resin A; CB is the weight ratio of resin B; and DA.CA+DB.CB is the density of the resin mixture).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to expanded particles of a crosslinked polyethylene resin, an expanded molded article using the expanded particles, and a bed core material comprising the expanded molded article.

[0002]

2. Description of the Related Art Crosslinked low-density polyethylene resin particles are dispersed in a closed container, and an inorganic gas type foaming agent such as carbon dioxide or an organic volatile foaming agent is heated to a temperature above the softening temperature of the resin particles at a pressure above its vapor pressure. It is known to obtain resin foamed particles by holding the same in the container and containing it, and then opening one end of the container and discharging the resin particles and the dispersion medium under an atmosphere at a pressure lower than that in the container.

Expanded molded articles obtained by using expanded particles of crosslinked low-density polyethylene resin have compression set, repeated compression set, and permanent compression set which are higher than those of expanded polystyrene, expanded polypropylene, non-crosslinked linear polyethylene resin expanded molded articles, and the like. It is excellent in distortion, etc. and has been utilized as various cushioning materials such as bed core materials and packing agents by taking advantage of these characteristics.

[0004]

However, when the base resin is low-density polyethylene, it has a drawback that the compressive strength is weak, and as the base resin, a low-density polyethylene resin having a high density or a medium-density polyethylene resin is used. Even when polyethylene resin is used alone, it is difficult to obtain a molded product having sufficient strength, and when high-density polyethylene resin is used alone, strength is increased but recovery physical properties such as compression set are deteriorated. However, a foam-molded article having a good balance between compression strength and recovery properties could not be obtained.

In particular, when an inorganic gas is used as a foaming agent, the secondary foamability during molding of the obtained expanded particles,
It was extremely difficult to obtain a molded product having excellent interparticle fusion and curability recovery and a small shrinkage ratio.

The present invention has been made to solve the above-mentioned conventional drawbacks, and has a smaller shrinkage rate of the molded product as compared with the conventional polyethylene resin foam-molded product and has both compression strength and recovery property. It is an object of the present invention to provide polyethylene resin foamed particles which can provide a foamed molded product having a good balance.

[0007]

The present invention has a density of 0.92.
0 to 0.935 g / cm ³ polyethylene resin (A)
50 to 90 wt% and a density of 0.940 to 0.970 g
10 to 50 wt./cm ³ of polyethylene resin (B)
% Of the mixed resin and the expanded resin obtained by cross-linking and expanding the mixed resin, wherein the relationship between the density and the mixing ratio of the resin (A) and the resin (B) in the mixed resin satisfies the following formula. Characterized crosslinked polyethylene resin foamed particles, 0.935 <D _A · C _A + D _B · C _B <0.945
(C _A + C _B = 1) D _A : Density of resin (A) (g / cm ³ ) D _B : Density of resin (B) (g / cm ³ ) C _A : Weight ratio of resin (A) C _B : Weight ratio of resin (B) D _A · C _A + D _B · C _B : Density of mixed resin (2) The above-mentioned (1), wherein the foamed particles are foamed using an inorganic gas-based foaming agent (3) foamed molded article, characterized in that it is formed by in-mold molding of the crosslinked polyethylene-based resin foamed particles according to (1) or (2) above, (3) above (3) A gist of a core material for a bed, which is characterized in that the foamed molded article described above is used.

In the present invention, the density is 0.920 to 0.9.
Examples of the 35 g / cm ³ resin (A) include low density polyethylene, and more specifically, branched low density polyethylene, linear low density polyethylene and the like. The melting point of the resin (A) is preferably 105 to 125 ° C.

The density is 0.940 to 0.970 g / c.
Examples of the resin (B) of m ³ include high density polyethylene. The melting point of the resin (B) is preferably 125 to 140 ° C.

The above melting point means that after about 5 mg of resin is heated to 200 ° C. at 10 ° C./minute by a differential scanning calorimeter,
Cool down to room temperature at 10 ° C / min and 200 again at 10 ° C / min
It means the temperature of the endothermic peak on the DSC curve obtained when the temperature is raised to ° C. When there are a plurality of endothermic peaks, the peak temperature is located on the highest temperature side.

When the value of D _A · C _A + D _B · C _B [the density of the mixed resin obtained by mixing the resin (A) and the resin (B)] is 0.935 g / cm ³ or less, foaming occurs. When molded, the molded product has a large shrinkage ratio and does not have sufficient compressive strength. Also 0.945 g / cm ³
When the above is the case, the molding temperature for obtaining the foamed molded product must be raised, and the resulting molded product has reduced recovery physical properties such as compression set. In the present invention, the mixed resin means a resin obtained by uniformly melting and kneading the resin (A) and the resin (B).

