Powder rubber - a new raw material generation for simplifying production.

For some years, almost all renowned tire manufacturers have actively pursued the development of novel processes for a simplified and, of course, more cost-efficient production of tires (refs. 4-6). These new processes - as far as they have been made known and published - as a rule turn away from today's discontinuously working machines which have remained almost unchanged for many years, tending to smaller, more flexible units and emphasize continuous production processes.

Successful implementation of these new processes accordingly still in the development phase certainly depends also on the development and introduction of new technologies for compound production.

The main reasons for adhering to the above mentioned batch process up to now are for the delivery of the rubber in bale form and for the high-energy and time consuming process of incorporating large quantities of reinforcing fillers into the polymer phase and their dispersion necessary for the later in-rubber properties. The heavy inflexible compounding units used today, such as internal mixers and rolls, were adequately adapted and optimized to this raw material form and processing technology over the years; now they even require them.

The development and introduction of new, even continuous, compounding processes (refs. 1 and 7), as they have been state of the art in the plastic industry for many years, however, require the availability of free-flowing rubber/filler batches in powder form, according to the concurring opinion of the experts (ref. 8).

One of the most promising approaches to moving a considerable step forward in this direction had already been the development of rubber/carbon black batches in powder form in the 1970s and 1980s (refs. 9 and 10). This technology, indeed considered as innovative at that time, could then, however, not find general acceptance. The reasons for this were both technical problems and the prohibitive product costs. What was more decisive, however - and history has taught us this again and again - is perhaps the fact that this idea was too advanced for its time.

Today it is the increasing interest of the tire industry in new technologies for the manufacture of tires. The realization that new compounding technologies have to be developed was evident two years ago. The idea of powder rubber technology is receiving new attention and this idea is to be adapted to today's requirements on raw material selection and to the available processing technologies (ref. 1). This article outlines the present status of development.

Objectives

The main objective is to produce powdery free-flowing rubber/filler batches (powder rubber, PR) which allow the introduction of a continuous compounding process in the rubber industry in the medium- to long-term future. For this purpose, it is necessary to have available all essential rubber/filler batches used in the rubber industry - of course graduated according to priorities - and parallel to this to develop continuous compounding processes (figure 1). These investigations were carriad out at Deutsches Institut fur Kautschuktechnologie (DIK) in close cooperation with renowned machine constructing companies, separately presented and published (ref. 2).

[ILLUSTRATION OMITTED]

Rubber/filler batches in powder form certainly offer advantages in today's discontinuous production processes compared to rubber bale technology. These advantages may refer to both the processes before production of the compound proper (e.g., storage, automatic dosing and weighing of all compound ingredients) and the compounding process itself (e.g., reduction of the mixing time and mixing energy, homogeneity, dispersion and vulcanizate performance). Environmental aspects (e.g., polymer-bonded carbon black, reduction of dust emissions) should not be disregarded, either.

The work for powder technology started on the E-SBR/carbon black system about two years ago. Introductory papers have been presented (refs. 2, 3 and 11-13). Further investigations are still being carried out. They are, however, supplemented by the first results obtained on the NR/carbon black system. The development work on this system commenced about one year ago.

Approach

The article presents a comparison of the systems E-SBR/carbon black and NR/carbon black, as it is known today, as part of the powder rubber concept. Both the E-SBR/carbon black and NR/carbon black compounds are often blended with parts of BR rubber to achieve the required final product properties. Since BR powder rubber is at present not yet available, the addition of bale rubber is still necessary for the time being. In a separate chapter, it is described and discussed in how far the production of compounds (i.e., E-SBR/CB or NR/CB resp.) using powder rubber and bale rubber as a blend is possible and what effects this will have on the compound data. For all process investigations, the dispersion and the achieved vulcanizate performance are the standard for the success of the development.

Characteristics, processing and application

E-SBR/carbon black

With an annual consumption of about three million tons (ref. 14), E-SBR (solid rubber) is by far the largest synthetic rubber in production. The two most important types, E-SBR 1500 and E-SBR 1712, are mainly used for automobile tire treads and in truck tire retreading, usually in blends with butadiene rubber.

The fillers used in these applications are almost exclusively carbon blacks of types N234 and N339 (N375) at fill factors of about 50-80 phr. Almost all E-SBR/carbon black compounds are mixed with rather large quantities of softening oils; in the case of E-SBR 1712, the oil is already contained in the bale rubber with 37.5 phr.

