GLASS TRANSITION IN FOOD INDUSTRY – CHARACTERISTIC PROPERTIES OF GLASS TRANSITION AND DETERMINATION TECHNIQUES

Semih Otles¹, Serkan Otles^2
1 Food Engineering Department, Faculty of Engineering, Ege University, Izmir, Turkey
² Izmir Institute of Technology, Chemical Engineering Department, Izmir, Turkey

ABSTRACT

In the last decade or two the phenomenon commonly known as the “glass transition” has become a topic of considerable interest in a wide range of scientific disciplines, ranging from biology to theoretical physics. In spite of this, there still appears to be a fairly widespread lack of understanding of the nature of the glass transition. This may be due in large part to the fact that the term “glass transition” is something of a misnomer, bringing to mind first order thermodynamic transitions such as the freezing of a liquid to form a crystal. In reality the glass transition is totally kinetic in nature and is due to the inability of the local structure of a liquid to equilibrate on an experimental timescale at low temperatures and high viscosities. The phenomenology and relaxational character of the glass transition will be described and discussed. Also discussed will be the interpretation of techniques, such as differential scanning calorimetry, commonly used to characterize the glass transition.

Key words: glass transition, food, physical properties.

INTRODUCTION

Glass transition theory applies to amorphous polymers and has also been found to be applicable to low molecular weight sugars. The glass transition temperature, Tg, is the temperature at which a glass to rubber transition takes place. Water, the most common plasticizer in foods, acts to decrease the glass transition temperature. The system properties above and below the glass transition temperature differ quite dramatically and such differences have been studied relative to the crystallization and the viscosity of sugar solutions. The relationship between the rate of chemical reactions which occur in food systems and the state of the system, either glassy or rubbery, has also been studied. The properties of glassy and rubbery systems may contribute to differences in chemical reaction rates in each of these states [1, 2, 3, 4, 5].

The traditional wisdom concerning Tg is that higher Tg’s confer several distinct benefits in a resin system:

• The higher the Tg, the lower the total amount of Z-direction movement when a PWB is heated (either in use or during process steps such as solder reflow). This translates into reduced stress on plated through holes (PTH’s) during processing, reduced risk of hidden or intermittent PTH defects, and therefore better board reliability [6].

• The higher the Tg the less likely it will be that rework will result in pads or lines detaching from the surface of the PWB. This was a major reason for using polyimide on almost all military PWB’s for many years where field repairs were often important [6, 7].

• The higher the Tg the less measling (separation of resin from glass weave at the knuckles believed due to differential expansion of the glass vs. the resin) will be experienced [7].

• The higher the Tg the better will be the long term thermal stability of a material [8].

The following physical properties undergo a drastic change at the glass transition temperature of any polymer:

• Hardness
• Volume
• Modulus (Young’s module)
• Percent elongation-to-break

Glass transition temperature measurement

There are several methods which are used to measure the glass transition temperature for a material. The most common method reported in the literature for glass transition determination in food is differential scanning calorimetry (DSC). DSC measures the change in heat capacity between the glassy and rubbery states and is indicated by a change in baseline in a DSC thermogram as shown below in Figure 2. DMTA on the other hand measures changes in elastic behavior at the transition. It is much more difficult to perform but has a much higher sensitivity.

Glass Transition is a method to characterize a property of a polymeric material. The glass transition is the temperature where the polymer goes from a hard, glass like state to a rubber like state. The best way to envision this type of transition is to put a rubber band (rubber like state, very flexible) into a container of liquid nitrogen. When removed the rubber band is solid and inflexible (glass state) and in fact the rubber band can be shattered. Upon standing and warming to room temperature the rubber band will again become flexible and rubbery (rubber like state) [12].

DSC defines the glass transition as a change in the heat capacity as the polymer matrix goes from the glass state to the rubber state. This is a second order endothermic transition (requires heat to go through the transition) so in the DSC the transition appears as a step transition and not a peak such as might be seen with a melting transition [12].

TMA defines the glass transition in terms of the change in the coefficient of thermal expansion (CTE) as the polymer goes from glass to rubber state with the associated change in free molecular volume [13].

Each of these techniques measures a different result of the change from glass to rubber. The DSC is measuring a heat effect, whereas the TMA is measuring a physical effect i.e. the CTE. Both techniques assume that the effect happens over a narrow range of a few degrees in temperature. If the glass transition is very broad it may not be seen with either approach. This is similar in some ways to comparing inorganic materials with thermoplastics in terms of their melting points: many inorganic materials have sharp melting points that take place within a fraction of a degree of temperature, while most polymers have a much wider melting range depending on the distribution of their molecular weight. Since thermosetting (crosslinked) resins such as the epoxies that are commonly used in our industry do not have melting points, we look at Tg as a measure of change of state, but like their thermoplastic brethren, thermoset resins for a variety reasons of do not all have sharp Tg transitions [14].

