By George M. Orlych, vice president of Technical Operations, MTI Polyexe, Inc.

The successful processing of plastic pellets into films and the subsequent converting of the film requires the addition of additives to the resin blend since, without adequate stabilization, the processing of polymers can lead to both mechanical and thermal degradation. In this article, the author will review the issues common to the production and converting of films, address the type of polymer additives used and discuss how the additives solve degradation issues, improve film handling and decrease winding defects.

Producers of polyolefin films (e.g., LDPE, HDPE, LLDPE, MDPE and PP), whether extruded blown film or cast film, obtain plastic resin in the form of pellets. Pellets, shipped in hopper trucks or railcars, are conveyed pneumatically and stored in resin silos at the manufacturing site. They then are transferred to the film production machines where they are dry-blended with other ingredients, extruded, melted and exit through a flat die (for cast films) or a circular annular die for extruded blown films. After exiting the die, the molten plastic is cooled, its edges typically are trimmed away and then it finally is wound on cores before packaging, labeling and shipment to converters or end users.

The successful processing of plastic pellets into films and the subsequent converting of the film require the addition of additives to the resin blend. This article will cover the issues common to the production and converting of films, the type of polymer additives utilized and how they solve these issues.

Antioxidants – primary and secondary

During film production, the extrusion conditions are designed to ensure complete melting of the plastic pellet (i.e., no gels or unmelts), homogeneous mixing (i.e., uniform color distribution) and steady state conveying of a uniformly mixed molten polymer to the extrusion die. In the extruder, polyolefins are subjected to mechanical shear and high pressure and are extruded at temperatures exceeding 400° F to 450° F.

Polyolefins are a series of long chains of hydrocarbon molecules, and without adequate stabilization, the processing of polymers can lead to both mechanical and thermal degradation. Degradation occurs when longer-chain polymers break down into shorter-chain fragments. The degrading of long-chain polymers into shorter chains is known as chain scission. These shorter polymer chains are reactive and will interact with the remaining longer-length polymer chains and degrade them. Degraded polymers can form hydrocarbon oils, which can build up on the extrusion dies, requiring frequent cleaning and loss of production. Degraded polymers also can discolor the film and show a loss of mechanical properties, such as tensile strength and elongation at break.

To prevent chain scission and other unwanted degradation, both process (melt) stabilizers and product stabilizers, or antioxidants, are added to the raw material resins. The term antioxidant (AO) collectively will be used to describe the chemical additives that inhibit the degradation of polymers due to thermal, mechanical and environmental factors.

Antioxidants fall into two main categories: primary and secondary. Primary antioxidants work by deactivating or inhibiting the shorter chain segments from causing chain scission. Secondary antioxidants work to interrupt the formation of reactive small-chain polymer segments and are referred to as preventative antioxidants. Both primary and secondary AOs are incorporated in the base resins by the resin producer and together they synergistically work to prevent polymer degradation.

For polyolefins, the most common types of antioxidants are hindered phenols (primary) and phosphite esters or sulfur compounds, such as thioesters (secondary). Table 1 shows the chemical structure of commonly used antioxidants.

Polyolefins are not equally stable to degradation. From least stable to most stable: PP < LDPE < LLDPE < MDPE < HDPE.

Polypropylene (PP) is more susceptible to degradation than other polyolefins and cannot be processed without added stabilizers. Low Density (LDPE), Linear Low Density (LLDPE) and Medium Density (MDPE) are less sensitive to degradation and require less stabilization. High Density (HDPE) polyethylene is the most stable out of all the polyolefins and requires the least amount of stabilizer.

The choice and amount of antioxidant depend on the polymer, extrusion conditions, processing temperature and final property requirements. Antioxidants typically are incorporated into the base polymers between 0.05% and 0.25% by weight.

Slip additives

When extruded, polyolefin films have a high coefficient of friction (COF) ~ 0.8 to 1.0. Instead of sliding over each other, films tend to stick to each other and to metal surfaces. This can cause issues during film production, such as sticking to rollers or dragging on surfaces, potentially causing wrinkles and generating unusable products. For producers of plastic bags that extrude and convert the film in-line, and users of form/fill/seal equipment, the COF of the film needs to be near 0.1 to 0.2. Incorporating slip additives during film production prevents these problems, increases output and improves product quality.

To be effective in reducing the COF of films, slip agents, by their nature, should have low compatibility with polyolefin resins. They function by migrating to the surface of the film, reducing the film’s coefficient of friction. During extrusion, the slip additives are dissolved completely in the molten polymer, but as the film cools and crystallizes, the slip additive migrates to the outside surface of the film. This process is shown in Figure 1.

The two criteria for slip agent performance are:

1. the speed of migration of the slip agent to the surface, and
2. the concentration of slip additive needed for minimum COF.

