Gelatine adhesives, also known as animal glues, are collagen-based water-soluble biopolymers derived from vertebrate connective tissues. One of the various fields in which gelatine adhesives are widely used is the conservation of cultural heritage such as decorated furniture and panel paintings. It is observed that, with time, the failure in these objects often occurs along the adhesive bondlines. Given the moisture and temperature sensitivity of these adhesives, obtaining knowledge of their long-term behaviour, when exposed to climate variations, is pivotal. Here, the influence of hygrothermal ageing (exposure to a combination of elevated temperature and relative humidity (RH) cycling) on the microstructure and macroscopic properties of four different types of gelatine adhesives is investigated. These adhesives were selected from different animal origins namely bovine, rabbit, and fish with different Bloom strengths. It was observed that ageing cycles interfere with the most critical structural feature of protein chains namely triple helices. A clear decay in triple helix content at the micro-scale, determined by Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) techniques, was observed which had implications on the macroscopic properties of these adhesives such as reduction of strain to failure and toughness (strain energy density to failure). The rate of decay in properties was revealed to be the highest in the adhesives with the lowest triple helix content. This study provides a scientific view of microstructure-property relationships in gelatinous adhesives as a function of environmental ageing, and stipulates the underlying mechanism of the degradation of mechanical properties as the loss of structural triple helices, regardless of the animal origin.@en

The surface treatment of plastics as well as metals or ceramics includes a thorough surface cleaning as an essential step prior to adhesive bonding and coating processes. Besides this, surface activation of polymers is often needed because their surface free energy is too low for durable adhesion of a coating or adhesive. In this chapter various types of UV/Ozone sources with different light spectra as well as the influences of spectra and ozone concentration are investigated and compared. Also the surface wetting and adhesive bond strength as a result of UV/Ozone, atmospheric plasma, or corona treatments on thermoset, thermoplastic, and rubber materials are presented. UV/Ozone treatment was found to show an excellent cleaning performance on all kinds of materials, and especially as a very useful technique for surface functionalisation of polymers, resulting in durable adhesion both for adhesives as well as coatings. This chapter is a condensed overview of over 30 years of experiments done with UV/Ozone treatments at The Delft University of Technology.

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Archaeological findings prove the appearance and use of birch bark tar since the Middle Palaeolithic. The production and use of birch bark tar and pine wood tar has overlapped since at least the late Neolithic, but probably for much longer. The reliable chemical identification of such archaeological tar residues can offer valuable insights into, for example, ancient technical complexity, trade and culture. In this context, the scarcity of these mainly organic residue findings in the archaeological record bears the need for non-destructive analytical tools. However, there is currently no systematic proposed way for this purpose. We aim here to verify the organic nature and test the reliability of the identification of archaeological pine wood tar and birch bark tar with a combination of SEM-EDS, FTIR microspectroscopy in reflectance mode and XRD. We examined a set of experimental adhesive replicas of pine tar and birch tar in pristine form, but also after a three-year-long weathering experiment. Additionally, we studied a set of archaeological samples, consisting of Mesolithic bone/antler points with adhering hafting residues, form the Dutch North Sea. This research shows that degradation negatively influences the reliable verification and identification of the organic residue constituents significantly. SEM-EDS as a starting point of analysis verifies the residue's organic nature, but it cannot be used to identify birch or pine tar. XRD can identify crystalline additives in the adhesive mixture, like ochre and wax, as well as phases related to the artefact's environment of burial and provenance. Micro-FTIR is also capable of verifying the organic matter of the residue constituents. The differentiation of birch and pine tars is hindered by vibrational modes occurring in neighbouring wavenumbers for both tars, and by the limited research on degradation markers indicative of thermal treatment to prove tar production. Until reference collections also account for degradation and include a wide variety of adhesives, results of FTIR collected in reflectance mode are best treated with some caution.

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In marine structures adhesive joint structures are often exposed to cyclic conditioning where the ambient humidity changes cyclically during their service. Though some comprehensive studies on the aging of adhesives exist, these researches mainly focus on monotonic aging conditions where the adhesive joints are exposed to a wet condition continuously for a long time. However, the few investigations performed on the cyclic moisture absorption of adhesive materials show that the parameters obtained in monotonic aging conditions are not suitable for estimation of the aging behaviour of adhesive exposed to alternating humidity. An important question that can arise is whether or not this frequency affects the mechanical behaviour of adhesive joints under cyclic aging condition. In this investigation bulk dogbone and square samples were manufactured, subjected to cyclic aging conditions with four different aging frequencies and tested. The results show that the moisture diffusion constant of adhesives exposed to higher aging frequencies increase more than those exposed to lower aging frequency conditions. In addition, the moisture content only affects the degradation of strength and stiffness of the tested adhesives in different aging frequencies.@en

