Note: Descriptions are shown in the official language in which they were submitted.
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ABRASION RESISTANT GRAPHITE-CONTAINING
EPOXY POWDER COATINGS
Field of the Invention
This invention relates to powder coating compositions and their use for
forming protective polymer coatings on metal pipe. More particularly, this
invention is
directed to a thermally conductive powder coating composition comprising a
highly
filled epoxy resin matrix that can be applied to pre-heated metal pipes to
provide a
fully cured abrasion resistant polymer coating alone or in combination with a
corrosion-barner undercoating.
Background and Summary of the Invention
Metal pipes designed for use in subterranean environments must be
protected from corrosion in order to ensure pipeline integrity, thereby
preventing
product leakage and the resultant environmental damage. Such corrosion
protection
has been typically accomplished by coating the metal pipe with a highly
adherent
thermosetting powder coating and use of cathodic protection on the installed
pipeline.
Breaches in the coating provide focused weak points where rapid corrosion can
occur.
The cathodic potential from buried anodes of more active metals or metal
alloys, or
from impressed negative electrical current, prevents corrosion at these sites.
In order
to achieve the optimum effectiveness and minimum operational cost of such
cathodic
protection, damaged areas in the coating must be kept to a minimum. Although
these
powder coatings are quite tough and durable, mechanical damage to the coating
can
occur from handling, during transportation, from installation, and while in
service.
Pipeline coatings are particularly susceptible to mechanical damage from
abrasion
when installed by the directional bore and pull-through technique. Powder
coatings
used as corrosion protective pipeline coatings typically contain relatively
low levels of
mineral fillers to assure optimum bonding to the pipe surface, and to provide
suffcient
flexibility for field bending of the pipe in cold climates during
installation. Such
corrosion-barner powder coatings are typically softer than highly filled
coatings due to
their lower mineral filler content, and they are more susceptible to
disruption by
scratching or gouging during handling or installation. Accordingly, it is
becoming
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more of interest to apply a more highly filled polymer coating over the
underlying
corrosion-barner coating to protect the underlying coating from damage prior
to or
during installation.
The outer coatings are typically applied in a thickness at least equal to
and often two or more times that of the underlying corrosion resistant coating
for
applications demanding greater protection. However, there are inherent
difficulties in
optimizing the thickness of the outer coating. Typically the inner and outer
powder
coating formulations are applied sequentially as part of a single coating
operation. The
pipe substrate is first heated to a temperature above the minimum cure
temperature of
the resin components but below the resin decomposition temperature. The
polymer
coating compositions are typically applied as a powder, preferably by
electrostatic
spray, to the preheated pipe. The applied powder coating compositions first
melt to
form a continuous polymer layer that is rapidly heated by the underlying
preheated pipe
to a temperature sufficient to melt and initiate polymerization and cure of
the
thermosetting resin component. The maximum thickness of the outer coating is
linuted
by the amount of latent heat in the preheated pipe and the efficiency of
transfer of heat
to the applied outer coating through the inner coating. It is important that
the whole
thickness cross-section of the outer coating is heated to sufficiently high
temperature
to assure complete cure of the applied outer coating. In other words, at
greater
coating thicknesses, it becomes more likely that some portion of the powder
resin
composition applied to form the important outer abrasion/impact resistant
coating is
not fully cured resulting in a coated pipe product having an outer coating
without the
requisite protective physical properties. While there can be subsequent post-
cure
reheat operations to complete the cure of the outer coating, such additional
operations
add significantly to the cost of polymer clad pipe products.
Thus, in accordance with one embodiment of the present invention
there is provided a thermally conductive powder coating composition that can
be
applied to preheated pipes, more particularly preheated pipes also precoated
with a
corrosion barrier polymer composition, such as a fission bonded epoxy, to
provide fizlly
cured high thickness, impact-resistant outer coatings. The composition
comprises a
mineral filled resin matrix in particulate form that includes an epoxy resin,
graphite and
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a mineral filler. In preferred embodiments the composition further includes an
epoxy
curing agent, a tertiary amine catalyst and flow control agents.
