The disclosed technology relates to halogenated polymer compounds, such as chlorinated polyvinyl chloride (CPVC) compounds, being free of heavy metals, particularly heavy metals in the form of stabilizer additives, the most common heavy metals being lead, tin, zinc, cadmium, and barium.
Halogen containing polymers tend to degrade or deteriorate when processed. Generally, the difference between the processing temperature and the degradation temperature is very small. Therefore, there is a risk that during the processing these halogen containing polymers will degrade. When such polymers degrade, it is believed that the halide acid generated by the polymer attacks the components of the processing equipment. Also, this acid further catalyzes de-hydrohalogenation reactions, such as HCl elimination, and additional degradation of the polymer.
Stabilizers have been developed to help deter such degradation. For example, heavy metal compounds such as tin are commonly used. However, heavy metal compounds are considered environmentally unfriendly, and there is a move to limit their use in many jurisdictions. In anticipation of increased regulation of heavy metal stabilizers in halogenated polymers, there is a desire in the industry to discover replacement stabilizers.
Zeolites and metal carboxylates have been taught as co-stabilizers in halogen containing polymers, generally in addition to heavy metal stabilizers, but also in combination with other types of stabilizers.
Zeolites are effective acid scavengers for halogen containing polymers and can enhance the polymer's thermal stability. Acid scavengers are compounds that react with acid groups in the polymer reaction mixture to form compounds that are typically chemically inert. Such acid scavengers are often required as co-stabilizers in halogenated polymers to neutralize the acid generated during melt processing. However, the use of zeolites as acid scavengers in halogen containing polymer compounds has been limited for several reasons. First, the zeolites generally have a large particle size, generally in the range of about 3 to about 6 microns. The large size of the zeolite particles is thought to not only cause surface blemishes on the finish of the end product made from such a polymer but also to diminish the physical properties, for example impact strength, of such polymers. Further, outgassing occurs frequently with polymers containing zeolites due to the evolution of water from the zeolite from the heat generated during melt processing. Such outgassing can cause foaming and affect the physical properties of the final product.
Metal carboxylates are also mentioned as potential co-stabilizers. However, when included at too high a concentration metal carboxylates have a tendency to plate-out (separate from the halogen polymer containing compound).
U.S. Pat. No. 4,371,656 describes an “improved” metal substituted zeolite covered by an organic substance and having a water content of 8 weight percent (wt %) or less for use as a stabilizer for halogen containing resins. The zeolite is taught to be suitable as it is, but that it may function more effectively when used in combination with other stabilizers.
A number of references teach the use of zeolite as a stabilizer in combination with other stabilizers. For example, U.S. Pat. No. 5,582,873 discloses a composition comprising a halogen containing polymer, a zeolite as the acid scavenger and a heat stabilizer selected from the group consisting of mixed metal stabilizers (such as calcium/zinc or calcium/barium), organotin stabilizers, lead stabilizers, metal free stabilizers or any combination thereof. Similarly, U.S. Pat. No. 6,414,071, to Wypart et al., issued Jul. 2, 2002 teaches a stabilized halogenated polymer wherein the stabilizer is a synthetic crystalline aluminosilicate of formula M2/nO.Al2O3.ySiO2.wH2O. For CPVC resins, the patent teaches and exemplifies the synthetic crystalline aluminosilicate in use with heavy metal stabilizers, including tin.
Other references teach the use of zeolite in combination with sodium carboxylate. However, no references have been found that do not include other stabilizers with the zeolite/sodium carboxylate combination. For example, EP2083044A1 to Reagens S.p.A. teaches a composition for stabilizing halogen-containing polymers comprising disodium adipate and a compound selected from the group consisting of M(ClO4) and (CF3SO3)nM. Zeolite and carboxylates are also taught as additives that can optionally be included in the EP′044 composition. However, the patent discourages the use of zeolites by noting that zeolites tend to release water, which prevents their use as stabilizers in some rigid or semi-rigid PVC applications. The patent does not teach or exemplify a formulation containing CPVC, a zeolite and a carboxylate. Similarly, EP2363431A1 to Reagens S.p.A. teaches a composition for stabilizing halogen-containing polymers comprising an alkali metal salt of adipic acid and an alkali metal salt of a carboxylic acid. Zeolite is taught as an optional additional additive in the EP′431 composition, in an amount from 0.1 to 3 parts by weight per 100 parts by weight of polymer. The patent does not teach or exemplify a formulation containing CPVC and a stabilizer package consisting of a zeolite and a carboxylate.
It would be beneficial to the industry to prepare an inexpensive and readily available alternative to current stabilizer systems for halogenated polymers, such as CPVC resins, without the need for a heavy metal stabilizer.