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Thermal degradation of polystyrene
Journal of Applied Polymer Science
Abstract
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.
... Due to the decrease in chain mobility caused by the dispersion of nanoparticles into the polymer matrices, the delay in polymer degradation as ZnO nanoparticle content rises in the samples can be explained. 41,42 Because inorganic nanoparticle fillers have very high surface areas and are nondegradable. The great dispersion of the nanoparticles in the polymer matrix acts as a barrier to the diffusion of the free radical, resulting in a delay in the breakdown of the rubber compounds. ...
This study examines the viability of functionalized, nano, and active zinc oxide (ZnO) as an alternative activator in styrene butadiene rubber–natural rubber (SBR–NR) blends filled with carbon black. Concerns about the environmental impact and toxicity of Zn leaching into aquatic organisms necessitate reducing the amount of ZnO in rubber compounding. The mechanical properties, crosslink density, and thermal and morphological qualities of the SBR/NR blends made with these ZnO (F2BB, N2BB, and A2BB) were studied and compared to conventional ZnO loaded blend (C5BB). Thermal stability was evaluated using thermogravimetric analysis and temperature scanning stress relaxation. A2BB showed the highest thermal stability, while F2BB exhibited a 35% increase in tensile strength and a 28% increase in elongation at break due to polysulfide linkages. N2BB and A2BB increased tensile strength by 28 and 16% compared to C5BB. Cytotoxicity of various ZnO concentrations on HT‐22 hippocampal cells was investigated, and active and nano ZnO were found to be less toxic. Using active and nano ZnO can improve the mechanical and thermal properties of the blend while reducing environmental toxicity, allowing for a 60% reduction in ZnO concentration in rubber compounds and resulting in cost savings and a sustainable alternative to conventional ZnO.
... He also discussed the possible breakage of C-C links under elongation flow. Murthy and Raghavendra (1981) concluded that increased degradation of polystyrene is caused by higher shearing in extrusion and pelletization processes, Inaba and Inoue (1981) investigated the changes of molecular weight of polystyrene degraded at 340-380° C. Deanin et al (1983 ) reported that by extrusion of HIPS for 5 cycles no further improvement was required. Francisco et al. (2006) stated that multiple processing and thermo-oxidation exhibit complex chemical and physical implications for the structure and properties of recycled HIPS. ...
High impact polystyrene (HIPS) was subjected to repeated thermal and shear stresses by different methods, using a high pressure capillary Rheometer, a laboratory extruder and a Brabender plasticorder. The results showed that the recycling of HIPS caused a decrease in the shear stresses and consequently in the viscosity at constant shear rate. The high pressure capillary Rheometer gave a greater decrease than with the Brabender plasticorder. Tensile strength values were progressively decreased with the laboratory extruder, but increased in the case of Brabender mixer; the elongation was decreased in both cases. This work was then applied in a local plant to compare experimental and actual results.
... The second model was developed during this study to describe the pyrolysis kinetics on a more fundamental basis. This model incorporates a statistical reaction pathway model, which has successfully been applied to Fuoss et al. (1964) 394 1 5.0 × 10 24 323 d Jellinek (1950) 348-400 0 10 13.1 188 a,d,e Kishore et al. (1976) 290-390 50-90 0 134 c,b Kokta et al. (1973) 200-500 1 100-140 g 200-500 1 190-230 g Kuroki et al. (1982) 310-390 1 1.8 × 10 11 152 b Madorsky (1953) 335-355 1 9.0 × 10 15 244 a Malhotra et al. (1975) 180-390 1 189-440 b Mehmet and Roche (1976) 200-700 1 10 14.5 -10 15 219-229 a,i Mertens et al. (1982) 500-800 1 92 c Risby et al. (1982) Mw 100.000 g/mol 200-600 1 3.6 × 10 13 176 Mw 390.000 g/mol 200-600 1 6.1 × 10 12 165 Sato et al. (1983) 100-600 0.75 3.5 × 10 11 177 b Wu et al. (1993) 367-487 0.5 5.0 × 10 10 173 c this study 365-400 70-90 1 3.3 × 10 13 204 a Measurements performed in vacuum environment. b Possible mass and heat transfer limitations. ...
