There are a number of papers published which discuss the yellowing or pinking (discoloration) of white polyethylene films. However, there is no reference nor sufficient explanation on the role of printing ink in such discoloration. Research presented herein is directed toward understanding the chemical interaction of the ink ingredients with film ingredients, which subsequently results in the film discoloration.
An accurate description of the role that different ink ingredients play in the film discoloration would be invaluable for industries where polyolefin films are either printed or coated.
Background “No science exists without observation.”
Discoloration of materials is not only undesirable, but sometimes requires disposition of product, which is costly. Discoloration is caused by a complex set of circumstances which can vary from problem to problem. Therefore, a single explanation to resolve the discoloration of films may not be possible. Discoloration is better explained on a case-by-case basis. In our observation, one or more of the following can cause these phenomena: ink, substrate, environment or even the processing conditions. For the scope of this paper, we have limited this presentation to yellowing/pinking of printed polyethylene films.
The discoloration of polyethylene film in packaging materials has been well documented by suppliers like Ciba-Geigy and DuPont. Although, their topic has some similarity for yellowing of films, it does not discuss printed materials nor has it covered the role of printing ink in discoloration. In the flexible packaging industry we have observed a number of incidences of discoloration of printed polyethylene in the non-print areas. This led us to undertake a complete investigation to understand the chemical and/or the physical interactions of ink, substrate, environment and processing conditions.
A systematic study of the discoloration phenomenon at one converter was undertaken and the results are presented herein. This study was conducted both in the laboratory and the field. We gathered data in steps to narrow our research toward a single cause-and-effect, so the presentation of results and discussions will follow in the same step-wise manner.
Round 1: Preliminary Data Collection
Areas of focus in the preliminary discussions with the converter and the suppliers were:
• Chemistry of the film resin from supplier.
• Additives used for processing of the film.
• Frequency and timing of the occurrence.
• Exposure conditions of printed materials.
• Type of inks being used.
The interviews and meetings with various suppliers and the converter were very important to precisely understand all the fundamentals. The preliminary information revealed that in many cases the film additive package had either BHT, or similar phenolic antioxidant, along with slip additives and anti-static agents. The discoloration was more pronounced with opaque polyethylene (containing TiO2 pigment) than with clear film. According to the converter, the occurrence of yellowing was higher in summer months or under warm storage conditions. The stain-like color was visible at the edges of the printed roll from top to the core in non-printed areas. Also, when these rolls were shrink-wrapped to avoid contamination of airborne particles and fumes, discoloration occurred. However, when identical printed material was converted to finish bags and stored in corrugated boxes under similar atmospheric conditions, no discoloration occurred. Also, rolls of film with no printing that were shrink-wrapped showed no discoloration. It was also noted that different inks from different ink manufacturers demonstrated varying degrees of discoloration. All of the inks were surface printing inks, based on nitrocellulose and polyamide resin-types.
Preliminary information led us to investigate the following possible causes of discoloration:
• Film additives.
• Relationship of elevated temperature.
• Storage of printed material in boxes versus shrink-wrapping.
• Ink ingredients.
Round 2: Design of Experiment I
Based on the above information, we requested the converter to provide us white polyethylene films with two levels of antioxidant and two types of antioxidant along with their standard film for our evaluations.
Laboratory investigations were conducted using inks colored with carbazole violet pigment because it is non-migratory in nature, thus eliminating migration of pigment as cause for discoloration.
For our cross-examination, these inks were made with varying ratios of nitrocellulose and polyamide from 0 percent to 100 percent. In the similar manner, we made inks with nitrocellulose and polyurethane. These are commonly used resins in the inks for surface printing applications for polyethylene films.
All of the above ink and film variables were flexo printed in the lab, leaving about two inches of non-printed film on both the sides of the print. Plain film samples were used as experimental controls.
Preliminary data from the converter demonstrated that discoloration occurred most often at elevated temperatures and under closed conditions (shrink-wrap). Therefore, we exposed all the samples individually in the manner outlined below:
• Room temperature in open.
• Room temperature in a closed glass jar.
• At 120°F in open.
• At 120°F in a closed glass jar.
The samples were exposed for 48 hours under the above conditions and then observed for discoloration.
