Polymer formulation Essay Example For Students

Polymer formulation Essay Outline1 Chapter 12 Introduction3 Chapter 24 LITERATURE REVIEW5 2.1. Nucleation and Crystallisation of Semi-Crystalline Polymers6 2.1.1. Crystallization Mechanisms7 2.1.2. Primary Nucleation8 2.1.2.1. Homogeneous Nucleation9 2.1.2.2. Heterogeneous Nucleation10 2.1.2.3. Orientation-Induced Nucleation11 2.1.3. Crystal Growth12 2.1.3.1. Primary Crystallization13 2.1.3.2. Secondary Crystallization14 2.1.4. Rate of Crystallization15 Crystal Growth Shape16 Nucleation Mode17 Avrami Exponent ( N )18 2.2. High Density Polyethylene ( HDPE )19 2.2.1. Chemical Structure, Crystallisation Rate and Morphology20 2.3. Organic Pigments21 2.3.1. Copper Phthalocyanine Pigments: Copper Phthalocyanine Green22 2.3.2. Consequence of Copper Phthalocyanine Green and Other Organic Pigments on Properties and Crystallisation Behaviour of Moulded Polyolefins23 2.4. Nucleating Agents24 2.4.1. Heterogeneous Nucleation of Polyethylene: Nucleating Agents Based on Potassium Stearate and Carboxylic Acid Salts25 2.5. So lving Organic Pigment Induced Shrinkage and Warpage in Polyolefins26 2.5.1. A Review of Possible Approaches27 2.5.2. Extinguishing Shrinking and Warpage by Incorporation of Nucleating Agents28 2.6. Word picture Techniques29 2.6.1. Differential Scaning Calorimetry ( DSC )30 2.6.2. Polarised Light Optical Microscopy ( PLOM )31 2.6.3. Shrinking Isotropy Measurements Chapter 1 Introduction Pigments are additives in a polymer preparation which provide countless possibilities to interior decorators who want to distinguish their merchandise. Legislation and uprising environmental consciousness has led to the gradual phasing out of heavy metal inorganic pigments and increased use of organic pigments. Despite their good heat stableness, light speed, tinctorial strength and low cost, certain organic pigments are widely known to do important warpage in polyethylene moldings ( even at pigment concentrations every bit low as 0.1 % wt ) . This phenomenon is particularly common in big thin-walled moldings such as palpebras, bottle crates and trays. It is by and large accepted that the warpage phenomenon is caused by the nucleating consequence these organic pigments have on polythene. They act as nucleating agents, increasing crystallization rate and changing the morphology of moldings. Morphologic alterations cause higher internal emphasis which leads to deformation. Adding on to the job, different organic pigments nucleate polythene to different grades, doing it impossible to bring forth moldings with indistinguishable dimensions utilizing indistinguishable processing conditions when a assortment of pigments are used. Numerous efforts have already been made, with normally moderate success, to work out organic pigment induced warpage. They range from seting procedure parametric quantities, mould design alterations, pre-treatment of pigments, to incorporation of extra additives. A reappraisal of literature in this research country showed that although some surveies have been conducted to look into the incorporation of nucleating agents to overrule nucleating effects of organic pigments on polypropene, limited information of this kind exists for polythene. The particular mechanism behind nucleating agents overruling nucleation by organic pigments is besides still ill-defined. Therefore, it is the purpose of this research to analyze the influence of nucleating agents, based on K stearate and carboxylic acid salts, on the crystallization and warpage behavior of high denseness polythene incorporating Cu phthalocyanine green pigment. Differential Scaning Calorimetry ( DSC ) and Optical Microscopy ( OM ) will be employed to follow the crystallization behavior of the preparations and correlativities between rate of crystallization and shrinking behavior will besides be made. Chapter 2 LITERATURE REVIEW 2.1. Nucleation and Crystallisation of Semi-Crystalline Polymers 2.1.1. Crystallization Mechanisms Crystallization involves the formation of an ordered construction from a broken stage, such as thaw or dilute solution. The crystallisation procedure of polymers is thermodynamically driven. It is governed by alteration in Gibbs free energy, ?G. ?G = ?H T?S ( 2-1 ) Where ?H is change in heat content, T is absolute temperature and ?S is change in information. When ?G is negative, crystallization is thermodynamically favorable. This occurs when loss of enthalpy upon crystallisation exceeds the loss of information multiplied by absolute temperature. It can hence be derived that as the absolute temperature of the system falls, the driving force of crystallization will increase. For a polymer to clear, it must conform to the undermentioned demands: Molecular construction must be regular plenty to let crystalline telling Crystallization temperature must be below runing point but non near to glaze passage temperature Nucleation must happen before crystallization Crystallization rate should be sufficiently high A hundred per centum crystallinity is non possible in polymers due to factors such as concatenation webs, syrupy retarding force and ramification. Thus they are termed ‘semi-crystalline . All semi-crystalline polymers exhibit a alone equilibrium runing temperature above which crystallites thaws and below which a molten polymer starts to clear. The crystallization of semi-crystalline polymers is a two-step procedure dwelling crystal nucleation and crystal growing. 