Solvents and propellants may make it easy to get liquid mounting adhesives out of an aerosol can, but these chemical agents hardly take it easy on the environment. "They tend to be greenhouse gasses," according to Russell Blette, a 3M technical specialist. Water-based adhesives, although friendlier to the atmosphere, have had their own problems with slower tack times and reduced adhesion. "The Holy Grail in our industry has been a water-based formulation that still meets the performance standards of solvent-based adhesives," Blette says.
Well, Hallelujah. Blette has invented a way to make up for the shortcomings of water-based adhesives by doing away with the traditional aerosol can, solvents, and propellants . His patented Pro-Spray system instead creates a quick-tacking aerosol spray through purely mechanical means: The system uses a bladder-filled plastic bottle to deliver the liquid adhesive to a nozzle, which then atomizes the adhesive with a blast of air. Both components fit a low-cost, handheld thermoplastic dispensing gun.
Replacing the solvents and propellants was a tough job given the crucial role both play in adhesive performance. Solvents allow chemists to render high-viscosity adhesives sprayable. And once the adhesive is applied, solvent flashes off quickly to promote faster tack times. Propellants, meanwhile, not only push the adhesive out of the can they also help atomize it as it runs through the pathways of the aerosol actuator. "Fluid particles are initially formed as a result of the vaporization of the propellant and the kinetic energy the propellant imparts to the liquid being dispensed," Blette explains. In the best performing adhesive sprays the particles have be smaller than 200 microns and uniform in size, he adds. Otherwise, the droplets will fall onto the substrate in tiny peaks and valleys, reducing the adhesive's effective bonding area.
Water-based adhesives in aerosol cans already use propellants. But without solvents to lower their viscosity, these adhesives usually have lower solids levels, which gives users less bang for their buck. "Lower solids means less deliverable adhesive for the substrate and less deliverable material in a given container," Blette says. And low-solids adhesives tend to be all wet for another reason. "Lower solids also mean there's more water to wet out the substrate," he says. And because water doesn't flash off as quickly as solvents, tack times increase.
The ProSpray's thermoplastic dispensing gun had to be designed for ease-of-manufacturing and ease-of-use in order for the system to compete with traditional aerosol cans. Its inventor, Russ Blette, managed to get the total parts count down to less than 25, excluding screws.
A blast of fresh air. In a traditional aerosol can, the solvent, propellant, and adhesive itself all collaborate to produce a spray. Blette's fundamental innovation was to separate the delivery and atomization functions into two mechanical devices. Pro-Spray's nozzle performs the atomizing role with a high-velocity stream of air that rushes through an annular gap and hits the adhesive at a 30 degrees -45 degrees angle as it exits at the center of the nozzle.
Blette borrowed the notion of an air-assisted nozzle from other applications. "Diesel fuel injectors and high-end paint guns have both used similar nozzles," he says. But applying the concept to the adhesive application required him to optimize several related design inputs-the size and shape of the gap, incoming air pressure, velocity of the air blast, and the angle at which air hits the adhesive. He ended up with a 0.015-inch gap that produces a 400 mph air velocity when the Pro-Spray is hooked up to a 35 psi source.
Because water-based materials exhibit more shear sensitivity than solvent-based adhesives, Blette also had to optimize the internal pathways through the nozzle. Standard aerosol actuators, for example, often have a 90 degrees bend between the container to the nozzle orifice. The Pro-Spray flow paths all have radiused transitions. And they deliver the material to the nozzle in a laminar flow to further promote atomization.
As for the delivery system, Blette adapted a proprietary container made by Exxel Atmos (Somerset, NJ). Taking the ship-in-a-bottle approach, these dispensers consist of a rubber bladder, or reservoir, suspended within a blow-molded container. As the bladder contracts, it simply squeezes the liquid out. A built-in valve controls the release of the liquid. When spraying low-viscosity fluids, these containers typically handle both the delivery and atomizing functions. For high-viscosity adhesives, Blette needed to combine the system with the air nozzle. "The Atmos container alone didn't have enough energy to break up the liquid into particles," he recalls.
Keeping it simple. Once he'd come up with the concept for the Pro-Spray's aerosol components, Blette turned his attention to two other design goals. With the Pro-Spray intended for use in commercial framing shops, it had to be easy to use with no special clean up or training. It also had to be easy to manufacture to keep costs low enough for the thrifty target market. "We needed to make the Pro-Spray as close to a push button aerosol as possible, " Blette says.
As Pro-Spray evolved, Blette fine-tuned the design to keep it as simple as possible. The canister, for example, currently installs in the dispensing gun with a single twist and secures with a snap-fit locking tab. Early designs iterations, by contrast, used a bayonet-style lock that required a more complex locking collar and alignment routine. And rather than add a separate shut-off system, Blette designed Pro-Spray to use the valve that's already part of the Atmos canister, whose back-and-forth motion within the Pro-Spray housing actuates the valve. "Adding a separate valve and actuator is a lot more complicated than simply moving the bottle and taking advantage of the canister's existing valve," he says.
To design Pro-Spray's housing, fastening, and sealing components-about 25 in all-Blette used a full complement of concurrent engineering tools. "3M has many internal resources," he says. "So we were able to use all the design tools that everybody is always talking about." That list included solid modeling in Pro/ENGINEER and mold filling analysis using Moldflow, SLA prototypes for form-and-fit models, and RTM parts for the working prototypes.
A fine spray. With Pro-Spray now coming out of field tests after a four-year development, Blette reports that it produces particles in the 20-micron range with a tight size distribution. "It gives us a much more efficient break up than a solvent-based adhesive," he boasts. So efficient, in fact, that the system enables water-based formulations with solids levels in the 17-20% range, or about a third more than water-based products in a can.
Taken together, the small particles, good distribution, and the high solids add up to some big advantages. For one, users can get more coverage from a smaller can. Blette notes a 10-oz Pro-Spray container provides about the same coverage as a 17-oz solvent-based spray can. For another, the system sprays faster with less of a billowing effect than a traditional aerosol. And unlike traditional aerosols, Pro-Spray's particle size distribution doesn't fluctuate as the propellant levels in the container taper off with use.
As for tack times, the system and a new waterborne adhesive that goes with it produce tack times very close, to those of solvent-based adhesives, Blette says, adding that water-based adhesives sprayed from a can usually take twice as long to tack. Blette believes the tack-speed improvements and other advantages could expand the market for aerosol adhesives into areas where productivity counts-such as assembling automotive interiors. "This system will change the way people spray aerosol adhesives," he says.
3M technical specialist Russell Blette holds a Mechanical Engineering degree from Kennedy Western University. He began his career as Engineerig Manager of Otto Engineering, a maker of automated dispensing systems which 3M purchased in 1989. While at 3M, Blette has developed dispensing systems and acoustic control systems. He currently works in the company's Technology Center where he focuses on hardware development.
A fifteen-year veteran of 3M and a mechanical engineer, Blette has 18 patents to his name. Three of those
patents deal with ways to deliver or spray water-based adhesives. The Pro-Spray system represents the end result of his work in that area. Blette's other patents involve diverse applications from carpet cleaner dispensing to solder dispensing for electronics.