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From corn to ethanol: improving the fermentation process with NIRS

From corn to ethanol: improving the fermentation process with NIRS

The production of biofuels from renewable feedstock has grown immensely in the past several years. Bioethanol is one of the most interesting alternatives for fossil fuels, since it can be produced from (renewable) raw materials rich in sugars and starch.

Fermenting corn starch to produce ethanol for fuel is a complex biochemical process that requires monitoring of several parameters to ensure optimal production. Measuring these parameters via traditional laboratory techniques takes about an hour to complete and is a limiting step for increasing plant capacity and efficiency. Near-infrared spectroscopy (NIRS) can replace routine laboratory analysis, decreasing operating costs and increasing plant efficiency and capacity.

Learn more about this fast, non-destructive analysis technique in our different series of blog posts, including the benefits of using NIRS and some frequently asked questions.

Producing high quality ethanol as a fuel additive

Ethanol is an increasingly important component in the global fuel market, with countries looking to secure domestic fuel supplies and reduce their greenhouse gas emissions relative to fossil fuels. The United States and Brazil lead world bioethanol production, accounting for 83% of the supply.

According to the Renewable Fuels Association, approximately 26 billion gallons (nearly 100 billion liters) of ethanol were produced globally in 2020 [1], slightly reduced from a 2019 peak due to the global pandemic crushing demand for gasoline and ethanol as well. Demand for corn to transform into ethanol is still likely to rise as the United States increases adoption of E15 blends (15% ethanol in gasoline) [2]. Ethanol for export is also likely to increase in demand, with countries such as China implementing a E10 fuel standard for motor vehicles.

One of the primary ways to meet increasing product demand while maintaining price competitiveness is to increase plant capacity. However, the standard laboratory analytical workflow for monitoring the different parts of the fermentation process can be a limiting factor for growing a production site or improving its efficiency. Another consideration is the seasonal, and even regional variation of feedstock quality, requiring ethanol producers to closely monitor the fermentation process to ensure the same quality product is achieved.

A report from the National Renewable Energy Laboratory estimated that nearly 40% of the production cost of fuel ethanol from corn comes from labor, supplies, overhead, and variable operating costs [3]. Optimization of these costs, which include routine quality checks of the fermentation broth, regular maintenance of the fermenters and distillation towers, and triaging process upsets in a timely manner, leads to higher profitability of the ethanol production facility.

To maximize bioethanol production and profitability, laboratory limitations must be overcome. Near-infrared (NIR) spectroscopy is a proven economical, rapid, and operator friendly way to overcome common laboratory limitations. First, a bit of background information about the production of bioethanol is needed before jumping into how to optimize the process.

Ethanol process: wet vs. dry milling

There are two main production processes when it comes to creating ethanol from sugars and starches from starting materials such as corn: the wet milling process and the dry milling process (shown in Figure 1). Nearly all ethanol produced for fuel in the U.S. (the largest bioethanol manufacturer in the world) is made using the dry mill process [2].

Figure 1. Schematic representation of the dry mill ethanol process.

Grains are first ground into smaller, more homogenous particles in the dry milling process, which allows the husk or shell to be more easily penetrated. Water and enzymes are then added to create a slurry called a «mash». To facilitate the conversion of starches to sugars, the mash is heated to specific temperatures, then cooled before yeast is added. The yeast performs the work of creating ethanol from the converted sugars via the process of fermentation. However, the percentage of ethanol is still quite low, and therefore the solution must be distilled and dehydrated to obtain the concentration and purity necessary for fuel additives.

Wet milling differs from this process by first soaking the grains before grinding and separating out the various components. The starches are then converted to sugars which are used for the fermentation process, just as with dry milling.

If you want to know more about the fermentation process, read our blog post about optimization of beer brewing.

Lab analysis shortfalls

The lab serves many functions, but one of the key ones is to monitor the progress of the fermentation in each fermentation tank. This typically requires many different technologies, because several parameters must be checked to ensure that a fermentation is on track. Tight monitoring and control over the various sugars present (e.g., glucose, maltose, DP3, etc.) throughout the fermentation process is necessary to understand the breakdown pathway of the starch (glucose generation) present in the mash and optimize ethanol production. Understanding this pathway enables the proper dosage of enzymes and yeast to the mash in the slurry tanks (Figure 1) to accelerate breakdown. Therefore, optimizing the enzyme and yeast blend is crucial for this process. These are the highest consumable costs for ethanol production and significantly affect the rate of production and final yield of ethanol.

