Effect of various artificial ageing procedures on adhesive joints for civil engineering applications

Journal Pre-proof Effect of Various Artificial Ageing Procedures on Adhesive Joints for Civil Engineering Applications K.V. Machalická, M. Vokáč, P. Pokorný, M. Pavlíková PII:

S0143-7496(19)30225-8

DOI:

https://doi.org/10.1016/j.ijadhadh.2019.102476

Reference:

JAAD 102476

To appear in:

International Journal of Adhesion and Adhesives

Please cite this article as: Machalická KV, Vokáč M, Pokorný P, Pavlíková M, Effect of Various Artificial Ageing Procedures on Adhesive Joints for Civil Engineering Applications, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2019.102476. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Effect of Various Artificial Ageing Procedures on Adhesive Joints for Civil Engineering Applications K.V. Machalická1*, M. Vokáč1, P. Pokorný1, M. Pavlíková2 *

1

corresponding author: email address: [email protected]

Czech Technical University in Prague, Klokner Institute, Šolínova 7, 166 08 Prague, CZ. 2

Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, 166 29 Prague, CZ.

Abstract For civil engineering applications of adhesive bonds, the service lifetime and environmental ageing is a fundamental question for safe design. The paper compares the effects of three different ageing procedures (immersion in warm water according to ETAG 002, neutral salt spray test according to ISO 9142, Procedure E4, and extended cataplasm test based on ISO 9142, Procedure E2) among each other on adhesives which are applicable in the civil engineering field. The research covers two different adhesives (two-part acrylate and silane terminated polymer, STP) applied in double lap shear joints composed of aluminium and Zn-electroplated steel substrates. Aluminium was used in two types of alloy and also as anodized aluminium due to its wide use in building facades. All mechanical test results were put in context with chemical analysis results to better understand the changes in the adhesive joint after environmental ageing. Based on infrared spectroscopy, we observed hydrolysis for both tested adhesives, which was more significant for the acrylate adhesive after immersion in water according to ETAG 002 and for the STP adhesive after the neutral salt spray test. Moreover, water and higher temperatures lead to the decomposition of the polymer structure in the case of the acrylate adhesive, which explains the reduction of significant mechanical properties

(often more than 60%) after all types of laboratory ageing procedures. The STP adhesive showed lower mechanical properties worsening (about 30%) than the acrylate adhesive caused by hydrolysis. Changes in polymers were futher examined by the water absorption test and the determination of the glass transition temperature by DMA. It was confirmed that the STP adhesive has a better water resistance than the acrylate adhesive due to lower water absoption rates and no significant changes in Tg after immersion. Keywords civil engineering, aluminium and alloys, steels, laboratory ageing, structural acrylics, mechanical properties of adhesives, water absorption, infrared spectroscopy, glass transition temperature

1.

Introduction

Adhesive bonding can be seen as a challenge and as an innovation in civil engineering – in applications such as building facades and claddings [1-5], glass load-bearing beams or columns [6], hybrid glass structures [7-9], steel and poorly weldable aluminium structures [10]. In the case of traditional mechanical joining methods, fundamental problems, such as high-stress concentrations, weakening of the cross-section in bolted connections, and residual stresses in welded joints, have to be solved. Adhesive bonding provides some important benefits such as a more uniform distribution of stresses along the connection (in dependence on the adhesive and adherend stiffness), no drilling holes for bolts increasing local stress peaks, weight reduction, joining dissimilar and/or thin materials and aesthetically pleasant smooth surfaces without any surface interruption by bolt heads.

On the other hand, adhesive joints for civil engineering applications often have to meet quite different requirements than joints for the automotive or aviation industries. A typical example of such a requirement is durability and design working life. Buildings are designed for at least 50 years of lifetime according to EN 1990:2002. However, replaceable structural parts of buildings, such as facades and their connections, are required to be durable for at least 10 to 25 years. The mechanical properties of adhesive joints are modified by the service environment and, thus, the scope of our research study is focused on building envelopes where joints are exposed to several environmental impacts such as high and low temperatures, humidity and water. All of them can affect adhesive joints by their influence on the adhesive or substrate material or the interface between the adhesive and the substrate. There are several artificial ageing procedures available in the codes or guideline documents, but their effect on particular adhesive joints is not mutually comparable and there is no direct relationship to real effects of natural ageing. Moreover, these regulations were developed primarily for the automotive or aeronautic industries and cannot be directly extended to civil engineering applications such as facade connections. There are some important differences, such as the joining materials, the operational environment, in-service conditions including loading configurations, as well as curing properties, joint geometry, manufacturing conditions and the above mentioned joint lifetime, which have to be taken into account when designing facade connections. For aeronautic structures, weight is a crucial objective achievable by using mostly carbon fibre reinforced composites, titanium, and chromic acid (CAA) or phosphoric acid anodized (PAA) aluminium held together by an adhesive, usually an epoxy/amine based thermoset resin [11,12]. Aeronautical structures are also

exposed to high environmental stresses, such as a combination of humid environments and temperatures ranging from -55 to 120 °C [12]. On the contrary, a building facade surface temperatures from -20 °C to +80 °C are generally regarded as the limits of the temperature range for facades according to EN 1991-1-5 Eurocode 1, Actions on Structures and also by the European Organisation of Technical Approvals in the guideline for European Technical Approval for Structural Sealant Glazing Systems (ETAG 002) [13]. Facade claddings can be designed from various types of materials while the load-bearing sub-frame is often made of hot-dipped galvanized steel, aluminium alloys (typically EN AW-5005, 6060, 6082 or others from 5xxx and 6xxx series), and sulphuric acid anodized aluminium (SAA) [14]. The material of the substrate and its treatment highly influence the adhesion. The morphology which favours mechanical interlocking and provides an increased area for interfacial interactions is clearly advantageous for the durability of adhesive bonds. For example, there are differences in anodized surfaces - CAA or PAA processes produce porous surfaces suitable for structural adhesive bonding due to the adhesive ability to penetrate before it cures completely. However, the SAA process also creates a porous surface, but the pores are narrower than in the case of CAA, so viscous adhesives cannot fully penetrate resulting in a relatively lower strength interface [15, 16]. Bjørgum et al. [11] studied hot AC anodizing, using either sulphuric or phosphoric acid electrolytes, in comparison with the standard DC sulphuric and phosphoric anodizing process for bonding by epoxy adhesives. Joints

with substrates pre-treated by the

standard SAA showed the lowest durability in wet conditions.

