Addition of Solid Particles

2.5.1 Fumed Silica

To overcome the soft nature of the heat cured vegetable oil polymers solid fillers could be added. The four commercial samples analysed (see section 2.3.4) contained particles of alumina, silica and barium sulphate, and silica and alumina are well known coating additives.[189,190] It has been shown that silica can hydrogen bond to the polymer matrix in epoxy resin-silica nanocomposites, this is shown by a decrease in vibrational frequency of C=O and SiO-H bonds in FT-IR spectroscopy.[191] Consequently, we chose to explore the use of added silica to increase the toughness of our coatings.

Fumed silica, also known as pyrogenic silica, is produced in a flame or an electric arc from silicon tetrachloride, the particles are large 3-dimentional aggregates. Fumed silica has a very large surface area and low density it is often used in coatings, used in cosmetics and an abrasive in toothpaste, so it is food safe. The silica used for this study was supplied by Wacker Chemie AG, a hydrophobic grade (HDK H20) was supplied which was functionalised with dimethylsiloxane.[192]

For the remainder of this investigation coating formulations will be based on epoxidised soybean oil as it showed the best properties of all of the heat cured plant oils. Epoxidised soybean oil is also commercially available and produced on a large scale as it is used as a plasticizer in PVC manufacture.[193]

65 Epoxidised soybean oil was mixed with fumed silica in volume ratios of 10:1, 5:1, 2:1 and 1:1 and 3 wt% TAS used as catalyst. The mixtures were coated onto ECCS with a 12 μm coating bar and cured at 400 °C for 60 s (Figure 2.18).

Figure 2.18. ESBO and fumed silica coatings on ECCS.

The addition of silica provided a great improvement to the surface properties, the samples were non tacky, were not damaged by a 5H pencil in hardness testing and passed adhesion and flexibility tests (Table 2.6). At this point a further adhesion test was used to screen the coatings, the Erichsen test;

Erichsen test, BS EN ISO 1520:2006 - measures adhesion to a substrate. The area where a cross hatch test has been performed is deformed into a dome 8 mm in height and tape is used to inspect for any removal of coating.

Volume ratio

Epoxy Soybean Oil Silica X-Hatch Flexibility Hardness Erichsen

10 1 100% Pass 4H 100%

5 1 100% Pass >5H 100%

2 1 100% Pass >5H 100%

1 1 100% Pass >5H 100%

Table 2.6. Industry tests of silica containing coatings.

In conclusion, the addition of fumed silica particles to the thermal improved the hardness and theother surface properties. A volume ratio of 5:1 (epoxidised oil: silica) produced a coating with sufficient hardness for a commercial coating, and

66 consequently, increasing the amount of silica would gain no advantage but increase the overall cost.

2.5.2 Addition of Pigments

Thermally cured samples are prone to discolouration (see 2.5.1 above), this is unattractive for a commercial product, so formulations often contain pigments. A common pigment used in cookware is carbon black, it is relatively cheap, safe for food use (recorded as E153 on food labelling)[194] _{and the dark colour disguises}

discolouration from repeated uses.

Carbon black was added to a formulation of epoxy soybean oil, fumed silica and TAS in amounts of 1, 2, 3, 4, 5 and 10 wt%, each mixture was coated onto an ECCS substrate, cured at 400 °C for 60 s, and then subjected to X-hatch, hardness, flexibility and Erichsen tests to evaluate their properties (Figure 2.19).

Figure 2.19. Coating formulations containing ESBO, fumed silica, carbon black and TAS.

The addition of carbon pigment did not affect surface hardness or adhesion properties until 10 wt% was included, where some coating is removed in the Erichsen test. However, the flexibility of the coatings were reduced significantly.

67

Carbon wt% Hardness Flexibility X-Hatch Erichsen

1 >5H Fail 100% 100% 2 >5H Pass 100% 100% 3 >5H Patchy 100% 100% 4 >5H Fail 100% 100% 5 >5H Fail 100% 100% 10 >5H Fail 100% < 100%

Table 2.7. Surface tests of coating formulation containing carbon black pigment.

Flexibility was adequate when 2 and 3 wt% of pigment was present, but a small amount of pigment was removed from the 3 wt% sample when folded through 180 °C, when folded again to give a 1T fold no coating was removed. In terms of aesthetics the 3 wt% coating performed better than 2 wt% as discolouration was less apparent and coating was more opaque. Thus the optimum amount of carbon black for this coating formulation would be ~3 wt% as more than this affects physical properties to much and any less does not give enough colouration.

2.5.3 Addition of PTFE

The optimum formulation so far (containing 5:1 epoxy soybean oil (ESBO):silica and 3 wt% carbon black) had a contact angle with water of 72° which was too hydrophilic for non-stick applications, (the commercial samples were around 90°). To increase this contact angle, the surface energy needed to be reduced, through the use of a release agent such as Teflon (PTFE).

PTFE is chemically inert due to the strength of the C-F bonds, this bond is highly polarised from the high electronegativity of fluorine. The polarisation adds some ionic character which shortens and strengthens the bond (C-F, length = 1.35 Å energy = 544 kJmol-1).[195] The chemical inertness and resistance to intermolecular forces gives a low coefficient of friction which creates non-stick behaviour.

68 Coating formulations of 5:1 volume ratios of ESBO and silica with 3 wt% carbon black were combined with 1, 2, 3, 4, 5, 10 and 20 wt% of PTFE powder. The formulations were coated with a 12 μm bar onto ECCS and cured at 400 °C for 60 s. For all samples there was failure of adhesion, the surface was rough with a powdery residue, which was easily removed exposing bare metal. It would appear that Teflon was not bonding to the ESBO binder which caused lack of adhesion between the binder and the surface. In commercial samples it is thought PTFE particles sinter together and create mechanical adhesion with the binder. The same curing process was used in our system so the particles should sinter together. It is possible that this failure might be due to either incompatibility between Teflon and epoxidised vegetable oils, or the oils are affecting the particle sizes of Teflon produced. More work and analysis would have to be undertaken to fully explain the reasons for the lack of adhesion, but at this point attention was turned elsewhere and not further work was undertaken