JPH10237211A - Manufacture of non-crosslinked polyethylene resin prefoamed beads - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture non-crosslinked polyethyene resin prefoamed beads having desired properties only using foaming agents which will not cause global environmental pollution. SOLUTION: Non-crosslinked polyethylene resin beads are dispersed in an aqueous medium in a tightly closed vessel, then an inorganic gas contg. nitrogen is introduced into the vessel to make the pressure in the vessel to 0.1-30kg/cm<2> G. Then the content is heated to not lower than the softening temp of the non- crosslinked polyethylene, and by releasing the content to an atmosphere with a lower pressure than the inner pressure of the tightly closed vessel through an orifice to obtain a low multiplicity foamed beads. These prefoamed beads are obtd. by giving foaming ability to the low multiplicity foamed beads and expanding their volume by heating, while the max. value αmax of circumference stress acting per unit cell membrane thickness is kept not greater than 10 [kg/cm<2> /μm].

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing non-crosslinked polyethylene resin pre-expanded particles. More particularly, the present invention relates to a method for producing non-crosslinked polyethylene resin pre-expanded particles which can be suitably used as a raw material of an in-mold foam molded article.

[0002]

2. Description of the Related Art Conventionally, when producing polyolefin resin pre-expanded particles, the resin particles are dispersed in an aqueous dispersion medium in a closed container,
Further, a method of introducing a volatile organic foaming agent, heating the resin particles to a temperature equal to or higher than the softening temperature of the polyolefin-based resin particles, and releasing the resin particles into an atmosphere at a pressure lower than the internal pressure of the closed container to form the foam is well known. Have been.

As volatile organic blowing agents used by this method, for example, aliphatic hydrocarbons such as propane, butane, pentane and hexane, and halogenated hydrocarbons such as methyl chloride, trichlorofluoromethane and dichlorodifluoromethane are used. It is used.

[0004] However, these volatile organic foaming agents are dangerous due to toxicity and flammability, or cause problems of global environmental pollution such as destruction of the ozone layer when released into the atmosphere. Yes, and some are expensive. Therefore, a production method using carbon dioxide as an alternative foaming agent as a foaming agent overcoming these disadvantages has been proposed (for example, Japanese Patent Publication No. 62-612).
27 publication).

[0005] Certainly, carbon dioxide is inexpensive compared to the volatile organic blowing agent, and has neither toxicity nor flammability,
Although it is relatively suitable as an alternative foaming agent, it can still promote global warming if released into the atmosphere in large quantities, and it has issues related to global environmental pollution, It easily dissolves in an aqueous medium in a closed container and becomes acidic, so that it is necessary to take measures against corrosion of the container, and as a result, there is also a disadvantage that the equipment cost is increased.

Accordingly, a method has been proposed in which a nitrogen-containing inorganic gas typified by air or water is used as a foaming agent (for example, Japanese Patent Publication No. 49-2183, Japanese Patent Publication No. Hei 4 (Kokai)).
-64334).

However, in the case of using a nitrogen-containing inorganic gas or water as a foaming agent in the production of pre-expanded particles of a polyethylene resin, the resin particles absorb and absorb the particles.
Because the amount that can be adsorbed is small, or the vapor pressure of the foaming agent in the range of suitable foaming conditions is low, or the permeability to the resin membrane is large, and the foaming agent rapidly escapes outside the resin particles during foaming. For this reason, only pre-expanded particles having a low expansion ratio of about 5 or less are obtained.

Further, in order to solve such a problem, there is known a method in which low-expansion pre-expanded particles of a polyolefin resin are produced, and the expanded particles are expanded stepwise to obtain expanded particles having a target expansion ratio (FIG. 1). For example, Japanese Patent Publication No. 61-11
253, JP-A-4-372630, etc.).

However, some of these conventional techniques use the above-mentioned volatile organic blowing agent or carbon dioxide as a blowing agent, and others use a crosslinked resin.

In addition, non-crosslinked polyolefin resins generally have a lower cell membrane strength than polystyrene resins and crosslinked polyolefin resins, and when a multi-stage foaming method as described above is used during the production of pre-expanded particles, foaming is not possible. Due to the low speed, the cell membrane breaks during volume expansion, resulting in an increase in the open cell ratio of the pre-expanded particles and a significant decrease in the mechanical strength of the molded article. Therefore, when producing non-crosslinked polyolefin resin pre-expanded particles, it is necessary to clearly define the physical properties of the low-magnification expanded particles and the multi-stage expansion conditions. There is no technology that has solved this problem.

