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Home > Material Profiles > Polyurethane

polyurethanePolyurethane

ASTM D1418, ISO 1629 Designations: AU, EU

ASTM D2000, SAE J200 Type/Class: BG (Millable)

Standard Color: Translucent Yellow

Trade Names:

Relative Cost: High

General Temperature Range: -60° to +180° F

Polyurethane is the toughest, most extrusion-resistant, and most abrasion-resistant of all elastomeric sealing materials. Polyurethane O-rings can withstand pressures up to 5,000 psi with a .010" extrusion gap. Polyurethane is also very resistant to explosive decompression and has excellent properties over a wide temperature range. Polyurethane O-rings are used in a wide variety of products, including quick-disconnect hydraulic fittings, hydraulic cylinders and valves, pneumatic tools, CO2 firearms, or for any application requiring extreme abrasion or extrusion resistance.

There are three primary components of any polyurethane mixture. The first is a low molecular weight (400-6000) hydroxyl-containing molecule with two or more hydroxyl groups per chain. Such a molecule is known as a polyol. This polyol has a low Tg (Glass Transition temperature) and is usually low melting (less than 70° C), thus lending the polyurethane compound a degree of rubber-like softness and flexibility.

PICKING A POLYOL

There are three main polyols in use today, each of which bestows different properties or levels of resistance on a polyurethane compound. Polyester polyols are the most widely used due to their good tear and abrasion resistance, as well as their resistance to oil and long-term heat. Polyester-based polyurethanes (designated under ASTM D1418 as AU) are, however, highly susceptible to degradation as a result of contact with the combination of heat and water (humidity). Moisture attacks their ester linkages, resulting in chain scission (division of the polymer chain into smaller, weaker segments). Over time, the polyurethane becomes soft and cheesy. This deterioration may take two years or more, but in hot, humid climates the material can degrade in a matter of weeks. Polyester urethanes are also subject to microbial (bacterial and fungal) attack in certain environments.

Polycaprolactone polyols, a special sub-group of polyesters, are the best choice for compounds to be molded into O-rings and other seals. Polycaprolactones have a reduced number of ester groups (the moisture attack sites) compared to the same molecular weight of standard (adipate) ester polyols. Polycaprolactones thus offer better water resistance than standard polyesters. They also provide better oil and thermal resistance than the polyethers.

Polyether-based polyurethanes (designated as EU) are of two very distinct types: polypropylene ether polyols (PPG type) and polytetramethylene ether glycol (PTMEG type). They offer increased water resistance, but they do not perform well in petroleum-based oils or solvents. Their low temperature properties are superior to polyester polyurethanes, but they are not as good as polyesters when exposed to long-term heat. The PPG type polyurethanes give very poor mechanical properties (such as abrasion resistance), whereas the PTMEG type is very close to esters in many mechanical properties (particularly in harder formulations above 80 Shore A). PTMEG-based polyurethanes have the highest resilience of any common formulation and are thus preferred where dynamic properties are paramount. For very soft formulations (less than 60 Shore A), esters are almost always superior to any ether in mechanical properties.

DETERMINING A DIISOCYANATE

The second primary component in any polyurethane is a diisocyanate, and the thermal properties of polyurethane are closely tied to the diisocyanate in use. The three diisocyanates that are most widely used in the production of polyurethane O-rings are MDI, TODI, and PPDI. MDI (diphenylmethane diisocyanate) is the workhorse of the polyurethane industry. As such, it is widely used in thermoplastic polyurethanes (TPUs). A complex molecule that is environmentally safer than other diisocyanates, MDI is one of the few diisocyanates used in formulations meeting NSF, FDA, USDA, and USP guidelines.

TODI (dimethylbiphenyl diisocyanate) polyurethanes have outstanding thermal properties (up to 225° F) and compression set resistance. TODI is rather costly, however, and difficult to obtain, being sole sourced from Japan.
PPDI (para-phenylene diisocyanate) polyurethanes offer outstanding dynamic properties, resilience, and heat resistance (up to 275° F). Unfortunately, PPDI is very expensive (approximately $30 per pound).

