Melt Free Radical Grafting of an Oxazoline Compound onto HDPE

Transcription

1 Bulg. J. Phys. 32 (2005) Melt Free Radical Grafting of an Oxazoline Compound onto HDPE T. L. Dimitrova 1, C. Colletti 2, F. P. La Mantia 2 1 University of Plovdiv Paissi Hilendarski, Tzar Assen Str. 24, 4000 Plovdiv, Bulgaria 2 Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze Palermo, Italy Received 23 August 2004 Abstract. Graft copolymers of polyolefines are widely used as compatibilizers in blends with engineering plastics. Although there are many polymers containing acid reactive groups, only a few polymers functionalized with basic groups have been used, because of their toxicity, difficulties in preparation, etc. The oxazoline group is a basic group, which presents high reactivity towards many other functional groups, such as acids, anhydrides, amines, etc. For this reason it is very suitable for the compatibilization of polymer alloys [1]. The aim of this work is to functionalize a high density polyethylene (HDPE) by grafting an oxazoline group ended compound, the ricinoloxazoline maleinate (OXA). The grafting reactions were carried out in an internal mixer. Many samples were made using various operative conditions. FTIR spectral analysis on the modified samples confirms the effectiveness of the grafting reaction. Two large bands at about 1730 and 1640 cm 1, characteristic of the oxazoline ring resonance, are present [2]. Their presence in the spectrum of the modified polymers was confirmed. The influence of operative conditions on the grafting yield was qualitatively evaluated using the same technique. The modified polyethylene was used as a compatibilizing agent in a previous work. PACS number: x; Bb 1 Introduction In recent years, increasing activities have been directed towards chemical modifications of existing polymers in order to obtain functional and/or engineering new materials [3,4]. The chemical modifications of existing polymers are important for at least two reasons: 1) they can be an inexpensive and rapid way of c 2005 Heron Press Ltd.

2 Melt Free Radical Grafting of an Oxazoline Compound onto HDPE obtaining new polymers without having to search for new monomers; 2) sometimes they may be the only way to synthesize polymer with end of use fitted properties. Among all possible chemical modification methods, free-radical grafting is probably the oldest, widely practiced and the most inexpensive one. Further advantages are gained with the use of batch mixers or screw extruders as chemical reactors, which allow the free-radical grafting reaction to occur without solvents [5]. As a chemical modification method, the free-radical grafting has been used to promote functional and/or mechanical properties of various types of polymers, in particular polyolefines, such as polyethylene (PE) and polypropylene (PP). These two polymers possess the most important volume share of the plastics industry due to their low cost, versatile properties and growing commercial applications. Although there are a number of commercially available polymers containing acid reactive groups [6], such as MA-grafted EPDM (Du Pont), SEBS (Shell), PP (Himont) and carboxylic-acid-functionalized polyethylene (Dow Chemical), PP (BP Performance Polymers), and poly(butadiene-co-acrilonitrile-co-acrylic acid (NBR) (Novacor), a few commercially available polymers purposely functionalized with basic reactive groups have been reported [7]. This is partly a result of the scarcity of suitable basic reactive monomers because of their toxicity, difficulties in preparation and handing, instabilities, cost-effectiveness, etc. However, there have been developmental polymers containing basic functionality. A well-known example of these is poly(styrene-co-vinyl oxazoline) (OPS) introduced by Dow Chemical. The introduction of OPS started an interesting period of research on the utilization of oxazoline functionality in the reactive compatibilization of polymer blends. Research efforts have also been made on the functionalizing of polymers with oxazolines and their use in interfacial reaction with other functional polymers. The aim of this work is to functionalize a high density polyethylene (HDPE) by grafting an oxazoline group ended compound, the ricinoloxazoline maleinate (OXA) and then use the funzionalized polymers as a compatibilizers in blends with polyetyleneterephtalate (PET). 2 Experimental 2.1 Materials and Preparation The material used in this work was a high density polyethylene, (HDPE, MFI = 6.5, density 96 kg/m 3 ) kindly supplied by Polimeri Europa, Italy. The functionalizing agent was a monomer containing some double bonds and an oxazoline end-group, Loxamid V-EP8515 (ricinoxazoline maleinated), kindly supplied by 205

