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BACKGROUND OF THE INVENTION
1. Field of the invention
This invention relates to an in-mold foam molding of a vinylidene chloride
type resin, and expandable particles and foam particles convenient for
preparation of the molding, more particularly to a novel vinylidene
chloride type resin in-mold foam molding having a broad cross-section and
a free shape which can be utilized as such for insulating material boards,
shock absorbing molded vessels, etc., and a series of foaming techniques
sufficient for completion thereof.
2. Description of the prior art
In recent years, abundant studies have been made about techniques for
expansion of synthetic resins. As a result, a large number of synthetic
resins have been made expandable, and individual technical fields have
developed depending on the type of resins used. However, a complete
technique for obtaining good foam molding which is satisfactory with
respect to cross-section, shape and dimension has not been developed for
vinylidene chloride type resins. Accordingly, there exists no foam molding
having a broad cross-sectional shape and a plate area which can be used as
such for an insulating material board.
The following reasons for this behavior may generally be contemplated for
vinylidene chloride type resins:
(1) The processing temperature for melt processing of the resin is so close
to the decomposition temperature at which decomposition will proceed that
thermal decomposition of the resin will occur in the extrusion processing
step;
(2) Decomposition of the resin is markedly accelerated when the resin comes
into contact with a metal such as iron or copper under a temperature
around the melting point of the resin;
(3) Due to high gas barrier property of the resin, the resin can be
impregnated only with a small amount of a blowing agent, and therefore it
can expand little by heating;
(4) The dependency of the rheological properties of the resin on the
temperature around the expansion temperature is so great that the
expanding conditions are difficult to control.
Thus, under the present situation, it is very difficult to obtain a highly
expanded homogeneous foam molding without causing thermal decomposition,
and no satisfactory foaming technique has been developed.
In the prior art, concerning foam moldings of vinylidene chloride type
resins and preparation thereof, for example, proposals have been made
about extrusion expansion by choice of special chemical blowing agents
(Japanese Patent Publications No. 3968/1964 and No. 16419/1967, U.S. Pat.
No. 2,948,048). However, these foam moldings are low in expansion ratio,
which is about 2- to 3-fold, and the final product cross-sections are
limited to small cross-sectional shapes, as represented only by artificial
bamboo blind, artificial bamboo blind core, ornamental threads, etc. The
purpose of expansion is also no more than controlling the surface lustre
or imparting flexibility.
Alternatively, as a technique for high expansion by use of a physical
blowing agent, the method has been also known, in which finely divided
vinylidene chloride type resin is mixed with a physical blowing agent, and
the resultant mixture is extrusion expanded at lower temperatures (about
120.degree. to 150.degree. C.) to give an extruded foam with a density of
about 240 Kg/m.sup.3 or less and cell sizes of about 0.1 to 1 mm (U.S.
Pat. No. 3,983,080). However, according to this method, it is difficult to
control thermal decomposition and thermal decomposition of the resin
proceeds, whereby it is possible to continue extrusion expansion to give
only strand-shaped foamed extrudate with uneven surface and markedly
irregular cell sizes.
Further, expandable unicellular spherical particles with diameters of about
1 to 50 um incuding a volatile liquid blowing agent within a thermoplastic
resin copolymer have also been proposed (Japanese Patent Publication No.
26524/1967 and Japanese Laid-open Patent Publication No. 59168/1974). In
this proposal, the definition of the thermoplastic resin is inclusive of
copolymers of vinylidene chloride with acrylonitrile or butyl acrylate,
and there is the description in a part of the examples that a foam-like
product can be formed through fusion of the particles by effecting heating
expansion. However, the above foam and the foam of the present invention
differ in principle of expansion, the structures of particles and foam,
performance and therefore use. The technical difference is explained to
draw clearly a line of demarcation between both.
