An overview on application of FTIR.
When infrared fight is passed through a sample of an organic compound, some of the frequencies are absorbed, while other frequencies are transmitted through the sample without being absorbed. If we plot absorbance or transmittance against frequency or wave number, the result is an infrared spectrum. For a nonlinear molecule (with n number of atoms), three degrees of freedom describe rotation and three describe translation. The remaining 3n - 6 degrees of freedom are vibrational degrees of freedom or fundamental vibration. Linear molecules have 3n - 5 vibrational degrees of freedom. In addition to fundamental vibrations, other frequencies can be generated by modulations. All the peaks of the spectrum are not of analytical importance. The characteristic peaks are of main interest.
On the basis of measurement of infrared frequencies, infrared spectrophotometers are divided into two basic classes. The first type is dispersive in nature, and the second is the interferometric type. In the former case, infrared light is separated into its individual frequencies by dispersion, using a grating monochromator. In the interferometer instrument, the infrared frequencies are allowed to interact to produce an interference pattern, and this pattern is then analyzed mathematically, using Fourier Transforms (ref. 1) to determine the individual frequencies and their intensities.
Infrared spectrophotometers consist of the following:
The main sources of infrared radiation used in the spectrophotometer are:
* A nichrome wire wound on a ceramic support;
* a Nernst glower, which is a filament containing uranium, thorium and cerium oxide held together by a binder; and
* the glower, a bonded silicon carbide rod.
These are heated electrically to a temperature range of 1,200 to 2,000[degrees]C.
Earlier, a prism was used to diffract the light, but now a diffraction grating is used. Light reflected from the grating is diffracted, interference arises at a certain angle, and so a specific wavelength appears with constitutive interference at a specific angle of reflection.
Most of the dispersive instruments use thermopile detectors. These consist of several thermocouples connected in series, so that their outputs are added together for greater sensitivity. In FTIR, a thermal detector is based on pyroelectric materials or on solid-state semiconductor devices using photovoltaic or photoconductive principles.
FTIR instruments are normally based on the Michelson's interferometer, in which the radiation from an infrared source is split into two beams by a half-silvered 45[degrees] mirror in such a way that the resulting beams are at right angles to each other. Michelson's interferometer is a device that can be used to measure length or change in length with great accuracy, by means of interference fringes.
Advantages of FTIR over IR
The advantages of FTIR over IR include high resolution that can be obtained in FTIR; for a FTIR, there is an improvement in signal-to-noise ratio, and it is easier to study small samples or materials with weak absorption; and in FTIR, time taken for a full spectral scan is less than one second. This makes it possible to obtain improved spectra by carrying out repetitive scans and averaging the collected signals.
Presently, most of the IR instruments are the interferometer type (FTIR). This variety of equipment is vastly used in qualitative analysis. It is also used in the rubber and polymer industry for both qualitative as well as quantitative purposes.
Applications in polymer and rubber industries
Polymer and rubber technologists have used FTIR for many characterization purposes. A good book on the application of FTIR in the characterization of polymers has been published (ref. 2).
The following sections specifically deal with the use of FTIR in the rubber field. A System 2000 Fourier Transformed Infrared Spectrometer (FT-IR) from Perkin Elmer has been used in the analysis.
Estimation of vinyl acetate content in EVA (polyethylene vinyl acetate)
EVA is extensively used in the rubber industry as a wrapping poly for different polymers like SBR and IIR. EVA in the form of a bag is also used as a weighing pouch for different rubber chemicals. The properties of EVA are very much dependent on vinyl acetate content. FTIR was used to estimate vinyl acetate content in EVA.
The methyl (-[CH.sub.3]) group of vinyl acetate absorbs in the IR region at about 1,370 [cm.sup.-1], and the paraffin group of the ethylene block (-CH2-) absorbs in the IR region at about 720 [cm.sup.-1] (ref. 3). The absorbance ratio ([A.sub.1,370]/[A.sub.720]) of standard materials of known composition was plotted against the ratio of vinyl acetate content (%) and ethylene content (%), and a straight line was obtained. From this graph, knowing the absorbance ratio, the vinyl acetate content of an unknown sample can be calculated.
The absorbance at about 1,370 [cm.sup.-1] and 720 [cm.sup.-1] of the standard EVA film was calculated after base line correction. The clean EVA sample film was held in the magnetic sample holder. The sample holder, along with the sample, was kept in the sample compartment for scanning. The spectra were recorded at 1,700-600 [cm.sup.-1] region at a resolution of 4 [cm.sup.-1]. The average of five scans was recorded for better signal to noise ratio. The absorbance at about 1,370 and 720 [cm.sup.-1] was calculated from the resulting spectra. The values were plotted against the corresponding vinyl acetate to ethylene content ratio.
