Thermal evaluation of the calcium stearate influence on polystyrene by in-line colorimetry

Abstract

This paper investigates evidence regarding the influence of a CaSt₂ residual additive in the optical properties of a commercial polystyrene. This work proposes an in-line optical detector operating by illuminating the polymer with a red monochromatic light in order to evaluate its thermal behavior during a heat cycling. The detection system was bench validated by measuring the light absorption of aqueous solutions of organic acid dyes. The system showed great sensitivity in detecting the contamination’s effect on the polymer, as a great impact on the polymer’s behavior was observed due to the presence of the additive during different thermal cycles. The detector system showed that the salt’s presence elevates absorbance by five times when the temperature is higher than its fusion temperature. The in-line optical detector has proved to be suitable to quantify the presence of additives that affects the polymer’s optical properties.

Keywords:
absorbance; calcium stearate; heat cycling; in-line optical detector; thermal characterization

1. Introduction

The color of a material is the result of the human visual perception of the combination of wavelengths not absorbed by a substance on which a beam of white light is impinged[¹]. Thus, color is not a specific property, depending how it interacts with the electromagnetic radiation. For instance, pure red color has its complementary color green and vice-versa, as shown in Ostwald’s cycle[²]. In other words, a red object is red by our perception because the material absorbs mainly in the green’s wavelength letting pass light in the red’s wavelength color region.

The addition of chromophore species, i.e., conjugate systems that absorb light at a specific frequency[³], may enhances the attenuation of the light beam by absorption[⁴,⁵]. This reduction in the intensity of the transmitted light beam (I) in relation to the initial intensity (${\text{I}}_0$) supplied to the sample is given as a ratio between them, known as absorbance (A), given by Equation 1.

$\text{A}=-\text{log}\left(\frac{\text{I}}{{\text{I}}_0}\right)$ (1)

Beer-Lambert's Law, Equation 2, establishes a relation between absorbance and the concentration of chromophore species[⁶]. This law is only valid for monochromatic light beams and only if the absorbent substance doesn’t change its physical state or chemical composition with the variation of concentration[⁷].

$\text{A}=\epsilon .\text{d}.\text{c}$ (2)

being ε the molar extinction coefficient (or molar absorptivity), d, the optical path length and, c, the concentration of the attenuating species, respectively.

Colorimetry is an established characterization tool in polymer science which has the light’s attenuation by absorption as object of study, being used by many scholars[⁸-¹⁵]. Ordinarily, researchers focus on the influence of distinct chromophore species on polymeric systems, such as dyes[⁸-¹²], pigments[¹³] and others additives[¹⁴,¹⁵], although few works take into account the effects of temperature variation, which is a frequent feature in polymer processing and may change the optical contribution of, for instance, existing additive in terms of absorbance. Phillips et al.[¹⁶] evaluated the thermal behavior of metal stearate lubricant mixtures for polymer compounding applications. It was observed rheological changes for all samples through cooling cycles. Zhang et al.[¹⁷] investigated the structural evolution of zinc stearate (ZnSt₂) and calcium stearate (CaSt₂) in a polypropylene random copolymer (PPR) upon heating and cooling. The CaSt₂ in PPR exhibited vitreous agglomerates with lamella ordered structure at low temperatures and transformed to diverse colloidal liquid crystal aggregates during heating, showing the temperature dependence of its structure. Antunes et al.[¹⁸] investigated the effect of manganese stearate (MnSt) concentrations on the thermo-oxidative degradation of high density polyethylene. Temperatures from 60 °C to 80 °C were performed. It was shown that the additives were able to accelerate the oxidative abiotic degradation of the polymer and promote the biodegradation of the material. Bensaad et al.[¹⁹] studied the effects of CaSt₂ as pro-oxidant agent on the degradation of polypropylene (PP) under natural exposure. The results showed that the concentration of 5% wt calcium stearate was sufficient to initiate the process of degradation. Despite these and other contributions, there is lack of studies focusing how temperature affect the impact of the polymer's additives in its optical properties. The applied temperatures are still far from those found in real polymer processing conditions. In this context, the aim of this work was to evaluate the influence of temperature in the polymer processing range in the absorbance of a polystyrene (PS) additivated with calcium stearate by quantifying in-line the transmitted light attenuation by absorption during a thermal cycle process with temperatures ranging in the typical processing conditions.

