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Evaluation of methods used for analysing maleic anhydride grafted onto polypropylene by reactive processing
Polymer Testing 19 (2000) 3–15
Test Method
Evaluation of methods used for analysing maleic anhydride grafted onto polypropylene by reactive processing S.H.P. Bettini, J.A.M. Agnelli* Departamento de Engenharia de Materiais da Universidade Federal de Sa˜o Carlos, Rodovia Washington Luiz, Km 235, Caixa Postal 676, 13.565-905 Sa˜o Carlos, Sa˜o Paulo, Brazil Received 20 June 1998; accepted 18 August 1998
Abstract Grafting of maleic anhydride onto polypropylene, in the presence of peroxides, was performed through reactive processing. The samples obtained were submitted to several analyses in order to check for conversion of the acid groups to anhydrides and whether purification was necessary. Quantification of reacted maleic anhydride was tested by titration of acid groups and Fourier transform infrared spectroscopy (FTIR). It was concluded that each type of processing requires verification of the necessity to purify the samples for removal of residual maleic anhydride. Spectroscopy was shown to be better for the quantification of reacted maleic anhydride, as long as the samples are submitted to thermal treatment at 130°C for at least 24 h. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Grafting of maleic anhydride onto polypropylene, in the presence of peroxides, in the melt has attracted a lot of commercial interest. These reactions have been conducted through reactive processing in continuous or batch mixing equipment that operate as chemical reactors [1]. The role of maleic anhydride functionalized polypropylene is to promote compatibilization between polar and apolar polymers, coupling between inorganic fillers and polypropylene as well as adhesion to metals [2–4]. According to several studies [5–7] graft polymerization of maleic anhydride onto polypropylene * Corresponding author. E-mail: [email protected] 0142-9418/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 6 6 - X
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S.H.P. Bettini, J.A.M. Agnelli / Polymer Testing 19 (2000) 3–15
in the melt, in the presence of peroxides, is accompanied by severe degradation of the polypropylene. Despite ease of operation and low costs involved in such processes, studies concerning maleic anhydride grafting onto polypropylene have presented difficulties in analysing the reacted maleic anhydride. Straightforward measurement of bound maleate is complicated by several factors, such as the presence of unreacted maleic anhydride in the reaction mass and the fact that the grafted succinic moieties may be in the acid or anhydride form [8]. Several analyses have been suggested, such as, titration [5,6,8,9], FTIR [8–12] and gravimetry [13]. Distinguished titration methods exist, namely: potentiometric titration [9], titration of a solution of maleic anhydride grafted polypropylene (PP-g-MA) dissolved in hot xylene by a base in non-aqueous medium [8] and titration of a PP-g-MA solution dissolved in water saturated xylene by a base in non-aqueous medium [6]. However, in order for the reacted maleic anhydride to be analysed by any of the aforementioned methods, it is required to verify if purification of the modified polypropylene is necessary for the removal of the unreacted maleic anhydride in the polymer mass. Several purification methods have been proposed: dissolution of modified polypropylene in xylene followed by precipitation in acetone [5,8], Soxhlet extraction with acetone [11–13] and evaporation of the maleic anhydride in a vacuum oven at temperatures above 100°C [11,12]. The scope of the current work was to test the several purification methods for removal of residual anhydride and analysis of the reacted maleic anhydride, show their advantages and drawbacks as well as to propose a more adequate methodology to analyse the reacted anhydride. 2. Experimental procedure 2.1. Materials Isotactic polypropylene was supplied by Polibrasil S.A. Indu´stria e Come´rcio (JE-6100) with melt flow index of 2.0 g/10 min. The maleic anhydride was supplied by Carbocloro Oxypar Indu´strias Quı´micas S.A., and the peroxide selected for this study was a 46.5% concentrate of 2,5dimethyl-2,5-di(t-butylperoxy)hexane in CaCO3, supplied by Elf Atochem Brasil Quı´mica Ltda. (Luperox 101 XL). 2.2. Reactive processing In order to investigate the effect of the type of processing on the analysis of reacted maleic anhydride the PP-g-MAs were processed in a torque rheometer and in a twin-screw extruder. The equipment used was a Haake System 90 torque rheometer, equipped with a Rheomix 600 mixing compartment and a Werner Pfleiderer ZSK-25 extruder. The processing conditions used in the rheometer were the following: process temperature: 180°C, rotor speed: 40 rpm, processing time: 15 min and nitrogen atmosphere. The concentrations used were 100/5/0.06 phr for polypropylene/maleic anhydride/peroxide, respectively. The reactive extrusion process was performed using the following conditions: temperature profiles: 180, 200, 200, 210, 210, 210 and 200°C; screw speed: 150 rpm and nitrogen atmosphere.
S.H.P. Bettini, J.A.M. Agnelli / Polymer Testing 19 (2000) 3–15
The following concentrations anhydride/peroxide, respectively.
