Recycling rubber products sensibly.

Recycling rubber products sensibly

The problem of scrap rubber wastes must be seriously confronted by all in our industry. With the decline in the number of rubber reclaiming plants, we are probably the only major industry that is recycling less today than we did twenty or thirty years ago (ref. 1).

Accumulations of scrap tires are growing, and landfill space for industrial rubber waste is becoming scarcer, more expensive and restrictive daily.

Using ground cured rubber as a compounding ingredient presents one viable alternative to solving this problem.

Ground rubber is as old as the rubber industry itself. Rubber products that were ground to size or buffed created rubber dust, which was usually discarded as waste. Some enterprising compounders found they could selectively use this dust, or even coarser grindings, as a compounding ingredient to reduce costs. Over time, an industry grew to actually grind rubber for this purpose (ref. 2).

Still, using such untreated ground rubber has its drawbacks: For one, it can only be added to a compound in low percentages of 5-15%, higher loadings result in severe degradation of the physical properties of the cured compound.

A workable solution has been developed to counter these drawbacks, Tirecycle and Fibrecycle, which offer the alternative of grinding used tires and other rubber wastes, separating the rubber, fiber and metal, thus allowing for complete recycling of tires and other industrial scrap (ref. 3).

The first of these products, Tirecycle, is a polymeric surface treatment which, when applied to cured rubber particulate, enhances its ability to chemically bond (crosslink) to an uncured elastomer matrix during vulcanization.

The second product, Fibrecycle, allows for the salvaged tire fiber to be treated and used in a similar manner.

Specialized treatments for such diverse polymers as SBR, NR, NBR, CR, EPDM, PBD, butyls, etc., are also available (ref. 4). There are treatments to allow for high rubber loadings of thermoplastics such as PE, PP and PVC. And a treatment can be customized further to give special desired physical properties and to accommodate specific production and compounding requirements.

Both of our proprietary products impact the environment in a sound and benign manner, as no chemicals are used in the grinding process, nor are any objectionable odors or gases generated. With the increasing strictness of environmental regulations, this method of recycling offers a viable alternative to expensive (and often prohibited) landfilling, or burning as fuel which requires expensive scrubbing equipment.

Our polymeric surface treated particulate product is a dry, free flowing powder-like material, similar in appearance to conventional ground rubber. It can be packed in bags, cartons or in bulk. Our salvaged treated fabric is packed in a similar manner and is also available in bale form.

Masterbatches are available for both products, allowing for compounding versatility at the consuming plant.

A complete compound can also be prepared or even presized into press-ready blanks, thus eliminating any down-stream "prep" work by a manufacturer. This is an additional, significant cost saving.

Processing and physical properties

For this article, data presented will be limited to industrial goods applications. Evaluations have been done on recapped tires and OTR tires, but since extensive testing is not complete, results will not be reported until a later date (ref. 5).

The data for this article were generated by using fabric and metal-free particulate obtained from grinding and separating whole tire scrap and then treating the grindings with our polymeric surface treated particulate. In this study, ground whole tires were usually used as feedstock because of availability, but in practice, factory scrap such as transfer pads, rejected moldings, buffings, cured flash, etc., can also be pulverized and used as the base material with similar results.

Mesh size

Our results indicate that uniformly finer particle sizes (under 30-mesh) do not necessarily improve performance commensurate with the added grinding costs (ref. 6). Grinding to a 10-, 20- or 30-mesh and retaining all the fines seems to result in cured physical properties very close or superior to those obtained with much smaller untreated meshes, and usually allows for higher rates of loading.

The advantages of finer particulates are in handling (milling and extrusion), and in camouflaging the presence of the particulate in the cured compound where surface appearance is absolutely critical.

The effects of adding 50% treated ground tire crumb at various mesh sizes, treated and untreated, are shown in table 1. The same general-purpose SBR compound was used throughout the experiment. Note that the tensile strength in the compound containing 50% treated 10-mesh crumb is higher than the one containing 50% untreated 30-mesh crumb.

