Note: Descriptions are shown in the official language in which they were submitted.
~1093909
"Metal substrate coated with mixture of epoxy resin and
imidazoline derivatives"
Thè învention relates to metal substrates in the form
Or pipe~ or containers coated with a hardened epoxy resin
composition and to a process for the manufacture thereof.
The invention i3 especially concerned with large pipes
coated with a coating composition of low toxicity which is
resistant to heat and chemic~l corrosion and which i8 also
~electrically insulating.
It has been proposed to coat metal pipes, e~pecially
large pipes, with an outer coating of bitumen ~r high pres-
qure polyethylene. Since high pressure polyethylene is very
sort, good protection from mechanical stress, in particular
blows, knocks and abrasions, is difficult to obtain. In
this respect, bitumen coatin~s have an even wor~e re~istan-
ce to such mechanical treatment. Damage to the coating
may cause corrosion of the metal and, fo~reasons of safety,
make~ large pipes unusable for the transporting of natural
ga~, mineral oil, petrochemicals, hot water, waYte water
and other gaseou~ and liquid chemical substances.
Furthermore, it has been proposed to coat pipes with
a layer of a hardenable resin, for example a mixture of
epoxy resin and coal tar asphalt in which coarse-grained
fillers are embedded. In addition a method has been pro-
posed for covering pipes with multiple coatings, in whicha preheated pipe rotating about i~s longitudinal axis is
provided with a thermosetting liquid coating mixture, and
is subsequently hardened in a separated hardenin~ rocess.
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lU~3909
It has al~o been proposed to use powdered hardenable
epoxy resin compositions containing, as curing agents, aro-
matic amines, acid anhydrides, dicyanodiamide or modified
dicyanodiamides, i.e. dicyanodiamides activated by small
quantities of accelerators. These so-called accelerators,
which a~ect the hardening rate of the curing agents, are
for example, mixtures o~ carboxylates of the metal lead,
iron, cobalt, manganese~ zinc or tin, with carboxylic
acids or anhydrides or adducts of epoxy re~ins ~ith imida-
zole derivatives, e.g. 2-methyl-4-ethyl-imidazole.
The hardenable epoxy resin compositions proposed
hitherto howeverhave the disadvantage that they have low
heat-resi3tance; moreover, when the epoxy layer i8 damaged,
the actio~of warm alkali solutions, hot water or hot steam,
leads to a los8 of adhesion at the junction of the metal
and the epo~y resin coating, resulting in corrosion under-
neath the coating. The sensitivity to alkalis is important
in many fields of application, for example in building
projects, where materials with an alkaline reaotion~ such
20~ as for example lime and cement, are usually pre~ent.
A coating moreover often contains pores, i.e. micros-
copic cavitie~ which extend as fas as the metal surface.
Such pores are formed, for example, by the pre~ence or for-
mation of gaseous and other products when the hardening pro-
cess is subject to side-reactions. These microscopic
cavities are very often the cause of so-called point corro-
sion of the metal wall. The above-mentioned disadvantages
..,
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lVg3909
are especially undesirable in the coating of large pipes,
~ince they lead to so-called delayed damage, which, for
economic and technical reasons, is unacceptable. In addi-
tion the epoxy resin coating materials used hitherto often
contain toxic components, such as aromatic amines as
hardener~ or lead-containing compounds as accelerators.
We have now found that metal pipes, containers and
other articles may be provided with a coating which is more
; durable than the aforementioned coatings and i~ non-toxic.
Thus according to the present invention we now provide an
article comprising a metal substrate in the ~orm of a pipe
or a container, said substrate being coated with a hardened
epoxy resin composition formed from A) a solid epoxy resin
based onepichlorohydrin and 4,4'-diphenylolpropane or 4,4'-
diphenylolmethane or both, B) from 1 to 12~ preferably from
3 to 9~ by weight based on the epoxy resin of a compound
of formula
! CH - N
~ - - R (I)
CH CH
\ /
Nl
H
; or of ~ormula
CH2 - N
~ R (II)
C ~ / H
N
H
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~09390~
(wherein R represents an alkyl group with 1 to 6 carbon
atoms, or an aromatic hydrocarbon residue with 6 to 10 car-
bon atoms), C) a flow agent and D) a thixotropic agent. The
term "aromatic hydrocarbon" includes phenyl, benzyl and
toluyl.
According to a further feature of the present invention
we provide a method oP coating a metal substrate which com-
prises heating said substrat~ , for example a pipe or con-
tainer, to a temperature above the melting temperature of the
epoxy resin component of an epoxy resin coating composition
as hereinbefore defined and sufficient to cure the 3aid
epoxy resin, ror example to a temperature in the range 250
to 330C, and powder-coating, conveniently electrostatically
or by powder-spraying, the said epoxy resin coating composi-
tion on to the hot surface of the substrate to provide a
substantially uni~orm coating thereon.
