Packaging applications advantages

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

This type of method offers several advantages over the pressure differential methods discussed previously. With little or no pressure differential there is less problem from leaks, no difficulty in supporting the test piece, and the situation is more like that in many packaging applications. Greater sensitivity is possible enabling very low permeability materials to be tested conveniently and the different transmission rates of the components of gas or vapour mixtures can be measured. The principal disadvantage is that the apparatus is relatively expensive. 

The alternative to using a dish is to form the material into a bag and this so-called pouch or sachet method is often used for plastics films. The advantages are that a larger surface area is exposed, leaks through the wax seal are eliminated, and the conditions are more similar to packaging applications. It is less attractive for rubbers because they are not often used in that sort of packaging application and an alternative to heat sealing the pouch would be necessary. 

Packaging applications are the most extensive. Meat, poultry, and egg containers arc thermoformed from extruded foamed polystyrene sheet. The fast-food market also accounts for a substantial amount of polystyrene for takeout containers where the insulation value of a foamed container is an advantage. Containers, tubs, and trays formed from extruded impact polystyrene sheets are used for packaging a large variety of food. Biaxially oriented polystyrene film is thermoformed into blister packs, meat trays, container lids, and cookie, candy, pastiy, and other food packages where clarity is required. 

While PLA was invented about 150 years ago, its moisture sensitivity and relatively high cost meant it did not have any packaging applications. Most uses of PLA were for medical applications where cost is not a harrier, and its biocompatibility is extremely important. When PET degrades, it produces lactic acid, which is also produced naturally in the human body (associated with muscle fatigue, for example). The advantages of something that can be implanted in the human body and then will slowly degrade and disappear over time are obvious. 

In the case of Solvent-based adhesives, the polymer or blend of polymers that will eventually form the bond is dissolved in a carrier solvent. Sometimes, as in many materials for packaging applications, the solvent is water, but since the bonds thus produced will be moisture-susceptible, the use of organic solvents is more usual. An added advantage of the use of organic solvents over water is that their lower latent heat of evaporation ensures faster drying. Against this, snch solvents are more expensive than water, are usually inflammable and are sometimes irritant or toxic. Growing concern about... 

This material has excellent barrier properties and is frequently used in food packaging applications. Films made from PVDC have good cling properties, which is an advantage for food wraps. Commercial polymers are all copolymers of vinylidene chloride with vinyl chloride, acrylates, or nitriles. Copolymerization of vinylidene chloride with other monomers reduces the melting point to allow easier processing. Corrosion-resistant materials should be considered for use when processing PVDC. 

Flexible printed wiring is used primarily in electronic packaging applications where low weight or volume is a prerequisite, or excessive vibration is likely to be encountered. The main advantage of flexible circuitry is its ability to be shaped into more than one plane or conform to an irregular shaped package. 

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