Protective polymer layer

Abstract

The present invention relates to a method of joining polymer coated steel pipes comprising the steps ofproviding polymer coated pipe segments with an uncoated length on both ends of the segments; welding the polymer coated pipe segments together; applying a curable polymer (A) onto the uncoated length of the welded pipe segments to form an first coating layer; andapplying a polymer composition (B) onto the first coating layer to form a topcoat layer with a thickness of 0.5 to 10 mm, wherein the polymer composition (B) has a melt flow rate MFR.sub.2 of 1.0 to 6.0 g/10 min, determined according to ISO 1133 at a temperature of 190 C. under a load of 2.16 kg, and includes a base resin comprising (B-1) a non-elastomeric polyethylene in an amount of 60 to 85 wt % of the total polymer composition being produced in a process using a single-site catalyst, and (B-2) an elastomer comprising a copolymer of ethylene and at least one polar comonomer, wherein component (B-1) or components (B-1) and (B-2) have been grafted with an acid grafting agent and the use of polymer composition (B) as topcoat layer with a thickness of 0.5 to 10 mm in a coating of coated steel pipe field-joints, a method of coating a steel pipe and the use of polymer composition (B) for the coating of steel pipe joints or steel pipes.

Claims

1. A method of joining polymer coated steel pipes comprising the steps of providing polymer coated pipe segments with an uncoated length on both ends of the segments; welding the polymer coated pipe segments together; applying a curable polymer (A) onto the uncoated length of the welded pipe segments to form a first coating layer; and applying a polymer composition (B) directly onto the first coating layer without any intermediate layers or adhesives, to form a topcoat layer with a thickness of 0.5 to 10 mm, wherein the polymer composition (B) has a melt flow rate MFR.sub.2 of 1.0 to 6.0 g/10 min, determined according to ISO 1133 at a temperature of 190 C. under a load of 2.16 kg, and includes a base resin comprising (B-1) a non-elastomeric polyethylene in an amount of 60 to 85 wt % of the total polymer composition being produced in a process using a single-site catalyst, and (B-2) an elastomer comprising a copolymer of ethylene and at least one polar comonomer, wherein component (B-1) or components (B-1) and (B-2) have been grafted with an acid grafting agent, and wherein the total number of polymer layers applied onto the welded pipe segments is two, consisting of the first coating layer and the topcoat layer.

2. The method according to claim 1, comprising the additional step of partly curing the curable polymer (A) or leaving the curable polymer (A) uncured at or after applying the curable polymer (A).

3. The method according to claim 2, wherein for applying the curable polymer (A) the welded pipe segments are heated to a first temperature being 1 to 20 C. above the melting temperature of the curable polymer (A) and for applying the polymer composition (B) onto the first coating layer to form a topcoat layer the welded pipe segments are heated to a second temperature being higher than the first temperature.

4. The method according to claim 1, wherein the curable polymer (A) comprises an epoxy resin which is applied in powder form or in liquid form.

5. The method according to claim 1, wherein the composition (B) comprises from 72 to 80 wt % of component (B-1) and from 20 to 28 wt % of component (B-2).

6. The method according to claim 1, wherein the first coating layer has a thickness of 0.01 to 0.5 mm.

7. The method according to claim 1, wherein the polymer composition (B) further comprises a pigment in an amount of 0.01 to 5 wt % wherein the pigment is selected from carbon black, azo-dyes, and titanium dioxide.

8. The method according to claim 1, wherein the non-elastomeric polyethylene (B-1) has a density of 925 kg/m.sup.3 to 945 kg/m.sup.3, determined according to ISO 1183.

9. The method according to claim 1, wherein the non-elastomeric polyethylene (B-1) is a copolymer of ethylene and at least one alpha-olefin comonomer with 3 to 20 carbon atoms.

10. The method according to claim 1, wherein the at least one polar comonomer in elastomer (B-2) is selected from alkylacrylates, alkylmethacrylates, and alkyl acetates.

11. The method according to claim 1, wherein the acid grafting agent is selected from unsaturated carboxylic acids and derivatives thereof such as anhydrides, esters and metallic and non-metallic salts.

12. The method according to claim 1, wherein the first coating layer and the topcoat layer define a coating; and wherein the coating applied to the uncoated length of the welded pipe segments has a peel strength of at least 250 N/cm, determined according to EN ISO 21809-1 at a temperature of 23 C.

