KR820002041B1 - Polyethylen blend foams having improved compressive strength - Google Patents

Abstract

Low-d, PE foam having a substantially closed-cell structure and good compressive strength were prepd. by extrusion foaming of 35-60 wt.% low-d, branched PE(I), 40-65 wt.% intermediate-d, liner I, and <= 0.12 g-mol/100 g resin of C1-4 fluoro carbon blowing agent. Thus, a foam made from a 50:50 blend of branched I and liner I in the presence of 8.2 1,2-dichloro tetrafluoroethane at gel temp.˜120≰C had d. 5.2 lb/ft3, open cells 1%, skin quality excellent and av. compressive strength 24.5 psi.

Description

Process for preparing polyethylene blend foams

The present invention is directed to improving the compressive strength of polyethylene blended foam materials.

Polyethylene foams are made from blends of low density polyethylene and intermediate density linear polyethylene. In particular, the present invention relates to a process for producing foams having a substantially closed-cell structure and having improved compressive strength.

As used herein, "compressive strength" refers to the properties of the foamed material measured according to the test method described in ASTM D-3575-77 and expressed in Lb / inch ² (psi).

Low density independent cell ethylenic polymer resin foam materials can be prepared by extrusion foaming of conventional solid thermoplastic ethylenic polymer resins such as low density side chain polyethylene. The resin is heated to make it plastic, and under pressure it is mixed with volatiles such as 1,2-dichlorotetra fluoroethane to form a flowable gelled material, followed by low pressure through shaping orifice or die opening. Extruded into a yarn. The volatiles evaporate to form a gaseous cell structure in the gel material which, when cooled, becomes a solid cell foamed material resin. It is preferable that these gas cells are uniformly formed in the foam in the same size and are independent of each other, that is, bubbles are separated from each other by membrane membranes of resin. Although the general principles are well known, most of the extrusion foaming techniques have not escaped the basic stage. Previous attempts to manufacture low density foamed products from medium or high density linear polyethylene have been challenging.

Polymeric foams made from polyethylene blends are well known. U.S. Pat. It describes a process for producing a foamed material having Japanese Patent Publication No. 47-56,497 dated June 8, 1982 describes a process for producing polyethylene foam material from two polyethylene blends of different densities. More specifically, two polyethylene resins having a density of <0.93 and a density of 0.94 were melt-blended and crushed, and then immersed in a liquid foaming agent, and foamed at a temperature <20 ° C higher than the arithmetic mean value of the two polyethylenes.

US Patent No. 3,793,239 describes a polymer blend of crystalline polyethylene with 1 to 15% by weight of thermoplastic polymers for extrusion foaming using two foaming mixtures. High density polyethylene and polypropylene blends are more preferred.

US Patent No. 3,351,569 describes a process for producing fine cell polyolefin foams from crystalline polymers of ethylene or propylene using liquid foaming agents and at least 10% by weight of finely divided solid inorganic nuclear attack agents. Two or more crystalline polymer mixtures of ethylene or propylene may be used.

Improved compression at a given foam density by extrusion foaming polyethylene blends made by mixing 35 to 60 weight percent of low density branched polyethylene with a density of about 0.910 to 0.930 and 40 to 65 weight percent of a medium density linear polyethylene having a density of about 0.931 to 0.940 It is possible to produce a foamed material resin product having strength and almost independent cell structure. The resulting polyethylene blended foam material has a density of about 3.0 to 15 Lb / ft ³ (0.048-0.240 g / cc) and preferably 3.0 to 7.0 pcf (0,048-0,112 g / cc). The average compressive strength of the foam material are substantially independent cell structure also has a 10% strain is about 7 to about 170psi (0.5 to about 12.0kg / cm ^2), preferably from 7 to 60psi (0.5 to 4.2kg / cm ²⁾ do.

As used herein, the term "average compressive strength" means the average value of the compressive strength values acting perpendicular to the direction of travel in the extrusion, expressed in psi or kg / cm ² .

It is a feature of the present invention to formulate low density foamed materials by combining medium density linear polyethylene with low density side chain polyethylene. Low density side chain polyethylene has a density of about 0.910 to 0.930 g / cc and a melt index of about 0.5 to 50 dg / min, and can be produced by conventional high pressure polymerization.

