Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-05-28
2002-11-12
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C264S176100, C528S176000, C528S196000, C528S198000, C528S271000, C528S272000, C528S332000, C528S480000, C528S50200C
Reexamination Certificate
active
06479625
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermoplastic polymers and methods for the crystallization of these polymers. More particularly, it relates to the discovery that the thermoplastic polymers may be expeditiously crystallized by the application of mechanical stress and heat to particles of the polymer without the hazard of stickiness and agglomeration of the polymer particles in the crystallizer.
2. Description of Prior Art
Crystallization of many thermoplastic polymers is necessary to make them workable in further processing steps toward the preparation of plastic end products. Crystallization is particularly important where the polymers are amorphous polymers, copolymers, or polymer blends which become sticky and agglomerate at relatively low temperatures. Without crystallization it is difficult to handle these polymer particles in blow molding, extrusion, and blending operations without problems of agglomeration of the polymer particles and sticking of the polymer to operating equipment. Polyesters of various types and copolymers containing polyesters usually must be crystallized before further processing, for example by “solid state” heating to increase melting points. Polyester particles tend to become sticky and agglomerate at temperatures much lower than their crystallization temperature, and are importantly typical of thermoplastic polymers in general.
Thermoplastic polymers have been crystallized by slowly heating the polymer particles through a sticky phase which occurs at temperatures somewhat lower than the crystallization temperature of the polymer. The polymer particles are slowly heated further to where the amorphous polymer chains are converted to crystals usually with the release of exothermic heat from the crystallization. It has been necessary to approach the exothermic phase slowly to prevent runaway temperature rises which can damage the physical properties and color of the polymers. With slow heating of the polymers long term elevated temperatures still can cause thermal damage to the polymers and yellowing of the product.
Even with slow heating using the present state of the art, parts of the thermoplastic particles become overheated, and semimelted so that the particles become sticky and agglomerate to form large masses in the crystallization equipment. This can damage the apparatus and the polymer properties, such as the intrinsic viscosities, melting points, and particle sizes.
The main prior art slow heat crystallization method requires the use of large vessel volumes per unit of production and the use of large amounts of inert gas, such as nitrogen, which is passed through the crystallizer to protect the hot thermoplastic from oxidation during the long hold up time required.
Tung et al in U.S. Pat. No. 5,919,872 disclosed that polyethylene terephthalate polyesters when coated with alkylene carbonates crystallize more quickly and at lower temperatures than the same uncoated polymers.
Herse et al in U.S. Pat. No. 5,663,290 disclosed crystallization of poly(ethylenenaphthalenedicarboxylate) and its copolymers by maintaining water content below a critical point throughout the crystallizing process so that a separate drying step is not required.
Keilert in U.S. Pat. No. 5,628,947 also disclosed a process for simultaneous drying and crystallization of crystallizable thermoplastics, by cooling strands of extruded polymer in 1.5 to 20 seconds.
Palmer in U.S. Pat. No. 6,344,539 discloses a process for crystallizing polyester granules providing a bi-component structure consisting of a thin crystalline skin and an amorphous interior. The crystalline skin prevents the granules from sticking in subsequent hot processing and the amorphous interior facilitates melting at lower temperatures in subsequent extrusion processes.
U.S. Pat. No. 3,544,523 to Mobil Oil teaches that polyester granules may be prevented from sticking by coating the granules with a small amount of anti-caking agent such as talc.
Al Ghatta et al in U.S. Pat. No. 5,714,571 disclose a method of continuous crystallization of polyester resin in a whirling fluid bed crystallizer where fluiding nitrogen gas enters at a temperature not lower than 195° C. and the average residence time of the polymers is higher than 5 minutes. Feeding the polymer from the fluid bed to a mechanical mixer, which moves the polymer longitudinally submitting the polymer to radial mixing, is disclosed. The fluid bed disclosed uses non-backmix piston type continuous flow to obtain uniform crystallization values in the product. The fluidization velocities of the nitrogen gas through the fluid bed is between 3 and 5 meters per second. The crystallinity degree of the polymer coming out of the fluid bed treatment is between 38 and 42 percent and is brought to 40 to 50 percent by subsequent crystallization processes which are conveniently carried out by moving longitudinally through a mechanical mixer at temperatures 10 to 30° C. higher than that of the polymer leaving the fluid bed. Another fluid bed may be used instead of a mechanical mixer.
Shelby et al in U.S. Pat. No. 6,159,406 disclose a process for introducing strain-induced crystallization to polyesters by sending amorphous polyesters directly from a melt phase reactor through a traditional strand die and then uniaxially stretching the extruded polymer strand resulting in an increased rate of crystallization than obtained with traditional thermal crystallization.
In summary, the prior art provides for the crystallization of thermoplastic polymers by: (1) an inert gas-fluidized bed of polymer particles with the use of large amounts of difficult to heat and expensive inert gas; (2) continuously stretching strands of polymer to increase the rate of thermal crystallization requiring a melt extruder and strand chopper at the site of the crystallization operation; (3) the use of a whirling fluid bed crystallizer heated by inert gas at more than 195° C. at a velocity of 3 to 5 meters per second to obtain 38 to 42 percent crystallization requiring the operation of a sophisticated industrial installation; and the addition of costly property changing coatings to the polymers.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to provide an effective, non-agglomerating, method of crystallizing crystallizable thermoplastic polymers in a simple mechanically fluidized crystallizer where the friction generated in the mechanical fluidization deforms the polymer particles to create sufficient stress in the polymer particles to orient the polymer molecules into crystals with the heat generated by the mechanical fluidization.
It is also an object to provide a method of crystallization that does not require the flow or heating of inert gases.
It is a further object to provide a method of crystallization that requires no additives.
It is a further object to provide a method which may be operated at locations remote from sophisticated industrial installations, not requiring melting, extrusion, stretching, quenching, chopping, drying, or solid state polymerizations of the polymers.
It is a further object to provide a method which allows the crystallization of the polymers accurately to the degree of crystallization desired.
It is a further object to provide a method which is particularly effective in crystallizing polyester polymers and copolymers which are difficult to crystallize without agglomeration.
SUMMARY OF THE INVENTION
I have now discovered that deformation of crystallizable thermoplastic polymer particles applied in a mechanically fluidized crystallizer creates heat of friction and mechanical stress on the particles sufficient to align the polymer molecules into crystallized polymers without agglomeration of the polymer, and I have also found that desired degrees of crystallinity may be obtained in the polymer particles by the accurate control of temperature and stress treatment time in a mechanically fluidized crystallizer. The discovery overcomes the limitations of the prior art and provides the objects of the in
Agri-Nutrients Technology Group, Inc.
Boykin Terressa M.
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