Vulcanization of elastomers.

This review will discuss the chemistry and technology involved in the vulcanization of rubber. The importance of insuring proper vulcanization to achieve desired properties in the final product cannot be overstated, yet its importance is frequently minimized or overlooked.

Unvulcanized, raw rubber is comprised of long chain molecules with molecular weights typically in the 150,000-500,000 range. In the unvulcanized state, rubber can easily be permanently deformed when subjected to a load, it typically exhibits relatively weak mechanical properties, and can be readily dissolved in suitable solvents. As such, unvulcanized rubber has limited commercial value.

What is vulcanization?

The term vulcanization or crosslinking refers to a chemical process in which the uncured, long chain rubber molecules are "tied" together into a three dimensional elastic network by the insertion of crosslinks (ref. 1). Vulcanization, per se, usually refers to the use of sulfur crosslinks, whereas crosslinking can involve other chemical species, i.e., peroxides, etc.

This three dimensional structure increases the strength and elasticity of the rubber, minimizes its tendency to permanently deform under load and reduces its sensitivity to heat and cold. Except for the initial choice of elastomer type, proper selection of the cure system and the curing process are the most important consideration in designing a rubber compound for a particular application.

Chemicals which impart this three dimensional network are the vulcanization or crosslinking agents. There is a limitless variety of combinations of these chemicals from which to choose, and the proper selection depends upon the type of elastomer and the performance properties desired. Sulfur is the most common vulcanizing agent because of its ability to react with carbon bonds present in the commercially important diene elastomers.

In addition to the vulcanizing agents, other auxiliary chemicals are typically used. These include:

* Accelerators - which function principally to increase the rate of reaction between sulfur and rubber.

* Activators - typically metal oxides (zinc oxide), and fatty acids (stearic acid). These materials facilitate vulcanization by forming rubber soluble complexes with the accelerators making them better able to react with sulfur to form crosslinks.

* Retarders - these chemicals are designed to delay the onset of vulcanization during processing of the rubber thereby minimizing the tendency to "scorch" or prematurely vulcanize.

The proper combination of each of the above chemicals comprise the elastomer cure system and each will be covered in greater detail.

History of vulcanization

Archeological evidence shows that natural rubber has been used for centuries in Central and South America for recreational, ceremonial and medicinal purposes (ref. 2). In the late 1700s, Priestly extensively investigated potential uses for rubber such as rainwear, erasers, etc., but it was not until 1839 that Charles Goodyear discovered that sulfur can be reacted with natural rubber to produce a material which did not become brittle when cold or sticky when hot. Hancock further refined this process and received a British patent in 1843 for sulfur vulcanization and for practical rubber processing techniques such as mastication (ref. 3).

Although sulfur alone will vulcanize rubber, it is an inefficient and slow process and the compound typically exhibits inferior properties. Inorganic materials such as zinc carbonate, litharge, magnesium oxide, etc., were commonly used in the 19th and early 20th centuries to improve sulfur vulcanization. In 1906, Oeslager discovered that aniline greatly increased the rate of cure, however aniline is toxic and caused severe health problems for those workers involved in open mill mixing and in the curing process. Within a couple of years it was discovered that the reaction product of aniline with carbon disulfide ([CS.sub.2]) produced an effective accelerator, thiocarbanilide, without the toxic side effects.

The period from circa 1910 to 1940 saw a flurry of excellent research in this area resulting in a wide range of chemicals for use as rubber accelerators. These developments paralleled the explosive growth of the rubber industry in developed countries around the world. A primary goal during this period was to improve productivity by speeding cure rate while also improving processing safety to minimize scrap. Figure 1 shows cure rate vs. scorch time for a variety of accelerators and when each of these was developed. Here, one can trace the chronology in attaining the cure/scorch balance culminating in the development of the sulfenamide class of accelerators in the late 1930s (ref. 4).

Except for a variety of specialty products for the newer high performance elastomers, no major new classes of vulcanizing agents, accelerators or activators have had wide acceptance in approximately 50 years. However, environmental concerns with some of the traditional chemicals has prompted new research in this area. The specific target today is a class of effective, non-nitrosamine forming vulcanization accelerators.

Vulcanization chemicals market information

Understanding the market for vulcanization chemicals requires an understanding of the rubber industry, itself. North American consumption of rubber was divided between natural rubber at 24% and all of the synthetic rubbers at 76% in 1991. Nearly all natural rubber is sulfur cured and most is used in tire applications. This results in significant use of accelerators designed for traditional sulfur cured compounds.

Tires account for 59% of synthetic rubber consumption worldwide with automotive mechanical goods accounting for 10% and non-automotive mechanical goods at 9% (ref. 5). Clearly this consumption pattern favors those accelerators used for the high speed, high productivity production of tires. And this favors the thiazole, and particularly, the sulfenamide classes of accelerators which comprise nearly 80% of accelerator consumption.

Opinions vary on where the market for rubber accelerators is heading today. On one hand, there is evidence that usage patterns will not significantly change in the near term. On the other hand, forces are in place to significantly alter these patterns. For example, growth rates are highest for high performance, non-tire elastomers which use cure systems which are different from the common types used in tires. Growth rates are also high for thermoplastic elastomers which do not require vulcanization (ref. 7). This growth trend towards TPEs is occurring in part due to the interest in recycling rubber articles; something that the TPE products can readily do while the traditional, vulcanized rubbers cannot.

