Process for preparing polyolefin resin pre-expanded particles

A process for preparing polyolefin resin pre-expanded particles comprising imparting expandability to polyolefin resin expanded particles having a crystal structure which shows two melting points in a DSC curve and heat expanding expanded particles having expandability, in which the relation between heat expansion temperature and the amount of a fusion preventing agent being attached to the surface of the polyolefin resin expanded particles is represented by the inequality: t-T.sub.V .ltoreq.49.1C-27.0 (t is heat expansion temperature, C is the amount of the fusion preventing agent being attached to the surface of the polyolefin resin expanded particles and T.sub.V is saddle temperature between two peaks showing two respective melting points in the DSC curve), and t is within the temperature range represented by the inequality: T.sub.L -30.ltoreq.t.ltoreq.T.sub.V (T.sub.V is the same as defined above and T.sub.L is lower melting point of the two melting points). According to the process, mutual fusion of the expanded particles and increase of open cell ratio of the pre-expanded particles can be prevented at the same time, expansion efficiency can be increased, and polyolefin resin pre-expanded particles having high expansion ratio can be prepared.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for preparing polyolefin resin 
pre-expanded particles, and more particularly to a process for preparing 
polyolefin resin pre-expanded particles which can be preferably used as, 
for instance, a raw material for an internal die expansion molded article. 
Conventionally, when polyolefin resin pre-expanded particles are prepared, 
there is well known a process for preparing polyolefin resin expanded 
particles, comprising dispersing resin particles together with a 
dispersant into an aqueous disperse medium in a closed container, 
introducing a volatile blowing agent thereto, then heating the resin 
particles to at lowest softening temperature of the polyolefin resin and 
releasing the resin particles into a lower pressure atmosphere than 
internal pressure of the closed container. 
For instance, there is already published a process for preparing polyolefin 
resin pre-expanded particles having a special crystal structure which 
shows two melting points in a DSC curve obtained by a differential 
scanning calorimetry, by using the above process for preparing polyolefin 
resin expanded particles (see, for instance, Japanese Unexamined Patent 
Publication No. 176336/1984 and Japanese Unexamined Patent Publication No. 
183832/1988). Furthermore, there is already published a method for 
improving expansion ratio of the above polyolefin resin pre-expanded 
particles having a special crystal structure with retaining a closed cell 
structure. In this method, expandability is imparted to the polyolefin 
resin pre-expanded particles and then, the particles are heated (see 
Japanese Unexamined Patent Publication No. 23428/1985 and Japanese 
Unexamined Patent Publication No. 90228/1985). 
In Japanese Unexamined Patent Publication No. 23428/1985, there is 
disclosed a process for preparing pre-expanded particles having higher 
expansion ratio than that of base non-crosslinked propylene random 
copolymer pre-expanded particles having a specific range of expansion 
ratio and cell number and having a crystal structure which shows higher 
temperature peak on a higher temperature point than a point of intrinsic 
peak of the base resin in a DSC curve. In this process, expandability is 
imparted to the non-crosslinked propylene random copolymer pre-expanded 
particles and then, the particles are heat expanded. 
In Japanese Unexamined Patent Publication No. 90228/1985, there is 
disclosed a method comprising a step of imparting expandability to 
polypropylene resin pre-expanded particles having the same special crystal 
structure as that of the pre-expanded particles disclosed in Japanese 
Unexamined Patent Publication Nos. 176336/1984 and 183832/1988 and having 
decreasing speed coefficient of internal pressure "k" of at most 0.30, a 
step of heating the polypropylene resin pre-expanded particles in a closed 
container to temperature "T" (.degree. C.) represented by the inequality: 
EQU Tm-65&lt;T&lt;Tm-30 
wherein Tm is melting finishing temperature of the base resin, a step of 
keeping as it is, a step of opening the edge of the container and a step 
of releasing the polypropylene resin pre-expanded particles into a lower 
pressure atmosphere than internal pressure of the container. 
However, all technologies in these methods relate to a polypropylene resin, 
in particular, ethylene-propylene random copolymer. Accordingly, 
technologies using the other polyolefin resin are not disclosed. 
As to heat temperature of the pre-expanded particles, as usual, it is 
described that vapor having vapor pressure of 0.8 to 1.5 kg/cm.sup.2 (G) 
(116.degree..about.127.degree. C.) or hot air having temperature of at 
lowest 100.degree. C. is preferably used (see Japanese Unexamined Patent 
Publication No. 23428/1985). Also, it is described that the pre-expanded 
particles are heated at temperature "T" (.degree. C.) represented by the 
inequality: 
EQU Tm-65&lt;T&lt;Tm-30 
wherein Tm is melting finishing temperature of the base resin (see Japanese 
Unexamined Patent Publication No. 90228/1985). However, although the 
pre-expanded particles having a special crystal structure are used, it is 
not disclosed that crystal characteristic of the pre-expanded particles is 
related to heat temperature of the pre-expanded particles to accomplish 
the objects. 
During heat expansion of the thus polyolefin resin expanded particles, in 
the case that heat temperature of the expanded particles is too high, open 
cell ratio of the obtained pre-expanded particles increases. In the case 
that the thus obtained pre-expanded particles are used for internal die 
mold, it is forecasted that mechanical strength of the produced molded 
article remarkably lowers or mutual fusion of the expanded particles is 
generated to cause short shot during feeding of the expanded particles 
into a molding machine. These phenomena are mentioned in the above 
Publications. 
Then, the inventors of the present invention have earnestly studied as to 
heat expansion and carried out the following examination. An 
ethylene-propylene random copolymer having the above temperature "Tm" of 
157.degree. C. was used as a base resin and subjected to the general 
procedure to give expanded particles. The obtained expanded particles were 
sufficiently washed with water and an acidic aqueous solution to give 
expanded particles having the above special crystal structure (expansion 
ratio: 11.3 times, average cell diameter: 300 .mu.m, decreasing speed 
coefficient of internal pressure "k": 0.15). The obtained expanded 
particles having the special crystal structure were heat expanded with 
vapor having vapor pressure of 1.5 kg/cm.sup.2 (G) (about 126.degree. C.) 
to give pre-expanded particles (expansion time: 30 seconds). However, 
although pre-expanded particles having high expansion ratio and a closed 
cell structure could be certainly prepared, mutual fusion of the expanded 
particles was generated to cause short shot of the expanded particles 
during molding. 
When pre-expanded particles are prepared by a heat expansion method, as one 
of the effective means for heightening expansion ratio of the obtained 
pre-expanded particles, it can be proposed that a resin layer on the 
surface of expanded particles is softened together with heightening heat 
temperature and internal pressure of the expanded particles. However, in 
the case that mutual fusion of the expanded particles is once generated as 
mentioned above, substantially, heat temperature cannot be heightened any 
more, so that expansion ratio of the obtained pre-expanded particles 
cannot be heightened. 