There are also different requirements for the mechanical properties of adhesives suitable for facade bonding. They have to be stiff and strong enough to transfer all mechanical loading applied on the facade panel through the joint to the load-bearing substructure. On the other hand, adhesives for facade bonding are needed to be flexible enough to accommodate the stresses arising from different thermal and moisture expansions of joining materials. Traditionally, elastic and durable silicone based adhesives were used for facades for fixing curtain walling panels or glazing which is generaly known as “Structural Silicone Glazing“ (SSG). Alternatively, another elastic bonding system (SikaTack-Panel) was successfully used in Germany for structural bonding of facade panels without an additional mechanical safety device [17]. In addition to elastic and relatively low strength silicone adhesives applied in linear joints, there is increased interest in adhesives providing higher strength, such as polyurethanes, acrylates or the transparent structural silicone adhesive (TSSA) developed for point fixing of glazing [18]. These higher strength adhesives can be applied in facades with larger cladding panels or in joints with smaller bonded areas. It enables increased transparency and/or the use of lightweight cross-sections for the sub-frame as it was examined by Pasternak and Ciupack [19]. Also, there are some relatively newly published research studies which are focused on the durability of adhesive joints for facades. Aßmus et al. [3] tested mechanical properties and resistance to ageing for silicone, polyurethane, epoxy and hybrid adhesives for a bioenergy facade with algae. Bues et al. [5] focused on two structural silicones, the influence of joint geometry and temperature. They observed a linear relationship between nominal shear strengths and temperature for a temperature interval from -20 to +80°C.

Nečasová et al. [20] tested polyurethane and MS polymer adhesives for Siberian larch and wood plastic composite facade panels. Joints were exposed to artificial (sudden temperature changes and frost resistance) and natural ageing with three years of exposure time. They found that selected artificial ageing methods can adequately simulate the effects of natural weather conditioning for selected adhesives and substrates. Ashcroft et al. [21] described the long-term durability of aluminium and titanium joints bonded by eight various adhesives, mostly modified epoxy and phenolic. The adhesive joints were exposed to hot-dry, hot-wet and temperate climates for six years and the results were compared with accelerated ageing. The hot-wet climate was assessed as the “worst-case” scenario to justify its frequent use in laboratory ageing. Ashcroft et al. [21] also pointed out the danger of triggering unrepresentative degradation by excessive temperatures and humidities. The technique of adhesive bonding for civil engineering and/or for facade applications is still not properly rooted in technical standards [19,20]. Certain brief information for designing adhesive joints is provided by Annex M of the Eurocode 9 EN 1999-1-1 (2007) and the ETAG 002 guideline is available for SSG systems [13]. ETAG 002 also provides information on durability testing of joints – tensile and shear tests under increased and decreased temperatures, tensile and shear tests after immersion in water for metal specimens and for glass specimens (with UV light), combination of humidity and NaCl and SO2 atmospheres (in accordance with ISO 9227 and ISO 3231). Unfortunatelly, the ETAG 002´s scope is focused on glazing and structural silicones only and it is out of date and needs revision from the perspective of design concepts used in Eurocodes according to [19].

This lack of standards for adhesive bonding in civil engineering and the

durability of joints is therefore often solved by the application of more or less modified procedures from the automotive and aviation industries. The problem with the choice of a proper ageing procedure is that we cannot define easily what kind of ageing procedure will be the most harmful for a particular adhesive [21, 22]. And the most harmful ageing scenario does not often provide the same or similar effect as real ageing. Thus, it is difficult to choose laboratory ageing to represent real ageing with the complex of consequences [12]. However, we can study particular adhesive systems under certain environmental conditions (e.g. moisture effect) and find out the weaknesses of the adhesive in relation to its possible real application, particular substrate and its pre-treatment. In the present work, the effects of three various laboratory ageing procedures on two adhesives with a different chemical base applied in Zn-electroplated steel and aluminium joints are compared because the substrate material and its treatment also have an important influence on durability. Moisture in bulk adhesive material tends to migrate to the adhesive-substrate interface and decrease adhesion forces there [12, 23, 24]. The effect of ageing was studied first by changes in mechanical properties and failure modes described in detail earlier [25, 26]. This article tries to explain the mechanical changes in context with modifications in the chemical structure and composition, and in context with the changes in the glass transition temperature and moisture absorption.

2.

Materials and Methods

2.1 Experimental programme Three different ageing scenarios (ETAG 002, Extended Cataplasm Test based on Procedure E2, EN ISO 9142, and neutral salt spray test according to Procedure E4, EN ISO 9142) were chosen for the study in order to compare their effect on particular adhesive joints. Two adhesives with a different chemical base, Silane Terminated Polymer (STP) and two-part acrylate, were applied in Zn-electroplated steel and aluminium substrates, which is described more precisely in section 2.2. Double lap shear tests for mechanical properties measurements and infrared spectroscopy to observe the chemical composition and its changes were carried out for the reference set of specimens and for each of three ageing sets of specimens. Moreover, the glass transition temperature for the reference set and after ageing according to ETAG 002 was examined by dynamic mechanical thermal analysis (DMA). After the ETAG test, leachates of both adhesives were examined by X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. The water absorption test was performed to compare water uptake for both adhesives in relation to their weathering resistance. The experimental programme is depicted in Figure 1.

STP adhesive / Acrylic adhesive

reference set initial shear strength glass transition temperature (DMTA) initial chemical composition (FTIR, XRF, XRD)

artificial ageing sets: ETAG 002

Extended Cataplasm Test

residual shear strengths chemical changes (FTIR, XRF, XRD) glass transition temperature (DMTA) after ETAG 002

Neutral Salt Sprey Test

water absorption test

Figure 1. Scheme of the experimental programme

2.2 Materials Acrylate and STP adhesives were chosen in regard to their possible usage in facade structural joints. Sikaflex 552 (Sika CZ, Brno, Czech Republic) is a one-component, weather and ageing resistant adhesive based on the silane terminated polymer (STP) technology. Sikafast 5215 NT (Sika CZ, Brno, Czech Republic) is a two-component structural adhesive based on the Sika’s Acrylic Double Performance (ADP) polymer technology. Both adhesives were applied in their optimal thickness recommended by the producer in manufacturer’s data sheets. The adhesive layer had a thickness of 0.8 mm for the ADP adhesive and 4.8 mm for the STP adhesive. Two substrate materials often used in facades (e.g. as a support frame) were selected for the study – blank aluminium and zinc-electroplated steel. Both materials were used with a smooth and roughened surface to investigate the possible influence of the surface

material and its roughness on weathering resistance. Moreover, sulphuric acid anodized aluminium was also chosen for the study because sulphuric acid anodizing is a widely used corrosion protection process of aluminium structures or parts of building facades. In the case of blank aluminium joints, two types of aluminium alloys, AlMgSi0.5 and AlMg1, were applied in specimens.