Lining techniques for the treatment of structurally damaged canvas paintings have been in use since at least the seventeenth century, with on-going invention, development, and refinement. These systems can be categorised based on their adhesive component – natural or synthetic – or by their application procedure, such as water-based, hot-melt, cold-lining, nap-bond lining, or mist-lining. The choice of lining system is often influenced by geographical practice and individual expertise rather than purely material-technical considerations, as comprehensive data for benchmarking different systems is limited in both literature and practice. This paper aims to address this gap in knowledge by comparing various lining adhesives and their application techniques. The adhesives under examination include glue-paste, wax-resin, BEVA® 371, Plextol® B500, and Dispersion K360:Plextol® D512. Mock-linings were designed to reduce variables and ensure standardisation. Previously reported recipes and descriptions of studio application techniques were used where possible to create the mock-linings. These were subsequently subjected to stress/strain through exposure to cyclic fluctuations in relative humidity (RH) and temperature (T) to simulate mechanical-physical ageing. Both unaged and aged samples underwent lap-shear and T-peel tests according to ASTM standards, as reported in earlier studies. The data presented here can assist conservators and scientists in establishing requirements and making informed decisions tailored to the specific needs of each painting. Results indicate that each lining technique has its own limitations, and the suitability of a given technique will depend on the type of treatment necessary for stabilising individual paintings.@en

Developing a bonded prestressed solution for strengthening structures utilizing an iron-based shape memory alloy (Fe-SMA), is of significant interest. This study is the first systematic investigation of adhesively-bonded Fe-SMA joints to achieve complete cohesive failure, which is an essential prerequisite for bond integrity. The Fe-SMA surface was prepared by combining UV/ozone exposure (UV), sol–gel (SG), and primers (P_C, P_NC), and the failure mode of the joint was investigated using the floating-roller peel test. Furthermore, the joint durability was studied through artificial aging using a salt spray cabinet. Cohesive failure was obtained for all investigated adhesives, and the effect of every surface preparation step was described. The application of sol–gel was found to be a crucial step in obtaining complete adhesive failure.

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Gelatine adhesives aka ‘animal glues’ are water-soluble biopolymers used in historic objects such as wooden cabinets and panel paintings since ancient times. This paper investigates the correlations between microstructural features, namely triple helices, and macroscopic properties of four different types of gelatine adhesives, prevalently used in conservation practices, irrespective of the animal origin. These adhesives include bovine bone, bovine skin, rabbit skin, and fish glues. Thin adhesive films were produced via solution casting methods in controlled climate conditions and their thermal and mechanical properties, and moisture sensitivity were investigated. XRD as a non-destructive characterisation method demonstrated good agreement with DSC in the quantification of gelatine adhesive (animal glue) triple helix content irrespective of the animal origin. Linear correlations between triple helices and gel (Bloom) strength and tensile strain energy to failure (toughness) were found for all adhesive types. Dynamic vapour sorption experiments demonstrated that lower triple helix content is correlated with higher moisture sensitivity of the adhesives. Moreover, the effect of environmental RH on the thermal behaviour of adhesives was investigated by DSC. The results demonstrated that the increase in environmental RH causes a reduction in the adhesives' glass transition and denaturation temperatures whilst triple helix content did not alter. Bovine bone glue with the lowest triple helix content showed the least toughness and highest moisture sensitivity, whilst fish glue with the highest triple helix content was identified as the most flexible glue.

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Adhesive joints are frequently exposed to cyclic ageing conditions during their service life, which can have a substantial impact on the mechanical properties of both the adhesive and the substrates. The safe life philosophy, commonly employed in the design of bonded joints, underscores the importance of obtaining an accurate estimate of the adhesive's durability. Therefore, it is essential to enhance the predictive capabilities of the adhesive's mechanical behavior under cyclic ageing conditions. This research aims to expand the use of quasi-static cohesive zone modelling (CZM) for damage and fracture analysis of dissimilar adhesive joints subjected to cyclic ageing environments. The first step involved measuring the mechanical properties of the adhesive through tensile tests on unaged and cyclically aged dogbone specimens, considering their moisture content and ageing cycles. Based on the results, a degradable CZM was developed. To validate the numerical model, dissimilar double cantilever beam specimens (DCBs) of glass fibre reinforced polymer (GFRP) and aluminium were manufactured and tested before and after ageing. The load-displacement curves of the bi-materials bonded joints were successfully predicted using the developed model where the properties of the material are defined as a function of the moisture uptake and ageing cycles at each material element. The obtained results showed that after 4 ageing cycles, the maximum load of DCB specimens decrease considerably.

