Method of producing a thermoplastic polymer-lined pipe

Process for lining a pipe with thermoplastic polymer using non-crosslinked, expandable thermoplastic liner which is uniformly heated after insertion into the pipe.

BACKGROUND OF THE INVENTION 
This invention relates to a method for lining tubular structures such as 
pipe, particularly, but not exclusively, long lengths of pipe in situ such 
as underground gas or water and sewer mains. 
Metal pipes may be lined with thermoplastic polymers to convey corrosive 
material such as acids, or the lining may be used simply to rehabilitate 
or prolong the life of old, existing water or gas supply pipes or other 
pipes which may carry relatively non-corrosive fluids. 
It has been proposed in U.S. Pat. No. 3,429,954 to line metal pipe by 
inserting into a pipe a longitudinally oriented heat shrinkable, 
irradiated thermoplastic polymer tubing, the tubing having in its 
unoriented state an outer diameter greater than the internal diameter of 
the metal pipe but slightly smaller than the internal diameter when 
oriented, and heating the tubing to a temperature above the crystalline 
melting point of the polymer to longitudinally shrink the polymer tubing 
and cause it to expand radially and fit tightly against the internal wall 
of the pipe. 
The polymer tubings employed in U.S. Pat. No. 3,429,954 were irradiated to 
cross-link the polymer. This technique may be satisfactory for small 
diameter pipe, which is normally obtained and installed from a large coil. 
However, polymeric pipe having a diameter of greater than 3 or 4 inches is 
generally available only in straight lengths of several feet each, which 
must be joined by butt fusion or butt solvent welding techniques. 
Cross-linked polymers cannot with confidence be joined by fusion or 
solvent welding and are inferior in this respect to non-crosslinked 
thermoplastic polymers. 
In the U.S. patent it is also suggested that the same polymer liners be 
expanded by heating using one of three methods. 
(a) application of hot air internally 
(b) passing the polymer lined pipe through a furnace so as to apply heat 
externally 
(c) immersing the polymer lined pipe in a propylene glycol bath. 
While methods (b) and (c) for obvious reasons are impractical for the 
relining of long, continuous lengths of underground pipe, method (a) is 
not effective either because in long pipe sections the air or any other 
gas or fluid will cool rapidly as it progresses along the pipe and there 
will be insufficient heat to raise the entire length of the polymer liners 
to the necessary temperatures. 
A further problem in the lining of long lengths of metal pipe is that to 
achieve the proper expansion of the polymer liners inside the metal pipe, 
the liner must be completely free to shrink in the longitudinal direction. 
If the shrinkage is prevented or only slightly hindered in any way, it 
will result in no or only a reduced and insufficient expansion, as well as 
undesirable axial tensile stresses in the pipe that may be harmful to the 
fused or solvent welded joint. 
In particular, where long, continuous lengths of pipe are to be lined, 
problems arise where the weight and associated friction drag of the long 
lengths of the lining itself, inside the metal pipe tends to prevent the 
required shrinkage. This situation may be further aggravated where the 
metal pipe is not completely straight and the liner becomes pinched or 
where the polymer liner during the expansion process itself expands 
unevenly or at a different rate either at various points along the length 
of the liner where it may tend to hang up, or around the periphery of the 
liner where this may cause the liner to curve in the length direction 
inside the metal pipe and thereby become pinched. 
It is the object of this invention to provide a method particularly for 
lining long lengths of underground pipe that will overcome all of the 
above mentioned difficulties and problems. 
SUMMARY OF THE INVENTION 
The present invention provides a process for lining a pipe with a 
thermoplastic polymer which comprises: 
(a) selecting a substantially non-crosslinked, expandable, thermoplastic 
tubular polymer liner having an outside diameter less than the internal 
diameter of the pipe and which, when fully expanded, would have an outside 
diameter greater than the internal diameter of the pipe; 
(b) inserting the tubular polymer liner into the pipe; and 
(c) heating the polymer liner sequentially from one point toward at least 
one free end of the pipe and uniformly about its circumference to a 
temperature sufficient to cause the liner to expand while applying 
positive axial pressure on the end of the liner toward which the liner is 
being heated.

