Curing spectroscopy

The principal techniques for determining the microstmcture of phenoHc resins include mass spectroscopy, proton, and C-nmr spectroscopy, as well as gc, Ic, and gpc. The softening and curing processes of phenoHc resins are effectively studied by using thermal and mechanical techniques, such as tga, dsc, and dynamic mechanical analysis (dma). Infrared (ir) and electron spectroscopy are also employed. 

Spectroscopy. Infrared spectroscopy (48) permits stmctural definition, eg, it resolves the 2,2 - from the 2,4 -methylene units in novolak resins. However, the broad bands and severely overlapping peaks present problems. For uncured resins, nmr rather than ir spectroscopy has become the technique of choice for microstmctural information. However, Fourier transform infrared (ftir) gives useful information on curing phenoHcs (49). Nevertheless, ir spectroscopy continues to be used as one of the detectors in the analysis of phenoHcs by gpc. 

Chemical Properties. MSA combines high acid strength with low molecular weight. Its pK (laser Raman spectroscopy) is —1.9, about twice the acid strength of HCl and half the strength of sulfuric acid. MSA finds use as catalyst for esterification, alkylation, and in the polymerisation and curing of coatings (402,404,405). The anhydrous acid is also usefijl as a solvent. 

No epoxy groups were detectable in the cured polymer by infrared spectroscopy. 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Lussier [71] has given an overview of Uniroyal Chemical s approach to the analysis of compounded elastomers (Scheme 2.2). Uncured compounds are first extracted with ethanol to remove oils for subsequent analysis, whereas cured compounds are best extracted with ETA (ethanol/toluene azeotrope). Uncured compounds are then dissolved in a low-boiling solvent (chloroform, toluene), and filler and CB are removed by filtration. When the compound is cured, extended treatment in o-dichlorobenzene (ODCB) (b.p. 180 °C) will usually suffice to dissolve enough polymer to allow its separation from filler and CB via hot filtration. Polymer identification was based on IR spectroscopy (key role), CB analysis followed ASTM D 297, filler analysis (after direct ashing at 550-600 °C in air) by means of IR, AAS and XRD. Antioxidant analysis proceeded by IR examination of the nonpolymer ethanol or ETA organic extracts. For unknown AO systems (preparative) TLC was used with IR, NMR or MS identification. Alternatively GC-MS was applied directly to the preparative TLC eluent. 

Transparent PVC plates were coated with a 70 pm thick film of an epoxy-acrylate resin containing 0.5 % of a benzotriazole UV absorber. They were first UV cured for one second and then exposed at 40°C to the low intensity radiations of a QUV accelerated weathering tester. The extent of the degradation was followed by Invisible spectroscopy, a very sensitive method that permits detec-... 

Gas chromatography-mass spectroscopy is used to obtain extensive compositional information on rubbers. It can identify minor components of the sample such as the breakdown products of the cure system and the antidegradants. 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

Polymerization Behavior. Both Fourier-transform infrared spectroscopy (FTIR) and differential scanning photocalorimetry (DPC) were used to characterize the polymerization behavior, curing time, and maximum double bond conversion in these systems. 

Bisphthalonitrile monomers were cured neat, with nucleophilic and redox co-reactants, or in combination with a reactive diluent. Dynamic mechanical measurements on the resulting polymers from -150 to +300°C turn up several differences attributable to differences in network structure. Rheovibron results were supplemented with solvent extraction, differential scanning calorimetry (DSC), vapor pressure osmometry, and infrared spectroscopy to characterize the state of cure. 

Figure 5. Monitoring of thermal cure by infrared spectroscopy.

Cole reviews the nse of Raman spectroscopy to monitor the curing of different thermoset composite polymer systems, thongh he cautions that it is not suitable for certain materials [215]. Cruz et al. studied the effect of elevated pressure on the cure rate and the degree of cnre of unsaturated polyester and other thermoset materials [216,217]. The approach worked well, though the experimental set up required addi-... 

A. Mank, E. Verstegen and H. Kloosterboer, In-situ curing analysis of photoreplicated objective lenses using Raman and IR spectroscopy, J. Appl. Polym. ScL, 99, 1287-1295 (2006). 

J.C. Cruz, T.A. Osswald and M. Kemper, Rrecise curing analysis of accelerated unsaturated polyester reactions under Raman spectroscopy. Proceedings, 64th Annual Technical Conference - Society of Plastics Engineers (ANTEC), Charlotte, NC, USA, May 7-11, 2006, Society of Plastics Engineers, Brookfield, CT, 2828-2832. 

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