Providing practical information to the converting and packaging industries…

Nanotechnology Functionality in Flexible Packaging; New Methods of Manufacturing
by Kent Coulter, Southwest Research Institute

    APPLICATION: By incorporating a lattice of one material with line dimensions of 100 nm between layers of similar or dissimilar materials, the mechanical and optical properties of a film can be modified to enhance stress balancing, directional reflectance, scattering, absorption, and interference can provide visual enhancement, shielding, sensing, or tracking.
Material multi-functionality is a key component in the development of smaller, lighter, and more powerful devices for application in a broad range of frequency dependent environments. One way to achieve this multi-functionality is to incorporate structure into a thin film device that has a frequency specific response for the material and a separate response to structure, i.e. visual transparency and microwave reflection. The pursuit of superior properties by synthesizing low dimensional architectures in engineered nanocomposites has led to a new class—meta-materials—that are being investigated as frequency selective surfaces, photonic band gap structures, and negative index materials. Only since 2000 have patterning processes such as lithography, nanoparticle assembly, and nanofabrication progressed to the point of experimental demonstration. Commercial products and industrial processes to make these unique structures are under development at national laboratories, start-up companies, and large technology companies. Structures have been built in the microwave and infrared regime, but the immediate challenge is to fabricate composite structures that possess spectral gaps at frequencies up to the optical region.

Development of new approaches that allow practical control of material structure and morphology at the nanometer scale is of tremendous importance. There are extensive applications in photonics, electronics, and catalysis where the inclusion of structure at dimensions below the optical wavelength of light will introduce order of magnitude improvements in material and device performance. Incorporating these nanostructures within traditional thin film layers provides a platform to achieve multi-functionality for miniaturization. Using the principles of structure zone modelling, patterns of nanometer dimensions have been deposited that offer a simple and robust approach to in-situ fabrication of nanostructured surfaces and thin-films with long range order.

Enhanced Economics of 100% Solids Adhesives
by Michael Leib, Rohm and Haas Co.

    APPLICATION: Avoiding use of solvent eliminates energy cost associated with running the dryers and energy and maintenance costs associated with running environmental controls. Additional savings with 100% solids adhesives include lower cost of capital, higher line speeds and efficiencies than conventional laminators with dryers, and improved worker safety.
Over the last four years the cost of natural gas has doubled, and the cost or crude oil has increased more than 160%. Energy prices will probably remain high due to the high demand for energy from China and India. In addition, the development of new oil is not matching consumption.

An adhesive using 100% solids is an effective route to reduce cost to converters. Such materials eliminate diluent solvent cost and natural gas cost. The dry adhesive applied cost of 100% solids adhesive is lower than that of solvent based and most aqueous laminating adhesives. The adhesives run at lower coating weights at higher line speeds. Since no emissions exist, energy cost associated with maintenance and running environmental controls is non-existent. The equipment for 100% solids lamination equipment is one-third to one-half the cost of a conventional laminator. Worker safety improves because workers no longer handling solvents or batch mixing adhesive because the adhesive is mixed using meter, mix, and dispense equipment.

100% solids adhesives are extensions of the urethane chemistry that is well known in the packaging industry. The primary difference between 100% solids adhesives and resins used in traditional solvent-based, dry-bond adhesives is the dry adhesive molecular weight. The molecular weight of 100% solids adhesives is significantly lower. Due to the lower molecular weight, initial bonds and shear strength are significantly lower than those obtained with solvent based analogs. Laminating equipment with sensitive tension control and rewind systems is therefore necessary to apply and laminate 100% solids adhesives. An adhesive at 100% solids can meet most lamination requirements.

Clarifiers for Polypropylene Film
by Chris Kerscher, Milliken & Company

    APPLICATION: Clarified polypropylene can provide improved shelf appeal, decreased MVTR, and reduced total manufacturing costs.
Today, clear polypropylene film occurs at the expense of manufacturing flexibility. While BOPP film has excellent optics, clear, blown polypropylene (PP) film would allow for more varied constructions. Advances in PP resin and additive technologies have resulted in higher melt strength resins, additive packages with better heat and color stability, and advanced clarifier technology. Combining these can create films with the optics of BOPP and the flexibility of the blown film process.

