CA1170416A - Method of producing open-cell foamed articles of cross-linked polyolefins - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An open cell foamed article of a cross-linked polyolefin haying a high open-cell ratio and a high degree of expansion is produced by a method which comprises the steps of preparing a foamable and cross-linkable polyolefin composition, forming the composition into a desired shape maintaining its gel percent at zero, decomposing the cross-linking agent and blowing agent concurrently by heating the composition under atmospheric pressure in such conditions that the peak of the ratio of the degree of cross-linking to the degree of decomposition of the blowing agent is not more than 20 and exerting a mechanical deformation to the resulting polyolefin foam to rupture the cell membranes.

Description

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Ij 1, ~ETHOD QF P~QDUCING

PEN~-CELL FO~ED ARTICLE~ OF CROSS-LXNKED pOLYOLEFINS
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'i E~CK~ROUND OF THE INVENTION
Il . , ¦ (1) Field of the InYention.
ll This invention relates to a method for the production ¦~of open-cell foamed articles of cross-linked polyolefins.
5 ¦1 (2) Description of the Prior Art:
ll Among the methods heretofore suggested for the manu-jifacture of cellular articles of cross-linked polyolefins, l~particularly of polyethylene, the methods which use the organic ¦I`decomposition type blowing agents are generally known in the art.
10 ¦IThese methods, as dlsclosed in Japanese Patent Publication ¦!NOS. 8840/1965, 18832/1967 and 22674/1968, generally comprise j~the steps of first cross-linking polyethylene by means of an !iorganic peroxide or by exposure to electron beams and thereafter ~ decomposing the blowing agent thereby imparting a cellular structure to the cross-linked polyethylene.~ In addition, there have been known a method in which a foamable and cross-linkable composition containing a polyolefin,~a blowing agent and a cross-linking agent is heated in a closed mold under increased pressure and thereafter the pressure applied to the composition in the mold is released resulting in the foamed cross-linked polyethylene, and also so-called "two-step" method, as disclosed in Japanese Patent Publication No. 29381/1970, in which the foamable and cross~linkable polyolefin compos~ti~on ~is heated in the s~e manner ~as aboye tQ paxtlai~ly decQmpose the blowin~ agent and thereafter further heated under atmospheric pressure to deco~pose the ~remainl~ng blowing agent. In the latter two methods, since the decomposition of bl~ing agent and cross-linking agent is effected' ¦ by heating t~e compos~tion in the closed mold under pressure, the cross-linking react~on of pol~ethylene takes place but the foam~ng is suppressed, and the expansion of polyethylene lloccurs only after the release of applied pressure. Therefore, 10 i the latter methods are same as the former methods in principle that polyethylene is first cross-linked and thereafter expanded.
The foamed products of cross-linked polyolefins obtained by the above mentioned methods have a closed cell structure. By I these methods, it will be difficult to obtain a foamed product 15 1l having an open cell structure. This is because, unlike the ~reactive foaming such as is involved in the production of poly-urethane foam, the foaming of cross-linked polyolefin according Ito the prior methods produces closed cells, and the membranes ¦¦enclosing these cells are so tough that, even under application ¦of compressive force, they will not be ruptured to transform such ¦closed cells into open cells and, even if the membranes are ¦forcibly ruptured somehow or other, the ruptured cell membranes ~will not be retained as they are. Owing to the melt elasticity ¦possessed commonly by polyolefins, such ruptured cell membranes cannot be reta~ned as they are, ~ithout reference to the particular type of blow~n~ agent and the presence or absence of cross-linking reaction. With the escape of the expanding gas r there ensues the .~heno~enon of..contxactl~on o,f.cell me~bxanes~ F occuX~ence of !lempty caYitie$. Thi:s ~henomen~n becomes more.conspicuous with ¦ the increasi~ng expans;~.on degree of polyolefin ~oam.
¦~ In the aboYe circums;tances! the ~reater part of the 5 ¦ii commercially a~ilable open-cell foamed articles is polyurethane ,,.foam. However, pol~olefins exhibit excellent weathering resis-¦Itance as compared with the soft urethane resin typifying those resins which are capable of producing open-cell foamed articles land also have very good resistance to chemicals and to water.
¦Thus, debut of an open-cell foamed article made from this resin ~has long been awaited.
So far, a few methods aimed at the production of open- ¦
cell foamed articles of polyolefins have been proposed, for ijinstance the method which comprises blending polyolefin with a 15 ll water-soluble powder such as starch and thereafter dissolving out ¦
¦the water-soluble powder from the mixture, and the sintering method in which the polyolefin powd.er is sintered. By these methods however, there are barely obtained cellular products of very low expansion ratio of the order of about 2 to 3 times the original volume.
Recently, there have been proposed methods which effect rupture of the membranes of closed cells of a foamed cross-linked . ¦polyethylene by the action of compressi~e force. One of these methods is disclosed in Japanese Patent Publication No. 10350/1974.
This method comprises cooling the foamed article of a thermoplastic resin having closed cells to a temperature below the second~order transition temperature (brittle temperature) of the thermoplastic ~resin and ~oll ~xessin~ th~ cooled ,f,o,a,m,ed 7axt~cle the~eby ~,producing a cellul~r arti,cle ,h,a,Yin~ open cells. ThIS ~ethod 1, ~,accom~lishes the trans-~o~m~tion of closed cells to open cells by l~
I~sacri~icing the strength o~ t~e thexmoplastic resin itself to 1, 5 i,some extent. Another of the methods is disclosed in Japanese , Patent Application la~d open to public inspection No. 63172/1979.
¦,This method cc,mprises producin~ a foamed article of polyethylene ¦Icontaining an inorganic filler and subjecting the formed article ! to compressive force there~y rupturing the membrances of closed 10 llicells and trans~orming the cells into open cells~ This method 'attains the transformation of closed cells into open cells by adding to the resin a large amount of the inorganic filler enough .
to lower the strength of the resin.
,, The former method, however, has a disadvantage that a 15 ¦I~very long time is required t~ cool the foamed article having ¦lextremely low thermal conductivity to a temperature below the 'brittle temperature (-100C) of the resin and the method, when desired to be carried through in a short period of time, is ',applicable only to foamed sheets of very small thickness.
~ The latter method also has a disadvantage that the method itself is hardly practicaL~,le and, if it is materialized by special technical efforts, the addition of the large amount of inorganic filler inevitably decreases the degree of expansion and increases the bulk density.
In any event, successful transformation of closed cells of a foamed cross-linked polyolefin to open cells on a commercial scale remains yet to be accomplished. This is because the poly-ethylene resin, etc. used as the raw material of the foamed _ 4 - , ,,, , ~, , .' ~17~
cross-linked polyolefin are so tough, b~ nature, that the membranes of closed cells in the foamed article will not be ruptured under application of compressive force and, even if the compressive force is great enough to rupture such membranes, the compressive force is transmitted only in the surface region of the foamed article. The compressive force transmitted to the deep portion of the ; foamed article is no longer great enough to rupture the membranes in that portion. Thus, the desired transformation of closed cells into open cells has not been accomplished to date.
SUMMARY OF T~IE INVENTION
An object of an aspect of this invention, therefore, is to provide a method for easily producing an open-cell foamed article of cross-linked polyolefin merely by imparting mechanical deformation to the foamed article without requir-ing any other special treatment or the addition of a filler.
An object of an aspect of this invention is to provide a method for the production of an open-cell foamed article of cross-linked polyolefin of a great thickness with a high degree of expansion.
An object of an aspect of this-invention is to provide open-cell foamed articles of cross-linked poly olefin possessing highly advantageous properties and having desired thicknesses with a high degree of expansion.
- To accomplish the objects described above and other objects, according to the present invention, there is provided ' ' ' , .

