Light-fastness and weather resistance in packaging printing

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Authors: Thomas Strohe, BASF Drucksysteme GmbH May 2000 

Dr. Hans-Jakob Kock, BASF Drucksysteme GmbH 

Light-fastness and weather resistance in packaging printing 

The market success of a product is determined (to an extent which should not be 

underestimated) by the design of the packaging and its presentation at the Point of 

Sale. Printing ink as an element in the packaging chain visualizes the creative ideas 

of the designer, communicates in a selective manner with the potential purchaser and 

influences his purchase decision to an important degree. 

In addition to the colour characteristic that is visually perceptible and a decisive factor 

for the retail customer, printing inks must reliably meet a large number of requirement 

profiles resulting from the specific processing, transport and storage conditions for 

the entire life cycle of the packaging with a continuing standard of quality. 

Apart form the technical properties of the printing process, inks for products to be 

stored for a long time in the open air and marketed directly from there have the 

requirement of two physical phenomena of pre-eminent importance. Namely lightfastness 

1 and weather resistance. 2 Fundamentally, it is to be noted that objects 

made from organic materials such as polyethylene and printing inks do not possess 

lasting resistance to irradiation and weathering. Just as polyethylene loses its 

mechanical properties with long-term irradiation due to light-induced decomposition 

processes (brittleness), a printing ink will change its colour under exposure of this 

nature. 

Anyone who frequently visits the big DIY stores or garden centres can see the 

problem at a glance. Large areas in the open air are stacked full with stretched or 

shrunken pallets of flower soil, turf, fertilizer or other gardening requisites. You can 

see from these products how long they have been standing there and what 

environmental stresses they have been exposed to. Bleached side-folds, algal or 

fungal infection, heavy surface dirt and contamination from the environment or torn 

packaging can often be seen. The printing ink must withstand all these influences 

and, if possible, retain the set and agreed colour for an unlimited period. The 

following remarks examine why such wishes are often beyond what is actually 

possible. 

The period of time in which a change in a printing ink becomes visible can be 

influenced to a certain but limited extent, however, by formulation, application of 

the printing ink to the substrate, contents of the packaging and 4further handling 

of the packaging. The four major parameters are explained below by an example 

1 Light-fastness: 

The light-fastness of printing is understood as its resistance to the action of daylight without the direct influence 

of weather (DIN 16525). The tests are carried out with light-fastness test equipment (xenon high-pressure lamps) 

that with an appropriate filter approximately simulate the spectral distribution of solar and sky radiation. The 

determination of the light-fastness of prints is carried out according to DIN 16525 using a light-fastness scale 

(wool scale: 1 -8) and a grey scale for the assessment of the change in colour as specified in DIN 54001. 

The details of the light-fastness of printing inks according to DIN 16519 refer to standard print specimens 

produced with a defined ink-layer thickness and depth of colour. 

2 Weather resistance: 

Weather resistance is understood as the resistance to the simultaneous action of radiation and atmospheric 

properties (climate: humidity/rain, ozone, waste gases)

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from the segment of the garden-soil packagings (extremely long storage in the open 

air). 

Formulation of printing ink 

For the light-fastness and weather resistance of printing inks, the colour fastness of 

the pigments used (in the complete packaging system) are of basic importance and 

are therefore at the centre of the following considerations. From the definition of lightfastness, 

the organic pigments used are assessed according to a scale of 1 (very 

slight) to 8 (outstanding). This characterization is carried out on the basis of 

measurements on standard proofs (DIN 16519) and is made available as basic 

information by the pigment manufacturers. These data serve as a major orientation 

aid for the formulation of the printing inks for garden-soil bags but it must always be 

remembered that account should be taken of the overall profile of the necessary lightfastness 

(including crease, scratch and abrasion resistance, low slip characteristics 

of printed flat film for heavy-duty bags) and that in addition it is essential to carry out 

your own tests of the weather resistance properties on practical proofs. 

Organic pigments are chemically changed by the action of light. Through this 

physico-chemical change, a continuous degradation of the colour properties takes 

place over a period of time. Ideas about the possible resistance to light in the course 

of time are frequently very imprecise. 

