Thermal Analysis UserCom 32

How can you quickly measure the quality of a sample of fish oil? What are the optimum storage conditions? Can DSC measurements under pressure (HP-DSC) give a quick answer to such questions? This article de - scribes DSC measurements performed on untreated and stabilized fish oils at increased temperatures under oxygen. The results are compared with classical test methods [1].

Introduction

Fish oils with a high content of essential omega-3 fatty acids are widely used as dietary supplements in foodstuffs. Unfortunately, the oils are very sensitive to oxidation as soon as they are extracted and rapidly become rancid. This makes their further use problematical (unpleasant taste and smell, presence of free radicals, etc.).

This problem can be overcome by adding antioxidants as stabilizers and using microencapsulation. This delays the oxidation process and improves the stability of the oils. Manufacturers of such fish oil products need a reliable test method to determine the quality of their products and to optimize storage conditions. The method should be rapid and easy to perform.

The aim of this study was to compare the results obtained from high-pressure DSC measurements with those from other analytical methods such as the peroxide value and gas phase extraction-gas chromatography.

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References

[1] M. Zongo, diploma work, Dept Life Technologies, HESSO-Valais (2009)

This article presents the new Flash DSC 1. This ultra-fast scanning DSC instrument is based on a sensor in the form of a chip which contains a complete miniaturized DSC. The Flash DSC 1 provides heating and cooling rates of several thousand Kelvin per second, that is, more than 100 000 K/min). The instrument can be used to analyze changes in the microstructure of metastable materials, to optimize the composition of materials, to simulate technological processes, and for the thermal analysis of very small samples.

Introduction

The commercialization of the DSC technique in the 1960s led to a rapid expansion of the use of this method for the thermal characterization of substances and materials. The great strength of DSC is that complex information can be quickly and easily obtained about physical transitions, the structure of materials, the kinetics and composition of chemical reactions, and other processes. Conventional DSC has therefore developed into a widely used standard method. Modern conventional DSC instruments have a signal time constant of about one second. Heating rates are between 0.1 K/min and 300 K/min, or about 3.5 decades.

Many materials such as semicrystalline polymers, polymorphic substances, composites or alloys are metastable. Their structure and hence their thermal, mechanical, electrical and magnetic properties depend on their thermal history. In particular, different cooling rates can lead to different metastable structures when the materials are cooled from the melt. The structures often change before melting when the materials are heated again. The reorganization processes involved are time dependent and the result of a DSC heating measurement depends on the heating rate. The reorganization often cannot be measured by DSC because it consists of exothermic and endothermic events that take place simultaneously.

Cooling rates of up to several hundred Kelvin per second are often used in the production of materials. Unfortunately, the heating and cooling rates of conventional DSC instruments such as the DSC 1 are too low to investigate the behavior of materials under these technologically interesting conditions. DSC instruments are needed that can achieve cooling rates of about 1000 K/s (60 000 K/min). In fact, heating and cooling rate ranges must be as wide as possible (about 6 decades) to study many aspects of structure formation and reorganization.

The main reason behind the development of the Flash DSC 1 was to make this heating and cooling rate range available for practical applications. The instrument is a commercial DSC based on new technology that allows heating and cooling rates of several thousand K/s.

This article presents the Flash DSC 1 and describes several interesting experiments.

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Fire behavior is a key property when choosing materials, especially in the electronics, electrotechnics, transport and construction industries. Due to their chemical structure, conventional and technical polymers are combustible and often even flammable and have to be modified by the addition of fire retardants. In the past, halogenated fire retardants have been widely used. In recent years, various phosphorus-based alternatives have been proposed.

Introduction

Phosphorus-containing fire retardants produce their effect through a number of different fire retarding mechanisms. The two most important mechanisms are flame poisoning in the gas phase (i.e. in the flame) and charring or carbonization in the condensed phase (i.e. in the pyrolysis zone).

The basic mechanism for flame poisoning is the release of phosphorus-containing pyrolysis products followed by the formation of PO radicals in the flame. Their reaction with the highly reactive H- and OH-radicals interferes with the chain mechanism of the oxidation of the hydrocarbons and so leads to a reduction of the heat released and flammability. Phosphorus-containing fire retardants can furthermore initiate or accelerate the charring or carbonization of the polymer matrix in the pyrolysis zone and thus prevent fuel or combustible material reaching the flame. In addition, the residues produced form a barrier against heat and mass transfer, which reduces fire risks. The barrier action is even the main mechanism in the case of intumescent systems, which nearly always contain phosphorus. These systems swell to a carbonaceous foam under the action of heat.

