Application of Fourier Transform Infrared Spectroscopy FTIR Instrument in Rubber Industry

Although almost all wavelengths of electromagnetic light are used in organic chemistry to study the structural properties of matter, our applications are mainly concentrated in three or four wavelength ranges of energy absorption: ultraviolet and visible light (UV-VIS); infrared (IR); microwave; and radio frequency absorption. Infrared spectroscopy (IR) is basically considered a vibrational spectrum. The infrared spectrum is divided into three regions: near infrared (harmonic region, 12500-4000cm-1), mid-infrared (vibration-rotation region, 4000-200cm-1) and far infrared (rotation region, 200-10cm-1).

When infrared light passes through a sample of an organic compound, some of its wavelengths are absorbed, while other wavelengths pass through the sample without being absorbed. If we plot the absorbance or transmittance against frequency or wavelength, we get an infrared spectrum. For a nonlinear molecule (with n atoms), three degrees of freedom are used to describe its rotation, and three degrees of freedom are used to describe its translation. The remaining 3n-6 degrees of freedom are used to describe the vibrational freedom or fundamental vibration. In addition, linear molecules have 3n-5 degrees of freedom for vibration. In addition to the fundamental vibration, the wavelengths of other frequencies are mainly obtained by modulation. Not all peaks in the spectrum are important in the analysis, only those characteristic peaks are of primary interest.

The instrument

is based on the measurement of infrared frequencies. There are two basic types of infrared spectrophotometers. The first is the natural scattering type and the other is the interferometer type. In the former type, the infrared light is scattered by a grating diffuser and is divided into individual frequencies. In the interferometer type instrument, the infrared light frequencies can interact with each other, thus producing interference light patterns. This type is then mathematically analyzed using Fourier transform technology to obtain the individual frequencies and their intensities. The

infrared spectrophotometer consists of the following parts:

◆ Infrared source

The main infrared light emitting sources used in the spectrophotometer are:

nickel-chromium-iron alloy wires protected by ceramic devices; Nernst luminescent elements, that is, a thin filament containing uranium, thorium and cerium oxides bonded together; silicon carbon rod filaments.

The above light sources are all electrically heated to 1200 to 2000 degrees Celsius when working

◆ In the early days, monochromators

mainly used prisms to separate scattered light, but now they use scattering gratings. The light path is dispersed after being reflected by the grating. When the grating is at a specific angle, interference occurs. Therefore, when a specific reflection angle is selected, the corresponding interference light is obtained, and a specific wavelength is obtained.

◆ DetectorMost

scattering instruments use thermopile detectors. This detector is composed of several thermocouples in parallel. The output obtained by the combination has greatly enhanced sensitivity. In Fourier transform infrared spectroscopy (FTIR), thermal detectors are mainly composed of pyroelectric materials or solid-state semiconductor devices, which use the principles of photoelectric generation or photoconduction. FTIR

instruments are generally based on Michelson interferometers. The light beam emitted by the infrared light source of this interferometer passes through a silver mirror at a 45-degree angle to form two beams of light, so that the beams formed are at right angles to each other. The Michelson interferometer is a very accurate device that can measure length or length changes through interference fringes.

Advantages of FTIR

Compared with ordinary infrared spectroscopy (IR), the advantages of FTIR include: high sensitivity; greatly improved signal-to-noise ratio; easy to study small amounts of samples and materials, even if their absorption is very weak; the time spent in a full-frequency spectrum scan is no more than one second, so multiple scans can be performed, and the collected signals can be averaged to obtain a better spectrum.

Most infrared spectrometers are currently of the interferometer type (i.e., Fourier transform infrared spectrometer). This instrument is widely used for qualitative analysis. But in the rubber and polymer industry, it is used not only for qualitative analysis, but also for quantitative analysis.

Application of FTIR in the polymer and rubber industry

In polymer and rubber production technology, FTIR is often used to characterize various properties. The following sections introduce the application of FTIR in the rubber field. The instrument used in all analyses is the Model 2000 Fourier transform infrared spectrometer produced by Platinum-Elmer.

◆ Determination of vinyl acetate content in EVA polyethylene-vinyl acetate

In the rubber industry, polyethylene-vinyl acetate is widely used as a packaging material for polymers such as styrene-butadiene rubber, butadiene rubber, etc. It is also used to make bags and package various rubber chemicals. The properties of EVA polyethylene-vinyl acetate strongly depend on its vinyl acetate content. FTIR can be used to determine the vinyl acetate content in EVA polyethylene-vinyl acetate.

In the infrared spectrum, the absorption peak of the methyl (-CH3) group in vinyl acetate is approximately around 1370cm-1, and the absorption peak of the vinyl group (-CH2-) in the alkane chain is approximately around 720cm-1. After obtaining the infrared spectrum of the standard substance with known content of each component, the ratio of the absorbance near 1370cm-1 to the absorbance near 720cm-1 (A1370/A720) is plotted against the ratio of the vinyl acetate content to the ethylene content, and a straight line will be obtained. Using the straight line graph obtained, if the absorption peak ratio (A1370/A720) of the unknown sample is known, its vinyl acetate content can also be calculated.

After baseline correction, the infrared spectrum of the standard EVA polyethylene-vinyl acetate sample film can obtain the absorbance near 1370cm-1 and the absorbance near 720cm-1. The clean and transparent EVA polyethylene-vinyl acetate sample film is placed on the electromagnetic sample holder. After the sample holder is loaded with the sample, it is placed in the sample cell for scanning. The recorded spectrum wavelength range is 1700-600cm-1, and the resolution is 4cm-1. A total of 5 scans are performed and the average is taken, so that a better signal-to-noise ratio can be obtained. In this way, the absorbance near 1370cm-1 and the absorbance near 720cm-1 can be calculated from the spectral results, and the obtained values ​​are plotted against the corresponding ratio of vinyl acetate content to ethylene content.

From the standard calibration graph, we can get the slope M and intercept C of the straight line. Then,

Y=MX+C

refers to the ratio of absorbance, and X refers to the ratio of vinyl acetate content to ethylene content (A/B). Knowing X (A/B) and A+B=100, the content of vinyl acetate can be calculated. [page]

James R. Parker et al. reported a similar application. They used photoacoustic Fourier transform infrared spectroscopy to quantitatively characterize the properties of EVA (polyethylene-vinyl acetate), EPDM (ethylene propylene diene monomer rubber), SBR (styrene-butadiene rubber) and NBR (nitrile butadiene rubber). This technique used to analyze mixed samples requires the background spectrum of carbon black filler materials to be obtained first.

The American Society for Materials Standard D3900 provides a method for determining the number of ethylene units in EPM (ethylene propylene diene monomer rubber) and EPDM (ethylene propylene diene monomer rubber). The combined technique of thermogravimetric analysis and Fourier transform infrared spectroscopy has also been used to quantitatively analyze EVA (polyethylene-vinyl acetate) and NBR (nitrile butadiene rubber).

◆ Use FTIR to determine the type of carbon atoms (CA, CP and CN) in rubber processing oil.

Rubber processing oil in the rubber industry mainly uses mineral oil. This method can be used to determine the type of carbon atoms in mineral oil (carbon atoms on aromatic rings, carbon atoms on straight-chain alkanes and carbon atoms on cycloalkanes). However, this treatment method is not suitable for materials containing water. The infrared spectrum records the wavelengths of two regions: 1750cm-1 to 1500cm-1 and 859cm-1 to 600cm-1.

The intensity of the peaks near 1600cm-1 and 720cm-1 can be used to calculate the content of aromatic ring carbon atoms and straight-chain alkane carbon atoms, respectively. Based on the difference, the carbon atoms on the cycloalkanes can also be determined. However, if the content of aromatic ring carbon atoms in mineral oil exceeds 20%, the carbon atoms on the straight-chain alkanes cannot be directly determined in this way. This is because the peaks near 720cm-1 may mask the absorption peak at 720cm-1. In this case, dilution techniques are required to analyze the carbon atom type. The

optical path length of the sample cell is determined by the following equation:

d=(n×10)/[2×(w1-w2)]

where n refers to the number of gratings between the wavelengths of w1 and w2.

If there is no grating, either the window of the sample cell is broken or the two beams of light are not parallel. After baseline correction, the absorbance of the corresponding peak position can be obtained:

CA=1.2+9.8×E
CP=29.9+6.6×E

E=A/c×d Here A=absorbance, d=length of the light path through the sample cell, and c=concentration factor (when it is undiluted oil, c=1)

CN=100 - (CA+CP)

Results below 10% are recorded to one decimal place.

In the case of a relatively high aromatic ring content in the oil (20%), it is necessary to add linear alkane mineral oil to dilute the aromatic ring content to 5%.

The CP of the blend can be calculated by the following equation:

CP=CP (blend) (s+d) - CP (diluent) d/s

S=the mass of the sample in the blend, d=the mass of the diluent in the blend.