The first visible changes in colour hue are already apparent after a relatively short 

exposure to light – depending on the light-fastness of the pigments. Even a pigment 

of light-fastness grade 6 (in the yellow - red colour range), i.e., a pigment classed as 

“very good” in resistance to light already displays recognizable changes in hue on 

standard prints after about 160 days (average daily exposure). Tests show that the 

resistance to light is approximately doubled when the light-fastness is raised by one 

grade.

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Fig. 1.: Average exposure to light (yearly average) up to a visible change in 

colour hue as a function of the light-fastness of the pigments 

In addition to the action of light, weather conditions have a major effect on the 

“visible” resistance to light of a printed image when it is stored in the open air. This is 

why for the area of garden-product bags pigments are selected which have a lightfastness 

(LF) value of ≥ 6 (according to the wool scale) and have been tested for 

their suitability for a weather-resistant printing ink. 

If, in turn, standard proofs are subjected to a period of exposure as a function of the 

seasonal radiation intensities, a dramatic reduction in the average value of 160 days 

to about 50 days is apparent when storage commences in mid-summer (on the basis 

of the number of hours of sunshine and its intensity).

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Fig. 2 : Recognizable changes in colour hue of a proof with a pigment of LF 

= 6 after the start of outdoor storage 

Special colours (spot colours), mixed from several pigments in the formulation of a 

printing ink, are often used in packaging printing. It must be noted here that it is 

always the pigment with the lowest light-fastness that determines the stability of the 

colour. It is therefore extremely dangerous to mix the colour required with only cost 

considerations in mind, as demonstrated by the example of the yellow hue HKS 4 

(Fig. 3).

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Fig. 3 : Light-fastness of HKS 4 mixed from yellow + red pigments of different 

light-fastness and different raw material prices 

Mixes of the hue with an LF of < 6 are not at all suitable for outdoor weather 

conditions. Even if the pigment price and the resulting ink price are exceptionally 

attractive, the complaints which will inevitably follow will rapidly eliminate this 

supposed advantage. 

Application 

All the dependencies listed so far are based on measurements of standard proofs 

(DIN 16519) with defined pigment concentrations and application quantity which 

naturally do not correspond to the actual relations pertaining in the manufacture of 

packaging material in practice (ink transfer, brightening). 

The light-fastness of a layer of printing ink increases with the depth of the colour, i.e., 

with the increasing pigment volume concentration. This dependence with a constant 

layer thickness can be very easily explained since the process of pigment 

degradation always takes place from “the top downwards”, i.e., always at the direct 

interface of ink and light. Since the pigment concentration in an ink-film element (= 

pigment surface concentration) with a marked depth of colour is correspondingly 

higher, the degradation which takes place continuously is only visible after fairly long 

exposure/weathering.

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Heavy extender or white brightenings (at a constant ink transfer quantity = thickness 

of wet or dry film) can lower the light-fastness by 1 - 2 grades, i.e., the light-fastness 

over a period of time to the first recognizable change in colour is halved or lowered 

even more clearly. 

Fig. 4: 

Influence of extension on light-fastness 

(NC ink) 

For technical reasons, some special hues in printing are often produced by 

superimposing basic colours (process colours). For example, the hue PMS 1795 C is 

produced by printing magenta over yellow, the layer thickness and pigment volume 

concentration of magenta being less than the layer of yellow underneath it. The same 

red colour can naturally also be obtained as a special colour with the corresponding 

mix of suitable hues, and printed on a separate printing unit. The different structure of 

the colour becomes clearly apparent when exposed to light or weather (Fig. 5).

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Fig. 5: 

Changes in colour hues by overprinting 1.)process colours 

and 2.) special colour (example PMS 1795) 

Since weathering begins on the top layer of the ink film and progresses through the 

layer, in the case of superimposed printing it is the magenta which is first changed or 

degraded so that the colour becomes yellowish relatively quickly. 

In addition to the repro and printing techniques, the corona pretreatment of PE film 

for heavy-duty bags has a further major influence on the light-fastness and especially 

water resistance of the packaging. A combination of excellent ink adhesion and very 

good water resistance can be achieved in a processing range of 38-42 mN/m. 

