4.24.1 Paraffin Waxes ……… 155 4.24.2 Microcrystalline Waxes ……….. 155
Non Linear Paraffin Waxes ………... 155
Ceresine Wax ……….. 155 4.24.3 Polyethylene Waxes ……….. 155
4.1 Accelerators
The rate of vulcanisation of a rubber compound is controllable by the choice of accelerator. The range of products offered to the rubber industry has been categorised historically into recognised classes. New developments have resulted in products that improve compound performance and which overcome dermatological problems, and do not generate nitrosamines and other extractable or volatile decomposition products.
Much of the accelerator volume used by the rubber industry is now supplied in treated forms to facilitate automatic weighing and reduce health contact problems.
4.1.1 Amines
Most of the materials in this category can be described as slow in character in their effect on vulcanisation. In most cases, they are seldom used on their own, only in combination with other accelerators being used as boosters.
4.1.2 Dithiocarbamates
Dithiocarbamates are generally classed as ‘ultra’ accelerators. When used alone they suffer from poor processing safety, due to a very rapid vulcanisation speed. They are usually used in combination with slower acting accelerators, except in the latex industry where elevated processing temperatures are not encountered.
Dithiocarbamates are polar in nature and thus will have a tendency to bloom when used in non- polar rubbers. The worst bloom problems will be encountered with the dimethyl derivatives; higher alkyl chain types have better solubility in non-polar rubbers. Some dithiocarbamates also confer antioxidant protection.
4.1.3 Dithiophosphates
Used in combination with other accelerators, the dithiophosphates reduce the risk of bloom and the formation of nitrosamines. Generally reversion stability is improved. The copper dithiophosphate will cause discolouration and contact staining.
4.1.4 Guanidines
Guanidines, when used alone, have a long scorch time and a long vulcanisation time requirement. Generally, when used as the sole accelerator, the properties of the compound, especially resistance to ageing, are poor. However, in combination with accelerators such as mercapto accelerators, they have a significant effect on the behaviour of the secondary accelerator increasing crosslink density and vulcanisation rate. They behave synergistically with thiurams, dithiocarbamates and to a lesser extent with sulphenamides giving considerable activation effects. Di-o-tolylguanidine can be used as a plasticiser for polychloroprene. The particle size of the guanidines can be critical; above 200 mesh they will not disperse readily.
4.1.5 Thiazoles
Thiazoles are classed as ‘semi-ultra’ accelerators. It is generally recognised that thiazole-type structures give better reversion resistance than the dithiocarbamates and thiurams. The term ‘sulphur donor’ was evolved to recognise that this type of sulphur containing accelerator could contribute to the chemical crosslinking process itself, by directly linking into the main polymer chain structure. It is also known that they confer some degree of antioxidant protection.
4.1.6 Mercapto Types
The mercapto accelerators are used in many rubber types and are very efficient. They confer good processing safety to the compounds, with intermediate vulcanisation rates, a broad vulcanisation plateau and very good ageing resistance. Used alone they give a relatively low crosslink density. In combination with other accelerators they often act synergistically giving faster vulcanisation and a considerably higher crosslink density. They do not usually confer optimum heat stability to products. In low sulphur compounds (semi-efficient) the mercapto accelerators are not very effective alone. In semi-efficient vulcanising systems in combination with bases they do, however, become very effective. However, such basic activation systems can have a detrimental effect on reversion stability in reversion prone compounds.
4.1.7 Benzothiazole Sulphenamide Types
The accelerators in this class become active as the amine groups are split off during vulcanisation. This accelerator type produces delayed commencement of vulcanisation, thus improving the compound’s processing character. Their thermal stability depends on the substituents on the nitrogen atom, giving various degrees of retardation. Vulcanisation with these accelerators allows for good compound flow time, with a rapid crosslink density increase after the initial delay. Problems can arise when compounds are stored for long periods as the chemical structure of these accelerator types can alter depending on their history, giving reduced processing safety and loss of the expected vulcanisation start delay. Secondary accelerators, such as thiurams and dithiocarbamates, synergistically increase the vulcanisation speed.
Benzothiazole sulphenamide accelerators are suitable for semi-efficient and efficient vulcanisation systems.
