Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Ti~le of lovention
PRl~aE~ t;:OlVlPOSlTlQN FOR NIET~L PIPES
Field o~ Invention
This invention relates to a primer composition, and more particularly
to an adhesive primer composition for pro~ecting metal pipes from
corrosion during storage and from corrosion and cathodic disbondment
while buried under0round.
Background of Invention
Metal pipes that are intended to be buried in -the ground as part of a
pipeline are subject to corrosion, both during the time o~ storage prior to
burial, and while buried underground. It is not uncommon that, metal
pipes may be stored for over one year prior to burial. This s~orage is
typically outdoors, thus the metal pipes will require protection against
corrosion during the storage period. ()ne of the usual methods used to
protect metal pipes during storage is to apply a rubber-based primer
coating to the pipes. Typically, the storage primer coating is not long
lasting in the sense that it need only serve its purpose for the time oF
storage. A different primer composition is used when the metal pipes are
buried.
Metal pipelines that are buried in the ground also require protection
against chemical and electrolytic corrosion. The usual method ef
providing such protection is to wrap the outer surface of the metal
pipeline with a protective coating so as to decrease the likelihood of
corrosion by reducing exposure of the metal pipeline to moisture and to
other sub-surf~ce chemicals. As well, the protective coatings used are
designed to insulate the metal pipeline from electrolytic corrosion.
Traditionally, the protective coatings applied to the buried metal
pipelines have included a coating of rubber-based primer composition ancl
one or more layers of adhesive tape outer-wrap. Notwithstanding these
layers of protective coatings, the metal of the pipeline inevitably is
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exposed to corrosive elements because oF imperfections in the prirner
composition and/or the adhesive tape, or, as is more likely, because the
protective coatings become perforated for some reason. These
perforations, referred to as "holidays", may be th~ result of rocks cutting ~:
5 the coating during back filling when the pipeline is buried in the ground, or
other back filling stresses. Holidays may also result from the movement
of the soil surrounding the pipeline due to changes in -temperature or
moisture content. Alternatively, rnovement of the pipeline itself due to
thermal expansion may result in perforation or other damage of the
0 protective coatings.
Primer compositions for use on metal pipes generally include a
mixture of a resin and a rubber component. For many years, the resins
used in the prim~r compositions were natural resins which had been ~;
reacted with zinc or calcium to form a resinate that was relatively stable
~5 and resistant to chernical degradation. Primer compositions made using
such resins provided some improvement in cathodic disbondment
resistance over the prior art, but nevertheless did not adequately eliminate
the problem.
It is preFerable, and in many cases it is an industry requirement,
20 that all major buried metal pipelines be cathodically protected against
corrosion. This is done by connecting the metal of the pipeline to a
sacrificial anode and inducing a direct current within the metal and anode
so that the rnetal pipeline becomes a cathode and therefore is subjected to
less material loss due to corrosion. Cathodic protection has been Found to
25 slow down corrosion caused by sub-surface chemicals and electrolytic
action at areas oF the pipelines where the holidays in the protective
coatings occur.
Notwithstanding cathodic protection oF buried metal pipelines,
corrosion where the holidays occur will nevertheless become significant
30 over time. A further problem caused by cathodic protection at holiday
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sites is an increased tendency oF the adhesion or bonding of the protective
coating, particularly ~he primer composition, to break down on the surface
of the pipeline where the holidays occur. This decrease in the adhesion or
bonding causes the primer composition and the adhesive tape to separate
from the metal pipeline. Accorclingly, the protective coating "disbonds" in
the area surrounding the holiday, thereby leading to even more corrosion
of the metal pipeline.
This phenomenon of decreased adhesion oF the protective coating
system in the area of the holidays is referred to as "cathodic
disbondment". It is believed that cathodic disbondment occurs because,
initially, there is a negative charge in the vicinity of the holiday. However,
with time this negative charge has the effect of creating metal oxides on
the metal surface and oF decreasing the adhesion of the primer
composition to the metal surface. This results in an increased area where
the metal surFace is exposed, and a corresponding increase in corrosion
and current flow around a holiday so that the problem of corrosion
becomes exaggerated over time.