2.3 Laboratory Ageing Procedures The specimens were exposed to three various ageing procedures to compare their effect on the adhesive joint. The first set of specimens was prepared as a reference set without the ageing exposure. The second set of specimens was prepared for immersion in warm demineralized water according to the ETAG 002 guideline. The specimens were immersed in 45 °C warm demineralized water for 21 days, followed by 1 day of conditioning at 23 °C and 50% relative humidity. Shear tests were performed immediately after conditioning. The third set of specimens was exposed to a corrosive environment in accordance with EN ISO 9142, Procedure E4. The specimens were exposed to neutral salt spray (NSS) at 35°C in the corrosive chamber for 21 days, then 95% of relative humidity at 40°C in the climatic chamber for 7 days followed. After the exposure to NSS and high relative humidity, the specimens were conditioned at 21 °C and 40% relative humidity for 10 days before shear tests were carried out. The whole procedure of exposure to NSS was performed at SVÚOM Ltd., Prague. Both the above mentioned ageing scenarios are described in detail in [25]. In comparison to the water absorption effect increased by

higher temperatures in the ETAG ageing regime, the neutral salt spray test examines the combination of high relative humidity and NaCl atmosphere. The last set of specimens was exposed to the cataplasm test based on ISO 9142, Procedure E2. Due to high loading of joints in building facades by the service environment (changes in temperatures and RH, high and freezing temperatures, high RH and UV radiation), the cataplasm test (7 days exposition to 70°C and 100% RH) was extended by immersion in demineralized water at a temperature of 20°C for 7 days and 1 day of dry heat (80°C) before 7 days of cataplasm conditions. After that, a freezing environment for 15 hr by shock cooling (within 3 minutes) down to -20°C and 1-day conditioning in room conditions (23°C, 50% RH) followed. Residual shear strength tests were performed within 6 hours after the termination of the Extended Cataplasm Test. The Extended Cataplasm Test is described in detail in [26]. We assume that the Extended Cataplasm Test can simulate the external environment in UV-protected joints (e.g. by suitable geometry and substrate material) by changing temperatures and relative humidity. Lin and Chen [27] and Viana et al. [28] published that if a water saturated adhesive is subjected to a dry environment, it loses all the water that was absorbed and due to cracks which were created or enlarged during ageing, the speed of subsequent diffusion sometimes increases. The process of moisture sorption–desorption–resorption can occur in real conditions as well as in the Extended Cataplasm Test in the immersion – drying – high relative humidity cataplasm part of the test.

2.4 Investigation Tools 2.4.1 Mechanical tests - double lap shear test The shear stress-strain analysis was carried out on double lap shear specimens. The adhesive was applied in two bonded areas 12 x 50 mm where the overlapping length was 12 mm, see Figure 2. In the case of blank aluminium specimens, the middle sheet was made of AlMgSi0,5 and sideways sheets of AlMg1 to compare adhesion to both alloys. The mechanical load was applied at a fixed displacement rate of 5 mm/min for the STP adhesive and 1 mm/min for the ADP adhesive, which creates similar strain rates for both adhesives applied in different thicknesses. Deformation was measured by four linear potentiometric transducers, thus each bonded area was measured by two transducers and the average value of deformation was calculated for the evaluation of the results.

Figure 2. Tested specimen with the depicted bonded area, dimensions in mm.

2.4.2 Water Absorption The water absorption ability of bulk adhesives can play an important role in the ageing resistance of adhesive joints. Water in the polymer of the adhesive can lead to chemical changes by hydrolysis and can change mechanical properties (softening and strength decrease), which is related to the glass transition temperature decrease. Absorbed water may move to the interfaces between the adhesive and the substrate where adhesion strength is lowered by the accumulation of water molecules and by the creation of a corrosion products layer [29, 30]. For that reason, water absorption is an important characteristic of adhesives in studying adhesive joint weathering resistance [31]. The water absorption of selected adhesives was found out in accordance with ISO 62:2008, Method 2. Three square-shaped specimens with dimensions of 60 x 60 mm and a thickness of 1 mm were prepared by moulding for each adhesive. The dimensions meet the criteria for the mould shape type D2 according to ISO 294-3:2002. Water sorption and desorption were performed by using the KERN ABJ 220-4NM analytical balance with a readability of 0.1 mg. First, the STP polymer specimens were dried in an oven at 50°C for 8 days, and the acrylate adhesive specimens for 11 days, with regular weighing every 24 hours until their weight was constant within ±0.1 mg. Then, the test specimens were immersed in boiling distilled water for 30 minutes. They were weighed after 15 minutes of cooling in water at room temperature and all drops of water on the specimen surface had been wiped out. The cycle of immersion and measurement was repeated 7-times for the STP polymer and 15-times for the acrylate adhesive until full saturation.

After immersion, final drying to recondition the test specimens was carried out. Final drying was performed in the same way as the initial drying described above.

2.4.3 Infrared spectroscopy After all three ageing scenarios, both tested adhesives were subjected to infrared spectroscopy to monitor the chemical structure of the adhesives and changes after ageing. The materials were tested with IR analysis using the FTIR Nicolet 6700 spectrometer (Thermo Scientific). The measuring device consists of a HeNe laser, the EverGloTM high - intensity IR source, an interferometer with a KBr beamsplitter, moving and static mirrors covered with gold, the HP DLaTGS pyroelectric detector, and an interchangeable sample compartment. The FTIR spectrometer operates in a 4 000 – 400 cm-1 spectral range with a 4 cm-1 spectral resolution. The spectra were collected after 32 scans in the absorbance mode using the attenuated total reflectance (ATR) diamond crystal. The spectral regions were normalised to enable the comparison of individual samples.

2.4.4 X-ray diffraction and X-ray fluorescence In order to find the presence of the inorganic phase, such as fillers or pigments, X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were performed on the specimens unexposed to ageing and on leachates (dry matter after evaporation of water) after the immersion test. The PANanalytical X´pert PRO X-ray diffractometer with Cu anodic material and the AXIOS XRF spectrometer with Rh anodic material were used for both analyses. The FTIR Nicolet 6700 spectrometer described above was used for the examination of leachates.

2.4.5 Dynamic mechanical thermal analysis (DMA) Amorphous polymers while heated change their state from glassy to rubbery one at a certain temperature. The elastic (energy stored) modulus E´ and the imaginary (energy loss, absorbed) E´´ modulus are recorded. The load-deformation response of the specimen is used to obtain information about the ratio of the loss and the storage modulus (E´´/E´), reported as the damping factor (tan δ), of the material over a temperature range. In the region of Tg, polymers behave significantly viscoelastically, which is shown by an increase in the damping factor tan δ. The elastic modulus E´ falls and the energy loss modulus E´´ shows a peak [32].

The main advantage of DMA with adhesives is

determining Tg since the storage and loss moduli show obvious changes in the Tg region and tan δ reaches its maximum. DMA is considered to be the most sensitive of the thermal analysis techniques for determining Tg [33, 34]. Dynamic mechanical thermal analysis (DMA) was used to determine the glass transition temperature (Tg) of both adhesives. Three specimens for each adhesive were tested before (reference set) and after immersion in water according to ETAG002. Specimens with dimensions of 30 x 5 x 1 mm were dynamically loaded in bending as a single cantilever at a fixed frequency of 1 Hz in a temperature interval from -80 to + 20 °C. The measurements were carried out by using DMA DX04T at the Department of Polymers of The University of Chemistry and Technology in Prague. After immersion according to ETAG 002, the specimens were immediately placed into plastic zipped bags to protect

them against the evaporation of moisture. The DMA measurement was carried out 7 days after the termination of ageing.