.-~ 6 ~7~4~6 a method for the production of open-cell foamed articles of cross-linked polyolefins comprising the ; steps of:
. blending with heating a polyolefin, a chemical blowing agent and a cross-linking agent to obtain a foamable and cross-linkable composition, :- forming the foamable and cross-linkable composition into a desired shape under pressure and at a temperature of from 115 to 155C and which is lower than the foaming temperature of said composition while maintaining its gel percent at zero, heating the shaped composition at a suitable foaming temperature under atmospheric pressure in such conditions that the peak of the ratio of the degree of cross-linking to the degree of decomposition of the blowing agent is not more than 20 to decompose the cross-linking agent and the blowing agent concurrently, thereby giving rise to a foamed product of cross-linked polyolefin havi.ng cells enclosed with very thin

2~ membranes capable of being easily ruptured by the action of mechanical force, and ~`
mechanically deforming said foamed product to cause the membranes of the cells to rupture;
wherein the gel percent is the ratio of the foamable and cross-linkable composition after extraction to that before extraction, the extraction .being carried out under reflux of trichloroethylene for 24 hours by means of a soxhlet extractor using a glass filter of from 45 to 50~ ; the degree of cross-lin~ing is the gel percent of the composition at a given heating time and the degree of decomposition of the blowing agent is the ratio of the degree of expansion at said given heating time to the final degree of expansion of the foamed composition.
BRIEF DESCRIPTION OF T~E DRAWING
'' The single figure is a graph showing the change of the ratio of the degree of cross-linking to the degree of decomposition of the blowing agent along the course of heating time at a suitable foaming temperature.

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DETAILED DESCRIPTION OF THE INVENTION
The method for the production of an open-cell foamed article of cross-linked polyolefin in accordance .; with the present invention consists primarily in utilizing the adjustment of the , .

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decompositiQn Xate o~ the blow~ing a~ent relatiYe to the xate of cxoss~link~n~ ~e~c~i~on, I
~n thè theory ~ the ~Qa~ing o~ cross-linked polyolefin, ' the so-called "preceding cross-linkin~ ~ ~ollowing foaming", as 5 ~ ~entioned a~ove, is common sense to those skilled in the art.
'In other words, ~ere has heen considere~ that the expanding gas ¦~is leaked out from the resin at the time of expansion, unless the yiscosity of xesln is increased by ~irtue of its cross-linking.l l However, it has been found that the dilation of precedently cross-¦
10 Ililinked resin is poor and thus it is hardly possible to produce a foamed product having cells enclosed with very thin membranes `suitable for obtaining an open-cell foamed article. Now it has been found, according to the present invention, that it is l possible to produce said foamed product by concurrently effecting ¦
15 ~the cross-linking reaction and the foaming of the foamable and ~¦cross-linkable composition being maintained at the state where its ,gel percent is zero.
1 Now, the meaning of the expression "concurrent decom-¦lposition of cross-linking agent and blowing agent" will be lexplained.
When the foamable and cross-linkable composition is heated under atmospheric pressure, the cross-linking reaction and the decomposition of blowing agent take place, and the cross-linking curve and the dec~mposition curve of blowing agent are respectiyely obtained. Now then, when the foamable and cross-linkable composition of which ~el percent is maintained at zero is heated ulder atmospheric pressure and th~ ratio (y) oE the .. .... ..
, ~L~7~416 ,deg,ree of, c,rjos,s,~linki~ng,to, the de~.ee o,f, decomposition o,f, the blow,~n~ a~ent agai~nst.the ~eatin~ time is plotted on.lo~axi.thmic ~graph paper! the.re can ~e o~tained the curye as shown in the ~iacc.ompanyin~ dra~ing. Howevex, in case:the ~oamable composition 5 I cro~s-linked in adyance as. in the prior methods is heated under ~,atmospheric pressure, the similar curve ~ay not be obtained.
¦, ... Degr.ee of. cross--l.inkin~
Y - .
, Degree of decomposition of blowing agent i Degree of cross-linking: Gel percent o~ resin at a certain 10 ll heating time Degree of decomposition of blowing agent: Ratio of the , degree of expansion at the same ,~ heating time as above to the final lll degree of expansion of the foamed 15 il product obtained.
Ii Where, the term "gel percent" means the ratio of the .weight of the sample after extraction to that before extraction, ¦~wherein the extraction is carried out under reflux of trichloro- "
lethylene used as a solvent during 24 hours by means of a soxhlet ¦extractor using a glass filter of from 40 to 50~ . The gel percent .
¦is calculated by the following equation. The degree of cross-linking is proportional to the increase of gel percent.

