Solutions to the rubber waste problem incorporating the use of recycled rubber.

The utilization of recycled rubber, especially in the form of reclaim, is a well-established practice in the rubber industry. At the beginning of the 20th century half of the rubber consumed was in the form of reclaim. This amounted to 20,000 metric tons of reclaim consumed by the United States (ref. 1). By the end of the 1950s, only about one fifth of the rubber hydrocarbon used by the United States and by Europe was reclaim. The drastic fall in reclaim consumption (ref. 2) was due to the reduction in cost and the increase in production capacity for natural and synthetic rubber; as well as to the more demanding technological developments of the following decade such as the introduction of steel belted tires and radial tires. By the middle of the 1980s less than 1% of worl-wide polymer consumption was in the form of reclaim. By that time environmental regulations (such as the U.S. Clean Air and Water Act of 1991, the German Bundesimmissionsschutzgesetz) were the primary factors influencing the near elimination of the reclaiming industry. During this era, most reclaim facilities chose to cease operation due to the high investments necessary to modernize their plants. Many of the remaining reclaimers now see opportunity in the waste management regulations enacted in some countries (in Europe, Germany and Holland having the most strict regulations).

The current rubber waste situation is illustrated in table 1. Available statistics indicate that roughly half of all rubber waste is re-used in the form of recycled materials or energy production. However, of this amount only a small percentage is recycled in the original sense of the word, that is the reutilization of the rubber waste in the form of crumb or reclaim. A large amount of crumb is used in road construction. In 1990, 25% (20,000 tons) of the crumb produced in the U.S. was used in road construction. In Europe, France has the leading role in this field. There, 250,000 tons of modified asphalt containing 37,500 tons of crumb was used in 1990 (ref. 3). The proportion of reclaim to virgin polymers used by the rubber industry, while still only about 1% is moving in an upward trend.

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The purpose of this article is to demonstrate that state-of-the-art recycled rubber products are valuable, high quality raw materials. However, their application certainly depends on their economic competitiveness with virgin raw materials. Cost aside, the utilization of reclaim and activated crumb provides a variety of processing advantages. Another benefit of the use of recycled rubber products coming to the fore the past few years is the positive effect on overall energy consumption. That is, much less energy is consumed through the production and utilization of these products than through the manufacture of virgin raw materials (table 2).

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Methodology

Base materials

The base materials utilized were the following industrial or post-consumer wastes:

* NR reclaim/activated crumb: Truck tire tread peelings, a by-product of the retreading process;

* SBR activated crumb: Passenger tire tread peelings;

* NBR activated crumb: Gaskets from automotive applications;

* EPDM reclaim/activated crumb: Profiles from automotive applications.

The base materials were ambiently ground on production scale to a maximum particle size of 40 mesh (0.425 mm) with an average particle size (D50-value) of 45 mesh (0.360 mm) before being reclaimed or surface-activated. Test methods are given in table 3.

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Results and discussion

Table 4 shows the different options afforded by the Vredestein reclaim process. The two principal methods of recycling elastomeric materials are:

* Grinding the rubber waste and re-using it in the form of granulate or surface-activated powder; or

* Treating the material through a thermo-mechanical process in order to generate a visco-elastic reclaim. This reclaim can then be used as a partial or (with the addition of a curing system) a total substitute for a virgin compound.

Table 4 - possible recycling products

               Reclaim     Surface      Crumb
                          activated
                          crumbs
IIR            x           --             T
NR             x            x             x
SBR            --           x             x
NR/SBR/(BR)    x            x             x
NBR            T            R             x
EPDM-S(1)      x            R             x
EPDM-S(2)      --           T             x
1. Sulfur-crosslinked  2. Peroxide-crosslinked;
T: the existing technology can be applied;
R: research project

Natural rubber reclaim

The reclaiming process consists of two primary phases: grinding followed by plasticizing. During the grinding phase, the feedstock (fiber-free truck tire tread peelings in the case of natural rubber reclaim is ground and separated from contaminants which include textile, metal, glass and stones. The resulting powder, having a maximum particle diameter of 40 mesh (0.425 mm) and an average diameter of 45 mesh (0.360 mm), is then thermally and mechanically broken down until a Mooney-viscosity ranging from 35 to 70 is achieved.