After ignition, polymers as a rule burn with a stable flame over the material surface. The flame is the reaction zone in which the material burns under the action of ox ygen. In contrast, in the oxygen-free condensed phase below the flame, the material is degraded through pyrolysis (i.e. decomposition in an oxygen-free environment) [1].

Characterization of the pyrolysis reaction is essential in order to understand the fire behavior of polymers and in particular the fire retardancy mechanisms of phosphorus-containing fire retardants (release of phosphorus, formation of charred or carbonized and/or inorganic residues). Thermogravimetric analysis (TGA) provides important information on fire behavior and the yield of charred or carbonized residue (the char yield) with a relatively small investment of time and money.

Further information on the pyrolysis reaction can only be obtained through the combination of different techniques, for example TGA with pyrolysis gas analysis using FTIR or mass spectrometry, analysis of the residue using FTIR, REM-EDX or solid state NMR) [2, 3]. Simultaneous TGA-FTIR analysis is an excellent approach and in comparison to the other methods does not require an enormous investment.

TGA measurements allow the number of degradation steps, the degradation temperatures, the mass loss during each degradation step and the amount of residue at different temperatures to be de - termined. When an FTIR spectrometer is coupled online to the TGA, the nature of the volatile decomposition products can be identified at any time during the measurement.

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References

[1] R.E. Lyon: Plastics and rubber, in C.A. Harper (Editor), Handbook of building materials for fire protection, McGraw- Hill New York (2004)
[2] U. Braun, B. Schartel, M.A. Fichera, C. Jäger, Flame retardancy mechanisms of aluminium phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced polyamide 6,6, Polymer Degradation and Stability, 92 (2007), S. 1528-1545
[3] U. Braun, B. Schartel, Flame Retardant Mechanisms of Red Phosphorus and Magnesium Hydroxide in High Impact Polystyrene, Macromolecular Chemistry and Physics, 205 (2004), S. 2185-2196

The characterization of the curing reaction and glass transition of a matrix resin is important for guaranteeing the properties and performance of a composite material. In this article, TOPEM^® and DMA are used to investigate a carbon-fiber reinforced epoxy composite.

Introduction

Composite materials are widely used as structural components in the aerospace, automobile and building industries. This has to do with their special properties such as high strength and stiffness to density ratio, low coefficient of thermal expansion, and favorable vibration-damping characteristics.

The strength and stiffness of a composite are mainly determined by the type of fiber used in the composite, the volume content of the fiber, and its orientation (e.g. unidirectional or as woven fabric, etc). Thermosetting resins are often used as the matrix component. The resin binds the fibers together in the right orientation to form a structure, transfers the forces between fibers and not least protects the fibers from chemical or physical damage.

It is therefore very important to characterize the matrix resin in order to adapt and optimize its properties to the production process, to control the quality, and analyze possible causes of failure. The glass transition and the curing behavior of the resin can have a major effect on the properties of a composite. These properties include impact resistance, brittleness, creep behavior, and solvent resistance.

Significant deviation of the glass transition temperature from normal values may indicate insufficient curing (under-cure) or a reaction that has gone too far (over-cure). This can for example result from an incorrect processing temperature or from temperature gradients in the production piece. Storage conditions (e.g. temperature and moisture) can also have an adverse effect on the matrix resin especially if the material is insufficiently cured. Postcuring during use of the composite can also adversely affect its properties.

The glass transition of a pure or a lightly filled resin can be easily determined by DSC by measuring the change in the specific heat capacity (Δc_p ). However, in a highly filled composite it may be difficult or even impossible to detect the glass transition because Δc_p is very small due to the dilution effect of the filler. In this case, a more sensitive technique has to be used, for example dynamic mechanical analysis (DMA). This method determines the glass transition by measuring changes in mechanical properties such as the elastic modulus or the damping factor.

Sometimes, changes in the glass transition are masked by other effects such as postcuring or the vaporization of constituents. Furthermore, thermal or mechanical pretreatment of the material can lead to relaxation effects. The measurement curve is then more difficult to interpret and evaluate. The determination of the glass transition temperature of the matrix resin by DSC may even become impossible. In such cases, temperature-modulated DSC, for example TOPEM^® [1], can be used to separate the cp changes of overlapping effects and obtain a better understanding of the transition.

In this article, we will show how carbon-fiber-reinforced epoxy resins can be characterized by TOPEM^® and DMA. These techniques can be used both in research and development and in process and quality control.

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References

[1] J. Schawe, The separation of sensible and latent heat flow using TOPEM, UserCom 22, 16–19.