. W, 1 ' { ~ ~1 ~ X, ) +, ,, ,~ } ,~o , , , Gel ,percent ~ - x 100 W _ { AT (l = x) + 0~7 T~X ~,.T ~
Where~
WO: Weight of the sample before extraction, ~7~4~

: r~l; Wei.~ht o~. the s~$a~1e af.ter. extr.action, ' T; Total weIght pa~ts of.the.co~ponents~, A: ~.e ght parts o~.the blowin~ ~yent,:.
Il C: ~ei~ht parts o$ the ~illers, 5 'I x: Decomposition.degree of the blowing agent, , AT (l-x)Wo^ Weight of the remaining blowing agent in the sample, I' ,.. .
-7 - ~T x WO:: Weight of the residue of decomposed blowing , agent in the sample, and 10 " T WO: Weight of the fillers in the sample.
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In the figure, the peak A of the curve indicates the . ratio (y) of the degree of cross-linking to the degree of , decomposition of the blowing agent at which the decomposition of ¦Iblowing agent most lags behind the cross-linking of resin compound 15 1i That is to say, at the heating time of this point A the distance ~ibetween the cross-linking cur~e and the decomposition curve of ! blowing agent is most wide. The greater value of this ratio (y) j at the peak means that the foaming is more delayed to the cross- ¦
linking accordingly. On the other hand, the smaller value of 20 said peak ratio means that the delay of the foaming to the cross-linking is little, that is to say, the cross-linking reaction and the foaming phenomenon of the foamable and cross-linkable . lcomposition concurrently took place.
Surprisingl~ it has now been disco~ered that there is a limit of peak in said ratio (y) for obtaining a foamed product having ~el membranes capable of being easily r~ptured by the . ... . ...~ , ....

