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.
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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.
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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°-45° 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°
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.
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