The Principle of Non-Drying Solvent-Free Composite Adhesive

Glue not drying is one of the common and tricky problems in the process of solvent-free lamination. Due to the various nodes, forms, and ways in which this issue can arise, it brings great trouble to the normal production of solvent-free lamination.

The phenomenon of glue not drying can be quite varied, including large areas of glue not drying, sporadic glue not drying in specific areas, continuous undrying in specific spots (such as the middle or edges), irregular undrying at certain stages within the film roll, and occasional undrying in some parts of the film roll, among many others.

In summary, glue not drying is a comprehensive challenge of solvent-free lamination and one of the key reasons for major quality accidents. If it cannot be timely regulated and prevented, it poses significant harm to the lamination production.

To solve the issue of glue not drying, one must first understand the principles of compound adhesives. The bi-component polyurethane adhesives used in flexible packaging compounding mainly consist of prepolymers with —NCO groups (Component A, the main agent) and polyols or polyamines with terminal hydroxyl (—OH) groups (Component B, the curing agent) prepared at a certain ratio (molar ratio).

For better understanding, we call this ratio the "R value," R value = [n(—NCO) / n(—OH)]. The amounts of the two components can be adjusted within a specific range, usually with the "R value" ≥ 1. During curing, a part of the —NCO groups participate in the curing reaction of the glue, producing adhesive force, and the excess —NCO groups can generate urethane and biuret with heating, increasing the degree of crosslinking and enhancing the cohesive strength and heat resistance of the glue layer.

For solvent-type bi-component polyurethane adhesives, the molecular weight of the main agent is relatively large, the initial adhesion performance is better, and the amounts of the two components can be adjusted over a wider range. The R value can be less than 1 or several times greater than 1. In contrast, for bi-component solvent-free polyurethane composite adhesives, since the initial molecular weights of the components are not large, the R value can only be equal to or slightly greater than 1, which is more conducive to complete curing.

In simple terms, in the design of solvent-free adhesives, the molar ratio (R value) of the main agent (isocyanate -NCO) to the curing agent (hydroxyl-OH) must be more precise, meaning the ratio of A/B components needs to be very precise to ensure the final stability of the adhesive.

In other words, the tolerance of solvent-free composite adhesives has narrowed compared to dry composite adhesives. For example, the composite process of dry composite adhesives could walk on an 8-lane road, while the composite process of solvent-free adhesives can only walk on a single-lane road, which raises higher requirements for operation and equipment!

Here it must be specifically mentioned that the isocyanate (-NCO) component in polyurethane adhesive is a relatively active substance and can easily chemically react with substances containing hydroxyl groups (-OH), amines, and epoxy compounds.

 
In the production process of flexible packaging, water vapor in the air precisely contains —OH groups (water molecule H₂O is composed of hydroxyl -OH); some additives, slip agents, antistatic agents in the film, such as oleamide, erucamide, etc., happen to contain amine substances.
Therefore, the isocyanate contained in polyurethane adhesives can react with hydroxyl and amine groups during the composite production. If not controlled during production, these hydroxyl and amine groups consume the isocyanate components in the adhesive, thereby directly disturbing the molar ratio "R value" of the adhesive, resulting in insufficient peel strength or the adhesive not drying.

 
Note 1

Reaction of isocyanate with water 2RNCO + H₂O → RNHCONHR + CO₂↑ (NCO)x + yH₂O → HO-(NHCOO-)x-NH₂,

 
A single water molecule reacts with two NCO groups to produce substituted urea. Water can be regarded as a chain extender or curing agent. This point has important guiding significance for the production and storage of polyurethane. Both raw materials and products need to strictly control moisture content.

 
The reaction of isocyanate with water releases carbon dioxide gas and can be used in the production of polyurethane foams, moisture-curing polyurethane adhesives, and coatings. The carbon dioxide (CO₂ gas and NH₂ gas released during the reaction in film lamination cause bubble formation, and the NH₂ generated is ammonia, where the pungent smell in solvent-free lamination is likely the smell of ammonia.

 
Note 2

Reaction of isocyanate with amine Reaction formula: RNCO + R′NH₂ → RNHCONHR′ RNCO (isocyanate) + R′NHR″ (amine) → RNHCONR′R″ (polyurea)  Due to the narrowing of the process path and the consumption of isocyanates in solvent-free lamination processes, the requirements and restrictions on the process are also very high. For example, if some variables during production consume —NCO, causing the A/B components to become unbalanced, or due to unstable factors such as glue supply stability, even mixing, and coating stability during production, the actual ratio of adhesive becomes unbalanced, leading to reduced peel strength or the phenomenon of glue not drying in severe cases.

