NO793568L - RESIN COMPOSITION SUITABLE FOR PREPARATION OF FOAM FROM POLYVINYL CHLORIDE RESIN. - Google Patents

Description

The present invention relates to a composition which is suitable

for the production of foam from a resin based on polyvinyl chloride.

It is previously known to produce resins based on polyvinyl chloride (hereinafter abbreviated to PVC resins) by means of various methods, some examples of which will be given below.

1) The resin is mixed or impregnated with a decomposable

foaming agent, which is a compound that is decomposable at a higher temperature with the evolution of a gas. The resin mixture is produced under heating using a technique that involves injection molding, extrusion or some other conventional molding, and whereby the resin expands into foam with the help of the gas that is developed during the decomposition of the foaming agent.

2) A so-called plastisol with a paste consistency is produced first

by mixing the resin with a plasticizer, and the plastisol is then converted into foam by mixing in air using a suitable mechanical device. Alternatively, the plastisol is further mixed with a decomposable foaming agent, whereby the mixture is subjected to heating, and whereby the foaming agent decomposes with the evolution of gas at the same time as the plastisol gelatinizes. 3) A resin mixture containing a decomposable foaming agent is first produced as shaped articles such as plates, blocks, rods, tubes or the like by rolling or some other mechanical method and at a temperature lower than the decomposition temperature of the foaming agent. The shaped article is then heated to cause expansion and formation of foam by the decomposition of the foaming agent. 4) A metal mold is filled with a resin mixture containing a decomposable foaming agent, if desired mixed with an evaporable foaming agent, further an organic solvent with which the resin is swellable and a plasticizer. The resin mixture is heated under pressure in the metal mold during melting and gelling. This is followed by cooling to room temperature under pressure and while the mixture remains in the metal form. This creates a shaped article, which is then again heated to a temperature that is above the softening temperature of the resin, whereby the resin mixture expands into foam with the help of the gas that is developed during the decomposition or evaporation of the foaming agents.

As will be clear from the above, the gas, which therefore causes expansion of the resin mixture and which also occurs in the foam of the PVC resin, is usually made up of either atmospheric air, which is mixed in by means of a mechanical device, or a gas which in turn forms a cleavage product of the decomposable foaming agent. However, the mechanical mixing of atmospheric air is not satisfactory as, even with complicated mixing equipment, it is difficult to obtain a fine and uniform cell structure under high expansion, and the use of a decomposable foaming agent is undesirable in the production of PVC resin with high whiteness due to the fact that they most of the decomposable foaming agents used in practice are azo compounds, which form colored decomposition products, which necessarily lead to the produced resin foam being colored yellow or brown. Furthermore, the cell structure of the resin foam, which is obtained by using a decomposable foaming agent, is not always satisfactory with respect to fineness and evenness, and especially in the production of resin foam with high expansion.

In addition to the above-mentioned disadvantages, the above-described methods 1) to 3) are not suitable for the production of hard or semi-hard foam with high expansion, i.e. that the methods are limited to the production of soft or flexible resin foam. The method 4) described above is also unfavorable in terms of efficiency and production costs, which is due to the fact that the process must be carried out in batches and that the time elapsed for the production of the foam is relatively long due to the complete nature of the process.

Another gas source for resin foam can be constituted by a volatile foaming agent consisting of a compound with a relatively low boiling point, and which easily changes into gas form when the resin mixture, which contains the aforementioned compound, is heated to a temperature above the softening point of the resin, and by which resin foam is obtained. This type of foaming agent is preferred because there are no colored decomposition products, which lead to coloring of the resin foam. The gas formed here is, as explained, not due to decomposition but to evaporation of the foaming agent.

The use of an evaporable foaming agent is successful in the production of various types of foam plastics such as polystyrene resins. However, no successful method of producing PVC foam has yet been developed, nor have the causes of the difficulties been fully investigated.

An aim of the present invention has thus been to produce a new composition which is suitable for the production of PVC foam plastic which contains a foaming agent of the evaporable type, and from which resin foam with high expansion, fine and uniform cell structure and without noticeable mis-staining.

The resin composition according to the present invention, which can be expanded into foam, comprises a PVC resin with an average degree of polymerization that does not exceed 0.20 ml per gram of resin, which resin further contains an evaporable foaming agent, which consists of a hydrocarbon or a halogenated hydrocarbon compound with a boiling point that does not exceed 90°C, and which is added to the above-mentioned PVC resin.

The resin composition described above can be expanded into foam with a high degree of expansion and, as a rule, with a fine and even cell structure. However, the cell structure is not always so fine and uniform when the degree of expansion is extremely high, as in the production of resin foam with a mass density of e.g.

0.10 g/cm 3 or less.

The inventors of the invention stated here have, through extensive experimental work, aimed to arrive at a resin composition which can be expanded into resin foam with an extremely high degree of expansion, which resin foam nevertheless has a fine and even cell structure. This purpose is achieved by using the above-described composition of PVC resin, which is impregnated with an evaporable foaming agent, and then by further adding from 0.5 to 30 parts by weight of foam-conditioned resin per 100 parts by weight of the first-mentioned mixture. The foam-conditioned resin can be either an acrylic resin or a styrene-based resin with a particularly low viscosity, which is at least 3.0 dl/g. The effect of the above-mentioned foam conditioning resin can be improved by the presence of small amounts of a nucleating agent in the resin composition, which agent can be a finely divided inorganic powder material or a combination of solid reactants which can evolve carbon dioxide gas by reaction in the resin composition.

The main component in the composition according to the present invention is a PVC resin, which is either a homopolymer or a copolymer of essentially vinyl chloride. When the PVC resin is a copolymer, it is recommended that the content of the monomer or monomers, which are copolymerized with vinyl chloride, does not exceed 40% by weight, or in other words that at least 60% by weight of the resin is based on vinyl chloride. This is because the resin foam obtained must have excellent flame retardant properties, high mechanical strength and other desirable properties associated with vinyl chloride resins.

The ethylenic unsaturated monomers, which are copolymerized with vinyl chloride, are well known in professional circles, and can e.g. consists of vinyl esters such as vinyl acetate and vinyl propionate, vinylidene halides such as vinylidene chloride and vinylidene fluoride, vinyl halides other than vinyl chloride such as vinyl fluoride, acrylic acid and its esters such as ethyl acrylate, methacrylic acid and its esters such as methyl methacrylate, acrylonitrile, meth-acrylonitrile, maleic acid as well as esters and anhydride thereof, fumaric acid and esters thereof and olefins such as ethylene and propylene.

