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A Guide to Shrink Wrap Materials |
This article will address shrink wrap materials as they relate to the packaging of individual rather than collation packs.It is valid to consider that shrink wrapping for transit and shrink wrapping of large loads fall into separate categories. As a consequence, these would need to be addressed in their own right. The wrapping of individual packs falls under the category of display packaging.
It is realistic to dispose of polyethylene relatively early on in this consideration since this material is not particularly suited to display. Its failures lie in a lack of strength that can only be compensated by appropriate thickness, poor clarity inherent in its chemical composition and a very soft surface which offers little or no sparkle or gloss, which at the same time scuffs and attracts dust. As a consequence, polyethylene is an unlikely candidate for display packaging although it would be wrong to assume that it is never used. From time to time, there are applications where the above disadvantages are not particularly significant in the context of the end use and as a consequence, it can be used. However, for the greatest part, display packaging involves the use of either PVC or polyolefin shrink film.
PVC - plasticised polyvinylchloride - has seen declining volumes over recent years as polyolefin films have been able to overcome machineability issues that make them, as a consequence, more likely candidates for use. PVC films are, without question, the easiest to seal and shrink in production but they continue to suffer from a number of issues that may be regarded by any end user as having greater or less significance. On sealing, PVC films degrade and small quantities of hydrogen chloride - HCL - are given off; these combine with any moisture to produce hydrochloric acid. This causes degradation of equipment and also acts as an irritant to any operator. Although there is no toxicity issue as such, there is an issue with regard to providing satisfactory working conditions and appropriate ventilation together with operator training and screening is necessary. Carbon deposits are also created on sealing and these require regular cleaning. Seal and shrink results are otherwise excellent although PVC seals are weak by comparison with those of polyolefin.
As PVC is plasticised, so it is necessary to store this film both before and after use in such a way as to avoid extremes of temperature. Although exceptionally high temperature is not particularly relevant to a wrapped pack, it is particularly relevant to PVC film on the reel before use. The material must be held at a temperature around 20 degrees centigrade or alternatively it is likely to suffer shrink damage, which will render it unusable.
In respect of extreme cold, the reverse applies. PVC film on the reel will not suffer from exposure to cold since provided the material is returned to room temperature it will be usable. However, wrapped packs will be prone to damage if they become particularly cold as the plasticiser will harden and the film will tend to shatter.
Last but not least, PVC has suffered from constant criticism by virtue of its chlorine atom, which is seen as environmentally unfriendly. Historically, there have also been other concerns raised with regard to vinyl chloride monomer and the type of plasticiser and stabiliser used although none of these have particular currency at this time.
PVC shrink film continues to occupy some of the market for display and continues to offer excellent machineability, which should not be ignored. It is generally not used in food shrink applications. It finds specialised application on over wrap equipment where a more rigid form is used - the wrapped pack can be shrunk to offer a particularly high quality finish. The plasticiser content of standard PVC shrink film can be varied to allow the production of slightly softer or slightly harder material. The softness imparts better strength whilst hardness provides a more rigid feel to the finished pack, which can be preferred on a given application.
Polyolefin films have continued to be developed to offer the best attributes of PVC - the ease of sealing and shrinking - without its demerits - the issues noted above with regard to use and storage. It must be noted that polyolefin shrink films also produce fumes on sealing and it is no less relevant to deal with these through appropriate ventilation. Polyolefin films now offer easy sealing such that specific grades will seal as readily as PVC. The shrink window with polyolefin is not quite as flat as with PVC and hence, equipment generally needs to be of quite a good standard to offer a good result. Nevertheless, polyolefin will offer stronger seals by comparison with PVC and good quality shrink down which is generally close to that offered by PVC - the latter will continue to offer better shrink results. However, there are no real issues with pre or post production storage. Overall, polyolefin represents a better mix of the best attributes and it is as a consequence of this that its market share has grown.
Environmental concerns are yet again another issue. Polyolefin materials are produced only from oil-based feed stocks - they use only non- renewable resources. In production or post-production, material cannot be recycled save into a lower form of plastic. In comparison, PVC’s principal constituent is common salt available from seawater whilst any scrap can be readily recycled back into PVC film. There are numerous formulations of polyolefin shrink film designed to offer attributes appropriate to the end use.
Films are invariably offered as multi-layer complexes. These layers comprise polyethylene and polypropylene in various forms. At the premium end of the market, irradiation is used to alter the molecular structure of polyethylene films and this gives the material particularly strong seal and puncture and tear resistance characteristics. However, irradiation militates against easy sealing and a consequence, due allowance must be given to this.
Polyolefin films are also formulated to offer low shrink force whilst they have also been extended into food use where both anti fog and barrier grades are available. Further enhancements include printing as well as pre-perforation which offers valuable benefits in manufacture. Advice should always be sought so that any end user ensures that the correct selection of material is made with attributes chosen appropriately at every point as well as costs being minimised. | | Author Info: About the author: Richard Jankel - http://www.kempner.co.uk - Shrink wrap expert at Kempner, the UK’s largest distributor of display shrink wrap film machinery and materials. If you plan to reproduce this article, please include the above link. |
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