Eco-friendly adhesives from vegetable oils

© Fraunhofer IMWS
Assessing the measured curve and determining tensile shear strength

Organic products are booming. Factory farms and fields drenched in chemical cocktails hold little appeal for the many consumers who prefer to see free-ranging animals in verdant pastures, eat untreated fruit and vegetables, and wear clothes made of eco-friendly, pesticide-free cotton. This need to go green is spilling over to other products. But simply replacing synthetic materials with wood, cork and the like alone is not the answer. For products to be genuinely sustainable, manufacturers must also use adhesives, paints and foamed plastics that are made of bio-based feedstocks.

Most legacy adhesives are made of petroleum-based thermosetting epoxy resins. These are synthetic resins that hold their shape once they have been heated. Monomers are the feedstock for epoxy resins. A curing agent or hardener causes the individual molecules to bind, creating a solid plastic that will not melt. Additives serve to fine-tune the properties to suit the given application. For example, epoxy resins can be made to take on a particular color, resist heat, sunlight and fire, and be easier to process. And added fillers cut the costs of these materials.

Vegetable oil epoxides with natural additives

Is there an eco-friendlier way of making epoxy resins? Adhesive manufacturers are certainly busy looking for more sustainable options. And they are aiming higher, for products that are better, cheaper and greener than their forebears. One new approach is showing promise – vegetable oil epoxides, an organic version of conventional epoxy resins. They are sourced from vegetable oils containing a high proportion of unsaturated fatty acids. These fatty acids are epoxidized; that is, their molecules are processed to create a three-membered ring consisting of two carbon atoms and one oxygen atom. Combining these vegetable oil epoxides with hardeners produces remarkably resilient plastics. And with that, sustainable adhesives, coatings and even foam resins are looking to be viable options.

There is a catch, though: The natural raw materials are extracted from vegetable seeds, so their chemical composition may vary greatly. And that variance is a problem for manufacturers.

Eco-friendly adhesives with enhanced properties

Researchers at the Fraunhofer Institute for Microstructures of Materials and Systems IMWS set out to tackle this problem by probing the properties of newly developed biogenic resins. This is a penetrating analysis: “We are investigating these resins from the micro to the macro level,” says Andreas Krombholz, group leader at IMWS. The first step is to determine how the varying feedstock affects the resins. Once the IMWS teams solve that puzzle, they can start enhancing and adapting resins to the given processing methods. Industry partners then usher these processes out of the lab and into the world, ramping them up for industrial-scale manufacturing.

Fraunhofer scientists are also using vegetable oil epoxides to develop novel adhesives. All formulations for these adhesives from the Fraunhofer labs are solvent-free. The researchers are digging deeper to learn what benefits this filler or that additive has to offer. High electrical conductivity, for example, comes in handy. If an adhesive layer is conductive, it can be heated from the inside out by applying an electrical voltage, thereby quickly and selectively curing it. Or an additive such as modified thyme oil could endow the adhesive with antibacterial properties.

Step by step toward greater sustainability

The scientists are also striving to make vegetable oil epoxides even more sustainable. The latest adhesives made of this material are 86 percent organic. A comparison best explains the significance of that figure: A material is deemed to be sustainable if it is 35 percent organic. So how did the researchers manage to put up those numbers? “To date, petrochemicals – that is, chemicals based on petroleum – have been used to epoxidize vegetable oils. Together with the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, we switched epoxidation over to enzymes for the first time. This means we can process vegetable oils without using petroleum-based chemicals. The treatment with enzymes takes place at 40 degrees Celsius rather than 100 degrees as in the past, so we are also conserving energy,” says Krombholz by way of explanation. The benefits do not end there. Industrial users in Europe had sourced the linseed oil for vegetable oil epoxides from Canada, which is not exactly eco-friendly considering the distance it has to travel. Fraunhofer scientists adapted the process to use Dragon's-head oil grown organically in Germany rather than linseed oil shipped in from overseas. This oil’s molecules have far more double bonds than those of linseed oil, so Dragon's-head oil not only has a greener environmental footprint. It also makes for a better epoxy. On top of that, the researchers found an eco-friendly replacement for the highly toxic hardener.

Foams made of sustainable materials

Vegetable oil-based epoxies’ sustainability-enhancing powers are not limited to adhesives. They can also serve to make greener foams such as those used in buildings’ insulating layers. But which resins work best for the various manufacturing processes? IMWS experts teamed up with industry partners to develop and fine-tune fast-curing, low-viscosity compositions of epoxidized linseed oils and hardeners. “The biggest challenge is to increase the biogenic share while improving the materials’ properties. Materials should also be easy to process,” says Krombholz. The Fraunhofer researchers’ latest foamed plastics have a biogenic by-weight share of 80 percent using lignin as the main additive, a by-product of cellulose production. Estimates put the annual production of lignin at around 20 billion metric tons.

Biological Transformation