Film & Foil
The importance of monomaterial polypropylene laminates in flexible packaging
By Stephen Langstaff, business development manager, packaging at Innovia.
L
ast years COP26 summit reminded us all the importance of reducing our carbon footprint. Food production is a major contributor to global warming and resource efficient packaging plays a vital role in reducing the carbon impact by reducing waste and extending shelf life.
Although the carbon footprint of packaging is important, it is usually comparatively small when compared to the product it is wrapping. This is one of the big advantages of flexible packaging. Due to its light weight, it is far more resource efficient than cans, bottles, tubs, or trays. However, in most countries flexible packaging is not collected and the infrastructure to recycle is restricted. This results in non-optimal end of life such as landfill, incineration, or waste to energy. The misconception that flexible packaging cannot be recycled is leading some to poor environmental choices, often leading to increases in food waste and packaging with higher Global Warming Potential than flexible alternatives.
Fortunately, things are changing. There are regular announcements in the press regarding
flexible packaging recycling investments and private initiatives to collect flexible packaging (e.g. UK supermarkets). As Governments start to mandate kerbside collections of flexible packaging this will accelerate the industry further.
Examination of the front of store collection of post-consumer flexible packaging in the UK, shows that around 70% of the material is either Polyethylene (PE) or Polypropylene (PP) and that 20% is mixed plastic and aluminium foil plastic laminates. Currently the PE fraction is downcycled to low grade PE film for use as garbage bags or supermarket carrier bags. The PP fraction tends to go into rigid uses, much of it outside the packaging industry. The laminate fraction generally goes to waste for energy. Technology exists to sort, wash and deink the PE and PP fractions to allow them to be “closed loop” recycled i.e. so they can converted back into high grade film applications.
Currently no large-scale efficient technology exists to recycle the mixed plastic laminates. The current approach is to try and remove them and replace with all PE or all PP structures which form most of the existing waste stream. Mixed polyolefin waste (PE and PP) has a market value, but if the objective is closed loop recycling back into high grade film, individual streams would be advantageous for film production. For many primary packaging applications, the high stiffness and transparency of PP has advantages over PE. As PP offers the option to go thinner and to run more efficiently on packaging machines. Both of which results in a lower carbon footprint.
Most food packaging laminates are formed using the best properties from two or more different materials to achieve the optimum pack performance. A common laminate is a biaxially oriented polyester (BOPET) laminated to a low-density polyethylene (LDPE). The BOPET used on the outside provides pack stiffness and gloss and the inner layer of LDPE
offers strong seals and hermeticity. A more sophisticated laminate is used in some pouch applications where barrier to moisture and oxygen is accomplished using an aluminium foil with a reverse printed PET on the outer face and a Cast PP used as the sealant web. Neither of these packs can be recycled back into film. Both give an excellent pack, which serves to protect the contents successfully. In both cases it would be extremely difficult to get an effective pack with only one film. To replace these laminate types, film manufacturers are developing new films with enhanced properties specifically for pouch applications. For example, the production of a more thermally stable biaxially oriented polypropylene (BOPP) to replace the BOPET. These products provide lower levels of shrinkage than traditional BOPPs and in some cases offer a non-sealing top skin to prevent jaw stick during packing.
A top web made from BOPP will never have the same temperature resistance as a PET film, therefore Cast PP seal webs need to be designed with a lower seal initiation temperature (SIT) to prevent serious distortion of the BOPP top web. Cast PP films are now available with SITs around 90°C. Options to replace the established PET/LDPE laminate mentioned above, are now available. The choice of barrier to replace the aluminium foil is highly dependant on the end application but several PP alternatives already exist. High barrier metallised films or AlOx and SiOx deposition layers can be used although the later is expensive.
Of growing significance is the use of BOPP/ EVOH coextrusions where newer products on the market show reduced humidity sensitivity which look likely to open new ground in retort packaging. Providing the content of EVOH/ METAL/SiOx/AlOx represents less than 5% of the overall pack structure then full recyclability will be maintained.
Xwww.innoviatech.com
18
April 2022
www.convertermag.com
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