3. Results 3.1 Mechanical properties of joints For each of the four tested batches of specimens related to ageing procedures (reference, ETAG 002, neutral salt spray test, and Extended Cataplasm test), engineering stressengineering strain diagrams are presented in Figure 3 a) for aluminium substrates and Figure 3 b) for zinc-electroplated steel substrates. The STP adhesive joints showed a significant drop in shear strength and the shear modulus after exposure to the Extended Cataplasm Test and after immersion in water according to ETAG 002. The neutral salt spray test caused a negligible worsening of mechanical properties for aluminium substrates, and a significant degradation of the joint was observed on Zn-electroplated steel specimens. The average values of the maximum achieved shear stress and the prevailing failure mode for all batches of specimens are presented in Table 1. The cohesive failure mode was observed in the reference set for all substrate material groups. After the ageing exposure, combined adhesive-cohesive failure was mainly visible, see Table 1 and Figures 4-7 for typical failure modes for all four batches of specimens. For aluminium joints, two alloys were used – AlMgSi0.5 for middle metal sheets and AlMg1 for sideways sheets of specimens. In the case of the STP adhesive, adhesive modes of combined failure were predominantly observed in sideways metal sheets after

ageing. Therefore, the AlMg1 alloy showed worse adhesion than AlMgSi0.5. Detailed results of the failure modes for aluminium alloys are available in Table 2.

Figure 3. Shear stress – shear strain diagram for the STP polymer applied in: a) aluminium joints, b) Zn-electroplated steel joints.

Table 1. Average shear strength values and failure modes for Silane Terminated Polymer, C – cohesive mode, A – adhesive mode, A-C – combined mode, where the letter in bold means the prevailing mode of failure.

reference set Type of specimen (substrate/treatment)

Immersion in water (ETAG 002)

Salt spray test (ISO 9142)

Extended Cataplasm Test

av. shear mode av. shear strength of strength [MPa] failure [MPa]

mode av. shear mode of of strength failure failure [MPa]

av. shear strength [MPa]

mode of failure

Roughened aluminium

1.87±0.13

C

1.25±0.24

A-C

1.82±0.08

A-C

1.27±0.11

A-C

Aluminium (smooth)

1.89±0.19

C

1.38±0.14

A-C

1.93±0.08

A-C

1.35±0.18

A-C

Anodized aluminium

1.87±0.18

C

1.65±0.12

C

1.95±0.07

A-C

1.48±0.09

A-C

Roughened galvanized steel 1.69±0.12

C

1.16±0.14

A-C

1.15±0.14

A-C

0.93±0.16

A-C

C

1.07±0.29

A-C

0.84±0.15

A-C

1.16±0.18

A-C

Galvanized steel (smooth)

1.72±0.11

Table 2. Prevailing adhesive mode of failure related to aluminium alloys used in STP adhesive joints. Silane Terminated Polymer set of specimen

adhesive failure - Al alloy

Note

REF

-

cohesive failure

ETAG

mostly AlMg1 (64% )

-

NSS

mostly AlMg1 (95% )

-

CP

mostly AlMg1 (78%)

-

Figure 4. STP adhesive: cohesive failure of the reference specimen.

Figure 5. STP adhesive: combined adhesive-cohesive failure of joints exposed to ETAG 002 ageing.

Figure 6. STP adhesive: combined adhesive-cohesive failure of joints exposed to the neutral salt spray test.

Figure 7. STP adhesive: combined adhesive-cohesive failure of joints exposed to the Extended Cataplasm Test.

Summarized results for the acrylate adhesive are presented as engineering stressengineering strain diagrams in Figure 8 a) for aluminium substrates and Figure 8 b) for zinc-electroplated steel substrates. The average values of the maximum achieved shear stress and the prevailing failure mode for all batches of specimens are shown in Table 3.

Figure 8. Shear stress – shear strain diagram for the ADP adhesive polymer applied in: a) aluminium joints, b) Zn-electroplated steel joints.

Table 3. Average shear strength values and failure modes for the ADP adhesive, C – cohesive mode, A – adhesive mode, A-C – combined mode, where the letter in bold means the prevailing mode of failure; * lower RH in the cataplasm part of the ageing procedure

reference set Type of specimen (substrate/treatment)

Roughened aluminium

Immersion in water (ETAG 002)

av. shear mode av. shear strength of strength [MPa] failure [MPa] 9.30±0.66

C

Salt spray test (ISO 9142)

mode av. shear mode of of strength failure failure [MPa]

Extended Cataplasm Test av. shear strength [MPa]

mode of failure

7.44±0.78

C

4.84±1.51

A-C

8.84±0.65*

C*

Aluminium (smooth)

8.43±0.87 A-C

5.78±1.69

A-C

3.36±0.50

A-C

6.80±1.67

A-C

Anodized aluminium

8.47±0.63

2.28±0.49

A

3.48±0.46

A

4.75±0.37

A

2.40±0.38

A-C

2.48±0.38

A-C

2.23±0.47

A-C

1.39±0.53

A

1.27±0.55

A

1.67±0.41

A-C

C

Roughened galvanized steel 6.24±1.18 A-C Galvanized steel (smooth)

4.68±0.79 A-C

The significant influence of the substrate material was observed in the reference set of specimens, where zinc-electroplated steel showed worse results than aluminium. Also, the positive effect of surface roughening on adhesion was observed for all batches of specimens. The effect of artificial ageing was significant for all three batches exposed to laboratory ageing procedures. A drop in strength depended on the substrate material. While the neutral salt spray test caused a considerable shear strength reduction of aluminium joints, immersion in water (ETAG 002) caused a significant worsening of anodized aluminium joints in strength. Furthermore, immersion in water caused a significant reduction of the shear modulus for all tested substrate materials. Roughened aluminium joints exposed to the Extended Cataplasm Test showed similar results as the reference set. It was caused by an accident during the cataplasm part of ageing (70 °C and 100% RH for 7 days), thus, the specimens were exposed lower relative humidity in this part. The other parts of the Extended Cataplasm Test (immersion, heating and freezing conditions), see paragraph 2.3, were performed accordingly.

The full cohesive mode of failure was observed only for roughened and anodized aluminium in the reference set of specimens. Zn-electroplated steel specimens were typically broken by a combined adhesive-cohesive mode of failure with a predominant adhesive manner. After ageing, adhesion forces were lowered and combined or the full adhesive failure mode was observed, typical failure modes are depicted in Figures 9-12.

Figure 9. ADP adhesive: combined cohesive-adhesive failure of the reference aluminium specimen.

Figure 10. ADP adhesive: combined adhesive-cohesive failure with the prevailing adhesive manner in a Zn-electroplated steel joint exposed to ETAG 002.

Figure 11. ADP adhesive: combined adhesive-cohesive failure with the prevailing adhesive manner in an aluminium joint exposed to the neutral salt spray test.

Figure 12. ADP adhesive: adhesive failure mode in an anodized aluminium joint exposed to the Extended Cataplasm Test.

