Rubber chemicals: an ongoing search for new products.
In 1839 Charles Goodyear added sulfur to raw rubber and became the world's first compounder. Since then compounders have added a staggering number of substances to rubber to improve performance, lower cost or both. Over time, certain classes of materials have become standard ingredients in rubber compounds. Fillers, oils, activators, antidegradants, vulcanizing agents and accelerators are present in nearly all rubber formulations.
Within each of these classes there are hundreds, if not thousands, of materials in use. To keep the length of this article manageable, only accelerators, antidegradants and prevulcanization inhibitors will be addressed.
Goodyear quickly learned that while heating rubber with sulfur resulted in significantly improved physical properties this was by no means a complete answer to the instability of rubber. The vulcanization process was slow and, therefore, expensive. Equally discouraging were the resulting unstable crosslinks which quickly broke down causing a loss in the physical properties of the final product. A variety of inorganic chemicals were tried to correct the shortcomings and to some degree these were successful. The real breakthrough occurred early in the 20th century with the addition of aniline to the rubber compound. The cure time was significantly shorter and the final product more stable. The toxicity of aniline did pose a problem and safer compounds were sought. By the late twenties thiazoles (MBT, MBTS, ZnMBT, etc.) were in common usage and cure times had dropped dramatically. Equally important, the finished product had a relatively stable crosslink network and product life increased.
The faster cure times made possible by the use of thiazole accelerators led to a new problem. Compounders were being asked to design compounds that would cure quickly but could still be processed at higher speeds in the factory without scorching. In the early '30s, Bayer demonstrated that a sulfenamide accelerator would offer longer delay and a faster cure rate than the thiazoles. The first commercial sulfenamide, cyclohexamine derived, was proposed by Monsanto in 1937. Since that time sulfenamides in their various forms have been the accelerator of choice for curing general purpose elastomers.
The improved crosslink structure modern accelerators made possible was only a partial solution to maintaining the properties of the finished rubber product.
Rubber chemists had noticed that the ability of natural rubber to resist aging varied widely from lot to lot. A naturally-occurring age resistor was present in differing levels in different batches of rubber. The search was on for materials that would duplicate the natural antioxidants.
The number of chemicals added to rubber compounds, with varying degrees of success, would have stocked a drugstore. The most effective seemed to belong to the aromatic amine class. These materials were relatively inexpensive, effective against heat and oxygen and they or their descendants are still in use.
Even with aromatic amines present, rubber articles were still failing more quickly than desired. Fatigue or flex failure accounted for some of the shortened life but another degrading action was also taking place. It wasn't until the '40s that the significance of atmospheric ozone on the life span of a rubber product was truly appreciated.
The first antiozonant was introduced in 1952. This product did offer protection from ozone but was highly fugitive and protection was relatively short term. Neither did it improve resistance to oxygen, heat or fatigue.
In 1941, Bayer patented an alkylaryl-paraphenylene diamine with a cyclohexanone alkyl group for use as an oxidation inhibitor. But it was not until 1954 that commercial production of the isopropyl derivative was begun. Now a compounder could add a single chemical and improve the resistance to heat, oxidation, ozone and fatigue.
Increased processing speeds and shorter cure cycles kept scorch a primary problem in rubber factories. The use of sulfenamides as primary accelerators had helped but more processing safety, without changing cure rate or final properties, was needed.
Retarders based on salicylic acid or phthalic anhydride were used but in general, they were not particularly effective at increasing the scorch delay of sulfenamides. In most cases, they also reduced the cure rate and final modulus.
A second material, n-nitrosodiphenylamine was effective to some degree with sulfenamides based on secondary amines, e.g. morpholine, but ineffective with primary amine sulfenamides.
In 1965, Monsanto introduced Santogard PVI which delayed the onset of cure (or scorch) without affecting other properties. Compounders were now able to tailor a compound to provide the scorch safety needed for high output processing and still enjoy the rapid cure rates offered by sulfenamide cure systems. This product is generally accepted as the last major technical innovation in the industry.
Before addressing the reasons for the lack of innovation and what unmet needs the rubber chemical industry still has to address, a discussion of that industry is appropriate.
Four companies are generally recognized as having global rubber chemical businesses: Monsanto and Uniroyal Chemical in North America, Bayer of W. Germany, and Akzo of The Netherlands. All four have extensive operations throughout the world.
