Thermal Degradation Of Polyurethanes Research Articles

The influence of silica nanoparticles on thermal degradation and mechanical properties of nanocomposites based on aliphatic polyurethanes

Nanocomposites based on aliphatic polyurethanes have recently attracted a lot of attention regarding economical and ecological aspects, due to their improved thermal and mechanical properties. The aim of this paper was to investigate the influence of silica nanoparticles, differing in size and specific surface, on thermal stability and degradation, lifetime and mechanical characteristics of the obtained nanocomposites. Two series of nanocomposites based on aliphatic polyurethanes were obtained by using a single-step procedure and by addition of silica nanoparticles of types A380 or N999 at different loadings (0,0 0.15, 0.5, 1,0 and 3.5 wt.%). It was found that the increase in heating rate caused shifting of the onset temperature to higher values (from 283 to 312 ?C). According to the shape of DTG curves, it was observed that the degradation mechanism of prepared nanocomposites consists of two overlapping processes, related to the scission of hard and soft building blocks. Based on DTG results, the addition of larger N999 silica nanoparticles induced lower thermal degradation, shifting the maximum rate temperatures of the first and second degradation stages to lower values, and caused the change in the degradation mechanism. Addition of smaller silica nanoparticles (A380) did not significantly affect the mechanism of the degradation reaction, indicating homogeneity of the obtained nanocomposites. The presence of A380 nanoparticles improved thermal stability of nanocomposites, by increasing the onset temperature from 286 ?C for the pristine elastomer to 303 ?C for the sample containing 3.5 wt.% of silica. Existence of interactions of A380 silica nanoparticles with hard and soft phases was observed, based on the increase in the maximum rates of the first and second degradation steps. The activation energy of thermal degradation of polyurethanes modified with A380 silica nanoparticles was obtained by using the Flyn-Wall and Toop models. Dependence of the activation energy and the lifetime of nanocomposites based on aliphatic polyurethanes on the silica content were estimated. The highest Ea values (determined for 1 and 5 % weight loss) were found for nanocomposites containing 0.5 and 0.15 wt. % of A380 silica nanoparticles (121 and 161.2 kJ/mol). A negative effect of the silica addition on mechanical properties of nanocomposites was observed. The polyurethanes containing smaller SiO2 particles (A380) had a higher tensile strength, elongation at break and hardness as compared to the elastomers filled with larger silica nanoparticles (N999).

Dry Sampling of Gas-Phase Isocyanates and Isocyanate Aerosols from Thermal Degradation of Polyurethane

The performance of a dry sampler, with an impregnated denuder in series with a glass fibre filter, using di-n-butylamine (DBA) for airborne isocyanates (200ml min(-1)) is investigated and compared with an impinger flask with a glass fibre filter in series (1 l min(-1)). An exposure chamber containing 1,6-hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and 2,4- and 2,6-toluene diisocyanate (TDI) in the concentration range of 5-205 μg m(-3) [0.7-33 p.p.b.; relative humidity (RH) 50%], generated by gas- and liquid-phase permeation, was used for the investigation. The precision for the dry sampling for five series with eight samplers were in the range of 2.0-6.1% with an average of 3.8%. During 120-min sampling (n = 4), no breakthrough was observed when analysing samplers in series. Sixty-four exposed samplers were analysed after storage for 0, 7, 14, and 21 days. No breakdown of isocyanate derivatives was observed. Twenty-eight samplers in groups of eight were collecting isocyanates during 0.5-32h. Virtually linear relationships were obtained with regard to sampling time and collected isocyanates with correlation coefficients in the range of 0.998-0.999 with the intercept close to the origin. Pre- or post-exposure to ambient air did not affect the result. Dry sampling (n = 48) with impinger-filter sampling (n = 48) of thermal decomposition product of polyurethane polymers, at RH 20, 40, 60, and 90%, was compared for 11 isocyanate compounds. The ratio between the different isocyanates collected with dry samplers and impinger-filter samplers was in the range of 0.80-1.14 for RH = 20%, 0.8-1.25 for RH = 40%, 0.76-1.4 for RH = 60%, and 0.72-3.7 for RH = 90%. Taking into account experimental errors, it seems clear that isocyanic acid DBA derivatives are found at higher levels in the dry samples compared with impinger-filter samplers at elevated humidity. The dry sampling using DBA as the reagent enables easy and robust sampling without the need of field extraction.