From the calibration graph, the slope (M) and intercept (C) were noted,
then, Y = MX + C
where Y is the absorbance ratio and X is the ratio of vinyl acetate to ethylene content (A/B). By knowing X (A/B) and the fact that A + B = 100, the percentage of vinyl acetate content can be calculated.
A similar type of application has been reported by James R. Parker, et al (ref. 3). They used photo-acoustic Fourier transform infrared spectroscopy for quantitative characterization of EVA, EPDM, SBR and NBR. The technique allows a background spectrum of carbon black filler to be run prior to analyzing the compounded sample.
ASTM D3900 (ref. 4) provides the test method for determination of ethylene units in ethylene propylene rubber (EPM) and EPDM rubber. The integrated TGA-FTIR technique has also been used for quantitative analysis of EVA and NBR (refs. 5 and 6).
Carbon type analysis ([C.sub.A], [C.sub.p] and [C.sub.N]) of rubber process oil by FTIR
This method (ref. 7) is used for determination of aromatic, paraffinic and naphthenic carbon in mineral base oils used in the rubber industry as rubber process oil. This procedure is not suitable for material containing water. The infrared spectrum of the sample is recorded in the two regions (1,750 [cm.sup.-1] - 1,500 [cm.sup.-1]) and (850 [cm.sup.-1] - 600 [cm.sup.-1]). The intensities of the peaks at about 1,600 [cm.sup.-1] and 720 [cm.sup.-1] are used to calculate the aromatic and paraffinic carbon contents respectively. The naphthenic carbon is determined by difference. The paraffinic carbon type cannot be determined directly if the aromatic content of the oil exceeds 20%. This is because neighboring peaks may mask the peak at 720 [cm.sup.-1]. In such cases, a dilution technique is used.
The path length of the cell is calculated from the formula: d = (n*10) / [2*([w.sub.1]-[w.sub.2])]
where, n is the number of fringes between two wave numbers [w.sub.1] and [w.sub.2]
If no fringes are obtained, either the cell windows are in bad condition or they are not parallel. The absorbance is measured at the corresponding peaks after base line correction.
[C.sub.A] = 1.2+9.8*E
[C.sub.p] = 29.9 + 6.6*E
E = A/c*d where A = absorbance, d = path length of the cell in mm, and c = concentration factor (1 for undiluted oils)
[C.sub.N] = 100 - ([C.sub.A]+ [C.sub.p])
Results below 10% are reported to one decimal.
In the case of highly aromatic oil (aromatic content more than 20%), it should be diluted with paraffinic oil in such a manner that the aromatic content becomes about 5%.
[C.sub.p] of the blend is calculated by means of the formula given below:
[C.sub.p] = [C.sub.p] (blend) (s + d) - [C.sub.p] (diluents) d/s where s = weight of the sample in the blend, and d = weight of the diluents in the blend.
A typical example of calculation of [C.sub.A], [C.sub.p], [C.sub.N] in paraffinic oil is given in table 1. In this case, the authors have taken low aromatic fraction ([C.sub.A]) containing paraffinic oil.
Determination of micro structure of rubber
The Indian standard IS 10016, part-4 (ref. 8) describes the method for the microstructure determination of polybutadiene rubber.
A few drops of polybutadiene rubber (BR) based cement were taken in potassium bromide (KBr) pellet, and a uniform film was prepared. It was dried for 15-20 minutes, and the spectrum was taken. Absorbance at 965 [cm.sup.-1], 910 [cm.sup.-1] and 735 [cm.sup.-1] represents the trans, vinyl and cis peaks, respectively.
The corrected absorption (after base line correction) at the specific wave number was measured. The relative concentration of the three components are given as follows--
[C.sub.C] = 1 * absorbance at the cis peak (735 [cm.sup.-1])
[C.sub.T] = 0.118 * absorbance at the trans peak (965 [cm.sup.-1])
[C.sub.V] = 0.164 * absorbance at the vinyl peak (910 [cm.sup.-1])
[C.sub.X] percent by mass = [C.sub.X] x 100/[C.sub.C + [C.SUB.T] + [C.sub.V]]
A typical example of calculation of micro structure of polybutadiene rubber (BR) is given in table 2.
