JP2007100016A - Method for producing polypropylene-based resin extrusion-foamed sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for highly efficiently producing a polypropylene-based resin extrusion-foamed sheet having light weight, capable of being suitably formed into a food container, etc., and making it possible that its heat formability is stabilized by a storage in a short time. <P>SOLUTION: This method for producing the polypropylene-based resin extrusion-foamed sheet comprises supplying an extruder with a polypropylene-based resin and propane, then kneading and cooling the supplied materials, so as to form a foamable gel, and further extruding the foamable gel through a die into a low-pressure region. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a polypropylene resin extruded foam sheet. More specifically, the present invention relates to a production method capable of efficiently producing a polypropylene resin extruded foam sheet that is lightweight and can be suitably molded into a food container or the like.

  Foamed sheets made of thermoplastic resins are generally lightweight, have good heat insulation and buffering properties against external stress, and can be easily obtained by heat forming such as vacuum forming. It is widely used in applications such as cushioning materials, food containers, heat insulating materials, and automotive parts, mainly polyethylene resins.

  However, polystyrene-based resin extruded foam sheets widely used for food containers and the like have the disadvantage of being inferior in heat resistance and oil resistance due to the nature of the polystyrene-based resin as a base material.

  On the other hand, polypropylene resin with excellent heat resistance and oil resistance has been considered difficult to obtain an extruded foam sheet due to low viscosity and tensile strength at the time of melting, but in recent years by irradiating the polypropylene resin with radiation A method using a resin in which long chain branching is introduced (for example, Patent Document 1) or a method using a resin obtained by modifying a polypropylene resin by reaction with a conjugated diene compound and a radical polymerization initiator (for example, Patent Document 1) 2) etc., it was found that an extruded foam sheet can be produced.

  In general, in an extruded foam sheet having a high closed cell ratio, the foaming agent is dissipated through the cell membrane immediately after foaming, and conversely, air flows into the cells. In the case of a polystyrene resin foam sheet, the thickness of the foaming agent is greatly increased at the time of heat molding by utilizing the slow dissipation of the foaming agent so that a certain amount of foaming agent remains. On the other hand, in the case of polypropylene resin foam sheets, the foaming agent dissipates quickly and it is difficult to control and leave a certain amount of foaming agent. Is usually stored. When the foaming agent remains in the bubbles of the polypropylene resin foam sheet during thermoforming, the thickness increases beyond the increase due to the thermal expansion of the normal air in the bubbles, and the foaming agent also acts as a plasticizer. Under the preheating conditions, excessive heating may occur, and flickering may occur on the surface. For this reason, stable thermoforming under a certain condition has been difficult.

However, in the production of a conventional polypropylene resin extruded foam sheet, volatile hydrocarbons having a relatively high boiling point such as isobutane, n-butane, isopentane, and n-pentane are used as a foaming agent for stable thermoforming. Since a storage period of at least about one day is required, stabilization in a shorter time has been demanded.
Japanese Patent No. 2521388 JP-A-11-228726

  An object of the present invention is to provide a production method that can stabilize a heat-moldability of a polypropylene resin extruded foam sheet that is lightweight and can be suitably molded into a food container, etc., and can be efficiently produced by short-term storage. It is in.

  As a result of intensive studies to solve such problems, the inventor of the present invention uses propane as a foaming agent, so that heat formability is stabilized by short-time storage. Has been found to be able to be produced, leading to the present invention.

That is, the present invention is characterized by supplying a polypropylene resin and propane to an extruder, forming a foamable gel while kneading and cooling, and extruding the foamable gel through a die into a low pressure region to obtain a foamed sheet. The method for producing a polypropylene resin extruded foam sheet (Claim 1) and the polypropylene resin extruded foam sheet have a density of 90 to 300 kg / m ³ and a closed cell ratio of 60% or more. It is related with the manufacturing method (Claim 2) of the polypropylene resin extrusion foam sheet of this.