Results and Observations
First, the plain film samples showed no discoloration. Second, the samples that discolored had both nitrocellulose and polyamide or nitrocellulose and polyurethane resins in the inks. Samples with only one resin in the ink showed no discoloration. Third, samples exposed in open containers did not discolor. Fourth, only samples printed on film containing BHT or other phenolic antioxidant (some Irganox) showed discoloration. Finally, printed samples on films with higher levels of phenolic antioxidant showed more intense discoloration than did the samples on films with lower levels of phenolic antioxidant. In summary, the following conditions need to be met for discoloration:
• Phenolic antioxidants must be present.
• Inks must contain both nitrocellulose and polyamide or nitrocellulose and polyurethane.
• Elevated temperatures.
• Closed environment.
The above results lead to the following conclusions:
1. Either the components of the ink are reacting to form a byproduct that can react with the film antioxidant to cause discoloration or the ink is reacting directly with the film antioxidant to cause discoloration.
2. The reaction with the film antioxidant is accelerated at elevated temperatures and in sealed systems.
3. The yellowing is directly related to the concentration of the antioxidant in the film.
Unresolved issues requiring further investigation:
• Direct or indirect reaction with the antioxidant.
• The role of ink pigment in the reaction.
• The role of TiO2 in the film for discoloration.
• Whether or not the byproduct is migratory or gaseous.
Round 3: Design of Experiment II
Inks were made in the laboratory with the same resin ratios as above but without any pigment. These clear varnishes were printed on opaque polyethylene along with a clear polyethylene film. Both the films were made with the same phenolic antioxidant package, which resulted in the greatest degree of discoloration in Round 2 above. Exposure conditions were limited to 120°F in sealed glass jars.
The following experiment was conducted to determine whether the reacting species were migrating through the polyethylene film or were gaseous in nature. A plain film sample of opaque polyethylene was placed in the bottom of a sealed jar, in which a printed sample of the same film was suspended from the lid of the jar. The jar was heated at 120°F.
Results and Observations
• The clear printed films exhibited discoloration, but at a lesser degree than printed samples on opaque film.
• The ink resin blend (varnish) results were identical to pigmented inks for discoloration on all samples.
• The plain opaque film sample which was placed in the same jar with the printed sample showed discoloration, even when not in direct contact with the print area.
It is well established that tints and colorants have a smaller visual effect on clear films than on opaque films. So the presence of discoloration, even at a lesser degree, in the clear films leads to the conclusion that TiO2 has no role in the discoloration of the films. Also, since non-pigmented varnishes discolor films identically to their pigmented counterparts studied earlier, the results indicate that pigments (except migratory dye-based) contained in inks take no active role in the discoloration.
Interestingly, the plain film sample placed in the same jar, but not in physical contact with the printed sample, also turns yellow, which indicates that there is a gaseous byproduct. It is evident that in sealed systems the concentration of a gaseous material will increase, but in an open system a gaseous material will diffuse and not become concentrated.
This conclusion is consistent with the field observation that printed samples that are shrink-wrapped discolored while those stored in corrugated boxes did not.
Round 4: Identifying the Byproduct and Reaction with Film
To complete our understanding of the discoloration process, we investigated the following:
• Reactions of the resins to form a reactive byproduct.
• Reaction of the byproduct with the film.
• Chemistry of the stain.
A large amount of resinous mixture was exposed at an elevated temperature in a sealed jar. The headspace above the mixture was analyzed. Analysis concluded that there were elevated levels of NOx compounds present. It is commonly believed that nitrocellulose decomposes to form NOx compounds (Figure 1).
Additionally, in the chemistry it is known that many oxidizing agents (such as NOx) react with phenolic antioxidants to form quinone structures, which are chromophoric agents. This reaction is reversible under UV light; therefore, the yellow color fades upon exposure to light (Figure 2).
The above investigation provides a complete explanation for the role of ink, substrate, environment and processing conditions in discoloration of printed films. This explanation also provided a temporary corrective action for the converter in which the shrink-wrap was perforated at several locations when placed over printed rolls to allow the NOx gases to escape.
Long-term corrective measures have since been employed. Through ink formulation, choosing the appropriate antioxidant for use in film production and proper levels of antioxidant in film production, the discoloration problem for the converter has been eliminated. The problem has been monitored for more than three years without a single recurrence of discoloration.
While experiments performed during this study indicated that discoloration occurs at elevated temperatures, it is important to note that at room temperature the same phenomena can occur, but over a longer time-scale.
Ciba Geigy, Technical bulletin, Textile chemicals, dyestuffs and chemical division. April 1979.
DuPont Co., Textile chemist and colorist, April 1983, vol. 15, No.4.