Sunday 20 January 2013

Shaping Operations


RUBBER MOULDING
Moulded rubber parts can be produced by different manufacturing methods. Major techniques are:
Compression moulding
Compression moulding is a process in which a compound is squeezed into a preheated mould taking a shape of the mould cavity and performing curing due to heat and pressure applied to the material. The method uses a split mould mounted in a hydraulic press Compression moulding process involves the following steps:
1. A pre-weighed amount of the compound is placed into the lower half of the mould. The compound may be in form of putty-like masses or pre-formed blanks.
2. The upper half of the mould moves downwards, pressing on the compound and forcing it to fill the mould cavity. The mould, equipped with a heating system, provides curing (cross-linking) of the
Compound
3. The mould is opened and the part is removed for necessary secondary operations

Injection moulding

Injection moulding is a process in which the compound is forced under high pressure into a mould cavity through an opening (sprue).The rubber material in form of strips is fed into an injection moulding machine. The material is then conveyed forward by a feeding screw and forced into a split mould, filling its cavity through a feeding system with sprue gate and runners. An injection moulding machine is similar to an extruder. The main difference between the two machines is in screw operation. In the extruder type the screw rotates continuously providing output of continuous long product (pipe, rod, and sheet).The screw of the injection moulding machine is called a reciprocating screw since it not only rotates but also moves forward and backward according to the steps of the moulding cycle. It acts as a ram in the filling step when the compound is injected into the mould and then it retracts backward in the moulding step. The mould is equipped with a heating system providing controlled heating and vulcanization of the material. The compound is held in the mould until the vulcanization has completed and then the mould opens and the part is removed from the mould. Injection moulding is a highly productive method providing high accuracy and control of shape of the manufactured parts. The method is profitable in mass production of large number of identical parts. A principal scheme of an injection moulding machine is shown here.
Transfer moulding

Transfer moulding is a process in which a pre-weighed amount of a compound is preheated in a separate chamber (transfer pot) and then forced into a preheated mould through a sprue, taking a shape of the mould cavity and performing curing due to heat and pressure applied to the material. The picture below illustrates the transfer moulding process. The method uses a split mould and a third plate equipped with a plunger mounted in a hydraulic press.
The method combines features of both compression moulding (hydraulic pressing) and injection moulding (ram-plunger and filling the mould through a sprue).The scrap left on the pot bottom (cull), in the sprue and in the channels is removed. Scrap of vulcanized rubber is not recyclable.
The transfer moulding cycle time is shorter than compression moulding cycle but longer than the injection moulding cycle. The method is capable to produce more complicated shapes than compression moulding but not as complicated as injection moulding.
RUBBER EXTRUSION
In the extrusion process of rubber, the compound including polymers, various types of additives and fills like curing agents, antioxidants, pigments are fed into the extruder. The extruder typically consists of a rotating screw inside a closely fitted heated barrel. The primary purpose of the extruder is to do three things, a) soften b) mix c) pressurize the rubber as it is fed continuously to the die at the extruder exit.

The die is a sort of metal disk that has a machined opening in the desired shape of the part that needs to be extruded. The rubber already softened by heating is then forced by the rotating screw through the die opening into the shape of the profile cut in the die. A typical phenomenon called die swell takes place as the rubber shape leaves the die. Because of this the part cross-section becomes larger than the die cross-section. The part cross-section depending on the material may rise up to several folds over the die.
Subsequently the processes of vulcanization or curing takes place as the last step in the extrusion process. This aids the rubber extruded profiles to maintain its shape and acquire necessary physical properties. Typical examples of extruded rubber parts are profiles, hoses, strips and cords.



CALENDERING

                The calendar is an important processing machine in polymer processing industry including both rubber and thermoplastic materials. Calendering is used to produce (1) sheet and film from thick polymer sections and (2) to embed polymeric materials into textile structures. A calendar comprises a large number of rolls or bowls held in a framework. The rolls rotate to produce sheeting and, by adjusting the distance apart of the rolls. The rubber industry normally uses three or four rolls and occasionally for rough gauge sheeting, a two roll calender. While two rolls are normally vertical, modern three roll calenders may have an offset top roll instead of vertical configuration. The offset top roll assists the feeding of the calender from a feed strip. Four roll calenders can have a vertical, an inverted L, or on the most modern calenders, a Z configuration on the rolls. Different Calender configurations are shown below.

