New views on using ground coal fillers.

New views on using ground coal fillers

There is increasing emphasis to improve performance and reduce the weight and cost of many rubber compounds. This modern trend is especially critical in automotive applications as well as industrial areas. This need has led to renewed investigations using ground coal as a functional filler to reduce weight and to improve certain performance parameters.

This article will discuss the concept of decreased compound specific gravity comparing ground coal to other common fillers. The concomitant change in certain physical properties, particularly the improvement in hysteresis and compression set, will be illustrated. Data derived from the addition of ground coal into practical compounds of SBR, EPDM, nitrile and fluoroelastomers will be used to demonstrate the stated premise.

Background

Ground bituminous coal is one of a family of inorganic fillers which has been widely used in the rubber industry. These fillers include calcium carbonates, clays, silicas, talc plus other mineral fillers. Table 1 lists the typical chemical and physical analysis of Mineral Black 325BA, the ground bituminous coal used for these studies. Figure 1 illustrates the relationship between ground coal and other mineral fillers by comparing the surface areas of several common fillers, surface area being indicative of effective particle size.

Table : Table 1 - chemical and physical properties of

                       mineral black 325BA
Specific gravity, raw material                  1.22
325 Mesh, wet screen, % through                99.9%
500 Mesh, wet screen, % through                99.5%
Average particle diameter              5-6 [micro] m
Moisture                                    1.0% Max
Volatile matter                           17.0-18.0%
Fixed carbon                                  75.00%
Ash                                         6.0-7.0%
Total sulfur                               .60-1.20%
Free sulfur                                    <.05%
Oil absorption                    50# Oil/100# 325BA
pH                                               7.0

Applications: Pound-volume relationships

The most common reason for using ground coal is its low cost and low specific gravity. Figure 2 is a comparison of the specific gravity of ground coal and other commonly used inorganic fillers. Examination of figure 2 shows that bituminous coal has the lowest specific gravity of the common mineral fillers.

Figure 3 shows the cost relationship in cents per pound between ground coal and some common mineral fillers including carbon black. Ground coal is near the low end of the chart with only whiting and clay being less expensive.

Figure 4 shows the pound-volume cost of ground coal in comparison to some mineral fillers including carbon black. As one can see, ground coal has a low pound-volume cost when compared to other materials.

In order to further investigate the concept of lower compound pound-volume costs a theoretical compound was mathematically analyzed. For the sake of simplicity the theoretical compound had two components, polymer and filler. The compound always contained 100 phr of SBR and varying amounts of filler, starting at 20 parts, then 40, 60, 80 and finally 100 parts of filler. Figure 5 shows the total specific gravity of the compounds, called system specific gravity, plotted against filler loadings. As can be seen, the total specific gravity of the compounds increases with the addition of any filler. However, the rate of increase is lowest with the addition of ground coal. The worst performers are clays and whitings, those fillers having the highest specific gravity.

Figure 6 shows the change in the cost per pound of the theoretical system as filler loadings are increased. In all cases the total cost per pound of the system was lowered with the addition of the filler.

Figure 7 shows the pound-volume cost for the system as filler loadings are increased. The compound with the lowest pound-volume cost is the compound with the ground coal additions. Figure 8 shows the pound-volume cost differences between ground coal and several mineral fillers. The greatest difference and hence the best area of applications is to incorporate ground coal as a substitute or extender for carbon black. This application technique will give the compounder the highest economic savings.

It can be concluded from this theoretical study that ground coal is the best economic choice when the filler system of a compound is considered. Although the costs per pound of other fillers may be lower, their inherent high specific gravities can in fact raise pound-volume costs to unacceptable levels.

Test results and discussion using actual compounds

Several practical compounds utilizing common polymers have been chosen to illustrate the application of mineral black in actual recipes. These recipes were selected because of their commercial or economic importance or because their physical properties were improved with the addition of ground coal.

SBR 1502

Ground bituminous coal was added to a practical SBR 1502 compound to lower pound-volume costs while maintaining usable physical properties. Table 2 shows the recipe used, the resulting costs per pound and pound-volume costs and the results of physical tests. In this case both pound-volume and pound costs are lowered enabling the compounder to produce a lighter weight and more cost effective compound while maintaining usable physical properties. Pound costs decreased by 6.5 % and pound-volume costs decreased by 13.2 % in compound A. In compound B pound costs were reduced by 16.1 % while pound-volume costs were reduced by 21.1 %.