Thermal degradation behavior of rigid and soft polyurethanes based on methylene diphenyl diisocyanate using evolved gas analysis-(gas chromatography)–mass spectrometry

Thermal degradation behavior and its mechanism of rigid and soft polyurethanes (PUs) were investigated by evolved gas analysis-mass spectrometry (EGA-MS) and evolved gas analysis-gas chromatography/mass spectrometry (EGA-GC/MS). Four steps of thermal degradation processes were found in EGA-MS analysis and evolved gases from each thermal zone were analyzed by EGA-GC/MS. From the results, it is suggested that thermal degradation of PUs might be occurred by increased bond strain of polymer chain due to increasing temperature and the sequence of thermal degradation in polymer chain is from hard segment to soft segment through four steps of thermal degradations.

Modelling of polyurethane foam thermal degradation within annular region subject to fire conditions

AbstractNuclear materials are placed in shielded, stainless steel packaging for storage or transport. These drum type packages often employ a layer of foam, honeycomb, wood or cement that is sandwiched between thin metal shells to provide impact and thermal protection during hypothetical accidents, as those prescribed in the Code of Federal Regulations (10 CFR 71·73). The present work discusses the modelling of the thermal degradation of polyurethane (PU) foam within an annular region during an 800°C fire. Measurements and analysis by Hobbs and Lemmon [M. L. Hobbs and G. H. Lemmon: ‘Polyurethane foam response to fire in practical geometries’, Polym. Degrad. Stab., 2004, 84, 183–197.] indicate that at elevated temperatures, PU foam exhibits a two-stage, endothermic degradation. The first stage produces a degraded solid and a combustible gas; the second stage reaction consumes the degraded solid and produces another combustible gas. As a result, during a prolonged fire, a gas filled void develops beside the...

Structural characteristics of UV‐irradiated polyurethane elastomers extended with α,ω‐alkane diols

AbstractA series of polyurethane (PU) elastomers were prepared by the reaction of poly (ε‐caprolactone) (PCL) and 4, 4′‐diphenylmethane diisocyanate (MDI), extended with a series of chain extenders (CEs) having 2‐10 methylene units in their structure. The synthesized samples were irradiated for 50, 100, and 200 h as such the spectral distribution of the light is good match for terrestrial solar radiation. Fourier transform infrared (FTIR) spectroscopy was used to study the chemistry of monomers and final PU samples. The structural changes in the PU samples upon ultraviolet (UV) irradiation were characterized using FTIR spectroscopic technique. In comparison with non irradiated samples, there was a continuous decrease of the bands upon long‐term UV irradiation. The results revealed that final PU sample extended with 1,6‐hexane diol (1,6‐HDO) to 1, 10 decane diol (1,10‐DDO) have been fully degraded at 200 h irradiation. Comparing PU sample extended with 1, 6‐HDO and 1, 10‐DDO, it was found that the degradation rate of later is higher than former. It was concluded that CEs length has considerable effect on thermal degradation of PU and by increasing CEs length thermal degradation upon UV irradiation shifted to lower temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Size-Separated Sampling and Analysis of Isocyanates in Workplace Aerosols—Part II: Aging of Aerosols from Thermal Degradation of Polyurethane

A new type of isocyanate sampler has been used to investigate aging aerosols generated during thermal degradation of polyurethane (PUR). The sampler consists of a denuder connected in series with a three-stage cascade impactor and a filter. The denuder collects gas-phase isocyanates. The three impactor stages had cut-off diameters (d(50)) of 2.5, 1.0 and 0.5 mum, respectively. The end filter collects particles <0.5 mum. For derivatization of isocyanates in the sampler, di-n-butylamine mixed with an equimolar amount of acetic acid was used for impregnation of the sampler stages. Consecutive sampling using three denuder-impactor samplers was performed in a test chamber, with a total sampling time of 9 min. Analysis of air samples was performed using liquid chromatography-mass spectrometry (LC-MS)/MS. Particle size measurements were performed using a scanning mobility particle sizer (SMPS). A time-dependent behavior was observed for aromatic diisocyanates during aging of the aerosol. Thermal degradation of different PUR materials showed different distribution of isocyanates between gas and particles. Aromatic diisocyanates (toluene diisocyanate (TDI) and methylene diphenyl diisocyanate) were initially in gas phase and associated to very small particles. After a few minutes most of these isocyanates were associated with particles <1 mum. Monoisocyanates and hexamethylene diisocyanate (HDI) were not found to be associated with particles.

