Thermal degradation of segmented polyurethanes

Thermal degradation of polyurethane samples was studied by a thermogravimetric method. The effect of soft-segment length and soft-segment concentration on activation energy of the degradation process was measured. Three methods of calculation gave activation energies at different stages of the very complex weight loss process. It was shown that at initial stages of the weight loss the process was dominated by hard-segment degradation. Activation energy of the whole process calculated by the Ozawa–Flynn method did not offer clear insight into the structure–stability relationship of polyurethanes. The second method showed that activation energy of the initial stage of degradation increased with decrease in hard-segment length. The Flynn method of calculating activation energy produced fairly constant activation energy of the first 40% degradation, at around 150 kJ/mol, for polymers in the C series. Generally, calculation of kinetic parameters of a complex degradation process as in polyurethanes gives results that are not easy to interpret. It has been shown qualitatively that polymers with higher soft-segment concentration display lower initial weight loss and higher onset temperatures of degradation. © 1994 John Wiley & Sons, Inc.     

Thermal degradation of cellulose model compounds in inert atmosphere

Thermal degradation of cellulose model compounds was studied using thermogravimetry (TG), electric spin resonance (ESR) spectroscopy, and gas chromatography–mass spectrometry (GS–MS). The molecular weight of samples did not influence the activation energy of gasification. In the case of catalytic influence of NaOH on cellulose thermolysis formation of gaseous products in the temperature range 250–350°C increased and a greater amount of residue was formed at 350–400°C compared to noncatalytic thermolysis. Residues of compounds with hemiacetal groups showed higher concentration of unpaired electrons than residues obtained from models with the blocked hemiacetal group.     

Study of the degradation of polyurethanes. I. The effect of various metal acetylacetonates on the photodegradation of polyurethanes

Photodegradation of polyurethanes based on polyester diol-diphenylmethane-p,′p-diisocyanate in the absence and presence of various metal acetylacetonates (Ti, V, Cr, Mn, Co(II), Co(III), Fe, Ni, Cu, Zn, Mg, Al, and Sn) was carried out; and the effect of these metal compounds on the degradation was examined by investigating the changes in the stress–strain properties and infrared- and ultraviolet-visible spectra of the polymer. The results obtained were as follows: Co(III), Co(II), Cu, Ti, and Sn acetylacetonates substantially enhanced the photo-oxidation of the polyurethane; and Al, Ni, and Zn acetylacetonates scarcely affected, or slightly retarded, it. The other metal acetylacetonates accelerated more or less the photo-oxidation of the polymer. Co(III), Co(II), and Cu acetylacetonates extremely degradation. Wavelength of the irradiated light was responsive to photochemical reactions of functional groups in the polymer. C? N and C? O bonds of urethane groups were very accessible to the photo-irradiation both without and with a glass filter, while methylene groups were rather stable to the irradiated light longer than 300 nm, through a glass filter. The effect of the metal compounds on the coloration of the polymer was also consistent with that of mechanical strength and infrared spectral changes.     

Thermal transition behavior of polybutadiene containing polyurethanes

The thermal transition behavior of a series of hydroxy terminated polybutadiene (HTPBD) containing segmented polyurethanes has been studied by differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA). Four transition regions are observed; the soft segment T_g, at ?74°C, two hard segment transitions T₁, at 40°C and T₂ at 103°C and a softening region by TMA at 180°C, presumed to arise from the dissociation of allophonate bonding, The low T_g, only 7°C higher than the T_g of free HTPBD, indicates nearly complete phase segregation despite the amorphous nature of the hard segment structure. The dependence of T₁, on hard segment length and thermal cycling suggests that it represents domains consisting primarily of shorter hard segments units. Factors contributing to the rather low mechanical properties of HTPBD polyurethanes are also discussed.     

A contribution to the UV degradation of polyurethanes

The effects of the nature of the polyols used in polyurethane (PUR) synthesis on the structural transformations after ageing by UV irradiation has been studied. The investigations were performed on PUR prepared from polyester and polyether diol oligomers. The characterization of the samples before and after ageing in view of the structural changes, which influence the course of the photooxidative degradation and photocrosslinking as well as in view of changes of mechanical properties has been done. Comparative investigations were performed by spectroscopic and viscometric measurements. The results show pronounced heterogeneity of the degradative reactions, including the existence of crosslinking processes and producing thus very inhomogeneous polymeric material. The course and the intensity of photooxidative degradation of PUR differ together with changes of mechanical properties depending whether polyester or polyether polyol have been used. The processes of photooxidative degradation is less expressed in polyesterurethane then in polyurethane based on polyether, under the same conditions of experiments. Different structures of polyester diols caused the various ageing behaviour of PUR too.     

