The Development of Rubber

In the early part of the eighteen-hundreds, the Americans made use of natural for the first time. First they made overshoes to keep their feet dry. Then came a certain Mr. Mackintosh, who made coats of cloth covered with natural rubber. But these first rubber overshoes and raincoats were all soft and sticky in summer, and hard and unelastic in the winter when it was cold, the rubber we have today is not sticky but soft and elastic, though very strong-even in the warmest summer and the coldest winter. There would be no automobiles such as we have today without it. But at that time every attempt to make rubber hard and strong come to nothing. The early overshoes and raincoats were simply not good enough and their makers went out of business.

Goodyear was living near some of these poor men and he got to work on this question of making rubber hard and strong. Once he started on this work, he was the sort of man who simply had to go on till he had overcome the trouble. First came the discovery that nitric acid (HNO3) made the rubber much better, and in a short time he was doing a small business in rubber shoes produced in this way.

Water, a friend of his had the idea that rubber might be made hard and strong if mixed with sulphur and put in the sun. Goodyear put his idea to the test , and saw that it did have more or less the desired effect, but only on the outside of the rubber. It is common knowledge now that the way to make rubber hard and strong-to“vulcanize” it ,as we say , is by heating it with sulphur. Goodyear spent another four years, in which things went very hardly with him, before he made the discovery how to vulcanize rubber completely. When at last he did it, he had nothing left at all. Everything of the smallest value had used to get money, even his children’s school-books.

Almost every discovery or invention has come a sort of story behind it though they are not all quite such unhappy stories.

The Part by Rubber in Modern Civilization

Rubber is indispensable to modern civilization. Its importance in every form of transport for tyres and tubes is self-evident and has been fully brought out. About 80 per sent of all rubbers is used in the motor industry, mainly for tyres. This applies to passangers, goods transport, and private cars. Without rubber, the rarious form of transportation could not run, the communication system would break down.

A large proportion of the remainder goes into what are known as mechanical goods, which include such articles as belting, packing, moulded good, hose and innumberable other tyres of products. Most of these articles are absolutely vital to the operation of the industry in general. Without them industry would come to a stand still.

Rubber is of the greatest possible importance in tyres on tractors for agricultural use. The change-over form steel wheels to pneumatlics has had a profound affect on agricultural development.

The electrical industry is the large consumer. Rubber is of outstanding significance in the electrical industry as insulation, particularly for eables. The reasons for its preeminent position are outstanding electrical properties, the good mechanical properties, and the ease with which it can be handled.

Natural Rubber 、Synthetic Rubber and Reclaimed Rubber

Natural Rubber is a high molecular weight hydrocarbon which comes from the latex of many plants and is obtained by coagulation with chemicals, by drying, electrical coagulation and other processes. The most important producer is Hevea Brasiliensis. The wild rubber produced form latex contains, besides the hydrocarbon, small quantities of protein, carbohydrates, resin-like substances, mineral salts and fatty acids. These other constituents act in part as natural accelerators and anti-oxidants and give the product processing properties which do not exist in the pure hydrocarbon.

［Natural rubber is cis-1, 4-polyisoprene with the empirical formular (C5H8)n］and the structural formular.

CH3

︳

[—CH2—C＝CH—CH2—]

When vulcanized, rubber becomes highly extensible, shows fast recovery, and is highly elastic.

“Synthetic rubber” is the common term, in fact incorrect, for chemically different polymers with rubber-like properties. Since the term rubber applies to the original product, i.c. polyisoprene, the term synthetic rubber should strictly lerased only for synthetic “Natural Rubber”(cisl,4-polyisoprence). For polymers, with rubbery properties, which do not have an identical structure to natural rubber, the terms artificial rubber, chemical rubber or synthesized rubber have been suggested. In modern usago the term rubber no longer refers to a chemically defined product but relates so physical properties, all synthetic polymers with properties similar to those of natural rubber may therefore be classified as properties rubbers.

In general, it may be stated tht synthetic rubber do not have all qualities of natural rubbers, but their special properties have made them more suitable than natural rubber for certain applications and they have achieved a phenomenal technical importance.

