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An eye toward safety.

An eye towards safety

In today's world of rubber products, demands are made on products that are requiring compounders to investigate and use materials that were only curiosities 20 to 25 years ago. Among these are peroxides.

Peroxides have long been known to be effective vulcanizing agents for rubber. They have also been recognized as being able to provide numerous benefits in the rubber compound. This includes improved age resistance, reduced compression set, excellent electrical properties and, in many cases, improved resistance to chemicals and oils. Unfortunately, since the chemistry of vulcanization is different, and historically peroxide selection was limited, usage was smaller than it could have been. The last decade has produced a significant change, however. The number of available peroxides has increased dramatically, allowing compounders to target cure temperature within a fairly narrow range. A partial list of these is shown in table 1. Likewise, coagents have been developed to enhance curative reactions and properties in the cured compounds. As a result, peroxide use has increased dramatically.

With the increased usage of these materials, questions of safety in storage and handling have gained significance also. Instead of the occasional bag of material in a warehouse, many facilities now are dealing with multiple tons on a normal basis. Since peroxides are reactive materials and, in some forms, quite unstable, a review of the pertinent safety procedures is in order. Lucidol Division of the Pennwalt Corp. has recently put together information specifically targeted at safe handling of peroxides, including a video presentation suitable for training. In this article, I will review a variety of the points made in that presentation.


Before beginning, it's probably worthwhile to review the basics of peroxide curing - specifically, how peroxides work.

Vulcanization using peroxides is accomplished by free radical dehydrogenation of a polymer followed by bonding of the polymer chains at that site. These are some big words that describe the reactions shown in figure 1. For illustration purposes, we will talk about dicumyl peroxide curing of polyethylene.

The first step in the reaction is the decomposition of the peroxide into cumyloxy free radicals. This is caused by heating of the compound. While the cumyloxy radical can subsequently decompose into acetophenone and a methyl free radical, the cumyloxy free radical predominates in the vulcanization.

The free radicals generated then remove a hydrogen from the polymer chain, creating a polymeric free radical. The polymeric free radicals then join to effectively crosslink at that site. The resultant bond is a direct carbon-carbon bond with no intermediary groups as occur with sulfur vulcanization.

This cure mechanism is of common use today with virtually all the polymer types available on the market, including silicone, fluoroelastomer, NBR, EPDM and all of the "workhorse" polymers.

As shown, the first step in the vulcanization process is the decomposition of the peroxide into free radicals. In figure 1, it was assumed that the peroxide was heated after being mixed into the compound. However, the same decomposition will occur whether or not the peroxide is mixed in a compound. Application of heat to the peroxide alone will initiate the generation of free radicals.

In addition, decomposition can occur if the peroxide is mixed with other reactive materials, such as acids. Once generated, free radicals will react with any material convenient. This may be rubber. Or it could be oxygen.

Different peroxides have different rates of reaction. Table 1 shows the recommended cure temperature for each of a variety of listed peroxides. This temperature relates to the relative reactivity of the peroxide. On a relative scale, Di-t-butyl peroxide is a relatively unreactive material while benzoyl peroxide is quite reactive. The Di-t-butyl peroxide is one of the general class of dialkyl peroxides (which includes dicumyl peroxide). All of these are characterized by high activation temperatures. Likewise, these peroxides also offer good storage stability and thermal stability during compounding and mixing. Many are available as pure peroxides (often liquids) or in the more common filler extended form.

The diperoxyketal peroxides, such as benzoyl peroxide, are somewhat more reactive as a class than the dialkyls. While they have less shelf stability than dialkyls, they offer the opportunity of curing at much lower curing temperatures.

What about safety?

Fortunately for us, as a class, the peroxides suitable for elastomer applications are among the least hazardous and most stable of the peroxides available. All are stable at room temperature and all are among the least sensitive to external conditions.

All peroxides will burn. However, filler extended forms of the peroxides are much safer than the pure forms. In conditions where the pure peroxide will ignite and burn vigorously, many of the filler extended forms will only smolder or burn slowly.

In spite of the relative safety of the filler extended forms, they must still be treated with respect. While safer, they will still burn. Because of this, they should always be kept away from sources of ignition. This includes sparks and static electricity as well as open flames. Likewise, strong acids should not be kept close to any peroxides.

Should a fire occur, appropriate extinguishing media would include water spray, water fog or dry chemical foam.

As with many other materials, peroxides should be stored in a cool location. Excess heat can result in loss of activity if nothing else. They should also be stored in such a manner that air can freely circulate around the material. Electrical switches in the storage area should be explosion proof.

When it's time to use the material, only enough should be removed from storage to cover the planned need. Don't take extra. Once removed from storage, don't return unused portions. This will alleviate potential contamination that could be dangerous. Also, it will minimize the amount of material in the work area that could be exposed to problems there.

Even under ideal conditions, peroxides will have a limited shelf life. Generally, they should be used within six months of receipt. First-in, first-out inventory management is recommended.

Contamination is one of the significant sources of problems with peroxides. Care should be taken to avoid all forms of contamination. Particular problems occur if contaminated with oxidizing agents, reducing agents, metal salts or mineral acids.

When handling the materials, always take precautions. Safety goggles and gloves are advisable. Even a face mask if breathing dusty particles is likely. Any prolonged contact with skin should be avoided. If it happens, wash the area immediately. If any material gets in the eyes, don't rub. Flush thoroughly with water for several minutes and see a doctor.

Any accidents involving peroxides, even the tame ones used in rubber, can be hazardous if ignored. Spills should be cleaned up right away using sparkproof equipment. Spills should be disposed of in a plastic lined barrel dedicated to peroxide disposal only. Don't mix it with other materials.

What about in mixing?

Once the peroxide reaches the compound area, continued caution is wise. Contamination and spill precautions still pertain. In addition, peroxides should be weighed separately from other compounding ingredients.

If mill mixing, avoid adding peroxides to a hot, empty mill. In a Banbury mixer, avoid adding material to an empty chamber. In both cases, addition should be made at the end of the normal mixing cycle. Batch temperature at the time of addition should be kept as low as possible and residence time in the mixer at temperature should be minimized.

All mixing equipment should be grounded (good standard practice). Likewise, batch sizes and mixing methods should be such that concentrated pockets of peroxide will not be formed.

If post curing is called for on a peroxide cured compound, be sure it is done in a well ventilated oven. Fumes from post curing can otherwise accumulate and can potentially be quite hazardous.

Summary Peroxides are finding wider and wider application in rubber products today. While these materials are safer and more manageable than in the past, they must still be treated with respect. When used properly and with reasonable caution, the potential for problems is minimal. There are probably many persons who have ignored a number of the procedures noted with no problems. However, ignoring the safety procedures always increases the risk of danger.

Train personnel to read provided literature and labels. Also, read the provided material safety data sheets. Then:

* store the material properly,

* handle the material properly,

* use appropriate in-plant precautions,

* encourage safety,

* dispose of properly.

When care is taken to keep the materials cool and clean, and when good housekeeping practices are followed, use of peroxide crosslinking agents will be problem-free and safe. [Tabular Data Omitted] [Figure 1 Omitted]
COPYRIGHT 1989 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Title Annotation:Tech Service
Author:Menough, Jon
Publication:Rubber World
Date:Jun 1, 1989
Next Article:Using reprocessed fluorocarbon elastomers for high heat and oil resistance.

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