Fire and Polymers - American Chemical Society

Chapter 15

Downloaded by UNIV OF GUELPH LIBRARY on September 17, 2012 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch015

Optimization of Processing Parameters for Fire-Retardant Polymer-Based Formulations L . Cartier, F . Carpentier, M. Le Bras, F . Poutch, and R . Delobel ENCL, GéPIFREM, UPRES EA 2698, BP 108, F 59652 Villeneuve d'Ascq, France

This work deals with the effect of the processing parameters on the flame retardant and the rheological properties of two ethylene-vinyl acetate ( E V A ) copolymer-based formulations. The two formulations are based on both the concept of intumescence and the concept of ceramitization. For the intumescent formulation, the polymer degradation, particularly the thickening caused by the cross-linking of the polymer chains relating to processing conditions, is discussed. Finally, a principal component analysis allows the proposal of a relationship between processing and properties variables, and shows that heat transfer is responsible for the thickening of the polymeric material when high mechanical strains reduces the delay for this behavior. For the formulation that creates a ceramic-like residue, a first attempt is made to explain the difference in flame-retardant properties observed in the mixing and extrusion process.

This research includes the development of ethylene-vinyl acetate copolymer (EVA)-based formulations utilized in many industrial applications and particularly in low voltage cable and wire manufacture (1-3). Unfortunately, these polymers are frequently flammable and so it becomes important to reduce their flammability to increase their applications. The typical flame-retardant

186

© 2001 American Chemical Society

In Fire and Polymers; Nelson, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


187 systems are based on halogenated compounds, which generate toxic by-products during their combustion. Thus, it is of importance to develop new halogen-free flame-retardant (FR) materials. One way to solve the problems consists of using additives in order to buildup an intumescent structure, i.e. a cellular charred layer on the surface which protects the underlying polymeric substrate from the action of heat flux or flame. Our laboratory has recently developed an intumescent formulation: E V A 8 / A P P / P A 6 with 40 wt.-% additive loading. The E V A 8 is the polymer matrix, ammonium polyphosphate the acid source and blowing agent, and polyamide 6 the carbonization agent (4). This formulation has not shown any interaction of the additives during processing. A second approach of developing new F R materials consists of identifying systems that are able to form a vitreous protective layer over the polymer matrix and show endothermic reactions which absorb the flame energy and limit the degradation process. The layer acts as an insulator and a mass transport barrier hindering the escape of very often flammable volatile combustion products. For this purpose, the formulation: EVA19/A1(0H) /FBZB with 65% additives was evaluated. The E V A 19 is the polymer matrix, aluminum hydroxide the mineral filler that decomposes into A 1 0 and water via an endothermic reaction, and zinc borate (FBZB) the synergistic agent and smoke suppressant (5). The change in viscosity of the formulations influences their transport during processing. It is important to ensure that the laboratory conditions correlate with the actual industrial processing parameters. Processing parameters, such as, residence time, temperature, flow rate, extrusion or mixing speed, could modify the rheological properties of the formulations (6, 7). Therefore, they have to be optimized. The rheological modifications are due to the thermo-mechanical treatments of the material during their processing, which subsequently influence the F R properties of the formulations. Indeed, it is now well-known that the first step of the modification of the polymer matrix (i.e. formation of unsaturated groups and intermolecular crosslinks (8, and references therein) can have an effect on char generation by the matrix degradation (9). This additional char formation is frequently accompanied by a reduction in yield and/or rate of formation of volatile fuels. Moreover, the char can act, in addition to the protective material formed from the additives, as a physical barrier against heat transmission and oxygen diffusion (10). A Brabender roller mixer and a counter-rotating twin-screw extruder were used to demonstrate the influences of residence time and temperature on both the F R performances and the rheological behavior of the formulations. The aim of the principal component analysis is to identify the respective effects of mechanical work and heat on the degradation process of the polymer during its processing and, to correlate their effect on the F R properties of the formulations. 3

