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NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 5

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 5

The history of polyurethanes

In 1937, the German chemist Dr. Otto Bayer (1902–1982) invented the versatile class of plastics we call polyurethanes. Polyurethanes are available in myriad forms—they are used in numerous products, from coatings and adhesives to shoe soles, mattresses, and foam insulation. Despite the variety in their characteristics, the underlying chemistry of these different forms is strikingly similar.

During World War II, the use of polyurethanes became popular as a replacement for rubber, which at the time was expensive and hard to obtain. Around the 1950s, polyurethanes began to be used in adhesives, elastomers, rigid foams, and flexible cushioning foams such as those used today.

Wallace Carothers, inventor of polyamide.
Dr. Otto Bayer was credited with inventing polyurethanes in 1937.

Nowadays, a life without polyurethane is difficult to imagine, as you can easily find it everywhere around you.

How is polyurethane created?

 

Polyurethanes are formed by reacting polyols (i.e., alcohols containing more than two reactive hydroxyl groups in each molecule) with di-isocyanates or polymeric isocyanates. Suitable catalysts and additives are used wherever necessary. Since both a variety of di-isocyanates and a wide range of polyols can be used to produce polyurethane, a large spectrum of polyurethane materials can be produced to meet the specific requirements for different applications. Polyurethanes can appear in a variety of forms including rigid foams, flexible foams, specialty adhesives, chemical-resistant coatings, sealants, and elastomers.

Figure 1. Molecular structures of isocyanates, polyols, and polyurethane.

Physical and chemical properties of polyurethanes

The properties of polyurethanes are highly dependent on their production process. When the polyol chain (Figure 1) is long and flexible, the final product will be soft and elastic. On the other hand, if the extent of cross-linking is very high, the final polyurethane product will be tough and rigid. The cross-linked structure of polyurethanes generally consists of three-dimensional networks which result in very high molecular weights. This structure also accounts for the thermosetting nature of the polymer since polyurethane typically does not soften or melt when exposed to heat.

One of the most popular forms of polyurethane is foam. This form is created by facilitating the production of carbon dioxide gas during the urethane polymerization process.

Typical applications of polyurethane

The primary application of polyurethane is in the production of foams (rigid and flexible). Other important applications and uses of polyurethane are listed below.

 

  • Low-density, flexible polyurethane foams are widely used in mattresses and automobile seats.
  • Bathroom and kitchen sponges are commonly made from polyurethane. It is also used in the manufacturing process of seat cushions and couches.
  • Polyurethane is also used to produce textiles used in some clothing and upholstery.
  • Due to its good insulating properties, polyurethane materials are commonly used in construction work.
  • Polyurethane moldings are also used in columns and door frames.
  • Flexible polyurethane is used in the manufacture of partially elastic straps and bands.
  • The low-density elastomers of polyurethane are widely used in the footwear industry.

In Table 1 a variety of polyurethane properties are compared to other conventional materials like rubber, metal, and plastic.

Table 1. Polyurethane in comparison with rubber, metal, and plastic.

PU vs. Rubber

PU vs. Metal

PU vs. Plastic

High abrasion resistance

Lightweight

High impact resistance

High cut and tear resistance

Noise reduction

Elastic memory

Superior load bearing

Abrasion resistance

Abrasion resistance

Thick section molding

Less expensive fabrication

Noise reduction

Colorability

Corrosion resistance

Variable coefficient of friction

Oil resistance

Resilience

Resilience

Ozone resistance

Impact resistance

Thick section molding

Radiation resistance

Flexibility

Lower cost tooling

Broader hardness range

Easily moldable

Low temperature resistance

Castable nature

Non-conductive

Cold flow resistance

Low pressure tooling

Non-sparking

Radiation resistance

Near-infrared spectroscopy as a tool to assess the quality of polyurethanes

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the polyurethane industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, a short overview of polyurethane applications is presented, followed by available turnkey solutions for polyurethane analysis developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first parts in this series? Find them here!

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Applications and parameters for polyurethanes with NIRS

When producing different types of polyurethanes, it is important to check certain parameters to guarantee the quality of the finished products. Typical parameters include hydroxyl number, acid number, moisture, and color in polyols as well as the NCO (isocyanates) content, (total) acid number, and moisture content in polyurethanes. The most relevant applications for NIRS analysis in polyurethane production are listed later in this article in in Table 2.

Where can NIRS be used in the polyurethane production process?

Figure 2 shows the individual steps from plastic producer via plastic compounder and plastic converter to plastic parts and foam producer.

Figure 2. Illustration of the production chain for polyurethanes.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of polyamides and offers a turnkey solution in the form of the DS2500 Polyol Analyzer. This instrument is a ready-to-use solution for the determination of multiple quality parameters in polyols and polyurethanes. For the analysis of polyurethane pellets and parts, the Metrohm DS2500 Solid Analyzer is recommended.

Figure 3. Turnkey solution for polyurethane analysis with the Metrohm DS2500 Polyol Analyzer.

Learn more about the possibilities of polymer analysis with Metrohm DS2500 Analyzers in our free brochure.

