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NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

Polyethylene terephthalate (PET): A brief introduction

PET is a very common plastic, mostly encountered in our lives as PET bottles and as a food packaging material. In this article you will learn how NIR spectroscopy can improve the efficiency of your PET analysis at different steps along the production cycle. Before getting into this, let’s introduce some background information about PET.

Polyethylene terephthalate (PET)

Polyethylene terephthalate (PET) is a general-purpose thermoplastic polymer which belongs to the polyester family. Polyester resins are known for their excellent combination of properties such as mechanical, thermal, and chemical resistance as well as dimensional stability.

PET is one of the most recycled thermoplastics and has the number 1 as its recycling symbol. Recycled PET can be converted into fibers, fabrics, sheets for packaging and for manufacturing automotive parts. PET is a highly flexible, colorless, and semi-crystalline resin in its natural state. Depending upon how it is processed, it can be semi-rigid to rigid. It exhibits good resistance to impact, moisture, alcohols, and solvents.

The chemical formula of PET is (C10H8O4)n and its molecular structure is shown in Figure 1.

Figure 1. Molecular structure of linear PET.

In addition to linear PET, there is also a branched version of the polymer. Branched PET is typically mixed with a small percent of isophthalic acid (C8H₆O4), because purified isophthalic acid (PIA, Figure 2) reduces the crystallinity of PET, serving to improve its clarity and increase the productivity of bottle manufacturing processes.

Diethylene glycol (DEG) as an additive also reduces the rate of crystallization of PET when crystallizing from the melt, isothermally and dynamically.

Figure 2. Molecular structure of isophthalic acid.
The key properties and advantages of PET resin are numerous:
  • very strong and lightweight, and therefore easy and efficient to transport
  • has good gas (oxygen, carbon dioxide) and moisture barrier properties, meaning low gas permeability (particularly against CO2)
  • exhibits excellent electrical insulating properties
  • broad range of use temperature (-60 to 130 °C)
  • high heat distortion temperature (HDT)
  • suitable for transparent application purposes
  • practically shatter-resistant – PET does not break or fracture and is used to replace glass in some applications
  • recyclable material
  • transparent to microwave radiation
  • very resistant to alcohols, aliphatic hydrocarbons, oils, greases, and diluted acids
  • moderately resistant to diluted alkalis, aromatic and halogenated hydrocarbons
  • PET is approved as safe for contact with foods and beverages by the FDA, Health Canada, EFSA, and other health agencies

What is polyethylene terephthalate (PET) used to make?

Polyethylene terephthalate is used in several types of packaging applications as shown in Figure 3. Due to its strength, light weight, and many other attractive properties, PET excels as a food packaging material.

Figure 3. PET is an ideal choice for many food packaging applications due to its strength to weight ratio.

Polyester makes up nearly two-thirds of synthetic fibers produced. There are many different types of polyester, but the type most often produced for use in textiles is PET. When used in a fabric, it is most often referred to as «polyester» or «poly» (Figure 4). This material costs very little to produce, which is the primary driver for its use in the textile industry.

Approximately 60% of the global PET production is used to make fibers for textiles while about 30% is used to make bottles for various purposes. Its ability to be recycled is especially attractive for manufacturers looking to save costs and operate in a greener manner.

Figure 4. PET makes up a significant portion of produced polyester fabric.

In the electronics industry, PET is chosen to replace less ideal materials due to its excellent electrical insulating properties and resistance to distortion even at high temperatures. PET is also used to manufacture many parts in the automotive industry (Figure 5).

Figure 5. PET is often used in the manufacturing of various automotive parts.

NIRS as a tool to assess the quality of PET

For over 30 years, near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the PET industry. Despite this, many producers still do not consistently consider the implementation of NIRS in their QA/QC labs. Limited experience regarding application possibilities or a general hesitation about implementing new methods are some of the reasons behind this.

The advantages of using NIR spectroscopy for QA/QC are numerous. One major advantage of NIRS is the determination of multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes NIRS a suitable method for the determination of several critical quality parameters in these polymers and many more.

In the remainder of this article, a short overview of PET applications is presented, followed by available turnkey solutions for PET, developed according the NIRS implementation 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 PET with NIRS

During production of PET it is important to check certain parameters to guarantee the quality. These parameters include the diethylene glycol content, isophthalic acid content, intrinsic viscosity (ASTM D4603), and the acid number (AN). Determination of these parameters is a lengthy and challenging process due to the limited solubility of the sample and the need to use different analytical methods.

