CA2183261A1

CA2183261A1 - Thermoplastic resin foam and method of production thereof

Info

Publication number: CA2183261A1
Application number: CA 2183261
Authority: CA
Inventors: Tomoyuki Kobayashi; Kenji Miyazaki; Michiaki Sasayama
Original assignee: Individual
Current assignee: Sekisui Chemical Co Ltd
Priority date: 1994-12-14
Filing date: 1994-12-14
Publication date: 1996-06-20

Abstract

An object of the present invention is to provide a foam molding which is light, excellent in bending strength, and excellent in balance between compressive strength and com-pression permanent strain, being a foam in which thermoplas-tic resin high expanded foams whose overall outer surfaces are covered with thermoplastic resin low-expanded-foam thin layers are thermally fused with each other through the aforementioned low-expanded-foam thin layers.

Description

1--~ 21~3261 SPECIFICATION
Thermoplastic Resin Foam and Method of Preparing the Same Technical Field The present invention relates to a thermoplastic resin foam and a method of preparing the same.
Background Technique A thermoplastic resin foam, which is light and excel-lent in heat insulation, flexibility and formability etc., is widely employed in various heat insulators such as a roof insulator or a floor insulator, a cushioning material, and various interior materials such as an interior material for a vehicle.
In general, a metho~ carrying out a step of pre-expand-ing thermoplastic reslns for obtaining expandable beads and then carrying out a step of charging the expandable beads in a mold for secondarily expanding the same and fusing the beads with each other thereby obtaining a molding is known as a method of preparing the aforementioned thermoplastic resin foam.
In the foam molding which is obtained by the aforemen-tioned method of preparing a thermoplastic resin foam, however, strength, particularly bendin~ strength is disad-vantageously insufficient. In the bead foam molding which is obtained from the aforementioned expandable beads, for example, a non-expandable thin film is formed on the surface 2183~61 of each bead. Therefore~ high compressive strength can be expected in the foam molding which is obtained by this method. In the bead foaM molding, however, the aforemen-tioned expandable beads are charged in a mold and thereafter 5 secondarily expanded and fused with each other to be formed, and hence fusion power between the beads is low. When a bending load is applied to the foam molding, therefore, separation and/or breakage are readily caused in the fusion interfaces between the beads, leading to insufficient bend-10 ing strength.
Japanese Patent Laid-Open No. 4-16330 discloses a method of improving the bending strength of a foam molding which is obtained by the aforementioned bead expanding method. In the method disclosed in this prior art, expan-15 dable beads which are obtained by pre-~7rr~n~; n~ thermoplas-tic resins are compressively charged in a mold and there-af ter ~ nS steam having a pressure which is higher by at least O . 2 kg/cm2 than that in the mold is introduced into the mold for secondarily ~lrr~n~1in~/fusing the same, thereby 20 improving fusiorl power between the beads. In the method descrlbed in this prior art, however, the foam molding obtained by forming is pressurized by the /~ .CR I ng steam.
In order to prevent reduction of steam flows between the expandable beads resulting ~rom excess ~compression, there-25 fore, the pressure of the degassing steam cannot be much increased. Thus, the fusion power between the beads cannot be suf f iciently increasea, and hence it has been still difficult to obtain a foam molding having sufficient bending strength .
Also in the method described in the aforementioned prior art, in addition, two steps~including that of pre-expandlng the expandable beads and secondarily F~r~nflin~ and fusing the pre~ rr;~nfl~fl ~xpandable beads in the mold must be executed, leading to a problem of low productivity.
Ob ~ects of the Invention A principal object of the present invention is to provide a thelmoplastic resin foam which is light, ,~x,-,11 f~nt in bending strength, and excellent in balance between com-pressive strength and compression permanent strain, and a method of preparing the same.
Another ob~ ect o the present invention is to provide a method of preparing a thermoplastic resin foam having homo-geneous and fine cells therein.
Disclosure of the Invention According to a wiae aspect of the present invention, provided is a foam which is characterized in that high expa-nded foams consisting of thermoplastic resins whose overall outer surfaces are covered with low-P-~r~nfl~fl-foam thin layers consisting of thermoplastic resins are ~h~rr~l ly fused through the low-expanded-foam thin layers.

1--~ 2183261 ., In the foam according to the present invention, expand-able thermoplastic resin pellets or annular substances are expanded and thermally fused with each other, as described later. At this time, voids may be formed between the low-5 expanded-foam thin layers, or holes may be formed as the case may be. However, t~le sizes thereof are generally not more than 7 mm, preferably not more than 5 mm, more prefera-bly less than 3 . 5~ mm, depending on the expansion ratios of the low-expanded-foam thin layers and the high expanded 10 foams, since homogeneity in thickness and shape holdability are inferior and various physical proper-ties are also re-duced if the same are oversized.
According to another wide aspect of the present inven-tion, provided is a method of preparing a foam comprising a 15 step of disseminating expandable thermoplastic resin pellets or annular substances containlng a foaming agent, and a step of expanding the l~xr~nflAhl e thermoplastic resin pellets or annular substances by heating the same to a temperature ,~xr~ flln~ the foaming temperature of t~le foaming agent which 20 is contained in the expandable thermoplastic resins.
The method of preparing a foam and the foam according to the present invention are now described in order.
Thermoplastic Resin In the method of preparing a foam according to the 25 present invention, expAnflAhl e thermoplastic resin pellets or '1--` 2183~
annular substances containing a foaming agent are first ~iclspm;ni~ted~ As to the thermoplastic resln forming the expandable thermoplastic resins containing a foaming agent, an arbitrary thermoplastic resin can be employed so far as 5 the same ls an expandable thermoplastic resin.
As to such an expandable thermoplastic resln, olefin-based resin such as low density polyethylene, high density polyethylene, linear low density polyethylene ( it is herein-after assumed that "polyethylene" ;n. lll~lPq low density lO polyethylene, high density polyethylene, linear low density polyethylene, or a mixture thereof ), random polypropylene, homopolypropylene and block polypropylene ( it is hereinafter assumed that "polypropylene" includes random polypropylene, homopolypropylene, block polypropylene with an extremely 15 small amount, generally not more than 5 percent by weight, of ethylene, or a mixture thereof ), polyvinyl chloride, chlorinated polyvinyl chloride, ABS resin, polystyrene, polycarbonate, polyamide, polyvinylidene fluoride, polyphenylene sulfide, polysulfone, polyether ether ketone 20 and copolymers thereof can be listed, for example, and these thermoplastic resins may be employed; nriPrPn~Pntly~ or two or more such materials may be ~ ; nP~l with each other.
Among the aforementioned thermoplastic resins, the olefin-based resin such as polyethylene or polypropylene, or 25 a mixture thereof is preferable in order to improve

2~3261 .
thermoformability of the obtained foam, and a mixture of high density polyethylene and homopolypropylene is particu-larly preferably employed, in particular.
It is necessary tha~ the thermoplastic resins for 5 forming the low-expanded foam thin layers and the thermo-plastic resins for forming the aforementioned high foams are the same type of resins.
Further, the aforementioned thermoplastic resins may be crosslinked at need. It is preferable to employ crnqql ;nkP~l 10 thermoplastic resins since the expansion ratios are in-creased and hence the obtained foam can be lightened while thermostability can also be improved in this case.
If the degree of crossl ;nkin~ of the aforementioned thermoplastic resins iB high, the expansion ratios are 15 reduced and thermofnrn~hl l; ty is reduced . When the degree of crosslinking of the thermoplastic resins is low, on the other hand, thermostability is reduced and cells are broken in P~rr;~nc;nn, and hence it is 1mrn~R;hlP to obtain homoge-neous foaming cells. rherefore, the aforementioned degree 20 of crn~l ;nk;n~ is preferably lO to 30 percent by weight in gel fraction serving as the index for the degree of crosslinking, and more preferably in the range of 10 to 20 percent by weight.
A method of crosq1 ;nk;n~ the aforementioned thermoplas-25 tic resins is not particular1y restricted, but ( 1 ) a ~ . ~
2183~61 crosslinking method employing an electron beam, ( 2 ) acroc~l~nk;n3 method employing an organic peroxide, and (3) a method of melting and kn~ ing silane modified thermoplastLc resins in/with thermoplastic resins and thereaf ter carrying 5 out a ~ater treatment fo~ cro~cl inkin~ can be listed, for example. In particular, the crosslinking method (3) employ-ing silane modified thermoplastic resins are preferably employed .
(1) The aforementioned method of cro~ nkin~ the 10 thermoplastic resins by an organic peroxide is described.
The aforementloned organic peroxide is not particularly restricted but dibutyl peroxide, dicumyl peroxide, tertiary butylcumyl peroxide, diisopropyl peroxide etc. can be list-ed, for example, 'while dicumyl peroxide and tertiary butyl-15 cumyl peroxide are preferable, and dicumyl peroxide ispreferable in particular.
The amount of the organic peroxide is preferably 0. 5 to 5 parts by weight, more preferably 1 to 3 parts by weight, with respect to 100 parts by weight of the thermoplastic 20 resins, since resin decomposition reaction progresses to color the obtained foam if the same is large while cr--c~l ~nkln~ of the thermoplastic resins is insufficient if the same is small.
( 2 ) The aforementior,ed method of applying an electron 25 beam for croecl inkin~ the thermoplastic resins is described.