Table : Table 2 - test results-SBR 1502 compound using

               325BA to lower lb-volume cost
Recipe               Control      A         B
SBR 1502(1)          100 PHR   100 PHR   100 PHR
N-550                  86.60        35        35
325BA                      -        35        70
Filler vol                48        48        77
Sp.gravity              1.24      1.14      1.17
$/lb                    0.31      0.29      0.26
$/lb-vol(2)             0.38      0.33      0.30
Hardness, Shore A         80        74        82
Tear, KNT/M            27.66     28.36     29.24
  (PPI)                (158)     (162)     (167)
Tensile, MPa           21.37     15.10     10.34
  (PSI)               (3100)    (2190)    (1500)
100% Modulus MPa       13.51      4.62      6.27
  (PSI)               (1960)     (670)     (910)
300% Modulus MPa           -     10.62      9.58
  (PSI)                         (1540)    (1390)
Elongation, %           170        410       340

(1) All recipes had the following added: ZnO - 3.0; stearic acid - 1.0; Santocure NS- 1.0; sulfur - 1.75. (2) Using measured specific gravity.

In this compound the tensile loss using ground coal was significant. However, it has been found in practical commercial compounds that levels of 60-90 phr of ground coal can be used and still retain useful physical properties.

Fluoroelastomer

Fluoroelastomers are high cost elastomers that are frequently used for their compression set capabilities. Often N-990 carbon black is added to the elastomer mix to further enhance compression set.

Table 3 shows recipes of Fluorel FC-2174 compounds where bituminous coal was used to replace or extend the N-990 carbon black additions. Shown are the pound and pound-volume costs and post cure physical property test results.

Table : Table 3 - test results - Fluorel FC-2174 compound

            using 325BA to improve costs and compression
                          set properties
Recipe                          Control      A         B
FC-2174                         100 PHR   100 PHR   100 PHR
N-990                                30         -        15
325BA                                 -        25        15
MgO                                   3         3         3
Ca(OH)[sub. 2]                        6         6         6
$/lb                              14.50     14.97     14.47
$/lb-vol(1)                       16.68     16.17     16.20

Physical properties, post cure - 20 hours @ 500[degrees] F:

Tensile, MPa                      15.99     10.34     12.69
  (PSI)                          (2320)    (1500)    (1840)
Elongation, %                       165       165       155
100% Modulus MPa                   7.58      6.31      7.83
  (PSI)                          (1100)     (915)    (1135)
Hardness, Shore A                    78        73        75

Compression set:

  70 Hrs @ 392 [degrees] F, %      17.1       7.8       7.4
  70 Hrs @ 70 [degrees] F, %        9.9       7.2       5.6

(1) Using calculated specific gravity.

Note the positive changes in compression set with the additions of bituminous coal filler with the concurrent decrease in pound-volume costs. Pound volume costs decreased by 3.1% in compound A and 2.9% in compound B. Compression set values after 70 hours at 392 [degrees] F decreased by 54% in compound A and 57% in compound B.

Tread base compound

An area of great interest to tire builders is to reduce rolling friction by decreasing the hysteresis of the rubber compound underlying the tread. A typical tread base or cushion stock compound was chosen for examination. Bituminous filler was added by reducing the amount of carbon black present in the compound on a volume-by-volume basis. The recipe and the test results are shown in table 4.

Table : Table 4 - test results-tread base compound using

             mineral black 325BA to lower pound-volume
                  costs and decrease hysteresis
Recipe                     Control    B        C
                           100%-     70%-     50%-
                           N-660     N-660    N-660
                                     30%-     50%-
                                     325BA    325BA
Nat. rubber RSS #1(1)       60 PHR   60 PHR   60 PHR
SBR 1778                        55       55       55
GPF N-660                       60       42       30
325 BA                           -     12.2    20.33
Sp. gravity                   1.14     1.12     1.10
$/lb                         .4144    .4056    .3994
$/lb-vol(2)                  .4724    .4502    .4353

