By Michael Molitor, President, Innovative Polymers, St. Johns, Michigan
With the recent advances in isocyanate and polyol chemistries, next-generation thermoset polyurethanes offer design engineers more flexibility in material selection than ever before.
Advanced thermoset polyurethanes offer new options for molding durable parts with unique combinations of performance characteristics, such as high impact strength with excellent heat resistance and good flexural properties. These next-generation polyurethanes can also exhibit outstanding tear strength and elongation, high resistance to chemical, oil, abrasion and wear, good UV stability and high impact strength. Some systems are also flame retardant and can be used at elevated temperatures and in moist environments.
A further benefit of many new polyurethanes is improved processing. The raw materials used to formulate next-generation products are typically liquids, rather than the conventional aromatic amine solids long associated with “hot pour” systems.
Today’s polyurethane components can be mixed and handled at room temperature, eliminating the need for expensive processing equipment. Moreover, low free-isocyanates are less volatile than their predecessors
With the proliferation of next-generation polyurethanes, selection of the best material for a project can prove as challenging as the parts for which they’re being used.
Thermoset polyurethanes are produced from a variety of raw materials combinations. The basic chemistry calls for the reaction of an isocyanate and a polyol to form a prepolymer. The prepolymer is then blended with a curative and, if needed, additives provide additional performance enhancements. The specific composition of a polyurethane system affects its gel time, processing temperature, hardness and mechanical characteristics such as strength and flexibility.
Isocyanates form one part of the prepolymer and affect the cure characteristics of the polyurethane as well as many of its end-properties. Isocyanates are typically based on either aromatic chemistries such as diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI). Alternatively, more expensive aliphatic chemistries, such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI), can be used to realize specific performance characteristics such as inhibition of yellowing in parts that will be exposed to sunlight. HDIs can improve weatherability and abrasion resistance of formulated systems as well.
Polyols also contribute to the function and mechanical properties of the thermoset system. These chemical components may include:
- Polyether polyols produce Shore A and Shore D hardness systems that, depending on additives used, feature high tear strength and elongation and outstanding Taber abrasion resistance. The materials can also be UV stable and water resistant.
- Polyester polyols are combined with isocyanates to form Shore A hardness polyurethanes with outstanding elongation characteristics as well as high tear strength and resistance to oils and chemicals. Depending on formulation, advanced polyester polyol systems can today be processed at temperatures between 100°F and 130°F (39°C to 54°C) and cure at 150°F (66°C), a major advantage over traditional high performance polyester polyol polyurethanes that may require processing at temperatures above 210°F (99°C).
- Polyester polyols may be more expensive than polyethers, and can be more viscous and difficult to handle. Polyurethanes based on polyesters withstand exposure to solvents and oils and provide improved abrasion and “cut” resistance when compared with polyether curatives.
- Polyol/amine hybrids, when reacted with an isocyanate (MDI or HDI prepolymer), produce polyureas. These Shore D hardness materials offer outstanding tensile strength, high flexural modulus, excellent impact strength and high heat deflection temperatures.
- Polyether polyol/polyurea hybrids accommodate the production of Shore A and Shore D hardness systems with high tear strength and good abrasion resistance.
Curatives produce the polymerization reactions between the isocyanate and polyol parts of the formulation, determining the structure and properties of the cured polyurethane.
Solid aromatic amines such as methylene-bis orthochloroaniline (MOCA/MBCA) were historically the most commonly used curing agents in polyurethanes formulated with TDI. However, the materials must be processed at 240°F (116°C) and have a number of environmental and safety/handling disadvantages. As a result, today, liquid aromatic amines are the preferred alternatives to MOCA/MBCA for their ease of processing and ability to cure at room temperature while maintaining or improving the end-performance properties of the polyurethane.
For MDI systems, diol curatives such as 1,4-butanediol (BDO) and hydroquinone diethyl ether (HQEE) are used. HQEE is often preferred to produce high hardness polyurethanes but it is more difficult to handle than BDO. HDI and IPDI are combined with polyols for curing and to attain outstanding light stability.
C.) Additives are incorporated in polyurethane formulations to modify physical properties. Plasticizers, for example, are used to develop very soft (below Shore 50A hardness) polyurethanes; other fillers help improve wear and abrasion resistance. And, to produce durable parts through reaction injection molding, polyurethanes are filled with glass fiber.
Pigmented pastes enable formulators to attain polyurethanes that consistently exhibit virtually any color in the spectrum. NOTE: The original prepolymer must be light in color to be effectively tinted.
As described above, next-generation polyurethanes are generally easy to mix because the low free-isocyanate polymers and curatives are liquids that require no heating before mixing either by hand or using automated meter/mix dispensing equipment. Gel times range from seconds to hours and many of the thermoset plastics cure at room temperature or slightly above. Parts can be demolded after 24 hours, or as little as 30 minutes for fast-setting systems.
As with virtually all two-part polyurethanes, the production of consistent, high-quality end products depends on precise ratio control, thorough mixing and maintenance of appropriate processing temperatures. As the liquid system components react, the polyurethane becomes increasingly viscous and, once cured, forms a solid mass. The reaction is exothermic, producing heat as the cure cycle proceeds.
With the many casting polyurethanes available on the market today, you can be certain that whatever hardness, elongation and flexural characteristics may be required for an application can be readily found among the various prepolymer/curative combinations. (See Table 1 for overview of various types of high-performance polyurethanes.)
