Abstract
A study of various performance characteristics of water based and solvent
based coatings was conducted. Typical chemistries involved in their
formulations and relevance in coatings were discussed. Effect of cross-linkers
and surface etching phenomena in enhancing the adhesion properties were
studied. A spherical X- Rite® spectrophotometer was used to determine
the color-shift properties. An MTS® tensile tester was used to measure
adhesion properties. A transfer press laminator was used to make laminated
cards and check adhesion of the coatings to various inked core stocks.
Other tests, blocking, and aging tests were also conducted to compare
and contrast the salient features of the coatings.
Introduction
Basic Definitions
Polymer: A macromolecule consisting of a large number of combining
units called monomers. In simple words, it is the film former that creates
the mechanism for cure and is the primary source of the coating's durability
and performance.
Adhesive: A substrate capable of holding materials together by surface
attachment.
Dispersion: A single-phase system consisting of polymer dissolved in
a liquid.
Polymers for water based coatings
There are two distinct formulation types of adhesives used for heat
activated adhesion of clear card overlays. Typically, water based coatings
for card manufacturing are based on cross linked urethane polymers.
This is due to their abrasion resistance, chemical resistance, flexibility,
impact resistance, and unusually high mechanical properties. However,
yellowing from ultraviolet radiation is a serious drawback to aromatic
type urethane coatings. Consequently, UV inhibitors are often included
in formulations. Another way to arrest yellowing is to use aliphatic
polyurethanes.
The prepolymer ionomer [Figure 1] when dispersed in water will form
extended chains and anionic polyurea-polyurethane.
Polymers for solvent based coatings
Solvents used in solvent based coatings are used to affect coating
thickness, application, the flow and the final uniformity of the coating
on the substrate. It is unusual for one solvent to successfully provide
all the desirable properties necessary for a coating, therefore, blends
are frequently used to satisfy all requirements. The main requirement
is the solvent or the blend must be compatible with the vehicle and
must have chemical properties that will not adversely affect the coating
or harm the substrate. Beyond that, toxicity, safety, odor, evaporation
rate,
solvency, viscosity, flash point, boiling point and of course, cost
must also be considered. The most commonly used solvents are aliphatic
and aromatic hydrocarbons and oxygenated compounds.
There are several solvent based coatings available for heat laminating
applications and the most common and widely accepted coatings are polyamides.
The basic formulation is solubilizing the resin using solvents such
as Toluene and Tetrahydrofuran [THF] and then coating the recipe onto
the film. To describe the adhesion chemistry in the polyamide; polymer
chains are bonded together by hydrogen bridges between the carboxylamide
groups. Typically, dimer fatty acids are used for better adhesion properties
[Figure 2].
Card manufacturers use a heat-lamination technique called planishing
for making glossy and durable secure or non-secure cards. There are
several parameters that affect the performance of the final card and
some of these critical properties are evaluated in the subsequent sections.
Experimental
A water based system and a solvent based system were selected. The
main ingredients in the water based system were urethane polymer dispersed
in water medium, cross-linker, and additives for anti-blocking and UV
inhibition. For the solvent based system the main ingredients were polyamide
base polymer solubilized in Toluene, THF and anti-blocking additives.
Both the formulations were coated on PVC clear film overlays and comparative
studies were done to evaluate their performance.
Adhesion
Three inked PVC core stocks, two blue and black litho inks and one
UV cured metallic silver were used to compare the adhesion properties
of the two coatings. Five repetitions were done for each test and the
average peel values were reported. ASTM D 903-93 test method was used
to determine the bond strength [Figure 3].
Color-shift
An X- Rite® SP68 Spectrophotometer was used to find the L, a, b
and Delta E values of the coatings.
Blocking
Ambient conditions of temperature and humidity can play an important
role in the blocking of heat seal coatings either in roll form or in
stacked sheet form [Ref.1]. Hence, the blocking studies were done in
a controlled environment of 70°F and 50% relative humidity. 5"
x 6" size samples of the coated overlays were stacked and tested
for blocking by placing a 50 lb weight on the samples for 3 days. Blocking
was measured by rating the noise level observed while separating the
sheets from one another; and by rating the noise level while unwinding
the rolls for coatings in roll form.
Aging Studies
Coatings were subjected to 150°F, 90% relative humidity environment
for 2 days [Figure 4]. Both the laminated cards as well as sheeted coated
overlay stacks were placed in the aging tester and peel values, blocking
and yellowing properties were evaluated.
