Rheological Behavior of Reaction Mixtures during Graft Copolymerization

Graft copolymerization of starch with water soluble monomers such as acrylic acid is a suitable chemical modification technique that helps adapt and improve its biopolymer properties. This in turn opens up flexibility in terms of applicability of the starch for non-food purposes. Unfortunately, formation of homopolymers during the homogeneous reaction is inherent and is accompanied by an exorbitant separation cost. Therefore, the development of an alternative process that evades this separation step would be quite ‘life saving’, and such would entail the use of the complete product as a thickening agent (see figure 1), therefore, demanding complete understanding of its rheological behavior. Polymerization results in the formation of higher molecular weight chains hence increase in viscosity thereby implying that the rheological behavior of the mixture is likely to have a significant effect on the kinetic behavior of the reaction. Unfortunately, little on the rheological behavior of the reaction mixture during graft copolymerization of acrylic acid onto gelatinized starch exists.

Groningen University researchers developed an alternative procedure that can overcome the inadequacies related to the current design process. The researchers purposed to use torque measurements hand-in-hand with apparent viscosity monitoring in their new method. Their work is now published in the research journal, Polymer Engineering and Science.

The research team commenced their studies by running all the graft copolymerization runs in a stainless steel batch reactor fitted with a water jacket to control reaction temperature. In addition, the team graft copolymerized pre-gelatinized cassava starch with acrylic acid using Fenton’s reagents as the initiating system. They then measured the torque as the reaction proceeded using a Heidolph overhead stirrer (figure 2a and 2b). Calibration Measurements were then undertaken in a closed system so as to prevent evaporation.

The authors observed that the apparent viscosity of the reaction mixture increased rapidly, thereby agreeing well with a first order exponential growth model. The maximum viscosity was seen at the intermediate monomer to starch ratio of 1.0, a phenomenon that was recorded to be in accordance with literature data. Starch thixotropy was then seen to be influenced by the grafting of poly-acrylic acid, until it had almost disappeared at the highest monomer to starch molar ratio of 2.0.

Recent measurements support an assumption made in the original paper. Measurements in a Mars-III Rheometer show that starch gels of 7.5% w/w have at least one thousand times higher viscosity (~ 10 Pa.s) then pure PAA (2-4 mPpa.s).  So, curvefitting on the torque values in the reactor with 1^st order functions indeed only applies to the viscosity development by the grafted starch, not to the growth of homopolymer. Viscosity measurements for the rheologic characterization of products grafted at different conditions are still in progress and will be published later.’

The Judy Witono et al study has shown that torque registration is a potentially useful tool that can be used to observe the rheology of reaction mixtures with starch during graft copolymerization with acrylic acid. In their work, an empirical correlation between torque and the apparent viscosity has been made. It has been seen that gelatinized cassava starch shows thixotropic behavior, which has been confirmed by a restoration of the apparent viscosity after a period of rest. These results provide a meaningful contribution to the needs of designing processes based on starch in an industrial scale.

The Groningen University study provided truly an economically feasible technique that can be employed to unlock the economic potential of cassava starch in non-food industrial applications, and in so doing, unlock the potential of many cassava growing tropical countries.