Reclaimed rubber is obtained by treating scrap vulcanized rubber by one of several methods, so that it becomes plastic again and can be handled on a mill and incorporated in a rubber compound, together with crude rubber.

Reclaimed rubber contains all the ingredients of the original compound, except for the uncombined, or free sulfur. It was first used for economical resean that is to reduce the manufacturing cost. However, it was found that has also other advantages in addition to its lower cost. It fancilitates mastication and has very beneficial effect on calendaring and extruding. The quantity used varies to a great extent and some compounds are based exclusively on reclaimed rubber. However, reclaimed rubber affects some of the mechanical properties of the vulcanize ate unfavorably. Therefore, it is not used in case where high abrasion of tear resistance or tensile strength is required.

Vulcanizing Agents、Accelerators and Activators

During vulcanization, the long chain of the rubber molecules become crosslinked by reactions with the vulcanizing agent to form three-dimensional structures. In pratice, sulfur is used almost exclusively for vulcanization of rubber. Other vulcanizing agents, such as sulfur chloride, selenium, tellurium, and thiuram disulfide, are used only in special cases.

Accelerations can increase the speed of reaction between sulphur and rubber, or lower the vulcanization temperature. They can also decrease the physical and technical properities of the vulcanizate.

Inorganic accelerators were discovered accidentally early in the history of the rubber industry when inorganic compounds were added go rubber as coloring agents and fillers. Among the basic salts and metal oxide recommended as accelerators, line, magnesium oxide, and litharge were the most common ones, litharge is still used to some extent, but its blackening effect on the vulcanizate limits its application to black compounds.

Oenslager’s discovery, in 1906, of the acceleration effect of aniline marked the beginning of a long line of organic accelerators which belong to many different groups.

A very small quantity, about 1 per cent, of these organic compounds has a remarkable accelerating effect. Organic accelerators were classified according to their technical effects and were placed in the following categories:

ultra-accelerators

semi-ultra-accelerators

medium-fast-accelerators

slow accelerators

Activators activate accelerators. Most accelerators are much more active in presence of activators than they are alone. The activator reacts with the accelerators. As a result, an intermediate complex is formed. The complex is more effective in activating the sulphur present in the rubber mixture.

There are two large classes of activators: metal oxides and fatty acids. Representatives of both groups must be present simultaneously. The most common member of the first group is zinc oxide which is in the proportion of about 3 to 5 percent as activator in almost every rubber compound. The second group includes oleic palmatic and especially stearic acid. They are used in amounts of 0.5 to 3 percent.

Fillers

Most rubber products contain fillers which belong to two groups: reinforcing and inert fillers. Generally speaking reinforcing fillers improve various physical properties of rubber compounds such as tensile strength, tear and abrasion resistance. Inert fillers impart certain processing properties, and reduce.

By far the most important filler is carbon black, whose introduction revolutionized the industry. It is used all rubber articles which have to meet severe service condition except light color products.

The carbon black used in rubber compounding are prepared by three different processes, each yielding a special class of carbon blacks. They are the channel process, the furnace process and thermal process.

From the practical angle, carbon blacks are classified as reinforcing and semi-reinforcing. Their qualities are indicated by initials, such as EPC(easy processing channel), SRF(semi-reinforcing furnace),HMF(high medulus furnace), etc. The variety used most extensively today is HAF (high abrasion furnace).

Inert fillers consists of fillers which can be incorporated in rubber compounds without substantially reducing their mechanical properties. Consequently, they make possible a lower manufacturing cost of rubber goods which need not be of very high quality. Many substances belong to the class of inert fillers such as whiting, barium sulfate, lithopone, etc.

Antioxidants

Antioxidants are used to reduce aging processes in vulcanizate. They function by slowing down the deterioration of rubber produces.

Rubber antioxidants are mostly phenols and amines, and more particularly their condensation products with other compounds. Usually 1 to 2 per cent is added to a rubber formulation. The most common one is phenyl naphthylamine.

Some classical mixtures of antioxidants are available and also several proprietary products the composition of which has not been disclosed. In general, it is preferable to use mixtures as they not only protect against oxygen, but have also other desirable properties. Some of them give resistance go cracking by flexing, others provide protection against heat or light, still others protect against the detrimental effects of metals, such copper, etc.

