OPTIMIZATION ASPECTS ON MODIFICATION OF STARCH USING ELECTRON BEAM IRRADIATION FOR THE SYNTHESIS OF WATER-SOLUBLE COPOLYMERS M. BRASOVEANU 1, E. KOLEVA,3, K. VUTOVA, L. KOLEVA 3, M.R. NEMȚANU 1 1 National Institte for Lasers, Plasma and Radiation Physics, Electron Accelerators Laboratory, P.O. Box MG-36, RO-07115, Bcharest-Măgrele, Romania, E-mail: mirela.brasovean@inflpr.ro, E-mail: monica.nemtan@inflpr.ro Institte of Electronics, Blgarian Academy of Sciences, 7 Tzarigradsko shossee blvd., Sofia 1784, Blgaria, E-mail: eligeorg@abv.bg, katia@van-compters.com 3 University of Chemical Technology and Metallrgy, 8 Kl. Ohridski blvd., Sofia 1756, Blgaria, E-mail: sra@abv.bg Received Febrary, 016 The acrylamide grafting onto starch by electron beam (EB) irradiation in order to synthesize water-solble copolymers having flocclation abilities was optimized by implementation of the Response srface methodology (RSM) and robst engineering design. The reslts showed different optimal regions of EB irradiation dose and dose rate depending on acrylamide/starch weight ratio. Key words: Grafting parameters optimization, electron beam irradiation, starch, acrylamide, response srface methodology. 1. INTRODUCTION The crrent environmental considerations impose rigoros protection rles and sstainable progress that minimize the impact of wastes on the environment. Accordingly, there is a strong demand to develop economically viable and ecofriendly replacements of conventional synthetic flocclants, based pon the renewable organic materials that are low cost and degrade natrally when are released in the environment [1]. Grafting is the most effective way of reglating the properties of natral polysaccharides, which can be tailor-made according to the needs and prodce high efficient graft copolymers [], having application as flocclating agents for treatment of different wastewaters. Rom. Jorn. Phys., Vol. 61, Nos. 9 10, P. 1519 159, Bcharest, 016
150 M. Brasovean et al. Electron beam (EB) grafting (modification of polymer sbstrates sing electron beam indced graft copolymerization) is an important process for creation of new fnctional materials with different applications [3]. EB irradiation grafting is sed to develop a wide variety of ion exchangers, polymer-ligand exchangers, chelating copolymers, hydrogels, affinity graft copolymers and polymer electrolytes, having varios applications in water treatment, chemical indstry, biotechnology, biomedicine, etc. [4]. The advantages of electron beam indced grafting process are connected with the ability to ionize polymers with limited reactivity in chemical processes, generation of polymer radicals is performed by clean non-chemical method, which can be precisely controlled and ses low energy. The electron beam irradiation can be easily modified and integrated into complete process lines. Electron beam irradiation can be implemented also in other processes with different effects reslting from the formation of free radicals besides grafting like cring, crosslinking and scission [5]. Dring the electron beam graft process the depth of EB energy deposition into materials can be controlled by the accelerating potential of the eqipment and the elemental composition and density of the material being irradiated. Acrylamide monomer gives a polar graft side chain reslting in hydrophilic copolymer. In order to estimate models, giving the relationship between the process parameters and the performance characteristics, a properly planned experimental design shold be performed. There are experiments dring which the variance is non-homogeneos over the factor (process parameters ) space and when the noise factors cannot be identified nor an experiment to stdy them can be condcted. The observations in this case are called heteroscedastic (variance varies with the factor levels). In this case regression models for the mean and the variance of the qality characteristics of the prodct, based on repeated observations, can be estimated. This circmstance gives possibility to implement robst (not sensitive to noises and errors) engineering approach for parameter optimization in terms of obtaining repeatability and qality improvement by minimization of variations in the qality characteristics. In this paper mlti-criteria optimization involving reqirements for economic efficiency, assrance of low toxicity, high copolymer efficiency in flocclation process and good solbility in water is presented. The optimization applies estimated regression models, describing the dependencies of the qality characteristics: y 1 monomer conversion coefficient (Conv, %), y residal monomer concentration (M r, %), y 3 apparent viscosity (η a, mpa s) and y 4 intrinsic viscosity ([η], dl/g) and y 5 Hggins constant (k H ) on the variation
