JPH0426301B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a crosslinked polyolefin foam, and more particularly to a method for producing a crosslinked polyolefin foam having uniform microcells.

(Prior art and problems to be solved) Conventionally, polyolefins are blended with a blowing agent, a blowing aid, and a crosslinking agent, and a part of the blowing agent is decomposed by applying pressure and heating for a certain period of time, and a crosslinking reaction is caused by the crosslinking agent. It is a well-known method to produce a low-density crosslinked polyolefin foam by two-step foaming, in which a primary foam is molded, and then heated under normal pressure to decompose the remaining foam. (For example, see Special Publication No. 45-29381). However, in this method, the second step of foaming is not put into a mold under normal pressure, but is heated in the absence of oxygen, such as in an oil bath with constant temperature nitrogen flow, to decompose and foam the remaining foaming agent. However, only products with uneven surfaces and large irregularities could be obtained. In order to improve this point, in the second stage foaming process, the primary foam has a cross-sectional shape that corresponds to the shape and dimensions of the final product.
It is also known to produce more uniform foams in a shorter time with better yields by placing the foam in an unsealed mold with dimensions and heating the mold externally (e.g., 57-191029). However, in these two-step foaming methods, in order to make the foamable composition match the internal volume of the primary mold, thin sheets of the foamable composition are usually stacked on top of each other, or several blocks are placed inside the mold. However, with this method, not only do the bubbles at the interface become rough and form weld lines, but also the bubbles are heated under pressure to decompose some of the blowing agent and form the primary foam. When manufacturing a primary foam, there are large burrs due to leakage at the corners or side surfaces of the resulting primary foam, and cracks are also observed due to friction with the mold wall surface. When the final molded body is produced by subjecting the body to a secondary foaming process, there is a drawback that it is difficult to obtain a foamed molded body in which bubbles are uniformly dispersed throughout the final molded body.

Therefore, the present inventors conducted various studies to improve the above-mentioned drawbacks, and found that by preforming the composition to obtain a primary foam, a final foamed molded product having uniform fine cells was obtained. The present invention was completed based on the discovery that it is possible to produce a crosslinked polyolefin foam having uniform fine cells.

(Means for Solving the Problems) That is, the present invention produces crosslinked polyolefin foam by two-step foaming of a composition in which a polyolefin resin is blended with a blowing agent, a crosslinking agent, and, if necessary, a foaming aid and other additives. In the method of manufacturing a body, the mold has a similar shape to the mold used in the primary foaming process, and its internal volume is 85 to
A method for producing a crosslinked polyolefin foam, characterized in that the composition is filled into a preforming mold having a volume of 95%, and the composition is preformed at a temperature at which the blowing agent and the crosslinking agent do not decompose. . Therefore, the crosslinked polyolefin foam in the present invention refers to a polyolefin foam having a degree of crosslinking of 70 to 80%, and includes, for example, low density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer, ethylene -Crosslinking by adding crosslinking agents such as dicumyl peroxide and di-t-butyl peroxide and blowing agents such as azodicarbonamide and dinitrosopentamethylenetetramine to polyolefins such as methyl methacrylate copolymers and chlorinated ethylene ionomers. It is cross-linked and foam-molded to a degree of 70 to 80%. In addition, it is preferable to use a foaming aid in this foamable composition depending on the type of foaming agent, and examples of the foaming aid include metal oxides such as zinc oxide and lead oxide, fatty acids or metal salts thereof, There are compounds whose main component is urea.
Furthermore, conventional compounding agents such as fillers and pigments can be added as necessary. In the present invention, as mentioned above, preforming is performed to improve the occurrence of weld lines, non-uniformity of bubbles, etc. The mold used for this preforming has a similar shape to the mold used for primary foaming. has. Also, its volume is
85% to 95% of the mold volume used for primary foaming,
If it is less than 85%, the generation of bubbles at the edges becomes rough, while if it is more than 95%, burrs will occur and a stable primary foam cannot be obtained. Therefore, at this stage, it is necessary to carry out the process under conditions where the foaming reaction and crosslinking reaction do not occur yet, that is, at a temperature at which the foaming agent and crosslinking agent do not decompose.The temperature range depends on the types of the foaming agent and crosslinking agent. It varies depending on the case, but usually 100~
The temperature is in the range of 120°C, during which time the composition is given fluidity and becomes a uniform mass having a shape similar to that of the primary mold. If even a portion of the blowing agent and crosslinking agent decomposes at this stage, it will cause insufficient foaming and uneven foaming in subsequent steps, making it impossible to obtain a uniform foamed molded product.