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action o~ ~echanical.~o~.ce, The.peak yalue o~ said r.atio (y) is ~n~luenced h.~ the ty~e ~ xe~ins.u~ed and.the ~ounts of the c~os;s-.link~ny agent or the ~lowin~ agent. HoweYer ! in spite of ' thes~e paXametexs, it has been ~ound that in case said peak ratio 5 ', is not more than 20, there can be obtained a ~oamed product 1. haying cell mem~ranes suita~le for manu~acturing the open-cell ¦ foamed art~cle. Sa;d Yalue "20" of the peak ratio is critical, ¦~ but it is preferable to control said peak ratio in the range of jl not more than 15, ~ecause in case o~ the particular type of lO i resins the rigidity is required for the reaction conditions, etc.
at the value close to 20.
Therefore, the expression"concurrent décomposition of ..cross-linking agent and blowing agent" as used herein means that Il the decomposition of cross-linking agent and blowing agent is 1 effected in such consitions that the peak of the ratio (y) is not more than 20. It will be a good practice to subject various polyolefins to a preliminary foaming to determine the range of the, ¦¦amounts of cross-linking agent, blowing agent, foaming aid, if ¦Irequired, and their optimum foaming temperatures which satisfy said conditions. In the actual operations, one can select the amounts of each components within the range thus determined.
To describe the present invention more specifically, a given polyolefin is mixed with a blowing agent, a cross-linking lagent and, if required, a foaming aid, a filler and a pigment, ¦and the resultant mixture is kneaded with a mixing roll heated or the like, Then, the obtained composition is placed in the mold ha~ing a desired cavity profile and, under the pressure , ~7(~16 ., ~
Il l ~pplied w~ th.a p~ess.! the~ally sha~ed.~t a t~mFeX~atu~e within . the ~an~e of ~xom 115C .to 155C! pxefe~ably fxo~ 12QC.to 140C, . and therea~ter removed ~xqm-the mold. In place of said shaping ,, at an eleyated temperature and under pressure,: the:composition 5 j after knea`d~n~ may be sIlaped by heating it in the mold to which . the pressure is not applied or by directly passing it through. I
an ext~uder or a calendering roll. HoweYer, since the heating in ¦
~I this shaping step rise the foama~le and cross-linkable composition ¦I to the thermally excited state and, as a result, contributes to 10 .~ the more smooth concurrent decomposition of the cross-linking .agent and the blowing agent in the following foaming and cross-;linking step, it is preferable to carry out the shaping of the . composition under the heating. For instance, in case the shaplng `,is carried out without heating and without applying a pressure, I
15 I~the cells of the foamed product obtained in the following foaming ¦
l~and cross-linking step are coarse and un-uniform, which is some-what undesirable. In this thermal shaping it is significant that , the foamable and cross-linkable composition should be shaped -i llmaintaining its gel percent at zero, namely at the predetermined 20 ¦¦ heating time and temperature whereby the cross-linking of ¦polyolefin will not occur. Therefore, the shaping temperature is required to be lower, preferably by more than 20C,than the jfoaming temperature in the following foaming and cross-linking Istep. If the cross-linking of polyolefin occurs in this thermal shaping step, as being apparent from the comparative examples described here~nafter, there will be obtained a final product having an open cell ratio of less than 50~ which cannot possibly -, 11 ~
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se~ s to he the open cell foamed pxoduct. In addition! if said shapin~ ~s ca~ied QUt at an eleyated tempe~atuXe and under pressure, as bein~ apparent ~rom Examples 11 to 15 described ~I hereinafter! t~e cell s-ize of the obtained foamed product becomes I
I more fine as the heatin~ time increases. Therefore, it is possible to delicatel~ vary the ap~eaxance and the tactile impression of I, the final foamed article with ~arying heating time. By the way, ,l in this thermal shaping step a ~ery few amounts of blowing agent ~I may be pre-decomposed, and as a result the shaped composition 10 iimay expand to the degree of about 2 times larger than the original il Yolume ~hen removed from a mold. However, this phenomenon is ' not within the concept of the foaming and/acceptable to the present invention. There can be considered that the above-, mentioned difference in cell size is due to the fact that the i nuclei for cells may be formed by this pre-decomposition of ' blowing agent.
The foamable and cross linkable composition shaped as ¦ above is then heated under atmospheric pressure thereby con-l¦currently decomposing the blowing agent and cross-linking agent.
¦ The meaning of the expression "concurrent decomposing of blowing ¦agent and cross-linking agent" and the condltions thexefore has ¦ already been explained. In this foaming and cross-linking step I the shaped composition is heated in an atmosphere of nitrogen or in a heating medium, for instance metal bath containing Rose's metal, ~ood~s metal or the like, oil bath, molten salt bath containing one or more of the salt such as sodium nitrate, potassium nitrate, potassium nitrite or the like. The shaped ~7~ 6 composltion ls ~re~e~ablx placed ~in an openable ~old o~ ~etal hox wfi~ch is not a~rti~g~t and ~eated in said heatin~ ~edium kept at a su~ta~le foam~ng temperatuxe~ ~therwise, t~e openable mold '1 or metal box Whic~ IS not airtI~ht may ~e provided with a heater 5 ,l on the s~rface o~ its ~etal plate or with a jacket through which a heating medium such as steam, heating oil, etc. is circulated.
I By the use of th~s openable mold~ the foamable composition is I indirectly heated ~y the heater or heating medium. Besldes, the ~i shaped composition may be coYered with a metal sheet, etc. capable 10 ,, of moying up and down and heated in such a state. After the heating for a predetermined period, the composition is cooled to obtain a cooled and foamed product. The foaming temperature is selected within the range of from 145C to 210C, preferably 1, from 160C to 190C, to suit the particular type of polyolefin 15 ¦1 actually used, and the heating time is within the period of from j 10 to 90 minutes, preferably from 15 to 40 minutes. Thus, there Il can be obtained a foamed article having closed cells which ¦~i membranes are able to be easily ruptured by exerting a mechanical ¦I deformation and the degree of cross-linking similar to that of ¦ the foamed product produced by the prior methods (up to about ¦ 95% of gel percent).
In accordance with the present invention, the heating in said foaming and cross-linking step may be carried ou-t in two steps. In this two-step process, the conditions for foaming and cross-~linkin~ polyolefin is mild~ and thus the decompositions of cross--l~nking agent and blowing agent can be accomplished more concurrently in two steps. By this two-step process, the ~17~3416 hete~ogeneous heat cond~cti~on in the dixectiQn o~ thickness of the ~oama~le and c~os~-l;nkable composition may he eliminated and the composition may~e'homo~eneously~ heated. As a result, there ~ill not arise'the phenomena such as the su~face cracking 5 ', resultin~ from the partial uneYenneSS of ~oamin~ in the composi-l~l tion, the collapse and the escaping of gas. Furthermore, it is ¦~ possible to ~ncrease'the expansion ratio of the foamed article 1' obtained up to about 70 times the original volume at will and ,i the thickness up to about 15~mm. Therefore, this two-step process!
`~ 10 1' is particularly su~table for producing thickex foamed articles or the foamed articles having higher expansion ratios more than 20 times the original ~olume.
To describe this two-step foaming and cross-linking .; ~ I
process more specifically, in the first step the foamable and 15 li cross-linkable composition shaped as mentioned above is heated ' in the same manner as mentioned hereinbefore, i.e. in the ~ atmosphere of nitrogen or in the metal bath, molten salt bath, I
¦~ etc. at a temperature of from 145C to 180C for a period of from ¦
1, 5 to 60 minutes, preferably from 10 to 45 minutes, and thereafter 20 ¦i the intermediate product is removed from the heating medium.
In the second step, the intermediate product is further heated in the same manner as mentioned hereinbefore at a temperature of from 170C to 210C for a period of from 5 to 50 minutes, l preferably from 15 to 40 minutes, and subsequently cooled to giye rise to a foamed art1cle with low density. In said first step, it is preferable to decompose from 5 to 70% of blowing agent, whereat the gel percent of resin composition is reached ` I - 14 -~7(~4~6 , I , to fxom ahout 20. to ~b.o;ut 8Q~ ..th.e de~xee of, decomposition of the blowin~ a~ent ~nd the ~el per:cent are ~exy hl~h, the aforement~Qned advantages of th~s two-step process will not be I deriYed.
5 , The ~oamed article obtained as aboYe is compressed by being passed between two rolls rotated at an equal speed, with the result that t~e compression so applied will rupture the membranes o~ closed cells ~f the ~oamed article and consequently Iconvert the closed cell structure to th.e open cell structure.
10 ill The open-cell foamed article obtained by the method of .this invention possessed outstanding properties fa~orably comparable with the properties of the foamed article of poly-urethane, and the open cell ratio thereof determined in the similar manner to Remington Pariser Method (ASTM D 1940-62T) is jequal or nearly equal to 100%.
The polyolefins which are preferably used in this linvention are low-density polyethylene, medium-density poly-.l~ethylene, high-density polyethylene, poly-1,2-butadiene, ethylene-Ilpropylene copolymer, ethylene-butene copolymer, ethylene-Yinyl 20 ¦1 acetate copolymer, copolymers of ethylene with up to 45~of l¦methyl-, ethyl-, propyl- or buthyl-acrylate or methacrylate, I
¦,chlorinated products of the above homopolymers or copolymers whichj ¦chlorine content being up to 60% by weight, mixtures of two or more lof the above polymers and mixtures of the above polymers with isotactic or atactic polypropylene.
To suit the purpose of.this invention,. the cross-linking .. agent ought to decompose in polyolefin at a temperature at least - 15 - . .... .... .....

~L7V~6 higher than the flow point of polyolefin. Organic peroxides which decompose upon being heated to liberate free radicals capable of giving rise to intermolecular or intramolecular cross-linked bonds and, therefore, serve advantageously as S radical generators meet this requirement. Examples of such organic peroxides include~ but are not limited to: dicumyl peroxide, l,l-ditertiary-butyl-peroxy-3,3,5-trimethyl cyclo-hexane, 2~5-dimethyl-2,5-ditertiary-butyl peroxyhexane, 2,5-dimethyl-2,5-ditertiary-butyl p~roxyhexane, ~ , ~
10 -ditertiary-butyl peroxydiisopropyl benzene, tertiary-butyl peroxy ketone, tertiary-butyl peroxy benzoate, etc. The organic peroxide which best suits the particular type of poleolefin used should be selected.
The blowing agents which are usable in this inven-tion are chemical blowing agents having the decomposition temperature higher than the melting point of poleolefin.
Examples of such chemical blowing agents include, but are not limited to: azo type compounds such as azodicarbona-mide and barium azodicarboxylate; nitroso type compounds such as dinitrosopentamethylene tetramine and trinitroso-trimethyl triamine; hydrazine type compounds such as p,p'-oxybis ~benzene sulfonyl hydrazide); sulfonyl semicarbazide type compounds such as p,p'-oxybis (benzene sulfonyl semi-carbazide3 and toluenesulfonyl semicarbazide, etc.

Besides the particular type of polyelefin used and the foaming temperature selected, the amounts of the cross-linking agent and the blowing agent are the significant factor exerting influence on the ratio Iy~
5 of the degree of cross-linking to the degree of decom-position of the blowing agent. If the amount of -16a-~7~6 cxQ~ nkin~ ;a~ent ~s~ tqo much o~ the amount of blowin~ a~ent is ve~y l~ttler the ~eak o~ s~aid X~tio ~ will e~sily exceed 20 and~ as a natural consequence, ~t is hardl~ possible to p~oduce ' an open-cell ~oamed article. There~re~ the amounts of the cross-5 , linking agent and the blow-in~ agent should be selected within the range in which the peak of sa;d ratio (y) will no exceed 20.
Besides the above factors, it is possible to control the peak of ratio (y) by adding a ~oaming aid (See Example 1 and IlComparative Example 1 described hereinafter). Thus, in the present invention, a foaming aid may be used depending on the ! particular type of blowing agent to be used. Examples of such aids include, but are not limited to: compounds having urea as a principal component; metal oxides such as zinc oxide and lead oxide; compounds having salicylic acid, stearic acid, etc~ as a 15 ,,principal component, i.e. higher fatty acids, metal compounds of ~higher fatty acids, etc.
i i In the present invention, to effect the improvement ~lof the properties of composition prepared and the reduction of ¦Icost, if desired, there may be added into the composition the 20 ¦~compounding additives or fillers which do not exert very harmful ¦influence on the cross-linkage of polyo-lefi~, for example metal ¦oxidessucllascarbon black, zinc oxide, titanium oxide, calcium oxide, magnesium oxide and silicon dioxide; carbonates such as magnesium car~onate and calcium carbonate; fibrous filler material such as pulp; various dyes; pigments; fluorescent materials; and rubber compounding ingredients commonly used.
UDlike the co~ven~ional technique w~hi-h ~,,~

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Il i ,iohtain xe~uixed ~educti~on o~ the st~en~th of the XeSin as b~
, cooli~n~ the closed-cell ~oame~ R~qduct to ~ tempexatu~e below the ,Ibrittle tempe~ature of the resin o~ addin~ lar~e amounts of inorganic ~lllers to the foaming composition, the method of this 5 ' invention utilizes the adjustment of the decomposition rate of the blow~ng agent relatiYe to the rate of cross-linking of the . i resin. Conse~uently, this inYent~on enables open-cell foamed ~articles of cross-linked polyolefins to be easily obtained llwithout impairing the advantageous properties of polyolefins.
10 ,~In addition, the final foamed articles can be obtained by this , inyention with high open-cell ratios falling within the range of from 97 to 100~ and in large thickness. The method of this ` invention has further advantages of easy operation, short working ~, , time and high productivity.
15 iI The open-cell foamed articles of cross-linked polyolefins obtained by the method of this invention can be suitably used for ,cushioning media, filters, heat insulating materials, coaters, etc.
,Particularly, ~hen said foamed articles are used in clothes, noise ,abating materials and heat insulating materials so far produced ~0 ¦by using soft polyurethane foams, they exhibit outstanding ¦resistance to weathering and chemicals and high flame retardance Iland, therefore, warrant safe use. I
¦l For more specific illustration of the present invention,l l the following examples are presented which are intended to be merely illustrative of and not in any sense limitative of the inyention.

