UV-Inkjet Printing on Food Packaging: State of the Art and Outlook
By Dr. Roel De Mondt
and Dr. Marc Graindourze
Agfa Graphics NV
Figure 1. Most common mechanisms of food contamination from ink compounds
Figure 2. Sustainability chart for direct print with LM inkjet inks on food packaging
Figure 3. Schematic representation of polymerization of liquid ink by UV-curing
Figure 4. Comparison standard UV-inkjet ink with LM UV-inkjet composition
Inkjet printing has become the major print technology for sign and display applications (posters, billboards, etc.). UV-inkjet printing is the fastest growing technology within this segment because of its combination of high speed and reliable printing, high image quality and high image durability (adhesion, scratch resistance and solvent resistance). UV-inkjet printing, however, is not limited to sign and display. It also is suited for many so-called industrial applications where the print is part of the product, e.g. an interior decoration panel or a phone cover.
Packaging printing is a growing business segment. There are a number of reasons for this. In order to stand out on the shelves, brand owners attach more and more importance to attractive designs. Also, packages need to contain lots of consumer information these days – just think of nutritional tables or allergen information. The diversification within the food and beverage families boosts packaging production, as do shorter product cycles and seasonal variations. Finally, personalization is on the rise. Traditional printing, however, is oriented at printing the same artwork in very high amounts, as it requires extensive pre-work, involving the creation of a master (print plate, flexo sleeve or gravure cylinder, for example), and thus takes considerable set-up time. This makes it less suitable for shorter run lengths and unsuitable for versioning and personalization. Digital printing is not hampered by these constraints.
The advantages of UV-inkjet printing in particular can deliver a strong answer to the functional requirements of packaging printing. UV printing is especially useful as it allows printing directly on the (plastic) packaging, without the need of a label or an ink-receiving layer or primer. Since packaging related to food and beverage accounts for approximately 50% of all packaging, it is very important that the food safety is guaranteed by the correct packaging material and printing techniques. Standard UV-curable inkjet inks are not suited for direct printing on food packaging (except for small ink amounts, e.g. only printing a batch code or expiration date). In contrast, low-migration UV-curable inkjet inks are suitable. Agfa has developed unique and patented low-migration ink concepts for printing directly on food packaging.
UV-inkjet inks need to have low viscosity to be able to be jetted through the small nozzles of the printhead; typical viscosity is 10-15 cP at 40°C. To achieve this, the standard UV-inkjet inks are based on reactive low molecular weight monomers and low molecular weight photoinitiators resulting in a curing degree not higher than 95% (conversion % of monomers to polymer). From the created cured ink layer, un-reacted monomers and photoinitiators can migrate through the substrate and/or set off to the food side when it gets in contact with the print side (e.g. by stacking printed food cups). For these reasons, standard UV-inkjet inks are not suited for printing directly on food packaging.
Food-safe packaging printing requires a controlled process. All the components of the printing process need to be combined correctly. The substrate (barrier quality) and the ink are key elements of the solution, in combination with the printing and process conditions, type of food etc. Low-migration (LM) UV-inkjet inks need to be designed to limit the potential migration (by all mechanisms) of ink compounds into the food; therefore, it is necessary to design the ink differently than the standard UV-inkjet inks. Innovative LM inks were developed and patented, which result in a very high curing degree while still having low viscosity tuned to the jetting process. A combination of low viscous, yet highly reactive monomers and diffusion-hindered photoinitiators results in a very high degree of curing, as derived from the measured amount of residual compounds via total extraction of printed samples. Diffusion-hindered photoinitiators are still low in viscosity but are not diffusing due to their high molecular weight combined with a unique molecular structure. They also can be built into the polymeric network that is formed during UV-curing of the liquid ink very shortly after jetting.
It has been proven that the LM ink can deliver food-safe packaging – such as PET beverage bottles printed on directly with LM ink – without the use of a label. Another example is direct print on plastic food tubs (rectangular, for olive oil, butter, etc.) and food and beverage cups (cylindrical or conic, for yogurt, cheese, dairy beverages and more). Also HDPE closure caps for beverage bottles were printed safely with these inks. This printing method delivers an answer to the current packaging trends as described above but also contributes to sustainable packaging by eliminating the label and by lowering waste, energy, transport, VOC emissions, etc.
Food packaging printing and UV-inkjet printing
There are many trends in food packaging that result in a decrease of the average print run length for food packaging printing. Brand owners consider packaging an extremely important element in building their brand identity: the values represented by the brand are reflected by the printing on the packaging. Another trend is the explosion of variations within a product family (e.g. different tastes), seasoning, private labels, etc., which results in a growing number of different packages printed at lower run length. A fast reaction time is essential to cater to these changing needs. Special designs of the packaging format, local trials of new products, etc. are other drivers towards shorter run packaging printing. Last but not least, the packaging cost often constitutes a large percentage of the total cost of the product. Long lead times – with large intermediate stock levels – result in a considerable supply cost and yield a complexity that needs to be managed.
