PVA (containing 86.5% to 89% degree of hydrolysis) were obtained from Kuraray Cooperative Limited, India. Maize starch containing 25% - 30% of amylose content was obtained from Sanstar Ltd. Glyoxal (∼40% content in water) was purchased from Sigma-Aldrich. Deionized water was used as the solvent in all experiments. Maize starch was dried to remove moisture.

First, Maize starch and PVA blends were mixed in water and poured into a kettle. Second, the kettle was closed and was sealed to make it air tight. Third, the seal tight kettle was heated to 60˚C temperature while mixing the sample at 175 rpm. The cross-linker was later added as per the amount given in Table 1. Subsequently, the temperature was raised to 90˚C - 95˚C keeping the constant stirring 2 hrs. After completion of cross-linking the solution was brought to room temperature for further analysis.

A bar applicator was used for casting films of 1000 µm. The films were then kept

at room temperature for curing for 24 hrs.

Formulation 1:

For formulation 1, the PVA/S blend was kept constant at 25/5 with composition of glyoxal varied from 0 to 1.5 (Table 1).

Formulation 2:

For formulation 1, the PVA/S blend was kept constant at 20/10 with composition of glyoxal varied from 0 to 1.5 (Table 1).

A Brookfield DV1 Viscometer was used to measure the viscosities of the formulations 1 and 2. All readings were taken at 30˚C.

Infrared spectra of crosslinked PVA and starch blends were measured on a PerkinElmer FTIR spectrum 100 instrument. Thin films were made from crosslinked blends prior to analysis.

A Perkin Elmer instrument Q100 DSC has been used for estimating the glass transition temperature (T_g).

Testing method employed to calculate pencil hardness was ASTM D 3363.

ATinus Olsen 5ST instrument was used for determining the ultimate stress of the films.

A Tinus Olsen H25KT instrument has been used for calculating the tensile strength. Two pieces of steamed beech wood were taken for determining the tensile strength. The adhesive was applied on one end (2.5 cm × 2.5 cm) of the two pieces and was held together for 2 and 24 hrs at room temperature. The cured wood samples were then tested using a controlled speed of 10 mm/min to obtain the tensile strength values.

The glyoxal acts as a cross-linker for PVA/S, which has led to subsequent viscosity

change as shown in Figure 2.

The effect of cross-linking between the hydroxyl groups of PVA/S is evident in Figure 2. The glyoxal has reacted with the hydroxyl groups and thus forming a cross-link. This cross-link increases the chain length of the PVA and S, thus the increase in viscosity. Since, the longer chain length increases the chances of entanglement, it has led to increase in viscosity as evident in Figure 2. Although, while comparing the formulation 1 and formulation 2 values, we get a much greater viscosity in formulation 2. This is due to presence of hydrolyzed starch which has contributed to increase in viscosity.

As, the concentration of cross-linker is increased, there is a movement of curve towards the lower wave number. This is observed since the cross-linker reduces the distance between chains by bridging between them, this bridging leads to lower H-bonding in the adhesive. The effect of decreased H-bonding can be seen in Figure 3 as the curve shifts towards lower wavenumber. Additionally, the increase in starch content also contributes to decrease in H-bonding (shown in Figure 3).

The glyoxal acts as a cross-linker for PVA/S, which has led to subsequent glass transition temperature change as shown in Figure 4.

The effect of cross-linking between the hydroxyl groups of PVA/S is evident in Figure 4. The glyoxal has reacted with the hydroxyl groups and thus forming a cross-link. This cross-link increases the chain length of the PVA and S, thus the increase in glass transition temperature. As evident from the FTIR curves, there is a decrease in H-bonding of the adhesive with subsequent increase in cross-linker. The similar trend is observed for Tg, there is an increase in Tg with increase in concentration of cross-linker. In comparing formulation 1 and formulation 2, there is slightly higher Tg for formulation 2 due to hydrolysis of starch.

Pencil hardness is a property dependent on the flexibility of the polymeric chains. The flexible chains of PVA are replaced partially by starch (which contains large 6 membered rings). The presence of rings in starch has contributed to increase in hardness of the formulations 2. Better cohesion in blends has also contributed to increase in hardness (as seen in Table 4). Both the formulations show a large difference in hardness with and without cross-linker. The cross-linking has clearly enhanced the hardness of the blends.

The glyoxal acts as a cross-linker for PVA/S, which has led to subsequent tensile strength change as shown in Figure 5.

As seen in the previous sections, there is an increase in viscosity, glass transition temperature and pencil hardness of the blends. The tensile strength shows a similar trend as there is decrease in free volume between the chains, there is an increase in number of hydroxyl groups at the surface, as the hydroxyl groups become abundant on the surface there is an increase in tensile strength. Since, more cross-linker causes this decrease in free volume more rapidly, the tensile strength is increased subsequently (Figure 5(a) and Figure 5(b)). The cohesion between chains has also increased which further reduces the chances of tensile failure.

The cross-linking of glyoxal has decreased the mobility of the polymer chains. This has led to increase in stress bearing capacity of the chains. Thereby, increasing the stress required to break the blends. Also while comparing the formulation 1 and formulation 2, the larger number of six membered ring in starch help in increasing the stress (as seen from Table 5).