Guowang Forum: Causes of and Solutions to Delamination in Laminated Products — A Record of the QQ Group Discussion

Planning: Editorial Department of Packaging Frontier

Execution: Shen Xiaoyun, Fan Yuechan, Huang Kaili

On the afternoon of June 22, the online technical exchange event themed "Causes and Solutions for Delamination of Laminated Products", organized by the Information Committee of the Guangdong Packaging Association and Packaging Frontier, and exclusively sponsored by Beijing Huateng (Guangdong Guowang), was held simultaneously in two QQ groups and three WeChat groups of Packaging Frontier, with more than 3,000 group members participating online.

The technical exchange was given by Mr. Zheng Fangming of Beijing Huateng (Guangdong Guowang). Mr. Zheng introduced various causes of lamination delamination from aspects such as low lamination strength, penetration damage by media, decay during storage, and metallization transfer, and gave corresponding solutions to prevent delamination.

I. Causes and Solutions for Delamination of Laminated Products — Presented by Zheng Fangming

Apart from lamination appearance and winding flatness, the main problem troubling flexible packaging laminators is the delamination of products in the later stage. Compared with surface white spots, bubbles, and winding flatness, delamination of laminated products is not easily detected immediately by operators.

In many cases, by the time a specific delamination condition is found, a large number of finished or semi-finished products have already been produced. Therefore, timely detection and taking preventive measures to reduce or avoid delamination are particularly critical.

(I) Preparations Required to Prevent Delamination

1. Analyze the substrates to be laminated

This includes plastic film materials, metallized films or aluminum foil, as well as inks and coatings on the film surface. According to the characteristics of different materials, and different types and quality conditions of inks, set the corresponding process parameters, such as adhesive application weight, lamination tension, machine speed, and the pressure of the drying tunnel and laminating rollers. Subsequent curing conditions should also be adjusted accordingly.

1. Processing requirements for the product in the later stage

For example, vacuum packaging, 100℃ water boiling, 121℃ retort, 135℃ retort, etc. According to the subsequent process requirements, select appropriate printing inks and determine whether an ink curing agent needs to be added. Also select suitable adhesives and appropriate mixing ratios, and ensure sufficient adhesive application weight to meet temperature resistance and hydrolysis resistance requirements.

1. Clearly understand the packaged contents

If the product to be packaged later contains acidic or alkaline media or strongly penetrating solvents, such as alcohol, acetic acid, methanol, toluene, or emulsifiable concentrate pesticides, or special food additives such as the flavor enhancer ethyl maltol, it is necessary to understand the types and specific contents of these substances in order to determine a suitable adhesive and set suitable process operating conditions.

These different types of substances have different effects on packaging bags. Solvent-based substances generally penetrate and migrate through the inner film. If they encounter high-barrier materials such as aluminum foil or metallized film, they will accumulate on their surface, swell the other plastic film and the adhesive, and "push open" the two films, thereby causing delamination.

Another case is that acidic or alkaline substances pass through the inner heat-seal film and come into contact with the adhesive, react with it, hydrolyze or degrade it. The adhesive after chain scission loses its original bonding performance and causes delamination.

1. Storage life of packaging films and bags

Apart from contents containing special media, food packaging products can basically fully meet the storage and sales cycle of food. However, for laminated products used in construction and decoration, as well as products placed outdoors for a long time, corresponding accelerated aging tests must be done to ensure that the products are suitable for the requirements under specific conditions.

Attention should be paid to the influence of environmental factors, such as high temperature, ultra-low temperature, or high humidity. For long-term storage at high temperature, the adhesive needs good temperature resistance and heat creep resistance. Similarly, at ultra-low temperatures, the adhesive becomes brittle and is also prone to delamination. High-humidity environments accelerate the hydrolysis of the adhesive, also leading to reduced strength or even delamination. Especially for metallized and aluminum foil products, the aluminum layer is easily oxidized and corroded gradually from the edges of the film or bag.

After all, polymer materials, including plastic films, printing inks, and adhesives, all have a process of aging and performance decay, including yellowing, embrittlement, and chalking. All these require us to have a certain understanding of the application occasions of the products.

1. Reasonable selection of ink

Before printing, it is necessary to determine whether general-purpose ink or temperature-resistant ink is needed.

When polyurethane reverse-print laminating inks first appeared, they all needed to be used with an appropriate amount of ink-specific curing agent. However, due to cost considerations, many ink manufacturers are using one-component reverse-print inks even for laminated products that require retorting.

In this way, the ink resin contains a large amount of active hydrogen substances, and the impact on subsequent lamination cannot be predicted. At the same time, factors such as the service life of the ink, the preservation and reuse of residual ink in the later stage, and the prevention of ink drying on the plate will also affect the choice of ink by color printing companies.

