A-Z Encyclopedia Outline

A

1. Abrasion Resistance

Also known as wear resistance, a character-isticdescribing theresistanceofamaterial towear, which takes place via the erosion ofsmall particles from the surface as a result ofthe frictional forces exerted by a slidingmember. A suitable measure of the rate ofwear is provided by the ratioV/m, whereV isthevolumeabradedperunit slidingdistanceand m is the coefficient of friction, whichcorresponds to the amount of abraded sub-stance per unit energy dissipated in sliding.The abrasion resistance is measured in anumber of different ways, according to theexpected service conditions of a particularproduct.Themostcommonmethodis touseabrasive papermounted on a rotating drum,allowing contact with a specimen subjectedto a constant load in order to bring about theformation of debris, which gives rise to lossof material by wear.

2. ABS

A blend or alloy consisting of a glassy styr-ene–acrylonitrile random copolymer matrixanddispersedmicroscopic rubberybutadiene–acrylonitrile random copolymer particles,which contain sub-micrometre glassy poly-mer inclusions. The three letters stand foracrylonitrile, butadiene and styrene. Acry-lonitrile–butadiene–styrene (ABS) poly-mers are produced either by blending astyrene–acrylonitrile (SAN) copolymer(glassy major component) with a butadie-ne–acrylonitrile rubber (NBR) copolymer(rubbery component), or by a specially de-signed bulk polymerization method. Thelatter consists of dissolving the rubbery

component in a mixture of the monomersfor the formation of the glassy matrix, fol-lowed by free-radical polymerization, whichproduces the glassy SAN matrix, graftedonto the diene elastomer. At any early stageof the polymerization, there is an inversionof phases, leading to the precipitation ofsmall glassy particles into the diene elasto-mer, which precipitate into larger particles,forming the characteristic morphologicalstructure of these toughened polymersystems. The different particle structuresobtained with the two methods of produc-tion are shown in the diagram.

Structure of rubber toughening particles in ABSpolymers. Left: System produced by blending ofemulsions. Right: System formed bymass polymer-ization. Source: Schmitt (1979).

There are many grades of ABS availablecommercially, differing in rigidity character-istics, which are determined by the amountof rubber used, and also in the degree ofplasticization of the glassy phase by theelastomer component. The glass transitiontemperature (Tg) values are in the region of100–110 �C for the glassy phase and around�40 to �50 �C for the rubbery component.The ordinary polymer grades are opaque,owing to the particle size of the precipitatedparticles being greater than 1mm and to the

Polymers in Industry from A–Z: A Concise Encyclopedia, First Edition. Leno Mascia.� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

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substantial difference in the refractive indexvalues for the two components.ABS is widely used in the automotive

industry, as it combines its intrinsic hightoughnesswith the ability to produce surfaceswith a high gloss, and also because of the easewith which parts can be metallized by con-ventional electroplating methods. It is alsoavailable in the form of blends with polycar-bonate, which exhibit better thermal oxida-tion stability and higher rigidity than conven-tional ABS grades. (See Styrene polymer.)

3. Absorbance

A coefficient denoting the fractional inten-sity of radiation absorbed by a body, that is,

absorbance a ¼ flux of radiation absorbedtotal incident flux

:

4. Accelerator

An additive used to speed up the rate ofcross-linking reactions in the curing ofthermosetting resins or elastomers. Forpolyester thermosetting resins, the acceler-ator is usually a cobalt naphthenate or cobaltoctoate in solution. For the case of elasto-mers, the accelerator is a sulfur-containingcompound, such as a thiourea, mercaptanand dithiocarbamate, or a non-sulfur-con-taining compound, such as a urea, guani-dine and aldehyde diamine. The chemicalstructures of typical accelerators for sulfurcuring of elastomers are shown.

In many cases, mixtures of acceleratorsare used to obtain a synergistic effect. Typi-cally, a thiazole type is used with smalleramounts of dithiocarbamate or an amine.

5. Acetal

A term used for poly(methylene oxide)(PMO), represented by the chemical formula�(CH2O)n�. Acetals are crystalline poly-mers with a melting point (Tm) in the regionof 180–190 �C and a Tg around�40 �C. Theyare also available in the form of randomcopolymers containing small amounts ofethylene oxide units. Acetals are widely usedin engineering applications for their highresistance to creep and wear under highloads. They also have a good resistance tosolvents and low water absorption character-istics, which makes them suitable for appli-cations requiring dimensional stability un-der moist environmental conditions. Blendswith up to about 30% polyurethane elasto-mers have been reported to display a goodbalance of engineering properties with re-spect to stiffness, strength, toughness andsolvent resistance.

6. Acid Number

A term used to denote the acid content of apolymer or resin, which is defined as theamount in milligrams (mg) of KOH (potas-sium hydroxide) required to neutralize100 g of polymer or resin.

2j 6 Acid Number

7. Acoustic Properties

Describe the response of amaterial to soundand particularly the absorption of sound. Inpolymers, theabsorptionofsoundtakesplacethrough molecular relaxations. The parame-ter that denotes the capability of a polymer todissipate mechanical energy through vibra-tions, known as the loss factor or dampingfactor (tan d), is also used to describe thesound absorption characteristics of poly-mers.(SeeViscoelasticity.)Themethodsusedto measure acoustic properties are broadlydivided into wave propagation, resonanceand forced vibration methods.Resonance and forced vibration methods

are used to measure the Young�s modulusand shear modulus as viscoelastic para-meters comprising an elastic and a losscomponent. Wave propagationmethods areused, on the other hand, to record the actualsound absorption characteristics of materi-als. Measurements are made by sendingacoustic pulses with duration less than 1msthrough the specimen immersed in a liq-uid, and detecting them by another trans-ducer on the opposite surface.

8. Acrylate Elastomer

(See Acrylic polymer.)

9. Acrylic

Ageneric term formonomers and polymerscontaining acrylate or methacrylate units.(See Acrylic polymer.)

10. Acrylic Polymer

Contains acrylic monomeric units in themolecular chains, represented by the gen-eral formula shown.

