Measurements under conditions of controlled relative humidity provide information that is crucial for understanding the effects that moisture content can have on the properties of a wide range of materials. TGA-Sorption analysis gives answers regarding
- Shelf life of products
- Structural properties
The TGA-Sorption System allows you to precondition samples at temperatures up to 150 °C and to increase or decrease the relative humidity.
In this Webinar, we will discuss the basic principles of the TGA-Sorption System and present some interesting applications.
The Webinar will cover the following topics:
- Basic Principles of TGA-Sorption
- Why Use TGA-Sorption?
- Relative Humidity Calibration
- Industries and Applications
- Application Examples
TGA-Sorption analysis yields valuable information to help study the effects of moisture on material properties.
The advantages of TGA-Sorption analysis
Product stability, processability and mechanical stability can be influenced by moisture and it is important to know how food products, powders or polymers react in a well-defined humid environment. The influence of humidity depends on the material or the constituents of the formulation or product. In general, the moisture absorbed by materials is classified either as free water or as chemically bound water.
Measurements under conditions of controlled relative humidity provide information that is crucial for understanding the effects that moisture content can have on the properties of a wide range of materials. TGA-Sorption analysis gives answers regarding processing, shelf life of products and structural properties.
Specifically, TGA-Sorption measurements can be used to determine sorption enthalpies as well as to calculate isotherms, BET and GAB plots, and surface coverage behavior. The method yields valuable information to help study the effects of moisture on material properties.
Slide 0: TGA-Sorption
Ladies and Gentlemen
Welcome to this seminar on TGA-Sorption.
I would like to present a high-performance analysis system that allows you to investigate the adsorption and desorption behavior of materials. The system consists of a conventional thermogravimetric analyzer coupled to a humidity generator via a special interface.
Slide 1: Contents
I will first explain the basic principles of sorption and describe the design of the METTLER TOLEDO TGA-Sorption System. In the second part of the seminar, I want to describe several interesting applications that illustrate the different possibilities of TGA-Sorption.
The main industries interested in measuring sorption behavior are the food, and pharmaceutical industries.
In the food industry, the focus is on the stability of food products and in particular on storage- and shelf-life.
In the pharmaceutical industry, testing for the clumping of powders and stability is very important.
With polymers, the effect of moisture on mechanical stability is the main topic of interest.
Slide 2: Why Use TGA-Sorption?
The next two slides summarize some of the reasons for performing TGA-Sorption measurements.
You can use TGA-Sorption to study drying processes or the adsorption and desorption behavior of moisture. The method also allows you to distinguish between strongly or weakly bound water and to determine the effect of moisture on the glass transition. In general, a lower glass transition temperature results in lumpiness, reduced crispness, faster crystallization or the softening of materials. This is of great importance in the food sector, with pharmaceutical products, and with polymers. The uptake of moisture can dramatically change the properties of materials. The effect of relative humidity on the stability of food products is also important for optimizing storage conditions and shelf life.
Slide 3: Why Use TGA-Sorption?
TGA-Sorption experiments allow you to derive sorption isotherms using specific models and equations. The Brunauer, Emmett and Teller (or BET model), and the Guggenheim, Anderson and deBoer (or GAB model) are the most well known. BET plots are used for monolayers, while GAB plots are more suitable for multilayers. This enables us to study how the moisture is adsorbed and determine whether water molecules are arranged in a monolayer or in several layers. Furthermore, we can also calculate sorption enthalpies.
Another advantage I should mention is that you can use the TGA as a conventional instrument without the humidity generator, or couple it to other instrumentation such as vacuum equipment, a mass spectrometer or an FTIR spectrometer. In each case, the conversion takes only a few minutes.
Slide 4: Introduction Basic principles of TGA-Sorption
The upper part of the slide shows a simplified schematic diagram of the TGA-Sorption System. The sample is usually exposed to a stepwise increase or decrease in relative humidity at constant temperature. The mass, or change in mass, is continuously measured by the TGA. This enables us to record the uptake or release of moisture under any desired conditions.
In a TGA-Sorption experiment, the TGA furnace is purged with a low flow of protective gas, typically about 5 to 10 milliliters per minute. The purge-gas flow-rate is low in order to prevent possible reduction of the relative humidity in the furnace. The diagram shows the inlet for the humidified gas and the gas outlet. The humidified gas normally flows at a rate of about 100 milliliters per minute into the furnace. The humidity generator shown on the right is connected to the TGA furnace via a transfer line.
