An integral part of bonding, casting, and painting
Biesterfeld
The industry thrives on further and new developments as well as improved manufacturing techniques. Lightweight construction is the current industry trend that can no longer be overlooked. Sustainability thru a reduction in CO₂ emissions and the longevity of products, as well as resource efficiency thru increased range, combined with product reliability, are at the forefront here. To keep up with these trends, increasingly diverse materials are being combined with each other. For example, today, electric cars already consist of a highly complex mix of various metals, plastics, fiber-reinforced composites, hybrid materials, and other components that need to be connected. In this context, conventional methods such as mechanical fastening or welding quickly reach their limits, not only in electromobility. To still meet the high demands depending on the respective industry, adhesives are increasingly establishing themselves in the market. This is accompanied by further challenges, such as correct execution, reproducibility, and quality control in adhesive technology. In welding technology alone, there exists a multitude of standards (as of 2018: over 60 standards) for welding steel, aluminum, weld seam preparation, occupational safety, and many others, to ensure that the various processes are uniformly defined.
It is therefore not surprising that the already well-known DIN 2304 "Adhesive Technology – Quality Requirements for Adhesive Processes" is now gaining increasing attention in various industries as a standard for the quality-compliant execution of adhesive bonds. First industries have ratified their own standards based on DIN 2304, specifically tailored to the safety classes of bonding, adhesive classes, or the material mix of the materials in the respective application case. In the rail vehicle industry, for example, the DIN 6701 standard series applies, which is content-wise analogous to DIN 2304. Even tho DIN 2304 does not have to be applied by law, many companies are already working according to this standard, organizing their adhesive processes professionally according to the current state of the art, or developing further guidelines with regard to adhesive technology, as is currently the case in shipbuilding. The advantages are obvious: easy reproducibility, higher product safety, and better-organized quality control.
SURFACE PRETREATMENT ESSENTIAL
Even in private life, we are confronted with the topic of gluing from a young age. From craft glue to adhesive bandages, everyone has had their individual, sometimes painful experiences. But why does a plaster, for example, stick excellently to a dry elbow, but not to an oil-smeared finger that got caught while changing a bicycle chain? Decisive for this are, on the one hand, the bonding forces between the joining part and the adhesive, called adhesion, and on the other hand, the internal bonding forces of the adhesive molecules (internal strength), called cohesion. The internal strength of an adhesive is significantly determined by the type of adhesive, the manufacturer's formulation, and environmental influences (such as temperature or humidity), so the properties of an adhesive must always be individually tailored to the application area. Users have a significantly greater influence on the adhesion of an adhesive. For a reliable connection of an assembled component, surface pre-treatment is essential, as factors such as surface roughness, surface tension, and substrate wetting significantly influence the quality. Most of the time, these factors are influenced by foreign contamination. This typically includes contaminants from production aids, release agents, or lubricants. In the electronics industry, rework processes are common, so even a fully cured adhesive on the substrate surface constitutes a contamination that must be completely removed before repairs can begin at that location. Furthermore, the tools and other aids used must also be free from contamination to prevent any negative impact on the adhesive bond. This plays a central role, especially in the field of electronics during potting processes. Furthermore, a surface pre-treatment thru cleaning or activation of the surface is required to consistently achieve the same quality in an adhesive bond. Particularly, the activation of the surface is required when substrates with very low surface energy are to be bonded. If the surface energy of the substrates is significantly lower than that of the adhesives, the adhesive cannot sufficiently wet the substrate, which is a prerequisite for bonding. Such substrates, like PP or PE, are well-known and can be quickly identified using test inks. Values for the surface tension of over 40 mN/m for clean and grease-free substrates have been established here.
INSIGHT INTO THE STATE OF THE ART – WHAT CLEANING AND ACTIVATION TECHNIQUES ARE AVAILABLE?
In fact, users in various industries have a wide range of cleaning agents and techniques available for cleaning and activating the substrate surface. The selection of the appropriate pre-treatment is highly dependent on the substrate or component to be bonded and the adhesive technology used. In practice, two cleaning technologies are leading: plasma cleaning/activation and wet-chemical cleaning/activation. In the plasma process, a plasma is generated under high voltage. Due to the high energy input of the plasma onto the substrate surface, surface contaminants are gently removed on the one hand, and on the other hand, polar molecular groups are embedded into the substrate surface thru the high energy, leading to a significant increase in the surface energy of the substrates. Plasma cleaning sometimes has a higher energy requirement; however, this is offset by easily automatable process techniques and a smaller space requirement. In wet-chemical cleaning and activation, products are used that are suitable for breaking molecular bonds or incorporating functional groups to increase the surface energy into a substrate. However, during the cleaning process, in addition to the surface contaminants that need to be removed, the substrate surface is also attacked. Especially with complex components, such as electronic circuit boards that are to be encapsulated or coated, individual components can be damaged as a result. For this reason, there is a wide range of products available for wet-chemical cleaning: There are cleaning and activation agents available that are precisely tailored to the adhesive and the substrate. These range from nonpolar/polar to aliphatic, neutral, strongly basic, and many other solutions. However, higher disposal costs may sometimes be incurred, or the substrate surface may be attacked.
ECONOMIC REWORK OF ADHESIVES
In the field of electronic assembly, post-processing is common, where highly alkaline cleaners are often used that break the molecular bonds in the polymer but also attack the substrate at the same time. Consequently, the right choice of cleaning agent must allow for the quick and substrate-friendly removal of contamination. Using silicones as an example, DOWSIL™ DS-2025 enables targeted and rapid destruction of the silicone polymer structure thru catalytically active substances and high basicity, ultimately leaving a silicone-free surface without damaging electronic components. A major advantage is that the product can be recycled, significantly reducing disposal costs.
FUTURE-ORIENTED TECHNOLOGY – CLEANING THRU LIFTOFF PROCESSES
For adhesive bonds based on MMA, epoxy, silicone, or PU adhesives, they can only be processed with great effort or with very aggressive media, if at all. The so-called lift-off processes, also known from semiconductor technology, represent a serious alternative. Cleaners that operate on this principle, such as the patented intelligent fluids®, work using Brownian molecular motion and Ostwald ripening. The interplay of both effects ensures a four-step cleaning process: First, innovative high-performance cleaning fluids penetrate the contamination, such as the adhesive, before the contamination is fragmented. Subsequently, the contamination is undermined, creating a bond break between the substrate and the contamination. Due to the physical mode of action of the product, however, it is not possible to break covalent bonds, which shows the limitations of this cleaning principle.
