Process for preparing polyolefin pre-expanded particles

Details Process for preparing polyolefin pre-expanded particles Process for preparing polyolefin pre-expanded particles

2001-07-30

2003-09-30

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S056000

Reexamination Certificate

active

06627668

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for preparing polyolefin resin pre-expanded particles. Specifically, the present invention relates to a process for preparing polyolefin resin pre-expanded particles which can be suitably used as a raw material for in-mold foamed articles. More specifically, the present invention relates to a process for preparing polyolefin resin pre-expanded particles which can be suitably used as a raw material for in-mold foamed articles and which have expansion ratio of 2 to 10 times, preferably 3 to 8 times in the lowest region.
A process for preparing polyolefin pre-expanded particles described below has been conventionally well known (for example, Japanese Unexamined Patent Publication No. 199125/1978). In the process, polyolefin resin particles are at first dispersed into an aqueous dispersion medium in a closed vessel. Next, after allowing the polyolefin resin particles to contain a volatile foaming agent, the particles are heated to at least the softening temperature of the above polyolefin resin. Thereafter by releasing the particles into a lower pressure atmosphere, polyolefin resin pre-expanded particles are prepared. However, when pre-expanded particles are prepared according to this method, the use of the volatile foaming agent results in cost increase. Furthermore, expansion ratio is difficult to be controlled due to large impregnation unevenness of the volatile foaming agent. Also, the expansion ratios of the particles are not uniform. For these reasons, excellent pre-expanded particles cannot be prepared. In addition, there are problems with safety such as combustion and explosion in case where a combustible gas such as propane or butane is used as the volatile foaming agent.
Japanese Unexamined Patent Publication No. 229936/1985 discloses a process for preparing pre-expanded particles wherein polyolefin resin particles are made to contain inorganic gas containing nitrogen as a foaming agent and the particles are released into a lower pressure atmosphere. The above method solves problems of cost increase and safety. However, pre-expanded particles uniform in particle size cannot be prepared according to the method due to fluctuation in expansion ratios caused by irregular temperature of the atmosphere into which the particles are released.
Also, Japanese Unexamined Patent Publication No. 221440/1985 discloses the following preparation process. First, resin particles of an ethylene-propylene random copolymer containing 1 to 12% by weight of ethylene are dispersed into water. Second, inorganic gas is introduced thereto and the particles are heated. Then it is released into a lower pressure atmosphere at an exit velocity of 200 to 500 m/second, thereby expanding the particles. According to the method, however, the exit velocity at the time of releasing the particles to the lower pressure atmosphere is too large to maintain the exit velocity for expansion. As a result, fluctuation in expansion ratios is increased among the pre-expanded particles to be obtained.
Alternatively, Japanese Unexamined Patent Publication No. 106546/1999 discloses the following preparation process. First, a polyolefin resin containing 100 parts by weight of a polyolefin resin and 0.05 to 20 parts by weight of a hydrophilic polymer is heated to obtain particles containing water. Thereafter, at the time of releasing the particles into a lower pressure atmosphere, the particles to be released are contacted with gas of at least 60° C. to prepare pre-expanded particles. However, the method causes to increase the fluctuation in expansion ratios of the pre-expanded particles to be obtained due to insufficient dispersion of the hydrophilic polymer.
SUMMARY OF THE INVENTION
From the viewpoint of the above prior arts, intense studies were conducted as to a process for preparing polyolefin resin pre-expanded particles which have small fluctuation in expansion ratios and uniform particle size at the same time by using water as a main foaming agent instead of a volatile foaming agent. As a result, it has been found that the above problems can be solved by a process for exposing particles to saturated vapor at the time of releasing particles into an atmosphere of lower pressure than the inner pressure of the closed vessel. According to this method, it is possible to obtain pre-expanded particles having an improved expansion ratio, less fluctuation in expansion ratios and more uniform particle size compared with the case where particles are not contacted with saturated vapor.
It has been also found that melt-kneading by using a multi-screw extruder makes it possible to prepare pre-expanded particles having less fluctuation in expansion ratios and more uniform particle size.
Furthermore, another finding is that, by previously preparing a masterbatch comprising a polyolefin resin and a hydrophilic polymer in which the concentration of the hydrophilic polymer is high, and by kneading the masterbatch with the polyolefin resin, dispersability of the hydrophilic polymer can be improved and the fluctuation in expansion ratios can be lowered.
The characteristics of the process for preparing pre-expanded particles of a polyolefin resin of the present invention is to contact particles to be released with saturated vapor at the time of pre-expanding the polyolefin resin by dispersing particles of the polyolefin resin into an aqueous dispersion medium in a closed vessel, heating the resin composition particles to temperature of at least the softening point of the polyolefin resin and releasing the particles from the closed vessel into an atmosphere of lower pressure than the inner pressure of the closed vessel.
The expansion ratio is preferably from 2 to 10 times.
It is preferable to perform expansion by releasing the particles after increasing the inner pressure of the closed vessel by introducing nitrogen, atmospheric air or inorganic gas containing the above nitrogen or atmospheric air as a main component.
The above polyolefin resin is preferably a polypropylene resin.
The temperature of the above saturated vapor is preferably at least the softening temperature to at most the melting point of the above polyolefin resin.
Preferably, the polyolefin resin particles comprise 100 parts by weight of a polyolefin resin and 0.05 to 20 parts by weight of a hydrophilic polymer.
It is preferable to melt-knead the above polyolefin resin composition comprising a polyolefin resin and a hydrophilic polymer by using a multi-screw extruder.
It is preferable that a masterbatch comprising a polyolefin resin and a hydrophilic polymer in which the concentration of the hydrophilic polymer is high is previously prepared and a polyolefin resin is melt-kneaded with the masterbacth by using an extruder.
DETAILED DESCRIPTION
The present invention is used for preparing pre-expanded particles from polyolefin resin particles.
In the present invention, pre-expanded particles of a polyolefin resin are prepared as follows. First, the above polyolefin resin particles are dispersed into an aqueous dispersion medium in a closed vessel. The above resin particles are then heated to the temperature higher than the softening point of the above polyolefin resin. Next, inorganic gas is preferably introduced to the closed vessel and the pressure inside the closed vessel is kept to 0.6 to 7.5 MPa. After that, the above resin is released into an atmosphere of lower pressure than the inner pressure of the closed vessel. The characteristic of the present invention is to contact the released particles with saturated vapor at this time.
Allowed to contact with the released particles, the above saturated vapor is used for reducing and preventing the contraction of particles to be expanded and producing pre-expanded particles having less fluctuation in expansion ratios and more uniform particle size. The temperature of the above saturated vapor is adjusted to preferably at most the melting point of the material resin used for particles to be pre-expanded, more preferably to the temperature range of 90 to 1

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