2.1.2. Primary Nucleation Primary nucleation can be defined as the formation of short-range ordered polymer collections in thaw which act as a focal Centre around which crystallisation can happen. There are three mechanisms of primary nucleation, viz. , homogenous nucleation, heterogenous nucleation and orientation induced nucleation. 2.1.2.1. Homogeneous Nucleation Homogeneous nucleation involves the self-generated creative activity of karyon in a semi-crystalline polymer thaw when it is cooled below its equilibrium thaw temperature. This procedure is termed every bit sporadic as karyons are formed in timely sequence. Creation of karyon occurs when statistical fluctuation within a polymer thaw consequences in the formation of ordered assemblies of concatenation sections larger than a critical size ; normally between 2-10nm. Below this critical size, the karyon are unstable and may be destroyed. By and large, super-cooling to between 50-100 °C below equilibrium thaw temperature is minimally required to accomplish true homogenous nucleation. The super-cooling is attributed to the energy barrier homogenous karyons are required to get the better of to make stableness. . When molecular sections pack following to each other to organize an embryo, there is a alteration in free energy, ?G, caused by two opposing mechanisms. The creative activity of new crystal surface additions free energy ( ?S is negative ) while the decrease in volume of the system decreases free energy ( ? ( U+pV ) ? ?H is negative ) . The two opposing mechanisms lead to a size-dependent free energy curve which defines critical karyon size. A little embryo has high surface to volume ratio and so ?G is positive ; in other words, crystal growing is non thermodynamically favorable. However as nuclei grow, the surface to volume ratio decreases up to a point where volume alteration outweighs the creative activity of new surface and alteration in free energy lessening ; crystal growing becomes progressively likely. This point is defined as critical karyon size and above this point, the energy barrier is overcome. Finally when ?G becomes negative, nuclei are thermodynamically stable, pav ing the manner for farther growing into gill or spherulites. The minimal figure of unit cells required to organize a stable karyon lessening when temperature lessening, due to a decrease in energy barrier. In other words, the rate of homogenous nucleation additions when temperature of the polymer decreases. 2.1.2.2. Heterogeneous Nucleation In pattern, one normally observes heterogenous nucleation and non homogenous nucleation. Heterogeneous nucleation involves the formation of karyon on the surface of foreign organic structures present in the liquefied stage of a semi-crystalline polymer. The foreign organic structures can take the signifier of adventitious drosss such as dust atoms or accelerator leftovers, nucleating agents added on intent or crystals of the same stuff already present in the liquefied stage ( self-seeding ) . The presence of foreign organic structures greatly reduces the energy barrier for the formation of stable karyon. This ground for this is, polymer molecules which solidify against preexistent surfaces of foreign organic structures create less new liquid/solid interface than the same volume of polymer molecules organizing a homogenous karyon. In bend, critical size of karyon is smaller in heterogenous nucleation as compared to homogeneous nucleation so that heterogenous nucleation ever occurs at lower supercooling. Foreign organic structures with crystallographic spacings fiting the semi-crystalline polymer are particularly effectual heterogenous nucleating agents. Favorable nucleation sites include crystal grain boundaries, clefts, discontinuities and pits. 2.1.2.3. Orientation-Induced Nucleation Orientation-induced nucleation is caused by some grade of molecular alliance in the liquefied stage of a semi-crystalline polymer. Molecular alliance reduces the information difference between the molten and crystalline province of the polymer. This sort of nucleation is of import in assorted procedures such as fibre melt-spinning, film-forming and injection molding. In these procedures, polymer thaw is sheared before and during crystallization. 