Cathodic disbondment has, thereFore, become a major concern in
the pipeline protection industry. As a result, an important feature of a
primer composition for buried metal pipelines is the ability of the primer
composition to remain bonded to the metal surFace and resist the
tendency of cathodic disbondmenlt despite the presense and formation of
holidays therein. The ability oF a primer composition to resist cathodic
disbondment is not necessarily directly related to its adhesive strength to
the metal surface. That is, a primer composition with strong aclhesion
may not have good resistance to cathodic disbondrnent.
United States Patent No. 5,108,809 issued to Patil provides a
surnnnary of the art devoted ~o the problem of cathodic disbondment in
buried rnetal pipelinas. Interestingly, all oF the prior art discussed in U.S.
Patent No. 5,108,809, including the primer composition disclosed in that
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patent, address the problem oF cathodic disbondmen~ by addition of a
"cathodic disbondment inhibitor" to the primer composition. None of the
prior art discloses the use of a rubber compound that, itself, also acts as a
cathodic disbondment inhibitor.
U.S. Patent No. 5,108,809 to Patil describes the use of a phenolic
resin as the cathodic disbondment inhibitor in primer compositions.
Phenolic resin is said to improve the bonding of the adhesive tape to the
metal pipe. The examples set out in U.S. Patent No. 5,108,809
demonstrate that the addition of the phenolic resin to a prirner
composition increases resistance to cathodic disbondment.
The rubber-based components o~ the primer compositions described
in U.S. Patent No. 5,108,809 include butyl rubber, chloro-butyl rubber
and therrno-plastic rubber. Although butyl rubber is a substantial
improvement over natural rubber compounds, it nevertheless has many
double bonds which are available For chemical action and thereFore it is
subject to chemical degradation over time. Chloro-butyl rubber does not
have as many double bonds. On the other hand, it has other side efFects,
including the potential release of chlorine ions which would react with the
metal pipeline and significantly increase corrosion.
Stable, synthetic rubber compounds which have superior resistance
to chemical degradation are known in the art. For instance, from U.S.
Patent Nos. 4,797,322 and 4,801,346 both issued to Huddleston, it is
known that compositions comprising a copolymer having styrene end
blocks and a poly~ethylene-butylene) mid-block (hereinafter raFerred to as
"S-EB-S") provides good resistance to chernical degradation in the
presence oF atmospheric weather and sunlight. However, S-EB-S
copolymer is relatively poor in its adhesion properties to metal. For
instance, tests have shown that the adhesive peel strength oF S-EB-S
copolymer on steel is approximately 60 to 80 ounces per inch oF width, ;
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whereas for purposes of a primer composition on pipelines, a peel
strength of at least 300 ounces per inch of width is required.
United States Patent No. 4,578,429 issued to Gergen discloses
block copolymers that have been modified by grafting an acid component
to the midblock. Examples of such modified copolymers are S-EB-S
copolymers on which anhydride functionalities have been graFted. One
example in Patent No. 4,578,429 indicates that the anhydride-modified S-
EB-S copolymer possesses the same resistance to chemical degradation as
the unmodified S-EB-S copolymer, but its adhesive strength to metal is
significantly enhanced.
United States Patent No. 4,801,346 issued to Huddleston discloses
a protective coating containing S-EB-S copolymer for use as a storage
protective system for metal pipes. Although the adhesion of S-EB-S
copolymer is insufficient for long-term use on buried pipelines, it was
found to provide sufficient protection during typical storage periods of the
metal pipes. A drawback with the use of the storage protective coating
disclosed in United States Patent No. 4,801,346, and indeed with all the
prior art, is that, when the metal pipe is ready to be buried underground,
the storage protective coating must be removed Itypically using a wire
brush or abrasive blasting) before the long-term primer composition and
~dhesive tape are applied to the metal pipelina. There are, therefore,
additional expenses incurred and time required to store metal pipes before
they are used in pipelines.
