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Recent advances in liquid urethane.

Recent advances in liquid urethane

In general, one considers as liquid urethane polymers all non-foam, 100% (or nearly 100%) solids, reactive polymer systems. Figure 1 shows some of the typical uses for urethane elastomers. The worldwide market for urethane liquid elastomers is somewhere about $1 BB. The market is usually divided into three major segments - adhesives and sealants, cast elastomers and coatings - of comparable size. The adhesives and sealants segment accounts for about 36% of the market, with the other two dividing the remaining 64%. The overall growth rate of the market is about 4-6%. Figures 2, 3 and 4 summarize these major segments in terms of types of materials processed and major end uses.

Urethane polymer systems are extremely diverse. The urethane systems in the market include durometer hardness from sub Shore A to + Shore D. One particular attraction of urethanes is their ease of processing on low capital cost machinery. Urethane polymer systems are processed by pouring, casting, spraying, troweling, injecting, brushing and other means. Typically urethane liquid polymer systems are comprised of an A and B side, one of which is a NCO terminated prepolymer and the other an amine, diol or polyol curative system. Another common system is the one "shot" or "inverted" system where the prepolymer is an alcohol or amine functional oligomer and the curative is a chain extended, polymeric or trimerized isocyanate. Urethane polymer systems are also available as one component systems where curing occurs by the reaction of moisture with available isocyanates or by the reaction of blocked isocyanates with available amine or alcohol.

Urethanes are selected because of their excellent toughness, abrasion resistance, broad service temperature range, environmental and fluid resistance, and excellent gloss and colorability.

Applications for urethane liquid polymers are extremely diverse. Many areas appear to be maturing, especially industrial rolls, wheels, bushings and bearings. Other areas such as urethane coatings have seen high growth but in select areas such as window laminates and abrasion resistant coatings for hoppers, hopper cars, pumps and piping.

RIM had rapid growth in the early 1980s but growth rate has begun to level off. Urethane adhesives have seen slow, steady growth, whereas urethane sealants have grown rapidly in several areas such as automotive and architectural glazing.

Numerous forces, both internal and external, have acted on the urethane industry resulting in slow and steady change over the past 10 to 15 years. Changes have occurred in materials, processing methods and in the markets served.

Forces for market change

Environmental, health and safety Both governmental regulation and a generally improved overall environmental and health consciousness have significantly impacted the urethane industry. In 1972 the Occupational Health and Safety Administration (OSHA) proposed regulations for 4,4' methylene-bis-(3-chloroaniline), (MOCA), curative which threatened to virtually cripple major sectors of this rapidly growing industry. Much development work through the 1970s and 1980s was directed toward finding replacements for MOCA. Regulation governing isocyanate exposure, especially tolylene diisocyanate (TDI), have fueled movement to methylene-di-p-phenyl diisocyanate (MDI) development of low TDI prepolymers and the use of chain extended or trimerized isocyanates in one shot systems.

Education of users The efforts of materials suppliers and trade organizations such as the Rubber Division of the American Chemical Society, the Polyurethane Manufacturers Association (PMA), the Society of the Plastics Industry (SPI) and others continue to increase the awareness of the utility of urethanes among engineers, designers and others who develop or specify material systems. This generally leads to increased interest in urethane product usage. However, because urethanes are such a diverse family of materials, and because the market is so heterogeneous, the task of educating the market is very great. The users must be educated not only in the overall benefits of urethanes, but also in the best urethane selections related to each application.

Quality and performance Making polyurethane parts with high quality and consistency is a great deal more complex than meets the eye. Market demands, competition and technological advances have lead to ever increasing improvements in the quality and performance of urethane parts. Lower residual monomer, narrower molecular weight distribution in polyols and prepolymers have lead to lower viscosities, easier processing and improved properties. Purer monomers and improved stabilizers have lead to improved color, light and environmental stability. Improvements in processing machinery and instrumentation have provided better ratio and temperature control and methods of quality measurement.

New materials technology

Isocyanates In the area of isocyanates the developments have included:

* 1, 4 cyclohexylene diisocyanate (CHDI);

* p phenylene diisocyanate (PPDI);

* m trimethyl xylene diisocyanate (TMXDI);

* new trimerized and chain extended isocyanates;

* better understanding of the uses of MDI; and

* new blocked isocyanates.

PPDI and CHDI are high performance diisocyanates earlier introduced by Akzo but then withdrawn. Currently Du Pont is in the commercial development stages of introducing PPDI and has CHDI in the planning stages. These two isocyanates have been demonstrated to provide exceptional dynamic and thermomechanical properties in castable and TPU systems.

Among the aliphatic isocyanates for coatings there has been continued growth in PPDI and trimerized HDI, along with the recent introduction of TMXDI

The development of new liquid MDI systems as polymeric chain extended MDI and MDI quasi-prepolymers has increased the prospects for high performance one shot systems.