2.1.3. Crystal Growth 2.1.3.1. Primary Crystallization Primary crystallization occurs when thaw of a semi-crystalline polymer is cooled below its equilibrium thaw temperature. It involves molecular sections lodging onto the turning face of crystallites or karyon. The attendant crystal growing occurs along the a and B axes, comparative to the polymer s unit cell. These add-ons of molecular sections can happen through two mechanisms: tight fold next re-entry or independent deposition ( illustrated in Figure 2.3 ) . Tight fold next re-entry requires that concatenation stems be laid down continuously from a individual polymer molecule in a series of hairpin decompression sicknesss until its length is exhausted. This individual molecule is thought to be ‘reeled in from environing liquefied stuff. This mechanism requires that molecular gestures along the polymer molecule s contour length to be several times faster than the rate of crystal growing. On the other manus, the independent deposition mechanism merely requires localised gesture of molecular sections. Molecular sections merely need to re-organise sufficiently to aline with molecular sections at the crystallite face. tight fold next re-entry independent deposition 2.1.3.2. Secondary Crystallization After a semi-crystalline polymer is cooled to room temperature, crystallization is still thermodynamically favorable but restricted by the low mobility of molecular sections in its formless parts. Over an drawn-out period of clip, which can cross from hours to hebdomads, re-arrangement of molecular sections within formless parts can take to farther crystal growing. This procedure is defined as secondary crystallization. Secondary crystallization can take two signifiers ; either thickener of preexistent crystallites by re-organisation of formless concatenation sections next to crystallite surface or creative activity of new crystallites by re-organisation of formless concatenation sections in interstitial parts between preexistent crystallites. 2.1.4. Rate of Crystallization The crystallization of semi-crystalline polymers is a two-step procedure and hence overall crystallization rate is governed by both nucleation rate and crystal growing rate. Both factors are extremely temperature dependant, as illustrated in Figure 2.4. When temperature is merely below equilibrium runing point, there exists a meta-stable part where rate of nucleation is low as karyon that are formed dissolve easy due to high thermic gestures. As super-cooling additions, thermodynamic conditions become more favorable and rate of nucleation additions and reaches a maximal near the glass passage temperature. On the other manus, kinetic conditions are less favorable as super-cooling causes viscousness to increase. This consequences in a displacement in maximal rate of crystal growing to higher temperatures where viscousness lessening is balanced by formation of karyon. Overall crystallization rate at a given temperature is normally expressed as the opposite of clip needed for half of the crystals to turn in the polymer ( 1/ t1/2 ) . When crystallization occurs under isothermal conditions, its advancement can be expressed by the Avrami equation: Ninety ( T ) = 1 exp ( -K.tn ) ( 2-2 ) Where Xc ( T ) is the fraction of stuff transformed at clip T, N is the Avrami advocate and K is the Avrami rate invariable. Equation ( 2-2 ) may besides be written as: ln ( -ln |1-Xc ( T ) | ) = n ln ( T ) + ln K ( 2-3 ) So that N and K may be obtained by plotting ln ( -ln |1-Xc ( T ) | ) against ln ( T ) ; n is the incline while ln K is the y-intercept. The value of the Avrami advocate, N, is dependent on mechanism of nucleation and geometry of crystal growing. Theoretical values of n matching to different nucleation manners and crystal growing form are tabulated in Table 2.1. Crystal Growth Shape Nucleation Mode Avrami Exponent ( N ) Rod Heterogeneous 1 Homogeneous 2 Phonograph record Heterogeneous 2 Homogeneous 3 Sphere Heterogeneous 3 Homogeneous 4 Table 2.1: Relation between Ns and nucleation manner / crystal growing form When crystallization occurs under constant-cooling-rate conditions, its advancement can be expressed by the Ozawa equation: Ninety ( T ) = 1 exp ( -? ( T ) / ?m ) ( 2-4 ) Where ? ( T ) is the Ozawa rate invariable, ? is the changeless chilling rate ( ?T/?t ) and m is the Ozawa advocate. Equation ( 2-4 ) may besides be written as: ln ( -ln |1-Xc ( T ) | ) = m ln ( T ) + ln ? ( T ) ( 2-5 ) So that m and ? ( T ) may be obtained by plotting ln ( -ln |1-Xc ( T ) | ) against ln ( T ) ; m is the incline while ln ? ( T ) is the y-intercept. Qiu et Al. combined the Avrami and Ozawa equations to do a connexion between the Avrami and Ozawa advocates: log ? = log F ( T ) a log T ( 2-6 ) Where a = n/m and the kinetic map F ( T ) = ( ? ( T ) / K ) 1/m. At a given grade of crystallinity, a secret plan of log ? against log T will give a and log F ( T ) as the incline and y-intercept severally. 