Waterborne Binder for Primer Systems
by Lorenzo Neil Petway
February 1, 2010
Since
the early days of epoxy chemistry, the standard primer based on a “1-type”
solid epoxy resin combined with a polyamidoamine has dominated the field of
corrosion protection. Besides the excellent corrosion protection properties,
several other attributes have led to the success of this type of
system:
- tolerant
mixing ratio;
- extremely
long pot life for all types of application techniques (brush, roller,
spray);
- very
good mechanical properties (impact resistance and
adhesion);
- relatively
short drying time (dust dry);
- good
optical aspect (flow and gloss); and
- cost
effectiveness.
These systems, however, require an induction
period, a period of time that allows the epoxy and curing agent (binder) to
react to some degree before the coating is applied. This increases the
compatibility of the binder components and results in improved performance of
the coating. The need for an induction period can be reduced or eliminated by
modification of the hardener (e.g., adducts).
From a performance point of view, there are few reasons to change this system.
The main drivers for new development today are legal or environmental aspects.
Solvents are a major contributor to formation of ground-level ozone (“smog”),
an air pollutant partially responsible for environmental and health problems.(1)
Unfortunately, this 1-type/polyamidoamine system requires a substantial amount
of solvent to be applied (approximately 50% v/v for spray application). The
EPA, California’s South Coast Air Quality Management District (SCAQMD) and many
European countries have established solvent reduction programs forcing
formulators and applicators to reduce the amount of VOC in their coatings.
Private companies and industry groups have also agreed to reduce their VOC
output respectively, and set limits for their product lines by evaluating
alternative environmentally friendly technologies such as: waterborne,
high-solids, solvent-free and powder coatings.
Everyone agrees that less VOC is better for the environment. However,
there are often technical reasons why this seemingly simple goal can be very
difficult, if not unlikely, to achieve. This paper presents a low-VOC,
waterborne, anticorrosive primer for heavy-duty corrosion protection.
Standard Epoxy Binder for Metal Primers
The
standard epoxy system is based on a low-molecular-weight (1-type) solid epoxy
resin. It contributes good adhesion and flexibility, good chemical resistance
as well as physical (‘lacquer”) drying performance to the coating. It is also
compatible with the polyamidoamine chemistry of the
hardener.
The traditional polyamidoamine hardener is a highly viscous material that is
typically used in solution form, usually in an aromatic solvent. The backbone
of this hardener is a dimerized fatty acid reacted with an ethyleneamine,
typically triethylenetetramine. This curing agent produces flexible coatings
with good adhesion, water resistance and excellent corrosion
protection.
A combination of the 1-type resin and
polyamidoamine hardener results in solventborne coatings with a pot life of
more than 8 hours; more than enough for an average working day. For improved
corrosion protection a slight excess of epoxy resin is often used. Due to the
solid and highly viscous components, additional solvents have to be used to
allow for coating application. For conventional spray application, the solvent
content is increased to lower viscosity resulting in a high VOC level in excess
of 450 g/L. The high level of solvent required to reach application viscosity
greatly improves the flow characteristics, ease of application and lengthens
the pot life.
VOC Reduction
It is obvious that reduction of the solvent (VOC)
content in such a binder system will result in a coating that would be very
difficult, if not impossible, to apply using conventional equipment. Apart from
using specialized application equipment designed to apply highly viscous
material, the most logical ways to reduce VOC is to use a less viscous resin
and/or hardener, use VOC-exempt solvents or use waterborne technology.
High-Solids Systems
The different stages of cure for thermoset systems
(dust dry, dry-to-touch, dry-to-handle, etc.) are proportional to the build up
in molecular weight through the formation of the polymer network. Traditional
1-type-based systems start this race of solidification at a relatively high
molecular weight. Solidification by solvent evaporation depends mainly on the
evaporation rate of the solvent. High-molecular-weight polymers lead to high
solution viscosity and a high level of solvent is required to achieve
application viscosity. Using lower-molecular-weight epoxy resins, such as basic
liquid epoxy resin (epoxy equivalent weight of 182-192 g/eq.), reduces the need
for high solvent levels at application viscosity. To achieve solidification of
the applied film, coatings based on liquid resin require chemical reaction with
the hardener component. This chemical reaction generally requires more time
than pure solvent evaporation. Therefore, these high-solids systems take longer
to reach the dust dry or dry-to-handle stage, especially at lower ambient
temperature. Liquid resins also have a much higher epoxy content compared to
solid resins, leading to shorter pot life compared to 1-type resin-based
systems.
