Photo: Rockwool, 3D Imaging Center

X-ray peek at curly stone fibres

Wednesday 06 Jun 18
by Morten Andersen


Carsten Gundlach
Senior Research Officer
DTU Physics
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Imaging technology is helping Rockwool ensure that its products meet the exact same specifications at all their factories.

The basic function of the Rockwool Group’s main product seems simple enough. Tiny stone fibres are woven together so that the wool contains pockets of air, giving the product its insulating properties. Yet inspiration drawn from modern brain research has helped the company understand its product even better.

“It’s well known that the orientation of the fibres in composite materials used in wind turbine blades has a major impact on tensile strength and a number of other properties. The same is true in stone wool, but things are actually much more complicated. You can give the stone fibres an overall orientation, but they also curl in quite chaotic ways,” explains Dorthe Lybye, Programme Manager at Rockwool.

A further challenge is that the company’s factories are different.

“There can sometimes even be differences between two production lines. First of all, we need to ensure that the relevant specifications are met by all the production lines. Secondly, we sometimes see that a particular production line has exceptionally good results. And then we want to know why, so we can transfer the same effect to the other production lines,” says Dorthe Lybye.

Much more than insulation

Insulation capacity is not the only material property that is important.

Rockwool markets a number of specialized products in addition to their well-known building insulation product. Examples include special products for industrial use, sound-insulating stone wool, high-strength products for floors, and highly wind-resistant products for exterior walls.

In fact, it is now possible to build a house’s exterior walling using only stone wool—the company has construction projects underway in the Netherlands and Denmark. There is also a special portfolio of products targeting commercial greenhouses. Horticulturists can control the supply of water and nutrients more precisely in stone wool than in potting soil.

In other words, there are plenty of properties to monitor. Rockwool therefore decided to join the LINX collaboration project (Linking Industry to Neutrons and X-rays) with a number of other companies. DTU is one of the academic partners.

“The timing was good, because DTU was just about to build a centre for imaging,” says Dorthe Lybye.

As a first step, Lucie Chapelle (MSc Eng) was employed as an industrial PhD student at Rockwool, in collaboration with DTU, where she had previously been part of a research project on complex materials.

Proximity to DTU an advantage

"The imaging staff at DTU believed they could essentially transfer the same methods that are used for wind turbine blades. But our products have proven to be more unique. It’s not too much to say that the project has been an academic challenge for them"
Dorthe Lybye, Rockwool

X-ray tomography was chosen as the investigation method.

This method involves irradiating an object from various angles. There will be differences in how the radiation is absorbed at different angles. An algorithm exploits these differences to construct a 3D image of the object. The image can be digitally split into a series of adjacent cross-sections.

X-ray tomography is a standard method used in healthcare. The method has only recently begun to be used in industrial materials studies.

“X-ray tomography is the right tool for us,” explains Lucie Chapelle. “We need to look at the fibres in very fine detail. However, if we chose the ultimate level of detail that you get from electron microscope imaging, we couldn’t be sure that the images would actually be representative of our product. In other words, we need both high resolution and the ability to study relatively large samples—preferably a few centimetres in length. X-ray tomography gives us the best possible compromise.”

However, the company has not found it necessary to purchase its own imaging equipment. The samples are taken to DTU in Lyngby instead. Often by Lucie Chapelle.

“I’m glad that it’s not too far away and that I can go along. I have built up extensive practical experience with the studies by now. The DTU researchers are experts in recording and analysing the images, but I’m an expert in the product. I’m often able to suggest an adjustment in the settings that leads to better results,” says Lucie Chapelle.

Entering the digital age

This kind of expert knowledge has proven more valuable anyone could have predicted.

“The imaging staff at DTU believed they could essentially transfer the same methods that are used for wind turbine blades. But our products have proven to be more unique. It’s not too much to say that the project has been an academic challenge for them,” says Dorthe Lybye.

One of the challenges has been that the fibres have different lengths, and that they weave in and out of the various cross-sections. The number of times each fibre adheres to other fibres also has a major impact on the material properties.

“We have lacked a tool to be able to follow the fibres precisely. In cooperation with DTU, we have managed to find the right method. In fact, we found that inspiration had to be drawn from brain research, where very complex samples also have to be investigated. We expect to have a software package ready later this year,” says Lucie Chapelle.

Supports sustainability

The importance of understanding materials is great. Rockwool has already undergone a series of conversions that make production more sustainable. First of all, quarried stone is no longer used exclusively as the raw material, but is supplemented by various types of building waste. The company is also working towards using more renewable energy in production.

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