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What is and How to Optimize ‘Glue Does Not Dry’ in Laminated Flexible Packaging: Key Factors and Comprehensive Solutions

“Glue does not dry” is sometimes identified as one of the causes of certain surface defects in the processing and application of composite flexible packaging, such as “local delamination” or “wrinkling.” Simultaneously, it is considered an indication of issues or instability in the quality of the adhesive used. Currently, the majority of laminated flexible packaging companies do not inspect whether the adhesive layer exhibits the “glue does not dry” phenomenon before delivering the composite products, nor do they impose any restrictive regulations on the degree of “non-drying of adhesive.” When downstream customers (e.g., food factories) make any quality complaints or return requests to the composite flexible packaging companies, they habitually first inspect whether the adhesive layer of the corresponding laminated products exhibits the “glue does not dry” phenomenon. If it is found, regardless of the degree, they will raise quality complaints to the adhesive supplier.



Table of Contents:

1. Causes of “glue does not dry” phenomenon

2. Various factors related to the 'glue does not dry' phenomenon:

(1) Manifestations of 'insufficient curing'

(2) Factors leading to 'insufficient curing agent'

(3) Factors leading to 'excessive curing agent'

3. Different Manifestations of “Glue does not dry” on Single Laminate Film and Film Rolls/ Laminate Web

(1) Factors leading to the “non-drying of adhesive” phenomenon on a single film

(2) Factors leading to the “non-drying of adhesive” phenomenon on the laminate web

4. Methods to Identify the Causes of “Non-drying of Adhesive”


1. Causes of “glue does not dry” phenomenon

In the processing of composite flexible packaging materials, solvent-based or solvent-free two-component reactive polyurethane adhesives (commonly known as adhesives or AB glues in some places) are widely used. One of the components is a prepolymer with hydroxyl (-OH) end groups, and the other component is a prepolymer with isocyanate (-NCO) end groups.

In the industry, the component with a relatively larger quantity among the two is called the main agent, while the component with a relatively smaller quantity is called the curing agent.

At room temperature, the main agent and the curing agent stored in sealed containers are both viscous fluids.

  • If the two components of the adhesive are exposed separately to the air, in the initial stage, both components appear as fluids with a certain degree of viscosity. With time passing, after the solvent is volatilized, the component with isocyanate end groups will turn into a solid with many internal bubbles (reaction products of moisture in the air and isocyanate groups), and the component with hydroxyl end groups will become a highly viscous elastic material.

  • l If the two components are mixed uniformly according to the ratio specified by the supplier, the solvent is driven out, and they are isolated from the surrounding humid air, and then subjected to a certain temperature for 'curing' for a period of time, a surface without stickiness and solid adhesive layer or film can be obtained.

  • It needs to be emphasized that during the curing process, the chemical cross-linking reaction between the main agent and the curing agent is gradually completed, and the stickiness on the surface of the adhesive layer gradually disappears as the reaction completion rate rises. If a sample of the adhesive film is taken/inspected at any time before the reaction completion rate reaches 100%, it will be found that the adhesive layer exhibits a certain degree of 'stickiness.'

  • If the two components are mixed in a 'random' proportion, even if the curing is carried out according to the conditions required by the supplier, the obtained adhesive layer or film will still be a solid or elastic material with a certain degree of stickiness on the surface.

In summary, the so-called 'glue does not dry' is a manifestation of the 'incomplete curing of adhesive.' In other words, the reason for the 'non-drying of adhesive' phenomenon is the 'incomplete curing of adhesive.'

 

2. Various factors related to the 'glue does not dry' phenomenon:

As mentioned above, the reason for the 'non-drying of adhesive' phenomenon is the 'incomplete curing of adhesive.' If we continue to analyze, three sub-reasons leading to 'incomplete curing of adhesive' can be found: insufficient curing, insufficient curing agent, and excessive curing agent.


(1) Manifestations of 'insufficient curing':

In the production process of laminate flexible packaging materials, the factors leading to 'insufficient curing' include the following: low temperature in the curing room; insufficient curing time; non-uniform temperature in the curing room; and large deviation in diameter of laminate web.

