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Electron‐microscopic examination discloses the existence of globular formations, or micelles, in thermosetting resins, varying in apparent diameter from 400–900 A. These micelles are seen in thin sections, in preshadowed carbon replicas, in shadowed plastic strips revealed in profile, and as shadowed individuals. Micelles are abundant in phenolic, diallyl phthalate, and epoxy resins, and comparatively rare in silicones. The existence of micelles is in accordance with the theories of Houwink and Stäger, who consider that polymerization proceeds most rapidly at specific points in the resin; and explains the discrepancies between theoretical and observed tensile strengths. At least in the case of phenolics, micelles appear to arise in filaments formed in the polymerization process. The occurrence of linear arrays is in accordance with this mechanism of micelle formation.

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