Polymers

Thermosetting resins Thermosetting resins are the materials that undergoes irreversible changes under the influence of heat from a fusible and soluble materials into infusible and insoluble materials through the formation of covalently cross-linked, thermally stable networks. The main characteristic of thermosets is that they require curing. Thermosets are generally used in liquid state and solidify and harden on curing. Some resins can be part cure and hold on to what is called B stage for completely curing at latter time. Some examples of thermosets include epoxies, polyesters and formaldehyde based resins. The formaldehyde based resins is classified into two classes, the condensation products obtained on reacting formaldehyde with phenol (or resorcinol) called phenolic resins or phenoplasts and the products obtained on reacting formaldehyde with urea (or melamine) called amino resins or amino plastics. Thermosetting resins have excellent resistance to solvents and corros

Molded plastics. The molded plastics are formed by injection molding. Here the molded plastics are formed by injecting molten liquid plastic into the mold, which is then allowed to stand sometime in order to cool after which it gets solidifies and take the shape of the mold. Plastic injection molding is extremely versatile method of producing parts and products. Not only is plastic injection molding simpler and more reliable, it is also extremely efficient. Many of the plastic parts that we use in everyday life are injection molded. These plastics are used as different shaped containers like shampoo bottles, water bottles and food containers etc. Molded plastics. They are also used as computer parts, car parts etc. Molded plastics. They are also used in medical application. Home decoration products and lawn decoration are constructed to large extend from molded plastics. Molded plastics   NEXT

Plastic Pipes. Plastic pipes are tubular sections or hollow pipes/cylinders made of plastics. They are usually but not necessarily of circular cross-section. The plastic pipes are used to convey substances that can flow such as liquids and gases (fluids), slurries, powder and small mass of solids. Slurries. They are used as hollow pipes. Hollow pipes are stiffer than solid pipes. A common example of plastic pipe is polyvinyl chloride (PVC) pipes made from combination of plastic and vinyl chloride. Polyvinyl chloride pipes. PVC pipes are durable, hard to damage and long-lasting. They do not rust, rot or wear overtime. Thus, these pipes are used in water system, underground wiring and sewer lines. The monomer used in production of PVC is vinyl chloride. Polyvinyl chloride formation. The polyvinyl chloride is produce by heating a water-emulsion of vinyl chloride in the presence of benzoyl peroxide or hydrogen peroxide in an autoclave under pressure. Next Auto

Polyelectrolytes. Polyelectrolytes are polymers that have dissociating groups in their repeating unit. They contain ionizable group along the chain, normally exhibit this property in solution that makes it different from non ionizable structures. Examples of polyelectrolytes. Like ordinary electrolytes are divided into acids, bases and salts, the polyelectrolytes are classified into polyanions, polycations and polysalts. Polysalts are made from reacting polyacids (polyanions) with monomeric base and vice versa. The polyelectrolytes behaves as both electrolytes and polymers. Like regular salts their solution are electrically conductive and like polymers their viscosity depends on molecular weight and polymer concentration. In non ionizing solvent -for example poly(acrylic acid) in dioxane- polyelectrolytes behave in completely normal fashion, but in aqueous solution they are ionised. Polyelectrolytes are used in water treatment as flocculation agents. Water treatment.

Cellular Polymers. Cellular materials widely occur in nature such as wood, cork, corals, sponge, human bones etc. These natural materials with cellular structure have been used for centuries. The idea of developing and manufacturing the broad range of cellular polymers arose when these natural cellular structures exhibited excellent properties such as high strength of wood, cushioning properties of cork and straw and good insulating properties of cork and balsa. Example of cellular polymers. The multifunctional cellular materials are widely used structurally for cushioning, thermal insulation, energy absorption from impacts, sound absorption and vibration dampening. The cellular polymers consist of two phase i.e solid and gas. Cellular rubbers are cellular polymer consisting rubber matrix containing a mass of cells. Some properties of cellular rubbers are;      1) Limited water absorption.      2) Weather resistant.      3) Resistant to chemical and acid. Cell

