Standards for Rubber Granulates and Powders 1

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1 RECYCLING UND UMWELT RECYCLING AND ENVIRONMENT Rubber powder Granulate Crumb Grinding Standardisation Classification Recycling Rubber powders (or granulates) from ground rubber scrap materials are used in many applications in different fields. These applications ask for different properties of the rubber powders. In this paper a number of suggestions to classify powders are given. A reference is being made toward ASTM D5603, that classifies powders with respect to feed stock and sieve analysis. It will be shown that the American situation does not comply as such to the European situation, due to different materials being used on both continents. Standards fuèr gemahlene und granulierte GummiabfaÈlle Gummimehl Granulat Crumb Grinding Standardisierung Klassifizierung Recycling Gemahlene und granulierte GummiabfaÈ lle werden fuè r viele Anwendungen eingesetzt. Verschiedene Eigenschaften sind fuè r diese Anwendungen wichtig. FuÈ r Klassifikationszwecke werden in diesem Artikel einige Empfehlungen gegeben. Es wird Bezug genommen auf ASTM D Dieser Standard klassifiziert Altgummi nach Rohstoff und Siebanalyse. An Hand einiger Beispiele wird deutlich gemacht, daû der amerikanische Standard fuè r Europa nicht zutrifft, da in Europa und den USA unterschiedliche Materialien eingesetzt werden. Standards for Rubber Granulates and Powders 1 H. J. Manuel, Maastricht (The Netherlands) With the manufacturing of rubber products, waste streams from production errors and end of life materials are generated. These wastes are dumped, burned, re-used and recycled. Recycling of tyres for instance (whereby tire components or whole tires are physically recycled), can take place by several processes, as shown in Table 1. Grinding rubbers to a granulate or a rubber powder is being done on an industrial scale, largely from tires, since they represent about 60 to 70 percent of all rubber being consumed. Therefore a wide variety of ground rubber powders is being offered on the market, that differ in properties, due to feed stock, grinding process, sieving process, defibering/deironing steps and logistics. By using a classification system it should become more clear what type of material the customer is dealing with. Applications with more demanding properties and/or consistency can then choose from a material that will live up to these requirements. For applications where properties are at a low level, or where the basic properties that come with the material are not as important, cheaper graded materials can be chosen from the proposed classification list. At this moment, as far as we know, there is only one Standard prescribing how to characterise the rubber powders: ASTM D5603. According to this Standard the ground rubber powders (denoted as ªparticulate rubberª) are to be classified with respect to feed stock and sieve analysis. The rubber powder should then comply to a range of chemical properties and a maximum allowable concentration of fibres and metal. Some examples will be given of the current Vredestein grades and their properties to supply data for a classification system. In a few examples it is shown how different powders can affect the properties of products. Discussion The materials used and the composition of rubber compounds in the USA are different from those in Europe. Due to different road conditions, constructions of cars, trends in tires (e. g. use of run-flat tires) and traffic laws for instance the chemical composition of tire compounds are different. Therefore, ASTM D5603 can not be used on the European continent in its present form. In Table 2 an overview is given of different rubber powders and granulates produced by Vredestein Rubber Resources, based on truck tire tread peelings and whole truck tires. As can be seen from the chemical specification of these materials, the concentrations of acetone ex- 1 Paper presented at the 7th ETRA Conf., Brussels, Belgium, March 29 ±31, Table 1. Different approaches used for the recycling of tires with some typical examples Process Example Retreading Retreaded truck tires and passenger car tires Use of tyres as a whole Artificial rafts; farmer foil weights Use of tyres in parts Tiles and building blocks from cut treads (sometimes with steel belt); wipers from inner liners Grinding Granulate and powder for e. g. tiles (with PU) or for use in fresh rubber compounds Back to feedstock processes Pyrolysis: to monomers or carbon black, steel and oil, tire derived fuel (TDF) Physical reclaiming Reclaim for mixing into fresh rubber compounds, like (under)tread, bead, apex, inner liner etc. KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 3/

2 Table 2. Specifications of commercial rubber powders and granulates from Vredestein Rubber Resources as compared to the values given in ASTM D ²: No PBN, PCB's detected in powder itself; Leaching of metals in water under government limits (building treaty) 102 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 3/2001