Uniroyal Chemical produces accelerators, antidegradants and specialty chemicals for the rubber industry in world scale plants in Geismar, LA; Elmyra, Canada; and Naugatuck, CT. It also has smaller plants, scaled to serve local markets, in Brazil and Taiwan; as well as minority interests in plants in Mexico and South Africa.
Monsanto, with large plants in Newport and Ruabon, UK; Sauget, IL; Nitro, WV; and Antwerp, Belgium is the major supplier to the rubber industry from this world-wide base. Its plants in Brazil, Argentina and Canada are smaller in scale as are the markets they serve. Joint ventures in Japan and Korea round out Monsanto's manufacturing base. As would be expected from Monsanto's close association with the tire industry, its product line leans heavily toward additives for general purpose elastomers. It also includes smaller volume specialty products such as a pre-vulcanization inhibitor and non-staining antioxidants.
Bayer is the largest rubber chemical producer in Europe. It has major plants in Leverkusen and Brunsbuttel in West Germany and in Antwerp, Belgium. The Bayer plants in the U.S. and Brazil are smaller and local market focused. In addition to the basic accelerators and antidegradants, the Rubber Chemical Group of Bayer manufactures and sells a wide variety of specialty products to the rubber industry.
The newest world player is Akzo, although its parts have been around for years. Akzo purchased Vulnax in 1987 (Vulnax was a joint venture of ICI and Rhone Poulenc to sell and manufacture rubber chemicals). Prior to this purchase, Akzo had been a major supplier of thiurams only. In rapid order the Stauffer insoluble sulfur business was acquired, then an accelerator plant in Italy. Rubber chemical manufacturing is concentrated in Europe, but its international sulfur business has given it a world-wide outlook.
To understand the future of the rubber chemical industry we need to look at the recent history of the rubber industry, more specifically tire production. Tremendous changes have occurred in the world-wide balance since 1978.
In North America there are now just two tire companies being traded on the NYSE. In 1978 there were eight or more. Tires are smaller and lasting longer. As a result there are, today, fewer tires produced and less rubber consumed than ten years ago. An obvious fallout has been the rationalization of the tire industry; but equally important has been the resulting shrinking of the rubber chemical market. The North American chemical industry was sized to serve a market which consumed 3,400,000 tons of rubber in 1978. By 1980 it was down to 2,600,000 tons. Without significant capacity reduction or a gain in market share, each supplier faced operating at marginal profit levels. The 1982-83 economic readjustment, when consumption dropped further to 2,400,000 tons, made the situation worse. Rubber fabricators responded by closing down several outdated plants, but as far as the rubber chemical industry was concerned, the fundamental issue remained very low consumption.
Europe experienced the same economic slowdown as North America, but had already undergone radialization and, because cars were smaller, tire downsizing was simply not an issue. Rubber consumption fell, but not to the extent that it did in the U.S. Chemical prices were, therefore, more stable. Increased raw material prices and lower volumes, however, did reduce margins and put a further squeeze on the profitability of the major rubber chemical suppliers.
The chemical producers attempted to recover the volume by exporting to the relatively faster-growing Pacific Rim. What they succeeded in exporting was the general instability and the price softness of the rest of the world. Prices of rubber chemicals dropped to their lowest levels in years in real dollars all over the world. The net effect of all this was several years of marginal economics for the rubber chemical producers.
Rubber chemical producers were now facing both lower volumes and lower margins. It became obvious that the lowest cost producer was in the best position to survive the days ahead. Technical resources that would have been employed to develop new products were diverted to process research in an industry-wide attempt to lower costs, and those individuals lost through attrition were seldom replaced.
Processes were indeed made more efficient, but no significant new products were introduced from the late '70s through the mid-'80s. The recovery of the world economy had started by then, and demand started approaching in-place capacity. Prices began to rise and historical margins were restored. Chemical suppliers began again to look for new products. Several changes have made this a difficult process. Research teams have to again shift emphasis and many necessary skills have been lost over the years to retirement and job changes. The environmental and safety regulations now in place make the introduction of new products more difficult and more time consuming.
The development and introduction of new products in this adverse atmosphere is almost prohibitively expensive. A chemist can no longer sit in the lab, dream up a molecule and then try to find a market for it. The relationship between customer and supplier must become a partnership. Joint programs with the customer identifying specific needs and the supplier devoting resources to satisfy these needs will become an integral part of product development.
