Flexible packaging: inks, coatings, curing and laminating

• Regulations
• Performance
• Economics

These areas will be examined during this particular chapter.

A number of previous chapters have so far referred to why UV-curable inks are now being used for flexible packaging: shorter runs, reduced delivery times, personalization, differentiation, etc. All factors which point to why narrow-to-mid-web flexible packaging technology presses are often the best solution for such applications. This can be amplified in the following diagram (Figure. 5.1)

This diagram provides a simplified illustration of what is happening nowadays in the market? Wide webs are primarily best for jobs of, say, 100,000 square metres, run at very high speeds. However, with the trend to shorter runs, personalization, differentiation, etc., a better suited technology for the short-run of less than 5,000 square meters can be narrow-web presses, while mid-web technology is better suited to run lengths between 5,000 and 50,000.

These figures are not related to future trends, they show what is happening now. It also explains why label converters, already well-used to printing shorter runs, versions and variations, see flexible package printing as a market opportunity. Certainly, when looking at the printing/ink technologies that are used nowadays on these two market sector applications, then UV-curing, widely used by label converters, is a technology to consider.

These trends, and with them the trend to move from long runs on wide webs to shorter runs of flexible packaging printed on narrower web presses using UV-curable inks, becomes highlighted by the fact that some 70 per cent of all flexible packaging is food related – it’s used to wrap food.

Therefore, the inks used to print the flexible packaging must be acceptable in terms of UV Food Contact Material (FCM) inks.

Historically, the term low migration inks was widely used. Today, there is an agreement between all the different ink manufacturers, including members of the UVFoodSafe Group, to no longer use the term ‘low migration ink’ but to all use ‘Food Contact Material’ inks. So how are these FCM inks defined?

Food Contact Material inks are inks that, if correctly applied and correctly cured, and with the right choice of packaging concept, the legal migration limits can be met. But, what are these legal migration levels?

• It’s 60 PPB for the overall migration level of substances.
• It refers to SML or a specific migration limit for those, let’s say, substances of which toxicological data are existing – and for those substance that exist are considered good evaluators.
• Where toxicological data are non-existing, the limit is 10 PPB.

How can PPB (Parts Per Billion) perhaps be more simply defined or made understandable? Well, one PPB can be compared with one tea spoon of water in an Olympic-sized swimming pool. So, if we are talking about PPB, this is maybe, a much easier way for it to be visualized. Essentially, we are looking at very, very, very small quantities when we discuss migration limits (Figure 5.2).

MIGRATION

Migration is undoubtedly an important consideration when talking about flexible packaging inks and their performance, but what are the different ways that migration can occur?

The first one is penetration migration. What does this mean? Essentially, the ink is able to go through the substrate. If the substrate is a plastic and it’s PP or PET, PVC, or whatever, it’s always some form of barrier, and some plastics are better barriers than others, like PET is a better barrier, but still it is nevertheless a non-full barrier. It’s a plastic, and all plastics are non-full barriers. They will allow some level of migration.

That’s the first kind of migration. The second area of migration, also referred to already a couple of times in previous chapters, is set-off migration in the reel, whether the set-off is from outside or inside contact. This is something that needs to be taken into account.

The third way that migration can occur is via vapour phase migration. That is volatilisation of compounds during cooking. This will have an effect on how the ink or coating can penetrate through substrates.

The last way that migration occurs is through condensation extraction – that is the condensation of critical components when cooking/sterilization takes place.

These different forms of migration relating to the safety of food packaging are illustrated in Figure 5.3.

Already mentioned earlier, the three overall requirements of UV-curable FCM inks are: Regulatory, Performance, Economical.

We need the Performance of the inks, especially after they are cured (and through all stages of production, forming, filling, distribution, and consumer usage). But, also, of course, by complying with the required Regulatory landscape. The regulatory landscape and the overall and various regulation and legislation requirements are driving a lot of what both the ink manufacturers and converters have to comply with today.

Overall, these regulatory requirements can be identified as follows:
No Specific European harmonized legislation for inks but several legislative instruments which impacts materials and articles for food

Some examples in the EU:

• Regulation EC 1935/2004.
• Regulation EC 2023/2006: rules on GMP
• Directive 10/2011 (Plastic Regulation)
• Swiss ordinance is the first specific and complete legislation on printing inks
• Nestlé Guidance Note on packaging inks.

The various regulations identified above oblige the whole industry, starting from the raw materials suppliers, or cell and ink manufacturer, the printer or converter, and even the brand owner, to comply with certain regulations.