Practical experience has enabled good reproducibility and the setting of defined 

plastic-film pretreatments to be obtained by an in-line coronary pretreatment in the 

printing process. The water resistance and the associated wet abrasion resistance 

are directly correlated with the weather resistance of printed products since in the 

open air the latter are permanently exposed to water. At pretreatment levels above 

the threshold value of 42mN/m or when an additional in-line pretreatment is used and 

despite a preceding pretreatment of the film during the extrusion, a rise in the 

pretreatment level causes a permanent deterioration in the water resistance of the 

packaging material. 

Influence of contents

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Especially in the case of garden-soil bags, a major contribution to the stabilization of 

the colours can be made by the formulation of the contents. When a high proportion 

of urea additives are contained in the soils and in combination with condensation and 

air in the side-fold area of the pallet, this causes one component inks and varnishes 

to start to dissolve. It is the condensation, which is basic to a greater or lesser extent, 

that is responsible for this phenomenon when the product is in a moist state. 

Another phenomenon that has been known in the market for several years is the 

bleaching out of blue (= copper phthalocyanine) colour constituents. Through an 

oxidative decomposition of the pigment triggered mainly by inadequately rotted 

compost in combination with light and moisture, a complete bleaching out of the 

normally very stable pigment (LF = WS 7) via a violet intermediate stage occurs. 

Fig. 6: Influence of the contents on colour stability 

(degradation of cyan) 

In practical tests, it has been demonstrated that when spot-colour hues of green 

(yellow/blue) in side-fold areas are changed to a basic green colour the phenomenon 

can be avoided completely. However, the mechanism that leads to the stabilization of 

the chemically very similar green pigment (chlorinated phthaloblue) has not so far 

been definitively clarified. On account of the variation in the basic colour used, a 

compromise between colour and light-fastness must consequently be accepted since 

not all hues of green can be obtained by such a mixture anyway. 

4 Storage/Transport

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The physical facts described above, that can only be influenced within a limited 

“scope”, call for an intensive consideration of the other processes in the marketing of 

the products. 

The filling of the bags with the soil usually takes place in campaigns long before they 

are actually needed by the final consumers. The critical point here is that the 

products are stored for very long times in the open air and exposed to the elements 

with scarcely any protection. When such storage areas are inspected, it can be found 

that the side areas in particular and the top layer of the pallet, i.e., the areas in 

constant, direct contact with the “weather”, are often faded and also exhibit lower 

weathering resistance. While there is no great technical problem in protecting the top 

layer from the action of light until sales actually take place (opaque covers), this is not 

so easy in the side area since this area, which often carries a lot of printing (multicolour 

halftone), is designed to attract consumers. When stretch covers are used, an 

improvement in light-fastness can be achieved when PE qualities featuring UV 

stabilizers (increase in service life of the plastic film) and white pigments (reflection of 

UV light, i.e., minimized exposure of the printing ink through a lower intensity of 

radiation) are employed. 

Summary 

Printing inks for outdoor use and especially for garden-soil bags must withstand 

extreme demands as regards colour stability and meet highly diverse user 

requirements. Since there are limits in time to light-fastness due to unchangeable 

physico-chemical laws of nature even when optimum use is made of all the 

possibilities in respect of formulation, all the groups involved in the packaging chain 

must make their technical contribution if the best-possible resistance to weathering is 

to be achieved. 

In addition to the selection of the most suitable raw materials where profitability may 

not be disregarded either, due account should be taken of the influences to be 

expected in the critical areas (side-folds, top layers) already at the stage of designing 

the packaging and the deciding on the colours. 

A significant increase in stability can be achieved by covering in a suitable manner 

goods that are stored on pallets in the open air. Until they are delivered to consumer 

markets, it would be sensible and also practical to put a cover over them which shuts 

out light completely so that the goods are not presented to the customers in an 

already damaged state. 

The many effects of the environment and the possible ways of influencing such 

effects in the packaging chain make it urgently necessary for all the organizations 

involved in this chain (packaging designers, repro companies, printing ink 

manufacturers, printers and marketing companies) to jointly devise solutions that 

ensure the best possible presentation of the packaging for the customer.

Light-fastness and weather resistance in packaging printing

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