Compression set data indicate that compounds containing untreated crumb are undercured, and that an adjustment in the cure system will be necessary to make them viable. Our experience would also indicate untreated compounds would probably be much too dry to process on standard rubber equipment.

Although the surface finish of moldings prepared with 50% 10-mesh treated polymeric surface treated particulate shows some roughness, it is acceptable for many applications. The compounds prepared with 20- and 30-mesh treated particulate produced parts with a very smooth surface.

In every case, the surface of compounds containing treated material is smoother than that of those containing untreated ground rubber of a similar mesh size. This we attribute to our treatment, which seems to lubricate each particle, allowing its better dispersion in the compound. Consequently, particles nearer the surface are pushed in to the mass of the molding and do not protrude from the surface as untreated particles tend to do. The lubricated surface of treated particles might also account for the improved mold flow observed when polymeric surface treated particulate is used.

Compounding

Compounding with our polymeric treated particulate is quite simple. Unlike other compounding ingredients, our product is added as an adjunct to the original compound without making any other adjustments to it. In practice, the batch size is reduced to accommodate the amount of polymeric treated particulate to be added. The base compound should be mixed in the normal manner and then the predetermined weight of our product is added late in the mix. As curatives are already present in our product, adding it early in the cycle could result in scorched stock. Dispersion is rapid and should not greatly affect cycle times.

To accommodate most existing tooling, we recommend blending our polymeric treated particulate with an existing standard compound at a 50:50 ratio. Compounds containing over 50% tend to be dry and bag on most mills and calenders. Using less than 50% may negate the economic advantage, unless a special or expensive base polymer stock is involved. Compounds using up to 80% stock can be accommodated, but require experience and special handling on the mills and downstream equipment.

As the polymeric treated particulate is a completely compounded product, it can be molded "as-is." Using 100% polymeric treated particulate from whole tires, a vulcanizate of approximately 60 durometer and 800 psi tensile was achieved. These properties may be acceptable for many applications, but special tooling is required for molding the blend by itself. Because of the low-bulk density of the crumb, molds must be designed to accept the higher bulk (volume) of material required in order to compress it into a finished part.

Loading study

Table 2 shows the effect that various levels of our polymeric particlate, treated and untreated 20-mesh ground tire scrap, have on Mooney viscosity and rheometer values of a good quality SBR compound. (T-609 is a standard treatment, while T-1209 has been modified for improved mold flow.) Generally, the Mooney viscosity will increase with higher loadings of ground rubber, but in all cases compounds of our product do not increase in viscosity as much as those with equal amounts of untreated rubber present. Also note cure times are shortened when using treated rubber, yet without reducing the scorch time.

Even though viscosity is usually increased with higher loadings of our polymeric treated particulate, probably due to the presence of cured rubber, we've found in actual factory processing the compounds tend to handle normally. In factory tests we have successfully injection- and transfer-molded parts using compounds containing 50%.

In most compounds there will be an initial loss of physical properties when low amounts of our product are added, (see figure 1). At about 20% loading, this loss levels out and loading of our product can be increased to as much as 80% without further reducing the base compound's physical properties significantly. Conversely, increasing the amount of untreated ground stock to a compound results in a steady deterioration of its properties proportional to the amount of load introduced.

Mineral fillers

One of the most common ways to reduce compound cost is by adding mineral fillers such as clay, etc. There are certain advantages to be gained by using a mineral filler in a compound, but if cost is the main consideration, our studies would indicate that our polymeric treated particulate is not only competitive on a pound-volume cost basis but offers other advantages as well.

Most low-cost fillers have a high specific gravity, and the specific gravity of the compound increases as more filler is added. This increase in weight must be considered when cost calculations are made. In addition to the extra weight of the compound that must be used to fill the mold, consideration must also be given to the extra freight costs involved, and loss of physical properties.