4.1.8 Thioureas
Thioureas mainly find use for the vulcanisation of CR, epichlorohydrin (ECO) and some ethylene propylene diene terpolymer (EPDM) compounds. They show high crosslinking activity, with usually adequate compound flow time before onset of the crosslinking. In EPDMs, the thioureas are used as activators for low activity third monomer types and, in the presence of calcium oxide desiccants, in free state vulcanisation of extrudates, etc. The use of thioureas can overcome the retardation caused by the desiccant. In this case some care must be taken otherwise overcompensation may occur. Thioureas are not used in food product applications and are a known health hazard, particularly for pregnant women.
4.1.9 Thiurams
Thiuram accelerators break down into the corresponding dithiocarbamate before becoming active. There is thus a delay in the onset of vulcanisation. Thiurams can be used over a wide range of sulphur systems. They can behave as system activators when used in combination with thiazoles, whilst acting as retarders for dithiocarbamate and xanthate systems.
A drawback for this class of accelerators is that secondary amines can split off, which may possibly then form nitrosamines.
In low or sulphurless systems, thiurams containing more than one sulphur atom act as sulphur donors. They also confer low tendency to reversion, and good heat stability.
4.10 Xanthates
Classed as ultra accelerators, xanthates are among the fastest of accelerators available to the rubber compounder. Their speed is such that they find only limited application in solid rubber product manufacture but they are used in low temperature curing of latex articles.
4.1.11 Miscellaneous Accelerators
A number of chemicals are used which do not fall into the general categories of the conventional accelerator types used by the rubber industry.
Chemicals such as magnesium and lead oxides, calcium hydroxide, antimony tri- and pentasulphide can be used as boosters for organic accelerators for some products, such as thick walled large volume articles, e.g., rollers.
4.1.12 Mixed Accelerator Packages
A number of accelerator systems used specifically for known processes are provided in blends often with an added processing additive.
4.2 Activators
4.2.1 Metallic Oxides Iron Oxide
Iron oxide is used for compounding silicone rubbers where it is used to improve heat stability. Iron oxide, mainly ferric oxide, also finds application as a pigment.
Lead Oxide
Lead monoxide or litharge is mainly used as an accelerator activator, and acid acceptor, and is especially useful in water resistant compounding. Litharge can also be used in long slow cures for such articles as ship’s fenders, bridge bearings, etc., where it enables the inside of the product to fully cure before the outside becomes overcured with the possibility of degradation or reversion of the outer layer of the product.
Litharge can also be used to provide compounds with high specific gravity and provides low permeability to, and protection from, radiation.
Magnesium Oxide
Magnesium oxide is used as a compounding ingredient for a number of purposes. Light magnesium oxide is used as an acid acceptor in the vulcanisation of halogen containing rubbers; heavy magnesium oxide, which has a larger particle size than the light grade, is less active as an acid acceptor. It is, however, used in heat resistant seal compounds. Magnesium oxide is also used as a cure modifier in halogenated butyl rubbers.
If stored for too long, magnesium oxide can convert to magnesium carbonate or hydroxide with resultant loss of activity as an acid acceptor. Magnesium oxide supplied in stick form overcomes this problem. Some forms of pelletised magnesium oxides can be too hard to break down in soft compounds and problems can arise from hard undispersed ‘ceramic’-like particles.
Some proprietary blends of magnesium oxide and zinc in the desired proportions for CR compounds are available with the addition of some processing additives to ensure the best dispersion characteristics.
Zinc Oxide
Zinc oxide plays a multi-functional role in rubber technology. Its main use is as an activator of the sulphur crosslinking reaction. Zinc oxide has been used as a pigmentation ingredient where it is particularly effective in absorbing ultraviolet rays. With high thermal conductivity and heat capacity in comparison to other compounding materials, zinc oxide can be used in some bulky product applications as an aid to heat conduction through a rubber mass to shorten cure times. It can, at reasonable addition levels, also assist with reduction of mould shrinkage values.
Zinc oxides are prepared from zinc metal by two main processes:
• the ‘direct’ or American process in which zinc containing ores are calcined in reducing
conditions with subsequent burning of the resultant zinc in air,
• the ‘indirect’ or French process wherein purified zinc is sublimed and burnt in air to form the oxide; this method gives zinc oxides of the highest purity.
Both these methods give zinc oxides of low activity. Zinc oxide from the American process can have a varying sulphur content, dependant upon the ore’s source, and unless known and allowed for, this can affect the compound vulcanisation rate.
