Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By gas forming or expanding
Reexamination Certificate
1999-07-30
2001-09-18
Foelak, Morton (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By gas forming or expanding
C425S00400R, C425S81700C, C521S079000, C521S081000, C521S142000
Reexamination Certificate
active
06290884
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing uncrosslinked polypropylene-based resin foam sheets which have fine foam cells and good appearance. More specifically, the present invention relates to a process for producing uncrosslinked polypropylene-based resin foam sheets, characterized in that the productivity in the production of high-expansion thin sheets is remarkably increased.
2. Description of the Related Art
A technique, in which extrusion foaming is carried out by interposing a gear pump between an extruder and a die, has been hitherto applied to a process for producing polystyrene and is described in, for example, “Plastic Technology 28 [2] 87 (1980)”.
The effects of using a gear pump in extrusion forming are as follows according to “Synthetic Resins 40 [9] 42 (1994)”.
(1) Effects brought about by the suppression of pressure rise in the extruder
reduction of energy consumption
prevention of rise in resin temperature
improvement of transfer efficiency (productivity)
reduction of retention time in the extruder
reduction of L/D of the extruder
(2) Influences of combination with die are eliminated.
The setting of temperature, pressure, and revolution number of the extruder becomes easier.
Irrespective of the shapes of die, the conditions set to the extruder do not need to be changed if the resin to be used and the production amount are unchanged.
The difference of product qualities due to difference of the abilities of operators is eliminated.
Unmanned operation and automated operation become possible.
(3) Stabilization in production
The influence of pressure change in the extruder is eliminated.
Even moistened raw materials can be used.
Temperature distribution becomes nearly constant.
Time required for the start up of production can be remarkably reduced.
However, all of the above-described effects are not necessarily obtained by using a gear pump because processing conditions suited for foaming vary depending on resins and because wavy or corrugated defects peculiar to foaming tend to occur at ends of sheets in the case of extrusion foaming.
In particular, in the case of an uncrosslinked polypropylene-based resin which is crystalline and whose viscosity sharply drops at a temperature higher than the melting point, it is impossible to produce high-expansion foam sheets having uniform fine foam cells by conventional extrusion foaming. Further, an attempt to increase delivery rate in order to increase productivity or to produce high-expansion foam sheets is associated with problems such as rise in resin pressure and temperature.
The loss due to wavy or corrugated defects at ends of sheets occurs when transverse expansion due to foaming is regulated by the width of die. This phenomenon is peculiar to foamed sheets, particularly a high-expansion foamed sheet by an expansion ratio of 2 or greater. In addition, the loss due to wavy or corrugated defects at ends of sheets occurs frequently when the resin temperature is high and the foaming gas tends to be leaked. Therefore, a need exists for means to reduce the above loss in the production of uncrosslinked polypropylene-based resin foam sheets.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a process for producing uncrosslinked polypropylene-based resin foam sheets, which process enables to produce foam sheets having fine foam cells and good appearance and is excellent in productivity and extrusion stability as well as in productivity improving effect in the production of high-expansion thin sheets and which process reduces wavy or corrugated defects at the ends of sheets, and to provide uncrosslinked polypropylene-based resin foam sheets.
The present inventors have made intense studies in order to solve the above-described problems in the production of uncrosslinked polypropylene-based resin foam sheets by interposing a gear pump between an extruder and a die. Specifically, they examined the relationship among the variables, namely the pressure (Pg) at the inlet of the gear pump, the theoretical expansion ratio &khgr; (theoretical expansion ratio based on the amount added of a foaming agent) when the foaming agent added is 100% held, and the coefficient of solubility H of the foaming gas in the molten resin. As a result, they found that it is possible to control the resin temperature within a range suited for foaming, to produce fine foam cells having good appearance, and to secure high productivity and extrusion stability if the above-mentioned variables are set within a specific relationship.
Accordingly, the present invention provides;
1. A process for producing uncrosslinked polypropylene-based resin foam sheets which is characterized by using an extrusion foaming apparatus having a gear pump which is interposed between the extruder and the die and which is linked by means of adapters, wherein the pressure (Pg) at the inlet of the gear pump is within the range specified by the following formula (Formula 1) and the extruder is controlled so that the value of Pg is constant.
Formula 1
Pc≦Pg(atm)≦Pc+90
PC=(&khgr;−1)/H
&khgr;: theoretical expansion ratio when the foaming agent added is 100% held
H: coefficient of solubility of the foaming gas in the resin at the resin temperature in the gear pump inlet (cm
3
(STP)/cm
3
·atm)
2. A process for producing uncrosslinked polypropylene-based resin foam sheets according to above 1, wherein the die is characterized by 5≦L/t≦12 where L (mm) stands for the length of lip land and t (mm) stands for the lip gap.
3. A process for producing uncrosslinked polypropylene-based resin foam sheets according to above 1 or 2, wherein the extruding apparatus is characterized by ¼≦A/B≦1and C/A≦10 where the B (mm) stands for the barrel diameter, C (mm) stands for the length of the adapter, and A (mm) stands for the adapter diameter.
4. The uncrosslinked polypropylene-based resin foam sheet produced by a process described in any one of above 1to 3.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integer or step.
Pc indicates a minimum pressure needed for dissolving the foaming gas in an amount corresponding to a predetermined expansion ratio. If the pressure Pg at the inlet of the gear pump is smaller than Pc, the foaming gas is not fully dissolved in the resin and therefore a gaseous phase is separated from the molten liquid phase. As a result, it is difficult to obtain a foam sheet having fine foam cells. On the other hand, if Pg is greater than (Pc+90), one of the important effects of the gear pump, namely the suppression of pressure rise in the extruder, is diminished. As a result, the purposes of preventing rise in resin temperature and enhancing productivity are adversely affected.
The extruder side is controlled so that the pressure Pg at the inlet of the gear pump is within the range of Pc≦Pg (atm)≦Pc+90 and so that Pg is constant. For making Pg constant, known controlling systems may be advantageously employed.
For example, feedback is made to the number of revolution of extruder screw or to the number of revolution of a metering feeder for charging raw materials to the hopper of the extruder. By this adjustment, since the pressure at outlet of the gear pump, the pressure a
Nagamatsu Tatsuhiro
Sakamoto Akinobu
Foelak Morton
Sughrue Mion Zinn Macpeak & Seas, PLLC
Sumitomo Chemical Company Limited
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