Structure And Use Of Polymers Engineering Essay

“ The word PLASTICS normally implies stuffs that have low strengths and stiffness ( comparative to metals ) , temperature restrictions as of their thermoplastic nature and that deform continuously under applied force.

The above characteristics are decidedly drawbacks when compared with more traditional stuffs like metal, wood or ceramic. Some advantages of PLASTICS are ;

Easy defining or modeling into complex forms with minimal fiction and coating

Low densenesss, i.e. low-weight merchandises

Thermal and electrical dielectrics

Other particular belongingss, e.g. frequently flexible, sometimes crystalline

New types of polymers and fiber-reinforced complexs exhibit high public presentation and long service life. Their most extended usage is in aircraft/aerospace applications non merely for military aircraft ( e.g. Stealth fighter- bomber is 65 % composed of polymer complexs ) , but besides in commercial air power. For illustration, in Boeing 767 about 3 % of the structural weight is due to polymer complexs, while in the forthcoming Boeing 777 about 10 % will be polymer complexs. ” [ 24 ]

2.1 – Polymer belongingss

Polymer belongingss are loosely divided into several categories based on the graduated table at which the belongings is defined every bit good as upon its physical footing. The most basic belongings of a polymer is the individuality of its component monomers. A 2nd set of belongingss, known as microstructure, basically describe the agreement of these monomers within the polymer at the graduated table of a individual concatenation. These basic structural belongingss play a major function in finding bulk physical belongingss of the polymer, which describe how the polymer behaves as a uninterrupted macroscopic stuff. Chemical belongingss, at the nano-scale, depict how the ironss interact through assorted physical forces. At the macro-scale, they describe how the majority polymer interacts with other chemicals and dissolvers.

2.1.1 – Monomers and reiterate units

The individuality of the monomer residues ( reiterate units ) consisting a polymer is its first and most of import property. Polymer terminology is by and large based upon the type of monomer residues consisting the polymer. Polymers that contain merely a individual type of repetition unit are known as homopolymers, while polymers incorporating a mixture of repetition units are known as copolymers. Poly ( cinnamene ) , for illustration, is composed merely of styrene monomer residues, and is hence classified as a homopolymer. Ethylene-vinyl ethanoate, on the other manus, contains more than one assortment of repetition unit and is therefore a copolymer. Some biological polymers are composed of a assortment of different but structurally related monomer residues ; for illustration, polynucleotides such as Deoxyribonucleic acid are composed of a assortment of nucleotide fractional monetary units.

A polymer molecule incorporating ionizable fractional monetary units is known as a polyelectrolyte or ionomer.

2.1.2 – Microstructure

The microstructure of a polymer ( sometimes called constellation ) relates to the physical agreement of monomer residues along the anchor of the concatenation. These are the elements of polymer construction that require the breakage of a covalent bond in order to alter. Structure has a strong influence on the other belongingss of a polymer. For illustration, two samples of natural gum elastic may exhibit different lastingness, even though their molecules comprise the same monomers.

2.1.2.1 – Polymer architecture

An of import microstructural characteristic finding polymer belongingss is the polymer architecture. The simplest polymer architecture is a additive concatenation: a individual anchor with no subdivisions. A related unbranched architecture is a pealing polymer. A bifurcate polymer molecule is composed of a chief concatenation with one or more substituent side ironss or subdivisions. Particular types of bifurcate polymers include star polymers, comb polymers, coppice polymers, dendronized polymers, ladders, and dendrimers.

Branching of polymer ironss affects the ability of ironss to skid past one another by changing intermolecular forces, in bend impacting bulk physical polymer belongingss.

Long concatenation subdivisions may increase polymer strength, stamina, and the glass passage temperature ( Tg ) due to an addition in the figure of webs per concatenation. The consequence of such long-chain subdivisions on the size of the polymer in solution is characterized by the ramification index. Random length and ataxic short ironss, on the other manus, may cut down polymer strength due to break of organisation and may likewise cut down the crystallinity of the polymer.

A good illustration of this consequence is related to the scope of physical properties of polythene. High-density polythene ( HDPE ) has a really low grade of ramification, is rather stiff, and is used in applications such as milk jugs. Low-density polythene ( LDPE ) , on the other manus, has important Numberss of both long and short subdivisions, is rather flexible, and is used in applications such as fictile movies.

Dendrimers are a particular instance of polymer where every monomer unit is branched. This tends to cut down intermolecular concatenation web and crystallisation. Alternatively, dendritic polymers are non absolutely branched but portion similar belongingss to dendrimers due to their high grade of ramification.

The architecture of the polymer is frequently physically determined by the functionality of the monomers from which it is formed. This belongings of a monomer is defined as the figure of reaction sites at which may organize chemical covalent bonds. The basic functionality required for organizing even a additive concatenation is two adhering sites. Higher functionality outputs branched or even crosslinked or networked polymer ironss.

An consequence related to ramification is chemical crosslinking – the formation of covalent bonds between ironss. Crosslinking tends to increase Tg and increase strength and stamina. Among other applications, this procedure is used to beef up gum elastics in a procedure known as vulcanisation, which is based on crosslinking by S. Car tyres, for illustration, are extremely crosslinked in order to cut down the leaking of air out of the tyre and to toughen their lastingness. Eraser gum elastic, on the other manus, is non crosslinked to let flaking of the gum elastic and prevent harm to the paper.