The compositions can be applied over a thermoplastic or thermosetting
corrosion barrier coat in an amount sufficient to provide a fully cured
protective
coating having an average total thickness of greater than 20 mils, more
typically about
25 to about 100 mils, and more than 40 mils in premium applications. The
combination of graphite with a high level of mineral filler enables a high
level of
thermoconductivity resulting in an exceptionally high efficiency transfer of
latent heat
from the heated pipe through the underlying corrosion barrier coating to
provide rapid,
complete cure of the abrasion/impact resistant outer coating in full thickness
cross-
section. Further, in circumstances where a post-cure reheat of the coated
surface is
required for complete cure, for example, at high coating thicknesses, when the
thermosetting polymer component has an unusually high thermoset initiation
temperature, or when coating over thin-walled pipe, the graphite-containing
resin
composition of this invention enables more efficient heat absorption and
transfer and
concomitantly facilitates the reheat cure of the outer polymer coating.
In another embodiment of the present invention there is provided an
improved method of forming a protective polymer coating on a metal pipe,
wherein the
protective polymer coating is formed by heating the pipe, applying a first
resin powder
coating composition to the heated pipe, and applying a second resin powder
coating
composition over the first coating composition. The present improvement of
that
method comprises the step of selecting for said second resin powder
composition a
resin powder coating composition of this invention comprising a mineral filled
resin
matrix in particulate form wherein said filled resin matrix comprises an epoxy
resin,
about 1 to about 10% by weight graphite, and about 40 to about 70% by weight
mineral filler.
In another embodiment of this invention there is provided an improved
method similar to that above except the method includes the step of reheating
the
coated pipe by exposing it to infra-red radiation for a predetermined time
sufficient to
cure the epoxy resin coating. The improvement in that reheat cure process is
the
selection and use of the above-described thermally conductive powder coating
composition. In preferred embodiments that composition includes an epoxy
resin,
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about 1 to about 10% by weight graphite, about 40 to about 70% by weight
mineral
filler, an epoxy curing agent and an effective amount of a tertiary amine
catalyst. Use
of that composition in the reheat process enables a reduction in the infra-red
radiation
exposure time for complete cure of the protective epoxy coating.
In another aspect of the present invention there is provided a method
for forming a fully cured protective abrasion-resistant resin coating on a
pipe wherein
the coating has an average thickness of about 20 to about 100 mils. The method
consists essentially of the steps of heating the pipe to a temperature above
the
temperature required to polymerize the resin but below the resin decomposition
temperature, applying a first thermosetting polymer composition to the heated
pipe to
form a first corrosion protective coating layer, and applying an amount of a
second
thermosetting polymer composition over the first composition to form a
second/outer
coating layer. The second resin composition comprises the present
graphite/mineral
filled epoxy resin matrix in particulate form. The second resin composition is
applied
in an amount sufficient to provide a fully cured protective coating having an
average
total thickness of about 20 to about 100 mils, all without reheating the pipe.
In still a fizrther embodiment of this invention there is provided an
abrasion-resistant, corrosion-inhibited pipe coated with a thermoset
multilaminate
epoxy coating. The outer layer of the coating is formed by application of the
present
highly filled, graphite-containing coating composition in powder form to a
preheated
pipe precoated with a corrosion-barrier coating composition resin having a
mineral
filler content of less than 40% by weight, optionally with graphite at 1 to
10% by
weight to enhance thermoconductivity.
Detailed Description of the Invention
In accordance with the present invention there is provided a highly
filled, thermally conductive powder coating composition that can be cured
through a
thermally induced cross-linking/polymerization mechanism to provide a fully
cured,
protective abrasion resistant resin coating on a metal pipe. The composition
is most
typically applied to a preheated metal pipe precoated with a thermosetting or
thermoplastic corrosion barrier coating. The present powder coating
composition
comprises a highly filled resin matrix that includes a thermosetting resin,
graphite, and
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about 40 to about 70% by weight, more typically about 50 to about 60% by
weight, of
a mineral filler. The resin matrix can also include a resin curing agent, a
tertiary amine
catalyst, a flow control agent and other additives to facilitate formulation
or
application of the coating composition. In one embodiment of this invention an
abrasion resistant, corrosion-inhibited pipe in accordance with this invention
is
prepared by heating the pipe to a temperature above a temperature required to
melt
and polymerize the resin but below the resin decomposition temperature,
applying a
first thermosetting resin composition to the heated metal pipe to form a first
inner
corrosion barrier coating, and applying the present powder coating composition
over
the first coating to provide a fizlly cured protective coating having an
average total
thickness of about 20 to about 100 mils. Total average thickness of the inner
and
outer coatings are more typically about 25 to about 80 mils, and more than
about 40
mils to about 80 mils in premium applications.
