CAKIN G STRENGTH REQUIREMENTS FOR THIS WORK

The strength of caked or lumped lactose powder is caused by either liquid bridges or solid crystalline bridges. The major difference between the humidity caking and

amorphous lactose related caking mechanisms is the viscosity of the liquid bridges and the likely size of the resultant crystalline bridges. In either case, some form of bridges are the major reason for the increased strength of lumped or caked bulk powder.

A liquid bridge has strength due to the presence of capillary forces. These are made up of surface tension forces and the force due to differences in pressure inside and outside the liquid bridge [Schubert 1 984] . Such forces are best measured as the tensile strength of the product. Schubert ( 1 975) defines tensile strength as the 'unidirectional maximum tensile force per unit of the plane cross-sectional area of the bulk material at right angles to the direction of the tension when a locally constant, purely tensile stress prevails in the fracture cross-section of the material regarded as a continuum'.

As such, the method of characterising the relative strength of product in different conditions needed to be able to measure the strength of the liquid bridges or solid bridges inherent in the bulk media. The technique decided upon, also needed to meet the requirement of being suitable for use in measuring the strength of material packed into the experimental temperature gradient rig described in Section 4 . 5 . 3 . 1 . This allowed the application of a temperature gradient to a packed lactose bed and the characterisation of the resulting strength profiles throughout the bed, due to moisture migration.

Any technique which required the removal of the product from the experimental rig would cause the disturbance of the liquid bridges giving the sample strent,>th and

therefore was not suited to this purpose. The technique also needed to be suitable for the strength determination of small samples which were equilibrated in desiccators at varying conditions.

5.4.2 AVAILABLE STRENGTH M EASUREMENT M ETHODS

Numerous methods can be found for the detennination of the bulk strength of particulate materials. These can be characterised into several main categories including, tensile strength, shear strength, crushing strength and other miscellaneous measurement methods.

5 . 4 . 2 . 1 _{TENSILE STRENGTH}

Tensile strength measurements have been made by Schubert ( 1 975), Schubert et al. ( 1 975) and Pietsch et al. ( 1 969) for finely divided solids. The advantage of tensile strength measurement is the availability of theoretical relationships between tensile strength and the various kinds of binding mechanisms which have been developed by Rumpf ( 1 96 1 ), Schubert ( 1 975) and Pietsch et al. ( 1 969).

Kawashima ( 1 99 1 ) summarises several other researchers tensile strength measurement devices as does Schubert ( 1975). Hasegawa et al. ( 1 985) detail two commercially

available tension testers. Most methods involve the measurement of strength when the powder is pulled apart. For example the split table method makes use of a horizontal

platform on to which the powder is packed. Part of the platform is movable and is pulled from the remaining part, causing a breakage in the powder. The force required to

separate the two portions is used as a measure of the tensile strength.

Each of the tension force methods are relatively simple, established techniques,

however they were not suitable to testing in situ in the experimental temperature gradient ng.

5.4 . 2.2 _{SHEAR STRENGTH AN D COHESION TESTS}

Shear strength measurements are often used as measures of cohesion or flow-ability in powder systems. Jenike ( 1 970) developed a shear cell which uses a split ring system into which the powder is packed. A loading force is applied to the top of the cell and the strength required to drag the top ring over the bottom ring is measured. Similarly an annular shear cell has also been developed [Walker 1 967] which allows the accurate measurement of shear forces at low normal stresses. For the purpose of measurement of food powders, the annular shear cell is preferred, particularly for low strength powders such as soup mix and onion powders [Schubert 1 987]. The annular shear cell is a complicated and expensive device however and simpler methods are often preferred by many researchers.

In spite of this, there are a number of researchers who have used shear measurements and many testers are commercially available. These include Nash et al. ( 1 965) who use a method similar to the Jenike tester for investigating the effectiveness of anti­

agglomerating agents in finely ground powders. Plinke et al. ( 1 994) used the method of Peschl ( 1 989) which is a rotational split level shear tester for measurement of shear strength in limestone. They found that the method agreed well with the more widely accepted Jenike and Gebhard techniques. Scoville and Peleg ( 1 98 1 ), Moreyra and Peleg ( 1 98 1 ) and Hollenbach et

al.

characterise the flow-ability of selected food powders. Hasegawa et al. ( 1 985) review the operation of three commercially available shear strength testers for use in

characterising powder beds.

The basic operation of any shear cell testers, effectively excludes the method for use in this work. All methods require one plane to be dragged across another plane of powder materia1. While such a setup is possible in small scale equilibrated samples, it i s

not easily achieved i n the experimental temperature gradient rig, without the physical disruption of the system.

5.4.2.3 COMPACTION AND COMPRESSIBI LITY M EASUREMENTS

Many researchers have used compression tests as a measure of the strength of particulate materials [Whynes and Dee 1 957, Moreyra and Peleg 1 98 1 , Scoville and Peleg 1 98 1 , Hollenbach et al. 1 982, 1 983, Abdel-Ghani et al. 1 99 1 , Riepma et al. 1 992 and Morishma et at. 1 993]. Each method is essentially a measurement of the crushing stren!:,>th of a standardised pellet or sample. While such measurements are common and the availability of instruments such as an Instron universal testing machine are high, the action of the method is to measure a combination of forces including shear, tension,

particle breakage and compressibility. This is likely to cloud the strength measurement which is aimed at the measurement of the strength provided by bridge formation. The other difficulty with these methods is that the instruments are not suitable for measuring strength profiles through the lactose bed in the temperature gradient experimental rig.

5 . 4 . 2 . 4 OTHE R M ETHODS

Several other alternative methods have been used in the past for the characterisation of the strength of powders. Abdel-Ghani e( al. ( 1 99 1 ) measured the effective elastic modulus by the method of indentation testing. Seville and Clift ( ] 984) showed the effect of thin layers of liquid on the fluidisation characteristics of particles. It was shown that the fluidisation characteristics were dependent on inter-particle forces and liquid bridges present in the system. Kono et al. ( 1 994) have used this as a basis for the measure of flow propel1ies of powers by measuring the minimum fluidisation velocity and minimum bubbling velocity of the powder bed. The results showed this technique to be better than the Jenike shear ceII which requires high normal loading to obtain accurate strength measurements

Penetrometry is a technique used extensively in the field of soil science for the characterisation of soil systems [McLaren and Cameron 1 990] . Such measurements simply involve recording the force required to push a probe into the sample. Because the size and shape of the probe will be influential on the force recorded, it becomes an empirical measure. A multi-point penetrometer has been used by Baker and Mai ( 1 982) for measuring the strength of top soil. The size of the pins which were pushed into the ground were approximately the same as the particulate system and the number of pins was increased until the force measurement device could accurately measure the applied force. In this way the forces broken by the penetrometer were the inter-particle forces under investigation rather than compressive strength commonly achieved with larger penetrometer probes.

Bagster ( 1 970a) summarised some of the early techniques which have been used to quantifY caking in powders. These range from largely qualitative observation based tests to the use of penetrometry. Irani et al. ( 1959) measured caking in fertilisers by

measuring the weight of fertiliser retained on a particular sized screen.

From this review of strength measuring techniques for bulk powders and the

unavailability of testing equipment capable of being utilised in the temperature gradient experimental rig, it was evident that a new strength measurement device or a

modification to one of those discussed above needed to be developed for use in this work. The multi point penetrometry method was considered to be the most likely technique for this work. This was developed further and is discussed below.