Kraft lignin in polyurethanes. III. Effects of the molecular weight of PEG on the properties of polyurethanes from a kraft lignin–PEG–MDI system

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Abstract
Polyurethane (PU) films were prepared by solution casting using a three‐component system consisting of a low molecular weight fraction of softwood kraft lignin (KL), polyethylene glycol (PEG) of various molecular weights, and crude diphenylmethane diisocyanate (MDI) as starting materials. All films were prepared using a constant NCO/OH ratio. The effects of KL content, PEG content, and molecular weight of PEG on the tensile, thermal, and swelling properties of the PUs were studied. The results showed that the crosslink density Tg and tensile properties were essentially unaffected by variations in the sequence length of the soft segment at a given PEG content. The properties of the PUs were found to depend mainly on the soft/hard segment ratio. The behavior of the glass transition temperature upon changes in the PU composition indicated that the soft and hard segments were extensively mixed.