Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1995-12-29
2002-03-26
Hoke, Veronica P. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S151000, C524S153000, C524S310000, C524S377000, C524S388000, C524S249000, C524S493000
Reexamination Certificate
active
06362258
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a stabilized thermoplastic polymer composition.
DESCRIPTION OF THE RELATED ART
It is well known to formulate polyolefin polymers with phosphorus-containing antioxidant-stabilizer additives, e.g., phosphites, phosphonites, to improve and preserve the properties of the polymer. It has been observed that silica-containing polyolefin polymers that are stabilized with phosphorus-containing additives exhibit a relatively rapid decrease in the amount of phosphorus-containing additive under conditions of elevated temperature and high relative humidity. The rapid disappearance of the phosphorus containing additive has given rise to a concern that the long term stability of phosphorus-containing additive stabilized polyolefin polymers compositions with respect to, e.g., melt index, may be compromised. What is needed is a technique by which the rate of disappearance of the phosphorus compound from the polyolefin polymer composition is slowed, without detrimentally affecting other properties, e.g., color, of the polyolefin polymer composition.
SUMMARY OF THE INVENTION
The present invention is directed to thermoplastic polymer composition, comprising:
a polyolefin polymer;
silica;
a phosphorus-containing antioxidant compound; and
a co-additive, said co-additive comprising one or more of (i) a polyalkylene glycol having a number average molecular weight of greater than or equal to 3,000, (ii) a polyalkylene oxide, (iii) a sterically hindered amine compound.
The thermoplastic polymer composition of the present invention exhibits low color and exhibits a substantial decrease, relative to analogous compositions that lack the co-additive of the composition of the present invention, in the rate at which the phosphorus-containing antioxidant compound is consumed, i.e., disappears, from the composition, particularly under conditions of elevated temperature, high relative humidity or elevated temperature and high relative humidity.
Another aspect of the present invention is directed to a method for decreasing the rate of consumption of a phosphorus-containing antioxidant compound in a polyolefin polymer composition, wherein said polyolefin polymer composition initially comprises the phosphorus-containing antioxidant compound and silica, comprising: adding a co-additive to the polymer composition, said co-additive comprising one or more of: (i) a polyalkylene glycol having a number average molecular weight of greater than or equal to 3,000, (ii) a polyalkylene oxide, (iii) a sterically hindered amine compound, to the polyolefin polymer composition.
DETAILED DESCRIPTION OF THE INVENTION
Polyolefin Polymer
Polyolefin polymers and methods of making polyolefin polymers are well known in the art Suitable polyolefin polymers include polyolefin homopolymers, such as, e.g., polyethylene, polypropylene, polyisobutylene, and polyolefin copolymers such as, e.g., poly(ethylene-propylene) copolymer, poly(ethylene-butylene) copolymer as well as blends thereof. In a preferred embodiment, the polyolefin polymer is a high density polyethylene polymer, i.e., a high density polyethylene homopolymer or a high density polyethylene copolymer.
Suitable high density polyethylene polymers are made, e.g., by a process wherein ethylene is polymerized in a slurry system at low pressure (2 to 4 megaPascals (MPa)) and moderate temperature (90 to 150° C.) in a low boiling hydrocarbon, e.g., isobutane or isopentane, the presence of a catalyst that includes chromium in the Cr(VI) state and a powdered substrate of high surface area, e.g., porous amorphous silica. A chromium compound and silica are mixed together and heated in a stream of dry air to activate the catalyst. Homopolymerization using the chromium catalyzed process produces a polymethylene or very high molecular weight &agr;-olefin. Typical homopolymer made by the chromium catalyzed process has a weight average molecular weight of about 40,000 to about 1,000,000, a melt index, as measured by ASTM Method D 1238-5T, of from 0.01 grams to 20 grams per 10 minutes (g/10 min) at 190° C., a density of about 0.960 to about 0.965 grams per cubic centimeter (g/cm
3
) and a crystallinity of about 94%. High density polyethylene copolymers containing units derived from other olefin monomers, e.g., 1-butene, 1-hexene, 1-octene, typically exhibit weight average molecular weights of from about 20,000 to about 1,000,000, melt indexes in the range of about 0.1 g to about 100 g/10 minutes and densities of from about 0.936 g/cm
3
to about 0.96 g/cm
3
.
Suitable high density polyethylenes are commercially available from a number of commercial sources, including, e.g., Phillips, Union Carbide, BASF and Paxon Polymer Company.
The composition of the present invention may, optionally, further include one or more other polymers blended with the polyolefin polymer, e.g., a fluoropolymers added for the purpose of enhancing processability, at a level of up to about 500 parts by weight (pbw) of the other polymer per million pbw of the polyolefin polymer.
Silica
In a preferred embodiment, silica is present in the composition of the present invention at a level of from 20 ppm to 1000 ppm silica, based on the amount of polymer.
In one embodiment of the present invention, the silica component is introduced as a support for a polymerization reaction catalyst used in making the polymer. Typical catalyst compounds used for polymerization reactions include, e.g., chromium compounds, vanadium compounds, titanium compounds. Suitable silica supported chromium catalysts include, e.g., chromium trioxide/silica, chromocene/silica, hydrated chromium acetate/silica, bis(triphenyl silyl) chromate/silica. Typically, the silica catalyst support residue is present at a level of from 20 ppm to 500 ppm silica, based on the amount of polymer, i.e., parts by weight per one million parts by weight polymer.
Silica may also be introduced to the composition of the present invention, in the form of fumed silica, as an anti-block additive. Suitable anti-block additives are commercially available, e.g., from Cabot Corporation. The silica anti-block additives are typically used at a level of from 50 ppm to 1000 ppm silica, based on the amount of polymer.
While not wishing to be bound by theory, it is believed that silica present in prior art polymers is involved in the undesirably rapid disappearance of the phosphorus compound from the polymer, particularly when the polymers are exposed to conditions of elevated temperature, high relative humidity or elevated temperature and high relative humidity.
Phosphorus-containing Antioxidant Compound
Phosphorus-containing compounds suitable for use as antioxidants and stabilizers in polymer compositions are well known in the art. Suitable phosphorus-containing antioxidant compounds include organo-phosphorus compounds, e.g., phosphites and phosphonites, such as, e.g., triphenyl phosphite, diphenylalkyl phosphites, phenyldialkyl phosphites, tris(nonyl-phenyl)phosphite, trilauryl phosphite, trioctadecyl phosphite, distearyl pentaerythritol diphosphite, tris(2,4-di-tert-butylphenyl)phosphite, diisodecyl pentaerythritol diphosphite, 2,2-ethylidene bis(4,6-di-tert-butylphenyl)fluorophosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, tristearyl sorbitol triphosphite, (2,4,6-tri-tert-butylphenyl)(2-butyl-2-ethyl-1,3-propanediol)phosphite, 2,2′,2″-nitrilo[triethyl-tris[3,3′,5,5′-tetra-tert-butyl-1,1′-biphenyl-2-2′-diyl]phosphite and tetrakis(2,4-di-tert-butylphenyl)4,4′-biphenylene diphosphonite.
In a preferred embodiment, the phosphorus compound is a phosphite compound and, more preferably, is tris(nonyl-phenyl)phosphite, (2,4,6-tri-tert-butylphenyl)(2-butyl-2-ethyl-1,3-propanediol)phosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite or distearyl pentaerythritol diphosphite.
The phosphorus compound is, at least initially, present in an amount effective to improve the oxidative stability of the polymer. In a preferred embodiment, the composition of the present i
Ashton Henry C.
Avakian Roger W.
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