On the other hand, in a process for producing a polyolefin resin expansion 
molded article comprising filling a molding machine with the above 
pre-expanded particles having the special crystal structure and subjecting 
the pre-expanded particles to heat fusion with vapor to give an expansion 
molded article having the desired shape, the amount of attachment on the 
surface of the pre-expanded particles is known as one of factors exerting 
an influence upon molding fusion property (see, for instance, Japanese 
Unexamined Patent Publication No. 57838/1992). 
That is, in a process for preparing polyolefin resin expanded particles 
comprising dispersing polyolefin resin particles into an aqueous disperse 
medium in a closed container, introducing a volatile blowing agent 
thereto, then heating the resin particles to at lowest softening 
temperature of the polyolefin resin and releasing the resin particles into 
a lower pressure atmosphere than internal pressure of the closed 
container, inorganic powder which is called dispersant or fusion 
preventing agent is added to the aqueous disperse medium together with the 
polyolefin resin particles, the inorganic powder is attached to the 
surface of the polyolefin resin particles to prevent mutual fusion of the 
resin particles in the aqueous disperse medium, and this surface 
attachment is washed and removed during or after preparation of 
pre-expanded particles to improve molding fusion property. 
For instance, in the process described in Japanese Unexamined Patent 
Publication No. 57838/11992, 3 parts (parts by weight, hereinafter 
referred to the same) of powdery tricalcium phosphate as a dispersant and 
0.12 part of sodium n-paraffinsulfonate are added to 100 parts of 
ethylene-propylene random copolymer pellet, and a method for washing 
pre-expanded particles just after preparation is changed, so that the 
amount of attachment (mainly, tricalcium phosphate) on the surface of the 
pre-expanded particles is changed. However, in the case that the amount of 
this attachment is at least 3300 ppm, there is a problem that molding 
fusion property remarkably lowers, so melt bonding ratio of the obtained 
molded article becomes 0%. This problem is pointed out in this 
Publication. 
Then, the inventors of the present invention have firstly considered that 
mutual fusion of expanded particles during heat expansion can be prevented 
or restrained in accordance with adjustment of the amount of attachment on 
the surface of the pre-expanded particles. So, by the inventors, there has 
been carried out heat expansion experiment comprising increasing the 
amount of attachment on the surface of the base expanded particles under 
the entirely same condition as in the case that mutual fusion of expanded 
particles has been generated at vapor pressure of 1.5 kg/cm.sup.2 (G) 
during the above heat expansion. As a result, it has been found that 
mutual fusion of expanded particles has not been generated even at vapor 
pressure of 3 kg/cm.sup.2 (G) (=143.degree. C.). 
In addition, by using the other polyolefin resin or composition thereof, 
such as ethylene-propylene random copolymer, ethylene-.alpha.-olefin 
copolymer or a resin composition prepared by adding an ethylene ionomer to 
ethylene-.alpha.-olefin copolymer, which has different melting finishing 
temperature each other, the same experiment as mentioned above has been 
carried out. As a result, it has been found that at lower temperature than 
T.sub.L -30 (.degree. C.) (T.sub.L : lower melting point of the two 
melting points of base pre-expanded particles), expansion ratio by heat 
expansion has been improved only a little. Also, it has been found that at 
higher temperature than T.sub.V (.degree. C.) (T.sub.V : saddle 
temperature between two peaks showing two respective melting points), 
mutual fusion of expanded particles has been generated having no relation 
to melting finishing temperature of base resin (composition) or no 
relation to the amount of attachment on the surface of base expanded 
particles, or open cell ratio of the obtained pre-expanded particles has 
become at least 20% by heat expansion. Accordingly, it has been found that 
retention of good moldability has been difficult. 
Furthermore, at the temperature range from T.sub.L -30 (.degree. C.) to 
T.sub.V (.degree. C.), there has been examined the relation between the 
amount of attachment on the surface of the expanded particles and t 
(.degree. C.) (t: heat expansion temperature at which mutual fusion of 
expanded particles has been generated). As a result, as to all kinds of 
polyolefin resin expanded particles, surprisingly, it has been found that 
there has been the linear relation between the upper limit of the 
temperature represented by t-T.sub.V (.degree. C.) (at the same 
temperature as this upper limit or higher temperature than this upper 
limit, mutual fusion of expanded particles has been generated) and the 
amount of a fusion preventing agent being attached to the surface of the 
expanded particles, so that the present invention has been completed. 
The present invention has been accomplished in consideration of the above 
knowledge. 
An object of the present invention is to provide a process for preparing 
polyolefin resin pre-expanded particles having high expansion ratio with 
retaining closeness of cell and without generation of mutual fusion of 
expanded particles in accordance with a heat expansion method by using the 
above polyolefin resin expanded particles having a special crystal 
structure. 
This and other objects of the present invention will become apparent from 
the description hereinafter. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided 
a process for preparing polyolefin resin pre-expanded particles comprising 
imparting expandability to polyolefin resin expanded particles having a 
crystal structure which shows two melting points in a DSC curve obtained 
by a differential scanning calorimetry and then, heat expanding expanded 
particles having expandability, 
which is characterized in that the relation between heat expansion 
temperature of the above expanded particles having expandability and the 
amount of a fusion preventing agent being attached to the surface of the 
polyolefin resin expanded particles is represented by the inequality (I): 
EQU t-T.sub.V .ltoreq.49.1C-27.0 (I) 
wherein t (.degree. C.) is heat expansion temperature, C (phr) is the 
amount of the fusion preventing agent being attached to the surface of the 
polyolefin resin expanded particles and T.sub.V (.degree. C.) is saddle 
temperature between two peaks showing two respective melting points in the 
DSC curve, and 
the heat expansion temperature is within the temperature range represented 
by the inequality (II): 
EQU T.sub.L -30 .ltoreq.t.ltoreq.T.sub.V (II) 
wherein t (.degree. C.) is heat expansion temperature, T.sub.V (.degree. 
C.) is saddle temperature between two peaks showing two respective melting 
points in the DSC curve and T.sub.L (.degree. C.) is lower melting point 
of the two melting points, 
the process as defined above, wherein the amount of the fusion preventing 
agent being attached to the surface of the polyolefin resin expanded 
particles is 0.001 to 0.3 phr, and 
the process as defined above, wherein the fusion preventing agent mainly 
consists of tricalcium phosphate.

