Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 This invention relates to method of overwrapping
thermoplastic articles with a synthetlc resin. ~ore
particulary, thls invention relates to a method oi
overwrapping thermoplastic articles ~or use in chemical
plants with a reinforced thermosetting resin.
In chemical plants, hlghly corrosive substances are
often conveyed in pipes, tubes or the like. These
substances vary from highly acidic to highly alkaline
10 and are understandably damaging to the pipes they are
conveyed in.
Use is widely made today of steel pipes, lined on the
inside with an inert plastic liner to convey corrosive
15 substances in chemical plants. This type of
arrangement suffers from various disadvantages.
For example, the costs of the steel pipes and the
seperately manufactured and inserted plastic liners are
often prohibitively high. Another disadvantage is the
20 fact that with this type of arrangement, only the
inside of the pipe is made corrosion resistant, and not
the outside. The corrosive environment of a chemical
plant in some areas therefore often causes the steel
pipe to corrode rapidly on the outside. A further
25 problem associated with the above arrangement lies in
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1 the dl~erence in coeff~cient o~ e~panslon between the
steel plpe and lts plastlc liner.
To use a thermoplastic pipe on lts own for the
aforesaid purposes also has its disadvantages. The
biggest problem wlth using a thermoplastic plpe such as
a polypropylene pipe lies in its relatively high
coefficient of expansion. The ooefficient of expansion
of a polypropylene pipe for e~ample, is about ten times
10 higher than that of a steel pipe and would have to be
stiffened in some way to prevent significant e~pænsion
and consequent sagging of the pipe when e.g. conveying
a heated substance. Another obvious problem with a
thermoplastic pipe is its relatively low strength and
15 pressure bearing capabilities compared to steel.
Various methods have been employed in the past to
overwrap a thermoplastic pipe with a layer of
reinforced themosetting resin in order to overcome the
20 above problems. These methods, however, suffer inter
alia, from the disadvantage that the thermosetting
layer does not bond well to the outer surface of the
pipe.
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1 It is accordingly an ob~ect of the present invention to
provide a method of overwrapping a thermoplastlc
article wlth a synthetic rasin which applicant believes
will overcome or at least minimize the problems and
disadvantages of the known arrangements.
According to one aspect of the invention, a method of
overwrapping a thermoplastic article with a
thermosetting resin comprises the steps of:-
- cooling the article to below ambient
temperature;
- applying a layer of thermosetting resin to
; 15 the outer surface of the article; and
- allowing the thermosetting resin to cure.
Preferably, a reinforced thermosetting resin is applied
20 to the outer surface of the article.
The article is preferably cooled to between 10C and
20C prior to applying the reinforced thermosetting
resin.
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1 Pre~erably also the method includes the ~urther step o~
strlating at least a portlon of the sur~ace of the
artic.le to be overwrapped prior to cooling the article
and applylne the reln~orced thermosetting resin.
~ith the above arrangement, the article contracts
slightly when cooled down. When the reinforced
thermosetting resln is therefore applled to the outer
surface of the article, the article is in a slightly
10 contracted state. An exothermic reaction occurs when
the thermosetting resin starts to cure and the heat
energy radiated as a result of the e~othermic reaction
is absorbed by the article. This causes the article
to expand slightly while the resin is curing. It will
15 be appreciated that with the above arrangement, the
expxnding outer surface of the article causes the
striations on the surface ko key permanently into the
resin, and when the resin has set, the outer surface
bears outwardly against the resin coating. It is
20 therei'ore not ordinarily possible -to remove the resin
overwrapping from the article, even if the article is
in a symetric shape, such as a length of straight
thermoplastic piping.
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1 The article may be striated by cuttlng at least ooe
groove into the outer surface thereof, and is
preferably striated by cutting a continuous hellcal
groove into the outer surface thereof by means of a
lathe and cutti~g tool arrangement or the like.
The article may be a length of thermoplastic pipe.
With this arrangement, a thermoplastic pipe such as a
10 polypropylene pipe may be overwrapped with a reinforced
thermosetting resin in order to render its corrosion
resistant, to curtail excessive linear expansion and
contraction of the pipe and to increase its strength
and pressure bearing capabilities.
The reinforced thermosetting resin may be applied to
the article in the form of a fibreglass filament
predipped in a thermosetting resin, the resin-carrying
filament in use being ~Irapped around the pipe.
The resi.n-carrying filament may initially be wound at
an angle of ninety degree~ to the longitudinal a~is of
the pipe, whereafter it may be wound to and fro at an
angle o fifty five degrees to the longitudinal a~is of
25 the pipe.
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1 The resin-carrying filament is preferably wrapped onto
the plpe until a layer of between 2mm and lOmm of
filament and resin is applied to the outer surface of
the pipe.
The thermosetting resin may be any one of a polyester-;
vinylester-; isothalic-, or bisphenolic resin.