Therefore, in recent years, air and / or
There is a demand for the development of a method capable of producing non-crosslinked polyethylene resin pre-expanded particles having a high expansion ratio and a low open cell ratio by using only a foaming agent which does not cause environmental pollution such as water.

[0012]

DISCLOSURE OF THE INVENTION In view of the above prior art, the present invention provides a method for producing non-crosslinked polyethylene resin pre-expanded particles having desired physical properties using only a foaming agent which does not cause global environmental pollution. It is made in order to provide a method that can be performed, non-crosslinked polyethylene resin particles are dispersed in an aqueous dispersion medium in a closed container, then nitrogen-containing inorganic gas is introduced into the closed container, and After the pressure is adjusted to 0.1 to 30 kg / cm ² G, the mixture is heated to a temperature equal to or higher than the softening temperature of the non-crosslinked polyethylene resin particles, and is discharged through an orifice into an atmosphere having a pressure lower than the internal pressure of the closed container. After obtaining the expanded foam particles, by imparting foaming ability to the low-expanded foam particles, by heating and volume expansion,
A method for producing pre-expanded particles, wherein the maximum value α _max of the circumferential stress acting on a unit cell film thickness of expanded particles during volume expansion by heating is set to 10 [kg / cm ² / μm] or less. The method for producing non-crosslinked polyethylene resin pre-expanded particles (Claim 1), and the orifice used for producing low-magnification expanded particles are circular, and the ratio L / D of diameter D to length L is 1. The present invention relates to a method for producing pre-expanded particles of a non-crosslinked polyethylene resin according to claim 1 which is 5 or more (claim 2).

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, first, non-crosslinked polyethylene resin particles are dispersed in an aqueous dispersion medium in a closed container, and then a nitrogen-containing inorganic gas is introduced into the closed container and the pressure in the closed container is increased. Is adjusted to 0.1 to 30 kg / cm ² G, heated to a temperature equal to or higher than the softening temperature of the non-crosslinked polyethylene-based resin particles, and discharged through an orifice into an atmosphere at a pressure lower than the internal pressure of the sealed container to obtain a low magnification. Expanded particles are produced.

The non-crosslinked polyethylene resin particles generally contain a non-crosslinked polyethylene resin, a filler and optionally used antioxidants, dispersants, lubricants, antistatic agents,
It is formed from a non-crosslinked polyethylene resin composition comprising a dye, a pigment, and the like, and has a size of 0.5 to 1
Particles of 0 mg / grain, more preferably about 0.5 to 5 mg / grain are used.

The non-crosslinked polyethylene resin is usually
The MI is preferably 0.5 to 30 g / 10 min at 190 ° C., more preferably 1 to 5 g / 10 min, and the flexural rigidity (ASTM D747) is 1000 to 8000.
kgf / cm ² , and even 2000-5000 kgf /
cm ² , melting point of 100 to 140 ° C., and further 11
Those having a temperature of 5 to 130 ° C are preferred. The MI is 0.5 g /
In the case of less than 10 minutes, the melt viscosity is too high to obtain pre-expanded particles having a high expansion ratio, and in the case of more than 30 g / 10 minutes, the melt viscosity with respect to the elongation of the resin during foaming is low and the foam is easily broken. This tends to make it difficult to obtain pre-expanded particles having a high expansion ratio. In addition, the flexural rigidity is 1000
When it is less than kgf / cm ² , the mechanical strength of the obtained molded article tends to be insufficient, and when it exceeds 8000 kgf / cm ² , the flexibility and cushioning property of the obtained molded article tend to be insufficient. When the melting point is less than 100 ° C., the heat resistance of the obtained molded body becomes insufficient, and when it exceeds 140 ° C., the molding pressure tends to be high, which is not preferable.

Specific examples of the non-crosslinked polyethylene resin include those having the above properties, such as low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer,
Examples include an ethylene-methyl methacrylate copolymer. These resins may be used alone or in combination of two or more. Among these non-crosslinked polyethylene resins, a density of 0.910 to 0.935 g / cm is particularly preferable.
The straight-chain low-density polyethylene of ³ is preferred from the viewpoint of good foamability.

The filler is a component used to easily form uniform bubbles when foaming the non-crosslinked polyethylene resin particles at a low magnification and then at a high magnification to obtain pre-expanded particles. . In particular, it affects the expansion ratio and average cell diameter of the low-expansion expanded particles, and affects the value of the circumferential stress acting per unit cell thickness through these physical properties. This will be described later.

The average particle diameter of the filler is preferably 50 μm or less, from the viewpoint of producing pre-expanded particles having uniform cells and producing a molded article having excellent mechanical strength and flexibility from the pre-expanded particles. It is preferably 20 μm or less. The lower limit is preferably 0.1 μm or more from the viewpoint of prevention of poor dispersion due to secondary aggregation and workability.

