CA1108350A - Expanded particulate material of polyolefin resin - Google Patents

Abstract

Abstract of the disclosure:
Expanded particles of a crosslinked polyolefin resin, each particle with uniformly spherical shape, hav-ing an average diameter of 1.4 to 5.5 mm, an average expansion ratio of 18 to 37 and specific compression coefficient of 1.6 x 10-3 to 4.0 x 10-3, are found to be produced by two-step foaming operations. They are useful for various purposes such as filtrating material, fillers in stuffed specimens and, especially for preparation of molded articles having constricted portions, giving excellent moldings having smooth surface without failure at corner or edge portions.

Description

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Thls invention relates to an expanded particulate material comprlsing expanded particles of a crosslinked polyolefin resin having improved characterist:icsa a process for producing the same and al~o to molded product produced ~- 5 there:from.
Expanded particles of a crosslinked polyolefin resin are at present useful primarily for preparation of molded products or cushioning materials. Recently, there are provided for use as particles for internal fillers in stuffed specimens or fillers in pillows or cushions.
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Furthermore, the expanded partlcles are permitted to float in a large amount in a solution9 thereby absorbing -~
solutes on the surface of the expanded particles to separate the solutes from the solvent~ and the expanded particles a~ter recovery of the solutes absorbed thereon are regenerated for repeated use. Various uses including filtration as mentioned above are now under development.
It is well known to produce expanded particles o~ a crosslinked polyolefin resin from a polyolefin resin ` 20 as base resin9 as disclosed by, for example a Japanese published unexamined patent application NoO 26435/1972.
It is also known to produce molded products by filling these expanded particles in a cavity and heating into a molded article corresponding to the shape of-the cavity~ as disclosed by UOSo Patent 35040683 Japanese published examined patent applications No~ 34391/1973 and No. 22951/1976.
The expanded particles prepared by the processes of prior art involve drawbacks such as

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difference in buoyancy between particlesg difference ir absorbed solutes between particles or depletion in ~iltrating ability due to ununiform pressure deformation of particlesg when applied in filtrating materials9 and local deformation as the lapse of time3 when applied as fillers in stuffed specimen. For th-Ls reasong applications in these fields are less advanced.
Furthermoreg when molded products are to be prepared by use of the expanded particles of prior artg fusion between particles in the inner portions of a part with greater thickness is poor~ while there are failures at corners or edges for a part with smaller thickness.
- Moreoverg it is entirely impossible to shorten the molding cycle for preparation of such molded products.
15An ob~ect of the present invention is to provlde expanded particles of a crosslinked polyolefin resin which can be provided for use as filtrating materialsg being excellent in filtrating ability with appropriate pressure deformation as well as absorption of solutesg sufficiently durable to repeated uses and easy of recovery of solutes by separationg when employed~
for exampleg in a filtrating machine which separates solutes from solvent khrough simple contack between the solution and the particles to thereby absorb the solutes in the solution on the surface of particles.
Another object of the present lnvention is to provide expanded pàrticles of a crosslinked poly-olefin which are substantially spher1~cal with uniformity in size and expansion ratio and have specific compression coefficient~ being useful as fillers for pillows or cushioning materials excellently contoured to human bodies without uneasy feelin~ and also as fillers for stuffed specimens, etc. free from shrlnkage or parkial deformation with lapse of time.
It is also another object of the present invention to provide expanded particles of a crosslinked polyolefin resin which can be molded into an article by a process capable of being designed more ef~iciently than ever in the prior art with shortened molding cycle9 said article yet having su~ficient cushioning ability even at thinner portions thereof with excellent mold reproducibility at corner or edge portions of the molded articleO
Still another object of the present invention is to provide a process for producing expanded particles of a crosslinked polyole~in resin which is capable of producing the novel expanded particles as mentioned above.
Further obJect of the present invention is to provide a process for producing an expanded molded article of a crosslinked polyolefin resin which is more efficient than prior art process and can give product with improved quality3 and also to provide such an expanded molded article produced by said process.
According to the present inventiong there is provided a particulate resin material comprising expanded crosslinked polyolefin resin particles which are substantially sphericalg resilient3 free-flowing~

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uniform in particle size and moldable in a cavity, each particle having a structure substantially constituted ofclosedcells without void~ and which have an average particle size ranging from 1.4 to 5.5 millimeters 9 an average expansion ratlo ranging from 18 to 37 based the original volume of unexpanded resln particles and a compression coefficlent ranging from 1.6 x 10~3 to 4.0 x 10 3 as determined from the following ~ormula~
S/(R x F) wherein S represents total energy for : ,, compression under pressure of' 1 kg/cm~, R average expansion ratio and F flowabilityg respectively~ of the expanded crosslinked polyolefin resin particles.
The expanded particles of a crosslinked polyolefin resin provided by the present invention are required to satisfy the following requirements-a) They should be spherically shaped particleswith substantially uniform si~e~
b) Their average size is critically within the range from 1.4 to 5.5 mm~
c) Each particle is filled internally with a number of closed cells and free from voidj d) The average expansion ratio o~ the particles ; based on the volume of starting unexpanded resin particles ls critically within the range from 18 to 37.
e) The compression coefficient of the particles ;~ is critically within the ran~e from 1.6 x 10 3 to ; 4.0 x 10 3.
The present ~nvention has been accomplished based on the discovery that the above requlrements ~L~f~