DETAILED DESCRIPTION OF THE INVENTION 
Thermoplastic polymers which can be used in the practice of this invention 
include any non-crosslinked, thermoplastic homopolymers and copolymers 
which are capable of maintaining a deformation brought about from being 
heated, stretched and cooled, and which return to their approximate 
original dimension up reheating. 
Specific examples include homopolymers such as polyethylene, polypropylene, 
polybutene; copolymers of ethylene and vinyl esters of aliphatic 
carboxylic acids such as vinyl acetate and vinyl propionate; copolymers of 
ethylene and alkyl acrylate and methyl methacrylate; copolymers of 
ethylene and other alpha-olefins such as propylene, butene, hexene and 
octene; copolymers of ethylene and ethylenically unsaturated carboxylic 
acids such as acrylic acid, methacrylic acid and the like. Still other 
classes of thermoplastic materials which can be used include polyacetals, 
polyvinyl chloride and chlorinated polyvinyl chloride, polyamides and 
acrylics such as polymethyl methacrylate. 
Preferred thermoplastic materials which can be used in the invention 
include polyethylene such as that manufactured by Du Pont under the Trade 
Mark ALATHON, ethylene-octene and ethylene-hexene copolymers. 
In a preferred method of preparing the liners used in the present 
invention, the polymer liner is first heated to a temperature within the 
normal annealing temperature range of the polymer, and preferably to a 
temperature which is about from 10 to 30 Centigrade degrees below the 
melting point of the polymer. The liner is then stretched to reduce the 
outer diameter sufficiently so that upon cooling to room temperature and 
removal of the tension, the dimensionally stable outside diameter is less 
than the internal diameter of the pipe to be lined. 
A polyethylene liner capable of expanding about from 8 to 15% of its 
diameter, for example, can be made from a tubular polyethylene billet with 
an outside diameter slightly larger than that of the internal diameter of 
the pipe by: 
(a) heating the billet to a temperature in the range 105.degree. to 
120.degree. C. or about 10.degree. to 20.degree. C. below its crystalline 
melting point 
(b) stretching the tubular billet at this temperature in the range 
1.3.times. to 1.7.times. its original length 
(c) cooling the stretched tube under tension to room temperature 
(d) relieving the tension and permitting the tube to relax for a period of 
time (usually 2 to 3 days) until the dimension remains stable. 
Preferably the outside diameter of the unexpanded liner is 5 to 12% smaller 
than the nominal inside diameter of the tubular structure so that one 
liner size may accommodate a narrow range of different pipe inside 
diameters. 
It is normally necessary to join a series of pieces of liner together to 
produce a sufficient continuous length. 
In the process of butt welding it is normal to clamp two pieces of pipe 
adjacent to each other, face off their adjacent end surfaces, apply heat 
to these end surfaces to affect some melting and then bring the two face 
surfaces or ends of the pipes together so as to butt weld the two portions 
together. If this is done with pipe which has been stretched such as the 
liners used in the present invention, the heat will cause circumferential 
expansion of the pipe or liner at the joint which will create an 
undesirable bulge at the joint. To avoid this, during the butt welding the 
two ends of liner or pipe must be clamped as near as possible to the 
junction and the diameters at this point must be slightly compressed or 
reduced to counteract the tendency of the pipe to expand as it is heated. 
By maintaining the clamp compression close to and on each side of the 
joint during the welding and until the joint has cooled to room 
temperature, it is possible to make a butt fusion without any diameter 
distortions or bulges. 
A preferred method of joining successive sections of the heat expandable 
liner by butt heat fusion involves 
(a) circumferentially compressing the end of each stretched liner section 
to be joined about from 1 to 5% by clamping each such end with a 
circumferential clamp; 
(b) positioning the clamps or stops on the equipment to permit the faced 
off ends of each liner section to protrude from the edge of each clamp 
about from 0.5 to 1.5 times the thickness of the liner; 
(c) heating the ends of the liner sections to be joined to the fusion 
temperature of the thermoplastic from which the liner is made; and 
(d) bringing the ends of the heated liner section into contact under 
pressure and maintaining the contact until the temperature of the ends 
falls to substantially ambient temperature. 
Typically the interfacial pressure applied during the fusion of the ends of 
the liner sections is about from 40 to 60 psi. 