Lamination
A custom engineered pilot 12"x19" transfer press laminator
was used to compare and contrast the coatings for color-shift, core
distortion, core flow, oozing, and adhesion properties.
Results & Discussion
Coating weight limitations
As discussed earlier, in the coating process one of many critical control
functions is adhesive and coating weight deposition. Coating weight
control is crucial to balancing the performance and economics of a coating
process. Solvent based coating's performance is very sensitive to their
coating deposition. If the coating weight is more than the threshold
coating weight, then the adhesive will tend to move the core stock causing
severe oozing, color-shift and processing problems during the lamination
stage. The reason for this phenomenon can be attributed to the thermoplastic
nature of the resin that tends to flow when exposed to elevated conditions
[Ref.2]. Un-crosslinked water based coatings will exhibit a similar
phenomenon whereas a crosslinked urethane will not.
Adhesion
Both systems enjoy good peel values beyond ISO specifications on a
variety of process inks when the proper formulations and coating techniques
are used [Figure 3]. The inferior adhesion properties of solvent based
coating against UV cured metallic inks [Figure 3] can be attributed
to the polyamide's brittleness and incompatibility towards the acid
groups of the acrylic based metallic inks. Another theory is the substantial
volume reduction during the curing stage of UV cure coatings. This loss
in volume leads to stresses in the film that, in effect, partly supplies
the force needed to pull the film from the substrate. Hence, less external
force is needed to remove the film. In other words, this leads to poor
adhesion.
On the other hand, water based coatings have better bonding to the
metallic ink substrates because of the nature of the multi-functional
amine crosslinker that can bite into the metallic coating by reacting
with the acid group functionalities in the metallic inks thereby resulting
in superior peel values. The crosslinker in the water based coating
not only forms a three dimensional thermoset network with the urethane
polymer but also has the tendency to bite into the inked substrates
depending on the functionality of the acidic groups in the inked substrate.
However, the stoichiometry of the crosslinker to the urethane polymer
and their compatibility to the other additives must be tailored in such
a way so that optimum results are achieved.
Color-shift
L, a, b and Delta E values of the coatings against full-bleed blue,
black and UV cured metallic silver inked core stock were studied after
laminating the cards using a transfer press laminator. Five cards of
each color were analyzed and the average values were reported. The coatings
were compared against standard laminations made from uncoated overlays
on the same inked core stocks to determine the Delta E value [Table
1].
Blocking
At controlled conditions, the blocking of a solvent based polyamide
coating is less severe than water based, urethane coatings. This is
due to polyamide resin coating's softness. Typically, water based coatings
have worse blocking than solvent based coating and are consistent with
the blocking rating irrespective of overlay film gauge. The reason for
this can be attributed to the anti-blocking additives used in these
coatings. For water based coating, the anti-blocking property mainly
comes from the particle size of the anti-blocking additive used in the
formulation, which tries to occupy the bumpy sites on the overlay [embossing]
and provides the anti-blocking barrier.
Aging - sheets
For a proper evaluation of an adhesive coating, it is important to
consider the effect of aging on its performance. Certain regulations
impact the adhesive performance over time. The final performance characteristics
need to be evaluated in terms of stress, heat, humidity or moisture
resistance. The aging tests results show severe yellowing and shrinkage
of the solvent based coatings. Neither yellowing nor significant shrinkage
was observed for the water based coating [Figure 5].
The reasons for this can be attributed to the UV inhibitors in the
water based coatings as well as the aliphatic urethane properties, which
can sustain yellowing. In the case of the solvent based coating, there
is a possibility of breakdown of the polyamide linkages at elevated
temperatures and in high humidity environment to release free amines,
which in turn cause yellowing. Further evaluations of the coatings indicate
significant blocking for both coatings. However, for the water based
coating, the overlays were separable without significant adhesive transfer
to the bottom layers and acceptable adhesion values were obtained on
the cards laminated using the aged overlay films. The solvent based
coated overlay samples were found to be blocked together and inseparable.
Aging - lamination
The peel value comparison between the aged [Figure 6] and un-aged [Figure
3] laminated cards show some decrease in the coatings' performance after
being subjected to an aging test. However, it was observed that the
aged laminated cards of water based coating showed higher adhesion strength
than solvent based coating [Figure 6].
A 2 year aged [at ambient conditions] stack of solvent based coating
and a 2 year aged stack of water based coating were checked for yellowing
against a 2 year aged stack of un-coated overlay [Figure 7].
