1. Introduction
Emulsion polymerisation is the most commonly used process for the production of water-borne latex polymers, and its importance in the industry keeps growing. It is a free radical polymerisation method carried out in a heterogenous reaction system, and commercially available polymer latex products usually contain around 40–55 wt% of solid content [
1].
Before a polymer latex can be transferred from a laboratory scale into production, the process needs to be thoroughly examined, for example, regarding reproducibility or heat transfer, and then successfully scaled up to a larger set-up. The following work focuses on the scale-up process of the emulsion polymerisation of vinyl acetate and Versa®10 from a 1 L scale to a 10 L and 100 L scale while achieving a high solid content of over 60 wt%. Moreover, this article examines the possibility of monitoring the scale-up process by using an inline method to measure the particle size.
A commonly used monomer in emulsion polymerisation is vinyl acetate, which is not only relevant in the industry but has also proved to be interesting for researchers [
2]. Emulsion polymerisation of vinyl acetate and Versa
®10 with a high solid content of over 60 wt% was successfully performed in the past and showed an excellent agreement between offline and inline particle size measurement methods up to a solid content of 36 wt% [
3]. However, agglomeration of particles at polymer content of 40–50 wt% created difficulties for further comparisons, which made a revision of the recipe necessary before considering upscaling. Pohn et al. developed a CFD model to simulate an upscaling from 1 L to 100 L and found it challenging to describe an emulsion polymerisation process in a 100 L stirred vessel with a turbine stirrer, leading to a laminar regime. They also came across the same issues regarding coagulation, resulting in a secondary population of larger particles [
1]. To avoid encountering the same difficulties, this work focused on improving the recipe on the one hand and modifying the set-up on the other hand. As a laminar regime does not seem suitable for the upscaling of emulsion polymerisation at high polymer contents, the process described in this work shows a set-up involving an anchor stirrer, instead of a turbine stirrer, leading to a turbulent regime.
1.1. Scale-Up
The scale-up process remains one of the major challenges of chemical engineering. It allows the building of a bridge between an innovation that occurred on a laboratory scale and an actual invention that works in an industrial environment. Upscaling a process can often reveal difficulties that were not detected on a smaller scale. Building a pilot plant is, therefore, a crucial step before transferring a new product into production. However, due to the numerous factors that can influence the product properties and process development, there is no standard recipe for a scale-up process, and success often lies in the hands of experience, successful ideas, and many mistakes, which in the end, can lead to the desired outcome [
4,
5]. Nonetheless, guidelines exist that are designed to offer a starting point in the planning of such a project. Firstly, it is recommended to avoid cross-influences by keeping the set-up as identical as possible regarding the measurements of the reactor, the dosing units, dosing time, or molar ratio of the components. The size and width of the stirrer, as well as its speed, are also important factors of a scale-up. Modifying the flow or mixing characteristics can have a huge impact on the success of the polymerisation process. Besides these aspects, there is a list of rules commonly used and accepted that can be considered for the scale-up process [
6]. The first one involves maintaining a constant stirring speed, regardless of the reactor size. This can, however, result in unobtainable power input and is therefore not adapted to all kinds of upscaling processes. The second and third rules indirectly apply to the size of the reactor, as they involve keeping either the stirring tip speed, which considers the size of the stirrer, or the circulation time constant, which depends on the diameter of the vessel. Another approach to the upscaling process involves keeping the Reynolds number constant. The Reynolds number is defined as a correlation between the density of the liquid, the viscosity, the stirring rate, and the diameter of the stirrer and is an important and dimensionless quantity used in fluid mechanics. It contributes to the description of the mixing quality and constitutes the main parameter of the scale-up process described in this work, as a turbulent regime was considered necessary. According to the fluid mechanics and when using an anchor stirrer, a turbulent regime is achieved when the Reynolds number of the stirring is higher than 10,000 [
7]. Further rules, which can be applied when planning a scale-up process, are keeping the power input constant, or controlling the mean energy dissipation. Each of these rules aims at keeping a constant similarity of the process, regardless of the final scale. Past experience and published articles suggest employing a certain rule, depending on the requirements [
8,
9,
10]. For example, according to Zhou and Kresta [
11,
12], for liquid–liquid dispersions, both energy dissipation and flow are the most important characteristics. Nonetheless, everyone agrees that a recipe that works for all processes does not exist.