Some of the most common analytical instruments and their use cases are listed in Table 1.

Table 1. Typical instruments and parameters that are measured during fermentation of corn to ethanol.
Parameter Measurement technique Analysis time (min) incl. sample prep.
Dissolved solids (°Bx) Refractometer 3–5
pH pH meter 3–5
Solids (non-volatiles) Infrared balance 15–20
Ethanol HPLC 30–45

Sugar profile 
(DP2, DP3, DP4+, glucose, total sugar)

HPLC 30–45
Glycerol HPLC 30–45
Lactic acid Ion chromatography 30–45
Acetic acid Ion chromatography 30–45
Water content Karl Fischer titration 5–10

If all the properties in Table 1 are to be measured, it can easily take an hour using six different pieces of equipment. Factor in conditioning steps and reference scans to ensure proper calibration, and the time for a routine fermentation analysis increases. For a single corn fermentation, this can take upwards of 55 hours—one hour to perform the analysis and six hours between each measurement. However, increasing the number of concurrent fermentations to four or six means that measurements from the different tanks will begin to overlap.

Overlapping instrument demand combined with long analysis times results in a number of different challenges for bioethanol producers. First, if scheduled sampling times overlap, then sampling must either be delayed or samples must age while waiting for analysis. Second, the long analysis time means that data is no longer current, but minimally one hour or older by the time it has been communicated to the plant control center, which decreases the ability to deal with deviations. Neither of these situations is ideal for manufacturers—time is money, after all.

Long laboratory analysis times and infrequent measurements reduce the ability to perform interventions or to adjust other critical parameters (e.g., enzyme addition rate or process temperature). Additionally, such long wait times can impede the decision to end a fermentation early and begin anew if the batch is judged to be beyond recovery.

Faster measurements equal higher profits

The most obvious way to overcome measurement time challenges is to increase the number of tools in the lab and/or to add automation. However, this approach has costs in time; twice the sample preparation increases operating expenses and still fails to give high-speed feedback to the plant operations team.

A better way to overcome measurement time delays is to deploy near-infrared spectroscopy (NIRS), which can make all of the traditional laboratory measurements with one piece of equipment, at the same time, in less than five minutes.

Figure 2 displays the average ethanol concentration from HPLC measurements during several fermentations from one plant. The data shows apparent discontinuities in the first 12 hours, with spikes in glucose and dissolved solids. It is also apparent that the total solids measurement at 48 hours is erroneous. However, because the lab data requires so much time to collect, this spike is ignored instead of retested.

Figure 2. Key parameters measured for corn fermentation to ethanol as reported by the primary analysis methods listed in Table 1.

The NIRS alternative to traditional measurements shown in Figure 3 is of a single fermentation monitored in near real time. This high-speed analysis is possible because sample preparation is trivial for NIRS. Compared to the combination of HPLC and other analytical methods that consume about 60 minutes of operator time per sample, NIRS measures the same parameters and produces a quality result in about a minute. The ability to collect many NIR spectra in the early stages of the fermentation process provides a higher fidelity picture, enabling more timely interventions to maximize ethanol production.

The NIRS alternative to traditional measurements shown in Figure 3 is of a single fermentation monitored in near real time. This high-speed analysis is possible because sample preparation is trivial for NIRS. Compared to the combination of HPLC and other analytical methods that consume about 60 minutes of operator time per sample, NIRS measures the same parameters and produces a quality result in about a minute. The ability to collect many NIR spectra in the early stages of the fermentation process provides a higher fidelity picture, enabling more timely interventions to maximize ethanol production.

Figure 3. Corn fermentation to ethanol as measured by near-infrared spectroscopy.

The higher speed NIRS analysis can be used to increase total plant throughput by growing the number of batches and revenue, as shown in Table 2. With the traditional analysis, the fermentation is allowed to run 62–65 hours, depending on the final laboratory results (Figure 2).