The amount of the filler is usually 0.001 part or more, preferably 0.005 part or more, based on 100 parts (parts by weight, the same applies hereinafter) of the non-crosslinked polyethylene resin. In molding, from the viewpoint of exhibiting excellent fusibility and obtaining a molded article excellent in mechanical strength and flexibility from the pre-expanded particles, 10 parts or less based on 100 parts of the non-crosslinked polyethylene resin, Above all, less than 5 parts.

The filler is roughly classified into an inorganic filler and an organic filler. Specific examples of the inorganic filler include talc, kaolin, clay, mica, calcium carbonate, and the like.
Calcium hydroxide and the like. These inorganic fillers may be used alone or in combination of two or more. Among these inorganic fillers, talc is relatively inexpensive, and low-magnification pre-expanded particles having uniform air bubbles are easily obtained. Therefore, the talc can be particularly preferably used in the present invention.

The organic filler can be used as long as it is in the form of a solid at a temperature not higher than the heating temperature at the time of foaming (the softening temperature of the non-crosslinked polyethylene resin + 50 ° C.). There is no particular limitation as far as there is. Specific examples of the organic filler include a fluororesin powder, a silicone resin powder, and a thermoplastic polyester resin powder. These organic fillers may be used alone or in combination of two or more.

The non-crosslinked polyethylene resin composition containing the non-crosslinked polyethylene resin and a filler, etc.
Usually, the mixture is melt-kneaded using an extruder, a kneader, a Banbury mixer, a roll, or the like, and then formed into a desired particle shape that can be easily used for foaming, such as a columnar shape, an elliptical columnar shape, a spherical shape, a cubic shape, and a rectangular solid shape.

The nitrogen-containing inorganic gas is a mixture of nitrogen, air, and nitrogen and an inorganic gas (for example, helium, argon, etc.) within a range that does not pollute the global environment. From the viewpoint of not polluting the environment, nitrogen or air is preferably used.

In the present invention, the non-crosslinked polyethylene resin particles are dispersed in an aqueous dispersion medium in a closed container, and a nitrogen-containing inorganic gas is introduced into the closed container until the heating is started. After the pressure is reduced to about 30 kg / cm ² G, the non-crosslinked polyethylene resin particles can be heated to a temperature higher than the softening temperature thereof. The nitrogen-containing inorganic gas can be sufficiently absorbed and adsorbed in addition to the dispersion medium, and the expansion ratio of the obtained low-expansion expanded particles can be controlled by these two types of blowing agents.

When the non-crosslinked polyethylene resin particles are dispersed in the aqueous dispersion medium in the closed container, as a dispersing agent, tribasic calcium phosphate, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, etc. Agents such as sodium dodecylbenzenesulfonate, n-
Sodium paraffin sulfonate, sodium α-olefin sulfonate and the like can be used in commonly used amounts.

As the aqueous dispersion medium, water is a typical example, but acetone, ethylene glycol, ethyl alcohol, t-butyl and the like may be used as necessary as long as they do not affect the global environment. It may contain a small amount of alcohol, glycerin and the like.

The pressure in the closed vessel after the introduction of the nitrogen-containing inorganic gas is determined by the pressure difference between the inside and the outside of the closed vessel at the time of release. Generally, the larger the pressure difference, the larger the expansion ratio of the expanded particles. 0.1-30 kg / cm
² G, and more preferably 1.0 to 30 kg / cm ² G. 30
When the pressure is applied in excess of kg / cm ² G, the internal pressure of the closed container after heating becomes too high, which puts an excessive load on the container and reduces the effect of improving the expansion ratio by pressurizing the container, which is not preferable. . Further, in order to control the expansion ratio of the obtained low-expansion expanded particles, it is preferable to adjust the release pressure with a nitrogen-containing inorganic gas immediately before release.
Release, in general, the softening temperature or more non-crosslinked polyethylene-based resin particles, carried out at the melting point + 20 ° C. or less, the pressure difference between the sealed container and out is generally 5 to 50 kg / cm ² G, more 6
It is performed at about 35 kg / cm ² G.

The softening temperature and melting point of the non-crosslinked polyethylene resin particles are usually determined by the fact that the particles are composed of a non-crosslinked polyethylene resin and a filler that is incompatible with the resin. Are substantially the same as the softening temperature and melting point of

In the process of releasing the non-crosslinked polyethylene resin particles from the closed vessel together with the aqueous medium and the nitrogen-containing inorganic gas, the pressure in the closed vessel increases as the volume fraction of the gas phase in the closed vessel increases due to the release. During the discharge, the pressure corresponding to this pressure drop is replenished by introducing a nitrogen-containing inorganic gas, etc., and the pressure drop during the discharge is reduced. It is preferred that the expansion ratio distribution of the low-expansion expanded particles be sharpened.