Polymers where R is anHatom are calledpolyacrylates, and those where R is amethylgroup are called polymethacrylates. Acrylicsare generally amorphous polymers and areavailable as linear polymers or blends (ther-moplastic), cross-linkable elastomers andcross-linkable adhesives and surface coat-ings. The properties of acrylic polymers arestrongly dependent on the nature of thesubstituent R and R0 groups, as illustratedin the table.

Density(g/cm3) Tg (�C)

Poly(acrylic acid) — 106Poly(methyl acrylate) 1.22 8Poly(ethyl acrylate) 1.12 �22Poly(n-butyl acrylate) <1.08 �54Poly(t-butyl acrylate) 1.00 43Poly(methacrylic acid) — 130Poly(methyl methacrylate) 1.17 105Poly(ethyl methacrylate) 1.12 65Poly(n-butyl methacrylate) 1.06 20Poly(n-hexyl methacrylate) 1.01 �5Polyacrylamide 1.30 165

Source: Data from Mascia (1989).

The data in the table clearly illustratethree important aspects of the structure–-property relationship in polymers, respec-tively chain stiffness (energy required torotate a group attached to the carbon atomin the backbone chain), internal plasticiza-tion (creating free volumes between poly-mer chains) and intermolecular forces. Onthis basis, one notes that polymethacrylateshave a higher Tg than the correspondingacrylates. Increasing the length of the alkyl-group acrylate or methacrylate polymersbrings about a large reduction in Tg throughinternal plasticization. The presence of a

10 Acrylic Polymer j3

carboxylic acid or an amide group causes theopposite effect through the increase in in-termolecular forces by the formation ofinter-chain hydrogenbonds. Themain char-acteristics of acrylic polymers available com-mercially are described below in alphabeti-cal order of the monomer units within thepolymer chains.

10.1 Acrylic Elastomer

Generally available as a random copolymeror terpolymer, typically poly(ethyl–butyleneacrylate) with Tg within the range �30 to�40 �C. These elastomers are well knownfor their resistance to oils, as well as for theiroxidative stability at high temperatures and

in UV light environment. They tend to ab-sorb large quantities of water via hydrolysisof the ester groups. Vulcanization can becarried out by either peroxide curatives, forsystems containing unsaturation in themainchains,orbymetalhydroxides throughsalt formation via carboxylic acid groups.

10.2 Acrylic Adhesive

(See Adhesive.)

10.3 Acrylic Fibre

Produced from polyacrylonitrile (PAN),whichisrepresentedbythechemicalformula

where the CN groups are organized in anatactic configuration, preventing themolec-ular chains from packing in a highly or-dered crystalline lattice. The fibres are pro-duced by solution spinning, because of thehigh viscosity and the rapid decompositionof the polymer at the temperatures thatwould be required for melt spinning. Ther-mal degradation takes place by cyclizationinvolving the pendent CN groups along thepolymer chains, which results in the forma-tion of infusible ladder polymers, as shownby the reaction scheme, and confers on thefibres a certain degree of fire retardancy

Moreover, this particular feature makesPAN fibres suitable for the production ofcarbon fibres due to the dimensional stabil-ity that they acquirewhile they are heated upto the high temperatures required forgraphitization. (See Carbon fibre.)

10.4 Acrylic Flocculant and Hydrogel

Based on polyacrylamide, a water-solublecrystalline polymer represented by the for-mula shown.

4j 10 Acrylic Polymer

10.5 Acrylic Impact Modifier

Based on methyl methacrylate–butadie-ne–styrene (MBS) terpolymer alloys, usedwidely as impact modifiers in rigid PVCformulations. They are produced mainlyfrom emulsions of a styrene–butadienerubber (SBR) elastomer and through graftpolymerization of methyl methacrylatechemically bonded on the surface of thepre-formed SBR particles. The tougheningaction in poly(vinyl chloride) (PVC) com-pounds arisesmostly from themiscibility ofthe acrylic outer layers of the dispersedparticles with the PVC matrix. These arepreferred to ABS impact modifier systemsin applications requiring superior resis-tance to UV light.

10.6 Acrylic Plastic

The most important polymer in this cate-gory is poly(methyl methacrylate) (PMMA),a glassy polymer available either as a ther-moplastic material or as a lightly cross-linked pre-formed product, such as sheetsor rod castings. PMMA is widely for its hightransparency and high resistance to UVlight. It is represented by the formula shownand is usually available as a homopolymergrade. (See Acrylic polymer.)

11. Activation Energy

A term in the Arrhenius equation widelyused to denote the sensitivity of the rate ofchemical or physical processes to changesin temperature. The activation energy (DH)is determined from the slope of the plot ofthe logarithm of the rate of reaction, or rateof physical change, against the reciprocal of

the absolute temperature (T ). This can bederived from the Arrhenius equation,

rate K ¼ A expð�DH=RTÞ;where A is a constant for the system R isthe universal gas constant. (See Arrheniusequation.)

12. Activation Volume

A term in the Eyring equation used todescribe the sensitivity of the yield strengthof materials to changes in applied strainrate. It is an adaptation of the Arrheniusequation for which the activation energyterm (DH) is replaced by the term (DH�ysY), where y is the activation volume andsY is the yield strength. The product ysY

represents the quantity by which the activa-tion energy has been reduced by the appli-cation of the stress required to induce yield(plastic) deformations. (See Yield failure.)The Eyring equation is written as

d«=dt ¼ B exp½ðDH�ysYÞ=RT �;

where d«/dt is the strain rate used in thetest, corresponding also to the rate at whichyielding deformations take place, and B is amaterial constant. (See Eyring equation.)

13. Activator

An additive used in conjunction with anaccelerator for the vulcanization of elasto-mers. Activators consist of metal oxides orsalts of lead, zinc ormagnesium, often usedin conjunction with stearic acid to enhancetheir solubility in the rubber mix.

14. Addition Polymerization

The conversion of monomer to polymerwithout the loss of other molecular species.This is contrary to the case of condensation

14 Addition Polymerization j5

polymerization, which takes place throughthe loss of small molecules, such as water.Typically, addition polymerization takesplace by a free-radicalmechanismor cationicpolymerization.