The method may include the further step of cleaning the
10 outer surface of the article with a suitable solvent
prior to applying the thermosetting resin to the outer
surface thereof.
The article is preferably cooled down to below ambient
15 temperature by placing it in a bath of solid C02.
The thermosetting resin may be cured by placing the
overwrapped article under infra-red heater lamps.
20 According to another aspect of the invention, a
thermoplastic article, overwrapped with a thermosetting
resin as described is provided.
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1 One embodlment o~ thls Invention will now be descrlbed
in detall wlth reYerence to the accompanyin~ drawings
ln which:-
Figure 1 - is a partly sectioned perspective vlew
o~ a striated pipe to be overwrapped
according to the invention;
Figures 2 & 3 are perspective views of the pipe of
figure 1 at different stages of
overwrapping; and
Figure 4 - is a partly sectioned perspective view
of the pipe of figure 1 after it has
beeo fully overwrapped according to the
method of the invention.
Referring now in particular to figure 1, a length of
polypropylene pipe lO ~ith a wall thickness of about 3mm
20 is provided and striated by placing the pipe 10 in a
lathe (not shown) and cutting a continuous helical
groove 11 into the outer surface of pipe 10. The
groove 11 is cut O.7mm deep by 1,2mm wide on a spiral
pitch oi' 2,5mm. The corners of the groove are not
25 entirely sharp but slightly rounded to minimise stress
points or lines from which cracks may propagate.
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1 The pipe 10 ls then thoroughly cleaned with a suitable
organic solvent (not ~hown).
After cleaning, the pipe 10 is placed in a cold bath
(not shown) containing dry blocks of C02 until the
temperature o e -the pipe 10 is between 12C and 15C.
The cooling operation usually lasts between 20 and 30
seconds, depending on the wall thickness and length of
the pipe. The cooling causes the pipe to contract or
10 shrink slightly.
The ccoled pipe 10 is -then located on a two a~is
controlled filament winding machine (not shown).
15 The controlled eilament winding machine is well known
and standard equipment in the art and is equipped with
a supply of fibreglass eilament and a reservoir Oe
thermosetting resin. The principle Oe operation Oe the
machine is to dip the fibreglass filament in the resin
20 and then to wrap the resin~carrying filament onto the
surface of the article to be overwrapped. The machine
can be set to wrap the filament at different angles
relative to the article.
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l In figure 2, a resin-carrying fibreglass filament 12 is
shown to be initially wo~lnd by means of the winding
machine (not shown) onto the pipe lO at an angle o~ 90
degrees to the longitudinal axis of the pipe lO. The
Eilament 12 th~s pena-trates and fills the groove ll.
Once the groove ll is filled with the resln and the
filament l2, the winding machine is set at an angle of
fifty five degrees to the longitudinal a~is of the pipe
and the filament 12 is wound to and fro along the
lO length of the pipe lO until a layer of 5 - 6 mm thick
of fibreglass filamen-t and resin is applied to the
outer surface of the pipe lO (see figure 3).
Once the desired layer thickness is attained, which is
15 in practice dependant on the temperature and pressure
bearing requirements of the final product, the
overwrapped pipe lO is removed from the winding
machine.
20 During and after the winding opera-tion, the resin
starts to cure on the pipe lO. The setting of the
resin, which may be any one of a polyester-,
vinylester-, isothalic-, or bisphenolic-resin, causes
an exothermic reaction to take place~ thereby radiating
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1 heat energy which is absorbed by the plpe 10. This
heat energy causes the temperature o~ the pipe 10 to
increase with a resultant e~pansion oi the pipe 10.
The striated outer suriace of the pipe 10 therefore
e~pands and keys permanently into the rein~orced resin
overwrapping, and bears firmly against the overwrapp~ng
when it is ~ully cured.
After winding, the pipe 10 is pre~cured under infra-red
10 heater lamps until the resin coating is dry enough to
be handled. After the pre-cure, a post-curing period
follows wherein the overwrapped pipe is left at ambient
temperature until fully cured. The post-curing
operation may alternatively also take place under
15 infra-red heater lamps.
Figure 4 shows an overwrapped pipe 10 wherein the
groove 11 is fully keyed into the layer 13 of
fibreglass filament and resin.
Once the overwrapping operation as described above is
completed, the layer 13 cannot be removed from the pipe
10. The overwrapping serves to stiffen the pipe 10,
increases its overall strength and pressure bearing
25 capabilities a~d curtails its e~pansion and
contractlon tendencles in use.
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1 It will be appreciated that many modifications and/or
variations oE the invention May be possible without
departing Erom the scope of the appended claims. For
e~ample, instead o~ striating the outer surface of the
pipe with a helical groove, circular grooves, notches,
knurls or burrs, etc., may be applied. Also, T-joints,
elbow joints or other types of pipe fittings may be
overwrapped by means of this method.
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