Powder rubber -- a new raw material generation for simplifying production - pt. 2.

(The first part of this article ran in the March, 2001 Rubber World)

Delivery form, conveying and storage

Appearance

Independent of the carbon black type and fill factor, the products are obtained as free-flowing, pourable and dustless products or pellets. The particle size distribution is adjustable within a fairly wide range through variation of the production parameters at least in the rubber-relevant fill factor area.

Figure 6 shows the particle size distribution of a product (E-SBR/carbon black) with a mean particle size of 700 [micro]m. A relatively narrow particle size distribution will be noted which can be obtained by a special precipitation technology in the PR process. The extremely low portion of small particles ([is less than] 200 [micro]m) reduces the dust raising, while the absence of large particles has a favorable effect on the dispersion of the material, especially in the continuous extruder mixer process (ref. 21). Oil absorption, too, is improved by the larger surface area of a small to medium-sized particle.

[GRAPH OMITTED]

The narrow particle size distribution, in addition, favorably affects the storage behavior of the product. The voids between the particles are retained, the contact area between the pellets is small. No separation into individual particle size fractions will occur during storage of the product.

Homogeneity

Depending on the climate, soil condition and the plant material used, deviations in the natural rubber bale are in part considerable and almost any experienced compounder is likely to know one or more problems with the fluctuations in the NR compounds. Since in powder rubber production, one has to start from the NR latex, it can be specified accordingly and - homogenized in large storage tanks - can certainly lead to a product consistency, and thus compound consistency, not known up to now.

By the powder rubber technology, it is possible to almost avoid any fluctuations in the fill factor within a production batch. Already during precipitation, the desired ratio of latex to filler determined by weighing is ideally established in each individual powder rubber particle.

Since the powder rubber particle is no longer subjected to high mechanical stress during finishing, this ratio is also retained in the finished product. Enrichments of the filler on the one hand and of rubber on the other hand practically do not occur.

Figure 7 shows a statistical evaluation of the fill factor determined by thermal gravimetry (TGA) for a production run of a product based on E-SBR/N234. During the entire production time, the desired fill factor could be adhered to within tight specification limits. Thus, reproducibility of the process seems to be ensured.

[GRAPH OMITTED]

If a section is made through a powder rubber particle and its central core is looked at using transmission electronic microscopy (TEM), the high degree of homogeneous dispersion of the carbon black filler in the rubber matrix will be noted. This shows that in each individual PR particle there is already a maximum degree of homogeneity and dispersion between rubber and filler before the rubber compounding process proper.

This fact and the above mentioned use of fluffy carbon black are in the final analysis responsible for saving a large amount of the mixing time in the compounding process with excellent dispersion properties being obtained.

Conveying and storage

From experience gained with these products, special attention is devoted to the conveying and storage behavior within the framework of the project. The aim is to reduce the tackiness of the products - in particular at a low fill factor - to a minimum by suitable measures. In today's PR production process, this is achieved by a filler layer mechanically anchored to the rubber phase (no subsequent dusting). This separation layer (layer thickness about 10-20 [micro]m) surrounds each individual particle and consists normally of the filler which is also contained in the batch. Thus, no undesirable foreign ingredients are added to the product which could later adversely affect the in-rubber properties.

The conveying and storage tests were carried out on an E-SBR/N234 (52 phr) in a pilot plant. Pneumatic conveying was carried out in a steel using he Fluidlift process (air velocity about 20.6-22.0 m/s) and the low-speed Fluidpulse process (air velocity about 4.3-5.5 m/s) which causes less strain on the product. It was possible to convey the product by both methods without any problems. In determining the particle size distribution before and after conveying, almost no differences were found. This shows that the applied separation layer anchored to the rubber phase even withstands the harsh conditions of Fluidlift conveying without appreciable increase in the dust content.

Testing of the conveying behavior of a product based on NR/carbon black is planned for the near future.

