Process for the production of polyolefin moldings

Details Process for the production of polyolefin moldings Process for the production of polyolefin moldings

1999-04-27

2001-06-05

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Extrusion molding

C264S477000, C264S210600, C264S210700, C264S290200, C264S210200

Reexamination Certificate

active

06241937

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for manufacture of polyolefin shaped articles having high strength and high modulus.
BACKGROUND ART
For manufacture of polyolefin having high strength and modulus, Japanese Patent Laying-Open No. Hei. 1-280013 discloses a method wherein ultra-high molecular weight polyethylene and a flow modifier such as paraffin wax are melt kneaded at a temperature above a melting point of the ultra-high molecular weight polyethylene and extruded from a die into a shaped article which is subsequently subjected to multi-stage stretching. The publication states that such a method permits the highly productive and simple manufacture of ultra-high molecular weight polyethylene having high strength and modulus on an industrial scale.
However, the method described in the above-identified publication requires the use of a solvent to remove the flow modifier either during or subsequent to the stretching process. This not only adds to operating time but also poses a problem of increased manufacturing cost resulting from the need to use the solvent for removal of the flow modifier.
DISCLOSURE OF THE INVENTION
An object of the present invention resides in its provision of a manufacturing method by which a polyolefin shaped article having high strength and modulus can be efficiently manufactured, thus obviating a need for a process of removing the flow modifier.
The present invention, as recited in claim
1
, is a method for manufacture of a polyolefin shaped article, which is characterized as including the steps of melt forming a composition principally containing 100 parts by weight of high-density polyolefin and 1-30 parts by weight of a polymerizable unsaturated compound having a preferred affinity for the high-density polyolefin into a sheet or film, calendering the sheet or film obtained, stretching the calendered sheet or film, and polymerizing the polymerizable unsaturated compound incorporated in the stretched sheet or film.
In the method for manufacture of a polyolefin shaped article in accordance with the present invention, high molecular weight polyolefin is preferably employed for the above-specified high-density polyolefin, as recited in claim
2
.
Also, in a particular aspect of the present invention, the aforementioned composition further contains a free-radical generator, as recited in claim
3
.
Preferably, a photoinitiator is employed for the above free-radical generator and the step of polymerizing the polymerizable unsaturated compound, which follows the stretching step, is carried out by ultraviolet irradiation, as recited in claim
4
.
Alternatively, the step of polymerizing the polymerizable unsaturated compound may be performed by electron beam irradiation, as recited in claim
5
.
Also, in a particular aspect of the present invention, the aforementioned polymerizable unsaturated compound consists at least partly of a trifunctional compound, as recited in claim
6
.
In the method for manufacture of a polyolefin shaped article in accordance with the present invention, in the step of calendering the aforementioned sheet or film, the calendering is performed at a calender ratio preferably within the range of 2-10, as recited in claim
7
.
It is also preferred that, in the step of stretching the aforementioned sheet or film, the stretching is performed at a stretch ratio preferably of at least 2, as recited in claim
8
.
In a narrower aspect of the present invention, high-density polyethylene having a weight average molecular weight within the range of 200,000-500,000 is employed for the aforementioned high-density polyolefin, as recited in claim
9
.
Where high-density polyethylene is employed as the aforementioned high-density polyolefin, it is preferred that, in the calendering step, the calendering is performed at a temperature within the range of 70-120° C., as recited in claim
10
. It is also preferred that, in the stretching step, the stretching is carried out at a temperature ranging from 70° C. to a melting point of high-density polyethylene, as recited in claim
11
.
Preferably, the method for manufacture of a polyolefin shaped article in accordance with the present invention further includes a step of subjecting the sheet or film, at least prior to being stretched, to heat treatment as recited in claim
12
.
Also, the above-specified stretching step may be a multi-stage stretching step wherein stretching is effected in the same direction as a calendering direction by two or more stages and wherein stretching at a final stage is effected equi-directionally at an orientation temperature within a range of 35° C. to 5° C. reduction from an orientation temperature used in stretching at a stage immediately before the final stage, as recited in claim
13
.
A detailed explanation of the present invention is given below.
The high-density polyolefin resin for use in the present invention is not particularly specified, so long as it is highly crystalline. Useful high-density polyolefin resins include homopolymers such as polyethylene, polypropylene, 1-butene, and 1-pentene; and copolymers containing not higher than 10 weight % of copolymeric components such as vinyl monomer units including vinyl acetate, vinyl alcohol, vinyl chloride and acrylic acid.
Preferably, high-molecular weight high-density polyolefin having a high weight-average molecular weight is suitably used for the high-density polyolefin resin, since it is able to impart increased mechanical properties, such as strength and modulus, to a resulting polyolefin shaped article. The high-molecular weight polyolefin, as used herein, refers to polyolefin which, like the below-described high-density high-molecular weight polyethylene, has a weight-average molecular weight of a level sufficiently high to impart increased mechanical properties, such as strength and modulus. Its weight-average molecular weight depends upon the particular type of polyolefin used, but is generally not lower than 200,000.
In the present invention, preferred among the above-listed resins is the high-density polyethylene resin.
A useful high-density polyethylene resin has a weight-average molecular weight preferably in the range of 200,000-500,000. If the weight-average molecular weight is below 200,000, stretching to orientate molecular chains may result in the failure to achieve desired improvements in strength and modulus. On the other hand, if the weight-average molecular weight exceeds 500,000, melt formation may fail, as is typically the case with ultra-high-molecular-weight polyethylene. Accordingly, the use of high-density high-molecular weight polyethylene having a weight-average molecular weight in the range of 200,000-500,000, as specified above, results in the further increased mechanical properties, such as strength and modulus, relative to using low molecular weight polyethylene.
Also, where a high-density polyethylene resin is used, its density is not lower than 0.94 g/cm
3
. If the density is low, stretching may result in the failure to achieve the desired improvements in strength and modulus. Also, its melt index (MI) is preferably in the range of 0.1-20, more preferably in the range of 0.1-10. If the melt index is lower than 0.1, an increased load may be imposed on a fabricating machine such as an extruder. If it exceeds 20, fabrication may become difficult.
In the present invention, a composition which contains 100 parts by weight of high-density polyolefin and 1-30 parts by weight of polymerizable unsaturated compound having a preferred affinity for the high-density polyolefin is used as a starting material. If the polymerizable unsaturated compound content is less than 1 part by weight, the incorporation thereof may not always be effective to achieve expected improvement in stretchability. On the other hand, if it exceeds 30 parts by weight, the polymerizable unsaturated compound may be caused to bleed on a surface of a shaped article, which not only lowers handling characteristics but also adversely affects stretchability.
The present invention will be now described in m

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