Generally, the thermally conductive powder coating compositions of
the present invention comprise a filled thermosetting resin matrix combined
with a
thermosetting resin, graphite, a mineral filler, a curing agent, and a
tertiary amine
catalyst. In one embodiment the thermosetting resin is an epoxy resin that
includes a
mineral filler and graphite. The epoxy resin is melt-blended with the curing
agent, the
catalyst, the graphite and the mineral filler to provide a melt-blended matrix
having the
graphite and filler uniformly dispersed and fully wetted with the epoxy resin.
Optional
additives can be employed to optimize chemical and physical characteristics of
the
coating composition. For example, the thermosetting resin matrix can include
coupling
agents and flow control agents for rheology control. The melt-mix is cooled
and
ground to a powder to provide the present thermally conductive powder coating
compositions that are useful for preparing protective polymer coatings for
metal pipe.
The coating composition of this invention comprises a thermosetting
resin. Preferably the thermosetting resin is an epoxy resin that is prepared
from
bisphenol A and an epoxide monomer, for example, epichlorohydrin. The nature
of the
monomer is not critical provided that the product epoxy resin has the
thermally
inducible cure profiles suitable for resin formulation and application to
preheated pipe
. surfaces. The preferred epoxy resin component of the present coating
composition has
an epoxide equivalent weight of about 650 to 2,300; more preferably, the
epoxide
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equivalent weight for the epoxy resin is about 900 to about 1,400. The melting
point
for the epoxy resin ranges from about 75 ° to about 130 ° C;
more preferably, the
melting point ranges from about 95 ° to about 120 ° C. The melt
viscosity of the epoxy
resin is about 40 to about 300 poise at 150°C.
The resin matrix of the present powder coating composition also
includes about 1% to about 10%, more typically about 2% to about 5% by weight
graphite. Either natural or synthetic forms of graphite can be included in the
filled
resin matrix. In one embodiment of this invention the graphite component has
an
average screen size of about 275 mesh to about 375 mesh. Preferably, the
graphite
component of the filled resin matrix is a naturally-occurring amorphous
graphite
having about 80% carbon content and a screen size of 99.8%-325 mesh. The
graphite
component is an excellent heat conductor, with a coefficient of thermal
conductivity
similar to that of many metals and much higher than that of the other formula
components. The graphite component also imparts other unique functionality to
the
1 S powder coating composition. Not only does it enable high radiant (infra-
red) heat
absorption and thermal conductivity, it also imparts lubricity and abrasion
resistance to
the coating with resultant enhanced protection of the underlying corrosion
barner
coating in the coated pipe products.
The present coating composition further includes about 40% to about
70% by weight mineral filler. In one preferred embodiment the matrix includes
about
50% to about 60% by weight filler and the weight ratio of mineral filler to
graphite is
about 4:1 to about 70:1. The mineral filler provides the desired impact and
abrasion
resistance to the protective coating and also contributes significantly to the
thermoconductivity of the present compositions that allows their use in
forming thick
fully cured resin coatings without tedious and expensive reheat operations.
Suitable
mineral fillers include calcium carbonate, magnesium silicate, aluminum
silicate, barytes
(barites), wollastonite, amorphous silica, crystalline silica, feldspar, and
mica. The
particulate mineral filler can also be selected from synthetic fillers such as
ZEEOSPHERES, from Zeelan Industries, or FIBER;FRAX from Carborundum. The
particulate mineral fillers have a median screen size of about 200 to about
400 mesh;
more preferably, the particulate mineral fillers have a median screen size of
about 275
to 350 mesh. In one embodiment the mineral fillers can have median particle
size of
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about 5 to about 20 microns. Coupling agents, such as the amine functional or
epoxy
functional silanes well-known in the art, can be used in the present
composition in
amounts effective to increase the wetting ability of the melted epoxy resin so
that the
resin can fully coat the filler particles.
