## 4 Tested Seal Materials and Tested Fluids
### Tested Seal Materials  
As seal materials, materials that are used today for modern seals operating under demanding operating conditions were used: FPM/FKM, POM, HPU Franc, NBR, HPU USI, EPDM, MVQ and PTFE I. Five samples of each material were made to ensure the reproducibility of the test results. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test Time**} & **Maximum** & **Maximum** & **Hardness** & **Maximum** \\  & **Volume Swell** & **Volume** & **Change Shore A** & **Tensile Strength** \\  & **[\%]** & **Shrinkage [\%]** & **[Points]** & **Change [\%]** \\ \hline
24 & 15 & \(-\)3 & \(\pm\)7 & \(-\)20 \\
70 & 15 & \(-\)3 & \(\pm\)7 & \(-\)20 \\
100 & 15 & \(-\)3 & \(\pm\)8 & \(-\)20 \\
250 & 15 & \(-\)4 & \(\pm\)8 & \(-\)20 \\
500 & 20 & \(-\)4 & \(\pm\)10 & \(-\)25 \\
1000 & 20 & \(-\)5 & \(\pm\)10 & \(-\)30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Recommended property changes’ limits for determining the compatibility of elastomer seals for industrial hydraulic-fluid application [11]. Fluorocarbon FPM/FKM (FPM designation according to ISO Standard, and FKM according to ASTM). The base material is the same in both cases: fluoro rubber, also known as Viton(r). This material it is used for U-shaped seals (lip seals), for wipers on piston rods and for other special seals. FPM/FKM is highly resistant to high temperatures and many chemicals and is suitable for use in various weather conditions. Regarding hydraulic fluids, it is compatible with sulphur-containing mineral oils and HFD-type hydraulic fluids, and almost with all phosphate esters and chlorinated hydrocarbons. According to the manufacturers, it is not resistant to anhydrous ammonia, amines, ketones, esters, hot water and low-molecular organic acids, which is good to know when using new types of hydraulic fluids. Polyoxymethylene POM is a polymer of extremely high strength, suitable for use in bearings, bushings and support rings and other applications in a wide range of engineering. POM is characterized by high strength, hardness, good sliding properties with low friction and the retention of dimensions down to very low temperatures (\(-40\)\({}^{\circ}\)C). It is characterized by a very stable crystal structure that enables high heat resistance, as well as excellent wear and solvent resistance. In the field of hydraulics, it is therefore used in hydraulic cylinders for support and guide rings. The thermoplastic polyurethane elastomer HPU(r) has a high resistance to hydrolysis, which makes it stable in water and in mineral oil. For this reason, it is used for both water-based hydraulic fluids used in mining, tunnelling equipment and presses (HFA- and HFB-type fluids). It is also suitable for use in cases of faster degradable hydraulic fluids of the HE type (vegetable oils and synthetic esters) and for equipment in the food industry. Nitrile butadiene rubber NBR is very popular and is also the most widely used elastomer for the manufacture of seals due to its excellent resistance to petroleum derivatives and its ability to be used in a wide temperature range. It is actually a complex family of unsaturated synthetic rubber copolymers of acrylonitrile (ACN) and butadiene, with both physical and chemical properties depending on the percentage of acrylonitrile in the base polymer. From the point of view of hydraulic fluids, NBR is generally suitable for use with all types of hydraulic fluids, for hydraulic mineral oils, fire-resistant fluids of the HFA, HFB and HFC types, silicone oils, oils of animal and vegetable origin and also for water, both hot and cold. Ethylene Propylene Rubber EPDM is an elastomer based on ethylene-propylene-diene rubber, which is used for various forms of seals in the field of hydraulics, mainly for static seals, but also as dynamic seals, but less often. It has good durability in the case of using hydraulic fluids based on phosphate ester and hydraulic fluids based on water. However, it is not resistant to mineral oils and other non-polar liquids.