Utilizing Fourier Transform Infrared for Development and Quality Assurance of Cure Chemistries
By Nasreen Khan, Technical Service Project Leader, and Shakher Puntambeker, Principal Chemist
Product development and quality assurance are paramount in the manufacture of quality engineered films. Fourier transform infrared spectroscopy (FTIR) is an analytical technique that assists development on the lab bench and assures quality during manufacturing. During the development process, it can be used to select optimal components. After the development phase, when chemistry and manufacturing processes are established, FTIR can help maintain quality of components and final product. This article will give a brief background on FTIR and describe how the authors company utilizes this tool to provide high-quality engineered films.
FTIR background and testing chemistries
Fourier transform infrared (FTIR) spectroscopy is an analytical technique that employs the infrared portion of the electromagnetic spectrum to probe certain material properties. Typically, the mid-IR range of the spectrum (2,500 to 25,000 nm) is used to identify materials whose molecules or functional groups vibrate as a response to IR energy. As the functional groups vibrate within their structure, a unique spectra or fingerprint of the material is generated. Samples can be any solid, liquid or gas that responds appropriately to IR energy. For plastics-related industries, including coaters and laminators, polymeric-based materials are frequently analyzed to determine a number of properties. From unknown identification to reaction monitoring, FTIR is an invaluable analysis tool in both the research and development and quality assurance realms in the manufacture of coatings, films and laminations.
Multipart chemistries have a variety of applications for coatings and laminations. From scratch resistance to adhesives in laminates, the FTIR is used to monitor chemical reactions and crosslinking systems, such as those in epoxies, polyurethanes and polyesters. The reactions are monitored over time or with the addition of heat or UV energy, and distinctive peaks are analyzed for change in peak height or even wavenumber shifts.
This article explores the cure progression of an isocyanate in adhesives. As adhesives cure, the multifunctional isocyanate groups form bonds with the free hydroxyl groups, amines or free moisture in the system. As the groups react and bond, the peak diminishes, and the peak height in the y-axis decreases. This peak occurs at approximately 2250 wavenumbers (cm-1).
Surface or bulk measurements can be taken with different FTIR setups. Laminates with a polyurethane or polyester adhesive can be measured in bulk with a transmission setup. In this mode, the IR beam will penetrate the entirety of the sample. When measuring in bulk, however, all of the layers and components present in the sample appear and overlap in the resulting spectra. Depending on the cross-linking chemistry and locations of the peaks in question, the spectra may need to be de-convoluted through subtraction methods with some of the individual layers or films. Fortunately, few (if any) other functional groups overlap with isocyanate at 2250 cm-1, and their curing is easier to analyze. All spectra have to be baseline-corrected so that the peak heights can be compared.
Development and cure monitoring
In development, after choosing an appropriate adhesive chemistry for the required laminate properties and film type, optimizing the formulation is the next step. The vendor-suggested proportions of components can be adjusted and iterated to achieve desired properties. When selecting an isocyanate crosslinking system, the isocyanate can be chosen based on properties, such UV stability, aliphatic or aromatic functionality, cure time and relative bond strength for laminates. The choice of chemistry also depends on the bonding substrates in the laminate. FTIR analysis plays a role during development to understand cure time, formulation and processing relationships to the desired properties. Depending on the requirements of the product, using too much cross-linker can result in a loss of certain properties, such as adhesion in a laminate, as it reacts to itself or to excess moisture in the environment. Not having enough of the isocyanate also may not achieve the desired properties. Therefore, selecting the appropriate formulation and processing for the product is imperative.
To demonstrate this principle, two isocyanates with different percent isocyanate content (% NCO) but similar functionality were mixed in the same proportion and with the same resin. After drawing down and creating the laminate at room temperature in the lab, the laminates were tested in the FTIR in transmission for up to seven days. The samples were tested at initial, one, two, three, six and seven days. As can be seen in Figure 1, the isocyanate with the slightly higher % NCO content (Chemistry 1 in black) takes longer to cure, as compared to that of Chemistry 2 in blue with lower % NCO content. This is expected because the isocyanate in Chemistry 1 started at a larger initial peak (at ~30% transmission) compared to Isocyanate 2 (at ~60% transmission) and remained higher at each time point. This also is consistent with the information provided by the vendor about cure speeds.
