Quantification of natural rubber blends by reflection/reflectance infrared and confocal Raman spectroscopy: a comparison of statistical methods

PASSERO, ALAN; FERREIRA, KARYN M.; DINIZ, MILTON F.; SANCHES, NATALIA B.; AMADO, JAVIER CARLOS Q.; DUTRA, RITA DE CÁSSIA L.

doi:10.1590/0001-3765202420230387

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ENGINEERING SCIENCES • An. Acad. Bras. Ciênc. 96 (3) • 2024 • https://doi.org/10.1590/0001-3765202420230387 copy

Quantification of natural rubber blends by reflection/reflectance infrared and confocal Raman spectroscopy: a comparison of statistical methods

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

The blend of butadiene and acrylonitrile copolymer (NBR) with natural poly-cis-isoprene (NR) shows increased resistance to swelling in solvents in comparison to the individual components. In aerospace, NBR rubber is used as thermal protection for rockets and shall not contain other polymers, even in low contents, otherwise, it can affect the protection performance and rocket safety by causing detachment of the elastomer/propellant interface; therefore, this investigation presents methodologies to determine the NR/NBR contents. This study explores different analytical techniques, such as Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy, in the mid-infrared (MIR) by reflection and in the near-infrared by reflectance (NIRA) modes, Furthermore, quantification strategies by univariate, bivariate and multivariate (chemometric) models are evaluated and compared. A proposed methodology, based on multivariate Raman microscopy with partial least squares regression (PLS), showed high linearity (R² > 0.99) and low error (< 0.82 %). The validation of FT-MIR data for the CH₃, which presented lower error (1.3%) than vinylidene band (6%), showed that both methodologies (reflection and NIRA reflectance) can be used for the quantification of NR in NR/NBR. These results constitute a contribution to the state of the art in researching industrial and aerospace elastomeric applications.

Key words
NBR; NIRA; NR; quantitication; spectroscopy; UATR

Methodology	Technique	Processing	Model	Regression	Analytical or relative band/rubber
A	FT-MIR/UATR	Median of spectra	Univariate	Linear	A₂₂₃₇ (*) (NBR)
B	FT-MIR/UATR	Median of spectra	Univariate	Linear	A₁₃₇₆ (*) (NR)
C	FT-MIR/UATR	Median of spectra	Univariate	Linear	A₈₈₇ (*) (NR)
D	FT-MIR/UATR	Median of spectra	Bivariate	Linear	Relative band: A887/A2237
E	FT-MIR/UATR	Median of spectra	Bivariate	Linear	Relative band: A1376/A2237
F	FT-MIR/UATR	Median of spectra	Bivariate	Polynomial	Relative band: A887/A2237
G	FT-MIR/UATR	Median of spectra	Bivariate	Polynomial	Relative band: A1376/A2237
H	FT-MIR/UATR	SNV Normalization	Multivariate	PLS	Multiples
I	Raman (514nm)	Derivative 1^st Order	Multivariate	PLS	Multiples

Methodology	Technique	Model	Regression	Band used	Error (%) in terms of precision
A	FT-MIR/UATR	Univariate	Linear	A₂₂₃₇	6.1
B	FT-MIR/UATR	Univariate	Linear	A₁₃₇₆	1.3
C	FT-MIR/UATR	Univariate	Linear	A₈₈₇	6.0
D	FT-MIR/UATR	Bivariate	Linear	A₈₈₇/A₂₂₃₇	7.8
E	FT-MIR/UATR	Bivariate	Linear	A₁₃₇₆/A₂₂₃₇	6.6
F	FT-MIR/UATR	Bivariate	Polynomial	A₈₈₇/A₂₂₃₇	7.8
G	FT-MIR/UATR	Bivariate	Polynomial	A₁₃₇₆/A₂₂₃₇	6.6
H	FT-MIR/UATR	Multivariate	PLS	Multiples	1.4
I	Raman 514nm	Multivariate	PLS	Multiples	0.8

