Multilayer Polypropylene Systems for Ultra High Operating Temperature
G P Guidetti, J A Kehr, V Welch
SYNOPSIS
As well head and operating temperature of oil and gas pipelines increase, so the boundaries of
technology are pushed to new limits.
Multi layer polypropylene systems are now readily accepted for high operating temperatures
(up to 120ºC). But even now higher temperatures of operation require practical, cost effective
solutions.
Modifications of Fusion Bonded Epoxy (FBE) and polypropylene technology indicate a
service limit of 150ºC is possible and commercially available for use by applicators without
modifications to existing 3 layer coating plants.
This paper will discuss the testing and development of this ultra high temperature system, it’s
limitations and advantages over existing solutions. It will also review the criteria for the
optimisation of the individual components of the multi-layer solution.
INTRODUCTION
As the demand for higher operating temperature pipelines increases so new solutions have to be
found and existing solutions extended to their practical limits. Existing world class coating such
as Fusion Bonded Epoxy (FBE), 3 layer polyethylene (PE) and 3 layer polypropylene (PP), all
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have current temperature limitations well defined in proposed European 1,2 and International
Standards3.
Existing 3 layer PP systems are proven up to 110ºC and occasionally used up to 120ºC 4. But
today solutions are often requested for operating temperatures of 200ºC and much research is
being carried out to reach these new limits. Some of these proposed solutions are beyond the
scope of this paper but modifications of proven PP technology can stretch the limits of organic
coating to around 150ºC.
This stretch requires modification of each individual component of the 3 layer PP system; the
FBE primer, the adhesive and the top coat. The selection criteria and rationale for each element
will be reviewed starting with the FBE.
CHARACTERISATION OF EPOXY
Two epoxy systems were evaluated (A and B) Table 1 with a commercially available resin as a
reference.
Table 1
A
B
(Reference)
Gel time /s (204ºC)
11.6
12
11.0
Cure time/s (204ºC)
150
154
90
Tg DSC
136
118
110
Tg DMA
166
141
117
Bend @ 23ºC
1.51º/ PD
2º/ PD
6º/ PD
The critical properties for long-term performance are considered to be the glass transition
temperature of the FBE. The limiting parameter for handling is flexibility. In general terms as
you increase the glass transition temperature (Tg) you decrease the flexibility.
The glass transition temperature (Tg) was measured by two methods; Differential Scanning
Calorimetry (DSC)5 and Dynamic Mechanical (Thermal) Analysis (DMA)6.
DSC is the traditional method of determining Tg but at higher temperatures of Tg the
inflexion from baseline is harder to accurately assess (Fig 1, Fig 2). DMA on the other hand
looks at a second order thermodynamic transition; the changes in modulus and damping (tan )
of the sample. Figures 3 and 4 show marked changes in modulus which are far easier to measure
than the inflexion mid point in DSC.
We believe this gives a more realistic value of Tg than DSC with excellent characterisation
but is a technique which does not lend itself readily to quality control, as DSC does.
Resin A has a higher cross link density and so has a higher resultant Tg but with reduced
flexibility.
CHARACTERISATION OF ADHESIVE AND TOPCOAT
The adhesive is a grafted co-polymer PP offering excellent adhesion to polar materials (ie epoxy,
steel, etc) Table 2.
2
Typical Properties
Physical properties
Melt flow rate (230ºC, 2.16 kg)
Specific gravity
Melting point
Mechanical properties
Flexural modulus
Tensile strength
Elongation, ultimate
Thermal properties
Vicat softening point (9.8 N)
Typical Properties
Physical properties
Melt flow rate (230ºC, 2.16 kg)
Specific gravity
Hardness, Rockwell
Melting point
Mechanical properties
Flexural modulus
Tensile strength
Elongation, ultimate
Notched Izod Impact strength at 23ºC
at -20ºC
Thermal properties
Vicat softening point (9.8 N)
Special characteristics
Fungi resistance
Bacteria resistance
Table 2
ASTM Method
Unit
Value
D 1238 L
D 792
D 3418
g/10 min
ºC
3
0.9
160
D 790
D 638
D 638
MPa
MPa
%
900
20
>400
D 1525
ºC
135
Table 3
ASTM Method
Unit
Value
D 1238 L
D 792
D 785
D 3418
g/10 min
R scale
C
0.8
0.9
79
160
D 790
D 638
D 638
D 256
D 256
MPa
MPa
%
J/m
J/m
1,000
23
>400
500
50
D 1525
ºC
145
G 21
G 22
no growth
no growth
The top coat is a UV stabilised PP co-polymer with good resistance to impact, indentation, stress
cracking and other environmental factors and low water absorption. Table 3.