Accordingly, it would be desirable if the storage protective coating
did not have to be removecl when the metal pipe is to be buried.
Preferably, significant advantages are achieved if the storage protective
coating is the same as the primer composition used on buried pipelines.
Since typical buried metal pipelines are protected with a layer of
prinler composition covered by one or more layers of an adhesive tape,
one of the functions of the primer composition is to form an adhesive ~ ;
r ,
bond between the inner adhesive tape and the surface of the metal pipe.
As well, it is preferable if the primer composition is capable of holding that
bond despite the formation of holiclays in the protec~ive coating. That is,
it is preferable if the primer cornposition has good resistance to ca~hodic
5 disbonding.
Additionally, the primer cornposition should also provide 3 relatively
impervious film to create an electrical barrier or insulation between the
metal pipeline and its surroundings. It is also desirable that the primer
composition be inert to chemical degradation for long periods of time
0 (normally in the order of 30 years), and that it have a degree of elasticity
so that it can withstand the stress of pressure due to longitudinal
movement of the pipeline relative to the surrounding soil.
Summary of the Invention
Accordingly, the present inventiol- provides a primer composition
15 for coating the outside surface of a metal pipe. The primer composition
comprises a mixture of solid components that includes a tackifying resin
and an effective amount of modifiad S-EB-S copolymer, and ona or more
solvents capable of solvation with the modified S-EB-S copolymer. The
modiFied S-EB-S copolymer has grafted onto it a functional group,
20 preferably an anhydride.
The present invention also provicles a metal pipeline systern
comprising a metal pipe having an outer surface, a layer of a primer
composition and at least one layer of adhesive tape over the layer oF
primer composition. The primer composition consists of a mixture of solid
25 components that includes a tackifying resin and an eFFective amount oF
anhydride-modiFied S-EB S copolymer.
~n~l~s:[lptLQn oF the Present InventiQ~
Primer compositions For coating metal pipes have been made by
combining a rubber-based cornpound having desired elasticity with a resin
30 designed to impart the necessary bonding and adhesive strength to the
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primer composition. It has been found that a suitable rubber compound
for use in primer compositions is reclaimed natural rubber made from inner
tubes and the like. These products are made with rubber that has been
~ulcanized and otherwise treated so that the rubber is relatively stable and
5 inert to chemical degradation for the requisite time period. Reclaimed
natural rubber continues to this day to be one of the preFerred rubber
compounds for use in primer compositions, however, the addition of
cathodic disbondment inhibitors are required.
It has been found, surprisingly, that primer compositions containing
10 anhydride-modified S-EB-S copolymer as their rubber components not only
have excellent adhesive properties, but also exhibit good resistance to
corrosion and to cathodic disbonding. As discussed above, S-EB-S
copoiymer has excellent resistance to chemical degradation, but its
adhesion or bonding to metal is relatively poor so that it is useful as a
15 short-term storage protective coating, but not in a primer composition for
buried pipelines. Modification of the S-EB-S copolymer by grafting
anhydride functionalities onto it results in a compound having a resistance
to chemical degradation comparable to S-EB-S copolymer, thus useful in a
storage protective coating. In addition, the anhydride-modified S-EB-S
20 copolymer exhibits increased adhesion and bonding to metal surfaces. It
has also been found by the invantor that primer compositions
incorporating anhydride-modified S-EB-S copolyrner are surprisingly
resistant to cathodic disbondment.
Typically, primer compositions for use on metal pipelines include a
25 resin component, a rubber component, and various additives such as
colorants, solvents, anti-03(idants, etc. The prior art has addressed ~he
problem of cathodic disbondment by including in the primer compositions
an additional component, generally described as a cathodic disbondment
inhibitor. The primer composition of the present invention, however, does
30 not include an additional component that acts as a cathodic disbondment
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inhibitor. Rather, the primer composition of the present invention has a
rubber matrix or backbone that, itself, inhibits cathodic disbondment.
