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The effect of process parameter for graft copolymerization of sorbic acid onto polypropylene film by full factorial experimental design.


Radiation induced grafting is known to be a very powerful method for surface modification of polymer materials. It offers versatile means for incorporating new functionalities into existing polymer because of its large penetration in polymer matrix, rapid and uniform formation of active sites for initiating grafting. However, the construction of grafted material by radiation-induced grafting not only depending on the formation of covalently bonded of an ionic component but other factors have to be taken into account to ensure perfect tailor making of grafted materials. Grafting depends on experimental parameters such as absorbed dose, dose rate, monomer concentration, temperature, time and atmosphere [1, 2]. Proper monitoring of these grafting parameters onto grafting yield is crucial in order to optimize the experimental parameter, which can result in the desired grafted polymer with specific properties.

The design of experiments has been applied successfully in various scientific and technical fields such as applied chemistry and physics, biochemistry and biological, chemical engineering, environmental protection and so on [3-5]. In recent years, the design of experiments has been also applied successfully in different areas of radiation induced grafting [6, 7]. The application of design of experiment is very valuable, not only for the determination of the optimal settings of an experiment, but also for the understanding of the effect of each factor at different levels, individually or in combination, to the final response. Although the use of experimental design is widely applied in a large area of polymer processing, its application in radiation induced grafting of antimicrobial agent (sorbic acid) onto polymer film is very rare. In fact, there is no report on radiation-induced grafting of sorbic acid onto PP film using this statistical approach. The main objective of this article is to investigate the individual and the interaction effects of four process parameters namely; absorbed dose, monomer concentration, reaction temperature and reaction time against percent grafting yield (%GY) for radiation induced grafting of sorbic acid onto polypropylene films by using full factorial experimental design.


2.1 Materials:

Polypropylene (PP) granules were purchased form Exxon Mobile Chemical Sdn Bhd. Sorbic acid was purchased from Sigma-Aldrich and used as received. Solvent and chemical reagents were of laboratory grade and were used without further purification.

2.2 Preparation of PP-g-SA:

PP granules were used to make 0.5mm thickness of PP film using Hot & Cold Press Machine model LP-S-50. The granules were pre-heated for 4 minutes followed by compression for 4 minutes at 180[degrees]C. Then, the film was subsequently cooled under pressure for 4 minutes. The PP film was then cut into pieces of known weight and ultrasonically washed in methanol solution for 30 minutes to remove surface contaminants. Subsequently the film was dried in a dry oven at 60[degrees]C until it reached constants weight. After that, PP films were placed in a zip-lock bag which was sealed after nitrogen flushing and irradiated under dry ice temperature using electron beam accelerator. The irradiated film was directly used for grafting reaction after irradiation. The grafting reaction was carried out in an ampoule, where the irradiated sample was placed inside the ampoule and immersed with sorbic acid solution of known concentration. The sealed glass ampoules were then put in a water bath, heated at temperature of 40 & 80[degrees]C and time of 1&5 hours to initiate grafting reaction. The samples were then removed from the solution and cleaned by sonicating for 30 minutes in methanol to remove the excess monomer. The weight of grafted sample was measured after drying overnight in an oven at 60[degrees]C. Effect of parameters evaluated against the grafting copolymerization was calculated by means of the grafting yield as defined below (Eq. 1):

Graftinq yield (%) = Final Weight (Wf)-Initial Weight(Wo)/ Initial Weight(Wo) x 100 (1)

Where, Wf and Wo is denote the weights of grafted and un-grafted samples, respectively.

2.3 Full factorial design of experiment method:

Graft copolymerization of sorbic acid onto polypropylene (PP) film was carried out using full factorial design. The absorbed dose, sorbic acid concentration, temperature and time were selected as the independent variables and percent grafting yield (%GY) was selected as the dependent variable. Each independent variable was investigated at two levels, high level (+1) and low level (-1). The condition of each parameter is listed in Table 1.The design of experiment and statistical analysis of the data was done employing Minitab software (version 16).


The significance of each parameter, both individual and upon interacting, was estimated using probability function analysis available in the Minitab software. Normal probability plot of effects is shown in Figure 3.1. Significant effects are represented by square symbol located away from the centre line, while circle symbols represent insignificant effect. The larger the significant effects, the further away they are from the straight line. From this result, significant effects that emerge are the main effects of B (monomer concentration), C (temperature), A (absorbed dose), BC (sorbic acid concentration and temperature), AC (absorbed dose and temperature), AB (absorbed dose and sorbic acid concentration) and ABC (absorbed dose, sorbic acid concentration and temperature) interactions. Among the effective parameters for this study is the sorbic acid concentration. Increases in sorbic acid concentration leads to increase of availability of the monomer to reach to the active sites on the polymer. The resulting interaction between absorbed dose and temperature can be explained by the fact that the amount of trapped radical is proportional to the absorbed dose and a high temperature enhance the increased molecular motion that accelerate the diffusion of monomer to the trapped radical for the polymerization. When polypropylene (PP) was irradiated, the trapped radicals are capable of initiating the grafting reaction. If a monomer is allowed to diffuse into trapped radical with the help of grafting temperature, grafting takes place during the diffusion process. This observation is well in line with Nho et al. in an attempt to graft 2,3-epoxypropyl methacrylate (EPMA) onto PP film [8].