The bigger challenge that the whole industry is facing now is that there is not one specific harmonized legislation for inks that exists today. But, there are several regulations that refer to them. They have just been mentioned. It’s the Regulation EC 1935/2004. It’s the Regulation EC 2023/2006 and there is the plastic regulation. Next to these, there is the Swiss ordinance, which was the first complete legislation on printing inks.

That’s why, more or less, the whole industry is always referring to the Swiss ordinance. It’s legislation that is really existing.

Then, of course, there are the guidance notes from Nestlé on the brand owner requirements, and there are also a couple of other brand owners that are working in this area and have their own guidance notes – which have both a positive and a negative list. The positive list meaning ingredients that the ink maker can use; the negative list meaning things that are not allowed to be used.

It is this regulatory landscape that is pushing the whole flexible packaging industry to be compliant with these various regulations and why ink makers have to undertake several things – the most important of these in order to be compliant is, first of all, raw materials selection.

RAW MATERIALS SELECTION

Ink makers that want to make an FCM ink, need to start with selecting the right raw materials. What does that mean? The raw materials should be Swiss listed and they should be compliant with all the required regulations.

Next to that, the ink maker needs to make sure that they know everything about the raw material being used. Does it have smaller molecules in there like stabilizers, like a catalyst; also any small ingredients that are there, impurities; everything needs to be known, all the constants. There are raw materials suppliers on the market today that have dedicated raw materials for this and they can be worked with. So, that’s very important.

Another thing, which is driven by EUPIA, the European Printing Ink Association, is that you cannot use CMRs: carcinogenic, mutagenic or reprotoxic cat 1A and 1B raw materials. So, each and every member of EUPIA has signed this policy to not use any CMRs.

The ink industry was also confronted in 2018 with some raw materials that were commonly used in UV inks shifting from being okay to being labeled as CMR. This resulted in a lot of work having to be done in terms of reformulating all these areas.

ECONOMICAL ASPECTS

Apart from the regulatory and technical performance requirements of FCM inks, it is also necessary to consider the economical aspect of the inks as well. The flexible packaging industry ideally wants FCM inks at the same price as non-FCM ink, so they need to also fit the purpose. It’s not just the regulatory and performance evaluations that, let’s say, give ink manufacturers a smaller opportunity, there are also the various economical considerations that have to be taken into account. For the ink manufacturer it is quite a challenging job to put all these different and varied requirements together.

GOOD MANUFACTURING PRACTICES

It is also important for an ink manufacturer to have the right production process for flexible packaging inks. They need good manufacturing practices (GMP). These are shown in Figure 5.4. and, as can be seen, include raw materials approval, raw materials storage, traceability, dedicated production area, log sheet layout and quality control.

So, what does good manufacturing practice actually mean?

On top of the ISO-9001 standard, it is necessary to undertake a number of things in order to be GMP approved; ink makers need to have an adapted raw materials approval process. All the raw materials being used need to be the right ones. They need to be stored in a dedicated area.
The production area also needs to be dedicated.

For instance, at Flint there are two floors; the second floor it’s only for FCM inks. The FCM inks are only made on that floor. Everything is completely separated.

There needs to be traceability, very important, at every stage, so that if there is any kind of problem each and every company in the value chain or in the supply chain is able to track back so that the problem can be identified and rectified. Traceability is therefore very, very important in the manufacture of FCM inks.

Next comes extended quality control that manufacturers also need to organize, record and document the GMP, which is not only value for the ink manufacturer, but also for people down the value chain. That’s why recommendations that we make to printers and converters need to be followed.

These recommendations are preferably to have dedicated presses to ease the handling, and avoid contamination of the press.

If not possible, very rigerous cleaning processes are required in between different print jobs. So, when a job which is a non-food, or a job which is indirect food, is done on the same press; clean it very, very well. It’s extremely important. If there are special products being produced the ink supplier can also provide guidance on the process – but, again, dedicated presses are the ideal case.

DESIGNING A UV FCM INK

Coming back to the raw materials and the regulations and the limitations that the regulations provide, as these get more and more stringent, the ink maker is targeting and designing for windows that get smaller every year. Designing a UV, FCM ink, is now within a design window that has shrunk considerably because of the regulations. These mean no CMR and so forth.

There are fewer building blocks available to make the ink (see left hand side of Figure 5.5). Yet, still, the technical performance should be the same as non-FCM inks, with regards to colour development, with regards to adhesion, printing speed and so.

INK CURING IS CRITICAL

The critical parameter in the whole UV, FCM ink process is the curing aspect. Everybody in the value chain, until the converter, can do a fantastic job in having good and proper and non-CMR raw materials, put them together in an ink – and then not process it following a proper Good Manufacturing Process (GMP) at the converter.

