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
1998-08-13
2003-03-04
Mulcahy, Peter D. (Department: 1713)
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...
C524S110000, C524S399000, C524S434000
Reexamination Certificate
active
06528566
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a heat stabilized halogen-containing polymer composition normally susceptible to heat-induced deterioration which comprises a halogen-containing polymer and the degradation products of a latent mercaptan present during processing of the composition at an elevated temperature, said products being formed during said processing and including a liberated mercaptan. The free mercaptan enhances the activity of metal-based heat stabilizers such as organotin carboxylates and mercaptides in the polymer composition. It particularly relates to the stabilization against heat of vinyl chloride polymer compositions and articles made thereof by a latent mercaptan selected from the group consisting of 2-S-(hydroxyalkylthio) tetrahydropyran, 5-S-(hydroxyalkylthio) tetrahydrofuran, and the carboxylic acid esters thereof in combination with very low levels of a metal-based heat stabilizer or certain Lewis acids. Said latent mercaptans are also referred to hereinafter as 2-S-(tetrahydropyranyl)-thioalkanol, 2-S-(tetrahydropyranyl)thioalkyl carboxylate, and their furanyl homologs, i.e., 5-S-(tetrahydrofuranyl)-thioalkanol and 5-S-(tetrahydrofuranyl)thioalkyl carboxylate.
This invention also relates to articles of manufacture such as rigid pipe and window profile, flexible film, and semi-rigid tubing that are prepared from such heat-stabilized vinyl chloride polymer compositions.
BACKGROUND OF THE INVENTION
It is well known that the physical properties of various organic polymers deteriorate and color changes take place during processing of the polymer and during exposure of formed polymer products to certain environments. Halogen-containing polymers are normally susceptible to heat-induced deterioration through autoxidation. The prime examples of such polymers are the vinyl and vinylidene polymers in which the halogen is attached directly to carbon atoms. Poly(vinyl chloride), copolymers of vinyl chloride and vinyl acetate, and poly(vinylidene chloride), the principal resin in self-clinging transparent food wraps, are the most familiar polymers which require stabilization for their survival during fabrication into pipe, window casings, siding, bottles, wall covering, packaging film, and the like. When such polymers are processed at elevated temperatures, undesirable color changes often occur within the first 5 to 10 minutes as well as during later stages of the processing. Haziness, which sometimes accompanies the color changes, is particularly undesirable where clear products are needed. The addition of heat stabilizers to such polymers has been absolutely essential to the wide-spread utility of the polymers. From a great deal of work in the development of more and more effective heat stabilizers there has emerged two principal classes: organotin compounds and mixed metal combinations. Organotin-based heat stabilizers are the most efficient and widely used stabilizers for rigid PVC. Synergistic combinations of alkyltin mercaptides and free mercaptans are particularly efficient heat stabilizers for rigid PVC during extrusion. They have not been entirely satisfactory, however, because of several failings on the part of the mercaptan synergist and are not used in flexible PVC. Many mercaptans give off an offensive odor even at room temperature and the odor grows worse at PVC processing temperatures. The oxidative stability of the mercaptans is very often very poor. Oxidation of the free mercaptans diminishes the synergism. A combination having an enhanced synergism would be welcomed especially by the flexible PVC industry. Also, because of the end-use of articles made from some polymers, many polymeric compositions require the presence of both biocides and heat stabilizers but the use of the organotin mercaptide/mercaptan combination in such a composition is often frustrated by the tendency of the free mercaptan to deactivate a biocide such as the much used OBPA (10, 10′-oxybisphenoxarsine).
Zinc salts in general have long been believed to be less satisfactory as heat stabilizers for halogen-containing polymers than the organotin-based stabilizers and, indeed, have lent their name to the catastrophic degradation known as zinc burn. In U.S. Pat. No. 3,660,331, Ludwig teaches the stabilization of vinyl halide resins by certain thioethers and thioesters of tetrahydropyran. Better heat stabilizer compositions are still needed, however. The thioether/low level metallic stabilizer combinations of this invention satisfy that need.
SUMMARY OF THE INVENTION
It has now been found that the activity of the 2-S-(tetrahydropyranyl)thioalkanol, the carboxylates thereof, and their furanyl homologs as heat stabilizers in halogen-containing polymer compositions is unexpectedly higher than that predicted on the basis of sulfur content when used in conjunction with very low levels of a metal-based stabilizer or a Lewis acid. Zinc salts are particularly valuable as synergists of latent mercaptans in their function as heat stabilizers for halogen-containing polymers. Zinc chloride, a Lewis acid, is of particular interest as such a synergist.
It is an object of this invention, therefore, to provide a heat stabilizer composition having the synergy of a mercaptan plus improved oxidative stability.
It is another object of this invention to provide a halogen-containing polymer composition stabilized against heat by 2-S-(tetrahydropyranyl)thioalkanols, carboxylates thereof, and their furanyl homologs in combination with a synergistic amount of a metal-based stabilizer or a Lewis acid.
It is another object of this invention to provide a PVC composition and article stabilized against heat by 2-S-(tetrahydropyranyl)thioalkanols, carboxylates thereof, and their furanyl homologs in combination with a synergistic amount of a metal-based stabilizer or a Lewis acid.
It is a related object of this invention to stabilize rigid, semi-rigid, and flexible PVC resin compositions with a heat stabilizer composition of this invention.
It is another object of this invention to provide a latent mercaptan-containing heat stabilizer composition which is substantially free from the offensive odor typically associated with mercaptans.
It is still another object of this invention to provide a flexible PVC composition and article stabilized against heat by a 2-S-(tetrahydropyranyl)thioalkyl carboxylate, its furanyl homolog, or a mixture thereof, in combination with a synergistic amount of a zinc salt.
These and other objects of the invention which will become apparent from the following description are achieved by adding a 2-S-(tetrahydropyranyl)thioalkanol, a carboxylate thereof, a furanyl homolog of either or both, or a mixture of two or more of said alkanols and esters, and a synergistic amount of a metal-based heat stabilizer or Lewis acid or a mixture of said metal-based heat stabilizer and Lewis acid to a halogen-containing polymer composition and processing the composition at an elevated temperature at which the latent mercaptan degrades to liberate a free mercaptan. The terms “latent mercaptan” and “blocked mercaptan” are used interchangeably herein.
Other products of the degradation of the blocked mercaptan are believed to include carbocations of the blocking moiety which are stabilized by a molecular structure in which the electron deficiency is shared by several groups. Resonance stabilization and neighboring group stabilization are two of the possible mechanisms by which the carbocations may be stabilized. The carbocations act as intermediates in the formation of stable compounds early in the hot processing of halogen-containing polymers. Although such mechanisms and the resultant carbocations are believed to be an impetus for the liberation of the active free mercaptan, this invention is in no way limited by the foregoing attempt to explain the working of the invention. Those skilled in the art will see the resonance stabilization and neighboring group stabilization that are possible in the following structures of the blocked mercaptan; other mechanisms may be at work in other blocke
Adams Paul B.
Duvall Tod C.
Norris Gene K.
Mulcahy Peter D.
Rohm and Haas Company
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