GB1568927A

GB1568927A - Process for producing crosslinked and foamed resin sheet

Info

Publication number: GB1568927A
Application number: GB861778A
Authority: GB
Original assignee: Sekisui Chemical Co Ltd
Current assignee: Sekisui Chemical Co Ltd
Priority date: 1978-03-03
Filing date: 1978-03-03
Publication date: 1980-06-11
Also published as: NL183195B; DE2809558C2; NL7802439A; NL183195C; DE2809558A1

Description

(54) PROCESS FOR PRODUCING CROSSLINKED AND FOAMED RESINSHEET (71) We, SEKISUI KAGAKU KOGYO KABUSHIKI KAISHA, a Japanese body corporate of 2-4-4 Nishitenma, Kita-ku, Osaka, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for producing a crosslinked and foamed resin sheet, and more specifically to a process for producing a crosslinked and foamed resin sheet composed basically of a synthetic thermoplastic resin, which is free from surface roughness, has a high surface strength, and can be satisfactorily processed by, for example, embossing.

Various processes for producing crosslinked and foamed resin sheets composed basically of a thermoplastic resin such as an olefin or vinyl chloride resin have been reported. Those generally employed in the art include, for example, a process for producing a crosslinked and foamed resin shaped article, which comprises mixing such a resin with a heat decomposable blowing agent and a crosslinking agent with or without a polyfunctional monomer, shaping the mixture into a desired shape such as a sheet, heating the resulting shaped article at a temperature above the decomposition temperature of the crosslinking agent but lower than the heat decomposition temperature of the blowing agent to crosslink the shaped article, and then heating the cross-linked article to the decomposition tempera ture of the blowing agent or to a higher temperature, or heating the shaped article to the heat-decomposition temperature or to a higher temperature to crosslink and foam the article simultaneously (see, for example, Japanese Patent Publications Nos. 17694/60, 14335/68, and 11375/68); and a process for producing a cross-linked and foamed resin shaped article, which comprises mixing a thermoplastic resin such as an olefin or vinyl chloride resin with a heat-decomposable blowing agent with or without a polyfunctional monomer, shaping the mixture into a desired shape such as a sheet, irradiating ionizing radiation to the resulting shaped article to crosslink the shaped article, and then heating the crosslinked article to the heat decomposition temperature of the blowing agent or to a higher temperature to foam the article (see, for example, British Patent 850,965, and Japanese Patent Publications Nos. 24131/64 and 29187/65).

The foamed resin articles obtained by these conventional processes, however, have the defect that because the expansion ratios of these articles are the same both at their inner and surface layers, the strength of the surface layer of the articles is weak; and consequently the articles readily undergo damages by an external force and cannot be used in applications which require strength higher than a certain point. Furthermore, since the resin articles have a low density also at their surface layer, an embossed design cannot be clearly repro duced when processing the article by embossing. In addition, the surface layer of the foamed article has a high ratio of expansion same as in the inner layer and is also weak to heat.When azodicarbonamide is used as the blowing agent, the resulting foamed shaped article has a colored surface, and to remove this coloration, the article must be heated. If it is heated until the color is removed, the resin on the surface of the shaped article melts and the surface becomes rough.

In an attempt to remove these defects of the prior techniques, methods have been suggested which involve bonding or heat-sealing a thermoplastic resin sheet or a low 9 > 'expansion foamed sheet of thermoplastic resin to the surface of such a crosslinked and foamed article. These methods, however, have the defect that the operation is complicated, and the cost of production is high.

Japanese Patent Publication No. 35,226/76 suggested a process for producing a crosslinked and foamed resin sheet having a cell density differing between its surface layer and inner layer, which comprises irradiating the surface of a sheet of a foam able thermoplastic resin containing a heat-decomposable blowing agent with ionizing radiation two or more times at different incidence angles with respect to the surface of the sheet thereby to cross-link the foamable thermplastic resin sheet, and then heating the crosslinked sheet to the decomposition temperature of the blowing agent or to a higher temperature to form the crosslinked foamble thermplastic resin sheet. To attain a high density selectively at the surface layer, the irradiating angle (the angle formed between the irradiated rays and the sheet surface) of ionizing radiation should be small.However, if this irradiating angle is smaller than 30 , reflection of the ionizing radiation increases, and its loss is great. Since the distance between the irradiation surface and the irradiation source increases at this time, the irradiated rays increasingly diffuse before they reach the sheet surface, and the loss of the irradiating rays is also great. For example, when the irradiating angle is 30 , the efficiency of irradiation is as low as 25neo, and 75% of the irradiated rays are wasted.If, on the other hand, the irradiating angle becomes larger than 30 , the ionizing radiation progressively reaches a farther point and finally the inner layer of the sheet, and therefore, the purpose of selectively providing a foamed surface layer having a higher density (i.e. a high degree of crosslinking) cannot be achieved.

