Structure Of Polyurethane Elastomers Research Articles

Investigating the Properties and Structure of Polyurethane Elastomers with Monoethylene Glycol Chain Extender

The effect of monoethylene glycol (mEG) acting as chain extender in polypropylene glycol (PPG-4000) and 4,4ʹ-diphenylmethane diisocyanate (MDI) reaction was investigated. Polyurethane elastomers (PUR) were changed from flexible to rigid materials by varying the mEG content. Results show that Shore A and D hardness values trend to increase with increasing mEG content. It appears that increasing the chain extender content increases the hard segment content in the polyurethane structure. Moreover, increasing the mEG content increases Young’s modulus and the tensile strength of PUR, while elongation at break decreases. The chemical structure of the hard segment of PUR was characterized by Infrared (IR) spectroscopy. IR spectra exhibited the bands typical for PUR consisting of –NH, CH2– and C=O groups. The spectra reveal a few interactions between the polymeric chains that appear to be responsible for the shift of transmittance peak and decrease of some peak intensity. This may be due to the hard segment aggregating more to form domains in the PUR when mEG content was increased.

Thermal decomposition of polyurethane elastomers containing antipyrene

Thermal analysis, IR and emission spectroscopy were used to study the influence of phosphorus and phosphorus/chlorine-containing antipyrenes on the thermal decomposition of polyurethane elastomers based on 4,4′-diphenylmethane diisocyanate and polytetraoxymethylene. The introduction of P-containing antipyrene into the structure of polyurethane elastomers induces changes in the mechanism and kinetics of thermal decomposition towards inactivation of the exothermal reactions of oxidative decomposition. It causes a shift of the first exothermal peak towards higher temperatures by 100° and helps the formation of temporarily stabilized structures. P/Cl-containing antipyrene has a smaller positive effect towards inactivation of the exothermal decomposition reactions and the formation of temporarily stabilized structures.

Structure of polyurethane elastomers: effect of chain extender length on the structure of MDI/diol hard segments

X-ray diffraction and conformational analyses have been used to study the structure of the hard segments in MDI/diol/PTMA polyurethanes, and to investigate the differences in structure that result from the use of different chain lengths for the aliphatic diol component. It is found that for butandiol and longer diol chain extenders (up to octandiol) the structure depends on whether the diol has an even or odd number of CH 2 groups. The even diol polymers adopt the lowest energy fully extended conformations that allow for hydrogen bonding in both directions perpendicular to the chain axis. Such a hydrogen bonding network would not be possible for the odd diol polymers in the extended conformation, and these adopt contracted, higher energy conformations. Both the odd and even diol polymers adopt staggered chain structures with triclinic unit cells, but the even diol polymers have higher crystalline order. The first two members of the homologous series, prepared using ethylene glycol and propandiol, are exceptions to the above behaviour and adopt contracted unstaggered structures. It is possible that these two diols are too short to permit packing of the MDI units in the same way as for the longer chain extenders. The results suggest the simple explanation for the better overall properties of the elastomers prepared using butandiol and the higher even diols. The hard segments of these polymers can crystallize more easily in the lowest energy extended conformation, and hence there is more of a driving force for phase separation.

Structure of polyurethane elastomers. X-ray diffraction and conformational analysis of MDI-propandiol and MDI-ethylene glycol hard segments

X-ray diffraction and conformational analysis have been used to investigate the structure of polyurethane hard segments prepared from diphenylmethane 4,4′-diisocyanate (MDI) with propandiol (PDO) and ethylene glycol (EDO) as chain extenders. The results are compared with those obtained previously for MDI-butandiol (BDO) hard segments. In the latter system, the poly(MDI-BDO) chains are fully extended with an all- trans conformation for the O(CH 2) 4O unit, and a monomer repeat of 18.95 Å. The unit cell is triclinic with a tilted base plane such that adjacent chains are staggered along the fibre axis. In contrast both poly(MDI-PDO) and poly(MDI-EDO) adopt unstaggered structures, i.e. the chains are in register and the unit cell base planes are perpendicular to the chain axis. The monomer repeats of 16.2 Å for poly(MDI-PDO) and 15.0 Å for poly(MDI-EDO) are shorter than the predicted repeats for fully-extended chains, indicating that these polymers have contracted conformations containing some gauche CH 2 groups. Conformational analysis shows that the 16.2 Å repeat for poly(MDI-PDO) can be achieved with the O(CH 2) 3O unit in the trans-gauche +- gauche +- trans or gauche +- trans-trans-gauche + conformations. Similarly the 15.0 Å repeat for poly(MDI-EDO) is predicted for the gauche +- trans-gauche − conformation for the O(CH 2) 2O unit. These conformations are of higher energy than the all- trans fully-extended chains. This may explain the higher crystalline perfection of the poly(MDI-BDO) hard segments, for which crystallization in the all- trans form will probably provide a greater driving force for phase separation.

