Rubber thermoset

The applications of rubber as an engineering material almost invariably involve bonding to a rigid substrate or reinforcement. In some instances these bonds need to be established to a fully cured thermoset rubber or a molded thermoplastic rubber, and a wide variety of adhesives suitable for this purpose are available. In... 

TPE s are more economical to produce than traditional thermoset materials because fewer steps are required to manufacture them than to manufacture and vulcanize thermoset rubber. An important property of these polymers is that they are recyclable. 

Advantages of Thermoplastic Elastomer over Thermoset Rubber Processing... 

A manufacturing method predominately used by the thermoplastic industry for the manufacture of hollow components, also used for thermosetting rubber. The polymer is forced against the inside of the mould surface by gas pressure applied either by a hollow needle or by the decomposition of a blowing agent. 

Sealing is evolving from thermoset rubbers towards TPEs, particularly TPVs. 

The contribution of Babu and Naskar gives a comprehensive overview on the recent developments of the thermoplastic vulcanizates (TPVs) with special reference to the technological advancement. TPVs combine high volume molding capability of thermoplastic with elastomeric properties of thermoset rubber. Therefore, they land themselves to broad area of applications in various fields. 

Some of the conditions used in rubber test methods may need modifying for application to thermoplastic elastomers because of their intrinsic thermoplastic nature. If the temperatures generally used in ageing and compression set tests on thermosetting rubbers were applied to thermoplastic materials they could appear to perform extremely badly. Whether this was significant would depend on the service temperature. Data sheets need to be checked as those for thermoplastic elastomers may have used much lower temperatures that would be found for conventional rubbers, and it is only too easy to get a misleading impression of performance. 

If a fully compounded thermosetting rubber is subjected to a plasticity measurement at a high enough temperature and for long enough, it will cure and, consequently, there is not always a clear distinction between a plasticity test and a test for scorch or rate of cure. For example, the Mooney viscometer is used to measure scorch, i.e. the onset of vulcanisation, and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure as well as the increase in stiffness as curing takes place. 

The reaction proceeds until all available monomer has reacted or until it is terminated by an acidic species. The gel point in these in situ polymerizations, as represented by the time of fixture, occurs witliin several seconds on strongly catalytic surfaces such as thermoset rubbers to several minutes on noncatalytic surfaces. 

Another type of rubber that is used frequently is thermoplastic rubber. Components are fabricated in a process that is similar to that used for common hard plastics, such as polyethylene or polystyrene, but the final product is an elastic material with properties otherwise equivalent to those of thermoset rubbers. No chemical reactions are involved in the processing of a thermoplastic rubber. The fabrication process consists of heating the rubber compound until it liquefies, injecting the liquid into a mold, cooling the mold, and finally removing the closure from the mold. The process is reversible. Closures can be remelted and remolded into different shapes or sizes as desired. 

Nearly all of the dynamically vulcanized EPDM/PP blend s (Santoprene ) growth has been at the expense of thermosetting rubbers due to the blend s easy melt processability and recyclability. Much of this growth has been in automotive applications such as rack and pinion steering boots, seat belt sleeves, and air ducts. EPDM/PP blend has also been used in window and door glazing seals, weather stripping and extrusion applications in the electricaE electronic industry. 

Various applications of the injection molding system have been developed outside the scope of the cure of rubbers, and a few examples are given, with the substitution of thermoset rubbers by thermoplastic elastomers a range of thermoplastic elastomer compounds were introduced and processed using reaction compounding technology. They are called reaction modified thermoplastic elastomers or ReMoTE [5]. 

Processing of TPEs is cheaper and simpler requiring fewer steps than does a thermoset rubber. A single step is required to shape TPEs into final article, whereas thermoset rubbers require mixing and vulcanization in addition to the shaping step. 

Processing times for TPEs are shorter since the long cycle time required for vulcanization is eliminated. Cycle time is typically seconds instead of the minutes required for thermoset rubbers. This enhances productivity and reduces costs. 

As a result of the fewer processing steps and shorter cycle time, lower energy is consumed in TPE processing than processing of thermoset rubbers. 

TPEs melt at elevated temperatures consequently, they are unsuitable for applications requiring even brief exposures to temperatures above their melting points. Many thermoset rubbers can withstand such a short exposure. 

Compression set is one of the most widely measured properties of rubber, featuring in a range of material and product specifications and finding use as a sensitive measure of the state of cure in both conventional thermoset rubbers and newer dynamically vulcanized thermoplastic rubbers. Comparative ease of determination has made compression set a popular test for sealing applications, although it is increasingly accepted that the property should not be used in place of the often more relevant stress relaxation. However, it can be relevant to seals where sufficient bodily movement occurs to open up a leakage channel before recovery of shape can take place. 

IPN TPEs offer physical and thermal properties of thermoset rubber, the processability of a thermoplastic, and a wider hardness range than available to other TPEs. 

---