Camphene/polystyrene solutions: A rheological approach for material processing industry

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is the peer reviewed version of the following article: Felix, M., Perez- Puyana, V., Paúl, A., Sepúlveda, R. (2019), Camphene/polystyrene solutions: A rheological approach for material processing industry. J Appl Polym Sci, 136, 47953. , which has been published in final form at https://doi.org/10.1002/app.47953 . This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited." 1 Camphene/polystyrene (PS) solutions: a rheological approach for material processing industry Manuel Felix1,*, Victor Perez-Puyana2, Antonio Paúl3, Ranier Sepúlveda3 1Departamento de Ingeniería Química, Escuela Politécnica Superior, Universidad de Sevilla, 41011 Sevilla, Spain. 2Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain. 3Dpto. de Ingeniería Mecánica y de los Materiales, Universidad de Sevilla, Isla Cartuja, 41092 Sevilla, Spain ------------------- *Manuel FELIX Escuela Politécnica Superior, Universidad de Sevilla, 41011 Sevilla, Spain. E-mail: [email protected] Phone: +34 954557179; fax: +34 954556447. 2 Abstract Camphene has received a great interest in material science since it has been postulated as an excellent candidate for the manufacture of porous materials by freeze casting technique. In this context, rheological properties of these systems are very important as they affect their stability and have many implications in the operation costs. This work elucidates the rheological properties of polystyrene (PS)-camphene solutions with industrial applications. Non‐ Newtonian behaviour plays a key role in the processing of these solutions. Unfortunately, the assessment of very low shear viscosities (below 0.005 Pa s) is still a challenge in camphenebased materials. Flow curves, stress growth tests and small amplitude oscillatory shear (SAOS) measurements were carried out as a function of PS concentration (0, 2.5, 5.0, 10 and 20 wt.% PS) to understand the rheological behaviour of these systems. Results indicate that liquid camphene has a shear-thinning behaviour (flow index = -0.99) when the increase of polymer concentration involves changes in the structure of these systems (flow index = -0.10 for 20 wt.% PS). An increase in PS concentration leads to a forwarding in the shear-thinning zone, and the Newtonian region (from ⁓1 s-1, for 0 wt.% PS, to 10-3 s-1 for 20 wt.% PS). Eventually, these results evidence the high dependence of 𝜂∞ on PS concentration (from ⁓1.16 ± 0.01 Pa·s, for 0 wt.% PS, to 631.5 ± 0.2 Pa·s for 20 wt.% PS). Keywords: Cox-Merz; Flow curves; Polymer solutions 3 1. Introduction Camphene is an organic component of formula C10H16, which belongs to the family of terpenes, that are the main component of essential oils of some plants and flowers. Their main application is related to the production of drugs and cosmetics. It behaves as plastic crystal at room temperature, generating dendrites during certain cooling conditions. Crystal growth has been previously studied, focusing on the control of the dendrite structural parameters, by using either constant temperature gradient or constant growth rate1. Nowadays, camphene is widely used since it exhibits interesting features: i) it is solid at room temperature2; ii) Its vapour pressure is below 1 atm in the solid and liquid phase (it can sublimate at room temperature); iii) it is able to dissolve some polymeric materials. In recent years, research has been focused on the use of camphene particle suspensions to develop porous materials. Araki and Halloran3 proposed the use of camphene to make interconnected Al2O3 porous material. Yoon et al.4 demonstrated the possibility of creating hydroxyapatite (HA) structures with large pore channels by freezing at temperatures near the solidification temperature of the HA/camphene slurry. Currently, researchers applied this method to make metallic materials. Thus, W-Ni porous probes were processed by freeze casting, using metallic oxide particles of WO3 and NiO powder as starting materials5. Sepúlveda et al 6 demonstrated the possibility to obtain directionally porous materials with nanometric Fe2O3 powder 20-40 nm in diameter. However, the use of metallic particles has been limited, due to its relatively high density, and the lack of affordable and contamination free submicron particle. Recently, Tibased porous material was developed by Xu et al7 using 14 m diameter Ti particles, and, despite the fact they use a water base suspension, authors increased the viscosity up to 40 cP to avoid powder settling. The settling effect of Mg particles (~44 m) in camphene was studied by increasing the solid phase contend and applying uniaxial press at room temperature8. 