Application example:

Pre-calibrations and starter model for the PU industry on the DS2500 Polyol Analyzer

The determination of the parameters listed below in Table 2 is a lengthy and challenging process with conventional laboratory methods. To measure them all, several different techniques are required which takes a significant amount of time, not only to analyze the sample, but also for the instrument management and upkeep.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PU samples.
Parameter Primary method Time to result (primary method) Relevant NIRS Application Notes NIRS benefits
Hydroxyl number in Polyols

Titration

90 min. preparation + 1 min. Viscometer

AN-NIR-068

AN-NIR-065

AN-NIR-035

AN-NIR-007

All three parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
NCO (Isocyanate) content in PU HPLC 20 min. preparation + 20 min. HPLC
Moisture content

Karl Fischer Titration

25 min. preparation + 5 min. KF Titration

 

The NIRS prediction models created for polyols are based on a large collection of real product spectra and are developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

To learn more about pre-calibrations for polyols, download our brochure and visit our website.

One example of a dedicated ASTM standard referring to NIRS is ASTM D6342-12 Standard Practice for Polyurethane Raw Materials: Determining Hydroxyl Number of Polyols by Near Infrared (NIR) Spectroscopy. The following application example demonstrates that the DS2500 Polyol Analyzer operating in the visible and near-infrared spectral region (Vis-NIR) provides a cost-efficient and fast solution for the determination of the hydroxyl number in polyols and the NCO (isocyanate) content in polyurethanes. With no sample preparation or chemicals required, Vis-NIR spectroscopy allows analysis of all three quality parameters listed in Table 2 in less than a minute. The results are shown in Figure 4 and Figure 5.

Figure 4. Turnkey solution for determination of hydroxyl number in polyols using the Metrohm DS2500 Polyol Analyzer. A: Sampling and analysis of polyols. B: NIRS results compared to a primary laboratory method along with the Figures of Merit (FOM).
Figure 5. Turnkey solution for determination of NCO content (Isocyanates) in polyurethane using the Metrohm DS2500 Polyol Analyzer. A: Sampling and analysis of polyurethane. B: NIRS results compared to a primary laboratory method along with the Figures of Merit (FOM).

This application example demonstrates that NIR spectroscopy is excellently suited for the analysis of multiple parameters in polyols and polyurethanes in less than one minute without sample preparation or using any chemical reagents. Visit our website to learn more about our variety of analytical solutions for the polymer industry!

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Fast and fundamental: influences on reliable electrochemical measurements

Fast and fundamental: influences on reliable electrochemical measurements

The ultimate goal of any researcher is to contribute to the progress of society by pioneering exploration beyond the known limits. Depending on the research type and application field, one way to fulfill this is to collect reliable experimental data on rapidly occurring processes (less than 1 ms).

Having insight into the fundamentals of these reaction mechanisms can ultimately lead to the discovery of new materials or the improvement of current solutions. In electrochemical research, reaction mechanisms and intermediates are investigated by measuring the kinetics and dynamics of the electrochemical processes happening at the surface of the electrode on a sub-ms timescale.

This article provides a short overview of the factors that have a direct influence on fast and ultra-fast electrochemical measurements from an experimental setup perspective.

Considering the following factors in the experimental design and execution is the first condition to obtain reliable experimental results for such measurements.

Additional challenges which researchers must be aware of when experimenting with «transient electrochemistry», i.e. doing electrochemical measurements at very low time scales, is presented in the featured article from E. Maisonhaute et al. [1].

Main factors that influence the reliability of fast electrochemical experimental results

The primary components of an electrochemical experimental setup are:

  • The electrochemical cell including the electrodes and electrolyte
  • The electrochemical instrument, i.e., the potentiostat/galvanostat (PGSTAT)

To perform reliable electrochemical experiments in general, and fast electrochemical measurements in particular, the specifications of the complete work system must be considered and the optimal settings must be used for all of the individual parts of the experimental setup.

Time constant of the electrochemical cell

The electrochemical cell and its specifications must be taken into account as it is an important element of the experimental setup.

Transient electrochemical experiments are not meaningful unless the cell time constant is small relative to the timescale of the measurement, regardless of the high-frequency characteristics of the control circuitry.

The cell time constant RuCdl (s) depends directly on the uncompensated resistance Ru (Ω) (i.e. the resistance of the electrolyte between the reference and the working electrode) and the double-layer capacitance Cdl (F) of the electrode [2].

As a consequence, when the potential is stepped or scanned rapidly, the true measured potential Etrue (V) lags behind the applied potential Eappl (V), according to the following equation:

Where RuCdl (s) is the time constant of the cell and t (s) is the time at which the measurement is taken.

Figure 1. Theoretical and true waveform applied to a real electrochemical cell [1].

For fast scan rates (i.e. when 𝑡 is much smaller than RuCdl ), the exponential term approaches 1 and significant errors in 𝐸true with respect to 𝐸appl can arise. For slow scan rates (i.e. when 𝑡 is much larger than RuCdl), the exponential approaches 0 and the errors become negligible.

