A good deal of interest is being taken in the theme of "Rubber Replacement". Before studying this subject in detail, it is necessary to define exactly what is meant by Rubber Replacement. This is not always the replacement of a particular rubber, but mainly the way in which it is compounded and processed. Before proceeding further, it is interesting to examine the history of the rubber industry.
The first rubber (now called natural rubber, NR) to be commercialised on any scale was that produced in Brazil in the early 1800's. It was observed that the native population played with a ball of rubber, which was made from the sap of a tree, later given the name Hevea Brasiliensis. They also chewed this rubber, which became the basis of the ever-popular chewing gum. The name rubber comes from the English word rubber or eraser, which was coined in the early 19th century and was used to rub out lead pencils. Other languages, with the exception of Dutch, use a derivation of the native name for the rubber tree, Caoutchoua. For over a hundred years, NR, as it became known, was the sole elastomer available, except for gutta-percha, a rubber containing an isomer of the main ingredient of NR, namely polyisoprene. Matthews and Harries also produced polyisoprene synthetically, as early as 1910, from dipentene. The main applications for NR were tyres, coated fabrics and low voltage cable insulation. The former two are still major applications for NR, while the latter has given way to PVC and polyethylene. It was in fact the advent of the motorised vehicle and the widespread installation of domestic electricity, which gave NR its impetus, in the early 20th century.
Prior to the Second World War, the United States government was concerned about the NR monopoly held mainly by the British and Dutch. Funds were made available to develop and produce a synthetic rubber replacement, mainly for tyres and military applications. The result was GRS, or as it is now known as, SBR (styrene-butadiene-rubber). GRS was not entirely successful in replacing NR due to its poorer properties.
The Germans had a similar problem and their answer was Buna-N (acrylonitrile-butadiene rubber or nitrile rubber) and their own version of SBR, Buna-S. An interesting side issue was the invention and development of polyvinyl chloride (PVC). The Germans primarily developed this thermoplastic to replace NR in certain less demanding applications. Examples were floor tiles, wire insulation and waterproof textiles. It was discovered that if PVC were to be mixed with certain plasticisers (e.g. adipates, phthalates or sebacates), the result was a flexible compound, which could be used as a NR replacement. An extra bonus was the discovery that rigid PVC could be used to replace metal pipes and profiles, thus saving valuable war materials.
Other rubbers of interest developed at the same time, were CR (chloroprene) and IIR (butyl rubber). The Russians also produced their own synthetic rubbers (sodium and potassium polybutadienes). By the end of the Second World War, the global production of synthetic rubber rose to over 1.1 million tones, versus 122,700 tonnes in 1940. NR production was about 18% of the tonnage of synthetic rubbers produced, in 1945. NR could be considered to have been replaced, if not in whole, then in part.
Table 1 shows the production of all rubbers, including natural rubber, for the period 1940 to 1950.
Styrene-Butadiene Rubber (SBR)
Sodium and Potassium Polybutadiene (Na/K-PB)
Butyl Rubber (IIR)
Nitrile Rubber (NBR)
Natural Rubber (NR)
% Natural Rubber
Table 1: Global Production of Major Synthetic Rubbers: 1940-1950 (thousand tonnes)
(Source: The History of Synthetic Rubber - Colin Barlow et al., Elsevier 1994 & IRSG)
It is interesting to note that though in 1945, NR would seem to have been almost replaced by synthetic rubber, by 1950 it had rebounded to quite a strong position, namely 71%. Since the end of the Second World War, until now, there are no less than 19 main types of synthetic rubbers produced throughout the world. The synthetic rubber production has now stabilised to about 58-59% of total rubber production, as shown in Table 2.
Table 2: Global Production and Consumption of Natural and Synthetic Rubbers: 2007-2009
(Source: International Rubber Study Group)
This growth owes itself primarily to the geo-political events of 1939-1945, when NR would have been in very short supply but highly in demand.
All of these synthetic and natural rubbers share in essence, the same processing technology. They are first prepared as un-vulcanised compounds, processed by extrusion or compression moulding and then vulcanised in a second stage at higher temperatures. It is possible to injection mould rubber, but not on standard thermoplastic injection moulding machines. In general, the production cycle time is many times longer than for thermoplastics due to the need to vulcanise as a secondary stage. Post-moulding scrap cannot be easily recycled, which adds to the cost. Rubber compounding requires a number of additives, which can have certain side effects. For instance, if peroxides are used for vulcanisation, they frequently generate unpleasantly smelling by-products.
The development of thermoplastic elastomers (TPE) came about as a suggested solution to the high costs of production and processing of rubber compounds. Synthetic rubbers had already been used extensively to increase the impact strength of certain thermoplastics. In the case of polypropylene (PP), this was of particular importance, since copolymerisation with ethylene, had not yet been perfected. The same was true with polystyrene (PS), which was mixed with SBR, in internal mixers to give PS some meaningful level of toughness. It was noted that increasing the percentage of rubber to more than the thermoplastic content brought about compounds which had certain elastomeric properties. In the case of PP mixed with EPDM, a new family of rubber-replacement materials evolved (TPE-O). One of the first real breakthroughs was replacing the SAAB 9 bumper, originally made from vulcanised EPDM, with a PP/EPDM blend. Such blends could be made softer by the addition of oil and harder by the addition of fillers such as talc.
Another major rubber replacement was the switch to PVC, in the 1960's, for the production of shoe soles. This easily processed thermoplastic rapidly replaced TSR. Another advantage of PVC was its ability to be foamed, reducing the high density of 1.2 to as low as 0.25 g/ml. About ten years later, development was being carried out in the USA to produce a unique class of styrene-butadiene copolymers, which acquired the new name of styrene block copolymers (SBC). These materials when mixed with PP, oil and filler caused a revolution in the shoe sole industry, reducing the costs dramatically. These were labelled as TPE-S. They also have a lower density in solid form than PVC. This was a serious rubber replacement, since up to then the only major alternative to TSR had been PVC. Today TPE-S based on SBS (styrene-butadiene-styrene) has the largest plastics material market share for soles, about 43%, while PVC usage is slowly declining.
Up until the 1980's, there was no serious challenge to thermoset rubbers, natural, or synthetic. Work pioneered by Monsanto and continued by AES, gave rise to a completely new family of TPE's. These materials were a continuation of the TPE-O compounds, except this time the EPDM component was cross-linked. They are now called thermoplastic vulcanisates or TPE-V. Parallel to this work, The SBS in TPE-S was replaced with a hydrogenated version, called SEBS (styrene-ethylene-butylene-styrene). This upgraded the TPE-S family considerably, presenting a serious challenge to thermoset rubbers. Today SEBS-based TPE-S and PP/EPDM-based TPE-V compounds are running neck and neck in growth rate. This situation as well as other interesting product developments will be discussed further in the second part of this article.