2010-10-18

More than "just" attractive surfaces

 

The surface finishing of molded parts made of polyurethane (PUR) doesn't just serve aesthetic purposes, but must also fulfill functional requirements such as mechanical, chemical resistance, printability and/or electrical conductivity.

 

In today’s molded devices, the technical function and the aesthetic design both play a significant role.  This applies not only for goods from the leisure and entertainment sectors, but also industrial, medical and other commercial applications. This key fact holds true regardless of whether the part is in a dialysis machine,  a fork lift truck or an industrial pump housing, the Paint of that plastic design plays a  decisive role in the success or failure of that product in the marketplace. 

 

For this reason, the material polyurethane (PUR) is being increasingly used, since it offers product designers and mechanical engineers diverse design options at comparably low production cost. Thanks to the favorable material properties of PUR, complex shapes can be realized with relatively low constructive expenditure so that attractive Paints and functional part requirements can be easily combined. The molded surface finish prior to paint that PUR provides plays an important role in obtaining an excellent finish.  With today's coating systems, not only aesthetic but also various functional requirements can be realized such as:


 

- Paint effect (color, gloss, effect image)
- mechanical resistance (abrasion, impacts, deformation)
- chemical resistance (bases, acids, solvents, hydraulic oils, cooling agents)
- climatic resistance (inner/outer room climate, outside air climate)
- special requirements (e.g. printable in pad or screen printing, can be welded over or glued)

 

Most of these properties can already be fulfilled with standard coating systems. Like the material PUR itself, the painting can be adapted exactly to the customer requirements or be adjusted to the intended purpose by "mixing" the desired properties into the paint. Of course, in the selection of the optimal coating system the composition of the substrate and certain constraints of the coating process must be taken into account. The most important PUR coating modes and their usage options are presented in the following section using the example of some molded parts and housings made of PUR, which were produced and finished in recent years at the factory of the plastic manufacturer Thieme in Teningen in the Baden region.

 

 

Micro-effect Paint: high abrasion resistance and no glaring effect
The high abrasion resistance and chemical resistance make this paint an ideal coating for stressed surfaces of high-quality consumption and investment goods, be it beverage dispensers, screen devices, boiler paneling or electrical cabinets. By reason of its weather resistance, the paint  can also be easily used outdoors (fig. 1). A further important advantage of the micro-effect paint, particularly in the industrial environment, is its absolute no glaring effect.

 

Not only the functionality, but rather also the Paint impression of the micro-effect paint can be varied according to customer demands: By mixing fine glass pearls into the paint, a self-structuring fine pearl effect is created, similar to an eroded surface. Depending on number and size of the pearls, the effect turns out finer or rougher.

 


Fig. 1: For the finishing of the electrical cabinet housing of a small sewage plant, Thieme recommended the usage of a micro-effect paint, since the customer required mechanical abrasion resistance (cleaning with high pressure cleaner) and weather resistance (usage outdoors) in addition to high-quality Paint appearance.

 

 

Structure Paint: diverse combination options of color, degree of gloss and structure
This coating mode offers diverse combination options of color, degree of gloss and structure effects and is thus particularly suited for decorative surfaces which must at the same time be robust and easy to care for. They are often used in industry, such as for measuring devices, medical devices or machine paneling (fig. 2). An important advantage of this paint is its high mechanical and chemical resistance so that the finished surfaces can be easily cleaned including aggressive agents. Structure coatings created in contrast to micro-effect paints, rougher structured surfaces - one also speaks of scars - or pearl structure effects, which is achieved at the end of the painting process (spraying) by means of rough color application. Thanks the distinct effect image of the paint which is created in this way, even slight unevenness can be covered, so that fewer scraping and sanding processes are required in the preparation of the substrate.

 


Fig. 2: The ergonomic control panel of a road finisher received a finish in the Thieme coating plant with a non-slip, abrasion resistant and contamination resistant structure coating, which is suitable for the daily "hard" usage at construction sites.

 

 

Smooth Paint: Visually high-quality impression
If the visual appearance is in the forefront, then the usage of smooth paint can be the best choice: This makes a very high-quality impression and at the same time emphasizes the design of the product. In addition to the "glossy" visual effect, the simple cleaning option of the smooth and chemically resistant surfaces is a further criterion for the usage of these coatings. Smooth paint is available in the designs semi-gloss and glossy and can be used in such various applications such as factory floor conveyer vehicles, solariums or medical devices (fig. 3).

 

 

Fig. 3: Thanks to the high gloss effect the paneling of the solarium makes a very high-quality appearance. In addition to the elegant visual appearance desired by Thieme customers, the easy cleaning of the smooth surface was a further criterion for the choice of a smooth coating.

 

 

Copper conductive paint: electrically shielding
For this coating type the function is the focus. By reason of its electrical conductivity, copper acrylate paint exhibits a high degree of electro-magnetic absorption across a wide frequency range and is therefore frequently used for measurement and medical devices in order to fulfill the specifications of the electromagnetic compatibility. In contrast to the other described coating modes, the copper conductive paint is applied to the inside of the PUR shaped parts. A precondition for sufficient electrical shielding is a certain minimum layer thickness of the coating film; furthermore contact points must be provided for multi-part PUR shaped parts.

 


Fig. 4: The dialysis device was coated with a chemically high-resistant structure paint on the outside in order to fulfill the requirements in the hospital. In the interior a copper conductive Paint was used for EMC shielding. At Thieme's suggestion water-based coatings were processed in both cases.

 

 

Printing and gluing: secure adhesion is guaranteed
Nearly all coating systems, from the structure to the smooth paint, can be printed or glued with suitable procedures. Special one- and two-component printing inks are used for the printing whereby the two-component inks exhibit higher resistance. The screen printing (for flat surfaces) and the pad printing (for slightly uneven surfaces) have proven themselves as printing processes.
If gluing of the molded PUR part is planned, then a coating structure should be selected which is not too rough, since this could affect the adhesion; when gluing with thin foils one should completely do without a structure of the coating. Like printing inks, adhesives are also available as one- and two-component systems, whereby the two-component adhesives exhibit better adhesive quality.

 

 

Environmentally friendly water-based painting
In addition to the desired visual and functional properties of the surface finishing, when coating molded PUR parts the increasing demands of environmental compatibility must be considered. Since 2007, the European VOC ordinance has applied, whereby the content of organic connections (VOC = Volatile Organic Compounds) must be restricted in paints. The water-based coating systems available today fulfill all specifications of the VOC ordinance and do not fall behind the conventional solvent-containing coatings in any way regarding the optical and functional properties. In order to conform to the legal provisions, Thieme has already in 2006 commissioned a fully automatic coating plant, which is optimized for the preparation of water-based coating systems.

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THIEME Corporation
3605 Swenson Avenue
St. Charles, IL 60174

Phone +1-630-513-1666
Fax     +1-630-513-1999
info@thieme-us.com