This research was funded through the Project HAR2011-29538 of the Spanish Ministry of Science and Innovation. We also acknowledge the assistance of the Laboratory of Analysis of Chemical/Stone Materials of the IRP (Instituto Universitario de Restauración del Patrimonio) of the UPV (Universitat Politècnica de València). authors wish to thank the group of researchers currently involved in the design and development of the experimental testing of materials: T. Mastroiacovo and G. Medda (Freelance, Italy), J. Orozco (Instituto Tecnología de los Materiales, UPV, Spain), M.Planes and J.L Moya López (Electron Microscopy Service at UPV), J. Grafiá (Instituto de Restauración del Patrimonio, UPV), L. Pérez (Physic, Universidad Complutense de Madrid) and V. Mira (Bateig Novelda Stone, Alicante).
1In recent years there is an increasing trend based on maintaining and/or minimizing agents and mechanisms that spoil artworks exposed outdoors. In most cases, the environmental control becomes complex and practically unavoidable. In this sense, the replacement of the original copy is a preventive conservation measure that allows safeguarding the artwork without altering its relationship with its original environment. These are situations of irreparable losses in sculptures and decorative stone material, which sole premise is to act under criteria of sustainability, efficacy and affinity with the materials of the monument. The most widespread method is the reproduction of sculptural works by ornamental organic matrices, through the technique of molding/bilayer system, resulting imprints faithful to the original ones.
Currently, the organic matrix composite mortars offer the possibility of creating parts of artworks that will be resistant to the most common environmental deterioration agents, enabling an improvement in their mechanical and structural characteristics, with notable saving of time and economic resources, while complying with the criteria of reversibility, compatibility, discernibility and respect for the original. This practice is intended to allow placing originals in indoor spaces with controlled environmental parameters, while exposing reproductions outdoors. Therefore, this physical- mechanical characterization of mortars will allow professionals choosing the most suitable for each case, and determine the amount of material needed to create the replacement part. Thus, sculptural copies will be made being aware of the strength of the material chosen and of how sunlight affects its mechanical properties over time. Similarly, this trial will allow the professional deciding whether the additive to be added to the mixture will decrease or increase the strength of the mortar. On the other hand, the professional knowing the minimum amount of material that will be necessary to achieve the desired strength will result in better management of the resources.
2Organic compounds mortars consist of a matrix of thermosetting resin that binds inorganic filler, being the aggregate of Tosca de Rocafort limestone the selected one for this case.
3We studied two types of synthetic thermohardening resins to be used as binders for aggregates, specifically a two-component epoxy resin ALY554 ® with a TETA hardening agent, and an unsaturated polyester resin Sintolit orthophalic Tritone ® with a MEKP catalyst agent were used, both provided by the CTS Spain SA company.
4Some mineral origin loads were chosen, as they were able to provide colour and texture according to the characteristics of the stone material under study. An inorganic aggregate –chemically inert-, was selected from the crushing of raw material Tosca de Rocafort (TR) limestone, specifically a micronized powdered calcite grain size 0,25-0,06mm.
5In this research three additives were studied, capable of improving performance against biological deterioration and enlightenment material. We used the Biotin® R biocide, from the CTS company, added in percentages of 3% and 5% as regards the binder phase of the mortar. The UV inhibitor Tinuvin® B75 from the Basf Company was added to the mortar binder phase in percentages of 0.1% and 0.3%, and the additive Hindered Amine Light Stabilizer (HALS) Tinuvin® 292, from the Kremer Company, added in percentages of 1% and 2% in the binder phase of the mortar.
