A review of recent infrared spectroscopy research for paper

Abstract

For many centuries, paper is the main material for recording cultural achievements all over the world. Infrared (IR) spectroscopy is an essential analytical tool for the structural analysis of paper and pulp chemistry. This review article introduced recent technical and scientific reports in terms of IR spectroscopy in the paper science and application, where interest had increased during the last couple decades (2000–2022). Five parts were described according to the application of IR spectroscopy: the analysis of cellulose and its derivatives, estimation the date of documents, identification the origins of fiber or paper, specification the chemical and physical properties of paper, and characterization the new materials of paper.

Keywords:

• Mid-infrared spectroscopy
• near-infrared spectroscopy
• paper

1. Introduction

Papermaking is considered to have originated in China.^[1] Since its invention, paper has covered almost every aspect of people’s life, including books, historical calligraphy, banknotes, invoices, suicide notes, tissues, and more. Because of the universality of its applications, paper as a carrier contains huge amount of information. Thus, paper is used as a key research object in archaeological traceability,^[2] cultural dissemination,^[3] forensic identification^[4] and other fields.

At first, paper is produced from tree bark and plant fibers, as well as from old rags and fishing nets. And then the paper is made from cellulose and hemicellulose from wood.^[5] Today, in order to satisfy the paper demands (for instance: whiteness, brightness, hardness, thickness, etc.) of various industries,^[6] in the process of papermaking, chemical compounds are involved, like sodium sulfide (Na₂S) or sodium hydroxide (NaOH), chlorine oxide (Cl₂O), calcium carbonate (CaCO₃), kaolinite (Al₂ (Si₂O₅) (OH)₄), ozone or oxygen along with metallic oxides, etc.^[7] This leads to the complexity of paper product research, which not only needs to consider the impact of the external environment,^[8] but also involves the internal chemical composition of paper products.^[9]

Infrared (IR) spectroscopy, has rapid, accurate, and nondestructive characteristics,^[10,11] not only shows great potential in the quality evaluation of agricultural products, pharmaceutical, soil, petrochemical, and the material assessment, but also presents major importance in the analysis of paper.^[1,12] Some researchers have reviewed IR spectroscopy research concerning paper products. A broad range of application from the 1980 to 2001 into pulp and paper materials, components and processed by infrared and Raman spectroscopy has been reviewed.^[13] Near-IR (NIR) applications for wood or paper science and industry were introduced, most of which have been published in latter half of the 1990s and the early 2000s.^[14] However, there is a lack of review research on IR in paper since 2000s.

The aim of the present review is to point out and focus on the paper researches regarding the use of IR spectroscopy. Five parts are presented in this survey: the analysis of cellulose and its derivatives, estimation the date of documents, identification the origins of fiber or paper, specification the chemical and physical properties of paper, and characterization the new materials of paper.

2. Content

2.1. The analysis of cellulose and its derivatives

Paper is a multi-component material, is mainly composed of cellulose, hemicellulose, and lignin, but can also include additives, such as starch, minerals, and synthetic polymers. Deterioration of paper is also caused by multiple factors: heat, light, acid hydrolysis, oxidative agent, even the presence of micro-organisms.^[15] IR spectroscopy plays an important role in observing the chemical changes occurring within the paper deterioration processes. Most of the researches focused on the mid-IR (MIR),^[16–24] only few of them use NIR.^[25] The related studies were summarized in Table 1.

Table 1. Studies on the degradation or aging of paper by IR spectroscopy.