The pyrolysis kinetics of low-density polyethylene, high-density polyethylene, polypropylene, and polystyrene has been studied at temperatures below 450 °C. In addition, a literature review on the low-temperature pyrolysis of these polymers has been conducted and has revealed that the scatter in the reported kinetic data is significant, which is most probably due to the use of simple first-order kinetic models to interpret the experimental data. This model type is only applicable in a small conversion range, but was used by many authors over a much wider conversion range. In this investigation the pyrolysis kinetics of the forementioned polymers and a mixture of polymers has been studied at temperatures below 450 °C by performing isothermal thermogravimetric analysis (TGA) experiments. The TGA experimental data was used to determine the kinetic parameters on the basis of a simple first-order model for high conversions (70−90%) and a model developed in the present study, termed the random chain dissociation (RCD) model, for the entire conversion range. The influence of important parameters, such as molecular weight, extent of branching and β-scission on the pyrolysis kinetics was studied with the RCD model. This model was also used to calculate the primary product spectrum of the pyrolysis process. The effect of the extent of branching and the initial molecular weight on the pyrolysis process was also studied experimentally. The effect of the extent of branching was found to be quite significant, but the effect of the initial molecular weight was minor. These results were found to agree quite well with the predictions obtained from the RCD model. Finally, the behavior of mixtures of the aforementioned polymers was studied and it was found that the pyrolysis kinetics of the polymers in the mixture remains unaltered in comparison with the pyrolysis kinetics of the pure polymers.
The utilization of natural fiber reinforced polymer composites is growing fast in numerous sectors. In this study, the effect of the addition of short date palm mat (DPM) fibers in polystyrene matrix on the physico-mechanical and thermal properties were studied. Short DPM fiber reinforced polystyrene composites were produced by compression moulding process and the fiber content was 5, 10, 15, 20 and 25 wt%. Physico-mechanical and thermal properties were examined. Scanning electron microscopy (SEM) and Fourier-transform infrared (FT-IR) analysis of the composites were also done. The findings from the investigates exposed that the composites with 10 % fiber content showed improved mechanical and thermal characteristics as compared to other composites. The morphological analysis also supported the results where good interfacial bonding among fiber and polystyrene matrix was found for the composites with 10 % fiber content. The optimized (10 % fiber content) composites were exposed to gamma radiation (2.5–7.5 kGy) and the best result was found at 5.0 kGy radiation dose. Degradation of gamma irradiated composites was conducted in four different media such as water, acid, base, and brine.
A kinetic study of the pyrolysis of the five main plastics contained in a typical municipal plastic waste (MPW) stream revealed that some interactions occur between the plastics during the mixture pyrolysis. A comparison of the kinetic parameters showed that the activation energy of each polymer in the mixture changed only slightly when compared with the single polymer pyrolysis. During the vacuum pyrolysis of MPW two aspects were considered: the pyrolysis of individual plastic and the pyrolysis of prepared MPW. The pyrolysis of polyolefins produced wax as a major product, followed by a small quantity of oil, and a low percentage of gas and solid residue. During the vacuum pyrolysis of commingled plastics, wax and oil were the major products. Two stage vacuum pyrolysis performed at 360° and 520°C enabled 99.55 wt % of the chlorine to be captured as HCl, followed by 62.03 wt % wax, 31.09 wt % oil, 1.0 wt % gas and 1.20 wt % solid residue (based on the initial material). The results suggested that the pyrolysis yields of the plastic mixtures were similar to those calculated based upon the pyrolysis behavior of the individual polymers. This is an abstract of a paper presented at the 98th AWMA Annual Conference and Exhibition (Minneapolis, MN 6/21-24/2005).
The kinetic process of polystyrene degradation was first studied by combining the non-isothermal differential and integral methods. The experiment results indicate that there has a mass loss stage for polystyrene degradation in N2. The temperature range is from 282.9°C to 398.2°C, and the maximum weight-loss rate is achieved at 372.4°C. Kinetic analysis results indicate the associated kinetic mechanism is first order chemical reaction: dα/dt = Ae(-E/RT)(1 - α). The average activation energy E is 199.2 kJ/mol and lnA is 36.56.
The paper is a literature review of the methods of polystyrene (PS) modification to increase its fire resistance. Various types of flame retardants are presented, with a focus on reactive flame retardants incorporated into the PS chain structure. In the industrial practice additive flame retardants are used to reduce the polystyrene flammability. Also, the main area of attention in the literature is physical modification. There are a few papers concerning the use of chemical modification, which can offer an interesting alternative. The problem of PS flame resistance is still present because of the necessity to phase out commonly used additive flame retardants according to European Community legislation. In the paper, combustion mechanism of polymers including thermal degradation of PS is discussed. The flammability test methods are described and the modifications of polystyrene with additive flame retardants are presented.
Thermal degradation of polystyrene was performed in a differential scanning calorimeter under nitrogen atmosphere, and the glass transition temperature was measured continuously. From random chain scission kinetics and Tg-molecular weight relationships, the rate constant for random scission was obtained. The rate constant was found to undergo a drastic change at a critical molecular weight (∽45,000), which corresponds to a similar observation in relaxation studies. A viscosity-dependent mechanism for radical chain end termination is thus suggested.
