JPH05287112A - Preexpanded particle of lightly crosslinked linear low-density polyethylene and its production - Google Patents

Abstract

PURPOSE:To obtain an in-mold expanded molded body well balanced in rigidity, dimensional stability, flexibility, in-mold moldability, etc. CONSTITUTION:A preexpanded particle of a lightly crosslinked linear low-density polyethylene is that of a crosslinked linear low-density polyethylene characterized by lightly crosslinking the linear low-density polyethylene and having >=0.05 to <6% gel fraction of the expanded particle.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to lightly cross-linked linear low density polyethylene pre-expanded particles and a process for their preparation. More specifically, a lightly cross-linked linear low-density polyethylene pre-expanded particle capable of providing an in-mold expanded molded article having a well-balanced rigidity, dimensional stability, flexibility and in-mold moldability, and Regarding the manufacturing method.

[0002]

2. Description of the Related Art Conventionally, a high-pressure low-density polyethylene resin (consisting of long-chain branches) has been used as a method for producing a polyethylene resin in-mold foam molded article.
In-mold foam moldings using foamed particles obtained by cross-linking and foaming have been put on the market and widely used as cushioning materials, heat insulating materials, surface protection materials, sundries and the like.

In addition, in recent years, it has been possible to obtain an in-mold foamed molded article excellent in dimensional stability, tensile strength, elongation and tear strength by using linear low density polyethylene without crosslinking other than the above-mentioned method. About JP-A-59-18703
It is proposed in No. 6 publication. In addition, this method is advantageous in terms of manufacturing cost because molding can be performed at a low molding pressure during in-mold molding, and the cross-linking step can be omitted. Furthermore, since it is non-cross-linked, it can be easily recycled. ,
Widely used in recent years.

However, this type of non-crosslinked linear low-density polyethylene-based in-mold molded article is excellent in tensile strength, elongation, and tear strength, but is inferior in flexibility of the molded article, so that the molded article is cracked. There is a problem that it is easy and the recovery rate after compacting the molded body is poor.

As a method for improving such drawbacks, a method of crosslinking linear low-density polyethylene (Japanese Patent Publication No. Sho 64-1498) has been proposed. Crosslinked linear chains having a gel fraction in these ranges have been proposed. The low-density polyethylene pre-expanded particles require a large amount of molding energy because the vapor pressure during foam molding in the mold is high. Further, when the degree of crosslinking is high, the degree of crystallinity is low, the rigidity is low, and it becomes difficult to obtain a high-magnification molded product. Further, since the degree of crosslinking is high, there is a problem in recyclability.

[0006]

DISCLOSURE OF THE INVENTION As a result of intensive studies to solve the above-mentioned drawbacks, the present inventors have found that non-crosslinked linear low-density polyethylene can be crosslinked relatively lightly to reduce the rigidity. In addition, it can improve the tensile strength, elongation, tear strength, crack resistance of the molded body, strain recovery after compression, and can obtain in-mold molded body without increasing the vapor pressure at the time of molding. Furthermore, they have found that they can be recycled sufficiently, and have completed the present invention.

That is, the present invention is a crosslinked linear low-density polyethylene pre-expanded particle obtained by crosslinking linear low-density polyethylene, and the gel fraction of the expanded particle is 0.05% or more 6
%, A lightly cross-linked linear low-density polyethylene pre-expanded particles and an aqueous dispersion of cross-linked linear low-density polyethylene particles, a dispersant and a volatile foaming agent in a pressure vessel. Melting point of the particles -10 ° C to + 35 ° C
While maintaining the temperature and pressure in the pressure vessel constant under a pressure higher than the vapor pressure of the volatile blowing agent at a temperature in the range of
The method for producing the pre-expanded particles is characterized in that the mixture of the particles and water is discharged under an atmosphere at a pressure lower than that in the container.