With NIRS analysis, this fermentation is shown to be complete in around 56 hours (Figure 3). Reducing fermentation time by six hours expands the potential number of batches by 13 over the course of a year, representing a potential plant capacity increase of 10%.

Table 2. Comparison of the apparent fermentation time based on primary lab analyses vs NIRS analysis.
Traditional Lab Analysis NIRS Analysis

Total measurement time

12 hours

5 hours

Number of measurements

12

62

Fermentation end point

~62 hours

56 hours

Batch capacity

37,850 L

37,850 L

Batches per year

129

142

Download our free White Paper to learn more.

Near-infrared spectroscopic solutions for ethanol producers

Metrohm offers several NIRS solutions for ethanol producers to make analysis easier and optimize production. The DS2500 Solid Analyzer (Figure 4) is ideal for rapid laboratory analysis of several critical quality parameters in the fermentation process.

Download our free Application Note below to learn more about how Metrohm NIRS laboratory instruments perform quality control measurements for the fermentation process.

Figure 4. The Metrohm DS2500 Solid Analyzer.

Additionally, Metrohm also manufactures NIRS instruments for measurements directly in the process, eliminating the need for removing samples and transporting them to the laboratory. Measurements taken in this way are the most representative of actual process conditions and therefore provide the highest quality data to operators. Learn more here about our different ranges of NIRS process analyzers and accessories.

Data communication between the process analyzer and the control room allows a direct overview of current conditions without delays and offers the possibility of integrating warnings when readings are out of specification or informing operators when the fermentation process is deemed to be complete.

For more information about the usage of NIRS for process analysis in bioethanol production, download our free Application Note.

Summary

Near-infrared analysis decreases measurement time for in-process fermentation samples by approximately 90%, from one hour to five minutes. Faster measurements allow the fermentation process to be followed much more closely, saving operator time to reduce costs and to optimize process conditions and plant operations. Capacity improvements of 10% are possible by being able to stop the fermentations based on rapid determination of the different parameters in the fermenter with NIRS rather than by slower traditional laboratory methods.

NIR methodology can provide benefits across the ethanol plant beyond fermentation monitoring to measure the performance of other plant components such as a centrifuge or dryer, making it a valuable tool to improve operations across the facility.

For more information about utilizing NIRS analysis in the bioethanol process as well as the available precalibrations for various quality parameters, download our free White Paper.

Free White Paper

Improving the corn to ethanol fermentation process with near-infrared spectroscopy (NIRS)

References

[1]  Annual Fuel Ethanol Production U.S. and World Ethanol Production. Renewable Fuels Association: Washington, DC, 2021. https://ethanolrfa.org/statistics/annual-ethanol-production/

[2]  Essential Energy: 2021 Ethanol Industry Outlook. Renewable Fuels Association: Washington, DC, 2021.  https://ethanolrfa.org/wp-content/uploads/2021/02/RFA_Outlook_2021_fin_low.pdf

[3]  Determining the Cost of Producing Ethanol from Corn Starch and Lignocellulosic Feedstocks. National Renewable Energy Laboratory (NREL): Golden, Colorado, USA, 2000. https://www.nrel.gov/docs/fy01osti/28893.pdf

Post written by Dr. Adam J. Hopkins (PM Spectroscopy at Metrohm USA, Riverview, FL) and Dr. Alyson Lanciki (Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland).

Chemical analysis of sourdough: pH and total titratable acidity (TTA)

Chemical analysis of sourdough: pH and total titratable acidity (TTA)

Like many, I am fascinated by the chemistry behind baking, and in this blog I want to talk about bread—sourdough in particular. There is a well-known saying in the baking industry: «The pH value bakes and the total titratable acidity tastes». Why are these two parameters important for baking bread, and how can they be determined in the best way? This is what I want to discuss here.

A brief history of sourdough

Bread has been part of the human diet for several thousand years, although not necessarily in the forms we are familiar with today. One exception to this is sourdough bread. Wild yeast and bacteria (lactobacilli) ferment the dough naturally, creating a tangy loaf full of crevices. Despite originating in the Fertile Crescent, one of the oldest physical examples (at nearly 6,000 years old) was excavated in Switzerland, showing how widely it spread by that point already.