Laminates. Laminating is the simple binding together of different layers of materials. Laminates or multilayer polymer films are used in industries for food and consumer product packaging. Multilayer films used for packaging have complex composition in order preserve their contents. The multilayer films used for packaging need to protect their contents and must be machinable at low cost. Each layer in multilayer films perform a unique barrier function in order to protect their contents from light, moisture, oxygen, chemicals and microbial materials. They are often refer to as reinforcing materials. Uses of Laminates. Typically, conventional polymers used for packaging are polypropylene (PP), polyethylene (PE), Polystyrene (PS) and polyethylene terepthalate (PET). The multilayer films are the materials which account for a significant proportion of material that end up at landfill sites or recycling plants. Landfill sites and recycling plants. Some of these material

Adhesives Any substance that is capable of holding materials together by surface attachment that resists separation is called adhesives. Cement, mucilage, glue and paste are interchangeable terms used for any organic substances that forms an adhesive bond. The stickiness of adhesives is caused by molecular bonds and its strength depends on how much stress is needed to pull those bonds apart. When the glue dries over a surface, it hardens, causing the molecule to stick together. On the other hand tape is made from mixing rubbery material with adhesives. Adhesives. Natural adhesives are made from organic sources such as vegetable starch, natural resins, or animals. They are often reffered to as bioadhesives. Synthetic adhesives are based on elastomers, thermoplastics and thermosets. Examples of thermosetting adhesives are epoxy, polyurethane, cyanoacrylate and acrylic polymers. Adhesives can contain hazardous solvents, plastics and preservatives. Many adhesives contain

Coatings. Painting and coating are interchangeable terms but basically painting is done for decoration and coating is done for protection. The polymeric coatings are applied to elastomers, metals, ceramics and other polymeric materials. Examples of polymeric coatings are natural and synthetic rubbers , urethanes, PVC, phenolic resins, acrylics, silicone and nitrocellulose. They are the polymers which provide surface that are both hydrophobic and oleophobic and make substances like adhesives and food ingredients non sticky. They are FDA approved. Polymeric coating are made from polymeric materials and are applied to different substrates using different techniques such as dispersion/ extrusion coating and solution application. These coating are done for corrosion resistance. They must adhere well to substrate and must not degrade by heat, moisture and chemicals. Acrylics and alkyds are used as corrosion resistant coating for farm equipment and industrial products. Polyure

Elastomers. The natural or synthetic polymers which have elastic properties is called Elastomers. For example; rubber.  Origin of rubber. The word 'rubber' is derived from its early application i.e the property of weeping tree to rub out pencil writing. Many materials were impregnated with rubber to make it waterproof. However, the performance of rubber was still quite poor because it was gummy at the time and fluctuation in temperature cause change in products. It was the year 1839, when Goodyear accidentally discovered the vulcanisation of rubber, which makes the rubber elastic and it can keep or preserve its characteristics with fluctuation in temperature. Types. There are two types of rubber. 1) Natural rubber: Natural rubber is produced from latex i.e the milky emulsion extracted from rubber tree or other plants, where the coagulated latex is transform into sticky, soft and plastic substance (Crude rubber) and then Vulcanised (Cured). The rubber is comp

Vulcanisation. The two disadvantages of rubber is that it becomes soft and sticky when heat and hard and solid when cold. In the 18th century these disadvantages were overcome, by the discovery of vulcanisation process accidently by Charles Goodyear. Vulcanisation process is defined as the process which involves chemical reaction between unsaturated rubber (polyisoprene) and small amount of sulphur at high temperature in the presence of activators and accelerators, to form crosslinking in free rubber molecule chains together to form a 3D elastic network. The vulcanisation process is also known as curing. The vulcanisation process and crosslinking reaction is shown in figure below. vulcanised and unvulcanised network. The vulcanisation process contains following principle stages: 1) Mixing of crude rubber with about 5-30% sulphur (crosslinking agent) and other additives such as accelerators, activators, antioxidants, color pigments etc. 2) Molding (Shaping) of the mi