a) through e) in combination are e~sentlal for achieving the excellent effect of the present invention. While being not by any theory, these parameters are necessary for the following reasons. For example, when the particles are not uniform in size as required in a), the particles are separated into classes with diEferent sizes during air conveying, whereby the width of variance is increased. The requirements b), c) and d) are minimum necessaxy conditions for the value of e) to fall within the specified range.
However, if -the requirements b~, c) and d) are satisfied, it does not necssarily follow that the value of e) will fall within the specified range. qlhus, the parameter of e) is a factor representing the structure of the expanded particles which has not so far been elucidated. To speak of the function of the compression coefficient of e) in detail, the particles with a compression coefficient less than l.6 x 10-3 will be liable to form particle bridges at the time of filling in a cavity, whereby the resultant molded product will contain vacant space at thin wall portion to lower cavity reproducihility at corner or edge portions of the molded product. Furthermore, a molded product prepared from such particles is lowered at thick wall portions in the strength of fusion between inner particles, failing to give good molded products with high cushioning ability.
On the other hand, with particles having a value exceeding 4.0 x 10-3, there ~;r c~
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is a tendency that the particles near the surface of the molded product have undergone fusion while expansion of khe inner particles is delayed~ whereby the resultant molded arkicle may suffer from such inconveniences as vacant space formed therein3 unfavorable changes in the strength of fusion between khe inner particles or shrinkage after cooling of the molded articleO Furthermore9 the particles with compression coefficient in the range from 1.6 x 10 3 to 4.0 x lO 3 -are found to be more advantageous, because molding can be completed within a short time even at a relatively lower temperature at the time of heat molding to enable shortening of molding cycle.
From the standpo~nt to obtain economically satisfactory results at the same time~ the expanded particles of the present invention may preferably have an average particle diameter from 2 to 4.5 mm~
an average expansion ratio from 23 to 32 and compression coefficient from 2.2 x lO 3 to 3.6 x lO 3. Using such particlesg it is possible to complete a molded article with complicaked shape~ for exampleg having a thin portion of about 3 to 6 mm~ in conformity with the ; desired shape of the mold cavi-ty, to a great and excellent advantage.
The true mechanism in which the compression coefficient acts on the molding in a cavity remains to be elucidated. As speculated from the above results~
the expanded particles at the time of filling in a cavity are required to be closely packed even in a narrow c~vlty through adequate de~ormation o~ the particles under compression. ~urthermore~ at the time of heat mclding~ the expanded particles are required to be adequately deforrned under relatively low pressure of the steam employed for heatingg thereby ~orming inter-stices between particles to permit passage o~ the steam deep into the cavity and effect simultaneous expansion of the expanded partlcles. Thus, the compression coefficient itself is the very criterion of the expanded particles for forming adequate deformation under a certain external force.
The present invention also provides a process for producing expanded particles of a crosslinked polyolefin resing which comprises first allowing particles of a crosslinked polyolefin resin containing a foaming agent to expand to an expansion ratio o~
from 3 to 9 and theng after imparting expandability to the thus pre-expanded particlesg further allowing said - pre-expanded particles to expand to an expansion ratio of from 13 to 37.
~ he above specified process of the present invention is characterized by the two~step expansion, namely (A) the primary expansion -in which cross]inked polyolefin resin particles are first expanded to an expansion ratio of about 3 to 9 and (B) the secondary expansion in which the above expanded particles after being endowed with expandability are further expanded to an expansion ratio o~ about 13 to 37 based on the original volume of unexpanded resins. It is intended by thls process to establ1sh a commercially applicableg economical process for production of highly expanded particles o.f a cro~slinked polyolefin resin wlth an expansion ratio of 13 to 379 whlch has been diff'i.cult iIl prior art process in s~tting foaming conditions~ for example~ for controlling the variance Or the expansion ratio of the resultant partlcles which i.s liable to occur even in the same lot. The present process is ~ound to enable production o~ SllC~I expanded particles as specified above by stabilizing foaming of particles w.ith sizes as small as 1.4 to 5O5 mm and expansion ratio of 18 to 379 which has been difficult in prior art9 and moreover accomplishing uniform foam1ng so as to make the compressicn coefficient of the expanded particles withln the range of 1.6 x lO 3 to 4.0 x lO 3.
In the primary expansion (A)~ if the expansion ratio is less than 3~ it takes too much time be~ore imparting e~p~ndability in the subsequent step to be uneconomicalO Furthermore3 highly expanded particles obtained from such a low extent of expansion suf~er from greater variancesO In view of the more strict requirement on the economy and the variance as mentioned above~ the expansion ratio in the primary expansion (A) is desirably within the range from about Ll to 7.
The extent of expansion from the expanded particles in (A) to those in (B) may suitably be selected depending on the desired expansion ratio of the particles to be obtai.ned in the step (B). From standpoint of making the variance smallest and e~fecting economically high degree of expansion~ the expanslon ratio to be selected for each of' the steps (A) and (B) should be not more than 10~ preferably from 3 to 8~
The expanded crosslinked polyolefin resin particles obtalned by the above process have closed cellular structuresg having a closed cell percentage af 85% or more with cell sizes ranging frorn 25 ~ 400 cells/mm .
There seems to be an intimate relation between the nature of the crosslinked polyolefin resin and the requisite steps of the invention to first forming expanded particles with lower expansion ratio and small extent of dispersion and then~ after imparting sufficient expandabillty theretog further exp~nd to uniform, highly expanded particles. Namely~ cross-linked polyolefin resin can poorly retain gaseous materials therein and crystalline in nature, thus having only a narrow temperature range for expansion. It is therefore difficult to lmpart expandability uniformly to the particles which will complet~ expansion of 10 times or higher at one ti~e or also diff.icult to convert expandability imparted to the to particles to expandlng force uniformly of 1~ times or higher.
Unexpectedlyg the present process is found to impart the compression coefficient entirely unknown in the art to the resultant expanded crosslinked poly~
olefin resin particlesg which are uniform in foamed cell distribution as well as in size. Furthermoreg the present process has made it possible to produce :