The application of thermal analysis in the study of reaction kinetics and reaction mechanisms in combination with presently available powerful analytical tools, in the sphere of materials with particular reference to high energy materials is presented and discussed. Also an attempt has been made to correlate the kinetic data obtained by TA with the performance characteristics, for some important materials.
The process of thermal destruction of styrene-divinylbenzene copolymers, studied in a derivatograph, proceeds in one stage, as indicated by the mass loss accompanied by a characteristic endothermic peak in the range 380–400°. The determined values of the kinetic characteristicsn, E, InA, ΔE andT
0 allow the conclusion that an increase in the divinylbenzene content of the monomer blend used to synthetize the divinylbenzene copolymers does not result in an increased thermal stability as compared to that of polystyrene.
STUDIES on potassium perchlorate/polystyrene (KP/PS) propellant systems have been carried out by using such techniques as thermogravimetry (TG), differential thermal analysis (DTA), and mass spectrometry (MS). It has been found that the thermal decomposition (TD) behavior of the KP/PS propellant is similar to that of the AP/PS propellant studied earlier.¹ It has also been observed that the TD of KP in the melt has a correlation with the burning rate (r) of KP/PS propellant at atmospheric pressure. © 1980 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.
To obtain a deeper insight into the thermal decomposition process of ammonium perchlorate (AP)-based composite propellants, a quantitative analysis of the thermal changes associated with the binder, oxidizer, and the propellant was conducted. The primary objectives were the enthalpy and kinetic estimations from the Differential Scanning Calorimetric (DSC) thermograms of polystyrene (PS), AP, and PS/AP propellant. Another objective was to compare the activation energies (E) of the thermal decomposition of PS and AP to that of the propellant.
The study concerned itself with the investigation of the kinetics of xylan in an inert atmosphere. It was found that there were two basic mechanisms for degradation in the temperature range studied, nucleation and diffusion. The range of temperatures studied was 200–290°C. By comparison to several theoretical models the correct theoretical interpretation was based on the best statistical fit of the data. In this way, it was discovered that a first order nucleation mechanism prevailed over the temperature range 200–290°C. A least squares program was developed from an IBM Statistical Package in which each model was subject to the same analysis.A Carter—Valensi model was obeyed from 240 to 255°C. This model is a diffusion type model. In this case, the product layer develops at the surface of the particle. Finally, an Avrami—Erofeyev equation was found as the best fit for the temperature range 270–290°C. It is surmised that polymer degradation in the solid state follows heterogeneous mechanisms rather than, as previously thought, by nth order homogeneous mechanisms.
Two pyrolysis experiments have been utilised to investigate reactions pertinent to flame retardant behaviour. Firstly, a mass spectrometric evolved gas analysis technique has been used to study the influence of the flame retardant decabromodiphenyl oxide (DBDPO) and the synergists antimony oxide and zinc borate on the activation energy for monomer (i.e. fuel) evolution from polystyrene composites. When the polystyrene was compounded with all three additives, a negative value of −82.5 ± 5.0 kJ mol−1 was obtained for the activation energy. This is explained in terms of a condensed phase reaction between the DBDPO and polymer radicals which reduces monomer evolution. The rate of the former reaction increases more rapidly than the latter with increasing temperature. As a consequence the rate of monomer evolution is decreased as the temperature is raised, hence the observed negative activation energy.Secondly, the extremely fast scan capability of the time-of-flight mass spectrometer has been utilised to monitor HCl and benzene evolution from various poly(vinyl chloride) (PVC) composites subjected to a very large and rapid temperature rise due to pulsed laser heating. The benzene evolution behaviour observed is correlated with smoke density data obtained from the same materials. It is found that a decrease in the smoke density value can be associated with an increase in benzene evolution. This may well indicate that additives which result in benzene rather than higher aromatics being evolved may well be used to produce PVC composites with lower smoke production in a fire.
We model the effect of the catalyst AlCl3 on polystyrene (PS). Detailed experimental studies were previously carried out on the effect of AlCl3 on PS, as part of an effort to understand how to minimize the degradation of PS during the Friedel–Crafts alkylation performed to obtain a graft copolymer from immiscible blends of PS and a polyolefin (PO). In the present work three mathematical models for the catalytic degradation of PS are proposed, all of which consider that reaction starts with the elimination of a phenyl group from the PS chain, followed by either chain scission or a change in the chain structure. The models vary in the way they consider the strength of the main chain bonds, or the reactivity of modified PS chains. Kinetic parameters for each model are estimated. Although the three proposed models could be used to represent our own experimental data, one is more accurate. Experimental data from other authors are used to evaluate its capabilities. Based on the predictions of the better model, we discuss conditions to minimize PS scission, such as operating at low temperatures and AlCl3 concentrations, and using short processing times.
