The Polymerization Stage

PTT is melt polymerized by either the transesterification of PDO with dimethyl terephthalate (DMT) or by the direct esterification of PDO with purified tereph-thalic acid (PTA). The process is similar to that for PET but with major differences, as follows  

Because of PDO s lower reactivity, more active catalysts based on titanium and tin, which would discolor PET, are used to polymerize PTT. 

PTT has different side reaction products. Instead of the acetaldehyde produced with PET, acrolein and allyl alcohol are the volatile byproducts of PTT production. The generation of acrolein is to be expected since it is one of the starting raw materials for making PDO. Acrolein is toxic and is a very strong lachrymator [22], and requires special handling and treatment. 

Instead of cyclic trimer, PTT produces a lower-melting cyclic dimer byproduct [23], 

Compared to 1,4-butanediol, which forms tetrahydrofuran byproduct in PBT polymerization, PDO is difficult to cyclize into oxetane because of the high ring strain. Oxetane was not found in the byproduct analysis [23], 

PTT is polymerized at a much lower temperature between 250 and 275 °C. Because of its higher melt degradation rate and a faster crystallization rate, it requires special consideration in polymerization, pelletizing and solid-state tteatment. 

Because TPA has a melting point of 300 °C and poor solubility in PDO, dhect esterification is preferably caiiied out in the presence of a heel under a pressure of 70-150kPa and at 250-270 °C for 100-140 min. A heel is an oligomeric PTT melt widi a degree of polymerization (DP) of 3 to 7, purposely left in die reaction vessel from a previous batch to improve TPA solubility and to serve as a reaction medium. The esterification step is self-catalyzed by TPA. 

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Acrylate polymerizations are markedly inhibited by oxygen therefore, considerable care is taken to exclude air during the polymerization stages of manufacturing. This inhibitory effect has been shown to be caused by copolymerization of oxygen with monomer, forming an alternating copolymer (81,82). 

The quahty of the water used in emulsion polymerization has long been known to affect the manufacture of ESBR. Water hardness and other ionic content can direcdy affect the chemical and mechanical stabiUty of the polymer emulsion (latex). Poor latex stabiUty results in the formation of coagulum in the polymerization stage as well as other parts of the latex handling system. 

The mixing vessels serve as buffers between the polymerization stage operated in batch mode and the separation units operated continuously. The capacity of each mixing vessel is three polymerization batches and the minimal hold-up is 0.1 polymerization batches to ensure a sufficient mixing effect. 

Options for assignment of the equipment The assignment of the units to the processing steps is fixed with respect to the stages of the plant but variable with respect to particular units within the stages (e.g., the reactors of the polymerization stage). 

Batch size The batch sizes in the preparation stage and in the polymerization stage are variable as batches may be split in the preparation stage and mixed in the polymerization stage. However, the concept of batches does not apply in the finishing stage. 

Changeovers No changeovers appear in the preparation stage and in the polymerization stage. The start-ups and shut-downs of the finishing lines are changeovers with certain set-up times which cause costs (see below). 

In the polymerization stage, the number of events, i.e., of the polymerization starts, N, is given and the events are identified by the index n = 1... N. Start times of polymerizations are represented by continuous variables in e [0, H] in with H denoting the given scheduling horizon. As an initial condition, the first polymerization is defined to start at tn= i = t°. 

The aggregated scheduling problem is subject to uncertainties in the following parameters (1) the capacity of the polymerization stage, i.e., a possibly reduced availability of polymerization reactors due to equipment failures, and (2) the demand profiles. 

The capacity of the polymerization stage limits the number of batches that can be produced in each period i. 

The dried mixed feed is now ready for the polymerization step, and catalysts can be added to the solution (solvent plus monomers) just prior to the polymerization stage or in the lead polymerization reactor. 

Free-radical polymerization processes are used to produce virtually all commercial methaerylie polymers. Usually free-radical initiators tqv > such as a/o compounds or ieroxides are used to initiate the polymerisations. Photochemical and radiation-initiated polymerizations are also well known. At it constant temperature, the initial rate of the hulk or solution radical polymerization of methaerylie monomers is first-order with respect to monomer eoneentration. anil one-half order with respect to the initiator concentration. Methacrylate polymerizations are markedly inhibited by-oxygen therefore considerable care is taken to exclude air during the polymerization stages of manufacturing. 

From the process-design perspective the versatility of applications of PVC demands a precise adjustment of material properties to quality requirements, which are for the most part determined during the polymerization stage, namely by molecular-weight distribution (MWD) and the morphology of particles. 

Copolymerizing the acrylate monomer with a monomer containing two or more polymerizable double bonds. Thereby some of the double bonds should survive the polymerization stage of the butyl acrylate and serve as grafting points in the SAN polymerization [14]. 

It is very important to stir the reaction at high speed during the polymerization stage. A standard household blender is a convenient method of mixing for interfacial polymerization, but should not be used with flammable solvents. 

According to this new postulated mechanism, all results obtained are readily explainable. It is reasonable that all EG was consumed at the beginning of the polymerization stage, and no EG residue is located at the terminal position of the polymer chains. 

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