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Zinc-free curing systems for silica.

Among inorganic fillers for rubber, fine particle precipitated silica provides the highest level of reinforcement in terms of tear strength, tensile properties and abrasion resistance. However, comparisons which include carbon black leave only tear strength (and the non-reinforcing properties of adhesion and heat resistance) as silica's major contribution of elastomer reinforcement. In particular, abrasion resistance, as measured by laboratory tests and highway tire wear, has been inferior to that provided by carbon black of comparable particle size.

A search for an explanation of silica's anomalous reinfocement behavior led to studies of the interactions of the normal cure activators, zinc oxide, stearic acid and glycols, with silica. This work indicated that the lack of abrasion resistance was due, not to a defect inherent in the silica, but to its surface modification by soluble zinc attachment (ref. 1 and 2). Thus the zinc was partially removed from its role of accelerator activator and, at the same time, became a barrier on the silica surface to prevent effective filler-polymer bonding. These studies also demonstrated that a state of cure could be achieved (by a thiazole-guanidine system) without zinc oxide, which was not only adequate but which produced vastly enhanced crosslinking and modulus values.

An early experiment with emulsion SBR 1502 reinforced by 50 phr Hi-Sil 233 illustrates this phenomenon (table 1). The three fold increase in modulus was accompanied by a large improvement in laboratory abrasion resistance. However, since the NBS abrasion test used at that time was, and remains, susceptible to the lubricating effects of free fatty acid (unreacted zinc oxide), confirmation of abrasion improvement was uncertain. Later work with emulsion and other polymer types using the Pico method confirmed that sizable increases in wear resistance properties actually occurred (table 2). Unfortunately, the presence of excess fatty acid as also an obstacle to the commercial development of zinc-free silica compounds. This was particularly the case for hard shoe soling where improved durability was off-set by a loss in adhesion. Solution polymers offer an obvious remedy to these problems.
Table 1 - emulsion SBR 1502 reinforced by 50
phr Hi-Sil 233
Zinc oxide 4 0
Stearic acid 2 0
Polyethylene glycol 3 0
300% modulus, MPa 2.8 8.7
Durometer 67 79
Table 2 - zinc free systmes ith emulsion
 300% modulus, Pico abrasion
Zinc oxide and MPa index
stearic acid Present Absent Present Absent
SBR 5102 3.1 3.5 83 101
SBR 1708 1.9 4.3 70 83
SBR 8107 1.9 4.9 104 132
BR 8407 3.7 6.9 175(1) 270(1)
SBR 1551 3.5 6.4 82 114
(1) NBS abrasion index
Formula: polymer - 100; Hi-Sil 210-50; oil - 5; ODPA - 1;
sulfur - 2; MBTS - 1.0; DOTG - 1.5; ZnO - 5; stearic acid
- 2.

The purpose of this article is to report the latest compounding technology involved in attaining highly abrasion resistant practical silica compounds based on zinc-free curing systems. In addition to polymer influence, the role of accelerator type, silane coupling agents, insoluble zinc oxide and sulfur concentration has been examined. Application of the zinc free system to carbon black reinforcement is also discussed.

Summary of zinc-free effects

A summary of the curing and vulcanizate properties of compounds based on zinc free and normal curing systems confirms the improvement in abrasion resistance and modulus found previously with emulsion SBR (tables 3 and 4). The low Pico weight losses of the BR 1220 blend are similar to those of comparable HAF compounds. Other properties which are altered by the exclusion of soluble zinc include rheometer crosslinks, hardness, viscosity and scorch activity (all increased). Changes in tan delta and elongation depend on polymer type. Tan delta is significantly reduced with SBR 1215 and slightly increased with BR 1220. In block SBR 1205 a temperature sweep showed an abrupt rise in tan delta above 50 [degrees] C in zinc-free compounds.
Table 3 - zinc-free cure effects with SBR 1215
Zinc oxide and stearic acid Present Absent
160 [degrees] C cure rate T90, minutes 33 29
130 [degrees] C scorch T5, minutes 30+ 10
ODRheometer crosslinks, dNm 60 90
Viscosity, ODR minimum 15 22
Durometer 70 80
Elongation, % 640 530
M300, MPA 3.2 9.0
Pico abrasion
 Weight loss, mg. 51 34
 Index 53 84
TAn delta, DMA 1 Hz, 60 [degrees] C .110 .067
Formula: 1215 - 100; Hi-Sil 210 - 50; Aro. resin - 10;
ODPA - 1; sulfure - 2; TBBS - 2; ZnO - 3 or 0; stearic acid
-2 or 0.
Table 4 - zinc free cure effects with BR 1220
Zinc oxide, stearic acid, PEG Present Absent
Cure ate 160 [degrees] C, T90 minutes 51 6.1
Mooney scorch 130 [degrees] C, T5 minutes 25 4
MDR crosslinks, dNm 22 27
Mooeny viscosity ML 100 140 158
Durometer 71 74
M20. MPa 1.06 1.22
M300, MPa 5.7 6.5
Tensile, MPa 12.5 18.8
Elongation, % 540 680
Pico abrasion, MG loss (index)
 At 23 [degrees] C 37 (70) 25 (102)
 At 100 [degrees] C 51 (68) 37 (92)
Pendulum rebound (Z), %
 At 23 [degrees] C 55.2 55.6
 At 100 [degrees] C 67.4 61.4
Compression set 3D 100 [degrees] C, % 78.4 81.0
DMA: 1 Hz, 30 [degrees] C
 E', MPa 19.5 24.5
 E'. MPa 1.70 2.37
 Tan delta .087 .097
Formula: BR1220 - 60; SBR1215 - 30; NR - 10; HI-SIL
210 - 50; Cl resin - 10; ODPA - 1; sulfur - 2; Zn0 - 3 or 0;
PEG - 1 or 0 ; stearic - 1 or 0;
TBBS 3 1
DOTG 0 1.5

