JPH03103449A - Foamed low-density polyethylene resin and production thereof - Google Patents

Abstract

PURPOSE:To obtain the subject foamed article having excellent processability and high quality and free from cavities by foaming a specific low-density polyeth ylene resin. CONSTITUTION:The low-density polyethylene resin(LDPE) to be used as a raw material for the subject resin has a density of 0.922-0.926g/cm<3>, a weight-average molecular weight of >=90,000 and a crystallization rate index A of <=10 {expressed by formula wherein a and b are the base length and the height of a triangle corresponding to a peak on a DSC curve measured by cooling 5+ or -0.1mg of the LDPE to 25 deg.C after heating to 180 deg.C at a heating rate of 10 deg.C/min and recorded at an amplifier range of 25mJ/sec and a chart speed of 10mm/min)} and is produced by blending two or more kinds of low-density polyethylenes having a crystallization temperature difference of >=3 deg.C. The LDPE is incorporated with a foaming agent such as propane, melted and kneaded in an extruder at 150-250 deg.C under 50-300kg/cm<2>G, temporarily maintained at 90-120 deg.C under 20-100kg/cm<2>G and extruded into a low-pressure atmosphere to obtain the objective foamed board having a thickness of 2-20cm and a density of 0.02-0.06g/cm<3>.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a low-density foam made from a low-density polyethylene resin and a method for producing the same.

[Conventional technology]

In general, polyolefin resin foams have the characteristics of good oil resistance, low compression set, and can withstand repeated compression, as well as other foam moldings such as excellent impact resistance and the fact that the hollow body does not break due to impact. It has excellent characteristics not found in other materials and is used as an important packaging material. In particular, low-density foams produced by extruding low-density polyethylene resins together with foaming agents have been widely used as cushioning packaging materials.

[Problem to be solved by the invention]

However, extruded foams made from low-density polyethylene resins may have problems with compressive strength and workability depending on the type of raw material, and when using raw materials that produce foams with good workability, However, there was a problem in that particles (i.e., giant bubbles) were likely to occur. In particular, particles inside the foam are difficult to discover, which poses an important problem in manufacturing.

Therefore, an object of the present invention is to provide a low-density polyethylene resin foam that has good processability and does not generate grains, and a method for producing the same.

[Means to solve the problem]

As a result of extensive research in order to solve the problems seen in the conventional technology, the inventors of the present invention have found that by using low-density polyethylene resin as a raw material, which has a high molecular weight, a fairly high density, and a slow crystallization rate. The inventors have discovered that the above object can be achieved, and have arrived at the present invention.

That is, according to the present invention, the density is 0.922 to 0.926.
g/d, a weight average molecular weight of 90,000 or more and a crystallization rate index A defined below of lo or less. is provided.

The crystallization rate index A was determined by differential scanning calorimetry using 5±0.1 mg of low density polyethylene resin at 10°C.
Draw a triangle from the peak on the DSC curve obtained when the temperature is raised to 180°C at a rate of 7 minutes and then lowered to 25°C at a rate of 10'Cl minutes, and the base is a and the height is b. In this case, it can be obtained from the following formula.

A = b / a However, the amplifier range for differential scanning calorimetry is 25 m
J/sec and chart speed is 10mm/min. Furthermore, according to the present invention, the density is 0.922 to 0.926 g.
/d, weight average molecular weight is 90,000 or more and claims (
A low-density polyethylene resin having a crystallization rate index A of 10 or less, which is defined in the same manner as in 1), is melt-kneaded together with a foaming agent at high temperature and high pressure, and then the obtained melt-kneaded product is once mixed under conditions that do not cause foaming. Provided is a method for producing a low-density polyethylene resin foam, which is characterized by holding the foam at a low pressure and then discharging it under low pressure.

According to research conducted by the present inventors, foams made from low-density polyethylene resin (hereinafter simply referred to as LDPE) have low compressive strength when low-density LDPE is used as a raw material, and low-molecular-weight LDPIE foams have low compressive strength. It was found that processability was poor when used as a raw material. Therefore, as raw material LDPE,
High density and high molecular weight are desirable.

