Polymer formulation Essay
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. 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.
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.
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 ] 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.
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.
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 ] 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 ] 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 ] 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 ] 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 ] 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 ]