Compounding Machines


Compounding is the operation of bringing together all the ingredients required to mix a batch of rubber compound. Each component has a different mix of ingredients according to the properties required for that component. Rubber compounding is generally carried out on open mills or internal mixers.
Open mill (Two roll mill)
An open mill consists of twin counter-rotating rolls, one serrated, that provide additional mechanical working to the rubber. The rolls can be heated or cooled as necessary. The rubber is placed on the rolls and mixing is achieved by the shearing action induced at the “nip” between the rolls. Additives are added in carefully weighed quantities during the mixing process. After the mixing operation is complete, the compound is removed from the mill in the form of sheet.

Internal mixer
Internal mixers are often equipped with two counter-rotating rotors in a large housing that shear the rubber charge along with the additives. The mixing can be done in three or four stages to incorporate the ingredients in the desired order. The shearing action generates considerable heat, so both rotor sand housing are water-cooled to maintain a temperature low enough to assure that vulcanization does not begin.

Compounding Ingredients


A rubber compound is obtained by mixing a base polymer or crude mixture with a series of additives. The choice of the base polymer and the additives is closely linked to the type of properties to be achieved. The resulting product is a non-vulcanized compound. The quantity of additives used varies for 20 to 130 percent as a percentage on the weight.A technical vulcanisate is made up from the following ingredients.


1.     Base polymer or blend of polymers
2.     Cross linking agents
3.     Accelerators of cross linking agents
4.     Accelerator modifiers (activators and retarders)
5.     Antidegradants (antioxidants, antiozonants, inhibitors of metal catalyzed oxidation, protective waxes)
6.     Reinforcing fillers (black, mineral and organic)
7.     Processing aids (chemical peptisers for polymers, softeners, plasticizers, dispersing aids, tackifiers, factice and lubricants)
8.     Diluents (inert mineral fillers, organic materials, and extending oils)
9.     Colouring materials (organic and inorganic)
10.                        Specific additives (blowing agents, fungicides, fibrous materials)
Crosslinking agents (Vulcanising agents)
Vulcanization is the conversion of rubber molecules into a network by formation of crosslinks. Vulcanizing agents are necessary for the crosslink formation. These vulcanizing agents are mostly sulphur or peroxide and sometimes other special vulcanizing agents or high energy radiation. Since vulcanization is the process of converting the gum-elastic raw material into the rubber-elastic end product, the ultimate properties like hardness and elasticity depend on the course of the vulcanization.
Accelerators
Accelerating agents increase the rate of the cross linking reaction and lower the sulphur content necessary to achieve optimum vulcanizate properties.
Activators
Like zinc-oxide and stearic acid. They activate the vulcanisation process and help the accelerators to achieve their full potential.
Antidegradants
These agents increase the resistance to attacks of ozone, UV light and oxygen.
Fillers
There are two types of fillers, reinforcing and non-reinforcing fillers. Carbon black is commonly used as reinforcing filler. This is also the reason why most rubbers are black. Calcium carbonate is an example of non-reinforcing filler.
Plasticizers
Besides fillers, plasticizers play the biggest quantitative role in building a rubber compound. The reasons for the use of plasticizers are: improvement of flow of the rubber during processing, improved filler dispersion, influence on the physical properties of the vulcanizate at low temperatures. Mineraloils and paraffins are widely used as a plasticizer.
Processing aids
These are chemicals added to the compound that improve the processability. Peptisers are used to increase the efficiency of mastication of rubbers. E.g.: Renacit IV (Zinc salt of pentachlorothiophenol). Factice are added to control die swell, improve surface quality and prevent distortion of shape, particularly in vulcanisation.
Colouring Materials-Pigments
Organic and inorganic pigments are used to colour rubber compounds. The colour pigments are also considered inactive fillers. Only silica’s have a reinforcing effect. Silicone can be coloured easily without loss of properties.
Specific additives
Blowing agents: DNPT, Sodium Carbonate etc.
Flame retardants: Antimony trioxide, Zinc borate etc.
Antistatic agents: Quaternary ammonium salts, Ethylene oxide condensate etc.

