Process for preparing expanded product of thermoplastic resin

A process for preparing an expanded product of a thermoplastic resin which includes a gas dissolving step of adding supercritical carbon dioxide and/or nitrogen as an blowing agent to a thermoplastic resin, and melting it in the thermoplastic resin, a cooling step of cooling the resulting molten resin composition under a pressure not less than a critical pressure of the blowing agent, a nuclei forming step of discharging the molten resin composition from a die to lower the pressure to a level not more than the critical pressure, and an expansion controlling step of cooling an expanded product of the thermoplastic resin to a level not more than a glass transition temperature or a crystallization temperature of the resin to control a cell diameter of the expanded product.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an expanded product of a thermoplastic 
resin having excellent physical properties and surface appearance, and 
more specifically, it relates to a process for preparing an expanded 
product of a thermoplastic resin having a high strength, a very small 
average cell diameter and a uniform average cell density by the use of 
carbon dioxide and/or nitrogen as an blowing agent and by extrusion. 
2. Description of the Related Art 
For the preparation of an expanded product of a thermoplastic resin, there 
is known a process for preparing the expanded product of the thermoplastic 
resin by the use of a chemical blowing agent or a physical blowing agent. 
A chemical expanding process usually comprises mixing a raw material resin 
with an organic blowing agent having a low molecular weight which can 
decompose at a molding temperature to generate a gas, and then heating the 
resulting mixture to a decomposition temperature of the blowing agent or 
more to carry out expansion molding. According to this process, the 
generation of the gas is sharply proportional to the molding temperature, 
and the decomposition temperature can easily be adjusted by adding an 
expanding aid or the like. Moreover, the expanded product having closed 
cells can also be obtained. 
However, with regard to this expanded product, its cost increases owing to 
the use of the special blowing agent, and what is worse, the discoloration 
of the expanded product, the generation of an odor, a problem of food 
sanitation occur owing to some remaining decomposed residues in the 
expanded product. In addition, there are problems such as smears of a 
molding machine with the chemical blowing agent and molding defects caused 
by such smears. 
On the other hand, a gas expanding process which is a physical expanding 
process comprises melting a resin in a molding machine, feeding an organic 
compound having a low boiling point such as butane, pentane or 
dichlorodifluoromethane to the molten resin, kneading them, and then 
releasing the mixture into a low-pressure zone to carry out expansion 
molding. The organic compound having the low boiling point which can be 
used in this process is compatible with the resin, and therefore it is 
excellent in solubility and retentivity, so that it has a feature which 
permits the formation of the expanded product having a high expansion 
ratio. 
However, these blowing agents are expensive and have dangerous problems 
such as inflammability and toxicity as well as the possibility of air 
pollution. In addition, there is a tendency that the use of Freon-based 
gases typified by dichlorodifluoromethane is wholly banned in view of an 
environmental problem such as the destruction of an ozonosphere. 
In order to solve such problems of the conventional processes, there have 
been suggested many processes in which an inert gas such as carbon dioxide 
gas or nitrogen is used as the blowing agent. However, the inert gas is 
less compatible with a resin and hence it is poor in solubility therein. 
In consequence, the obtained expanded product has large and uneven cell 
diameters and a low cell density, so that it is insufficient in points of 
appearance, mechanical strength, heat insulating properties and expansion 
ratio. 
As a technique for solving these problems, U.S. Pat. No. 4,473,665 has 
disclosed a preparation process for obtaining an expansion-molded article 
in which fine cells having a diameter of 2 to 25 m are uniformly 
dispersed. In this process, a thermoplastic resin sheet is first 
impregnated under high pressure with an inert gas until it has been 
saturated. Afterward, the sheet is heated up to a glass transition 
temperature of the thermoplastic resin, and then depressurized so that the 
gas may be supersaturated in the resin to form cell nuclei. Next, the 
sheet is quenched to control the growth of the cells. Furthermore, another 
preparation process utilizing extrusion or injection molding has been 
exemplified which comprises heating and melting a thermoplastic resin 
beforehand saturated with an inert gas under pressure, shaping the molten 
resin under pressure, cooling and depressuring the resin to form cell 
nuclei, and then cooling the resin to control a cell diameter. According 
to the above-mentioned processes, the expanded products containing many 
fine cells therein can be obtained, but the inert gas is less compatible 
with the resin, and therefore ten and several hours are required to fully 
impregnate the resin with the gas, which makes it difficult to 
industrially practice these processes. 
