Polymerization in MelamineFormaldehyde Molded Resins d
H. P. WOHNSIEDLER Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.
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HE degree of polymerization (DP) and structure of molded, thermosetting resins and plastics are of considerable importance in connection with their mechanical, thermal, and electrical properties. Various theoretical interpretations of their polymeric structure have been made. No experimental contributions dealing with the degree of reaction in the molded state a n d interpretation of structure based on such data for either the phenol-formaldehyde or amino resin systems have been found in the literature. By providing some experimental data for one of t h e important groups of thermosetting resins, the melamineformaldehyde type, the number of possible structure interpretations can be narrowed down, a t least for this system. Melamine-formaldehyde and urea-formaldehyde polymers belong to the general class designated as condensation polymers. As such they might be expected t o adhere t o the principles governing condensation polymerizations. This latter subject has been reviewed by Flory ( 5 ) . The theoretical kinetics of these condensations have received advanced development against a background of experimental data, chiefly for polyesterifications. In treating Condensations which lead t o linear polymers and certain condensations which lead t o branched, together with cross-linked, polymers, all functional groups are considered as having equal reactivity. The reaction of these groups is regarded as a purely random matter being independent of the number and configuration of other groups as well as the size of the reacting polymer molecules (1,8 ) . From these considerations it has become possible t o derive statistically the average polymer size distribution in relation to the degree of reaction for reacting units of different functionalities (8, 4, 12). I n this treatment intramolecular reactions, those giving rise t o cross linking of branches of individual chains, are neglected, since only a small part of the reaction is of this type (9). I n condensations leading t o three dimensional polymers, where all functional groups do not have the same reactivity, the theoretical kinetics become complicated. Such a situation holds for glycerol as a reactant in a polyesterification, for example. Here the different reactivities of primary and secondary hydroxyl become a factor requiring special consideration (8). The same situation enters in the melamine-forrflaldehyde condensation t o be dealt with, where -NH2 and -K" are potentially reactive groups. I n the case of polyesterifications the linking mechanism is that of ester formation, with a minor pkoportion of anhydride linkages also appearing (9). I n the formation of melamine-formaldehyde polymers linking has been attributed t o a number of possible mechanisms, and their relative importance in actuality remains to be assigned. Both the questions of linking mechanism and group reactivity require clarification in order t o permit a purely kinetic approach in solving the question of average degree of polymerization and structure in the melamine-formaldehyde system. The present method adopts a different approach and one in which the kinetics are purposely assigned a minor role. A number of idealized polymer types are derived, including straight or branched chains, cross-linked chains, and others which vary in the degree of cross linking. For each of the polymer types an expression is derived which relates the average degree of polymeriza-
tion to afactor, Lm,for the extent of reaction. These expressions are also represented graphically. By reference t o these data experimentally determined values for Lm for the molded state aru converted t o average degree of polymerization values for the different idealized polymer systems. From the various values obtained, interpretations are made of the most likely polymer structures and degree of polymerization in the molded state. The resins under consideration are limited t o those derived from a molar ratio of formaldehyde t o melamine of 2 t o 1. RELATIONSHIP OF CONDENSATION WATER TO DEGREE O F POLYMERIZATION
DETERMINATION OF CONDENSATION WATER. Nalter and Gewing (14) were among the first t o formulate possible ureaformaldehyde polymer structures in a systematic manner. They recognized the relationship between condensation water expressed as moles water mole urea and the number of units in the polymer for different types of polymer structure. The possibility of azomethine groups forming was ruled out, and all polymer structures carried the methylene bridge as the sole linking unit. These investigators were more concerned with their technique for resinification and the effect of condensing agents on different starting materials than interpretation of average polymer structure and size. Gams, Widmer, and Fisch (6, 7 )in their study of the melamineformaldehyde system determined the condensation water which was split out in the course of heating of various resins in thin layers. The resinification element was expressed as moles water mole melamine and for values