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Experimental Materials E V A 8 (Evatane) and E V A 1 9 (Escorene Ultra) with 8% and 19% (wt/wt) vinyl acetate content were supplied by Elf-Atochem and Exxon Chemicals respectively. Ammonium polyphosphate (APP), ( ( N H P 0 ) , n=700, Exolit 422) was supplied by Hoechst. Polyamide 6 (PA6) was obtained from Nyltech. Aluminum hydroxide (Al(OH) ) was supplied by Alcan (ΑΤΗ SF-07/E) and zinc borate (Firebrake F B Z B : 2 Z n 0 . 3 B 0 . 3 . 5 H 0 ) was obtained from U S Borax. Formulations were processed using a Brabender Plasti Corder PL2000 roller mixer and a Brabender D S K 42/7 intermeshing counter-rotating twinscrew extruder. Limiting oxygen index (LOI) (//) was measured using a Stanton Redcroft instrument on sheets (120x10x3 mm ). The U L 94 test (12) was carried out on sheets (125x13x1.6 mm ). Rheological measurements were carried out on samples (25x25x1.6 mm ) using a Rheometric Scientific Ares-20A thermal scanning rheometer in a parallel plates configuration. The conditions of viscosity testing were: strain value (3°), angular rate (0.5 rad.s" ), heating rate (20°C/min), force exerted on the samples (0.5 N). Solid state N M R experiments were performed. C N M R measurements were carried out on a Brucker ASX100 at 25.2 M H z (2.35 T) with magic angle spinning (MAS), high power *H decoupling (DD) and H - C cross polarization (CP) using a 7 mm probe. A repetition time of 10 s and a spinning speed of 5 kHz were u s e d . A l and B N M R measurements were performed on a Bruker ASX400 at respectively 24.5 M H z and 128.3 M H z (9.4 T) with M A S using a 4 mm probe. A repetition time of 10 s and a spinning speed of 15 kHz were used. 4

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Principal components analysis The principal components analysis was carried out using the STAT-ITCF computational method (Institut Technique des Céréales et des Fourrages, Paris). It is a descriptive method: from the obtained data, the relationships between the variables are derived directly from the computations; the neighboring variables may be explained by the existence of a relationship between two variables and that orthogonal positions may corresponds to the absence of relationships. In this study, ten variables were used for the drawing of the correlation circles. Six centered variables which are: temperature gradient, G T ; set temperature, Tc; thickening temperature, T E ; heat required to mix the material, Q; mixer's rotor speed, V ; and mechanical work required to mix the material, W M E C ; and four supplementary "guest" variables that are not used in the computation of the principal axes: torque at the thickening point, C E ; activation


189 energy, E ; constant, C O ; torque at 1/T=0, and from the calculation of the activation energy, time to reach the thickening point, TPS. A

Results


Flame-retardant properties of the intumescent formulation E V A 8 exhibits poor heat resistance; its limiting oxygen index (LOI =17 Vol.-%) is low and it is not classed (NC) by the U L 94 test. Thus, the polymer requires the incorporation of additives to achieve F R properties. The intumescent formulation E V A 8 (60 wt.-%)/APP (33 wt.-%)/PA6 (7 wt.-%) exhibits good flame-retardant properties after processing at 250°C using the twin-screw extruder at 50 rpm (1 rpm = 2πRad min" ). It shows L O I = 30 V o l . - % (the experimental error on the LOI measurements is ±0.5 Vol.-%) and a V 0 U L 94 rating, i.e. self-extinction and no dripping. The L O I values, considered as the F R properties vary as a function of both the screw speed and the processing temperature (Figure 1). 1

Shear stress (1/2π rad-1)

Figure 1: LOI values of EVA8/APP/PA6processed using a counter-rotating twin-screw extruder at various temperatures and screw speeds. In the low shear stress range (rotation < 35 rpm), the L O I decreases when the processing temperature increases. The maximum value is reached when the rotation of the screw is 50 rpm in the 250-270°C processing temperature range. Such behavior is not easy to understand at this stage of this study. It may be presumed that the variation of the measured L O I values results from chemical and/or physical changes of the intumescent formulation, arising from the thermo-mechanical effect of the process. It is obvious that such transformations modify the formulation's flow properties during the process. Therefore, it is necessary to study the formulation's rheological variations as a


190 function of time and temperature to optimize the F R properties and to insure that the material can be processed in the optimum manner.

Flame-retardant properties of the ceramic-like formulations The flame-retardant properties of E V A 19 (LOI = 22 V o L - % and N C by the U L 94 test) are significantly improved at 65 wt.-% total loading of A l ( O H ) (LOI = 44 V o l . - % and V I U L 94 rating). These properties are reinforced by substituting a small amount of aluminum hydroxide by zinc borate which reveals a synergistic effect for the EVA19/A1(0H) /FBZB formulations in the case of both a mixing and an extrusion process (Figure 2). This effect is optimum at 5 wt.-% of F B Z B . Moreover, in this last case and for smaller Z B content, a V I rating is still achieved whereas a higher F B Z B content leads to a N C rating.


