Is peroxide/coagent curing for you?

As new technologies emerge and develop in the rubber industry, compounders are discovering that sulfur cure systems cannot achieve the physical and chemical performance properties or meet the critical specifications the new applications demand. Consequently, compounders are looking to other curing systems to provide the technical advantages necessary to achieve these required physical and chemical properties. Among the number of sulfur cure alternatives are three peroxide curing technologies: peroxide-only, peroxide/coagent and peroxide/scorch retarded coagent cure systems.

Peroxide/coagent curing technology

Peroxide-only cure systems generally result in slow cure rates and low crosslink density. Peroxide/coagent cure systems, on the other hand, increase the cure rate and increase the crosslink density. Such an increase in the cure rate results in a reduced scorch time and may lead to undesirable scorchy behavior where elevated processing temperatures are required, such as in mixing, injection molding and extrusion.

Peroxide/scorch retarded coagent systems minimize scorchy behavior by increasing the Mooney scorch time without affecting cure rate and crosslink density. Coagent technology for increasing scorch time is based on non-nitroso scorch retarders which are safe and effective for robber compounding and include several solid and liquid scorch retarded coagents.

A key feature of all peroxide/coagent cure technology is the superior strength of the C-C bond compared to the S-S or C-S bonds in sulfur curing. The C-C bonds result from a chain reaction of free radicals that are produced by healing peroxides (figure 1). These free radicals readily abstract hydrogen atoms from the polymer backbone to form polymer radicals. The polymer radicals in turn combine with each other to form very strong and stable C-C crosslinks.

How strong are these C-C crosslinks in elastomers? Consider the energy it takes to break them down. 42% more calories are requirod to degrade a C-C bond than it takes to degrade a perfect C-S monosulfide bond. And it takes 72% more calories to degrade a C-C bond than an S-S bond that results from a normal sulfur vulcanization. This strength is one of the main reasons why peroxide/coagent cured elastomer products exhibit superior heat resistance characteristics to sulfur cured systems.

Peroxide/coagent cure product benefits

When considering the merits of a peroxide/coagent cure, elastomer performance benefits, as illustrated in figures 2-4, first come to mind. However, other coagent benefits such as excellent scorch safety, fast cure rate, nitroso free, low Mooney viscosity, and straightforward formulation characteristics make them particularly easy to work with in the manufacturing process.

Seemingly contradictory production and performance capabilities such as low viscosity and increased product hardness are attainable at the same time with the use of acrylic coagents. Because the coagents act as plasticizers, the more that is added, the lower the viscosity becomes and the easier it is to process. Upon curing, however, precisely the opposite effect takes place. In crosslinking, the coagent liquids literally become part of a newly created polymeric system, enhancing the performance properties of most elastomers.

By varying the type and amount of acrylic coagent in a formulation, the elastomer compounder can easily enhance the following performance parameters: Wide hardness range; low compression set; high modulus; high resiliency; high elongation; low Mooney viscosity; high tensile strength; good scorch safety; excellent heat resistance; excellent processability; improved aging characteristics; and good oil and water resistance

Acrylic liquid coagents, in general, give good crosslink density, low Mooney viscosity and low compression set. Acrylic solid coagents give good load bearing properties, good tear resistance and high tensile and elongation.

Mixtures of the above Coagents can be employed to specifically tailor three properties to meet different product specifications. A general rule of thumb is to mix acrylates with acrylates, and methacrylates with methacrylates.

Overcoming scorch problems

The value of using acrylic coagents as "vulcanizable plasticizers" with peroxides has been generally understood throughout the rubber industry. However, while reducing the Mooney viscosity and ultimately curing finished products, many coagents are hampered by the uncontrollable onset of scorch. Most coagents are not scorchy during the processing and molding of elastomer compounds.

Generally, the addition of any amount of acrylic coagent reduces the Mooney scorch time drastically. Even in simple compression molding, difficulty has been experienced in filling all the cavities of a mold before curing begins to take place. Typical results from adding coagents to the peroxide compound are increases in scorch time as much as 10 fold at [250 degrees F].

