Quality analysis and control strategies for epoxy resin and prepreg

Trends in Analytical Chemistry 74 (2015) 68–78

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Trends in Analytical Chemistry j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t r a c

Quality analysis and control strategies for epoxy resin and prepreg B. Jiang a,*, Y.D. Huang a, S. He a, L.X. Xing a,b, H.L. Wang a a Polymer Materials and Engineering Department, School of Chemical Engineering and Technology, Harbin Institute of Technology, P.O.Box:1254, Harbin 150001, China b Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA

A R T I C L E

I N F O

Keywords: Analysis Differential scanning calorimetry Epoxy resin Micro-computed tomography Microstructure control Near-infrared spectroscopy On-line analysis Polymer composite Prepreg Quality control

A B S T R A C T

Epoxy resin and prepreg have been used to fabricate high-performance materials for application in industries, such as energy, aircraft, automobile, and sports equipment, which require controlled epoxyresin matrix and prepreg quality during preparation. Near-infrared (NIR) spectroscopy, microcomputed tomography (Micro-CT), β-ray technology and differential scanning calorimetry are used for analysis of prepreg quality. We review the advantages and the drawbacks of various analytical strategies for quality and on-line control of prepreg. In this review, we present an overview of the NIR spectroscopy and Micro-CT methods, which are new methods for the on-line analysis and microstructure control of prepreg. These analytical methods can direct the quality control of epoxy resin and prepreg. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4. 5. 6.

Introduction ........................................................................................................................................................................................................................................................... 68 The curing process and epoxy resin/prepreg-quality analysis using NIR spectra ......................................................................................................................... 69 2.1. Analysis of the theoretical basis behind the NIR spectrum of epoxy resin/prepreg ...................................................................................................... 69 2.2. Dynamics analysis of the epoxy-resin cure reaction via NIR spectroscopy ...................................................................................................................... 69 2.2.1. Study of the cure reaction of epoxy resin (bisphenol A) using an amine by NIR spectroscopy ............................................................... 69 2.2.2. Study of epoxy-resin cure reaction with diaminodiphenylsulfone (DDS), acrylates and other curing agents via NIR spectroscopy ........................................................................................................................................................................................................................... 71 2.3. Study of the on-line control of epoxy resin/prepreg using NIR spectroscopy ................................................................................................................. 71 Microstructural characterization of prepreg using Micro-CT ............................................................................................................................................................... 73 3.1. Microstructural features of prepreg based on a reinforced glass cloth .............................................................................................................................. 74 3.2. Microstructural features of prepreg based on carbon-cloth reinforcement ..................................................................................................................... 76 Analysis of resin in prepreg using β-ray technology ............................................................................................................................................................................... 76 DSC for studying the degree of cure of prepreg ....................................................................................................................................................................................... 76 Conclusions ............................................................................................................................................................................................................................................................ 76 Acknowledgments ............................................................................................................................................................................................................................................... 77 Appendix: Supplementary material ............................................................................................................................................................................................................ 77 References .............................................................................................................................................................................................................................................................. 77

1. Introduction Epoxy-resin composites are widely used for daily life, such as kitchenware, and cutting-edge technologies, such as the aerospace industry, because of their excellent mechanical and anticorrosive properties. The influence of the composite quality on the

* Corresponding author. Tel.: +86 451 8641 4806; fax: +86 451 8641 8270. E-mail address: [email protected] (B. Jiang). http://dx.doi.org/10.1016/j.trac.2015.03.028 0165-9936/© 2015 Elsevier B.V. All rights reserved.

resin properties during preparation justifies studying the on-line control of the production of epoxy-resin composites. Different curing times and temperatures also greatly impact the mechanical properties of the polymer [1]. The cure rate and mechanism for epoxy composites have been widely researched [2–4], and this exhaustive knowledge of both characteristics can significantly facilitate controlling the reaction to obtain a resin-matrix composite with the required property. Manipulating the reaction process enables more efficient raw material and resource usage, reduces unnecessary energy consumption and yields products with superior chemical and physical properties.

