Urethanes, curing

RIM—Reaction injection-molded urethane, cure t5 min, 80°C. Thermoplastic=BF Goodrich Estane 5701, fuse at 175 C. 1-Component—Dow Corning Urethane Bond , cure 3 days, room temperature. A—Age in boiling water, 2 h B —age in boiling water, 5 h C=cohesive bilure in polymer, >30 N/cm. 

Estimates (4) show that only 18% of the present RIM machines have multiple-stream capability. Although success has been demonstrated with the three-stream IMR approach, the auto industry expressed a desire to obtain a two-stream release agent. This two-stream approach would allow the RIM molder to gain release by a simple addition of the IMR agent into the A or B side. A fluid had to be found which would have the proper balance of incompatibility and reactivity in order to obtain the desired release and paintability, yet not impair the urethane cure. Several experimental fluids which have shown promise for release, paintability, processability and physical property retention as two stream candidates have been developed. 

CURE SPEED. Cure speed Is very Important In those applications that require high output. TDI based urethanes cure faster than IPDI based urethanes (Table II). Variations In the polyol also Impact cure speed. Linear polyether linkages In a polyol have previously been shown to enhance cure speed.This effect Is Illustrated in Table V, composition N. 

Uses Carrier in inks hard resin additive in overprint varnishes reactive polyol in melamine- and urethane-cured coatings for wood and metal intermediate for polyester and alkyd coatings cure accelerator, hardener, gloss aid, chem. resist, aid in coatings Properties Wh. pastilles sol. ( 25% solids) in Arcosolv soivs., butanol, NMP insol. in water, propylene glycol, and ethylene glycol m.w. 2500 (wt. avg.) bulk dens. 5.556 Ib/gal acid no. < 0.1 hyd. no. 255 VOC 0.10%... 

Urethane curing systems have been long used as solid propellants. Recently, a triazole curing system has been introduced into a new binder recipe in which azide groups in the polymer react with triple bonds of a dipolarophile curing agent [101]. 

The nitroso systems also have common features with the so-called urethane-curing systems dealt with in the next section. 

Cargill 3000 (saturated OH functional polyester) Cargill 2400 (blocked urethane curing agent) Modaflow Powder if R900 Ti02 ... 

Pentaerythritol and trimethylolpropane acryUc esters are usefiti in solventiess lacquer formulations for radiation curing (qv), providing a cross-linking capabihty for the main film component, which is usually an acryUc ester of urethane, epoxy, or polyester. Some specialty films utilize dipentaerythritol and ditrimethylolpropane (94,95). 

A large number of polymeric compounds have been investigated, but most modem propellants utilize prepolymers that ate hydroxy-functional polybutadienes (HTPB), carboxy-functional polybutadienes (CTPB), or a family of polyethylene oxides (PEGs) to form urethanes. Typical cure reactions... 

Epoxy Resins. Urethane and ester-extended hydantoia epoxy resins cured with several compounds seem to have better properties than the previous ones (98). These resins are prepared from hydantoias such as (21) (99,100). 

For many moderate-duty films for operating temperatures below 80 to 120°C, M0S2 is used in combination with acryflcs, alkyds, vinyls, and acetate room temperature curing resins. For improved wear life and temperatures up to 150—300°C, baked coatings are commonly used with thermosetting resins, eg, phenohcs, epoxies, alkyds, siUcones, polyimides, and urethanes. Of these, the MlL-L-8937 phenoHc type is being appHed most extensively. 

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

OxazoHdines, formed by reaction of alkanolamines with aldehydes, ate useful as leather tanning agents and ate effective curing agents for proteins, phenoHc resins, moisture-cure urethanes, etc. They also find use as antimicrobial agents. 

In general, however, the vulcanizates suffer from poor low temperature crystallization performance compared to a conventional sulfur cure, and also have inferior tensile and tear properties. Urethane cross-linking systems (37), eg, Novor 950 (see Table 3) are also extremely heat resistant, but exhibit inferior tensile and dynamic properties compared to conventional sulfur-cured vulcanizates. One added virtue is that they can be used in conjunction with sulfur systems to produce an exceUent compromise according to the ratios used (38). 

One-part urethane sealants (Table 3) are more compHcated to formulate on account of an undesirable side reaction between the prepolymer s isocyanate end and water vapor which generates carbon dioxide. If this occurs, the sealant may develop voids or bubbles. One way to avoid this reaction is to block the isocyanate end with phenol and use a diketamine to initiate cure. Once exposed to moisture, the diketamine forms a diamine and a ketone. The diamine reacts with the isocyanate end on the prepolymer, creating a cross-link (10). Other blocking agents, such as ethyl malonate, are also used (11). Catalysts commonly used in urethane formulations are tin carboxylates and bismuth salts. Mercury salt catalysts were popular in early formulations, but have been replaced by tin and bismuth compounds. 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

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