Reclaimed tire rubber in TPE compounds.

Every year in North America some 280 million tires are discarded. These scrap tires, not suitable for recapping, have created huge scrap piles which are estimated by the EPA to be from 3 to 6 billion units. A scrap tire unit typically weighs 20 lbs. Michelin Tire has defined the average composition of a scrap tire as 14% fabric, 16% steel and 70% rubber.

These mean averages yield a recoverable rubber potential of some 3.9 billion lbs. from the annual flow alone. Most states now have enacted legislation on proper disposal procedures intended to divert whole scrap tires from limited landfill space as well as produce safer stockpiling methods with regard to fire danger. Current attempts to use this material in mass have revolved around burning scrap tire chips or shreds as a fuel supplement. Scrap tire fuel falls into two primary categories: Tire derived fuel (TDF), tire chips with up to 95% of the steel removed and a heat value of approximately 16,000 Btu per lb. and tire cord fuel (TCF), tire chips with the steel and fabric left intact yielding a heat value of approximately 13,000 Btu per lb.

TDF and TCF represent a value of only $40 per ton or $0.40 per scrap tire unit. Pyrolic recovery of the oils, steel and carbon black represent another method to recover value from the scrap tire flow. These attempts have proved economically unsuccessful due to the low price of crude oil and weak markets for the vast amount of carbon black, (30%) produced. Pyrolysis plants remain unproved and produce toxic waste water as a byproduct of operation. The manufacture and use of crumb rubber provides another avenue for the use of scrap tire material. Most crumb rubber, however, is produced from the grinding of tire buffings, a by-product of tire retreading. The production of whole scrap tire derived crumb rubber is now increasing, but brings the perplexing problem of quality control due to the variety rubber polymers that are incorporated to manufacture a tire. Exact tire formulations are held as proprietary, but include NR, SBR, BR and IR. These elastomers are used in various portions of the tire with the tread being different from the sidewall and inner lining. The thermoplastic applications for crumb rubber are held proprietary by individual manufacturers. Specific uses for ground truck tire tread stock include a compound additive for new tire tread stock with thermoset rubbers and low end filler for plastic compounds.

Activated tire rubber (ATR), developed by Texas Encore Corporation, is a patented surface modified crumb rubber product. ATR is designed to allow the transfer of the crumb rubber's physical elastomeric properties into thermoplastics. The surface modification of the scrap tire crumb allows for the use of either crumb rubber produced from tread or whole scrap tires to act as an effective, easy to use modifier or extender for thermoplastic or thermoplastic elastomers.

Experimental

Compound ingredients

Advanced Elastomer System's Santoprene 203-50 was chosen as the parent thennoplastic elastogier for the experimental compounds. Santoprene 203-50 has the least elastomeric properties of the various grades commercially available. Crumb rubber stocks employed were derived from a standard tread stock produced from truck tire buffings and a whole scrap tire stock including the sidewall and tread. The crumb rubber was ambiently produced by cracker mill. The rubber particle mesh size of 40 was used in all compounds.

Compounding equipment

The Instamelt rotary extrusion system was used to produce the compounds. This system was used due to heat transfer characteristics, simplicity and uniform shear. Standard twin screw extruders can be utilized to produce "ATR" thermoplastic compounds. Extrusion data are given in table 1.

Table 1 - compound extrusion data
Extruder RPM                                    216
Manifold pressure                           125 psi
Gear pump outlet pressurre                  650 psi
Die pressure                                650 psi
Inlet resin temperature                     Ambient
Exit inlet zone 2 temperature         460[degrees]F
Exit compression zone 3 temperature   460[degrees]F
Exit outlet zone 4 temperature        475[degrees]F
Manifold temperature                  475[degrees]F
Gear pump temperature                 420[degrees]F
Die temperature                       340[degrees]F

The Instamelt extruder was equipped as a strand extruder and employed a standard water bath cooling system. The compound description is in table 2. Comparisons of these compounds were made to virgin Santoprene thermoplastic rubber. Results are given in table 3. (An unexpected observation during injection molding was that the extruder pressure was dropped from 500 psi to 300 psi when the 50% ATR compounds were produced. This was required to reduce flanges on the part mold.