were
used
100/4/0.1
phr
for
5
polypropylene/maleic
2.3. Purification methods 2.3.1. Solubilization in xylene and precipitation in acetone (PPT) A total of 4 g PP-g-MA was dissolved in 400 ml xylene under reflux at 130°C for 1 h. The temperature was lowered to 45°C and acetone was added. The precipitate was vacuum filtered and washed several times with acetone. The sample was then left in a vacuum oven for solvent removal. 2.3.2. Soxhlet extraction with acetone The PP-g-MA samples were placed in a Soxhlet for maleic anhydride extraction with acetone for 7 h and 30 min (SOX1) and 12 h (SOX2). Next, the samples were placed in a vacuum oven for acetone removal. 2.3.3. Heat treatment at 130°C Pellet samples of PP-g-MA were placed in a vacuum oven at 130°C for 6 h (TT6) and 12 h (TT12). The purified and non-purified samples were hot pressed at 200°C and analysed by FTIR. The same samples, however, without hot pressing, were titrated for analysis of reacted maleic anhydride. The non-purified samples were submitted to heat treatment for 96 h at 130°C, in order to verify conversion of the acid groups to anhydride. Samples were withdrawn every 24 h of heat treatment and analysed by FTIR. The formulations processed in the torque rheometer were submitted to all aforementioned purification methodologies, whereas the formulation processed in the extruder was only submitted to purification by dissolving in xylene and precipitation in acetone. In order to verify the efficiency of purifying of the samples processed by reactive extrusion thermogravimetric analyses were performed on the purified and non-purified samples. The equipment used was a TGA 2050 thermogravimetric analyser. Analysis conditions were: heating from 30 to 550°C at a heating rate of 10°C/min in nitrogen atmosphere. 2.4. Quantification of reacted anhydride 2.4.1. Titration Determination of reacted maleic anhydride via titration was carried out based on the technique developed by Gaylord and Mishra [6]. Titrations were performed in a 665 Dosimat Metrohm Dosing. The procedure adopted was as follows: 2 g PP-g-MA was dissolved in 400 ml xylene saturated with water. Aliquots of 100 ml were transferred to an Erlenmeyer flask and titrated with 0.05 N ethanolic potassium hydroxide (KOH), using thymol blue in dimethylformamide (DMF) as indicator. As soon as the solution became dark blue it was back titrated to a yellow end-point by the addition of 0.05 N isopropanolic hydrochloric acid (HCl).
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2.4.2. FTIR The hot pressed films were submitted to transmitting infrared analysis in a Nicolet Magna IR750 spectrophotometer with resolution of 2 cm−1 and 128 scans per spectrum. The wavelengths of interest were 1713 cm−1, characteristic of carbonyls from carboxylic dimer acids, 1790 and 1867 cm−1, characteristic of five-membered cyclic anhydride carbonyls, and 1167 cm−1, characteristic of CH3 groups. Using these bands the carbonyl index (CI) was calculated: CI ⫽
A1790 A1167
where A1790 is absorbance at 1790 cm−1, characteristic of carbonyls from five-membered cyclic anhydrides; A1167 is absorbance at 1167 cm−1, characteristic of CH3 groups, proportional to the amount of PP. As this is a relative measurement of the amount of reacted maleic anhydride, a calibration curve should be constructed. 2.4.3. Calibration curves Two types of anhydrides were tested to construct the calibration curve: succinic anhydride and dodecenyl succinic anhydride. The polypropylene and anhydride blends were prepared in the Haake torque rheometer and analysed by FTIR at the following conditions: blending temperature of 180°C, rotation speed of 30 rpm and mixing time of 5 min.
3. Results 3.1. Analysis of the purification methodologies and conversion of the acid groups to anhydrides In order to check for the necessity to purify the PP-g-MA samples prepared by reactive processing the samples grafted in the torque rheometer were submitted to several purification methodologies and analysed by FTIR. The results are presented in Fig. 1. As shown by Fig. 1 the CI of the non purified samples and those purified by the several methodologies presented little variation. Visual observation of the graph does not allow to make any statement on these differences. Therefore, statistical techniques were used to compare the averages through variance analysis and Tukey test for a significance level of 5%. The statistical analyses showed that there are no significant differences between the non-purified samples and those purified by the several methodologies. This may be explained by the fact that, for the system used in the torque rheometer, the major part of the residual anhydride must have been evaporated and removed together with N2, since the punner that seals off the system is only lowered on loading and N2 enters and leaves the system through an adapted system. Since the films analysed by infrared spectroscopy were hot pressed at 200°C some amount of unreacted maleic anhydride may have been eliminated, as this temperature is very close to the boiling point of maleic anhydride (202°C). From this analysis it was concluded that, for the system under investigation, purification of the samples tested in the Haake rheometer was not necessary.
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Fig. 1. Comparison of the purification methods through carbonyl index measurements.
After analysis of the purification methodologies, the following stage was to verify whether the samples required heat treatment in a vacuum oven for the conversion of possible acid groups, resulting from the ring opening of the anhydride, to anhydrides. Therefore, the non-purified samples were placed in a vacuum oven for 96 h and analysed by FTIR every 24 h. Conversion of dicarboxylic acids to anhydrides was monitored by the variation of the absorbance at 1713 cm−1 and 1790 cm−1, characteristic of the acid carbonyls and anhydride carbonyls, respectively. The results of this heat treatment are presented in Figs. 2 and 3. As can be seen in Figs. 2 and 3 the amount of carbonyls increased significantly due to the anhydride groups, when the samples were submitted to heat treatments of 24 h. This increase might be justified by the conversion of acid groups to anhydrides. Once again statistical tests were performed to check whether the differences in heat treatment times were significant or not for the non-purified samples. These tests showed that the differences are significant for the samples with and without heat treatment. The averages did not present significant differences, at a significance level of 5%, for heat treatments from 24 to 96 h. Acid group conversion to anhydrides was confirmed through infrared spectra, presented in Fig. 3, where one may observe the increase in the cyclic anhydride carbonyl band at 1790 cm−1, when the sample is submitted to heat treatment. In the case of the 1725 cm−1 band, narrowing in the 1713 cm−1 region is seen with the increase in heat treatment time. According to Wilhelm and Gardette [14], who studied polypropylene degradation through infrared spectroscopy, the dimer carboxylic acids absorb radiation at about 1710 cm−1 and ketones at about 1720 cm−1. So, narrowing of the band in the 1713 cm−1 absorption region confirms the conversion of the acid maleates to anhydride. The 1725 cm−1 band may be attributed to the degradation of poly-
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Fig. 2. Carbonyl index variation with heat treatment time (temperature of 130°C, under vacuum) of the non-purified sample.
