May 11, 2010

Midwest Biopolymers & Biocomposites Workshop

 

Sophie Morneau

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Ultrasonic Cutting of Biodegradable Polylactic Acid (PLA) Films

Ultrasonic cutting and welding are commonly used in the packaging industry. The cutting knife/edge vibrates at frequencies typically between 20-40 kHz, heating the substrate during the cutting and simultaneously sealing the cut edge. With the growing use of polylactic acid (PLA), a biodegradable thermoplastic material derived from starch, in the packaging industry there is a corresponding growing need to cut and seal this material. However, PLA is relatively brittle compared to other commonly used packaging materials, such as polyethylene terephthalate (PET). It has been anecdotally reported that this issue is enhanced during mechanical cutting.

This study used a Branson Ultrasonic’s system to cut and seal PLA films. The system had a knife edge fixture and a flat faced horn. For the cutting experiments, the equipment was operated in a “ground detect mode.” During the welding experiments, a flat faced horn was used with a raised fixture design. Welded films were cut both mechanically and with ultrasonics. In addition, virgin films were cut both mechanically and with ultrasonics. The weld and base material properties were compared including tensile strength and toughness. It was found that micro-cracks were less common in the samples cut with ultrasonics, leading to samples with a higher toughness. In addition, it was found that ultrasonics could weld various thicknesses of PLA film in less than 500 mS and achieve strengths matching the base material strength. When welded samples were cut (after welding) with ultrasonics, the strength was increased by a full magnitude compared to samples cut after welding with a mechanical shear. When virgin samples (unwelded samples) were cut, it was found that while there was little difference in the tensile strength, there was a significant increase in toughness when ultrasonics were used compared to mechanical shears.

Sophie Morneau

Sophie Morneau

Manager, Plastics Joining Laboratory & ATG Customer Service

Branson Ultrasonics

Sophie.Morneau@
emerson.com

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