E-SBR (BR)/carbon black compounds are in most cases mixed in two stages, less frequently in three stages. The overall mixing time is about 5-6 minutes. E-SBR/carbon black compounds are considered to be processible without any problems, and the output per mixer line is correspondingly high.

NR/carbon black

As a natural product, natural rubber, which is today usually produced by small farmers, necessarily exhibits larger product variations than a standardized synthetic rubber produced by chemical reaction (ref. 15). Because of its special vulcanizate properties, such as high strength values and excellent dynamic data, which are unique due to its chemical structure (crystallization tendency), this rubber is, in combination with carbon black, still indispensable, especially in the fabrication of truck tires. Here, mainly carbon black types N234 and N115 are used in the tread, in the tire carcass often N660 and in the adhesion compound N326 (automobile and truck). In the mechanical rubber goods (MRG) area, still other carbon blacks, such as types N375, N339 and N550 have to be considered.

NR compounds used in truck tires normally have fill factors of about 50 phr. Softening oils are not used or only in low concentrations.

Apart from silica-containing compounds, the natural rubber/carbon black system is one of the most demanding combinations in the rubber industry as regards the mixing expenditure. The number of mixing stages is larger, but also the mixing time is markedly longer than that for the E-SBR/carbon black system. In addition, a so-called mastication stage (ref. 16) precedes the compounding process proper, in which the initial viscosity of the natural rubber of about 90-110 Mooney points has to be reduced to values of about 70 points tolerable from the processing point of view.

NR/carbon black systems are often mixed in 3-4 stages including this mastication stage, the total mixing time is then about 10 minutes.

The description of both systems, as they are used today in the rubber industry as standard, serves thus for orientation and as specification for the development of the specific powder rubber systems. It establishes the benchmark both for the raw materials to be used and their employment in today's production process in the rubber industry. To overcome this benchmark, i.e., to present the advantages of the powder rubber compared with today's technology, would be of significance for the successful introduction of powder rubber in the present mixing rooms of the rubber industry - independent of the development of new future production processes.

Raw material concept

Rubber

In both systems, an aqueous latex emulsion is the starting material for the production of powder rubber. Treatment of the polymerizate, such as coagulation, mechanical water separation and thermal drying, as well as pressing of the rubber crumb into bales and their packaging, can be skipped. In the case of natural rubber, stabilization of the material by smoking is not necessary, either. This reduces the impact on the environment and the natural resources are conserved. While it is relatively easy in the case of the E-SBR system to obtain customized products (e.g., adjustment of viscosity, styrene content) from the polymer producer, the natural rubber latex system is much more complicated. Therefore, some remarks on this special raw material are to be made in this context.

NR types, impurities, stability

For the powder rubber production process, two basic variants are available. Field latex is the raw rubber latex obtained during tapping of the rubber tree which flows with a solid content of about 30% into the collecting cup provided for this purpose. As a natural product, this raw material contains rather large quantities of foreign matter consisting mainly of salts, resins and proteins, which may account for as much as 5% to 8% of the solid content.

Centrifuged latex is a field latex concentrated to about 60% solid content. In this process, large portions of impurities are removed; however, low amounts of rubber substances of high molecular weight are also lost. Determination of the molecular weight distribution of field and centrifuged latex by gel permeation chromatography (GPC) clearly shows these differences in chain length (figure 2).

[GRAPH OMITTED]

In the powder rubber technology, both latex types may be used so that the decision for one or the other product has to be made according to economic and transport aspects, as well as to issues of availability. Both products - whether field or centrifuged latex - have to be protected from the attack by bacteria. In the case of centrifuged latex, various methods proved successful in practice are known and also in use. High-ammonia types are exclusively stabilized with ammonium (about 0.8%), but require high amounts of precipitants in the precipitation process, whereas low-ammonia types are mixed with low amounts of [NH.sub.3] (~0.25%). For sufficient stability, further additives are, however, required, such as ZnO and TMTD, although in very low quantities.

Field latex is normally not stored for a prolonged period of time, but is either processed to bale rubber or centrifuged latex. Therefore, relatively little experience with long-time stability has been gained, when in addition, it is to be assumed that its storage life is shorter than that of the centrifuged latex due to its high content of impurities.

In intensive tests, the issue of stabilization and storage of field latex was therefore investigated. As a result of these tests, it is now possible to handle field latex without any problems and to store it for prolonged periods of time. The latex types used in the powder rubber process do not contain any N-nitrosamine-forming substances for stabilization, such as TMTD, and thus conform to the environmental requirements of the rubber industry.