Structure–Property Relationship of Castor Oil-Based PU–PBMA Interpenetrating Polymer Networks

Castor oil-based polyurethane (PU) and polybutyl methacrylate (PBMA) interpenetrating polymer networks (IPNs) (PU/PBMA; 80/20, 60/40, 50/50, 40/60 and 20/80) have been prepared by sequential polymerization. The prepared IPNs have been characterized by physico-mechanical and optical properties. The thermal behaviour of IPNs were performed using thermo gravimetric analyzer (TGA). TGA thermogram shows that the thermal degradation of PU–PBMA IPN was found to proceed in three different steps. The microcrystalline parameters such as crystal size (hNi) and lattice disorder (g) of IPNs have been estimated using wide angle X-ray scattering studies (WAXS), and quantification of the polymer network has been carried on the basis of these parameters.

Determination of isocyanates, aminoisocyanates and amines in air formed during the thermal degradation of polyurethane.

An air sampling method for the determination of isocyanates, aminoisocyanates and amines formed during the thermal degradation of polyurethane (PUR) is presented. The method is based on the collection of air samples using impinger flasks containing di-n-butylamine (DBA) in toluene with a glass fibre filter in series. Isocyanates are derivatized with DBA to urea derivatives, and amines are derivatized in a subsequent work-up procedure with ethyl chloroformate (ET) to carbamate esters. Amine, aminoisocyanate and isocyanate derivatives were characterized using liquid chromatography-time of flight mass spectrometry (LC-TOFMS) and liquid chromatography-chemiluminescent nitrogen detection (LC-CLND). Quantification was performed by LC-MS, monitoring molecular ions [MH]+ in the electrospray mode. The instrumental detection limits for amines, aminoisocyanates and isocyanates were in the ranges 30-40, 2-3 and 3-70 fmol, respectively. Thermal degradation products of PUR were observed in high concentrations during welding in district heating pipes and PUR-coated metal sheets. Eleven isocyanates, three amines and five aminoisocyanates were identified. The concentrations of isocyanates, aminoisocyanates and amines in samples collected in the smoke close to the welding spot were in the ranges 150-650, 4-290 and 1-70 ppb, respectively. In samples collected in the breathing zone, isocyanates and aminoisocyanates were observed in the ranges 9-120 and 4-19 ppb, respectively. The compounds were present in both gas and particle phases. Volatile compounds dominated in the gas phase, whereas less volatile compounds dominated in the particle phase. The method presented makes it possible to sample and determine amines and aminoisocyanates, in addition to isocyanates. The need to monitor these compounds is clearly illustrated by the high concentrations found during the thermal degradation of PUR.

Biological monitoring of aromatic diisocyanates in workers exposed to thermal degradation products of polyurethanes.

Exposure to diisocyanates was assessed by biological monitoring among workers exposed to the thermal degradation products of polyurethanes (PURs) in five PUR-processing environments. The processes included grinding and welding in car repair shops, milling and turning of PUR-coated metal cylinders, injection moulding of thermoplastic PUR, welding and cutting of PUR-insulated district heating pipes during installation and joint welding, and heat-flexing of PUR floor covering. Isocyanate-derived amines in acid-hydrolysed urine samples were analysed as perfluoroacylated derivatives by gas chromatography mass spectrometry in negative chemical ionisation mode. The limits of quantification (LOQs) for the aromatic diamines 2,4- and 2,6-toluenediamine (2,4- and 2,6-TDA) and 4,4'-methylenedianiline (4,4'-MDA) were 0.25 nmol l(-1), 0.25 nmol l(-1) and 0.15 nmol l(-1), respectively. The LOQ for the aliphatic diamines hexamethylenediamine (HDA), isophoronediamine (IpDA) and 4,4'-diaminodicyclohexyl methane (4,4'-DDHM) was 5 nmol l(-1). TDA and MDA were detected in urine samples from workers in car repair shops and MDA in samples from workers welding district heating pipes. The 2,4-TDA isomer accounted for about 80% of the total TDA detected. No 2.6-TDA was found in the urine of non-exposed workers. The highest measured urinary TDA and MDA concentrations were 0.79 nmol mmol(-1) creatinine and 3.1 nmol mmol(-1) creatinine, respectively. The concentrations found among non-exposed workers were 0.08 nmol mmol(-1) creatinine for TDA and 0.05 nmol mmol(-1) creatinine for MDA (arithmetic means). Exposure to diisocyanates originating from the thermal degradation of PURs are often intermittent and of short duration. Nevertheless, exposure to aromatic diisocyanates can be identified by monitoring diisocyanate-derived amines in acid-hydrolysed urine samples.