1. Selection of adhesive

In the entire lamination production process, the cost proportion of adhesive should be the smallest. However, the role it plays is particularly important.

For specific performance requirements, it is necessary to select an adhesive that can meet the requirements.

(II) Common Types of Delamination and Solutions

1. Delamination found immediately off the machine, or low strength and no initial adhesion

For example, in solventless lamination, or lamination with some aluminum foil-specific adhesives with relatively large molecular weights, after the solvent is dried, the surface state appears relatively "dry". If it is laminated with a thicker second substrate, such as RCPP, a "delaminated" state with no initial adhesion may occur. This situation is relatively common in winter when the temperature is low.

In this case, samples can be taken and cured/baked at about 50℃. Generally, the strength improves significantly after ten-odd minutes. Otherwise, check whether the adhesive quality is normal or whether the surface treatment of the film is normal.

1. Delamination after curing

This includes cases where the peel strength is extremely low after curing. First, determine which film the adhesive is on. If it is difficult to judge with the naked eye, qualitative analysis can be carried out with an infrared spectrometer.

After determining the delaminated film, analyze according to the characteristics of the film whether it is a surface treatment problem, insufficient adhesive bonding strength, or caused by the precipitation of additives such as slip agents in the film (such as PE).

1. Delamination of laminated film or bags after storage at room temperature

Generally, for laminated products, counting from the completion of lamination, the adhesive is considered to be nearly 100% cured after 7 days, and the peel strength tends to stabilize.

However, laminated films, inks, and adhesives will undergo aging and decay to varying degrees under different temperature, humidity, and light conditions. Long-term storage may lead to a decline in peel strength or even delamination.

For general packaging contents, if delamination occurs after short-term storage, and on the premise of excluding the possibility that the contents contain special penetrating media, it is necessary to determine whether the storage environment has an influence and whether the aging stability of the adhesive meets the requirements.

1. Delamination during bag making, commonly known as heat-seal delamination

This situation is mostly seen when the adhesive has not been fully cured. During heat sealing, the high temperature softens the adhesive, and displacement under stress easily leads to delamination.

In this case, it is necessary to consider several aspects: whether the adhesive mixing ratio is appropriate, whether the quality of the printing ink is reliable, whether the solvent purity is qualified, and whether the solvent residue of printing and lamination meets the standard.

1. Delamination after water boiling or retorting

There are also two cases here: water-boiled and retort packaging for ordinary contents, and water-boiled and retort packaging for contents containing special media.

First, it is necessary to determine whether the choice of adhesive is appropriate and whether it is used according to the recommendations of the adhesive supplier. For products with temperature resistance requirements, especially products containing aluminum foil, the production process requirements are also stricter than those for ordinary products, such as controlling the solvent residue of printing and lamination, ensuring sufficient adhesive application weight, and setting suitable curing conditions including curing temperature and sufficient curing time.

For products whose contents contain special additives or penetrating solvents, preliminary testing work must also be done. These media are numerous and have different degrees of influence, so more testing is needed before mass production.

1. Delamination after storage of contents containing special media

After packaging specific contents, or after corresponding water-boiling or retort sterilization operations, delamination occurs only after being left for a period of time. For such cases, aging storage tests must be carried out to ensure that the packaging bag can withstand the long-term penetration effect of the contents.

1. Delamination of metallized laminated film and solutions

Delamination of metallized laminated film, also often called metallization peeling or metallization transfer, has always troubled flexible packaging companies. Its direct manifestation is that the freshly laminated product has good initial adhesion strength, but after curing the peel strength drops significantly, and when peeled open, the metallized layer is found to have transferred entirely or mostly to the other substrate.

This phenomenon mostly occurs in PET printed film laminated to VMPET with PE film as the heat-seal backing, and PET printed film or OPP printed film laminated to VMCPP also has a relatively high probability of occurrence.

The delamination of metallized laminated film is closely related to the quality of the metallized film itself. Poor-quality metallized film is very likely to cause this type of quality problem. In addition, there is also a view that if the adhesive has large reaction internal stress and crystallinity, it is also easy to cause damage to the metallized film and lead to delamination.

When dealing with the problem of metallization transfer, the common approach is to improve the flexibility of the adhesive film, so that the stress formed at the metallized interface that damages the coating is absorbed and released, thereby slowing down metallization peeling. The most common practice is to reduce the amount of curing agent so that the crystallinity of the adhesive layer is reduced and it becomes relatively soft. However, when using this practice, it is recommended to do sufficient testing to ensure the later peel strength and avoid wrinkling and tunneling caused by the adhesive not being dry.

In addition, shortening the curing time is also conducive to the slow release of the reaction internal stress of the adhesive. However, to ensure a good curing effect, it is recommended to shorten the curing time while appropriately increasing the curing temperature.

1. Heat-seal delamination of laminated film

This is the delamination phenomenon during bag making. The general reasons are as follows:

① Poor heat resistance of the adhesive.