In one embodiment of the present invention the resin matrix is an epoxy
resin matrix formulated to contain about 0.1% to about 1.5% by weight of a
tertiary
amine catalyst. Commercially available catalysts suitable for use in the
present
invention include, but are not limited to, imidazole, 2-methyl imidazole, 2-
propyl
imidazole, 2-phenyl imidazole, 2-ethyl-4-methyl imidazole, 2-methyl imidazole
epoxy
resin adduct, 2-propyl imidazole epoxy resin adduct, dimethyl amino methyl
phenol,
2,4,6-tri(dimethyl amino methyl) phenol, 4-dimethylaminopyridine, 4-(4-methyl-
1-
piperdinyl)pyridine, benzyldimethylamine, triethylenediamine, 2-phenyl
imidazole. The
amine catalysts are effective to promote polyrr~rization of the epoxy resin at
elevated
temperatures, typically above 3 5 0 ° F ( 199 ° C) , but not at
ambient temperatures.
Preferred epoxy resin matrices for the present powder coating
composition further comprise about 0.5 to about 18% by weight of one or more
curing
agents. A wide variety of curing agents are commercially available. Typical
curing
agents include primary and secondary amines, aromatic amines, carboxylic acids
and
anhydrides. The curing agents also can include di-functional primary and
secondary
polyamines, phenolic-poly (hydroxy ethers), phenol-formaldehyde resins or urea-
formaldehyde resins. Specific examples include, but are not limited to,
dicyandiamide,
low molecular weight phenol-formaldehyde condensates, and linear phenolic
resins
having a free phenolic hydroxyl functionality.
The thermosetting resin formulations of this invention can include other
additives to complement the physical and chemical properties of the thermally
conductive powder coating composition. Thus, for example, as mentioned above,
one
or more commercially available coupling agents can be employed to facilitate
wetting
of the graphite and particulate mineral filler with the epoxy resin
composition.
Coupling agents, for example amino functional or epoxy functional silane
coupling
agents sold by Witco, can be employed, often in trace amounts, to facilitate
the resin
formulation. Flow control agents, such as 2 ethyl hexyl acrylate polymer or
acrylate
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co-polymers, can be included in the coating composition neat or absorbed onto
a
mineral substrate to facilitate uniform coverage of the pipe with the melted
resin.
The thermally conductive powder coating composition of the present
invention is prepared by dry blending the epoxy resin, the mineral filler, the
graphite,
the curing agent, and the catalyst to provide a dry-mixed powder composition
which is
then transferred via a hopper into an extruder for melt-blending to disperse
the solid
components and wet the mineral filler and graphite with melted epoxy resin.
The
extruder can be any extruder commonly used in the art such as a single screw
extruder
or a twin-screw extruder. It is important that the melt-blended matrix be
processed
through the extruder at a rate/temperature selected to limit the
polymerization of the
epoxy resin while the melted matrix is in the extruder. The melted resin blend
is
extruded through a die, through a chilled roller, and cooled on a cooling belt
in a flat
continuous solid matrix sheet. The solid matrix is then chipped to provide a
granulated
composition which is then ground into a powder and size classified to provide
a
thermally conductive powder coating composition having an median or average
particle size of about 30 microns to about 100 microns, more typically about
40 to
about 70 microns.
The thermally conductive powder composition according to the present
invention is applied to a metal pipe, most typically over a preapplied
thermoplastic or
thermosetting resin corrosion barrier coating, to provide a protective polymer
coating.
The protective powder coating composition may be applied by any of the methods
commonly known in the art. For example, a metal pipe can be heated and
thereafter
immersed in a fluidized bed of the powder coating composition, or the powder
coating
composition can be electrostatically applied to the heated metal pipe. In that
later
method, the thermally conductive powder composition is applied to the metal
pipe as
the pipe is advanced through a series of powder spray stations. The pipe is
typically
rotated about its longitudinal axis to ensure that the entire exterior of the
pipe is
uniformly coated with the coating composition.
Prior to application of the inner corrosion barrier coating the metal pipe
is washed to remove soils, lubricants, or other contaminants. The pipe is then
advanced to a blast-cleaning station where it is abrasively cleaned. For
example, the
pipe can be subjected to grit blasting to provide a surface profile for
optimum adhesion
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of the applied resin. The pipe is then advanced to a heating station where it
is heated
to a pre-determined temperature that is above the temperature needed to cure
the
thermally conductive powder coating composition but below the temperature at
which
the powder coating composition decomposes. The method of heating is not
critical.
