Hemicellulose adhesive

The hemicellulose adhesives were evaluated on 50- by 100-mm2 A-flute corrugated board samples. Commercial single-faced board (125 to 160 g/m2) bonded with a maize starch adhesive and liner (160 g/m2) was obtained. The liner and fluting material were prepared from virgin kraft pulp and were not surface-sized with starch. The single-faced material was cut into samples (50 x 100 m2) with the flutes in the 50-mm direction. Liner samples were cut in 60- by 110-m2 samples. 

Dissolved hemicellulose adhesive. Finely ground hemicellulose was heated with borax (3.8% on solids) and sodium hydroxide (1.3% on solids) to 70 °C for 15 minutes. The solution was cooled and used as the cooked hemicellulose adhesive. 

Partially dissolved/suspended hemicellulose adhesive. One-sixth of the hemicellulose used for this preparation was heated at 70 °C for 15 min with sodium hydroxide (1.3% on total solids) and half the water required for the whole mixture. After being cooled to ambient temperature, the remainder of the water, hemicellulose, and borax (3.8% on solids) was added. In some cases (Table I), the borax and sodium hydroxide contents were varied. 

Starch control. The starch control was prepared by the same procedure used to prepare the partially dissolved hemicellulose adhesive. 

The effect of borax on the strength of the hemicellulose adhesive was subsequently investigated. A factorial experiment was done employing five concentrations of borax and five concentrations of sodium hydroxide. The sodium hydroxide variable was included, since formation of the diol-boron complex is pH sensitive 12). 

The results listed in Table I indicate that the ply adhesion of the hemicellulose adhesive increased with increasing amounts of borax and sodium hydroxide. The increase in strength, however, leveled off after an optimum was reached. Optimum strength results were obtained with 3.8% borax and 1.3% NaOH on hemicellulose solids. These quantities of additives were subsequently used in all further experiments. 

The hemicellulose adhesive used consisted of one-sixth of the hemicellulose in the dissolved (cooked) state and the remainder in a suspended (raw) state. This preparation was subsequently compared with a hemicellulose adhesive with all the powder dissolved (70 °C treatment) and with a preparation with all the hemicelullose powder suspended (raw). The same amounts of borax and NaOH were used in each case. A starch adhesive prepared according to the Steinhall procedure 13) was used as control. 

The set time of each adhesive was also determined. According to the method employed, these values can only be used for comparative purposes. The set times of all the adhesives were between 4 and 5 seconds under the conditions used. This indicates that little difference exists in the set time of the hemicellulose adhesives and the starch control. The only exception is the longer set times... 

The ply adhesion values of test samples glued with the suspended hemicellulose adhesive leveled off at consistencies above 16% solids. The strength of this adhesive above 16% solids was similar to the strength values of industrial corrugated board samples. The industrial board was collected from the mill that supplied the single-faced and liner material. 

The ply adhesion of the dissolved hemicellulose adhesive was high at low solids content but decreased somewhat at higher solids contents (Figure 3). The set time of this adhesive was, however, long at low consistencies. 

The strength of the suspended/dissolved hemicellulose adhesive increased when the solids content was increased and leveled off at 13% solids (Figure 3). The raw-cooked hemicellulose adhesive thus reached the plateau of maximum strength at a lower solids content than the raw hemicellulose adhesive. 

The viscosities of the three hemicellulose adhesives and the starch control are given in Figure 4. The viscosity of the suspended hemicellulose adhesive was lower when compared at different solids contents than that of the starch control. It was, on the other hand, much higher than the viscosity of a raw starch suspension (30 cP at 35% solids). The higher viscosity of the raw hemicellulose adhesive is probably caused by the gel-like outer layer of the raw hemicellulose-suspended particles. The suspended starch particles showed no such surface gel effect, but remained totally undissolved. The viscosities of the cooked and raw-cooked hemicellulose adhesive were similar to those of the starch adhesive (Figure 4). 

The stability of the hemicellulose adiiesives and the effect of particle size were subsequently assessed. Hemicellulose adhesives were prepared with a portion (one-sixth) cooked and the remainder raw with borax and sodium hydroxide as before. Different particle sizes of hemicellulose were used for the suspended portion. The mixtures were used to glue corrugated board test samples after 4 hours or 4 days standing. The results (also on citrus box material) shown in Table III clearly indicate that the ply adhesion strength of the adhesives was not affected by standing or by particle size. 

XA one-sixth portion of the hemicellulose adhesive was dissolved (cooked) while the remainder was used as a suspended powder (raw) of different particle sizes. 

The studies on the ply adhesion of the hemicellulose adhesive showed that the addition of 2 to 3% of borax on hemicellulose led to substantial improvements in the strength of the adhesive. A hemicellulose solution was subsequently prepared containing 2% borax (on hemicellulose) at pH 8.5. The solution was... 

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