~18326:~
The amount of application of the electron beam is preferably 1 to 20 Mrad, and 3 to 10 Mrad is particularly preferable since the expansion ratio of the obtained foam is reduced due to excess application of cr~ql ink;n!J if the 5 same is large while ther~ostability is reduced and foaming cells are so broken that homogeneous foaming cells cannot be obtained if the same is small.
The method of applying the electron beam is not partic-ularly restricted, but a method of employing two electron 10 beam generators for passing the thermoplastic resins there-between thereby irradiating the thermoplastic resins with the electron beam can be listed, for example.
Then, (3) the cros,e:llnk;n~ method employlng a silane modified thermoplastic resin, which is most preferable, is 15 descrlbed.
The aforementioned silane modified thermoplastic resins can be employed with no particular restriction so far as the same are employed in general. As to such silane modified thermoplastic resins, silane modified thermoplastic resins 20 of polyethylene, silane modified thermoplastic resins of polypropylene, silane modified thermoplastic resins of an ethylene-vinyl acetate copolymer, and silane modif ied ther-moplastic resins of polystyrene can be listed, for example.
Due to high f~xri~n.1iqh~ 1; t~, the silane modified thermoplastic 25 resins of poIyethylene, the silane modified thermoplastic resins of polypropylene and silane modified thermoplastic resins of polystyrene are preferable, while the silane modiiiQd thermoplastic resins of polyethylene and the silane modified thermoplastic resins of polypropylene are more 5 preferable.
The silane modified thermoplastio resins are prepared by graft-modifying thermoplastic resir~s with an unsaturated silane compound, for example.
The aforementioned unsaturated silane compound indi-10 cates a compound which is expressed in a general formula RlSiRZmY3-m -In the formula, organic functional groups such as alkenyl groups such as a vinyl group, an allyl group, a propenyl group, a cyclohe~enyl group etc.; a glycidyl group;
15 an amino group, a methacryl group; and halogenized alkyl groups such as a y-chloroethyl group and a y-bromoethyl group can be listed as the aforementioned Rl.