That is, the anhydride-modified S-EB-S copolymer is, in effect, the rubber
component and the cathodic disbondment inhibitor of the primer
5 composition. This is a significant advantage over the primer compositions
of the prior art. Since the rubber component is itself a cathodic
disbondment inhibitor, there is no need to acld a separate cathodic
disbondment inhibitor to the primer composition, thereby eliminating any
disadvantages encountered when compounds are added to a mixture.
In order to obtain the required adhesive qualities for primer
compositions used in pipeline coating, it is necessary-to create a
strengthened bonding between the primer composition and the metal
pipeline. The adhesive and bonding characteristics of S-EB-S copolymer
are increased by the addition or graFting of an anhydride functionality to
the S-EB-S copolyrner. The anhydride functionality can be graFted onto
the S-EB-S copolymer by reaction with maleic anhydride. This process
and the resulting modified S-EB-S copolymer has been described in U.S.
Patent No. 4,578,429 issued to Gergen, and the anhydride-modified S-EB-
S copolymer is commercially available from Shell Chemical Company
under the trade-marks l<RATC)N 1901X and KRATON 1924. The
anhydride modified S-EB-S copolymer is said to exhibit increased adhesive `
strength to metal surFaces as compared to the unmodiFied S-EB-S
copolymer.
Patent No. 4,578,4~9 does not disclose, or even contemplate, the
use of anhydride-modiFied S-EB-S copolymer in primer compositions for
use on metal pipes. In particular, the surprising improvement in cathodic
disbondment oF primer compositions incorporating modified S-EB-S
copolymer is not contemplated.
U.S. Patent No. 4,578,429 discloses that the Functional group
grafted onto S-EB-S copolymer need not be limited to an anhydride
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functional group. F;a~her, the class of preferred functional groups that can
be grafted onto S-EB-S copolymer include carboxylic acid groups and their
salts, anhydrides, esters, imide groups, amide groups, acid chlorides and
the like. The preferred modifying functional groups include anhydrides
5 such as acrylic anhydride, maleic anhydride, itaconic anhydride and
citraconic anhydride.
The addition of the anhydride functionality to the S-EB-S copolymer
is believed to provide sites for creating an effective bond between the
copolymer and the iron in the metal pipe. That is, the anhydride
10 functionality on the S-EB-S copolymer provides chemically active sites
which promote the formation of bonds with the iron on the surface of the
rnetal pipe. As a result of the use of the anhydride-modified S-EB-S
copolymer, the primer composition of the present invention has been
found to have excellent adhesion strength iover 350 ounces per inch oF
15 width).
The primer composition of the present invention comprises a
mixture of solid components and at least one solvent. The solid
components include a tackifying resin component and the anhydride-
modified S-EB-S copolymer component. The solvents useful in the
20 present invention are those capable of solvation with the mixture of solid
components and the anhydride-modilFied S-EB-S copolymer.
Other components commonly used in primer compositions may also
be used in the primer composition of the present invention. For example,
various colorants may be used, the main function bein0 to provide colour
2s to the primer composition so as to enable the user to ensure that the
surface oF the metal pipe is completely covered with primer composition.
For this purpose, carbon black is the best known and least expensive
coloran t .
Typically, the adhesive tape used For protectin~ metal pipes has a
30 backing material made olF a blend of polyolefins. The adhesive layer
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usually is composed oF butyl and butyl-reclaimed rubber that is tackified
with petroleum resin and polybutene, and filled with clay. Examples of
formulations for the adhesive layer are found in United States Patent Nos.
4,687,794 and 4,692,352, both issued to Huddleston. For example, a
5 suitable adhesive layer comprises the following:
_ _
butyl rubber 76 Ibs.
_ _ _
reclaimecl butyl rubber 145 Ibs.
polybutene tackifier 63.5 Ibs.
clay 220 Ibs.
I . . _
¦ anti-oxidan-t 2 . 5 Ibs .
