Humidity, particularly water vapor in the air, is an "invisible degrader" that threatens the chemical stability and final performance of hot melt adhesives. Its impact permeates the entire process of storage, transportation, and use.

1. The "Natural Enemy" of PUR: Irreversible Chemical Deactivation
For moisture-curing polyurethane hot melt adhesives, ambient humidity is a necessary condition for chemical crosslinking but also its greatest foe during storage.

• Risk: Any intrusion of trace moisture into the packaging will pre-react with the isocyanate end groups in the PUR prepolymer. This reaction consumes the active components, directly leading to "shortened pot life, reduced bond strength, or even complete failure" during use. Once it occurs, the process is irreversible.

• Manifestation: Moisture-compromised PUR may exhibit abnormally high viscosity, surface skinning, bubble formation, or gel particles.

2. Balance Disruption in Water-based Systems: Attenuation of Physical Properties
The stability of water-based adhesives, exemplified by aqueous polyurethane dispersions, relies on a precise formulation balance.

• Risk:

  • Water Evaporation: In dry environments, poor packaging seals can lead to water loss, causing "increased solid content, higher viscosity, and deviation in application performance."

  • Excessive Moisture Absorption: In high-humidity environments, the adhesive film dries slowly and may absorb excess water, leading to "reduced water resistance and weakened initial tack."

  • Microbial Growth: Warm, humid conditions can cause spoilage and deterioration of water-based products.

3. Potential Risks for EVA/PSA: Interface Bonding and Storage Hazards
Although EVA and PSA are not sensitive to moisture, humidity can still have indirect effects.

• Risk:

  • Substrate Impact: High humidity increases the moisture content of porous substrates (e.g., wood, leather). When the adhesive cures on them, water evaporation may form a weak boundary layer or create stress leading to debonding.

  • Packaging Corrosion: Long-term exposure to highly humid environments can weaken cardboard boxes and cause rusting of metal components, indirectly jeopardizing product safety.

Solutions: Building a Dynamic "Dry Zone" Protection System

1. Active Moisture-proofing in Packaging:

   • For PUR products, the gold standard is aluminum composite barrier bags + sufficient desiccant + vacuum/nitrogen flushing sealing.

   • For all products, use inner bags with good barrier properties and ensure heat-sealed edges are complete and defect-free.

2. Humidity Management in Logistics and Warehousing:

   • Use "container desiccants" inside shipping containers to absorb "container rain" generated by day-night temperature differences during transit.

   • Stipulate in contracts or select warehouses with "temperature and humidity monitoring" capabilities for transit and storage.

   • Warehouses should be well-ventilated and kept away from water sources and steam pipes.

3. Enhanced Awareness at the User End:

   • Clearly mark "Keep Dry," "Store Sealed," and the ideal humidity range (e.g., 30%-60% RH) on product labels and outer boxes.

   • Instruct customers that after opening PUR packaging, any unused portion must be "air squeezed out and tightly resealed" immediately using the original packaging, and recommend use with a dedicated hopper dryer.