However, in terms of production, LDPE generally tends to have a low molecular weight when high density is desired, and low density when high molecular weight is desired. Therefore, LDPE with high density and high molecular weight is not found in commercial products. Among commercially available products, it is possible to select one with the highest possible density and high molecular weight. However, such LDPs with high density and high molecular weight tend to
All of the commercially available products E have a fast crystallization rate and are not satisfactory in that respect.

The cause of fog inside the foam is that crystallized substances are generated in the extruder and are formed as nuclei, which develop into giant bubbles. Foaming is performed by lowering the temperature to just below the crystallization temperature and extruding. If the temperature is higher than that, the resin will be soft and the cells will not be fixed and will shrink. On the other hand, if the temperature is lower than the above-mentioned temperature, crystallization occurs, and hard lumps and giant bubbles (i.e., particles) are generated.

Further, the resin temperature within the extruder is not uniform in the strict sense, and varies by about ±0.5°C during continuous operation. Therefore, in the case of a resin that has a high crystallization rate and a suitable foaming temperature range of only about 0.5° C., it is estimated that a portion of the resin will fall below the crystallization temperature and a crystallized product will be generated.

For the above reasons, in the present invention, LDPE having a specific density, molecular weight, and crystallization rate index is used as a raw material. Although this raw material LDPE cannot be produced by devising a polymerization method, it is not possible to produce it as a commercially available product. It is expensive because it is not inside. Therefore, it is usually obtained by blending two or more types of low density polyethylenes having different crystallization temperatures.

When two types of low density polyethylenes having different crystallization temperatures are blended, an LDPE having a crystallization temperature intermediate between the two and a slow crystallization rate can be obtained. In this case, the difference in crystallization temperature of the raw material low density polyethylene to be blended is preferably 3°C or more, particularly preferably 5°C or more. Of course, the blend may be of three or more types, and the mixing ratio is preferably such that a large amount of raw materials with a small crystallization rate index is used.

The raw material low-density polyethylene to be blended is a branched or linear polymer mainly composed of ethylene and has a density of 0.920 to 0.930 g/aJ at normal temperature and pressure. Also included are acrylates and copolymers with propylene, butene, pentene, hexene, and other vinyl monomers that can be copolymerized with ethylene.

In the present invention, low density ethylene has a density of 0.922
~0.926g/cd, preferably 0.923-0.9
26g/a#, particularly preferably 0.924-0.926
g/al.

When the LDPE is a blended product as described above, the average density after blending may be within the above range.

Usually, the density of low density polyethylene is O. fl10-0.
Since it is 930 g/ad, it can be said that the raw material LDPE of the present invention tends to have a high density. Since the LDPE foam has high density as a raw material, the LDPE foam of the present invention has excellent compressive strength and creep properties. Raw material LDP
If the density of H is less than 0.920 g/ad, the foam will have low compressive strength.

Furthermore, the LDPE in the present invention has a weight average molecular weight of 9.
0,000 or more, preferably 120,000 or more. When the LDPE is a blended product as described above, the weight average molecular weight after blending may be within the above range. Since high molecular weight LDPE is used as a raw material, the LDPE foam of the present invention has excellent processability (particularly punching processability) and brittleness resistance. If the weight average molecular weight of the raw material LDPH is less than 90,000, the foam will have poor processability.

Furthermore, the LDPE in the present invention has a slow crystallization rate,
That is, the crystallization rate index is 10 or less, preferably 8.
or less (for blended products, the value after blending is 10 or less,
preferably 8 or less). L with slow crystallization rate
Since DPE is used as a raw material, the LDPE foam of the present invention does not generate viscosity. If the crystallization rate index of LDPH exceeds 10, the foam becomes electrically charged.

