# Moisture absorption comparison between bulk adhesives and adhesively bonded joints in different environments

Arij Fawaz, Emilie Lepretre, Xavier Chapeleau, Sylvain Chataigner

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Arij Fawaz, Emilie Lepretre, Xavier Chapeleau, Sylvain Chataigner. Moisture absorption comparison between bulk adhesives and adhesively bonded joints in different environments. ECCM21 – 21st European Conference on Composite Materials, Jul 2024, Nantes, France. hal-05394975

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Distributed under a Creative Commons Attribution 4.0 International License
MOISTURE ABSORPTION COMPARISON BETWEEN BULK
ADHESIVES AND ADHESIVELY BONDED JOINTS IN DIFFERENT
ENVIRONMENTS

Arij FAWAZ¹, Emilie LEPRETRE², Xavier CHAPELEAU³ and Sylvain CHATAIGNER⁴

¹MAST/SMC, COSYS/SII-I4S, Université Gustave Eiffel, Bouguenais 44344, France

Email: arij.fawaz@univ-eiffel.fr

²MAST/SMC, Université Gustave Eiffel, Bouguenais 44344, France

Email: emilie.lepretre@univ-eiffel.fr

³COSYS/SII-I4S, Université Gustave Eiffel, Inria, Bouguenais 44344, France

Email: xavier.chapeleau@univ-eiffel.fr

⁴MAST/SMC, Université Gustave Eiffel, Bouguenais 44344, France

Email: sylvain.chataigner@univ-eiffel.fr

**Keywords:** Adhesively bonded joints, Epoxy adhesive, Durability, Moisture diffusion, Interphase.

**Abstract**

The use of adhesively bonded composite reinforcements is increasing in various industries for which they encounter diverse environmental conditions throughout their operational lifespan. Moisture absorption stands as a common aging factor leading to the deterioration and weakening of mechanical properties in these materials. Moreover, there is limited knowledge regarding the durability of bonded joints in wet environments. Consequently, moisture diffusion continues to hinder their widespread application across multiple fields. This study will focus on studying the durability of bulk adhesives and adhesively bonded joints under various wet conditions: exposure to 98% relative humidity, immersion in artificial sea water as well as in demineralized water.

In this study, water diffusion was first examined in bulk adhesives, and subsequently in two bonded assemblies having different adhesive thicknesses. Gravimetric measurements were carried out to obtain the corresponding diffusion coefficients. The experimental results revealed Fickian behavior for the studied epoxy bulk adhesives, while a non-Fickian diffusion was obtained for the studied methacrylate adhesive. Regarding the bonded assemblies, those with a 1 mm adhesive thickness exhibit faster diffusion compared to those with a 3 mm adhesive thickness.

**1. Introduction**

Adhesively bonded joints are increasingly being used in several fields such as aerospace, transport and marine sectors. Their widespread use in many civil engineering applications is due to several advantages. Indeed, compared to the traditional joining techniques (like fastening and riveting) they provide lightweight structural assemblies and allow dissimilar materials joining with no damage during manufacturing process. Moreover, they also permit to obtain more uniform stress distributions along the bonded zone.

Bonded assemblies are exposed to several environmental conditions such as high humidity levels specially in marine environment. Thereby, they should be designed to withstand such environmental conditions. Hence, predicting the long-term behavior of adhesively bonded joints in different environmental conditions is of crucial interest.

Water sorption by polymers is considered as one of the major setbacks in adhesive's application under humid conditions. Water and humid air are known to cause a reduction in strength, durability and service
life of adhesives joints [1]. Moisture degradation of adhesives can result in a decrease in their mechanical properties and can cause both reversible effects, such as plasticization and swelling, as well as irreversible ones, like hydrolysis and oxidation. The detrimental effects are more significant in adhesive joints since degradation of the interface between adhesive and adherend could initiate an interfacial failure. Some authors in the literature found that water penetration is faster in bonded joints than in bulk adhesives [2,3]. This phenomenon was attributed to the presence of interphase and/or interface between adherends and adhesive within the bonded assembly. Furthermore, it was found that in thinner joints, where the ratio of the interphase's thickness to adhesive's thickness is larger than that for thicker joints, there is a greater uptake of water.