A cross-link suggests a subdivision point from which four or more distinguishable ironss emanate. A polymer molecule with a high grade of crosslinking is referred to as a polymer web. Sufficiently high crosslink concentrations may take to the formation of an infinite web, besides known as a gel, in which webs of ironss are of limitless extent-essentially all ironss have linked into one molecule.

2.1.2.2 – Chain length

The physical belongingss of a polymer are strongly dependent on the size or length of the polymer chain.. For illustration, as concatenation length is increased, runing and boiling temperatures increase rapidly. Impact opposition besides tends to increase with concatenation length, as does the viscousness, or opposition to flux, of the polymer in its thaw province. Chain length is related to run viscousness approximately as 1:103.2, so that a ten-fold addition in polymer concatenation length consequences in a viscousness addition of over 1000 times. Increasing concatenation length moreover tends to diminish concatenation mobility, addition strength and stamina, and increase the glass passage temperature ( Tg ) . This is a consequence of the addition in concatenation interactions such as Van der Waals attractive forces and webs that come with increased concatenation length. These interactions tend to repair the person chains more strongly in place and resist distortions and matrix dissolution, both at higher emphasiss and higher temperatures.

A common agency of showing the length of a concatenation is the grade of polymerisation, which quantifies the figure of monomers incorporated into the concatenation. As with other molecules, a polymer ‘s size may besides be expressed in footings of molecular weight. Since man-made polymerisation techniques typically yield a polymer merchandise including a scope of molecular weights, the weight is frequently expressed statistically to depict the distribution of concatenation lengths nowadays in the same. Common illustrations are the figure mean molecular weight and weight norm molecular weight. The ratio of these two values is the polydispersity index, normally used to show the “ width ” of the molecular weight distribution. A concluding measuring is contour length, which can be understood as the length of the concatenation anchor in its to the full extended province.

The flexibleness of an unbranching concatenation polymer is characterized by its continuity length.

2.1.2.3 – Monomer agreement in copolymers

Monomers within a copolymer may be organized along the anchor in a assortment of ways.

Alternating copolymers possess on a regular basis jumping monomer residues: [ AB… ] N.

Periodic copolymers have monomer residue types arranged in a repeating sequence: [ AnBm… ] thousand being different from N.

Statistical copolymers have monomer residues arranged harmonizing to a known statistical regulation. A statistical copolymer in which the chance of happening a peculiar type of monomer residue at an peculiar point in the concatenation is independent of the types of environing monomer residue may be referred to as a truly random copolymer.

Block copolymers have two or more homopolymer fractional monetary units linked by covalent bonds. Polymers with two or three blocks of two distinguishable chemical species ( e.g. , A and B ) are called diblock copolymers and triblock copolymers, severally. Polymers with three blocks, each of a different chemical species ( e.g. , A, B, and C ) are termed triblock terpolymers.

Graft or grafted copolymers contain side ironss that have a different composing or constellation than the chief concatenation.

2.1.2.4 – Tacticity

Tacticity describes the comparative stereochemistry of chiral centres in adjacent structural units within a supermolecule. There are three types: isotactic ( all substituents on the same side ) , ataxic ( random arrangement of substituents ) , and syndiotactic ( jumping arrangement of substituents ) .

2.1.3 – Polymer morphology

Polymer morphology by and large describes the agreement of ironss in infinite and microscopic ordination of many polymer ironss.

2.1.3.1 – Crystallinity

When applied to polymers, the term crystalline has a slightly equivocal use. In some instances, the term crystalline finds indistinguishable use to that used in conventional crystallography. For illustration, the construction of a crystalline protein or polynucleotide, such as a sample prepared for x-ray crystallography, may be defined in footings of a conventional unit cell composed of one or more polymer molecules with cell dimensions of 100s of As or more.

A man-made polymer may be lightly described as crystalline if it contains parts of 3-dimensional telling on atomic ( instead than macromolecular ) length graduated tables, normally originating from intramolecular turn uping and/or stacking of next ironss. Man-made polymers may dwell of both crystalline and formless parts ; the grade of crystallinity may be expressed in footings of a weight fraction or volume fraction of crystalline stuff. Few man-made polymers are wholly crystalline.

The crystallinity of polymers is characterized by their grade of crystallinity, runing from nothing for a wholly non-crystalline polymer to one for a theoretical wholly crystalline polymer. Polymers with microcrystalline parts are by and large tougher ( can be bent more without interrupting ) and more impact-resistant than wholly formless polymers.

Polymers with a grade of crystallinity nearing zero or one will be given to be crystalline, while polymers with intermediate grades of crystallinity will be given to be opaque due to light dispersing by crystalline or glassy parts. Thus for many polymers, reduced crystallinity may besides be associated with increased transparence.

2.1.3.2 – Chain conformation

The infinite occupied by a polymer molecule is by and large expressed in footings of radius of rotation, which is an mean distance from the centre of mass of the concatenation to the concatenation itself. Alternatively, it may be expressed in footings of pervaded volume, which is the volume of solution spanned by the polymer concatenation and graduated tables with the regular hexahedron of the radius of rotation.