Among the above-mentioned comonomers, vinyl acetate is particularly advantageous, as a copolymer of vinyl chloride and vinyl acetate is not only amenable to impregnation with a foaming agent, but also has a markedly reduced melt viscosity, so that smooth and quiet foaming is ensured which leads to the formation of resin foam with a further improved fine and even cell structure.

In order for such beneficial effects to be achieved upon application

of a copolymer, the content of vinyl acetate in the copolymer - the resin should ideally be at least 3% by weight and with an upper limit, as previously mentioned, at 40% by weight due to consideration of the flame retardant and mechanical properties of the resin foam.

The important parameters for the PVC resins used in the resin composition according to the present invention are the average degree of polymerization and the pore volume. The average degree of polymerization, which can be easily determined by measuring the liquid viscosity of the resin, preferably does not exceed 2000, as a PVC resin with an average degree of polymerization greater than 2000 has an extremely high melt viscosity as well as poor gelatinization ability, so that it is difficult to obtain foam with high expansion even with a larger amount of foaming agent impregnated in the resin. On the other hand, the lower limit of the average degree of polymerization is determined by the mechanical properties of the resin foam produced from the resin. For example, a PVC resin with an average polymer-isolation degree less than 300 can only produce resin foams with poor mechanical properties.

Another important parameter for the PVC resin used in the present resin composition is the pore volume, which should not exceed 0.20 ml/g, or preferably not 0.10 ml/g, which values are very small compared to ordinary PVC resins which have a pore volume of approx. 0.25 ml/g or more when the resin is a homopolymer of vinyl chloride. The pore volume is determined using a mercury pressure pore meter, whereby the mercury pressure is increased from 1 to 100 kp/cm 2 , so that mercury is pressed into the pores of the resin particles, which have a pore diameter of approx. 3 0 / loo. or less.

The above-mentioned limitation of the pore volume is very important

as a PVC resin with a large pore volume exhibits poor foaming agent retention as well as loss of foaming agent not only during storage of the resin composition, which is impregnated with the foaming agent, but also during the molding process, whereby the resin composition is formed into articles of resin foam, whereby it becomes difficult to obtain objects with high foam expansion.

The PVC resins which can satisfy the above-mentioned requirements are obtained by suspension polymerization of vinyl chloride monomer or a monomer mixture which essentially consists of vinyl chloride monomer in an aqueous medium, which contains a suspending agent, and furthermore there is a free radical polymerization initiator which is soluble in the monomeric phase.

The evaporable foaming agent, which is to be impregnated in the above-described PVC resin, is, as mentioned above, a hydrocarbon or a halogenated hydrocarbon with a boiling point that does not exceed 90°C, or preferably 70°C. When using a foaming agent with a boiling point above 90°C, the already expanded resin foam will show a noticeable shrinkage on standing, so that the resulting foam body will have a cell structure that is not satisfactory with regard to fineness and uniformity.

Hydrocarbon or halogenated hydrocarbon compounds which are suitable for use as foaming agents can e.g. be propane, butane, isobutane, pentane, neopentane, n-hexane, isohexane, n-heptane, methyl chloride, methylene chloride, chloroform, carbon tetrachloride, ethylidene chloride, ethylidene fluoride, trichloroethylene, 1,2-dichloroethane, trichlorofluoromethane, dichlorodifluoromethane, chlorotrifluoromethane, bromine -trifluoromethane, tetrafluoromethane, dichlorofluoromethane, chlorodifluoromethane, trifluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, dibromotetrafluoroethane, chloropentafluoroethane, hexafluoro-

ethane, 1-chloro-1,1-difluoroethane and the like. If desired, these evaporable foaming agents can of course also be used in combination - two or more - with each other.

The quantity of the above-mentioned evaporable foaming agent, which is used in the impregnation of the PVC resin, depends on the desired degree of expansion of the resin foam. It is obvious that the degree of impregnation must be increased when a foam body with a high degree of expansion is desired. When a foam body with a low degree of expansion is desired, it may be sufficient to impregnate with a quantity as small as 3% by weight or less with foaming agent. The amount of foaming agent is in most cases in the range of 1 to 30% by weight or more when it is desired to obtain foam bodies with high expansion.

The impregnation of the PVC resin with the above-mentioned foaming agent is carried out in principle by bringing the components into contact with each other. Especially when the PVC resin is in powder form, it can only be mixed with the foaming agent, so that the foaming agent is absorbed into the resin particles. When the foaming agent is present as a gas at room temperature under atmospheric pressure, a simple impregnation method is to add the PVC resin, water and dispersant to a pressure vessel, e.g. an autoclave,

which is equipped with a stirrer for forming a suspension of the resin powder in the aqueous medium. The foaming agent is then added to the suspension under pressure, after which the mixture is stirred while increasing the temperature to 30 to 90°C for 3 to 20 hours. After absorption equilibrium is established inside the container and the mixture is cooled to room temperature, the resin which has absorbed the foaming agent is removed from the container, dehydrated by a suitable means such as centrifugation and dried in an air stream at a relatively low temperature, e.g. 50°C

or lower, thus giving the desired PVC resin impregnated with foaming agent.

The resin composition thus produced, which is impregnated

with evaporable foaming agent, as such can be directly manufactured into shaped articles of resin foam by means of known techniques such as injection molding and extrusion molding as well as compression molding in a metal mold, whereby gelatinization of the resin as well as expansion of the gelatinized resin by means of it by evaporation of the foaming agent produced gas takes place at the same time. The resin composition can, if desired, be mixed with certain types of additives, which are usually used when casting PVC resins, such as plasticizers, flame retardants, anti-oxidants, antistatic agents and the like at a relatively low temperature to prevent premature evaporation, before manufacturing. of the foaming agent.

As previously mentioned, one of the most difficult problems in the production of a foam body from PVC resins consists in ensuring a fine and even cell structure in the resin foam, and especially when the degree of expansion of the foam is extremely high, or when said foam has a mass density of f .ex. 0.10 g/cm 3 or less. Naturally, depending on the manufacturing conditions, the foam may have

a mass density of approx. 0.30 g/cm 3. The inventors of the present invention have carried out experiments with the aim of arriving at an agent for conditioning the cell structure by adding a foam conditioning agent to the resin composition, and they thereby came to the conclusion that the addition of certain types of thermoplastic resins are effective for this purpose.

