Melt polycondensation, combined polymers

The usefulness of piperazine and its derivatives for the preparation of polyamides has been known for some time, and it is one of the few disecondary diamines which can be employed to prepare high-melting polycondensates. Piperazine polyamides are especially useful as hot melt adhesives (68USP3377303), since they display unusually good adhesion to vinyl-based polymers such as poly(vinyl chloride). Piperazine is also the preferred diamine for the preparation of thermoplastic polyamides which exhibit the combined properties of low melt viscosity, high softening point and impact resistance at low temperatures (80USP4218351). 

Scheme 1. Melt polycondensation for the preparation of combined LC polymers.

Moon, S.I., Kimura, Y., 2003. Melt polycondensation of L-lactic add to poly(L-lactic acid) with Sn(II) catalysts combined with various metal alkoxides. Polymer International 52,299—303. 

Hoftyzer and van Krevelen [100] investigated the combination of mass transfer together with chemical reactions in polycondensation, and deduced the ratedetermining factors from the description of gas absorption processes. They proposed three possible cases for poly condensation reactions, i.e. (1) the polycondensation takes place in the bulk of the polymer melt and the volatile compound produced has to be removed by a physical desorption process, (2) the polycondensation takes place exclusively in the vicinity of the interface at a rate determined by both reaction and diffusion, and (3) the reaction zone is located close to the interface and mass transport of the reactants to this zone is the rate-determining step. 

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes. Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate), UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below). 

For most applications, PBT is not used in the pure form which comes out from the polycondensation equipment, but as compounded materials produced by mixing and homogenizing PBT melt with specific additives in specially designed extruders. Sometimes, only a small amount of additive is combined with the PBT. In other cases, high loadings of a variety of ingredients can lower PBT content to below 30 %. Characteristic for all those cases is that the PBT is still the continuous phase. Like many crystalline polymers, the low melt viscosity of PBT and its ability to mix with many types of filler and with other polymers make it very amenable to the formation of composites and blends. 

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