ISO 21561 (ref. 9) specifies procedures for the quantitative determination of the microstructure of the butadiene units and the content of the styrene units in solution-polymerized SBR (S-SBR) by 1H-NMR spectroscopy as an absolute method and by IR spectroscopy as a relative method. A spectroscopic study of the structure of butyl and halobutyl rubber has been reported (ref. 10). A comparative method based on the FTIR has been established taking 1H-NMR as an absolute method. It has been reported that about 6% of the isoprene units undergo 1,2 addition. An FTIR method has been developed to measure the isoprene unit in butyl and halobutyl rubber.
Identification of polymer/polymer blends in vulcanizate (ref. 11)
Because of different modes of presentation of spectra, it is strongly recommended that a set of reference spectra on the same instrument should be prepared before proceeding to an unknown sample. The following absorptions, when they occur, are of no diagnostic value and should not be used for rubber identification: 3,330 [cm.sup.-1], 2,860 [cm.sup.-1], 1,700 [cm.sup.-1] and 1,450 [cm.sup.-1].
ASTM D3677 specifies the method for identification of rubber from a vulcanizate by FTIR. It involves extraction followed by pyrolysis. The principle absorption bands of some rubber, in order of diagnostic value, are given in table 3.
The pyrolyzate of chloroprene rubber can give a variable spectrum that also tends to be lacking in characteristic features. The most characteristic absorption is that at 820 [cm.sup.-1], but this is rather broad and often not very intense. A weak absorption at 747 [cm.sup.-1] sometimes fails to appear, while the stronger absorption at 885 [cm.sup.-1] is common in some degree to all other polymers. Some of the principal absorption in the spectrum obtained from the pyrolyzate of polybutadiene rubber is closely similar both in wave number and intensity to those found in the spectrum of chlorosulphonated polyethylene. The results of a test for chlorine should be taken into account when deciding between these two rubbers. The spectrum of a polybutadiene pyrolyzate differs from that of a styrene butadiene pyrolyzate in that the absorption due to an aromatic constituent is absent or much reduced.
The infrared examination of pyrolyzate can identify a mixture of two types of rubber in the range from 80% major component to 20% minor component. It will not distinguish between emulsion and solution polymerized rubber. It will also not measure the ratio of acrylonitrile to butadiene. A pyrolysis procedure will not distinguish between butyl rubber and its halogenated forms. Different grades of fluoroelastomer cannot be identified by FTIR.
A blend of natural or synthetic isoprene (20%) and chloroprene (80%) may create difficulties, and identification of the minor component may be achieved only when its amount is equal to or more than 30% in the blend. Likewise, a blend of styrene butadiene rubber (80%) and high cis- polybutadiene rubber (20%) may present difficulties, and identification of the minor component may only be achieved when it is equal to or more than 30% in the blend. Ethylene-propylene rubber in blends with other rubbers presents difficulties when its content is in the range from 20% to 40 %.
O'Keefe reported a good collection of spectra for identification purposes (ref. 12). Frisone and co-workers (ref. 13) reported a multi-laboratory study of the qualitative and quantitative analysis of polymer blends using several techniques including FTIR. Characterization of binary/tertiary blends of SBR, NBR and poly vinyl chloride (PVC) by IR spectroscopy has also been reported by Ghebremeskel and co-workers (ref. 14). Bhatt and co-workers (ref. 15) reported a correlation study of polymer ratio in NR/SBR and NR/BR blends by TGA and FTIR. The miscibility of IR and BR with various vinyl contents has been investigated by using DSC and temperaturedependent FTIR (ref. 16).
Identification/characterization of different chemicals used in the rubber industry (ref. 17)
The FTIR provides a basis for producer-consumer agreement. The ability to superimpose the infrared spectrum of the test specimen upon that of a reference specimen is evidence that the two are identical. If the spectra are not super-imposable, the location, shape and relative absorbance of every absorption band in each of the spectra is compared. If the material is identical, these factors should agree. ASTM 2702 (ref. 17) describes the details about the sampling technique for rubber chemicals. The spectra of some rubber chemicals that were examined are given in figure 1. Figures 1a and 1b are the spectra of the control and experimental 6PPD (1,3 dimethyl butyl para-phenylene diamine is used as an anti-degrading agent in the rubber industry). Although the spectrum is given separately for better clarity, the software of the 2000 FTIR system has the option to overlap or superimpose the spectra to compare them.
