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Spray Painting and Coating
Waste Reduction Alternatives
Many products require some type
of coating, such as paint, lacquer or varnish. By incorporating
waste reduction techniques into coating application
processes, companies can realize great savings.
By reducing waste, large-volume painting
operations may be able to avoid reaching the threshold
level for chemical use which requires reporting emissions
to the Minnesota Emergency Response Commission and compliance
under the Minnesota Toxic Pollution Prevention Act.
The U.S. Environmental Protection Agency (EPA) is proposing
new air emission rules which will apply to many painting
processes. Painting operations that adopt waste reduction
practices now will have an advantage if stricter air
emission standards must be met in the future.
Opportunities for reducing waste exist
throughout the coating process, from product design
through manufacturing, coating and cleaning. Look for
opportunities to reduce waste when selecting coating
equipment and coatings. Operator technique and cleanup
when jobs are finished are also areas waste can be reduced.
This fact sheet discusses how businesses can incorporate
waste reduction strategies into their coating processes
to reduce or eliminate paint-related wastes and emissions.
The following sections list select
options available for spray painting and coating. MnTAP
can provide additional information on these topics and
vendors can provide detailed product information and
hands-on training.
Surface Preparation
Many products require a preparation
step prior to painting. This step is commonly called
pretreatment for new products and paint stripping for
products that need rework.
Pretreatment/Cleaning
Cleanliness of parts.
The first step for reducing waste in pretreatment is
assessing the cleanliness of parts. Determine the sources
of contaminates to reduce or eliminate them. Consider
to what degree surfaces become contaminated with substances
such as oil from machining, dirt from the manufacturing
environment and oil from people's skin.
Cleanliness of the process.
Next, determine the cleanliness standard needed to satisfy
the pretreatment process. Once contaminant sources are
identified and cleanliness standards are set, determine
which contaminants can be eliminated. If contaminants
cannot be reduced sufficiently through process changes,
assess the cleaning methods used.
MnTAP developed Get
It Plated Right, a series of fact sheets which covers
a variety of difficult part cleaning issues. This series
is available online.
Phosphatizing. Phosphatizing
prepares the surface of metal parts for coating. The
volume of water used to maintain the phosphatizing bath
solution can be reduced by analyzing and controlling
each bath’s temperature, chemical concentration
and pH level; and by recirculating the solution or rinse
water to other baths where possible. An added benefit
is the potential for reduced chemical use. Additional
information on cleaning and phosphatizing can be found
in the fact sheet Metal
Phosphatizing Operations [#64].
Paint Stripping
Evaluate the problem. Usually, old paint must
be removed before a new coat of paint is applied. Assess
what caused the need for repainting. Inadequate initial
part preparation, defects in the coating, application
equipment problems and coating damage due to improper
handling can all result in the need for repainting.
While no process is perfect, reducing the need for repainting
directly affects the volume of waste from paint removal.
Consider other approaches.
Once the need for paint stripping is minimized, consider
alternative paint-stripping approaches. Outsourcing
paint-stripping work may be cost effective. Consider
the advantages of reduced environmental liability, avoiding
employee exposure to paint-stripping hazards, and eliminating
expenses for purchasing, operating and maintaining stripping
equipment. Because outside stripping operations specialize
in paint stripping, they are more efficient and can
reduce the problems associated with stripping.
If you choose to maintain an
on-site paint removal operation, key factors to consider
are the characteristics of the substrate being stripped,
the type of paint being removed, and the volume and
type of waste produced. Chemical stripping has commonly
been used in a number of applications, but less toxic
and less costly alternatives are available. For example,
chemical stripping can often be replaced with mechanical
stripping using metal and nylon brushes. Alternatives
to chemical paint-stripping include:
- abrasive blasting with a variety
of materials
- mechanical removal using scrapers,
wire brushes or sand paper
- pyrolysis: vaporizing the paint
coating in a furnace or molten salt bath
- cryogenics: freezing the paint
off
- extreme high-pressure water or
air
Often, removed paint and chemical
stripper combinations require disposal as hazardous
wastes. Waste type and volume can have a major impact
on cost.
Transfer Efficiency
The type of coating material and application method
used impact transfer efficiency. When discussing spray
application equipment, transfer efficiency measures
how much paint makes it from the container onto the
surface being painted. More importantly, the overall
transfer efficiency of a specific coating refers to
the amount of coating needed to get the proper dry film
thickness. Overall transfer efficiency accounts for
the amount of coating lost to evaporation and emissions
relative to the amount of solids that coat the part.
