The Application Notebook
Traditionally, simple analytical characteristics (for example, content of epoxy groups, chlorine, α-glycol groups, viscosity or softening point) are used to identify epoxy resins.
Epoxy resins are an important group of synthetic resins. They find applications in a wide variety of industries, and are used for the production of coatings, adhesives, laminates, casting materials, etc.
Traditionally, simple analytical characteristics (for example, content of epoxy groups, chlorine, α-glycol groups, viscosity or softening point) are used to identify epoxy resins. But these quantities are often unable to distinguish among particular samples and to find relations between the epoxy structure and its properties.
For virtually all synthetic resins, there are no suitable size-exclusion chromatography (SEC) calibration standards available. The preparation of standards by fractional precipitation is laborious and frequently leads to unsatisfactory results. Polystyrene calibration standards are used universally, but are far from the true values of the resins they purport to mimic, so the absolute analysis can only be completed by DAWN or miniDAWN multi-angle light scattering (MALS) coupled to SEC.
Figure 1: Light scattering (top) and RI (bottom) chromatograms from SEC of bisphenol A based epoxy resin. Mn (SEC-MALS) = 1520 g/mol, Mn (VPO) = 1470 g/mol.
In this application note, the chromatograms recorded from the SEC analysis of an epoxy resin sample are depicted in Figure 1. The light scattering signal proves conclusively that the miniDAWN MALS detector can be used to detect even very low molar masses of the epoxy. In fact, the Mn value determined by SEC–MALS agrees with those determined by vapour pressure osmometry (VPO), which measures absolute Mn directly.
Figure 2: Differential molar mass distributions of two epoxy resin samples with equal epoxide equivalent weights of 1670.
Figure 2 contrasts differential molar mass distribution curves of two epoxy resin samples with identical epoxide equivalent weights. It demonstrates that the capabilities brought to SEC by MALS is the most direct way to correctly determine the molar mass of synthetic resins. The MALS detection reveals differences that are not seen by traditional analytical methods or by using SEC with column calibration and RI detection alone.
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