1.2. Inline Monitoring
The successful upscaling of a process is not limited to the size of the set-up alone but also includes maintaining the desired product properties. This is why a thorough analysis of the dispersion during the scale-up process is crucial. Among other properties, the latex particle size and its distribution, for example, have an influence on the surface properties of the dried polymer film or, alternatively, can provide information about the kinetics of the reaction as well as the number of radicals per particle [
13,
14]. To obtain knowledge and control of the particle size, it is essential to use the right analytical methods [
15,
16]. The most common methods for particle size measurements are offline analysis, for example, dynamic light scattering (DLS) or disc centrifuge (DC). Offline analysis has many disadvantages, which can be traced back to the sampling and sample preparation, as most methods exclude high solid contents. However, a promising inline measurement method has gained interest in recent years, as it is able to monitor the particle size and scattering coefficient during the emulsion polymerisation process [
17,
18,
19,
20,
21,
22]. Photon density wave (PDW) spectroscopy has already been established in the fields of biochemistry and food chemistry but has lately proven to be a useful tool in the process control of emulsion polymerisation [
3,
23]. Studies on a laboratory scale showed a successful comparison of particle size measurements between PDW spectroscopy and common offline measurements methods, such as DLS [
24] and sedimentation analysis by means of a DC [
3]. Furthermore, Schlappa et al. [
23] showed that the inline measurement of the reduced scattering coefficient could be useful for the real-time detection of modifications occurring in the process. For example, gelation could be recognised instantly, and appropriate countermeasures could be initiated immediately in order to save the product. This could also prove to be useful for a scale-up process.
PDW spectroscopy is an inline Process Analysis Technology (PAT) that can be used to monitor the reaction progress by measuring the optical properties of the dispersion, i.e., the absorption coefficient and the reduced scattering coefficient [
17,
18,
19,
20,
22]. The measurement method uses the correlation between the reduced scattering coefficient and the particle size to determine the latter in real time in the dispersion by measuring the reduced scattering coefficient [
22,
25,
26,
27,
28].
2. Materials and Methods
All chemicals were used directly without further purification. Each component of the reaction was flushed with nitrogen for at least 40 min, and the nitrogen flow inside the reactor was maintained during the whole process. The gas flow was fed into the reactor through a metal pipe with a diameter of 3 mm that was immersed into the initial charge. The amount of water evacuated by the continuous nitrogen flow was not taken into account for the 1 L process, as it was determined to be less than 1%. However, the amount of water evacuated during the 10 L and 100 L processes was calculated theoretically and confirmed by test runs. The amount of water lost at the end of the reaction was considered when determining the amount of monomer needed to achieve the desired polymer content. The latter was also confirmed via solid content determination (see method descriptions below).
For safety reasons, the first emulsion polymerisations were performed in a 1.8 L RC1e reaction calorimeter (Mettler Toledo) with a double jacket steel reactor (HP60, Mettler Toledo) to determine the heat generation during the process. The vessel had a diameter of 10.3 cm and was sealed with a non-heated steel lid. An anchor stirrer with a span of 9 cm was used to stir the dispersion.
The scale-up emulsion polymerisations were carried out in three different reactors of, respectively, 1 L, 10 L, and 100 L. The 1 L reactor was a double-jacket glass vessel, whereas the 10 L and 100 L reactors were double-jacket reactors made of steel. Each reactor had a dish-like bottom with the sample outlet in the middle. For the sake of comparability, all three set-ups were kept as identical as possible. In scale-up processes, it is common to keep a constant height to diameter ratio. The three reactors used for the experiments described in this work were not customised and therefore cannot fully meet this criterion. However, while H usually refers to the total height of the vessel, when considering the filling height at the beginning of the reaction, the H
1/D (cf.
