Polyurethanes solvent selection

Although the use of lignin as an additive to polyurethanes is not new (15-20), even the most judicious selection of lignin isolation or modification schemes has not allowed researchers to overcome the incorporation limit of 25 to 40 weight percent of lignin as an active component in polyurethanes. Solvent fractionation allows for the isolation of lignin fractions with well defined solubilities and functionalities (21,22). Both of these features are critical for the practical inclusion of lignin into liquid polyol systems. 

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes. Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate), UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below). 

The effectiveness of the carboxylic acid as adhesion promoter is governed by its compatibility with the polyurethane and the solubility in the organic solvent selected to dissolve the polyurethane. It has been established (Pastor-Sempere et al. 1995) that the mechanism by... 

Polycarbonate primers in Europe are 2K-isocyanate curing or IK-nonreac-tive products based on fairly flexible linear polyurethane or polyester resins. Thus the low glass transition temperature (Tg) of these flexible resins maintains the low-temperature impact resistance of the plastic. This resin selection is in addition to the proper solvent selection. In general, PC painting is not considered difficult when the coatings are formulated consistent with that discussed previously. 

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Yoon el al. [112] reported an all-solid-state sensor for blood analysis. The sensor consists of a set of ion-selective membranes for the measurement of H+, K+, Na+, Ca2+, and Cl. The metal electrodes were patterned on a ceramic substrate and covered with a layer of solvent-processible polyurethane (PU) membrane. However, the pH measurement was reported to suffer severe unstable drift due to the permeation of water vapor and carbon dioxide through the membrane to the membrane-electrode interface. For conducting polymer-modified electrodes, the adhesion of conducting polymer to the membrane has been improved by introducing an adhesion layer. For example, polypyrrole (PPy) to membrane adhesion is improved by using an adhesion layer, such as Nafion [60] or a composite of PPy and Nafion [117],... 

Luo et al. have fabricated a conductive polyurethane/MWNT composite thin film with a selective vapor-induced sensing performance through in-situ dispersion polymerization (120). This kind of thin films yielded higher responsivity to some solvent vapors like benzene, toluene, acetone and chloroform than others such as carbon tetrachloride, cyclohexane, methanol and formaldehyde. 

Dickert and co-workers [18] have made an innovative advance by applying polyurethane-based MIPs to quartz crystals for the selective detection of solvent vapours (see also Section 20.2.5.8. and Chapter 21). Electrodes may be attached to quartz crystals to form quartz crystal microbalances (QCMs), wherein minute increases in the mass of the device (for example, upon adsorption of solvent vapour) result in a decrease of the resonant frequency of the crystal. For crystals... 

When the analytes are to be retained in a sorbent, the sample (which can be solid, semi-solid, liquid or gaseous) is inserted in the solid state into the extraction cell. Samples in the latter three forms are supported on an appropriate material in order to ensure effective attack by the supercritical fluid. Solid supports are not used for liquid, gaseous and semi-solid samples only, however. Some research work conducted so far on solid samples has involved not natural samples but synthetic ones prepared from a selected sorbent (a natural matrix where the presence of the analytes of interest was previously excluded or a synthetic support such as polyurethane foam or glass wool) with which a solution containing the analytes was homogenized. Quantitative evaporation of the analyte solvent is mandatory as any residual solvent may alter the polarity of the supercritical fluid and hence its action to an extent dependent on the particular fluid and solvent properties, and also on the amount of solvent retained. 

Pb was sorbed on a resin [27] and polyurethane foam [28] modified with the crown ether, 18-crown-6, and its derivatives. The sorption of Pb (and other metals) on a chelating resin containing Arsenazo I was studied in water-organic solvent media [29]. A silica gel, modified with 2-mercaptobenzothiazole [30] or 2-mercaptobenzimidazole [31 ] was used for concentration of Pb and other heavy metals from aqueous solutions. Pb was selectively separated from Al, Mg, and Fe(III) on a cation exchanger modified with tin(IV) antimonate [32]. 

The reaction could not be carried out with the usual sulfonic acid ion-exchange resin, because its maximum use temperature was 120°C. The product cannot be made directly from acetone and aniline owing to the formation of a dihydroquinoline by-product (from two molecules of acetone and one of aniline). A shape-selective zeolite might allow the reaction to take place without formation of this byproduct. An inexpensive way of making this diamine from acetone and aniline, similar to the preparation of bisphenol A from acetone and phenol, could lead to new families of polyamides, polyimides, polyureas, polyurethanes, and epoxy resins. A palladium catalyst supported on Nafion dimerized ethylene much faster in water than in organic solvents. The butene was easy to separate.18... 

Dimethylformamide (DMF) has been known since 1893, but since the 1950s, it has evolved as an important solvent. Its main uses are as a solvent for spinning acrylic fibers, polyurethane and polyamide coatings and films, PVC, polyacrylonitrile, extraction of aromatics from petroleum, selective solvent for removal of acid gases from natural gas, solvent for dyes, electrolyses in galvanization processes, and paint remover and cleaner [16]. By 1980, the worldwide production of DMF had grown equal to the production of formic acid at 220,000 metric tons per year [18]. By 1993 the U.S. production of formic acid was 30 to 35 mm pounds and DMF production had grown to 60 to 65 mm pounds. 

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