MIXING THE COMPOUND

Though other strategies are possible, the most popular method of polyurethane mixing combines a diisocyanate with one of the aforementioned polyols. The result is known as a pre-polymer, and this liquid or waxy solid is then combined with the third main polyurethane component: a chain extender. This extender acts much like a cross-linking or vulcanizing agent used to cure rubber.
The crystalline combination of a diisocyanate and chain extender gives the compound a degree of plastic-like hardness and rigidity. The toughness of polyurethane elastomers is almost entirely due to the crystalline areas phase separating from the polyol segments in the polymer chain. The softening of the polyurethane as temperature is raised is dependent on the melting point of these crystalline hard segments.

PROCESSING THE COMPOUND

Polyurethane O-rings are usually processed in one of three ways: 1) as a compression-molded millable gum, 2) as a molded castable thermoset, or 3) as an injection-molded thermoplastic. Millable gum O-rings have somewhat better abrasion and extrusion resistance than those made from standard hydrocarbon elastomers, but compression set can sometimes be a problem as temperatures increase. The recommended upper temperature limit for millable gum polyurethanes is approximately 200° F. Millable gum polyurethanes offer good low temperature flexibility, as well as resistance to sunlight, ozone, hydrocarbon fuels, and petroleum-based oils. As already noted, these properties will depend on the polyol and diisocyanate in use.

Processed much like rubber, millable gum polyurethanes develop hardness and modulus primarily through the addition of fillers. (Phase separation, while present, is secondary.) There are limits, however, as to how much both hardness and modulus can be improved. Millable gums rely on chemical crosslinks (actually, covalent bonds) to hold the macromolecular chains together. We at RL Hudson offer a variety of millable gum polyurethanes (both polyether-based and polyester-based) in 70, 80, and 90 Shore A. These are typically either black or natural (translucent yellow), but other colors and hardness levels can be formulated with the addition of pigments and fillers.

Cast, thermoset polyurethanes offer the best balance of properties. In addition to tensile strength, abrasion resistance, and extrusion resistance, cast polyurethanes have heat resistance up to 225° F. Cast polyurethanes outperform millable gum polyurethanes in compression set resistance and elongation. Cast polyurethanes offer strong resistance to aliphatic solvents, alcohols, ether, petroleum products, and mineral-based oils. They can also be used around weak acids, weak bases (ether-types only), and mixtures with less than 80% aromatic constituents. O-rings molded from cast polyurethanes are slightly more expensive than those made from millable gums.

Rather than through the fillers required by millable gums, cast polyurethanes gain their hardness and modulus as a result of phase separation between the hard and soft segments in the polymer chains. Agglomeration of the hard segments (diisocyanate and chain extender) provides large pockets of crystalline particles. These particles act much like the reinforcing fillers used in rubber or millable gums. The polymer chains themselves are held together by hydrogen bonds. Though they are not individually as strong as the covalent bonds in millable gums, these hydrogen bonds are present in numbers great enough to become a significant source of strength and rigidity for the material.

REWRITING HISTORY

High performance cast, thermoset polyurethanes combine polycaprolactone and TODI. This beneficial combination provides excellent heat and compression set resistance. These compounds also have much better resistance to hydrolysis (chemical decomposition as a result of contact with water) than polyester-based polyurethanes. RL Hudson offers two standard polyurethane O-ring compounds: 9272 and 9292, which are 70 and 90 Shore A, respectively. Both proprietary compounds are recommended for continuous service up to 200° F and intermittent use up to 250° F. On the low temperature side, they are generally good in static applications down to -65° F. Instead of using commercial urethanes, our materials are reacted at the plant. This allows us to provide a wide variety of specialty compounds ranging from 60 to 95 Shore A.

Though very widely used, injection-molded polyurethane O-rings have historically offered the least-favorable properties, but times are changing. A new PPDI-based compound offers an excellent combination of dynamic properties, resilience, heat resistance (up to 275° F), and compression set resistance. This new material crosses into the property domain only previously achievable using cast urethanes. RL Hudson’s PPDI-based compound 9800 is available in 92 Shore A. The only real drawback to injection-molded O-rings: gate marks left on the seal’s outside diameter (O.D.) can become leak paths. In many cases, such gate marks can be adjusted so that they do not make contact with mating surfaces.

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