Foam conditioning resins of the present invention include acrylic resins and styrene-based resins, and these resins are particularly effective when they have a viscosity as low as at least 3.0 dl/g, measured in a chloroform solution with a concentration of 0.1 g/100 ml at 25°C. These foam conditioning resins are mixed with the PVC resin impregnated with the evaporable foaming agent in an amount of from 0.5 to 30 parts by weight

per 100 parts by weight of foaming agent impregnated PVC resin,

and said admixture is carried out before the resin composition is molded into shaped articles of resin foam.

The acrylic resin that is suitable as a foam conditioning resin

is either a polymethyl methacrylate or a copolymer resin which mainly consists of methyl methacrylate and one or more acrylic acid esters such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate and the like. The acrylic resin can also contain other than the above-mentioned comonomers, namely comonomers such as styrene, acrylonitrile, vinyl esters as well as esters of methacrylic acid such as ethyl methacrylate but not methyl methacrylate, further n-butyl methacrylate, 2-ethylhexyl methacrylate and the like, provided that the content is limited. The content of methyl methacrylate in the acrylic resin is preferably in the range from 60 to 95% by weight.

It is desirable that the acrylic resin as foam conditioning resin has a viscosity as low as at least 3.0 dl/g, or preferably at least 5.0 dl/g measured in a chloroform solution at a concentration of 0.1 g/100 ml at 25 °C as mentioned above.

It is recommended to use acrylic resin with a further reduced viscosity, or in other words with a higher average degree of polymerization when the average degree of polymerization of the PVC resin is very large, i.e. approaches the upper limit of 2000. Furthermore, it is preferred to use an acrylic resin prepared by emulsion polymerization of the acrylic monomers because further improvements are obtained by the use of such a resin. The aforementioned improvements consist, among other things, of easy-flowing feeding into the casting machines with the resulting reduced risk of blocking the inlet with the added resin composition, further acceleration of a uniform gelatinization of the resin composition and increased expandability of the gelatinized and melted resin composition.

The amount of the added foam conditioning acrylic resin

is from 0.5 to 30 parts by weight, or preferably from 3 to 20

parts by weight per 100 parts by weight of foaming agent impregnated PVC resin. Higher amounts of acrylic resin than those specified above do not result in any further improvement, but on the contrary in undesirable effects in relation to the PVC resin's natural properties such as the flame retardant properties.

Styrene-based resins belong to another class of foam conditioning resins. The styrene-based resin may be a homopolymer of styrene, but it is preferred that the styrene-based resin is a copolymer consisting mainly of styrene and a smaller amount of acrylonitrile as comonomer. The copolymer can, if desired, naturally comprise one or more other comonomers which can be copolymerized with styrene and acrylonitrile. The styrene-based resin has a reduced viscosity of at least 3.0 dl/g,

and then measured in a chloroform solution at a concentration of 0.10 g/100 ml at 25°C. It is recommended that the styrene-based resin has the greatest possible reduced viscosity in the event that the vinyl chloride-based resin has such a high degree of polymerization that it approaches the upper limit of approx. 2000.

The above-mentioned comonomers, which are copolymerizable with

styrene and acrylonitrile, are e.g. esters of acrylic acid such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate and the like, esters of methacrylic acid such as methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate and the like, maleic acid and fumaric acid as well as esters thereof and maleic anhydride.

The above-described styrene-based resins are obtained using a known polymerization method, but it is recommended that the resin be produced by emulsion polymerization in an aqueous medium.

The amount of the styrene-based resin to be used as foam conditioner may be the same as for acrylic resin, i.e. in the range from 0.5 to 30 parts by weight, or preferably from 3 to 20 parts by weight per 100 parts by weight of the PVC resin impregnated with the evaporable foaming agent.

The reason for the marked improvement achieved with respect to the cell structure of the resin foam by the addition of foam conditioning resin is presumably that the gelatinization of the PVC resin is accelerated by the foam conditioning resin; that the melt viscosity of the PVC resin during the molding process is adequately regulated or increased, so that the expandability of the foam is facilitated; that the cell walls in the foam are strengthened and thus have a higher resistance to coalescence or collapse of the foam; and that shrinkage

of the once formed foam at a higher temperature is prevented due to improved retention of the gas developed by the evaporation of the foaming agent.

It was further found that the foam conditioning effect described above, which was achieved by adding a foam conditioning resin, can be further enhanced by the presence of certain types of nucleating agents in combination with the foam conditioning resin. According to the present invention, suitable nucleating agents are an inorganic fine powder material such as calcium carbonate, talc, barium sulphate, finely divided silica, titanium dioxide, clay, aluminum oxide, bentonite, diatomaceous earth and the like with an average particle diameter of 30/m² or less, or preferably 10/m .or less, as coarser particles of these inorganic powder compounds affect the fluidity of the molten resin during the melting process in an unfavorable direction, so that the surface condition of the foam bodies obtained by mixing such a coarse powder becomes poor in terms of gloss, and streaks appear as well as poor uniformity in the cell structure.

Another type of nucleating agent consists of a combination of equivalent amounts of an acid such as boric acid and organic acids, e.g. citric acid, tartaric acid and oxalic acid as well as a carbonate or hydrogen carbonate of sodium, potassium or ammonium such as sodium carbonate, sodium hydrogen carbonate, potassium carbonate, ammonium hydrogen carbonate and the like.

The amount of nucleating agent to be added in combination with the foam conditioning resin ranges from 0.01

to 20 vpkf--^pl Ar no. 100 vpVt-Hpl per mother) slfiimmpmi rirlpl -i Tnnre»rfnÉ»r+-

PVC resin. When the amount of nucleating agent exceeds 20 parts by weight, the degree of expansion of the foam resin is reduced, and the resulting foam body has poor properties and a less smooth and attractive surface.

It is optional whether the resin composition according to the present invention is mixed with a known decomposable foaming agent, which in that case will be limited to an amount of approx. 5 parts by weight or less per 100 parts by weight PVC resin impregnated with the evaporable foaming agent. The decomposable foaming agent suitable for use can e.g. be azobisisobutyronitrile, diazoaminobenzene, diethylazodicarboxylate, diisopropylazodicarboxylate and the like, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, N,N'-dimethyl-N,N<1->dinitroso-terephthalamide and the like, and sulfonylhydrazide compounds such as benzenesulfonylhydrazide, toluenesulfonylhydrazide . -(benzenesulfonyl hydrazide) and the like as well as sodium hydrogen carbonate.

