JPH0672193B2 - Method for producing crosslinked polyolefin resin foam - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a crosslinked polyolefin resin foam. More specifically, it relates to a method for producing a crosslinked polyolefin resin foam, which is obtained by foaming a styrene-impregnated polymerized crosslinked polyolefin resin obtained by impregnating a specific amount of styrene monomer into a specific crosslinked polyolefin resin and polymerizing the resin. [0002] Styrene-impregnated polymerized polyolefin resin foams are superior in strength as compared with polyolefin resin foams, and the dimensional accuracy of the molded body when processed into a molded body is high. Good, the molded product has advantages such as good cushioning performance when used as a packaging material. However, due to the rigidity of the styrene component, the softness and elasticity, which are the characteristics of the polyolefin resin foam, are deteriorated, and the sliced surface of the molded body becomes rough and has a rough feel. For example, it is not preferable to wrap items such as cameras, watches, and printers that are susceptible to shocks or high-quality painted items whose surface is easily damaged. It is presumed that such a defect can be improved by uniformly refining the cells of the foam, and as a method therefor, styrene-impregnated polymerization and cross-linking using a specific polyethylene having a low softening temperature is disclosed in JP-B-51-46138. A polyolefin resin foam and a styrene-impregnated polymerized uncrosslinked polyolefin resin foam have been proposed in JP-B-52-10150. However, the foam obtained by the above method is 1 mm
² contains 60 to 150 cells, the cell diameter is not yet miniaturized and the cell diameter fluctuates greatly, and since large cells are mixed, the rough texture does not disappear, softness and elasticity Is also insufficient. Further, since the specific polyethylene or uncrosslinked polyolefin resin having a low softening temperature is inferior in heat resistance, the styrene-impregnated polymerized polyolefin resin foam obtained from such a resin is also inferior in heat resistance. As a method for solving the above problems and using a polyolefin having an arbitrary softening temperature and further refining the cell, Japanese Patent Publication No. 51-46138 discloses a method of impregnating a polyolefin resin with a styrene monomer for polymerization. A method of cross-linking such a polyolefin-based resin after being heated is disclosed. However, in this method, since any styrene-based monomer is impregnated and polymerized in the molecular gaps in the amorphous region of the polyolefin-based resin, the styrene-based polymer component in the obtained styrene-impregnated cross-linked polyolefin-based resin is small. It was found that as a result, the number of the cells formed by foaming was large and small. In view of the above circumstances, the inventors of the present invention include a styrene-impregnated polymerized polyolefin system containing a uniformly miniaturized cell and excellent in strength, dimensional accuracy of a molded product and buffering performance. As a result of earnest research for the purpose of developing a resin foam, it was found that when a specific crosslinked polyolefin resin is used as the polyolefin resin and a method is carried out in which the resin is impregnated with a specific amount of styrene monomer and polymerized. Since the molecular voids in the amorphous region of the resin are divided into a large number of fine voids by cross-linking, only a small amount of the styrene monomer to be impregnated enters the fine voids. The styrenic polymer component in the polyolefin resin exists as a large number of finely dispersed particles, and the foam has a large number of uniformly fine particles without adding a nucleating agent. The inventors have found that they include cells, and have completed the present invention. That is, the present invention was obtained by impregnating 100 parts of a polyolefin resin having a degree of cross-linking of 15 to 80% (parts by weight, the same hereinafter) with 10 to 300 parts of a styrene monomer, polymerizing the resin, and then polymerizing. Styrene-impregnated polymerized cross-linked polyolefin resin is impregnated with a foaming agent, and the method for producing a cross-linked polyolefin resin foam is characterized by foaming.For this method, 100 parts of polyolefin resin having a cross-linking degree of 15 to 80%, A crosslinked polyolefin-based resin foam is produced, which comprises a polyolefin-based resin impregnated with 10 to 300 parts of a styrene-based monomer as a base material and contains 200 or more cells in 1 mm ² . The shape of the foam produced by the method of the present invention is not limited and may be particles. The cross-linking degree of the polyolefin resin having a cross-linking degree of 15 to 80% (hereinafter, also referred to as cross-linking polyolefin-based resin) is 200
The sample was placed in a mesh wire net, immersed in boiling xylene for 24 hours for extraction, and then measured as insoluble matter. Examples Crosslinked polyolefin resins used in the present invention include, for example, ethylene, propylene, butene-1, pentene-1,3-methylbutene-1,4-methylbutene-1,4-methylhexene-1. Homo- or copolymers of mono-olefins such as ethylene-vinyl acetate copolymers,
