Developing a heat-resistant coating

and Xigao Jian, Dalian University of Technology, Dalian, China

Working with the Dalian University of Technology, PetroChina Pipeline Company has developed a new heat-resistant coating which has exhibited good thermal and anti-corrosion properties. Based on a poly-phthalazinone ether sulfone ketone material, this coating can be successfully used on high-temperature pipeline systems.

Known as PPESK for short, this material was synthesized in our laboratory, and has shown good solubility and very high glass transition temperature (Tg), in the range of 263±305˚C, by the virtue of the introduction of twist non-coplanar phthalazinone moieties into the polymer’s backbone. The resin was dissolved in organic solvent, and baked to form varnish. The effects of polymer’s sulfone/ketone molar ratios (S/K) and intrinsic viscosity on the coating’s properties were also studied.

The result showed that the varnish has excellent adhesion to metal, impact resistance and flexibility properties when S/K is 8/2 and the polymer’s intrinsic viscosity is about 0.50~0.60dL/g. The pigments were ulteriorly mixed into varnish to form color coating. The coating has excellent heat resistant and anti-corrosion properties, and can be used in the high temperature system. The goal here is to present our research and findings on the newly developed PPESK coating.

Heat-resistant materials

With the progress of aviation and chemical industry, novel heat resistance materials, which can survive chemically and thermally hostile environments, must be developed to meet the needs of industry and new technology. Today, although remarkable advances have been made in the development of high temperature thermoplastic, the most high temperature thermoplastics, such as PEEK, have poor solubility and can seldom be made into coatings. Although PES resin can be used as coating, it can only be used at 200˚C.

The PPESK material has superior mechanical strength, excellent thermal stability and high glass transition temperature (Tg) in the range of 263±305˚C (Figure 1). The material can be soluble in some apronic solvent such as N-methyl-2-pyrrolidone and N,N-dimethylacetamide. These properties are very important for their applications in the coating. Solvents used were industrial grade. The PPESK coatings were formed following the relevant literature. The carbon black was FW200 material made by Degussa corporation. The conduct of the research and the experimental studies is described below.

Varnish and coating

Varnish with the solid content of 15% (wt,%) and different boiling point solvents were brushed on tinplate panels (50x120x0.2~0.3mm) to a thickness of 30±5mm. Tested panels were previously sandblasted to Sa 2 1/2(SIS 05 59 00-67) attaining 20±4mm maximum roughness and then degreased with toluene. At last, the coating was cured at 80˚C, 120˚C, 150˚C, 180˚C for 15 minutes, respectively. After mixing for 30 minutes, varnish and carbon black with the ratio of 10:3(w%) were coated to form black coating. The material was then baked at 80˚C, 120˚C, 150˚C, and 180˚C for 15 minutes, respectively.

Measurements

The intrinsic viscosities (h) of polymers were measured using an Ubbelohde viscometer at a concentration of 0.5 g/dL in N,N-dimethylacetamide (DMAc) at 25˚C. Film adhesions, flexibilities and impact resistances were measured referencing to Chinese National Standards GB/T 1720-89, GB/T 1731-93, and GB/T 1732-93, respectively.

Experimental results

When the polymer’s intrinsic viscosities were about 0. 50~0.60dL/g, the coating properties of PPESK coatings with different sulfone/ketone molar ratio S/K were measured at the same conditions.

As shown in Table 1, the result indicated that the insulated coating has excellent mechanical properties when sulfone/ketone was equal and above 8/2(mol, ratio). The film adhesion to metal increased with the increasing of sulfone content in the polymer chain. The appetency of sulfone group to metal was much stronger than that of carbonyl, as the result that the polymer with more sulfone group was much easer gained electron, which made electron on the metal shift to coating film with the result of forming double electricity layer to produce static gravitation.

The table indicated that film flexibility and impact resistance were not lining to sulfone/ketone. The reasons are complex. On the one hand, film flexibility increased with the increasing of ketone content in the polymer chain because the polymer chain containing carbonyl group was much more flexible than that of sulfone group; on the other hand, the flexibility and impact resistance of the coating were simultaneously affected by the coating adhesion to metal. By the above result, the content of sulfone of the polymer and the mechanical properties of the coating have a correlation. PPESKs with the S/K ratio of 8/2 were selected as the experiment object to carry through in the next experiment.

Viscosity is the main physical property of fluid, which is very important to coating properties. According to the definition of intrinsic viscosity, the magnitude is decided by the structure and molecular weight of polymer in standard concentration and solvent. Polymer’s molecular weight directly influences coating’s viscosity and properties. During the development of the coating’s prescription, we expect to receive the highest solid content in the request of flowing viscosity to use as low as polymer’s molecular weight. Meanwhile, we hope to increase the extent of molecule chain to enhance the film properties for the thermoplastics coating. The research showed that the intrinsic viscosity significantly effects the flexibility and impact resistance of the film coating (Table 2). With the increasing of intrinsic viscosity, the flexibility and impact resistance of film coating increased. The reason may be that the attracting force between polymer and metal increased with the extent of polymer molecule chain, so as to improve the adhesion to metal. Considering film properties only, higher polymer molecule weight should be selected as film material.

To test the heat resistances of PPESK coating, the coating was placed into muffle. The results are shown in Table 3. The coating could not show cracks at 270˚C for 364 hours, and could endure at 330˚C for 24 hours. PPESK coating has good thermal properties. The anti-corrosion properties of PPESK coatings were also tested. The results showed that the PPESK coatings could endure 15% hydrochloric acid and 30% sodium hydroxide solution’s corrosion for more than sixty days.

Conclusion

A new heat resistant PPESK coating, which is superior to conventional PES coating, was successfully developed. The polymer’s intrinsic viscosity and the sulfone/ketone molar ratio were very important factors for the coating properties. The PPESK coating exhibits good thermal and anti-corrosion properties.

Acknowledgment

Based on a paper presented at the NACE CORROSION 2008 Conference & Expo, held in New Orleans, Louisiana, March 16-20, 2008.