Improved transfer efficiency uses
less paint per finished product. Because less paint
is used, air emissions from solvents in the paint are
reduced. It also reduces the rate of paint entering
booth filters and landing on the floor and walls of
the spray booth. High transfer efficiency rates reduce
the amount of paint wasted while minimizing solid and
liquid wastes, and air emissions.
Spray Application
Equipment
Equipment Available
To achieve the best transfer efficiency assess the application
equipment available and evaluate equipment performance
using each coating material considered appropriate for
your application. Each combination of application equipment
and coating creates its own characteristics. Weigh these
results against the specifications set for your finished
product. For example, if increasing transfer efficiency
results in a coating film thickness that is greater
than specified, materials will be wasted.
Conventional
Spray
In use for over 50 years, conventional, or atomized
spray, uses air at high pressure (40 to 70 pounds per
square inch [psi]) to atomize a liquefied stream of
paint. The high-energy air stream finely atomizes paint
making it easy to apply and yielding very good finishes
with high-quality visible characteristics.
A disadvantage to conventional spray
is that a high degree of atomization is accompanied
by a very fine spray that is highly susceptible to overspray.
The result is more paint waste and low transfer efficiency.
High-volume/Low-pressure
(HVLP)
As the name suggests, a high volume of air at low pressure
is used to atomize paint. The air-pressure limit for
HVLP is 10 psi at the spray gun air cap. The lower energy
level reduces overspray and improves transfer efficiency.
Generally, fluid-delivery rates up to 10 ounces per
minute with low viscosities work best with the HVLP
gun. At higher fluid delivery rates or with heavier
materials, HVLP may not atomize well enough to achieve
an acceptable finish.
Airless
This method of atomizing paint does not use compressed
air. Paint is pumped at high pressure through a small
opening at the spray tip to achieve atomization. Changes
in airless spraying are made by adjusting the coating's
viscosity or the system's pressure. This method has
higher transfer efficiency than conventional spray.
Many high-viscosity coatings can be applied without
costly solvent thinning. Also, this method allows for
rapidly applying a heavy paint coat—useful for
keeping up with a fast-moving paint line.
Air-assisted
This spraying system “assists” airless systems
by using supplemental air jets to guide the paint spray
and boost the level of atomization. Air-assisted airless
technology combines the best characteristics of both
air and airless spray. Benefits include: substantial
material savings and reduced overspray when compared
to conventional spray and improved transfer efficiency
and finishing appearance when compared to airless technology.
The ability to lower the fluid pressure from airless
systems is the primary factor in increased finish quality.
Also, operator technique is enhanced as the application
rate is slowed, making product coating easier.
Electrostatics
The electrostatics method gives the paint and the part
opposite electrical charges. As paint is sprayed the
electrical charge draws it to the part, which results
in higher transfer efficiency. In this process, paint
spray is less susceptible to drafts and air currents.
Another advantage is that electrostatic coating helps
wrap coatings around part edges. For safety when using
electrostatics, the part should be grounded or prepped
with a solution that will provide a ground path. With
special equipment, waterbased paints can also be electrostatically
applied.
Rotary Atomization
This application system atomizes paint by dropping a
stream of liquid on to a disk or bell-shaped object
spinning at high speed. Rotary atomizers use electrostatics
to attract paint to the part. This process is useful
for high-viscosity paints and can create a spray without
the use of a thinner, while also maintaining high transfer
efficiency. The equipment needed for this type of application
is very specialized and usually requires a major conversion
of a paint line.
Other Equipment
Spray Booths
Dry booths and wet booths are the two basic types of
enclosures used in most painting applications. The key
difference is that the dry booth depends on a filter
of paper, fiberglass, Styrofoam or metal to collect
overspray, while the wet booth uses water with chemical
additives. Decisions about this equipment should be
made based on the type and volume of painting done and
the type and volume of waste generated by the booth
itself.
Dry filter booths generally meet the
requirements of small-volume painting operations because
of their low purchase cost. One disadvantage to using
a dry filter booth is waste disposal. Often the majority
of waste is the filter media itself, contaminated by
a relatively small amount of paint. Reusable filters
may decrease waste volume and reduce disposal cost.
In some applications, overspray can be collected for
reuse.
