Low-odor polyethylene blends

Details Low-odor polyethylene blends Low-odor polyethylene blends

2001-01-17

2002-10-08

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...

C525S240000, C428S035700, C428S500000, C264S331110

Reexamination Certificate

active

06462135

ABSTRACT:

The present invention relates to low-odor polyethylene blends comprising (a) from 30 to 90% by weight of a high-molecular-weight ethylene copolymer with a melt flow rate MFR 190/21.6≦1.5 g/10 min, a density ≦0.950 g/cm
3
, a weight-average molecular weight M
w
≦300,000 g/mol and a polydispersity Mw/Mn of from 1 to 10, and (b) from 10 to 70% by weight of a low-molecular-weight ethylene homopolymer or ethylene copolymer with a melt flow rate MFR 190/2.16 of from 20 to 100 g/10 min, a density ≦0.95 g/cm
3
, a weight-average molecular weight M
w
of from 8000 to 80,000 g/mol and a polydispersity M
w
/M
n
of from 2.5 to 12, where the Al content of the high-molecular-weight component is from 5 to 60 ppm, the Al content of the low-molecular-weight component is from 0 to 5 ppm and the Al content of the blend is from 1 to 55 ppm. It further relates to a process for preparing polyethylene blends of this type, and also to their use for producing moldings, in particular hollow articles and pressure pipes.
Ever higher requirements are being placed upon the mechanical load-bearing capacity of moldings made from polyethylene. In particular, there is a requirement for highly stress-cracking resistant, impact-resistant and stiff products which are particularly suitable for the production of hollow articles, and also of pressure pipes. The requirement for good stress-cracking resistance at the same time as stiffness is not easy to fulfil, since these properties counteract one another. Whereas stiffness increases with the density of the polyethylene, stress-cracking resistance decreases with increasing density.
For hollow articles and pressure pipes it has therefore proven to be advantageous to use blends made from a high-molecular-weight, low-density ethylene copolymer and from a low-molecular-weight, high-density ethylene homopolymer. These are described, for example, by L. L. Böhm et al., Adv. Mater. 4, (1992) 234-238. Similar polyethylene blends have been disclosed in EP-A 100 843, EP-A 533 154, EP-A 533 155, EP-A 533 156, EP-A 533 160 and U.S. Pat. No. 5,380,807.
Pressure pipes made from polyethylene are used increasingly for conveying drinking water. For this application, besides high stiffness and high creep rupture strength at high pressures it is important that the polyethylene has very low odor and is very taste-neutral. The odor level of a material may be determined by various methods. A necessary, but not sufficient, criterion for a good odor level is a very low proportion of volatile carbon compounds in the material. For example, the test known as the VW Audi test (test specification 3341, Verband Deutscher Automobilbauer, Recommended Standard No. 277), determines volatile carbon fractions of a material at 120° C. However, it is also essential to assess odor. According to DIN 10955 and 10951 Al or EN 1622 a number of test personnel assess the odor of a material on a scale from 0 to 4. The “electronic nose” records volatile constituents of the material by means of various conductivity measurements.
Polymeric Materials Encyclopedia, Ed. J. P. Salamone, CRC Press, New York, 1996, pages 5997-98 lists possible causes for odor in polyethylene. The odor of polyethylene is generally caused by oxidation of the polymer or by catalyst residues, e.g. the triethylaluminum used as cocatalyst in Ziegler catalysts. Other possible causes are additives, e.g. Ca stearate or Zn stearate, and especially decomposition products of these. These additives are used, for example, to bind HCl deriving from Ziegler catalysts. For drinking water applications it is therefore frequently necessary to add odor-trapping additives to the polyethylene or to carry out additional steps, such as deodorization via aeration.
Low-molecular-weight components are a particular problem in the preparation of low-odor polyethylene blends, since their molecular weight should be very low to ensure sufficiently high density/stiffness in the blend. If, however, the low-molecular-weight component has a broad molecular weight distribution there is the risk that the blend will comprise too many oligomers which could cause odor. U.S. Pat. No. 5,350,807 has therefore disclosed polyethylene where a degree of polymerization of only 9 to 125 is a preferred range for the low-molecular-weight fraction. In addition, impact strength of the blend is adversely affected by too high a proportion of oligomers. The molecular weight distribution of the low-molecular-weight component should therefore be very narrow.
Narrow molecular weight distributions are preferably achieved with the traditional Ziegler catalysts and metallocene catalysts. However, these require large amounts of free organic Al cocatalysts, which adversely affect the odor of the polyethylene. The free cocatalysts may moreover form ethylene oligomers via the molecular-weight-increase reaction described by Ziegler, and these increase the proportion of volatile compounds in the polymer. In addition, Ziegler catalysts have only low productivity in the preparation of low-molecular-weight polyethylene, and therefore a large amount of catalyst is required, and therefore also a large amount of cocatalyst. Phillips catalysts give only broad molecular weight distributions and likewise require large amounts of cocatalysts in order to achieve low molecular weights. The problem of preparing low-odor, low-molecular-weight polyethylene components for ethylene polymer blends with high-productivity catalysts has not yet been solved.
None of the abovementioned disclosures encompasses ethylene polymer blends which have, besides good mechanical properties, a low odor level and low intrinsic taste.
It is an object of the present invention to provide improved blends of this type.
We have found that this object is achieved by means of the blends defined at the outset. A process for preparing blends of this type has also been found, as has their use for hollow articles and pipes.
The polyethylene blend of the present invention comprises two components.
The low-molecular-weight component (b) is composed of an ethylene homopolymer or ethylene copolymer with a weight-average molecular weight of from 8,000 to 80,000 g/mol, preferably from 20,000 to 70,000 g/mol and particularly preferably from 30,000 to 60,000 g/mol. The polydispersity M
w
/M
n
is from 2.5 to 12, preferably from 3 to 10 and particularly preferably from 5 to 8. The melt flow rate MFR 190/2.16 of the ethylene homopolymer or ethylene copolymer is from 15 to 100 g/10 min, preferably from 20 to 60 g/min and particularly preferably from 25 to 40 g/l0 min. The density is at least 0.95 g/cm
3
, preferably from 0.95 to 0.97 g/cm
3
, particularly preferably from 0.96 to 0.97 g/cm
3
.
Besides ethylene, the low-molecular-weight component may also comprise comonomers. The comonomer is selected taking account of the properties desired. However, comonomers preferably used are 1-olefins, particularly preferably propene, 1-butene, 1-pentene, 1-hexene, 1-octene or 4-methylpentene. The amount of the comonomer used is likewise selected taking into account the properties desired, but the amount is preferably not more than 1 mol %, based on the amount of all the monomers used.
The low-molecular-weight polyethylene component of the present invention has only small fractions of volatile carbon compounds. The proportion of volatile carbon, measured at 120° C. by the abovementioned VW-Audi test is not more than 80 mg/kg, preferably not more than 70 mg/kg.
The low-molecular-weight component preferably comprises no aluminum. However, the present invention also encompasses components which comprise traces of Al. Traces of this type may result, for example, from preparation, transport or storage in vessels comprising Al. The Al content of the low-molecular-weight component is, however, never greater than 5 mg/kg, based on polyethylene.
The high-molecular component (a) is composed of an ethylene copolymer with a weight average molecular weight ≧300,000 gimol, preferably from 350,000 to 700,000 g/mol and particularly preferably from 400,000 to 600,000 g/mol. T

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