Simulations of the initial distribution of volatiles from pyrolysis of polystyrene were based on propagation rate constants estimated by thermochemical kinetic procedures. The voluminous database exhibits a disturbing lack of consistency with respect to effects of conversion level, temperature, and reactor type. It therefore remains difficult to assign the true primary distribution of the major products, styrene, 2,4-diphenyl-1-butene (“dimer”), 2,4,6-triphenyl-1-hexene (“trimer”), 1,3-diphenylpropane, and toluene, and its dependence on conditions. Probable perturbations by secondary reactions and selective evaporation are considered. The rate constant for 1,3-hydrogen shift appears much too small to accommodate the commonly proposed “back-biting” mechanism for dimer formation. Dimer more likely arises by addition of benzyl radical to olefinic chain-ends, followed by β-scission, although ambiguities remain in assigning rate constants for the addition and β-scission steps. With this modification, the major products can be successfully associated with decay of the sec-benzylic chain-end radical. In contrast, the minimal formation of allylbenzene, 2,4-diphenyl-1-pentene, and 2,4,6-triphenyl-1-heptene suggests a minimal chain-propagating role for the prim chain-end radical. Compared with polyethylene, the much enhanced “unzipping” to form monomer from polystyrene and the more limited depth of “back-biting” into the chain arise from an enthalpy-driven acceleration of β-scission coupled with a kinetically driven deceleration of intramolecular hydrogen transfer. In contrast, the greater “unzipping” of poly(isobutylene) compared with polyethylene is proposed to result from relief of steric strain.
Poly(styrene peroxide) (PSP) initiated photopolymerization of vinyl monomers has been carried out using benzoyl peroxide or di-tert-butyl peroxide as a reference photoinitiator with radiations of wavelength ⩾300 nm. Comparative rates of PSP degradation in the vinyl monomers are examined to assess its efficiency of radical generation. The faster rate of PSP decomposition in acrylamide and acrylonitrile has been attributed to their catalytic effects on PSP decomposition. The obtained polymers were characterized by molecular weight, 1H-NMR and thermal studies. Interestingly, it revealed the absence of attached PSP segments in poly(acrylamide) and poly(acrylonitrile) unlike in poly(methyl methacrylate) (PMMA) and polystyrene (PS) where PSP blocks were reported to be present near their chain ends. Presence of PSP block does not drastically affect their thermal stabilities. The average concentrations of PSP blocks in PMMA–b-PSP and PSP–PS–PSP, determined by 1H-NMR, DSC, TG and GPC were in reasonably good agreement, and it is related to the extent of PSP decomposition in methyl methacrylate and styrene. The molecular weight and yield of the polymers, obtained with PSP, were always higher and lower, respectively, compared to simple peroxides in all the monomers except in AA where they are roughly comparable, and this was attributed to the differences in the concentration of the primary radicals formed during irradiation.
The effect of cobalt salicylate on the oxidative degradation and ignition of polystyrene has been studied. It was found that cobalt salicylate sensitizes both the degradation and ignition of polystyrene by facilitating electron-transfer processes in the propagation step. From thermochemical and kinetic studies it was found that the cobalt ion, owing to its ability to exist in variable valence states, promotes electron transfer in the propagation step of polymer degradation, increasing the rate of propagation and consequently the overall rate. Using solid-phase thermal ignition theory, an attempt has been made to explain the sensitization of ignition by the cobalt ion.
Difference equation method has been suggested to evaluate the kinetic parameter of the degradation of polymers under isothermal condition. The method is simple and applicable to degradation process for any value ofn, the reaction order. By using one simple method all the kinetic parameters cannot be obtained in other methods only by difference equation.
Summary A correlation has been established between the heat of depolymerization (?H) of vinyl polymers for going from solid polymer state to gaseous monomer state and the activation energy (E) of degradation. On this basis it has been shown that the rate controlling step in the degradation lies in the initiation step. Attempt has been made to correlate theE and ?H with glass transition temperature (Tg) and melting temperature (Tm) of the polymers.[/ p]
Decomposition and combustion of three chlorinated polyethylenes (C1PE-25%, C1PE-36% and C1PE-42%) and a high density polyethylene (HDPE) under medium heating rates (11.5–67.5°C (s−1) in an oxidant gas flow was investigated using a Du Pont 951 thermogravimetric analyzer (TGA). The decomposition rate constants prior to ignition and during combustion were obtained using an Avrami-Erofee'v type equation, [− In(1 − C)]= kt + A where n was chosen equal to four. The results showed that the rate constant was sensitive to the isothermal furnace temperature only if a small sample mass (about 2.40 mg) was used. Increasing O2 concentration in the gas flow greatly promoted the decomposition rate prior to ignition, but was less significant in the combustion stage. The rate of decomposition for HDPE was much higher than that of the chlorinated polyethylenes, indicating the effectiveness of substituted chlorine in suppressing the decomposition rate of the polymers. On the other hand, increasing the chlorine content from 36 to 42% did not reduce the decomposition rate further under our conditions.
