Glass transitions and enthalpy relaxation in coals

Glass transitions and enthalpy relaxation in coals

Glass transitions in coals Alexander J. Mackinnon, and enthalpy M. Mirari Antxustegi relaxation and Peter J. Hall Department of Pure and Applie...

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Glass transitions in coals Alexander

J. Mackinnon,

and enthalpy

M. Mirari

Antxustegi

relaxation

and Peter J. Hall

Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow Gl lXL, UK (Received 14 May 1992; revised 14 January 1993)

Differential scanning calorimetry was used to investigate the glass-to-rubber transition at 383 K of Illinois No. 6 coal dried at 383 K and thermally treated to 523 K. The intensity of the transition is 0.2 J g- ’ K- I, similar in magnitude to that for polypyrrole and polystyrene. The temperatures of the glass transitions in the coal and the polymers are also very similar. It is concluded that the glass transition of the coal represents a significant change in structure. Annealing experiments were conducted on the coal at temperatures just below the glass transition. Enthalpy relaxation occurred after thermal treatment to 553 K. (Keywords:

macromolecular

structure,

coal; differential

scanning

At room temperature, coals are glassy solids’. Glasses are non-crystalline materials in which molecular motion is severely restricted. Diffusion through glassy solids is slow and this means that the kinetics of chemical reactions tend to be diffusion-limited’. A number of claims have been made for glass-to-rubber transitions in coals at -600 K3*4. The existence of rubbery states is important because diffusion through rubbers is generally orders of magnitude faster than in the corresponding glass. Such transitions are therefore important in overcoming mass transfer limitations in coal chemistry. The nature and importance of these transitions has been criticized by a number of authors. Hall’ pointed out that for some coals the transition is accompanied by a decrease in specific heat capacity, and this is not typical for a glass transition. Hall and Mackinnon6 have measured the intensities of these transitions and performed numerous repeat measurements on different samples of the same coal. From the intensities, it was concluded that these transitions, if real, do not represent a significant change in coal structure. The reproducibility of the transitions was not good. Yun and Suuberg’ have reported that these transitions are accompanied by some irreversible change to coal structure and are therefore not true glass transitions. It is therefore concluded that these transitions are not true glass-to-rubber transitions. More recently, Mackinnon and Hall8 observed a glass-to-rubber transition in Illinois No. 6 coal that had been heated to 523K and then quenched to room temperature. It was shown to be possible to cycle through this transition a number of times. The existence of this transition was confirmed by low-frequency dielectric loss measurements. A large peak in dielectric loss was observed for dried Illinois No. 6 coal that had been thermally treated as in the differential scanning calorimetric (d.s.c.) experiments. Mackinnon and Hall8 did not present quantitative data, so it was not possible to assess 001~2361/94/01/0113X13 8x2 1994 Butterworth-Heinemann

Ltd

calorimetry;

coal)

the significance of this transition as regards changes in coal structure. EXPERIMENTAL A Mettler DSC 30 system was used, with temperature calibration from the melting points of indium, lead and zinc standards, and enthalpy calibration by integration of the melting endotherm of an indium standard. Temperatures were accurate to kO.5 K, and it was estimated that enthalpies were accurate to f0.05 J g- ‘. Standard aluminium pans were used, with two pinholes to allow evaporation of water. The coal sample, IOmg, was spread in a monolayer over the base of the aluminium pans to maximize heat transfer. D.s.c. was performed at lOKmin_’ although no difference was noted in the results if the heating rate was varied between 0.5 and 30Kmin-‘. Nitrogen was used as a carrier gas. Illinois No. 6 coal was obtained from the Argonne Premium Coal Sample Program. For comparison, d.s.c. was also performed on samples of polypyrrole and polystyrene. The preparation of the polypyrrole has been described by Doyle et ~1.~The polystyrene was a Polymer Labs standard, of number-average molecular weight 42 200. The order of the d.s.c. experiments was as follows. A sample of coal was weighed into an aluminium pan and was dried for 30min in the d.s.c. cell under N, at 383 K. The sample was cooled to 303 K and reweighed, and d.s.c. was performed to a temperature of 523K at a rate of 10 K min- ‘. This is referred to as ‘fresh coal’ in Figure 1. The sample was then cooled to 303 K at a nominal quenching rate of 100 K min- ’ and reweighed. D.s.c. was re-run to 473 K at 10 K min- ‘. This is referred to as ‘run 2’ in Figure 1. The sample was then cooled to 303 K.