The storability for both product classes was determined by the Jahnike test. The results obtained in both cases show relatively good flowing properties. For prolonged storage times, a recirculation line is required for moving the product at regular intervals. In the meantime, however, an improvement in long-term storability has been achieved by further product optimization, in particular by optimizing the particle size distribution (figure 6) and by subsequent treatment of the material during the drying process. Thus, under certain circumstances, the recirculation line may be dispensed with. Further tests are necessary to confirm this assumption.

PR in the discontinuous compounding process

In comparison with the conventional production process, some marked changes will have to be made, even for kneader compounds, when using powder rubber. The prior weighing and size reduction of the rubber bale are eliminated. Instead of storing the fillers, the silos now accommodate the powder rubber. All compound ingredients can thus be fed to the mixing unit via an automatic weighing and dosing system. The handling of free carbon black is avoided, the dust emission reduced.

Figure 8 shows the various possibilities of using powder rubber in the discontinuous compounding process. A part from the direct processing of the products in the internal mixer and on an open mill, the powder rubber process offers the additional possibility of mixing all compound ingredients first in a turbomixer and then performing the plasticizing step with the compound now containing all ingredients, as usually performed in the mixer or on an open mill.

[ILLUSTRATION OMITTED]

The advantage of this procedure is certainly that all compound ingredients are already present homogeneously dispersed in the powdery compound - even when a high amount of oil is added, the compound retains its powder form - so that the mentioned subsequent plasticizing and dispersion step is not unnecessarily extended by compound ingredients difficult to incorporate (e.g., ZnO, sulfur).

Dosing of powder rubber

Rubber tests have shown that the reinforcing behavior of powder rubber is markedly higher compared to that of the standard compound with equal filler loading. This manifests itself primarily through a higher modulus, higher hardness and compound viscosity.

According to the present state of development, this is due to two effects:

* Fluffy effect - higher surface activity of the fluffy compared with the equivalent pelletized carbon black;

* powder rubber effect - higher in-rubber structure of the filler due to the structure-stabilizing effect of the penetrating polymer chains.

The first effect was already described in the raw material section. The more pronounced powder rubber effect, on the other hand, is determined by the powder rubber process proper. During precipitation, the filler particles and rubber chains are first merely brought more closely together through the surface interaction. In the compounding process (internal mixer, rolls, extruder), this interaction is enhanced in the plasticizing stage by penetration of the polymer chains into the filler structure. It is well known that both carbon blacks and silicas have a certain fractality in structure (DBP number). This means that the initial structure of the filler is reduced in the kneading process under pressure until the penetrating highly viscous polymer builds up a certain counter-pressure and supports the structure from the inside. This dependence on the pressure of the filler structure is often expressed by the 24 M4 DBP number which simply indicates that the measured initial structure does not correspond to the structure present in the rubber. Since in the powder rubber process the filler has already been incorporated in the rubber in a manner causing no strain on the structure because there is no compressive stress, the rubber molecules already attached to the filler particles can, during kneading, immediately penetrate into the initial structure, still almost completely retained, and support it. The rubber-relevant result therefore assigns the carbon black in the powder rubber product a higher in-rubber structure which manifests itself in the same way as if the carbon black with a higher DBP number had been used from the outset. These findings are confirmed by measurement of the bound rubber content. For the powder rubber product, these values are in all cases higher than those for the comparable standard product (figure 9).

[GRAPH OMITTED]

Various measures may be taken to counteract these two effects, if necessary and desired at all.

* The first possibility is the reduction of the fill factor. An adaption of the fill factor to 76 phr N234 has proved successful, for example, for a standard product E-SBR/N234, 80 phr in the corresponding powder rubber. When the user, as suggested, keeps both a product with a high fill factor and a product with a low fill factor in stock, he can individually establish his desired adjustment without any problems by blending.

* A second possible measure to compensate the two above mentioned effects is the addition of increased amounts of softening oils. This means that when the fill factor is equal to that of the standard compound (e.g. 80 phr), more oil can be added to the powder rubber. The compound costs are reduced within the framework given by achievement of the required application in-rubber properties

It is, of course, also possible to combine both methods and thus adapt its formulation to the specified provisions.

