Quantification of the Vulcanizing System of Rubber in Industrial Tire

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Quantification of the vulcanizing system of rubber in industrial tire rubber production by laser-induced breakdown spectroscopy (LIBS) Stefan Trautner, Johannes Lackner, Wolfgang Spendelhofer, Norbert Huber, and Johannes D. Pedarnig Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05879 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Analytical Chemistry

Quantification of the vulcanizing system of rubber in industrial tire rubber production by laser-induced breakdown spectroscopy (LIBS) Stefan Trautner,† Johannes Lackner,‡ Wolfgang Spendelhofer,‡ Norbert Huber,† and Johannes D. Pedarnig*,† †Institute

of Applied Physics, Johannes Kepler University, Altenberger Strasse 69, A-4040 Linz, Austria Austria GmbH & Co. KG, Webersdorf 11, A-5132 Geretsberg, Austria

‡KRAIBURG

ABSTRACT: The properties of natural and synthetic rubber critically depend on the concentration of the vulcanizing system, among others. Sulfur and zinc oxide are typically used as crosslinking and activating agents for the vulcanization reaction (0-3 wt%). We present an advanced spectroscopic method to chemically analyze the vulcanizing system in rubber under ambient conditions and we demonstrate a novel application to measure the elements in-line of industrial rubber production. The laserinduced breakdown spectroscopy (LIBS) technique is optimized to ablate material from the surface of produced rubber sheets and to measure the optical emission of S and Zn from the rubber plasma in air. The sulfur lines in the near-infrared range are masked by molecular emission bands of the C-N radical and spectrally interfered by atomic lines of O. Plasma excitation in collinear doublepulse geometry and detection of plasma emission with time-gated detectors suppresses the spectroscopic overlays and enables to resolve the sulfur lines. For the determination of ZnO the weak Zn lines in the ultraviolet range are measured due to their superior intensity stability compared to the much stronger lines in the deeper UV. S and ZnO are quantified in three different rubber materials prepared from the most important polymers used in rubber production. The mean error of prediction of concentrations RMSEP is ≤0.07 wt% for S and ≤0.33 wt% for ZnO for all polymer types. Our results demonstrate that the vulcanizing system of rubber can be quantified under ambient conditions with LIBS. Other chemical elements could be analyzed also and the rubber production could be controlled employing this multi-element detection technique as process analytical sensor.

rubber. These methods measure the thermomechanical properties of the sample material and enable to check if the properties comply with pre-defined specifications. However, such methods do not provide direct chemical information on the concentration of elements and the materials homogeneity. For detailed chemical analysis of selected rubber samples other methods such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), laser ablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES), electrothermal vaporization (ETV) coupled with ICP-MS or ICP-OES, and flame atomic absorption spectrometry (AAS) can be employed. These methods allow for element analysis with high accuracy. However, the amount of material measured per analysis is small (in the range of mg to g) and the analytical results are not necessarily representative for the lot (rubber sheet) considering the complex chemical composition of rubber and the materials production process. Furthermore, the off-line analysis methods require time-consuming sample preparation and measurements which results in poor sampling statistics considering the throughput of material produced on industrial scale. Therefore, there is a need for process analytical technology (PAT) that provides direct measurements of the relevant physical and chemical properties during materials fabrication.4-6 Fast in-line analysis with good sampling statistics could be used to establish a feedback loop in order to control the industrial rubber production and to improve the process efficiency and rubber quality. A key parameter for the fabrication of rubber is the concentration of the vulcanization agents. Sulfur and zinc

INTRODUCTION Rubber has outstanding materials properties such as mechanical elasticity, viscoelasticity, dielectric strength, thermal stability, resisting power against chemicals, morphological flexibility, and durability, and is used in a wide range of applications.1-2 The annual global production of more than 28 million tons (2017) is used for tire production and retreading (70%), latex (12%), technical products (8%), storage of food (5%), adhesives (3%) and other applications (2%).3 Rubber is manufactured from polymers (e.g., based on caoutchouc), fillers (e.g., carbon black), plasticizers (e.g., oil), various additives (e.g., anti-aging agents, stabilizers against degradation by UV light, ozone, and weathering), and a vulcanizing system (e.g., sulfur, zinc oxide). Depending on the required rubber materials properties and the aimed application the type and concentration of substances added to the polymers is varying. Typically, the various components are weighted, thoroughly mixed in a mechanical heavy-duty batch mixer, and then milled and/or extruded. The batch process is thus transformed into a continuous process yielding a rubber sheet that is further processed later on, e.g. by morphological shaping, vulcanization, and cutting. In order to control the production of rubber a piece is punched out of the rubber sheet and tested in the laboratory. The material is typically analyzed by rheometry employing a temperature ramp to the sample during measurements to check the viscoelastic properties and the vulcanization process. Thermogravimetry and differential scanning calorimetry can be also employed to determine the thermal stability of the 1

