# Near-Infrared Spectroscopic Method for Monitoring Water Content in Epoxy Resins and Fiber-Reinforced Composites

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Section

#### 2.1. Materials

#### 2.2. Methods

^{3}and 2.54 g/cm

^{3}, respectively. The density of the composite (${\rho}_{comp}$) was determined to be 1.97 g/cm

^{3}by measuring the mass and dimensions of a large composite block. The volume and mass fractions of neat resin were calculated using Equations (1) and (2), respectively.

^{−1}using 32 scans per spectrum with a resolution of 4 cm

^{−1}. A spectrum of the driest (in this case, dried) sample was subtracted from the spectrum of the sample of interest. Then, the line connecting spectrum points at 5400 and 4900 cm

^{−1}was constructed. The slope and the intercept of this line were obtained. Subsequently, baseline correction was performed by subtracting the obtained line from the spectrum of interest.

## 3. Results and Discussion

#### 3.1. Water Uptake, Drying and Conditioning in Air Experiments of Neat Resin

#### 3.2. Reversible Drop in Ultimate Tensile Strength of Neat Resin with Water Content

#### 3.3. The Method for Monitoring Water Content in Neat Resin

^{−1}. This band changed depending on the water content, as shown in Figure 3. Based on study by Falk et al. [35], this band corresponds to a combination mode of stretching and bending of the water molecule’s OH group. This observation is also consistent with a recent study by Muroga et al. on spectroscopic evaluation of water content in polylactide (PLA) and with a novel work on water monitoring methods in PMMA by Wiedemair et al. [21,36]. Thus, this absorption band is chosen to monitor the water content of the epoxy resin. The wavenumber of the water absorption band shifts to lower values as the water content increases. True water content and the corresponding wavenumber values at the absorption peak are reported in Table 1.

^{2}= 0.9466):

^{2}= 0.9466, which indicates that this model equation accounts for 89.61% of the variation in absorbance band maximum values in the dataset. The remaining 10.39% of variation not explained by the equation is expected to be due to the sample thickness having a tolerance of 5%, as well as some possible drying in air during the collection of spectra, since spectra are taken in ambient conditions at room temperature in air atmosphere. Such low variation even within a 5% thickness tolerance of samples indicates this method as being precise and efficient for monitoring water content of neat resin. Thus, this method can be used as an indicator of the water concentration-dependent drop of the mechanical properties of the material.

#### 3.4. Extension of the Method to Samples of Varying Thickness

^{2}= 0.8480.

^{2}= 0.8480). Using model equation (Equation (10)), it is known that ${\epsilon}^{\ast}{\delta}_{2mm}$ equals 0.1247. From this, taking into account a 5% thickness tolerance, the attenuation coefficient is 0.0624 ± 0.0031${\%}^{-1}\xb7{\mathrm{mm}}^{-1}$. The values are within the standard deviation. Since the attenuation coefficient ${\epsilon}^{\ast}$ in our case has units of ${\%}^{-1}\xb7{\mathrm{mm}}^{-1}$, in order to obtain the molar attenuation coefficient $\epsilon $, calculation of water molar concentration is required. Using the definition of true water content (Equation (9)), the relationship between the molar concentration of the diffusant (water) and the true water content can be written as Equation (14).

^{−1}for the resin of interest at full saturation (${W}_{max}^{\ast}=3.44\%$) for a sample of 2 mm in thickness, using Equation (14), the molar concentration is 2.29 ± 0.12 M, and using Equation (15), the molar attenuation coefficient $\epsilon $ is equal to 1.01 ± 0.06 $\frac{\mathrm{L}}{\mathrm{mol}\xb7\mathrm{cm}}$. The low value of the molar attenuation coefficient is explained by the fact that resin media has a relatively high light attenuation itself. Note that molar attenuation coefficient for water in epoxy media $\epsilon $ is obtained using the difference spectra with respect to the spectrum of the dried neat resin.

#### 3.5. Extension of the Method to Composite Systems

^{2}of 0.9329, meaning that the developed model accounts for 87.03% of the variation in the absorbance band’s maximum values in the dataset. The best linear regression fit (R

^{2}= 0.9908) has only a slightly higher slope than the model, as shown in Figure 6.

#### 3.6. Final Remarks on the Results

## 4. Conclusions

^{−1}in the NIR combination mode region correlated with the true water content. The method provides a benefit over the conventional gravimetric analysis providing the possibility to deduce the mass of an absolutely dry material and subsequently the true water content, which is an important indicator of water content-dependent properties. Based on extensive measurements of neat resin and composite samples of varying water content and thickness, regression was performed, and the quantitative absorbance dependence on water content in the materials was successfully established. The model equations for monitoring water content in epoxy resin and composite material samples were obtained and experimentally validated. The model was related to the Beer–Lambert law and explained in such terms. The details of the method were reported, allowing the use of the method in practical applications.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 3.**Difference of spectra with respect to the dried epoxy of the water absorbance band in epoxy resin samples of varying water content. The baseline corresponds to the driest sample (in this case, dried).

**Figure 5.**True water content curves of neat resin and glass fiber-reinforced composite (scaled by the resin mass fraction).

True Water Content, W* (%) | Wavenumber (cm^{−1}) |
---|---|

0.26 | 5230 |

0.61 | 5225 |

1.68 | 5219 |

1.88 | 5214 |

2.84 | 5214 |

2.94 | 5208 |

3.34 | 5208 |

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**MDPI and ACS Style**

Krauklis, A.E.; Gagani, A.I.; Echtermeyer, A.T.
Near-Infrared Spectroscopic Method for Monitoring Water Content in Epoxy Resins and Fiber-Reinforced Composites. *Materials* **2018**, *11*, 586.
https://doi.org/10.3390/ma11040586

**AMA Style**

Krauklis AE, Gagani AI, Echtermeyer AT.
Near-Infrared Spectroscopic Method for Monitoring Water Content in Epoxy Resins and Fiber-Reinforced Composites. *Materials*. 2018; 11(4):586.
https://doi.org/10.3390/ma11040586

**Chicago/Turabian Style**

Krauklis, Andrey E., Abedin I. Gagani, and Andreas T. Echtermeyer.
2018. "Near-Infrared Spectroscopic Method for Monitoring Water Content in Epoxy Resins and Fiber-Reinforced Composites" *Materials* 11, no. 4: 586.
https://doi.org/10.3390/ma11040586