Fig. 3. Infrared spectra of the non-purified samples submitted to heat treatment at 130°C for 96 h in a vacuum oven.
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propylene, not necessarily from the reactive processing, but rather from aging with storage. This will be confirmed at a later stage, when spectra of pure polypropylene processed in the torque rheometer are presented. According to studies by Wilhelm and Gardette [14] and Kelen [15], oxidative degradation of polypropylene may be accompanied through infrared analyses by the appearance of carbonyls from carboxylic acids, ketones, aldehydes and esters, in the range from 1690 to 1750 cm−1. From this item we may conclude that the samples submitted to reactive processing in the torque rheometer do not need purification. However, it is necessary to submit the films to heat treatment in a vacuum oven at 130°C for 24 h, to convert the acid groups of the reacted maleates to anhydride groups. Since reactive processing in the torque rheometer and in the twin-screw extruder are very different, the samples processed by reactive extrusion were also analysed to check whether purification was necessary. Therefore, the sample prepared by reactive extrusion was purified by dissolving in xylene and precipitation in acetone, placed in a vacuum oven to extract solvent, hot pressed and heat treated at 130°C for 24 h to convert the acid groups to anhydrides. This sample was compared with the non-purified sample, hot pressed and submitted to the same heat treatment as before. The results are presented in Fig. 4. As can be seen in Fig. 4, there is a significant difference between the purified and non-purified samples prepared by reactive extrusion. This means that the amount of unreacted maleic anhy-
Fig. 4. Effect of purification on the carbonyl index of the sample processed by reactive extrusion.
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dride, but only incorporated into the polymer bulk, during reactive extrusion, is much higher than in processing in the torque rheometer. This behavior is due to the fact that the process in the extruder is a closed process, where the pellets, proceeding from the dosing apparatus, are transported to the extruder funnel through a plastic pipe, and nitrogen passes through the extruder, exiting at the end. No vacuum was applied to the extruder barrel, but a gap at the end of the barrel was maintained open, through which residual maleic anhydride could leave the polymer bulk. As at this gap the pressure is atmospheric, the efficiency of anhydride extraction is low, which explains the large amount of incorporated maleic anhydride. It is, therefore, necessary to purify the samples processed by reactive extrusion. In order to ensure the efficiency of the purification performed after extrusion, the extruded samples, without purification and purified by dissolution in xylene and precipitation in acetone, were submitted to thermogravimetric analysis. The results of these analyses are presented in Fig. 5. As maleic anhydride has a boiling point of 202°C, the weight loss was measured at this temperature. The percentage weight loss for the non-purified and purified sample was 2.064 and 0.022%, respectively. These values show that the purification methodology employed was efficacious in the removal of unreacted maleic anhydride. 3.2. Analysis of the quantification methodologies of reacted maleic anhydride As previously discussed, one of the major difficulties in studies on the grafting of maleic anhydride onto polypropylene is its characterization, and especially, the quantification of reacted maleic anhydride. This analysis should be carried out after having checked for the necessity to
Fig. 5. Thermogravimetric analysis of the purified and non-purified samples processed by reactive extrusion.
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purify the samples, because as was seen in the previous section, this depends on the process employed. In our investigation we used the titrating of acid groups developed by Gaylord and Mishra [6], as well as FTIR. Each technique has advantages and disadvantages. One should therefore be judicious on deciding which to select. The titration proposed by Gaylord and Mishra [6] assumes that the acid groups of the sample analysed are originated only from the conversion of the anhydrides, that reacted with polypropylene, to acids. For FTIR analysis one should have ensured that all the reacted maleic anhydride is in the anhydride state. 3.2.1. Titration of acid groups The main advantage of the titration technique is that it is an absolute measurement, i.e. quantification of reacted maleic anhydride is determined straightforwardly, without the need for standards. However, some difficulties exist in executing this technique, which may make utilization impracticable, such as: 쐌 Difficulty in visualizing the transition point. Two indicators were tested: thymol blue in DMF and phenolphtalein. 쐌 Alteration of the time elapsed between titration with KOH and back titration with hydrochloric acid alters the result. Besides the procedure difficulties of this titration, an important factor regarding the reliability of this technique is the guarantee that the titrated groups are originated only from the conversion of anhydrides to acids. The existence of other acid groups, for instance, originated from polypropylene degradation, leads to errors when analysing the results. Titrations were first carried out with the samples tested for the purification analysis. The results are presented in Table 1. Analysis of the titration results, performed for the several purification methods and for the nonpurified sample, showed that the amount of grafted maleic anhydride is much higher in relation to the results presented by other authors for similar peroxide and maleic anhydride levels. It was, therefore, decided to submit these samples to FTIR analysis for a better comprehension of the results.
Table 1 Results of the titrations performed with purified and non-purified samples Samples
Percentage of maleic anhydride
Non-purified PPT Heat treatment 6 h (TT6) Heat treatment 12 h (TT12) Soxhlet 7 h 30 min (SOX1) Soxhlet 12 h (SOX2)
0.96 0.76 0.77 0.87 0.98 1.05
PP/MA/peroxide: 100/5/0.06 phr.
