CHARACTERIZATION OF MICRONIZED RUBBER POWDERS WITH COST EFFECTIVE PERFORMANCE BENEFITS IN RUBBER COMPOUNDS

Tom Rosenmayer, Ravi Ayyer, Frank Papp Lehigh Technologies Inc, Tucker, GA 30084

Abstract similarities and differences between the cryogenic and ambient grind particles. Micronized Rubber Powder (MRP) is classified as dry, powdered elastomeric compound in which a The second objectives is to attempt to correlate the results significant proportion of particles are less than 100 of the characterization work to the processing and microns. It is used as a compound extender to offset the properties of rubber compounds made with MRP. spiraling prices of and synthetic polymers. Previous work [6, 7, 8] in MRP-containing rubber It improved the sustainability and in some case the composites indicated a dependence on both processing performance of the compounds in which it is used. MRP and properties; in this work we attempt to relate those is typically made from cured elastomer feedstock, properties back to a fundamental analysis of the MRP typically chips from end-of-life , via a cryogenic surface. process at a temperature below the Tg of the polymer. A better understanding of MRP surface properties is needed to facilitate efforts to utilize the material as a high value, Materials sustainable material for use in various industrial and consumer rubber products. An update on MRP MicroDyne™ is a cryogenically ground micronized characterization is presented, including surface rubber powder (MRP) produced by Lehigh Technologies morphology by SEM, surface chemistry by XPS, surface Inc. It is a free flowing, black rubber powder produced area by Kr BET, and particle size distribution by laser from end-of-life tires that easily disperses into a multitude diffraction. An example is given that demonstrates how of systems and applications (e.g., roof coatings, adhesives, the surface chemistry can be used to explain the effect of asphalt, resins, sealants, etc.). Several products MRP on cure and physical properties in rubber were used for characterization in this study. For compounds. comparison, ambient ground derived rubber powders were also included in the study. Introduction Material Symbol Description The objectives of this paper are twofold. The first objective involves characterizing the micronized rubber MicroDyne™ 400 µm < 10% MD400 powders (MRPs) using various physical and chemical MRP (-400 µm) > 90% techniques that is essential to understanding its physical 177 µm < 10% and chemical structure and morphology. It enables its cost MicroDyne™ MD180 (105-177 µm) ~ 60 % effective application and further optimization in various MRP markets such as high performance tires & rubber goods, (-105 µm) > 30% paints/coatings, asphalts and . The MRPs are 105 µm < 10% MicroDyne™ produced using a cryogenic turbo mill technology from MD105 (75 -105 µm) ~ 60 % MRP 400 µm down to 50 µm size range. A majority of earlier (-75 µm) > 30% investigations [1,2,3] used large size (180 – 400 µm) MicroDyne™ 50 µm < 10% MD50 ambient, ambient-wet or thermally regenerated ground tire MRP (-50 µm) > 90% rubber (GTR) powders in plastics. The commercial 400 µm ~ 40% ambient processes can routinely produce 400 µm Ambient Rubber Amb400 250 µm ~ 40% particles, however at particle sizes less than 100 µm throughputs are low. To understand the effects of grinding process on the rubber particles, an ambient sample with Description of rubber compounds 400 µm nominal size is also included in the study. . Previous research suggested that ambient grind particles Experimental have a rougher texture and higher surface area compared to cryogenic grind particles at same nominal size [4, 5]. Thermo-gravimetric analysis (TGA) – TGA on all the The present study shows deeper insight into the rubber powder samples was performed to evaluate the compositions of the constituents in the ground rubber. the samples. The weight loss around 300 °C is considered Rubber powders typically constitute process oils, rubber to be due to low molecular weight additives and process hydrocarbons, carbon black, and inorganic fillers. The oil in the rubber sample. The polymer undergoes thermal experiments were conducted using Perkin Elmer Pyris-1 degradation resulting in weight loss of approximately 55 TGA analyzer. A typical cycle consists of heating up from % up to 530 °C. The step loss seen on the graph at around 30 °C to 530 °C under nitrogen blanket at 10 °C/min. The 600 °C was attributed to the carbon black in the rubber cycle is held for 10 min at 530 °C. The gas is switched sample. From the Table, it was consistently in the range of from nitrogen to oxygen. The sample is then further 30 %. heated up to 850 °C at 10 °C/min. This method enables quantification of all the constituents in a single TGA cycle.

