Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2002-09-12
2004-05-25
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C528S298000, C528S301000, C528S302000, C528S307000, C528S308000, C528S308600, C528S503000, C428S034100
Reexamination Certificate
active
06740377
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to polyester compositions, and particularly to poly(cyclohexylenedimethylene terephthalate) (PCT) copolyester formulations having improved crystallization behavior and articles made therefrom exhibiting improved shatter resistance. More particularly, the invention relates to PCT copolyester formulations prepared by the process of solid state polymerization and have greater than 70 mole % 1,4-cyclohexanedimethanol, a crystallization halftime, as defined herein, of between 2 minutes and 10 minutes, and an inherent viscosity of greater than 0.90 dL/g, and to extrusion blow molded articles made from the PCT copolyester formulations. Surprisingly, these articles have improved shatter resistance over articles made from poly(ethyleneterephthalate) (PET) and copolyesters containing terephthalic acid, ethylene glycol and less than about 40 mole % 1,4-cyclohexanedimethanol (CHDM).
2. Background of the Invention
Extrusion blow molding is a common process for creating hollow articles from polymeric materials. A typical extrusion blow-molding manufacturing process involves: 1) melting the resin in an extruder; 2) extruding the molten resin through a die to form a parison having a uniform wall thickness; 3) clamping a mold having the desired finished shape around the parison; 4) blowing air into the parison, causing the extrudate to stretch and expand to fill the mold; 5) cooling the molded article; and 6) ejecting the article from the mold.
The hollow articles generated by extrusion blow molding are often used to contain solid or liquid products. The container must, therefore, be sufficiently tough to protect the product and prevent it from leaking or spilling after an accidental drop or impact. Toughness of the blow molded article is related to several factors, including part design, wall thickness, size of the container, and material. For filled articles, size of the container affects toughness greatly, as the weight of the contents produces the impact weight. Larger containers will hold heavier masses that will produce a higher impact load. In order to compensate for these higher impact loads, wall thickness must be increased or a tougher material must be selected. Unfortunately it is not always possible to increase wall thickness due to melt strength limitations and cost. Thus, the preferred solution is usually to extrusion blow mold the containers from a tougher material.
For many applications, such as non-carbonated beverage bottles and other containers, the polymer used in the extrusion blow molded articles also need to be amorphous so that the blow molded article will be transparent. This limits the number of polymers that can be utilized. Polyethylene terephthalate (PET) and copolymers based on PET are often used for extrusion blow-molding hollow, amorphous transparent articles.
Unfortunately, the low toughness of PET based polyesters and PET copolyesters containing up to 40% CHDM restricts container design and utility. It is possible to add impact modifiers to these materials, but the resultant blends are generally opaque. There are other clear extrusion blow moldable materials available, such as PVC and polycarbonate. However, these resins can have problems with chemical resistance, toughness, resin cost and environmental concerns. Thus, there is a need for a copolyester having greater toughness than PET and PET copolymers that can be extrusion blow molded into hollow, amorphous transparent articles.
It is known that injection molded articles made from copolyesters of terephthalic acid with ethylene glycol and greater than 50 mole % 1,4-cyclohexanedimethanol, such as those described in U.S. Pat. No. 2,901,466, show improved toughness over injection molded articles made from PET and PET copolymers. Typically, these copolyesters have inherent viscosities (I.V.) less than 0.90 dL/g, and require high melt processing temperatures and fast quenching to avoid crystallization related problems. This combination of low I.V. and high melt temperature processing leads to a low melt strength in the polyester or copolyester. Additionally, articles that crystallize during the extrusion blow molding process are either totally or partially white. Such white or hazy parts are unacceptable in applications where clarity and transparency are required.
In order to form good quality containers that have uniform side wall thickness and to prevent tearing (blow-out) of the parison during expansion, the polymer extrudate must have good molten dimensional stability. Dimensional stability is related to the polymer's melt strength. Generally, melt strength has been determined in accordance with ASTM D3835 by extruding the molten polymer downward through a die 0.1 inch (0.254 cm) in diameter and 0.25 inch (0.635 cm) long at a shear rate of 20 seconds
−1
using an Instron rheometer and allowing the extrudate to fall freely. A material having high melt strength has a tendency to resist stretching and flowing as a result of gravitational force when in the softened or molten state. Shorter flow lengths indicate better melt strength and, consequently, less sag of the parison. Thus, materials with high melt strength perform better in the extrusion blow molding process.
The melt strength of a polymer is directly related to its melt viscosity measured at 1 radian/second on a rotary melt rheometer. Polymers that have melt strengths high enough to be extrusion blow molded typically have melt viscosities of greater than 30,000 poise measured at 1 rad/sec and at the melt temperature of a typical parison.
Because of the high viscosity requirements particular to extrusion blow molding, special grades of PET and PET copolymers must be used. The inherent viscosity of a PET based polyester does not exceed 0.90 dL/g when made in a typical commercial melt phase polymerization reactor. In order to obtain the high melt viscosities required for the extrusion blow molding process, a PET based polyester with an inherent viscosity of less than 0.90 dL/g must be processed at a relatively cold temperature. For example, a melt phase PET copolyester having terephthalic acid with ethylene glycol and containing between 20 and 50 mole % 1,4-cyclohexanedimethanol as a modifying diol needs to be extrusion blow molded at a parison melt temperature of between 210° C. and 230° C. Fortunately, this copolyester has a low melting temperature and very slow crystallization halftime.
In contrast, if PET composed substantially of terephthalic acid and ethylene glycol or PET with low levels, i.e., less than about 10 mole % and preferably less than about 5 mole % of a secondary comonomer such as a diacid, a diol or combinations thereof, is processed at this low a temperature, it can crystallize in the feed section or barrel of the extruder, or in the parison. Crystallization in the extruder can halt the process or lead to imperfections in the final article. Crystallization in the parison can lead to undesirable opacity or embrittlement of the final article, or will lead to a parison that can not be blown into the desired final shape. Thus, other methods must be used to raise the melt viscosity of crystallizable polyesters so that they can be processed at temperatures above their melt temperature. One method is to add a branching agent to the composition. Another method is to raise the molecular weight through solid state polymerization, referred to herein as solid state or stating polymerization processing.
The solid state polymerization process is well known. Typically, amorphous precursor pellets that have been prepared by melt phase polymerization are first crystallized at a temperature 10°-100° C. below their melt temperature and then moved to a second stage where they are held at a temperature at least 10° C. below their melt temperature for a sufficiently long time (2-40 hours) in a vacuum to increase the inherent viscosity of the polymer.
One unique problem with polyesters and copolyesters is that they must be both solid state polymerization processed and the
Dobbs Harold Eugene
Pecorini Thomas Joseph
Acquah Samuel A.
Boshears B. J.
Eastman Chemical Company
Graves, Jr. Bernard J.
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