For aluminium joints, the adhesive mode of combined failure was predominantly observed for sideways metal sheets (AlMgSi0.5) both for the reference set and the sets after ageing. Therefore, the AlMgSi0.5 alloy showed worse adhesion than AlMg1, which is in contrast to the STP adhesive. Detailed results of the failure modes for aluminium alloys are available in Table 4. Table 4. Prevailing adhesive mode of failure related to aluminium alloys used in acrylate adhesive joints. Acrylate adhesive set of specimens

adhesive failure - Al alloy

Note

REF

mostly AlMgSi0.5 (70%)

in the case of smooth Al only

ETAG

AlMgSi0.5 (100%)

in the case of smooth Al only

NSS

mostly AlMgSi0.5 (66%)

-

CP

AlMgSi0.5 (100%)

in the case of smooth Al only

3.2 X-ray diffraction and X-ray fluorescence before ageing tests

The presence and quantification of elements obtained by X-ray fluorescence analysis for the Silane Terminated Polymer adhesive are shown in Table 5 and the corresponding diffraction pattern of the phase presence in Figure 13. The STP adhesive proved a significant content of TiO2 – a white stable pigment, which was also visible in the

leachate after the immersion test according to ETAG002, and CaCO3,which provides more readily a rheological function. The STP adhesive contains a considerable portion of the amorphous phase, which is probably composed of finely ground SiO2 and iron oxides. Compared to the STP adhesive, the ADP adhesive contains only CaCO3 and a considerable portion of the amorphous phase. Calcium carbonate if it contains cementbase impurities, or the amorphous content (a mixture of substances) may give the adhesive grey colour. The X-ray fluorescence results of the ADP adhesive are shown in Table 6. and the results of diffraction in Figure 13.

Table 5. Presence and quantification of elements for STP adhesive element presence [wt. %] Si 0.12 Ca 31.05 Ti 7.06 Fe 0.07 summarized 38.40

Table 6. Presence and quantification of elements for ADP adhesive element presence [wt. %] Si 0.01 Ca 29.85 Ti Fe 0.33 Mn 0.02 Zn 0.08 summarized 30.40

Figure 13. XRD spectral analysis.

3.3 Water Absorption

For each test specimen, the change in mass c relative to the initial mass was calculated by using the formula:
=

  

∗ 100%,

(1)

where m1 is the mass of the test specimen after initial drying and before immersion, m2 is the mass of the test specimen after immersion, and m3 is the mass of specimens after immersion and final drying. Both adhesives showed a water-soluble matter loss during immersion which was defined as the difference between the initial mass and the mass after final drying. The experimental data were fitted with the Fickian diffusion model. The Fickian diffusion coefficients D for both adhesives were obtained using the procedure described in ISO62:2008, and are shown in Table 7 together with the arithmetic means of the change in mass c relative to the initial mass and the water-soluble matter loss.

Table 7. Key properties (arithmethic means) for both adhesive´s diffusion behaviour Property Change in mass c relative to the initial mass Water soluble matter loss Diffusion coefficient D

STP adhesive

ADP adhesive

Units

2.5

6.2

%

0.5 8.18 x 10-5

1.8 10.12 x 10-5

% mm2/s

We can notice that the ADP adhesive is able to absorb almost a 2.5-times higher content of water (calculated without the influence of water-soluble matter) in a similar or a slightly shorter time than the STP adhesive. Moreover, we can observe a higher amount of water soluble matter in the ADP adhesive. These properties of both adhesive´s

diffusion

behaviour indicate a lower resistance of the ADP adhesive to wet

environmental conditions.

3.4 Infrared spectroscopy

Firstly, the material major absorption bands identification and assignment were performed, the results are given in Tables 8 and 9. In order to find out how three different ageing procedures and the absorption test involve the material structure and the composition the tested adhesives spectra were compared with the reference one, Figures 14 and 15.

Table 8. Assignments of the major absorption bands of Silane Terminated Polymer (STP) [35]. Wavenumber/cm-1 3444, 3316 2967 2929 2850 1730 1436 1290 1113 875 720 712

Assignment ν (O-H) in Si-OH νas (C-H) in –CH3 νs (C-H) in –CH3 νs ( C-H) in –CH2 ν (C=O) δ (C=C-H) ρ (C-H) in Si-CH2 νas (Si-O-Si) ρ (C-H) in –CH3 ν (Si-O-C) ρ (C-H) in –CH2

Table 9. Assignments of the major absorption bands of the two-part acrylate adhesive (ADP) [36]. Wavenumber/cm-1 3395 2953 2922

Assignment ν (O-H) νas (C-H) in –CH3 νs (C-H) in –CH3

2872 1729 1448 1240 1153 1086 1019 875 712

νs ( C-H) in –CH2 ν (C=O) δ (C-H) in –CH2 νs (C-O-C) νas (C-O-C) δ (C-H) νs (C-O-C) ρ (C-H) in –CH3 ρ (C-H) in –CH2

Figure 14. Comparison of the reference and aged samples of Silane Terminated Polymer (STP)

Figure 15. Comparison of the reference and aged samples of the two-part acrylate adhesive (ADP).

As we can see, both tested adhesives can absorb water groups O-H (hydrolysis) during ageing, which corresponds to higher intensities at 3316 and 3395 cm-1 for the STP or ADP adhesive, respectively. In the case of the STP adhesive, the neutral salt spray test caused hydrolysis. The water groups O-H bonding and the destabilisation of C=C-H bonds are clearly visible. For the ADP adhesive, great changes can be seen in the ETAG and NSS ageing tests, the long-term influence of water and higher temperatures lead to the decomposition of the polymeric structure.

3.5 Infrared spectroscopy and X-ray fluorescence after the ageing test A fine precipitate formed in the bath during the ageing test according to ETAG 002 in the case of both adhesives. The STP adhesive gave a white sediment in the bath while the ADP adhesive gave a light brownish sediment. Both sediments were exposed to

evaporation to gain a dry matter which was subjected to X-ray fluorescence and infrared spectroscopy. The STP adhesive X-ray fluorescence results are summarized in Table 10 and the ADP adhesive results in Table 11. The infrared spectroscopy analysis of sediments (dry matter) of both adhesives is shown in Figure 16.

Table 10. Presence and quantification of elements for the STP adhesive (dry matter after evaporation) gained by XRF.

element Si Ca Ti Fe summarized

presence [wt. %] 0.01 9.67 1.41 0.01 11.10

Table 11. Presence and quantification of elements for the ADP adhesive (dry matter after evaporation) gained by XRF. element Si Ca Ti Fe Mn Zn summarized

presence [wt. %] 13.83 0.14 0.02 13.99

Figure 16. FTIR spectra of both adhesives dry matter (after immersion).

The results show that in the case of the STP adhesive, mainly CaCO3 (in the minority also TiO2 from the adhesive structure) is separated after the exposure. In the case of the

ADP adhesive, CaCO3 is also separated, but the infrared spectroscopy results show that C-H fragments from short aliphatic chains were found in the sediment (dry matter). It confirms a higher level of degradation (decomposition) of this adhesive.

3.6 Glass transition temperature determination by DMA The value of Tg corresponds to the maximum of the damping factor (tan δ), which is depicted in Figures 17 a) and 18 a). Figures 17 b) and 18 b) show the corresponding elastic moduli E´ for all tested specimens. The average values of the glass transition temperature Tg for all tested sets of specimens are summarized in Table 12. Immersion in warm water for three weeks induced a small decrease of the glass transition temperature and a decrease of tan δ in the ADP adhesive. Furthermore, the evolution of tan δ versus temperature changed from a single to double peak shape. The presence of two maxima may result from inhomogenous absorption of water [23]. In the case of aged specimens, the modulus E´ decrease was observed in a much wider temperature interval (from -60 to 0°C) than for the reference set of specimens. As a result, it can be stated that the transition interval was significantly extended, which could be the result of plasticization by water uptake. Almost no changes in Tg were observed for the STP adhesive after ageing. The immersion test caused a decrease of Tg by an average of 1°C, but no changes in the modulus E´ were observed. It indicates a much better resistance of the STP adhesive to the presence of water than the ADP adhesive.