[FIGURE 1 OMITTED]
Identification of oil/plasticizer type in a rubber compound
The authors have used the FTIR to characterize the oil type in a rubber compound. The acetone extract was separated using column chromatography with different solvents. The toluene fraction was taken for the IR study. The spectrum so obtained was compared with the library spectrum that was made in the same condition. Figure 2a is the spectrum of di-butyl phthalate plasticizer. Figure 2b is the spectrum of the toluene fraction of the extract from the nitrile rubber compound (typical oil seal compound). By comparing the spectra, it was concluded that the experimental spectrum is of the phthalate type.
[FIGURE 2 OMITTED]
Use of FTIR in the reaction mechanism study
FTIR and NMR studies on the crosslinking reaction between chlorosulfonated polyethylene and epoxidized natural rubber have been reported (ref. 18). Thomas and co-workers (ref. 19) have studied the epoxidation mechanism of natural rubber by FTIR along with NMR. An FTIR study on the curing of brominated poly (isobutylene-co-4-methyl styrene) has been published (ref. 20) and the curing with different zinc salts was investigated by FTIR. The residual vulcanization accelerator in natural rubber latex film was determined by using FTIR (ref. 21). FTIR has been used in many ways to study different mechanisms (refs. 22-24). Sung Joon Oh reported the study of the peroxide curing of poly butadiene and zinc diacrylate blends by FTIR (ref. 25). The thermal autoxidation of cis-1,4-polybutadiene was studied by FTIR at various temperatures (ref. 26).
Miscellaneous application of FTIR
Gui-Yang Li reported the FTIR mapping of polymer swelling and solvent segregation in a benzene/cyclohexane/polyisoprene rubber system (ref. 27). An FTIR investigation of the molecular structure at the crack interface in unfilled and silicafilled polyisoprene has been reported (ref. 28). The same kind of analysis was also reported by Kralevich, et al, in a silicafilled natural rubber compound (ref. 29). FTIR was also used in the characterization of clay-SBR nanocomposite materials (ref. 30). An examination of diffusing materials on rubber surfaces by photo-acoustic FfIR (PAS-FFIR) has been reported by Peck, et al (31).
The main advantages of FTIR over other techniques are the ease of instrument operation, economy of running the instrument, interpretation of data, high resolution, high speed and ease of sample preparation. However, being a relative method, it requires correlation with some of the absolute techniques in quantitative work. For qualitative work, this technique is excellent to identify materials if a data bank is available. The rubber technologist can use this technique as a problem-solving tool. Yet, a lot of attention is required for the proper application of FTIR in the rubber field. The technique has a wide scope of capabilities.
(1.) A.L. Smith, Applied Infrared Spectroscopy, Wiley-Interscience, NY, 1979.
(2.) Hatsuo Ishida, Symposium on "Fourier Transform Infrared Characterization of Polymers", ACS Div. of Polymer Chemistry, Plenum Publ. Corp., October 1987.
(3.) James R. Parker and Walter H. Waddell, Journal of Elastomers and Plastics, vol.28, p. 140, 1996.
(4.) ASTM D3900, 2000.
(5.) R. Schonherr, Kautschuk Gummi Kunststoffe, vol. 50, p. 564, August, 1997.
(6.) R. Schonherr, Kautschuk Gummi Kunststoffe, vol. 49, p. 737, November, 1996.
(7.) Indian Standard IS 13155-1991.
(8.) Indian Standard, IS 10016 (Part-4), 1984.
(9.) ISO/FDIS 21561, 2005.
(10.) D. Cheng, I. Gardner, C. Frederick, A. Dekmezian, P. Hous and H. Wang, Rubber Chem. and Tech., vol. 63-2, p. 265, 1990.
(11.) ASTM D 3677, 2000.
(12.) Jerome F. O'Keefe, Rubber World, June 2004.
(13.) G.J. Frisone, D.L. Schwarz and R.A. Ludwigsen, Rubber and Plastics News, vol. 21, 1997.
(14.) G.N. Ghebremeskel and S.R. Shield, Rubber World, January, 2003.
(15.) J. Bhatt, B.K. Roy, A.K. Chandra, S.K. Mustafi and P.K. Mohamed, Rubber India, September, 2003.
(16.) Kazuhiro Yamada and Yoshinori Funayama, Rubber Chem. and Tech., vol. 63, p. 669, 1990.
(17.) ASTM D 2702, 1995 (re-approved 1998).
