Modification of crumb rubber to enhance physical properties of recycled rubber products.

Crumb rubber from scrap tires (crumb) is being used as a cost-beneficial extender in some rubber products. In such products, about 20% to 50% of crumb rubber is utilized. This amount of inert crumb, i.e. untreated crumb, significantly reduces the physical strength properties of the rubber product. Thus, untreated crumb is used in applications where the product is not stressed in a dynamic sense and where high strength is not required. Such applications include mats, road markers, pads, etc.

If a rubber compound incorporating more than 30% crumb had a higher physical strength, e.g. tensile strength of 15 MPa or more, and were low cost, there would be many more applications for crumb.

The principal reason that there is reduced strength when crumb is incorporated, is because there is little interfacial bonding between the crumb add the virgin matrix elastomer. Thus there is the need to develop chemical processes to modify the surface of crumb, and compounding technology, to enhance the rebonding of crumb.

Also, if the crumb could be devulcanized in whole or at least within a layer around the surface of the particle it could be rebonded directly using established compounding technology.

Background and related developments

Over the past 70 years, several researchers have investigated potential commercial processes for modifying the surface of crumb rubber particles in order to promote their rebonding and incorporation into virgin elastomer compounds.

F.J. Stark of Minnesota has patented (ref. 1) the use of certain oligomers and functional liquid polymers as agents for promoting adhesion of crumb particles to a virgin elastomer matrix. Presumably, these liquid binders act as a softening agent at the crumb particle surface and serve as a covulcanizable bridge to bond crumb to the elastomer matrix. When a standard SBR compound is modified by addition of 30 phr crumb, treated with 3% liquid polymer, products with tensile strength from 9 to 12 MPa and 300% to 380% elongation at break are obtained. Stark has described the application of this compound in products such as roof shingles, carpet backing and molded goods.

D. Mahlke of Huls AG, Germany has shown (ref 2) that untreated tire crumb can be rebonded directly by a similar technique using liquid polyoctenamer. With this binder and some extender oil added as a softener, rebonded crumb exhibits tensile strength of 8 Mpa and 300% elongation at break. Of interest is Mahlke's finding that better physical properties are obtained using larger size crumb particles. This can provide some additional cost-benefit by obviating the need to employ cryogenic grinding for generation of the crumb.

Another approach to rebonding crumb rubber is to change the chemical nature of the particle surface. Tire crumb is essentially hydrocarbon and non-polar and is therefore not compatible with polar binders and elastomers such as polyurethanes, epoxy resins, etc. Bernard Baumann of Composite Particles Inc. has developed (ref. 3) an efficient process for modifying the surface of crumb particles using oxidizing gases. The resultant treated particles, sold commercially as Vistamer materials, can be used as cost-beneficial extenders in elastomeric polyurethanes for applications such as industrial tires, wheels, automotive components, etc.

The use of liquid polymers and chemically modified crumb are somewhat expensive treatments for application of crumb in conventional rubber products.

B.C. Sekhar and others (ref 4) have recently announced that they are commercializing a process for direct devulcanization of crumb by addition of 7 phr of a patented devulcanization agent, and masticating the mass at 70[degrees]C for 10 minutes in the presence of 2 phr plasticizer plus 6 phr of virgin natural rubber. This treated crumb can be directly revulcanized at 150[degrees] in 10 minutes. Tensile strength is in the range of 10 to 15 MPa with about 300% elongation at break.

Albeit that this process is a timely contribution to the problem of re-utilizing scrap rubber, the concept is the subject of several earlier studies and patents.

F.B. Menadue has described a process (ref. 5) whereby the crumbed scrap from vulcanized tire tread compound was masticated on a mill with the addition of 1% sulphur, 0.5% diphenyl guanidine and 1% zinc oxide. The milled mass was revulcanized at 60 psig steam for 15 minutes resulting in a product with 15 MPa tensile strength.

