Hot-melt adhesive film is a solvent-free adhesive material, either with or without a substrate. Its core value lies not in being a "universal adhesive," but rather in three key aspects: zero VOC emissions, precisely controllable adhesive application, and no requirement for complex curing processes. These three characteristics confer distinct advantages in terms of environmental compliance, delivery lead times, and cost-effectiveness.
Why has the composite process shifted from solvent-based adhesives to hot-melt adhesive films?
Solvent-based PU adhesives present three key challenges: the requirement for drying and solvent evaporation, which leads to high energy consumption and potential VOC hazards; adhesive application volume being influenced by viscosity and the scraping blade, resulting in significant batch-to-batch variability; and difficulties in controlling residual solvent levels.
Hot-melt adhesive films address these issues comprehensively: the adhesive application amount is determined by the material itself, resulting in minimal variation; no drying chamber is required during lamination, enabling continuous hot pressing; there is no VOC emission, and extended curing time is unnecessary. However, these films impose stricter requirements on the temperature uniformity of the equipment.
Whether it is worth switching depends primarily on three factors:
1. The customer has VOC or odor requirements;
2. After compounding, the tactile feel should be optimized by adjusting the adhesive layer thickness;
3. The increased material costs can be recouped within 1–2 years.
If any one of these three conditions is met, it typically possesses switching value.
Comparison of Three Main Types of Adhesive Coatings
Material | Softening/Melting Point Range | Advantages | Limitations | Typical Applications
TPU exhibits good elasticity at approximately 80–130 °C, remains soft at low temperatures, and demonstrates excellent flexural resistance; it can be compounded with apparel fabrics and TPU films. However, its production cost is relatively high, and its hydrolysis resistance is slightly limited. Applications include functional apparel, footwear materials, and automotive interior components.
EVA: temperature range approximately 70–100 °C; low cost; good fluidity; wide heat sealing temperature range; moderate strength; limited washability and heat resistance; suitable for packaging, temporary lamination, and general composite applications.
PES: approximately 110–150 °C; resistant to water washing, dry cleaning, and low temperatures; exhibits high interfacial strength; however, it has a relatively narrow temperature window and imposes stringent requirements on equipment. Applications: lining fabrics, automotive interiors, and wash-resistant products.
PA: melting point approximately 120–160 °C; resistant to dry cleaning and high temperatures; offers extensive adjustability in hand feel after moisture absorption; requires humidity control; sensitive to drying conditions; suitable for thick fabrics and high-performance composites.
Grammage and Thickness: How to Choose?
The grammage of hot-melt adhesive film typically ranges from 15–200 g/㎡, corresponding to a thickness of approximately 0.015–0.2 mm.
selection principle:
• For porous or coarsely woven fabrics, the GSM may be appropriately increased;
• For lightweight fabrics, or when emphasizing breathability and softness, the fabric weight should be reduced;
• As the weight increases, bonding strength typically improves, but the hand-feel becomes firmer.
Process window: temperature, pressure, and time
1. temperature
Temperature is the most critical parameter. If the temperature is too low, the adhesive film may not melt sufficiently, resulting in poor interfacial wettability and a high risk of delamination; if the temperature is too high, the adhesive layer may become overly fluid, potentially leading to adhesive leakage and hardening of the substrate.
A common approach is to conduct temperature gradient tests-such as grouped testing at 110 °C, 120 °C, 130 °C, and 140 °C-followed by evaluation based on peel strength and tactile feel. The thermal efficiency of different equipment varies significantly; therefore, production should not be based solely on the temperature recommendations provided by the manufacturer.
2. Stress
The pressure should ensure sufficient contact between the adhesive film and the substrate; however, higher pressure is not necessarily better. Excessive pressure may cause the adhesive layer to be over-compressed, leading to adhesive leakage or hardening of the substrate.
3. Time
The processing time is determined by the combined linear velocity. If the time is too short, bonding will be insufficient; if the time is too long, the material may undergo excessive heating.
Peel strength: Key acceptance criterion
Peel strength is the primary acceptance criterion for hot-melt adhesive films; common standards include GB/T 2791, ISO 11339, and ASTM D1876, with units expressed in N/cm or N/25 mm.
When conducting the test, please pay attention to the following three points:
1. The numerical variation at different temperatures shall be kept within 20% as much as possible;
2. Record the failure mode: whether it is interfacial debonding or failure of the material itself;
3. The test conditions shall include the initial state, the state after rinsing, and the state after high-temperature aging.
If only the "peel strength of 8 N/cm" is cited without specifying the test conditions, this value lacks acceptance significance.
Common Issues Resolution
Why is the performance of the composite of TPU hot-melt adhesive film and TPU film unstable?
The two materials belong to similar chemical families, but their design objectives differ: TPU films prioritize mechanical properties, whereas TPU hot-melt adhesive films prioritize flow properties and adhesion performance. Replacing hot-melt adhesive films with TPU films can easily lead to unstable adhesion.
What should I do if bubbles appear after mixing?
First, check the moisture content of the materials and their surface contamination; then, examine the pressure, temperature, and line speed. In most cases, bubbles originate from inadequate evacuation of air at the interface, rather than from issues with the rubber film itself.
epilogue
The selection of hot-melt adhesive films can be summarized in three key points:
1. The material selection shall be based on the operating temperature, washing requirements, and substrate conditions;
2. The process window must be verified through actual measurements; manufacturer-provided parameters may not be directly adopted.
3. The acceptance inspection must document the test conditions, the condition of the substrate, and the data after aging.
Hot-melt adhesive films have evolved from auxiliary materials to design variables; both procurement and engineering teams must jointly assume acceptance responsibilities.
