Polymer formulation Essay (7503 words)

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 ) . [ 1,2 ] This phenomenon is particularly common in big thin-walled moldings such as palpebras, bottle crates and trays. [ 3 ]

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. [ 2 ] 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. [ 4 ]

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. [ 5 ] The crystallisation procedure of polymers is thermodynamically driven. It is governed by alteration in Gibbs free energy, ?G. [ 6 ]

?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. [ 7 ]

For a polymer to clear, it must conform to the undermentioned demands: [ 8 ]

• 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. [ 6 ]

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. [ 9 ] There are three mechanisms of primary nucleation, viz. , homogenous nucleation, heterogenous nucleation and orientation induced nucleation. [ 10 ]

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. [ 7 ] This procedure is termed every bit sporadic as karyons are formed in timely sequence. [ 11 ] 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 [ 7 ] ; normally between 2-10nm. [ 11 ] Below this critical size, the karyon are unstable and may be destroyed. [ 11 ]

By and large, super-cooling to between 50-100°C below equilibrium thaw temperature is minimally required to accomplish true homogenous nucleation. [ 12 ] The super-cooling is attributed to the energy barrier homogenous karyons are required to get the better of to make stableness. [ 7 ] .

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. [ 13 ] A little embryo has high surface to volume ratio and so ?G is positive ; in other words, crystal growing is non thermodynamically favorable. [ 13,14 ] 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. [ 13,14 ] Finally when ?G becomes negative, nuclei are thermodynamically stable, paving the manner for farther growing into gill or spherulites. [ 14 ]

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. [ 7 ]

2.1.2.2. Heterogeneous Nucleation

In pattern, one normally observes heterogenous nucleation and non homogenous nucleation. [ 15 ]

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 ) . [ 7,8 ]

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. [ 6 ] 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. [ 16 ]

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. [ 7 ]

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. [ 8,17 ]

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 ) . [ 6 ] 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. [ 7 ] 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. [ 6 ]

1. tight fold next re-entry
2. independent deposition [ 6 ]

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. [ 6 ]

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. [ 8 ] 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. [ 8,18 ]

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 ) . [ 8 ]

When crystallization occurs under isothermal conditions, its advancement can be expressed by the Avrami equation: [ 8 ]

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: [ 19 ]

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. [ 19 ]

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. [ 19 ]

Table 2.1: Relation between Ns and nucleation manner / crystal growing form [ 19 ]

When crystallization occurs under constant-cooling-rate conditions, its advancement can be expressed by the Ozawa equation: [ 8 ]

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: [ 20 ]

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. [ 20 ]

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 ) . [ 7 ]

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. [ 7 ]

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. [ 7 ] Bank and Krim [ 21 ] 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.

1. extraneous position,
2. position along c-axis [ 7 ]

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. [ 7 ]

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. [ 7 ]

vitamin D ) composite theoretical account [ 7 ]

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 ne’er 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 ) . [ 22 ]

2.3. Organic Pigments

2.3.1. Copper Phthalocyanine Pigments: Copper Phthalocyanine Green

Copper phthalocyanines are a category of organic pigments which dominate the sectors of bluish and green colour of polymers. This laterality can be attributed to desirable belongingss such as high tinctorial strength, bright chromaticities, first-class visible radiation and conditions speed first-class heat stableness and good chemical opposition. [ 23 ] In add-on, in malice of its structural complexness, this category of pigments is cheap as they are manufactured in high output from low cost starting stuffs. [ 24 ]

The parent compound of Cu phthalocyanine pigments is highly easy to fix ; a phthalic acid derived function is condensed with a beginning of N such as urea and a Cu salt such as cupric chloride in the presence of a metal accelerator such as V or Mo. This is normally done in organic dissolvers, at elevated temperatures ( about 200°C ) and sometimes under increased force per unit area. The attendant petroleum Cu phthalocyanine ( outputs of over 90 % ) is purified commercially by one of several procedures ; salt abrasion, solvent-free salt abrasion, acerb pasting and acid puffiness. [ 3,25 ]

Figure 2.11 illustrates the chemical construction of the Cu phthalocyanine parent compound. It consists of a tetrabenzoporphyrazine karyon incorporating a cardinal Cu atom. The planar molecule is in the signifier of a quadratic form with length and thickness of 1.3nm and 0.34nm severally. [ 27 ] This parent Cu phthalocyanine compound, which is characterised by unsubstituted benzine rings, is used as bluish pigment. Copper phthalocyanine blue is polymorphic and exists in five crystal signifiers. Out of the five, the two of commercial importance are the alpha and beta signifiers while the other three are distorted ‘signifiers. [ 27 ] Different crystal signifiers bring about a fluctuation in the blue shadiness. Alpha crystals exhibit a bright-red-shade blue while beta crystals exhibit a green-shade blue. [ 26 ]

