International NACE Paper
Fast Curing Polycyclamine Cured Tank Linings
Paper No.
11040
2011
Fast Curing Polycyclamine Cured Tank Linings
Michael Winter
International Paint LLC., Protective Coatings
6001 Antoine Drive
Houston, TX 77091
USA
ABSTRACT
Traditional epoxy tank linings are limited in their applications due to minimum curing temperatures and
the need for long cure times at low temperatures before being returned to service. Polycyclamine cured
epoxies allow for the production of very high solid content, fast curing single coat tank linings systems
that can be both applied at low substrate temperatures and cure rapidly at low temperatures to allow for
a fast return to service which minimizes downtime for the asset owner.
Furthermore, the use of this technology does not compromise chemical resistance or high service
temperature resistance, unlike other low temperature curing systems.
This paper discusses the chemistry and performance characteristics of these systems and
demonstrates how the use of such systems can save an owner time and money.
Key Words: Tank Linings, Chemical resistance, Epoxy, Novolac, Polycyclamine, fast return to service
©2011 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE
International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are
solely those of the author(s) and are not necessarily endorsed by the Association.
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INTRODUCTION
Epoxy resin based coatings have been used for many years to line the inside of chemical
storage tanks and process vessels. They are particularly suited for services that require resistance to
hydrocarbons, water immersion, caustics and certain mineral acids. Generically the coatings are often
identified by the terms “epoxy”, “epoxy phenolic (or phenolic epoxy)” and novolac epoxy phenolic”,
although is there is no accepted definition of what this means1. The term “phenolic epoxy” is especially
abused and has been used for coatings containing Bisphenol A, Bisphenol F and novolac epoxy resins,
or combinations thereof.
Apart from the inaccurate use of generic terms for epoxy resins, the use of this terminology
misses out on a key part of an epoxy lining formulation – the curing agent. There is a relative handful
of epoxy resins to choose from when formulating an epoxy coating, but there is a huge choice of curing
agents. The experienced tank lining formulator knows that he or she can change the coating properties
more through the choice of curing agent than by choice of epoxy resin. Curing agent selection plays a
critical role in the chemical resistance, speed of curing, temperature of curing, ease of application and
mechanical performance of the finished product.
EPOXY RESIN CHEMISTRY
There are three types of epoxy resin typically used in tank lining formulations – those based on
Bisphenol A, Bisphenol F and epoxy phenol novolac resins. All of these resin types are available in a
range of molecular weights (& therefore viscosities) and functionalities. Bisphenol F resins have a
slightly higher functionality (typically 2.1) and a lower viscosity than equivalent Bisphenol A epoxy
resins. Epoxy Phenol Novolac resins have a higher functionality – and therefore can give more highly
crosslinked cured films – but also have higher viscosities which makes formulating high solids or
solvent free paints more difficult.
Figure 1: Bisphenol A based epoxy resin
Figure 2: Bisphenol F based epoxy resin
Figure 3: Epoxy phenol novolac resin
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In principal, increasing the crosslink density will increase the chemical resistance (while
reducing flexibility), at least to solvents, of the cured film, so that typically the chemical resistance
imparted by these resins is as follows: epoxy phenol novolac resins > Bisphenol F resins > Bisphenol
A. However, the reality of chemical resistance is much more complicated than this and cannot be
simply defined according to generic epoxy resin types.
There are certain chemical exposures where the epoxy resin type plays a key role – one
example is in concentrated sulfuric acid resistance. The possibility of Bisphenol A epoxy resins to form
a stabilized tertiary carbocation in strong acidic conditions reduces their resistance compared to
Bisphenol F or epoxy phenol novolac resins – see Figure 4.
Figure 4: Concentrated acid resistance of Bis-A (top) vs Bis-F and
Novolac resins (bottom).
CURING AGENTS FOR EPOXY RESINS
While there is only three main classes of epoxy resin used in coatings, the formulator can
choose from a huge array of curing agents2. The original epoxy curing agents were unmodified
aliphatic amines, such as diethylene triamine (DETA – see Figure 5), triethylene tetramine (TETA – see
Figure 6) and triethylene pentamine (TEPA – see Figure 7). These curing agents can provide high
levels of chemical resistance, but when used unmodified give films that are rather brittle and the curing
agents have poor compatibility with epoxy resins, potentially causing problems with amine blush. Prereacting these amines with epoxy resins to create adducts improves the compatibility, but has little
effect on film properties.
