Rapid Estimation of Comonomer Content in Polyolefins by FTIR
Dr G Sivalingam, Assistant Vice President - Technology, Reliance Technology Group - Polymers (RIL)
Madhumita Saroop, Senior Manager, Reliance Technology Group (RIL)

Rapid Estimation of comonomer content in polyolefins is the need of polyolefins industry and has been demonstrated by using Fourier Transform Infrared Spectroscopy (FTIR). The developed method could successfully predict the ethylene content in Polypropylene Random Copolymer (PP-RCP) and ethylene & 1-butene content in PP terpolymer. Methods for quantitative estimation of short chain branching in Linear Low Density Polyethylene (LLDPE) have also been established in this article.

Infrared (IR) spectroscopy is a powerful tool and is of fundamental importance for the determination of molecular structure, composition, configuration and stereo regularity of polymers [1]. Polyolefin IR spectra are dominated by methylene bands (polyethylene, PE) and methyl and methylene bands (polypropylene, PP). The simplest polymer structure ie, polyethylene, would be a chain of methylene units terminated on each end by methyl groups. Since PE is composed almost completely of methylene groups, its infrared spectrum would be expected to consist solely of methylene stretches and bends. Typical polyolefin FTIR spectrum is given in Figure 1. It can be seen from the figure that there are four sharp peaks which dominate the spectrum: The methylene stretches at 2,920 and 2,850 cm-1 and the methylene deformations at 1,464 and 719 cm-1. Due to the crystallinity of polyethylene, the 1,464 and 719 cm-1 peaks are split, and additional peaks are seen at 1,473 and 731 cm-1. For PP, the addition of a methyl side group on every other carbon atom in polyethylene complicates the infrared spectrum. In polypropylene, in addition to the methylene, methyl and methine groups are also present. The methyl peaks appear at 2,962/2,952 (split peak), 2,868 and 1,377 cm-1. A methyl deformation gets overlapped with the methylene deformation, and the peak gets shifted slightly to 1,458 cm-1.

Polyolefins are produced in many forms of copolymers and blends. The presence of comonomer and short chain branching disrupt crystallinity of the methylene backbone and results in modified (The article continues on page 52) physical properties, such as lower melting point, lower modulus, and higher impact strength[2]. In order to understand the effect of comonomer type and its content on the physico-mechanical properties of these commercially important polyolefins, qualitative and quantitative estimation of comonomer present is of great importance [3].

Since FTIR spectroscopy is a relatively simple and fast technique and can have quantitative approach, we have explored the option of using FTIR for the quantitative determination of comonomer/short chain branching in polyolefin. In this paper, we report methods developed for determination of ethylene content in PP-RCP, ethylene and 1-butene content in PP terpolymer and short chain branching in LLDPE.

Experimental Details
FTIR scans of pressed film specimens (~250 micron thickness) of PP-RCP samples (standard and validation samples), PP terpolymer samples (~100 micron thickness; standard and validation samples) and LLDPE samples (~100 micron thickness), were recorded in absorbance versus wave numbers (cm-1) in Perkin Elmer FTIR model SPECTRUM-100 spectrophotometer in the range of 4500 cm-1 to 650 cm-1. The FTIR method is typically applicable to samples which appear pure and clear, do not show significant interferences from residual catalysts, additives or artifacts.

Results and Discussion
Ethylene content in PP Random Copolymer: A method has been developed for the determination of ethylene content (%C2) in propylene- ethylene semi-crystalline copolymers including random grades over the range 0.5 to 13 wt. % ethylene using FTIR. The spectrum of a PP-RCP show characteristic band in the range of 790 – 660 cm-1, which is due to methylene sequences [-(CH2)- n rocking vibrations, where n = 1,2,…5] and a sharp prominent peak at ~725 cm-1, indicates presence of ethylene comonomer. There is a large band at 4,482 - 3950 cm-1 because of the combined absorption of methyl and methylene groups. Thus the ratio of area of peak at 725 cm-1 to area of absorption bands between 4482 – 3950 cm-1 should be able to predict the ethylene content in the polymer. For demonstration of this, standard samples with ethylene content in the range of 1.5 to 6 wt% have been chosen and empirical correlation was developed.

Figure 3 shows the validity of ratios of ethylene peak to overall other peaks for predicting the ethylene content in RCP. The equation obtained was:

Where, Ac2 is the area under the peak 725 cm-1
At is the total area under peak 4482-3950 cm-1
Et (wt%) is the ethylene content
To assess the reliability of the Eq1, FTIR spectra of other PP-RCP samples were recorded. The spectra were analysed according to the above mentioned method and AC2/ At values were measured and are shown in Table 1. Using the measured AC2/At values for each sample, ethylene contents were determined from the Eq1 and were compared with plant values. It can be seen from the table 1 that ethylene content estimated using developed equation closely represents the plant values measured through established methods indicating applicability of the correlation for random copolymer.

