Injection moulding process using polymer blends containing high

Details Injection moulding process using polymer blends containing high Injection moulding process using polymer blends containing high

1980-06-20

1982-09-14

Pavelko, Thomas P.

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Shaping against forming surface

2643281, B29F 100, B29B 100

Patent

active

043495044

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for injection moulding at high pressures exceeding 250 MPa (300 MPa) of blends of high molecular polyethylene having high density (HMWHDPE) with a polymeric material with which HMWHDPE has a limited compatibility (solubility) for producing an injection moulded article, wherein HMWHDPE forms a fibre phase in a matrix of the other polymeric material.
From the Swedish Patent Specification No. 401,129 it is previously known that an essential increase of the pressure in the mould cavity in the processing of thermoplastic resins by means of injection moulding results in a number of property improvements of the moulded parts and at the same time certain technical process advantages can be obtained. Thus, said Swedish Patent Specification discloses an injection moulding process wherein the pressure in connection with the injection and subsequent solidification of the plastic melt exceeds 250 MPa. Said pressure range shall be compared with the pressure range 50-150 MPa which is the normal pressure range in injection moulding.
Among the improvements of the properties of the moulded part which are provided with process pressures exceeding 250 MPa there is to be noted a general increase of the level as regards the stiffness (modulus of elasticity) and strength parameters of the product as has been obtained in experiments with polyethylene of varying density and with polypropylene. Furthermore, in measurements on both crystalline as amorphous polymers a substantial decrease in the mould shrinkage has been observed. In certain cases said change was so large that the mould shrinkage assumed negative values.
An additional property improvement which has been observed in experiments of this type has been a substantial decrease of the internal stress level. As is well known internal stresses are a defect of normal injection mouldings; they i.a. can result in warpage, undesirable formation of crazing and cracking etc. Their reduction, in certain cases complete elimination, can be traced back to an increase of the melting and softening point, respectively of the plastic with the pressure. Thus, a pressure within the range 250-1000 MPa has been found to increase these critical temperatures so that a practically momentary solidification of the melted plastic material injected in the mould cavity is obtained. This prevents the formation of an internal stress structure, which, as is well known, is due to a gradually solidification of the melt and to the forces which the already solidified and the still liquid portions of the moulded part exert on each other.
As regards the improvement of the property profile of the moulded part, especially as regards the mechanical parameters, the effects obtained when increasing the process pressure are dependent on the structure of the plastic used. Thus, the greatest changes have been observed with crystalline plastics, especially polyethylene, wherein they have been traced back to changes in the structure of the crystalline phase. In the crystallization of a polyethylene melt under pressure exceeding about 300 MPa a crystal modification is obtained wherein the folded lamellar structure present at normal pressures is converted to a structure consisting of straight chains (extended chain-structure) to an extent increasing with the pressure. The improved property profile can be explained by the presence of said new structure. The new modification with straight chains i.a. gives a more organized package of the molecules in the space.
Especially remarkable results are obtained with HD-polyethylene having high molecular weight, which refers to polyethylene having a molecular weight of from 200.000 up to 1.5 millions and higher (corresponding to a melt index of less than 1 g/10 minutes at 190.degree. C./2 kg load) hereinafter designated HMWHDPE. As compared to the improvements of modulus and strength obtained with this and similar qualities the corresponding values obtained with HDPE of normal molecular weight were relatively small. As an illustration of the

REFERENCES:
patent: 4237089 (1980-12-01), Kubat
Chemical Abstracts 85:64204, 86:91157.

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