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Prepreg is a semi-finished product during composite-material manufacture and is also known as a high-performance material for thermostructural and mechanical applications [5–7]. Prepreg quality has an important effect on composite properties. Some analytical technologies, such as spectroscopy [8], gamma-rays [9], ultrasonics [10], dynamic mechanical analysis [11], and differential scanning calorimetry (DSC) [12,13], have been used to analyze prepreg quality. In this review, we focus on analyses of the quality for epoxy resin/ prepreg using these methods. Near-infrared (NIR) spectroscopy and micro-computed tomography (Micro-CT) are especially promising candidates. 2. The curing process and epoxy resin/prepreg-quality analysis using NIR spectra NIR spectroscopy is a rapid, accurate and non-destructive method for simultaneously measuring different constituents in various products. The rapid growth of computer technologies has facilitated digitization and chemometrics of analytical instruments because chemometrics can extract spectral information from background interference when coupled to the particular characteristics of NIR testing techniques. Currently, NIR spectroscopy is used in polymer manufacture [14–16]. 2.1. Analysis of the theoretical basis behind the NIR spectrum of epoxy resin/prepreg NIR absorption caused by molecular overtones or molecular combination vibrations for C-H, O-H, N-H and C-O bands are characteristic of polymers in the NIR spectra and could be used to determine polymer composition. In addition, the NIR spectrum contains information related to polymer properties, such as the conformation and crystallinity; therefore, it can be used for various polymer analyses. Epoxy resin composites contain hydrogen-containing, epoxy, carbonyl and benzene groups, which can all be analyzed by NIR spectroscopy. The mode of action for the NIR light on the samples is shown in Fig. 1. NIR diffuse-reflection light is focused on the surfaces of samples, where light undergoes much reflection in the samples and returns to the surface. NIR absorption is greatly influenced by the components and molecular structure of the epoxy resin composite (prepreg), denoted Cj in the following equation:

m

C j = b0 + b1 A1 + b2 A2 +  + bn An or C j = b0 + ∑ bij Ai

(1)

i =1

where Cj are influence factors; b0 is constant; bij is coefficient; and, Ai is linear function value at wavelength i. A quantitative relation was found between the influencing factors and NIR spectral features for samples using statistical methods. NIR spectroscopy is an indirect analytical technology that uses a calibration model developed to analyze unknown samples quantitatively and qualitatively, as shown in Fig. 2. 2.2. Dynamics analysis of the epoxy-resin cure reaction via NIR spectroscopy Epoxy resins are widely used as an industrial material due to their heat resistance, electrical insulation, dimensional stability and chemical resistance [17,18]. The curing process for the resin directly influences the manufacturing process, quality of composite product and physical properties of the final polymer. NIR spectroscopy shows good potential for on-line, in situ monitoring of the monomer conversion and average molecular weight of the polymer resin [19]. 2.2.1. Study of the cure reaction of epoxy resin (bisphenol A) using an amine by NIR spectroscopy The bisphenol A epoxy-resin system includes the diglycidyl ether of bisphenol A (DGEBA), dicyandiamide and an accelerator. Different cure-time studies indicate that initiation at 80°C (100 min, 150 min, 200 min) produced a lower degree of cure than initiating at 90°C (40 min, 60 min, 90 min) for a post-cure temperature of 110°C (120 min). These divergences in the primary amine conversion indicated that a higher conversion rate improved the production rate [20]. The curing reaction for DGEBA using a triamine was studied for epoxy conversion by Fourier-transform (FT)-NIR spectroscopy, DSC and size-exclusion chromatography (SEC). The results from these three technologies (FT-NIR, DSC, and SEC) were close; however, NIR had unique advantages for rapid on-line control and was accurate for the epoxy conversion [21,22]. For the DGEBA system (DGEBA with an anhydride hardener and N-benzylpyrazinium hexafluoroantimonate catalyst), the epoxy conversion was monitored using NIR spectroscopy, and the epoxy-group changes were analyzed (4530 cm−1) [23]. The NIR spectra could distinguish between the reaction dynamics of the primary and secondary amines. By evaluating the twodimensional hetero-spectral correlations in the NIR spectra, the sequential change order in the ether group and polyamine could detail the complex epoxy-curing reaction for both primary and secondary amines [24]. Equation (1) was used to calculate the epoxy and -NH2 conversion during the curing process for the epoxyresin composite using FT-NIR [7,25]:

aNH2 = 1 −

[ A5072,t A5072,o ] [ A5991,t A5991,o ]

aepoxy = 1 −

Fig. 1. Mode of action of non-ionizing radiation (NIR) light on the sample.