Although it has been reported in the literature that water diffusion is often governed by Fick's laws, several authors have demonstrated deviations from Fickian diffusion. Other models have been developed for these cases, such as the Langmuir model [4], the dual Fickian diffusion [5] and the delayed dual Fickian [6].

This paper initially outlines the experimental investigation of water sorption behavior for three structural bulk adhesives using a high-precision balance. Then, the same procedure is also carried out for bonded assemblies with steel substrates and epoxy adhesive, where a comparison of the water sorption profiles for different bonded joint thicknesses is conducted. Additionally, another comparison is made between different humid environments.

## 2. Experimental methods

### 2.1. Materials and sample preparation

In this study, three commercial adhesives were used: two epoxy-based adhesives labeled as E1 and E2, and a methacrylate adhesive marked as M. The selection of these materials was driven by their industrial interest and also by their frequent applications in marine environment, which represents the case and condition of this research.

Bulk adhesive discs have been manufactured by pouring the adhesives into circular molds. To facilitate the demolding process, the used molds were made from non-adherent materials: Teflon molds for the M and E1 adhesives and modified polyethylene molds for the E2 adhesive. The discs made for both adhesives E2 and M have the same diameter *d* of 80 mm and the same thickness *e* of 1 mm (± 0.05 mm). For the adhesive E1, the discs have a diameter of 70 mm and a thickness of 2 mm (± 0.09 mm). The dimensions were selected with an aim to consider a 1D surface diffusion i.e., along the thickness direction (*d* >> *e*), this allows to simplify the studied diffusion phenomena (figure 1).

The E2 discs were cured at 80°C for 3 hours, the E1 discs were cured at 80°C for 16 hours and the methacrylate adhesive was cured at 25°C for 24 hours according to their technical data sheets. According to [7], taking a slightly higher curing time than that suggested in technical data sheet is much more preferable.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** 1D water diffusion through the thickness *e* of bulk adhesive discs.</figcaption></figure>
Regarding the bonded assemblies, E1 was used as an adhesive, with stainless steel plates serving as adherends. Stainless steel was specifically chosen to prevent corrosion, which could complicate the studied mechanisms of water diffusion. Before bonding, the metallic substrates were sanded using a sandblasting machine, achieving an average roughness of 3.5 µm (± 0.5 µm). Then, they were cleaned with acetone. The bonded joints were manufactured in two different thicknesses of 1 mm (± 0.1 mm) and 3 mm (± 0.5 mm) as shown in figure 2-B. Two temporary steel wedges with these thicknesses were used to ensure a consistent adhesive thickness throughout the bonded assembly. All the bonded joints have the same length of 80 mm (± 7 mm) and the same width of 5 mm (± 0.5 mm), as shown in figure 2-A. These dimensions allow for the adoption of the assumption of 1D surface diffusion, i.e., along the width (where width << length).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Dimensions of bonded assemblies: (A) 5 mm width and 80 mm length, and (B) two different thicknesses of 1 mm and 3 mm.</figcaption></figure>

## 2.2. Aging conditions and gravimetric measurements

Before aging, methacrylate samples were dried in an oven at 50 °C until a constant mass was obtained, while the epoxy adhesives discs were placed in a desiccation chamber for 4 weeks (figure 3). No drying process was conducted for the bonded assemblies since the mass loss of E1 bulk specimens during drying was found to be minimal. Thus, the bonded joints were immediately submerged in water after curing to prevent any humidity absorption.