15. Additive

A substance added to a polymer formula-tion in minor amounts to improve ormodify specific characteristics related tomanufacture and/or end use of products.

16. Additive Concentrate

Amixture of additive and a polymer powder(usually) where the amount of additive pres-ent is much larger than the quantity re-quired in the final formulation. The concen-trate ismixedwith the neat polymer or resinto bring down the amount of additive to therequired level. (See Master batch.)

17. Adherend

A term often used to denote the componentin contact with the adhesive layer.

18. Adhesive

Substance used to stick (bond) two or morecomponents of a product or structure.Adhesives can be divided into �hot-meltadhesives�, based on thermoplastic poly-mers, �curable adhesives�, based on thermo-setting resin, and �pressure-sensitive ad-hesives�, based on elastomeric polymers.Apart from the bonding requirements, anydifference in molecular structure or formu-lation details between polymer composi-tions used for �bulk� components and thoseused for adhesives arises from differencesin the way the two systems have to be

processed or applied. The table lists somepolymers used for adhesives.

Commercial name Chemical nature

Thermoplastic hot meltsPolyesters andpolyamides

Low-crystallinity systems

Phenoxy Poly(glycidyl hydroxyl ether)EVA Ethylene–vinyl acetate

copolymerEEA Ethylene–ethyl acrylate

copolymer

Thermosetting (cold cure and heat curable)Phenolics Mostly modified with NBREpoxy Mostly DGEBA resinsAcrylics Several types available

Pressure-sensitive adhesives (PSAs)Most uncuredelastomers

Contain tackifiers

Water-borne emulsionsPoly(vinyl acetate) Partially hydrolysed

The chemical compositions of polymersused for hot-melt adhesives are similar tothose used for plastics, but they have lowermolecular weights to meet the low-viscosityrequirements for manufacturing purposes.Adhesive grades are usually copolymerscontaining small amounts of carboxylic acidor hydroxyl groups to enhance their affinityand bonding characteristics for the morecommon adherends. Oligomeric systemsused for curable adhesives usually containcomplex mixtures of auxiliary ingredients,and often have a heterogeneous morpho-logy, as ameans of obtaining a good balanceof properties and also to satisfy specificrequirements for a multitude of applica-tions. Acrylic adhesives are usually basedon low-volatility monomers containing per-oxide initiators with a very long half-life.Some very innovative systems have beendeveloped that take advantage of prevailingconditions to accelerate or induce curing in

6j 18 Adhesive

the adhesive. An interesting example is theanionic polymerization of cyanoacrylate ad-hesives, which takes advantage of the slight-ly basic nature ofmany inorganic substratesand the presence of water adsorbed on thesurface to promote cure reactionswithin theadhesives.

19. Adhesive Test

Usually refers to a mechanical test for mea-suring the strength of bonded specimens,which is normally defined as the force re-quired to separate the adherends of a joint,divided by the overall bonded area. Underideal conditions, that is, those in which theactual intrinsic strength of the adhesive is tobemeasured, failure has to take placewithinthe bulk of the adhesive (cohesive failure)and not at the interface between adherendand adhesive (interface failure). This is anessential requirement of structural adhe-sive, and it is for this reason that the surfaceof the adherend is often treated to increasethe interfacial bond strength to the levelrequired to ensure that failure takes placewithin the adhesive. The preparation of thesurface of the adherend usually entails thegeneration of chemical groups that canreact with the adhesive to produce chemicalbonds across the interface. If the adhesive ismechanically strong (i.e. it requires highstresses for fracture), it is unlikely thatcohesive failures can be achieved onlythrough physical bonds at the interface,even when these are strong types such ashydrogen bonds, bearing inmind that thesemay beweakened through the absorption of

water. Nevertheless, the strength values ob-tained depend on geometric factors, such asthe dimensions of the bonded area and thethickness of the adhesive layer. Conse-quently, they cannot be used as fundamen-tal parameters for use in theoretically baseddesign procedures and have to be consid-ered primarily as data for �quality controland specifications�.

19.1 Tensile Test

Typical bonded specimens, known as buttjoints, are shown. These are pulled at con-stant strain rate up to fracture. The bondstrength is calculated by dividing the load tofracture over the bonded area.

Typical axial loaded �butt-joint� specimens:(a) wood-to-wood bond; (b) metal-to-metal bond.

19.2 Shear Test

Also known as �lap shear� to describe thetype of specimen used. Typical specimenstypes are shown.

19 Adhesive Test j7

These are the more widely used tests,mainly because both the specimens and theexperimental procedure are simple. Thestrength of the bonded specimen, knownas the �lap shear strength�, is expressed asthe load to fracture divided by the bondedarea. In these tests the adhesive layer issubjected to a combination of tensile andshear stresses at the interface andwithin theadhesive layer, which vary in the oppositemanner along the bonded length of thelapped area, as shown. The shear stress inthe adhesive layer decreases from a maxi-mum at the edge of the lapped area to zeroin the middle when the bond line is suffi-ciently long. Completely the opposite to thisis the change in axial tensile stress, which ismaximum in the centre and smallest (zero)at the outer edges.

19.3 Fracture Toughness Test: General

The fracture toughness of bonded speci-mens is normally measured using double

cantilever specimens. These are specimenswith a very long bond line subjected to a�crack opening� fracturemode by pulling thebonded cantilevers away from each otherand recording the load during fracture prop-agation along the �bonded line�. The value ofthe critical strain energy release rate Gc iscalculated from a fundamentally derivedequation, using the appropriate values fordC/da (i.e. the rate of increase of specimencompliance C with crack length a). (SeeCompliance and Fracture mechanics.)Two types of specimens are usually used:

. the uniform double cantilever beam(UDCB) specimen, which is known alsoas the thin strip test;

. the tapered double cantilever beam(TDCB) specimen, which is known alsoas the wedged specimen.

The description and geometry of thesetwo specimen types are given below.