In U.S. Pat. No. 5,158,986, there has been disclosed a technique for 
obtaining an expanded product having an extremely fine cell diameter and a 
high cell density by using a supercritical fluid as the blowing agent and 
impregnating a thermoplastic resin with this supercritical fluid. Since 
the supercritical fluid has an excellent solubility similar to that of a 
liquid and an excellent diffusibility close to that of a gas, the 
supercritical fluid has a high solubility in the resin. In addition, since 
the supercritical fluid has a high diffusion rate in the resin, the 
impregnation of the resin with the blowing agent can be accomplished in a 
short period of time. In this U.S. patent publication, there have been 
proposed a process for obtaining an expanded product which comprises 
forming the thermoplastic resin into a sheet by an extruder, introducing 
the sheet into a pressurized chamber filled with carbon dioxide in a 
supercritical state to impregnate the sheet with carbon dioxide, and then 
heating the sheet in an expanding chamber under atmospheric pressure to 
allow the sheet to expand, and another process for obtaining an expanded 
product which comprises melting the resin in an extruder, impregnating the 
molten resin with carbon dioxide in a supercritical state, extruding the 
thus impregnated resin into a sheet-like product, introducing the 
sheet-like product into a pressurized chamber, in which cell nuclei are 
formed owing to a pressure difference, and then heating and cooling the 
sheet to control the diameter and the density of the cells. 
In both the processes, however, large-scale high-pressure facilities are 
necessary and hence an enormous facilities cost is required, and a work 
efficiency is also poor. In consequence, it is difficult to practice these 
processes on an industrial scale. Furthermore, in the former process, a 
long time is taken to fully impregnate the sheet-like product with carbon 
dioxide, because the sheet-like product is directly impregnated therewith. 
On the other hand, in the latter process, the molten resin is impregnated 
with carbon dioxide, and hence the impregnation rate of carbon dioxide is 
higher than in the former process, but it is difficult to carry out the 
dissolution of carbon dioxide and the formation of many cell nuclei only 
by the kneading of one extruder, so that the expanded product having many 
fine cells can scarcely be obtained. 
The present inventors have proposed, in Japanese Patent Application 
Laid-Open No. 11190/1996, a process for preparing an expanded 
thermoplastic resin product containing many fine cells in a uniform state 
by expansion extrusion which comprises a gas dissolving step of 
impregnating a molten thermoplastic resin with an inert gas as an blowing 
agent in a first extruder and an adapter having a mixing portion connected 
to the first extruder to form a compatible state of the thermoplastic 
resin and the inert gas, a cooling step of lowering the temperature of the 
molten resin in a second extruder, maintaining a pressurizing state, a 
nucleus-forming step of forming many cell nuclei owing to the abrupt drop 
in pressure, and an expansion controlling step of controlling the diameter 
of the cells. 
According to this preparation process, it is possible to continuously 
perform the production of the expanded product which can scarcely 
practically be prepared by the preparation process disclosed in U.S. Pat. 
No. 4,473,665 or U.S. Pat. No. 5,158,986. However, from further researches 
by the present inventors, it has been confirmed that this preparation 
process has a limit to an effect of imparting strength to the extruded 
articles. 
The expanded product of the thermoplastic resin having the excellent 
strength is required to have an optimum cell structure in points of a cell 
diameter distribution, a cell density and the like, and in order to obtain 
the expanded product of the thermoplastic resin having the excellent 
strength, it is an essential requirement to prepare the expanded product 
of the thermoplastic resin having the optimum cell structure. 
The expanded product of the thermoplastic resin having the excellent 
strength is required to have the cell structure having a fine cell 
diameter and a very large number of cells, but in order to obtain such a 
cell structure, it is necessary that the opening of a die lip should be 
extremely decreased so as to apply a high shear to the molten resin. 
However, if the opening of the die lip is extremely decreased, the 
expanded product is thin and the expansion ratio is low, so that the 
desired expanded product cannot be obtained. 
To the contrary, if the opening of the die lip is increased in order to 
prepare the thick expanded product, the high shear cannot be applied to 
the molten resin, so that the expanded product having the fine cell 
structure cannot be obtained. 
Furthermore, in the case that a usual extrusion expanding process using the 
chemical blowing agent or the organic compound having a low boiling point 
as the blowing agent is employed, the expansion ratio and the thickness of 
the expanded sheet can be controlled only by adjusting the opening of the 
die lip. However, this expanding process has a limit to the amount of the 
blowing agent to be added, in contrast to the expanding process in which 
the supercritical fluid is used. In the usual extrusion expanding process, 
therefore, it is difficult to form many cells, and the increase of the 
cell diameter due to the increase of the opening of the die lip cannot be 
avoided. In consequence, the expanded product having the high strength 
cannot be prepared. 
