The challenge of the “asymmetric substrate”
This aspect represents the primary criticality and is directly related to the structure of the membrane, which is not a homogeneous substrate but a system composed of two profoundly different surfaces, designed to perform opposite functions.
- Side A (concrete side): nonwoven polypropylene fleece. The surface, fibrous and irregular, is intrinsically non-printable and is intended for mechanical anchorage to fresh concrete. Any attempt to print on this side would be unacceptable: the ink would penetrate into the fleece with uncontrolled bleeding, the fibers would interfere with the correct operation of the printing press, and the mechanical bonding capacity would be compromised.
- Side B (soil side): smooth multilayer polyolefin sheet. This surface constitutes the actual waterproof barrier and is the only one suitable for printing. From a chemical-physical standpoint, it is a plastic material with low surface energy.
The ideal flexographic solution
Flexography is, by its nature, a single-sided process. The membrane is conveyed between the impression cylinder and the plate cylinder: the former provides rigid and uniform support to the printable polyolefin side, while the fleece side remains protected and does not come into contact with the inking system. Flexography is therefore mechanically and functionally suited to handling asymmetric substrates without compromising the integrity of the non-printable surface.
2. Protection of the hydro-reactive core
The hydro-expansive polymer layer represents the functional core of the membrane. During production, the main critical issue is the risk of premature activation caused by moisture or chemical contamination.
Conventional inks may contain water or volatile organic compounds that, over time, could migrate through the polyolefin layer or introduce substances capable of compromising the long-term performance of the expansive polymer. For this reason, the printing and drying process must be extremely fast and chemically inert.
UV flexography
Inline printing for this application is based almost exclusively on the use of UV-cured flexographic inks.
These are 100% solids formulations, free of water and solvents, composed of monomers, oligomers, and photoinitiators. After deposition on the polyolefin surface, the ink is immediately exposed to high-intensity UV lamps, which trigger rapid crosslinking, transforming it into a hard and chemically stable plastic film within a fraction of a second.
Main advantages:
- Instant curing: complete absence of smearing or ink transfer even at high line speeds.
- No contaminants: the absence of solvents eliminates the risk of migration of volatile substances toward the hydro-reactive core.
- Waterproof print: the cured UV ink forms a solid, non-porous layer that increases the overall robustness of the surface.
3. Precision printing
The membrane is not a standard product, but a highly engineered system. The printed information does not serve a merely aesthetic or branding function, but constitutes essential operating instructions for correct installation.
Requirements:
- Self-adhesive edge: the line “overlap up to this line” must be perfectly aligned with the adhesive strip applied at the factory. Even minimal deviations can generate incorrect joints and potential leakage points.
- Reference grid: the cutting grids used on site must ensure high dimensional accuracy and perfect repeatability over the entire length of the roll.
- Functional instructions: markings such as “this side toward concrete” must be clearly legible and correctly oriented.
Modern flexographic lines, equipped with electronic registration systems and optical sensors, allow continuous control of print position, ensuring accuracy on the order of tenths of a millimeter even at high production speeds. In this way, functional markings become reliable tools to ensure correct and safe installation.
4. A print designed to last
The value of the membrane lies in its high durability, with a service life designed to extend over several decades. Consequently, the printed information must also maintain integrity and legibility during installation and service life.
The printed surface must withstand abrasive stresses, the highly alkaline environment of fresh concrete, and exposure to weathering before and after backfilling. The combination of surface treatment of the substrate by corona discharge and the use of UV-cured inks makes it possible to achieve high and virtually permanent adhesion. The ink forms a crosslinked polymeric film, extremely tenacious and chemically bonded to the polyolefin.
For this specific multicomponent hydro-reactive membrane, flexography is not only the highest-performing solution, but a true enabling technology.
Written by Sergio C. | Team Giugni®