⫾ ⫾ ⫾ ⫾ ⫾ ⫾
0.014 0.062 0.024 0.006 0.003 0.047
12
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Fig. 6 presents the spectra of the non-purified samples, processed in the Haake torque rheometer, of: pure polypropylene; polypropylene and peroxide; and polypropylene, peroxide and maleic anhydride. All these spectra present a peak in the 1690 to 1750 cm−1 region, characteristic of carbonyls. As mentioned before, carboxylic acid carbonyls absorb energy in the 1713 cm−1 region in the infrared spectra, which allow us to state that acid groups, probably originated from the degradation of polypropylene, are present in the samples. Therefore, the amounts of maleic anhydride evidenced through titration, may be justified by the presence of other acid groups in the samples analysed. We concluded that in our study measuring the amount of grafted maleic anhydride should not be done by means of titration, as this may lead to error, since other acid groups are present in the grafted material, not only originated from the reacted maleic anhydrides. 3.2.2. FTIR Spectroscopy In virtue of the difficulties encountered in the titration of acid groups, discussed previously, FTIR analysis was tested. Quantification of maleic anhydride was performed through measurements of the carbonyl index (CI), as described in the experimental procedure. As this is a relative measurement a calibration curve should be constructed to obtain the absolute values of reacted maleic anhydride. The use of succinic anhydride as standard was tested. Since the temperature for mixing polypropylene with succinic anhydride had to be higher than 160°C, to accomplish fusion of the polymer, on encountering the rotors sublimation of anhydride was observed. This phenomenon was confirmed by FTIR analyses, of which the spectra did not present carbonyl peaks in the region close to 1790 cm−1.
Fig. 6.
FTIR spectra of the non-purified samples processed in the Haake torque rheometer.
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To overcome this problem a succinic anhydride bound to a hydrocarbon chain was used, increasing in this way its boiling point, and consequently avoiding sublimation at the blend temperature. The anhydride used for calibration was dodecenyl succinic anhydride. The spectrum of one of the PP/dodecenyl succinic anhydride blends is shown in Fig. 7. It can be seen that the absorption bands of the standard and of the samples grafted with MA coincide. Utilization of this anhydride for calibration is, therefore, appropriate. The calibration curve is presented in Fig. 8. The results obtained through FTIR could be fitted by a straight line passing through zero with correlation coefficient of 0.99986. The results obtained through FTIR and calibration curve are presented in Table 2 (purified and non-purified samples grafted in the torque rheometer submitted to heat treatment in a vacuum oven at 130°C for 24 h). In some works calibration curves have been constructed by titrating the samples submitted to infrared spectroscopy. This procedure is valid when one has ascertained that the acid groups in the sample are originated solely from the conversion of the anhydride groups. The advantage of the calibration curve proposed in our study is that, purifying when necessary and converting the acid groups to anhydrides, the amount of reacted maleic anhydride can be measured for any PP-g-MA sample. 4. Conclusion This investigation showed that PP-g-MA samples obtained by different types of reactive processing should be analysed to check for the necessity of purification to eliminate residual maleic
Fig. 7. FTIR spectrum of a PP/dodecenyl succinic anhydride blend and of a PP-g-MA obtained by reative extrusion.
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Fig. 8.
Calibration curve for the determination of the percentage of reacted maleic anhydride.
Table 2 Results obtained through FTIR and calibration curve Samples Non-purified PPT Heat treatment 6 h (TT6) Heat treatment 12 h (TT12) Soxhlet 7 h 30 min (SOX1) Soxhlet 12 h (SOX2)
A1790 A1167 0.06597 0.05933 0.06798 0.06963 0.06983 0.06587
⫾ ⫾ ⫾ ⫾ ⫾ ⫾
Percentage of anhydride maleic 0.0016 0.0070 0.0059 0.0029 0.0028 0.014
0.37 0.33 0.38 0.39 0.39 0.38
⫾ ⫾ ⫾ ⫾ ⫾ ⫾
0.009 0.039 0.033 0.016 0.016 0.078
Purified and non-purified samples grafted in the torque reometer submitted to heat treatment in a vacuum oven at 130°C for 24 h.
anhydride. In our studies the samples processed in the Haake torque rheometer did not need purification due to the open conduction process. In the case of reactive extruding purification of the samples was required. Titration of acid groups should only be employed when one has ascertained that all acid groups present are originated solely from the conversion of reacted anhydride groups. For FTIR analysis the samples should be submitted to heat treatment to convert the acid groups to anhydrides. This treatment should be conducted in a vacuum oven at 130°C for at least 24 h. The calibration curve, constructed using dodecenyl succinic anhydride was shown to be adequate to quantify maleic anhydride in absolute terms.
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Acknowledgements This research was supported in part by Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES) and in part by Polibrasil S.A. Indu´stria e Come´rcio. The authors thank Professor Dr Dilson Cardoso and Professor Dr Luı´s Fernando de Moura of the Chemical Engineering Department (UFSCar). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Lambla M. Congresso Brasileiro de Polı´meros, vol. 1. Sa˜o Paulo, 5–7 November 1991. p. 474–481. Laguna O, Vigo JP, Taranco J, Oteo JL, Collar EP. Rev. Plast. Mod. 1988;56(390):878–84. Singh RP. Prog. Polym. Sci. 1992;17:251–81. Khunova V, Zamorsky Z. Polym.-Plast. Technol. Eng. 1993;32(4):289–98. Callais PA, Kazmierczak RT. ANTEC, Dallas, TX, 7–11 May 1990. Gaylord NG, Mishra MK. J. Polym. Sci.: Polym. Lett. 1983;21:23–30. Brown SB. In: Xanthos M, editor. Reactive extrusion: principles and practice. New York: Oxford University Press, 1992. p. 75–199. Kozel HT, Kazmierczak RT. In: ANTEC, vol. 49. Proceedings. Montreal, 1991. Hogt A. In: ANTEC. Proceedings. 1988. p. 1478–1480. Ho RM, Su AC, Wu CH, Chen SI. Polymer 1993;34(15):3264–9. Fujiyama M, Wakino T, Wachi H, Tani K. Kobunshi Ronbunshu 1992;49(2):87–95. Fujiyama M, Wakino T, Wachi H, Tani K. Kobunshi Ronbunshu 1992;49(2):97–104. Sathe SN, Rao GSS, Devi S. J. Appl. Polym. Sci. 1994;53:239–45. Wilhelm C, Gardette JL. J. Appl. Polym. Sci. 1994;51:1411–20. Kelen T. Polymer Degradation. New York: Van Nostrand Reinhold, 1983 [211 pp.].
