Determination of Binder Content Using Low-Field Nuclear Magnetic Resonance (LF-NMR)

During the mixing process, rubber macromolecules form strong chemical and physical bonds with the surface of active fillers such as carbon black. As a result, part of the rubber becomes tightly bound to the surface of the carbon black particles and can no longer be dissolved in organic solvents. This portion is known as bound rubber.

The formation of bound rubber helps promote the breakdown and even dispersion of carbon black agglomerates during mixing. However, if too much bound rubber is generated too early—before the carbon black–rubber clusters have fully dispersed—it may form a stiff coating around the agglomerates, making them harder to break down. This is especially true for high-unsaturation diene rubbers, particularly natural rubber. Therefore, during early mixing, conditions must be carefully controlled to avoid excessive temperature rise and limit premature bound rubber formation.

Determining bound rubber content has long been a challenge in the industry. Traditional chemical methods have low accuracy and are prone to subjective errors. With NMR technology, the relaxation decay curve of elastomer materials varies with internal composition. This allows bound rubber content to be measured quickly and non-destructively using NMR relaxation techniques.

The relaxation decay curve of elastomers changes according to the internal state of the sample. Low-field NMR takes advantage of the fact that different components within the rubber exhibit different relaxation times. By analyzing these variations, the bound rubber content can be quantified.

Low-field NMR places no specific requirements on sample shape or color—any sample that fits within the detection probe can be measured. The technique offers fast and accurate quantification of bound rubber content without the need for destructive preparation.