INTRODUCTION AND LITERATURE SURVEY - Shodhganga

1 QfuxptemINTRODUCTION ANDLITERATURE Introduction_ ___ _ _ 7 - ~~ _-_,.. V .7 ___ . ~ ,, ,.. :_-- ; Thermoset Phenolic Modification of phenolic Fiber reinforced Scope and objectives of the workCfiapter IntroductionThe development of new and improved polymers and their application innovel areas have led to innumerable new products. Polymers differ in characteristicsbecause of their unique structural complexities. Eventhough polymers are broadlyclassified as elastomers, plastics and fibers, the properties of individual members ofthe same group vary widely.

2 A change in the nature of pendant groups, arrangementof the monomeric units or change in the reaction conditions, catalysts etc. cantremendously change the properties of individual the last five decades researchers have been extensively exploring allpossibilities to discover novel polymers. But now, a large amount of work is aimed atmodification of existing polymers [1-'7]. This has been achieved by the modificationof monomers in the initial stage, blending different types of polymers together so thatthe properties of one polymer offset the drawbacks of the other or by othertechniques like addition of filler or reinforcements [8-I l].

3 The general classification of polymers mentioned earlier is according to certaincommon pattems of behaviour. Elastomers are characterized by large deformability,lack of rigidity, large energy-storage capacity, nonlinear stress-stain curves andcompressibility of the same order of magnitude as most liquids [12]. Certainelastomeric materials possess additional useful characteristics to a relative degree, suchas corrosion and chemical resistance, oil resistance, ozone resistance, temperatureresistance and resistance to other environmental conditions.

4 The rubbery polymers arecharacterized by low molar cohesion (<2 kcal/ g mol per SA chain length) [13].Of the three different types, polymers show greatest strength in the fiberform. Fibers are characterized by high mechanical strength. They exhibit molarcohesion in the range of 4-lOkcal/g mol per SA chain usually possess greater strength than rubbers. Some of them are hard,homy, rigid, stiff and dimensionally stable while others may be soft and and LITERATURE surveylastics usually exhibit molar cohesion in the range of 2-5 kcal/g mol per 5/3tchainrngth.

5 Plastics are generally classified as thennoplastics and thermosetting shows some of the important thermoplastics and their Important thermoplastics and their properties [12, 14]ThermoplasticsPropertiesPolyethyleneP olypropylenePoly vinyl chloride(PVC) ._ ' .. _ ..1 Poly methylmethacrylatePolyamidesPolycarbonat esPolyacetalsAc lOnimle_ Good processability, appearance, lower cost along with aBugldienastyrene l good balance of engineering properties including, low(ABS) processability, excellent electrical insulationproperties and chemical resistance, toughenss andflexibility even at low temperatures, low water vapourpermeability and reasonable clarity of thin appearance, low density sterilisability, enviromnentalstress cracking resistance and good heat resistance andexcellent flex PVC has moderate heat resistance and goodchemical, weather and flame resistance, good electricalinsulation properties and good resistance to electrical insulation properties, low water absorption, ieasy processability.

6 Good barrier properties, gooddimensional properties, reasonable chemical resistance and llow thermal transparency, good weathering resistance, goodoptical properties and moderate impact toughness, rigidity, abrasion resistance, gooddrocarbon resistance and reasonable heat , good toughness, excellent transparency, goodinsulation properties, virtually self-extinguishing andphysiological mechanical strength, predictable dimensional ibehaviour, chemical and corrosion resistance, light weight,acceptability for food contact applications and ease ofcreep, good dimensional stability and high strength andrigidity3(jfiapter -1 _Thermosetting plastics [12], as the name implies, can be cured, set or hardenedinto a permanent shape.)

7 Curing is an irreversible chemical reaction also known ascrosslinking. It usually occurs under the influence of heat and/or catalyst and leads toa three dimensional rigidized chemical structure. For some thermosetting materials,curing is initiated or completed even at room temperature. Although the cured partcan be softened by heat, it cannot be remelted or restored to the flowable state thatexisted before curing. Continued heating for long times leads to degradation load-bearing qualities of plastics can be improved by reinforcing withfibers.

8 Such fiber-reinforced plastics have replaced conventional structural materials,such as wood and steel, in a variety of engineering applications because of a highstrength-to-weight ratio, excellent chemical resistance, weatherability, and versatilityof product formaldehyde (PF) resins are a type of thermosets which are, interalia, used as a matrix for fibre reinforced plastics (FRP). The widespread use ofthese resins is due to their low cost, ease of processing, excellent wettingproperties with reinforcements, weathering resistance, dimensional stability,thermal resistance, chemical resistance and ablative properties [l5].

9 The mostimportant properties, however, that differentiate phenolics from other plasticcomposites are excellent creep resistance at high temperatures and ablativeproperties. This particular study aims at the modification of PF resins forimproved mechanical Thermoset resinsThermosetting resins change irreversibly under the influence of heat and/orcatalyst into infusible and insoluble materials by the formation of covalently cross linked, thermally stable networks. Preparation of themiosetting polymers is a twostage process involving the formation of long chain molecules with reactive the second stage these chains are crosslinked by heat and/or the addition of4I ntrocfuction and LITERATURE surveycuratives.

10 Sometimes irradiation is employed to achieve cross-linking [16]. Importantthermosetting resins are phenolics, aminoplasts, epoxy resins, unsaturated polyesters,alkyds, vinyl esters, allyl resins, polyurethanes, silicones, furfurals etc. [17]. Phenolic resinsPhenolic or PF resins were the first polymeric resins produced commerciallyfrom simple low molecular weight compounds [15]. These are prepared by thepolycondensation between phenol and formaldehyde in the presence of either an acidor a base catalyst. The nature of the product is dependent on the type of catalyst andthe mole ratio of reactants.