The most relevant applications for NIRS analysis of PET are listed in Table 1.

Table 1Available application notes for use of NIR for PET
Polymer Parameter Related NIRS Application Notes
Polyethylene terephthalate (PET)

Diethylene glycol, Intrinsic Viscosity, Acid number, Isophthalic acid

AN-NIR-023

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

Figure 6 shows the individual production steps from the plastic producer via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PET 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 6. Illustration of the polyethylene terephthalate production chain.

Easy implementation of NIR spectroscopy for plastic producers

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

Figure 7. Turnkey solution for PET analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

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

Due to the limited solubility of polyethylene terephthalate and the need to use several different analytical methods, the determination of the parameters listed in Table 2 is a lengthy and challenging process with conventional laboratory techniques.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PET samples.
Parameter Primary method Time to result (primary method) NIRS benefits
Diethylene Glycol content

Extraction + HPLC-MS

45 min. preparation + 40 min. HPLC-MS All four parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
Isophthalic acid content Dissolve + HPLC 45 min. preparation + 40 min. HPLC
Intrinsic viscosity Dissolve + Viscometer 90 min. preparation + 1 min. Viscometer
Acid number Dissolve + Titration 90 min. preparation + 10 min. Titrator

The NIRS prediction models created for PET 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 PET, download our brochure and visit our website.

The result of this turnkey solution for rapid non-destructive determination of the key quality parameters for PET listed in Table 2  is shown in Figure 8.

Figure 8. Turnkey solution for diethylene glycol, isophthalic acid, intrinsic viscosity and acid number in PET using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PET granulate. 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 the feasibility of NIR spectroscopy for the analysis of multiple parameters in PET in less than one minute without sample preparation or using any chemical reagents.  Learn more about the procedure in our free Application Note!

Future installments in this series

This article is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for the analysis of polyethylene terephthalate (PET). Future installments in this series will be dedicated to:

 

  • Polyamide (PA)
  • 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.

Supercharge your battery research – Part 2

Supercharge your battery research – Part 2

Battery technology has come a long way since the rudimentary voltaic pile was developed over two centuries ago. The breakthrough innovation of the lithium ion battery and its subsequent improvements has increased the use and accessibility of electronics, particularly in the consumer market. Electronics are more portable, affordable, and thanks to rechargeable or secondary batteries, they are becoming more sustainable.

Expansion of application possibilities is another reason that energy storage research, particularly batteries, is currently a hot topic. For example, only a decade ago drones were the domain of the military industrial complex, and now a drone with a camera is a standard part of nearly any successful photographer or influencer’s gear. Thanks to an improved battery life and more cost-efficient materials, a drone now has an affordable price tag for a larger segment of the civilian population.

This kind of disruption is happening in larger, more profitable markets as well. Tesla, a newsworthy brand thanks to their technological innovations and public relations, still has a small, albeit increasing, market share of the overall automotive market. Their success has challenged other established brands to recognize that a change from conventional combustion engines can be lucrative. Volvo and Ford are committed to be «fully electric» by 2030 [1]. General Motors (GM) has committed to not only be electric by 2035, but for their business to be carbon neutral by 2040 [2].

The automotive market is a high profile example of an industry that will drastically retool their sector—from manufacturing to sales—and this will happen across many other industries as there is a greater focus on climate change and renewable energy sources from governments and consumers alike. Accurate and scalable R&D will be required to make these transformations possible, and the hunt for improved energy storage solutions is at the heart of these changes.

Electrochemistry was the key to the discovery of energy storage and is the natural technique of choice for future innovations.

Electrochemical characterization techniques for lithium ion batteries

In part one of this series, we introduced various techniques to analyze the composition and purity of electrode materials and lithium salts, in addition to accurate water content determination in the battery materials.

In this article we highlight techniques that will allow the characterization of multiple attributes of the electrochemical behavior of Li-ion batteries using a high precision potentiostat/galvanostat. In some cases, the difference between techniques is due to performing the experiment in a different mode (i.e., potentiostatic or galvanostatic), and the additional information gathered provides a more complete picture of battery behavior.

