CHARACTERIZATION OF PELLETS 1 Size:

2.4.1.1 Sieve analysis

A nest o f British Standard sieves were used to size the pellets. The series followed a V2 progression with mesh apertures from 0.5 mm upto 4 mm, and these were placed

in ascending order on top o f a base sieve. After placing a weighed quantity o f pellets on the top sieve, the sieve nest was mechanically agitated for 10 minutes using an Endecotts test sieve shaker [Endecotts Ltd, London, UK].

The 1 .0 - 1 .4mm sieve fraction o f pellets was collected for use in all further work for the duration o f this project.

2.4.1.2 Ring gap sizing

The instrument used was as described by Nystrom and Stanley-Wood [1976] (see section 1.2). However, in this study the material was fed manually, ensuring that individual particles were delivered onto the sizing table. The sizing table was mounted on an electromagnetic vibrator [Vibro EM-03, Dynapac AB, Sweden] imparting a vertical vibration o f frequency 6000/min. The voltage applied controls the amplitude o f vibration ; the optimum setting was found to be at 140V and thus all subsequent experiments were carried out at this voltage.

A 4 gram sample o f pellets from each batch was measured and repeated. During the process the sample was separated into collection tubes, each with a size range o f its own according to its position around the sizing table, and then each fraction was weighed (± O.Olg).

Studies were carried out on both uncoated and final coated pellets (thickest coat 5). The size distributions o f each batch were plotted on graphs o f mean minimum dimension (thickness) in microns against cumulative percentage undersize. From these graphs the interquartile range and the median value o f thickness were obtained.

2.4.1.3 Image analysis:

2.4.1.3.1 Dimensions of pellets

The image analysis system as described in section 2.4.2.4 was used to obtain measurements o f length, breadth and thickness for 30 pellets randomly selected from each batch (for both uncoated and final coated (coat 5) pellets). This was achieved by attaching the pellets to a cubic holder and measuring length and breadth from one profile, following which the holder was rotated through 90° and length and breadth were measured again. The lowest value was then taken as the thickness o f the particle.

2.4.1.3.2 Coat thickness

Coated pellets were photographed in cross-section using an epifluorescence microscope [Axioplan, Zeiss, Germany]. The cross-sections were made by hand using a simple micro-lathe under a microscope. The photographs were taken at a magnification o f 200x with a lens o f 0.5 numerical aperture, on 160T colour film. A scale was also photographed under the same conditions. This scale was then placed under the camera o f the image analysis system described in section 2.4.2.3 for calibration purposes, and the negatives were used for measurements o f coat thickness. Three different samples per batch o f each coat thickness were looked at, and five measurements were taken from each sample.

2.4.2 Shape:

2.4.2.1 Visual assessment

After production, the pellets were visually classed as being either round, oval, dumbbell shaped or cylindrical.

2.4.2.2 Scanning electron microscopy

Pictures o f the pellets and their surfaces were taken using this technique. Sample pellets were mounted on aluminium stubs using double-sided adhesive tape and then gold sputter coated for ~ 2 minutes [Emitech 550]. The scanning electron microscope used was a Phillips XL20 [Phillips Analytical, U.K.].

2.4.2.3 Two-dimensional image analysis using shape factor cr

The measurements were carried out using an Image Analyser [Seescan Solitaire 512, Seescan, Cambridge, UK] connected to a black and white camera [CCD-4 miniature video camera module, Rengo Co. Ltd., Toyohashi, Japan] and a zoom lens [18- 108/2.5, Olympus, Hamburg, Germany] as described by Podczeck and Nevyton [1994].

For each batch 30 pellets were chosen randomly and fixed on a clean microscope slide with the aid o f double-sided adhesive tape. Working in a darkened room, the slide was placed on a light box under the camera attached to the image analyser and the zoom lens brought into focus. The system was calibrated using a calibration standard and

then the threshold level (contrast between the image and the background) was adjusted so that the best possible image was seen. Next, the option 'kill small' was selected on the programme to erase the image o f any small particles that may have been stuck to the sellotape. The image o f each pellet was then selected in turn, which prompted the programme to measure parameters such as length, breadth, perimeter and radii at 1 ° angles from the centre o f gravity o f the pellet to its perimeter. From these measurements the mean radius and standard deviation, the variation coefficient o f the radius (an indication o f roughness), aspect ratio, perimeter and e^ shape factor were calculated for each batch and subsequently printed out on the attached printer.