2.1.4 – mechanical belongingss

The majority belongingss of a polymer are those most frequently of end-use involvement. These are the belongingss that dictate how the polymer really behaves on a macroscopic graduated table.

Tensile strength

The tensile strength of a stuff quantifies how much emphasis the stuff will digest before enduring lasting distortion. This is really of import in applications that rely upon a polymer ‘s physical strength or lastingness. For illustration, a gum elastic set with a higher tensile strength will keep a greater weight before snarling. In general, tensile strength additions with polymer concatenation length and crosslinking of polymer ironss.

Young ‘s modulus of snap

Young ‘s Modulus quantifies the snap of the polymer. It is defined, for little strains, as the ratio of rate of alteration of emphasis to strive. Like tensile strength, this is extremely relevant in polymer applications affecting the physical belongingss of polymers, such as gum elastic sets. The modulus is strongly dependent on temperature.

Conveyance belongingss

Conveyance belongingss such as diffusivity relate to how quickly molecules move through the polymer matrix. These are really of import in many applications of polymers for movies and membranes.

2.1.5 – Phase behaviour

Melting point

The term runing point, when applied to polymers, suggests non a solid-liquid stage passage but a passage from a crystalline or semi-crystalline stage to a solid formless stage. Though abbreviated as merely Tm, the belongings in inquiry is more decently called the crystalline thaw temperature. Among man-made polymers, crystalline thaw is merely discussed with respects to thermoplastics, as thermosetting polymers will break up at high temperatures instead than thaws.

Boiling point

The boiling point of a polymeric stuff is strongly dependent on concatenation length. High polymers with a big grade of polymerisation do non exhibit a boiling point because they decompose before making theoretical boiling temperatures. For shorter oligomers, a boiling passage may be observed and will by and large increase quickly as concatenation length is increased.

Glass passage temperature

A parametric quantity of peculiar involvement in man-made polymer fabrication is the glass

passage temperature ( Tg ) , which describes the temperature at which formless polymers undergo a second-order stage passage from a rubbery, syrupy formless solid, or from a crystalline solid ( depending on the grade of crystallisation ) to a toffee, glassy formless solid. The glass passage temperature may be engineered by changing the grade of ramification or crosslinking in the polymer or by the add-on of plasticiser.

Blending behaviour

In general, polymeric mixtures are far less mixable than mixtures of little molecule stuffs. This consequence consequences from the fact that the drive force for commixture is normally entropy, non interaction energy. In other words, mixable stuffs normally form a solution non because their interaction with each other is more favourable than their self-interaction, but because of an addition in information and therefore free energy associated with increasing the sum of volume available to each constituent. This addition in entropy graduated tables with the figure of atoms ( or moles ) being mixed. Since polymeric molecules are much larger and therefore by and large have much higher specific volumes than little molecules, the figure of molecules involved in a polymeric mixture is far smaller than the figure in a little molecule mixture of equal volume. The energetics of commixture, on the other manus, is comparable on a per volume footing for polymeric and little molecule mixtures. This tends to increase the free energy of blending for polymer solutions and therefore do solvation less favourable. Therefore, concentrated solutions of polymers are far rarer than those of little molecules.

Furthermore, the stage behaviour of polymer solutions and mixtures is more complex than that of little molecule mixtures. Whereas most little molecule solutions exhibit merely an upper critical solution temperature stage passage, at which stage separation occurs with chilling, polymer mixtures normally exhibit a lower critical solution temperature stage passage, at which stage separation occurs with warming.

In dilute solution, the belongingss of the polymer are characterized by the interaction between the dissolver and the polymer. In a good dissolver, the polymer appears conceited and occupies a big volume. In this scenario, intermolecular forces between the dissolver and monomer fractional monetary units dominate over intramolecular interactions. In a bad dissolver or hapless dissolver, intramolecular forces dominate and the concatenation contracts. In the theta dissolver ( ) , or the province of the polymer solution where the value of the 2nd virial coefficient becomes 0, the intermolecular polymer-solvent repulsive force balances precisely the intramolecular monomer-monomer attractive force. Under the theta status ( besides called the Flory status ) , the polymer behaves like an ideal random spiral.

Inclusion of plasticisers

Inclusion of plasticisers tends to take down Tg and increase polymer flexibleness. Plasticizers are by and large little molecules that are chemically similar to the polymer and make spreads between polymer ironss for greater mobility and decreased interchain interactions. A good illustration of the action of plasticisers is related to polyvinylchlorides or PVCs. A uPVC, or unplasticized polyvinylchloride, is used for things such as pipes. A pipe has no plasticisers in it, because it needs to stay strong and heat-resistant. Plasticized PVC is used for vesture for a flexible quality. Plasticizers are besides put in some types of cleaving movie to do the polymer more flexible.

2.1.6 – Chemical belongingss

The attractive forces between polymer ironss play a big portion in finding a polymer ‘s belongingss. Because polymer ironss are so long, these interchained forces are amplified far beyond the attractive forces between conventional molecules. Different side groups on the polymer can impart the polymer to ionic bonding or H bonding between its ain ironss. These stronger forces typically result in higher tensile strength and higher crystalline runing points.