Scanning Electron Microscopy (SEM) – The morphology of the samples was observed using a Zeiss scanning electron microscope. The samples were gold coated to avoid charging.

Sieve Analysis – The sieve analysis of all samples was done using TYLER equivalent meshes with a portable ROTAP sieve shaker per ASTM D-5603-01. Typically about 100 gm of the sample was placed in the ROTAP Figure 1. Thermo-gravimetric charts showing the weight and shaken for 10 min. The weight of the particles on loss of MRP samples with temperature each screen was measured and percentage of weight retained on the screen was determined to obtain the The remaining residue was due to the inorganic fillers particle size distribution. such as silica in the sample. All the samples showed approximately 5.5 to 6 % of ash content except the MD50 sample. The weight loss for MD50 is pushed upward by Surface area measurement – The surface area of the amount of additional ash content compared to the rest of samples was determined using multipoint Krypton ‘BET’ the samples. This was attributed to the processing aid adsorption isotherm method. The experiments were added during ROTAP analysis for avoiding agglomeration conducted on Qunatachrome’s Autosorb-1. It was of the rubber particles. The size range of the processing reproduced three times and an average value was taken for aid is below 10 µm, which allowed it to pass through all interpretation. the larger size screens. TGA results of MD400 and Amb400 samples Electron spectroscopy for chemical analysis (XPS/ESCA) were about the same. – XPS spectra were obtained by using a Thermo-K-Alpha spectrometer. The X-ray was the monochromated Al K- Table II. Thermo-gravimetric summary table of GTR and alpha line (1486 eV). The analyzer pass energy was MRP . typically 50 eV with 200 watts X-ray power (spot size Polymer Carbon -7 Extractables Ash 200 µm). The vacuum was recorded as 1 x 10 mbar. The Sample RHC Black % % charge neutralization was active during the acquisition of % % data. The samples were used as produced without any pre- treatment. All the experiments were done at room MD400 7.2 57.7 29.9 5.2 temperature. The binding energy assignments for MD180 7.5 56.8 30.0 5.6 particular peaks were made using known literature. MD105 7.7 55.3 30.1 6.6 MD50 5.6 52.1 29.6 12.4 Amb400 6.4 57.3 30.5 5.8

I - Characterization of Rubber Powders Particle Size Distributions

Thermo-gravimetric analysis The particle size distribution of the powders prepared by ambient and cryo grinding was examined in their “as The thermo-gravimetric charts representing percent produced” form. Figure 2 (a) compares the size weight loss with temperature were shown in Figure 1. distributions of rubber samples having nominal particle Table II represents the breakdown of the constituents in size of 400 microns (40mesh). Clearly the distributions depend on the production plant’s mode of grinding and on GTR/PP composites. A systematic investigation has the sieving technique. It was observed that the been made of the effect of grinding process on the surface cryogenically ground powder had larger amount of fine area of the particles produced. All the samples were as particles than the corresponding size ambient grind produced commercially, the distribution curves of which powder. In the cryogenic method, freezing the rubber were shown in Figure 2. The surface area of all the GTR below its embrittlement point followed by fracturing in a samples was determined using multipoint Krypton BET high speed turbo mill evidently produces larger amount of adsorption isotherm. The experiments were repeated to fines. It suggested that the cryogenic distributions should obtain consistent results. Previously it has been reported have higher surface area as compared to the ambient that the ambient-ground rubber had somewhat higher distributions of the same nominal mesh size. surface area than the cryo-ground rubber for the same Figure 2 (b) compares GTR samples produced by nominal particle size [9]. It was thought to be a reflection cryogenic method. The cryogenic turbo mill technology of the porous or rough morphology of ambient ground produces fine rubber particles below 50 µm. The chart materials. In some instances the higher surface area was compares particle size distributions for nominal 400 µm believed to be due to higher fiber content in the ambient (40mesh), 180 µm (80 mesh), 105 µm (140 mesh) and 50 samples. µm (300 mesh) size particles. For all the powder samples, Figure 3 shows comparison of cryogenic grind large amount of fines was observed relative to their and ambient grind particles. For reference a model based nominal mesh sizes. The distribution curve was found to on spherical particles was also included. Both ambient and depend on many processing variables that included cryogenic particles nitrogen ratio, milling rpm, mill gap, and sieving technique. It was seen that by adjusting the parameters, the particle size distribution can be controlled as needed. The size distribution data essentially suggested very high surface area particles at 50 µm as compared to 400 µm GTR.