4 Therefore, new studies directed to altering the rheological properties of these systems are essential for the understanding of their stability and industrial processing9. Nowadays, the industrial use and scientific research of particle suspensions in PS is growing. Polymer nanocomposites (PNCs) are a new class material with novel functional applications, as they merge the functionality of nanoparticles with the properties of the polymer host. In some cases, PS is dissolved in methyl ethyl ketone (MEK)10, xylene11, or toluene12–14, to disperse the particles. Therefore, camphene can be a suitable candidate to be used as a less harmful solvent with a much easier removal process. In addition, it can be easily recovered (and reused) by installing a vapor trap. Despite the fact that many applications relay on the understanding of the mechanical behaviour of the liquid phase, very few studies have been dedicated to the understanding of their rheological properties, which, together with particle size and surface charges, will control the suspension stability15–17. In this sense, Jen et al.18 studied the viscosity of camphene/UHMWPE solutions in order to stablish its gelation point. Araki and Halloran19 performed a rheological study based on camphene/alumina slurries at different solid particle contents (33.1-50.8 vol%) without any polymer addition. Moreover, Yang and Perng20 compared the viscosity behavior of 3 different solvents, including camphene, to dissolve polypropylene (PP) in a temperature range of 110-160ºC. As a result, the use of camphene significantly reduced the PP entanglement strength, showing the lowest viscosity amongst other solvents (paraffin oil and decalin) consequently reducing the energy required for pumping these slurries. Rheology has been widely used for the study of small deformations and flow properties of polymeric materials. In fact, results obtained were related with the internal microstructure of these systems21. Many rheological research was dedicated to several polymer melt or solutions, such as: polystyrene (PS), polyvinyl alcohol (PVA), polyolefin elastomer (POE), or polycaprolactone (PCL) among others. Unfortunately, the rheological scientific background of 5 camphene-polymer solutions is scarce, being limited to the determination of its viscosity in large deformations22. Knowledge of their rheological properties will help understanding the behaviour of these systems. For instance, an increase on camphene viscosity could reduce the particle sedimentation effect, although operation costs for these systems will be higher 23 due to load loss. The aim of this work is the evaluation of rheological properties of PS-camphene solutions, allowing the understanding of their stability as well as their use in future industrial applications, since no similar commercial materials are available, to our knowledge. To achieve this objective, five PS-camphene solutions with varying PS concentration (0, 2.5, 5.0, 10 and 20 wt.% PS) were studied. Sample preparation and rheological characterization were performed at 60⁰C (under Tg) and no polymerization was induced. Thus, the tendency to PS aggregate formation was reduced and their countereffects were negligible. The rheological assessment was based on the analysis of non-linear properties by means of flow curves and stress growth tests. Moreover, their linear behaviour was studied by using small amplitude oscillatory measurements (SAOS). All these measurements were carried out in a high-sensitive stress-controlled hybrid rheometer (DHR-3, TA Instruments). 2. Material and methods 2.1. Materials All Polystyrene (PS) solutions were prepared using camphene (C10H16, melting point 51 ºC) as solvent. Camphene was purchased from Sigma-Aldrich (St. Louis, Missouri, USA) with a 95% assay . PS, from Sigma-Aldrich had a molecular weight (Mw) of 350 kDa (Tg  95 ⁰C, melt index: 2.0-4.0 g/10 min at 200 ºC). 