I'm looking at the 3-D Clear Organizer Cube as a 3-1-1 bag. I bought a cheaper, well constructed REI bag, but after one or two trips the plastic turned opaque. The plastic used in the cube is listed as "urethane". The plastic in the REI bag is listed as "thermoplastic urethane". Is that different? Has anyone had any issue with the plastic on the cube turning opaque?