INSTRUMENTATION
DRIERITE NEW DRYING UNIT for AIR & GASES AT PRESSURES up to 90 psig
Figure 3. Du Pont 900 Differential Thermal Analyzer
haling 9,'Γ - l î ° :
min.
Figure 4. Thermogram of a polyethylene terephthalate
2s/8"O.D.xll3/8" LABORATORY 6AS DRYING UNIT NOW AVAILABLE IN ACRYLIC PLASTIC The redesigned unit will provide exit dew points of —100 ° F. and has a heavy duty aluminum cap. Flow rates may be 200 liters per hour. Indicating D R I E R I T E changes from blue to pink when exhausted. (Specify the plastic model from your Laboratory Supply Dealer.) Write for Bulletin 68.
W. A. HAMMOND DRIERITE CO. XENIA, OHIO Circle No. 51 on Readers Service Card 104 A
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ANALYTICAL CHEMISTRY
tion, and melting and freezing points of crystalline solids. Clues to the existence of transitions between crys talline phases are also obtained. By indicating changes in heat capacity, the technique gives information on the existence and temperature of secondorder or so-called glass transitions which occur when new internal degrees of freedom become available for the absorption of heat energy. These transitions often are the reason for sharp variations in stiffness or tough ness of materials of construction (e. g. "Melting" of polystyrene, tough versus brittle plastic). Among chemical changes which can be followed are the polymerization of monomers; or pre-polymers may be detected or measured by the magnitude of the heat liberated in the D.T.A. experiment. Pre-curing of thermo setting resins may be measured by comparing the size of the curing exotherms with known standards. Reac tion kinetics have been studied in detail and physical reactions such as rates of crystallization. The beginning of
thermal degradation of a material is indicated by the endothermal break when the material is heated in an inert atmosphere. By substituting an oxidiz ing atmosphere such as air in the deg radation experiment, sample oxidation can be studied. D.T.A. has been used at constant temperature to measure the induction period and rate of oxidation of polyethylene. This furnished a means of evaluating the effectiveness of various anti-oxidants. Aside from the identification and measurement of specific transitions, D.T.A. provides unique thermal fin ger-prints, useful, along with spectro scopic techniques, in identifying ma terials. A D.T.A. adaptation of the familiar organic qualitative analysis technique of preparing derivatives pro vides a wealth of information. When the reactants are mixed in the D.T.A. cell, the heat effects associated with the reaction are followed as the tempera ture is increased. The reaction, melt ing, boiling, and decomposition temper atures of the derivative are quickly and simply obtained.
