Ionic liquids as additives for thermoplastics - CiteSeerX

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Ionic liquids as additives for thermoplastics K.I. Park and M. Xanthos* Material Science & Engineering Program and * Otto H. York Department of Chemical Engineering, New Jersey Institute of Technology, Newark, NJ Abstract In attempts to develop new process modifiers for thermoplastics, two ionic liquids with long chain hydrophobic cations and different anions were introduced in a biodegradable polymer. Methods of incorporation included melt blending, solvent casting and microencapsulation from w/o/w systems at concentrations up to 10 wt%. The modified polymers were characterized rheologically and by TGA to determine process and thermal stability, respectively, and by DSC to determine miscibility and types of the polymer-ionic liquid interactions. Potential applications in plasticization, lubrication and emulsification are discussed for selected polymer-ionic liquid combinations.

Introduction Plasticizers function by embedding themselves between the chains of polymers, space them apart, increasing the free volume, and thus significantly lowering the glass transition temperature of the polymer and making it softer. Some conventional plasticizers volatilize and tend to concentrate in enclosed spaces. Recently, ionic liquids (ILs) have been shown to be potentially environmentally-benign solvents due to their low volatility, low melting points, a broad temperature liquid range, high-temperature stablity, low flammablity and compatiblity with organic and inorganic materials [1]. So far, ILs have been mostly used as alternative solvents for chemical synthesis [2]. ILs have been recently evaluated as non-volatile plasticizers and as external or internal lubricants in several polymers including PVC [3], PMMA [4] and polyamides [5]. In this article, an amorphous PLA (polylactide) polymer is blended with two phosphoniumbased ILs at various ratios by melt-blending, solution casting and a microencapsulation technique in order to investigate polymer/IL miscibility and its role in the development of a modified polymer with novel rheological, mechanical and thermal characteristics. Results of thermogravimetric analysis, differential scanning calorimetry, capillary rheometry, batch mixer torque measurements and optical microscopy are discussed.

Experimental Materials

Polylactide polymer pellets 4060D were purchased from Natureworks®, Minnetonka, MN. Trihexyl tetradecyl phosphonium decanoate, [THTDP][DE], (IL-1), MW 655.13, Trihexyl tetradecyl phosphonium tetrafluoroborate, [THTDP][BF4], (IL-2), MW 570.68, Methylene chloride (CAS No 75-09-2) were obtained from Sigma-Aldrich. Polyvinyl Alcohol (98.8% fully hydrolyzed, CAS No 9002-89 -5) was purchased from J. T. Baker. Structures of IL-1 and IL-2 are shown in Figure 1.

Sample preparation methods 1. Melt-blending PLA/ IL-1 and PLA/ IL-2 compounds were melt blended by two methods using a counter rotating Brabender batch mixer (PL2000, C.W. Brabender) at 50 rpm and 160°C under nitrogen. The resin was pre-dried at 70°C for 24 hrs. Method 1: PLA pellets were first processed until steady torque was achieved. IL-1 or IL-2 were then added at three different concentrations up to 5wt% and mixed for 10 minutes. Method 2: IL-1 or IL-2 was dissolved in EtOH/H2O (4:1) and PLA pellets were added in this solution until thoroughly immersed. The solvent was removed by heating at 70°C for 24hrs. The concentrations of IL-1 and IL-2 in the solution were selected so that the dried pre-mix contained 10wt% IL. In order to produce 1wt% and 5wt% of IL-1 or IL-2 concentrations, proper amounts of PLA pellets were added along with the precoated pellets in the mixer bowl and processed until the torque stabilized. All blends were then compressed to films or ground to fine powders for characterizations. 2. Solvent casting PLA/IL-2 blends containing up to 10wt% IL-2 were prepared from methylene chloride as a common solvent. Clear films were produced by solvent evaporation. PLA/IL-1 blends could not be produced by this method since no common solvent was identified. 3. Microencapsulation PLA/IL-2 microspheres were prepared by the emulsion solvent evaporation method [6]. PLA (2 g) was dissolved in methylene chloride (20 ml), then IL-2 (0.1g or 0.2g) was dissolved in the polymer solution. An aqueous solution containing 2.5wt% of PVA was used as emulsifier. The polymer solution was mixed with 200 ml PVA solution and stirred at room temperature at 900 rpm for 2 h until the methylene chloride was completely evaporated. The produced PLA/IL-2 microspheres were

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then filtered and rinsed three times with distilled water and dried at 70°C for 12h.