Physical properties

Hardness, Shore A               64       59       57
Tensile, MPa                 21.17    20.00    18.41
  (PSI)                     (3070)   (2900)   (2670)
100% modulus, MPa             3.65     2.76     2.34
  (PSI)                      (530)    (400)    (340)
200% modulus, MPa             9.10     6.20     4.69
  (PSI)                     (1320)    (900)    (680)
300% modulus, MPa            15.58    10.41     7.79
  (PSI)                     (2260)   (1510)   (1130)
Tear strength, KNT/M         40.28    36.42    36.42
  (PPI)                      (230)    (208)    (208)

Goodyear healy rebound 

  Durometer, Shore A            64       61       58
  Rebound, %                  66.0     69.9       71

Goodrich flexometer

  Durometer, Shore A            63       59       57
  Temp rise, [degrees] F        24       17       14
  Time to equil, min             9        9        9
  Set, %                       1.0      0.9      0.9

Yerzley resilience 

  Resilience, %               66.7     67.4      72.6
  Hysteresis, %               33.3     32.6      27.4
  Frequency, Hz                3.9      3.8       3.6
  Dynamic Mod., MPA           7.93     6.27      5.58
  (PSI)                     (1150)    (910)     (810)

Garvey die rating

  Swelling                       3        4         4
  30 [degrees] Edge              2        3         3
  Surface                        3        4         4
  Corners                        2        4         4
  Total                         10       15        15

(1) All recipes had the following added: Flectol H - 2.0; ZnO - 5.0; stearic acid - 1.5; Santocure - 1.2; sulfur - 2.2 (2) Using measured specific gravity

The results show several significant changes in the compound:

* Heat build-up was reduced dramatically, the control compound had a 24 [degrees] F temperature rise while the ground coal compounds had a 17 [degrees] F and 14 [degrees] F rise respectively.

* Hysteresis was markedly decreased with the addition of ground coal. Hysteresis decreased by 2.1% and 17.7% respectively.

* Economic factors were improved. Pound costs decreased by 2.1% and 3.6% with the additions of ground coal while pound-volume costs dropped by 4.7% and 7.8% respectively.

* Garvey Die Ratings were also improved with the ground coal additions.

EPDM hose compound

Ground bituminous coal filler was added to an EPDM radiator and coolant hose compound. The filled compound was compared to a control compound containing 70 phr of N-650 carbon black and 130 phr of N-762 carbon black. Two experimental compounds were evaluated. In compound A, 50 phr of N-762 was replaced with an equal volume of bituminous filler.

In compound B the N-762 volume was totally replaced with the ground bituminous material. The critical parameters evaluated were physical properties, compression set, cure rate and processing behavior. The test results are shown in table 5.

Table : Table 5 - test results-EPDM coolant hose compound

              using 325BA to lower compound costs
Recipe                Control     B        C
                      100%-     50%-      0%-
                      N-762     N-762     N-762
                                50%-      100%-
                                325BA     325BA
EPSYN 5508(1)         100 PHR   100 PHR   100 PHR
N-762                     130        65         -
325BA                       -        47        94
Sp. gravity              1.17      1.12      1.10
$/lb                    .4330     .4258     .4190
$/lb-vol(2)             .5066     .4769     .4609

Physical properties

Elongation, %             390       390       610
100% modulus, MPa        3.52      3.38      2.00
  (PSI)                 (510)     (490)     (290)
300% modulus, MPa        9.66      9.38      3.59
  (PSI)                (1400)    (1360)     (520)
Tensile, MPa            11.52     11.03      8.28
  (PSI)                (1670)    (1600)    (1200)
Shore A durometer          71        68        56

Compression set

70 Hrs. @ 125 [degrees] C, 25% deflection, 1/2 hr. recovery

Compression set, % 72.2 73.6 72.9

Rheometer data

Max torque, NT-M         5.98      4.93      4.21
  (lb-in)              (52.9)    (43.7)    (37.3)
Min torque, NT-M         1.46      1.15      1.13
  (lb-in)              (12.9)    (10.2)    (10.0)
Scorch time, min          5.6       6.1       7.1
Cure time, Tc 50%         7.9       9.1      10.1
Cure time, Tc 90%        12.0     14.25      15.4

Tack rating

0 = None 4 = High

After

  24 Hours                  2         2         3
  72 Hours                  1         1         2
  96 Hours                  1         1         1

(1) All recipes had the following phr added: Sunpar 2280 - 130; AC 617A - 5.0; ZnO - 5.0; Stearic acid - 1.0; NBC - 1.0; Sulfur - 1.25; TMTD -0.8; Sulfads - 0.8; Altax - 1.0; TDEC - 0.8; N650 Carbon black - 70.0. (2) Using measured specific gravity.