2.4.2.4 Three-dimensional image analysis using the shape factor ec3

The three-dimensional shape evaluations employed the same equipment as in the two- dimensional studies [see section 2.4.2.3], except the light source was changed to a toplight rather than a lightbox. However there were also some differences in the measuring procedure and in the computer software used to calculate the shape factor. Firstly the sample preparation differed; in this case the pellets were mounted on a black rectangular holder (50 x 2 x 2 mm) with the help o f an adhesive. Care was taken to use the minimum amount o f adhesive possible so as not to affect the outline o f the image. Also the pellets had to be stuck in their most stable resting positions, and not lying beyond the edge o f the holder. Fifteen pellets were attached on each holder and 30 pellets were measured per batch. The measuring process basically followed the same steps as in the two-dimensional technique, only this time the analysis was carried out twice for each particle; first with the holder positioned so that the particles were facing upwards (directly under the light and camera), then rotated through 90° so that the particles had their side profiles exposed to the camera above (see figure 2.1). The measurements obtained included two sets o f length, breadth, aspect ratio, perimeter and estimated perimeter. From these the value o f the roughness e„, ellipticity ei, and shape factor Cc3 were calculated for each batch.

camera direction

\ /

\ /

■

Figure 2.1 The two positions from which the image analysis measurements were made by turning the sample holder through 90°.

2.4.3 Density:

2.4.3.1 Bulk/tapped density studies

A mechanical tapping device or jolting volumeter [Eberhard Bauer D7300, Germany], was used to follow the change in packing volume which takes place when void space decreases and consolidation occurs.

Weighed pellets (approximately 80 g) were placed in a 100 ml measuring cylinder with an inner diameter o f 25 mm, and tapped by means o f a constant velocity rotating cam. The starting volume was recorded, followed by the volume at 10, 50, 100, 500 and 1000 taps. From this, the increases from an initial bulk density Do, to a final bulk density Df, were calculated using D = M/V.

For each batch o f pellets the study was carried out twice, on uncoated pellets as well as the equivalent coated batch with the thickest coat (coat 5).

Statistical analysis o f the results was undertaken; for each batch, the values o f bulk density for coated and uncoated pellets were compared using a simple t-test (p = 0.05). In addition, an analysis o f variance between all the batches (coated and uncoated) using the F-test was carried out.

2.4.3.2 Apparent density studies

The apparent/true densities o f the pellets were determined using the technique o f helium pycnometry. A weighed amount o f pellets/powder [electronic balance HA-

180M, A&D Co. Ltd., Japan] were placed in the sample cell o f a fully automated gas displacement pycnometer [AccuPyc 1330, Micromeritics, Norcross, USA]. For each batch, two samples were weighed out and analysed. The equipment was programmed to perform ten purges (sample clean up and air removal from within the chamber) prior to carrying out five runs to collect data. Values for the volume measured on each run, the corresponding calculated density and deviation, as well as the average volume, density and standard deviation were recorded for each sample. Mean values were then calculated from the results o f both samples o f pellets.

Measurements were carried out on all uncoated pellets, first intact and then crushed using a mortar and pestle, as well as on final coated pellets (coat 5) o f all batches. In addition, the apparent densities o f the raw materials in the pellet formulation were measured as well as a powder mix o f the materials in the proportions used in the coating formulation (3 samples each).

2.4.3.3 Effective density studies

A measure o f the effective density o f the pellets was obtained during the mercury penetration experiments for determining the porosity o f the pellets. These are described in section 2.4.4.

2.4.4 Porosity measurements

The principle o f mercury intrusion was used in determining the porous properties o f the uncoated pellets. This technique involves applying pressure to force mercury into inter- and intra-particular voids/pores in the pellet sample under vacuum. The pycnometer apparatus used was as described by Strickland et al [1956]. It consisted o f a calibrated precision stem o f a glass sample cell with a ground glass joint and a tap. One end o f the stem was dipped into a container o f mercury and the other end was connected with rubber tubing to a vacuum pump and a simple manometer (see figure

S A M P L E CHAMBER