  According to the method for producing a polypropylene resin extruded foam sheet of the present invention, even in a polypropylene resin extruded foam sheet having a high closed cell rate, the heat moldability of the polypropylene resin extruded foam sheet is stabilized by short-time storage. , It is possible to ship immediately after manufacturing without substantially storing in the warehouse.

  In the production method of the present invention, by using propane as a foaming agent, the foaming agent dissipates in a shorter time than in the case of hydrocarbons having a large number of carbon atoms such as butane and pentane, and the resulting polypropylene resin extruded foam sheet is shorter. It is characterized by being able to be thermoformed by storage of time.

  In the present invention, a polypropylene resin and a foaming agent are supplied to an extruder, melted and kneaded, then cooled to a foaming temperature to form a foamable gel, and extruded through a die under a low pressure. Manufacture sheets. As a method for forming an extruded foam sheet, for example, a circular die having an annular lip is used, and a cylindrical foam is obtained by extruding from the lip of the die into atmospheric pressure, and then the cylindrical foam is taken up. On the other hand, there is a method in which a sheet is formed after being drawn and cooled by a forming process using a cooling cylinder (mandrel) and then cut into a sheet. For the formation of the foamable gel, a conventional extrusion foaming apparatus such as a tandem extruder in which a plurality of extruders such as a single screw extruder or a twin screw extruder are connected, or a gear pump connected to the tip of the extruder is used. Can be used.

  On the other hand, when the closed cell ratio of the polypropylene resin extruded foam sheet is low, the foaming agent dissipates quickly, so even if conventional butane or pentane is used, the thickness variation in thermoforming converges quickly, but as described later The object of the present invention is not achieved because the moldability and the rigidity of the resulting molded article are impaired.

  In the production method of the present invention, propane is pressurized and fed as a liquid phase under pressure using a diaphragm pump or the like as in the case of conventional foaming agents. In contrast, ethane and methane, which are hydrocarbons with fewer carbon atoms, have a high vapor pressure at room temperature when used as a blowing agent, making it difficult to pump in the liquid phase, and the supply equipment is versatile. Therefore, it is not preferable.

  However, in the production method of the present invention, if necessary, hydrocarbons other than propane, ethers, alcohols, ketones, halogenated hydrocarbons, carbon dioxide, nitrogen as long as the effects of the present invention are not impaired. A foaming agent such as water may be used in combination.

  The amount of propane used in the present invention varies depending on the target density of the extruded foam sheet, but is preferably in the range of 0.5 to 3 parts by weight with respect to 100 parts by weight of the polypropylene resin.

  The polypropylene resin used in the present invention is a propylene homopolymer, or a random copolymer or block copolymer of propylene with one or more copolymer components selected from ethylene and an α-olefin having 4 or more carbon atoms. It is a polymer.

  The melt flow rate at 230 ° C. of the polypropylene-based resin is preferably 10 g / 10 min or less, more preferably 5 g / 10 min or less, because pressure holding with a die is easy in extrusion foaming. preferable.

  The melt tension at 230 ° C. of the polypropylene resin is preferably 5 g or more because it is easy to produce an extruded foam sheet having a desired density or closed cell ratio.

  The melt tension was measured using a Toyo Seiki melt tension tester, and a polypropylene resin heated to 230 ° C. was extruded from an orifice having a diameter of 1 mm, a length of 10 mm, and an inflow angle of 45 degrees at a speed of 1 m / min. The extrudate is wound while passing through a tension detection pulley and accelerated from a speed of 1 m / min, and the tension value when the extrudate is cut is measured.

  As the polypropylene resin having a high melt tension, a linear polypropylene resin (hereinafter, this polypropylene resin may be referred to as “raw polypropylene resin”) is irradiated with an electron beam to introduce long chain branching. And a modified polypropylene resin obtained by melt-kneading a raw material polypropylene resin, an isoprene monomer and a radical polymerization initiator, and the like. In particular, a modified polypropylene resin obtained by melt-kneading a raw material polypropylene resin, an isoprene monomer and a radical polymerization initiator is preferred because it is easy to produce and economically advantageous.