Custom reclaim - three principal production factors influence the physical properties of reclaim: the chemical composition (table 5), the duration of breakdown and the ratio of thermal to mechanical breakdown. Varying the duration and ratio of the different breakdown steps allows the production of custom reclaims differing in viscosity, tensile strength and other related properties (figure 1).

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An explanation of this effect is found in the selectiveness of the mechanical breakdown step: it is primarily the carbon-carbon backbones of the network which are broken down, and preferably the longer chains will be broken. This leads to a more narrow molecular mass distribution. The therm-chemical breakdown step is random; as a result, the percentage of low molecular weight-polymer, acting as a peptizer and having no reinforcing effect on the network, increases, and the tensile strength of the cured reclaim decreases (ref. 4). These differences in reclaim quality influence the properties of a compound containing the different reclaims (see figure 3).

Modern recycling technology offers the possibility to produce custom reclaims. The advantage to the customer is that specific requirements for quality, price and chemical composition can be met. There is even the possibility for manufacturers to re-use their, own production-waste by introducing a closed-loop for this material (ref. 5) (see loop 2 in figure 3). This solution to the production waste problem is becoming more and more attractive as raw material prices and landfilling and incineration fees rise. To calculate the break-even point, the costs of transportation and the production of the reclaim have to be compared with the costs of raw materials and the fee for landfilling or incinerating.

Advantages and limitations of reclaim utilization - The well-known rule that natural rubber reclaim improves processing behavior is still valid. The main advantages derived from the use of reclaim concern the processing behavior of the compound. These include: lower power consumption and other processing costs resulting from shorter mixing cycles; low calendering, mixing and extrusion temperature resulting in fast and uniform calendering and extrusion; improved penetration of fabric and cord; increased tack with minimal effect on temperature variation; and low swelling and shrinking during extrusion and calendering.

Other important advantages include: lower raw material costs; improved stability during curing in hot air or open steam; better air venting properties; and improved reversion and aging performance of natural rubber compounds (ozone, UV)

Reclaim can be applied to the entire range of rubber products including tires, technical rubber goods (not only for the automotive industry), conveyor belts, shoe soles and industrial coatings. Compound content of reclaim can vary from a few percent to 100%; the average content 5-15%. The preferred application for products consisting of 100% reclaim are conveyor belts, mats and sheets.

The influence of natural rubber reclaim RNR/B91 on the properties of a truck tire tread compound (for the recipe, see table 6) is illustrated in figures 4 through 7. For these examples, the reclaim has been added on top of the existing compound with adjustment to the curing system. Within the typical range of concentration (up to 20%) of reclaim, no difference in properties of the compound due to the compounding method (added on top in comparison to adjustment to rubber hydrocarbon- and carbon black-content) is observed.

Table 6 - test compund
Truck tire tread                          phr
Nr                                       100.00
Carbon black                              50.00
Aromatic oil                               5.00
Zinc oxide                                 5.00
Stearic acid                               2.00
IPPD                                       1.00
TMQ                                        2.00
Par. wax                                   2.50
TBBS                                       1.50
Sulfur                                     1.50
Reclaim                                      *
                                         171.00

The addition of reclaim to the compound results in a slight influence on tensile strength, as illustrated in figure 4. The influence of the addition of 1% of B91 reclaim on other properties like tear strength (figure 5), rebound resilience and compression set (figure 6) is in the same order of magnitude (approximately 0.7% for low concentrations in this compound). For higher loadings the effect on these properties is even less. There is no influence on modulus (figure 4), abrasion resistance (figure 5), hardness (62, Shore A) and density.

The aging resistance of this truck tire tread compound is only slightly influenced by the addition of reclaim. (Aging conditions: 212[degrees]F (100[degrees]C), 4 days). See figure 7 for the influence of aging on tensile strength, (For the recipe, see table 6.)