In the present invention, the melt index of each of the resins (A) and (B) used in the mixed resin is not particularly specified, but it is preferable that both are 1 to 20 g / 10 minutes. The melt index values of the resin (A) and the resin (B) should be similar to each other.

The expanded beads in the present invention can be obtained by using the following production method, for example. That is, each of the above resins (A) and (B) is charged into an extruder, various additives are added to the resin if necessary, and these are heated and melted in the extruder, and then the molten mixture is formed into a strand from the extruder. After extruding and cooling, cut into pellets with a pelletizer or the like. Then, the granular resin is irradiated with an electron beam, or a dispersant is added to a dispersion medium such as water, and the mixture is put into a closed container together with the organic peroxide and heated to a temperature not lower than the decomposition temperature of the organic peroxide while stirring. Crosslinking is performed by heating and then cooling. Then, in another closed container, the obtained crosslinked mixed resin particles, a dispersant, a dispersion medium such as water and a foaming agent are charged, and after heating in a closed container to a predetermined temperature (foaming temperature), Foamed particles can be obtained by discharging from a closed container under atmospheric pressure. The closed container for cross-linking the mixed resin and the closed container for producing the expanded beads may be the same. In that case, after the crosslinking of the mixed resin is substantially completed in the closed container, the temperature inside the container is adjusted to the foaming temperature, the foaming agent is pressed into the container, and the contents of the container are stabilized when the pressure inside the container is stable. Are released under atmospheric pressure to obtain expanded particles.

In the above-mentioned production method, the degree of crosslinking of the crosslinked granular resin is such that about 1 g of the crosslinked granular resin or expanded particles is boiled in 100 cc of boiling xylene for 8 hours, and then a stainless steel 200-mesh wire net is used. The degree of crosslinking is preferably such that the weight after filtering and drying the insoluble matter at 80 ° C. for 12 hours is 20 to 70% of the weight before crosslinking.

Further, in the above method, as the foaming agent used for foaming the crosslinked granular resin, nitrogen, oxygen,
It is desirable to use an inorganic gas type such as air argon, but a conventional organic volatile foaming agent may be used as long as there is no risk of ozone layer destruction. The amount of the foaming agent used is determined depending on the density of the foamed particles to be obtained and the pressure resistance of the closed container, but is usually 1 to 50 parts by weight per 100 parts by weight of the granular resin after crosslinking.

In the above method, the temperature for foaming the granular resin after crosslinking is preferably in the range of [melting point] to [melting point + 80 ° C.] of the granular resin (mixed resin) before crosslinking. The above melting point means that after about 5 mg of the mixed resin is heated to 200 ° C. at 10 ° C./minute by a differential scanning calorimeter, −
Cool down to room temperature at 10 ° C / min and 200 again at 10 ° C / min
It means the temperature of the endothermic peak on the DSC curve obtained when the temperature is raised to ° C. When there are a plurality of endothermic peaks, the peak temperature is located on the highest temperature side.

The expanded beads of the present invention further have the following effects. That is, when the foamed particles are molded in-mold to obtain a foamed molded article, the foamed particles of the present invention are compared with conventional foamed particles,
Both the filling time of the expanded particles in the mold and the cooling time after heating with steam can be shortened (in comparison, the mold shapes are the same). This effect has a large volume (50,000 cm ³
The above is particularly remarkable when an in-mold molded product having 100,000 cm ³ or more) is to be obtained.

Next, the foamed molded article of the present invention will be described. The foamed molded product of the present invention is formed by in-mold molding of the above-mentioned crosslinked polyethylene resin foamed particles. As the method of in-mold molding, after filling the foamed particles into a mold having a predetermined shape, the temperature above the softening temperature of the foamed particles with steam,
A conventionally known method such as heating to a temperature within the range of [melting point + 30 ° C.] to fuse and integrate the expanded particles in the mold is used.

Since the foamed molded article of the present invention is constructed as described above, it has excellent fusion bonding properties between the foamed particles, has well-balanced recovery physical properties and compressive strength, and has a shrinkage ratio. It is small and has excellent dimensional accuracy. Further, in the case of the conventional crosslinked polyethylene resin foamed particles obtained by using the inorganic gas type foaming agent, when this is molded in the mold, the shrinkage ratio of the obtained foamed molded article tends to be large. The expanded particles composed of the specific cross-linked mixed resin particles described in 1., even if the expanded particles are obtained by using an inorganic gas type foaming agent, the shrinkage ratio of the foamed molded product obtained by in-molding this Can be maintained at a low level. The foamed molded product of the present invention having the above-mentioned properties can be very suitably used as, for example, a bed core material.