Thermal stability of polyurethanes with carbon-carbon crosslinks

Thermal properties of polyurethanes based on 4,4′-diphenylmethane diisocyanate and poly(ethylene adipate) crosslinked only by C? C linkages were studied by isothermal and non-isothermal thermogravimetry carried out in nitrogen atmosphere. Previously studied polyurethanes crosslinked only with allophanate groups were used for comparison. It was found that the structure of crosslinks determines the initial stage of degradation process.     

Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms

The products of the thermal degradation of polydimethylsiloxane (PDMS) are determined by the heating conditions, since two competing mechanisms are involved.Cyclic oligomers are formed in the low degradation temperature range and during slow heating in programmed degradation. This involves molecular splitting of oligomers from loop conformations of the PDMS chain favoured by its flexibility, and assistance on the part of empty silicon d-orbitals.Methane and oligomers are formed in the high temperature range and during fast heating. This shows that homolytic scission of Si-CH₃ also takes place and is followed by hydrogen abstraction.     

Melamine - formaldehyde resins II.—Thermal degradation of model compounds and resins

The thermal degradation of a series of cured melamine-formaldehyde resins and of two relevant model compounds has been investigated using thermogravimetry accompanied by the analysis of volatile products. Low temperature degradation reactions which occur up to ～350°C involve the loss of water and formaldehyde, the latter arising from reversible demethylolation reactions. It is proposed that ether linkages, which model compound studies indicate would yield formaldehyde in the low temperature region, are not present to any significant degree in cured resins. Deep-seated rupture of the polycondensate system takes place at temperatures above ～375°C when a number of independent reactions involving both side-chain and ring degradation give rise to products among which ammonia, hydrogen cyanide, carbon monoxide and melamine have been examined.     

Thermal stability of polyurethanes based on vegetable oils

A series of polyurethanes from polyols derived from soybean, corn, safflower, sunflower, peanut, olive, canola, and castor oil were prepared, and their thermal stability in air and nitrogen assessed by thermogravimetric analysis, FTIR, and GC/MS. Oil‐based polyurethanes generally had better initial thermal stability (below 10% weight loss) in air than the polypropylene oxide‐based polyurethane, while the latter was more stable in nitrogen at the initial stage of degradation. If weight loss at a higher conversion is taken as the criterion of stability, then oil polyurethanes have better thermal stability both in air and in nitrogen. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1723–1734, 2000     

Thermal degradation of phenolic resins. V

The effect of varying the nature of crosslinks in phenolic resins has bee studied with the use of resins made from phenol, p-cresol, and bisphenol A. Thermal analysis showed that the methylene bridge was stronger under nonoxidative conditions than the isopropylidene linkage. The result is somewhat affected by the degree of crosslinking.     

Chemical degradation of polyurethanes. II. Degradation of microporous polyurethane elastomer by phosphoric acid esters

Microporous polyurethane elastomer, based on 4,4′‐diphenylmethane diisocyanate and polyester polyol Bayflex 2003 (Bayer AG), was degraded by phosphoric acid esters  (CH₃CH₂O)₃P(O) and (ClCH₂CH₂O)₃P(O) at 180°C. Structure of degraded products was investigated by means of ¹H‐, ¹³C‐, and ³¹P‐NMR spectroscopy. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 886–893, 2000     

Effect of silicon and phosphorus on the degradation of polyurethanes

A series of polyurethanes containing silicon and phosphorus was prepared from 4,4′‐diphenylmethane diisocyanate (MDI), poly(tetramethylene ether glycol) (PTMG), diphenylsilanediol (DSiD), and methylphosponic acid (MPA). ¹H‐NMR spectra determined the qualitative and quantitative characteristics of these polymers. The thermal stability and activation energy for thermal degradation of these polymers were measured by thermogravimtry and compared with pure polyurethane (PU). The DSiD incorporated into the main chain of the polymer improved the thermal stability of PU, while the phosphorus‐containing polyurethane (P–PU) displayed a lower thermal stability than that of pure PU. The activation energies at various degradation stages for the pure PU, silicon‐containing polyurethane (Si–PU), and P–PU polymers were calculated by the Ozawa method. The activation energies of the Si–PU polymers were higher than were those of pure PU and increased according to the increase in the DSiD content. However, the P–PU polymers' activation energies were smaller than were those of pure PU, and they decreased with increasing phosphorus content in the range of 0.1 ≤ conversion ≤ 0.5, whereas the reverse was true between 0.6 and 0.9. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 881–899, 2001     