1. C.I. pigment green 7, B ) C.I. pigment green 36 ( 3y ) ,
2. C.I. pigment green 36 ( 6y ) [ 28 ]

Copper phthalocyanine viridity, the pigment of involvement in this undertaking, is produced from the Cu phthalocyanine blue by replacing the Hs on the four benzine rings with halogens. Unlike its bluish opposite number, where fluctuation of shadiness is achieved by alteration of crystal signifier, fluctuation in the green shadiness is controlled by grade of chlorination or bromination. Copper phthalocyanine viridity merely has one known crystal signifier. [ 26 ] The two types of Cu phthalocyanine green pigments are color index ( C.I. ) pigment green 7 and color index ( C.I. ) pigment green 36. C.I. pigment green 7 is a blue-shade viridity made by presenting 13 to fifteen Cl atoms to replace Hs in the benzine ring of the Cu phthalocyanine blue molecule ( illustrated in Figure 2.12 ( a ) ) . C.I. pigment green 36 is a yellow-shade viridity made by gradual replacing of Cl atoms in C.I. pigment green 7 with Br atoms. The most brominated C.I. pigment green 36, known as 3y, has an utmost xanthous shadiness ( illustrated in Figure 2.12 ( degree Celsius ) ) while the least brominated C.I. pigment green 36, 6y, has a much more blue shadiness ( illustrated in Figure 2.12 ( B ) ) . [ 28 ]

The outstanding tinctorial and speed belongingss of both Cu phthalocyanine green pigments allow their application under the harshest conditions. They can be used efficaciously in masstone shades and sunglassess down to the really palest deepness. Both green pigments can be processed at temperatures in surplus of 260°C with small color alteration. They have even better chemical and coloring material stableness than Cu phthalocyanine blues. On comparing, C.I. pigment green 7 is preferred over C.I. pigment green 36. The latter is weaker and more opaque and histories for less than 5 % of Cu phthalocyanine leafy vegetables used in the polymer industry. [ 3 ]

2.3.2. Consequence of Copper Phthalocyanine Green and Other Organic Pigments on Properties and Crystallisation Behaviour of Moulded Polyolefins

Although the combination of dramatic public presentation and low cost make Cu phthalocyanine green ideal pigments, its usage is non without challenges. It is widely known that Cu phthalocyanine viridity and a few other pigments can do unacceptable degrees of shrinking and warpage in moulded parts of polyolefins. [ 2,29 ] The job persists even at pigment concentrations every bit low as 0.1 % wt. [ 2 ] Shrinking can be described as decrease in moulded portion dimensions in mention to model pit dimensions. [ 30 ] Warpage is a step of out-of-plane deformation and commonly arises from the relaxation of imbalanced residuary emphasis in a moulded portion or imbalanced shrinking in flow and cross way. [ 30 ]

The early work of Turturro et Al. [ 2 ] demonstrated that this shrinking and warpage phenomenon is merely limited to organic pigments. It was reported that no deformation occurred in HDPE moldings incorporating inorganic pigments such as BBS red ( cadmium selenide ) , 21 M yellow ( blend of PbCrO4, PbSO4 and PbMoO4 ) and 500 L yellow ( complex of Ni and Ti ) . Findingss from ulterior surveies by Bugnon et Al. [ 31 ] and Suzuki & A ; Mizuguchi [ 29 ] are in good understanding. Suzuki & A ; Mizuguchi [ 29 ] reported similar observations when they incorporated inorganic pigments, TiO2, Fe2O3 and Cd Y into HDPE and PP. Using scanning negatron microscopy, Bugnon et Al. [ 31 ] were able to demo that when inorganic pigments such as Cadmiums or CrTiO4 are incorporated into HDPE, there is no interaction between pigment surface and polymer. The polymer basically builds a pit around the pigment. On the other manus, an organic pigment of diketo-pyrrolo-pyrrole chemical science was found to intermix into the HDPE matrix. This led them to suggest that inorganic pigments do non bring on shrinking and warpage as their chemical fundamental laws and polar hydrophilic surfaces have no interactions with polymers and do non act upon their crystallization behavior.

