STA Measurements with Steel Furnace

Introduction

The Platform Concept at NETZSCH-Gerätebau GmbH

Our platform concept is currently comprised of three basic instruments (DSC, STA and TMA), each of which comes in two different models (F1 and F3 ). All electronic components necessary for operating these instruments along with the gas supply unit are contained in a single, integrative housing. The furnaces and sample holders can be quickly and easily exchanged by the operator. This modular setup not only lends a uniform appearance to the instruments, but also capacitates maximum flexibility for adjusting to changing analytical situations and for facilitating the implementation of any consequent modifications necessary in instrument operating conditions. Figure 1 depicts the various instrument versions comprising the platform concept. 

A steel furnace is available for all three instrument types. This allows coverage of a temperature range of -150°C to 1000°C at the sample. This application note will discuss measurement results typical in this temperature range for polymers (thermoplastics, elastomers) and crystalline organic substances, such as sugar.

STA 449 F1 Jupiter^® with Steel Furnace

In addition to the instrument variations mentioned above, a number of add-ons can be supplied for simultaneous thermal analysis (STA), such as coupling methods, PulseTA^® or the water-vapor generator. There are presently nine furnace systems available for the STA 449, covering a temperature range of -150°C to 2400°C at the sample (figure 2).

Measurement Conditions

The measurement results for a polymer film made of polyethylene terphthalate (PET), two elastomer samples, and sorbitol – a C6 sugar – are presented in this application note. Standard conditions were employed for all investigations; these are summarized in table 1.

Table 1: Measurement Conditions

Measurement Results

In order to characterize elastomers, it is necessar to carry out the analyses in a range below room temperature. Since elastomers do not have any crystalline portions, no Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting point or melting range exists for these substances. Elastomers are purely amorphous solids; i.e., ones which have solidified in an unstructured way. By means of DSC, however, important information about the material properties can be attained – for example, by determining the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature. At this temperature, the mechanical properties of the sample change dramatically. At temperatures below the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature (Tg), an amorphous material is brittle and fragile; above the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature, on the other hand, it is elastic and flexible. This mechanical property change can be measured very easily with mechanical test methods such as DIL, TMA or DMA. Because the specific heat of a sample also changes during this mechanical property change, a caloric method such as differential scanning calorimetry (DSC) can also be used to determine the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature. In the DSC measurement results, the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature can be observed as a step; the step height is a direct indication of the change in specific heat, in units of J/gK. 

In the investigation of polyisoprene (NR, natural rubber), the glass transition is expected to occur at a temperature of approximately -50°C. This Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature, however, can vary depending on the rubber mixture and the selection of additives such as plasticizers, and can therefore be adjusted to the corresponding application requirements. Figure 3 shows the results of the determination of the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature for two elastomer samples. 

For semi-crystalline materials, amorphous regions exist alongside crystalline ones (domains). The amorphous regions are characterized by means of the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature as described above, whereas the crystalline regions are characterized by their melting behavior. Since mechanical and thermal treatment steps can alter the ratio of the amorphous to the crystalline region, DSC investigations usually involve the comparison of two heating segments. Between these two heating runs, the samples undergo linear cooling in the DSC instrument by means of a controlled cooling program in order to avoid subjecting the material to any new states of StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress. Figure 4 depicts the comparison of these two heating segments (red: 1^st heating, green: 2^nd heating), along with the cooling segment (blue) that was carried out between the two heating runs. 

It can be clearly seen that the transparent PET film was largely amorphous prior to the first heating and that it was characterized by a higher crystalline proportion following the controlled cooling which took place at a rate of 10 K/min.

A typical temperature-time-profile for such cyclic treatment of a sample is depicted in figure 5 applied for the investigation of sorbitol.

The measurement results for sorbitol are presented in figure 6. The substance was fully crystalline before the investigation, which is why no glass transition was observed during the first heating (red) in the range around 0°C. Sample melting was detected at a peak temperature of 101°C. During the cooling of the liquid sorbitol sample (blue) no CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization was observed; instead, the sample solidified amorphously, as indicated by detection of the glass transition at -3.6°C (midpoint). During the second heating (green), the glass transition was detected again (midpoint: -0,3°C); by then, the sample was completely amorphous and thus exhibited no melting. The cyclic temperature treatment at heating and cooling rates of 10 K/min caused the sample to change from a completely crystalline to a completely amorphous state.

Summary

The measurement examples demonstrate that even an STA – which is designed primarily for the high-temperature range – is capable of analyzing samples for which a DSC 204 F1 Phoenix^® or a DSC 200 F3 Maia would normally be used, simply by virtue of changing the furnace.