Figue 3. Comparison of surface areas of rubber particles produced by cryognic and ambient method. The black line represents a model for spherical particles.

showed high surface areas relative to the spherical particle model. The cryogenic particles showed two orders of magnitude increase in surface area from Cr400 to Cr50. It was observed that the cryogenic particles had higher surface area than the ambient particles at all the nominal particle sizes. This was contrary to previous observations and arguments about cryogenic rubber powders [1,9]. Comparing ambient and cryo-grind powders, the surface area of cryogenic powders increased by a factor of 1.2X at 400 microns to about 2 to 2.2X for 180 and 200 micron particles respectively, Table III. The observations correlate with higher content of fines in the cryogenic Figure 2. Comparing particle size distributions charts of powders. (a) ambient and cryogenic GTR (b) cryogenic GTR of Table III. Effect of grinding process on the GTR surface large and small nominal size particles area 2 Sample Mesh Micron Surface area (m /g)

MD400 40 400 0.066 Surface area measurements MD180 80 177 0.482 This study was focused on understanding the effects of MD105 140 105 0.58 grinding process and of the surface areas of the particles MD75 200 75 2.47 MD50 300 48 4.41 Amb400 40 400 0.0563

Morphology of Rubber Powders

The surface morphology of the rubber particles is expected to depend on method of production. The size, shape and morphological features on particle surface can affect the properties of the polymer/GTR composites. The Figure 4. Scanning electron micrographs of MD400 and differences in surfaces of cryogenic and ambient grind Amb400 samples at low and high magnifications. rubber particles were studied by scanning electron microscopy. Chemical analysis of the Micronized Rubber Powders

The effect of grinding process on particle morphology is It is known that polymer- interfacial adhesion is a compared in Figure 4. For simplicity, as produced rubber key factor in improving the composites mechanical samples of 400 µm nominal size are shown. Figures 4 (a) properties. Very little is known about surface chemistry to (c) show surfaces at low and high magnifications for and morphology of GTR particles. X-ray photoelectron MD400; while (e) to (f) represent corresponding surfaces spectroscopy (XPS) of the rubber particles was performed for Amb400 rubber. Figure 4(a) showed distribution of for determining the surface elemental compositions. The cryo particles with size scales in the range of 200 to 400 study investigated for presence of any oxidized species µm. Larger ambient particles in the size scale of 400 µm and/or unsaturation that can potentially react with the were observed in Figure (d). The cryogenic particles and/or compatibilizers. largely demonstrated smooth surface morphology. The ambient particles showed two distinct morphologies; in Figure 5 compares the XPS spectra of MD400 and first the surface was covered in porous, rough nodules and Amb400 rubber samples. The major peaks observed in in another the surface was smooth. Previously it was both the spectra were attributed to following elements. reported that the ambient particles exclusively form Si peaks – at 103 and 153 eV porous, rough texture [4,10]. However our observations of C1s peak – 285 eV amb400 and other ambient and wet-ambient grind samples O1s peak – 532 eV indicated that it consisted of both types of morphology, Zn peaks – 1022 and 1045 eV along with presence of fibers. From Figure 4 (d) the composition of the two textures appeared to be in equal proportions. Figure (e) shows texture of one of the smooth ambient particles; it appears very similar to that of corresponding cryogenic particle image in Figure 5 (b). Further magnification of about 50KX showed that the details on the surface of both ambient and cryogenic smooth particles look about the same. The morphology of the rough ambient particles was comparable with that reported in the literature [4, 10].