The present invention provides a process for producing a crosslinked and foamed resin shaped article which is free from the aforesaid defects. Crosslinked and foamed resin shaped articles produced in accordance with the invention are free from surface roughness, have a high surface strength, can be processed satisfactorily by, for example, embossing, and have a surface layer of a density (i.e. a high degree of crosslinking) and an inner portion of a low density (i.e. a low degree of crosslinking and a high expansion ratio).

According to this invention, there is provided a process for producing a crosslinked and foamed resin sheet which comprises shaping a resinous mixture comprising a thermoplastic synthetic resin and a heat-decomposable blowing agent into a sheet form at a temperature above the melting or softening temperature of the resin and below the decomposition temperature of the blowing agent, applying a high-energy ionizing radiation to the resulting foamable resin sheet to crosslink the resin sheet, and then heating the resulting crosslinked foamable resin sheet to the decomposition temperature of the blowing agent or to a higher temperature, wherein the high-energy ionizing radiation has a range of at least 0.7 J d g/cm2in which ? is the density in g/cm3 of the foamable resin sheet and d is the thickness in cm of the foamble resin sheet, and a low-energy ionizing radiation with a range of not more than 0.6W d g/cm2 is applied to the foamble resin sheet before, during or after the application of the high-energy ionizing radiation.

The thermoplastic synthetic resin may be any type which can be shaped into a sheet form and has the property of self-crosslinking upon the application of ionizing radiation. Thermoplastic synthetic resins which can be especially suitably used in this invention include olefin resins and vinyl chloride resins.

The term "olefin resin", as used in the present specification and the appended claims, is meant to include not only a homopolymer of an olefin, but also an olefin compolymer composed of at least 50 mole%, preferably at least 70 mole%, of at least one olefin unit and the remainder being another monomer copolymerizable with the olefin, and a blend of at least 50% by weight, preferably at least 60% by weight, of the olefin homopolymer or copolymer with another polymer.

Examples of the olefin are ethylene, propylene, butylene, and hexene, and lower olefins having not more than 6, preferably not more than 4, carbon atoms are preferred. Examples of monomers copolymerizable with the olefin are vinyl acetate, vinyl chloride, acrylic acid and its esters, and methacrylic acid and its esters. Typical examples of the olefin resin that can be used in this invention include polyethylene, polypropylene, polybutylene, and ethylene/propylene copolymer, an ethylene/vinyl acetate copolymer, and an ethylene/vinyl chloride copolymer. They can be used either alone or as a blend or two or more.

The other polymer that can be blended with the olefin homopolymer or copolymer may be any polymer compatible with it. Examples are polybutadiene, polyisoprene, polychloroprene, chlorinated polyethylene, polyvinyl chloride, a styrene/butadiene copolymer, a vinyl acetate/ethylene copolymer, an acrylonitrile/butadiene copolymer, a vinyl chloride/vinyl acetate copolymer, and butadiene copolymers.

Likewise, the term "vinyl chloride resin", as used in the present specification and the appended claims, is meant to include not only a homopolymer of vinyl chloride, but also a vinyl chloride copolymer composed of at least 50 mole%, preferably at least 70 mole%, of a vinyl chloride unit and a minor proportion of another monomer copolymerizable with vinyl chloride, and a blend of at least 50% by weight, preferably 60% by weight, of the vinyl :duchloride polymer or copolymer with another polymer.

Typical examples of the monomer copolymerizable with vinyl chloride are ethylene, vinyl r ZXacetate, and vinylidene chloride. Examples of the other polymer that can be blended with the vinyl chloride homopolymer or copolymer are typically polyethylene, polyvinyl acetate and an ethylene/propylene copolymer.

An ethylene resin is especially suitable as the thermoplastic resin for use in this invention.