Processing of silica reinforced solution polymers without soluble zinc was quite difficult. Although SBR 1205 was helpful in reducing roughness (at the expense of increased set), only the addition of 10 phr natural rubber (or polyisoprene) produced compounds which could be considered suitable for factory operations. A second important feature supplied by natural rubber was a significant increase in cure rate. The available polymer evaluations indicate that a blend with suitable processing characteristics would include BR-60, SSBR-30 and NR- 10.


Early studies of insoluble zinc oxide cure systems were carried out with thiazole-guanidine combinations, usually MBTS-1 and DOTG-1.5. Current work which examined the performance of individual accelerators at a concentration of 2 phr revealed a wide range of efficiency (table 5). In respect to Pico abrasion (expressed as weight loss at 23 [degrees] C and 70 [degrees] C) and 300% modulus only TBBS, DPTH, DPG and MBTS provided a satisfactory level of reinforcement in the BR/SSBR/NR blend. Among these, MBTS appears to be somewhat deficient in crosslinking and modulus, and efficient performance of DPG (and DOTG) was accompanied by prohibitively low scorch safety. Lower compression set and heat build-up were obtained with DPTH. A sinlilar evaluation of TBBS, TCS and TBTD in SBR 1215 again showed TBBS to be most effective in terms of modulus and abrasion resistance.


Thus, at this point in the program either TBBS or DPTH can be recommended as accelerators most suitable for the zinc-free system.

These data allow a general statement that only thiazole or guanidine accelerators can function without soluble zinc oxide; thiuram types, except for those with bulky amine groups, require insoluble metal oxide activation. Further evidence of this basic difference in acceleration mechanism is discussed below.


Variation in sulfur concentration from 0.5 to 4 phr in BR/SBR/NR accelerated with TBBS produced the expected increases in crosslinking, hardness and modulus; compression set and flexometer heat build-up declined. However, at 0.5 phr, Pico abrasion losses failed to attain the normal 24 mg. value characteristic of efficient zinc-free systems with I to 4 phr sulfur. When sulfur donor DPTH replaced TBBS, these sulfur related trends were barely discernible. With DPTH at 3 phr, a sulfur level of 0.5 phr provided satisfactory abrasion resistance and, more importantly, a significant improvement in heat resistance (table 6).


Insoluble zinc oxide

As long as a solubilizing fatty acid or polyethylene glycol is absent, zinc oxide will not react with silica and can therefore be used to activate those accelerators which are otherwise unsuitable in a (soluble) zinc-free system. As previously noted, natural rubber at 10 phr increased cure rate. The presence of polyisoprene or natural rubber also influences the effect of insoluble zinc oxide. Without NR, insoluble zinc oxide provides small improvements in abrasion with TBBS acceleration, but none with NR present. However, with accelerators which failed to achieve a full cure (table 5), insoluble zinc or lead oxides produce drarnatic improvements. This is particularly true for ZBDC where added insoluble lead oxide increased modulus and abrasion resistance to values equal to the best attained with TBBS and, in addition, reduced compression set from 85% to 61%. A similar set reduction occurred with DPTH (table 6). Unfortunately, scorchiness may preclude practical application (table 7). A less dramatic example of insoluble zinc oxide activation occurs with TCS accelerated SBR 1215/NR which benefitted in respect to cure rate, modulus and Pico abrasion, but sustained a precipitous rise in viscosity. In most cases, lead oxide (litharge) and zinc oxide produce similar results. In all cases, the addition of polyethylene glycol, benzoic acid and similar additives reduced abrasion resistance.
Table 7 - insoluble oxide activation
ZBDC 2 2
Lead oxide - 2
Cure rate 160 [degrees] C T90, minutes 12 2
MDR crosslinks, dNm 13 29
Moooney scorch 130 [degrees] C T5, minutes 23 5
23 [degrees] C Pico abrasion, mg loss 49 21
70 [degrees] C Pico abrasion, mg loss 71 34
300% modulus, MPa 2.4 7.2
Compression set 3D 100 [degrees] C, % 93 61
Formula" BR1220 - 50' SBR1250 - 40; NR - 10; Hi-Sil
210 - 50; Cl resin - 12; ODPA - 1; sulfur - 2

A significant fact which emerges from these results is that zinc or lead oxides do not require added fatty acid to activate thiuram accelerators or react with sulfur. Since abrasion resistance has improved, we can conclude that the metal-sulfur-accelerator complex does not react with surface silanols to impede silica-polymer bonding. Reaction of insoluble zinc oxide with sulfur (to form zinc sulfide) evidently reduces the proportion of polysulfide crosslinks and thereby produces lower compression set and markedly improved heat aging resistance (table 6).