3 T.L. Dimitrova, C. Colletti, F.P. La Mantia Figure 1. Chemical formula of the oxazoline monomer. Henkel KgaA. This substance is in the liquid state at room temperature and shows a low volatility (its boiling point is 250 C at the pressure of 0.1 mbar) and will be referred to in this work as OXA. Its chemical structure is shown on Figure 1. To increase the grafting reaction kinetic of OXA onto the polyethylene in some cases dicumil peroxide (dcp) was used as radical initiator. It has a medium time of life t 1/2 = 60 s at a temperature of 180 C. In this work was used a solid product, containing 40% by wt of dicumil peroxide, kindly supplied by Elf Atochem, Italy. 2.2 Blending Procedure and Samples Preparation The radical grafting reaction of HDPE with OXA was carried in a co-rotating batch mixer (Brabender Plasticorder mod. PLE330, capacity 50 cm 3 ). To examine the effects of processing conditions on the reaction effectiveness, different temperatures, screw speeds and processing times were used. Table 1 shows the name codes and the processing conditions of the samples. The samples were purified in boiling xylene for 30 min about and after that in acetone for elimination of the remaining non reacting oxazoline and other reaction products. Both films and sheets were prepared from the obtained modified polyethylenes in a molding press at a temperature of 180 C. 2.3 Characterization Mechanical properties were measured using an Instron machine mod Rheological tests were performed with a rheometer Rheometrics RDA II with parallel plates. The samples were cut from the sheets obtained by compression molding and the tests were performed at 270 C in the frequency range of s 1. A FTIR (Perkin Elmer) was used for the analysis of the spectra. The calorimetric analysis was performed using a DSC7 Perkin Elmer technique. 206

4 Melt Free Radical Grafting of an Oxazoline Compound onto HDPE Table 3.1. Codes Name and processing conditions of the samples. The symbols have got the next meanings: H HDPE; Hg HDPE-g-OXA. The number after the symbol means the type of the variable (1 without peroxide; 2 with dcp; 3 different speed of mixing; 4 different temperature; 5 different quantity of dcp); and the number before the symbol is the value of the variable. Code of Time of the mixing Temperature OXA Speed of the Perosside the sample (min) ( C) (phr) mixing (rpm) dcp (phr) H 7H H H ,3 3Hg Hg Hg Hg Hg ,3 7Hg ,3 15Hg ,3 30Hg ,3 32Hg Hg Hg Hg Hg Hg Results and Discussion The kinetics of the grafting reaction was studied through analysis of the torque of the mixing, FTIR infrared spectra analysis and test of the solubility. 3.1 Mixing Procedure and Torque Analysis The following procedure of the mixing was applied: putting the polyethylene (45 g) in the camera at selected temperature; mixing on to the obtaining of a stable torque (about 1.5 min); adding of dcp (when needed); adding of the oxazoline in the side of the camera (time zero); From Figure 2 is seen that the reaction occurs very rapidly and completes in 7 10 minutes. The long mixing time results in a progressive degradation of both modifier and non modifier polymers. Adding the oxazoline monomer the torque 207

5 T.L. Dimitrova, C. Colletti, F.P. La Mantia Figure 2. Mixing torque of the samples HDPE verging and modified without dcp (samples 30H1 and 30Hg1) and the same materials in the presents of dcp (samples 30H2 and 30Hg2) at 180 C and 64 rpm. increases because a grafting reaction occurs. The increasing of the torque is more pronounced when peroxide is used. The dcp acts as a radical initiator, but also causes some entanglement of the polymer. The influence of the temperature (at 160 C, 180 C and 200 C) and of mixing speed (32 rpm, 64 rpm and 128 rpm) was also studied. The higher the processing temperature and the screw speed, the greater the thermal and the mechanical degradation of the modified polymers. 3.2 FTIR Spectral Analysis Two typical bands at 1730 cm 1 and 1640 cm 1, corresponding respectively to C=N bond and to the oxazoline s ring are observed in the oxazoline s spectra (sample 1 in Figure 3). As seen from the same figure, this bands of absorption are absent in the polyethylene s spectrum (sample 2). In the same time both bands typical for the oxazoline appears in the spectra of the modified polyethylenes (sample 3 and 4). These bands also present after the purification of the modified polymers (sample 3). This fact means that the presence of the oxazoline bands is due exclusively to the grafting of the oxazoline groups onto the polyethylene macromolecules. In Figure 4 is shown the influence of the FTIR spectra of the samples by the mixing duration (a), the temperature (b), and the mixing speed (c), and the time in the presence of the dcp initiator (d). To quantify the grafting yield, the intensity ratio of the oxazoline s band at 1730 cm 1 and 1640 cm 1 to polyethylene s peaks at 1370 cm 1 and 2200 cm 1 are used. The values of the reports are presented in the Table