2. Materials and Methods

2.1 Polymer

A commercial grade of polystyrene (PS) provided by Innova (Brazil), code N2560, with the presence of a single residual additive: Calcium stearate (CaSt₂), was used in this work. Polystyrene is commonly additivated with low quantities of CaSt₂, which acts as lubricant and releasing agent during molding[²⁰]. In addition, it modifies the filler surface reducing water absorption, preventing agglomeration of the filler particles, and reduces the surface energy helping its dispersion[²¹]. Also the calcium stearate has a wide melting temperature range of T_m = 130 °C up to 180 °C[²²].

2.2 Acid dyes

Water soluble organic acid dyes with the colors: Red (N-2R 200), Green (N-3G 200) and Blue (N-2R 200) provided by Quimanil Produtos Químicos Ltda (Brazil) were used in the previous stages of this work to evaluate the efficiency of the proposed detection system. The dyes chemical structures are presented in Figure 1.

Figure 1
Chemical structure of the organic acid dyes: (a) red, (b) green and (c) blue.

2.3 Thermal stage system

An optical microscope Leica DMRXP (Figure 2a) with a hot stage system CSS450 from Linkam Scientific Instruments (Figure 2b) was used for thermo-optical characterization. This system constitutes of a hot chamber with two parallel quartz windows between which the specimen is positioned (Figure 2c), allowing efficient temperature control (-50 °C to 450 °C) at defined different heating rates.

Figure 2
(a) The optical microscope (Leica DMRXP) fitted with the hot stage system (CSS450 from Linkam Scientific Instruments); (b) Hot stage front closed view; and (c) Front opened view.

2.4 In-line optical detector system

The in-line optical detector is placed over the two parallel quartz windows in the hot stage system. For this system, in one side is positioned the light emitting diode LED of red (630 nm) monochromatic wavelength and at the other side there is a single phototransistor in order to collect the transmitted light’s intensity. The light exiting the LED passes through the first transparent window, through the molten polymer stream, in which is attenuated by light absorption, exit through the second transparent window and, lastly, reaches the phototransistor which quantifies the transmitted light intensity. Changes in it are collected by software, developed in LabVIEW 8.6 (National Instruments^TM) platform for data collection, real-time calculation, screen presentation and data archiving. The adopted optical configuration is in agreement with previous set-ups[¹⁵,²³].

2.5 Experimental procedure

2.5.1 Bench validation of the optical detector with aqueous solutions

The in-line optical detector was tested and its response validated quantitatively by using aqueous solutions of the reference dyes. To avoid metal oxidation a slit-die made of Teflon was constructed and used for the bench validation. A schematic representation of the complete validation system is presented in Figure 3.

Figure 3
Sketch of the bench validation system of the in-line optical detector using a closed water flowing loop, pumped through a Teflon slit-die.

The validation set-up consists of a water circulating closed-loop, fed by a large beaker containing 1.0 Liter of filtered water, pumped by a submersed aquarium plastic pump, circulating through the Teflon slit-die, having it being fitted with the in-line optical detector. A concentrated mother water solution was prepared and stepwise added to the circulating system in known volumes via a syringe, increasing their content progressively. The concentrated solution is produced with 1.0 g of dye placed in 250 mL of water. After each addition of the concentrated solution, a 3 min minimum period is waited for complete mixing, and then the detector signal is recorded, measuring the transmitted light intensity and averaging during a 3 min time interval.

2.5.2 Optical evolution during the thermal cycling of the PS/CaSt₂

The in-line optical detector was allocated in the Linkam thermal stage, and a complete thermal cyclic process was applied at a constant rate of 10 °C min^-1, cooling from 210 °C down to 80 °C and heating back again to 210 °C. Each optical characterization analysis was repeated in triplicates for better accuracy of the results. Pellets of the polystyrene as received and pellets of the polystyrene after an extraction process were added between the two parallel quartz windows of the Linkam system, with a constant gap of 1.5 mm. The extraction process was done solubilizing it with chloroform and then precipitating the solubilized polymer with ethanol. A period of one week was waited for the completed evaporation of residual solvents. Samples of PS after the additive extraction were obtained and the extracted residue was collected. During each thermal cycle, absorbance intensity measurements were collected as a function of the analysis temperature. By comparing the absorbance variations, information regarding the influence of the additive on the optical response of the polystyrene is quantified.