Figure 5: Diethylene triamine
Figure 6: Triethylene tetramine
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Figure 7: Tetraethylene pentamine
The brittleness of films produced from aliphatic amine curing agents resulted in the development
of polyamides curing agents. These are produced by condensing an aliphatic amine with a dimer acid.
Although the resulting resins are polyamides, they retain amine functionality which is reactive towards
epoxy resins (see Figure 8).
Figure 8: Example of polyamide curing agent
While polyamide curing agents give an increase in flexibility and water resistance compared to
aliphatic amine films, they have a lesser resistance to strong solvents and acids and their high viscosity
makes it hard to formulate low VOC/high solids coatings.
A later development of polyamide chemistry was to use a monofunctional acid in place of a
dimer acid to produce an amidoamine condensation product (see Figure 9). The principal advantage of
amidoamines over polyamides is their somewhat lower viscosity that allows for formulation for higher
solids systems.
Figure 9: Example of amidoamine curing agent
Aromatic amine curing agents, such as methylene dianiline (MDA – see Figure10) were used for
many years as curing agents for epoxy tank linings due to the very high chemical resistant properties
that could be obtained. The aromatic backbone of MDA (and other aromatic amines) contributes to the
high Tg (glass transition temperature) as well as the excellent acid and solvent resistance that can be
obtained using these materials. Aromatic usually require a higher curing temperature to achieve full
crosslinking.
Unfortunately, MDA is a suspect carcinogen and the toxicological concerns over the use of this
and other aromatic amines has lead to them being phased out and formulators have had to search for
alternative curing agents to provide the best chemical resistance.
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Figure 10: Methylene dianiline
One curing agent that has proved useful in formulating chemical resistant coatings without using
aromatic amines is meta xylene diamine (MXDA – see Figure 11). This combines some features of
aromatic and aliphatic amines as it has an aromatic ring, but the amine groups are separated from this
by methylene groups. Good performance and handling properties are usually achieved by adducting
with epoxy resin.
Figure 11: Metaxylyene diamine
Today the most commonly used class of curing agents for formulating high solids and solvent
free chemical resistant coatings are cycloaliphatic amines. Examples are isophorone diamina (IPD –
see Figure 12), cyclohexane diamine (see Figure 13) and 4,4 – bis (para amino cyclohexyl) methane
(see Figure 14). Cycloaliphatic amines are very useful for formulating very low VOC and high solids
epoxy coatings with high chemical resistance. It is possible to make coatings with chemical resistance
properties approaching those of aromatic amine cured systems, but with improved cure speed and
lower temperature curing capability. The downside of these materials can be tendency for amine
blushing.
Figure 12: Isophorone diamine
Figure 13: Cyclohexane diamine
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Figure 14: 4,4 bis (para amino cyclohexane)
A more recent development has been the introduction of polycycloaliphatic polyamines
(polycyclamines for short), see for example Figure 15. These amines have multiple reaction sites
spaced along a cycloaliphatic or partially aromatic backbone and when adducted or otherwise modified
can provide even greater performance and cure response than the conventional cycloaliphatic amine
curatives. Coatings based on these materials have been reported to show excellent performance in
Canadian oil field environments3. These curing agents can also be used, either as adducts or blended
with other curing agents such as cycloaliphatics, to produce rapid curing, fast return to service tank
linings with high level of chemical and temperature resistance.
Figure 15: Examples of polycycloaliphatic polyamine (polycyclamine) curing agents
CROSSLINK DENSITY
In the section on epoxy resins an example was given on how the chemistry of the polymer
backbone affects chemical resistance. However, the resistance to certain chemicals, especially strong
solvents, is largely dependent on the crosslink density in the cured film. In principal, the higher the
crosslink density of a cured film, then the greater the resistance to permeation and swelling from
solvents or water (permeation can also be modified based on the choice of pigmentation used in the
coating). Figure 16 shows how the resistance to methylene chloride for a particular system increases
as the degree of cure and therefore crosslink density increases. In this case, the same system was
cured at three different temperatures – higher curing temperatures give rise to a higher degree of cure
and a higher glass transition temperature for the cured film. The lesser cured films showed a rapid
absorption of methylene chloride, followed by a weight loss as some of the unreacted components
leach out of the coating film. The more highly crosslinked sample gives a much slower weight gain,
indicating a greater permeation resistance.
