H. Hespe

On the physical crosslinking of amine-extended polyurethane urea elastomers: A crystallographic analysis of bis-urea from diphenyl methane-4-isocyanate and 1,4-butane diamine*)**)

The substance under investigation is taken as a model for hard segments consisting of MDI 1) and 1,4-butane diamine. It crystallizes in the triclinic space group P¯1 witha=4.6297;b=5.8259;c=25.286 Å;α=90.721;β=91.580;γ=102.90 degrees andZ=1. Bond distances and angles are given, together with some data on the conformation.The most significant finding is that neighbouring molecules in one and the same plane are linked to one another by means of bifurcated hydrogen bonds.A comparison with the corresponding data for an analogous bis-urethane provides a plausible explanation of the main differences between diamine and glycol extension in polyurethane elastomers.

The physical crosslinking of polyurethane elastomers studied by X-ray investigation of model urethanes

Polyurethane elastomers extended with aliphatic glycols or diamines, show a characteristic fluctuation in their thermal and hydrothermal properties which is dependent on the number of CH2 groups in the chain extender (“even” or “odd”).The causes of this behavior are examined in the following report using X-ray crystallographic analyses of single crystals of model urethanes. These model compounds were prepared by reaction between diphenylmethane -4-mono-isocyanate and glycoles of the HO-(CH2)n-OH structure withn=2 to 6.Whilst strain-free hydrogen bonds can form between neighboring molecules in urethanes with “even” chain extenders, significant strains occur in urethanes containing “odd” chain extenders which result in reduced stability of the physical crosslinking system.

Zum Einfluß von Härte, chemischer Vernetzung und Vorverlängerung auf das Erweichungsverhalten von Polyurethan-Elastomeren mit 1,4-Butandiol als Kettenverlängerer

ZusammenfassungMit wachsendem Isocyanatgehalt steigt das Modulniveau des kautschukelastischen Zustandes, der Glasübergang wird zunehmend asymmetrisch zu höheren Temperaturen verschmiert. Der letztgenannte Effekt tritt nicht auf im spezifischen Volumen und in der spezifischen Wärme. Aus diesen Befunden wird auf eine zunehmende Durchdringung der Weichsegmente mit harten Bereichen bei wachsendem Isocyanatgehalt und auf eine starke Zerklüftung der Phasengrenzflächen geschlossen.Die Erweichung der Hartsegmentbereiche besteht aus mehreren Einzelprozessen, von denen die beiden mit den höchsten Erweichungstemperaturen dem Aufschmelzen von kristallinen und parakristallinen Hartsegmentaggregaten zugeordnet werden. Eine Deutung der übrigen Umwandlungen war nicht möglich.Neben dem Einfluß des Isocyanatgehaltes werden die Auswirkungen unterschiedlicher Kennzahl (NCO∶OH) und wachsender Vorverlängerung diskutiert.SummaryThe influence of the isocyanate content on the mechanical and thermal behaviour of polyurethane elastomers is studied by differential scanning calorimetry, thermomechanical analysis, and by dynamic mechanical measurements. It is concluded that the separation between hard and soft segment domains becomes very uncomplete with increasing hardness. This effect causes the very broad glass transition of hard PU-elastomers.The melting behaviour of the hard segment regions is discussed, and the influence of the NCO/OH ratio on the properties of the elastomers is investigated.

Irreversible deformation of semicrystalline PUR-elastomers —a novel concept

A description of irreversible quasi-isothermal deformation of PUR-elastomers to large strains is developed on the basis of the classical thermodynamics of irreversible processes. This approach is shown by representing stress-strain cycles at different constant strain rates and at two temperatures.The discussion implicates the description of birefringence data and first stretchingcalorimetric results.