First, as the greatest difference in technique, the foam of the present
invention is prepared by expanding particles containing a blowing agent
impregnated (dissolved) therein to give multi-cellular foam particles
enriched in recovery and resilience. The multi-cellular particles are in
turn formed into a mass according to the in-mold molding technique. As a
consequence, it is possible to obtain a foam with a high closed cell
percentage excellent in mechanical strength. In contrast, the expandable
particles according to the aforesaid proposal are so called micro-baloons,
in which liquid blowing agent is included within small baloon-like
entities made of a resin, and therefore, even when these may be fused by
heating expansion, the resultant foam molding is a mass consisting of
units of baloon-like unicellular bubbles, with a low close cell
percentage, and also inferior in mechanical characteristics. Further, the
microbaloons are used primarily as a mixture with inks or paints for
applying relief patterns on wall papers, etc., and their particle sizes
are very small, as small 1 to 50 um, as mentioned above. Therefore, when
it is attempted to mold the particles, they cannot be filled uniformly
within a mold or steam cannot be passed to inner portions of the molding.
Thus, they are basically different from the expandable resin particles of
the present invention in that they cannot be formed into a mass according
to the in-mold expansion molding as intended by the present invention.
Also, the expandable resin particles of the present invention can be
extrusion expanded to give a homogeneous good extrusion expanded board
with a large cross-section and high closed cell percentage. On the other
hand, when the expandable resin particles according to the above proposal
are attempted to be extrusion expanded, the shells of the resin containing
the liquid blowing agent will be broken to form an unhomogeneous mixture
of the resin and the blowing agent, which can be discharged through the
nozzle only to result in evaporation of the blowing agent, with the resin
substantially failing to be expanded. Also, in this respect, the
expandable resin particles of this invention are fundamentally different
from the expandable particles according to the above proposal.
SUMMARY OF THE INVENTION
The present invention has been accomplished under such a situation, and its
first object is to provide a foam of a vinylidene chloride type resin
fully enjoying the characteristics possessed by the vinylidene chloride
type resin (e.g. flame retardancy, oil resistance, chemical resistance,
gas barrier characteristic, mechanical strength, etc.) with a
cross-section and a dimension, which can be used as such in, for example,
a board for insulating material.
A second object is to provide expandable vinylidene chloride type resin
particles and foam particles prepared by pre-expansion thereof, which are
advantageous in accomplishing the first object.
Further, a third object is to provide advantageous processes for producing
the expandable particles, foam particles and foam moldings prepared
therefrom, respectively, to be used for accomplishing the above first and
second objects.
In the first place, the summary of the present invention as a whole, namely
a series of foaming techniques may be set forth in terms of the relation
between the main claim and the above objects as follows.
The first object of the present invention can be accomplished readily by
practicing an embodiment of the present invention, namely a vinylidene
chloride type resin in-mold foam molding, comprising a large number of
multi-cellular foam particles made of a substantially non-crystalline
vinylidene chloride type resin with an average particle size of 0.08 to 25
mm which are closely fused together mutually with adjacent particles
thereby forming a foam with an expansion ratio of 4 to 150. The vinylidene
chloride type resin foam satisfying the above first object is a novel
foam, the emergence of which has heretofore been expected of its
emergence, but which existed nowhere in the world. The present inventors
have accomplished this for the first time by utilizing the expandable
particles or the pre-expanded foam particles satisfying the second object
of the invention, namely by preparing expandable vinylidene chloride type
resin particles, comprising a volatile organic blowing agent incorporated
in substantially non-crystalline vinylidene chloride type resin particles
with an average particle size of 0.05 to 5 mm, said particles having
smooth surfaces without interstice, filling said particles directly in a
mold and permitting them to expand to form a molding, or pre-expanding the
expandable particles once into foam particles, namely vinylidene chloride
type foam particles, which are multi-cellular particles with a closed cell
percentage of 65% or more obtained by expansion of the substantially
non-crystalline vinylidene chloride type resin particles to 4 to 150-fold
and include a volatile organic blowing agent gas within the particles,
followed by filling of the foam particles in a mold for expansion into a
foam molding.
