1.0. MATERIALS (COMPOUNDS) USED FOR HYDRAULIC AND PNEUMATIC SEAL PRODUCTION
1.1. Plastomers (Thermoplastics)
These materials with macro molecular structure are called thermoplastics or engineering plastics.
On examination of the molecular architecture, it can be seen that, unlike elastomers, there are no cross links present. Although this product is resilient under system pressure, that is to say it is able to return to its original form, its performance is lower compared to elastomers.
Polyacetal, polyamide and PTFE (Teflon) materials are generally used plastomers in sealing elements.
1.1.1. Polyacetal (POM) (Polyoxymethylene)
These are used as back-up rings or guidening element materials in hydraulic and pneumatic systems.
There are two types of POM, filled with or without fibreglass, depending on their use. Despite working temperatures ranging from -40°C to +140°C, real dimensions can not remain
stable in excess of 100°C.
It can be used safely with mineral oils and HFA, HFB type oils. Types with added fibreglass provide longer lasting contact pressure resistance.
1.1.2. Polyamide (PA6, PA6, 6)
This material can be used at peak temperature of 140°C and can retain its dimensions between -40°C and 100°C.
Like Polyacetals, Polyamids can be used as back-up rings and guidening elements in Hydraulic-Pneumatic cylinders. They are available with or without fibreglass.
1.1.3. PTFE (Teflon)
In 1938, in the DUPONT Laboratories, Dr. Roy J. Plunket Freon happened upon a white waxy substance whilst carrying out some tests. On further analysis, the culmination of the research revealed that the substance contained important qualities for use in industry.
In production technology, PTFE, whose production technique resembles that of powder metallurgy, in its powdered form is moulded as rods or tube and sintered in owen. However, as
a result of technological progress over the last 15-20 years, injection and extrusion methods are used on materials such as thermoplastics, which have become less popular, and therefore less widely used.
Teflon can be used as virgin, or used within fillers in order to improve mechanical performance. The fillers’ effects on PTFE are to improve extrusion resistance, reduce friction and form changes under extreme heat conditions. In addition, it increases resilience to attrition and hardening.
The most important fillers are;
-Fibreglass,
-Carbon-graphite,
-Colouring pigments,
-Bronze and molybdenum disulfide.
1.1.3.1. Fibreglass
Fibreglass filler provides chemical resistance in a wide range of working temperatures. It is especially resistant to powerful alkaline solutions and hydrofluoric acids.
1.1.3.2. Carbon-Graphite
It improves resistance to wear and hardness. Teflon filled with Carbon-Graphite prevents quick wear on tool inserts during machining and provides high pressure resistance.
1.2.5. Neoprene (CR)
CR is a cholorobutadiene polymer. It is resistant to air, ozone and flame. It has good resistance to mineral oils with high aniline points and greases. Thus, it is used in applications which need to be resistant both to oils and air and ozone at the same time.
For example; rod and rod end bellows in automotive sector are produced from neoprene materials. Working Temperature Range: -45°C to +105°C.
1.2.6. Ethylene Propylene Rubber (EPDM)
It has excellent resistance to phosphate, ester fluids, automotive break fluids, water and steam. Working Temperature Range: -40°C to +145°C
1.2.7. Styrene Butadiene Rubber (SBR)
It has good resistance to glycol based automotive break oils, inorganic acids, base chemicals and alcohol. Often used with other elastomers e.g. natural rubber. Working Temperature Range: -50°C to +100°C
1.2.8. Polyurethane (PU)
PU is moulded with injection techniques. It has excellent mechanical properties such as high tensile strength, wear resistance, tear strength, and extrusion resistance. It offers a good resistance to air, ozone, mineral oils, grease and aliphatic hydro carbons. Working Temperature Range: -30°C to 100°C.
1.2.9. Natural Rubber (NR)
NR is produced from rubber tree. It can be used when high mechanical resistance and elasticity is necessary (for example, vibration absorber blocks etc.). Working Temperature Range: -60°C to 100°C.
1.3. THERMOPLASTIC ELASTOMERS (TPE)
TPE materials are widely used in hydraulic-pneumatic applications with a variety of combinations due to their high resistance to mineral oils and high tensile strength. Working temperature is between -40°C and +120°C
1.1.3.3. Molybdenum di Sulphite
Improves hardness and reduces friction force. It is used with the other fillers.
1.1.3.4. Bronze
Improves thermal resistance, high pressure stability and extrusion resistance. It is the most preferred filler for hydraulic applications.
1.2. ELASTOMERS
These differ from plastomers in the respect that the macro molecular structure comprises of cross links. Due to this characteristic, these are highly superior visco-elastic products. These products need to contain the properties of both liquidity and elasticity. Elastomers are derived from the sap of the rubber tree, which is extracted by penetrating the bark, and is collected and modified with sulphur.
Materials which can be stretched to twice their original size before returning to their original dimensions at room temperature are called polymeric elastomer products.
Macro molecular cross links can be produced with the effect of heat and pressure. This process is called vulcanization (cross link reaction) and gives the material its perfect elastic properties.
The most commonly used elastomers are explained below;
1.2.1. Nitrile (NBR)
Nitrile is a widely used nitrile butadiene and acrylo nitrile polymer.
Alp Seals NBR formulas contain between 30% and 50% acrylo nitrile (ACN). Our NBR which has a working temperature between -30°C and +105°C (peak temp. at +130°C) show extremely high resistance under different working conditions such as mineral oils (H, H-L and H-LP type lubricating oils) and grease and mineral based fire resistance oils (HFB and HFC group oils), vegetable and organic oils, and aliphatic hydrocarbons (propane, butane, petrol )
Strongly not recommended for aromatic hydrocarbons (benzene, trichloroethylene, HFD type fire resistance oils), glycol and ether contained break hydraulics.
1.2.2. Hydrogenated Nitrile Butadiene Rubber (HNBR)
HNBR rubber, is a fully or partial hydrogenated standard NBR polymer with double linked butadiene. Its chemical resistance is similar to NBR. If it is vulcanized with peroxides, working temperature and mechanical durability can be extended. Working Temperature Range is between -30°C to +130°C.
1.2.3. Viton (FKM) (FLUOREL ELASTOMER)
It is resistant to any type of grease oil and solvent between -30°C and +225°C. Better results can be obtained in low gas transfer systems. It has a good resistance to a range of different chemicals (especially greases, fuels, aliphatic and aromatic hydrocarbons, some fire resistant oils).
1.2.4. Silicone (MVQ)
Silicone which is highly resistant to ozone, air and oil, protects its characteristics between -60°C and +200°C. Yet, it has poor resistance to oxidised oils, some of hypoid and E.P type oils).