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the VIS and the efficient laser ablation of the rubber material. The pulse energy E was set to the maximum level of the laser system to measure analytical signals with high signal / noise ratio (SNR). The inter-pulse delay time was selected for maximum line intensity.13 The laser beam was focused onto the sample surface and the irradiance and fluence was 3.6 109 W/cm2 and 18 J/cm2, respectively (spot area 1 mm2, lens with 200 mm focal length). The radiation of laser-induced plasma was collected in collinear geometry, spectrally separated by dichroic mirrors, focused into quartz glass fibers (400 µm diameter), and guided to two compact Czerny Turner spectrometers. This optical layout employing two optical detection paths was advantageous for simultaneous and independent measurements of LIBS spectra in the UV/VIS and NIR ranges. The spectrometers (Avantes AvaBench 2048XL ULS, entrance slit width 25 µm) were equipped with CCD detector arrays. The UV/VIS spectrometer covered the spectral range Δλ = 180 460 nm with an optical resolution δλ = 0.29 nm and had a minimum integration time of tg = 2 ms. The corresponding parameters of the NIR spectrometer were Δλ = 860 - 1160 nm, δλ = 0.4 nm, and tg = 2 µs. The mirrors and beamsplitters were placed in adjustable mounts to adjust each optical path separately. The spectrometers were triggered and synchronized with each other and to the laser using a LabView program and a PC in the control unit. For the measurements the LIBS sensor head was mounted above the conveyor belt of the production line. The distance to the sample was adjusted manually ( 0.2 mm with respect to the focal plane) before the start of measurements. A pressurized gas flush (1.5 bar) between measurement head and conveyor was installed to remove vapor from the hot rubber and residual ablated material from the optical beam path. The distance sensor monitored the distance between measurement head and rubber samples to detect variations in the process, e.g. changes of the rubber sheet thickness. Spectra Collection and Processing. For the quantification of analytes, 30 rubber samples were prepared in the form of sheets (approx. 1.5 m  0.5 m  10 mm) and measured by LIBS while being transported on a conveyor (v ≈ 15 cm/s). For each sample, 1000 to 2000 double-pulse (DP) LIBS measurements were performed. We averaged over 50 measurements to reduce shot-to-shot signal fluctuations due to variations of laser energy, laser-sample distance, and sample surface and obtained 20 to 40 spectra in the UV/VIS and NIR ranges per sample (3.3 sec/spectrum). The first step of spectra processing was the extraction of spectral signals IA for the analyte elements and the background signals BA. The analyte emission lines were spectrally integrated to obtain peak areas and the background intensities close to the element lines were integrated in spectral ranges without noticeable features. As second step, the extreme value distributions (EVD) of the analyte and background signals IA and BA and the normalized signal IA / BA were calculated for each sample and element. The EVD histograms typically showed normal distributions for the signals and the normalized signals. Finally, calibration curves for the analytes S and ZnO were determined from normalized signals, the limits of detection (LOD) and quantification (LOQ) were calculated, and the results validated.25

oxide (ZnO) are added as cross-linking agent and activator for the vulcanization of the polymer chains, respectively. The mechanical properties of the material (e.g., elasticity, stiffness, wear) depend on the amount of S and ZnO added and for the production of rubber of high quality the concentrations have to be controlled precisely in the process. X-ray fluorescence spectrometry (XRF) and Prompt gamma neutron activation analysis (PGNAA) are employed for in-line element analysis in cement and coal industry.7-8 In the rubber production, XRF and PGNAA in-line measurements would be very difficult to perform due to the required labor-intense human interaction in the processes and the corresponding radiation hazards. XRF using low energy x-rays ( 0.98 (Fig. 6). For ZnO in NR and SBR the correlation was r2 ≈ 0.97 and 0.96, respectively. Also the fit parameters a0 and a1 were significantly different for the three polymers used in the rubber sample production. This indicates a significant influence of the rubber matrix on the measured Zn signals. The reduced linearity of the ZnO calibration is subject of further investigations.