3 Optimization aspects on modification of starch 151 of the process parameters: acrylamide/starch (AMD/St) weight ratio, concentration of AMD and St, respectively, electron beam irradiation dose and dose rate.. EXPERIMENTAL Experiments for the modification of starch by grafting acrylamide sing electron beam irradiation were performed in order to synthesize water-solble copolymers having flocclation abilities. The synthesis of graft copolymers was performed by two steps: (1) preparation of soltions containing starch and monomer; () irradiation of soltions by electron beam. 1. Starch aqeos soltions were prepared by dissolving corn starch in distilled water. Acrylamide was added to starch aqeos soltion with frther stirring, reslting in varios acrylamide/starch (AMD/St weight ratios) homogenos aqeos soltions.. Therefore, homogenos aqeos soltions prepared in step 1 were exposed to electron beam irradiation. The irradiations were carried ot at ambient temperatre and pressre by sing linear electron accelerators of mean energy of 6.3 MeV with different irradiation doses and dose rates. The synthesized graft copolymers were characterized by monomer conversion coefficient, Conv [%]; residal monomer concentration, M r [%]; apparent viscosity, η a [mpa s] and intrinsic viscosity, [η] [dl/g]; and Hggins constant, k H. The variation regions [z min z max ] of the process parameters were: for EB irradiation dose (z 1 ) [0.65 5.50 kgy]; the dose rate (z ) [0.41 1.50 kgy/min] and the concentration of AMD (z 3 ) [9.8 19.6 %]. The concentration of St for this experiment was constant and is 1.8%. The (AMD/St) weight ratio varies from 5.56 to 11.11. There were 19 process parameter experimental sets and for each set the means y and the variances s of the qality characteristics of graft copolymers: monomer conversion coefficient, residal monomer concentration, apparent viscosity, intrinsic viscosity and Hggins constant were estimated sing three measrements and the following eqations: y 1 n n i1 y i s 1 n 1 n i1 y y 1,,..., N, where n is the nmber of replications, N is the nmber of experimental sets (N = 19). i (1)
15 M. Brasovean et al. 4 3. MODELS OF THE MEAN AND THE VARIANCE OF THE QUALITY CHARACTERISTICS The estimated vales of the means y and the variances s can be considered as two responses at the design points and ordinary least sqares method can be sed to fit two polynomial regression models for each qality characteristic [6]: Model of the mean vale: k y y x ˆ ( ) f ( x) () i1 yi yi Model of the variance: k s x ˆ f x ln (3) i1 i i where ˆ yi and ˆ i are estimates of the regression coefficients, and f yi and f i are known fnctions of the process parameters. The variance of normally distribted observations has a distribtion. The se of the logarithm transformation of the variance fnction makes it approximately normally distribted, which improves the efficiency of the estimates of the regression coefficients. The models are estimated for coded in the region [ 11] vales of the process parameters, sing the following eqation: x i z z z )/( z z ), (4) ( i i, max i,min i,max i, min where x i and z i are the coded and the natral vales of the process parameter, correspondingly, z i,min and z i,max are the minimal and the maximal vales of the parameter experimental region. The (AMD/St) weight ratio and the concentration of AMD, when coded, have eqal levels, since St concentration was constant dring the experiment. Their inflence on the variation of the qality characteristics cannot be distingished in this case and only one of these factors shold be sed in the estimated models. The dependencies of the means and the variances of the prodct qality characteristics: y 1 residal monomer concentration (M r, %), y monomer conversion coefficient (Conv, %), y 3 apparent viscosity (η, mpa s) and y 4 intrinsic viscosity ([η], dl/g) and y 5 Hggins constant (k H ) on the variation of the process parameters: x 1 electron beam irradiation dose, x electron beam
5 Optimization aspects on modification of starch 153 irradiation dose rate and x 3 concentration of AMD are estimated. The obtained regression models are presented in Table 1 and Table, together with the vales of the corresponding mltiple correlation coefficients R. These coefficients are tested for significance and their vales are measres of the accracy of the estimated models (the closer to 1 is the vale of R, the better the model describes the variations of the qality characteristics as a fnction of the process parameters). Table 1 Models for the means of the prodct qality characteristics Model R y 1( x 0.01.9960x ) 1 +1.355x +8.1993x 1 +3.455x 1 x 3 + 0.9173 + 0.587x x 3 y ( x ) 99.7648+30.90x 1 8.8640x +3.8197x 3 46.4803x 1 0.987 y 3( x 4.3536+.769x ) 1 +0.6985x +0.9575x 3 5.390x 3 1.0758x 3 1 x 3 1.5789x 3 0.8499 1 x y 4( x ) 3.3567+.6973x 3 +3.8957x 1 1.4537x 1 x +0.6490x x 3 0.874 lny 5 ( x ) 1.5068+0.6948x 3 +1.330x 4 +.64538x 1 0.7396 Figre 1 shows the contor plots of the variation of the investigated qality characteristics, depending on the EB irradiation dose (x 1 ) and the dose rate (x ) at constant AMD concentration z 3 = 14.7% (or AMD/St weight ratio 8.335), which are calclated by the