Figure 2. Properties that affect the filling of a die.


Shear testing, a more traditional technique, determines the shear behaviour of consolidated powders. Properties measured include cohesion, unconfined yield strength and internal angle of friction. Shear testing measures the onset of flow, the transition from the static to dynamic state and is especially useful for understanding how powders behave in hoppers. It also adds value during the selection of an appropriate material of construction for process equipment by quantifying friction and adhesion between the powder and the contact surface.


Bulk properties include key design parameters such as bulk density, permeability and compressibility. Bulk density can be measured on conditioned or consolidated powders; permeability and compressibility are both measured as a function of applied normal stress. Each bulk property complements shear and dynamic data by providing insight that can be used to rationalise observed behavioural trends. In combination these techniques present a powerful portfolio for process related studies, for formulators, designers and manufacturers.


USING POWDER PROPERTIES


For formulators, powder testers generate a set of descriptive parameters for each candidate formulation. One way of using these data is to consider them within the context of the processing environment. For example, during die filling a powder flows under gravity induced, relatively low stress conditions. Comparing appropriate flow energy measurements for different formulations, Aerated Energy data, will therefore suggest a ranking for their relative success in this step. During storage on the other hand a powder may be subject to higher stresses; consolidation both by direct compression and vibration is a possibility. Here measuring the changes in powder flowability that arise from de-aeration and consolidation is illuminating.


This approach is productive but requires an understanding of the conditions to which the powder is exposed in the process, so it is often easier to use measured properties to rationalise process experience. Quantifying the properties that dominate performance in a given unit operation provides the information


Figure 3. Using powder properties to rationalise experience.


needed to develop new formulations with intrinsically better characteristics for that application. In this way manufacturing experience, and pilot plant observations, can be presented so as to guide formulation. Evaluating the impact of decisions taken early on in the project, on process design and on manufacturing, becomes more straightforward.


Returning to the example of tableting, analysis of the process suggests that for the die filling operation, cohesion, mechanical friction and interlocking, and permeability will all be important factors (see Figure 2). Ensuring these properties are optimised will result in consistent flow of the blend into the die and the timely release of entrained air, ensuring complete filling. The question is what are the ‘optimum’ values in this case?


Within the operational environment there is often knowledge of which formulations tablet well and which do not. Analysis of these samples forms a database that can be mined to define the properties of an easily processed blend. Formulation A may process easily to high product quality, B may process, with care, at an acceptable rate and with satisfactory yield, while C may give rise to plant stoppages and an inconsistent product. Measuring these formulations defines upper and lower limits of acceptability for process dominating properties. It becomes clear that aerated energy (a measure of cohesion) should lie between AE1 and AE2, specific energy (a measure of mechanical friction /interlocking) should lie between SE1 and SE2 and permeability must be in the range P1 to P2. The multiple benefits of this methodology are fully illustrated in Figure 3. An analogous approach is relevant during more systematic pilot scale studies.


Figure 4 shows a plot that could be easily generated during formulation studies. Two base systems are being blended with a flow additive to improve flow properties. The shape of the flow energy - additive concentration curve is, in part, dependent on the surface roughness of the particles. With this data alone, the formulator can see how to reduce flow energy i.e. make the blend more free-flowing and where the optimum concentration lies.


Figure 4. The impact of flow additive concentration on flow properties.


MOVING FORWARD


QbD demands more detailed consideration of the manufacturing process much earlier in the development cycle. Using appropriate powder properties to rationalise existing manufacturing experience of different blends defines the ‘design space’ for specific unit operations.


It identifies those properties that dictate performance and quantifies upper and lower limits for acceptable processability. Pilot scale experience is equally valuable, with more scope for experimentation.


Injecting this understanding into the formulation process makes it easier to tailor properties towards successful manufacture. It simultaneously highlights areas where the process design will be especially critical. Such an approach therefore brings together the often discrete functions of formulation, design and manufacture promoting a more holistic development workflow that addresses some of the challenges of implementing QbD.


Modern powder testing instrumentation has moved on from single number measurement to offer the industry the comprehensive portfolio of techniques today’s challenges demand. Dynamic, shear and bulk property measurement are a complementary and productive trio for process-related investigation in support of QbD goals. The intelligent use of these techniques will help the industry to more efficiently develop processes that work reliably and economically throughout the lifetime of the product.


References


[1] Freeman, R.F. (2007) Measuring the flow properties of consolidated, conditioned and aerated powders - A comparative study using a powder rheometer and a rotational shear cell. Powder Technology, Vol. 174, Issues 1-2, Pages 25-33.


Contamination Control Solutions Reduce the Carbon Footprint of Cleanrooms


Pressure to reduce energy usage and carbon footprints together with eliminating waste, are now targets for many organisations and Dycem is no exception. Dycem contamination control surfaces are designed to be fully reusable and have a life expectancy of between three and five years. They combine a unique polymeric compound and advanced processing technology to create a smooth, washable surface that can be cleaned with a damp mop and squeegeed dry, leaving the floor covering immediately ready for further use, without loss of performance in the capacity to trap contaminants. The alternative products to Dycem contamination control surfaces are peel-off mats. These mats are layered, adhesive coated, polyethylene sheets that are ripped up when dirty and then disposed of as consumer waste.


In a paper by Gerry Prout of Kennet Bioservices, comparison of the use over a two-year period of Dycem flooring and peel-off mats is reported. Dycem flooring uses 18 tonnes less of raw materials, saves over 3 million MJ of energy in manufacture and use, while reducing emission of greenhouse gases by over 120 tonnes. A single peel-off mat can produce a ball of waste 20cm (8”) in diameter that is disposed of directly to landfill sites. Deborah Hoffet, QA manager and microbiologist at the Rogosin Institute’s Xenia Division in Ohio, USA, swapped the traditional disposable peel-off mats for Dycem flooring as part of a larger green initiative and now finds that Dycem flooring has proven to be ‘substantially more effective at retaining the particulates, and it can be washed and lasts as long as five years’.


Both suppliers and end users now have a responsibility to consider green initiatives and environmentally friendly options. As Dycem Managing Director, Mark Dalziel, said: “It is vital for manufacturers of disposable products to pay attention to the environment and at Dycem, our design team has this in their briefs for new and improved solutions for our users.” At the end of their working life Dycem flooring and mats are 85% recyclable and do not contribute to landfill waste.


Circle no. 68


Laboratory Products Focus


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