3. Results and Discussions

3.1 Bench validation of the optical detector with aqueous solutions

The response of the optical detector was tested for each colored organic acid dye dissolved in an aqueous solution. For that, the absorption of each dye with red illumination color was measured using pure filtered water as reference medium. Increasing stepwise the dye concentration one can obtain curves of absorbance (as normalized transmitted light intensity) as a function of dye concentration in the aqueous solution. Figure 4 shows the curves and, as expected[⁷], the results are linear crescent curves.

Figure 4
Absorbance as normalized transmitted light intensity as a function of the dyes’ color and concentration, in aqueous solutions, using monochromatic red light illumination.

The aqueous solution with the red dye absorbs weakly the red light illumination, which is in agreement with literature[¹], since absorption of a specific color wavelength by a sample of the same color is minimal. On the other hand, as observed from the performance of the solution with the green dye, showing a significant absorption under red illumination, the absorption of a specific color wavelength by a sample of the complementary color is maximal. The blue dye, whose complementary color is yellow, has a significant absorption under red illumination, which lead us to conclude that the blue acid dye used here is not a pure color (referring to the Ostwald’s cycle), but an intermediate color instead.

The optical detector operating as an in-line colorimeter has shown: i) linear response to the increase dye concentration, obeying Beer-Lambert’s law[⁶], and ii) correct response under the respective color illumination. This performance has indicated to us the proper function of the in-line colorimeter under light absorption, validating it.

3.2 Optical evolution during the thermal cycling of the PS/CaSt₂

The optical characterization of the polystyrene, measured as absorbance curves as a function of temperature is shown in Figure 5.

Figure 5
Absorbance as a function of temperature for the PS, heat cycled between 210 °C and 80 °C, at 10 °C min^-1.

A molten pure polymer, particularly polystyrene, is expected to does not absorb visible light, i.e., be transparent. During its heating thermo-oxidative degradation can occur, which produces chromophores groups, contributing to color changes due to the light absorbance of these degradation products. The presence of the strong hysteresis in Figure 5, set between 80 °C and 170 °C, let clear that another effect beyond the contribution of these chromophores groups is contributing for this optical behavior and, the presence of the calcium stearate, CaSt₂, may be this factor. Calcium stearate has a wide melting temperature range of T_m = 130 °C up to 180 °C[²²], which coincides to the sigmoidal change in the absorbance curves’ during heating, shown in Figure 6a.

Figure 6
Light absorbance during five heat/cooling cycles of the polystyrene sample as received, ranging from 80 °C to 210 °C. In (a) the heating curves and in (b) the cooling curves. Dashed lines correspond the melting/crystallization temperature ranges of the CaSt₂, 129 - 180 °C and 180 - 120 °C, respectively.

In order to further investigate the optical influence of the CaSt₂ observed in this commercial polystyrene sample, another experiment was performed. Initially, a polystyrene sample as received was thermally cycled from 210 ºC down to 80 ºC at a constant cooling/heating rate of 10 ºC min^-1 for five consecutive cycles. The resultant changes in the transmitted light intensity curves are presented in Figure 6, separated into two groups, that due to the heating (Figure 6a) and cooling (Figure 6b) portions.

The results observed in Figure 6 show that the additive strongly affects the light absorbance of the polymer, in which, for each additional cycle, the additive’s contribution is stronger, featuring a possible transformation of this additive[¹⁷,²⁴] which leads to higher passage of light through the sample, implying bigger negative values of absorbance. The normalized absorbance was arbitrarily set zero at 210 °C.

The next step was to evaluate the influence of the additive in the absorbance in isotherms under and above its melting temperature. Isotherms at fixed temperatures, seen in Figure 7, were done in the Linkam System CSS450 for pure polystyrene.

