IS THERE REALLY A DEVULCANIZATION
METHOD THAT WORKS AND IS ECONOMICAL?

Non-Chemical Devulcanization

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INTRODUCTION

One of the biggest problems in the world today is “What to do with rubber generated as scrap?” (Both post manufactured and post consumer scrap) Do we landfill material until we can no longer landfill, or do we start doing something about it now? Most people are aware of the problems that we have and will be facing in years to come. The amount of scrap rubber being generated accounts for approximately 5-7% percent of all rubber products made today. For years, scrap has served one basic purpose – to be collected and discarded in landfills. With landfill space disappearing, emphasis has been placed on recycling of scrap elastomers.

Processes such as cryogenic & ambient grinding, chemical devulcanization and ultrasonic devulcanization have been introduced to eliminate some of the scrap issues at hand. The process being used for silicone has been in the industry for years, but has never been expanded in the way that we are now using it.

Recycling of various elastomers has a certain niche in the industry. It has always fallen under the impression that rubber scrap is nothing but garbage. Different names have cropped up such as recovered, reclaimed, devulcanized, ground, etc. When a chemist or Technical Director hears these names or words, he/she cringes at the use of these materials. These types of materials are good for only limited applications. There is another process defined at TRC Industries that can be of use in place of the above mentioned processes. It will be demonstrated in this paper that elastomers that have been devulcanized can be used in a wide range of applications.

CURRENT PROCESSES

Cryogenic & ambient grinding of scrap elastomers can produce some desired results. The smaller the particle size, the better. In this form, the crumb elastomer is used as a filler, thus only a certain percentage can be used. Once a certain level of crumb has been added, the finished compound takes on a dryer, rougher texture and is difficult to hold together without adding additional ingredients, thus changing the original compound.

Chemical devulcanization of scrap elastomer is done on finely ground material crumb. The finer the grind, the better the surface treatment. Usually chemically treated elastomer can be added at a much higher level than untreated crumb. The chemically treated material holds together and gives more plasticity in the finished compound. One thing that has to be considered in a chemically treated material is, “Is the chemical compatible with the original elastomer?”

Ultrasonic devulcanization is another method that has been proven in a laboratory environment. One drawback to ultrasonic devulcanization is the expense required to set up this process in a production environment. Once in a factory environment, I am sure this would offer another alternative to the scrap issue at hand.

Our process used has been in the rubber industry for a number of years. What TRC has done, is taken this process, added some modifications (which are patent-pending) and have taken the reclaiming to another level. One thing to keep in mind if you are planning to use a devulcanized material; ask a few questions:

1. What do I want out of a recycled or reclaimed material?
2. Do I want to reuse it back into the same compound?
3. What kind of physical or dynamic property drop-off is considered okay?

The kind of preparation or thinking used to formulate the virgin compound should be used in reclaiming that cured compound. The process starts with the collection of available scrap in the fabricator’s facility. The collection of scrap is very important to the customer of devulcanized material. Like materials should be collected and not contaminated with other types of elastomers. The process is very material oriented. When the scrap arrives it is completely inspected for ontamination. It is put through a sampling process and then allowed to enter production. The sampling process entails taking scrap out of each container and running it through the devulcanization process. The sampling process is the same as the production process but on a smaller scale.

Once approved, the scrap is ground to a size of 10 mesh – ¼” at ambient temperature. It is subjected to a certain amount of time and temperature, depending upon the elastomer to be devulcanized. Once the appropriate amount of the time has been added, then the devulcanized material is milled and checked for Mooney viscosity and dispersion of elastomer. This is checked against a present viscosity, established in the initial testing set by the customer. The material is run over processing equipment and packaged to a pre-determined weight.

Depending on the compound, customers can generally blend between 10% to 60% percent of devulcanized material to achieve the desired results. One unique feature to our devulcanized elastomer is we add no chemicals to devulcanize or surface treat our product. Devulcanized elastomers, blended with virgin elastomer compounds, can be utilized in molded, extruded, and calendered applications. It is recommended that our devulcanized material be used as a direct polymer substitute rather than filler. In this respect, it can be used as a raw polymer.

DEVULCANIZATION

A truly devulcanized elastomer can be characterized as an elastomer that is as pliable and as processable as its virgin counterpart, maintaining as much physical and structural integrity as possible. To prove the degree of devulcanization of the TRC processed elastomer, we picked two elastomer compounds currently devulcanized and had them tested for a percentage of devulcanization.

One elastomer was a compounded silicone elastomer (peroxide cured) used in an after market, spark plug boot application. The other, an EPDM (sulfur cured) compounded for a spark plug boot application as well. On each elastomer, two samples were submitted for evaluation. First samples consisted of cured parts (control). Second samples (end product) run through the heat process. A standard cross-linking density test, using the swelling method, was performed on each set of samples.

The percentage of devulcanization was considered substantial. The EPDM was approximately 25% lower in devulcanization rate compared to the silicone elastomer. We decided to pursue a higher percentage rate of devulcanization on the EPDM elastomer. Some modifications were made in regards to time and temperature and two additional samples were submitted for re-evaluation on percentage of devulcanization.