In the formula, R2 represents an aliphatic saturated hydrocarbon group or an aromatic hydrocarbon group, and a 20 methyl group, an ethyl group, a propyl group, a decyl group or a phenyl group can be listed, for e2~ample.
Further, m represents 0, 1 or 2.
In the formula, Y represents a hydrolyzable organic group, and a methoxy group, an ethoxy group, a formyloxy _ g _ ~ 32~1 group, an acetoxy group, a propionoxy group, an alkyl group or an arylamino group can be listed, for example, while Y
may be identical to or different from each other when m is 0 or 1.
As to the aforementioned unsaturated silane compound, that expressed in a general formula CH2 = CHSi(OA)3 is pref-erable. In the formula, A is a hydrocarbon group preferably having a carbon number of 1 to 8, more preferably 1 to 4, and vinyl trimethoxysilane, vinyl triethoxysilane, vinyl triacetoxysilane etc. can be listed as preferable unsaturat-ed compounds, for example.
When the aforementioned silane modified thermoplastic resins have methoxy group, the methoxy group come into contact with water and hydrolyzed, to form hydroxyl group.
The hydroxyl group and hydroxyl group of other mole-cules react with each other to form Si-O-Si bonds, whereby silane modified thermoplastic resins are crncq-l ;nkPrl with each other. At this tims, a silane croqql ;nk;n~ catalyst is preferably employed.
The gel fraction of the silane modified thermoplastic resins after croqql; nk; n~ is preferably in the range of 60 to 85 percent by weight since the crosql i nki n~ density is reduced to reduce exp~n(l;~h; l; ty of the expandable thermo-plastic resins if the same is reduced, and more preferably in the range of 70 to 80 percent by weight, in a viewpoint 2183~1 of illlpLUV~ t of ~xr~nq~on stability.
The gel fraction in the present invention indicates weight percentage of the residual weight of the resins af ter dipping in xylene of 120C for 24 hours to the weight of the resins befora the xylene dipping.
The silane modified thermoplastic resins preferably have such a melt index tl~at difference between the same and the melt index of the thermoplastic resins ln and with which the silane modified thermoplastic resins are melted and kneaded is not more than 15 g/10 min.
This is because the silane modified thermoplastic resins cannot be homogeneously dissolved in the thermoplas-tic resins if the difference is greater than 1 g/10 min.
The amount of the silane modified thermoplastic resins is preferably in the range of 1 to 50 parts by weight, more preferably 5 to 40 parts by weight, and further preferably 10 to 30 parts by weight, with respect to 100 parts by weight of the thermoplastic reslns, since cr~-qql ;nkin$
density is increased to reduce the expansion ratio of the obtained f oam and deteriorate lightweightness if the amount is large while an ~xr~n-l; hle resin composition has no shear viscosity which is necessary for expansion in heat-expansion and expansion stability is reduced if the amount is too small .
In order to crosslink the thermoplastic resins by the 32~ I
silane modified thermoplastic resins, a silane cr~sq~;nkin~
catalyst is employed at l~eed.
The aforementioned silane cr~csl inkin~ catalyst is not particularly restricted so far as the same facilitates cro.c~l; nk; n~ reaction between silane modified thermoplastic resins, and dibutyl tin diacetate, dibutyl tin dilaurate, dioctyl tin dilaurate, tin octanoate, tin oleate, lead octanoate, zinc 2-ethylhexanoate, cobalt octanoate, lead naphthenate, zinc caprylate, zinc stearate etc. can be listed, for example.
The amount of the aforementioned silane cro.cc:7 ;nk;ns catalyst is preferably in the range of 0 . 001 to 2 . 5 parts by weight, more preferably 0 . 01 to 2 parts by weight, further preferably 0 . l to l . 5 parts by weight with respect to 100 parts by weight of the thermoplastic resins since expan-dability of the expandable thermoplastic reslns ls reduced if t~le amount is increased, while the cro~l ;nking reactlon rate of the silane modified thermoplastic reslns is reduced i f the amount is reduced .
A method of adding the silane modified thermoplastic resins to the thermoplastic resins is not particularly restricted so far as the same is a method capable of homoge-neously adding the resins, and a method of supplying thermo-plastic reslns and silane modified thermoplastic resins to a single screw or twin screw extruder and then solvent-knead-~ 21~3261 ing the same, a method of solvent kn~A~in~ with a roll, and a method of solvent knf~A~lin~ with a kneader can be listed, for example Methods of water treatments include a method of expos-5 ing the resins to water vapor in addition to a method ofdipping the resins in the water, and in this case, the methods may be carried o~lt under pressurization when the treatmants are made at temperatures which are higher than 100C.
The temperatures of the water and the water vapor for the water treatments are preferably 50 to 130C, and 90 to 120C are particularly preferable since the cro~link;n~
reaction rate is reduced if the temperatures are low while the expandable thermoplastic resin pellets or annular sub-15 stances are fused with each other if the same are high.
The time for the water treatment is preferably 5 min-utes to 12 hours since the crosslinking reaction may not completely progress if the time is short while the expand-able thermoplastic resin pellets or annular substances are 20 fused with each other if the same is long.
Foaming Agent The aforementioned foaming agent is not particularly restricted so far as the same has a foaming temperature which is higher than the melting temperatures of the em-25 ployed thermoplastic resins, and inorganic pyrolytic foaming -~-- ~18326 7 agents such as sodium bicarbonate, ammonium carbonate, ammonium bicarbonate, an azldo compound and sodium borohydride; and organic pyrolytic foaming agents such as azodicarbonamide, n7:~h~ S~ cobutyronitrile, N, N ' -dinitroso-pentamethylenetetramine, P, P ' -dinitrosopentamethylene-tetramine, P,P'-oxybisbenzenesulfonylhydrazide, barium azodicarbonate and trihydrazinotriazine can be listed, for example, while azodicarbonamide which is f~rP~ nt in sani-tary is preferable due to simplicity in adjustment of the decomposltion temperature and the decomposition rate and a large amount of gas generation. In the present invention, "foaming temperature" indicates the decomposition tempera-ture of the d~c , ~ tion type foaming agent.
The amount of the foaming agent is preferably 1 to 25 parts by weight, and more preferably assumed to be 5 to 15 parts by weight with respect to lO0 parts by weight of the thermoplastic reslns, slnce no homogeneous cells are formed due to cell breaking if the same is large while no expansion is attained if the same is small.
Preferred Example of Expandable Thermoplastic Resin In the preparation method according to the present invention, f~-rr~n~l~hl e thermoplastic resin pellets or annular substances containing the aforementioned foaming agent are employed, while those prepared by hl~n~1in~ two types of non-croq~1~ nk~ thermoplastic resins which are hardly compa ible with each other and a sllane modified thermoplastic resln composltion employing thermoplastic resins of the same type as one of the non-crrlcql ;nkP~1 thermoplastlc reslns are preferably employed as the aforementioned expandable thermo-5 plastic resins. Since the rsslns contain the two types ofnon-croqql inkP~ thermoplastic resins which are hardly com-patible with each other, a island-sea structure descrlbed later is formed when the resins are crr~qql inkPrl by the aforementioned sllane modified thermoplastic resins.
When the reslns contain the aforementioned two types of non-crosslinked thermoplastic resins which are hardly com-patible with each other, they are preferably mixed with each other in the range of 3:7 to 7:3 in weight ratio, more preferably in the range of 4:6 to 6:4, and further prefera-15 bly in the range of 5:5, in order to obtaln a foam of a high expansion ratio having e~cellent surface smoothness ln whlch one ls homogeneously dispersed in the other.
When the dif ference between melt index of the afore-mentioned two types of non-crosslinked thermoplastic resins 20 is increased, an extremely rough lsland-sea structure is formed and a foam of a high expansion ratio cannot be ob-talned. When the difference between the melt index is reduced, on the other hand, no homogeneous island-sea struc-ture is formed and a foam of a high expansion ratio cannot 5 be obtained. Therefore, the aforementioned difference ~ 21~3~6~
between the melt indices is preferably in the range of 3 to 25 g/10 min. so that homogeneous bubble structures having fine grain sizes can be implemented, while 5 to 13 g/lO min.
are more preferable and 7 to 11 g/10 min. are further pref-erable in order to attain a foam of a higher expansion ratio .
In the method of preparing a foam according to the present invention, more l?referably, P- r~n~hle thermoplastic resins in which l to 50 parts by weight of the aforemen-tioned silane modified thermoplastic resins which are of the same type as one non-cr~q~l; nkf~l thermoplastic resin of the two types of non-crosslinked thermoplastic resins, the aforementioned silane crosslinking catalyst and the afore-mentioned foaming agent are blended with lO0 parts by weight of the thermoplastlc reslns containing the aforementioned two types of non-cro~cl i nki:.rl thermoplastic resins are em-ployed .
Since the aforementioned two types of non-croc:cl i nk thermoplastic resins are hardly compatible each other, an ~n~l~ ly micro island-sea structure in which one non-cro~ nkl:~rl thermoplastic resin is homogeneously and finely dispersed in the other non-croc~l i nkf~ thermoplastic resin is taken when the aforementioned expandable thermoplastic resin composition is blended by an extruder or the like.
Further, the silane modified thermoplastic resins ` 2~3261 employing the thermoplastic resins of the same type as the one non-crn~sl inkP~ thernoplastic resin are preferentially dissolved in the one non~croqcl ;nkP~ thermoplastic resin since the thermoplastic resins are of the same types. In 5 addition, the silane modified thermoplastic resins can be homogeneously diffused in the expandable resln composition whether the same are dissolved in the non-cro~f~l i nk~ ther-moplastic resin forming the sea or those forming islands, since the two types of t~lermoplastic resins take the ex-lO tremely micro island-sea structure.
When silane modified thermoplastic resins of the same type as the non-crosslinked thermoplastic resin forming the sea are employed so that the silane modified thermoplastic resins are preferentially dissolved in the non-crnsel ink 15 thermoplastic resln forming the sea, the silane modified thermoplastic resins are croscl inkP~ with each other by performance of a water treatment so that a crnscl inkPd structure is preferentially introduced into a continuous layer ( sea part ) and the sea which is the continuous layer 20 extends in expansion, whereby the aforementioned expandable thermoplastic resins have melt viscosity which is suitable for ~ r~nq~ nn as a whole.
When silane modified thermoplastic reslns of the same type as the non-croqCl inkP~l thermoplastic resins forming the 25 islands are employed, the silane modified thermoplastic ~ 2183~
resins are preferentially dissolved in the non-crr1qql i nk~
thermoplastic resin forming the islands, and a water treat-ment is performed so that the silane modified thermoplastic resins are crosslinked with each other and a crosslinked 5 structure is preferentially introduced into non-continuous layers ( island parts ), on the other hand, it is assumed that the re8ins become suitable for expansion due to ~he follow-ing action, although the same is not clearly worked out.
The non-crr~qql i nk~l thermoplastic resins forming the 10 islands are homogeneously and finely dispersed and the grain sizes of the thermoplastic resins forming the islands are generally ~ LL~ 1 Y small as compared with the diameters of gases generated by decomposition of the pyrolytic foaming agent while the spaces between the islands are ~ LLI ly 15 small as compared with the gas diameters, whereby the gases generated by decomposition of the foaming agent are substan-tially continuously enclosed with the non-crosslinked ther-moplastic resins forming the islands ln a macro view.
Therefore, the gases are in states enclosed with layers 20 having viscosity which is suitable for expansion and break no f oams, whereby the expandable resin composition employed in the present invention attains melt viscosity which is suitable for expansion as a whole.
A foam which is obtained from the aforementioned ex-25 pAn~Ahl e thermoplastic resins has portions partially having 2~8326 ~
low croqqllnk;n~ denslty, and hence the same ls Px~pll~nt in thermof~rr-h; 1 i ty since such portions have flowability in forming .
Further, the portio]ls having low cr~Sql i nki n~ denslty 5 can be re-melted, while portions having high cross-linking denslty can be employed as a kind of fillers and are avail-able for recycling.
On the other hand, it is possible to reduce the amount of the silane modified t~lermoplastic resins to not more than 50 parts by welght with respect to 100 parts by weight of the thermoplastic resin composition so that internal stress ln forming resultlng frorl croqqlinkin~ can be reduced, and a foam which is obtained b~ Pxr~n~lins this expandable resin composition is PX~-~l lPnt in ~ q~n~l stability The melt viscosity of the composition forming the aforementloned PXrRnr1~hl ~ thermoplastic resln pellets or annular substances is assumed to be in tlle range of 5000 polse to 20000 poise at a temperature of 190 C. In the case of not more than 5000 poise, the vlscoslty ls too much 20 reduced and a foam of a hlgh expansion ratio cannot be obtalned slnce the same cannot wlthstand a foamlng pressure in PXr~nq~ ~n and is easlly foam-broken. In the case of at least 20000 polse, on the other hand, the viscoslty is too much increased and a foam of a high expansion ratio cannot 25 be obtained since a foamlng pressure by foam gases generated 2183~61 .
by decomposltion of the foaming agent is insufficient. ~he melt viscosity in this specification is a value measured in accordance with JIS K7199.
In addition, the gel fraction of the resins in the 5 aforementioned expandable thermoplastic resins is set to be preferably 10 to 30 percent by weight, more preferably lO to 20 percent by weight. If the same is lower than 10 percent by weight, thermostabili~y is reduced, foaming cells are foam-broken in PXr:~n~l on, and homogeneous foam clearances 10 cannot be obtained. Whell the gel fraction is higher than 30 percent by weight, on the other hand, the Pxr~n~ ~m ratio is reduced and thermofnrr-hi 1 i ty is also reduced due to excess progress of cro~ 1 ~ nk; n3, Pellets or Annular Substances A method of preparing the expandable thermoplastic resin pellets or annular substances which are employed in the present invention themselves is not particularly re-stricted but a method of supplying the thermoplastic resins, the pyrolytic foaming ag~nt etc. forming the expandable 20 thermoplastic resin pell~ts or annular substances ( hereinaf -ter "pellets or annular substances" are referred to as "granular materials" ) to an extruder, melting and kn~lin~
the same at a temperature which is lower than the decomposi-tion temperature of the pyrolytic foaming agent, thereafter 5 extruding the same in the form of a sheet and cooling the 21832~1 same, and cutting the same for preparing ~oxr;~n~hle thermo-plastic resin gra-nular m~terials, and ( 2 ) a method of 8Up-plying the thermoplastic resins, the pyrolytic foaming agent etc. to an extruder, melting and knP~in~ the same at a 5 temperature which is lower than the decomposition tempera-ture of the pyrolytic foaming agent, thereafter extruding the same in the form of a strand and cooling the same, and cutting the same for preparing expandable thermoplastic resin pellets or annular substances can be listed, for 10 example.
The shape of the expandable thermoplastic resin pellets or annular substances is not particularly restricted either but may be in the form of a hexagon, a circular cylinder or a spherical body, for example, while the hexagon is most 15 preferable since the expandable thermoplastic resin granular materlals will not roll ~ihen the same are disseminated.
When the expandable thermoplastic resin pellets or annular substances are in the form of hf~gl~n~, the length of one edge of each hexagon is not particularly restricted 20 but preferably 0.1 mm to 50 mm, and particularly preferably O . 5 mm to 20 mm, since the ratio of low-l~xr~n~ -foam thin layers is reduced in the prepared foam molding and bending strength is reduced if the same is too long while escape of foam gases ls increased if the same is too short Dissemination - 21 -~ 21g3~ ~
According to the present invention, the expandable thermoplastic resin pellets or annular substances which are prepared in the aforementioned manner are disseminated.
This dissemination may be performed on a moved surface such 5 as on a .,ullv~iy~J~ belt in a preparation apparatus shown in Fig. 1 described later, for example, or on a static surface.
Further, the expandable thermoplastic resin pellets or annular substances may be disseminated so that the pellets or annular substances will not overlap along the thickness 10 direction, i . e., to form a single layer, or so that the ~lrr~n~1~hl e thermoplastic resin pellets or annular substances overlap with each other along the thickness direction.
As to preferable methods of disseminating the expand-able thermoplastic resin pellets or annular substances so 15 that the same will not overlap with each other along the thickness direction, ( 1 ) a method of disseminating a plural-ity of expandable thermoplastic resin granular materials in the form of rectangular parallelopipeds which are provided with minimum edges having substantially identical heights 20 and bringing the same into a single layer by high frequency vibration while making the minimum edges along the thickness direction, ( 2 ) a method of disseminating a plurality of expandable thermoplastic granular materials in the form of cubes having substantially identical heights and bringing 25 the same into a single layer by high freguency vibration, ~ 218326 L
and ( 3 ) a method of bringing a plurality of expandable thermoplastic resin granular materials in the form of cubes having substantially identical heights which are disseminat-ed in an overlapping manner into a single layer by a coater 5 or the like can be preferably listed, for example, while a method of arranging respective expandable thermoplastic resin granular materials having substantially identical heights not to overlap with each other is also employed as the case may be.
~he heights herein referred to indicate the heights of respective expandable thermoplastic resin granular materials which are directed when the ~xrAn~ hl e thermoplastic resin granular materials are disseminated.
In the aforQmentioned dissemination, the aforementioned 15 pellets or annular substances are preferably so disseminated that downwardly pro3 ected areas of the e~cpandable thermo-plastic resln pellets or annular substances being dissemi-nated occupy lO to 75 96 of regions enclosed with outer edges of the regions where the pellets or annular substances are 20 disseminated.
It is possible to ensure a deaeration space in expan-sion of the f~xr;~n~hl e resin granular materials by limiting the sum of the pro3 ected areas of the expandable resin granular materlals after dissemination to lO to 75 9~ of sums 25 of the pro;~ected area of the aforementioned regions, so that ~ 21832fil a foam having no voids can be prepared by l'XrAn~li n~ the respective :~xrAn~lAhle resln granular materials while repel-ling air from clearances therebetween. Further, the respec-tive ~ rAn~ hl e resin granular materials can be 5 fused/integrated with each other with no insufficient expan-sion spaces, whereby it is possible to prevent the foam as prepared from formation of holes.
In addition, it is L~ossible to obtain a foam attaining compatibility of bending strength and flexibility by limit-10 ing the sum of the pro~ected areas of the l~xrAnrlAhl P thermo-plastic resin pellets or annular substances after fiic5F~m~nA-tion to 10 to 75 % of the total pro~ected area of the afore-mentioned r-egions thereby limiting the ranges of the expan-sion ratios and the thicknesses of the low-expanded-foam 15 thin layers.
Expansion Forming According to the inventive preparation method, the aforementioned pellets or annular substances are disseminat-ed and thereaf ter the pe~ lets or annular substances are 20 heated to a temperature exceeding the foaming temperature of the ioaming agent which is contained in the aforementioned f~xrAnrl~hle thermoplastic resins, so that the pellets or annular substances are expanded.
When the expandable thermoplastic resin pellets or 25 annular substances are expanded by heating, surfaces and the --~ ~1832~1 interiors of the respective pellets or annular substances become low-expanded-foam-resin thin layers and high Pxr~nfl.ofl foams while the low-expanded-foam-resin thin layers of the surfaces are fused to be integrated with each other while 5 being expanded, whereby it is possible to obtain a thin layer molding in which thermoplastic resin high expanded foams covered with the aforementioned thermoplastic resin low-~xr~nfll~fl-foam thin layers on the overall outer surfaces are thermally fused with each other through the low-expand-10 ed-foam thin layers.
The inventive preparation method, which can obtain a foam by an extremely simple method of disseminating expand-able thermoplastic resin granular materials and heating and ~xr~nfl~n~ to fuse the same, is a preparation method having 15 excellent productivity w~lich can omit a step of pre-expand-ing the expandable thermoplastic resin pellets or annular substances .
The aforementioned heating method is not particularly restricted but arbitrary heating means can be employed so 20 far as- the same can heat the granular materials to a temper-ature l~x~ flin~ the foaming temperature of the foaming agent. For example, it is possible to heat the granular materlals by employing an electric heater, a far infrared ray heater, a heating apparatus formed by circulating a 25 heating medium such as heated oil or air or the like.