When the orifice used for producing the low-magnification expanded particles is a circular orifice, the ratio L / D of the diameter D to the length L is 1.5 or more, and more preferably 1.5 or more and 50 or less. Is to increase the average cell diameter compared to the expansion ratio,
Since the cell film can be made thicker, it is preferable in that it has the effect of reducing α _max under the same heating volume expansion condition.

In the present invention, pre-expanded particles are produced by imparting expandability to the thus-obtained low-magnification expanded particles and then heating.

The provision of the foaming ability means that the low-magnification expanded particles are preliminarily treated so as to easily cause volume expansion by heating. The low-magnification expanded particles are pressurized with a nitrogen-containing inorganic gas or the like in a pressure vessel. To increase the internal pressure of the low-magnification expanded particles. Specifically, the low-magnification expanded particles are mixed with a nitrogen-containing inorganic gas in a pressure vessel at 1 to 30 kg / cm.
² G, further pressurized at a pressure of 1 to ²⁰ kg / cm ² G,
This means increasing the internal pressure of the low-magnification expanded particles to about 1 to 20 kg / cm ² G, and more preferably about 1 to 15 kg / cm ² G. Pressing may be performed in a room temperature atmosphere, but when performed in a heating atmosphere within a range that does not affect the crystal characteristics of the non-crosslinked polyethylene resin composition, the internal pressure of the low-magnification expanded particles quickly reaches an equilibrium pressure. This method may be used.

The heating method is conventionally known, and is not particularly limited as long as there is no concern about global environmental pollution. Examples thereof include steam heating, hot air heating, infrared heating, and ultrasonic heating. Of these, steam heating is preferably used in terms of equipment cost and heating efficiency.

The high magnification by heating volume expansion in the present invention, the maximum value of circumferential stress acting on the unit cell layer per thickness of the expanded beads in a volume expansion by heating alpha _max is 10 [kg /
cm ² / μm] or less, preferably 7 [kg / cm ² / μ
m] or less, more preferably 5 [kg / cm ² / μm]
It is performed under the following conditions.

As long as the α _max is 10 [kg / cm ² / μm] or less, the magnification may be increased by the above-mentioned overheated volume expansion any number of times. Meanwhile, α _max is 10 [kg / cm ² / μm]
It is difficult to:

Next, the maximum value α _max of the circumferential stress acting on the unit cell film thickness will be described.

The volume expansion of the low-magnification expanded particles due to heating is as follows:
Usually, it is widely used as a method for producing polystyrene-based resin pre-expanded particles or crosslinked polyolefin-based resin pre-expanded particles, but is used as a method for producing non-crosslinked polyolefin-based resin pre-expanded particles, particularly non-crosslinked polyethylene-based resin pre-expanded particles. In this case, the cell membrane constituting the cell is more likely to be broken by tensile deformation during the heating volume expansion than the case of the pre-expanded particles of polystyrene resin and the pre-expanded particles of cross-linked polyolefin resin. The open cell ratio of the expanded particles is increased, and as a result, the mechanical strength of the obtained molded article is liable to be remarkably reduced. Therefore, it is necessary to delicately control the physical properties of the low-magnification expanded particles subjected to the heating volume expansion, the degree of imparting the foaming ability, the degree of heating, and the like. However, conventionally known techniques are based on the premise that some of these factors are fixed, and some only disclose the conditions independently regarding the low-magnification expanded particle physical properties, and some only regarding the degree of heating. Absent. In the present invention, a new theory that takes into account all the factors for suppressing cell membrane breakage during such heating volume expansion is newly constructed, and the case where the theory is applied to the case of a non-crosslinked polyethylene resin will be described below. .

(Calculation of α _max ) Although the cells of the foam are not strictly spheres, they are usually regarded as spheres, and the cell size is indicated by the average cell diameter. Assuming that all cells of the foam are spherical, the formula:

[0039]

(Equation 1)

(Where V _s is the volume fraction of the resin (composition), ρ _f , ρ _g , and ρ _s are the foam, the gas in the cell,
Resin density, d is the diameter of a sphere as a cell, t is the cell thickness)
("Plastic Form Handbook" Nikkan Kogyo Shimbun, February 28, 1973, first edition, 222
page).