EPDM reclaim - sulfur-cured EPDM reclaim can be produced by the above-described process combining a thermochemical and mechanical breakdown step. Investigations with EPDM profile waste from different sources have shown that the quality of the reclaim significantly depends on the feedstock used. This implies that EPDM reclaim is preferably produced in a closed loop.

The Vredestein reclaim process allows the production of reclaim with a tensile strength of 50% of that of the feedstock. Testing of this reclaim in different compounds has shown that the utilization of it in seals, cooling hoses and automotive profiles is possible.

Butyl reclaim - the feedstock for butyl reclaim is inner tubes, and most of it is used in inner tubes and inner liner production. Another application is in cable filling compounds.

Figure 8 shows the influence to be expected when reclaim is added to an inner liner compound based on chlorobutyl rubber and bromobutyl rubber. The effect of doing so results in a remarkable increase in flex fatigue stability after aging.

Surface-activated crumb rubber

Surcrum is the trade name of a surface-activated crumb rubber developed by Vredestein. It is designed to be used as an active filler with the characteristics of a vulcanized rubber.

The surface-activation is based on two processes: the grinding step and the activation step. During the grinding step a fine powder with an average particle diameter of 45 mesh and having a high specific surface area (1 - 2 [m.sup.2]/g) is produced (ref. 6). The surface of this crumb is very reactive.

During the second step a crosslinkable surface layer consisting of a polymer and a curing system is added to the crumb. Unlike untreated crumb this activated powder is able to crosslink with the surrounding matrix. The effect of this coating is a doubling in tensile strength: for natural rubber based Surcrum curing of the untreated powder results in a vulcanizate with a tensile strength of 3-4 MPa. The tensile strength of the cured Surcrum is at least 8 MPa (table 7).

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Different types of Surcrum have been developed (table 4). The first one, introduced four years ago, was the NR-based Surcrum. Meanwhile a coating for SBR-, NBR- and EPDM-based crumb has been developed and the first production runs have been performed. The coating of all these types of Surcrum is specific to the base polymer.

Surcrum is suitable for use as a compound substitute or extender as well as for curing as a stand-alone.

Products consisting of 100% Surcrum - These rubber goods can be produced either continuously (on a roto-cure or double-belt press) or in batches by compression molding. Both methods result in products which meet the specification for Surcrum (see table 8 for NR based Surcrum). The curing conditions for Surcrum are 12 minutes at a temperature of 302[degrees]F for a product thickness of 2 mm. The pressure necessary to achieve a sufficient compression is 10-12 bar. This is in the upper working range limit for a roto-cure press and a common pressure when working with a double-belt press

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Examples of product applications containing 100% Surcrum include conveyor belts, mats and damping sheets. It is also possible to produce profiled or multiple-ply articles. An additional advantage of this processing method is a reduction in energy and equipment costs because no mixing or calendering steps are necessary.

Surcrum as a compound substitute

No significant difference in the influence on compound properties is found between Surcrum and reclaim, except that Surcrum results in an increase in Mooney viscosity. Figure 9 illustrates the influence of 20% natural rubber based Surcrum (feedstock: truck tire tread peelings) on the properties of a truck tread compound (for the recipe see table 9). Figure 10 shows the influence of Surcrum on a passenger tire tread compound (for the recipe see table 10). Compounding with Surcrum requires no adjustment to other compound ingredients, as Surcrum contains its own curing system.

Table 9 - test compound
                       Truck tire tread     Passenger car
                                 phr        tire tread phr
NR                          100.00
SBR 1500                                     100.00
Carbon black                 55.00            55.00
Zinc oxide                    5.00             5.00
Stearic acid                  1.00             1.00
GPPD                          1.50             1.50
TMQ                           1.50             1.50
Prot. wax                     2.00             2.00
CBS                           1.00             1.00
Sulfur                        2.50             2.50
PVI                            0.5              0.5
Surcrum                          *                *
                            175.00           175.00

As can be seen in figures 9 and 10, a significant difference in the properties of the compound containing untreated and treated crumb is found. Adding 20% of crumb results in an approximate loss in tensile strength of 13% in this SBR compound and 22% in this NR compound; the utilization of surface-treated powder results in an influence of only 8% and 15% respectively. The positive effect of the surface-activation increases when the concentration of Surcrum increases. Some properties such as the abrasion resistance of the NR- and SBR-based compound, as well as the tear strength of the SBR compound, are not negatively influenced at all.

