Glass Fiber Length Degradation in Thermoplastics Rocessing

Abstract

Abstract Glass fiber length is of great concern to both the compounder and molder of fiberglass reinforced thermoplastics. We know that the fiber attrition is considerable, but how short is it and where in the process does it occur? Can it be affected by using the longer length chopped strands? These are the types of questions that have gone unanswered because of the considerable time and effort necessary to quantify fiber length and length distributions. Burning the resin off, taking a photomicrograph of the glass, and physically measuring the lengths with a scale takes a great deal of time and money. A second method sometimes employed is a sieve analysis. This involves a series of screens which classify the fiber length by the size of the openings. This method, however, can lead to higher than actual averages due to a matting effect of the longer fibers which sometimes can trap the shorter fibers. Fibers that pass through the screen openings lengthwise can also distort the reported value. There is now available fast and accurate computerized, electronic image analyzing equipment that facilitates fiber length measurement. To summarize, the average fiber length in molded bars from extrusion compound with 1/8, 3/16, and 1/4 in. chopped strands were a close 0.009, 0.010, and 0.010 in., respectively, for PBT. For PP, the values were 0.025,0.026, and 0.027 in., respectively. Only slight shifts in distributions were evident. The majority of the fiber attrition occurred in the extruder, with further damage in the injection unit of the molding machine. No further damage was evident during injection of extrusion compound through runners and gates. There was, however, further damage to the fibers upon injection through runners and gates in direct molded PP. Average fiber lengths in this case were 0.026, 0.023, and 0.028 in. with 1/8, 3/16, and 1/4 in. input lengths, respectively. There also was a shift in length distribution toward longer fibers in direct molding. This shows up as higher impact values.