DETAILED DESCRIPTION 
The process for preparing polyolefin resin pre-expanded particles of the 
present invention comprises, as mentioned above, imparting expandability 
to polyolefin resin expanded particles having a crystal structure which 
shows two melting points in a DSC curve obtained by a differential 
scanning calorimetry and then, heat expanding expanded particles having 
expandability, and is characterized in that the relation between heat 
expansion temperature of the above expanded particles having expandability 
and the amount of a fusion preventing agent being attached to the surface 
of the polyolefin resin expanded particles is represented by the 
inequality (I): 
EQU t-T.sub.V .ltoreq.49.1C-27.0 (I) 
wherein t (.degree. C.) is heat expansion temperature, C (phr) is the 
amount of the fusion preventing agent being attached to the surface of the 
polyolefin resin expanded particles and T.sub.V (.degree. C.) is saddle 
temperature between two peaks showing two respective melting points in the 
DSC curve, and the heat expansion temperature is within the temperature 
range represented by the inequality (II): 
EQU T.sub.L -30.ltoreq.t .ltoreq.T.sub.V (II) 
wherein t (.degree. C.) is heat expansion temperature, T.sub.V (.degree. 
C.) is saddle temperature between two peaks showing two respective melting 
points in the DSC curve and T.sub.L (.degree. C.) is lower melting point 
of the two melting points. 
In the present invention, the above differential scanning calorimetry is 
carried out in the same manner as in, for instance, Japanese Unexamined 
Patent Publication No. 23428/1985, Japanese Unexamined Patent Publication 
No. 90228/1985 and the like. In accordance with the differential scanning 
calorimetry, crystallization property of expanded particles is examined by 
using a differential scanning calorimeter with raising the temperature of 
the expanded particles to 220.degree. C. at a rate of 10.degree. C./min. 
The polyolefin resin expanded particles used in the present invention have, 
as shown in a graph of FIG. 2, two endothermic peaks (melting points) 
owing to crystallization in the DSC curve obtained by the above 
differential scanning calorimetry. As to these two peaks, when lower 
temperature peak (melting point) is represented as "T.sub.L " (.degree. 
C.) (mark ".alpha." in FIG. 2) and higher temperature peak (melting point) 
is represented as "T.sub.H " (.degree. C.) (mark ".beta." in FIG. 2), as 
usual, the difference between these two melting points "T.sub.H -T.sub.L " 
is 5.degree. to 20.degree. C. or so. Also, as shown in FIG. 2, a curve 
consisting of these two peaks looks like a saddle of horse as a whole. In 
this saddle, the other extreme value appears on the most heat releasing 
point of the DSC curve. In the present invention, the temperature at which 
this extreme value appears is defined as saddle temperature "T.sub.V " 
(.degree. C.) (mark ".gamma." in FIG. 2). 
The polyolefin resin expanded particles having the above specific crystal 
structure can be prepared by expanding polyolefin resin particles for 
expansion with a blowing agent. 
Examples of the above polyolefin resin are, for instance, a propylene resin 
such as polypropylene, ethylene-propylene random copolymer, 
ethylene-propylene block copolymer or ethylene-propylene-.alpha.-olefin 
terpolymer; an ethylene resin such as linear low density polyethylene, low 
density polyethylene or high density polyethylene; and the like. In order 
to retain mechanical strength of molded articles produced by using the 
pre-expanded particles, there is suitably used a polyolefin resin having 
the endothermic amount in the whole of the peaks containing two melting 
points of at least 30 J/g, preferably at least 50 J/g and showing 
crystallinity in accordance with the differential scanning calorimetry of 
the expanded particles used for heat expansion. Also, in consideration of 
balance of physical properties other than mechanical strength of the 
molded articles, and fusion property and the extent of proper range during 
molding, ethylene-propylene random copolymer and linear low density 
polyethylene can be, in particular, preferably used. 
As far as physical properties of the obtained expanded particles, 
pre-expanded particles and molded articles are not remarkably lowered, as 
an expansion nucleating agent, hydrophilic substances such as hydrophilic 
inorganics and hydrophilic organic compounds can be added to the 
polyolefin resin. 
As mentioned below, during heat expansion, in the case that water and/or 
alcohol are included in the expanded particles and utilized as an 
auxiliary blowing agent, heat expansion efficiency can be more improved. 
Because the polyolefin resin shows hydrophobic property, in the case that 
the above hydrophilic substances are added to the polyolefin resin, water 
and/or alcohol which are auxiliary blowing agents can be easily included 
in the expanded particles. 
Examples of the hydrophilic inorganics are, for instance, talc, silica, 
borax, sodium phosphate and the like. Examples of the hydrophilic organic 
compounds are, for instance, a water absorbing polymer such as crosslinked 
poly(sodium acrylate), an ethylene ionomer and the like. It is desired 
that the amount of the hydrophilic substances is 0.001 to 20 parts or so 
based on 100 parts of the polyolefin resin. 
Polyolefin resin particles for expansion can be prepared, for instance, by 
adding the hydrophilic substances to the polyolefin resin and melt 
kneading them by means of a single screw extruder or a twin screw 
extruder. 
Then, a blowing agent and a fusion preventing agent are added to the resin 
particles for expansion and then, these components are fed into a closed 
container and expanded under suitable condition, so that there can be 
prepared polyolefin resin expanded particles having the above specific 
crystal structure. The fusion preventing agent is attached to the surface 
of the polyolefin resin expanded particles. 
As the blowing agent, there can be exemplified a blowing agent used for 
imparting expandability to the polyolefin resin expanded particles as 
mentioned below. The amount of the blowing agent is suitably adjusted so 
that the polyolefin resin expanded particles have the desired expansion 
ratio. For instance, it is desired that the amount of the blowing agent is 
0.1 to 30 parts or so based on 100 parts of the resin particles for 
expansion. 
As the fusion preventing agent, there can be exemplified every substance 
which is conventionally used as a dispersant or a fusion preventing agent. 
Examples of the fusion preventing agent are, for instance, fine granular 
aluminium oxide, tricalcium phophate, calcium carbonate, magnesium 
carbonate, kaolin, bentonite and the like. These can be used alone or in 
an admixture thereof. Among them, tricalcium phophate is particularly 
preferable because fusion preventing effect is excellent. The amount of 
the fusion preventing agent is suitably adjusted so that the amount of the 
fusion preventing agent being attached to the surface of the polyolefin 
resin expanded particles is within the range as mentioned below. 
To the resin particles for expansion, there can be suitably added, for 
instance, an auxiliary dispersant such as sodium n-paraffinsulfonate, 
sodium dodecylbenzenesulfonate, benzalkonium chloride or 
alkyltrimethylammonium chloride. 
In order to sufficiently exhibit mutual fusion preventing effect of the 
expanded particles within the suitable temperature range for heat 
expansion, it is desired that the amount of the fusion preventing agent 
being attached to the surface of the thus obtained polyolefin resin 
expanded particles is at least 0.001 phr, preferably at least 0.005 phr. 