 
After understanding the principle of the glue, the solvent-free lamination operation process can be reevaluated to avoid the phenomenon of glue not drying. Various factors affecting the R value of solvent-free adhesives exist in several stages in flexible packaging enterprises, mainly reflected in the following three aspects:

 
Stability of Adhesive Supply by Mixing Institutions

The mixing institution ensures the storage and mixing supply of A/B component adhesives. The precision and reliability of the equipment determine the final coating performance of the adhesive. It can be said that most glue not drying issues arise during the mixing stage, which has a high correlation with process control, the quality of mixing machines, and operational standards.
The mixing institution can be divided into eight parts: A/B adhesive tanks, metering pumps, glue transport pipes, glue dispensing head, static mixing pipe, glue head swing device, liquid level sensor, and control PLC. The mixing institution senses the adhesive level in the coating tank through the liquid level sensor, transmits signals to the mixing PLC, and notifies the metering pump to supply glue from the adhesive tank to the composite coating tank. The glue needs to be mixed accurately through the transport pipes, dispensing head, and mixing pipe to coat the adhesive evenly. 

A/B Adhesive Tanks

The A/B adhesive tanks are stainless steel storage tanks used to store A/B component adhesives separately. As the adhesives are continuously consumed during production, the adhesive levels in the tanks need to be timely replenished and added. To ensure the stability of the production coating, the temperature of the adhesive should be maintained between 35-60 degrees. In the process of adding adhesives, three issues may arise that could affect the final coating performance:

The added adhesive has a temperature difference with the glue in the tank. Especially in winter, adhesives directly taken from the warehouse have lower temperatures, and adding cold adhesives into the tank can cause the glue in the tank to stay at a low temperature for a long time. Low temperatures impact the adhesive in two ways: the viscosity of the adhesive increases rapidly, and the leveling wetting is reduced, making it difficult for the adhesive to spread on the film surface, potentially causing coating shrinkage and white spots.

 
The increased viscosity of the adhesive at low temperatures also causes changes in the metering of the metering pump, reducing the amount of adhesive delivered. Whether reducing A or B adhesive, the output ratio of glue becomes unbalanced, affecting the R value, leading to incomplete adhesive reaction, and resulting in localized undrying or peeling drop-off in composite products.

Additionally, the high viscosity of the adhesive under low temperatures increases the resistance in the metering pump, glue transport pipes, mixing plates, and mixing pipes, causing pressure on the pipeline walls and increasing resistance. Prolonged abnormal glue supply causes the adhesive to adhere to the inner wall of the transport pipes, partially blocking the pipes, leading to inaccurate metering or glue ratio, and significantly troubling the composite process.

 
To solve the temperature difference issue, adhesives required for production should be preheated in an oven for 12-24 hours beforehand to match the temperature of the working adhesive in the tank, avoiding deviations due to temperature differences and reducing instability factors during production.

The adhesive tank's inadequate sealing during add adhesive, resulting in a large amount of air mixing in the tank. During high humidity, water vapor in the air reacts with the NCO- component in the adhesive, causing an imbalance in the actual A/B adhesive ratio and deviation in the mixing ratio. The polyurethane formed by the reaction creates skinning inside the tank, potentially blocking the tank and the entire glue supply system, leading to poor glue supply from the mixing institution.

The adhesive tank should maintain a high level to reduce the presence of air. Apart from necessary adhesive additions, the tank lid should remain sealed to prevent air entry. Desiccants should be regularly checked to ensure they are not ineffective. The appearance of the desiccant usually indicates whether it can still be used. Used desiccants can be reused by baking in an oven until they revert from red to blue.

During adhesive addition, operators may mistakenly add A adhesive into the B adhesive tank or vice versa, causing cross-reactions and cross-linking within the tanks. Upon discovery, substantial losses have already occurred, and the increased viscosity due to A/B adhesive reactions can severely clog the tanks and entire glue supply system, leading to system paralysis and potential scrapping.

 
To prevent such low-level human errors, in addition to managerial controls, the following measures can be taken:

• Use the same distinguishable color markings for adhesive tanks and A/B component adhesives. For example, if A adhesive barrels are blue, paint the adhesive inlet of the A adhesive tank blue to alert operators against errors.
  
  

• During small adhesive barrel additions, drill holes in the adhesive barrel lid for faster adhesive addition. Quickly add the adhesive and seal the tank immediately to prevent excessive air entry. Minimize the time A adhesive barrels containing isocyanates (NCO-) are exposed to air to prevent reactions with moisture causing polymerization and skinning.
  
  

• Large barrel sealed adhesive addition can avoid temperature differences during addition and air contact issues, also minimizing hazardous waste from used adhesive barrels.