During drying, specimens were periodically weighed on a 10⁻³ g resolution balance (Kern PLJ 2000-3A having a maximum weighing range of 2.1 kg) for the E2 and M discs, and a 10⁻⁴ g resolution balance (with 610 g of maximum weighing range) was used for the bonded joints and E1 discs. The water content $M_t$ (%) is determined using equation (1) where $m(t)$ is the mass of the sample at a time $t$ while $m_0$ corresponds to the weight of totally dried specimen (i.e., for $M_t = 0$).

$$M_t (\%) = \frac{m(t)-m_0}{m_0} \times 100 \qquad (1)$$

Following drying, the E1 disc specimens were investigated under three different aging environments, which include immersion in demineralized water (DW), immersion in seawater (SW), and exposure to 98% relative humidity (RH) (figure 4). The bonded assemblies, E2 and M discs were placed only in DW and SW. The artificial seawater was prepared by dissolving sodium chloride (NaCl) into pure water following the reference salinity value of 3.5% provided by the ASTM D 1141 standards [8].

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Drying process of (A) Epoxy adhesives in desiccation chamber, and (B) Methacrylate adhesive in an oven at 50°C.</figcaption></figure>
In each environment, a repeatability study was generated using five disc samples and five bonded assemblies (table 1). This was done to enhance confidence in the results. The total number of produced bulk adhesive discs was 45 (three adhesives, three wet environments and five specimens), while the number of bonded assemblies was 20 (two thicknesses of bonded joints, two wet environments and five specimens).

<table><caption>**Table 1.** Summary of the number of samples used for each sample type and for various aging environments</caption><thead><tr><th rowspan="2"></th><th colspan="3">Bulk adhesives</th><th colspan="2">Bonded joints with E1 adhesive</th></tr><tr><th>E1 adhesive</th><th>E2 adhesive</th><th>M adhesive</th><th>1 mm adhesive thickness</th><th>3 mm adhesive thickness</th></tr></thead><tbody><tr><td>SW</td><td>5 samples</td><td>5 samples</td><td>5 samples</td><td>5 samples</td><td>5 samples</td></tr><tr><td>DW</td><td>5 samples</td><td>5 samples</td><td>5 samples</td><td>5 samples</td><td>5 samples</td></tr><tr><td>98% RH</td><td>5 samples</td><td>5 samples</td><td>5 samples</td><td>-</td><td>-</td></tr></tbody></table>

To ensure the contact of all discs specimens' faces with water and achieve uniform diffusion, a small support (blue element in figure 3-B) was used to maintain the specimens vertically. During this wet aging process, the temperature was maintained at 50°C by placing all the specimens in a hygrothermal chamber (figure 4). This temperature is at least 20°C below the glass transition temperature Tg of all the adhesives used. The primary purpose of increasing the temperature is to accelerate diffusion, as highlighted in the study of [9].

<figure><img src="image_5.png" /><figcaption>**Figure 4.** E1 discs and bonded assemblies placed in DW and SW, with E1 discs alone subjected to 98% RH. All the samples were in a hygrothermal chamber at 50°C.</figcaption></figure>

After placing all the specimens (bulk adhesives and bonded joints) in the previously mentioned humid environments, they were taken off at different times, wiped off with tissues to dry their surfaces and weighed using the aforementioned balances. Thereby, the water gain can be calculated using equation (1) where $m_0$ is defined as the initial dry mass before aging. Once the saturation is reached, $M_t$ will be equal to the saturation moisture content $M_s$.
Regarding the calculation of diffusion coefficient, the formula proposed by Crank [10], who defines a solution for the Fick model, is used. This formula corresponds to a 1D diffusion that takes place along the sample thickness *e* for the discs and along the sample's width for bonded assemblies.