The test results showed:

* At high levels ground bituminous fillers enhance tack and processing in the first days after mixing. However, this effect diminished with time.

* The ground bituminous can replace the carbon black levels up to 50-60% by weight and have little serious effect on physical properties.

* Compression set is unaffected by the use of ground coal.

* Ground bituminous has been known to retard the bloom of sulfur and sulfonating accelerator fragments in compounds. None of these compounds showed bloom. This could be of significant practical benefit for other EPDM compounds that are susceptible to this condition.

* The results showed that once again the economic factors of the compound were improved with both $/lb. and $-lb-volume decreasing. The $-lb.-volume decreased by 6% and 9% respectively.

Because of the major economic benefits realized, the examination of ground bituminous filler in this EPDM compound further suggests that partial replacement of the carbon black component, in automotive coolant systems, weatherstripping and other industrial molded goods is possible and should be strongly considered by the compounder for these applications.

Nitrile compound

In a typical high quality nitrile rubber compound ground bituminous coal was substituted for N-550, a highly reinforcing carbon black. Ground coal substituted for 25% and 50% of the weight of the carbon black while the filler volume was kept constant. Table 6 summarizes the test data.

Since N-550 is a fairly reinforcing carbon black, there is an immediate loss of physical properties with the substitution of ground coal. The compression set remains the same or is reduced slightly upon the substitution of the mineral black for the N-550 material. The data would suggest that 10-20% of the N-550 could be replaced with bituminous coal allowing the compounder to decrease the pound and pound-volume costs while still maintaining usable properties.

Table : Table 6 - test results - nitrile compound using

            325BA to substitute for N-550 carbon black
                      Control     A         B
                        100%-    75%-       50%-
                        N-550    N-550      N-550
                                 25%-       50%-
                                 325BA      325BA
Hycar VT-335(1)       100 PHR   100 PHR   100 PHR
N-550                      50      37.5      25.0
325BA                       -       9.1      18.1
Sp. gravity              1.18      1.16      1.14
$/lb                    .8185     .8196     .8213
$/lb-vol(2)             .9658     .9507     .9363

Physical properties

Elongation, %             600       650       700
100% modulus, MPa        2.14      1.72      1.86
  (PSI)                 (310)     (250)     (270)
300% modulus, MPa        8.14      5.93      4.34
  (PSI)                (1180)     (860)     (630)
500% modulus, MPa       17.59     11.86      8.48
  (PSI)                (2550)    (1720)    (1230)
Tensile, MPa            22.07     19.10     16.21
  (PSI)                (3200)    (2770)    (2350)
Durometer, Shore A         63        59        60

Compression set:

70 Hrs. @ 150 [degrees] C, 25% deflection, 1/2 hr. recovery

Compression set % 76.0 73.3 74.6 (1) All recipes had the following phr added: Paraplex G-50 - 10; Stearic acid - 0.5; Sulfur - 1.5; ZnO - 5.0; Nonox B - 2.0; Altax - 1.5.

Summary

Ground bituminous coal, in this case Mineral Black 325BA, can be the best economic choice when a filler system is being considered. Although the cost per pound of ground coal is higher than most other extender fillers, its inherently low specific gravity results in significant decreases in the pound cost and $-lb.-volume cost of the final compound.

In addition to the economic advantages realized when ground coal is added to a compound certain physical properties can be enhanced, particularly hysteresis and compression set. These property changes can translate into lower heat build-up in tires resulting in lower rolling resistance and in better compression set in sealing compounds. EPDM hose compounds are especially suitable for additions of ground coal for economic improvement and weight savings.

PHOTO : Figure 1 - surface areas of fillers

PHOTO : Figure 2 - specific gravity of fillers

PHOTO : Figure 3 - cost comparisons for rubber fillers

PHOTO : Figure 4 - pound volume cost for fillers

PHOTO : Figure 5 - system specific gravity

PHOTO : Figure 6 - system dollar per pound

PHOTO : Figure 7 - dollar/pound volume cost

PHOTO : Figure 8 - pound-volume cost differences (fillers compared to bituminous coal)