French process material, in general, tends to have a blockier particle shape with a relatively narrow particle size range in comparison with the American process product. The French process oxides also exhibit a finer particle size and hence have a higher surface area.
Zinc oxides from wet chemical routes can be prepared to fineness values which control and enhance their reactivity in the rubber compound, with consequent higher activity rating. This route involves the initial precipitation of zinc carbonate followed by drying and calcination to remove water and carbon dioxide. The resultant zinc oxide is often characterised by having a high specific area. These grades do have a high tendency to agglomerate together and thus are often supplied in dispersed form for rapid and adequate dispersion in the rubber.
Zinc oxides can be prepared from chemical industry by-product sources and from zinc soaps from a variety of industrial processes. These grades are generally off-coloured and consequently considered of lower grade and offered at lower cost and are confined to use in black compounds. These grades can also vary in consistency, batch to batch, causing cure variation in compounds containing them.
According to the source of the zinc oxide there can be a problem with the inclusion of a small percentage of lead. Zinc and lead are found in the same ores in nature and unless correctly refined there can be some lead as a contaminant in the zinc oxide. Even 1% of lead in a zinc oxide is sufficient to cause discolouration of light coloured compounds.
Zinc oxide in storage will slowly absorb carbon dioxide and therefore cannot be regarded as an inert material. Old material may show different reactivity if the storage time exceeds 12 months, thus affecting cure characteristics of compounds.
Compounds containing fine particle zinc oxides can show increased viscosity. When an increased dosage of these fine particle materials is required there can be considerable stiffening of the compound, which can be used to advantage to give good dimensional stability to extrudates such as profiles and hoses and to stabilise open steam cured goods in general.
Extremely fine particle size zinc oxides, are easily incorporated into the rubber and have little pigmenting power and thus can be used in transparent vulcanisates without creating opaqueness. These grades can be used at as low a dosage as 0.8 phr giving the required degree of activation. Due to the ease of dispersion and high specific surface in contact with the rubber, the dosage level required for these grades is significantly lower than with conventional materials. This type of zinc oxide can also be used in the range of 20-50 phr as a reinforcing agent for products requiring high resilience for use in engineering products such as springs, etc., and for the production of food quality compounds.
Zinc Peroxide
Zinc peroxide is used as the curative for carboxylated acrylonitrile-butadiene rubber (XNBR) compounds. It confers better scorch safety than does zinc oxide. It is usually added in a masterbatch form.
4.2.2 Organic Crosslinking Activators
The crosslinking efficiency of many peroxide-initiated free radicals is low. These labile radicals can be converted to more stable radicals by contact in situ with polyfunctional monomers to form a three-dimensional network. Crosslinking efficiency is thus increased by some 20%. In addition, these materials act as plasticisers during processing and in some cases also act as hardening agents.
4.2.3 Silane Coupling Agents
Silane coupling agents improve properties of compounds containing silica and silicate fillers by forming chemical bonds across the filler and the rubber interface. They can also give improved properties with other materials, such as carbon blacks. They can be used with clays which do not form strong chemical bonds with the polymer, thus bringing them into the general category of useful processing additives. The improved compound properties result from a better compatibility and linkage of the rubber and filler. The coupling agent can be added direct to the mixer with the filler, or may be purchased already in place on the intended filler.
The silane coupling agents which are frequently used are bis-(3-triethoxysilylpropyl)tetrasulphane and 3-thio-cyanatopropyl triethoxy silane.
The effect of coupling agents on the physical properties of the compound is to: lower compression set,
reduce heat build-up under dynamic conditions, reduce tan delta,
improve crosslink stability,
improve tear related characteristics, and improve resistance to swelling by water.
The addition of a coupling agent can dramatically improve the abrasion resistance of compounds containing silica reinforcement. The effect is dependant on the surface area of the silica.
4.2.4 Titanate and Zirconate Coupling Agents
Organometallic titanate and zirconate coupling agents form monomolecular layers on most materials, such as metals, metal oxides, carbonates, sulphides, sulphates, siliceous materials, carbon black, some synthetic fibres, dispersed dyes and organic pigments. They render the substrate hydrophobic (moisture free), organophilic (rubber compatible and reactive), organofunctional (e.g., phosphato flame retardant functionality to provide controlled intumesence) and catalytically reactive with the polymer phase.
