Author: Georg Schubert, business director automotive adhesives within the transport and metal business, Henkel Adhesive Technologies Liquid gasketing sealants have been used for many years in automotive engineering as an alternative to solid gaskets. For instance, gasketing products suitable for flange sealing applications are deployed in engine construction and in the drive train generally. In addition to anaerobic-curing acrylates for high-rigidity flanges, RTV silicones and – a new development – polyacrylates that likewise cure at room temperature for sealing flexible flange connections are available. Nowadays products have to satisfy both performance specifications and increasingly sustainability criteria as well. [caption id="attachment_27746" align="alignright" width="239"]Fig. 1: Reliable gasketing systems are key to new engine development which today is geared not only to generating high performance with reduced fuel consumption and low CO2 emissions, but also to ever more compact dimensions Fig. 1: Reliable gasketing systems are key to new engine development which today is geared not only to generating high performance with reduced fuel consumption and low CO2 emissions, but also to ever more compact dimensions[/caption] The purpose of all sealing systems in the automotive industry is to prevent leakage and thus avoid the dreaded 'oil patch under the car'. The sealing process is performed either by material bonding or by compression (non-positive joint) using a flexible solid gasket and an appropriate bolting system. A major trend in engine manufacture is the use of gasketing systems in engines that involve adhesive bonding to create an integral joint (fig. 1). In combination with a tailored metering and application system, Henkel is able to offer an automated process that is also suitable for large production quantities (fig. 2).

Sealing rigid flange connections

[caption id="attachment_27749" align="alignright" width="244"]Fig. 2: Automated application of liquid gasketing sealants Fig. 2: Automated application of liquid gasketing sealants[/caption] The ideal solution for rigid flange connections is an anaerobic acrylate gasketing compound. This single-component sealant based on dimethacrylate ester is particularly suitable for torsionally stiff metal flange assemblies. Curing takes place in the absence of air and thus following the mating of the two flange faces. Because of its reaction mechanism the material is only suitable for metallic flanges. In addition, the brittleness of this high-strength solution has so far limited its use in more flexible constructions. Based on an innovative formulation, a flexible anaerobic acrylate adhesive has now been developed. The new flange gasketing sealant (Loctite 5189) exhibits significantly improved elastic properties – particularly after temperature aging – making it suitable for less rigid joints and connections (fig. 3 and 4). [caption id="attachment_27751" align="alignright" width="300"]Fig. 3: Elastic properties of Loctite 5189 Fig. 3: Elastic properties of Loctite 5189[/caption] This solution is already being applied in various Ford engines. For example, Loctite 5189 is used to seal the cam caps of the 1.0 litre EcoBoost engine, and also highly stressed joints in the 1.6 litre version from this OEM. The 125 PS (123 BHP/91 kW) single-litre engine, which is installed in the Ford models Focus and Fiesta (among others), was also the overall winner of the 2013 International Engine of the Year Award.

Silicones for flexible flanges

[caption id="attachment_27753" align="alignright" width="300"]Fig. 4: Elongation properties of Loctite 5189 Fig. 4: Elongation properties of Loctite 5189[/caption] Room temperature vulcanising (RTV) silicone sealants are also used in engine construction as an alternative to anaerobic-curing acrylate gasketing compounds. This silicone group is characterised by high temperature and good chemical resistance. The high elasticity of silicone means that it is suitable for sealing flexible flanges subjected to large relative movement, and also surfaces exhibiting significant roughness or gaps. In addition, silicones adhere well to different plastic surfaces which, due to the increasing trend towards lightweight technologies, are being encountered more frequently in place of their metal equivalents. With many hot oil-resistant RTV silicones, the curing process gives rise to the chemical compound methyl ethyl ketoxime (also known as MEKO) which is suspected of being hazardous to health. [caption id="attachment_27756" align="alignright" width="300"]Fig. 5: Stressed joints of the Ford 1.0 liter and 1.6 liter EcoBoost engines are sealed with the anaerobic FIP flange gasketing compound Loctite 5189 Fig. 5: Stressed joints of the Ford 1.0 liter and 1.6 liter EcoBoost engines are sealed with the anaerobic FIP flange gasketing compound Loctite 5189[/caption] This led to the development of the first oxime-free RTV silicone (Loctite 5970). This product has no risk classification whatsoever (white material safety data sheet) and fulfills the ambition in sustainable development, occupational health and safety. While these oxime-free silicones are already in widespread use right across Europe, the switch-over in America and Asia is still in its infancy. Here too, however, manufacturers are beginning to turn their back on oxime-containing solutions.

Sealing solution to aggressive chemicals

In addition to anaerobic gasketing sealant systems and silicones, there is even a third, advanced technology: liquid polyacrylates. The new RTV polyacrylate Loctite 5810, which is very similar in appearance and in application to its silicone counterparts, can be regarded as a better form of silicone because it also exhibits higher resistance to aggressive, synthetic transmission and hydraulic oils and is free of silicones and oximes. [caption id="attachment_27758" align="alignright" width="300"]Fig. 6: Permeation of Loctite 5810 compared to silicone Fig. 6: Permeation of Loctite 5810 compared to silicone[/caption] With its chemical advantages over silicones, RTV polyacrylates thus open up new possibilities for engine construction. Unlike silicone-based gasketing sealants, which over time break down the foam-inhibiting additives in engine oils, polyacrylate-based sealing applications exhibit a neutral behaviour, leaving the properties of the additives unimpaired. In addition to outstanding oil resistance, polyacrylate also exhibits a significantly lower degree of permeation to hydrocarbons (fig. 6). This means that gaskets and seals in the engine intake area can be made significantly more gas-tight, thus supporting manufacturers in the achievement of their own sustainability targets. The advent of polyacrylate gasketing sealants, which are applied in liquid form, also opens up new areas of application in transmission unit manufacturing because liquid silicones are not sufficiently resistant to the new synthetic transmission oils. Until now, therefore, expensive solid gaskets have had to be used.

A glance into the future

The pressure in automotive engineering to meet general emission targets is also very apparent in engine manufacturing. More and more metal drive train components are being replaced by lightweight solutions involving plastics. In addition, new synthetic low-friction oils enable a further increase in engine fuel efficiency. One of the side effects of this is that demands being placed on the associated gasketing sealant systems are increasing. In order to support and drive these trends, it requires the continuous development of new liquid gasketing materials to fulfill even future sustainability efforts.