Material & Material Properties
Which requirements does the material place on the technology?
The number of adhesives, sealants and casting compounds that can be dispensed is constantly growing. The use of epoxy, polyurethane, silicone or other material types in a process depends on the requirements of the end product. Factors such as viscosity and filler concentration, as well as curing and any necessary pre-treatment, must be taken into account during process development and already predetermine various processes that must be integrated into a comprehensive process engineering concept in terms of system and process technology.
Polyurethanes are highly versatile and can replace silicones or epoxy resins in many applications. Depending on the requirements and application, the properties of polyurethanes can be modified more than with any other material type by means of suitable components and additives. Due to their lower price, they are the most commonly used 2-component material. Polyurethanes are preferred for insulation and corrosion protection of electrical components. Their balanced range of properties and low shrinkage are particularly favorable. Since they are only heat-resistant up to a temperature of around 130°C, PU casting resins are less suitable for applications with high temperature requirements.
Acrylates are potting materials which are characterized by reactive acrylic groups. They are frequently used as 1C materials and are often characterized by a light-induced polymerization reaction. Due to their amorphous structure, some polyacrylates are transparent and are used in this form, for example, as plexiglass (PMMA) or in optical bonding. However, the use of acrylates in the adhesives industry has declined significantly in recent years and is increasingly being replaced by silicones.
Silicone-based resins play an important role due to their excellent electrical insulation properties. Their very good hydrophobic properties and high elasticity make these materials interesting for demanding sealing applications in the automotive industry or lighting technology. Cured silicone casting resins are characterized by their high heat resistance in a temperature range between approx. -40°C to >250°C. For this reason, they are often used when the temperature requirements can no longer be met with PU. Compared to epoxy resins and polyurethanes, however, the use of silicones is associated with additional costs due to their superior properties.
Based on a carbon skeleton, silane-modified polymers are characterized by terminal silane groups, which enable the bonding of plastics with metals by coupling with organofunctional starting groups. The carbon skeleton is mostly based on epoxides or polyethers/polyurethanes. Depending on the organofunctional group, one end of the molecular chain initiates bonding with a plastic, while the other reacts with the surface of a metal by means of a silicone group. The latter takes place at room temperature with the aid of a catalyst and air humidity and is based on the fact that metals are covered on their surface with reactive hydroxide groups. A disadvantage of MS polymers is their sensitivity to humidity. Furthermore, the selection of the correct sealing material must be correctly coordinated due to its chemical reactivity.
These resins are used as versatile construction adhesives, but also for potting and bonding printed circuit boards and especially for encapsulating ignition coils. They offer very high mechanical strength, hardness and abrasion resistance. They also adhere very well to almost all surfaces, have good electrical properties, good chemical resistance, low flammability, high resistance to glow heat and good temperature stability. By using suitable fillers, the usual shrinkage of these resins during hardening can also be minimized. Epoxy resins are more expensive than polyurethanes, for example. When processing them, their usually high viscosity and considerable heat generation during curing must also be taken into account. The latter in particular can lead to malfunctions in sensitive assemblies.
Rheology describes the flow behavior of a fluid at defined shear rates. Due to the shifting of individual molecular chains or particles relative to each other, different flow properties develop for different substances. The shear force and viscosity of a fluid can be determined by means of rheological analyses. Due to different properties in the flowability of substances, these can be categorized into different viscosity classes. Each of these viscosity classes has a certain flow behaviour, which influences the application of a potting material. The viscosity is a measure for the flow resistance, the flowability and the internal friction of a fluid, which can be categorized from low to high viscosity areas. The viscosity also depends on the temperature. The warmer a liquid, the lower its viscosity. (Rule of thumb: +10°C = ½ Viscosity). With certain temperature-curing materials, however, heating has the opposite effect. In the dispensing process, a lower viscosity of the potting material enables faster processing. Furthermore, air bubbles in liquid media rise faster, which simplifies evacuation. With filled, low-viscosity media, however, the sedimentation behaviour is also accelerated. For uniform temperature control, the entire process should be heated.
One-component potting media (1C)
In the case of 1C potting materials, the ready-to-use resin is used directly. It reacts by changing the ambient conditions. This can be done, for example, by increasing the temperature, adding air humidity, removing atmospheric oxygen or making contact with the substrate surface. Consequently, a reaction without a concrete hardener takes place. In certain cases, however, the hardener component is encapsulated in the resin component. Due to strong shear stress, the hardener can be decapsulated and the chemical reaction begins. With 1K materials, a less complex system machinery is required.
Two-component potting media (2C)
With 2C resins, two specific starting materials consisting of different monomers react to form a polymeric product. By optimally mixing both components, the reaction is initialized and can then proceed according to the given specifications. 2C potting materials enable better control of the properties of the end product than would be possible with 1C materials. This is achieved by varying reactive groups of both components and enables a more precise adaptation of the end product to its end function. Other advantages include shorter curing times and a reduction in VOC emissions as well as environmental impact. Today, many increased requirements in the electronics industry can only be met with 2C materials. In addition, they sometimes allow simpler and less expensive material handling.