21832~ 1 Further, a method of heating and Pxr~nrl;n~ to fuse the expandable thermoplastic resln pellets or annular substances while restricting th; rkn~s~es thereof is not particularly restricted either. For ~xample, it is possible to dissemi-5 nate the aforementioned expandable thermoplastic resinpellets or annular substances between two plate bodies for r~xrnn~l;nS the same while keeping the space between the plate bodies~ constant. Alternatively, it is also possible to disseminate the l~xr~n~l~hl e thermoplastic resin pellets or 10 annular substances, thereafter hold the same between two plate bodies, and thereafter expand the same while increas-ing the space between the plate bodies up to a certain size.
Further, it is also possible to disseminate the expandable thermoplastic resin pellets or annular substances, thereaf-15 ter hold the same between two plate bodies, and thereafterincrease the space between the plate bodies on both sides through expansion pressures in expansion of the expandable thermoplastic resin pellets or annular substances.
Arbitrary plate members such as iron plates, steel 20 plates or tetrafluoroeth~lene belts, ior example, can be employed as the aforementioned plate bodies.
Further, it is preferable to arrange composite sheets which are thermoplastic resin sheets which are reinforced with fiber, for example, between the aforementioned plate 25 bodies and the expandable thermoplastic resin pellets or ~ 2183~61 annular substances. In this case, the aforementioned ther-moplastic resin sheet-type substance may be arranged between the plate body and the expandable thermoplastic resin pel-lets or annular substances only on one side thereof.
When the aforementioned composite sheet is employed, the aforementioned expandable thermoplastic resin pellets or annular substances are p~eferably previously disseminated on the composite sheet so tllat the composite sheet is integrat-ed with the obtained foam. In this method, a second compos-10 ite sheet is superposed on the disseminated ~-~rr~n~ hl e thermoplastic resin pellets or annular substances, to obtain a lamlnate. Then, this laminate is heated to a temperature exceeding the foaming temperature of the foaming agent, so that the P~r;ln~hl e thermoplastic resin pellets or annular 15 substances are expanded and a foam in which the composlte sheets are integrated with the foam is obtained.
In the foam which is integrally formed with the thermo-plastic resln sheet type substance on at least one surface as hereinabove descrlbed, the bending strength of the over-20 all foam is effectively improved by the thermoplastic resinsheet type substance.
In addition to the aforementioned composite sheet, various materlals such as glass paper and a chopped strand mat ( t~lese are generlcally called as "reinforcing sheet" ) 25 can be employed.

~183~1 If the weight of glass fiber which is employed for the glass paper and the chopped strand mat is heavy, however, it is impossible to attain weight reduction of the obtained foam. If the weight of the glass fiber is light, on the 5 other hand, it is impossible to attain il..~ V~ t in strength of the obtained foam. Therefore, preferably that of 10 to 500 g/m~, more preferably that of 50 to 300 g/m~, is employed as the aforementioned glass fiber.
Further, the thermo]?lastic resin which is employed for 10 the afDrementioned composlte sheet is not particularly restricted either, but p~lyethylene, polypropylene, polyeth-ylene terephthalate etc. can be listed, for example. In order to improve adhesion between the composite sheet and the foam part, further, a composite sheet employing a ther-15 moplastic resin of the same type as the thermoplastic resinsemployed for the foam is preferably employed.
As the fiber which is employed for the composite sheet, further, inorganic fiber such as carbon fiber, organic fiber such as aramid fiber or nylon fiber, metal fiber etc., for 20 example, can be listed in addition to the glass fiber, and either woven fabric or nonwoven fabric of such fiber is available .
It is impossible to attain weight reduction of the obtained foam if the thickness of the aforementioned rein-25 forcing sheet is large, while reinforcement and strengthe--- 2~832~
ning are insufficient if the same is small. Therefore, the~h;rknf~qq of the composite sheet is assumed to be preferably 0.05 to 1 mm, more preferably 0.1 to 0.5 mm.
In order to obtain a foam by the inventive preparation 5 method with employment of the Pxrf~nr~ hle thermoplastic resin which is a preferable example prepared by bll~n~lin~ the aforementioned silane modified thermoplastic resins, the silane modified thermoplastic resins are cr~-qql ;nkf~l with each other by a water treatment and thereaf ter heated to a 10 level exceeding the foaming temperature of the foaming agent, whereby the foam is obtained.
A reinforcing material such as glass short fiber, carbon short fiber or polyester short ~fiber, and a filler such as calcium carbonate, aluminum hydroxide or glass 15 powder may be added to t~le thermoplastic resin employed for the aforementioned f~xrFln~l~hl e thermoplastic resins at need, in order to improve the bending strength of the foam.
When short fiber is added as the reinforcing material, 1 to 20 parts by weight is preferable and 3 to 10 parts by 20 weight is particularly pI~eferable with respect to 100 parts by weight of the thermoplastic resin, since cells are broken in expanslon and a high expansion ratio cannot be attained if the amount is large, while no effect of reinforcing the strength of the obtained foam is attained if the same is 25 small.

