GB2053227A - Process for manufacturing an olefin resin foam - Google Patents

Abstract

A process for manufacturing a polyolefin resin foam comprising adding to and mixing with a base polyolefin resin a cell control agent and a volatile organic blowing agent and extrusion-expanding the resultant mixture, wherein the cell control agent comprises amorphous silica of pH 3.5 to 4.5 in the form of nonporous microspheres with an average particle size of 5 to 100 m mu and is added in an amount of 0.1 to 5 percent by weight of the base polyolefin resin.

Description

SPECIFICATION Process for manufacturing an olefin resin foam Polyolefin resin foams have a wide range of industrial applications such as cushioning materials, etc.

because of their closed-cell structure, flexibility and other features. However, since the polyolefin resin is crystalline in nature, its cell size has not been easily controllable in the extrusion-expansion process.

Generally, in manufacturing an extruded polyolefin resin foam, a small quantity of inorganic fine powder (such as talc) is added to a polyolefin resin before being extrusion-expanded into a foam in order to produce small, uniform cells. However, such fine powder of a conventional type is highly susceptible to conditions such as extrusion-expanding temperature, the quantity of a volatile blowing agent used and other factors, and, thus, even the smallest fluctuations of these conditions in the extrusion-expanding process may cause the cell size to vary so remarkably that the resultant foam does not have a desired thickness and appearance.

Also, it has been difficult to accurately correct or compensate varying factors and conditions. Further, it requires rather complicated operations and equipment to control the quantity of a cell control agent correspondingly to the specific conditions involved for otherwise correcting or preventing the variation in the quality of the resultant foam. Besides, in such a method as cited immediately above, defective or unacceptable products produced until the quantity of the cell control agent is properly adjusted to the specific conditions may often amount to a large loss of product.

The present invention provides a process for manufacturing a polyolefin resin foam by adding to and mixing with a base polyolefin resin a cell control agent and a volatile blowing agent and extrusionexpanding the resultant mixture, characterized in that the cell control agent comprises amorphous silica of pH 3.5 to 4.5 in the form of nonporous microspheres with an average particle size of 5 to 100mll and is added in an amount of 0.1 to 5 percent by weight of the base polyolefin resin.

If the content of the amorphous silica cell control agent is lower than 0.1 percent by weight or exceeds 5.0 percent by weight, it is difficuit to obtain a foamed-product having small, uniform cells.

The pH of the unique cell control agent herein referred to is measured by the method in accordance with ASTM E-70-52T. Preferably the cell control agent contains at least 98 percent by weight of silicon dioxide.

The amorphous silica usable in the present invention is obtained by a method which is generally known as a dry process. The object of the present invention cannot be achieved by using any such porous nonspherical silica that is generally used in the art, nor by using any such silica having an average particle size outside the range of 5 - 100mIr. and is difficult to obtain with any silica other than amorphous silica containing at least 98 percent by weight of silicon dioxide.The reason for this is not known, but this technical breakthrough may possibly be attributed to dispersibility of the cell control agent, the spherical surface conditions of the agent and compatibility of the volatile blowing agent with the base resin at the time when the resin, the blowing agent and the cell control agent are kneaded with each other within an extruder at a high temperature under high pressure.

To add the cell control agent of the present invention to the base resin, it may be either dry blended or masterbatched with the latter.

The polyolefin resin herein referred to includes, for example, low-density, medium-density and high-density polyethylene, polypropylene, polyethylene-propylene copolymer, polyethylene-vinyl acetate copolymer, polyethylene-acrylic acid copolymer and metal salts of polyethylene-acrylic acid copolymer.

These may be used either singly or in mixture.

Useful volatile organic blowing agents include those having a boiling point equal to or lower than the Vicat softening point of the base resin as, for example, propane, butane, dichlorodifluoromethane, dichlorotetrafluoroethane, dichlorofluoromethane, trichlorofluoromethane, chlorodifluoroethane or difluoroethane.

These may be used either singly or in mixture. The use of chemical blowing agents is not desirable for the present invention because the proportion of closed cells in the extrusion-expanded foam will be lowered thereby.

