Thermoplastic polymers with improved infrared reheat properties

The invention relates to a method for using spinel pigments to increase the infrared heat-up rates of thermoplastic resins and specifically polyester bottle resins. In particular, the method comprises adding spinel pigments to polymerized resins to increase the reheat rates of the resulting polyester pre-forms. When uniformly distributed, these spinel pigments absorb applied energy and thereupon transfer the energy to the polyester.

DETAILED DESCRIPTION OF THE INVENTION The invention is a method for improving the reheat characteristics of thermoplastic resins that are responsive to IR radiation. More specifically, the invention uses spinel pigments, particularly calcinated compounds of copper, chromium, iron, and nickel, to modify the specific heat properties of polyester pre-forms. In a first embodiment, polyester is synthesized either from ethylene glycol and terephthalic acid, or from ethylene glycol and dimethyl terephthalate. To achieve desired heat transfer characteristics in the resulting polyester, a spinel pigment is added to the reactants. Thereafter, a polyester bottle pre-form is injection molded from the spinel-containing polyester. The preferred polyester, polyethylene terephthalate, may be synthesized from dimethyl terephthalate and ethylene glycol by a two-step ester exchange reaction. The initial esterification step, which forms low molecular weight polyester prepolymer, typically proceeds while the reactants are in solution. The subsequent melt polymerization step proceeds at a temperature above the melting point of the polyethylene terephthalate polymer. The reaction kinetics of the melt polymerization are improved by continually removing ethylene glycol. It will be well-understood by those of ordinary skill in this art, of course, that polyester resins are frequently formed of copolymers that include either additional dicarboxylic acids (e.g. isophthalic acid) or other dihydroxy alcohols (e.g. diethylene glycol) or both. The invention described herein is just as suitable for use with such copolymers as it is with polyethylene terephthalate homopolymer. Accordingly, as used herein, phrases such as “synthesizing polyester from a dicarboxylic acid and a dihydroxy alcohol” will be understood to include the synthesis of copolymers that contain additional diacids or dialcohols or both. Alternatively, in a preferred method of synthesizing polyethylene terephthalate, terephthalic acid is reacted with excess ethylene glycol to form an esterified intermediate. This is then polymerized to form a homopolymer by way of a condensation reaction. To promote the polymerization of the polyester, ethylene glycol should be continually removed to manipulate favorably the polymerization kinetics. The synthesis of polyesters by applying either of these techniques is well-known to those of skill in the art, and is discussed—by way of example and not limitation—in Odian's Principles of Polymerization (Second Edition, 1981), which is published by Wiley-Interscience. The diacid and dialcohol reactants may be obtained as purchased chemicals or, alternatively, the polyester can be obtained from resource recovery techniques. To the extent the invention is practiced using recycled polyester, the polyester must satisfy standards for packaging food products. Using only virgin polyester polymer does not generally create a purity problem. One high-purity recycling method is fully described in the co-pending application “Food Quality Polyester Recycling,” Ser. No. 08/703,491, filed Aug. 27, 1996, and now U.S. Pat. No. ______, which is commonly assigned with this application. This effective polyester recycling technique includes first cleaning comminuted pieces of post-consumer polyester to remove surface contaminants. Then, the post-consumer polyester pieces are heated to form a polyester melt, which is then extruded and blended with a melt of virgin polyester prepolymer. The blended melt is then cooled such that the post-consumer polyester solidifies while the virgin polyester prepolymer remains as a prepolymer. The post-consumer polyester is pelletized and then polymerized in the solid state. Alternatively, a melt of post-consumer polyester and virgin polyester may first be pelletized, after which a blend of the respective pellets is polymerized in the solid state. In a second embodiment, a polyester melt is obtained by heating either virgin or recycled polyester, or a combination of both. Likewise, to promote beneficial heat transfer properties in the resulting polyester product, a spinel pigment is added to the polyester melt. A polyester pre-form is then injection molded from the spinel-containing polyester. Whether polyester is synthesized from reactants or polyester is obtained in polymer form, the methods for practicing the invention are essentially the same regardless of the nature of the polyester starting materials. The spinel pigments that are suitable in practicing the invention include both natural spinels and synthetic spinels. See Lewis, Hawley's Condensed Chemical Dictionary (12 th Edition), p. 1081. In a preferred embodiment, however, the spinel pigment is either copper chromite black spinel or chrome iron nickel black spinel. These preferred spinels are available as products designated “Black 1” and “Black 376,” respectively, from the Shepherd Color Company of Cincinnati, Ohio. Including an amount of a spine pigment—especially copper chromite black spinel or chrome iron nickel black spinel—such that the resulting spinel-containing polyester has a spinel concentration of between about 10 parts per million (ppm) by weight and about 100 ppm by weight leads to a polyester pre-form having excellent reheat properties. A spinel concentration