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highly expan~ed small particles o:~ crosslinked poly-olerine resin which has been deemed to be difficult -~
in the art.
In perform:Lng the primary expansion in the step (A) or imparting the exparldability in the step (B)~ there may be employed an inorganic gas principally composed of nltrogeng typically air or nitrogen~ or a volatlle organic blowing agent such as hydrocarbons or halogenatecl hydrocarbonsg which may be contalned (e.g. permitted to be impregnated in the particles with heating under pressure) in the particles to impart : expandability theretog followed by expansion to achieve desired foaming. It is more preferred in the present process to carry out the primary expansion in the step (A) by impregnating the resin partlcles with a liquid organic blowing agent to have the organic blowing agent contained therein and then foaming with heating the thus :~
impregnated particles to form pre-expanded particlesO
On the other handg expandability may preferably be lmparted to such pre-expanded particles in the subsequent step (B) by holding the pre-expanded particles in an atmosphere of an inorganic gas under high pressure (e.g. about 5 kg/cm2-G) at a high temperature (eOgO
about 80C)g thereby pressure charging an inorganic gas into cells of the pre-expanded particlesg which particles are then subjected to heating expansionO
By use of the different foaming methods for the step (A) and the step (B)g respectively, as described above there can be obtained more favorable results.

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Perhaps, this is due to the fact that in the step (A) a liquid organic blowin~ agent can be lmpregnated deep into the core portions o~ rigid particles to enable uniform foaming, while in the step (B) foaming is completed under the conditions substantially free from influences caused by latent heatg etcO
As compared with the process of the present invention as described above~ the processes of prior art are not satis~actory in commercial appllcation.
For exampleg in a commercial scale operation wherein a number ~f large scale vessels with capaclty of 20 m3 are arranged and connected with lines therebetween for air conveying of the base resin particles and expanded particles~ there are several inconveniences.
For instance, in the step of bag packaging expanded particlesg the bags in which said particles are packaged in equal weights as they are taken out suffer from great changes in volumeg resulting in molded products having a wide range of densities. In the step for producing molded products in a system wherein storage tank of expanded particles (or tank for imparting expandability) and molding cavity are connected with a line, the resultant molded products are greatly changed in density3 failing to give desired cushioning property.
In generalg the entrapped gaseous material in the polyolefin resin cannot be held therein under foaming conditions as different from a polystyrene resin9 but dissipated therefrom in a very short lapse o~ time.
This characteristic is dependent also on the distribution - 12 _ of the ga,seous materials in the particles (~or exampleg distribution among i~dividual particles or distribution along the cross sectional area of` each particle)g and therefore it is required io have gaseous materials 5 contained with a distribution as unlform as possible by suitable selection of condltlons for impregnating a polyolefin resin with gaseous materials and expanding said resin. On the other handg as polyolefln resins are c~ystalline, the r~nge of' temperatures showing an optimu~ viscosity suitable for foaming the resin particles is very narrow. The temperature range ; cannot remarkably be broadened by crosslinking the poly-olefin resins. Accordingly~ the gas for foaming cannot be utilized effectively if the resins are foamed under very severe conditions as compared with the expansion of polystyrene resins, The af'orementioned narrowness in the range of temperatures brings about adverse eff'ects on distribution with regard to resin expansion~
e.g. distribution of expansion among particles and distribution in cell size in each particles.
There seems to be little attention paid on these considerations in the methods of prior art as mentioned above with the result that variances in density and particle volume of the expanded particles obtained are further increased through separation into classes of' particles with different particle sizes and densities during air conveying. Such a variance ln particle size or density has for the first time been recognized as a serious problem and in~estigated when produc-tion of expanded particles is performed on a large scale. This problem has been found to be mos-t conspicuous when gaseous materials are introduced under a pressure of 10 kg/cm2 or more and expansion is effected to an expansion ratio of more than 10 at one time. ~he process according to the present lnvention as described above is f'ree from such an inconvenience and therefore can advantageously be utilized for commercial applicationO
The expanded particles of a crosslinked poly-olefin resin accordin~ to the present invention are found to be advantageously used for molding in a cavity with ease to produce excellent molded products. By use of the expanded particles of the invention as described aboveg after imparting expandability thereto, there can be produced novel expanded moldings by molding in a cavity under heating according to conventional procedures. For example, as described in Example 3 of UOS~ Patent 3950490689 the expanded polyolefin particles may be subjected to shrinkage under pressure with heating to 100C ~r higher~ and the shrinked particles are filled in a cavity under pressure, followed by release of the pressure to atmospheric to allow expansion of the particlesg whereby there is formed a molded product t through fusion between the particles. Alternativelyg as described on column 69 line 55 to column 79 line 7 in said Patentg heated expanded polyolefin particles are filled in a cavity and then the pressure in the cavity is increased to compress said particlesg followed by reduction in volume of the cavity simultaneously wlth .

