Dr. Duane Priddy, Plastic Failure Labs
Cracked plastic mouthwash tubes were received along with freshly molded empty tubes that had not yet been used. Tests were performed on the tubes to determine the following: material composition (FTIR), filler content (TGA), molecular weight (MW) and molecular weight distribution (MWD) of the resin (HT-SEC). The tubes are also being analyzed for manufacturing defects using optical microscopy.
FTIR showed that the plastic material used to make the tubes was polyethylene containing an inorganic filler (titanium dioxide). TGA showed that the filler content is 3.5%. The MW and MWD values for the resin in the tubes indicated that the amount of regrind material in the tubes was >30%. Optical microscopy showed that the unfilled tubes contain incipient cracks. These incipient cracks open up to become open cracks after the tubes are filled with mouthwash. Examination of the cracks showed that the material in the region of the crack is contaminated with debris, most likely from the use of contaminated regrind material. Also the tube wall thickness in the crack region is significantly thinned compared to the wall thickness on the tube wall opposite the crack. Large variation in wall thickness causes stress concentration on the material cross-section in the thinnest regions of the tube.
The root cause of failure of the tubes is two-fold:
- the material used to make the tubes is contaminated with debris (due to the extensive use of regrind) which act as defects in the material to initiate the cracking; and
- the wall thickness around the circumference of the tube is not uniform.
Above Image: Top is a the appearance of the freshly molded bottle before filling with mouthwash. Bottom shows a bottle that cracked after filling/sealing.