Pressed Compositions

In firework manufacture the use of gunpowder for priming pressed compositions containing chlorates can also be questioned on the grounds of chemical incompatibility. Sulfuric acid is known to react with chlorates to form chloric acid, HCIO3, which would be avoided by the use of sulfurless gunpowder. 

On ignition of the firework a reaction zone moves down the pressed composition and the components of the gunpowder react producing heat and hot gases. The latter entrain the titanium particles and eject them from the firework tube. During this process the metal particles are... 

The main species responsible for the blue flame from such a composition is cuprous chloride, CuCI hence the use of this salt together with the chlorine producer (hexachloroethane) and a source of extra copper (pyrotechnic copper powder). The cellulose dust acts as a moderator to control the burning rate of the pressed composition. 

BINDER A substance such as varnish, shellac or gum arable that is used to bind together the components of a pressed composition. 

BURNING RATE The regression of a reaction zone of a pressed composition or fuse, usually expressed in millimetres per second. The volume burning rate is expressed in cubic centimetres per second while the mass burning rate, which is the product of the composition density and the volume burning rate, is expressed in grams per second. 

A reaetion that produces a whistling effect is burning intermittently from layer to layer in the pressed composition. A whistling reaction is on the verge of an explosion, so these mixtures must be cautiously prepared and carefully loaded into tubes. Large quantities of bulk powder should be avoided, and they should never... 

There were relatively large regions, consisting only of HAP in the sinter section, which strongly suggests the possibility of a hot-pressed composite to maintain excellent biocompatibility. 

PBXs also include some pressed compositions where the level of nitramine is in excess of 94% (by weight). The explosives are only encapsulated by a minimum amount of binder such as Viton or Estane. The physical properties and safety characteristics of these formulations are not as good as those of castable PBXs but the performance is increased. 

Variations in the density of the pressed compositions do occur, particularly near to the surface resulting in an anisotropic product. Pressing in incremental stages or using two pistons can reduce these variations as shown in Figures 7.4 and 7.5, respectively. 

Laurent, C., Peigney, A., Dumortier, O., Rousset, A., (1998), Carbon nanotubes-Fe-alumina nanocomposites. Part II Microstructure and mechanical properties of the hot-pressed composites , J. Eur. Ceram. Soc., 18, 2005-2013. 

Zhang et al. studied the effect of conductive network formation in a polymer melt on the conductivity of MWNT/TPU composite systems (91). An extremely low percolation threshold of 0.13 wt% was achieved in hot-pressed composite film samples, whereas a much higher CNT concentration (3-4 wt%) is needed to form a conductive network in extruded composite strands. This was explained in terms of the dynamic percolation behavior of the CNT network in the polymer melt. The conductivity of extruded strand showed a hopping resistivity dominated behavior at low concentrations and a dynamic percolation induced network dominated behavior at higher concentrations. It was shown that a higher temperature can reduce the filler concentration required for the dynamic percolation to take effect. 

Where neither a load-carrying component nor a less critical component can be completely manufactured from the reservoir material, a useful alternative is to bond, rivet or press composite onto the surface of a metal component, or to incorporate inserts of the composite in a machine component. Many applications of this technique have been developed, and a few examples will serve to illustrate it. 

Suzuki et al ° made hot-pressed composites of molybdenum disulphide 80%, molybdenum dioxide 10% and niobium 10%. They were pressed at 25 MPa and 1500°C in carbon dies, and the flexural strength was 59 - 63 MPa and the elastic modulus 27.9 MPa. To improve the strength they added 5% of 304 stainless steel, and this gave a flexural strength of 69 - 80 MPa and elastic modulus 43.8 MPa. The coefficient of friction of the latter compact was 0.07 to 0.18 and the specific wear rate 2.2 x 10" mm Nm at 450°C in vacuum. 

A survey of European Space Tribology Laboratory vacuum testing on satellite slip-rings reported that slip-rings designed for satellite solar-array mechanisms had operated satisfactorily over long periods with a variety of Ag/Cu/MoSj compacts . The relationship between brush wear rate and molybdenum disulphide content is shown in Figure 12.5 for some hot-pressed composites in silver with 1% of copper. 

When a composition which contains too much water or solvent is pressed under a high pressure, the water or solvent is squeezed out, and the pressed composition cracks perpendicularly to its axis. Therefore the amount of the water or solvent must be suited to the pressure. 

In practice we take the ratio H/D less than 0.8, where H is the length of the pressed composition and D the diameter. To avoid such an inclined distribution of the density another mould like in Fig.65 may be used. The mould has two plungers,A and C, and a spacer D. On pressing D is removed and the composition is pressed from both sides, A and C, but this type of mould is not so popular, because it is somewhat inconvenient for... 

When the composition contains air which is not perfectly driven out during pressing, many cracks occur in the pressed composition perpendicular to the axis(Fig.70). A proper clearance between the tube and plunger is important. 

Figure 2 shows TEM image of the PE nanocomposite containing 1 vol. % of MMT. The dark features in the micrograph correspond to the exfoliated monolayers and nanostacks of MMT distributed throughout the PE matrix. It can be seen that the nanoscale MMT layers lack any sort of orientation in the matrix of the pressed composite. Moreover, the exfoliated MMT particles exhibit a very high aspect ratio (longitudinal size thickness). 

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