When the amount of the above fusion preventing agent is too much, although 
mutual fusion of the expanded particles during heat expansion can be 
almost completely prevented, fusion property during molding of the 
obtained pre-expanded particles is remarkably lowered, so that the 
pre-expanded particles must be washed before molding. Accordingly, in 
order to remove the fear of washing the pre-expanded particles, it is 
desired that the amount of the fusion preventing agent being attached to 
the surface of the polyolefin resin expanded particles is at most 0.3 phr, 
preferably at most 0.15 phr, more preferably at most 0.13 phr. 
As to T.sub.L, T.sub.H and T.sub.V in the DSC curve of the above polyolefin 
resin expanded particles, it is desired that, as usual, T.sub.L is 
90.degree. to 160.degree. C. or so, T.sub.H is 110.degree. to 180.degree. 
C. or so and T.sub.V is 95.degree. to 175.degree. C. or so. Also, as to 
the polyolefin resin expanded particles, it is desired that expansion 
ratio is 2 to 30 times or so and average cell diameter is 10 to 500 .mu.m 
or so. 
Then, expandability is imparted to the polyolefin resin expanded particles. 
A method for imparting expandability to the expanded particles is not 
particularly limited. In accordance with the conventionally known methods, 
expandability can be imparted to the expanded particles. For instance, in 
the polyolefin resin expanded particles to be used for heat expansion is 
included a blowing agent of which boiling point is lower than heat 
expansion temperature and which cannot dissolve the polyolefin resin in a 
suitable amount, so that expandability can be imparted to the polyolefin 
resin expanded particles. As the blowing agent, for instance, air and an 
inorganic gas such as nitrogen gas or carbonic acid gas are particularly 
preferable because these have generality, no combustibility and no 
toxicity, and because these are gases during heat expansion, so it is easy 
to retain high internal pressure of the expanded particles. Also, there 
can be utilized various compounds which are conventionally widely used for 
preparing polyolefin resin pre-expanded particles, such as lower aliphatic 
hydrocarbons such as propane, butane, isobutane, pentane, isopentane and 
cyclopentane; and halogenated hydrocarbons. In consideration of, in 
particular, prevention of destruction of ozone layer, among the 
halogenated hydrocarbons, a compound having no chlorine in its molecule, 
so-called third generation flon, is preferable. The amount of the blowing 
agent is not particularly limited. It is desired that, as usual, the 
amount of the blowing agent is suitably adjusted so that the internal 
pressure of the expanded particles during heat expansion is 0.1 to 30 
kg/cm.sup.2 (G) or so. 
In the present invention, water and/or alcohol can be suitably used as an 
auxiliary blowing agent. Although vapor pressure during heat expansion is 
low, expansion efficiency can be more heightened when water and/or alcohol 
are utilized as an auxiliary blowing agent. 
Then, the thus obtained expanded particles having expandability are heated 
to give polyolefin resin pre-expanded particles. 
As a method for heat expanding the expanded particles having expandability, 
there can be employed, for instance, a method comprising using hot air or 
vapor. As disclosed in Japanese Unexamined Patent Publication No. 
133233/1984, a method comprising using vapor is particularly preferable 
because quantity of heat can be quickly imparted to the expanded particles 
having expandability and productivity can be improved. When vapor is used, 
heating time may be, as usual, one minute or so. 
The process of the present invention is greatly characterized in that the 
relation between heat expansion temperature of the expanded particles 
having expandability and the amount of the fusion preventing agent being 
attached to the surface of the polyolefin resin expanded particles is 
specific, and the relation between the heat expansion temperature and 
"T.sub.V " and "T.sub.L " is specific. 
The relation between the heat expansion temperature of the expanded 
particles having expandability and the amount of the fusion preventing 
agent being attached to the surface of the polyolefin resin expanded 
particles is, as mentioned above, represented by the inequality (I): 
EQU t-T.sub.V .ltoreq.49.1C-27.0 (I) 
wherein t (.degree. C.) is heat expansion temperature, C (phr) is the 
amount of the fusion preventing agent being attached to the surface of the 
polyolefin resin expanded particles and T.sub.V (.degree. C.) is saddle 
temperature between two peaks showing two respective melting points in the 
DSC curve, and the above heat expansion temperature "t" is within the 
temperature range represented by the inequality (II): 
EQU T.sub.L -30.ltoreq.t.ltoreq.T.sub.V (II) 
wherein T.sub.V is the same as defined above and T.sub.L (.degree. C.) is 
lower melting point of the above two melting points. 
The heat expansion temperature "t" is "T.sub.L -30" (.degree. C.) or higher 
than "T.sub.L -30" (.degree. C.), preferably "T.sub.L -25" (.degree. C.) 
or higher than "T.sub.L 25" (.degree. C.), and "t" is "T.sub.V " (.degree. 
C.) or lower than "T.sub.V " (.degree. C.), preferably "T.sub.V -2" 
(.degree. C.) or lower than "T.sub.V -2" (.degree. C.). 
It is not desired that "t" is lower than the above lower limit "T.sub.L 30" 
because resin layer which composes the expanded particles is hard, 
expansion efficiency ".epsilon." lowers and the ratio of expansion ratio 
before heat expansion to expansion ratio after heat expansion becomes 
small when heat temperature is too low. 
Expansion efficiency ".delta." is calculated in accordance with the 
following equation (III): 
##EQU1## 
wherein K.sub.O (times) is expansion ratio of expanded particles before 
heat expansion, K (times) is expansion ratio of pre-expanded particles 
after heat expansion, P.sub.O atm (abs)! is internal pressure of the 
expanded particles at room temperature (23.degree. C.) just before heat 
expansion, T (t+273.2(.degree. C.)) is heat expansion temperature and Tr 
(.degree. C.) is room temperature (=23.degree. C.) during measuring 
P.sub.O. 
That is, when the expansion ratio after heat expansion is the same as that 
before heat expansion (K.sub.O =K), .epsilon.=0. Also, when cubic 
expansion of the expanded particles during the change from increased 
internal pressure by temperature of the given internal pressure of 
expanded particles to atmospheric pressure owing to the heat expansion is 
equal to cubic expansion of the expanded particles during the heat 
expansion, .epsilon.=1. Accordingly, when effect of an auxiliary blowing 
agent such as water, water vapor or alcohol is exhibited, there is 
possibility of .epsilon.&gt;1. 
In the case that the heat expansion temperature was too low, that is "t" 
was lower than "T.sub.L -30" (T&lt;T.sub.L -30), expansion efficiency lowered 
(.epsilon.&lt;0.1). Accordingly, on the basis of the above equation (III), in 
order to increase K, P.sub.O must be heightened. It is not desired that 
P.sub.O is heightened because pressure during imparting expandability to 
the polyolefin resin expanded particles must be heightented or time for 
imparting expandability to the expanded particles must be lengthened. 
It is not desired that "t" is higher than "T.sub.V " because mutual fusion 
and/or open cell of the expanded particles are generated. 
