Polyol-cured polyurethanes

More recently, systems have been reported which thermally generate isocyanates without the evolution of side products. When these thermal reactions are carried out in the presence of polyols, cured polyurethanes are said to be produced. One such system is based on bis(cyclic ureas). 

Example 11. One-Component, Moisture-Cure Polyurethane Sealant. This example is of a low-hardness, high-elongation, moisture-curable polyurethane sealant. The material is based on a low-%NCO prepolymer made from 4,4 -MDI and a low-unsaturation (low-monol-content) Acclaim polyol from Bayer. It is adapted from (a) J. Lear et al., Adhesives Age, February 1999, pp. 18-23 and (b) B. Lawrey, et al., presented at UTECH 2000, The Hague, The Netherlands, March 30, 2000, Crain Communications London, 2000. 

Fully cured polyurethanes present no health hazard they are chemically inert and insoluble in water and most organic solvents. Dust can be generated in fabrication, and inhalation of the dust should be avoided. Polyether-based polyurethanes are not degraded in the human body, and are therefore used in biomedical applications. Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

In many adhesives formulations, the resin portion is the same or similar and what determines its properties is the hardener or catalyst that is used to cure the resin. Thus, depending on the hardener, epoxy adhesives may be referred to as amine-cured, anhydride-cured, polyamide-cured, or novolac-cured. Polyurethanes may be polyol-cured or hydroxypolyester-cured. 

A one-part, heat-curing polyurethane adhesive has recently been introduced in which the curative is incompatible, and hence nonreactive, with an isocyanate prepolymer. Upon heating, the finely dispersed polyol curative becomes reactive towards the prepolymer and curing takes place. 

Polyols are multifunctional alcohols with a polyether, polyester, or polycarbonate backbone. They are typically viscous liquids with molecular weights ranging from 200 to 5000g/mol. The polyol component of the polyurethane significantly affects the degradation rate of the final cured polyurethane. Typically, the rate of hydrolytic degradation of polyols observes the order polycarbonate[Pg.482]

Donnely synthesized mono- or bisglucoside-modified polytetrahydrofuran (PTHF) polyols and cured them with MDI. When the glucoside-modified PTHF was cured with MDI in the presence of ethylene glycol as a chain extender, the cured polyurethanes exhibited good mechanical properties comparable to commercial reaction injection molding polyurethanes. Kurita and... 

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

Polyurethane foam Natural oil polyol, water, and surfactants were weighed into a 1 quart metal cup and premixed for 15 seconds at 1800 rpm using a pin type mixer. The catalyst was added and the mixture stirred an additional 15 seconds at 1800 rpm. The polyisocyanate was then added and the mixture stirred at 2400 rpm for 3 seconds and immediately transferred to a 15 x 15 x 10 wooden box lined with a polyethylene bag. The buns were allowed to cure overnight before testing. 

Polyurethanes (PU s). SL and 4,4 -diphenylmethane diisocyanate (MDI) were dissolved in tetrahydrofuran (THF), and the solution was stirred for 1 hr at 60°C. A THF solution of polyethylene glycol (PEG 400) and diethyl bis(2-hydroxyethyl)aminomethylphosphonate (polyol containing phosphorous) was added to the reaction mixture, and the reaction time was extended for 1 hr. In all reactions, the molar ratio of the total amount of isocyanate groups to the total amount of hydroxyl groups (NCO/OH) was maintained at 1.2. The lignin content in PU was 20 wt%. Each solution was drawn on a glass plate, and allowed to dry for 48 hr. The residual solvent in a sample was removed under vacuum and curing of each PU film was carried out at 120°C for 3 hr under a pressure of 50 kg/cm2. 

This method relies on the fact that the isotopes of certain atoms have electrons that will flip under certain conditions. This change in state can be detected, and how they are connected can be shown. The H and 13C isotopes are able to produce resonance spectra for polyurethane raw materials and cured samples. ASTM method D4273 details a method to determine the primary hydroxyl content of polyether polyols. 

The addition of small amounts of fluorinated polyols to conventional aqueous polyurethane coatings can improve their water resistance and mechanical properties considerably 47 FEVE resins can be applied as water-based coatings and cured at 160°C (320°F) for 25 min to produce coatings with high gloss and good water and weather resistance.48 They may also be cured with water-based hardeners.49... 

Reaction injection molding (RIM) is a fast, low-pressure, low-temperature, low-cost process for one-step conversion of reactive liquids into large finished solid plastic products. Liquid polyol and liquid diisocyanate are mixed by impingement, pumped instantly to fill a large mold cavity, and polymerize/ cure rapidly to form a thermoset polyurethane product. The cured polymer may be a stiffly flexible product such as automotive bumper covers, front ends, and trim or a rigid foamed product such as furniture and housings (cabinets) for computers, business machines, TY and radio. 

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