Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-06-15
2003-02-11
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S323000, C524S492000, C524S493000, C524S494000, C524S401000, C524S080000
Reexamination Certificate
active
06518341
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to polyamides, and more particularly to an improvement in thermally stabilized polyamide molding resins and compositions. Still more particularly, this invention is directed to thermally stabilized, impact modified polyamide injection molding formulations having a reduced tendency toward burning and discoloration in the molded part. The invention may be further described as directed to a method for reducing mold burning of injection molding formulations comprising impact modified, partially aromatic polyamides containing a copper halide heat stabilizer.
Polyamides generally exhibit a balance of thermal, strength and stiffness properties which make them suitable for many applications. The resins are particularly attractive for use in applications where resistance to chemical and thermal attack is required. Aliphatic polyamides, termed nylons, generally are readily processed thermally and have gained wide acceptance in the molding arts and in the extrusion arts, including fiber spinning and film extrusion.
Partially aromatic polyamides and copolyamides have been developed for use in high temperature applications, and crystalline and semi-crystalline copolyamides comprising at least about 40 mole % partially aliphatic terephthalamide units are known for their particularly good thermal properties and performance in demanding environments. However, such polyamides typically have relatively high melting points e.g., about 290° C. or higher, and the degradation temperatures for some do not greatly exceed their melting points; accordingly, requirements for melt processing and molding these polyamides are more rigorous and complex than those for polyamides such as nylon 6,6, melting at about 260-265° C.
Fabricating high temperature, partially aromatic polyamides for the production of molded articles, extruded profile goods, laminates or the like, particularly when filled, requires processing the resin at temperatures very near the resin decomposition temperature, together with severe shear stress during molding or extrusion operations. Good resin thermal stability is critically important to attaining good properties in the molded part as well as to maintaining those properties in a variety of uses, particularly in demanding environments.
The art of stabilizing resins against deterioration through exposure to thermal oxidative environments is well-developed. The decomposition of aliphatic polyamides has been the subject of a great many studies, and numerous additives have been proposed for improving their thermal oxidative resistance, both during processing and while in use. Stabilizers act to inhibit the oxidation processes, preserving the aliphatic polyamide chain intact. Stabilizer compositions comprising copper(I) halide and an alkali metal halide are described in the art for use with polyamides, and the use of complex compounds comprising copper salts and diamines has also been disclosed. Heat stabilizers comprising combinations of copper halides, alkali metal halides and phosphorus compounds have been employed for use in polyamide molding resins and the like, as shown for example in U.S. Pat. No. 4,937,276.
The aliphatic segments of high temperature, partially aromatic polyamides are subject to the same thermal-oxidative decomposition processes, and the thermal stabilizers for aliphatic polyamides have also been found useful with these polyamides. However, partially aromatic polyamides generally require higher processing temperatures and otherwise are likely to be subjected to more severe conditions. Stabilizers commonly employed with aliphatic polyamides may partially decompose thermally during processing at these elevated temperatures and form gaseous products that detrimentally affect the properties or appearance of molded and extruded goods. Where substantial degradation occurs, these byproducts may also result in splay formation in molded articles. Methods and compositions for overcoming these difficulties are disclosed in the art, for example in U.S. Pat. No. 5,763,561.
Partially aromatic polyamide molding resin formulations may further include additional resin components such as impact modifiers, flow improvers and the like. Modifiers in wide commercial use for these purposes include polyolefins and modified polyolefins such as, for example, those described in U.S. Pat. No. 5,436,294. Polyolefin modifiers are known to be sensitive to thermal oxidative attack, becoming crosslinked and severely degraded when subjected to the high temperatures generally employed when molding partially aromatic polyamides. Moreover, copper compounds such as are found in the heat stabilizers commonly used with polyamides tend to accelerate the thermal decomposition of olefin-based resin modifiers.
Importantly, injection molding of intricate, detailed parts requires the use of cavity molds with many small or narrow spaces. The molds are supplied with vents positioned to allow air displaced by the molten resin, as well as any entrained gases and gaseous decomposition products, to escape from these areas of the mold cavity so that the molten resin may completely fill the mold. Thermal decomposition byproducts, together with small amounts of other volatile components, for example, processing aids, stabilizers or other low molecular weight solids, may be entrained by the escaping gases, carried into the vents and become deposited on cooler surfaces within these passages. In the molding of copper stabilized, high temperature polyamide formulations, this problem appears to be particularly exacerbated by the presence of olefinic modifiers.
During extended molding operations using copper stabilized, impact modified polyamides these deposits build up over time and form a solid, intractable residue within the vent passages that clogs or plugs the vents. Escape of the hot gases and decomposition byproducts is then blocked, causing adiabatic compression of the polyamide resin in the final feed zone, raising the mold temperature. The resin then becomes carbonized and darkened, a condition called “gas burning”, resulting in visible imperfections including discoloration or “burn” marks and other unacceptable cosmetic flaws in the molded parts. The molding operation must then be halted while the mold is cleaned. Moreover, because the refractory, intractable residues produced in molding these polyamide formulations are difficult to remove without damage to the cavity surfaces, a costly replating or other refinishing of the mold cavity may then be necessary.
Methods and compositions for providing copper stabilized, impact modified, high temperature polyamide resin formulations that will undergo injection molding without producing unacceptable levels of vent-clogging deposits are clearly needed by the resin molding art. Such improvement in polyamide molding formulations would provide a significant reduction in costs by significantly extending the time between mold cleanings, thereby avoiding the need for frequent shutdown of the molding operation.
SUMMARY OF THE INVENTION
This invention is directed to methods for providing copper stabilized, impact modified polyamide molding resin formulations comprising a high temperature, partially aromatic polyamide and to improved impact-modified polyamide injection molding resin formulations having a reduced tendency toward burning and discoloration in the molded part.
Injection molding formulations according to the invention exhibit excellent thermal stability, together with a reduced rate of accumulation of mold deposits, thereby increasing the time between mold cleanings and reducing the occurrence of burned or discolored molded goods. The invented formulations are particularly useful in the manufacture of injection molded and extruded goods intended for extended use in demanding environments and at elevated temperatures and where resistance to chemical and thermal attack is an important consideration.
DETAILED DESCRIPTION OF THE INVENTION
The impact modified polyamide injection molding resins and compositions of this inve
Bersted Bruce H.
Hurley James M.
McDermott & Will & Emery
Rajguru U. K
Seidleck James J.
Solvay Advanced Polymers, LLC
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