The time constant of the cell can be reduced in three ways:

  • Reduce Ru via increasing the conductivity of the electrolyte by either increasing concentration of supporting electrolyte or decreasing viscosity
  • Reduce the size of the working electrode (e.g., by using microelectrodes) so that Cdl will be minimized
  • Move the reference electrode as close as possible to the working electrode (e.g., by using a Luggin capillary) so that Ru will be minimized

The electrochemical instrument: potentiostat/galvanostat (PGSTAT)

The potentiostat/galvanostat (PGSTAT) is used to accurately control the applied signal (potential or current) and measure the response (current or potential, respectively) from the electrochemical cell. The accurate control of the applied signals is achieved by using a control loop (or feedback loop) circuit.

When fast electrochemical measurements are executed, the following specifications will have a direct influence on the results and must be considered.

Bandwidth of the control loop of the PGSTAT

In general terms, bandwidth can be described as the parameter that defines how fast the instrument is able to react to any changes in the signal.

In electrochemical terms, the bandwidth is the frequency beyond which the performance of the system is degraded.

The bandwidth of the control loop of the PGSTAT (i.e. bandwidth of the instrument) indicates how fast the applied signal is controlled through the feedback loop.

Higher bandwidth means that the instrument uses a faster control loop (faster feedback). As a result, the applied signal will reach the desired set point faster, and in ideal circumstances the output signal will be identical to the theoretical waveform. However, depending on the properties of the electrochemical cell connected to the instrument, the applied signal might overshoot. In extreme cases, the instrument feedback loop might get out of control causing the potentiostat to oscillate. This is more likely when high-capacitance electrochemical cells are connected to the PGSTAT.

When a Lower bandwidth is used, the overall stability of the PGSTAT increases by reducing the speed of the control loop. In this case, the consequence is that at very high measurement speeds, the output of the applied signal may be slightly less accurate due to a slower slew rate. Nevertheless, when measuring fast transients is not within the scope of the experiment, using the instrument with a lower bandwidth setting is recommended for highly accurate experimental results.

Figure 2. Schematic representation of the applied signal when Low bandwidth (Low speed) and High bandwidth (High speed) settings are used compared with the theoretical response.

Therefore, it is important to choose the control loop bandwidth settings according to the type of the measurement. For ultra-high speed measurements, a higher bandwidth setting must be used with the following considerations:

  • The higher the bandwidth, the higher the noise and the probability that the control loop will go out of control and oscillate.
  • When working with a High bandwidth setting, it is necessary to pay special attention and use adequate cell shielding and electrode connectors. The use of a Faraday cage is recommended in these cases.
  • The use of a high impedance reference electrode (RE) (e.g., double junction reference electrode, a salt bridge with frit) in combination with a High bandwidth of the control loop might lead to instability of the PGSTAT and even to oscillations.
Bandwidth of the current sensor (current range)

The measurement of the current response of an electrochemical cell (in potentiostatic mode) and the control of the applied current value (in galvanostatic mode) is executed with specially designed current sensors. In order to achieve the best sensitivity and resolution for the measurement, individual current sensors are used depending on the magnitude of the measured (or applied) current.

Each current sensor circuit (which corresponds to a current range) has a specific bandwidth or response time. Therefore for the most accurate results (especially important for fast, time resolved experiments), the current range must be selected so that the bandwidth of the current sensor will not be the limiting factor for the time response (speed) of the measurement.

In general, the lower the measured currents, the lower the bandwidth of the current sensor.

Data sampling interval vs the timescale of the investigated transient signal

The measured electrochemical response can have a complex shape with components at many frequencies. The highest frequency component of the measured or applied signal determines the bandwidth of that signal. The bandwidth of the signal should not be higher than the bandwidth of the measuring device.

If the highest frequency component of the signal is fSIGNAL, then according to the Nyquist Theorem [3] the sampling rate fSAMPLE must be at least 2 fSIGNAL (i.e. two times higher than the highest frequency component of the signal).

Figure 3. Effect of the sampling frequency of an ideal sinusoidal signal [3]. Shown here are the theoretical signal (dashed line), sample points, and resulting measured signal (orange line).

In other words, the data sampling interval must be lower than the timescale in which the time resolved (transient) measurement from the investigated electrochemical process is expected to occur. There is a practical correlation between the sampling interval and instrument bandwidth. When the sampling interval is:

  • higher than 100 μs: the 10 kHz (High Stability) bandwidth should be selected.
  • between 10–100 μs: the 100 kHz (Fast) bandwidth should be selected.
  • smaller than 10 μs: the 1 MHz bandwidth (Ultra-Fast) should be selected.

Summary

To measure reliable experimental data, all elements of the experimental setup must be considered with their own specifications and limitations. The overview above highlights the main factors and parameters which can have a direct influence on fast electrochemical measurements.

Fast measurements start here!

Visit our website to learn more about the variety of potentiostats/galvanostats from Metrohm Autolab.

