reduced levels of zinc oxide in the vulcanization of rubber compounds

Reduced levels of zinc oxide in the

vulcanization of rubber compounds

Engº Luis A. Tormento28/06/2016

Introduction The vulcanization process, or the process by which the rubber is heated with

sulfur to create a network of cross chemical bonds, was invented by Charles Goodyear already in 1839. The vulcanized product:

• It is not sticky like raw rubber,• It does not harden in cold weather nor soften at high temperatures;• It is resilient, returning to its original shape after deformation or application loads

and is highly abrasion resistant.

The process, a technological breakthrough, has been and is being improved since then. The rubber industry had to wait until the first decades of the 20th century, when three discoveries greatly improved the quality and durability of rubber:

• organic accelerators,• Reinforcement by carbon black and silica, and• Activators such as lead oxide, magnesium oxide and calcium oxide.

Introduction• The discovery by Oenslager of the first organic accelerator in 1906, however,

gave a great improvement in the rubber vulcanization process, later considered a major advance in the rubber industry.

• The non-accelerated vulcanizing process uses elemental sulfur in the amount of 8 per 100 parts of rubber (phr), with a temperature of 142° C for 6 hours. With the addition of some parts of the organic accelerator, vulcanization time was greatly reduced. Furthermore, it was found that zinc oxide (ZnO) acts as an activator, increasing the effect of most organic accelerators.

• The use of accelerators in combination with zinc activators, has a marked effect on the vulcanization rate and distribution of crosslinks formed. The advantage of organic accelerators lies not only in shortening the vulcanization time, but also in reducing curing temperature and reducing the required amount of sulfur. Furthermore, the durability of rubber articles as well as the physical properties, particularly tensile strength, are improved by the use of these organic accelerators.

Introduction Small amounts of zinc can enter in the environment during the manufacturing

process of ZnO, through specific air emissions or waste or used waters. Certain aquatic species have been shown to be sensitive to low levels of zinc, particularly the soluble zinc compounds, which are classified as ecotoxic to aquatic organisms. ZnO is also likely to be classified as such, which may lead to restrictions. ZnO can be used safely in many applications, while the excess release is controlled. However, if there are spills in waterways, with ZnO in powder form, the dispersion index was such order, there would be no way to immediately control the toxic effect.

Introduction In view of future legislation and eco-labeling requirements for vehicle tires, for

example, it can be said that it is desirable to keep the ZnO content in the lower rubber compounds possible, not only for the environment but also for economic reasons . The growing concern on environmental protection leads us to assume that soon there will be restrictions on the use of ZnO in rubber compounds.

• The objective of this work is to show the best and new routes to reduce the harmful environmental impact of rubber compounds, with special emphasis on activators. The focus is on nano ZnO, because in the rubber industry, is well known the fact that derivatives of zinc are the best activators for sulfur vulcanization. The complete elimination of zinc compounds would be very ambitious. This paper presents an alternative vulcanization system using a new production process of ZnO - nanotechnology - where the produced ZnO has a structure that allows it to be used in such small amounts as those found in the environment; and even at that level, it provides significant improvements in the physical properties of rubber products and important advances in curing speed.

Environment Zinc occurs naturally in soil, sediments, water and air, where it is picked to fill

out essential metabolic functions. As such, distribution, transport and effects (bioavailability) of zinc in the environment, it will largely depend on the specific physical and chemical characteristics and the body. For these reasons, to be meaningful, the availability of zinc in the environment must take into account these factors.

While the environmental quality standards for zinc in water and soil range from 20- 100 µg / 40-100 mg / kg dry weight, respectively, the forecast of the environmental concentration of intended uses for nano ZnO result in concentrations of at least an order of magnitude below these standards.

Environment Studies have shown that the nano ZnO has the same environmental profile of

the bulk ZnO. As such, the Risk Assessment Report in Europe concluded that: "current use of nano zinc and zinc compounds, alone, would not lead to regional higher levels than those found in surface water and sediments".