This procedure was repeated five times. After this set of experiments, the sample was thermally

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Glass transitions and enthalpy relaxation in coals: A. J. Mackinnon

annealed in the d.s.c. cell at 373 K for 50 min. D.s.c. was then run at 10 K min- ’ from 303 to 473 K. For the final set of experiments, the sample was heated at lOKmin_l to 553 K. Cycled d.s.c. runs were then made between 353 and 473 K. Similar experiments were performed on six different samples of Illinois No. 6 coal, taken from three different Argonne sample bottles. D.s.c. on the polypyrrole and the polystyrene was performed under the same conditions as for the coal, at 10 K min- ’ to 523 K. The polypyrrole was dried at 373 K for 30min; the polystyrene was not dried.

Figure 1 shows the results of the first two d.s.c. runs. As expected, C, increases with increasing temperature. The fresh coal shows a broad endotherm centred at -408 K. This has been observed previously for Illinois No. 6 and other dried coal&*. It was originally thought that this might be caused by evaporation of residual water or some mild pyrolysis. However, there was no detectable weight loss during the run. It has been reported that a small amount of water may be bound to coal at temperatures up to 423 K”. If there was any weight loss during the first run due to desorption of tightly bound water, it was less than the limit of detection of the weighing apparatus, i.e. < 0.05 mg, or <0.5%. The loss of such a small amount of material would not be expected to cause such a large endothermic peak as observed for the fresh coal in Figure 1. Therefore the broad endotherm can be due only to some physical change in the structure. No weight loss was detected in any of the succeeding d.s.c. runs. Run 2 in Figure 1 shows quite distinct behaviour. Over the region 33@388 K, C, is lower than for the fresh coal. There is an intense and relatively sharp second-order phase transition. The characteristics of the transition were evaluated using an unpublished algorithm developed by Mettler Toledo Ltd (Leicester, UK). The glass transition temperature (T,) is at 385 K. The intensity (AH) of this transition is 0.22 J g- ’ K- ‘, the onset is at 384 K and the end of the T, region is 389 K. It was shown previously that it was possible to cycle through this transition a number of times. This is consistent with a glass-to-rubber transition. As mentioned in the introduction, Mackinnon and Hall* have shown that this transition is accompanied by a large dielectric loss peak. For comparison, the glass transition intensities for samples of polypyrrole and polystyrene were determined. CP Wg.W 1.9 1 1.8

Fresh Coal

1.7 1.6 1.5 1.4

. 1.33



3;o

4;o Temperature

Figure 1 D.s.c. curves for Illinois No. and thermally treated to 453K (‘run 2’)

114

1

I

450

300

CP (J1g.K) 1.8

500

(K) 6 coal

dried

Fuel 1994 Volume 73 Number 1

(‘fresh coal’)

.-

1

,I6: _a-____,_-*-

1.6

Ilj#&Z&; 300 Figure 2

RESULTS AND DISCUSSION

et al.

