Network changes in recycled rubber.

The disposal and re-use of scrap tires is a serious environmental concern. Due to environmental pressure and for economic reasons, the impetus towards developing viable recycling methods for used commercial tires is growing. In view of this situation, this article is based on one of many recycling techniques.

The recycling process used in this work is the Sekhar-Kormer-Sotnikova reaction known as the De-Link process. The process causes uncoupling of sulfur crosslinks in vulcanized natural and synthetic rubber. The modifications engendered in the rubber network by the De-Link process produces elastomeric materials that can be re-vulcanized without the need for additional vulcanizing agents. The process does not generate disposal hazards and can be directly carried out in the product manufacturer's factory. The scrap rubber used for this method is finely ground crumb available commercially, which currently is the major way of utilization of rubber waste. The crumb can be produced either by ambient temperature mechanical grinding or by cryogenic shattering. It can then be combined with virgin material to reduce cost.

A tire vulcanizate has to meet strict requirements in mechanical properties and performance. In general, the addition of crumb reduces the properties. However, using the De-Link process, up to 30% of recycled material can be incorporated back into the virgin compound with minimal property changes. An essential aspect of recycling would be to characterize the concomitant changes that take place in the rubber network when subjected to repeated recycling. This work reports and analyzes the changes that occur in a typical tire tread compound after repeated recycling by the De-Link process. Mechanical properties of vulcanizates were investigated and sol-gel analysis was used to characterize the rubber network.

Based on the ease of reprocessing, all polymeric materials can be divided into two categories, including: thermoplastic and thermoset. Thermoplastic materials can be softened and reshaped repeatedly with the assistance of heat and pressure, and can be recycled using established processing techniques. On the other hand, a material classified as a thermoset is one that attains a degree of permanence, and therefore, cannot be recycled in the normal way.

Rubber is a thermoset. Once valuable resources (raw materials and energy) have been invested in a rubber product, they are effectively locked-in by the vulcanization process. Vulcanization, or curing of rubber, leads to the formation of a three-dimensional crosslinked structure, which gives rubber its unique properties, but at the same time makes the vulcanization process irreversible. This leads to difficulties once the product reaches its end of life.

As a result, a significant level of rubber scrap is accumulated every year worldwide. Seventy percent of the scrap is generated from used tires. Current disposal methods cannot deal with the growing used tire waste and account for only 30% of the total accumulated waste. In addition, these methods have their limitations due to environmental concern and their effectiveness. Under new EU legislation, landfilling and stockpiling that is currently one of the main ways of dealing with used tires is going to be banned this year, giving an additional one million metric tons of used tires that have to be disposed by other environmentally acceptable methods.

Rubber crumb that is recovered from tires by current conventional methods is used mainly as cheap filler, processing aid, etc. Using untreated crumb back into new tires is limited due to significant loss in strength properties. Generally, there is 1% deterioration in properties for every percent addition of rubber crumb. This can be explained by poor interfacial bonding between the crumb and the virgin matrix of elastomer. To improve properties and increase the amount of crumb that can be used, it is necessary to chemically treat or devulcanize the rubber in preparation for re-vulcanization.

The result of an extensive literature review on recycling methods identified the De-Link process to be one of the most promising devulcanization methods, offering an effective solution in utilizing rubber scrap, including tires. The method uses harmless chemicals already established in the tire industry to devulcanize rubber waste during the first stage of the process. During the second stage, the compound can be revulcanized without the need for additional vulcanizing agent. It can be used on its own or blended with virgin materials in order to reduce compound costs.

Experimental

Materials

Compounding ingredients used for this work were chosen in the light of information acquired from the literature search (R. Kohler and J. O'Neill [ref. 1]). All materials used were of commercial grade and were used as received without further purification. The De-Link R, a patented proprietary reactant, was received from Guthrie Symington Limited, UK. The standard compound formulation (NR-S) is shown in table 1 and recycled compounds in table 2. Three recycling stages were performed in this work, compounds NR-1, NR-2 and NR-3.