Currently, one of the places most well-known for its sourdough bread is San Francisco, in California. Why California? Bakers from France brought their techniques there during the Gold Rush in the mid 1800’s, and it has since become ubiquitous with the city. In fact, San Francisco has its own eponymous strain of sourdough bacteria: Fructilactobacillus sanfranciscensis.

Sourdough bread loaf full of crevices
Figure 1. Cross section of a sourdough bread loaf.

Many home bakers try to make sourdough at some point, since the ingredients are simple and no leavening agent is used, except for what nature provides. However, with so many people at home during 2020–2021, it was an ideal time for many people to see what they could produce. The development of the starter is of key importance—if there is not sufficient wild yeast and bacteria (or they do not have enough nutrients), then the dough will not rise, and you are left with a dense, chewy result. (While much has been written about how to make the best homemade sourdough, I cannot contribute to this topic, as my own baking spree focused on the Swiss Butterzopf.)

Click here to download the recipe and try it out yourself!

Lactobacilli: helpful bacteria

As their name suggests, lactobacilli produce lactic acid (Figure 2) and also acetic acid, and these give the sourdough bread its characteristic tangy, sour taste. The sourness of the bread also has positive effects on its shelf life, making it possible for our ancestors to preserve the bread for a longer time to supplement their diet.

There is another reason why the presence of this helpful bacteria is important. Without the lactic and acetic acid, it would be impossible to bake bread made from rye flour, which is commonly used in sourdough bread of northern Europe. How come?

Figure 2. Chemical structure of lactic acid.

Starch is the key component within bread and influences the shape, crumb consistency, and overall flavor. During the baking process, gelatinization occurs between the starch within the flour and the water added to the dough. However, flour also contains the enzyme amylase, which catalyzes the hydrolysis of starch into sugar. During the gelatinization process, starch is more prone to hydrolysis by amylase. Strong amylase activity at this point will have detrimental effects on the bread crumb. For wheat, the amylase is already denatured at the temperature gelatinization begins within the dough. This is not the case for rye, which gelatinizes at a lower temperature when amylase activity happens to be the highest [1]. By making an acidic (sour) dough, the amylase activity is inhibited and it becomes possible to bake bread made from rye flour.

So how much acid is necessary and when is it too much? This question brings us back to the two key parameters, pH value and total titratable acidity (TTA), I mentioned in the introduction.

Figure 3. Fermenting sourdough starter in a glass jar.

pH value regulates enzyme activity

The pH value is important to inhibit amylase in an  optimal manner. Every enzyme has an optimal pH range in which it functions the most efficiently. For amylase, the optimal pH value (highest enzyme activity) ranges from pH 5.4 to 5.8. At a lower pH value its activity will be reduced.

The pH value can be easily measured using a pH electrode. For dough analysis, an electrode such as the Spearhead electrode which can pierce into the sample is the best sensor. As the pH value is temperature dependent, the sensor measures the temperature as well.

What is the pH value?

The pH value is the negative logarithm of the hydronium concentration. Therefore, the smaller the pH value, the higher the hydronium concentration.

Pure water itself contains a small amount of free hydronium ions, and its pH value is therefore 7.

As acids release hydronium ions when they are in solution (dissociation), acidic solutions have pH values between 0 and 7. 

Contrary to this, alkaline solutions and products have even less hydronium ions than pure water. They have pH values ranging from 7 to 14. An example of an alkaline solution is lye, which is used to produce lye rolls.

For more information on pH measurement check out our other blog posts «Avoiding the most common mistakes in pH measurement» and «FAQ: All about pH calibration».

Total titratable acidity helps assess the taste

Why do we need to determine the total titratable acidity (TTA) if measuring and controlling the pH value is sufficient to regulate the amylase activity? This is because the pH value does not provide any information about the ratio of lactic acid and acetic acid present in the dough. While the amylase activity is not dependent on the ratio of the two, the composition is important for the taste. For optimal sourdough flavor, the ratio of lactic acid to acetic acid should lie between 3:1 and 4:1. If the ratio shifts towards containing more acetic acid, the taste usually becomes too sour.