within the spinel-containing polyester of between about 20 ppm by weight and about 50 ppm by weight is more preferred, and a spinel concentration of about 50 ppm by weight is most preferred. The spinel pigments are easier to handle than carbon black or metal particles, have a more consistent size distribution, and can be measured more consistently than carbon black or metal particles. Regardless of whether the starting materials are glycol and diacid reagents, or polyester polymer, the polyester bottle pre-forms may be injection molded directly from a spinel-containing polyester melt. Alternatively, such a spinel-containing polyester melt may be first crystallized to promote solid state polymerization or solidified to accommodate inventory requirements. In brief, the spinel-containing polyester melt may be solidified, and later heated just prior to the step of injection molding a polyester bottle pre-form. In another aspect, the invention also includes blow molding a polyester pre-form to produce a polyester bottle. This may be accomplished by forcing air into a still hot polyester pre-form, or reheating a cooled polyester pre-form to facilitate the blow molding process. Again, the capability of cooling and reheating the pre-form facilitates inventory control and helps to overcome process limitations. The inclusion of the spinel pigments in the polyester pre-form increases reheat rates, a most desirable characteristic in the bottle manufacturing art. Furthermore, the spinel concentration within the polyester pre-form is easily controlled. Thus, steady-state manufacturing processes are more easily maintained; this improves bottle production throughput and product quality. The infrared heating can be carried out in any appropriate or conventional fashion, including those techniques presently used to reheat pre-forms. The term “infrared” (IR) is generally used to refer to that portion of the electromagnetic spectrum between the visible and microwave ranges. Although the boundaries of the region are informal rather than absolute, the IR frequencies are usually considered to be from about 0.78 microns (&mgr;) to about 300 &mgr; (e.g., Lewis, supra, at page 635) or even to about 1000 &mgr; (e.g., Sze, Physics of Semiconductor Devices, 2 d Ed. (1981) at page 683). As known to those familiar with analytical spectroscopy, many of the functional groups present in organic molecules (including thermoplastic polymers) respond vibrationally to IR frequencies, thus—in the case of the pre-forms—generating the energy required for the reheating process. When polyester is produced using glycols and acids, the spinel pigments may be added at various points in the polyester synthesis. For example, the spinel pigment may be added directly to these reactants in solution at the start of chemical synthesis. Alternatively, the spinel pigments may be added to be pre-polymer melt during esterification or to the polymer melt during condensation polymerization, or just prior to injection molding. The timing of the spinel pigment addition is largely one of convenience, however, given that the spinel pigments themselves do not react or otherwise interfere with the polyester polymerization. In other words, when the spinel pigment is added to the admixture is immaterial provided that the spinel pigments become fully distributed throughout the polyester resin. Thus, the heat transfer efficacy of the spinel pigments, especially that of the copper chromite black spinel and chrome iron nickel black spinel, is unaffected by the standard polyester synthesis, which may include solution esterification, melt polymerization, crystallization, and solid state polymerization. The presence of the well-distributed spinel pigments favorably increases the reheat rate of the polyester bottle pre-forms. Practicing the invention as herein described will result in a polyester bottle pre-form having between about 10 ppm by weight and about 100 ppm by weight of spinel pigment. In preferred embodiments, such polyester pre-forms will contain between about 20 ppm by weight and about 50 ppm by weight of the spinel pigment, and in most preferred embodiments the polyester pre-form will contain about 50 ppm by weight of the spine pigment. As noted previously, copper chromite black spinel and chrome iron nickel black spinel are the most preferred spinel pigments. Finally, in practicing the invention, polyethylene terephthalate is the preferred polyester. FIG. 1 illustrates some of the advantages of the invention in graphical fashion. FIG. 1 plots the pre-form surface temperature in degrees centigrade against the reheating lamp power for four (4) samples. Line 1 represents an unmodified control polyester. Lines 2 - 4 represent a polyester prepared at pilot scale using varying amounts (including zero) of reheat additives. The composition of Lines 3 and 4 , which as FIG. 1 illustrates have the best reheat properties of the pilot polyesters, respectively include 50 ppm of the Shepherd's “Black 376 ” and “Black 1 ” pigments referred to above. Line 2 represents a composition with 20 ppm of the Shepherd's “Black 1 ”. Comparing lines 1 - 4 makes it evident that the use of the spinel pigment according to the present invention provides significantly improved reheat results. It will be understood by those of skill in the art that the invention as herein disclosed is not limited to polyester homopolymers; the invention may also be practiced using polyester that is copolymerized or blended with another kind of polymer. Moreover, the specification has disclosed typical embodiments of the invention. In doing so, however, terms have been used only in a generic and descriptive sense, and not for purposes of limitation. The scope of the invention is set forth in the following claims.