If overall painting volume can justify
the investment, a wet booth may work to your advantage.
This type of booth eliminates the need for disposal
of filter media and reduces waste in weight and volume.
This is achieved by chemically separating the paint
from the water then settling, drying, or using a centrifuge
or cyclone to remove the solids for disposal.
Paint Heaters
Paint viscosity may need adjusting before spraying.
Most often this is accomplished by thinning the paint
with organic solvents. But, using solvents for thinning
requires the purchase of additional materials and increases
air emissions. An alternative method for reducing viscosity
is to use heat. Paint heaters use less solvent, have
lower solvent emissions, create more consistent viscosities
and produce faster curing rates. Consult your paint
supplier to determine if paint heaters can be used with
your coating.
Coatings
Organic Solvent-based
This is the traditional type of painting material, typically
containing about 40 percent solids with a relatively
high organic-solvent content. While this coating material
is one of the most versatile, its low solids content
and high percentage of solvent carrier can cause low
overall transfer efficiency. To get the required dry
film thickness more material must be sprayed compared
to coatings with higher solids content and lower volatile
organic compound
(VOC) emissions.
High-solids
This paint type has a higher percentage of paint solids
and a lower percentage of solvent carrier. Overall transfer
efficiency is usually better than traditional solvent-based
paint. The increased solids content means that the application
rate can increase or fewer applications are needed to
get the required film thickness. Air emissions from
the solvent are generally less due to reduced organic
solvent content. A paint heater may be required to reduce
viscosity. Also, the film thickness is more difficult
to control with high solid paints.
Waterbased
These paint types typically have a high solids content,
use water as a solvent and have very low or no organic-solvent
content. Advantages of these paint types include reduced
VOC emissions, reduced fire hazard, minimized or eliminated
hazardous waste disposal and easy cleanup. Some companies
have so drastically reduced air emissions from painting
with waterbased coatings that they no longer need an
air emissions permit or need to report releases on the
EPA's Form R reports. Using a waterbased coating may
require a cleaner surface, longer drying times, increased
oven temperatures and a temperature-controlled paint
storage area. The switch to waterbased materials must
be done carefully. Waterbased coating technology is
the fastest changing in the market today.
Catalyzed or
Two-component
These coatings are created by mixing two low-viscosity
liquids just before entering the application system.
One liquid contains reactive resins, and the other contains
a catalyst that promotes resins to polymerize. These
coatings eliminate or reduce solvents and cure at low
temperatures.
The catalysts and paint components
may be hazardous themselves and create a different set
of emission and exposure problems than those of organic
solvents. Catalyzed painting also means more material
may be wasted if pot life is neglected.
Powder Coating
Producing no VOC emissions, hazardous overspray wastes
or wastewater sludges, powder coatings are 100 percent
resin in dry, powdered form which when cured in an oven
produce a high-quality, durable, corrosion-resistant
coating. Collecting and reusing dry coating material
that does not stick to the part is possible. Reuse allows
powder coaters to achieve very high overall transfer
efficiencies.
Powder coating requires specialized
application equipment using electrostatic charges to
apply the material. The substrate must be able to tolerate
the oven's curing temperature (typically 300 to 450°
F). Advancements in powder coating formulas are occurring
at a rapid pace. New powder coatings are becoming available
to meet special manufacturing needs.
Radiation Cured
Ultraviolet (UV), electron beam (EB) and infrared (IR)
coatings use electromagnetic radiation to cure. These
coatings typically have a lower VOC content than conventional
coatings, require smaller ovens and allow for increased
production rates due to a shorter curing period. The
shape of the part will affect curing—flat surfaces
are easiest to cure. Capital investments are higher
than for conventional ovens and the cost of the coating
material may be higher depending on the application
method used and transfer efficiency.
Equipment Cleaning
Equipment cleaning may be required when painting is
completed, a color change is needed or maintenance is
required during operation. Equipment cleaning offers
opportunities for reducing waste and air emissions.
When assessing the cleaning process,
all cleaning tasks should be reviewed to determine if
cleaning is necessary. While many assume that spray
guns, tips and lines must be cleaned for reuse, cleaning
some low-cost items may not be advisable. Costs from
cleaning-solvent purchases, solvent waste disposal and
solvent emissions could be higher than simply replacing
the item being cleaned.