A novel approach based on dilute-solution viscometry was developed particularly for monitoring the slow rates of random chain scission reactions. It was applied to study the kinetics of the thermal degradation of bulk polystyrene (PS), in the presence of N2 at 180 °C. Results on the rate constant obtained from two PS samples, which conform to a Schulz distribution and a Wesslau distribution function, are in good agreement. Special features of these functions were exploited to make the present method more attractive and practical.
Poly(p-phenylene sulphide), poly(2-methyl phenylene sulphide) and poly(2,6-dimethyl phenylene sulphide) have been subjected to comparative thermal analysis utilising thermogravimetry and isothermal weight loss studies in air and nitrogen. Comparative thermal stabilities have been evaluated and related to polymer structure. Activation energies for decomposition have been evaluated and are discussed.The related poly(p-phenylene sulphone) and poly(2-methyl phenylene sulphone) have also been studied and their behaviour analysed in the light of their possible formation during thermal oxidative degradation and air curing of the parent poly(phenylene sulphides).
High-resolution (M/DELTAM = 7000) time-of-flight secondary-ion mass spectrometry (TOF-SIMS) was used to study the mechanism of fragment-ion formation for several polystyrenes. Ions are formed from neutral fragments resulting from chain cleavage and are cationized with a silver ion. Spectra of the polystyrenes consist of repeating patterns of clusters of invidual peaks extending to m/z > 3000. Fragmentation of the polymer backbone can be explained by limited chain fragmentation, consistent with mechanisms proposed for other methods. Most polystyrenes show five clusters in their repeat pattern except for poly(alpha-methylstyrene) which differs considerably. This difference is discussed. Cluster structure can be explained by the contributing species having zero, one, or two rings and/or double bonds. The results of this study support the idea that chain fragmentation proceeds through a cyclic intermediate.
A conical spouted bed reactor (CSBR) has been used for the kinetic study of polystyrene pyrolysis in the 723-823 K range and the results have been compared with those obtained by thermogravimetry (TGA) and in a microreactor (MR) of very high sample heating rate. The comparison proves the advantages of the gas-solid contact of this new reactor for the kinetic study of pyrolysis of plastics at high temperature, which stem from the high heat transfer rate between gas and solid and from the fact that particle agglomeration is avoided. It has been proven that, in the temperature range required for maximizing the yield of styrene, heat and mass transfer limitations within the particle are important when polystyrene particles of 1 mm, are fed into the reactor. A high yield of styrene is obtained (64.5 wt.%) in the 723-773 K range. (C) 2002 Elsevier Science B.V. All rights reserved.
Thermal decomposition studies on double-base propellants have been carried out in air and vacuum using differential scanning calorimetric, thermogravimetric and mass spectrometric techniques. Low temperature (90 °C–170 °C) decomposition seems to involve diffusion controlled process, the mechanism in air and vacuum being different. High temperature (170 °–205 °C) decomposition involves bond breakage.
The thermal stability and degradation kinetics of several polystyrenes and styrene–alkyl methacrylate copolymers and terpolymers with a number-average molecular weight (M̄n) of 6000–250,000 g/mole have been studied using dynamic thermogravimetry (TG). The degradation kinetics of each polymer sample have been successfully attributed to a sample first-order reaction expression. The results indicate that the thermal stability and degradation kinetics of the polymers are independent of the size of the molecules within the molecular weight range investigated. The steric hindrance effects of the pendent groups appear to be responsible for the improved thermal stability and resistance of CC bond scission in the styrene–alkyl methacrylate copolymers and terpolymers.
