Leon P.B.M. Janssen

Water absorption, retention and the swelling characteristics of cassava starch grafted with polyacrylic acid

An important application of starch grafted with copolymers from unsaturated organic acids is the use as water absorbent. Although much research has been published in recent years, the kinetics of water absorption and the swelling behavior of starch based superabsorbents are relatively unexplored. Also, water retention under mechanical strain is usually not reported. Cassava starch was used since it has considerable economic potential in Asia. The gelatinized starch was grafted with acrylic acid and Fentons initiator and crosslinked with N,N-methylenebisacrylamide (MBAM). Besides a good initial absorption capacity, the product could retain up to 63 g H2O/g under severe suction. The material thus combines a good absorption capacity with sufficient gel strength. The mathematical analysis of the absorption kinetics shows that at conditions of practical interest, the rate of water penetration into the gel is determined by polymer chain relaxations and not by osmotic driven diffusion.

The Acetylation of Starch by Reactive Extrusion

Potato starch has been acetylated in a counter rotating twin screw extruder using vinylacetate and sodium hydroxide. The desired starch acetylation reaction is accompanied by an undesired parallel base catalysed hydrolysis reaction of vinylacetate and a consecutive hydrolysis reaction of the acetylated starch. Also deacetylation may occur. During the extrusion process sodium hydroxide, vinylacetate, granular potato starch and water were supplied to the extruder. Conversions up to 100 % could be achieved. The degree of substitution could be varied from 0.05 to 0.2. Selectivities from vinylacetate towards starch of up to 80 % could be achieved. From reaction kinetics and experiments it could be concluded that the deacetylation reaction could be neglected. The degree of gelatinisation appeared to be a major key parameter. Increasing the gelatinisation in an early stage of the extrusion process, at high starch concentrations and temperatures, increases the selectivity and degree of substitution. The gelatinisation degree can be increased by applying high shear and temperatures to the starch granule and by using screws with modified geometries.

Improved homopolymer separation to enable the application of 1H NMR and HPLC for the determination of the reaction parameters of the graft copolymerization of acrylic acid onto starch

Graft copolymers of starch with acrylic acid are a promising green, bio based material with many potential applications. The grafting of acrylic acid onto cassava starch in an aqueous medium initiated by Fentons reagent has been studied. Common grafting result parameters are add-on (yield) and graft efficiency (selectivity). However, the analysis of the reaction products and an accurate determination of these parameters stand or fall with a complete separation of the entangled but ungrafted homopolymer from the grafted product. Therefore, this separation is the core of the newly developed analytical procedure. An appropriate solvent has been selected with dedicated testing from the range methanol, ethanol, acetone, dioxane, 2-propanol, and 1-propanol. Acetone showed the best performance in many respects. It has a high dissolving power for the homopolymer, as well as the highest yield of precipitation for the starch derivatives and it is the most economical in use. After the successful separation, the precipitated graft copolymers could be analyzed quantitatively by nuclear magnetic resonance. The liquid with homopolymer and unreacted monomer was analyzed by high pressure liquid chromatography. Proof of grafting has been found by FTIR and TGA analyses. The mass balance calculation shows a systematic error which appears fairly consistent: 18.0±2.5 wt%. This was used as a correction factor in the calculation of the grafting parameters but more importantly, it means that the method we developed has a high level of repeatability, in the order of 97%.

Material distribution in the partially filled zone of a twin-screw extruder

Simulation of monomeric material transport in reactive extrusion revealed that two different situations can be distinguished in the partially filled zone of a twin-screw extruder. Material is transported from the bottom of the barrel to the top of the screw by drag or material remains on the bottom of the channel because of gravity. Which process takes place can be predicted when the ratio between the Froude number (gravitation forces) and the Reynolds number (viscous forces) is known. From experiments in a perspex model of a corotating, self-wiping, twin-screw extruder, using glycerol and silicone oil as model fluids, it could be observed which situation can be expected at different filling degrees, screw speeds, throughputs, and viscosities. In the case of a constant filling degree, material moves up with the screw flight at a certain value of Fr/Re. At higher Fr/Re ratios, a larger filling degree is needed for the screws to drag material up with the screw flights. When the filling degree reaches one-half, material moves up with both the pushing and the pulling screw flight. On the basis of these observations, a model is proposed to determine the backflow in the partially filled zone of a corotating, self-wiping, twin-screw extruder, dependent on viscosity and screw speed.

Insights in starch acetylation in sub- and supercritical CO2.

An in-depth study on the acetylation of starch with acetic anhydride (Ac(2)O) and sodium acetate (NaOAc) as the catalyst in pressurized carbon dioxide (scCO(2)) in a broad pressure range (8-25MPa) and a temperature of 90°C is provided. Highest degrees of substitution (DS) of 0.29 (1h reaction time) and 0.62 (24h reaction time) were found near the critical point of the mixture (15MPa). The phase behavior of the system CO(2), starch and acetic anhydride (Ac(2)O) was studied in a high pressure view cell. The critical points were a clear function of the temperature and increased from the range of 9.4-10MPa to 14.5-14.8MPa when going from 50 to 90°C (Ac(2)O mole fraction at the critical point in the range of 0.08-0.09). Acetylation experiments with a range of starch particles sizes showed a clear relation between the DS and the particle size.