For the first time, the molecular weight distribution and extent of side reactions have been quantified in polyurethane prepolymers. A combination of NMR, size exclusion chromatography, and mass spectrometry has been used to measure the molecular weight distribution. The values obtained are consistent with Flory's step polymerization theory. The type and amount of side reactions as a function of reaction temperature have also been investigated. At low temperature of reaction, the amount of branches arising from allophanate linkages is negligible. With only a relatively modest change in reaction temperature, this type of side reaction increases dramatically, so that at 145 °C as much as 10% of the nitrogens participate in allophanate linkages. The effect of this reaction on the molecular weight distribution was also measured. Upon formation of these side products, the molecular weight distribution broadens and the average increases. In this prepolymer system, the reactivity is independent of molecular weight.

Hot-melt adhesives facilitate fast production processes because the adhesives set simply by cooling. Formulations contain polymers to provide strength and hot tack (resistance to separation while adhesive is hot), and tackifiers and/or oils to dilute the polymer entanglement network, adjust the glass-transition temperature, lower the viscosity, and improve wet-out (molecular contact of the adhesive with the substrate over the entire bonding area). Some adhesives also contain waxes to speed setting, lower viscosity, and improve heat resistance. Obtaining adequate strength and heat resistance from nonreactive hot melts requires that some component of the hot melt separate out into a dispersed but interconnected hard-phase network upon cooling. The hard phases are commonly either glassy styrene domains (for adhesives based on styrenic block copolymers) or organic crystallites (for adhesives based on waxes, olefinic copolymers, or ethylene copolymers). This article will describe first the material properties relevant to the processing and performance of hot-melt adhesives, then the chemistry and function of the specific raw materials used in hot melts, and will conclude with illustrative application examples and corresponding formulations.

The morphological development of crystallizable polymer blends has been investigated using optical microscopy and infrared and Raman spectroscopy. Both binary and ternary blends were studied. In each case, a crystallizable polyester, either poly(hexamethylene adipate) (PHMA) or poly(hexamethylene sebacate) (PHMS), is mixed with noncrystallizable polyether, poly(propylene glycol) (PPG). Although they possess similar chemical structures, PHMA and PHMS exhibit very different miscibility behavior. In ternary blends, an acrylate, poly(methyl methacrylate and n-butyl methacrylate) [P(MMAnBMA)], is also incorporated in the mixture. With the high spatial resolution achievable (∼1 µm 2 ), the composition distribution can be carried out using a micro-Raman instrument. Specific Raman features associated with polyesters have been established. For immiscible PPG/PHMA blends, the composition and distribution within PHMA-rich and PHMA-poor phases are characterized. The exact composition in each phase has been obtained by analyzing Raman data obtained. Additionally, on the basis of the measured intensity for conformation-sensitive Raman peaks, the distribution of crystallites within each phase has been characterized. The third relative immobile acrylate component is extremely effective in changing the overall blend morphology.

Infrared spectroscopy has been used to study the evolution of polyurethane foam structure, providing measures of relative reaction kinetics, hard segment growth, the onset of phase separation, the formation of order, and the development of final morphology. Changes in free, monodentate, and bidentate hydrogen-bonded urea groups dominate the organization of the entire ensemble. Hard segments formed by reaction of 2,6-toluene diisocyanate (2,6-TDI) and by a mixture of 80% 2,4-TDI and 20% 2,6-TDI displayed very different local segmental alignment, a factor crucial in the development of morphology. Phase separation occurred faster, with fewer and shorter hard segments, in the systems with well-ordered straight chains. The formation and time evolution of monodentate ureas suggest that phase development may be incomplete, or trapped, in systems with ill-defined urea structures. A low degree of spatial order exists in the systems containing these structures.

Using time-resolved Fourier transform infrared spectroscopy, the reaction kinetics of ternary blends consisting of crystallizable polyester [poly(hexamethylene adipate) (PHMA)], polyether [poly(propylene glycol) (PPG)], and poly(methyl methacrylate-co-n-butyl methacrylate) [P(MMA-co-nBMA)] has been characterized. As the polyester and polyether have reactive isocyanate (NCO) units, they are able to react with water vapor in the environment. A specially designed cell was constructed to obtain reaction kinetics for samples of varying thickness at different relative humidity and temperature. Without catalysts, the reaction kinetics obtained is significantly slower than expected for the diffusion-limited mechanism of a homogeneous medium, indicating that the reaction-limited mechanism controls primarily curing in these thin films. As shown previously, the miscibility behavior of these blends at various temperatures is complex. The morphological features, which have been characterized by vibrational spectroscopy, optical microscopy, and atomic force microscopy, depend on thermal history and initial phase behavior. Reaction rates were shown to be highly dependent on sample morphology, being faster with smaller phase-separated domains and lower degrees of crystallinity.