Lamp black carbon black for EPDM applications. (Tech Service).

Approximately 95% of all carbon blacks are produced by the furnace process nowadays. It is the standard process for the various ASTM carbon blacks which are commonly used in the rubber industry.

Despite the great flexibility of the furnace process, Degussa still uses some special carbon black manufacturing processes, because the carbon blacks so produced show properties which cannot be obtained by the furnace technology. The lamp black process is one of those special technologies applied to produce D-0 whose specific properties are very interesting for the manufacturing of rubber goods. The purpose of this article is to highlight the special properties of lamp black CB, to point out its advantages for certain special applications and to explain the reasons for its unique properties profile.

Manufacture

Figure 1 shows two carbon black production processes used, the furnace black process and the lamp black process. Both processes are based on thermal oxidative cracking, but differ considerably in processing.

[FIGURE 1 OMITTED]

In the furnace process, oil and air are atomized and partially combusted. Accordingly, very high gas velocities are reached (up to 500 m/s) and the carbon black formation takes place in a narrow space within a narrow temperature range. Different carbon black types can be produced by varying the process parameters.

D-0 is manufactured by the lamp black process by burning feedstock in open pans. Highly aromatic oils are burned in open cast iron pans with a diameter of 2-3 meters. The air flows in from outside. The carbon black forms in the region of the hood. The residence time spectrum of the gases, as well as the temperature range of the carbon black formation, are considerably wider than those in the furnace process. Standard data for both processes are listed in table 1 (for more details about carbon black production processes please consult ref. 1).

The different conditions in both processes lead to two different carbon blacks. A relatively narrow distribution of primary particle diameter and aggregate sizes is achieved by the furnace process, while a very wide distribution is produced by the lamp black process (figures 2 and 3).

[FIGURES 2-3 OMITTED]

Table 2 shows characteristic data of D-0 and two furnace blacks with apparently similar structure and surface area. It is noticeable that, in the case of the lamp black CB, the difference between the original structure (DBP number) and the structure alter breakdown (CDBP number) is unusually large. The extent of the structural breakdown of D-0 under compression is depicted in figure 4. This CB already shows a structural breakdown after the first mechanical stress of 24,000 psi (=24M1) which results in a smaller structure than for the furnace blacks. In the literature, a strong structural breakdown under compression is often associated with good dispersibility.

[FIGURE 4 OMITTED]

D-0 is characterized by high purity due to the nature of the process: Frequent sources of impurities do not arise, such as non-volatile asphaltenes or crude oil impurities, badly atomized oil droplets, oil droplets which have hit the reactor wall or particles of the reactor lining.

Because of its high chemical purity, D-0 meets the requirements of the law with respect to rubber applications in the food sector (BGA).

Lamp black CB in EPDM

Processing behavior

A simple extrusion compound was selected to compare lamp blacks with furnace blacks (table 3). The carbon blacks were dosed differently in order to produce compounds with the same hardness. Three compounds with a hardness of approximately 60[degrees] durometer A were selected for detailed studies.

Special properties of D-0 are already apparent in the uncured compounds. At low shear velocities, as in the measurement of Mooney viscosity (figure 5), no deviation can be observed. At high shear speeds, however, such as those occurring in extrusion or injection molding, there are clear advantages for D-0 (figure 6).

[FIGURES 5-6 OMITTED]

Its excellent flow characteristics at high shear rates can be used to particular advantage for the production of articles with a high hardness.

A further advantage in extrusion compounds is the low die swell due to the special reinforcement properties of D-0.

Further studies (figure 7) confirm that the die swell of D-0 is considerably lower compared to similar furnace blacks, even at different mixing times. In production, a reduced die swell means that the profile is less dependent on the extrusion speed and that greater reliability can therefore be achieved. No significant differences between these carbon blacks can be observed in their vulcanization properties.

[FIGURE 7 OMITTED]

Vulcanizate properties

Clear differences are also seen in the vulcanizate between compounds with D-0 and compounds with furnace blacks. At similar hardness, high values are obtained for rebound elasticity with D-0. As can be seen in figure 8, the elasticity of the mixture with D-0 is 7 points higher than that of the mixture with P-HS 45.

[FIGURE 8 OMITTED]

Although the dynamic properties of EPDM articles are secondary in importance, use is made of the expression snappiness, which is influenced by elasticity. Snappiness describes the ability of a profile to regain its original form quickly after deformation, thus ensuring the quick sealing of gaps. This is particularly good with mixtures based on D-0.

The reinforcement properties of this lamp black show deviations compared to common furnace blacks. In the case of small deformations (<100%), the stress values are still comparable. At larger deformations, however, the D-0 moduli are considerably lower than those with the furnace blacks. This is clearly shown in figure 9.

[FIGURE 9 OMITTED]

The particular tensile properties of D-0 become apparent when the entire curve is shown (figure 10). At small deformations, the graphs still run parallel At larger deformations, the stress values with D-0 are considerably lower than those of the furnace blacks. Elongation at break is at the same level for all three carbon blacks.

[FIGURE 10 OMITTED]

In practical application, this means that, because of the lower stress values, complicated hoses are easier to remove from the vulcanization molds (thorns). This special reinforcement characteristic in the case of minor mad major deformation is a specification requirement for shaped hoses of some major automobile manufacturers.

In this application, D-0 cannot be adequately substituted by furnace blacks. Its special properties profile, here in particular its stress-strain properties, can hardly be matched by other fillers without breaching other specification limits.