The resistance to cracking on repeated flexing (called flex cracking) has been extensively studied. This phenomenon may be considered dynamic aging of rubber while oxidation at rest may be regarded as static aging. Finally, the best means found so far is adding waxy substances to rubber compounds. After vulcanization, these have a tendency to bloom on the surface of the rubber article, thus forming a protective film.

Plasticizers or Softeners

Any material that can be added to rubber to either aid mixing, promote greater elasticity, produce tack, or extend (or replace) a portion of the rubber hydrocarbon (without a loss in physical properties), can be classified as a softener.

Making rubber plastic is an expensive operation that can be facililated by the use of plasticizers or softeners. Many substances, such as vegetable and mineral oils, waxes, paraffins, pitch, tar and resins have been used for this purpose. The principle plasticizers are the fatty acid, expecially stearic acid, pine tar, and the so-called mineral rubbers, which are natural asphalts.

The most important effects of plasticizers or softeners are as follows:

They reduce the cost of mechanical treatment necessary for plasticizing the rubber.

They facilitate the addition and uniform distribution of fillers.

They allow mastication performed at lower temperature, thus avoiding scorching of the compounded rubber.

They make highly filled rubber compounds sufficiently soft for moulding.

They increase the adhesiveness of the compound and give tackiness when this is desirable.

In the use of plasticizers of softeners, many factors should be considered so that the desired degree of plasticity and adhesiveness can be achieved, and sticking of the compound to rolls and its excess ive softening at elevated temperature can be avoided.

Also, too much plasticizer has a harmful effect on the mechanical and aging properties of the finished product. Therefore, plasticization is not a simple problem. Sometime several plasticizers are combined to obtain satisfactory results.

Softrners are used in relatively small quantities, 0.5 to 5.0 percent on the rubber, except in some very special cases where the mechanical properties are sacrificed in order to obtain other advantages.

In addition to the before mentioned types of softeners, new varieties have been developed form petroleum. They are found, in general, in the aromatic fractions obtained from refining highgrade lubricating oil by selective solvent methods, or by vacuum distillation. These softeners may be used in larger quantities without causing excessive reducing in the mechanical properties.

The Effect of Rubber Compounding on Rubber Properties

We have just discussed the various compounding ingredients and have indicated their prinecipal affects, new we shall look at rubber compounding from a different angle. We shall describe how the properties of the vulcanizate can be modified and how the rubber compound can adapt to manufacturing operations by the addition of suitable ingredients.

Tensile Strength This property, which is seldom encountered in the service conditions of rubber, is often used as a measure of their quality.

We have noted the effect of reinforcing and inert fillers on the tensile strength. We have also mentioned that addition of reclaimed rubber, factice, crumb rubber and plasticizers would adversely affect the tensile strength, Besides these factors the tensile strength to a certain degree.

Modulus A high value of the modulus indicators that the vulcanizate is hard and stiff and a low value points to a soft, flexible product. The modulus can be controlled best by proper choice of accelerator and fillers.

Hardness Rubber article seldom call for a lower hardness than that of a pure gum stock. If necessary, the hardness can be reduced by addition of large quantities (up to 80 PHR) or factices or softeners but this also causes reduction in the mechanical product will be.

Tear And Abrasion Resistance These two properties vary in substantially the same manner within certain limits. If the highest possible values are sought, carbon black should be used. Additional improvement in these properties can be achieved by proper choice of the accelerator MBT and its derivatives being especially recommended. Reclaimed rubber, factices, and curmb rubber cause a pronounced decrease in these properties.

Aging And Fatigue Resistance For resistance to repeated deformations, overfilled, or highly pigmented compounds should be avoided; also, it is imperative to use optimum vulcanization conditions.

For heat resistance, large quantities of fillers of plasticizers and sulfur should be used, The EV and SEV systems have been employed recently in many heatresistant compounds and prove successfully due to higher bond energy of mono and disulfur linkages.