The lower part of the diagram displays typical curves obtained from a TGA-Sorption experiment. The ordinate on the left is the TGA curve that records the sample mass, and the ordinate on the right the relative humidity. TGA-Sorption measurements are usually performed under isothermal conditions at the temperature of interest.
Slide 5: Introduction Furnace cross-section
Now let me describe the design of the system.
In the upper right part of the diagram, we see a TGA/DSC 1 equipped with a large furnace.
The lower part of the diagram shows an enlarged cross-section of the TGA furnace with the transfer line to the humidity generator and the gas inlet. The diagram also shows the position of the relative humidity temperature sensor in the TGA furnace. This is introduced through the gas outlet into the furnace chamber. The sensor is close to the TGA sensor, but of course must not touch it. The red and white sample and reference crucibles are positioned on the TGA sensor.
Slide 6: Modular Humidity Generator MHG
The next slide shows the Modular Humidity Generator or MHG for short.
The MHG can be operated manually, or under software control with triggering. In the latter mode, the MHG receives a signal at the start of the TGA measurement, and simultaneously begins its program. This degree of automation provides convenient and user-friendly operation and allows several samples to be analyzed consecutively.
The MHG has an integrated water tank. The relative humidity temperature sensor is shown on the left of the instrument and the mixing chamber on the right.
Slide 7: Humidity Generator Operating principle of the MHG
I will now explain the design of the MHG in more detail.
The slide shows a schematic diagram of the TGA-Sorption System with the TGA on the left and the humidity generator on the right.
The TGA balance is purged with protective gas at a flow rate of about 10 milliliters per minute in order to prevent any possible condensation of moisture in the balance chamber. The MHG uses a single sensor positioned close to the sample in the TGA furnace. The sensor simultaneously measures both relative humidity and temperature. Gas of the required relative humidity passes into the furnace via the interface. The gas outlet is directly above it.
The right part of the diagram shows details of the internal construction of the MHG. The flow control unit monitors the flow rates of the gases and liquids. These flow separately into the mixing chamber, which is directly attached to the external wall of the interface. In the mixing chamber, gas of precisely defined relative humidity is produced. This then passes on into the TGA.
Production of humidified gas therefore occurs in the mixing chamber and not in the sorption instrument itself. This means that the transfer lines do not have to be heated and can be long, depending on requirements. It also guarantees that the relative humidity does not change, which might well be the case if the gas were transported through long lines. There is no dead volume in the inlets and no condensation effects occur in the inlets.
Slide 8: Humidity Generator Mixing chamber
The slide shows the closed mixing chamber on the left and the internal moist absorbent fleece on the right.
Water is supplied by a pump and mixed with the dry gas to produce gas of the desired relative humidity. The humidified gas then passes into the TGA. Excess water is pumped out of the mixing chamber so that no condensation occurs.
Slide 9: Humidity Generator Interfacing the MHG to the TGA
Here we see an enlarged view of the mixing chamber with the gas and liquid lines on the left, the TGA interface in the middle, and the relative humidity and temperature sensor on the right. The sensor is inserted through the TGA interface and measures the relative humidity and temperature close to the sample.
Slide 10: Relative Humidity Calibration
The relative humidity in the TGA-Sorption Analyzer System is calibrated by making use of the phenomenon of deliquescence. Deliquescence is the process in which a substance absorbs water vapor from the air to form a solution. This occurs when the partial pressure of the water vapor in the air is greater than the vapor pressure of the solution. The response is an increase in mass.
The relative humidity at which deliquescence begins is called the deliquescence point. Below the deliquescence point, the material is stable and absorbs practically no moisture.
The table lists salts such as lithium chloride, magnesium chloride hexahydrate, magnesium nitrate hexahydrate and their deliquescence points at various temperatures.
Slide 11: Relative Humidity Calibration
The deliquescence point of a salt can be determined in a TGA experiment by increasing or decreasing the relative humidity.
In the first example, the relative humidity was increased in steps starting at a value below the deliquescence point. The experiment was performed isothermally at 27.5 degrees Celsius.
The blue curve shows the relative humidity and the red curve the simultaneously recorded TGA curve. The deliquescence point is observed as a sudden increase in weight and is evaluated as the onset.
Slide 12: Relative Humidity Calibration
This slide illustrates the second method for determining the deliquescence point.
In this example, the relative humidity was initially set to a value above the deliquescence point and gradually decreased in steps as shown in the blue curve. The temperature was 27.5 degrees Celsius, the same as in the previous example. The relative humidity was held constant for at least 20 minutes after each change in relative humidity so that equilibrium could be reached. The simultaneously recorded red TGA curve shows the initial uptake and later release of water. The maximum in the curve indicates the deliquescence point as the point at which the loss of water or drying begins.