The density of the polypropylene resin extruded foam sheet obtained by the production method of the present invention is preferably 90 to 300 kg / m ³ . If the density of the polypropylene resin extruded foam sheet is less than 90 kg / m ³ , the rigidity of the resulting molded product may be impaired. If the density exceeds 300 kg / m ³ , the heat insulating property of the obtained molded product may be impaired. is there.

  The closed cell ratio of the polypropylene resin extruded foam sheet obtained by the production method of the present invention is preferably 60% or more, and more preferably 70% or more. If the closed cell ratio of the polypropylene resin extruded foam sheet is less than 60%, the heat moldability and the rigidity of the resulting molded product may be impaired.

  The closed cell ratio of the polypropylene resin extruded foam sheet can be easily achieved by adjusting the resin temperature immediately before discharging from the die, as is generally done. For example, when a polypropylene resin having a melt tension at 230 ° C. of 5 g or more is used and an extruded foam sheet is produced by the above method, if the closed cell ratio of the obtained extruded foam sheet is low, an extruder or die Can be easily adjusted to the above range.

  In the production method of the present invention, for the purpose of controlling the number of bubbles, a nucleating agent such as sodium bicarbonate-citric acid mixture, monosodium citrate, or talc may be used in combination. The addition amount of the nucleating agent is not particularly limited, but is usually preferably 0.01 to 3 parts by weight with respect to 100 parts by weight of the polypropylene resin composition.

  In the present invention, a normal polypropylene resin and other thermoplastic resins may be mixed within a range that does not impair the foamability of the polypropylene resin.

  In the present invention, if necessary, stabilizers such as antioxidants, metal deactivators, phosphorus processing stabilizers, UV absorbers, UV stabilizers, fluorescent brighteners, metal soaps, antacid adsorbents, Alternatively, additives such as a crosslinking agent, a chain transfer agent, a nucleating agent, a lubricant, a plasticizer, a filler, a reinforcing material, a pigment, a dye, a flame retardant, and an antistatic agent may be added.

  Moreover, you may laminate | stack the non-foamed layer which consists of resin extruded from the film or T-die etc. as needed to the polypropylene resin extruded foam sheet obtained with the manufacturing method of this invention. The resin forming the non-foamed layer is preferably a polyolefin-based resin, and more preferably a polypropylene-based resin, from the viewpoint of adhesion to an extruded foam sheet, heat resistance, oil resistance, and the like.

  Since the polypropylene resin extruded foam sheet obtained by the production method of the present invention is excellent in heat moldability such as plug molding, vacuum molding, and pressure molding, it is possible to obtain a molded article with little thickness unevenness and a beautiful appearance.

  Examples of thermoforming include plug molding, matched mold molding, straight molding, drape molding, plug assist molding, plug assist / reverse draw molding, air slip molding, snapback molding, reverse draw molding, free drawing molding, plug Examples include AND ridge molding and ridge molding.

  EXAMPLES Next, although this invention is demonstrated in detail based on an Example, this invention is not limited to this Example.

(Measurement of density of extruded foam sheet)
Based on JIS-K6767, the extrusion foam sheet | seat of 48 hours after extrusion was measured.

(Measurement of closed cell ratio of extruded foam sheet)
In accordance with the method described in ASTM D2856, an extruded foam sheet 48 hours after extrusion was measured using an air pycnometer.

(Evaluation of thermoformability of extruded foam sheet)
For the evaluation, an extruded foam sheet of 1 hour after extrusion, 12 hours after extrusion, and 24 hours after extrusion was used.

