Vulcanization Chemistry. -- Early in his career, Coran initiated a program of research which lead the way to a basic understanding of delayed action accelerated sulfur vulcanization, a process which is essential in the production of tires and other rubber articles. The purpose of the work was to answer many questions and to solve some of the long-standing mysteries associated with the use of the delayed action vulcanization process. The approaches used were imaginative and the results were definitive.
The chemistry of delayed action vulcanization was sufficiently defined to give a rationale for a delayed onset of crosslinking. Monomeric polysulfidic reaction products of sulfur and accelerator inhibit the onset of rapid crosslink formation; this inhibition gives the delayed action (safety) needed for high speed processing without the occurrence of premature vulcanization. Dr. Coran's world-class research in the field of vulcanization chemistry has become classic.
Vulcanization Control. -- Dr. Coran's early work on delayed actionvulcanization led to his invention of new classes of vulcanization-controlling agents and to a major new product, N-cyclohexylthiophthalimide (Santogard® PVI). This product is now used throughout the world to prevent premature vulcanization. One value of Santogard PVI is its ability to reduce or eliminate waste due to premature curing or scorching of rubber stocks before they can be molded. The use of the product also permits more severe, and hence, more rapid treatment of rubber stocks in the forming and shaping prior to molding; thus it expands plant utilization, Santogard PVI is recognized as the most significant product innovation for vulcanization control since the introduction of sulfenamide accelerators, over 60 years ago. The product has won two awards for international industrial excellence.
Thermoplastic Elastomers (TPEs). -- Dr. Coran and his group discovered a new route to thermoplastic elastomers based on dynamic vulcanization, the process of vulcanizing an elastomer during its melt-mixing with molten plastic. The resulting compositions comprise completely vulcanized micron-size particles of rubber dispersed in a thermoplastic matrix. The elastomer phase can be quite voluminous; thus, very rubbery products can be produced which can be processed as thermoplastic materials. Commercial elastomers which were the result of the work are Santoprene® and Geolast® thermoplastic elastomers. Such products are called TPVs (for "thermoplastic vulcanizates," a term coined by Dr. Coran).
Thermoplastic elastomers are processed into finished parts with far less expense than are conventional, thermoset elastomers, They are not vulcanized In the mold. (Conventional, "in-the-mold" vulcanization requires long durations of time for the vulcanization process, or crosslinking, to take place. During this time expensive molding equipment is engaged.) The Santoprene and Geolast thermoplastic elastomers are rapidly fabricated, directly to finished parts, by using techniques (injection molding, calendaring, extrusion, etc.) which are generally used with thermoplastics (e.g. polyethylene, polypropylene, poly(vinyl chloride), polystyrene, etc.). Markets for these materials have been established and are rapidly growing.
The Dispersion of Carbon Black into rubber. -- Carbon black is the most important reinforcing filler for rubbers. The incorporation of carbon black into rubber vulcanizates generally gives improved strength, extensibility, fatigue resistance, abrasion resistance, etc. However, poor dispersion can in itself give rise to certain detrimental effects. Large agglomerates, which can act as failure-initiating flaws, are responsible for decreases in ultimate tensile strength, energy to break, tearing energy, fatigue resistance, etc.
For the above reasons, Dr. Coran has initiated new research in order to develop an improved understanding of the carbon-black dispersion process, including the understanding of factors which affect the kinetics of dispersion. Estimates of the degree of dispersion were obtained by using a simple, improved method for preparing surfaces of carbon-black filled rubber vulcanizates for dark-field optical microscopic examination. The method could even be adapted to the preparation of surfaces of unvulcanized rubber stocks and masterbatches. The dispersion ratings correlated very well with ultimate tensile strength and other performance-related properties such as fatigue resistance.
Other Achievements. -- Dr. Coran has contributed to many aspects of science and technology of rubber and plastics; he is an inventor of an accelerator (Vocol) for vulcanizing EPDM rubber and reinforcing systems for rubber, based on specially treated cellulose fiber (Santoweb).
It should also be noted that Dr. Coran made a significant contribution to the understanding of thermodynamic interactions (and the requirements for thermodynamic compatibility) between polymers and diluents (solvents and plasticizers). The work was published about 40 years ago but it is still considered relevant. Dr. Coran's more recent work has been concerned with blends of elastomers with other elastomers or plastics. A goal of this work was to develop understanding and technology to enable the development of high-performance blends wherein the effects of thermodynamic incompatibility, cure-rate incompatibility and morphological anisotropy are minimized. Some of this work has been related to the evolution of phase morphology during the mixing of blends. Other aspects of the work relate to the chemical modification of polymers and other methods to make polymers more technologically compatible with one another. Some of the work has been directed to the recycling of used rubber and other polymers in the form of useful blends. |
