Full definition
Plasticizers are critical additives used in the formulation of rubber compounds, allowing for enhanced physical properties and ease of processing. Typically present in concentrations ranging from 5 to over 30 parts per hundred rubber (phr), these compounds serve to reduce hardness, improve flexibility at lower temperatures, and facilitate the manufacturing process by lowering Mooney viscosity. This reduction in viscosity is particularly beneficial during mixing, extrusion, and calendering operations, enabling better flow characteristics and reduced energy consumption in production. By incorporating plasticizers, manufacturers can also partially replace more expensive polymers, thereby reducing overall material costs while maintaining performance standards.
There are several types of plasticizers available, each with unique characteristics that make them suitable for different rubber types. Paraffinic oils are commonly used with EPDM, IIR, NR, and SBR due to their non-staining properties and general compatibility. Naphthenic oils are versatile and compatible with NR and SBR, providing moderate solvency. Meanwhile, aromatic oils, although offering high solvency and reinforcement for SBR, are restricted under EU REACH regulations due to their polycyclic aromatic hydrocarbon (PAH) content, leading to the development of alternatives such as TDAE, MES, and RAE. Ester plasticizers, including DOP, DIOA, and DOS, are primarily used for NBR and CR, enhancing low-temperature flexibility. Furthermore, synthetic oils like polyalphaolefin and polyisobutylene are utilized for specialty applications requiring specific performance criteria.
When selecting a plasticizer, several criteria should be considered: compatibility with the rubber polymer, desired changes in properties, volatility (as higher molecular weight indicates lower volatility and migration), and adherence to regulatory standards set by organizations such as the FDA and EU. However, excessive use of plasticizers can lead to blooming, a phenomenon where the additive migrates to the rubber's surface, resulting in a decrease in strength and performance. According to ASTM D2226, petroleum oil classification is essential for assessing the suitability of plasticizers in rubber formulations. Additionally, regulations dictate that aromatic oils should not exceed a DMSO extract of 3% as per EU Directive 76/769, ensuring safety and compliance in industrial applications.