In the lamination process of flexible packaging, air humidity is a critical environmental factor that directly affects material performance, adhesive curing and final product quality. Relative humidity (RH) and absolute humidity (AH) are two core indicators for measuring atmospheric water vapor content. Closely correlated yet conceptually distinct, these two parameters fundamentally determine the production stability and yield rate of flexible packaging lamination. This article elaborates on the internal relationship between the two humidity indicators, analyzes their specific influences on lamination quality, and provides targeted process control solutions.
1. Definitions and Correlation of Absolute Humidity and Relative Humidity
1.1 Absolute Humidity (AH)
Absolute humidity is an absolute quantitative indicator that reflects the actual water vapor mass in unit volume of air, independent of ambient temperature. It is commonly measured in g/m³ in the flexible packaging industry. As a direct reflection of the real water vapor content in the air, AH serves as a core reference for precise process adjustment.
Formula: AH = Mass of water vapor / Volume of air
1.2 Relative Humidity (RH)
Relative humidity is arelative ratio indicator representing the degree of air moisture. It refers to the percentage ratio of actual water vapor pressure to saturated water vapor pressure at the same temperature, with the unit of %RH. RH is a dynamic value that fluctuates with temperature changes and is widely adopted for daily workshop humidity monitoring.
Formula: RH = (e/es) × 100% Where: e = actual water vapor pressure; es = saturated water vapor pressure at the current temperature
1.3 Core Correlation Between AH and RH
The saturated water vapor pressure of air rises with temperature increase, forming a dynamic balance between the two humidity parameters. Under a constant absolute humidity condition, rising temperature increases the air's water-holding capacity, which reduces relative humidity. Conversely, falling temperature decreases saturated water vapor pressure and raises relative humidity.
In short, absolute humidity reflects the fixed and objective water vapor content per unit air volume, while relative humidity is susceptible to temperature interference. Therefore, production control cannot rely solely on RH data. Combining AH values to confirm the real atmospheric moisture content is essential for accurate process optimization.
2. Impacts of Humidity on Flexible Packaging Lamination Quality
2.1 Adhesive Curing and Bonding Performance
In high-humidity environments (RH ＞ 70%), free water vapor in the air participates in the chemical reaction of polyurethane adhesives, releasing carbon dioxide. This causes internal bubbles and incomplete curing in laminated films. Meanwhile, excessive moisture residual in solvent-based adhesives reduces the interlayer peeling strength, leading to delamination risks.
Low-humidity environments (RH ＜ 40%) cause premature surface drying of adhesives, resulting in poor leveling performance, uneven coating and pinhole defects. For solvent-free lamination, insufficient reaction of component A adhesive frequently occurs under dry conditions.
It is critical to distinguish AH and RH in low-temperature workshops. High RH at low temperature does not necessarily mean high absolute humidity. Curing agent dosage should be adjusted according to AH values rather than RH readings. For PET/VMPET aluminum-plated structures, reduced curing agent dosage for special aluminum-plated adhesive is required under low AH to avoid aluminum layer transfer and poor peeling strength. For nylon-containing and aluminum foil structures applicable to boiling and retort packaging, increased curing agent dosage is necessary under high AH to ensure complete curing and meet high-temperature resistance and strength requirements.
2.2 Substrate Moisture Absorption, Deformation and Delamination
Nylon (NY) and other common flexible packaging substrates feature strong moisture absorption. Under high RH conditions, nylon films absorb moisture and expand. After lamination, subsequent humidity changes cause substrate shrinkage, resulting in film curling, interlayer dislocation and delamination. In addition, high absolute humidity increases substrate moisture content. Water evaporation during the lamination process forms interfacial bubbles, which destroy the structural stability of laminated films.
2.3 Solvent Volatilization and Residual Solvent Risk
During printing and coating processes, high ambient humidity slows down organic solvent volatilization, leading to excessive residual solvent that fails the safety standards for food and pharmaceutical packaging. Moreover, excessive water vapor in high-AH air may mix with adhesive liquid, causing turbidity and pre-gelation, and further compromising coating uniformity and final film performance.
2.4 Static Accumulation and Production Hazards
In dry and cold winter conditions, low absolute humidity in workshops inhibits static dissipation. High-speed friction of plastic films generates accumulated static electricity, which causes dust adsorption and irregular lightning-like marks on film surfaces, damaging visual quality. In severe cases, excessive static electricity may ignite organic solvents, posing major safety hazards.
3. Optimized Process Control Recommendations
3.1 Precise Workshop Temperature and Humidity Control
Lamination workshops shall implement constant temperature and humidity management. The recommended standard working condition is 23±2℃ with RH 50±5%. Dehumidifiers and central air conditioning systems are applied to stabilize absolute humidity and avoid drastic environmental fluctuations.
3.2 Pre-treatment and Moisture Protection for Substrates and Adhesives
Highly hygroscopic substrates such as nylon shall be pre-dried in a 50℃ oven to control the moisture content below 0.1%. Adhesives and curing agents must be tightly sealed during storage to prevent moisture absorption, deterioration and failure.
3.3 Dynamic Process Adjustment Based on Humidity Parameters
Establish a humidity-linked process adjustment system. Increase curing agent proportion under high absolute humidity to ensure full adhesive curing. Under low AH conditions, strengthen static elimination and optimize adhesive leveling and curing parameters to adapt to dry production environments.
4. Conclusion
Absolute humidity determines the total actual water vapor content in the air, while relative humidity reflects the real-time moisture level of the working environment. Jointly, these two parameters dominate the lamination quality, substrate stability and solvent volatilization efficiency of flexible packaging products. Abnormal humidity causes common defects including bubbles, delamination, insufficient bonding strength, excessive residual solvent and static-related flaws. Accurate monitoring and dual control of temperature and humidity, together with dynamic process optimization, can effectively eliminate lamination defects, stabilize product quality, and improve production yield and operational safety.