Typical specific gravity of polymeric treated particulate in tire stock is 1.17. While more expensive than the usual mineral filler, our product overcomes this extra cost by the reduced weight of the finished part involved, i.e., lower pound/volume cost, reduced freight and packaging costs and better physical properties.

To maintain a good state of cure, the compound must be adjusted to accommodate an increased mineral filler load. The addition of accelerators is usually necessary. As this is not necessary with compounds of our product, these often hidden extra costs are avoided.

Table 3 compares loaded compounds with mineral fillers against those with polymeric treated particulate. Note that compounds containing mineral filler have additional plasticizer added to maintain durometer, and also added curatives in an attempt to maintain a reasonably high state of cure. Tensiles steadily decrease as the load increases, unlike the Tirecycle compound which steadies after its initial loss. Even with adjustment to the original cure system, a mineral filled compound did not show as good compression set as did the polymeric treated particulate, indicating a higher state of cure in the compound of our product.

The above compounds were tested for abrasion resistance using the NBS test method. Results from our polymeric treated particulate were far superior to the compounds using high loadings of mineral fillers.

Non-SBR compounds

To help offset increasing costs of specialty polymers, work has been done to assess the effect of our polymeric treated particulate treated elastomers in high performance compounds. An example is presented in table 4. N-1300, a Tirecycle treated 20-mesh ground chloroprene stock, and N-1301, a treated 10-mesh stock, were added to a general purpose chloroprene recipe. Even at the rate of 40%, the cured physicals were not significantly affected. Surface finish on both compounds was excellent. Work done on other non-commodity-type elastomers, such as EPDM, butyl, etc., has yielded similar results.

Evaluating polymeric treated particulate in the laboratory can be a relatively simple process. A standard compound should be banded on the lab mill, and the polymeric treated particulate stock added until just before the batch becomes too baggy to handle. Molding the test slab is carried out in the normal fashion. The viscosity of the compound modified with our polymeric treated particulate will be higher than that of the original, but should not present any difficulties in the mold. Slightly lower tensiles will be noted, but in most cases the part should function as well as one molded with the original compound.

Summary

Tirecycle and Fibrecycle are not the total solution to our industry's tire recycling problem, but it does offer promise that it can contribute in a major way by presenting a workable method of recycling rubber waste in an economical and environmentally acceptable manner.

In addition, the polymeric treated particulate process lends itself to recycling post-industrial cured rubber scraps. Not only are cost advantages involved, but a molder can have his own scrap custom-ground and treated, thus insuring him control not only of its content, but also any chemical resistance properties he may have compounded into the original recipe.

Contrary to what one might expect, compounds with high loadings can produce parts with an acceptable surface finish for most industrial parts.

Another advantage is in the many forms in which the Tirecycle/Fibrecycle products come: a free flowing particulate, a masterbatch, a compound, or preformed for direct introduction to a mold.

All data presented here were generated in the G&E laboratory in Wadsworth, Ohio and are based on tests we believe to be reliable. They are presented as examples only and not as guaranteed results of operations not under our direct control. [Tabular Data 1 to 4 Omitted] [Figure 1 Omitted]

References

[1] "Reclaiming never recovered from radialization," Rubber & Plastics News, May 28, 1990. [2] Swor, R.A. and Newton, H.V., "R-20: New recycling technology for the 1980s," Rubber India, 1981. [3] Stark, Fred J. Jr., "A new method for using recycled particulate rubber in rubber compounding." Paper no. 67, ACS Rubber Division, Philadelphia, 1982. [4] Stark, Fred J. Jr., "A new method for using recycled particulate polychloroprene, nitrile and EPDM in rubber compounding." Paper no. 10, ACS Rubber Division, Chicago, October, 1982. [5] Stark, John, "Tread compounds containing high concentrations of surface treated particulate," Rubber World, April, 1989. [6] Stark, Fred J. Jr., and Wagner, Donald P., "Variations of compounding polymer surface treated particulate ground rubber." Poster session, ACS Rubber Division, Dallas, TX, April, 1988.