THREE LAYER EPOXY/ POLYETHYLENE SIDE EXTRUDED COATINGS FOR PIPE FOR HIGH TEMPERATURE APPLICATION.

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1 International Pipeline Conference Volume II ASME 1998 IPC THREE LAYER EPOXY/ POLYETHYLENE SIDE EXTRUDED COATINGS FOR PIPE FOR HIGH TEMPERATURE APPLICATION. Mike Alexander CARNEAU INC *" Street Nisku, Alberta T9E 7X4 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 the purpose to evaluate 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. Copyright 1998 by ASME

2 INTRODUCTION Perfect pipe coating should be resistant to damage from elements, such as water and 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 the 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 the coating plant under strict process conditions. Two of the 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 mechnical and handling damage. The second coating is based on extruded polyolefins, typically polyethylene, which offers toughness and good damage resistance and has 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 uses fusion bond epoxy as a primer 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 system has excellent damage resistance, it is much easier to handle than FBE, and the incidence of holidays compared to FBE is 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 the 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 French standard NF A is being used to a smaller extent. Canadian Standard CSA Z 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 to be two to three times thinner than the DIN Standard. The rationale behind it is in the fact that 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.

3 There are many new three layer polyethylene standards being worked upon; the European EN standard, international ISO, and American ASTM. All of these new standards are at various stages of completion at the moment 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 coatings will have properties similar to those in Table 1. TABLE 1: Properties Of Well Designed Three Layer Polyethylene Coating Property Peel adhesion Cathodic disbondment Water boil adhesion Direct impact resistance Flexibility Shear resistance Test result High at wide temp range Excellent over wide temp range High after prolonged water boil High at low and high temp Very good at low temperature 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 an immediate effect on the quality of the final product therefore, a lot of work always goes 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 Z Therefore, 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 into two distinctive groups: - A physical benefits 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, temperatures, thickness, and different bending characteristics. Generally, the trend in the industry is 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

4 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 D1003LD EP Tgl Tg Delta II Gel Time (Sec) 180 Deg. C Deg. C Impact (in lbs.) At-23 C At Ambient Adhesion Hot Water, 48 hrsat 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) ADHESIVES Over the last ten years, adhesive technology has developed and advanced from the early days of hot melt adhesives, based on EVA, EAA, and EEA 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 aie 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: First of all, 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 consists 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 to their chemical make up or due to the molecular weight distribution. There are many specifications for various projects and proper selection of the adhesive may make the difference as to whether or not the particular product will meet the specification.

5 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 to which is commonly used now. The respective merits of these two types of polyethylene are the subject of many arguments. However, there is 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 the 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 ASTMD Compound Density ASTM D Base Density ASTM D Melt Flow Ratio Internal Nova 90 Cold Temp. Brittleness ASTM D 746 <-75C ESCR, F50 ASTM D ,000 Hours 10% Solution, S0 C Hardness, shore D ASTM D Vicat Softening Point ASTM D C Carbon Black Content ASTM D % Oxidative Induction Time, 220eC ASTM D 3895 > 15 minutes Oxygen Gas Tensile Strength at Yield ASTM D MPa Elongation at Break ASTMD % SIDE EXTRUSION PROCESS Side extrusion is the most widely used process for coating large diameter pipe. In this process, pipe travels spirally through the FBE booth and past both the adhesive and 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 virtually no distinction or separation between the polyethylene layers, the number of which can vary from a few to approximately ten, depending on the die gap opening and total coating thickness. Table 4 shows some typical properties of the adhesion between the layers. TABLE 4: H. D. P. E., Black Compound Tensile strength, Tensile strength, Elongation, % circumferential longitudinal No Seam Seam Circumferential

6 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 a good process control and to inspect the process at all stages. As a minimum requirement, the coating applicator should inspect and critically assess materials and processing 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 HIGH TEMPERATURE TESTS One of the main applications for three layer polyethylene coated pipe is at elevated temperatures 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 85C. 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, the Vicat softening point of adhesive and polyethylene. Glass transition temperature of FBE is between 100 and I05C, 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 105C and 124C, respectively. If we subtract 20C safety region as dictated by the good industrial practice, we will arrive at the highest operating temperature of the whole system as 85C. It is usually recommended not to exceed this temperature for a long period of time as prolonged exposure to temperatures in excess of 85C will contribute to premature failure of the coating system. At temperatures above 105C, 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 95C shows that the coating becomes somewhat prone to attack, disbonded radius is usually twice as big as at 65C. Typical disbonded radii are shown in Table 5.

7 TABLE 5: Cathodic Disbondment Radius At Different Temperatures Test Duration Tem perature Voltage Disbonded Radius 30 days 23C 1.5V 5mm 30 days 65C 1.5 V 15mm 30 days 95C 1.5V 25mm High temperature peel test indicate that the peel phenomenon for thermoplastic materials, as shown adhesion becomes gradually lower as the in Table 6. temperature increases which is a typical Table 6: Peel Strength At Elevated Tem perature Peel tem perature Cross head speed (tensilometer) Peel value (kg/25mm strip) 23C 10mm/min 15 40C 10mm/min 10 60C 10mm/min 7 80C 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 W ater Soak Followed By Peel Adhesion Sample immersed in boiling w ater 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 tests were developed to predict behaviour of coating exposed to high temperature in high soil stress areas, such as muskeg, heavy clay, and permafrost. Typically, it involves compression loading and lateral loading to simulate real life forces acting on 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. Typical values are in Table 8. ABLE 8: Shear Test Values At Elevated Tem perature Test Temperature Compression Force Lateral Force Movement 85C 40 kg 19 kg 0 mm 90C 40 kg 19 kg 0.2 mm 95C 40 kg 19 kg 0.5 mm

8 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, applicator and 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 Gameau, Inc. for the permission to present this paper. All tests were conducted in Gameau Inc. s testing lab. During ther period from 1995 to 1997, unless indicated otherwise.