* The third possibility is to reduce the initial structure of the carbon blacks to be used in the powder rubber process somewhat from the beginning so that the in-rubber structure finally corresponds approximately to that of the standard compound. In how far this is reasonable and desirable is, however, to be discussed with the users.

Kneader fill factor, temperature build-up and mixing sequence

In particular in the tangent internal mixer, it is necessary, due to the higher density (smaller volume) of the powder rubber, to increase the degree of loading of the kneader by about 10-15%. In this way, the mixing capacity, at least in the first stage, can be enhanced by the above mentioned percentage by merely using powder rubber instead of a conventional mixing method.

Figure 10 shows the kneader diagrams of a conventionally produced rubber compound and of a rubber compound produced by using a comparable powder rubber.

[GRAPH OMITTED]

Due to the fact that the fillers have already been incorporated in the rubber phase, energy consumption and temperature build-up markedly differ from those in the conventional compounding process.

While in the classic compounding process the temperature gradually rises to the desired discharge temperature, the speed of the kneader remaining high, a steep temperature curve will be noted in the powder rubber process. In order not to appreciably exceed the desired discharge temperature, it is therefore necessary to reduce the speed of the mixer in due time.

The addition of large quantities of oil - as they are certainly necessary only in the E-SBR/carbon black system - should be effected in a stage in the mixing sequence in which the powder rubber is still present in powder form. The large surface area of the powder and the absorbent carbon black splitting layer are then able to also take up fairly large concentrations of softeners without slipping of the kneader blades.

In particular in the case of the NR/carbon black system, it has proved advantageous to add the anti-aging agent, especially those of the class of the N-p-phenylenediamines, to the compound at a later time in the mixing sequence, e.g., at the end of the premixing stage. Phenylenediamines seem to impede the reduction of viscosity of the powder rubber. To achieve low viscosities in the case of the powder rubber based on NR/carbon black, also with short mixing times, this procedure is therefore recommended. The other compound ingredients may be added to the powder mixture as usual. The possibility of first premixing a complete rubber compound in the powder mixer and then processing it mechanically has already been mentioned elsewhere. The above recommendations of the addition anti-aging agents are, of course, also valid for this case.

Mixing time, stages, energy and capacity

The fact that in the process of using powder rubber the filler is already incorporated in the rubber matrix reduces the mixing process in the mixer and on the mill, respectively, to a short plasticizing and dispersion process. This means that the time which would be otherwise required for incorporation of the filler during mixing can be saved. By using fluffy carbon black, a form of filler was employed at the same time which is characterized by special particle fineness, and thus also promotes dispersion.

Figure 11 shows the influence of the mixing time (E-SBR/ 80 phr N234) on dispersion (ref. 22), and the particle size of the filler in the matrix in comparison with that of a compound produced by the standard method. The results were obtained on a 1.5-L lab-mixer.

[GRAPH OMITTED]

It turns out that the powder rubber compound still exhibits excellent dispersion values, even after an extremely short mixing time, the carbon black particles being in all cases considerably smaller than in the standard compounds

In the meantime, mixing tests on E-SBR/carbon black were performed in a 45-L E-mixer. These tests were to ensure a better transferability of the results to the kneader size usually employed in production. The product E-SBR/N234 80 phr (standard compound) was compared with powder rubber E-SBR/N234 76 phr. For the standard compound, a two-stage mixing sequence was chosen, starting from bale rubber and carbon black filler. The total mixing time was 2.5 minutes, the mixing time in the prestage being 1.5 minutes, and one minute in the finishing stage. This short mixing time - certainly not the rule in today's production of E-SBR/carbon black compounds in the tire industry - was deliberately chosen to clearly show that it is possible with the powder rubber technology even then to considerably reduce the mixing time with excellent dispersion.

On the basis of this comparison, each manufacturer of E-SBR/carbon black compounds will see by his compounding method by how much he can possibly shorten the mixing time. Table 4 shows the results of this mixing study conducted on a 45 L mixer in summary.