The final, very important step in the process, is the curing. Other things are also important, but if the ink is not properly cured the chances of having migration occur are higher than when it’s properly cured. So, why is curing so critical?

First of all, there is a higher risk of migration and this is demonstrated by facts. There is a consortium in the industry, called the UVFoodSafe Group, which includes ink manufacturer, materials suppliers, and press manufacturers. This Group undertook a trial together which demonstrated that the less curing that takes place, the higher the migration.

When people say they don’t want migration, it should be understood that, at science levels, there is always going to be some migration. It’s better to control that migration and to know what is migrating because, in practice, there is always a little bit taking place. It’s better to know which specific substance is migrating, to know what the limit is, to control this. It’s much more important than to say the aim is zero migration.

Without proper curing in FCM applications there will be less good scratch and solvent resistance.

So, what do printers/converters need to do to ensure proper curing?
•
Monitor the printing speed. The higher the speed is, the higher the risk of having higher migration. Each and every aspect needs to be taken into account. Next to the curing there is also the thickness of the ink. There is the substrate, there is the adhesion. They have multiple parameters, but the UV curing speed is important.

•
Monitor chill roller temperatures. If you have a very thin substrate, heat sensitive, it needs to be cooled. Fully understandable is that heat helps the curing process. So, it is necessary to find the right balance between how far to cool or heat the substrate up a little bit – so that the substrate is not damaged.

•
Monitor the ink thickness. The thicker the ink, the more difficult it is to cure. So, that’s another aspect that can be monitored.

•
Check the UV lamp intensity and check the lifetime of the UV lamps because intensity of a UV lamp is going to go down in time. That is also something that needs to be monitored.

•
Check UV lamp reflectors. Make sure they are clean and doing their job effectively.

Now, let’s consider the sources that produce UV light. There are two UV light sources nowadays that are predominately used on the market. One that has already been there for many, many years, which is the UV Mercury lamp. It’s very well known by the label printers. Then, more recently in the last couple of years, there has also been a big increase in the use of UV LED lamps.
As can be seen in Figure 5.6., UV light Mercury has an output wavelength between 200 and 400 nanometers, while UV LED is only at 385 and 395 nanometers.. It is monochromatic light that is shining on the ink. The wavelength and the light intensity dictate the chemistry.

The UV Mercury is referred to as conventional UV - and it should be noted that conventional UV inks will not cure when exposed to UV LED light.
So, from an ink perspective, if label or flexible packaging printing customers go to UV LED, there is a different chemistry that is used. This means that at Flint for example, they have FCM inks for UV Mercury and they have also UV LED FCM inks that are available.

It should also be noted that there are different UV LED lamp suppliers on the market. Some of these use 385 nanometers, others are 395 nanometers. It is therefore important for the printer/converter to know and understand all the parameters that could influence the curing. These include:

• The irradiance output
• The distance between the ink and the lamp
• The angle of the lamp to the web.

As previously stated, it is also important to know the press speeds and so forth. All the parameters that could influence the curing need to be checked, with job printing conditions documented very well.

BARRIERS, BARRIER COATINGS AND LAMINATED WEBS

Chapter 2 of this book looked in quite some detail at the substrates used for flexible packaging, and stating that there is no perfect material, which means that many materials used for flexible packaging involve the use of multi-layers which are created by bonding together (laminating) two or more materials, whether paper, plastic or foil, to create the required barrier.

Mention was also made in Chapter 4 of barrier coatings that can be applied to flexible packaging to prevent migration of ink, adhesive, or other substance through the face material. Barrier coatings provide barriers for food packaging requirements, and may include protection against oxygen and aromas, liquid water and water vapor, oils, and grease.

An effective barrier can prevent both losses from the packaged product, and penetration into the package, both of which can affect quality, and shorten product shelf life.

Packaged food products are being maintained fresh longer as a result of new materials, and food processing developments. For example, O. scavengers are now being used that work within a sealed package to limit O. reaction with a food product. Combined with effective O. barrier packaging, food packagers have the ability to improve shelf life, preserve product appearance, and flavor, while minimizing preservative use.
Knowledge around migration and the various methods of barrier protection is therefore crucial for servicing the flexible packaging food markets.

Suppliers must be able to provide the right barrier or seal for a given application and be able to support printers and converters with ‘fitness for use’ testing.

But, just to be clear, what do we actually mean when we talk about a barrier?
Let’s use Figure 5.7. to explain.

In this, we can say that a pane of glass is a barrier. Try putting your face to a pane of glass at home. You won’t get through it. It’s an effective barrier. Aluminium of a certain thickness of say, 12 micron and above, is also an effective barrier.