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release of the pressure in the cavity to atmosphericg whereby sald particles are expanded and fused into a molded product. These methods can give only molded products with considerably lower expansion ratio (i.e~
higher density) than the expa,nded particles employed.
Furthermoreg due to transfer or compression of the heated particlesg t)le resultant molded products have bad appearance ~uality and it is also impossible to obtain molded products havlng complicated shapes or good cushioning ability.
Another well known method for molding in a cavity, as disclosed in Japanese published examined patent application No. 22951/197~9 comprises holding expanded particles of a crosslinked polyolefin resin in an inorganic gas atmosphere at high temperature under hlgh pressure to thereby incorporate inorganic gas into cells of said expanded particles and increase the inner pressure in the cells (to impart expandability)g which are then taken out for cooling and immediately ~; 20 (inner pressure being required to be maintained~ filled in a cavity~ followed by heating of the cavity to expand said particles to obtain molded product. ~'his methodg however~ is practically disadvanta~eous for molding of such a resin as crosslinked polyolefin~
from which gases incorporated therein are readily dissipatedO Forg in most commercial operations~ the capacity in the process for imparting expandability does not necessarily coincide with that în the molding process for consuming the particles. Thusg it is often required to ~tore the particles having imparted expnad-abllity as stock~ whereby there is required a great labor or cost for maintenance of the expandability. Furthermore~
bulk production on a commercial scale is made substantially impossible by requirement to arrange the process for impart-ing expandability and the molding process wlthin a short distanceO Moreoverg the vital defect Or thls method is difficulty in recognizing the extent of expandabllity imparted (or remained). For this reason, lt is very di~ficult to control the foaming in the cavity which depends largely on the expandability~ resulting in considerable dlspersion of the foamed products obtained.
Ir. accordance with the present invention~
there is also provided a process for producing molded products from the expanded particles of a crosslinked polyolefin as described above. The process of the present invention comprises compressing the expanded particles of a crosslinked polyolefin resin to l~o to 80% of the orlginal particles volume thereof with heating or a~ normal temperature9 filling the thus compressed `
particleæ in a cavity (pressure in the cavity after filling may either be atmospheric or slightly higher) and then heating directly the particles in the cavity ; with steam (at about 110 to 130C). In some cases;
the resultant molded products may be allowed to reside in a drying chamber adjusted at a suitable temperature.
The advankages of the aforesaid process for producing molded product over the prior art are as follows~

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(1) Molding cycle can be improved because molding is possible at a lower temperature for a short time;
(2) The molding procedure is simple to afford more econornical and efficient combination of stepsg since the expanded particles are only compressed and filled in a cavity immediately before moldin~
(3) The molded products are uniform in quallty because there is no periodical change in expandabili-ty of the expanded par~icles~
(4~ Moldin~ of products with smaller thickness or complicated shapes is possible because the expanded particles under compressed state are filled in a cavity without heating;
(5) Due to excellent uniformity and fusion characteristlc of the particles filled in the cavityg the resultant product is excellent in cushioning property.
These ad~antages can be made more complete by suitable selection of the compression coefficlent of the expanded particles to be employed. That isg by the :
pressure of the steam used for directly heatin~ the expanded particles in a cavity at about 110 to 130Cg the expanded particles themselves are appropriately def'ormed under pressure to permit passage of the steam to the core portion in the cavityg whereby whole of' the expanded particles in the cavity receives substantially equal heat content to effect s:imultane~us `~ expansion in a short time. Furt,herg lower temperature .

conditions can be appliecl ~or heating of the mold to shorten also the cooling timeO
The expanded particles of a crosslinked poly-olefin resin of the present invention can be molded int~
a product which is also novel~ comprising expanded partlcles of a crosslinked polyolefin resln integrally closely bonded between the expanded particles, sa-ld molded product having an average density of 0.12 to .028 g/cm3 ~nd a compression strength ~25% deformationg kg~cm2) per its density (g/cm3) of 14 to 18 and being substantlally smooth on its surface with substantially .
no failure at corners or edges even at constricted portions (e.g. in a molded particle having at least a part with thickness of about 3 to 6 mm).
The polyolefin resin re~erred to in the present invention includes ethylene homopolymers such as high density palyethylene9 medi~m density polyethylene or low density polyethylene or a mixture thereo~ and ethylenic copolymers having ethylene content of 80% or moreg e.g. ethylene~vinyl acetate copolymerg ethylene-acrylic acid ester copolymerg ethylene-methacrylic acid ester copolymerg etcO The polyolefin resin to be employed in the present invention may have a mèlt inde;c which is not specifically limited but generally within the range from 1.0 to 45.
There may be used any conventional proeedure known in the art for crosslinking the polyolefin resin for preparation of the starting crosslinked polyolefin resin particlesg using a crosslinlcing agent such as . .
~ - 18 -' ' .

