NR elastomer composite: Process, material and performance characteristics.

The standard method for compounding elastomers, referred to as dry mixing, is to combine dry, baled solid rubber, carbon black (CB) and other ingredients in an internal mixer. The internal mixer serves to incorporate and disperse the CB and other ingredients. However, there is a limit to the dispersion that can be achieved without degradation of the polymer, which can limit the use and performance of reinforcing fillers.

The latex master-batch method is an alternative to the conventional dry-mixing method. In this case, a CB slurry is exposed to intense milling to break up CB agglomerates. The milled slurry is then combined with the elastomer latex, mixed and then acid coagulated into a solid wet crumb. The wet crumb is then dried and formed into bales of CB/rubber masterbatch (MB). The tire and industrial product manufacturers can incorporate additional ingredients such as antioxidants, curatives, etc., into the MB in a shortened dry mix cycle.

The primary advantages of latex-carbon black wet mixing include increased overall efficiency of CB mixing measured in terms of reduced power consumption/lb., of compound and increased throughput. In addition, significantly better dispersion is achieved, particularly for high surface area/low structure carbon black, which is difficult to disperse. The primary reason for this is that a large portion of the CB incorporation and dispersion occurs while the CB and rubber are both in the fluid phase.

For natural rubber, the situation is quite different. Whereas dry mixed NR masterbatch is an industry standard, wet masterbatch methods have never been successfully commercialized. NR lattices are much less stable than synthetic lattices due to potential premature coagulation and therefore come with considerable handling difficulties. This same characteristic means that it is also difficult to achieve the desired level of dispersion of CB into the latex prior to the onset of coagulation.

In general, NR latex can be coagulated by three different methods or their combinations: Chemical, mechanical and thermal. Of these, acid coagulation, with or without electrolytes, is the most prevalent. Acid (pH 4 to 5) and/or electrolytes destabilize the surface layer of the latex particle, which is comprised of proteins and lipids. Mechanical methods to destabilize the latex particles are based on shear force, which must be high enough to destabilize the protein/lipid protective layer on the latex particle.

Excellent control of mixing and coagulation is a core requirement for the successful development of a NR latex based masterbatching process. This article describes an innovative and controllable mixing approach for latex based natural and synthetic rubber.

Elastomer composite process

A new process using a mechanical approach to coagulation has been developed (refs. 1 and 2). This new process can continuously produce emulsion based elastomer composites (EC) of similar composition to conventional masterbatch. While this new process can be equally applied to natural rubber and synthetic lattices, current commercialization efforts focus on natural rubber.

The process is conceptually described in figure 1. It is a continuous process that simultaneously feeds an elastomer latex and a CB slurry stream to the mixing zone of a proprietary designed coagulum reactor. The elastomer latex may be either natural or synthetic, and the particle slurry can be comprised of carbon black or other particulate fillers. Under high speed and turbulent conditions, the CB/latex mixture completely coagulates prior to discharge. The wet coagulated mixture is dewatered, dried and then masticated.

[Figure 1 ILLUSTRATION OMITTED]

This proprietary technology represents a breakthrough versus conventional dry mixing of NR/CB compounds. It is the first true continuous liquid phase mixing/coagulation technology successfully applied to a NR latex system. This new process produces an elastomer composite with superior dispersive and distributive filler dispersion (figure 2).

[Figure 2 ILLUSTRATION OMITTED]

In the conventional dry mixing of carbon black into natural rubber, with increasing mixing time the quality of dispersion of the carbon black will improve, but not without breakdown of the compound and degradation of two key measures of compound performance: Its molecular weight (MW) and polymer-filler interaction as indicated by bound rubber. While the importance of these may vary depending on the specific application, for tread compounds, tread life (wear and cut/chip) and fuel efficiency performance are believed to improve with higher molecular weight, bound rubber and dispersion. As subsequently described, each of these is attainable through the EC technology.

This new process offers the following advantages:

* Dramatic improvement in mixing plant productivity;

* increased grade flexibility in mixing plants and minimized CB handling;

* improved cleanliness, consistency, quality and dimensional control;

* deferred capital investment in mixing;

* simplified supply chain from the rubber plantation to the end user;

* minimized use of hot rooms; and

* capability to effectively disperse and therefore use a wider range of CBs, including CBs with high surface area/low structure.

In addition, this new technology fits with the new modular tire making technology now being commercialized. Notwithstanding, growth in masterbatch is also being driven by environmental issues growing around the use and handling of carbon black.