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The L O I values strongly depend on the process. Indeed, the samples processed using the mixer at 190°C, 7 min residence time and 40 rpm screw speed lead to an optimal L O I of 46 V o l . - % which corresponds to a synergistic effect of 2 V o l . - % . The values obtained from extruded samples are generally higher and a maximum performance (LOI = 50 V o l . - % , synergistic effect of 6 Vol.-%) is reached at a processing temperature of 190°C, 7 min residence time and 40 rpm screw speed. The explanation for this difference in L O I values under comparable processing conditions for the mixer and extruder could be due to fundamental differences between the mixing mechanisms: 1) mixing time; the mixing time in the mixer is about three times longer than in the extruder, 2) shear stress strength; for the same shear rate, the mechanical constraint is much higher in the extruder, 3) the presence of oxygen in the system; in the case of an extrusion


191 process, the pressure gradient along the screw retains the air near the feeding zone while in the mixer the air is always in contact with the polymer formulation. This evolution of the two systems versus the zinc borate content may be correlated to the rheological and/or chemical modifications that are investigated in this paper.


Rheological variation of E V A 8 versus the processing conditions If chemical transformations of the material result in a significant change of the flow properties, they can be studied using both the mixer or the extruder at processing parameters such as torque, temperature, viscosity, and the activation energies. The two factors susceptible to the influence of the polymer's chemical degradation during its processing are the mechanical and the thermal constraints on the material (13). The extruder allows good shear stress control but it is quite impossible to control the temperature change of the molten polymeric material during the process, and to correctly measure this variable. Consequently, the material's viscosity may not be deduced from the material flow in dies. A roller mixer allows a nearly correct measurement of the two variables. The Brabender torque measured using a roller mixer (assumed to be a cylinder rheometer) may be related to the viscosity η of the thermoplastic material by: Torque = Κ η = Κ η exp (E /RT) (with η the pre-exponential factor and E the activation energy) when the temperature range is not too large (14, 15). Plots of the logarithm of the torque versus T give values of Κ η and of E which are characteristic constants of a thermoplastic material; change of these values points out the modifications of the material. Figure 3 shows the change of E V A 8 during the mixing process under air when the temperature of the melt increases (heating rate: 5°C min' ). Linearity in the first zone corresponds to the thermoplastic behavior of E V A 8 . Change of the slope at about 200°C may be explained by the chemical change of E V A 8 to form unsaturated groups with evolution of acetic acid, the decomposition product of vinyl acetate groups. In the range 205-220°C (zone 2), the degraded material is thermoplastic. In die 220-250°C region (zone 3), this linearity is not observed; a relative increase of the viscosity takes place with a computed difference in viscosities of 300 Pa.s. This phenomenon can be explained by a cross-linking effect of the polymer, the increase in torque as a function of time being a characteristic of a thermoset resin. Finally in the last zone, the modified resin again shows the behavior of a thermoplastic. This shows that the chemical degradation modifies the flow properties of the material. 0

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Figure 3: Values of viscosity as a function of the processing temperature (heating rate: 5°C/min) using a mixer (rotation speed: 50 rpm). The variation in viscosity is observed when two consecutive temperature gradients are applied (respectively 5°C min" and -5°C min" ) to the material (Figure 4). The two viscosity curves are superimposed when the temperature is above about 245°C. Thus, the irreversible chemical degradation of the E V A 8 ends at 245°C after the application of the first temperature gradient. 1

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Figure 4: Viscosity of the EVA8 as a function of temperature, at mixer speed of 50 rpm. Two successive temperature gradients were applied between 150°C and 260°C and between 260°C and 150°C. E V A 8 mixing under nitrogen reveals that chemical modifications of the material are identical to those observed previously with the temperature gradient under air. This demonstrates that the modification of the E V A 8 is not related to a chemical degradation due to the oxidation of the material; the chemical modification o f the E V A 8 is initiated by the thermo-mechanical constraints exerted on the material.


