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Analysis of moisture content is critical to ensuring material quality

1st April 2013


Moisture content analysis is a critical component of material quality and essentially a function of quality control in most production and laboratory facilities. From biological research organisations, pharmaceutical manufacturers, to food producers and packers, moisture content control greatly influences the physical properties and product quality of nearly all substances and materials at all stages of processing and final product existence. Paul Wesolowski reports.

Currently, many moisture analysis methods are available for commercial purposes. The primary methods of water content determination include spectroscopic, chemical, conductivity and thermogravimetric analysis. For this technical review, the information will focus on the thermogravimetric method of moisture analysis, and the use of halogen heating as the source of thermal radiation. To understand the thermogravimetric moisture analysis principle, it is important to understand what moisture is, the definition of moisture content and the effects of moisture on material processing and formulation.

Moisture is simply water diffused in a relatively small quantity. Nearly all materials contain at least a diminutive volume of moisture as a component of the molecular makeup. Moisture is a given in the mass of materials, however the relative percentage is dynamic and therefore not constant.

Materials from the perspective relation to moisture content, generally a sample of material will continually increase or decrease in weight because of hygroscopic action. Hygroscopic action is the amount of moisture a material will absorb relative to ambient temperature and humidity conditions. Temperature and humidity can be controlled in laboratory environments. However in processing, transportation and storage facilities they are impractical to control. If random sampling is correctly administered, the representation will closely reflect the properties of the whole batch.

Moisture content can be thought of as the amount of water in a material or substance. Generally, H2O content is difficult to measure because of the complex intermolecular bonding properties within the substance matrix. For measurable water content to be justifiably determined, and proper levels sustainable in a processing environment, water content of a sample material is then referred to as moisture content in the testing and evaluation process of moisture analysis.

Excessive or deficient moisture content of a substance can adversely impact the physical properties of a material. Weight, thermal expansion, amalgamation and electrical conductivity are only a partial example of the properties that can be altered by even a minute presence, or conversely an abundance of moisture. The presence of moisture in a processing environment is unavoidable, and usually necessary for successful processing, however control parameters must be in place. Understanding and maintaining the correct moisture content of a material is essential for manufacturing processors and testing laboratories. A processed material's moisture content will define, for example, the shelf life of processed foods and provisions, the reactivity of chemical compounds in inventory, or the binding properties of bulk materials. As a result, the ability to accurately identify and control moisture content levels during processing procedures is paramount to the success of countless commercial scientific operations. Similarly, the identification and efficient operation of reliable moisture analysis equipment is an essential component of any production or laboratory environment. The thermogravimetric method is universally recognised as an efficient, reliable and cost-effective method for determining moisture content, and can be utilised in virtually any such environment.

The principle of the thermogravimetric method of moisture content determination is defined as the weight loss of mass that occurs as the material is heated. The sample weight is taken prior to heating and again after reaching a steady-state mass subsequent to drying. Various thermogravimetric methods and technologies can be used for sample drying; for example, the halogen technological thermo-radiation drying method is a universally applicable, highly efficient and practical test procedure for in-process testing. The thermogravimetric drying process has many advantages, notably this type of testing is simple and generally does not require high capital investment.

In combining state-of-the-art thermogravimetric drying and halogen heating with highly accurate weighing technology, for example, the Ohaus MB45 moisture analyser delivers a fast, precise method of completing a wide variety of moisture analysis procedures. The MB45 is suitable for sampling applications in the food processing, pharmaceutical and chemical industries, providing loss on drying results for volatile content in powders, pastesand/or liquids.

The measure of moisture content during thermogravimetric analysis defines moisture as the loss of mass of a substance when heated, by the process of water vapourisation. The substance difference is continually calculated and recorded by a precision balance. Sample substance mass is measured before and after the drying process for final moisture determination on percentage basis.

Thermogravimetric analysis is a complete, accurate and responsive method of moisture determination. Several drying methods are classified as thermogravimetric. To be effective, moisture determination methods must be fast, repeatable, and precise. Two fundamental thermogravimetric moisture content measuring technologies are halogen and infra red drying. Other traditional methods include oven drying and microwave drying processes.

Oven drying methods for moisture determination are commonly used for commercial purposes. The oven-drying methodology consists of heating by convection with forced or circulating hot air. The accuracy and moisture range that oven-drying techniques offer is very similar to other thermogravimetric methods. Primary disadvantages of using oven-drying methods include time andnon-portability. For complete and accurate results with oven drying moisture analysis techniques, the ramp up and soaking times are rather long. This practice may hinder actual production value with lengthy material quality feedback for quick decision making at the point of production.