The probabilistic assessment of the environmental effects of nano ZnO, estimates that contributions to ambient concentrations of intended uses would not result in risks to organisms in the water or soil.

Reports of work of ZOPA (Zinc Oxide Producers Association), EFSA (European Food Safety Authority), EWG (Environmental Working Group), and IZA (International Zinc Association).

Activators effect on the vulcanization accelerated by sulfur

Elastomers and rubbers play a very important role in modern technology. They constitute a class of materials such as metals, fibers, wood, plastic or glass.

The properties exhibited by a rubber are determined by the composition of the compound, a mixture of chemicals and additives, and the curing process, the latter process one of the key factors in rubber technology.

Although vulcanization by sulfur is a very old industrial process, considerable research has been conducted to study this process over the last decade; but a complete understanding of this complex chemical process remains a scientific challenge. The difficulties encountered in this research field are credited to a range of chemical reactions that take place more or less simultaneously during the vulcanization process, ranging from reactions on the surface of metal oxides for chemical radicals.

Effect of activators in the accelerated sulfur vulcanization

Reduction of vulcanization time through yearsComponent/year 1850 1880 1905 1920

Natural rubber 100 100 100 100

Sulfur 8 8 6 3

ZnO - 5 5 5

Aniline - - 2 -

Stearic acid - - - 1

MBT - - - 1

Vulcanization 142ºC, minutes 360 300 180 30

Vulcanization and the role of activators and accelerators

Vulcanizing agents are mostly based on sulfur or peroxide and sometimes other special agents or high-energy radiation. The vulcanization by sulfur can only be applied to rubbers with unsaturation in its chain or unsaturation on side (pending) groups. The type of crosslinking formed in the case of sulfur vulcanization largely depends on the vulcanization system. Crosslinking may be predominantly mono-, di- or polisulfidcs. The distribution of the length and type of these crosslinks is important, since it affects the thermal stability and physical properties of the vulcanizate.

The vulcanization may be characterized fairly by measuring the evolution of crosslinking as a function of time, usually called curing curve with an oscillating disk rheometer (ODR) or oscillating cavity rheometer (MDR) or processing analyzer of rubber (RPA). With these rheometer, torque is measured and the shear modulus as a function of time at a given temperature. It is implicitly assumed that the module is proportional to the concentration evolution of crosslinking.

Vulcanization and the role of activators and accelerators

Parallel to the development of accelerators, activators were discovered. It is reported in 1905 that ZnO serving as a reinforcing filler for rubber compounds. Since 1912, however, ZnO was gradually replaced in tire by a high level of carbon black to develop suitable physical properties. Activators / inorganic accelerators such as lead oxide, magnesium oxide and calcium oxide, were added to reduce the vulcanization time. In the early 1920s the role of activators was discovered. It was found that ZnO combined with stearic acid (SA), reduced time of vulcanization and improved the properties of rubber, even if not accelerate vulcanization. Fatty acids such as stearic acid, are used to solubilize the zinc and releasing zinc ions in the system to form complexes with accelerators. In various special rubbers MgO, Ca (OH) 2 and PbO are used.

A subcategory accelerated sulfur vulcanizing system is the sulfur donor systems. In these systems, the sulfur necessary for the formation of the network is provided by the accelerator which functions as both an accelerator and a sulfur donor. It should be noted that while not accelerated sulfur systems are no longer commercially important, they are of interest as a starting point to understand the accelerated sulfur vulcanization systems.

Vulcanização e o papel dos ativadores e aceleradores

The complexity of vulcanization by sulfur above mentioned, has given rise to many publications on the vulcanization process reaction mechanisms. The situation is further complicated by the interaction of accelerators and activators, each component influencing the reactivity of the other, and these interactions affect the curing mechanism. The proposed mechanisms ranged from radical to ionic. Several researchers concluded that both radical and ionic mechanisms are present.

