Thermal Analysis UserCom 30

In this article, we investigate the process of vitrification and devitrification during the non-isothermal curing of a resin system. The measurements were performed using TOPEM^® , a new temperature-modulated technique developed by METTLER TOLEDO [1]. A previous article (see UserCom 29) dealt with vitrification during isothermal curing [2].

Introduction

When an epoxy resin cures, the resin molecules react and form a highly crosslinked network. The physical properties of the material change drastically: before the curing reaction, the resin is liquid whereas after the curing process it is a highly crosslinked solid. In the initial stages of the curing reaction, the rate of cure is controlled by the kinetics of the chemical reaction. As the reaction proceeds and the degree of crosslinking increases, the glass transition temperature (T_g) of the system continuously increases from its initial value.

If the epoxy system is heated at a rate that is sufficiently high, the momentary sample temperature will always be higher than the glass transition temperature of the system. At the end of such an experiment, the system is completely cured (the degree of cure, α , that was initially 0, is now 1) and the glass transition temperature reaches its highest value (T_g∞).

On the other hand, if the system is heated at a low heating rate, at some point, the momentary glass transition temperature of the system can reach the momentary sample temperature. In this case, the system changes to a glassy state (vitrifies) as a result of the chemical reaction. From this point onward, the rate of the curing reaction is no longer limited by the kinetics of the reaction but by the mobility of the molecules involved in the reaction.

Following vitrification, the molecules lose their mobility almost completely; the curing reaction slows abruptly and practically comes to a standstill. The same process occurs in isothermal curing if the system is cured at a temperature below T_g∞ [2, 3].

However, in non-isothermal curing (in contrast to isothermal curing) vitrification is followed by devitrification. This occurs when the sample temperature once again becomes higher than the glass transition temperature. In this case, the molecules regain their mobility and the reaction can proceed to completion.

In the past, the non-isothermal curing of resins has been measured by temperature-modulated DSC at a single frequency [4-6]. The frequency behavior of vitrification and devitrification has been studied in detail by dielectric relaxation spectrometry [7] or mechanical spectrometry (e.g. by torsional braid analysis [8]).

The frequency dependence of vitrification and devitrification can also be measured by conventional temperature-modulated DSC (ADSC).

This is, however, very time consuming because a new experiment has to be performed for each frequency, usually at a very low heating rate and with sample and blank runs. Furthermore, each experiment needs a fresh mixture of the resin and the crosslinking agent because the samples are reactive systems and cannot be stored. Each new mixing process inevitably leads to small variations in the composition of samples, which in turn contributes to experimental uncertainty. In this article, we show how the frequency behavior of vitrification and devitrification in non-isothermal curing processes can be investigated by TOPEM^® . Knowledge of the frequency behavior of vitrification and devitrification allows the temperature dependence of the relaxation time to be determined. This is of fundamental interest for understanding the glass transition.

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Literature

[1] Schawe, J.E.K.; Hütter, T.; Heitz, C.; Alig, I.; Lellinger, D.: Stochastic temperature modulation: A new technique in temperature-modulated DSC. Thermochim. Acta 446 (2006) 147−155.
[2] Fraga, I.; Montserrat, S.; Hutchinson, J.: Vitrification during the isothermal curing of a thermoset studied by TOPEM^® . UserCom 29, 17–20.
[3] Fraga, I.; Montserrat, S.; Hutchinson, J.: Vitrification during the isothermal cure of thermosets. Part I. An investigation using TOPEM^® , a new temperature modulated technique. J. Thermal Anal. Calorim. 91 (2008) 687−695.
[4] Van Assche, G.; Van Hemelrijck, A.; Rahier, H.; Van Mele, B.: Modulated differential scanning calorimetry: non-isothermal cure, vitrification, and devitrification of thermosetting systems. Thermochim. Acta 286 (1996) 209−224.
[5] Montserrat, S.; Calventus, Y.; Colomer, P.: Vitrification and devitrification phenomena in the dynamic curing of an epoxy resin with ADSC. Usercom 11, 17−19.
[6] Montserrat, S.; Martín, J. G.: Non-isothermal curing of a diepoxidecycloaliphatic diamine system by temperature-modulated differential scanning calorimetry. Thermochim. Acta 388 (2002) 343−354.
[7] Montserrat, S.; Roman, F.; Colomer, P.: Vitrification, devitrification, and dielectric relaxations during the non-isothermal curing of a diepoxy-cycloaliphatic diamine. J. Appl. Polym. Sci. 102 (2006) 558−563.
[8] Wisanrakkit, G.; Gillham, J. K.: Continuous-heating transformation (CHT) cure diagram of an aromatic amine/epoxy system at constant heating rates. J. Appl. Polym. Sci. 42 (1991) 2453−2463.

The processing temperature and the degree of gelation have a decisive influence on the mechanical stability and fracture behavior of hard PVC. The degree of gelation is mainly determined by the enthalpy of fusion of the crystallites that melt below the maximum processing temperature, T_p . Due to the effect of different recrystallization processes, there are fewer crystallites with a melting point around T_p. This is observed as a so-called melting gap in the DSC melting curve. Differential scanning calorimetry, DSC, is a well-established and convenient method for determining both T_p and the degree of gelation. The ISO 18373 standard specifies how to take samples from rigid PVC pipes and measure and evaluate the DSC measurement curves; other materials can be tested in a similar way.

Introduction

Polyvinylchloride (PVC) has been used for over 60 years for pipes, window frames, films, electrical insulation material, and countless other applications. Hard PVC (PVC-U) is especially suitable for the manufacture of longlasting products such as water pipes. More than 28 million tons of PVC was produced worldwide in 2001 [1]. In Europe, about half of all the different types of pipes (e.g. water, wastewater, and gas pipes) are made of hard PVC. Arguments for the use of PVC, for example for pipes are [2]:

• PVC has very long-lasting material properties and is continuously being developed.
  