Thus, in the case that the heat expansion temperature "t" is within the 
temperature range represented by the inequality (II), there is realized 
the linear relation between the upper limit temperature (.degree. C.) of 
"t-T.sub.V " and the amount of the fusion preventing agent "C" (phr) being 
attached to the surface of the polyolefin resin expanded particles. That 
is, the relation represented by the inequality (I): 
EQU t-T.sub.V .ltoreq.49.1C-27.0 (I) 
is realized. 
When the specific amount of the fusion preventing agent "C" being attached 
to the surface of the expanded particles is fixed, on the basis of the 
fixed "C" and "T.sub.V ", there is univocally fixed the upper limit of the 
temperature "t" at which excellent heat expansion can be carried out 
without mutual fusion of the expanded particles. The upper limit of the 
temperature "t" can be obtained in accordance with heat expansion 
evaluation experiment at heat temperature in several levels. The heat 
expansion evaluation experiment is carried out for respective several 
expanded particles. The several expanded particles show respective 
different values as to "T.sub.V " and "C". 
The relation between the above upper limit temperature "t-T.sub.V " 
(.degree. C.) and the amount of the fusion preventing agent "C" (phr) is 
shown in FIG. 1. As is clear from FIG. 1, the linear relation between 
"t-T.sub.V " and "C" is realized. No mutual fusion was generated under the 
condition within the right lower area surrounded with open cell critical 
line A and fusion critical line B in FIG. 1. Accordingly this right lower 
area is preferable area. On the contrary, mutual fusion was generated 
under the condition within the left upper area in FIG. 1. Also, under the 
condition within the area satisfying the inequalities "t-T.sub.V &gt;0" and 
"t-T.sub.V &lt;49.1C-27.0" (area over the open cell critical line A), 
although mutual fusion of the expanded particles was not generated, open 
cell ratio of the obtained pre-expanded particles was higher than 20%. 
As mentioned above, by adjusting the amount of the fusion preventing agent 
to be attached to the surface of the polyolefin resin expanded particles 
and the heat expansion temperature of the expanded particles having 
expandability so as to satisfy the conditions represented by the 
inequality (I) and the inequality (II), the polyolefin resin pre-expanded 
particles having high expansion ratio can be prepared with retaining 
closeness of cell and without generation of mutual fusion of expanded 
particles. 
According to the process of the present invention, mutual fusion of 
expanded particles and increase of open cell ratio of pre-expanded 
particles owing to breaking of cell membrane during heat expansion can be 
prevented at the same time, heat expansion efficiency can be heightened, 
and polyolefin resin pre-expanded particles having high expansion ratio 
can be prepared. 
The polyolefin resin pre-expanded particles prepared by the process of the 
present invention can be easily molded in accordance with conventionally 
known molding methods and suitably used for cushioning materials. 
The process for preparing polyolefin resin pre-expanded particles of the 
present invention is more specifically described and explained by means of 
the following Examples. It is to be understood that the present invention 
is not limited to the Examples, and various changes and modifications may 
be made in the invention without departing from the spirit and scope 
thereof. 
Melting point and melting finishing temperature of polyolefin resin (base 
resin) used in the following Examples and Comparative Examples were 
measured as follows. Base resin particles in an amount of about 1 to 10 mg 
were exactly weighed and subjected to differential scanning calorimetry 
with raising the temperature thereof from room temperature to 220.degree. 
C. at a rate of 10.degree. C./min. by using differential scanning 
calorimeter SSC5200 commercially available from Seiko Instruments Inc. to 
give a DSC curve. Endothermic peak in the DSC curve was defined as melting 
point. Melting finishing temperature was measured in accordance with a 
method described in Japanese Unexamined Patent Publication No. 23428/1985. 
Example 1 and Comparative Examples 1 to 4 
Into a single screw extruder (50 mm.o slashed., L/D=3) were fed 
ethylene-propylene random copolymer (melting point: 137.degree. C., 
melting finishing temperature: 157.degree. C.) as a polyolefin resin and 
talc in an amount of 0.005 phr as an expansion nucleating agent to give 
resin particles for expansion in a particle weight of about 1.8 mg. 
Into a first closed container were fed 100 parts of the obtained resin 
particles for expansion, 300 parts of water, 1.4 parts of tricalcium 
phosphate as a fusion preventing agent and 0.03 part of sodium 
n-paraffinsulfonate as an auxiliary dispersant. Then, thereto was added 13 
parts of isobutane as a blowing agent, and the resin particles for 
expansion were expanded through a circular orifice having a diameter of 4 
mm under the condition at expansion temperature of 138.degree. C. and 
expansion pressure of 17 kg/cm.sup.2 (G) to give polyolefin resin expanded 
particles. 
As properties of the obtained polyolefin resin expanded particles, 
expansion ratio and average cell diameter were measured in accordance with 
the following methods. As a result, the polyolefin resin expanded 
particles were expanded particles having an expansion ratio of 11.3 times, 
an average cell diameter of 300 .mu.m and a closed cell structure. 
The polyolefin resin expanded particles showed two melting points in a DSC 
curve obtained by differential scanning calorimetry using the differential 
scanning calorimeter SSC5200 commercially available from Seiko Instruments 
Inc. and had a special crystal structure. As to T.sub.L, T.sub.V and 
T.sub.H in the DSC curve, T.sub.L, T.sub.V and T.sub.H were 136.8.degree. 
C., 149.4.degree. C. and 156.7.degree. C, respectively. 
The polyolefin resin expanded particles were sufficiently washed with 
hydrochloric acid showing pH 3 and water, and dried. Then, the amount of 
the fusion preventing agent being attached to the surface of the dried 
expanded particles was measured in accordance with the following method. 
As a result, the amount of the fusion preventing agent was 0.05 phr. Then, 
this expanded particles were fed into a second closed container, and the 
internal pressure of the second closed container was adjusted to 5 
kg/cm.sup.2 (G) with air. The expanded particles were allowed to stand at 
room temperature for about 24 hours, and expandability was imparted to the 
expanded particles. After the obtained expanded particles having 
expandability were taken out from the second closed container, decreasing 
speed coefficient of internal pressure was measured. The decreasing speed 
coefficient of internal pressure was 0.15. The internal pressure at 
23.degree. C. of the expanded particles having expandability just before 
heat expansion was 3.9 atm (abs). 
After the above expanded particles having expandability were fed into a 
third closed container, heat expansion was carried out under the condition 
shown in Table 1 to give polyolefin resin pre-expanded particles. 
As properties of the obtained polyolefin resin pre-expanded particles, 
expansion ratio, expansion efficiency, open cell ratio and mutual fusion 
were examined in accordance with the following methods. The results are 
shown in Table 1. 
Polyolefin resin expanded particles! 
(1) Expansion ratio (K.sub.O) 
Expanded particles in an amount of about 2 g were exactly weighed and 
volume of the expanded particles was measured by a dipping method with 
water. True specific gravity of the expanded particles was calculated. 