As will be understood, prior to coating the metal sub-
strates ~hould be thoroughly cleaned, for example by sand-
blasting.
The method of coating should be 80 carried out that the
composition melts to~orm as even film over the surface of the
substrate and i5 immediately hardened, no further processing
steps being required. The heat capacity of the hot metal
substrate is itself generally sufficient with large metallic
articles to cross-link the epoxy resin in a short time, for
example less than a minute, without any further heating. It
i3 surprising that despite the high temperature of the sub-
strate during the application of the coating composition and
the hardening thereof, no cracking of the
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3909
coating occurs and neither a deterioration of the properties.
The epoxy resins used as component A) of the coating compo-
sitions preferably have an epoxy equivalent weight in the range 600 to
2000, more preferably 700 to 1500 and particularly 875 to 1100. It may
be advantageous to use mixtures of epoxy resins with different epoxy
equivalent weights in order to improve the mechanical or other properties
of the coating. Then the content of epoxy resins having an epoxy equiva-
lent weight in the range 1500 to 2000 is preferably above 5 and not more
than 20% by weight to avoid a deleterious effect on the flow properties.
The curing agent (component B) of the coating composition)
is preferably a compound of formula I or II wherein R represents a
methyl, ethyl, n-propyl, n-butyl, n-hexyl, isopropyl, isobutyl, tert.-
butyl, phenyl, benzyl group or any of the various toluyl groups, the R
substituent being preferably in the 2-position. Aryl substituted
imidazolines of general formula II are particularly preferred since
these compounds retain their ability to cross-link epoxy resins at the
high baking temperatures used and moreover due to their chemical stabil-
ity improve the storability of the powdered coating composition. In
particular 2-phenyl-2-imidazoline has been demonstrated to be an especially
suitable curing agent. The baking temperatures generally used in~the
manufacture of the resin coated metal substrates according to the in-
vention, such as pipes and containers, are generally in the range 250
to 330C and the curing agent selected should
~r~
,~A
. ,. . . .
.. . .
... . . .. . . ~ ~; .. .
...... . ~ . .. .
390~
therefore be one which retains its ability to harden the resin at these
temperatures. On the other hand dicyanodiamide, a conventional curing
agent which is frequently used in other hardening processes, melts at
211C under decomposition and its properties as a hardener are therefore
seriously impaired above this temperature.
The flow agent ~component C) of the coating composition) serves
to improve the flow properties of the composition and at the same time to
facilitate the wetting of the substrate and of any pigments which may be
present. The quantity used is generally up to 5% preferably from 3 to
3.75% by weight based on the epoxy resin. Preferred flow agents include
for example polyvinylbutyral, silicone oils, silicone resin or a poly-
acrylate, for example a concentrate of an epoxy resin and a polyacrylate
marketed by Monsanto under the trade name Modaflow*.
The coating composition according to the invention also contains
a thixotropic agent which serves to increase the viscosity of the molten
composition on the substrate and thus produce an even coating. The presence
of fillers is in addition made redundant. The thixotropic agent is con-
veniently present in an amount of 1 to 5%, preferably 2 to 3% by weight
based on the total composition. A preferred thixotropic agent is finely
divided silicon dioxide.
The coating may if desired additionally contain pigments, for
example non-toxic pigments which are conventionally used in coatings for
metal pipes. Such pigments, preferably used in low concentrations, are
preferably lead-free,
*Trade Mark
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1(~93909
resistant to high temperatures and to chemical attack,for
example by water, acids, alkalis, soil, lime and cement.
Titanium dioxide and chromium oxide green are two examples
of pigments which may be present in the coating compositions
used in the present invention. The total pigment content is
generally not more than 40%, preferably not more than 20%
by weight of the total composition. The pigment concentration
i~ gen~rally selected so as to affect the mechanical proper-
ties of the coating as¦little as possible. Pigment concentra-
tions of more than 40% by weight are generally to be aboidedsince excessively high pigment contents result in poor resi-
stance of the coating to mechanical stresses, poor wetting
of the pigment and inhomogeneity of the coating.
The epoxy resin coating compositions used according to
the invention are conveniently prepared rrom finely-divi~ed
starting materials so that the homogeneity of the starting
mixture is improved and at least partial addition of the
curin ~gent, for example the imidazoline, to the epoxy resin
i3 pos~ible during the molten-liquid stage. The starting ma-
terials are mixed, for example in an extruder suited to duro-
plastic materials. After extrusion, which usually takes
several minutes, e.g. 1 or 2 minutes, the reaction is imme-
diately terminated by means of intensive cooling to prevent
further enlargement of the molecules. After cooling the epoxy
resin composition is ground to a fine particle ~ize, for
example to a powder with a maximum particle size of 60 to
100 microns.
