JP2003291196A - Production method for polypropylene resin foam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a stable production of a polypropylene resin foam by stably injecting a carbon dioxide into the molten resin in an extruder. <P>SOLUTION: In an extrusion expansion molding wherein a carbon dioxide is fed into the extruder by a liquid supply pump to form a foaming gel by kneading the molten polypropylene resin with the carbon dioxide and cooling them, extruding the foaming gel into a low pressure region through a die to obtain a foam having an expansion ratio of 2 to 15 times, the pressure in the pipe between the liquid supply pump and the extruder is maintained at 5 MPa or more, while the suction part and the discharge part of the liquid supply pump is cooled down to -5°C or less. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polypropylene resin foam which can be suitably used as a buffer packaging material or a food packaging material. More specifically, the present invention relates to a method for producing a polypropylene resin foam while using carbon dioxide having high environmental compatibility as a foaming agent and ensuring high production stability. [0002] In extrusion foaming of a polypropylene resin, a physical foaming agent represented by a hydrocarbon such as butane or pentane is generally used. For example, a foaming agent in a liquid form using a liquid feed pump is used. It is used by sending from a storage tank to an extruder. [0003] In addition to the fact that carbon dioxide does not require an explosion-proof device, carbon dioxide is said to have a smaller effect on global warming than hydrocarbons, and its use as a foaming agent is being studied. [0004] However, carbon dioxide has a lower boiling point and a lower critical temperature than conventional blowing agents such as hydrocarbons, and has a property of being easily solidified (dry ice) when exposed to a low pressure from a high-pressure liquid state. For this reason, cavitation occurs in the liquid sending pump, the liquid becomes a compressible gas in the pipe, dry ice is generated in the pipe or the liquid sending pump, etc.
In some cases, the liquid transfer itself may not be possible. In order to solve these problems, Japanese Patent Laid-Open Publication No.
In the method of manufacturing a thermoplastic resin foam by extrusion foaming using a foaming agent containing carbon dioxide gas, Japanese Patent Application Laid-Open No. 1-291318 describes (A) filling the inside of a pipe with a liquid or a high-pressure gas before starting liquid supply. (B) pressurizing a storage tank of carbon dioxide; and (C) cooling a liquid sending pump to a temperature lower than air temperature.
However, these are suitable for producing an alkenyl aromatic resin foam having an expansion ratio of 15 times or more, but when producing a polypropylene resin foam having an expansion ratio of 2 to 15 times, the supply of carbon dioxide is not sufficient. There was a problem that stable supply was difficult due to the small amount. In recent years, attempts have been made to use carbon dioxide in a supercritical state as a blowing agent.
968 discloses a technique in which the pressure of liquid carbon dioxide is increased to a critical pressure or higher, and further raised to a critical temperature or higher to form supercritical carbon dioxide, and then added to the molten thermoplastic resin. However, there is a problem that the apparatus is complicated and the operation becomes complicated, for example, the pressure is adjusted not only by the pump but also the pressure is adjusted by the pressure holding valve, and the heater is heated by the heater to make it supercritical. SUMMARY OF THE INVENTION An object of the present invention is to stabilize an extruded polypropylene resin foam by stably injecting carbon dioxide into a molten resin in an extruder with a simple device. To achieve strategic production. Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, when extruding carbon dioxide into a molten resin by sending carbon dioxide in an extrusion foaming method. The present inventors have found that the carbon dioxide can be stably supplied by maintaining the pressure in the front pipe at a certain level or more and cooling the suction part and the discharge part of the liquid sending pump to −5 ° C. or less, thereby completing the present invention. Reached. That is, according to the present invention, carbon dioxide is supplied into an extruder by a liquid feed pump, a foamable gel is formed while kneading and cooling with a molten polypropylene resin, and the foamable gel is extruded through a die to a low pressure region. And expansion ratio 2-1
In extrusion foaming to obtain a 5-fold foam, maintain the pressure in the pipe between the liquid pump and the extruder at 5 MPa or more, and cool the suction and discharge sections of the liquid pump to -5 ° C or less. And a method for producing a polypropylene resin foam. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a polypropylene resin foam having an expansion ratio of 2 to 15 times by an extrusion foaming method using a foaming apparatus using a conventional extruder. It is intended to provide a method for stably injecting a certain amount of carbon dioxide even in a small amount. The polypropylene resin used in the present invention is a homopolymer of propylene, a random copolymer of one or two or more copolymer components selected from ethylene and an α-olefin having 4 or more carbon atoms and a random copolymer of propylene or It is a block copolymer. The melt flow rate of the polypropylene resin at 230 ° C. is preferably 10 g / 10 min or less, and more preferably 8 g / 10 min or less, because it is easy to hold the pressure with a die in extrusion foaming. . Further, the temperature of the polypropylene-based resin is 230 ° C.