2.2. High Density Polyethylene ( HDPE ) 2.2.1. Chemical Structure, Crystallisation Rate and Morphology High denseness polythene, HDPE, is a semi-crystalline polymer made up of repetition units ( C2H4 ) N and has a general signifier as illustrated in Figure 2.5. It consists chiefly of unbranching molecules with really few defects to interrupt its one-dimensionality or hinder crystalline wadding. As such, HDPE has a high rate of crystallization, grade of crystallinity and denseness ( 0.94-0.97 g/cm3 ) . Bing a semi-crystalline polymer, HDPE exhibits a three-phase morphology dwelling of submicroscopic crystals surrounded by a non-crystalline stage consisting a partly ordered bed adjacent to the crystals and disordered stuff in the intervening infinites. This is illustrated in Figure 2.6. The unit cell of HDPE, defined as the smallest agreement of its concatenation sections that can reiterate in three dimensions to organize a crystalline matrix, is orthorhombic ; a cuboid with each of its axes holding different lengths while the angles of bordering faces are all 90 ° . Each unit cell is made up of two ethylene repetition units ; a complete unit from one concatenation section and parts of four others from environing concatenation sections. Bank and Krim reported that the a, B and degree Celsius axes of a polyethylene unit cell are of dimensions 7.417, 4.945 and 2.547A severally. This is illustrated in Figure 2.7. extraneous position, position along c-axis HDPE unit cells pack together in a three dimensional array to organize little crystals known as crystallites. Most normally, crystallites of HDPE take the signifier of ‘lamellae ; crystallites with a and B dimensions that are much greater than their hundred dimensions. Lamellae thicknesses are normally between 50 to 200A while sidelong dimensions can run from a few hundred As to several millimeters. Figure 2.8 illustrates a HDPE gill. Assorted theoretical accounts have been proposed to explicate the agreement of molecular ironss in gill. They include next re-entry with tight creases, patchboard, loose cringles and a theoretical account with combined characteristics ( illustrated in Figure 2.9 ) . As molecular length of HDPE is known to be many times greater than lamellae thickness, all theoretical accounts indicate some signifier of concatenation folding. However, they differ in their specific nature of turn uping. vitamin D ) composite theoretical account In HDPE, the most common big scale-structures composed of crystalline and non-crystalline parts are known as ‘spherulites . A spherulite consists of lamellae turning outward radially from a common nucleation site. As this growing progress into formless liquefied polymer, local inhomogeneities in concentrations of crystallisable sections will be encountered. This causes the folded concatenation filaments to inevitable turn and subdivision. As illustrated in Figure 2.10a, a spherulite will resemble a bundle in its early phase of development. Faning out of the turning gill will later bring forth a spherical construction but true spherical symmetricalness is neer achieved due to encroachment of neighboring spherulites. This growing of spherulites besides involves the segregation of non-crystalline stuffs into parts between lamellar threads. Thus the overall construction of a spherulite consists of distorted and branched gill with polymer ironss largely perpendicular to their long axis and formless parts ( illustrated in Figure 2.10b ) . Macbeth - Supernatural Forces EssayTomlins et Al. experimented with the accommodations of injection modeling treating parametric quantities in effort to better the dimensional stableness of HDPE moldings incorporated with organic pigments. Out of the five procedure parametric quantities investigated ( keeping clip, keeping force per unit area, injection velocity, melt temperature and mould temperature ) , they found that high keeping force per unit area had the largest influence on shrinking and warpage. High keeping force per unit area improves dimensional stableness. It was besides observed that increasing the velocity at which the mold pit fills besides reduces out-of-plane deformation. This can be achieved by cut downing injection clip, increasing melt temperature or increasing mould temperature. In add-on, they noted that keeping clip does non hold any influence on out-of-plane deformation. This is interesting as it is common pattern to widen keeping clip to temper out intern al emphasiss in moldings that may do warpage. Other additives in a pigmented polyolefin preparation may besides overrule the negative nucleating consequence of the organic pigments. In their survey of the influence of organic pigments on HDPE mechanical belongingss, Lodeiro et Al. observed that the bearer ( LLDPE ) and/or the wetting agent in the organic pigment masterbatches they used serves to cut down the the negative nucleating consequence of the pigments. The debut of nucleating agents into pigmented polyolefin preparations may besides get the better of the unfavourable nucleating consequence of the organic pigments. This is the attack adopted in this undertaking and will be discussed in item following this subdivision. Tomlins suggested that little alterations in mold design characteristics may function to supply more dimensional stableness to moulded parts. He mentioned that the add-on of stiffening ribs, right arrangement