Powder Handling: Make the Most of Flow Additives

Table 1. Three commercially available magnesium stearate products underwent testing.

Used effectively, flow additives can substantially increase manufacturing efficiency by maintaining consistent flow through the process and preventing unplanned shutdowns due to machine blockages. They also can impart superior performance to a finished product, either directly, e.g., better flow properties for a fine milk powder, or indirectly, such as consistent tablet weight resulting from a smoothly operating tableting press. To select the most-appropriate flow additive, powder processors must consider a wide range of variables. In some applications, very specific end-product requirements may dictate choice. For instance, certain grades of hydrophobic silica aren't permitted in food applications — so, in such cases, choosing an appropriate additive relies on a detailed understanding of final product use and the regulatory framework that governs it.However, a processor also should take many other factors into consideration. Indeed, making the optimum choice requires fully determining how an additive will affect the manufacturing process and influence critical quality attributes of the final product. While most flow additives are selected on the basis of their ability to improve flow, the question remains as to what constitutes "improved flow" in a given process. Furthermore, an additive simultaneously and unintentionally may substantially affect a range of other powder properties that contribute to process performance. Compressibility, permeability, response to consolidation and, indeed, the ability to aerate, all may change with the inclusion of just small quantities of flow additive; these potentially may impact, possibly detrimentally, many aspects of behavior in a process. Therefore, optimizing the use of flow additives requires a comprehensive understanding of exactly how the additive will influence both process performance and product quality. MULTIVARIATE POWDER ANALYSISIt is helpful to consider powders as multicomponent systems comprising solids (the particles), gases (air entrained between the particles) and water, often in the form of moisture. Powder behavior depends upon complex interactions between these components as well as external variables, such as the environmental conditions experienced during processing. (For specifics on the impact of moisture, see: "Optimize Humidity for Effective Powder Handling.") Individual processes subject powders to various stresses and flow regimes, making it important to identify the properties that dictate performance in any specific operation. For example, the low-stress dynamic conditions that prevail in a fluidized bed differ dramatically from the high-stress static conditions imposed on a powder stored under its own weight in a hopper. Therefore, a unique set of powder properties will define performance in each case. Comprehensively defining how a flow additive will influence powder behavior consequently requires a multivariate approach to powder analysis. Dynamic powder properties directly quantify flowability and have proven application in optimizing manufacturing practices. Dynamic measurement involves rotating a precision-engineered blade through an accurately controlled volume of powder along a prescribed path. Measuring the axial and rotational forces acting on the powder determine its resistance to the motion of the blade. The resulting data then are used to generate flow parameters such as basic flowability energy (BFE) and specific energy (SE). Dynamic properties can be measured for powders in consolidated, moderate-stress, aerated or even fluidized states, allowing the generation of data that directly relate to a specific process. Equally important, the repeatability, reproducibility and sensitivity of dynamic measurement enable the user to identify and quantify even subtle differences in powder behavior, making the technique a valuable tool for detailed flow additive studies.

Figure 1. All three flow additives cut the microcrystalline cellulose's SE by more than half to a value that remains uniform across a range of low concentrations.

Universal powder testers, such as the FT4 powder rheometer, complement dynamic powder testing with other valuable methods such as bulk property measurement and shear analysis. As a result, they can provide the multifaceted approach to powder characterization required for success with flow additives.FACTORS TO CONSIDERWhile the need for a flow additive may be clear, the right additive isn't always immediately evident. Incorporating a flow additive requires careful consideration of a number of factors, including:• which additive to use, including its specification and grade;• the amount of additive necessary for optimal effect; and• whether the resulting blend will perform as needed under specific process conditions.Considering each of these points in turn helps provide guidance for the development of an optimal blend.Selecting the type and grade of additive. Many commercially available flow additives come in different chemical forms or in various grades. At a minimum, there's usually a choice of suppliers. Therefore, it's important to assess whether all available versions of the chosen additive deliver identical performance. Table 1 provides details of three commercially available magnesium stearate (MgSt) products that can serve as flow additives.Figure 1 shows the impact of each type of MgSt on the SE of a commercially available microcrystalline cellulose (Avicel PH-101). SE measures the ease with which a powder flows in an unconfined low-stress state. The three grades all have a similar impact, decreasing the SE of Avicel PH-101 by over 50%, from approximately 9.4 to 4.5 mJ/g. However, the three grades differently affect the aerated energy (AE) of Avicel PH-101 (Figure 2). The AE of a powder quantifies the flow energy as air flows through the sample and is a reliable indicator of the absolute levels of cohesion in the powder. Figure 2 shows the dihydrate grade results in very different performance compared with the monohydrate and Stear-o-Wet.

Figure 2. The AE of the cellulose depends upon the grade of MgSt employed and shows a nonlinear response with respect to concentration.

