FOCUS ARTICLE ASSESSING THERMAL STABILITY - THE CHALLENGE OF POWDERS. Swati Umbrajkar, Ph.D., CSP

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1 FOCUS ARTICLE ASSESSING THERMAL STABILITY - THE CHALLENGE OF POWDERS Swati Umbrajkar, Ph.D., CSP When it comes to solid materials, thermal instability can result not only from self-reaction or molecular fragmentation (decomposition), but also from oxidation reaction of the solid with atmospheric oxygen. As the median particle size of a product decreases, the surface area to volume ratio increases and this can accelerate the self heating process smoldering / glowing possibly leading to flaming combustion from modest temperatures. This article examines the challenges in assessing the thermal stability of powders and demonstrates how traditional thermal techniques such as Differential Scanning Calorimeter (DSC) and Differential Thermal Analysis (DTA) can fail to diagnose self-heating phenomena. Specific techniques for powder thermal stability assessment (such as the Diffusion Cell, Aerated Cell and Basket Tests) are summarized, and a selection of thermal stability data on solids with different behaviors are presented to highlight the shortcomings of pursuing a one-dimensional DSC or DTA program for the thermal stability assessment of solids.

2 The objective of this article is to highlight a rigorous strategy for thermal stability assessment, specifically in relation to powders. INTRODUCTION The assessment and understanding of thermal instability associated with process materials is a prerequisite for safe manufacturing. This is particularly true for materials exposed to elevated temperatures during processing. Therefore, information gathering on thermal stability should commence as early as possible in the process development lifecycle. Studies to accurately define safe exposure temperatures should then be performed using laboratory equipment and methods specifically appropriate to represent the plant or processing technique selected, or envisaged, for scale-up. Many incidents arise from exothermic and / or gas generating decomposition and oxidation (self-heating) where the thermal stability limits of process materials and products are not understood, or respected. For materials that may be handled as powders, irrespective of whether events are detected in the DSC or DTA screening tests, some additional powder thermal stability testing is strongly recommended since oxidative events may occur which are not observed in the contained and oxygen-starved environment of the smallscale DSC and DTA tests. The expertise to perform specific powder self-heating tests, or even to consider the necessity for such tests, is not common. For liquids (or solids with a melting point below maximum temperature of process T process (max) ), if it is seen that difference between onset temperature and safety factor (ΔT safety ) is significantly above T process (max), then further testing is not usually required the risk posed by the material is considered to be tolerable under the specific processing conditions. However, if this is not the case (i.e. if (To - ΔT safety ) < T process(max) ), then specialist adiabatic calorimetry is required to more accurately/reliably assess the thermal stability of the material. THERMAL STABILITY TESTS DEVELOPED FOR POWDERS Almost all pharmaceutical and fine chemical final products as well as other materials such as foodstuffs are powders. In most cases, these will be dried at elevated temperature in a potentially wide variety of drying equipment. Powders are generally produced with a small particle size to improve and facilitate handling and conveying. This small particle size produces a very large surface area to volume ratio which could accelerate oxidation. Although some drying takes place under an inert or vacuum atmosphere, other operations are performed in air where oxidative events may become possible. Even if the drying operation itself is performed under an inert atmosphere, discharge of hot powder, storage of hot / warm powder or inadvertent loss of the inert atmosphere during drying needs to be considered so that maximum safe exposure temperatures can be identified. This can only be achieved using the appropriate test methodology. In most cases, conservative drying temperatures are selected for drying operations. However, through appropriate tests, it may be possible to increase drying temperatures and safely achieve greater throughput and productivity. Most test methods devised specifically for powders are based on ensuring that the oxygen availability of the plant scale operation is reflected in the laboratory test. Table 1 highlights the appropriate test methods and plant procedures to which they would relate. Test Name Diffusion Cell Test Aerated Cell Test Air Over Layer Test Basket Tests Applicable Plant Situations Bulk drying / storage (up to 1 m 3 ) Rotary drying Slumped fluid bed or spray drier bed Tray drying (thick layers > 30 mm) Fluid bed drying Band drying Tray drying (thin layers < 30 mm) Spray / pneumatic conveying (flash) drying Layer deposits in / on equipment Bulk storage (any scale) Bulk drying (any scale) Table 1: Powder Test Methods* and Their Applicability *Abbott, J. A. Prevention of Fires and Explosions in Dryers: A User Guide, 2nd ed.; Institution of Chemical Engineers: Rugby, UK, 1990.

3 Figure 1: Strategy for Assessing Thermal Stability In all of these tests, an initial screening (temperature ramped) test is usually followed by a series of isothermal tests to provide optimum process temperature and process exposure times.