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. , . : . , 1~L7~L6 ., I . .
Exam~le l; , ' A compos~ti,on con,~i,stin~ o,f, ethylene-yinyl acetate I copolymer ~proprietary product of Mitsui Polychemical co. ! Ltd., marketed under trade name o~ "EYerflex P-14Q3"~ YAC 14% by 5 i`weight), 17 parts ~y~ weight per hundred parts by weight of resin ,'(phr~ of azodicarbonamide Cproprietar~ product of Eiwa Chemical ~,Industry Co., Ltd., marketed under trade name of "Vinyhol , f AC#50s~ 0.83 phr o~ dicum~l peroxide and 0.5 phr of zinc oxide `'was kneaded in a mixing roll at 85C. the resultant blend was 10 I,charged in a mold (150 X 150 X 7mm) within a press kept at 126C
and heated under increased pressure for 30 minutes to form a foamable and cross-linkable sheet. The gel percent of this sheet Iwas zero. The sheet obtained was then heated for 40 minutes in a llmetal bath kept at 170C to obtain an intermediary foamed product 15 l in which 30.5% of the blowing agent was decomposed. Thereafter, the intermediary foamed product was further heated in a metal bath ¦kept at 190C for 20 minutes to obtain a foamed product in which ~he remaining blowing agent was completely decomposed. The peak ¦value of the ratio(y) in said foaming and cross-linking step was 20 ¦10.4. After the cooling, the foamed product was passed between two rolls separated by a space of 3mm and rotated at an equal speed to rupture the cell membranes. The foamed article obtained had a thickness of 23~0mm, a bulk density of 0.03g/cm3 and an open : ¦cell ratio of 100%.
25Wherein, the open cell ratio was measured in a similar manner to ~em~ngton Par~ser-method ~ASTM D 1940-62T) and I ^~ ;rrc7Jcs ~rrks .

, 3 ~7~94~6 ~, i l i.
~deter~ined bx the ~Qllo~in~ c~lculation ~ox~ula.

.. '(V.s'~ ~). ~ (~' ~ Y~
j Open cell r~tio ~ 100 ~$ ~ ~R

~ ~s = ~Y X lQQ
~s ~ ~R

~here, i~ !
Vs: Yolume of sample, , VR: Volume of resin ~atrix (= weight of sampls Ws/
~ density of resin)/ and 10 ~ aV: Increase in Yolume.

Examples 2 - 4:

The procedure oE Example 1 was repeated using the varying amounts of zinc oxide and dLcumyl peroxide shown in Table ~ l. In each Example the complete open-cell foamed article havingi 'the open cell ratio of 100% was obtained. The degree of decompo-sition of the blowing agent in the intermediary foamed product ~¦w~as respectively 51.7% in Example 2, 69.0% in Example 3 and 11 ¦lin Example 4, and the peak value of the ratio (y) was 5.0 in ¦Example 2, 1.27 in Example 3 and 4.0 in Example 4.

20 ~I Comparative Example 1:

l'he procedure of Example 1 was repeated without using zinc oxide. The peak Yalue of the ratio (y) in the ~oaming and cross-linkin~ step was 20.5. The foamed article obtained had an open cell ratio of 55.3~, which exhibits that the rupture of , , . -, , ~ , ~ ,, ~7(~416 i. , .
cell ;me~h~ane~ waS e~ected only pa~ti~all~.

, i , . .
EXample 5s A foamed art~cle ~as producea ~x~m a composition consisting of ethylene-~inyl acetate copolymer (pxoprietary 'product of ~itsubishi Petrochemical Co., Ltd. marketed under tradename of "Yukalon EVA-41H~ AC 16% b~ weight), 17 phr of ~azodicarbonamiae and 0.53 phr of dicumyl peroxlde in the same manner and under the same conditions as in Example 1. The open cell ratio of the foamed article o~tained was 100%.

Example 6:

Il A composition consisting of the same resin as used in 'I Example 5, 17phr of azodicarbonamide, 0.08phr`of zinc oxide and ,0.73phr of dicumyl peroxide was kneaded in the same manner as in iiExample 1. The resultant blend was~rged in a mold (140 X 140 X
15 ~l28mm) within A press kept at 126C and heated under increased pressure for 30 minutes to form a foamable block. The foamable ¦block obtained was then heated in a metal bath kept at 170C for 40 minutes to obtain an intermediary foamed product in which 27%
lof blowing agent was decomposed. Thereafter, the intermediary Ifoamed product was placed in an openable mold (370 X 370 X llOmm) ¦which is not air-ti~ht, and heated in a metal bath kept at 190C

for 30 minutes to decompose the remaining blowin~ agent completely.
After the cooling, the ~oamed product was removed from the mold.
The foamed product was passed between two rolls separated I

t~ ~ J~ ~narks ~1L7~
, I

hy a space of lO~m and ~otated at an equal speed to ~uptuxe the I cell membxanes. The thick open-cell foamed article haying a thickness of lOQmm! a ~ulk dens-ity of 0.03 g/cm3 and an open cell ratio of 100% was obtained.