The melt tension is 5 g or more,
It is preferable because a foam having a desired expansion ratio can be easily produced. The melt tension was measured using a melt tension tester manufactured by Toyo Seiki Co., Ltd., and a polypropylene resin heated to 230 ° C. was weighed 1 mm in diameter and 10 m in length.
m, extruded from an orifice having an inflow angle of 45 degrees at a speed of 1 m / min, and the extruded product is passed through a pulley for tension detection and wound up while being accelerated from a speed of 1 m / min to cut the extruded product. Is measured. The liquid feed pump used in the present invention is a pump used for pressurizing carbon dioxide and for injecting a foaming agent into an extruder. Any known pump such as a diaphragm type or a plunger type can be used. In addition, these pumps are preferably two or three pumps because they can suppress fluctuations in the pressure in the piping. In the present invention, it is essential to maintain the pressure in the pipe between the liquid feed pump and the extruder at 5 MPa or more in order to stably supply carbon dioxide to the extruder for a long time. When the pressure is lower than 5 MPa, the supply amount of carbon dioxide is reduced due to cavitation or the like, and in some cases, dry ice is generated and the piping is blocked, so that there is a possibility that the liquid cannot be sent. The adjustment of the pressure in the pipe can be easily achieved by adjusting the resin pressure in the extruder at the site where carbon dioxide is injected. In the present invention, it is essential to cool the suction part and the discharge part of the liquid sending pump to -5 ° C. or less in order to stably supply carbon dioxide to the extruder for a long time. When the temperature exceeds −5 ° C., the supply amount of carbon dioxide is reduced due to cavitation or the like, and in some cases, dry ice is generated and the pipe is blocked, so that there is a possibility that the liquid cannot be sent. The temperature of the part to be cooled is preferably the temperature of the liquefied carbon dioxide in the liquid feed pump or the piping, but may be the surface temperature of the apparatus if it is difficult to install a thermocouple in the structure. Further, it is preferable to cool not only the suction part and the discharge part of the liquid sending pump but also the head part in order to further stabilize the supply of the foaming agent. However, when cooling the head in the diaphragm type pump, it is necessary to keep the temperature of the drive oil for the diaphragm from falling below the proper use temperature. As a method of cooling the suction part and the discharge part of the liquid sending pump, there is a method of surrounding the part with a jacket or the like and circulating a refrigerant in the jacket or putting dry ice therein. In the present invention, the supply amount of carbon dioxide is arbitrarily selected depending on the expansion ratio of the foam to be produced. Although it depends on the efficiency of the foaming apparatus used, it is preferably 0.1 to 5 parts by weight based on 100 parts by weight of the polypropylene resin. The method of the present invention discloses a method using carbon dioxide alone as a blowing agent. However, a blowing agent other than carbon dioxide may be used in combination with another line or as a mixture with carbon dioxide. . When the carbon dioxide and another blowing agent are used in combination as a mixture, foaming which does not solidify at the cooling temperature, flow rate and pressure of the suction part and discharge part of the liquid sending pump is required for cooling by the method of the present invention. It must be an agent. Preferred blowing agents for use with carbon dioxide include, for example, hydrocarbons such as propane, n-butane, i-butane, n-pentane, i-pentane and neopentane, 1,1,1,2-tetrafluoroethane,
One or two or more of hydrofluorocarbons such as 1,1-difluoroethane and difluoromethane, alcohols such as methyl alcohol and ethyl alcohol, ethers such as dimethyl ether and the like can be mentioned. Of these, propane, n-butane, i-butane, n-pentane and i-pentane are more preferred. In the present invention, the apparatus for producing the foam is not particularly limited, and includes a single-screw extruder, a twin-screw extruder, a tandem extruder in which a plurality of extruders are connected, a device in which a gear pump is connected to the extruder tip, and the like. , Conventional foaming equipment can be used. Of these, the tandem extruder or the extruder can be easily adjusted by adjusting the resin pressure in the extruder at the site where the foaming agent is injected, so that the piping pressure between the liquid feed pump and the extruder can be easily adjusted. A device in which a gear pump is connected to the tip is preferable. According to the above method, carbon dioxide can be stably supplied to the extruder. When starting to feed carbon dioxide using the liquid sending pump, it is necessary to fill the pipe from the carbon dioxide storage tank to a position immediately before the extruder press-fitting section with a liquid or a high-pressure gas at the start of the liquid sending. This is preferable because the generation of dry ice due to rapid pressure fluctuation can be suppressed. The liquid or gas previously filled in the pipe may be one or more of hydrocarbon, hydrofluorocarbon, alcohol, ether, carbon dioxide, nitrogen, air and the like. Next, a method for producing an extruded foam according to the present invention will be described based on examples and comparative examples, but the present invention is not limited to only these examples. Example 1 100 parts by weight of a propylene homopolymer (melt flow rate at 230 ° C .: 0.5 g / 10 min, melt tension at 230 ° C .: 12 g) and a baking soda-citric acid mixture 0 as a nucleating agent .12 parts by weight in a ribbon blender, and then the mixture was treated with 50 kg / h at 6 kg / h.