A copolymer of a monoolefin such as an ethylene-vinyl chloride copolymer or an ethylene-methyl methacrylate copolymer with another polymerizable vinyl monomer or a mixture thereof, having any density, MI and softening point Dicumyl peroxide, 2,5-t-butyl perbenzoate, organic peroxides such as benzoyl peroxide and α-rays and β-rays are crosslinked according to a conventional method by irradiation with radiation to give a crosslinking degree of 15 to 80. % Is preferable. If the degree of cross-linking is less than 15%, the impregnation of the styrene-based monomer will be non-uniform, resulting in open cells and voids, and the dispersion of styrene polymer particles will be poor, resulting in uniform and fine cells. It is hard to get it. When the degree of cross-linking is further reduced, the heat resistance of the styrene-impregnated polymer resin becomes poor, and the foam also becomes poor in heat resistance. On the other hand, if the degree of cross-linking exceeds 80%, there is no problem in obtaining fine cells, but a large amount of cross-linking agent is required, and if the expanded particles are heat-fused to form a foamed molded product, the inter-particle fusion It is difficult to wear, and neither is preferable. Examples of the styrene-based monomer which is impregnated in the cross-linked polyolefin resin and polymerized (impregnated and polymerized) include, for example, styrene, α-methylstyrene, p-methylstyrene, and other nucleus-substituted styrene.
As a method of performing impregnation polymerization, a crosslinked polyolefin resin is dispersed in an aqueous dispersion system containing an inorganic dispersant such as calcium phosphate, magnesium pyrophosphate, calcium carbonate, etc., and a polymerization catalyst such as benzoyl peroxide is dissolved therein. Method of impregnating and polymerizing by adding styrene monomer at once or gradually, impregnating crosslinked polyolefin resin with styrene monomer by adding only styrene monomer to the system at once or gradually Examples of the method include adding a polymerization catalyst after polymerization, and polymerizing. The amount of the styrene-based monomer impregnated and polymerized is 10 to 300 parts with respect to 100 parts of the crosslinked polyolefin resin, in other words, 9 to 75% is preferably contained in the obtained styrene-impregnated and crosslinked polyolefin resin. If the impregnation polymerization amount of the styrene-based monomer is less than 10 parts, the cell diameter does not become sufficiently small,
On the other hand, if it exceeds 300 parts, on the contrary, the cell diameter becomes large, and a foam having a smooth touch cannot be obtained. The styrene-containing polymerized crosslinked polyolefin resin particles having a particle size of 0.25 to 10 mm thus obtained are dispersed as they are or in water according to a conventional method, and an expandable particle is obtained by impregnating with a low boiling point organic blowing agent under pressure. Create. Examples of the low-boiling organic blowing agent include aliphatic hydrocarbons such as propane, butane, pentane and hexane, cyclopropane, cyclobutane, cyclopentane,
Examples include cycloaliphatic hydrocarbons such as cyclohexane, halogenated hydrocarbons such as methyl chloride, ethyl chloride, trichlorofluoromethane, dichlorotetrafluoroethane, tetrachlorodifluoroethane, etc., but aliphatic hydrocarbons or cycloaliphatic When a group hydrocarbon is used as a foaming agent, uniform fine cells cannot be obtained unless the expandable particles are cured after the impregnation of the foaming agent, so a low-boiling halogenated hydrocarbon that can be foamed as it is without curing. It is preferred to use the class. The expandable particles impregnated with a foaming agent can be foamed by a known method such as a method of foaming by heating with steam, or a method of releasing the impregnated state under pressure at once to atmospheric pressure for foaming. The crosslinked polyolefin resin foam according to the present invention thus obtained contains 200 or more cells per 1 mm ² . The number of cells contained in 1 mm ² referred to in this specification is
It is an average value obtained by cutting at least 100 foamed particles and calculating the number of cells found in the central area 1 mm ² of 10 foamed particles, which are considered to have an average cell diameter. The number of cells was calculated at the center of the expanded particles. When the expanded particles were left as they were, there was a large difference in the cell density between the central part and the peripheral part except for the skin part near the outermost cells 2 and 3 of the expanded particles. But not
If the molding conditions are improper when the foamed particles are filled in the mold and the molding conditions are inadequate, the state of the cells in the periphery may be disturbed. It is a calculated number. In order to eliminate the rough feeling of the sliced surface of the foam and improve the surface property, it is preferable to make the cells uniformly fine until 200 cells or more are contained in 1 mm ² , but it is smoother and softer. To give it a feel,
An equation containing more than 200 cells in 1 mm ² and related to the expansion ratio: [Equation 1] It is preferable that the number of cells represented by the formula (d represents a foam density (g / cc), and 0.02≤d≤0.08) is contained in 1 mm ² . The foam density d (g / cc) is obtained by measuring the capacity of the foam by the submersion method.
