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Characteristics Of Flexible Food Packaging Materials
by W. Scott Whiteside, Clemson University

APPLICATION: Flexible packaging converters, designers, and users need a thorough understanding of the specific needs of each packaged food product to design flexible materials that will provide maximum shelf life.
Flexible food packaging materials can have a significant and complex effect on food product quality and shelf life. Various biochemical, chemical, physical, and boilogical changes that occur during storage or processing that can undergo influence by the properties of the flexible packaging material. Some common types of changes that can take place in food products are changes in texture, flavor, color, appearance, and nutrition value.

Texture changes can often be observed as softening or toughening of a food product. Softening can frequently result from the activity of various endogenous enzymes found in certain food products. The oxygen barrier properties of the packaging material can have a major impact on the rate of these oxygen dependent reactions. The water vapor barrier properties of flexible packaging materials can also influence food product texture through moisture loss or gain. Oxygen ingress can initiate and accelerate oxidative rancidity in food products creating a multitude of unacceptable flavors. Aromas and other volatile compounds outside the food packages can also permeate the food package contributing to additional unwanted flavor changes.

The barrier properties of a packaging material can influence retention of desired volatile flavor compounds. Oxygen barrier properties of packaging materials can have a significant effect on the oxidation of heme (red meat) and carotenoid pigments found in many food products leading to changes in color.

Enzymatic browning can also be affected by the oxygen barrier properties of the flexible packaging material. Alteration in food particle size and shape generally due to breakage is often a major aspect of food product quality that flexible packaging has placed considerable effort to resolve. This is particularly important in brittle food such as snack foods where rigid packaging forms can often provide protection from external compressive forces. The nutritive value of vitamins, proteins, and lipids can be affected by permeability of flexible packaging materials to oxygen, light, and moisture. Finally, the capacity of flexible packaging materials to impact food product temperatures can be a critical factor in all the aforementioned changes in food product quality.

The paper provides a brief overview of some significant characteristics for various food products and their relation to flexible food packaging materials by examining meat products, fruits and vegetables, dairy products, and snack foods. It also includes a section on the method of processing and packaging.

New Studies Toward Understanding Adhesion Of Polyolefins To Primed Substrates
by Bruce W. Foster, Mica Corp.

In extrusion lamination of roll stock for flexible packaging, good adhesion consists of two components—sufficient intimate contact between the substrate and extrudate and subsequent chemical bond formation between the two surfaces. Although some mechanical interlocking may occur when the substrate is porous or semi-porous such as paper or non-wovens, this mechanical bond is usually not sufficient to survive the down-stream handling and use of the structure. Chemical bonding in the form of covalent or ionic linkage can provide the chemistry to insure package integrity.

In some extrusion coating and laminating operations, primers provide the substrate surface modification necessary to achieve these strong chemical bonds. Once the film is successfully primed, extrusion variables will significantly affect the final bond quality. Some key extrusion variables are oxidation in the air-gap via heat and air-gap distance, heat via thickness and temperature, and choice of polymer. In this paper, the author limits polymer choice to a low-density extrusion coating grade with a melt index of about 8 and density of about 0.917.

This work measured the level of surface oxidation on polyethylene extrudate by the following method. The polyethylene was extrusion coated onto release paper slip-sheets. The polyethylene film was removed and the exposed surface was coated with a crosslinked PEI based primer using a #3 wire-wound rod. The primer was then cured for one to two minutes with a hair dryer. The excess primer was washed off under a faucet with room temperature water. The sample was patted dry and further dried with the hair dryer for one minute.

A 2% aqueous solution of Eosin B stain was coated over the sample with a #3 wire wound rod. After about 30 seconds, the stain was washed off under flowing room temperature water. The sample was again patted dry with a paper towel and dried with a hair dryer. The delta E was measured relative to the background (stained with eosin but no primer) using a colorimeter.

The experimental work in this paper demonstrates the utility of a new method for quantifying the stable surface oxidation on a polyethylene melt that is available for bonding to a primed film. The data also supports the idea that every set of process conditions has an optimum combination of melt temperature and air gap time to achieve good bonding. Further work is necessary to determine if die shear rate is a significant contributor to oxidation reactions.

The Effect Of Calcium Carbonate Concentrate Base Resin Melt Index On Linear Low Density Film Properties
by Amy B. Hitchcock, Eastman Chemical Company

APPLICATION: As packaging applications become more demanding and resin prices continue to increase, film extruders are searching for ways to improve film properties without increasing cost.
Calcium carbonate concentrate is often added by many film producers in applications where dart impact strength is an important property and film clarity is relatively unimportant. Typically, these applications include end products such as trash bags, pigmented merchandise bags, and heavy duty shipping sacks. The purpose of this study was to examine the effect of the melt index of the calcium carbonate base resin on film properties and to identify if possible the best base resin to achieve optimum film properties.

For applications where clarity and tensile strength are relatively unimportant, calcium carbonate concentrate can be added to films to improve dart impact strength. This can occur without significantly affecting tear and sealing properties or the stiffness of the film. The addition of calcium carbonate may be useful for markets such as can liners and pigmented merchandise bags. For film applications, the base resin used for the calcium carbonate concentrate (provided it is the same type base resin as the resin used as the primary component, i.e., LLDPE with LLDPE, LDPE with LDPE) does not appear to have a strong effect on film properties.

The negative effect of calcium carbonate addition regardless of the base resin can be observed in the film tensile strength and the optical properties. The tensile strength decreased 27% to 47% in the TD and 32% to 52% in the MD for films produced using 1.0 MI, 0.920 g/cc resin. For films produced with the 0.5 MI, 0.917 g/cc resin, tensile strength decreased 10% to 40% in the TD and 15% to 49% in the MD. For applications where tensile strength is a key property such as mulch and landscape supply bags, addition of calcium carbonate at any level is not recommended.

Selection Of Packaging Materials Based On Understanding Basic Concepts Related To Permeation
by Kay Cooksey and Duncan O. Darby, Clemson University

APPLICATION: Permeation, diffusion, solubility, and sorption are terms important to understanding fundamentals of package/product interactions. This paper covers the basics of permeation, concepts such as Fickian and NonFickian behavior, and how these concepts can determine shelf life.
The type of food, chemical composition, size, storage conditions, expected shelf life, moisture content, aroma/flavor, and appearance are only some characteristics that require consideration when selecting the proper material for a food product. A continuing trend in food packaging is the design of packages to extend the shelf life of foods while maintaining fresh-like quality. This places a high demand on selecting materials that not only provide the needed properties to maintain the quality of the food but also offer a cost effective price. The permeability of a packaging material is a very critical feature of a package for influencing the quality of a food product. Materials can be selected to provide a very long shelf life, but one must ask whether there is a need for the best barrier. Furthermore, does the extension of the shelf life justify the cost of the material and the quality of the food? Therefore, knowing the important factors for material selection based on permeability is an essential part of the package design process.

When determining what material works best for specific food applications, one must first understand the terminology used to describe materials. If prediction using equations is necessary, choose the equation that best fits the data provided and the information desired. Consider factors such as polymer characteristics and chemical interaction with characteristics of the permeant. Finally, determine the conditions (relative humidity and temperature) under which the material will be used. Taking all these known factors under consideration will not guarantee the best possible material for the application but will definitely provide for a more informed decision that could save time and money in the future.

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