The use of these decomposable foaming agents is desired to further improve the fineness and uniformity of the cell structure of the resin foam, and to reduce the shrinkage of the foam body so that its shape is better retained. However, too much decomposable foaming agent is undesirable as the foam body is colored by the colored decomposition products, and as foam bodies with uneven surface properties are obtained. It is also recommended to add a decomposition activator, which may be known, such as certain types of zinc compounds, copper compounds and the like to accelerate the decomposition of the decomposable foaming agent with accompanying gas evolution at a temperature lower than that prevailing at the casting of the resin composition.

The above-described expandable resin composition

with the addition of the foam conditioning resin is very advantageous in the production of shaped objects from PVC resin foam, as the composition gives resin foam with a high degree of expansion and with a fine and uniform cell structure independent of the stiffness of the desired foam products, and which can vary from soft and flexible products to hard and rigid ones.

The aforementioned shaped objects are assumed here to be produced using conventional techniques such as extrusion-moulding, injection-moulding, compression-moulding and the like.

In the following, the present invention will be explained in more detail

by means of examples, where i.a. parts means weight—parts. The methods for determining the amount of impregnated evaporable foaming agent in the PVC resin as well as the resin's pore volume will appear below.

The PVC resin impregnated with evaporable foaming agent is heated in an air oven at 130°C for 2 hours, and the amount of impregnating agent is calculated according to the formula: (W^ - Wx 100 (%), where W 1 and W_ are the weight before and after heating, respectively .

Regarding the pore volume, the following method was used. The pore volume was determined using a model 70H mercury pressure porosimeter manufactured by CARL ERBA Co., where the mercury pressure was increased from 1 to 100 kp/cm 2 and expressed in ml per grams of resin.

Example 1. (Includes trials no. 1 - 13)

To a 5 l autoclave, which was equipped with stirrers, was added 1000 g of vinyl chloride homopolymer or a copolymer resin consisting of vinyl chloride and vinyl acetate as indicated in Table 1 below, where P and Vp stand for the average degree of polymerization and average pore volume, respectively in the resin,

2000 g purified water, 150 g trichlorofluoromethane and 200 g butane,

which was added by pressing it in. The temperature was then increased to 70°C with stirring, and at this temperature it was stirred for 8 hours to impregnate the resin with trichlorofluoromethane and butane as the evaporable foaming agents.

After cooling to room temperature and exhausting the excess

of foaming agents from the autoclave, the thus impregnated resin was taken out of the autoclave, dehydrated and dried in air flow at 40 to 50 °C for approx. 8 hours.

The amount of impregnating agent, i.e. foaming agent in the resin

was determined immediately after manufacture and after one week of storage at 20 °C, and the results are shown in Table 1.

100 parts of the resin obtained, which was impregnated with the foaming agents, was mixed with 2 parts of a tin-containing stabilizer and with 1 part of calcium stearate. From the resin mixture, a foam body in the form of a cylindrical rod was produced by extrusion molding with an extrusion machine under the conditions stated below.

The mass density of the foam body obtained in each of the experiments appears in the table.

The operating conditions for the extrusion machine were:

The foam body obtained in trial no. 9 was very fragile. Self

if the foam body obtained in test no. 11 had a high degree of expansion, then it had poor fire-retardant properties.

Example 2. (Includes trials no. 14 to 13)

To the same autoclave that was used in example 1, 1000 g of copolymer resin was added, which was composed of 88% by weight vinyl chloride and 12% by weight vinyl acetate and with a pore volume Vp of 0.010 ml/g and an average degree of polymerization P of approx. . 650, 2000 g purified water, 1.0 g partially saponified, polyvinyl alcohol and further one or two types of evaporable foaming agents as indicated in table 2, and in the quantities that also appear in table 2. The mentioned components are added, if necessary, by pressing , and the whole is then stirred for 8 hours at 70°C to impregnate the resin with the foaming agent.

The names of the evaporable foaming agents used

in this example is as shown below, and the designations will also be used in connection with the other examples.

PR: propane

PE: pentane

HE: n-hexane

TCFM: trichlorofluoromethane

MC: methyl chloride

BU: butane

MEC: methylene chloride

DCTFE: dichlorotetrafluoroethane

DCDFM: dichlorodifluoromethane

DCFM: dichlorofluoromethane,

TCE: 1,1,2-trichloroethane

TCDFE: tetrachlorodifluoroethane

ISO: isooctane

The amount of impregnation with the foaming agents. and the mass density of the foam body with a cylindrical rod shape, which was produced in the same way as stated in example 1, is shown in table 2. The foam bodies from trials no. 24 to no. 26 showed great shrinkage after casting.

Example 3 (Includes trial nos. 27 to 33)

To a 100 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 30 kg of the same copolymer resin of vinyl chloride and vinyl acetate as used in Example 2,

50 kg of purified water and 15 g of a partially saponified polyvinyl alcohol. Furthermore, a mixed foaming agent, which was composed of butane and trichlorofluoromethane in a ratio of 2:1, and in a quantity that is indicated in table 3 below, was added while pressing in. Stirring then followed for 8 hours at a temperature, which also appears from

the table, to impregnate the resin with the foaming agent. The dehydration and drying of the resin which was impregnated with the foaming agent was carried out in the same way as stated in example 1. Amount of impregnating agent used, i.e. foaming agent, shown in table 3-

Resin mixtures were prepared, whereby 100 parts of the resin obtained above impregnated with the foaming agent, 2 parts of a tin-containing stabilizer and 1 part of calcium stearate were used for each resin mixture. From the resin mixture, a foam body in the form of a plate was produced by extrusion molding with an extrusion machine, which was operated under the conditions stated below. The foam bodies thus obtained were subjected to determination of mass density, thermal conductivity measured at 20°C according to the method described in JIS A 1413 and compression strength measured at 20°C according to the method described in ASTM

D 1621. The results appear in table 3.

Operating conditions for the extrusion machine:

Example 4. (Includes trials no. 34 to no. 43)

The procedure for the production of copolymer resin of vinyl chloride and vinyl acetate, which was impregnated with a mixed foaming agent of trichlorofluoromethane and butane, was the same as stated in example 1, where the resin used had a content of vinyl acetate. The average degree of polymerization P and pore volume Vp is shown in table 4 below. The amount of impregnation with foaming agent, as determined immediately after production and after storage for one week at 20°C, is shown in table 4.

Expandable resin compositions were prepared, whereby in each case a mixture was used consisting of 100 parts of the copolymer resin prepared above impregnated with the mixed foaming agent, 2 parts of a tin-containing stabilizer and 1 part of calcium stearate together with or without the addition of 1 part talc as nucleating agent, and further 1 part of a decomposable foaming agent of an azodicarbonamide compound, which is obtained under the name Celmic 133 from Sankyo Kasei Co., Japan, as well as one of the acrylic resins E-I or E-2 was added and then in a amount as shown in table 4.