Rubber Processing

The raw material of rubber manufacture is crude or raw rubber in which various ingredients are incorporated to prepare a compound. This compound is brought into the desired frm and the finished products is obtained by vulcanization or cure.

Rubber articles are manufactured by several operations, the most important ones being as follws.

Physical treatment of the raw rubber makes it suitable for incorporation of compounding ingredients. This operation is called plasticization of mastication, which is carried out on a mill, in an interal mixer, or in a “plastlentor”----a highspeed screw extruding machine.

Incorporation of various substances especially fillers into rubber is called mixing, which is also effected on a mill or in an interal mixer.

Pretreatment of the mix to make it suitable for the preparation of the desired product may consist of calemdering, extrusion, impregnating or spreading, depending on the article to be made.

Forming the product to be manufactured consists. in general, of the use of a mold with the desired shape and dimension. This step is called molding.

Vulcanization or curing the molded article will give the rubber properties which will meet the specifications. This operation is carried out in an autoclave or press.

Plasticization (1)

Raw rubber, especially natural rubber, has to be plasticized to allow incorporation of the ingredients. The main mechanism of plasticization is chain scission, molecular weight reduction and plasticity increase .It makes the material more easily processable.

There are three ways to masticate rubber as follow.

Mechanical Plasticization

This can be carried out on a mixing mill or in an internal mixer, or in a special machine designed for this purpose called plasticator. This is a force chemical process. A thigh temperature and in the presence of air. Rubber down primarily by oxidative degradation. At room temperature the main mechanism of molecular weight reduction is mechanical chain scission.

A mixing mill consists essentially of two horizontal side by side rolls which are rotating at different speeds in opposite opposite directions and can be heated or cooled. The gap between the rolls is adjustable and this gap is called the “nip “in rubber terminology. Raw rubber is placed between the rolls which usually have a diameter of 10-20 inch and are water cooled to maintain their temperature at about 140F

The two rolls which are made of special steel are mounted on a support made of last iron or molded steel. In genernal, the bearings of the rear roll are stationary and those of the front roll movable, so that the clearance between the rolls can be varied. The rolls are driven by an electric motor and gear boxes. The front roll is rotated at a speed of about 15-25 r .p .m. The speed of the rear roll is higher, the ratio being 1:1.2 to 1:1.5.

Passing the raw rubber repeatedly between the two rolls of the mill will change its physical properties. The hard and compact raw rubber gradually becomes soft. It loses its elestioity and becomes plastic. After a given tim period, which varies with the condition of working from some 10 to 20 minutes, the raw rubber pieces form a soft and plastic strip which rotates together with the front roll. the raw rubber is then considered to be plasticized and is suitable for taking up fillers and other ingredients.

Plasticization (2)

Mechanical plasticization can also be performed in an interal mixer. An internal mixer consists of two horizontal rotors with nogs or protrusions, encased by a jacket. The Bride ‘Banbury mixer’ has a friction ratio between the rotors, and the work is carried out between the rotors and the jacket. In the Francis Shaw ‘intermix’, however, the rotors run at even speed, and the nogs are designed to produce a friction ratio between the rotors. The work in this machine is done between the rotors, rather than between rotors and jacket. Internal mixer is fitted with a pneumatically operated ram to ensure that the rubbers and the rotors are in contact.

This is a very powerful machine which plasticize at a high speed (40 r.p.m.) and high temperature (270-320 oF). The operation can be completed in a few minutes. The capacity of a large internal mixer is about 500 pounds of rubber, which is much higher than that of a two roll mill. The plasticized rubber is discharged at the bottom of the machine through a drop door which opens automatically when the operation is completed.

It is effected in an oven for about 24 hours at 300-400oF in the presence of air at atmosphere. The process may be made continuous by modifying the oven to the form of chutes or tunnels whose output may amount to several tons an hour.

This term is used to cover the softening of rubber by peptizing agent. It can be carried out both on mixing mills and internal mixers. The peptizing or softening effect takes place only at temperature about 175 oF. It is more rapid and economical than mechanical plasticizing under certain conditions.

