THREE LAYER EPOXY/POLYETHYLENE SIDE EXTRUDED COATINGS FOR PIPE FOR HIGH TEMPERATURE APPLICATION (ASME 98)
Author
Mike Alexander
GARNEAU INC.
Abstract:
The author will focus on the properties of three layer epoxy/ polyethylene coating for pipe, based on the experience developed in the lab, coating plant and in the field.
The demands of the respective Canadian and other international standards will be looked at with pointing out the respective merits of various specifications.
Special attention will be paid to the properties of the coating involving pipelines operating at elevated temperature, especially running through permanently wet areas, such as permafrost.
Lab results will be correlated with the real life experience.
Three layer Epoxy/ Polyethylene coatings will be compared to other commonly used coatings in the industry with the object to assess respective benefits and projected longevity versus cost.
INTRODUCTION
Perfect pipe coating should be resistant to damage from elements, such as water, UV, resistant to corrosion attack, able to operate at elevated temperature, but at the same time resistant to low temperature and not susceptible to handling damage in storage, transportation or in the field.
Pipeline coatings belong to several broad groups, which can be generically called extruded polyethylene coatings and fusion bond epoxy coatings, if we ignore the presence of older coatings, such as coal tar, bitumen coatings and tapes.
Coating technology has changed a lot within last few decades. After the Second World War the coating of choice was coal tar, today coal tar is still being used mostly in underdeveloped countries. Nowadays most coatings are made from synthetic resins, applied in a coating plant under strict process conditions.
Two most popular coatings of choice are the Fusion Bond Epoxy, which is a thermosetting powder coating applied directly to the clean steel surface with excellent adhesion to steel. The biggest problem with this coating is that it is prone to mechanical and handling damage. The second coating is based on extruded polyolefins, typically polyethylene, which offers toughness and good damage resistance and have excellent moisture permeation resistance. The problem with this type of coating is that it has essentially no adhesion to steel. It needs, therefore, a primer layer, in most cases epoxy, and an adhesive tie layer to join epoxy with polyethylene.
Side extruded epoxy/polyethylene coating described in this paper is uses fusion bond epoxy as a primer layer, however there are other coating systems on the market, which use liquid epoxies as well. Further, the coating described in this paper is uses extruded polymeric adhesive tie layer, however some coatings on the market use a tie layer applied by spray. The outer polyethylene layer described in this paper is applied by side extrusion, however there are systems on the market, where the outer layer is applied in the powder form by spray.
This type of coating has excellent damage resistance, it is much easier to handle than FBE and the incidences of holidays, compared to FBE, are greatly reduced.
Three layer side extruded coatings were originally developed in 1980's in Europe. The first coatings of this kind did not use epoxy primer, they were designed usually as two layer systems and were susceptible to the phenomenon known as cathodic disbondment, causing runaway disbondment of the coating from the steel surface in the presence of holiday under cathodic protection conditions. This problem was solved with the addition of epoxy primer layer.
STANDARDS AND SPECIFICATIONS
The three layer coatings are described in several national standards. The oldest and still most widely used is German Standard DIN 30670. French standard NF A 49-710 is being used to a lesser extent. Canadian Standard CSA Z245.21 is gaining international acceptance over last few years, since it was first published in the early 1990’s.
There are very major differences between these national standards, not only in the properties, quality control, process, testing, but also in the underlying philosophy of the respective standards.
The biggest weakness of the DIN Standard is that it does not require the coating applicator to use the epoxy primer, it does not call for the cathodic disbondment testing and the specified peel adhesion value is very low.
The French NF Standard addresses many of the weaknesses of the DIN Standard and goes into much higher level of detail in specifying the material selection and performance criteria.
Canadian CSA Standard calls for the polyethylene layer two to three times thinner than the DIN Standard. The rationale behind it is in the fact that the Canadian Standard specifies High Density Polyethylene, which is much tougher to damage, than Low Density Polyethylene, used typically in Germany. Despite the lower coating thickness, impact and damage resistance of both coatings, thicker (3mm) DIN and thinner (1.5mm) CSA, is very similar.
There are many new three layer polyethylene standards being worked upon, just to mention the European EN standard, international ISO and American ASTM. All these new standards are at various stages of completion at the time of writing this paper.
Numerous consulting, engineering and petroleum companies issued their own specifications for the three layer polyethylene systems, which are usually a modification of major international standards.
Well specified and well produced three layer Polyethylene coating will have properties similar to those in Table 1.