Then, the expansion ratio "K.sub.O " of the expanded particles was 
calculated by dividing the true specific gravity of the resin 
(composition) by the true specific gravity of the expanded particles. 
(2) Average cell diameter 
Average cell diameter of the expanded particles was measured by observing 
the cross section of the expanded particles with a microscope. 
(3) Amount of a fusion preventing agent (tricalcium phosphate) being 
attached to the surface of the expanded particles (C) 
To a conical beaker were added 50.0 ml of an aqueous solution (calorimetric 
solution) containing 0.022% by weight of ammonium metavanadate, 0.54% by 
weight of ammonium molybdate and 3% by weight of nitric acid, and "W" (g) 
of the expanded particles. They were stirred for one minute and then, 
allowed to stand for 10 minutes. The liquid phase in the obtained mixture 
was poured into a fused silica cell having an optical path length of 1.0 
cm, and absorbance "A" of the liquid phase was measured at a wavelength of 
410 nm by a spectrophotometer. 
Using the same calorimetric solution as mentioned above, absorptivity 
".mu." (g/L cm) of tricalcium phosphate had been previously measured at a 
wavelength of 410 nm. On the bases of ".mu.", "A", "W", the amount of the 
fusion preventing agent "C" (phr) was calculated in accordance with the 
following equation. 
##EQU2## 
Polyolefin resin pre-expanded particles!(1) Expansion ratio (K) 
Expansion ratio "K" of the pre-expanded particles was calculated in the 
same manner as the expansion ratio "K.sub.O " of the above polyolefin 
resin expanded particles. 
(2) Expansion efficiency (.epsilon.) 
Expansion efficiency ".epsilon." of the pre-expanded particles was 
calculated in accordance with the following equation (III): 
##EQU3## 
wherein K.sub.O, K, P.sub.O, T and T.sub.r were the same as defined above, 
respectively. 
(3) Open cell ratio 
Closed cell volume of the pre-expanded particles was measured by using air 
comparative hydrometer 1000 type commercially available from TOKYO SCIENCE 
INC. Then, closed cell ratio (%) of the pre-expanded particles was 
calculated by dividing the closed cell volume by apparent volume of the 
pre-expanded particles previously measured by the dipping method with 
water. Open cell ratio (%) was calculated by subtracting the closed cell 
ratio from 100. 
(4) Mutual fusion 
The existence of mutual fusion was examined by observing the pre-expanded 
particles with the naked eyes. 
Example 2 
Polyolefin resin pre-expanded particles were prepared in the same manner as 
in Comparative Example 3 except that the amount of the fusion preventing 
agent was changed from 1.4 parts to 2.5 parts, the obtained polyolefin 
resin expanded particles just after expansion through the orifice of the 
first closed container were sufficiently washed with water, and the 
polyolefin resin expanded particles were not washed with hydrochloric 
acid. 
The properties of the polyolefin resin expanded particles and polyolefin 
resin pre-expanded particles were examined in the same manner as in 
Example 1. The results are shown in Table 1. 
Examples 3 to 5 and Comparative Example 5 
Polyolefin resin pre-expanded particles were prepared in the same manner as 
in Example 1 except that ethylene-propylene random copolymer (melting 
point: 145.degree. C., melting finishing temperature: 161.degree. C.) was 
used as a polyolefin resin instead of the random copolymer used in Example 
1, the amount of talc was changed from 0.005 phr to 0.01 phr and the 
polyolefin resin expanded particles were not washed with hydrochloric 
acid. The internal pressure at 23.degree. C. of the expanded particles 
having expandability just before heat expansion was 4.3 atm (abs). 
The properties of the polyolefin resin expanded particles and polyolefin 
resin pre-expanded particles were examined in the same manner as in 
Example 1. The results are shown in Table 1. 
Example 6 and Comparative Examples 6 to 7 
To 100 parts of a resin mixture containing 98% by weight of the 
ethylene-propylene random copolymer used in Examples 3 to 5 and 
Comparative Example 5, as a polyolefin resin, and 2% by weight of 
HIMILAN.TM. #1707 commercially available from MITSUI-DUPONT POLYCHEMICAL 
INC., as an ethylene ionomer, was added 1 part of talc. The obtained 
mixture was fed into the single screw extruder used in Example 1 to give 
resin particles for expansion. The melting point and the melting finishing 
temperature of the resin particles for expansion were 147.degree. C. and 
159C., respectively. 
Then, polyolefin resin expanded particles were prepared in the same manner 
as in Example 1 except that the blowing agent was changed from isobutane 
to water, the expansion temperature was changed from 138.degree. C. to 
155.degree. C. and the expansion pressure was changed from 17 kg/cm.sup.2 
(G) to 30 kg/cm.sup.2 (G) (pressed with nitrogen gas) when the resin 
particles for expansion were expanded through the orifice of the first 
closed container. The polyolefin resin expanded particles were expanded 
particles having an expansion ratio of 9.8 times and an average cell 
diameter of 150 .mu.m. As to T.sub.L and T.sub.V in the DSC curve of the 
polyolefin resin expanded particles, T.sub.L and T.sub.V were 
143.5.degree. C. and 155.6.degree. C., respectively. 
The polyolefin resin expanded particles were sufficiently washed with 
hydrochloric acid showing pH 1 and fed into the second closed container. 
The internal pressure of the second closed container was adjusted to 8 
kg/cm.sup.2 (G) with nitrogen gas. The expanded particles were allowed to 
stand in water bath at 80.degree. C. for 3 hours, and expandability was 
imparted to the expanded particles. The decreasing speed coefficient of 
internal pressure of the expanded particles having expandability was 1.7. 
The internal pressure at 23.degree. C. of the expanded particles having 
expandability just before heat expansion was 5.0 atm (abs). 
After the above expanded particles having expandability were fed into the 
third closed container, heat expansion was carried out under the condition 
shown in Table 1 to give polyolefin resin pre-expanded particles. 
The properties of the polyolefin resin expanded particles and polyolefin 
resin pre-expanded particles were examined in the same manner as in 
Example 1. The results are shown in Table 1. 
Examples 7 to 8 and Comparative Example 8 
Resin particles for expansion were prepared in the same manner as in 
Example 1 except that linear low density polyethylene (melting point: 
120.degree. C., melting finishing temperature: 132.degree. C.) was used as 
a polyolefin resin instead of ethylene-propylene random copolymer and the 
amount of talc was changed from 0.005 phr to 0.01 phr. 
Then, polyolefin resin expanded particles were prepared in the same manner 
as in Example 1 except that the blowing agent was changed from isobutane 
to water, the expansion temperature was changed from 138.degree. C. to 
125.degree. C., the expansion pressure was changed from 17 kg/cm.sup.2 (G) 
to 35 kg/cm.sup.2 (G) (pressed with air) and the amount of tricalcium 
phosphate as a fusion preventing agent was changed from 1.4 parts to 4.0 
parts when the resin particles for expansion were expanded through the 
orifice of the first closed container. 