~1~32~ ~
The length of the short fiber is preferably 1 to 20 mm, and particularly preferably 3 to 5 mm since weight reduction of the obtained foam cannot be attained if the same is large while no effect of reinforcing the strength of the obtained foam ls attained if the same is small.
When a filler is added, 10 to lO0 parts by weight is pref erable and 30 to 50 parts by weight is particularly preferable with respect to lO0 parts by weight of the ther-moplastic resin, since weight reduction of the obtained foam cannot be attained if the same is much while no effect of reinforcing strength of the obtained foam is attained if the same is less.
Foam The foam according to the present invention is a thin layer molding in which t~lermoplastic resin high expanded foams whose overall outer surfaces are covered with low -foam thin layers consisting of thermoplastic resins are thermally fused with each other through the low-ex-panded-foam thin layers.
Since the high expanded foams are thl~rr~l ly fused with each other through the low-~xrAn~lP~l-foam thin layers, the respective low-expanded-foam thin layers provide compressive strength and the high expanded foams provide flexibility, while high heat insulation performance is also provided since the low-~xrAn~ l-foam thin layers are i~x~An~d~ Fur--- 2183~6~
ther, the high PxrAn~-~rl foams whose overall outer surfaces are covered with the respective low-expanded-foam thin layers forming the foam are thermally fused to be strongly integrated with each other through the low-expanded-foam 5 thin layers by the foaming pressure in ~r~n~ n, whereby the same are not separat~d and/or broken in thermal fusion interfaces and are excellent in fl~x;h~1;ty while having high bending strength.
The aforementioned L~roper thermoplastic resins can be 10 employed as the thermoplastic resins forming the aforemen-tioned high ~Xr~n~ foal[Ls and low-expanded-foam thin lay-ers, and it is possible to obtain the inventlve foam by employing the expandable th~ E 1 ~ ctic resln which is pre-pared by mixing a foaming agent and other arbitrary compo-15 nents with the thermoplastic resins.
The high expanded foams whose overall outer surfacesare covered with the low-expanded-foam t~lin layers may be formed as a single layer not to overlap with each other along the th i ~.knf~s direction, and in this case, the high 20 expanded foams are thermally fused with each other through the low-~xr~n~ 9-foam thin layers along the transverse directions, i.e., in the longitudinal directlon and the cross directlon. In addition to the aforementioned flexi-billty of the foam, therefore, the foam enters a pseudo-25 truss structure and the bending strength is further in-~1~326 1 creased since the low-expanded-foam thin layers extend along the thickness direction ~f the foam and the low-expanded-foam thin layers are homogeneously formed along the thick-ness direction.
On the other hand, the foam may have a multilayer structure in which the high expanded foams are stacked in plural along the thil-.knl~q direction, and in this case, the foam is not separated and/or broken in the fusion interfaces when a~ bending load is applied to the foam. Therefore, it is possible to obtain a Eoam which is excellent in flexibil-ity .
When the thermoplastic resin has preferentially crr~ ; nkf~fl portions having high cr-)ssl; nki n~ density and hardly crosslinked portions havlng low crosq1 i nki n~ density and these have island-sea structures, the portions having low crr~ ;nk;n~ density have flowability in forming, where-by the foam is excellent in thermoformability.
When the expansion ratios of the aforementioned low-P~r~nfl~fl-foam thin layers are low, flexibility of the foam is reduced while heat conductivity is increased. If the expansion ratios of the low-expanded-foam thin layers are high, on the other hand, a foam having high bending strength cannot be obtained. Therefore, the ~- r~n~ n ratios of the low-expanded-foam thin layers are preferably 1.1 to 10 times, more preferably 1. 2 to 7 times, and particularly ~83~6 ~
preferably 1.2 to 5 times.
Weight reduction of the foam cannot be attained if the low-~xrRn'i~-foam thin L~yers are thick, while a foam having hiyh bending strength cannot be obtained if the same are 5 thin. Therefore, the lo~ xrAn~lPfl-foam thin layers are preferably 30 um to 500 ~m, more preferably 40 ,um to 400 rum, and particularly preferably 50 llm to 400 ,um.
The th i ~.knf~c:s:~c of the low-expanded-foam thin layers may not be homogeneous, I~ut may be heterogeneous.
The thicknesses of the low-expanded-foam thin layers herein referred to indicate average thi~knF~scF~s of the low-f~xr;ln~ -foam thin layers along the cross-sectional direc-tion of the foam.
According to the present invention, the ~xrAn~ n 15 ratios~ and the thicknesses of the aforementioned low-expand-ed-foam thin layers are preferably l.1 to lO times and 30 ,um to 500 ,um since weight reduction and bending strength of the foam are compatible, more preferably 1. 2 times to 7 times and 40 ,um to 400 ~m, and particularly preferably 1.2 times 20 to 5 times and 50 ,um to ~lO0 ,um.
If the expanslon ratios of the high expanded foams are low, it is difficult to attain weight reduction and the heat conductivity of the foam is increased to reduce the heat insulation property of t~le obtained foam, while a foam 25 having high bending strerlgth cannot be obtained if the same 2~832~1 are high. Therefore, the expansion ratios of the high expa-nded foams are preferably 20 to 50 times, more preferably 5 to 50 times, particularly preferably 10 to 35 times.
If the sizes of the high Pxr~n~pd foams are large, the 5 bending strength of the obtained foam is reduced, while surface smoothness of the obtained foam is reduced if the same are small. Therefore, the sizes of the high expanded foams are preferably 5 to 10 mm, and particularly preferably 7 to 5 0 mm .
The sizes of the high PXrF~n~P~l foams may not be homoge-neous, but may be heterogeneous.
The sizes of the high PXr~ntlP~l foams herein referred to indicate the maximum values among dimensions of respective directions in cross sections.
The foam according to the present invention consists of such a structure that the high PXrF~n~lP~l foams are thermally fused with each other thl-ough the low-Pxr~ntlP~l-foam thin layers, and the same is generally in t~e form of a sheet.
In the foam according to the present invention, a 20 composite sheet consisting of the aforementioned reinforcing fiber and the thermoplastic resin may be stacked at least on its one surface, whereby it is poqq~ hl P to further improve bending strength.
A method of preparing the " foam" according to the 5 present invention is not particularly restricted to the ~o ~
21832fi~
.
aforementioned inventive L,l~aldLion method, but (1) a method of disseminating expandable thermoplastic resin granular materials, heating, Pxr~n~ins and thermally fusing the same for preparing a foam can be preferably listed, 5 while it is also possible to employ ( 2 ) a method of previ-ously preparing a thermoplastic granular foams and stacking the same by thermal fusion for preparing a ~oam, as the case may be.
Effect of the Inven~ion In the inventive foam, as hereinabove described, the thermoplastic resin high expanded foams whose overall outer surfaces are covered Wit~l the thermoplastic resin low-ex-panded-foam thin layers are thermally fused with each other through the aforementioned low-Pxr~n~lP-l-foam thin layers.
15 The low-PxrAn~P~-foam thi n layers can improve compressive strength of the foam whi3 e providing hig~l heat insulation performance due to low expanded foaming. In addition, the low-expanded-foam thin layers strongly fuse to integrate the respective high foams with each other, whereby the same also 20 provide sufficient flPx;h~lity which cannot be attained in a foam obtained by a conventional bead P~rr;in.C~ ~-n forming.
According to the inventive method of preparing the aforementioned foam, further, it is possible to readily prepare a foam having the aforementioned PX~'Pl 1 Pnt effects 25 with high productivity.

