Photolysis of segmented polyurethanes. The role of hard‐segment content and hydrogen bonding

Abstract

The photodegradation of segmented polyurethanes based on methylene 4, 4′‐diphenyldiisocyanate (MDI) is shown to be dependent on the physical structure of the polymer. As the hard segment content of the polyurethane is increased, the photodegradation efficiency is lowered. In particular, the extent of the photolytic decomposition is inversely dependent on the degree of hydrogen bonding in the aryl carbamate groups in the polyurethane backbone. Utilizing appropriate model compounds for comparison, the formation of the ortho photo‐Fries rearrangement product, as detected by fluorescene spectroscopy, is also shown to be dependent onthe degree of hydrogen bonding. In general, the restrictive mobility imposed by hydrogen bonding is a critical factor which must be considered in the photochemistry of segmented polyurethanes.

Degradation mechanism of poly(ether‐urethane) Estane® induced by high‐energy radiation. II. Oxidation effects

“…Otherwise, hard segments oxidation also occurred in the central methylene of MDI as described by mechanism II and induced formation of benzoic acid derivatives 3. Mechanism III indicated diimide diquinones formation from hydroperoxide for wavelengths higher than 340 nm 6–9…”

“…In aromatic PURs with methylene bis(p‐phenyl isocyanate) (MDI) hard segments, three main mechanisms described in Figure 3 could induced HS degradation. Firstly, the energy absorption induced urethane scissions at CN bonds leads to photo‐Fries rearrangement,6 as illustrated by mechanism I. The photo‐oxidation of photo‐Fries products leads to carboxylic acids.…”

“…Several studies tried to observe radicals issued from PUR hard segments and soft segments degradation using Electron Paramagnetic Resonance (EPR) 6, 15–17. In a poly(ester‐urethane), the signal observed under irradiation in anaerobic conditions fastly decreased under oxygen and changed into a anisotropic signal attributed either to peroxy or nitroxyl radical, that were very stable in time under oxygen at 300 K. This signal was attributed to the superposition of three signals: a peroxy signal, an alkyl signal and a broad singlet (Δ H pp = 3.6 mT) attributed to radical • C(O) O(CH 2 ) n or φNH • CO 6…”

Preparation and properties of MDI/H12MDI‐based water‐borne poly(urethane‐urea)s—Effects of MDI content and radiant exposure

“…The humidity and temperature of the chamber were probably the factors that drove up the yellowness of the screened part. These behaviors are probably attributable to MDI's photosensitivity, thought to be caused by aromatic diurethane bridge 10, 13–15. The YI of samples increased in the order of WBPU‐50 > WBPU‐25 > WBPU‐0 (not changed), indicating the increase of photo oxidation with increasing MDI content.…”

“…The secondary factors are atmospheric pollutants, biological agents, acts of nature, high‐energy radiations, and mechanical stress (accelerates physical damage). A number of authors have investigated the harmful effects of UV exposure on polyurethanes 9–13. For polyurethanes based on MDI, the photosensitivity is thought to be due to an aromatic diurethane bridge.…”