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[ AE ,t AR ,o ] [ AE ,0 AR ,t ]

(2)

(3)

where AE,0 and AR,0 are the initial absorption-peak areas at the epoxy and phenyl groups, respectively, and the corresponding values at time t were AE,t and AR,t, respectively; A5072,0 and A5991,0 are the initial absorption-peak areas for the primary and aromatic CH overtone peaks, respectively, and their corresponding values at time t are A5072,t and A5991,t, respectively. We studied the epoxy-resin curing process using FT-NIR spectroscopy [26]. Increasing the curing time (1 min, 3 min and 10 min) gradually weakened the combination and overtone bands for the

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Collect of the typical samples

Scan the spectra of the samples

Measure of the calibration values From reference values From spectral characteristic From exterior feature

Select of the samples

Classification for the calibration and prediction samples

Calibration model part

Developing of the chemometrics method

Selection of the wavelength Prediction model part hDevelopment of the primary model

h Selection the pretreatment method

Optimizing of the model and remove of the outlier

Ideal

Ideal

Unsatisfactory Test model

Predict the chemisty values

Unsatisfactory Analyze the unknown samples Evaluation index

Fig. 2. Design of calibration model developed.

epoxy (-CHOCH2: 4522 cm−1 and 5909 cm−1) and amine groups (-NH2: 5086 cm−1 and 6579 cm−1) and stabilized the absorption peak for the phenyl groups (5086 cm−1). This curing process is shown in Fig. 3. Larrech [27] studied a model reaction between phenyl-glycidyl ether and aniline at 95°C. The NIR spectrum recorded the reaction

process, which occurred in two stages with rate constants that overlapped. The multivariate curve resolution-alternating least squares (MCR-ALS) was used to study the reaction and compute the bands associated with the found solutions, which indicated that the model reaction occurred in two non-sequential stages. Analyzing the

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5

4522 cm-1

Absorbance units/ a.u.

4625 cm-1 -1

4

5909 cm

5086 cm-1

6579 cm-1

3 1 min

2 3 min

1 10 min

0 8000

7000

6000 5000 Wavenumber/cm-1

4000

Fig. 3. Non-ionizing radiation (NIR) spectra of epoxy resin for several curing times at 100°C.

spectral changes throughout the reaction showed the primary amine bands at 5053 cm−1 reducing as the reaction progressed. This technique seems to be more efficient than using the mid-IR spectra where the N-H bands overlap [28]. Fischer [29] analyzed the curing kinetics for an epoxy resin with an amine using NIR. The final sample experienced an uneven reaction rate, and the NIR spectrum for the epoxy amine at various intervals can be obtained via InGaAs-camera imaging. The chemical conversion of the amine-curing agent was observed by NIR spectroscopy at key points thermally identified in the physical development [30]. These changes were then used to analyze the NIR spectra for the curing process to determine the underlying network structure of the epoxy-amine system throughout the curing reaction. The NIR spectral information can be used to optimize the curing process and formulation while further enhancing the utilization of molecular dynamic simulations. The MCR-ALS method was successfully applied to the NIR data to monitor the epoxy-resin reactions. NIR spectroscopy was a suitable technique for studying the curing reactions of various epoxy resins with amine hardeners. 2.2.2. Study of epoxy-resin cure reaction with diaminodiphenylsulfone (DDS), acrylates and other curing agents via NIR spectroscopy Musto [31,32] monitored the curing of epoxy resin [system composed of tetraglycidyl-4,4-diaminodiphenylmethane (TGDDM) using 4,4-diaminodiphenylsulfone (DDS) as curing agent] via FT-NIR spectroscopy and dynamic-mechanical analysis, which analyzed the polymerization mechanism, and detected water within the resin. Li [2] discussed the isothermal curing kinetics parameters for 5 wt% polysulfone TGDDM/DDS nanofibrous membranes and toughened TGDDM/DDS films using DSC and NIR. These data were used to analyze the curing kinetics for different functional groups related to the epoxy-curing process and further illustrate the reaction mechanisms. An NIR-ultrasonic set-up was used to investigate the photopolymerization of different resins (e.g. epoxy acrylates and acrylated polyurethanes). The chemical conversion of acrylates was derived from the decreased area for the characteristic acrylate CH2 = CH- stretching band at 6169 cm−1 [33]. Simultaneously measuring the modulus and conversion provided extensive knowledge on the interdependence of the network formation and chemical conversion.