A foamed molded article is produced from the thus obtained preformed article in a conventional two-step process. That is, the preform obtained as described above is placed in a primary foaming mold, and the blowing agent is gasified at the same time as the crosslinking agent decomposes or after the crosslinking agent starts to decompose, so that the undecomposed blowing agent becomes 60% Primary foaming is performed until ~75% remains, and the resulting primary foam is heated in a metal bath, oil bath under normal pressure, or directly heated with steam, or if it cannot be sealed. A secondary foaming process is performed in which the metal plate of the mold is heated directly or indirectly from the outside in the mold to decompose the remaining foaming agent and foam it to form a low-density foam.

Next, the present invention will be explained using examples.

Example 100 parts by weight of polyethylene, Yucalon LM (Melt Index 8, manufactured by Mitsubishi Yuka), 17 parts by weight of azodicarbonamide (manufactured by Sankyo Kasei) as a blowing agent,
0.5 parts by weight of urea-based foaming aid (Selton NP) 0.5 parts by weight of zinc stearate, 0.5 parts by weight of stearic acid and 0.15 parts by weight of active zinc as foam regulators, and 0.7 parts by weight and 1.1 parts of dicumyl peroxide as crosslinking agents. After blending bis-tertiary butyl peroxide and 0.84 to 1 part by weight of 3,3,5-trimethylcyclohexane and kneading to uniformly disperse the mixture, the lower side is 260 mm,
It is filled into a preforming mold with a trapezoidal cross section with a top side of 295 mm and a thickness of 27.5 mm, sealed with an external pressure of 5.0 Kg/cm ² or more, and heated at 120°C for 10 minutes to preform. One side of the lower side of the preform obtained in this way is
Fill a primary mold with a diameter of 270 mm, a top side of 305 mm, and a thickness of 27.5 mm (filling rate: 93%), seal with external pressure, and heat to 155°C.
was heated for 30 minutes to carry out a crosslinking reaction, and at the same time decompose a portion of the foaming agent and cause foaming. Next, this primary foam is placed in a mold that can be closed, but not sealed, with an internal volume of 85 mm x 850 x 850 mm, and is heated and pressurized using a press molding machine to obtain a foamed molded product. The resulting foam had no weld marks, the size of the bubbles was uniform in the center and edges, and
Its physical properties were as follows.

Density 0.028g/cc Tensile strength 2.2Kg/cm ² Elongation rate 116% Compressive strength 5% 0.19Kg/cm ² 25% 0.45Kg/cm ^2Comparative example As a preforming mold, one side of the bottom side is 245mm, one side of the top side is 280mm The procedure was the same as in the example except that a preforming mold having a trapezoidal cross section and a thickness of 27.5 mm was used, and the filling rate of the primary mold was 83%. The resulting foam had large cell diameters and was rough both at the ends and in the center. Furthermore, the physical properties of the foam differ depending on the part.

(Effects) As described above, in producing a crosslinked polyolefin foam by two-stage foaming, the present invention
By performing preforming before performing the first preforming step, a crosslinked polyolefin foam having uniform fine cells can be produced.

Claims (1)

[Claims] 1 Polyolefin resin with foaming agent and crosslinking agent,
Furthermore, in a method for producing a crosslinked polyolefin foam by two-stage foaming of a composition containing foaming aids and other compounding agents as necessary, the composition has a shape similar to that of the mold used in the primary foaming step. , filling the composition into a preforming mold whose internal volume is 85 to 95% of the volume of the primary foaming mold, and preforming the composition at a temperature at which the blowing agent and crosslinking agent do not decompose. A method for producing a crosslinked polyolefin foam, characterized by: