Physical Structure of Phenoplasts
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OR many years the The rate of net-work formation, when cured phenolic films are swollen in acetone, i s c h e m i s t has been helpful in investigating phenoplasts in advanced states of cure. This technique has been applied in an investigation of the effect of catalyst concentration, reactant ratios, temfamiliar with the resinous products resultperature of initial reaction, and temperature of cure on the degree of cure in the final hardened resin. The existing concepts of resin structure are reviewed in the light of this ing from the condensations of aldehydes with additional experimental evidence. A modlfied concept of phenoplast structure which Dhenols. H e is equally more closely conforms with the experimental facta is proposed. iamiiiar with t h e domin a n t p o s i t i o n which phenoplasts have assumed in the synthetic resin field since their R . A . Barkhug,Jr., and S . Carswell introduction in the 6rst decade of this century. Despite their MONSANTO CHEMICAL COMPANY, SPRINGFIELD, MASS. favorable economic position, relatively little is known about the chemical nature and the physical structure of the thermosetting resins in general and the phenoplasts in particular. This gel point for threedimensional glyptal-type resins from a stasituation has resulted from two factors: (a) T h e immediate tistical study of the reaction mechanism. However, as will be requirements of industry, in the past, have been satisfied withseen later, such a postulated mechanism does not appear capable out the neceasity of a great deal of fundamental research. of accounting for all of the experimental facts in this complex (b) The cured resinoid does not permit usual methods of inphenolic system. vestigation, such as viscosity or osmotic measurements in soluHetenyi (8) provided evidence for this type of structure, tion and x-ray diffraction patterns. based on its elastic behavior. He found that slightly cured Despite these difficulties, investigators such as Bender, Rassections of thermosetting resins, when subjected to stresses a t chig, Granger, Megson, Koebner, and Zinke, to name only a few, llOo C. and then slowly cooled, retain their elastic deformation have made important contributions to our knowledge of the on the removal of the load. Such a test piece may then be sewed organic chemistry of the condensation products. Investigaand viewed in polarized light. An isochromatic fringe pattern, tions into t h e physical structure of these resinoids have been corresponding t o an elastic state of stress, may then be obmore fragmentary but have led t o the isogel theory of Houwink served. H e explains this phenomenon on the basis of a gelatinous and others and to the sphero-colloid theory, which was recently threedimensional network, which acts elastically, and an inelaborated by Stager. ternal viscous phase. At elevated temperatures this viscous phase is quite fluid, but, upon cooling, the viscosity increases ISOGEL THEORY until a t room temperature it is so great that i t prevents the d e formed elastic network from returning to its original shape. The initial reaction between phenols and aldehydes leads t o This is but one of the many arguments supporting the sponge the formation of small, two-dimensional products joined by structure for these thermosetting resins. methylene linkages. Through further polycondensations, the Houwink’s studies (6)on high elasticity observed in B-stage molecules grow in size in two or three dimensions until they phenolic resins form another line of evidence for the sponge reach the ill-defined range termed “colloidal”. At this point, structure. Houwink showed further (6) that the tensile strength after dehydration, the resin is in the state usually referred to aa increases as cure proceeds and as the elastic skeleton grows. isocolloidal. This state, in essence, is composed of condensed It is well known that the tensile strength of such resins never molecules having a rather limited range of particle size. These approaches that calculated from either primary or sccondary are dispersed in a medium of their lower homologs; the convalence bonds; this has been demonstrated by the work of densing agent or catalyst is often viewed as the stabilizing Houwink (6) and de Boer (1). This discrepancy is often exmedium. plained by the presence of hcheretdlen-that is, gaps in the Under the influence of heat this dispersed phase grows in primary bond structure which serve as flaws at which stresssize at the expense of the dispersing medium until a critical concentrations build up. point is reached at which gelation takes place. According t o the theory of Houwink and others, once the gel point is reached, SPHERO-COLLOID THEORY the material is considered t o be an isogel which is composed of Another school of thought holds that a somewhat opposite infinitely large macromolecules which form a spongelike skeleton structure exists. The more highly polymerized three-dimenthroughout the mass. Trapped in the capillaries and pores of sional network forms small clumps, or sphero-colloids, which are the sponge are the unreacted ingredients and resin particles of a surrounded by a continuous phase of the lower-molecular-weight lower degree of condensation. This portion is a more mobile, viscous material which is often referred to as Harzbrei. T h e viscous Harzbrei. According to this viewpoint, the isocolloid structure is retained throughout the curing of the resin and is to spongelike skeleton portion of the structure becomes increasbe found in the final cured mass. I n this theory there is no ingly predominant as condensation progresses. With further need to assume t h a t the isocolloidal particles are fully linked by condensation the Harzbrei becomes more closely identified with the sponge skeleton until ultimately the greater majority of the primary valence bonds t o give a rigid sponge skeleton. Gelation is assumed t o be the result of association and interpenctration available linkages are employed in the tight three-dimensional of the molecules. network. However, this ultimate state of cure is apparently Stiiger and co-workers (7,8,9), in an effort t o elucidate further seldom reached in the molding of commercial phenolic resins. the structure of the phenoplasts, conducted an extensive inMany of the mechanical properties of the phenoplasts seem vestigation of broad significance during 1936-40. Swelling to support the isogel structure. The elastic nature of the resin phenomena resulting from the action of solvents on the cured is attributed t o the rigid macromolecular skeleton, whereas its phenoplasts were especially studied in this work, and an attempt viscous flow characteristics are attributed t o the Harzbrei. was made to correlate t h e observations with the mechanical Flory (9) and Stockmeyer (IO) presented one of the strongest properties of the phenoplasts. Stiiger interprets his results as arguments for the isogel structure in their ability to predict the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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