JPH0446977B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to foamed polyethylene resin particles that can be applied to a method of molding the foamed particles by heating with a heating medium such as steam in a mold that can be closed, but not sealed, to form a molded article, and a method for producing the same. As a method for manufacturing a foamed molded product by in-mold molding of polyolefin resin, a general method is to pre-foam resin particles containing a volatile foaming agent by heating them with steam or the like, but in the case of polyethylene resin, The melt viscosity of the resin decreases significantly near the melting point, making it extremely difficult to obtain foamed particles with little shrinkage at high magnification, and it is essential to crosslink the polyethylene resin. In addition, high-pressure low-density polyethylene is exclusively used as a raw material for these cross-linked polyethylene foaming agents because of its good cross-linking properties, and although it has excellent flexibility and cushioning properties, it has poor heat resistance and It had the disadvantage that it had to be used at a relatively low expansion ratio because of its lack of rigidity. Therefore, the present inventors have developed a foam that has improved heat resistance and can be used at a higher expansion ratio without sacrificing the flexibility and cushioning properties that are characteristic of conventional cross-linked high-pressure low-density polyethylene foams. In addition to getting
As a result of extensive research in order to obtain a polyethylene foam that has excellent foam moldability even without crosslinking, we have found that by using a specific polyethylene resin and using a specific pre-foaming method, it has good moldability. The inventors have discovered that it is possible to produce a foam that has excellent heat resistance and mechanical properties, can be used at higher expansion ratios, and has good flexibility and cushioning properties, and has thus arrived at the present invention. That is, the present invention has a melt index (MI; JIS K6760) of 0.1 to 50 g/10 minutes and a density of
Polyethylene resin foam particles whose base resin is polyethylene made of a copolymer of ethylene and α-olefin having 4 to 20 carbon atoms and having a melting point of 0.910 to 0.940 g/cm ³ and 110 to 130°C, and their production method, i.e., the melt index is
0.1-50g/10min, density 0.910-0.940g/ ^cm2 and melting point 110-130℃, ethylene and carbon number 4~
Polyethylene resin particles mainly composed of polyethylene made of a copolymer with 20 α-olefins and a volatile blowing agent are dispersed in water in the presence of a dispersant,
The foaming agent is impregnated into the resin particles by heating to a temperature in the range of -25°C to +10°C from the melting point of the resin particles, and the temperature inside the container is increased under pressure higher than the vapor pressure of the volatile foaming agent. The present invention provides a method for producing expanded polyethylene resin particles, characterized in that a mixture of the particles and water is discharged into an atmosphere at a lower pressure than the inside of a container while maintaining a constant pressure. The specific polyethylene resin used in the present invention has a melt index of 0.1 to 50 g/
10 minutes, the density is 0.910-0.940g/ ^cm3 and the melting point is
α of ethylene and carbon number 4 to 20, which is 110 to 130℃
- It is a copolymer with olefin. If the melt index is less than 0.1 g/10 minutes, the fluidity during foaming will be poor and foaming will be difficult, and if it exceeds 50 g/10 minutes, the fluidity will be too high, making it difficult to increase the foaming ratio and causing shrinkage. It becomes easier. If the density is less than 0.910g/ ^cm3 , the resin becomes too soft and tends to shrink.
If it exceeds 0.940 g/cm ³ , it becomes close to high-density polyethylene and becomes difficult to mold as will be described later. If the melting point is less than 110℃, the heat resistance of the foam will be insufficient, and the temperature will exceed 130℃.
If it exceeds this value, it becomes close to high-density polyethylene and becomes difficult to mold. Carbon number 4 to be used as comonomer
Examples of the α-olefin of 20 include 1-butene, 1-pentene, 1-hexene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene,
Examples include one or more selected from 4,4-dimethyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-octadecene, and the like. In order for the density of the entire copolymer to be within the above range, the content of the comonomer is usually about 3 to 12% by weight, although it varies depending on the type of α-olefin. The melting point of the polyethylene was determined using a differential scanning calorimeter (DSC).
