In general, when we think of oriented substrate films for packaging or industrial uses, we are focused on clear films – or perhaps films with various surface appearances, such as shiny or glossy, or a matte surface, ranging from a little hazy to a paper-like surface. Film surface appearance generally is controlled by skin resin selection – and sometimes by additives, such as silica-based particles, for matte surfaces. In opaque films, we only find films that are white in color with perhaps 30% light transmission and aluminized films with light transmissions of 2% or less. Seldom do we come across pink, green or, say, black films that are sold directly to end users or converters. There are several reasons for this.

Limitations of color

One of the primary reasons for not producing pigmented films is that the market size for any particular color is very limited, and inventory control is difficult. Modern orienters are producing films at rates of 10,000 lbs/hr, and many are significantly above that. Only a few market segments can support such large production rates, such as those of the opaque films and metallized products, and then only because of the more universal properties designed into the film, not necessarily because of the color. Inventory control was behind Henry Ford’s first car-color selection comment that customers could have any color they wanted, as long as it was black. When we were developing “white” films, it was for a candy company with white packaging. We expected that they would be able to eliminate their white ink coverage. However, the white color of the film was not the correct white for the company’s product specifications, so the white ink was printed over the white film anyway.

Variations in white

So what controls the “whiteness” of “white opaque” films? When studied, variations in the white appearance are found between manufacturers due to variations in cavitation technologies and the impact of the cavitating agents on the “whiteness” or hue of the film. This is well documented in the KODAK patent literature for photographic “paper” made from laminations with cavitated films. This variation in film “white color” is due to light absorption by the cavitating agents, such as calcium carbonate, which shifts the white color hue of the film in conjunction with the light-scattering reflections of the cavitated void structures. Many producers try to adjust for this by adding a TiO² pigment to the film because TiO² is perhaps a more universal white pigment for many producers of white products, such as papers, paints and inks.

There also are cultural variations in what is an acceptable “white” to various populations. I have been told that while Americans like a “blue” or “cool” white, other populations prefer a “reddish” or “warm” white.

Achieving customer standards

So, it generally is left to the end user or converter to produce colored films by coating or printing. This also is cost-effective because each customer will have its own standard “white” or “yellow” or “red” color that has been chosen specifically for its brand identity, and much time is spent at the beginning of a converting or printing run in matching the “red” to the customer’s standard “red” quality standard.

This is what I meant above when I referenced the difficulty of inventory control. If a film producer had to make various lots of red film for a range of customers, a greatly expanded warehouse would be needed. Also, the time spent doing the quality checks during changeovers between red for customer one and red for customer two would produce tons of film that is not “red” for anybody and would not be useable, perhaps even as reclaim in “red films.”

Another significant manufacturing cost and source of waste would be purging between colors, say red to green, and also back to clear films. With machines of the size and scale of orienteering lines, the time required for a color change is long and difficult. Generally, it is best to institute a complete line shutdown to drain resin hoppers and resin transport systems, while the pigments are purged slowly from the extruder and melt system. This is sustainable in white films due to large sales volumes, which minimize the need for the color change back to clear or into white film production.

The problem with minerals

Another unappreciated reason to avoid pigments in films is that the pigments generally are mineral-based, leading to several effects seen in the film production. First, most minerals will cavitate films during stretching. Cavitation by particles is controlled by particle-size selection and the narrowness of the particle-size distribution. In general, we want cavitating particles to be about 1 to 4 microns and less than 10 microns in size. Below particle sizes of perhaps 0.5 and ideally below 0.1 micron, i.e. submicron particles, any film cavitation is minimized or not effective due to the stretching polymer’s ability to stretch around or perhaps rotate the particle without a significant stress riser, which initiates the cavity formation. Consequently, there is a somewhat arbitrary – but performance-based – delineation that pigment particle sizes are submicron, i.e. 1 micron or less, and cavitation particles are 1 micron or more.

Second, minerals often are a catalyst for polymer degradation. Each pigment’s chemical composition and perhaps crystal structure will interact chemically with the polymer in the high-temperature and -pressure environment of the extruder. For instance, in TiO², there are three primary crystal forms: anatase, rutile and brookite. While I only have experience with the rutile and anatase forms, there is a marked difference in polymer degradation between them during polymer extrusion.

So, in closing, pigmenting films have a lot against them in terms of manufacturing efficiencies, inventory control and, consequently, cost. If there was only one universally accepted meaning of “red” or “green” or any other colors of the rainbow, we might be able to manufacture them in a cost-effective way. However, if we look around us, we find many hues of each color and many colors not prominent in the rainbow, so pigmenting a film acceptable for any two users would be difficult. 

Eldridge M. Mount, Ph.D.
Consulting Technical Editor
585-223-3996, emmount@msn.com
ARC Member