What is not a barrier? Well, paper and plastics films. Even if you have a PET or polypropylene films from 40 or 38 micron, they are not an effective barrier. There are always voids in film. It is never 100% closed. Migration is able to occur. The thicker the plastic, the longer it will be before migration takes place – but it will still occur.

Providing barrier layers, whether through laminate constructions or the use of barrier coating, are properly constructed and applied, and everything is done as it should be, there should be absolutely nothing to worry about.

MIGRATION TESTING

To build further on migration and all the related regulatory, production, performance and curing requirements, how is migration testing carried out? The overall requirements are illustrated in Figure 5.8.

 Migration testing is the process in which we are measuring the amount of migration from a cured sample into a migration cell: the printed package is brought into contact with a simulating solvent. It’s not the ink itself that is in contact, it’s the other side of the print. The cell can also bring in different temperatures. It can even be kept for different types of printed packaging at specific temperatures. The third part is to use the right analytical techniques.

Depending on the food type that is going to be wrapped and the plastic that Is being printed, and depending on the food type being packaged, the right simulant needs to be chosen. This can be examined further by studying Figure 5.9.

As can be seen, for fatty foods the simulant needs to be a vegetable oil (in water): for dairy products it is 50 per cent ethanol (in water), and so forth. Because of this wide variety, the industry accepted to have one a universal simulant, which is 95 percent ethanol (5% water).

The next element of the test is how long should the non-printed side of the sample be in contact with this simulant in the cell? How is the choice of exposure test conditions determined? This depends on the way the food is going to be used.

Will it be used for example, at room temperature, in the refrigerator or in the microwave? The answer will have an impact on the temperature and the time that is needed need to bring the non-printed side of the sample into contact within the cell. This is shown in Figure 5.10.

The most common condition used is 40 degrees Celsius for ten days. This is the generally accepted and most widely used whole industry test method to do migration testing. Bring the non-printed side of the sample in contact with 95 percent of ethanol for 10 days at 40 degrees Celsius.

After 10 days, analytical techniques are used to examine what’s inside the cell. The institutes that are performing migration tests will therefore need to know what to look for; a statement of the composition of the ink is always given to the institute doing the testing. They will use two techniques;

• Liquid Chromatography with Mass Spectrometer (LCMS) and Gas Chromatography with a Mass Spectrometer detector (GC-MS)
• to look for possible migrants that are coming from the ink. Figure 5.11.amplifies the way that identification and quantification of migrants is undertaken

CONCLUSION

Within the flexible packaging industry, both in Europe and outside of Europe, there are printers and converters using UV Mercury and UV LED FCM inks. They are safe and working to produce UV printed flexible packaging, and there are even big brand owners and small brand owners that are already using and accepting this technology for indirect food containers.

The same applies to shrink sleeves. Do not forget that the shrink sleeves are printed on the inside of the sleeve. However, if the bottle is a plastic, ink migration can take place through the bottle. Yes, the bottle is normally thicker and the chance of migration is lower, but in principle, scientifically speaking, microscopic levels can still be there.

Nevertheless it should be emphasised that UV curing is a demonstrated technology that’s been working for flexible packaging. To continue this technology’s success, it is very important that there is an open collaborative spirit throughout the whole supply or failure value chain. See Figure 5.12.

It’s important that everyone in the supply chain works with each other in an open way, starting with oversight of the supplies.

Supplies of the raw materials to the ink manufacturers are very important. Then, the ink manufacturer in turn, needs to be giving full information to the printers and the converters and on the food companies, as well as full information to the institutes that do the migration testing. All the different possible migrants and all the different ingredients in the ink need to be given to the institutes so that when they receive a sample from a printer to do migration testing, they know exactly what to look for.

It is also recommended that the printer/converter also implements GMP if possible – dedicated presses, monitoring of the ink, the ink thickness, the temperature of the chill roll, the substrate being used. The UV curing; whether this is LED or Mercury, and also very important that they know what the printed packaging is going to be used for. What is happening after printing? Will the pack be going through sterilization; is there sealing and so forth?

The message is to only do the things in the right way, promote UV technology to the brand owners, and to perceive that it is a safe technology when undertaken correctly.

WHY PRINT PACKAGING IN NARROW WEB?

• What is the typical run length / job?
• Printers using wide web presses
• Printers using narrow web presses
• Too expensive
• Best for jobs
• > 100,000 sqm
• Best for jobs
• < 5,000 sqm
• Mid-web best for
• jobs between 5,000 - 50,000+ sqm

and here more and more packaging jobs end up here.