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Mr. Navnath V. Karche, Ms. Priyanka N. Gavhankar, Mr. Bhupendra Dansena DKTE’S Textile and Engineering Institute, Ichalkaranji, Kolhapur. 416115
Solid State Polymerization and enhancing Crystallinity & Molecular weight to improve tensile properties of Polyester yarn
A
bstract: Solid State Polymerization (SSP) is a process in which the polymer chain lengths are increased by heat in the absence of oxygen and water, by means of either vacuum or purging with an inert gas to drive off the by-products of reactions. The reaction is driven by temperature, pressure, and the diffusion of by-products from the interior of the pellet to the surface. SSP is an important step frequently used after melt-polymerization for the purpose of enhancing the mechanical and rheological properties of polymers before injection blow molding or extruding. The SSP technique is widely applied in industrial manufacture of bottle-grade PET, films, and superior industrial fibers. Keyword: SSP- Solid State Polymerization, Tg Glass Transition temperature, Tm Melt temperature, IV Intransic Viscosity, After polymerization in the melt phase, the molecular weight of polyester can be further increased by polycondensation of PET chips in the solid state and this process is called solid state polymerization (SSP). This process enables higher molecular weights to be reached which are either technically or commercially not feasible in the melt phase. Solid state polymerization in the form of chips or powder, pre-extrusion SSP is a com-
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mon approach in the production of high molecular weight polymers for molded products. Polycondensation reaction is exothermic, therefore, it can also be driven forward to give polymer with higher molecular weight if the reaction is carried out at a lower temperature. At lower temperatures of polycondensation, equilibrium shifts forward to higher completion enabling formation of higher molecular weight of the product. Since temperatures cannot be lowered in melt polymerization, the best solution is to carry out the reaction in solid state, wherein temperatures lower than melting point of the polymer can be used. At these temperatures, the kinetics of the reaction is slow and it takes a long time to complete the reaction. The molecular weight can be increased up to 27000 (IV, 0.90) for bottle grade and as high as 38000 (IV, 1.20) for technical applications such as tyre cord, seal belts and air bags The first step before carrying out the solid state polymerization is precrystallization of chips. The water quenched polymer chips obtained from melt polycondensation stage are almost wholly amorphous. Therefore, the chips are first intensely dried and annealed at a gradually increasing temperature up to the point of
maximum crystallization rate to impart high level of crystallinity. This raises the glass transition temperature of the chips and prevents their sticking to each other during solid state polymerization. The SSP is performed at temperatures between 220 and 235°C for PET, which lie above the glass transition temperature and below the crystalline melting point of PET. Under these conditions, polymer chain end groups are mobilized sufficiently to undergo polycondensation. The annealed chips are heated in a stream of hot inert gas or by agitating in a vacuum drier to remove small traces of EG and other volatile by-products. As the reaction is carried out in the solid phase, volatile by-products can diffuse easily to the surface of each polymer chip and are instantly carried away by the gas flow or high vacuum. The main advantages of increasing molecular weight in solid state compared to melt phase are: 1. Problems associated with stirring of viscous melt are eliminated. 2. Degradation and side reactions are limited in the solid state due to lower processing temperature used. Solid state polycondensation process favours polymerization process compared to depolymerization. 3. Best means of achieving PET with
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June 2017