Polyol crystallization

Beilstein Handbook Reference) AI3-09048 BRN 0211560 CCRIS 6202 CP 626 EINECS 204-171-4 HSDB 2922 NCI-C61585 Niagathal NSC 1484 Phthalic anhydride, tetrachloro- Tetrachloro-phthalic anhydride Tetrathal, Flame retardant used with polyester resins and polyols. Crystals mp = 254.5" bp = 349-354" sublimes d = 1.49 km = 298 nm (MeOH) slightly soluble in EtzO. Monsanto Co. 

A problem in producing transparent polyurethanes arises from the tendency of some polyols to crystallize (cold-harden) on ageing. This is a particular problem with soft and flexible urethanes as these require the use of high molecular weight polyols and there is an increase in polyol crystallization tendency with molecular weight increase. 

The cohesive polyester-polyol crystals of commercially available PU adhesives melt at about 50 °C - the lamination adhesive is thermally activated and becomes soft and capable of heat-sealing (Fig. 8-26). During coohng, the polyester-polyol segments re-crystaUize, resulting in a rapid increase in the internal strength of the adhesive film. 

A hst of polyol producers is shown in Table 6. Each producer has a varied line of PPO and EOPO copolymers for polyurethane use. Polyols are usually produced in a semibatch mode in stainless steel autoclaves using basic catalysis. Autoclaves in use range from one gallon (3.785 L) size in research faciUties to 20,000 gallon (75.7 m ) commercial vessels. In semibatch operation, starter and catalyst are charged to the reactor and the water formed is removed under vacuum. Sometimes an intermediate is made and stored because a 30—100 dilution of starter with PO would require an extraordinary reactor to provide adequate stirring. PO and/or EO are added continuously until the desired OH No. is reached the reaction is stopped and the catalyst is removed. A uniform addition rate and temperature profile is required to keep unsaturation the same from batch to batch. The KOH catalyst can be removed by absorbent treatment (140), extraction into water (141), neutralization and/or crystallization of the salt (142—147), and ion exchange (148—150). 

The polyols used are of three types polyether, polyester, and polybutadiene. The polyether diols range from 400 to about 10,000 g/mol. The most common polyethers are based on ethylene oxide, propylene oxide, and tetrahydrofuran or their copolymers. The ether link provides low temperature flexibility and low viscosity. Ethylene oxide is the most hydrophilic and thus can increase the rate of ingress of water and consequently the cure rate. However, it will crystallize slowly above about 600 g/mol. Propylene oxide is hydrophobic due to hindered access to the ether link, but still provides high permeability to small molecules like water. Tetrahydrofuran is between these two in hydrophobicity, but somewhat more expensive. Propylene oxide based diols are the most common. 

The most common non-crystallizing soft segment in urethane adhesives is based on poly(oxypropylene) polyols, shown in Table 2. Most non-crystalline soft... 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

The presence of particles in the brains of experimental rats and humans exposed to asbestos has been reported (Pontefiart and Cunningham, 1973 Auerbach et al., 1980). In experimental studies, particles of Teflon, a reflux paste, enter the brain via intravascular transport when injected into the bladder (Aaronson et al., 1993). Encephalitic reactions to accumulated calcium oxalate crystals in the brain as a result of infusions of glucose surrogate polyol solutions have been described (PciflPcr etal., 1984). Such studies indicate the capacity of particulates to enter the brain and thus pose a potential pathological threat to the functioning of the central nervous system (CNS). 

Whatever the precursor, the formation of an intermediate solid phase was always observed. It can be inferred from X-ray diffraction (Fig. 9.2.7) and infrared spectroscopy that this intermediate phase shows a lamellar, incompletely ordered structure (turbostratic structure) built up with parallel and equidistant sheets like those involved in the lamellar structure of the well-crystallized hydroxides Ni(OH)2 or Co(OH)2, these sheets are disoriented with intercalation of polyol molecules and partial substitution of hydroxide ions by alkoxy ions (29). The dissolution of this solid phase, which acts as a reservoir for the M(I1) solvated species, controls the concentration of these species and then plays a significant role in the control of the nucleation of the metal particles and therefore of their final morphological characteristics. For instance, starting from cobalt or nickel hydroxide as precursor in ethylene glycol, the reaction proceeds according to the following scheme (8) ... 

Lyoprotectants can affect enzyme stability in both stages of lyophilization the freezing and the drying stages. In the freezing stage of lyophilization, ice crystals form and have been shown to be a cause of enzyme denaturation. Studies have shown that when added as a lyoprotectant, the amorphous polyol mannitol stabi-... 

Starch-PVA films plasticized with glycerol embrittled with age. A study of various polyols was undertaken to identity plasticizers which would reduce this tendency.131 It was demonstrated that sorbitol and glycol glycoside performed well. When sorbitol alone was used, it migrated to the surface and crystallized. When sorbitol and glycerol were used in a 3 1 w/w ratio, the films were stable and showed no evidence of sorbitol crystallization. Substituting plasticizer with PVA did not adversely affect the films when starch was substituted for plasticizer, the films became more susceptible to aging. 

---