Compounding (1)

In conventional rubbers such as natural rubber or the synthetic polyisoprenes, prenes, polybutadiene, styrene-butadiene (SBR) in contrast with plastic, it is unusual to apply the materials in the natural unadulterated state (except for a little colouring dyestuff or pigment ). It is nearly always essential to mix the phur, fatty acid, wax, zinc compounds and vulcanization accelerators. This mixing of the ingredients together with the raw rubber, or the plasticized rubber is called ‘compound’, and the final homogeneous mix is referred to as the ‘rubber compound’.

Compounding Recipes

In order to aid in the development of a rubber compound the various ingredients to be used are compiled into a ‘recipe’. Every recipe contains a number of components, each having a specific function either in the processing, shaping, vulcanization, or end use of the product.

In general, from the data given in the compounding recipes the following information can be obtained.

• All the ingredients used are normally given in amount based on a total of 100 parts of the rubber or combinations of rubbers (or masterbatches) used. This notation is general listed as PHR (parts per hundred of rubber). Thus the effect of varying any ingredient used are easily recognized when the physical properties or processing characteristics are compared.
• Although the function of each component is never indicated in industrial or laboratory recipes, it is apparent that many different materials with specific purposes are used in every recipe.
• In many recipes, the materials are listed in the general order that they are mixed into the rubber during processing. This method aids compounder in setting up his mixing schedules for processing various compounds and for the preparation of special masterbatches which may be used in many different products.

Weighing and Measuring

The ingredients of the compound have to be weighed, before they are mixed with rubber. The quantities of weighed ingredients are be ordered in the compounding recipe.

Powders are being weighed by automatic scales equipped with pans of known weight. In large rubber factories, fully automatic devices feed the Banbury mixers. With such charging equipment, dust is avoided, as the entire operation is carried out in closed containers. This is a great advantage, especially in plants where carbon black are used.

In modern plants, volumetric measurements is used for liquids rather than weighing. Special pumps of the type used in gas stations meter out the desired quantity of liquid automatically.

Compounding (2)

Preparation of the Rubber Compound

The compound is prepared on a mill or in a mixer of similar construction to those used for mastication.

Mill MixingThe rubber is first added to the nip so that it forms a band on the front roll with a rolling ‘bank’ of rubber in the nip or gap form by the two rolls. In this nip, the rolls revolve the rubber ’bank’ which consists of rubber compound brought back by the rotation of the front roll, and which will be carried along again by the roll. The size of the bank can be controlled by controlling the nip opening. It is obvious that the less the clearance between the two rolls, thinner will be the band of rubber on the front roll and thus the bulkyer will be the bank of rubber waiting to be seized again by the rolls. It is obvious that just as in plasticizing and even more so, the operator plays a very important part. With his special knife, he cuts off put back , and transfers pieces of rubber from the band to the bank in order to obtain a very uniform compound . Depending on the skill of the operator, the resulting compound may be excellent or very bad. Operating a mixing mill is hard work. Handing the hot compound demands much physical efforts. In addition, the operator works in a dusty atmosphere, with powdered substances suspended in the air. Therefore, mixing mills are often equipped with a hood for exhausting the vapors and powder particles .The process usually takes between 20-40 minutes, but may take longer. At the end of the process, the rubber band is cut and peeled off roller.

Internal Mixer MixingInternal mixers enable large quantities of rubber compound to be made than can be conveniently handed on mills alone. Internal mixer are indispensable equipment in the large tyre plants which have to turn out enormous quantities of compounds. Other advantage of the internal mixer are that it can be automated, and it is much cleaner than straight mill of their pounding. They are better known as Banbury mixers from the name of their inventor.

The Banbury mixer also has two rolls, but their surfaces are not smooth. They are provided with deep grooves of such design as to circulate the compound not only vertically but also horizontally, paralle to be axis of the rolls. These rolls, or rotors, are enclosed in a chamber and the rubber is worked between the two rotors as well as between the rotors and the walls of the chamber.

The raw rubber and the various compounding ingredients are introduced into the mixer through a gravity-feed hopper, and forced into the mixing zone by an air-operated plunger.

