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In gradual growth polymerization, the degree of polymerization strictly depends on the conversion rate and is very sensitive to impurities and molar imbalance, especially when the conversion rate is high.
The monomer imbalance may prevent the formation of long polymer chains (resulting in molecular weight stagnation) and promote the occurrence of undesirable secondary reactions (resulting in chain branching, gelation, and/or chain degradation).
The development and implementation of programs for online monitoring and control of conversion rate and weight average molecular weight (Mw) are very important to avoid gel formation, early Mw stagnation, and polymer chain degradation.
Online process measurement tools such as near infrared process spectroscopy (NIRS) and rheology (including viscosity measurement) are ideal for determining the physical properties of polymers.
Guided Wave and Sofraser have collaborated to develop a dual measurement system that can be mounted on a single shared flange, which can be mounted directly on the side of the reactor for real-time process control.
Figure 1. Double probe flange developed by Guided Wave and Sofraser. Allows real-time viscosity and NIR measurements at the same location in the reactor. Image Source: Guided Wave
By performing NIRS measurements and dynamic viscosity measurements at the same time, process engineers can make informed decisions based on real-time trends in polymer construction, monomer reduction, and weight average molecular weight. In addition, the dual-probe configuration allows two measurements under the same local flow and sample conditions.
In other words, if the viscometer and NIRS probe are mounted on separate flanges at different locations in the reactor, the transient flow in the reactor may introduce deviations between the two technologies.
Polyurethane, like other similar step-growth polymerizations, is usually produced in a two-step process.
In the first step, a low-average molecular weight polymer material (prepolymer) is produced by reacting a polyol with a large excess of diisocyanate, usually with a 2:1 feed molar ratio. Near-infrared spectroscopy can measure the number of active hydroxyl groups (OH) on polyols.
The number of OH directly affects the number of urethane bonds, which greatly affects the physical properties of the final polyurethane product. Therefore, the OH value is an important parameter to be monitored and controlled during the production of polyols.
In addition, the laboratory methods commonly used for hydroxyl value determination are time-consuming and involve the use of hazardous materials. In-situ NIR transmission probes have faster throughput and reduce occupational exposure to hazardous materials required for offline testing.
In the second step, the polymer chain is extended by the reaction of the prepolymer with a low molecular weight diol or polyol (chain extender). Usually, the main goal is to produce a high molecular weight polymer resin at the end of the second reaction step. In order to achieve this goal, some secondary goals should be pursued.
First, the monomer conversion rate and composition should be strictly controlled in the first reaction step. Secondly, the amount of polyol added to the reaction vessel in the second step should be strictly controlled.
These secondary control objectives are to avoid monomer imbalance, which may lead to the production of low molecular weight polymers and ultimately the loss of bulk products.
Finally, the evolution of the weight average molecular weight should be accurately monitored by process viscometry and spectroscopy. The output of these process monitoring tools can then be used to control the weight average molecular weight and other parameters during the polyurethane synthesis chain extension step.
The control logic shown below can be adjusted as needed to meet specific product requirements. In the first step, process spectroscopy is used to monitor monomer conversion and weight average molecular weight.
Other parameters, such as the concentration or ratio of diisocyanate, excess water concentration, or the ratio of ethylene glycol reaction products, can also be measured using process spectroscopy.
Figure 2. The above flow chart shows how the combined measurement of process viscometry and spectroscopy can be used to control gradual growth copolymerization. Image Source: Guided Wave
During the second step of the reaction, the average polymer molecular weight was monitored by process spectroscopy and viscometry. According to the trend of molecular weight, the feed rate of reactant is adjusted or metered based on NIR and MIVI real-time data.
Double measurement is the best way to control the polymerization rate to meet the specification endpoint.
It should also be noted that as the monomer conversion rate increases, the accuracy of the average molecular weight measured by the NIR analyzer (such as the NIRO full spectrum analyzer) begins to decrease.
Therefore, in the final stage of crosslinking and chain extension, a process viscometer (such as MIVI) is used to more accurately measure the average molecular weight.
The onset of gelation can be determined by process spectroscopy and viscometry. If NIR determines that monomer conversion has occurred without a change in average molecular weight, gelation may have occurred.
Viscosity measurement can be used as a secondary confirmation of gelation. If gelation occurs, the viscometer will measure sudden disturbances due to agglomeration or branching of the polymer.
The dual-probe method allows process engineers to react quickly to the gel and start adding inhibitors to slow down the cross-linking speed. The concentration of the inhibitor, such as hydrochloric acid, can be measured by process spectroscopy.
Once the reaction trend returns to normal, the amount of inhibitor can be reduced and the metered feed rate of reactants (such as 1,4 butanediol) can be increased. The process control scheme allows the process engineer to guide the reaction to the desired molecular weight trajectory and ensure that the product meets the specifications.
Once the target molecular weight is reached, process engineers can transfer the polyurethane that meets the specifications to any post-molding processing, such as extrusion.
Bulk polymer production itself is a fairly conventional chemical process. However, in order to optimize production and therefore reduce manufacturing costs, real-time process analysis is required.
Both near-infrared process spectroscopy and viscometry, including viscosity measurement, are ideal for determining the physical properties of polymers.
The development of modern process monitoring tools can directly measure chemical reactions in real time, such as monomer conversion, molecular weight, acid value, and even the formation of side branches and gels.
Dual probes and subsequent process control schemes can be used to increase the yield of batch copolymerization. The probe was designed by Elementale (Texas, USA) and was developed in cooperation with Guided Wave and Sofraser.
This information is derived from materials provided by Guided Wave and has been reviewed and adapted.
For more information on this source, please visit Guided Wave.
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