Navigating Ink Migration

27 May 2014

An extract from the latest edition of Sun Chemical’s Best Practice Guide to printing food packaging and sensitive goods with UV & EB curing and conventional offset inks and coatings.

In 2004, Sun Chemical introduced the first edition of Print for Packaging - A Printing Low Migration Best Practice Guide for printer converters and end-users. The guide was updated in 2007 and now a third edition has been prepared to reflect the current packaging market situation. This 3rd edition represents a summary of the key challenges faced by the packaging development supply chain and has been prepared by Sun Chemical using inhouse know-how, and after discussion with many experts in the printing and packaging industries. The following is an edited extract covering the problem of ink migration in food packaging.

Printing inks, coatings and adhesives, unless specifically designed for the purpose, should not under normal circumstances
come into direct contact with packaged foodstuffs. Therefore printed food packaging should be printed in such a manner that set-off during and after the printing process is avoided as far as is practically possible in order to ensure that the surface of the packaging in contact with the packaged product is free of inks and coating.

If it is to be fit for purpose, designing packaging that is suitable for food requires specific information to be known. As required by Good Manufacturing Practice, the chances of success are greatly improved if all the stakeholders in the production of the packaging are connected. That is not always straightforward, as some of the steps between the brand-owner specifying the packaging and the consumer receiving the packaged goods may not be directly connected.

The nature of the foodstuff dictates how it must be packaged and, once that is determined, the choice of substrate, inks and coatings can be reasonably assessed. However, every package is different, so the outcomes may differ according to the perceived risk of transfer of material from the packaging to the food and the barrier properties of the packaging materials used. End use properties may also influence the choice, for example if the food is to be hot filled or heated in the packaging, which might lead to additional risks of migration.

The ink film on a package is extremely thin (of the order of 1-3 g/m2), consequently the total quantity of ink involved is likewise
very small indeed. Inks and coatings for food and sensitive packaging are specially made for this purpose and are based on
materials guidance issued by the European industry bodies representing printing inks: EuPIA and CEPE.

Where there is a risk of "set-off" on the reverse side of the print, a functional barrier should be included in the package design. Unless using a specially designed coating, use of a coating or over-print varnish will not normally prevent migration. Set-off can be controlled to some extent by adopting simple procedures, and by ink and coating selection. When conventional oil based inks are used, spray powder and/or water-based coating can help reduce the risk. With UV curable inks and coatings, the best possible cure should be achieved. Reducing the temperature in the stack will also reduce the risk of volatile component migration, which may be reduced further by 'fanning' or airing the stack.

In web printing, ensuring the maximum drying or curing and keeping the temperature as low as possible before re-reeling can help reduce the risk of setoff. If practical, lowering web tension to minimise pressure in the reel may also help reduce set-off.

A finished package is produced from >97% substrate and about 0.5% ink and up to 1.5% of coating. The consequence of this is that the substrate plays a key role in both the organoleptic performance of the package and the potential result in migration testing. In general terms, for cartonboard based packaging for food, 'pure' cellulose-based board (GZ type) is usually preferred to that containing groundwood or some recycled material content (GC, GD, GT), but the substrate supplier should be consulted for definitive information and advice.

Reverse side cartonboard printing
Under normal circumstances, printing on the inside of the packaging (that is to say, printing the 'non-printing' side of the substrate) should be avoided. In most cases this is undesirable on technical grounds as the reverse side of many folding carton substrates is uncoated and not intended as a printing surface. There is pressure from 'marketing' in some cases to increase the appeal of the packed product by introducing text or images on the normally non-printed side, for example, for competitions or special offers.

Any intentional printing on the inside of a carton or box increases the risk of migration by putting the packed product in very intimate contact with print and/or coating in a confined space. In addition, the ink will penetrate the reverse side of the board during the printing process, more than the outer surface, which is designed for printing. When printing on the uncoated surface, a part of the ink vehicle normally included in the dried or cured matrix (conventional or UV curing) will be preferentially absorbed into the body of the board, and so become available for transfer by migration. Partly dried or cured ink greatly increases the risk of migration and can lead to organoleptic changes in the packaged goods.

If reverse printing cannot be avoided, use of low migration inks and coatings is recommended, and the location of the print within the pack should be such that the risk of migration is minimised. Furthermore, it should be noted that the smaller the printed area, the lower the risk. The print should always be as fully dried or cured as possible. Some substrate suppliers now offer two-side printing board, which helps overcome issues of ink and coating absorption, reducing the risk of migration.

Direct food contact inks
In rare instances, the print on the packaging is intended to be in very close or direct contact with the packaged goods. In these
circumstances, it is worth considering whether the printing should be conducted using the same type of ink as used for printing food itself. Such products are specifically designed and formulated from materials that are edible, though the colour range is somewhat limited. These inks may be sourced from specialist suppliers, who also manufacture them in food production compliant conditions.

Microwave and ovenable packaging An increasing proportion of convenience packaging is now printed in some way, and there are growing concerns about the design of such packaging, particularly the risk of potential impact of the packaging on the contained foodstuffs during the cooking process. It is always a minimum and mandatory requirement to ensure consumer safety when selecting materials for packaging. An additional cooking process being included in a pack design adds to that demand. Foodstuffs packaged in boxes or trays, that are to be cooked by microwaving or in an oven, can be assumed to be subject to a number of conditions, including:

Close proximity of print to foodstuff

Long term storage (extended shelf-life products)

A wide variety of (uncontrollable) cooking times and temperatures

Exposure to temperatures in excess of 200°C when cooked in an oven (Note also, the potential effect of air circulation in a fan oven

Localised heating in a microwave oven, especially if the packaging includes a susceptor

Under these conditions, careful attention must be given to packaging design and selection of materials, since there is increased potential for migration from the packaging due to the high temperatures which can be attained during microwave and oven cooking.

Potential migrants
include thermal breakdown products from pigments, volatile components from the ink and coating vehicle systems, low molecular
weight components of inks, coatings, adhesives and the substrate, and by-products from the UV curing process.

Selecting colours with care
Not all colours are applicable for high temperature applications and only those based on heat stable pigments are recommended. Even then, exposure to temperatures above 200°C for periods in excess of 30min should be avoided. Low migration coatings should be used with the inks where gloss, controlled slip properties and print protection are required. In some cases, a cross-linkable water based coating may be used, but standard water based coatings should be avoided, as they can both melt and break down in elevated temperatures.

Watch for potential hazards
There have been a small number of reported instances of a potential fire hazard when containers printed with a printing in incorporating carbon black pigment are heated in a microwave oven. Although these incidents appear to be rare, they have not been subjected to definitive technical evaluation. Consequently, products printed with carbon black containing inks, intended for microwave applications, should be assessed under appropriate conditions of use to ensure they are fit for that specific purpose.

Packaging safety responsibilities
Knowledge regarding the performance of different types of printed material in elevated temperature applications is far from complete. It is always recommended that packaging produced for elevated temperature applications is tested to ensure that it complies with legal requirements. It is the printer converters' and packaging distributors' responsibility to ensure the packaging has been fully assessed for risk and that it meets regulatory requirements for its end use. Therefore, migration testing under appropriate conditions of use is
strongly recommended before proceeding with commercial printing of packaging for microwave or ovenable applications.

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