Thermal degradation of polymers. XVII. Thermal analysis of polyquinazolones and related systems

The thermal characterization of a series of polyquinazolones, poly(quinazolone diones), and polybenzoxazinones by thermal analytical techniques (TG, DSC) is described. Comparative thermal stability measurements by dynamic and isothermal TG in air and N₂ are critically discussed. Kinetic studies by isothermal TG in air and nitrogen leading to activation energies are described. The inherent difficulties in comparative thermal stability studies on complex polymer systems are discussed in terms of their structural and compositional variables and their effect on the assessment parameters used.     

Thermal degradation and fire performance of wood treated with PMUF resin and boron compounds

Plantation Chinese fir wood was modified by low molecular weight phenol melamine urea formaldehyde (PMUF) resin, boron compounds (BB), and the mixture of PMUF/BB (PMUF‐BB), followed by a curing step. The fire performance and thermal degradation of wood was measured by limiting oxygen index instrument, cone calorimeter, and simultaneous thermal analysis. The results showed that the limiting oxygen index increased to 50.7%, 43.5%, and 55.0% for BB, PMUF, and PMUF‐BB samples, respectively. The PMUF resin decreased the heat release rate of wood but increased the total heat release compared with the control samples. The thermal analysis results demonstrated that PMUF resin enhanced the thermal stability of wood, however, had little impact on the residual chars. Combinative treatment with boron compounds could substantially reduce the fire risk for PMUF‐modified wood, making them especially useful for application in public settings.     

Oxidative degradation of polyolefins. I. Thermal oxidative degradation of atactic polypropylene in solution

Atactic polypropylene was oxidized in solution with radical initiator in the absence and presence of metal catalyst to considerably high conversion over the temperature range of 70° to 125°C, and oxidation products and the change in molecular weight distribution were measured. The unoxidized polypropylene had no ultraviolet absorption at 253.7 nm, whereas the oxidized polypropylene showed distinct absorption over a wide range of molecular weights. It was found that oxygen was incorporated into the polymer chain as hydroperoxide, acid, carbonyl, and hydroxy groups. Much of the absorbed oxygen was found to be involved in smaller fraction of low molecular weight products. Although the average molecular weight of the oxidized polypropylene decreased significantly, the formation of low-boiling products was quite small.     

Thermal degradation of polystyrene

The importance of the study of thermal degradation of polymeric fuels arises from their role in the combustion of solid propellants. Estimation of the condensed-phase heat release during combustion can be facilitated by the knowledge of the enthalpy change associated with the polymer degradation process. Differential scanning calorimetry has been used to obtain enthalpy data. Kinetic studies on the polymeric degradation process have been carried out with the following objectives. The literature values of activation energies are quite diverse and differ from author to author. The present study has tried to locate possible reasons for the divergence in the reported activation energy values. A value of 30 kcal has been obtained and found to be independent of the technique employed. The present data on the kinetics support to chain-end initiation and unzipping process. The activation energies are further found to be independent of the atmosphere in which the degradation of polymer fuel is carried out. The degradation in air, N₂, and O₂ all yield a value of 30 kcal/mole for the activation energies.     

Thermal degradation of perfluoropolyphenylenes

The thermal degradation of meta- and para-linked perfluoropolyphenylenes in vacuo and in oxygen has been studied. Rates of breakdown were determined thermogravimetrically and products of breakdown in vacuum analyzed by using a mass spectrometer. The thermal stability in vacuo was comparable with that of polyphenylene, and that in oxygen was rather inferior to that of polytetrafluoroethylene. The higher molecular weight polymers gave as the main volatile degradation products silicon tetrafluoride and carbon dioxide, together with a carbonized residue containing virtually no fluorine.     

聚碳酸酯的热降解

综述了聚碳酸酯的热稳定性及其影响因素,并详细论述了一些添加剂如偶联剂甲基三甲氧基硅烷、钛酸四丁酯、阻燃剂二苯砜磺酸钾(KSS)、聚氨丙基苯基倍半硅氧烷(PAPSQ)等对聚碳酸酯热稳定性的影响,阐述了Kissinger法和Ozawa法等聚合物热降解动力学的分析方法。重点介绍了近期国内外有关聚碳酸酯热降解产物和热降解机理方面的研究成果。