Polymerization is a chemical process in which smaller molecules, known as monomers, combine to form larger molecules, known as polymers. This process is fundamental to the production of many plastics and other materials. A key aspect of polymerization is the degree of polymerization, which indicates how many monomer units are linked together in a polymer molecule. The degree of polymerization has a decisive influence on the physical properties of the resulting polymer, such as strength, flexibility and temperature resistance.
LiquiSonic® measuring systems in polymerization
LiquiSonic® is an inline analysis system that measures the concentration in the polymerization directly in the process without any time delay. The device is based on the high-precision measurement of the absolute sonic velocity and process temperature and thus allows processes and complex reactions to be tracked.
The sensor design of the LiquiSonic® measuring devices enables uncomplicated cleaning of the devices, which means that the process does not have to be interrupted by time-consuming cleaning work and can run as efficiently as possible.
In the field of polymerization, LiquiSonic® offers the user numerous advantages:
Real-time monitoring: the technology enables continuous monitoring of the polymerization process in real time. This allows changes to be detected and reacted to immediately, ensuring consistent product quality.
No sampling required: As the system measures directly in the process, no manual sampling is required. This minimizes the risk of contamination and process interruptions.
Robust and low-maintenance technology: LiquiSonic® measuring devices are designed for continuous use in industrial environments. They are resistant to aggressive media and high temperatures, resulting in an extended service life and lower maintenance costs.
Optimization of processes: By closely monitoring the polymerization reaction, users can finely control the process, resulting in higher yields and lower production costs.
The LiquiSonic® system can therefore be used for high-precision concentration determination as well as for phase detection and process monitoring (crystallization). An internal limit value monitoring system signals when limits are exceeded or not reached and sends real-time information to the process control system.
Fast and accurate monitoring of polymerization, the degree of polymerization and the concentration of monomers and macromolecules is therefore possible. This monitoring ensures that the optimum product quality is achieved throughout the polymerization of caprolactam to PA6.
Precise knowledge of the polymerization process and the ratio of monomers to macromolecules is particularly important in order to minimize product losses and maximize the efficiency of the process. By precisely determining the concentration of monomers and macromolecules throughout the process, the user can ensure that the final product meets the desired specifications.
LiquiSonic® ensures high-precision analysis of the caprolactam concentration with permanent data recording. The measuring system is also successfully used for phase separation between caprolactam and ammonium sulphate in a matter of seconds.
Sensor design of LiquiSonic®
The robust sensor design and the choice of special materials, such as HC2000 or PFA, ensure a long process service life for the system. SensoTech also offers sensors with corresponding ATEX, IECEx and FM certification.
LiquiSonic® reduces the concentration of back-caprolactam (residual monomer) to a minimum, thus optimizing system productivity.
The LiquiSonic® immersion sensors can be easily installed in the feed and transport lines. No bypass is necessary when installing the LiquiSonic®sensors and dead spaces are avoided.
The LiquiSonic® Controller 30 can be connected to up to 4 sensors. This makes it possible to monitor several measuring points at the same time.
Typical measuring ranges
Caprolactam concentration range: 70 to 100 m%
Temperature range: 80 to 130 °C
Caprolactam concentration range: 0 to 10 m%
Temperature range: 20 to 70 °C
Incoming goods: Concentration range oleum : 0 to 30 m%
Temperature range: 10 to 60 °C
Basics of polymerization
Definition of polymerization
Polymerization is a chemical process in which monomers (individual molecules) are combined to form a macromolecule (polymer).
The determination of conversion in chemical reactions in general and in polymerization reactions in particular is of great importance with regard to process monitoring, process control and process control.
Just like concentration measurement, the importance of monitoring polymerization in all areas of the economy is increasing enormously at present. High economic effects such as material and energy savings as well as quality improvements are possible.
There are a number of measurement methods for concentration and conversion measurements, such as density measurement, refractive index measurement, conductivity measurement, color, turbidity and viscosity measurement, all of which have their physical and technological application limits.
The possibility of determining concentrations by measuring the speed of sound has been known for some time and has established itself as a standard measurement method.
Physical principles of polymerization
The propagation velocity v of ultrasound in liquids depends on their density and adiabatic compressibility via the following relationship:
v = speed of sound
ρ = density
βad = adiabatic compressibility
Compressibility is a determining factor for the speed of sound. This means that as the speed of sound increases, the density and compressibility can be in opposite directions. As a result, large differences in the speed of sound can occur with small or small differences in density. The reverse case occurs very rarely.
The speed of sound is determined by the structure of the substance, i.e. by atomic and molecular groups, isomers or chain lengths. This relationship makes it possible to characterize substances using ultrasound.
The sonic velocity v of some selected monomers and polymers at 20 °C is shown in the following table.
The structure of the macromolecule, which is produced by the polymerization of monomers, influences the sonic velocity, as it is determined by the arrangement of the atomic and molecular groups, isomers and chain lengths.
For monomer-polymer systems, it is generally true that the differences in the speed of sound between monomer and polymer are primarily determined by the chain length and the degree of branching and cross-linking. The table clearly shows that the differences occurring between monomer and polymer and thus between the start and end of the polymerization reaction are sometimes very large.
Measuring methods in polymerization
Various measuring methods are used to determine the degree of polymerization in order to monitor the progress and quality of the process. Common methods include viscosity measurements, concentration measurements, gravimetry and calorimetry.