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Figure 16: Methylene chloride resistance as a function of degree of cure .
It would therefore stand to reason that choosing epoxy resins and curing agents with the highest
functionality and the shortest distance between crosslinking sites would result in a cured film with the
highest crosslink density and therefore the highest degree of chemical resistance. If only it was that
simple……
With high functional resins curing at normal ambient temperatures, there is a tendency for the
coating film to gel before complete reaction has occurred. When this happens, the film is left in a
partially cured state, with a significant amount of unreacted epoxide and amine functionality. In addition
to not achieving a fully crosslinked state, the unreacted groups may further compromise chemical
resistance by being available to chemical attack from certain chemical species. Figure 17 gives
graphical depiction of a coating film becoming “frozen” before full crosslinking has occurred.
Figure 17: Gelation of a coating before full reaction.
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To get around this problem, experienced coating formulators know how to modify formulations
using various reactive and non reactive diluents (these may be benzyl alcohol, nonylphenol and various
low molecular weight glycidyl ethers) to increase molecular mobility during curing of the coating film
(see Figure 18). There are those in the industry that maintain that using diluents decreases chemical
resistance or otherwise impacts long term performance – indeed, inappropriate use of diluent, use of
the wrong diluent or adding too much may have a deleterious effect – but the judicious use of the right
material may significantly aid in the development of a crosslinked network and therefore help to obtain
the maximum degree of chemical resistance. At the end of the day, the properties of the total
formulation as applied and cured are most important, rather than the individual components and the
coating formulator must understand how to maximize the benefits that may be obtained from a certain
material, such as a polycyclamine curing agent, by using the correct type and amount of other
formulation components.
Figure 18: Effect of diluents on cross linking.
TANK LININGS – RETURN TO SERVICE
To work properly, a tank lining must be resistant to the chemicals that will be stored or
processed in the vessel. However, an owner/operator and a painting contractor may also have other
requirements that have to be taken into consideration.
Outside of coating selection, the most important factor affecting performance is surface
preparation. For chemical tank linings it is critical that the surface is prepared to the required degree of
cleanliness, with the correct surface profile for the coating being installed. For aggressive
environments, such as hot water services, it may be necessary to wash the surface several times in
order to achieve the right degree of cleanliness.
The coating system must be practical in install in the minimum time period possible to allow the
equipment to be returned back into service, thus minimizing costly downtime. Historically tank linings
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were left to cure for periods of 7-10 days at temperatures of 25ºC and above before being returned to
service. Over time, formulators learnt how to reduce this time period to 3-4 days at 25ºC. Now, using
well formulated coatings based on polycyclamine curing agents, it is possible to return linings into
service after 24 hours or less at 25ºC. Extreme chemical services may still require a longer cure time,
or a force cure schedule, but a large amount of chemical services can now be handled with epoxy lining
systems that are applied in a single coat one day and are placed in service the following day. With the
correct selection of epoxy resin type and/or blending with other curing agents, polycyclamine curing
agents can be used to produce coatings with resistance to strong solvents, such as methanol or
concentrated mineral acids.
The maximum benefit of fast return to service systems is obtained when the coatings are
formulated to be applied in a single coat. This requires the coating to be very high solids or solvent free
to minimize solvent entrapment when applying a thick film, which may be from 16 to 60 mils thick,
depending on product and service environment.
FAST CURE POLYCYLAMINE SYSTEMS – LABORATORY PERFORMANCE
Polycyclamine cured systems have been tested in the laboratory against a wide range of
chemicals. The chemical resistance profile of an individual coating depends upon all the formulation
components, but polycyclamines have been used to create coatings with some of the following
interesting chemical resistance properties:
A polyclamine based coating was placed in hot (95⁰C) deionized water immersion for one year.
After one year the coating showed no blisters or other defects (see Figure 19) and no sub film corrosion
(see Figure 20).