(18.) A. Roychoudhury, P.P. De, N. Dutta, N. Roychoudhury, B. Haidar and A. Vidal, Rubber Chem. and Tech., vol. 66, p. 230, 1993.
(19.) G. V. Thomas and M.R.G. Nair, Kautschuk Gummi Kunststoffe, vol. 50, p. 399, May 1997.
(20.) Donghang Xie and Hsien Wang, Rubber Chem. and Tech., vol. 77-1, p. 161, 2004.
(21.) A. Temel, R. Schaller, M. Hchtl and W. Kern, Rubber Chem. and Tech., vol. 78-1, p. 28, 2005.
(22.) G.K. Jana and C.K. Das, "Progress in rubber," Plastics and Recycling Technology, Vol. 21, no.3, 2005.
(23.) S.L. Agrawal, S.K. Mandot, N. Mandal, S. Bandyopadhyay, R. Mukhopadhyaya, A.S. Deuri, R. Mallik and A.K. Bhowmick, Rubber Plastics and Recycling Technology, vol. 21, no. 3, 2005.
(24.) A. Mousa and M. Mekewi, ES. El- Mosallamy and E. Baer, Rubber Chem. and Tech., vol. 77-2, p. 278, 2004
(25.) S.J. Oh and J. L. Koenig, Rubber Chem. and Tech., vol. 73-1, p. 74, 2000.
(26.) R.L. Pecsok, P.C. Painter, J.R. Shelton and J.L. Koenig, Rubber Chem. and Tech., vol. 49-4, p. 1,010, 1976.
(27.) G-Y Li and J.L. Koenig, Journal of Elastomers and Plastics, vol. 36, p.33, 2004.
(28.) J. Glime and J.L. Koenig, Rubber Chem. and Tech., vol. 73-1, p. 47, 2000.
(29.) M. Kralevich and J.L. Koenig, Rubber Chem. and Tech., vol. 71-2, p. 300, 1998.
(30.) S. Sadhu and A.K. Bhowmick, Rubber Chem. and Tech., vol. 76-4, p. 860, 2003.
(31.) M.C. Peck, M. Samus, P. Killgoar and R. Carter, Rubber Chem. and Tech., vol. 64-4, p. 610, 1991.
N. Mandal, S. Chakraborty, R. Ameta, S. Bandyopadhyay, S. Dasgupta and R. Mukhopadhyay, Hari Shankar Singhania Elastomer and Tyre Research Institute (HASETRI) and A.S. Deuri, J.K. Tyre
Table 1--calculation of [C.sub.A], [C.sub.P], [C.sub.N] of a typical paraffinic oil Calculation of cell path Wave length range: 2,271 [cm.sup.-1] to length (d) 491 [cm.sup.-1], number of fringes: 27 d = 0.0758 mm Calculation of [C.sub.A] Peak position: 1,604.22 [cm.sup.-1], corrected height: 0.0865 absorption unit (A), [C.sub.A] = 12.4% Calculation of [C.sub.P] Peak position: 724.57 [cm.sup.-1], corrected height: 0.3592 absorption unit (A), [C.sub.P] = 61.2% Calculation of [C.sub.N] [C.sub.N] = (100 -12.4 = 61.2)% = 26.4% Table 2--calculation of [C.sub.C], [C.sub.T], [C.sub.V] of a typical polybutadiene (BR) sample Calculation of [C.sub.C] Peak position: 739.07[cm.sup.-1], corrected height: 0.6246 A [C.sub.C] = 0.6246 Calculation of [C.sub.T] Peak position: 967.43 [cm.sup.-1], corrected height: 0.0269 A [C.sub.T] = 0.0032 Calculation of [C.sub.V] Peak position: 912.40 [cm.sup.-1], corrected height: 0.0498 A [C.sub.V] = 0.0082 Cis % by mass Cis % by mass = 98.22% Table 3--characteristic absorption band of some rubber Rubber Wave number Intensity ([cm.sup.1]) Polyisoprene rubber 885 Very strong 1,370 Strong 800 Medium Styrene-butadiene rubber 699 Very strong 775 Strong 1,490 Medium 909 Strong 990 Strong Acrylonitrile butadiene rubber 2,220 Medium strong 962 Medium 1,610 Medium 1,590 Medium Chloroprene rubber 820 Medium 885 Medium 699 Medium 769 Weak Butyl rubber 1,370 Strong 1,390 Strong 885 Strong Polybutadiene rubber 909 Strong 962 Strong 990 Medium 813 Weak 695 Weak
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