Then, into the second closed container were fed the polyolefin resin 
expanded particles without washing, and the internal pressure of the 
second closed container was adjusted to 8 kg/cm.sup.2 (G) with air. The 
expanded particles were allowed to stand at room temperature for 18 hours, 
and expandability was imparted to the expanded particles. The decreasing 
speed coefficient of internal pressure of the expanded particles having 
expandability was 0.15. The internal pressure at 23.degree. C. of the 
expanded particles having expandability just before heat expansion was 5.0 
atm (abs). 
After the above expanded particles having expandability were fed into the 
third closed container, heat expansion was carried out under the condition 
shown in Table 1 to give polyolefin resin pre-expanded particles. 
The properties of the polyolefin resin expanded particles and polyolefin 
resin pre-expanded particles were examined in the same manner as in 
Example 1. The results are shown in Table 1. 
The DSC curve of the polyolefin resin expanded particles used in Examples 7 
to 8 and Comparative Example 8, which was obtained by the differential 
scanning calorimetry is shown in FIG. 2. 
Examples 9 to 11 and Comparative Examples 9 to 10 
To 100 parts of a resin mixture containing 95% by weight of the linear low 
density polyethylene used in Examples 7 to 8 and Comparative Example 8, as 
a polyolefin resin, and 5% by weight of HIMILAN.TM. #1856 commercially 
available from MITSUI-DUPONT POLYCHEMICAL INC., as an ethylene ionomer, 
was added 0.1 part of talc. The obtained mixture was fed into the single 
screw extruder used in Example 1 to give resin particles for expansion. 
Then, using the above resin particles for expansion, polyolefin resin 
expanded particles were prepared in the same manner as in Examples 7 to 8 
and Comparative Example 8. The polyolefin resin expanded particles were 
expanded particles having an expansion ratio of 3.1 times, an average cell 
diameter of 160 .mu.m and a closed cell structure. As to T.sub.L and 
T.sub.V in the DSC curve of the polyolefin resin expanded particles, 
T.sub.L and T.sub.V were 108.6.degree. C. and 118.7.degree. C., 
respectively. 
Expandability was imparted to the polyolefin resin expanded particles in 
the same manner as in Examples 7 to 8 and Comparative Example 8, and the 
obtained expanded particles having expandability were heat expanded under 
the condition shown in Table 1 to give polyolefin resin pre-expanded 
particles. 
The properties of the polyolefin resin expanded particles and polyolefin 
resin pre-expanded particles were examined in the same manner as in 
Example 1. The results are shown in Table 1. 
In Table 1, "T.sub.L " and "T.sub.V " of every polyolefin resin expanded 
particles, "t-T.sub.L " and "t-T.sub.V " are shown. 
TABLE 1 
__________________________________________________________________________ 
Properties of polyolefin resin expanded particles 
Condition of 
Average 
Amount of 
heat expansion 
Expansion 
cell fusion 
Heat vapor 
Heat 
Example ratio diameter 
preventing 
pressure 
temperature 
No. T.sub.L (.degree.C.) 
T.sub.V (.degree.C.) 
K.sub.O (times) 
(.mu.m) 
agent C (phr) 
(kg/cm.sup.2 (G)) 
t (.degree.C.) 
__________________________________________________________________________ 
Ex. 1 136.8 
149.4 
11.3 300 0.05 1.0 119.6 
Com. Ex. 1 
136.8 
149.4 
11.3 300 0.05 1.5 126.4 
2 136.8 
149.4 
11.3 300 0.05 2.15 134.5 
3 136.8 
149.4 
11.3 300 0.05 3.05 143.5 
4 136.8 
149.4 
11.3 300 0.05 4.00 151.1 
Ex. 2 136.8 
149.4 
11.3 300 0.49 3.05 143.5 
Ex. 3 140.4 
154.6 
9.2 320 0.28 1.1 121.1 
4 140.4 
154.6 
9.2 320 0.28 2.0 132.9 
5 140.4 
154.6 
9.2 320 0.28 2.4 136.9 
Com. Ex. 5 
140.4 
154.6 
9.2 320 0.28 4.0 151.1 
Ex. 6 143.5 
155.6 
9.8 150 0.006 1.0 119.6 
Com. Ex. 6 
143.5 
155.6 
9.8 150 0.006 0.4 108.7 
7 143.5 
155.6 
9.8 150 0.006 1.8 130.2 
Ex. 7 108.6 
118.7 
3.8 100 0.765 0.6 112.7 
8 108.6 
118.7 
3.8 100 0.765 0.8 116.3 
Com. Ex. 8 
108.6 
118.7 
3.8 100 0.765 1.0 119.6 
Ex. 9 108.6 
118.7 
3.1 160 0.771 0.4 108.7 
10 108.6 
118.7 
3.1 160 0.771 0.6 112.7 
11 108.6 
118.7 
3.1 160 0.771 0.8 116.3 
Com. Ex. 9 
108.6 
118.7 
3.1 160 0.771 1.0 119.6 
10 108.6 
118.7 
3.1 160 0.771 1.2 122.7 
__________________________________________________________________________ 
Properties of polyolefin resin pre-expanded particles 
Expansion 
Expansion 
Example 
t-T.sub.L 
t-T.sub.V 
ratio efficiency 
Open cell 
Mutual 
No. (.degree.C.) 
(.degree.C.) 
K (times) 
.epsilon. (-) 
ratio (%) 
fusion 
__________________________________________________________________________ 
Ex. 1 -17.2 
-29.8 
21.3 0.21 0.7 None 
Com. Ex. 1 
-10.4 
-23.0 
26.1 0.30 0.7 Existence 
2 -2.3 -14.9 
34.0 0.46 0.9 Existence 
3 +6.7 -5.9 
42.2 0.61 1.6 Existence 
4 +14.3 
+1.7 
53.1 0.81 25.1 Existence 
Ex. 2 +6.7 -5.9 
45.0 0.66 1.5 None 
Ex. 3 -19.2 
-33.4 
18.5 0.21 0.9 None 
4 -7.5 -21.7 
31.6 0.50 1.1 None 
5 -3.5 -17.7 
47.2 0.82 6.3 None 
Com. Exp. 5 
+10.7 
-3.5 
54.7 0.96 17.4 Existence 
Ex. 6 -23.9 
-36.0 
14.5 0.15 1.0 None 
Com. Ex. 6 
-34.8 
-46.9 
10.5 0.07 0.2 None 
7 -13.3 
-25.4 
19.4 0.22 1.6 Existence 
Ex. 7 +4.1 -6.0 
22.7 0.90 6.4 None 
8 +7.7 -2.4 
24.6 1.16 9.2 None 
Com. Ex. 8 
+11.0 
+0.9 
24.8 1.16 41.7 None 
Ex. 9 +0.1 -10.0 
10.5 0.43 11.6 None 
10 +4.1 -6.0 
12.4 0.54 11.7 None 
11 +7.7 -2.4 
16.7 0.78 12.8 None 
Com. Ex. 9 
+11.0 
+0.9 
19.2 0.92 21.1 None 
10 +14.1 
+4.0 
22.5 1.10 30.7 None 
__________________________________________________________________________ 
From the results shown in Table 1, the following can be understood. 
