Effect of caustic soda concentration on the green bond strength of Stein-Hall type corrugating starch adhesives and rheological properties of their gelatinized pastes

This study investigated the rheological properties of Stein-Hall type corrugating adhesives (SH-adhesives) composed of carrier and main portions, which were needed for enhancing initial bond (green bond) strength to make the corrugated fiberboard at high speed on the double backer. Starch slurries were individually prepared from corn, high-amylose maize (hylon-5), waxy maize and wheat starch. Main portions of SH-adhesives were individually prepared from corn or wheat starch. All carrier portions were prepared from cooked corn starch paste only. Initial shear rigidity, maximum shear stress, shear stress-strain pattern and viscosity of these gelatinized pastes from slurries and SH-adhesives were measured by the Couett type viscometer at 85degC. In the concentration of caustic soda up to 0.9%, the rheological properties of 4 species pastes from slurries were organized into 3 types. As for type-1, the waxy pastes always behaved in the low rigidity less than 0.4 kPa and the large strain at maximum stress as 9.6-10.8, except no borax addition. As for type-2, hylon-5 pastes behaved in the high rigidity and the small strain at maximum stress under 0.7. On the other hand, as for type-3, the rigidity of corn and wheat pastes remarkably decreased to 0.3 kPa from 6.8 kPa, and the strain at maximum stress increased to 3.8 from 0.3 with increasing caustic soda concentration. In the same way, gelatinized pastes from the SH-adhesives composing the main portion of suspended corn or wheat reduced their rigidity and increased their strain at maximum stress with increasing caustic concentration. However, the green bond strength of adhesion samples laminated by table test with those corn starch SH-adhesives showed the strongest point at some caustic concentration. By SEM, glue lines of those adhesion samples delaminated in the measurement of the green bond strength were observed, and it was shown that clear shape swelling starch granules reduced with caustic increase, and the gelatinization in that strongest point was proceeded to swelling the starch granule until unclear shape. These results suggested that the balance of fluidity and rigidity behavior as the rheological properties of gelatinized SH-adhesives pastes was required to enhance the green bond strength on the corrugator.