VOC-Exempt Solvents
Reduction
of VOC using VOC-exempt solvents is not as simple as it seems, and formulating
with them can be challenging. These solvents will usually bring higher costs
when compared to traditional solvents. Also, some have very low flash points,
which lead to increased shipping costs and safety issues. Differences in
solubility, incompatibility with amine hardeners and high evaporation rates are
factors critical to performance that must be addressed properly, otherwise poor
performance and application properties may occur.(1)
Waterborne Systems
Quite possibly the best way to reduce the VOC level is
to eliminate most, if not all, of the solvent by using an inexpensive, non-VOC
carrier such as water. Waterborne epoxy emulsions offer the opportunity to use
high-molecular-weight solid epoxy resins used in the 1-type/polyamidoamine
systems discussed previously, without much concern over viscosity. In these
solid resin emulsions the resin is dispersed in water using a surfactant. The
surfactant allows the formation of discrete microscopic particles on the order
of 1 µm. These particles require some solvent (a VOC) to facilitate coalescence
of the particles to form a continuous film.
Waterborne coating
formulations can be very sensitive to changes in raw materials and differences
in formulations when compared to robust solvent-based systems. Minor changes
can cause a significant drop in performance. Therefore optimization of the
formulation and choice of raw materials is essential to ensure stable
performance of a waterborne coating. A poor choice of waterborne hardener can
also lead to poor performance and has led to some skepticism of waterborne
epoxy technology for critical applications. Formulators need to be made aware
of the technologies and commercial products available to help them optimize
formulations and meet the demanding performance criteria that exist today.
Current Requirements
Over
the last few years new requirements have been introduced for primer systems.
Many of these changes are driven by regulatory requirements and/or economics.
Some examples are listed below.
- Improved
adhesion to various substrates, especially poorly prepared and difficult
substrates such as untreated cold-rolled steel. This lowers the project costs
by minimizing surface preparation.
- Cost
improvement by reduction of maintenance work and extension of warranty
(improved corrosion resistance).
- Faster
cure, without sacrificing pot life.
- Improved
operator safety by providing hardeners that do not contain corrosive volatile
amines. This may reduce the potential for skin and eye irritation, as well as
sensitization effects.
- Broadened
application window for waterborne systems for applications below 15
°C.
All of these properties are mainly influenced by the technical profile and
capabilities of the curing agent. The latest trends also reveal a demand for
very low-VOC formulations that offer the same drying profile as solid
resin-based coatings.
Waterborne Epoxy Primers
Waterborne 2K epoxy primers have been used
successfully for more than 15 years in industrial applications. During the last
few years, growth in demand for waterborne coating systems has significantly
increased due to new American and European legislation regarding the protection
of the environment and reduced VOC levels for coatings. For example, SCAQMD instituted a VOC limit of
100g/L for primers effective July 1, 2006.(2)
Initially there were some restrictions for the use of waterborne
products in certain applications because of their inability to meet the
required performance properties. However, the performance of waterborne systems
has improved dramatically over the past few years with the introduction of
specialized products for applications on metal surfaces. These developments
have drastically reduced the limitations for waterborne products as a
replacement for solventborne (and high-solids) systems in anti-corrosive primer
applications, with the exception of the physical restrictions of using water.
The storage and transportation issue of waterborne products in cold environments
is limited to the epoxy resin emulsion as its freeze-thaw stability is
typically poor. This issue can be completely avoided by using systems based on
basic liquid epoxy resin. However the waterborne curing agent must be able to
emulsify the liquid epoxy resin. Optimally, the curing agent will also provide
adequate pot life.
New Epoxy Binder Technology
In
response to the current performance requirements for high-performance
VOC-compliant primers, a new waterborne amine curing agent (Aradur®
3986 epoxy curing agent) and a new solid epoxy resin emulsion (Araldite®
PZ 3961-1 epoxy resin) were developed. The curing agent Aradur® 3986
epoxy curing agent is a solution of a formulated polyamine adduct at 40% (w/w)
solids in water. It is believed that the development of Aradur® 3986
epoxy curing agent is a significant step forward in waterborne epoxy primer
technology for the following reasons:
- no
volatile amines;
- no
solvents;
- long
pot life;
- non-critical
mixing ratio;
- fast
drying, even when using liquid epoxy resins;
- outstanding
compatibility with both waterborne epoxy emulsions and liquid epoxy resins;
and
- excellent
adhesion to difficult substrates, outperforming traditional solventborne
solutions.
The newly developed emulsion Araldite® PZ 3961-1 epoxy resin offers
the desirable performance of a solid resin without the limitation of a high
solvent level. Some of the main features of Araldite® PZ 3961-1 epoxy
resin are: relatively low VOC level, approximately 140g/L; fast “lacquer” dry
properties; excellent adhesion to difficult substrates; and high flash
point.