Examples of the ethylene resin that can be advantageously employed in the invention are low-, medium- and high-density polyethylenes, an ethylene/vinyl acetate copolymer, an ethylene/propylene copolymer, a blend of polyethylene and polypropylene, a blend of polyethylene and an ethylene/vinyl acetate copolymer, and a blend of polyethylene and an ethylene/propylene copolymer. Of these, the medium-density polyethylene, low-density polyethylene and the ethylene/propylene copolymer are especially suitable.

Preferably, the ethylene resins have a softening point of less than 1300C, and a melt index of 2.0 to 20, a number average molecular weight of 20,000 to 60,000, an intrinsic viscosity, at75 C. inxylene, of 0.8 to 1.1,and density of 0.910 to 0.940.

The heat-decomposable blowing agent used in this invention preferably is solid at ordi nary operating temperatures and has a higher decomposition temperature than the melt shaping temperature of the resin with which it is mixed. It further has the property of decomposing with the evolution of gases such as nitrogen gas, carbon dioxide gas or ammonia gas when heated to its decomposition temperature or to a higher temperature.

Any conventional heat-decomposable blowing agents used heretofore in the production of thermoplastic resin foams can be used in the process of the present invention. Especially preferred blowing agents are those which decompose at a temperature of more than 140"C., preferably between 170 and 220"C., to generate gases. Specific examples include azodicarbonamide and its metal salts, hydrazodicarbonamide, N,N' dinitrosopentamethylene tetramine, azobisisobutyronitrile, bis-benzene-sulfonyl hydrazide, 4,4'-hydroxy-bis(benzenesulfonyl semi-carbazide), and toluenesulfonyl hydrazide.

Azodicarbonamide is especially advantageous because of its good thermal stability and suitable decomposition temperature. These blowing agents can be used either alone or as mixtures. The amount of the blowing agent can be varied over a wide range according, for example, to the degree of expansion required of the final foamed shaped article. Usually, its amount is at least 0.1 part by weight, preferably 1 to 100 parts by weight, most preferably 2 to 80 parts by weight, per 100 parts by weight of the thermoplastic resin.

The blowing agent is mixed with the thermoplastic synthetic resin. As required, to facili tate the expansion of the resin by the heat decomposition of the blowing agent, the mixture may be incorporated with conventional foaming assistants such as oxalic acid, boric acid, zinc oxide, zinc stearate, lead stearate and zinc octanoate. The amount of such a foaming assistant is not critical, and can be varied broadly according to the type of the blowing agent.

Generally, the amount of the assistant is 0.01 to 50 parts by weight, preferably 0.1 to 10 parts by weight, per 100 parts by weight of the thermoplastic resin.

If desired, the mixture may contain conventional additives such as coloring agents (e.g.

cadmium yellow, quinacridone red, cobalt blue, cadmium red, red iron oxide, titanium oxide, zinc oxide, and carbon black); nucleating agents (e.g. talc, diatomaceous earth, calcium carbonate, zinc stearate, and aluminum stearate); lubricants (e.g. paraffin, or stearic acid); stabilizers (e.g. 2-hydroxy-4-methoxybenzophenone, or 2,6-ditert-butyl hyd roxytoluene); fire retardants (e.g. antimony oxide, and chlorinated paraffin); fillers (e.g.

calcium oxide, magnesium oxide, sodium carbonate, potassium carbonate, strontium car bonate, barium sulfate, lithopone, magnesium carbonate, calcium carbonate, silica, kaolin clay, and talc); plasticizers (e.g. dioctyl phthalate, dioctyl adipate and di-isodecyl phtha late); antistatic agents (e.g. a dialkyl phosphate salt, and an alkylaryl sulfonate); and deterioration inhibitors (e.g. t-butyl p-cresol and dilauryl thiopropionate) in the amounts usually employed in the art.

According to the process of the invention, the resinous mixture comprising the thermop lastic synthetic resin and the heat-decomposable blowing agent, with or without the optional ingredients described above, are shaped, preferably after melt-kneading, at a temperature lower than the decomposition temperature of the heat-decomposable blowing agent.

Mixing of the ingredients can be performed by conventional means, for example, using a kneader, blender or mixer. The melt-kneading can be carried out using, for example, an extruder, roll, or injection molding machine in which the thermoplastic resin is melted by heat and kneaded with the other ingredients.