Calendering

Calendering mainly are rubber sheet calendaring and coating fabrics. Rubber sheet are calendered to uniform thickness and width. For most purpose the rubber mixing is next taken to the calendaring rolls. The simplest possible form of calendar consists of two perfectly true and smooth rolls one superimposed upon the other, the mass of mixed rubber being fed in between the rolls on one side and passing out in the form of a sheet on the other.

Most calenders consist of three rollsThe middle (intermediate or central) roll bearing is stationary, the other two are movable along the machine stand and the nip of the rolls with respect to the central can be adjusted. The rubber sheet is brought to uniform thickness in two steps. The sheet leaving the first pair of rolls (top and intermediate) is immediately seized by the second pair (intermediate and bottom), whose nip is smaller than that of the first pair. The sheet obtained is, of course, more even and regular than would be produced by one pair of rolls only. The calender rolls are bored for steamheating as when warm the rubber is, of course, softer and more plastic.

A calender operator should attempt to produce a flat sheet of material regular in thickness across its width and length. Many factors are involved in achieving this object, including stock temperature of the feed, roll temperature, float of the rolls in the their bearings, speed and condition of the gears. A regular supply of feed stock is required, warm up as nearly as possible to the calender temperature, and the supply to the nip must be consistent in quantity and temperature. All variations tend to bad production. A regular bank clearly gives a regular product, and blended batches aid this end.

The three-roll calender is used also coating fabrics, that is for spreading a rubber layer over them. Skim coating or topping is generally applied to fabrics which have been treated to remove moisture, and the fibers treated with a suitable adhesive to ‘bond’ to the hot coat or skim of rubber applied on the calender. The rubber layer is prepared by the first pair of rolls and the second pair presses it against the fabric surface. Frictionin gis confined to three-roll calenders. In contrast to the topping process, it is for spreading rubber into fabric under shearing action.

The three-roll calender is suitable for coating only one surface of the fabric. For coating both sides, a four-roll calender is used, which has a third pair of roll for preparing a second rubber layer.

Extrusion

Extrusion is a very economical and wikely used method of processing rubber both for making blanks for molding and for forming rods, tubes, strips, channels and gaskets of a wide variety of sizes and shapes.

The rubber compound is first warmed on a mill and then fed directly to extruder in a continuous strip. An extruder consists substantially of a cylindrical piece or barrel in which a screw is rotating. A rubber strip introduced into the extruder is seized by the screw whose threads take along and push it toward the end of the cylinder, which has an orifice or die of the desire shape. The rubber compound, warmed up by the energetic processing and ejected underpressure, has the continuous shape. Obviously, a great variety of articles can be made by extrusion. The best example is the tyre tread, which leaves the extruder flat. It is cut to proper lengths and assembled on the drum of tyre building machine with the other components, such as cord, fabric plies, beads, etc.

If a hollow product, for example a hose, is to be made, then the extruder head will be equipped with a core mounted in the center of its circular orifice, or die. To prevent sticking of the hose to itself if it collapses after leaving the extruder, the core is also hollow and through its opening tale is spread in the interior of the hose.

The rubber compound swell on extrusion by an amount which depends on the consistency of the stock, the pressure and the rate of extrusion, the temperature, and other factors, so that the size and contour of the die and the operating conditions must be adjusted in order to obtain a product having the desired shape and dimensions.

Extruder screw are made of special steel. They are hollow to permit cooling and their pitch is usually variable. Their diameter are 2 to 10 inches. They of treatment than at constant speed. Temperature control is very important, especially for the head die, which shoud have sutomatic control.

The extruder is continuously and, in most cases, automaticly charged with a preheated rubber compound.

Vulcanization

Vulcanization is still the most important operation in producing a usable rubble article. Various compounding ingredient merely modify properties, and the use of accelerators, of course, makes volume production possible. It is interesting to note that, even though vulcanization has been known and used for over 150 years, there has been little change in the basic process. Sulfur is still the most important curing ingredient, and steam is still the most important source of controlled temperature.