All solvents should be stored in covered
containers when not in use. Leaving solvents in the
open air creates unnecessary solvent waste and VOC emissions.
A standard should be set to assure that used solvent
is disposed of or recycled only when it loses its cleaning
effectiveness, not just because it looks dirty.
For equipment that requires cleaning,
methods that minimize solvent use and reduce evaporation
are ideal. For example, a gun washer is a piece of equipment
similar to a dishwasher designed to hold a number of
spray guns and related equipment. It cleans by circulating
solvent inside a closed chamber. The result is rapid
cleaning and extended solvent cleaning-life while reducing
solvent waste and the emissions from evaporation. For
more information about automatic cleaning systems see
MnTAP's reference list Spray
Gun and Equipment Cleaning System Suppliers [#79].
One method for line cleaning introduces
turbulence to the solvent going through the line during
cleaning by alternating pulses of solvent and compressed
air. Payback on this equipment can come from increased
production output due to faster color changes and from
material savings through decreased solvent use.
Solvent Reuse
Alternatives
On-site recycling of used solvent is another way to
reduce waste and save money. Savings come from reducing
the amount of solvent purchased, and decreasing disposal
cost by reducing the volume of spent solvent that must
be sent off-site. Three common methods of solvent recycling
are settling, filtering and distilling.
Settling is putting the used solvent
in a container and letting the particulate matter settle
out. The container should be designed to allow for the
removal of the solvent without shaking up the sludge
which has settled. Filtering equipment which removes
the particulate matter from solvents is also available.
Distilling is an attractive option
for many organic solvent users. Equipment is available
in a variety of sizes, one-gallon capacity and larger.
For more information, request reference materials on
solvent recycling and selecting a still from MnTAP.
Alternative
Solvents
Due to the increased need to reduce VOC and hazardous
air pollutant emissions, alternative cleaning solvents
are being used. Alternatives include formulas containing
acetone, dibasic esters (DBE) and terpenes. Although
acetone is not considered a VOC, it is extremely flammable.
For more information see MnTAP's reference list Safer
Stripping and Cleaning Chemicals for Coatings and Polymers
[#55].
Operator Technique
An operator's technique affects transfer efficiency
and if improved upon or corrected will reduce waste.
When using spray equipment, maintaining the distance
between the gun tip and painting surface according to
equipment specifications will help assure the proper
film thickness. For electrostatic
applications, the distance between the spray tip and
the part also affects the charge the paint maintains.
Gun angle relative to the painting
surface also affects transfer efficiency. Keeping the
spray gun perpendicular to the painting surface helps
avoid uneven coverage that might otherwise require more
paint than necessary to produce an acceptable finish.
One tool to assist spray painters
is Laser Touch. Mounted on a spray gun, the Laser Touch
unit emits two laser beams that converge into one dot
when the gun is properly positioned. The visual signal
of both lasers coming together on a part lets operators
instantly know if they have proper aim, gun-to-part
distance and gun angle. Call MnTAP for more information
about the Laser Touch.
A new tool to improve operator technique
is virtual reality spray painting. Virtual reality software
and equipment can be customized to match the parts painted
at your company along with the coating viscosities,
fluid pressures, air pressures and spray equipment you
have. Painters can be trained virtually with no mess,
waste or reject parts. Virtual reality spray painting
was developed by the Johnson Center for Virtual Reality
and Pine Technical Collage. Call MnTAP for more information.
Triggering the spray gun on and off
at the appropriate time will minimize overspray and
improve finish quality. Also important are proper spray
overlap and the speed of the stroke.
Training and experience will provide
operators with a knowledge of the various painting techniques
needed to paint parts of different configurations. Different
techniques are helpful when painting inside and outside
corners, or slender, round, flat, large and small parts.
Training options are available through trade associations,
material suppliers and equipment vendors.
Additional
Resources
Additional printed resources about coating and related
industries are available from MnTAP. Call to request
the checklist Resources
for Minnesota's Coating Industry [#35]. Resources
also can be found on the Painting & Wood Finishing
page on MnTAP's Web site, or search the site for topics
of concern to your business.
For More Information
MnTAP has a variety of technical assistance services available to help Minnesota businesses implement industry-tailored solutions that maximize resource efficiency, prevent pollution, increase energy efficiency, and reduce costs.Our information resources
are available online. Or, call MnTAP at 612.624.1300
or 800.247.0015 from greater Minnesota
for personal assistance.
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