Table 1 records a typical example of calculating CA, CP, and CN in linear alkane oil. From the table, it can be seen that the CA in linear alkane oil is relatively low. Table 1. Optical path length (d) of CA, CP and CN sample cells

in typical straight-chain alkane oils Wavelength range: 2271 cm-1 to 491 cm-1, grating number: 27 d = 0.0758 mm CA value peak position: 1604.22 cm-1, correction height: 0.0865 Absorbance (A), CA = 12.4% CP value peak position: 724.57 cm-1, correction height: 0.3592 Absorbance (A), CP = 61.2% CN value CN = (100-12.4-61.2)% = 26.4% Determination of rubber microstructure Indian Standard IS10016, Part 4 describes the method for determining the microstructure of polybutadiene rubber. A few drops of polybutadiene rubber are glued to a potassium bromide (KBr) transparent disc to obtain a uniform disc. After drying for 15 to 20 minutes, the test is carried out to obtain an infrared spectrum. The absorption at 965cm-1, 910cm-1 and 735cm-1 represents the cis, ethylene and trans absorption peaks, respectively. After baseline correction, the absorbance at each characteristic wavelength is measured. The relative concentrations of the three components are given by the following equations: CC = 1 × absorbance of the trans absorption peak at 735cm-1 CT = 0.118 × absorbance of the cis absorption peak at 965cm-1 CV = 0.164 × absorbance of the ethylene absorption peak at 910cm-1 The mass percentage of CX = CX × 100 / (CC + CT + CV) Table 2 shows a typical example of calculating the microstructure of polybutadiene rubber (BR). Table 2. CC, CT and CV in polybutadiene rubber CC value peak position: 739.07 cm-1, corrected height: 0.6246 Absorbance (A), CA = 0.6246 CT value peak position: 967.43 cm-1, corrected height: 0.0269 Absorbance (A), CT = 0.0032 CV value peak position: 912.40 cm-1, corrected height: 0.0498 Absorbance (A), CT = 0.0082 Cis% of total Cis 98.22% Standard No. 21561 formulated by the International Organization for Standardization details the determination of the butadiene microstructure and styrene content in styrene-butadiene rubber obtained by solution polymerization. It uses one-dimensional nuclear magnetic resonance hydrogen spectrum as an absolute method and infrared spectrum as a relative method. There are literature reports on the spectroscopic research method of butyl synthetic rubber and halogenated butyl synthetic rubber. Using one-dimensional nuclear magnetic resonance hydrogen spectrum as an absolute test method, a relative method based on Fourier transform infrared spectroscopy was established. There are also reports that 6% isoprene (rubber monomer) undergoes a 1,2 addition reaction. The method used is to use Fourier transform infrared spectroscopy to determine the content of rubber monomer and isoprene in butyl synthetic rubber and halogenated butyl synthetic rubber. ◆ Identification of vulcanized rubber polymer-polymer blends Due to the differences in different spectral expression patterns, it is strongly recommended to make a series of reference sample spectra on the same machine before making an unknown sample. If the following absorption bands appear, they are of no analytical diagnostic significance and cannot be used for rubber identification: 3330cm-1, 2861cm-1, 1700cm-1 and 1450cm-1. American Materials Association Standard D3677 details the method of using FTIR to identify vulcanized rubber. The samples involved must first undergo high-temperature decomposition. Some of the characteristic absorption bands of rubber listed therein can be used as a standard for judgment. The spectrum given by the thermal decomposition product of chloroprene rubber is uncertain and lacks obvious characteristic absorption values. The most typical absorption is at 820cm-1, but it is usually very broad and the signal is not strong. There is a weak absorption at 747cm-1, but it is often not seen, while the strong absorption at 885cm-1 is in some ways no different from other polymers. Some characteristic absorption bands of polybutadiene pyrolysis products are very similar to those of chlorosulfonated polyethylene in terms of wavenumber and intensity. Therefore, after obtaining the test results of chlorinated rubber, if you want to make a choice between two such rubbers, you must consider it carefully and should consider the test results of chlorinated rubber. The main difference between the spectrum of the pyrolysis product of polybutadiene rubber and the spectrum of the pyrolysis product of styrene butadiene rubber is the difference in the amount of absorption, which is caused by the absence or lack of aromatic functional groups in the former. Infrared testing of pyrolysis products can also identify mixtures of the two rubbers, from 80% of the main component to 20% of the minor component. However, it is impossible to distinguish whether it is a microemulsion polymerized or solution polymerized rubber. Similarly, the ratio of acrylonitrile to butadiene cannot be measured. This method of testing pyrolysis products cannot distinguish between butyl rubber and its halogenated rubbers. In addition, FTIR cannot distinguish different grades of fluorinated rubber. [page] If a blend contains 20% natural and synthetic butadiene and 80% chloroprene, this method cannot be used to identify it, unless the content of the minor component reaches or exceeds 30%. Similarly, if a blend contains 80% styrene butadiene rubber and 20% highly trans-polybutadiene rubber, this method will also be difficult to use, unless the content of the minor component reaches or exceeds 30%. If the content of EPDM in the blend is between 20% and 40%, this method will not work.








































