Process for producing olefinic resin foams
This invention relates to a process for producing olefinic resin foams. 
     Olefinic resin foams are currently produced for the most part by (A) a method which comprises melting and kneading an olefinic resin together with a blowing agent (mainly a volatile blowing agent) in an extruder, and extruding the molten mixture through a die to obtain a non-crosslinked foam, or (B) a method which comprises melting and kneading an olefinic resin together with a chemical blowing agent and a crosslinking agent in an extruder, extruding the mixture through a die to form an unfoamed and non-crosslinked crosslinked extrudate, and then heating the extrudate in a heating furnace to perform its foaming or its foaming and crosslinking. The non-crosslinked foam produced by method (A) has unsatisfactory thermal stability and mechanical strength. In the production of an olefinic resin foam by method (B), for example by the procedure suggested in Japanese Patent Publication No. 18232/77, the chemical blowing agent remains partly undecomposed to give a yellow foam. Since the colored foam greatly impairs its merchandise value, a special measure needs to be taken to decolor such a product. 
     It is an object of this invention therefore to eliminate the defect of the prior art, and to provide a process for producing an olefinic resin foam which is white and has superior properties and a high expansion ratio. 
     The object of the invention is achieved by an improved process for producing an olefinic resin foam which comprises the steps of foaming a composition comprising an olefinic resin, a chemical blowing agent and a crosslinking agent into an unfoamed sheet-like or rod-like shape, and foaming it by heating, said foaming step being carried out in an oxygen-free atmosphere or an atmosphere containing not more than 10% of oxygen. 
     The olefinic resin used as a starting material in this invention includes polymers or copolymers of aliphatic monolefins such as polyethylene produced by a high-pressure, medium-pressure or low-pressure process, polypropylene, polybutene, an ethylene/propylene copolymer, an ethylene/butene copolymer, an ethylene/vinyl acetate copolymer and an ethylene/ethyl acrylate copolymer, and mixtures of these with rubbers, styrene-type resins or polyacrylonitrile. 
     Examples of suitable crosslinking agents are peroxides such as dicumyl peroxide, 2,5-dimethyl-2,5-di(tertiary butyl peroxy)hexane, di-tertiary butyl perterephthalate, tertiary butyl hydroperoxide, and 1,3-bis(tertiary butyl peroxyisopropyl)benzene. 
     The chemical blowing agent used in this invention is a so-called decomposable blowing agent which is an organic compound that decomposes at a temperature of at least 140° C. to generate gases. Examples are azodicarbonamide (decomposition temperature 190°-230° C.), benzene-1,3-disulfohydrazide (146° C.), diphenylsulfone-3,3&#39;-disulfohydrazide (148°), diphenyl oxide-4,4&#39;-disulfohydrazide (150° C.), and N,N&#39;-dinitrosopentamethylene tetramine (160° to 200° C.). 
     The unfoamed shaped article of olefinic resin composition to be foamed by heating in the process of this invention (to be referred to as an intermediate shaped article) may be in the non-crosslinked or crosslinked state. It may be of various shapes such as a sheet, rod or pillar. The intermediate shaped article is prepared by mixing 100 parts by weight of an olefinic resin, 0.01 to 5 parts by weight of a crosslinking agent and 0.01 to 35 parts by weight of a blowing agent, and shaping the mixture. Generally, it is preferred to form a sheet-like shaped article. The sheet may be obtained by kneading the ingredients by a heated roll and thus making them into a sheet form. Alternatively, after roll kneading, the mixture may be extruded through an extruder at a temperature of 110° to 150° C. In an especially preferred embodiment, the uncooled, crosslinked or non-crosslinked, unfoamed sheet extruded from the extruder is directly introduced into a heating furnace. The thickness of the sheet may be in the range of 0.1 to 50 mm. 
     The resulting intermediate shaped article is then heated in a heating furnace in an oxygen-free atmosphere or an atmosphere having an oxygen concentration of not more than 10%, preferably not more than 5%, more preferably not more than 2%. When the oxygen concentration of the heating atmosphere is more than 10%, a white beautiful olefinic resin foam cannot be obtained. The lower the oxygen concentration, the more the degree of whiteness and the higher the expansion ratio. An oxygen-free atmosphere is best suited. 
     Generally, an atmosphere having an oxygen concentration of not more than 10% is provided most simply and conveniently by substituting an oxygen-free inactive gas such as N 2  or CO 2  for the air in the heating furnace. It would be feasible also to remove oxygen by absorption or adsorption using an oxygen absorbent or adsorbent. 
     The heating temperature in the heating furnace is above the decomposition temperature of the chemical blowing agent. Generally, it is 200° to 260° C., preferably 220° to 240° C. The heating time varies according to the thickness of the intermediate shaped article, the heating temperature, the oxygen concentration of the heating atmosphere, etc. Periods of about 3 to 20 minutes are generally preferred. 
     The heating furnace used in this invention may be of the type which is adapted to heat the resin sheet while conveying it continuously using a conveyor belt, wire gauze belt or the like. Heating in the heating furnace can be performed, for example, by radiation using a nichrome wire, infrared rays, etc., or by forced convection of a heated inactive gas such as nitrogen and carbon dioxide gas within the furnace. 
     The olefinic resin foam in accordance with this invention may contain various additives such as antioxidants, pigments, fire-retarding agents, antistatic agents and auxiliary agents. 
     The process of this invention enables the continuous production of a white olefinic resin foam having a high expansion ratio by a simple operation. 
    
