The stepwise regression design is an exploratory method in which the independent variables are continuously and sequentially varied until a model fitting with all the response variables is found [20, 21]. Evaluation of the experimental data for the purpose of selecting the appropriate model was performed using software program, Design expert, version 18.104.22.168. The software model is generated by fitting a quadratic polynomial model to the data set. The RSM targeted model was developed with AraG software comprising of a quadratic polynomial expression of the following form (6):
The 32 full factorial design is considered as the middle point in between center point and factorial points. The model generated is referred to as a central composite design. The software program utilizes star points instead of the variables in order to increase the experimental throughput. The factorial design allows combinations of settings inside the levels. 32star points can be seen as a trade-off between the center-point design and the full-factorial design. The design points occur in a hexagonal pattern. The center point and factorial points are at six and the star points at four. The number of factorial points is identical to that of star points. The use of this factorial design permits the ratios of factorial points and star points to be adjusted at the design stage. The software program used to experimental design given in the table. The space allowed for factorial points in the table is 5-40 at two levels with center. The order of the experiment is A-C where A is center point and B, C and D are factorial points. Each response was evaluated in triplicate.
where b1 to b22 and c0 to c24 are regression coefficients of the model. The observed and predicted values for all the responses were tabulated in table 1, 2 and 3. A correlation coefficient of significant level 0.05 between observed and predicted values was computed using Excel statistical software package. The change in form of response surface with respect to concentration of HEC HN 13 and PMMA is shown in tables 4 and 5. 7211a4ac4a