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organic peroxides or irradiation of electron beam~
It is preferred in commercial application to use a crosslinking agent selected from organic peroxidesg including dicumyl peroxide; 2~5-dimethyl(2~5-di-t-butyl peroxy)hexene-3~2~ ~-dimethyl-N~rnethyl-~-ethyl benzyl peroxide and the like. The amount of such a crosslinking agent is suitably selectecl depending on the reaction conditions, the polyolefin resin employed and various characteristics required for the expanded product~ but - 10 it generally ~alls within the range of from 0.35 to 1.5%
by weight based on the resin. The crosslinking reaction may be performed in conventional manner known in the art~ for exampleg by dispersing and heating resin particles containing organic peroxide in an aqueous medium. The resin particles suhjected to crosslinking are spherical or shaped in pellets which can be converted to spherical shapes by heatingg their average size in terms of spherical diameter being generally from 0.5 to 2.1 mm. The gel content of the resultant crosslinked resin is desired to be from 30 to 70% from standpoint of desirable foaming charac'ceristics.
The expanded particles of a crosslinked poly-olefin resin of the present invention and molded products prepared therefrom may further contain pigments or other additives dispersed therein, as incorporated in the starting resins or deposited or coated on the surfaces of the expanded particles or molded products~
In the following~ definitions of the terms and the methods for evaluation or measurement are explained in detail~
1) Particle diameter Expanded particles are pro~jected (x 10) and the diameter of the outer circle externally contacted with the projected image for each of 100 or more particles is measured and calculated as an average : diameter.
2) _mpression coefficlent This is defined by the followin~ formula:
Compression S
coefficient R x F
S (Total ener~y for compression under pressure Or 1~ kgicm21.
~: Expanded particles are dipped into water in a measuring cylinder which can be pres~urized with khe air to measure the volume~VO) of the expanded partlcles.
Theng the air is pressurized at 9 rOr exampleg 0.3 kg/cm2 or 0.5 kg/cm2 into ~ the measuring cylinder and the air i pressure(P) and the compressed volume(V) of the expanded particles are measured.
By repeating slmilar procedures by increasing the pressure (P) at some re~ular interva]s9 the relation between the compressed pressure (P kg/cm2) and compresslon degree (VO - V)/V0 is determined to obtain the S S curve as shown in Fig. 19 and then total energy for compression under pressure of 1 kg/cm2 is measured by - integration corresponding to the area S on this curve (Fig. 1 shows one example in which the curve is made by varying the compressed pressure P at lntervals of' 0.5 kg/cm2) ~ :
Welght o:f expanded particles W(g) is precisely measured and said particles are dipped into water in a messcylinder to measure the volume of the expanded particles V(cc). Bulk densiky is determined from p~=W/V. Sald particles are heated in nitrogen atmosphere at 160C for 30 minutès to d'ëtermlne the density p~ of degassed resin. Expansion ratio is calculated from Po/Pl~ counting fractions of 0.5 or over as a whole number and disregarding the rest.
F (Flowability of resin)o Expanded particles are heated in nitrogen ~ atmosphere at 160C for 30 minutes. The ~ 20 degassed re~ins obtained are sub~ected to ' measurement using a flow tester with diameter of 1 mm and length of 6 mm (flat entry) under load of 150 kg at 180C (pre-heated ~or 5 minutes). Flowability o~ the,resin i9 given 25 ~ as falling speed o~ plunger ~ .
Flowability of resin is characteristic value representing a tendency to be deformed of the resin fllm constituting the expanded partlcles by external force~ especi.ally ..

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a tendency -to be deformed by the temperature ancl dynclmic force of heating medium in cav:Lty molding, In Examples~ the ~low ~ester employed ls produced by Sh~mazu Manufacturing Co.g Japan.
3) Mo]dability There ls prepared a t~st mold shaped ln a box of 300x600x80(mm), with thickne~.s at bott~om ~f 8 mm and thickness of external walls of 25.n~l, having partitioningr walls (one in longitudinal dire~tion and 24 in lateral direction) each wlth thickness of 6 mm and height of 25 mmO The de~ree of filling in the partitioned portions and filling in the edge portions of the box bottom as well as the time for heat moldlng are evalua~ed.
.~ 15 ~illin~ at.,narrow portions:.
Samples cut at 10 mm from khe upp~r face of the parti~ioned portions in the abo~e test mold are dlpped into water to measure their :~ volumes.and the percentages relative to .
theoret.ical mold volume are determined. ~.
The re~lts are rated by the following criterl~.
Rank ~ 'L~, ID'C'~tlC'~
o 98% or more ~ less than 98%9 90% or rnore x less than 90%
;: Filling at ed~e portions:
: The number of failures with 2 mm or more - per 300 mm of edge line is. counted and ' rated by the follo~lng cr~teria.

Rank F~lling degree (number of failures/30 mm) o less than 10 ~ 10 or more, less than 25 x 25 or more ~1~ timeO
Molding is carried out by vary-lng the total heating time for one slde heatlng (maximum steam pressure = 0.3 kg/cm2~) and both side heatin~ (maximum steam pressure ~ 1.0 kg/cm2-G) to determine the minumum mold heating time before defects such as shrinkage or slnk marks appear in the molded product and rated by the criteria set forth belowO
The presence of sinking is noted when the . ratio of the volume of the molded product measured after being left to stand for 24 hours afker molding to cavity volume is less than o.8.
Ran~ Mold heating time(sec.) o less than 15 15 or more~ less than Z0 x 20 or more

4) Quality of molded product The molded product is evaluated for its appearance~ internal fusion and .fusion at narrow portions.
Appearance The number of fallures with depth of 2 mm or more are counted on the flat portion of 3~

the molded product and eva3uated as follows.