Table 1 - Properties of well designed three layer polyethylene coating:
|
Property
|
Test result
|
|
Peel adhesion
|
High at wide temp range
|
|
Cathodic disbondment
|
Excellent over wide temp range
|
|
Water boil adhesion
|
High after prolonged water boil
|
|
Direct impact resistance
|
High at low and high temp
|
|
Flexibility
|
Very good at low temperature
|
|
Shear resistance
|
Almost no movement at high temp
|
COATING MATERIALS AND COATING PROCESS
The raw materials such as fusion bond epoxy powder, adhesive co-polymer, polyethylene, grit and shot constitute a major component of the cost of the final coating. They also have immediate effect on the quality of the final product; therefore a lot of work goes always into evaluation and selection of the best possible materials.
Here, in Canada, we are blessed with some of the best materials used in this kind of coating system. We can be proud that Canadian-made polymeric adhesive and Canadian made polyethylene are considered one of the best in the world and are being used in many countries around the globe, sometimes in far away places, such as Asia and South America.
Steel Pipe Surface
Steel pipe surface must be prepared to near-white metal condition, or better, by using steel shot and steel grit, or a mixture of shot and grit. The preferred anchor pattern depth should not exceed 60 micrometers, but sometimes it can be as deep as 100 micrometers, if it is compensated by higher FBE thickness. The dust contamination should not exceed 30%, when measured according to CSA Z245.20, so it is very important that the bag house system is working properly in the pipe cleaning process.
Chemical Surface Treatment
Some pipeline owners specify use of chromate or phosphoric acid or combination of both, phosphoric acid and chromate rinse, after the abrasive steel blasting. The benefits of using chemical surface treatment are well known in improving final properties of the product, but they can be categorized in two distinctive groups:
A physical benefit due to rinsing with liquid and therefore removing steel dust from the surface
A chemical benefit due to formation of crystalline network of phosphate or chromate on the surface, which activates the surface chemically and improves adhesion. This last benefit is especially visible in the wet testing, such as cathodic disbondment or hot water soak.
Fusion Bond Epoxy
Epoxy powders used in the three layer coatings can belong to two different groups: primer quality and coating quality. There are some important differences between these two materials, both are applied under different application conditions, at different temperatures, at different thickness and have different bending characteristics. Generally, the trend in the industry has been recently to move away from primer type powders to coating-type FBEs as a primary layer. There are several important reasons for doing that, but generally coating type FBE allows the end user to specify higher thickness of the first layer and, therefore, achieve better properties of the whole system. FBE selection and application, including pipe preheat temperature, is arguably the most important part of the successful three layer polyethylene final product. Table 2 lists some of the most important properties of two widely used FBE powders.
TABLE 2
|
FBE POWDER TESTS
(source: suppliers’ data sheet and tests at Garneau Lab) |
||
|
|
D1003LD
|
EP971197
|
|
Tg1
|
62.52
|
59.65
|
|
Tg2
|
101.99
|
101.76
|
|
Delta II
|
54.714
|
65.405
|
|
Gel Time (sec)
|
|
|
|
180°C
|
25
|
30
|
|
200°C
|
17
|
19
|
|
Impact (in lbs.) At -23°C
|
160
|
160
|
|
At Ambient
|
160
|
160
|
|
Adhesion Hot Water, 48 hrs at 88°C
|
v.good, #2
|
v.good, #2
|
|
Coating Thickness
|
425 micron
|
400 micron
|
|
CD 48 hrs at 85°C
|
1 mm
|
2 mm
|
|
Coating Thickness
|
450 micron
|
400 micron
|
|
Hardness (BUCHHOLZ)
|
100
|
83
|
ADHESIVES
Over last ten years adhesive technology has developed and advanced from the early days of hot melt adhesives, based on EVA (ethylene-vinyl acetate), EAA (ethylene-acrylic acid), and EEA (ethylene-ethyl acrylate) through terpolymers to grafted polyethylenes. In most countries grafted polyethylenes are the widest used adhesives for the three layer coatings, as they provide the best overall properties for these systems. Adhesives needed for three layer coating systems are usually co-polymers of grafted polyethylene with active maleic anhydride groups or similar and are well known in most countries.
The adhesives fulfill dual purpose:
Firstly, adhesives bond chemically to the uncured groups in the epoxy powder and, providing that the FBE is not cured at the moment of contact with the adhesive, form a strong bond, which cannot be separated under normal peel test.
Secondly, the adhesive bonds physically to the outer polyethylene jacket by forming a chain entanglement between adhesive layer and polyethylene layer. There is a strong chemical affinity between the adhesive and polyethylene: over 95% of the adhesive consist of polyethylene, so quite obviously they both bond together physically very well, especially in the molten state.