Galvanostatic Intermittent Titration Technique (GITT)

One of the first techniques available to researchers exploring the properties of battery electrode materials is the Galvanostatic Intermittent Titration Technique (GITT). Usually conducted on a half-cell, this technique is a series of current perturbations followed by a relaxation time, which provides information about the thermodynamic properties and electrode materials including the critical diffusion co-efficient. All of this information gives a better understanding of the electrochemical behavior that can be expected by the materials.

If you’re looking for more information on this subject, download our free application note AN-BAT-003.

Potentiostatic Intermittent Titration Technique (PITT)

Potentiostatic Intermittent Titration Technique (PITT) is similar to the GITT technique detailed above, but the PGSTAT is operated in potentiostatic mode. A series of potential step perturbations is applied to the system, and current is measured as a function of time. Both GITT and PITT are capable of accurately determining the diffusion coefficient.

When using a PGSTAT in galvanostatic mode you can also characterize the performance of Li-ion batteries by using different current rates and charging and discharging during various cycles, known colloquially as «cycling». With this technique researchers can understand the rate performance of the Li-ion battery, its capacity, and the associated power and energy density. This is the most commonly used technique in battery research. A constant current constant voltage (CCCV) procedure is usually applied in order to make sure that a battery is fully charged, while avoiding any battery overcharge.

Learn more about characterizing the performance of lithium ion batteries with cycling by downloading our free application note AN-BAT-002.

CCCV is the industry standard for Li-ion battery charging, and PGSTAT operates in both galvanostatic and potentiostatic mode for this measurement. Galvanostatic cycling is performed within a safe potential window at which the electrolyte is stable. Any slight deviation from the potential cutoff may result in poor cycle life.

Voltage profile of a 18650 Li-ion battery, cycled at ~ C/15 (left), and its corresponding dQ/dV versus V plot (right). The corresponding peaks and plateaus are marked in the figures.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) provides additional data and therefore greater insight into the battery’s behavior and potential performance by using galvanostatic charge/discharge cycling and then adding in the most powerful technique that is being used extensively in current battery research. With EIS, the highly dynamic behavior of a battery and the diffusion of ions at the interfaces can be characterized. In a single experimental procedure encompassing a broad range of frequencies, the influence of the governing physical and chemical phenomena may be isolated and distinguished at a given frequency range and state of charge. With EIS it is possible to measure the internal impedance of the battery and model it using the equivalent circuit and understand the contribution of the battery components to the total impedance of the cell.

For EIS determination of batteries it is important to use 4-terminal sensing to avoid the contribution of wires to overall impedance. This is important for any low impedance electrochemical system. Learn more about this research by downloading our free Application Notes AN-EC-013 and AN-BAT-008.

With EIS, it is possible to determine the through-plane tortuosity of battery electrodes, which along with overall electrolyte conductivity, the transference number of a Li-ion of battery electrolyte, and diffusion coefficient of electrolyte gives a good indication of the practicality of certain battery chemistry for high power applications. In addition, the mass transport limitation of the battery separator and its ionic conductivity plays a crucial role in overall performance of the batteries.

By determining the MacMullin number, researchers can determine the quality of the separators for their application in certain Li-ion cells.

Nyquist plot: Negative imaginary part of impedance as a function of the real part of impedance for an 18650 cell.

Download our free Application Notes below for more information on these subjects.

Download our free white paper «A Guide to Li-ion Battery Research and Development» written by electrochemical instrument innovators at Metrohm Autolab. This white paper provides additional information about applicable electrochemical techniques and provides useful definitions to terminologies that are relevant to Li-ion battery research and development.

 

Free white paper:

A Guide to Li-ion Battery Research and Development

Post written by Dr. Reza Fathi, Product Specialist at Metrohm Autolab, Utrecht, The Netherlands.

Validation of titration methods

Validation of titration methods

Manufacturing products of the highest quality is a must, especially in the pharmaceutical and food industries. This requires accurate, reproducible, and simple analysis methods that eliminate human errors as much as possible. Automated titration is one such solution that offers additional time and cost savings to laboratories.

After applying automation to a titration method, how can you ensure that the chosen method also delivers a reliable result? And how do you know that it is suitable for the analysis of your analyte(s)? This requires method validation of a titration, which includes standardization of the titrant as well as determination of accuracy and precision, linearity, and specificity.

