As with all raw materials and finished products, there are large differences in quality between different polyurethanes on the market. The polyurethane (PUR) block copolymer, which consists of linked segments of hard and soft PUR, has superior properties compared to many other plastics. The hard segments provide extremely high mechanical stability, while at the same time the soft segments give the material high flexibility and dynamic load capacity. The use of specific PUR compounds to create industrial components can therefore result in more efficient and durable hoses.
By Mark Bobonick, Director of Sales & Operations, NORRES
PUR Properties
Polyurethane is a polymer composed of organic units joined by carbamate links. The use of ester-polyurethane or ether-polyurethane to manufacture industrial hoses provides the product with a number of technical advantages. High tensile strength, values for elongation at break, very high elasticity, and unusually high creep resistance result in static and dynamic load capacities that far exceed the values of many other rubber-like materials.
PUR hoses handle overstraining in the bending radius and repeated rhythmic stress from intermittent operation well due to their good resilience and low compression set. These properties allow for the development of special hoses with good performance under opposing conditions, as they can handle high tensile and compression loads despite their low weight. The high resistance to tear propagation means, for example, that a hose with cut damage can be patched and continue to be used without the need to relieve the axial forces. This is rarely possible with hoses made from other materials.
Like all organic materials, polyurethane is also combustible. The toxicity of the combustion gases and smoke density are usually measured according to DIN 53436. Judged on the basis of this leading international standard, the potential release of hazardous substances (acute inhalation toxicity) at 1,472°F is not more critical than that of natural products such as wood, wool or leather.
Flame-resistant polyurethane hoses for the wood processing industry are typically type-tested according to DIN 4102-B1 and meet the safety requirements of the Holz BG. Some hoses, such as NORRES Pre-PUR® Polyurethane products, are also tested according to UL94 and many products achieve the classification HB, V2 or V0.
Practical Advantages
According to standard tests, the abrasion resistance of Polyurethanes, such as Pre-PUR® Polyurethane, is about two and a half to five times higher than for many rubber raw materials; it is roughly three to four times higher than that of soft PVC. The differences in the field are frequently even greater, since the excellent damping and impact resilience of polyurethane does not come into play with the standard test methods.
While many polyurethanes can remain in water at 68°F to 104°F for years, polyester polyurethanes in contact with warm water or steam, roughly 140°F or higher, undergo irreversible degradation (hydrolysis) of the polyester chains. Stabilization and extremely high molecular weight can, however, combat this process. Hoses made from ether Pre PUR are not sensitive to hydrolytic degradation and are an even better solution in situations where a hose may be exposed to higher temperature liquids.
Polyurethane hoses also show good resistance to dilute acids and bases. Although they are subject to attack by concentrated solutions and strongly oxidizing acids, ASTM test methods are petroleum industry standards accepted worldwide for quality and reliability. These test methods are for petroleum trace analysis and oil condition tests. ASTM levels 1-3 cause no loss of strength at 68°F.
Minor Drawbacks
Intensive, long-term weathering will lead to yellowing and a loss of mechanical properties. In such cases, additional UV stabilizers or coloring pigments should be added. Un-stabilized Ester Thermoplastic polyurethane in long-term contact with soils and similar substances or heavy contamination under conditions favorable to microbes can be destroyed by enzymes from the organisms which attack the chemical bonds. Under very unfavorable conditions the first signs of damage can be seen after 8 to 24 weeks. Ether polyurethanes are inherently resistant to microbiological attack over long periods.
Closing Remarks
The high-quality raw materials with special stabilizers demonstrate considerably better stability and significantly longer service life than can be obtained with many other products.