Starch is a natural polymer found in many processes either as an adhesive or a thickener. Following paper production, corrugated board is the second largest application of non-food starches globally, where it it used as an adhesive between the fluting and liners. The control of the adhesive viscosity during process and storage is critical. However, despite further developments regarding the formulation of starch adhesives, the viscosity is commonly not stable enough over extended periods of time, in particular over weekend storage. In this first of a series of blog posts with the corrugated boards application as the example, I will give an introduction to this problem, and the new technology of Exilva, a microfibrillated cellulose, to solve it.
Starch adhesives are either prepared cold from pre-manufactured starch adhesive powder or cooked together with caustic soda and borax/boron to make an opaque glue. This adhesive is applied to the tips of the fluted paper, which is accordingly pressed to another flat paper, called the liner. During drying under heat the starch adhesive gelatinizes, creating a strong bonding of the corrugated cardboard.
Starch can be differentiated in two types of glucose polymers; the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight. The amylose fractions are never truly soluble in water and in time they will form crystalline aggregates by hydrogen bonding - a process known as retrogradation, or setback. Retrogradation is the cause of viscosity instability in starch adhesives. Amylopectin is more soluble and less prone to retrogradation.
Like starch, cellulose is a polymer of glucose. However, the way the glucose monomers are connected is different, which makes cellulose much stronger, non-water soluble and non-digestible. This is the basis for the formation of Exilva microfibrillated cellulose, which builds a strong three-dimensional networks of cellulose fibrils in polar solvent systems, stabilized by hydrogen bonds. Possessing a highly entangled network and a large surface area with the similar functional groups as starch; if added to the starch adhesive, will the interaction between the cellulose fibrils and starch be strong enough to stabilize the amylose chains, preventing them to realign themselves during storage?
Upon addition of Exilva to the starch adhesive, rheology measurements show an increase in the storage modulus of the adhesive. Thus, being directly related to a higher extent of interactions, the Exilva technology should be able to trap starch polymers and granules within a stronger structure.
Earlier on the Exilva blog, we have demonstrated the alkaline stability of MFC, even at extremely high pH conditions. Manufactured at harsh process conditions (pH 11-12), may the Exilva technology be the one solution to improve the viscosity stability of the starch adhesives for corrugating industry?
In my next post, I will look into some results from corrugated board factory trials with Exilva in the starch adhesive.
Meanwhile, if you would like to talk to us about Exilva in starch based adhesives, give us a note here: