There are many articles discussing the yellowing or reddening (discoloration) of white polyethylene films. However, these articles do not provide sufficient reference or explanation for the role of printing inks in such discoloration phenomena. Here, the authors studied the chemical interaction between the ink composition and the film composition, which is the cause of discoloration of the film.
Accurate description of the role of different ink components in the discoloration of the film will be of great value for printing or coating polyolefin films.
Discoloration of background materials is not only annoying, but it also costs unnecessary money to manage the product. The causes of discoloration vary, so it is almost impossible to solve the film discoloration by a single interpretation.
Discoloration is best explained through a case study. According to our observations, one or several of the following factors may cause this phenomenon: ink, substrate (film), environment or even process conditions. This article only limits the scope of the discussion to the yellowing/reddening of printed polyethylene films.
Ciba-Geigy and DuPont and other suppliers have elaborated on the discoloration of polyethylene film in packaging materials. Although their topics are somewhat similar to film yellowing, they do not address the discoloration of printing inks and printing inks. The role. We observed some special cases of discoloration of printed polyethylene films in the flexible packaging industry, which led us to resolve to undertake a complete investigation to explore the physical and/or chemical interactions between inks, substrates, environment and process conditions.
Project Example We conducted a systematic study of discoloration at a processing company. The study was conducted separately in the laboratory and the production site. We try to collect data in a direction that narrows the scope of research so as to achieve a single “cause-resultâ€. Therefore, the presentation and discussion of research results follow the same pattern of steps.
Stage 1: Preliminary data collection The preliminary discussions between the processors and suppliers are as follows:
â—† The principle of supplier film resin;
â—† additives used in film processing;
â—† (discoloration) frequency and time;
â—† Exposed conditions of printed materials;
â—† The type of ink used.
It is important to talk to different suppliers and processors so that you can understand all the basic situations accurately. Preliminary information indicates that in many cases the film additives do not contain BHT (2,6-di-tert-butyl-p-cresol) or contain similar phenolic antioxidants, as well as slip agents and antistatic agents. Opaque polyethylene films (containing titanium pigments) discolor more than transparent films. According to the processor, films tend to yellow more easily in summer or at higher temperatures. This rust-like color is visible from the outer edge to the inner core at the edge of the film web in the non-printing area. In addition, discoloration can also occur when the film roll is shrink-wrapped to avoid dust particles and smoke. However, discoloration does not occur when the same printed material is processed into bags or stored in corrugated boxes under similar air conditions. Similarly, a rolled film without printed content does not show discoloration when it is shrink-wrapped. Processors also point out that ink products from different ink manufacturers can cause varying degrees of discoloration. Here, all inks are surface printing inks based on nitrocellulose and polyamide resins.
Conclusion Preliminary information allows us to determine the possible causes of discoloration to be investigated next:
â—† film additives;
â—† relationship between elevated temperature;
â—† Comparison of two storage methods: printed film box packing and shrink packing;
â—† Ink composition.
Stage 2: Design of Experiment 1 Based on the above information, we asked the processor to provide us with a white polyethylene film containing two levels of antioxidants and two types of antioxidants with their standard films for our evaluation.
In the experiments, inks colored with carbazole violet pigments were used, because the inks themselves did not migrate, thus eliminating the need to place the samples for pigment migration in the following manner:
â—† Open at room temperature;
â—† placed in a closed glass jar at room temperature;
â—† Open at 120 oF;
â—† Placed in a closed glass jar at 120 oF.
The sample was left under the above conditions for 48 hours, and then the discoloration was observed. (To be continued)