The shaping temperature is above the melting temperature or softening temperature of the thermoplastic resin used and below the decomposition temperature of the heat decomposable blowing agent. Accordingly, it is difficult to determine the shaping tempera ture unequivocally, and the temperature is properly determined according to the type of the bonding agent used or the type of the thermoplastic resin used. Usually, the shaping temperature is preferably 110 to 1500C., more preferably 120 to 1400C. It is sufficient to perform mixing for 1 to 10 minutes at this temperature.

The mixed compositon is shaped into a sheet form. The shaping can be performed by a known method such as extrusion molding, injection molding or roll molding.

In the manner described hereinabove, a foamable resin sheet can be obtained. The thickness of the resin sheet can be varied over a wide range according to the usage of the final product. Generally, its thickness is at least 0.1 mm, preferably 0.2 to 30 mm, more preferably 0.3 to 20 mm. The density of the foamable resin sheet is mole critical, and can be changed broadly according to the usage of the final product. Generally, the density is at least 0.5 g/cm3, preferably 0.8 to 2 g/cm3, especially 0.9 to 1.5 g/cm3.

The foamable resin sheet is then exposed to high-energy ionizing radiation to crosslink the thermoplastic resin constituting the sheet, and to render the sheet suitable for foaming.

The term "ionizing radiation", as used in this specification, denotes electro-magnetic waves or charged particles having the ability to interact directly or indirectly with a substance and consequently ionizing the substance. The present invention, all sources of ionizing radiation which are usually employed to crosslink synthetic resins are feasible. Examples of such sources are electron beams and p-rays.

The high-energy ionizing radiation has such a strength as to permeate into the inside of the foamable resin sheet, and is applied under such irradiating conditions that crosslinking occurs over the entire thickness of the foamable resin sheet. The high-energy ionizing radiation has a range of at least 0.7up d g/cm2, which can vary depending upon the thickness (d cm) and density (Q g/cm3) of the foamable resin sheet used. (For a discussion on the range of ionising radiation see "Effects of Ionizing Radiation of NaturaI and Synthetic High Polyers" by Frank A. Bovey, Interscience Publishers (1958), pages 8, 9, 20 to 23 and 30 to 35.) There is no special upper limit to the energy range, but too strong an energy will be wastful.Hence, the high-energy ionizing radiation preferably has a range of 0.8 to 10 9 d g/cm2, especially 0.9 to 6.0cup d g/cm2. The dosage of the high-energy ionizing radia- tion depends upon the intensity of the radiation source, the type of the resin, the thickness of the sheet, and cannot be determined definitely. Generally, the irradiation dosage is at least 0.1 Mrad, preferably 0.5 to 40 Mrads, more preferably 1 to 20 Mrads.

The high-energy ionizing radiation can be applied until the thermoplastic resin in the foamable resin sheet crosslinks to the desired degree of crosslinking. The degree of crosslinking is expressed by the gel content of the resin, and it is advantageous to perform the ionizing radiation until the foamable resin sheet has a gel content of at least 5% by weight, usually 15 to 60%byweight.

The greatest characteristic of the present invention is that before, during or after the application of the high energy ionizing radiation, a low-energy ionizing radiation is applied to the foamable resin sheet.

The purpose of applying the low-energy ionizing radiation is to crosslink only the surface layer of the foamable resin sheet. Accordingly, the ionizing radiation used for this purpose has a lower level of energy than the high-energy ionizing radiation which is used to crosslink the foamable resin sheet entirely in its thickness direction. The low-energy ionizing radiation used in this invention has a range of not more than 0.62 d g/cm2, although the range can be varied according to the thickness (d cm) and density (g g/cm3) of the foamable resin sheet. Preferably, it may have a range of 0.1 to 0.5 dg/cm2, more preferably 0.15 to 0.4 9dg/cm2.

The dosage of the low-energy ionizing radiation is not critical, and depends upon the type of the resin constituting the sheet, the thickness of the sheet, etc. Advantageously, the dosage is generally 0.1 to 60 Mrads, preferably 0.5 to 40 Mrads, and more preferably 1 to 30 Mrads.

The high-energy and low-energy ionizing radiations can be applied by an ordinary radiation devices such as a nuclear reactor, a cobalt 60 y-ray irradiating device, and an electron beam accelerator (e.g., the Van de Graaff type, resonance type, or straight line type).

The low-energy ionizing radiation can be applied before or during the application of the high-energy ionizing radiation. Or this can be performed after the application of the highenergy ionizing radiation and before the heating step to be described. Or the irradiation may be performed through two or more stages mentioned. For example, it can be performed continuously before and during the application of the high-energy ionizing radiation, or during and after the application of the high-energy ionizing radiation.