Viscosity, molecular weight, molecular weight reduction

Compared with synthetic rubber types, natural rubber has a very high molecular weight (ref. 16) which is still increased during storage by intermolecular condensation reactions. The Mooney viscosities of about 90-110 Mooney points measured in the NR bale, therefore, often require a mechanical reduction before the filler addition proper. This reduction is camed out in the so-called mastication stage on the mill or in the internal mixer. Normally, Mooney viscosities of about 70 Mooney points are obtained in the process. This may be performed with or without using the so-called peptizers (ref. 17). In the first case, the time of the mastication process is shortened.

With the finished powder rubber, which already contains the total amount of fillers, such a mastication stage is not possible. However, since it is, among other things, the objective of the powder rubber technology to reduce the mixing times and number of mixing stages (refs. 1 and 3) and render continuous compounding processes possible, special attention is to be focused on the viscosity of the NR latex used in the powder rubber.

During mastication of the natural rubber bale, as carried out in the internal mixer or on the rolls, both mechanical and oxidative degradation processes take place (ref. 17). In the mechanical process, the chains are broken up by the shear stress produced during mixing. Thus, radicals are formed which have the tendency to recombine. This is, however, prevented to a large extent by its saturation with the stable biradical (air) oxygen.

Oxygen does, however, not only promote the mechanical break-up of the rubber, but is in turn the cause of the oxidative degradation which takes place in a complex radical chain reaction (refs. 17-19), which is not to be dealt with in detail in this context.

The mechanical degradation process is especially efficient at low temperatures (negative temperature coefficient) since the possibility of applying shear energy and using it for rupturing the chains decreases with rising temperature. The mechanical process prefers the rupture of the longer molecules on average more firmly bonded since the smaller ones "slip through" more easily. In contrast, the oxidative degradation as "normal" chemical reaction exhibits a positive temperature coefficient; this means that with rising temperature this process takes place at a faster rate. Since the place of bond breaking is only controlled by the impact probability, the oxidative degradation process is not selective in choosing the chains to be ruptured, but statistical. Such radical chain reactions proceeding oxidatively have always to depend on starter radicals which in turn are created by mechanical rupture of the bonds.

Already mentioned, the mechanical degradation process is not possible for the NR latex in the powdered rubber process. Thus, only the oxidative process can be used in this case, which is normally initiated at elevated temperatures by the addition of radical-forming agents.

Figure 3 shows the molecular weight distribution of a product masticated on a roll and a product decomposed chemically. In both cases, the Mooney viscosity is about 75 Mooney points.

[GRAPH OMITTED]

The lower selectivity of the chemical decomposition results in a wide molecular weight distribution and a larger number of short chains.

In the practice of rubber application (table 1), these differences in molecular weight distribution have a marked effect. The chemically degraded NR product proves inferior to the product broken up mechanically in the mixer, both as regards the strength values and the dynamic properties. This means that a purely oxidative decomposition process for adjustment of the Mooney viscosity of the natural rubber in the powder rubber does not come into consideration for application reasons.

Table 1 - comparison of the vulcanizate data of
a mechanically and a chemically decomposed
NR rubber filled with 50 phr N234 (for molecular
weight distribution, see figure 3)

                                   NR-bale/N234    NR/N234 PR
                                      50 phr         50 phr
                                   masticated on   decomposed
                                      a roll       chemically
                                      ML4~75         ML4~75

ML 1+4 (100 [degrees] C)   (MU)          63              63
Dmax-Dmin                  (dNm)      14.81           12.24
Tensile strength           (MPa)       24.6            18.0
Modulus 300%               (MPa)       12.2            10.5
Durometer (A)
  hardness                  (-)          66              64
Ball-rebound                (%)        59.7            58.2

With the latex material used in the PR process today, it has become possible to manufacture NR/carbon black products by using special low-viscous NR latices in combination with suitable peptizers and/or processing aids which result in low compound viscosities even with short mixing times. Despite the high molecular weight of about 800,000, the mastication process can be thus dispensed with. The higher molecular weight in turn is the basis for the later excellent in-rubber properties.

Carbon black fillers

Both in the E-SBR/carbon black system and NR/carbon black system, fluffy carbon black is the form of carbon black used in the PR process. After production in the furnace reactor, fluffy carbon black is obtained in finely divided voluminous form. In this form, however, it can not be processed in the rubber industry, but has to be pelletized in water by means of a pelletizing agent and subsequently dried again.

Under the same conditions, N234 fluffy carbon black, as used in the PR process, is markedly more finely divided than the wet-pelletized N234. Moreover, the N234 fluffy carbon exhibits a narrower particle size distribution.