The method of the present invention is applicable particularly advantageously to the manufacture of extruded foams having a density ranging from 15 to 100 kg/m3. Also, in the method according to the present invention, such additives as thermal stabilizer, lubricants, antistatic agents, flame retardants, etc. may be used in such an amount of 0.1 - 5 percent by weight that is ordinarily used in the art.

The present invention is further described in detail by way of the preferred embodiments and comparative examples thereof, in which the cell control agents as listed in Table I below were used.

TABLE I Cell control Average agents SiO2 particle in trade names Manufacturer Surface profile pH Content(%) size (m ) Preferred embodiments: Aerosil 300 Nippon Aerosil Co., Ltd. Nonporous 3.6 ~ 4.3 99.8 < Approx. 7 Aerosil OX-50 Nippon Aerosil Co., Ltd. Nonporous 3.8 ~ 4.5 99.8 < Approx. 40 Comparative Examples: Silene EF Fuji Talc Co., Ltd. Porous 9 ~ 10 64 Approx. 30 Bitasil 220 Tagi Fertilizer Manufacturing Co., Ltd. Porous 8 ~ 9 81 ~ 85 Approx. 30 Silmos T Shiraishi Kogyo Kaisha Ltd.Porous 9 ~ 9.5 57.5 Approx. 50 Talc LMR Fuji Talc Co., Ltd Porous 9 ~ 10 61.7 Approx. 1 x 103 In the drawings, Figure lisa chart plotting the average cell size obtained by the cell control agent of the present invention and by another cell control agent against the density thereof; and Figure 2 is a chart plotting the average cell size against the content of specific cell control agents used.

Example 1 and Comparative Example No. 1 100 Parts byweight of low-density polyethylene (F-2130with MI of 3.0, produced by Asahi-Dow Limited), a varied amount of a volatile blowing agent, namely 21 - 32 parts by weight of dichlorotetrafluoroethane, 0.3 part by weight of barium stearate as lubricant and 0.6 part by weight of Aerosil OX-50 or Talc LMR as a cell control agent were fed, respectively, into an extruder with an inside barrel diameter of 65 mm heated at 200"C and evenly kneaded therein. The thus kneaded mixtures were cooled down to 100 C by a cooler succeeding to the extruder, and then extruded into foams 2 mm thick, respectively.Properties and qualities of the resultant foams are summarized in Table II. Also, the average cell size of the resultant foams are plotted against the density thereof in Figure 1.

TABLE II Density of Average Cell control agent foam (Kg/m3) cell size (mm) Quality of foam Example 1 Aerosil OX-50 26.0 1.15 No thickness variation, evenly distributed cells, good appearance 31.5 1.10 36.0 1.10 41.0 1.10 Comparative Example No. 1: Talc LMR 26.0 0.1 or below Heavy corrugate marks and thickness variation in lateral direction, uneven cell distri bution, inferior appearance 33.5 Approx. 0.62 35.0 Approx. 0.82 Light corrugate marks and thickness variation, rather uneven cell distribution and inferior appearance 41.0 1.19 No corrugate mark and thickness variation, good appearance with evenly distributed cells As can easily be seen from the results obtained using the products of Example 1 and Comparative Example No. 1 shown in Table II, the results obtained with the product of the invention were much improved in that the average cell size of the resultant foam did not change even when the content of volatile blowing agent was varied and, therefore, foamed sheets of stable quality could be obtained.

Example No. 2 and Comparative Example No. 2 Except that 24 parts by weight of chlorodifluoroethane were used as a volatile blowing agent instead of dichlorotetrafluoroethane, the same formulations as those of Example 1 and Comparative Example No. 1 were subjected to extrusion-expansion, respectively, in the same manner as in Example 1 and Comparative Example No. 1 but the extrusion-expansion temperature at a point just preceding to the circular die was set at three levels of 102"C, 104"C and 106"C, respectively.

The resultant foams were 2 mm thick, respectively, and showed properties and qualities as summarized in Table Ill below. Table Ill also shows that, in the method according to the present invention, the average cell size of the resultant foam did not change even when the extrusion-expansion temperature was varied to a small extent and foamed sheet of a stable quality can be obtained.