SAMPLE PREPARATION
Metal coupons were sand blasted to Sa 2.5, Rz = 50µm. The plates were heated to 200ºC and
coated with 150µm of epoxy A or B. 300µm of adhesive was then extruded on top of the epoxy
with a melt temperature of 230ºC. 700µm of top coat was then extruded and applied by manual
pressure roller then allowed to air cool.
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SYSTEM CHARACTERISATION
The principle properties used in production to evaluate the performance of a 3 layer coating
system are its resistance to peel, peel test, and its resistance to Cathodic Disbondment (CD), CD
test, which are both well established test methods7. Table 4.
Table 4
Steel Temp
Epoxy
(ºC)
A
200
A
230
B
200
(*)
No peeling
(o)
Peeling and necking
CD @ 65ºC
2 days (mm)
1.8
0.5
Peeling @
23ºC
(N/mm)
> 40 (*)
20-30
> 40 (*)
CD @ 65ºC
7 days (mm)
2.1
2.1
Peeling @
150ºC
(N/mm)
> 10 (o)
> 10 (o)
> 10 (o)
SELECTION CRITERIA
Commercial epoxies with good peel adhesion at 150ºC are available but their long term
environmental stability while operating well above ( 40ºC) their Tg is unproven. Questions
arise regarding its creep resistance, resistance to water vapour penetration, water softening and
epoxy delamination. All of which can affect long term performance.
Under this uncertainty the general rule of operating below the Tg of the epoxy is a sensible
one which is more or less obeyed.
INDUSTRIAL APPLICATION
A specification with an operating temperature of 130ºC was required for pipe for the Stolt
Comex/Elf OOMBO operating in 102m deep water.
Looking at the production pipe for the Stolt Comex project production records show that
168.3mm, X42 pipe with wall thickness of 9.9mm and 12.7mm was coated with an average of
130µm of resin A, 267µm of adhesive and in excess of 3mm of top coat.
Product Peel and CD data are seen in Table 5.
Table 5
Average/mm
3.3
CD Test 28 days 3%
Min N/mm
13.6
Peel test 100ºC
4
Max N/mm
18
Bend test evaluation showed that the FBE coating had at least 1% flexibility at 0ºC*.
º/ PD = 2 x % strain x 57.3
= 2 x 0.01 x 57.3
= 1.146 º/ PD
and 1.719º/ PD at 23ºC
This compares favourably with the experimental values in Table 1.
*[and 1.5% at 23ºC]
CONCLUSION
The optimisation of the epoxy is concerned with the limit of flexibility as the cross linking
density and Tg increases. The selection of the PP is based on maintaining environmental stability
at elevated operating temperature.
Thicker coatings are used to reduce the surface temperature which reduces oxygen and water
vapour permeation. All of these factors increase the practical life of the coating.
By modification to existing technology organic coatings can be pushed to new operational
limits. Provided the penalty of reduced flexibility can be accepted then an operating temperature
of 150ºC can be achieved.
By careful use of materials designed for specific applications the integrity of the pipeline
asset can continue to be managed effectively as ever increasing demands are made on all
concerned.
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ACKNOWLEDGEMENTS
The Authors would like to thank Mr B Cavalier, Isotub Coating for his assistance in this paper.
REFERENCES
1.
Draft European Standard 029016; Steel tubes and fittings for on and offshore pipelines External three layer extruded polyethylene based coatings.
2.
Draft European Standard 029063; Steel tubes and fittings for on and offshore pipelines External three layer extruded polyethylene based coatings.
3.
Draft International Standard ISO/DP10800; Steel pipes and fittings used for buried or
submerged pipelines - External epoxy powder coatings.
4.
Guidetti, G P, Locatelli, R, Marzola, R and Rigosi, G L (1987) Heat resistant
polypropylene coating for pipelines in “Proceedings of the 7th International Conference on
the Internal and External Protection of Pipes” (ed R Galka), The Fluid Engineering
Centre, Cranfield, pp 203-10.
5.
ASTM D3418-88; Standard Test Method for Transition Temperatures of Polymers by
Thermal Analysis.
6.
ASTM D5026-93; Standard Test Method for Measuring the Dynamic Mechanical
properties of plastics in Tension.
7.
NFA 49-711; Steel tubes Three layer External Coating Based on Polypropylene.
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