[0008]

EXAMPLES The linear low-density polyethylene (hereinafter referred to as LLDPE) referred to in the present invention means ethylene having 4 to 8 carbon atoms.
Is a copolymer of α-olefins having a resin density of 0.915 to 0.940 g / cm ³ , preferably 0.918 to 0.940 g / cm ^3.
It is more preferably 0.935 g / cm ³ . The LLDP
If the resin density of E is less than 0.915 g / cm ³ , the rigidity of the finally obtained molded product will be insufficient, and it will be difficult to obtain a high magnification, and if the resin density exceeds 0.940 g / cm ^3. In the meantime, although there is an advantage that the finally obtained molded body has high rigidity and high magnification, a large amount of steam energy is required during in-mold molding.

The cross-linked LLDPE pre-expanded particles in the present invention can be obtained by cross-linking and pre-foaming the LLDPE particles, and the degree of cross-linking is determined by the gel fraction of the expanded particles.

The gel fraction is measured by replacing the blowing agent in the expanded particles with air and then, when the expanded particles are boiled in boiling xylene for 3 hours, the ratio of the weight of the xylene-insoluble matter to the initial weight of the expanded particles. Is a value expressed in%.

The gel fraction in the present invention is 0.05% or more and 6% or more.
It is preferably less than 0.1%, more preferably 0.1 to 5%.
When the gel content of the expanded beads is less than 0.05%, the effect of making the molded article less prone to cracking decreases, and conversely, when the gel content of the expanded particles becomes 6% or more, the effect of making the molded article less prone to cracking is obtained. However, conversely, the crystallinity decreases, the rigidity decreases, and a large amount of steam energy is required at the time of molding.

The cross-linking of the LLDPE in the present invention comprises
It may be performed before the production of the expanded beads or at the same time as the process of producing the expanded beads.

The cross-linking method of LLDPE includes a method in which a chemical cross-linking agent and LLDPE are dispersed and heated to cross-link, a method in which the chemical cross-linking agent and LLDPE are kneaded and heated in an extruder to cross-link, and an electron beam is irradiated. Examples include a method of crosslinking.

The particle size of the LLDPE in the present invention is not particularly limited, but usually the sphere volume conversion diameter is 0.25 to 10.0 mm.
Those having a diameter of 0.5 to 6.0 mm are preferably used for in-mold molding.

The expansion ratio of the LLDPE pre-expanded particles in the present invention is in the range of 5 to 50 times.

[0016] The measurement of the expansion ratio (X) is the density of the pre-expanded particles measured from the volume of the immersed the pre-expanded particles in ethanol and [rho, wherein when the density of the material was [rho _o:

[0017]

[Equation 1]

It is a value represented by

The ratio of the molded product in the present invention varies depending on the ratio of the pre-expanded particles and the shrinkage ratio of the molded product.
The range is up to 60 times. The compact ratio (X) is JI
When the apparent density measured by SK 6767 is ρ and the density of the raw material is ρ _o , the formula:

[0020]

[Equation 2]

It is a value represented by

Next, an example of the method for producing pre-expanded particles of the present invention and the method for producing an in-mold molded article from the expanded particles will be described.

First, 0.05 to 0.3 part of a chemical crosslinking agent is dispersed in a dispersion medium with respect to 100 parts of LLDPE particles (parts by weight; the same applies hereinafter), and the LLDPE particles are crosslinked.

Examples of chemical cross-linking agents include dicumyl peroxide, t-butyl cumyl peroxide and 2,5-dimethyl-
2,5-di (t-butylperoxy) hexyne-3, 2,2-bis (t-butylperoxy) butane, di-t-butylperoxide and the like are used.

The dispersion medium may be any solvent which does not dissolve the resin, such as water, methanol and glycerin, but water is usually used.

The heating temperature at the time of crosslinking varies depending on the melting point of LLDPE and the type of chemical crosslinking agent, but is usually 130 to 170 ° C.
Is. The heating time is about 0.5 to 3 hours, and crosslinked LLDPE particles having an arbitrary degree of crosslinking can be obtained.