It is by and large agreed that the shrinking and warpage of polyolefins induced by Cu phthalocyanine viridity and other organic pigments is associated with the nucleating consequence these compounds have on the polymers. [ 2,29,31 ] These compounds provide a foreign surface that reduces the free energy of formation of a new polymer karyon. [ 27 ] Vonk [ 32 ] was one of the first few persons who pointed out that organic pigments can move as nucleating agents for polythene. The nucleating consequence of organic pigments on polyolefins has since been the focal point of intensive surveies over the old ages. The cardinal literature identified from this research country is that produced by Koh [ 33 ] for Clariant ( Singapore ) Pte Ltd. Koh [ 33 ] studied the influence of C.I. pigment green 7 and C.I. pigment green 36 on the crystallization and belongingss of HDPE. It was reported that the high degree of differential shrinking in HDPE moldings incorporated with Cu phthalocyanine leafy vegetables was accompanied by increased crystallization rate, increased peak / onset crystallisation temperature and decreased spherulite size. These findings clearly indicate that Cu phthalocyanine viridity can move as a nucleating agent for HDPE. It was besides reported that increasing pigment concentration will do an addition in crystallization rate and degree of differential shrinking.

Koh ‘s [ 33 ] findings are in line with those from similar surveies carried out by Turturro et Al. [ 2 ] , Suzuki & A ; Mizuguchi [ 29 ] and Silberman et Al. [ 34 ] Turturro et Al. [ 2 ] observed a similar nucleating consequence of Cu phthalocyanine viridity on HDPE with the assistance of depolarization and dilatometry techniques. In add-on, they found that the Avrami exponent value of HDPE decreases with increasing concentration of Cu phthalocyanine viridity ; which indicates a displacement in morphology, off from the spherulitic one feature of pure polythene. They proposed that the strong nucleating consequence of Cu phthalocyanine green causes merely the development of filaments in HDPE, which later do non organize into spherulites. Interestingly, they besides found that pigments do non impact the absolute degree of crystallinity in HDPE ; connoting that these compounds affect merely the dynamicss and non the thermodynamics of the crystallization procedure. [ 2 ] Suzuki & A ; Mizuguchi [ 29 ] and Silberman et Al. [ 34 ] showed that, apart from HDPE, Cu phthalocyanine viridity can besides move as a nucleating agent for PP. Furthermore, Silberman et Al. [ 34 ] found that the add-on of Cu phthalocyanine green into PP would increase its lamellar size and diminish the activation energy ( Uact ) of its crystallization procedure. The account they put frontward for these observations was based on the specific chemical construction of the pigment. The symmetricalness of N in the Cu phthalocyanine green molecule, with an absence of complex constructions was thought to advance the dynamic surface assimilation of PP molecules on the pigment surface and the subsequent crystallization procedure. This will take to the formation of a perfect crystal construction of big lamellar size. Together, the plants from all three writers demonstrated that, besides Cu phthalocyanine viridity, organic pigments of anthraquinone, perylene, quinacridone, Cu phthalocyanine blue and condense azo chemical sciences can besides move as nucleating agents for polyolefins. [ 2,29,34 ]

At this point, with the assistance of assorted documents, it is established that shrinking and warpage of polyolefins induced by Cu phthalocyanine viridity and other organic pigments are associated with these pigments functioning as nucleating agents for the polymer. However the specific mechanism correlating nucleating consequence and shrinking or warpage has yet to be discussed. Both Turturro et Al. [ 2 ] and Suzuki & A ; Mizuguchi [ 29 ] proposed the same account for this phenomenon. In a modeling procedure such as injection molding, the quench rate is non the same at different parts of the polymer. Polymer melt in contact with mold walls crystallise and ‘freeze ‘ really rapidly, which consequences in crystals of low flawlessness with polymer ironss oriented in the way of flow. This bed of imperfect crystals in bend impedes heat exchange between polymer thaw in the nucleus parts and the mold walls. As a consequence, polymer thaw in the nucleus parts cool easy and give rise to regular crystals. As the surface ‘freezes ‘ really rapidly, contraction in the nucleus parts due to crystallization will bring forth emphasis in the ‘frozen ‘ outer bed and cause deformation. In add-on, relaxation of orientated parts after remotion of polymer from the mold will besides do internal emphasis and lead to deformation. The presence of a strong nucleating agent such as Cu phthalocyanine viridity will restrict the clip available for orientated ironss to retrieve during chilling and besides increase the thickness of the tegument bed. Both factors will take to more marked deformation. [ 2,29 ]

