Thermal Analysis UserCom 9

In principle, three important properties of PVC samples can be investigated with thermoanalytical methods. They are:

• the thermal stability,
  
• the temperature and form of the glass transition
  
• the degree of gelation.

In practice two important methods of thermal analysis are used[1]: Thermogravimetric Analysis (TGA) and Differential Scanning calorimetry (DSC).

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Literature

[1] DIN 51005: Thermische Analyse (TA), Begriffe

The investigation of printed cicuit boards is used as an example to show how the results from different thermoanalytical techniques can be evaluated to make a comprehensive assessment of the product.

Introduction

Printed circuit boards

Printed circuit board (PCB) is the term used to describe nonconducting sheets of material that serve as supports for printed circuits of electronic components. The PCB is a laminate material made by pressing together several layers of a support material with a curable plastic as binder (the matrix resin). The outermost layer consists of a copper foil that forms the circuits after the redundant areas have been etched away.

Resin-bonded paper known as “FR2” is made using paper as the support material. For industrial electronics high performance PCBs known as “FR4” are used. These are made from fiberglass and a special epoxy resin as the matrix resin.

Requirements for a PCB

• A certain degree of structural rigidity and above all dimensional accuracy
  
• Low thermal expansion (soldering, operating temperatures): PCBs are anisotropic; they exhibit quite different coefficients of expansion of length (x), width (y) and thickness (z).
  
• Sufficiently high softening temperatures since the mechanical and dielectric properties deteriorate when the matrix resin softens (glass transition).
  
• Adequate thermal durability of the matrix resin to resist the soldering bath temperature and possible heat accumulation in later operation. Any initial degradation of the matrix resin is accompanied by the formation of gases which can force open the layers (delamination) and thereby destroy the laminate.
  
• Finally, the PCB must be flame resistant (FR). The high performance “FR4” standard material is flame resistant due to its brominated monomer building blocks (tetrabrom-bisphenol A). Because of this, corrosive decomposition products are formed on heating. Up until now, it has not been possible to achieve adequate flame resistance with ecologically more friendly additives.

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Introduction

Butter is made by the mechanical blending of droplets of fat from cow’s milk. Household butter is a water-in-oil emulsion containing about 15% water. Water-free butterfat is not only used for roasting but also for the manufacture of foodstuffs. Besides its typical buttery taste, the rheological properties, the texture and the appearance of the foodstuff are also influenced by the seasonal variation of its composition.

In contrast to many fats of plant origin, which consist mainly of three or four fatty acids (as glycerides), butter contains about 10 important fatty acids with 4 to 24 carbon atoms. The changing fatty acid content originating from the seasonal variations in the animal feed influences the crystallization of butter and through this its physical properties. For example, summer butter (at the same temperature) is appreciably softer than winter butter, which is the reason why butter taken directly from the refrigerator in winter is more difficult to spread. The foodstuffs industry in particular is very interested in standardized butterfats. These can be produced by fractional crystallization. The water-free molten butter is cooled down with gentle stirring until crystallization begins. The (high melting) crystals are filtered off and the remaining melt cooled further. The next crystal fraction is then obtained in the same way. Finally, the native butter has been separated into several fractions, which, depending on market requirements, can be mixed to give standard products. Such special products are required for the manufacture of ice cream, chocolate and pastry, to mention just a few. Margarine is produced by emulsifying water with plant fats (formerly also with animal fats).

The crystalline composition of butter and margarine can be investigated using various analytical techniques:

• Isothermal NMR at different temperatures*)
  
• Isothermal X-ray diffraction at different temperatures*)
  
• Differential Scanning Calorimetry

*) normally from 0 °C to 40 °C in steps of 5 °C

Of these methods, DSC is the most important since almost any temperature program (heating, cooling or isothermal) can be performed automatically and reproducibly. Crystallization is observed as DSC of butter fats and margarine Daniel Dalemans, F&E SA N. Corman, Goé, Belgium an exothermic peak and melting as an endothermic peak..

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TGA measurements are normally per- formed at atmospheric pressure. It can of course happen that a step in a TGA curve is the result of two overlapping processes. For instance, solvent or plasticizers can evaporate at the same time as the polymer has already begun to decompose. In some cases the overlapping processes can be separated by changing the heating rate, either manually or automatically (MaxRes). If this is not possible, there is another possibility that will be described in more detail below: measurements at reduced pressure. While the atmospheric pressure generally has little or no efffect on the course of a decomposition reaction, this is quite different for processes involving evaporation. In such cases, a reduction of pressure shifts the vaporization process to lower temperatures. The separation of the two overlapping effects (one of which is a vaporization process and the other a decomposition reaction) should therefore be possible by working at reduced pressure with the thermobalance. But how can we reduce the pressure in a thermobalance and yet still measure properly?

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Differential scanning calorimetry (DSC) is together with thermogravimetry (TG) the most important thermoanalytical method for the analysis of damage to plastic components or for the investigation of manufacturing problems. With the aid of this technique, possible causes of damage can be identified or eliminated. It helps the interpretation of results from other methods such as infrared spectrometry, viscosity determinations and mechanical testing. In many cases damage could have been avoided if DSC measurements had been routinely performed as part of the quality assurance of incoming goods. In some cases the results of DSC measurements have led to the integration of DSC in the goods-in control. In damage analysis, DSC is used primarily to investigate the following problems:

• is the plastic part made of the prescribed polymer?
• has the type of pellet been delivered that was ordered?
• is the material contaminated with another type of plastic?
• are any internal stresses “frozen” in the parts?
• has the plastic been thermally damaged during processing in use?
• is the material sufficiently stabilized?
• has the material been completely cured?

A number of relevant examples will now be discussed in order to illustrate the important role that DSC plays in this type of work.

The DSC20 measuring cell with standard sensor and TC10A controller were used for all measurements. Standard aluminum crucibles with pierced lids and nitrogen or air purging were used for sampling. The heating rate was normally 10 K/min. The sample weight ranged from 5 mg to 25 mg according to the problem concerned, whereby particular attention was paid to good contact with the bottom of the crucible [1]. The curves measured during this work were imported into a PC with STAR^e software.

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Literature

[1] J. Vogel „DDK - Einführung in die Messtechnik, Fehlervermeidung“, Thermische Analyse an Kunststoffen - Methoden und Anwendungen, LabTalk-Seminar der Fa. Mettler-Toledo 25.11.97