13. The method according to claim 1, wherein the topcoat layer has an environmental stress crack resistance ESCR (F20) of more than 2000 h, determined according to ASTM D 1693-A in 10% Igepal.

14. The method according to claim 1, wherein the topcoat layer has Vicat A softening point of more than 95 C., determined according to ISO 306.

15. The method according to claim 1, wherein the topcoat layer has a Shore D hardness of more than 45, determined according to ASTM D 2240.

16. A method of coating steel pipes comprising the steps of applying a curable polymer (A) onto the surface of the steel pipe to form a first coating layer; and applying a polymer composition (B) directly onto the first coating layer without any intermediate layers or adhesives, to form a topcoat layer with a thickness of 0.5 to 10 mm, wherein the polymer composition (B) has a melt flow rate MFR.sub.2 of 1.0 to 6.0 g/10 min, determined according to ISO 1133 at a temperature of 190 C. under a load of 2.16 kg, and includes a base resin comprising (B-1) a non-elastomeric polyethylene in an amount of 60 to 85 wt % of the total polymer composition being produced in a process using a single-site catalyst, and (B-2) an elastomer comprising a copolymer of ethylene and at least one polar comonomer, wherein component (B-1) or components (B-1) and (B-2) have been grafted with an acid grafting agent, and wherein the total number of polymer layers applied onto the welded pipe segments is two, consisting of the first coating layer and the topcoat layer.

17. A method for coating steel pipe field-joints or steel pipes comprising the step of providing a polymer composition having a melt flow rate MFR.sub.2 of 1.0 to 6.0 g/10 min, determined according to ISO 1133 at a temperature of 190 C. under a load of 2.16 kg, and including a base resin comprising (B-1) a non-elastomeric polyethylene in an amount of 60 to 85 wt % of the total polymer composition being produced in a process using a single-site catalyst, and (B-2) an elastomer comprising a copolymer of ethylene and at least one polar comonomer, wherein component (B-1) or components (B-1) and (B-2) have been grafted with an acid grafting agent, as a topcoat layer with a thickness of 0.5 to 10 mm, and wherein the topcoat layer is applied directly onto a first coating layer without any intermediate layers or adhesives, so that the total number of polymer layers applied onto the steel pipe field-joints or steel pipes is two.

18. The method according to claim 1, wherein the pipe segments are heated to a first temperature of 110 C. to 170 C. to cure polymer (A), and heated to a second temperature of 170 C. to 200 C. to cure polymer composition (B).

19. The method according to claim 7, wherein the pigment is present in an amount of greater than 3 wt %.

Description

1. METHODS

a) Density

(1) Density of the polymer was measured according to ISO 1183-1:2004 Method A on compression moulded specimen prepared according to EN ISO 1872-2 (February 2007) and is given in kg/m.sup.3.

b) Melt Flow Rate

(2) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR.sub.2 for polyethylene was determined at 190 C. under a load of 2.16 kg.

c) Peel Strength

(3) Adhesion of polymer on steel was tested by Instron 1122 peel strength test equipment according to EN-ISO 21809-1 (DIN 30670). The test was conducted at a temperature of 23 C.: A strip of 3 cm width is cut of the coating layer. The other end of the strip is fastened to pulling equipment and the pulling strength is measured during the peeling of the strip from the steel with a pulling speed of 10 mm/min. The results are expressed as N per cm.

d) Shore Hardness

(4) Shore D hardness was determined according to ISO 868-2003. Test specimens 1010 mm were milled out of a compression moulded sheet of thickness 4 mm. the compression moulding was done at molding temperature of 200 C. Material was pre-heated by applying light contact pressure for 10 min. Then full pressure was applied for 1 minute, after which material was cooled with a cooling rate of 15 C./min. Demolding temperature was 40 C.

e) Vicat a Softening Point

(5) The Vicat A test was conducted according to ISO 306 method A50 using a load of 10 N and a heating rate of 50 C./h. Test specimens 1010 mm were milled out of a compression moulded sheet of thickness 4 mm. the compression moulding was done at molding temperature of 200 C. Material was pre-heated by applying light contact pressure for 10 min. Then full pressure was applied for 1 minute, after which material was cooled with a cooling rate of 15 C./min. Demolding temperature was 40 C.

f) Environmental Stress Crack Resistance

(6) ESCR was conducted according to ASTM D 1693 (50 C., 10% Igepal CO630).

