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Thermoforming primer: The secret to clear PP packaging

Although polystyrene (PS) has traditionally been the most widely used polymer in the thermoforming industry, and polyethylene terephthalate (PET) is a popular choice for cold drink cups and deli containers, PP is rapidly gaining ground. This paper works to explain the advantages of the material, the challenges to forming it, and offers some proven solutions.

(Editor's note: The following primer on thermoforming PP was submitted by Adam Watson and Brian Burkhart, global market managers at Milliken Chemical, a supplier of nucleating and clarifying agents.)

This trend is the result of PS and PET disadvantages, including their higher density and cost and poor heat stability, versus PP advantages:  particularly lower density/cost, less environmental impact and better economics. McDonald's and Starbucks are among the food service giants that have switched to clarified PP for cost and environmental reasons.

When thermoforming PP, high-performance nucleating agents help improve processing speed, physical properties and part aesthetics; and new clarifying agents provide clarity approaching that of glass and amorphous polymers. High-performance nucleating agents improve the quality of extruded PP sheet feedstock, giving thermoformers the material needed to produce high-quality packaging and other parts. Further, powerful clarifying agents for PP enable this polymer to replace PS and PET in see-through parts, particularly shallow-draw lids and containers and high-end applications. These technologies are driving the adoption of both medium- and high-clarity PP in food packaging and containers.

Polypropylene vs. polystyrene and polyethylene terephthalate
Polypropylene is a semi-crystalline resin with low density; outstanding heat and chemical resistance; good mechanical properties, including a balance of stiffness, impact resistance and chemical inertness; excellent moisture barrier properties; low odor and taste; and, when clarified, beautiful aesthetics. Homopolymer PP is widely used in sheet extrusion and thermoforming applications because of its stiffness, clarity and cost-effectiveness. Random copolymer PP can offer even higher levels of clarity and gloss and better cold temperature impact properties.

From an environmental standpoint, PP's low density helps reduce the amount of material needed as well as overall packaging weight, which helps minimize landfilling and cuts fuel usage during shipping. It also has a low carbon footprint, thanks to fewer emissions released during manufacture. Polypropylene is easily recycled in existing waste management systems, allowing the material to be reused indefinitely. Plus, it does not exhibit undesirable side effects during recycling, such as crosslinking and forming a gel, or outgassing.

Polystyrene is an amorphous polymer that exhibits high stiffness and good dimensional stability. General-purpose polystyrene (GPPS), also referred to as crystal polystyrene, offers exceptional clarity and rigidity. High-impact polystyrene (HIPS) is made by adding butadiene rubber to styrene during the polymerization process. HIPS is opaque and provides improved impact properties while maintaining good stiffness.

Disadvantages of PS include brittleness and unsuitability for microwave applications, as well as a higher density that raises material costs. PS is not easily recycled because of its low scrap value. It is generally not accepted in curbside collection recycling programs.

Polyethylene terephthalate (PET) is a linear thermoplastic polymer in the polyester family. It is strong, impact resistant and transparent. Plastic processors use thermoformed, non-oriented PET for blister packs and trays, and crystallizable PET for oven and freezer packaging. Although PET is lightweight, PP has a 36% lower density. Another PET drawback is that non-oriented PET cannot be microwaved. Also, thermoformed PET packaging is not recycled due to its wide variation in viscosity.

PP extrusion and thermoforming:  processing and aesthetic challenges
In spite of PP's superior properties, its widespread use for extruded and thermoformed parts has been hampered by several processing related issues. PP has low melt strength, which can result in sagging and sheet thinning during thermoforming. Further, the material has a narrow temperature window (15°C vs. 30°C for PS & PET) for successful thermoforming that requires careful monitoring. The amount of time required for PP to fully crystallize can lengthen production cycles and reduce productivity. Finally, as it cools and crystallizes, PP can shrink unevenly, causing warpage of the final part. This is commonly seen in oval rather than perfectly round lids and containers.

Increasing PP's crystallinity level and reducing the size of spherulitic structures improve thermoforming results. High crystallinity enhances modulus, increasing the strength and self-supporting properties of the sheet to minimize sagging during the heating phase of thermoforming, and improving overall strength and dimensional stability of the final part. The presence of small spherulites improves clarity by scattering less light than larger ones, and also improves mechanical property balance. However, balancing these two factors during extrusion is tricky, because higher chill-roll temperatures that promote higher crystallinity also produce larger spherulitic structures with lower impact strength and poor clarity.

The other key challenge to broader use of PP is its inherent haze. Especially in food packaging and containers, clarity is equated with quality and safety, as well as ease of use. In the past, PP simply could not compete with the high clarity of PS, PET and other materials.

Nucleating agents optimize processing speed, quality and performance
Nucleating agents are additives used with PP to increase its crystallization rate by providing additional sites for crystal growth. They target crystallization behavior in processing to improve cycle time or line speed, as well as crystallinity and morphology in the final product, improving optical properties and mechanical characteristics such as stiffness. Original classes of nucleating agents were designed to improve stiffness, and advanced classes made further gains in mechanical properties and productivity/processing, but are hindered by dimensional characteristics.

Recent breakthroughs in PP nucleation technology have led to the creation of high performance nucleating agents that provide additional advantages, including shorter cycle times, reduced warpage, optimized dimensions and better aesthetics. Although these additives tend to be more expensive than older types of nucleating agents, they are effective in very low concentrations (typically less than 1000 ppm), helping to compensate for the higher price. Also, the add-on cost of high performance nucleators is far outweighed by productivity gains of up to 30% and significantly reduced scrap rates.