The ionizing radiation may be applied to one or both surfaces of the foamable resin sheet.

When it is desired to obtain a foamed and crosslinked resin sheet whose both surface layers are crosslinked to a high degree, the low-energy ionizing radiation should be applied to both surfaces of the foamable resin sheet.

The procedure described above affords a foamable resin sheet in which at least a part of its surface layer has been crosslinked to a higher degree than its inner layer. The crosslinked formable resin sheet is then heated to the decomposition temperature of the heatdecomposable blowing agent or to a higher temperature to foam it.

The heating temperature varies according to the type of the blowing agent. Usually, however, it is at least 1800 C, preferably 200 to 300"C. Foaming can be completed generally within 1 to 20 minutes.

The heating can be performed by a conventional method, for example in a heating oven.

Since according to the process of this invention, a low-energy ionizing radiation is applied to at least one surface of the foamable resin sheet to crosslink only the surface layer of the sheet in addition to the application of a high-energy ionizing radiation for the crosslinking of the entire sheet, the inner layer of the foamable sheet is crosslinked to a degree suitable for foaming, and its surface layer is crosslinked to a higher degree than the inner layer.

After the end of foaming, therefore, a foamed and crosslinked resin sheet can be obtained which has a surface layer having a lower expansion ratio than the inner layer and a high physical strength.

In addition, the sheet obtained by this invention is free from surface roughness, and its surface is cross-linked at a high crosslinking density. Hence, a clear embossed design can be imparted to the surface of the resulting sheet.

The crosslinked and foamed resin sheet prepared by the present invention can be used in various applications, for example as interior finishing materials (wall materials) for buildings or motor vehicles, heat-reistant materials, mats, and cushioning materials.

The following Examples and Comparative Examples illustrate the present invention specifically.

Example 1 A mixture consisting of 100 parts by weight of low-density polyethylene (having a melt index of 7.0 and a density of 0.920) and 13 parts by weight of azodicarbonamide was fed into a monoaxial extruder to form a foamable sheet having a density of 0.98 g/cm3 and a thickness of 3.0 mm.

The sheet was fed to an electron beam accelerator, and an ionizing radiation having a high energy with a range of 0.8 g/cm2 was applied to the top and bottom surfaces of the sheet in a dose of 3 Mrads. Then, an ionizing radiation of low energy with a range of 0.08 g/cm2 was applied to the top and bottom surfaces of the sheet by using the same electron beam accelerator in each of the doses indicated in Table 1 (Runs No.s 1 to 5). The sheet was then fed into a heat-foaming oven, and heated at 230"C. for 4 minutes to afford a foamed sheet.

The thickness of the surface layer of the foamed sheet, the cell diameters at the surface layer and the inner layer of the sheet, the apparent density and the surface strength of the sheet were measured, and its surface condition was also observed. The results are shown in Table 1.

The resulting foamed sheets were free from surface roughness, and cells on their surface layers were very compact and the surface strength was high. The resulting resin sheets could be satisfactorily embossed.

Comparative Example 1 The same resinous mixture as used in Example 1 was formed into a foamable sheet in the same way as in Example 1, and the sheet was crosslinked by application of a high-energy ionizing radiation. Application of a low-energy ionizing radiation was omitted. The crosslinked sheet was fed into a heat-foaming oven, and heated at 2300C for 4 minutes to afford a foamed sheet.

The foamed sheet was evaluated in the same way as in Example 1, and the results are shown in Table 1 (Run No. 6).

Table 1 Comparative Example I Example I Run No. 1 2 3 4 5 6 Dose of the low energy ionizing 2 4 6 8 10 0 radiation (Mrads) Thickness (,u) of the surface 250 200 150 150 150 0 layer Cell dia meter ( ) less less ie,a Surface 100-200 30-60 than than than 350-450 layer 10 10 10 hyer 300-400 300400 300-400 300-400 300-400 300400 layer A apparent density (g/cm3) Surface layer 0.080 0.175 0.286 0.303 0.313 0.035 Inner 0.030 0.032 0.031 0.030 0.030 0.036 layer Entire sheet 0.036 0.038 0.042 0.040 0.042 0.035 Surface strength 0.5 1.0 1.3 1.5 1.5 0.1 (kg) Surface roughness None None None None None Slight The surface strength was determined as follows: A rounded stainless steel nail with its tip having a radius of 0.5 mm was applied to a sample foamed sheet at an angle to 75". A load was exerted on the tip of the nail, and the sample was pulled at a speed of 5 meters/min. The load at which the surface of the sample was broken was measured, and made the surface strength.