6To prepare the mortar specimens, different phases were followed as described: 1) Preparation of aggregates in the type and dosage corresponding to the stone material studied; 2) Preparation of the binder phase required for the production of the specimens to be performed; 3) Gradual pouring of the aggregate in the matrix (TR epoxy organic mortar 2:1 and TR polyester organic mortar 4:1 –two or four parts of the arid and one of the binder), kneading continuously; 4) Addition of the catalyst agent; 5) Pouring of the mixture obtained in the mold and maintenance and the exhaust hood until sufficiently hardened and 6) Unmolding and acronym signing of each specimen for recognition and classification according to the test performed. The specimens were prepared using a RTV silicone mold size (55 ±2 x 10 ±2 x 4 ±0.2)mm, taken from the UNE-EN ISO 3673-2:2013 for the epoxy resin and UNE-EN ISO 3672-2:2002 for polyester resin. 70 specimens were prepared in all.
7Three specimens of each type were subjected to accelerated aging with ultraviolet irradiation. The ultraviolet irradiation test light is particularly important in the field of organic materials, because this class of polymers is disclosed to be particularly sensitive to photo-light deterioration. It aims to subject the materials studied to accelerated aging, according to the UNE-EN ISO 4892-3:2006, determining, approximately, the intensity of damage and alterations suffered by them under stress conditions. For this research we have used an OSRAM L36/37 fluorescent lamp that emits UV light with the power of 36 W, with a wavelength of 313nm, at a distance of 10 cm, for an exposure time of 1000 hours.
8The aged specimens were then subjected to a flexural cracking test. The flexural test compares the tensile strength of the mortar, obtaining comparable data. These data were obtained as graphs of force (N) and elongation (mm). For this purpose, the TENSILE 2KN DEBEN tester Microtest (Gafan) with software for data DEBEN Microtest V5.3.53 was used.
9SEM was used for morphological examinations of mortars allows observing imperceptible features with the binocular magnifier. In this study, a scanning electronic microscope Jeol JSM 6300 with an x-ray Link-Oxford-Isis microanalysis system was used, with analytical conditions of filament tension of 10-20kV, with the power of 2x10-9 A and a working distance of 15mm.
10The figure 1 show Force/strain curves of the mortar specimens before and after being subjected to the UV test. The mortars behave in different ways according to the type of resin employed, so that those made of polyester turn into more rigid and frail, while those of resin epoxy acquire ductility and absorb mechanical efforts in a better way. It is observed that the amount and size of the aggregates influence strength, as well as the fracture types (Figure 2. A-F). In all the cases, the stress of the materials is favored by the presence of microspores that are developed during the preparation of the specimens.
Fig. 1 Force (MPa)/Strain (%) curves of the mortar specimens
Force (MPa)/Strain (%) curves of the mortar specimens TRI-TR, TRI-TR-T292 1%, ALY-TR, ALY-TR-BIO R 5%, ALY-TR-T292 1% before and after being subjected to the UV test.
Fig. 2 Observation of the fracture
It is observe the fracture types during the influence of the tensile strength (A. TRI-TR, B. TRI-TR-T292 1%, C. TRI-TR-T292 1% (after uv), D. ALY-TR, E. ALY-TR-BIO R 5% and F. ALY-TR-BIO R 5% (after uv).
11The specimen TRI-TR-T292 1% (Figure 3. A, before aging and B, after aging) does not offer changes on texture after UV aging, however, the specimen ALY-TR-BIO R 5% (C, before aging and D, after aging) show the presence of microcrackings and fissures as a consequence of the degradation.
Fig. 3 Sight of photochemical degradation
(a-d) Sight of photochemical degradation of the surface and cracking area of the resins and mortars specimens through SEM.
12All the resins and organic mortars specimens, subjected to accelerated aging tests through 1080h (45 cycles) of ultraviolet light irradiation, show significant differences on tensile test trials. The results of the specimens have been compared to natural stones specimens, showing that those of resin without additives or aggregates are the most resistant, followed by mortar specimens and, finally, by those of natural stone. Moreover, it has been demonstrated that flexion strength varies considerably depending on the type of additive used, showing those prepared with the inhibitor UV Tinuvin 292 on 1% the higher stability.