Sample	Technique	Goal	Relevant information	Refs.
White office paper	IR and Raman	Analyzed the degraded papers and evaluated the differentiation	Carbonyl and carboxyl were formed; 2 D correlation was a useful to show the differences of peaks; the changes of cellulose happened before fillers.	^[^16^]
Paper	FT-IR, FT-Raman	How the presence of lignin polymer influences degradation process for pure cellulose	Degradation was monitored by C = O; some changes in C = C; at least three new carbonyl species were formed; rearrangement of hydrogen bond network.	^[^17^]
Transformer paper	FT-IR	Identified changes in the chemical structure of transformer paper aged in mineral oil.	The intensity of the O-H decreased but the C-H and C = O increased with aging. Built the correlation lines to predict the degree of aging.	^[^18^]
Antique washi paper	FT-NIR	The diffusion process of deuterated water in washi paper.	Four absorption bands were identified in the NIR spectral range from 7200 to 6000 cm^−1	^[^25^]
Moroccan paper	ATR-FTIR, XRD, EDXRF	Assessed the aging process after the aqueous de-acidification treatment	FT-IR identified the major modifications, including the hydrogen bond; the residual water; the decreased and the crystallinity of cellulose.	^[^20^]
Moroccan paper	ATR-FTIR, XRD, SEM-EDS	Evaluated the conservation restoration process with the alkaline treatment	The de-acidification procedure proved by the rise of the alkaline reserve content allowing the preservation of paper.	^[^19^]
Newsprint paper	FT-IR DRIFT	Monitored the processes after the application of Bookkeeper.	The changes in O–H at 3700 cm^−1; multiple coordination of OH occur; the higher consumption of carbonyl compounds at higher humidity.	^[^21^]
Kraft paper	FT-IR	Proposed the interrelated mechanisms to explain the increase in paper insulation life.	A slower aging rate for cellulose insulation in natural ester dielectric fluid.	^[^22^]
Foxing stains on papers	IR, XRF, LIBS	The influence of the iron valence and paper aging in the formation of stains	The foxing was related to oxidation of the cellulose chain; carbonyl content was related with the aging.	^[^23^]

Energy dispersive X-ray fluorescence spectrometry (EDXRF); Scanning electron microscopy energy dispersive spectrometry (SEM–EDS); FTIR sample techniques—diffuse reflectance (DRIFT); Laser induced breakdown spectroscopy (LIBS).

During the aging process of paper, Figure 1 displayed the Fourier transform-IR (FT-IR) spectra of hydro-thermally degradation (humidity 100%, temperature 100 °C). The reaction was monitored mainly based on C = O stretching vibrations formed during the process of carbonyl compounds (1665 and 1739 cm⁻¹); some changes in C = C stretching vibrations were also noticed at 1694 cm⁻¹; as well as re-arrangement of existing hydrogen bond network (at 1640 cm⁻¹).^[17] Besides, crystallinity might be expected to change and, as a result of oxidation, carbonyl and carboxyl groups were formed, which were visible in the MIR spectra. Pointed that the changes in the structure of cellulose that took place first followed by the modifications in fillers.^[16] Transformer paper aged in mineral oil was performed. FT-IR results showed that the peak absorption intensity of the O-H decreased but the C-H and C = O increased with aging.^[18] O-H groups in the amorphous, semi-crystalline and two types of crystalline regions of cellulose were identified in the NIR spectral range from 7200 to 6000 cm⁻¹ by using a deuterium exchange method.^[25]

Figure 1. FT-IR spectra of hydrothermally degradation (humidity 100%, temperature = 100 °C). Sample: (a) starting sample; (b) after 2 days; (c) after 8 days; (d) after 10 days; (e) after 12 days; (f) after 15 days. Figure modified from^[17] with permissions of Elsevier.

Paper manuscript is subjected to numerous degradation factors affecting their conservation state. Some researches represented an attempt to evaluate the conservation restoration process after several treatments, such as: the alkaline, the aqueous de-acidification, the application of bookkeeper reagents, and the microorganism.

Four restored Moroccan manuscript papers were employed to assess the aging process in the aqueous de-acidification treatment. The alteration of hydrogen bond network (3000–3600 cm⁻¹); the desorption of paper residual water (1641–1647 cm⁻¹); the significantly decreased of the crystallinity of cellulose (1425, 1370, 900 cm⁻¹), indicating the effectiveness of de-acidification procedure proved by the rise of the alkaline reserve content allowing the long-term preservation of paper.^[19,20] The application of Bookkeeper reagents caused significant changes mostly in the region of 3700 cm⁻¹, assigned to the coordination of stretching vibrations of OH groups coordinated with Mg²⁺. With the relative humidity and the rise of temperature, accelerating the transformation of carbonyls to carboxylates from 1740–1720 and 1630–1650 cm⁻¹.^[21] Compared with the conventional transformer oil/kraft paper system, the natural ester fluid had greater affinity for water, could modify the cellulose structure by transesterification.^[22] Besides, while exploring the influence of oxidation and paper aging in the formation of paper stains, it was stated that the foxing phenomenon was related to a strong oxidation of the cellulose chain. Carbonyl content varied between samples treated with iron (III) and (II) ions, especially in the relation with the kind of aging.^[23]

