How X-ray helps build better milk cartons

Modern society requires innumerable packaging solutions and they must be cheap, but when we take a carton of milk or juice from the fridge few of us stop to consider that we are holding a state-of-the-art sandwich material in our hands. In addition to the actual cardboard, the packaging consists of thin layers of polymers and aluminium foil, ensuring watertight and odour-sealing properties.

A typical packaging comprises five to seven layers of material and each state-of-the-art package takes less than one tenth of a second to manufacture — a single machine in the Tetra Pak production hall can spit out 40,000 milk cartons per hour. This extremely fast production process poses a challenge to developers.

Christel Andersson, development engineer at Tetra Pak’s materials development office, explains, “This is only possible thanks to the stringent control of materials and understanding the changes they undergo during processing. X-ray technology is a natural tool for us. We constantly try to reduce material consumption in the individual packaging both for financial reasons and to reduce our environmental footprint and the consumption of resources.”

The key to achieving the best possible result quickly and with the least material resources is to be found in the right combination of materials, design, processing and pretreatment. It requires in-depth, highly detailed knowledge.

“Polymer layers have the same thickness as human hair, so we need extremely high resolution to get the results we want,” said Eskil Andreasson, technology specialist at Tetra Pak Packaging Solutions.

The company uses electron microscopy — SEM (scanning electron microscopy) — for these detailed studies.

“While we will continue to use SEM, the one drawback of the technology is that we need to cut samples of the material. This means that there will always be a small margin of doubt as to whether the cutting sample has changed structure and other characteristics. Conversely, the advantage of X-ray radiation is that you can see the characteristics of the material from the outside. Also, you don’t change the sample, enabling you to subsequently perform measurements on the same sample.”

Tetra Pak has three X-ray equipment units. The company has also recently partnered with DTU and several other Danish research institutions and enterprises* in the LINX project — Linking Industry to Neutrons and X-rays.

“We’re pleased with our own X-ray measurements, but they are limited to taking snapshots. Reality isn’t static, but dynamic,” explained Andreasson. “In order to ‘film’ the changes in the structure in the high resolution we need, you have to have greater X-ray sources. We need DTU to help us make this transition.”

An added bonus of LINX is the cooperation with the Danish companies involved in the project.

“Even though a company such as Velux operates with significantly larger dimensions when manufacturing its windows, some of the basic issues regarding composite material processing remain the same,” said Andreasson. “You wouldn’t think that Tetra Pak and Novo Nordisk share common challenges, but in fact Novo Nordisk uses much of the same simulation technology as we do. In this way, the LINX project also acts as a ‘hub’ for experience exchange. There’s a lot of new X-ray tomography hardware and software currently available. In fact, I think it’s an advantage that the companies are so different. We aren’t competitors, which makes it safe to share knowledge.

“X-ray tomography is relevant to all our development work — packaging design, pretreatment, processing and the choice of closing mechanisms. We have only seen the tip of the iceberg in terms of what this the technology has to offer.”

Among its research partners, DTU Compute and DTU Physics are of particular interest to Tetra Pak.

“One thing is setting up the experiments so you get your results, but it’s also important to limit yourself so you don’t end up processing all too large volumes of data. This is where DTU can really help us,” said Andreasson.

“DTU researchers are also skilled at imaging,” added Andersson. “As an experienced user you can get something out of the raw data, but it’s a basic human need to see with your own eyes what is happening in the material.

“The vision is that one day we will reach the point where we no longer only verify the effects of innovative material solutions, but actually turn things upside down so that we use visualisations on the basis of X-ray tomography to design the materials at the molecular level,” said Andersson. “This is an area undergoing rapid development — so-called molecular dynamics.”

Initially, Tetra Pak has chosen to limit its involvement in material studies to the LINX project, but clearly it will make sense to include other areas later, concluded Eskil Andreasson.

*In addition to DTU, the University of Copenhagen and Aarhus University are also participating in the project. The participating regions are the Capital Region of Denmark and the Central Denmark Region. The companies are: Biomodics, CO-RO, CPH Inventures, Exruptive, Frichs Ecotech, Grundfos, LM Wind Power, Novo Nordisk, Novozymes, Rockwool, TEGnology, Tetra Pak (Sweden), Velux, Xnovo Technology, and Aalborg Portland.