From Figure 7, we can see the accelerating aging effect of solvent
based polyamide coating and its susceptibility to UV light. Delta E
values of the solvent based coating and the water based coating against
uncoated overlay as standard are 5.24 and 1.86 respectively.
Lamination
Due to the thermoplastic nature of the solvent based coatings, oozing
/ flowing of adhesive or the core-stock distortion is common during
the lamination process. This problem also causes a color-shift. Solvent
based coating users have developed process techniques to combat this
phenomenon including using a border etched silver sheet to hold the
distortion. Sometimes, this precaution may not be sufficient especially
if the coating weight is more than the upper specification limit. From
Table 1, it was evident that the Delta E value on blue inked core stock
using solvent based coating was 1.59 (> 1.0), which was considered
a perceivable color shift
to the normal eye.
Other Issues
With regard to environmental issues involved, polyamide resin being
polar in nature with a solubility parameter of 17.5 (J/cm3)1/2, similar
polar solvents, such as Toluene [18.2 (J/cm3)1/2] and THF [18.6 (J/cm3)1/2]
are typically used to solubilize the resin [Ref. 2, 3]. Toluene has
been identified by USEPA as a toxic air pollutant or Hazardous Air Pollutant
[HAP]. Hence, some card manufacturers, especially from countries with
strict environment regulations, raise concerns on the residual solvent
content in the solvent based coatings.
The excellent adhesion properties of polyamide solvent based coating
were attributed to the surface etching which actually increases the
effective surface energy of the substrate, and the polarity contributing
to the hydrogen bonding between the carboxylamide groups. However, their
resistance to most industrial solvents precludes their widespread use
as coatings. Water based coatings show superior chemical resistance
compared to the solvent based coatings. The reason is the stable thermoset
formation through the cross linking reaction between the amide and urethane
polymer. Moreover, the urethane and urea functional groups provide very
strong hydrogen bonding between neighboring functional groups which
results in the good solvent and chemical resistance to the polyurethanes.
In other words, the better the stoichiometry between the cross linking
agent and the polymer, the better the solvent resistance will be.
In the field, there are reported overlay processing differences between
the two systems. Processing issues such as mag-stripe layers becoming
tacky after running the solvent based coated rolls, residual solvent
smell, film tackiness during days with high humidity, and adhesive build
up at the razor blades needing frequent clean-up are some of the major
issues for the card manufacturers when working with solvent based adhesives.
Blocking and coating opacity are major issues with water based coatings.
The chemistry and physics of these drawbacks have
been described in this paper.
Conclusions
The pros and cons of water based and solvent based coating in card
manufacturing have been demonstrated. The data presented shows that
solvent based coatings show excellent adhesion properties on PVC core
stock with litho inks and show inferior properties on UV cured metallic
inks. It was also shown that solvent based coatings were more susceptible
to severe environmental conditions than the water based coatings. Oozing,
blocking, yellowing, color-shift and environmental issues were discussed
and reasons for solvent based coatings' handicap in these areas in comparison
with water based coatings were attributed to the basic resin chemistry,
resin properties and toxicity of the solvents involved in the formulations.
Finally, the versatility of polymer characteristics is only one part
of the potential variety that can be chosen in the development of any
adhesive. Additives such as rheology modifiers, colorants, etc., greatly
influence the performance of an adhesive. Every adhesive formulation
has its own personality. This uniqueness of the bonding process reminds
the author of the well-known statement in the industry that there is
no one universal adhesive that can meet all the
requirements of the ever changing needs of the customer. I would suggest
four main factors considered when evaluating a coating selection. They
are bonding method /process, coating weight, performance and cost. There
are always choices and tradeoffs with compromises to be made. For example,
adhesion vs. cohesion, cost vs. performance, etc. Therefore, it is up
to the customer to weigh all the factors in light of what is important
to them and select a coating accordingly.
Acknowledgements
I would like to thank Tod Gregory and William J. Crawford, for technical
suggestions.
References
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Part II; Paper Film Foil Converter, November, 2000.
2. Kirk, R.E.; Othmer, D.F.; Encyclopedia of Chemical Technology, Vol.
6, IV edition, A Wiley-Interscience Publication, 1995.
3. Stoye, D.; Freitag, W.; Resins for Coatings: Chemistry, Properties,
and Applications, Hanser Publishers, Cincinnati, OH, 1996.