Variation of strain in the adhesive layer and the two adherends in a single lap shear test.

8j 19 Adhesive Test

19.4 Fracture Toughness Test: UDCBSpecimen

Double cantilever beam (thin strip) test. The load can be applied either directly at the edge of the strips orthrough a wedge at the open end of the bonded strips.

In UDCB testing, it is important that thetwo adherends undergo only elastic defor-mation during the test to ensure that all theenergy imposed is used only to inducefracture of the adhesive layer. Note that, fora double cantilever beam, the compliance Cis given by the expression

C ¼ D

P¼ 64a3

EWB3;

where D is the deflection, P is the load, a isthe non-bonded distance (cf. crack length),E is the Young�s modulus of the adherend,W is the width of the specimen and B is thethickness of the beam. Differentiating theabove equation with respect to a gives anexpression for dC/da that can be substitutedin the generic equation for Gc, which canthen be calculated from the recorded load tofracture (Pf), that is, the maximum loadrecorded in the test. From the above equa-tion one obtains

Gc ¼ 96P2f a

2

EW2B3:

Note that, when the adherend is veryflexible (compliant), the test is known asthe �T-peel test�. If, on the other hand, one ofthe adherends is very rigid (i.e. very thickand/or the material has a very high modu-lus) and the other is very compliant (i.e. verythin and/or the material has a very lowmodulus), the test is known as the �L- peeltest�. Both tests can be used to measure the

fracture toughness in terms of theGc value,which is given by the formula

Gc ¼ ðPf=BÞð1þ «Þ;whereB is thewidth of theflexible adherendstrip and « is the tensile strain («¼P/BWE).When « is very small, the value of Gc isobtained directly from the ratio Pf/B. Theunit N/m corresponding to this ratio isequal to J/m2, which is the appropriate unitfor Gc.

19.5 Fracture Toughness Test: TDCBSpecimen

Tapered double cantilever specimen for fracturetoughness testing of adhesives.

For TDCB specimens the value of dC/dais given by the expression

dC=da ¼ 6m=EB;

where B is the thickness, E is the Young�smodulus of the beam andm is a function ofboth the height h of the beam and cracklength a, that is,mh3� (1 þ y)h2� 4a2¼ 0,

19 Adhesive Test j9

where y is the Poisson ratio of the beam.The taper of the outer edge of the specimenis estimated by maintaining the value of mconstant so that the height (depth) of thebeam becomes larger as the non-bonded(crack) length increases. The solution ofm gives a slightly concave contour for theouter edge, with the curvature becomingmore pronounced as the non-bonded dis-tance gets smaller. Therefore, by using largevalues of a, the contour of the specimenbecomes approximately linear, that is, thegradient is constant. Typically, if the gradi-ent angle is 11�, then

dC=da ¼ 90=EBh0;

where h0 is the height (depth) of the beam atthe crack tip. This simpler geometry makesit easier to produce specimens for testing.The advantage of the TDCB specimen isthat the fracture load (Pf) remains constantduring fracture propagation, making it pos-sible to measure the rate of crack propaga-tion along the bond line.A slip–stick type of crack propagation

occurs if fracture takes place in a mixedmode, involving cleavage through the bulkof the adhesive layer and debonding at theinterface with the adherend.

Typical force–deformation curves obtained in testswith TDCB specimens.

The TDCB test is particularly useful forfractures induced under fatigue (cyclic load-ing) conditions, so that the crack growth rate

can be measured as a function of the fre-quency and magnitude of the applied load.Using UDCB specimens, on the otherhand, the load to fracture decreases aftercrack initiation because the value of dC/dabecomes smaller with increasing cracklength. For many systems, the fracturetoughness of structural adhesives has beenmeasured also with respect to the long-termstatic and dynamic (fatigue) behaviour,through measurements of the increase incrack length with time and/or loadingcycles.

20. Adhesive Wetting

Wettability is an essential characteristic ofan adhesive to ensure that it completelycovers the surface of the adherend. Thiscondition is satisfied by ensuring that theadhesive and adherend have a similar sur-face energy. In quantitative terms, this re-quirement is to make sure that the so-called�work of adhesion�,WA, is very small. This isrelated to the surface energies, g, betweenthe various phases by the equation

WA ¼ gLV þgSV þgLS;

where the subscripts LV, SV and LS denotethe three interfaces concerned, respectivelyliquid–vapour, solid–vapour and liquid–solid. The surface energy for the solid–liquid interface is lowest when the respec-tive surface energies of the solid (substrate)and the liquid (adhesive) are very similar.Surface energies depend on the chemicalconstitution. Low polarity will give lowvalues for the surface energy. Polytetrafluor-oethylene (PTFE) has the lowest surfaceenergy because of the absence of net dipolesin the structure. Polyethylene (PE) also has avery low surface energy, owing to the ab-sence of net dipoles, but it is not as low as forPTFE. Polymers with very strong dipoles,such as those containing COOHorOH sidegroups or NH groups in the main chains

10j 20 Adhesive Wetting

(e.g. polyamides and polyurethanes), havevery high surface energies. (See Surfaceenergy.) Surface oxidation of PE or PTFEcan introduce polar groups and increaseaccordingly the surface energy. These treat-ments would be used, therefore, to improvethebondingofpolaradhesives,suchas thosebased on epoxy resins. Chemical reactionsacross the interface bring about an increasein the interfacial bonding between adhesiveand adherend, which represents the highestlevel of bonding that can be achieved.

21. Adiabatic Heating

A term used to describe the self-heating of apolymer taking place during intensive mix-ing and extrusion, resulting from the shear-ing action of the rotors of the mixer or thescrew of an extruder.

22. Air-Slip Forming

A technique used in vacuum forming (alsothermoforming) by which air is injectedthrough fine orifices in the cavity of themould in order to prevent sticking and rapidcooling of the polymer sheet during draw-ing. (See Thermoforming.)

23. Aliphatic

A term used in chemistry to describe thelinear connection of carbon atoms to eachother. This implies the absence of benzenerings, a compound containing which wouldbe referred to as �aromatic�. (See Aliphaticpolymer.)