Inkjet printing is a digital printing technology that is very well suited to fulfill the requirements related to all these trends. Inkjet printing has a very short set-up time, and the first print is OK. As it is a digital technology, the run length can be defined very precisely, which determines the cost. Thanks to these qualities just-in-time printing becomes possible, with no intermediate stocks and the possibility to print on the final packaging unit. This printing step can be offline (separate printing process in a nearby location, but not linked to the packaging line), near-line (printing in-house at the packaging company, but the day before or some hours before the packaging step) or inline with the packaging (no intermediate printing stock).
Inkjet printing using UV-curable inks is especially suited because it is the most reliable industrial inkjet printing technology today, even at high print speeds (up to 50-75 m/min full width in single pass, and increasing every year), with high resolution and high consistency. UV-inkjet printing is also very interesting for printing directly on the plastic substrate because of the compatibility with many types of substrates. Depending on the type of the substrate, only a pretreatment (corona, plasma or flame) is needed for perfect ink wetting and adhesion.
UV-inkjet printing on food and beverage packaging is suited to print directly on plastic food containers or other types of plastic food packaging material (plastic foil, aluminum sealing material, pouch, laminate comprising plastic, closure cap, etc.). However, low-migration UV-inkjet ink is needed to deliver food-safe packaging. The ink components can contaminate the food by different mechanisms, in general, as indicated by migration. There are several mechanisms whereby the food can be contaminated by ink compounds, the two most important being migration of ink compounds through the substrate and set-off of ink compounds by transfer from the print side to the other side (food contact side) after printing (e.g. by rolling up printed plastic foils or by stacking printed containers). This is illustrated in Figure 1. It is not only a matter of LM ink, but also that all components of the printing solution need to be aligned to each other. A food-safe print is obtained by the right combination of the substrate, the UV ink, the printing process and conditions, the type of food and the conditions at which the food is treated, stored and used by the customer. For example, the printing on a substrate with very high barrier quality (e.g. an aluminum-based substrate) poses no or very little risk of migration through the substrate but can pose a problem when set-off is possible during the production process (rolling up or stacking). Type of food plays an important role in the contamination also. For example, chocolate is known to easily “absorb” migratables from printed packaging.
Because of the public health risks, many initiatives from governments worldwide have been targeting guidelines and regulations for food-safe packaging, including the printing of food packaging. Unfortunately, the guidelines differ from region to region – sometimes even from country to country (inside Europe). However, there is mostly a common ground in these legislations, which is the target that the (printing of) packaging should not endanger human health, change the food or alter its organoleptic properties (taste, sight, smell and touch). An important initiative is the Swiss Ordinance 817.023.21, which comprises positive lists of compounds that are allowed to be used in inks for food packaging. List A comprises compounds for which there is a good assessment of the health and safety risk and, thereby, a specific migration limit (SML) is defined for each compound of List A. List B comprises allowed compounds for which the current information is insufficient to define an SML, but that are allowed for use in printing inks for food packaging provided they are not detected using an analytical technique with a detection limit of 10ppb.
This 10ppb value refers, in fact, to 10µg of migrated compounds from a 6dm² packaging area (6dm² is used as standard packaging area for 1kg of food). This 10ppb value is found as a “rule-of-thumb” in many guidelines and regulations related to maximum allowed migration of each ink compound used for food packaging printing, especially when only limited toxicity data are available. Within Europe, Germany is preparing a regulatory initiative similar to the Swiss. Implementation and date of coming into force have, however, been postponed many times and are still unclear.
Other guidelines and regulations, as well as methods of migration testing, need to be considered when designing LM inks. In Europe especially, compliance with plastic packaging regulation EC 10/2011 also needs to be checked. The principles of good manufacturing practices also apply to the production of food packaging inks, substrates and the total package. In addition, some brand owners have defined their own guidelines for food packaging printing, including sometimes company-specific lists of restricted substances. A thorough discussion of these legal and “paralegal” aspects of printing inks is, however, not the scope of this article.
Sustainable digital printing
Direct print on food packaging delivers a very large improvement in sustainability over traditional printing on labels, which then are applied to the food packaging. This is valid for aspects of supply chain, the printer and the end user. These three appear to be connected to the respective 3 P’s of sustainability: profit, planet and people. This is demonstrated in Figure 2.
At supply chain level, there is no need for production of the label material, transport of printed label material to the packaging company, warehousing, recycling or scrapping of overstock or waste of label material and printed labels.