Medium density linear polystyrene has a density of 0.931 to 0.940 g / cc and a melt index of 0.5 to 50 dg / min, and a medium density linear polyethylene having such characteristics can be produced by Ziegler type low pressure polymerization method.

If the resulting blend can only be extruded, a small amount of high density linear polyethylene having a density greater than 0.940 can be used in the starting polyethylene blend.

In the present invention, the polyethylene blend is 35 to 60% by weight of low density branched polyethylene, preferably 40 to 55% by weight, most preferably about 50% by weight and 40 to 65% by weight of medium density linear polyethylene, preferably 45 to 60% by weight, Most preferred can be prepared by mixing with about 50% by weight. These polyethylenes can be blended by various known methods already known, and the mixing method is not a problem as long as the two polyethylenes are completely mixed.

In particular, the blends of polyethylene used must have a melt index difference between the polyethylenes of less than about 3.0. Foaming agents used to prepare polyethylene blended foams are well known and solids and volatile liquids that decompose into a cast composition at extrusion temperatures may be used. Especially preferred are halogenated hydrocarbon compounds having 1 to 4 carbon atoms. Examples of such blowing agents include dichlorodifluoromethane, trichlorofluoromethane, chlorodifluoromethane, 1,2-dichlorotetrafluoroethane, 1-chloro-1,1-difluoroethane, 1,1 -Dichlorotrifluoroethane, 1,1-difluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane, 1-chloro-1,1,2,2-tetrafluoroethane, There are 2,2-difluoropropane and 1,1,1-trifluoropropane, of which 1,2-dichlorotetrafluoroethane is most preferred. Although the use of a sole blowing agent is preferred, it may also be used as two or more blowing admixtures.

The blowing agent is mixed with the starting polyethylene blend in proportion to the degree of expansion required for the foam cell product to be produced, and typically produces about 20 volume expansions to produce products with a lower foam density of about 0.048 g / cc (about 3.0 pcf). Is done. The foamed product obtained according to the initial dose of blowing agent will have a density of about 3.0 to 15.0 pcf (0.048 to 0.240 g / cc), preferably 3.0 to 7.0 pcf (0.048 to 0.112 g / cc).

The blowing agent may be mixed with the polyethylene blend by conventional methods to form a flowable gel material, but a preferred method is to use a continuous method. By continuous method is meant mixing in an extruder using heating to plasticize the resin blend, pressurization to prevent casing of the blowing agent and mechanical treatments for complete mixing of the resin blend and the blowing agent. The obtained gelled material is cooled if necessary, extruded through a suitable dioripiece into a low pressure chamber at normal atmospheric pressure and expanded to a low density cell forming material. Once the foamed extrudate is formed, it is separated from the extruder, cooled, and then transported for later processing such as storage or use.

The obtained polystyrene-containing foamed material has a substantially independent cell structure and has good flexibility in bending and molding. The compressive strength measured at 10% deformation of this foam material is a significant improvement when compared to foam materials having comparable foam density made from low density polyethylene alone.

The gas space in the polyethylene blended foam cell was initially filled with the volatile foaming agent used to produce the foamed material, but over time the foaming agent is released from the foamed material and gradually replaced as air entering the foamed material cell. . The gas space of the foam is inevitably completely filled with air.

Fine grinding of solids such as calcium silicate, zinc stearate, magnesium silicate and similar materials effectively mixes with the polymer blend or gelled material to serve to control the cas bubble cell size in the pre-expansion stage. This material is typically used up to 1% of the polymer weight.

Foam materials can also be prepared by mixing polyethylene blends with numerous fillers, pigments, lubricants, antioxidants and other known additives.

The following examples illustrate the invention in detail. All 100 Parts and percentages are by weight unless otherwise specified.

Example 1

Polyethylene blended foams are typically produced by continuous extrusion into a conventional screw extruder consisting of a continuous feed, compression and melting zone, refinement zone and mixing zone. The extrusion machine has a conventional heater and gauge for temperature control of the stage. Control of the flow rate and supply of liquid blowing agent under pressure is carried out between the weighbridge and the mixing zone. The blending stage of the extruder mixer stage is connected to the die orifice having a rectangular configuration via a cooler.

