HDPE polymer compostion

Details HDPE polymer compostion HDPE polymer compostion

2003-02-06

2004-10-26

Nutter, Nathan M. (Department: 1711)

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

Synthetic resins

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

C525S216000, C525S240000

Reexamination Certificate

active

06809154

ABSTRACT:

This invention relates to high density polyethylene (HDPE) compositions and to their use in the production of moulded articles.
Monomodal HDPE, produced by the use of Ziegler-Natta or chromium-based catalysts, is a well known material for the production of articles such as packaging or containers by injection- and blow-moulding.
It is important that such articles be resistant to stress-cracking, since they are frequently used to contain materials whose uncontrolled release would be environmentally undesirable. The assessing of such resistance is by the Environmental Stress Crack Resistance test (“ESCR”), the procedure for which is laid down in ASTM D 1693. Cond. B.
It is also important that they possess good stiffness, so as not to deform when stacked during transportation or storage. Stiffness in this context is expressed as Tensile Modulus (also referred to as “E-modulus”), the procedure for assessing which is laid down in ISO 527-2.
It has not proved possible with existing HDPE formulations to achieve both adequate stiffness and adequate ESCR in demanding applications such as manufacture of bottles for detergents. Increasing the density of the HDPE has been found to increase the stiffness, but only at the expense of decreasing the ESCR.
It has now been found that by using bimodal HDPE in such formulations and adding a nucleating agent to the polymer there may be obtained an HDPE moulding composition of increased stiffness and undiminished ESCR.
According to the present invention, therefore, an HDPE composition for producing moulded articles with increased E-modulus and high ESCR comprises an ethylene homopolymer or an ethylene alpha-olefin copolymer which is a bimodal polymer produced in a multistage process and comprising
i) a low molecular weight ethylene polymer and
ii) a high molecular weight ethylene polymer or copolymer and a nucleating agent.
The effectiveness of the nucleation is surprising, since although nucleation has been widely practised in manufacturing polypropylene-based compositions this has not been the case with polyethylene because it is inherently such a rapidly crystallizing polymer. The effect of nucleation on bimodal HDPE could therefore not have been foreseen.
The alpha-olefin of the aforesaid ethylene alpha-olefin copolymer is suitably selected -from propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene and cyclic olefins, its content being typically between 0.1 and 10 weight %. The density of the composition is preferably between 910 and 980 kg/m
3
, more preferably between 945 and 975 kg/m
3
, while its melt index MFR
21.6
, measured at 190° C. under 21.6 kg load, is preferably between 1 and 1000 g/10 min.
Different ranges of melt index within this range may be selected for particular applications. Thus for manufacture of household and industrial containers MFR
21.6
is advantageously between 15 and 50; for injection moulding it should be between 120 and 770; and for manufacture of large cans an MFR
21.6
between 1 and 10 is appropriate.
The overall composition suitably comprises 5-95% of the low molecular weight ethylene polymer or copolymer and 95-5% of the high molecular weight ethylene polymer, preferably 30-70% of the former and 70-30% of the latter. Its number average molecular weight M
n
is suitably between 1,000 and 20,000 g/mole, while its weight average molecular weight M
w
is suitably between 50,000 and 500,000 g/mole. Its polydispersity index M
w
/M
n
is generally between 5 and 75, although for injection moulding applications it is preferably between 10 and 20, whilst for blow moulding applications it is preferably between 20 and 60.
The most effective nucleating agents are those containing an organic group and a polar group and which are insoluble in the HDPE, such as mono- or polycarboxylic aromatic acids or salts thereof A particularly effective nucleating agent is sodium benzoate. The nucleating agent is usually present in an amount of between 1 and 10,000 ppm, advantageously between 10 and 5000 ppm; preferably between 50 and 2000 ppm, and in certain favoured embodiments between 400 and 1200 ppm.
The HDPE according to the invention is a bimodal or even multimodal polymer. By “bimodal” is meant that the polymer comprises at least two components, one of which has a relatively low molecular weight and a relatively high density and another of which has a relatively high molecular weight and a relatively low density. Typically the molecular weight distribution (MWD) of a polymer produced in a single polymerization stage using a single monomer mixture, a single polymerization catalyst and a single set of process conditions will show a single maximum, the breadth of which will depend on catalyst choice, reactor choice, process conditions, etc; i.e. such a polymer will be monomodal.
The bimodal (or multimodal) polyethylene is produced by polymerization using conditions which create a bimodal or multimodal polymer product, eg using a catalyst system or mixture with two or more different catalytic sites, using two or more stage polymerization processes with different process conditions in the different stages (eg different temperatures, pressures, polymerization media, hydrogen partial pressures, etc).
Such bimodal (or multimodal) HDPE may be produced relatively simply by a multistage ethylene polymerization, eg using a series of reactors, with comonomer addition in only the reactor(s) used for production of the higher/highest molecular weight component(s). Examples of bimodal PE production are given in EP-A-778,289 and WO92/12182.
If an ethylene homopolymer component is produced by slurry polymerization involving use of recycled diluent, that diluent may contain small amounts of higher &agr;-olefins as contaminants. Likewise where an earlier polymerization stage has produced an ethylene copolymer component, small amounts of comonomer may be carried over to an ethylene homopolymerization stage. Accordingly, by ethylene homopolymer is meant herein a polymer preferably containing at least 99.9% by weight of ethylene units. Likewise as in a multistage/multireactor polymerization using more than one catalyst system, the homopolymerization catalysts may be at least partially active during the copolymerization reaction, any copolymer component making up less than 5% by weight of the total polymer shall not be considered to be the lowest molecular weight component in an HDPE according to the invention.
The polymerization reactions used to produce the HDPE of the invention may involve conventional ethylene homopolymerization or copolymerization reactions, eg gas-phase, slurry phase, liquid phase polymerizations, using conventional reactors, eg loop reactors, gas phase reactors, batch reactors etc. (see for example WO97/44371 and WO96/18662). The catalyst systems used may likewise be any conventional systems, eg chromium catalysts, Ziegler-Natta and metallocene or metallocene:aluminoxane catalysts, either homogeneous or more preferably heterogeneous catalysts, eg catalysts supported on inorganic or organic particulates, in particular on magnesium halides or inorganic oxides such as silica, alumina or silica-alumina. For the preparation of the high molecular weight component in particular it is especially desirable to use supported Ziegler-Natta catalyst as the molecular weight can then conveniently be controlled using hydrogen. It is also possible to use supported metallocene catalysts as it is particularly straightforward to select desired molecular weights by appropriate selection of particular metallocenes. The metallocenes used will typically be group IVa to VIa metals (in particular Zr or Hf) complexed by optionally substituted cyclopentadienyl groups, eg groups carrying pendant or fused substituents optionally linked together by bridging groups. Suitable metallocenes and aluminoxane cocatalysts are widely described in the literature, eg the patent publications of Borealis, Hoechst, Exxon, etc.
Typically however the HDPE will be prepared using multistage polymerization using a single catalyst system or a plurality

Affiliated with

Also associated with

Rating

HDPE polymer compostion does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with HDPE polymer compostion, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and HDPE polymer compostion will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3261041