Test Method
Evaluation of methods used for analysing maleic anhydride grafted onto polypropylene by reactive processing S.H.P. Bettini, J.A.M. Agnelli* Departamento de Engenharia de Materiais da Universidade Federal de Sa˜o Carlos, Rodovia Washington Luiz, Km 235, Caixa Postal 676, 13.565-905 Sa˜o Carlos, Sa˜o Paulo, Brazil Received 20 June 1998; accepted 18 August 1998
Abstract Grafting of maleic anhydride onto polypropylene, in the presence of peroxides, was performed through reactive processing. The samples obtained were submitted to several analyses in order to check for conversion of the acid groups to anhydrides and whether purification was necessary. Quantification of reacted maleic anhydride was tested by titration of acid groups and Fourier transform infrared spectroscopy (FTIR). It was concluded that each type of processing requires verification of the necessity to purify the samples for removal of residual maleic anhydride. Spectroscopy was shown to be better for the quantification of reacted maleic anhydride, as long as the samples are submitted to thermal treatment at 130°C for at least 24 h. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Grafting of maleic anhydride onto polypropylene, in the presence of peroxides, in the melt has attracted a lot of commercial interest. These reactions have been conducted through reactive processing in continuous or batch mixing equipment that operate as chemical reactors [1]. The role of maleic anhydride functionalized polypropylene is to promote compatibilization between polar and apolar polymers, coupling between inorganic fillers and polypropylene as well as adhesion to metals [2–4]. According to several studies [5–7] graft polymerization of maleic anhydride onto polypropylene * Corresponding author. E-mail: [email protected] 0142-9418/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 6 6 - X
4
S.H.P. Bettini, J.A.M. Agnelli / Polymer Testing 19 (2000) 3–15
in the melt, in the presence of peroxides, is accompanied by severe degradation of the polypropylene. Despite ease of operation and low costs involved in such processes, studies concerning maleic anhydride grafting onto polypropylene have presented difficulties in analysing the reacted maleic anhydride. Straightforward measurement of bound maleate is complicated by several factors, such as the presence of unreacted maleic anhydride in the reaction mass and the fact that the grafted succinic moieties may be in the acid or anhydride form [8]. Several analyses have been suggested, such as, titration [5,6,8,9], FTIR [8–12] and gravimetry [13]. Distinguished titration methods exist, namely: potentiometric titration [9], titration of a solution of maleic anhydride grafted polypropylene (PP-g-MA) dissolved in hot xylene by a base in non-aqueous medium [8] and titration of a PP-g-MA solution dissolved in water saturated xylene by a base in non-aqueous medium [6]. However, in order for the reacted maleic anhydride to be analysed by any of the aforementioned methods, it is required to verify if purification of the modified polypropylene is necessary for the removal of the unreacted maleic anhydride in the polymer mass. Several purification methods have been proposed: dissolution of modified polypropylene in xylene followed by precipitation in acetone [5,8], Soxhlet extraction with acetone [11–13] and evaporation of the maleic anhydride in a vacuum oven at temperatures above 100°C [11,12]. The scope of the current work was to test the several purification methods for removal of residual anhydride and analysis of the reacted maleic anhydride, show their advantages and drawbacks as well as to propose a more adequate methodology to analyse the reacted anhydride. 2. Experimental procedure 2.1. Materials Isotactic polypropylene was supplied by Polibrasil S.A. Indu´stria e Come´rcio (JE-6100) with melt flow index of 2.0 g/10 min. The maleic anhydride was supplied by Carbocloro Oxypar Indu´strias Quı´micas S.A., and the peroxide selected for this study was a 46.5% concentrate of 2,5dimethyl-2,5-di(t-butylperoxy)hexane in CaCO3, supplied by Elf Atochem Brasil Quı´mica Ltda. (Luperox 101 XL). 2.2. Reactive processing In order to investigate the effect of the type of processing on the analysis of reacted maleic anhydride the PP-g-MAs were processed in a torque rheometer and in a twin-screw extruder. The equipment used was a Haake System 90 torque rheometer, equipped with a Rheomix 600 mixing compartment and a Werner Pfleiderer ZSK-25 extruder. The processing conditions used in the rheometer were the following: process temperature: 180°C, rotor speed: 40 rpm, processing time: 15 min and nitrogen atmosphere. The concentrations used were 100/5/0.06 phr for polypropylene/maleic anhydride/peroxide, respectively. The reactive extrusion process was performed using the following conditions: temperature profiles: 180, 200, 200, 210, 210, 210 and 200°C; screw speed: 150 rpm and nitrogen atmosphere.