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Chen and Cook [34] reported the curing kinetics and morphology for interpenetrating polymer networks (IPNs) obtained from a mixture of a rigid epoxy resin and flexible dimethacrylate resin. Their findings indicated that the photopolymerization of dimethacrylate occurred more rapidly than the epoxy curing. NIR spectroscopy also confirmed that combined thermal and photochemical methods yielded virtually independent cures. Kortaberria [35] explored the curing process and curing kinetics for an epoxy resin modified with poly(methylmethacrylate) based on the NIR spectra. The analytical results indicated increasing the curing temperature made the reaction faster and reached the maximum extent earlier. NIR spectroscopy can also be used to investigate other epoxyresin-based curing processes, for example, the curing kinetics of an epoxy-vinyl ester resin based on an azobis(isobutyronitrile)initiated vinyl ester mixed with a diamine-cured or imidazolecured epoxy resin, to associate the curing movement also with the dynamic and steady shear rheology as the curing system undergoes gelation and vitrification [36]. The absorption species rapidly decreased in styrene (6135 cm−1) and methacrylate (6166 cm−1) for the initial 25 min of the isothermal cure at 70°C, where the epoxy absorption decreased less rapidly due to its lower polymerization rate. However, after a post cure at 160°C, the epoxy reaction approached completion. Garrido [37,38] studied the kinetic analysis for silicon-based epoxy-resin reactions and probed the phenyl glycidyl thioether and aniline using the concentration and spectral profiles via NIR spectroscopy to obtain the kinetic-rate constants. 2.3. Study of the on-line control of epoxy resin/prepreg using NIR spectroscopy Carbon or glass fabric was preheated through a drying tower to remove water and surface coating. Next, the fabric was pulled into the epoxy-resin solution. The impregnated fabric was then fed through nip rollers to meter the cloth to a solution ratio before entering another drying tower to remove excess solvent and to form the epoxy resin/prepreg. Poor impregnation could decrease the mechanical properties of the composite. Excess resin would induce flow and cause waste. A good relationship between storage, aging time and curing conversion was obtained, and the isothermal transformation of the epoxy-resin prepreg was controlled by NIR spectroscopy [7]. The influence of different temperatures on the prepreg-conversion rate and the degree of curing was studied [39]. The results indicated the epoxy did not completely convert at low temperatures. The initial epoxy-conversion rate between the two glass plates of the cured

Fig. 4. Sketch of the on-line control of prepreg using near-infrared (NIR) spectroscopy.

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3.5

Predicted value (%)

3.0

Calibration sample Validation sample

2.5

2.0

1.5

2

1

1.0 1.0

1.5

2.0

2.5

3.0

3.5

Actual value (%)

38

Calibration sample Validation sample

7

Predicted value (%)

Predicted value (%)

8

6 5 4

36

34

32

30

3 3 3

4

5

6

7

Calibration sample Validation sample

4 30

8

32

34

36

38

Actual values (%)

Actual value (%)

Predicted values (%)

(a)

36 34 32 30

Resin content

6 5 Volatile content

4 3 2

Pre-curing degree

1 0

20

40

60

80

100

Time (min) (b) Fig. 5. Calibration model developed and on-line control of the preparation process of prepreg using near-infrared (NIR) spectroscopy: a-1) prepreg NIR spectra; a-2) volatile content; a-3) pre-curing degree; a-4) resin content; and, b) on-line control of the volatile content, pre-curing degree and resin content.

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Table 1 (a) Statistical results of the parameter of the calibration and prediction models. (b) Statistical results of the evaluation index of the calibration and prediction models (a) Quality indexes

Classification

Number

Maximum (%)

Minimum (%)

Mean (%)

Volatile content

Calibration samples Prediction samples Calibration samples Prediction samples Calibration samples Prediction samples

132 44 138 46 96 32

7.863 7.221 38.100 36.730 3.218 2.912

3.790 3.921 30.050 30.470 1.198 1.278

5.451 5.421 33.790 33.690 2.103 2.036

Resin content Pre-cure degree (b) Quality indexes

Pre-processing routines

Spectral ranges (cm-1)