Proccesing of Rubber


Raw elastomers as received from the manufacturer or from plantation in the case of natural rubber have few uses as such, because they all have poor physical properties and chemical properties. The levels of properties such as resilience, abrasion resistance, hardness, and tensile strength that raw elastomers possess are completely inadequate for such typical end applications such as tyre tread, fuel hose, radiator hose, or an oil seal. To develop the required level of such elastomers have to be mixed with various compounding ingredients, shaped and cured. These three components, mixing, shaping, and curing together constitute rubber processing.
Mixing
Mixing is the first and most critical process. If each of the various components rubbers, fillers, oils and chemicals is not thoroughly distributed and dispersed through the mass of the compound, then problems cascade down through the subsequent process of shaping and curing and result in less than optimum physical properties in the end product.
A compound is formulated first to meet the requirements of the end application. Before mixing, there is a mastication step. In this step, the reduction of viscosity and increase of plasticity is brought about by mechanical milling and working.
Mixing process is carried out in any of the two basic machines: two roll mill and the internal mixer. Among these, internal mixer is widely used, particularly for large scale production. The compounding ingredients are mixed and uniformly distributed though intensive shearing action of rolls. After mixing, the compound is either stored for maturation or taken to shaping operations.
Shaping
Shaping operation is carried out after mixing. There are three general shaping processes: extrusion, calendaring and moulding. The first two are followed by a curing stage, often an inline continuous system, whereas in moulding, whether compression, transfer or injection, curing takes place in the mould. In shaping operation, the rubber compound is formed into the shape required by the end product. After shaping, the product is either transferred to dispatch or to curing stage.
Curing (Vulcanising)
                The final step in processing is the vulcanization or curing of the shaped product. Basically, the process is that of applying heat at a certain temperature for a certain time. In curing stage, the curing or vulcanizing agents incorporated in the rubber compound reacts with rubber and forms crosslinks, which will convert the compound into the final product with required properties.   

Classification of Rubber


Natural Rubber

Natural rubber is a solid product obtained through coagulating the latex produced by certain plants, particularly the Brazilian rubber-tree (Hevea Brasiliensis). This raw material is usually tapped from the rubber tree, which is native to Amazonia. Although there a large number of species that exude secretions similar to latex when the bark is cut, only a few produce sufficient quantities of a quality adequate for exploitation on economic bases.

The structural formula for the molecule of natural rubber may be represented by the simple unit C5H8 multiplied many 1000 times. Rubber  is an addition polymer of a diene  monomer(a hydrocarbon molecule containing two double bonds) called isoprene or 2-methyl-1,3-butadiene with the following formula:

CH2=C(CH3)-CH=CH2
The repeating unit shown above would, therefore, be called the monomer.  On polymerization of isoprene, we get polyisoprene, which is the chemical name of natural rubber

Poly isoprene

Synthetic rubbers


SOME EXAMPLES FOR SYNTHETIC RUBBER:

SBR(STYRENE BUTADEINE  RUBBER)


Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of  synthetic rubbers derived from styrene and butadiene. The co monomers for SBR production, styrene and butadiene, are today invariably produced from petroleum sources.

STRUCTURE OF SBR
The bulk of SBR is produced by free radically initiated emulsion polymerisation. Solution polymerization is also used for preparation of SBR. Copolymers prepared by use of lithium catalyst systems first came into existence in early 1960 and varieties of types are now commercially available. Polymerization is carried out in solution and with these systems the reactivity ratios are quite different to observed with free radical polymerizations.

Properties

Property
S-SBR
E-SBR
Tensile strength (MPa)
18
19
Elongation at tear (%)
565
635
Mooney viscosity (100 °C)
48.0
51.5
Glass transition temperature (°C)
-65
-50
Polydispersity
2.1
4.5

Applications

The elastomer is used widely in pneumatic tires, shoe heels and soles, gaskets and even chewing gum. It is a commodity material which competes with natural rubber. Latex (emulsion) SBR is extensively used in coated papers, being one of the most cost-effective resins to bind pigmented coatings. It is also used in building applications, as a sealing and binding agent behind renders as an alternative to PVA, but is more expensive. In the latter application, it offers better durability, reduced shrinkage and increased flexibility, as well as being resistant to emulsification in damp conditions. SBR can be used to 'tank' damp rooms or surfaces, a process in which the rubber is painted onto the entire surface (sometimes both the walls, floor and ceiling) forming a continuous, seamless damp proof liner; a typical example would be a basement.