S.H.P. Bettini, J.A.M. Agnelli / Polymer Testing 19 (2000) 3–15
The following concentrations anhydride/peroxide, respectively.
were
used
100/4/0.1
phr
for
5
polypropylene/maleic
2.3. Purification methods 2.3.1. Solubilization in xylene and precipitation in acetone (PPT) A total of 4 g PP-g-MA was dissolved in 400 ml xylene under reflux at 130°C for 1 h. The temperature was lowered to 45°C and acetone was added. The precipitate was vacuum filtered and washed several times with acetone. The sample was then left in a vacuum oven for solvent removal. 2.3.2. Soxhlet extraction with acetone The PP-g-MA samples were placed in a Soxhlet for maleic anhydride extraction with acetone for 7 h and 30 min (SOX1) and 12 h (SOX2). Next, the samples were placed in a vacuum oven for acetone removal. 2.3.3. Heat treatment at 130°C Pellet samples of PP-g-MA were placed in a vacuum oven at 130°C for 6 h (TT6) and 12 h (TT12). The purified and non-purified samples were hot pressed at 200°C and analysed by FTIR. The same samples, however, without hot pressing, were titrated for analysis of reacted maleic anhydride. The non-purified samples were submitted to heat treatment for 96 h at 130°C, in order to verify conversion of the acid groups to anhydride. Samples were withdrawn every 24 h of heat treatment and analysed by FTIR. The formulations processed in the torque rheometer were submitted to all aforementioned purification methodologies, whereas the formulation processed in the extruder was only submitted to purification by dissolving in xylene and precipitation in acetone. In order to verify the efficiency of purifying of the samples processed by reactive extrusion thermogravimetric analyses were performed on the purified and non-purified samples. The equipment used was a TGA 2050 thermogravimetric analyser. Analysis conditions were: heating from 30 to 550°C at a heating rate of 10°C/min in nitrogen atmosphere. 2.4. Quantification of reacted anhydride 2.4.1. Titration Determination of reacted maleic anhydride via titration was carried out based on the technique developed by Gaylord and Mishra [6]. Titrations were performed in a 665 Dosimat Metrohm Dosing. The procedure adopted was as follows: 2 g PP-g-MA was dissolved in 400 ml xylene saturated with water. Aliquots of 100 ml were transferred to an Erlenmeyer flask and titrated with 0.05 N ethanolic potassium hydroxide (KOH), using thymol blue in dimethylformamide (DMF) as indicator. As soon as the solution became dark blue it was back titrated to a yellow end-point by the addition of 0.05 N isopropanolic hydrochloric acid (HCl).
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2.4.2. FTIR The hot pressed films were submitted to transmitting infrared analysis in a Nicolet Magna IR750 spectrophotometer with resolution of 2 cm−1 and 128 scans per spectrum. The wavelengths of interest were 1713 cm−1, characteristic of carbonyls from carboxylic dimer acids, 1790 and 1867 cm−1, characteristic of five-membered cyclic anhydride carbonyls, and 1167 cm−1, characteristic of CH3 groups. Using these bands the carbonyl index (CI) was calculated: CI ⫽
A1790 A1167
where A1790 is absorbance at 1790 cm−1, characteristic of carbonyls from five-membered cyclic anhydrides; A1167 is absorbance at 1167 cm−1, characteristic of CH3 groups, proportional to the amount of PP. As this is a relative measurement of the amount of reacted maleic anhydride, a calibration curve should be constructed. 2.4.3. Calibration curves Two types of anhydrides were tested to construct the calibration curve: succinic anhydride and dodecenyl succinic anhydride. The polypropylene and anhydride blends were prepared in the Haake torque rheometer and analysed by FTIR at the following conditions: blending temperature of 180°C, rotation speed of 30 rpm and mixing time of 5 min.
3. Results 3.1. Analysis of the purification methodologies and conversion of the acid groups to anhydrides In order to check for the necessity to purify the PP-g-MA samples prepared by reactive processing the samples grafted in the torque rheometer were submitted to several purification methodologies and analysed by FTIR. The results are presented in Fig. 1. As shown by Fig. 1 the CI of the non purified samples and those purified by the several methodologies presented little variation. Visual observation of the graph does not allow to make any statement on these differences. Therefore, statistical techniques were used to compare the averages through variance analysis and Tukey test for a significance level of 5%. The statistical analyses showed that there are no significant differences between the non-purified samples and those purified by the several methodologies. This may be explained by the fact that, for the system used in the torque rheometer, the major part of the residual anhydride must have been evaporated and removed together with N2, since the punner that seals off the system is only lowered on loading and N2 enters and leaves the system through an adapted system. Since the films analysed by infrared spectroscopy were hot pressed at 200°C some amount of unreacted maleic anhydride may have been eliminated, as this temperature is very close to the boiling point of maleic anhydride (202°C). From this analysis it was concluded that, for the system under investigation, purification of the samples tested in the Haake rheometer was not necessary.
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Fig. 1. Comparison of the purification methods through carbonyl index measurements.