(7) Test specimens according to ASTM D 1693 condition A were prepared through compression molding of sheets of thickness 1.85 mm. Compression molding was done according to ISO 1872-2 at molding temperature of 200 C. Material was pre-heated by applying light contact pressure for 10 min. Then full pressure was applied for 1 minute, after which material was cooled with a cooling rate of 15 C./min. Demolding temperature was 40 C. The specimens (38.62.5 mm130.8 mm) were cut out of the sheets, and notched according to ASTM D 1693 Table 1, condition A

g) Melting Temperature

(8) The Melting Temperature (T.sub.m) is measured with Mettler TA820 differential scanning calorimeter (DSC) on 30.5 mg samples. The melting curves were obtained during 10 C./min cooling and heating scans between 10-200 C. Melting temperatures were taken as the peaks of endotherms.

h) GPC

(9) The weight average molecular weight Mw and the molecular weight distribution (MWD=Mw/Mn wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) is measured by a method based on ISO 16014-4:2003 and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with refractive index detector and online viscosimeter was used with 2GMHXL-HT and 1G7000H columns from Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 140 C. and at a constant flow rate of 1 mL/min. 209.5 L of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with 15 narrow MWD polystyrene (PS) standards in the range of 1 kg/mol to 12 000 kg/mol. Mark Houwink constants were used for polystyrene and polyethylene (K: 1910.sup.3 mL/g and a: 0.655 for PS, and K: 3910.sup.3 mL/g and a: 0.725 for PE). All samples were prepared by dissolving 0.5-3.5 mg of polymer in 4 mL (at 140 C.) of stabilized TCB (same as mobile phase) and keeping for max. 3 hours at 160 C. with continuous shaking prior sampling in into the GPC instrument.

2. EXAMPLES

a) Preparation of Polymer Composition (B)

(10) Polymer Composition (B)-Inventive (B-Inv)

(11) A medium density polyethylene was produced using a single-site catalyst prepared according to example 1 of WO 95/12622. The polymerisation was carried out in a slurry loop reactor. The temperature in the reactor was 85 to 100 C. Isobutane was used as a diluent and the pressure in the slurry loop reactor was approximately 40 bar. 1-hexene was used as a comonomer and the feed ratio was 4.8 to 5.2 kg comonomer/100 kg ethylene. The incorporation of 1-hexene was around 3.3 to 3.5% of the total weight in the medium density polyethylene. Small amounts of hydrogen were used to control the MFR (0.33-0.37 Nm.sup.3/ton ethylene) with almost 100% conversion.

(12) The adhesive blend compositions in the following examples were grafted in Werner & Pfleiderer ZSK 32-mm co-rotating, twin-screw, extruder.

(13) The single-site medium-density polyethylene, characterised of no long chain branches, has a density of 934 kg/m.sup.3 and an MFR.sub.2 value of 6 g/10 min. The M.sub.w/M.sub.n value of the single-site medium-density polyethylene is about 2. The ethylene butyl acrylate elastomer, having a butyl acrylate content of 27% by weight a density of 926 kg/m.sup.3 and an MFR.sub.2 value of 4 g/10 min, was added.

(14) The blend composition was grafted in Werner & Pfleiderer ZSK 32-mm co-rotating, twin-screw, extruder.

(15) In the extruder 74.99 wt % single-site medium-density polyethylene, 23.5 wt % ethylene butyl acrylate elastomer, 1 wt % of a pigment masterbatch containing 75.95% by weight polyethylene with a MFR.sub.2 of 2.2 g/10 min and a density of 920 kg/m.sup.3, 1.25% by weight Chromophthal yellow GRP (pigment Y95), distributed by BASF Pigments EU, 8.65% by weight Versal Yellow 6G (pigment Y94), distributed by Synthesia, 4.0% by weight of Tiona 188, distributed by Quimicoplasticos, or Tioxide R-FC, distributed by Huntsman, 10% by weight Chimassorb 119 FL and 0.15% by weight Irganox 1076 FD, both distributed by Ciba Specialty Chemical, now BASF SE are added. 0.4 wt % of antioxidant Irganox B 225, distributed by Ciba Specialty Chemical, now BASF SE, was added as stabiliser and the graft was achieved by adding 0.1 wt % of maleic anhydride. 0.01 wt % peroxide initiator (Perkadox 14S-fl, distributed by Akzo Nobel) was dissolved in isododecane. The temperature in the extruder was varied between 170 and 210 C. and the screw speed was set at 200 rpm. The resulting grafted blend composition had a MFR.sub.2 value of 4.6 g/10 min.