Thermal decomposition of commingled plastics comprising high and low density polyethylene, polystyrene, polypropylene and poly(vinyl chloride) was investigated under vacuum conditions by dynamic thermogravimetric analysis in the temperature range of 25–600°C. The mixtures were representative of the major polymeric materials found in a municipal plastic waste (MPW) stream. The study focused on the role of each polymer in the stabilization or destabilization of other interacting plastics, as well as the fate of chlorine produced from PVC. First, the decomposition of each single plastic was investigated. Then the binary combination of all above polymers and finally the multicomponent mixture similar to a MPW sample were studied. Two approaches have been applied to study the interaction between components in the commingled plastics. The first approach involved a comparison of the experimental curve of the decomposition of the mixture with a calculated non-interacting component decomposition curve based on the behavior of the individual polymers. The second approach involved a comparison of the kinetic parameters of each plastic in the mixture as determined by two different methods: (1) fitting the single polymer decomposition curve and (2) fitting the peak corresponding to the pyrolysis of one particular polymer on the mixture pyrolysis curve. The results obtained indicate that some interactions occurred during MPW pyrolysis, mainly at high temperatures (>375°C). PS and PVC appear to be the plastics responsible for the interactions through their intermediate pyrolysis products. The chlorine from PVC in the mixture is released almost completely before 375°C, when the conversion of the MPW to pyrolysis products has reached ca. 13%.
The decomposition of basic nickel carbonate in the presence and absence of NaNO3 (sodium nitrate)–KNO3 (potassium nitrate) eutectic melt has been studied by employing isothermal TG and dynamic TG–DSC techniques. The rate constants for the decomposition in the presence of eutectic melt were found to be higher than when carbonate alone was heated. In both cases, nickel oxide was found to be the end product. Catalytic activities of nickel oxide obtained by the two methods were tested for their efficiency in decomposition of polystyrene (PS). It was found that at higher temperature (330 and 360°C), the nickel oxide obtained from eutectic melt was more active than that obtained by direct decomposition.
Polyurethane ionomers based on either poly(propylene glycol), PPG, or poly(ethylene glycol), PEO, and di- (4-isocyanatophenyl) methane, MDI, butane-1,4-diol and cis-2-butene-1,4-diol have been synthesized and characterized. Attempts to convert the unsaturated polyurethanes directly to ionomers by reaction with N-chlorosulphonyl isocyanate, CSI, followed by hydrolysis, failed due to the more favoured reaction of CSI with the labile hydrogen on the urethane linkage. Therefore, the cis-2-butene-1,4-diol was first converted to the dibenzoate which was then treated with CSI to form the β-lactam derivatives. Subsequent hydrolysis of these derivatives with tetra-n-butyl ammonium hydroxide solution produced ionic derivatives whilst at the same time regenerating the terminal -OH groups. These ionic diol intermediates were then used for synthesizing the polyurethane ionomers.
Ammonium perchlorate (AP) has been coated with polystyrene (PS), cellulose acetate (CA), Novolak resin and polymethylmethacrylate (PMMA) by a solvent/nonsolvent method which makes use of the coacervation principle. The effect of polymer coating on AP decomposition has been studied using thermogravimetry (TG) and differential thermal analysis (DTA). Polymer coating results in the desensitization of AP decomposition. The observed effect has been attributed to the thermophysical and thermochemical properties of the polymer used for coating. The effect of polystyrene coating on thermal decomposition of aluminium perchlorate trihydrazinate and ammonium nitrate as well as on the combustion of AP-CTPB composite propellants has been studied.
Reproducible results for the thermal decomposition of sodium azidc have been obtained by (i) precipitating the salt as a fine powder from aqueous solutions acidified with hydrazoic acid and (ii) decomposing it in the form of small pellets, preferably under argon. When these precautions are observed the kinetics obey the Avrami-Erofeev equation with n = 2. An alternative method of finding the activation energy of the reaction without fitting the results to a kinetic equation is introduced. Both methods yield a value of 38 kcal./mole for the activation energy for the thermal decomposition of sodium azide. OH - and Cl- ions retard the reaction but Fe3+ ions catalyse it. A mechanism for the reaction is postulated which explains these and previously observed facts.
