A Communication from the PLACE Div. of TAPPI
- Published: May 31, 2005, By David J. Bentley, Jr., Contributing Editor
Providing practical information to the converting and packaging industries…
EXECUTIVE SUMMARY
Easy-Opening Packaging Technology
by Inge Roucourt, Johan De Clippeleir, and Spencer Hirata, Basell Polyolefins
Today’s consumer is increasingly demanding the convenience of user-friendly, easy-opening packaging. This paper discusses relationships between structure, morphology, and properties using recent laboratory results and shows how these factors influence the easy-opening performance. Easy-opening technology today uses a cohesive failure mechanism delivering unique characteristics that are sometimes not fully exploited. The paper also reviews some common ethylene vinyl acetate based seal/peel systems regarding the basic parameters that control the easy-opening effect. These include adhesion of a dispersed phase toward the matrix with reference to the peel quality obtained at an interface.
Polybutene-1 is a polymer with an intrinsic high shear sensitivity and low heat of fusion that has a melting point similar to polyethylene. These are the unique properties that the packaging industry uses to produce easily opened packaging seals. Although the easy-opening concept using polybutene-1 has wide use in the packaging industry, its use is not always sufficiently exploited.
The paper shows that the degree of incompatibility between polybutene-1 and polyethylene that gives a polyethylene matrix with polybutene-1 in a primarily dispersed phase controls peel performance. The mixing capacity of processing equipment, the melt flow properties and concentration of the components used, and the strength of the weak bonds introduced control this dispersion quality. The type of polyethylene components used, the crystalline morphology obtained, and the molecular weight of the blend components used influence the strength or adherence of the weak bonds. Laboratory results showed that low vinyl acetate content resins seemed to deliver a similar balance of compatibility and dispersability compared with similar melt flow rate low density polyethylene resins. Increasing the percentage of vinyl acetate seems only to increase the compatibility with polybutene-1.
EXECUTIVE SUMMARY
Using Thermal Process Imaging For Extruder Analysis And Improvement
by Andrew W. Christie, Optex Process Solutions, LLC
Recently, improved infra-red temperature sensing tools have become readily available to provide a more direct real time measurement of melt temperature variations. These tools analyze cross web temperature uniformity with and without a static mixer. This paper expands on earlier work by using this technology to understand better the complex process interactions in melt temperature development on a production extrusion coating line.
The challenge for all production operations today is to improve product quality and production efficiency. For many operations, shorter production runs and more varied products complicate this challenge. How do you assure that you process a high melt index acid copolymer and a fractional melt linear low density polyethylene with equal consistency? How do you establish an optimum extruder temperature profile? Do you increase back pressure to increase work in the screw? Do you add a static mixer? These and many other questions confront the managers of extruder operations daily and often require decision making with limited or misleading data.
Although the work in this paper is not a controlled experiment, the results indicate that machine modifications can significantly improve process stability and uniformity. An infra-red process imager is a useful tool for analyzing and understanding the temperature field variation in extrusion processing. This tool allows simultaneous monitoring of cross web (position dependent) temperature and down web (time dependent) temperature variation. Coupling this process data with simulation analysis greatly enhances the capability of understanding and interacting with extrusion processes for process improvement and optimization.
EXECUTIVE SUMMARY
A Novel Seal-Peel System
by Alex Hayden, Amcor Flexibles Healthcare
A new co-extruded product is available to make packages peelable. The product falls into the cohesive failure category of peelable materials but offers additional advantages to the converter and end-user. Unlike conventional cohesive failure systems, this product separates the seal and peel functions into two distinct polymeric layers and buries the peel layer inside the co-extrusion. Some benefits afforded by this new system include a wider seal window, a more consistent peel strength, and reduced particulate generation when peeled.
Among the advantages afforded to a converter and end users by this new technology are a wider seal window, more consistent peel strength, and improved performance when used with porous substrates. These benefits are primarily due to the separation of the seal and peel functions into two distinct polymeric layers. The material can be produced by co-extrusion via blown, cast, or coating production methods. For simplicity, the paper focuses on co-extrusion coating.
This new peelable material has a multi-layer construction. When co-extrusion coating, a polymeric coating is applied to a carrier base web. The carrier web can range from paper to polyester to barrier laminates depending on end-use requirements. The co-extrusion coating consists of three distinct layers including a backing layer, a peel layer, and a seal layer. The backing layer provides most of the strength characteristics of the coating as well as providing good adhesion to the carrier film. The formulation of the peel layer controls the peel strength. The composition of this layer is critical to the performance of the peelable system. The design of the seal layer provides a weld or permanent seal to the mated web. The polymer choice is critical since it must not only achieve a high seal strength to the mating web but also exhibit low elongation at break to achieve a clean peel initiation.
This seal-peel system offers several advantages over conventional peelable systems. Its seal layer can be formulated differently to achieve a weld type seal strength to a wide variety of substrates. The peel layer can provide varying seal strength levels. The buried cohesive failure offers a consistent seal strength. When sealed to a porous substrate or compared with a coated product, the particulate generation is substantially less. In some cases, the system can eliminate roadblocks to increasing package throughput rates.
EXECUTIVE SUMMARY
The Effects Of Resin Selection And Extrusion Variables On Peelable Seal Performance
by Shaun Pirtle, Laura Mergenhagen, Jeff Wooster, and Rob Cotton, The Dow Chemical Company
This paper addresses variations in peelable seal performance and attempts to understand the mechanisms behind these inconsistencies. These inconsistencies can range from the inability of the seal to be opened with reasonable force through premature failure of the seal that results in a loss of package integrity. Variations in seal strength between extrusion lines or production facilities are common. These dictate that the peelable seal formulation and fabrication conditions must be as robust as possible. In many situations, more than one variable may be responsible for inconsistencies in peel performance. An overall understanding of the peelable seal mechanism is therefore necessary.
A key performance requirement for most peelable seals is a consistent opening mechanism. A peelable seal is a heat seal with a defined force generally in the range of 1 to 10 lb/in for flexible packaging. More specifically for cereal packaging, 1 to 3 lb/in is common although the target varies according to individual manufacturer requirements. These peel forces must occur with a broad sealing window generally in the sealing range of 100°C to 130°C. Controlled peelable heat seals are desirable in flexible packaging applications because they offer product integrity (hermetic seals) while allowing easy access to the product without the use of tools or the destruction of the package. Typical products that use peelable seal technology are cereal box liners, cake mix liners, cracker tubes, medical packaging, and single serving snack packages.
The scope of this paper covers polyolefin based peelable seal based structures. The majority of cereal packaging consists of polyolefin based films that are typically coextrusions of high density polyethylene with a peelable seal layer composed of a 14% to 18% by weight vinyl acetate content ethylene vinyl acetate copolymer blended with polybutylene. Structures using polyolefin plastomer based peelable seals are also available. They offer some advantages over PB based systems such as lack of aging, improved organoliptics, improved COF control, and better economics.
The proper selection of resins and extrusion variables are critical when formulating peelable seal structures. Understanding the viscosity relationship between the continuous and contaminating phases is critical in determining how to produce a cohesive failure peelable seal formulation based on polyolefin plastomers and polypropylene. For structures using cohesive failure technology, blends of polyolefin plastomers and polypropylene resins incorporating between 25% to 35% of polypropylene by weight give excellent peelable seal performance.
For information about the PLACE Division of TAPPI, access the TAPPI web page at tappi.org. To obtain the complete papers whose expanded summaries appear in this section, go to the TAPPI web site at tappi.org., then click on "the PLACE" in the section designated Journals.
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