Low-Coat-Weight Spray Technology for Breathable Backsheet Lamination
Breathable backsheet films (microporous polyethylene) are used in hygiene products to allow water vapor to escape while retaining liquids. Laminating these films to nonwoven outer covers requires a hot melt adhesive that does not block the pores. The adhesive must be applied in an open pattern (spiral or dot) at very low coat weight (1-3 gsm). If the coat weight is too high or the pattern too continuous, the breathability (MVTR – moisture vapor transmission rate) drops significantly. Therefore, hygiene hot melt coating machines use specialized spiral spray nozzles that produce a fine, non-woven-like adhesive web. The adhesive is applied to the nonwoven side, then the film is laminated with minimal pressure to avoid squeezing adhesive into the pores.
The spiral spray technology: A nozzle with a central orifice for adhesive and two angled air jets that swirl the adhesive filament. The resulting pattern is a hollow cone or figure-eight that expands into a thin, irregular web. The open area (ratio of uncovered to covered) can be as high as 80-90% at 2 gsm. The adhesive forms small droplets or fine fibers. To achieve this, the adhesive viscosity must be low (500-1500 mPa·s at 150°C). The air pressure (0.5-1.5 bar) and adhesive flow are balanced. The distance from nozzle to web (20-50 mm) affects pattern width (10-150 mm). The nozzle is heated to 140-170°C. The valve cycles intermittently to create patterns that match the product length. The machine may have up to 20 nozzles across the width for full coverage.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Coat weight control at such low levels is challenging. The gear pump must have high precision (e.g., 0.5 cc/rev) and run at low speed. The adhesive supply should be filtered to 100 mesh to prevent nozzle clogging. The line speed (300-500 m/min) means the adhesive deposition time per product is milliseconds. The pump speed is set based on the target coat weight and product geometry. For example, a 400 mm wide product at 400 m/min with 2 gsm coat weight requires a flow rate of about 19 L/h. Small fluctuations cause significant percentage changes. Therefore, many machines use a closed-loop weight control based on a non-contact sensor (infrared absorption), though direct measurement of 2 gsm is difficult. Often, operators rely on gravimetric sampling every 30 minutes.
Breathability testing is essential. The laminated composite is tested for MVTR according to ASTM E96 (inverted cup method). A drop of more than 30% from the uncoated film indicates excessive adhesive coverage. The hot melt coating machine should allow adjustment of pattern density by varying air pressure, nozzle height, or adhesive flow. Increasing air pressure breaks the adhesive into finer fibers, increasing open area but also increasing overspray (waste). Overspray deposits adhesive on machine parts, requiring more frequent cleaning. Some machines use electrostatic assist to guide the fibers toward the web, reducing overspray. The nozzles should be cleaned every 2-4 hours; carbonized adhesive on the nozzle tip disrupts the pattern.
Defects specific to low-coat-weight breathable lamination: “Pinholing” – large open areas without adhesive leading to film-nonwoven detachment. Increase coat weight or adjust pattern. “Adhesive strike-through” – adhesive penetrates the nonwoven and contacts the film, reducing breathability; increase viscosity (lower temperature) or use a denser nonwoven. “Nozzle spitting” – large drops of adhesive causing sticky spots; reduce air pressure or clean nozzle. “Pattern misalignment” relative to product; check encoder and valve timing. The machine should have a strobe light for visual inspection of the spray pattern. A camera with UV illumination (if adhesive has fluorescent tracer) can detect pattern coverage in real time.
Maintenance: Nozzles are consumable items; replace every 500-1000 hours. The air supply must be clean and dry; use filters and dryers. The melt tank should be drained and cleaned weekly to prevent degraded adhesive from affecting viscosity. For sanitary napkins, which are thinner than diapers, even lower coat weights (1-2 gsm) are used to maintain softness. Some lines use a “semi-random pattern” slot die with very shallow dots. By optimizing low-coat-weight spray technology, hygiene manufacturers achieve excellent laminating strength while preserving the breathability and comfort required for premium products.