ZnO as vulcanization activator ZnO is still known as the best activator for vulcanization by sulfur. As mentioned earlier,

there is a growing concern about the potential environmental and health effects in the release of zinc compounds into the environment of rubber products or rubber compounding.

Numerous articles about the effect of ZnO in curing rubber were edited, In many vulcanization systems, ZnO is a precursor of zinc-accelerator complex. It reacts with most highly active accelerators forming a zinc salt. The formation of the zinc ion from the complex with different accelerators is critical for efficient cure. The above reaction with stearic acid forms a soluble salt hydrocarbons and releases water before the crosslinking.

The crosslinking efficiency of the natural rubber (NR) and rubber polyisoprene (IR) is more pronounced when ZnO is present. ZnO appears to cause a small increase in cure time and process safety (Scorch).

ZnO as vulcanization activator In addition to its role as an activator for sulfur vulcanization, there is also evidence that

the addition of ZnO in a compound reduces heat generation and improves tire wear resistance. ZnO acts as a "heat sink" that absorbs the energy generated by friction, without a large increase in internal temperature. It has also been discovered that ZnO improves the heat resistance of the vulcanizates, and its resistance to dynamic loading action. The high thermal conductivity of the ZnO helps to dissipate the heat taking place at concentrations that could affect the properties of the rubber. The ZnO properties are particularly beneficial in applications such as vibrators and shock absorbers where the load and cyclic stresses might generate heat and degrade the rubber. ZnO is also necessary for the binding of rubber to steel cord tire and the bond between the metal and rubber in technical products.

In addition to improving the properties of vulcanized rubbers, ZnO also assists in the processing of the uncured rubber. The ZnO is added to rubber compositions to reduce the shrinkage of molded articles of rubber and keep molds clean. This increases the productivity by increasing the number of molding cycles and cleaning. In the past zinc compounds were added as a filler and to absorb harmful reaction products. The use of ZnO as the filler is not economically feasible due to its high cost; for this reason, it was replaced by cheaper fillers (carbon black or silicates).

ZnO as vulcanization activator In rubber formulations for other types of vulcanization systems, for example peroxides,

ZnO is often also present, although not activate peroxides. The role of ZnO in a peroxide cured compound is as an acid acceptor for acid materials present in fillers or polymers containing halogens. ZnO is also added to improve the aging resistance, possibly by reaction with acids of the by products generated by crosslinking reaction. It is also a crosslinking agent in polymers containing halogen as well as those containing carboxyl groups.

In another mechanism, it is assumed that the ZnO is distributed in the form of crystalline particles in rubber mixtures. Molecules of sulfur accelerators and fatty acids diffuse through the rubber chain and are adsorbed on the ZnO intermediate, with the formation of complexes. The most important control parameter here is the dispersion of inorganic ZnO in the rubber.

Effects of the level of ZnO Traditionally rubber compounds have 5-8 parts of ZnO. The levels depend on specific

applications and are optimized based on performance requirements. Over the years, lower levels have been tested and according to various researchers, it appears that ZnO levels can be reduced to a minimum of about 2 phr to activate rubber compounds. A small reduction in the module can be compensated by increasing the accelerator level. Higher levels of ZnO are typically used for EPDM.

A parameter that is referred to and affects the activity of ZnO is its surface area. In so-called highly active zinc oxide, the average particle size is reduced and increased specific surface area, which results in easier and better dispersion. Active zinc oxides are usually prepared with specific surfaces (BET) ranging from 30 to 70 m2 / g, compared with ~ 6 m2 / g in the conventional ZnO red seal. The highest activity is probably due to increased accessibility of Zn2 + ions on the surface, compared to the conventional size particles.

Efeitos do nível de ZnO One of the most recent developments in this field is the use of so-called nano ZnO.