(1) Comparison between Example 1 and Comparative Examples 1 to 4 
In the case that t, T.sub.V and C satisfy the relation represented by the 
inequality (I), and t, T.sub.V and T.sub.L satisfy the relation 
represented by the inequality (II) as in Example 1, there can be prepared 
excellent pre-expanded particles having high expansion ratio, high 
expansion efficiency and low open cell ratio, and showing no mutual 
fusion. On the contrary, in the case that t, T.sub.V and C do not satisfy 
the relation represented by the inequality (I) as in Comparative Examples 
1 to 3, mutual fusion is generated. Also, in the case that t, T.sub.V and 
C do not satisfy the relation represented by the inequality (I), and t, 
T.sub.V, and TL do not satisfy the relation represented by the inequality 
(II) as in Comparative Example 4, mutual fusion is generated and open cell 
ratio becomes extremely high. 
It can be understood that when the amount of the fusion preventing agent 
being attached to the surface of the expanded particles is little such as 
0.05 phr, mutual fusion of the expanded particles is generated at 
relatively low heat temperature, so that it is impossible to heighten heat 
expansion temperature. 
(2) Comparison between Example 2 and Comparative Example 3 
In the case that C is increased to 0.49 phr, t, T.sub.V and C satisfy the 
relation represented by the inequality (I), and t, T.sub.V and T.sub.L 
satisfy the relation represented by the inequality (II) as in Example 2, 
there can be prepared excellent pre-expanded particles having high 
expansion ratio, high expansion efficiency and low open cell ratio, and 
showing no mutual fusion. 
(3) Comparison between Examples 3 to 5 and Comparative Example 5 
In the case that t, T.sub.V and C satisfy the relation represented by the 
inequality (I), and t, T.sub.V and T.sub.L. satisfy the relation 
represented by the inequality (II) as in Examples 3 to 5, there can be 
prepared excellent pre-expanded particles having high expansion ratio, 
high expansion efficiency and low open cell ratio, and showing no mutual 
fusion. On the contrary, in the case that t, T.sub.V and C do not satisfy 
the relation represented by the inequality (I) as in Comparative Example 
5, mutual fusion is generated. 
It can be understood that when the amount of the fusion preventing agent 
being attached to the surface of the expanded particles is much such as 
0.28 phr, mutual fusion of the expanded particles is not generated till 
the expansion temperature is heightened to relatively high temperature, so 
that it is possible to heighten expansion efficiency and expansion ratio. 
(4) Comparison between Example 6 and Comparative Examples 6 to 7 
In the case that t, T.sub.V and C satisfy the relation represented by the 
inequality (I), and t, T.sub.V and T.sub.L. satisfy the relation 
represented by the inequality (II) as in Example 6, there can be prepared 
excellent pre-expanded particles having high expansion ratio, high 
expansion efficiency and low open cell ratio, and showing no mutual 
fusion. On the contrary, in the case that t, T.sub.V and T.sub.L. do not 
satisfy the relation represented by the inequality (II) as in Comparative 
Example 6, expansion efficiency remarkably lowers. Also, in the case that 
t, T.sub.V and C do not satisfy the relation represented by the inequality 
(I) as in Comparative Example 7, mutual fusion is generated. 
It can be understood that when the amount of the fusion preventing agent 
being attached to the surface of the expanded particles is too little 
during heat expansion, mutual fusion of the expanded particles is easily 
generated, so that suitable range of heat temperature becomes narrow. 
(5) Comparison between Examples 7 to 8 and Comparative Example 8 
In the case that t, T.sub.V and C satisfy the relation represented by the 
inequality (I), and t, T.sub.V and T.sub.L satisfy the relation 
represented by the inequality (II) as in Examples 7 to 8, there can be 
prepared excellent pre-expanded particles having high expansion ratio, 
high expansion efficiency and low open cell ratio, and showing no mutual 
fusion. On the contrary, in the case that heat temperature is too high, 
and t, T.sub.V and T.sub.L do not satisfy the relation represented by the 
inequality (II) as in Comparative Example 8, open cell ratio becomes 
remarkably high such as 41.7%, so that the pre-expanded particles are not 
expanded particles having a closed cell structure. 
In the case that the amount of the fusion preventing agent is much such as 
0.765 phr as in Examples 7 to 8 and Comparative Example 8, molding fusion 
property during internal die mold lowers. Accordingly, in Examples 7 to 8 
and Comparative Example 8, the polyolefin resin expanded particles were 
sufficiently washed with the acidic aqueous solution before internal die 
mold so that the amount of the fusion preventing agent during internal die 
mold is at most 0.3 phr. 
(6) Comparison between Examples 9 to 11 and Comparative Examples 9 to 10 
In the case that t, T.sub.V and C satisfy the relation represented by the 
inequality (I), and t, T.sub.V and T.sub.L satisfy the relation 
represented by the inequality (II) as in Examples 9 to 11, there can be 
prepared excellent pre-expanded particles having high expansion ratio, 
high expansion efficiency and low open cell ratio, and showing no mutual 
fusion. On the contrary, in the case that heat temperature is too high, 
and t, T.sub.V and T.sub.L do not satisfy the relation represented by the 
inequality (II) as in Comparative Examples 9 to 10, open cell ratio 
becomes remarkably high such as higher than 20%, so that the pre-expanded 
particles are not expanded particles having a closed cell structure. 
It can be understood that when the amount of the fusion preventing agent 
being attached to the surface of the expanded particles is much such as 
0.771 phr as in Examples 9 to 11, mutual fusion of the expanded particles 
is not generated at each level, so that expansion efficiency of the 
pre-expanded particles can be fixed at about 0.4 to 0.8 and expansion 
ratio of the pre-expanded particles can be suitably adjusted. 
In the case that the amount of the fusion preventing agent is much such as 
0.771 phr as in Examples 9 to 11 and Comparative Examples 9 to 10, molding 
fusion property during internal die mold lowers. Accordingly, in Examples 
9 to 11 and Comparative Examples 9 to 10, the polyolefin resin expanded 
particles were sufficiently washed with the acidic aqueous solution before 
internal die mold so that the amount of the fusion preventing agent during 
internal die mold is at most 0.3 phr. 
In addition to the ingredients used in the Examples, other ingredients can 
be used in the Examples as set forth in the specification to obtain 
substantially the same results.