. . .: .
10~3909
Pipes and containers are~generally coated on the outer
sur~aces only for economic reasons, but it is also possible
to coat only the inner surface~ or to coat both the outer
and the inner surfaces. Thus the properties required of
the coating depend on the particular application: in the
case of internal coatings only the durability and possibly
the resistance to temperatures up to, for example 130 or
140C, are of importance, the mechanical strength being
less critical since knocks and blows will occur only rarely.
We have found that a coating thickness of 100 to 2000
microns is generally adequate. However, thicker coatings
may also be applied.
The invention relates in particular to the coating of
large pipes, for example such pipes having an internal dia-
meter of more than 100 mm and more usually pipes having adiameter of 300 to 1600 mm, Such pipes are often used for
transporting petrochemicals and other ga~e~us and liquid
chemicals at various temperatures and are laid above;or
below ground or under water. They may be made of various
metals , but especially from iron and its alloys.
Coated metal substrates according to the invention have
been subjected to a number of tests, the reQults of which
are shown in the following Table. The test articles consi-
ted of resin coated pipe sections and steel sheets mea-
suring 300 x 100 x 10 mm. The metal substrates were rirstlpretreated with steel shots STS 20 to remove rust to grade
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109390~
1 according to DIN 18 364. Subsequently they were heated by
gas burners to a surface temperature of 290 + 10C, after
which th~owdered coating composition was electrostatically
applied and hardened.
The following tests were carried out using test articles
of sheet steel measuring 300 x 100 x 10 mm coated with a 300
micron thick epoxy resin composition prepared according to
Example 1 which follows. In tests A) to D) the coating was
scored by the lattice cut method according to DIN 53 151,
10 The!score marks produced were not sealed.
Test A): Storage in lN NaOH solution at 50C
The test articles were stored for 6 months at 50C
in a lN NaOH solution. Afterwards the loss of adhesion at
the point contact of the steel and the coating was estimated
(evaluated according to DIN 53151).
Test B): Boiling test, in distilled water
The test was carried out as an alternating boiling test
over 10 cycles, each cycle comprising heating for 20 hours
at boiling temperature and storing at room temperature for
2~ 4 hours. The formation of bubbles (evaluated according to
DIN 53209) and the loss o~ adhesion at the contact Or steel
and coating (evaluated according to DIN 53151) were estimated.
Test C): Boilin~ test in tap water
~ . . _ . . . _
The test was carried out analogously to Test B) except
25 that instead of distilled water tap water of pH 6.9 with a
carbonate hardness of 13.7dH and a constant hardness of
10.9dH was used.
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10~3~05~
Test D): Bending test according to DIN 53152 and DIN_1605
Bending pegs of 20, 30 and 40 mm diameter were used.
The tests were carried out at -5C, +23C, ~50C and+ 130C.
In the further tests, articles with unscored coatings
5 were used.
- Tbst E): Presence of pores according to preliminary standard
DIN 30670
_ _
The test articles were scanned with an electrode at a
test voltage of 5kV + 5kV per mm thickness of coating. The ;~ -~
10 presence of pores on the coated surface rather than the cut
edge was studied.
Test F): Impact resistance according to preliminary standard
~; DIN 30670 tsecond 3.2.1_and 5.6)
;~ For a coating thickness of 300 micron, an impact energy
15 of 3 Nm was used:
`~ The following Examples illustrate the production of
articles according to the present invention. T indicates
parts by weight.
Example 1
20 Preparation of a powdered epoxy resin coating composition.
73.0 T o~ a coarsely ground epoxy resin (maximum particle
size approximately l mm) based on 4,4'-diphenyiolpropane and
epichlorohydrin ~ softening point (according to Durrans):93C
to 104C, epoxy equivalent weight: 875 - 1000; viscosity:
25 430 630 cP in 40 % solution (measured in ethylene glycol
dibutyl ether at 250~) 7, 3.0 T of a flow agent concentrate
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lU9390~
(MODAFLOW manufactured by Monsanto) consisting of the above-
mentioned epoxy resin and a polyacrylate in a weight ratio
of 9:1, 4.0 T of 2-phenyl-2-imidazoline (melting point 101C -
103C, determined according to the capillary method), 13.0 T
5 of titanium dioxide Kronos RN 57 P (Kronos is a trade mark,
manufactured by Kronos Titan_gesellschaft mbH), 5.0 T of
chromium oxide GX (manufactured by Bayer AG ) and 2.0
highly dispersed silicon dioxide were mixed in a sealed, fast-
ro;tating mixer, firstly for 1 minute at 800 rpm (revolutions
lO per minute)~ then for 1 minute at 1600 rpm and finally for 30
seconds at 800 rpm, the mixer being simultaneously cooled with
water.
