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Discoloration Resistant Polyolefin Films

Following is an expanded summary of a complete paper that is available on the TAPPI web site at tappi.org. On the page, click "the PLACE" in the section designated "Journals."

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Application: In the film industry, a selected set of circumstances and certain types of stabilizers can lead to discoloration due to inadequate stabilizer concentrations, harsh processing conditions, improper selection of white pigments, prolonged storage in a environment containing oxides of nitrogen (pollution), and even lengthy exposure to fluorescent lighting. A new approach to the problem with “phenol free” stabilization systems can suppress post extrusion discoloration in clear and white pigmented films.

Manufacturers of film grade polyolefins and the downstream fabricators of film products often look for new advances in polymer stabilization to ensure that they can provide the best products with the most value while minimizing costs. Additives or combinations of additives are useful tools for adjusting film product performance to give improved physical and aesthetic properties. Examples of such improvements might include better maintenance of melt flow rates, lower YI color, improved long term thermal stability, inhibition of gas fade discoloration, enhanced additive compatibility, reduced taste and odor, resistance to irradiation induced oxidation, and the suppression of “gels” and other extrusion related imperfections.

Over the last several decades, a variety of phenolic antioxidant and phosphite melt processing stabilizers have become available to meet these new opportunities. Used in different ratios and concentrations, blends of phenolic antioxidants and phosphites can provide optimal performance based on the nature of the polymer, the extrusion temperatures, and the shear rates experienced during various types of extrusion processes. These traditional phenol based systems have had successful use in various polyolefin applications for the last 20 years. They provide good melt flow control and acceptable maintenance of color in routine film applications.

As new applications for polyolefins have developed, opportunities for “higher” performance stabilization systems have become necessary. “Higher” performance may mean better melt flow maintenance under more demanding extrusion conditions such as high temperatures and shear. For these applications, one can use a substitution strategy such as changing the phenolic antioxidant, the phosphate, or both and then finely tuning the system by changing the ratio of these components. For the most demanding applications, new stabilizer chemistries using hydroxylamine or benzofuranone (lactone) can supplement the performance of the base stabilization system.

In some cases, “higher performance” stabilization was necessary to provide improved maintenance of color. The “improved maintenance” of color can mean lower color during repeated heat histories (recycle of edge trim and scrap), minimizing discoloration during warehouse storage (gas fading), maintaining low color during railcar shipment during hot and humid conditions, or resistance to discoloration from oxidizing environments such as corona treatment or gamma irradiation. Each higher performance stabilization system described above used the concept of a phenolic antioxidant as one component of the stabilization building blocks. While phenolic antioxidants have a proven track record, they can also be prone to discoloration when they over-oxidize. For an increasing number of “color critical” film applications especially in white films, this has become an issue. Several approaches can minimize discoloration.

Minimizing Discoloration
For improved initial color and color maintenance, a variety of approaches are possible such as the following:

  • Change the nature (propensity to discolor) of the phenolic antioxidant
  • Change the phosphite (color inhibition by alleviating the workload on the phenolic)
  • Change the phenolic (lower) : phosphite (higher) ratio
  • Examine “hyperactive” stabilizers (hydroxylamines; benzofuranones) as a booster for traditional binary blends
  • Change the acid acceptor (create a more “pH neutral” environment in the matrix)
  • Eliminate the phenolic antioxidant (“phenol-free” stabilization).

Our laboratories have pioneered the concept of “phenol free” stabilization with the development and commercial introduction of several products over the last decade. These “phenol free” stabilization systems usually use a combination of hindered amines that provide light stability and good long term thermal stability. The systems also use powerful melt processing stabilizers such as phosphites, hydroxylamines, tocopherols, benzofuranones (lactones), or selected combinations of these melt processing stabilizer chemistries.

Since their introduction in 1995, “phenol free” systems have had successful use in applications where melt flow control is important and where the initial color and maintenance of color are critical. The most prevalent use has been in fiber applications such as polypropylene or high density polyethylene where any type of discoloration is not acceptable. Recently, other color critical applications have also used this approach such as polypropylene homopolymers and copolymers. Although the “phenol free” systems have consistently solved most discoloration problems, adoption of this product concept in high density polyethylene and linear low density polyethylene has been slow compared with polypropylene and high density polyethylene fibers.