24. Aliphatic Polymer

A polymer containing aliphatic carbonatoms along the backbone of the molecularchains, for example, polypropylene.

25. Alkyd

A type of polyester resin. The term is de-rived from a combination of the wordsalcohol and acid to indicate that it is aproduct resulting from the reaction of analcohol (multifunctional) and a dicarboxylicacid. The latter is usually a mixture of natu-rally occurring fatty acids and aromatictypes. Alkyds are normally divided into oxi-dizing and non-oxidizing types. The oxidiz-ing types are produced from unsaturatedfatty acids, whose double bonds can reactwith oxygen from the atmosphere to pro-duce free-radical species that can causecross-linking reactions. The non-oxidizingtypes are cross-linked by reactions of thefree OH groups in the chains with a ureaformaldehyde or melamine formaldehyderesin, or with multifunctional isocyanates.

26. Allophonate

A type of chemical group formed fromreaction between an isocyanate and a ure-thane group.

27. Alloy

(See Polymer blend.)

28. Alpha Transition Temperature

The alpha (a) transition temperature Ta isthermodynamically classified as a second-ary transition, denoting the temperature atwhich thefirst partial derivative of a primaryfunction, such as the volumeor the enthalpy(@V/@Tor@H/@T), shows a discontinuity.Tais also known as the glass transition tem-perature (Tg) insofar as it represents a ref-erence temperature for the change in thedeformational behaviour of a polymer fromthe glassy state to the rubbery state, which

28 Alpha Transition Temperature j11

entails a reduction inmodulus by a factor of103–104 with increasing temperature.

29. Alternating Current (AC)

A current resulting from the application of acyclic (sinusoidal)voltage.Thecyclicvariationof voltage and current takes place at specificfrequencies. (See Dielectric properties.)

30. Aluminium Trihydrate Al2O3�3H2O

A functional filler used to impart flame-retardant and antitracking characteristics toa polymer. Particle size is in the range2–20mm and surface area in the region of0.1–6m2/g. The amount of water corre-sponds to 34.5wt%, which volatilizes veryrapidly above 220 �Cand reaches about 80%completion at around 300 �C. (See Flameretardant, Antitracking and Filler.)

31. Amine-TerminatedButadiene–Acrylonitrile (ATBN)

An oligomer, also known as liquid rubber,amine-terminated butadiene–acrylonitrile(ATBN) is used for the toughening of epoxyresins. Commercial systems are availablewith molecular weight between 2000 and5000 and with acrylonitrile content around25–35%. (See Epoxy resin, subsection�Reactive toughening modifiers�.)

32. Amino Resin

A resin produced from the reaction of amultifunctional amine and formaldehyde.The most commercially important aminoresins are urea formaldehyde (UF) andmel-amine formaldehyde (MF) resins. Both re-sins are water clear, unlike phenol formal-dehyde (PF) systems. Thenetwork structureof the cured UF resins and the reactionsinvolved are shown schematically.

These are formed from condensationreactions between �CH2OH of the basicresin to produce oxymethylene bridges, asshown in the scheme.

Condensation reactions in the curing of UF resins.

Dense network structure of cured urea formaldehyde (UF) resin.

12j 32 Amino Resin

Similar reactions take place in the cross-linking of MF resins, as shown.

A large number of these oxymethylene bridgesundergo a loss of formaldehyde to produce a densernetwork.

The moulding compounds of both UFand MF resins contain cellulose fillers andadditives, such as pigments, curing cata-lysts, external lubricants and sometimesalso small amounts of plasticizer. For whiteformulations, the cellulose filler is usually ableached variety in order to eliminate thepossibility of producing undesirable tintsfrom impurities. Sometimes MF resins arebutylated in order to increase their shelf-lifewhen used as aqueous solutions or micro-suspensions. In this case the condensationreactions will be preceded by the lossof butanol. For both UF and MF resins,however, the curing reactions rarely go to

completion, so that there will be a substan-tial number of CH2OH groups presentin addition to �NCH2�O�CH2N� and�NCH2N� groups.The use of cellulosic fillers for moulding

powder or paper for laminates not onlyprovides an efficient reinforcing and tough-ening function but also ensures that thewater produced does not result in the for-mation of voids. The strong affinity of waterwith the structural units of amine resins, viahydrogen bonds, allows a substantialamount of water to remain dissolved in thenetwork.

33. Amnesia

A term (jargon) used in the technology ofheat-shrinkable polymers to denote the ex-tent by which the product fails to reach thedimensions it had before being stretched.(See Permanent set.)

34. Amorphous Polymer

The term is used to denote the lack of orderat the supramolecular level. This implies

Network of a highly cross-linked melamine formaldehyde (MF) resin. Source: Ehrenstein (2001).

34 Amorphous Polymer j13

that the molecular chains (thermoplastics)or the macromolecular networks (thermo-sets or cross-linked rubber) have a randomconfiguration, as illustrated in the sche-matic diagram.

35. Anatase

A crystalline form of titanium dioxide (tita-nia) used as the basic component of whitepigments. (See Titanium dioxide.)

36. Anionic Polymerization

A type of polymerization that takes place viathe growth of chains containing an anion inequilibrium with a small-sized counter-ion(cation) originating from the initiator, usu-ally a metal alkyl (e.g. lithium butyl) or analkyl metalamide (e.g. potassamide). Thesteps involved in the polymerization are asfollows.

Step 1. Initiation: Addition of the anionfrom the initiator onto the monomer,

LiBuþH2C¼CHX!Bu�H2

C� CHX�Liþ

Step 2. Propagation: Rapid addition of alarge number of monomer molecules,

Bu�H2C� CHX� Liþ

þ nH2C¼CHX!Bu�H2C�CHX� ðH2C� CHXÞ�n Liþ

Step 3. Termination: Polymer chains reachfull size when the monomer is exhausted orthrough secondary reactions with other spe-cies present, such as solvent.

37. Anionic Surfactant

(See Surfactant.)

38. Anisotropic

A product or specimen exhibiting aniso-tropy. (See Anisotropy.)