In addition, the crystallization rate index in this case is determined by differential scanning calorimetry when LDPES±0.1 mg is heated at 10°C.
Draw a triangle from the peak on the DSC curve obtained when the temperature is raised to 180°C at a rate of 10°C/min and then lowered to 25°C at a rate of 10°C/min, the lower side is a, and the height is b. In this case, A can be found by the following formula. A=b/a However, the amplifier range in differential scanning calorimetry is 25mJ
/sec and the chart speed is 10 mm/min.

To explain this measurement method using drawings, in Fig. 1, the curve shows that 5 ± 0.1 mg of LDPH was heated to 180°C at a heating rate of 10°C for 7 minutes using a differential scanning calorimeter, and then 10°C/min. This is the DSC curve obtained when measuring the temperature by decreasing the temperature to 25 ° C at a rate of
The crystallization rate index A is determined as the b/a value.

In addition, from the viewpoint of the product shape, the melt index (hereinafter simply referred to as MI) of the raw material LDPE is 1.0 to 3.5 g/1.
It is desirable that the value be in the range of 0 minutes (in the case of blended products, it is desirable that the value after blending be within this range).
When using a product with an MI of less than 1.0 g/10 minutes,
During the foaming process, the flow at both ends is poor, resulting in folds, and conversely, 3
．． If more than 5 g/10 minutes is used, the foam will flow too well during the foaming process and the width and thickness will be small. Note 5
The MI referred to in this) is measured by the method described in JIS K6760.

The LDPE foam of the present invention is produced by melt-mixing the LDPE with a blowing agent under high temperature and high pressure, then holding the obtained melt-kneaded product under conditions that do not cause foaming, and then discharging it under low pressure. obtained by doing.

Therefore, in the production method of the present invention, a foam is produced using a foaming device having a structure in which an extruder is combined with an accumulator. That is, in a pressurized cylinder of an extruder, LDP E and a blowing agent are heated under conditions of a temperature of 150 to 250°C and a pressure of 50 to 300 kg/cnfG, together with an anti-shrinkage agent and other additives as necessary. The mixture is melt-kneaded to obtain a foamable melt-kneaded product. Next, the melt-kneaded product was heated at a temperature and pressure (90 to 120 °C and 2
0 to 100 kg/cJG) into an accumulator. Then, the obtained extruded melt is intermittently discharged under low pressure from a die orifice provided at the tip of the accumulator by the pressure of a movable ram at a discharge rate of more than g per second and a cross-sectional area of the die orifice of 1 cm2. The resulting foam, which can be foamed and molded into any shape to obtain a low density foam, typically has a thickness of 2-20c+
m, and is formed into a plate shape with a density of 0.02-0.06 g/a.

As the blowing agent used in the present invention, fatty acid hydrocarbons, halogenated hydrocarbons, or fluorocarbon gases are used alone or in combination. Specific examples of aliphatic hydrocarbons include propyne, butane, isobutane, pentane, isopentane, etc., and examples of halogenated hydrocarbons include chlorine- or bromine-substituted products of these aliphatic hydrocarbons.

In addition, as chlorofluorocarbon gas, chlordifluoromethane,
Trifluoromethane, 1,2,2,2-tetrafluoroethane, 1-chloro-1,1-difluoroethane, 1.
1-cyfluoroethane, 1-chloro-1.2,2.2-
Tetrafluoroethane, trichlorofluoromethane, dichlorodifluoromethane, 1,2-dichloro-1.2,
Examples include 2.2-tetrafluoroethane. These can be used alone or mixed for at least 2 seconds. The amount of blowing agent used is usually 2 to 2 parts by weight of LDPEIOO.
It is 0 parts by weight. In addition, examples of the anti-shrinkage agent include polyoxyethylene monomyristate, polyoxypropylene monomyristate, polyoxyethylene monopalmitate, polyoxypropylene monopalmitate, polyoxyethylene monostearate, and polyoxypropylene monomyristate. Stearate, polyoxyethylene distearate, monolauric acid glyceride, monomyristic acid glyceride, monopalmitic acid glyceride, monostearic acid glyceride, monoarachic acid glyceride, dilauric acid glyceride, dipalmitic acid glyceride, distearic acid glyceride, ■-palmitate- Examples include various aliphatic esters such as 2-stearic acid glyceride, 1-stearo-2-myristic acid glyceride, and tristearic acid glyceride. These may be used alone or in combination of two or more, and may be 0.5 to 0.5 to 0.5 parts by weight of LDPEIOO.
It is blended in a proportion of 5 parts by weight.

