Compositions – Preservative agents – Anti-oxidants or chemical change inhibitants
Reexamination Certificate
2000-04-28
2003-07-22
Thexton, Matthew A. (Department: 1714)
Compositions
Preservative agents
Anti-oxidants or chemical change inhibitants
C252S400530, C252S403000, C252S407000
Reexamination Certificate
active
06596198
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to pellets comprising an additive system comprising at least a stabilizer and a mold release agent. The stabilizer comprises less than 50 wt. % the combined total weight of the stabilizer and the mold release agent.
BACKGROUND OF THE INVENTION
Organic polymers, in particular polyolefins such as polyethylene and polypropylene, commonly are known as “plastics.” Various additive systems are used during the processing of plastics in order to assure that the plastic product has long term stability and desired service properties. Additives and stabilizers prevent the plastic product from being damaged by light, heat, and by residues of the catalyst system used to produce the plastic.
Many commercially available additives are effective to protect against polymer discoloration and thermooxidative degradation. In the past, the protective additives were supplied in powder form, which led to difficulty in handling the additives. Powder additives are difficult to convey in the systems most commonly used in the plastics industry, and significant losses of the powders occur during conveying. Powder additives also are dusty, presenting a potential for dust explosion and potential health risks to workers, who either inhale the dust or whose skin comes into contact with the dust. Additionally, powders are difficult to meter accurately.
Forming pellets from the powders alleviates the problems described above; however, the operation of a pellet mill subjects the powdered components to shear forces. Increased temperatures caused by shear forces can cause the stabilizer to melt and fuse in the pellet mill, preventing the production of pellets. In order to avoid melting and fusing of the stabilizer in the pellet material, fatty acids, fatty alcohols, and/or their derivatives (sometimes collectively called “fatty acids”) having a relatively low melting point are added to the stabilizer. Certain fatty acids also function as mold release agents during polymer part forming processes, such as injection molding, thermoforming, and the like.
Fatty acids produce acceptable product quality even at relatively high concentrations in most host plastics, with about 4000 ppm by weight to about 7000 ppm by weight generally being acceptable. However, acceptable product quality is achieved only using lower concentrations of the stabilizer—generally, from about 100 ppm by weight to as much as 4000 ppm by weight.
Pelleted additive systems are needed which have adequate pellet properties and which contain a relatively smaller concentration of stabilizer
SUMMARY OF THE INVENTION
The present invention provides a stabilizer additive system for a polymer host comprising pellets. The pellets comprise an amount of at least one stabilizer having a first melting point. The stabilizer is selected from the group consisting of an antioxidant, a UV light stabilizer, and a combination thereof. The pellets also comprise a quantity of at least one processing aid selected from the group consisting of a mold release agent, a lubricant, and a combination thereof. The processing aid has a second melting temperature lower than the first melting temperature. The amount of the stabilizer and the quantity of the processing aid comprise a combined total weight. The amount of the stabilizer is from a minimum amount effective to stabilize the polymer host to less than 50 wt. % of the combined total weight.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides pellets of an additive system for stabilizing polymeric materials against ultraviolet light and thermooxidative deterioration. The pellets contain a “reduced amount” of stabilizer in order to increase product quality. As used herein, the phrase “reduced amount” when used to refer to the stabilizer is defined to mean a concentration of less than 50 wt. % based on the combined weight of the mold release agent and the stabilizer. The pellets may be designed to produce a variety of concentrations and ratios of stabilizer/mold release agent in a host plastic. Preferably, the pellets comprise a concentration of stabilizer based on the combined weight of the mold release agent and the stabilizer of less than about 50 wt. %, preferably from about 5 wt. % to less than about 50 wt. %, more preferably from about 5 wt. % to about 40 wt. %, and most preferably from about 5 wt. % to about 10 wt. %. In a most preferred embodiment, the pellets comprise a compressed powder blend in which the weight ratio of the mold release agent(s) to the stabilizer(s) is from greater than about 1:1 to about 70:1, most preferably about 1:1 to about 10:1.
As used herein, the term “pellet” generally refers to granules made using extrusion techniques. “Pellets” may have substantially any shape, but typically are small, columnar or cylindrical bodies of a sterically hindered phenol antioxidant, and may have flat surfaces such as cubes, rectangular parallelepipeds, etc. The pellets are formed from a homogeneous mixture of powders of at least one stabilizing compound, a mold release agent, and may comprise other desired additives. Optional additives known in the art include, but are not necessarily limited to nucleating agents, clarifiers, pigments and colorants, metal deactivators, and acid neutralizers. The pellets are coherent enough to provide sufficient abrasion resistance during handling, but insufficiently coherent to interfere with dispersion of the additive system in a polymer host.
In order to form pellets, the powder blend is pressed through a die extruder to form strands. In a preferred embodiment for making the pellets, the powdered stabilizer, the powdered mold release agent, and any powdered additive are dry blended to form a homogeneous powder blend. The powder blend then is fed to a suitable Pellet Mill at a maximum rate that achieves an acceptable pellet yield. As used herein, the term “pellet yield” refers to the wt. % yield of pellets of nominally about 3 mm diameter. Suitable pellet yields are about 80 wt. % or more, preferably about 90 wt. % or more, more preferably about 95 wt. % or more.
Persons of ordinary skill in the art recognize that a variety of factors affect the temperature of extrusion, including but not necessarily limited to powder composition, rotor speed, feed rate, solvent, type of pellet mill, etc. The “aspect ratio,” or the ratio of the diameter to the length of the holes in the die plate, is of particular importance. The smaller the “aspect ratio,” the cooler the temperature of extrusion. In the present invention, the temperature of extrusion is maintained below the melting temperature of the stabilizer, preferably at about 100° C. or less, depending upon the stabilizer, most preferably about 70° C. or less. Typically, an aspect ratio of from about 3 to 1 to about 5 to 1 is required to maintain an extrusion temperature of about 70° C. or less. The feed rate of the powder blend preferably is as high as possible while still maintaining this temperature. Using a Kahl Model 14-175 pellet press, the feed rate preferably is at least about 45 lb/hr, more preferably at least about 60 lb/hr, and most preferably about 100 lb/hr or greater.
Examples of pelleting equipment suitable for adaptation and use in the present invention include, but are not necessarily limited to those described in the following U.S. Patents, which are incorporated herein by reference: U.S. Pat. Nos. 4,446,086; 4,670,181; 4,902,210; 5,292,461. A preferred pellet press is a Kahl Model 14-175 Pellet Press equipped with a die plate containing holes of from about 2 to about 6 mm diameter, preferably about 3 mm diameter, which runs at from about 25 lb/hr to about 150 lb/hr. The length at which the strand-like product breaks after leaving the die is determined by a number of factors, including but not necessarily limited to the composition, the temperature, the extrusion pressure, the speed of the revolutions, and the distance between the cutters and the bottom of the die plate. The press operates at a rotor speed of nominally from about 80 to about 250 rpm, pr
Albemarle Corporation
Pippenger Philip M.
Thexton Matthew A.
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