By having both a NIRS measurement and a dynamic viscosity measurement, process engineers can make informed decisions based on real-time trends of polymer building, monomer reduction, and weight-average molecular weight. Additionally, the dual probe configuration allows both measurements to occur under the same localized flow and sample conditions. In other words, if the viscometer and NIRS probes were installed on separate flanges, in different locations on the reactor, then transient flow within the reactor could introduce a bias between the two techniques.

Role of PAT in Polyurethane Production

Polyurethanes, like other similar step-growth polymerizations, are usually produced in two-step processes. In the first step, polymer materials of low average molecular weights (prepolymer) are produced through the reaction of a polyol and a large excess of diisocyanate, typically using a feed molar ratio of 2:1. NIR Spectroscopy can measure the number of reactive hydroxyl groups (OH) on the polyol. The OH number directly impacts the number of urethane linkages, which greatly influences the physical properties of the final polyurethane product. OH Number is therefore, an important parameter to monitor and control during polyol production. Additionally, the laboratory method that is commonplace for hydroxyl number determination is both time-consuming and involves the use of hazardous materials. An In situ NIR transmission probe has faster throughput and reduces occupational exposure to the hazardous materials required for offline testing.

• In the second step, the polymer chain is extended through the reaction of the prepolymer with a diol or polyol of low molecular weight (chain extender). Normally, the main objective is the production of polymer resins of large molecular weights at the end of the second reaction step. To achieve this objective, some secondary objectives should be pursued.
• First, monomer conversion and composition should be tightly controlled during the first reaction step. Second, the amount of polyol fed into the reaction vessel during the second step should be rigorously controlled. 
• These secondary control objectives are required to avoid monomer imbalance, which may lead to the production of polymer of low molecular weight and eventually cause the loss of the batch product.
• Finally, the evolution of weight average molecular weights 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 weights and other parameters during the chain extension step of polyurethane synthesis.

Purposed Process Viscometry and Spectroscopy Control Scheme for Polyurethane Synthesis

The control logic shown below can be adapted as needed to meet the specific product requirements. During the first step, process spectroscopy is used to monitor the monomer conversion and weight average molecular weight. Other parameters such as concentration or ratio of diisocyanates, excess water concentration, or ratio of glycol reaction products can also be measured with process spectroscopy.