USP General Chapter <1225> Validation of Compendial Procedures and ICH Guidance Q2(R1) Validation of Analytical Procedures: Text and Methodology define the validation elements – some of the most important ones are described in the following article.

These include (click to go to each section):

Standardization

Dilution and weighing errors as well as the constant aging of all titrants lead to changes in the concentration of the titrant. To obtain results that are as reliable as possible, the most accurate titrant concentration is a prerequisite. Standardization of the titrant is therefore an integral part of a titration method validation. The standardization procedures for various titrants are described in the Volumetric Solution section of USP – NF as well as in the Metrohm Application Bulletin AB-206 regarding the titer determination in potentiometry.

The titrant to be used in the validation must first be standardized against a primary standard or a pre-standardized titrant. It is important that the standardization step and the sample titration are carried out at the same temperature.

Primary standards are characterized by the following properties:

  • high purity and stability
  • low hygroscopicity (to minimize weight changes)
  • high molecular weight (to minimize weighing errors)

The use of a standard substance (primary standard) allows accuracy to be assessed.

For more information about titrant standardization, check out our blog posts «What to consider when standardizing titrant» (for potentiometric titration) and «Titer determination in Karl Fischer titration».

Accuracy and precision

Accuracy is defined as the proximity of the result to the true value. Therefore, it provides information about the bias of a method under validation. Accuracy should be determined over the entire concentration range.

Precision is usually expressed as the standard deviation (SD) or relative standard deviation (RSD). It expresses how well the individual results agree within an analysis of a homogeneous sample. Here, it is important that not only the analysis itself but also all sample preparation steps are performed independently for each analysis.

Precision is evaluated in three levels:

  1. Repeatability: the precision achieved by a single analyst for the same sample in a short period of time using the same equipment for all determinations.
  2. Intermediate precision: analysis of the same sample on different days, by different analysts and with different equipment, if possible, within the same laboratory.
  3. Reproducibility: precision obtained by analyzing the same sample in different laboratories.

Determination of both accuracy and precision is necessary, as only the combination of both factors ensures correct results (Figure 1).

Figure 1. Only when both precision and accuracy are high can correct results be obtained, as high precision does not necessarily mean good accuracy, and vice versa.

For titration, accuracy and repeatability are usually determined together. At least two to three determinations at three different concentration levels (in total six to nine determinations) are recommended. For assays, the recommendation is to use a concentration range of 80% to 120% of the intended sample weight.

Linearity

Linearity expresses whether a particular method gives the correct results over the concentration range of interest. Since titration is an absolute method, linearity can usually be determined directly by varying the sample size and thus the analyte concentration.

To determine the linearity of a titration method in the range of interest, titrate at least five different sample sizes and plot a linear regression of the sample volume against the titration volume consumed (Figure 2). The coefficient of determination (R2) is used to assess linearity. The recommendation is to use a concentration range of 80% to 120% of the intended sample weight.

Figure 2. Linear regression curve for the assay of potassium bicarbonate.

Specificity

Impurities, excipients, or degradation products are among the many components that may be present in a sample. Specificity is the ability to evaluate the analyte without interference from these other components. Therefore, it is necessary to demonstrate that the analytical procedure is not affected by such compounds. This is the case when either the equivalence point (EP) found is not shifted by the added impurities or excipients, or in the event it is shifted that a second EP corresponding to these added components can be observed when a potentiometric sensor is used for indication.

Specificity may be achieved by using suitable solvents (e.g., non-aqueous titration instead of aqueous titration for acid-base titration) or titration at a suitable pH value (e.g., complexometric titration of calcium at pH 12, where magnesium precipitates as magnesium hydroxide).

How can this be implemented in practice? The titrimetric determination of potassium bicarbonate with hydrochloric acid will serve as an example here.

In this case, potassium carbonate is expected as an impurity with pkb values of 8.3 and 3.89. This makes it possible to separate the two species during the acid-base titration. Figure 3 shows the comparison of a curve overlay of the titration curves of potassium bicarbonate with and without added potassium carbonate.

Figure 3. Curve overlay of the specificity test using 1 g KHCO3 with and without 0.5 g K2CO3 (green and orange = no K2CO3 added; blue and yellow = K2CO3 added). Click to enlarge.

