Can you imagine a life without polymers? It is not for nothing that polymers are, by a fair distance, the most widely used basic materials known to mankind. They possess such an unbeatable combination of functional properties, strength-to-weight ratios, convenience and costs that they have become the work-horse materials that drive and support the entire economy.
Since 1983, when they overtook metals as mankind’s most widely used basic materials, their usage has literally exploded and the yawning gap between them and other materials has become a gigantic chasm that can never be bridged. The fact that their specific gravities are so much lower than other materials has delivered the lowest resource usage and highest sustainability that we can ever imagine. (1 square meter of iron or steel will weigh 7.5 time a square meter of plastic; 1 square meters of aluminum will weigh 2.71 times a square meter of plastic of the same thickness.)
They are also endlessly recyclable but even a fraction of this is not achieved on the ground due to overall apathy, human habits like littering and the abject failure of municipal bodies and civic authorities to set up and efficiently operate systems for collection, segregation and disposal or recycling of post-consumer waste and to undertake mass consumer education. The easy way out seems to be deeming these as extended producer responsibility rather than formulating a pro-active and responsibility-delivering action plan by the powers that be. Admittedly, industry also needs to step up and address recycling of mixed plastics and printed and metallized waste on a war footing and developing technologies for doing this.
Be that as it may, my intention here is to highlight some of the unique properties that polymers bring to the table to help deliver packaging solutions that no other materials can effectively provide. Innumerable Life Cycle Analysis (LCA) studies on packaging have been carried out and every one of them arrives at one or more of the following conclusions:
- substituting polymers with other non-polymer materials results in a quantum increase in usage of basic materials/resources,
- the usage of energy in the process is much higher,
- damage to the environment goes up substantially and carbon footprints are adversely affected,
- costs go up by unaffordable proportions,
- weight of packages and resultant transportation costs (during production and procurement of basic materials, in-plant storage/movement and distribution across the supply chain, waste collection and disposal) sky-rocket, and
- in many cases, substitution is not even possible.
So, let us look at some of these unique properties and discuss some case-studies which clearly establish the utility of polymers. These properties are all individually analyzed below.
This is a unique polymer/plastic property. Heat-sealability and the ability to produce leak-proof, hermetic high-strength heatseals and cost-effective high-speed machine operation are not provided by any other material except, to some extent, wax. Even here, the wax seals are usually too weak to be used commercially and the sealing formulation has to be buttressed by the addition of polymeric materials like EVA (ethylene-vinyl acetate) to get strong and leak-proof seals.
Needless to say, hermetic seals are absolutely imperative for providing adequate shelf-life (prevention/reduction of permeation of moisture/gases both into the package and egress from its contents), leak-proofing, tamper evidence and convenient opening of the package. The vast majority of flexible packaging could not be used without heat-sealability. What is even more useful is that polymers enable the seal strength and openability to be precisely tailored (low/high strength seals, partially or fully peelable seals, directional and easy to tear seals). Even effective hot-melt seals are provided only by polymers.
Let us look at some specific examples.
One of the most exacting and difficult packaging specifications to produce is the aluminum foil used to package small cubes or portions of processed cheese. The requirement is that the cheese has to be filled hot and sealed without the application of external heat through heatseal jaws. The pack consists of two components – a body which encloses and holds the cheese and a lid which goes on top of the cheese and is sealed on its edges to the body to provide hermetic seals and tamper-proofing. The seal has to be effected using only the heat of the molten cheese that is filled into the formed body.
This is made possible by using a plasticized vinyl coating on the aluminum foil that seals at a low temperature. The risk is that such a low-temperature heat-seal coating could have blocking problems in the foil rolls during transportation where ambient temperatures could go up even beyond 50 degrees celsius; blocked rolls will not unwind on the wrapping machine. The trick is to formulate the polymer coating to get both heat-seal and not block at transportation temperatures. Only a polymer can satisfy both these criteria. Like-wise, in the packaging of cheese slices, the wrap has to provide both heat-sealability and peelability of the seal as well as release from the cheese, which only a polymer film can provide.
Technically, a metal can will provide both these properties but the can cost for a 10 gm-20 gm pack would far exceed even the product cost apart from not providing the convenience of easy opening.
As can also be seen, heat-sealable flexible packs are absolutely necessary for small unit-dose or single-use packages as other non-polymer rigid or semi-rigid systems like metal cans, cartons, tubes or bottles would be prohibitively expensive. Packs with polymer heat-seals are cost effective for the full range of products (solids, powders, liquids, pastes, agglomerates/granules and mixed formulations).
In our next installment, we will look at more special and unique properties of polymers for packaging applications.