'low temperature in the curing room' refers to the situation where the temperature in the curing room or curing box used by the customer is lower than the curing temperature specified by the supplier. It can be divided into two states:

  •  The set temperature value of the curing room is lower than the specified value:  the customer intentionally sets the temperature of the curing room lower than the temperature value required by the supplier for a certain purpose.

  •  The actual measured temperature value in the curing room is lower than the set value of the temperature controller:  the temperature value measured by a mercury thermometer or an infrared thermometer in the curing room is lower  than the temperature value displayed by the temperature controller in the curing room.

In this case, there are two possibilities:

First, the temperature controller is not accurate (which means that the temperature controller needs to be recalibrated);

Second, the placement position of the thermocouple in the curing room is unreasonable (the hanging position is too high or too close to the heat source) and needs to be repositioned.

 'Insufficient curing time' means that the packaging material processing company shortened the curing time for the sake of rushing the goods.

 'Non-uniform temperature in the curing room' refers to the situation where the 'range' of the actual measured temperature values at different positions in the same curing room or curing box is relatively large, meaning that there is a significant difference between the highest and lowest temperature values measured. This phenomenon usually indicates that the design of the curing room or curing box is unreasonable.

④ 'Large deviation in diameter of the laminate web' means that for the same batch of laminate products, the diameter of some rolls is larger, for example, reaching 1 meter, while the diameter of some rolls is smaller, for example, only 0.4 meters. Under the same curing (temperature and time) conditions, the temperature values that can be reached in the central part of different rolls will have significant differences, resulting in differences in curing results.


(2) Factors leading to 'insufficient curing agent':

In the production process of laminate flexible packaging materials, the factors leading to 'insufficient curing agent' are as follows:

① No curing agent added or insufficient amount of curing agent added:  In laminate flexible packaging material processing companies, the specific manifestation is that the main agent of the same batch of purchases has been completely used up, but there is still some amount of curing agent left in the original curing agent container.

② Abnormal consumption of the curing agent: means that the amount of curing agent used by the processing company is according to the requirement, but the final result still shows “glue does not dry”.  That means the curing agent is abnormally consumpted during the production process.

        Causal Factors of “Abnormal consumption of the curing agent”:

      • Active hydrogen and moisture brought into the glue solution by the diluent;

      • Condensation of moisture into the glue solution from humid air;

      • The moisture adsorbed by the composite substrate from the air;

      • Active hydrogen and moisture brought into the ink by the diluent;

      • Moisture condensed into the ink from the ambient air;

      • The active hydrogen contained in the ink itself.


(3) Factors leading to 'excessive curing agent':

① The amount of curing agent added is excessive: It refers to the customer deliberately increasing the proportion of curing agent for a certain purpose when dispensing glue.

② The factors that consume the curing agent are abnormally reduced: It refers to the factors that consume the curing agent under normal conditions do not show their impact. For example, weather conditions changed rapidly, the amount of moisture condensed from the air into ink and glue is greatly reduced.  It can take the following forms: the quality of the diluent used is better than the standard or other regions; The temperature and humidity of the environment are relatively low; Add curing agent to the ink; There is less active hydrogen in the inks used; The volatilization of solvents in glue trays and ink trays is effectively suppressed.

 

3. Different Manifestations of “Glue does not dry” on Single Laminate Film and Film Rolls/ Laminate Web

The phenomenon of “glue does not dry” can be classified into two categories: “localized non-drying of adhesive” and “overall non-drying of adhesive” :

localized non-drying of adhesive

overall non-drying of adhesive

a single laminate film

the non-drying of adhesive is present only in specific areas of the film, such as the inked parts.

the non-drying of adhesive is present on 100% of the film's surface.

batch-produced laminate films (multiple film rolls)

the non-drying of adhesive being present only on certain portions of the laminate films, such as the inked areas, but not affecting all rolls equally.

the same non-drying of adhesive phenomenon is present on all laminate films.