Table 4 - compounding tests on the E-SBR/carbon
black system in a 45-L internal mixer

                          Unit   Standard   Powder    rubber

Stages                              2         2          1
Fill factor                        0.7       0.8       0.73
Mixing time
1. stage                  sec       83        52        95
2. stage                  sec       66        57        --
Mixing energy
1. stage                   kJ     18,808    15,544    12,238
2. stage                   kJ     10,279     9,247      --
Specific mixing energy   kJ/kg     738.3     549.7     297.8
Dispersion
DIK-method (ref. 23)       %      ca. 80    ca. 95   ca. 90-92
Phillips                            6         8         7-8
Peak area                  %       8.4       1.1        2.2

In this mixing test series, a reduction of the mixing time by about 45-50% was achieved with powder rubber beyond the already short mixing times of the standard compounding process. This results in savings of energy up to 60% and an increase in capacity of 45-50%, which is due to a higher fill factor of the mixer and the shorter mixing time of the powder rubber product.

The dispersion properties of the powder rubber compounds are in all cases appreciably better than those of the standard compound or, in other words, to achieve the dispersion rating of the powder rubber compounds, the standard compound has to be mixed for a considerably longer period than was the case in the test series. Comparable tests on the NR/carbon black powder rubber system have up to now only been carried out in a laboratory mixer (1.5 L). Results obtained are in table 5. The formulation corresponds to a truck tire tread compound (NR/N234 50 phr).

Table 5 - compounding comparison between a standard NR/carbon
black compound and a comparable powder rubber

                            Unit       Standard        Powder rubber
                                    NR/N234 50 phr     NR/N234 47 phr

                                                      1       2
Mastication time             min       8-10            --      --
ML 1+4 (100 [degrees] C)            ~95 [right
                                    arrow] 72 at
                                    150 [degrees] C
Mixing time stage          1 min.        4(*)         4(**)   2(***)
ML 1+4 (100 [degrees] C)                 85           82      91
Mixing time stage          2 min.        2.5          2.5     2.5
ML 1+4 (100 [degrees] C)                 68           67      74
Dispersion
Phillips                                 7            8       8
Peak area                    %           1.2          0.4     0.2

(*) Mastication of the NR bale was carried out on a WP
kneader mixer

(**) 6PPD (2phr) was added after mixing time of 2 min.

(***) 6PPD (2phr) was added after mixing time of 1 min.

The results clearly show that also with a powder rubber based on NR/carbon black, appreciable reductions in the mixing time can be achieved. The order of magnitude is likely to correspond at least to that in the E-SBR/carbon black system. Thus, the entire mastication step is eliminated when using NR/carbon black powder rubber, regardless of the fact that the user performs the mastication of the NR as a separate mixing stage or integrates this stage with the incorporation of at least part of the filler. Compound manufacture for the NR/carbon black system certainly differs among users with regard to mixing time and mixing stages more than for the E-SBR/carbon black system, so that it is difficult to make generally valid statements about the mixing time and mixing stage reduction when using powder rubber. Each compound manufacturer should, therefore, evaluate the data in table 5 and draw conclusions on the basis of their own conditions prevailing locally.

It is, however, important to point out once again that anti-aging agents, as in this example 6PPD, should be added later to the basic compound. For powder rubber compound 1, the addition was effected after two minutes, for compound 2 after one minute. In the latter case, addition of the agent still later in the process, e.g., at the end of the prestage for reducing the viscosity towards the standard compound would have probably been of benefit if the mixing time is reduced further. The addition of small amounts of zinc soaps (about 2 phr) may also be recommended as processing aid and/or the addition of commercial NR peptizer agents in the usual concentrations (0.1-0.3 phr). This is, however, only necessary with really short mixing times which, due to the properties of the powder rubber, are still sufficient to disperse the carbon black well, but too short to also bring the viscosity of the NR fully to the level of the masticated NR bale in the standard compound.