The establishment of these partnerships will benefit the customer by solving his problems quicker. The supplier will benefit from the reduced risk inherent in the introduction of new products as well as more rapid product introduction.
The product needs identified by rubber fabricators fall into three categories. The first area includes products which will replace current chemicals with known environmental concerns. Concerns over nitroso-amines are growing and there is a need for products which do not produce or liberate secondary amines such as morpholene or dimethylamine. The pressure for new chemicals in this area is somewhat less than would be expected. In many, if not most cases, other products with equivalent performance are available. With minor formula changes, sulfenamides based upon tertiary-butylamine or cyclohexylamine can replace a morpholine based sulfenamide. In other cases, other systems can be used that will result in function or performance equivalence. Currently, sulfenamides dominate the accelerator market. They have excellent cure characteristics and, when combined with a pre-vulcanization inhibitor, they have an almost infinitely variable delay. It seems unlikely that anything to replace them will be coming out of the laboratories in the near future.
The second area of need is in long term endurance. Tire manufacturers have improved tires five-fold in the last 10 years through radialization, engineering and compounding improvements. The industry is at the point now where sidewall life is becoming a factor, particularly in truck tires with their multiple retreads. Paraphenylene diamines are powerful antidegradants but eventually they are consumed and the rubber article becomes vulnerable to degradation.
The approaches to this problem are multiple. Constructing sidewalls of a saturated polymer or a mixture of a saturated polymer and a general purpose polymer would result in a sidewall with inherent resistance to oxygen and ozone. Unfortunately, flex resistance would still be required. Mixtures of saturated and unsaturated polymers are difficult to make and cure uniformly. A need then exists for chemicals which will improve fatigue but not be consumed. Additives which would promote mixing and curing uniformity or improve the adhesion between saturated and unsaturated polymers would offer another route to the solution of the long term endurance problem.
A third major need is for greater throughput in the factory. Processing aids are common, but all suffer from one or another shortcoming. What seems to be needed is a totally new approach to processing. The industry needs to understand the properties that impact processing. Once we are able to measure them reliably, we will be in a better position to modify those properties and optimize processability.
More rapid cure rate is a subset of greater throughput; shorter cure times mean more tires. Initially at least, sulfenamides enabled the compounder to reduce cure times, but the pressure remained to further increase productivity. Higher mold temperatures are also used, but both methods have apparently reached their limits with the current state of the art.
The opportunity exists for a chemical that could significantly improve cure rate, allowing for even faster cures at the mold temperatures of today. Increased hot tear strength would allow for higher mold temperatures and shorter cure times.
These new products will be almost by definition low volume, high performance products. The small volume and high development costs will dictate that only those products that offer an advantage for which the customer is willing to pay will survive.
The present day rubber chemicals, accelerators, antidegradants and prevulcanization inhibitors will be around for a long time. They are proven performers that producers know how to make well and products that rubber fabricators know how to use.
An exception to this general statement will occur if environmental or health problems are discovered. At that point, the industry might be forced to a more expensive or less efficient replacement.
Before concluding this article, a few words about the future of the market itself would be appropriate.
Rubber consumption has grown since the early '80s and some rubber chemical capacity has been removed from production. Presently, chemical supply is well matched to the demand. Historically, the industry would have rushed large new capacity into place and a spiral of price reductions would have begun. The combination of slow market growth and high investment costs of a world scale plant seems to have dictated this behavior.
What we are seeing is small investments in de-bottlenecking projects that are increasing capacity as needed. Small investments in emerging economies are also mitigating the need for large investments. Eventually, of course, an investment in a new large plant will have to be made. At that time, we might see a joint venture within the industry to manufacture a key raw material or even the final product. This would help spread the risk inherent in large capacity expansions. Cross licensing of new products is another way to reduce the cost and the risk of new product development.
The rubber chemical industry is alive and well after a several year illness caused by the sudden and drastic downsizing of the U.S. market. Our future health depends upon our ability to keep supply and demand balanced. We must also find and produce a continuing stream of new products. New products that offer clear advantages to our customers, and are profitable for the producers. I have no doubt we will.
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|Title Annotation:||Rubber World 100th anniversary|
|Date:||Oct 1, 1989|
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