ROSEMONT, ILL. — For caps producers, nucleation can provide a practical solution to eliminate shrinkage variations in high density polyethylene beverage closures.
Nucleating agents can be utilized to help processors control the process.
The potential of nucleation as a solution to this issue was addressed when Milliken Chemical Senior Development Engineer Bernard Vermeersch addressed attendees Sept. 11 during Plastics Caps & Closures 2014, held in Rosemont, just outside of Chicago.
Vermeersch discussed the benefits of Milliken's Hyperform nucleating agents in eliminating shrinkage variations in HDPE beverage closures.
“Closure features all have a function and dimensions are critical,” he said.
Liability can be a key issue. Fillers, with their high-speed lines, require caps that do not jam. Incorrect diameters or faulty seals can lead to customer complaints and cause high costs.
“While there is an increasing focus on closures for branding, design today is far more than a question of appearance,” Vermeersch noted.
Properties of the polymers, including optical and physical characteristics, depend on the end size of the spherulite structures and the crystalline orientation in the matrix.
In nucleated polypropylene, crystallization occurs earlier in the cooling process and happens at a faster rate. This allows decreased cooling time of the polymer. Also, nucleation density is much higher and crystal spherulite size is much smaller.
According to Vermeersch, polypropylene is a relatively easy material to nucleate because the rate of crystallization is low enough to allow nucleating agents to have a direct impact on the nucleation density.
Apart from the thermal mechanism, nucleation is strongly affected by impurities, dyes, plasticizers, fillers and other additives in the polymer. This is also referred to as heterogeneous nucleation.
The issues, including shrinkage, can be more difficult to control in HDPE compared to PP. Other issues can included caps that are cocked or sit too high or loose on the bottle neck. They also can have torque issues as well as broken bridges, flashing and leakage.
Pigments affect the closure dimensions differently in PP and HDPE, in large part because they nucleate at different temperatures Vermeersch pointed out that nucleating agents can be used to address these issues.
“Nucleating agents can increase the crystallization rate of polypropylene,” Vermeersch pointed out. “This means that the resin will freeze more quickly and enable improvements in the part quality in most conversion processes.”
Nucleation has traditionally been used to improve physical properties of PP and is now bringing PE to the next level of performance. Nucleating agents can reduce cycle times, improve warpage and aid dimensional control.
Hyperform HPN-20E, a nucleating agent for PE, allows for consistent processing, improved quality and cycle time reduction in the production of caps and closures — all while remaining true to Milliken's commitment to sustainability.
An organic salt, HPN-20E nucleates both PP and PE and has gained major food approvals across the globe.
“This provides substantial benefits in several processes to improve barrier properties, ESCR, shrinkage control, aesthetics, part quality and productivity,” Vermeersch said. “It can be successfully incorporated into a resin or masterbatch. You must achieve a good dispersion in the final part to obtain a good performance.”
Ultimately, nucleation can increate closure quality and performance with no effect on organoleptic or physical properties.
“You can adjust cycle times and masterbatches that have been treated to counteract the effects of pigment nucleation,” Vermeersch explained. “The masterbatches will be more expensive, but there will be less waste and fewer problems.”