Paradigm shift in the use of adhesives

With the activation of a new CLT production, Martinsons of Bygdsiljum decided to use a different adhesive. In the former production, which was started in 2003, a MUF system had been used. With the start of the new production line, the former one has been shut down. Today, Henkel’s Loctite Purbond adhesive is used instead. Jon Martinson, responsible for technical matters, and product manager Daniel Wilded explain the reasons for this change of system.

Many years of experience with MUF adhesives

When it comes to the production of cross-laminated timber, Scandinavia is a somewhat untapped region. Martinsons, however, is a pioneer in this field. The company has been producing CLT in central Sweden since 2003 and is thus an expert in the sector. At the time, the annual production was about 6,000 m³ of mainly small-sized CLT elements. In 2017, the company produced some 20,000 m³ thanks to the new production line which has been in operation since the end of 2016.

A standardised product

The new production line has brought about a change not only in sizes (up to 3 metres in breadth and 16 metres in length) but also in adhesive technology. By Scandinavian standards, the latter one represents a unique selling proposition. Martinsons, however, did not want to be in an isolated position and thus decided to use a one-component polyurethane adhesive (PUR) for its CLT production.

“When you look at the international CLT markets, almost all renowned producers use PUR systems in their production”, explains Wilded with regard to the company’s motives. Tough this seems to have been a rather “uncreative” decision, there were factors which led to it. “To this day, there are no harmonised product standards for CLT in Europe like the ones that exist for glulam. This can pose some problems in practice. Every company produces its own products with various lamination thicknesses and adhesives, which then leads to different product characteristics. When CLT is to be used for the construction of a building, one would have to do separate calculations for the CLT of every single company. The market for MUF-glued cross-laminated timber is smaller than the one for PUR-glued CLT. Also, the end product has different characteristics, which makes it necessary to do separate calculations for buildings. This is why we decided to standardise our product as much as possible. It gives us access to a bigger market.”

One-component system

This was not the only reason for changing the system, though. “Henkel’s Loctite Purbond is a one-component system and from a transformation point of view, this an invaluable advantage for us”, explains Jon Martinson, son of the CEO.

This system makes the application of the adhesive much easier. Depending on whether one produces three- or five-layer panels, adhesives with different wait times are required. With no curing agent needed, the company does not have to order and store this component, which in turn optimises production processes.

Less maintenance

“The PUR adhesive is easy to handle, it cures at room temperature and hardly pollutes the machines. With our old CLT-line, the elements were brought in an energy-intensive high-frequency press for curing. When we used a MUF adhesive, we had to clean the glue applicators at least once during every shift. Thanks to the PUR system, we have to clean them only once every six months. Additionally, the minimal cleaning effort has reduced glue waste”, explains Martinson.

No emissions

Another advantage, according to Martinson and Wilded, is the lack of formaldehyde emissions. “It is better to use a formaldehyde-free adhesive. The emissions are still lower compared to those of one cubic metre of pine wood, but this topic is a rather emotional one and it is hard to change people’s minds”, says Wilded.

“The situation is similar when it comes to fire performance. The concerns regarding PUR systems’ fire performance are unfounded. Here, you can easily make adaptations in calculations by raising the charring rate. We know that there are discussions about these aspects, but we don’t consider them a big problem”, he adds.

A higher charring rate to increase safety

“When the fire reaches the adhesive joint, the high temperatures can destroy it. There is a possibility that the isolating charred layer of wood, which forms in the process, might come off, thereby increasing the charring. This is a relatively rare phenomenon that can be taken into account by calculating a higher charring rate”, explains Christian Lehringer, Head of Engineered Wood Europe at Henkel, and adds: “Scientific studies done by the ETH Zurich have shown that in case of fire, the dimensions of cross-laminated timber are not essential for the usual situations, independently of the certified adhesive system used.”

The decision for PUR

The first meeting with Henkel’s Engineered Wood Team took place in the summer of 2015. “At the beginning of the discussion, we had to decide whether we would continue to use a MUF adhesive or opt for a PUR-system. This fundamental decision had an influence on the entire planning of our new CLT production. Eventually, we opted for polyurethane. Henkel had worked for a number of customers before and being the biggest supplier in this area, we found the competent partner we were looking for”, explains Wilded.

Martinsons is satisfied with its choice. After signing the contract, Henkel continued to support the company during the start-up phase in December 2016. Pieter Janssen, project manager of Engineered Wood in Scandinavia, and one of Henkel’s technicians were on site on a regular basis and made the necessary adaptations to processes. The system works flawlessly ever since.

A high demand for CLT in the future

Wilded expects a positive order situation in the future. Especially, when it comes to multi-storey residential buildings, he expects a considerable growth despite the need of further efforts to convince the public of the advantages of CLT. The fact that one of Martinsons’ competitors will start producing CLT in Sweden in the near future does not worry Wilded, on the contrary. The higher the level of awareness and acceptance of CLT structures, the more can sales go up, and the market is big enough for various manufacturers.

“We would be very satisfied if only 5% of all orders for multi-storey wooden buildings were placed at our company”, comments Wilded. Martinsons wants to focus on projects which use glulam and CLT, since the company can rely on their in-house expertise.