2.2. Methods 6 2.2.1. Sample preparation 50 g of PS/camphene solution samples were prepared as follows: Each solid camphene and PS pellets mixture (Table 1) were introduced on individual seal glass container to prevent any camphene loss. Then, each container was heated up to 60 ºC by using a sonicated bath at 45 kHz frequency (VWR USC500Th) until complete PS dissolution was achieved. The time until complete PS dissolution increased with its concentration, solutions with 2.5, 5.0, 10 and 20 wt.% PS, needed about 0.3, 1.3, 7.3, and 15.3 h respectively. Once no trace of solid PS was observed, samples were removed from the bath and cooled down freely to room temperature. Longer time was necessary to achieve full PS dissolution, the temperature was keept at 60 ºC in order to reduce any polymer degradation, also to replicate experimental conditions reported previously for the fabrication of porous materials3–5. PS/camphene solution were stored at 20 ºC, and tested within 24 h of its preparation. Since camphene is a non-polar solvent, no pH measurement is expected. However, the it was checked that PS dissolution did not modifie pH. . 2.2.2. Rheological characterisation All rheological measurements were carried out using a high-sensitive stress-controlled rheometer (DHR-3, TA Instruments, USA). This rheometer is equipped with a second generation magnetic bearing (Hybrid rheometer) that increase the sensibility of the measurements, especially for low-torque values. The testing temperature was fixed at 60 ºC using a peltier system (samples were maintained at this temperature for 30 min before the rheological analysis). To avoid solvent evaporation a solvent trap was used. Furthermore, different setups were used for the different rheological tests carried out: 7 Flow curves Steady-state flow curves were obtained for camphene solutions containing 0, 0.60, 1.25, 2.5, 5.0, 10 and 20 wt.% PS. These experiments were carried out selecting the stress-controlled mode. Shear viscosity values were recorded from 1 to 150 s-1. The geometry selected for these tests was 15mm concentric cylinder coupled with a peltier accessory. Oscillatory measurements Mechanical spectra were obtained for camphene solutions containing 0, 2.5, 5.0, 10 and 20 wt.% PS by means of frequency sweeps in a frequency range from 0.05 to 1.25 Hz. All these measurements were carried out within the linear viscoelastic range (LVR) which was previously determined for each sample by means of strain sweep tests (from 1·10-4 to 1·10-1 Pa) at two different frequencies (0.1 and 1.0 Hz). Low inertia (60 mm) parallel plates were used for these experiments, where 500 μm constant gap was fixed. Peak hold tests Transient flow tests were performed for camphene solutions containing 2.5, 5.0, 10 and 20 wt.% PS by 15 mm couette flow equipped with peltier concentric cylinder accessory. Eight different constant shear rates were applied to these solutions (1·10-4, 5·10-4, 1·10-3, 5·10-3, 1·10-2, 5·10- 2, 1·10-1 and 5·10-1 s-1) whereas transient viscosity was recorder over 1800 s. 2.3. Statistical analysis At least three replicates of each measurement were carried out. Results showed are the mean of three replicates. ANOVA tests were performed using Excel from the Microsoft Office 2016 statistical package to determine significant differences (p < 0.05). Uncertainty was expressed as standard deviation (SD). 8 3. Results and discussion 3.1 Flow curves Figure 1 shows the dependence of viscosity on shear rate obtained for camphene-PS solutions as a function of polymer concentration (0, 0.60, 1.25, 2.5, 5.0, 10 and 20 wt.% PS). These results indicate that the shear viscosity of the camphene system is very low, which justify the industrial interest above-mentioned. Although the scientific literature related with the rheological characterisation of polymer/camphene solutions is scarce, these results agree with previous results obtained for camphene viscosity in absence of polymers22. Moreover, this plot evidences that, within the shear rate interval studied, these polymer solutions exhibit a Newtonian behaviour since shear viscosity does not depend on shear rate. Similar results were obtained for polyethylene terephthalate (PET) in CHCl3/DMF cosolvent 24. However, this behaviour does not correspond to the viscoelastic response found for poly(vinyl alcohol) (PVA)/water solutions, where a shear-thinning behaviour was found 25. In any case, both studies agree on the dependence of viscosity on polymer content which is also observed in Figure 1. This effect was for other polymer solutions where the viscosity of these solutions is controlled by their