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

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

The history of polyurethanes

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

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

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

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

How is polyurethane created?

 

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

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

Physical and chemical properties of polyurethanes

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

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

Typical applications of polyurethane

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

 

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

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

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

PU vs. Rubber

PU vs. Metal

PU vs. Plastic

High abrasion resistance

Lightweight

High impact resistance

High cut and tear resistance

Noise reduction

Elastic memory

Superior load bearing

Abrasion resistance

Abrasion resistance

Thick section molding

Less expensive fabrication

Noise reduction

Colorability

Corrosion resistance

Variable coefficient of friction

Oil resistance

Resilience

Resilience

Ozone resistance

Impact resistance

Thick section molding

Radiation resistance

Flexibility

Lower cost tooling

Broader hardness range

Easily moldable

Low temperature resistance

Castable nature

Non-conductive

Cold flow resistance

Low pressure tooling

Non-sparking

Radiation resistance

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

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

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

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

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

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

Applications and parameters for polyurethanes with NIRS

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

Where can NIRS be used in the polyurethane production process?

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

Figure 2. Illustration of the production chain for polyurethanes.

Easy implementation of NIR spectroscopy for plastic producers

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

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

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

Application example:

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

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

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

Titration

90 min. preparation + 1 min. Viscometer

AN-NIR-068

AN-NIR-065

AN-NIR-035

AN-NIR-007

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

Karl Fischer Titration

25 min. preparation + 5 min. KF Titration

 

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

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

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

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

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

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

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

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

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

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

Polyamide (Nylon): A brief introduction

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

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

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

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

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

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

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

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

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

Figure 3. Molecular structure of Polyamide 66.

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

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

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

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

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

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

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

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

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

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

Applications and parameters for polyamides with NIRS

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

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

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

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

Figure 4. Illustration of the production chain for polyamides.

Easy implementation of NIR spectroscopy for plastic producers

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

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

Application example:

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

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

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

Viscosity

90 min. preparation + 1 min. Viscometer

AN-NIR-077

AN-NIR-060

AN-NIR-005

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

Titration

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

Titration

90 min. preparation + 20 min. Titration
Moisture content

Karl Fischer Titration (oven)

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

 

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

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

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

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

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

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

Future installments in this series

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

 

  • Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

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

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

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.

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.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

Undoubtedly, there is a trend nowadays towards stricter quality assurance and quality control in production processes, such as in the polymer industry. At the same time, this trend is accompanied by a stronger focus on cost-saving and time-efficient methods so that performing more testing will not automatically result in higher costs. 

Major driving factors for companies to voluntarily implement more testing and quality practices include: 

  • Cost pressure. Testing can reveal out-of-specification products, allowing production to be stopped in plenty of time, eliminating excess manufacturing costs.
  • Increased competition. Quality practices provide a competitive edge and can be used as a marketing tool to raise brand value.
  • Scarcity of resources. Qualified staff are difficult to find; therefore, checks that can be carried out by non-specialists are invaluable.

Near-infrared (NIR) spectroscopy is an analytical method that addresses the above drivers and is particularly suited for making quality control more efficient and cost-effective as shown in this article. A short overview of NIRS is presented, followed by application examples for the quality control of polymers, concluding with indications and examples regarding how polymer producing and processing companies can benefit from the utilization of NIRS.

NIR technology overview

The interaction between light and matter is a well-known process—just recall the last time your skin was sunburned. Depending on the applied light intensity and energy, the interaction can be destructive (as with a sunburn) or harmless (like radio waves). Light used in spectroscopic methods is typically not described by the applied energy, but in many cases by the wavelength or wavenumbers.

A NIR spectrometer such as the Metrohm DS2500 Polymer Analyzer measures this interaction between light and matter to generate spectra as displayed in Figure 1. NIRS is especially sensitive to the presence of certain functional groups including -CH, -NH, -OH, and -SH. Therefore, NIR spectroscopy is an ideal method to quantify chemical parameters like water content (moisture), hydroxyl value, acid number, and amine content, just to name a few. Furthermore, the interaction is also dependent upon the matrix of the sample itself, which allows the detection of physical and rheological parameters like density, intrinsic viscosity, and melt flow rate.

Figure 1. Nylon and polyethylene spectra resulting from the interaction of NIR light with the respective samples.

All this information is contained in just one spectrum, making this method suitable for quick multiparameter analysis. Solid samples, such as powders, are secured within an appropriate container or vial (Figure 2a) then placed as-is on the analyzer. Heterogeneous samples, such as polymer pellets, can be analyzed using larger measurement cups (Figure 2b).

Figure 2. Solid sample placement for NIR spectra measurement. A) Direct measurement of powders in a vial. B) Large heterogeneous sample such as pellets can be analyzed using large sample cups.

Learn more about the DS2500 Polymer Analyzer on our website!

