Molecular weights between resins

The molecular weight between crosslinks (Me) was determined for each epoxy/amine ratio of the neat resin from the rubbery plateau region of the modulus curve following the Tg region. This can be seen in Figure 13 for several epoxy/amine ratios. The Me values were calculated from the following equation ... 

Figure lA. Glass transition temperature (Tg) and molecular weight between crosslinks (Me) as a function of epoxy/amine ratio for C-stage cured neat resin. 

Condensation in acid medium gives soluble, fusible phenolic resins, with an average molecular weight between 600 and 1,500, and a structure consisting essentially of phenol residues linked by methylene groups in the ortho- and para-positions they are called Novolaks. No further condensation occurs on heating... 

The commercially most important epoxy resins are those prepared from 4,4 -isopropylidenediphenol (bisphenol A) and epichlorohydrin. They have molecular weights between 450 and 4000 [n in formula (II) between 1 and 12] and softening points between 30 and 155 °C. Such epoxy resins are still soluble, but become insoluble and infusible through subsequent crosslinking reactions. 

These data are consistent with the observation O) that increased crosslinking (i.e., epoxy resins with lower EEW s) will result in a decreased propensity for the cured polymer to absorb solvents. Thus, the solvent resistence of an organic coating can be controlled by the formulator via variations in the molecular weight between crosslinks. 

Epoxy resins are made with a reaction producing a resin with a molecular weight between 350 and 6000. These resins are terminated with an epoxide group. Figure 5.4 illustrates a basic unhardened epoxy resin. 

Crosslink density may be defined as the number of effective crosslinks per unit volume. The crosslink density is a key parameter in determining the properties of an epoxy resin after cure. It is dependent on the number of reactive sites (functionality), the molecular distance and chain mobility between functional sites, and the percentage of these sites that enter into reaction. Crosslink density is inversely related to the molecular weight between crosslinks Mc. 

FIGURE 3.12 Effect of molecular weight between crosslinks on the physical state of epoxy resins. 

Figures 14 and 15 show the relations between the amount of iron arene initiator, the reaction enthalpy (AHj and the glass transition temperature Tg of the polymerized Bisphenol-A diglycidylether (cf. Table 2, structure I, x = 0.15) and the oligomer product based on the former compound (cf. Table 2, structure I, x = 11.8). The maximum polymerization heat per mole of epoxide is observed ivith an initiator concentration of 1.5-2.5% (w/w). At this concentration, Tg of the crosslinked resin is about 115 °C for the polymerized low-molecular-weight expoxide and about 80 "C for the polymerized high-molecular-weight epoxide resin.

In many applications of low molecular weight hydrocarbon resins, including flooring, adhesives, rubber compounds, inks, and coatings, the best performance is often associated with plasticizers that are marginal solvents rather than perfect ones. The difference between the resin parameter and the plasticizer parameter indicates the place of the system in the Flory-Huggins phase diagram. The separation of phases is responsible for the improved physical properties. While the difference of the parameters readily explains the behavior, the parameters for many industrial materials are not sufficiently well defined, and specific solubility tests must be used to control both resin and plasticizer. 

The molecular weight of the polymer formed is a function of the reaction temperature used during the polymerization. Lower temperatures favor increased molecular weights. The relationship between polymerization temperature and molecular weight of resin (as expressed by relative viscosity) is shown in Figure 6. In commercial practice, the molecular weight is controlled by reaction temperature, and the reaction rate is controlled by the selection of the initiator and its concentration. 

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