A TMS 30001 species of cassava was used for the study. It was obtained from the International Institute of Tropical Agriculture, Ibadan. Also Sodium Tetraborate (Borax) [Viscosity enhancer], Sodium Hydroxide, and Hydrochloric Acid [Gelatinization Modifiers] were used.

Two different sets of experiments were carried out to determine the effects of varying concentrations of reactants and process parameters on the quality of the adhesive produced.

Experiment 1

In experiment 1, a response surface (central composite) design with 2 factors was used to relate the effects of processing parameters, namely the temperature of heating mantle, and Concentration of Borax on the quality of the adhesive produced for the levels and factors used. A face-centered design (in which α = 1) was used, thereby producing 3 levels. The number of replicates made was two.

Experiment 2

In experiment 2, a response surface (central composite) design with 2 factors was used to relate the effects of processing parameters, namely Concentration of NaOH and Concentration of Borax on the quality of the adhesive produced for the level and factors used. A face-centered design (in which α = 1) was also used, thereby producing 3 levels as in the previous experiment. Also, two replicates were made.

The cassava roots were peeled and washed, after which they were grated into a pulp. The cassava starch was then washed out of the mix of cassava pulp and clean water, passing through a fine sieve. The starch slurry was then allowed to settle, and the fibre and clean water were decanted. The starch particles recovered were then sun dried on an elevated platform, after which they were milled into fine powder using a hammer mill.

Production of adhesive using HCl as a gelatinization modifier: 6 g of dry cassava starch was dissolved in 100 ml of 0.01 M HCl and stirred while it was being heated on a hot plate. The mixture (i.e. of cassava starch and HCl) was heated to varying temperatures ranging from 65˚C - 85˚C, after which Borax was added at varying mass (concentrations) ranging from 8% - 20% mass of Borax/mass of Starch while stirring until it became sticky. The product (adhesive) was then allowed to cool and qualitative analysis was carried out on it.

Production of adhesive using NaOH as a gelatinization modifier: 6 g of dry cassava starch was dissolved in 100 ml of varying concentrations of NaOH (ranging from 0.01 - 0.05 M) and stirred while it was being heated to a known temperature (which is the optimum temperature 85˚C gotten from experiment 1) using a hot plate. Varying concentrations of Borax were then added to the mixture (i.e. of cassava starch and NaOH) at varying mass (ranging from 8% - 20% mass of Borax/mass of starch), until the mixture was sticky. The product (adhesive) was then allowed to cool and qualitative analysis was carried out on it.

To determine the optimum process conditions for the production of the adhesive, firstly, the process was carried out at a constant concentration of gelatinization modifier (HCl) while varying mass % borax/starch and temperature so as to determine the optimum temperature for the production (i.e. using Minitab software), after which using the determined optimum temperature, the concentration of the second gelatinization modifier (NaOH) was varied alongside the % mass borax/starch so as to determine the best gelatinization modifier. The quality parameters, namely: density, bond strength, viscosity, and pH of the best quality adhesive were discussed.

The physico-chemical properties of the adhesives were determined using the standardized procedures. The viscosity of the starch adhesive was determined using an NDJ-5S digital rotary viscometer. The density was measured using the ASTM method D1875, which involves using a density cup. The pH was determined using a pH meter (Jenco model no. 6173). While the bond strength was determined using a Thwing-Albert QC-3A Universal testing machine according to the ASTM D 1876, commonly called the T-peel test.

The effects of varying temperature and Concentration of viscosity enhancer (Borax) on the viscosity, density, pH and bond strength of the adhesives are shown in Figures 1-4 respectively (Table 1).

Viscosity

As observed in Figure 1: Interaction plot of Viscosity (Factors: Temperature, Mass % borax/starch), the viscosity of a starch based liquid increases with temperature increase until the starch attains gelatinization temperature, after which a decline is observed [5] . This increase in viscosity is due to the swelling of the starch granules as the reaction temperature is increased [6] . This trend is observed in which as temperature is increased, the viscosity of the adhesive also increases.

Meanwhile, since borax is a viscosity enhancer, addition of more of it to the adhesive would increase its viscosity. Borax forms complexes with starch molecules which are negatively charged, and they further crosslink to form higher molecular weight starch complexes causing a large increase in viscosity [7] . This trend is observed in which as the concentration of borax/starch is increased, the viscosity of the adhesive also increases. A similar result was observed in the work of Jackson [3] .

Density

As shown in Figure 2: Interaction plot of Density (Factors: Temperature, Mass % borax/starch), the density of the adhesive decreases with temperature because of the swelling of the starch granules, which cause an increase in the adhesive volume. However, when gelatinization temperature is reached and the starch granules swell to their maximum capacity and disintegrate [8] , the excess moisture which is yet to be absorbed is evaporated, resulting in a drop in volume, and a corresponding increase in density. This explains the decrease in/constant density of adhesive between 65˚C and 75˚C and the increase at 85˚C.