The acrylic resin that was used as a foam conditioner in the experiments and designated E-I or E-2 in Table 4 had the following composition: E-I: A copolymer resin composed of 90% by weight methyl methacrylate and 10% by weight ethyl acrylate, and which had a reduced viscosity of 10 dl/g at 25°C in 0.1 g/100 ml chloroform solution.

E-2: A commercial acrylic resin which was available under

trade name Paraloid K-120 from Rohm&Haas Co.

From the expandable resin compositions, foam bodies were produced by extrusion molding using the same extrusion machine and the same operating conditions as stated in Example 1, and the foam bodies were examined with regard to mass density and cell structure. The results appear in table 4. Assessment of the cell structure, as indicated in the table, was determined according to the following standards: Cell structure A: The diameter of the cells did not exceed 500 µm

Cell structure B: The diameters of the cells were from 500 µm to 2000 µm.

The foam body obtained in trial no. 40 was noticeably brittle. In trial No. 43, premature expansion of the resin composition took place. so that you got expansion in the nozzle, and this

led £il flow marks on the surface of the foam body.

Example 5. (Includes trials no. 46 to no. 56)

To the same autoclave as used in example 4 was added 1000 g of the same copolymer resin as used in example 2, which was composed of vinyl chloride and vinyl acetate, 2000 g of purified water, 1.0 g of partially saponified polyvinyl alcohol and one or a combination of two types of the evaporable foaming agents listed in table 5 below, and then in quantities that also

are indicated in this table. The foaming agent or foaming agents were added under pressure and then the mixture was subjected to stirring for 8 hours at 70°C to impregnate the resin with the foaming agent or foaming agents. The amount of impregnating agent or foaming agent used appears in table 5.

Each of the expandable resin compositions was prepared by mixing 100 parts of the copolymer resin impregnated with the evaporable foaming agent, 1 part (trial nos. 46 to 48) or 3 parts (trial nos. 49 to 56) nucleating agent, as indicated in Table 5, and then together with or without the addition of 6 parts acrylic resin (Metablen P551, which is a product of Mitshubishi Rayon Co., Japan) as a foam conditioner.

The nucleating agents used in the experiments were the following:

Orben: An organic complex of a colloidally hydrated aluminum silicate with an average particle diameter of approx.

0.5 µm, and the product is manufactured by Shiraishi Calcium Co., Japan.

Hakuenka 0: A calcium carbonate filler with an average particle diameter of 0.02 to 0.03 µm. The product is manufactured by Shiraishi Calcium Co., Japan.

Titanium dioxide A-100: A titanium dioxide filler with an average particle diameter of approx. 0.15 to 0.25 µm, and the manufacturer is Ishihara Sangyo Co., Japan.

Aerosil 200: A fumed silicon dioxide filler with a specific surface area of approx. 200 m 2/g and an average particle diameter of approx. 0.012 µm. A product of Nippon Aerosil Co., Japan. Aerosil 380: A fumed silicon dioxide filler with a specific surface area of approx. 380 m 2/g and an average particle diameter of approx. 0.002 um. A product of Nippon Aerosil Co., Japan.

A1203C: An alumina filler with an average particle diameter of approx. 0.005 to 0.02 µm. A product of Nippon Aerosil Co., Japan.

Barium Sulphate #100: A product of Sakai Chemical Co., Japan, and which has an average particle diameter of approx. 0.6 um.

Santenton No. 5: A clay product from Tsuchiya Kaolin Co., Japan.

3S talc: A talc product from Nitto Funka Kogyo Co., Japan.

From the thus produced expandable resin compositions, foam bodies in the form of a cylindrical rod were produced by extrusion molding with the same extrusion machine and using the same operating conditions as stated in example 4. Foam body mehe was examined with. consideration of mass density, and the results appear in table 5 below.

Example 6 (Includes trial nos. 58 to 71)

To a 100 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 30 kg of a copolymer resin, which was composed of 88% by weight vinyl chloride and 12% by weight vinyl acetate, and which had an average degree of polymerization P of about 850 and a pore volume Vp of 0.<*>015 ml/g. Furthermore, 50 kg of purified water were added, 15 g of a partially saponified polyvinyl alcohol, 6 kg of trichlorofluoromethane and a further 3 kg of butane were added under pressure. Then followed stirring for 8 hours at 70°C to impregnate the resin with trichlorofluoromethane and butane as the evaporable foaming agents. After completion of impregnation, cooling to room temperature and draining of excess foaming agent, the resin was dehydrated by centrifugation and drying in air flow at 40 to 50°C. The total amount of foaming agents with which the resin was impregnated was 11.8% by weight.

Each of the expandable resin compositions was prepared by mixing 100 parts of the above-mentioned copolymer resin impregnated with evaporable foaming agents, 2 parts of tin-containing stabilizer, 1 part of calcium stearate with or without the addition of talc (experiments except No. 62)

in an amount specified in table 6 below or in combination with 0.5 parts sodium bicarbonate and 0.4 parts citric acid (experiment no. 62) as a nucleating agent, a decomposable foaming agent and an acrylic resin as a foam conditioning agent as specified in table 6.

The designations used in table 6 for the decomposable foaming agents and acrylic resins are as follows:

AIBN: 3<-,o<J -azobisisobutyronitrile

PTS: p-toluenesulfonyl hydrazide

ATTENTION: 4,4'-oxy-bis(benzenesulfonylhydrazide)

DNM: dinitrosopentamethylenetetramine

Celmic 133: see example 4

E-3: A copolymer resin composed of 85% by weight methyl methacrylate, 10% by weight butyl acrylate and 10% by weight ethyl acrylate, and with a reduced viscosity of 5.5 dl/g at 25°C.

E-4: A copolymer resin composed of 85% by weight methyl methacrylate and 15% by weight butyl acrylate, and with a reduced viscosity of 5.0 dl/g at 25°C.

E-5: A commercial acrylic resin available under the trade name

Paraloid K-125 from Rohm&Haas Co.

E-6: A commercial acrylic resin available under the trade name Paraloid K-125 from Rohm&Haas Co.