PLS factors

R2

RMSEC

RMSEP

Volatile content

First derivative with vector normalization

8

0.990

0.07350

0.153

Resin content

Multiplicative Scattering correction Min-Max normalization

12000-5492.3 4801.9-4000 12000-4597.5

8

0.980

0.225

0.452

6

0.960

0.0957

0.152

Pre-cure degree

12000-5392 4801.9-4000

epoxy prepreg was lower than that of the directly cured epoxy prepreg; however, the activation energy was higher than for direct curing. Resin content, pre-cure degree and volatiles content are keys to preparing a prepreg. The traditional analytical method involves solvent extraction, weighing and burn-off. However, this method is not ideal for analyzing prepreg quality due to the time requirements, material waste and sample destruction. Jiang [40–42] and Li [43] developed an on-line control for preparing the prepreg (Fig. 4). NIR spectroscopy could be used to predict simultaneously resin content, pre-cure degree and volatile content. Fig. 5(a) shows the on-line collection spectra for the samples using the calibration model developed via PLS. The correlation coefficient (R2), root mean square error of calibration (RMSEC) and root mean square error of prediction (RMSEP) were calculated to evaluate the models and to determine the best calibration model. Table 1 shows the results for the calibration models developed.

For the resin content, pre-cure degree and volatile content, R2 was 0.98, 0.96, and 0.99, respectively; RMSEC was 0.225, 0.0957, and 0.0735, respectively; and RMSEP was 0.452, 0.152, and 0.153, respectively. Furthermore, the prepreg was analyzed in 2 min without any sample destruction. The technological parameters for the prepreg could be adjusted according to the results of NIR spectroscopy (Fig. 5-b). Fig. 6 shows this on-line control strategy, which enhances production efficiency and economizes on the time and the material required by industry. 3. Microstructural characterization of prepreg using Micro-CT Micro-CT is a non-destructive method to obtain accurate 3-D images constructed via the digital manipulation of multiple X-ray projections [44]. This method characterizes the 3-D sample microstructure. As a medical recognition technique, Micro-CT has been

The monitoring quality of prepreg by NIR

Control

Update and optimal of the model process

NIR analysis (calibration model) Update of the model

system Reference analysis Optimal model

Moving velocity

Temperature of the dry tower

Distance of the press roller

Solution concentration

Preparation process of prepreg

Fig. 6. The quantification-analysis process of near-infrared (NIR) spectra.

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Fig. 7. Scanning electron microscopy (SEM) of the prepreg cross-section: (a) normal temperature; and, (b) 100°C.

the prepreg cross-section. In Fig. 7 (a), the cross-section was damaged during prepreg preparation: the resin concentrated at the surface of the glass fabric. In this case, the internal prepreg structure was not scanned via electron microscopy. Fig. 7 (b) shows the pre-cure prepreg cross-section at 100°C for 10 min. The glass fiber was partially broken, which indicates the prepreg structure was damaged during preparation. This result affected the prepreg quality analysis. The degree of prepreg impregnation needs to be assessed in 3-D due to the complex internal structure. Micro-CT enables the full 3-D visualization for analyzing the internal structure for various

successfully applied to composite materials. For example, Centea [45] examined composite microstructures for laminate processes via Micro-CT, and Tsukrov [46] observed microcracking within a 3D woven carbon-fiber/resin via Micro-CT. 3.1. Microstructural features of prepreg based on a reinforced glass cloth Optical and scanning electron microscopy reveal only surfaces or 2-D structures rather than internal damage. Fig. 7 (a,b) shows

Sample

Reconstructed 10mm

a

b

Prepreg

Defect c

d

Fig. 8. Micro-CT image of 3-D prepreg: (a) reconstructed sample; (b) different impregnation degree of the prepreg; (c) reconstructed microstructural features of carboncloth prepreg; and, (d) amplification of microstructural features from (c).

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Fig. 9. Micro-CT images and CT intensity of the divided prepreg: (1) a – top region; b – region between top and middle; c – region between middle and bottom; and, d – bottom region; (2) CT intensity of sample with mark line regions: a – top region; b – region between top and middle; c – region between middle and bottom; and, d – bottom region.