The measuring mode is referred to as «diffuse reflection», generally an appropriate procedure for analyzing granules, fibers, flakes, and both coarse and fine powders. For diffuse reflection (Figure 3), the NIR light comes from below the sample, penetrating and interacting with it, while being partially absorbed. Unabsorbed NIR light reflects to the detector. In less than 1 minute, the measurement is completed, and the results are displayed.

Figure 3. Schematic display of the light path interacting with a sample during diffuse reflection.

The procedure to obtain NIR spectra already highlights two major advantages of NIR spectroscopy: simplicity regarding sample measurement and speed.

  • Fast technique with results in less than a minute.
  • No sample preparation required – measure sample as-is.
  • Low cost per sample – no chemicals or solvents needed.
  • Environmentally friendly technique – no waste generated.
  • Non-destructive – precious samples can be reused after analysis.
  • Easy to operate – inexperienced users are immediately successful.
Read our previous blog posts to learn more about NIRS as a secondary technique.

What kinds of polymer manufacturers in the production chain might benefit from using NIR spectroscopy?

Figure 4 illustrates 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 pure polymers are produced, and their purity requires confirmation. NIRS is also a very useful technique for the next step, when polymers are compounded into products to be used for further processing. 

Figure 4. Simplified illustration of the polymer production chain.

A plastic part producer, typically an injection molding or extrusion company, assesses the quality of the received polymer batches. In many cases, the certificate from the supplier is trusted without any further verification. However, a rapidly growing number of companies that create products for the medical industry or that produce parts of high value or in high quantities have started to assess the important rheological quality parameters of each polymer batch before using it for injection molding or extrusion. Feeding an out-of-specification polymer to the production process leads to costly standstill of the equipment and its time-consuming cleaning.

Here, a quick pre-check of the starting polymer material used in the process would be ideal to avoid such risks and potential downtime. For this purpose, NIRS is the ideal solution because it is fast, has low running costs, and can be operated by personnel without any extensive chemical education.

When the final part is created at the end of the production process, it can also be subjected to NIR spectroscopic investigations for quality control. This is useful for assessing the homogeneity or thickness of bottles or sheets of material, for example.

What kinds of polymer applications and parameters are possible with NIRS in general?

In principle, NIRS analysis is more suitable for measuring bulk materials and not for trace analysis. Furthermore, polymer samples should contain no more than 3% carbon black and a reference method should be available. When complying with these prerequisites, NIR spectroscopy can be used as a fast and cost-saving alternative measurement technology.

Metrohm Application Bulletin 414 describes several applications for the polymer industry that can be carried out with the aid of NIRS instruments. This document contains analyses of a wide range of parameters in a very large array of samples.

Examples for use of NIR spectroscopy for selected polymers are indicated in Table 1.

Table 1. Examples for use of NIRS for selected polymers.
Polymer type Parameter Conventional analysis method Advantage of using NIRS Related NIRS Application Note
Polyethylene (HDPE/LDPE) Density Densimeter Results within 30 seconds AN-NIR-003
AN-NIR-081
Melt Flow Index MFI apparatus

Time-saving

No cleaning of equipment

AN-NIR-083
Polypropylene (PP) Melt Flow Index MFI apparatus

Time-saving

No cleaning of equipment

AN-NIR-004
AN-NIR-082
AN-NIR-083
Polyamide (PA) Intrinsic Viscosity Ubbelohde viscosimeter

No time-consuming dissolution in hazardous chemicals

No waste

Cost savings

AN-NIR-005
AN-NIR-060
COOH, NH2, Moisture Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-077
Polyethylene terephthalate (PET) Intrinsic Viscosity Ubbelohde viscosimeter

No time-consuming dissolution in hazardous chemicals

No waste

Cost savings

AN-NIR-023
Acid number Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

Isophtalic acid HPLC

No eluent solvents needed

Time & cost savings

No chemically educated operator needed

Polyurethane (PU) OH of polyols Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-006
AN-NIR-007
Isocyanate content Titration AN-NIR-035
AN-NIR-065
AN-NIR-068
Polyvinyl Alcohol (PVA) Degree of alcoholysis Titration

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-076
Silicone Rubber Vinyl content Gas Chromatography

Time & cost savings

No chemicals needed

No chemically educated operator needed

AN-NIR-084
Polyvinylidene Chloride (PVDC) Sheet thickness Weighing

Time-saving

Reduced user error risk

AN-NIR-092

Save up to 90% on running costs with NIRS

Underestimation of quality control processes is one of the major factors leading to internal and external product failure, which have been reported to cause a loss of turnover between 10–30%. As a result, many different norms are put in place to support manufacturers with their QC process. However, time to result and the associated costs for chemicals can be quite excessive, leading many companies to implement near-infrared spectroscopy in their QC process.

Our free white paper illustrates the potential of NIRS and displays cost saving potentials up to 90%.

Future installments in this series

This article is only a general overview of the use of NIR spectroscopy as the ideal QC tool for the polymer industry. Future installments will be dedicated to the most commonly produced and commercially important polymers and will include much more detailed information. These polymers are:

  • 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.