The specific features of the processes for readily obtaining these
expandable particles, foam particles and in-mold foam moldings, namely the
processes satisfying the third object of the present invention reside in
employing substantially non-crystalline vinylidene chloride type resins
with small particle sizes and adopting the so called contact impregnation
method, in which the resin is impregnated by contact with a volatile
organic blowing agent under the temperature conditions where impregnation
can be effected rapidly; utilizing the ability of retaining the volatile
blowing agent possessed by the non-crystalline vinylidene chloride type
resin and expandability of the impregnated resin on heating to a
multi-cellular product with a high closed cell percentage; and adopting
the heating expansion in-mold molding method employing a cavity, which can
be closed but is not sealed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(A) shows an electron microscope photograph of the non-crystalline
vinylidene chloride type resin particles according to the present
invention;
FIG. 1(B) shows an electron microscope photograph of crystalline vinylidene
chloride type resin particles for a comparative purpose;
FIG. 2 is a graph showing the relationship between the particle size of the
non-crystalline vinylidene chloride type resin particles and the maximum
expansion ratio;
FIG. 3 is a graph showing retentivity of blowing agent of the expandable
non-crystalline vinylidene chloride type resin particles according to the
present invention and expandable polystyrene particles for comparative
purpose;
FIG. 4 is a graph showing the cumulative expansion ratio when the
expandable non-crystalline vinylidene chloride type resin particles are
expanded in three steps;
FIG. 5 is a graph showing the changes of the secondary expansion ratio with
lapse of time of the pre-expanded foam particles of the non-crystalline
vinylidene chloride type according to the present invention and the
polystyrene pre-expanded foam particles for comparative purpose;
FIG. 6 is an electron microscope photograph of the fractured surface of the
foam molding of the present invention;
FIG. 7 is a graph showing the relationship between foam density and 5%
compression strength of the foam molding of the present invention;
FIG. 8 is a graph showing the changes in thermal conductivity with lapse of
time of the expansion molded board of the vinylidene chloride type resin
of the present invention and the polystyrene extrusion expanded board for
comparative purpose;
FIG. 9 is a graph showing the relationship between the vinylidene chloride
content in the resin in the case of a copolymer resin of vinylidene
chloride and methyl methacrylate and the oxygen index; and
FIG. 10 is a graph showing the relationship between the blowing agent
composition ratio of Freon 11 to Freon 12 and the maximum expansion ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To describe now in detail about the present invention, by referring to the
novel aspects of the inmold foam molding of the present invention with
respect to the salient features in processes for production thereof, which
have rendered possible the matters impossible in the prior art, the
essential requirements for the foam molding of the present invention are
to be clarified below.
The salient features in processes for accomplishing the foam molding of the
present invention may be summarized in the combination of the three points
(1), (2) and (3) shown below:
(1) employment of a substantially non-crystalline vinylidene chloride type
resin as the base resin;
(2) selection of a volatile organic blowing agent as the blowing agent, and
employment of the contact impregnation with the blowing agent utilizing
large magnitude of the specific surface area possessed by the fine resin
particles as obtained by the suspension polymerization method for
incorporation thereof in the resin; and
(3) employment of the known molding method practiced for the in-mold
expansion molding method with expandable resin particles (typically
polystyrene expandable particles) for formation of the foam molding.
For convenience of description, the reasons for respective items (1), (2)
and (3) are to be described below in this order.
First, the requirement (1) is essential because the use of a
non-crystalline vinylidene chloride type resin surprisingly enables
impregnation of a large amount of a blowing agent into the resin
particles, and also allows the surface condition of the resin particles
(including the internal structure) to permit the blowing agent to expand
the resin particles into multi-cellular foam particles, and further
permits the flowing viscoelastic characteristics of the resin near the
expansion temperature to take a state suitable for expansion.
Referring now to FIG. 1(A) and FIG. 1(B), such a situation is to be
described plainly. FIG. 1(A) and 1(B) are electron microscope photographs
showing the surface states of vinylidene chloride type resin particles to
be used for base resins, (A) being the substantially non-crystalline resin
as mentioned in the present invention, while (B) being the crystalline
resin for comparative purposes.
As can clearly be seen from a comparison between FIG. 1(A) and FIG. 1(B),
the non-crystalline resin is free of interstices or cracks and relatively
smooth on its surface, while the crystalline resin is formed of a mass of
blocks having uneven surfaces to be gathered as a whole into a spherical
mass, and interstices or cracks can be seen therein. Such states of both
may be estimated to be the same also in the internal structure of the
particles. Formation of the above-mentioned blocks may be considered to be
due to crystallinity of the resin.
