Laura Gascho
Expert in flexible packaging
Gerhard Schubert GmbH

When it comes to sustainability, there is rarely a simple either/or choice: from energy generation to packaging materials, most solutions call for thoughtful consideration. Their advantages, after all, vary depending on the context. The same applies to paper and plastic. Both can be suitable as recyclable packaging materials, yet each one influences processing and packaging styles in its own way.

Paper and plastic are as different as night and day – and they differ not only in their composition and physical properties. The materials also polarise public perception: whereas paper, a fibre-based and therefore seemingly natural packaging material, is often regarded as ‘green’, synthetically produced plastic has long been associated with the image of a potentially harmful substance – for both the environment and human health.

Thanks to advances in material development, the reality is that certain types of paper and plastic can be used reliably and sustainably. Mono-plastics made from a single type of polymer are suitable as efficient packaging for a wide range of products and subsequently for recycling. During recycling, they do not form countless fragments that would first have to be laboriously separated and processed. They have nothing in common with the composite films that are not recyclable under current conditions, and which will disappear from the packaging mix as part of the EU Packaging Regulation, which becomes legally binding from August 2026. Paper-based films are also suitable for the circular economy, provided that laminated or dispersed barrier coatings comply with certain limit values – and are therefore not an obstacle to recycling.

But being recyclable does not mean that the two materials can be used indiscriminately. On the contrary, they are fundamentally different: anyone wishing to use them strategically and in an environmentally sound manner needs to consider not only the applicable regulations but also products, packaging styles and processes, so that each material can be used to its full potential.

All photos: Gerhard Schubert GmbH

The barrier benefits of mono-polymers

A single type of plastic instead of many: mono-films are increasingly making a name for themselves. And for good reason, as they can achieve what was long considered impossible: returning plastic to the cycle almost entirely. As flow-wrapped or stand-up pouches, mono-materials now protect a remarkable range of sensitive consumer goods – from chocolate bars to coffee or a portion of peanuts. This is achieved through the clever combination of carrier materials such as PP or PET and small amounts of barrier substances such as ethylene-vinyl alcohol copolymer (EVOH). For example, mono-polymers reliably keep water vapour, oxygen and UV rays from penetrating. They are easy to process and, thanks to their high flexibility, are suitable for a variety of packaging styles. 

It is also true that mono-films require more gentle sealing processes than composite plastics. As a rule, thermoplastics seal well under heat. Under the influence of heat, the structure of their sealing layers changes. However, tight, uniform seal seams will only form if the sealing unit applies neither too much nor too little heat to the films. At excessively high temperatures, the plastic melts through or burns; if the heat input is too low, the molecules of the sealing layers do not bond together. The seal therefore remains ineffective, preventing the packaging from properly protecting its contents. This results in waste, which manufacturers should strive to avoid for environmental reasons alone.

And this is precisely where the challenge lies with mono-films: as all polymer layers have the same melting point, they must be heated as gently as possible. Ultrasonic sealing has proven effective for this purpose, especially with PP and PET. Mono-PE proves to be more challenging, as it absorbs mechanical vibrations more strongly, which can cause the energy transfer to become too intense and melt through the material. Especially with ultrasonic sealing, however, this transfer must be successful: the process generates localised heat through the vibration of a so-called sonotrode. This vibrating sealing tool comes into contact with the packaging material only briefly, but it creates a tight seal in a very short time and without the use of high temperatures. 

The tools used only get warm to the touch – and require significantly less energy than heat-sealing systems. Although the latter remain an option, they require more rigorous control of film speed and temperature to reduce the risks mentioned. However, especially on pick & place lines, the line’s feed speed – and with it the sealing speed – varies considerably, making heat sealing extremely challenging. Ultrasonic technology is a more suitable option for a simple, lower-risk sealing process for mono-plastics.

Due to their high barrier properties, mono-materials such as PP or PET are suitable even for sensitive products such as coffee or chocolate.

Fibrous material with integrated protection

It is similar when sealing paper-based film, even though the material is generally suitable for other packaging purposes. Paper does have a barrier layer, which can be co-extruded, laminated or applied as a dispersion. However, it is only suitable to a limited extent for highly sensitive products such as coffee – as the barrier function varies depending on the layers used. Laminated solutions consist of separately manufactured films made of plastic, or less commonly of bio-based polymers, which are subsequently laminated onto the paper. There are also co-extruded layers, which are applied directly to the base layer during production and are not subsequently bonded or coated. In both cases, this layer can have very good barrier properties, but it often makes the material more difficult to recycle.

Dispersed barriers, on the other hand, are applied directly to the paper as a liquid solution. This allows for the production of more recyclable and thinner coatings, although these generally do not offer the same barrier performance as laminated or coated solutions. Paper-based primary packaging with a dispersed barrier is often unsuitable for highly sensitive, fatty or moist foods; in high humidity, for example, the performance of paper-based packaging materials declines. These paper films come into their own with dry foods, technical items or as outer packaging for multipacks – where plastic films will become less attractive from a regulatory perspective from 2030 onwards. 

As with mono-films, manufacturers need to ensure gentle sealing processes when using paper. Paper has an insulating effect, so it generally requires longer sealing times than mono-plastic to activate the sealing medium. This also applies to high-barrier papers for highly sensitive products: compared to papers with medium or low barrier properties, these variants contain significantly more plastic in the coating. It is necessary to heat this coating thoroughly to create airtight seal seams. The longer the sealing time, the greater the risk of performance losses in the line and irreversible damage to the packaging material – especially when paper is to be heat-sealed.  

Ultrasonic sealing is more material-friendly and faster, as vibrations and localised heat input achieve optimal sealing results even with fibre-based packaging materials. The technology produces tight seal seams, even with varying film speeds, such as in the pick & place process.  

Paper-based films protect dry products such as biscuits from oxygen.

Implications for the process itself

Beyond the immediate sealing process, paper presents further specific challenges for which manufacturers will need to adapt their packaging processes. Due to paper’s insulating properties, longer sealing times are required, which in turn affect the performance of a packaging line. In direct comparison with flowpacks made from mono-plastics, fewer units per minute can be produced from paper. Paper is also stiffer and less malleable than plastic; as a result, flowpacks formed from paper can be correspondingly larger. Their dimensions, in turn, influence the dimensions of secondary and tertiary packaging. However, special forming shoulders often allow dimensions comparable to those of flowpacks made from mono-plastics to be achieved. With this in mind, consumer goods manufacturers switching to paper should thoroughly assess all packaging formats and adapt their designs at an early stage. 

Anyone who has ever folded a sheet of paper also knows how easily the material creases. It is no different with paper films: the film feed within the machines can cause crease breaks, which impair the aesthetics and impact of the packaging on the retail shelf. To avoid this, paper-processing systems require special elements adapted to the forming behaviour of the paper, for example on the forming shoulders. And because paper is abrasive, components of the packaging line that come into contact with the product need to be coated with a special finish to prevent premature wear. Additional drives on the film rolls can also help guide the paper more gently through the line, thereby preventing tears before sealing.

Unlike composite plastics (below), mono-films (above) consist of a single type of plastic such as polypropylene (PP) or polyethylene terephthalate (PET).

As the examples above illustrate, successful sustainable packaging depends on more than simply the right packaging material. Only when materials, sealing technologies and other elements of packaging lines complement one another effectively can reliable, recyclable packaging be achieved – to meet the needs of products, the environment and current regulations.