The extruded foam sheet was cut into 540 mm × 540 mm squares, and squares of 500 mm × 500 mm and 420 mm × 420 mm whose diagonals coincided with this were drawn. Using a single molding machine (VAS-66-45T, manufactured by SENBA System Co., Ltd.), after fixing it to a frame with an internal dimension of 500 × 500 mm so as to coincide with a previously drawn line of 500 mm × 500 mm, an ambient temperature of 200 ° C. For 50 seconds.
Using 10 points on the 420 mm × 420 mm line as thickness measurement points, the average thickness before and after heating was calculated, and the molding magnification was calculated based on the following formula.
(Molding magnification) = (Average thickness after heating) / (Average thickness before heating)

Moreover, the surface state of the extruded foam sheet after heating was evaluated according to the following criteria.
○: There is no fuzzy burn or trace of foam breakage on the surface of the extruded foam sheet. ×: There is a fuzzy burn or trace of foam breakage on the surface of the extruded foam sheet.

From the evaluation of the molding magnification and the surface condition after heating, the heat moldability was determined according to the following criteria.
○: Molding magnification is stable within 12 hours after extrusion, and the surface state after heating becomes ◯. X: Molding magnification is not stable or surface state after heating is not within 12 hours after extrusion.

(Rigidity evaluation of molded body)
Using the single molding machine used in the heat moldability evaluation, the extruded foam sheet for 48 hours after extrusion was heated for 50 seconds in a heating furnace adjusted to an atmospheric temperature of 200 ° C., and then the matched mold molding method (mold temperature) 25 ° C.), a tray-shaped molded body having an opening inner dimension of 175 mm × 198 mm, a bottom inner dimension of 150 mm × 165 mm, a depth of 30 mm, and a flange portion having a width of 5 mm around the opening. About 10 molded bodies, using a DIGITAL FORCE GAUGE (MODEL No.MD-2000 (SV-2)) manufactured by IMADA, the lip center portion on the long side of the molded body at the opening of 198 mm was fixed. Compressed at a compression speed of 10 mm / second so that the dimension of 175 mm direction was compressed, the load when compressed by 10 mm was measured, and the average value of 10 was evaluated according to the following criteria.
○: Load is 4.0N or more Δ: Load is 3.5N or more and less than 4.0N ×: Load is less than 3.5N

(Insulation evaluation of molded body)
200 g of water was put into the molded body, and the outer wall temperature at the bottom of the container after the contents were heated to 90 ° C. using a microwave oven was measured.
○: Less than 65 ° C Δ: 65 ° C or more, less than 70 ° C x: 70 ° C or more

Example 1
100 parts by weight of a propylene homopolymer having a melt flow rate of 1.4 g / 10 min (manufactured by Grand Polymer Co., Ltd., J101) and 0.2 part of t-butyl peroxybenzoate (manufactured by NOF Corporation, Perbutyl Z) The mixture stirred and mixed with a ribbon blender is supplied to a twin screw extruder using a measuring feeder, and 0.4 part of isoprene is supplied from the middle of the extruder using a liquid pump, and melted in the twin screw extruder. By kneading, pellets of the modified polypropylene resin were obtained. The melt tension of the obtained modified polypropylene resin was 14.2 g.
The twin-screw extruder was the same-direction twin-screw type, the screw diameter was 44 mmφ, and the maximum screw effective length (L / D) was 38. The set temperature of the cylinder part of the twin-screw extruder was 180 ° C. up to the isoprene press-in position, 200 ° C. after the isoprene press-in position, and the screw rotation speed was set at 150 rpm.