This is the peak temperature when the endothermic curve was measured at a temperature increase of 10°C/min. Although the polyethylene resin used in the present invention can be suitably foamed and molded by the method of the present invention even if it is non-crosslinked, it may also be crosslinked by organic peroxide, electron beam irradiation, etc. do not have. In the present invention, the specific polyethylene resin is used as a base resin, and less than 50% by weight of other polyolefins, such as high-pressure low-density polyethylene, high-density polyethylene, polypropylene, ethylene, etc. - One type or two or more types of propylene copolymers etc. may be mixed. In addition, ultraviolet absorbers, antistatic agents, heat stabilizers,
Additives such as flame retardants, colorants, and inorganic fine powders may also be added as appropriate depending on the purpose. Next, a method for producing expanded polyethylene resin particles of the present invention will be explained. Conventionally, polyolefin resin particles and a volatile blowing agent are dispersed in water in a pressure-resistant container such as an autoclave, and after this is brought to a high temperature and high pressure state,
A method of pre-foaming polyolefin resin particles by discharging them into a low pressure region is known, for example, as disclosed in West German Patent Publication No. 2107683 and Japanese Patent Publication No. 1344/1983.
There is a description in the issue etc. This method has great difficulties in the following three points when applied to polyethylene resins. The first problem is that when foamed particles are subjected to molding, with commonly used polyethylenes such as high-pressure low density polyethylene and high density polyethylene, if the heating temperature during molding is low, the particles will not fuse together, and if the heating temperature is high, the particles will not fuse together. shrinks, and the range of heating conditions required to obtain a satisfactory molded product is extremely narrow, making molding difficult. The second problem is that when a mixture of resin particles and water under high temperature and high pressure is released into a low pressure atmosphere, highly foamed pre-expanded particles can be obtained, but fusion (blocking) of the particles occurs, resulting in a single particle. It is extremely difficult to obtain a shaped foam and cannot be subjected to molding. The third problem is that the expansion ratio of the foamed particles obtained by releasing a mixture of resin particles and water into a low-pressure atmosphere varies widely, and the weight and physical properties of the molded product obtained by molding are inconsistent, and the appearance The quality of the product also deteriorates, and the value of the product is greatly impaired. In order to overcome these problems, the present inventors conducted intensive research and found that the specific polyethylene resin described above was used and a mixture of resin particles and water was released from the pressure container into a low pressure area. The three problems mentioned above were successfully solved by discharging through a small-diameter opening and strictly controlling the temperature and pressure inside the container during discharge. First, we will discuss the first problem, the polyethylene type. The pre-foaming method, in which a mixture of resin particles impregnated with a volatile blowing agent and water under high temperature and high pressure is discharged into a low-pressure region to foam, can control the temperature of the resin during foaming within a narrow range, making it possible to control the temperature of the resin within a narrow range. Compared to the method of pre-foaming using water vapor or the like, foaming is also possible using a resin such as high-density polyethylene, which has a narrow temperature range that exhibits the optimum viscoelasticity for foaming. However, in the molding process in which foamed particles are filled into a mold and heated, with polyethylene commonly used such as high-pressure low-density polyethylene and high-density polyethylene, if the heating temperature during molding is low, the particles will not fuse together, and the heating temperature will be too low. If the temperature is too high, the particles will shrink, and the range of heating conditions required to obtain a satisfactory molded product will be extremely narrow, making molding difficult. However, surprisingly, the specific polyethylene resin of the present invention, that is, the melt index is 0.1 to 50 g/10 minutes, and the density is
0.910 to 0.940 g/cm ³ and a melting point of 110 to 130°C. It has been found that by using a polyethylene resin made of a copolymer of ethylene and an α-olefin having 4 to 20 carbon atoms as the base resin, the range of heating conditions during molding can be widened and molding can be facilitated. The reason for this has not yet been fully elucidated, but
In the specific polyethylene resin of the present invention, the crystallization temperature range of the expanded particles is wide as observed by DSC etc., and the temperature range where the expanded particles exhibit maximum foaming in the mold and the crystallization temperature range of the expanded particles. This is thought to be due to the fact that the temperatures at which the particles fuse together are close to each other. Next, regarding the second problem, fusion (blocking) of particles during pre-foaming, when a mixture of polyethylene resin particles and water is released into a low pressure area, the diameter of the particles in terms of spherical volume By ejecting through one or more apertures having an aperture of 1.2 times or more and 3 times or less, it was possible to obtain foamed particles with no blocking at all. If the opening is too small, particles cannot pass through the opening, causing blockage; if the opening is too large, many particles will pass through the opening at the same time and be released into the low pressure area, causing the particles to pass through the opening. During or after passage (during foaming), particles fuse together, causing blocking. The shape of the opening is usually circular or elliptical, but polygonal shapes can be used depending on the case. The size of each pore is determined by the particle size of the polyethylene resin particles used, but for normal in-mold molding it is 0.5~
Particles with a diameter of about 6 mm (converted to spherical volume) are used, so the size of the openings is 0.3 to 250 mm ² in terms of area.