D.L. Twiss et al of Dunlop Ltd. have patented (ref. 6) processes wherein the crumb is softened with a plasticizer such as pine tar, and devalcanized by the addition of 0.5% to 2% of mercaptobenzothiazole and an organic acid catalyst. Tire treads made from the revulcanized crumb gave about 75% of the properties of the original compounds.

H. Hildebrandt (ref. 7), S. Yamashita (ref. 8) and E. Stalinski (ref. 9) have developed patented processes for devulcanization of crumb using a softening agent, amines and a catalyst.

A comprehensive summary review of methods of devulcanization has been presented to the ACS Rubber Division by W. Warner (ref 10).

General method of devulcanization in this study

We have re-examined the above approaches to devulcanization of the crumb particle in whole or at least at the surface in order to improve the physical strength properties of compounded blends of treated crumb with some virgin elastomers. The concept involves a four step process.

The first step is to add one or more liquid chemicals to the crumb in order to swell and soften the surface in order to allow penetration of the reactive chemicals into the particle. Plasticizers such as terpenes and pine oil are very compatible with natural rubber and SBR.

The second step involves stirring into the softened mass a reactive chemical that can cause scission of the sulfar-sulfur bonds to devulcanize the rubber, or at least to reduce the cross-link density significantly. This step can be accelerated by the addition of catalysts and neutralizing agents to limit unwanted reactions.

The third step is to masticate the treated crumb on a mill to make the devulcanized crumb as uniform as possible, etc.

The fourth step is to recompound the treated crumb with some virgin elastomer and additional curatives to obtain the final compound for product manufacture. Rubtec Research and Recycle Inc. has developed a proprietary formulation in the above process to devulcanize scrap tire rubber crumb in a brief time cycle with minimum energy input. This has been tested in the laboratory at Canadian Rubber Testing and Development and found to be technically effective. The examples cited below are from these formulations unless otherwise indicated .

Experimental procedures

The crumb used in these studies was prepared by both ambient and cryogenic grinding processes. Particle size ranged from as small as 30 mesh minus to as large as 5 to 8 mesh.

Preparation of devulcanized rubber (DVR)

For example, the treatment of the crumb was conducted in a two liter stainless steel vessel equipped with a cooling jacket since the process is exothermic. The crumb was added to the vessel with stirring at room temperature. The plasticizers and reactive chemicals were added to the crumb and the mass stirred for one to two minutes. The catalyst, dispersed in a wood by-product, was added with continuous stirring. The temperature was maintained at 80[degrees]C to 100[degrees]C by cooling and the mass stirred for an additional two to three minutes before the mass is discharged from the vessel and allowed to cool to room temperature.

The rubber compounds were mixed on both an internal mixer and on a 10 cm x 20 cm two roll mill. The virgin elastomer, DVR, and fillers were mixed in the mixer to make master-batches. The activators and curatives were mixed in on the mill. If a fully compounded formulation were to be mixed the entire mixing was done in the mixer. The cure characteristics were determined using an oscillating disc rheometer. Physical properties were measured using a tensile tester and the procedures of ASTM D412 method. The volume swell determinations were made after immersion in xylene for 48 hours.

Treatment of crumb with liquid polymers

The addition of liquid polymers to modify the surface of crumb was conducted in a two liter stainless steel vessel utilizing a heating tape to increase the temperature of the mass. Crumb was heated at 100[degrees]C for a period of one hour. A 25/75 blend of the liquid polymer in processing oil was also heated separately for 30 minutes at 100[degrees]C. The crumb, activators and curatives were combined in the vessel and stored with heating to maintain the temperature in the range 60[degrees]C to 80[degrees]C. After one to two minutes the liquid polymer in oil was added and the mass was stiffed for an additional five minutes. The treated crumb was discharged from the vessel, allowed to cool and stored for three to four days before being mixed and compounded with virgin elastomer. The compounds were prepared on the mill, characterized and cured as described above.