After joining successive sections, the beads on the inside and outside of 
the pipe are typically cleaned by conventional means. Internal cleaning is 
described, for example, in Brandt, U.S. Pat. No. 3,805,311, hereby 
incorporated by reference. 
Thus by successively joining sections it is possible to insert into the 
metal pipe a total length of heat expandable liner which should typically 
be about from 10 to 35% longer than the metal pipe to allow for 
longitudinal shrinkage which will vary depending on the required 
expansion. 
To expand the liner it is preferably heated from the inside of the liner 
with a radiant infra-red heater. The polymer liner should be heated 
uniformly about its circumference and sequentially from one point toward 
at least one free end of the pipe. Accordingly, the radiant heater may be 
inserted into the liner, either to an intermediate point or at the end of 
the pipe, and moved through the pipe towards an open end from said 
intermediate point or from said one end at a rate such as to relieve the 
internal strain applied by the stretching and to allow for full expansion 
of the polymer liner into contact with the internal surface of the pipe. 
The method of moving the heating zone from an intermediate point or from 
one end is to ensure that the longitudinal shrinkage which is associated 
with radial expansion can take place unhindered. 
To obtain the best expansion and fit with the internal diameter of the pipe 
to be lined and to protect the fused or welded joints, an axial 
compressive force is preferably applied at the free end of the polymer 
pipe toward which the heater is moving so as to overcome frictional drag 
forces from the weight of the not yet expanded section of the liner, to 
overcome the resistance forces from pinch points along the pipe and in 
this way assist in the movement of the unexpanded liner section to 
accomplish the required axial shrinkage. Preferably this axial force 
should produce some axial compression in the zone of the expansion during 
the entire operation. Preferably, the axial pressure applied is equal to 
about from 100 to 400 psi of the cross-sectional area of the pipe wall. 
The radiant heater may, for example, be in the form of a rod or bar 
supported at one end on a carriage which is shaped to conform to the shape 
of the liner and is slid along the liner. A preferred heater which can be 
used in the present invention, having anterior and posterior ends, 
comprises: 
(a) a platform; 
(b) at least one radiant heating element mounted on the platform; 
(c) a spiral air tube mounted on the platform to encircle the heating 
element at least once in a direction axial to the liner, the air tube 
having a plurality of apertures formed therein positioned to create a 
circumferential air flow within the pipe; 
(d) electrical and pressurized air sources functionally connected to the 
radiant heating element and air tube, respectively; 
(e) means for moving the platform through the pipe; and 
(f) a diametric gasket positioned anterior to the heating element to 
separate the apparatus from the portion of the pipe anterior to the 
heating element. 
In the installation of pipe liner according to the present invention, the 
liner may not conform in minute detail to the inside of the pipe which may 
be rough, uneven or pitted. Further, if good conformation of the pipe 
liner and the pipe wall is initially obtained, temperature variations in 
the pipeline can cause a gap between the liner and the pipe wall. In the 
event that a leak in the system should develop, detection and repair of 
the leak would be difficult. 
Accordingly, in another preferred embodiment of the invention, the pipe 
liner has a coating of about from 10 to 100 mils on the outer surface 
thereof of a thermoplastic elastomer hot melt adhesive, the thermoplastic 
elastomer having a flexural modulus of about from 500 to 10,000 psi and a 
melting point of about from 40.degree. to 120.degree. C. Preferably, the 
polymer has a melting point of about from 50.degree. to 100.degree. C., 
and melting points of about from 65.degree. to 85.degree. C. are 
especially preferred. 
As the heater is moved through the liner, the outside surface of the liner 
reaches temperatures that will melt or soften the thermoplastic 
elastomeric melt adhesive. As the liner simultaneously expands and presses 
against the internal surface of the pipe, a good bond between the pipe and 
the liner is assured. 
Thermoplastic elastomers which can be used in a coating for the pipe liner 
include, for example, ethylene vinyl esters, ethylene acrylate copolymers, 
ethylene olefinic copolymers such as ethylene propylene rubbers, styrene 
isoprene, styrene butadiene or styrene ethylene propylene block 
copolymers. These materials can be functionalized with a comonomer which 
gives good adhesion to metals, such as maleic anhydride, the half-ester of 
maleic anhydride, acids such as methacrylic acid, acrylic acid or itaconic 
acid or epoxy functionalized or saturated monomers such as glycidyl 
methacrylate or acrylate. By controlling the comonomer content and 
selecting the proper comonomer, these materials can adhere well to the 
polymer of the pipe liner by direct copolymerization or by a grafting 
reaction. 
