Osmotic Blistering

CARDANOL CHEMICALS N.V.
International distributor of Cardolite Corporation
Offerlaan 116-118
B-9000 Gent
Belgium
Osmotic Blistering
By:
Nit BaetsM, Cardanol Chemicals
Summary:
Osmotic blistering is a regular seen
phenomenon when humid concrete is
coated. This article describes shortly the
most important parameters involved.
Suggestions are made to decrease the risk
of blistering, followed by an accelerated
test method. We prove that by anticipating
on the physico-chemical aspects of this
natural behaviour we were able to
overcome the repelling effect of the water.
Introduction:
The osmotic effect installs itself in
presence of a semi-permeable membrane, a
diluent - in our case water -, constituent(s)
for which the membrane are not permeable
and concentration differences. If one of
these preconditions is missing there no
such thing as osmosis.
1) Adhesion:
The adhesion of the primer is the most
important parameter in the development of
blisters. A well-penetrated and good
adhering primer is very likely to course
trouble.
2) Semi-permeable membrane:
The semi-permeable membrane can be
situated at different levels. The upper part
of the concrete substrate may act as one
because of carbonatation in the pores,
penetration of the primer, pore size
distribution depending on the type of
concrete or a concentration of high
molecular admixtures who migrated to the
surface during the cure of the concrete. The
sizes of the pores and the pore distribution
induce a capirarity in all cases.
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The mimer or other layers of the coating
may act as well as a membrane. The
chemical composition of the layers defines
its susceptibility towards this phenomenon.
3) Blister-fluid:
Analyses of the blister-fluids indicated the
presence of organic and inorganic
constituents:
0 Inorganic constituents are mainly salts
coming from the mineral substrate.
• Organic substances are related to the
primer. These can be flexibilisers as
Benzylalcohol, accelerators as
Nonylphenol or solvents.
Not all molecules have the same influence
in the osmotic effect; larger molecules
have a smaller influence on the pressure.
4) Stability of the constituents:
In case constituents of the primer react
with the mineral substrate, decomposition
materials are formed which take part in the
osmotic effect (Ref.: 1,2, 3).
5) Pre-treatment and application:
Assuring a good adhesion starts with a
proper pre-treatment of the concrete
substrate. The pre-treatment necessary
depends on the state of the concrete and the
desired status before application.
When a two component system is not
properly blended, we may find amines.
Improper blending is mostly accorded by
local differences in mechanical properties,
which turn these spots in to preferential
areas of delimitation.
Reducing the risk of osmotic blistering:
The risk of delimitation of the coating is
minimised when:
Date issued:
Document #:
1999.05
c-5.51/1/B
Article regarding NC 558
Revision:
0