After analysis of the purification methodologies, the following stage was to verify whether the samples required heat treatment in a vacuum oven for the conversion of possible acid groups, resulting from the ring opening of the anhydride, to anhydrides. Therefore, the non-purified samples were placed in a vacuum oven for 96 h and analysed by FTIR every 24 h. Conversion of dicarboxylic acids to anhydrides was monitored by the variation of the absorbance at 1713 cm−1 and 1790 cm−1, characteristic of the acid carbonyls and anhydride carbonyls, respectively. The results of this heat treatment are presented in Figs. 2 and 3. As can be seen in Figs. 2 and 3 the amount of carbonyls increased significantly due to the anhydride groups, when the samples were submitted to heat treatments of 24 h. This increase might be justified by the conversion of acid groups to anhydrides. Once again statistical tests were performed to check whether the differences in heat treatment times were significant or not for the non-purified samples. These tests showed that the differences are significant for the samples with and without heat treatment. The averages did not present significant differences, at a significance level of 5%, for heat treatments from 24 to 96 h. Acid group conversion to anhydrides was confirmed through infrared spectra, presented in Fig. 3, where one may observe the increase in the cyclic anhydride carbonyl band at 1790 cm−1, when the sample is submitted to heat treatment. In the case of the 1725 cm−1 band, narrowing in the 1713 cm−1 region is seen with the increase in heat treatment time. According to Wilhelm and Gardette [14], who studied polypropylene degradation through infrared spectroscopy, the dimer carboxylic acids absorb radiation at about 1710 cm−1 and ketones at about 1720 cm−1. So, narrowing of the band in the 1713 cm−1 absorption region confirms the conversion of the acid maleates to anhydride. The 1725 cm−1 band may be attributed to the degradation of poly-
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Fig. 2. Carbonyl index variation with heat treatment time (temperature of 130°C, under vacuum) of the non-purified sample.
Fig. 3. Infrared spectra of the non-purified samples submitted to heat treatment at 130°C for 96 h in a vacuum oven.
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propylene, not necessarily from the reactive processing, but rather from aging with storage. This will be confirmed at a later stage, when spectra of pure polypropylene processed in the torque rheometer are presented. According to studies by Wilhelm and Gardette [14] and Kelen [15], oxidative degradation of polypropylene may be accompanied through infrared analyses by the appearance of carbonyls from carboxylic acids, ketones, aldehydes and esters, in the range from 1690 to 1750 cm−1. From this item we may conclude that the samples submitted to reactive processing in the torque rheometer do not need purification. However, it is necessary to submit the films to heat treatment in a vacuum oven at 130°C for 24 h, to convert the acid groups of the reacted maleates to anhydride groups. Since reactive processing in the torque rheometer and in the twin-screw extruder are very different, the samples processed by reactive extrusion were also analysed to check whether purification was necessary. Therefore, the sample prepared by reactive extrusion was purified by dissolving in xylene and precipitation in acetone, placed in a vacuum oven to extract solvent, hot pressed and heat treated at 130°C for 24 h to convert the acid groups to anhydrides. This sample was compared with the non-purified sample, hot pressed and submitted to the same heat treatment as before. The results are presented in Fig. 4. As can be seen in Fig. 4, there is a significant difference between the purified and non-purified samples prepared by reactive extrusion. This means that the amount of unreacted maleic anhy-
Fig. 4. Effect of purification on the carbonyl index of the sample processed by reactive extrusion.
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dride, but only incorporated into the polymer bulk, during reactive extrusion, is much higher than in processing in the torque rheometer. This behavior is due to the fact that the process in the extruder is a closed process, where the pellets, proceeding from the dosing apparatus, are transported to the extruder funnel through a plastic pipe, and nitrogen passes through the extruder, exiting at the end. No vacuum was applied to the extruder barrel, but a gap at the end of the barrel was maintained open, through which residual maleic anhydride could leave the polymer bulk. As at this gap the pressure is atmospheric, the efficiency of anhydride extraction is low, which explains the large amount of incorporated maleic anhydride. It is, therefore, necessary to purify the samples processed by reactive extrusion. In order to ensure the efficiency of the purification performed after extrusion, the extruded samples, without purification and purified by dissolution in xylene and precipitation in acetone, were submitted to thermogravimetric analysis. The results of these analyses are presented in Fig. 5. As maleic anhydride has a boiling point of 202°C, the weight loss was measured at this temperature. The percentage weight loss for the non-purified and purified sample was 2.064 and 0.022%, respectively. These values show that the purification methodology employed was efficacious in the removal of unreacted maleic anhydride. 3.2. Analysis of the quantification methodologies of reacted maleic anhydride As previously discussed, one of the major difficulties in studies on the grafting of maleic anhydride onto polypropylene is its characterization, and especially, the quantification of reacted maleic anhydride. This analysis should be carried out after having checked for the necessity to
Fig. 5. Thermogravimetric analysis of the purified and non-purified samples processed by reactive extrusion.
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purify the samples, because as was seen in the previous section, this depends on the process employed. In our investigation we used the titrating of acid groups developed by Gaylord and Mishra [6], as well as FTIR. Each technique has advantages and disadvantages. One should therefore be judicious on deciding which to select. The titration proposed by Gaylord and Mishra [6] assumes that the acid groups of the sample analysed are originated only from the conversion of the anhydrides, that reacted with polypropylene, to acids. For FTIR analysis one should have ensured that all the reacted maleic anhydride is in the anhydride state. 3.2.1. Titration of acid groups The main advantage of the titration technique is that it is an absolute measurement, i.e. quantification of reacted maleic anhydride is determined straightforwardly, without the need for standards. However, some difficulties exist in executing this technique, which may make utilization impracticable, such as: 쐌 Difficulty in visualizing the transition point. Two indicators were tested: thymol blue in DMF and phenolphtalein. 쐌 Alteration of the time elapsed between titration with KOH and back titration with hydrochloric acid alters the result. Besides the procedure difficulties of this titration, an important factor regarding the reliability of this technique is the guarantee that the titrated groups are originated only from the conversion of anhydrides to acids. The existence of other acid groups, for instance, originated from polypropylene degradation, leads to errors when analysing the results. Titrations were first carried out with the samples tested for the purification analysis. The results are presented in Table 1. Analysis of the titration results, performed for the several purification methods and for the nonpurified sample, showed that the amount of grafted maleic anhydride is much higher in relation to the results presented by other authors for similar peroxide and maleic anhydride levels. It was, therefore, decided to submit these samples to FTIR analysis for a better comprehension of the results.