Weaker acids such as lactic acid and acetic acid do not completely dissociate, meaning not all acid molecules present will release their hydrogen ion. As lactic acid is a stronger acid in comparison to acetic acid, more lactic acid will dissociate and thus contribute more to the pH value. By determining the TTA, it is possible to find out what the total amount of acids is within the dough.

For the determination of the TTA, dough is homogenized with water to obtain a suspension. It is then titrated to a pH value of 8.5 with 0.1 molar sodium hydroxide solution. The use of an automated titrator provides reliable results without human interference (Figure 4).

Figure 4. A robust and compact titrator for the determination of the total titratable acidity

Before the titration starts, the pH value of the suspension can be determined easily, so you get both parameters (pH value and TTA) without needing to do double the work. For more detailed information on how the analysis is done, download our free Application Note.

pH value and TTA for the perfect sourdough quality

By combining the information about the pH value and TTA it becomes possible to assess the quality of sourdough and thus maintain a constant quality in the final product, especially if delays in the production process occur.

It also becomes possible to detect changes in the sourdough starter which might occur if storage conditions cannot be maintained, and thus provide critical information when it is time to prepare a new starter.

Table 1. Common pH and TTA values for various kind of breads [1].
Bread type pH value TTA
Wheat bread 5.4–6.0 4–6
Wheat mixed bread 5.0–5.3 6–8
Rye mixed bread 4.5–4.8 7–9
Rye bread 4.3–4.7 8–10
Rye bread (coarsely ground) 4.2–4.6 9–14

Lessons learned

I hope this blog post on the chemistry of sourdough has given you some new insights on this fascinating kind of bread.

As for myself, I will probably not venture into the sourdough baking arena but stick with my homemade Butterzopf.

Figure 5Butterzopf (made by the author): a traditional Swiss bread usually consumed on Sundays for brunch.

If you are interested in other blog articles related to yeast, check out our post about beer brewing: «Making a better beer with chemistry». If you have more of a sweet tooth then read our blog post on the «Chemistry of chocolate».

The chemistry of bread

Straightforward determination of pH value and total titratable acidity (TTA) in dough

Post written by Lucia Meier, Technical Editor at Metrohm International Headquarters, Herisau, Switzerland.

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.

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

Nonaqueous acid-base titrations are widely used in several industries, including the petrochemical  and pharmaceutical sectors. Whether you are determining the acid or base number (AN or BN) in oils or fats, titrating substances that are insoluble in water, or quantifying products with different strengths of acidity or alkalinity separately, nonaqueous acid-base titration is the method of choice.

If you already have some experience performing nonaqueous acid-base titrations, you may remember that there are several challenges to overcome in comparison to aqueous acid-base titrations.

In this blog post, I would like to cover some of the most typical issues that could pop up during nonaqueous acid-base titrations and discuss how to best avoid them. An important point to note is that there is no single solution regarding how to perform any nonaqueous acid-base titration correctly. The right procedure depends highly on the solvent and titrant used.

What is a nonaqueous acid-base titration?

Before discussing nonaqueous titrations, first let’s talk a little bit about aqueous acid-base titrations.

Here, a sample is dissolved in water, and depending of the nature of the sample (whether it is acidic or basic) a titration is performed either using aqueous base or aqueous acid as titrant. For indication, a glass pH electrode is used.

However, sometimes due to the nature of the sample, aqueous titration is not possible. Nonaqueous acid-base titration is used when:

  • the substance of interest is not soluble in water
  • samples are fats or oils
  • components of mixtures of acids or bases have to be determined separately by titration

In these cases, a suitable organic solvent is used to dissolve the sample instead of water. The solvent:

  • should dissolve the sample and not react with it
  • permits the determination of components in a mixture
  • if possible, should not be toxic

The solvents that are most often used include ethanol, methanol, isopropanol, toluene, and glacial acetic acid (or a mixture of these). Titrants are not prepared with water but rather in solvent. Frequently used nonaqueous basic titrants are potassium hydroxide in isopropyl alcohol or sodium hydroxide in ethanol, and a common nonaqueous acidic titrant is perchloric acid in glacial acetic acid.

