Influence of chemical composition on the rheological behavior of condensation reaction resins

In this paper, the chemorheological and dynamic mechanical behavior of melamine-formaldehyde (MF) resins of four different formaldehyde/melamine (F/M) molar ratios (1.25, 1.5 1.75 and 2.00) are Investigated. ME resins polymerize via a polycondensation reaction involving formation of up to 10 wt% of [H.sub.2]O on cure. This typically results in rapid and extensive foaming of the resin when it is cured under atmospheric pressure. Experimental adaptation for the foaming behavior of ME resins is used to gather rheological Information concerning the curing kinetics and the mechanical response of neat ME resins of different molar ratios. Likewise, the procedures developed allow curing of the resins under atmospheric pressure, hence allowing volatile evacuation as occurs during venting procedures (commonly used during compression molding of ME molding compounds) or as a result of absorption by hydrophilic fillers or substrates. The results show that increased moisture content in the B-stage leads to faster reaction rates and greater foaming. Gelation and vitrification times are identified for each molar ratio, and are found to increase with decreasing molar ratio. The dynamic mechanical behavior of carefully molded neat ME samples of different molar ratios is studied using DMTA. [T.sub.g] is found to be 200°C for the resin with the lowest formaldehyde content (F/M = 1.25), and around 230°C for the other resins. The storage shear modulus above [T.sub.g] is studied, and the results show that the crosslink density increases with increasing molar ratio.
INTRODUCTION Polymers based on melamine (1,3,5-triamino-2,4,6-triazine) and formaldehyde have been used commercially for over 60 years. Originally used as wood adhesives, they are now mainly found in flooring and decorative [...]