Figure 3. Calibration curve of Sulfur in natural rubber.

Figure 5. Calibration curve of Sulfur in synthetic rubber BR.

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Figure 6. Calibration curve of Zinc oxide in synthetic rubber BR.

Figure 7. Validation of S and ZnO in different rubbers (NR, SBR, BR) measured in-line by LIBS.

The LOD and LOQ values were determined from 95% confidence and prediction bands of the calibration.25 For sulfur in NR, SBR, and BR the LOD and LOQ values were 0.14-0.17 wt% and 0.28-0.32 wt%, respectively (Tab. 1). For ZnO the respective limits were higher. Estimations of LOD and LOQ by 3σ / a1 and 10σ / a1, respectively, gave values that were similar to the calculations with the prediction band method (σ the standard deviation of signal intensity for the blank sample). The normalized signals for S and for ZnO, IS / BS and IZn / BZn, were checked also against possible cross-sensitivity for the other analyte, ZnO and S, respectively. The signals for S and ZnO did not show a significant dependence on the concentration of the respective other analyte. A possible influence of rubber sheet temperature on the measured analyte signals was investigated as well. The calibration curves for S and ZnO in NR/SBR matrix were the same for samples at 20 and at 95 °C. In order to validate the calibrations and to estimate the accuracy of predictions for unknown samples, a leave-one-out cross validation (LOOCV) was performed. The predicted concentration for S was close to the nominal concentration for all polymers and samples (Fig. 7). Also for ZnO in BR the prediction had good accuracy. The root mean square of error of prediction (RMSEP) was calculated from the predicted analyte concentrations CP,i and the nominal concentrations CN,i using the n samples of the calibration data sets RMSEP =

1 𝑛 ∑ (𝐶 ― 𝐶𝑁,𝑖)2 𝑛 𝑖 = 1 𝑃, 𝑖

Initial attempts on multivariate analysis of measured LIBS spectra did not provide satisfying results on predicting the S and ZnO concentrations. Optimization of spectra preprocessing and spectral window selection might be necessary to obtain chemometric models for accurate prediction. The accuracy of predicted concentrations fulfills the demands in the production. It is therefore promising to apply the LIBS method for in-line measurements and monitoring of the production process. In industrial rubber production the major components and additives are mixed in different ratios depending on the technical specifications of the rubber in the final product or appliance (“rubber recipe”). Continuous inline measurements could provide quasi real-time element analyses of the rubber being produced and enable to close a feedback loop to the materials supply chain to control the production process (Fig. 8). Table 1. Vulcanizing system in tire rubber quantified by LIBSa Polymer

.

The prediction of concentrations was better for sulfur (RMSEP < 0.1 wt%) than for ZnO reflecting the better linearity of the S calibration curves (Table 1).

Analyte

LOD

LOQ

RMSEP

wt%

wt%

wt%

NR

S

0.16

0.32

0.03

SBR

S

0.17

0.32

0.07

BR

S

0.14

0.28

0.07

NR

ZnO

0.67

1.29

0.17

SBR

ZnO

0.85

1.62

0.33

BR

ZnO

0.39

0.75

0.09

aSamples

prepared and measured in-line at rubber production site of Kraiburg Austria in Geretsberg in June 2018.

We have performed two extended LIBS in-line measurement campaigns at the rubber production facility in 2017 and 2018 to test the LIBS method for process monitoring. More than 15 million LIBS spectra were accumulated. The results of long-term tests showed stable LIBS signals for S and ZnO that were reproducible when 5



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of concentrations predicted from measured LIBS calibration data are promising for applications. Using our developed double-pulse LIBS system the feasibility of in-line measurements in industrial rubber production was demonstrated. An improved rubber production and the development of specific rubber materials can be envisioned using a fast in-line LIBS analyzer for process control. Furthermore, the measurement method could be diversified to detect sulfur in the production of technical rubbers that are used for many different technical products, e.g. conveyor belts, and to quantify S also in other materials, e.g. in oil and coal.

comparing rubber sheets that were produced with the same “recipe” (materials composition and procedure) at different days. The LIBS signals measured on the same rubber sheet did not show systematic variations with time except of shot-toshot fluctuations for S and ZnO and of rare detection of spectral lines from other elements (Si, Mg, Ti, Al, Ca).