Underside of adhesion test dollies
Figure 19: Polycyclamine novolac epoxy after 1 year in 95⁰C deionized water, showing results
of adhesion pull tests indicating good post test adhesion.
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FIigure 20: No sub-film corrosion after 1 year in 95 C deionized water
The same polycyclamine cured novolac epoxy coating was tested in an autoclave with
deionized water at 150⁰C/5 bar pressure for 30 days (see Figure 21). The coating showed no blistering,
flaking or other defects and excellent post-test adhesion.
Underside of adhesion test dollies
Figure 21: Polycyclamine novolac epoxy after 30 days in deionized water at 150⁰C/5 bar
A different polyclamine cured novolac epoxy was immersed for 12 months in concentrated
(98%) sulfuric acid at 40⁰C for one year. This coating was post cured at 50⁰C to achieve maximum
chemical resistance. After one year immersion, the coating had discolored, but was otherwise in
excellent shape. A similar coating was immersed in 10% sulfuric acid at 60⁰C for one year without post
curing and showed excellent performance, with only a slight discoloration of the coating noticed.
Another different formulation of a polycyclamine cured novolac epoxy was immersed in
methanol at 40⁰C for 2 years without any adverse effect. Again, this particular coating was post cured
to achieve maximum chemical resistance.
FAST CURE POLYCYLAMINE SYSTEMS – FIELD HISTORY
As mentioned above, polycyclamine curing agents can be formulated together with Bisphenol A,
Bisphenol F or novolac epoxy resins (or combinations thereof) depending on the desired properties of
the product. Some real life case histories described below illustrate how the fast curing characteristics,
extreme chemical resistance and high temperature immersion resistance of polycyclamines can be
effectively utilized in practice:
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A Bisphenol A epoxy/polycyclamine system that is formulated for fast return to service and good
resistance to hydrocarbon solvents, gasolines and ethanol was used to coat an ethanol storage tank.
During the application the ambient temperature fluctuated from a high of 55ºF (13ºC) during the
daytime down to 35ºF (2ºC) at night. Despite the cool conditions, the lining was cured sufficiently to be
placed back in service after only 3 days.
A novolac epoxy/polycyclamine system formulated for good resistance to sulfuric acid was used
to line the internals of a tank used to store 93% sulfuric acid. After 4 years, the lining was inspected
and found to be in perfect condition.
A novolac epoxy/polycyclamine coating formulated for high temperature immersion service was
used to line produced water vessels that see service temperatures of up to 190-200 ºF (88 - 93ºC).
These vessels were placed back in service 24 hours after being coated. The lining was inspected after
18 months service and found to be in perfect condition (see Figure 22).
An epoxy/polycyclamine coating formulated for acid resistance and high temperature immersion
resistance was applied to a deionizer vessel, operating at 215–225 ºF (101-107ºC) and periodically
dosed with 7% HCl and 20% NaOH. After 6 years the lining was inspected and found to be in excellent
condition (see Figure 23).
Figure 22: Polycyclamine cured novolac epoxy after
18 months service in hot produced water
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Figure 23: Polycyclamine cured epoxy after 6 years in deionizer vessel.
Black streaks are tar balls on the surface of the coating.
CONCLUSIONS
Raw material developments have provided coatings formulators with a large tool box of
ingredients from which to work. The use of traditional generic descriptions of epoxy resins to
categorize performance of chemical resistant linings is a gross oversimplification. Curing agent
chemistry, together with other formula ingredients, plays a large role in determining the performance of
a particular coating. The use of the most recent advances in curing agent chemistry, such as
polycyclamines, can offer significant advantages to an owner in both chemical resistance and rapid
return to service when properly formulated into a coating and supported by the correct test data and
field experience.
REFERENCES
1
See, for example, “Ask the Coatings Experts – Understanding Epoxies and Where to Use Them”
Edited by Lou Vincent, Materials Performance Vol 49, No 5, May 2010
2
See, for example, “Protective Coatings, Fundamentals of Chemistry and Composition” Clive Hare,
Chapter 15 Epoxy systems
3
“Field Performance versus Laboratory Testing: A Study of Epoxy Tank and Vessel Linings used in the
Canadian Oil Patch”, Mike O’Donoghue, Ron Garrett, Ron Graham and V.J. Datta
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