References

[1] Maisonhaute, E.; et al. Transient electrochemistry: beyond simply temporal resolution, Chem.Commun., 2016, 52, 251—263. doi:10.1039/C5CC07953E

[2] Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2001, 2nd ed. Russian Journal of Electrochemistry, 2002, 38, 1364–1365. doi:10.1023/A:1021637209564

[3] Keim, R. The Nyquist–Shannon Theorem: Understanding Sampled Systems. All About Circuits, May 26, 2020. https://www.allaboutcircuits.com/technical-articles/nyquist-shannon-theorem-understanding-sampled-systems/ 

Post written by Dr. Iosif Fromondi, Product Manager and Head of Marketing and Sales Support at Metrohm Autolab, Utrecht, The Netherlands.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 4

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 4

Polyamide (Nylon): A brief introduction

Wallace Carothers (1896–1937), the creator of polyamide.

Polyamide, more commonly known as Nylon, was first synthesized by Wallace Hume Carothers, an American organic chemist working for the DuPont chemical company. In 1935, he developed the formula known as PA66, or Nylon 66.

Just a few years later in 1938, Paul Schlack, a German chemist working at IG Farben, developed PA6 (also known as Nylon 6), a different molecule based on the organic compound caprolactam. Both types of polyamides are well-suited for many kinds of applications. The use of PA6 or PA66 depends on the technical requirements needed as well as the economical constraints.

The two most widely used polyamides are by far PA66 and PA6. These polyamides are most often manufactured into fibers for the textile industry or blown into films used for the packaging industry. Polyamides are also used to produce parts for numerous industries.

Polyamides with the highest performances are PPA (Polyphthalamide or high-performance polyamide) and PA46. Polyamides with these qualities are often used as a replacement for metal materials or for very specific applications where the polymer is exposed to extreme conditions, e.g. automotive structural parts or safety helmets.

Differences between Polyamide 6 (PA6 / Nylon 6) and Polyamide 66 (PA66 / Nylon 66)

Polyamide 6 (PA6) is also known as Nylon 6 or Polycaprolactam. It is one of the most commonly used compounds in the polyamide family. PA6 is synthesized via the ring-opening polymerization of caprolactam.

Figure 1. Molecular structure of caprolactam.
Figure 2. Molecular structure of Polyamide 6.

Polyamide 66 (PA66), also known as Nylon 66, is one of the most popular thermoplastics for engineering purposes and is primarily used as a metal replacement for various applications. Nylon 66 is synthesized via the polycondensation of hexamethylenediamine and adipic acid (two monomers containing six carbon atoms each).

Figure 3. Molecular structure of Polyamide 66.

The differences between both PA6 and PA66 come down to a lot of little things. While both are cost effective, Nylon 6 is typically around 30% cheaper than Nylon 66. A comparison of different factors is made for the two polymers in Table 1.

Table 1. Comparison of PA6 and PA66
Parameter PA6 PA66
Machinability – low tool wear and surface finish Good Better
Mold shrinkage Lower Higher
Water absorption rate Higher Lower
Tensile strength 6.2 × 104 kPa (Good) 8.2 × 104 kPa (Better)
Crystalline melting point 225 °C 265 °C
Density 1.15 g/mL 1.2 g/mL
Typical molding shrinkage ratio 1.2 % 1.5 %
Key properties of PA66 and PA6

As stated earlier, Polyamide 66 (PA66) and Polyamide 6 (PA6) are used in so many different applications because of their excellent performance and relatively low cost. Some of the most important properties of these polyamides are listed below:

  • High strength and rigidity at high temperatures
  • Good impact strength, even at low temperatures
  • Good abrasion and wear resistance
  • Excellent resistance to fuels and oils
  • Good fatigue resistance
  • Very good flow for easy processing
  • PA6 has excellent surface appearance and better processability than PA66 due to its very low viscosity
  • Good electrical insulating properties
  • High affinity for water absorption can limit the applications and usage
  • Low dimensional stability (water absorption results in dimensional change)

Near-infrared spectroscopy as a tool to assess the quality of polyamides

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the polyamide industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, a short overview of polyamide applications is presented, followed by available turnkey solutions for polyamide analysis developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first parts in this series? Find them here!

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Applications and parameters for polyamides with NIRS

Polyamide production requires that certain important quality parameters be checked on a regular basis. Typical parameters are relative viscosity as well as the amine and carboxylic end groups, and moisture content. Functional group and viscosity analysis of polyamides is normally a lengthy and challenging process due to the limited solubility of the sample and the need to use different analytical methods. Furthermore caprolactam, an important precursor for polyamide production, is very hygroscopic and water soluble—therefore it is crucial to have a reliable analysis technique for determination of water content. Otherwise the quality of the final product could be compromised.

The most relevant applications for NIRS analysis of PA quality parameters are indicated later in this article in Table 2.

Where can NIRS be used in the production process of polyamides?

Figure 4 shows the individual steps from plastic producer via plastic compounder and plastic converter to plastic parts and textile producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PA are produced, and their purity needs to be confirmed. NIRS is also a very useful technique during the next step where polymers are compounded into intermediate products to be used for further processing.

Figure 4. Illustration of the production chain for polyamides.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of polyamides and offers a turnkey solution in the form of the DS2500 Polymer Analyzer. This instrument is a ready-to-use solution for the determination of multiple quality parameters in different polyamides.