Apart from changing the shrinking and warpage behavior of polyolefins, the nucleating consequence of Cu phthalocyanine viridity and other organic pigments is thought to besides hold a pronounced influence on the mechanical belongingss of polyolefins. An probe of how certain organic pigments affect the mechanical belongingss of HDPE was undertaken by Lodeiro et Al. [ 1 ] They found that tried pigments, Cu phthalocyanine blue and irgalite xanthous do impact the chief mechanical belongingss of HDPE. In peculiar, it was observed that the presence of little sums of phthalocyanine blue in HDPE is sufficient to do an addition in ductileness, decrease in Young ‘s modulus ( up to 10 % ) , decrease in output emphasis and addition in failure strain. They attributed these effects to smaller and more legion spherulites induced by the pigment ; smaller spherulites in larger Numberss, each surrounded by formless stuff, consequences in a polymer that will deform more readily and hold lower output emphasis and higher failure strain.

2.4. Nucleating Agents

2.4.1. Heterogeneous Nucleation of Polyethylene: Nucleating Agents Based on Potassium Stearate and Carboxylic Acid Salts

Nucleating agents have traditionally been added to semi-crystalline polymers to heighten processing and stop merchandise features. The incorporation of these compounds consequences in shorter rhythm clip as they increase the crystallisation rate of semi-crystalline polymers, guaranting faster hardening from the thaw upon chilling. Their add-on besides consequences in the formation of smaller spherulites in semi-crystalline polymers. This alteration in spherulite size improves mechanical belongingss ( such as tensile strength, hardness and modulus ) and optical belongingss ( such as haze and transparence ) . [ 8,35 ]

Polyethylene, and in peculiar high denseness polythene, has an highly fast rate of crystallisation, which makes it really hard to nucleate. [ 8,35 ] This is likely the ground why little has been published on its nucleating agents. That being said, a smattering of nucleating agents have been identified to day of the month. Together, the plants of Solti et Al. and Ge et Al. showed that benzoic acid, talc and Na2CO3 can efficaciously nucleate polythene. [ 8 ] Besides the usage of particulate or low molecular weight nucleating agents, polythene can besides be nucleated by epitaxial crystallisation on another polymer substrate. Loos et Al. was able to show the thaw crystallization of LLDPE on oriented HDPE. [ 8 ]

Potassium stearate is another nucleating agent that has been shown to be effectual for polythene. This compound is one of the two nucleating agent chemical sciences that are of involvement in this undertaking. Potassium stearate is an organic acid metal salt with molecular expression, C18H35KO2, and a chemical construction as illustrated in Figure 2.13. [ 36 ] It is prepared from get downing stuffs potassium hydrated oxide ( KOH ) and stearic acid ( CH2 ( CH2 ) 16COOH ) . [ 37 ] In their research, Narh et Al. [ 37 ] generated obliging informations that clearly suggests that K stearate can efficaciously nucleate HDPE. They reported detecting an addition in crystallization temperature from 113.9°C to 116.9°C and a lessening in concluding spherulite radius from 47µm to 13.3µm when K stearate was incorporated into HDPE. In add-on, they besides noticed an addition in nucleation denseness from 1.94×104/cm3 to 8.56×105/cm3.

The 2nd nucleating agent chemical science of involvement in the undertaking is that of carboxylic acid salts. A carboxylic acid salt is formed when a carboxylic acid reacts with a base. [ 38 ] The most common and good known nucleating agent of this chemical science is sodium benzoate, an aromatic carboxylic acid salt. Apart from Na benzoate, salts of mono- or polycarboxylic acids such as Na succinate, Na glurate, Na cinnamate, Na capronate, sodium-4-methyl valerate, aluminum phenyl ethanoate are besides known nucleating agents. The above-named carboxylic acid salts are reasonably effectual in nucleating polythene. [ 39,40 ]