Methodology	Technique	Model	Regression	Band used	Error (%) in terms of accuracy
A	FT-MIR/UATR	Univariate	Linear	A₂₂₃₇	8,5
B	FT-MIR/UATR	Univariate	Linear	A₁₃₇₆	8.2
C	FT-MIR/UATR	Univariate	Linear	A₈₈₇	15.7
D	FT-MIR/UATR	Bivariate	Linear	A₈₈₇/A₂₂₃₇	4.7
E	FT-MIR/UATR	Bivariate	Linear	A₁₃₇₆/A₂₂₃₇	9.6
F	FT-MIR/UATR	Bivariate	Polynomial	A₈₈₇/A₂₂₃₇	5.3
G	FT-MIR/UATR	Bivariate	Polynomial	A₁₃₇₆/A₂₂₃₇	5.7
H	FT-MIR/UATR	Multivariate	PLS	Multiples	1.9
I	Raman 514nm	Multivariate	PLS	Multiples	1.6

Met.	Technique	Model	Regression	Band used	Precision error (%)	Accuracy error (%)	Linearity (R²)	General Evaluation
A	FT-MIR	Univariate	Linear	A₂₂₃₇	6.1 (A)	8.5 (A)	0.95 (A)	Adequate, with restrictions
B	FT-MIR	Univariate	Linear	A₁₃₇₆	1.3 (E)	8.2 (A)	0.96 (E)	Adequate, with restrictions
C	FT-MIR	Univariate	Linear	A₈₈₇	6.0 (A)	15.7 (I)	0.13 (I)	Inadequate
D	FT-MIR	Bivariate	Linear	A₈₈₇/A₂₂₃₇	7.8 (A)	4.7 (A)	0.91 (A)	Adequate, with restrictions
E	FT-MIR	Bivariate	Linear	A₁₃₇₆/A₂₂₃₇	6.6 (A)	9.6 (I)	0.80 (I)	Inadequate
F	FT-MIR	Bivariate	Polynomial	A₈₈₇/A₂₂₃₇	7.8 (A)	5.3 (A)	0.99 (E)	Adequate, with restrictions
G	FT-MIR	Bivariate	Polynomial	A₁₃₇₆/A₂₂₃₇	6.6 (A)	5.7 (A)	0.98 (E)	Adequate, with restrictions
H	FT-MIR	Multivariate	PLS	Multiples	1.4 (E)	1.9 (E)	0.99 (E)	Excellent
I	Raman (514nm)	Multivariate	PLS	Multiples	0.8 (E)	1.6 (E)	0.99 (E)	Excellent

Sample NR/NBR (NR %)	ANALYTICAL BAND A₄₀₆₃ (NR)	MEDIAN A₄₀₆₃	STANDARD DEVIATION	AVERAGE STANDARD DEVIATION	RELATIVE DEVIATION (RD) (%)	METHODOLOGY ERROR (%)
10	0.060	0.060	0.006	0.003	5.00	1.95
	0.054
	0.057
	0.069
	0.067
20	0.067	0.067	0.002	0.001	1.49
	0.067
	0.073
	0.072
	0.067
30	0.075	0.079	0.003	0.001	1.26
	0.079
	0.082
	0.081
	0.076
40	0.096	0.096	0.006	0.003	3.12
	0.101
	0.088
	0.088
	0.103
50	0.104	0.107	0.004	0.002	1.87
	0.109
	0.099
	0.109
	0.107
60	0.123	0.121	0.008	0.004	3.30
	0.134
	0.121
	0.116
	0.120
70	0.143	0,138	0,004	0,002	1,45
	0.135
	0.138
	0.138
	0.134
80	0.133	0.146	0.007	0.003	2.05
	0.150
	0.146
	0.143
	0.150
90	0.155	0.154	0.006	0.003	1.95
	0.154
	0.162
	0.153
	0.149

Sample NR/NBR (% NR)	A₁₃₇₆ (median values) Analytical Band MIR/UATR	A₄₀₆₃(median values) Analytical Band NIR/NIRA
90	8.890	0.154
80	7.800	0.146
70	8.170	0.138
60	7.130	0.121
50	6.930	0.107
40	6.100	0.096
30	6.020	0.079
20	5.430	0.067
10	5.050	0.060

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[1] Correspondence to: Rita de Cássia Lazzarini Dutra E-mail: ritacld@ita.br