High-performing nucleators kick-start the crystallization process as the PP resin cools following extrusion, permitting higher extruder output and reduced cycle times during the forming process. For example, Hyperform, a nucleating agent our company supplies, has the highest PP crystallization onset temperature (Tc) available, enabling crystallization to begin earlier in the cooling process. A higher Tc means that less cooling time is required. This acceleration can increase productivity by up to 40% compared to older technologies. For the thermoformer, high performing nucleating agents contribute to faster throughput and greater productivity. Further, the significant speed advantage from these nucleators gives thermoformers the flexibility to slow down processing as needed to control shrinkage.

Illig Machinenbau of Heilbron, Germany, a manufacturer of thermoforming equipment for packaging, ran trials of melt-phase thermoforming using a homopolymer PP with Milliken's Hyperform HPN-68L agent. The company documented a 23% increase in machine productivity, along with better dimensional stability of the rectangular trays that were produced.

In another example, a 9g margarine container produced by in-line thermoforming showed the following benefits from the addition of Milliken's Hyperform HPN-600ei agent to the PP resin: a productivity increase from 19.1 to 20.8 cycles per minute; improved stiffness that enabled easier stacking of parts, also contributing to faster cycle times; and improved gloss level.

Isotropic shrinkage to avoid warping
In an ideal world, a thermoformed part would shrink to the same degree in both the machine direction (flow direction) and transverse direction (perpendicular to flow), creating a uniform shape. In reality, PP can shrink anisotropically due to certain pigments, contaminants and even older classes of nucleating agents, causing warpage of the finished part or an oval shape instead of a circle. This issue is a particular problem for lids, which may not fit the container, and for cups that need to be stacked. Poorly controlled shrinkage leads to high scrap rates and quality problems. It also may require processing conditions to be adjusted for each pigment type to produce identical parts in different colors.

By accelerating crystallization and promoting isotropic shrinkage, high-performance nucleating agents, such as Milliken's Hyperform HPN-600ei, improve part uniformity and quality and enhance dimensional stability. Further, adding an agent to various pigmented formulations makes the crystallization temperature and shrinkage performance of the pigments much more consistent, reducing the amount of processing adjustment that is required. Reduction in warpage and scrap helps to raise quality and cut costs. This is particularly important for pigmented materials or recycling systems where process variation is especially high.

In another example, the use of Milliken's Hyperform HPN-600ei agent to replace an older-generation nucleating agent reduced warpage on the bottom of a deli cup made with a homopolymer PP, while also raising productivity by 13% and providing better aesthetics and easier processing. 

Enhanced mechanical properties
In the past, nucleating agents often raised a dilemma: while improving the stiffness of PP, they reduced its impact resistance. Both of these attributes are important in applications such as cold drink cups, which need to be stacked high without crushing the cups on the bottom (topload performance) and also offer a substantial, high-quality "feel" to the consumer (stiffness).

The addition of high-performing nucleating agents such as Milliken's Hyperform HPN-600ei solves this problem by providing higher stiffness without sacrificing impact performance. With a high performance nucleating agent, PP can successfully replace PET and other materials in cold drink cups.

Clarifying agents open new opportunities for PP
High performance nucleating agents improve the clarity of PP, making it suitable for a number of applications. However, there are situations where greater clarity is needed. These include high-end packaging and containers that demand exceptional transparency, and shallow-draw items such as cup lids, clamshell containers and blister packs.

In the latter case, shallow configuration means the part does not benefit from drawing out the PP into an elongated shape and aligning the molecules, which would enhance the clarity provided by the high-performance nucleator. Therefore, while a high-performing nucleating agent can provide sufficient PP clarity for everyday use, additional technology is needed to allow this material to be used in super-clear applications. That is where a clarifying agent comes into play.

Millad NX8000 clarifying agent from Milliken Chemical provides a typical 50% reduction in haze versus the current industry standard. This advanced technology produces excellent aesthetics for shallow draw packaging using thermoformed PP, including a high level of clarity and gloss. Millad NX8000 makes PP suitable for shallow-draw parts, opening new opportunities for thermoformers in this sector.

Highly clarified PP can successfully compete with traditional materials because it offers lower density for reduced cost and lower resource usage; fast and easy processing for high productivity; and recyclability. Millad NX8000 also delivers performance benefits including increased material strength and rigidity, and improved heat and chemical resistance.

McDonald's USA, which initially launched its premium beverage line in amorphous PET (APET) cups, identified the opportunity to achieve cost savings and provide positive environmental benefits while maintaining performance and premium product image by transitioning to clarified PP. The reduction in package weight delivers significant savings in the amount of resin used and reduces energy in transportation of the raw materials and finished packaging throughout the supply chain. Consumer research conducted prior to launch confirmed that differences between the two packages in appearance and performance, including sidewall rigidity and clarity, were negligible.

To reduce the eco footprint of its Vivanno cold cups, Starbucks switched from industry-standard PET to PP cups, which can be mixed (or co-mingled) together with other PP containers without impacting recyclability. PP cups use 15% less plastic than PET cups and emit 45% less greenhouse gases during their production. Starbucks uses more than a billion cold drink cups each year.

Many trends are driving the need for replacement of traditional clear packaging materials. Along with economic pressures to reduce costs throughout the production cycle, manufacturers are listening to consumer demands for sustainability - less material, easier recyclability - as well as safety and quality. In contrast to PET, PS, and other materials, optimized PP checks all the boxes on the wish lists of consumers, thermoformers and manufacturers. Overall, PP offers a cost-effective, high-performance, aesthetically pleasing and sustainable solution for a wide array of applications.

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