Characterization Thermal properties: PLA, ionic liquids and their compounds were characterized, a) by differential scanning calorimetry, DSC, (TA Instruments, QA 100 analyzer) from 0˚C to 150˚Cat a heating rate of 20˚C /min to determine glass transition temperature and b) by thermogravimetric analysis, TGA, (TA Instruments, QA 50 analyzer) from room temperature to 500˚C, at a heating rate of 20˚C /min in a nitrogen atmosphere to determine thermal stability. Capillary rheometry: Apparent viscosities of PLA/IL-1, PLA/IL-2 compounds were determined using a capillary rheometer (Instron, Model 4204) in the shear rate range of 15 s-1 to 38 s-1 at 190°C. A melt flow indexer (Tinius Olsen, Model UE-4-78, Willow Grove, PA) was also used to measure flow characteristics at 2060g, 190°C. Optical microscopy: Size of PLA microspheres containing 5 wt% and 10 wt% of IL-2 and their size distributions were examined using an optical microscope (Olympus PM-10AK, Japan).

Results and Discussion Melt blending: Experiments conducted according to Method 1, where components were added separately, resulted in a decrease of the torque to zero when the ILs were added, regardless of the concentration used, 5 or 2 wt%. This suggested phase separation and concentration of the ionic liquid at the bowl/blade-walls. As a result, mixing was poor. Experiments conducted according to Method 2 produced different results. At 5wt% IL-1 torque dropped to zero, 2 min after addition. At 5wt% IL-2 torque increased somewhat after addition and remained more or less steady throughout the mixing period. At all times, it was lower than the torque levels attained by the PLA alone. These results are shown in Figure 2. At 10wt% IL-1 concentrations, torque was practically zero at the time of addition and remained zero until the end of the mixing period. At 10wt% of IL-2, torque decreased gradually from the time of addition but was still measurable. At 1wt% of IL, torque values were measurable for both ILs, however torque decreased much more rapidly in the case of IL-1. These experiments show significant differences of the effects of the two ionic liquids on the flow characteristics of PLA suggesting lubrication, plasticization or even degradation, at much more pronounced levels in the case of IL-1. Apparent viscosity: PLA/IL-2 samples prepared by method 2 showed important differences in their viscosity behavior as shown in Figure 3. The apparent viscosity of PLA/IL-2 (10 wt%) is 57.3 Pa·s while that of PLA is 531.6 Pa·s at 384 s-1, 190 oC. While PLA/IL-2 (10wt%) exhibits a marked negative dependence of apparent viscosity vs. apparent shear rate in the range relevant for processing, the viscosity of PLA decreases only slightly

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when the shear rate increases. Viscosity reduction was also observed for PLA/IL-1 (1wt%) in agreement with the torque/time behavior discussed above. Viscosity measurements of PLA/IL-1 at any of the IL-1 concentrations used were not possible, even with a smaller diameter die. Thermal properties: Tg determination experiments for the PLA/IL blends suggest that both ILs can be considered as plasticizers. This is related to the decrease of the PLA Tg from 60 oC to lower values depending on the type and concentration of ILs as shown in Figure 4. 10 wt% IL-1 lowers Tg to about 45 oC, which is at the same level as that attained with PEG conventional plasticizers.[7] The effect of IL-2 on the Tg of PLA is less pronounced. Miscibility and plasticization can be confirmed from the transparency of flexible plasticized films prepared by the melt blending Method 2 and in the case of PLA/IL-2 by the solvent casting method. Figures 5 and 6 compare thermal stabilities of single component and blends produced by Method 2. PLA/IL-1 blends have lower thermal stability than PLA/IL-2 blends (Fig. 5). This may due to the inherent lower thermal stability of IL-1 (see Figure 6) or a catalytic degradative effect, since the PLA degradation curve appears to lie to the right of all blends containing ionic liquids (Compare Figs 5 and 6) . It should be noted that IL-1 has been reported to catalyze PVC degradation [8]. Microencapsulation: In these experiments, the objective was to investigate a low temperature blending method that would minimize potential thermal degradation problems associated with PLA or PLA/ionic liquid blends produced in the melt. The TGA results shown in Figure 7 suggest that a combination of PLA with IL-2 has lower thermal stability than either component, an indication of degradation of the polymer or severe contamination with the EVA emulsifier. An interesting effect of increasing concentration of IL-2 is the reduction of particle size. The average size and standard deviation of microspheres containing 5 wt%, 10wt% of IL-2 are 266µm/±69.7 and 177µm/±44.8 respectively. This may be due to the complementary emulsification effect of IL-2 to that of EVA alone and/or the decreased viscosity of IL-2/PLA solution yield ing smaller microspheres since lower shear forces are necessary for droplet disruption [9]. Figures 8, 9 show optical micrographs of microspheres which have different IL-2 concentrations in PLA.