Table 1 Results of the titrations performed with purified and non-purified samples Samples
Percentage of maleic anhydride
Non-purified PPT Heat treatment 6 h (TT6) Heat treatment 12 h (TT12) Soxhlet 7 h 30 min (SOX1) Soxhlet 12 h (SOX2)
0.96 0.76 0.77 0.87 0.98 1.05
PP/MA/peroxide: 100/5/0.06 phr.
⫾ ⫾ ⫾ ⫾ ⫾ ⫾
0.014 0.062 0.024 0.006 0.003 0.047
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Fig. 6 presents the spectra of the non-purified samples, processed in the Haake torque rheometer, of: pure polypropylene; polypropylene and peroxide; and polypropylene, peroxide and maleic anhydride. All these spectra present a peak in the 1690 to 1750 cm−1 region, characteristic of carbonyls. As mentioned before, carboxylic acid carbonyls absorb energy in the 1713 cm−1 region in the infrared spectra, which allow us to state that acid groups, probably originated from the degradation of polypropylene, are present in the samples. Therefore, the amounts of maleic anhydride evidenced through titration, may be justified by the presence of other acid groups in the samples analysed. We concluded that in our study measuring the amount of grafted maleic anhydride should not be done by means of titration, as this may lead to error, since other acid groups are present in the grafted material, not only originated from the reacted maleic anhydrides. 3.2.2. FTIR Spectroscopy In virtue of the difficulties encountered in the titration of acid groups, discussed previously, FTIR analysis was tested. Quantification of maleic anhydride was performed through measurements of the carbonyl index (CI), as described in the experimental procedure. As this is a relative measurement a calibration curve should be constructed to obtain the absolute values of reacted maleic anhydride. The use of succinic anhydride as standard was tested. Since the temperature for mixing polypropylene with succinic anhydride had to be higher than 160°C, to accomplish fusion of the polymer, on encountering the rotors sublimation of anhydride was observed. This phenomenon was confirmed by FTIR analyses, of which the spectra did not present carbonyl peaks in the region close to 1790 cm−1.
Fig. 6.
FTIR spectra of the non-purified samples processed in the Haake torque rheometer.
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To overcome this problem a succinic anhydride bound to a hydrocarbon chain was used, increasing in this way its boiling point, and consequently avoiding sublimation at the blend temperature. The anhydride used for calibration was dodecenyl succinic anhydride. The spectrum of one of the PP/dodecenyl succinic anhydride blends is shown in Fig. 7. It can be seen that the absorption bands of the standard and of the samples grafted with MA coincide. Utilization of this anhydride for calibration is, therefore, appropriate. The calibration curve is presented in Fig. 8. The results obtained through FTIR could be fitted by a straight line passing through zero with correlation coefficient of 0.99986. The results obtained through FTIR and calibration curve are presented in Table 2 (purified and non-purified samples grafted in the torque rheometer submitted to heat treatment in a vacuum oven at 130°C for 24 h). In some works calibration curves have been constructed by titrating the samples submitted to infrared spectroscopy. This procedure is valid when one has ascertained that the acid groups in the sample are originated solely from the conversion of the anhydride groups. The advantage of the calibration curve proposed in our study is that, purifying when necessary and converting the acid groups to anhydrides, the amount of reacted maleic anhydride can be measured for any PP-g-MA sample. 4. Conclusion This investigation showed that PP-g-MA samples obtained by different types of reactive processing should be analysed to check for the necessity of purification to eliminate residual maleic
Fig. 7. FTIR spectrum of a PP/dodecenyl succinic anhydride blend and of a PP-g-MA obtained by reative extrusion.
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Fig. 8.
Calibration curve for the determination of the percentage of reacted maleic anhydride.
Table 2 Results obtained through FTIR and calibration curve Samples Non-purified PPT Heat treatment 6 h (TT6) Heat treatment 12 h (TT12) Soxhlet 7 h 30 min (SOX1) Soxhlet 12 h (SOX2)
A1790 A1167 0.06597 0.05933 0.06798 0.06963 0.06983 0.06587
⫾ ⫾ ⫾ ⫾ ⫾ ⫾
Percentage of anhydride maleic 0.0016 0.0070 0.0059 0.0029 0.0028 0.014
0.37 0.33 0.38 0.39 0.39 0.38
⫾ ⫾ ⫾ ⫾ ⫾ ⫾
0.009 0.039 0.033 0.016 0.016 0.078
Purified and non-purified samples grafted in the torque reometer submitted to heat treatment in a vacuum oven at 130°C for 24 h.
anhydride. In our studies the samples processed in the Haake torque rheometer did not need purification due to the open conduction process. In the case of reactive extruding purification of the samples was required. Titration of acid groups should only be employed when one has ascertained that all acid groups present are originated solely from the conversion of reacted anhydride groups. For FTIR analysis the samples should be submitted to heat treatment to convert the acid groups to anhydrides. This treatment should be conducted in a vacuum oven at 130°C for at least 24 h. The calibration curve, constructed using dodecenyl succinic anhydride was shown to be adequate to quantify maleic anhydride in absolute terms.
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Acknowledgements This research was supported in part by Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES) and in part by Polibrasil S.A. Indu´stria e Come´rcio. The authors thank Professor Dr Dilson Cardoso and Professor Dr Luı´s Fernando de Moura of the Chemical Engineering Department (UFSCar). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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