Key benefits of many of the new high-performance polyurethanes are property combinations that were previously not available including: high heat, high impact, high modulus products as well as systems that can withstand exposure to gouging, cutting, chemicals, oils and solvents. Some high-performance products are also flame retardant.
Abrasion resistance is, in next-generation polyurethanes, a quantifiable property according to industry-standard Taber abrasion testing. While polyurethanes have long been recognized for their ability to resist wear in challenging applications, advanced polyurethanes exhibit wear loss in the 5 to 20 mg range.
Tear strength of new polyurethanes is also excellent compared with conventional hand-casting systems. Super-tough, new polyurethanes and polyurea hybrids offer tear strengths from 225 up to nearly 650 pli. Similarly, notched Izod impact strengths of next-generation polyurethanes is as high as 6.5 ft-lb/in. compared with < 1.0 ft-lb/in. for traditional products. Heat deflection temperatures are now in the 248°F (120°C) to 346°F (174°C) range.
A. Sporting Equipment, such as skateboard wheels, have been made with polyurethanes for many years. Today, MDI ether-based polyurethanes are, however, producing wheels with greater resilience, good rebound and excellent abrasion resistance. As a result, skate boarders realize improved speed performance with a smoother ride. The variety of hardness ranges available with high-performance polyurethanes is an added advantage. For example, small wheels with a durometer hardness of around Shore 75A are preferred for street skating while ramp skaters require larger, harder wheels that perform at high speeds without sliding.
B. Automotive components such as fender flairs and bumper covers must be durable and tough enough to withstand vibration and shock from traveling on roads and rough terrains. The parts must also endure weather and temperature extremes without cracking. A new TPO-like polyurethane based on advanced crosslinked technologies gives vehicle manufacturers the reliable performance they need. The same material is also easy to process, even on large vehicle parts.
C. Oilfield pigs and components are easier to mold and longer lasting with new MDI ester-based polyurethanes. The next-generation polyurethanes have a low viscosity that accommodates hand-batch or meter mixing and can be molded in low-cost epoxy tooling built to the broad variety of sizes and shapes required for the oil field application. (Aluminum molds machined to the numerous sizes needed would be prohibitively expensive and require long lead times that would not allow pig manufacturers to respond to customer needs in a timely manner.) The new polyurethanes cure at 150°F (66°C) which is considerably lower than the temperatures required by “hot pour” TDI prepolymers, producing energy savings. Once cured, MDI ester-based polyurethanes exhibit good dimensional stability, ultra-high abrasion resistance, and excellent oil and chemical resistance.
D. Mining equipment coated with or cast using high-performance MDI and TDI ether-based polyurethanes requires less maintenance and replacement to substantially reduce operating costs. These polyurethanes are available in a range of durometers, from Shore A to Shore D hardnesses, making them suitable for use in nearly all mining environments. Other important properties for long-term durability include resistance to sliding and impact abrasion as well as cut and tear resistance. Typical applications include steel and wire-mesh screens coated with polyurethane, metal pump, chute and valve bodies lined with polyurethane, and cast polyurethane impellers.
E. Agricultural, manufacturing and other industrial parts represent an ever- growing market for high-performance polyurethanes. The most abrasive and harsh environments, including grain handling and textile manufacturing, benefit from MDI polyester polyol-based polyurethane-lined equipment for extended service life. In addition, for parts that come in contact with oats, wheat and cereals, some MDI ester-based polyurethanes are FDA-approved for dry food contact. TDI polyester-based, Shore A hardness polyurethanes exhibit high solvent resistance and mechanical properties for use in printing rollers.
MDI ether-based prepolymers mixed with diol curatives offer excellent hydrolysis resistance for casting agitators and other components produced for laundry equipment. TDI ether-based prepolymers combine hydrolysis resistance with hardness and dynamic stability even when exposed to changing operating temperatures. As a result, the materials handle use in components such as paper processing rollers. TDI ether amine-based polyurethanes exhibit outstanding abrasion resistance and high load bearing capacity for power transmission guide rollers.
Determining the best material for a specific project requires a thorough understanding of desired part performance properties as well as processing and handling criteria.
A. Part performance including anticipated mechanical stress, required dimensional tolerances, desired surface finish, and typical service environment/ temperatures in which the part will be exposed are keys to polyurethane selection. When a number of properties are important, the performance criteria should be handled in order of priority. If abrasion resistance is the primary requirement, this should be the starting point followed by hardness, elongation, impact strength or ability to handle a load or high temperatures.
If a polyurethane must match a specific color, that must be considered at the beginning of the selection process. Some materials are naturally dark in color and cannot, therefore, be pigmented to attain light color shades.
When there is little information about required performance properties, anticipated service conditions can provide a beginning step in material selection. For example, if a component is being molded for outdoor equipment, the selected polyurethane must be able to withstand exposure to temperature extremes, humidity and rain, and the destructive effects of ultraviolet rays from the sun.
B. Processing/handling characteristics are the second important criteria in material selection. Among the factors to consider in selecting a polyurethane is its work life. The gel time of the material must be long enough to provide for complete mold filling before the part begins to cure. NOTE: Remember that shop temperature affects work life; higher than 77°F (25°C) temperatures will decrease gel time while lower than 77°F (25°C) will increase gel time and affect the cure cycle.
Molding process is also important. If a part will be rotomolded, a material should be selected that features a low viscosity with gel time long enough to coat mold surfaces before the system begins to set. Specific parts may require degassing or pressure casting. Or, if being processed via meter/mix equipment, the polyurethane may be subjected to higher pressures and temperatures than when being hand cast in a silicone rubber mold.