Investigations of the nano ZnO use in compounds of NR / BR showed significantly improved physical properties, particularly abrasion resistance, heat generation and tera resistance. The most promising role in the use of nano ZnO is as a reduction of ZnO content in rubber compounds, which is highly desirable from an ecological point of view. The US Environmental Protection Agency considers ZnO a toxic chemical to aquatic organisms. The European Union classifies ZnO as a dangerous chemical and states that its application in rubber technology should be reduced and controlled. Keep the ZnO content in the rubber as low as possible helps alleviate the difficulty of disposing of waste zinc (such as ZnS), in the compounds.

Compared with the traditional zinc oxide, nano ZnO has a large specific surface area, high chemical activity, high purity; the particle size can be adjusted as needed; the product has unique properties, such as the photochemical effect and better performance in protecting to ultraviolet - UV protection rate is up to 98%; It has antibacterial effect, and anti-enzyme effect.

Effects of the level of ZnO O nano ZnO é esférico, com distribuição uniforme de tamanho de partículas e com um

tamanho médio de partícula menor que 60 nanômetros. Os testes de área superficial específica e análise do tamanho dos poros mostraram que a superfície BET do nano-ZnO é cerca de 35m2/g.

Use of nano ZnO in Rubber Use in Natural Rubber

Ingredient (phr) ZnO Conventional ZnO NanoNR (SMR-5) 100 100ZnO 5 2Stearic acid 2 2Carbon black HAF 40 40TDAE 8 8TMQ 1 1CBS 0,6 0,6TMTD 0,1 0,1Sulfur 2,5 2,5Physical Properties (6 minutes @ 150ºC) Hardness (Shore A) 66 65Tensile Strength (MPa) 31,04 31,48Elongation at Break (%) 893 849Modulus at 300% (MPa) 6,32 6,41Tear Strength (N/mm) 105 104Compression Set (%) 39,31 33,82Abrasion Resistance (cm3/hr) 3,71 3,23Heat build-up (∆T) ºC 4,4 3,2Toluene immersion (Flory equation)Density of reticulation (10-5/kg) 66 65

NR compound and properties with common ZnO and nano ZnO


In particular, the properties of natural rubber depend on the compound formula and vulcanization.

Testing the effect of a low content of ZnO in the vulcanization of natural rubber were prepared containing a series of vulcanizates 0.25; 0.3; 0.4; 0.5; 1, or 2 phr of nano ZnO. Study of the cure characteristics of these vulcanizates and curing characteristics to 5phr of normal ZnO at 150 ° C:

The low loading nano ZnO (0.25, 0.3, or 0,4phr) was not effective for vulcanizing. Charging with 0.5 phr of nano ZnO resulted in in a marginal decrease in the scorch time

- in fact, the vulcanization in the presence of 0.5 phr of ZnO nano was faster than the vulcanization in the presence of higher percentages of nano ZnO and normal ZnO (phr 1 or 2).

These results indicate that the nano ZnO behaves as an effective curing agent and cure activator for natural rubber.


The cure rate of vulcanizates containing 0.5 phr of nano ZnO was significantly higher than that of the vulcanizate containing 5 phr of conventional ZnO, suggesting that the natural rubber becomes crosslinked in the presence of nano ZnO. Because of nano ZnO has a larger surface area than the area of conventional ZnO, the reaction with stearic acid and accelerators is very fast. It accelerates the vulcanization process, allowing the insertion of sulfur in the polymer chain faster.

In summary, it was found that, in addition to the ecological advantages of reducing the content of ZnO in rubber, the use of nano ZnO in the vulcanization of natural rubber offers several advantages. Values from 1 to 2 phr were sufficient for vulcanization of some natural rubber compounds, showing a significant reduction in the ZnO content in the formulations, and as a rule these values fall below the value present in the soil and the environment, which assures that is not an environmental pollutant.

Use of nano ZnO in Rubber Use in Chloroprene Rubber

The polychloroprene, commonly known as Neoprene is a polar rubber, and is usually regarded as a special purpose rubber used for applications requiring oil and solvent resistance. For example, oil-resistant materials are very important in the automotive industry for a variety of components that come in contact with some fluids.