We see that both methods can be used to determine the deliquescence points. In practice, it is easier to use decreasing relative humidity because maxima are more easily evaluated than onsets.
Slide 13: Industries and Applications
Now that I have explained the basic principles of TGA-Sorption, I would like to summarize the industries and areas in which TGA-Sorption is likely to be of interest.
The list shows that TGA-Sorption is widely used, for example to test the stability and softening behavior of products, to determine optimum storage conditions, and to detect the influence of moisture on product performance. Furthermore, TGA-Sorption can yield information about the loss of water by drying, and be used to determine sorption isotherms.
I will now present some typical application examples.
Slide 14: Application 1 Uptake of water by polyamide 6 at 95% RH
Water can have a dramatic effect on the physical and chemical properties of polymers. One of the main effects is a reduction in mechanical strength due to softening. In many cases, water acts as a plasticizer and shifts the glass transition to lower temperatures.
Polyamides adsorb more moisture than most other types of plastics. The slide shows the TGA and relative humidity curves of a sample of polyamide 6. The sample was first allowed to equilibrate for several hours at a relative humidity of 10%. The relative humidity was then increased in one step to 95%. The graph shows that after about 10 hours at this high relative humidity, the mass had increased by about 8.3%. After absorption of water, the polyamide would be expected to exhibit a lower glass transition point and be relatively soft at room temperature.
Slide 15: Application 2.1 Free water and bound water
When we investigate the water content of materials, we have to differentiate between free, and bound water.
Free water is adsorbed from the environment and is not chemically bound to a material. It can also easily be desorbed at slightly higher temperatures.
Bound water is chemically bound as water of crystallization or water of hydration. The water forms part of the crystal structure of a substance. Consequently, it has an impact on the physical appearance and properties of a material.
Both free and bound water influence the bioactivity of food, for example rice or wheat starch. In general, foodstuffs are often affected by microbial growth above a certain water concentration or water activity as it is called. The aim is therefore to determine optimum storage conditions in order to prevent the food products from deteriorating and becoming spoiled. The water content of products can be easily determined by thermal analysis methods such as TGA and sorption analysis.
Slide 16: Application 2.2 Dynamic sorption curve of amiloride hydrochloride dihydrate
The next example shows how free water can be distinguished from bound water.
The diagram displays the uptake and release of moisture as a function of relative humidity. The sample was amiloride hydrochloride dihydrate, a drug substance used to treat high blood pressure. It was first dried at 125 degrees Celsius and converted to the anhydride. The dashed black line shows the temperature program and the red TGA curve the loss of water.
After drying, the relative humidity (RH) was increased in steps of 10% allowing sufficient time for equilibrium to be reached at each step.
The blue curve shows the humidity steps and the red curve the resulting increase in mass. At a relative humidity of about 50%, the sample has regained its original water of crystallization. A further increase in relative humidity results in the uptake of free water, which is released when the relative humidity is decreased. This process is clearly shown on the right side of the slide. At 25 degrees Celsius, the water of crystallization remains bound in the sample and can only be liberated by increasing the temperature.
Slide 17: Application 3 Sorption enthalpy of maltodextrin
The METTLER TOLEDO TGA-Sorption System also allows you to measure sorption enthalpies. This is illustrated using Maltodextrin as an example. Maltodextrin is a mixture of monomers, dimers, oligomers and polymers of glucose and is produced by the hydrolysis of starch. It is used as an excipient in pharmaceutical products, or as a stabilizer in the food industry.
From top to bottom, the diagram shows the TGA curve in red, the relative humidity of the furnace atmosphere in blue, and the DSC curve in green. The DSC curve clearly shows the effect that a dynamic sorption program has on the enthalpy of the sample. The curve exhibits exothermic peaks that gradually tail off after each change in relative humidity. The integral of the area under the middle peak yields a value of about 42 Joules per gram, which is typical for starch products.
The correlation between sorption enthalpies and water content allows the temperature dependence of water sorption to be described at any humidity. This possibility offers many advantages for the characterization of products.
Slide 18: Application 4.1 Sorption behavior
Before I present the next application, I would like to explain a few important terms.
The term sorption behavior refers to the ability of a hydroscopic product to adsorb or desorb water vapor at a particular temperature until equilibrium is reached.
Sorption isotherm is the graphical description of the sorption behavior of a substance. It represents the relationship between the water content of a product and the relative humidity of ambient air at a particular temperature under equilibrium conditions.
Sorption isotherms are presented as water content versus water activity or relative humidity.
Slide 19: Application 4.2 Water activity
Now let me explain what we mean by water activity.