Rubber crumb

Regardless of whether one incorporates untreated, surface-modified or devulcanized scrap into virgin compound, the process starts with grinding the scrap to crumb. It can be done at ambient temperature or cryogenically. In each case, producing very fine crumb is essential. As stated by R. Kohler and J. O'Neill (ref. 1), to produce a satisfactory product, untreated filler crumb must be at least 40-60 mesh, or preferably even finer. The effect of particle size (10 to 80 mesh) investigated by R. Kohler and J. O'Neill showed that the size of the crumb had very little effect on tensile strength, but elongation at break and Mooney viscosity improved with smaller crumb. They also investigated the influence of particle morphology. This was performed because crumb produced at ambient temperatures has a rough surface, while cryogenically ground crumb is more crystalline in nature with flat planes and straight-line fractures. They concluded that the surface morphology of the devulcanized crumb has no influence on the final properties of vulcanizates, and therefore, the choice of grinding method depends mostly on availability.

For the purpose of this study, crumb was obtained via cryogenic grinding using an SPEX Freezer Mill 6800, which is a cryogenic laboratory mill. It chills samples to liquid nitrogen temperature (-195.8[degrees]C) before grinding them by impact milling. The low temperature preserves structural and compositional aspects and is an ideal process for grinding materials that resist conventional milling techniques.

Electronic controls allow an entire chilling and grinding sequence to be programmed. A typical milling cycle after precooling consists of several grinding periods interrupted by pauses for re-cooling.

The crumbing technique:

* First stage: Vulcanized rubber sheets were first reduced to approximately 1x1 mm pieces using a cutting mill (model SM 2000) with the standard hopper and parallel-cut rotor.

* Second stage: Rubber pieces were ground in a freezer mill under liquid nitrogen conditions. The machine settings were based on the general recommendations for rubber from the manufacturer of the mill. The parameters and settings were modified during the grinding of the standard formulation compound NR-S and were used for the subsequent compounds NR-1 and NR-2. The details are shown in table 3.

Particle distribution

Cryogenic grinding gives quite a wide particle size distribution compared to an ambient temperature process, and therefore it is important to determine the size and distribution of rubber particles to ensure that similar material is produced each time. This was done by using analytical sieve shakers.

It can be seen from figure 2 that all three crumbed materials (NR-S, NR-1 and NR-2) had similar particle size distribution. To prepare compounds NR-1, NR-2 and NR-3, only crumb with mesh size 30 and above was used.

Devulcanization of crumb

Rubber crumb obtained via cryogenic grinding, was devulcanized on a two-roll mill by adding the chemical reactant, De-Link R. In general, the amount of the reactant required depends on the polymer type and content, and amount of sulfur in the original material. The devulcanization process is a mechano-chemical process, based on the Sekhar-Kormer-Sotnikova (SKS) reaction (ref. 2). The devulcanized crumb is called De-Vulc. To improve the handling of the devulcanized crumb, a process aid and low viscosity raw rubber were used as additives.

Compounding

All compounds were prepared on a two-roll mill (6" x 12", double geared, double ratio laboratory mill) with a friction ratio 1:1.25.

De-Vulc was prepared using the following method (table 4):

* Rubber crumb was divided into three equal portions of approximately 50 gm. Using the same two-roll laboratory mill each portion was mixed with the appropriate amount of reactant De-Link R on a tight nip setting with cold water running. Mixing time was approximately seven minutes.

* The three portions of devulcanized crumb were then combined together and mixing was continued, with addition of plasticizer Struktol NS60 and raw natural rubber for a further 13 minutes.

* The De-Vulc material was sheeted, cooled and ready for mixing with virgin compound.

Compounds NR-1, NR-2 and NR-3 (table 2) were prepared using the following technique:

* Virgin material was mixed with De-Vulc compound for approximately two minutes, crosscut and end-passed several times.

