POWDER PROPERTIES LABORATORY

Transcription

1 Ground Rules POWDER PROPERTIES LABORATORY You will work as a team of no more than 6 students. At the end of this laboratory session each team will turn in a single report. The report will be reviewed, and all members of the team will be given the same grade for completing the assignment if it is acceptable. It is therefore essential that you cooperate with each other. Your team report is to be turned in before you leave the laboratory. All the data should be reported, calculations shown, graphs drawn and questions answered. The final report should have a coversheet with the printed name of each team member and their signature. If a signature is missing no credit will be awarded to that person. As a pharmacist you are expected to be able to communicate in writing. Accordingly, an organized, legible and thorough report will receive a higher grade than a disorganized, scruffy or cursory one. Similarly, pharmacies are expected to be neat, tidy and have a professional appearance. The same is true of your bench and shared areas, particularly after the laboratory session is over. Do not ignore this hint! Part I: SIEVE ANALYSIS INTRODUCTION Dry sieving allows the fractionation of relatively coarse powders and granules. Sieves are stacked ("nested") with the largest apertures at the top and the smallest at the bottom (Figure 1). A sample of powder is placed on the top sieve and shaken for a fixed period of time at a given amplitude and pulse frequency. The weight of powder on each sieve can then be calculated and the particle size distribution obtained. Particles must have a two dimensional profile smaller than the sieve aperture in order to pass through a particular sieve. A mean sieved diameter is calculated. Since the weight of particles on each sieve is determined the mean sieved diameter represents a mass distribution. The size of the apertures in each sieve are denoted by a mesh number. The mesh number is the number of wire strands (of constant diameter) per inch used to weave the square mesh pattern. The side length of the aperture in microns is inversely related to the mesh number. Figure 1 1

2 EXPERIMENTAL METHOD You will determine the particle size distribution of a sample of coarse powder using a nest of sieves shaken in a Sonic Sifter. A TA will instruct your group when it is your turn to use the equipment. 1. Using at least a three decimal place electronic balance, record the weight of each empty sieve and the collection pan. Also record the sieve size. 2. Arrange the sieves in a sequential nest: smallest mesh number (largest aperture) at the top, largest mesh number (smallest aperture) at the bottom. Add the collection pan to the bottom of the nest. 3. Add approximately 5 g of accurately weighed coarse powder to the top sieve, and cover with the rubber cap. 4. A TA will insert the nest of sieves into the Sonic Sifter. The sample will be shaken for 5 minutes with a sieve Amplitude greater than Reweigh each sieve and the collection pan. Calculate the weight and percentage of powder on each sieve and in the collection pan. Then calculate the cumulative weight percentage of powder that is finer than the aperture, see tables below 6. Obtain the sieve data from the TA on the fine powder samples. 7. Use the probability paper, last page, to calculate the mean diameter and standard deviation (If the TA suggest it you may want to calculate the geometric mean and standard deviation) for the coarse and fine powder particles. RESULTS TABLES Mesh Number Aperture Sieve Weight (g) Size (um) Empty (Initially) With Sieved Powder Pan --- 2

3 Aperture Amount of Powder in Range Size (um) Mass retained (g) % Finer than d TA s Data % Finer --- Total = Total = Total = QUESTION ON SIEVING 1. Determine the mean sieved diameter and standard deviation (normal or log normal, see TA for directions) of your powder sample and with the data supplied by the TA, using the probability paper given on the last page. 2. Would you expect to get the same mean sieved diameter if you performed the experiment described above, but made the following changes? Justify your answer. a. Increased the sieving time to 10 minutes. b. Decreased the sieve amplitude to setting Dry sieving is a useful technique for particles down to around ~25 µm. Why is it usually impossible to get a reliable particles size estimate with smaller particles by this method? 4. Describe as many limitations as you can think of for particle size determination by sieving. What types of particles could not be sized by sieving. 5. If a large percentage of powder were deposited on the top sieve or the bottom pan, is the particle size you determined representative of the powder sample? Justify your answer. 3