39. Anisotropy

Thecharacteristicofaspecimenorproduct inwhichthevalueofagivenpropertyisdifferentinmagnitudewhenmeasured in twoperpen-dicular directions. In the case of cylindricalgeometries such as rods, fibres or tubes, thetwo perpendicular directions of anisotropicbehaviour are the longitudinal and circum-ferential directions. Anisotropy in polymerproducts results from the preferential align-ment of polymer chains and crystallites in aspecificdirection. Inmost cases anisotropy isdeliberately introduced to enhance the prop-erties in one specific direction. In the case offibresandtapes,theorientationisdeliberatelyinducedintheaxial,or longitudinal,directionas a means of increasing the mechanicalstrength in thedirection inwhich theproductwould be subjected to mechanical forces in

Linear polymers (left) and cross-linked polymers (right).

14j 39 Anisotropy

service. (See Orientation.) In the case ofcomposites,anisotropyarisesfromthediffer-ent degree of fibre alignment in two perpen-dicular directions.

40. Annealing

Atermdenoting the thermal treatment (usu-allyat constant temperature followedbyslowcooling)ofaspecimenorproductwithaviewto releasing the internal stresses set up dur-ing moulding or to developing the highestpossible degree of crystallinity. This thermaltreatment serves also to stabilize the dimen-sions and properties of a product prior to itbeing used in service. Annealing is carriedout at temperatures very close to the glasstransition temperature (Tg value) of thepoly-mer if amorphous or just below its meltingpoint (Tm) if the polymer is crystalline. Note,however, that annealingmay induce embrit-tlement of products made from glassy poly-mers through physical ageing and thosemade fromcrystallinepolymers through thethickening of the lamellae.

41. Anodizing

(See Metallization.)

42. Antiageing

(See Antioxidant, Stabilizer and UVstabilizer.)

43. Antibacterial Agent

(See Antifouling additive andAntimicrobialagent.)

44. Antiblocking Agent

An additive that produces roughness on thesurfaces of a soft polymer product, prevent-

ing these from sticking to another throughinterfacial attraction forces. This phenome-non is particularly problematic in the case offlexible films used for plastic bags, as itmakes it difficult to open them. The mostwidely used antiblocking additives consistof fine particles 20–50 nm diameter, usuallyinorganic fillers such as silica, talc, kaolin,calcium carbonate and zeolites.

45. Antidegradant

Ageneric nameused primarily in relation torubber formulations to denote an additivethat improves the resistance to ageing. (SeeDegradation and Stabilizer.)

46. Antifoaming Agent

An additive used to prevent the formation offoams during the mixing of liquid systems.They are also known as defoamers or foamsuppressants. Early systems were primarilyvegetable or mineral oils. Nowadays anti-foaming agents are complexmixtures in theform of hydrophobic solids containing ac-tive ingredients, consisting of a variety ofcompounds derived from water-solublepolymers. They normally contain a liquid-phase component such as mineral or vege-table oils, poly(ethylene glycol), silicone oils(polydimethylsiloxanes) and fluorosilicones(polytrifluoropropylmethylsiloxanes). Theseare particularly effective in non-aqueoussystem particles because of their low surfaceenergy and immiscibility. The other compo-nent is a solid consisting of hydrophobicsilica or hydrocarbon waxes. Antifoamingagents may also contain ancillary ingredi-ents consisting of surfactants, couplingagents, stabilizers and carriers. The latterhave the function of holding the ingredientstogether. In general, the defoamer has asurface energy lower than that of the foam-ing medium and should be immiscible and

46 Antifoaming Agent j15

readily dispersible. The main uses of defoa-mers inpolymer systemsare in coating, latexand emulsion formulations.

47. Antifouling Additive

Known also as biocides and bactericides,these contain chemicals that are toxic tomicroorganisms. These include zinc andbarium salts, as well as mixtures of zincoxide or barium metaborate with thiazolesor imidazole compounds. They are widelyused for marine coatings. More recently,extensive use has been made of the incor-poration of sub-micrometre particles of sil-ver, particularly for furniture and appliancesfor clinical application. (See Antimicrobialagent.)

48. Antimicrobial Agent (Biocidal Agent)

An additive with the ability to inhibit thegrowth of a broad range of microbes, suchas bacteria, moulds, fungi, viruses andyeasts. Antimicrobial activity in polymerscan be attained either by the incorporationof additives or through modifications of thechemical structure. Additives that can beused as antimicrobial agents include anti-biotics, heavy-metal ions (silver or copper),cationic surfactants (quaternary ammo-nium salts with long hydrocarbon chains),phenols and oxidizing agents. An additivethat is frequently used as an antimicrobialagent in polymers is metallic silver, in theform of fine (nano-sized) particles, eventhough it does not release metal ions aseasily as copper. The use of copper isavoided, however, as it can have some dev-astating effects on the heat stability of thepolymer. In order to increase the rate ofrelease of ions, silver salts are sometimesembedded in hydrophilic carrier particles,such as silver-substituted zeolites. Macro-molecular antimicrobial agents used in

polymers include poly(2-t-butylaminoethylmethacrylate) (PTBAEMA) and N-hala-mines grafted onto polymer chains throughacid or anhydride functionalization of thelatter. N-halamines are compounds inwhich one chlorine atom is attached to thenitrogen atom. In both cases the antimicro-bial activity derives from the formation of aquaternary ammonium salt, while the poly-meric nature prevents them from beingeasily extracted by water.

49. Antimony Oxide

Corresponds to Sb2O3 and is used as awhitepigment as a result of its high scatteringpower resulting from its high density (5.7 g/cm3) and as a functional filler in conjunc-tion with chlorinated or brominatedcompounds to impart fire-retardant charac-teristics to polymers. Optimal tinting andflame-retardant properties are achievedwith particle size in the range of 0.2 to 1mm.These systems are generally considered toproduce toxic products under burning con-ditions. (See Flame retardant.)