implementation of the estimated models in Table 1. Table Models for the variance of the prodct qality characteristics Model R 3.8165 1.8301x ln s1 x 1 1.8470x +.4135x 3 0.7819.759x 1 x +0.41x x 3 +0.7634x 1 x 3 ln s x 1.0867 1.393x +1.31x 3 0.8401x 1 x +0.944x x 3 0.73x 1 x 3 0.8590 ln s3 x 3.843 0.750x +1.6698x 3 +3.166x 1 -.0835x 1.716x 1 x 0.8009 6.547 0.1703x ln s4 x 1 0.9083x +1.0830x 3 +3.4344x 1 1.8865x 1 x 0.851 0.5980x 1 x 3 The standard deviations of the prodct characteristics are calclated by the eqation: ) ln( s x e i i s. (5) Visalization of the standard deviation dependence on the process parameter vales for each one of the prodct characteristics is presented in Figre. It can be seen that the standard deviation s x of the process parameter monomer conversion coefficient (y, %) has the highest vales within the experimental region.
154 M. Brasovean et al. 6 a) y f ( z, ) b) y f ( z, ) 1 1 z 1 z c) y f ( z, ) d) y f ( z, ) 3 1 z 4 1 z e) y f ( z, ) 5 1 z Fig. 1 Contor plots of the mean vales of the qality characteristics at z 3 = 14.7% (or AMD/St weight ratio 8.335).
7 Optimization aspects on modification of starch 155 a) b) c) d) Fig. Contor plots of the standard deviations of prodct characteristics depending on EB irradiation dose (x 1 ) and the dose rate (x ) at constant AMD concentration z 3 = 14.7% (or AMD/St weight ratio 8.335): a) x x x x. ; b) ; c) s 1 s ; d) s 3 s 4 4. OPTIMIZATION Mlti-criteria optimization nifying reqirements for economic efficiency, assrance of low toxicity, high copolymer efficiency in flocclation process, good solbility in water, as well as the repeatability of the obtained reslts is performed. Methods based on graphical optimization and on Pareto-optimization are implemented for solving this task. The set of reqirements is the following: residal monomer concentration: y 1( x ) < 5% => assrance of low toxicity; monomer conversion coefficient: y ( x ) > 90% => economic efficiency; apparent viscosity: y ( ) > 3 mpa s => copolymer efficiency in flocclation process; 3 x
156 M. Brasovean et al. 8 intrinsic viscosity: y 4 ( x ) > 6 dl/g => copolymer efficiency in flocclation process; Hggins constant: 0.3 ( x y 5 ) 1 (or 1.0397 lny ( ) 5 x 0) => good solbility in water. Graphical optimization is a method for mlti-criteria optimization, applicable in cases with formlated one- or two-sided constrains for the prodct qality characteristics. It is condcted in order to find the regions of the process parameters, working at which the reqirements for the qality characteristics are flfilled simltaneosly. The optimal regions are obtained by sperimposing the contor plots of the calclated limit vales of the characteristics, ths finding the section of all the admissible vales of the process parameters. In Fig. 3 the optimal regions of the process parameters vales EB irradiation dose (z 1 ) and dose rate (z ), for different concentrations of AMD (z 3 ): 14.7% and 19.6% (or AMD/St weight ratio correspondingly eqal to 8.335 and 11.11) are presented. For the lowest AMD concentration z 3 = 9.8% (or AMD/St weight ratio 5.56) there is no allowable region for the process parameters, where the optimization reqirements are simltaneosly flfilled. It can be seen that at lower concentration of acrylamide z 3 = 14.7% (or lower acrylamide/starch (AMD/St) weight ratio), the EB irradiation doses shold be chosen in the region 4.8 5. kgy and the dose rates shold be between 0.5 and 1.0 kgy/min. The increase of the AMD concentration to z 3 = 19.6 % reqires EB irradiation doses in a wider range from.3 to 4.8 kgy and dose rates higher than 0.6 and less than 1.3 kgy/min. Critical restrictions are connected with Hggins constant (y 5 ) and good solbility in water; intrinsic (y 4 ) viscosity and copolymer efficiency in flocclation process; as well as monomer conversion coefficient (y ) and economic efficiency. The reqirements for the qality characteristics residal monomer concentration y 1( x ) < 5% connected with the assrance of low toxicity as well as the apparent viscosity (y 3 ) are flfilled for wider regime conditions within the experimental region. For improving the reprodcibility of the obtained reslts the variation of the prodct qality characteristics shold be minimized. This pts additional reqirements for the soltions that are obtained throgh graphical optimization. In order to minimize the variances of the prodct parameters, mlti-criterion optimization is performed and compromise Pareto-optimal soltions are obtained (Table 3 and Table 4). If these compromise soltions are compared two by two, one can note the property of these soltions some of the obtained optimal vales are better bt at least one vale is worse than that in another compromise soltion. Every variance fnction has its minimm at different set of process parameters and a compromise can be chosen among many by some additional criteria. In the tables are presented only six Pareto-optimal soltions. The minimal vales for each standard deviation are bolded.