The pipe can be heated in a furnace, with an induction coil or a forced air
heater. The
pipe is heated to a temperature of about 3 50 ° F to about 5 50
° F ( 199 ° C to about
3 24 ° C), more preferably about 400 ° F to about 5 00 °
F (204 ° C to about 260 ° C).
In one preferred embodiment the hot metal pipe is grounded and
advanced to a spray station where a first epoxy powder coating composition (a
corrosion barrier coat) is electrostatically applied to the hot pipe to
provide an inner
coating having a thickness of about 8 to about 15 mils. The epoxy powder
composition may be applied by one or more spray heads. Preferably the pipe is
continually rotated along its longitudinal axis to ensure that the
electrostatic
application of the powdered resin provides a uniform coating of the resin
composition
to provide a corrosion barrier. The heated pipe retains sufficient residual
thermal
energy to melt and polymerize the applied powdered resin to provide a cured
epoxy
resin coating.
A second epoxy coating is applied immediately after application of the
first coating. Typically the pipe is advanced to a second spray station where
a second
epoxy resin coating composition is applied over the first epoxy coating
composition.
Use of two separate spray stations allows the over-spray from each
electrostatic
spraying operation to be separately recovered and recycled.
The second powder coating composition comprises a mineral filled
resin matrix in particulate form. The resin matrix comprises an epoxy resin
and about
1% to about 10% by weight graphite, and about 40% to about 70% by weight
mineral
filler. The metal pipe, already coated with an inner corrosion barrier
coating, retains
sufficient thermal energy to melt and fully cure the after-applied outer
coating
composition without necessity of re-heating the coated metal pipe, even when
it is
applied at a rate sui~icient to provide total average coating thicknesses of
about 20 to
about 100 mils, more typically about 25 to about 80 mils (or about 45 to about
80 mils
in certain premium applications). Typically the ratio of the thickness of the
outer
coating to the thickness of the inner coating is generally greater than 1:1,
more
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typically greater than 2:1. Application of a second epoxy resin coating
composition
over a first epoxy coating composition on a hot metal pipe according to the
present
invention provides a fully cured protective polymer coating that is
heterogeneous in
thickness cross-section.
Following application of the present thermally conductive powder
coating composition, the coating is allowed to cure for about 1 to about 10
minutes,
more preferably 2 to about 5 minutes before the pipe is advanced to a cooling
station
where the coated pipe is typically either quenched/cooled with water or
ambient air.
In another embodiment of the present invention, a protective polymer
coating is formed on a metal pipe. Typically a first epoxy resin powder is
applied to a
hot pipe to provide a corrosion-resistant coating, and thereafter a second
highly filled
powder coating composition is applied over the first/inner coating. Preferably
the
second resin powder coating composition is a mineral filled epoxy resin matrix
in
particulate form. The resin matrix comprises an epoxy resin, about 1 to about
10% by
weight graphite, and about 40 to about 70% by weight mineral filler. In a
preferred
embodiment, the second powder coating composition is applied in an amount
sufficient
to provide a fizlly cured protective polymer coating having an average total
thickness
of about 20 to about 100 mils without re-heating the coated pipe. In one
embodiment
the average total thickness of the protective polymer coating is about 25 to
about 80
mils.
In yet another embodiment of this invention there is provided an
improvement in a process where a protective polymer coating is applied to a
metal
pipe by heating the pipe, applying a first epoxy resin powder coating to the
surface of
the heated metal pipe, applying a second thermosetting resin over the first
coating
composition and re-heating the pipe to fully cure the resin coatings by
exposing the
pipe to infra-red radiation for a predetermined time sufficient to cure the
epoxy resin
coating. By selecting the second resin powder coating as a thermally
conductive
powder coating composition comprising a mineral filled resin matrix in
particulate form
that includes an epoxy resin, about 1 to about 10% by weight graphite, and
about 40%
to about 70% by weight mineral filler, the coated pipe can be exposed to infra-
red
radiation for a time less than the pre-determined time and still provide a
fizlly cured
protective epoxy coating on the pipe.