(16) Polymer Composition (B)-Comparative (B-Comp)

(17) Polymer composition (B-comp) was produced similar to polymer composition (B-inv) except that the composition did not contain an ethylene butyl acrylate elastomer.

(18) In the extruder 98.49 wt % single-site medium-density polyethylene, 1 wt % pigment masterbatch, 0.4 wt % of antioxidant Irganox B 225, 0.1 wt % of maleic anhydride, and 0.01 wt % peroxide initiator dissolved in isododecane were compounded. The resulting grafted blend composition had a MFR.sub.2 value of 5.0 g/10 min

b) Preparation of HDPE Composition

(19) A high density polyethylene resin was polymerized according to example 1 of EP 1 865 037. As polymerization catalyst the Ziegler-Natta catalyst produced according to example 3 of EP 0 688 794 A has been used. The resin was compounded with 0.4 wt % of antioxidant Irganox B 225 (distributed by Ciba Specialty Chemical, now BASF SE) and 2.25 wt % carbon black.

c) Pipe Coating

(20) Coating 1

(21) The ends of two steel pipe segments with a diameter of 114 mm were cleaned to remove the excess material from the surface in the area to be welded. The segments were then welded together. The uncoated area was then heated to 110 C. Epoxy powder (Infralit EP/PE 8087-18) was then sprayed onto the pipe surface so that the thickness of the epoxy layer was 135 m. Then the pipe was heated to 180 C. and the polymer composition (B-inv) as prepared above was extruded onto the epoxy layer by using the equipment described in paragraphs [0072] to [0076] of EP 2 181 832. The temperature of the melt was about 230 C. The topcoat layer had a thickness of about 4.5 mm. The thus obtained coating is referred to as Coating 1 below. The coating was smooth and free of cracks.

(22) Coating 2

(23) Coating 2 was produced as a three-layer coating comprising the epoxy, an adhesion layer and the HDPE composition as described above as top coat as described in paragraph [0037] of EP 1 865 037. The epoxy primer layer had a thickness of about 100 m, the adhesive layer had a thickness of about 250 m and the HDPE layer had a thickness of about 3.2 mm. The thus obtained coating is referred to as Coating 2 below. The coating was smooth and free of cracks. While such a three-layer coating gives good properties its application, in field conditions is not applicable and it is thus suitable as factory coating only.

(24) Coating 3

(25) Coating 3 was produced in a similar manner as Coating 1 but in place of polymer composition (B-inv) the polymer composition (B-comp) was used. When the coating was inspected, cracks were observed in the coating. Therefore it was not feasible to analyze the coating further.

(26) Coating 4

(27) Coating 4 was produced as in a similar manner Coating 1 but instead of Composition (B-inv) the HDPE composition as described above was used. Because of the absence of the adhesion layer the coating did not adhere to the epoxy layer and no coating was obtained.

(28) d) Properties of the Coatings

(29) Table 1 discloses the properties of the coating according to the method of the invention (Coating 1) using the polymer composition (B-inv) of example (a) and the three layer HDPE coating (Coating 2) according to the state of the art using the high density polyethylene composition of example (b). It can be seen that surprisingly the coating according to the invention (Coating A) shows not only excellent peel strength but that also polymer composition (B-inv) shows sufficient mechanical strength to meet the requirements of Class A three-layer polyolefin coatings according to ISO 21809-1. In regard of ESCR and peel strength even the requirements of Class B three-layer polyolefin coatings according to ISO 21809-1 are met.

(30) TABLE-US-00001 TABLE 1 ISO 21809-1 ISO 21809-1 Coating 1 Coating 2 Class B Class A MFR.sub.2 [g/10 min] 4.6 0.48 Melting temperature 121 128 [ C.] Vicat A [ C.] 104 120 >110 >95 Shore D 49.8 60 >55 >45 ESCR [h] >5000 >5000 >1000 >300 Peel strength [N/cm] 370 >500 >150 >100