Experience shows that, with conventional binder technology, it is not possible to formulate high-solids paints with fewer than 250 g/L VOC. Simply reducing the solvent content in existing medium-solids paints to meet legal requirements results in a highly viscous paint which cannot be applied. To reduce the non-volatile components (NVCs) requires major changes in the molecular structure of the binder.
Reduction in molecular weight, improved dilutability of the resin in common solvents, and lower intermolecular interaction are just some of the fundamental demands on innovative high-solids binders. The challenge is to increase the NVC at working viscosity to comply with the regulations, without impairing drying (both dry-to-touch and complete cure), optical appearance, or mechanical and chemical resistance.
One way to achieve high NVC at low-processing viscosity is with 2-pack paints – in particular, isocyanate-curing silicone hybrid resins. Silicone hybrid resins combine a broad range of properties which result in high quality paints. The polyester component enables, for example, a high crosslinking density in the fully cured film which delivers very good mechanical and chemical properties.
The silicone component in the resin molecule lowers viscosity. This effect is attributed to the free rotation of the silicone chains ~[Si(CH3)2 – O – Si(CH3)2] and their low tendency to interact. The silicone component also reduces the UV-yellowing tendency of the polymer.
Figure 1 compares two silicone hybrid resins with a high-solids polyester resin. It is obvious that the new type of silicone hybrid resins possess a significantly higher NVC at comparable intrinsic viscosities of the polymer solutions. At approximately 7500 mPa·s the conventional polyester resin solution has a NVC of 76%, whereas the new silicone hybrid resin has a NVC of 90%. This enormous difference gives the paint formulator great scope regarding other constituents in the formulation such as rheological additives or solvents.
The properties of the aforementioned resins were compared using formulated 2-pack paints. Two variants of the new silicone hybrid resins were compared with a commercial high-solids polyester:
• A Silicone hybrid resin with more flexible segments;
• B Silicone hybrid resin with rigid segments; and
• C High-solids polyester.
A typical test formulation for the silicone hybrid resin is shown in Table 1. Non-volatile data and VOC content of the paints is shown in Table 2.
After paint application and curing, resistance to liquids was tested according to ÖNORM EN 12720 “Furniture – assessment of resistance of surfaces to cold liquids.” The resistance was tested according to ÖNORM A 1605-12 within Class 1-B1; test results are shown in Figure 2. The assessment criteria for determining chemical resistance is noted in Table 3.
Silicone hybrid B shows immediately – and particularly after a recovery of 24 hours – optimum resistance to the liquids mentioned above. This is attributable to the higher rigid segment content in the silicone hybrid.
Silicone hybrid resin B exhibits outstanding resistance to various liquids in tests carried out according to DIN EN ISO 2812-4 (Table 4).
Salt Spray Test
The use of the new type of silicone hybrid results in paint films with a very high crosslinking density. This, together with skillful formulation of pigments and fillers in the paint, is an important influence in withstanding the salt spray test. A 2-pack epoxy primer was applied to a sand-blasted steel substrate. The primer was then overcoated with a white topcoat based either on the new type of silicone hybrid resins or on conventional polyester C. The test was carried out in accordance with DIN EN ISO 12944 Category C5 I and M. The images (Figure 3) show results after 1440 hours.
Stone Impact Test
Resistance to stone impact was tested according to DIN EN ISO 20567 1. Metal panels were prepared by painting them with 2-pack epoxy primer and topcoats based on silicone hybrid resins (A, B) and polyester (C). After a period of recovery, testing was carried out on the coated panels.
The results (Table 5 and Figure 4) show clearly that silicone hybrid resin A, with its increased toughness and resiliency, contributes to an improvement in the index for stone impact resistance.
To assess the working window of the resins, the paint was applied wedge-wise using a pneumatic spray apparatus (Figure 5).
Here the favorable influence of the silicone unit in the polymer is particularly evident. Compared with the polyester, the silicone hybrid resin shows about 30% less pinhole formation in a comparable cured-coating thickness. Silicone hybrid resins permit trouble-free application of greater coating thicknesses.
The reduced tendency of silicone hybrid resins to form pinholes is especially beneficial for the optical appearance of the applied coating, again showing the positive effect of silicone in the paint (Figure 6).
Contact Angle Measurement
Apart from when high-temperature-resistant coatings are required, there are a number of reservations in the coatings industry concerning the use of silicone raw materials. As the new silicone hybrid resins are also well suited for formulating primers, contact angles of the cured coatings were measured (Table 6). The higher the contact angle, the more difficult it is to wet the surface with a subsequently applied coating layer (Figure 7).
It can be seen that the silicone does not affect the wettability; hence, the use of silicone hybrid resins in primers does not lead to any problems.
Silicone units in the polymer do not affect adhesion. Primers were formulated with silicone hybrid resins, applied and subsequently overcoated with conventional top coats. Additionally, coatings formulated with silicone hybrid resins were overcoated using the same paint. Adhesion tests, carried out to DIN EN ISO 20567, gave a characteristic value of GT 0-1 in all cases, as with comparable binders (Figure 8).
The impact strength of the new silicone hybrid resins is clearly shown in the Impact Test - DIN EN ISO 6272-1/2. Figure 9 shows the results of a 1 kg weight falling from a height of 1 m onto the coated specimen. The upper deformation is the result of the reverse impact test.
This property of silicone hybrid resins leads to good deformability and adhesion of the coating on different substrates.
The compatibility of silicone hybrid resins in current binders is also very good and well balanced. Compatibility with various resins is shown in Table 7.
It should be noted that silicone hybrid resins have very good pigment wetting properties both in tinting with pigment pastes and in direct grinding with mixed pigmentation. This property promotes the corrosion resistance of the formulations. However, wetting and dispersing additives are necessary to formulate high-gloss paints with the lowest haze. In artificial weathering to ISO 4892-2 both silicone hybrid resins show color change ΔE < 1.0 after 6000 hours which permits good long-term color stability of the paint.
The new silicone hybrid resins are particularly suitable in topcoat applications for corrosion protection covering a range of uses from transportation to marine applications. Coats up to 200 μm can be applied in one step. Their combination of good adhesion and flexibility makes them suitable for coating plastics and for coil coatings.
In addition to excellent corrosion protection, minimal tendency to yellowing and weathering, chemical- and mechanical-resistance, silicone hybrid resins offer exceptional benefits in processability and optical appearance. An overview of the silicone hybrid resin properties is presented in Table 8. With silicone hybrid resins having a non-volatile content of 90% and concurrent low intrinsic viscosity, formulation of paints with extremely low VOC content (200 – 100 g/L) is possible. Additionally, the silicone hybrid resins enable simple handling during paint manufacture and application making these materials user-friendly. Primers, topcoats and direct-to-metal (DTM) paints can all be formulated with silicone hybrid resins.