Wood pulp fibers can be used as reinforcement in plastics for increased stiffness. A combined analytical-experiential method has been used to assess the stiffness reinforcement of different wood pulp fibers. A large variety of composites based on various wood pulp fibers in an epoxy vinyl ester matrix were manufactured. The fiber investigated should reflect the most commonly used types in the pulp and paper industry for different paper and board applications. The longitudinal Young's moduls of the fibers was determined as the parameter that minimizes the difference between measured composite moduli and the calculated values from a micromechanical composite model. One finding is that industrially produced fibers show consistently lower stiffness than laboratory produced fibers and that mechanically defibrated fibers have significantly lower stiffness than chemically pulped fibers. The results indicate that a mild defib-ration process should be used that does not damage the cell wall structure so that the inherent high stiffness of the native fibers can be retained. It was also found that unbleached fibers are more suitable than bleached fibers for use as reinforcement in polymers in terms of stiffness. An optimal lignin content range exists for softwood kraft fibers where the stiffest fibers can be extracted. The stiffness of the fibers deteriorates below and above a kappa number of 30 to 50 for laboratory-made softwood kraft fibers. Moreover industrially pulped hardwood fibers had higher stiffness than corresponding softwood fibers. Estimated values for the longitudinal Young's modulus were in agreement with values for similar cellulosic fibers found in the literature, e.g., 40 GPa for spruce kraft fibers, 32 GPa for pine kraft fibers, 36 GPa for birch kraft fibers, and 37 GPa eucalyptus kraft fibers.
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