Monitoring Styrene Butadiene Rubber: ACOMP Application Note 004

Online monitoring of styrene butadiene copolymerization, including a final coupling reaction

 

Authors:
Michael F. Drenski, Wayne F. Reed, Fluence Analytics
Piotr Kozminski, Terry Hogan, Bridgestone Research Americas (Akron, OH)

 

Abstract

Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) was used to continuously measure the anionic polymerization of styrene butadiene rubber (SBR) at the 20 L scale. The following application note details ACOMP successfully demonstrating that it can track conversion, along with weight average molecular weight (Mw), and low and high shear reduced viscosities (RV). After the initial reaction, the effects of the coupling agent were directly monitored by ACOMP. This showed a doubling in Mw over time and increases in RV commensurate with what one expects for random coils in a good solvent.

ACOMP yields continuous data on these important characteristics of the reaction and resulting polymers. These polymer properties include conversion, molecular weight, reduced viscosity, and shear thinning viscosity. It is important to note that ACOMP is not a chromatographic method. Thus, it yields continuous average values, such as Mw, rather than the intermittent molecular weight distributions (MWD) provided by GPC. The work below, conducted in conjunction with Bridgestone Research Americas, outlines results obtained from the application of ACOMP to SBR reactions.

This work outlines results obtained from the application of ACOMP to SBR reactions.

Reaction information on styrene butadiene rubber

The styrene butadiene rubber reaction monitored was a conventional anionic polymerization of styrene (127.01 grams, 1215.3 mmol) and butadiene (508.02 grams, 9390 mmol) performed in hexanes at 14 wt% monomer using a catalyst of n-BuLi (4.4602 mmol) and ditetrahydrofurylpropane (2.933 mmol). After the polymerization was complete, to generate the base product, tin tetrachloride (1.100 mmol) was added to the reactor to couple the polymer chains.

Results

Figure 1 shows raw detector signals for UV, SLS, and high and low shear viscometers, corresponding to the left-hand y-scale.

The UV signal in Figure 1 does not return to its solvent value, due to scattering and absorption by the polymer that has formed. The true fractional conversion, f, is found by a procedure developed by Fluence Analytics, and the true conversion found by this procedure is shown on the right-hand y-axis. The signals in Figure 1 are all normalized to a scale of 1. The decay of the UV 265 nm signal shows the conversion of the styrene into polymer. The light scattering at 90o increases with respect to the increasing polymer mass, and it reaches a plateau during the first phase. When the reaction begins, both viscosity signals increase until a plateau is reached. The addition of the coupling agent causes strong increases in both viscometers.

Figure 1
Figure 1: Raw ACOMP detector signals and corresponding fractional conversion
Figure 2
Figure 2: Mw and low and high shear RV from the data of Figure 1

Figure 2 shows Mw and low and high shear RV as a function of time, including the coupling reaction. Mw was obtained from angular extrapolation (five angles; 45, 65, 90, 115, 135) to zero angle. Figure 2 reveals several features. First, Mw and both low and high shear RV increase monotonically in the first phase. In contrast, in free radical reactions, chains are initiated, propagate, and terminate quickly, so that Mw decreases versus time. Examining the first and second plateau values gives Mw,2/Mw,1=1.94 +/- 0.06. Similarly, high and low shear RV increase in the first phase, again exhibiting ‘living’ type behavior of the reaction and increase after coupling.

Two sample aliquots were taken before and after the coupling agent was added to the reactor for characterization by conventional GPC standard calibration. The elugrams of each sample revealed two peaks of low and high molecular weight. The base product, shown in Figure 3, was determined to have a Mw of 132,000 g/mol for the low molecular weight peak and 300,000 g/mol for the high molecular weight peak.

Figure 3
Figure 3: Elugram of Base product before coupling agent

Conclusion:

ACOMP has proven the capability of directly monitoring and characterizing polymer conversion, Mw, and RV of the polymers produced during a living, anionic solution based SBR polymerization process. This information is directly important with respect to characterizing the macromolecular polymer properties of Mw and RV, and it is also useful in providing insights into production rates and efficiencies, such as improving cycle time and yields while achieving product consistency from batch to batch. This information also provides complementary information on the polymers produced which may lead to further correlations to important rheological properties of polymer end products such as Mooney and tanδ. The ultimate goal of utilizing ACOMP data is to monitor and control polymer manufacturing at the industrial scale, leading to improved production efficiencies and minimum off-specification product.

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