Due to the nature of nonaqueous solvents, they are normally poor conductors and do not buffer well. This makes indication a bit challenging because the electrode must be suitable for such sample types. Therefore, Metrohm offers the Solvotrode which is developed specifically for nonaqueous titrations.

This pH electrode offers the following advantages over a standard pH electrode:

  • Large membrane surface and a small membrane resistance for accurate reading, also in poorly buffered solutions
  • A flexible ground-joint diaphragm which can easily be cleaned even when contaminated with oily or sticky samples, additionally it offers a symmetrical outflow for outstanding reproducibility
  • The electrode is shielded and is therefore less sensitive to electrostatic interferences
  • It can be used with any nonaqueous electrolyte such as lithium chloride in ethanol

In the following sections I will discuss the most common mistakes when performing potentiometric nonaqueous acid-base titrations and how you can avoid them.

Electrostatic effects

The influence of electrostatic effects during analysis is normally negligible. However, maybe you have once seen a curve like the one below which looks relatively normal until suddenly a spike occurs.

Figure 1. Titration curve with a spike which might have occurred from an electrostatic interference.

This is then an indication of an electrostatic effect. However, where does it come from and how can we overcome this?

Electrostatic charge can be generated from many sources, such as friction. For example, while walking across a surface you will generate an electrostatic charge which will be stored in your body. You have probably touched the doorknob after walking across a carpeted space in your socks and obtained a small electric shock—this is the discharge of built up electrostatic charge. If we now assume that you are electrostatically charged and then you approach an electrode that is currently measuring (in use), this will result in a spike (Figure 1). Therefore, it is essential to make sure that you are either properly discharged or that you do not approach the electrode during measurement. You can avoid this issue by wearing the appropriate clothes. ESD (electrostatic discharge) clothes and shoes are mostly recommended when performing nonaqueous titrations.

Blocked diaphragm

A blocked diaphragm is another point which occurs more regularly during nonaqueous titrations. Due to the oily and sticky sample, you might have seen that the electrode diaphragm is clogged and cannot be opened anymore. What should you do then?

In most cases, you can place the electrode in a beaker of warm water overnight. This treatment often helps to loosen the diaphragm. To completely prevent the diaphragm from clogging, a Solvotrode with easyClean technology should be used. With this electrode, electrolyte is released by pressing the head ensuring that the diaphragm is not blocked.

Choice of electrolyte and storage solution

We recommend two types of electrolyte for nonaqueous titrations.

For titrations with alkaline titrants: tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene glycol

For titrations with acidic titrants: lithium chloride c(LiCl) = 2 mol/L in ethanol

Please make sure to store the electrode in the same electrolyte with which it is filled.

Checking the electrode according to ASTM D664

To check whether the Solvotrode is still in good working condition, perform a test according to ASTM D664 using aqueous buffer solutions of pH 4 and 7. The procedure is as follows:

  • Measure the potential of buffer pH 4.0 while stirring and note the value after 1 minute
  • Remove the electrode and rinse it well with deionized water
  • Measure the potential of buffer pH 7.0 while stirring and note the value after 1 minute
  • Calculate the mV difference between the reading of buffers 4.0 and 7.0
  • The difference must be larger than 162 mV (20–25 °C) to indicate an electrode in good shape

If the measured potential difference is less than 162 mV, the electrode requires maintenance. Lift the flexible sleeve of the ground-joint diaphragm to let some electrolyte flow out. Repeat the measurement according to the steps above. If the value is still less than 162 mV, clean the electrode or replace it.

Proper rinsing and cleaning

Proper rinsing is essential if you want to obtain reliable results. Otherwise, the curve might flatten and the equivalence points are no longer recognizable. Figure 2 illustrates this phenomenon well.

Figure 2. Different determinations according to ASTM D664. With time, the start potential of the curves shifts which indicates an unsuitable cleaning procedure.

The sample is the same, however, you see that the equivalence point and starting potential begin to shift and the curves become flatter. This indicates an improper cleaning procedure between measurements. The corresponding electrode is shown in Figure 3.

Figure 3. Appearance of the electrode used in Figure 2 after five measurements.