It will be about. In the method of the present invention, the size of one pore is determined by the particle size of the polyolefin resin particles used, so the release rate can be controlled by increasing or decreasing the number of pores.
An example of providing the openings is a method in which a pressure-resistant orifice plate having one or more of the aforementioned openings is inserted behind the discharge valve via a flange. Next, the third problem, the variation in the expansion ratio of expanded particles, will be described. In the present invention, when the mixture of resin particles and water is discharged into a low pressure region, it is necessary to maintain constant the temperature and pressure inside the container, and more preferably, the partial pressure of the volatile blowing agent in the gas phase inside the container. is necessary. Since the expansion ratio of the expanded particles varies greatly due to temperature fluctuations, it is necessary to control the temperature as strictly as possible. For example, it is preferable that the temperature fluctuation from the start of discharge to the end of discharge is within 5°C. Temperature control can be easily carried out by using, for example, a commonly used pressure-resistant container with a jacket. As the mixture of resin particles and water inside the container is released, the head space inside the container increases and the pressure inside the container decreases, and the degree of foaming of the released foamed particles decreases, so the pressure inside the container increases at the time of release. It is necessary to maintain a constant value, but as the time required for release increases, the expansion ratio of foamed particles will decrease significantly if the pressure is only maintained by pressurizing an inert gas such as N ₂ or air. This is because even if the head space, which increases with release, is kept under pressure by pressurizing an inert gas such as _N2 or air, the partial pressure of the volatile blowing agent in the gas phase inside the container decreases, causing the resin to release. This is because the volatile blowing agent is discharged and the amount of blowing agent impregnated into the resin decreases. Therefore, it is preferable to keep not only the total pressure inside the container constant but also the partial pressure of the volatile blowing agent in the gas phase inside the container. This can be achieved by either reducing the head space in the container by the amount that increases as the head space increases as the volume of the volatile foam increases, or keeping the head space constant while increasing the head space. The agent may be introduced into the container from the outside. (Either of the above methods is necessary when the volatile blowing agent in the space inside the container is in an unsaturated state and the partial pressure of the volatile blowing agent in the gas phase inside the container decreases as it is released. (If the volatile blowing agent is saturated in the container, for example if it is in liquid form and exists in excess in the container, introduction from the outside is not necessary.) The former method complicates the process. Therefore, the latter method is more preferable; for example, the volatile blowing agent is introduced in liquid form continuously or discontinuously through a regulating valve or the like while controlling the pressure to keep the total pressure in the container constant. do it. Note that the term "maintaining the pressure constant" as used in the present invention includes adjusting and maintaining the pressure within a pressure range corresponding to the range of permissible magnification fluctuation of the foamed particles. In this way, it is possible to obtain pre-expanded particles with extremely small fluctuations in expansion ratio even if the time required for release becomes long. Volatile blowing agents used in the present invention include hydrocarbons or halogenated hydrocarbons with a boiling point of -50 to 120°C, or propane, butane, pentane, hexane, heptane, cyclopentane, cyclohexane, monochloromethane, dichloromethane, monochloromethane, etc. Examples include ethane, trichloromonofluoromethane, dichlorodifluoromethane, dichloromonofluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, etc., and these may be used alone or in combination of two or more. good. The amount of these volatile blowing agents to be charged should be determined by considering the type of blowing agent, the desired expansion ratio, and the ratio of the amount of resin in the container to the space volume inside the container, so that the amount impregnated in the polyethylene resin is 5 to 40% by weight. It is decided to become a department. In the present invention, when dispersing polyethylene resin particles in water, it is desirable to use a small amount of a dispersant to prevent agglomeration of resin particles during heating. As the dispersant, used are water-soluble polymers such as polyvinyl alcohol, methyl cellulose, and N-polyvinylpyrrolidone, and fine powders of poorly water-soluble inorganic substances such as calcium phosphate, magnesium pyrophosphate, zinc carbonate, titanium oxide, and aluminum oxide. Water-soluble polymers have problems with drainage measures,
It is preferable to use a fine powder of poorly water-soluble inorganic substances, but if the amount used is too large, the fusion between the pre-expanded particles during molding will be poor, so when using these inorganic substances, it is necessary to use them as a dispersion aid. It is preferable to use a small amount of an anionic surfactant such as sodium alkylbenzenesulfonate, sodium α-olefin sulfonate, or sodium alkylsulfonate to reduce the amount of inorganic substances used. In this case, approximately 0.1 to 3 parts by weight of the poorly water-soluble inorganic substance fine powder and 0.001 to 0.5 parts by weight of the anionic surfactant are used per 100 parts by weight of the resin. The heating temperature in the method of the present invention varies mainly depending on the type of volatile blowing agent used and the desired expansion ratio, and is preferably a temperature in the range of -25°C to +10°C above the melting point of the polyethylene resin used, preferably above the melting point. The temperature ranges from -20°C to +5°C. For example, if the melting point is 120℃, the heating temperature should be 95℃ to 125℃.