**3. Results and discussion**

**3.1. Gravimetric measurements for bulk adhesives**

Using equation (1), the variation of the macroscopic water content $M(t)$ can be graphically represented for M, E2, and E1 discs. These are respectively depicted in figures 5-A, 5-B and 6 that illustrate the evolution of weight gain with the square root of time over the adhesive thickness. Note, measurements for the E1 discs are still under progress as saturation was not obtained for the moment.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Sorption curves in SW and DW for the (A) M and (B) E2 discs.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Sorption curves for the E1 discs under three wet conditions: immersion in SW, immersion in DW, and exposure to 98% RH.</figcaption></figure>

These curves depict the absorption process for the epoxy adhesives E1 and E2, revealing two distinct stages. Initially, water uptake results in a linear increase in weight gain relative to the square root of time across the adhesive thickness. Subsequently, a plateau emerges, indicating saturation (figures 5-B and 6). For the methacrylate adhesive, the water absorption curve denotes that after immersion in water, the samples' mass increases linearly with the square root of time in the initial stage. Then, the water uptake begins to decrease after 10 days of wet aging in both SW and DW environments (figure 5-A). A small dispersion in results was obtained for all the disc adhesives.

Results of the gravimetric measurements reveal that the water uptake curves of the adhesives E1 and E2 follow Fickian diffusion, indicating non-anomalous water diffusion behavior. Indeed, the theoretical Fickian curve is distinguished by two phases: initially, a linear increase in water uptake over the square root of time is observed, succeeded by a slower absorption rate leading to a plateau, indicating saturation.
However, the water absorption profile displays a non-Fickian behavior for the M adhesive, as evidenced by a loss of mass observed after 10 days of wet aging. According to [11], this can be attributed to an initiation of material dissolution within the samples, altering the obtained weight change profile to be lower than the theoretical Fick curve. Indeed, a hydrolysis reaction occurred, causing a break in the macromolecular chain and resulting in a reduction in the rate of cross-linking. This reaction also led to the release of molecules into the water, leading to a loss of mass. This is considered as an irreversible consequence of the water diffusion within adhesives.

From the diffusion curves below, the diffusion coefficients can be calculated for the E2 and M adhesives in SW and DW. Table 2 presents the mean values of the obtained diffusion coefficients D and the mean water content at saturation $M_s$.

<table><caption>**Table 2.** Diffusion parameters for the M and E2 adhesives in two wet conditions (DW and SW).</caption><thead><tr><th rowspan="2">Aging environment</th><th colspan="2">M adhesive</th><th colspan="2">E2 adhesive</th></tr><tr><th>DW</th><th>SW</th><th>DW</th><th>SW</th></tr></thead><tbody><tr><td>D (mm²/s) 10<sup>-7</sup></td><td>92.0 ± 6.20</td><td>78.1 ± 5.20</td><td>3.82 ± 0.23</td><td>3.62 ± 0.12</td></tr><tr><td>M<sub>s</sub> (%)</td><td>2.92 ± 0.16</td><td>1.73 ± 0.03</td><td>2.22 ± 0.05</td><td>2.09 ± 0.03</td></tr></tbody></table>

In both SW and DW environments, the mean diffusion coefficient *D* of the methacrylate adhesive exceeds that of the epoxy adhesive E2. Additionally, as indicated in **Table 2**, the diffusion coefficient is slightly larger in DW than in SW for the E2 adhesive (5.5% difference). However, a more significant contrast in diffusion coefficients between SW and DW is observed for the M adhesive, where *D* is 17.8% larger in DW compared to SW. For the E1, the samples are not saturated yet, but based on **figure 6**, it can be noticed, from a first inspection, that diffusion exhibits a slightly faster rate in DW compared to SW, with SW showing faster diffusion than in the 98% RH environment.

To summarize, the diffusion coefficient for epoxy adhesive E2 is slightly greater in DW compared to SW. A more pronounced contrast emerges with the methacrylate adhesive, consistently showing a higher diffusion coefficient in DW than in SW. It is important to note that the diffusion coefficient of these adhesives is primarily influenced by the chemical nature of these polymers. For the E1 adhesive, diffusion is ongoing, but initial findings suggest that the diffusion coefficient is higher in DW than in SW. Moreover, it appears to be greater in SW than in a 98% RH environment. These results deal with some authors in the literature who found that moisture ingress is more important in deionized water than in seawater [12].