    
     The following Examples illustrate the invention more specifically. 
     EXAMPLE 
     One hundred parts by weight of low-density polyethylene (NUC-9026, a trademark for a product of Nippon Unicar Co., Ltd.), 20 parts by weight of azodicarbonamide and 0.8 part by weight of dicumyl peroxide were kneaded by a roll heated at 115° C. The resulting sheet was pelletized by a pelletizer, and introduced into an extruder. The pellets were extruded through a T-die at a maximum extrusion temperature of 125° C. to form an unfoamed, non-crosslinked sheet (intermediate shaped article) having a thickness of 2 mm and a width of 200 mm. The intermediate shaped article was heated in a heating furnace in an atmosphere having each of the various oxygen concentrations tabulated below. The expansion ratios and the degrees of whiteness of the products were measured, and the results are tabulated below. 
     
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Relation between oxygen concentration                                     
and expansion ratio                                                       
Oxygen      Heating temperature                                           
concentration                                                             
            240° C.                                                
                       230°C.                                      
                                  220° C.                          
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21%         10.3       17.2       18.5                                    
15%         14         18.2       22.2                                    
10%         20         24.2       24                                      
 5%         25.7       26.2       25.7                                    
 1%         31.2       33.2       31.2                                    
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Relation between oxygen concentration and                                 
the degree of whiteness (Hunter whiteness)                                
Oxygen      Heating temperature                                           
concentration                                                             
            240° C.                                                
                       230° C.                                     
                                  220° C.                          
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21%         53         53.2       55.2                                    
15%         54.5       55.2       55.4                                    
10%         62.5       60.4       61                                      
 5%         66.7       66.4       65.5                                    
 1%         68.2       69.2       69.4                                    
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     The Hunter whiteness was measured by a digital color difference meter (ND-101D type, a product of Nippon Denshoku Kogyo K.K.), and expressed by comparison with the whiteness of a vacuum-deposited magnesium oxide coating on a mirror surface which was taken as 100. It is seen from the above results that the Hunter whiteness can be maintained at 60 or more by adjusting the oxygen concentration to not more than 10%. Products having a Hunter whiteness of less than 60 were clearly found to be colored when observed with unaided eyes. 
     The physical properties of the product of this invention were compared with those of a non-crosslinked polyolefin foam sheet obtained by extrusion foaming. The results are shown below. 
     
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                             Tensile Tear                                 
            Expan-  Thermal  strength                                     
                                     strength                             
Base        sion    stabil-  (kg/cm.sup.2)                                
                                     (Kg/cm.sup.2)                        
Sample resin    ratio   ity (° C.)                                 
                               LD   TD   LD   TD                          
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Conven-                                                                   
       Low-                                                               
tional density  30      60     1.2  0.9  0.6  0.5                         
proudct                                                                   
       poly-                                                              
       ethylene                                                           
Product                                                                   
       Low-                                                               
of the density  30      80     4.1  3.2  2.2  2.4                         
inven- poly-                                                              
tion   ethylene                                                           
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     The tensile strength and tear strength were measured in accordance with JIS K-6767. LD stands for &#34;longitudinal direction&#34;, and TD, &#34;transverse direction&#34;.