Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2000-11-20
2001-11-20
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C525S437000, C525S444000, C524S048000, C524S129000, C524S284000, C428S036900
Reexamination Certificate
active
06319576
ABSTRACT:
FIELD OF THE INVENTION
This is invention relates to a method for crystallizing polyester compositions. In particular, this invention relates to increasing the crystallization rate of poly(ethylene terephthalate) (PET) with hyperfunctional nucleation agents.
BACKGROUND OF THE INVENTION
Thermoplastic polyester materials have many commercial applications and are commonly used in fabrics, films, and containers. Polyesters are popular due the their mechanical strength, flexural characteristics, clarity, thermal stability, impact strength, and permeability characteristics. In the beverage industry, in particular, poly(ethylene terephthalate) (PET) has emerged as a major material for bottling carbonated as well as non-carbonated beverages.
In spite of these positive characteristics, PET possesses some significant limitations. One of these limitations is related to its rate of crystallization. The rate of crystallization of PET is slow relative to some other plastics, such as poly(butylene terephthalate) (PBT) and high density polyethylene (IIDPE). A consequence of this relatively slow rate of crystallization is that relatively long cycle times are required to achieve crystallinity in PET, and when achieved, crystallinity is often accompanied by opacity, due to the relatively large size of crystallites formed by thennal crystallization. Crystallinity itself is often desirable in molded parts, due to the higher thermal and mechanical stability associated with crystallinity. Crystallinity is especially desirable when parts or containers will be subjected to elevated temperatures.
Prior art methods to increase the rate of crystallization of PET have been directed toward incorporation of inorganic compounds, polyolefins, and salts into PET to act as nucleation aids. The use of these inorganic compounds and salts, while having a positive effect on the crystallization rate, have other adverse consequences. In particular, inorganic compounds and salts typically lower the thermal stability of PET. Salts such as sodium chlorobenzoate react with the polyester, and adversely affect the molecular weight. In addition, their effectiveness is time dependent; that is, the effectiveness of a given amount of sodium chlorobenzoate is dependent on the processing time in the polymer melt. Polyolefin nucleation agents, while increasing the rate of crystallization of PET, are relatively ineffective (requiring high loading levels), inherently affect clarity, and are thermally unstable at normal polyester processing conditions.
Thus, there exists a need in the art for a method to accelerate the rate of crystallization of PET that will allow crystalline PET parts to be manufactured without incurring long cycle times. In particular, there exists a need for a method to accelerate the rate of crystallization of PET that does not suffer the limitations of prior art methods.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a method of crystallizing a polyester composition to produce a polyester article that is highly crystalline.
Another object of the present invention is to provide a method of crystallizing a polyester composition to produce a highly crystalline polyester article at high rates and without multiple processing steps.
Another object of the present invention is to provide a method of crystallizing a polyester composition that has improved clarity over prior art inventions.
Another object of the present invention is to provide a method of crystallizing a polyester composition that does not impair the thermal stability of the polyester.
This invention solves the above-described problems in the art by providing a method for increasing the crystallization rate of a crystallizable polyester comprising incorporating into the polyester an effective amount of a hyperfunctional nucleation agent. The hyperfunctional nucleation agent comprises a plurality of functional groups and is capable of forming covalent bonds with polyester chains. It is believed that, when the hyperfunctional nucleation agent is incorporated into the polyester, there is an increased tendency for the polymer chain to cluster, with a concomitant reduction in the entropy of the polyester in the vicinity of the hyperfunctional nucleation agent. Thus, the hyperfunctional nucleation agent is a stable nuclei in the polyester around which further crystallization occurs.
This invention also encompasses a method for crystallizing polyester comprising thermally forming a mixture including polyester and an effective amount of a hyperfunctional nucleation agent. Suitable methods of thermoforming include melt extrusion, injection molding and blow molding. Thus, this invention also encompasses articles made by thermally forming the mixture including polyester and the hyperfunctional nucleation agent. For example, the method of this invention is useful to make containers such as bottles.
These and other objects, features, and advantages of the invention will become apparent in the following detailed description and claims.
DETAILED DESCRIPTION OF THE INVENTION
In the crystallization of polymers, there are two separate events that occur. The first event is the formation of nuclei in the polymer matrix. The second event is growth of the crystallite around these nuclei. The overall rate of crystallization of the polymer is dependent, therefore, on the equilibrium concentration of nuclei in the polymer matrix, and on the rate of growth of crystallites around these nuclei.
Typically, at high temperatures, the rate of growth of crystallites is fast, while the steady state concentration of nuclei is relatively low. Conversely, at lower temperatures, the steady state concentration of nuclei is higher, but the rate of crystallite growth is slower. A consequence of this two-step process is that at high temperatures (relatively near the melting point) and at low temperatures (relatively near the glass transition temperature) the rate of crystallization is slow, while the rate is relatively faster at temperatures intermediate between these two transition temperatures. Another consequence is that at high temperatures there are relatively few, large crystallites formed, while at low temperatures, there are relatively more numerous, smaller crystallites formed.
In order to increase the rate of crystallization of a polymer, then, one must increase either the steady-state concentration of nuclei in the polymer matrix, or increase the rate of growth of crystallites. Increasing the rate of crystal growth a low temperature is difficult to achieve without changing the polymer composition itself, since the rate of crystal growth is inherently linked to the rate of motion of the polymer chains, which in turn is directly related to the glass transition temperature. Conversely, the steady-state concentration of the nuclei can, in principle, be readily adjusted, if stable surrogate nuclei can be incorporated into the polymer matrix. If this can be achieved, then the rate of crystallization will increase with increasing temperature, rather than going through a maximum rate at intermediate temperatures. This effect would be exhibited by a shift in the temperature of maximum crystallization rate toward higher temperatures.
While the number of crystallites formed is directly related to the number of nuclei present, the ultimate total crystallinity achieved at a given temperature will be approximately the same, regardless of the number of nuclei. Thus, if there are relatively few or relatively large nuclei, the resulting crystallites will be relatively large. Conversely, if there are a relatively large number of nuclei and each nucleus is relatively small, the resulting crystallites will also be relatively small. Large crystallites have a greater tendency to scatter incident light than small crystallites; therefore, when large crystallites are present, the polymer matrix will be more hazy or opaque than when small crystallites are present (at the same total level of crystallinity). Therefore, by use of effective nucleation agents that promote t
Rule Mark
Shi Yu
Acquah Samuel A.
Sutherland & Asbill & Brennan LLP
The Coca-Cola Company
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