Low-field NMR method for studying ABS emulsion polymerization and rubber content

Emulsion polymerization refers to the process where monomers are dispersed in water using an emulsifier and mechanical stirring to form an emulsion, followed by the addition of an initiator to trigger polymerization. When using emulsion polymerization to produce synthetic rubber, in addition to the four main components—monomers, water, emulsifiers, and initiators—other additives such as buffers (to maintain constant pH), activators (to form redox systems), regulators (to adjust molecular weight and suppress gel formation), and antioxidants (to prevent aging of raw and vulcanized rubber) are often added. Industrial examples include emulsion styrene-butadiene rubber (SBR) and polyacrylate emulsions.

The ABS emulsion polymerization process produces latex particles, which are a type of polymer dispersion in water. The process of ABS emulsion polymerization can be studied using low-field nuclear magnetic resonance (NMR), which helps monitor the molecular mobility of the reaction process in real-time.

Heat-resistant ABS resin typically has a heat distortion temperature between 90°C and 105°C. It exhibits good heat resistance, toughness, and fluidity. This material is used in the production of automotive parts, such as door panels and rear wheel covers, as well as in household appliances like microwaves, rice cookers, and hairdryers. The heat resistance of ABS can be enhanced by reducing the rubber content, increasing the molecular weight of SAN (styrene-acrylonitrile), and increasing the acrylonitrile content.

The rubber content in ABS directly affects the material’s physical and chemical properties. Therefore, controlling the rubber content is critical. In industrial production, measuring the rubber content of ABS materials is essential for optimizing processes and controlling product quality, ensuring the quality and stability of the final product.

The relaxation decay of the matrix’s NMR signal is very rapid, typically decaying to zero within microseconds. In contrast, the NMR signal from rubber fillers decays much more slowly, often lasting for tens or hundreds of milliseconds. By appropriately sampling the NMR signal, only the rubber’s NMR signal can be captured, allowing for quantitative measurement. The image below shows the free induction decay (FID) signal detected after a 90-degree pulse. Prior to testing, a standard curve is established to correlate the NMR signal intensity with the rubber content, enabling measurement within 30 seconds to 2 minutes.