Example 2 A mixture consisting of 100 parts by weight of low-density polyethylene (having a melt index of 2.0 and a density of 0.924) and 15 parts by weight of azodicarbonamide was formed into a foamable sheet with a thickness of 3 mm and a density of 0.99 g/cm3 in the same way as in Example 1. A high-energy ionizing radiation having a range of 0.8 g/cm2 was applied to the top and bottom surfaces of the sheet in a dose of 5 Mrads using an electron beam accelerator. A low-energy ionizing radiation with each of the ranges shown in Table 2 was applied in a dose of 10 Mrads to the top and bottom surfaces of the sheet by the same electron beam accelerator. The sheet was then fed into a heat-foaming oven, and heated at 2300C. for 4 minutes.

The density, surface strength and embossability of each of the resulting foamed sheets were determined, and the results are given in Table 2.

The results show that when a short-range ionizing radiation (low-energy ionizing radiation) was applied additionally, the cells on the surface layer were compact, and the strength of the surface layer was high; and that the product exhibited good embossability. However, when only the long-range ionizing radiation (high energy ionizing radiation) as in Comparative Example 2 given below was applied, the entire sheet was crosslinked to almost the same extent, and the cells on the surface were not compact. Thus, the product had a low surface strength, and poor embossibility.

Comparative Example 2 The same resinous mixture as in Example 2 was shaped into a foamable sheet in the same way as in Example 2. The sheet was crosslinked by a high-energy ionizing radiation in the same way as in Example 2. The sheet was further exposed to a high-energy ionizing radiation (with a range of 0.8 g/cm2) under the same conditions as the crosslinking in a dose of 10 Mrads. The sheet was treated in the same way as in Example 2 to afford a foamed sheet.

The foamed sheet was evaluated in the same way as in Example 2, and the results are shown in Table 2.

Comparative Example 3 The same resinous mixture as used in Example 2 was shaped into a foamable sheet. The sheet was exposed to a high-energy ionizing radiation in the same way as in Example 2 (the application of a low-energy ionizing radiation was omitted), and immediately then, heated in a heat-foaming oven in the same way as in Example 2 to afford a foamed sheet.

Table 2 Example 2 Comparative Comparative 1 2 Example 2 Example 3 Crosslinking by the Range (g/cm2 0.05 0.10 0.80* low-energy Not applied ionizing Dose (Mrads) 10.0 10.0 10.0 radiation Surface layer 0.304 0.098 0.043 0.029 Density Inner layer 0.030 0.030 0.062 0.034 (g/cm3) Entire sheet 0.038 0.036 0.056 0.032 Surface strength (kg) 1.6 0.7 0.1 0.1 Embossability Good Good Poor Poor Crosslinked by application of high-energy ionizing radiation Example 3 Polyvinyl chloride (average degree of polymerization, 1000) 100 parts by weight Dioctyl phthalate 36 Azodicarbonamide 15 Calcium stearate 1 Zinc stearate 1 Trimethylolpropane trimethethacrylate (vulcanization accelerator) 2 Dioctyltin maleate 0.5 A mixture of the above ingredients was fed into a biaxial extruder to form a foamable resin sheet having a thickness of 2 mm and a density of 1.35 g/cm3.

The sheet was fed into a cobalt 60 irradiating device, and cobalt 60 was irradiated to each of the top and bottom surfaces of the sheet in a dose of 3 Mrads at a dose rate of 0.15 Mrad/hr (range 11.5 g/cm2). Then, an ionizing radiation of a low energy with a range of 0.08 g/cm2 was applied in a dose of 5 Mrads to each of the top and bottom surfaces of the sheet. The sheet was then fed into a heat-foaming oven, and heated at 2300C. for 10 minutes to afford a foamed sheet.

The sheet had the following characteristics.