The lower titration curve corresponds to the solution containing both potassium bicarbonate and potassium carbonate. Two EPs are found here: the first EP can be assigned to the added potassium carbonate, while the second corresponds to the sum of potassium bicarbonate and potassium carbonate. The curve at the top of the figure clearly shows only one EP for the potassium bicarbonate solution without impurities.

Find out more about the proper recognition of endpoints (EP) in our previous blog post.

Conclusion

If you follow the recommendations above, you will be ready for titration method validation – and now it`s time to get started!

Using potentiometric autotitration instead of manual titration increases the accuracy and reliability of your results. In addition, the use of an autotitrator ensures that critical regulatory compliance requirements, such as data integrity are met.

Right from the start, Metrohm products provide peace of mind and confidence in the quality of the data you produce with proper IQ/OQ.

If you would like to learn more about Metrohm Analytical Instrument Qualification, have a look at our two blog posts dedicated to this important topic.

Additional security is also provided, e.g., by Metrohm Buret Calibration which ensures that the accuracy and precision of your dosing device are within the required tolerances. Traceable monitoring of the performance and function of the instrument through regular re-qualifications and tests is therefore a given.

Watch our free webinar

available on demand!

How to convert from manual to automated titration procedures

Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

Polypropylene and polyethylene: A brief introduction

Did you know that polypropylene (PP) and polyethylene (PE) are the most produced plastics in the world? Products made out of PP and PE are so ubiquitous that every single one of us encounters them several times per day. In this article you will learn how NIR spectroscopy can improve the efficiency of your PP and PE analysis along different steps along the production cycle. But first, let’s get a little bit of background information about PP and PE.

Polypropylene (PP)

Polypropylene (also known as polypropene or PP) has a chemical formula of (C3H6)n. It is a thermoplastic polymer mainly produced from propylene monomers. PP is a versatile plastic commodity that also functions as a fiber. In 1954, it was first polymerized simultaneously by the Italian chemist, professor, and Nobel laureate Giulio Natta and Karl Rehn, a German chemist.

Polypropylene has the unique ability that it can be manufactured via several different methods and be utilized in many applications like packaging, injection molding, and fibers. This plastic commodity is the second most popular in the world, only preceded by polythene.

Polyethylene (PE)

Polyethylene (or polythene, PE) is also a polymer, but it is made from ethylene monomers and has the chemical formula (C2H4)n. The first synthesis of PE in 1898 by the German scientist Hans von Pechmann was accidental. Similar to PP, PE is also a thermoplastic.

PE is the most used plastic worldwide. Polythene is very stable and is a good electrical insulator. It has a very low melting point and is used in large amounts for the automotive and food packaging industries. Approximately 70% of PE is utilized in food packages, food containers, pallets, and even in crates and bottles.

Polyethylene is available in many different types:

  • Ultra-High-Molecular-Weight Polyethylene (UHMWPE)
  • Ultra-Low-Molecular-Weight Polyethylene (ULMWPE or PE-WAX)
  • High-Molecular-Weight Polyethylene (HMWPE)
  • High-Density Polyethylene (HDPE)
  • High-Density Cross-Linked Polyethylene (HDXLPE)
  • Cross-Linked Polyethylene (PEX or XLPE)
  • Medium-Density Polyethylene (MDPE)
  • Linear Low-Density Polyethylene (LLDPE)
  • Low-Density Polyethylene (LDPE)
  • Very-Low-Density Polyethylene (VLDPE)
  • Chlorinated Polyethylene (CPE)
Figure 1. Molecular structures of PE and PP.

Differences between polypropylene and polyethylene

Which is better, polypropylene or polyethylene? It all depends on the application! For what purpose are they being used? Both polymers are considered «commodity plastics». These are plastics that are used in high volumes for a wide range of applications.

Let’s compare some of the properties of each.

Table 1. Comparison chart of polypropylene vs. polyethylene.
Polypropylene (PP) Polyethylene (PE)
Chemical Properties

Semi-crystalline

Polypropylene bag

Inert, translucent

Polyethylene bag

Electrical Properties

High static charge

Poor insulator

Low static charge

Good insulator

Melting Point
130–171 °C 115–135 °C
Chemical Formula
(C3H6)n (C2H4)n
Uses
Fibers, films, caps, hinges, synthetic paper Plastic bags, bottles, food containers, pallets, geomembranes, films made of plastic, crates, etc.
Density

0.855 g/cm3 amorphous

0.946 g/cm3 crystalline

0.88–0.96 g/cm3
Relative Cost
Low Medium

NIRS as a tool to assess the quality of PP and PE

For over 30 years, near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the PP/PE industry. Despite this, many producers still do not consistently consider the implementation of NIRS in their QA/QC labs. Limited experience regarding application possibilities or a general hesitation about implementing new methods are some of the reasons behind this.