193 Chemical modification in the ceramic-like formulations The processing parameters (temperature of 190°C, 7 min residence time and 40 rpm screw speed) used for die formulations E V A 19/Al(OH) /FBZB processed using the mixer and the extruder are much less severe than those chosen for E V A 8 (Figure 2). A degradation of E V A 19 that causes the formulation to thicken is not expected. Indeed, C N M R measurements confirm that E V A 19 involved in the formulations is not modified during the mixing or the extrusion process. The spectra o f A l N M R also show no differences between the A l ( O H ) alone and the formulations made in the mixer and the extruder. The aluminum hydroxide is not modified during processing and does not interact with the other constituents of the formulation E V A 19/Al(OH) /FBZB. The B N M R spectra of F B Z B and the formulations processed using the mixer are very similar, implying that there is no major modification of F B Z B during the mixing process. However, concerning the extrusion process, an evolution of the peaks is observed which corresponds to degradation of F B Z B . The simulation of the spectra allows a proposal of two species with isotropic chemical shifts of about 1 and 18 ppm, which are assigned to B 0 and B 0 units respectively (16-18). The ratio B 0 / B 0 versus F B Z B content changes and is optimized at 5 wt.-% F B Z B , which indicates that F B Z B transforms during the extrusion process (Figure 5). 3

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Rheological variation of E V A 1 9 / A l ( O H ) / F B Z B versus temperature 3

The work on the intumescent formulations reveals that the chemical transformations of E V A 8 during processing directly affect its rheological behaviour and influence the F R performance of the material. Since chemical



194 modification of F B Z B is demonstrated, the rheological fluctuations of the ceramic-like formulations need to be evaluated (Figure 6). The viscosity measurements are performed in the temperature range from 200°C to 500°C for the formulations containing different amounts of F B Z B and processed using the mixer and the extruder with similar conditions. Since no change is observed above 360°C, Figure 6 shows the formulations' rheological variations at low temperatures. The zinc borate content has a significant influence on the viscosity for all of the processes that have been utilized. Indeed, the viscosity increases to reach a maximum for a value of 5 wt.-% of F B Z B before decreasing with the addition of larger zinc borate quantities in the formulations. However, it is important to note that the extrusion process leads to a higher viscosity compared to that obtained from the mixer. These observations should help understand the F R performance evolution which depends on both the substitution in F B Z B of the formulations EVA19/A1(0H) /FBZB and the two different processes. 3

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Discussion Intumescent formulation E V A 8 / A P P / P A 6 In this section, we will simply assume that the rheological and F R performances of the intumescent material are explained only in the E V A 8 changes. The presence of the additives in the molten matrix does not significantly affect its properties during processing.



195 The chemical modification of the E V A 8 is not due to its thermal oxidation, but rather to a thermo-mechanical-radical effect which relates to either the heating of the material or the mechanical work. In order to optimize the processing conditions of EVA8-based formulations, the influence of both the thickening mechanism parameters and exposure time on the constraints have to be identified. For this purpose, the principal components analysis is performed following a procedure developed by Heermann (19). A part of the required mechanical work to mix the material is converted into heat that depended on shear stress rate. The difference between the set and processing temperatures becomes smaller when the latter is higher. The thermoplastic behavior of the material where viscosity decreases with temperature generates less friction at high temperature, thus explaining the heat change versus shear stress. In the analysis, the variable Q and W M E C are systematically corrected for this effect. The relationships between all of the variables are then tested. Two significant correlation circles are computed (Figures 7-a and 7-b) using three principal axes. Axes 1 and 2 correspond respectively to the heat data (Tc, T E and Q) and the mechanical and thermal constraints that modify the viscosity of the formulation (GT and W M E C ) . Axis 4 corresponds to the global effect of less represented variables (GT, V and W M E C ) . The activation energy E , characteristic of the material's chemical transformations (Figure 7), increases with the heat Q. The chemical modification of the material is favored by a large import of heat thus, a high setting temperature. The increase in the activation energy related to Q causes a decrease in viscosity according to the viscosity law. Logically, the torque C E (which is proportional to the decrease in viscosity) decreases when the activation energy E increases. Alternatively, the activation energy decreases when the rotor speed increases. Indeed, this rheological behavior of the material, where viscosity decreases with high rotor speed, is predominant over the decrease in viscosity related to temperature. It seems that at high rotor speeds, the thickening process is not favored or more precisely, it is hidden due to the breaking of the crosslinked chains as soon as they are generated. The activation energy is independent of the thickening time TPS, the temperature gradient G T and, above all, the mechanical work W M E C . In addition, the thickening time is correlated to both the mechanical work and the temperature gradient. Therefore, the thickening process is related to the heat provided to the system and the application time of the thermal constraints, which is the residence time in the mixer. The mechanical work increases with the thickening time and decreases when the rotor speed increases (Figure 7). A s discussed previously, the high rotor speed partially hides the thickening process due to the polymer's rheological characteristics. The result is a decrease in viscosity and/or the breaking of the polymeric chains. A