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Arising from questions concerning the long-term stability of the mortar, this research establishes the composition of the epoxy resin mortar used for the construction of the dalle de verre lantern of Liverpool Metropolitan Cathedral. Water ingress had manifested itself early in the life of the lantern as a result of flaws in the mortar. These defects are described and illustrated herein. The paper also describes the process of characterising the epoxy resin, quartz flour, and carbon black in the lantern mortar in order to prepare mock-ups for weathering assessments to gauge the mortar’s future behaviour. The mortar specification was accomplished by archival research and personal interviews, complemented by laboratory analyses of lantern mortar samples using Fourier-transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and thermogravimetric analysis. The difficulties in sourcing sufficiently detailed documentary information on the original mortar components are discussed. @en

The adhesive layer in the adhesive joints can experience different modes of loading. Although the fracture energy of adhesive is generally considered to be a material parameter, it is found to be a function of the joint configuration too. Thus, to accurately simulate the behaviour of bonded joints, it is recommended to obtain the fracture energy of the joints using the same substrate(s) as in real applications. In some applications, it is necessary to join dissimilar substrates using adhesives. However, for pure mode fracture tests it is essential to reach the desired loading mode even in a dissimilar joint. Not only the joint configuration but also the environmental conditions need to be considered in fracture tests. In this condition, due to the aging, the stiffness of substrates and adhesive layer might change, and as a result, the adhesive may experience a mixed-mode loading condition. The current study aims to investigate the variation of the mode mixity for dissimilar double cantilever beam adhesive joints with composite/metal substrates subjected to cyclic aging. At different stages of the aging cycles, the mode mixity was calculated during the test using displacement fields obtained by digital image correlation and based on the Williams series expansion. In addition, the variation of flexural stiffness of polymer matrix composite substrates after cyclic aging was investigated using a three-point bending test Finally, based on the variation of composite substrate flexural stiffness and using the finite element method, the variation of the mode-mixity ratio was calculated numerically and compared to the experimental results. The obtained results show that during the cyclic aging the moisture diffusion decreases flexural stiffness of polymer matrix composite substrates significantly, but the variation of substrate flexural stiffness deviates the mode mixity in the aged double cantilever beam specimens.

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Canvas paintings are prone to environmental ageing. Hence, the structural conservation of canvas paintings may require lining, a process in which a secondary canvas is adhered to the reverse of the damaged original canvas to provide additional support. Choosing the optimum adhesive in combination with a lining method is crucial and yet challenging, as they should preferably be mechanically and chemically stable and reversible for at least 100 years. Comprehensive data on thermal and long term mechanical behaviour of prevalently used adhesives and their bonded assemblies to canvas is scarce and yet necessary to enable conservators for a proper choice of the materials in terms of durability. In this study, four prevalently used adhesives in the conservation of canvas paintings are investigated and their creep performance is evaluated and benchmarked at three different temperatures and environmental relative humidities (RHs). These adhesives are either bio-based (animal glue-starch paste and beeswax-dammar resin mixtures), or synthetic (BEVA® 371 and an aqueous Plextol™ D540/K360 dispersion mixture). Differential Scanning Calorimetry (DSC) technique is used to study the thermal transitions at different RHs. T-peel and lap shear tests are performed to determine the fracture behaviour and shear strength respectively. An in-house built creep set-up equipped with environmental control is developed which allows investigation of the mechanical creep for different canvas bonded assemblies. The results demonstrate the effects of temperature and relative humidity on the creep behaviour of lined canvases, which are related to their physical response. Moreover, the animal glue-starch paste shows the best creep mechanical performance for this application, while the Plextol^TM acrylic dispersion mixture in combination with Mist-Lining is a better alternative when both environment and reversibility are considered.