0.08 is preferable. If it is less than 0.02, it becomes difficult to stably generate particles having uniformly fine cells, and if it exceeds 0.08, the expansion ratio is low and the foam becomes stiff and has a soft feel. All of them are not preferable because they cannot be obtained. When such a crosslinked polyolefin resin foam is further foam-molded and used as a molded article, it has a good buffering performance like a styrene-impregnated polymerized polyolefin foam obtained by a known method, withstands repeated compression, and has chemical resistance. It retains its excellent properties such as excellent, and the surface when sliced is beautiful, smooth and soft. Furthermore, since the cell diameters are made uniform and fine, cell shrinkage and buckling are less likely to occur, and a high-foaming material is obtained, which is rich in elasticity, and exhibits strong characteristics against compression and bending. As described above, the foam according to the present invention has a very high utility value as a packaging material for articles that are weak against impact or whose surface is easily scratched. The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited to these examples. Examples 1 to 4 Ethylene-vinyl acetate copolymer (vinyl acetate content 15%
(% By weight, the same below), MI 1.5, Picat softening point 67
℃, density 0.93 (g / cc)) crosslinked with dicumyl peroxide 100% particles having a crosslinking degree of 77% and 0.1 g of dicumyl peroxide dissolved styrene monomer 100g, 1 liter reactor, Then add 2.0 g of calcium phosphate and 0.1 g of sodium dodecylbenzene sulfonate as an emulsifier, and add 50
Add 0 g of pure water, stir and disperse, and then raise the temperature to 130 ° C.
Hold at 0 ° C for 4 hours to crosslink styrene monomer with ethylene-
By impregnating and polymerizing with a vinyl acetate copolymer, styrene-impregnated polymer particles having an average particle size of about 1.2 mm were obtained. The obtained styrene-impregnated polymer particles are cut,
FIG. 1 is a sketch drawing of a photograph (10000 times) of a cross section taken by an electron microscope after being immersed in tetrahydrofuran for 24 hours to extract almost all of styrene. As shown in FIG. 1, the styrene polymer was dispersed as a large number of very small particles. The styrene-impregnated polymer particles were impregnated with dichlorodifluoromethane at 50 ° C. in a saturated dichlorodifluoromethane atmosphere to give expandable particles, which were then subjected to 0.4 kg / cm ² G using a known pressure batch type pre-expanding machine. By heating for 4 seconds, 6 seconds, 8 seconds, and 10 seconds under the water vapor pressure, the foamed particles were obtained as shown in Table 1 to have foamed particles having four kinds of densities. Measure the number of cells of the obtained expanded particles,
The results are shown in Table 1. The cells of the obtained expanded particles were all uniformly fine, and the sliced surface was soft and beautiful without any rough feel. [Table 1] Comparative Examples 1 and 2 100 g of the same ethylene-vinyl acetate copolymer as used in Examples 1 to 4 and a styrene monomer 10 in which 2 g of dicumyl peroxide and 0.5 g of benzoyl peroxide were dissolved.
0 g was placed in a 1 liter reactor, then 1 g of magnesium pyrophosphate and 0.4 g of sodium dodecylbenzenesulfonate as an emulsifier were added, and 500 g of pure water was added and dispersed by stirring. It was kept for a period of time to impregnate and polymerize the styrene monomer. Then, the temperature was raised to 140 ° C. and the temperature was maintained for 3 hours to carry out crosslinking to obtain styrene-impregnated polymer particles having an average particle size of about 1.2 mm. FIG. 2 is a sketch drawing of a photograph (10000 times) of the cut surface of the obtained styrene-impregnated polymerized particles, taken by an electron microscope in the same manner as in Examples 1 to 4. By comparison with FIG. 1, it can be seen that the styrene polymer particles have a non-uniform particle size, a large number of large particles, and a small number. Then, dichlorodifluoromethane was impregnated in the same manner as in Examples 1 to 4 and then heated under a steam pressure of 0.5 kg / cm ² G for 4 seconds and 7 seconds, respectively, to cause foaming. Seeded expanded particles were obtained and their cell number was measured. The results are shown in Table 2. The cells of the foamed particles were all large and visually observable, nonuniform and few in number.