The intermolecular forces in polymers can be affected by dipoles in the monomer units. Polymers incorporating amide or carbonyl groups can organize H bonds between next ironss ; the partly positively charged H atoms in N-H groups of one concatenation are strongly attracted to the partly negatively charged O atoms in C=O groups on another. These strong H bonds, for illustration, consequence in the high tensile strength and runing point of polymers incorporating urethane or urea linkages. Polyesters have dipole-dipole bonding between the O atoms in C=O groups and the H atoms in H-C groups. Dipole bonding is non every bit strong as H bonding, so a polyester ‘s thaw point and strength are lower than Kevlar ‘s, but polyesters have greater flexibleness.

Ethene, nevertheless, has no lasting dipole. The attractive forces between polythene ironss arise from weak new wave der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative negatrons. As two polymer ironss approach, their negatron clouds repel one another. This has the consequence of take downing the negatron denseness on one side of a polymer concatenation, making a little positive dipole on this side. This charge is adequate to pull the 2nd polymer concatenation. Van der Waals forces are rather weak, nevertheless, so polyethylene can hold a lower thaw temperature compared to other polymers.

2.2 – Polypropylene

Polypropylene ( PP ) was foremost produced by G. Natta, following the work of K. Ziegler,

by the polymerization of propylene monomer in 1954 ( Figure 2.1 ) . The supermolecule of PP contains 10,000 to 20,000 monomer units. The steric agreement of the methyl groups attached to every 2nd C atom in the concatenation may change ( see Figure 1.2 ) . If all the methyl groups are on the same side of the tortuous spiral concatenation molecule, the merchandise is referred to as isotactic PP. A PP construction where pendent methylene groups are attached to the polymer anchor concatenation in an alternating mode is known as syndiotactic PP. The construction where pendent groups are located in a random mode on the polymer anchor is the ataxic signifier.

Figure 2.1 Propylene Monomer

Figure 2.2 PP polymer molecule in isotactic, syndiotactic and ataxic signifiers.

Merely isotactic PP has the needed belongingss required for a utile plastic stuff. Stereospecific or Ziegler-Natta accelerators are used to polymerize PP in this signifier. The pendent methylene group in PP is replaced by a Cl atom in polyvinyl chloride ( PVC ) , by a benzine ring in polystyrene ( PS ) and by a H atom in polythene ( PE ) . The pendent group significantly affects the belongingss of the polymer, and accordingly the belongingss of PP are really different from other trade good plastics such as PE, PVC and PS.

In 1957, PP was commercially produced by Montecatini as Moplen. Recently, metallocenes have attracted widespread attending as the new coevals of olefin polymerization accelerators. Metallocene accelerators provide enhanced control over the molecular make up of PP, and classs with highly high isotacticity and narrow molecular weight distribution ( MWD ) are possible.

Major Advantages

PP is really popular as a high-volume trade good plastic. However, it is referred to as a low-priced technology plastic. Higher stiffness at lower denseness and opposition to higher temperatures when non subjected to mechanical emphasis ( peculiarly in comparing to high and low denseness PE ( HDPE and LDPE ) ) are the cardinal belongingss. In add-on to this, PP offers good weariness opposition, good chemical opposition, good environmental emphasis checking opposition, good detergent opposition, good hardness and contact transparence and easiness of machining, together with good processibility by injection molding and bulge.

Table 2.1Comparison of unmodified PP with other stuffs: Advantages

Property

PP

LDPE

HDPE

Hip

Polyvinyl chloride

Flexural modulus ( GPa )

1.5

0.3

1.3

2.1

3

Tensile strength ( MPa )

33

10

32

42

51

Specific denseness

0.91

0.92

0.96

1.08

1.4

Specific modulus ( GPa )

1.66

0.33

1.35

1.94

2.14

HDT at 0.45 MPa. ( A°C )

105

50

75

85

70

Maximum uninterrupted usage temperature ( A°C )

100

50

55

50

50

Surface hardness

RR90

SD48

SD68

RM30

RR110

Cost ( ?/tonne )

660

730

660

875

905

Modulus per unit cost

( MPa/? )

2.27

0.41

1.97

2.4

3.31

ABS = acrylonitrile butadiene cinnamene RM = Rockwell R

HIPS = high impact polystyrene SD = Shore Durometer

The belongingss of unmodified PP are compared with other competitory thermoplastics in Table 2.1. It can be seen from the tabular array that PP offers advantages over most of its

competitory stuffs on the footing of specific modulus ( modulus to density ratio ) , heat warp temperature ( HDT ) , maximal uninterrupted usage temperature or modulus to cost.

