Conductive Coatings - byk.com

1 JANUARY 2012 | In recent years, requirements for modern coat-ings have dramatically increased. Besides optical appearance, new functionalities like improved mechanical properties , anti-static behavior or electrical conductivity are desired. To achieve this, several materials can be incorporated into the coat-ing film. For enhanced electrical conductivity, substances like copper or silver particles, Conductive organic poly-mers, or carbon black can be used. In many cases, such materials have to be used in the form of nanoparticles to achieve even distribution inside the film and low percola-tion thresholds. There are several synthesis routes known to produce such nanoparticles, but they often result in either low yields or in materials that need further processing to show optimum performance.

2 This can lead to high material prices, and in combination with increasing raw material costs, the overall costs can become prohibitive for some applications. An example of this would be the use of silver. The average price of silver has been steadily increasing during the last few years, and silver nanoparticles are even more expensive. For Coatings applications, most companies now focus on non-metallic compounds, like carbon materials, to achieve electrical exists as different allotropes, , diamond, graphite, fullerenes and carbon nanotubes. While dia-mond consists of sp3 hybridized carbon with a cubic crys-tal lattice, all other known allotropic forms contain sp2 hybridized carbon atoms, and thus they are preferred for electrical conductivity. Since their observation in 1991 by Iijima, carbon nanotubes have been the focus of consider-able Scientists have since reported remarkable physical and mechanical properties for this fascinating allotrope of By T.

3 Tinthoff, N. Hanitzsch, M. Pickave, J. Tecklenburg, N. Willing and M. Berkei | BYK-Chemie GmbH, Wesel, GermanyConductive Coatings Using carbon NanotubesTABLE 2 | One-component clearcoat based on Worleesol 61 A .MaterialContent [g]Worleesol 61 327* glycol* : * Premix curing conditions, 20 min flash-off, 30 min at 130 C, adjust pH to to 1 | Two-component clearcoat based on Bayhydrol A [g]Component ABayhydrol A Y K- BBayhydur : Ratio A:B = :1, adjust pH to to & Coatings INDUSTRY 17 From unique electronic properties to mechanical proper-ties that exceed any current material, carbon nanotubes offer tremendous opportunities for the development of new material systems. In particular, the excellent electrical con-ductivity of carbon nanotubes combined with their high aspect ratio offer potential for the development of functional Coatings .

4 This article reports on recent advances in using additives based on carbon nanotubes to enhance the elec-trical conductivity of several coating All Coatings were applied on glass panels or PET foils with wire bars at 75 m wet film thickness. After a curing pro-cedure, all panels were stored for 24 h in an acclimatized chamber, and surface resistivity was measured three times at each formulations are shown in Tables 1-3. Multi-wall carbon nanotubes from different suppliers were used in the form of aqueous and DiscussionTo determine the performance of multiwall carbon nano-tubes (MWCNT) in different aqueous coating systems, we used a two-component clearcoat based on Bayhydrol A 145, and two one-component baking systems based on Worleesol 61 A or Bayhydrol D155, respectively. The A 145 system started curing immediately after applica-tion, while curing of the baking systems was initiated by elevated temperature.

5 The two baking systems offer different resin chemistries; alkyd/melamine in Worleesol 61 A and polyester/melamine in Bayhydrol D155. The difference between the three systems had an inf luence on the distribution of MWCNT inside the coating, resulting in different percolation all Coatings , dosages of to MWCNT (cal-culated on solid resin) were incorporated, and films were applied on glass and PET substrates using wire bars to achieve 75 m dry film thickness. Films with to 2% MWCNT were already quite dark but still translucent, while higher MWCNT concentrations resulted in non-transparent Coatings (Figure 1).As seen in Figure 2, the lowest percolation thresholds were observed in the D155 system, followed by the Wor-leesol 61 A system. Even at low MWCNT concentrations of to 2%, anti-static properties were achieved, and concentrations of 2% to 4% MWCNT resulted in electro- Conductive Coatings with surface resistivity below 105 possible explanation for this finding follows.