4 Part II: POWDER FLOW PROPERTIES During many pharmaceutical production processes it is necessary to transfer large quantities of powder from one location to another in a controlled manner. For example: 1 Powder blending 2 Powder filling into containers (e.g. dusting powders) 3 Powder flow into capsules 4 Powder filling into the dies of a tablet press Figure 2 One method of assessing flow properties is the Angle of Repose. Powder is allowed to flow freely through a funnel onto the center of an upturned petri dish of known radius (Figure 2). When the powder reaches the side of the petri dish the height of the cylindrical cone is determined. From the petri dish radius (r, cm) and cone height (h, cm) the angle of repose (between the petri dish and base of the powder cone) can be calculated. Flow rate can also be determined by measuring how fast a powder flows through an aperture. Free flowing powders exhibit a high flow rate and a smaller angle of repose. Angle of repose and flow rate depend on particle size, shape and surface roughness. Flow properties are frequently enhanced by the use of glidents. EXPERIMENTAL METHOD θ 1. Measure the external diameter of the petri dish supplied to you. Position the bottom of a funnel or paper cone about 5 to 15 cm above the center of the upturned petri dish using a ring stand. 2. Make sure there is a piece of paper under the petri dish so you can pick up the powder and reuse the powder for all your replicates. 3. Slowly pour the coarse powder sample into the funnel, tapping the funnel as necessary to ensure that powder flows through the hole. Continue this process until the bottom of the powder pile just begins to fall over the edge of the petri dish. Measure the height of the pile using a ruler. If the powder is lumpy, sieve it prior to beginning the experiment. 4. Repeat Step 2 until you consistently obtain the same answer. Calculate the mean height of the coarse powder pile and the mean angle of repose (θ). Remember: Tan θ = Opposite / Adjacent, therefore Tan θ = h / r. 5. Repeat Steps 2 and 3 using both fine powder and fine powder with glident. 4

5 RESULTS TABLES Sample Run # Height of Conical Powder Pile (cm) Coarse Powder Fine Powder Fine Powder with Glident Petri Dish Radius = cm Sample Coarse Powder Fine Powder Fine Powder and Glident a see below for definition Angle of Repose ( 0 ) (From Your Experiment) Carr Index a QUESTIONS ON FLOW AND ANGLE OF REPOSE 1. Plot angle of repose (X-axis) against Carr Index (Y-axis), see below. Is there any correlation between these two measures of powder flow? How about particle size 2. Name a typical glident used in tableting. What properties would you expect this material to have? 5

6 3. Why is granulation (the process of producing larger aggregated particles from smaller individual particles) important in tableting? 4. If a batch of granules for compression showed unusually poor flow properties, how would you expect this to affect the uniformity of tablet weight? Part III: REAL, TAPPED and BULK DENSITY The true density (ρ t ) of a powder sample is the weight per unit volume of the material with no air spaces between particles. Therefore, if a material has a true density of 1 g cm -3, 100 g of material will occupy 100 ml assuming individual particles fit together exactly. In practice most powders do not fit together very well. Therefore, if one fills a graduated cylinder to 100 ml with a powder, the weight of powder required may only be 70 g. This apparent density is known as the bulk or expanded density (ρ b, 0.7 g cm -3 ). If the100 ml cylinder is then tapped, the particles slide past each other and become consolidated. The 70 g of particles which once occupied 100 ml may now only occupy 80 ml. They have an apparent packed or tapped density (ρ p ) of g cm -3. Carr's index is a measure of interparticulate forces. If the interparticulate forces are high, powders will have a low bulk density because bridging will occur between particles. This results in a large Carr's index and a large change in volume caused by tapping. If the interparticulate forces are low, particles will have little affinity for one another, and will compact spontaneously. Under these circumstances, Carr's index is small and little change in apparent density is induced by tapping. ρ p - ρ b Carr's Index = ρ p Porosity is the volume ratio occupied by air spaces (voids) between particles of a powder sample. ρ p Porosity = (Packed) and (Expanded) ρ t ρ t ρ b 6

7 EXPERIMENTAL METHOD 1. The true density of powder is 1.6 g cm Determine the weight of a 100 ml graduated cylinder (supplied by the TA's). Without tapping, use a powder funnel to fill the cylinder to 100 ml with coarse powder. Record the weight of the cylinder and powder. 3. Give your filled cylinder to a TA for tapping using an automated tap density apparatus. Record the volume occupied by the sample after 100 taps. 4. Repeat Steps 1 and 2 using the fine powder sample. RESULTS TABLE Sample Coarse Powder Weight of 100 ml Graduated Cylinder (g) Empty Filled to 100 ml Volume occupied by powder after 100 taps (ml) Fine Powder Sample Coarse Powder Bulk Density (g cm -3 ) Packed Density (g cm -3 ) Porosity Carr's Index Bulk Packed Fine Powder 7

8 Questions on REAL, TAPPED and BULK DENSITY 1. A granulation has been prepared with a bulk density of 0.73 g cm -3. If the granulation is tableted with 10 mm diameter, flat faced tooling (circular), and the lower punch drops to a depth of 8 mm in the die cavity, what will be the theoretical weight of the resulting tablet? 2. Give reasons why the actual tablet weight might deviate from the theoretical weight. Probability Paper

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