NBR


Nitrile rubber, also known as Buna-N, Perbunan, or NBR, is a synthetic rubber copolymer of acrylonitrile(ACN) and butadiene. Trade names include Nipol, Krynac and Europrene.

STRUCTURE OF NBR
Nitrile butadiene rubber (NBR) is a family of unsaturated copolymers of 2-propenenitrile and various butadiene monomers (1,2-butadiene and 1,3-butadiene). Although its physical and chemical properties vary depending on the polymer’s composition of nitrile, this form of synthetic rubber is generally resistant to oil, fuel, and other chemicals (the more nitrile within the polymer, the higher the resistance to oils but the lower the flexibility of the material).

Production

Emulsifier (soap), 2-propenenitrile, various butadiene monomers (including 1,3-butadiene, 1,2-butadiene), radical generating activators, and a catalyst are added to polymerization vessels in the production of hot NBR. Water serves as the reaction medium within the vessel. The tanks are heated to 30–40 °C to facilitate the polymerization reaction and to promote branch formation in the polymer. Because several monomers capable of propagating the reaction are involved in the production of nitrile rubber the composition of each polymer can vary (depending on the concentrations of each monomer added to the polymerization tank and the conditions within the tank). One repeating unit found throughout the entire polymer may not exist. For this reason there is also no IUPAC name for the general polymer. The reaction for one possible portion of the polymer is shown below:
1,3-butadiene + 1,3-butadiene + 2-propenenitrile + 1,3-butadiene + 1,2-butadiene → nitrile butadiene rubber
Monomers are usually permitted to react for 5 to 12 hours. Polymerization is allowed to proceed to ~70% conversion before a “shortstop” agent (such as dimethyldithioarbamate and diethyl hydroxylamine) is added to react with the remaining free radicals. Once the resultant latex has “shortstopped”, the unreacted monomers are removed through a steam in a slurry stripper. Recovery of unreacted monomers is close to 100%. After monomer recovery, latex is sent through a series of filters to remove unwanted solids and then sent to the blending tanks where it is stabilized with an antioxidant. The yielded polymer latex is coagulated using calcium nitrate, aluminium sulfate, and other coagulating agents in an aluminium tank. The coagulated substance is then washed and dried into crumb rubber.
The process for the production of cold NBR is very similar to that of hot NBR. Polymerization tanks are heated to 5–15 °C instead of 30–40 °C. Under lower temperature conditions, less branching will form on polymers (the amount of branching distinguishes cold NBR from hot NBR).

Applications

The uses of nitrile rubber include non-latex gloves for the healthcare industry, automotive transmission belts, hoses, O rings, gaskets, oil seals, V belts, synthetic leather, printer's roller, and as cable jacketing; NBR latex can also be used in the preparation of adhesives and as a pigment binder.
Unlike polymers meant for ingestion, where small inconsistencies in chemical composition/structure can have a pronounced effect on the body, the general properties of NBR are not altered by minor structural/compositional differences. The production process itself is not overly complex; the polymerization, monomer recovery, and coagulation processes require some additives and equipment, but they are typical of the production of most rubbers. The necessary apparatus is simple and easy to obtain. For these reasons, the substance is widely produced in poorer countries where labour is relatively cheap. Among the highest producers of NBR are mainland China and Taiwan.

BUTYL RUBBER


Butyl rubber is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for Isobutylene Isoprene Rubber. Polyisobutylene, also known as "PIB" or polyisobutene, (C4H8)n, is the homopolymer of isobutylene, or 2-methyl-1-propene, on which butyl rubber is based. Butyl rubber is produced by polymerization of about 98% of isobutylene with about 2% of isoprene. Structurally, polyisobutylene resembles polypropylene, having two methyl groups substituted on every other carbon atom. Polyisobutylene is a colourless to light yellow viscoelastic material. It is generally odourless and tasteless, though it may exhibit a slight characteristic odour.
Butyl rubber has excellent impermeability, and the long polyisobutylene segments of its polymer chains give it good flex properties.
The formula for PIB is: –(–CH2–C(CH3)2–)n
The formula for IIR is: 
It can be made from the monomer isobutylene or CH2=C(CH3)2 only via cationic addition polymerization.

APPLICATIONS
Butyl rubber and halogenated butyl rubber are used for the inner liner that holds the air in the tire.