In general, flow additives consist of very fine particles that coat the relatively larger particles of the substrate in a blend, lubricating interparticulate movement in a manner analogous to ball bearings. SE primarily depends upon interparticulate friction and mechanical interlocking, which typically is a consequence of rough or irregularly shaped particles. The results presented here suggest all three grades comparably impact these two aspects of flowability.In contrast, AE provides a measure of the strength of the cohesive bonds between particles of the powder. While the additive lubricates physical interactions between substrate particles, it simultaneously might increase cohesive bond strength if the agglomerates of Avicel/MgSt themselves are attracted to one another in low-stress conditions. The AE results suggest these different grades generate agglomerates of varying cohesive strength, which in turn may reflect their intrinsic cohesivity.Establishing the optimal concentration of additive. Flow additives often impart functionality at relatively low concentrations. While many people assume that more flow additive will equate to better flow properties, this assumption frequently is wrong — making it important to assess thoroughly how the performance of a blend varies with flow additive concentration. The data shown in Figure 1 indicate that with this system SE effectively is independent of MgSt concentration and grade at low levels. The amount of additive used, above the threshold value applied in these tests of 0.1% w/w, has no appreciable effect. However, Figure 2 indicates AE clearly depends upon additive concentration. The AE of the Avicel PH-101 blended with monohydrate or Stear-O-Wet rises steadily with concentration while the analogous curve for the dihydrate grade passes through a maximum. As far as AE is concerned, it's likely that cohesivity in the overall system is affected by two factors: the relative strength of substrate-additive and additive-additive cohesivity; and the extent to which the additive distributes over the substrate as concentration increases. It's impossible to quantify the individual impact and the combined effects of these factors but the data clearly support the previous observation that increasing flow additive content doesn't necessarily improve all flow properties. If a process heavily depends upon the aeration properties of a blend, then these data suggest the introduction of a flow additive isn't always beneficial or the results easily predicted.

Figure 3. MgSt, regardless of grade, increases the WFA of sorbitol but decreases the WFA of the microcrystalline cellulose.

Ensuring the formulated blend is well suited to processing conditions. While the preceding issues focus very much on the flow additive — grade and concentration — it's important to recognize that each blend for each application presents a unique optimization task. Changes induced by an additive will depend upon the substrate to which it is being added. Furthermore, the additive may alter a number of aspects of powder behavior, not just flowability. Understanding all possible effects is crucial. The data shown in Figures 3 and 4 for wall friction angle (WFA) and AE, respectively, illustrate this point well. These tests used a fixed concentration of MgSt, 0.3 % w/w, on two commonly used excipients, Avicel PH-101 (which has a nominal particle size of 50 µm) and C*Sorbidex (sorbitol with a nominal particle size of 190–200 µm).The shear property WFA routinely is measured to support hopper design activities. However, more generally, it indicates how easily a powder moves across a given surface. In this case, WFA was measured using a type-316L stainless steel test coupon with a surface finish of 1.2 µm Ra; so the results show to what extent the MgSt changes interactions between the individual substrates and the grade of stainless steel. The first conclusion that can be drawn from the WFA results (Figure 3) is that all three additive grades appear to influence each substrate in a similar way. All three increase the WFA of sorbitol and decrease the WFA of Avicel PH-101. A rise in WFA indicates greater resistance between the substrate and equipment surface. This means that inclusion of MgSt detrimentally impacts sorbitol if the aim is to ease movement across a stainless steel surface. In contrast, MgSt lowers the WFA for Avicel PH-101, suggesting it might successfully serve as a lubricant, for example, in a tableting process. This result is interesting because it not only shows the effects of MgSt aren't consistent for each substrate but also that the additive doesn't deliver the expected lubrication for the sorbitol/stainless-steel combination.The results for AE (Figure 4) show a very different response. The inclusion of 0.3% MgSt raises the AE of the Avicel PH-101 but lowers the AE of the sorbitol. The two substrates again respond differently to the inclusion of the additives. However, in this case, the changes observed aren't uniform for all grades of MgSt. The dihydrate more significantly affects the flow properties of both substrates than the other grades, halving the AE of sorbitol and doubling that of the Avicel PH-101.

Figure 4. The MgSt raises the AE of the cellulose but lowers the AE of the sorbitol, with dihydrate inducing a more pronounced change than other grades.

The parameters presented here — WFA and AE — reflect behavior in different processing environments. Therefore, the contrasting results demonstrate the importance of measuring properties that accurately reflect a specific aspect of a process or application. MgSt used with Avicel PH-101 should ease flow at certain concentrations and provide lubrication in stainless steel equipment. However, there's also a risk of increasing overall cohesivity in the bulk, which could have implications, for example, in filling applications. In contrast, MgSt isn't necessarily an ideal lubricant for sorbitol contacting stainless steel but can reduce overall cohesivity within the bulk powder.OPTIMIZE ADDITIVE USEChoosing the most-appropriate flow additive to resolve a processing issue and incorporating it at an optimal level require a comprehensive approach to powder analysis. Testing must accurately reflect the impact of the additive on the specific substrate being used and represent conditions that directly relate to the manufacturing process or final application. Dynamic powder testing, in combination with shear and bulk property measurements, is a proven approach to addressing this type of complex powder-handling challenge. The experimental data presented here demonstrate the capability of such powder characterization tools in optimizing the application of flow additives.

BRIAN ARMSTRONG is a powder technologist and JAMIE CLAYTON is operations director for Freeman Technology, Tewkesbury, U.K. E-mail them at [email protected] and [email protected].