350

400 Temperature

D.s.c. curves for polypyrrole

450

500

(K) and polystyrene

(MW 42200)

Hall et al.’ ’ have argued that polypyrrole serves as a good model for coal in a number of respects. Figure 2 shows d.s.c. curves for the two polymers. A glass transition is evident at 378 K for the polypyrrole and at 377 K for the polystyrene. The intensity of the glass transition for both polymer samples is 0.2 J g- ’ K- ‘. The d.s.c. signal for the polypyrrole has more noise than the others because of the small sample weight used. It was possible to cycle through these transitions a number of times. The intensities are slightly lower than for the Illinois No. 6 and the transitions occur at a slightly lower temperature. The coincidence of the intensity and temperature of the polypyrrole glass transition with those of the coal further illustrates the utility of this polymer as a model for coal. The magnitude of the transition for the coal therefore shows that it represents a significant change in structure. The first-order process in the fresh coal of Figure 1 could therefore be caused by some relaxation process in which the coal is transformed into a rubbery state. This is a common phenomenon l2 for glasses that are formed from quenched polymer melts. In effect, the quenching freezes-in some entropy, which is released when the glass goes into a rubbery state. Therefore the enthalpy decrease between the fresh coal and run 2 of Figure 1 below the glass transition might represent some excess entropy produced during the coalification process. The integral of the difference in the two curves below the glass transition is 5.8 J g- I. Experiments on solvent-induced swelling in coal blocks by pyridine l3 show that fresh coal undergoes a certain amount of irreversible swelling. Subsequent swellings are observed to be reversible. This kind of behaviour is consistent with a relaxation from a non-equilibrium fresh state to a more energy-minimized state after solvent interaction. Such relaxation is caused by disruption of hydrogen bonding by the solvent. The present work has demonstrated that it is possible to induce a similar relaxation thermally. It would be of interest to investigate the swelling properties of thermally relaxed coals. When coals are formed, they are subject to stress from overlying rocks. This may prevent them from developing in an energy-minimized state, and it stores stress in the coal. There are a number of possible explanations for the existence of the glass transition. The conventional explanation would be that at 384 K the free volume in the coal exceeds a certain fraction of the total volume, and macromolecular freedom of motion increases correspondingly14. A second explanation has been provided by E. M. Suuberg (personal communication): hydrogen bonds could dissociate thermally at N 384 K and macromolecular freedom of motion increase as the effective

Glass transitions CP

(J&K)

1.9 1.8

---__--.

Run 2

-

Ann

300

350

50

min

400 Temperature

450

500

(K)

Figure 3 D.s.c. curves for Illinois No. 6 coal thermally 523 K and the same sample annealed at 373 K for 50min CP

treated

to

(J&K) 1.91 J

1.81.71.61.5-

Heat

1.4 1.3

I

I

300

350

to 553K.

553K,

Run

2

-----

553K.

Run

3

I 400 Temperature

Figure 4 Cycled thermal treatment

treated

-..-.-...’

d.s.c. measurements to 553 K

Run

and enthalpy

relaxation

in coals: A. J. Mackinnon

et al.

There are a number of explanations for enthalpy relaxation. The simplest is in terms of free volume. Enthalpy relaxation occurs when the free volume is less than predicted by the Doolittle equation15. This can be brought about either by annealing or by thermal treatment and cooling. The Doolittle equation predicts that glass transitions occur when the free volume exceeds a certain theoretical amount. Thus the glass transition occurs at a higher temperature than that of the equilibrium state. After the glass transition the system can relax into its equilibrium state. The C, peak occurs when the system absorbs energy to create free volume. The thermal treatment to 553 K could therefore perturb the coal macromolecular structure in such a way as to decrease the free volume. When the glass transition is finally reached, the system can relax to an equilibrium state. Runs 2 and 3 suggest that the relaxation is not complete, because there are still C, peaks and the glass transition temperature is progressively decreasing. The C, peak in run 3 is of low intensity and the glass transition temperature is only 5 K above the ‘equilibrium’ value of Figures 1 and 3. This suggests that the system is close to equilibrium after run 3.