50. Antioxidant

An additive within the general class of anti-ageing additives or stabilizers. An antioxi-dant reduces the rate of degradation ofpolymers resulting from the action of oxy-gen in the atmosphere on defective sites of amolecular chain. This creates free radicals,which initiate and propagate a series ofreactions resulting in chain scission,cross-linking and formation of oxygen-con-taining chromophore groups, such as car-bonyls. These reactions cause discolora-tions and embrittlement of the product. Anantioxidant reacts with the free radicals toproduce inert compounds, thereby prevent-ing the propagation of degradation reac-tions. The antioxidant activity is regenerated

16j 50 Antioxidant

through secondary reactions so that theycan act as efficient stabilizers even at verylow concentrations (0.1–0.5%). Typical anti-oxidants used in polymers are ortho- andpara-substituted tertiary butyl phenols oraromatic amines. Amine antioxidants areless widely used, as they impart a darkdiscoloration to the products and they arerather toxic, which makes them less attrac-tive for general uses. Typical examples ofphenolic stabilizers are shown.

Example of phenolic antioxidants.

Note that the second example of phenolicantioxidant contains a phosphite group,which provides a synergistic effect with thephenolic unit by acting as a peroxide de-composer. Themore common types of anti-oxidants do not have a built-in peroxidedecomposer and, therefore, they require thepresence of another additive to exert thisfunction. These are typically phosphite andmercaptan compounds, which acceleratethe decomposition of hydroperoxides anddeactivates them through the formation ofstable products.Antioxidants intervene in the degrada-

tion reactions caused by the formation offree radicals in the polymer chains (P)through a series of reactions with oxygen,starting at sites with the weakest CH bond,such as tertiary or allylic carbons. The�initiation� reaction.

PHþO2 !POOH!PO. þ .OH

is followed by rapid propagation reactions

P. þO2 !POO.

POO. þPH!POOHþP.

PHþH. !H2 þP.

P. þP0H!PHþP0.

PHþ .OOH!P. þH2O2

OH. þPH!P. þH2O

Propagation reactions are the most dam-aging reactions insofar as they affect a largenumber of polymer molecules. The antioxi-dant intervenes predominantly at this stageby reacting with the free radicals in thepolymer chains, forming non-reactive pro-ducts, a phenomenon known as quenching.Denoting by AH the antioxidant molecule,either amine or phenol type, the quenchingreaction can be written as

P. þAH!PHþA�

where A� represents a stable (non-reactive)radical. The mechanism for the loss ofreactivity of the A� radicals for the case ofa phenolic antioxidants is as shown.

Note that the stability of the radicals isdue to internal delocalization, making itinaccessible for interacting with polymerchains.The reaction scheme for the action of

secondary stabilizers as peroxide decompo-sers is:

50 Antioxidant j17

Examples of mercaptan and phosphitecompounds used as secondary stabilizersare dilaurylthiodipropanoate (DLTDP) anddistearylthiodipropanoate (DSTDP).

The structures of two phosphite second-ary stabilizers are shown. These are knowncommercially as Weston 618 (left) andUltranox 626 (right).

51. Antiozonant

An additive used primarily in diene elasto-mers to improve the resistance to attackfromozone in the environment, particularlythe ozone generated in electrical motorsand other devices through corona dis-charges. An antiozonant is often a waxcapable of migrating to the surface to forman oxidation-resistant layer.

52. Antiplasticization

Although the term denotes a phenomenonwith the opposite effect to plasticization,

this must not be interpreted in terms of anincrease in the glass transition temperature.Antiplasticization is a phenomenon thatbrings about an increase in modulus at lowtemperatures, as well as embrittlement at

ambient temperatures. In PVC, this isbrought about by the incorporation of smallamounts of plasticizers (typically 5–10 partsper hundred) capable of exerting strongphysical interaction, such asH�bondswiththe Cl atoms in the chains. These interac-tions cause a depression of short-rangerelaxations, normally associated with rota-tional movements of side groups in a poly-mer chain. The effects of the addition of

18j 52 Antiplasticization

small amounts of plasticizer in PVC com-pounds on the variation of modulus withtemperature, shown in the diagram, pro-vides evidence that antiplasticization isessentially a phenomenon occurring at lowtemperatures and that normal plasticizationtakes places at higher temperatures.

Plot of dynamic shearmodulusagainst temperaturefor a PVC sample (curve on the right, triangles) anda sample containing 9wt% tricresyl phosphate(curve on the left, circles). Source: Mascia (1978).

Illustration of effects of antiplasticization of a PVCcompound. Upper curve: unplasticized PVC. Lowercurve: PVC plasticized with 9wt% tricresyl phos-phate. Source: Mascia et al. (1989).

The embrittlement effect due to antiplas-ticization is illustrated in the second dia-gram in terms of the reduction in �criticalcrazing strain� as a function of the CH3OH/H2O ratio, representing the relative affinityof the environmental agent for the polargroups in the polymer.

53. Antistatic Agent

An additive used in polymer formulationsto reduce the build-up of static charges onthe surface of products or structures. Anti-static agents are additives capable ofmigrat-

ing to the surface of a product, forming aconductive path through ionization result-ing from the absorption of moisture in the

53 Antistatic Agent j19

atmosphere. These are usually surfactantsconsisting of long-chain aliphatic amines,amides or quaternary ammonium salts,alkyl aryl sulfonates and alkyl hydrogenpho-sphates. Among other compounds used asantistatic agents are water-soluble ionicallyconductive polymeric compounds, such ashexadecyl ethers of poly(ethylene glycol).(See Surface resistivity.)

54. Antitracking

Preventing the formation of surface trackson the surface of a dielectric, or insulator,subjected to a high voltage. The surfacetracks consist of carbonaceous paths thatare formed as a result of the chemicaldegradation of the polymer, thereby produc-ing very conductive channels capable ofleaking a current to ground. Tracking ofinsulators is often prevented by depositingsuitable oils or greases on the surface, suchas silicone types, as they are thermally stableand do not form carbonaceous residues.