 

(1) Factors leading to the “non-drying of adhesive” phenomenon on a single film:

For single laminate films, the “overall influencing factors” that may cause the entire film to exhibit the “non-drying of adhesive” phenomenon are:

① Insufficient curing; 

② Lack of curing agent or insufficient amount of curing agent added; 

③ Active hydrogen and moisture brought into the adhesive by diluents; 

④ Moisture condensation from humid air entering the adhesive; 

⑤ Moisture on the dampened substrate; 

⑥ Excessive curing agent.


For single composite films, the “localized influencing factors” that may cause specific areas of the film to exhibit the “non-drying of adhesive” phenomenon are:

① Active hydrogen present in the ink itself; 

② Active hydrogen and moisture brought into the ink by diluents; 

③ Moisture condensation from humid air entering the ink; 

④ Moisture on the dampened substrate; 

⑤ Excessive curing agent.


(2) Factors leading to the “non-drying of adhesive” phenomenon on the laminate web:

For batch film rolls, the “integral influencing factors” that may lead to the phenomenon of “glue does not dry” in the whole batch of fim roll are:

① insufficient curing;

② No curing agent added or insufficient amount of curing agent;

③ Active hydrogen and moisture brought into the glue by the diluent;

④ moisture on moisture-affected substrates;

⑤ the active hydrogen contained in the ink itself;

⑥ Active hydrogen and moisture into the ink by the diluent;

⑦ Excess curing agent


For batch film rolls, the “local influencing factors” that may cause a part of the whole batch of laminate film roll to have the phenomenon of “glue does not dry” are:

① Uneven temperature in the curing room;

② Large diameter deviation of film rolls; ③ Moisture condensation from humid air entering the adhesive; ④ Moisture condensation from humid air entering the ink.

It is worth noticing that:

① The factor of “dampened substrate” will be expressed to varying degrees in PA, aluminum foil, and aluminum-coated thin films (including VMPET, VMCPP, VMCPE). In other words, besides PA film, aluminum foil, and aluminum-coated films also have moisture-absorbing properties.

② The moisture-absorbing properties of PA, aluminum foil, and aluminum-coated films will only manifest when these substrates are not well-packed to prevent moisture and in high-temperature and high-humidity environments. Therefore, some customers may report that there is no “glue does not dry” phenomenon in composite materials containing aluminum foil during winter, but the phenomenon appears during summer.

③ The different “dampened” substrates mentioned above will have different contributions to the “non-drying of adhesive” phenomenon on single composite films and batch-produced film rolls.

④ Curing agents containing isocyanate are in a fluid state under normal conditions (when not in contact with active hydrogen, base, or moisture). When an “excessive curing agent” situation occurs, the excess curing agent can also cause the adhesive layer to exhibit a certain degree of stickiness. However, when the composite film is peeled off, and the adhesive layer containing excessive curing agent is exposed to humid air, the curing agent on the adhesive layer's surface will rapidly undergo a chemical reaction with the moisture in the air, causing the stickiness to gradually disappear. This explains why some composite film samples may initially exhibit stickiness in the adhesive layer but lose the stickiness after repeated adhesion or storage.


4. Methods to Identify the Causes of “Non-drying of Adhesive”:

(1) Repeatedly adhere the film sample (already peeled off) that exhibits the “non-drying of adhesive” phenomenon. If the stickiness gradually disappears, it indicates an “excessive curing agent.” If the stickiness remains unchanged, it indicates an “insufficient curing agent” or “insufficient curing.”

(2) Breathe on or leave the peeled surface of the film sample showing the “non-drying of adhesive” phenomenon in an “open” environment for several hours. If the stickiness disappears, it indicates an “excessive curing agent.” If the stickiness remains unchanged, it indicates an “insufficient curing agent” or “insufficient curing.”

(3) Subject the film sample (not yet peeled off) that exhibits the “non-drying of adhesive” phenomenon to a secondary curing process in an 80°C electric oven for several hours. If the stickiness disappears or weakens, it indicates “insufficient curing.” If the stickiness remains unchanged, it indicates an “insufficient curing agent” or “excessive curing agent.”

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