Discussing the Impact of Solvent-Free Lamination on Heat Seal Temperature

In production practice, some customers have observed that products made with solvent-free lamination often affect the heat seal temperature. Commonly used heat seal materials include PE and CPP, with the issue of affected heat seal temperature being more prevalent in PE structures. This article uses PE structures as an example to explore the problem.

I. Cause Analysis

The three key factors for heat sealing are temperature, pressure, and heat seal time.

We have found that, under identical conditions—i.e., with the same pressure and heat seal time (bag-making speed)—the heat seal temperature of solvent-free laminated products tends to rise compared to solvent-based dry-laminated products, sometimes by as much as 15–20°C. However, this does not occur with all solvent-free laminated products; it is a probabilistic event and also shows some seasonal correlation.

To identify the cause, we conducted a parallel comparative experiment:

Test Materials

Several composite films with elevated heat seal temperatures;

Dry-laminated samples with the same structure and thickness;

Normal solvent-free laminated samples with the same structure and thickness.

Test Equipment

Cotton swabs, ethyl acetate, and alcohol;

Single-edge razor blades;

Fourier transform infrared spectrometer;

Tensile tester;

Heat sealer;

High-magnification electron microscope.

Test Methods and Procedures

(1) Three sets of sample films, each 15 mm wide, were taken from the same location on the problematic composite film, labeled as Sample 1, Sample 2, and Sample 3.

(2) Dry-laminated samples of the same size were labeled as Sample 4, and normal solvent-free laminated samples as Sample 5.

(3) Each sample was first wiped with a cotton swab, then lightly scraped with a razor blade.

(4) Sample 1 was left untreated; Sample 2 was thoroughly wiped with ethyl acetate on the heat seal area, and Sample 3 was thoroughly wiped with alcohol on the heat seal area.

(5) The heat sealer parameters were set based on film thickness: 420 N, 0.1 s, starting at 120°C with 5°C increments. All five samples were heat-sealed simultaneously.

(6) The heat seal strength of each sample was measured using a tensile tester, and data were recorded in a table.

(7) The heat-sealed cross-sections of each sample were observed under a 2500x electron microscope.

(8) Fourier transform infrared spectroscopy was performed on the heat-sealed surfaces of the three sample groups, and the spectra were analyzed.

(Due to certain reasons, the data are not presented here; only the experimental process and methods are described.)

Test Results

Samples with elevated heat seal temperatures had more powdery residue when scraped compared to other samples.

Compared to dry-laminated samples, solvent-free products with normal heat seal temperatures showed a slight increase, which could be considered equivalent.

Products with elevated heat seal temperatures returned to normal heat seal temperatures after being wiped with either alcohol or ethyl acetate.

Under high-magnification observation, samples with no issues showed complete fusion between the two PE layers at normal heat seal temperatures, with no visible interface. However, problematic samples exhibited an obvious interface between the two PE layers at the same temperature. After increasing the temperature or solvent wiping, the PE layers fused completely.

Due to PE interference, the infrared spectra could not accurately identify additional substances.

Test Conclusions

The rise in heat seal temperature is caused by the introduction of a new substance on the PE heat seal surface. This substance can be easily removed with solvents, but existing test equipment could not precisely identify it.

Given limitations, conducting expensive tests like mass spectrometry was impractical, so theoretical support was sought.

Research indicates that polyethylene molecular chains contain branches, and the number of branches varies significantly depending on the polymerization method. The spatial arrangement of polyethylene molecular chains is planar zigzag, with a bond angle of 109.3° and a tooth distance of 2.534 × 10⁻¹⁰ m.

In flexible packaging, the commonly used PE is LDPE (low-density polyethylene), which has a branched structure. High-pressure LDPE contains more branches than low-pressure HDPE (high-density polyethylene). In addition to terminal methyl groups, some branches include ethyl, butyl, or longer chains formed during polymerization due to chain transfer. These branches affect molecular folding and packing density, reducing crystallinity and density.

Due to these structural characteristics, PE films are less dense.

Additionally, during processing, heating causes thermal expansion, increasing molecular spacing and reducing density further during curing.

Another consensus is that solvent-free adhesives have much smaller molecular weights compared to solvent-based adhesives used in dry lamination. In other words, before cross-linking, solvent-free adhesive monomers are much smaller. Increased van der Waals motion upon heating facilitates the penetration of small molecules through the PE film.

Reviewing the reaction mechanism of polyurethane adhesives, a side reaction—poorly controlled production conditions—can cause NCO components to react with water, forming polyurea and carbon dioxide gas.

Polyurea lacks heat-sealing properties. Some argue that if polyurea forms, heat sealing would be impossible, not just require higher temperatures.

However, this is not entirely accurate. For example, dust lacks adhesiveness. If a tape's adhesive surface is covered with dust, it cannot stick to anything. But if the dust proportion is small, adhesion remains, albeit with reduced peel strength. Similarly, if polyurea is abundant at the film interface, heat sealing fails. But if only a thin layer floats on the heat seal surface, normal heat seal temperatures may not suffice because the polyurea prevents instant fusion. Increasing the temperature fully melts the PE, allowing the non-melting polyurea to sink below the interface under pressure, restoring pure PE conditions and enabling heat sealing.

Normal heat seal temperatures require only a thin molten layer at the interface. Without higher temperatures, non-melting polyurea cannot sink, preventing heat sealing.

In summary: The rise in heat seal temperature is caused by polyurea. While slip agents in PE exist in both dry-laminated and problem-free solvent-free laminated products, testing the same PE formulation shows that dry lamination rarely exhibits this issue, whereas solvent-free lamination does. This is because dry-lamination adhesives have larger molecular weights, lower penetration probabilities, and faster reactions in ovens, leaving little opportunity for NCO monomer penetration. Solvent-free adhesives lack these traits.

Customer feedback indicates this phenomenon occurs more frequently in winter than summer. The primary difference between seasons, aside from minor PE variations, is temperature. In winter, the temperature difference when films enter and exit the curing room creates invisible condensation layers on the film surface. This water layer enables migrated NCO components to react and form polyurea.

To confirm that polyurea forms from migrated NCO components reacting with water rather than water entering and reacting first, consider this: If the reaction occurred between layers, dry and solvent-free lamination would yield similar results, as water migration conditions through PE layers are identical under identical structures and materials. Additionally, externally introduced water would lead to uncontrolled local concentrations, potentially causing excessive or rapid CO₂ formation and bubbles—yet no such feedback has been reported.

Thus, we can infer that polyurea forms when NCO components migrate to the PE heat seal surface and react with water.

II. Solutions

Knowing the probable cause, how can we resolve or prevent it? For prevention, address the root issue. Since the primary cause is insufficient gas barrier properties due to PE molecular spacing, focus on improving PE by selecting suitable formulations, increasing molecular steric hindrance, and filling molecular gaps with appropriate additives. Some clients have already validated this approach.

Production experience also shows that not all solvent-free lamination exhibits this issue, indirectly indicating a relationship with adhesive formulation—e.g., molecular weight distribution, small-molecule proportion, or branching.

The analysis also clarifies that condensation layers on films trigger this problem, so production must avoid such conditions.

If the issue arises, products need not be scrapped—they can be salvaged before bag-making. For example, running them through a dry laminator with a rotating brush in the adhesive unit to scrub the heat seal surface, then increasing oven airflow and temperature to fully volatilize solvents, can revive such products. Alternatively, convincing customers to adjust packaging temperatures may help, though many are reluctant.