Rapid Quality Control in Polyurethane Production

Rapid Quality Control in Polyurethane Production: Hydroxyl Number in Polyols and Isocyanate Content in Polyurethane Under One Minute with Vis-NIRS

Introduction

Plastics and polymers have become an integral part of modern lifestyles today. Depending on their physical structure, variable parameters they contain, and different degrees of flexibility they possess, they can be transformed into virtually any shape. Manufacturing processes for plastics can vary according to their intended uses, and today they can be found in the content of a wide range of products spanning from aircraft and car parts to packaging, from medical device products to textiles (and even many other vital components necessary for us to sustain our lives). Polymer production is a challenging process in which high-purity raw materials undergo complex reactions and are transformed into polymers, fibers, resins, and rubbers. To ensure these products meet expected specifications, quality control analyses must be applied at numerous points, from raw material intake through reaction management, intermediate product optimization, to final product quality and even energy consumption, recycling, and sustainability. Given the importance of consumption rate and product quality, the ability to conduct quality control analyses with rigor, accuracy, speed, and low cost is of high importance to manufacturers.

Polyurethanes (PU)

Polyurethanes, invented in 1937 by German chemist Dr. Otto Bayer (1902–1982), appear as a sub-class of polymers. During World War II, the use of polyurethanes became popular by replacing rubber, which was then expensive and difficult to obtain, and from the 1950s onwards they began to be used in adhesives, elastomers, rigid foams, and flexible cushioning foams. [caption id="attachment_152645" align="aligncenter"] Figure 1: Polyurethane in shoe soles
[/caption] Today, polyurethanes can be found in countless forms and can be used in a wide range of everyday products, from coatings and adhesives to shoe soles, insoles, and foam insulation. Polyurethanes are obtained by reacting polyols (that is, alcohols containing more than two reactive hydroxyl groups per molecule) with di-isocyanates or polymeric isocyanates. Appropriate catalysts and additives can also be used when necessary. Since both various di-isocyanates and a wide variety of polyols can be used to produce polyurethane, a wide range of polyurethane materials can be produced to meet specific requirements for different applications. Polyurethanes can be found in various forms, including rigid foams, flexible foams, specialty adhesives, chemically resistant coatings, sealants, and elastomers. The properties of polyurethanes are greatly dependent on their manufacturing processes. When the polyol chain is long and flexible, the final product will be soft and elastic. On the other hand, if the degree of cross-linking is very high, the final polyurethane product will have a hard and rigid structure. The cross-linked structure of polyurethanes typically consists of three-dimensional networks resulting in very high molecular weights. This structure also reveals the thermosetting nature of the polymer, as polyurethane typically does not soften or melt when exposed to heat. The most popular and primary application of polyurethanes is foam production (rigid and flexible). This form is obtained by facilitating the production of carbon dioxide gas during the urethane polymerization process. The properties of polyurethanes can typically be compared with other conventional materials such as rubber, metal, and plastic. Various standard analytical techniques and methodologies are used for parameters monitored in polyurethane raw material and quality control processes; titrimetric techniques for hydroxyl number measurements in polyols, HPLC analyses for NCO (isocyanate) measurements in polyurethane, and Karl Fischer titration for moisture determination stand out as the leading ones. In addition to the fact that many of these techniques require lengthy and challenging sample preparation stages, potential errors that may occur in these processes can cause significant production delays and, consequently, financial losses based on incorrect production and total turnover. Within the scope of this review article, we aim to examine some of the analyses performed during raw material and quality control in polyurethane production and to provide detailed information about the advantages that Near-Infrared Spectroscopy (NIRS) techniques can provide to these processes.

Near-Infrared Spectroscopy (NIRS)

Near-Infrared Spectroscopy (NIRS) is a technique based on the principle of molecule and light interaction; it provides significant contributions to the practical, rapid, and accurate monitoring of critical parameters compared to conventional methods in quality control analyses. NIRS technique, in addition to being a non-destructive method, allows you to perform faster, more environmentally friendly, and cheaper analyses compared to wet chemistry methods by not requiring any sample pre-treatment. A NIR spectrometer measures the interaction between light and matter to create a spectrum. Near-infrared spectroscopy is particularly sensitive to the presence of specific functional groups such as -CH, -NH, -OH, and -SH. This makes NIR spectroscopy an ideal method for measuring many chemical parameters such as water content (moisture), hydroxyl number, and NCO. In addition to chemical parameters, since the interaction between matter and light also depends on the matrix possessed by the sample, it is also possible to determine the physical and rheological parameters of polymers. Near-Infrared Spectroscopy (NIRS) has become an established method for both rapid and reliable quality control analyses in the polyurethane industry for over 30 years.