Figure 5. Turnkey solution for PA analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Pre-calibrations available for the polyamide industry on the DS2500 Polymer Analyzer

The determination of the parameters listed below in Table 2 is a lengthy and challenging process with conventional laboratory methods. To measure them all, several different techniques are required which takes a significant amount of time, not only to analyze the sample (which has limited solubility, further complicating the situation), but also for the instrument management and upkeep.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PA samples.
Parameter Primary method Time to result (primary method) Relevant NIRS Application Notes NIRS benefits
Relative viscosity

Viscosity

90 min. preparation + 1 min. Viscometer

AN-NIR-077

AN-NIR-060

AN-NIR-005

All four parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
Carboxyl end groups

Titration

90 min. preparation + 20 min. Titration
Amine end groups

Titration

90 min. preparation + 20 min. Titration
Moisture content

Karl Fischer Titration (oven)

2 min. preparation + 15 min. KF Titration (oven)

 

The NIRS prediction models created for polyamides are based on a large collection of real product spectra and is developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

To learn more about pre-calibrations for polyamides, download our brochure and visit our website.

Figure 6 shows the results of the Metrohm turnkey solution for non-destructive determination of several quality parameters in PA listed in Table 2.

Figure 6. Turnkey solution for relative viscosity (RV), amine end groups, carboxyl end groups, and moisture in nylon (PA6) using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PA6. B: Results of the four analyses from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for each analysis.

This solution demonstrates that NIR spectroscopy is very suitable for the analysis of multiple parameters in polyamide in less than one minute without sample preparation or using any chemical reagents. Learn more about the procedure in our free Application Note!

The examples shown above refer to PA6 and PA66, but NIRS is undoubtedly a great tool for the rapid screening and QC of polyamides with different chain lengths.

Future installments in this series

This blog is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for Polyamide 6 (PA6) and Polyamide 66 (PA66). The last installment of this blog series will be dedicated to:

 

  • Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Chemistry of Fireworks

Chemistry of Fireworks

Developed nearly two millennia ago in ancient China, fireworks are increasingly used in cultural celebrations around the world and enjoyed by nearly all ages. As one of the most entertaining forms of chemistry, fireworks appeal to our senses of sight and sound, offering a staggering variety of colors, sizes, shapes, sounds, and so on. We love to watch fireworks because they take our breath away with their magnificence and mystery.

However it is not all fun and games. The business of fireworks (and the field of pyrotechnics in general) is very serious since they should be made as safe as possible to use and also environmentally friendly. Beyond fireworks, other pyrotechnics are found in all kinds of entertainment, like in concerts, movies, and more serious applications for defense and security (e.g., safety measures like flares and signal lights).

What are fireworks made of?

Early fireworks were quite dangerous and were used for protection rather than for celebrations, and hardly resemble the ones we are now familiar with.

It all began back in Ancient China with the invention of gunpowder, which was created from a mixture of charcoal, sulfur, and saltpeter (potassium nitrate). Eventually, as new developments were made to increase the safety and predictability of using these early fireworks, experimentation with colors began and people started using them more for nonviolent purposes. Now there is an entire industry devoted to the development of all kinds of fireworks for consumers and professionals alike.

Learn more about the history of fireworks in the links below:

A firework, or aerial shell as it is also known, basically consists of three main parts aside from the housing: gunpowder and an igniter to make the rocket explode, and inside of the transported capsule on the top there are small garniture pods usually called «stars» (despite being shaped like spheres or cylinders) that include various chemicals for the desired effects. Stars consist of a colorant, a fuel, an oxidizer (oxygen providing substance, e.g., chlorates or nitrates), and a binder to hold the ingredient mixture together in a compact briquette.

The industry has spent a significant amount of time in development to make fireworks explode in shapes like stars and stripes, hearts, or even more complex forms like a cartoon figure, or letters and numbers if timed correctly.

Cross-sectional diagram of a firework capsule filled with star garnitures (72) and igniter (70). [1]

Forming a rainbow of colors

The vibrant colors of fireworks come from the combustion of metal ions which make up to 20% of the components. Metals have been used to color flames even before the invention of modern fireworks (e.g. Bengal fire). Chemically speaking, these metal ions change their electronic state by heating (addition of energy) and then going back to a lower energy state before emitting light of a certain color.

Table 1. List of metals used in pyrotechnics and their colors [2].

Color

Metal

Example compounds

Red

Strontium (intense red)

SrCO3 (strontium carbonate)

Lithium (medium red)

Li2CO3 (lithium carbonate)

LiCl (lithium chloride)

Orange

Calcium

CaCl2 (calcium chloride)

Yellow

Sodium

NaNO3 (sodium nitrate)

Green

Barium

BaCl2 (barium chloride)

B3N3 (boron nitride)

Blue

Copper halides

CuCl2 (copper chloride), at low temperature

Indigo

Cesium

CsNO3 (cesium nitrate)

Violet

Potassium

KNO3 (potassium nitrate)

Rubidium (violet-red)

RbNO3 (rubidium nitrate)

Gold

Charcoal, iron, or carbon black

 

White

Titanium, aluminum, beryllium, or magnesium powders

 

Very prominent here is the yellow color from sodium which is also seen in older street lightbulbs in some countries. Unfortunately, the most vibrant colors formed are also the most toxic for the environment, like strontium (red) and barium (green). These contaminants can be measured in the air, water, and even in the soil—but more on that later.