Conclusions In this study, two phosphonium cation based ILs with different anions were evaluated as potential plasticizers and/or lubricants for PLA. Both ILs lowered the glass transition temperature of PLA and modified its rheological characteristics as evidenced from reduced viscosities and apparent phase separation and lubrication. The effects were much more pronounced for the IL-1 containing a hydrophobic decanoate anion, presumably as

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a result of higher overall compatibility with the matrix vs. the IL-2 containing a hydrophilic BF4 anion. Thermogravimetric analysis suggested that the ILs could also be responsible for the decrease of the degradation temperature of the PLA. PLA/IL microspheres were sucessfuly prepared and the prescence of IL appeared to affect the particle size of the microspheres. IL-2

Acknowledgements Partial funding was provided by the US Army Corrosion Office, Picatinny, NJ through the Smart Coatings Program at the NJ Institute of Technology. The assistance of Dr. V. Tan and staff of the Polymer Processing Institute (NJ) is very much appreciated.

Fig. 1. Structures of ionic liquids, IL-1 and IL-2. 1600 1400

1. J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker and R.D. Rogers, Green Chem 3 (4), pp. 156–164, 2001. 2. B. Weyershausen and K. Lehmann, Royal Society of Chemistry, 7, 15-18, 2005. 3. M. Rahman and C. S. Brazel, Polym. Prepr. (Am. Chem. Soc.), 45, 1, 301, 2004. 4. M.P. Scott, M.G. Benton, M. Rahman and C.S. Brazel, ACS Sym. Ser. 856 468, 2003. 5. F. G. Schmidt, F. M. Petrat, A. Pawlik, H. Haeger and B. Weyershausen, “Polymeric compositions containing ionic liquids as plasticizers”, PCT Int. Appl., WO04/005391, 15.1.2004, Creavis GmbH. 6. S. J. Park and S. H. Kim, Journal of Colloid and Interface Science, 271, 336-341, 2004. 7. M. Sheth, R. A. Kumar, V. Dave´, R. A. Gross, S. P. McCarthy, Journal of Applied Polymer Science, 66, 1495-1505, 1997. 8. M. Rahman, C. S. Brazel, Polymer Degradation and Stability, pp.1-12, 2006 9. R. Jeyanthi, R.C. Mehta, B.C. Thanoo, P.P. DeLuca, J. Microencapsulation 14, 163–174, 1997.

Key words Ionic liquids, Polylactic acid, Plasticizers, Lubricants, Microspheres

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Fig. 2. Batch mixer torque curves of PLA and PLA containing 5wt% IL-1 or IL-2 100000 P LA P LA+1wt% IL-2 P LA+10wt% IL-2

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Fig. 3. Apparent viscosity vs apparent shear rate curves of PLA and IL-2/PLA blends.

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Fig. 4. DSC comparison of PLA vs. PLA containing different wt% of IL-1 and IL-2.

Fig. 5. TGA comparison of PLA containing different wt% of IL-1 and IL-2.

Fig. 6. TGA comparison of IL-1, IL-2 and PLA.

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Fig. 7. TGA comparison of PLA and PLA/IL-2 microspheres.

Fig. 8. Optical micrograph of PLA microspheres containing 5wt% of IL-2.

Fig. 9. Optical micrograph of PLA microspheres containing 10wt% of IL-2.

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