The non-crystalline resin particles of the above FIG. 1(A) can be
impregnated with a large amount of blowing agent, and when they are
expanded by heating with steam, a large number of bubble nuclei are formed
to give highly expanded multi-cellular particles, and extrusion expansion
thereof can give a highly expanded homogeneous expanded board enriched in
closed cells. In contrast, the crystalline resin particles of FIG. 1(B)
can be impregnated with only a small amount of blowing agent, and when
they are expanded by, for example, heating with steam, they can be changed
only a little and to an extent which cannot be said to be expanded.
Such a difference in phenomenon may probably be due to the fact that, in
the non-crystalline resin particles, the blowing agent is impregnated in
the form of being dissolved within said resin, while, in the crystalline
resin particles, the blowing agent is contained through the voids or
cracks. Accordingly, when the crystalline resin particles are attempted to
be expanded by heating, the blowing agent will be dissipated through such
cracks in a large amount, whereby the expanding ability of the blowing
agent cannot sufficiently be utilized, and also crystallinity of the resin
will interfere with the flowing elongation of the resin during the
expansion process to make formation and growth of cells difficult.
Thus, the vinylidene chloride type resin to be used as the base resin for
the expandable resin particles of the present invention is required to be
substantially non-crystalline.
Next, FIG. 2 is a graph of experimental examples showing the relationship
between the particle size of the resin to be used in the present invention
and the maximum expansion ratio.
What is meant partially by FIG. 2 is that, in addition to employment of the
non-crystalline resin of the above item (1), the above item (2), namely
the contact impregnation method with the use of particles of small sizes,
is also required.
Generally speaking, vinylidene chloride type resins have higher barrier
characteristics for volatile blowing agents, particularly Freon type
organic blowing agents, and it has been considered difficult to impregnate
these resins with such blowing agents so as to obtain homogeneous
multi-cellular products. Whereas, in the present invention, by selection
of the resin as described above, and also by appropriately selecting the
particle sizes of the resin particles and the temperature condition for
impregnation, it is possible to accomplish impregnation of the blowing
agent capable of high expansion under commercial conditions as shown in
FIG. 2.
Also, as in the above item (2), by use of a volatile organic blowing agent,
a large amount of the blowing agent can be impregnated into the resin
particles, whereby a high degree of expansion was realized. In contrast, a
decomposition type chemical blowing agent can be mixed with difficulty and
dispersed uniformly in the resin particles. Further, by use of the contact
impregnation between the resin particles and the volatile organic blowing
agent, impregnation of the blowing agent can be effected efficiently at
normal temperature to around the softening point of the resin particles
and therefore deterioration or decomposition of the vinylydene chloride
resin during impregnation can be avoided. Whereas, in the extrusion
impregnation method, which is another useful method, decomposition of the
resin will inevitably occur, because the resin is once molten by heating.
FIG. 3 is a graph showing an example of retentivity (continuation) of the
bowing agent (expandability) in the expandable resin particles of the
present invention. The graph is noted as a phenomenon in which the
presumption is denied that the comtinuation of expandability is improbable
since the amount of the blowing agent dissipated will be greater in
proportion to the specific surface area of the resin particles, if
impregnation of the resin particles with the blowing agent is assumed to
be effected only through the largeness of the specific surface area of the
particles.
Further, FIG. 4 is a graph showing the expansion ratio in the respective
steps in cumulative forms when the expandable resin particles of the
present invention are expanded in three steps. This Figure shows that the
blowing agent impregnated into the expandable resin particles can remain
within the pre-expanded particles as the blowing agent unconsumed in the
initial heating expansion, and also that the resin employed for the
expandable resin particles has rheological properties which can stand
expansion in multiple steps. Such a continuation of expandability may be
estimated to be due to a phenomenon based on the special gas barrier
characteristic possessed by the resin.