8326 ~
According to a pref~erred aspect of the present inven-tion, further, an expandable resin composition having melt viscosity which is in the range of 3000 poise to 20000 poise at 190C is employed, whereby expanslon stability is further improved and hence it is possible to provide a foam of high expansion having a homogeneous bubble structure and ex-cellent bending strength.
( 1 ) When the gel fraction of the resins in the expand-able thermoplastic resins is made 10 to 30 percent by weight, proper crnqf:l ink;n$ is applied to the resins and it is possible to provide a foam which is balanced in formabil-ity and thermostability .
( 2 ) When Pxr~n~;~hl e thermoplastic resins containing specific silane modified thermoplastic resins, a silane cros-:l i nkin~ catalyst and a foaming agent in those mixed with a prescribed amount of specific non-crnc~l i nkP~l thermo-plastic resins are employed, portions having low crosslink-ing density are provided and hence such portions have flowability in forming and are excellent in thermoforming, and it is possible to obtain homogeneous and delicate foam-ing cells which are PxrPl l Pnt in thermostability since crnqql ;nk;n~ is properly applied as a whole and having proper melt viscosity, while the bending strength of the obtained foam also becomes PX~Pl 1Pnt.
According to the present invention, therefore, it is ~83~ ~
p,)c~ hl e to provide a foam which is suitable for various heat insulators including a roof insulator and a floor insulator, a cushioning material and various interior mate-rials including a vehicle interior material.
5 Brief Descriptlon of the Drawings Fig. 1 is a front elevational view showing an example of an apparatus which is employed for a method of preparing a foam according to the present invention.
Fig. 2 is a front elevational view showing another 10 example of an apparatus ~hich is employed for the method of preparing a foam according to the present invention.
Fig. 3 is an electron microphotograph enlarging a high expanded portion of a foam obtained in Example 23 to 20,000 times .
15 Best Mode for Carrying Out the Invention Fig. l is a front elevational view of an apparatus example which is employed for a method of preparing a foam according to the present invention, and l and 2 denote reinforcing sheets, 3 denotes thermoplastic resin pellets or 20 annular substances containing a foaming agent, 5 and 6 denote ~:UI~V~yUL belts, 9 and 9 denote heating apparatuses, 10 and 10 denote cooling apparatuses, and 11 denotes a foam.
The reinforcing sheet 1 is fed to the ~;UIIV~yUL belt 5, so that the ~rrRnflRhl e thermoplastic resin pellets or annu-25 lar substances 3 are disseminated on the reinforcing sheet 1 ~ 326~
by a pellet or annular substance disseminator 4 which isprovided halfway the same. Then, the reinforcing sheet 2 is fed to~ the conveyor belt 6 and superposed on the pellets or annular substances 3, so that the same is successively fed 5 to preheating apparatuses 8 and 8, the heating apparatuses 9 and 9 and the coo-ling ap~aratuses 10 and 10. 7 denotes a vibration apparatus, which vibrates the reinforcing sheet 1 for homogenizing the dlsseminated f~rAn~hl e thermoplastic resin pellets or annular substances 3.
The ~r;ln~hl e thermoplastic resin pellets or annular substances 3 are preheated in the preheating apparatuses 8 and 8 and heated in the heating apparatuses 9 and 9 to a level ~r~ lin~ the foaming temperature of the foaming agent, so that the foaming agent is decomposed and the thermoplastic resin pellets or annular substances 3 are melted/expanded and the resin pellets or annular substances are fused with each other and fused on surfaces of the reinforcing sheets l and 2.
The heating temperature in the heating apparatuses 9 and 9 is generally assumed to be at least the foaming tem-perature of the foaming agent and not more than the foaming temperature + 20C, such as about 200C, for example.
The expanded and fused superposed sheets are cooled in the cooling apparatuses ( the cooling temperature is about 30C, for example) 10 and 10, to be suppressed in expansion ~ 21~326~
and ad~usted to prescribed thir~kn~ces~
In the aforementioned heating apparatuses 9 and 9 and the cooling apparatuses 10 and 10, it is preferable to provide plural vacuum su~tion grooves 92 and 92 and 102 and 102 on heating surfaces 91 and 91 and cooling surfaces 101 and 101 thereof so that homogenelty of thi ~-.knf~cq~ and surface smoothness can be retained.
Thus, the foam 11 consisting of a foam layer 112 formed by ~-rrAnq~l~n of the thermoplastic resin pellets or annular substances 3 and reinforcing sheet layers formed to be integrally fused on both surfaces thereof is obtained.
Fig. 2 shows an apparatus of another example for carrv-ing out the method of preparing a foam according to the present invention. Here, a reinforcing sheet lb is fed onto a ~,UllV~y~L belt 5a. On an Inrl ;n~(~ portion of this rein-forcing sheet lb, ~- rAn~lAhl e thermoplastic resin pellets or annular substances 3a are disseminated from a pellet dis-seminator 4a. In the 1nr.1 in~ surface portion of the afore-mentioned thermoplastic resin sheet lb, a hot platen 8a which is heated to a temperature of 200 C to 210 C is pro-vided. Therefore, the surface of the reinforcing sheet lb is brought into a melted state, whereby the expandable thermoplastic resin pellets 3a adhere to the reinforcing sheet lb, and excess pellets are downwardly dropped. Thus, the dropped pellets can be used again.

21832~:~
.
Then, a second reinforcing sheet 2a is superposed on the adhering thermoplastic resin pellets 3a, so that the expandable thermoplastic resin pellets or annular substances 3a are heated and expanded by the hot platen ~a which is 5 heated to the temperature of about Z00 C . Thus, a molding lla is obtained.
Concrete Examples of the method of preparing a foam and the foam according to the present invention are now de-scribed .
I0 Preparation of Expandable Thermoplastic Resin Granular Materials High density polyetllylene (by Mitsubishi PetrorhPmtri~l Co., Ltd., trade name: EY40H, melt index (hereinafter re-ferred to as MI); 1.5 g/10 min. ), polypropylene (1) (by 15 Mitsubishi Petrorh~m;r~l Co., Ltd., trade name: MA3, MI; 11 g/10 min. ), polypropylene (2) (by Mitsubishi Petrochemical Co., Ltd., trade name: M~{ 8, MI; 0.3 g/10 min. ), polypropylene (3) (by Mitsubishi PetrorhPm;r~l Co., Ltd., trade name: MA2A, MI; 25 g/10 min. ) and silane modified 20 polypropylene (by Mitsubishi Petrochemical Co., Ltd., trade name: LINKLON XPM800HM, MI; 11 g/10 min., gel fraction after oro~ nk; n~ 80 percent by weight ) were weighed in the rates shown in the following Tables 1 to 3, further mixed with 0.1 part by weight of dibutyl tin dilaurate serving as a 25 crf)c~l ;nk;n~ catalyst and 5 parts by weight of A70~1;r.Flr-21~26 L
bonamide (by Otsuka rh~mlri~l Co., Ltd, trade name: S0-20, foaming temperature = 210C), thereafter supplied to a l~iaxlal extruder of 30 mm in aiameter, melted/kneadea at a temperature of 180C, an-~ extruded in the form of sheets 5 having thi~knPq~es &hown in Tables 1 to 3 and widths of 300 mm. Thereafter the sheets were cooled and cut into dimen-sions of 5 mm in width by 5 mm in length, dipped in water of 98C for 2 hours and thereafter dried, thereby obtaining l~xr~n~1~hle thermoplastic resin granular materials A.
Examples 1, 3, 5, 9, 11, 13, 15, 17, 19, 21 and 23 The expandable thermoplastic resin granular materials A
obtained in the aforementioned manner were employed and the preparation apparatus shown in Fig. 1 was utilized to obtain foams. In the preparation apparatus shown in Fig. 1, howev-15 ~r, the thermoplastic resin sheet substances 1 and 2 werenot employed but the aforementioned expandable thermoplastic resin granular materials A were directly ~licc~m~n~ted on the conveyor belt 5. This d}ssemination was so carried out that the rates (~ s~omin~tion pro;~ected area ratios) of sums of 20 downwardly proJected areas of portions where the ~7rr~n~hl e thermoplastic resin granular materials A were actually disseminated to total pro~ected areas of portions enclosed with outer edges of the regions where the ~rAn~hle thermo-plastic resin granular materials were disseminated reached 25 the ratios shown ln Tables 1 to 3 and became double layers.

`--' 2183261 ~he heating temperature for the expandable thermoplastic resin granular materials A by the heating apparatuses 9 and 9 was assumed to be 210C, and the granular materials were to ~xrAnri~ by heating for 10 minutes and thereafter cooled in the cooling apparatuses 10 and 10 for 10 minutes, to obtain foams of 5 mm in thickness.
~xrAnqlnn ratios of the obtained foams, th;(-kni~qqes of low-~xrAn~o~i-foam thin layers, low-~x~An~ -foam thin layer expansion ratios, high expanded foam f~xrAnq~r~n ratios, states of the foams, foam clearance states, bending strength values, 25 96 compressive strength values, compression perma-nent strain, thermoforma~ility levels and thermostability levels were measured in the following methods. These re-sults are also shown in Tables 1 to 3.
(Expansion Ratio) Measured in accordance wlth JIS K6767.
(Low-expanded-foam thin layer Thirkn~qq) Sections of the formed foams were observed and measured with a microscope provided with a scale.
( Low-expanded-foam hin layer Expansion Ratio ) The low-expanded-foam thin layers were cut out from the foams, and measured in accordance with JIS K6767.
( High Expanded Foam Expansion Ratio ) The high f'XrAn~ foams were cut ou-t from the foams, ana measured in accordance with JIS K6767.