From each of the resin compositions, a foam body in the form of a plate was produced by extrusion molding with the same extrusion machine and under the same operating conditions as stated in example 3. The foam bodies were examined with regard to mass density, cell structure, appearance, compressive strength and other mechanical properties as well as thermal conductivity. The determination of the compressive strength was carried out according to the method described in ASTM D 1621, and the thermal conductivity was determined according to the method described in JIS A 1413. The results appear in table 6.

In trials 70 and 71, a premature expansion of the resin composition took place, and then in the nozzle, which led to foam breakage and the appearance of flow marks on the surface of the foam body. The appearance of the foam body obtained in trial no. 69 was also less satisfactory. The cell structure of the foam bodies was satisfactorily good in all trials except trial no. 68, where the foam body showed less uniformity in the cell structure.

Example 7. (Includes trials no. 72 to no. 92)

To a 5 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 1000 g of homopolymer polyvinyl chloride resin or a copolymer resin composed of vinyl chloride and vinyl acetate, as indicated in Table 7, 200 g of purified water as well as one or a combination of two types of evaporable foaming agents, as indicated in table 7 and quantities that also appear in this table. The addition took place, if necessary, under pressure. Then followed stirring for 8 hours at 70°C to impregnate the resin with the evaporable foaming agent. After completion of the impregnation, cooling to room temperature and exhausting the excess of the foaming agents, the resin was dehydrated by filtration and dried in air flow at 40 to 50°C for approx. 5 hours. The resins thus obtained were examined with regard to the content of the amount of foaming agent, and the results appear in Table 7. Furthermore, the resins were allowed to stand for a week at 20°C to examine losses, i.e. loss during storage. The reduction in the amount of foaming agent lay

in the range from 6 to 9% for each of the resins.

Each of the expandable resin compositions was prepared by mixing 100 parts of the above-obtained resin impregnated with a volatile foaming agent, 2 parts of a tin-containing stabilizer and 1 part of calcium stearate together with or without the addition of a nucleating agent, a decomposable foaming agent and an acrylic resin ($-1) as a foam conditioner, as indicated in Table 7 below. From each of the resin compositions, a foam body in the form of a cylindrical rod was produced by extrusion molding. The operating conditions for the extrusion machine were as follows:

The foam bodies thus obtained were examined with regard to mass density and the nature of the cell structure. The results appear in table 7.

Regarding the designations for the evaporable and decomposable foaming agents, reference is made to previous examples.

The criteria for the cell structure A and B appear from example 4, and the cell structure C indicates that the cells of the foam have a diameter exceeding 1 mm, and the structure is rough and not uniform.

The foam body obtained in experiment no. 87 was noticeably brittle, and the foam body from trial no. 91 and No. 92 showed noticeable shrinkage after casting.

Example 8. (Includes trials no. 96 to no. 109)

To a 100 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 30 kg of a copolymer resin, which consisted of 90% by weight vinyl chloride and 10% by weight vinyl acetate, and which had an average degree of polymerization of 1050 and a pore volume of 0.023 ml/g, 50 kg of purified water, 15 g of a partially saponified polyvinyl alcohol and 6 kg of trichlorofluoromethane, and. further 3 kg of butane were added under pressure. Then followed stirring for 8 hours at 70°C to impregnate the resin with trifluoromethane and butane as evaporable foaming agents. After complete impregnation, cooling to room temperature and exhausting excess foaming agents, the resin was dehydrated by centrifugation and drying in an air stream at 40 to 50°C. The total amount of trichlorofluoromethane and butane in the thus impregnated resin was 12.0% by weight.

Each of the expandable resin compositions was prepared by mixing 100 parts of the above-prepared resin impregnated with evaporable foaming agents, 2 parts tin-containing stabilizer and 1 part calcium stearate together with or without the addition of 1 part talc as a nucleating agent, 0, 5 parts Celmic 133 as decomposable foaming agent and one of the acrylic resins E-7 to E-13 in a quantity ratio that appears in table 8. From the resin compositions, foam bodies in the form of a plate were produced using an extrusion machine, which was run under the conditions specified below.

The germination agent was omitted in experiment no. 106 and No. 107, and the decomposable foaming agent was omitted in trials 106 and No. 108.

Acrylic resins:

E-7: A copolymer resin which was composed of 90% by weight methyl methacrylate and 10% by weight ethyl acrylate, and which had a reduced viscosity of 4.5 dl/g at 25°C.

E-8: A copolymer resin composed of 90% by weight methyl methacrylate and 10% by weight ethyl acrylate, and which had a reduced viscosity of 7.0 dl/g at 25°C.

E-9: A copolymer resin composed of 90% by weight methyl methacrylate and 10% by weight ethyl acrylate, and which had a reduced viscosity of 11.0 dl/g at 25°C.

E-10: A copolymer resin composed of 90% by weight methyl methacrylate and 10% by weight ethyl acrylate, and which had a reduced viscosity of 15.3 dl/g at 25°C.

E-ll: A copolymer resin composed of 95% by weight methyl methacrylate and 20% by weight ethyl acrylate, and which had a reduced viscosity of 10.7 dl/g at 25°C.

E-12: A copolymer resin composed of 80% by weight methyl methacrylate, 5% by weight ethyl acrylate, 5% by weight butyl acrylate and .10% by weight butyl methacrylate, and which had a reduced

viscosity of 11.0 dl/g at 25°C.

E-13: A copolymer resin composed of 80% by weight methyl methacrylate and 20% by weight ethyl acrylate, and which had a reduced viscosity of 2.0 dl/g at 25°C.

Operating conditions for the extrusion machine:

The foam bodies thus obtained were examined with regard to density, cell structure, compressive strength, which was determined according to the method described in ASTM D 1621, and flexural strength, which was determined according to the method described in ISO R 1209. The results appear from table. 8.

In experiment no. 104 and no. 105, a premature expansion of the resin composition took place, and then while this was still in the die. This led to a large breakage of foam as well as shrinkage of the foam body after casting and a less uniform cell structure. The foam bodies obtained in experiments No. 108 and No. 109 also had a less uniform cell structure, even if premature expansion of the resin composition had not taken place.

As will be understood from the results of the table below, an acrylic resin with a high reduced viscosity can provide advantages with regard to the possibility of reducing the amount of acrylic resin as well as increased retention of gas, which is needed to build up foam. Furthermore, such an acrylic resin will exhibit stable foam cells and reduced shrinkage. When an acrylic resin with a slightly reduced viscosity is used or when the amount of acrylic resin is insufficient, on the other hand, the foam will break to a large extent, and you get a large shrinkage of the foam after casting as well as a coarser cell structure in the foam.