* After that, each rubber stock was sheeted off at 2.5 mm thickness and left to cool.

Vulcanization

Vulcanization characteristics of the rubber mixes were determined on an oscillating disc rheometer 100 at two temperature settings, 140[degrees]C and 150[degrees]C, with a motor speed of 60 minutes. The curing characteristics are presented in table 5.

It can be observed from the data in table 5 that, although scorch time decreased from five to two minutes after the first recycle, it did not change after subsequent recycling stages.

Test procedures

Mechanical properties

To assess the mechanical properties, the following tests were performed: Tensile, tear, hardness, compression set and rebound resilience. Test specimens were prepared by compression molding, and tests were carried out using relevant standards.

Sol-gel analysis

The method involves determination of the fraction of rubber remaining unconnected in the network (soluble rubber) and crosslink density (gel portion). Swelling measurements employed in this work were based on the technique described in (ref. 3). It was assumed that the swelling was isotropic. From the swelling experiment, the linear swelling ratio, sol fraction and the degree of swelling were determined.

The volume fraction of the polymer was determined from the following equation:

(1) [[phi].sub.PV] = Vp/Vc

Where Vp is the volume of the polymer and Vc is the volume of all ingredients in a recipe. The volume of an ingredient is a ratio of its mass to its density (ref. 4). Values of densities of ingredients were taken from (ref. 4).

The volume fraction of unextractable ingredients ([phi]unexV) was found using the following equation:

(2) [phi]unexV = Vc -Vp/Vc

The effective crosslink density was calculated by means of the Flory-Rehner equation (ref. 5).

(3) N = -1/2Vs x ln(1-Vr) + Vr + [chi]V[r.sup.2]/[Vr.sup.1/3] - Vr/2

Where, N is the effective crosslink density, in moles of crosslink per unit volume, Vs = molar volume of the swelling solvent, Vr = volume fraction of polymer in the swollen gel (the degree of swelling found earlier), [chi] polymer-solvent interaction parameter (it was assumed that carbon black did not influence it). The polymer-solvent parameter was taken from (ref. 6) and had a value of 0.393 for the toluene/NR system. The molar volume &the swelling solvent was taken as a ratio of relative molecular mass of toluene and its density. It was found to have a value of 106.4 [cm.sup.3]/mol.

Results and discussion

Mechanical properties--tensile strength

Tensile strength is an important property of any rubber compound, particularly tires, where maintaining good strength properties is vital for their performance.

Figure 3 demonstrates that tensile strength was slightly affected by recycling and resulted in 7% loss of tensile strength after the first, and 13% after the third recycle. However, even after repeated recycles, this compound had the potential to meet the requirements for tensile strength (between 20 and 27 MPa) for an NR truck tire compound. In addition, the results achieved by the De-Link process were significantly better compared to other competitive recycling methods. For example, one of the most promising among chemically-treated rubber crumb technologies, Surcrum, resulted in a 15% loss in tensile strength, when 20% of recycled crumb was used in a similar application (ref. 7). Another example is the mechano-chemical technology. Using 25% of rubber crumb, devulcanized by this method in an industrial tire formulation, resulted in a 24% loss in tensile properties (ref. 8). No data reporting tensile strength after repeated recycles, have been found for these methods. Moreover, in both examples, a lesser amount of crumb was added to virgin material than in this project using the De-Link process.

Modulus

The effect of the number of recycles on modulus is shown in figure 4. It can be seen from the results that the changes in modulus at different elongations show a similar behavior.

Comparing the last recycled compound with the virgin compound, it can be seen that the modulus did not change. This behavior is in agreement with the results after one recycling, as described by Kohler and O'Neill (ref. 1). It can therefore be concluded that modulus is not influenced by recycling. For comparing mechanical properties of rubber compounds, it is preferable to have the same modulus levels, because different moduli would certainly lead to changes in other mechanical properties, and hence obscure the results. Clearly in the present work, as the moduli did not change, the mechanical properties can be compared legitimately. From a practical point of view, this means that the recycled compounds would not require the additional expense of incorporating additives in order to correct their moduli.