3 tractables, carbon black, polymer and ash are different from ASTM D5603. The data show that, in general, acetone extracts are lower and ash and polymer content are higher than in the States, while the amount of natural rubber used in treads and the tire as a whole is considerably higher. The second classification aspect that is mentioned in ASTM D5603 is the sieve analysis. This can be performed on a mechanical sieve shaker, such as the ro-tap sieve shaker. According to the Standard the powder is denoted to be of a certain mesh size if less than 10 % (5 % for 10 and 20 mesh size) is retained on the ªdesignation screenº and nothing on the foregoing sieve, the ªzero percent retained screenº. This is a simple and quick test method and can be used for most powders and granulates that are produced by grinding all or most of the material to a certain size. However, in many operations materials are produced by separating a single stream of ground materials on consecutive sieves, or in a cascade sieve, to create a wider product range. These materials have sieve limits both at the top as well as at the bottom end of the sieve fractions. If these materials were to be denoted with ASTM D5603, the performance of the materials can be quite different from powders which have been sifted on one top sieve. An example where this might lead to problems is where bulky equipment with relatively large clearances in bearings is used to handle the material. A granulate without fines (i. e. sieved on top and bottom sieves) with appropriate dimensions will not clog the bearings, whereas the one-sieve powders might lead to enhanced maintenance (and thus costs). Another area where this is of importance can be found amongst manufacturers of crumb-polyurethane (PU) products. These products are made by mixing the crumb with a PU system and pressing the end product in a mould. Examples of products that can be made this way are shown in Table 3. For these applications the amount of PU used depends on the amount of fines present in the powder. More fines mean more PU most be used to create the products, and thus the costs increase with the amount of fines present. For these Table 3. Examples of products made from rubber granulates and polyurethanes Stable mats and tiles Tiles for playing grounds Noise dampening tiles or mats Speed ramps Road furniture Rail road-crossing blocks Sport track/hall or tennis court underground Playing ground furniture particular applications the specific raw material source of the powders is less important than for other applications, as long as it is a consistent stream. Also, a small concentration of e.g. fibrous material is not likely to cause major problems, since it will be embedded in a PU matrix. A quick method that will give some indication on the amount of PU needed to coat an amount of powder is the determination of the pour density and the compacted density. The compacted density, in combination with the specific gravity of the rubber can be used as an indication of what is needed to fill up a certain volume, i.e. how much powder and PU is needed to make the end product. Apart from the powders differing in origin and sieve analysis, the grinding technology also can have an influence on the performance of the material in end products. In a comparative test several different powders with different mesh sizes, origins and grinding techniques were used to produce surface activated crumb, according to a technique developed by Vredestein [1, 2, 3]. From the figure 1. it can be seen that the properties primarily are determined by the raw material source, while sieve analysis and grinding technique do not play a role in this case. In another test, however, SBR rubber scrap material was ground cryogenically and ambiently. Here it was found that the cryogenic material performed less (had a lower tensile stress and elongation) than the ambient material. In an article by Gibala et al. [4] a single cryogenic and ambiently ground rubber particle was embedded in a tensile test bar. After rupturing the test bar the break surface was investigated. It was found that the crack had passed through the ambiently ground particle, whereas it had followed the boundary between the cryogenic particle and the rubber matrix, thereby exposing the clean surface of the cryogenic particle. This indicates that the ambient powders usually will be the best choice for high performance applications. The surface of powders can be measured analogously to carbon black. In the literature some articles have described the use of these techniques as applied to rubber powders [5, 6]. It appears that the CTAB technique is one of the best suited techniques to use in practice. From a comparison of literature data it can be concluded that the cryogenic powders have lower surface/mass ratios, roughly up to half that of ambient powders (compared at equal mesh size). In a comparison done by Vredestein a 40 mesh ambient powder and a 80 mesh wet ground powder gave values of 14 and 17 m 2 /g respectively. The ambient powder value compared close to a literature value on another ambiently ground 40 mesh powder: 12 m 2 /g. By multiplying these values with the median particle size a comparison between different techniques can be made (since equally processed material with half the Figure 1. Tensile strength and elongation at break of ground rubber powders treated with a surface activating process, after which they were vulcanised in a press (t 90 ˆ 150 8C). The ground rubber powders, differing in mesh size were produced by cryogenic, wet and ambient grinding from different feed stock materials. KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 3/