Application in-rubber properties

Table 6 shows a survey of common rubber laboratory data of carbon black-filled powder rubber compounds based on E-SBR in comparison with the corresponding standard compounds manufactured by the conventional method. In the case of the E-SBR compounds, the tilt factor of 70 phr N234 was obtained by blending with a product of a fill factor of 60 phr and 80 phr in the mixer.

Table 6 - in-rubber data of various PR products based
on E-SBR/N234

Method         Unit     60 phr N 234    70 phr N 234    80 phr N 234
                        Conv.   PR(*)   Conv.   PR(*)   Conv.   PR(*)

[Delta]
  torque      (Nm)       13.7    13.6    13.8    13.4    14.4    13.8
[t.sub.10%]   (min.)      3.1     2.8     2.9     2.7     2.7     2.6
[t.sub.90%]   (min.)      8.0     7.6     8.1     7.9     7.9     8.1
Tensile
  strength    (MPa)      19.9    20.7    18.2    19.2    17.9    17.8
modulus
  300%        (MPa)       7.5     8.2     8.4     8.3     8.9     8.8
Elongation
  at break    (%)         570     570     510     550     480     510
Durom. A
  hardness    --           63      65      66      65      68      68
E' 0 [deg-
  rees] C     (MPa)      21.2    22.6    29.3    28.9    38.3    38.1
E" 0 [deg-
  rees] C     (MPa)       7.7     8.1    11.6    11.0    15.6    14.8
tan [Delta]
  0 [deg-
  rees] C     --        0.365   0.360   0.390   0.380   0.406   0.390
E' 60 [deg-
  rees] C     (MPa)       8.3     8.9     9.5     9.7    10.3    11.2
E" 60 [deg-
  rees] C     (MPa)       1.9     2.1     2.4     2.4     2.9     3.1
tan [Delta]
  60 [deg-
  rees] C     --        0.224   0.230   0.250   0.250   0.277   0.274
DIN
  abrasion    ([mm.
              sup.3])      82      69      70      69      73      63

(*) Fill factor 58; 67; 76 phr

The compound composition corresponds to a pure E-SBR compound with the usual portions of activators, anti-aging agents and accelerators. The oil quantity (20, 25 and 30 phr) was adjusted to a durometer hardness of about 65A according to the fill factor (rheometer and vulcanization temperature 165 [degrees] C). In the case of the powder rubber compounds, the mixing time was shortened by half (first stage).

After correction of the fill factor, the rheological, static and dynamic in-rubber properties of the powder rubber compounds approximately correspond to those of the respective standard compounds, however with the mixing markedly reduced. Advantages in the DIN abrasion could, in addition, suggest a better road abrasion behavior (good dispersion) of the powder rubber compounds.

Analogous laboratory tests were carried out on the NR/N234 system (standard 50 phr). Table 7 shows the vulcanizate data as determined on the compounds described in table 5.

Table 7 - comparison of the in-rubber properties of
NR/N234 powder rubber with the standard compound

                           Unit      Standard       Powder rubber
                                  NR/N234 50 phr   NR/N234 47 phr

Mixing time stage 1        min.           4             4       2
Mixing time stage 2        min.         2.5           2.5     2.5
ML 1+4 (100 [degrees] C)                 68            67      74
[Delta] torque             Nm          15.7          15.4    15.1
Tensile strength           MPa         25.8          26.2    27.1
Modulus 300%               MPa         12.3          12.3    12.0
Elongation at break        %            520           530     540
Durom. A hardness          -             67            67      68
Tear resistance            N/mm          23            24      25
Ball rebound               %           59.8          64.9    62.2
E' (60 [degrees] C)        MPa          7.9           7.7     7.9
E" (60 [degrees] C)        MPa          1.2           1.0     1.0
tan [Delta] (60
  [degrees] C)             -          0.152         0.127   0.126

First rubber tests on the NR/carbon black powder rubber system show that the in-rubber properties are at least equal to those of the standard compound, independent of the mixing time.

Blending of powder rubber with bale rubber

The rubber industry often uses blends of various rubber types to achieve the technical requirements. The combination of E-SBR/BR rubber is state-of-the-art in the automobile tire tread sector. In track tire treads, a blend of NR/BR is frequently used to obtain the required product properties (e.g., road wear).

