Recycling of vulcanized factory waste.

Most vulcanizing processes produce significant amounts of waste and rejects. On average, this is 5-15% of rubber consumed and is comparable in value to the total profits of the company. A process which can economically reduce this waste to zero must therefore be of great interest to every wide-awake management. This article outlines such a process.

The simple fact is that the scrap must be rendered soluble in the compounded stock in order to be recyclable. Only then can it be dispersed to the molecular level and thereby give a stock with elastic and viscous properties comparable to the stock alone.

Within a few years of his discovery of vulcanization in 1839, Goodyear had taken out his first patent on reclaim. The succeeding years have produced an extensive history of chain- and crosslink-oxidizing acid and alkali processes to produce it. However, when incorporated back into fresh recipes, the properties of the subsequent product have been drastically reduced. Reclaim processes have virtually disappeared in industrialized countries. Resort today has been made to crumbing the vulcanizate and this only applies to small markets for tire crumb relative to the production of scrap tires.

All this was explicable over 40 years ago from the then knowledge of a vulcanizate[ref. 1]. Physically, it is a mass of intertwining rubber chains of a molecular weight of around 200,000 which are joined together randomly on average about 20 times into one gigantic molecule. To get down to the Flory gel point for a random average of 20 crosslinks per chain, the chain segments have to be split 19 times. This gives the length of the degraded chain segments as 10,000. At this stage, the material is a gum, with all its elasticity and viscosity gone and it acting as just a plasticizer.

The same argument applies if the chemical process is rupturing of the chain segment at a crosslink.

The exception is if the crosslink itself is split and the chain segment is not. Then, each split doubles the length of the chain segment. The common disulfic or polysulfidic crosslink can conceivably be ruptured. This might be done, for example, with a small-molecule disulfide, mercaptan or alkyl iodide[ref. 2]. However, the process would leave a scorch-reactive chemical in the mix and is in any case impracticable in a rubber factory. In summary, all the efforts in the past were doomed to failure, as are likely all future chemical degradation processes -- even without considering their cost.

The one process we foresaw as promising is a mechanochemical process. The most important such process discovered is the cold-mastication of a rubber to rupture its polymer chains in central regions to free radicals and these to be terminated by adventitious oxygen or other radical-acceptor[ref. 3] present. The process relies on getting sufficient elongational shear at these central areas of the chain. It is independent of the chemistry of the rubber, from polybutadiene to polybutylene. No chemicals are added.

There are now literally hundreds of materials which have been subjected to mechanochemical reaction[ref. 4]. The vital trick has been how to apply the mechanical energy to activate the reaction. Small molecules have been reacted by high shear rate, as in lubrication and wear of shafts in bearings or pistons in cylinders. For polymers, ball mills and drills have been used.

What is certain is that a conventional internal mixer is not a suitable machine. Otherwise, its solubilizing action would have been empirically discovered years ago. This is predictable as the spaces between the moving parts result in any crosslinked particles merely being moved within a rubber matrix.

The mechanism proposed is summarized in the following equation, where a crosslink is ruptured to give two free radicals and the average length of the chain segment between two crosslinks is doubled. If this degradative reaction proceeds beyond the Flory gel point, soluble and high-molecular-weight rubber chains will be produced. In air, the radicals will be terminated. Under nitrogen or other inert atmosphere, some or all of the radicals will recombine or attach to polymer, thus giving a material of higher molecular weight.

[ILLUSTRATION OMITTED]

A suitable high shear mixer (HSM)

We designed an HSM by intuition and empirically tested it to see whether it could solubilize pieces of rubber vulcanizate in conjunction with added material to form a rubbery matrix. The thinking was that gaps between the moving parts needed to be of the order of a fraction of a mm. A controlled proportion of the energy should be imparted as elongational shear. The free radicals produced needed to be terminated other than within the crosslinked network and this with the other requirements above suggested addition of a thermoplastic material, preferably the raw rubber or stock from which the vulcanizate had been made, to direct the shear. Cooling had to be an order of magnitude better than that of a conventional internal mixer, requiring a much higher metal-surface/rubber-charge ratio.

It is not wholly clear to us why our HSM is so effective. However, the fact is that we have not failed to solubilize any rubber vulcanizate we have tried. This has included scrap from all the general-purpose and tire rubbers and the common specialty rubbers (table 1). The time taken has generally been less than 10 minutes, with the mix keeping almost to ambient temperatures. The speed of rotation of our laboratory and pilot-scale machines has been less than 50 rpm and 15 rpm, respectively. Energy consumption is less than for an internal mixer.

Table 1 - vulcanizates solubilized

SBR with reinforcing black and sulfur cures
NR with reinforcing black and sulfur cures
Industrial tire crumb (NR)
Automobile tire crumb (SBR)
Gum NR with C-C bonds
Polychloroprene radiator hose
Polacrylonitrile hose
EPDM moldings

A special feature of our process, as predicted, is that flushing air from the cavity of the machine and blanketing it with nitrogen generally gives a stiffer and less tacky soluble mass. It is likely to confer higher elastic properties to the final product than an air-present process, but that among other expectations has yet to be investigated.

The shearing principle of the mixer is portrayed in figure 1. This is a straightened-out section through the upper static and lower rotating member. On a quarter rotation, the different conformation of the cavity is shown. The material has thereby to be elongationally stretched in the narrow gaps between the lands and turbulently mixed in the grooves. The lands are claw-shaped, figure 2, thus moving the material at the surfaces to the center, where it has to well up and force its way out with rolling-bank mixing.

[FIGURES 1 & 2 ILLUSTRATION OMITTED]

Recycled-rubber into fresh vulcanizates

Only two sets of data have been evaluated so far. They at least establish that the level of fresh vulcanizates is not drastically lowered. These fresh vulcanizates were made under the unsatisfactory conditions of having to handle charges of only c 100g and no optimizing was practicable.

The vulcanizates and added 25% compounded stock were of industrial-tire and automobile-stock, respectively, and the results are in table 2.

Table 2 - properties of fresh cured stock with 25% added recyclable material

                     Industrial-tire    Auto-tire
                     formulation        formulation
                       0     25%         0     25%
ts MPa                21       16        16     11
eb %                 470       470      470    370
Mod. 100 Mpa         2.0       1.4      1.6    1.5

A continuous high shear mixer

The principle of figure 1 can be applied to a continuous mixer of the extruder conformation. The upper profile can be scrolled on the inside of the barrel and the opposing lower profile on the screw.

This has not been constructed yet, but is worth mentioning as it will provide us with a continuous HSM of again chosen amounts of high and turbulent shear by design of lands and grooves.

We can then foresee its use. One clear-cut use is to recycle large amounts of vulcanized rubber, notably used-tire rubber. The tread and apex rubber should be scalped off. It should be fed into a large extruder of the form of the conceptual drawing of figure 4. There appears no limit to the throughput of the mixer. It will perforce be of the order of 100,000 tons per year to make a serious bite into the tire mountains.

[FIGURES 3 & 4 ILLUSTRATION OMITTED]

References

[1.] L. Bateman, ed., The Chemistry and Physics of Rubber-Like Substances, Maclaren Press, 1963.

[2.] W.C Warner, Rubber Chemistry and Technology, 1994, 67, 559.

[3.] M. Pike and W. F. Watson, J. Polym Sci, 1952, 9, 229.

[4.] A. Casale and R.S. Porter, Polymer Stress Reactions, Acad. Press, 1978.