Liquid prepolymers

Few non.chemists know exactly what an epoxide is. but practically everyone has used an "epoxy glue for household repairs or an epoxy resin for a protective coating. Epoxy resins and adhesives generally consist of two components that are mixed just prior to use. One component is a liquid "prepolymer/ and the second is a "curing agent" that reacts with the prepolymer and causes it to solidify. 

Step growth polymerization can also yield highly crosslinked polymer systems via a prepolymer process. In this process, we create a prepolymer through a step growth reaction mechanism on two of the sites of a trifunctional monomer. The third site, which is chemically different, can then react with another monomer that is added to the liquid prepolymer to create the crosslinked species. We often use heat to initiate the second reaction. We can use this method to directly create finished items by injecting a mixture of the liquid prepolymer and additional monomer into a mold where they polymerize to create the desired, final shape. Cultured marble countertops and some automotive body panels are created in this manner. 

The reaction actually involves the sodium salt of bisphenol A since polymerization is carried out in the presence of an equivalent of sodium hydroxide. Reaction temperatures are in the range 50-95°C. Side reactions (hydrolysis of epichlorohydrin, reaction of epichlorohydrin with hydroxyl groups of polymer or impurities) as well as the stoichiometric ratio need to be controlled to produce a prepolymer with two epoxide end groups. Either liquid or solid prepolymers are produced by control of molecular weight typical values of n are less than 1 for liquid prepolymers and in the range 2-30 for solid prepolymers. 

Urethanes. Urethane elastomers are prepared by the reaction of an isocyanate molecule with a high molecular weight ester or ether molecule. The result is either an elastomeric mbber form or a liquid prepolymer that can be vulcanized with an amine or a hydroxyl molecule (see Urethane... 

Urethanes are processed as rubber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The liquid prepolymers are cast using automatic metered casting machines, and the thermoplastic pellets are processed like all thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensile, and resistance to many oils is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

Reactivity of Functional Groups. The reactivity of the functional groups of liquid prepolymers significantly affects the processing, cure behavior, and the ultimate mechanical properties of the cured binder and propellant. The reactivity of carboxyl groups of CTPB can be determined by the rate of reaction with n-butyl alcohol. The rate of esterification is measured from the rate of water evolution from the alcohol—carboxylic acid reaction, and a plot of water evolved vs. time then permits the calculation of the corresponding rate constants. 

A rich variety of materials (including precursors) have been incorporated into the template-directed synthesis.33 169 170 Curable liquid prepolymers are particularly attractive because they have been widely used in replica molding with resolutions better than a few nanometers. By judicially selecting a monomer, the volume shrinkage involved in... 

An especially interesting degradation is that of polymer 2.33 (This polymer was synthesized by the route in equation 8. The isocyanate prepolymer is Hypol 2000, and the diol prepolymer is PEG-1000.) The solid polymer was readily degraded by ambient laboratory light over the course of 5 h, and as degradation occurred the solid polymer was converted back into the liquid prepolymers ... 

A vinyl-terminated silicone prepolymer reacts with active hydrogen in another silicone prepolymer. The two separately stored liquid prepolymers are mixed together, and the mixture can be cast or molded. There is no byproduct hence, these polymers are preferably used in biomedical applications. 

Resin Certain liquid prepolymer products, such as epoxies and unsaturated polyesters, which are subsequently cross-linked to form hardened polymer. 

All the samples made from solid or semi-solid prepolymers had the same fine structure, with the size of the discontinuous phase in the range from 15 nm to 25 nm, whereas the samples made from liquid prepolymers had slightly larger discontinuous phases in the range from 25 nm to 50 nm. 

For systems having a slower rate of reaction (such as samples of Series A, sample E-1, sample E-2, and sample F-2, which were made from liquid prepolymers) the higher diffusivity... 

Epoxy-amine liquid prepolymers are extensively appHed to metallic substrates and cured to obtain painted materials or adhesively bonded stractures. Overall performances of such systems depend on the interphase created between the organic layer and the substrates. When epoxy-amine Hquid mixtures are appHed to a more or hydrated metaUic oxide layer (such as Al, Ti, Sn, Zn, Fe, Cr, Cu, Ag, Ni, Mg, or E-glass), amine chemical sorption concomitant with metaUic surface dissolution appear, leading to the organometaUic complex or chelate formation [1, 2]. Furthermore, when the solubility product is exceeded, organometaUic complexes may crystaUize. These crystals induce changes of mechanical properties (effective Young s modulus, residual stresses, practical adhesion, durability, etc.). 

In previous studies [3, 4], we had pointed out that the interphase formation mechanisms result from dissolution of the metallic surface layers, concomitantly with ion diffusion through the liquid prepolymer. In order to detect the dissolution phenomenon, pure amine (either DETA or IPDA) was previously applied to chemically etched metallic sheets (either A1 or Ti alloys were used, and had hydroxidic surfaces). After 3 h, the metallic surfaces were scraped with a PTFE spatula. The modified amine (i.e., the amine reacted with the metal) was analyzed. Whatever the natures of the amine and the metal were, metal ions were detected in the modified amines by ICP analysis and new peaks were detected by infrared spectroscopy [5]. To indicate hydroxide dissolution, a very thin layer of liquid amine was applied to chemically etched aluminum, and Infrared Reflection - Absorption Spectroscopy (IRRAS) spectra were recorded every 5 min (the hydroxide band intensity variation at ca. 3430 cm was followed). The OH group peak intensity decreased when the amine-metal contact time increased [5]. Conversely, if pure DGEBA monomer was apphed to the metal surfaces, even after 3 h in contact with the metallic surfaces, no metal ion was detected by ICP in the DGEBA recovered, and the infrared spectra remained identical before and after the contact with the metal. Finally, if pure amine monomer was applied to gold-coated substrates, no chemical reaction was observed (by either IGP or FTIR analyses). 

Casting liquid prepolymers on microfabricated molds is the simplest and cheapest method of replication. Masters can be fabricated as discussed above or by simply defining structures in photoresist on a variety of substrate materials. Once the masters have been fabricated, they are often treated with a mold release agent prior to use. The liquid polymer is poured over the molds and placed under vacuum in order to completely fill the features and remove any air bubbles. Finally, the material is thermally cured, and the replica is cut or peeled from the mold. This process is schematically represented in Fig. 3. 

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