1

CONCLUSIONS

I

1

450

500

on Illinois

No. 6 coal

(K) after

degree of cross-linking decreased; the hydrogen bonds could reform on cooling. It is unclear whether the temperatures in the d.s.c. experiments were high enough to disrupt hydrogen bonding. As yet, no experimental data are available to distinguish between these possibilities. Figure 3 shows the d.s.c. curve after annealing at 373 K for 50min and, for reference, run 2. The effect of this annealing is to increase the enthalpy below the glass transition. The annealing may also increase the entropy of the coal, with a consequent decrease in the intensity of the glass transition. This is unusual behaviour. Hatakeyama et ~1.‘~ observed a decrease in C, after annealing of dioxane lignin. The reasons for this decrease are unclear. Figure 4 shows the d.s.c. results after thermal treatment to 553 K. The glass transition temperature has increased from 385K in run 2 of Figure 3 to 400K in run 1 of Figure 4, but it decreases in run 2 to 395 K and in run 3 to 390K. Run 1 also shows a broad endotherm following the glass transition. This is a common feature associated with glass transitions. Similar peaks are evident in Figure 2 for polypyrrole and polystyrene. There are a number of hypotheses to account for this kind of behaviour. Hatakeyama et ~1.” have conducted d.s.c. investigations on dioxane lignin, (DL), which has a complex disordered, cross-linked structure. Ha1116 has shown that the specific heat capacity can be modelled using two-component Einstein specific-heat models in a similar way to coal. Structurally, lignin has many similarities to coal (it is also, of course, a coal precursor). The d.s.c. investigations of Hatakeyama et al. also displayed C, peaks after the glass transitions. They have been shown to be associated with enthalpy relaxation in glasses.

The glass transitions previously observed for dried, thermally treated Illinois No. 6 coal have an intensity of 0.22 J g-’ K- ‘. This value is similar to that for polypyrrole and 42 200 polystyrene. The temperatures of the glass transitions are also similar. The glass transitions represent a significant change in the coal structure. Thermal treatment of the coal to 553 K induces enthalpy relaxation. ACKNOWLEDGEMENTS This work was funded by SERC grant GR/H18821. The authors are grateful to Professors R. A. Pethrick and E. M. Suuberg for helpful suggestions and discussions, and to Dr K. Vorres for providing samples of Illinois No. 6. REFERENCES

5 6 7 8 9 10 11 12 13 14 15 16

Green, T. K., Kovac, J., Brenner, D. and Larsen, J. W. in ‘Coal Structure’ (Ed. R. A. Meyers), Academic Press, New York, 1982 Otake, Y. and Suuberg, E. M. Fuel 1989, 68, 1609 Lucht, L. M., Larson, J. M. and Peppas, N. A. Energy Fuels 1987, 1, 56 Peppas, N. A. and Lucht, M. L. ‘Investigations of the Crosslinked Macromolecular Nature of Bituminous Coals’, Final Report, DOE Contract No. DE-F622-78E, 12279, US Department of Energy, 1980 Hall, P. J. Fuel 1991, 70, 899 Hall, P. J. and Mackinnon, A. J. Am. Chem. Sot. Div. Fuel Chem. Preprints 1992, 37, 872 Yun, Y. and Suuberg, E. M. Am. Chem. Sot. Div. Fuel Chem. Preprints 1992, 37. 856 Mackinnon, A. J. and Hall, P. J. Fuel 1992, 71, 974 Doyle, S. E. Mahoubin Jones, M. G. B. and Pethrick, R. A. Polym. Commun. 1985,26, 262 Deevi. S. C. and Suubere. E. M. Fuel 1987.66454 Hall, P. J. Mackinnon, L’J. and Pethrick, d.~A: Am. Chem. Sot. Div. Fuel Chem. Preprints 1992, 37, 893 Ferry, J. D. ‘Viscoelastic Properties of Polymers’, Wiley, New York, 1980 Cody, G. D., Jr, Larsen, J. W. and Siskin, M. Energy Fuds 1988, 2, 340 Petrie, S. E. B. J. Polym. Sci. A-2 1972, 10, 1255 Hatakeyama, H., Nakamura. K. and Hatakeyama, H. Polymer 1982, 23, 1801 Hall, P. J. Polymer 1992, 33, 2222

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