Many scorch aids have been tried in the past but with accompanying negative side effects of slower cure rate and poorer physical properties. Some new developments eliminate the negative processing characteristics particularly with regard to scorch. Saret 517, for example, provides the best of all worlds...fast cure rates and long Mooney scorch times without any negative impact on performance parameters. Direct substitution of ordinary acrylic coagents with Saret 517 in test formulations managed to increase the Mooney scorch from 4.4 minutes to 45 minutes without the loss of any product performance (table 1).

[TABLE OMITTED FROM ORIGINAL PUBLICATION]

Injection molding represents the most demanding processing condition sensitive to scorch problems. Therefore, the unusual combination of good scorch safety and low Mooney viscosity obtainable with crosslinking coagents such as Sartomer's Saret 517 makes them of particular interest to injection molders. They are referred to as an injection molder's dream, by not being scorchy at 250*F processing temperatures and not retarding at 300*F curing temperature range.

Dos and don'ts of peroxide/coagent cure

Not all polymer types are compatible with peroxide curing. In some cases, while crosslinking is taking place, the polymers are simultaneously undergoing chain scission. The end net effect is that no crosslinking results. The following list provides a general guideline of what polymers are recommended for peroxide/coagent curing.

Use peroxide/coagents to enhance the properties of the following elastomers: Natural; polyisoprene; polybutadiene; styrene butadiene; acrylonitfile butadiene; polychloroprene; polyurethane; chlorosulfonyl polyethylene; chlorinated polyethylene; ethylene-propylene co- and terpolymers; silicone; hydrogenated nitriles; polysulfide; polyethylene; ethylene vinylacetate co-polymers; ethylene butylacrylate; ethylene acrylic; acrylonitrile butadiene styrene terpolymers; flouroelastomers; bromobutyl; butyl modified with divinylbenzene; and elastomer blends.

All of the above are compatible with peroxide/coagent curing. In every case, the process creates stronger, high-density C-C bonds for improved high performance.

Do not use peroxide coagents to enhance the properties of: Polyisobutene; butyl; epichlorohydrin; polypropylene oxide; and polypropylene.

These materials do not form strong C-C crosslinks due to simultaneous chain scission during the crosslink process. The net result is that little crosslinking takes place, and, in some instances, degradation of the polymer may occur.

Suffur cure elastomers Sulfur cure requires that elastomers contain unsaturation to affect the cure. Many high technology elastomers are free of unsaturation and therefore cannot be sulfur cured. This limits the field of sulfur curable elastomers which include: Polyethylene; chlorinated polyethylene; ethylene vinylacetate copolymers; ethylene acrylate copolymers; ethylene butylacrylate copolymers; fluoroelastomers; ethylene propylene copolymers; and silicones.

In addition, mixtures of saturated and unsaturated elastomers are peroxide cured to ensure that both elastomeric systems are cured. This added flexibility is another advantage that peroxide/coagent cure systems have over sulfur.

Cost

Cost, of course, is always important. But again, experience has shown that traditional vulcanizers often inadvertently overemphasize comparisons of coagents and peroxide raw material chemical costs vs. sulfur raw material chemical costs. In reality, superficial comparisons are only valid in the most rudimentary applications. In more sophisticated applications, other factors are involved. For example, in some sulfur cure processes, as many as five expensive chemical accelerators may be used to help overcome performance weaknesses in the sulfur cure formulation. These accelerators add to formulation costs, chemical expenses and processing costs for weighing, handling and mixing materials. These added factors increase total production costs as well as the risk of production errors.

Inevitable comparisons of peroxide vs. sulfur

Any discussion of peroxide/coagents among compounders inevitably comes to a competitive comparison with sulfur. Sulfur is being questioned more and more as compounders are becoming more familiar with the safety and ease of pcroxide/coagent curing. Problems in the past with sulfur cure, which include bloom, nitroso by-product considerations, poor color, odor, high Mooney viscosity and poor heat age performance, have caused compounders to rethink their formulations in favor of peroxide/coagent cure.

Actual comparisons of total production costs show that elastomers should not always assume that sulfur is always the low cost solution. Manufacturing costs of production, compounding complexity and quality control costs may add up to favor peroxide/coagent curing on a cost basis.

Performance comparisons demonstrated previously favor a peroxide/coagent cure whenever high heat resistance and low compression set are requirements. Finally, many elastomers simply cannot be sulfur cured. The versatility of the peroxide/coagent cure system provides the ability to "dealin" performance with the proper selection of Saret coagent.