Paul Mills and Andrew Stecher, Plasmatreat USA, Elgin, IL
Abstract
UV curable coatings are becoming an increasingly popular alternative to conventional coatings. Compared with thermal
coatings, UV coatings provide a number of benefits to plastic
part manufacturers including enhanced appearance, improved
performance, and various process worker safety, and environmental advantages. But, same high speed curing and highly-crosslinked chemistry that underlie these benefits can also make
adhesion failures more likely. This paper examines the problem
of adhesion common to UV curable liquid and powder coatings,
and describes some tradeoffs associated with popular methods
to reduce adhesion problems. Atmospheric plasma provides an
especially attractive method of enhancing adhesion of UV cure
coatings to a wide range of plastic materials.
Background
Since the early 1970s, UV curable coatings have gained slow,
but steady acceptance as an alternative finishing technique for
a wide range of substrates from wood flooring, to glass optical
components, and from pipe and tube to plastic cosmetics containers. Today for example, nearly all polycarbonate headlight
lenses, most plastic commercial eyewear, and a large percentage
of plastic consumer electronic devices are UV coated.
A number of factors are believed to be responsible for the success of UV coatings growth (Cohen, 2012). First, UV curing is
an extremely rapid process compared with conventional thermal
baking and curing. UV formulations cure almost instantaneously
when exposed to ultraviolet light (Walton, 2012). By contrast,
conventional waterborne and solvent borne systems require
substantial oven dwell times and relatively higher temperatures.
This makes UV curing attractive for coating applications such as
graphic arts and printing, optical fiber coating, wood molding
and panel finishing, and similar high speed applications.
This speed advantage is even more impressive when com-
paring traditional thermoset powder coatings to UV curable
powders. Once applied, traditional thermoset powder coatings
require a substantial amount of time in order for the powder
to melt and flow to produce a smooth, continuous film. This
melt/flow process is important to achieve both aesthetic and
performance qualities. After flowing, thermoset powder coat-
ings also require substantial dwell time at high temperatures
to achieve the complete polymer cross-linking needed for full
performance. Taken together, this two-stage process routinely
requires between 20 and 60 minutes (Walton, 2012). By con-
trast, UV curable powder coatings also use thermal energy to
melt and flow the powder coating, but rely on ultraviolet light
instead of heat to achieve crosslinking. With UV cure powders,
total process times of less than 10 minutes have been reported
(Schwarb and Knoblauch, 2011) .
Another feature of UV coatings is their tough surface properties; particularly high scratch and mar resistance. It is these
properties that make UV coatings especially well-suited for applications such as hardwood flooring, optical coatings, and CD/
DVD coatings where surfaces often must take a good deal of
abuse. The surface durability of UV coatings stems from the high
cross-link density common to UV formulations particularly those
using popular (meth)acrylate chemistry (Schwalm, 2006). But
while high crosslink density provides tough surfaces, it has some
drawbacks as well. For example, physical shrinkage is closely associated with the high crosslink density found in UV films. The
acrylate monomers and oligomers shrink considerably as longer-distance Van der Waals forces are replaced by strong but shorter
covalent bonds. Schwalm (2006) suggest that shrinkage as high
as 35% can occur in UV formulations. Shrinkage causes internal
stress that can result in defects and dimensional changes leading
to decreased adhesion (Jian et al. 2013).
A third feature of UV cure liquid coatings is that instead of
conventional organic solvents, UV formulas frequently employ
low molecular weight additives such as monomers and other reactive diluents. These are fully consumed in the curing process,
leading to the notion that some UV coatings are “100% solids”
materials. This feature has attracted the attention of government
regulatory agencies and environmentally conscious manufacturers (Loof, 2001). But again, while eliminating solvents results in
fewer hazardous air pollutants and lower VOCs, removing these
solvents presents a challenge to attaining adhesions, since solvents help wet-out the surface of the part. Powder coatings emit
virtually no VOCs or hazardous air pollutants, and contain no
Overcoming Adhesion
Failures of UV Coatings with
Atmospheric Plasma Treatment
