_MNL_35-2004.pdf

The Basics of Testing Plastics:IVIechanical Properties,Flame Exposure,and General Guidelines Stephen Burke Driscoll,editor ASTM Manual Series:MNL35 ASTM Stock#:MNL35 ABS-f-PCABS+PVCABS+PUPC+PETPVC-I-PMMA Crystallinity This is considered one of the three important attributes of a polymer The extent of crystallinity will affect processability and end-use properties as does the effect of molecular weight 4 MANUAL ON THE BASICS OF TESTING PLASTICS UJ a.O cc Q.PROPERTY,K MOLECULAR WEIGHT MOLECULAR WEIGHT(DP)FIG.1-1.Effect of molecular weight and distribution of processability and performance.-CCrr linear branched crosslinked FIG.1-2.Basic polymer structures.ladder RANDOM BLOCK GRAFT ABAABABBABAABABBA A A A A A A A A A A A A-B B B B B B B B B B B-A A A A A A A A A A-B B B B B B B B B B B B B B B B B B B B B B B B B-A A A AAA AAA AAA AAA I D E A L(A L T E R N A T I N G)ABABABABABABABABABA FIG.1-3.Structures of co-and terpolymers.and MWD.Starting with a completely amorphous(no crys-tallinity)polymer to steadily increasing crystalline content(percent crystallinity),the majority of physical properties will improve,but at the expense of processability.Modulus,strength,heat,and chemical resistance will increase,while impact resistance,clarity,and directional shrinkage will suf-fer.Additionally,the melting point and heat-sealing temper-atures must be increased with increasing percent crystallin-ity.Crystallinity can be promoted by selection of catalyst,me-chanical design and processing,and thermal treatments(an-nealing versus quenching).Crystallinity is a function of the summation of bonding forces that combine to align the poly-mer chains in a very specific pattern or configuration.The macromolecular chains pack in closely and form a regular structure,resulting in a crystalline site.This can be com-pared to many forks or spoons stacked neatly together This close packing promotes crystalline growth,although the smaller the size of the crystalline site,the better the clarity.Finally,crystallinity is often referred to as thermally revers-ible cross-links since when heated they disappear,yet reform when cooled slowly.Chemical Formulations Polymers are generally grouped into two major subdivisions based on how the reactants combine and if a by-product were produced:Polycondensation A.(poly)condensation:A+BPoly C-I-by-product.In the condensation scheme,two or more reactants or feed stocks combine,forming the desired polymer AND a by-product,which is usually water These polymers can be either thermoplastic or thermosetting.Examples of condensation polymers include:CLASSIFICATIONS AND DEFINITIONS 5 1.thermoplastic polyesters(PET)and polyamides(nylons),which generate water as the by-product.2.thermoplastic polycarbonate,which yields HCl as the by-product.When preparing to process a condensation polymer,it is very important that it is dried properly.Otherwise,the poly-mer can depolymerize at the elevated processing tempera-tures;a small amount of surface or trapped moisture(often less than 0.3%)can lead to a significant loss of molecular weight and,consequently,functional properties,including impact behavior.The majority of the second generation of engineering or high-performance thermoplastics is based on condensation chemistry.A third example of a polycondensation resin is an epoxy,which is based on the reaction of bisphenol-A(BPA)with epichlorohydrin,yielding HCl as the by-product.Epoxies are thermosetting resins,and all thermosets are based on the condensation reaction mechanism.Other commonly used thermosets include phenolics,aminos,unsaturated polyes-ters,and silicones.Polyaddition B.poly)addition:monomer A polymerized to Polymer A;there is no by-product The addition-type polymers are thermoplastic;the most commonly used include members of the olefin,vinyl,and styrenic families.Polymerization Engineering Regardless of how many starting monomers,the structure of the polymer,or the molecular weight or its percent ciys-tallinity,there are other classifications schemes,including design of reactor equipment,which can be used.For exam-ple,the reaction might be based on acidic or alkaline con-ditions or use either t5e of catalyst,the reaction can be anionic or cationic,and the reactor can be either the sus-pension,emulsion,mass/bulk,or solution lypQ.In cill cases,polymers made using one set of these param-eters will yield entirely different products than those based on other combinations.PVC can be produced using these four different polymerization techniques or methods.Al-though the vinyl chloride monomer(VCM)is common to all four,the fined product does reflect major differences in how it can be used.Suspension-grade PVC is a freely flowing white powder having the consistency of granulated sugar.This particle size makes this grade ideal for injection mold-ing,extrusion,and calendering.The emulsion grade PVC,which has a finer pEirticle size similar to baking flour,is more suitable for coating applications,including plastisols and