For the corresponding PR product in the compounding process, this means that the filler is here in a stage of small particle size. An accordingly short mixing can then be chosen to achieve the final dispersion of the filler. The use of fluffy carbon black in the powder rubber process is thus an essential precondition for shortening the mixing time.

The determination of the carbon black surface area ([J.sub.2]-adsorption) and carbon black structure (DBP number) practically did not show any difference between fluffy carbon black and the corresponding wet-pelletized carbon black. However, when both carbon blacks are compared, the fluffy carbon black exhibits a higher reinforcement behavior (modulus), but also a higher hardness and compound viscosity.

This behavior can only be explained by the higher surface activity of the fluffy carbon black. It is generally known that the pelletizing process, in particular the high temperatures during drying of the carbon black, cause a certain breakdown of active centers on the carbon black surface important for reinforcement. Since the powder rubber production process, in particular drying of the product, takes place at moderate temperatures, the fluffy carbon black properties are retained and manifest themselves in the rubber data (fluffy effect in the powder rubber). Another important difference of the fluffy carbon black from wet-pelletized carbon black is its purity. Its grit content is, according to experience, very low. In addition, the fluffy carbon black does not contain any pelletizing agents. The use of fluffy carbon black in the PR process would, therefore, have a favorable effect on the reject rate, in particular in thin-walled extruded articles (e.g., profiles). Straining of sensitive compounds could be markedly reduced.

Production

For both systems, the first step in the powder rubber process is the separate production of a stable latex emulsion and a homogeneous filler suspension in water. Since the carbon black suspension influences the precipitation and product properties, and the specific latex exhibits a different behavior, preparation of the filler suspension of the two rubber systems varies at least in detail. The special properties of the two types have also to be considered in handling the latex. While, for example, the E-SBR latex shows a relatively fair behavior as regards its stirring stability, special precautions have to be taken for the NR latex, which prevent premature uncontrolled precipitation of the rubber. This can be achieved by certain additives and auxiliary agents which are added to the latex shortly after its collection. Manufacture of the batches in powder form is affected subsequently by joint precipitation of the two components. Both Broensted acids and Lewis acids may be used as precipitants. The precipitation parameters for both systems were adjusted so that both types can be produced in one plant from the equipment point of view. The precipitating method proper is, however, fundamentally different for both systems and takes into account the different properties and behavior of the two latex types (precipitation behavior), but also the different requirements on the finished powder rubber product. In this context, the tackiness of the products is to be mentioned, among other things, which is less pronounced in the E-SBR than in the NR. Adjustments are also necessary due to the fact that NR products have normally lower filler loadings than the products based on E-SBR.

The powder rubber process is a process in which the coagulating latex surrounds the filler particles without uncontrolled grain growth, i.e., without undesired clotting of the product. A separately added filler fraction, moreover, forms an effective separating layer around each powder rubber particle so that already after precipitation, the finished powder rubber is obtained in water with its later particle size distribution.

If necessary, the product may be subjected to a maturing process of several hours in water in a downstream homogenization tank. Subsequently, the major part of the water is mechanically separated, and finally the product is dried to a moisture content of about 1%. During the overall treatment process, the product has to be handled with care. Drying is effected under permanent moving of the powder to avoid clotting (figure 5).

[ILLUSTRATION OMITTED]

Products

Latex and carbon black qualities, fill factors, softeners

Basically, all common E-SBR types and NR latices, as well as the usual carbon black used in the rubber industry, can be converted to powder rubber. The same applies to filler loadings in the rubber-relevant area. In the final analysis, the user will decide which products and product compositions prevail on the market. When introducing a new generation of the raw material, it is of course necessary to consider early on - and here also the cooperation of the user is required - how far all rubber and carbon black types, which found wide acceptance in the industry, are really needed in the future for satisfying the requirements on the compound quality. A certain limitation of the raw material range could already in the early stages contribute to a simplified stock keeping at the user and help to reduce the costs in this area. Selection of the raw materials for E-SBR is certainly determined by the solid rubber types mainly used today. These are E-SBR 1500 and E-SBR 1712. As regards the carbon blacks, primarily N234 and N339 are to be mentioned in the automobile tire tread sector, the largest field of application of E-SBR.

As far as the fill factor adjustment is concerned, it is conceivable to offer two filler loadings per E-SBR and carbon black type whose lower limit could be about 60, possibly 50 phr, and the upper limit 80 phr. The compound manufacturer would then be able to adjust all fill factors within this range by blending two products.