From the above researches, the process of paper degradation has been modeled as a combination of cellulose acidic hydrolysis, oxidation, and the presence of micro-organisms.^[24] During the aging process, it was found through the analysis of IR spectroscopy: the significantly decreased of the crystallinity of cellulose,^[16,20,26] the hydrogen bond has changed,^{[17,18,20,26]} C = O stretching vibrations formed during the process of carbonyl compounds.^[16–18] Especially, pointed that the changes in the structure of cellulose that took place first followed by the modifications in fillers.^[16]

2.2. Estimation the date of documents

Document dating is still a major challenge in many examination fields. There are some researches in the exploration of the document dating (Table 2). At first, simulated the aging process in the form of artificial aging, by building the model between artificial aging time and spectra, and then predicted the artificial time. For instance, FT-IR and NIR have been used to characterize the aging of cellulose paper, gave a correlation of 0.99 between the spectra and aging time, with an error of prediction of 95 h for samples up to 3000 h of aging.^[27] The correlation lines between the functional groups (3329, 2922, 2854, and 1745 cm⁻¹) and the average number of chain scissions of transformer paper were built to predict the degree of aging.^[18]

Table 2. Studies on the estimation of the paper date by IR techniques.

Sample	Technique	Goal	Relevant information	Refs.
Cellulosic paper	FT-IR and NIR	Assess the condition of paper insulation	Activation energy was 98 kJ/mol (a carbonyl band); model with an error of 95 h.	^[^27^]
Chinese calligraphies and paintings	FT-IR, XRD	The chemical characteristics of rice paper	FT-IR differentiated the era of rice paper.	^[^26^]
Reports	FT-IR	Analyze the complexity of document dating problems and sample variability	Although the inorganic compounds were influencing the models, RMSEP was around 4 years.	^[^28^,^29^]
Publications	FT-IR	Estimate the date of paper	CNN gave an accuracy of 98.77%; the important variables focused on the 1700–1400 cm^−1	^[^31^]
Publications	FT-IR	Estimate the date of paper	The accuracy of LS-SVM was 99.26%; the selected variables focused on the peaks of inorganic components and cellulose.	^[^30^]

Recently, with the development of instruments and computers, more and more information could be obtained from IR spectra and algorithms. Some researchers have established the relationship between the natural aging time and the IR spectra. The chemical characteristics of rice paper, pigments, and seals on Chinese calligraphies and paintings were studied. This could be the base of estimating the age of rice paper according to the FT-IR differentiated.^[26] FT-IR was employed to estimate the date of documents from different years (Figure 2). Partial least squares regression (PLSR) models were built by employing generalized least squares weighting (GLSW) and orthogonal least squares filters (OSC). Although the inorganic compounds were still influencing the models, acceptable values for root mean square error of prediction (RMSEP) were around 4 years.^[28,29] Similar conclusion was acquired in our previous study, FT-IR spectra of journals dated from 1940 to 1980 were collected, and the recognition accuracy was 99.26%. The selected variables were mostly focused on cellulose and inorganic components.^[30] In order to improve the interpretability of the model, convolutional neural network (CNN) was employed, with the accuracy 98.77%. The active variables from CNN model was concentrated on the 1700–1400 cm⁻¹, corresponding to the cellulose crystallinity, which was consisted with the aging processing by other techniques.^[31]

Figure 2. The chemometric approaches for document dating. Figure taken from^[28] with permissions of Elsevier.

The above studies demonstrated the potential of IR spectroscopy and chemometrics to assess the date of documents. This is extremely important because the method can estimate the date of the documents even doesn’t know the chemical compositions of paper. It can provide the prospect of implementing advanced analytical methodologies in scientific police laboratories.