Major Disadvantages

The major disadvantages of unmodified PP compared with other competitory

thermoplastics are apparent from Table 2.2. It can be seen that PP has significantly higher mould shrinking, higher thermic enlargement and lower impact strength, peculiarly at sub-ambient temperatures, than HIPS, PVC and ABS. However, PP has lower mould shrinking and thermic enlargement coefficient than HDPE and LDPE. Poor UV opposition and hapless oxidative opposition in the presence of certain metals such as Cu are other disadvantages of PP. As any semi-crystalline stuff, PP besides suffers from high weirdo under sustained burden in comparing to an formless plastic such as ABS or PVC. Other disadvantages of PP are hard solvent and adhesive bonding, hapless flammability, warpage, limited transparence, hapless wear belongingss, unsuitableness for frictional applications and hapless opposition to gamma radiation. However, most of these disadvantages could be overcome, either wholly or to a certain grade, by proper choice of stuff, reasonable design and good processing. The processing of PP by thermoforming and blow molding is hard. Vacuum forming of PP is besides hard.

Table 2.2Comparison of unmodified PP with other stuffs: Disadvantages

Property

PP

LDPE

HDPE

Hip

Polyvinyl chloride

Mould shrinking ( % )

1.9

3

3

0.5

0.4

Thermal enlargement ( x10-5 )

10

20

12

7

6

Notched Izod impact

strength ( kJ/m ) at 23 A°C

0.07

& gt ; 1.06

0.15

0.1

0.08

PP is non risky to wellness, nevertheless, it can let go of volatile organic compounds ( VOCs ) into the environing air during high-temperature processing. Workers at the processing works can be subjected to these VOCs through inspiration or tegument contact. Good airing utilizing exhaust fans can understate the exposure. Residual monomer and accelerators present in the rosin can increase the toxicity.

2.3 – Rheology

2.3.1 – Melt Rheology

Thermoplastics, by the very definition of the word, denotes polymeric stuffs which can be made to soften and take on new forms by the application of heat and force per unit area. In their original natural stuff signifier, thermoplastics are available in solid province as french friess, granules, or pulverization. These are melted and reshaped to organize assorted plastic merchandises.

Melt rheology is concerned with the description of the distortion of the stuff under the influence of emphasiss. Distortion and flux of course exist when the thermoplastics are melted and so reformed into solid merchandises of assorted forms. All polymer thaws are viscoelastic stuffs ; that is, their response to external burden lies in changing extent between that of a syrupy liquid and an elastic solid. In an ideal syrupy liquid, the energy of distortion is dissipated in the signifier of heat and can non be recovered merely by let go ofing the external forces ; whereas, in an ideal elastic solid, the distortion is to the full recovered when the emphasiss are released.

A polymer thaw represents a bunch of embroiled, flexible strings of changing lengths. Molecular weight or the grade of polymerisation signifies the length of the twine, whereas the molecular-weight distribution signifies the extent of length fluctuation in the bunch. If the ethene molecule were magnified 100 million times, so its length would be about 1 centimeter. On the same graduated table, the polymer molecule of LDPE would be 15 m long, whereas that of ultrahigh- molecular-weight polythene ( UHMWPE ) would be about 3-8 kilometer in length. It is obvious that these ironss can non be found in drawn-out signifier, but they exist alternatively in an embroiled and distorted province. It is these webs that provide the opposition to distortion and, hence, with increasing molecular weight, the thaw viscousness goes up, processibility worsens although, of class, mechanical belongingss improve. The sensitiveness of rheological trials is chiefly due to concatenation webs ensuing in big differences in flow behaviour even for little differences in concatenation length or ramification. Change in distortion rate shows alterations in flow behaviour. Rheologic measurings are frequently used as an effectual tool for ;

Quality control of natural stuffs, fabricating process/final merchandise

Predicting stuff public presentation

The sensitiveness of the rheological belongingss to structural differences in stuffs forms a ready to hand assistance to the quality control applied scientist when make up one’s minding whether to accept or reject an incoming stuff.

A typical viscousness versus shear rate curve would be like the one shown in Fig. 2.3. There is a Newtonian part in the low shear where the viscousness does non alter with shear rate. At some critical shear rate, there is a uninterrupted bead off of viscousness with shear rate. The slump of viscousness with shear rate would happen earlier if the molecular-weight distribution is widened. This is because the shorter molecular ironss are of lower viscousness and do the viscousness from the Newtonian part to diminish Oklahoman. However, at higher shear rates, the viscousness of the polymer holding a broad molecular-weight distribution would be higher. The molecular ironss that are really long be given to move doggedly and elastically at higher shear rates, giving higher viscousness. Therefore, the incline of the viscousness versus shear rate curve can be used as a tool to depict the molecular-weight distribution. The tallness of the low shear ( Newtonian ) viscousness can associate to the mean molecular weight. However, these relationships are well affected by influences such as ramification, cross-linking, and molecular web.

The followers is slightly of a behavioural presentation of how branched, cross-linking, or web will impact the polymer viscousness versus shear rate map presuming the molecular weight and molecular-weight distribution remain changeless. Higher degrees of ramification, cross-linking, or web can give the visual aspect of holding a higher mean molecular weight at low-shear- rate proving in comparing to a mention polymer holding low web and a little sum of ramifying and/or cross-linking. As the shear rate is increased, the viscousness drops off Oklahoman. At higher shear rates, the viscousness will be much higher than that of the mention stuff holding small ramification, web, or cross-linking.

Due to the dependance of rheology on the construction and the basic built-in chemical science of the polymers, rheological informations can be used efficaciously to command material parametric quantities like molecular weight, molecular-weight distribution, ramification, cross-linking, and so away so that the right pick of the polymer to be processed can be made under a given set of processing conditions.