Araldite® PZ 3961-1 epoxy resin contains a fraction of the VOC level
typically present in a standard solvent-based solid epoxy resin, but cannot be
used to prepare zero-VOC primers. Zero-VOC primers can be prepared using liquid
epoxy resin since the need for coalescing solvents is eliminated. However, as
previously mentioned, high-solids and solvent-free systems typically do not
possess the fast touch dry capability of solid epoxy
resins.
When formulated with liquid epoxy resins
(and waterborne emulsions thereof), Aradur® 3986 epoxy curing agent
offers the ability to formulate zero-VOC primers and coatings with 3-4 hours of
pot life at 23 °C. Furthermore, depending on the solids content of the
formulation, they can offer a visible end of the pot life, indicated by an
increase in viscosity and ultimately gelation. A key benefit of Aradur®
3986 epoxy curing agent is its ability to offer a fast dry to handle with
liquid resins. Table 1 offers a comparison of a similarly formulated primer
based on Araldite® PZ 3961-1 epoxy resin and Araldite® GY
6010 epoxy resin. The physical drying (dust-dry) time is about 1 hour longer,
but its dry-to-handle time is approximately the same as the coating based on
Araldite® PZ 3961-1 epoxy resin.
Excellent adhesion to difficult substrates provides
formulators with the tools to formulate surface tolerant primers. To illustrate
this feature, cold-rolled steel panels were coated with a basic primer formula
based on Aradur® 3986 epoxy curing agent with Araldite®
PZ 3961-1 epoxy resin and another with Araldite® GY 6010 epoxy resin
using only inert pigment and fillers. A standard solventborne primer based on
1-type solid epoxy resin (SER)/polyamidoamine (PAA) was used as a control. The
solventborne formulation contained a zinc phosphate anti-corrosion pigment. All
three formulations used 80% curing agent calculated on theoretical
stoichiometry, which tends to improve corrosion resistance. As shown in Figure
1, the waterborne primer based on Araldite® GY 6010 epoxy resin
provided better corrosion resistance after 500 hours of salt fog exposure (by
ASTM B 117) than the solvent-based control.
The lowest amount of
creep at the scribe was obtained using Aradur® 3986 epoxy curing
agent with Araldite® PZ 3961-1 epoxy resin. It should be pointed out
that the purpose of this testing program was to highlight the performance
properties of the respective binders, not to optimize the formulation. It is
anticipated that the optimization of these formulations using anti-corrosive
pigments or corrosion inhibitors and adhesion promoters (i.e., silanes) would
result in improved performance.
Performance Under Low Temperature Conditions
It has been shown that a liquid epoxy resin binder
system can approach the dry times of a solid epoxy resin emulsion binder when
used with Aradur® 3986 epoxy curing agent. Another positive feature
of this combination is the low minimum film forming temperature (MFT), which
allows for applications as low as 10 °C without problems. Although the MFT of
the Araldite® GY 6010 epoxy resin coating is less than 10 °C, it did
not form a continuous film given less than three hours induction time. The
results shown in Figure 2 were obtained in non-pigmented
lacquers.
As
expected, the surface hardness development at 10 °C was much slower, but
achieved about the same level of hardness as the 23 °C cured coatings after an
additional cure time of 2 days at 23 °C (Figure 3).
To verify that an Araldite® GY 6010 epoxy resin/Aradur®
3986 epoxy curing agent-based primer cured at low ambient temperature
conditions could still provide a high degree of corrosion resistance, it was
applied onto grit-blasted steel panels and cured at 10 °C and 80% relative
humidity (RH) and at 23 °C and 50% RH for comparison. To simulate the
application process recommended for solid resin dispersion-based coatings,
three layers of the primer [3 mil (75 µm) dry film thickness each] were applied
within 8 hours (simulating a normal working day) at 23 °C. All panels were
then cured at the relevant temperature for 7 days before starting the salt spray
test exposure.
After 8,000 hours of salt spray test, no significant
differences in the performance level of the coatings could be found, indicating
those cured at 10 °C achieved a similar level of corrosion resistance as those
cured at 23 °C (Figures 4A and 4B).
Low-VOC Waterborne Anticorrosive Primer
Formulating a low-VOC waterborne anticorrosive primer
that could meet the performance standards and application properties of a
standard high-VOC 1-type/polyamidoamine system was the driving force behind the
development of Aradur® 3986 epoxy curing agent. Ideally, one should
be able to formulate a high-performance anti-corrosive primer free from heavy
metals and hazardous air pollutants (HAPs). Such a primer should be user-friendly
for applicators as well as the environment. It is believed that these
objectives were achieved with the development of formulation
A-2.
Formulation A-2 was
designed to match the main advantages of a solventborne epoxy system for a
metal primer: corrosion resistance and protection of difficult and poorly
prepared substrates, non-critical mixing ratio, good application properties and
rapid cure with long pot life. In addition, the waterborne system was able to
address some important limitations of the standard system including high VOC
level, high flammability and heavy metal and/or HAP content. Formulation A-2
meets the stringent SCAQMD VOC requirements for industrial maintenance and the
demanding performance requirements of anti-corrosive primers.