In the equation, if V _s ≒ ρ _f / ρ _s , the cell thickness t is given by the following equation:

[0042]

(Equation 2)

(Where K is the expansion ratio (= ρ _s / ρ _f ≒ 1)
/ V _s ), and d is the same as described above).

The average film thickness σ _m of the circumferential stress applied to the thin spherical shell subjected to the internal pressure is given by the following equation:

[0045]

(Equation 3)

(Where P is the pressure (gauge) in the cell, d,
t is the same as described above) (edited by the High Pressure Gas Safety Association,
"Class K Machinery Manufacturing Security Officer Training Text High-Pressure Gas Safety Technology" April 8, 1994, Revised Edition, 3rd edition, 140
page). Thus, from the equation, the equation, σ _m is:

[0047]

(Equation 4)

## EQU5 ## Further, assuming that there is no escape of gas in the cell during the two-stage foaming, the following equation is obtained: K (P + 1) = const. Holds.

Further, it is assumed that the rise of the heating temperature in the initial stage of volume expansion due to heating is step-like, and that the temperature inside the expanded particles becomes a desired temperature simultaneously with the start of heating.
Internal pressure (gauge) P of expanded particles at the start of volume expansion by heating
_in is the formula:

[0050]

(Equation 5)

(Where P ₀ is the internal pressure of the expanded beads immediately after the start of heating after application of the foaming ability (room temperature; gauge), T _s is the heating temperature (° C.), and Tr is room temperature (° C.)).

When the expansion ratio K increases with the volume expansion of the foamed particles, P _in is obtained from the formula, and the internal pressure P of the foamed particles at that time is obtained from the formula, and σ _m is obtained from the formula. The cell diameter is obtained from the product of the cell diameter of the expanded foam particles and the volume expansion ratio to the 1/3 power, and t is obtained from the formula.

Here, the circumferential stress α acting per unit cell film thickness is defined as in the following equation.

Α = σ _m / t α can be determined for the expansion ratio at any time during the heating volume expansion by the above-described procedure, and various parameters are appropriately set and α is set to K When plotted against, a behavior is shown in which the value is maximized at a specific expansion ratio and then decreases. Let the maximum value of α be α _m . Therefore, α _max = α _m in a state having a particularly good volume expansion property. However, there is a case where the volume expansion stops before α becomes maximum as K increases. This case, the volume expansion is stopped (is finally e) with the calculated value alpha in the expansion ratio of the pre-expanded particles to alpha _max.

The relationship between α _max calculated as described above and the open cell ratio of the pre-expanded particles obtained was summarized.
It was found that there was a clear positive correlation between the two (FIG. 1). That is, as the maximum value of the circumferential stress acting on the unit cell film thickness during the heating volume expansion is larger, the number of sites where the cell film breaks during the heating volume expansion increases, and the open cell rate of the obtained pre-expanded particles is higher. It became clear that it would be. α _max is calculated for the first time when the expansion ratio and cell diameter are used as physical properties of the low-magnification expanded particles; room temperature is used as the experimental environment; the internal pressure of the expanded particles immediately before heating is used as the degree of foaming capability; It is possible to That is, it has been clarified that the closed cell structure of the obtained pre-expanded particles is maintained only when all the above factors are appropriately set.

Then, for the low-magnification expanded particles of the non-crosslinked polyethylene resin, the pressure unit is kg / cm ² , the cell diameter and the cell thickness are μm, and σ _m [kg / c
m ² ], t [μm], α was obtained by the equation, and the relationship between α _max and open cell rate was plotted on a graph according to the above-described procedure. As a result, the open cell rate of the pre-expanded particles was 20% or less. It has been found that it is necessary to set α _max to 10 [kg / cm ² / μm] or less in order to prevent a decrease in mechanical strength after molding.

As described above, since α _max is determined by many factors related to heating volume expansion, for example, only the expansion ratio and cell diameter of the low-expansion expanded particles suitable for this are independent of other conditions. I can't decide,
To further clarify the present invention, "Ultzex 2022L" (density 0.920 g / cm ³ ) manufactured by Mitsui Petrochemical Industries, Ltd. was used as a non-crosslinked polyethylene resin.
The internal pressure of the foamed particles immediately before heating is 5 [atm (abs)], and the heating volume expansion is 0.8 kg / cm ² G steam (# 116
℃), when α _max is 10 [kg /
The expansion ratio and the cell diameter range of cm ² / [mu] m] become low magnification expanded particles, shown in FIG. 2 for the case of changing the room temperature 5 ° C. and 30 ° C.. In the present invention, the range in which the heating volume expansion property is good and the open cell ratio of the pre-expanded particles is low,
It is on the upper side of the curve shown in FIG. As described above, for the first time, the range disclosed in the present invention defines the range of the physical properties of the expanded low-expanded particles having a good heating volume expansion property quantitatively and clearly. In addition, when the low-expanded foamed particles having an expansion ratio of 10 are expanded by heating and volume expansion due to a change in room temperature, the winter time (room temperature
(° C.), the cell diameter needs to be about 300 μm or more, whereas in summer (room temperature 30 ° C.), the cell diameter may be about 260 μm or more. Thus, the seasonal difference in the heating volume expansion property can be explained.