After additional testing on the EPDM compound, the percentage of devulcanization increased from 64% to 76%. Further modifications to process rendered scrap EPDM elastomer useless in desired application.

Certain questions may arise in regards to the molecular chain incision (breaking the rubber chains) of an elastomer compound. To answer some of the questions asked internally, we embarked upon a series of tests to prove or disapprove the chain incision theory. For these series of tests we have chosen two virgin elastomers, one a silicone base, and the other an EPDM base.

For testing purpose, a Wacker Silicones base was chosen for this evaluation.

Silicone formula: Parts per hundred
Wacker R401/40 base 100
Dow Corning STI-T 1

Ten pounds of the above material was mixed on a laboratory 2 roll rubber mill. An ASTM 6”x6”x.07 +/- .005 slab was press cured for 10 minutes at 250 F for the control. Then the five pounds of material was cured in a hot air oven for 2 hours at 400 F. After verifying cure condition of the silicone base, the material was allowed to sit for 24 hours to achieve ambient temperature. The cured base was ground at ambient temperature to a nominal 1/8” – ¼” particle size. At this time the ground base and the same base without peroxide were subjected to time and temperature according to parameters developed in the TRC process. Upon completion of the devulcanization process, a series of tests were run according to established ASTM procedures. The results are as follows:

A standard cross-link density test using a swelling method was run on the cured and devulcanized base. A devulcanized rate of 80% was achieved.

OBSERVATIONS

The question to chain incision may be answered by asking other questions such as, “Does the vinyl content of the devulcanized silicone base have a bearing on physical results?” Again does the vinyl content have a bearing on the cure state of elastomer? The following observations were noted based on the test completed on the silicone base blend with devulcanized base:

Devulcanized base w/peroxide:

The durometer and tensile properties degraded slightly, but elongation and tear values were up. The more devulcanized silicone added, the tougher or more green strength the formula had. Hot tear of material improved with the addition or more devulcanized silicone base.

Devulcanized base w/o peroxide:

The durometer and tensile properties degraded a little more rapidly and as noted above, elongation and tear values increased. The rapid degradation of tensile may be associated with the lower vinyl presence in the formula. The green strength was not enhanced due to any presence of a certain percentage of residual cross-linked base.

These questions and others may be answered as we move forward with additional testing that has not been completed as of this date. Additional testing will be done throughout the year with a further update in the upcoming ACS meeting, in October 1998.

Second series of tests were run on an established spark plug boot application using EPDM. Ten pounds of a mixed compound was received without its cure package being added. The cure was added to five pounds of material on a laboratory 2 roll mill with a .100” nip opening. The catalyzed material was then tested to achieve the standard physicals. The remaining material was then cured and then allowed to sit for a period of 24 hours at ambient temperature. The cured EPDM was then ground to a 1/8-inch particle size and placed in a vessel for devulcanization under a set time and temperature. After that period of time the material was then removed and allowed to cool to ambient temperature. The material was then milled on a laboratory mill, with a .075” nip opening, to transform ground material into a sheet form. Upon completion of this process a standard set of testing was run on this material.
Press cured: 10 minutes at 320 F 6”x6” x .075”

ASTM slabs were used.

The following observations were noted on the EPDM elastomer. We saw stability between specific gravity and durometer. A degradation of tensile and elongation properties were noted. However, tear properties did increase. Keep in mind that the original cured EPDM was devulcanized to a rate of only 76% using a solvent swelling method. You may ask yourself, “Were the drop off in properties due to residual crosslink left on the particles or an actual incision of the molecular chain?” Further tests will be completed throughout the upcoming year. Results will be published in the upcoming ACS rubber division meeting, in October 1998. Several other projects are under way including post consumer products such as scrap tires in the form of crumb, tire bladders, radiator hoses, etc. Again, as we move forth, findings and results will be released at later meeting dates.

ADDITIONAL TESTING

Following is some physical testing run on several silicone formulas to establish specifications in the automotive and military industry. Along with these are physical tests run on some EPDM formulas.

COST ADVANTAGES

There is a definite cost saving’s in utilizing devulcanized elastomers in your formulas. When dealing with specific applications, the amount of devulcanized material used may vary depending on the dynamic and physical properties that need to be achieved.

The devulcanized elastomer can be used as a direct virgin polymer substitute, to achieve the desired effect, rather than a filler to be added on top of total weight, thus producing more of a cost savings advantage.

CONCLUSION

When dealing with scrap elastomers, they must be treated similar to a virgn elastomer compound. The devulcanization of the individual scrap elastomer must be custom tailored to fit each application without increasing cost, having to add to the processing of an elastomer compound to make radical changes in formulas, and minimal effect on the finished parts or end product. As we move towards the future, the need to recycle scrap elastomers will increase. A viable alternative has been demonstrated to allow the ease of incorporating your scrap elastomers without any chemical being added. What you devulcanize is what you get back. Recycling of scrap elastomers is here to stay.