After rubber compounds have been properly mixed and shaped into blanks or fabricated into a composite item, they must be vulcanized by one of many processes. During vulcanization, the following changes occur:

• The long chain of the rubber molecules become crosslinked by reactions with the vulcanizing agent to form three-dimensional structures.
• The rubber loses its tackiness and becomes insoluble in solvents and is more resistant to deterioration normally caused by heat, light and aging.
• The mechanical properties of compound have been increased.

Finished articles can be cured by different techniques. The four most common methods of hot vulcanization are briefly outlined as follows:

• In moulds under hydraulic pressure between steam or electrically heated platens in a ‘daylight’ press.
• In moulds under hydraulic pressure entirely surrounded by steam in an autoclave press.
• Wrapped in cloth, or embedded in an inert powder such as talc, in live steam under pressure; the so-called ‘open steam’ cure.
• Directly in hot air, at atmospheric or raised pressure, the ‘dry-heat’ method of vulcanization.

Air has relatively low thermal capacity capacity as compared with steam. Its thermal conductivity also much lower. Heat is therefore not transferred quickly by hot air to the rubber articles to be vulcanized. A given compound normally takes about twice as long to cure in hot air as in saturaled steam. On the other than mould curing. Because of the slowness of vulcanization accelerators in compounds that are vulcanized in hot air should be active at a relatively low temperature in order that the surface of the articles acquires some stability through an early onset of vulca nization.

Tyres

The first pneumatic tyre was invented in 1845 by R. W. Thompson (B.p.10990) and consisted of a rubber tube with a leather covering. J.B. Dunlop rediscovered the hollow tyre in 1888. It consisted of a rubber tube covered with textile and rubber. The cord weave was introduced between 1916 and 1919 the overall performance of a tyre depends on its construction (tread pattern, choice of cord fibre, etc.) just as such as on the rubber used. It will be interesting to note the main trend of the past two decades: from tube to tubeless, from bias ply to tadial ply, from rayon to nylon, steel or polyester cords.

A pneumatic tyre is a complex arrangement consisting of steel bead wires and steel or texlile casing cord surrounded by a number of different rubber compounds. Even for a small car tyre, these comprise of tread, sidewall, casing, inner liner and head compound. The formulation for these rubber compounds are chosen according to the major demands required of them in service-abrasion resistance and grip for treads; flexibility for sidewall; heat resistance for casing; impermeability to air for inner liners etc.

The steel or textile cord in the casing supply much of the strength of the tyre and allow high internal pressures to be used. Cotton cord material has been replaced by rayon, polyester, glass, brass plated steel cord. Now Fibre B (Kevlar) is been introduced, a synthetic textile based on an aromatic amide with an exceptionally high modulus of elasticity.

Each rubber compound in a tyre contains rubber polymers (elastomers) cross-linked (vulcanized) with sulfur. The more commonly used elastomers are natural rubber and its synthetic equivalent cis-polyisoprene, styene butadiene copllymer and polybutadiene.

Only about 40~50% of the rubber compound is elastomer, the rest been made up of various carbon black (30%), mineral oils, and waxes, pine tars, reins, various organic antioxidants, accelerators, retarders and peptizers, together with sulfur, zine oxide and stearic acid. Choosing the types and relations of these so-called compounding ingredients to achieve the physical properties required is the duty of tyre compounders.

Rubber Belts

Rubber is employed for a number of perposes in belts. The leading uses are:

• conveyor belts;
• Flat transmission belts;
• V-belts.

The fabric used for belts must be such that (1) The warp thread are strong enough to carry the required loads; (2) The weft threads must be sufficiently strong so that belt fasteners will be held firmly. For both conveyor belts and transmission belts the initial stage of preparation are very mush the same. The fabric must first be dried either by passing over a series of hot rollers or through a drying chamber. The fractioning process is carried out while the fabrics still warm. The milled compound is fed between the top and middle bowls and runs on the middle bowl. The fabric is fed between the bottom and middle bowls. The compound is forced into the interstices of the fabric, the fractioned fabric being cooled. The operation is repeated for the other side. Using a liner to prevent sticking.

Conveyor belts

Rubber conveyor belts are used for conveying every conceivable type of material. They are consisted of : (1)duck fractioned with rubber; (2)a tough rubber cover which protects the duck against corrosion and mechanical attack.

