Effect of isocyanates on the crystallinity and thermal stability of polyurethanes

Abstract

SYNOPSISPolyurethanes were synthesized from polyester and butanediol with three different diisocyanates, i.e., 4,4'-diphenylmethane diisocyanate (MDI), m-xylene diisocyanate (XDI), and 2,4-toluene diisocyanate (TDI). The effect of chemical structures of diisocyanate compounds on the degree of crystallinity and the thermal stability were observed. Differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) were used to determine the degree of crystallinity of the hard segment. The thermal degradation of polyurethanes was studied by the thermogravimetric method. It has been shown that the polyurethane hardsegment crystallinity decreases in the following order: MDI > XDI > TDI. The experimental results also indicated that polyurethanes with aralkyl diisocyanates, i.e., XDI, had the best thermal stability. The polyurethanes synthesized from aromatic diisocyanates, i.e., MDI and TDI, had worse thermal stability than from XDI. However, owing to the higher degree of hard-segment crystallinity for polyurethanes from MDI, these polyurethanes had a better thermal stability than those based on TDI. 0 1996 John Wiley & Sons, Inc. I NTRO DUCT10 NProperties of segmented polyurethanes depend on the structure of hard and soft segments, their length, and concentration, as well as the interaction of both phases. Usually the soft segment is based on polytetramethylene oxide or aliphatic polyester and the hard segment on butanediol (BD) and diisocyanate compounds such as diphenylmethane diisocyanate (MDI), xylene diisocyanate (XDI), and toluene diisocyanate (TDI). Being of different chemical nature from the soft segments, the hard segment tend to separate in the matrix due to immiscibility of both phases and tend to associate into "domains" forming physical crosslinks.'-6 While hard segment is glassy or semicrystalline at room temperature, the soft segment phase is in the rubbery state. Polyesters synthesized from adipic acid and low molecular weight glycols are crystalline homopolymers having melting points around 50°C. However, polyester soft segments in polyurethanes lose the ability to crystallize for short segment length, due to steric hindrances. Because crystallization of the soft segments is an undesirable property, it can be avoided by copolymerization or, as in some commercial prepolymers, by inserting TDI or other irregularities into the soft segments. Generally speaking, polyurethanes are thermally very stable polymers. The onset degradation temperature of the urethane bond depends on the type of isocyanate and glycol used. It is a general rule that the more easily formed urethanes are less stable, i.e., more easily dissociated than are the more difficult ones. Thus, the highest degradation temperature, above 250°C, is observed for a urethane formed from alkyl isocyanate and alkyl alcohol, followed by aryl isocyanate-alkyl alcohol, alkyl isocyanate-aryl alcohol, and an aryl isocyanate-aryl alcohol corn bin at ion^.^,^ In this work, crystallinity and thermal stability of segmented polyurethanes based o...

A Novel Approach for the Utilization of Biocellulose Nanofibres in Polyurethane Nanocomposites for Potential Applications in Bone Tissue Implants

“…This is observed in another study that shows that, in a polycaprolactone (PCL) and MDI PU, PCL crystallinity dominates for low HS/SS values and MDI crystallinity dominates for high HS/SS values [34]. It should be noted that crystallinity is also dependent on the types of HS applied [35,36]. Decrease of phase mixing or increase of phase mixing may, thus, result in crystallinity increase due do domain dominance as may be observed in the results of Heijkants et al [37].…”

Thermoplastic polyurethane synthesis with modified montmorillonite prepared by torque rheometry: Investigation of morphological, thermal, chemical, and physical properties

“…The TGA analysis revealed that the incorporation of the nanofiller promoted a decrease in the initial degradation temperature for the rigid (urethane) and flexible (polyol) phases of TPU. A likely explanation for the decrease in the thermal stability of the first mass loss with an increased quantity of montmorillonite, may be an increased number of easily formed, less stable urethane groups, which results in a decrease in the thermal stability, as the initial degradation temperature for the urethane bond is dependent on the type of isocyanate and glycol used . The presence of the organophilic clay may result in a higher ordering in the hard segments and the formation of more inter‐urethane hydrogen bonds .…”

Mechanical and Thermal Properties of Waste Bio-Polymer Compound by Hot Compression Molding Technique

“…Figure 6 shows that the first curve of the BPC occurs at 74°C for volatile material such as gaseous matter (Song, Chen, Yu, Linliu, & Tseng, 1996) and usually this volatile material is not to be taken as the first decomposition temperature. The first decomposition only occurs on hard segments of BPC at 260°C and continues to 371°C for the second decomposition, indicated as the soft segment of the BPC structure.…”

“…Meanwhile, Figure 7 shows the thermogram of the epoxy material used to laminate the BPC, with hard segment decomposition at 351°C and volatile material with 1 distinct region. In Figure 8, the BPC laminated with epoxy shows that the decomposition only started in the hard segment at 263°C for the cross-linking agent, the second decomposition occurred at 351°C for the epoxy, and the third occurred at 416°C, according to (Song et al, 1996). The tabulated result of decomposition temperatures is shown in Table 1.…”