The next step beyond masterbatch is the radical modification to the compound supply chain via fit-for-use compounds. These possibilities are within the capabilities of materials produced using this process.

EC compound characteristics

Macro-dispersion

A series of NR latex based elastomer composites was made with carbon black with morphological properties (structure and surface area) of regions 1, 2 and 3, as shown in figure 3. In order to demonstrate the benefits of this process, both macro-dispersion quality and molecular weight of the sol portion of the latex based masterbatch were compared to conventional dry-mix.

[Figure 3 ILLUSTRATION OMITTED]

The macro-dispersion quality was characterized by using a computerized imaging analysis system (refs. 3 and 4). The % undispersed area is a parameter used to quantify the macro-dispersion quality, as shown in figures 4 and 5. The sol portion MW was measured by GPC after extraction.

[Figures 4-5 ILLUSTRATION OMITTED]

In general, carbon blacks with morphological properties in region 1 are most difficult to disperse using conventional dry mixing methods. As shown in figure 4, the "% undispersed area" of those blacks in masterbatch made from dry mixing methods is above 1% over a wide range of MW. These results were generated from different mixing techniques. On the other hand, the elastomer composite dispersion quality falls below 0.3%. Furthermore, the dispersion quality is independent of molecular weight. Carbon blacks having morphological properties in region 2 are relatively easy to disperse with dry mixing techniques compared with carbon blacks in region 1.

As shown in figure 5, the macro-dispersion quality can reach 0.4% undispersed area, but the MW sol is lower than 0.4 x [10.sup.6] due to the MW breakdown during dry mixing. The carbon blacks in region 2 in general have high structure, which results in high compound viscosity during mixing. The mastication is much more severe due to high shear force (and high temperature) generated from high viscosity. When the MW sol of dry masterbatch is between 0.35 x [10.sup.6] and 1.1 x [10.sup.6], the dispersion quality suffers. In contrast, the dispersion quality of the latex based elastomer composite retains a MW sol in a range that remains excellent. Elastomer composites made with carbon blacks in region 2 can preserve high MW and offer superior macro-dispersion quality. This also is valid for region 3. In summary, this new latex based mixing process can produce composites with superior macro-dispersion quality and preserve the molecular weight of the polymer. Furthermore, the carbon black dispersion quality does not depend on the carbon black morphological properties.

Micro-dispersion

The micro-dispersion quality of rubber compounds has been characterized by TEM. According to the electron micrographs, figures 6 and 7, the latex based composite shows more uniform distribution of CB aggregates in the rubber matrix than does the dry mixed masterbatch.

[Figures 6-7 ILLUSTRATION OMITTED]

The superior micro-dispersion quality has also been demonstrated indirectly by examining the "Payne effect" as measured by dynamic strain sweep. The Payne effect is closely related to filler-filler interaction. A large Payne effect indicates poorer micro-dispersion. As shown in figure 8, the elastomer composite shows much lower dynamic modulus (G') at low strain; therefore, a smaller Payne effect, compared with dry mixed material. As described in Wang, et al (ref. 5), this is an important influence on properties that must be managed during processing to give the desired levels of performance.

[Figure 8 ILLUSTRATION OMITTED]

Molecular weight

In addition to superior dispersion quality, high molecular weight is also a unique characteristic of the elastomer composite. The typical weight average MW of NR latex is about 1.0 x [10.sup.6] to 2.5 x [10.sup.6], whereas the MW of the latex-based elastomer composite falls to 0.4 x [10.sup.6], as shown in figures 4, 5 and 9.

[Figure 9 ILLUSTRATION OMITTED]

Bound rubber

One of the key measures of compound performance is believed to be bound rubber. The amount of bound rubber is only an indicator of the degree of interaction between the filler and polymer chain. The nature of the interaction can be physical and/or chemical, depending on filler surface activity and chemical functionality, and the chemical composition and functionality of the elastomer (ref. 9). According to classical bound rubber theory, the high MW should produce high bound rubber (refs. 7 and 8). This can be attributed to the fact that no significant polymer breakdown occurs even under high mechanical shear.

The bound rubber of the elastomer composite is higher than that of the dry-mix counterpart measured at 50 [degrees] C in toluene. However, the amount of bound rubber will depend on the type of solvent used, the time of extraction, and the temperature. The key drawbacks of this extraction method are that it cannot differentiate bound rubber and rubber gel particles. Also, it cannot reveal the detailed structure of the bound rubber. More recently, researchers have been using NMR techniques to investigate the detailed structure of bound rubber (refs. 10-15). Future work on these latex based elastomer composites is focused on the mechanism of bound rubber formation in the elastomer composite process and the bound rubber structure/strength.