of chilling channels, right choice of gate with mention to portion geometry and the usage of rounded corners alternatively of square border corners all assist in bettering dimensional stableness. Although non a executable solution, it is interesting to observe that the work of Broda indicated that organic pigments do non take part in the nucleation procedure of PP when a high grade of orientation is induced. He reasoned that under high degrees of orientation, really effectual row karyons are formed in PP, and in the presence of such karyons, heterogenous karyon formed from pigment crystals become undistinguished and the crystallization procedure merely occur on row karyon. In this instance, crystallization is besides no longer spherulitic. Comparing the different attacks, it is apparent that some are more practical than others. Surface intervention of organic pigments is effectual but there is an built-in job associated with this attack ; It is extremely possible that new bare organic pigment surfaces will be formed ( e.g. by agencies of shearing ) when pigmented polyolefins are processed into concluding articles. Changing of treating parametric quantities is the simplest and most cost effectual method, but it is by the writers ain admittance that because organic pigments promote really marked anisotropic shrinking, wholly extinguishing deformations by merely seting procedure parametric quantities is really hard. Furthermore, , when a assortment of pigments are used, it is to practical to alter a new set of treating parametric quantities each clip the pigment is varied. Changing of mould design to counterbalance for shrinking and warpage is non ever possible while bring oning high degrees of orientation is non at all executable. It would be ideal if the negative nucleating consequence of organic pigments can be negated merely by adding nucleating agents. 2.5.2. Extinguishing Shrinking and Warpage by Incorporation of Nucleating Agents To day of the month, some work has been done to demo that the incorporation of strong nucleating agents can work out or at least cut down pigment induced shrinking and warpage. These probes have chiefly been conducted on polypropene. In a survey that closely relates to the present research, Tomlins et Al. investigated the influence of nucleating agents on the dimensional stableness of pigmented PP moldings. Although the nucleating agents and pigments examined in their survey were non revealed due to commercial sensitiveness, several of import decisions can be drawn from their findings. Their work showed that add-on of nucleating agents can well cut down in-plane warpage in pigmented PP and the consequence becomes more important with increasing nucleating agent concentration. Decrease in out-of-plane warpage is non as important. It was besides demonstrated that difference in anisotropic shrinking ratio ( ratio between flow way and cross way shrinking ) that exists between PP moldings incorporating different pigments can be reduced by nucleating agent incorporation. This decrease in difference is nevertheless, insensitive to nucleating agent concentration but depends more on nucleating agent type. Many commercial nucleating agents available in the market have been alleged to work out the shrinking and warpage job caused by organic pigments. Halstead and Jones showed that the carboxylic acid salt based nucleating agent from Milliken and Company, Hyperform ® HPN-68L, promotes comparatively isotropous shrinking in PP moldings as compared to other nucleating agents such as Na benzoate. They attributed this to the plate-like atom form of HPN-68L holding no preferred flow orientation whereas in the instance of Na benzoate, ruler-shaped atoms orientate in the way of flow ( illustrated in Figure 2.15 ) . Their work demonstrated that when HPN-68L is incorporated into pigmented PP moldings, the isotropic shrinking behavior associated with this nucleating agent will still prevail while the warpage behavior associated with the integrated organic pigment will basically be cancelled out. It was besides indicated that incorporation of HPN-68L would function to ‘level out the diffe rence in anisotropic shrinking ratio and crystallization temperature that exists between PP moldings incorporating different organic pigments. These findings clearly suggest that HPN-68L has nucleation power that overrides the nucleating ability of organic pigments. This overruling power allows for the production of moldings with indistinguishable dimensions utilizing indistinguishable processing conditions even though they contain different organic pigments. Apart from Milliken and Company, BASF , Borealis and Ampacet besides claim that their several nucleating agents have the ability to overrule the negative nucleating consequence of organic pigments. The nucleating agent from BASF is of Zn monoglycerolate chemical science ( tradenamed Irgastab ® NA 287 ) while that from Borealis is made from a particular reactor technique where accelerator is pre-polymerised