When the heating volume expansion is repeated several times, if the first heating volume expansion does not cause cell union and the number of cells does not change, if the heating volume expansion does not change, Cell diameter is 1 / the volume expansion ratio
It will be proportional to the cube. As shown in FIG. 2, in the technique disclosed in the present invention, the lower limit curve of the range preferably used is proportional to the expansion ratio to about the 2.1 power, so that the curve before the first heating volume expansion is obtained. Even if the pre-expanded particles are in the range of physical properties suitably used in the present invention, they may not be suitable for re-heating volume expansion after heating volume expansion. This means that, for example, under the conditions used in the preparation of FIG. 2, the low-expanded foamed particles having a foaming ratio of 5 times and a cell diameter of 100 μm sufficiently within the range of the present invention are heated and expanded in volume, and the volume is not expanded. Can be understood from the fact that the expansion ratio becomes 10 times and the cell diameter becomes 126 μm, which is not suitable for the heating volume expansion again.

As is clear from the above description, when the heating volume expansion condition is determined, and when the expansion ratio of the low-magnification expanded particles as the raw material is constant, the larger the average cell diameter, the larger the heating ratio. Α _max during volume expansion becomes smaller. In order to increase the average cell diameter, any of conventionally known methods such as adjustment of the type and amount of the filler to be added can be applied, but if such a formulation is changed, the present invention can be obtained. In the present invention, a circular orifice is used as the orifice used in the production of the low-magnification expanded particles, since the surface properties, the cushioning properties, the mechanical strength, etc., of the molded body produced using the pre-expanded particles may be affected. Preferably, the ratio L / D of the diameter D to the length L is preferably 1.5 or more. As described above, when a circular orifice having a large value of L / D is used, particularly when water and / or a nitrogen-containing inorganic gas is used as a foaming agent as in the present invention, the pressure is released. Although the foaming ratio is the same or slightly lower because of the compatibility between the speed and the dissolution of the foaming agent from the resin and the vaporization rate, the effect of increasing the average cell diameter is particularly large, and as a result, it is used as a raw material for heating volume expansion. Since more suitable low-magnification expanded particles can be obtained, the range of the heating volume expansion condition is widened, or the open cell ratio of the pre-expanded particles under the same conditions is reduced, which is preferable.

[0060]

Next, the method of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to only these Examples.

The expansion ratio, average cell diameter and open cell ratio of the low-ratio expanded particles and the pre-expanded particles in the following examples were determined by the following methods.

(Expansion Ratio) Take about 3 to 10 g of expanded particles, precisely weigh the weight w, immerse them in an ethanol solution in a 100 ml measuring cylinder, measure the volume v from the meniscus before and after immersion, and determine the true volume of the expanded particles. The specific gravity ρ _b = w / v is determined, and the expansion ratio K = ρ _{r is determined} from the ratio with the density ρ _r of the raw resin composition.
/ Ρ _b was determined.

(Average Cell Diameter) Thirty cells were arbitrarily taken out of the expanded particles, the cell diameter was measured in accordance with ASTM D3576, and the average cell diameter d was calculated.

(Open cell ratio) The closed cell volume of the obtained expanded particles was determined using an air-comparison hydrometer (model 1000, manufactured by Tokyo Science Co., Ltd.), and the volume was determined separately by the submersion method. It was calculated by subtracting the closed cell volume and dividing by the apparent volume.

Example 1 Linear low-density polyethylene ("Ultzex 3021F" manufactured by Mitsui Petrochemical Industries, Ltd., density 0.930 g / c)
m ³ , MI 2.1 g / 10 min, flexural rigidity 4000 k
gf / cm ² , softening temperature 112 ° C, melting point 122 ° C) 10
0 parts of talc (Hayashi Kasei Co., Ltd. “TA
LCAN POWDER PKZ ”, average particle size about 10μ
m) The mixture was fed together with 0.12 parts to a single screw extruder for mixing (50φ), extruded from a cylindrical die having a diameter of 2φ, and the obtained cylindrical strand was cooled with water and then cut to obtain a columnar non-crosslinked polyethylene resin pellet ( 1.8 mg / pellet, softening temperature 112 ° C, melting point 122 ° C).