EC mixing characteristics

In theory, better compound performance is expected from superior dispersion, high bound rubber and high MW sol. However, these result in a high compound Mooney viscosity which is not favorable to downstream processing, such as mixing and extrusion.

The compound that results from the EC process has unique processing characteristics. Compounders must take this into account in downstream processing. Although the mechanism of this effect is not known with certainty yet (mechanical versus thermal oxidation), past experience indicates that mastication of the EC based elastomer composite is much more efficient than the dry-mixed counterpart under the similar conditions (ref. 16).

Mixing plant throughput

In the typical NR mixing facility, most of the mixing resources are focused on two operations: Incorporation and dispersion of carbon black, and regulation of Mooney viscosity. Since EC material contains the carbon black predispersed, and due to the above noted efficiency in viscosity reduction, much of the time and energy expended in these operations is no longer needed. As a result, mixing plant throughput can be greatly improved. Table 1 illustrates this improvement on the laboratory scale.

Table 1 - mixing plant throughout using elastomer composite

                                  Dry-mix

                        Batch wt. (gr)   Cycle time
                           (grams)         (min.)

Sta 1/premass               1,150
Black incorporation         1,335           3.5
Final                       1,335           5.0
                                            1.5
Final compound (gr/h)       8,010
% original throughput           -

                                     EC-1

                        Batch wt. (gr)   Cycle time
                           (grams)         (min.)

Sta 1/premass               1,468           3.5
Black incorporation                         n/a
Final                       1,468           1.5
Final compound (gr/h)       17,616
% original throughput          220

                                    EC-2

                        Batch wL (gr)   Cycle time
                           (grams)        (min.)

Sta 1/premass                              n/a
Black incorporation                        n/a
Final                       1,468          1.5
Final compound (gr/h)      58,720
% original throughput         733

A typical natural rubber mixing cycle consists of a premastication stage, lasting approximately 3.5 minutes, a second five minute stage in which additional ingredients are added, and a 1.5 minute final stage, in which curatives are added (ref. 17). Based on a typical 75% fill factor, a "00C" size internal mixer is capable of mixing six final batches per hour, a throughput rate of approximately 8,000 grams of finished natural rubber tread compound. Using elastomer composite material of exactly the same formulation, a 3.5 minute first stage is required to incorporate activators and other ingredients, and the same 1.5 minute final stage is used to incorporate curatives. Since the carbon black is already dispersed in the EC material, the batch may be mixed with approximately 10% increased batch size without dispersion effects, therefore the same internal mixer is capable of mixing approximately 17,000 grams per hour, more than 200% of the original throughput.

Experimental elastomer composite materials have been produced which do not require the 3.5 minute ingredient incorporation stage. Only the 1.5 minute curative incorporation stage is needed. With such a material, the mixing throughput per hour is 40 batches or just under 58,000 grams per hour, 733% of the original throughput.

This set of experiments has been verified on production-sized equipment, with much the same results. The obvious benefits in mixing throughput are of significant economic impact to rubber product manufacturers who are willing to use this technology.

The authors believe that the logical extension of this technology is the ability to process this material without using current internal mixing technology, possibly using a continuous mixing operation, with even greater efficiency and throughput.

EC performance characteristics

A mapping study to compare the compound performances between EC and dry-mix counterparts was conducted with carbon blacks with a wide range of morphological properties, as shown in table 2. The test compound is based on the NR truck tread formulation presented in table 3.

Table 2 - carbon black morphological properties (CTAB AND CBCP) and loading of carbon blacks selected in mapping study

Recipe: NR/CB/oil = 100/variable/5

                    CTAB          CDBP      Loading
CB type         ([m.sup.2]/g)   (cc/100g)    (phr)

N234                 119           100        45-65
Regal 660            110            60        50-90
N330                  83            89        60-80
N326                  83            69        65-85
Sterling 6740         54            85        65-90
Regal 250             50            50        90-110
Table 3 - NR truck tread formulation

Masterbatch for EC and dry-mix
Ingredient                       Phr        Example
NR                               100
Carbon black                     Variable   50
Sundex 790                       5
Antioxidant                      0.7
Total                                       155.7
                                            (Masterbatch)

Final mix for EC and dry-mix
Masterbatch                      Variable   155.7
AZO 66                           4.0
Hysteree 5016                    2.0
Santoflex 13                     2.0
Sunproof Improved                2.0
Wingstay 100                     1.0
Santocure NS                     1.8
Sulfur                           1.0
Total                                       169.5