Technical Challenges
Overall, the retention of physical properties is probably the most important measure during the use of a film product. For example, “cheap” film products that break open during hauling have left indelible memories in the minds of young trash disposers trying to earn an allowance. The traumatic days of “cheap” plastic bags are over. Serious advances have addressed most concerns regarding durability. The adjustment of the molecular architecture of a polymer via new catalyst systems and polymerization process have brought high strength linear low density polyethylenes to the market with exceptional properties. Conventional stabilizers such as phenols and phosphites find use routinely to protect the retention of physical properties.

With the progress of optimizing the strength and toughness of film products, other ways to improve film products have arisen. The focus over the last several years has been to minimize product discoloration during storage to improve visual and aesthetic appeal. Molecular architecture can do little to influence visual perception other than minimizing haze and improving clarity. From this point forward, the selection of the stabilization system really influences the aesthetic appeal. Accordingly, considering the consequences of selecting the components that comprise this system is important.

The continuing challenge in polymer stabilization has been to select the best combination and concentration of stabilizers from the chemistries that are commercially available. The primary goal of stabilization is to shut down the negative aspects of polymer degradation associated with free radical chemistry and preserve the molecular architecture designed into the polymer via the catalyst system and polymerization process. This results in improved maintenance of the physical and aesthetic properties of the polymer. For long term thermal stability, one can choose from different hindered phenolic antioxidants or hindered amines. For melt processing stability, one can select melt processing stabilizers based on phenolic, phosphite, hydroxylamine, or lactone type chemistries.

In the past, most polyolefin film product stabilization systems used a combination of phenol and phosphite type stabilization systems. Under certain circumstances, the inadvertent over-oxidation of the phenolic component can lead to an undesirable discoloration of film products. This tendency to discolor has not necessarily been observed at the reactor during polymer pelletization or noticed during loading of a railcar. The inclination to discolor has primarily been at film fabricators where the films undergo a color shift over a relatively short period. This is sometimes quickly but typically in less than two to four weeks.

Most applications for linear low density polyethylene film grade resins involve packaging for consumer items. The physical properties necessary for these products are virtually built in via the molecular weight or molecular weight distribution of a polymer. The aesthetic properties are typically of less concern since most products are “single use.” In this competitive arena, creating value via additive technology is not necessarily the norm. Aside from these volume markets, some niches exist where aesthetics play a key role — higher end packaging. Where retention of aesthetics is important, additives can create differentiated value.

This study is a continuation of our previous work initiated two years ago by a well-known and respected film manufacturer. The base polymer chosen for this work was a blown film grade linear low density polyethylene. The polymer was devoid of stabilization and was kept in cool dark storage until use. Two grades of titanium dioxide came from a manufacturer of white pigments. The TiO2 was compounded into commercial concentrates

The first part of this work examined three stabilization systems. The first was a blank without stabilizers. Although often overlooked, the author considers this an important control experiment. The second was a traditional blend of a phenolic antioxidant and a phosphite. The third was a phenol free stabilization system based on a hindered amine for long term thermal stability and a 1:2 blend of a melt processing blend based on a hydroxylamine and a phosphite.

The complete paper shows the extensive results measured with the systems using the combinations of different variables for the following properties:

  • Melt flow rate
  • YI color data
  • Gas fade aging
  • Oven aging
  • Resistance to fluorescent lighting.

Melt flow rates can be controlled by either “phenol” or “phenol free” stabilization systems containing a powerful melt processing stabilizer. Good initial color occurs, and some color development occurs with additional extrusion passes. The most differentiating measure is gas fade aging especially in the presence of titanium dioxide. The work clearly shows that the “phenol free” stabilization systems offer good melt flow control, low initial color, and good color maintenance during gas fade aging.

In general, phenol free stabilization appears very practical for those polyethylene applications that have moderate demands on melt processing stability and more stringent demands on maintaining low YI color and good YI color maintenance during extrusion and ambient storage. This study shows that going phenol free is not necessarily as easy as it looks. It requires the use of the most powerful melt processing stabilizer chemistries to fulfill the demanding requirements of melt flow rate control. A more demanding polymer in regard to maintaining melt flow rates — susceptibility to molecular weight enlargement, crosslinking, or chain scission reactions — requires access to more potent process stabilizer chemistries.

Accordingly, a clearly rational and reasonable trend has developed regarding the adoption of “phenol free” stabilization systems for a variety of polyolefins. This technology has already achieved commercial success in several polypropylene homopolymer and copolymer applications. In addition, success occurred in representative examples of commercial products based on phenol free stabilization systems in different types of high density polyethylene and linear low density polyethylene.

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