(a)

(b)

Figure 17. DMA results for the STP adhesive: (a) damping factor versus temperature, (b) elastic modulus E´ versus temperature.

(a)

(b)

Figure 18. DMA results for the ADP adhesive: (a) damping factor versus temperature, (b) elastic modulus E´ versus temperature.

Table 12. Summary of glass transition temperature measurements – average values.

Set of specimens STP adhesive- reference STP adhesive – ETAG ADP adhesive - reference ADP adhesive - ETAG

average value of Tg [°C] -59 -60 -53 -55/-6

4. Discussion and interpretation of results 4.1 Mechanical properties Both chosen adhesives exposed to three different ageing scenarios showed a different effect on their mechanical properties. Also, the influence of the substrate material and its roughness was observed. In order to better identify the degradation effect of ageing on the mechanical properties of all kinds of tested specimens, the strength reduction (SR) and the initial shear modulus reduction (GR) were determined, using the following equation:

% =

  ∙ 

100,

(2)

where τ0 is the mean value of shear strength of a particular batch of specimens of the reference set and τA is the average value of shear strength of the corresponding batch of specimens exposed to laboratory ageing; and % =

  

∙ 100,

(3)

where G0 is the mean value of the initial shear modulus of a particular batch of specimens unexposed to ageing and Gt is the average value of initial shear strength of the corresponding batch of specimens exposed to laboratory ageing. The initial shear modulus was calculated as a secant shear modulus with an engineering strain range of 0.01 and 0.05. The initial shear strain value 0.01 was chosen to eliminate inaccuracies at the beginning of the loading procedure. Figure 19 a), b) shows that the Extended Cataplasm Test had a serious degrading effect on the mechanical properties of the STP adhesive, however, immersion in water according to ETAG 002 proved almost the same deterioration for aluminium and Znelectroplated steel substrates. For anodized aluminium, a greater drop in shear strength

was observed in specimens exposed to the Extended Cataplasm Test only, while the shear modulus reduction was similar for both procedures. The neutral salt spray test proved the reduction of the average shear strength by 32% for roughened and 51% for smooth Zn-electroplated steel substrates, which was probably caused by an extensive creation of corrosion products at the interface, which significantly lowered adhesive strength and led to an overall strength reduction of the joint. The shear strength and shear modulus reductions together with failure modes, presented in Table 1, indicate a relatively strong interphase in the case of anodized aluminium specimens and the STP adhesive. A full cohesive or prevailing cohesive failure mode was observed and less strength reduction was measured while the adhesive was influenced by the water absorption rise from laboratory ageing. Also, we can observe almost no deterioration of shear strength for blank and anodized aluminium after NSS and shear modulus reduction together with hydrolysis. Some adhesives can even show increased mechanical properties as Bjørgum [11] observed. It can be explained by the creation of a stronger interphase according to research published by Arslanov and Kalashnikova et al. [37, 38] and not by additional hardening of the adhesive by ageing, as the strength increase is sometimes explained.

Figure 19. Comparison of shear strength reduction and initial shear modulus reduction for all ageing procedures.

Figure 19 c) and d) shows the shear modulus reduction of the ADP adhesive by more than 70% for all kinds of substrate after immersion in water. Also, a significant reduction of shear strength (more than 60%) for Zn-electroplated steel and anodized aluminium, together with a full adhesive or prevailing adhesive mode of failure (see Table 3), was observed. For this reason, immersion in water according to ETAG 002 was found as the most harmful ageing scenario for the two-part acrylate adhesive, however, all three ageing procedures caused a significant deterioration. For example, the neutral salt spray test reduced the shear strength of smooth aluminium by 60% and its shear modulus by 57%. A similar deterioration was also observed for anodized aluminium specimens exposed to NSS. A weak interphase, observed by the adhesive failure mode and a

significant reduction of shear strength due to ageing was created in all anodized aluminium specimens exposed to all three ageing procedures. It can be most probably caused by an insufficient interlocking effect by the viscous and rigid ADP adhesive as Kinloch and Dillard [15,16] described. Moreover, a higher extent of the adhesive failure and lower strength for sulphuric acid anodized specimens can be explained by the presence of sulphates in SAA oxides. Sulphates are known to be less stable in wet conditions [11], but it does not explain a good adhesion of the STP adhesive to anodized aluminium.

4.2 Comparison of ageing procedures effect on both adhesives FTIR analysis generally proved the least degrading effect of the Extended Cataplasm Test on the structure of both adhesives. Hydrolysis, the formation of hydrogen chains and substitution and/or addition of OH- groups, is shown primarily after NSS and ETAG for both adhesives. However, the greatest degree of hydrolysis damage is evident after the ETAG test for the two-part acrylate adhesive. A significant rate of distilled water hydrolysis in the acrylate polymer together with a higher water absorption rate have been discussed earlier in [39]. The ADP adhesive also suffers by the decomposition of the polymer structure. FTIR spectra showed C-H fragments from short aliphatic chains in the sediment after the immersion test, which confirms a higher level of degradation (decomposition) of the adhesive. The degradation of the polymer structure was accompanied by a significant drop in mechanical properties after the ETAG 002 test. Especially the shear modulus was reduced by more than 70% for all types of substrates.

Moisture-induced plasticization is often accompanied by the lowering of adhesive´s Tg [28]. The ADP adhesive showed changes in the glass transition interval, where the double peak of the damping factor (tan δ) at temperatures -55°C and -6°C was formed together with the extension of the temperature interval of the modulus E´ decrease than in the reference specimens. It indicates the extension of the transition interval as a result of water uptake. According to literature [28,40], Tg normally decreases with water absorption, but in some cases it can slightly increase due to the creation of secondary cross-linking between the main polymer and water molecules. In the case of the ADP adhesive, changes of the chemical structure resposible for the extension of the glass transition temperature interval obviously occurred. The effect of chloride anions in NSS ageing on the hydrolysis reaction of the ADP adhesive is probably very minimal. On the contrary to the ADP adhesive, NSS caused a significant formation of hydrolysis as was shown in the FTIR spectra in the case of the STP adhesive. It appears that the presence of chloride anions facilitates hydrolysis, probably due to their higher selective absorption for this adhesive and the polarity of Cl[41, 42]. Consequently, the shear stiffness of the STP adhesive was reduced after NSS by 20% and its shear strength was reduced by more than 30% for roughened and 50% for smooth Zn-electroplated steel substrates. Furthermore, we observed that hydrolysis firstly starts on the surface of the adhesive layer, i.e., at the adhesive/metal interface, whereas in the bulk, hydrolysis has not begun yet, as Calvez [23] similarly observed. The FTIR spectra of adhesives after all ageing procedures were examined close to the edge of the bonded area, i.e., in places where hydrolysis had begun. It means, after 3 weeks in NSS, hydrolysis of the STP adhesive