The advantages of using NIR spectroscopy for QA/QC are numerous. One major advantage of NIRS is the determination of multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes NIRS a suitable method for the determination of several critical quality parameters in these polymers and many more.

In the remainder of this article, a short overview on PE and PP applications are presented, followed by available turnkey solutions for PE and PP, developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first part in this series about NIRS as the ideal QC tool for the polymer industry? Find it here!

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

Applications and parameters for PE and PP with NIRS

During production of PE and PP it is important to check certain parameters to guarantee the quality. These parameters include the density to classify the PE type, copolymer level to enhance certain properties like strength and solvent resistance, and melt flow rate to make sure PP can be formed to the intended shape.

The most relevant applications for NIRS analysis of PE and PP are listed in Table 2.

Table 2Available application notes for use of NIRS for PE and PP
Polymer Parameter Related NIRS Application Notes
Polyethylene (HDPE/LDPE)

Identification, Density, Melt Flow Index, Copolymer level

AN-NIR-083

AN-NIR-081

AN-NIR-034

AN-NIR-003

Polypropylene (PP)

Identification, Melt Flow Index, Additives

AN-NIR-083

AN-NIR-082

AN-NIR-034

AN-NIR-004

Where can NIRS be used in the production process of PE and PP?

Figure 2 shows the individual production steps from the plastic producer via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PE and PP 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 2. Illustration of the polyethylene/polypropylene production chain.

Easy implementation of NIR spectroscopy for plastic producers

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

Figure 3. Turnkey solutions for PE and PP analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Turnkey solution for the determination of Melt Flow Rate (MFR) of PP

The Melt Flow Rate of polypropylene pellets is an important parameter to measure so that PP can be formed in the intended shape. The model created with the chemometric software is 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 PP, download our brochure and visit our website.

The result of this turnkey solution for the non-destructive determination of Melt Flow Rate of PP without rheological tests is shown in Figure 4.

Figure 4. Turnkey solution for Melt Flow Rate of PP using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PP pellets. B: Results of MFR from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for this analysis.

This solution demonstrates the feasibility of NIR spectroscopy for the analysis of MFR in polypropylene samples. The standard procedure (ASTM D1238) requires a significant amount of work including packing the sample, preheating, and cleaning. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows the analysis of MFR in less than a minute.

Learn more about the procedure in our free Application Note!

Future installments in this series

This article is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for the analysis of polypropylene and polyethylene. Future installments in this series will be dedicated to:

  • Polyamide (PA)
  • 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.

How much do pipes rust in a year?

How much do pipes rust in a year?

Why is corrosion important?

According to the Association of Materials Protection and Performance (AMPP) the total estimated annual cost of corrosion is as high as 3.5% of a country’s GDP [1]. An AMPP international study [2] found that in the United States alone, the corrosion related cost can be as high as $1.4 billion USD annually in the oil and gas exploration and production sector. This figure climbs even higher, up to $40 billion USD for gas and drinking water distribution plus sewer systems. This is an unavoidable problem with a high cost to bear.

Even though the corrosion itself isn’t unavoidable, it can be controlled by using the right material in the right place. Using a reliable test method that evaluates the material’s resistance against corrosion and predicts its potential failure is of the utmost importance. This test method should also be cost-effective and practicable.

What is corrosion?

Corrosion refers to a naturally occurring process that involves the deterioration or degradation of metals and alloys through a chemical reaction. The corrosion rate is highly dependent on the type of material, ambient temperature, contaminants/impurities, and other environmental factors. Most corrosion phenomena are electrochemical in nature and consist of at least two reactions on the surface of the metals or alloys.

For example:

These electrochemical process require three main elements:

  • Anode: where the metal corrosion occurs.
  • Cathode: the electrical conductor, which is not consumed during the corrosion process in the real-life electrochemical cell configuration.
  • Electrolyte: the corrosive medium that enables the transfer of electrons between the anode and the cathode.

