This history of coatings for plastics, like that of paint
itself, is an evolutionary story. Paint used by cavemen on the walls of their
caves or decorating the faces of Indian warriors was formulated for decorative
purposes. As more durable goods were developed, some paint also filled a more
functional purpose. Today’s automotive paints combine spectacular pigments and
resin systems for good looks and durability. The rusted out car is a thing of
the past thanks in part to better coatings.
Coatings for plastics have followed this same arc – from
decorative and functional purposes to the latest coatings that combine the best
of both. Today’s cell phone for example is a fashion statement for many teen
owners; but its hard, clear topcoat is engineered to withstand scratching by
coins and keys in their pockets at the same time.
New applications that rely on both the beauty and brawn of
coatings continue to evolve. A case in point is the intense interest in coatings
used in metal sputtering or physical vapor deposition (PVD). This hubbub of
activity is occurring because hexavalent chrome, used in the chrome plating
process, has been found to be a health risk and is being eliminated from many
uses. The resulting search for "chrome alternatives" has led to PVD. What’s
more, plastics can be made to look like their metal counterparts, thereby
reducing weight and opening up new designs for a wide range of automotive,
household, and industrial products.
The quest to find a suitable chrome alternative has led to a
process combining metal deposition with a sandwich of base coats (that provide a
smoother surface), and top coats that provide a hard, durable finish – once
again, providing both strength and beauty. But is there a chance to be more than
just a chrome "replacement"? More generally, can coatings be more than just
decorative paints and durable surfaces? Today’s chemistry is allowing paint
formulators to provide added value to paint – transforming the role of coatings
from decoration and protection to an even higher status in a product’s function.
Some of these product attributes include the following:
1. Easy-to-clean coatings. Shiny, high-gloss products look
great. A TV bezel painted piano black looks great on the plasma TV but not so
attractive when it’s smudged. Oleophobic coatings designed to resist oils from
the skin not only make this surface harder to smudge, but the oils wipe off
almost effortlessly. Anti-fingerprint and fingerprint-resistant coatings are
attractive for PVD items, kitchen appliances, car trim, and consumer electronics
where retaining appearance on shiny surfaces is important.
2. Antimicrobial coatings. Not only can the surface of the
countertop toaster look prettier, or the handle on the fridge look like chrome –
but the PVD/coating system can help fight invisible germs by incorporating
antimicrobial additives into the top coating. Efficacy testing reliably
concludes that these coatings can be effective in helping to kill organisms that
commonly grow on these surfaces.
3. Extreme scratch/mar resistance. New additive technology
and advanced resin systems now facilitate top coats that provide surfaces that
are much harder and scratch-resistant than ever before.
4. Anti-theft coatings. Coatings can now contain additives
that make them easily identifiable as OEM-applied materials. Technology that is
nearly impossible for counterfeiters to replicate helps protect brand identity
and offers consumer safety.
Developments Leading to PVD
Slow to adopt new technology, it has been frustratingly
difficult to convert end users from the traditional high-solvent paints in favor
of waterborne, UV and other new materials despite cost and performance benefits
that might be expected to sway manufacturers to use them.
Until their hand is forced by regulations, many manufacturers
hold fast to the old methods of painting. Even as production shifts to Asia and
a chance to begin anew, we see children’s toys arrive coated with decades-old
Now, two forces have collided which have spurred interest in
new coating systems. The insatiable consumer demand for "bright metal" finishes
combined with environmental pressure by regulatory authorities has kicked off a
search for chrome alternatives in markets ranging from alloy wheels to cell
phones. The following factors have contributed to this current situation:
• Hexavalent chrome has been labeled a potential carcinogen
and regulations to eliminate its use. Hard chrome plating has faced enormous
pressure due to health and safety concerns.
• Strong consumer demand for chrome appearance. The
proliferation of personal electronics and teletronics has created new markets
for attractive goods where bright metal appearance is desired.
• Migration from metallic substrates to plastics for greater
design flexibility and weight reduction, but where heat-sensitive substrates
limit the processing options.
• The drive to lower cost. Pressure to find ways to make
parts cheaper is at odds with plating processes that are notoriously
energy-intensive and inefficient.
Manufacturers faced with the need to eliminate hexavalent
chrome from their manufacturing have explored a range of alternatives. These
alternatives include minimizing hex chrome with new, improved methods: Trivalent
chrome systems; PVD with no top coating; and ‘chrome-colored’ paints. Without
addressing the details, each of these alternatives has its own merits and
limitations. But as alternatives have been explored, the use of PVD
metallization has gained a lot of traction and there has been significant
commercial success in combining PVD and specialized coatings to provide an
acceptable chrome replacement.
Basecoats: Adhesion and Leveling
While it would be desirable to metallize directly to the
plastics, this has not been practiced in most applications since, even when
produced with high quality molds, plastic surfaces are too rough for direct
metallization. The variability of the substrate (sometimes made of virgin resin,
but often a blend of regrind and virgin resin) along with noticeable defects in
the molding process make metallizing bare parts too difficult. The tolerance and
maintenance of molds smooth enough for producing parts that can be metallized
directly is prohibitive.
Instead, a base coat is applied to the substrate. This base
coat frequently provides three benefits:
It ensures good adhesion of the metal layer.
It provides a smooth, level surface upon which to
It provides added protection to the underlying substrate
against effects such as UV degradation of the material.
