Efficient and Sustainable Pre-Treatment Before Painting. Ideal Preparation for Coating

Whether metal or plastic products are involved – pre-treatment is the basis for high quality painting. Consequently, industry is working on solutions from both a material and a system standpoint, which provide for greater economy and improved sustainability, along with optimised quality.

As a rule, all products which have to be coated require pre-treatment which is matched to the material and the product’s intended purpose. Importance is of course also placed upon maximised process reliability in this respect, because pre-treatment has a decisive effect on the functional and optical characteristics of the painted or coated surface. At the same time, global competition necessitates ever more efficient and sustainable pre-treatment and cleaning.

More Ecological Iron Phosphating

In order to provide metallic surfaces with effective corrosion protection and ideal paint adhesion, they’re subjected to conventional iron phosphating during pre-treatment in numerous industry sectors. This is accomplished by means of spraying, immersion or manual high-pressure application, usually at working temperatures ranging from 40 to 60° C, with degreasing and phosphating taking place in a single step. More recently developed processes are already effective as of a temperature of 30 to 35° C. Furthermore, single-component fluoride-free phosphating is also available in the meantime, which can be used in both spray and immersion applications. This offers ecological as well as economic advantages.

Multi-Metal Compatible, Nano-Ceramic Pre-Treatment Without Sludge

In order to surpass the quality of iron phosphatising with regard to corrosion protection and paint adhesion, zinc phosphatising is used for parts included in high quality industrial goods such as those required for the production of motor vehicles, construction equipment and farming machinery. Despite continuous process optimisation, significant disadvantages remain. These include heavy-metal content, complicated process control and large amounts of sludge.

As a result, the trend is moving towards so-called nano-ceramic pre-treatment processes which can be easily implemented with existing systems technology. This multi-metal compatible alternative to conventional iron and zinc phosphating offers economic, ecological and process engineering advantages. The media, which can be used for spraying, immersion and coil coating, are free of heavy metals which significantly reduces the effort and costs involved with wastewater treatment, disposal, equipment cleaning and system maintenance. Furthermore, conversion coatings can be generated on steel, aluminium and zinc by means of nano-ceramic technology with just a single bath adjustment, whose corrosion protection characteristics are comparable to or exceed those provided by zinc phosphatising. The deposited layers are extremely thin and have a very large surface area. This results in optimised paint adhesion as compared with zinc phosphatising. In the case of steel, the originally grey surface has a gold coloured or blue to light violet appearance after pre-treatment. This results from the fact that the pH value of the steel is shifted due to a slight pickling effect, and because ceramic is deposited onto the surface with layer thicknesses in the nano range. The pre-treatment materials do not contain any nano particles.

On the one hand, a process engineering advantage results from the simpler process sequence as opposed to zinc phosphatising, which ideally includes 5 zones: 1) alkaline degreasing 2) rinsing 3) rinsing with deionised water 4) generation of the nano-ceramic conversion coating at a pH value of 4 to 6 usually within 30 to 120 seconds at room temperature 5) rinsing with ionised water.

The concentration of the pH value in the nano-ceramic bath can be monitored and readjusted either manually or automatically. Monitoring costs are reduced. Consumption of fresh water, chemicals and energy, as well as the amount of wastewater, can be reduced by means of a cascade scrubber for the process water beginning with the last rinse up through alkaline degreasing. Beyond this, there’s no more phosphate sludge to be disposed of.

As is also the case with iron phosphatising, nano-ceramic processes can be set up as single-bath systems with simultaneous degreasing and pre-treatment, or existing systems can be accordingly converted.

Degreasing and Pickling with Renewable Raw Materials

In the meantime, products based on renewable raw materials have also been developed for the pre-treatment of components made of steel, iron and aluminium. Non-ionic tensides (alkyl polyglycosides and amphoteric tensides) can be produced from, for example, starch and glucose syrup on the basis of betaines or sultaines. They can be found in powdery, mildly alkaline and borate-free cleaning agents. Amongst other materials, sugar beet scraps are used as a raw material for the production of environmentally-friendly, non-toxic pickling solutions for aluminium, steel and iron.

Snow for Cleanliness

Parts made of plastics and fibre-reinforced plastics are pre-treated conventionally by means of a power-washing system with an aqueous cleaning agent and a downstream retained water dryer. However, this cost, space and energy intensive variant is being replaced more and more frequently with alternative processes such as CO₂ snow-jet cleaning or plasma processes.