began close to the adhesive layer surface, whereas, in the bulk adhesive, hydrolysis probably had not begun. It was also accompanied by an adhesive mode of failure caused by the formation of corrosion products in locations close to the edges of the adhesive layer, whereas the centre of the adhesive layer showed a cohesive failure mode. Probably, after a longer exposition time to NSS, the STP adhesive in aluminium joints would show a worsening of mechanical properties and adhesion to the substrate due to further formation of corrosion products and progressing hydrolysis. The comparison of three various ageing procedures focused on hydrothermal ageing is quite a complicated task due to different effects on the chosen adhesives given by the different chemical nature of the adhesives. Generally, we can say that the most labourious procedure – the Extended Cataplasm Test, due to several changes in ageing conditions within the whole procedure, was expected as the most suitable for the simulation of real ageing conditions without exposure to UV light. The Extended Cataplasm Test is a promising procedure thanks to the combination of high and freezing temperatures, including quick changes causing thermal stresses [28], high relative humidity, immersion in water, and drying, it provided a slightly less harmfull effect on both adhesives than ETAG ageing or NSS procedures. However, its effect on the STP adhesive´s mechanical properties was also significant. The most harmful effect on both adhesive´s chemical structure was that of the ETAG procedure together with NSS, which was shown by FTIR analysis. The NSS ageing test can be undoubtebly useful for joints in seaside locations where Cl- ionts can influence the moisture degradation process. All the three chosen ageing procedures can create a weak interphase depending on the adhesive and the substrate and its pre-treatment. This

confirms that the substrate, pre-treatment and the adhesive have to be considered as a complex, interactive and dynamic system [44]. The effect of labortory ageing procedures should be compared with real environmanetal ageing as the next step of assessment to determine the most suitable method for the simulation of a real environment for building facade joints. Furthermore, the water uptake behaviour of both adhesives was examined in boiling water. It is known that the water uptake behaviour of adhesives depends greatly on the environmental conditions [28] and thus direct comparison with joints exposed to a real environment is complicated. Moisture and water uptake of the adhesive also depends on its stress-state and real joints are always subjected to some kind of stress [43].

5

Conclusion

The effect of three various ageing procedures (immersion according to ETAG 002, neutral salt spray test according to EN ISO 9142 and Extended Cataplasm Test) was studied through changes in mechanical properties and failure modes together with chemical changes investigated by infrared spectroscopy, X-ray fluorescence, X-ray diffraction, water absorption and glass transition temperature changes. The main results are summarized below: •

STP adhesive provides relatively good water/moisture resistance in comparison to the ADP adhesive;

•

STP adhesive showed lower water absorption than the ADP adhesive – the average change in mass relative to the initial mass was almost 2.5 times higher for the ADP adhesive;

•

neutral salt spray test (chloride anions) facilitates hydrolysis of the STP adhesive in contrast to the ADP adhesive;

•

adhesion to various metals (alloys AlMg1, AlMgSi0.5, Zn-electroplated steel, sulphuric acid anodized AlMgSi0.5) was observed for reference and ageing test specimens: the AlMg1 alloy showed worse adhesion to the STP adhesive, AlMgSi0.5 showed worse adhesion to the ADP adhesive, Zn-electroplated steel proved lower ageing resistance due to the formation of corrosion products and it also showed worse adhesion to the ADP adhesive;

•

ADP adhesive has lower ageing resistance due to the decomposition of the polymer structure together with hydrolysis, which was observed after immersion according to ETAG 002;

•

ADP adhesive’s low moisture/water resistance was also confirmed by changes in the glass transition temperature interval;

•

ETAG 002 and NSS ageing procedures caused the most harmful degradation both of joints and the polymer structure;

•

substrate, its pre-treatment and the adhesive have to be considered as a complex system in given environmental conditions.

Acknowledgement The authors gratefully acknowledge funding from the Czech Science Foundation, under the grant GA18-10907S.

References [1] Nečasová B, Liška P, Kelar J, Šlanhof J. Comparison of Adhesive Properties of Polyurethane Adhesive System and Wood-plastic Composites with Different Polymers after Mechanical, Chemical and Physical Surface Treatment. Polymers. 2019;11(3):397. https://doi.org/10.3390/polym11030397 [2] Ciupack Y, Pasternak H, Mette C, Stammen E, Dilger K. Adhesive Bonding in Steel Construction - Challenge and Innovation. Procedia Engineering. 2017;172:186-193. https://doi.org/10.1016/j.proeng.2017.02.048 [3] Aßmus E, Popp C, Weller B. Permanent Hydrothermal Exposure on Load-bearing Adhesives in Glass Constructions. In: Challenging Glass Conference Proceedings, vol. 6. Challenging

Glass

6.

Delft:

TU

Delft;

2018.

pp.

299-308.

https://doi.org/10.7480/cgc.6.2154 [4] Nicklish F, Weller B. Adhesive Bonding of Timber and Glass in Load-bearing Facades – Evaluation of the Ageing Behaviour. In: WCTE 2016 CD-ROM Proceedings World Conference on Timber Engineering WCTE 2016. Wien: Vienna University of Technology, 2016 [5] Bues M, Schuler C, Albiez M, Ummenhofer T, Fricke H, Vallée T. Load bearing and failure behaviour of adhesively bonded glass-metal joints in facade structures. The Journal

of

Adhesion.

http://doi.org/10.1080/00218464.2019.1570158

2019;95(5-7):653-674.

[6] Kalamar R, Bedon C, Eliášová M. Experimental investigation for the structural performance assessment of square hollow glass columns. Engineering Structures. 2016;113:1-15. https://doi.org/10.1016/j.engstruct.2016.01.028 [7] Dias V, Odenbreit C. Investigation of hybrid steel-glass beams with adhesive silicone shear

connection.

Steel

Construction.

2016;9(3):207-221.

https://doi.org/

10.1002/stco.201400004 [8] Overend M, Jin Q, Watson J. The selection and performance of adhesives for a steel– glass connection. International Journal of Adhesion and Adhesives. 2011;31(7):587-597. https:/doi.org/10.1016/j.ijadhadh.2011.06.001 [9]

Kozłowski M, Kadela M, Hulimka J. Numerical Investigation of Structural

Behaviour of Timber-Glass Composite Beams. Procedia Engineering. 2016;161:9901000. https://doi.org/10.1016/j.proeng.2016.08.838 [10] Albiez M, Vallée T, Fricke H, Ummenhofer T. Adhesively bonded steel tubes — Part I: Experimental investigations. International Journal of Adhesion and Adhesives. 2019;90:199-210. https://doi.org/10.1016/j.ijadhadh.2018.02.005 [11] Bjørgum A, Lapique F, Walmsley J, Redford K. Anodising as pre-treatment for structural bonding. International Journal of Adhesion and Adhesives. 2003;23(5):401412. https://doi.org/10.1016/S0143-7496(03)00071-X [12] Delozanne J, Desgardin N, Coulaud M, Cuvillier N, Richaud E. Failure of epoxies bonded assemblies: comparison of thermal and humid ageing. The Journal of Adhesion. 2018;:1-24. https://doi.org/10.1080/00218464.2018.1547198 [13] ETAG 002. Guidelines for European Technical Approval for Structural Glazing System, EOTA European Organisation Technical Approvals, Brussels, 1999 [14] Specifications for the QUALANOD quality label for sulphuric acid-based anodizing of aluminium, edition 1.1.2019, Qualanod, Zurich, 2019

[15] Dillard D. Advances in structural adhesive bonding. Woodhead Publishing; 2010, pp. 196-202 [16] Kinloch A, Little M, Watts J. The role of the interphase in the environmental failure of

adhesive

joints.

