Prepolymer reactor

For the next stage, the concentrated lactic acid is transferred into a prepolymer reactor. The prepolymer reactor is actually a second evaporator, which further removes water from the lactic acid. At the same time, the condensation polymerization takes... 

The prepolymer is separated from the water by spray drying and then formed into cylindrical pellets of uniform size (159). At this point additives can be added to the porous pellets from solution or suspension. These pellets are then placed in a soHd-phase condensation reactor where they are heated to 260°C for up to 4 h under nitrogen, with a small amount of water added. The pressure is maintained close to atmospheric pressure. At the end, x > n. 

The isocyanurate reaction can be both beneficial and troublesome. It can be the bane of production engineers. Low levels of alkaline impurities present in urethane raw materials such as polyols, tackifiers, etc., can cause problems in prepolymer production, resulting in high viscosity products at levels of 5 ppm or less. At higher levels of alkaline impurities, more serious problems can occur, including poor shelf life, poor caulkability, or poor sag resistance. At levels of 15 ppm or higher, the alkalinity can cause an isocyanurate reaction in a prepolymer that can result in a gelled reactor. 

Continuous polymerization processes for PA-6,6 have been reported for over 30 years.5,6,28 Prepolymerization in tubular (Fig. 3.21) or baffled reactors is particularly well suited to continuous polymerization. The polymerization of prepolymers to high-molecular-weight materials in a continuous process is more difficult to control as small differences is molecular weights result in large differences in viscosities. Viscosity differences result in different hold-up times in die reactor and thus nonhomogeneous products. 

The precondensation can be earned out continuously with the use of a tubular reactor at a temperature of 290-310°C.56 The tubular reactor is a 4-m-long coiled pipe with a diameter of 4 mm which is heated at 300°C. At the end of the pipe is a valve which is regulated so that the pressure is 1.5 bar. The residence time in the pipe is only seconds. The prepolymer obtained can be postcondensed in the solid state to a high molecular weight. 

At higher water concentrations (at higher pressure) (Fig. 3.24), the reaction is considerably more rapid. The prepolymer obtained is further polymerized in another reactor, which has a working pressure of 1 bar or less. The total reaction time of the prepolymerization can be considerably shortened by processing it in an autoclave.812 28 In a laboratory, PA-6 can be synthesized in several ways from m-aminocaprolic acid, from lactam and from lactam and water, and anionically. 

In typical industrial operations, TPA is not dissolved in EG or BHET but in prepolymer. The latter contains PET oligomers with one to approximately six to eight repeat units and a significant concentration of carboxyl end groups of between 200 and llOOmmol/kg. It was found [94] that the solubility of TPA in prepolymer is much higher than indicated by the values given in the literature. Nevertheless, the esterification reactor still contains a three-phase system and only the dissolved TPA may react with EG in a homogenous liquid-phase... 

The monomers TPA and EG are mixed upstream to the esterification reactor in a jacketed slurry preparation unit equipped with a stirrer for highly viscous fluids (e.g. Intermig ). The typical molar ratio of EG to TPA lies between 1.1 and 1.3. The esterification temperature and the molar ratio of monomers are the main controlling factors for the average degree of polycondensation of the esterification product (prepolymer), as well as for its content of carboxyl end groups and DEG. The latter mainly occurs as randomly distributed units of the polymer molecules. 

The esterification reactor is usually not emptied completely after a batch is finished and a small amount of prepolymer is retained in the reactor. The reason for this is the solubility of TPA in EG and BHET, as discussed earlier in Section 3.1. During operation, the batch-wise prepared slurry is fed continuously into the esterification reactor while the esterification is already proceeding. For a significant part of the process time, the batch esterification reactor is operating semi-continuously. 

The feed silo for the prepolymer, located at the top level of the plant, allows the reactor to be filled via gravity. Finally, a blending silo is needed to maintain a homogenous and constant quality with respect to the intrinsic viscosity of the product. 

The activity of Ti catalysts in SSP depends on the kind of stabilizer fed into the reactor. In the production of PET, phosphorous-containing chemicals are commonly added as stabilizers. These products improve the thermal stability, particularly in processing, which results in reduced degradation and discoloration and are therefore of importance with respect to quality. Such materials are added during the production of the prepolymer. These stabilizers are mainly based on phosphoric or phosphonic (phosphorous) acids or their esters. 

Oil-Based SINs. The SINs produced were based on a castor oil polyester-urethane and styrene crosslinked with 1 mole percent of technical grade (55%) divinyl benzene (DVB) (7). This structure may be written poly[(castor oil, sebacic acid, TDI)-SIN-(Styfene, DVB)], poly[(CO,SA,TDI)-SIN-(S,DVB)]. Benzoyl peroxide (BP) (0.48%) was used as the free radical initiator for the styrene and 1,4-tolylene-diisocyanate (TDI) was used as the crosslinker for the polyester prepolymer. A 500 ml resin kettle equipped with a N inlet, condenser, thermometer, and high torque stirrer was used as the polymerization reactor. 