Relation to carbon black

In keeping with the objective of attaining abrasion reinforcement equal to that of carbon black, comparisons between zinc-free silica and conventional HAF compounds were made in various polymers. Results obtained in a BR-SBR-NR blend indicated that abrasion equality was present at 23'C but not at 700C (table 8). Although rheometer crosslinking with silica exceeded that with HAF, modulus was lower. A similar anomaly occurred between flexometer heat build-up and tan delta where a 50% reduction in thc latter contrasts to a slight increase in the fonner.


Equally interesting results were seen in applying the zinc-free system to HAF compounds. Without zinc oxide and stearic acid, increased crosslinking and modulus were accompanied by reduced tan delta and heat build-up. Addition of insoluble zinc augmented these trends and produced a remarkable reduction in compression set. These generally favorable results indicate that combined silica and black reinforcement would benefit from zinc-free curing systems.

Silane coupling effects

The silane coupling of silica in zinc-free systems effects further increases in modulus and abrasion resistance as well as the customary reductions in compression set, heat build-up and viscosity. Generally, less silane is required to produce these effects in zinc-free than in normal soluble zinc curing systems. For example, a mercapto silane concentration of only 2% (silica basis) has provided reinforcement properties significantly above those of a comparable HAF control in BR/SSBR blends. A normal cure system would involve three to five percent to attain the same effect.

The addition of the non-coupling methyl silane (A163) for possible viscosity reduction produced higher abrasion losses and a general decay in other properties.

The unusual properties of mercapto silane coupled zincfree silica compounds include dynamic mechanical behavior as well as abrasion reinforcement. An equal hardness comparison of Ciptane (Hi-Sil 210 treated with silane A189) with HAF in SBR 1215 illustrates the vastly superior dynamic properties which can be achieved by silane coupling in a zinc-free environment (table 9). The combination of high loss modulus E" at 0 [degrees] C, stiffness retention at 90 [degrees] C (E'), and extremely low tan delta at 60 [degrees] C is noteworthy.


Compounds reinforced with silica from which the free water has been removed by drying generally exhibit improvement in abrasion resistance and in properties related to cure state. Thus it could be predicted that more efficient coupling would also occur in the absence of silica surface water.

Results obtained with a BR/SSBR formula which included insoluble zinc oxide confirm a slight but consistent trend to more effective coupling with dry silica, and demonstrate that water must be regarded as an additional barrier to silica-polymer bonding.

Electron microseopy

Previous work on silica reinforcement included the use of scanning electron microscopy to reveal filler structure in compounds with and without silane coupling (ref. 3). These studies showed that silica polymer coupling produced a reduction in silica agglomerate size. Similar treatment of zinc-free vulcanizates revealed a corresponding de-agglomeration (figure 1). The procedure included freeze fracturing under liquid nitrogen, coating with approximately 200 angstrom platinum via ion beam microsputtering and mounting with silver adhesive. Surfaces were exaniined in mirror fracture origin areas and photomicrographs taken. Among 15 views, over half show a distinct size difference between zincfree and soluble zinc compounds. Surface relief is lower in the absence of zinc.

Generally, filler agglomerate reduction can be pictured in terms of a change in the ratio of filler-filler to filler-polymer bond strengths. As the ratio shifts in favor of the latter, more de-agglomeration occurs during mixing. The accompanying improvement in reinforcement can be attributed to both smaller filler units and stronger filler-polymer bonds. As previously noted, zinc-free vulcanizates benefit from increased modulus, tensile and abrasion properties. Since these changes parallel those associated with silane coupling, it can be inferred that the mechanism is similar, that is, an increase in silica-polymer bond strength, which results from elimination of the zinc barrier on thc silica surface.


Studies of zinc-free curing systems for precipitated silica reinforced solution polymers have demonstrated that abrasion resistance equal to that imparted by HAF carbon black can be achieved without the use of silane coupling. Practical compounds were based on acceleration by TBBS (N-T-butyl-2-benzothiazyl sulfenamide) or DPTH (dipentamethylene thiuram hexasulfide). Application of the zinc-free concept to all-black compounds produced significant reductions in tan delta, compression set and heat build-up.


[1.] Bachinann et al, Rubber Chein. Technol. 32, 1286 1959). [2.] H.J. Wartmann and C.R. Strauss, "Analysis of cure parameters to define vulcanization and reinforcement," paper presented at Spring meeting of the Rubber Division, ACS, Miami Beach, May 1965. [3.] N.L. Hewitt, Rubber World, Max, 1990.
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Author:Hewitt, N.L.
Publication:Rubber World
Date:Sep 1, 1992
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