Adhesives and sealants
Typically 1wt.% based on the formulation is a good initial starting point. However, you need to optimize that through further testing.
Typically amino- and mercapto-functional silanes work best on metals.
Building protection and water repellents
If water-repellent comes into contact with a glass surface, what should we do? In general it is recommended to avoid contact between Dynasylan®, Protectosil® CHEM-TRETE®, Protectosil® AQUA-TRETE®, and products and glass surfaces. Especially Protectosil® CHEM-TRETE® BSM, Protectosil® CHEM-TRETE® 40 D, Protectosil® CHEM-TRETE® PB VOC, Protectosil® WS 405, Protectosil® WS 700 P, Protectosil ANTIGRAFFITI®, Dynasylan® 100 W, and Dynasylan® 100 NK will leave residue on windows and must be removed immediately if contact occurs. If a silane-based water repellent should contact a glass surface, remove it as soon as possible with a solvent. Water-based emulsions such as Protectosil® AQUA-TRETE® Emulsion ESM, Protectosil® AQUA-TRETE® 20, Protectosil® AQUA-TRETE® 40, Protectosil® WS 405, Protectosil® WS 700 P and Protectosil ANTIGRAFFITI® can usually be removed from windows with standard window-cleaning products, if done promptly after application. Tar and/or bug remover designed for automobiles may be used to remove overspray from vehicles.
Filler and pigment treatment
Typically the neat silane is sprayed on an agitated filler in a mixer. Temperature, surface moisture, by-products, and dwell times need to be monitored carefully.
Usually 1wt.% based on the filler is used for fillers with a surface area of less than 20 m2/g. Higher surface area fillers require a higher silane dosage.
Silanes require active sites, preferably hydroxyl groups, on the filler surface to react with. Therefore all silicate-type fillers, inorganic metal oxides and hydroxides can be surface treated.
Wire and cable
It is possible to crosslink mineral filled thermoplastic compounds (e.g. halogen free flame retardant cable insulation) with silanes. One of the biggest challenges here is the chemical modification of such compounds by means of peroxide/vinyl silane mixtures without initiating any pre-scorch. The type of compound showes a strong influence in crosslinking quality.
Peroxide and e-beam crosslinking are carried out by initiation of PE-radicals and their recombination to form stable C-C- bonds. In case of peroxide crosslinking, a suitable peroxide is compounded into the polymer during the production step. The process temperature must be kept carefully below the specific decomposition temperature of the peroxide used. The finished compounds are then crosslinked with heat (mostly by means of steam equipment) directly after the shaping step. When using e-beam technology, crosslinking of the polymer chains occur under the influence of high-energy beams. The main difference between peroxide and e-beam crosslinking is the need for expensive e-beam equipment to initiate radicals. The crucial difference of silane technology compared to the peroxide/e-beam process is, that the polymer is chemically modified in the first step. Generally, this reaction is carried out by grafting a vinylsilane onto the polymer chain with smaller amounts of peroxide. After the grafting step, the polymer is still thermoplastic and can be used in multiple ways. The crosslinking itself always takes place outside the extruder, and is initiated by water (water bath, steam chamber or ambient conditions). As a summary, silane technology has proven to be the most economic way to crosslink PE as well as providing the additional benefits of adhesion promotion to inorganic fillers. So it can be described as a highly attractive and multidimensional solution for the challenges within the wire & cable and pipe industries.
Besides the grafting level, other significant parameters are: the catalyst (type/amount) and the crosslinking environment (humidity, temperature). Water is essential for the final crosslinking step. The water molecules have to penetrate the polymer and must inititate the hydrolysis reaction which is followed by condensation of silanol species (crosslinking through chemically stable Si-O-Si bonds). The highest possible amount of water molecules on the polymer surface in combination with high temperatures are an optimal solution for the penetration of water and the crosslinking reaction. As a logical result, the most effective and fastest way is the use of a waterbath (highest possible amount of water molecules) at elevated temperatures (80-90°C, or ambient pressure). By using higher pressure the speed can be increased significantly. However, this method needs some extra investment. Another option is using steam. This way is slower than the water bath system (higher temperatures but fewer water molecules). Crosslinking under ambient conditions is the most cost effective process with regards to equipment . In this case, the amount of water (e.g. at 23°C/ 50% rel. Humidity) is significantly lower compared to a water bath. As the temperatures are lowest, too, it is definitely the slowest way to cure the polymer product. It is also possible to enhance the crosslinking speed by increasing the amount of catalyst. But this philosophy showes some limitations due to undesired pre-crosslinking reactions during the process. One recent success story from Degussa is Dynasylan® SILFIN 63 that meets this challenge. Using this new product, a significant increase in crosslinking speed (especially under ambient conditions) is achievable compared to standard silane mixtures.
Which polymers might be chemically modified using silanes ? PE (polyethylene) as well as PE-based copolymers (e.g. EVA) are the most commonly used polymer grades for silane crosslinking technology. When using peroxide, the main question is the sensitivity of the polymer towards free radicals. Once formed, these can either recombine, add vinyl products, or even degrade. For this reason, a polymer such as PP isn‘t suitable for this technique. Depending on the polymer structure a wide variety of Dynasylan® products may be suitable for a chemical modification.
Through crosslinking PE, the thermodimensional stability of this base material can be significantly increased. This enables the use of PE-X products at higher temperatures, exceeding the temperature limitations of their original thermoplastic counterparts. In end applications like underfloor heating pipes or power cables the polymer is permanently/often confronted with elevated temperatures. The standard PE grades show limitations because of their thermoplastic nature. A temporary temperature increase would lead to a more or less complete failure.