Example 7:
i ' .1 .
A foamable sheet was obtained from a composition consisting of the same resin as used in Example 1, 17 phr of azodicarbonamide, 0.2 phr of zinc oxide and 0.63 phr of dicumyl peroxide in the same ~anner and under the same conditions as in Examplel. The foamable sheet obtained was heated in a metal bath kept at 190C for 15 minutes to completely decompose the blowing agent and cross-linking agent in a single stage, resulting ~ in a foamed product. After the cooling, the foamed product was !i converted to an open~cell foamed article ha~ing an open-cell ratio~
15 l, of 100~ by passing between two rolls rotated at an equal speed in ¦Ithe same manner as in Example 1.

¦~ Example 8: 1 An open-cell foamed article was produced from a l¦composition consisting of the same resin as used in Example 5, 20 1!35 phr of azodicarbonamide, 0.53 phr of dicumyl peroxide under the same conditions as in Example 1. The open-cell foamed article highly expanded had an open-cell ratio of 100%, a thickness of 30mm and a bulk density of 0.019 g/cm3.
. I

1~7(~4~6 i li . .
- ~` Ex~mple 9;

A composi,tLon cons~i~sting of low-density pQlyethylene ,~ (proprietary product o~ Mi,ts,u~i$h~ Petrochemical Co.~ Ltd.
-~marketed under the tradename of "~ukalon LK-30," density;
0.918 g/cm3, MFR 40), 17 phr of azodicarbonamide and 0.2 phr of , ~zinc oxide was intimately kneaded in a mixing roll at 100C.
The resultant blend was placed in a mold (150 X lS0 X 7mm) within a press kept at 136C and heated under increased pressure for ,, i ,,30 minutes to form a foamable sheet. This sheet was cross-linked and expanded under the same conditions as in Example,l, and "
thereafter passed between two rolls rotated at an equal speed in the same manner as in Example 1.~ An open-cell foamed article having an open-cell ratio of 100% and a thickness of 23mm was thus ,lobtained.

' Example 10:

An open-cell foamed article was produced from a compo-,1 . i ¦Isition consisting of low density polyethylene (proprietary product, ¦of Mitusbishi Petrochemical Co., Ltd. mark~ted under the tradename ¦¦of "Yukalon HE-30", density; 0.92 g/cm3, MFR 0.5), 17 phr of 20 llazodicarbonamide and 0.13 phr of dicumyl peroxide under the same jlconditions as in Example 9. The open-cell foamed article had an ¦lopen-cell ratio of 100~ and a thickness of 23mm.

Examplesl1 ~ 15:
-The procedure of Example 1 was repeated using the same -~ ~a~ m~ks ~17V4~6 :.
., , 'composition as in Ex~ple 1 ,a,nd the y~xying heatin~ t~me within a, press ~ l! 5! lQJ 20 ~a,nd 3Q ~i~nutes. Ea,ch ~oamed article had ` an open-cell ratio o~ lQQ%~ and the thickness of shaped article and ~ts appearance was unchanged regardless of the heating time.
Howeyerr the`cell size was reduced ~ith the increase of heating '~time.
i , Comparative Examples 2 and 3:

The foamed articles were produced from the same composition as used in Example 5, under the same conditions as in Example 5 except that the heating temperature within a press was changed to 145C (comparatlve example 2) and 151C (comparative ~' ` example 3) respectively. After the step of compressing, the ,foamed articles were found by test to possess the open-cell ratios llof 47.2~ and 45.2~ respectively, which exhibit that the rupture 15 ,of cell membranes was obtained partially and the foamed articles will not function as an open-cell`foamed article. Under the , conditions, the gel percents of foamable sheets removed from a Ilmold within a press were 20.0% at 145C and 31.2~ at 151C

¦Irespectively. I

20 1~ The operating conditions involved in the examples and comparatives examples cited above and the results respectively obtained are shown in the following Table 1. And the relation between the cell size of the final foamed product and the heating Itime in the shaping step in examples 11 to 15 is shown in ITable 2.

- ` -1~7~6 Tahle 1 ~

1 CQ~POS~I~TIQN AND SHAPIN~ CONDI~TION$

`, No. of Composit~on (ph~) Shapiny Gel 1~ -- Conditlons ! examplesi: Foamlng Cross- under of shaped i and ~lo~ing l~n~Ln~ pressure sheet 1 ccmparatiye Res~n agent (z~nc a~ent Temp. Time (%) ~les* ~ADCA) oxide) (DC~? (c) (min. ) 1E~ "P-1403" 17 0.08 0.83126 30 o 'I P-1403 17 0.20 0.83126 30 0

3 P-1403 17 2.50 0.83126 30 0

4 P-1403 17 0 0.33126 30 0 5EV~ "EV~-41H" 17 0 0.53126 30 0 ` 6EV~-4IH 17 0.08 0.73126 30 0 7EV~ "P-1403" 17 0.2 0.63126 30 0 8EV~ "EY~-41H" 35 0 0.53126 30 0 9LDPE "LK-30" 35 0.2 0.53136 30 0 10LDPE "HE-30" 35 o ~ 0.13136 30 0 11EV~ "P-1403" 35 0.08 0.83126 1 0 12 P-1403 35 0.08 0.83126 5 0 13 P-1403 35 0.08 0.83126 10 0 14 P-1403 35 0.08 0.83126 20 0 15 P-1403 35 0.08 0.83126 30 0 I
l*EY~ "P-1403" 17 O 0.83126 30 0 2*EV~ "EY~-41H'i 17 0 0.53145 3020.0 3*Ey~-4IH 17 0 0.53- 151 3031.2 ~ L7~3~L3L~ ~