~ 32~~
( State of Foam ) Surfaces and sections of the foams were observed.
( Foam Clearance State ) Sections of the foams were observed with a microscope 5 provided with a scale to measure maximum inner diameter values of clearances such as voids and holes and foaming cells etc., for making four-stage evaluation with the fol-lowing reference as foam clearance states.
~: The maximum inner diameter value of foaming cells 10 was less than 2 mm, and clearances were substantially ab-sent .
0: The maximum inner diameter value of clearances was less than 3.5 mm.
~ : The maximum inner diameter value of clearances was 15 at least 3 . 5 mm and less than 7 mm.
X: The maximum inner diameter of clearances was at least 7 mm.
( Bending Strength ) The foams were cut into 50 mm by 150 mm by 50 mm, and a three-point bending test was made with application of bend-ing loads under conditions of a spun of 100 mm, a pressing speed of 50 mm/mln., R of a press$ng shaft = 5 and n = 5, to measure bending strength values.
( Broken Portion ) Broken portions upon application of the aforementioned -- 21832~ ~
bending loads were visually observed.
( 25 ~ Compressive Strength ) Measured in accordance with JIS K6767.
( Compression permanent strain ) Measured in accordance with JIS K6767.
( Thermoforr~h; l; ty) Respective opening portions of several types of bot-tomed cylindrical bodies having opening end portions R = 5, diameters = 70 mm and prescribed depths were covered with foams of 20 mm by 20 mm by 5 mm which were heated to 180C, then the heated foams were forced into opening concave portions with a cylindrical member of 70 mm in diameter, depths h (mm) of the foams which were forced into the open-ing concave portions were measured when the foams started to be broken, drawing ratios were obtained by the following equation, and thereafter five-stage evaluation was carried out with the following reference as thermof~rr~h;lity:
Drawing Ratio (96) = (h/80) x 100 Thermoformability;
drawing ratio of at least 75 % ............................ 5 drawing ratio of at least 60 % and less than 75 96 ... 4 drawing ratio of at least 50 % and less than 60 %
... 3 drawing ratio of at least 30 96 and less than 50 %

21832~1 ... 2 drawing ratio of not more than 30 % ... 1 ( Thermostability ) The foams were cut into 20 mm by 20 mm by 5 mm, allowed 5 to stand under a 210C atmosphere for 5 minutes and thereaf-ter cooled at 23C, and volumes V (mm3) were measured with an underwater substitution densimeter to obtain volume changes (%) through the following eS~uation, and thereafter four-stage evaluation was made with the following reference 10 as thermostablity:
Volume Change (~6) = {(2000 - V)}/2000 x 100 Th~ ~ ~dbility;
volume change of not more than 20 % ... ~) volume change of at least 20 % and less than 30 %
... 0 volume change of at least 30 % and less than 50 %
...
volume change of at least 50 % ... X
Examples 2, 4, 6, 10, 12, 14, 16, 18, 20, 22 and 24 The P~r~n~l~hl ~ thermoplastic resin granular materials A
obtained in the aforementioned manner were disseminated as shown in Tables 1 to 3 not to overlap with each other in the ~h~l-knPqq directions, to obtain foams through the prepara-tion apparatus shown in rig. I similarly to Example 1.

-- 21~326 ~
However, the temperature for heating the thermoplastic resin granular materials A by the heating apparatuses 9 and 9 was assumed to be 210C, and the granular materials were heated for 10 minutes, e7rr~nrle~, thereafter transferred to the cooling apparatuses 10 and 10 which were set at 30C and cooled for 10 minutes, to obtain foams of lO mm in thick-nesS-As to the obtained foams, respective characteristics were measured similarly to Example 1, and the results were shown in the following Tables 1 to 3.
Example 7 The expandable thermoplastic resin granular materials A
obtained in the aforementioned manner were employed and the preparation apparatus shown in Fig. 1 was employed to obtain a foam. In Example 7, a composite sheet belng a high densi-ty polyethylene (by Mitsubishi Petro~hPmi--~l Co., Ltd., trade name: JX10, MI; 20 g/10 min. ) 90 g/m2 sheet reinforced with glass paper (by ORIBEST, trade name: FVP-045) 45/m2 was employed as the reinforcing sheet 1 on the ~:UllV~yUL belt 5 of the preparation apparatus shown in Fig. 1. The aforemen-tioned Pl-pPnfl~hl ~ thermoplastic resin granular materials A
were disseminated on this composite sheet to attain the ~i ccc~m~ tion pro~ected area ratio shown in Table 1 and a multilayer .
Thereafter a sheet consisting of the same material as ~ 21~32~ ~
the aforementioned composite sheet was employed as the reinforcing sheet 2 shown in Fig. 1, to expand and form the ;~ln~1~hl ~ thermoplastic resin granular materials A between the composite sheets. In this case, the heating temperature 5 by the heating apparatuses 9 and 9 was assumed to be 210C, and the materials were l~2r;~ntlP~l by heating for 10 minutes, thereaiter transferred to the cooling apparatuses 10 and 10 which were set at 30DC and cooled for 10 minutes, thereby obtaining a foam of 10 mm in thl r~.knl~qs .
As to the obtained foam, respective characteristics were measured similarly to Example 1. The results are also shown in the following Table 1.
Example 8 A foam was obtained similarly to Example 7. In Example 15 8, however, disseminatio~l of the l~xr;~n~hle thermoplastic resin ~ranular materials onto a fiber-reinforced thermoplas-tic resin sheet was in a rate shown in Table 1, while the ~xr~nr~hl e thermoplastic resin granular materials were disseminated not to overlap with each other along the thick-20 ness direction. Forming was carried out similarly to Exam-ple 7 as to other points, to obtain a foam of 10 mm in th~ ~-.knl~qq .
Characteristics o~ the obtained foam were measured similarly to Example 1. The results are also shown in Table 25 1.

-- 218326~
Comparative Example 1 The F~rr~n(l~hl e thermoplastic resln granular materials A
obtained in the aforementioned manner were expanded by heating in a gear oven of 210C for 10 minutes and there-5 after air-cooled, to obtain foaming granular materials. The foaming granular materials were charged in a mold having inner ~i q1r~r~c of 10 mm by 200 mm by 200 mm, heated and fused with a hand press of 170C for 5 minutes, thereafter transferred to a cooling apparatus which was set at 30C and lO cooled for lO minutes, to obtain a foam of 10 mm in thick-ness .:
Characteristics of the obtained foam were measured similarly to Example l. The results are also shown in the followlng Table l.

~ 2183~61 .
I ~ o ~" N N ~ ~o ~o N O X IL _ ~ N O
0 a~ o CO N NO, ~ ~ N 'J N C ~ O o O - r~ N O
~ ~ N ~ U7 U7 N ~ C O o o ~ ~o O
U~ O N O -- U~ '~t N ~ X O IL ~ ~ N O
~1' 0 ~ O ~ ~ ~ ~ N ~ C~J ~q tD O N o In N O
O ) Q --' N ~ ~ O o O ID U) N O
N , cOo O ~ N~ O o O ~ 3 0 N o ~ ~ N O
~ D O N o ~ N O
E a~ E ~ E E $ C E
I ~ I~_ ~ Cl ~ ~ ~ 2 ~ ~ 2 O E~ 0.~ E E~
_ ~q _ --~ 218326 ~
CO ~ ~ ~ "0~ ~ ~q tD --o ~ ,l ~ o æ O ~ ~ O ~ ,~ ~ ~ 0~ ~ - ~ O
~ ~ ~ ~ ~ U~ l C o ~) E '`' ~ ~,, x _ ~ ~ ~ C o C~ X
æ ~ O ~0 ~ I o LL ' ~
- _ æO~oc~ CIXo EOOr I I
o ~ g IL
æo~OI~ g XO oE
E ~ 3 G C ~ E w E ~ ~e 2 ~ E ~( 2 C
o~ ._ _ O E~ o~ c E ~E~
- 5~ --,. . .. _ _ . , . _ .. .

2~83261 oO ~ o ~ o ~ _ o U~ ~ ~ ~ --gC ~ C ~ ~ 3 LO
~I O ~ C L~ C c ~ O '5 C~ O
~o 8 ~ ~, o ~ C`l C C ~ o E ~o o o O O c~l O ~ o ~ ~ o LL _ C~ <1 , ~o o c~l ~ c --c ~, o E ~ ~ ~ o g ~ C`~ ~ l C C ~ O ~ ~D ~ O
C l o ~ l ~ ~ C ", O C~ 0~ 3 ~ <I
O o C~l O' ~~ N ~ ~ C ID O o o C`~
.~ ~ E ~ E ~ E o E "~ o~ E E ~, E -- E ~0 ~ ~j 9 o ~ 2 u, E~ o~cg ~ Es E~
-- 51 ~
, . _ . . . . _ ..