The coating of thermoplastic elastomers can be applied to the surface of 
the pipe line either continuously or intermittently. The coating can be 
applied by conventional coating means or in the form of a tape that is 
wrapped around the pipe and subsequently heat fused to provide an integral 
coating. 
The invention may be more fully understood by reference to the Figures. 
In FIG. 1 is shown an iron gas main 10 which is located underground and may 
be several feet below ground level 11. 
The gas main is to be lined with a polyethylene pipe liner 12. 
The length of prestressed polymeric pipe liner 12 is chosen to be of larger 
length than the pipe 10. The outer diameter of the polymeric liner is not 
more than 10% less than the internal diameter of the pipe 10. 
One end 13 of the iron pipe is exposed by excavating a cavity in the 
ground, the cavity being sufficiently large to feed through a long length 
of polyethylene pipe liner which may, for example, be about 10 to 12 
meters in length. 
The polymeric pipe liner is then inserted into the pipe 10 leaving a 
portion of the length of the polymeric pipe liner exposed so as to allow 
for axial shrinkage of the liner during the subsequent radial expansion 
within the pipe. 
A radiant heater 14 which is preferably in the form of an elongated tube is 
then inserted into the liner on a carriage 15. The carriage 15 is shaped 
to conform to the shape of the liner so that it will slide freely along 
the liner and has a post 16 at one end of which the heater 14 is 
cantilevered. 
At the other end is a second post 17 to which is attached a rope or cable 
18 which may incorporate the wires carrying the power supply to the heater 
14. Alternatively separate wires may be provided to the heater 14. 
The rope or cable 18 passes around the pulley 19 and to a motor 20 which 
can provide a steady gentle pull on the rope 18 so as to pull the carriage 
or sledge 15 at a predetermined steady rate through the pipe. The motor 20 
is preferably variable in speed so as to adjust the speed at which the 
carriage 15 moves through the pipe. 
It has been found in practice that it is desirable to position the radiant 
heater 14 so that it is below the mid-level of the liner to ensure that 
the portion of the liner at the lowermost part of the pipe receives 
sufficient heat. 
Since the liner is most likely resting on the bottom of the metal pipe this 
is where the more important heat sink is. For this reason this area should 
receive the highest heat intensity from the infrared heater. Since the top 
of the liner is not initially in contact with the steel pipe--and 
therefore, has no heat sink--and at the same time is favored as far as 
convective heat transfer is concerned, this area should receive the lowest 
heat intensity from the radiant heater. This is achievable with the type 
of improved heating arrangement shown in FIG. 4. 
A central support tube 25 carries the electric wiring supplying four 
radiant rod heaters 27, 29, 31, 33 of the infrared type, each being 
provided with a reflector 26, 28, 30, 32 respectively. The reflector 32 
and heater rod 33 are located much further from the center line of the 
pipe than the other heaters are. 
The infra-red heaters preferably operate in the range 2 to 3 u., i.e., in 
the medium range of infrared spectrum. Preferably, the stretched liner is 
heated in the presence of an inert gas. 
An incentive for using infrared heating is that polyethylene over a large 
range of wave lengths is somewhat transparent to infra-red radiation. This 
means that a significant proportion of the infra-red energy penetrates 
into the polyethylene liner wall directly causing it to heat up faster 
than if it was heated up by other means which depend solely on conduction 
through the pipe wall. 
The heater 14 is then pulled gently through the pipe and liner so as to 
cause the liner to expand and attempt to return to its original diameter 
thus making an effective tight seal against the inner circumference of the 
pipe 10. 
Because of drag and weight of the liner when such long lengths of liner are 
employed, the expansion may not occur uniformly and fully. To overcome 
this problem an axial thrust or force is preferably applied to the liner 
12, for example, by means of a block or bar 21 connected by rods 22 to 
jack devices 23 which may be set to provide a steady pull on the block 21 
so as to overcome any drag or inertia of the liner as it proceeds into the 
pipe. 
