Use of MDI (methyl diphenyl diisocyanate) binders in OSB production can be traced back to 1985 when the first OSB mill converted to using this product, in North America.

By the year 2000, according to Huntsman Polyurethanes which is one of the main producers of MDI, around 30 OSB mills globally had switched over to using this product in at least some part of their panel. Seven of those mills use MDI in both face and core layers, while the rest use it in the core layer only, often using a conventional phenol formaldehyde (PF) or melamine urea phenol formaldehyde (MUPF) resin in the face layers.

Being derived from oil in a complex process with a high entry cost which does not lend itself to small-scale production, there are only a few suppliers of MDI worldwide.

The main reason in the past for not utilising MDI in the face of OSB is that it is a very effective adhesive – and not just for wood. Without proper precautions, mills can find the faces of their OSB panel stuck as firmly to the press platens/belts as they are to the core layer. However, there are new ways which solve that ‘sticky’ problem completely, which we will come to later.

Another reason can be the perceived higher cost of MDI versus PF or MUPF resins – another subject we will return to.

Meanwhile, the benefits which Huntsman claims for its ‘Suprasec’ and ‘Rubinate’ MDI binders are:

* Better panel moisture resistance in the field

* Increased mill productivity levels

* Increased wood species variety and tolerance

* Potential increase in the panels’ physical property performance

* Increased ability to manufacture speciality products

* The creation of better mill operating parameters and savings in drying, blending and pressing

* Decreased blender cleaning

* More cost-effective binding

Following the initial success of MDI for composite wood products in the US, in the early 1990s chemical giant ICI set up its composite wood products division, within its polyurethanes division, in Europe.

That polyurethanes division, together with ICI’s titanium dioxide business, was bought by Huntsman, a family-owned chemical business headquartered in Houston, Texas, US, in 1999; Huntsman promptly doubled in size ‘overnight’.

The administrative and research and development (R&D) parts of Huntsman occupy the former ICI premises in Everberg on the outskirts of Brussels. This is also the home of Huntsman’s dedicated wood team in Europe and where small-scale trials can be carried out for customers, to address their specific needs.

Simon Baker is the commercial director of the composite wood products division.

“Our MDI binders are primarily used in the OSB sector and this represents a significant proportion of our business in panels, although it can also be used in MDF, particleboard and lightweight wood fibre insulation products,” he said.

“The real advantage of MDI is the ability to create a strong bond with wood – it forms a bond with the lignin in the wood and achieves a good distribution on fibres, due partly to its low viscosity, to give a homogenous product,” explained Mr Baker. “It also allows shorter press times than other resins and works at lower temperatures.”

Huntsman works with the major panel-making machinery manufacturers in the development of new products.

“We are not just here to supply the resins. For instance we developed a special non-standard resin for the new Siempelkamp insulation board line and that is a good example of what we are trying to do,” continued Mr Baker. “We believe the wood sector has a very bright future. It is a sustainable construction method and offers good carbon capture to help combat global warming. We want to help the industry to grow and to support that growth through the development of new products.

“The wood industry generally needs to push its positive message – it is a great material. The Wood Panel Industry Federation in the UK and the European Panel Federation are doing their best for the sector, but we all need to do more.”

Examples of panel products other than OSB where MDI has been used are to be found in specialist grades of MDF and particleboard, such as for exterior use, in so-called zero formaldehyde boards and in fire retardant panels. However, these are never likely to be big volume markets – that area is likely to be dominated by OSB for the foreseeable future.

Huntsman Polyurethanes does however supply polyurethane glues for I-beam manufacture in the US as an alternative to resorcinol and is interested to see how this business develops in Europe.

An advantage for MDI in such applications is that it is colourless, where glue lines can be very visible with other resins such as PF.

“We account for less than 1% of glue used in the wood industry,” admits Mr Baker, who also recognises that the price of MDI is higher than competitor resins. “It is if you just compare the price [per litre], but in terms of cost of use, you have lower dosage, shorter press times. You must truly understand the economics – we are not going for a commodity market but the higher specification market.”

So what are the often-cited downsides to MDI and Huntsman’s answers to those criticisms? I asked Mr Baker.

First, that it sticks to press platens/belts.

“Historically, some mills applied sacrificial paper faces to the panels which were sanded off after pressing, but for some time we have offered a liquid agent which we developed and which is spray-applied to the platens/belts before pressing. This has had a big uptake by users of continuous presses,” he said.

Secondly that MDI is a ‘dangerous’ chemical to humans, potentially causing respiratory problems and sensitisation.

“I will answer that in two ways: Firstly, MDI, like any product, needs to be treated in the right way and following the advice we give, then there is no reason why there should be any risk; secondly, We take our environmental health and safety very seriously and there is a team of industrial hygiene specialists who go out to customers to ensure they understand how to manage not only the resins, but also dust and other by-products of panel production. They discuss storage of MDI, handling and blending.”

The third ‘accusation’ is that lack of tack is a problem with MDI as the mat does not hold together well before the press.

“Lack of tack can be a problem on some of the older multi-daylight particleboard presses which we are addressing through R&D. It can be an issue, though not really on continuous lines, and we are close to finding a solution for those older lines as well.

“MDI has some very real advantages, particularly in niche and demanding applications,” said Mr Baker.

There are of course ways to achieve resins which do not contribute to formaldehyde emissions other than using MDI, such as the use of formaldehyde catchers and so-called E0 resins, but the disadvantages of E0 resins can be loss of production speed and relatively high resin loadings. A combination of these resins with special grades of MDI in a hybrid system can apparently be a solution to compensate for production and strength loss.

Another group – the bioresins – also offer a formaldehyde-free bonding system.

“It is interesting to see these other [bio] resins out there but the question is can they be taken from laboratory scale to industrial scale?” asked Mr Baker.

There are other panel products which need bonding, but do not use wood. Agricultural fibre-based boards are notoriously difficult to stick together due to wax in the case of wheat straw, for instance.

“We have had a lot of success in improving the technology and have made panels from a wide variety of materials, and have done a lot of work with Compak [a specialist in straw-based board manufacture],” said Mr Baker.

Huntsman has three global production centres for its MDI: Rotterdam for the European market; Louisiana for the US, Canadian and Mexican markets; and Shanghai for Asia.