2.3. Specification the chemical or physical properties of paper

In actual production, the performance of paper, which is determined by its chemical and physical properties, directly affects the quality of print products. Therefore, it is essential to measure the chemical or physical properties of paper. Related researches were summed up in Table 3.

Table 3. Studies on the chemical or physical properties of paper by IR techniques.

Sample	Technique	Goal	Relevant information	Refs.
Japanese papers	FT-IR	Classified the paper	The deconvolution of FT-IR spectra enabled the recognition of hemicelluloses, chemical pulps, inorganic fillers, and lignin.	^[^32^]
Coniferous pulp paper	FT-NIR	Monitored kinetics and estimated the biodegradation rate of coniferous pulp paper.	Starch and resin adhesives improved mechanical properties; the R^2 between spectra and breaking length was 0.75, and the RMSEP was 0.78 km.	^[^33^]
Insulating paper	NIR	Determined the DP	NIR combined with the PLSR to determine the DP, and the RMSEP was 83.	^[^34^]
Wood and paper	NIR	Analyzed the molecular deformation	It might be possible to predict stress levels in cellulose materials nondestructively using NIR.	^[^35^]
Paper	NIR, MIR	Determined the paper properties	Satisfactory reliability was achieved for all properties but aluminum content.	^[^36^]
Rice straw pulps	NIR	Determined the key properties of rice straw pulp.	The correlation was strong for four of them (breaking length, stretch, tear index and brightness), addition of cationic.	^[^37^]
Magnetic paper	ATR-FTIR, FTIR-PAS	Determined the loading degree of the obtained magnetic papers.	ATR-FTIR(R^2=0.99, RMSEP = 1.22) was more accurate in predicting the loading degree than FTIR-PAS (R^2=0.91, RMSEP = 2.83).	^[^38^]
Paper	IR	Studied the integrity of IR based online moisture measurement systems.	The observed measurement error was 2.9% at the 95% confidence level.	^[^39^]
Bacterial cellulose and paper	FT-IR	Studied the disperse bacterial cellulose wet membranes; test of the physical properties of the sheet.	With the increasing of bacterial fibers, the tensile index, tear index, burst index, and stiffness improve, while the porosity and the water absorption decreased.	^[^40^]

About the chemical properties, the deconvolution of FT-IR spectra from old and modern Japanese papers enabled the recognition of hemicelluloses, chemical pulps, inorganic fillers and lignin.^[32] Otherwise, the addition of cationic starch and resin adhesives improved the mechanical properties of paper. FT-NIR spectra were most varied in bands of 4283, 4400, 4742, and 4808 cm⁻¹, which corresponded to C-H, and O-H functional groups of cellulose and hemicellulose.^[33] For the degree of polymerization (DP), NIR combined with PLSR to predict the DP of the paper, and the RMSEP was 83.^[34] And for the molecular deformation of wood and paper. Peak positions of both bands at 6286 ± 5 cm⁻¹ and 6470 ± 10 cm⁻¹ were linearly correlated with strain levels, suggested that it might be possible to predict stress levels in cellulose materials nondestructively by using NIR.^[35] With the use of a considerable sample set, combined with PLSR algorithm (Figure 3), the coefficient of determination (R²) of ash content, lignin content, DP of cellulose, and pH were all above 0.96, only the R² of aluminum was about 0.87.^[36]

Figure 3. PLS calibrations for estimation of ash content, lignin content, pH, DP, and aluminum content of historical paper. Reprinted (adapted) with permission from^[36]. Copyright (2007) American Chemical Society.

As for the physical properties, the potential of NIR to determine the key properties of rice straw pulp has been evaluated. Out of the five paper properties considered, the correlation was strong for four of them (breaking length, stretch, tear index and brightness), with the R² of cross-validation close to 1.00.^[37] It was discovered that the elevated loading degree increased the IR absorption, and reduced the tensile strength of the paper. The PLS analysis showed the ATR-FTIR data were strongly correlated with the degree of loading (R² = 0.99, RMSEP = 1.22).^[38] For moisture, the observed measurement error was 2.90% in the first-time calibrations at the 95% confidence level. And error could be reduced to under 0.50% by applying traceable methods in the basic calibration.^[39] For the study of bacterial cellulose wet membranes and physical properties by IR and thermal analysis, with the increasing dosage of bacterial fibers, the properties of tensile index, tear index, burst index, and stiffness greatly improved, while the porosity and the relative water absorption decreased. Expected that bacterial cellulose may partly replace plant fiber in the pulp and paper industry, as a green energy-saving material.^[40]

