Getting the right surface properties has never been more important

4 min read

The use of composites has ballooned in the last few years and the material’s exponential rise shows no sign of slowing down. The lightweight but strong material is providing a viable solution for many transport industries, which are seeking to reduce emissions with minimum compromise.

However, while composites provide a solution to the overarching problem of fuel efficiency, it creates more difficulties in the practicality of its application. The problem with composites is that part production is labour intensive and the process is not easily automated.

Most advanced materials, like carbon fibre composites, need some form of surface treatment to ensure good adhesion with paints, lacquers and adhesives. A major worry for automotive OEMs is delamination of coatings and paints that result in a product having limited cosmetic value. Consumers often use the paint work of a vehicle as a marker of overall car quality. So, if the paint work is poor, people assume the rest of the car is engineered to the same low standard.

However, many options suffer from limitations, for example hand sanding is highly labour intensive, time consuming and generates a significant amount of dust. It is a tough trade-off for engineers to contend with.

One UK start-up is determined to change this, however, and has recently been spun-out from research in to this very problem. Oxford Advanced Surfaces has developed a range of surface treatments to prepare advanced materials for coating and bonding. Based on its patented reactive chemistry platform, Onto has the ability to form a bond with most materials, making it useful as an adhesion promoter.

Failure to properly treat surfaces can cause delamination and cracking

The attraction of Onto is particularly useful in the automotive industry. Here, the slower transition to composites and other advanced materials has meant multi-material assemblies are much more prevalent, and enabling good adhesion between many dissimilar materials is vital. It is also vital when painting, as different materials on the same assembly might react differently to coatings and paints. To have the ability to alter the surface chemistry and make the surface properties uniform has therefore become essential in getting a good paint finish.

Onto is made from a water/alcohol base and unlike most adhesion promoters for plastics they contain no aromatic hydrocarbon solvents and are suitable for a wide range of plastics, reinforced composites, metals and other advanced materials.

Surface preparation is not only important for applying the finishing coatings like paintwork to composites, it is also fundamental to ensuring good adhesion between different materials. Onto allows fast and convenient substrate priming and is effective at being used to improve the adhesive bond between various plastics, metals and composites. The improvement to this bond reduces the risk of delamination at weaker interfaces, leading to improvements to bonded assemblies and a reduction of mechanical fasteners.

Onto provides adhesion promotion uniformly across a coated substrate and does away with sanding, meaning no dust is generated during production and assembly and the bulk properties of the substrate remain unaffected.

The Onto treatment has a long shelf life, and is reportedly quicker and easier than other common processes. Its formulations can also be tailored for conventional coating techniques such as spray, dip and even brush coating.

Plasma Treatment

Surface adhesion is not just an issue with composites and the automotive industry. Those using polymers elsewhere will find that printing inks on the surface might easily rub off, gaskets won’t bond and coating will not adhere. It is common that no matter how much cleaning or abrasion of the surface, certain materials remain difficult or often impossible to paint, coat, print or bond to without using harsh solvent based primers or high temperature flame torch treatments.

A common option currently used by many in industry for achieving good adhesion levels of paints, coatings, sealants and glues to traditionally ‘non-stick’ plastics comes from plasma treatment equipment supplier, Dyne Technology. The Staffs based firm has a great deal of experience in achieving strong adhesion to what many consider ‘non-stick’ materials such as Polypropylene (PP), Polyethylene (PE), High-Density Polyethylene (HDPE) and EPDM, all of which have a low surface energy.

There are well known methods of increasing the surface energy of plastics and rubbers that include hard, environmentally damaging wet chemical treatments, high temperature flame torch treatment, high voltage corona discharge and the plasma treatment of plastics.

Plasma Treating plastics is also referred to as Plasma Surface Activation, and is a highly effective, long lasting method of increasing the surface energy of plastics and rubbers, such as PP and PE.

During Plasma Surface Activation, the component undergoes an environmentally friendly process, which does not alter the bulk properties to the treated part and does not mark, discolour or damage the component in any way.

While many might want to overlook surface treatment, it is a vital step if the designed quality finish is to be achieved and the integrity of joints is to meet the specified standards.

Loctite chosen for arduous, sub-zero conditions

The Prinroth Beast is built for supreme performance in the harshest of environments. Powered by a 12.5L Caterpillar C13 Acert turbo diesel engine, the vehicle is responsible for grooming mountain slopes for rejuvenated skiers the following day.

Operating in temperatures around -30°C and on slopes that can be over 45° steep, The Beast relies extensively on Henkel’s Loctite engineering adhesives to secure components and keep vital hydraulic components sealed.

For sealing hoses Prinroth uses Loctite 572 to create an instant seal that cures to burst strength. Loctite 638 is also used, typically to retain ice studs and aluminium track blades. As well as being able to cope with pressures up to 550bar, the product has to remain unaffected by the freezing temperatures.

Loctite 638 provides resistance to dynamic, axial and radial loads and is one of Henkel’s high strength products, capable of carrying high loads and filling all voids to prevent corrosion and fretting.

Loctite 243 is the predominant thread locking product used to prevent self-loosening and secure any threaded fasteners against vibration and shock loads. A typical application on The Beast is on M20 bolts tightened to 365Nm to secure the track-drive wheel.