Precautions for Solvent-Based Adhesive Preparation in Dry Lamination

In the dry lamination process for plastic packaging, adhesive preparation is one of the most critical control points. Adhesive preparation refers to mixing the adhesive base, curing agent, and solvent according to specified ratios and correct operating procedures to form a homogeneous adhesive solution usable on production machines.

Quality accidents in lamination caused by improper adhesive preparation are common in printing and packaging enterprises. Examples include severe under-curing of the adhesive layer due to insufficient or no curing agent added; incomplete curing caused by unbalanced adhesive ratios, resulting in edge curling, aluminum delamination, or leakage at heat-sealed areas during bag making; insufficient coating weight and substandard peel strength caused by excessive solvent addition; and poor lamination due to non-standard reuse of leftover adhesive, among others.

The following summarizes the preparation methods and precautions for commonly used two-component solvent-based dry lamination adhesives, for reference by operators and frontline managers.

I. Importance of Adhesive Technical Data (Adhesive Instruction Manual)

Technical data is an essential reference before using adhesives. To prepare adhesive correctly, one must first obtain the technical documentation provided by the adhesive manufacturer, which lists standard mixing ratios for different working concentrations. These ratios are usually determined through laboratory testing by the manufacturer, based on the full reaction of base resin and curing agent, with a slight excess of isocyanate groups from the curing agent added to compensate for consumption by external uncertainties.

Most enterprises issue process sheets for specific products in the lamination process, which specify adhesive mixing ratios based on the manufacturer's technical data and other influencing factors. Some enterprises also establish Standard Operating Procedures for Adhesive Preparation, which clearly define ratios for all adhesives used in-house.

In general, adhesive preparation must follow standard specifications. Any ratio adjustment under special conditions must be based on these standard formulas. In particular, changes to the base-to-curing-agent ratio require consideration of complex influencing factors and must not be arbitrarily adjusted by the operator. The tables below show excerpts from adhesive technical data and enterprise lamination process specifications.

Relationship Between Solid Content and Diluent During Dilution

Table 1

Table 2

II. Production Process Requirements

The working concentration (solid content of the mixture) is determined by the required coating weight and the line count of the coating roller, in accordance with production process requirements. The mixing ratio is then confirmed by referring to the adhesive instruction manual or production process sheet.

Different product types - such as ordinary two-layer structures, aluminum foil-containing structures, or high-temperature retort structures - have different requirements for adhesive type and coating weight. These differences directly affect the required adhesive mixing ratio.

Production processes are established by comprehensively considering adhesive technical data, product type, service environment, testing data, and other factors. Therefore, the mixing ratio before use must comply with production process requirements, while adhesive technical data remains an important reference.

III. Influences of Environmental and Operational Conditions

Adhesive mixing ratios are affected by several objective conditions, mainly as follows:

1. Requirements for Lamination Peel Strength

Different product types typically have specific peel strength requirements. Besides reasonably designing the coating weight, this can also be achieved by fine-tuning the base-to-curing-agent ratio. For example, for general aluminum-coated films not using dedicated aluminum-coating adhesive, reducing the curing agent ratio can alleviate aluminum delamination and improve peel strength (other quality risks from this practice must be evaluated). The curing agent dosage is usually reduced by 10%–20% based on the standard amount. For instance, a standard ratio of base:curing agent = 5:1 may be adjusted to 5:0.8. To ensure product quality, the actual reduction ratio must be verified through testing and documented in standard technical data.

For high-temperature retort products, to reduce the decay of peel strength after retorting, the curing agent ratio is usually increased by 3%–5% to enhance adhesive layer hardness and resist strength degradation. Under ultra-high temperature and humidity (humidity > 90%), production of retort packages is not recommended even with increased curing agent.

2. Line Count of Coating Rollers

Many packaging manufacturers have a limited range of dry lamination coating rollers, sometimes using a single roller for all products. When the line count is inappropriate but a stable coating weight is required, the working concentration of the adhesive solution must be adjusted. A coating roller with fine lines delivers lower coating weight, so a lower working concentration is needed by increasing the solvent ratio. A roller with coarse lines delivers higher coating weight, requiring higher working concentration and thus less solvent.

Adjusting ratios to compensate for roller limitations often increases costs (solvent evaporation loss) or causes quality defects such as white spots due to anilox blockage. Within the limitations of coating rollers, working concentration is adjusted by varying solvent addition to control coating weight.

3. Seasonal Changes and Ambient Temperature & Humidity

Seasonal and weather variations - such as temperature and humidity differences between winter and summer, or short humid spells in southern China - significantly affect adhesive ratios. The ideal environment is 25±5℃ and 50%–60% RH. Maintaining these conditions year-round is difficult in non-temperature-controlled, non-enclosed workshops.