Acta

Materialia.

2000;48(18-19):4543-4553.

https://doi.org/10.1016/S1359-6454(00)00240-8 [17] Krüger G, Schneider R, An eleastic adhesion system for structural bonding of facade panels.

Otto-Graf-Journal,

1999,

vol.

10,

pp.

87-98.

available

online

at:

https://www.mpa.unistuttgart.de/publikationen/otto_graf_journal/ogj_1999/beitrag_krueger.pdf

(accessed

5.9.2019) [18] Sitte S, Brasseur M, Carbary L, Wolf A, Wolf A, Dean S. Preliminary Evaluation of the Mechanical Properties and Durability of Transparent Structural Silicone Adhesive (TSSA) for Point Fixing in Glazing. Journal of ASTM International. 2011;8(10):104084. https://doi.org/10.1520/JAI104084 [19] Pasternak H, Ciupack Y. Development of Eurocode-based design rules for adhesive bonded joints. International Journal of Adhesion and Adhesives. 2014;53:97-106. https://doi.org/10.1016/j.ijadhadh.2014.01.011 [20]

Nečasová B, Liška P, Novotný M. Ageing of Adhesive Joints for Facade

Applications – Comparison of Artificial and Real Weathering Conditions. MATEC Web of Conferences. 2019;279:02013. https://doi.org/10.1051/matecconf/201927902013 [21]

Ashcroft I, Digby R, Shaw S. A Comparison of Laboratory-conditioned and

Naturally-weathered Bonded Joints. The Journal of Adhesion. 2001;75(2):175-201. https://doi.org/10.1080/00218460108029600 [22]

Van Lancker B, Dispersyn J, De Corte W, Belis J. Durability of adhesive glass-

metal connections for structural applications. Engineering Structures. 2016;126:237-251. https://doi.org/10.1016/j.engstruct.2016.07.024

[23]

Calvez P, Bistac S, Brogly M, Richard J, Verchère D. Mechanisms of Interfacial

Degradation of Epoxy Adhesive/Galvanized Steel Assemblies: Relevance to Durability. The

Journal

of

Adhesion.

2012;88(2):145-170

https://doi.org/10.1080/00218464.2012.648067 [24]

Bowditch M. The durability of adhesive joints in the presence of water.

International

Journal

of

Adhesion

and

Adhesives.

1996;16(2):73-79.

https://doi.org/10.1016/0143-7496(96)00001-2 [25]

Machalická K, Vokáč M, Eliášová M. Influence of artificial aging on structural

adhesive connections for facade applications. International Journal of Adhesion and Adhesives. 2018;83:168-177. https://doi.org/10.1016/j.ijadhadh.2018.02.022 [26]

Machalická K, Vokáč M, Kostelecká M, Eliášová M. Durability of structural

adhesive joints for facade applications exposed to the extended cataplasm test. The Journal of Adhesion. 2019;1-21. https://doi.org/10.1080/00218464.2019.1570157 [27] Lin Y, Chen X. Moisture sorption–desorption–resorption characteristics and its effect on the mechanical behavior of the epoxy system. Polymer. 2005;46(25):1199412003. https://doi.org/10.1016/j.polymer.2005.10.002 [28] Viana G, Costa M, Banea M, da Silva L. A review on the temperature and moisture degradation of adhesive joints. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. 2016;231(5):488-501. https://doi.org/10.1177/1464420716671503 [29]

Adams R, Comyn J, Wake W. Structural adhesive joints in engineering. 2nd ed.

London: Chapman & Hall; 1997, pp. 211-220 [30]

Kinloch A. Adhesion and Adhesives - Science and technology. Chapman and

Hall: Springer Netherlands; 1987, pp. 353-362

[31]

Sato C. Influences of Water on the Adhesive Properties. In: da Silva LFM, Sato C,

editors. Design of Adhesive Joints Under Humid Conditions, Advanced Structural Materials 25, Berlin Heiderberg: Springer-Verlag; 2013, pp. 77-94. [32]

Menard K. Dynamic mechanical analysis. Boca Raton: CRC Press; 1999

[33]

Petrie E. Handbook of adhesives and sealants. New York: McGraw-Hill; 2009

[34]

Comym J. Thermal Properties of Adhesives. In: da Silva LFM, Öchsner A,

Adams RD, editors. Handbook of adhesion technology. Heidelberg: Springer; 2011, pp. 415-442 [35]

Huberab M. P.,

Kelch S., Berke H. FTIR investigations on hydrolysis and

condensation reactions of alkoxysilane terminated polymers for use in adhesives and sealants.

International Journal of Adhesion and Adhesives. 2016;64:153-162.

https:/doi.org/ 10.1016/j.ijadhadh.2015.10.014 [36]

López M., Fuentes G., González R., Peón E., Toledo C. PMMA/Ca2+ Bone

Cements. Parts I. Physico Chemical and Thermoanalytical Characterization. Latin American Applied Research. 2008;38(3):227-234 [37]

Arslanov V, Kalashnikova I. Spontaneous recovery of the strength of polymer-

aluminum adhesive systems in aqueous medium: I. Water transport and surface reactions. Colloid journal. 1996;58(6):697 – 706 [38] Kalashnikova I, Matveev V, Arslanov V. Spontaneous recovery of the strength of polymer-aluminum adhesive systems in aqueous medium: II. Structure-morphological and chemical transformations in the transition region. Colloid journal. 1996;58(6):722 729 [39] Weldon D. G. Failure Analysis of Paints and Coatings. Wiley (second edition), USA (2009)

[40] Zhou J, Lucas J. Hygrothermal effects of epoxy resin. Part II: variations of glass transition temperature. Polymer. 1999;40(20):5513-5522. https://doi.org/10.1016/S00323861(98)00791-5 [41] Owen, M. J., Dvornic P. R., Silicone Surface Science. Springer, USA (2012) [42] Syzcher M., Syzcher´s Handbook of Polyurethanes, CRC Press, London (2012) [43] Liljedahl C, Crocombe A, Wahab M, Ashcroft I. The effect of residual strains on the progressive damage modelling of environmentally degraded adhesive joints. Journal of

Adhesion

Science

and

Technology.

2005;19(7):525-547.

https://doi.org/10.1163/1568561054352513 [44] Loh W, Crocombe A, Abdel Wahab M, Ashcroft I. Modelling anomalous moisture uptake, swelling and thermal characteristics of a rubber toughened epoxy adhesive. International

Journal

of

Adhesion

https://doi.org/10.1016/j.ijadhadh.2004.02.002

and

Adhesives.

2005;25(1):1-12.