Polyols The workhorses of the castable industry continue to be adipate esters. Caprolactone esters have established a strong following in select areas though their true distinction from butylene adipates may in large part be their high and consistent quality. Poly (tetramethylene ether) glycols have continued to carve out market share against polyesters where superior low temperature, dynamic and environmental (microbial/hydrolytic) resistance is important.

Ethylene oxide (EO) capped propylene oxide ether glycols (PPGs) have established a small base in the cast elastomer market, providing moderate properties compared to other urethanes but still greatly improved toughness and abrasion resistance compared to conventional rubber. One particular strength of EO capped PPGs comes to light in one component moisture cure systems. Due to their high hydrophillicity compared to PTMEG or polyesters; sealants, coatings and adhesives containing EO capped PPG moisture cure much faster. A new development in the area are the new high molecular weight, low unsaturated PPGs and EO capped PPGs. Targeted first for the sealants market, these 2,000-6,000 molecular weight diols provide improved elasticity and toughness with good extendibility with fillers.

Polycarbonate polyols continue to make inroads as high performance coating systems due to their excellent combination of U.V. resistance, toughness and flexibility compared to acrylics.

Curatives Driven by market demands for improved curatives with:

* low toxicity, non carcinogenic, environmentally safe;

* liquid at room temperature;

* broad stochiometric tolerance;

* manageable gel times with rapid cures; and

* competitive cost/performance, a number of new amine curatives have been offered over the past 10 to 15 years. Some have carved out niches, some have been dropped. Among the amine curatives available today are:

* MOCA - Due to its long gel time and broad process tolerance, MOCA remains the dominant curative for TDI prepolymer systems above 80-85A.

* Ethacure - These are room temperature liquid substituted aromatic diamines. Ethacure 100 has very short gel times, consequently its greatest utility is in RIM systems. Ethacure 300 is a moderately fast curative which provides good to excellent properties with both MDI and TDI prepolymers.

* Unilink - These are room temperature liquid secondary aromatic diamines with a bulky alkyl substituted on the amine which results in moderate reactivity and reduced hardness. These curatives are used alone and in conjunction with other curatives finding utility in flexible elastomer systems, coatings and adhesives.

* Polacure/polamine - These are p amino benzoate esters of various chain length diols. These materials have relatively low toxicity in urethane systems and are a Federal Drug Administration (FDA) approved ingredient for certain applications. The Polamine 740 has the disadvantage of high melting point but produces urethanes of high durometer with excellent properties. The Polamine 250 is a low melting solid and 650 is a liquid. These curatives produce moderate to high durometer systems with good to excellent properties. Higher molecular weight Polamines 1000 and 2000 are used for soft coatings and adhesives or may replace the prepolymer in "inverted" systems, i.e. systems where the polamine acts as an amine terminated prepolymer and liquid MDI acts as a curative.

* Others - A large number of aliphatic diamine and amino alcohols are used to cure the much less reactive aliphatic isocyanate systems. Examples are: piperazine, propylene diamine, isophonone diamine, mono ethanol amine and di ethanol amine.

* Diol curatives - Butane diol, hydroquinone di (B hydroxy ethyl ether), (HQEE) and butanediol (BDO), together with minor amounts of 2-ethyl-2-(hydroxy methyl)-1,3 propane diol (TMP), are the main curatives for moderate and high durometer MDI prepolymer systems. Together with MDI, these systems provide superior resilience, environmental stability and toughness. For flexible systems, a broad range of diols, triols and polyols, together with a catalyst, are formulated into proprietary cure systems for a range of prepolymers.

Prepolymers Two trends stand out in the castable market. First, due to their low NCO vapor pressure and excellent properties with non-MOCA curatives, MDI based prepolymers continue to grow against TDI. Due to regulatory and health concerns, suppliers of TDI systems began moving slowly toward low free TDI prepolymers. However, now as the market begins to recognize the performance benefits of the combination of low free TDI and low molecular weight distribution, the move to low free TDI systems is accelerating. 1,4 trans cyclohexylene diisocyanate (CHDI) prepolymers have been marketed on a very limited basis. Here the cast elastomer industry came to know what the coatings industry knew all along, that is, the outstanding chemical resistance and hydrolytic stability of urethanes made from aliphatic isocyanates. CHDI, however, also provides mechanical properties equal to or better than the best aromatic isocyanates. CHDI's unique combination of excellent chemical resistance, toughness and high resilience is of particular interest for rolls in wet paper processing and for down hole oil applications. Following the successful start-up of their PPDI business, Du Pont is soon expected to bring CHDI to market. Du Pont has recently introduced p-phenylene diisocyanate (PPDI) as a commercially available product. This has quickly been followed by the commercialization of PPDI based prepolymers by several of the high performance prepolymer suppliers. PPDI is a very compact, rigid and symmetrical molecule. Its first benefit is its ease of handling. It is available in a free flowing flake form. Due to its non-symmetrical reactivity, i.e., the first NCO is 12 times more reactive than its second, PPDI readily produces low free PPDI prepolymer with low oligomer content. Cast elastomers and TPUs with exceptional dynamic mechanical properties, thermomechanicals and cut growth resistance have recently been produced.