with monomers ( tradenamed BNT ) . The nucleating agent from Ampacet can be found in materbatches Ampacet 103003 and Ampacet 103004 but its chemical science is unrevealed. Similar to Milliken and Company, Ampacet besides mentioned that the plate-like atom form of their nucleating agent may be the ground why it can supply better dimensional stableness to pigmented moldings. In 2006 and 2010, Milliken and Company filed for two patents on nucleating agent blends ; U.S. Patent Application Number 11/078,003 and U.S. Patent Number 7,659,336 B2 severally. The former is a blend of Hyperform ® HPN-68L with a phosphate ester salt based nucleating agent, NA-11 ( from Asahi Denka Kogyo K.K. ) , while the latter involves a blend of Hyperform ® HPN-68L and HPN-20E. It was reported in both instances that a blend of two nucleating agents does non ensue in one compound dominating or overruling the nucleating consequence of the other. Alternatively, beyond the outlooks of the writers, the nucleating consequence of co-nucleants both contributed to resultant physical belongingss of the polymer and even yielded interactive effects in some instances. Two of import decisions can be interpreted from the patents. First, it has been discussed that for nucleating agents to extinguish shrinking or warpage in pigmented polyolefins, they are required to overrule the nucleat ing effects of the pigments. However, it is shown in these two patents that even a strong nucleator such as Hyperform ® HPN-68L does non needfully ever override other nucleating agents. In other words, there is a possibility that Hyperform ® HPN-68L may non be able to contradict the negative nucleating effects of all organic pigments. Second, it is besides shown in these two patents that the usage of two nucleating agents may give interactive belongingss and this could be applied to the present research. A blend of K stearate and a carboxylic acid salt could be tested to look into if it imparts interactive betterments to dimensional stableness. 2.6. Word picture Techniques 2.6.1. Differential Scaning Calorimetry ( DSC ) Differential scanning calorimetry, DSC, measures the heat flow into or out of a polymer specimen as a map of either clip or temperature. Intergration of extremums in a DSC hint ( heat flow vs. clip or temperature ) gives the heat content alteration in a specimen. When there is heat flow into the specimen, the heat content alteration is endothermal and when the specimen releases heat, the heat content alteration is exothermal. The two chief types of commercial DSC instruments are the ‘heat-flux and ‘power compensated types. A ‘power compensated DSC instrument measures the difference in power supplied to a polymer sample and a mention, in order to maintain their temperatures the same ( illustrated in Figure 2.16a ) . In a ‘heat flux DSC instrument, one individual warmer is used to increase the temperature of both the sample and mention cell ( illustrated in Figure 2.16b ) . Temperature difference between sample and mention pan happening due to exothermic and endothermal effects in the polymer are recorded. ‘heat flux DSC instrument , ‘power compensated DSC instrument A reappraisal of literature affecting the usage of differential scanning calorimetry in polymer crystallization surveies revealed that the value of this technique lies in its ability to quantify assorted facets of the procedure. Non-isothermal differential scanning calorimetry ( changeless chilling rate ) can give the undermentioned information: Onset crystallization temperature ( Tc, oncoming ) Peak crystallization temperature ( Tc, extremum ) Percentage crystallinity , which is given by, % Crystallinity = ( ?Hf ° / ?Hf ) x 100 % Where ?Hf is measured heat of merger and ?Hf ° is heat of merger of the polymer when 100 % crystalline Ozawa advocate ( m ) and rate invariable ( ? ( T ) ) by utilizing equation ( 2-5 ) Isothermal differential scanning calorimetry ( changeless crystallization temperature ) can give the undermentioned information: Crystallization half-time ( t1/2 ) , which is the clip taken for 50 % of entire crystallization to happen Rate of crystallization , which is given by, Rate of crystallization = 1/t1/2 Avrami advocate ( n ) and rate invariable ( K ) by utilizing equation ( 2-3 ) Tomlins and Richardson showed that kinetic parametric quantities from the Avrami equation, N and K, can be farther used for computation of activation energy for crystallization and spherulitic growing rate. 2.6.2. Polarised Light Optical Microscopy ( PLOM ) Polarised light optical microscopy, is the most widely used method to characterize the supermolecular construction ( e.g. spherulites ) of semi-crystalline polymers. The popularity of this word picture technique can be attributed to ease of usage, simple sample readying and low cost. As illustrated in Figure 2.17, a polarised light optical microscope adds two polarizing filters to an ordinary optical microscope. These polarizing filters cause visible radiation that passes through it to vibrate