In addition to the above phenomena, a surprising phenomenon is shown in
FIG. 5. FIG. 5 is a graph showing the change of re-expandability of the
particles exhibited with lapse of time, when the resin foam particles as
mentioned in the present invention which have been once pre-expanded are
maintained in the air. The phenomenon exhibited by FIG. 5 may be
considered to be a phenomenon in which the inner pressure of the blowing
agent within the cells which must have been used in the initial expansion
is restored to a pressure greater than the original pressure through the
action of entraining the air, which is a very useful characteristic when
obtaining expanded particles of a high expansion retio or in the case of
molding an in-mold expansion molding. It is a phenomenon which deserves
special mention in the vinylidene chloride type resins clarified as a
result of the investigation by the present inventors.
Whereas, the expandability possessed by the expandable particles as shown
in FIG. 4 or the reexpandability possessed by the foam particles as shown
in FIG. 5 is nothing but the ability which governs heating foaming
(heating expansion) of the particles within a mold in the in-mold foam
molding and close fusion among particles accompanied therewith, and
clarification of the expandability has enabled employment of the in-mold
foam molding method. Further, by virtue of such an expandability, foam
moldings of various shapes and structures, with various densities, can be
obtained with ease.
The above item (3), namely the use of the in-mold foam molding method is
essentially required, because the vinylidene chloride type resin, which is
liable to be thermally decomposed, can be heated uniformly and very
efficiently at lower temperature and within a shorter period without
bringing about a broad residence time distribution, thereby avoiding
complete thermal decomposition of the resin, to convert the resin into a
foam molding.
The in-mold molding employed herein is the method, in which a mold cavity
(known and called generally as the mold cavity which can be closed but is
not sealed) is filled with expandable resin particles or pre-expanded foam
particles, which are heated externally of the mold walls with a fluid such
as steam through small perforations to be expanded, thereby filling the
voids among the particles to effect fusion, followed by quenching, to form
a molding.
During these operations, since the expandable resin particles of this
invention employ a non-crystalline resin as shown in the above (1), having
a Vicat softening point lower by about 50.degree. to 60.degree. C. or more
than the crystalline resin of the prior art, heating molding with steam of
120.degree. C. or lower conventonally used in in-mold molding is fairly
possible, and the foaming temperature can be set at a temperature far
lower than the decomposition temperature of the resin.
Further, in the expandable resin particles of the present invention, as
shown by the above item (2), fine resin particles obtained by suspension
polymerization are employed and impregnated with a blowing agent according
to the contact impregnation method, whereby no such heating melting or
mechanical shear as required in the extrusion impregnation method is
necessary to cause substantially no denaturation or thermal decomposition
of the resin. Also, it is not necessary to add plasticizers or thermal
stabilizers, conventionally used for prevention of such denaturation or
thermal decomposition. As the result, foam moldings can be obtained having
inherent properties substantially unchanged from the vinylidene chloride
type resin such as gas barrier property or flame retardancy.
As described above, the in-mold foam molding of the present invention is a
novel product, which has been completed according to the preparation
method as summarized by the above items (1), (2) and (3).
The molding of the present invention is described in detail below.
FIG. 6 is an enlarged schematic illustration of the cross-section of the
molding of the present invention, which is shown by the electron
microscope photograph of the cross-section when it is fractured, for
better understanding of its structure.
The molding of the present invention as shown in FIG. 6 is a mass of a
large number of multi-cellular particles (particles formed by expansion of
expandable particles and foam particles) made of a substantially
non-crystalline vinylidene chloride type resin as the base resin, having a
structure such that said particles are fused closely to the outer surfaces
of the adjacent particles to be integrated into a foam molding. This
structure is nothing but the correct expression of the specific features
of the in-mold molding method as described in detail above which has
completed the molding of the present invention.
FIGS. 7, 8 and 9 are typical examples of the characteristics exhibited by
the molding of the present invention, FIG. 7 showing an exemplary graph of
the relationship between foam density and compression stress necessary for
5% compression, FIG. 8 an exemplary graph of the retentivity of the
insulating performance exhibited by the molding of the present invention
and FIG. 9 an exemplary graph of flame retardancy performance (oxygen
index) exhibited by the base resin of the present invention.
