HIGH-RESISTANCE, INK-JETTABLE, SPECIALITY COATINGS FOR LONG-TERM OUTDOOR EXPOSURE

Momentive Performance Materials is a market leader in the field of speciality coatings for use on plastic substrates. Plastic parts are widely used in highly demanding interior and exterior applications for a variety of industries. These include automotive, architectural or consumer electronics. SilFORT hardcoats help to overcome the inherent weaknesses of plastic substrates. Additionally, the speciality coatings meet high performance criteria, such as high scratch and abrasion resistance, protection against chemical attack, long-term weatherability, excellent optical appearance and easy-to-clean properties. 

In the last issue of 2023, Specialist Printing Worldwide, Momentive Performance Materials introduced the SilFORT UV-curable hardcoat product portfolio. The performance of SilFORT UVHC9000 hardcoat was highlighted. This is a speciality coating which is ink jettable, with a low-gloss coating. It is used mainly in exterior architectural applications to protect an underlying printed décor or image from weathering. Besides UV-curable coatings, Momentive Performance Materials also offers a wide range of heat-curable SilFORT hardcoats. These hardcoats provide best-in-class performance regarding scratch/abrasion resistance and long-term weatherability. 

Today, heat-curable SilFORT hardcoats are used as protective coatings for exterior plastic applications as shown in Figure 1. Especially at the front of the vehicle, polycarbonate is widely used. A protective coating is required to fulfill demanding automotive OEM specifications. The speciality coating technology enables the innovative use of plastics in a variety of automotive applications including forward lighting, exterior pillar and trim or glazing.

A current example of a front-of-car application is shown in Figure 2. The unique grill design of the electric vehicle offers new and exciting possibilities for the implementation of lighting features and/or sensor integration.

“SilFORT hardcoats help to overcome the inherent weaknesses of plastic substrates”

The standard process to apply the hardcoats on plastic parts, in an industrial coating line, is achieved either by spray, flow or dip coating. Each of these methods has its advantages and disadvantages. Spray coating – the most widely used method – offers a flexible coating process independent of the shape and size of the part. In return, a high amount of overspray is created and lowers the efficiency of the coating process.

Digital ink-jet printing is seen as a favourable method to apply not only UV-curing coatings, but also thermally cured hardcoats such as SilFORT PHC587C or AS4700. These speciality coatings establish an efficient and precise technique for automotive and non-automotive applications.

Momentive has over 30 years’ experience in the field of development and optimisation of heat-curable, high-abrasion and scratch-resistant siloxane resin coatings (sol-gel). The chemical principle is shown in Figure 3.

“SilFORT heat-curable hardcoats offer an outstanding, glass-like scratch performance”

SilFORT heat-curable hardcoats offer an outstanding, glass-like scratch performance according to established OEM and Tier 1-specific test methods. The performance in an automotive car-wash simulation (ISO 20566) is shown in Figure 4. Data are determined on black polycarbonate specimens. SilFORT UVHC5000 – a typical Momentive UV-cured headlamp coating – was tested along with heat-cure sol-gel coatings for general exterior use.

As a new application method, ink jetting of Momentive heat-curable coatings was recently introduced to customers. Firstly, printing trials were carried out with Xaar 1002/1003 printheads. The viscosity of the coating fluids was adapted to the printhead-specific parameters (Figure 5). 

Figure 6 summarises the general coating parameters using Xaar printheads with Momentive UV- and thermally cured coatings. Optimal viscosity ranges are similar for both coating grades at temperatures between 20–60°C. 

“Coatings can also be applied by multi-pass jetting technology”

All coating viscosities can be adapted to the optimum values by dilution with suitable solvents. This can also compensate for rheological effects caused by the use of nano-particle as filler, such as in the SilFORT UVHC series. Coatings with higher viscosities or visco-elastic properties can be printed with alternative inkjet printheads applying the Micro- or Jet-Valve technology with droplet sizes >1nl. The latter applies to the SilFORT SHP401, SHP470FT2050 and SHP470, as well as solvent-package adapted SilFORT SHP products.

The process of the ink-jet application of Momentive siloxane resin coatings needs a flash-off phase at ambient or optionally elevated temperatures which can be realised by IR- or convection oven heat as shown in Figure 7. Subsequently, the cure of the coating is carried out between 85–130°C depending on the product.

Coatings can also be applied by multi-pass jetting technology. Gloss can be reduced by utilising matting agents, such as Syloid® ED40 (W.R. Grace & Co), Momentive SilFORT MHC Matt or other silica-matting agents. In this case, clogging of the ink-jet nozzles has to be taken into account or suitable inkjet heads with a nozzle diameter of >20µm must be selected.

Momentive speciality coatings help customers to meet performance and design requirements for a variety of automotive applications. The company’s portfolio includes thermal and UV-cured hardcoats. Besides these, heat-cured, sol-gel coatings can be applied by the ink-jet technology, using commercially available printheads for industrial use. Momentive’s application development teams are ready to support customers with dedicated trials and prototyping in its global, pilot-line facilities. 

SilFORT is a Trademark of Momentive Performance Materials