Figure 2.3 A typical viscousness versus shear rate curve for a thermoplastic thaw, demoing the consequence of physical structural alterations during flow.

2.3.2 – Processing

Polymer processing is defined as the technology forte concerned with operations carried out on polymeric stuffs or systems to increase their public-service corporation [ 1 ] . The topic of polymer processing has been traditionally and continuously discussed [ 2 ] in footings of the prevailing processing methods which are viewed as unit-shaping operations to change over the polymeric stuffs into utile merchandises. Some of these treating methods such as injection casting, compaction casting, calendering, bulge, thermoforming, blow casting, transportation casting, and so on are briefly described below.

Injection Modeling

Injection casting is a cyclic procedure for bring forthing distinct indistinguishable articles from a hollow cast which is suitably shaped to give the concluding merchandise profile. Injection modeling equipment consists of two parts: ( 1 ) the injection unit and ( 2 ) the clamping unit. The map of the injection unit is to run, conveyance, and shoot the polymer into the cast, whereas the clamping unit is involved with the map of automatically opening and shuting the cast and chuck outing the finished merchandise.

The procedure begins by dropping the thermoplastic natural stuff in the signifier of pulverization or pellet from a hopper into the het chamber of the injection-molding unit where it is melted or softened. On making molding temperature, the extremely syrupy polymer thaw is pushed with a hydraulic random-access memory through a little nose of the injection unit into a cooled cast. The cast is made up of at least two pieces held together by a mechanical toggle clinch. Once the cast is filled with the thaw and hardening Begins, so an extra sum of thaw is packed into the cast to countervail the material shrinking that occurs during hardening. When the thaw cools in the cast to a temperature that is suited for taking the merchandise without deformation, the clinch keeping the two cast halves is released and the finished article is ejected.

In some injection-molding units, the hydraulic random-access memory is replaced by a rotating prison guard, which transports the polymeric stuff into a reservoir instantly behind the nose. During the injection procedure, the prison guard stops revolving and Acts of the Apostless as a random-access memory to force the thaw into the cast.

Compaction Modeling

Compaction modeling consists of forcibly squashing a preweighed mixture of modeling pulverization or a prepelletized charge into the coveted form of a het cast pit, non by shooting it into a closed cast, but by shuting one-half of the cast on the other. Heat is conducted from the hot cast walls to the polymer. The platens of the cast have proviso for warming and chilling so that portion remotion from the cast can be accomplished. About all thermoplastics can be compaction molded and really often mechanical belongings ratings are carried out in the research lab utilizing compression-molded specimens. The thermoplastic which is most frequently compaction molded is UHMWPE. Many forte polymers are besides at times compaction molded because their casting temperatures are excessively high to allow processing in conventional injection-molding machines.

Compaction casting has certain advantages over injection casting. The casts are simpler than those used in injection casting, fillers remain comparatively undamaged, and there is minimal material wastage. The disadvantage is that the procedure is slower and there are geometrical restrictions with regard to the complexness of the parts that can be molded.

Calendering

Calendering is a high-production-rate procedure for fabricating fictile sheets and movies. It is peculiarly used in treating stiff every bit good as plasticized PVC, as it prevents polymer debasement during the fabrication procedure.

The calender normally consists of four axial rotations which are configured in different ways. The most common constellation used is the upside-down L form. The polymeric stuff is fed between the shot of the first two axial rotations and so transferred to the following two shots formed by axial rotation Numberss 2, 3, and 3, 4, severally. The first shot helps to command the provender rate while the 2nd and 3rd shot is utilised to put the sheet or movie thickness. The temperature, velocity, and surface coating of the axial rotations are the variables for carry throughing good merchandise quality.

Bulge

Bulge is a procedure for doing uninterrupted fictile objects such as tubings, pipes, rods, overseas telegrams, wires, and a assortment of profiles which include fibrils, movies, and sheets. The procedure is best described by the really word “ bulge, ” which is derived from the Latin words ex intending “ out ” and trudere intending “ to force. ” The polymer pulverization or pellet is melted or softened in a het barrel and conveyed forward, plasticated, homogenized, and pressurized by a revolving prison guard under high shear into a metal dice holding a form that is similar to the form of the coveted article. The chief operating variables are the frequence of screw rotary motion and the barrel temperature profile which is controlled by thermocouples placed inside the metal barrel wall. Sections of the barrel are at times cooled to take inordinate heat generated by syrupy dissipation. The dice coninuously shapes the thaw into the coveted signifier and the merchandise is formed which is infinite in one way. The molten profile produced is cooled either by air or H2O quench or by running it over iciness axial rotations.

Thermoforming

Thermoforming is the procedure by which level polymer sheets or movies are shaped into reasonably deep-drawn container signifiers. The procedure involves warming of the level stock, which is firmly clamped along the margin, to a temperature a small above the thaw or glass-transition temperature and so using vacuity or force per unit area to coerce the deformable sheet to conform to the mold form. The polymer used for thermoforming should non be prone to crawl, as the unsupported sheet must non droop during the warming procedure. It is for this ground that the most normally thermoformed polymers are ABS and HIPS.

