The nature of bonds in paper and the behaviour of paper under mechanical strain
Work done during the past twenty years on the physics of fibre bonds in paper and the behaviour of paper under mechanical strain is reviewed. The first major section of the article illustrates the chemical nature of the cohesion in paper and reports the results of direct observations on fibre-to-fibre bonds. A statistical theory of the geometry of fibrous networks is shown to be a basis for the characterization of fibre bonding in paper. The second major section is concerned with the mechanical behaviour of paper. This is shown to be a complex subject because paper exhibits almost every known rheological property. Direct, conventional rheological methods, molecular theories and the concept of elastic networks are discussed in this context. Further complications arise from any changes in the water content of the environment. Such effects are conveniently thought of as a competitive process wherein cellulose and water compete for hydrogen bonds for their hydroxyl groups. Not least among the rheological phenomena that paper exhibits is mechanical fracture. This is a discontinuous process that is found to be intimately connected with the mechanical work done on the specimen and is principally governed by its geometrical distortion.