Redox system and process

Details Redox system and process Redox system and process

2001-11-20

2004-03-30

Cheung, William (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...

C502S300000, C526S219600, C526S229000, C526S093000, C526S094000, C526S230000

Reexamination Certificate

active

06713557

ABSTRACT:

This invention relates to a redox initiator system for preparing emulsion polymers and aqueous dispersion polymers having low residual monomer content. More particularly, it relates to a process for reducing the residual monomer content in an aqueous emulsion polymer prepared from two or more ethylenically unsaturated monomers using a free radical redox initiator system, the redox system including at least one oxidant, at least one reductant and at least one metal-chelate promoter complex.
It is often desirable to employ redox systems as initiators for preparing aqueous emulsion polymers from ethylenically unsaturated monomers, particularly if polymerization at temperatures lower than those at which conventional thermal initiation systems provide an effective level of free radical production, namely temperatures below 85° C. However, some redox systems employing metal promoters have limited utility in preparing aqueous emulsion polymers and controlling residual monomer content. For example, the hydrogen peroxide-ferrous salt system catalyzes emulsion polymerization by the addition of iron ions, yet does not have utility in the preparation of all aqueous emulsion polymers, particularly those in which the pH of the aqueous phase is above 7, because the iron ions aggregate and precipitate at high pH. In such cases, it is a common practice to add chelating ligands such as ethylene diamine tetraacetic acid (EDTA) and its salts in order to maintain metal ion solubility and the polymerization is initiated by the free radicals formed by reaction between the complexed metal ion and peroxide. Conventional redox initiator systems employing metal salts such as iron(II), iron (III), cobalt (II) and Ce(IV) in which the metal ion is complexed with polyaminocarboxylic acids have been described in publications of Brown et al in the Journal of Applied Polymer Science, Vol. 18, pp. 1269-1277 (1974); Hsu et al in the Journal of Polymer Science, Part A, Polymer Chemistry, Vol. 31, pp. 3213-3222 (1993); and Wang et al in the journal Polymer, Vol. 37, pp. 2509-2516 (1996). However, some redox initiator systems are less efficient for reducing the residual monomer content than others, particularly for reducing levels of certain monomers. In addition, ligand complexation affects both the thermodynamics and kinetics of the redox cycle in an emulsion polymerization. The behavior of such redox initiator systems over a range of pH conditions can be complicated due to a number of factors, such as the interplay between the solubility of the metal ion or metal-chelate complex, competition among various potential ligands in the solution and the kinetics of various metal-mediated and direct reactions necessary to produce initiating radicals.
Conventional redox initiator systems used in preparing aqueous emulsion polymer dispersions of two or more ethylenically unsaturated monomers include an oxidant, sometimes referred to as a “catalyst”, a reductant, often referred to as an “activator” and, optionally, a soluble transition metal ion, referred to as a “promoter”. However, much of the use of redox initiator systems in emulsion polymerization is based on empirical observations and “rules of thumb”. A number of different techniques have been employed to improve the solubility of the catalyst or promoter, but each technique has its limitations. For example, since metal ion solubility decreases as the pH of the aqueous phase of an emulsion polymerization increases, it is sometimes helpful to add a chelating agent such as EDTA to maintain solubility of the metal promoter. However, at high pH a chelating agent such as EDTA has a negative impact on reaction kinetics. The present invention provides a novel redox system which has utility in reducing residual monomer content in emulsion polymerization processes involving aqueous dispersions of monomers as compared to processes using alternative redox initiator systems.
U.S. Pat. No. 5,087,676 discloses a process for reducing the content of olefinically unsaturated monomers in an aqueous polymer dispersion using a free radical redox initiator system which is soluble and consists of an oxidizing agent, a reducing agent and a combination of iron and vanadium salts. Examples of suitable complexes which are added to keep the metals in solution under the reaction conditions disclosed are alkali metal salts of tartaric acid, citric acid, pyrophosphoric acid and ethylene diamine tetraacetic acid (EDTA).
Alternative redox initiator systems for reducing residual monomer content in aqueous dispersions of ethylenically unsaturated monomers using complexing agents which do not have unfavorable kinetics in reducing monomer levels at pH >7 and which can be employed for reducing monomer levels at temperatures less than 85° C., temperatures that exist for example in processing or storage tanks, are still desired. In addition, aqueous emulsion polymerization processes of two or more ethylenically unsaturated monomers using such redox initiator systems in reducing residual monomer content are desired. The inventors have surprisingly discovered that residual monomer content in aqueous emulsions of two or more ethylenically unsaturated monomers can be effectively reduced when certain free radical redox initiator systems are employed under the polymerization conditions, the redox initiator systems including an oxidant, a reductant and a metal promoter complex, wherein the metal promoter complex includes at least one soluble metal salt and at least one chelating ligand having fewer than six groups coordinated to the metal salt. Improvements have been discovered in reducing residual monomer content at the end of an emulsion polymerization, during the emulsion polymerization and at lower monomer chasing temperatures.
Inventors have discovered a strong compositional and pH dependence on the efficiency of various redox initiator pairs involving certain chelating ligands and metal promoters for controlling residual monomer levels in aqueous dispersion polymers prepared from one or more ethylenically unsaturated monomers. The effect of chelating ligands on the efficacy of monomer chasing using redox initiator systems is dependent on the mechanism of the redox initiator systems. Redox initiator systems which follow a conventional redox mechanism are strongly affected by the presence and nature of chelating ligands in aqueous emulsions of ethylenically unsaturated monomers under the condition of the polymerization. Inventors have further discovered that metal promoter complexes employing chelating ligands with lower metal coordination numbers (CN), such as tetradentate and pentadentate aminocarboxylate ligands (CN=4 and 5 respectively) have improved rates of reducing monomer levels as compared to metal promoter complexes employing hexadentate chelating ligands (CN=6) and maintain metal ion solubility over a range of pH that is sufficient for most practical emulsion processes, especially at lower monomer chasing temperatures. In addition, inventors have identified that some common by-products or oxidation products in an emulsion polymerization process can also function as weak metal ion chelating ligands. Inventors have discovered a process for the efficient reduction of residual monomer levels in an emulsion polymerization or an aqueous polymer dispersion. By employing this process, inventors demonstrate a range of chelating ligands having utility in that it is apparent such chelating ligands can not bind to all the coordination sites on the metal ion, leaving reactive sites on the metal ion accessible for radical generation and further catalysis. The process further improves the efficiency of redox initiator systems in new and existing aqueous polymer dispersions.
According to a first aspect of the present invention there is provided a redox initiator system for reducing residual monomer content in an aqueous emulsion polymer prepared from one or more ethylenically unsaturated monomers which includes at least one oxidant, at least one reductant and at least one metal promoter complex

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