Quality Control Points with NIRS in the PU Production Chain

PU quality control analyses are important and necessary at numerous specific steps in the process from the production of pure polymers to the processed final product form. While some steps in the production chain are optional (additives), the operational steps typically ranging from raw plastic producers to plastic part manufacturers are as follows: The first point where near-infrared laboratory equipment can be used is where raw material (polyol, isocyanate) qualities are verified and pure polymers such as PU are produced. NIRS is also a highly useful technique for the following processing step where polymers are combined with additives to create intermediates for later use in subsequent steps. Typically, a plastic part manufacturer with extrusion, injection molding, or film blowing processes evaluates the quality of the polymer batch received to achieve efficient production. In many cases, without verification, reliance is placed on the certificate received from the supplier. On the other hand, the rapidly increasing number of companies manufacturing for the medical industry or producing high-value or large-quantity parts has also brought about the need to begin monitoring important rheological quality parameters for each polymer batch before use in injection molding, extrusion, or film blowing processes. Feeding an out-of-specification polymer into the production process leads to costly equipment downtime as well as time-consuming cleaning procedures. Rapid pre-screening of starting polymer materials used in the process is an ideal solution to prevent such risks and possible time losses. In this context, NIRS stands out with its high speed, low operating costs, and usability by personnel who do not need to have detailed chemistry knowledge. When the final part is produced at the end of the production process, quality control analyses can again be performed using NIRS. During the production of different polyurethane types, it is very important to control certain parameters to ensure quality product output. These quality parameters typically include hydroxyl number, acid number, water content, and color in polyols, and NCO (isocyanate) content, moisture content, and acid number in polyurethanes. Due to the need to use different analytical methods and various measuring equipment that come with them for measurement, the determination of typical parameters listed in Table 2 can involve lengthy and challenging processes with traditional laboratory techniques. With Metrohm's new Polyol Analyzer system, which combines visible (Vis) and near-infrared (NIR) regions, it is possible to measure various chemical and physical quality control parameters in less than one minute without any sample preparation, non-destructively, without chemicals, and without the need for consumables, using techniques developed in accordance with ASTM E1655-17 NIRS guidelines.

Experimental Content and Workflow

Within our review, repeated measurements were performed using a Metrohm DS2500 Polyol Analyzer in transmission mode, covering the entire Vis-NIR wavelength range (400-2500 nm), for hydroxyl number analyses in polyol samples and isocyanate content analyses in polyurethane samples.   All samples were placed in 8 mm single-use vials and reproducible spectra were obtained at a constant temperature of 30°C using the integrated temperature control of the DS2500 Polyol Analyzer. As can be seen in Figure 5, all measurements were performed without any sample pre-treatment.

Method Development

The Vision Air Complete software package was used for data collection, model creation, and quantification method development processes. Thanks to this triple package software that takes the user interface experience to a whole new level, all instrument management, including the assignment of operation procedures, is applied from within the office with the help of Vision Air Manager Network, while all actual measurements are performed in the quality control laboratory with the Vision Air Routine interface. The obtained spectra were automatically uploaded to the database and made directly accessible for method development. Quantitative method development was performed using Vision Chemometrics Software.

Analysis Results

The obtained Vis-NIR spectra (see Figure 6 and Figure 7) were used to create prediction models that would be used in the quantitative determination of hydroxyl number in polyol samples and isocyanate content in polyurethane samples. The quality of the prediction models was evaluated within correlation graphs showing the relationship between values obtained from laboratory primary methods and Vis-NIR data (see Figure 8 and Figure 9). Reliability data (FOM) regarding expected data accuracy during routine analysis are also shown in these figures.

Conclusion

This review demonstrates that it is possible to perform hydroxyl number analyses in polyol samples in accordance with ASTM D6342-12 using near-infrared spectroscopy (NIRS) technique. NIRS technique also allows for rapid detection of isocyanate content in polyurethane samples. Compared to wet chemistry methods, fast and cost-effective analyses represent the most striking advantage of NIRS technique. In particular, the ability to determine hydroxyl number in less than one minute and without using hazardous chemicals, unlike the methodology applied in ASTM D4274-16 guidelines, makes it possible to achieve superior time, high occupational safety, and cost savings especially in the polymer and chemical sector. As Metrohm, in addition to being the global sector leader in primary methodologies (potentiometric titration & Karl Fischer titration) that form the basis of NIRS analysis techniques in the polymer sector, we also provide fully integrated, fast, precise, and reliable solutions directly to your preference through our Vis-NIRS system solutions and software compatible with international standards under one roof.   References 1. Metrohm Blog, NIR Spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1 2. Metrohm Blog, NIR Spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 5 3. Metrohm Application Note, AN-NIR-035 Quality Control of Polyols 4. Metrohm Application Note, AN-NIR-068 Quality Control of Isocyanates 5. Metrohm Brochure, 80005287EN Polymer and plastics analysis – Quality control for polymer production 6. Metrohm White Paper, WP-036 Lean manufacturing of polyurethane, assisted by near-infrared (NIR) and Raman spectroscopy   Yeliz Yavuz Çevik Head of Electrochemistry / Spectroscopy Product Group Metrohm Turkey