2~ 83261 .~, In ~:x~mrle~q 1 to 24, every ~l;qq~m~n~tion was assumed to be 250 g/mZ in dissemlnation of the thermoplastic resin granular materials A. While omitted in Tables l to 3, expansion ratios of the overall foam moldings as obtalned are 20 tlmes in every one of Examples 1 to 24 and compara-tive example l.
As obvious from Tables l to 3, it ls understood that bendlng strength values were ~ I l ~ 1 y improved to at least 0.12 kg/cm2 in the foams of ~ l Pq l to 24 although the bending strength of the obtalned foam molding was low at 0 . 076 kg/cmZ ln comparative example employlng a bead expan-slon/formlng method of carrylng out pre-p~r~nq~mn and maklng Pxr~nq~mn again ln formi~lg. Thls ls because the foams were obtained by the inventive preparation method in Examples l to 6 and Examples 9 to 24, and the overall outer surfaces of the high expanded foams ~ere covered with the low-PxrAn~lP~-foam thin layers in the Coams so that the low density foams were thermally fused Wit~l each other through the low-expand-ed-foam thin layers.
Further, it is understood that bending strength values are higher in the foams in which high f~xr~n~lPd foams were formed not to overlap with each other along the thickness directions as compared with the multilayer foams consisting of the same compositions. Comparing Example l and Example 2 with each other, for example, it is understood that Example -- 2~L832fil 2 in which the high expanded foams are not superposed with each other along the thickness direction has higher bending strength as compared with E:xample 1. Similarly, it is understood that F:l~^~rlPR 4, 6, 8, lO, 12, 14, 16, 18, 20, 22 and 24 have higher bending strength values as compared with Examples 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23.
Further, it ls understood that bending strength values are higher in Examples 5 to 8 and E , l P.~ 13 to 24 in which the ~-~rr~n~ n ratios of the low-expanded-foam thin layers are in the range of 1.1 to 10 times and the thi l-kn~':Sf~':
thereof are in the range of 30 to 500 ,um as compared with Examples 1 to 4 and Examples 9 to 12, and hence foams havlng higher bending strength values were obtained while effectu-ating lightweightness.
In addition, it is understood that composite sheets consisting of reinforcing fiber and the thermosetting resins are stacked on both surfaces in Examples 7 and 8 and hence the bending strength values are effectively improved as 0 . 51 kg/cmZ and 0 . 57 kg/cm~ respectively.
In Examples 9 to 12, on the other hand, bending strength values were low as compared witll the r~- I n; n~
Examples since the tliss~m~n~tion proJected area rates of the thermosetting resin granular materials A were out of the range of 10 to ;75 96. Therefore, it is~ understood that a foam having high bending strengt~l can be obtained by bring--- 21~326 1 ing the dissemination pro~ected area ratio of the th, ,c,,t-ting resin granular materials A into the range of 10 to 75 %.
Further, lt is understood that foaming cell states are excellent in Examples 15, 16 and 19 to 24 in which melt viscosity values of the expandable thermoplastic resins employed for obtaining the thermoplastic resin granular materials A are in the range of 5000 to 20000 poise at 190C
and hence homogeneous and fine foam cells are h~ g~nl~nusly 10 dispersed. Thus, it is understood that foams having higher bending strength values are obtained. For example, Example 13 and Example 15 were e~ecuted similarly to each other except that the melt viscosity values and the gel fractions of the aforementioned thermosetting resins were changed, 15 while the bending streng~h was ~, ~.v~d to 0.29 kg/cmZ in Example 15 although that of Example 13 was 0 . 27 kg/cm2 .
Similarly, the bending strength was improved to 0.30 kg/cm2 in Example 16 although t~le bending strength was 0 . 29 kg/cmZ
in Example 1~. Comparing Example 17 and Example 19 with 20 each other, further, it is understood that the bending strength was improved to 0 . 29 kg/cm2 in Example 19 while that of Example 17 was 0 . 28 kg/cm2, although only viscosity values of the '~-rrAn~Ahl e thermosetting resins were dif-ferent. Therefore, it is understood that foaming cells are 25 further homogeneously and finely formed and a foam which is -- ~18326~
improved in bending strength is obtained by bringing the melt viscosity of the expandable thermoplastic resins into the aforementioned specific range.
Further, it is understood that foams which are excel-5 lent in balance between thermoforr~hi l i ty and thermos-tability can be obtained in Examples 17 to 24 in which the gel fractions of the ~.xr~nfl;~hl ~ thermoplastic resins are in the range of 10 to 30 percent by weight. Namely, thermofor-mability levels are in evaluation of at least 3 in Examples lO 17 to 24, while thermostability levels maintain evaluation of at least ^. In Examples 1 to 16, on the other hand, results capable of satisfying both of thermofr~rr-hi l; ty and thermostability as described above are not obtained.
In addition, it is understood that Examples 23 and 24 15 in which two types of non-croF:~l ink~fl thermoplastic resins being hardly compatible l~ith each other are blended in rates in the range of 3: 7 to 7: 3 in weight ratio are Pxr~ nt in both of thermoformability and thermostability while bending strength values are furt~er lmproved as 0.30 kg/cm2 and 0.31 20 kg/cm2.
Fig. 3 is an electron microphotograph enlarging a part of a low density foam of a foam according to the present invention comprising high ~xri~nfl~d foams and low-expanded-foam thin layers covering outer surfaces thereof to 20, 000 25 times, and as understood from this photograph, it can be -confirmed that portlons having high crc~ccl ~nk~n~ density belng made crnqql ;nkin~ preferential, i.e., portions having high density in the photograph, and hardly crncRl ;nkf.~
portions having low crnqcl;nkin~ density, i.e., portions 5 having low denslty in the photograph, form a island-sea structure. Since the hardly crosælinked portions having low crnccl; nki n~ density are thus present and these portions have flowability in forming, the foam according to the present invention is rendered excellent in thermofor-10 mability.
While it is said that 25 96 compressive strength must beat least O . 6 kgf /cm2 and the compression set must be not more than 10 96 in a roof insulator, 25 96 compressive strength values are at least O . 6 kgf /cm2 and compression sets are rendered not more than 10 96 in Examples l to 24.
Namely, it is understood that foams well-balanced between compressive strength and compression set are obtalned.

Claims

(1) A foam comprising:
high expanded foams consisting of thermoplastic resins;
and low-expanded-foam thin layers consisting of thermoplas-tic resins covering outer surfaces of said high expanded foams consisting of said thermoplastic resins, a plurality of said high expanded foams being thermally fused with each other through said low-expanded-foam thin layer.

(2) The foam in accordance with claim 1, wherein said high expanded foams consisting of said thermoplastic resins being covered with said low-expanded-foam thin layers in overall said outer surfaces are arranged as a single layer along the thickness direction not to overlap with each other, and thermally fused with each other through said low-expanded-foam thin layers along the transverse direction.

(3) The foam in accordance with claim 1 or 2, wherein the expansion ratios of said low-expanded-foam thin layers are 1.1 to 10 times, and the thicknesses thereof are 30 to 500 µm.

(4) The foam in accordance with claim 1 or 2, wherein the expansion ratios of said high expanded foams are 20 to 50 times.

(5) The foam in accordance with any of claims 1 to 4, further comprising a composite sheet being stacked on at least one surface, and consisting of reinforcing fiber and a thermoplastic resin.

(6) The foam in accordance with claim 1 or 2, wherein said thermoplastic resins have preferentially crosslinked por-tions having high crosslinking density and hardly crosslinked portions having low crosslinking density, said portions forming a island-sea structure.

(7) A method of preparing a foam comprising a step of disseminating expandable thermoplastic resin pellets or annular substances containing a foaming agent, and a step of expanding said expandable thermoplastic resin pellets or annular substances by heating the same to a temperature exceeding the foaming temperature of said foaming agent being contained in said expandable thermoplastic resins.

(8) The method of preparing a foam in accordance with claim 7, wherein said expandable thermoplastic resin pellets or annular substances are disseminated in a single layer not to overlap with each other along the thickness direction.

(9) The method of preparing a foam in accordance with claim 7 or 8, wherein said expandable thermoplastic resin pellets or annular substances are so disseminated that downwardly projected areas of said expandable thermoplastic resin pellets or annular substances occupy 10 to 75 % of downward-ly projected areas of portions being enclosed with outer edges of regions where said pellets or annular substances are disseminated.

(10) The method of preparing a foam in accordance with claim 7 or 8, wherein melt viscosity of said expandable thermoplastic resin pellets or annular substances is 5000 to 20000 poise at 190°C.

(11) The method of preparing a foam in accordance with claim 7 or 8, wherein the resin gel fraction in said expand-able thermoplastic resins is 10 to 30 percent by weight.

(12) The method of preparing a foam in accordance with claims 7 to 8, wherein said expandable thermoplastic resins contain 100 parts by weight of a thermoplastic resin compo-sition containing two types of non-crosslinked thermoplastic resins being hardly compatible with each other and being prepared by mixing said two types of non-crosslinked thermo-plastic resins in the range of 3:7 to 7:3 in weight ratio,

1 to 50 parts by weight of silane modified thermoplas-tic resins employing thermoplastic resins being of the same type as one of said non-crosslinked thermoplastic resins, a silane crosslinking catalyst, and a foaming agent.

(13) A method of preparing a foam, comprising:
a step of disseminating expandable thermoplastic resin pellets or annular substances on a composite sheet consist-ing of reinforcing fiber and said thermoplastic resins;
a step of stacking a second composite sheet consisting of reinforcing fiber and thermoplastic resins on disseminat-ed said expandable thermoplastic resin pellets or annular substances thereby obtaining a laminate; and a step of heating said laminate to a temperature ex-ceeding the foaming temperature of said foaming agent there-by expanding said expandable thermoplastic resin pellets or annular substances for integrating said laminate.