Powder dispensing is not as well understood as liquid dispensing because powder dispensing involves a two-phase fluid containing a compressible gas and solid particles. Even powders dispensed by gravity or by shaking a canister have air mixed with the solid particles. Flowability of a powder is believed to be influenced by multiple factors, including the size and shape of the particles, the tendency for particles to stick to each other, the density of particles, and the volume of air between particles. Particles may stick to each other due to electrostatic attraction as well as adhesive forces. Moisture absorbent powders in particular are prone to caking and resist flow when moisture is sufficiently absorbed. Therefore, moisture absorbing powders are typically contained in relatively air-tight dispensers so that they remain flowable for dispensing after being stored for extended periods in the presence of moist ambient air, such as often exists in a bathroom.
Moisture absorbent powders are useful in maintaining body surfaces dry and feeling soft. Where body surfaces are substantially smooth and upward facing, it is relatively easy to shake a powder from a canister onto the surface and distribute the powder evenly by using one's fingers. However, delivering powder to a body surface having hair or which faces substantially horizontal or downward is benefited by a delivery system which effectively squirts a pattern of powder onto the surface without the need for finger distribution of the powder. Spray type dispensers are generally preferred for such applications.
Squeeze type powder sprayers are known in the art. In one version a resilient bulb is squeezed to cause a burst of air to flow past a container of powder. The powder is drawn into and mixed with the airstream, presumably because the movement of the air generates a low pressure zone adjacent the powder surface. Bulb type powder dispensers are typically limited to very low powder doses.
Squeezebottles which contain powder and have an air headspace are another version, wherein powder is discharged by squeezing the bottle to cause headspace air to push a portion of powder and air out of the bottle. Air pressure may force the powder out an open orifice as in U.S. Pat. No. 2,450,205 to Rose or U.S. Pat. No. 2,840,277 to Bach. One disadvantage of these prior art dispensers is that their discharge orifices are always open, thereby exposing contained powder to moisture. Prior art references show removable closures for powder dispensers, but such closures may not be replaced after spraying, and therefore are not fool-proof In the liquid dispensing art there are found references having squeezebottle dispensers with resilient self-sealing valves. An example is U.S. Pat. No. 4,749,108 to Dornsbusch et al. Dornsbusch et al. show a normally concave-shaped resilient slit valve which inverts under sufficient internally developed pressure. Because this slit valve is intended to seal the container from liquid dripping, the slit must close tightly. Valve inversion causes the slit to close more tightly until it finally opens. Thus, a liquid head in a downward pointing bottle would not result in valve leakage. However, the need for the valve to invert generally requires that a high internal pressure be developed. If such a valve were used with powder instead of liquid, a virtual explosion of powder would occur once the valve opened because compressed air would burst through with the powder. Where targeted delivery of a fine powder is desired, such an explosion is disadvantageous because it results in a significant "cloud of dust" or wasted, non-targeted powder.
Other references, such as publication WO 90/14893, dealing with compressible fluid spray nozzles other than for squeezebottle dispensers, show resilient flat valves, which operate at much higher pressures than inverting valves. If such a valve could be applied to a squeezebottle powder dispenser, both self-sealing and targeted powder delivery could possibly be achieved.
Diptubes are used with squeezebottle powder dispensers intended for upright dispensing. Although powder flowing through a diptube adds to the internal air pressure needed to be developed in a squeezebottle to initiate powder dispensing, the diptube is necessary to provide a path for powder to flow upward to the discharge opening. Without a diptube, air compressed in the squeezebottle headspace would merely exit without carrying powder with it. However, diptubes provide an opportunity for powder plugging if a powder should become compacted in the diptube. The ideal powder has the ability to easily mix with air and flow through the diptube without compacting for proper functioning.
The hereinbefore mentioned Bach reference has a diptube leading from the bottom of the bottle to a mixing chamber just ahead of a discharge orifice. The mixing chamber has a separate opening for a portion of the headspace air to enter the mixing chamber. Although a mixing chamber is advantageous for obtaining a uniform distribution of powder in air, a separate opening to the mixing chamber is believed disadvantageous because it enables headspace air to exit the dispenser without first pushing powder up the diptube. Substantial variation in powder volume dispensed may result from each squeeze actuation of the dispenser if headspace air can escape other than through the bulk of the powder in the bottle.