Functionalised Micronized Rubber Powder (FMRP) finds applications in tyres, conveyor belting, hose and other technical rubber goods. FMRP is a cryogenically ground rubber powder produced exclusively from pre-selected end-of-life whole truck tyres and undergoes a patented chemical surface treatment in order to (re)-activate (or “functionalise”) its ability to form crosslinks during vulcanisation.

As such the rubber powder becomes a bonded and integral part of the new rubber compound, allowing the rubber industry to produce more sustainable and high performance products.

Recycled Vulcanizate Particulate Rubber is important in rubber compounding. It allows the rubber compounder to add a certain percentage of rubber filler back into its parent or similar compounds or to use the material as substitute or stand-alone compound. Its use may lower compound costs or improve performance characteristics, or both.

Several grades of recycled vulcanizate particulate rubber exist. Their classification is based on two major characteristics: particle size distribution and the polymer type found in the parent rubber from which the recycled vulcanizate particulate rubber was derived.

The term Micronized Rubber Powder (MRP) refers to products with designations smaller (finer) than 425 μm (40 mesh). These materials typically range in particle sizes from less than 300 μm (50 mesh) to less than 75 μm (200 mesh) regardless of polymer type or method of processing. A common particle size is less than 177μm (80 mesh) which provides a useful balance between performance and its cost of production.

Parent compounds (from which MRPs are produced) range from whole tyres (e.g. truck, passenger car, bus), tyre tread, tyre sidewall, tyre retread buffings and non-tyre rubber compounds.

The term Functionalized Micronized Rubber Powder (FMRP) and its characterisation is not specifically covered by ASTM D5603. The term has been used loosely within the industry for a few years to describe any MRP whose surface activity/energy has been increased by whatever means in order to promote marketing traction. For example MRPs that are ground at ambient or higher temperature are assumed to have a higher surface energy and rougher surface than MRPs that are cryogenically ground, therefore having a higher level of chemical and surface interaction with the host compound into which they are mixed – with some small level of improved in-rubber performance. There is evidence to support this. In essence the performance of current state-of-the-art MRPs (and those claiming to be “FMRPs”) main limiting factors restricting large-scale re-use in tyres and technical rubber goods are twofold:

1. Size reduction in reality is limited due to energy efficiencies and other technological limitations to about 180 microns (80 Mesh) (D95 percentile)

2. Low surface energy: powders do not chemically bond/co-vulcanize into new rubber compounds because there are insufficient active chemical groups on the surface of the powder to create chemical crosslinks (covalent bonds)

The net effect of these limiting factors is that the addition of small percentages (~2 – 6 wt.%) of MRPs into new rubber compounds yields unacceptably low mechanical reinforcement (particularly with regards to tear strength and abrasion resistance). Additionally, the dynamic performance of compounds containing MRP is compromised, manifesting in an increased tan delta value and Payne effect, leading to an increase in hysteresis, heat build-up and rolling resistance of the tire. This reduces the fuel efficiency of a tyre.

It was speculated that reactivation of the vulcanization potential of the MRP by chemical treatment would significantly improve both the mechanical and dynamic performance of MRPs in rubber compounds in a similar way that silica fillers used in tyre manufacturing are activated by organosilanes, thereby opening the door to large scale re-use within the tyre and TRG sectors. Put another way, silica is used as a reinforcing filler in tyres with global volumes of around two million tons. Without surface activation by organosilane it would not be possible to use silica in tyres in significant volumes in much the same way as MRP cannot be used in large scale volumes in tyres whereas FMRP can for precisely the same technical reasons.

The “true” principle of “functionalization” is to form a high density of chemical crosslinks between the surface of the vulcanized MRP and the unvulcanized rubber compound during vulcanization. As such the rubber powder no longer presents as “discreet and disruptive” particles within the new matrix, instead it becomes a chemically bonded and integral part of a much more homogenous and uniform matrix. This dramatically improves its in- rubber performance and facilitates the possibility of adding much higher concentrations than would be otherwise possible with non-functionalized MRP. In turn this leads to greater cost reduction potential.

The global micronized rubber powder industry is segmented into tyres, technical rubber goods (TRG), thermoplastic vulcanizates (TPVs) and asphalt applications. Essentially FMRP finds potential application in many different rubber polymers, particularly in natural rubber, styrene butadiene rubber (SBR) and polybutadiene rubber (BR) applications (e.g. tyres, conveyor belts, moulded and extruded products) but can also find use in virtually any other kind of rubber polymer that is vulcanized by regular sulphur cure systems (e.g. EPDM, NBR, HIIR)

FMRP can therefore be used in widespread applications where properties such as crack suppression (flex fatigue), tear strength, modulus (stiffness), tensile strength and visco-elastic properties such as heat build-up and hysteresis are important. Investigations are currently ongoing to determine the effects of FMRP in asphalt as it is anticipated that its addition will improve the interfacial adhesion with the asphalt, thereby improving road performance such as crack resistance over a wider range of temperature fluctuations, as well as leading to sound reduction (already demonstrated with untreated MRP). Additionally, FMRP provides significant cost savings without increasing the weight of the new product (due to its low density) and provides an excellent sustainable and circular solution by allowing significant concentrations of waste tyres to be added into new tyres whilst potentially improving tyre performance.