- June 01, 2007, David J. Bentley, Jr., Editor
Nucleation Of Polyethylene Blown Film
by Heather Dolan, Milliken & Co.
APPLICATION: Nucleation is a common practice with polypropylene resins. A new nucleator specifically developed for polyethylene provides significant value in coextruded lamination films.
Polyolefins can be nucleated using an insoluble or soluble nucleator. Without a nucleator, a polymer melt cools slowly forming large spherulite structures and a hazy material. When a nucleator is present, it serves as an initiation point for these crystals to start growing. If a soluble nucleator is dissolved in the melt, it “crashes out” as the melt cools and forms an infinite network of initiation points for crystal growth. The changes in crystal size are observable on the surface using Atomic Force Microscopy (AFM) techniques.
Changing crystallization structure can influence many important attributes for flexible packaging. These include tear strength, impact resistance, processing ease, optical properties, barrier, stiffness, and seal characteristics. Many market segments of the film industry want improved optical and physical properties including shrink films, overwrap films, lamination films, food packaging, greenhouse films, and stretch films.
Nucleation improves shelf appeal while maintaining key physical properties of LLDPE. Compared with standard LLDPE, haze decreases by an average of 40% while gloss increases by 70%. This becomes comparable with the optics of LL/LDPE blends. Where nucleation provides benefits is high optical performance combined with better physical properties. Compared to an LL/LDPE blend, nucleation can improve machine direction tear by up to 50%. Similarly, impact properties of pure LLDPE can be restored as opposed to the loss of up to 47% of impact performance with a LL/LDPE blend. Nucleation of LLDPE has also consistently demonstrated improvements in stiffness that may lead to downgauging opportunities.
Nucleation enables a unique balance of optical, physical, and barrier properties in polyethylene film. Nucleation can promote crystal growth at higher temperatures and at faster rates. Crystal size and orientation impact physical property performance. These changes manifest as benefits to shelf appeal, productivity, tear and impact performance, hot tack strength, and moisture vapor barrier.
The Unique Performance of PVdC — A Proven Material Displays Its Innovative Potential
by Matthias Huter, SolVin GmbH & Co.
APPLICATION: Continuously improved PVDC-copolymers provide high barrier properties against humidity, gases, and aromas and can bring other benefits to flexible packaging including heat sealability, oil and fat resistance, transparency, gloss or anti-mist properties.
Polyvinylidene chloride (PVdC) resins and coatings have been a part of the flexible packaging world for more than 50 years by providing a unique combination of functional characteristics that has found numerous applications. According to 2004 estimates, approximately 160,000 metric tons of PVdC are used annually around the world in extruded, coextruded, coated, and laminated structures. According to a 2005 study of high barrier packaging films, PVdC remains the leading barrier polymer in high performance packaging films. In the United States for dry food packaging, PVdC-coated BOPP films hold a 53% share of barrier films. Forecasted growth for PVdC-coated PET films is a healthy 9% per year driven primarily by new applications including lidstock for prepared foods, retort applications, and clear stand-up pouches.
In the area of polymer developments, new grades of PVdC latex and resins have been introduced in recent years that offer improved barrier vs. standard grades that have been in use for many years. Reductions in oxygen transmission rates of approximately 70-75% for both latex materials and resins have been achieved. These allow flexible packaging converters the ability to offer higher barrier, lighter-weight, and less costly packaging solutions. PVdC can also be combined with other barrier materials resulting in very high barrier packaging films. For example, PVdC coatings can be applied to metallized PET films to give not only improved barrier but also better flex resistance. Advanced polymerization technology has resulted in new PVdC latex grades that do not require the use of a primer to achieve good adhesion to certain substrates. This results in simpler converting processes, less complex structures, and improved barrier performance.
To obtain the complete version of these papers, go to www.tappi.org, and select “the PLACE” under the Publications/Bookstore heading.