[0020]

EXAMPLES Next, the present invention will be described in more detail with reference to specific examples. Examples 1 to 3 and Comparative Examples 1 to 9 First, the resins selected from those shown in Table 1 were blended in the combinations and the mixing ratios shown in Table 2 (there are some used alone in Comparative Examples), and the resulting mixture was mixed in an extruder. After kneading and melting, the mixture was extruded into water in the form of strands, cut with a pelletizer, and granulated into mini-pellets having a length of 4 mm, a cross-sectional diameter of 0.8 mm, and a weight of 7 mg. Next, a predetermined amount of dicumyl peroxide was added as a crosslinking agent to an autoclave having a capacity of 5 liters, 5 g of kaolin as a dispersant, 0.25 g of sodium dodecylbenzenesulfonate as an emulsifier, and 1 kg of the above pellets as a foaming agent of 85 g.
3000cc water with g carbon dioxide (dry ice)
Was dispersed in the autoclave, heated to 155 ° C. with stirring, held for a certain period of time to perform crosslinking and impregnation with a foaming agent, and then introduce carbon dioxide gas having a pressure equal to the pressure in the autoclave at that time into the autoclave. While maintaining the pressure, one end of the autoclave was opened to release the pellets and water at the same time to foam the pellets to obtain crosslinked expanded particles. The value of D _A · C _A + D _B · C _B of the mixed resin and the expansion ratio of the obtained expanded particles are also shown in Table 2.

The expanded particles obtained in each of the above Examples and Comparative Examples were molded into a mold (internal dimension: thickness 50 mm × 300 mm × 300 m).
m) was filled and heated with steam having a pressure of 1.2 to 3.0 kg / cm ² G to obtain a molded body. The obtained molded product was evaluated for fusion property, dimensional accuracy, compressive strength, and recovered physical properties. The results are also shown in Table 2. The measuring methods and evaluation criteria of the above physical properties are as follows.

(1) Measuring method of fusion property and evaluation standard As shown in FIG. 1, thickness 10 mm × width 50 mm × length 1
The test body 1 obtained by cutting the foamed molded body to a length of 00 mm was pulled and broken in the length direction (direction a in the figure). The fracture surface F was observed and evaluated based on the following evaluation criteria. ○: Material fracture of fracture surface is 60% or more △: Material fracture of fracture surface is 40% or more and less than 60% × ・ ・ ・ Material fracture of fracture surface is less than 40%

(2) Measuring Method of Dimensional Accuracy and Evaluation Criteria The foamed molded body immediately after molding was aged in an oven at 60 ° C. for 24 hours and then left at room temperature for 24 hours, and then the surface direction of the foamed molded body was measured. The shrinkage ratio was measured. The shrinkage rate was calculated by the following formula. Shrinkage rate = [(die size-molded product size) / die size] x 1
00 (%) The value of the shrinkage rate obtained by the measurement was evaluated based on the following evaluation criteria. ◯: Shrinkage in each direction of 300 mm × 300 mm in the surface direction is less than 3% Δ: Shrinkage in each direction of 300 mm × 300 mm in the surface direction is 3% or more and less than 4% ×: Surface direction Shrinkage in both directions of 300 mm x 300 mm is 4% or more

(3) Method for measuring compressive strength and evaluation criteria 25% compressive strength S _{2 of the} foamed molded product (however, thickness 25 mm
X 50 mm long x 50 mm wide test piece 10 mm / mi
The stress at the time of compressing until it is distorted by 25% with n) is measured, and the S ₂ and the conventional foamed molded product (having resin 1 as a base material) having the same density as the density d (before compression) of the test body Compressive strength S ₁
Was calculated and evaluated based on the following evaluation criteria. ◯ ... S ₂ ≧ 1.6S ₁ Δ ... S ₁ <S ₂ <1.6S ₁ × ... S ₂ ≦ S ₁ Note that S ₁ can be expressed as the equation of d, and S ₁ = 209
It is a d ² + 3.55d + 0.266. Figure 2 is a diagram graphed S _1, further shows the S ₁ to the graph the range of S ₂ corresponding to the range of the criteria, as a region.

(4) Method for measuring recovery properties and evaluation criteria Test piece 2 having a thickness of 25 mm × a length of 50 mm × a width of 50 mm
3 is sandwiched between the pressing plates 3 as shown in FIG.
After narrowing the gap between the pressing plates at 0 mm / min as shown by the arrow b and compressing it to 25% of the original thickness (compressing until the strain amount becomes 75% of the original thickness), immediately at the same speed as indicated by the arrow c. Thus, the gap between the pressing plates was widened to remove the distortion. The amount (%) of residual strain in the pressing direction when the strain was removed and the compressive stress became 0 was measured and evaluated based on the following evaluation criteria. ◯: Residual strain rate is less than 20% ×: Residual strain rate is 20% or more FIG. 3B shows the above test when the strain is removed after the test body is strained by the above method. It is a graph showing the example of the change of the internal stress with respect to the change of the strain rate of a body. In the figure, L represents a stress-strain curve, K represents a recovery curve, and S represents a residual strain amount (%).