The mixture was plasticised and homogenised under the
following conditions in a Buss-Ko-Kneader PR 46 (manufactured
15 by Buss AG., Basle, Switæerland). Temperature of screw:
100C~ temperature of central part of housing: 105C;
temperature of outlet part of housing: 105C; temperature
of nozzle: 100C; temperature of the molten homogenised
mixture: llOaC; revolution rate of the proportioning screw:
20 18 rpm; revolution rate of the kneading screw: 62 rpm. The
molten homogenised epoxy resin material was rolled out flat
on two rollers, rotating in opposite directions and filled
with cooling brine, whereby it was intensively cooled, and
then was passed via an outlet caterpillar track on to a
25 water-cooled steel belt, where an additional countercurrent
of cold air was blown on to the epoxy resin material
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~V5~3909
from above. The cooled epoxy resin material was coarsely
ground, in known manner, e.g. in a blade mill (maximum particle
size 4 - 5 mm). It was then finely ground in a sifter mill while
being classified at the same time. The maximum particle size
5 of the powdered composition was in the range 60 to 100 micron.
b) Manufacture of coated p pes
Large pipes were heated by means of ring or line burners
arranged in a star shape. The pipes were moved along and
rotated until they showed a constant temperature of 290 ~ 104C
10 over the,total length of the pipe. The subsequent electrostatic
application of the powdered coating composition to form a
layer 300 microns thlck was carried out according to conventional
methods. The heat capacity of the large heated pipes was quite
'adequate to effect chemical cross-linking. The properties of
15 the coating were tested as described hereinbefore and the
results are given in the following Table.
Example 2
~ An epoxy resln coating composition was prepared ,~
analogously to Example la) except that the ratio of expoy resin:
20 2-phenyl-2-imidazoline was 74T:3T. Large pipes were coated with
this composition analogously to Example lb) and the same good
test values of the coatings were abtained as in Example 1.
Example 3
An epoxy resin coating composition was prepared analogously
25 to Example la) except that the ratio of epoxy resin:
2-phenyl-2-imidazoline was 71 T : 6 T. Large pipes were
coated with this composition analogously to Example lb)
and the same ~ood test values o~ the coatin,~,,s were obtained
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~0~3909
as in Example 1.
Example 4
An epoxy resin coating composition was prepared analogouslyto Example la) except that an epoxy resin mixture containing
590 % by weight of the epoxy resin used in Example la) and 10 %
by weight of an epoxy resin with an epoxy equivalent weight of
1500 to 2000 was used, and the ratio o~ epoxy resin: 2-phenyl-
2-imidazoline was 73.4T:3.6T. Pipes were coated with this
composition analogously to Example lb) and the same good test
0values of the coatings were obtained as in Example 1.
Example 5
An epoxy resin coating composition was prepared analogously
to Example la) except that the ratio o~ epoxy resin: 2-phenyl-
2-imidazoline was 71T:6T, no pigment was used and the amount
15 of silicon dioxide was increased to 3 % by-weight. Coated pipes
extraordinarily high impact strength.
Comparison Example 1
An epoxy resin coating composition was prepared analogously
to Éxample la) but using only 61T o~ epoxy re8in and~instead o~
20 the 2-phenyl-2-imidazoline, 16T o~ 2-acetyl-glycerine esteriried
in the 1- and 3-positions with one molecule o~ trimellitic
anhydride. Pipes were coated with this composition analogously
to Example lb) andlthe test values of the coating are given
in the Table.
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.,
~093909
Comparison Example 2
An epoxy resin coating composition was prepared
analogously to Example la) but using as component B) instead
of 2-phenyl-2-imidazoline, a modified dicyanodiamide containing
5 85 T of dicyanodiamide, 9.5 T of 2-ethyl-4-methyl-imidazole and
5.3 T of an epoxy resin based on Bisphenol A with an epox~
number of 190.
_BLE
Test methods for
10 large iaminated pipes Exarnple 1 Compar. 1 Compar,?
A) Storage in 1 N NaOH solution
at 50C
` Duration of test: 6 months 0 3 3
B) Boi.ling test in distilled
water
Duration of test: 240 hours 0 3 3
C) Boiling test in tap water
Duration of test: 240 hours 0 3 3
D) Bendin~ test according to
DIN 53152 and DIN 1605 0 1 2 `
E) Absence of pores according
to preliminary standard`
DIN 30670
~) Impact reslstance accor-
dingtO preliminarY standard
DIN 30670 0 1 2
Evaluation:
(according to DIN 53230 0 = best mark
5 = worst mark
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