Oven drying techniques are normally implemented away from the actual production area further adding lag time and ambient changes to the test process. Most oven drying methods do not contain a precision weighing balance-type instrument to measure and record mass changes. Mass determinations for before and after measurement recording must be implemented manually by the technician, increasing the chance for error. Most oven drying methods do not allow the continuous referencing, graphing and recording of substance mass changes creating a void of data between the test start point and dry state status.

The technology of halogen drying is an excellent method for moisture content determination. Halogen moisture analysis can be used for determining the moisture content of virtually any substance. The technology uses a halogen radiator in conjunction with an integrated precision balance for detection, measurement and recording of sample weight during H2O vaporisation from the test procedure.

Halogen drying is up to twice as fast as traditional infra red technology (drying time may vary depending on substance type). Halogen radiators are more responsive in heating and cooling requirement cycles than infrared heaters; the accelerated responsiveness saves actual test cycle time (Fig.1). Halogen operating advantages are more efficient than other methods because of the technology of small intense heating elements used as the substrate for the thermoradiator. The glass sheath design encases the inert halogen gas for quick heat conduction, full heating power can be reached within seconds of the test start.

The thermogravimetric method of infra red drying uses concentrated infrared radiation for heating. The process of infra red drying is based on the principle of infrared radiation heat being absorbed by a substance sample. Infra red radiation converts to heat energy at the point of contact with the surface of the test specimen (Fig. 2). During the absorption process, infrared energy is molecularly conducted from the surface through the sample substance. The rate of absorption is critical to the drying time, if the dielectric properties of the sample are high, the drying test time will increase.

Halogen and infra red drying methods both operate with similar accuracy. Generally, if testing parameters of the manufacturer are followed, accuracy levels of 0.1 per cent to 0.5 per cent can be attained. The moisture test range for both methods ranges from 0.5 per cent to 99 per cent of the sample substance. The primary difference in the performance of the two technologies is in the temperature to time ratio of the test. Halogen drying technology reaches full temperature parameters in two seconds, infra red technology is slower in reaching peak temperature because of the time lag of energy absorption into the sample and partial deflection of ray wave energy (Fig. 2). With a standard capacity of 45g, the Ohaus MB45 moisture analyser has a readability of 0.01 per cent or 0.001g. The halogen heating technology performs up to 40 per cent faster than infra red drying technology, and allows users to heat samples from 50oC to 180oC in under a minute.0 In addition, the MB45 incorporates newly developed software applications designed to simplify operation, save time and produce accurate results. The graphical LCD works in tandem with the software to display per centage moisture, per centage solids, temperature, time, a real time drying curve and more.

Using thermogravimetric drying procedures for moisture content determination is a very efficient and effective method in production processing and quality control applications. To fully understand the drying process of moisture vapourisation, users should be aware of ancillary fundamentals associated with the process for optimum results. Two important fundamentals of thermogravimetric drying are substance vaporisation and regain.

Substance vaporisation is an ancillary effect of thermogravimetric drying. In addition to H2O vapourisation, thermogravimetric drying methods do not distinguish weight loss of arbitrary compounds during the test cycle. Also in addition, if the drying temperature is set to high, sample decomposition may result. For the purpose of moisture content relative to drying, regain is the process of a material sample reabsorbing moisture as the drying process is concluded. Regain can be thought of as a natural occurrence dependent on the ambient test environment and test parameters being implemented. Regain test methods are applicable for determining natural absorption properties of a material from a dry to natural state.

Moisture content determination accuracy for substance samples identifying a larger bulk volume will be as precise as the representative sample quality. When choosing material samples from a batch, the area of sample selection is crucial. For reproducible test results, a test sample must be truly representative of the entire homogenous batch being tested. For example, a bulk powder mixing process must be thoroughly blended before extracting a representative sample. In the bulk mixing process a common condition occurs where as a greater concentration of moisture exists away from the surface and edges of the material. A test sample gathered from the top will not be a true representation of the batch.

Optimum sample weight resolution is important for moisture content determination when using the thermogravimetric drying process. A test sample's weight and placement can influence the accuracy and test cycle time. To keep test cycle time to a minimum, a small sample weight should be used. If a sample size is excessive, a larger volume of mass must be vaporised decreasing the test performance and overall accuracy. Test sample weights are correlated with repeatability. As with excessive sample size, a too small sample will interfere with repeatability results due to a minute per centage concentration of moisture in the reference sample.

The key for successful moisture determination, efficient productivity, and superior product quality is fast test determination without lengthy pauses in production processes. Successful moisture analysis means testing at the point of processing, and making adjustments before product integrity is effected.

ENQUIRY No 70

Paul Wesolowski is with Ohaus Europe, Nanikon, Switzerland. www.ohaus.com






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