PPG based prepolymers which provide the good mechanical properties of urethanes but at a lower cost differential compared to conventional elastomers are slowly establishing a market share. New technology in the area of PPG and EO PPG polyols may significantly enhance the performance of elastomers made from these polyols. As the market for urethane coatings has grown, so has interest in prepolymers based on aliphatic isocyanates. These are, however, low volume specialties.

One component systems One component systems are typically either moisture cure systems or heat activated blocked isocyanate systems. Moisture cure systems are used extensively for adhesives, coatings and sealants. In these systems curing occurs by available NCO groups reacting with ambient moisture. Heat activated block isocyanate systems are used extensively in baked coatings and in a variety of heat cured castable systems.

One shot or inverse systems Here the prepolymer is replaced by hydroxy or amine functional oligomer and the curative is a quasi prepolymer, a trimerized isocyanate, a chain extended isocyanate or a polymeric isocyanate. This approach to formulating a liquid urethane system completely side steps the issue of high melting point amine curatives, residual free isocyanate and viscous prepolymers.

Growth in this area has been enhanced by the development of new chain extended MDIs and the availability of high NCO quasi prepolymers. Because they are simple mixtures and not reactor products, one shot systems are often economically formulated to meet exact requirements of a particular end use.

The development of Polamine 650, 1000 and 2000 has provided new opportunity for one shot systems. These are amine functionalized polyols where the chain end is p amino benzoate. The inductive effects of the para ester provides an aromatic amine functionality with sufficiently low reactivity to be used with liquid MDIs. This combination provides a room temperature processible system with management gel times. The resulting systems have very good dynamic and mechanical properties over a range of hardnesses.

New market opportunities

Design and styling changes in cars and trucks, and the rapid growth in recreational vehicles have provided excellent opportunities for polyurethane foam, RIM, elastomers and coatings. Emphasis on cost and productivity, plus better recognition of performance, have provided major inroads in mining, agriculture and related materials handling. The recent boom in office machinery has provided opportunities for elastomeric belts, rolls, guides and RIM housings. In the rapidly growing health care market, tubing, catheters, transdermal drug delivery systems, wound dressings, surgical drapes and cardiovascular repair parts have all made major inroads.

Rapid growth in leisure and sports, along with rapidly changing trends, have created a wide range of opportunities. Notably, footwear, skate blades, motor sports, skiing continue to be areas of high growth of rapid change.


Among the developments a few appear to be most interesting:

* Moca - MOCA continues to be with us as a major curative. There have been no studies linking MOCA to human cancer and effective monitoring programs have been developed to monitor exposure. Recently OSHA issued a regulation limiting MOCA to 20 parts per billion (ppb). With the improved handling systems available today these limits are deemed to be tolerable by most programs.

Only a few of the many MOCA replacements even came close to meeting the industry's processing and performance needs while being low in toxicity and affordable. Of those currently available are Ethacure 300, Polacure/Polamine and Unilink. While these curatives have found a number of uses, none of these matches MOCA's long gel times and excellent properties, especially with TDI systems.

* Urethane tires - There continues to be hope for the development of a urethane tire. The latest successes are with technology developed by Uniroyal-Goodrich. Through licenses this technology is being introduced into materials handling wheels by Industrial Tire Ltd. and Urethane Products Inc. The latest development in automotive tires is that NHTSA has approved a mini spare based on this technology. It is not clear what models or year of car will be selected for introduction of this tire.

* CHDI and PPDI - PPDI is back and CHDI is coming back. The new PPDI from Du Pont is significantly higher quality than previous materials and has been demonstrated to provide greatly superior dynamic and thermomechanical properties to MDI in both castable and TPU systems.

* Inverse systems based on polamine - This is potentially a very interesting development. Combined with liquid MDI systems, it provides the only two component room temperature processible polyurea system available.

This system has the potential for excellent processibility, dynamic properties and thermomechanical properties in the moderate to high durometer range. Cured with prepolymers or quasi prepolymers, these products provide low durometer systems with excellent properties.

* "Perfect" prepolymers - These are TDI systems with low oligomer and low free TDI content. These are not only attractive as a means of reducing health risks and meeting OSHA TDI exposure limits, but have been found to increase the dynamic properties of TDI/MOCA systems.

* One shot systems - The availability of new liquid MDIs which provide increasingly higher properties continues to support growth in the area of one shot systems. These systems are attractive due to low viscosity, low free isocyanate and often lower cost than comparable prepolymer systems.
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Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Russell, David
Publication:Rubber World
Date:Sep 1, 1991
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