Next, 100 parts of the obtained pellets together with 300 parts of water, 4.0 parts of tribasic calcium phosphate and 0.8 parts of n-paraffin sodium sulfonate were put into a pressure-resistant closed vessel (internal volume 10 L). At room temperature (22 ° C.), the pressure was increased to 15 kg / cm ² G by nitrogen gas. Thereafter, when the mixture was heated to the foaming temperature of 125 ° C., the internal pressure of the sealed container became about 22 kg / cm ² G. Further, nitrogen gas was added to adjust the internal pressure of the closed vessel to 35 kg / cm ² G, and then discharged into a normal pressure atmosphere through a circular orifice having a diameter of 4 × 60 L (L / D = 15) to obtain low-magnification expanded particles. During the discharge, nitrogen gas was appropriately introduced and the pressure was maintained so that the internal pressure of the container did not decrease. The obtained low-magnification expanded particles had an expansion ratio of 2.35 and an average cell diameter of 96 μm.
Having a closed cell structure.

Next, the obtained low-magnification foamed particles were transferred into another pressure-resistant sealed container, and then exposed to air at a temperature of 60 ° C. and air.
The pressure was increased to kg / cm ² G, and the mixture was allowed to stand for 3 hours to impart foaming power to the low-magnification foamed particles. After cooling for about 1 hour, release the pressure, leave at room temperature and normal pressure, and measure the internal pressure of the foamed particles beforehand measured at 5 atm (abs) (about 3 minutes).
After that, it is supplied to another pressure vessel, and after sealing, steam (0.8
kg / cm ² G (≒ 116 ° C.)) for about 30 seconds,
Heat to volume expansion. At this time, the room temperature was 22 ° C.
The obtained pre-expanded particles contracted after the completion of the heating volume expansion. Therefore, the air was further pressurized in a pressure vessel at room temperature and 8 kg / cm ² for about 18 hours so that the surface of the expanded particles was free of wrinkles, and then left at normal pressure. Then, while the internal pressure of the particles was stable, the expansion ratio, cell diameter and open cell ratio were measured. As a result, the pre-expanded particles had an expansion ratio of 6.2 times and an average cell diameter of 145.
The pre-expanded particles had a uniform cell structure of 1.1 μm and an open cell ratio of 1.1%. Under the above conditions, the maximum value α _max of the circumferential stress acting per unit cell film thickness was calculated. As a result, α _max = 1.30 [kg / cm ² / μm].

Example 2 The water vapor pressure during heating volume expansion was set to 1.1 kg / cm ² G ( ²
Except that the temperature was changed to 121 ° C., low-expanded expanded particles and pre-expanded particles were produced in the same manner as in Example 1. The obtained pre-expanded particles had an expansion ratio of 12.5 times and an average cell diameter of 159 μm.
m, the open cell rate is 0.8%, and α _max =
It was 1.36 [kg / cm ² / μm].

Example 3 A non-crosslinked polyethylene resin was replaced with linear low-density polyethylene ("Ultzex 2022" manufactured by Mitsui Petrochemical Industries, Ltd.).
L ", density 0.920 g / cm ³ , MI 2.1 g / 1
0 min, flexural rigidity 3000 kgf / cm ² , softening temperature 1
02 ° C, melting point 120 ° C) and low-expanded foamed particles and pre-expanded particles were produced in the same manner as in Example 1 except that the foaming temperature for producing the low-expanded foamed particles was changed to 124 ° C. The obtained non-crosslinked polyethylene resin pellets had a softening temperature of 102 ° C and a melting point of 120 ° C. The obtained low-magnification expanded particles had an expansion ratio of 3.81 times and an average cell diameter of 97 μm.
m, pre-expanded particles: expansion ratio 23.4 times, average cell diameter 1
74 μm, open cell rate 9.8%, α calculated
_max = 3.74 [kg / cm ² / μm].

Example 4 Low magnification expanded particles and pre-expanded particles were produced in the same manner as in Example 3 except that the amount of talc added was changed to 0.1 part.
The obtained non-crosslinked polyethylene resin pellets had a softening temperature of 102 ° C and a melting point of 120 ° C. The obtained low-magnification expanded particles had an expansion ratio of 3.0 times and an average cell diameter of 80 μm.
m, pre-expanded particles: expansion ratio 24.8 times, average cell diameter 1
63 μm, open cell rate 4.5%, α calculated
_max = 2.73 [kg / cm ² / μm].

