Pyrosil® Treatment: Flame-Deposited Silicate Adhesion Layer
Pyrosil® treatment makes difficult surfaces bondable by depositing an ultra-thin, highly reactive layer of amorphous silicon dioxide (SiO2), 5 to 100 nanometers thick, by flame-based Combustion Chemical Vapor Deposition (CCVD). The part passes through a laminar gas flame doped with a silicon-containing precursor, much like conventional flame treatment, at atmospheric pressure and inline. Because the flame contact is brief, the process is safe for temperature-sensitive plastics.
The deposited silicate carries a dense population of Si-OH groups and nanoscale roughness, which gives adhesives, inks and coatings a defined hydrophilic surface to bond to. That is why Pyrosil® succeeds where plasma, corona and flame fall short: on PTFE and other fluoropolymers and on silicones, where essentially no other atmospheric method achieves durable adhesion, and on PEEK, PPS and PBT, as well as glass, ceramics and metals. Because the layer does not reorient the way an oxidized polymer surface does, the activation typically stays usable for weeks to months rather than hours or days. SURA Instruments GmbH developed the equipment and processes in the 1980s; Sabreen specifies, trials and implements Pyrosil® on production lines.
For the test data, including water contact angles on PBT, PEEK and PPS compared with atmospheric plasma, see the technical paper Pyrosil® Treatment for Improving Polymer Adhesion. To check whether a treated surface is ready for your ink or adhesive, use the surface energy and dyne level calculator.
Combustion Chemical Vapor Deposition (CCVD) is a chemical process by which thin-film coatings are deposited onto substrates in the open atmosphere. In the CCVD process, a precursor compound is added to the burning gas. The flame is moved closely above the surface to be coated. The high energy within the flame converts the precursors into highly reactive intermediates, which readily react with the substrate, forming a firmly adhering deposit. The microstructure and thickness of the deposited layer are controlled by process parameters including speed of substrate or flame, number of passes, substrate temperature and distance between flame and substrate. Silicon dioxide layers are the most commonly deposited layers. Freshly deposited layers are highly reactive and can thus serve as adhesion promoting layers for polymer coatings and bonding.
PYROSIL Versus Flame Treatment
PYROSIL and traditional flame plasma treatment have process similarities, but the flame chemistry and surface functionality mechanism are different. Both methods are gas-phase surface oxidation processes in which the combustion of a hydrocarbon fuel, under controlled conditions, generates the flame plasma that modifies the substrate surface. Premixed laminar flames produce an exothermic reaction. The PYROSIL flame is pinkish on color while the traditional flame treatment color is bluish in color. Reference Photo 1. The main difference is that PYROSIL produces SiO2 and flame treatment does not. Consequently, a defined hydrophilic surface is built due to the high density of Si–OH groups on the surface of the deposited SiO2 particles. Similar combustion equipment, ribbon burners and process setup/control are utilized. The three main process control variables are flame chemistry, distance of the substrate from the flame and dwell time of treatment. Treatment must take place in the main reaction luminous zone or oxidizing.
For PYROSIL, one equation for the combustion of organosilicon compound tetramethysilane “TMS” and the formation of SiO2 is:
Si(CH3)4 + 13 O2 + C3H8 — SiO2 + 10 H2O + 7 CO2
For natural gas flame treatment, the following equation describes the combustion reaction:
CH4 + 2O2 + 8N2 — CO2 + 2H2O + 8N2
Both combustion reactions produce CO2 and H2O water byproducts.