Previous theories of bulk degradation of vinyl polymers in vacuo have been based on the assumption that the initial chain lengths were constant (homodisperse model), even though experimental studies rarely meet this requirement. Advantages accrue from the assumption of a chain-length distribution which is of exponential type: (i) this corresponds most closely to the polymers actually handled, (ii) the mathematical theories become simpler. Provided initiation is confined to the chain ends, and the monomer produced in the "zipping" reaction is removed by distillation, the initial exponential chain-length distribution remains invariant throughout degradation. If, as is commonly assumed, the initiation rate is first order in the overall chain-end concentration, the mass decay by monomer distillation is first order, too. Invariance of the mean chain length (DPn) and first-order decay of the mass have been observed for polymethyl methacrylate (PMMA) of low molecular weight in the past. (Unaccountably, the degradation behaviour of high molecular weight PMMA, which should obey an exponential distribution law, accords better with theories for a homodisperse model.) The first-order mass decay observed in the past with numerous polymers becomes theoretically intelligible. An improved theory is given for the steady-state treatment of degradation. Previous data on the initial degradation rates of PMMA as a function of DPn still fit well the asymptotic solutions for high and low DPn, but are now seen to diverge largely from the solution for the intermediate region. Because of the simplicity of the solution for exponentially distributed polymer, a small random scission component, superposed on chain-end activated zipping, can be treated with ample accuracy to fit the data in the literature for DPn-decay during degradation of polystyrene. The data are not sufficiently accurate to distinguish between random splitting, weak-link scission, or scission following chain transfer to polymer. The rate curves on low-molecular-weight polystyrene at high temperature by Madorsky can be fitted to the improved theory assuming termination by disproportionation, another rate curve by Grassie and Kerr for high-molecular-weight polystyrene at low temperature can be fitted assuming first-order radical termination. In accordance with theory, Madorsky's rate curves at various temperatures can be superposed with suitable scale factors, which obey the appropriate Arrhenius law. Grassie and Kerr's observation of superposition of the DPn-decay curves at various temperatures can be plausibly explained. Attention is drawn to the importance of bulk degradation studies in tactic polymers.
Principally by the application of number-average molecular-weight measurements, certain of the details of the mechanism of the thermal degradation of polystyrene have been clarified. The initial steep fall in molecular weight is due to a disproportionation reaction occurring at "weak links" in the polymer chains and results in stable molecules - not radicals as previously suggested. Volatile products are liberated in a radical reaction initiated exclusively at chain ends and having a kinetic chain length of the order of 103. There is no evidence for significant amounts of intermolecular transfer at any stage and the non-monomeric volatile products (dimer to pentamer) are produced by intramolecular transfer near chain ends.
Differential scanning calorimetric studies on ammonium perchlorate have been carried out. The enthalpy values for the phase transition endotherm and the two exotherms have been reported in the present communication. A new method has been developed for the estimation of kinetic parameters from DSC the mograms. The values for activation energy as calculated by the above method for low temperature and high temperature exotherms are in close agreement with literature values. The present studies also confirm the presence of small exothermic peaks at the initial stages of high temperature exotherm. Explanation for the same has been given.
A method is presented for the estimation of kinetic parameters during polymer degradation from two DTA traces. By this method, changes in mechanism with conversion may be detected. The method is applied to polyethylene, polypropylene, polystyrene, and polytetrafluoroethylene (Teflon). The agreement between observed and reported values of kinetic parameters is good. Advantages and disadvantages of the method are discussed.
A method of calculating regression rates over(r, ̇) of simple polymer fuels has been worked out and applied to poly(methyl methacrylate), polystyrene, and polyethylene. The method depends upon (a) invoking the concept of a critical fragment size (c.f.s.) and (b) the use of a first-order rate equation -dn/dt=kn, for describing the scission of backbone bonds in the polymer. The c.f.s. is the chain length of the volatile product of chain degradation, above which size it is energetically more economical to continue breaking backbone C-C bonds than to remove the fragment from its environment. From random scission of these bounds, we have derived a rate of loss-in-weight law m1/m0=1-exp (-ikt)[i+1-exp (-kt)], in which i is the maximum chain-length of the c.f.s. For other mechanisms of chain degradation, the average chain-length of the c.f.s. was used and a rate of loss-in-weight law m1/m0=1-exp (-kt) was involved. The first-order rate equation, above, was shown to be consistent with these laws. Agreement with measured over(r, ̇) values is very good, being off by not more than a factor of 2, but depends upon the application of the appropriate surface temperature Ts. Extension of this method to composite fuels does not give results in agreement with experiment, even when limiting values of Ts are used, and KClO4 or NH4ClO4 is the oxidizing filler. The conclusion was drawn that the two-temperature model of a burning composite fuel was more likely to succeed. It was shown that a crystallite surface temperature for NH4ClO4 of 1010°K could be calculated without invoking a solid-phase reaction.
The rapid decrease in molecular weight which occurs in the initial stages of the thermal degradation of polystyrene at 325°C occurs at the same rate whether the reaction is carried out in bulk polymer or in solution in naphthalene or tetralin. On the other hand, the radical chain depolymerization process which occurs in bulk polymer and in naphthalene solution, and results in the evolution of volatile products, principally monomer, dimer and trimer, is completely inhibited in tetralin solution. Thus it is deduced that volatilization and molecular weight decrease are manifestations of two distinct processes and this together with previous evidence is taken as proof that the decrease in molecular weight is due to scission of weak links rather than to an intermolecular process involving the depolymerizing free radicals.