Crosslinking and mechanical property of liquid rubber. III. Curative effect of aralkyl and alicyclic diols

This is the third report on the relationship between the properties and the structure of polyurethane elastomers derived from hydroxyl-terminated liquid polybutadiene (HT-BD). The curative effect of low molecular weight diols was investigated. Previously it was found that the use of aromatic diols presented a processing problem because of its high melting point. To solve this problem, the use of aralkyl diols and alicyclic diols was examined in this paper. The processability was significantly improved by the use of these diols, and the elastomers obtained retained considerably high mechanical properties.

Crosslinking and mechanical property of liquid rubber. II. Curative effect of aromatic diols

AbstractThe relationships between the properties and structure of polyurethane elastomers derived from hydroxyl‐terminated liquid polybutadiene (HT‐BD), several low molecular weight aromatic diols, and diisocyanates were studied. The curative effects of the diols were examined by such means as tensile properties, dynamic viscoelasticity, x‐ray diffraction, and others. It was found that the properties of the elastomers derived from HT‐BD were greatly affected by the structure of low molecular weight chain extender diol. These results were interpreted by the steric effect, crystallinity, and the concentration of polar groups.

Studies on the structure of polyurethane elastomers, 4. NMR spectra of linear polyurethane based on 2,4‐tolylenediisocyanate

AbstractIt is shown that urea and carbamoyl (urethane) groups in derivatives of 2,4‐tolylenediisocyanate (1) can be differentiated by their chemical shifts when the NMR measurements are carried out in N,N‐dimethylacetamide (DMA) which contains lithium chloride. This method can further be applied to the analysis of linear polyurethane prepolymers prepared from α‐(hydroxytetramethylene)‐ω‐hydroxypoly(oxytetramethylene) (2) and 1.

Mechanical Properties of High Density Cellular Urethanes

The use of cellular polyurethanes either as microporous foams in shoe upper materials (poromerics) or in closed cell form as soling materials has increased rapidly during the last few years in the footwear industry. Compared with vulcanised rubbers, these materials have high strength over an extended temperature range, high set and good resistance to cut growth. The research now presented.has been concerned with determining the reasons for these mechanical properties of polyurethane as compared with compounded conventional vulcanised rubbers. The supplementary contribution to the thesis presents a review of earlier work on the strength and reinforcement of crystalline, amorphous and filled vulcanised rubbers and includes a short section on the effect of crosslinking on ultimate failure properties of natural rubber. This study has been extended by the author into the effect of chain branching in polyurethane elastomers on the failure properties. The viscoelastic properties of natural and artificial leathers are also discussed in order to demonstrate similarities between the different materials and show how a cellular polyurethane sheet has hysteresial properties similar to those of a natural material of fibrous structure. It is shown that a cubical lattice model can be applied to explain the differences between such mechanical properties as modulus, tensile and tear strength of a cellular polyurethane and the corresponding solid material of the same polymer. This model, which has previously been applied to the mechanical properties of a natural rubber latex foam, indicates that the strength of cellular polyurethanes is due to the very high strength of the solid material. An extensive investigation into the effect of time and temperature on the tensile properties of cellular and solid polyurethanes is presented in order to show that polyurethanes of the type used in poromerics have a very broad relaxation spectrum extending over 18 decades of time. Because of this response to deformation, the failure properties remain fairly constant over the temperature range from 21 – 160°c. Above 160°c, the tensile properties fall quite markedly. Stress softening in these polyurethanes is very high and can only be reversed by heating to temperatures above 160°c. The cut growth and fatigue properties of cellular and solid polyurethanes are considered. Following a brief review of the investigation on cut growth and fatigue of vulcanised rubbers involving the use of tearing energy theory, it is shown that cut growth and hysteresis properties of vulcanised .rubbers can be correlated. The lower limit of tearing energy (To) below which no cut growth takes place in the absence of chemical effects is found to be higher for polyurethanes than for vulcanised rubbers. Fatigue failure of cellular polyurethanes is found to be due to cut growth from the largest pore in the sample. These data are also compared with measurements on other two phase elastomer systems such as styrene butadiene copolymer vulcanisates with high styrene content and polystyrene-polybutadiene thermoplastic rubbers. From an extensive review of the literature on the structure of polyurethane elastomers, it is deduced that polyurethanes of the type used in poromerics consist of a segmented structure of long polyester chains connected to very minute (25A) hard urethane segments. The cohesion of the hard segments is primarily due to hydrogen bonding and other physical forces. It is concluded that the high strength, good cut growth resistance and broad relaxation spectrum of polyurethanes are due to the reinforcement given by the hard urethane segments which act as well dispersed minute filler particles in the polyester rubber matrix. The hydrogen bonding between the hard segments dissociates at approximately 170°c so giving a degree of thermoplasticity which produces a very high permanent set. An appendix discusses some of the practical applications in the footwear industry of the work presented, such as forming of poromerics, tearing from stitch-holes and flex cracking of solings