Results of physical testing

The tabled data show the formulations, rheometric and cure characteristics of the mixed compounds and the physical properties of cured ASTM slabs made from these compounds.

DVR compounds

Table 1 shows the physical properties of the slabs prepared directly from re-vulcanized DVR, without any virgin elastomer included in the compound, with variation in crumb origin and curatives concentration. The DVR of examples 1, 2 and 3 was prepared from large size crumb, 5 to 8 mesh, and which had been presheeted on a min. Examples 4 and 5 used fine particle crumb, 30 mesh minus, mixed directly with the other ingredients.

[TABULAR DATA 1 OMITTED]

The properties of all compounds are similar. The size of the crumb particle apparently has no significant effect on final properties. The maximum torque in the rheometer is relatively high suggesting that the state of vulcanization (crosslink density) of the total mass is high. The viscosity increases as the sulfur concentration increases to 3 phr. The minimum torque in the rheometer is considerably lower for the DVR prepared from larger particle size crumb. The actual physical properties are quite consistent with hardness from 68 to 72 points on the Shore A scale, tensile strength 6.7 to 8.2 MPa, and elongation 160% to 225%. The tear strength is quite high at up to 325 N/cm.

Tables 2 and 3 show the properties of blends of DVR with natural rubber without any addition of reinforcing fillers, or extenders.

[TABULAR DATA 2 & 3 OMITTED]

For the compounds of table 2 the conventional natural rubber accelerator DBBS was incorporated. In the second set of compounds listed in table 3 a low sulfur and sulfur donor type accelerator was used. With DBBS accelerator the cure rate is slower but the actual state of cure as indicated from the rheometer data is higher. This is reflected by the higher level of modulus and tensile strength of the compounds made with DBBS accelerator. The hardness and modulus increases as the concentration of DVR in the compound increases whereas the tensile and tear strength properties decrease.

In general, compounds with 40 phr of DVR give a good balance of physical properties for many rubber product applications.

The data in table 4 are for a blend of 70% DVR with natural rubber but also including additional carbon black, plasticizers and various curatives. All compounds exhibit similar hardness in the range 57 to 65 Shore A, tensile strength in excess of 15 MPa with elongation at break up to 485%, and tear strength from 345 to 515 N/cm. The cure times are relatively long, but could be reduced by curing at a higher temperature.

[TABULAR DATA 4 OMITTED]

Compound 3 with CBS accelerator exhibits a good balance of properties, high tear strength and short cure time. This compound should lower cost and be useful in molded goods.

The compound formulation shown in table 5 with a higher level of carbon black and mineral fillers should be useful as a lower cost molding compound. The cure time of 10 minutes is short and there is a good balance of physical properties.

[TABULAR DATA 5 OMITTED]

The data of table 6 show that high hardness rubber can be obtained using DVR in blends with natural rubber and SBR. The properties of compound I are adequate for special applications for high hardness products. The filler and extender are low cost materials.

[TABULAR DATA 6 OMITTED]

NR compounds with crumb treated with liquid polymers

The data in table 7 show the formulations and comparative properties of blends of natural rubber containing about 33 phr of crumb for both untreated crumb and crumb treated with Vestenamer plus oil. It should be noted that there is also 32 phr of carbon black added to this compound. All of the physical properties of the blend are improved, particularly the tear strength, when the crumb is treated.

[TABULAR DATA 7 OMITTED] The data of table 8 show a similar comparison for crumb treated with the Ricon liquid PBD polymer. However, in this case there is no carbon black added to the formulations. Again, the treated crumb exhibits about a 30% increase in physical strength and tear resistance.

[TABULAR DATA 8 OMITTED]

Discussion

DVR

The results described indicate that the proprietary process to devulcanize crumb tire rubber is effective and that the DVR can be used alone and in blends with natural rubber to give rubber products with useful properties.

