Discoloration mechanism of printed polyethylene film (2)

Observations and Experimental Results First of all, blank samples did not show discoloration. Second, the discolored sample ink contains nitrocellulose and polyamide (or nitrocellulose and polyurethane). Samples containing only one resin in the ink did not show discoloration. Third, the open sample did not change color. Fourth, only the samples printed with BHT or other phenolic antioxidant inks show discoloration. Finally, the discoloration of the sample with higher phenolic antioxidant content in the ink was greater than that of the sample with lower content. In summary, the following conditions need to be included in the cause of discoloration:
â—† must contain phenolic antioxidants;
â—† The ink must contain nitrocellulose and polyamide (or nitrocellulose and polyurethane);
â—† Increased temperature;
â—† Closed environment.
Conclusion The above results can lead to the following conclusions:
1. By-products react between the ink components and react with the film anti-oxidant to cause discoloration; or the ink reacts directly with the film anti-oxidant to cause discoloration.
2. The reaction between the ink or by-product and the film antioxidant is accelerated in an elevated temperature or in a closed system.
3. The yellowing of the film is directly related to the concentration of antioxidant in the film.
For unresolved issues, further investigations are required:
â—† Direct or indirect reactions with antioxidants;
â—† The role of ink pigments in the reaction;
â—† The role of titanium dioxide in the film in discoloration;
â—† Whether the by-product is migratory or gaseous.

Stage 3: The design of Experiment 2 without any pigment. These clear varnishes were printed on opaque polyethylene films and compared to transparent polyethylene films. Both films were added with the same phenolic antioxidant, the one that resulted in the deepest discoloration in stage 2. Exposure conditions are limited to 120 oF in closed glass jars.
The following experiments were used to determine whether the reactive material will migrate from the polyethylene film or whether it is gaseous. A blank polyethylene film sample was placed on the bottom of a closed storage tank while a printed film sample was hung under the lid of the storage tank. The tank was heated to 120 oF.
Observations and Experimental Results â—† Transparent printed films showed discoloration, but to a lesser extent than opaque printed films.
â—† The result of using the ink resin mixture (varnish) is the same as the result of the coloring of the pigment, all resulting in the discoloration of all the samples.
â—† Blank opaque film samples placed in the same tank as printed film samples also showed discoloration, even if they were not in direct contact with the printed film.
Conclusion There is enough evidence to show that the visual effects of colorants and colorants on transparent films are less than those on opaque films. Therefore, even if the degree of fading within the transparent film is relatively low, it can be concluded that titanium dioxide does not play a role in the fading of the film. Similarly, based on the results of the foregoing study, since unpigmented varnish causes discoloration of the film as well as pigmented printing ink, it can be concluded that the pigment contained in the ink (except the migrating dye) discolors in the film. There is no positive effect.
Interestingly, although the blank film sample in the same chamber as the printed film did not have any physical contact with the printed film, yellowing occurred, indicating the presence of gaseous by-products. It is clear that the concentration of gaseous materials will increase in a closed system, but in an open system the gaseous materials will dissipate without concentration.
This conclusion is consistent with the observations made at the production site: The printed samples shrink-wrapped on site are discolored, while the samples stored in corrugated boxes are not discolored.

Phase 4: Determination of by-products and their reaction with the film In order to fully understand the discoloration process, we conducted the following investigations:
â—† Resin reaction forms active byproducts;
â—† Reactions between by-products and thin films;
â—† The formation mechanism of stains.
When the temperature rises, a large amount of fatty mixture will be produced in the closed storage tank. We analyzed the space above the mixture. The analysis results showed that the content of NOx compounds increased. It is known that nitrocellulose decomposes to form NOx compounds.
In addition, it is known in chemistry that many oxidants (such as NOx) react with phenolic antioxidants to form a quinone structure, which is a color former. This reaction is reversible under UV light, so the yellowish color will retreat under light.
Conclusion The above investigation provides a complete explanation of the role played by inks, (substrate) substrates, environmental and process conditions in discoloration of printed films. It also pointed out to the processor a temporary corrective measure to prevent the discoloration of the film, namely to put several holes in different positions of the shrink wrap film so that NOx gas can escape when it is placed on the print film roll.
But in addition, long-term corrective measures must be taken. By selecting the appropriate antioxidant in the ink formulation and its content in the film production, the occurrence of discoloration can be eliminated. After this measure was taken, it was monitored for more than three years. There has been no case of discoloration.
The experiments in this study show that discoloration occurs at elevated temperatures, so care must be taken that the same phenomenon may occur at room temperature, but over a longer period of time.