Find out more information about how fireworks get their colors in the links below:

Safety first

Safety is always a critical issue when discussing fireworks, whether concerning their construction, their use, or their storage. Too many serious accidents have happened over the years involving fireworks.

Learn more about how to handle fireworks in a safe manner here:

Among one of the largest fireworks disasters recorded in Europe was in Enschede (The Netherlands) in 2000. This explosion occurred in the warehouse of the S.E. Fireworks factory, which was located in the center of a residential area as the city grew and continued to build homes around it. An entire neighborhood was razed and the largest of the explosions was felt up to 30 kilometers away.

Because of this incident, sales of larger fireworks in most European countries is only allowed outdoors. Accumulating fireworks at home in preparation for celebrations should be avoided at least in confined environments like basements or apartments. It is better to store them in a ventilated shed or car parking to avoid problems in the case of a fire. Also do not store fireworks for long periods, since most of commercial fireworks are meant to be used within 3–6 months after production because the paper contents can get humid, ionic substances can dissolve and recrystallize, and therefore the likelihood of a failure increases.

In the event of a firework failure: Never have a look immediately! Wait at least 15 minutes at a proper distance and then use a tool to confine it afterwards—never touch it with your bare hands, especially when dealing with exploding fireworks or rockets.

Having said this, fireworks have integrated some safety features over the last several years to work more properly and reliably. For instance, the propellants have been modified from containing black powder to using technology from rockets such as plasticizers for better burning performance during launch, also resulting in less smoke and dust on the ground. A dedicated chain of reactions has to be followed, otherwise it will burn in a harmless way.

Knowledge is power: Prevent accidents with proper analytical testing

In order to help prevent fireworks accidents such as the one in Enschede and countless others, it is crucial to closely monitor different quality parameters including the water content of paper-based fireworks, grain size of the metal particles, and the purity and composition of the colorant, just to mention a few. Adequate quality control provides an entertaining, but safe fireworks experience even in the hands of the general public, when proper protocols are followed.

Metrohm offers several analytical technologies and related applications for this area of research. Analyses can be performed for a wide variety of substances and quality parameters as well as trace materials in the laboratory, on the street, and in the air either via wet chemical methods (e.g., Karl Fischer titration, ion chromatography, voltammetry) or spectroscopic techniques (e.g., near-infrared spectroscopy [NIRS] and Raman spectroscopy).

As mentioned earlier, moisture is an important quality parameter when discussing the safety of explosive materials. Metrohm offers two different techniques for accurate analysis of water content in a variety of matrices which are outlined in the following blog posts.

When it comes to determining the individual concentrations of the main constituents, some wet chemical techniques really stand out. Suppressed anion chromatography is ideal for measuring the ionic components of e.g., firecracker powder, other explosive material, and even in explosion residues for forensic purposes. Coupling an ion chromatograph to a mass spectrometer (IC-MS) opens up even more analysis possibilities. Read more about these studies (and more) by downloading our free Application Notes.

The use of several different metal salts to create the vibrant colors of fireworks can be beautiful but also harmful to our health and that of our environment. Voltammetry (VA) is an electrochemical method suitable for the determination of trace and ultratrace concentrations of heavy metals and other electrochemically active substances. Not only is VA excellent at determining these substances in the laboratory, but also in the field such as for measuring the after effects of a fireworks display or an undesired event. Check out our selection of VA instruments and applications on our website.

Spectroscopic techniques like Raman can help to determine the presence of dangerous explosive materials even when keeping a safe distance by using different instrument attachments. Read our free White Paper about how to use MIRA DS from Metrohm Raman for the purpose of identifying explosives safely.

Environmentally friendly fireworks – a contradiction?

Although fireworks are a very spectacular form of entertainment, there is quite an environmental impact after big cultural events or national holidays. The general atmospheric pollution after a fireworks display has been set off can be seen in an increase of dust and smoke, but also heavy metal content in the air as most contemporary fireworks use these for coloring.

The unburnt material still contains a significant amount of heavy metals. After falling to the ground, this material can dissolve and enter the ground water after it rains. Plastics materials that covered the fireworks for safety reasons are found again as broken shell shrapnel or as microplastics. The combustion of the compounds inside the fireworks leads to increased air pollution in form of aerosols that can be measured and evaluated resulting in heavy metals in the air, fine dust, and even nanoparticles which are extremely harmful for our lungs.

Metrohm Process Analytics has developed the 2060 MARGA (Monitor for AeRosols and Gases in ambient Air) which is used by official agencies and research bodies worldwide to monitor the air quality fully autonomously. This instrument is based on the analytical technique of ion chromatography and can be used as a dedicated continuous air monitoring device that can be left unattended for several weeks at a time, or as a research instrument that can be used for other projects when not monitoring the air quality.

Learn more about the 2060 MARGA and its capabilities in our blog post.

To find out more about the use of Metrohm instruments to monitor the air quality, check out this selection of peer-reviewed articles.

A new «green» firework generation is being developed for both professional and indoor use to try to minimize the heavy metal content and also reduce aerosol forming agents. This makes them more suitable for indoor pyrotechnic shows and for movie production. In regular outdoor shows (e.g. at theme parks), the gunpowder for transport of the capsule has mostly been substituted with an air pressure gun mechanism.