The plate-shaped foam obtained as described above is processed into various shapes as necessary and used as packaging cushioning materials, heat insulating materials, buoyancy materials, etc.

In the production method of the present invention, as mentioned above, since LDPE with a slow crystallization rate is used as a raw material, LDPH
The temperature range suitable for extrusion foaming is wide, which makes industrial production very easy.

〔Effect of the invention〕

Since the low-density polyethylene resin foam of the present invention has the above-mentioned structure, it exhibits the following outstanding effects.

(a) Since LDP[E, which has a slow crystallization rate, is used as a raw material, crystallization does not easily occur during extrusion foaming, and it is an excellent product that does not generate voids or gas due to crystallization.

(b) Since it is made from high-density LDPE, it has excellent compressive strength and creep properties.

(c) Since high molecular weight LDPE is used as a raw material, it has excellent brittleness resistance and punching workability.

In addition, in the method for producing a low-density polyethylene resin foam according to claim (2), since the crystallization rate of the raw material LDPE is slow,
The temperature range suitable for extrusion foaming of the resin is wide, and it has the outstanding effect of making industrial production extremely easy.

〔Example〕

Next, the present invention will be explained in more detail with reference to examples. Examples 1 to 4 and Comparative Examples 1 to 9 100 parts by weight of various branched low-density polyethylenes shown in Table-1 and 1.0 parts by weight of blowing agents and monostearic acid glyceride of the types and amounts shown in Table-1, In a tandem extruder (screw diameters 60 mm and 90 m, respectively),
After melt-kneading at a temperature of 200°C and a pressure of 170 kg/aJG, it was extruded into an accumulator set at a temperature of 105°C and a pressure of 40 kg/aJG. Next, the melted and kneaded material was discharged from this accumulator to atmospheric pressure through die orifices provided in the accumulator at a discharge rate of 7 g/see per die orifice, and a thickness of 50+ a.
A plate-shaped foam body with a width of 650 m, a width of 650 m, and a length of 2000 m was obtained. Table 1 also shows the properties of the foam obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the method for measuring the crystallization rate index A used in the present invention.

Claims (15)

