Pictures of an aerosol can being tested for droplet size distribution by laser diffraction under normal test conditions. Left image is with room lights on, right is with room lights off to visually enhance the laser path and measurement zone. January 2017 Spray 33 is. If a graphical shoulder is seen on the small droplet size side, it indicates the product is producing fine droplets at a higher rate than larger droplets. This would likely give a drier feel to the spray. If a graphical shoulder is seen on the large droplet size side, it often indicates incomplete atomization. This shoulder means the product is producing more large droplets and will likely give a wetter feel to the spray. The impact of atomization on the product use should be paired with sensory goals and overall product usage to ensure the best match of formulation and dispensing is met. The second curve produced is called the Undersize or Cumulative distribution curve. This curve is the integral of the frequency curve and traces the distribution from 0% to 100% of the spray. The most common use for this curve is in overlaying multiple tests to ensure consistent dispensing. While the frequency curve can also be used in overlaying results, the Undersize curve often provides a clearer picture, as there is a simple S-shaped curve displayed for each sample. As the S-curve moves to the left of the graph, the droplet size becomes smaller; as it moves to the right of the graph, it becomes larger. From a comparison between different packaging solutions standpoints, this is the most efficient way to observe an impact, if it is present. The final discussion point of graphical reporting relates expectation of results and the frequency graph. In this, if a distinctly bimodal distribution is reported when it is unexpected, the methodology used may need to be addressed. Most products are not intended to dispense two different droplet sizes, so obtaining such a result should raise questions when looking at reports. It is important to note the numerical values discussed previously are all derived from the graphs discussed here. Key testing methodology concepts Above, the values and graphs used to quantify product performance and safety were discussed. It is important to note that these values must be used only once a quality method has been developed and investigated for the product under examination. Unfortunately, there is no single method that will work for all delivery devices and formulations. While there are commonalities in testing all sprays, there should be some methodology investigation for each product to ensure the best results are generated. The first, and simplest, common theme to testing is related to ensuring the testing is fit for product use. The product in question has a desired usage for the consumer. This may involve both shaking and dispensing at a distance. As the goal of testing is to pair the testing with sensory impact, the product under examination should be tested under conditions likely experienced by the consumer. This allows a straightfoward approach to tying the test data to the consumer reaction. A second important concept relates to the testing of the device and how to quantify results. Sprays evolve and travel through the measurement zone (laser beam). This indicates a time-related result is present based on the spray and its evolution. As the testing instruments provide triggers, the concept for best practices is based primarily on how long the spray is measured for and usually ensuring the entire spray is included in the final reporting (there may be some desire to isolate product initiation and shut down droplet size measurements in specific applications). Making sure there is consistency in timing for the spray measurement is important from one test to the next to obtain the best reproducibility in data. The third theme to testing relates to a phenomenon called “beam steering.” Beam steering is caused by propellants involved in aerosol dispensing. As the system uses a laser to measure the light scattered by the droplets, the laser is aligned to a detection system very specifically. As the spray occurs, the laser measures scattered light of everything that is sent through it. If not accounted for, the propellant can cause an apparent population of large droplets that are not actually being dispensed. This artifact must be eliminated by processing existing data or setting the test method up to automatically eliminate it. The beam steering phenomenon is present in aerosol testing but it is not necessarily the exact same impact on each sample. For that reason, each new product or packaging solution should have some investigation to ensure the testing method is ideal. The final theme to best practices relates to cleanliness of the measurement system. As stated above, the system operates with a laser and a detection field. The instruments record a background measurement prior to the test. This background should be looked at to ensure the system is clean and there is no material being sprayed through the system during this measurement and there is no material that has deposited onto the windows on either side of the instrument. If there is found to be a polluted background, the system windows can be cleaned as one would clean their eyeglasses. Only once the system is clean (as dictated by the background measurement) can quality data be acquired. Summarizing droplet size testing by laser diffraction Laser diffraction is a quick and easy method used to characterize droplet size distribution in sprayed products of all types. Aerosol products provide a few critical testing methodology concepts that must be dealt with to ensure proper testing. Once methodology is optimized for a specific product, initial data review should concentrate initially on graphical evidence of how the spray is developed. Only once best lab practices are used and the results are understood on a graphical basis should the values discussed above be investigated and specifications be put into place. By following this experimental flow, the user can help minimize time consuming work related to inconsistent results caused by inefficient testing methodology and technique. When utilized and understood properly, laser diffraction droplet size testing can help tie consumer sensory response to specific data parameters that can help develop new products, improve existing products and ensure product and packaging are fit for the markets intended. Spray
Spray January 2017
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