All of these characteristics are exhibited here without alteration of the
characteristics of the vinylidene chloride type resin by thermal
decomposition or denaturation, as reflected by the specific features of
the process of the present invention selected to give such effects, thus
making the molding of the present invention useful in industry.
More specifically, FIG. 7 shows that the molding of the present invention
can be provided as a foam with a broad density range, indicating the
possibility of complying with the requirement of various compression
strengths which are different depending on uses. Also, the excellent
compression strength per density is a characteristic which has been
achieved by the molding of the present invention, which is a mass of
multi-cellular particles.
FIG. 8 illustrates an example, showing usefulness of the molding of the
present invention when employed as a board for thermal insulation. For
comparative purposes, a polystyrene extruded foam board which is reputed
to be excellent in insulating performance is also shown, but it can be
seen that the molding of the present invention can retain excellent
insulating characteristics for a long term. The insulating performance may
be variable in its level of absolute value depending on the sizes of the
cells constituting the molding or the gas held in the foam molding, but it
has been confirmed according to experiments by the present inventors that
a molding having insulation up to 0.018 to 0.028 [kcal/m.hr..degree. C.]
in terms of thermal conductivity at density around 40 Kg/m.sup.3 can be
obtained.
FIG. 9 suggests the advantage that a foam molding having flame retardancy
can provide without using specifically a flame retardant for preparation
of the molding of the present invention. This is one of the advantages
obtained as the result of utilizing the characteristics possessed by the
base resin as such.
Further, still another advantage derived from the method for preparation of
the molding of the present invention is the advantage of latitude in
setting the thickness or dimension, the surface area of a cross-section
and the shape of the molding. According to the experiments by the present
inventors, a foam molding, for which a mold cavity can effectively be
prepared, for example, with dimensions of about 3 mm or longer and a
cross-sectional area of 9 mm.sup.2 or larger, can freely be produced. Even
within the scope of the experiments, a block with a thickness of 100 mm, a
width of 900 mm and a length of 1800 mm can be easily molded, thus
indicating a possibility that a molding with freely selected dimensions
and shapes can be prepared depending on the design of the mold.
In the following, some details about the molding, the expandable particles,
foam particles as mentioned in the present invention and preparations
thereof are to be supplemented.
The vinylidene chloride type resin as herein mentioned refers
comprehensively to copolymeric resins of vinylidene chloride with at least
one of the comonomer components copolymerizable therewith. The
copolymerizable comonomers are disclosed in Polymer Handbook, 2nd edition,
edited by Brandrup & Immergut, including, for example, styrene, vinyl
acetate, vinyl chloride, vinyl bromide, acrylonitrile, methacrylonitrile,
acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, butyl
acrylate, methyl methacrylate, ethyl methacrylate, ethylene,
methylpropylene, methylbutene and others.
In general, the designation "vinylidene chloride type" is commonly accepted
to refer to copolymers in which vinylidene choride units exist at a
proportion of 50 wt. % or more, because the characteristics of the
vinylidene chloride units as the main component will dominate the
characteristics of the copolymer resin itself. In such a sense, among the
vinylidene chloride type resins to be used for the foam molding of the
present invention, those containing 50 wt. % or more of vinylidene
chloride units are preferable resins, since the resultant foam moldings
can effectively exhibit the characteristics such as insulating performance
and flame retardancy. However, in the foam molding of the present
invention, even when the base resin contains a small amount of vinylidene
chloride units, specifically about 10 wt. % (desirably 30 wt. % or more),
the resultant foam molding can be endowed with flame retardancy and
insulating performance, as compared with the resin containing no such
component. Thus, from such facts, the concept of the vinylidene type resin
is defined comprehensively so as to extend far beyond the range as
commonly accepted.
Further, the vinylidene chloride type resin to be used in the present
invention is required to be non-crystalline. The term "non-crystalline" is
the antonym against the ordinary "crystalline", and a non-crystalline
resin may be defined, for example, as a resin which exhibits no peak value
as exibited by the temperature for crystal melting or crystal formation
when the crystal temperature of the resin is measured according to the DSC
(Differential Scanning Calorimetry). However, in the present invention,
the resins incorporating a small amount of crystalline components or
additives incorporated in the non-crystalline resins, for the purpose of,
for example, controlling cell sizes of the foam particles or other
purposes, are also included within the substantially non-crystalline resin
as mentioned in the present invention, provided that they do not alter the
technical thought of "utilizing the characteristics of the non-crystalline
resin for completion of expandable resin particles".