6 The MWCNT were incorporated into the Coatings in the form of dispersions in which the carbon nanotubes were homo-geneously dispersed and stabilized by wetting and dis-persing additives. This perfect distribution was trans-ferred into the coating system. In the two-component system, curing started directly after application, which kept the carbon nanotubes in a very well-dispersed state. It resulted in a higher percolation threshold since many MWCNT were in an isolated position and were not con-tributing to the percolation network. The baking systems started curing after the f lash off time and while applying heat. This enabled the MWCNT to agglomerate on a very small scale to achieve percolation. It can also be described as a kind of self-organization, and it seems that the resin chemistry also contributed to this process.

7 Unfortunately, we could not yet clearly see differences in distribution by REM microscopy to prove this theory. In addition to the inf luence from the resin chemistry or curing process, there was a significant contribution due to the particle size, aspect ratio and surface chemistry of the Conductive pigment itself. We used two different kinds of multiwall carbon nanotubes. The first one, MWCNT 1, exhibited some structural defects and some oxygen-containing groups on the surface. The second one, MWCNT TABLE 3 | One-component clearcoat based on Bayhydrol [g]Bayhydrol D 3277. 8 Butyl : Curing conditions 20 min flash-off, 30 min at 130 C, adjust pH to to 1 | Clear Coatings based on Bayhydrol D155 with different con-centrations of MWCNT (0%, , 1%, 2% from left to right). + + + + + + + + + + + + + + + 1 Content in %Surface Resistivity Worleesol 61 ABayhydrol A 145 Bayhydrol D + + + + + + + + + + + + + +13 ControlCarbon BlackMWCNT 1 MWCNT 2200 nm200 nm1 mSurface Resistivity FIGURE 2 | Surface resistivity of different clear Coatings with different concentrations of MWCNT Coatings Using carbon NanotubesJANUARY 2012 | 2, had less structural defects and a less polar surface, as well as a slightly longer particle length.

8 As a comparison, we used a Conductive carbon black, which offered a polar surface and particle agglomerates with low aspect ratios compared to MWCNT. Figure 3 shows that MWCNT 2 offered lower percolation thresholds compared to MWCNT 1 in the D155 system. To achieve the same level of surface resistivity, one would have to use between 8% to 10% car-bon black, MWCNT 1 or MWCNT 2. This effect becomes even more predominant in the Bayhydrol A 145 system where MWCNT 1 gave almost no surface resistivity improvement. Due to their more polar particle surface, the carbon nanotubes kept their well-dispersed status inside the two-component clearcoat system and even at high concentrations of 8%, there was almost no improvement in conductivity. The MWCNT 2 had a higher aspect ratio and resulted in a percolating network, exhibiting surface resistivity of 106 W at a MWCNT 2 concentration of 6%.

9 It is obvious that the performance of carbon nano-tubes as well as any other Conductive pigment strongly depends on several parameters. First, is the interaction between the Conductive pigment and the coating matrix. If the affinity of the resin to the Conductive pigment is too high and the Conductive pigment stays in a perfectly dispersed status, the formation of a percolating network is hindered. Therefore, the Conductive pigments should be used in the form of dispersions to guarantee a good initial dispersion status. In addition, the wetting and dispersing additives used have to guarantee good stabilization as well as suitable compatibility towards the coating matrix. Percolation will take place when the Conductive pigments are allowed to agglomerate on a molecular state, still keeping a good distribution.

10 Strong f locculation has to be avoided; otherwise the necessary amount of conduc-tive pigments would be too high. There has to be a good balance between optimum distribution and minimum second point to be considered is to use Conductive pigments with high aspect ratios to achieve percolation at low concentrations. Therefore we believe that carbon nanotubes can offer a strong benefit compared to carbon blacks due to their structural uniqueness (Figure 4).Compared to organic anti-statics (for example, those based on quaternary ammonium salts), Conductive pig-ments offer permanent anti-static or electro- Conductive behavior to Coatings . Quaternary ammonium salts can create a thin water film on the surface due to their high polarity, but their ions can migrate out of the coat-ing matrix, often resulting in decreasing performance over time.