4 The results from the Diffusion cell and Aerated cell tests can be applied to powder masses of up to 1000 kg. Above this, other methods for extrapolating safe conditions are required, the most applicable of which is the Basket test method. In essence, Go/No Go (onset for self-heating) boundary conditions of the powder are closely defined in a range of special cubic baskets of different sizes (usually, 15 cm3, 125 cm3 and 1000 cm3). A plot of Log (volume/surface area) vs 1/temperature should provide a straight line which can be extrapolated to reasonably large bulks and any shape of container. This allows accurate prediction of maximum allowable storage temperature conditions. When studying any powder, potential large scale handling and drying conditions must be examined by specialists to identify the most appropriate test method. CONCLUSION Understanding the thermal stability of all processed products (raw materials, intermediates, final products, by-products and waste streams), particularly those exposed to elevated temperature during processing, is a pre-requisite for safe processing. The thermal limits of all materials must be assessed and understood such that prevention and mitigation of decomposition, self-reaction and oxidation (self-heating) can be assured. Although liquid materials can be adequately assessed through traditional thermal techniques such as DSC and DTA, these techniques often fail to diagnose self-heating phenomena for powders. Specific techniques for powder thermal stability assessment (such as the Diffusion Cell, Aerated Cell and Basket Tests) are available and can unveil startling information on oxidative self-heating. The test method selected for any study must be appropriate for the processing conditions in the plant. Most importantly, provision of appropriate tests for materials handled as powders should be made. When interpreting data from any test, there are a wide variety of factors which should be taken into account including: mode of test, ramp rate, sample mass, temperature range of tests, materials of construction of test cell and study of potential contaminants. Due to the complexity of the factors affecting the thermal stability test data, review of such data by an expert and guidance on the application of this data to the process conditions is highly recommended. For further information regarding testing for Chemical Reaction Hazard Assessments and evaluation and classification of energetic materials, please contact Swati Umbrajkar, Ph.D., Manager Chemical Process Evaluation Group at Tel: , Fax: , safety-usa@chilworthglobal.com, or you may also visit our website at:

5 SWATI UMBRAJKAR Swati Umbrajkar, Ph.D. is the Manager of the Chemical Process Evaluation Group. Dr. Umbrajkar received her Doctorate from the New Jersey Institute of Technology. Her research interests include the synthesis of metal/metal oxide nanocomposites; analysis of highly energetic materials using X-ray diffraction, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and a number of post analysis techniques to characterize the thermodynamic and kinetic parameters of a test system. Dr. Umbrajkar consults with clients on a variety of process safety issues including but not limited to high-pressure DSC cell tests, adiabatic calorimetry (ARC and ADC), reaction calorimetry (RC-1), all of which allow for the safe scale-up of batch and semi-batch processes. She has expertise in determining self-acceleration decomposition temperature (SADT) and time to maximum rate (TMR), which are critical issues associated with the storage of bulk materials. As the Manager and Consultant in the Chemical Process Evaluations Laboratory, she is proficient in the interpretation of data for a wide variety of process safety scenarios. She has authored several articles in the fields of, Synthesis and Analysis of Highly Energetic Materials and Chemical Process Safety. She received the Excellence Award from NASA for her services in NASA s SHARP student program in 2005 and Research Experience for Undergraduate (REU) students at the New Jersey Center for Engineered Particulates (NJCEP) in She was awarded the Best Graduate Student Research Overall presented at the Graduate Student Research Day on November 6, She is also the recipient of the best presenter award at the AIAA (American Institute of Aeronautics and Astronautics) Young Professionals in Science and Engineering Conference (Northeast Section) in November She is a member of the American Institute of Chemical Engineers. ABOUT DEKRA INSIGHT DEKRA Insight is the global leader in safety at work. We specialize in helping clients evolve both their organizational culture and their operational environment, empowering them to reduce injuries, save lives, protect assets and in the process, achieve higher performance. Our integrated solutions have been honed over decades and are proven to reduce risk and enhance organizational cultures: Safety strategy Building your roadmap for long-term safety improvement Culture & leadership Building high-performance teams Behavioral reliability Assuring unwavering execution of safety systems and processes Governance & capabilities Providing the framework for safety execution and results Safety Resource Optimization Putting your resources to work for safety Management Systems Developing and aligning the systems that drive safety excellence Data Analytics & Metrics Information and insight that drive results Process Safety Lab Testing Precise data, analysis and tools for process safety decision and action Process Safety Engineering Engineering and advice for process safety excellence everywhere DEKRA Insight represents the collective expertise of our legacy businesses and partners, each an institution in safety: BST, Chilworth, Optimus Seventh Generation, RCI Safety, RoundTheClock Resources, and Russell Consulting. To contact us: > France : info-fr@chilworthglobal.com > Netherlands : info-nl@chilworthglobal.com > India : info-in@chilworthglobal.com > Italy : info-it@chilworthglobal.com > Germany : exam-info@dekra.com > Spain : info-es@chilworthglobal.com > UK : info-uk@chilworthglobal.com > USA : safety-usa@chilworthglobal.com > China : info-cn@chilworthglobal.com > Wallonia : info-be@chilworthglobal.com 2016 DEKRA Insight. All rights reserved. All trademarks are owned by DEKRA Insight, reg. U.S. Pat. & Tm. Off.; reg. OHIM and other countries as listed on our website.