Selected in the range of °C. If the heating temperature is lower than this range, the expansion ratio will drop significantly, and if it is higher than this range, the closed cell ratio of the expanded particles will be undesirably low. According to the above-described method of the present invention, pre-expanded polyolefin resin particles can be obtained without any blocking between particles and with extremely little variation in expansion ratio, and can be easily molded using a wide range of heating conditions by known methods. It can be molded into a molded product with good fusion between particles, a beautiful appearance, and a uniform density distribution. Compared to conventional cross-linked high-pressure density polyethylene molded products, the molded products obtained in this way have higher expansion ratios (lower density) and equivalent cushioning performance, are superior in heat resistance and toughness, and are suitable for use as cushioning materials. , suitable for use in packaging materials, insulation materials, containers, etc. Regarding the method for molding the expanded polyolefin resin particles of the present invention, for example, the obtained expanded particles can be molded into a mold immediately, or after curing and drying for an appropriate period of time, as they are, or after imparting foaming ability to the expanded particles. Fill it and heat it with a heating medium such as steam.
Molding can be performed at a heating temperature of about 105 to 130°C and a heating time of about 3 seconds to 2 minutes. The foaming ability can be further imparted to the foamed particles by impregnating the cells of the foamed particles with an inorganic gas such as _N2 or air to increase the internal pressure of the cells, or by compressing the foamed particles with pressurized air, etc. There is a method to increase the internal pressure of the bubble. Also, it does not have foaming ability.
Alternatively, after filling the applied foam particles into the mold,
A method of forming by narrowing the mold by compression is also used. The present invention will be explained in more detail below with reference to Examples. Example 1 In a pressure-resistant container with an internal volume of 1000 and a stirrer, 4
- Copolymerized methyl-1-pentene, melting point 120℃,
Density 0.920g/cm ³ , Melt index 2.1g/10
polyethylene particles (spherical volume equivalent particle diameter approximately 2
mm) 100 parts by weight (225 kg) were dispersed in 300 parts by weight of water using 0.5 parts by weight of powdered basic tribasic calcium phosphate and 0.006 parts by weight of sodium dodecylbenzenesulfonate as dispersants, and dichlorofluoromethane was added while stirring. 30 parts by weight was added and the temperature was raised to 117°C. The internal pressure of the pressure vessel at this time was 27 kg/cm ² (gauge pressure). Next, liquid dichlorodifluoromethane was injected while adjusting it with a valve, and while maintaining the internal pressure at 27 kg/cm ² (gauge pressure), the discharge valve at the bottom of the pressure container was opened, and the orifice installed after the valve was opened. The particle and water mixture was released into the atmospheric pressure atmosphere through one circular hole of 4 mm internal diameter in the plate. The time required for ejection was approximately 30 minutes, and the pre-expanded particles obtained during ejection had no blocking between particles, and the average expansion ratio was 26.7 times, with most of the expanded particles in the range of 24 times to 28 times. , and there is extremely little variation in foaming ratio.
