(722ak) Polyurethanes Based on Polyols from Castor Oil, Starch Granules and Starch-Derived Glycol and Glycerol Glycosides: Morphology, Synthesis, Chemical, Mechanical, and Thermal Properties

A series of polyols were prepared from castor oil (CO) and cassava starch (S) by two routes in order to obtain products with high hydroxyl content:

a) Incorporation of starch granules into the original castor oil and pentaerythritol- modified castor oil.

b) Modification of starch by glycosylation and transesterification with castor oil and modified castor oil.

The resultant polyols were characterized by hydroxyl value according to ASTM D1957-86; Fourier transform infrared spectroscopy (FTIR) to identify structure changes of the different obtained products; Matrix Assisted Laser Desorption/Ionization Time of Flight Mass Spectroscopy (MALDI TOF MS) was also used to detect and characterize the components of castor oil and to determine the products of the transesterification reaction. In this work we also used MALDI TOF MS to characterize the components of starch glycosylation products.

Several polyurethanes (PU) were synthesized by reacting the different polyols derived from castor oil and cassava starch with 4,4-diphenyl methane diisocyanate (MDI) and isophorone diisocyanate (IPDI). The structure of the polyurethanes was confirmed by FTIR spectroscopy using KBr pellets. The results of spectral analyses were used to identify physical crosslinking (hydrogen bonding) in the polyurethane. The inter or intramolecular interactions play a significant role in the miscibility of polymer components and properties of the macromolecules. Thermal stability of these polymers was determined by thermogravimetric analysis (TGA). The higher the hydroxyl value of the obtained modified castor oil, the higher the density of rigid segments of the corresponding polyurethane, the higher the thermal stability. Chemical and solvent resistances were measured according to ASTM D543-67 method. Each polymer specimen of known dimensions was immersed in solvents with increasing order of solubility parameter d from 8.2 to 14.5 (cal/cm3)1/2. This procedure was continued till equilibrium swelling was achieved. The swelling behavior of the polyurethanes in various solvents was investigated and the solubility parameter was determined based on the ?average structure? of each polyurethane repeating unit, as determined by the method of matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI TOF MS). The crosslink density of polyurethanes was determined by swelling tests using the Flory-Rehner's equation. The variation of the crosslink density of polyurethanes was studied based on the hydroxyl value of the polyol utilized in the synthesis. The stress-strain properties were determined using an Instron Universal Testing machine according to ASTM D-527 procedure and hardness measurements were made on a Shore-A hardness test apparatus using ASTM-785 method. Dynamic mechanical thermal analyses (DMTA) of selected polyurethanes were carried out on a Rheometrics DMTA V. The storage modulus (E'), the loss modulus (E'') and loss factor (tand) were measured. The temperature corresponding of the maximum of the tand peak position was taken as glass transition temperature (Tg). Scanning electron micrographs were obtained on a JEOL JSM-6400 electron microscope. The samples were previously coated with a gold layer and the morphology of the materials was studied through their fractures after immersion into liquid nitrogen.

The effect of polyol modification on the mechanical, chemical and thermal properties of the resulting polyurethanes was evaluated as a function of the percent of modifier agent (pentaerythritol, starch or starch-derived glycosides). The properties of the elastomeric polyurethanes were studied related to their chemical structure and morphology. There were also evaluated the effects due to the structure of the soft segments, consisting on the polyol, and the hard segments, consisting on the polyfunctional isocyanate and crosslinker. The morphology of polyurethane was investigated as related to compatibility and microphase segregation between the two segments. The change in the crosslink density and physical interactions in the polyurethane were studied in function of the chemical modification of castor oil and starch.

We found that the incorporation of starch into the polyurethane structure results in a higher physical crosslinking and higher hard segment content. The SEM image of the castor oil polyurethane with starch granules showed the starch granules immersed in the matrix of polyurethane, which implies phase separation between starch granules and polyurethane phase. The starch granules were dispersed in the polyurethane system without shape modification of the granule in the continuous polyurethane phase. Mechanical properties of the polyurethane obtained when the starch is incorporated to the polyols derived from castor oil modified by transesterification reaction with pentaerythritol, are found to be much better than the ones observed in the polyurethanes synthesized from the incorporation of starch to unmodified castor oil. The synergistic effect in the mechanical properties of the polyurethanes can be attributed to the physical interpenetration between starch and polyurethane based on modified castor oil, since interpenetration increases the cohesion and makes intimate contact between the two phases.

Without prior modification, starch cannot be reacted with the diisocyanate to obtain polyurethane. So, starch was reacted with ethylene glycol to form glycol glycoside and with glycerol to form glycerol glycoside. Glycol glycoside and glycerol glycoside were analyzed by FTIR and MALDI TOF MS. Then, two types of polyol-glycosides were synthesized by transesterification reaction of the obtained starch glycosides with castor oil and with pentaerythritol-modified castor oil.

Polyurethanes were synthesized from the polyol-glycosides by reaction with isophorone diisocyanate (IPDI). Different amounts of glycosides were used to study the effect of the glycoside content on the final polyurethane. Polyurethane networks prepared from IPDI and castor oil modified by transesterification with pentaerythritol and by reaction with starch glycosides showed excellent chemical resistance, hardness and tensile strength properties compared to the ones synthesized from unmodified castor oil. All materials prepared showed a uniform polyurethane phase without any presence of starch particles which indicates that polyol-glycosides of high hydroxyl content have been effectively reached and reacted to obtain a network formation in which starch glycosides are chemically incorporated. The degree of crosslinking in these networks can be controlled according to the type of polyol-glycoside used, which gives higher levels when glycerol is employed instead of ethylene glycol.