Figure 1 and
Table 1) ratio is 0.25 in all three reactor sizes. The exact measurements are summarised in
Table 1 and represented in
Figure 1 for a better understanding.
In order to achieve a better mixing of the components at all stages of the reaction, a stainless-steel anchor stirrer was used in combination with two baffles [
29]. To avoid forming a water swirl at the beginning of the reaction, when only water and emulsifier were present, baffles were inserted into the reactor to interrupt the flow and to enable a better mixing of the components. The width of the baffles amounted to 1/12 of the reactor diameter and they were placed at a distance of 1/72 from the tank wall [
30,
31]. The anchor stirrer had a width of 0.87.D of the respective vessel, and 0.95.D when deducting the baffles [
32,
33]. The stirring rate is summarised in
Table 2 and determined by the Reynolds number (cf.
Table 3).
The emulsion polymerisations were carried out as a semi-batch, closed-loop, controlled and starved–fed process at 1 bar. The initial charge contained demineralised water with a conductivity of 0.8 uS.cm
−1 (see
Table 4 for exact amount), 0.20 g.L
−1 ammonium iron (III) sulphate (Merck KGaA) as a catalyst, and 50.4 g.L
−1 Mowiol 4–88 (Sigma-Aldrich, St. Louis, MO, USA) as an emulsifier, resulting in a final emulsifier fraction of 2.44 wt% based on the total amount of monomer.
All experiments, including the test runs in the reaction calorimeter, were performed identically as described hereafter. The initial charge and feeding rates were summarised in
Table 4.
The temperature of the reaction solution was brought to 60 °C within, respectively, 30, 60, and 120 min while flushing its content with nitrogen for at least 40 min before starting the reaction. The temperature of the 1 L, 10 L and 100 L reactor was regulated, respectively, by a Julabo Cryo Compact F30-C thermostat, a Huber Unistat Tango Thermostat, and a Huber Unistat 405wl Thermostat with constant jacket temperature.
Two dosing units were used; the first contained the monomer mixture, consisting of vinyl acetate and neodecanoic acid vinyl ester (Versa
®10, Wacker Chemie AG, Burghausen, Germany) in a molar ratio of 9:1. The feeding rate can be found in
Table 4.
The second dosing unit contained a premixed 3.4 wt% solution of, respectively, the reducing and the oxidising agents, L-ascorbic acid (AsAc, Sigma-Aldrich, St. Louis, MO, USA) and tert-butyl hydroperoxide (tBHP, Sigma-Aldrich, St. Louis, MO, USA), cf.
Table 5. The feeding rate can be found in
Table 4.
The dosing occurred through PTFE hoses with an inner diameter of 3 mm. The end of each hose was immersed into the reaction solution so that the dosing substance would be immediately stirred into the reactor content. For the 1 L reactor, the oxidising and reducing agents were fed with a syringe pump from kdScientific and using polyethylene syringes from Henke-Sass, Wolf GmbH. The monomer feed for the 1 L reactor and both feeds for the 10 L and 100 L reactor were performed with a precision SyrDos syringe pump from Hitech Zang using 2.5 mL syringes.
The feeding of the reducing and oxidising agents was started first. After 5 min, the dosing of the monomer mixture was started and stopped after achieving a polymer content of 63–67 wt% based on the total mass. The polymer content was checked via microwave analyser. The feeding of the reducing and oxidising agents was continued for another 10 min before they were stopped. The catalyst was added to the reaction solution diluted in 1 mL demineralised water by using a polyethylene syringe from Henke-Sass, Wolf GmbH, just before starting the monomer dosing unit. The total dosing time was 7.33 h.