Studies on the structure of polyurethane elastomers. III. Characterization of linear polyurethanes by NMR‐spectroscopy: Components and molecular weight

AbstractThe NMR method was applied to the analysis of linear polyurethanes and also for their molecular weight determination. The number‐average molecular weight can be calculated from the ratio of NMR absorption intensity corresponding to chain segments of polymers to that of end groups.

Infrared Spectra and Structure of Polyurethane Elastomers from Polytetrahydrofuran, Diphenylmethane-4, 4′-diisocyanate, and Ethylenediamine

Abstract The infrared spectra (4000–650 cm−1) of a series of polyurethane elastomers synthesized from polytetrahydrofuran, diphenylmethane-4,4′-diisocyanate, and ethylenediamine were examined. By comparing the spectra with those of the constituent polytetrahydrofuran and of the model compounds for the hard segments of the elastomers containing the urea, urethane, and diphenylmethane groups, most of the stronger bands could be assigned with reasonable assurance to the vibrations of each constituent part. The frequencies of the urea and the urethane characteristic bands correspond to complete association of the polar groups in the polymers. The spectral changes produced by uniaxially stretching polyurethane films can be interpreted as due to the induced crystallization of the polyether blocks in the elastomer, which remain amorphous in the undistorted state. Polarization measurements on stretched samples confirmed that the polyether chains are oriented parallel to the direction of the stretch.

Studies on the structure of polyurethane elastomers. II. High resolution NMR spectroscopic determination of allophanate and biuret linkages in the cured polyurethane elastomer: Degradation by amine

AbstractThe high resolution NMR method which had been proved to be useful for the analysis of the linear polyurethanes was further applied to determine the allophanate and the biuret linkages in the cured polyurethane elastomers. The polyurethane elastomer cured with glycol or diamine was made soluble into DMSO, DMA etc. by degrading the allophanate and the biuret linkages with aliphatic primary amine. The NMR investigation of degradation products thus obtained gave the quantitative informations about the cross‐linkages present before the degradation.

Spectral studies on the structure of polyurethane elastomers

Spectral studies on the structure of polyurethane elastomers

Studies on the structure of polyurethane elastomers. I. NMR spectra of the model compounds and some linear polyurethanes

AbstractThe NMR spectra of a series of compounds were taken in order to identify the nitrogen‐containing linkages which had been considered to be present in polyurethane elastomers. It has been found that the urea, the urethane, the allophanate and the biuret can be discriminated each other by their different chemical shifts when the measurement is made in polar solvent like N,N‐dimethyl acetamide, dimethyl sulfoxide, etc. The method was further applied to the analysis of the uncured linear prepolymer prepared from polypropylene glycol and diphenylmethane‐4,4′‐diisocyanate or tolylene diisocyanate.