6 Melt Free Radical Grafting of an Oxazoline Compound onto HDPE Figure 3. Comparison between the FTIR spectra of oxazoline monomer (OXA), HDPE (H) and the modified polyethylene purified (7Hg purified) and non purified (7Hg non purified). From this analysis it results that the quantity of the grafting oxazoline depends strongly on the processing condition. In particular, the ratio between the intensity of the peaks I 1730/ I 1370, I 1730/ I 2020, I 1640/ I 1370 and I 1640/ I 2020 increases with the mixing time. That indicates an increasing of the quantity of Table 3.2. The values of the rations of the typical oxazoline pikes and the typical pikes of the polyethilene. Code of the sample I 1730 I 1730 I 1640 I 1640 I 1730 I 1730 I 1370 I 2020 I 1370 I 2020 I 1640 I 1640 theoretical 7H1 0,06 3Hg1 0,47 1,10 0,22 0,51 2,16 2,03 7Hg1 0,58 1,29 0,26 0,58 2,25 2,12 15Hg1 0,67 1,49 0,29 0,65 2,29 2,2 30Hg1 1,1 2,5 0,41 0,93 2,7 2,62 3Hg2 0,74 1,66 0,21 0,46 3,63 7Hg2 0,78 2,04 0,20 0,52 3,9 15Hg2 0,89 2,21 0,20 0,50 4,44 30Hg2 1,16 2,3 0,20 0,53 4,34 32Hg3 0,48 1,22 0,22 0,56 2,19 1,92 128Hg3 1,07 2,57 0,39 0,94 2,74 2,67 160Hg4 0,54 1,22 0,25 0,26 2,19 1,92 200Hg4 0,86 1,96 0,34 0,77 2,55 2,47 3Hg5 0,45 0,99 0,21 0,47 2,11 9Hg5 0,43 1,05 0,19 0,46 2,28 209

7 T.L. Dimitrova, C. Colletti, F.P. La Mantia Figure 4. FTIR spectra of the samples at different times of the mixing (a), temperatures (b), speeds of the mixing (c) and times of mixing in the presence of the dcp initiator (d). the grafting oxazoline with the mixing time. We have to mention that the reports I 1730/ I 1370 and I 1730/ I 2020 increase more rapidly than the reports I 1640/ I 1370 and I 1640/ I It is, probably, due to the oxidation phenomenon of the polyethylene. This ipotesis is confirmed also from the fact that the report I 1730/ I 1640 increases also with the time, due to the same reason, but probably also of opening of the oxazoline s ring. Indeed, the value of the report I 1730/ I 1640 for the oxazoline monomer is For the verging polyethylene the value I 1730/ I 1370 is If the peak at 1730 cm 1 has a contribution due to the oxidation, it is possible to calculate the theoretical values of the report I 1730/ I 1640 for the modified polymers. Those values are presented in the Table 2 and there it is seen, they are all significantly higher than 1.45 and increase with the time of the mixing. As mentioned first, this fact can be explained by a probable opening of the oxazoline s ring. Because the long time of the mixing results in a bigger degradation of the polymer, confirmed also from the torque, and the reaction is completed in less than 10 minutes, a mixing time of 7 minutes was chosen to study the other processing parameters. The ratio between the intensity of the oxazoline peaks and the polyethylene 210