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This study demonstrates that the mechanical constraints play two roles in the system. The first may be explained by the generation of free radicals that initiate the crosslinking reaction when the application time of the mechanical constraints or residence time is long enough. The second is the large friction to the material that leads to an increase in the processing temperature. To resume, the thermal phenomenon is responsible for the thickening of E V A 8 . The shear stress accelerates the initiation of the phenomenon at temperature below 200°C. Therefore, it is possible to Theologically modify a material according two different procedures: a) The material is exposed to a thermal constraint for a period of time which corresponds to a major part of the residence time in the extruder; and b) The material is exposed to a high mechanical constraint that generates a local elevation of temperature, which is the initiating factor of the process. The evolution of the F R performance versus the processing conditions (temperature and shear stress using the extruder) may now be explained from these results. The residence time of the polymeric materials in the extruder is low (from 30 to 120 s., a function of the screw rotation speed). A t 220°C and low screw rotation speeds, the degradation of the vinyl acetate groups of E V A 8 is not complete when the high temperature allows degradation. In a fire, release of the acetate allows the evolution of acetic acid, which has a dilution effect in the flame. Temperatures higher than 220°C in this rotation range give an easily flammable unsaturated polymer. Using a screw rotation of 50 rpm at all temperatures allows intermolecular crosslinking of the polymer chains to occur whose charring gives comparatively high F R performances. Finally, in the highest rotation speed range, the breaking of the polymeric chains occurs which leads to easily flammable low molecular weight species.


197 Ceramic-like formulation E V A 1 9 / A l ( O H ) / F B Z B 3

In the previous sections, it is shown that the rheological and F R properties of the EVA19/A1(0H) /FBZB formulations depend on both the zinc borate transformation and the process. Indeed, the later influences the F R performances since different maximum LOI values of 46 V o l . - % and 50 V o l . - % are found for mixing and extrusion, respectively. The mechanism of action of zinc borates in such formulations has been recently proposed (5). During the heating of the material, the polymer degrades and forms a cross-linked network and a carbonaceous char, aluminum hydroxide decomposes into A 1 0 and F B Z B undergoes a transformation that leads to a vitreous protective coating made of a boron-based glassy phase which allows a better retention of the char. The mechanically stable vitreous coating formation is evidenced by the increase of the B 0 / B 0 ratio that reaches a plateau with time. In the formulations discussed in this paper, the degradation of F B Z B during the process is more important in the case of extrusion than mixing. The good L O I results of the extruded samples may be caused by this small degradation of F B Z B . Boron oxide may act as an initiator and its incorporation (from 0.5 to 1 wt.-%) in the formulation during the process may increase the kinetics of boron oxide formation that would lead to better F R performances. The boron-based glassy coating may be also of a better quality. B N M R studies at different degradation time of the formulations processed using the mixer and the extruder should be performed to confirm this hypothesis. Also, the mechanical constraint, which is much stronger in the extruder, may reduce the particle size of the components of the blend. This behavior generally has an impact on the F R performance of the materials. Experiments with particle sizes of 0.5 μηι for aluminium hydroxyde and 3 μηι for F B Z B (instead of respectively 0.7 μηι and 7 μηι used in this paper) are under evaluation. Particular attention will be paid on the impact of the smaller zinc borate particle sizes on the B 0 / B 0 ratio. The low temperature viscosity measurement is correlated to the L O I test Indeed, for both process, i.e. extrusion and mixing, the L O I values vary as a function of the viscosity measurements (Figures 2 and 6). In the L O I test, the samples are placed in a vertical position and the protective layer when formed and generated during the combustion may flow down due to the lack of resistance of the melted polymer phase supporting the protective layer. In other words, the support of the protective layer on the surface of the sample depends on the viscosity of the material. The higher is the viscosity the better is the L O I (20), thus explaining the high LOI values observed at a 5 wt.-% F B Z B loading, which is the content that leads to the highest viscosities. The two different processes that are used lead to the same U L 94


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198 classification. The formulations with a total loading of F B Z B below and above 5 wt.-% exhibit a V I and a N C rank, respectively. In this case, the difference in the viscosity measurement between the mixing and the extrusion process is not large enough to compensate for the force exerted by the weight of the samples.