TOTAL: 507 Ibs.
Various-tackifying agents, anti-oxidants and solvents which are
already known in the art may be used in the primer composition of the
0 present invention, so long as they are compatible with the anhydride-
modified S EB-S copolymer rubber base and the other ingredients.
Examples of various tackifying resins that are known and marketecl to the
adhesive industry include -the following: resins such as gum, wood or-tall
oil resins; rnodified resins, for example polymerized resin or hydrogenated
15 resin; resin esters such as pentaerythri-tol-wood resin; glycerin-
hydrogenated resin, glycerin-highly stabilized resin, and a pentaery-thritol-
highly stabilized resin; polymerized petroleum hydrocarbons, for example
cycloaliphatic hydrogenated olefins, olefins, aliphatic petroleum
hydrocarbons, modified aromatic hydrocarbons, dicyclopentadiene, mb(ed
~0 olefins, alkyl-aroma~ic petroleum hydrocarbons, modified aromatic
hydrocarbons; polymerized terpenes such as alpha-pinene, d-limonene,
beta-pinene, terpene, etc.; miscellaneous resins such as alpha-
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methylstyrene, vinyi-toluene, alpha-me~hylstyrene, styrene, terp~ne
phenolic, etc.; and metallic resinates such as mixed calcium/zinc and zinc
resinates.
As discussed in U.S. Patent No. 4,578,429, the modified S-EB-S
copolymer will usually contain from 0.02% wt. to 20% wt., preferably
0.1% wt. to 10% wt., and most preferably 0.2% wt. to 5% wt. of the
functional group. It has been found that the preferred range for the primer
composition of the present invention is from about 0.07% wt. to 3.3%
wt. maleic anhydride based on the total weight of the mixture of solid
components. In other words, the anhydride component of the anhydride-
modified S-EB-S copolymer makes up about 0.07% wt. to 3.3% wt. of
the total mixture of solid components. Preferably, the mixture of solid
components will have about 2% wt. of maleic anhydride.
In the primer composition of the present invention, the rnixture of
solid components will be from about 6% wt. to about 30% wt. of the
total primer composition. In the preferred primer compositions, the
mixture of solid components will comprise about 12% wt., about 19%
wt., or about 27% wt. of the total primer composition.
The relative amounts of tackifying resin and anhydride-modified S-
EB-S copolymer in the primer composition oF the present invention is also
important. Too much tackifying resin results in a procluct that is brittle
when dried, whereas too much anhydride-modified S-EB-S copolymer
results in a product that has lower adhesion to metal. It has been found
that the ratio of tackifying resin to anhydride-modified S-EB-S copolymer
should be in the rangs of about 20 to about 175 parts resin to 100 parts
of anhydride-rnodified S-EB-S copolymer, by weight. Preferablv, the ratio
is in the range oF about 75 to about 1~5 parts resin to 100 parts
anhydride-modified S-EB-S copolymer, by weight.
The primer composition of the present invention is, therefore, useful
to protect metal pipes from corrosion during storage. It is also useful on
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buried pipelines because of its increased adhesion and its superior
resistance to chemical disbondrnent. One advantage of the primer
composition of the present invention is that it does not have to be
removed from stored metal pipes prior to burial underground. Rather,
5 when the metal pipes are removed from storage, and prepared for burial,
all that needs to be done is the addition of a fresh layer of primer
composition over the existing layer, and then wrapping the adhesive tape
around the pipe. The need to remove the storage protective coating is
eliminated, thus saving substantial time and expense.
The rnetal pipeline system oF the present invention includes a metal
pipe whose outer surface is coated with the primer composition of the
present invention. One or more layers of adhesive tape is then applied
over the primer composition.