55. Antitracking Additive

An additive that imparts antitracking char-acteristics to a polymer. These are sub-stances that can release large quantities ofwater when the outer surface layers of theinsulator reach temperatures in the regionof 300–400 �C, as a result of the heat pro-duced by the arcs generated by the appliedhigh voltage. Arcs contain very reactive oxy-gen ions, which give rise to extremely rapiddegradation reactions. The most widelyused antitracking additive is aluminiumtrihydrate, Al2O3�3H2O, which loses allthree H2O molecules at around 350 �C,corresponding to about 35wt% weight loss.(See Flame retardant.)

56. Apparent Shear Rate

The shear rate calculated on the basis thatthe polymer melt behaves as a Newtonianliquid. The value of the apparent shear rate,_ga, at the wall can be calculated from theflow rate Q and the dimensions of thechannels, as follows.

. Circular channel: _ga ¼ 4Q=pR3, whereRis the radius of the channel.

. Rectangular channel: _ga ¼ 6Q=WH2,where W is the width of the channel andH is the depth, valid for shallow channels,that is, when W � H.

A shape factor, S, has to be introduced forother situations. Correction has to be madebymultiplying the value calculated from theabove equation by the appropriate shapefactor, S¼ 1 – 0.65(H/W). (See True shearrate and Non-Newtonian behaviour.)

57. Apparent Viscosity

The value of the viscosity (ha) calculated onthe basis that the polymermelt behaves as aNewtonian liquid, that is ha ¼ t= _ga, wheret is the shear stress and _ga is the value of theapparent shear rate.

58. Aprotic Polar Solvent

A solvent without the capability of undergo-ing hydrogen-bonding interactions throughthe involvement of protons.

59. Araldite

A tradename for a variety of epoxy resins.

20j 60 Araldite

60. Aramid

A term used to denote aromatic polyamidefibres, an example of which is shown.

Example of an aramid fibre displaying fibrillationcharacteristics. Source: Ehrenstein (2001).

The chemical structure of a typical poly-amide used for the production of aramidfibre is shown.

These are often identified by the trade-name Kevlar. The molecular chains of aro-matic polyamides are very rigid and areoriented in the axial direction of the fibres,forming strong attractionswith other neigh-bouringmoleculesviahydrogenbonds.Thisprovidesahighstrengthandmodulus,withahighlevelofductility, relativetocarbonfibres(which are extremely brittle) and even glassfibres. Aramids have a very high meltingpoint (around 500 �C) but are susceptible toUV-induced degradation. (See Composite.)

61. Aromatic Anhydride

Frequently used compounds for the produc-tion of polyesters and polyimides and some-times used as curing agents for epoxyresins.

62. Arrhenius Equation

The name of an equation derived by thechemist Arrhenius to describe the variationof the rate of a chemical reaction (K ) as a

function of the absolute temperature (T ).This is written as

K ¼ A expð�DE=RTÞ;where A is a characteristic constant, DE isthe activation energy for the reaction and Ris the universal gas constant. This equationhas been found to apply equally well tophysical processes that involve movementsof some structural constituents, for instancein the case of electronic or ionic conductionand gas diffusion.

63. Asbestos

A magnesium silicate fibre occurring natu-rally in four different forms. The �chrysotile�variety has been used for reinforcement ofthermosetting resins, particularly phenolictypes, in engineering applications such asbrake pads. It consists of fine fibrils packedinto bundles for its high reinforcing effi-ciency due to the high modulus (see themicrograph).

SEM micrograph of chrysotile asbestos fibres.Source: Wypych (1993).

Owing to toxicity issues, asbestos hasbeen largely replaced by other high-perfor-mance reinforcing fibres. (See Composite.)

64. Aspect Ratio

The ratio of the length to diameter of fibresused in composites. Sometimes used also

64 Aspect Ratio j21

for platelet particles as the ratio of thenominal width to the thickness.

65. Atactic Polymer

A term used to denote the lack of a specificorder in the chemical structure of a polymer,such as polypropylene. A polymer with anatactic chemical structure. (See Isotacticpolymer and Tacticity.)

66. Atomic Force Microscopy (AFM)

A microscopic technique that producesimages of the surface topology via the tip ofa scanning probe that measures the atomicforces (attractive or repulsive) against thesurface under examination. Atomic forcemicroscopy (AFM) is particularly useful forexamining surface features of very smalldimensions, for example 10–1000nm. Thetip is placed on a cantilever that will deflect asa result of the atomic surface forces, asshown in the diagram.

The movements are detected by the re-flection of a suitable focused laser beam. Intapping-mode AFM the cantilever is oscil-lated at a certain frequency with an ampli-tude that allows the tip to come into close

proximitywith the surfacewithout touchingit. In most cases a feedback mechanism isemployed to adjust the tip-to-sample dis-tance to maintain a constant force betweenthe tip and the surface of the sample.

67. Attenuated Total ReflectanceSpectroscopy (ATR Spectroscopy)

An infrared spectroscopy technique used toidentify specific chemical groups present onthe surface of a sample, supported on aprism with a very high refractive index,which reflects the incident infrared radia-tion transmitted through the thin film incontact with the prism. (See Infraredspectroscopy.)

68. Attenuation

(See Damping.)

69. Avrami Equation

Originally developed to model the crystalli-zation rate ofmetals during solidification, ithas been found to apply equally well to the

Schematic diagram of the operating principle of an atomic force microscope. Source: Lavorgna (2009).

22j 69 Avrami Equation

crystallization of polymers. The Avramiequation is usually written as

lnð1�wtÞ ¼ Ztn;

where wt is the volume fraction of polymercrystals (also known as the degree of crys-tallinity) at time t, while Z and n are char-acteristic constants for the polymer.

70. a/W Ratio

The ratio of the length of the crack (ornotch), a, to the width of the specimen,

W, used in the evaluation of the toughnessof materials using fracture mechanics prin-ciples. (See Fracture mechanics.)

71. Azeotropic Copolymerization

Conditions in which the composition of themolecular chains of a copolymer (i.e. theratio and position of the two monomerunits) remains the same throughout thepolymerization process.

71 Azeotropic Copolymerization j23