Thermal degradation of polyurethane and polyurethane/expandable graphite coatings

This study deals with the thermal degradation of polyurethane (PU) and polyurethane/expandable graphite (PU/EG) coatings. EG represents a new generation of intumescent additives which provides good fire retardancy to various materials and in particular to PU. The thermal analyses have shown that the thermo-oxidative degradation as well as the pyrolysis of PU are not affected by the presence of EG. On the other hand, they have enabled the determination of the temperatures which correspond to different steps of the development of the intumescence : formation, stabilisation and degradation of the intumescent shield. Spectroscopic analysis of the residues resulting from each of these steps has demonstrated that the structure of the char is similar for PU and PU/EG and consists in a carbonaceous polyaromatic species. However, the results have suggested that the char formed from PU/EG may contained trapped free radicals.

Thermal degradation of polyurethanes containing lignin studied by TG-FTIR

Thermal degradation of polyurethanes (PUs) having various kraft lignin (KL) contents in nitrogen and also in air was studied by simultaneous thermogravimetry and Fourier transform infrared spectroscopy TG-FTIR. Changes of volatile products at various temperatures were analysed. Differences in TG curves and FTIR spectra of PUs and raw materials were observed, suggesting that the thermal degradation mechanism of PUs is different from that of raw materials. The amounts of thermal degradation products having characteristic functional groups changed according to the KL contents in the PUs. Thermal degradation in nitrogen apparently proceeded in one step, while it proceeded in two steps in air. This marked difference suggested that an oxidation reaction occurred during the thermal degradation of PUs in air. © 1998 Society of Chemical Industry

Determination of complex mixtures of airborne isocyanates and amines. Part 3. Methylenediphenyl diisocyanate, methylenediphenylamino isocyanate and methylenediphenyldiamine and structural analogues after thermal degradation of polyurethane.

A method is presented for the determination of isocyanates in polymeric methylenediphenyl diisocyanate (MDI) and related compounds formed during the thermal decomposition of polyurethane (PUR). Derivatization of isocyanates was performed in impinger flasks containing dibutylamine (DBA) with the formation of urea derivatives. Compounds containing amine groups were then derivatized with ethyl chloroformate (ET to give urethane derivatives. Reversed-phase liquid chromatography, with a gradient flow rate of 40 milligrams min-1 and mass spectrometry in the electrospray mode monitoring positive ions was studied. Injection volumes of up to 10 milligrams of the sample were made possible by using column focusing. 1,5-Naphthyldiisocyanate-DBA and 1,5-naphthyldiamine-ET derivatives were used as internal standards. Virtually linear calibration curves were obtained for 4,4'-MDI-DBA and 4,4'-methylenediphenyldiamine-ET (MDA-ET) and the correlation coefficients were 0.9952-0.9964 (n = 14). The precision for five injections of samples spiked with 4,4'-MDA-ET, and 4,4'-MDI-DBA ar concentrations of 50 nmol ml-1 was 2.76 and 2.55%, respectively. The instrumental detection limit, defined as three times the noise, was 4 fmol of MDI-DBA and 50 fmol of MDA-ET injected. In chromatograms of polymeric MDI derivatized with diethylamine, dipropylamine and DBA, the presence of several structural isomers and analogues in polymeric MDI was demonstrated. In the chromatograms of thermal decomposition products of MDI-PUR, in addition to isocyanates, related amino isocyanates and amines were also observed.

Effect of isocyanates on the crystallinity and thermal stability of polyurethanes

Polyurethanes 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 diiscynate (TDI). The effect of chemical structures of diisocyate 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 hard-segment 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. © 1996 John Wiley & Sons, Inc.

Effect of isocyanates on the crystallinity and thermal stability of polyurethanes

Polyurethanes 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 diiscynate (TDI). The effect of chemical structures of diisocyate 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 hard-segment 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. © 1996 John Wiley & Sons, Inc.

Biomarkers of exposure, antibodies, and respiratory symptoms in workers heating polyurethane glue.