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materials, and allows the 3-D microstructure to be characterized (see Video S1). Micro-CT images for the prepreg samples were reconstructed using a 3.8791(x) × 3.9904(y) × 0.9221(z) mm3 cylindrical region of interest (ROI) for a sample of 10-mm diameter. Fig. 8 shows the 3-D microstructure profile and prepreg shape; the resin did not fully impregnate the glass cloth, and a void existed in the fasciculus. The internal tomographic images were obtained as a 3-D microstructure. To analyze the internal prepreg structure, the sample was divided into four sections from top to bottom in Fig. 9-1 (a, b, c, d); the corresponding CT intensities between the prepreg and the defect for each section were also analyzed. The x-axis showed the sample length, and the y-axis represented the CT intensity. Fig. 9-2 (a, b, c, d) shows the change in CT intensity observed when the concentration was higher for the prepreg than the defects. The defects were regions where the resin did not impregnate the glass cloth. Three orthogonal slices from the CT data yielded the degree of internal impregnation of the prepreg. The analytical images mentioned above provide an essential theoretical basis for analyzing the degree of impregnation. The total and prepreg volumes in the reconstructed region (Fig. 8) and divided sections (Fig. 9-1 a,b,c,d) were calculated using advanced bone-analysis software (MicroView, GE Healthcare Software Support, London, ON, Canada). The impregnation degree was the ratio between the prepreg and total volumes. The impregnation degree for the reconstructed and divided sections was 54.23% (whole), 44.95% (a), 82.58% (b), 71.78% (c), 12.05% (d), respectively. These analytical results indicated that the impregnation degree was higher in the middle than the top and bottom of the prepreg. This conclusion agreed with the 3-D CT images. The above analysis showed the prepreg sample had defects and the impregnation degree in the reconstructed region was 54.23%, so the other 45.77% was not impregnated.

3.2. Microstructural features of prepreg based on carbon-cloth reinforcement

Amplifier Photomultiplier Scintillant Prepreg

I I0 β-ray source

Fig. 10. Monitoring resin content of prepreg by β-ray technique.

5. DSC for studying the degree of cure of prepreg Differential scanning calorimetry (DSC) is a thermoanalytical technique that analyzes sample changes based on temperature change. This method has been used extensively to study the cure behavior of polymer composites [49–51]. The degree of cure of prepreg directly influences the product-manufacturing process and is a determining step in the preparation of fiber-reinforced thermoset composites. The composite quality is controlled to a great extent by the cure-cycle parameters [52,53]. Fig. 11 shows the TG-DSC curves for the cure process (HD03 epoxy resin, amine curing agent and acetone diluents). The epoxy-resin cure process underwent intramolecular and intermolecular condensation reactions below 200°C. Small molecules and water were initially volatilized during the prepreg cure. A high degree of curing degree induces a crisp prepreg, whereas a low degree of curing yields a viscous prepreg. Thus, the best pre-cure temperature for the prepreg was 160170°C for 15 min during the preparation. 6. Conclusions

A prepreg was reconstructed using a 3.6724(x) × 3.3227(y) × 0.8426(z) mm3 box as shown in Fig. 8(c). The dark voids are defects as marked in Fig. 8(d). The region of the segmentation between the prepreg and void defects is clearly identified. The yellow and offwhite boxes in Fig. 8(d) represent the prepreg and void defects, respectively. The ratio of prepreg to defects was approximately 2.37:1.

The above analyses used NIR spectroscopy, Micro-CT, β-rays and DSC for epoxy resin and prepreg. NIR spectroscopy could simultaneously determine the resin content, pre-curing degree and volatile content, and the prepreg could be analyzed in 2 min without requiring sample destruction. However, NIR spectroscopy has certain defects as a secondary method in predicting values. In addition, NIR

4. Analysis of resin in prepreg using β-ray technology Beta particles are high-energy, high-speed electrons or positrons emitted by certain radioactive nuclei. These emitted beta particles are a form of ionizing radiation also known as β-rays [47,48]. β-ray monitoring systems are theoretically based on the β-ray energy being attenuated when it penetrates the prepreg specimen (Fig. 10). The β-ray energy attenuation corresponds to an exponential relationship:

I = I0 exp ( − μρχ )

(4)

where I0 and I are the energy before and after the β-rays penetrate the material, respectively, and μ, ρ and χ are absorption coefficient, density and thickness for the materials, respectively. During the resin-content monitoring, particles emitted by a β-ray source penetrate the pre-coated paper and are received by a detector installed above the pre-coated paper. Calibrations between the resin content and β-ray output have been established. The resin content was controlled based on the β-ray analysis with an error range of ±2%.

Fig. 11. The curve of the prepreg-cure process using thermogravimetry-differential scanning calorimetry (TG-DSC).