- Rank ~ arance Number2~~7~ 7~
100 cm .
o less than 3 ~ lJ to 2 S x 21 or more ~
Internal fusion- `
The external wall portion of the af'oresaid test mold is cut and dipped in water to the depth of 5 cm for 24 hoursg thereafter taken out, washed with ethanol on its surfaceg dried at 35C for one hour and weighed. The degree of water absorbed per sample volume is calculated and evaluated as follows.
Rank Water absorption__(vol.%) o less than 0O4%
0.4% or more~ less than 1.2%
x 1O2% or more Fuslon at narrow portions:
The partitioned sections of the molded product is cut OUtg its tensile strength measured and evaluated as followsO
Rank Variance of tensile stren~th(n-10) - o less than 10%
~ 10 to 20%
x more than 20%

* Variance = Maxin =~100 - ~4 -3~`~

5) Overall evaluation Rank (~ ..... 0 The marks o for all items o ..... .~ot more than 3 marks Q with no mark x .Ø 4 or more marks ~ with no mark x x .Ø. at least one mark x

6) Gel co t The resin particles are dipped in toluene and refluxed with heating for 24 hours.
The extract residue is represented in weight percentage.

7~ Cell slze of expanded particles The expanded particle is cut and its cross-section is observed with mlcroscope and the number of cells per 1 mm2 at 5 places are counted. The average value is calculated from the number of cells.

8) Percenta~e of closed cells The expanded particles are dipped in an aqueous solutlon of which surface tension is weakened with addition of sur~actant at 23C for 24 hoursg then washed with ethyl alcohol to remove water adhered on the surface followed by drying and measured for the change in weight.
Percentage of ~W
closed cells (l V - (W~d) ) x lOO

.
- . ` ''' ': ' ~ ~

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W weiKht gain after dipping in water(g) V : volume of' particles(cc) W original weight o~ particles(g) d : resin density of particles(~/cc)

9) Melt Index ~M.I.) According to ASTM D-1238-65T

10) Variance in expansion ratio of ex~anded particles Samples optionally collected from the lot~
each being 50 gg are subjected to classification by sieve type particle diameter distribution measuring instrument. The average expansion ratio(T) of the particles on the sleve on which maximum amount of particles remaing the ave~age expansio~ ratio(M~ of the whole expanded particles larger than said particles remained and the average expansion ratio(N) of the whole expandea particles smaller than said particles remalned are determined.
Variance in = _ - N x 100 (%) expansion ratio T
Example 1 One hundred (100) parts of a low density polyethylene pellet (density = 0.921~ M.Io = 2.5) and 0.45 parts of dicumyl peroxide are dispersed in water in the presence of a dispersion stabilizer. The dispersion is elevated to 160C over 2 hours and heating is continued at 160C for 30 minutes to produce crosslinked polyethylene resin particles which are substantially spherical with particle diameter of 0.7 mm9 having gel conterlt of 55%.
The thus prepared partic]es are then treated with an excessive amount of dichlorodifluoromethane at 80C under 27 at~. ror 30 minutes to have 15~ dichloro-difluoromethane impregnated therein. The impregnatedparticles are allowed to expand by heating with steam at 120C for 14 seconds to give pre-expanded particles of crosslinked polyethylene wlth expansion ratio of Ll.
The pre--expanded particles are held ln an air-pressurized atmosphere of 9 atm. at 70C for 4 hoursgthereby pressure charging the air into the pre-expanded particles~ followed by heating at 107C with steam for 12 seconds for expansion to give expanded particles with expansion ratio of 23.
The above expanded particlesa when measured after belng left to stand for one week at normal temperature under normal pressureg is found to have compression coe:~ficient of 3.5 x 10 3 ~S-value = 0.25, flowability of resin - 3.1)9 their particle diameter being 2 mm.
The expanded particles are compressed to 65%
of original particles volume immediately before being filled ln a cavity and sub~ected to heat molding while being filled in the cavity under compressed state.
Moldability and the quality of the molded product are evaluated. As the molding machine~ ECH0-120 Type machine (produced by Toyo Metal & Machinery Co. 9 Japan) is employed. As the resultg the molded product has an average expansion ratio of 249 filling ratio at ` -narrow portion of` 98%9 filling degree at edge portion o~ 5 failuresj molding time being 10 seconds (maximum steam pressure 1.0 kg/cm2 gauge)g appearance quality ln terms of the number of failures being 39 internal fusion accounting for 0,2% (water absorption~ and fusion at narrow portion for tensile strength of 3.4+0.15 kg~cm2. The results of evaluation are listed as No. 1 in Table 1.
Table 1 also shows the properties and the ~esùlt~ of evaluation of varlous expanded partlcles which are prepared similarly as described above by varying the size of the crosslinked polyethylene resin particles and the expansion ratio of the pre-expanded particles. In Experiments Nos. 49 5, 6~ 89 10 and 11 howeverg a low density polyethylene (density - 0.915, .
M.I. = 20) is employed as the starting polyethylene.
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Compar:Lson example 1 To 100 parts of crosslinked polyethylene resin particles with particle diameter of 1.5 mm prepared by the same method as in Example 1 are added 20 parts of dichlorodifluoromethane to carry out impregnation treatment at 80C for one hour~ whereby there are obtained expandable particles containing 15% of' dichlorodifluoromethane.
These particles are heated with steam at 125C
for 14 seconds to produce expanded particles with expansion ratio of 13 ahd particle diameter of 305 mm.
The resultant expanded particles have a co~pression coefficient of 1.2 x 10 3 (S=0.0489 flowability of resin =3.1) and evaluation test of the moldability thereof and quallty of the molded product give the results. filling capacity at narrow portion~98%~ number of failures at edge portion=10; molding time=13 seconds; appearance - quality in terms of number of failures=4~ internal fusion-0.6% (water absorption)g fusion at narrow ~;
portion-tensile strength of 3.1'0.2 kg/cm2. The results of evaluation are set forth in Table 29 No. 1.
Various expanded particles are prepared similarly as described above by varying the size of`
the crosslinkecl resin particles and the results of evaluation fO~ these particles are also shown in Table 2. In ~xperiments Nos. 5, 6, 7, 8 and 9g the start~ng polyethylene has a density of 0.915 and M.I.
of 20. The expanded particles of Nos. 5 and 7 are prepared by the two-step expansion similarly as described in Example 1.