In practice, polyethylene resin particles are fed to the extruder through a feed hopper. The weighbridge is kept in the range of 180 ± 20 ° C. The blowing agent is pumped into the inlet between the weighbridge and the mixing zone at a pre-measured rate under pressure that can be maintained in the liquid state. The mixed mass of polyethylene blend and blowing agent melted in the mixing zone is cooled in a nearly uniformly adjusted temperature control that is slightly above the temperature at which the solid polymer precipitates as crystals in the gelled material. In the case of the polyethylene mixture used in this example, when the temperature is adjusted to about 115 ° to 119 ° C., it easily passes through the diori piece. The polymer extruded to atmospheric pressure through the diolith expands to form a cell foam material which is produced and discharged continuously from the die at the same time. It is cooled and cured into a strip of porous and flexible solid resin foam.

In the experiments described below, the polyethylenes listed in Table I are used. Specific formulations and blending ratios of these polyethylene blends are shown in Table II. It was mixed. Also blowing agents such as dichlorodifluorotan (FC-12) or 1,2-dichloro tetrafluoroethane (FC-114) were fed to the extruder in a substantially constant proportion.

Various physical properties of the obtained extruded foam products were measured and the results are shown in Table II.

TABLE I

TABLE II

(1) See Table 1.

(2) the weight percent of each polyethylene in the blend

(3) blowing agent supply rate in parts per hundred

(4) Supply rate of magnesium silicate and calcium stearate in parts per hundred.

(5) Density of foam measured within about 5 minutes after extrusion

(6) the amount of open cells in the structure of the foamed material measured according to ASTM D-2856-A

(7) Compressive compressive strength at 10% strain measured according to ASTM D-3475-77

ND = no measurement

* = Not an embodiment of the invention

From the data in Table II, it can be seen that the polyethylene blend foams prepared according to the present invention showed a very good improvement in compressive strength at 10% strain when compared to polyethylene foams having comparable foam density made from low density polyethylene alone.

Open cell% measurement and subjective evaluation of surface properties are good indicators of the polyethylene compound formation process. A smooth, homogeneous surface of the foamed material as well as a low open cell percentage is desirable. An open cell concentration of 15-30% points to a manufacturing process problem and an open cell concentration greater than 30% cannot be tolerated in the product. Although it is still an acceptable manufacturing process under this criterion, Experiment No. 1.6 indicates that boundaries are recognized at a concentration of 3.0 pcf of foam material. As shown in Test No. 1.7, the attempt to lower the density of the foaming material to 3.0 pcf or less with the use of excess blowing agent resulted in the loss of all bubbles in the die. Test No. 1.8 indicates that FC-12 can be substituted for FC-114 as a blowing agent.

Example 2

The foamed materials were prepared from the various low density and linear polyethylene blends listed in the table according to the continuous extrusion process of Example 1. The process characteristics of the foamed materials listed in this example are described in Table III.

TABLE III

(1) (2) (3) and (5) (6) (7) are the same as Table II

(4) feed rate of magnesium silicate and amine stearate in parts per hundred

* = Not an embodiment of the invention

The results described in Table II indicate that the polyethylene blends that can be used from the standpoint of the manufacturing process consist of a mixture of 40 to 55% by weight low density polyethylene and 45 to 60% by weight medium density linear polyethylene. The best manufacturing process can be obtained from blends with low density polyethylenes with the same melt index as the linear polyethylene (example 5.0), and the worst manufacturing process is obtained from polyethylene blends with large differences in the melt index. Are described in Test Nos. 2.3, 2.5 and 2.6.

Claims (1)

After heating and plasticizing a conventional solid thermoplastic resin and mixing a blowing agent into a gel, it is extruded into a low pressure chamber through die opening and an extruded body to produce a processable resinous body. Polymer formulation by foaming method In the foam production process, thermoplastic resins are (1) 35 to 65% by weight low density side chain polyethylene with a density of 0.910 to 0.930 and (2) 40 to 65% by weight linear density polyethylene with a density of 0.931 to 0.940. It is composed of a polyethylene compound prepared by the method, the density of the polyethylene compound foam material prepared in this way is about 3.0 to 15.0 pcf (0.048 to 0.240 g / cc), and almost independent cell structure (Substantailly closed-cell structure) and 10 An improved method for producing a polymer blended foam material having a% strain compressive strength of about 7 to 170 psi (0.5 to 12.0 kg / cm ² ).