Byline: Liz White
How so you make polyurethane parts that are more renewable? This is a question that many PU businesses would like an asnswer to, and Sonderhoff Chemicals, based in Cologne, Germany, has been busy devising one route.
Sonderhoff claims its new approach, using natural fibre and polyurethane (PU/NF), gives a part with 67 percent ‘green’ content, of which 40 percent is flax fibre, and the rest is polyols derived from natural oils (NOPs).
Volker Welsch, who is head of development at Sonderhoff Chemicals, said natural fibres give low mechanical strength compared to glass fibre-reinforced thermoplastic polyamides or epoxy-fibreglass composites, but their additional advantage lies in their lightness.
Natural fibres have comparatively low density, which for flax fibres is about 1.45g/cm³or 37 percent lighter than fibreglass (2.3g/cm³).
Natural fibres leave no sharp edges when they break, unlike fibreglass and carbon fibres, which is important in vehicle crash performance, Welsch said, in a presentation at the 6-7 Oct 2011 meeting of the FSK, the German association for makers of foamed plastic and polyurethane, held in Wolfsburg, Germany.
In addition, energy costs for making natural fibres are significantly lower than for fibreglass. Since CO2 is bound in the fibres, this leads to a much better CO2-balance with natural fibres.
Sonderhoff worked with Reimer Modellbau as a process specialist with many years experience in processing natural fibres together with thermosetting materials.
Welsch said a drawback of natural fibres is that the quality changes as the weather alters, so they are not as consistent a product as synthetic fibres.
This has a direct impact on mechanical stability, which is correspondingly lower than for synthetic fibres.
Also, natural fibres can absorb a high amount of water, which may make upstream drying of the fibres necessary, and also requires resin systems which are tolerant to residual moisture, Welsch noted.
When used in thermoplastics, the low thermal stability of natural fibres is a problem, according to the Sonderhoff representative. Thermoset epoxy and polyurethane systems can, however, chemically bond with hydroxy-functional cellulose with no need for additives.
For this work, Sonderhoff chose flat fibrous flax to give the best mechanical properties. Flax is widely produced in Europe, with the corresponding fabric also available, said Welsch.
Natural fibres in auto use already
Reinforcement using natural fibres for automotive parts is already established in series production — in parcel shelves, interior door panels and spare wheel covers, he said. These are mostly compression-moulded PP with natural fibre felts, but Welsch said some PU parts containing natural-fibres are also used.
Sonderhoff aimed to develop a polyurethane system giving comparable processability, with final properties close to those of an epoxy resin-infusion system, and using a polyol containing a very high proportion of renewable raw materials.
The result is a low viscosity, hydrophobic system with high mechanical strength.
Low viscosity was important for quick and rapid penetration of the natural fibre fabric. Such high impregnating is a prerequisite to achieve good mechanical values of the fibre bundles, Welsch noted. In addition, the system is resistant to hydrolysis and UV resistant. MDI (methylene diphenyl diisocyanate) was used.
Important properties are:
- Mix viscosity 150 mPas. at 23°C
- Tensile strength: 42 MPa
- Elongation at break: 6%
- Hardness of 78 Shore D
Welsch compared the properties of PU/NF in with those of epoxy resin + natural fibre-fabric (EP/NF) and epoxy resin + fibreglass fabric (EP/GF), using flax fibre fabrics, at fibre levels of 40 percent.
The results show that PU/NF against EP/NF has benefits and the distance against EP/GF has shrunk considerably.
Sonderhoff expects that use of a suitable sizing will further improve the mechanical properties of PU/NF, Welsch said.
The final aim is for the PU/NF materials to replace glass-fibre-reinforced epoxy compounds in various applications.
Foamed window profiles
Energeto windows with foamed polyurethane insulation in the profiles are the “best in class,” according to Dr Gerhard Schuhmann, head of product development and R&D at window maker Aluplast GmbH.
As in the automotive sector, energy savings and reducing CO2 emissions in construction are strong aims of the German federal government in recent years, so for the construction industry, particularly for window components, good insulation is vital, Schuhmann indicated.
In a typical building, the windows form only 8 percent of the building envelope but still represent 40 percent of the energy loss, said Schuhmann.
In 2010, Germany built 12.6 million window units (21 million m²). Of these, 7.2 million were plastic windows.
The focus on renovation is “stronger than ever, with a lot of old houses getting new windows,” requiring small lot sizes in very different styles, said Schuhmann. New windows, in contrast, have to be wide with huge surfaces. This increases the need for thermal insulation and also “means we have a weight problem,” since some models, using triple glass, weigh 150 kg, which can be hard to handle, Schuhmann added.
Plastic window frames traditionally have steel elements as the core, with a PVC cover, which, “in energetic terms is catastrophic for energy loss,” he said.
In 2004, Aluplast developed a novel adhesive system to use between the glass and profile, which eliminates the heat bridging found with steel.
Then, in 2008, the Karlsruhe, Germany-based company took the steel out of the frame and replaced it with reinforced nylon with high glass-fibre content, to give the necessary strength.
A special feature is a piece of rubber that holds glass into frame and helps bonding. The company has also automated putting the glass into frame and added adhesive robots.
The latest development is to insulate the hollow plastic profiles of the frames with PU foam.
”This is a process we developed on our own, and with Cannon also, Schuhmann said. Aluplast can supply both machines and systems for foaming; “Users can buy the entire package from our company.”
A specially selected foam type is injected at high pressure into the profile chambers, in an amount is carefully calculated in advance for the specific profile dimensions and volume.
Instead of foaming window profiles in advance, and then installing the foamed profiles separately, Aluplast’s system allows all kinds of chambers to be filled, on the site of the window maker itself.
Schuhmann pointed out that it is highly important that “we always foam them completely,” so users must know the exact dimensions.
The final effect is a lot better U values, so that when it is -5°C outside, it can be 20°C inside, with U values dropping from 1.2 to 0.89 W/m2K — a 30-percent improvement in insulation, Schuhmann said
Home owners will get no condensation with these windows, Schuhmann stressed.
Aluplast’s “foam-inside” product is coming onto the market in Germany, France, Slovenia and Belgium.
In Germany the issue is U values while in other countries it is important to build houses which need no added heating, Schuhmann noted.
France still allows U values of 1.4 W/m2K, for new houses which is “far away from German standards,” he commented.
Asked why Aluplast does not foam the profiles at its plant, Schuhmann said this is partly because not all frames are foamed.
Some companies make 5000 units a week and foam only about 500 of them, he said.