Fiber after treatment. The production of fabric from fiber involves various intermediate steps between spinning and weaving. They must be scoured, lubricated, sized, dyed and may undergo various treatments. Scouring. The removal of impurities from textile materials is called scouring. This is done by surface active agents such as soap and synthetic detergents. On the surface of fibres oils,fats etc are present naturally or purposely added during various process. The principle function of this process is removing these impurities. Lubrication. In order to reduce friction against themselves and against the processing machinery, lubrication is done. The lubricants are generally mineral or vegetable oil or suitably refined petroleum products. Sizing. Size is surface coating in order to protect the yarn during weaving. It makes the yarn smooth by binding the protruding fibres onto the core of yarn. Starch is used to size cotton and to some extend wool and rayon. Pro

Dry spinning. In dry spinning the polymer is dissolved in its solvent and then extruded, as the fibre emerge from the spinneret the solvent is evaporated off by hot air. The solvent in most cases is collected and reused. The polymer used in this method are acetate, triacetate, acrylics, modacrylics, spandex, Vinyon and PBI. The process is illustrated in fig below. Mechanism of dry spinning. Wet spinning. This is the oldest method. This type of spinning is required by a polymer which require dissolving in a solvent to be spun. This method is named wet spinning because the polymer is extruded directly into the liquid bath. Since the polymer is extruded directly into the liquid, the filament experience a greater drag force than that of polymer extruded in air, due to which speed of this process is reduced as compare to the melt and dry spinning. This process is based on precipitation. The polymer fibre spun using this method include acrylics, spandex, aramid and

Spinning. The bulk polymer is converted into fiber form by spinning. Spinning usually requires solution or melt of polymers . If a polymer can be melted under reasonable condition, the melt spinning is preferred over other solution process for production of fibre. The various spinning process are described below: 1) Melt Spinning: It is the convienent and economic process for manufacturing polymer fibre at industrial scale. In melt spinning, the polymer chips are melted . The melted polymer chips are then pumped through a spinneret into an air chamber. Spinneret. The polymer extruded is then cool and solidifies into a continuous filament which is then drawn out from the chamber, processed or twisted or both further and subsequently wound into spools. The polymer fibre manufacture from this method are polyester,nylon, polypropylene etc. This process is illustrated from figure below. Mechanism of melt spinning.

Properties of textile fibres. Esthetic factor. Silk is an outstanding example with good esthetic properties. Filament acetate ranks next. Polyesters appear wool like and are most pleasing. Example of esthetic. Comfort. Comfort of fibre is most important but little understood property. It is related to the structure of the fabric in determining it's ventilation and heat insulating characteristics and twisting properties. The comfort is not, however, depend on high moisture absorption in the fabric. Crease resistance. The polyester, nylon and acrylics are good in this respect. Cellulosics are not good in crease resistance, however they may be modified by resin finishes to give good crease resistance. Example of crease. Fabric stability. The stability of shape and dimension of synthetic fibres are outstandingly good. Wool retain well shape in garments but shrinks on wet treatment. A phenomenon called pilling is important in synthetics. A nodule or pill is f

Properties of textile fibres Electrical properties Fibres are usually not used in electrical application, however their property of great interest is their resistivity. Too high resistivity leads to the development of static charge, which cause the fabric to cling and to be difficult to clean. Mechanical properties The mechanical properties of fibre are quite complex. The stressed textile fibre is a visco-elastic complex system in which number of irreversible process takes place. On the basis of fibre stress strain curve, it is divided into two types; silk like curve and wool like curve. Moisture regain The moisture regain of synthetic polyesters, nylon and acrylics are lower than that of natural fibres, that the synthetics are classed as hydrophobic materials. This is an advantage for rapid drying. Dyeability The hydrophobic materials are difficult to dye. Acrylics are bad in this respect. Polyesters and nylon are intemediate in dyeability. While cellulose and cellu

Fibres. One most common definition of fibres require that its length should be atleast 100 times its diameter. Fibre refers to the unit that can be spun into a yarn or can be made into a fabric by various method. Among the natural fibres; wool,cotton and flax usually have length 1000-3000 times their diameter and coarse fibre such as jute,ramie and hemp have length 100-1000 times their diameter. The textile fibre is the basic element of fabric and other textile structures. On the basis of origin; fibres are classified into three types viz 1) Natural fibres. 2) Manufactured/ man made fibres. 3) Mineral fibres. Natural fibres. Fibres that can grow or develop and come from natural resources i.e plants and animals is called natural fibres. For example; wool, cotton and silk etc. Manufactured (or man made) fibres. The fibres which are obtained from industrial process i.e whether by action of chemical reagents on natural polymers or through polymers obtained from ch