② Imbalance of the adhesive mixing ratio. This includes several situations: first, whether the curing agent has deteriorated and become ineffective to a certain degree; whether there is a mixing error in production operations; whether the purity, alcohol content, and moisture content of the solvent exceed the standard; whether the solvent residue of printing, the quality of the printing ink, and the residual active hydrogen substances exceed the standard; and environmental factors such as whether the humidity is too high.

③ Poor curing state. Consider whether there is sufficient time to cure at a suitable temperature.

④ The influence of solvent residue. Whether in the printing process or the lamination process, if the amount of residual solvent is too large, between the film layers, on the one hand it has a swelling effect on both the adhesive and the film, and on the other hand, volume expansion when heated will also cause delamination to a certain extent.

1. Ink peeling (transfer) and delamination of printed film

Printed films generally use BOPP, PET, or nylon film.

For ink transfer on BOPP film, it is generally because the polarity of PP material is relatively low, the adhesion of the printing ink on its surface is insufficient, and it is transferred to the inner film by the adhesive.

For laminated products using BOPP as the printed film, the peel strength requirement for this layer is generally not too high, and BOPP film is also easy to break when pulled.

However, for PET printed film, this film has strong polarity. Even without treatment, its surface tension is above 42dyn, and it basically has good adhesion to most reverse-print inks.

But after coating with a two-component polyurethane adhesive and laminating with other films, especially with flexible inner films such as PE film, during the peel test, it is sometimes found that the printing ink layer transfers to the inner film. The thicker the inner film, the more obvious this situation is.

Why does this happen? This is actually closely related to the characteristics of PET film. This film has good barrier properties and strong penetration resistance, and similarly its permeability to printing solvents is also poor. When the solvent residue in the printing process is too high, the solvent in the lamination process will more easily penetrate into the ink layer, and the solvent will be more difficult to volatilize completely.

Various solvents accumulated between the printed film layers wander, penetrate, and swell between the film layers, resulting in poor peel strength of the printed film layer in the later stage, and even delamination.

To avoid this kind of ink transfer problem, pay attention during printing and lamination:

(1) Control the printing solvent residue well, and thoroughly dry the ink layer to prevent false drying of the ink layer. If limited by the working conditions of the printing press and the drying effect is poor, the printed film can be rewound once on the laminator and dried through the drying tunnel of the laminator to ensure that the printed film can release the solvent completely.

(2) The same applies to the lamination process. The solvent in the adhesive should volatilize as much as possible when passing through the drying tunnel, ensuring that the heating and exhaust systems of the laminator's drying tunnel are in good working condition.

After all, lamination delamination has various forms, and the handling methods of different color printing companies are not the same. Companies should investigate according to their actual production conditions and continuously summarize experience, and they will certainly be able to solve the lamination delamination problem well.

II. Free Exchange Session

1. Solvent residue

Xiaoyang from Zhongtuo: Generally, at what solvent residue level does a problem easily occur?

Zheng Fangming from Guangdong Guowang:

There is no very definite number. Different materials and different ink thicknesses will give different results. Generally, the requirements for sampling lamination are still used as the benchmark.

It is recommended to use a good printed sample as the standard sample, and after testing, use that value as the standard. According to our summary of solvent residue testing for color printing plants over the years, generally the printing solvent residue can be controlled below 2mg/㎡. This is sampled and analyzed immediately off the machine; of course, in many cases it can be made even lower.

Xiaoyang from Zhongtuo:

Yes, this requires long-term data collection. At present, many companies have a lot of ink, especially old ink, and there are some loopholes in storage methods, which lead to ink aging and often result in low strength after printing and lamination. For PET/VMPET/PE and CPP, regarding heat-seal materials, are there any points that need attention or control to alleviate the metallization transfer effect?

Zheng Fangming from Guangdong Guowang:

This mainly depends on the quality of the metallized film. Moreover, with generally thick backing films, the phenomenon of metallization transfer occurs more often.

Minimize the adhesive application weight when laminating the printed film, and reduce the tensile deformation of the backing film.

Zheng Fangming from Guangdong Guowang:

Adjusting the adhesive mixing ratio is also relatively common.

I hope everyone will pay attention to improving the coating condition.

Xiaoyang from Zhongtuo:

The aspects involved in metallization transfer are indeed quite broad, from ink to solvent residue, adhesive application weight, adhesive mixing method, drying tunnel temperature, winding tension, and curing conditions, etc., and they are also inseparable from metallization strength.

1. Xiao Zhang from Mingyou Packaging: Our new order is MOPP19//(VMPET-PET)12//PET12//CPP65. Among them, MOPP19//(VMPET-PET)12 is planned to use water-based adhesive, (VMPET-PET)12//PET12 is planned to use solvent-based adhesive, and PET12//CPP65 is to be laminated with solventless adhesive. This order has not been produced yet. This structure is being made for the first time, and it is a bag with different front and back designs. For the MOPP/(VMPET-PET) layer, will the peel strength be poor if water-based adhesive is used? Will solventless lamination give slightly better peel strength than water-based adhesive?