Problems with viscosity measurement
Viscosity measurements are common, but they can be problematic. In particular, they are influenced by temperature fluctuations, shear rates and the presence of impurities, which can change the viscosity of the polymer mixture and therefore provide inaccurate measurement results. In addition, viscosity is difficult to measure at very high or very low molecular weights.
The presence of impurities can result in unreliable measurement results and then requires an intensive cleaning process, which negatively affects the effectiveness of the process.
Advantages of concentration measurement
In contrast to viscosity measurement, concentration measurements are less susceptible to interference factors. They offer a direct measurement of the monomer concentration and are not dependent on the physical properties of the polymers. This leads to more accurate and reliable data on the progress of polymerization.
Processes
Polymerization can take place through a wide variety of reaction mechanisms, whereby the monomers react to form longer chains or branched structures, the macromolecules. Depending on the reaction mechanism, polymerizations are divided into
Solution polymerization
Emulsion polymerization
suspension polymerization
polycondensation
Depending on the number of copolymers and product-changing additives, the change in sonic velocity shows a characteristic curve. Typically, the sonic velocity of all components involved is determined as a function of temperature in order to compensate for this later. The course of the reaction can then be derived from the sonic velocity over time and the material conversion can be calculated.
In the following description, this is explained as an example for the emulsion polymerization of styrene-butadiene latex. Parameters such as concentration, degree of polymerization, etc. are determined in the same way for the other polymerization types.
Emulsion polymerization of styrene-butadiene latex For the reaction system
emulsion polymerization of butadiene-styrene, the individual components and the latexes were investigated.
The following figure shows that the sonic velocity of the monomers differs significantly from that of the polymers.
The speed of sound and the concentration are directly related. Furthermore, the degree of polymerization, which reflects the proportion of the polymer in the monomer, correlates with the concentration. It is therefore possible to determine the concentration and the degree of polymerization using ultrasonic measurement technology. The following figure illustrates this relationship in a polymerization of butadiene styrene.
In the case of emulsion polymerization of butadiene and styrene, the degree of polymerization can be determined with an accuracy of 0.1 %.
Applications
Based on our experience of over 20 years, we have accumulated a great deal of knowledge in the field of polymerization, which has been absorbed through applications at the customer's premises and in the company's own technical centre. This knowledge flows into new projects, whereby customer data is always treated confidentially.
During polymerization, not only macromolecules but also monomers come into the focus of monitoring in order to ensure the exact course of the reaction and product quality.
The following secondary literature on various manufacturing processes is available from SensoTech:
Optimization of polyamide production
Optimization of polyurethane production
Safe and efficient styrene-butadiene latex (SBR) production
The applications investigated so far include
Caprolactam polymerization
Styrene-butadiene latex
Phenol-formaldehyde resin
Poly-methyl-meta-acrylate PMMA
Polyvinyl acetate PVA
Polyvinyl chloride PVC
Polyamide PA
Polyvinylidene chloride PVdC
Epoxy resin
Polystyrene PS
Polycarbonate PC
Polyester PE
polyethylene
Formaldehyde-urea resin
elastane
Aldol in acetaldehyde
Polyurethane PU
Polysiloxane
Isoprene rubber IR
Methyl silicone resin
Silicone acrylate
Potassium methyl silicone resin
silicone resin
Polysulfide polymer
Paraphenylene terephthalamides PPTA
Hindered Amine Light Stabilizers HALS
Methacrylamide MAA
customized compositions
The LiquiSonic® measuring device enables the monitoring and control of different reactions, especially in batch processes. Depending on the process and process liquid, catalytic and enzymatic reactions as well as polymerization, crystallization and mixing processes can be optimized and the quality of the end product guaranteed.
The general rule for monomer-polymer systems is that the differences in sonic velocity between monomer and polymer are primarily determined by the chain length and the degree of branching and cross-linking.
The table shows that the differences occurring in the sonic velocity between monomer and polymer and thus between the start and end of the polymerization reaction are very large.
The speed of sound and concentration are directly related. Furthermore, the degree of polymerization, which reflects the proportion of polymer in the monomer, correlates with the concentration. For this reason, the concentration and the degree of polymerization can be determined with the LiquiSonic® measuring technology.
Caprolactam production application example
One of the world's most important polyamides is PA6, known as Perlon, which is produced by polymerizing the monomer caprolactam (CPL). Due to the complexity of the manufacturing process, it is divided into 4 areas:
Synthesis of crude caprolactam
Separation and crystallization of ammonium sulphate
Purification and preparation of the crude caprolactam
Polymerization to PA6
In caprolactam production, the base material cyclohexanone oxime is first produced from cyclohexanone, hydroxylamine and H2SO4. Crude caprolactam is produced by adding oleum and ammonia, which is separated from the ammonium sulphate phase. The monomer caprolactam is then purified and concentrated by extraction and crystallization. After polymerization, the polymer is finally separated from the residual monomer and purified.
Macromolecules, polymers and plastics are ubiquitous products and must meet the highest standards. The processes developed for their production often run under high process pressures and temperatures. Due to these boundary conditions, the monitoring and control of these processes must meet the highest safety requirements.
Polymerisations-Überwachung
Die Umsatzbestimmungen bei chemischen Reaktionen besitzen allgemein und insbesondere bei Polymerisationsreaktionen eine hohe Notwendigkeit hinsichtlich Prozessverfolgung, Prozesskontrolle sowie Prozesssteuerung.
Polymerisations-Überwachung | 11 MB