Examples of factors for controlling the cell diameter and the cell density 
of the expanded product of the thermoplastic resin include temperatures at 
the times of the cell nuclei formation and the cell nuclei growth, a 
viscosity, a resin pressure, the shear rate of the molten resin 
composition (hereinafter referred to simply as "the shear rate"), a gas 
concentration and the surface energy of the thermoplastic resin. The 
viscosity and the surface energy depend largely on a selected material, 
and the expansion ratio of the expanded product to be prepared depend 
largely on a working temperature and the concentration of the selected 
blowing agent. 
That is to say, in order to obtain the expanded product of the 
thermoplastic resin having the necessary cell structure, it has heretofore 
been necessary that an optimum die shape and optimum molding conditions 
for the desired products are found, and each time the quality of the 
products is altered, the die is changed. 
Furthermore, in the process disclosed in Japanese Patent Application 
Laid-Open No. 11190/1996, it is essential conditions that the shear rate 
in the die is increased by causing a rapid pressure loss in the die 
portion, and therefore if the shear rate is excessively increased, a flow 
velocity distribution in a slit portion within the die increases, so that 
a discharge rate is liable to be unstable. Accordingly, in the case that 
the expanded products of the thermoplastic resin having many fine cells in 
the uniform state are mass-produced, the discharge rate of the resin is 
not stable, so that the uneven discharge of the resin easily occurs and 
hence it is difficult to stably obtain the continuously extruded expanded 
products having a good surface appearance. 
SUMMARY OF THE INVENTION 
An object of the present invention is to obtain an extruded expanded 
product having a more excellent mechanical strength than an expanded 
product of a thermoplastic resin prepared by a usual extrusion expanding 
process. 
Another object of the present invention is to obtain an expanded product of 
a thermoplastic resin having a fine cell structure for expressing an 
excellent strength by an extrusion expanding process in which a 
supercritical fluid is used as an blowing agent. 
A still further object of the present invention is to prepare an expanded 
product of a thermoplastic resin having a high strength which can be used 
in a use of a structural member or the like in which the strength is 
required. 
The present invention has been developed to provide a process for preparing 
an expanded product of a thermoplastic resin which is characterized by 
adjusting a resin pressure and a shear rate by a shear rate adjusting 
section 12 arranged in a die 4 at the time of expanding the resin in the 
vicinity of an outlet of the die 4 to optionally control a cell diameter 
and a cell density for expressing an excellent strength. 
Furthermore, the present invention has been developed to provide a process 
for preparing an expanded product of a thermoplastic resin which can 
stably and evenly extrude the expanded product of the thermoplastic resin 
having a cell structure containing many fine cells in a uniform state and 
a good surface appearance. 
The present inventors have intensively researched on a process for 
preparing an expanded product of a thermoplastic resin, and as a result, 
the present invention has been attained. That is to say, the present 
invention is directed to a process for preparing an expanded product of a 
thermoplastic resin by extrusion molding which comprises: 
(I) a gas dissolving step of melting the thermoplastic resin at 100.degree. 
to 450.degree. C. in a continuous plasticator having a line for feeding an 
blowing agent, and adding supercritical carbon dioxide and/or nitrogen in 
a ratio of 0.1 to 30 parts by weight with respect to 100 parts by weight 
of the thermoplastic resin to form a molten resin composition in which the 
thermoplastic resin is compatible with supercritical carbon dioxide and/or 
nitrogen, 
(II) a cooling step of cooling, the molten resin composition to a 
temperature of 50.degree. to 300.degree. C. at a tip portion of the 
continuous plasticator, while maintaining a pressure not less than a 
critical pressure of supercritical carbon dioxide and/or nitrogen, 
(III) a nuclei cell forming step of discharging the molten resin 
composition from a die 4 connected to the tip portion of the continuous 
plasticator and set to an optimum expansion temperature of the molten 
resin composition, whereby the pressure is lowered to a level not more 
than the critical pressure of supercritical carbon dioxide and/or nitrogen 
to form cell nuclei, and 
(IV) an expansion controlling step of rapidly cooling the extruded product 
of the thermoplastic resin to a level not more than a glass transition 
temperature or a crystallization temperature of the resin to control a 
cell diameter of the expanded product, wherein 
the die in the step (III) has a flow velocity distribution adjusting 
section 13 having a function of lowering the shear rate on the downstream 
side of a shear rate adjusting section 12 having a function of generating 
the shear rate of the resin in the range of 10.sup.2 to 10 .sup.4 
sec.sup.-1 ; this shear rate adjusting section includes a portion (an 
average sectional area A) in which a sectional area of a section vertical 
to a resin flow direction on the upstream, side of a resin flow path is 
narrowed; the flow velocity distribution adjusting section includes a 
portion (an average sectional area B) in which the same section is spread 
on the downstream side of the flow path; and B/A is in the range of 1.3 to 
20.