9 Optimization aspects on modification of starch 157 a) b) Fig. 3 Contor plot of the optimal regions of the process parameters EB irradiation dose (z 1 ) and dose rate (z ), for different concentrations of AMD: a) z 3 =14.7 % (or AMD/St weight ratio 8.335); b) z 3 = 19.6 % (or AMD/St weight ratio 11.11). Table 3 Pareto-optimization optimal process parameters and standard deviations x of prodct characteristics z 1 z z 3 s 1 x s x s 3 x x 1 4.7815 1.3038 16.8766 0.0463 1.314 0.1635 0.0547 4.68 1.3887 18.177 0.0764 1.616 0.1071 0.0405 3 4.387 1.3811 17.9708 0.0671 1.5350 0.115 0.0419 4 4.7686 1.3066 16.991 0.0470 1.38 0.1615 0.0541 5 4.5346 1.3604 17.6355 0.0580 1.4430 0.151 0.0450 6 4.3741 1.3790 17.9746 0.0683 1.5435 0.1130 0.0419 s i s 4
158 M. Brasovean et al. 10 Table 4 Pareto-optimization optimal process parameters and the vales of the constrained means yi ( x ) of prodct characteristics z 1 z z 3 y ( ) y ( ) y ( ) y ( ) y ( ) 1 x 1 4.7815 1.3038 16.8766 3.9473 94.533 4.759 6.0138 0.9909 4.68 1.3887 18.177.6813 99.4131 5.7490 6.004 0.9803 3 4.387 1.3811 17.9708 3.0080 98.5060 5.5014 6.014 0.9963 4 4.7686 1.3066 16.991 3.906 94.7094 4.348 6.0193 0.9881 5 4.5346 1.3604 17.6355 3.3787 97.313 5.1139 6.05 0.9910 6 4.3741 1.3790 17.9746.957 98.6489 5.555 6.0089 0.9798 x 3 x 4 x 5 x 5. CONCLUSIONS Regression models for the mean vales and the variances of the investigated prodct qality characteristics depending on the variations of the grafting process parameters dring the modification of starch by grafting acrylamide sing electron beam irradiation in order to synthesize water-solble copolymers having flocclation properties were estimated. Optimal regions of the grafting process parameters that satisfy specified optimization criteria based on the estimated models for the mean vales of the prodct parameters were obtained by implementation of graphical optimization. Ths, different optimal regions of EB irradiation dose and dose rate depending on AMD/St weight ratio were identified. The optimal regions were fond to be 4.8 5. kgy for irradiation doses and 0.5 1.0 kgy/min for dose rates, at AMD/St weight ratio of 8.335. Increase of acrylamide concentration (AMD/St weight ratio of 11.11) reqired different regions of irradiation doses (.3 4.8 kgy) and dose rates (0.6 1.3 kgy/min). Compromise Pareto-optimal soltions flfilling the formlated restrictions and at the same time simltaneosly minimizing all the variances of the qality characteristics were received. This additional reqirement for robstness (insensitivity) in respect of variations in inpt parameters and noise factors of the process lead to better reprodcibility of the reslts, together with the implementation of the reqirements for economic efficiency, low toxicity, efficient copolymer in flocclation processes and good water solbility. Acknowledgements. Bilateral joint research project between Blgarian Academy of Sciences (BAS) and Romanian Academy (RA) is acknowledged. Blgarian co-athors wold like to acknowledge the spport of the BNF Scientific Investigations nder the project DO0-17 (BIn- 5/009). This work was also partially spported by a grant of the Romanian National Athority for Scientific Research, CNDI UEFISCDI, project nmber 64/01.
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