Adhesive can be applied either by extrusion or by spray, in the powder form. Both systems differ dramatically in the property called melt flow index, which is a measure of viscosity of polyethylene, or, in other words, is a reflection of the polyethylene chains molecular mass.
It is very important to prequalify the adhesive properly in the form of plant trials, as different adhesives differ a lot in terms of application characteristics, moisture absorption, oxidation time, extrudability and properties of the final product. For example, some commercial adhesives perform well at room temperature, but lose drastically the peel adhesion values at elevated temperature, due the their chemical make up or due to the molecular weight distribution.
There are many specifications for various projects and proper selection of the adhesive might make the difference, as to whether the particular product will meet the specification, or not.
POLYETHYLENE
The polyethylene extruded on top of the adhesive can belong to several groups of density, molecular weight distribution and lineality. In the past, low density polyethylene was used a lot. Over the years, however, with new and improved polyethylene manufacturing processes, polyethylene density increased from approximately 0.925 to 0.945, as per ASTM D792, commonly used now. The respective merits of these two types of polyethylene are subject of many arguments. However there is a strong evidence that high density polyethylene with narrow molecular weight distribution provides a much tougher coating with less mechanical damage, than its low density counterpart. There is a notable trend in the industry to switch to higher density polyethylene over last few years. Typically, however, there is a limit of density not exceeding 0.95, as above this value polyethylene seems to be more prone to environmental stress cracking. Table 3 shows typical properties of high density polyethylene.
Table 3
|
PROPERTY
|
TEST METHOD
|
Canadian HDPE
|
|
Melt Index
|
ASTM D 1238
|
0.32
|
|
Compound Density
|
ASTM D 792
|
0.95
|
|
Base Density
|
ASTM D 792
|
0.941
|
|
Melt Flow Ratio
|
Internal Nova
|
90
|
|
Cold Temp. Brittleness
|
ASTM D 746
|
<-75°C
|
|
ESCR, F50 10% Solution, 50°C
|
ASTM D 1693
|
1,000 Hours
|
|
Hardness, shore D
|
ASTM D 2240
|
62
|
|
Vicat Softening Point
|
ASTM D 1525
|
122°C
|
|
Carbon Black Content
|
ASTM D 1603
|
2.2%
|
|
Oxidative Induction Time, 220°C Oxygen Gas
|
ASTM D 3895
|
> 15 minutes
|
|
Tensile Strength at Yield
|
ASTM D 638
|
21 MPa
|
|
Elongation at Break
|
ASTM D 638
|
850%
|
Source: Novacor lab
SIDE EXTRUSION PROCESS
Side extrusion is the most widely used process for coating large diameter pipe. In this process pipe travels spirally past through the FBE booth and past the adhesive and past the polyethylene die. Several layers of molten polyethylene are applied in a flat sheet form and squeezed together by a silicone rubber roller. The main purpose of the roller is to improve interlayer adhesion and to eliminate air trapment between the layers. The side extrusion technology provides a uniform coating with virtual no distinction and no separation between the polyethylene layers, the number of which can vary from few to approximately ten, depending on the die gap opening and total coating thickness.
Table 4 - Typical properties of the adhesion between the layers.
| H. D. P. E. , black compound | |||
| Tensile strength, circumferential (MPa) | Tensile strength, longitudinal (MPa) | Elongation, % | |
| No Seam | 18.6 | 29.0 | 960 |
| Seam | 18.0 | 27.6 | 896 |
| Circumferential | 18.3 | 28.7 | 920 |
QUALITY PROGRAM AND INSPECTION
Some standards, such as Canadian CSA Z245.21 require the coating applicator to have a registered quality program in accordance with the ISO 9000 series or similar. While the merits of ISO 9000 are not disputable, it is still very important for the coating applicator to have good process control and to inspect the process at all stages. As a minimum, the coating applicator should inspect and critically assess materials and process for acceptance or rejection at the following crucial points:
- Receiving of raw materials
- Testing of raw materials
- Pipe surface preparation
- Pipe surface chemical treatment
- Pipe heating temperature
- FBE application
- Adhesive extrusion
- Polyethylene extrusion
- Holiday inspection
- Film thickness at pipe body and weld
- Laboratory testing
- Cutback
- Pipe storage
- Loading and shipping.
Some of the inspection steps are defined as special processes and have to follow special process procedure in accordance with ISO 9000.
