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Elastomers And Rubber Compounding Materials Pdf

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An elastomer is a polymer with viscoelasticity i. Elastomers are amorphous polymers maintained above their glass transition temperature , so that considerable molecular reconformation , without breaking of covalent bonds , is feasible. Their primary uses are for seals , adhesives and molded flexible parts. Application areas for different types of rubber are manifold and cover segments as diverse as tires, soles for shoes , and damping and insulating elements. IUPAC defines the term "elastomer" as a " polymer that displays rubber-like elasticity.

Rubber Technology

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A short summary of this paper. IntroductionThe basic properties of rubber products are highly dependent on the elastomer s used in their manufacture. However, these properties can be modified favourably through the appropriate choice of compounding ingredients.

Some are added to facilitate or accelerate crosslinking, others improve processability and others to improve the properties of the finished rubber product. Excluding mineral-based fillers these compounding ingredients can be classified as follows: vulcanising agents, vulcanisation accelerators, activators of vulcanisation, retarders and inhibitors of vulcanisation, antidegradants, antireversion agents, plasticisers and softeners, and miscellaneous ingredients. Each class will be dealt with in turn in the following sections.

Vulcanising agentsMaterials that are able to form crosslinks between polymer chains may be generally classified as vulcanising agents. Listed below, and described in some detail, are the curing agents in common use together with a category of less frequently used materials. SulphurElemental sulphur is the most widely used vulcanisation agent in the rubber industry and is effective in elastomers containing some degree of unsaturation. Ground sulphur is most widely used, often referred to as rhombic sulphur or rubber makers' sulphur.

The molecular structure of rhombic sulphur comprises an eight membered ring and is crystalline in nature. The relatively low solubility of sulphur in rubber at ambient temperature is the cause of so-called 'sulphur bloom'. It appears as an off-white powdery coating on the surface of the uncured compound due to migration from the bulk compound when the limit of solubility is exceeded. If present in excess it has an unfavourable effect on the building tack of green components.

Sulphur bloom can also occur in vulcanisates but here the disadvantage is largely cosmetic. Datta and F. InghamSulphur bloom can be prevented by substituting rubber makers' sulphur with so called insoluble sulphur. This is a crystalline, polymeric form of sulphur [1] and is insoluble in solvents and elastomers.

During vulcanisation it is converted into rhombic sulphur allowing the vulcanisation process to proceed as normal. Sulphur donorsThe term 'sulphur donor' is a common designation for organic disulphides and higher sulphides that are capable of providing active sulphur during the vulcanisation process thereby generating sulphidic crosslinks.

Sulphur donors can be categorised into those that are applied as a direct substitute for free sulphur, with no major change in vulcanisation characteristics, and those that act simultaneously as vulcanisation accelerators see Section 6. The chemical structures of these sulphur donors are shown in Figure 6. The prime function of TBzTD is as a secondary accelerator but at higher loading it can also function as a sulphur donor [3]. Some additional materials are available that are capable of acting as sulphur donors; for example, alkyl phenol polysulphide [4], bis 3-triethoxy silyl propyl tetrasulphide [5] TESPT , and dialkyl dithiophosphate polysulphide [6].

PeroxidesCrosslinking with peroxides has been known since when Ostromyslenski disclosed that natural rubber could be transformed into a crosslinked state with dibenzoyl peroxide [7].

However, there was little interest in peroxide crosslinking until the development of fully saturated ethylenepropylene copolymers in the early s. In order to meet these requirements peroxides containing tertiary carbon atoms are most suitable [8], whilst peroxy groups bonded to primary and secondary carbon atoms are less stable. Organic peroxides that are suitable for crosslinking elastomers are shown in Figure 6. In addition to the symmetrical peroxides, asymmetrical peroxides are also in use, as for example tert-butyl perbenzoate, tert-butylcumyl peroxide and some polymeric peroxides [9].