2.4. Identification the origin of fiber or paper

The possibility to discriminate the paper can be of considerable importance in questioned document examinations. The analysis of paper can help to connect or relate different documents. Document frauds can be carried out substituting a page of an agreement without the consent of all parties.^[41] In this instance, identification of the origin of the fiber or paper would be a strong point to support or exclude forgery. Related studies were summarized in Table 4.

Table 4. Studies on the identification of fibers or papers by IR techniques.

Sample	Technique	Goal	Relevant information	Refs.
Office paper	FT-IR, XRD	Discriminate paper	19 similar types of office paper could be differentiated.	^[^41^]
Paper relic	FTIR	Discriminate the paper relics	PLS-LDA and LS-SVM were effective techniques with 100% classification accuracy.	^[^42^]
Office paper	ATR-FTIR	The application of ATR-FTIR for the discrimination of paper	The discrimination of chemometrics analysis (99.64%) was better than qualitative features (97.83%).	^[^43^]
Office paper	UV-VIS-NIR	Differentiated 20 office paper	Based on the difference of spectral peaks, all the possible sample pairs could be discriminated.	^[^44^]
Printer paper	Pro-NIR	Identified paper	Eight kinds of papers were discriminant.	^[^45^]
Document paper	MIR, NIR	Determined the spectral region and data pre-processing with discriminating ability	MIR had higher discriminating ability than the NIR. The first derivative and two ratios produced the best classifications.	^[^46^]
Paper finishes	FT-IR, NIR	Affording the accurate classification and identification of paper finishes.	Paper samples from various manufactures afforded nearly 100% success. And could classify the coated samples into three subgroups.	^[^47^]
Euro banknote	FT-IR	Establishing the authenticity of Euro banknotes	FT-IR was suitable for the identification of counterfeits.	^[^48^]
Banknote	FT-IR	Identified the forged banknotes	The method will work equally well for the other denominations.	^[^49^]
Counterfeit banknotes	Multispectral images (400–1000 nm)	Developed an algorithm to detect counterfeit banknotes.	Provided 99.8% accuracy for genuine and 100% for counterfeit banknotes.	^[^50^]
Euro banknotes	NIR camera	Detected counterfeits of Euro banknotes.	Allowed to recognize the value and forgery of banknotes.	^[^53^]
Banknotes	Pro-NIR	Authenticity assessment of banknotes.	Pro-NIR with SPA-LDA was effective to identify the authenticity of banknotes.	^[^51^]
Banknotes	VSC, ATR-FTIR	Discrimination the counterfeit banknotes	The PCA and PLS-DA discriminated the genuine and counterfeit banknotes successfully, the counterfeit samples failed to discriminate into separate groups.	^[^52^]
Historical banknotes	HIS, FT-IR, XRD	Detected the material differences in historical banknotes.	Making important observations for materials differences.	^[^54^]

Hyperspectral imaging (HSI).

19 similar types of office paper were characterized by FT-IR to individuate the most discriminating features that could be measured by these techniques, all the samples could be differentiated.^[41] In the application of FT-IR technique in the discrimination of 15 types of paper, PLS-LDA and LS-SVM were effective with 100% classification accuracy.^[42] 24 types of office paper brands were subjected to ATR-FTIR from 400–4000 cm⁻¹, results displayed that the accuracy (99.64%) was better achieved by principal component analysis (PCA) rather than matching spectra peaks (97.83%).^[43] 20 samples were divided into many sample pairs, and all the possible samples pairs could be discriminated according to the difference of ultraviolet–visible-NIR (UV-VIS-NIR) spectral peaks.^[44] A portable NIR could categorize the eight types of printer paper.^[45] Six document papers were classified by the MIR (2500–4000 cm⁻¹) and NIR (4000–9000 cm⁻¹). It was found that MIR had higher discriminative ability than the NIR.^[46] For the identification of paper finishes, the k-nearest neighbors (kNN) method provided a nearly 100% accuracy for paper samples with different finishes, even could classify the coated samples into three subgroups.^[47]

Except for forged documents, the number of counterfeit banknotes in the world is increasing every year. It is much easier to forge banknote with a similar visual characteristic as the original than to produce a banknote with the identical chemical composition as the original. Therefore, it is necessary to do chemical analysis for distinguishing the genuine and counterfeit banknote. Wherein, FT-IR spectroscopy is a great solution to confirm the chemical components on banknote, and then quickly identify the counterfeit banknotes.