As long as the BR/carbon black system is not yet available in powder form, the question arises in how far it will be possible to use bale rubber with or without additional carbon black addition as a blending component when employing powder rubber.

In the first case, the process is started with a powder rubber which has a fill factor that is higher than the desired fill factor. By blending it with pure bale rubber, this fill factor is finally reduced to the required level. An example is the use of an E-SBR/N234 80 phr powder rubber blended with 20 parts of BR bale. The achieved fill factor would then be 60 phr N234. Corresponding tests have already been presented (ref. 3) and confirm the feasibility of this procedure. In the second case, bale rubber (eg., BR) and carbon black are added simultaneously to the corresponding powder rubber system. The fill factors of the powder rubber and the overall compound are retained and the carbon black used mainly serves as filler for the bale rubber. The tests were performed on a 45-L-mixer (table 8). Table 9 shows the most important rubber data determined in the above described tests.

Table 8 - test on blending powder rubber based
on E-SBR/N234 76 phr with BR rubber and
extra addition of N234

                             Standard   Powder rubber
E-SBR 1500                      80            -
BR cis>96%                      20            20
N234                            80            16
PR (E-SBR/N234 76 phr)          -            141

Other ingredients (phr) oil 30; ZnO RS 3; stearic acid 2;
6PPD 2; wax 1; CBS 1.5; sulfur 1.5

Mixing stages                   2             2
Mixing time stage 1 (sec.)      81            58
Mixing time stage 2 (sec.)      65            62
Mixing time (sec.)             166           120
Mixing energy (kJ)            27,565       23,793
Table 9 - vulcanize data using a blend of
powder rubber, BR-bales and additional
carbon black

                               Unit         Standard      PR

ML 1+4 (100 [degrees] C)        -                 76      77
Tensile strength               MPa              16.2    18.0
Modulus 300%                   MPa               7.8     7.5
Elongation at break             %                520     570
Energy at break                 J              121.1   144.5
Shore A hardness                -                 69      69
Tear resistance                N/mm               44      49
tan [Delta] (0 [degrees] C)     -              0.362   0.355
tan [Delta] (60 [degrees] C)    -              0.282   0.283
DIN abrasion                   [mm.sup.3]         65      62
Dispersion
Phillips                        -                  6       8
Peak area                       %                8.5     1.7

The tests show that the described procedure of adding bale rubber and carbon black to the powder rubber is possible. As expected, the effects are less pronounced both in the compound production (reduction of mixing time and savings in mixing energy) and in the vulcanizate data than in the case of using powder rubber only. In the final analysis, the development of a BR/carbon black powder rubber is also necessary for the discontinuous compounding process to utilize all synergies of the powder rubber technology.

Summary

The intention of the rubber industry to fundamentally simplify its production processes led to the resumption of the development work on rubber/filler batches in powder form.

The development starts from a new raw material concept which is based on using the raw materials in the PR manufacturing process as soon as possible after their production, i.e., without energy- and cost-intensive conditioning steps.

On the basis of the present results, the production of carbon black-filled E-SBR and NR/carbon black types of high quality and consistency can be guaranteed. The initial conveying and storage tests have shown that it is possible to convey the products by various conveying methods without any problems and, with suitable design of the silo geometry, store them and empty the silo reliably and without difficulties.

The initial mixing tests in the batch mixer process show an excellent compound quality even after very short mixing times. There seem to also be advantages in the in-rubber data. Here should be mentioned the excellent dispersion and the overall better static in-rubber properties. The simplification of the production processes and introduction of a continuous compounding process in the rubber industry seem to be realizable in the future on the basis of these promising results.

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/USA, 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, Oriando, 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. 192 ff.

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

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

(21.) R. Uphus, O. Skibba and R. Schuster, paper presented at the ACS-Rubber Division Meeting, Dallas, TX, April, 2000.

(22.) H. Geisler, lecture on the occasion of a workshop at DIK, Hannover, Germany, Oct. 1997.