Horrocks and Kerscher [ 10 ] showed that nucleating agents from Milliken and Company based on dicarboxylic acid salts ( Hyperform® HPN-68L and HPN-20E ) are extremely effectual in nucleating polythene. They demonstrated that HPN-20E in peculiar exhibits superior nucleation public presentation. It was reported that the inclusion of HPN-20E reduced peak crystallization temperature of LLDPE, mLLDPE and HDPE by 5.4°C, 4.4°C and 3°C severally. This was accompanied by important lessening in spherulite size and crystallization half-time. For illustration, HPN-20E inclusion reduced crystallisation half-time of LLDPE from 57 proceedingss to 5.5 proceedingss. In add-on, they besides showed that HPN-20E incorporation has a positive impact on barrier belongingss, lucidity and physical belongingss of LLDPE blown movies. Hyperform® HPN-68L is of disodium bike [ 2.2.1 ] heptane-2,3-dicarboxylate chemical science as disclosed within U.S. Patent Number 6,465,551 [ 42 ] while Hyperform® HPN-20E is of Ca hexahydrophthalic acid chemical science as disclosed within U.S. Patent Application Number 11/952,372. [ 43 ] The chemical constructions of both compounds as disclosed within U.S. Patent Number 7,659,336 B2 [ 41 ] are illustrated in Figure 2.14.

2.5. Solving Organic Pigment Induced Shrinkage and Warpage in Polyolefins

2.5.1. A Review of Possible Approaches

To day of the month, assorted attacks have been proposed to get the better of the job of organic pigment induced shrinking and warpage in polyolefins. They range from seting procedure parametric quantities, mould design alterations, pre-treatment of pigments, to incorporation of extra additives.

Bugnon et Al. [ 31 ] outlined that unlike organic pigments, which have lipotropic and non-polar surfaces, inorganic pigments do non bring on shrinking and warpage in polyolefins because their hydrophilic and polar surfaces present no interactions with the polymers. Based on this statement, they investigated the feasibly of imitating an inorganic pigment surface on organic pigments by surface interventions in order to contradict their negative nucleation consequence. They found that precipitating a bed of metal oxide ( e.g. zirconium oxide, silicon oxide or aluminum oxide ) or a bed of polar polymer ( e.g. PVOH or cullulose derived functions ) onto the surface of organic pigments will cut down their negative nucleating consequence and the attendant warpage of pigmented HDPE. The betterment is particularly apparent for the latter method.

Tomlins et Al. [ 44 ] 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 internal 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. [ 1 ] 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 [ 30 ] 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 [ 27 ] 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. [ 31 ] 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. [ 44 ] 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. [ 45 ] 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 [ 4 ] 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. [ 4 ] 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 difference 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 [ 46 ] , Borealis [ 47 ] and Ampacet [ 48 ] 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 ) [ 49 ] while that from Borealis is made from a particular reactor technique where accelerator is pre-polymerised with monomers ( tradenamed BNT ) [ 50 ] . 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. [ 48 ]

In 2006 and 2010, Milliken and Company filed for two patents on nucleating agent blends ; U.S. Patent Application Number 11/078,003 [ 51 ] and U.S. Patent Number 7,659,336 B2 [ 41 ] 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. [ 41,51 ] 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. [ 41,51 ] 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 nucleating 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. [ 52 ]

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 ) . [ 53 ] 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. [ 53 ]

1. ‘heat flux ‘ DSC instrument [ 54 ] ,
2. ‘power compensated ‘ DSC instrument [ 55 ]

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 ) [ 66 ]
• Peak crystallization temperature ( Tc, extremum ) [ 66 ]
• Percentage crystallinity [ 56 ] , 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 ) [ 57 ] , which is the clip taken for 50 % of entire crystallization to happen
• Rate of crystallization [ 8 ] , 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 [ 57 ] 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. [ 58 ] The popularity of this word picture technique can be attributed to ease of usage, simple sample readying and low cost. [ 59 ]

As illustrated in Figure 2.17, a polarised light optical microscope adds two polarizing filters to an ordinary optical microscope. [ 60 ] These polarizing filters cause visible radiation that passes through it to vibrate in merely one plane. [ 61 ] 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. [ 60 ]

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. [ 62 ]

polarised light optical microscope [ 63 ]

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 [ 2,64,65 ] :

• 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 ) . [ 64 ]
• Krumme [ 65 ] 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.

1. normal visible radiation,
2. polarised visible radiation,
3. after binary transition [ 65 ]

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. [ 59 ] If thermic history is non of import, thin movies can be obtained either by solvent projecting [ 66 ] ( 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 [ 2 ] ( 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 5µm 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. [ 62 ]
• Cooling rate of commercial hot-stages ( max 20°C/min ) can non forestall crystallization from get downing before isothermal crystallization temperature is reached. [ 65 ]

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. [ 30 ] In this research, the former criterion will be adopted.

ASTM D955 [ 67 ] 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 [ 4 ] and Koh [ 33 ] 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. [ 4 ]