A further limitation with regard to the suitability of peroxides concerns the efficiency of crosslinking. Higher efficiencies are observed for those peroxides that form one of the following radicals during homolytic decomposition [10] see Figure 6. Halflife values can be estimated in solution utilising the technique of differential thermal analysis. These values, or more precisely the temperatures at which their half-life is equivalent, provide an indication of practical vulcanisation temperatures [11] see Table 6.

The materials are sold under the trade name of Novor, the most popular grade being Novor There are several reasons for this: they are expensive, they exhibit little scorch resistance and the materials possess limited storage stability, absorbing moisture under humid conditions. Metal oxidesVulcanisation of CR rubber is often carried out with a combination of zinc and magnesium oxides.

Lead oxides can also be used particularly to obtain vulcanisates with low water absorption. Polysulphide rubbers Thiokol and chlorosulphonated polyethylene Hypalon are also vulcanised using metal oxides. Other vulcanising agentsQuinone dioxime is used for the vulcanisation of butyl rubber. The presence of an oxidising agent in the vulcanising system is necessary to convert the dioxime into p-nitrosobenzene, the actual vulcanising agent. Diamines with blocked amino groups, e.

Vulcanisation acceleratorsThe reaction of rubber with sulphur is slow even at high temperatures. It is not surprising then that for the past years efforts have been made to speed up the vulcanisation process.

The first such cure acceleration was accomplished by adding metal oxides to the compound, such as those of lead, calcium and magnesium. The improvement was rather limited however and it was not until Oenslager [15] discovered organic accelerators in that significant improvements in the rate of vulcanisation were feasible [16].

Aniline was the first of such organic accelerators but owing to its toxicity was soon replaced by the reaction product of aniline and carbon disulphide [17]. A significant step forward was the discovery of 2-mercaptobenzothiazole MBT in [19].

Following the discovery of MBT, the disulphide, MBTS, was found to provide greater scorch premature vulcanisation generally due to excessive heat history safety at higher temperatures. A further important step was the discovery of sulphenamide type accelerators. They represent the products of the oxidative condensation of thiazoles with an amine. The application of these more effective accelerators has allowed, in general, a reduction of compound sulphur levels in addition to increased rates of vulcanisation.

This more efficient use of sulphur has led to improved vulcanisate properties. Triazine-based accelerators have been introduced that provide even higher crosslinking efficiency. However, they have not established themselves as an important class of accelerators possibly due to an unfavourable price versus performance.

There are a wide variety of accelerators available to the compounder. These include accelerator blends of which there are well over In order to rationalise this extensive range of materials it is useful to classify them in terms of their generic chemical designation listed below: Sulphenamides Thiazoles Guanidines Thiurams Dithocarbamates Dithiophosphates Miscellaneous Some specific chemical structures belonging to each class are shown in Sections 6.

SulphenamidesA significant step forward with regard to the development of accelerators occurred in the s when Zaucker and Bagemann found that sulphenamides provided a delayed action to the vulcanisation process [20]. Today there are a number of commercially available suphenamide-based accelerators differing in amine moiety, thereby providing control over scorch time and cure rate. The more basic the attached amine, the shorter is the scorch time and the higher is the cure rate.

Perhaps the most widely applied sulphenamide is Ncyclohexylbenzothiazolesulphenamide CBS due to a good balance between scorch safety and cure rate. If greater scorch safety is required the sulphenamide of choice should be Ntert-butylbenzothiazolesulphenamide TBBS or 2- 4-morpholinothio benzothiazole MBS.

TBBS not only provides greater scorch delay than CBS but also produces a vulcanisate of higher modulus, permitting greater economy of use at equal modulus [21]. Typical cure characteristics of these sulphenamides are shown in Figure 6. Sulphenamides are the preferred accelerators for steelcord skim stocks where long scorch times are required to ensure adequate build up of the interfacial copper sulphide layer on the surface of the brass coated steelcord prior to the onset of vulcanisation.