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In some industrial applications, adhesive joints are cyclically exposed to a moist environment, where cyclic moisture absorption and desorption can significantly alter the fracture energy of the bonded joints. Most previous studies are based on monotonic aging conditions, while the performance of bonded joints under cyclic aging is not well explored The aim of the current study is to investigate the effect of cyclic aging on mode I fracture energy of dissimilar DCB (double cantilever beam) adhesive joints. Accordingly, bulk adhesive plates were manufactured and exposed to 4 aging cycles. After the aging process, at different exposure times, the aged adhesive plates were used to bond dissimilar Al/GFRP substrates using a secondary adhesive. Then the prepared DCBs were tested and subsequently the mode I fracture energy of the adhesive was determined. Meanwhile, using gravimetrical tests and numerical simulation, moisture diffusion of the adhesive layer in different exposure times was analysed. Using experimental and numerical results, the variation of fracture energy as a function of moisture uptake was studied. In addition, glass transition temperature (T_g) and chemical bonding of the aged adhesive were analysed in different aging cycles. The results showed that by increasing the number of aging cycles, the reduction rate of mode I fracture energy between the aging cycles decreases.

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Glass bottles having a metal closure are preferred for oxygen sensitive beverages e.g. beer. Using thinner closure which comprise polyvinyl chloride-free component is more sustainable. However, protecting the sealing performance as a result of metal closure thickness reduction is challenging. Here we show the relation between the leakage in beer bottles and surface roughness of three different thermoplastic elastomer seals. Compression relaxation and creep-recovery behavior of seals have been analyzed by using a dynamic mechanical analyzer. The results showed that metal downsizing was possible with Liner A (low density polyethylene (LDPE)/styrene-ethylene-butylene-styrene) and B (LDPE/ styrene-butadiene-styrene), but not with Liner C (high-density polyethylene/butyl rubber). Optimizing smaller surface topography parameters such as the surface roughness depth R_z, kurtosis S_ku, average void volume V_vv, arithmetic mean peak curvature S_pc, the density of surface peaks S_pd and higher peak material volume V_mp, peak material portion S_mr resulted in a better sealing performance. Liner C was found to show an increased leakage risk, since there was a high level of stress relaxation leading to a reduced sealing force. The sealing liner material with low relaxation, low elastic modulus and high creep recovery compliance was found to ensure better sealing when thinner metal closures are used.

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Collagen-based bioadhesives, also known as animal glues, are derived from animal parts (e.g. skin, bone, and cartilage). They have been widely used in the production of decorative wood working, paintings, bookbinding, and other historical objects for centuries as well as for their restoration and preservation [1]. Collagen, in which the term (kólla) is derived from the Greek word for glue, is a fibrous structural protein with a hierarchical microstructure at different length scales. Collagen is water insoluble and can be turned into water-soluble gelatine through the denaturation process. Upon denaturation, the triple helix structure in collagen unwinds and turns into random coils giving gelatinous animal glue. Gelatinous animal glue can be dissolved in water and during gelling and subsequent drying, the random protein coils undergo partial renaturation back into triple helices. These triple helices in the adhesive act as physical cross-links leading to a continuous three-dimensional network structure [2]. These microstructural features affect the physical and mechanical performance of the glue at micro and macro scales. In this study four different adhesives from different animal species both mammalian and fish are investigated. These adhesives are bovine bone, bovine skin, rabbit skin, and fish. For this study, thin films of these adhesives were manufactured using the solution casting method. Physical and mechanical behaviour of the adhesives were characterized using Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Bloom strength evaluation, and uniaxial tensile tests. @en

High performance structures require the use of different materials to meet their demanding requirements. Especially fibre reinforced polymer composites are nowadays often bonded to metals in order to take the most advantage of the materials properties and to minimize their disadvantages. However, the interface in such bi-material assemblies often represents the weakest point and thus has to be carefully addressed to ensure structural integrity. This review paper presents an overview of the research on bi-material interface crack problems over the past 30 years. Three categories of the research are discussed: mechanical testing, crack driving force and mode partitioning. The literature reveals that the key element to the fracture analysis of the bi-material interface crack is how to perform the mode partitioning. The proposed theories for mode partitioning by many researchers are meaningful yet underdeveloped and need further experimental validation.

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In this study, different surface pretreatments were applied to clean and activate titanium alloy surfaces. The samples were subjected to grit blasting treatments using two different pressures and afterwards, a UV/Ozone treatment was applied at different times to study the wettability and surface oxidation of the titanium samples. Scanning electron microscopy and laser confocal microscopy showed the surface morphology and the increased roughness with grit blasting pressure. X-ray Photoelectron Spectroscopy revealed that titanium was increasingly oxidized with increasing UV/Ozone treatment time, which leads to a reduced contact angle and a better adhesive performance in a butt tension test proving the effectivity of this surface treatment for titanium. Furthermore, the addition of sol-gel AC-120 and corrosion inhibition primer BR 6747 showed to be an additional improvement in the initial adhesion and after different degrees of aging by exposure to salt-spray, making the surface treatment techniques used in this research, a promising environmental friendly alternative to improve adhesive bonding performance.

