Polymers are especially susceptible to swings in temperature. When they get hot, some are liable to melt and when they cold, other polymers may shatter. Knowing the optimal temperature for plastics manufacturing can ensure the strongest, most durable products. Differential Scanning Calorimetry (DSC) is an analysis tool that measures the performance and degradation of polymers when exposed to a wide range of temperatures, including extreme heat and cold. Also, the thermal stability of a material can be measured using the DSC device. In this article, DSC and OIT thermal analysis are briefly described. Keywords: Differential Scanning Calorimetry (DSC), Oxidation induction time (OIT).
Differential scanning calorimetry (DSC) is a technique used to investigate the response of polymers to heating. DSC can be used to study the melting of a crystalline polymer or the glass transition. The DSC set-up is composed of a measurement chamber and a computer. Two pans are heated in the measurement chamber. The sample pan contains the material being investigated. A second pan, which is typically empty, is used as a reference. The computer is used to monitor the temperature and regulate the rate at which the temperature of the pans changes. The rate of temperature change for a given amount of heat will differ between the two pans. This difference depends on the composition of the pan contents as well as physical changes such as phase changes. For the heat flux DSC used in this lab course, the system varies the heat provided to one of the pans in order to keep the temperature of both pans the same. The difference in heat output of the two heaters is recorded. The result is a plot of the difference in heat (q) versus temperature (T). Figure 1, shows the internal layout of a DSC instrument.
where t is time. The heating rate is the time rate change of temperature: Equation (2):
where ∆T is the change in temperature. One can obtain the heat capacity from these quantities:
If the Cp of a material is constant over some temperature range, then the plot of heat flow against temperature will be a line with zero slope as shown in Figure 2. If the heating rate is constant then the distance between the line and the x axis is proportional to the heat capacity. If heat is plotted against temperature then the heat capacity is found from the slope.
If a polymer in its molten state is cooled it will at some point reach its glass transition temperature (Tg). At this point the mechanical properties of the polymer change from those of an elastic material to those of a brittle one due to changes in chain mobility. A typical example of a heat flow versus temperature plot at a glass transition temperature is shown in Figure 3. The heat capacity of the polymer is different before and after the glass transition temperature. The heat capacity Cp of polymers is usually higher above Tg. DSC is a valuable method to determine Tg. It is important to note that the transition does not occur suddenly at one unique temperature but rather over a range of temperatures. The temperature in the middle of the inclined region is taken as the Tg.
Above the glass transition temperature the polymer chains have high mobility. At some temperature above Tg the chains have enough energy to form ordered arrangements and undergo crystallization. Crystallization is an exothermic process, so heat is released to the surroundings. Less heat is needed to keep the heating rate of the sample pan the same as that of the reference pan. This results in a decrease in the recorded heat flow. If the convention of ‘exothermic – down’ is used then the result is a dip in the plot of heat flow versus temperature as seen in Figure 4. Such a crystallization peak can be used to confirm that crystallization occurs in the sample, find the crystallization temperature (Tc) and determine the latent heat of crystallization. The crystallization temperature is defined as the lowest point of the dip. The latent heat (enthalpy) of crystallization is determined from the area under the curve.
The polymer chains are able to move around freely at the melting temperature (Tm) and thus do not have ordered arrangements. Melting is an endothermic process, requiring the absorption of heat. The temperature remains constant during melting despite continued heating. The energy added during this time is used to melt the crystalline regions and does not increase the average kinetic energy of the chains that are already in the melt. In a plot of heat against temperature this appears as a jump discontinuity at the melting point as seen in Figure 5B. The heat added to the system during the melting process is the latent heat of melting. It can be calculated from the area of a melting peak observed in a plot of heat flow against temperature, such as the one in Figure 5A. The Tm is defined as the temperature at the peak apex. After melting the temperature again increases with heating. However, the heat capacity of a polymer in the melt is higher than that of a solid crystalline polymer. This means the temperature increases at a slower rate than before.
The DSC diagram containing the glass transition temperature, crystallization peak, and melting peak is shown in Fig.6.