Example 5 Examples 3 and 4 were repeated except that the amount of talc added was changed to 1.0 part.
In the same manner as described above, low-magnification expanded particles and pre-expanded particles were produced. The obtained non-crosslinked polyethylene resin pellets had a softening temperature of 102 ° C and a melting point of 120 ° C. The obtained low-expanded foamed particles had an expansion ratio of 3.4 times and an average cell diameter of 37 μm, and the pre-expanded particles had an expansion ratio of 20.9 times, an average cell diameter of 78 μm and an open cell ratio of 17.9%, and were calculated. α _max = 7.72 [kg / cm ² / μm].

Comparative Example 1 The same procedure as in Example 5 was repeated except that the circular orifice used for producing the low-magnification expanded particles was 4φ × 5 L (L / D = 1.25). Pre-expanded particles were produced. The obtained low-expansion foamed particles have an expansion ratio of 3.
6 times, average cell diameter 26 μm, pre-expanded particles 2
It was 3.4 times and the average cell diameter was 63 μm.
8.2%, which was not a good pre-expanded particle. Also, the calculated α _max = 12.42 [kg
/ Cm ² / μm].

Comparative Example 2 The pre-expanded particles obtained in Example 3 (expansion ratio 2
(3.4 times, average cell diameter 174 μm, open cell rate 16.8%)
And the steam pressure is 0.8 kg / cm ² G (Ｇ1
16 ° C.) and another heating volume expansion was performed. The obtained pre-expanded particles had an expansion ratio of 36.9 times and an average cell diameter of 3
Although it was 64 μm, the open cell ratio was 50.7%, and it could not be said to be good pre-expanded particles. Further, α _max = 78.08 [kg / cm ² / μm] obtained by calculation.

FIG. 1 shows the correlation between the open cell ratio of the pre-expanded particles obtained in each Example and Comparative Example and α _max obtained by calculation. There is a strong positive correlation between α _max and the open cell rate of the pre-expanded particles. To obtain pre-expanded particles having an open cell rate of 20% or less, α _{max is set} to 10 [kg / cm ² / μ].
m] or less.

[0075]

According to the present invention, a volatile organic blowing agent such as a conventional aliphatic hydrocarbon such as butane or a halogenated hydrocarbon, and a blowing agent such as carbon dioxide are not used.
Non-crosslinked polyethylene resin pre-expanded particles having a high closed cell ratio and a small variation in magnification can be obtained. In addition, since flammable gas such as butane is not used, there is no danger of fire or explosion.Halogenated hydrocarbons such as chlorofluorocarbons are not used. There is no fear of destruction of the global environment such as global warming. Further, unlike the foaming agent, the non-crosslinked polyethylene resin particles can be highly foamed with only a nitrogen-containing inorganic gas such as air and water, so that equipment costs, variable costs, and the like can be significantly reduced, which is also advantageous in terms of economy. It is.

[Brief description of the drawings]

FIG. 1 is a graph showing the correlation between the open cell ratio of pre-expanded particles obtained in each of Examples and Comparative Examples and α _max obtained by calculation.

FIG. 2 is an illustration of “Ultzex 2022L” manufactured by Mitsui Petrochemical Industries, Ltd. as a non-crosslinked polyethylene resin.
(Density 0.920 g / cm ³ ), the internal pressure of the expanded beads immediately before heating was 5 [atm (abs)], and the heating volume expansion was 0.1 mm.
A graph showing the relationship between the expansion ratio and cell diameter of low-expansion expanded particles having an α _max of 10 [kg / cm ² / μm] when performed using 8 kg / cm ² G steam (Ｇ116 ° C.). It is.

Claims (2)

[Claims] A non-crosslinked polyethylene resin particle is dispersed in an aqueous dispersion medium in a closed container, and a nitrogen-containing inorganic gas is introduced into the closed container to reduce the pressure in the closed container to 0.1 to 1.
After heating to a temperature equal to or higher than the softening temperature of the non-crosslinked polyethylene resin particles after releasing to 30 kg / cm ² G, the non-crosslinked polyethylene resin particles are discharged through an orifice into an atmosphere at a pressure lower than the internal pressure of the closed container to obtain low magnification expanded particles. A method of producing pre-expanded particles by imparting foaming ability to the low-magnification expanded particles and expanding the volume by heating, wherein the maximum of the circumferential stress acting per unit cell film thickness of the expanded particles during the heating volume expansion. Value α
_A method for producing non-crosslinked polyethylene resin pre-expanded particles, wherein _max is 10 [kg / cm ² / μm] or less. 2. The non-crosslinked polyethylene resin according to claim 1, wherein the orifice used for producing the low-magnification expanded particles has a circular shape, and the ratio L / D of the diameter D to the length L is 1.5 or more. A method for producing expanded particles.