How to physically and chemically distinguish polyester-based and ether-based thermoplastic polyurethane?

Question

Accepted Answer

Thermoplastic polyurethane (TPU) is a linear block copolymer composed of soft segments from polyols and hard segments from diisocyanates and chain extenders. TPU can be classified into polyester-based and ether-based types. Here are reliable methods to identify them:

Physical Methods:

Density Test: Measure the density of the TPU sample. Ether-based TPU typically ranges from 1.13 to 1.18 g/cm³, while polyester-based TPU ranges from 1.18 to 1.22 g/cm³.
Tear Test: Tear the TPU film. If the resulting tear has obvious jagged edges, it is polyester-based TPU. If the jagged edges are minimal, it is ether-based TPU.

Chemical Methods:

Colorimetric Reaction Method 1: Dissolve the TPU sample in 5–10 mL of glacial acetic acid. If insoluble, use other solvents such as dimethylformamide (DMF) or o-cresol. Drop the solution into approximately 0.1 g of p-aminobenzaldehyde reagent. Polyester-based TPU will develop a yellow color within a few minutes. Ether-based TPU does not show this reaction.
Colorimetric Reaction Method 2: Mix about 5 g of TPU with phenolphthalein in methanol and add a few drops of 2 M potassium hydroxide. Then add a few drops of hydroxylamine hydrochloride saturated solution in methanol. Heat to 50°C briefly, acidify with 1 M hydrochloric acid, and add one drop of 3% ferric chloride solution. Polyester-based TPU will immediately display a purple or reddish-purple color, while ether-based TPU shows no color change.

Physical methods are simple and easy to perform, while chemical methods are more complex but provide higher accuracy. Choose the method that best suits your available resources.