Table 2 - compound descriptions
ATRTS activated tire rubber/tread stock
ATRWT activated tire rubber/whole tire
Four compounds were prepared with the following
crumb rubber loading
1. ATRTS 30   30% tire crumb (tread stock)
2. ATRWT 30   30% tire crumb (whole tire)
3. ATRTS 50   50% tire crumb (tread stock)
4. ATRWT 50   50% tire crumb (whole tire)
Table 3 - typical properties of ATR
Color                               Black, black/white
Product description                 Granular powder
Shore A                             50 - 55
Specific gravity                    1.1 +/-.05
Specific heat                       .40 - .45
Tensile strength                    2,000 psi
Low temp. Brittle Point [degree]F   -80
Resistance to:
  Heat aging (212[degrees]C         G
  Abrasion                          E
  Impact                            E
  Tear                              F
  Oxidation                         F
  Sound                             E
  Acid, dilute/conc.                F/F
  Alkali, dilute/conc.              F/F

Discussion

The various ATR modified Santoprene compounds were not specifically intended to fill an existing specification. The purpose was to demonstrate the feasibility of ground scrap tire rubber as a modifying ingredient in thermoplastic elastomers.

The most unanticipated result was the overall performance of whole scrap tire (ATRWT) compounds versus the tread stock (ATRTS) compounds. In all areas of comparison the whole tire compounds were equal or superior. This is important for both environmental and available feedstock supply considerations.

A second unexpected occurrence was the ease of producing a 50% loaded compound with a strand extruder. Other attempts have had difficulty exceeding 35% with a strand extruder.

The reduction of pressure, from 500 psi to 300 psi, required while injection molding the test parts, was also surprising. Normal exceptions would be for a greater amount of pressure to maintain mold flow.

Though tensile substantially dropped in all compounds from the parent material, when compared to the softer more elastomeric Santoprene compounds (55 and 64 Shore A hardness), the attained tensiles were comparable (tables 4 and 5). Tire rubber typically has a Shore A Hardness ranging from 50-55 and it is anticipated that ATR would fit well into TPE compounds having a similar Shore hardness. The loss of elongation, though disappointing, is anticipated to improve dramatically with the softer more elastomeric compounds. Tear strength was impressive when compared with Shore hardness ranges to 80A. ATR has received U.S. patent allowance for use with plastics, recycled plastics, thermoplastics, thermoplastic elastomers, polyacrylics, asphalt and latex. ATR+ has shown spectacular results five grade increase) as a hot mix asphalt binder modifier. Testing is done under SHRP performance criteria.

[TABULAR DATA 4&5 OMITTED]

Summary

The physical properties that tire rubber brings to compounds - resistance to abrasion, excellent temperature stability, high carbon black content (30%) and resistance to impact - are all beneficial qualities. The specific gravity of ground tire rubber is higher than most TPEs, and depending on loading, will slightly raise the gravity of established TPE compounds.

The most beneficial impact of ground tire rubber on TPE compounds may be economic. The modified ground tire rubber used in these compounds is expected to cost $0.30-$O.40 per lb. The TPE compounds compared are priced at $2.00/lb. and greater. Assuming a 25% loading of ground tire rubber and a specific gravity increase by .03 the economics of scrap tire rubber extended TPEs are shown in table 6.

Table 6 - assumptions
                            TPE              ATR
Material cost           $2.00 per lb.    $0.40 per lb.
Specific gravity            0.98               1.1
Compound percent              75                 25
Modified compound
  Material cost ($2 x 75%) + ($.40 x 25%) = $1.60
  Adjustment for sp. gr. $1.60 x 1.01 + $1.62

This represents a $0.38 per lb. savings on the modified compound. Additional compounding cost is estimated at $0.08 per lb., yielding a net savings of $0.30 per lb.

There is a vast quantity of processable scrap tire feedstock located throughout North America. Should market specifications be met by compounding modified scrap tire rubber with thermoplastic elastomers, both economic and environmental concerns benefit.

High value TPE compounds are distinguished by excellent temperature stability and the ability to be injection molded. Ground tire rubber being composed of thermoset elastomers have inherent temperature stability. Converting ground scrap tire rubber to a component capable of being injection molded opens a new raw material for the TPE industry.