CO₂ snow-jet cleaning has established itself in numerous applications, for example in the automotive industry and the automotive supply sector. On the one hand, this is due to savings amounting to as much as 50% for investment costs, 20% for operating costs and up to 80% for floor space requirements. Since the non-toxic, incombustible snow is made of recycled liquid carbon dioxide, CO₂ snow blasting is an environmentally sound process. Particulates as well as film-like contamination can be gently removed from products made of various plastics and composites in this way.

The liquid carbon dioxide is expanded as it passes through a nozzle and is accelerated with compressed air to ultrasonic speeds. The cleaning effect is based on a combination of mechanical, thermal and chemical characteristics, by means of which the CO₂ snow removes contamination in a dry and residue-free fashion – even from very small gaps. The inline capabilities and minimal space requirements associated with this process allow for direct integration of cleaning into the painting process, thus ruling out the possibility of renewed contamination of the component, for example during transport or storage. A further advantage of the dry process results from increased freedom in designing the respective components because, for example, water retaining geometries no longer carry media over into the painting process.

Cleaning and Activation in a Single Step

Plasma cleaning is also a dry process. Differentiation is made between low-pressure and atmospheric plasmas. In the case of low-pressure plasma, treatment is conducted in sealed chambers in a vacuum. This makes it possible to clean workpieces with complicated shapes as bulk goods or individual parts. The use of a great variety of process gases is also possible, because treatment takes place in an evacuated, sealed space. Direct and indirect corona discharge (dielectric barrier discharge) functions under ambient pressure. With the first variant, the discharge (plasma) strikes the workpiece directly. In the case of indirect atmospheric-pressure plasma, which makes use of so-called plasma heads (nozzles), discharge takes place at the plasma head and is directed to the surface to be processed by means of compressed air. Thanks to simpler systems technology without vacuum components, the investment costs are lower and atmospheric-pressure plasma systems can also be more easily incorporated into automated production lines.

Above all thin organic contamination can be effectively removed with plasmas. In most cases air is used as the process gas for cleaning before painting processes, and the material removal rate increases along with oxygen concentration.

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 the case of both low-pressure as well as atmospheric-pressure plasma, organic contamination is broken down into short, volatile chains, and is oxidised into water and carbon dioxide by means of chemical reaction with the oxygen. At the same time, free ions and electrons react with the surface, thus forming polar groups. Consequently, surface tension is adjusted to an ideal value for the subsequent painting process. Surface tensions of greater than 72 mN/m can be achieved in this way by means of plasma treatment. As a result, the surface becomes highly wettable which assures ideal painting conditions when coating difficult to paint plastics, thus contributing to reduced scrap rates.

PaintExpo – Leading International Trade Fair for Industrial Coating Technology

PaintExpo covers the entire process sequence in the field of coating technology and offers a comprehensive overview of the latest developments from pre-treatment right on up to quality control for liquid painting, powder coating and coil coating. The exhibition programme covers equipment and application technology, paints, drying and cross-linking systems, conveyor systems, automation solutions and painting robots, pre-treatment, measuring and test equipment, quality control, environmental engineering, filtration technology, accessories, consumable materials, services, paint stripping and technical literature. Nearly all renowned suppliers will participate at the leading international trade fair for industrial coating technology. Comprehensive, representative offerings will allow visitors to gather detailed information in a targeted fashion, and to make direct comparisons of various systems and processes at a single location. PaintExpo will take place at the exhibition centre in Karlsruhe, Germany, from the 8^th through the 11^th of April, 2014. www.paintexpo.com

Author: Doris Schulz

Pre-treatment is the basis for high quality coatings.

Image source: Woerwag

Degreasing with products based on renewable raw materials shows identically results compared to conventional degreasers.

Image source: NABU-Oberflächentechnik

After having undergone 1,000 hours of acetic salt-spray testing, the nano-ceramically pre-treated and single-layer powder-coated aluminium plate has not shown any signs of infiltration.

Image source: NABU-Oberflächentechnik

Particulates as well as film-like contamination can be gently removed from products made of various plastics and composites with CO₂ snow-jet cleaning.

Image source: Venjakob

CO₂ snow-jet cleaning can be easily automated and integrated into the painting process.

Image source: acp

Based on PaintExpo information