Depending on the materials and environment, corrosion can occur in different ways, such as uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, or microbiologically induced corrosion to name just a few. Learn more about the different types of corrosion in our free white paper.

This white paper also includes details about relevant electrochemical techniques including Linear Sweep Voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS), and Electrochemical Noise (ECN or ZRA). These techniques allow for the exploration of corrosion mechanisms, the behavior of different materials, the rate at which corrosion occurs, and also to determine the suitability of the corrosion protection solutions such as protective coatings and inhibitors, among others.

Find out more about these subjects individually with our selection of free Application Notes (AN).
Calculation of corrosion parameters with NOVA – Tafel plot corresponding to corrosion behavior of iron in seawater. (Click to enlarge)

Creating pipe-flow conditions in your corrosion laboratory

Internal corrosion is the most problematic cause of pipeline failure. To understand the fundamentals about corrosion failure and its root causes within pipelines, a similar environment should be created in the lab.

The Rotating Cylinder Electrode (RCE) is an integral part of creating hydrodynamic electrochemical experiments in the lab that create turbulent flow conditions which realistically simulate the situation for liquids flowing through pipes. The RCE can be used with most electrochemical techniques such as chronoamperometry, chronopotentiometry, and potential sweep.

Study of the corrosion rate as a function of rotation speed (convective flux) is one of the most common applications for the RCE. Corrosion studies can be performed using linear or cyclic polarization measurements (LP, DPD, CP), electrochemical impedance spectroscopy (EIS), and electrochemical noise (ECN) with respect to the rotation speed.

Results obtained by electrochemical methods are more accurate and are obtained much faster than conventional corrosion investigation methods (e.g. salt spray), providing more efficiency and productivity to any corrosion measurement laboratory. Learn about the RCE and how to simulate realistic pipe-flow conditions in the lab combined with electrochemical corrosion techniques in our free white paper.

One typical method in electrochemical corrosion studies is linear polarization (LP). With this method, it is possible to evaluate the corrosion behavior of a sample under pipe-flow (i.e. turbulent flow) conditions and learn about the corrosion rate of the sample at a specific flow rate.

Metrohm offers two Application Notes that use this technique specifically:

The Tafel plot obtained from LP measurement gives an indication of the corrosion potential. Using dedicated analysis tools in the NOVA software from Metrohm Autolab, the corrosion rate analysis can be performed and corrosion rate can be calculated, giving an indication of how much the pipe will rust in a year (in mm/year) under given conditions. Once this information is available for a certain material, a more corrosion resistive environment can be developed by applying a certain coating or a corrosion inhibitor.

Tafel plot created by Metrohm Autolab’s NOVA software. Blue line is measured without corrosion inhibitor and red line is measured with corrosion inhibitor.
Tafel plot created by Metrohm Autolab’s NOVA software corresponding to the measurements done in quiescent electrolyte (blue) and under 500 RPM rotation rate (red). All other experimental parameters were kept the same.

A second evaluation can be performed to learn how much the pipe will rust in a year, under these resistive conditions. In the example below, under standard conditions, the corrosion rate of carbon steel is measured at 0.25 mm/yr. However, when a specific corrosion inhibitor is used (tryptamine in this case), the performance is significantly improved and the corrosion rate drops to 0.065 mm/yr. These results can be achieved in a matter of minutes by using electrochemical methods, whereas by conventional methods (e.g., salt spray chamber combined with weight loss analysis), it takes up to a few months to conclude the results. That is a huge difference in efficiency!

Corrosion Parameter No Inhibitor With Inhibitor
Ecorr (V) from linear regression -0.479 -0.392
Ecorr (V) from Tafel analysis -0.482 -0.396
Rp (Ω) from linear regression 42.62 135.96
Rp (Ω) from Tafel analysis 43.32 136.39
Corrosion rate (mm/year) from Tafel analysis 0.25 0.065
Linear regression and Tafel analysis data resulting from experiments with and without corrosion inhibitor.

Summary

Understanding the corrosion behavior of a material under real-life conditions helps manufacturers to more quickly optimize the material design in terms of corrosion resistance, either by using a more suitable material for the pipes or by using adequate corrosion protection methods (i.e., coatings or corrosion inhibitors), which results in significant cost savings and safer operation.

Post written by Dr. Reza Fathi, Product Specialist at Metrohm Autolab, Utrecht, The Netherlands.