The DSC and TMA often give results that differ from one another by 5-10°C when used to test a polymer. Moreover, some polymers are more amenable to DSC (epoxies for instance) or to TMA (i.e. some of the first generation polyimides such as Kerimid 601), because the transition is easier to observe using one technique over the other. For example if a polymer has a very large CTE above the glass transition, the polymer may be easier to test with the TMA than with the DSC, where the glass transition may be almost invisible because it is very broad or does not absorb an amount of heat easily detectable by the DSC [14].

Points to ponder:

1. DSC is the classic and “official” way to determine Tg even though in some cases there are polymeric materials that do not exhibit a sharp Tg by DSC [15].

2. Tg and Melt Point are distinctly separate phenomena and even when looking at thermoplastic materials such as are part of many laminates (PPO, PPE) their Tg’s should not be construed as being melt points and vice-versa [15].

3. PTFE for instance has a minor second order crystalline transition at about 19°C that results in a minor hiccup in the curve of dielectric constant vs. temperature, but is neither a melt point nor a real Glass Transition.

4. Most polymers have Tg’s but technically its measurement depends on a crystalline transition so if a polymer is largely or totally amorphous in nature it may not have (or readily exhibit) a Tg [15, 17].

Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry is a technique that is used to study the thermal transition of a polymer. Thermal transitions are the changes that take place when a polymer is heated. The melting of a crystalline polymer and the glass transition (Tg) are examples of thermal transitions. The device that is used to measure Tg and other thermal transitions (such as the melting and crystallization temperatures) is shown in the Figure 3 [18].

This device consists of two pans. In one pan, the polymer sample is loaded. The other pan is the reference pan and is normally left empty. These two pans are located on top of a heater. The computer assembly will turn on the heaters and the heating rates (q*t^-1) of the two pans are accurately controlled (about 10 °C per minute). The computer programs the heating rate to stay exactly the same through out the experiment. It is also important to note that the two separate pans, with their heaters are heated at the same rate as each other [19].

The presence of polymer material in the sample pan results in an uptake of more heat in order to keep the temperature of the sample pan increasing at the same rate as the reference pan. This means that the heater underneath the sample pan will provide more heat energy than the heater under the reference pan. The DSC experiment is all about the measurement of how musch heat that the sample pan heater has to put out as compared to the reference pan heater. In DSC experiments the data of temperature increase (T) are plotted against the difference in heat output of the two heaters at a given temperature [16, 19].

The computer will provide plots of the difference in heat output of the two heaters against temperature. This means that the above plot is the heat absorbed by the polymer against temperature.

Heat/time = q*t^-1 = heat flow [16]

Temperature increase/time = ∆T*t^-1 = heating rate

When we divide the heat flow q*t^-1 by the rate of heating ∆T*t^-1. We obtain:

(q*t^-1)/( ∆T*t^-1) = q*∆T^-1 = Cp = Heat Capacity

The amount of heat it takes to get a certain temperature increase in a material is called heat capacity, or Cp. Heat capacity is obtained by dividing the heat supplied by the the temperature increase. Therefore DSC plot gives Cp.

Point of Tg Determination

In the literature, researchers report glass transition temperature data as either the midpoint or the onset Tg. As shown below in Figure 4, the point chosen for the determination of Tg can affect the value of the glass transition temperature. The onset Tg is generally considered the most appropriate temperature to report. However, many researchers report midpoint Tg values since a plot of the first derivative of the glass transition curve shows a peak at the midpoint glass transition temperature making this point easy to identify [16].

The graph below (Fig. 5) shows results using the DSC at 10°C*min^-1 and determining the midpoint temperature for native and pre-gelatinized starch.

CONCLUSION

The traditional wisdom concerning Tg is that higher Tg’s confer several distinct benefits in a resin system such as; The higher the Tg, the lower the total amount of Z-direction movement when a PWB is heated, the less likely it will be that rework will result in pads or lines detaching from the surface of the PWB and the less measling.

Also hardness, volume, modulus (Young’s module) and percent elongation-to-break undergo a drastic change at the glass transition temperature of any polymer.

DSC defines the glass transition as a change in the heat capacity as the polymer matrix goes from the glass state to the rubber state. TMA defines the glass transition in terms of the change in the coefficient of thermal expansion (CTE) as the polymer goes from glass to rubber state with the associated change in free molecular volume. Each of these techniques measures a different result of the change from glass to rubber.

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