Elongation at break

It can be observed in figure 5 that elongation at break decreased marginally with the number of recycles. Generally, for rubber compounds, it is not unusual that a decrease in tensile strength (for the same modulus) is always accompanied by a decrease in elongation at break.

Tearing energy

Tearing energy was determined using relevant standards and a trouser test piece. It can be seen from the test results (figure 6), that there was a slight decrease in tear properties after the first recycling (around 6%), which is in agreement with the work that has been previously carried out on the De-Link process (ref. 1). In general, reduction in tear properties occurs with addition of recycled rubber. However, if one compares the results of this work with other competitive recycling methods, one can deduce that De-Link offers the best results. For example, addition of only 10% of surface-modified crumb, Vistamer, into a virgin compound, resulted in 8% loss of tear properties (ref. 9), and incorporating 20% of Surcrum led to 10% loss (ref. 7).

Hardness

Hardness was measured using a dead load hardness tester, with a ball indentor of 2.5 mm acting under a total force of 5.7N.

Hardness increased slightly after the first recycle and stabilized thereafter. This could be due to rubber crumb acting as filler: and the addition of filler generally leads to an increase in hardness.

Compression set

This test is intended to reflect the ability of the rubber to recover after prolonged periods under compression. A lower value of compression set indicates an enhanced ability of rubber to recover. The thickness of the specimens was measured before and after 25% compression for seven days at 120[degrees]C and used to obtain the compression set.

The compression set improved after the first recycle and did not change during subsequent recycles. Other recycling methods tend to show an increase in compression set, i.e., reduced ability to recover.

Rebound resilience

Rebound resilience gives an indication of the dynamic behavior of rubber and is an important characteristic for tires. The test was carried out on a Dunlop Trypsometer.

It can be seen from the results in figure 9 that rebound resilience sharply increased after the first recycle and then stabilized thereafter.

Sol-gel analysis

The plot in figure 10 shows an increase in a sol fraction with the number of recycles. This is a consequence of the devulcanization and revulcanization reactions that occur during the de-linking process. The fact that a sol fraction is increasing can explain some decrease in fracture properties (tensile strength, tear energy) of the recycled compounds. Sol fraction characterizes an ineffective portion of the network structure. In comparison, a gel cannot be fractured without breaking chemical bonds, and the strength is higher at the gel point since chemical bonds must be ruptured to create fracture surface. Therefore, an increase in the amount of the sol portion of the rubber network will lead to a deterioration of the strength properties.

A gel portion can be characterized by the effective crosslink density. Elastomers have an optimum crosslink density range for practical use. It has to be high enough to prevent failure by viscous flow, but low enough to avoid brittle failure. It can be observed from figure 11 that the crosslink density of the first recycled compound decreased marginally and then increased in the case of the second recycled compound. It did not change from the second compound to the third. In general, the values of effective crosslink densities of the virgin and recycled compounds are quite close, and this can explain some of their similar properties (for example modulus at 100%, 200%, 300% and 400% elongation). The fact that the values for the second and third recycled compounds are the same can be used to explain the similar values for properties such as compression set, rebound resilience and hardness. Improved set and rebound resilience in recycled compounds can be explained by the high degree of crosslinks maintained through the recycling stages with the De-Link process. Another explanation could be the structural and crosslink type changes in recycled compounds. As indicated by Ishiaku (ref. 10), De-Link R devulcanizes rubber to some extent (some crosslinks still remain unbroken). When blended with virgin compound and re-vulcanized, these remaining crosslinks, as well as new crosslinks, will influence the properties of the recycled compound. After repeated recycling, the structure of the compound becomes even more complex, having some fraction of undissociated crosslinks from the previous recycling stages. Taking into account that the crosslinks formed are also shorter (ref. 7), this can lead to improvement in the above-mentioned properties.