In this way, the Rubber Additives -Compounding Ingredients adhesion between compound and steelcord is achieved. Whilst MBS exhibits the slowest cure rate and the longest scorch time, this accelerator is now out of favour in the tyre industry due to toxicological concerns related to nitrosamine formation. Their chemical structures are shown in Figure 6. TBSI exhibits long scorch delay in addition to a slow cure rate and as such is used increasingly in steelcord skim stocks. An additional benefit is its increased resistance to hydrolysis compared to the sulphenamides, a noteworthy consideration when used in high temperature, high humidity conditions.

This results in an increase in crosslinking efficiency of the cure system, but also a decrease in processing safety see Table 6. Although as yet unavailable commercially, excellent processing safety combined with a high cure rate and high crosslinking efficiency can be achieved by the use of a bis sulphenamide and pyrimidine sulphenamide [22] Figure 6 S B C r u h p l u S G P D D T M T 6. In addition it exhibits a high cure rate approaching that of an ultra accelerator [23].

The general structure of thiocarbamyl sulphenamide is shown in Figure 6. ThiazolesThiazoles are by far the most commonly applied accelerators. The most important accelerator of the group, MBT, was introduced in and its subsequent impact on the rubber industry has been remarkable.

The zinc salt is rarely used in dry compounding but is commonly used in the natural rubber latex industry, particularly in the manufacture of elastic thread. MBT provides a medium vulcanisation rate giving relatively low modulus vulcanisates, both in NR and synthetic elastomers.

It has a tendency, mainly in NR, to scorch during the processing and storage of the green compound. In natural rubber, MBT acts as a peptising agent at elevated temperatures. It is common practice to use a secondary accelerator, a so-called booster or kicker, in combination with MBT. With DPG, for example, a rapid vulcanisation rate with considerable increase in modulus can be achieved. MBT can be boosted with thiuram disulphides or dithiocarbamates to provide shorter cure times, but at the expense of scorch safety.

It does, however, provide increased processing safety. It can be activated in a similar manner to MBT. ZMBT too gives vulcanisates of low modulus, but it is used mainly in latex applications. The guanidines are slow with regard to rate of vulcanisation but relatively safe accelerators in terms of processing.

They are rarely used as primary accelerators due to their slow cure rates, although this can be put to good use in the cure of large sectioned articles. Their main use is as a secondary accelerator in thiazole or sulphenamide accelerated NR or SBR compounds. DPG is also used in speciality rubbers such as CR and polysulphide rubber where it acts as a peptiser [24]. In silica containing compounds DPG is employed as a so-called cure activator.

Rubber Additives -Compounding Ingredients

Bartholomew, G. Richard Eykamp, W. Rubber Chemistry and Technology 1 November ; 32 5 : — While special purpose elastomers for high temperature applications have been available since the introduction of the polyacrylates and silicones in the early 's, the intensive development and study of such materials really began in the early 's. Prior to that period, the major effort in the field of synthetic elastomers was directed toward development of general purpose polymers, principally for such applications as military and civilian tires, wire and cable, and other large volume uses. The extensive synthetic rubber development program sponsored by the U. Government during and following World War II is a matter of common knowledge to every rubber technologist and is well covered by Whitby.

Elastomers and Rubber Compounding Materials reviews the properties of elastomers and particular groups of ingredients and chemicals mixed into the basic elastomer to form a rubber compound. After introducing the history of rubber industry and the general properties of rubber, the book discusses the properties, classification, concentration, stabilization, modification, application, transport, and storage of latex. It presents as well the methods of production, composition, physical properties, and chemical reactions of dry rubber. The book then focuses on the production and classification of different synthetic rubbers, such as styrene-butadiene, isoprene, butadiene, ethylene-propylene, and chloroprene. It also discusses the production, properties, and applications of elastomers, vulcanization chemicals, fillers, stabilizers, plasticizers, blowing agents, and textile reinforcing materials used in formulating rubber compounds. This book will be of great value not only to those who are in the rubber industry, but also to students of polymer science and rubber technology. Preface Chapter 1.

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Elastomers and Rubber Compounding Materials

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