volume in very diluted solutions (as well as for typical solutes) 26. However, polymer chains can interact increasing the complexity of these systems as polymer concentration increases, so that a non- Newtonian behaviour can be observed 27. Nevertheless, Figure 1 indicates that all camphene-PS solutions studied in this work behaved in that manner at the shear rate interval studied. Moreover, the viscosity depends on PS concentration. Table 2 illustrates viscosity values obtained for these systems at 10 s-1 shear rate. Interestingly, viscosity and polymer concentration exhibit an exponential dependence that can be fitted to 𝜂 = 𝜂0𝑒𝑎·[𝑐] (1) 15 the financial support granted to carry out this work (Ref. MAT2016-76713-P MINECO/FEDER/EU). 5. References 1. Çadırlı, E.; Maraslı, N.; Bayender, B.; Gündüz, M. Mater. Res. Bull. 2000, 35, 985. 2. Smith, H. A.; Fuzek, J. F.; Meriwether, H. T. J. Am. Chem. Soc. 1949, 71, 3765. 3. Araki, K.; Halloran, J. W. J. Am. Ceram. Soc. 2005, 88, 1108. 4. Yoon, B. H.; Koh, Y. H.; Park, C. S.; Kim, H. E. J. Am. Ceram. Soc. 2007, 90, 1744. 5. Park, S. H.; Park, S.-M.; Park, S.-J.; Park, B.-Y.; Oh, S.-T. Korean J. Mater. Res. 2018, 28, 108. 6. Sepúlveda, R.; Plunk, A. A.; Dunand, D. C. Mater. Lett. 2015, 142. 7. Xu, Q.; Gabbitas, B.; Matthews, S.; Zhang, D. J. Mater. Process. Technol. 2013, 213, 1440. 8. Zou, N.; Li, Q. JOM 2018, 70, 650. 9. Roussel, N. 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Flow properties of polymer melts; Plastics Institute London, 1970. 18 Figure 1 Figure 1: Steady-state curves obtained for camphene-PS solutions as a function of polymer concentration using 15 mm concentric cylinder geometry. Temperature was kept at 60 ºC using a peltier system. Solvent evaporation was avoided using a solvent trap accesory. Experiments were carried out selecting the stress-controlled mode, recording shear viscosity ranges from c.a. 1 to 150 s-1. 19 Figure 2 Figure 2: Mechanical spectra obtained by means of frequency sweep tests for camphene solutions containing: (A) 0% PS; (B) 2.5% PS; (C) 5% PS; (D) 10% PS and (E) 20% PS. Low inertia (60 mm) parallel plates were used for these experiments Temperature was fixed at 60 ºC using a peltier system. The solvent evaporation was avoided using the solvent tramp accesory. Experiments were carried out within the linear viscoelastic range (LVR) from 0.05 to 1.25 Hz selecting the stress-controlled mode. 20 Figure 3 Figure 3: Steady-state values of shear viscosity, η, (obtained by means of flow experiments) and complex viscosity, η*, (obtained by means of SAOS measurements) as a function of shear rate (for η) or angular frequency (for η*). Temperature was fixed at 60 ºC using a peltier system, whereas the solvent evaporation was avoided using the solvent tramp accesory. 21 Figure 4 Figure 4: Stress growth measurements performed at six different shear rate (1·10-4, 5·10-4, 1·10-3, 5·10-3, 1·10-2, 5·10-2, 1·10-1 and 5·10-1 s-1, in the strain-controlled mode) as a function of PS concentration: (A) 0% PS, (B) 2.5% PS, (C) 5% PS, (D) 10% PS and (E) 20% PS. Temperature was fixed at 60 ºC using a peltier system. Solvent evaporation was avoided using a solvent trap. 15 mm couette flow geometry was used to determine the transient viscosity over 1800 s. 22 Figure 5 Figure 5: Steady-state flow curves obtained by means of stress growth measurements (η), SAOS tests (η*) and steady-state flow curves (η). Temperature was fixed at 60 ºC using a peltier system, whereas the solvent evaporation was avoided using the solvent tramp accesory. 23 Table 1 System PS wt% vol% mol% 0% PS 0,00 0.000 0,0E+00 0.60% PS 0,60 0,486 2,3E-04 1.25% PS 1,25 0,202 9,8E-05 2.5% PS 2,50 2,033 1,0E-03 5% PS 5,00 4,087 2,0E-03 10% PS 10,00 8,252 4,3E-03 20% PS 20.00 16,832 9,7E-03 Table 1. Weight, volume and molar polystyrene concentrations in PS-camphene solutions. 24 Table 2 Table 2: Shear viscosity values obtained from steady-state flow curves for camphene-PS solutions at 10 s-1 shear rate as a function of polymer concentration (0, 0.60, 1.25, 2.5, 5.0, 10 and 20 wt.% PS). Temperature was fixed at 60 ºC using a peltier system. The solvent evaporation was avoided using the solvent tramp accesory. Different letters within a column indicate significant differences (p>0.05) System η10 (mPa·s) 0% PS 0.60% PS 1.25% PS 2.5% PS 1.16 ± 0.01a 1.53 ± 0.01b 2.09 ± 0.01c 4.04 ± 0.02d 5% PS 7.61 ± 0.05e 10% PS 35.0 ± 0.03f 20% PS 631.5 ± 0.2g