Again, the elongation at break is comparable for all three compounds, the only possible disadvantage of the compound containing the lamp black being its reduced tensile strength. The reduced tensile strength is the logical consequence of the reduced reinforcement at great deformations. However, in many product specifications, a minimum limit is specified for the tensile strength which can still be easily achieved by compounds containing D-0.

It is essential in the case of hoses that the products are free from fault, especially when they are to be stressed in use. For carbon blacks, this means that the sieve residue should be as small as possible and, in any case, free of coarser residues. This is achieved by all three carbon blacks shown here.

In the manufacture of profiles and printer rollers, the demands on the raw materials are in many cases even higher, since the visual appearance of the products or their surface finish are important quality criteria.

The low sieve residue mentioned above, evidence of the high product purity, is one precondition, but good dispersion of the carbon black is also necessary in order to obtain products with perfect surfaces. A common method of characterizing the surface of extrudates is visual evaluation on the basis of a comparative scale. This is known as the Phillips rating, shown in figure 11. By this method of evaluation, the compound containing D-0 shows the best possible results compared to the compounds containing furnace blacks.

[FIGURE 11 OMITTED]

The filler dispersion can be evaluated in a more objective way by measuring the surface roughness of a vulcanizate using the topographic method (ref. 3). Briefly, this method consists of scanning the freshly prepared cut surface (5 [mm.sup.2]) of a vulcanizate with a needle (as in a record player). The data obtained can either be used to generate photorealistic images, or they can be converted into numerical values which allow an objective evaluation of the dispersion (figure 12).

[FIGURE 12 OMITTED]

The results show that both furnace blacks exhibit good dispersion (6% would correspond to average dispersion, while poorer dispersion would show more than 10% peak area). The dispersion of the lamp black, however, is even better than the two very well dispersed furnace blacks, probably because of the special structure of D-0, as already mentioned earlier.

A special feature of D-0 in compounds is its matte appearance. Matte surfaces are preferred to the "cheap and shiny plastic" look.

Summary

Due to their different manufacturing processes, furnace blacks and the lamp black D-0 show differences which are not reflected in a simple comparison of their colloidal properties, such as structure and surface area. Almost all requirements for the manufacture and the specifications of rubber articles can be fulfilled by the appropriate compounding and processing of furnace blacks. There are applications, however, in which the use of D-0 offers particular advantages. The ability of lamp blacks to act as a problem solver results from the following characteristic profile:

* Because of its chemical purity, D-0 meets the European requirements for rubber-based articles used in the food sector;

* compounds containing D-0 have silk glossy to matte surfaces.

* compounds containing D-0 are easy to extrude, while at the same time exhibiting high collapse resistance which makes them suitable for complicated profiles;

* because of its particularly low viscosity at high shear velocities, D-0 is excellent for hard extrusion articles;

* compounds containing D-0 are very easy to inject, so they are particularly suited for injection molded parts with a complex structure.

* because of its excellent dispersion properties, D-0 is best suited for soft compounds; and

* because of its excellent dispersion characteristics and its high purity, D-0 is well suited for articles with high surface finish demands, such as rollers and profiles.

Table 1--manufacturing parameters for carbon
blacks

Grade           Residence time (s)   Temperature ([degrees]C)

Lamp black             0.7                  100-1,500
Furnace black
  N 774 (SRF)          0.9                    1,400
  N 330 (HAF)          0.3                    1,550
  N 115 (SAF)         0.008                   1,650

Table 2--typical values for carbon blacks

Black   Type         Surface area               Structure
                  Iodine       CTAB           DBP        CDBP
                  number      number        number      number
                  (mg/g)   ([m.sup.2]/g)   (ml/100g)   (ml/100g)

P-HS    Furnace     43          42            121         86
45      black
P-HS    Furnace     29          29            124         83
25      black
D-0     Lamp        25          18           (140)        64
        black

Black     Sieve
        residue at
         325 mesh
          (ppm)

P-HS       <20
45
P-HS       <20
25
D-0        <10

Table 3--test formulations

First step            (Phr)     (Phr)     (Phr)

Buna EP G 5455         150       150       150
Purex HS 45          130-140     --        --
Purex HS 25            --      130-146     --
Durex 0                --        --      120-154
Zinc oxide              5         5         5
Stearic acid            2         2         2
Lipoxol 4000 (PEG)      5         5         5
Paraffin oil           50        50        50
Second step
Batch first step       100       100       100
MBT                     1         1         1
TBzTD                  1.2       1.2       1.2
ZBPD                    2         2         2
                       1.5

References

(1.) "What is carbon black?" Degussa publication.

(2.) "Large particle oil furnace black. A new type of carbon black," P.H. Johnson and W.T. Cooper, Rub. Chem. Tech. Vol. 45, no.1, p. 145, March 1972.

(3.) Formulation (all in phr): Keltan 812 -100; carbon black--var.; paraffinic oil-88; ZnO-5; stearic acid-1; calcium oxide-5.6; CaC[O.sub.3]-40; CBS-1.5; TMTD-0.6; DPTT-0.8; ZDBC-1.6; sulfur-1.5. Data from "Rhombus line--specialty blacks for improved processing in the MRG industry," Degussa Report PA501_E_3_2001.

(4.) "Filler dispersion analysis by topography measurements," A. Wehmeier, Technical Report from Degussa TR 820.1E, 2001.