The plies are assembled on ling tables, being passed through squeeze rolls to consolidate them.

The rubber cover may be applied in several ways. Calendered sheets of the right thickness are rolled on to the built up fabric, first on the one side, then the other, and the assembly run through squeeze rolls.. Extruded strips are then applied to the edges. The belt is vulcanized in long presses which may be 30 feet long by 6 feet wide. The belt is being stretched during vulcanization.

Transmission Belts

Transmission belts are generally much narrower than conveyor belts. the fractioned canvas is slit into strips of the requisite width. These are then assembled in long table, pass through squeeze rells and finally wrapped by a ply of fractioned duck.

The another type of transmission belts are made by assembling the requisite member of full width of fractioned canvas, vulcanizing, and then cutting to the desired width. The edge are sealed with a coating of rubber solution.

V-belts

V-belts are generally constructed of a top layer of rubber ,a cord section, situated at the neutral axis, and a base or cushion rubber, the whole assembly being surrounded by a fabric jacket. The shorter belts are manufactured on rotable collapsible drum formers. The belts are subsequently vulcanized in multi-ring mould in open steam.

The long-length belts are built on twin-drum building machines. The belts are vulcanized endlessly by moulding in a press under controlled stretch conditions.

After manufacture, the belts are checked for length and made up into matching sets.

Rubber Hose

Rubber plays an important part as hose for conveying fluids or gases. There are a number of different types used depending on the nature of the fluid and the conditions of use. Hose will differ in construction depending on whether it is used at ordinary pressure, under high pressure, or rubber and fabric; or rubber, fabric and metal.

For present consideration, however, they may be divided into the following classes, according to the method adopted for reinforcement;

• Hose reinforced with fabric used for conveying fluids under moderate pressure.
• Hose reinforced with braided yarn and used for conveying fluids under moderate pressure.
• Hose reinforced with wire, used under heavy pressure or strong suction.

For high duty hose, reinforcement with fabric or braided yarn is inadequate. For high pressure or suction work which might cause the hose to collapse, it is necessary to use still stronger reinforcing material such as galvanized steel spiral or braided steel wire.

In a case of suction hose, the steel wire is built in as closely as possible to the interior diameter and most of the fabric is outside this wire, in order to reinforce the hose effectively.

Hand-Assembled Footwear

The footwear industry is an important outlet for rubber. Hand-assembled footwear is one of its products.

Although this method is labour intensive, it still has its advantages in that it allows for greater flexibility in changing product design without the need for acquiring new and sometimes expensive equipment. Some all-rubber industrial and Wellington boots and canvas-topped sports shorts are still made by this method.

A pair of all-rubber industrial boots may consist of 40 rubber or rubberized fabric component parts, each prepared and cut to the required shape prior to assembly. These parts consist of the outer rubber leg, outer rubber vamp, fabric leg lining, fabric vamp lining, insole, toe and heel reinforcements, foxing strip, sole and heel.

The calendered sheet for the production of outsoles is produced on a four roll calendar where the fourth roll is engraved with the necessary sole pattern. Any required sole design and thickness is obtained by changing the fourth roll.

The component, freshened by a solvent wipe or thin rubber solution, are assembled in a specified sequence, tightly on the metal last and then thoroughly rolled to ensure good bonding.

For canvas-topped footwear, the fabric components are die cut and machined together to form the upper, and on to which eyelets and heelpieces have been fitted. This is lasted on to the last, and the necessary rubber components attached.

Vulcaniation is carried out in a large cylindrical autoclave heated mternally by steam coils fitted in the base and around the sides. The boots or shoes are loaded on to carries and wheeled into the autoclave. Immediately the autoclave doors are closed, air, at a pressure of some 3kg/cm2, is admitted to offset any blistering of blowing of the outer rubber parts. The temperature gradually rises to 135oC, where it is then maintained for the remainder of the vulcanizing period, which is normally 60~65min, in total.

Because of the different thickness of rubber components and the hot-air method of vulcanization, the compound need to have a plateau-type curing system to avoid degradation. Accelerator system based on thiazoles with TMTD as a booster have been proved effective.