Levan as an Adhesive: Summary
INTRODUCTION: Levan has strong adhesive properties. While the entangled branches contribute to cohesive strength, the large number of hydroxyl groups (hydroxyl number 89 mg KOH/gm) helps form adhesive bonds with a variety of substrates. Safe for users and the environment, levan is a "green" adhesive of particular value where a temporary bond is desired. Dried levan adhesive can be removed with water.
BENEFITS OF LEVAN-BASED ADHESIVES:
- Produced from renewable resource, sugar.
- No petroleum or natural gas derivatives in product.
- No VOCs, HAPs or other toxic emissions.
- Water-based with no solvents.
- No health issues for users.
- No dermal irritation.
- No allergic contact sensitization.
- Reduce regulatory burden.
- Long term storage as powder.
HOW TO USE LEVAN-BASED ADHESIVE: For application to aluminum, optimal viscosity is obtained around 20-25% solids. The adhesive may be diluted with water and low levels of a water miscible solvent may be added. On rough wood the low-viscosity levan may need a thickener included or as much as 30% solids. Cross-linked levan is effective on at least one plastic. Other substrates have not been tested. Curing is accomplished by drying. As water is removed and the solids rise above 45%, entanglement of branches radiating from the sphere surface contributes to the cohesive strength. After curing, the adhesive bond can be broken, moistened and re-joined.
ADHESIVE STRENGTH: It should be noted that all test results reported here were obtained using levan dissolved in water with no additives. It may be anticipated that formulated glues based on levan would have a superior performance.
The following table shows the tensile strength of levan compared with other natural polymers when used to bond bare aluminum coupons.
Tensile strength is average of 9 or 10 replicates using bare aluminum coupons.
Levan can be used as a wood adhesive where the wood often fails before the adhesive bond. Test methods and results are available in the paper from the International Journal of Adhesives and Adhesion found in the Reference section below.
On aluminum, levan has a tensile strength up to 10.3 MPa (1500 psi) depending on the surface finish and preparation method. The following graph shows the shear strength for various materials bonded with levan. Although the levan in water formulation did not adhere as strongly to plastics, it was noted that cross-linked levan formed a very strong bond on the smooth sides of the fiberglass composite, Garolite. Other plastics were not tested with the cross-linked levan (see water resistant paragraph below).
EXCEPTIONALLY LOW INTRINSIC VISCOSITY: Levan and inulin are the only polysaccharides forming spheres, 50-200 nm in diameter. The spheres offer little resistance to flow resulting in exceptionally low intrinsic viscosities. Indeed, the intrinsic viscosity of levan is only 0.14 dl/gm. By way of comparison, materials used as thickeners such as CMC may approach 100 dl/gm intrinsic viscosity. Levan can form a thin bond line. Good coverage was seen on aluminum at only 40 gm/m2. Unlike most polysaccharides, levan does not swell in water.
MICROBIAL GROWTH NOT ENCOURAGED: Unlike some natural adhesives, levan does not encourage growth of fungi and bacteria. Cultures of Aspergillus sp., Cladosporium sp., Brevibacillus sp., Staphylococcus epidermidis, and Micrococcus sp. were grown on levan coated agar plates and on treated construction materials. Organism growth was not visibly different from that of most untreated controls. It should be noted that levan did not hinder the growth of existing bacteria and fungi.
RESISTANCE TO SOLVENTS: Levan is very resistant to the effects of organic solvents such as jet fuel, methylethylketone and d-limonene.
WATER RESISTANT VERSION: Levan is water soluble. This is useful in some applications, but turning it into a water resistant form is essential for other applications. This is not a straightforward problem. The dilemma is that the same hydroxyl moieties responsible for adhesive properties are also responsible for water susceptibility. The difficulty is to make levan water resistant while retaining the adhesive properties.
Levan may be made water insoluble by tying up a portion of the available
hydroxyl groups. The most successful approach found to date has involved
cross-linking. One method uses an emulsifiable diphenylmethane diisocyanate
(MDI) such as one sold by Huntsman called Rubinate. A hexamethylene
diisocyanate (HDI) will also cross-link levan but there is considerable
foaming and the open time is very short. Another avenue is to derivatize
levan for increased water resistance.