Water-based adhesive generally does not cause delamination. In addition, the PET inside should use double corona treatment. To prevent metallization transfer, if solvent-based adhesive is used, first, apply less adhesive; second, cure at as low a temperature as possible; third, reduce the tensile deformation of the film.

Xiao Zhang from Mingyou Packaging:

Different colors of ink also have different peel strengths, but our company's standard for peel strength is only a single value, without distinguishing the peel values of each color, so we often have to go through the review process. For structures like BOPA15/PE52 (jelly sticks, frozen products), what is the general peel standard?

It is normal for different colors of ink to have different peel strengths; it is only a matter of the size of the difference.

Zheng Fangming from Guangdong Guowang:

NY laminated with PE can generally achieve relatively high strength. Are you more concerned about its cold storage resistance?

Xiao Zhang from Mingyou Packaging:

We make BOPA/PE jelly stick products, and the peel strength of different batches is sometimes good and sometimes bad, unstable. We previously received complaints about jelly stick bag breakage, so we are more concerned about the peel issue.

1. If water-based ink is used for printing and then solventless lamination is carried out with some high-barrier films, what issues need attention?

Zheng Fangming from Guangdong Guowang:

The biggest concern with water-based ink printing is the drying effect of the solvent (water). Compared with esters and alcohols, water is more difficult to volatilize. Solventless adhesive contains a large number of NCO groups, which react with water molecules to produce CO2 gas.

When doing lamination, one may not necessarily understand the adhesives of every manufacturer, but it is best to be relatively clear about the common adhesive types and performance classifications on the market. This will help choose suitable adhesives for different laminated products.

Zheng Fangming from Guangdong Guowang:

Yes, in this way an appropriate adhesive can be comprehensively selected from the aspects of application performance and cost.

1. Sanding Packaging: What causes the adhesive to turn white after mixing with solvent?

The adhesive turning white may be because alcohol solvent was mistakenly added to the ester-soluble adhesive, causing the resin and solvent to become incompatible and turn white.

Zheng Fangming from Guangdong Guowang:

This situation does indeed occur, and in severe cases it may even clump. Generally speaking, when the adhesive is first opened/mixed, the adhesive is in a high-viscosity state; ethyl acetate has low viscosity and relatively low molecular weight. During this period, phase separation may occur, and the state presented is whitening. Even if no curing agent is added, this state can occur, and it is more obvious for aluminum foil adhesive or high-temperature retort adhesive.

But if it is still white after stirring evenly, then other factors need to be considered.

It should still be white after dilution.

Zheng Fangming from Guangdong Guowang:

Then consider whether the solvent is impure, the wrong solvent was used, or the adhesive quality is the problem, and check them one by one.

1. Zhang from Fangda Packaging: We use solvent-based adhesive. What is the reason for the delamination at the spot heat-sealed zipper position of a three-layer laminate? The structure is PA//VMPET//PE.

Zheng Fangming from Guangdong Guowang:

If the middle layer is metallized, it is relatively more troublesome. The heat-sealed position is subjected to high temperature and a relatively large degree of stretching.

Zhang from Fangda Packaging:

Now it shows decay and delamination.

Lü Minghua from Chongqing Jute:

The delamination should be at the NY/MPET layer, right? If it is the outer layer, confirm whether it is ink transfer or metallization transfer.

Zheng Fangming from Guangdong Guowang:

First observe the state of delamination. If it only appears at the zipper position, it is often because the curing state is poor. I see that this bag is printed with full-coverage ink, so also pay attention to the curing effect of the ink layer. Check whether the outer layer is tacky.

If possible, for ink with a backing layer, add ink-specific curing agent to that ink. Of course, adding more curing agent to the adhesive can also alleviate it.

Fangda Packaging:

The outer layer is laminated on a dry laminator, and the inner layer is solventless lamination. The outer layer has become tacky.

Lü Minghua from Chongqing Jute:

Generally, when NY is used as the outer layer, the stability of the metallized laminated structure is relatively poor, and the metallization transfer phenomenon is also relatively serious. On the one hand, this is because NY has strong polarity; on the other hand, NY as the surface layer has relatively large shrinkage after moisture absorption, which easily causes metallization transfer or strength decay. For this type of structure, it is recommended to use enhanced metallized film.

Fangda Packaging:

Okay, thank you.

Waterborne polyurethane adhesive can effectively solve the metallization transfer problem and can be used for heat-seal bag making and zipper sealing.

Alvin from Jiahe Wansheng:

Aluminum-plastic laminating adhesive can also be used.