CARDANOL CHEMICALS N.V.
International distributor of Cardolite Corporation
Offerlaan 116-118
B-9000 Gent
Belgium
• the pre-treated carried out properly
• the application is made in the right way
• the primer is solvent free
The coating is for most paint companies
the only parameter in control. We have to
make sure that the primer is not subject of
blistering:
• Adhesion: The most important factor is
the adhesion on the substrate. A low
viscous primer that spreads out well
and penetrates the concrete profound is
less likely to provoke blisters.
• Flexibility: The flexibility is not only
of importance in regard to the final use
of the coating. The flexibility and the
influence of temperature changes
correlates directly with the visual
aspect of blisters.
• Water soluble constituents: Research
has proven that humid concrete
substrates coated with a solvent free
primer - and preserved under humid
conditions - stay free of blister even
after several years (Ref.: 4, 5). Solvent
containing primers provoked blisters
under the same conditions (Ref.: 6).
The primer is stable under alkali
conditions and has no water dilutable
decomposition products.
Primer formulation:
We developed a concrete primer
complying too the above guidelines, A low
viscous Bisphenol A resin diluted with a
Constituents Primer 1 Primer 2
Part A
Bisphenol A (Eurepox 80 80
730)
Necires EPXL-2 20 20
Part B
Cardolite@ NC 558 50 50
Euredur 13 0 2
DFT (CL) 300 300
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Article regarding NC 558
hydrocarbon resin is used as A component.
Cardolitb NC 558, a low viscous
Phenalkamine hardener, cross-links the
system. Euredur 13 is used to adjust the
cure at lower temperatures (5”C/95%
R.H.). The primer is applied at 300~ film
thickness.
Necirez EPXL-2 is a non-reactive
flexibiliser but contains aromatic OHgroups
that bound in the hydrocarbon resin
to the rest of the primer. The dense crosslinking
obtained with Cardolite@ NC 558
further minimises migration of the
flexibiliser. The concentration of
accelerator is kept limited hence its
influence on the flexibility and
hydrophobicity of the primer. The water
vapour transmission rate of this primer is
extremely low (1.33 lo-” mih) and assures
a complete seal off the substrate.
Two types of humid concrete are used as
substrate:
• High performance concrete: The high
performance concrete (62 N/mm2) is
since its dense structure subject of an
increased capillarity and a less
profound penetration.
• Regular concrete: Concrete with
compression strength of 30 N/mm* is
more accessible for the epoxy primer
but may contain more water.
Accelerated test method:
Sandblasted concrete tiles, of which the
four edges are primed, are totally
submerged for 2 weeks in water. Three to
four hours prior to the application of the
primer they are taken out and dried with a
cloth. The water content of the substrate
varies between 5.5% and 6.8%, depending
on the type of concrete.
Date issued: 1999.05
Document #:
C-5.51/2/B
Revision: 0

CARDANOL CHEMICALS N.V.
International distributor of Cardolite Corporation
Offerlaan 116-118
B-9000 Gent
Belgium
The tiles are partially submerged in water
directly after application and are exposed
to IR-light (150 W) during 8 hours a day.
The primed surface reaches a temperature
of 38 “C during radiation. This heat
fluctuation of about 20 “C accelerates the
osmosis.
In this stage, it would be premature stating
this primer to be the overall solution. A
record of accomplishment of field
applications needs to be set out to support
our results, it is however looking
promising.
References:
1: Wisser, S.: Verhalten van liisemittelfieien Epoxidharz-
Systemen auf wassersaugenden betontichen. In:
Kunstharz Nachtichten, 41
2: Missfeld, H . : Zur Wechselwirking zwichen
betonuntergrund und Beschichtungen. In: Bauforschung
Baupraxis, 171
3: Ettel, W.-P.; Ablner, M.: Blasenbildungen bei
Epoxiedharzbeschichtungen und Ihre Ursachen. I N :
Baustofflndustrie 22, Nr 6
4: Sasse, H.R.; Littmann, K.: Entwickhmg van
Anforderungs Kriterien f% d i e Dauerhaftigkeit van
Fahrbahnabdichhmgen in Trog- und Tunnelbauwerktxv
Forschungsbericht F 345. “on Instituut fiir Bauforschung,
Aachen.
5: Lithnann, K: Beschichtung van jungem beton mit
Epoxidharzen nach den TL-BEL-EP. Im vorliegendes
Tagungsband.
6: Feibrich, M.: Kunststoftbeschichtungen auf stidig
durchfeuchtetem beton. Forschungsbericht Nr F 184 van
das Instutut fiir Bauforschung der RWTH Aachen
Each primer is applied on both concrete
substrates and exposed for 3 months. The
primers showed no single osmotic
blistering.
Conclusion:
Lab results have proven that a low viscous,
truly solvent free, well adhering concrete
primer enables to overcome osmotic
blistering under the given circumstances.
We hereby confirm the out come of the
number of researches done in this delicate
field.
CAFDOLITE Date issued: 1999.05
ARTICLES Document #: c-5.51/3/B
Article regarding NC 558 Revision: 0