UV stabilizers  UV radiation is the radiation having energy in the range 100-72 kcal or corresponds to UV region (280-400 nm). This radiation have sufficient energy to cleave the polymer linkages resulting in yellowing and embrittlement of polymers. Polyethylene,PVC,Polystyrene,Polyesters and Polypropylene shows degradation at wavelength 300,310,319,325 and 370 nm of UV radiation respectively. Thus these degradation demands the use of UV stabilizers such as phenyl salicylate. Phenyl salicylate in the presence of UV radiation rearranges to form 2,2' dihydroxybenzophenone which act as energy transfer agents i.e they absorb energy to form chelate and then release energy at longer wavelength to give quinone derivatives. Phenyl salicylate. The other UV stabilizers are benzotriazole, substituted acrylonitrile, metallic complex and pigments such as carbon black. The pigments absorb UV radiation and act as screening agents. Colorants. Colorants which provide color in

Heat stabilizers. Heat stabilizers are added to polymers to prevent their degradation due to heating. These substances are added into a material to protect them when subjected to intense thermal process. In case of chlorine containing polymers such as PVC on heating loses HCl i.e dehydrohalogenation and forms chromophoric conjugated polyenes structure as shown in figure below. Degradation of PVC on heating. These allylic chlorides are unstable and degradation continuous as an unzipping type of chain reaction. This degradation is accelerated by the presence of iron salts, oxygen and hydrogen chloride. The addition of organic phosphites such as mixed alkyl and aryl phosphites or triphenyl phosphites, form complexes and prevent the formation of insoluble chlorides. Toxic heat stabilizers such as Lead, Cadmium and Barium salts act as HCl scavengers. Epoxy group act as HCl scavenger. Less toxic epoxidised, unsaturated oils such as soybean oil act as HCl scavenger. Addit

Antioxidants (AO) The role of antioxidants is to prevent the oxidation of polymers. These are the substances which easily get self oxidised, although in some cases antioxidants act by combining with the oxidised polymer to give a stable product. Antioxidants generally belong to the class of phenols, aromatic amines and salts and condensation products form by reacting aminophenols and amines with aldehyde, ketone and thiocompounds. On the basis of protection mechanism antioxidants are classified into two categories viz 1) Kinetic chain breaking antioxidants. 2) Peroxide decomposers. 1) Kinetic chain breaking antioxidants These are also called chain terminators or chain scavengers. These antioxidants scavenges some of all low molecular free radical and polymeric radical. 2) Peroxide decomposers These decompose the hydroperoxy group present in the polymer. Peroxide decomposers. On commercial basis antioxidants are classified into two types. 1) Primar

Plasticizers Plasticizers are organic substances of low volatility which when added to plastic compounds improves its flexibility, extensibility and processibility. Thus in polymers the flexibility of polymer is achieved by internal and external plasticization. Internal plasticization is produced by copolymerisation which result in increase in flexibility of polymer chain or by grafting another polymer into the given polymer chain. Thus Poly(vinylchloride-co-vinylacetate) is internally plasticized because of increase in its flexibility. The large bulky groups increase the polymer motion and placement of such groups by grafting on polymer chain act as an internal plasticizers. In external plasticization, plasticizers are incorporated into polymers by heating or mixing or both. Plasticizers must be non volatile, inert, non mobile, cheap and compatible with the system to be plasticized. Plasticizers are classified into primary and secondary plasticizers. Primary plasticizer

Additives The foreign substances added to polymers intentionally enhances or modify their properties and are called additives. Thus addition of additives to polymer enhances its mechanical, optical, electrical and acoustic properties. The additives include fillers, stabilizers , plasticizers , colorants and flame retardants. Fillers Among the naturally occurring fillers are cellulosics (such as α-cellulose,starch, wood flour) and proteinaceous fillers (such as soyabean residues). The fillers are divided into two categories, those that reinforce the polymer and enhances its mechanical performance and those that are used to take up space and thus reduce the amount of actual resin to produce a part - is reffered as extenders. The third less common category of filled polymers is that in which filler is dispersed in the polymer to increase its electrical conductivity. The polymer which contain filler that reinforce its mechanical performance is often called composites.