The two most common slip agents for polyolefins are fatty acid amides: oleamide and erucamide (see Table 2).

Comparing the two slip additives, erucamide migrates slower to the surface than oleamide, but it ultimately provides a lower COF than oleamide. Because of the slower migration, it will migrate to the surface after the roll is wound onto a core. This can be an advantage as rolls that are too slippery can lead to winding issues, such as telescoping (a sideways shifting of the material close to the core). The slower-blooming erucamide is utilized if high extrusion temperatures are involved, as it is less likely to volatize. Finally, if the film requires in-line surface treatment, such as corona or flame treatment, the slower-blooming erucamide allows films to retain their treatment level.

For in-line converting processes that require low COF, like bag manufacturing, fast-blooming slip additives, such as oleamide, are the additive of choice. Adding too much slip additive can have the negative effect of interfering with surface treatment and can transfer to surfaces, causing issues with adhesion, printing and laminating operations. Too much slip also can make the film appear hazy. The concentration of slip additives needs to be determined for optimal performance and cost, but a general industry loading level is 900 to 2,000 ppm.

Antiblocks

Polyolefin films tend to stick together, often making it difficult to separate film layers. Blocking occurs when two adjacent film layers stick to one another. Film blocking can cause various issues. For film converters, the most common blocking issue is seen as a high unwinding force. Tightly wound rolls can be blocked near the core, causing unwinding issues, such as film stretching and wrinkling. If the unwind force is too high, the roll may not be able to be unwound at all and become unusable.

Exactly how antiblocks work is not yet understood, but the current theory is that antiblocks create a space between the interface of two film surfaces by creating micro-roughness on the film’s surface. Since blocking is a surface adhesion effect, additives that create a micro-rough surface would reduce the adhesion between layers. High surface roughness decreases the blocking force: the lower the blocking force, the better the performance of the antiblock. This is illustrated in Figure 2.

The two important considerations that will affect how well an antiblock performs are:

1. The number of antiblock particles on the film surface, and
2. The size of the antiblock particle.

The most common antiblocking additives are inorganic minerals, as shown in Table 3.

Three-dimensional particles, such as silicas and silicates, are more effective than two-dimensional platy antiblocks, such as talc. The rule of thumb regarding antiblock particle size is in the range of 6% to 20% of the film thickness.

Note: Because slip agents also modify film surfaces, they sometimes are used as antiblocks and are sold as slip/antiblock combinations.

Conclusion

Additives are as essential to film manufacturing and subsequent converting as the plastic resin. For the film producer, the use of primary and secondary antioxidants incorporated into the plastic resin mitigates degradation due to thermal and mechanical shear.

The incorporation of slip agents and antiblocks into films improves film handling by allowing films to slide easily over equipment surfaces and rollers and are used to improve roll condition by decreasing winding defects, such as roll telescoping and blocking. 

Resources

1. Al-Malaika, S,; Antioxidants; A Review, In Plastic Additives: An A-Z Reference; Pritchard, Geoffrey, Editor,; Chapman and Hall,1988; pp 55-64.
2. Wylin, F.; Slip Additives; In Plastic Additives Handbook; Zweifel, H.; Maier, Ralph D.; Schiller, M.; Editors,; Hanser, 2000; pp 629 – 634.
3. Keck-Antoine, K.; Levens, E.; Bayer, J.; Mara, J.; Jung, D.; Jung, S.; Additives to design and improve the performance of multilayer flexible packaging,; In Multilayer Flexible Packaging; Wagner, John R.; Editor,; Elsevier, 2010; pp 37 – 56.
4. Kormminga, T,; Van Esche, G,; Anti-blocking Additives,; In Plastic Additives Handbook; Zweifel, Hans; Maier, Ralph D.; Schiller, Michael,; Editors,; Hanser, 2000; pp 613 – 628.
5. Sobottka, R,; Feltham, E,; Anti-blocking or polymer films,; In Plastic Additives: An A-Z Reference; Pritchard, Geoffrey, Editor,; Chapman and Hall,1988; pp 50-54.

George M. Orlych has a BS in Chemical Engineering from the University of Illinois (1985) and an MS in Polymer Science from the Illinois Institute of Technology (1990). He has spent most of his career in the developing and manufacturing of plastic films, paper coatings and silicones for the release liner industry. He has worked for HP Smith (now Loparex), Akrosil (now Mondi Packaging) and currently is the vice president of Technical Operations at MTI Polyexe, Inc. MTI Polyexe is a leading manufacturer of blown polyolefins films and silicone-coated release liners. He is a member of ACS, SPE and Tappi. Orlych can be reached at email: gorlych@mtipolyexe.com; www.mtipolyexe.com.