SOLVENT-BASED, SCREEN-PRINTING INKS WITH ULTRA-SHORT CURING PROPERTIES

Solvent-based, screen-printing inks are used for many decorative and industrial printing applications worldwide. Solvent-based, two-component, screen-printing inks offer excellent chemical and mechanical resistances and, in many cases, even outdoor stability.

The challenge with two-component, screen-printing inks is the long curing time for the completion of the chemical reaction. This results in the final properties of the ink film. When drying printed or decorated components with jet dryers or drying cabinets, a rest period of up to 24 hours is often necessary before the next step. This varies depending on the drying parameters and the respective ink system. 

“Proell has redesigned the curing and drying behaviour of the NoriPUR ink system”

From a chemical perspective, a two-component reaction will theoretically continue until all functional groups have reacted. This reaction can take place several days after drying – referred to as ‘post-curing’.

To speed up process chains, Proell has redesigned the curing and drying behaviour of the NoriPUR ink system. With Hardener 047 – newly released in 2024 – it is now possible to cure and dry the NoriPUR ink system with NIR technology within seconds.

NIR drying is suitable for curing selected screen- and pad-printing inks, coatings and printing inks on a wide range of substrates. Both physical drying and chemical curing can be strongly accelerated. NIR refers to the region of the electromagnetic spectrum that extends beyond the visible range to longer wavelengths. It represents the high-energy region of the infrared (IR) spectrum.

This range of IR light extends from 780–3,000nm and comprises the spectral ranges IR-A (780–1400nm) and IR-B (1,400–3,000nm). In this NIR range, the wavelength of the electromagnetic radiation achieves a very high energy density. This density enables the curing and drying of special screen-printing inks at impressive speeds. 

Due to the short wavelength and the resulting high energy density, NIR radiation can penetrate very deeply into the printed film. Adequate drying is particularly important in the interface area to achieve good adhesion.

NIR drying leads to very effective and uniform solvent evaporation. This allows the coating to dry and cure evenly and quickly. However, not every ink system responds equally well to NIR technology.

“NIR radiation can penetrate very deeply into the printed film”

With conventional thermal-forced drying – hot-air technologies – many ink systems absorb a large proportion of the energy at the surface. As a result, these systems emit heat via a gradient towards the substrate. In contrast, NIR penetrates deep into the printed film and causes a direct, sharp rise in temperature in the ink film.

When using the NIR process, drying takes place simultaneously and axially through the entire layer thickness. Problems such as skin and blister formation – or pinholes caused by the solvents – rarely occur and can be avoided by controlling the parameters.

NIR curing enables extremely fast further processing of suitable solvent-based two-component ink systems. By using NIR technology to cure screen-printing inks, mechanical and chemical resistance can be achieved within a very short time (2–12 seconds in the case of NoriPUR).

This versatile ink system can be used as a one- or two-component ink for printing on a wide range of materials. These include PVC, pre-treated polyester, acrylic, polycarbonate, wood and metals.

Processed as a two-component ink, NoriPUR has excellent chemical and mechanical resistances. In combination with Hardener 047, a nearly complete curing with NIR technology is possible within a few seconds. Immediate, problem-free further processing is achievable. Ultra-fast drying can be attained with all colour shades in the NoriPUR series.

The NoriPUR basic colours and highly opaque colour shades passed the toy standard test (DIN EN 71, part 3). A mixture of NoriPUR 945 and 948 has passed the USP Class VI standard by the Toxikon, Inc test.

Instead of a conventional hardener, NoriPUR inks can be processed with an NIR hardener (Hardener 047) that can be activated. This is specifically adapted to the NIR-drying process.

The use of this NIR hardener significantly accelerates the drying of the two-component ink layers. The hardener can be used with all NoriPUR colour shades.

This accelerated curing/drying can be advantageous for decorating front panels (control panels for white goods) and beverage crates, as well as metals and many other substrates. As immediate further processing is possible, this is particularly suitable for bulk goods.

In a world full of dynamic just-in-time processes, NoriPUR – processed with Hardener 047 – makes it possible to complete the curing process in just a few seconds. The enormous time advantage over conventional drying is impressive and opens new fields of application in screen printing. Proell has successfully tested this concept with the NoriPUR ink system to ensure that customers can apply even faster and more efficient process chains.

“Ultra-fast drying can be attained with all colour shades in the NoriPUR series”

Highly opaque white inks – such as NoriPUR 944/013 – have been specially created for printing onto beverage crates. Even these screen-printing inks, applied in thick layers, can be cured within a few seconds thanks to the targeted and intensive NIR drying process. Subsequent storage of the voluminous beverage crates is therefore completely unnecessary. 

Polyethylene (PE) or polypropylene (PP) – for example PE-HD – are difficult printing substrates. Therefore, flame treatment of the surface is essential for good ink adhesion. This is very common in industrial practice. However, a different pre-treatment can also be used. Untreated PP-HD has a surface-free energy of 32.6mN/mm. The surface treatment achieves a value of 41.5mN/mm. A surface energy above 40mN/mm often results in good wettability and good adhesion of the ink and thus higher mechanical resistance. The surface finish also plays a role. In industrial applications, flame pre-treatment is usually carried out inline and automatically, immediately before printing.

“The enormous time advantage over conventional drying is impressive”

NoriPUR 945 (standard white) and NoriPUR 944/013 (white, highly opaque) with 10% Hardener 047 were printed (single print) with a 77-55 screen fabric on a flame pre-treated PE-HD beverage crate.

In one example, the ink layers were cured with a total NIR irradiation time of 12 seconds. Drying was carried out with an NIR module on a conveyor belt for demonstration purposes. The combination with hot air also optimises the evaporation of solvents. It should be noted that white colour shades require the longest exposure time. A series of radiation cycles were tested. The samples were hand-dry within seconds. The irradiation time must be adapted to the colour shade and can also be controlled via intensity.

The subsequent adhesion and scratch tests were even passed immediately after irradiation (see Table 1). The adhesion test was carried out with a cross-cutter and a Tesa tear-off test seconds after drying. The scratch-resistance test was carried out manually. Necessary parameters vary depending on the NIR device and must be determined individually.

The printed ink layers showed excellent chemical resistance to alcohol, alkalis, water and other common test substances. All customer requirements were met and further processing of the crates was possible within a very short time. A major advantage of the fast NIR-curing process is that there is no need to dry/store the crates in large halls. The crates can be further processed within minutes and can be stored outdoors due to the weather resistance of the printed NoriPUR film.

“The NoriPUR ink system is ideally suited for the application of NIR-drying technology"

It has been shown that the NoriPUR ink system is ideally suited for the application of NIR-drying technology. By using the specially developed Hardener 047, a drying time of just a few seconds can be achieved. As a result, longer waiting times are eliminated and process chains are effectively optimised. Due to the accelerated drying and curing, no negative effects on the ink surface and substrates are obtained. Proell has shown that special, two-component, screen-printing systems – such as NoriPUR – can be cured extremely quickly.

NoriPUR is a registered Trademark of Proell