Flexing fatigue life

The undispersed carbon black (poor macro-dispersion) in rubber compounds can act as defects, which can initiate cracks and promote crack growth. This eventually leads to fatigue failure. A rotary-type of flexing fatigue tester was used to evaluate crack growth rate (CGR) of NR compounds (ref. 21). As shown in figure 10, the crack growth rate of dry-mix compounds is reduced as the MW sol increases. This suggests that a higher MW favors crack growth resistance. Furthermore, the crack growth rate can be further reduced through improving the dispersion quality, as shown to dry mixed compounds at the same MW sol. As shown in figure 11, crack growth reduction is much more pronounced for the carbon blacks in region I (N326, Regal 660 and Black Pearls 800), which is the region that EC compounds demonstrate the most significant improvement in macro-dispersion quality over dry-mix compounds, as previously discussed.

[Figures 10-11 ILLUSTRATION OMITTED]

Hysteresis

The origin of filler contribution to compound hysteresis is from filler-filler interaction. In the rubber industry, either the tangent delta (Tan D) at 60 [degrees] C and/or rebound at room temperature have been commonly used to measure the compound hysteresis properties. Both have a strong correlation to the rolling resistance of tires. It has also been reported that rebound has a linear relationship with CTAB [multiplied by] [??.sup.2][Rho] over a wide range of CTAB, CDBP and loading of reinforcing carbon blacks in SBR compounds (ref. 18). In this study on NR latex based composites, the same behavior has been observed. The rebound decreases with increasing CTAB [multiplied] [??.sup.2]. As shown in figure 12, the rebound of EC compounds in general were four to five points higher than the rebound of dry mix counterparts under the same CTAB[??.sup.2].

[Figure 12 ILLUSTRATION OMITTED]

Abrasion wear

Although wear resistance is one of the most important tire performance requirements, the tire wear mechanisms for natural rubber are still not well understood. Mechanisms such as mechano-thermal degradation and fatigue fracture (roll formation and delamination) have been widely discussed (refs. 19-22). However, from a material point of view, it is believed that several factors can have an impact on wear resistance of NR compounds, such as filler type, polymer type, polymer-filler interaction and filler-filler interaction.

For the filler, the filler morphology, surface activity, loading and dispersion quality are critical to the wear performance (refs. 23-25). The superior dispersion quality of the EC based compounds should therefore improve wear resistance. On the other hand, unlike dispersion quality, the impact of MW on wear resistance is not clear. It is likely that there is an optimum MW for wear resistance, if fatigue fracture is one of the mechanisms. More study in this area is needed.

Although lab results are encouraging, the actual wear performance is being validated in road tests, which is not a trivial task since many other factors affect tire performance.

Summary

Using a mechanical shear approach, a true continuous process has been developed to produce emulsion based elastomer composites with superior filler dispersion quality at macro- and micro- levels with high molecular weight. This proprietary technology is a breakthrough versus conventional dry mixing, with superior dispersion to synthetic wet masterbatch technology. It is the first true continuous liquid phase mixing/coagulation technology successfully applied to a natural rubber latex system without adding acid/electrolyte. Since the carbon black dispersion quality of EC product does not depend on the carbon black morphological properties, it provides the potential to use carbon blacks even with low structure, which are not used in rubber due to very poor dispersibility.

In addition, higher bound rubber is also a unique material characteristic of EC products. Bound rubber formation in EC process has been discussed. With a high MW and superior carbon black dispersion, EC products can have higher bound rubber than in dry-mixed compounds.

The inherently different molecular weight and bound rubber structure in EC leads to faster mixing cycles and improved mixing plant efficiency. Based on observations of laboratory and commercial scale mixing processes, mixing throughput compared to typical dry mix formulations can be at least doubled through use of elastomer composite material, and experimental materials promise to give even greater improvements. Theoretically, improved compound performance is expected from superior dispersion, high bound rubber and high molecular weight. Mixing and downstream processing conditions must be monitored to ensure optimal compound performance and maintain plant processability.

The NR compound performance of EC and similarly compounded dry-mix counterparts were compared. Laboratory results indicate that EC compounds may demonstrate performance advantages over dry-mix counterparts in many aspects such as lower hysteresis, better fatigue life and improved abrasion wear as a result of its unique characteristics of dispersion, bound rubber formation and molecular weight. Actual tire performance must be verified in actual road tests under various operating conditions.

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