SHRINKAGE OF SHORT PP AND PAN FIBERS UNDER HOT-STAGE MICROSCOPE

Marked shrinkage behavior when heated is typical of semicrystalline po lymer fibers such as polypropylene (PP) and polyacrylonitrile (PAN). S hrinkage of PP and PAN fibers may give the possibility to control the spalling tendency of fiber concrete under the heat exposure of fire. C ut staple fibers are normally delivered for concrete reinforcement. Mo dern methods for continuous fibers cannot be used by the end-user for shrinkage determination of commercial staple fiber grades. The shrinka ge of five different commercial staple fibers specially designed for c oncrete reinforcement was studied under a hot-stage microscope. Signif icant differences in cumulative shrinkages of the various PP and PAN f ibers were detected, shrinkages being 3-15% with PP fibers and 6-7% wi th PAN fibers at a temperature of 150-170 degrees C. At about 160-165 degrees C, PP fibers melt, whereas PAN fibers continue shrinking. Hot- stage microscopy provides a simple and a relatively accurate method fo r estimating thermal shrinkage of staple PP and PAN fibers, the deviat ions from measured average values remaining typically at 10-15%. (C) 1 995 John Wiley and Sons, Inc.