Laboratory Testing of Three Above Ground Paint Systems for Canadian Applications

Laboratory Testing of Three Above Ground Paint Systems for Canadian Applications Amal Al-Borno, Mick Brown Charter Coating Service (2000) Ltd. #6, 4604 13 th Street N.E. Calgary, AB, Canada T2E 6P1 Robert Worthingham, Matt Cetiner TransCanada Pipelines Ltd. 450-1st Street S.W. Calgary, AB, Canada T2P 5H1

TransCanada were specifying and using a 3-coat paint system for above ground structures. The introduction of two-coat alternatives with claimed performance benefits as well as being quicker to apply led to an interest in comparing the two types of coating. Discussion with coating suppliers led to the selection of two 2- coat systems; one from the same manufacturer as the 3-coat and one from a competitive supplier.

Identification Primer Mid-coat Topcoat VOC g/l of Topcoat Coating 1 Metallic Zn-rich epoxy High build epoxy Aliphatic polyurethane 311 Coating 2 Metallic Zn-rich epoxy None Acrylic modified polysiloxane 218 Coating 3 Metallic Zn-rich epoxy None Engineered polysiloxane 84

The 3 coat system: Zinc rich epoxy primer a thin layer that gives good adhesion and helps prevent corrosion at damage sites. Intermediate epoxy giving build, cohesive strength and damage resistance. Aliphatic polyurethane topcoat providing colour and gloss with resistance to weathering.

The 2 coat system: Zinc rich epoxy primer similar to the 3-coat system. Polysiloxane chemistry based topcoat giving build, adhesion, and appearance that is resistant to weathering conditions.

Appeared in 1990 s as an alternative to polyurethanes. The higher Si-O (108 Kcal/mole) vs C-C (83 Kcal/mole) bond strength and the reduced tendency to oxidation suggests better resistance to weathering. Lower VOC s compared to polyurethanes. Not combustible Reduced application time and costs due to being in only two coats.

Evaluate and compare performance of polysiloxane candidate coatings versus currently used 3-coat system by examining: Flexibility Impact resistance Adhesion Hot water soak adhesion Prohesion / Accelerated weathering testing

Coating Coating 1 Coating 2 Coating 3 Components Metallic Zinc-rich Epoxy /Epoxy/Aliphatic Polyurethane Metallic Zinc-rich Epoxy /Acrylic Modified Polysiloxane Metallic Zinc-rich Epoxy /Engineered Polysiloxane Film Thickness (mils) Degree of Flexibility (degree of bend per pipe diameter length) 21 C 0 C -30 C 13.5-14.6 1.3 0.6 0.6 9.4-11.3 2.0 2.0 0.7 8.5-9.7 1.3 0.7 0.5 Coating 2 showed the most flexibility Reduced flexibility at lower temperatures

Coating Coating 1 Coating 2 Coating 3 Components Metallic Zinc-rich Epoxy /Epoxy/Aliphatic Polyurethane Metallic Zinc-rich Epoxy /Acrylic Modified Polysiloxane Metallic Zinc-rich Epoxy /Engineered Polysiloxane Film Thickness (mils) Resistance to Impact 21 C 0 C -30 C 13.5-14.6 Pass Pass Fail 9.4-11.3 Pass Pass Fail 8.5-9.7 Pass Pass Fail Mixed results of pass and fail indicating that this was close to the performance limits of Coatings 1 and 3 All coatings showed reduced impact resistance at low temperatures

Coating Components Adhesion Results X-scribe Pull-Off Failure Location Coating 1 Metallic Zinc-rich Epoxy /Epoxy/Aliphatic Polyurethane 5A - Pass 2562 Various locations in system Coating 2 Metallic Zinc-rich Epoxy /Acrylic Modified Polysiloxane 5A - Pass 2385 Between fixture and glue Coating 3 Metallic Zinc-rich Epoxy /Engineered Polysiloxane 5A - Pass 2480 Between fixture and glue Coating 1 had cohesive strength close to 2600 psi The polysiloxanes had higher adhesive and cohesive strength than the pull-off values

Coating Components Film Thickness (mils) 2-Day Exposure 7-Day Exposure Coating 1 Coating 2 Coating 3 Metallic Zinc-rich Epoxy /Epoxy/Aliphatic Polyurethane Metallic Zinc-rich Epoxy /Acrylic Modified Polysiloxane Metallic Zinc-rich Epoxy /Engineered Polysiloxane 12.8-14.7 1 - Pass 1 - Pass 9.2-10.6 1 - Pass 1 Pass 8.2-9.2 1 - Pass 1 Pass In the 7 day test Coating 2 showed good adhesion but blistering to the substrate. This coating is not designed for immersion service.

Cyclic Testing Using: UV-Light/Condensation (72 hours): Using cyclic 4 hours UVB light @ 60 C / 4 hours condensation @ 50 C. Cold Exposure (24 hours) @ -30 C. Salt Spray (72 hours) ASTM G85, Annex 5 - wet/dry cycling: NaCl + (NH 4 ) 2 SO 4 solution / warm air @ 35 C.

Coating Coating 1 Coating 2 Coating 3 Time (hours) Parallel Scribe Adhesion X- Scribe Pull-Off (psi) Blisters or Checking Rusting Undercreep (mm) Colour Change 500 A 4A 2732 None 9 1.17 None 500 A 4A 2323 None 9 2.05 None 1000 A 5A 2650 None 8 2.53 None 1000 A 5A 2242 None 9 2.31 None 500 A 4A 2568 None 7 1.75 None 500 A 4A 2732 None 8 1.66 None 1000 A 5A 2548 None 8 4.57 None 1000 A 5A 2221 None 9 3.40 None 500 A 4A 2691 None 8 0.85 Slight 500 A 4A 2732 None 7 0.92 Slight 1000 A 5A 2773 None 9 2.93 Slight 1000 A 5A 2752 None 9 2.83 Slight

No significant reduction in adhesion of any of the coatings. No checking or blisters. All samples showed rusting but this was a surface phenomenon with no visible rusting of the substrate. Colour change for Coating 3 was yellowing of a white coating. Coating 2 was a grey colour which may have helped prevent colour change. Slight undercreep for all; least for 3-coat system.

The cold environments in Canada put extra demands on coatings which showed lower flexibility and impact resistance at -30 C. All coatings passed impact test at 21 C and 0 C but all failed at -30 C. Coating 2 acrylic modified polysiloxane showed greatest flexibility. All coatings showed excellent adhesion but the polysiloxanes had better cohesive strength. All coatings showed excellent adhesion in the hot water soak adhesion test but Coating 2 acrylic modified polysiloxane showed blistering after seven (7) days since no blistering was observed in 1000 hours of weathering this is not considered a problem.

Prohesion/accelerated weather testing resulted in the following: No significant reduction in adhesion Surface rusting No chalking Slight yellowing of Coating 3 Coating 2 was grey and this may have reduced potential colour change. All had slight undercreep with rusting from the X-scribe steel the 3-coat system was overall most resistant to long term undercreep.

Although there were some minor differences in the performance of the coating systems the study indicates that replacement of the currently used three-coat zinc / epoxy / urethane system with the tested zinc/polysiloxane two-coat systems would not result in significant reduction in coating performance or expected service life.