in merely one plane. The first is located below the microscope phase to polarize light supplied by a light beginning ( e.g. halogen or curve lamp ) and the 2nd serves to analyze the polarization of visible radiation after it passes through a specimen ( therefore it is besides known as an ‘analyser ) . By and large, the polarisers are oriented such that their polarization waies are at right angles and no light base on ballss through the system. However, when a birefringent or optically anisotropic specimen is placed on the sample phase and rotated through 360 grades, they will be illuminated at some angles during the rotary motion. For an optically isotropous specimen, the field of position will stay dark at all angles of rotary motion. Spherulites can be view utilizing a polarising light optical microscope because they are ‘spherically birefringent objects with two alone refractile indices ; radial ( nr ) and digressive ( n? ) . The difference in these two indices gives rise to the ‘Maltese cross form as illustrated in Figure 2.18. Polyethylene spherulites have larger refractile index in the digressive way nr lt ; n? ; negative spherulites. polarised light optical microscope A study of literature affecting the usage of polarised light microscopy in polymer crystallization surveies showed that this technique can give the following valuable information : Spherulite size, nucleation denseness and overall spherulitic texture The full crystallization procedure can be followed and onset crystallization temperature can be determined with the assistance of a hot phase and a picture camera mounted on the microscope. Besides with the assistance of a hot phase and mounted picture camera, micrographs can be taken at fixed clip periods to mensurate radius of spherulites ( R ) . Measured radius can be used to find spherulitic growing rate ( dR/dt ) . Krumme showed that by utilizing binary transition and pel numeration, it is possible to cipher per centum crystallinity, nucleation denseness, Avrami advocate ( n ) and Avrami rate invariable ( K ) from a micrograph taken at a specific clip ( T ) . Binary transition basically involves change overing formless parts in a micrograph into black coloring material and crystalline parts into white coloring materials so that their countries can be used for farther computations. normal visible radiation, polarised visible radiation, after binary transition To look into spherulitic texture utilizing polarised visible radiation microscopy, polymer specimens must be in the signifier of thin movies. If thermic history of the polymer is of importance, thin movie specimens can be prepared by microtomy. If thermic history is non of import, thin movies can be obtained either by solvent projecting ( fade out polymer in a dissolver and topographic point a bead of the solution on a glass slide and let dissolver to vaporize ) or by melt pressing ( between microscope glass slide and cover-slip at elevated temperatures ) . Although there are many advantages in utilizing polarised visible radiation microscopy, this technique does hold its disadvantages: In general, merely spherulites with diameters above 5 µm can be resolved and utilize for computations utilizing this technique. Smaller spherulites, such as that produced by the add-on of nucleating agents, may non be clearly resolved and required the usage of other techniques such as negatron microscopy or little angle light dispersing. Cooling rate of commercial hot-stages ( max 20 °C/min ) can non forestall crystallization from get downing before isothermal crystallization temperature is reached. 2.6.3. Shrinking Isotropy Measurements Normally used criterions to mensurate shrinking from mould pit to moulded dimensions of thermoplastics include ASTM D955 and ISO 294. In this research, the former criterion will be adopted. ASTM D955 describes three different geometries that are applicable for shrinking measurings ; Type D2 60x60x2mm square plaque, Type A 12.7x127x3.2 rectangular bars and Type B discoid specimens with 100mm diameter and 3.2mm thickness. It is mentioned that Type A specimens are more applicable when shrinking in machine way is expected while Type D2 specimens are more applicable when shrinking is expected in both machine and cross waies. Harmonizing to the criterion, mould shrinking is calculated and reported for both the machine way, MD, and the cross way, TD: Percentage mould shrinkage in MD = ( mould dimension in MD specimen dimension in MD ) x 100 % mould dimension in MD Percentage mould shrinkage in TD = ( mould dimension in TD specimen dimension in TD ) x 100 % mould dimension in TD In their work, Halstead A ; Jones and Koh both used a shrinking symmetry ratio to characterize the differential shrinking in moulded specimens: Shrinking symmetry = per centum mold shrinking in MD per centum mould shrinkage in TD A shrinking symmetry ratio of 1 would bespeak unvarying shrinking. As the value moves farther off from 1, it becomes more likely that the moulded specimen will falsify.