## 3.2. Gravimetric measurements for bonded assemblies

The water absorption profiles for bonded assemblies, placed in SW and DW, with 1 mm and 3 mm thicknesses of adhesive, are illustrated in figures 7 and 8, respectively.

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Water sorption curves of bonded assemblies with 1 mm adhesive thickness in SW and DW.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Water sorption curves of bonded assemblies with 3 mm adhesive thickness in SW and DW.</figcaption></figure>

The stainless steel adherends of the bonded assemblies placed in SW began to corrode after 10 days of immersion (figures 7 and 8). After 25 days, corrosion became more significant, leading to a noticeable loss of mass in the water absorption profile. This loss of material due to corrosion exceeded the mass of water absorbed by the bonded joint. Consequently, it is not possible to determine a conclusive value of the diffusion coefficient for these bonded assemblies immersed in SW. However, normalization with respect to the volume of adhesive in each bonded assembly (considering the exposure surface and the primary diffusion-driving distance, which is the width for bonded joints) should be performed to be able to compare the diffusion rate between DW and SW for the 3 mm joints as well as the diffusion rate between bulk adhesives and bonded assemblies. This normalization is intended to account for uncertainties in the dimensions of the bonded assemblies.

In contrast, no corrosion was observed in the bonded joints placed in DW. Hence, only results for bonded assemblies in DW can be adopted and analyzed further. However, since diffusion is still active, the value of the diffusion coefficient for these bonded assemblies cannot be determined at this stage. The corresponding macroscopic absorption curves are depicted in figure 9. Nonetheless, it is evident that diffusion occurs slightly faster in the thinner adhesive joint (figure 9), despite the observed dispersion. This dispersion can primarily be attributed to uncertainties in the dimensions of the bonded joints. Further data is required for a more accurate comparison where the exact value of diffusion coefficient can be calculated.

<figure><img src="image_10.png" /><figcaption>**Figure 9.** Water sorption curves of bonded assemblies in DW with 1 mm and 3 mm adhesive thicknesses.</figcaption></figure>

**4. Conclusion**

In this study, the diffusion kinetics were compared at the same temperature of 50°C for three distinct adhesives: two epoxy resins, labeled as E1 and E2, and a methacrylate adhesive, labeled M. Bulk adhesives and bonded assemblies were investigated. Gravimetric measurements demonstrate that the water absorption within bulk epoxy adhesives obeyed the Fickian diffusion. This means that water sorption occurs in the polymer matrix without considering specific interactions between water and polymer chain. However, a degradation happened to the bulk methacrylate adhesive due to the water
penetration and a loss of mass initiates after 10 days of immersion. A faster diffusion is observed in the E2 bulk adhesive with respect to that of the methacrylate adhesive. Moreover, diffusion occurs more rapidly in DW than in SW for all the adhesives, consistently with existing literature.

Since adhesives are rarely used alone in real-life applications, ungoing investigations are carried out to study the diffusion process in bonded assemblies, where the interfaces (the weak points of bonded joints affecting their durability) become relevant. At this stage, slightly faster diffusion was observed in thinner joints. However, the diffusion process is still active, so further data is needed to calculate and compare diffusion rates more accurately.

The achievements of this study could be extended to predict the water uptake in bonded joints and the remaining mechanical strength in long-term aged adhesively bonded joints. For this, mechanical investigations are under investigations to correlate diffusion kinetics to mechanical degradation.

## Acknowledgments

The authors would like to acknowledge the technical teams of the MAST/SMC laboratory and COSYS/SII laboratory at Université Gustave Eiffel, as well as the 'Région Pays de La Loire' for co-funding this study.

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