,, ~

Table 1 (2) FOAMING CONDITIONS AND PROPERTIES OF FINAL PRODUCT
j:
of one step Conditions of tw~ Thickness Bulk Open-cell i e~amples . step foaming of flnal r ~ :
; foam mg Of Lmal ratio and openrcell open-cell .
. first step .second step foam %
I cGmparatlve T T- foam ( ) examples* (C)P ~in )Temp. Tlme Temp. Time (mm) (g/cm ) - 170 40 lgo20 23 0.030 loo !
2 - ~ 170 40 lgo20 23 0.030 loo 3 - - 170 40 lgo20 23 0.030 loo ` 4 - - 170 40 19020 23 0.030 loo - - 170 40 lgo20 23 0Oo30 loo 6 - - 170 40 l9o30 loo 0.030 loo 7 lgo 15 - - ~ 23 0.030 loo ~ 8 - - 170 40 lgo20 30 o.olg loo I -`' 9 - - 170 40 lgo20 23 0.030 loo i lo - - 170 40 lgo20 23 0.030 loo !
- - 170 40 lgo20 23 0.030 loo 2 _ - 170 40 lgo20 23 0.030 loo 3 - - 170 40 190~ 20 23 0.030 loo 4 - - 170 40 lgo20 23 0.030 loo - - 170 40 l9o20 23 0.030 loo * - - 170 40 lgo20 23 0.030 55.3 2* - - 170 40 lgo20 20 0.030 47.2 3* - - 170 40 lgo20 20 0.030 45.2 1 - 26- . . .~

. , ' , .

117V4~L~

. ' ' Table 2 ,, '.

No. of Heating time in examples shaping step Cell size ,~ (min.) (mm) f I . ~
ll 1 1.5 12 5 0.9 . 13 10 0.8 . 14 20 0.6 ` 15 30 0.6 . ~ .
;l !
i, I~ f , ; ~ .

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A method for the production of open-cell foamed articles of cross-linked polyolefins comprising the steps of:
blending with heating a polyolefin, a chemical blowing agent and a cross-linking agent to obtain a foamable and cross-linkable composition, forming the foamable and cross-linkable composition into a desired shape under pressure and at a temperature of from 115 to 155°C and which is lower than the foaming temperature of said composition while maintaining its gel percent at zero, heating the shaped composition at a suitable foaming temperature under atmospheric pressure in such conditions that the peak of the ratio of the degree of cross-linking to the degree of decomposition of the blowing agent is not more than 20 to decompose the cross-linking agent and the blowing agent concurrently, thereby giving rise to a foamed product of cross-linked polyolefin having cells enclosed with very thin membranes capable of being easily ruptured by the action of mechanical force, and mechanically deforming said foamed product to cause the membranes of the cells to rupture;
wherein the gel percent is the ratio of the foamable and cross-linkable composition after extraction to that before extraction, the extraction being carried out under reflux of trichloroethylene for 24 hours by means of a soxhlet extractor using a glass filter of from 45 to 50µ; the degree of cross-linking is the gel percent of the composition at a given heating time and the degree of decomposition of the blowing agent is the ratio of the degree of expansion at said given heating time to the final degree of expansion of the foamed composition.

2. The method according to claim 1, wherein the decomposition of the cross-linking agent and blowing agent in the shaped composition is effected more concurrently by the two step heating; in the primary heating from 5 to 70% by weight of the blowing agent originally present in said composition being decompo-sed, and in the secondary heating the undecomposed blowing agent and cross-linking agent remaining in the primary foamed product being decomposed at a higher temperature than that in the primary heating.

3. The method according to claim 1 or 2, wherein the decomposition of the cross-linking agent and blow-ing agent in the shaped composition is effected by heating the composition in a bath selected from the group consisting of metal bath, oil bath and molten salt bath or in an atmosphere of nitrogen gas.

4. The method according to claim 1, wherein the shaped composition is placed in an openable mold which is not airtight and provided with a heater or a jacket through which a heating medium is circula-ted, and the decomposition of the cross-linking agent are blowing agent in the shaped composition is effected by the indirect heating with the heater or the heating medium.

5. The method according to claim 1, wherein the foaming temperature of the shaped composition falls within the range of from 145 to 210°C.

6. The method according to claim 2, wherein the foaming temperature of the shaped composition falls within the range of from 145 to 180°C in the primary heating and within the range of from 170 to 210°C in the secondary heating.

7. The method according to claim 1, wherein the shaping of the foamable and cross-linkable composi-tion is effected by use of an extruder or a calender-ing roll.

8. The method according to claim 1, wherein the mechanical deformation is effected by means of compres-sion exerted with two rolls rotated at an equal speed.

9. The method according to claim 1, wherein the foamable and cross-linkable composition contains a foaming aid.

10. The method according to claim 1, wherein the foam-able and cross-linkable composition contains a compound-ing agent or filler, such as metal oxides, carbonates, fibrous filler materials, dyes, pigments, fluorescent materials and rubber compounding ingredients.

11. The method according to claim 1, wherein the poly-olefin is selected from the group of high-density polyethylene, medium-density polyethylene, low-density polyethylene, poly-1,2-butadiene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-vinyl acetate copolymer, copolymers of ethylene with up to 45% of methyl-, ethyl-, propyl- or butyl-acrylates or methacrylates, chlorinated products of the above homo-polymers or copolymers having clorine content of up to 60% by weight, mixtures of two or more of the above mentioned polymers, and mixtures of the above mentioned polymers with atactic or isotactic poly-propylene.

12. The method according to claim 1, wherein the cross-linking agent is an organic peroxide having a decomposition temperature higher than the flow temperature of polyolefin.

13. The method according to claim 1, wherein the blowing agent is selected from the group of azo type compounds, nitroso type compounds, hydrazide type compounds and sulfonyl semicarbazide type compounds possessed decomposition temperatures exceeding the melting temperature of polyolefin.