Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-02-25
2001-08-28
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S210000, C525S211000, C525S216000, C525S232000, C525S240000, C524S528000
Reexamination Certificate
active
06281290
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to masterbatches and their method of use for producing polyolefin articles, particularly films. High density polyethylene can be incorporated into the masterbatch alone or in combination with a hydrocarbon resin and/or a polyolefin and when used in combination with a hydrocarbon resin causes the masterbatch to solidify more rapidly and be pelletized more efficiently (improved compounding efficiency). These masterbatches ultimately result in extruded polyolefin articles, such as cast polypropylene films, of optimal stiffness and ductility.
2. Discussion of Background
Polyolefins are plastic materials useful for making a wide variety of valued products due to their combination of stiffness, ductility, barrier properties, temperature resistance, optical properties, availability, and low cost. Being a semi-crystalline polymer, a number of these important properties such as stiffness, barrier properties, temperature resistance, and optical properties, depend on the ability of the polyolefin to crystallize in the most effective manner, and to the desired degree.
The process for forming a polyolefin product strongly affects the crystallization behavior of the material and its ultimate properties. For instance, when polypropylene is cast into thin film, the polymer cools so quickly that the ultimate level of crystallinity is reduced by this “quenching” process, and correspondingly the stiffness of the film is reduced. Cast polypropylene films typically exhibit a stiffness, measured as tensile modulus, of nominally 100 Kpsi. Highly oriented polypropylene (OPP) films typically-exhibit modulus values 2-4 times higher than the values for cast polypropylene film while non-oriented thick molded articles typically exhibit modulus values nominally 50% to 100% higher then cast polypropylene film. Also when making cast film, it is important that the polypropylene melt solidify quickly to promote high production rates, and also that the crystalline regions which are formed are not so large in size that they confer haze to the film.
Other molded polyolefin articles, particularly thin gauge products made by thermoforming, injection molding, or blow molding, are subject to similar constraints. Faster crystallization which permits rapid demolding and stiffer products is desired, as well as good optical properties promoted by small crystalline domain size.
As a means for improving the stiffness of polyolefins, the addition of a high softening point hydrocarbon resin to polyolefins, such as polypropylene, is known. The composition of the hydrocarbon resin must be such that it exhibits a significantly higher glass transition temperature (Tg) than the amorphous regions of the polypropylene (Tg around −10° C.), and the hydrocarbon resin must be highly compatible in the polypropylene. It is believed that the effect of the hydrocarbon resin is to increase the Tg of the amorphous polypropylene fraction and by doing so increase its tensile modulus at temperatures below 38° C.
The hydrocarbon resins described above are friable solids which exhibit very low melt viscosity at the temperatures normally used to process polyolefin. An effective way to blend hydrocarbon resin into polyolefin is in a separate compounding step prior to the final use of the blend. It is difficult to incorporate hydrocarbon resin into polypropylene during an actual conversion step (for example film casting, sheet extrusion, etc.) because of the hydrocarbon resins dusting characteristics and low melt viscosity. A more effective way to incorporate hydrocarbon resin into polyolefin during the conversion step is to add the resin in concentrate form as a mixture of resin with polyolefin. U.S. Pat. No. 5,213,744 describes a process of forming a concentrate consisting of a simple binary mixture of hydrocarbon resin and polyolefin, and using this concentrate as a more effective way of incorporating hydrocarbon resin into a polyolefin film formulation at a level of 5 wt. % to 30 wt. %.
Although the stiffening caused by adding hydrocarbon resin is desirable, it can be achieved only by adding high levels of hydrocarbon resin (typically at or above 5 wt. %) to the total polyolefin formulation, and only if the softening point of the hydrocarbon resin is 100° C. or higher, the stiffening effect increases as the hydrocarbon resin content and softening point increase. While the stiffening effect caused by the addition of hydrocarbon resin to polyolefins, is desirable, adding high levels of resin (e.g., above 5 wt. %), has a negative impact on ductility and results in increased formulation cost. Therefore, it would be highly desirable to enhance polyolefin stiffness by addition of hydrocarbon resin to the polypropylene at levels below 5 wt. % and preferably below 3 wt. %.
SUMMARY OF THE INVENTION
Before further discussion, a definition of the following terms will aid in the understanding of the present invention.
Masterbatch—a mixture of 2 or more ingredients which simplifies adding these ingredients to a material as a blend, rather than as a plurality of individual ingredients. In the present case masterbatch is defined as a blend of one or more ingredients (additives) in the proper proportion by weight with a polymer or mixture of polymers, where the total formulation is ultimately added to a second polymer, which is either the same or different than the polymer or mixture of polymers which comprise the masterbatch, as the means of incorporating the additives into the second polymer.
Additive—An additive is typically a substance added to a polymer which is non-polymeric in nature, or if it is polymeric in nature is substantially different in type and character from the polymer to which it is added. In the present case, additive refers to both the hydrocarbon resin and high density polyethylene (HDPE) which are ultimately blended with polyolefin. By our definition, HDPE is considered an additive although it could also be considered a polymer in the context of the masterbatch definition.
Polyolefin Blend—the final formulation resulting from the combination of a masterbatch with a polyolefin polymer or mixture of polymers. Accordingly, the polyolefin polymer or polymers into which the masterbatch is included is termed the blend polyolefin.
Hydrocarbon Resin—refers to low molecular weight resin products of about 10,000 number average molecular weight (Mn) or less derived from polymerizing feedstocks from the coal or petrochemical industries, resin products derived from terpene, rosin, or other feedstocks. This term will not be used to refer to high molecular weight polymer products of about 50,000 number average molecular weight or more.
As discussed above, adding a high softening point hydrocarbon resin to a polyolefin, such as polypropylene, will increase the glass transition temperature (Tg) of the amorphous phase of the polyolefin and modify its properties. One effect of hydrocarbon resin addition is greater stiffness. However to achieve significant property modification the hydrocarbon resin must be added at levels at or above 5 wt. % of the total polyolefin blend. Adding high levels of hydrocarbon resin has a negative impact on ductility and impact properties, increases formulation cost, and slows down the crystallization rate of the polyolefin. It would therefore be desirable to achieve the favorable effects of hydrocarbon resin addition at lower hydrocarbon resin add levels.
The present inventor has discovered that adding low levels of high density polyethylene (HDPE) to a polyolefin, such as polypropylene, accelerates the crystallization rate of the polyolefin when the high density polyethylene (HDPE) is adequately dispersed into the polyolefin and the ingredients are effectively added. It appears that under fast cooling conditions the HDPE crystallizes faster than the polyolefin and as the HDPE begins to crystallize it acts as a nucleator for subsequent crystallization of the polyolefin.
Accordingly, the present invention is directed to masterbatches for modifyi
Boshears E. J.
Eastman Chemical Company
Graves B. J.
Wilson Donald R.
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