Starch is extensively used as an ingredient in adhesives for paper and paperboard, particularly in the manufacturing of corrugated board. Dextrins, which are obtained from potato or corn starch, are utilized as adhesives for laminated items and as gums for envelope construction and sealing.
Lamination of Paper
Paper can be bonded to itself or other materials for packaging and labeling purposes. Glue made of starch or modified starch is often used for this purpose. Low-quality paper can be laminated with a fine paper for printing. Label papers are coated on one side for printing and treated with an adhesive on the other. Glue is applied using roller systems and good contact is achieved with pressure. Theories about adhesion have been proposed, but good bonding depends on wetting of the substrate by the adhesive and the establishment of intermolecular bonds through a boundary layer. Improved bonding occurs with acid/base interaction due to the presence of polar groups. The strength of the glue bond is measured by the peeling force required for separation.
The Corrugator for Paperboard
The process of creating corrugated board involves combining two unbleached Kraft products – corrugating medium and linerboard. The medium is the fluted or corrugated portion, and the liner is the surface sheet onto which the fluted tips are bonded.
The corrugating process involves softening the medium by heat and steam, fluting it between two corrugating cylinders, applying adhesive to the tips of the flutes, and bonding it to the preheated liner by pressure and heat. The equipment is operated at high temperatures using high-pressure steam.
The corrugating adhesive is made up of corn starch, which contains a partially gelatinized carrier starch and ungelatinized raw starch. Carrier starch is prepared in the primary mixer by suspension in water, addition of sodium hydroxide, and heating, while raw starch and borax are suspended in the secondary mixer and heated.
The carrier portion of the adhesive provides the viscosity needed to hold the raw starch in suspension and to retain water during application.
Borax reacts with raw starch when it gelatinizes on the glue line, providing high viscosity and aiding in bond formation.
After starch application to the flutes, it must be gelatinized/pasted and dehydrated to form a minimum green-bond strength, which is necessary to resist delamination during subsequent mechanical operations.
The bond between medium and liner is formed in situ by heat and pressure.
Besides the conventional corrugating adhesive, various modifications are in use.
Starch Selection for Use in Corrugation and Lamination
Starch is commonly used in corrugated board production. Specialty starches are available for faster production or improved board quality. Modified starches such as high-amylose or waxy maize starch can improve bond strength or water retention. Wheat starch is used when cost is a factor. Water-resistant starch formulations are required for refrigerated boxes or outside storage. Starch-based hot melts are used in packaging applications. Adhesive viscosity is measured using a brass cup viscometer.