It is necessary to prevent fusion between the resins because fusion occurs between the resins during the crosslinking. As the dispersant used at this time, a poorly water-soluble inorganic substance such as calcium phosphate, magnesium pyrophosphate, titanium oxide or aluminum oxide is used. Further, a small amount of anionic surfactant such as sodium alkylbenzene sulfonate or sodium α-olefin sulfonate is used as a dispersion aid. In this case, the amount of the slightly water-soluble inorganic substance fine powder used per 100 parts of the resin is 0.1 to 3 parts, and the amount of the anionic surfactant used is about 0.001 to 0.3 parts.

Next, a method of pre-expanding the crosslinked particles will be described.

The crosslinked particles and the volatile foaming agent are dispersed in water in the presence of a dispersant, and the melting point of the crosslinked particles is -10 to +35.
The crosslinked particles are impregnated with a volatile foaming agent by heating to a temperature in the range of ° C, and the temperature and pressure in the container are maintained so that the pressure and the temperature are constant or higher than the vapor pressure of the volatile foaming agent. However, it can be obtained by discharging the mixture of the crosslinked particles and water under an atmosphere at a pressure lower than that in the container.

As the volatile blowing agent used in the present invention, hydrocarbons or halogenated hydrocarbons having a boiling point of -50 to + 120 ° C., such as propane, butane, pentane,
Examples include hexane, heptane, cyclohexane, monochloromethane, dichloromethane, monochloroethane, trichloromonofluoromethane, dichlorodifluoroethane, dichloromonofluoroethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, etc. These may be used alone or in combination of two or more. You may use it.

The amount of these volatile foaming agents used is the amount of impregnation in 100 parts of the crosslinked particles in consideration of the type of the foaming agent, the desired expansion ratio, the ratio of the resin amount in the container and the space volume in the container. To be 5 to 40 parts.

The pre-expanded particles obtained as described above are filled in the mold without substituting internal pressure after replacing the blowing agent in the expanded particles with air and then 1.0 to 2.5 kg / cm 3.
By performing in-mold molding with a water vapor pressure of about ² (G), a molded product in which particles are fused to each other can be obtained.

The pre-expanded particles of the present invention will be described below based on examples.

Examples 1 to 3 and Comparative Examples 1 to 3 In a 10 L pressure vessel, the resin density was 0.930 / cm ³ , and the MI was 2.
1 (melting point 122 ℃) LLDPE pellets (4.5mg / grain)
200 parts of water per 100 parts (2.4 kg) of dicumyl peroxide and 100 parts of the resin particles,
Cross-linked particles were obtained by charging 0.5 parts of tricalcium phosphate and 0.006 parts of normal paraffin sodium sulfonate as a dispersant, heating the mixture from room temperature to 150 ° C. for about 2 hours, keeping it for 2 hours, and then cooling and taking it out. .

100 parts of the obtained crosslinked particles, 300 parts of water, 0.5 part of tricalcium phosphate as a dispersant and 0.006 part of normal paraffin sodium sulfonate, and a predetermined amount of isobutane were charged in a 10 L pressure vessel and stirred, and the results are shown in Table 1. After maintaining the foaming temperature and foaming pressure for 30 minutes, the mixture of crosslinked particles and water was discharged under atmospheric pressure through a 5 mmφ orifice provided at the bottom of the container while maintaining the inside of the container at the above foaming pressure with nitrogen gas. , Foamed particles were obtained.

After the foamed particles were aged at atmospheric pressure for 24 hours, the gel fraction of the expanded particles and the magnification of the expanded particles were measured.

The obtained foamed particles are 450 mm × 300 mm × 50 mm
The mold was filled in, and the expanded particles were heated and fused with steam to obtain an in-mold molded body. The molded body taken out from the mold is 80
After curing for 24 hours in a dryer at ℃, various physical properties of the molded body were measured. The results are shown in Table 1.

(Molding pressure) Molding in which a notch having a depth of 2 mm is made from the surface of the molded product, and this part breaks at the back and does not break at the interface of the particles, and the ratio of breaking at the particles themselves is 60% or more. The pressure (water vapor pressure) was determined and judged according to the following criteria.

○: Less than 1.5 kg / cm ² △: 1.5 kg / cm ² or more and less than 2.0 kg / cm ² ×: 2.0 kg / cm ² or more (height of cracked compact) Thickness 20 mm, width 40 mm, length 250 mm The test piece of No. 1 was cut out from the molded body, a notch having a depth of 5 mm was made from the surface at the center in the length direction, and a hard ball (321 g (42.5 mmφ)) was dropped from the arbitrary height to the surface opposite to this part. After that, measure the remaining thickness of the test piece, and the remaining thickness is 8 mm.
The following heights were determined and judged according to the following criteria.

O: 40 cm or more B: 25 cm or more and less than 40 cm X: Less than 25 cm (molded product compressive strength) The strength at 50% compression was determined according to JIS K 6767 and judged according to the following criteria.

[0041] ○: 1.35 kg / cm ² or more △: 1.30 or 1.35 kg / cm ² less ×: 1.30 kg / cm of less than ² (compression unrecovered strain) After compressing the plate-like test piece having a thickness of 25mm up to 75% The thickness of the test piece when the pressure was released and the compressive stress became 0 was measured, and the unrecovered strain after compression was obtained by the following formula and judged according to the following criteria.

[0042]

[Equation 3]

○: Less than 12% ×: 12% or more

[0044]

[Table 1]

Examples 4 to 6 and Comparative Examples 4 to 6 Experiments were conducted in the same manner as in Example 1 except that LLDPE having a resin density of 0.920 g / cm ³ and MI 1.0 (melting point 121 ° C.) was used. The results are shown in Table 2.

[0046]

[Table 2]

Examples 7-9 and Comparative Examples 7-9 Experiments were conducted in the same manner as in Example 1 except that LLDPE having a resin density of 0.935 g / cm ³ and MI 2.1 (melting point 123 ° C.) was used. The results are shown in Table 3.

[0048]

[Table 3]

As described above, only by lightly crosslinking LLDPE, flexibility is increased and cracking is less likely to occur as compared with non-crosslinked LLDPE. Further, compared with the case of highly cross-linking, the amount of steam energy used can be reduced because the molding can be performed at a lower molding pressure. Moreover, LLDPE has the same flexibility as that of a conventional high-pressure low-density polyethylene crosslinked product, and since LLDPE has rigidity, it has the advantage of increasing the molded product magnification.

As described above, the reason why a well-formed in-mold molded product can be obtained by only lightly crosslinking the non-crosslinked LLDPE is that LLDPE has a entanglement of molecular chains in a molten state as compared with high-pressure low density polyethylene. Since it is large, it is considered that the cross-linking effect can be exhibited by the formation of a few cross-links.

[0051]

EFFECT OF THE INVENTION By using the pre-expanded particles of the present invention, it is possible to obtain an in-mold expanded molded article having a well-balanced rigidity, dimensional stability, flexibility and in-mold moldability.

Claims (2)

[Claims] 1. Crosslinked linear low density polyethylene pre-expanded particles obtained by crosslinking linear low density polyethylene, characterized in that the gel fraction of the expanded particles is 0.05% or more and less than 6%. Linear low density polyethylene pre-expanded particles cross-linked to. 2. A volatilization of an aqueous dispersion comprising crosslinked linear low density polyethylene particles, a dispersant and a volatile foaming agent in a pressure vessel at a temperature in the range of −10 ° C. to + 35 ° C. of the particles. A mixture of the particles and water is discharged under an atmosphere at a pressure lower than that in the container while the temperature and pressure in the pressure resistant container are kept constant under a pressure higher than the vapor pressure of the organic foaming agent. Item 2. A method for producing pre-expanded particles according to Item 1.