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Protective coatings on metal pipe in accordance with this invention are
evaluated for imperfections such as holidays, or discontinuities in the
protective
coating. Highly charged electrical probes trail the exterior surface of the
pipe. If the
underlying metal pipe is exposed through a holiday or imperfection in the
polymer
coating, the probe provides an electronic signal and indicates the location of
the
imperfection. If a holiday or discontinuity is detected, the imperfection in
the
protective coating is typically repaired using a 2-component epoxy patch/glue
or
application of the powder coating components with localized heating.
The following non-limiting examples are provided to illustrate thermally
conductive powder coating compositions of the present invention. The
ingredients
listed in Examples 1 and 2 are compounded in a mixer to uniformly disperse the
solid
components of the powder composition. The dry mixed components are fed into an
extruder to provide a melt-blended matrix that is forced through a die and
then cooled
on a chilled roller and cooling belt to provide a solid resin matrix
composition in the
form of a continuous sheet. The solid sheet-shaped resin matrix is chopped
into
granules and then ground to provide the powder coating composition.
ExamJ~le 1
Weight Percent
Epoxy resin 34.01
Phenolic hardener 6.43
Amine catalyst 0.16
Flow control agent 1.00
Mica 5.12
Barium sulfate 51.21
Graphite 2.05
Total 100
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Example 2
Weieht Percent
Epoxy resin 45.02
Phenolic hardener 6.43
Dicyandiamide 0.53
Catalyst 0.25
Flow control agent 1.00
Quartz silica 42.50
Mica 4.50
Gra~ 2.00
hite
,
Total 100
Example 3
Weisht Percent
Epoxy resin 3 8.60
Dicyandiamide 0.90
Amine catalyst 0.15
Flow control agent 1.00
Wollastonite 57.3 5
Graphite 2.00
Total 100
The protective polymer coating prepared using the formulation listed in
Example 3 was evaluated for its impact and abrasion resistance. The coating
exhibited
excellent resistance to impingement from a concrete mixture containing either
iron ore
or crushed stone. Ring samples were cut from the coated pipe and the ring
samples
were evaluated for low temperature elongation, cathodic disbonding, hot water
adhesion. The results of the tests are tabulated in Table 1.
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TABLE 1
TEST PROCEDURE TOTAL THICKNESS RESULTS
(mils)
Elongation 4-point, -30C' 68-75 1.9/PD
Elongation 4-point, 0 C' 66-76 1.7/PD
Elongation 4-point, RT' 68-77 2.5 /PD
Elongation 4-point, RT' 36-38 4.9/PD
Impact ASTM G14 68-75 160 in.-Ibs
Adhesion CAN/CSA-2245.2068-75 1 Rating
CDT ASTM G95 68-75 0 mm disbondment
Hardness ASTM D 25832 nr 40
Gouge TISI Gouge Test335 24 kg
Gouge TISI Gouge Test350 26 kg
1. Four Point Bend Test at the specified temperature 2. Test performed using a
Barcol Impressor Model # GYZJ934-1 produced by the Barber Colman Co. and sold
by Paul Gardner Associates 3. TISI Gouge Test
Four Point Bend Test: The degree of bending that the coated metal pipe having
a pipe
diameter "PD" can withstand without the protective polymer cracking or
disbonding
from the pipe is measured on a Four Point Bender. A strap cut from a coated
metal
pipe having a thickness "t" including the polymer coating is supported on top
of two
parallel pins that are spaced a distance "d" from each other. Two additional
pins are
placed on top of the metal strap at a distance greater than "d" from each
other so that
the two pins under the strap are centered between fhe two pins on top of the
metal
strap when viewed vertically. The two pins on top of the strap are rigidly
held in
place. The two pins under the metal strap are mounted on a movable block that
moves
in a vertical direction and forces the strap to bend. After the strap is bent,
it is
released from the apparatus and the angle "A" that the strap remains bent from
linear is
measured. The angle °/PD that a pipe having a pipe diameter of "PD" can
be bent
without the protective polymer cracking or disbonding is determined by the
equation:
_At
°/PD - d
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Technical Inspection Services. Inc. Gouge Test: The abrasion resistance of the
protective polymer coating prepared using the formula listed in Example 2 was
evaluate by moving a weighted gouge along the longitudinal direction of the
coated
pipe at a rate of 10 inches per minute. The amount of force, determined by the
weight
on the gouge, required to expose the bare metal pipe as discerned using a
holiday
detector is listed in Table 1.