The jacks may be automatic devices which provide a constant pull or they 
may be operated manually and the force applied may be adjusted to suit 
particular lengths of liner and weights of liner and drag situation. 
In FIG. 3 an alternative method is shown of applying the axial force to the 
liner. A clamp 24 is used which again may be attached via rods 22a to 
suitable jacks similar to 23. By attaching the clamp 24 at a point fairly 
close to the entrance 13 of the pipe 10 a better control can be achieved 
on the axial force applied without distorting the liner. 
A preferred heater for use in the present invention is shown in FIG. 5. In 
that Figure, platform 50 has two radiant heating elements 51 mounted 
ontthe platform through support means 52. Spiral air tube 53 is similarly 
mounted on the platform to encircle the heating element. Apertures 54 are 
formed in the air tubes in such positions as to create a circumferential 
air flow within the pipe liner. Blower 55 provides a pressurized air 
source connected to the air tube. The blower is powered by a motor 56. The 
motor and radiant heating elements are respectively connected to 
electrical sources 57 and 58. Diametric gasket 59 is positioned anterior 
to the heating element to separate the apparatus from the portion of the 
pipe preceding the heater. Optional posterior diametric gasket 60 
similarly isolates the portion of the pipe behind the heating apparatus. 
The heating apparatus can be either drawn or pushed through the pipe liner 
by attaching means 61. 
It has been found to be particularly convenient to mount the air tube and 
the heating element on a common suspension, as shown in the Figure. While 
a single radiant heater can be used, particularly with smaller sized pipe, 
it is preferred to position two or more heaters, as shown, substantially 
equidistant from the center of the pipe. 
The present invention provides a method for lining pipe that not only 
effectively seals decayed pipe, but can be efficiently operated by a 
relatively small work force. Example 
An iron gas pipe 10 which had lain in the ground for 50 years was to be 
lined with a polythene liner so as to extend the life of the original 
pipe. It was important to retain the volume flow through the pipe and so 
the liner was selected to cause the minimum reduction cross-sectional area 
of the original pipe and had to be brought into firm contact with internal 
surface of the pipe 10. 
The pipe 10 was a 20 ft (6 m) 8 inch nominal diameter cast iron main with 
an average inside diameter of 8.4 inches (21.4 cm). 
A polyethylene liner was selected with an outside diameter of 8.95 inches 
(22.7 cm) and a wall thickness of 0.29 inches (0.75 cm) produced from an 
ethylene octene-1 copolymer having a melt index of 1 and a density of 
0.937 g/cc containing antioxidants and pigment. 
The polyethylene liner was heated to a temperature of 113.degree. C. over a 
period of 1 hour and subsequently stretched 1.5.times.. After the liner 
had cooled to room temperature in this state the tension was released and 
the liner was permitted to relax. After three days, no further dimensional 
changes were noticable and the outside diameter of the liner remained 7.65 
inches (19.4 cm). 
A 25 ft (7.72 cm) long section of this liner was inserted into the 20 ft (6 
m) long cast iron pipe in such a way that the liner and pipe were flush at 
one end with the longer length liner extending 5 ft out of the other pipe 
end 13. 
A heater, of the design shown in FIG. 5, using four 1 ft long, 3/8 inch 
diameter 1 kw cartridge heaters was slid into the liner so that the heater 
elements extended just outside the far end, with the carriage section 
supported inside the liner. 
After the current was turned on and the heater cartridges had reached their 
maximum temperature, the motor was started and the heater carriage 
progressivly moved along the pipe from right to left as shown in FIG. 1. 
As soon as the end of the heater was about 6 inches inside the liner, the 
end of the liner was plugged and the air source was turned on. At the same 
time, axial pressure of 100 pounds per square inch of liner wall thickness 
was applied by means of the mechanism shown in FIG. 1. By moving the 
heater carriage at a rate of 4 to 5 inches/minute along the liner, the 
liner expanded to provide a good fit with the internal diameter of the 
cast iron pipe. The axial pressure ensured that full radial expansion took 
place. In the course of the radial expansion and axial contraction the 
portion of the liner extending outside of the pipe contracted into the 
pipe.