Moreover, the texture of those sliced surfaces was hard and rough. [Table 2] Examples 5 to 7 and Comparative Example 3 Low density polyethylene (MI 1.5, Picut softening point 95 ° C.,
The density of 0.924 (g / cc)) is shown in Table 3, and 140 g of particles crosslinked with four kinds of added amounts of dicumyl peroxide and 60 g of styrene monomer in which 0.3 g of benzoyl peroxide are dissolved are used in Examples 1 to 1. In the same manner as in 4, each was dispersed in a 1 liter reactor, impregnated with styrene monomer at 90 ° C for 2 hours, then heated to 130 ° C and kept for 4 hours to polymerize the styrene monomer. The average particle size is about 2.
7 mm styrene impregnated polymer particles were obtained. The styrene-impregnated polymer particles thus obtained were impregnated with dichlorodifluoromethane at 60 ° C. in a saturated dichlorodifluoromethane atmosphere to give expandable particles, and 1.2 kg / cm ² G was obtained using a known pressure batch type pre-expanding machine. Each of them was heated for 12 seconds under the steam pressure of 4 to obtain 4 kinds of expanded particles. The degree of crosslinking of the obtained expanded particles, the expanded particle density, and the number of cells were measured, and the results are shown in Table 3. [Table 3] As is clear from Table 3, as the degree of cross-linking becomes lower, the cells become open-celled, the expanded particles shrink, and the fluctuation range of the cell becomes larger. Therefore, the lower limit of the degree of cross-linking is about 15%. I understand. Example 8 Low density polyethylene (MI 1.5, Picutt softening point 95 ° C.,
0.5% to 100 parts of 50% crosslinked particles with a density of 0.921 (g / cc))
As shown in Table 4, 0, 10, 15, 20, and 30 parts of the styrene monomer having benzoyl peroxide dissolved therein were respectively impregnated and then polymerized in the same manner as in Examples 5 to 7 to give an average particle size of 2 mm styrene impregnated polymerized particles were obtained.
Then, the particles were impregnated with dichlorodifluoromethane in the same manner as in Examples 5 to 7 using a pressure batch type prefoaming machine.
Five kinds of expanded particles having a density of 0.039 to 0.038 g / cc were obtained by adjusting the heating time in the range of 10 to 15 seconds under a steam pressure of 1.2 kg / cm ² G. After examining the number of cells of the obtained expanded particles, the particles were aged for 24 hours and filled in a mold of 40 mm × 150 mm × 200 mm which can be closed but not closed, and the pressure was 1.0 kg / cm ² G A molded body was produced by heating the steam for 20 seconds.
The molded body was dried at 70 ° C. for 5 hours and then sliced with a knife, and the sliced surface was examined. The results are shown in Table 4. [Table 4] When the amount of styrene impregnated was 10 parts or more, there was almost no variation in cell diameter between particles and within particles, and the appearance of the sliced surface was such that the cells were uniformly beautiful and the surface feel was very good. Examples 9 to 11 and Comparative Example 4 Ethylene-vinyl acetate copolymer (vinyl acetate content 10%,
Particles with a cross-linking degree of 52.4% obtained by cross-linking MI 1.5, Picut softening point 73 ° C, density 0.93 (g / cc)) with dicumyl peroxide.
Into a 40 liter reactor, 50 g of magnesium pyrophosphate and 10 g of sodium dodecylbenzenesulfonate were added, and 25 liters of pure water were added to disperse them. 1.8kg, 3.6kg, 5.4k of styrene monomer was added to the mixture.
