In blow molding processes, molten resin must form into stable parisons for a time long enough to permit a mold to enclose the parison. If these molten resins do not possess sufficient "melt strength" or melt viscosity, the parisons will tend to elongate or draw under their own weight and either not be blow moldable or result in blow molded articles which have non-uniform wall thicknesses, low surface gloss, poorly defined sample shape, and a large number of pitmarks.
Polymers such as polyesters, polyamides, polyethers, and polyamines when melted generally form thin liquids having low melt viscosities and poor melt strengths. These low melt viscosity materials are unsuited or are only poorly suited for the manufacture of extruded shapes, tubes, deep-drawn articles, and large blow molded articles. In order to overcome this disadvantage and to convert these polymers to a form better suited for the above-mentioned manufacturing techniques it is known to add compounds to the plastics which will increase their melt viscosities. The materials which are added to increase the melt viscosity of the plastics are generally cross linking agents, as described, for example, in U.S. Pat. No. 3,378,532. Such cross linking agents may be added during the condensation reaction by which the plastics are formed, and/or to the plastics after their formation (prior to, or during their melting). Examples of cross linking agents which may be added to the plastics after their formation and before or after their melting in order to increase the melt viscosity include compounds containing at least two epoxy or isocyanate groups in the molecule, organic phosphorus compounds, peroxides, bishaloalkylaryl compounds, and polyesters of carbonic acid.
These known cross linking agents which are added to increase the melt viscosity of the polymer are not completely satisfactory. They may, for instance, cause an excessively rapid and large increase in viscosity or form reaction products which have an adverse influence on the quality of the plastics. Furthermore, the results obtained with the use of these known cross linking agents are not always uniform or reproducible. For example, when polyesters of carbonic acid are used to increase the melt viscosity, the degree of viscosity increase is generally dependent not only upon the amount of additive used but also upon its molecular weight and on the stage of the polycondensation reaction at which the addition takes place.
Besides having sufficient melt viscosity or "melt strength", polymers which are to be used in blow molding and related applications should also possess sufficient die swell, i.e., the molten polymer should expand as it is released from the extrusion die. This die swell is important for blow molding applications since (a) the larger the diameter of the extruded polymer, the easier it is for air to be blown into the parison, and (b) the greater the die swell the greater the expansion of the molten polymer to fit the particular mold.
Polyesters having low intrinsic viscosities are particularly difficult to blow mold. The prior art illustrates the use of numerous additives to modify various properties of polyesters. For example, U.S. Pat. No. 3,376,272 discloses a process for the preparation of branched chain, high molecular weight thermoplastic polyesters having a multiplicity of linear non-cross linked polyester branched chains from dicarboxylic acid anhydrides, monoepoxides, and an alcohol compound by reacting these compounds at a temperature below 150.degree. C. However, the polyesters described in this patent are formed from anhydrides and are therefore not crystalline. Non-crystalline polymers tend to take longer time to set up in a mold and thus are not suited or are only poorly suited for blow molding and related applications.
As indicated above, compounds containing epoxy groups in the molecule have been used to increase the melt viscosity of polyesters (see, for example, U.S. Pat. No. 2,830,031). But although the use of epoxies as cross linking agents for polyesters is known, little appears known about the use of epoxies as reactants which promote the branching (and hence increase the "melt strength") but not the cross linking of polyesters.
U.S. Pat. No. 3,547,873 discloses the production of thermoplastic molding compositions from linear saturated polyesters and polyfunctional epoxides. This process, however, also yields products which lack the melt strength and die swell necessary for blow molding applications.
Furthermore, U.S. Pat. No. 3,692,744 discloses the preparation of polyester molding materials which can be injection molded by having present in the polyesterification mixture, besides the terephthalic acid and diol components, 0.05-3 moles percent, on the acid component, of a compound containing at least three ester forming groups such as a polycarboxylic acid, a polyhydric alcohol, or a hydroxy carboxylic acid. The use of epoxy compounds is not disclosed, however.
Copending United States patent application Ser. No. 669,066, which was filed on Mar. 22, 1976 and which is also assigned to the assignee of the present invention, generically discloses and claims a process for preparing branched chain thermoplastic polymers having increased melt strength and which are useful in extrusion applications. Said copending application is entitled "Improved Polymers for Extrusion Applications" and is filed in the name of John R. Costanza and Frank M. Berardinelli. This process comprises reacting at least one thermoplastic polymer which is in the molten state with at least one branching agent which may be selected from the group consisting of epoxy having a functionality of at least two and isocyanate having a functionality of at least three. Branched chain, high melt strength thermoplastic polymers useful in extrusion applications are obtained from this process.
In the above described process, reaction between the thermoplastic polymer and the isocyanate branching agent is sufficiently rapid that catalysts are not necessary. However, the reaction between the thermoplastic polymer and the epoxy branching agent requires the use of a catalyst. Typical acid acceptor type catalysts such as diethylamine tend to rapidly produce the improved melt strength polymer products, but these products, while acceptable for many uses, in some instances tend to be rather brittle and somewhat colored.