Normally, borax has the ability to increase the density of the adhesive. Borax forms complexes with starch molecules which are negatively charged, and they further crosslink to form higher molecular weight starch complexes [7] . So as the concentration of borax is increased the molecular weight of the starch complexes also increases, causing an increase in density. This trend was observed at all temperatures.

pH

The pH of the starch adhesive would increase towards alkalinity as noticed in Figure 3: Interaction plot of pH (Factors: Temperature, Mass % borax/starch) because as the temperature is increased the unreacted gelatinization modifier is gradually evaporated, leaving behind the dissolved starch and borax which are more basic. This results in a pH which tends towards basicity with temperature increase.

Basically, borax is a salt with a pH of about 9. It has a neutralizing effect on the starch adhesive which is made from an acidic gelatinization modifier. This is why the pH of the starch adhesive tended towards basicity with an increase in the concentration of the viscosity enhancer (borax) from 8% - 20%.

Bond Strength

The bond strength of the starch adhesive starts increasing just before reaching gelatinization temperature as seen in Figure 4: Interaction plot of Bond strength (Factors: Temperature, Mass % borax/starch) because of the gelatinization of the starch, after which there is a decline as a result of a completion of gelatinization [9] . This trend is followed in which there is an increase in bond strength with temperature.

As borax is increased, there is an increase in viscosity, which in turn leads to an increase in bond strength. Borax has an effect of increasing tackiness [10] . Therefore, an increase in the concentration of Borax would in effect cause an increase in the bond strength of the starch adhesive. This trend was observed in the samples prepared at 65˚C and partially in the samples prepared at 85˚C. The samples prepared at 75˚C however were problematic. The fluctuations observed at 75˚C and 85˚C might be due to errors during testing.

From the first set of experiments, the data obtained as responses (i.e. viscosity, density, pH, drying time, bond strength) were used to optimize the factors (i.e. temperature, and mass % borax/starch) used to obtain these responses using a statistical tool (Minitab). The optimized factors were then used to carry out the second set of experiments. As shown in the diagram Figure 5: Optimization Plot, the optimized factors are a temperature of 85˚C and 20 mass % borax/starch.

The effect of varying the concentration of the gelatinization modifier (NaOH) and mass % borax/starch on the response viscosity, density, pH, bond strength are shown in Figures 6-9 respectively (Table 2).

Viscosity

The viscosity of the starch adhesive increases with increasing concentration of NaOH as observed in Figure 6: Interaction plot of Viscosity (Factors: Concentration of NaOH, Mass % borax/starch). This is due to the fact that increasing the concentration of NaOH of the starch adhesive causes swelling which in turn causes an increment in viscosity [11] . This trend is observed between the 0.01 and 0.05 M NaOH solutions. Though, this wasn’t in 0.03 M solution probably due to errors with the 0.03 M solutions.

Density

The density of the starch adhesive was observed to decrease with increases in the concentration of NaOH as seen in Figure 7: Interaction plot of Density (Factors: Concentration of NaOH, Mass % borax/starch). This is due to the swelling effect of NaOH [11] on the adhesive which causes its volume to increase with an increment of the concentration of NaOH. Since density is mass per unit volume, and the mass remains constant while the volume is increasing, which explains the decrease in density with an increase in the concentration of NaOH.

pH

The pH of the starch adhesive increases towards basicity as the concentration of NaOH is increased as shown in Figure 8: Interaction plot of pH (Factors: Concentration of NaOH, Mass % borax/starch) since pH is the negative logarithm of H⁺ concentration [12] , and the higher the concentration of OH⁻ the lower the concentration of H⁺ (since as OH⁻ concentration increases, H⁺

concentration decreases making the value of the –Log greater). This is explained by the fact that the higher the concentration of a base in a solution, the higher it causes the pH of the solution it reacts with to be. The pH tended towards a value of 10 on the pH scale.

Bond Strength

The bond strength of the starch adhesive increased with higher concentrations of NaOH as observed in Figure 9: Interaction plot of Bond strength (Factors: Concentration of NaOH, Mass % borax/starch). This is attributable to the demonstrated direct correlation between NAOH concentration and adhesive tackiness [10] . Tackiness is the ability of an adhesive to glue two or more surfaces together, and the stronger they are held together, the higher the bond strength. Therefore, a higher tack means higher bond strength. This explains why an increase in the NaOH concentration of a starch adhesive increases the bond strength of the adhesive. This trend is observed in the experiments carried out on bond strength.

Preferred Choice of Starch-based Adhesive

Based on the data from the first set of experiments, using HCl of 0.01 M, the optimum process conditions are a temperature of 85˚C and 20% mass of borax/starch. The value of the various properties of the adhesives produced such as viscosity, density, pH and bond strength are 73.44, 1.066, 4.55, and 22.31 respectively. However, for the second set of experiments, varying NaOH concentration and mass % borax/starch, the optimum process condition using 0.01 M NaOH to produce the adhesive at a temperature of 85˚C was 8% mass borax/starch. The values of the various properties of the adhesives produced such as viscosity, density, pH and bond strength were 48.61, 1.052, 8.28, and 11.60 respectively.

Comparing the adhesives produced using HCl and NaOH, it was established that the adhesive produced with HCl was of a higher quality as characterized by a higher viscosity value, superior density, a desirable pH value between 4.0 and 7.5, and a much higher bond strength (which is one of the most important properties of adhesives).