Technology and Equipment

Greengum – a Decisive Breakthrough in the Recovery of Waste Rubber

( d –r Lev Beirakh)
Two trends dominate the market of rubber materials in recent years:
• A sharp rise in prices for natural and synthetic rubbers. The price of natural rubber rose from $ 800 per ton at the turn of the century up to $ 6,000 per ton at present.
• Oversupply of market by crushed coarse crumb as a result of the adoption of environmental legislation prohibiting dumping of waste tires in landfills in the World.

At the present time most of the tires are supplied for processing into crumb. Thousands of plants for recycling of used tires and other rubber waste aroused in the World. In general, these plants are engaged in collecting, sorting, primary breakdown and fragmentation of tires to crumb size.
A small portion of the crushed crumbs fed to incineration, pyrolisis, and similar applications. As for the burning of oil, Mendeleyev said prophetically in the late 19th century, when oil was considered only as a fuel: "Oil is not fuel. You can burn and banknotes".
The main purpose of recycling has been and continues to be the processing the crushed chips into high-quality rubber material, suitable for repeated use in the rubber industry.
The two main areas for further processing of crushed rubber crumb into secondary raw materials in the recovery industry are dominated by rubber waste:
1. Fine grinding crumb rubber and the production of ever more finely ground crumb.


2. Devulcanization of crushed rubber crumb to produce a plastic material, which is well compatible with the components of rubber compounds.
Consider the first direction.
Using a very fine fraction of rubber crumbs\powder leads not only to a sharp increase in the cost of material, but also to a deterioration of the physical properties of rubber compounds containing the powder.
The rationale for such an assertion, and a comprehensive analysis of factors affecting the properties of the fine powder and rubber compounds containing the powder, are given in an analytical review of Greengum.
Below are summarized the main factors from this analytical review, leading to a decrease in quality when using ultra - fine rubber crumb:
1. Low compatibility with the components of the particles of vulcanized raw rubber mixtures. No further refinement will make the surface of these particles more compatible. At any grade of dispersity crushed rubber powder remains to be an inert filler.
2. The increase in the strength of the particles during grinding.
3. Agglomeration of finely ground crumbs.
4. Contamination of crumb by products of the interaction between the working parts of tools during grinding.
5. Stratification of the crumbs on fractions along the height of receptacle.
Due to combined effect of these factors, ultra-fine grinding of rubber particles leads only to a sharp increase in the cost of material and does not solve the problem. Rubber crumb, whatever way and to what size it was ground, does not have the necessary adhesion and compatibility with rubber. So, for example, the American standard limits the allowable proportion of finely ground crumb mixture in the tire to no more than 2.3%.
We turn now to the second direction - devulcanization of crushed rubber crumb.
The oldest and still most widely-spread method of devulcanization in the World (especially in Asia and Eastern Europe) is so-called "autoclave" or "pan" technology. Rubber regenerates obtained by the autoclave technology continue to dominate the world market of recycled materials. They are flexible, well-mixed with other ingredients of rubber compounds.
However, the technology of autoclave devulcanization causes profound destruction of rubber molecules. Exposure to chemical agents at high temperature and pressure on the vulcanized rubber particles leads, along with rupture of sulfur bonds, to a deep cleavage carbon bonds in the main chain, because the energy of the C - S (66 kcal / mol) is too close to the energy of C - C bond (80 kcal / mol).
The splitting of the main chains during this process along with the devulcanization leads to sharp falling of the strength of the recycled rubber. Suffice it to say that if the tensile strength of the original vulcanizate is 200-250 kg/cm2, the strength of the regenerate after autoclaving is mainly in the range 25 - 45 kg/cm2. Similar considerations apply to other technologies devulcanization carried out at high temperatures: processes Krieg and Alsdorf, different variants of devulcanization in the extruder (dispergators, twin-screw extruders, the Chicago process SSSE), etc.
Thus, the two discussed areas for further processing of the crushed crumb: ultra-fine grinding and devulcanization at high temperatures do not solve the problem of obtaining high-quality rubber compounds with a high content of secondary rubber materials.
This situation has caused an urgent need to establish a modern technology for the processing of crushed chips in high-quality secondary materials while preserving the original properties of the vulcanizates.
A number of discoveries of rubber chemists in recent decades have contributed to solving this problem.
• In the 80s of the 20th century Krebs found that the reaction of splitting bonds S - C and S - S, which usually occurs only at an elevated temperature, in the presence of amines is sharply accelerated and occurs even at room temperature. Scheele later extended this effect to the sulfide derivatives. Their publications have given impetus to the use of amine and sulfide derivatives in the process of devulcanization. It has made to be possible to carry out the devulcanization process at moderate temperatures, which contributed to the protection of main chains of elastomers from excessive degradation.


• A significant step towards the industrial realization of Krebs and Scheele discoveries made a group of Japanese chemists (Okamato with co-workers), who were first to develop grinding and simultaneous devulcanization process at industrial roll mill. They put forward the concept that the amine and sulfide agents interact with the short-living free radicals, which are formed at the time of grinding.