To better serve the requirements of a different laminate, the same chemistry as in Chemistry 1 (same % NCO content) was used but in a higher proportion of the isocyanate to the resin. Doubling the proportion of the isocyanate Chemistry 3 (Figure 2, blue) compared to the original Chemistry 1 (red) proved again that having a greater proportion of isocyanate in the chemistry increased the time needed to cure. Chemistry 3 started with the peak at ~20% T, while Chemistry 1 started at ~40% T. The black line represents the film used in the laminate. After seven days of cure, Chemistry 1 is close to fully cured, while Chemistry 3 is not. However, increasing the proportion of the isocyanate component in the mixture allows for slightly different bulk properties, as needed for the requirements of the product. Ultimately there is a balance between formulation, processing and final product for two-part chemistries, which can be monitored using FTIR.
Quality assurance and feedback using FTIR
Once a formula and processing steps are established, analyzing the isocyanate peak in a laminate during production provides another level of feedback to control the process. As seen in Figure 3, two laminates were made from the same batch of chemistry (Chemistry 1) in the lab; however, one drawdown (Chemistry 1 Sample 2 in green) was made incorrectly. The isocyanate peak height was higher initially, even though the batch of chemistry was the same. Visually, the quality of the laminate was poor, there were bubbles, and the laydown was not smooth. Given the initially high isocyanate peak height, as expected, the sample was not cured in seven days. This highlights the importance of controlling the processing of two-part chemistries. If processing is different, even with the same chemical mixture and proportions, the curing time and quality can be affected. Measuring and identifying the approximate range of isocyanate peak height and associating it with acceptable product performance provides another test method to assure quality during production.
The spectra depicted thus far represent samples made in the lab; however, the same principles apply to production samples of two-part chemistries in laminates. Figure 4 shows historical FTIR data taken of samples of a laminate right off the production line. Pieces of the sample were placed in various conditions and aged to understand the effects of storage conditions and temperature on the cure time. The isocyanate peaks at 2250 cm-1 from bottom to top represent storage for 24 hours in a refrigerator, 24 hours in the lab/warehouse, 24 hours in 100°F oven and a laminate aged for two months. This implies that, as more heat or time is added to the laminate, the cure rate is increased and the time needed for a full cure is decreased. This provides an idea of post-production processing and storage conditions with respect to curing. As needed, changing the processing conditions including additional storage time before successive manufacturing steps can help deliver quality products.
FTIR can be used to determine the degree of cure of UV curing materials. The determination of the extent of cure is important for quality control and also can be used for optimizing the manufacturing process.
For experimental purposes, a hard-coat formulation that cures by radical polymerization due to UV exposure was coated on a PET film. For curing purposes, laboratory UV curing equipment from American Ultraviolet with a mercury lamp was used. The curing was performed at 125 WPI and 90 FPM with multiple passes to illustrate degree of cure.
A Bruker Alpha FTIR spectrometer with a diamond ATR accessory was used for collecting spectra after each pass. The ATR technique measures IR reflectance from the surface of the coating and minimizes the contribution of the base film to the IR spectrum. The spectrum was taken by pressing the coated side of the film on the diamond crystal. For each experiment, 32 scans were taken.
For evaluating cure of the hard-coat, the peak at 810 cm-1 related to the acrylic moiety was used. As the coating cures, the amount of the acrylate decreases and so the intensity of the 810 cm-1.decreases. This can be seen in Figure 5. By referencing another peak that does not change during curing, a method can be developed to correlate the intensity of the 810 cm-1 to the degree of cure.
In developing, formulating and ensuring quality engineered films, coatings and laminates, the FTIR is a powerful and dynamic tool. Particularly in the process of iterating formulations of two-part chemistries such as isocyanate-based adhesives monitoring the cure reaction based on the isocyanate peak can help choose and identify appropriate proportions for a desired product. It also can provide a check during production and help determine appropriate processing and storage after production. In addition to chemistries that cure over time, chemistries that involve UV curing also can be studied and their processing perfected with the help of the FTIR. Depending on surface or bulk measurements, multiple setups can be used to glean the information needed for different sample types. Beyond the scope of this article, the FTIR can help identify contamination and be utilized for incoming inspection and bulk unknown chemical identification. Ultimately, the FTIR is a powerful analytic tool to ensure a great quality product.
The author would like to acknowledge Robin Kobren for providing historical data included in the article as well as experimental guidance.
- Radice, S. and Bradley. M. (2007) Time-Based FT-IR Analysis of Curing of Polyurethanes (Application Note# 51255). Retrieved from Thermo Fischer Scientific website.
- Bruker Optics Inc. (2010). Determination of the degree of cure of a varnish (Application Note AN# 103). Retrieved from Bruker Optics Inc website.
- Tracton, A. (2005). Coatings Technology Handbook, Third Edition. CRC Press.