[Claims] (1) Made by foaming a low-density polyethylene resin having a density of 0.922 to 0.926 g/cm^3, a weight average molecular weight of 90,000 or more, and a crystallization rate index A defined below of 10 or less. A low-density polyethylene resin foam characterized by: The crystallization rate index A was determined by differential scanning calorimetry using 5±0.1 mg of low density polyethylene resin at 10°C.
Draw a triangle from the peak on the DSC curve obtained when the temperature is raised to 180°C at a rate of 10°C/min and then lowered to 25°C at a rate of 10°C/min, and the base is a and the height is b. In this case, it can be obtained from the following formula. A=b/a However, the amplifier range for differential scanning calorimetry is 25m
J/sec and the chart speed is 10 mm/min. (2) In claim 1, low density polyethylene resin M
A low-density polyethylene resin foam, characterized in that I is 1.0 to 3.5 g/10 minutes. (3) The low-density polyethylene resin foam according to claim 1 or 2, wherein the low-density polyethylene resin is a mixture of two types of low-density polyethylene resins. (4) The low-density polyethylene resin foam according to claim 3, wherein the difference between the crystallization temperature of one low-density polyethylene resin and the other low-density polyethylene resin is 3° C. or more. (5) The low-density polyethylene resin foam according to claim 3 or 4, wherein the low-density polyethylene resin is branched low-density polyethylene. (6) In any one of claims 1 to 5, the thickness is 2 to 20
A low-density polyethylene resin foam characterized by being plate-shaped and having a density of 0.02 to 0.06 g/cm^3. (7) The density is 0.922 to 0.926 g/cm^3, the weight average molecular weight is 90,000 or more, and the crystallization rate index A defined in the same manner as in claim (1) is 1
0 or less is melt-kneaded together with a blowing agent under high temperature and high pressure, and then the obtained melt-kneaded product is temporarily held under conditions where no foaming occurs, and then it is discharged under low pressure. A method for producing a characteristic low-density polyethylene resin foam. (8) The method for producing a low-density polyethylene resin foam according to claim 7, wherein the MI of the low-density polyethylene resin is 1.0 to 3.5 g/10 minutes. (9) The method for producing a low-density polyethylene resin foam according to claim 7 or 8, wherein the low-density polyethylene resin is a mixture of two types of low-density polyethylene resins. (10) According to claim 9, the low density polyethylene resin foam is characterized in that the difference between the crystallization temperature of one low density polyethylene resin and the crystallization temperature of the other low density polyethylene resin is 3°C or more. Production method. (11) The method for producing a low-density polyethylene resin foam according to claim 9 or 10, wherein the low-density polyethylene resin is branched low-density polyethylene. (12) In any one of claims 7 to 11, the blowing agent is propane, butane, isobutane, pentane, isopentane, chlordifluoromethane, trifluoromethane,
1,2,2,2-tetrafluoroethene, 1-chloro-
1,1-difluoroethane, 1,1-difluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane, trichlorofluoromethane, dichlorodifluoromethane, 1,2-dichloro-1,2,2,2 - A method for producing a low-density polyethylene resin foam, characterized in that it is one type or a mixture of two or more types selected from tetrafluoroethane and the like. (13) In any one of claims 7 to 12, the low density polyethylene resin is melt-kneaded together with a blowing agent in an extruder at a temperature of 150 to 250°C and a pressure of 50 to 300 kg/cm^2G, and then The obtained melt-kneaded product is once heated at a temperature of 90 to 120°C and at a rate of 20 to 100 kg/cm^
It is extruded into an accumulator maintained under a pressure condition of 2G, and then it is extruded from a die orifice provided at the tip of the accumulator by the pressure of a movable ram with a die orifice cross-sectional area of 1 cm^2 and a discharge amount of 150 g or more per second. 1. A method for producing a low-density polyethylene resin foam, characterized by discharging the foam. (14) In any one of claims 7 to 13, polyoxyethylene monomyristate, polyoxyethylene monomyristate, polyoxypropylene monomyristate, polyoxyethylene monopalmitate, polyoxypropylene is added to the melt-kneaded product. Polyoxyalkylene higher fatty acid esters such as palmitate, polyoxyethylene monostearate, polyoxypropylene monostearate, polyoxyethylene distearate, monolauric acid glyceride, monomyristic acid glyceride, monopalmitic acid glyceride, monostearic acid glyceride , higher fatty acid glycerides such as monoarachic acid glyceride, dilauric acid glyceride, dipalmitic acid glyceride, distearic acid glyceride, 1-palmito-2-stearic acid glyceride, 1-stearo-2-myristic acid glyceride, tristearic acid glyceride, etc., stearin 100 weight of a low-density polyethylene resin is added with a shrinkage stopper consisting of one or a mixture of two or more selected from acid amide, palmitic acid amide, stearic acid diethanolamide, stearic acid monoethanolamide, hydroxyethylamine, hydroxypropylamine, etc. A method for producing a low-density polyethylene resin foam, characterized in that the foam is blended at a ratio of 0.5 to 5 parts by weight. (15) In any one of claims 7 to 14, the thickness is 2 to 2.
A method for producing a low-density polyethylene resin foam, characterized in that it is formed into a plate shape of 20 cm and a density of 0.02 to 0.06 g/cm^3.