The mechanical behavior of adhesives is strongly influenced by a large number of variables, relating to a complex interaction of mechanical-physical-chemical factors, such as its loading direction (shear, peel), the temperature and the environmental relative humidity (RH). These variables can have a large influence on the durability of restored art objects where thermoplastic adhesives have been used as a consolidant. This study aims to characterise the mechanical and physical behavior of some adhesives commonly used polymers by conservators as consolidants to restore cultural objects such as canvas paintings or historic wooden furniture. Twelve commercially available natural and synthetic adhesive materials were tested. The influence of RH at room temperature on the mechanical and physical properties of the adhesives was investigated. Shear and peel experiments were performed on adhesively bonded wood and canvas coupon to establish mechanical characterisation. The physical properties of the adhesives were determined by performing moisture adsorption measurements and Differential Scanning Calorimetry (DSC). The results of this study demonstrate that synthetic adhesive products are able to resist higher shear and peel loads than natural types. Moreover, the influence of important changes in RH on the mechanical properties of the adhesives was demonstrated. Reflecting on the combined data derived from shear and peel tests with the adhesive's sensitivity to moisture will help conservators to select the most suitable adhesives for their applications to achieve optimal durability and the best mechanical performance in versatile environmental conditions.

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Materials, both natural and synthetic, are used as adhesives or consolidants during the conservation of cultural objects. However, few adhesives have been formulated specifically for the needs of cultural heritage. This article describes a study of the stability of ethylene butyl acrylate copolymers as well as blends of these polymers with low molecular weight resins, which may be used as consolidating adhesives for flaking and fragile paint layers. The materials were aged under simulated museum conditions. Fourier-transform infrared spectroscopy and size exclusion chromatography were used to analyze changes in chemical structure and molecular weight distribution, respectively. The degree of degradation was found to be dependent on the specific blend composition. With the addition of a hindered amine light stabilizer, minimal changes occurred in the molecular weight distribution and no loss in polymer solubility were noted. Therefore, it is suggested to incorporate a stabilizer into ethylene butyl acrylate polymer blends to extend their life and safe use in cultural heritage. Differential scanning calorimetry was employed to investigate the miscibility of the binary blends and the temperature window of their application. Furthermore, adhesive peel strength for the individual components and the binary mixtures revealed values comparable to that of Paraloid™ B-72, a frequently used consolidant. Consolidation tests on the blends were done by professional conservators on mock up and dummy paintings.@en

Surface morphology of adherends is an important factor to take into consideration when studying and improving the performance of an adhesive bonded joint. In this study, the adhesion performance three different surface morphologies of Selective Laser Melted (SLM) Ti6Al4V was studied. The three surface morphologies were created by manufacturing the adherends with different build directions (0, 45 and 90°). Scanning electron microscopy and laser confocal microscopy were used to assess the obtained morphology and roughness of the printed surface areas to be bonded. Those surfaces were subjected to 40 min of UV/Ozone treatment to remove organic contamination traces on the surface which lead to a reduced apparent contact angle and improved adhesive strength. The samples printed at 45°, which showed the highest surface roughness, presented the best adhesive performance during the tensile tests. The addition of sol-gel AC-120 and corrosion inhibition water-based primer BR 6747-1 showed an effective improvement in aging behaviour after 6 weeks of salt spray exposure.@en

To improve their adhesion strength, polymeric surfaces are usually modified through different treatments. This study investigates the effect of mechanical, chemical, and energetic treatments on the bonding strength of ethylene propylene diene methylene (EPDM), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS) materials. Three adhesives based on different chemical compositions, namely silicone, polyurethane, and modified-silane (MS) polymer, were considered. Results show that the effect of the applied treatments on the adhesion strength of EPDM surfaces is insignificant. Only a slight improvement is obtained in the case of polyurethane-based adhesive, while the failure modes remained adhesive. As for PVC, most treatments were effective in the case of the silicone-based adhesive, especially grit blasting, primer, and UV/ozone treatments. Only UV/ozone treatment improved the adhesion strength and altered the failure mechanisms of this material when polyurethane and MS-based adhesives are used. The adhesion of ABS increased and the failure modes changed from adhesive to cohesive for most treatments. Particularly, a significant improvement is obtained when primer coating and UV/ozone radiation are applied. This comparative study paves the way for the design of polymeric joints with highly enhanced adhesion performance.@en