The DVR can be vulcanized with the conventional curatives and processed in standard mixers and on a two roll mill. The DVR had sufficient plasticity that it did not shred or crumble on the mill and thus other ingredients could be realy mixed in on the mill. In general, the strength of the DVR compounds without any virgin elastomer added are only modest.

DVR blends with natural rubber

It is shown that good physical properties are obtained when DVR is blended with natural rubber. When the natural rubber is the major portion of the blend, i.e. >50%, the properties are very good. Although no laboratory study has been made, it is speculated that the cross-link density of the revulcanized DVR is actually higher than that of the virgin natural rubber matrix which might contain only a small amount of the DVR material. It is possible that the stress load in the blend is differentiated and that the strain in the matrix phase exceeds its breaking point when the overall strain on the whole sample is at a lower value.

Better physical properties are obtained for blends when the conventional DBBS accelerator is used. This accelerator gives rise to long polysulfidic cross-links, i.e. -Sx-. Such longer cross-links might give rise to more intermolecular mixing and cross-linking at the DVR/virgin elastomer interface. In contrast the sulfur donor curative system is known to give many monosulfidic cross-links, i.e. -S-, thus giving higher cross-link density, less interphasial crosslinking and less molecular chain mobility.

Potential commercial molding compounds

The data of tables 4, 5 and 6 show that complete formulations for blends of 50 to 70 phr DVR in natural with moderate or high levels of filler and extender loading provided useful compounds of varying hardness, modulus and tensile strength.

The experimental results indicate that, as with virgin rubber compounding, it is necessary to optimize the curatives system in order to optimize the processing and physical properties of the DVR blend.

Surface modification with liquid polymers

As indicated in an earlier study by Stark and others, the use of liquid polymer additives to crumb does enhance the properties of compounded blends with virgin elastomers, particularly the tear strength. This suggests that the interphase bonding at the interface of the crumb and virgin matrix is enhanced. The use of the liquid polybutadiene in the extender oil reduces the rheometric viscosity of the treated crumb mass significantly. This feature might be utilized in preparation of crumb blend compounds with elastomers that have intrinsically high rheometric viscosity to obtain easier processing recycled rubber products.

Conclusion

Crumb rubber from scrap tires can be devulcanized by a relatively simple process using chemical additives to soften the crumb because scission of the sulfur-sulfur cross-links. This devulcanized rubber can be be re-vulcanized to give rubber materials of moderate properties. It also can be blended and compounded with natural rubber to give lower cost rubber products with good physical properties.

References

(1.) F.J. Stark, U.S. Patent 4,481,335, Nov. 6, 1984. (2.) D. Mahlke, Recycling of waste rubber by sur ace modification, Nordic Rubber Conference, Helsinki, May 1993. (3.) B.D. Baumann, High value engineering materials from scrap rubber, ACS Rubber Division Meeting, April 20, 1994. (4.) Presentation on devulcanization -- using the Delink process, STI-K Polymers SDN.BHD, Brickendonbury, UK, Apr. 27,1995. (5.) F.B. Menadue, Some technical aspects of rubber recycling, Rubberage, 56,511-512 (1945). (6.) D.F. Twiss et al, Dunlop Ltd., Regenerating vulcanized rubber, Brit. Patents 577,868, June 4, 1946 and 578,482, July 1, 1946. (7.) H. Hildebrandt, Continental G-W, A.G., Regeneration of synthetic rubber vulcanizates, Ger. Patent 1,244,390, July 13,1967. (8.) S. Yamashita, Kyoto Inst. Tech., Reclamation of vulcanized rubber - Japan-USSR Polymers Symposium (Proc. Eng.) 1976,355-364. (9.) E. Stalinski, Reclaiming of natural or synthetic rubber, France Patent 1, 1517,694, March 22, 1968. (10.) W.C Warner, Rubber World, Methods of devulcanization, ACS Rubber Division Meeting, October 1993.