Conclusions

Mechanical properties such as tensile strength, elongation at break and tearing energy decreased slightly with the number of recycles. However, the results still meet the property specifications for tire compounds, even after the third recycle. These properties are very dependent on the size of the crumb and can be further improved by using very fine crumb.

Hardness increased marginally after the first recycle, but remained stable for the subsequent recycled compounds.

Modulus at 100%, 200%, 300% and 400% elongation was not significantly affected by repeated recycling. Scorch time was seen to reduce after the first recycle, but there was no further decrease and it remained stable for subsequent recycles.

Compression set and rebound resilience improved after the first recycle and maintained their values with further recycles.

Swelling measurements showed correlation between increase in sol fraction (with the number of recycles) and reduction in strength properties. The value of effective crosslink density that characterizes a gel portion of the rubber network was not affected by recycling and, therefore, some properties were similar before and after recycling. In addition, the properties proved to be better compared to other competitive methods, such as surface-treated crumb (Surcrum) and a mechano-chemical devulcanization method using a high shear mixer.

This work has demonstrated that 30% of recycled tire compound, devulcanized by the De-Link method, can be added back to virgin tire compound repeatedly. Therefore, it can offer an economical and ecological solution in dealing with used tires.

References

(1.) Rudi Kohler, John O'Neill, "New technologies for the devulcanization of sulfur-cured scrap elastomers," Rubber Division, ACS, Louisville, KY, Oct. 8-11, 1996.

(2.) Ares N. Theodore, "Cure and mechanical behavior of elastomeric compounds containing devulcanized materials, " Rubber World, May, 1998.

(3.) A. Anagon, "Evaluation of chain scission during mixing of filled compounds," Rubber Chemistry & Technology, Nov.-Dec., 1996.

(4.) W. Kleemann, K. Weber, Elastomer Processing: Formulas and Tables, Hanser Publishers, Munich, 1998.

(5.) A.N. Gent, Engineering with Rubber, Hanser Publishers, 1992, p. 20.

(6.) J.A. Brydson, Rubbery Materials and Their Compounds, Elsevier Applied Science, London, New York, 1988, pp. 48-55.

(7.) I.W. Dierkes, "Solutions to the rubber waste problem incorporating the use of recycled rubber," Rubber World, May 1996.

(8.) C.J. Brown, W.F. Watson, "Recycling of vulcanized factory waste, "Rubber World, May 1998.

(9.) "Rubber in the environmental age-progress in recycling," Rapra Technology Limited, Shawbury, Shropshire, UK, November 18, 1996.

(10.) Ishiaku, Chong, H. Ismail, "Determination of optimum De-Link R concentration in a recycled rubber compound," Polymer Testing, v. 18, Dec. 1998, pp. 621-633.

Table 1--NR-S standard
compound formulation

Component                        Amount
                                  phr

Natural rubber                    100
  SMR-20
Carbon black                       45
  N220
Zinc oxide                          5
Stearic acid                        2
Santoflex 13                       1.5
TBBS                               0.7
Sulfur                             2.5

Table 2--recycled compound formulation

NR-1                      NR-2                      NR-3

70% NR-S virgin           70% NR-S virgin           70% NR-S virgin
  compound                  compound                  compound
30% NR-S                  30% NR-1                  30% NR-2
  crumbed and de-         crumbed and de-           crumbed and de-
  vulcanized by De-         vulcanized by De-         vulcanized by De-
  Link process              Link process              Link process

Table 3--settings for
grinding in a freezer
mill

Grinding process               NR-1

Pre-cooling, minutes             5
Run, minutes                     2
Cooling between run,             2
  minutes
Rate, beats per second          14
Number of cycles                 4

Table 4--De-Vulc
compound formulation

Component                   Amount
                               phr

Crumb                          100
De-Link R                        5
Struktol 60 NS                   2
Natural rubber                   6
  (SMR 20)