Examining the specific reasons for that chemical resistance polymers withstand chemical resistance, can be observed that the lateral polar groups tend to provide good resistance to swelling in hydrocarbon oils, induce a slight load electronegative in electronegative atom with a slight positivity around carbon atom. The dipole moment produced tends to provide oil resistance.

Chloroprene rubbers are usually cured using metal oxides. Among them the main linking agent is zinc oxide, which is used together with magnesium oxide. Lead oxides are still used at some point when low water absorption is required.


Ingredient (phr) ZnO Conventional ZnO NanoNeoprene W 100 100Maglite D 4 4Stearic acid 1 1HAF carbon black 40 40DOP 8 8TMQ 1 1ZnO 5 2ETU 0.5 0.5Physical Properties (20 minutes @ 150ºC) Hardness (Shore A) 67 69Tensile Strength (MPa) 19.82 19.48Elongation at breal (%) 489 468Modulus at 300% (MPa) 8.42 8.91Tera Strength (N/mm) 53 60Compression Set (%) 19.12 17.36Abrasion resistance (cm3/hr) 2.68 2.41Heat build-up (∆T) ºC 13.2 13.3Toluene immersion (Flory equation)Density of reticulation (10-5/kg) 7.38 7.94

CR compound and properties with common ZnO and nano ZnO


It was noted in the study of the oxides, that nano ZnO in dienic rubbers improved mechanical and reinforcing properties. The use of nano ZnO compared to the conventional ZnO, gives the following results:Comparing the same level of activation, compounds of 2phr of conventional ZnO show significant reduction in the torque values, tensile properties and other properties, compared with conventional 5phr ZnO.Vulcanizates lower dosage (2phr) nano ZnO, have a similar cure rate and tensile properties as compared with vulcanizates with a higher dosage (5phr) conventional ZnO.Tear strength values are higher for vulcanizates with 2phr of nano ZnO with respect to the conventional vulcanizates 5phr ZnO.Hardness, compression set, heat buildup and resistance to swelling, are superior with 2phr of nano ZnO than with 5phr of conventional ZnO.The crosslink density of the vulcanizates with 2phr of nano ZnO is slightly higher than the vulcanizates with 5phr of conventional ZnO

Use of nano ZnO in Rubber Use in SBR

SBR have less unsaturation than NR and its double bonds are chemically less active than the double bonds of natural rubber isoprenoid units. The SBR composition is done with a similarity comparable to NR and other unsaturated hydrocarbon rubbers. SBR requires more accelerators because of lower unsaturation.


Ingredient (phr) ZnO Conventional ZnO NanoSBR 1502 100 100ZnO 5 2Stearic acid 2 2HAF carbon black 40 40TDAE oil 7 7TMQ 1 1CBS 1 1TMTD 0.2 0.2Sulfur 2 2Physical Properties(20 minutes @ 150ºC) Hardness (Shore A) 69 69Tensile Strength (MPa) 11.8 12.0Elongation at Break (%) 402 390Tear Resistance (N/mm) 47.5 50Compression Set (%) 22.85 20.45Abrasion Resistance (cm3/hr) 2.64 2.60Heat build-up (∆T) ºC 12.9 8.8Toluene Immersion ( Flory equation)Density of Crosslink (10-5/kg) 7.16 7.36

Compounds of SBR and properties with common ZnO and nano ZnO


Compounds of 5phr of conventional ZnO have superior cure characteristics, tensile properties and other properties in comparison with compounds with 2phr of nano ZnO.

Compounds with low dose (2phr) ZnO (nano) showed similar curing characteristics, at high doses (5phr) conventional ZnO.

Tensile properties of compounds with optimal dosage of nano ZnO are comparable to those cured with 5phr of conventional ZnO.

Compression properties, loss of abrasion, heat build-up are low for vulcanizates with optimum dosage of nano ZnO.

The retention of tensile properties of vulcanizates with low dosage of nano ZnO after thermal aging is higher as compared with vulcanizates with 5phr of conventional ZnO.

