HERRAMIENTAS PARA EL DESARROLLO DE APLICACIONES.

The additives chosen were both hydrophil fumed silica with a primary particle size of about 120 Â according to the supplier (BDH Chemicals Ltd., Poole, England). Although the particles were very small, they agglomerated extensively due to their electrostatic charges and high surface area of 200 m^/g (according to supplier). One type of fumed silica (which will be referred to as SI fumed silica in the thesis) had a skeletal density of 3998 kg/m^ and this value was 2700 kg/nP for the other type (M5 CAB-O- SIL). They both had a bulk density of 70 kg/m^.

3.2. Equipments and Methods

3.2.1. O bservation of Powders bv SEM

Scanning Electron Microscopy (SEM) was used to study particle shape and size as well as their surface morphology specially in mixtures. In SEM a fine beam of electrons of medium energy (5-50 keV) is caused to scan across a series of parallel tracks. These electrons interact with the sample, producing secondary electron emission, back- scattered electrons, light and X-rays. Each of these signals can be detected and displayed on the screen of a cathode ray tube like a television picture. Examinations are generally made on photographic records of the screen. The SEM is considerably faster and gives more three dimensional details then transmission electron microscopy (TEM).

Samples as large as 25 mm x 25 mm can be accommodated and parts viewed at magnifications varying from 20 to 100,000x at resolutions of 15-20 nm as compared to 0.3-0.5 nm for the TEM (Allen, 1990). Because of its great depth of focus, the SEM can provide considerable information about the form of a particle and its surface morphology. Its depth of focus is nearly 300 times of the optical microscope.

Chapter 3; I Observation of Powders by SEM_____________________ pg

Sample powders were dusted on an aluminium stub which had already been covered with a thin layer of adhesive. With an Emitech K550 gold sputter, a thin gold coating was formed onto the particles to dissipate charge caused by the electron beam of the microscope. The stubs were then placed in the specimen holder ensuring that the top of the stubs were level with the surface of the holder. The samples were then placed in the vacuum chamber of the SEM and the chamber vented. This was signified once the current meter light was ignited. The voltage of the electron beam was then entered and the filament current control rotated clockwise (usually 70-80 jdA) until two peak deflections of the trace were observed. After one or two other minor adjustments, the surface of the sample was viewed at various magnifications and appropriate photomicrographs taken.

It should be noted that the SEM will only provide good images if the sample conducts electricity. The gold coating must be thick enough to ensure conductivity, but not so thick as to obscure the surface features. Also, some materials are easily damaged by very high coating currents. This was avoided by lowering the coating currents and lengthening the deposition times.

Electron micrographs in Section 4.1 show FCC, Silica and kieselguhr particles with and without additives at various magnifications. Higher magnifications show the surfaces of the particles in more details. They will be discussed in more detail in Section 4.1.

3.2.2. Sample Preparation

The powders were dried at 120 °C overnight for all the tests. Mixtures were prepared by weighing out the appropriate amount of ultrafines and host material. The total mixture was then blended using an impeller with two blades which were coated with rubber to avoid particle breakage during blending. Mixing time was normally five minutes, when an evenly dispersed and homogeneously mixed powder was obtained. The mixing would be continued when any agglomerate was observed by naked eye. In some experiments drying temperature and mixing time were increased to 150 “C and 40 minutes respectively but no significant changes were observed in the final results. The blending was carried out at ambient temperature, pressure and humidity. Samples were left in sealed containers for a few days before use.

SEM pictures were taken from the host materials before and after blending in the same manner as the mixtures were produced. No difference in particle size distribution was detected; therefore the mixing procedure did not alter the size distribution of the host materials.

M5 CA B-O -SIL was added to FCC, silica and kiesel-2 while SI fumed silica was added to kiesel-1. Samples of silica and/or FCC were prepared in concentrations of 1, 2.5, 5, 10 and 15% weight ratio of ultrafines to the total weight of the additive and the host material. Samples of kieselguhr and ultrafines were in concentrations of 1, 2, 3, 4 and 5% wt.

3.2.3. Particle Size Measurements

SEM pictures provide a good indication of particle shape and size. However, some other methods were also used for particle size measurements depending on the particle size of the material in order to quantify their size distribution. Each technique will be described below :

3 .2 .3 .1 . Sieving

Sieving which is the easiest, cheapest and probably the oldest method for powder sizing, is applicable for liquid suspensions (wet analysis) as well as for analysis of dry powders. In this technique, a number of sieves with different standard meshes are nested one on top of another in a descending order with the largest aperture size sieve on the top. The stack forms a cylinder which normally sits on a pan called receiver. Agitation results in an increase in sieving efficiency.

The sieve apertures are classified as coarse (4 to 1(X) mm), medium (0.2 to 4 mm) and fine (less than 0.2 mm), which are mainly woven wire or punched plate sieves. Fine sieves below 0.04 mm or microsieves, however, are manufactured electrolytically from perforated metal foils. The weight of the sample depends on the density of test particles; a powder mass of 20 to 60 g of fine particles and 60 to 150 g of coarse ones are recommended for a sieve of 20 cm diameter. Sieve size is based on the median average of the particle sizes on the central sieve.

The appropriate sieving time is on the basis of different recommendations. BS1796 (1952) suggests that the sieving classification should be continued until less than 2% of the particles pass through in any 5 minutes of sieving process. In general 20 minutes

Chapter 3; Particle Size Measurements________________________________ 9 ^

sieve classification time is sufficient for a successful dry analysis. Fine particles are likely to be aggregated during the process and the effect of this problem can be reduced by vibration.

The main problems associated with sieving are blinding and damage to the apertures due to vibration. Also, different particle shapes and surface patterns may result in analytical errors due to specific geometries or cohesive forces, respectively. Prolonged test runs is also a disadvantage specifically as an on-line particle size analyzer.

Particle size of FCC was measured by a nest of 20 cm Tyler sieves in the range of 300- 53 pim using a shaker. 150 g of the powder was sieved for 25 minutes and the weight of the powder in different sieves and the receiver was measured. The powder was then sieved for a further 5 minutes and collected sample on each sieve was weighed again. There was little difference between the two sieve analysis (less than 2%). Therefore 30 minutes was found to be the optimum time for sieving. Mean particle size of the sample, dgm» was then determined by equation (2.8).

In order to measure the size of silica particles, 100 g of the powder was used with the same nest of sieves and shaker; sieving time was 30 minutes.

3.2.3.2. M alvern

SEM pictures showed that particle size of kieselguhr was below 53 pim^ the smallest sieve available. Therefore, sieving was not the appropriate method to determine the particle size distribution. In this case a Malvern 3600E sizer was used which was interfaced with a personal computer to data acqusition. Malvern 3600E system consists of a complete laser particle sizer optimized for the measurement of powders dispersed in