Fig. 3 presents the optical images of microbubble in a cellulose acetate film before and after plasticizer removed and its corresponding SEM image. It can be seen from Fig. 3a and 3b that once the plasticizer is cleaned using the combination of optical microscopy and stainless-steel needle, the original handwriting is revealed. Although the plasticizer is cleaned in some microbubbles, the image is not enough clear. SEM was used to observe the morphology of microbubble internal surface after removing the protective layer and plasticizer (Fig. 3c). The internal surface of microbubble is rough due to the formation of microbubbles.
Previous literature has confirmed that the shortening of incident light pathway is the fundamental factor leading to blurring of the image [30]. From Fig. 3b, it is seen that a number of small dark spots are observed inside the microbubble. Light scattering increases when light passes through the film due to the uneven interface in the microbubble. Filling materials were selected to permeate inside the microbubble to reduce the scattering attributed to the rough interface.
Conservation applications of EC
Polymer materials are commonly used for the protection cultural heritage objects [31], ethyl cellulose is the most commonly employed filling material and butan-1-ol is used as the solvent. As shown in Table 1, the 3% butan-1-ol solution of EC has the shortest permeation time. The permeation time can be seen to increase with increasing concentrate of EC. Fig. 4 presents the optical images of the interface using different concentration of butan-1-ol solution of EC. The clarity of the image improves with the EC (Fig. 4b-c), whereas it decreases with EC concentrations of more than 5% due to increased viscosity and the decreased permeability (Fig. 4d-e). Based on these results, the 5% butan-1-ol solution of EC was selected as the filling material.
Table 1 The concentration selection of filling material
|
concentration
|
EC
|
w. 3%
|
w. 5%
|
w. 7%
|
w. 10%
|
Permeation time(s)
|
< 30
|
30-50
|
>60
|
>100
|
Accelerated ageing tests are used to study the long-term behavior of a variety of filling materials. Hydrolysis and oxidation are the main pathway for the degradation of filling materials [32-36]. Temperature, humidity and ultraviolet light were chosen as parameters to investigate the filling material durability (Fig. 5). Folding endurance and tensile strength are important indicators that characterize the mechanical properties of film [32]. The effects of dry heat, UV and hydrothermal aged on tensile strength and folding endurance of samples before and after treatment are shown in Fig. 5a and 5b. It can be seen that the tensile strength and folding endurance of treated samples are increased by 1.5% and 2.8%, respectively. The tensile strength of untreated samples decreased by 28.6%, 16.9% and 40.3% after dry heat, UV and hydrothermal ageing 21 days, while treated samples decreased by 12.4%, 3.4% and 23.9% respectively. The folding endurance of untreated samples decreased by 46.7%, 33.6% and 56.1% after dry heat, UV and hydrothermal ageing 21 days, while treated samples decreased by 30%, 12.7% and 45.4% respectively. The tensile strength and folding endurance of untreated samples are significantly decreased after ageing compared to the treated samples. The effect of hydrothermal aging was greater than that of dry heat and ultraviolet exposure, implying that relative humidity and temperature are the main factors that influence aging. The treated samples exhibited a high level of stability to humidity, temperature and UV and exhibit smaller loss of film strength compared to untreated samples. From Fig. 5c, it can be seen that the optical density is not changed before and after treated filling materials. The transmittance of the untreated samples is decreased more than that of the treated sample with the increase of the aged times. The result indicates that the filling materials have the good durability.
As outlined in the above-mentioned method displayed in Fig. 2, about 0.2 ml 5% EC in butan-1-ol as filling materials was infiltrated into the interior of the cleaned microbubble to obtain a clearer image (Fig. 6a). The SEM image of the interface shows a uniform and flat surface (Fig. 6b). It is proposed that the filling materials form an EC film that adheres to the rough surface of the microbubble. In addition, the difference between the refractive index of rough the interfacial particles and filling materials decreases leading less light scattering and enhancing the clarity of the image [30].
The Keyence VK-X250K shape analysis and laser confocal microscopy are used to characterize the internal surface morphology and roughness of the microbubbles before and after filled the addition of filling materials. Surface geometric features are described by morphology parameters. The most commonly used parameter is surface roughness, which is represented by the contour curve of a section on the surface [37]. The smaller value of surface roughness, the smoother surface. As shown in Fig. 7, the internal surface of microbubble after cleaning has a morphology with significant flotations with the height varying from 18.51-45.52 µm, while it is relatively uniform and smooth after filling with the height of -2.46-8.13 µm. The sample surface roughness parameters were measured by using Keyence software VK-H1XAC after corrected, and the measured area is ~ 268 × 358 µm as shown in Table 2. The measured mean surface arithmetical height (Sa) of the internal interface of microbubble after cleaned was 0.971 µm, whereas it becomes 0.367 µm after filled with filling materials. Sa decreased by 62.2% compared to untreated samples. The maximum height (Sz) at the internal surface of microbubble was 27.014 µm after cleaned, while it decreased to 10.589 µm after filled. The ratio of height to width of surface properties (Str) of the microbubble interface is 0.612, whereas it decreased to 0.172 after filled. Str is closer to 0, indicating the fringes emerge on the surface, while it is closer to 1, indicating that the surface does not depend on direction. The experimental result is further confirmation that the internal surface of microbubble is not uniform. The arithmetic means peak curvature (Spc) of the internal surface of microbubble is 4167.165 mm-1 after cleaning, whereas it is reduced to 1299.603 mm-1 after filled, indicating that the sharpness of surface peaks is reduced. The area ratio of interface expansion (Sdr) of microbubble interface is 0.455 after cleaned, whereas it decreases to 0.076 after filled. Based on the above discussion, the internal surface of the cleaned microbubble is more uniform and smoother.
Table 2 Surface roughness test results of film performed before (a) and after (b) the addition of filling materials
|
Sa (µm)
|
Sz (µm)
|
Str (1/mm)
|
Spc
|
Sdr
|
a
|
0.971
|
27.014
|
0.612
|
4167.165
|
0.455
|
b
|
0.367
|
10.589
|
0.172
|
1299.603
|
0.076
|
relative decrease (%)
|
62.2
|
60.8
|
71.9
|
68.8
|
83.3
|
The light scattering characteristics of rough surface inside in the microbubble are directly related to reflectivity, which is determined by surface reflectivity within the visible light range [38]. Fig. 8 shows that the reflectivity of the internal surface of microbubble after cleaning greater than that of the filled surface. The combined results shown in Fig. 7 and 8, reveal that the internal surface of microbubble is very rough after cleaning, enhancing the diffuse reflection of visible light.
On the basis of above analysis, we propose the mechanism of action about filling materials and the corresponding schematic illustration is drawn in Fig. 9. The image observed using optical microscopy depends on the transmission of light. The diameter of microbubble is 0.5-2 mm and it is filled with plasticizer crystals. We observe the black dot, it is mainly because visible light is majorly reflected by the microbubble. The surface is rough after the plasticizer inside in the microbubble has been cleaned, leading to increased diffuse reflection and resulting in blurred images. This phenomenon can be explained by the theory of light scattering whereby the rough surface scatters the incident light [39]. The scattering layer shortens the pathlength of the incident light and weakens the transmitted light of image, resulting in unclear image and black spots [40-41]. The refractive index difference between the space and filling material is decreased when filling materials of higher refractive index is filled into the microbubble. The light scattering is decreased significantly, increasing the amount of transmitted light.