They may also act as reactive super plasticisers to increase rubber flow while increasing the mechanical properties of the rubber. Viscosity reduction or polymer solvation and higher filler loading can be accomplished with less plasticiser. Flow is achieved through molecular rearrangement and not average molecular weight reduction of the rubber.
Titanates can, in contact with ingredients containing phenolic functionality, produce colouration. Zirconates generally do not produce colour when in contact with phenols.
Neoalkoxy zirconates also provide novel opportunities for the adhesion of fluorinated polymers to metal substrates because the introduction of a zirconate at the interface results in a metal oxygen zirconium VI organo fluoride.
4.3 Antidegradants
The unsaturated structure of the diene hydrocarbon rubbers makes them susceptible to attack by both oxygen and ozone. Oxidative degradation of all rubbers, irrespective of their structures, is inevitable as the energy associated with incident natural light is approximately three times that of a typical carbon-carbon or carbon-hydrogen bond.
Retardation of oxidative degeneration and the effects of ozone attack can be mitigated but not totally overcome by the use of chemicals which, unfortunately, in the case of the most effective types, carry the penalty of causing staining of the rubber compound or surfaces with which it comes into contact.
4.3.1 Antioxidants
Oxidative ageing of rubbers is limited by the rate of diffusion of oxygen into the rubber product and is usually confined to the outer 3 mm. Antioxidants are used to protect rubbers from the effects of thermal oxidation and the vast majority of compounds will contain one or more. Peroxide vulcanisates are usually protected with dihydroquinolines. Other antioxidants react adversely with the peroxide inhibiting the crosslinking reaction.
The durability of the antioxidant can be affected by a number of factors, the most important being the amount present in the compound. The process of vulcanisation can result in the loss of migratory or volatile antioxidants. Over long periods of service, the antioxidant will decrease in concentration at ambient temperatures due to reaction with oxygen. Loss by volatility is usually only a problem with antioxidants such as the mono-phenolics.
The most effective antioxidants for light coloured rubber compounds are the hindered bisphenols, but these offer little ozone and flex cracking resistance.
Strongly Staining Antioxidants
Aryl naphthylamine derivatives are good general antioxidants with moderate volatility and negligible effect on cure. These give a small degree of fatigue protection in natural and polyisoprene rubbers, but little in styrene-butadiene and butadiene vulcanisates.
Diphenylamine derivatives are very good antioxidants and provide fatigue activity and good metal poison protection.
Moderately Staining Antioxidants
Dihydroquinoline derivatives offer moderate antioxidant and flex cracking activity with excellent metal poison protection.
Non-Staining Antioxidants
Benzimidazole derivatives are excellent antioxidants, with limited fatigue activity. They offer excellent metal poison protection.
Bisphenol derivatives show very good antioxidant activity and are the best materials for light coloured articles. Selected materials are FDA approved. These derivatives have low volatility and no effect on cure rate and do not give a bloom. A slight pink discoloration can occur after prolonged exposure to light in white or light coloured products.
Hydroquinones show very good antioxidant activity, but offer limited fatigue and metal poison protection.
Alkylphenol derivatives show medium antioxidant activity, but provide limited fatigue and metal poison activity.
4.3.2 Antiozonants
Ozone attack on rubbers takes the form of cracking which takes place perpendicular to the direction of the strain.
Ozone attack occurs mainly at the olefinic double bond of a diene rubber and, if not protected against, will result in loss of physical integrity for thin sectioned articles and surface cracking on larger mass products.
Too high a dosage of antiozonant can result in the formation of unsightly blooms on the rubber surface. Too little antiozonant can lead to worse attack than when none is present.
Strongly Staining Antiozonants
Dialkyl-p-phenylenediamine derivatives show good antioxidant activity with very good antiozonant characteristics, with low volatility and at normal dosage levels give no visible bloom to the rubber surface. They give good to moderate flex cracking resistance and increase the critical stress required to promote cracking. The dialkyl-p-phenylenediamines are liquids; their use reduces compound scorch time and they can have a relatively short durability due to their high reactivity. Alkyl carbon contents of less than 7 are not used as they can induce dermatitis.
Aryl-alkyl-phenylenediamines have very good antioxidant activity with very good antiozonant characteristics, with low to moderate volatility and no visible bloom to the rubber surface. They give very good flex cracking resistance and have no effect on critical stress but reduce the rate of cut-growth. These are the most widely used antiozonants as they have high activity, with minimal