4 particle diameter must have twice the surface per gram). By this method a 80 mesh cryogenic, 80 mesh wet and 40 mesh ambient powder gave values of 2, 3 and 6 respectively, reflecting the difference in the process. Another technique to discern between different processes and surface/mass ratios is microscopy. Cryogenic powders will have a more edgy, flat surface as compared to the rough, undefined surface of ambient powders. In some laboratory trials indeed we saw differences between rough or brittle surfaces. A further quantification is difficult to achieve, although some literature does describe light-microscopy-imaging techniques aided by computers [7]. If a qualifying scheme could be set up, it might lead to a quick method to classify materials. Another aspect of powders that will influence its application range is concerned with the degree of contamination by metals, fibres and others. Applications with higher demands on the properties will generally ask for materials with very low levels of contamination, whereas other applications are not so meticulous. An example where it is not so important that the rubber powder is completely free of textile is the road building industry. Here rubber powders are used to incorporate elastomeric properties in rubberised asphalt. The material is completely immersed in a tacky compound and a small (though consistent) amount of textile and metal can be allowed for, thus leading to a better price/performance ratio. However, the handling conditions of the material will determine the requirements for the allowable concentrations of (especially) fibres. If big bags of materials are to be handled in free air, risk will have to be avoided that fibrous material pollutes air or ends up in the lungs of employees. It is therefore open to discussion if a Standard should incorporate materials with lower contamination levels but which are safe to handle. Another example where contaminations are less important is the cement industry. Here the rubber is used for its calorific value and the iron even is needed in the process. A defined sieve analysis hardly is needed for this material, while fibrous material will simply be burned. However, the same rules for a safe handling of the powder will still apply to this application (although the problems are probably not big, since the cement industry can handle roughly ground granulate that will have fibres embedded in larger rubber chunks, that will not float around as in finer mesh materials). Last but not least, there is the matter of the possible leaching of detrimental chemicals from the powders to the environment, for certain applications. Examples could be the use of ground rubber mixed with soil to create a certain structure, the use of tyres cut in half under golf fields to serve as water carriers and the use of crumb in dry processed road constructions (where crumb replaces part of the aggregate). In these applications the crumb has contact with water, that might leach out some of the hazardous ingredients in the powders. As was shown in the table, the Vredestein powders are free from nitrosamines, PCB's, PBN and did not leach heavy metals above government limits in a test conducted for possible use in aforementioned applications. This is caused by the fact that the raw materials used by Vredestein primarily are gathered in Western Europe. For other parts of the world this will, most likely, not be true yet. Obviously this will have an impact on usability of powdered material in other parts of the world. The ASTM Standard does not give any limits for concentrations of hazardous chemicals. However, due to trends in rubber compounding, such as reformulating to nitrosamine free recipes, will most probably be bound to different regions, it is probably best to specify the region from which the tyres originate to have an indication on its usability. For special cases producer and consumer can probably best agree on further analyses to be done. Suggestions for standardisation In the discussion above some rubber powder properties have been linked with their impact on the processing of the powders and the end properties of several products. Based upon this review we would like to make some suggestions for a standardisation of rubber powders: * ASTM D5603 can serve as a starting point for a European Standard. We feel it should be modified and expanded with the following: * We propose D5603 to be modified with respect to the