The printing step can be at the packaging company or by a nearby partner. There is a short set-up time because there is no need of a master. The run length can be fully variable with regard to the exact demand of each print, thus eliminating waste – also because the first print is already OK. Printing at the last possible moment becomes possible. Also, the UV-curing is very energy efficient and there are no VOCs released from the ink during the curing step. UV-LED is preferable over curing by Hg bulb because of the absence of toxic Hg waste, low ozone creation, low energy, low heat, instant on/off and almost constant dose over time.
Last but not least, the brand owner and the consumer profit from digital printing by having improved versioning, easy communication in different languages, health information and personalization.
Overall, shortening of the print process holds the potential to limit food waste, as it eliminates losses due to the printing step and an overall shorter chain of food production, including packaging. Of course, by using the LM inkjet inks, food-safe packaging printing can be guaranteed, thus removing human health concerns among consumers.
Design of low-migration UV-inkjet inks
First, it is important to comply with the guidelines and regulations, e.g. the Swiss Ordinance Lists, by selecting only ink compounds that are allowed for food packaging printing. Of course, ink formulations also need to be compliant with TSCA (USA) and REACH regulations. Second, the purity of the ink compounds is an important factor in the LM ink formulation. It is manageable to control the ink compounds so that they are free of solvents, heavy metals, etc.
Next, it is important to select ink compounds in view of low toxicity, low skin irritation, etc. This is especially important when selecting monomers and photoinitiators. For inkjet printing, the constraint of low viscosity for the jetting process through the small nozzles of the printhead is an important selection criterion, limiting the use of high molecular weight compounds in UV-inkjet inks.
The design of the LM UV-inkjet inks needs to aim for a combination of a high degree of curing, in order to have low residual amounts in the cured ink of ink compounds that can migrate, with the use of compounds that are low in viscosity but cannot migrate. The proprietary LM UV-inkjet ink formulations take all these restrictions into account and are even defined by them. This is illustrated in Figure 3.
LM UV-inkjet inks need the combination of low viscous but highly reactive monomers with diffusion-hindered photoinitiators. Highly reactive low viscous monomers are, for instance, monomers with two different polymerizable functions – of which the first is an acrylate and the second an ethylenically unsaturated polymerizable group, preferably selected from the group consisting of a vinyl ether group, an allyl ether group and an allyl ester group.
Diffusion-hindered photoinitiators and co-initiators are compounds that are designed to result in low migration levels, and – for inkjet – also are designed to have low viscosity. Different types of approaches are possible. Oligomeric and polymeric – as well as polymerizable initiators and co-initiators – have proven to be useful for formulating LM UV-inkjet inks. Each type has specific advantages towards the inkjet ink formulation and/or the lowest possible migration.
It is the combination of the above monomers and diffusion-hindered photoinitiators that results in a high degree of curing and a low amount of compounds that can migrate from the cured layer. The LM ink concept is summarized in Figure 4.
Printing with LM UV-inkjet inks
Designing the LM UV-inkjet inks while taking into account the food safety aspect is only one important task, however. Next to that, the LM inkjet ink needs to be designed to deliver robust industrial printing reliability yielding a constant image quality day in, day out, in an industrial environment; therefore, the inks need to be tuned to the printhead, the curing system and all other elements of the printing system and process. Ideally, the printing process also is tuned, to a certain extent, toward the inks for maximal system performance.
Moreover, the cured LM UV-inkjet inks also need to fulfill the functional requirements linked to the type of packaging, especially the physical properties of the printed packaging. Each application requires a different adhesion, scratch resistance, flexibility, solvent, water resistance, etc. The LM ink formulation needs to be tuned to the specific applications without jeopardizing the food safety aspects.
The new LM UV-inkjet inks are being used for many different types of direct printing – on plastic food containers, caps, foils, etc. A good example is printing on PET beverage bottles. The LM ink design is tuned to deliver the required image quality when printing directly on PET (no label, no pre-treatment), with good physical properties (adhesion, flexibility, scratch resistance), food safety (low migration, even for very thin PET bottles that become more and more popular), and allowing for recycling (recycling to PET beverage bottles within the current recycling processes and installations). This is illustrated in Figure 5.
Other examples include direct print on plastic food containers (polypropylene substrate that has low barrier quality, taking into account stacking of the printed containers shortly after printing, as well as adhesion and scratch resistance), and direct print on HDPE closure caps (immediate adhesion on the caps, low migration, high image quality). Each of these applications requires an LM ink that meets the specific combination of demanding functional requirements. This is possible by fine-tuning the LM ink formulation. Food-safe packaging could be obtained in this way by combination of print system design and ink design, resulting in production printing equipment, with output up to 36,000 printed packages/hour.