Processing-Structure-Property Relationships in Ultra High Molecular Weight Polyethylene Fibers

Christopher K. Henry

doi:10.17918/00000339

Lightweight and robust materials are of interest for many applications. Ultra-high molecular weight polyethylene (UHMWPE) fibers are currently capable of tensile properties comparable to common industrial materials while also providing a low density. A few manufacturers have begun to utilize these materials in various applications, yet fundamental questions regarding the development and cause of the remarkable properties remain unanswered. In this study, the processing and structure of UHMWPE fibers from all stages of fabrication were analyzed to establish relationships with mechanical performance. The goal of this study was to determine which parameters were most critical in the development of the tensile response of the fiber, and then, if possible, how the processing could be altered to generate improved properties. A gel-spinning apparatus was developed to investigate the fiber spinning process. It was found that the strain rate during the spinning process, normalized by the relaxation time and represented via the Weissenberg number (Wi), correlates to the crystalline structure of the fiber, and that velocity ratio (the common method of characterization in these processes) is insufficient in classifying the structure of the spun fiber. A modified extensional rheometer was used to investigate the post drawing of the characterized fibers. Both draw temperature, draw velocity, and starting crystalline structure were studied. Draw velocity and temperature were found to control the sizes observed in the drawn structures. The initial morphology, when drawn at identical velocity and temperature, provides relatively constant structural sizes, but causes changes in the proportions of the structural features and the percent crystallinity. With this data set, we conclude that the crystalline structure is unrelated to mechanical performance. However the intermediate Wi fiber exhibits increased tensile strength and stiffness compared to the other spun fibers. This improved performance is attributed to differences in the initial crystallinity, or distribution of the different crystalline morphologies, of the samples. Differences in starting crystallinity are thought to alter the necessary amount of drawing required to reach the same mechanical response. While draw temperature and velocity do not directly improve the ultimate tensile properties, they do play a role in maximizing the possible deformation. Failure during drawing is observed to occur at a constant stress ceiling, which is independent of all observed parameters. This critical stress indicates a failure defect size of 0.93 nm through Griffith's Theory analysis. When maximum draw ratio is observed as a function of initial fiber diameter it is found that thinner fibers result in higher ultimate draw ratios. Controlling initial diameter, and to a lesser extent temperature and velocity, adjusts the strain at which this critical stress is reached providing higher draw ratios and ultimately higher crystallinity. Crystallinity is further found to be the ultimate determining factor in tensile property behavior, as seen through Fusion theory.

Processing-Structure-Property Relationships in Ultra High Molecular Weight Polyethylene Fibers

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Processing-Structure-Property Relationships in Ultra High Molecular Weight Polyethylene Fibers

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