Blow Modeling

Blow casting is the procedure by which hollow articles such as bottles are manufactured. A liquefied parison is foremost prepared by the procedure of bulge and so this preshaped arm is trapped between two mold halves and air is blown into it to enable the parison to take the form of the cast. When the polymer comes in contact with the cold cast, it solidifies, thereby enabling the finished article to be ejected.

Transportation Modeling

Transportation casting is a kind of intercrossed procedure between compaction and injection casting. The polymeric mass is foremost melted in a separate reservoir which fundamentally is a portion of the het cast. The cast is so closed by a random-access memory which causes the liquefied polymer to reassign from the reservoir into the mold pit through smugglers and Gatess. The form of the article to be made is determined by the profile of the mold pit. Transfer-molding rhythms are shorter than those of compaction casting and the easy flow of premelted charge allows larger, more intricate parts to be manufactured without much warpage. Therefore, one of the major restrictions of compaction casting are overcome by utilizing transportation casting.

Projecting

Certain low-viscosity systems such as a polymer dispersed in a sufficient sum of plasticisers can be easy cast into a cast holding the shape/profile of the concluding finished merchandise. In fact, this is a common method by which extremely plasticized PVC is processed to organize flexible merchandises.

Slush Modeling

Slush casting is a simple fluctuation of the casting procedure. The low-viscosity stuff, such as, plastisol is poured into a hot cast and after a thick casting is formed on the wall of the cast pit, the extra stuff is poured out. In order to guarantee unvarying coating, the cast is frequently rotated.

Rotational Molding

Measured measure of polymer pulverization is poured into the cast, which is so rotated about two axes in a het oven to organize the finished merchandise upon chilling.

Blending, Compounding, and Blending

Blending, combination, and blending are procedures which do non straight ensue in any concluding plastic merchandise, but, however, they could be included under the class of polymer processing operations. The three footings are frequently synonymously or interchangeably used, and although assorted research workers have defined these footings, one is at times faced with the quandary of nomenclature [ 3 ] . In the present instance, definitions of the footings are given as applicable to the capable affair and therefore excludes any other intensions of the footings.

Blending, in this context, is defined as a procedure in which two or more constituents or ingredients are physically intermingled without breeding any important alteration in the physical province of the constituents [ 4 ] . The constituents in this present instance are polymers to organize polymer blends.

On the other manus, combination is the term used for those instances wherein polymers are softened, melted, and intermingled with solid fillers and other liquid additives to organize filled polymer systems.

The word commixture is applied to both the procedures of blending and combination and describes the procedure of confidant intermingling of two polymers or polymers with fillers/additives without any specific limitations. The of import facet in commixture is to measure the quality of mixtures [ 5 ] or the goodness of blending [ 6 ] . The two belongingss which are utile in declaring the goodness of blending are the graduated table and strength of segregation.

Scale of segregation is a step of the mean separation between governments comprised of the same constituent and may be correlated to the mean striation thickness, which is the mean distance between similar interfaces in a mixture [ 7 ] . On the other manus, strength of segregation is a step of concentration at any point from the average concentration [ 7 ] .

The procedure of blending involves the interrupting down of the single constituents into smaller elements and so scattering of the elements of one constituent in the infinite occupied by the other [ 8 ] . In the instance of filled systems, the procedure includes the interrupting down of agglomerates, their separation, and segregation, until a concluding random distribution is achieved of each constituent in the system. The intent of commixture is to achieve an acceptable grade of homogeneousness or uniformity of composing, assessed through the appropriate graduated table of examination, which is defined [ 7,9,10 ] as the minimal size of the segregation part that would do the mixture to be imperfect for the intended intent. Therefore, it is sensible to specify the perfect alloy as the province in which no fluctuations in composing or morphology are observed at the relevant intimacy of scrutiny.

2.3.2 – Rheology and Processing Link

Polymer processing is non an art as looked upon by the enterpriser, but it is a scientific discipline associating to the flow and distortion of the polymeric stuffs, viz. , rheology. It is through the assorted treating operations described before that a myriad of merchandises are developed for assorted applications. However, non all the polymeric stuff that is processed gets formed into the concluding merchandise for commercial usage. The ground for lower stuff output is the inability to keep merchandise quality, frequently due to the ignorance of the polymer processor with respect to the procedure and the natural stuff features. Some of the common processing jobs along with the likely causes are given in Table 2.4. Such are the jobs which adversely affect productiveness and merchandise quality ensuing in economic loss. For maximising net income, it is of import to better merchandise quality, minimise material waste, cut down production downtime, and optimise the procedure. All these are within the control of the polymer processor who has a good apprehension of the polymer thaw rheology and its relationship to the polymer procedure.