When taking a sample from the 1 L and 10 L or 100 L reactor, the reaction was stopped with, respectively, 1 mL and 10 mL of a 1.7 m% solution of hydroquinone (Sigma-Aldrich, St. Louis, MO, USA). A sample was taken at, respectively, 10, 20, 30, 40, 50, 60, and 63 wt% polymer content to be able to determine the course of the particle size as a function of the polymer content and to verify a complete conversion at all times.
Each emulsion polymerisation considered in the following work was replicated three times to ensure a reproducible outcome.
2.1. Determination of Yield
Samples were taken through the outlet at the bottom of the reactor (after discarding enough dispersion to compensate for the dead volume of the outlet) and measured by gas chromatography (GC) as well as by microwave analyser in order to determine the yield of the reaction. In addition to the offline determination methods, inline monitoring of the monomer amount was performed by Raman spectroscopy throughout the entire reaction.
2.1.1. Gas Chromatography (GC)
The samples were measured with an Agilent 7820A using hydrogen as a carrier gas (column: CP-Sil 5CB fused silica, 30 m, 1.0 µm, detector: FID, injector temperature: 200 °C, detector temperature: 250 °C, sample volume: 0.4 μL).
Then, 500 mg of each sample was withdrawn with a 100–1000 µL Eppendorf research micropipette, weighed, and then dissolved in 5 mL of N,N-dimethylacetamide. Finally, 70 mg of toluol was added as an internal standard and also weighed. Once fully prepared and dissolved, 1.5 mL of the solution was transferred into an amber glass vial and sealed with a PTFE/silicone septum and measured.
2.1.2. Microwave Analyser
The samples were measured with a Smart System 5 device from CEM. The microwave analyser measured the total solid content of the sample. Approximately 3 g of sample was weighed, dried at a temperature up to 120 °C and then weighed again by the device, which then determined the weight difference. A CEM glass fibre sample pad was used as a sample carrier and tared beforehand. The drying process was temperature-controlled through microwave radiation. This method of analysis allows the determination of the total solid content of the sample. The polymer content can then be determined by subtracting all other solid components, such as Mowiol 4–88 and the initiator. Analysis via microwave analyser provides the solid content within minutes. However, the total conversion was confirmed later by determining the residual monomer by GC.
2.1.3. Raman Spectroscopy
Raman spectroscopy without internal standard or calibration is a qualitative measuring method. Small changes in laser intensity or exposure time affect the intensity of the signal directly. Raman spectroscopy was therefore used solely for safety reasons in order to be able to detect a significant accumulation of the monomer at an early stage. The aim was, therefore, not to make quantitative statements, but merely to show that the monomer content remains constant over the entire process time.
Raman Spectroscopy was measured with a RamanRxn1-785 system from Kaiser Optical Systems (IO-1/2S-NIR probe, laser power 387 mW at the probe, Software: ICRaman). A new measurement was acquired every 32 s with an integration time of 30 s. The monomer content was tracked by monitoring the intensity of the peak at 1650 cm
−1, which shows an overlap of the C=C bond of both monomer, vinyl acetate and Versa
®10 [
34]. The 12 mm probe was immersed into the initial charge at the opposite side of the PDW spectroscopy probe to avoid light interferences.
2.2. Inline Particle Size Measurement
PDW spectroscopy measurements were carried out inline, immersing the stainless-steel probe (2.5 cm diameter) directly into the reaction solution during the process. The probe was passed vertically into the vessel and maintained at equal distance between the stirrer and the wall of the reactor. The probe was far enough within the vessel for the optical fibres to protrude approximately 0.5 cm into the initial charge.
Three different wavelengths were used: 638 nm, 778 nm, and 855 nm. The device used was a Mini-PDW-spectrometer from the company PDW Analytics GmbH in collaboration with InnoFSPEC of the University of Potsdam. The measurements were processed with software based on Labview 2016. The refractive index and density of the copolymer were determined in a previous work and were implemented in the software [
3].