Example 9 (Includes trials no. 110 to no. 117)

To a 10 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added .3 kg of a homopolymer polyvinyl chloride resin or a copolymer resin of vinyl chloride and vinyl acetate, where the content of vinyl acetate in the resin is indicated in Table 9 below, and which resin had an average degree of polymerization and a pore volume which is also indicated in the table. Furthermore, 5 kg of purified water, 1.5 g of a partially saponified polyvinyl alcohol and 600 g of trichlorofluoromethane were added and further 300 g of butane were added under pressure. Then followed stirring for 8 hours at 70°C to impregnate the resin with trichlorofluoromethane and butane as evaporable foaming agents. After completion of impregnation, cooling to room temperature and draining of excess foaming agents, the resin was dehydrated by filtration and dried in an air stream at 50°C for 5 hours. The total amount of foaming agents in the thus impregnated resin is indicated in the table.

Each of the expandable resin compositions was prepared

by mixing 100 parts of the resin obtained above, which was impregnated with the evaporable foaming agents, 2 parts of tin-containing stabilizer, 1 part of calcium stearate, 1 part of talc as a nucleating agent and 10 parts of one of the acrylic resins E-7,^E- 10

and E-13 (see Example 8) as foam conditioner. From the resin compositions, foam bodies in the form of a cylindrical rod were produced in the manner indicated in example 7. Mass density and the nature of the cell structure in these foam bodies can be seen from table 9. Those in experiment no. 115 and No. 116 the obtained foam bodies showed some shrinkage after molding.

When the vinyl chloride-based resin has a high degree of polymerization, it will be understood from the results from Table 9 that the use of an acrylic resin with a correspondingly greatly reduced viscosity will be desirable. The foam bodies with high expansion and uniform cell structure can also be obtained with a resin with a lower content of vinyl acetate, which otherwise requires a relatively high production temperature, by choosing a suitable acrylic resin as foam conditioner.

Example 10 (Includes trials no. 118 to no. 133)

To a 5 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 1000 g of a homopolymer polyvinyl resin or copolymer resin of vinyl chloride and vinyl acetate, whereby the content of vinyl acetate is indicated in the following table 10 , and which resin had an average degree of polymerization and a pore volume as indicated in the same table.Furthermore, 2000 g of purified water, 1.0 g of a partially saponified polyvinyl alcohol and 150 g of trichlorofluoromethane were added, and further 100 g of butane was added under pressure. This was followed by stirring for 8 hours at 70°C to impregnate the resin with trichlorofluoromethane and butane as evaporable foaming agents. After complete impregnation, cooling to room temperature and exhausting excess foaming agents, the resin was dehydrated by filtration and dried in an air stream at 40 to 50°C for about 5 hours.

The total amount of foaming agents in the resin, which served as an impregnating agent, was determined and the results appear in Table 10. The loss of foaming agents by shrinkage during storage at 20°C for one week was from 6 to 9% for each resin.

Next, the expandable resin compositions were prepared by mixing 100 parts of the resin obtained above, which had been impregnated with foaming agents, 2 parts of tin-containing stabilizer and 1 part of calcium stearate together with or without the addition of a nucleating agent of the type indicated in table 10, and in the amount shown in the same table. Furthermore, a decomposable foaming agent was added, which is also indicated in the table, as well as a copolymer resin S-1, which is composed of 70% by weight styrene and 30% by weight acrylonitrile, and which has a reduced viscosity of 12.0 dl/g at 25°C, as one styrene-based foam conditioning resin, and in an amount indicated in the table. From the resin compositions, foam bodies were made in the form of a cylindrical rod by extrusion molding. The operating conditions for the extrusion machine were essentially the same as stated in section 7. The foam bodies thus obtained were not increased with respect to density and cell structure, and the results appear in table 10.

Example 11. (Includes no. 134 to no. 143)

To a 5 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 1000 g of a copolymer resin, which was composed of 90% by weight vinyl chloride and 10% by weight vinyl acetate, and which had an average degree of polymerization of 850 and a pore volume of 0.015 ml/g. 2,000 g of purified water and 1.0 g of a partially saponified polyvinyl alcohol as well as one or a combination of two types of evaporable foaming agents, which were added or fed under impingement as indicated in Table 11. This was followed by stirring at 70°C for 8 hours to impregnate the resin with the foaming agent.

After completion of impregnation, cooling to room temperature and emptying of excess foaming agent, the resin was dehydrated by centrifugation and dried in an air stream. The amount of foaming agent with which the resin was impregnated is shown in table 11.

Each of the expandable resin compositions was prepared by mixing 100 parts of the above-obtained resin, which was impregnation, of the foaming agent, with talc as a nucleating agent, Celmic 133 as a decomposable foaming agent, and styrene-based copolymer resin S-1 as a foam conditioner, and the quantities of each component are given in table 11. From the resin compositions, foam bodies were produced in the same way as in the previous example. Mass density of the thus obtained foam bodies is shown in table 11. The foam bodies in experiment no. 141 and No. 142 show significant shrinkage after casting.

Example 12. (Includes trials no. 144 to 150)

To a 100 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 30% of a copolymer resin, which was composed of 90% by weight vinyl chloride and 10% by weight vinyl acetate, and which had an average degree of polymerization of 1050 and a pore volume of 0.023 ml/g, 50 kg of purified water, 15 g of a partially saponified polyvinyl alcohol and 6 kg of trichlorofluoromethane, and a further 3 kg of butane were added under impregnation. Then followed stirring at 70°C for 8 hours to impregnate the resin with trichlorofluoromethane and butane as the evaporable foaming agents. After completion of impregnation, cooling to room temperature and draining of excess foaming agents, the resin was dehydrated by centrifugation and dried in an air stream at 40 to 50°C. The total amount of foaming agent with which the resin was thus impregnated was 12.0% by weight.

Each expandable resin composition was prepared by mixing 100 parts of the copolymer resin obtained above, which was impregnated with the foaming agents, 2 parts tin-containing stabilizer, 1 part calcium stearate, 1 part talc as a nucleating agent, 0.5 parts Celmic 133 as a decomposable foaming agent and one of the styrene-based copolymer resins S-2 to S-5 as a foaming agent, which resins are described below, and the amounts of which are indicated in table 12 below. Foam bodies were produced from these resin compositions in the form of a plate by extrusion molding with an extrusion machine, which was run under the same operating conditions as indicated in Example 8.

The foam bodies thus obtained were examined with regard to mass density, cell structure, compressive strength and flexural strength, and the results appear in table 12. In trials 149 and no. 150, a premature expansion of the resin composition took place, and while this was still was in the die, which led to foam breakage as well as significant shrinkage of the foam bodies after casting.

