Elemental analysis answers simple questions with hard numbers. How much sulfur? Is there fluorine, and how much? Any chlorine, bromine or silicon?
Sulfur
Measured by high-temperature combustion with infrared detection of sulfur dioxide. A sulfur-cured rubber shows a few per cent; a peroxide-cured one shows very little. Correct for non-sulfur-powder sources before reporting. See the sulfur correction.
Fluorine
Fluoropolymers are identified by combustion followed by ion chromatography, or by oxygen flask combustion and an ion-selective electrode. Fluorine content (weight per cent) is a key discriminator in FKM.
| FKM family (approximate) | Fluorine content |
|---|---|
| Standard VDF/HFP copolymers | about 66 % |
| VDF/HFP/TFE terpolymers | about 68 to 70 % |
| High-fluorine grades, some peroxide-curable types | about 70 % and above |
| Low-temperature grades with ether monomers | varies, often lower |
| Perfluoroelastomers (FFKM) | above about 72 %, up to the fully fluorinated limit |
Higher fluorine generally means better chemical and fluid resistance and poorer low-temperature flexibility, unless special monomers are used to lower Tg. This is the central trade-off for HPLT fluoroelastomers.
Chlorine and bromine
Found by combustion with ion chromatography or X-ray fluorescence. Chlorine indicates chloroprene, chlorosulfonated polyethylene, chlorinated polyethylene, chlorobutyl, epichlorohydrin or PVC in a blend. Bromine points to bromobutyl or brominated cure sites.
Silicon
Silicon with a strong Si–O infrared signature means a silicone rubber. Silicon in the ash without a silicone polymer points to silica filler.
Nitrogen
Total nitrogen supports the acrylonitrile content estimate in NBR and HNBR. Divide the nitrogen percentage by the nitrogen fraction of acrylonitrile (about 26.4 %) to get acrylonitrile as a percentage of the polymer. Correct for nitrogen in antioxidants and accelerators when they are significant.
A note on safety
Burning halogenated rubber releases corrosive acid gases. The equipment, scrubbing and venting must be designed for it.
