As a rule yellowing is usually caused by light and increased temperature (UV or heat yellowing). Brilliant white papers are particularly susceptible to yellowing here. One particular form of yellowing is termed contact yellowing.
This occurs when a sheet is placed in contact with a surface that has already been printed in a stack. Printing colour components, but also decomposition products, permeate the surface of the paper opposite causing yellow discolouration. The material itself may be yellow, or react with coating colour components to form yellow compounds. In terms of chemistry, this is usually caused by aldehydes which destroy the brightening agent or which combine with the amino-groups present in coating colour components. Casein, which was often utilised in former times, is particularly susceptible to this.
Both the composition of the printing colour and the coating colour itself can considerably improve or worsen the process of yellowing. In the case of critical print objects, we recommend that materials are tested and matched in advance.
Thick layers of colour, a high percentage of binding agents drying by oxidation in the printing colour and additional siccatives, are disadvantageous because they can result in the creation of colour formers.
Reduced-odour printing colours and early airing of the stack can help to reduce yellowing potential. Dispersion varnishes do not tend to yellow. This is why they are to be recommended instead of printing varnishes.
