Properties Related to the Polymer Structure

Essentially, the high thermal and chemical stability of fluorocarbon elastomers, as of any fluoropolymers, is related to the high bond energy of the C-F bond, and to the high bond energy of the C-C and C-H links, caused by the presence of fluorine [6]. 

Copolymers of VDF and HFP, completely amorphous polymers, are obtained when the amount of HFP is higher than 19% to 20% on the molar base [7]. The elastomeric region of terpolymers based on VDF/HFP/TFE is defined by the monomer ratios. Commercially, VDF-based fluorocarbon elastomers have been, and still are, the most successful among fluoroelastomers [8]. The chemistry involved in the preparation of fluorocarbon elastomers is discussed in some detail in Chapter 2. 

Swelling resistance of fluoroelastomers is directly related to the fluorine content in the molecule. This is demonstrated by data in Table 5.3 [9]. For example, when the fluorine content is increased by mere 6% (from 65% to 71%), the volume swelling in benzene drops from 20% to 3%. Copolymers of VDF and HFP have excellent resistance to oils, fuels, and aliphatic and aromatic hydrocarbons, but they exhibit a relatively high swelling in low-molecular-weight esters, ketones, and amines, which is due to the presence of the VDF in their structure [10]. VDF-based fluoroelastomers 

Note HFP, hexafluoropropylene PMVE, perfluoromethylvinyl ether CTFE, chlorotrifluoroethylene P, propylene HPFP, hydropentafluoropropane VDF, vinylidene V) 

Note FKM, fluorocarbon elastomer VDF, vinylidene fluoride HFP, hexafluoropropyl-ene TFE, tetrafluoroethylene CSM, cure site monomer PMVE, perfluoro-methylvinyl ether. 

---