Figure 7
Absorbance as a function of time (s) for polystyrene as received. Isotherms of 140 °C, 150 °C, 160 °C and 170 °C were evaluated.

The results presented in Figure 7 show the optical polymer’s response for each temperature. The absorbance is negative above the additive’s fusion temperature and positive below it, namely, the additive absorbs less light when molten.

To show this influence of the additive, it was necessary to repeat the same hot stage measures, but with the PS without it, i.e., after the extraction process previously prepared. First, the extracted residue was submitted to FTIR analysis to ratify its identity as stearic acid calcium salt. The results are presented in Figure 8.

Figure 8
Transmittance (%) versus wavelength (cm^-1) for the extracted additive.

In the spectra, it’s observed two main groups of bands around 1500 and 2900 cm^-1. The peak values lead us to conclude, according to previous works available on literature[²⁵], that the obtained residue is, in fact, calcium stearate, since, such additive presents characteristic bands at 1542 and 1575 cm^-1 due to antisymmetric stretching bands for unidendate and bidendate association with calcium ions and at 1472 cm^-1 due to methylene scissoring band and 2850, and 2914 cm^-1 symmetric and antisymmetric methylene stretching band respectively.

The PS after the extraction was submitted to thermal cycling from 210 °C down to 80 °C at a constant heating rate of 10 °C min^-1 for five consecutive cycles. The results are presented in Figure 9.

Figure 9
Absorbance as a function of time (s) for polystyrene after the additive extraction. Five heat cycles were evaluated from 210 °C to 80 °C.

The results presented in Figure 9 display the polymer’s response for continuous thermal cycles. The absorbance is now increasing after each cycle, contrasting with the previous results obtained before the additive extraction. It’s intuitive to conclude that the additive was impacting the optical properties of the polymer and pushing the collected absorbance values to a negative range.

The next step was to repeat the isothermal measurements with the PS samples after the extraction process. Figure 10 shows the differences of maximum absorbance intensity achieved in each isotherm for the commercial PS before and after the additive extraction process.

Figure 10
Maximum absorbance intensity for polystyrene as received and after the additive extraction process. Isotherms of 140 °C, 150 °C, 160 °C and 170 °C were evaluated.

The results showed in Figure 10 corroborate the data presented in Figure 9 in which the influence of the stearic acid calcium salt is confirmed as the cause of the negative absorbance values. The salt’s presence elevates the collected light intensity by the phototransistor when the temperatures are higher than its fusion temperature. On the other hand, in its absence, the light intensity doesn’t change considerably, being five times less intense as before. The weak deviation from the zero-line in Figure 10 are due to thermal degradation.

Investigating the values in Figure 10, it’s verified a remarkable reduction in the absorbance intensity after the thermal cycles. This fact can be explained by the removal of not only the residual additive previously existent in the polymer system, but also the removal of possible degrading agents such as free radicals and residual catalyst, which suggests a PS more resistant to degradation over time after the extraction process.

4. Conclusions

A thermal cycling method has been proposed for quantify changes in the absorbance of a polymer matrix with residual additivities in high temperatures. This work showed that the resulting absorbance is a function of the pair polymer and wavelength illumination and, its behavior changes with the presence of a contaminant additive, leading even to negative values. The additive’s extraction process lead to a more resistant polymer to degradation, showing that besides additives are important in the polymer manufacturing, sometimes, depending on the type of analysis, they can have negative effects. Optical properties, such as absorbance analyzed in this paper, as they are more sensitive to internal changes in the polymeric system, are affected by the presence of an additive and, consequently, its structural changes with temperature.

6. Acknowledgements

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 scholarship (00889834001-08) and the authors would also acknowledge the Programa de Pós-Graduação em Ciência e Engenharia de Materiais (PPG-CEM) of Federal University of São Carlos for providing the laboratorial facilities.

