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 1950's, polyurethanes began to be used in adhesives, elastomers, rigid foams, and flexible cushioning foams such as those used today.
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.
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.
|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|
|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.
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.
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.
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.
|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||
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 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free White Paper below.
To learn more about pre-calibrations for polyols, download our brochure and visit our dedicated webpage.
One example of a dedicated ASTM standard referring to NIRS is ASTM D6342 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.
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.