As in the bale technology, softening oils may be added in addition in the powder rubber production. Due to the high viscosity of the E-SBR 1712, this should be done in any case. Analogously to the bale rubber technology, here, too, an oil quantity of 37.5 phr could be chosen for a 80 phr carbon black loaded material. In the case of a 60 phr loaded product, however, a lower oil content (i.e., 25 phr) is more suitable to avoid tackiness problems. The following products should then be sufficient to largely cover all requirements in the automobile tire tread area and in the truck tire retreading sector (table 2).

Table 2 - possible PR products based on E-SBR/carbon black

E-SBR-type   CB-type     Phr     Oil

1712          N234       60      25 phr
1712          N234       80      37.5 phr
1712          N339       60      25 phr
1712          N339       80      37.5 phr
1500          N234     60 (50)     --
1500          N234       80        --
1500          N339       60        --
1500          N339       80        --

For the NR/carbon black system, the carbon blacks N234 and N115, with two fill factors each (40 and 60 phr), are possible for use in truck tire treads. For the tire carcass, N326 are to be used in adhesion compounds, and N660 for sidewall and base compounds are still to be mentioned. The products (table 3) would thus have to be provided for introduction of the powder rubber concept on the basis of NR/carbon black in the tire. The addition of softeners is likely not to be considered in NR compounds.

Table 3 - possible powder rubber products based on NR/carbon black

Application   CB     Phr

LKW-LF        N234   40
                     60
              N115   40
                     60
Base          N660   50
Sidewall      N660   60
Adhesion      N326   50

As regards the use of powder rubber in the mechanical rubber goods sector, detailed talks with the specific users concerning the composition would certainly be necessary so that here a proposal for a range of products seems to be premature.

(Part 2 of this article will appear next month.)

Acknowledgements

"Practical rheology of rubber compounds" is based on a paper given at the April 2000 meeting of the Rubber Division.

"Processing promoters - keys to successful compounding" is based on a paper given at the March 2000 meeting of the Southern Rubber Group.

"Powder rubber - a new raw material generation for simplifying production" is based on a paper given at the April 2000 meeting of the Rubber Division.

References

(1.) U. Gorl and K.H. Nordsiek. Kautsch. Gummi Kunstst. 51 (1998) 200.

(2.) R. Uphus, O. Skibba, R. Schuster and U. Gorl, paper presented at the ACS-Meeting, Rubber Division, Orlando, FL, Sept. 1999.

(3.) U. Gorl and H. Lauer, paper presented at the ACS-Meeting, Rubber Division, Orlando, FL, Sept. 1999.

(4.) Neue Reifen Zeitung, 12/99 and 01/00.

(5.) European Rubber Journal, May 1996.

(6.) Neue Reifen Zeitung 01/00, EJU Rubber Trends 2nd quarter '97.

(7.) E.T. Italiander, Gummi, Fasern, Kunstst. 50 (1997) 456.

(8.) Delphi-Report, Kunftige Herstellungsverfahren in der Gummiindustrie, Rubber J. 154 (1972) 20; Kautsch. Gummi Kunstst. 26 (1973) 127.

(9.) K.H. Nordsiek and G. Berg, Kautsch. Gummi Kunstst. 28 (1975) 397.

(10.) K.H. Nordsiek, paper presented at the UNIDO-Conference, Phuket, Thailand, May 1982.

(11.) U. Gorl and H. Lauer, paper presented at the DKG-Meeting in Bad Neuenahr, Germany, Oct. 1999.

(12.) U. Gorl, paper presented at the Worldwide Amazon Rubber Conference, Manaus, Brazil, Nov. 1999.

(13.) R. Uphus, O. Skibba and R. Schuster, paper presented at the Worldwide Amazon Rubber Conference, Manaus/Brazil, Nov. 1999.

(14.) Worldwide Rubber Statistics 1996, International Institute of Synthetic Rubber Producers (IISRP).

(15.) J.S. Dick, paper presented at the ACS-Rubber Division Meeting, Orlando, FL, Sept. 1999.

(16.) A. Subramaniam, Rubber Res. Inst. Malaysia, Technol. Bull. No. 4 (1980) 1.

(17.) Th. Kempermann, Plastics, Paint and Rubber. 21, 9-35. (18.) W. Schnabel, Polymer Degradation, Hanse Verlag, 1981, S. 192ff.

(19.) H.H.G. Jellinek, Degradation and Stabilization of Polymers, Elsevier, 1983, S. 337ff.

(20.) Degussa-Huls Brochure - Was ist Russ?