Chloroprene rubber


 Among the speciality elastomers polychloroprene [poly(2-chloro-1,3-butadiene)] is one of the most important.  First production was in 1932 by DuPont (“Duprene”, later “Neoprene”) and since then CR has an outstanding position due to its favourable combination of technical properties.


  
The basic polymerization scheme leads to incorporation of the monomer into a polymer consisting of different structural units. The physical, chemical and rheological properties of the different grades of commercial polychloroprene are dependent on the ability to change the molecular structure by changing polymerization conditions, e.g. polymerization temperature or monomer conversion, polymerization aids (comonomers, type and amount of molecular weight modifier and emulsifier) and conditions during finishing.
The high amount of trans-1,4-units in the polymer (about 90 % at standard polymerization conditions) leads to synthetic rubber, which has crystallization as an inherent property.



Properties

CR is not characterised by one outstanding property, but its balance of properties is unique among the synthetic elastomers. It has:
Good mechanical strength
High ozone and weather resistance
Good aging resistance
Low flammability
Good resistance toward chemicals
Moderate oil and fuel resistance
Adhesion to many substrates

Polychloroprene can be vulcanized by using various accelerator systems over a wide temperature range.
 Application areas in the elastomer field are widely spread, such as moulded goods, cables, transmission belts, conveyor belts, profiles etc.

Rubber


Rubber belongs to the class of substances termed ‘polymers’: high molecular weight compounds, predominantly organic, consisting of  long –chain molecules made up repeating units usually on a back bone of carbon atoms. These high molecular weight polymers have a lower temperature limit to their rubbery state.  At the so called glass transition temperature Tg, there is a fairly abrupt change to a glassy state. Materials in the class of polymers which are, at normal temperatures, plastics, become rubber like as the temperature is raised above their Tg.
In rubbery state, polymers behave in many ways like viscous liquids, because the links in the long chain are freely rotating and enable flow and distortion of material to occur under stress. Because of the chain length and the presence of side groups on the chain, their molecular freedom is restricted and they show both time-dependent viscous and elastic properties, and are said to be viscoelastic.
Mention must be made of the phenomenon of crystallisation, which is more complex in rubbers than in ordinary low molecular weight substances   crystallisation of rubbers takes place by local rearrangement of portions of molecules to form crystallites. 

HISTORY OF RUBBER


Rubber was known to the indigenous peoples of the Americas long before the arrival of European explorers. The first scientific study of rubber was undertaken by Charles de la Condamine, when he encountered it during his trip to Peru in 1735.
The first use for rubber was an eraser. It was Magellan, a descendent of the famous Portuguese navigator, who suggested this use. In England, Priestley popularized it to the extent that it became known as India Rubber.
In 1815, Hancock had invented a rubber mattress and through an association with MacIntosh he produced the famous waterproof coat known as the "macintosh". Furthermore, he discovered how to cut, roll and press rubber on an industrial scale. He also noted the importance of heat during the pressing process, and built a machine for this purpose. Finally, in 1842, Hancock came into possession of vulcanized rubber produced by Goodyear.
In 1845, R.W. Thomson invented the pneumatic tire, the inner tube and even the textured tread. In 1850 rubber toys were being made, as well as solid and hollow balls for golf and tennis.
         South America remained the main source of the limited amounts of latex rubber that were used during much of the 19th century. In 1876, Henry Wickham gathered thousands of para rubber tree seeds from Brazil, and these were germinated in Kew Gardens, England. The seedlings were then sent to India, Ceylon (Sri Lanka), Indonesia, Singapore and British Malaya. Malaya (now Malaysia) was later to become the biggest producer of rubber. In the early 1900s, the Congo Free State in Africa was also a significant source of natural rubber latex, mostly gathered by forced labor. Liberia and Nigeria also started production of rubber.

In India, commercial cultivation of natural rubber was introduced by the British planters, although the experimental efforts to grow rubber on a commercial scale in India were initiated as early as 1873 at the Botanical Gardens, Calcutta. The first commercial Hevea plantations in India were established at Thattekadu in Kerala in 1902. In the 19th and early 20th century, it was often called "India rubber." In 2010, India's natural rubber consumption stood at 978 thousand tons per year, with production at 893 thousand tons; the rest was imported with an import duty of 20%.