As OEMs strive to develop cleaner and more efficient vehicles and increasingly turn to lightweight materials, the finishing industry is developing systems to keep the quality up to metal body standards

Today, plastic parts account for 15 to 20% of a car’s weight. Many of these parts, in exterior and interior applications, are given their respective visual, haptic and functional characteristics by means of a paint coating. The painting technology industry is responding to ever-stricter demands with regard to quality, economy and the efficient use of resources with new material and process solutions and exciting technology and application developments.

In order to compensate for increasing vehicle weight due to electric drives and fuel-efficient engine technology, the proportion of lightweight parts fitted to many vehicles will increase from 30% to roughly 70% by 2030; this is the prognosis of a current McKinsey study.

In addition to high-strength steels and light alloys, plastics are also playing an important role in ‘lightweighting’ future vehicle programmes. They often make it possible to manufacture components of minimal weight and/or to produce parts less expensively, or to integrate the functions of several workpieces into a single component.

Adding protection and beauty
Through painting and other surface treatments, plastic parts can be provided with protection against mechanical, chemical and physical influences such as scratches, stone chipping, weathering and UV radiation, as well as resistance to ingredients used in cosmetics and cleaning agents. However, paints and coatings are significantly more than just technical utilities: they create surfaces that are appealing to all of the senses, generate an emotional link to the product and can increase its perceived value. The quality of the coating has to fulfil very strict requirements in order to assure that this alluring effect on the human senses is properly achieved. At the same time, global competitive pressure necessitates more and more economic, sustainable and flexible painting processes. To this end, the painting industry is working continuously on optimising existing solutions and developing new ones.

Faster finishes with fewer steps
The trend towards fewer steps in the painting process with the help of filler-free painting systems is enjoying more and more popularity, for components included in body styling kits made of plastic as well. With good reason: eliminating the intermediate drying step reduces energy consumption by 15 to 20%. At the same time, material consumption and solvent (typically VOC) emissions are also reduced. Beyond this, less production floor space is required for the painting system. Corresponding painting solutions are available for, amongst other components, high-gloss injection moulded interior parts such as trim moulding made of an ABS-PC blend (ABS: acrylonitrile-butadiene-styrene, PC: polycarbonate).

With just a single-coat painting process, paint can provide components with a distinguished, high gloss finish. Even greater potential savings are offered by an innovative 2-component paint system which makes it possible to paint components made of polypropylene (PP) and thermoplastic polyolefins (TPO) without the need for pre-treatment or a primer.

After applying the single coat of paint, the parts demonstrate excellent UV stability, as well as resistance to scratching and chemicals, and sun creams and mosquito repellents. 2-component paint is currently the only paint coating system in the world which fulfils the specifications of the automobile manufacturers for painting interior parts made of PP and TPO without pre-treatment/flaming and primer (it is the only corresponding product in the GM MATSPC list and Toyota’s TSH 3130G, for all classes). It has also been approved for worldwide use by GM in accordance with the new GMW 14867 standard. Toyota has already been using 2-component paints for quite some time for painting interior parts on its Aurion (a Camry-derived saloon sold in Australia, New Zealand and some parts of Asia).

UV technology joins a new clearcoat
UV paints cured under ultraviolet radiation combine ecological and economic advantages: for example shortened processing times and smaller painting systems, as well as reduced emissions and energy consumption. These benefits are augmented by qualitative aspects for the painted surfaces as well, such as higher gloss levels, durability and resistance to scratching.

A solvent-free, 100% UV-curing clear coat has recently been made available for coating plastic interior parts made of ABS (acrylonitrile-butadiene-styrene). This new coating meets the quality standards of various OEMs: Renault, TL 226 Volkswagen and DBL 7384 Daimler.

Corresponding tests have revealed that considerably better resistance to scratching is achieved with this 100% monocure system than with the aqueous or solventcontaining systems, as well as the dual-cure systems, which are currently in use. Curing takes place in an atmosphere with reduced oxygen content, so-called inert gas curing, at room temperature and in a matter of a few seconds. The oxygen reduced atmosphere prevents the radicals, which are necessary for polymerization, reacting with the oxygen in the air and thus the occurrence of so-called oxygen inhibition. This allows for increased curing distances, and areas that are less exposed to the UV radiation are homogeneously hardened.

Solvent still a solution
So far, solvent-based and waterborne paints have been used in most cases for coating plastic parts in the automotive industry. Solvent-based dual-cure UV-systems have also been in use for some years. They allow for good delustering and metallic hues. However, it’s not possible to produce all metallic hues with UV-curing paints. 2-coat systems are available for these applications, which are comprised of a waterborne base coat and a UV-curing clear coat. 100% UV-curing clear coats with piano-lacquer look, as well as 2-component dual-cure systems, have recently been used to an increasing extent for painting plastics, with the former demonstrating significantly greater resistance to chemicals. Good adhesion values are achieved without pretreatment on many common plastic substrates such as ABS, SAN (styrene acrylonitrile), PS (polystyrene) and ABS/ PC (polycarbonate). If the surface does not respond well without a pre-treatment, adhesion characteristics can be often be optimised by activating the surface with the help of a flaming process.

All that glitters is not chrome
Plastic surfaces with a chrome-like appearance are still in great demand. Plastic components are chrome plated by means of either conventional electroplating or PVD coating. Conventional pigmented systems and UV-curing paints are available for protecting the metal layer after the coating process, and for providing it with an individualised appearance.

Perfection in preparation
Cleaning and pre-treatment of plastic components is essential for good painting results, and dictates the quality of finish to an even greater extent than with painted metal surfaces. Traditionally, a power washing system with an aqueous cleaning agent and a downstream retained water dryer are used to achieve the required level of surface integrity. However, this cost, space and energy intensive approach is being replaced more and more frequently with alternative cleaning processes such as CO2 snow-jet cleaning, plasma processes or a newly developed steam cleaning method.

CO2 snow-jet cleaning has established itself in numerous applications in the automotive industry and the automotive supply sector. The non-toxic, non-flammable snow made of recycled carbon dioxide removes particulates and filmlike contamination from exterior and interior parts made of various plastics and composites, and is gentle on the material’s surface. It also makes it possible to reliably remove contamination from very small gaps.

A further advantage of the dry process is increased freedom in designing the respective components because, for example, water retaining geometries can be incorporated (do not have to be designed-out) as they will no longer carry media over into the painting process.

Plasma processes, which are broken down into lowpressure and atmospheric-pressure plasmas, are used primarily for the removal of thin layers of inorganic contamination. The surface is simultaneously cleaned and activated during plasma treatment. This dual function is based on the physical and chemical characteristics of the process. In addition to a clean surface, increased surface tension is achieved in this way as well, thus assuring ideal conditions for the painting process downstream.

Steam heat
Good results obtained with steam cleaning for the removal of many types of contamination such as oil, grease, release agents, dust and fingerprints is based on the interaction of saturated steam with a high-speed air jet. Steam is generated by means of a flow heater: pressurised water flows through a tubing system which is equipped with heater coils, and is heated up to a temperature of between 135 and 280°C, depending on the cleaning task.

The amount of liquid in the steam can also be matched to the cleaning task, as well as the type of contamination. Wet steam is used to remove oil, which changes the viscosity of the oil such that it is atomised into extremely fine droplets. These are then blown off of the surface of the part, along with particulate contamination, by the air jet. The highspeed air jet is also used for drying.