Ethylene and 1-Butene Content in PP Terpolymer: The robustness of the approach can be demonstrated by extending it to terpolymers. The ethylene and 1-butene contents in 1-butene-ethylene-propylene random terpolymers (terpolymer) have been simultaneously predicted over the wide range of concentrations.

The spectrum of terpolymer show characteristic peaks at ~730 cm-1 and at 770 cm-1 which are due to rocking vibrations of methylene sequences of ethylene component and ethyl branches of butylene component respectively. There is large band at 4482 – 3950 cm-1 that is due to the combined absorption of methyl and methylene groups. Similar to the method developed for estimation of ethylene content in random copolymer, empirical correlations have been developed to simultaneously predict the ethylene content and 1-butene in terpolymers. The ethylene content can be obtained from the ratio of area of peak at 730 cm-1 to area of peaks at 4482-3950 cm-1 and 1-butene content can be obtained from the ratio of area of the peak at 770 cm-1 to area of peaks at 4482-3950 cm-1.

The empirical correlations obtained were:

Ac2 is the area under the peak 730 cm-1
Ac4 is the area under the peak 770 cm-1
At is the total area under peak 4482-3950 cm-1
Et (wt%) is the ethylene content
1Bt (wt%) is the 1-butene content

Figure 4 shows the validity of ratio of ethylene peak to overall other peaks for predicting the ethylene content and 1-butene peak to overall peaks for predicting the 1-butene content in terpolymers.

To assess the reliability of Eqs 2 and 3, FTIR spectra of unknown terpolymer samples were recorded. From the spectra, ethylene and 1-butene contents for each sample were predicted using equations 2 and 3 respectively and are shown in Table 2 along with measured values. It can be seen from the Table 2 that developed methods were able to satisfactorily predict the ethylene and 1-butene content.

Comonomer content in Linear Low Density Polyethylene: Further, this approach has been extended to polyethylene such as Low Density Polyethylene (LDPE) and LLDPE. LLDPE can be made with various copolymers such as 1-butene, 1-hexene, 1-octene and 4-methyl- 1 –pentene [4]. The qualitative analysis of short chain branching (SCB) in polyethylene is important for the correlation of molecular structure with physical properties [5].

FTIR spectrum of LDPE/ LLDPE show characteristic peaks at 2850-3000 cm-1 due to C-H stretching of methylene and methyl group, at 1,464 cm-1 due to methylene deformation (– CH scissoring), at ~ 1390 - 1370 cm-1 due to methyl deformation (- CH bending); at 720-725 cm-1 due to methylene rocking and in the region 1000-750 cm-1 due to methyl (long chain) rocking bands [2]. The methyl and methylene rocking bands, which are more characteristic of short chain branch type, were found to be more useful. Methyl, ethyl, butyl, isobutyl, and hexyl branches are qualitatively and quantitatively characterised in LLDPE copolymers by FTIR spectroscopy. Ethyl, butyl and hexyl type SCB in LLDPEs are readily distinguishable [4, 6] by the methyl rocking band positions (886-894 cm-l). The absorption bands at 887 cm-1, 893 cm-1 and 888 cm-l can be used to identify 1-butene, 1-hexene and 1-octene copolymers respectively. An absorbance band at 770 cm-1 has also been used to identify the ethyl branches in 1-butene LLDPE [7].

In the spectrum of LLDPE, we have considered the peak at 1465 cm-1 and peaks in the region 886-894 cm-1, which are due to methylene deformation (–C-H scissoring) and methyl (long chain) rocking bands respectively. The type of comonomer present is identified from the peak position in the range of 894-886 cm-1, ie, 770 cm-1 for 1-butene, 893 cm-1 for 1-hexene and 888 cm-1 for 1-octene.

For quantitative estimation of the comonomer, following measurements were made from the spectrum of standard LLDPE samples and the data on the same is given in Table 3. A correlation between ratio of peak areas at 894-886 cm-1 and 1465 cm-1 and that of comonomer content was found as:

The calculated comonomer content (%) as per equation 4 and the corresponding plant values of LLDPE samples are also given in Table 3 (on previous page). This unequivocally demonstrates the applicability of the method developed for polyolefin. It can be seen from Table 3 that the variation of comonomer percentage between calculated value and that of plant value is less than 5 per cent and transfer function could actually predict various types of comonomers in LLDPE.

Conventional transmission FTIR spectroscopy can be applied to rapidly analyse comonomer contents in polyolefin. Simple transfer functions can be generated to determine ethylene content in PP random copolymer and ethylene and butylene content in PP terpolymer with < 5 % variation. The FTIR technique was also successfully applied to identify and determine the comonomer content (1-hexene, 1-octene) in LLDPE. The methods developed in this work reinforce FTIR as a reliable and convenient technique for determination of qualitative and quantitative estimation of comonomer in polyolefin.

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