This electrode was certainly not cleaned properly! Anyone who performs a nonaqueous titration must consider which solvent might best dissolve the residue—this is not an issue that other analysts can easily solve due to the nature of each individual sample. However, do not ignore an electrode with such an appearance.

Conditioning the glass membrane correctly

As you may remember from our previous blog post about pH measurement, it is essential that the hydration layer of the glass membrane stays intact. Nonaqueous solvents dehydrate the glass membrane rather quickly. A change in the hydration layer can have an impact on the measured potential, therefore it is important that the hydration layer is always in the same state before starting a titration to achieve the most reproducible results.

Proper electrode immersion depth

This can be established with a conditioning step of the glass membrane to rebuild the hydration layer. However, if the solvent is able to remove the hydration layer faster than it takes to perform a titration, this can lead to ghost equivalence points. Therefore, the electrode should be completely dehydrated and kept like this for all further titrations.

 

Polar solvents (e.g., ethanol, acetone, isopropyl alcohol, or mixtures with toluene)

Water-free solvents (e.g., dimethylformamide, acetonitrile, acetic anhydride, or mixtures of these)

Preparation of electrode

Store only the pH membrane (not the diaphragm) in deionized water overnight to build up a proper hydration layer.

Lift the flexible sleeve to allow some electrolyte to flow out.

Dehydrate the pH membrane by placing only the pH membrane (not the diaphragm) in the solvent you will use afterwards for titration.

Lift the flexible sleeve to allow some electrolyte to flow out.

Conditioning of glass membrane Place the pH membrane (bulb only) into deionized water for 1 minute. Place the pH membrane (bulb only) into the corresponding solvent for 1 minute.
Rinsing procedure Rinse electrode with 50–70% ethanol. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with 50–70% ethanol. Rinse electrode with glacial acetic acid. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with glacial acetic acid.
Remarks Make sure to always keep the bulb of the electrode in deionized water for the same time duration, otherwise the thickness of the hydrated layer (and therefore the response) may vary. Avoid any contact of the electrode with water as this can induce a reaction with the solvent causing ghost equivalence points and irreproducible results.

Maintenance of burets

It is not only the electrode that needs some special attention when performing nonaqueous titrations, but also the electrical buret. Some special maintenance is required since alkaline nonaqueous titrants are especially aggressive and they tend to crystallize, therefore leakage of the buret is likely.

The buret must be maintained on a regular basis according to the manufacturer’s instructions. Metrohm recommends the following procedure:

  • For shorter titration breaks, it is recommended to refill the cylinder with titrant (especially with OMNIS)
  • Clean the buret with deionized water at the end of the day
  • Lubricate the cylinder unit on the centering tube and on the cylinder disc

Also check the corresponding manual of the buret. The most important points are mentioned there which will lead to a longer working life of the buret.

Thermometric titration as an alternative

One alternative to using potentiometric nonaqueous acid-base titration is thermometric titration (TET), depending on the sample and analyte to be measured. Thermometric titration monitors the endothermic or exothermic reaction of a sample with the titrant using a very sensitive thermistor.

The benefit of TET over potentiometric titration is clearly the maintenance-free sensor which does not require any conditioning nor electrolyte refilling. More information about thermometric titration can be found in our previous blog posts below.

Summary

Hopefully this article has provided you with information about the main problems encountered during nonaqueous titrations. First, make sure that all electrostatic influences are eliminated. This will save a significant amount of troubleshooting. Then prepare and treat your electrode correctly before, during, and after titration. Make sure to condition the electrode right before your first measurement!

Of special importance here is the solvent you plan to use. If it is a polar solvent, the electrode should be conditioned in deionized water. If nonpolar solvents like acetic anhydride are used, the electrode should be dehydrated first. Between measurements, the electrode should be cleaned with a suitable solvent and the diaphragm should be opened on occasion.

Last but not least, take care of your buret. Maintain it regularly and replace it whenever necessary. With this advice, performing nonaqueous titrations should be a breeze!

For more information

about nonaqueous titrations, download our monograph:

Nonaqueous titration of acids and bases with potentiometric endpoint indication

Post written by Iris Kalkman (Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland) and Dr. Sabrina Gschwind (Head of R&D at Metroglas, Affoltern, Switzerland).

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.

Other 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 is dedicated to:

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.