For simplicity, water activity can be defined as water that is freely available and not bound. It should not be confused with the total water content of the sample.
Water activity can have a value between 0 and 1. Pure water has a value of 1, while a product with no free water has a value of 0. Bacterial growth ceases at low water activity, so the value of the water activity is directly related to the shelf life of food products. The larger the value, the faster a food or pharmaceutical product is likely to deteriorate.
Slide 20: Application 4.3 BET and GAB models
Specific model equations have been proposed to characterize sorption isotherms. Few models are however able to fit the entire range of relative humidity.
As I mentioned earlier, BET and GAB are the two most commonly used models. BET is used for water activities of less than 0.5 and to characterize monolayers. GAB covers a wider water activity range than BET and is used for water activities up to 0.9 and for multilayers.
Slide 21: Application 4.4 Sorption isotherm of basmati rice
Many products in the food industry are sensitive to moisture. This can strongly affect the storage conditions and shelf life of the food product.
Rice in particular, presents special problems during transportation because it is not dry when it is shipped. If the moisture content of rice becomes too high, there is an increased risk of fermentation, mold formation, loss of quality, or agglomeration.
The example in the slide shows the sorption isotherm of a grain of basmati rice at 24 degrees Celsius derived from sorption curves using the BET model. The sorption isotherm shows that the critical water content of 15% is not reached even at a relative humidity of 95% so the quality of the rice is not at risk. The analysis therefore provides important information for optimizing the storage and transportation conditions for the rice.
Slide 22: Application 5.1 Sorption behavior of wheat starch
Here we see the dynamic sorption curve of wheat starch at 30 degrees Celsius. The relative humidity was first increased in steps from 5 to 85% and then decreased back down to 5%. The equilibrium weight at the end of each step allows a sorption isotherm to be calculated that shows how much water is absorbed on the surface.
In the next two slides, I want to discuss different sorption isotherms in more detail using wheat starch as an example.
Slide 23: Application 5.2 Sorption behavior of wheat starch
The slide displays the sorption and desorption isotherms obtained from the previous measurement.
As I mentioned earlier, the isotherms can be calculated from the equilibrium weight at the end of each relative humidity step. The information allows us to calculate the water activity, which, as we have seen, is a measure of the bioactivity of products. For simplicity, the water activity can also be called freely available water.
Slide 24: Application 5.3 Sorption behavior of wheat starch
This slide shows a typical evaluation of wheat starch using the GAB model. The information allows you to calculate the weight of water needed to form a monolayer. Curve fitting yields the necessary parameters. In this example, the result is a water content of 7.5% based on the dry mass.
Slide 25: Application 6: Sorption behavior of lactose in milk powder
Milk powder consists of a high percentage of lactose in addition to proteins, fats, minerals, and water. The lactose is amorphous due to the processing conditions.
The slide displays an isothermal measurement of lactose at 30 degrees Celsius in which the relative humidity was first increased in 10% steps from 0 to 90% and then decreased again.
The black curve shows the programmed relative humidity and the blue curve the measured relative humidity. The TGA curve is colored red and shows the increase or decrease in weight of the sample. When the relative humidity reaches a certain value, the amorphous lactose crystallizes. Crystalline lactose is however much less hygroscopic than the amorphous lactose. At this point, the excess water evaporates. The process is observed as a weight loss in the TGA curve. The lactose crystallizes above the glass transition temperature and can cause an undesired change in the taste of the product. Storage conditions and ambient relative humidity are therefore of great importance
Slide 26: Summary: TGA-Sorption
TGA-Sorption analysis is an excellent method for characterizing drying processes and investigating the sorption or adsorption behavior of samples. The technique allows free water to be distinguished from bound water.
TGA-Sorption measurements can be used to determine sorption enthalpies as well as to calculate isotherms, BET and GAB plots, and surface coverage behavior. The method yields valuable information to help study the effects of moisture on material properties.
The METTLER TOLEDO TGA/DSC 1 can also be used as a conventional standalone instrument without the humidity generator.
Slides 27: For More Information
Finally, I would like to draw your attention to information about TGA-Sorption that you can download from the Internet.
Thermal analysis articles and applications from widely different fields are published twice a year in UserCom, the well-known METTLER TOLEDO biannual technical customer magazine. You can download UserCom articles at www.mt.com/ta-usercoms .
Slides 28: For More Information
Additional information about our application handbooks, webinars or thermal analysis in general can be downloaded from the internet addresses given on the slide.
Slide 29: Thank You
This concludes my presentation on TGA-Sorption. Thank you for your interest and attention.