Top Coats: Protection and Added Appearance Options
A smooth base coat topped with a microscopically thin
layer of bright metal has a beautiful lustrous appearance – but little in the
way of durability. The time, expense, and inherent difficulties of depositing
metal PVD layers typically results in metal films only angstroms thick,
certainly much thinner than a typical paint film. This does not provide an
adequate, weatherable barrier nor is it sufficiently tough enough to stand up to
To protect the metal layer, a protective top coat is
ordinarily applied. The top coat provides the necessary properties common to a
wide range of painted goods. When the bright metal PVD layer has the desired
appearance, the top coat might be formulated to be a water-clear coating,
providing a protective but transparent barrier for the fragile metal layer.
The top coat also can be formulated with tints, metallic
flake, and other effects that alter and enhance the appearance of the PVD film.
Top coats can provide color shifts, tinted chrome appearance and sparkle to an
otherwise lustrous finish. Along with modifications to the target material and
process gases used in the PVD process, the top coat can provide another tool for
designers looking for unique appearances.
Many PVD applications benefit from top coats that can be
applied as thin as possible while still achieving the needed performance
specifications. Sometimes this is due to functional aspects of parts, which must
fit other mating parts. Generally a thinner coating provides the greatest bright
metallic effect at the lowest manufacturing cost.
Processors have turned to ultraviolet-cured (UV) coatings for
thin top coats. UV coating chemistry is ideal for providing exceptionally clear,
mar resistance at lower film builds. UV coatings provide several other
Clarity. UV coatings are used for coating optical devices
from eyeglass lenses to precision optics. These water-clear coatings are a
perfect complement to bight metal PVD layers where luster is desired.
Scratch and mar resistance. UV coatings are noted for
tough surface properties stemming from the very high crosslink density common in
UV curing. Hardwood flooring is a common example of material that utilizes UV
coatings to resist scratches.
Speed of cure. UV coatings cure nearly instantaneously.
An obvious advantage of fast cure is productivity and reduced floor space
required for production. But importantly, time also means dirt. The longer a
mirror-like surface is exposed to airborne contaminants the more likely is
contamination. UV permits a lean production cell where components are metallized
and then moved directly to a compact UV coating line where only moments later
the part is coated and fully cured.
Low energy requirements. While energy consumption was a
‘soft’ cost issue only a few years ago, the current skyrocketing costs of energy
have many producers revisiting the low energy cure requirements of UV coatings.
But while UV top coats offer significant thin film benefits,
simultaneous demands for the coating (scratch, mar, clarity, flexibility, deep
colors, anti-fingerprint, anti-microbial, hydrophobicity, and anti-theft) force
formulators to consider tradeoffs between various properties or compromises on
the customer’s wish list.
Introducing the Mid-Coat: Opening New Doors
Constantly asking the top coat to do more has its
limits. Tradeoffs exist between competing properties such as extreme scratch
resistance and enhancing deeper, high chrome colors or special effect
appearances. A solution to this formulation tug-of-war is to introduce another
coating layer or ‘mid-coat’ to the paint process.
A mid-coat can provide another layer of opportunity to
achieve looks and functionality difficult to produce in a single protective top
coat. Use of the mid-coat system has already produced commercial successes
unavailable from conventional two coat systems. For example, top coats can be
formulated with extreme scratch-resistant properties, while mid-coats can be
formulated to enhance the appearance of deeper metallized color choices. The
color palette has been expanded significantly while the surface properties are
As consumers continue to clamor for products that sport
bright, chrome appearance in a wide range of industries, producers actively seek
alternatives to a chrome process being regulated out of existence. Other
technologies are being trialed, but one of the most promising may be a
combination of physical vapor deposition and a system of base and top coatings
which offers both appearance and performance.
A groundswell of development in nanotechnology, new coating
additives, and resin systems now enables coatings to provide a range of
specialized functions. Coatings can resist water, fingerprints, and bacteria.
Not only will future parts satisfy the cosmetic and durability requirements of
customers but also, they may provide functions not attainable with any other
The combination of a base coat to provide adhesion and a
mirror smooth surface for metallization along with a clear or tinted top coat
can provide a part that has a lustrous bright metal appearance along with robust
The latest introduction of three-coat systems utilizing
mid-coats provides manufactures with greater flexibility and choice. By freeing
the formulator to develop coating properties difficult to achieve in a single
layer, a greater range of color, gloss, physical performance, flexibility, and
other cosmetic and physical properties can be provided. But even more exciting
is that in addition to PVD/coating systems providing a bona fide "chrome
replacement", they also can provide new functionality. Coated parts not only can
have a beautiful and tough finish but also, can fight germs and resist water,
fingerprints, and dirt. These coatings can provide new special effects or
provide value such as brand identity protection. In the 2000s, paint continues
to evolve from the decorative purposes that date back to cave walls to a truly
integral aspect of a part’s form and function.
Kalcor Coatings Company, Willoughby, Ohio, specializes in custom paint
formulations for a range of automotive and industrial markets including a
complete line of decorative coatings for plastics. Kalcor also is the designated
North American partner of Cashew Company Ltd. of Japan, a global supplier of
coatings for plastics. For more information about Kalcor, call Pam Thrall at
(440) 946-4700, email email@example.com
or visit www.kalcor.com.