classification by parent compound. Rather than using a grade 1 to 6 classification we feel it would be clearer to make a distinction in truck versus passenger car tire versus other rubber sources as indicated in Table 4. * Due to the fact that tires from different areas will have different compositions we feel that the region from which the tires originate should be mentioned when materials are specified as well. As discussed, this will also give some indication on the concentrations of hazardous chemicals present in the powder, that might leach to the environment. * We propose D5603 to be modified with respect to the chemical composition for proposal grade 1, 2 and 3 as indicated in Table 5. For grades 4, 5 and 6, data is lacking at the moment, that should be gathered to fill the gaps. * D5603 should further be expanded with sieve limits for top and bottom sieve for powders produced in a cascading sieving procedure. For top and bottom sieves the same sort of classification scheme can be used as mentioned in D5603. * The mesh size of the powders will give an indication of the surface/mass ratio, i. e. that finer powders ground with the same technology will have higher ratios. As discussed this ratio differs between techniques. Since this is of importance for applications where the powder should be firmly embedded Table 4. Proposed grades for a classification scheme compared to the existing ASTM D5603 norm Proposal grade ASTM D5603 grade Grade 1: Truck tire tread (Part of) grade 2 Grade 2: Truck tire buffing (Part of) grade 3 Grade 3: Whole truck tire (Part of) grade 1 Grade 4: Passenger car tire tread (Part of) grade 2 Grade 5: Passenger car tire buffing (Part of) grade 3 Grade 6: Whole passenger car tire (Part of) grade 1 Grade 7: Other tire (e. g. full rubber tire) Grade 5 Grade 8: Technical rubber goods Grade KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 3/2001

5 Table 5. Proposed limits for chemical properties of ground rubber powders and granulates from truck tyres and treads (proposed grades 1, 2 and 3) Chemical property Proposed limits Acetone extract 4±13 % Ash content 3±20 % Carbon black 25±34 % Polymer content 45±60 % NR % of total RHC 45±80 % (or NR % of total compound) (20 ±50 %) Table 6. Proposal for a classification of ground rubber powder and granulates by different grinding techniques Proposal for classification of grinding techniques Cryogenic grinding Ambient grinding(< 130 8C) High temperature grinding (> 130 8C) Wet grinding Buffing (sieving, without further grinding) Other techniques in a (rubber) matrix we propose that the grinding technology is mentioned in the Standard. A proposal for a distinction in categories is given in Table 6. * As discussed, it is possible to measure the surface of the powder directly with a CTAB technique. Since it is not a simple and cheap technique, it would have to be discussed whether or not this should be part of a Standard. * Concerning allowable concentrations of contamination present in the powders, we feel that D5603 could have more stringent limits. It is logical that the tread grades should not be allowed to have any fibrous content. The limits mentioned for the other grades, namely 0.5 % seems to be rather high. Surely, with modern processing equipment this limit can be set to 0.1 % or even lower. Several market investigations have shown this to be possible. As mentioned in the discussion above some applications are not that meticulous about contamination, as long as it can be handled safely. It is probably best if a broader industrial debate is set up, to discuss the need for a proposal incorporating grades with broader limits. Materials with higher concentrations of contaminating agents, that can be used for specific applications can then be included as well. Conclusion Standardisation is necessary for a better understanding between suppliers of rubber powders and producers of rubber end products that use these powders [8]. In this presentation data on commercially available rubber powders from Vredestein Rubber Resources were discussed in comparison to the ASTM Standard. With this suggestions for a Standard were given that will help set up a European Standard. At present a CEN workshop has already been installed to work on this subject. References [1] European Patent (1996), R.A.J. Smits. [2] W. Dierkes, GAK 48 (1995) 398. [3] W. Dierkes, ACS ± Rubber Division Meeting, Philadelphia, Pennsylvania, USA, May 2±5, [4] D. Gibala, G.R. Hamed and J. Zhao, Rubber Chem. & Technol. 71 (1998) 861. [5] R.P. Burford and M. Pittolo, Rubber Chem & Technol. 55 (1982) [6] G.W. Holland, B. Hu and S. Holland, Rubber World, May 1994, 29. [7] A. Spittel and R.H. Schuster, Kautsch. Gummi, Kunstst. 47 (1994) 896. [8] J.L.B. de Groot, C.N. Smit, E. Mot and B.L. van der Ven, Kunststof en Rubber , 51. The author Henk Jan Manuel is Manager R&D at Vredestein Rubber Resources. Corresponding author Henk Jan Manuel, Vredestein Rubber Resources, P.O. Box 4 37, 6200 AK Maastricht, The Netherlands. KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 3/