Pyrolysis reproducibility parameters

To a first approximation, good interlaboratory reproducibility of the pyrolysis profile is obtainable however, intralaboratory matchings have been disappointing. Several major parameters influence pyrolysis reproducibility ... 

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

As indicated above, to achieve control of the pyrolysis course in flash pyrolysis, it is necessary for the sample to be reproducibly heated. Ideally, the total decomposition of the sample should occur over the same temperature range. The reason for a precise temperature control is illustrated in an example shown in Figure 4.1.3. This figure gives the weight variation of a sample where the pyrolytic processes may occur following two independent reaction kinetics, both of the first order process (1) with E = 100.7 kJ/mol and A = 9.6 10 sec and process (2) with E = 65 kJ/mol and A = 5.5 10 sec (the kinetics parameters were selected from data indicated for cellulose pyrolysis). 

The reproducibility of the results for heated filament pyrolysers (CDS Pyroprobe 1000) and Curie point pyrolysers (Horizon Instruments) was reported for several samples [34]. This included several synthetic polymers, dammar resin, chitin, an insect cuticle, a hardwood (cherry), a seed coat (water lily), lycopod cuticle (fossil Eskdalia), as well as several organic geological samples. All samples were pyrolysed at 610° C for 5 s in a flow of helium. The residence time in the pyrolyser before pyrolysis was kept constant and the temperature of the sample housing was 250° C. Other parameters such as the temperature of the transfer line to the analytical instrument were also the same. Both systems were connected to a GC/MS system for the pyrolysates analysis. 

One of the most important characteristics of pyrolysis is the temperature pattern of sample heating. Therefore, the changes of the filament temperature with time is an essential characteristic of pyrolysers. The most significant parameters are as follows (1) filament (sample) heating time (2) reproducibility of the kinetic heating curve and (3) constancy and stability of the maintained temperature. 

Cases where eqn. 3.1 is variable are also considered, of course. The next step is to select one or more optimal relationships among the experimentally obtained ones, similar to eqn. 1 [101], i.e., most clearly defined (specific), reproducible and providing for maximum accuracy of calculation. This approach must be applied in considering the effects of different experimental parameters on the composition of the pyrolysis products. The possibility of using a computer at this stage was also examined by Kullik et al. [102, 103]. 

In order to obtain reproducible results and characteristic pyrograms, one must define the optimal experimental parameters, which must then be strictly standardized, as the thermal degradation of a polymer is often sensitive to even minor changes in the pyrolysis conditions. Apart from the cell type, the determining experimental parameters are (1) the pyrolysis temperature and time, (2) the sample size and shape, (3) the nature and velocity of the carrier gas and (4) the chromatographic separation conditions. Let us now consider in greater detail the effect of the above factors on the yield of pyrolysis products and the specificity of pyrolysis. 

Pyroprobe 1000. This provides complete choice of thermal processing parameters. Pulse pyrolysis is permitted at rates up to 20 °C per second to temperatures as high as 1400 °C. In addition to accuracy and reproducibility, the temperature versatility allows for uninterrupted sequential runs on the same sample under different thermal conditions without removing the sample probe. Two probe designs are available coil element for solid polymers and ribbon element for polymer film and solvent deposits on polymers. 

In the case of filament pyrolysers, the parameters which pertain specifically to the sample were identified [533]. These factors are method and uniformity of sample deposition, region of sample deposition, sample thickness, sample-to-filament contact, but also catalytic effects, nature of carrier gas, flowrate, pyrolysis chamber temperature and purity of solvents used in sample deposition. Important parameters are also the temperature-time profile (TTP), which depends upon TRT, THT as well as Teq. Reproducibility is enhanced if the entire sample experiences the same TTP and if the primary products... 

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