Low-shrinkage polyethylene moulding compounds

Details Low-shrinkage polyethylene moulding compounds Low-shrinkage polyethylene moulding compounds

1997-12-11

1999-11-09

Wilson, Donald R.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

215329, 526348, 526352, C08L 2304

Patent

active

059816643

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to molding compositions based on polymers of ethylene made from range from 0.948 to 0.964 g/cm.sup.3, a melt flow index (MFI) of from 6 to 20 g/10 min and a mean molar mass distribution M.sub.w /M.sub.n in the range from 2 to 5, range from 0.935 to 0.953 g/cm.sup.3, a melt flow index (MFI) of from 0.1 to 0.35 g/10 min and a mean molar mass distribution M.sub.w /M.sub.n in the range from 6 to 20, and 0.957 g/cm.sup.3, the melt flow index is in the range from 1.0 to 2.0 g/10 min, the mean molar mass distribution M.sub.w /M.sub.n is in the range from 3 to 10 and the difference of the densities of components A and B (.DELTA.D=D.sub.(A) -D.sub.(B)) is in the range from 0 to 0.029 g/cm.sup.3.
The present invention also relates to a process for preparing these molding compositions by melt extrusion, the use of the molding compositions for producing films, fibers and shaped articles, to films, fibers and shaped articles made from these molding compositions and to screw closures made from the molding compositions.
Mixtures based on ethylene polymers, termed polyethylene blends below, are known and are used, for example, for producing shaped articles which are resistant to stress cracking, as described in DE-C 34 37 116.
In more recent times, polyethylene blends have been used in injection molding to produce many types of screw closure. It is advantageous if, after the injection molding procedure, i.e. during cooling, the screw closures retain their dimensions and shape, i.e. do not shrink (low shrinkage). It generally leads to easier injection molding if the polyethylene molding compositions have good flowability in the melt. Low shrinkage coupled with retention of shape represents an important property of plastics which are to be used, for example, to produce screw closures with accurate fit.
The known molding compositions are not fully satisfactory with respect to the combined properties of dimensional stability and retention of shape (shrinkage behavior) on the one hand and good flowability on the other hand.
It is an object of the present invention to overcome these shortcomings and to provide, in particular, polyethylene molding compositions which have both good flowability and good dimensional stability and retention of shape (low shrinkage).
We have found that this object is achieved by means of the molding compositions defined at the outset, a process for their preparation, their use for producing films, fibers and shaped articles, by films, fibers and shaped articles made from these molding compositions, and by screw closures made from these molding compositions.
The novel molding compositions contain, as component (A), from 40 to 65% by weight, preferably from 55 to 60% by weight, based on the ethylene polymers, of an ethylene polymer whose density measured according to DIN 53 479 is from 0.948 to 0.964 g/cm.sup.3, preferably from 0.959 to 0.964 g/cm.sup.3, whose melt flow index, measured according to DIN 53 735 at 190.degree. C. and 2.16 kg load, is from 6 to 20 g/10 min, preferably from 6 to 10 g/10 min, and whose mean molar mass distribution M.sub.w /Mn, measured by the GPC method at 135.degree. C. in 1,2,4-trichlorobenzene (polyethylene standard), is in the range from 2 to 5, in particular from 3 to 4.
Component (A) is usually obtained by polymerizing ethylene or by copolymerizing ethylene with C.sub.3 -C.sub.12 -alk-1-enes in the presence of Ziegler catalysts known to the person skilled in the art, for example from DE-A 34 33 468, especially Example 1. The ethylene-alk-1-ene copolymers contain up to 1% by weight, preferably up to 0.5% by weight, of structural units derived from the alk-1-ene. It is preferable to use one or more comonomers selected from the class consisting of the C.sub.3 -C.sub.8 -alk-1-enes, such as propene, 1-butene, 1-pentene, 1-hexene and 1-octene.
The novel molding compositions contain, as component (B), from 35 to 60% by weight, preferably from 40 to 45% by weight, based on the ethylene polymers, of an ethylene polymer whose density, measured accor

REFERENCES:
patent: 4461873 (1984-07-01), Bailey et al.
patent: 4937299 (1990-06-01), Ewen et al.
patent: 5110685 (1992-05-01), Cross et al.
patent: 5284613 (1994-02-01), Ali et al.

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