In high-temperature and high-humidity environments (humidity > 85%, temperature > 35℃), solvent evaporates during adhesive circulation in the glue tank. The cooler adhesive absorbs moisture, and condensed water may mix in, consuming part of the curing agent. Therefore, when humidity exceeds 85%, the curing agent dosage is typically increased by 5%–10% to offset moisture-related consumption.

At very low temperatures, even at the same working concentration, viscosity increases, hindering adhesive transfer. To improve transfer efficiency, solvent proportion may be increased by approximately 10% to reduce viscosity by 2–3 seconds. However, actual coating weight also depends on anilox line count, cell depth, roller type, machine speed, pressure, doctor blade pressure, and film corona treatment level. Performance must be verified to ensure target coating weight is achieved. Alternatively, high-solid, low-viscosity adhesives can be used for low-temperature conditions.

4. Viscosity Stability

Adhesive ratios must be determined based on the solid content of base and curing agent and the required working concentration. As noted earlier, many factors affect whether the process-designed coating weight is achieved, among which viscosity is critical.

Viscosity is influenced by adhesive properties, mixing ratio (solvent dosage), and ambient temperature. Stable coating requires stable adhesive viscosity under consistent conditions.

During operation, continuous solvent evaporation increases viscosity and typically raises coating weight if solvent is not replenished. Therefore, during regular use, viscosity must be stabilized by manually adding solvent at intervals or via automatic viscosity controllers. Stable working concentration ensures stable coating weight. Maintaining a target viscosity for consistent application will alter the adhesive-solvent ratio.

5. Abnormal Material Quality or Material Properties

Material abnormalities refer to known issues such as excessive alcohol-based residual solvents in printed films or substandard solvent purity before adhesive preparation. For example, if isopropyl alcohol (or ethanol) in printed films exceeds 5 mg/m², curing agent dosage is usually increased by 1%–3%. If recycled ethyl acetate is used with purity below 99.5% - especially when total alcohol and moisture exceed 0.2% - curing agent is increased by 3%–5% to counteract consumption of isocyanate groups. A standard ratio of 5:1 may become 5:1.05. Such films carry high quality risks, including potential insufficient peel strength and persistent high residual solvents.

Note: This ratio adjustment is only an emergency remedy, not a routine practice. The root cause - excessive residual solvents and low solvent purity - must be addressed at the source.

Regarding material properties: nylon film is highly hygroscopic. For lamination involving nylon, curing agent is typically increased by 3%–5%, or 5%–10% under high humidity (>85% RH).

6. Reuse of Leftover Adhesive

Leftover adhesive generally refers to prepared adhesive recovered from the glue tank (unmixed adhesive in sealed original containers can usually be used normally). Shelf life of leftover adhesive is normally no more than 48 hours, or 12 hours for fast-curing adhesives. Even stored at 4℃, it may emulsify and turn white over time; severely emulsified adhesive must be discarded.

For cost savings, short-term stored leftover adhesive can be reused by blending with fresh adhesive. The mixture is adjusted to the required working concentration according to the fresh adhesive ratio, fully stirred, and its viscosity measured. Leftover adhesive is blended with fresh adhesive at 5%–30% by weight, stirred thoroughly, and rechecked for viscosity.

If viscosity is lower than fresh adhesive, add base and curing agent proportionally to restore initial viscosity (often caused by excess diluent from anilox cleaning).

If viscosity is higher, add fresh solvent to reach target viscosity (caused by excessive solvent evaporation and high solid content).

The blending ratio depends on product type: acceptable for general products, not recommended for retort packages. The more severe the emulsification, the lower the proportion. Any addition of leftover adhesive changes the adhesive-solvent ratio.

Reusing leftover adhesive involves quality risks and must be based on product type and operational experience. Importantly, leftover adhesive may only be mixed with fresh adhesive of the same manufacturer and model. Adhesives from different manufacturers cannot be mixed. For different models from the same manufacturer, follow the supplier's instructions.

IV. Standardized Management

Given the importance of proper adhesive preparation and stable ratios, standardized management is necessary in practice. Standard Operating Procedures for Dry Lamination Adhesive Preparation should cover operational requirements and risks to avoid, as referenced in the table below.

Table 3

V.

Conclusion

Adhesive preparation is vital in the dry lamination process. The key is to follow process specifications while considering product structure, production environment, adhesive performance, and service conditions. Human errors must be strictly avoided, such as arbitrary adjustment of curing agent dosage, insufficient mixing time, or accidental omission of curing agent.

Proper adhesive preparation is fundamental to ensuring dry laminated product quality and must not be neglected. Specialized dry lamination adhesive mixing systems are now available, enabling automatic or semi-automatic preparation. These systems better ensure stable mixing ratios, minimize human error in manual preparation, control solvent costs, and further support consistent lamination quality.