Table 2.4 Processing Problems and Causes

Processing jobs

Probable causes

Weld lines in PVC pipes

Degradation and improper spider leg design

Stain in PP merchandises

Thermal debasement

White flow lines in clear acrylic molded parts

Low or inhomogeneous temperature profile

Pressure buildup taking to decease explosion

High viscousness and improper dice design

Thin wall sections/holes in blow-molded PE bottles

Choice of incorrect class of polymer

Table 2.5 Possible Remedies to Processing Problems

Proper equipment selection/design

Adequate instrumentation/control

Proper stuff handling ( drying/feeding )

Quality control on the input natural stuff

Use of proper additives

Table 2.6 Extrusion Die Head Pressure [ Ref. 11 ]

Profile constellation

Melt force per unit area ( x105dyn/cm2 )

Cast film/pipe/sheet

0.35-1.03

Monofilament

0.69-2.07

Wire coating

1.03-5.52

Flat movie

1.38-4.14

If one takes a expression at the possible redresss given in Table 2.5 for rectifying the processing jobs described in Table 2.4, so one can easy set up a unequivocal nexus between rheology and processing. A figure of other illustrations can be cited to demo the rheology-processing nexus, as discussed below.

Based on the profile constellation, the bulge die caput force per unit areas are different [ 11 ] , as shown in Table 2.6 ; hence, the flow response of the polymer is different in each of these state of affairss.

Bulge of polymer in the signifier of a uninterrupted profile requires sufficient thaw strength so that the liquefied merchandise coming from the hot dice in the unsupported signifier is strong plenty to retain its form until it is cooled to a point of significant strength. Besides, in bulge, unequal streamlining of the dice may take to nonlaminar flow ensuing in thaw break. The rapid acceleration of the surface beds at the die issue may bring on defects of the shark tegument type. Correct geometry and size of the dice is necessary to avoid flow defects. For illustration, it is known that LDPE melt exhibits big whirls in the dice entryway country, whereas HDPE thaw does non ; hence, attention has to be taken to right plan the dice in these two instances.

In injection casting, on the other manus, the liquefied concluding merchandise is good supported on all sides by the cool cast and does non necessitate high thaw strength. However, a proper gate design is desirable to avoid jetting flows. When managing low-viscosity stuffs, a part of shallow flights of the prison guard in the forepart is necessary to supply the necessary opposition to guarantee that air is excluded. Similarly, a reversed tapering nose, trapezoidal half-round smuggler, diaphragm gate at the border, or spring-loaded pin opposite the gate would assist every bit good. The flow way of the liquefied polymer determines place and extent of splay grade and weld line, and from an application point of view, it besides dictates whether the constituent is subjected to hoop emphasiss or transverse/longitudinal emphasiss.

Thermoforming, movie blowing, and fiber whirling affect the ability of a polymer sheet, movie, or fiber to defy high mechanical emphasiss and license considerable extension without failure in the semisolid province. The ideal response for a stuff in the thermoforming procedure is the 1 which would let easy extension at an mean strain and so rapid stiffening after that. In blow- casting procedure, if the thaw is more elastic, it will blow up more uniformly and give a more even merchandise.

In the calendering operation, a large-diameter calender is preferred so that a higher magnitude of the stretching constituent is achieved. During processing, monitoring of the viscousness versus shear rate map allows one to obtain a better apprehension of how the polymer is being affected during the procedure. If a job arises, rheological measuring can find whether the cause of the job is in the procedure or in the stuff, as illustrated by the last two instances considered in Table 2.4.

Design of a processing equipment, excessively, can be made on an optimal footing through the apprehension of rheology. The compaction ratio of an extruder prison guard and the length of the assorted zones -plasticating, compaction, and metering- are all chosen depending on the rheology of the polymer being processed. For illustration, a polythene movie bulge line can non be indiscretely used for HDPE every bit good as LDPE. Due to the built-in structural differences between the two types of polythenes, for better movie quality and greater production flexibleness, the general pattern is to take a HDPE class that has a viscousness which is 70-100 % higher than the LDPE class at the same temperature. The high viscousness of HDPE evidently leads to a greater syrupy dissipation during runing and therefore needs higher torsion for the thrust. Therefore, a LDPE line can non be adapted to treat HDPE because of the torque restriction. Further, cut downing the shear rate is the lone manner to cut down heat coevals. Therefore, a LDPE line can hold a maximal screw velocity every bit high as 150 revolutions per minute, whereas the maximal screw velocity of a HDPE extruder has to be 100 revolutions per minute or less.

A good cast design for commanding merchandise quality has to be done besides on the footing of the rheology of the system. For illustration, the optical lucidity in a poly ( methyl methacrylate ) merchandise is lost due to thermic debasement. When the thaw enters the cast through the gate, it experiences high shear rates, taking to syrupy heat coevals in localised topographic points whereby the polymer degrades and leaves white flow lines in the merchandise. The syrupy heat coevals is relative to viscousness ; a proper cast design which could cut down the shear rate or supply localised warming in the high shear part would work out the job cognizing the viscousness versus shear rate map for the polymer. A figure of such practical illustrations can be cited wherein the usage of rheology can be made.

The quality of the solid polymer merchandise, excessively, can be controlled through rheological measurings by set uping a correlativity between viscoelastic informations and public presentation belongingss like impact strength, service temperature, and thermic stableness. Rheology therefore acts as a span for associating the viscoelastic behaviour of polymers to cardinal features such as molecular weight, molecular- weight distribution, and long-chain ramification on one manus, and to practical affairs such as processibility and mechanical belongingss of the finished merchandise on the other.