Light scattering techniques (static and dynamic) are commonly used to analyze the particle size distributions (PSDs) of polymer emulsions. Both techniques need specialized mathematical algorithms to invert either the scattered light time profiles or the scattered light patterns to obtain the PSDs. As a result of the ensemble nature of these techniques, they are low in resolution and sensitivity making them vulnerable to artifacts and instabilities. This article describes the ability of the Nicomp algorithm - employed only in the Nicomp 380 - to provide accurate and realistic PSDs for emulsion samples that have comparatively wide distributions.
Limitations of Light Scattering Techniques
Analyzing emulsions is a challenging task for light scattering techniques. In general, the production objective is to create an emulsion of a specific mean diameter and a relatively narrow width or polydispersity. Light scattering techniques can quickly and accurately measure the mean diameter, except for emulsions that have significant amounts of large particles. These large particles tend to ‘jerk’ the distribution to larger mean diameters, leading to inaccurate results.
The size of nearly 99% of the solids fraction has been reduced by the homogenization process. However, the emulsion still has 1-2% of the solids fraction in particles larger than 1µm. It has been demonstrated by numerous studies that such a small percentage of solids fraction can still lead to stability problems. Nevertheless, the amount of material in the tail is not adequate to be detected by light scattering techniques.
Capabilities of Single Particle Optical Sizing (SPOS) Technique
SPOS is a powerful and highly sensitive particle sizing technique capable of detecting trace amounts of large particles. This technique is utilized by the AccuSizer 780, which can provide more accurate and well-defined results for the largest globules in the emulsion. These globules are intimately related to emulsion safety and stability.
SPOS is a single particle counter capable of counting particles down to 0.5µm. The AccuSizer 780 is ideal for applications demanding the quantification of the coarse particle tail of a primarily sub-micron distribution. Unlike light scattering ensemble techniques, the AccuSizer 780 does not assume the shape of a PSD.
The SPOS technique constructs the PSD by counting and sizing hundreds of thousands of particles and placing them into very narrow size bins. It can handle broad distributions as it is not an ensemble technique. Furthermore, no artifacts are generated as the distribution is not computed.
Figure 1a consists of the Volume-Weighted PSD acquired by the Nicomp 380 on a PVD emulsion, showing a single Gaussian peak with a mean diameter of roughly 0.68µm and width of 18%. Figure 1b consists of the Volume-Weighted results from the same sample acquired by the AccuSizer 780.
Figure 1. a. Volume-Weighted PSD of PVC Emulsion from Nicomp 380; b. Volume-Weighted PSD of PVC Emulsion from Accusizer 780.
The results obtained from the 380 and 780 Accusizer demonstrate the mean diameter of the main peak. The main peak produced by the 380 was slightly broadened due to the inferior resolution of the 380 model. Conversely, the 780 model detected particles as large as 30µm, which were not detected by the 380 model owing to the low sensitivity inherent in light scattering methods.
Figure 2 shows the results obtained from an unstable emulsion with a broad distribution. Figure 2a consists of the Volume-Weighted 380 results obtained from the second emulsion. The analysis generated two peaks, one at 0.27µm and a second reading at 1.3µm. The first peak can be identified with the emulsion peak, while the second peak is a proxy peak for the aggregate tail. Such a distribution can pose challenges for an ensemble measurement.
Figure 2. a. Volume-Weighted PSD of Broad PVC Emulsion from Nicomp 380; b. Volume-Weighted PSD of Broad PVC Emulsion from Accusizer 780.
The larger particles have the tendency to obscure or shift the contribution from the main peak. Nonetheless, the Nicomp algorithm can ‘separate out’ the contribution from the larger particles, thereby allowing the stabilization of the main peak to the correct mean diameter. The analysis yields a mean diameter that is in correlation with the expected value. Figure 2b consists of the Volume-weighted PSD acquired by the AccuSizer 780. The true particle size is represented by the second peak observed by the 380 model, further validating the accuracy of the Nicomp algorithm and its ability to deal with complex distributions.
The results clearly demonstrate the ability of the Nicomp 380 model to accurately size both narrow and polydispersed PVC emulsions using the patented Nicomp fitting algorithm. The Nicomp 380 model was able to correctly determine the two modes present in the polydispersed emulsion. Nevertheless, only the single particle counter ‘AccuSizer 780’ was able to determine the largest particles present.
This information has been sourced, reviewed and adapted from materials provided by Entegris
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