A significant amount of research has gone into substituting heavy metal-based colorants with more environmentally benign substances by increasing the luminosity of lithium derivatives by substituting them for strontium, or by using boron instead of barium or chlorinated compounds.

Finally, the plastic parts commonly used to surround fireworks are planned to be substituted by microcrystalline cellulose mixtures with better plasticizing binders. This leads to a similar stability compared to the current plastic materials, but the cellulose-based containers burn up completely and do not leave harmful materials scattered on the ground.

The future of fireworks shows

All safety measures increase the joy of fireworks not only during, but also after the event—being green and being safe. Foretelling the future, some of these celebrations may now use a cadre of lighted drones in a choreographed dance. This has been happening more steadily as drones fall in price and increase in their handling and programming capabilities. However, fireworks have already been with us for a couple of thousand years, and probably will not disappear any time soon.

Download our free Application Notes

and White Papers related to explosives and propellants

Post written by Dr. Norbert Mayr (Ph.D. in the field of HEDM, pyrotechnics, propellants, and oxidizers), Marketing Specialist & Product Training at Metrohm International Headquarters, Herisau, Switzerland.

Green hydrogen, future fuel: Using potentiostats to develop new catalysts for hydrogen production

Green hydrogen, future fuel: Using potentiostats to develop new catalysts for hydrogen production

Hydrogen – clean and green?

Due to its high gravimetric energy density and zero pollution emission, «green hydrogen» is a clean and sustainable energy carrier which is expected to become one of the fuels of the future. Green hydrogen is produced with renewable energy sources, and it can help to mitigate global warming by using cleaner fuels.

Producing green hydrogen with electrolysis

The most favorable way to produce green hydrogen is via water splitting electrolysis, where water (H2O) is broken down into its counterparts by using a direct electric current. Electrolysis is a sophisticated technique that has been used for many decades in industry. When using this technique for the production of hydrogen, drawbacks are the sluggish reaction kinetics when using inexpensive catalysts or the high costs for more optimal catalysts (e.g. platinum). To produce hydrogen in an efficient and economical manner, the goal for researchers around the world is to develop catalysts for this purpose which are highly active, inexpensive, and stable over long periods.

This article explains in more detail how Metrohm potentiostats can be used to characterize recently developed catalysts for electrochemical hydrogen production.

Electrode reactions

Considering alkaline solutions, the water splitting reaction can be described by two half reactions (Figure 1):

  • Hydrogen Evolution Reaction (HER) at the cathode
  • Oxygen Evolution Reaction (OER) at the anode
Figure 1. Water splitting reaction with the respective half reactions at the cathode and anode.

A critical issue for water splitting is the slow reaction kinetics of the half reactions. To overcome this, electrocatalysts which decrease the activation energy to an acceptable value need to be designed. For an ideal catalyst, a voltage of 1.23 V (at 20 °C and 1013 mbar) would have to be applied to begin hydrogen and oxygen evolution between the electrodes. Unfortunately, when using catalysts in real life situations, voltages above 1.8 V need to be applied.

Requirements for suitable catalysts

Catalysts for the production of green hydrogen from electrochemical water splitting reactions must be (click to go directly to each segment):

The first three properties can be determined by electrochemical measuring techniques using a potentiostat.

Activity

The activity of a catalyst is characterized by three values (click to go directly to each segment):

which can be obtained from the polarization curve as displayed in Figure 2.

Figure 2. Polarization curves of an ideal catalyst (orange) and a real catalyst (dark blue) considering the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER).
Linear polarization

To record a polarization curve (Figure 2), a potentiostat is used in combination with a three-electrode setup. The working electrode is coated with the catalyst which is to be characterized. A typical setup for this purpose is shown in Figure 3.

Figure 3. Potentiostat from Metrohm Autolab combined with a three-electrode setup using a Rotating Disc Electrode (RDE) as working electrode to keep mass transport under defined conditions.

During the measurement, the potential (beginning at a defined initial potential value) is swept to the end potential in a linear fashion over a certain time interval (Figure 4). For HER-catalysts, the potential is swept in the negative direction compared to the initial potential; catalysts used for OER purposes are swept to positive potentials with respect to the initial potential.

The overpotential (η) is a very important parameter to evaluate the activity of the catalyst; its value is affected by the kinetic barrier for the reaction. To overcome this barrier, a higher potential than the thermodynamic potential (1.23 V for OER, or 0 V for HER) – the overpotential – has to be applied to reach the same current densities. The more active the catalyst, the lower the overpotential.

Figure 4. General diagram of linear sweep voltammetry.
Tafel analysis

Aside from the overpotential, the Tafel slope and exchange current density are two additional parameters that help characterize the activity of the catalyst. They can be accessed by plotting the logarithm of the kinetic current density versus overpotential to create the so-called «Tafel plot» (Figure 5).

Figure 5. Tafel plot of OER for two different catalyst materials.