The crosslink density of compounds with optimal dosage of nano ZnO is slightly higher than the vulcanizates with 5phr of conventional ZnO.

The swelling values are considered equivalent for the vulcanizates of 2phr of nano ZnO and 5phr of conventional ZnO.

Use of nano ZnO in Rubber Use in XNBR

Carboxylated nitrile rubber (XNBR) can be crosslinked with sulfur in the presence of accelerators; however, the most suitable method is the application of divalent metal oxides, in particular zinc oxide (ZnO). The crosslinking of the elastomer takes place by reaction of its carboxyl groups with zinc oxide, resulting in the formation of ionic carboxylic salts, considered crosslinkers. In contrast to the covalent crosslinks formed during the vulcanization of conventional sulfur systems / accelerator or peroxides, and are multifunctional.

Salts of the carboxylic group form clusters of six to eight members of dipolar ions forming larger multiplets, which are dispersed in the elastomer matrix without forming a separated phase. These multiplets have a considerable impact on the glass transition temperature of the elastomer and its sensitivity to water. These clusters are considered as ionic aggregates, immersed in an elastomer matrix.


The presence of ionic clusters is responsible for the improved physical properties of ionic elastomers even without the addition of load, compared with those conventionally crosslinked with sulfur and accelerators. The proportion of ionic crosslinks present in the form of agglomerates or multiplets in the elastomer network, depends on the nature and structure of the elastomer macromolecules, as well as the chemical nature and concentration of the carboxylic salt group. According to studies, the clusters are formed by multiplets association. This association is caused by electrostatic interactions between multiplets, and is hampered by the elastic forces of shrinkage of the polymer chain. The restricted mobility of the elastomer chain in the vicinity of ion clusters results in the formation of a hard phase.


Zinc oxide is very effective and used as the crosslinking agent for carboxylated elastomers. It can be used for the production of vulcanizates with high: tensile strength, tear strength, hardness and hysteresis.

The improved mechanical properties of ionic elastomers are mainly a result of its high capacity for relaxation of tensions, due to the chain of the elastomer sliding over the ionic cluster and the reform of ionic bonds under external deformation. Furthermore, the ionic elastomers have a thermoplastic character and can be processed in the molten state as a thermoplastic polymer. However, there are some disadvantages on the carboxylic elastomers cured by zinc. The most important are: low scorch, low bending properties and high permanent deformation. In order to overcome these characteristics, the carboxylated nitrile rubber is crosslinked with zinc peroxide or a system blended of znc peroxide / zinc oxide. The XNBR vulcanization with zinc peroxide leads to the formation of ionic crosslinking. However, the necessary vulcanization times are higher for those vulcanizates and withstand excellent traction and crosslink density comparable to vulcanizates crosslinked with zinc oxide.


The vulcanization times obtained are considerably higher compared with those of XNBR crosslinked with zinc oxide. So surpass the scorch problems. Taking into account the fact that during the crosslinking process, the zinc oxide reacts with the carboxylic groups of the elastomer, which leads to the formation of carboxylic salts (ionic crosslinking). The most important parameters which influence the activity of zinc oxide is its surface area, particle size and morphology. These parameters determine the size of the interface between the crosslinking agent and the elastomer chains.

The size of the zinc oxide particles is the main parameter influencing the activity of ZnO. The reduced particle size and increased specific surface area of the zinc oxide, providing better contact between the particles of the crosslinking agent and the elastomer strands is the main advantage of the nano ZnO in the crosslinking of XNBR. Nano ZnO achieves higher level of crosslinking and formation of denser links between the polymer chains; This contributes to the formation of compounds with less permanent deformation, higher heat resistance and improved resistance to oils and solvents.

Thank You

Thank you

Luis A. TormentoNPD DirectorLT Químicos

Tel: +55 (11) 5581-0708E-Mail: [email protected]

mailto:[email protected]

reduced levels of zinc oxide in the vulcanization of rubber compounds

Engineering