Surface layer thickness: 150 EL Cell diameter Surface layer: less than 10 it Inner layer: 300 to 400 u Apparent density Surface layer: 0.306 g/cm3 Inner layer: 0.029 glcm3 Entire sheet: 0.037 glcm3 Surface strength: 1.5 kg Surface roughness: None WHAT WE CLAIM IS: 1. A process for producing a crosslinked and foamed resin sheet which comprises shaping a resinous mixture comprising a thermoplastic synthetic resin and a heatdecomposable blowing agent into a sheet form at a temperature above the melting or softening temperature of the resin and below the decomposition temperature of the blowing agent, applying a high-energy ionizing radiation (the term ionizing radiation" being as defined herein) to the resulting foamable resin sheet to crosslink the resin sheet, and then heating the resulting crosslinked foamable resin sheet to the decomposition temperature of the blowing agent or to a higher temperature; wherein the high-energy ionizing radiation

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Crosslinked by application of high-energy ionizing radiation Example 3 Polyvinyl chloride (average degree of polymerization, 1000) 100 parts by weight Dioctyl phthalate 36 Azodicarbonamide 15 Calcium stearate 1 Zinc stearate 1 Trimethylolpropane trimethethacrylate (vulcanization accelerator) 2 Dioctyltin maleate 0.5 A mixture of the above ingredients was fed into a biaxial extruder to form a foamable resin sheet having a thickness of 2 mm and a density of 1.35 g/cm3.

The sheet was fed into a cobalt 60 irradiating device, and cobalt 60 was irradiated to each of the top and bottom surfaces of the sheet in a dose of 3 Mrads at a dose rate of 0.15 Mrad/hr (range 11.5 g/cm2). Then, an ionizing radiation of a low energy with a range of 0.08 g/cm2 was applied in a dose of 5 Mrads to each of the top and bottom surfaces of the sheet. The sheet was then fed into a heat-foaming oven, and heated at 2300C. for 10 minutes to afford a foamed sheet.

The sheet had the following characteristics.

Surface layer thickness: 150 EL Cell diameter Surface layer: less than 10 it Inner layer: 300 to 400 u Apparent density Surface layer: 0.306 g/cm3 Inner layer: 0.029 glcm3 Entire sheet: 0.037 glcm3 Surface strength: 1.5 kg Surface roughness: None WHAT WE CLAIM IS: 1.A process for producing a crosslinked and foamed resin sheet which comprises shaping a resinous mixture comprising a thermoplastic synthetic resin and a heatdecomposable blowing agent into a sheet form at a temperature above the melting or softening temperature of the resin and below the decomposition temperature of the blowing agent, applying a high-energy ionizing radiation (the term ionizing radiation" being as defined herein) to the resulting foamable resin sheet to crosslink the resin sheet, and then heating the resulting crosslinked foamable resin sheet to the decomposition temperature of the blowing agent or to a higher temperature; wherein the high-energy ionizing radiation

has a range ofat least 0.7 vP d g/cm' in which d is the thickness in cm of the foamable resin sheet, and 2 is the density in g/cm3 of the foamable resin sheet, and a low-energy ionizing radiation with a range of not more than 0.6 5 d g/cm2 in which d and kP are as defined is applied to the foamable resin sheet before, during or after the application of the highenergy ionizing radiation.
2. A process according to claim 1 wherein the high-energy ionizing radiation has a range of 0.85 dto 10 Q dg/cm2.
3. A process according to claim 1 or 2 wherein the low-energy ionizing radiation had a range of 0.1# d to 0.5 # d g/cm2.
4. A process according to claim 1, 2 or 3 wherein the source of the ionizing radiation is an electron beam or rays.
5. A process according to any one of the preceding claims wherein the low-energy ionizing radiation is applied to the foamable resin after the application of the high-energy ionizing radiation.
6. A process according to any one of the preceding claims wherein the high-energy ionizing radiation is applied in a dose of 0.1 to 40 Mrads.
7. A process according to any one of the preceding claims wherein the low-energy ionizing radiation is applied in a dose of 0.1 to 60 Mrads.
8. A process according to any one of the preceding claims wherein the foamable resin sheet has a thickness of 0.1 to 30 mm and a density of 0.8 to 2 g/cm3.
9. A process according to any one of the preceding claims wherein the thermoplastic synthetic resin is an olefin resin or a vinyl chloride resin.
10. A process according to claim 9 wherein the thermoplastic synthetic resin is an ethylene resin.
11. A process according to any one of the preceding claims wherein the heatdecomposable blowing agent has a decomposition temperature of 170 to 2200C.
12. A process according to claim 1 substantially as described in any one of the Examples.
13. A cross-linked and foamed resin sheet produced by a process as claimed in any one of the preceding claims.