May 11, 2010

Midwest Biopolymers & Biocomposites Workshop


Poster Abstracts

110: Ultrasonic Welding and Cutting of Polylactic Acid (PLA) Films

David Grewell1 and Julius Vogel2

1Department of Agricultural and Biosystems Engineering and 2Mechanical Engineering, Iowa State University, Ames, Iowa

In this work, the weldability of polylactic acid (PLA), a biodegradable polymer derived from corn starch was examined. Samples of biaxial oriented PLA films of various thicknesses were impulse and ultrasonic welded at various processing parameters. It was found that depending on the process and operating parameters, PLA could be welded with weld strengths equal to parent material strengths. Relatively high weld strengths were achieved with impulse welding over a wide range of welding parameters. In addition, ultrasonic welding produced high weld strengths with relatively short cycle times while the weld strength was more sensitive to the welding parameters. In detail, ultrasonic welded samples had a weld factor (weld strength/base material strength) of one at cycle times of 0.25 s. The weld factor was significantly lower at shorter weld times as well as weld times above 0.35 s. However, with ultrasonic welding, the standard deviation of the weld factor was relatively high at ~30%. In contrast, with impulse welding of samples with a thickness of 100 µm for 2 to 3 s had a weld factor of one and a standard deviation of only ± 5%.

Ultrasonic cutting is often used in industry to cut products that are difficult to cut with standard mechanical systems. The cutting knife/edge typically vibrates at a frequency of 20-40 kHz, heating the substrate during the cutting and possibly simultaneously sealing the cut edges. PLA is relatively brittle compared to polyethylene terephthalate (PET), which has similar mechanical properties. This brittleness is enhanced during mechanical cutting which produces micro-cracks further embrittling PLA. In this study, a Branson Ultrasonic equipment was used to cut PLA films. The system had a knife fixed to the anvil and a horn with a flat surface that engaged the knife. The equipment operated in a ground detect mode. After cutting, the samples were tested for mechanical properties such as tensile strength, toughness, and tensile elongation and compared with samples cut using a mechanical shear, as well as optically examined for micro cracks. It was found that ultrasonic cut samples had higher toughness and elongation, and the edges did not have micro-cracks compared to samples cut with mechanical shears.