Explicit expressions are obtained for the surface regression rates of pyrolyzing vinyl polymers. Thermal degradation of the polymer in a subsurface reaction zone is assumed rate limiting. Emphasis is placed on determination of the proper degradation mechanism (e. g. , mode of initiation and magnitude of the kinetic chain length compared to the degree of polymerization), development of kinetic data, and derivation of corresponding expressions for the rate of mass loss. The method of calculation is based on a matched asymptotic expansion scheme, with the nondimensional activation energy treated as the expansion parameter.
In the general theory of chain depolymerization reactions two parameters are important: the kinetic chain length 1/ε and the transfer constant σ. In terms of these the molecular weight distribution, average molecular weights and rates of decomposition can be discussed. These rates decrease monotonously with conversion when the kinetic chain length is large in comparison with the D.P., regardless of the initiation mechanism. When it is small, there is a maximum rate, if the initiation is random or transfer is pronounced. If initiation is occurring only at the chain ends, the reaction becomes of zero order. Although the general theory for any value of the kinetic chain length has been developed, numerical solutions are so far available only for extreme cases. However, the initial rate has been given for all conditions. The theory is compared with experimental results on molecular weight change and rate of degradation for polystyrene and polymethylmethacrylate, respectively. In the former instance, an estimate of the kinetic chain length is made from the observed monomer yields. In the latter, ε and the rate constant for initiation are obtained from molecular weight and rate curves. The observed molecular weight decrease in methyl methacrylate is more pronounced for the larger molecular weights and higher degrees of conversion, than the theory neglecting chain transfer would predict. This has been tentatively ascribed to chain transfer becoming more important at later stages of the reaction. Thus, it would appear that the general state of affairs in thermal depolymerization of addition polymers is accounted for.2 However, discrepancies remain which can at least qualitatively be explained in the framework of the general theory. Their quantitative interpretation requires a refinement of the reaction mechanism considered, in particular the initiation step, and further numerical evaluation of the rate equations. The theory was developed in order to interpret the thermal and photodecomposition of chain polymers. There are degradative processes which exhibit a striking similarity to the phenomena discussed here, although the detailed chemistry is, of course, entirely different. In the deterioration of textiles, some agents attack the fabric in such a way, as to leave the tensile strength of the residue unimpaired, while others produce a gradual decay in the tensile properties. Also, the breakdown of proteins by proloolytic enzymes seems to occur principally in two ways, one leaving intact material and small species, the other producing molecules of intermediate sizes.3 A kinetic description of these reactions could be similar to ours, which involves, in one extreme a slow process, followed by more rapid ones, and in the other a single, moderately rapid step.
The nonisothermal method of Freeman and Carroll is used to investigate the kinetics of the thermal degradation of polyethylene and polystyrene. The reactions were studied thermogravimetrically under a vacuum of 1 mm Hg. Decomposition appears to occur in stages. The rate parameters were determined for each region of reaction. Mechanisms of degradation are suggested. The results of this investigation are compared with the kinetic parameters reported by other investigators for the decompostion of these polymers.
The decrease of the limiting viscosities of unfractionated and fractionated polystyrene with time has been studied over a range of temperatures from 248 to 340°C. in vacuo. Distribution curves of several degraded samples have been determined. It has been established that the theory of random breaking of links is not applicable in the case of polystyrene. Three major deviations from the random theory have been found. The experimental results have been evaluated employing the weak link theory. The mechanisms proposed account satisfactorily for the deviations from the random theory, and give a fair representation of the experimental results, only the experimental distribution curves still showing deviations from those predicted by theory. The elementary steps for the degradation in vacuo have been discussed and the role of oxygen during polymerization has been indicated.
When heated in a vaccum at 325° to 375°C., polystyrene yields mainly the monomer and some dimer, trimer, tetramer, etc., indicating that the breaks in the molecular chains occur largely at the ends. Polyethylene, under similar conditions, but at a some-what higher temperature, yields fragments of an average molecular weight of about 700, indicating that the molecular chains break, mostly at random positions. A study was made of rates of thermal degradation of these two polymers by measuring rates of loss of weight of samples by means of a very sensitive tungsten spring balance enclosed in a vaccum. The samples were limited to 5–6 mg. in order to avoid spattering during degradation. Rate curves plotted against per cent loss of weight by volatilization indicate that in the case of polystyrene the process is intermediate between a zero and first order reaction, while in the case of polyethylene, the process approximates a first order reaction. Activation energies were calculated on the basis of rates of degradation at various temperatures. The values thus obtained for polystyrene and polyethylene are 58 and 68 kilocalories, respectively.