After feeding to a 5 mm-90 mm φ tandem extruder and plasticizing at 220 ° C. in a first stage extruder (65 mm φ),
Carbon dioxide is fed from the extruder tip to propylene homopolymer 1
1.3 parts by weight with respect to 00 parts by weight and kneaded with the molten resin, and when the resin temperature is 1 in a second stage extruder (90 mmφ).
Cool down to 63 ° C and use a circular die (75 m
mφ), extruded into the atmosphere, outer diameter 200 mm, length 25
The sheet was drawn while being stretched and cooled in a 0 mm cooling cylinder to obtain a sheet-like foam having an expansion ratio of 5.2. The carbon dioxide supply device is composed of a siphon-type cylinder filled with liquefied carbon dioxide, a double-diaphragm-type liquid feed pump, and a mass flow meter. A pressure gauge was provided in between, and the measured value of the mass flow meter was fed back to the rotation speed of the liquid sending pump to automatically control the supply amount of carbon dioxide. Then, the surface temperatures of the suction part, the discharge part, and the head part of the liquid sending pump are −1.
After cooling with dry ice below 0 ° C. and adjusting the screw rotation speed of the second stage extruder so that the resin pressure in the extruder press-fitting section becomes 12 MPa, the liquid feed pump is started and the valve in the extruder press-fitting section is started. Was released. When the supply of carbon dioxide was started, the resin pressure at the extruder press-fitting portion, and the display of the pressure gauge between the mass flow meter and the extruder became 11 MPa,
The screw speed of the second extruder was adjusted so as to maintain this pressure. After the foam was taken out, it was operated for 8 hours. During that time, every 30 minutes, the supply amount of carbon dioxide was monitored, and the foam was sampled to measure the expansion ratio.
1.3 parts by weight of carbon dioxide is always supplied to 100 parts by weight of the resin, and the expansion ratio is stable between 5.1 and 5.3 times, so that industrial production is possible. Was determined. The monitoring of the carbon dioxide supply was carried out by placing a carbon dioxide cylinder on a weigher and measuring the weight loss. Example 2 The supply amount of carbon dioxide was changed to 1.8 parts by weight, and the pressure of carbon dioxide between the mass flow meter and the extruder was changed to 8 MPa.
A sheet foam having an expansion ratio of 7.8 was obtained in the same manner as in Example 1 except that the temperature was maintained at a, and thereafter the operation was continued for 8 hours. 1.8 parts by weight of carbon dioxide is always supplied with respect to 100 parts by weight of the resin, and the expansion ratio is 7.6 ~.
It was stable between 7.9 times. Comparative Example 1 A sheet-like foam having an expansion ratio of 5.2 was manufactured in the same manner as in Example 1, and was used for cooling the suction part, discharge part, and head part of the liquid sending pump. When the supply of dry ice was stopped, the surface temperature of each part of the liquid sending pump exceeded -3 ° C. The supply amount is reduced to 1.1 parts by weight based on 100 parts by weight of the resin,
Since the expansion ratio of the obtained foam also decreased to 4.3 times, it was judged that stable foam production was difficult, and the experiment was stopped. Comparative Example 2 The pressure of carbon dioxide between the mass flow meter and the extruder was adjusted to 4MP by adjusting the screw rotation speed of the second stage extruder.
A sheet-like foam was obtained in the same manner as in Example 1 except that a was maintained at a. The expansion ratio of the foam was 4.9 times,
Two hours after the start of the operation, the supply amount of carbon dioxide was increased even though the rotation speed of the liquid sending pump reached the upper limit.
The amount was reduced to 1.0 part by weight, and the expansion ratio of the obtained foam was also reduced to 3.7 times. Therefore, it was judged that stable foam production was difficult, and the experiment was stopped. According to the present invention, using a simple device,
By stably sending carbon dioxide to the molten resin in the extruder and press-fitting it into the extruder, it is possible to stably produce an extruded polypropylene resin foam.

[Brief description of the drawings] FIG. 1 is an explanatory diagram of a manufacturing method of the present invention. [Explanation of symbols] 1 liquefied carbon dioxide storage tank (siphon cylinder) 2 Liquid sending pump 3 Pressure gauge 4 Extruder 5 Suction section 6 Discharge section 7 Head 8 Mass flow meter

Claims (1)

Claims: 1. Carbon dioxide is supplied into an extruder by a liquid feed pump, and is kneaded with a molten polypropylene resin to form a foaming gel while cooling the foaming gel. In the extrusion foaming to obtain a foam having an expansion ratio of 2 to 15 times by extruding into a region, the pressure in the pipe between the liquid feed pump and the extruder is maintained at 5 MPa or more, and the suction part and the discharge part of the liquid feed pump Is cooled to -5 ° C or lower.