8 Melt Free Radical Grafting of an Oxazoline Compound onto HDPE peaks increases also with the increasing of the temperature and the speed of the mixing. But in the same time increase also the oxidation and degradation phenomenon. The phenomenon of the opening of the oxazoline s ring is also presented. A temperature of 180 C and a speed of mixing of 64 rpm and a mixing time of 7 minutes ware chosen as moderate processing parameters that permeate a good grafting reaction without big negative effects. The concentration of the OXA doesn t depend significantly on the quantity of the grating oxazoline. A concentration of 6 phr was found to be better. The presence of the peroxide initiator accelerates the grafting reaction for the low mixing time (less than 10 minutes). For the long mixing time the effect of the oxidation is significant. The reports corresponding to the peak I 1640 are quite constant, the reports corresponding to the peaks I 1730 increase with the mixing time. It is possible that an opening of the same part of the grafted oxazoline rings occurs, due to their reactions with the active groups, which are results from the oxidation. The entanglement of the polymer increases with the quantity of dcp, confirmed also from the increasing of the torque. The concentration of 0.3 phr was found to accelerate the grafting reaction with out significant entanglement. Starting from these results, the sample 7Hg1, obtained at a temperature of 180 C and screw speed of 64 rpm for a mixing time of 7 min in the absence of the peroxide, was found to be better. This sample was successfully used as a compatibilizer for the PET/HDPE blends. 3.3 Rheological Characterization In Figure 5 are demonstrated the flow curves of the HDPE verging (sample H), grafted (sample 7Hg1) and mixed in the same conditions without any additives (sample 7H1). The curve of the sample 7H1 is near to those of the verging polymer at low frequencies decreases slowly at high frequencies. This means that the effects of the thermo-mechanical degradation aren t significant. A different behavior was noticed for the modified polymer. The viscosity is higher than those of the sample 7H1 at low frequencies, but the non-newtonian behavior is more pronounced. This behavior is due to the grafting reaction. The oxazoline monomer consists of an aliphatic chain with about 20 carbon atoms containing insaturations and an oxazoline ring. During the reaction onto the chain of the polyethylene the ramifications with a medium length were formed and for this reason an increasing of the viscosity and a non-newtonian behavior was demonstrated. 211

9 T.L. Dimitrova, C. Colletti, F.P. La Mantia Figure 5. The viscosity of the samples H, 7H1 and 7Hg Calorimetric and Mechanical Characterization The DSC curves of the three samples are similar and there aren t any differences from the melting points and the heat of the melting. The main mechanical properties of the HDPE samples and of the modified polymer are comprised in Table 3. There isn t any significant difference of the mechanical properties. Table 3.3. Mechanical properties of the samples H, 7H1 and 7Hg. E (GPa) TS (MPa) EB (%) Sample min avg max min avg max min avg max H H Hg Conclusions An oxazoline modified polyethylene has been produced by melt free radical grafting in a batch mixer. The functionalizing agent was a low molecular weight compound (rinocsazoline maleinate, Loxamid V-EP 8515, by Henkel KGaA). The functionalized polymer has been purified to remove any trace of reactants and then analyzed by FTIR. Two peaks at 1670 cm 1 and 1725 cm 1, attributed to the oxazoline ring resonance, were chosen to demonstrate the presence of oxazoline. The results show that functionalization of the polyethylene can be achieved by melt blending with rinocsazoline maleinate even in the absence of free radical initiators. This minimizes the risk of gelification due to polyethylene 212

10 Melt Free Radical Grafting of an Oxazoline Compound onto HDPE crosslinking. The modified polyethylene is suitable as compatibilizing agent for a large spectrum of polymer blends. References [1] Liu, Baker (1996) Reactive Modifiers for Polymers, ed. S. Al-Malaika (Chapman & Hall, London). [2] R. Scaffaro, C. Colletti, F. P. La Mantia, A. Valenza, M. Paci, P.L. Maganini, S. Filippi, Proocedings of XIV Italian Conference of AIM, Sptember 1999, Salerno, Italy, p [3] M. Lambla (1993) Reactive processing of thermoplastic polymers, in Comprehensive Polymer Science 1 st supplement (eds G. Allen and J. C. Bevington), Pergamon, New York, ch. 21. [4] R. Scaffaro, C. Colletti, F.P. La Mantia, XIV Convegno Italiano di Scienza e tecnologia delle macromolecole, settembre 1999, Salerno, p [5] G.-H. Hu and M. Lambla, in Encyclopedia of Materials Science and Tecnology Vol. 18 (in press). [6] T.L. Dimitrova, F.P. La Mantia, F. Pilati, A. Valenza, A.M. Visco (2000) Polymer [7] Down Chemical Co. (1985) Reactive Polystyrene, product literature (Midland, MI). 213