Products made from either natural or synthetic rubber are a vital part of everyday living. Synthetic rubbers offer superior thermal stability and resistance to oxidizing agents and oils. One synthetic rubber uses Bromobutyl (BIIR), a copolymer of isobutylene and small amounts of brominated isoprene that provides unsaturated vulcanization sites. Bromobutyl rubber is derived from halogenating butyl rubber with bromine in a continuous process. This elastomer has many of the attributes of butyl rubber, but the addition of bromine improves adhesion to other rubbers and metals, resulting in substantially faster cure rates (i.e., lower amounts of curative agents are required).

Usually, the determination of the bromine content and other quality parameters (e.g., Mooney viscosity, volatile content, calcium stearate content, and functional bromide) requires various reagents and time-consuming analytical methods. However, near-infrared spectroscopy (NIRS) offers rapid and reliable simultaneous quantification of those parameters in Bromobutyl rubber without the use of chemicals.

A total of 68 samples of Bromo Isobutylene Isoprene rubber (BIIR, Bromobutyl rubber) were collected to create a prediction model for quantification of several quality control parameters including Mooney viscosity, bromine content, volatile matter content, calcium stearate content, and functional bromide. All samples were measured with a Metrohm NIRS DS2500 Liquid Analyzer (400–2500 nm, Figure 1) in transmission mode with an 8 mm sample holder. Reproducible spectrum acquisition was achieved using the built-in temperature control set at 50 °C. For convenience, disposable vials with a pathlength of 8 mm were used, which made cleaning of the sample vessels unnecessary. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.

All measured Vis-NIR spectra (Figure 2) were used to create a prediction model for quantification of the key quality parameters of BIIR. The quality of the prediction model was evaluated using correlation diagrams which display a very high correlation between the Vis-NIR prediction and the reference values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figures 3–7).

This Application Note demonstrates the feasibility to determine multiple key parameters for the quality control of Bromobutyl rubber with NIR spectroscopy. Vis-NIR spectroscopy enables a fast alternative with high accuracy, and therefore represents a suitable alternative to the standard methods (Table 2). No chemicals are required with near-infrared spectroscopic analysis, and cleanup is quick and easy when using disposable sample vials as shown in this study.

Internal reference: AW NIR CN-0019-112022