Blister mixers

Figure 8.23 Schematic of blister ring mixers a) a blister mixer positioned in a metering section, b) blister section positioned at the discharge end of the screw near the tip, and c) the cross-sectional view showing the region of annular flow between the screw and barrel wall (courtesy of ieff A. Myers of Robert Barr, Inc)...

The shear stress that the material experiences for flow across a blister mixer can be estimated using Eqs. 8.12 and 8.13. For this calculation, the flow in the axial direction is ignored to simplify the mathematics, and thus only the angular flow is considered. Ignoring the axial flow causes the calculated shear rate to be lower than in the actual channel. 

Figure 15.9 Schematic for a blister and gear mixing section. The blisters are used to adjust the specific rate of the process, and the gear mixer and second blister are used to distribute and disperse the stripping agent into the resin...

You may wonder what the source of these bubbles or blisters may be. Blisters are caused by one of two things-water or air. If the substrate is at all porous, it contains air. Or there may be some water inside the concrete, too deep to see. When the hot asphalt is applied, the heat, transmitted into the concrete, causes the air to expand or the water to vaporize. In either case, it tries to escape, and in doing so, causes bubbles. An alternative possible source is the kettle. If a few drops of rain get into the kettle, and the pot tender has not stirred it all out, some foam may have gotten into the pail the mechanic has just used-and as this hot material is spread, a bubble formed. If the pot is stirred with a power mixer, or even vigorously by hand, the surface can trap air which can get into the applied hot asphalt membrane in the same manner. A few random blisters can be repaired without too much trouble-but if there are more than one per square foot this problem should be corrected before proceeding. 

Materials have been mixed with a high-speed mixer, entrapping air in the resin. After the surface has been troweled in place and finished, air bleeds out as blisters or tiny bubbles. A paddle mixer moving at more than 350 rpm can cause this. 

Mixing screws — S-shaped kneader, Eagle mixing. Turbine mixing, Cohen double-wave, or Maillefer screws with pin-, blister ring-, fluted-, barrier-screw. Dynamic Extentional Flow Mixer (DEEM), etc. 

Some of the more widely used high-performance screws include the Energy Transfer (ET) screw [19], the Variable Barrier Energy Transfer (VBET) screw [20], the Double Wave screw [21], Stratablend [22] and Stratablend II [23] screws, and the Unimix [24] screw. Other devices can, however, act as a trap for solid materials from the melting section. These devices include Maddock-style mixers [25], spiral dams, and blisters. They will be briefly discussed in this section as many of them have been the subject of many articles on secondary mixing processes. 

Figure 12.11 Production mixer with central bypass, (a) Shows entry slots before flow resisting blister, outlet pipe and liquid injector passing through stator flange (b) View showing central bypass outlet. (Photographs bt G. Lawley. Rapra Technology)...

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