FT-IR spectra of counterfeit banknote showed almost identical peak positions regardless of the part of the banknote, while the peaks of genuine banknote pointed to different processes in the original production indicating several printing processes and different materials being applied (Figure 4). CaCO₃ commonly used as a filler in the papermaking industry which was not used in the production of banknote paper. Therefore, the absorption peaks of CaCO₃ (2900, 1500–1250 and 875 cm⁻¹, circled by the blue ellipses in Figure 5) could be an indicator to confirm the counterfeit banknote.^[48] Otherwise, the differences between the spectra of forged and genuine notes were observed in the O-H (3500 cm⁻¹), C-H (2900 cm⁻¹) and C = O (1750 cm⁻¹) regions of the MIR spectra recorded for the polymer film covering the holographic strip.^[49] By using low resolution multi-spectral images (400–1000 nm), provided 99.80% classification accuracy for genuine banknotes and 100% detection accuracy for counterfeit banknotes.^[50] For authentic banknotes and counterfeits, successive projections algorithm (SPA) combined with linear discriminant analysis (LDA) could correctly classify into their respective class by pro-NIR.^[51] Analyzed the genuine and confiscated counterfeit Malaysian banknotes, ATR-FTIR showed marked differences in the spectra, particularly in the region of 1800–650 cm⁻¹. PCA and PLS-discriminant analysis (PLS-DA) also could discriminate successfully.^[52] Both hardware and software components were described to detect counterfeit and the value of Euro banknotes. The effectiveness of the proposed solution has been properly tested on a dataset composed by genuine and fake Euro banknotes provided by Italy’s central bank.^[53] A research approach was presented for detecting material differences in historical banknotes. The changes experienced by the paper substrates during micro fade testing also provided a way for discriminating between two groups of banknotes.^[54]

Figure 4. FT-IR spectra of 200 Euro banknote (back side). (a) The spectra of a counterfeit banknote; (b) the spectra of a genuine banknote. Modified by the figure in^[48].

Figure 5. FT-IR spectra of paper substrate of a counterfeit (red line), and genuine (black line) 200 Euro banknote. Modified by the figure in^[48].

2.5. Characterization the related materials of paper

Chemical structural fragments of molecules tend to absorb IR radiation in the same frequency range regardless of the structure of the rest of the molecule that the functional group is in. This correlation between the structure of a molecule and the frequencies at which it absorbs IR radiation allows the structure of unknown molecules to be identifies and structural or chemical changes of the molecule to be followed.^[55] That’s why IR is particularly suitable for the characterization of paper materials. Studies on the characterization of related paper materials by IR techniques were compiled in Table 5.

Table 5. Studies on the characterization of related paper materials by IR techniques.

Sample	Technique	Goal	Relevant information	Refs.
Coating filter paper	MIR and NIR	Developed the novel coating method.	Coating filter papers had high hydrophobicity.	^[^1^]
Kraft pulp	FT-IR, XRD	Investigated the effects of reaction conditions on the content of aldehyde groups and yield loss of the oxidized CNC.	The use of oxidized CNC improved the wet and dry strength of paper.	^[^56^]
Thin cellulose films	FT-IR	The adhesion between cellulose and hemicellulose surfaces.	The model system enabled the evaluation of different bonding mechanisms of pulp and paper.	^[^57^]
Cellulose paper	FT-IR, XRD	Prepared Ag NPs and surface-coated onto the cellulose papers.	The coated paper exhibited a high effectiveness of the antibacterial activity against E. coli or S. aureus.	^[^58^]
Filter paper	FE-SEM, FT-IR, XRD	Fabricated cellulose-based filter paper composite scaffolds.	Scaffolds showed good antibacterial properties against S. Aureus and E. Coli.	^[^59^]
Photocatalytic paper	IR, XRD	Improved the performance of the photocatalytic paper.	The photocatalytic activity of the new paper improved; and was rather stable under UV light.	^[^60^]
Insulation paper	FT-IR, XRD, SEM	Further improved anti-aging performance of transformer insulation paper.	The mechanical and electrical properties of the modified insulation paper were better.	^[^61^]

