Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-04-27
2001-04-24
Copenheaver, Blaine (Department: 1771)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S096000, C521S099000, C521S149000
Reexamination Certificate
active
06221925
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to foaming of high density polyethylene (HDPE) to form articles having a low final density and a fine closed cell structure. The HDPE resin is altered with respect to low shear viscosity and elasticity to improve melt strength and thereby foamability.
BACKGROUND OF THE INVENTION
Poor melt strength of high density polyethylene is a major factor which makes it difficult to produce foamed articles therefrom. In the past, polyethylene foams have been routinely made from high pressure-low density polyethylene (LDPE) which exhibits superior melt strength compared to that of the HDPE, at the same viscosities. The melt strength of HDPE can be increased simply by increasing the HDPE molecular weight. However, the increase in molecular weight is accompanied by an increase in melt viscosity, which interferes with processability and contributes to cell collapse in the foaming process. The highly branched nature and low viscosity at high shear of the LDPE provides for a desirable environment for foaming, as compared to the normally linear HDPE molecule.
Therefore, it is an object of the present invention to provide an HDPE suitable for foaming.
SUMMARY OF THE INVENTION
The invention provides an HDPE resin with high melt strength relative to its melt viscosity and is thus similar to high pressure low density polyethylene. The invention also provides foams of the HDPE and articles of manufacture produced therefrom. The process of the present invention comprises treating the HDPE with low levels of high temperature peroxide and/or electron beam radiation to induce long chain branching of the previously linear HDPE molecules. Melt strength and elasticity are improved dramatically without a large increase in viscosity, thereby improving the foaming characteristics of the HDPE resin.
The invention therefore includes a process for forming articles consisting essentially of foamed high density polyethylene having an original density, prior to foaming, of at least 0.94 g/cc, comprising treating said high density polyethylene, prior to foaming, with a peroxide to provide a peroxide treated high density polyethylene having a low shear viscosity which is at least about 1.25 times the viscosity of the untreated high density polyethylene measured at 0.1 rad./sec. and a high shear viscosity which is less than about 3.0 times the viscosity of the untreated high density polyethylene measured at 100 rad./sec.;
admixing the peroxide treated high density polyethylene with a blowing agent; and
foaming the peroxide treated high density polyethylene to form a closed cell foam product which exhibits a density reduction, as a result of foaming, of over 20%.
DETAILED DESCRIPTION OF THE INVENTION
The foamed products of the present invention are produced from an HDPE resin which is generally, at least 95% HDPE. The process of the present invention comprises compounding, or admixing, the HDPE with a peroxide and/or exposing to electron beam radiation.
The starting HDPE material has a density of at least 0.94 g/cc as measured by ASTM D-792. The HDPE may be the product of gas phase, slurry or solution polymerization. Polymerization can be conducted in the presence of metallocene, or metallocene based catalysts, as well as with chromium or Ziegler catalysts. The HDPE can be a homopolymer of ethylene or modified to contain small amounts of comonomer selected from an alpha olefin containing 3 to 10 carbon atoms, preferably 4 to 10 carbon atoms; in these instances the polymer resin will contain greater than 95% of its weight as ethylene units.
In accordance with the invention, it has been discovered that the control of melt strength and melt viscosity may be achieved in a single operation. In accordance with the invention, the untreated or unmodified HDPE may be subjected to peroxide modification at elevated temperatures, which are above ambient. The level of peroxide added to the HDPE is generally in the range of from about 50 to about 5000 ppm. The temperature of the peroxide treatment is generally in the range of from about 150° to about 260° C.
The peroxides used in the present invention are high temperature peroxides that may undergo almost complete decomposition at normal compounding temperatures (200° to 260° C.). The half life temperature of the peroxides used in the present invention at 0.1 hours is greater than 130° C. Half life temperature at a given time is the temperature at which one half of the peroxide has decomposed. Suitable peroxides include but are not limited to dicumyl peroxide, 2,5-dimethyl-2,5-di-(tert butyl peroxy), hexane, tert-butyl cumyl peroxide, di-(2-tert-butylperoxyisopropyl) benzene, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexyne-3, cumene hydroperoxide these contain 2 to 20 carbon atoms. The peroxide may be preblended with the HDPE or introduced separately as a liquid feed, such as in a mineral oil carrier and compounded using conventional compounding methods.
Treatment of the HDPE is preferably in a nitrogen atmosphere. Nitrogen is introduced to the zone of HDPE treatment in accordance with the invention at the feed throat of the compounding extruder so as to minimize exposure to oxygen. Compounding under this condition significantly enhances the crosslinking efficiency of the peroxide. The resultant peroxide treated HDPE retains its thermoplastic properties. This property is valuable for recycling.
The HDPE may also be compounded or admixed with at least one, preferably two, antioxidants. The role of antioxidant stabilizers in HDPE is to protect the polymer from oxidative degradation after compounding or admixing and thus preserve its strength properties. The mechanism for degradation of HDPE via oxidation is an autocatalyzed, free radical chain process. During this process, hydroperoxides are formed which decompose into radicals and accelerate the degradation. Antioxidants prevent this degradation by (1) scavenging radicals to interrupt the oxidative chain reaction resulting from hydroperoxide decomposition and (2) consuming hydroperoxides.
The antioxidants contain one or more reactive hydrogen atoms which tie up free radicals, particularly peroxy radicals, forming a polymeric hydroperoxide group and relatively stable antioxidant species. The mixture of the primary antioxidant and/or secondary antioxidant in the HDPE is in the range of from about 300 to about 3000 ppm based on the desired level of oxidative stability desired in the final foamed product. The phenolic antioxidants are the largest selling antioxidant used in plastics today; they include simple phenols, bisphenols, thiobisphenols and polyphenols. Primary antioxidants include hindered phenols such as Ciba Geigy's Irganox 1076, 1010 and Ethyl 330.
Secondary antioxidants include phosphorus-based antioxidants, generally phosphites. The phosphite acts by converting hydroperoxides to non-chain propagating alcohols, while the phosphite itself is oxidized to phosphates. These secondary antioxidants are used when processing stability is of concern. Trisnonylphenyl phosphite is one of the most widely used phosphites. Other suitable secondary antioxidants are GE's Weston TNPP, Ciba Geigy's Ultranox 626 and Irgafos 168. An exhaustive list of primary and secondary antioxidants can be founds in the reference [
Chemical Additives for the Plastics Industry
, Radian Corporation, Noyes Data Corporation, N.J., 1987].
The resultant HDPE can be characterized by its large increase in low shear viscosity (measured at 0.1 rad/sec) and small increase in high shear viscosity (measured at 100 rad/sec). The resultant HDPE also exhibits a significant increase in elasticity for a given melt index. Increasing elasticity indicates increased melt strength. Elasticity is defined as the ratio of G′ to G″, the elastic modulus to the storage modulus. The ratio of viscosity of peroxide treated HDPE to untreated HDPE ranges from 1.25 to 40 at low shear, preferably 1.8 to 30, and from 1.0 to 3.0 at high shear, preferably 1.0 to 2.2.
The peroxide treated HDPE of
Constant David R.
Poloso Anthony
Copenheaver Blaine
Cuomo Lori F.
Mobil Oil Corporation
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