Cohesion testers

The Cohesion Tester has been used in industry, mainly as a quality control-type test. It can be useful as an aid to assessment of flow properties and of power requirements in mixers, bulk conveyors and feeders. The original developers of the tester, Warren Spring Laboratories35, have even tried to correlate the cohesion value with unconfined yield stress determined with the Jenike shear cell and found a good correlation for some powders (fc = 6 x C). The tester is, quite obviously, only useful with fine, cohesive powders because ... 

In understanding, the greatest question to be yet resolved is what is the best, yet simple, test to be used to monitor the cohesivity or flowability of powders There are, of course, the direct (or almost direct) ways of measuring it (the cohesion tester or the tensile testers) but neither the tests nor the equipment are really simple. Angles of repose or spatula are simple but, unfortunately, become quite meaningless as the powder gets more cohesive. I personally think that the way powders compact can be directly related to cohesivity the recent evidence of the usefulness of the Hausner ratio in many different applications points in this direction. The compaction test should also be better defined and more widely used. 

ISSA TB-139. 1990. Classifying emulsified asphalt/aggregate mixture systems by modified cohesion tester measurement of set and cure characteristics. Annapolis, MD International Slurry Seal... 

AJAX Equipment (Bolton) Ltd Warren Springs Laboratory cohesion tester It is appropriate to include in this chapter an instrument which was one of the first attempts to measure the cohesion of powders directly. It was initially developed at Warren Springs Laboratory, Stevenage, but is now commercially available from AJAX Equipment (Bolton) Ltd. 

Figure 1,21 AJAX (Bolton) and Warren Springs Laboratory (WSL) cohesion tester.

An alternative procedure, to ensure no external force is applied to the powder bed by the vaned paddle, is to place the compacted sample on a balance and when the paddle is immersed in the powder to raise the vaned head slowly until the balance reading is zero. This dynamic method of bulk powder characterisation is allied to the rheological method for measurement of the viscosity of non-Newtonian fluids and suspensions. Commercial instruments based on the WSL cohesion tester are now available in the form of the FT4 Powder Rheometer (Freeman Technology) and the Stable Micro Systems Powder Flow Analyser (Stable Micro Systems). 

In order to develop the proper dow pattern, knowledge of a material s dow properties is essential. Standard test equipment and procedures for evaluating sohds dow properties are available (6). Direct shear tests, mn to measure a material s friction and cohesive properties, allow determination of hopper wall angles for mass dow and the opening size required to prevent arching. Other devices available to evaluate sohds dowabiUty include biaxial and rotary shear testers. 

A material s flow function is usually measured on the same tester as the wall friction angle, although the cell arrangement is somewhat different (Fig. 6). ConsoHdation values are easily controUed, and the cohesive strength of the bulk soHd is determined by measuring interparticle shear stresses while some predeterrnined normal pressure is being appHed. 

The Jenike shear cell tester is classified as a direct shear tester that is capable of providing information on a solids cohesive strength as well as its wall friction properties. The tester allows us to measure the strength of a powder blend as a function of pressure applied to it. These are two main considerations when design a bin or hopper to ensure reliable material flow. The tester consists of a base, a moveable shear ring resting on top of the base, and a top cover lid (Fig. 7.3).61 The base is fixed while the lid rotates at a constant low rate. Powder blend is placed in the ring and base and a... 

Yuko Tsuya also used electron diffraction in studying the progress of running-in with an unconsolidated "floated" film initially about 0.5/ym thick, using an oscillatory friction tester with a stroke of 20 mm and load 8 kg. She found a gradual transition from a layer of randomly-oriented particles with little adhesion or cohesion to a smooth cohesive layer with the molybdenum disulphide crystallites almost fully oriented with the (001) basal planes parallel to the substrate surface. The coefficient... 

The flow through an orifice is an excellenf mefhod fo compare materials with good flow. Pharmacy, however, most often deals with cohesive powders that do not readily flow. The flow of these materials is increased by employing force feeders, resulting in a dynamic process. Angle-of-repose measurements that do not utilize force and that are often a measurement of stagnant powder have therefore limited application. Shear testers, that measure the frictional characteristics... 

Adhesive characteristics of thin SPI-100 films could not be measured directly because adhesive forces are generally larger than cohesive forces. To obtain some information on adhesion values, thin high molecular weight SPI-100 films on substrates were overcoated with about. 1 mm of a commercially available polyimide (Product A) to provide greater cohesive strength than can be obtained with SPI-100 alone. The combined layers were then pulled in an Instron tester giving the results shown in Table II which also includes the values for commercial products A and B. 

Research has shown that the cubical triaxial tester is useful for investigating anisotropy of cohesive and noncohesive powders and the effect of particle shape and sample deposition methods. For anisotropic materials such as wheat flour, the strains in three principal directions are statistically different (Li and Puri, 1996). Wheat flour was used for the calibration of the triaxial tester to determine parameters related to a hopper design (Kamath, 1996). 

Li, F. and Puri, V.M. 1996. Measurement of anisotropic behavior of dry cohesive and cohesionless powders using a cubical triaxial tester. Powder Technol. 89, 197-207. 

Bouzakis K-D, Michailidis N, Lontos A, Siganos A, Hadjiyiannis S, Giannopoulos G, Maliaris G, Erkens G (2001) Characterization of cohesion, adhesion and creep-properties of dynamically loaded coatings through the impact tester. Z Metallkd Mater Res Adv Tech92(10) 1180-1185... 

Split cell testers Tensile strength measurements may also evaluate internal adhesion properties, independent of the mechanical interaction of shear plane, because T is measured at zero shear. Warren Springs Laboratories (WSL) designed two instruments one to measure the cohesive strength of powders at varied states of compaction and the other to measure tensile strength. The tensile strength measurement equipment, developed by WSL, was the WSL tensile tester, a diametrically divided split shallow circular cell. 

One way which relies upon the fundamental knowledge of the stress-strain-volume behaviour of bulk solids is dependent upon the development of testers such as the biaxial and triaxial shear testers as well as the now universally accepted Jenike shear cell, or the standard shear test tester. Other instruments, such as the annular shear cells and the cross-sectional Peschl and Colijn (1977) tester, use the same stress-strain-volume principle. These annular shear cells may also be used to evaluate a bulk powder flow function. The powder flow function, having been discussed previously, still requires a family of yield loci before cohesion can be evaluated. 

The second way relies upon the development of specific powder testers, which will permit the straightforward measurement of cohesiveness and/or tensile strength of difficult flowing and poorly mixed cohesive powders. 

Cohesion of a powder is defined as the shear strength at zero normal load. The tester is thus designed to assess the cohesive strength of powder samples at varied states of compaction, from lightly settled conditions to firm uniaxial compressed compacts. The bench-moimted equipment consists of a radially finned vane mounted on a vertical spindle, which in turn is carried on low friction bearings (Figure 1.21). 

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