Development of Biobased Poly(ester-urethane) Coatings with Excellent Hydrophobicity from Tomato Byproducts

Cutin from tomato byproducts is used to synthesize poly(ester-urethane) (PEU) coatings by a two-step reaction. In the first step, polyester polyols (POLs) are prepared by esterification with 1,4-butanediol (BDO) of the omega-hydroxy acid mixture from cutin (HC). A design of experiment (DOE) approach is used to optimize this reaction step. HC and POLs are characterized by H-1, C-13 NMR, attenuated total reflection (ATR) infrared spectroscopies, acid-base titration, titration of the hydroxyl groups, and size-exclusion chromatography (SEC). Two different POLs obtained with 1:1 or 1:3 BDO/HC weight ratio are then used to prepare PEU coatings by reaction with isophorone diisocyanate (IPDI) or hexamethylene diisocyanate (HDI). The four resulting PEUs are characterized by ATR, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) and tested on paper, glass, and wood substrates. With IPDI the formation of amorphous coatings with smooth surfaces by scanning electron microscopy (SEM) is observed. On the contrary, HDI gives semicrystalline coatings having rough surfaces and water contact angles larger than 100 degrees. All prepared PEUs show good hydrophobicity on all tested substrates, with the lowest (similar to 72 degrees) contact angle values observed on polyurethane prepared with IPDI. Adhesion by the Scotch test was very good on paper, worst on glass, and intermediate on wood, thus showing these coatings suitable for cellulose-based substrates.