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spectroscopy requires numerous samples to develop a calibration model. Micro-CT can analyze the internal microstructure of prepreg and calculate the defect degree. β-ray analyses require a calibration curve and can analyze the resin content on-line. DSC can analyze the pre-curing degree. However, Micro-CT, β-ray analysis and DSC require significant time to prepare and to analyze the samples and are destructive tests that are ill-suited to rapid on-line analysis. For future studies, NIR spectroscopy is a suitable on-line, rapid method, and Micro-CT is excellent for microstructure analysis of prepreg. These analytical methods are significant in studying the quality control of composite materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 50903025) and the Fundamental Research Funds for the Central Universities (Grants No. HIT NSRIF 2013046 and HIT IBRSEM 2011 10). Appendix: Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.trac.2015.03.028. References [1] H. Howland, S.W. Hoag, Analysis of curing of a sustained release coating formulation by application of NIR spectroscopy to monitor changes physical– mechanical properties, Int. J. Pharm. 452 (2013) 82–91. [2] G. Li, Z.B. Huang, P. Li, C.L. Xin, X.L. Jia, Curing kinetics and mechanisms of polysulfonenanofibrous membranestoughened epoxy/amine systems using isothermal DSC and NIR, Thermochim. Acta 497 (2010) 27–34. [3] F.R. Mustata, T. Nita, B. Ioan, Epoxy resins cross-linked with bisphenol A/methylenedianiline novolac resin type: curing and thermal behavior study, Eng. Chem. Res. 51 (2012) 8415–8424. [4] L. Xu, J.H. Fu, J.R. Schlup, In situ near-infrared spectroscopic investigation of epoxy resin-aromatic amine cure mechanisms, J. Am. Chem. Soc. 116 (1994) 2821–2826. [5] G. Czél, M.R. Wisnom, Demonstration of pseudo-ductility in high performance glass/epoxy composites by hybridisation with thin-ply carbon prepreg, Compos. Part A 52 (2013) 23–30. [6] T. Ogasawara, S.Y. Moona, Y. Inoue, Y. Shimamura, Mechanical properties of aligned multi-walled carbon nanotube/epoxy composites processed using a hot-melt prepreg method, Compos. Sci. Technol. 71 (2011) 1826–1833. [7] Y.F. Yu, H.H. Su, W.J. Gan, Effects of storage aging on the properties of epoxy prepregs, Ind. Eng. Chem. Res. 48 (2009) 4340–4345. [8] R.W. Jones, N. Yeow, J.F. McLelland, Monitoring ambient-temperature aging of a carbon-fiber/epoxy composite prepreg with photoacoustic spectroscopy, Compos. Part A 39 (2008) 965–970. [9] Y.H. Zhang, Y.D. Huang, L. Liu, Surface modification of aramid fibers with gamma-Ray radiation for improving interfacial bonding strength with epoxy resin, J. Appl. Polym. Sci. 106 (2007) 2251–2262. [10] L. Liu, Y.D. Huang, Z.Q. Zhang, Z.X. Jiang, L.N. Wu, Ultrasonic treatment of aramid fiber surface and its effect on the interface of aramid/epoxy composites, Appl. Surf. Sci. 254 (2008) 2594–2599. [11] M. Xie, Z.G. Zhang, Y.Z. Gu, M. Li, Y.Q. Su, A new method to characterize the cure state of epoxy prepreg by dynamic mechanical analysis, Thermochim. Acta 487 (2009) 8–17. [12] M.L. Costa, Monitoring of cure kinetic prepreg and cure cycle modeling, J. Mater. Sci. 41 (2006) 4349–4356. [13] M. Hayaty, M.H. Beheshty, M. Esfandeh, Cure kinetics of a glass/epoxy prepreg by dynamic differential scanning calorimetry, J. Appl. Polym. Sci. 120 (2011) 62–69. [14] Q. Zou, G.D. Deng, X.D. Guo, W. Jiang, F.S. Li, A green analytical tool for in-process determination of RDX contentof propellant using the NIR system, ACS Sustain. Chem. Eng. 1 (2013) 1506–1510. [15] L.A. Rodrı′guez-Guadarrama, Application of online near infrared spectroscopy to study the kinetics of anionic polymerization of butadiene, E Polym. 43 (2007) 928–937. [16] J.M. Barbas, A.V. Machado, J.A. Covas, In-line near-infrared spectroscopy for the characterization of dispersion in polymer-clay nanocomposites, Polym. Test. 31 (2012) 527–536.

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