Adding advanced clarifiers to blown PP film generates multiple benefits including high shelf appeal, low cost constructions, easy manufacturing, long shelf life, and down gauging opportunities. These have particular interest for food packaging and stand-up pouches.

Clarifier technology trials on lines ranging from pilot scale to commercial production equipment have shown the same trends in haze and gloss improvements. These include 60% haze reduction in homopolymer polypropylene and 85% haze reduction in random copolymer polypropylene. Clarifiers improve the optical and physical properties of film by increasing the crystallinity by approximately 2% as calculated by refractometry. Throughput is not compromised. Actually, an increase in peak crystallization temperature results in a lower frost line height that suggests the ability to increase line speed. Tear and impact performance for PP film remains poor, but other layers within a structure can provide these properties. Clarification also increases stiffness by 14% in the machine direction and 30% in the transverse direction. This increased stiffness could translate into savings through down gauging or easier down stream converting.

Polypropylene can easily run on existing polyolefin blown film lines with general purpose screws. An extrusion temperature of 225°C will work well. If the line is dedicated to PP, a screw designed specifically for PP, a high pressure die, and a dual lip air ring should be considered.

Significant shelf appeal improvements are possible when using clarifiers. A clarified random copolymer readout replaced an unclaimed random copolymer PP layer in a seven-layer, 5 mil thick, high barrier, high stiffness pouch construction of PP/tie resin/Nylon 6/EVOH/Nylon 6/tie resin/PP. Adding a clarifier decreased the haze by more than half from 29% to 12% and increased the 60° gloss from 78% to 123%. Additional improvements are possible by clarifying both the PE and PP layers in a multilayer construction such as PP/tie resin/EVOH/tie resin/PE with a starting haze of 24.2%. Clarifying the PP layer reduced the haze to 13.7%. Clarifying both olefin layers improved the haze to 7.6%.

Coextruded HMW-HDPE Film Structures With Improved Bubble Stability
by Brian Wolfe, Equistar Chemicals APPLICATION: High ratios of LLDPE can change the molecular weight of HMW-HDPE resin to improve bubble stability in coextruded films. The American Plastics Council states that 1.6 billion pounds of HMW-HDPE was sold in North America in 2005. Approximately 63% or slightly more than one billion pounds of this was for grocery sacks. Another 26% was for institutional can liners. In recent years, cost pressure has significantly increased for these market segments. Film converters have seen more pressure to recapitalize to higher output equipment. In most cases, this has resulted in an influx of coextrusion capacity capable of producing multilayer films for these markets.

This was especially true for the grocery sack industry. Recent industry reports suggest that approximately 50% of the grocery sack industry uses coextrusion equipment. At the film gauges typical for grocery sacks of 0.7 mil or 17 microns, few physical property improvements are possible by coextruding or blending lower density resins into the HMW-HDPE at concentrations up to 20%. Converters have nevertheless seen increased gauge and physical property variation as LLDPE concentrations increased above 20%. This increase placed a physical limit on the amount of LLDPE that can be reasonably incorporated into HMW-HDPE films.

This study explored the use of higher molecular weight HMW-HDPE resins in coextrusions to improve melt strength and reduce gauge variation. It also examined the impact of increased LLDPE concentration at levels greater than 30%. Finally, it used new analytical methods to correlate the rheological properties of various HMW-HDPE resins with their ability to provide increased melt strength.

Using HMW-HDPEs with higher molecular weights in thin film applications provides many benefits. The use of higher molecular weight resins lowers gauge variation in unblended coextrusions of HMW-HDPE and LLDPE. Some process changes are necessary to offset the decrease in physical properties seen with the higher molecular weight. While the increased incorporation of LLDPE does not dramatically improve the impact or tear properties of the films, the improved melt strength of polymers might allow for greater addition of LLDPE to improve optical properties and broaden heat sealing windows. Using other resins that provide slightly improved gauge variation might offer a benefit with an increase in modulus and tensile properties at negligible changes in impact and tear properties. Analytical techniques such as SER and Rheotens can discriminate relative melt strengths of HMW-HDPE resins and would be useful in making resin selections.

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