2.3. Offline Determination of the Mean Particle Size
In addition to the inline measurement via PDW spectroscopy, the mean particle size was also determined by sedimentation analysis by means of DC and DLS. The refractive index and density of the copolymer were also implemented in the software of both measurement methods [
3].
2.3.1. Disc Centrifuge (DC)
Measuring the particle size by sedimentation analysis requires a gradient, which is why 0.2 mL of methanol was injected into the DC while at a halt. Then, the motor was started, and when reaching the maximum speed, 15 g of demineralised water was added steadily. One drop of the sample was diluted in 0.3 mL demineralised water and 0.1 mL methanol, and then 0.1 mL of the diluted sample was injected into the DC. The device used was a Disc Centrifuge DC24000 of the brand CPS. The given accuracy and repeatability lie at ±0.5% [
35].
2.3.2. Dynamic Light Scattering (DLS)
The mean particle size was also determined by dynamic light scattering using a Zetasizer Nano ZS from Malvern Instruments. One drop of the same sample, used previously, was diluted in 5 mL demineralised water and then measured in a polyethylene UV-cuvette. Each given particle size was obtained through a triple determination, each consisting of 18 measurements at 25 °C. A previous work provided a calibration of the device, which was used to correct the DLS measurements [
3].
2.4. Determination of the Zeta Potential
The zeta potential was determined by a Zetasizer Ultra from Malvern Panalytical and using the ZS XPLORER software. The sample was diluted 200 times with MilliQ water (0.055 µS.cm−1 at 25 °C) and then measured in a disposable folded capillary cell (DTS1070) at 25 °C.
2.5. Determination of the Theoretical Particle Size
The determination of the theoretical particle size was based on the previously calculated number of particles. The number of particles (Np) was calculated based on the conversion and on the average particle size (dp) of the first sample with 10 wt% polymer content, measured by DC, as this was considered the reference measurement. The following equation was used:
where χ is the monomer conversion determined by GC, g
Monomer is the amount of monomer dosed into the dispersion at the time of the sampling (g), ρ is the density of the copolymer (g.cm
−3) [
36], and dp is the particle size (nm) measured by DC.
It was assumed that the number of particles would stay constant throughout the reaction. The theoretical size of the other samples was then calculated accordingly.
4. Conclusions
The emulsion polymerisation of vinyl acetate and Versa®10 is known to tend to agglomerate at high solid contents. For this polymerisation system, a scale-up process up to a polymer content of 67 wt% was successfully and reproducibly achieved in a scale-up process from 1 L, through 10 L, and up to 100 L via a redox-initiated starved–fed semi-batch process. Safety precautions, such as inline monitoring of monomer content via Raman spectroscopy (qualitative monomer accumulation) and calorimetric studies, were implemented to ensure the possibility of identifying any resulting risk and simultaneously enable a quick intervention to avoid accidents. In addition, the recipe was optimised to ensure a stable and reproducible process, with an increase in the reaction temperature to a maximum of 2 °C and no lump formation.
PDW spectroscopy proved to be a reliable and precise measuring method for the inline monitoring of particle size, even at a higher scale and with polymer contents up to 67 wt%, showing a clear advantage in comparison to DLS or DC. All determined mean particle diameters concurred with the DLS and sedimentation analysis measurements up to a polymer content of 40 wt%. PDW spectroscopy continues to concur with the calculated theoretical particle size at even higher polymer contents up to 67 wt%, while DLS and DC showed difficulties, which could be due to post-process alteration of the dispersion.
PDW spectroscopy also displayed the reproducibility of the process in all reactor sizes and even offered reliable information on the mean particle size of creamy and slightly lumpy emulsions. The measurement method has therefore proven to be promising not only on a laboratory scale but also on an industrial scale. However, further investigations should be carried out in order to emphasise the possibility of obtaining even more information by analysing the scattering coefficient also measured by PDW spectroscopy.
Future work could also investigate whether the dispersion is subject to post-process or cooling agglomeration and whether this is a reversible process.