• How to cite: Bernando, F., & Canevarolo Junior, S. V. (2024). Thermal evaluation of the calcium stearate influence on polystyrene by in-line colorimetry. Polímeros: Ciência e Tecnologia, 34(2), e20240017. https://doi.org/10.1590/0104-1428.20230130

7. References

• ¹
  Ohta, N., & Robertson, A. R. (2005). Colorimetry fundamentals and applications. New York: John Wiley & Sons. http://doi.org/10.1002/0470094745
  » http://doi.org/10.1002/0470094745
• ²
  Jacobson, E. (1948). Basic color: an introduction of the Ostwald Color System Chicago: Paul Theobald.
• ³
  Lindon, J. C., Tranter, G. E., & Koppenaal, D. W. (Eds.). (2017). Encyclopedia of spectroscopy and spectrometry. Oxford: Academic Press.
• ⁴
  Born, M., & Wolf, E. (1999). Principles of optics: electromagnetic theory of propagation, interference and diffraction of light. London: Cambridge University Press. http://doi.org/10.1017/CBO9781139644181
  » http://doi.org/10.1017/CBO9781139644181
• ⁵
  Schlatter, G., Serra, C., Bouquey, M., Muller, R., & Terrisse, J. (2002). Online light scattering measurements: A method to assess morphology development of polymer blends in a twin-screw extruder. Polymer Engineering and Science, 42(10), 1965-1975. http://doi.org/10.1002/pen.11089
  » http://doi.org/10.1002/pen.11089
• ⁶
  Strong, F. C. (1952). Theoretical basis of Bouguer-Beer law of radiation absorption. Analytical Chemistry, 24(2), 338-342. http://doi.org/10.1021/ac60062a020
  » http://doi.org/10.1021/ac60062a020
• ⁷
  van de Hulst, H. C. (1981). Light scattering by small particles New York: Dover Publications.
• ⁸
  Kyzas, G. Z., Bikiaris, D. N., & Lazaridis, N. K. (2009). Selective separation of basic and reactive dyes by molecularly imprinted polymers (MIPs). Chemical Engineering Journal, 149(1-3), 263-272. http://doi.org/10.1016/j.cej.2008.11.002
  » http://doi.org/10.1016/j.cej.2008.11.002
• ⁹
  Gold, M. H., Glenn, J. K., & Alic, M. (1988). Use of polymeric dyes in lignin biodegradation assays. Methods in Enzymology, 161, 74-78. http://doi.org/10.1016/0076-6879(88)61011-1
  » http://doi.org/10.1016/0076-6879(88)61011-1
• ¹⁰
  Gelover-Santiago, A., Fowler, M. A., Yip, J., Duhamel, J., Burillo, G., & Rivera, E. (2012). Unexpected absorbance enhancement upon clustering dyes in a polymer matrix. The Journal of Physical Chemistry B, 116(21), 6203-6214. http://doi.org/10.1021/jp208646b PMid:22594837.
  » http://doi.org/10.1021/jp208646b
• ¹¹
  Liu, Y., & Claus, R. O. (1999). Strong enhancement of optical absorbance from ionic self-assembled multilayer thin films of nanocluster Pt and polymer dye. Journal of Applied Physics, 85(1), 419-424. http://doi.org/10.1063/1.369401
  » http://doi.org/10.1063/1.369401
• ¹²
  Fleischmann, C., Lievenbrück, M., & Ritter, H. (2015). Polymers and dyes: developments and applications. Polymers, 7(4), 717-746. http://doi.org/10.3390/polym7040717
  » http://doi.org/10.3390/polym7040717
• ¹³
  Gilmor, C., Balke, S. T., Calidonio, F., & Rom-Roginski, A. (2003). In-line color monitoring of polymers during extrusion using a charge-coupled device spectrometer: color changeovers and residence time distributions. Polymer Engineering and Science, 43(2), 356-368. http://doi.org/10.1002/pen.10030
  » http://doi.org/10.1002/pen.10030
• ¹⁴
  Fornes, T. D., Yoon, P. J., & Paul, D. R. (2003). Polymer matrix degradation and color formation in melt processed nylon 6/clay nanocomposites. Polymer, 44(24), 7545-7556. http://doi.org/10.1016/j.polymer.2003.09.034