TABLE III Extrusionexpansion Average Temperature cell Density Cell control agent ( C) Size (mm) (Kg/m3) Quality of foam Example 1 Aerosil OX-50 102 1.13 35 No thickness variation, evenly distributed cells, good appearance 104 1.15 36 " 106 1.15 36 " Comparative Example No. 2: Talc LMR 102 0.80 35 Light crrugate mark and thickness variation inn lateral direction, rather uneven cell distribution and inferior appearance 104 1.16 36 No thickness variation, evenly distributed cells and good appearance 106 1.45 36 No thickness variation, evenly distributed cells, but appearance lacked smooth feeling Example 3 and Comparative Example No. 3 The same procedure as described in Example 1 and Comparative Example No. 1 was repeated except that 24 parts by weight of dichlorodifluoroethane was used as volatile blowing agent and except that Aerosil 300, Bitasil 220, Silene EF, Siloms T and Talc LMT are added individually in the quantities of 0.3, 0.6 and 0.9 parts by weight to obtain mixtures for extrusion-expansion, without using any lubricant. The resultant foams showed properties as shown in Figure 2, respectively. As obviously understood from the results plotted in Figure 2, the average cell size of the resultant foams is hardly affected by such a small variation in the quantity of a cell control agent as appearing in Example 3 so long as a cell control agent according to the present invention is used. In the observations as summarized in Tables II and Ill and shown in Figures 1 and 2, cell sizes were measured on the cross-section of a foam magnified by 20 times and algebraic mean of their maximum diameters was given as their average cell size.

Naturally, the invention includes, a resin composition for forming a polyolefin foam comprising a base polyolefin and, as a cell-control agent, from 0.1 to 5% by weight of the base polyolefin of amorphous silica of pH 3.5 to 4.5 in the form of nonporous microspheres with an average particle size of 5 to 100 mu.

Also the invention includes a polyolefin foam having dispersed therein from 0.1 to 5% by weight of the polyolefin of amorphous silica of pH 3.5 to 4.5 in the form of nonporous microspheres with an average particle size of 5 to 100 mu.

Claims (8)

1. A process for manufacturing a polyolefin resin foam comprising adding to and mixing with a base polyolefin resin a cell control agent and a volatile organic blowing agent and extrusion-expanding the resultant mixture, wherein the cell control agent comprises amorphous silica of pH 3.5 to 4.5 in the form of nonporous microspheres with an average particle size of 5 to 100 mu and is added in an amount of 0.1 to 5 percent by weight of the base polyolefin resin.

2. A process for manufacturing a polyolefin resin foam as claimed in claim 1, wherein the cell control agent comprises amorphous silica containing at least 98 percent by weight of silicon dioxide.

3. A process as claimed in claim 1 or claim 2 wherein the base polyolefin is polyethylene, polypropylene, polyethylene-propylene copolymer, polyethylene-vinyl acetate copolymer, polyethylene-acrylic acid copolymer or a metal salt of a polyethylene acrylic acid copolymer.

4. A process as claimed in any preceding claim wherein the blowing agent is propane, butane, dichlorodifluoromethane, dichlorotetrafluoroethane, dichlorofluoromethane, trichlorofluoromethane, chlorodifluoroethane, or difluoroethane.

5. A process for manufacturing a polyolefin resin foam substantially as hereinbefore described in any one of Examples 1 to 3.

6. A polyolefin foam when produced by a method claimed in any one of the preceding claims.

7. A resin composition for forming a polyolefin foam comprising a base polyolefin and, as a cell-control agent, from 0.1 to 5% by weight of the base polyolefin of amorphous silica of pH 3.5 to 4.5 in the form of non-porous microspheres with an average particle size of 5 to 100 myt.

8. A polyolefin foam having dispersed therein from 0.1 to 5% by weight of the polyolefin of amorphous silica of pH 3.5 to 4.5 in the form of non-porous microspheres with an average particle size of 5 to 100 mu.