O'Keefe reported a good method for collecting spectral signals for rubber identification. Frisone and co-workers reported the results of their joint laboratory using a variety of techniques to quantitatively and qualitatively analyze polymer blends, including Fourier transform infrared spectroscopy. Ghebremeskel and co-workers used infrared spectroscopy to characterize binary/ternary blends of SBR (styrene butadiene rubber), NBR (nitrile butadiene rubber) and PVC (polyvinyl chloride). Bhatt and co-workers reported on the study of polymer ratios in NR (natural rubber)/SBR (styrene butadiene rubber) and NR (natural rubber)/BR (polybutadiene rubber) blends using thermogravimetric analysis and FTIR. The compatibility of IR (polyisoprene rubber) and BR (polybutadiene rubber) blends with different vinyl contents was studied using differential scanning calorimetry and temperature-dependent FTIR.

◆ Identification/Characterization of Various Chemicals in the Rubber Industry

FTIR provides a basis for agreement between producers and consumers. If the infrared spectra of the test sample and the reference sample overlap, it proves that they are the same material. If the infrared spectra do not overlap, the position, shape and relative absorbance of each absorption band can be compared. If it is the same material, these conditions will match. ASTM Standard 2702 describes the details of sampling techniques for rubber chemicals. The infrared spectra of some rubber chemicals are shown in Figures 1 and 2. Figures 1 and 2 are control and experimental 6PPD (1,3 dimethylbutyl phenylenediamine, used as an antidegradant in the rubber industry). Although the spectra are provided separately, there is an option in the FTIR2000 system software to directly overlay or add spectra for comparison.

Figure 1. Common 6PPD spectra obtained from literature

Figure 2. 6PPD spectrum obtained from the laboratory

◆ Identification of oil/plasticizer in rubber mixture

The author of this article used FTIR to characterize the type of oil in the rubber mixture. The components can be separated by column chromatography of the acetone extract using different solvents. The components separated in toluene were tested by infrared. The obtained infrared spectrum was compared with the standard spectrum in the literature (same experimental conditions). Figure 3 is the plasticizer dibutyl phthalate, and Figure 4 is the infrared spectrum of the separated components obtained from the mixture of nitrile rubber (typical oil seal) in toluene. Comparing the two infrared spectra, it can be concluded that the experimental sample is a phthalate type plasticizer.

Figure 3. Spectrum of dibutyl phthalate reported in literature

Figure 4. Infrared spectrum of nitrile rubber mixture in toluene

◆ Application of FTIR in the study of reaction mechanisms The

use of FTIR and NMR to study crosslinking reactions between chlorosulfonated polyethylene and epoxidized natural rubber has been reported. Thomas and his co-workers studied the mechanism of epoxidation of natural rubber, also using FTIR and NMR spectroscopy. Recently, the use of FTIR to study the cure reaction of brominated poly(isobutylene-4-methylstyrene) has been reported, and a variety of different zinc salts that can be used for the cure reaction have been discovered using this method. FTIR has also been used to determine the amount of residual vulcanization accelerator in latex films. FTIR has also been used to study various reaction mechanisms. Sung Joon Oh used FTIR to study the peroxidation cure reaction mechanism of polybutadiene and zinc diacrylate. FTIR has also been used to study the thermal auto-oxidation of trans-1,4-polybutadiene at different temperatures.

◆ Other Hybrid Applications of FTIR

Gui-Yang Li used FTIR to describe polymer swelling and solvent phase separation in a benzene/cyclohexylamine/polybutadiene rubber system. The molecular structure of crack surfaces in unfilled and silica-filled polybutadiene rubber can also be studied using FTIR. The same type of analysis is reported in the work of Kralevich et al., where the material studied was a silica-filled natural rubber mixture. FTIR has also been used to study layered styrene-butadiene rubber nanocomposites. Pcck et al. reported the use of photoacoustic FTIR to detect dispersed substances on the rubber surface.

Conclusion

Compared with other techniques, the main advantages of FTIR are: easy instrument operation, economical and feasible machine operation, easy-to-interpret data structure, high resolution, fast and time-saving, and convenient sample preparation. Nevertheless, as a relative method, absolute measurement methods are still needed in quantitative analysis. In qualitative analysis, if data is available, this technology can be used well to identify materials. Rubber technology experts often use it as a problem-solving tool. To date, the appropriate application of FTIR in the rubber field still needs more attention. I believe that this technology will have a wider application. (end)