• This concept was evolved by two chemists De and Adhikari from the Indian Rubber Institute of Kharagpur. They suggested the mechanism of chemical reaction by means of free radicals at moderate temperatures.


• In the late 20th century, several companies, primarily Dutch company Vredestein, patented technologies and products based on the concept of surface devulcanization. The Dutch company offered the market a rubber raw material Surcrum first, and then B01 and similar surface-devulcanized materials.
• In those same years, B. Sekhar (Malaysia) and VA Kormer with co-workers (Russia) have published a patent, and entered the market with technology De-Link, which is another realization of the concept of devulcanization at moderate temperatures with amino-sulfide modifier on rollers. The technology has commercial applications, and was purchased by a number of consumers.


• Earned recognition in the developing world has Israeli company LEVGUM. Levgum technology developed for the devulcanization on a roll mill, uses a two-component modifier. The basic fundamental differences modifier Levgum of the then existing technologies are: a) the modifier consists of particles of solid powders to facilitate grinding crumb rubber; b) the fraction of the modifier was reduced to 2.5-3%, compared to 5-6% in De-Link, which cheapens the process.
Technology Levgum showed good results in the application of the consumers. At the same time Levgum technology, as well as the Technology De-Link, require repeated passing of the material between the rollers, which complicates the process and requires high production costs.
Israeli company GREENGUM represents further development of technologies and modifiers for fine grinding with simultaneous surface devulcanization at moderate temperatures using the above concepts described above.
Based on the opening of the Krebs - Scheele and mechanism of the De - Adhikari, they have developed a number of recipes of modifiers operating at moderate temperatures.
By using the concept of surface devulcanization, chemists of GREENGUM proceeded from the assumption that the presence of surface-devulcanized layer of small thickness is sufficient to ensure compatibility. Remaining intact core of particles ensures high retention of physical properties of the vulcanized rubber, while the surface layer of devulcanized particles ensures compatibility. Surface devulcanization increases adhesion between the particles of crushed rubber crumbs and crushed between the compounds and components. Increased adhesion allows using a higher percentage of reclaim without sacrificing mechanical properties.

To ensure the quality, the degree of devulcanization should be neither too small, because it will lead to a deterioration of compatibility with the components of the compound, nor too large, as this deep degradation of the main circuit includes a significant amount of particles, which reduces the physical properties of rubber. Consequently, the degree of devulcanization must be regulated by a system of influencing factors.
In addition to the above concepts, the GREENGUM developers also attracted the concept of Moscow Professor G. Bloch (Russia), which he developed in the 70th years of the 20th century. According to this concept, the reaction of curing and devulcanization are mutually reversible. Speed of direct and reverse reactions depends on the acidity of the medium - the higher the acidity, the lower the rate of vulcanization and faster the rate of devulcanization. Bloch has shown that vulcanization of a typical tire compound is completely stopped at pH less than 3.2. This means that in order to prevent re-vulcanization, modifier must contain a fixed contents of organic acid, which supports the acidity of the medium at the proper level.
These concepts and considerations were used in the development of technology and the Israeli firm Greengum modifiers.
The development of GREENGUM included the following:


1. Investigation of physical and mathematical relationships and properties reclaim containing compounds from its defining parameters (temperature, clearance, friction, velocity, or rollers on the machine rotor LIM, acidity, etc.) and the construction of mathematical models for these dependencies.
2. Development of methods for fine regulation of the degree of devulcanization using monitoring and dose changes influencing factors on the basis of mathematical models.
3. Creating a 23 modifier formulations for different applications, which include some brand new components that make by modifiers were significantly more effective than previously used.

4. Design of specialized 2-roll mill suited to Greengum process by speeds, friction, nip adjustment, temperature maintenance, feedback conveyor and other features.
5. The creation of fundamentally new equipment for fine grinding process with simultaneous surface devulcanization surface at moderate temperatures in a single pass - machine LIM.
6. Development of formulations and process conditions based on the preparation of compounds based on GREENGUM-reclaim for 50 different applications.
7. Equipping both processes by monitors and controllers to regulate the degree of devulcanization with feedback.
The results of development are reflected in the slides, which are an integral part of the presentation material.
The materials presented on the slides, clearly show that:

v  Technology, Equipment and GREENGUM modifiers are best suited to the needs of modern rubber industry.

v  Compounds based GREENGUM-reclaim exhibit the high retention of the original properties even at high content of reclaim in compound. Thus, retention of such important parameter as Tensile Strength at addition of 60 weight parts of Greengum reclaim per 100 weight parts of natural rubber comprises 80% from the Strength of virgin tire compound. It is not reachable by any industrial technology.

v  Cost efficiency of products and compounds based on GREENGUM-reclaim is far superior as compared with similar products produced by other technologies.
Recycling of rubber waste into high-quality secondary materials is the future of the global rubber industry. This technological breakthrough will dramatically reduce the economic costs and effectively protect the environment.

Enclosed: 5 slides-shows including 30 slides.