Figure 3.2 shows the Pareto graph for each parameter that influences grafting yield (GY). Bar length are proportional to the absolute value of the estimated effects, which helps to compare relative importance of the effects. Red line shows limit of the experimental error. It can be also seen that three main effects have a significant effect on the response (%GY), which are: A, B, C and four significant interaction; BC, AC, AB and ABC. This means that the grafting yield is strongly dependent on the investigated reaction parameters, which support earlier result in Figure 3.1. On the other hand, raising the value of reaction time (D) from the lowest (1 hour) to the highest (5 hour) leads to negative effect on GY. The grafting loss above 1 hour may be attributed to the depletion of the monomer or decrease in the active sites available for grafting. Thus, the reaction time (D) and its interaction with other parameters may therefore be eliminated from the model.

The term main effect is the average of all of the responses produced by changing the level of a factor. A main effect plot could be used to determine which factors influence the response and to compare the relative strength of the effects. Main effect plot for each independent parameter of GY are depicted in Figure 3.3. The plot indicates that the sorbic acid concentration has a significant effect on the grafting yield (GY). Upon changing sorbic acid concentration values from low to high level, the grafting yield increased up to 8.1%. Thus, these parameters are considered to be positive. In contrast, when variation from low to high absorbed dose and reaction temperature took place, a decrease of GY was observed. It should be noted that physical properties of trunk polymer are often deteriorated by radiation induced reaction such as crosslinking and degradation. According to Cleland et al., 10kGy absorbed dose should be enough to carry out graft process since PP starts to degrade at absorbed dose beyond 100kGy[9]. Furthermore, Melendez-Ortiz et al. reported that when absorbed dose is greater than 30kGy, macro-radical recombination starts to predominate which causes GY to plateau [10]. Also, the effect of reaction time is not noticeable on the GY since the lines on the plot are almost parallel.

The interaction effects plots provide the mean response of two factors at all possible combinations of their settings. If the lines are not parallel, there is interaction between the two factors. The effect of studied parameters and interaction effect between parameters on radiation induced grafting of sorbic acid onto PP film are presented comparatively in Figure 3.4. The interaction plot show that the maximum interaction effect was between sorbic acid concentration (B) and temperature (C). The steep effect lines obtained for the BC implies that there was strong two ways interaction between the main effect of B and C. The flat effect line attained for the rest of the main effects reveals their insignificant effect on GY since their effect lines are nearly parallel. In this analysis, interaction effect between grafting time was very low and thus can be neglected.

The model equation related to the level of parameters and percentage of grafting obtained from the experiment is shown in the equation model in coded unit as below:

GY(%) = 1.387 - 1.177[C] + 1.161 [B] - 0.705[A] - 1.224[BC] + 0.810[AC] - 0.776[AB] + 0.759[ABC]

where C is reaction temperature, B is monomer concentration and A is absorbed dose.


This study showed that full factorial experimental design approach is an excellent tool and could provide better understanding of main effect parameters and interaction between parameters for graft copolymerization of sorbic acid onto polypropylene film. As observed, the most effective parameteris monomer concentration (B). The absorbed dose (A) and reaction temperature (C) have a positive effect, while the reaction time (D) had a negative effect on the grafting yield. The most influencing interaction was between BC, AB, AC and ABC. These results are important since the classical parameter design is complicated with a large number of experiments required to be conducted when the number of the process parameters increased. For this reason, the full factorial design of experiment is a useful tool to study the significant parameter and interactions between two or more variables at reduced number of experimental trials.


Article history:

Received 12 March 2015

Accepted 28 April 2015

Available online 6 June 2015


The authors wish to thank to International Atomic Energy Agency (IAEA) for granting Coordinate Research Programme (CRP) fund under the vote number of RC17457.


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(1,2) Nor AzwinShukri, (1) Zulkafli Ghazali, (1) Nor Azillah Fatimah Othman, (2) Mat Uzir Wahit

(1) Radiation Processing Technology, Malaysian Nuclear Agency(Nuclear Malaysia), Bangi, 43000,Kajang, Selangor, Malaysia

(2) 1Center for Composites.UniversitiTeknologiMalayia, 81310, UTM Johor Bahru, Johor, Malaysia

Corresponding Author: Nor AzwinShukri, Radiation Processing Technology, Malaysian Nuclear Agency (Nuclear Malaysia), Bangi, 43000,Kajang, Selangor, Malaysia

Table 1: Experimental ranges and levels of the factors used in the
full factorial design

Parameter         Notation   Units        Low   High

Absorbed dose     A          kGy          10    100
Sorbic acid       B          % (w/v)      1     10
Temperature       C          [degrees]C   40    60
Time              D          hour         1     5
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Author:AzwinShukri, Nor; Ghazali, Zulkafli; Othman, Nor Azillah Fatimah; Wahit, Mat Uzir
Publication:Advances in Environmental Biology
Article Type:Report
Date:Jun 30, 2015
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