The pathogenic basis of respiratory disorders associated with isocyanates are still obscure. One reason for this is the lack of good estimates of human exposure. In this study exposure was estimated by measurement of isocyanate metabolites in biological samples. In a factory using isocyanate based polyurethane (PUR) glue, isocyanate concentrations in air were measured by liquid chromatography. Samples from 174 employees were analysed for metabolites of 4,4'-methylene diphenyl diisocyanate (MDI) in plasma (P-MDX) and urine (U-MDX). After hydrolysis, 4,4'-methylenedianiline was measured by gas chromatography-mass spectrometry (GC-MS). The employees were screened for work related respiratory symptoms and tested for specific immunoglobulin E (IgE) and IgG antibodies directed against isocyanate conjugated to human serum albumin. The time weighted isocyanate concentrations in air were low (MDI < 0.2-7; hexamethylene diisocyanate (HDI) < 0.1-0.7; 2,6-toluene diisocyanate (TDI) < 0.1 microgram/m3). All subjects had detectable P-MDX and U-MDX. There were significant associations between the estimates of exposure to thermal degradation products of an MDI based glue and P-MDX (range < or = 0.10-5.5 micrograms/l); and U-MDX (< or = 0.04-5.0 micrograms/g creatinine); in cases of heavy exposure. P-MDX and U-MDX were associated with each other (r = 0.64; P = 0.0001), work related symptoms (P-MDX: P = 0.03; Mann-Whitney U test), and serum concentrations of MDI specific IgG antibodies (r = 0.26; P = 0.0007). Unexpectedly, high P-MDX and U-MDX concentrations were also encountered in workers cutting textile (P-MDX 2.4-4.5 micrograms/l; U-MDX 0.81-3.8 micrograms/g creatinine); the reason is still unknown. Equally unexpected, there were significant negative associations between P-MDX and liver function tests. The results clearly show the value of biomarkers for isocyanate exposure; in particular, P-MDX is useful. Further, these results show the risk connected with thermal degradation of PUR.

Unusual effect of filler (CaCO3) on thermal degradation of polyurethane

AbstractThe thermal degradation of polyurethane is exothermic in nature. The addition of CaCO3 inverts the exothermicity to endothermic behavior in the presence of air. © 1994 John Wiley &amp; Sons, Inc.

Determination of Isocyanate and Aromatic Amine Emissions from Thermally Degraded Polyurethanes in Foundries

Thermal degradation of polyurethanes can cause emissions of isocyanates and aromatic amines into work atmospheres. Results from laboratory studies on the use of 4,4′-methylenediphenyl isocyanate (MDI)-based polyurethane bound foundry core materials showed that anilines and phenyl isocyanates are major degradation products, but the possibility for emissions of MDI and its corresponding diamine, 4,4′-methylenedianiline, also was demonstrated. Results from field investigations in two different foundries showed that of these degradation products, mainly anilines and phenyl isocyanates are found in foundry work atmospheres, whereas MDI and methylenedianiline concentrations were negligible. The selection of air sampling and analytical procedures for, as well as interpretation of results from, simultaneous determination of isocyanates and their corresponding amines are discussed.

Determination of Isocyanate and Aromatic Amine Emissions from Thermally Degraded Polyurethanes in Foundries

Thermal degradation of polyurethanes can cause emissions of isocyanates and aromatic amines into work atmospheres. Results from laboratory studies on the use of 4,4′-methylenediphenyl isocyanate (MDI)-based polyurethane bound foundry core materials showed that anilines and phenyl isocyanates are major degradation products, but the possibility for emissions of MDI and its corresponding diamine, 4,4′-methylenedianiline, also was demonstrated. Results from field investigations in two different foundries showed that of these degradation products, mainly anilines and phenyl isocyanates are found in foundry work atmospheres, whereas MDI and methylenedianiline concentrations were negligible. The selection of air sampling and analytical procedures for, as well as interpretation of results from, simultaneous determination of isocyanates and their corresponding amines are discussed.

Thermal degradation of polyurethanes. Part VI. Influence of segment structure of polyurethanes on their thermal stability

Thermal degradation of polyurethanes. Part VI. Influence of segment structure of polyurethanes on their thermal stability

Evolution of HCN during high‐temperature thermal degradation of polyurethanes

AbstractThe thermal degradation of an aliphatic, aromatic, and secondary polyurethane has been studied over a range of temperatures from 500 to 1000°C with respect to the kinetics and mechanism of HCN evolution. This evolution shows a rapid linear increase of evolved HCN with time, passing into a plateau when the polymer has been consumed. In high temperature ranges, thermal decomposition of the evolved HCN also takes place, and in some temperature regions the degradation is diffusion controlled. Overall, the process is governed by a depolycondensation reaction which is the slowest step in a sequence of reactions. The reaction sequence is kinetically evaluated and the respective rate constants, diffusion constants, and energies of activation are presented. The reaction mechanism accounts well for the experimental results in spite of some simplifications which had necessarily to be applied.