It was claimed before that ambient grinding gives higher surface area attributing to the rough texture as compared to cryogenic grinding [4]. Additionally it was suggested that the rougher texture may nullify the effect of more fines in cryogenic powders [9]. As observed previously in Table III, the surface areas of cryogenic particles were consistently higher the ambient grind particles. Also we Figure 5. Comparison of XPS spectra of MD400 and observed that not all particles are rough in ambient grind Amb400 rubber particle as claimed; but it consisted of significant amount of smooth particles as well. The XPS of other cryogenic particles revealed similar spectra. The relative intensities of different atoms present on the rubber surface are summarized in Table IV. Ambient particles showed presence of somewhat more oxygen and less silica as compared to cryogenic particles.

Table IV. Chemical Analysis of the Rubber powders using XPS

Interestingly, no sulphur was observed on the surface. Previous researchers noted presence about 0.34-0.66 atomic wt. % sulphur on ambient GTR particles [4].

II – Processing of Rubber Compounds with MRP Figure 6. DOE response curves for constant T10 cure times for various MRP and BBTS accelerator levels. A designed experiment was conducted on a standard SBR/BR tread compound to understand the effects of MRP addition. In the design, MRP content was varied from 2.0 to 14.0 percent, accelerator (BBTS) was varied from 0.2 to 1.4 percent, and sulfur content was varied from 1.5 to 4.0 percent. The processing was a standard two-pass mix, with the MRP added in the master mix along with the carbon black. Tensile plaques were cured at 160C for 20 minutes. Cure times were measured @ 160 C on an Alpha Technologies’ Rheometer MDR 2000 per ASTM D5289. Tensile properties were measured per ASTM D412.

A sample of the process dependence is shown in Figure 6. MRP content is on the x-axis and BBTS accelerator content on the y-axis. The iso-lines shown are the cure times (T10 – time to 10% cure) in minutes. It is clear that increasing the MRP content at constant accelerator levels results in reduced cure times. Alternatively, constant cure rates can be achieved with increasing MRP content by reducing the amount of accelerator added. This effect is consistent with the XPS results that indicate the presence of accelerator compounds (N) on the MRP surface.

Figure 7. DOE response curves for constant 300% The property dependence is shown in Figure 7. MRP modulus for various sulfur and MRP levels. content is on the x-axis and sulfur content on the y-axis.

In this case the iso-lines shown are for the modulus (300% modulus) in MPa. In this case, increasing the MRP Conclusions content at constant sulfur has the effect of depressing modulus. Alternatively, increasing the S content with Micronized rubber powder (MRP) is a valuable resource increasing MRP content can be done to maintain modulus. for improving environmental quality, reducing These results are consistent with the XPS results that did dependence on imported raw materials, and hedging not indicate the presence of sulfur on the MRP surface. volatile commodities costs. Its successful usage depends on a proper understanding of the surface science of MRP and how that affects the processing and properties of the materials in which it is used. The MRP materials in this study have a cost of approximately half that of prevailing virgin rubber compound prices. Thus proper understanding and use of these materials can result in significant economic and environmental benefit.

Acknowledgements

The support of the National Science Foundation for the characterization portion of this project is gratefully appreciated.

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