Blow molding Polystyrene was the first synthetic polymer used for blow molding during second world war. Blow molding is a process in which heated hollow thermoplastic tube (parison) is inflated into a closed mold confirming the shape of the cavity of the mold. Blow molding is a manufacturing process by which hollow plastic parts are formed. There are three principle techniques of blow molding based on the method by which parison is formed and these are a) Extrusion blow molding (EBM) b) Injection blow molding (IBM) c) Stretch blow molding (SBM) Extrusion blow molding Extrusion blow molding is the simplest type of blow molding. A hot plastic material, called parison is dropped from the extruder and captured in water cooled mold. Once the mold are closed,air is injected through the top of the container as if one were blowing up a balloon. Injection blow molding Here Parison is injected molded . Injection mold is opened and parison is transferred to a bl

Extrusion molding. Extrusion is a method by which molten plastic is pushed through a two dimensional opening. Example of extrusion can be seen when toothpaste is squeeze out of a tube. Thermoplastics such as PVC,LDPE, HDPE,PP can all be extruded. In this method the polymer in form of pellets are fed into the extruder through a Hopper. Then the material are pushed forward by a feeding screw which is then passed out through the orifice giving continuous polymer product. The heaters present over the extruder make the polymer soft and melts it. Here cross-linking occur during heating and melting of polymer material. This process continuous as long as raw pellets are supplied. This method is used to produce long shape polymers with constant cross-section. Extruding are usually used for frames like window profile. It is also used to make pipes, drinking straws and rods etc. Check out injection molding also Extrusion molding.

Injection molding. Injection molding is a process in which molten polymer is forced under high pressure into the mold cavity through an opening called sprue. In this process the pelletized material is put into the cylinder through a Hopper, from where charge passes into the heating chamber where it melts and then it passes into the mold cavity through the nozzle where the polymer attains its shape. The cylinder is provided with a screw which is also called receprocating screw since it not only rotates but also moves forward and backward according to the polymer molding step. The characteristics of this process is that the time cycle of this process is very small. This process is highly productive and polymer of accurate shape and size are formed. (Check out Extrusion molding also) This process is generally used for thermoplastics. The thermoplastics used in injection molding process are polypropylene (PP), polycarbonate (PC), acrylonitrile butadiene styrene (ABS) and nylon

Compression molding. Molding Molding is a technique generally use for thermoplastic and thermosetting materials. This technique is used to produce sheet like, hollow and solid materials, from very small to very large materials. The various molding process are Compression molding. Injection molding. Extrusion molding . Blow molding. Compression molding This technique is widely used in automotive (vehicle motors) industries to produce parts that are large, thin,strong,stiff. This process is also used in household goods and electrical industries. The polymers processed from compression molding are epoxies (EP), phenolics(PF), melamine formaldehyde (MF) and urea formaldehyde (UF). Compression molding occurs by the following steps: Compression molding. A pre-weighed polymer with additives and fillers (called charge) is placed in the lower half of the mold. The charge may be powder, pellets,putty like masses. The charge are heated before placing on the lowe

Inorganic polymers Inorganic polymers are the polymers which do not contain any carbon atom in their backbones. The monomers in inorganic polymer are linked via covalent bonds. The polymer containing organic and inorganic constituents are called hybrid polymers and most 'so called' inorganic polymers are hybrid polymers. Polyphosphates, Polyphosphazines and silicones are example of inorganic polymers. Similarities between organic and inorganic polymers. 1) The monomers in both organic and inorganic polymers are linked via covalent bond. 2) Both organic and inorganic polymers are macromolecules with high molecular masses. Difference between organic and inorganic polymers. 1) Organic polymers contain carbon atoms in their backbone while inorganic polymers do not contain any carbon atoms in their backbones. This is the main difference between organic and inorganic polymers. 2) Organic polymers have simple structure while inorganic polymers have highly branched c