Emphasizing Compounding Requirements for Low Temperature and Low Compression Set Applications Based on NBR, CR, CSM, ACM, AEM, and FKM
Synthetic specialty rubbers are extensively utilized across a vast range of applications through various processing methods. The structure-property relationship plays a crucial role in governing their processing and applications. Within the same ASTM-classified rubber, manufacturers design various grades to fulfil specific applications. Special attention is required in selecting materials and additives to meet the specific requirements of formulations and compound designs.
Key properties such as molecular weight, molecular weight distribution, low-temperature properties, and compression or tension set are significantly influenced by structure-property relationships and the additives used in compounds. Understanding the effect of compounding variables on viscosity, elasticity, and flow is essential for better compound design. This chapter provides an exhaustive discussion with practical compounding guidelines, focusing on NBR, CR, CSM, ACM, AEM, and FKM rubbers.
Pranab Kumar Bhattacharyya
Founder – AI Chat Rubber Polymers, Pune, Maharashtra, India
Introduction
Synthetic specialty rubbers are extensively used across a vast range of applications through various processing methods. The structure-property relationship plays a crucial role in governing their processing and applications. Within the same ASTM-classified rubber, manufacturers design various grades to fulfil specific applications. Special attention is required in selecting materials and additives to meet the specific requirements of formulations and compound designs.
Key properties such as molecular weight, molecular weight distribution and low-temperature properties are significantly influenced by structure-property relationships and the additives used in compounds. Understanding the effect of compounding variables on viscosity, elasticity, and flow is essential for better compound design. This chapter provides an exhaustive discussion with practical compounding guidelines, focusing on NBR, HNBR, CR, CSM, ACM, AEM, and FKM rubbers. in mixing, compression moulding , injection moulding, extrusion, and calendering. The chapter lies in discussion on effect of these fundamental properties like viscosity a measure of molecular weight on gas permeability. Solubility, adhesion strength of adhesives, green strength n of extrudate , mould shrinkage and Die swell, migration, and physical properties like Tensile, elongation , abrasion on the vulcanisate,
It is understood that, for any individual polymer, the value of a material property is a function of molecular weight at low molecular weights but attains a plateau value at some ‘high’ molecular weight which varies with polymer type. It is also usually presumed that the transition from dependence to independence of property on molecular weight corresponds to the transition from non-brittle to brittle behaviour for an inherently flexible material. Most properties are presumed to be independent of molecular weight at higher molecular weight.
Low temperature properties are crucial when developing components with specialty elastomers like NBR, ACM, and Viton. Elastomers exhibit three types of low-temperature behaviour: crystallization, thermal stiffening, and glass transition.
- Crystallization: This occurs due to the reorientation of the molecular structure at low temperatures and is time-dependent. As the elastomer cools, its molecules align into a more ordered structure, which can affect the material’s properties over time.
- Thermal Stiffening: This is an almost instantaneous hardening of the material, defined to occur when the material’s modulus reaches 10,000 psi (68,900 kPa). For instance, thermal stiffening temperatures for natural rubber and neoprene are approximately -62°F (-52°C) and -37°F (-38°C), respectively.
- Glass Transition: The glass transition temperature is the point at which the material loses its viscous, rate-dependent characteristics, leading to a brittle state where the elastomer can no longer absorb shock effectively. Below this temperature, rubber materials may shatter upon impact.
Low temperature applications must meet a wide range of standards covering versatile applications beyond international automotive standards and EN standards for aerospace industries. Even, low temperature performance is essential for exporting joint gaskets to certain countries with specific climatic conditions.
This chapter focuses on selecting synthetic specialty elastomers for low temperature applications. It includes an in-depth discussion on how blends, fillers, plasticisers, and cure systems influence the low temperature properties of rubbers such as NBR, HNBR, ACM, AEM, and FKM. This information will aid technologists in designing effective rubber compounds for low temperature environments.
Contents covered in this chapter include:
- Molecular Weight and Polydispersity: Understanding the molecular weight and Molecular Weight Distribution (poly-dispersibility) of polymers.
- Viscosity average Molecular weight and its laboratory measurement, relationship of Molecular weight and Solution Viscosity of Elastomer.
- Mooney viscosity versus Molecular weight and Comparative result of Mooney Viscosity among Massed and Unmassed CR, NBR, SSBR, ACM in a laboratory test.
- The advantages and disadvantages of Molecular weight and Molecular Weight Distribution in Compression, Injection, and Extrusion processes, with emphasis on NBR, ACM, and CR rubber.
- Influence of Mooney Viscosity on Gas permeability, the Migration ability of additives, solubility, and physical properties like Tensile, Elongation, Abrasion, and Tear resistance.
- Specific Volume and Glass Transition Temperature: Significance of these properties on elastomeric components.
- Low-Temperature Properties: Understanding TR10 and brittleness temperature of rubber and their behaviour in different types of rubber components.
- Influence of Grades, Fillers, Plasticizers, and Cure Systems: Selecting the appropriate grades and additives for low-temperature applications based on NBR, HNBR, ACM and FKM rubber.
- Special emphasis is placed on the aerospace application of NBR to satisfy EN specifications at different hardness.
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