HIGH TEMPERATURE TESTS
One of the main applications for three layer polyethylene coated pipe is service at elevated temperature in the wet areas, such as in Canadian muskeg or permafrost. This type of coating performs in these tough operating conditions much better than other conventional coatings. Three layer polyethylene coating has been used now successfully for almost eight years in wet, boggy, gas gathering fields, operating at 85°C. In one instance the coating used previously on 80km of gathering lines had to be totally replaced, as it failed after only two years in service.
In order to approve three layer coatings for high temperature wet service; we have to look at several aspects, including:
- Material properties
- Short and long term lab testing
- Field performance
Material properties.
Properties of the raw material can be assessed based on the glass transition of FBE and based on the Vicat softening point of adhesive and polyethylene. Glass transition temperature of FBE is between 100 and 105°C, which is a good indication that the FBE does not undergo thermal structural changes up to this temperature.
Similarly, Vicat softening points of adhesive and polyethylene are 105°C and 124°C, respectively. If we subtract 20°C safety region as dictated by the good industrial practice, we will arrive at the highest operating temperature of the whole system as 85°C. It is usually recommended not to exceed this temperature for a long period of time, as prolonged exposure to temperatures in excess of 85°C will contribute to premature failure of the coating system. At temperatures above 105°C polyethylene will not only age much faster, but also become soft and pliable, be prone to damage and can even start flowing off the pipe.
Short and long term lab testing.
High temperature lab testing conducted on physical pipe samples coated with three layer polyethylene includes
- cathodic disbondment,
- high temperature peel test,
- hot water test and
- High temperature shear test.
Cathodic disbondment conducted at 95°C shows that the coating becomes somewhat prone to attack, disbonded radius is usually twice as big as at 65°C. Typical disbonded radii are shown in Table 5.
Table 5 - Cathodic disbondment radius at different temperatures.
|
Test duration
|
Temperature
|
Voltage
|
Disbonded Radius
|
|
30 days
|
23C
|
1.5V
|
5mm
|
|
30 days
|
65C
|
1.5V
|
15mm
|
|
30 days
|
95C
|
1.5V
|
25mm
|
High temperature peel test indicates that the peel adhesion becomes gradually lower, as the temperature increases, which is typical phenomenon for thermoplastic materials, as shown in Table 6.
Table 6 - Peel strength at elevated temperature.
|
Peel temperature
|
Cross head speed (tensilometer)
|
Peel value (kg/25mm strip)
|
|
23°C
|
10mm/min
|
15
|
|
40°C
|
10mm/min
|
10
|
|
60°C
|
10mm/min
|
7
|
|
80°C
|
10mm/min
|
4
|
Hot water adhesion is a test in which a coated sample is immersed in boiling water for a period of time ranging from 24 hours up to several months. After removal from the hot water bath the coating is tested for peel adhesion. This test indicates that the adhesion values become gradually lower after prolonged exposure to hot or boiling water, as shown in Table 7.
Table 7 - Hot water soak followed by peel adhesion.
|
Sample immersed in boiling water for
|
Peel adhesion value
|
|
1 day
|
10 kg/25mm
|
|
7 days
|
8 kg/25mm
|
|
14 days
|
6 kg/25mm
|
|
30 days
|
5 kg/25mm
|
Shear test was developed to predict behaviour of coating exposed to high temperature in high soil stress area, such as muskeg, heavy clay, and permafrost. Typically, it involves compression load and lateral load to simulate real life forces acting onto the buried pipeline. This test is conducted at several temperatures to indicate the limit at which the coating starts to lose adhesion to the pipe, as shown in table 8.
Table 8 - Shear test values at elevated temperature.
|
Test temperature
|
Compression force
|
Lateral force
|
Movement
|
|
85°C
|
40 kg
|
19 kg
|
0 mm
|
|
90°C
|
40 kg
|
19 kg
|
0.2 mm
|
|
95°C
|
40 kg
|
19 kg
|
0.5 mm
|
CONCLUSIONS
- Three layer polyethylene coating can be used quite successfully at elevated temperatures.
- Physical temperature limit is determined by the materials and test properties of the product. Three layer polyethylene coating offers many challenges to the coating specifier, to the applicator and to the inspector.
- A well written specification is very important for the successful application of this coating.
- The technical risk involved is quite high for the coating applicator.
- The coating applicator must adhere strictly to all crucial steps in application process.
- More emphasis is needed by the industry to develop coating test methods, which do not require cutting the pipe.
ACKNOWLEDGMENT
The author wishes to express gratitude to the management of Garneau, Inc. for the permission to present this paper.
All tests were conducted in Garneau Inc.'s testing lab during the period from 1995 to 1997, unless indicated otherwise.