The above non-crystalline vinylidene chloride type resin can be produced
according to various method such as mass polymerization, emulsion
polymerization, suspension polymerization, etc. but it is preferred as
mentioned above to use suspension polymerization, from the standpoint of
avoiding thermal decomposition or preparing readily resin particles with
particles sizes necessary for obtaining the expandable resin particles of
the present invention, namely an average particle size of 0.05 to 5 mm.
In the vinylidene chloride type resins, transition from crystallinity to
non-crystallinity occurs as the ratio of the comonomer units relative to
vinylidene chloride units is increased. The ratio of the comonomer units
at which such a transition occurs depends on the comonomer employed, but
in the non-crystalline vinylidene choride type resins of the present
invention, the comonomer units may be contained generally at a proportion
of 5 to 10 wt. % or more, or 30 wt. % or more at the highest.
And, as the vinylidene chloride type resin to be used in the present
invention, it is preferred to use a copolymer of vinylidene chloride and
an acrylic monomer. When the target foam molding is desired to be high in
expansion ratio and enriched in rigidity and heat resistance, it is
desirable to use a copolymer of vinylidene chloride and methyl
methacrylate with a content of methyl methacrylate in the copolymer of 30
wt. to 90 wt. %.
Further, by using as the crosslinking component, for example,
divinylbenzene or a polyethylene glycol esterified at both ends with
acrylic acid, or by incorporating glycidyl methacrylate and methacrylic
acid as a part of the monomer components, a crosslinked non-crystalline
vinylidene chloride type resin can be obtained. When such resin particles
are used, in-mold moldability is good and the resultant foam molding is
enriched in closed cellular structure to be improved in compression
strength and thermal conductivity.
The resin particles to be used for the expandable resin particles of this
invention may be have the shapes of the resin particles as such obtained
by suspension polymerization as mentioned above, and they are ordinarily
spherical or approximate thereto. Their sizes should be chosen in
connection with the impregnation rate of the blowing agent, retentivity of
expandability in the expandable resin particles, the in-mold dimensional
shape to be employed and the in-mold heating efficiency, and those with an
average particle size from 0.05 to 5 mm are used in the present invention.
In view of making the above connection well balanced as a whole, it is
desirable to use particles which are as regular as possible with an
average particle size from 0.1 to 1 mm.
Further, from the standpoint of wishing to choose a resin suited for
in-mold steam expansion molding from among substantially non-crystalline
resins obtained, it is desirable to use the Vicat softening point of the
resin as one of the indices. Those resins generally have Vicat softening
points of 120.degree. C. or lower, but the use of a resin having a Vicat
softening point within the range of from 60.degree. to 100.degree. C. is
preferred, because the fusing force among particles in the molding is
dense and a molding with excellent surface smoothness can be obtained.
The blowing agent to be used for the expandable resin particles of the
present invention is a volatile organic blowing agent having a boiling
point lower than the softening point of the resin employed. The blowing
agent may be determined in view of the compatibility with the resin, the
vapor pressure at the foaming temperature, and the boiling point of the
blowing agent. When the target conditions cannot be satisfied by one
blowing agent, two or more blowing agents can be mixed to prepare a
blowing agent suitable for expansion of the resin. In this case, from the
point of view of the compression strength or the elasticity of the foam
molding, it is preferred to use an aliphatic hydrocarbon or an aliphatic
halogenated hydrocarbon having a boiling point of 60.degree. C. or lower.
On the other hand, from the point of view of compatibility between the
resin and the blowing agent, it is preferred to use a blowing agent having
an average solubility parameter (SP value) within the range of 5.7 to 7.0.
Examples of blowing agents may include aliphatic hydrocarbons such as
propane (SP value=6.4), butane (6.8), isobutane (6.8), pentane (7.0),
isopentane (6.7), ne | | |