  » http://doi.org/10.1016/j.polymer.2003.09.034
• ¹⁵
  Yoon, P. J., Hunter, D. L., & Paul, D. R. (2003). Polycarbonate nanocomposites: part 2. Degradation and color formation. Polymer, 44(18), 5341-5354. http://doi.org/10.1016/S0032-3861(03)00523-8
  » http://doi.org/10.1016/S0032-3861(03)00523-8
• ¹⁶
  Phillips, J., Weldhagen, M., Mhlabeni, T., Radebe, L., Ramjee, S., Wesley-Smith, J., Atanasova, M., & Focke, W. W. (2021). Thermal characterization of metal stearate lubricant mixtures for polymer compounding applications. Thermochimica Acta, 699, 178906. http://doi.org/10.1016/j.tca.2021.178906
  » http://doi.org/10.1016/j.tca.2021.178906
• ¹⁷
  Zhang, X., Wei, F., Wang, Z., Li, G., Yang, S., & Feng, J. (2023). Comparative investigation of the structural evolution of zinc stearate and calcium stearate in a polypropylene random copolymer upon heating and cooling. Polymer, 267, 125646. http://doi.org/10.1016/j.polymer.2022.125646
  » http://doi.org/10.1016/j.polymer.2022.125646
• ¹⁸
  Antunes, M. C., Agnelli, J. A. M., Babetto, A. S., Bonse, B. C., & Bettini, S. H. P. (2017). Abiotic thermo-oxidative degradation of high-density polyethylene: effect of manganese stearate concentration. Polymer Degradation & Stability, 143, 95-103. http://doi.org/10.1016/j.polymdegradstab.2017.06.012
  » http://doi.org/10.1016/j.polymdegradstab.2017.06.012
• ¹⁹
  Bensaad, F., & Belhaneche-Bensemra, N. (2018). Effects of calcium stearate as pro-oxidant agent on the natural aging of polypropylene. Journal of Polymer Engineering, 38(8), 715-721. http://doi.org/10.1515/polyeng-2017-0391
  » http://doi.org/10.1515/polyeng-2017-0391
• ²⁰
  Chukwu, M. N., Madufor, I. C., Ayo, M. D., & Ekebafe, L. O. (2011). Effect of stearic acid level on the physical properties of natural rubber vulcanisate. Pacific Journal of Science and Technology, 12(1), 344-350.
• ²¹
  Nguyen, T. T., Nguyen, V. K., Pham, T. T. H., Pham, T. T., & Nguyen, T. D. (2021). Effects of surface modification with stearic acid on the dispersion of some inorganic fillers in PE matrix. Journal of Composites Science, 5(10), 270. http://doi.org/10.3390/jcs5100270
  » http://doi.org/10.3390/jcs5100270
• ²²
  Larrañaga, M. D., Lewis, R. J., & Lewis, R. A. (2016). Hawley’s condensed chemical dictionary (13th ed.). New Jersey: John Wiley & Sons. http://doi.org/10.1002/9781119312468
  » http://doi.org/10.1002/9781119312468
• ²³
  Bernardo, F. O. C., Silva, J. M., & Canevarolo, S. V. (2018). Dispersed particle size characterization by in-line turbidimetry during polymer extrusion. Polymer Testing, 70, 449-457. http://doi.org/10.1016/j.polymertesting.2018.08.005
  » http://doi.org/10.1016/j.polymertesting.2018.08.005
• ²⁴
  Vold, R. D., Grandine, J. D., 2nd, & Vold, M. J. (1948). Polymorphic transformations of calcium stearate and calcium stearate monohydrate. Journal of Colloid Science, 3(4), 339-361. http://doi.org/10.1016/0095-8522(48)90021-X PMid:18877000.
  » http://doi.org/10.1016/0095-8522(48)90021-X
• ²⁵
  Lu, Y., & Miller, J. D. (2002). Carboxyl stretching vibrations of spontaneously adsorbed and LB-transferred calcium carboxylates as determined by FTIR internal reflection spectroscopy. Journal of Colloid and Interface Science, 256(1), 41-52. http://doi.org/10.1006/jcis.2001.8112
  » http://doi.org/10.1006/jcis.2001.8112

Publication Dates

• Publication in this collection
  27 May 2024
• Date of issue
  2024

History

• Received
  09 Jan 2024
• Reviewed
  15 Mar 2024
• Accepted
  31 Mar 2024