[0026]

[Table 1]

[0027]

[Table 2]

As can be seen from Tables 1 and 2, the foamed molded article obtained by in-mold molding using the cross-linked polyethylene resin foamed particles of the present invention is compared with the one using conventional polyethylene resin foamed particles. The foamed particles have excellent fusion bonding properties, and have well-balanced recovery physical properties and compressive strength.
Moreover, it has excellent dimensional accuracy.

Next, using the expanded particles of Example 1 and the expanded particles of Comparative Example 1, respectively, 120 cm in length and width,
A bed core material having a thickness of 20 cm (the volume is about 192000 cm ³ because a plurality of meat reliefs are formed in the thickness direction) is molded in a mold, and at that time, time for filling the foamed particles into the mold, steam heating time, Each cooling time was measured. The results are shown in Table 3. It should be noted that the filling and cooling conditions were the same, and only the heating conditions were such that the pressure was 1.8 k when the expanded beads of Example 1 were used.
Water vapor of g / cm ² G was used, and when the expanded particles of Comparative Example 1 were used, water vapor of pressure 1.2 kg / cm ² G was used.

[0030]

[Table 3]

As can be seen from Table 3, the expanded beads of the crosslinked polyethylene resin of the present invention are compared with the conventional expanded particles.
When molding in the mold, the time required for filling the foamed particles in the mold and the cooling time after heating with steam can be shortened.

[0032]

As described above, the crosslinked polyethylene resin expanded particles of the present invention have a density of 0.920 to 0.93.
50 to 90 g of polyethylene resin (A) of 5 g / cm ³
wt% and a density 0.940 to 0.970 g / polyethylene of cm ³ Resin (B) the range of 10 to 50 wt% and the mixed density 0.935 g / cm ³ greater, 0.945 g / cm
^Since it is formed by cross-linking and foaming the mixed resin so as to be less than ^3, by using the expanded particles, while maintaining good fusion bond between the expanded particles, compared with the conventional foam molded article In this case, the compressive strength is higher than that of a molded product of expanded particles made of a low-density polyethylene resin alone as a base resin,
In addition, the shrinkage rate of the molded product is small and the dimensional accuracy is excellent compared to the molded product of expanded particles made of low-density polyethylene or medium-density polyethylene resin alone as the base resin. It is possible to obtain a foamed molded product having excellent recovery properties as compared with a molded product of particles, that is, having a good balance of both recovery physical properties and compression strength while having good fusion properties, and further, as a foaming agent, an inorganic material. Even if a gas is used, there is an effect that it is possible to obtain a foamed molded product having a small shrinkage rate and excellent dimensional accuracy.

Further, since the foamed molded article of the present invention is obtained by in-mold molding of the above-mentioned cross-linked polyethylene resin foamed particles, compared to the conventional in-mold foamed molded article, the foamed particles are not It has good fusion properties, good balance between recovery properties and compressive strength, small shrinkage rate and excellent dimensional accuracy, and has an effect that it can be used very suitably as a bed core material, for example.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a method for measuring the fusion bondability of a foam molded article.

FIG. 2 is a diagram for explaining a method for measuring the compressive strength of a foamed molded product.

FIG. 3 (a) is a diagram for explaining a method for measuring recovery physical properties of a foamed molded article, and FIG. 3 (b) is an example of a stress-strain curve and a recovery curve recorded when measuring recovery physical properties of a test body. .

[Explanation of symbols]

 1, 2 Test body sampled from foamed body 3 Press plate L Stress-strain curve K Recovery curve S Residual strain amount (%)

Claims (4)

[Claims] 1. A density of 0.920 to 0.935 g / cm ^3.
Of 50 to 90 wt% of the polyethylene resin (A) and 10 to 50 wt% of the polyethylene resin (B) having a density of 0.940 to 0.970 g / cm ³ are crosslinked and foamed. A crosslinked polyethylene resin foamed particle, which is a particle, wherein the relationship between the density and the mixing ratio of the resin (A) and the resin (B) in the above mixed resin satisfies the following formula. 0.935 <D _A · C _A + D _B · C _B <0.945
(C _A + C _B = 1) D _A : Density of resin (A) (g / cm ³ ) D _B : Density of resin (B) (g / cm ³ ) C _A : Weight ratio of resin (A) C _B : Resin (B) weight ratio D _A · C _A + D _B · C _B : Density of mixed resin 2. The crosslinked polyethylene resin foamed particles according to claim 1, wherein the foamed particles are foamed using an inorganic gas type foaming agent. 3. A foamed molded article obtained by molding the crosslinked polyethylene resin foamed particles according to claim 1 or 2 in a mold. 4. A bed core material comprising the foamed molded article according to claim 3.