Conclusion The study shows that F R Properties of intumescent E V A 8 / A P P / P A 6 formulations depend on their processing conditions. A relationship between high F R ratings and cross-linking (thickening) of the E V A 8 phase in the material is discussed. It is shown that the increase in viscosity can be assigned to the thermal treatment. Moreover, shear stress, i.e. mechanical constraint, plays two distinct parts; it allows the formation of the species which initiate the thickening process and contributes also to the breaking of intermolecular links and to the degradation of the polymer chains. The difference in flame retardant properties for the ceramic-like formulations E V A 19/Al(OH) /FBZB processed using the mixer and the extruder was discussed. In a first approach, it is shown that this difference may be explained by a decrease in die F B Z B particle size due to higher mechanical constraints that occur in the extrusion process compared to the mixing process. 3

References 1. 2.

Ray, I.; Khastgir, D . J. Appl. Polym. Sci. 1994, 53, 297-307. Ray, I.; Khastgir, D . Plastics, Rubber and Composites Processing and Applications 1994, 22, 37-45. 3. Fedor, A . R . Int. Wire and Cable Symp. Processings 1998, 183-187. 4. Siat, C . Ph.D thesis, Université d’Artois, Lens, France, 2000. 5. Bourbigot, S.; Le Bras, M.; Leeuwendal, R.; Shen, K.K.; Schubert, D . Polym. Deg. Stab. 1999, 64, 419-425. 6. Seibel, S.R.; Papazoglou, E. Annu. Tech. Conf.- Soc.Plast. Eng. 1995, 53, 366-370. 7. Beecher, E . ; Grillo, J.; Papazoglou, E . ; Seibel, S.R. Annu. Tech. ConfSoc.Plast. Eng. 1994, 52, 1626-1630. 8. Pécoul, Ν . Ph.D thesis, U S T L , Lille, France, 1996. 9. Balabanovich, A.I.; Schnabel, W.; Levchik, G.F.; Levchik, S.V.; Wilkie, C . A . In “Fire Retardancy of Polymers - The Use of Intumescence”, Le Bras M . ; Camino, G.; Bourbigot, S.; Delobel, R. Eds., The Royal Society of Chemistry, Cambridge, U K , 1998, 236-251. 10. Carty, P.; White, S. Polymer 1994, 35, 343-347.



199 11. Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-like Combustion of Plastics, ASTM D 2863-ISO 4589. * It is generally assumed that the higher the L O I value is the better is the F R behavior. 12. Tests for Flammability of Plastic Materials for Part in Devices and Appliances, Underwriters Laboratory Safety Standard 94, ASTM D 568ISO 1210:1992. 13. Zaikov, G.E.; Goldberg, V. In Encyclopedia of Fluid Mechanics, Gulf Publishing Company, Houston, Texas, 1990, 9, 13, 403-423. 14. Rothenpieler, A . In Mesure des Propriétés de Transformation des Hauts Polymères avec le P L A S T I - C O R D E R Brabender, Réunion Commune AFICEP-SPE-GFP, Paris, 23 mars 1973. 15. Poutch, F. Mémoire CNAM (available at G E P I F R E M , E.N.S.C.L), Lille, France, 1998. 16. Turner, G.L.; Smith, K.A.; Kirkpatrick, R.J.; Oldfield, R.J. J. Magn. Reson. 1986, 67, 544-550. 17. Bray, P.J.; Edwards, J.O.; O’Keefe, J.G.; Ross, V . F . , Tatsuzaki, I. J. Chem. Phys. 1961, 35, 435-442. 18. Kriz, H . M . ; Bishop, S.B.; Bray, P.J. J. Chem. Phys. 1968, 49, 557-561. 19. Heermann, E.F. Psych. 1963, 28, 161-172. 20. Carpentier F. Ph. D . thesis, U S T L , Lille, France, 2000.


Fire and Polymers - American Chemical Society

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