• PVC pipes are lighter than pipes made of other materials but are nevertheless strong, durable and flexible enough to bend without breaking. They are easy to install and are compatible with other materials used in pipe networks.
  
• PVC is resistant to corrosion. Pipes have low flow resistance due to their smooth surface.
  
• PVC products have a good price/performance ratio.
  
• PVC can be recycled and used in new products; its material properties remain intact. Great efforts are being made to process used material directly to make new products. If this succeeds, the life cycle assessment is good. Thermal recycling (burning) however remains a difficult problem.

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Literature

[1] Rogério P. Marques, José A. Covas, Processing Characteristics of U-PVC Compounds, Firmenschrift von CIRES S.A. (Companhia Industrial de Resinas Sintéticas), Estarreja, Portugal, 2003.
[2] www.pvc4pipes.org

Besides food safety regulations, multilayer packaging films used in the food industry have to fulfill a wide variety of technical and commercial requirements. These involve shelf life, transportation, tear resistance, barrier properties, sales appeal, and so on. Modern packaging materials consist of several different thin polymer films bonded together in different ways to form a multilayer film. The polymers most widely used are polyethylene, polypropylene or polyamides. A multilayer film provides a range of properties which cannot be obtained from a monolayer film alone. Since the polymers exhibit different melting behavior, information about the different layers can be obtained from differential scanning calorimetry, DSC, and thermomechanical analysis, TMA. This article presents the results of the analysis of a 0.1-mm multilayer film. The TMA measurement conditions were optimized to obtain precise information about the different layers, the nature of the polymers and the thickness of each layer. The latter was also measured by light microscopy to verify the TMA results.

Introduction

The following sections describe the analysis of a colorless, transparent multilayer polymer film. The melting behavior and thickness of the individual film layers were the main points of interest. The techniques used were DSC, TMA and visible light microscopy. The results from the different techniques showed good agreement.

Polyethylene (PE) is a thermoplastic polymer produced through polymerization of ethylene. It is a polyolefin and is widely used for cable insulation, packaging materials and many other applications. Polyethylenes in multilayer films ensure mechanical and chemical stability and prevent the uptake of moisture. Besides high molecular weight and ultra-high molecular weight polyethylene, there are three main types of polyethylene. They exhibit different melting behavior [1]:

• PE-LD: strongly branched polymer chains, low density, melting point 110 °C.
  
• PE-LLD: linear low density PE, melting point 123 °C.
  
• PE-HD: weakly branched polymer chains, high density, melting point: 135 °C.

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Literature

[1] Adolf Franck-Kunststoffkompendium, Vogel Fachbuch, 2000

This article describes how the kinetics of crystallization of nanograins in an amorphous Fe-Si-B-alloy was investigated. The structure of the Fe-Si nanocrystallites determines the soft magnetic properties of the material. The measurements were performed by DSC and evaluated using the Advanced Model Free Kinetics (AMFK) software. This allowed the apparent activation energy as a function of the conversion to be determined. From this, conclusions could be drawn about the complex multi-stage crystallization process.

Introduction

Soft magnetic materials are ferromagnetic substances that are weakly magnetized in a magnetic field and thus exhibit slight hysteresis. They are used in new innovative electrical devices such as compact low-loss switch-mode power supplies, magnetic amplifiers, transformers and chokes.

Ferromagnetic materials can acquire good soft magnetic properties when ultrafine microcrystalline structures are formed. Fe-Si-crystallites with a grain size limit of 10−15 nm are a good example. Such structures are produced when amorphous Fe-Si-B alloys, to which small amounts of Cu and Nb have been added, crystallize [1].

The originally proposed alloy ha s the nominal composition Fe_73.5Si_13.5B₉Cu₁Nb₃ (at %). Rapid solidification yields an amorphous metallic glass in the form of a ribbon with a thickness of about 20 μm.

The nanocrystalline structure is obtained by annealing between 500 and 600 °C. In this process, Fe-Si grains are produced with a typical particle size of 10−15 nm in an amorphous matrix. The distance bet ween the crystallites is about 1 to 2 nm [1].

Knowledge of the crystallization kinetics of amorphous alloys offers the possibility of producing specific crystalline structures and hence for optimizing the properties of the alloys.

The processes involved in the crystallization of amorphous metal alloys are complex. This makes it impossible to describe the nanocrystallization using a model-fitting method such as the Avrami crystallization model [2]. For this reason, in this article, model free kinetics is used to investigate the crystallization of the nanocrystallites. A detailed discussion of the results and a comparison with different crystallization models have been published in reference [2].

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Literature

[1] G. Herzer in K.H.J. Buschow (Ed.) Hanbook of Magnetic Materials, Vol. 10, Elsevier, 1997, 415−462.
[2] H. A. Shivaee, H.R.M. Hosseini, Thermochimica Acta, 494 (2009) 80–85.

The article describes how a quality problem involving a plain bearing was solved using differential scanning calorimetry, DSC, and thermogravimetric analysis, TGA.

Introduction

A consumer goods manufacturer suddenly began to receive an increasing number of complaints about a product. The cause was traced back to an injection-molded part made of plastic that was used as a plain bearing. Detailed investigations showed that nothing had changed in the product except that the supplier of the plain bearings was new. Preliminary DSC measurements in an external laboratory indicated that the material used for the production of the good and the bad plain bearings was the same polymer. Other factors were therefore thought to be the cause of the problem. A quick solution to the problem did not seem possible.

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