SIN s were prepared by adding the proper amount of oil prepolymer to the reactor and heating to 80°C. The styrene/DVB/BP mixture was charged to the reactor followed by purging with gas for 10 minutes. Next, the required amount of TDI needed to react with the remaining hydroxyl groups was added. The TDI crosslinks the reactive oligomers into a three-dimensional network. The reaction was carried out under a flow of N gas of 40 cm /min and the temperature maintained at 80°C. 

The process begins in a prepolymerizer, which is a water-jacketed reactor with a mixer in it. See Figure 23—12.) The styrene is partially polymerized by adding the peroxide initiator and heating to 240—250°F for about four hours. About 30% of the styrene polymerizes and the reactor contents become syrupy goo. Thats about as far as the prepolymer step can go—30% conversion— because the mixing and heat transfer gets very inefficient as the goo gets thicker, and the polymerization becomes hard to control. 

A reactor was charged with poly[(R)-3-hydroxybutyrate], diethylene glycol, dibutyltin dilaurate, and diglyme and then heated overnight and the telechelic hydroxylated prepolymer isolated and used without further purification. 

Figure 4.2. Flowsheet of production of cast polyurethane elastomer articles from a prepolymer by the continuous method 1 - vessel to store prepolymer 2 - vessel to prepare prepolymer 3 - reactor for curing agent 4 -transfer pump 5 - metering pump 6 - mixing device 7- mold.

When preparing a batch of material, the reactor and the prepolymer are hot. Suitable heat-resistant gloves must be worn. It may be necessary to wear a pair of impervious gloves under the heat-resistant gloves to give both chemical and heat protection. 

Because of the physical size of the reactor, it is normally mounted with the top of the reactor at a mezzanine floor level. This allows easy access to the inlet ports. The reactor can be mounted at this point on a number of load cells. The load cells must be of such capacity to take the whole weight of the reactor when it is full of prepolymer. In this case, all connectors to the reactor must be flexible. 

Keeping the walls clean of buildup is important, because buildup would cause heat transfer problems as the production proceeds. The polyurethane prepolymer builds up in viscosity as the chain length increases and tends to hang onto the wall of the reactor. The heat of the wall will increase the speed of the reaction and hence increase the reaction rate and viscosity. Viscosities in the reactor will be generally between 30 and 4,000 MPa.s at a temperature of approximately 90°C. 

If a pump is fitted to the discharge of the reactor, it can be used to fast-circulate the hot prepolymer through cooling tubes to rapidly reduce the temperature. When the temperature probes indicate a too-rapid temperature rise, the addition of polyol must be slowed down immediately. If the temperature still rises too quickly, the addition of the polyol must be stopped until the temperature is brought back under control. The temperature probes must be positioned so that the temperature of the prepolymer indicated is unaffected by the wall temperature of the reactor. A second thermocouple positioned elsewhere in the reactor can be used to detect runaway reactions. 

Depending on the smoothness of the reactor and heat uniformity, solid polyurethane is sometimes formed and may be discharged with the material. These lumps must be removed by passing the material through a free-flowing filter. The filter material must be resistant to both the temperature and prepolymer. Polypropylene filter material has been successfully used. The filters must be cleaned after use. 

The reactor needs to be kept clean to keep the heat transfer optimal and to prevent solid material in the prepolymer. The method employed is to use an appropriate solvent such as methyl ethyl ketone (MEK), methylene chloride, or m-pyrol (NMP). To prevent an explosive vapor mixture from being formed when the solvent is added to the reactor, the air must be replaced by nitrogen gas. The solvent needs to be heated to just above its boiling point and kept there until the solid material has been softened and removed from the metal. A second rinse with clean solvent may be needed. 

In production units where polyurethane prepolymers are used, liquid nitrogen is an option to provide an inert atmosphere in the reactors. The unit should be set up professionally by a supplier and the liquid nitrogen unit properly maintained. 

The prepolymer may be finished in the melt phase with UIF s DISCAGE reactor or in a solid-stating unit to obtain the required end-product features. 

A level of agitation above the minimum shear can be used to reduce the rubber particle size. However, agitation is not always effective in reducing the rubber particle size (see section 5.2) and there is also an upper limit based on hardware limitations (torque). Another aspect that plays a role related to shear is the feed rate. Increasing the feed rate means that the residence time in the reactor(s) is shorter. A lower amount of shear is transferred to the prepolymer, giving in general larger rubber particles in the end product. 

A reactive extruder may be considered to be a horizontal reactor with one or two internal screws for conveying reactant polymer or monomer in the form of a solid or slurry, melt, or liquid. The most common reactants are polymer or prepolymer melts and gaseous, liquid, or molten low molecular weight compounds. 

The slurry polymerization process with prepolymer has been scaled up from bench-scale (100 grams) to pilot-plant operation (0.5-ton batches) without major difficulties. An important aspect of this process is the absence of reactor fouling. 

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