In practice, metal pipes are prepared as follows before storage and
15 before burial in the ground. The outer surface o~ the metal pipe is treated
by a cleaning process such as sand blasting or wire brushing. The primer
composition of the present invention is then applied onto the outer surface
of the pipe, typically by spraying or brushing. The metal pipe can then be
stored until needed. When the metal pipe is to be used in a metal pipeline
20 system, a fresh layer of primer composition is applied by spraying or
brushing over the existing layer of primer composition. Once the solvents
in the primer composition have evaporated, typically about 30 seconds,
one or more layers of adhesive tape are helically wound onto the metal
pipe. The adhesive tape wrapping used typically consists of a spiral
25 overlapped wrapping. The tape is made oF a baclcing of polyethylene and
an internal layer oF butyl rubber, tackifiers, fillers and anti-oxidants.
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A primer composition for coating the outsida surFace of a metal
30 pipe was prepared having the Following composition:
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_ . _
Component Parts by Weight
_ _
KRATON 1901X 100
ESCOREZ 2101 100
. .
Toluene 200
Heptane 520
Isopropyl Alcohol 80
_
TOTAL~ oo
, ,, . .~_ _ . . . ~ .
In the above example, KRATON 1901 X is the commercial trade-
mark for the S-EB-S block copolymer as rnodiFied with maleic anhydride.
5 KRATON 1901X is available from Shell Chemical Company and includes
2% wt. maleic anhydride.
ESCOREZ 2101 ~a trade-rnark) is a petroleum-based tacki~ying resin
available ~rom Exxon Corporation. Toluene, heptane and isopropyl alcohol
are solvents capable of dissolving the styrene end-blocks, the ethylene-
0 butylene mid-blocks and the anhydride functionality of the anhydride-
modified S-EB-S copolymer.
The standard cathodic disbondment test was carried out on a metal
pipe having a protective coating that included tha primer composition
described above. In this test, an 18 inch metal pipe oF 3 1/2 inches
15 outside diameter was coated with the primer composition and overlaid
with a layer oF adhesive tape. The pipe section had been preparsd by
abrasive blas~ing to near-white cleanliness rating with a 2 mil profile. The
primer composition was applied by brushing and allowed to dry about 15
minutes before applying a 4 inch wide black innerwrap with a 50%
20 overwrap. A 1/4 inch diameter holiday was drilled through the protective
'2. ~ 5
14
coating. The specimen was immersed to a level of 12 inches in a 3%
sodium chloride solution, and the solution was heated to 1 50F. A
potential of 1.5 volts was applied to a platinum wire anode. These test
conditions were maintained for 28 days.
Upon termination of the test, the specimen was allowed to cool to
room temperature, and the adhesion near the holiday was evaluated.
Resistance to cathodic disbondment was determined by rneASuring the
radius from the edge of the intentionally created holiday to the point
where the coating exhibitecl good adhesion. It was found that the
cathodic disbondment radius averaged approximately 5 millimetres.
Typically, disbondment radii with other primer compositions of the prior
art are in the order of 17 to 20 millimetres.
The cathodic disbondment radius obtained using the primer
composition of the present invention was approximately the same as ~he
cathodic disbondment radius reported in U.S. Patent No. 5,108,809,
where phenolic resins were added as cathodic disbondrnent inhibitors.
Accordingly, the use of anhydride-modified S-EB-S copolymer as the
rubber component of the primer composition of the present invention
provides sirhilar results in terms of cathodic disbondment to the addition
of a phenolic resin to the primer composition.
Isopropyl alcohol was included in the primer composition of the
above Example as it aids in dissolving the anhydride functionality of the
modified S-EB-S copolymer. Isopropyl alcohol also serves to combine
with moisture on the surface structure of the metal pipe, thereby
promoting the wettability of the steel by the solvent which carries the
mixture of solid components of the primer composition of the present
invention.
Tha use of anhyclride-modified S-EB-S copolymer as the rubber
component of a primer composition has, therefore, been found to provide
a primer composition with chemical stability, superior adhesion qualities to
metal, and superior resistance to cathodic disbondment. It is useful as a
component of a protective system during both storage and burial of the
metal pipe.
Modifications and variations of the present inven~ion are possible
5 within the sphere and scope of the claims appended hereto.