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From the crosslinked resin particles, there are produced various pre-expanded partlcles with dir.ferent expansion ratios whlch are then subjected to further expansion~
respectively, to prepare expanded particles. The expansion ratios of respective expanded particles, variances of expanslon ratios and compression coefficients thereof are shown in I'able 3Q
As apparently seen from Table 3g it is prefer-: able to first pre~expand the particles to an expansion ratio Or 3 to 9 and then expand the pre-expanded : 15 particles to an expansion ratio of 13 to 37 in order to make the variance in expansion ratio smaller; more preferably first to an expansion ratio o~ 4 to 7 and then to an expansion ratio of 18 to 37.

Table 3 Expansion Exp. ratio in No. pre-expansion_ Expanded particles __ Compresslon Expansion Variance coefficlent ratio _ (%) _ (x103) 2 10 35 3~3 3. 0 3 ~.0 37 20 4.0 4 35 18 3 . 6 " 32 11 2. 5 6 7.0 37 1ll 4.0 7 '~ 32 10 3. 5 23 8.5 2.2 9 " 18 9.2 3.0 I~,o 23 9.3 2.6

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Example 3 Using various polyolefin resinsg the following expanded particles are prepared.
(1) A high density polyethylene (density=0.~51, M~Io=10) is formed into substantlally spherical particles. Said particles are irradiated with electron beam to prepare crosslinked polyethylene partlcles with gel content o~ 40%. Subsequentlyg dichlorotetrafluoro-ethane ls impregnated into the crosslinked polyethylene particles at 40C' under pressure for one hourg followed ~ .

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by heating with steam at 140C for 20 seconds to give primarily expanded particles with expansion ratio of 7. These primarily expanded particles are held in an air-pressurlzed atmosphere o~ 9.5 atm., at 90C for 8 hours, thereby pressure charging the air lnto the expanded particles~ followed by heatlng with steam at 140C for 15 seconds to prepare secondarily expanded particles with expansion ratio of 24. The resultant expanded particles have the properties QS shown in Table 4.
(2) Using the resins as shown in Table 4g various expanded particles are prepa~ed according to the same procedure ~s descri~ed in (13. The heating conditions for expanslon o~ the respective resins are as follows:

Polyethylene(density=0.915gM,IO=10) 120Cg 10 to 20 sec.
density=0.921,M.I,-3,5) " tdensity=o~9269M~I.=2o) Methyl acrylate(10%~-ethylene(90%) =3,0) 125Cg Vinyl acetate(10%)-ethylene(90%) 90Cg 10 to (M.I.=2.5~ 15 sec, ~'$~3~3 Table 4 Particle Expan- Compressi.on diameter sion Vari- coerficient Polymer _(mm) ratio ance (x103) Polyethylene (d=0.951, M.I.=10) 3.5 24 17 1.8 Polyethylene (d=0.915~ M.I.=lQ3 3.5 24 8.5 2.3 Polyethylene (d-0.921, M.I,=3.5) 3.5 24 8.3 2.6 Polyethylene (d-0,9269 M.I.=20) 3.5 24 8,l~ 2.4 Methyl acrylate(10%)-ethylene(90%) copolymer(M.I.=3.0)3.5 24 9.2 1.6 Vinyl acetate(10%)-ethylene(90%) copolymer(M.I.=2.5~3.5 24 20 3.6 -Example 4 The pre~expanded partlcles prepared in the same manner as in Example 1 are subjecked to treatment under the conditions set forth below for imparting expandability thereto, and then secondarily expanded particles are prepared.

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Table 5 Exp. No.
1 2 3 _ ~ 5 Conditions for ~~
imparting expandability Added gas Dichloro- Pro- Dichloro~ Nitro- Air difluoro- pane difluoro- gen methane methane (20%) Nitrogen ( ~0~ ) Pressure 10 10 10 ].0 10 ~kg/cm2-G) Temperature tC)45 28 80 80 80 Time (hour) 6 8 4 4 4 Secondar,~ on Heated steam pressure~kg/cm2 G) 0.5 0.5 0.33 0.32 0.31 Heating time(sec.) 30 30 3 25 24 Quality o~ expanded product Expansion ratio25 25 30 30 30 Variance ~%) 35 30 10.0 9.5 9.3 ~ompression coefficient (x103) 1.8 1.6 2.g 2.8 2.8 As clearly shown from Table 59 when an organic gas is used as foaming gas3 heat content necessary for expansion is greater due to latent heat for evaporation of an organic gasg whereby dispersion tends to be increased. Furtherg in such a case, the expanded particles to which expandability is imparted differ in thermal conductivity to form different cellular structures at the time of secondary expansion3 resulting i.n decreasecl compression coefficient.
~x2mple 5 Uslng the secondarily expanded particles prepared in the same manner as in ~xample 1, there are prepared several compression moldings under the following conditions to give the results as shown in Table 6. The molding cavity employed is shaped in a box having outer dimensions of 300x300xlO0 ~mm), thickness of outer wall of 25~m, and also having inner partitioning walls (2x2 sheets) with thick-ness of 9mm. Compression molding is carried out undermaximum steam pressure of 1.0 kg/cm2-G at the time of heat molding. In preparing the expanded particles of Experiments Nos. 1 and 2, in order to prevent loss of foaming ability ~.
through diffusion of the gases added into the particles, the expanded particles after being endowed w;.th expandability :~
are taken out successively portionwise corresponding to the amount to ~e molded and then immediately filled in the molding cavity for heat molding~