No decrease in foaming ratio was observed in the latter half of the discharge operation. After drying the expanded particles at 60℃ for 24 hours,
Pressurized with air at Kg/cm ³ (gauge pressure) for 2 hours,
Air is impregnated into the bubbles of the foamed particles, and then 290
A mold of 270 x 50 mm was filled into a mold and heated ^for 15 seconds with steam at 1.5 kg/cm ² (gauge pressure). The molded product was smooth and extremely good. Table 1 shows a comparison of the physical properties of this molded product with that of a crosslinked high-pressure low density polyethylene molded product (manufactured by Kanebuchi Kagaku Kogyo, trade name: Eperan). It can be seen that the 38x molded product exhibits the same flexibility and cushioning properties as the 27x crosslinked polyethylene molded product.

[Table] Comparative Example 1 100 parts by weight (900 g) of the high-pressure low-density polyethylene and high-density polyethylene particles (particle diameter approximately 2 mm) shown in Table 2 were powdered as a dispersant in a pressure-resistant container with an internal volume of 4 and equipped with a stirrer. Disperse 0.5 parts by weight of basic calcium phosphate and 0.006 parts by weight of sodium dodecylbenzenesulfonate in 300 parts by weight of water, add 30 parts by weight of dichlorodifluoromethane with stirring, and raise the temperature to the temperature shown in Table 2. Pressurization
Pressurely inject _N2 and maintain the internal pressure of the container at that time.
The mixture of particles and water was discharged into the atmospheric pressure atmosphere through an orifice plate having one 4 mm diameter opening. The obtained expanded particles had the expansion ratio shown in Table 2, and when they were molded by impregnating air into the cells of the expanded particles in the same manner as in Example 1, the results were satisfactory as shown in Table 2. No suitable molded body was obtained.

[Table] Comparative Example 2 In Example 1, without using the orifice plate,
Foaming was carried out under the same conditions as in Example 1, except that the mixture of pellets and water was discharged directly from a discharge valve with an internal diameter of 25 mm. The foamed particles obtained contained a large number of blocked particles of about 2 to 20 particles, and could not withstand subsequent use. Comparative Example 3 In Example 1, instead of liquid dichlorodifluoromethane, pressurized N ₂ was injected while adjusting it with a valve to maintain the internal pressure of the pressure container at 27 Kg/cm ² (gauge pressure). It was released using the same procedure as 1. The average foaming ratio during the 30 minute discharge time is
It decreased from 27.2 times to 15.6 times. Example 2 Table 3 also shows the results of pre-foaming and molding polyethylene particles (spherical volume equivalent particle diameter of about 2 mm) copolymerized with the comonomers shown in Table 3 in the same manner as in Comparative Example 1.

【table】

Claims (1)

[Claims] 1. Melt index (MI) is 0.1 to 50g/10
minute, density 0.910-0.940g/ ^cm3 and melting point 110
~130℃, ethylene and α- with 4 to 20 carbon atoms
Polyethylene resin foam particles whose base resin is polyethylene made of a copolymer with olefin. 2 Melt index is 0.1 to 0.1 in a pressure container.
50g/10min, density 0.910~0.940g/ ^cm3 and melting point 110~130℃, ethylene and carbon number 4~20
Polyethylene resin particles whose base resin is polyethylene made of a copolymer with α-olefin and a volatile blowing agent are dispersed in water in the presence of a dispersant,
The foaming agent is impregnated into the resin particles by heating to a temperature in the range of -25°C to +10°C from the melting point of the resin particles, and the temperature inside the container is maintained at a pressure higher than the vapor pressure of the volatile foaming agent. A method for producing foamed polyethylene resin particles, which comprises discharging a mixture of the particles and water into an atmosphere at a lower pressure than the inside of a container while maintaining a constant pressure. 3. While maintaining the temperature and pressure within the container constant and also maintaining the partial pressure of the blowing agent in the gas phase within the container constant, the mixture of particles and water is released into an atmosphere at a lower pressure than the inside of the container. A method for producing expanded polyethylene resin particles according to claim 2. 4 When releasing the mixture of the particles and water into an atmosphere at a lower pressure than in the container, the mixture is released through one or more openings having a diameter of 1.2 times or more and 3 times or less of the spherical volume equivalent diameter of the particles. A method for producing expanded polyethylene resin particles according to claim 2.