A 90-125 mmφ tandem extruder was prepared by mixing 100 parts by weight of the modified polypropylene resin and 0.5 parts by weight of a cell nucleating agent master batch (manufactured by Dainichi Seika Kogyo Co., Ltd., PO510K) with a ribbon blender. And then melted in a first stage extruder (90 mmφ) set at a cylinder temperature of 200 ° C., and then 1.5 parts by weight of propane as a blowing agent with respect to 100 parts by weight of the modified polypropylene resin. And cooled in a second stage extruder (125 mmφ) set at 155 ° C. and discharged from a circular die (127 mmφ) set at 160 ° C. under atmospheric pressure to a cooling cylinder having an outer diameter of 335 mm and a body length of 800 mm. While being molded, it is stretched and cooled while being drawn at 6.4 m / min to obtain a cylindrical foam, and this is cut with a cutter to obtain a thickness of 2 0 mm, to obtain an extruded foam sheet of width 1040 mm. The discharge rate was 120 kg / hour. The obtained extruded foam sheet was stored at room temperature (25 to 35 ° C.).
The evaluation results of the obtained extruded foam sheet are shown in Table 1.

(Example 2)
Extruded foam sheets were obtained in the same manner as in Example 1, except that the amount of propane was 2.4 parts by weight, the set temperature of the second stage extruder was 153 ° C., and the take-up speed was changed to 9.6 m / min. . The evaluation results of the obtained extruded foam sheet are shown in Table 1.

(Example 3)
Instead of the modified polypropylene resin, propylene homopolymer PF-814 (melt tension: 9.8 g) manufactured by Basel Co., Ltd. was used, the propane amount was 0.8 parts by weight, and the set temperature of the second stage extruder was 158. Extruded foam sheets were obtained in the same manner as in Example 1 except that the temperature was set to ° C and the take-up speed was changed to 3.5 m / min. The evaluation results of the obtained extruded foam sheet are shown in Table 1.

Example 4
Extruded foam sheets were obtained in the same manner as in Example 3, except that the amount of propane was 0.7 parts by weight and the take-up speed was changed to 3.1 m / min. The evaluation results of the obtained extruded foam sheet are shown in Table 1.

(Example 5)
Extruded foam sheets were obtained in the same manner as in Example 2 except that the amount of propane was 2.8 parts by weight and the take-up speed was changed to 12.0 m / min. The evaluation results of the obtained extruded foam sheet are shown in Table 1.

(Comparative Example 1)
An extruded foam sheet was obtained in the same manner as in Example 1 except that 2.1 parts of isobutane was used as a foaming agent with respect to 100 parts by weight of the modified polypropylene resin. The evaluation results of the obtained extruded foam sheet are shown in Table 1.

(Comparative Example 2)
An extruded foam sheet was obtained in the same manner as in Example 1 except that 2.6 parts of isopentane was used as a foaming agent with respect to 100 parts by weight of the modified polypropylene resin. The evaluation results of the obtained extruded foam sheet are shown in Table 1.

(Comparative Example 3)
An extruded foam sheet was obtained in the same manner as in Comparative Example 1 except that the set temperature of the second stage extruder was changed to 159 ° C. The evaluation results of the obtained extruded foam sheet are shown in Table 1.

  The extruded foam sheet in the examples has stable heat moldability after storage for a short time after extrusion foaming, and the resulting molded article is also excellent in rigidity and heat insulation, and particularly has a density and closed cell ratio of Claim 2. It can be seen that the extruded foam sheet is excellent in the balance between rigidity and heat insulation. On the other hand, it can be seen that the extruded foam sheets of Comparative Examples 1 and 2 require long-term storage in order to stabilize the heat moldability. Since the extruded foam sheet of Comparative Example 3 has a low closed cell ratio, it can be seen that although the heat moldability is stabilized in a short time, the obtained molded product is inferior in rigidity and heat insulation.

Claims (2)

  Polypropylene resin extrusion characterized by supplying a polypropylene resin and propane to an extruder, forming a foamable gel while kneading and cooling, and extruding the foamable gel into a low pressure region through a die to obtain a foamed sheet. A method for producing a foam sheet. The method for producing a polypropylene resin extruded foam sheet according to claim 1, wherein the polypropylene resin extruded foam sheet has a density of 90 to 300 kg / m ³ and a closed cell ratio of 60% or more.