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Example 6 In this Example~ the following tests are conducted for evaluation of the ranges for various parameters required for moldability.
There is employed a box-shaped cavity for testing of which outer dimensions are 200x400xlOO(mm) with outer wall thickness of 20 m~ partitioning walls with length of 160 mm, height of 50 mm and thicknesses of 2~ 3g 4, 5g 69 ~ 10 and 15 mm `being arranged at intervals o~ 34 mm in said cavity. Heating for each moldin~ iscarried out under the condition optimized `~
for each of the expanded particles. Thickness of the partitioning wall in which filling percentage o~ the particles is 98% or h~gher, the number of ~ailures per 300 mm of inner edge of the wall surfaceg the number of failures per 100 cm2 of the box bottom and compression strength (25% compression~ kg/cm ) per denslty ~g/cc) of' the molded p~oduct are de~ermined to give the results as shown in Table 7.
Table 7 clearly sho~s that the products obtalned by use of the expanded particles of the present invention are more flexible than the expanded moldings of prior art~ and particularly that even the articles having smaller thickness portions with excellent quality can be molded by compression molding.

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Claims (7)

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

1. A particulate resin material comprising expanded crosslinked polyolefin resin particles which are substantially spherical, resilient, free-flowing, uniform in particle size and moldable in a cavity, each particle having a structure substantially constituted of contiguous closed cells, and which have an average particle size ranging from 1.4 to 5.5 millimeters, an average expansion ratio ranging from 18 to 37 based on the original volume of unexpanded resin particles and a compression coefficient ranging from 1.6 x 10-3 to 4.0 x 10-3 as determined from the following formula: S/(R x F) wherein S represents total energy for compression under pressure of 1 kg/cm2, R average expansion ratio and F flowability, respectively, of the expanded crosslinked polyolefin resin particles.

2. A particulate resin material as in Claim 1, wherein the polyolefin resin is polyethylene.

3. A particulate resin material as in Claim 2, wherein the polyethylene is a low density polyethylene having a density of from 0.910 to 0.930.

4. A particulate resin material as in any of Claims 1 to 3, wherein the average particle size is in the range from 2.0 to 4.5 mm, the average expansion ratio is in the range from 23 to 32 and the compression coefficient is in the range from 2.2 x 10-3 to 3.6 x 10-3.

5. A process for producing expanded crosslinked polyolefin resin particles which are substantially spherical, resilient, free-flowing, uniform in particle size and moldable in a cavity, each particle having a structure substantially constituted of contiguous closed cells, and which have an average particle size ranging from 1.4 to 5.5 millimeters, an average expansion ratio ranging from 18 to 37 based on the original volume of unedpanded resin particles and a compression coefficient ranging from 1.6 x 10-3 to 4.0 x 10-3 as determined from the following formula: S/(R x F) wherein S represents total energy for compression under pressure of 1 kg/cm2, R
average expansion ratio and F flowability, respectively, of the expanded crosslinked polyolefin resin particles, which comprises first allowing particles of a crosslinked polyolefin resin containing a foaming agent to expand to an average expansion ratio of from 3 to 9 and then, after imparting expandability to the thus pre-expanded particles, further allowing said pre-expanded particles to expand to an average expansion ratio of from 13 to 37, said average expansion ratio being based on the original volume of unexpanded resin particles.

6. A process for producing expanded crosslinked polyolefin resin particles as in Claim 5, wherein expandability is imparted to the pre-expanded particles by impregnating the pre-expanded particles with an inorganic gas principally composed of nitrogen under a pressurized atmosphere.

7. A process for producing a molded article from expanded crosslinked polyolefin resin particles which are substantially spherical, resilient, free-flowing, uniform in particle size and moldable in a cavity, each particle having a structure substantially constituted of contiguous closed cells, and which have an average particle size ranging from 1.4 to 5.5 millimeters, an average expansion ratio ranging from 18 to 37 based on the original volume of unexpanded resin particles and a compression coefficient ranging from 1.6 x 10-3 to 4.0 X 10-3 as determined from the following formula: S/(R x F) wherein S represents total energy for compression under pressure of 1 kg/cm2, R average expansion ratio and f flowability, respectively, of the expanded crosslinked polyolefin resin particles, which comprises the steps of compressing the expanded particles to 40 to 80% of their volume, charging the thus compressed particles in the compressed state to a mold and thereafter heating the particles to effect expansion of the particles to fill the mold and to integrally bond.