A comparison of formulation A-2 with a
commercial solventborne, surface-tolerant primer based on liquid epoxy
resin/polyamidoamine chemistry can be found in Table 2.
The commercial solventborne primer used here serves to
illustrate several points discussed earlier. Some of the performance properties
that this commercial primer is designed to achieve are high corrosion
resistance, surface tolerance and rapid cure along with a lower VOC. The
polyamidoamine curing agent contributes to the high level of corrosion
resistance and surface tolerance, but also increases the solvent demand.
Since it uses liquid epoxy resin, high solids can be
reached, but at the expense of longer touch-dry and dry-to-handle time, shorter
pot life and lower impact resistance. Adhesion and corrosion protection of a
difficult substrate, such as cold-rolled steel, are diminished compared to
using a 1-type solid resin.
In
addition to its significant advantage in VOC, formulation A-2 also has
advantages in pot life, dry times, adhesion to cold-rolled steel and impact
resistance. The corrosion protection of A-2 proved to be slightly better than
that of the commercial primer on both grit-blasted steel and on cold-rolled
steel, a difficult substrate.
The
corrosion resistance test panels of each primer at 1000- and 2000-hours of salt
fog exposure are shown in Figures 5 and 6, respectively. The corresponding
corrosion resistance ratings are shown in Tables 3 and 4.
After 1000 hours of salt fog exposure, formulation A-2 has shown better
corrosion resistance on untreated cold-rolled steel than the solvent-based
control, as indicated by scribe rating and creep. This result confirms that A-2
has better adhesion to this relatively smooth substrate than the control, a key
property of a primer coating. Formulation A-2 also performs better on
grit-blasted steel in terms on scribe rating and creep. Despite its lower film
thickness, the A-2 primer has the same blister rating as the
control.
The 2000-hour results on cold-rolled steel are very poor for both primers and
are only included for comparison purposes. The grit-blasted panels coated with
formulation A-2 show a higher rate of attack in the second 1000-hour period of
exposure compared to the control; however the scribe rating and creep are still
somewhat better. Again, the waterborne primer seems to be slightly more
resistant to blistering than the solventborne control, having only a few small
blisters; the blisters on the control are much larger in size, but also few in
number. These results demonstrate the ability of the Araldite® PZ
3961-1 epoxy resin/Aradur® 3986 epoxy curing agent binder system to
provide a high level of corrosion protection without compromising other
required performance properties.
Conclusion
For decades the standard solvent-based, 1-type, solid
epoxy resin/polyamidoamine system has dominated the field of corrosion
protection for a variety of reasons including its excellent corrosion
resistance and low cost. Unfortunately, the associated high VOC is gradually
forcing coating manufacturers to formulate away from this aging technology.
Reduction of the VOC level can acceptably be achieved by few methods.
Waterborne technology offers the benefit of low VOC and can provide the high
performance required today.
A new waterborne epoxy binder system designed to meet the challenges
facing the formulators of corrosion-resistant primers has been presented in
this paper. A starting-point formulation that has out-performed a commercial
solvent-based anti-corrosive primer has also been
presented.
Formulators
using Aradur® 3986 epoxy curing agent benefit from high-quality
corrosion-resistant primers suited for a range of different and very specific
applications. The main benefits of Aradur® 3986 epoxy curing agent
are its excellent adhesion to difficult substrates (i.e., poorly prepared or
untreated steel, cold rolled steel, galvanized steel, aluminum etc.) and its
outstanding compatibility with liquid epoxy resin and waterborne emulsions of
both liquid and solid resins. Combination with the solid resin emulsion
Araldite® PZ 3961-1 epoxy resin helps achieve the best adhesion to
the difficult substrates described above.
Using Aradur® 3986 epoxy curing agent, formulators can design
primers with very low (even zero) VOC, low sensitization potential and good
application properties. Aradur® 3986 epoxy curing agent may be used
for industrial anti-corrosion primer applications, as well as heavy-duty
primers on blasted substrates — even when used at temperatures as low as
10 °C with 80% relative humidity. Consequently, formulators can benefit
from its versatility by using only one pigmented hardener base to cover many
different primer applications. This versatility provides the tangible benefits
of high performance, improved economics and logistics without the restrictions
imposed by current VOC regulations.
Acknowledgement
The
author thanks Messrs. Dong Le, Martin Gerlitz and Alwin Krotzek for their hard
work, expertise and support of this project.
This
paper was presented at The Waterborne Symposium, sponsored by The University of
Southern Mississippi School of Polymers and High Performance Materials and The
Southern Society for Coatings Technology, 2009, New Orleans, LA.
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