The composition of the styrene-based copolymer resins:

S-2: A copolymer resin composed of 70% by weight styrene and

30% by weight of acrylonitrile, and which has a reduced viscosity of 2.0 dl/g at 25°C.

S-3: A copolymer resin composed of 70% by weight styrene and

30% by weight of acrylonitrile, and which has a reduced viscosity of 4.0 dl/g at 25°C.

S-4: A copolymer resin composed of 70% by weight styrene and

30% by weight of acrylonitrile, and which has a reduced viscosity of 10.0 dl/g at 25°C.

S-5: A copolymer resin composed of 75% by weight styrene and

25% by weight of acrylonitrile, and which has a reduced viscosity of 14.6 dl/g at 25°C. As Table 12 shows, a styrene-based copolymer resin with a more reduced viscosity than the foam conditioner can result in benefits such as improved gas retention or foam building, foam stabilization, and reduced shrinkage even when the amount of resin incorporated is relatively small, while, on the other hand, the use of resin with a less reduced viscosity leads to foam breakage and increased shrinkage after casting, which results in a coarser cell structure, and then especially when the added amount is insufficient.

Example 13. (Includes trials no. 151 to no. 156)

To a 10 liter autoclave, which was made of stainless steel and equipped with a stirrer, was added 3 kg of a homopolymer polyvinyl chloride resin or a copolymer resin of vinyl chloride and vinyl acetate, whereby the content of vinyl acetate is indicated in the following table 13 , and which had an average degree of polymerization and a pore volume which also appears from the table, 5 kg of purified water, 1.5 g of a partially saponified polyvinyl alcohol, 600 g of trichlorofluoromethane and further 200 g of butane, which were added during impregnation. Then followed stirring at 70°C for 8 hours to impregnate the resin with trichlorofluoromethane and butane as the evaporable foaming agents. After completion of impregnation, cooling to room temperature and draining of excess foaming agent, the resin was dehydrated by filtration and dried in air flow at 50°C for 5 hours. The total amount of foaming agent, which served as an impregnating agent, is shown in table 13.

The expandable resin compositions were each prepared by

mixing 100 parts of the resin obtained above, which was impregnated with evaporable foaming agents, 2 parts tin-containing stabilizer, 1 part calcium stearate, 1 part talc as nucleating agent, 0.5 parts Celmic 133 as decomposable foaming agent. and styrene-based resin of the type that appears in the table as a foam conditioner, and in an amount that also appears in the table. From the resin compositions, foam bodies were produced by extrusion molding in the same way as stated in example 10. Mass density and the nature of the cell structure of the foam bodies can be seen from table 13. The foam body according to experiment no. 155 showed some degree of shrinkage during casting. At trial no. 156 there was significant foam breakage and the foam bodies in this experiment were subject to a strong degree of shrinkage after casting.

As can be seen from the results in table 13, the styrene-based resin as a foam conditioning agent should ideally have a more reduced viscosity when the vinyl chloride-based resin has a relatively low degree of polymerization. A foam body with high expansion and with a uniform cell structure can be obtained even with a vinyl chloride resin, which contains nothing or a relatively small amount of vinyl acetate, and which otherwise requires a relatively high production temperature when a suitable foam conditioner is selected.

On the other hand, a copolymer resin with a low degree of polymerization can provide a foam body with high expansion even with a styrene-based resin as a foam conditioner, thereby obtaining a

slightly reduced viscosity when the content of vinyl acetate in the copolymer resin is high. However, a foam body can . with high expansion is difficult to obtain with a copolymer resin where the content of vinyl acetate is low.

Claims (14)

1. A resin composition which is expandable to form a foam body upon heating, characterized in that it consists of (a) 100 parts by weight of a polyvinyl chloride-based resin with an average degree of polymerization not exceeding 2000 and a pore volume not exceeding 0.20 ml/g, and (b) at least 1 part by weight of an evaporable foaming agent selected from the group consisting of aliphatic hydrocarbon compounds and aliphatic, halogenated hydrocarbon compounds with a boiling point not exceeding 90°C, which constitutes impregnating agent for said polyvinyl chloride resin. 2. A resin composition according to claim 1, characterized in that it further contains (c) from 0.5 to 30 parts by weight of a foam conditioning agent selected from the group consisting of acrylic resins and on styrene-based resins and (d) from 0.01 to 20 parts by weight of a nucleating agent. 3. A resin composition according to claim 1 or 2, characterized in that the polyvinyl chloride-based resin is a copolymer resin consisting of from 60 to 97% by weight vinyl chloride and from 40 to 3% by weight vinyl acetate. 4. A resin composition according to claim 1 or 2, characterized in that the polyvinyl chloride-based resin has an average degree of polymerization of at least 300. 5. A resin composition according to claim 2, characterized in that the foam conditioner has a reduced viscosity of at least 3.0 dl/g, which viscosity is measured in chloroform solution with a concentration of 0.1 g/100 ml at 25°C. 6. A resin composition according to claim 2, characterized in that the nucleating agent is an inorganic powder material with an average particle diameter that does not exceed 30 µm. 7. A resin composition according to claim 2, characterized in that the nucleating agent is a combination of an acid, which is a solid at room temperature, and a carbonate or bicarbonate of sodium, potassium or ammonium. 8. A resin composition according to claim 7, characterized in that the acid, which is a solid at room temperature, is boric acid or an organic acid selected from the group consisting of citric acid, tartaric acid and oxalic acid. 9. A resin composition according to claim 2, characterized in that the acrylic resin is a polymethyl methacrylate. 10. A resin composition according to claim 2, characterized in that the acrylic resin is a copolymer resin consisting of methyl methacrylate and at least one acrylic ester selected from the group consisting of alkyl acrylates and alkyl methacrylates excluding methyl methacrylate. 11. A resin composition according to claim 10, characterized in that the content of methyl methacrylate in the copolymer resin is in the range from 60 to 95% by weight. 12. A resin composition according to claim 2, characterized in that it. on the styrene-based resin is a copolymer resin consisting of from 90 to 40% by weight of styrene and from 10 to 60% by weight of a comonomer which. is copolymerizable with the styrenes 13. A resin composition according to claim 12, characterized in that the comonomer which is copolymerizable with styrene is acrylonitrile. 14. A resin composition according to claim 2, characterized in that it further consists of a decomposable foaming agent in an amount that does not exceed 5 parts by weight.