Surcrum can be applied as a compound ingredient in a variety of rubber products including tires, conveyor beltings and technical rubber goods. The advantages of using Surcrum include compound cost reduction and improvement in processing behavior (such as lower die-swell and better air-venting during vulcanization). Substituting Surcrum for untreated crumb in compounds results in improved product properties when using the same loading, and no change in properties in the case of higher loadings.

Conclusion and summary

In recent years the rubber industry has demonstrated a growing interest in recycling. It was this phenomenon which led to the introduction of Vredestein's recycling concept: the recycling and re-use of production waste in a closed loop. The first close loops are now functioning and the results have been positive.

Vredestein's continuous-process method of reclaiming offers the possibility to produce custom reclaims in terms of reclaim composition, viscosity and feedstock.

The addition of reclaim to a compound only requires an adjustment to the curing system. The primary advantages of using reclaim are cost reduction and an improvement in compound processing behavior. Reclaim is mainly used as a compound substitute, but can as well be used as a ready-to-cure batch when combined with a curing system.

Another group of recycled-materials is the surface-treated crumbs. A special activating process has been developed, which can be applied to different kinds of rubber. The coating is adjusted to the polymer of the base material. NR- and SBR-based Surcrum have been commercialized; NBR and EPDM Surcrum are under development.

These activated crumbs can be used as a partial or a total substitute of a compound. The latter application offers the possibility to produce flat products like conveyor belts and mats in a very economical way.

[Figure 1 - thermal and mechanical breakdown

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[Figure 2 - different qualities of reclaim

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[Figure 3 - the closed-loop recycling concept]

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[Figure 4 - tensile strength, elongation at break]

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[Figure 5 - tear stength and abrassion resistance]

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[Figure 6 - rebound resilience and compression set]

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[Figure 7 - tensile strength]

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[Figure 8 - most important properties of an innerliner]

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[Figure 9 - NR crumb and Surcrum in an NR compound]

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[Figure 10 - NR crumb and Surcrum in an SBR compound]

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Literature

[1.] B. LaGrone, consultant to U.S. Rubber Reclaiming, Inc.; Reclaimed rubbers, paper presented at Rubber Division ACS Educational Seminar, May 22-24, 1995; Akron, OH. [2.] Dr. K. Vohwinkel, consultant to Vredestein Rubber Recycling; internal information; Nov. 1993. [3.] J. Jonckers; Scrap tire utilization in the U.S. and Europe; European Asphalt magazine, (3) 1993. [4.] J. Laumans, G. Nijman, L. Smeets, Vredestein; Een nieuwe generatie reclaim rubber van Vredestein, technical paper, Maastricht, October 1988. [5.] W. Dierkes, Vredestein; Ein Ansatz zur Losung der Restgtoffproblematik: die Wiederverwendung von Produktionsabfallen im Rahmen des Recycling-Konzeptes; presented at the 2. Dutch-German Rubber symposium; Maastricht, April 25-27, 1995. [6.] W Dierkes, Vredestein; Surcrum A new development in rubber recycling; presented at the 127th meeting of the Rubber Division; Philadelphia, Pennsylvania; May 2-5, 1995.

Acknowledgements

"Solutions to the rubber waste problem incorporating the use of recycled rubber" is based on a paper given at the October 1995 Rubber Division meeting. "Improved rubber properties created with sulfenimide accelerators" is based on a paper given at the October 1994 Rubber Division meeting.

"Modification of crumb rubber to enhance physical properties of recycled rubber products is based on a paper given at the October 1995 Rubber Division meeting.