4.3. Ageing resistance
The stability (ageing resistance) is an index that we must take into account to evaluate the application value of the prepared copolymer. The energy possessed by the ultraviolet light reaching the ground can disrupt the internal chemical chains of polymer structures, and this photoinduced chemical degradation will aggravate accompanying by the oxidation reaction. During the ultraviolet radiation process, the average molecular weight of the copolymer will be reduced because of the radiation degradation. Therefore, the mass loss rate (ML) was used to characterize the ageing process during the UV-aging test, which was defined as:
Where m0 is the mass of the prepared copolymer before conducting the UV-aging test; m1 is the mass of the prepared copolymer after ultraviolet radiation in one cycle.
To find out the optimum ratio, the prepared copolymers with adding different HFMA contents (0, 9.08%, 16.65%, 23.05%, 28.54% and 33.30%) were adopted to conduct the UV-aging test, which runs for one month and the mass of the prepared copolymer was monitored every day. The mass loss rates were shown in Fig. 6. The mass of the six copolymers all decrease with the increase of the ultraviolet radiation time. The mass loss rates exhibit two obvious stages, the first one is the rapid growth stage and the second one is the stationary stage. For the copolymers with 0% and 16.65% HFMA content, the turning point is at about 10 days; for the copolymers with 9.08% and 23.05% HFMA content, the turning point is at about 15 days; and for the copolymers with 28.54% and 33.30% HFMA content, the turning point is at about 20 days. Additionally, Fig. 7 indicates that the maximum mass loss rate is not completely proportional to the additive amount of HFMA. Specially, the copolymer with 16.65% HFMA content possesses the minimum mass loss rate, which enlightens us that adding moderate HFMA will help to the ageing resistance of prepared copolymer, or at least that the synthetic copolymer will not be fast aging and lost its utility value.
4.4. Contact angle of the copolymer
To investigate the water repellency of the prepared copolymer when applied to the conservation of stone building heritages, the contact angle test of the copolymer film were conducted. Previous scholars have confirmed that − CF3 group is capable of enhancing the hydrophobicity of a copolymer film surface[28, 35–37]. This is because the polar OH − groups of the stone surface are replaced by the non-polar and thermally stable –CF3 groups originating from the fluorinated copolymer, impelling the formation of a hydrophobic surface on the stone building heritage.
However, ascertaining the proper HFMA proportion to maximize the hydrophobicity of the obtained copolymer is needed. And Fig. 8 exhibits the changes of contact angle with the increasing HFMA contents. As a whole, the contact angle increases with the increase of HFMA content, which confirms the consensus that the fluorinated monomer can modify the wettability of the prepared copolymers. This is due to that the fluorine atoms migrate freely to the surface of the film during its curing process, decreasing the surface free energy and advancing the hydrophobicity. Nevertheless, the surface hydrophobicity could not be further improved or even be weakened when increasing HFMA content after 16.65%, this phenomenon reveals that the fluorine atoms have been saturated and the hydrophobicity could not be further improved by adding more HFMA. Additionally, the adhesion between the fluorinated copolymer and stone surface is negatively related to the hydrophobicity. The adhesion will be reduced by increasing HFMA content after the fluorine atoms are saturated on the stone surface, hereat, the rainwater will penetrate into the stone through the space between the stone surface and the fluorinated copolymer film. This circumstance avianizes the hydrophobic performance of the stone building heritage coating by fluorinated copolymer.
4.2. Surface characteristics
The surface morphology of the stone samples and prepared copolymer were investigated by using SEM analysis (Fig. 6). After experiencing a long-term weathering, the surface structure of the stone building heritage has been changed. Apparently, as shown in Fig. 6a and Fig. 6b, the surface of weathered stone samples becomes more loose and rough, resulting in the generating of massive secondary pores. These pores are belonged to macropores[39, 40], which provide pathways for water flow or infiltration and solute transport. On the one hand, the increased porosity will cause the weathered stones to be weakened and lose their strength directly, and on the other hand, it will aggravates this damage by driving the reaction of minerals with rainwater penetrating through the secondary pore structure.
Figure 9c and Fig. 9d unfold the surface structure of the prepared copolymer, they are obviously more dense, compact and smooth than that of weathered stone samples. However, comparing to Fig. 9d, some fissures are developed on the surface of copolymer film exhibited in Fig. 9c, indicating that HFMA is beneficial to the formation of the copolymerization emulsion in this paper. And HFMA changes the surface structure of the prepared copolymer and makes it more smooth and firm, which may be come down to two reasons: (1) the fluorocarbon groups with a low surface energy are arranged to dense networks on the coating surface; (2) fluorinated groups increase the stiffness of coating surface and lower the possibility of cracking.
4.5. Wettability alteration of the stone building heritage surface by coating fluorinated copolymer
Exposing to natural environments (illumination, rainfall and temperature cyclic changes, etc.) for a long time, historical stone buildings will be relaxedly weared out, especially for which without any effective conservation measures. As stated by previous scholars[43, 44], water, either alone or in combination with other environmental elements, will increase the damage of the stones existing in the atmosphere. Therefore, cutting off the contact of water and stone is requisite to reduce the progressive deterioration of the stone building heritages.
Section 4.4 has certified the copolymer with 16.65% HFMA content possesses the strongest hydrophobic features. As suggested by Licchelli et al. that the contact angle measurement is related to an “instant” water repellence behaviour, while the long-term water resistance can be better assessed by evaluating the water absorbed through the treated stone surface during the capillary absorption test. Therefore, to ascertain the wettability modifications of the un-coated and coated stone samples by this kind of fluorinated a copolymer, both the contact angle and spontaneous imbibition tests of copolymer film were performed.
To guarantee the repeatability of results, the contact angle tests were conducted at least three times for each stone sample. The contact angles and test errors of the stone samples before and after fluorinated acrylate copolymer emulsion coating were shown in Fig. 10. The contact angles of the stone samples before coating fall in the range of 18.80o-69.05o, contrarily, those of the stone samples after coating vary from 91.90o-119.80o. Apparently, the contact angles of the stone surface are greatly increased by 58.35%-428.95% via coating the fluorinated acrylate copolymer emulsion. This shows that the fluorinated monomer can strongly improve the hydrophobicity of stone surface, as reported by other researhers[46–49].