Metals suffer from corrosion, and while plastics are immune to corrosion, they also are prone to degradation such as oxidation. Polymer producer normally add stabilizer to improve the resistance of susceptible polymers to oxidative degradation. Polyethylene, for example, can suffer decomposition in the air at approximately 200 ◦C, whereas, in the absence of oxygen, in a nitrogen atmosphere, it undergoes thermal degradation at approximately 400 ◦C, therefore, antioxidants are added to protect against oxidation in applications. Depending on the specific application, plastic materials have to be stabilized to a greater or lesser extent against oxidation and environmental influences. A simple method to check the efficiency of the stabilizers or stabilizing systems used is to determine the oxidation induction time (OIT) or oxidation induction temperature (OIT∗) of the molten material. OIT is a standardized test performance using a DSC. Especially for polyolefins, OIT and/or OIT∗ measurements are well established for quality control purposes as a quick screening method to check the activity of the stabilization system used. The OIT measurement is most popular in this case. Many publications deal with this in detail.
The test essentially subjects the specimen to an accelerated oxidation environment, while monitoring for occurrence of exothermic and endothermic reactions. The test is performed in two ways: oxidation induction temperature and oxidation induction time.
The sequence of a standardized OIT measurement using the DSC method according to EN 728 is outlined in Figure 8. A sample of the polymer (approximately 15 mg) is placed in a clean aluminum pan. After positioning the uncovered sample pan together with an empty reference pan in a calibrated DSC oven, a nitrogen atmosphere is established in the measuring cell. Then, the sample and the reference are heated rapidly (at least_20 K/min) to the temperature at which the OIT value is to be determined. When the required temperature is reached for the first time an isothermal step of 3 min follows. After reaching this point (indicated as t1 in Figure 8) the atmosphere is switched to oxygen and the DSC oven is held at the same temperature until an exothermal signal (oxidation) can be recognized. The onset of this oxidation signal corresponds to a time t2. The OIT value can now be determined as the time between t1 and t2, as described in Figure 8. According to EN 728, the flow of nitrogen and oxygen should be adjusted to 50 ml/min±10% during the entire measuring procedure. In this case the evaluation of t2 can still take place because a deviation from the base line, defined in advance, is taken as the time t2. It should be pointed out, however, that the two methods yield different OIT values of t2. Thus, comparative series of measurements must always be performed with the same evaluation method in order to ensure comparability of results. Finding a suitable measuring temperature for the isothermal phase often causes further difficulties with OIT measurements. If the temperature is too low there is a substantial increase in the duration of the measurement. On the other hand, if the temperatures are too high, oxidation takes place immediately after the introduction of oxygen. The onset temperature of the decomposition signal (t2) can no longer be determined. Acceptable OIT times (range from 30 to 60 min) are frequently obtained with polyolefins at 200 or 210 °C. Usually, however, extensive preliminary tests are necessary to optimize the test conditions for OIT measurements of unknown samples. In this connection, the dynamic OIT∗ measurement described below requires substantially less effort.
The oxidation induction temperature (OIT∗) is evaluated in accordance with Figure 9. The sample is heated up continuously (i.e. 10 °C/min) under a pure oxygen (or air) gas flow. A change of gases at a defined time, as stated under OIT measurement, is not necessary. OIT∗ is determined as that point in the thermogram where the onset of the decomposition signal results. OIT∗ is usually more clearly pronounced as the onset time t2 in OIT measurements (t2 is necessary for the determination of the OIT values). It is obvious that the OIT∗ method needs less effort in setting up the measurements and in the majority of cases it delivers clearly defined onset points.
DSC is a powerful method for identifying the structure of chemical substances and thermal analysis of materials. Differential scanning calorimetry is for quantitative measurement of energy change. This method can be used to measure the melting temperature, the latent heat of melting, and to investigation of the glass transition and crystallization temperature. Also, according to the mentioned cases, it can be concluded that the OIT test is one of the important factors in determining the efficiency of polymer materials and parts when exposed to high temperatures. This test is used to check the raw materials and also to check the final product quality, so that if the OIT of the raw materials is low, it indicates low thermal resistance and the material cannot be processed; Or there is a possibility that during processing, the material will lose its stability and be destroyed.
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