By evaluating the Tafel plot, these two important kinetic parameters can be extracted. One is the Tafel slope (b) which can be expressed by the following equation:

η = a + b log i

  • η = overpotential
  • i = current density

The other parameter is the exchange current density (i0), which can be obtained by extrapolating the current at zero overpotential (Figure 6).   

The Tafel slope is related to the catalytic reaction mechanism in terms of electron-transfer kinetics. For example, faster electrocatalytic reaction kinetics lead to a smaller Tafel slope which is shown in a significant current density increment as a function of the overpotential change (Figure 6).

Koutecký and Levich analysis

The exchange current density (i kin0) describes the charge transfer under equilibrium conditions. A higher exchange current density means a higher charge transfer rate and a lower reaction barrier. As explained earlier in this article, for a better electrocatalyst, a lower Tafel slope and a higher exchange current density are expected.

When performing real experiments, mass transport is the limiting factor at higher overpotentials (Figure 5) which leads to a nonlinear slope of the Tafel plot.

To overcome the undesirable impacts of mass transport (e.g. diffusion), a Rotating Disc Electrode (RDE) (Figure 3) is used. The current signal is therefore measured at different rotation rates. From this data set, it is possible to extract the pure kinetic current (i kin0) according to Koutecký and Levich (Figure 6).

Figure 6. Tafel plot of OER for two different catalysts. The exchange current density (i kin0) is determined by extrapolation to η = 0.

You can find more information about Koutecký and Levich analysis by downloading our free Application Note:  

Impedance analysis

Another measuring technique can be used to determine the reaction kinetics of catalysts: electrochemical impedance spectroscopy (EIS). One of the most important advantages of impedance spectroscopy is that it is a non-destructive and non-invasive measuring technique. This enables consecutive measurements to be performed on the same sample, such as experiments at different temperatures or at different current densities. Also, the aging effects of catalysts can easily be determined this way.

Impedance spectroscopy is an alternating electric current (AC) technique, where an AC voltage with a very small amplitude of a few millivolts is applied to the electrode which responds with an AC current. The values of the applied voltage signal and the values of the corresponding current signal are used to calculate the AC resistance—the impedance. A broad frequency range of several decades is applied and this enables the identification of the kinetic and transport processes that take place on the electrode over different time scales.

Displaying the impedance spectra in a Nyquist plot (Figure 7) exhibits the charge transfer resistance of the reaction as a semicircle. The diameter of this semicircle corresponds to the reaction kinetics: the smaller the diameter, the faster the kinetics of the reaction.

Figure 7. Example of a Nyquist plot of two different catalysts with different activities.

Electrochemical impedance spectroscopy does not only provide kinetic information to researchers, it also gives insight into mass transport effects and conductivity of electrolytes and membranes.

To find out more about EIS, check out our selection of free Application Notes:

Stability

For industrial use, a catalyst should exhibit an extremely low degradation rate. It needs to be stable for many operating hours. At the development stage, stability is an important factor to determine whether a catalyst has the potential for use in practical applications. Stability can be characterized by the changes of the overpotential or current over time by using chronoamperometry, chronopotentiometry, and cyclic voltammetry. These are described in the following sections.
Chrono methods (chronoamperometry and chronopotentiometry)

When using a chronoamperometry method, a constant voltage is applied to the catalyst and the corresponding current signal is sampled and plotted as an i/t curve (Figure 8).

Figure 8. Chronoamperometry measurement of two different catalysts for OER.

Conversely, when using a chronopotentiometry method, a constant current is applied to the catalyst and the voltage response is measured and plotted as an E/t curve (Figure 9). For this measurement, the longer the tested current or potential remains constant, the better the stability of the catalyst.

Figure 9. Chronopotentiometry measurement of two different catalysts for OER.
Cyclic Voltammetry

Cyclic voltammetry is a technique that measures the current density by cycling the potential of the working electrode linearly versus time (Figure 10) . In contrast to linear sweep voltammetry (Figure 4), after the end potential is reached, the potential in a CV experiment is scanned in the opposite direction to return to the initial potential.

Figure 10. Example of a cyclic voltammetry (CV) diagram with one cycle shown.

To determine the degradation rate of a tested catalyst, usually more than 5000 cycles must be executed with a scan rate between 50–100 mV/s. Before and after CV cycling, linear sweep voltammetry (LSV) is used to examine the overpotential shift at a specific current density.

The smaller the change of the overpotential, the better the electrocatalyst’s stability.

Efficiency

The efficiency (η) can be determined by the faradaic efficiency (or coulombic / coulombian efficiency) in terms of experimental results compared to theoretical predictions.

To be able to calculate the theoretical hydrogen volume via Faraday’s Law, the total charge is needed. This value is measured by the potentiostat using the chronocoulometry method which records the total charge over time (Figure 11).

Figure 11. Example of a chronocoulometry plot.

Conclusion

Choosing the right technique for the analysis of activity, stability, and efficiency of catalysts depends on the specific research and development project focus. Luckily, Metrohm offers a wide range of solutions that will meet all kinds of research requirements.

Visit our website

and learn more about our wide range of solutions for electrochemical research!

Post written by Sandro Haug, Product Manager Electrochemistry at Deutsche METROHM GmbH & Co. KG, Filderstadt, Germany.