g, 7.2 kg was added and each was impregnated with a styrene monomer by heating at 80 ° C. for 3 hours. Next, the styrene monomer in which 3% benzoyl peroxide was dissolved
0.2kg, 0.4kg, 0.6kg, 0.8kg each was added over 1 hour while maintaining at 80 ° C, then heated to 110 ° C and kept for 5 hours to polymerize, and each average particle size was about 2
mm styrene impregnated crosslinked polyethylene particles were obtained. The obtained particles were heated at 60 ° C in a saturated dichlorofluoromethane atmosphere,
Each was impregnated with dichlorofluoromethane to give expandable particles. The expandable particles were heated in a pressurized batch type foaming machine under a steam pressure of 0.8 to 1.2 kg / cm ² G for 17 seconds to obtain 4
A particle having a seed density of 0.024 (g / cc) was obtained. The results of examining the number of cells are shown in Table 5. [Table 5] The cell diameter increased as the styrene impregnation amount increased, and the upper limit was about 300 parts. Examples 12 to 16 Ethylene-vinyl acetate copolymer (vinyl acetate content 5%,
MI2, Picutt softening point 83 ℃, density 0.92 (g / cc)) crosslinked with dicumyl peroxide, particles with cross-linking degree of 64.7% 10kg
And 40 kg of styrene monomer in an 80 liter polymerization machine
After adding g of calcium phosphate and 8 g of sodium dodecylbenzene sulfonate and adding 40 liters of pure water to disperse, the temperature was raised to 140 ° C and kept for 5 hours to impregnate the crosslinked particles with styrene monomer. With residual dicumyl peroxide, the styrene monomer is polymerized and the particle size is 0.25-10 mm.
To obtain styrene-impregnated crosslinked polyethylene particles. To the particles
Using a pressure batch type foaming machine impregnated with the foaming agent at 60 ° C under a saturated gas atmosphere of the foaming agent shown in Table 6, 0.5 kg /
By heating for 8 seconds under a steam pressure of cm ² G, 5 types of expanded particles were obtained. The density and the number of cells of the expanded particles thus obtained were examined, and the results are shown in Table 6. [Table 6] When n-butane or iso-butane is used as a foaming agent, the cells will not be foamed by continuous foaming unless impregnated with the foaming agent and then cured. It was Particles produced by using dichlorofluoromethane as a foaming agent were placed in a mold that can close but not close to form a molded body. The density of the obtained molded body is 0.0268 (g
/ cc), soft touch, rich elasticity, excellent surface properties, and smooth sliced surface. FIG. 3 is a sketch drawing of a microscope photograph (15 times) of a sliced surface of the molded product, and FIG. 4 is a commercially available product obtained by impregnating a polyethylene-vinyl acetate copolymer with a styrene monomer by a known method and crosslinking it. Foamed molded products (trade name: Pioflex,
It is a sketch drawing of a microscope photograph (15 times) of a sliced surface of Sekisui Plastics Co., Ltd. It is clear that in the molded body obtained from the foam according to the method of the invention shown in FIG. 3, the cells are much more uniform and finer than the cells of the molded body shown in FIG. The crosslinked polyolefin resin foam produced by the method of the present invention is foam-molded and used as a molded article.
Good cushioning performance like styrene-impregnated polymerized polyolefin foam obtained by known method, withstands repeated compression,
Maintains characteristics such as excellent chemical resistance, and the surface when sliced is beautiful, smooth and soft. Furthermore, since the cell diameter is made uniform and fine, shrinkage and buckling of the cell do not easily occur, and a high-foamed material is obtained, which is rich in elasticity and has a strong property against compression and bending. As described above, the foam produced by the method of the present invention has a very high utility value as a packaging material for articles that are susceptible to impact or whose surface is easily scratched.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sketch drawing of an electron micrograph (10000 times) of a cross-sectional structure of styrene-impregnated polymer particles obtained in an example of the present invention. FIG. 2 is a sketch drawing of an electron micrograph (10000 times) of a cross-sectional structure of styrene-impregnated polymer particles obtained by a conventional method. FIG. 3 is a sketch drawing of a micrograph (15 times) of a cross-sectional structure of a molded body produced from the foam obtained in the example of the present invention. FIG. 4 is a sketch drawing of a micrograph (15 times) of a cross-sectional structure of a commercially available foamed molded product obtained by a conventional method.

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Claims (1)

[Claims] 1. 100 parts by weight of a polyolefin resin having a degree of crosslinking of 15 to 80% is impregnated with 10 to 300 parts by weight of a styrene monomer, polymerized, and then obtained styrene-impregnated crosslinked polyolefin resin. A method for producing a crosslinked polyolefin-based resin foam, which comprises impregnating a foaming agent with a foaming agent to cause foaming. 2. The method according to claim 1, wherein the blowing agent is a low boiling point halogenated hydrocarbon.