Med-Tech Innovation Materials analysis


I make sure I keep it consistent? Virtually all routes for creating a shaped or coated end product from powder involve a “powder in liquid” suspension and this is true for most industrial sectors, not just medical devices. Often the suspension stage arises early on in the production cycle, meaning that failure to optimise at this point has a knock-on effect for yields and quality in intermediate as well as final products. For example, inorganic spray-dried granulates, which can be used as a coating on implants, are created by using a nozzle or rotating disc to create droplets from suspensions, a process called atomisation. The droplets are immediately dried to create granulate. It is important to control the rheology of the suspension prior to atomisation. Often, a high solids content suspension with a low viscosity is desirable. This calls for the use of surfactants, with zeta potential (or powder surface charge) measurements providing essential parallel information to rheology. Failure to use this information to optimise the suspension can lead to problems such as spray drier nozzle blockage, inappropriate granulate size or hollow granulates that perform badly in plasma spraying.


Q3: How can the powders used in my manufacturing processes be characterised and what can that information be used for?


By gaining a better understanding of powder characteristics medical device manufacturers can agree a robust powder specification with their suppliers. Characterisation of powders can be divided into two broad areas: techniques that tell you specifically about the powder and techniques that tell you about how the powder performs during subsequent processing.


A measurement that is often overlooked is determination of the shape of the particle. Particle size analysis often uses mathematics to define a “sphere equivalent” diameter, but exact knowledge of the shape can provide useful correlations to particle packing and flow. Various imaging approaches to determine particle shape are available. In one approach, 2D images of tumbling particles are collected and measured for deviation from classic basic shapes (spheres, needles etc.). Putting the above two analyses together, a graph of shape versus particle size can be particularly informative. Figure 1 relates to two powder samples with comparable particle size (as measured by laser diffraction). The graph shows that, in terms of particle shape, there are significant differences. Powder 2 exhibits a higher level of sphericity in the coarser fraction and this was shown to correlate strongly to the final sintered microstructure and mechanical properties.


Other useful techniques to characterise powders


include: • density and surface area analysis to check for open and closed porosity in powder particles


• X-ray fluorescence spectrometry (XRF) for bulk chemical analysis in inorganic species


• inductively coupled plasma spectroscopy (ICP)/atomic adsorption spectroscopy (AA) for detection of impurities down to ppb levels


• Fourier transform infrared analysis (FTIR), this can provide information on organic and inorganic species present


• nuclear magnetic resonance (NMR) for detailed organic analysis


• gel permeability chromatography (GPC) to determine molecular weight for organic polymer powder species. In terms of tests for on-going processing, powder flow is a useful measurement. As indicated above, this is relevant to how a powder will flow and pack in a die (prior to pressing a shape). It can also inform behaviour during transport and during storage in silos. Flow measurement techniques range from simple tests such as angle of repose (literally what angle exists in powders poured from a fixed height onto a surface) through to dry powder rheometers featuring torque measurements made as a rotating spindle is plunged into a powder bed. Other tests tend to be mechanical, for example


Two powder samples with


comparable particle size, but significant differences in particle shape


Taking characterisation of the powder itself first, it is


probably fair to say that particle size is one of the most important properties measured, with laser diffraction being a common approach to particle sizing. A standard mistake, however, is to rely on the so-called “D50” measurement (the particle size at which 50 volume% of the powder has a lower particle size). In reality, the particle size distribution is critical; for example, two powders with an identical D50 can show contrasting flow behaviour (such as into a die prior to pressing) with the presence of fines often retarding good flow.


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compressive or flexural strength of bars pressed from a feedstock powder, with performance often related back to the properties of the initial powders. Factorial experimental design can be used to determine trends between powder properties and process variables. This can be used to improve quality control tests, associated pass/fail limits and, ultimately, final product yields and properties.


Phil Jackson is Business Development Manager, Medical Materials,


at Ceram, Queens Road, Penkhull, Stoke-on- Trent ST4 7LQ, UK, tel. +44 (0)1782 764 428 e-mail: enquiries@ceram.com www.ceram.com/medical


September/October 2013 ¦ 35


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