The paper, covered by new produced coatings which were characterized by using NIR and MIR, had high hydrophobicity.^[1] FT-IR was employed to confirm the generation of aldehyde groups. The oxidized cellulose nanocrystal (CNC) was used as a strength additive to paper, and the results indicated that both the dry and wet tensile index were improved with the oxidized CNC.^[56] The chemistry and the thickness of the cellulose films were analyzed by FT-IR. Results displayed that the model system enabled the evaluation of different bonding mechanisms discussed in pulp and paper research.^[57] FT-IR was utilized to confirm whether the silver nano-particles (Ag NPs) were immobilized onto the CNC. Experimental results demonstrated that the coated paper made from the Ag NPs-loaded CNC nano-composites exhibited a high effectiveness of the antibacterial activity.^[58,59] The poly-dopamine-loaded cellulose fiber (PLCF) was used as raw material to make photocatalytic paper. The loading of PLCF was verified by IR. As observed, it was confirmed that the occurrence of PLCF significantly improved the photocatalytic performance of paper.^[60] The new insulation paper was prepared by the modified CNC. The tensile index, breakdown strength and DP of insulation paper containing modified CNC was increased than the reference one.^[61]

2.6. Other related research

In addition to the researches described above, there are some other related studies (Table 6), including the reuse of waste paper,^[62,63] the visualization of adhesives and varnish on the printed paper,^[64] and research on the penetration of the sizing chemicals in paper.^[65]

Table 6. Studies on the other paper-related researches by IR techniques.

Sample	Technique	Goal	Conclusion	Refs.
Waste paper	FT-IR, XRD, SEM	Synthesized water-soluble and organic-soluble CEC from waste paper.	Waste paper have the capacity to produce upgraded and high quality cyanoethyl cellulose for applications.	^[^62^]
Office waste paper	FT-IR, XRD	Isolated CNC from office waste paper	Ink and calcite were almost totally removed after NaOH treatments; the crystallinity index value increased.	^[^63^]
Printed paper	NIR-HIS	Visualized the synthetic material on the surface of paper	The distribution of the synthetic material on printed paper could be visualized by NIR-HIS.	^[^64^]
Paper	ATR-FTIR	The penetration of the sizing chemicals into the paper structure.	The presence of the copolymers on the paper top surfaces, despite the only small amount of chemicals.	^[^65^]

Water-soluble and organic-soluble cyanoethyl cellulose (CEC) could synthesize from waste paper, which provided an alternative to paper recycling.^[62] CNC was isolated from office waste paper using an alkali solution and a subsequent acid hydrolysis process. FT-IR and XRD results illustrated that ink and calcite were almost totally removed after alkali treatments. The crystallinity index value increased with respect to initial office waste paper.^[63] Compared with the conventional methods, NIR-HIS at wavelength of 1.0–2.35 um was proposed as a new method to visualize the adhesives and varnish on the printed paper.^[64] A study on the penetration of the sizing chemicals in paper was carried out by using FT-IR. It was possible to detect the presence of the copolymers on the paper top surfaces, despite the application of only small amounts of these chemicals in the surface sizing.^[65]

3. Summary and outlook

This review introduced the application of IR techniques in paper, mainly mentioned five aspects according to the different fields of application, and finally gave a deeper perspective on the trends of the development on the IR spectra in the application of paper. We hope that this paper will help researchers develop new efficient methods to address the exiting problems of paper, which could significantly promote the development of IR techniques.

Disclosure statement

The authors declare no conflict of interest.

Additional information

Funding

FundingThis work was supported by Key research and development program in Xinjiang Uygur Autonomous Region (2022B03002-2).