ASSOCIATED CONTENT Supporting Information The file “Supporting information_LIBS of rubber.pdf” includes a list with the concentrations of S and ZnO in the rubber samples produced for the in-line measurements, a schematic of the LIBS measurement setup developed, and a table with the spectroscopic parameters of all relevant atomic emission lines and molecular emission bands.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: +43 732 2468-9403. Fax: +43 732 2468-9402. ORCID Johannes D. Pedarnig: 0000-0002-7842-3922 Figure 8. Schematic of process control in rubber production by in-line LIBS element analysis.

Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

The constant LIBS signals indicated that all the rubber batches used to produce a long sheet had the same concentration of the vulcanizing system and that the production process was stable. The rarely detected elements (Si and Mg, Fig. S2. Al, Ca, and Ti, Fig. S3) were impurities from other rubber mixtures and by-materials used in the production, e.g. Mg and Si originated most likely from talcum powder and silica gel, respectively.

ACKNOWLEDGMENT Financial support by the Austrian Research Promotion Agency FFG is gratefully acknowledged (K-project imPACts 843546). The authors wish to thank the team members in the Technikum and in the production of KRAIBURG for their assistance in sample preparation and the in-line measurements. We also want to thank Dr. Hubert Duchaczek of voestalpine Stahl (Austria) and Prof. Matthias Otto and Dr. Daniela Bauer of TU Bergakademie Freiberg (Germany) for the laboratory analyses of rubber samples. The multivariate analysis of LIBS spectra by Verena Haunschmid and Dr. Thomas Natschläger from Software Competence Center Hagenberg SCCH (Austria) is appreciated.

CONCLUSIONS We present here the first quantitative LIBS measurements of the vulcanizing system of rubber in ambient air and in-line of the rubber production. So far, S was measured in vacuum or inert gas background like He and Ar and the rubber was analyzed off-line in a laboratory. We measured S and ZnO in various types of rubber containing different polymers (natural rubber and synthetic rubber). Despite the complexity of the spectra measured in the UV/VIS and NIR ranges, the analytes S and Zn were measured with good signal-to-background ratio and without spectral interference by employing advanced LIBS measurement technology and spectra evaluation procedures. Molecular emission bands and atomic lines that were overlaying the faint NIR emission lines of sulfur can be excluded from measurements by appropriate timing of the spectrometer and detector and by applying double-pulse plasma excitation. We have investigated the UV/VIS and NIR LIBS spectra of rubber, identified a spectral fingerprint in the NIR as the C-N red system (confirmed by modelling of spectra)24, and determined calibration curves of the analytes. The low errors

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Table of Contents artwork


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Figure 1. Emission lines of zinc and carbon neutral atoms (I) and of Zn ions (II) in double-pulse laserinduced plasma of natural rubber of different ZnO concentration. 290x203mm (300 x 300 DPI)


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Figure 2. Emission lines of sulfur, carbon, oxygen, and nitrogen neutral atoms in double-pulse laser-induced plasma of natural rubber of different S concentration measured in air. Measurements in Helium background gas (inset). 272x208mm (300 x 300 DPI)



Figure 3. Calibration curve of Sulfur in natural rubber. 289x202mm (300 x 300 DPI)


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Figure 4. Calibration curve of Sulfur in synthetic rubber SBR. 289x202mm (300 x 300 DPI)



Figure 5. Calibration curve of Sulfur in synthetic rubber BR. 289x202mm (300 x 300 DPI)


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Figure 6. Calibration curve of Zinc oxide in synthetic rubber BR. 289x202mm (300 x 300 DPI)



Figure 7. Validation of S and ZnO in different rubbers (NR, SBR, BR) measured in-line by LIBS. 272x208mm (300 x 300 DPI)


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Figure 8. Schematic of process control in rubber production by in-line LIBS element analysis. 190x338mm (96 x 96 DPI)


Quantification of the Vulcanizing System of Rubber in Industrial Tire

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