Environmental issues could also influence accelerator use patterns. Some common types have been classified as nitrosamine producers with reported toxicological implications. Clearly, cost effective replacements for these materials are needed and are the subject of research today.

Effect of vulcanization on physical properties

As previously stated, vulcanization is a chemical process to improve an elastomer compound's physical properties. However, in most cases, not all of the desired properties reach their optimum levels simultaneously. It is one of the rubber compounder's key responsibilities to achieve a balance of the most important requirements by the proper selection of cure system (chemical) and time/temperature cure cycle (physical).

Frequently, the curing equipment available, i.e., presses, autoclaves, LCM lines, etc., do not allow the curing conditions to be varied as desired, so the compounder must design his or her cure system to be compatible with the existing equipment while also meeting the compound performance requirements.

Measuring vulcanization

The formation of a three dimensional structure during vulcanization increases the stiffness, or modulus, of the compound. Therefore, following the increase of modulus versus cure time provides a continuous picture of the vulcanization process. Oscillating disk rheometers, available for nearly 30 years, provide a useful method to do this (ref. 8).

In this test, a preweighed sample of uncured rubber is placed into a preheated cavity containing a conical shaped rotor. The cavity is closed and the rotor is set to oscillate within the rubber sample. As vulcanization proceeds, the compound's resistance to rotor movement increases and this resistance is followed as a function of time, thereby a continuous profile of cure behavior is obtained. These cure curves, shown in figure 2, provide the compounder with information on: 1 - scorch safety or cure induction time; 2 - cure rate; 3 - time state of cure, or stiffness developed; 4 - optimum cure time, or time when modulus no longer increases significantly; and 5- reversion, or the tendency to degrade upon overcure.

Effect on physical properties

Sulfur vulcanization is a complex reaction during which both the number (crosslink density) and type (crosslink structure) are continuously changing as the reaction proceeds. The chemical structures formed at any given time during cure may favor one set of properties such as tear strength while not being optimum for others such as hysteresis and set. A cure system designed to minimize these tradeoffs is always a goal of the compounder.

Layer (ref. 9) has summarized the effect of both number and crosslink length (Sx) on the most common physical properties (table 1).

Property                         Number            Length
Tensile strength                > Then <              >
Elongation                          <                 -
Modulus                             >                 -
Flex life                           <                 >
Set                                 <                 >
Hysteresis                          <                 >
Resilience                          >                 <
Thermal stability                   -                 <
Oxidative stability                 -                 <
Rolling resistance                  <
Tear strength                   > Then <              >
Abrasion                            >                 >

These trends can be explained by the following. Modulus or stiffness will increase as the number of crosslinks increases because the 3-D structure which forms becomes more resistant to deformation under load thereby requiring greater force to reach a given elongation. With overcure, crosslink structure either degrades and modulus drops as in natural rubber, or it continues to build resulting in marching modulus as in SBR and polybutadiene.

Tensile and tear strengths and fatigue life frequently pass through a maximum as cure progresses. These ultimate properties are highly dependent on the presence of flaws in the material. As crosslinking proceeds and modulus builds, any applied load acting on a flaw (due to poor dispersion, small nicks or tears, localized overcure, etc.) will concentrate the stresses at the flaw and provide a mechanism for tensile and tear propagation of poor fatigue life.

Permanent set and low hysteresis properties depend upon minimizing the viscous or plastic component of modulus. Since crosslinking increases elasticity, a high state of cure typically provides the best set and heat buildup properties.

Heat resistance is influenced by both the type and extent of cure. The greater the strength of the chemical bonds in the crosslink, the better is the compound's heat resistance. Peroxide cure systems, which result in carbon-carbon bonds offer the best potential heat resistance. Sulfur based systems result in a range of sulfur crosslinks varying from 1 to >30 sulfur atoms per crosslink, and heat resistance improves as the number of more thermally stable short crosslinks predominates. This is an important factor in designing the desired cure system.

Some properties are not significantly dependent upon crosslinking and remain nearly invariant as cure progresses. These include thermal conductivity, electrical properties and low temperature brittleness.

Another cure system consideration is the compound scorch behavior. Prior to vulcanization, rubber is plastic-like and can be processed into desired shapes such as tires, hoses, belts or other articles. The time available to accomplish this processing depends largely upon the cure system and is referred to as the scorch time. If a compound cures prematurely during the processing step, it usually becomes useless scrap. Therefore, a key requirement of the vulcanization step is to minimize premature vulcanization or scorch.

Bibliography

(1.) Coran, A.Y., "Vulcanization," Internal Technical Review, Monsanto Chemical Company, 1988. (2.) Thompson, R.S., "Rubber in America before 1492," Godfrey Cabot Inc., Boston, 1950. (3.) Shaw, D., "People who made it possible," European Rubber Journal, 25, June 1989. (4.) Warrach, W., Miles, Inc., Personal communication, Akron, OH, 1991. (5.) International Institute of Synthetic Rubber Producers, 1990. (6.) Personal communication, Miles, Inc., 1992. (7.) Chemical Market Reporter, October, 1991. (8.) Rader, C.P., Vulcanization, Intermediate Correspondence Course, Rubber Division, ACS, Akron, OH, 1985. (9.) Layer, R. W., Elastomerics, May, 1988.

(Editors note: This is the first part of a four-part series on vulcanization. Subsequent parts will cover vulcanizing agents, chemicals, cure systems and cure system design.)