Contact Angles of Viscoelastic-Thermal Compression (VTC) Modified Paraserianthes falcataria (L.) Laminas

Wood is recognized as hygroscopic material, which tends to absorb moisture from surrounding, thus affecting both physical and mechanical properties of the material itself. The aim of this study was to evaluate the effect of viscoelastic-thermal compression (VTC) on the contact angles of modified laminas of Paraserianthes falcataria (L.), in correlation with density and wettability of the wood. This low-density wood species was subjected to densification treatment in order to improve its density as well as mechanical properties. VTC is a densification treatment which involved pre-steaming for softening purpose and compression via hot pressing. There were four different pre-steaming durations alongside one control (NS/D: no pre-steaming (control); S1/D: 10 minutes; S2/D: 20 minutes; S3/D: 30 minutes). The laminas underwent contact angle test (sessile drop method) by referring to ASTM D7334-08: Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement. In addition, basic morphological feature of the laminas was determined by using Scanning Electron Microscope (SEM). The contact angle of S1/D (10 minutes) laminas indicated the lowest degree of contact angle, which means it had better wettability; while S3/D (30 minutes) laminas recorded the highest degree of contact angle, therefore having poor wettability.


Introduction
Wettability is defined as molecular interaction betweeen solid and liquid, which involves direct contact between them [1]. This interaction formed contact angle, which commonly measured to determine the wettability of adhesives and indicate the bonding strength when it is applied to substrates [1,2]. Contact angle measurement is a method to evaluate the wettability of adhesives when applied to solid surface, whereby contact angle > 90˚ indicated poor wettability, while < 90˚ is evaluated as good wettability [1][2][3][4][5][6][7][8]. Historically, wettability of wood was measured for the first time by using inclined plate method, whereby is indicated based on the cosine of angle of the liquid with the solid surface, which was further developed by Adam [9].
In general, Phenol-Resorcinol Formaldehyde (PRF) is commonly used as the liquid for contact angle measurement, due to its wide utilization in the wood-based industry [1]. However, American Society for Testing and Materials (ASTM) International suggested that water can also be selected as the testing liquid as it is used to determine the wetting properties of that liquid on the coating surface or to determine the hydrophilicity and hydrophobcity of the surface itself [10].
Wood densification is a process of enhancing the density of wood, as well as its mechanical properties [11][12][13]. In history, densification was first implemented about a century ago, producing densified wood product such as "Lignostone" and "Staypak". In depth, densification makes the wood cells to deform plastically, thus being able to prevent fracture of the cells [14][15][16].
The wood species used for this study was Paraserianthes falcataria (L.), or generally known as Batai wood, which had average density of 360 kg/m³, and can be found in Southeast Asia countries such as Malaysia and Indonesia [17][18][19]. It is also widely known as fast growing tree species, whereby it can reach 30-50 meters in height within 9-10 years [17][18][19]. This particular wood species was mainly utilized in pulp and paper industry, as well as furniture component. However, Batai wood are not suitable for structural application due to its poor mechanical strength. Densification of low-density wood species was proven to produce wood that possess enhanced mechanical strength, which capable to be an alternative for existing high-density wood species [11][12][13][14][15][16].
In this study, a densification method known as viscoelastic-thermal compression (VTC) was applied on Paraserianthes falcataria laminas, whereby it consisted of pre-steaming (softening) and compression by hot-pressing.

Viscoelastic-Thermal Compression (VTC)
The undensified lamina replicates were pre-steamed at different time durations by using a steamer, as stated in Table 1. The transition time from the steamer to hot-press machine upon completion of presteaming was 10 seconds.
The densified lamina were replicated with final thickness of 10 mm, and cut into small size (50 mm (length) x 50 mm (width)) for contact angle measurement. 20 μL of liquid was dropped on the surface of replicate. Images were captured immediately after the liquid drop was in contact with the solid surface (indicated as 0 second), followed by 10 second, 20 second and 30 second in order to observe the changes of contact angle within that specific time period. The images were then analysed by using ImageJ software, whereby all the images were converted into black and white for contact angle measurement via Low Bond Axisymmetric Drop Shape Analysis (LBADSA) plugin.

Scanning Electron Microscopy (SEM)
The laminas were observed via scanning electron microscope in order to observe basic morphological feature of the VTC modified laminas. Small cubes (5 mm (length) x 5 mm (width) x 5 mm (thickness)) were prepared and observed via the high-end microscope. 800x magnification was used to observe the cell lumens. The image were captured and further analysed via ImageJ software, whereby the area of lumens were calculated.

Results and Discussion
Non-pre-steamed (NS/D) indicated the lowest average of contact angle, while 30 minutes pre-steamed (S3/D) recorded the highest average of contact angle. Figure 1 showed the changes in contact angle for 30 seconds. Both image and average area of cell lumen for the non-pre-steamed (NS/D) were shown in Figure 2 and Table 2 indicated that most of the lumens were not completely collapsed during densification, due to absence of steam at high temperature inside the wood [1,15]. The absence of steam means the wood was not softened, which reduced the compressibility of the lamina; hence causing most of the cell lumen not being collapsed, by which the mean area of cell lumen for non-pre-steamed (NS/D) was the highest (136.446 μm 2 ). Moreover, this condition influenced the hydrophilicity of the wood, whereby water tends to diffused inside the capillary of wood [1]. This interaction caused non-pre-steamed (NS/D) to have low mean value of contact angle.  However, 10 minutes pre-steamed (S1/D) indicated the lowest mean value of contact angle, even though the result displayed in Table 2 indicated that S1/D had the second highest mean value of cell lumen area (89.77 μm 2 ). The significant difference of mean contact angle between S1/D and NS/D might be influenced by lesser amount of extractive in S1/D compared to NS/D, by which previous studies reported that presence of extractives reduced the wettability of wood, as well as its adhesion properties [20,21]. 20 minutes pre-steamed (S2/D) laminas with average density of 636.07 kg/m 3 , recorded cell lumen area mean value of 65.512 μm 2 , which was lower than NS/D and S1/D. 30 minutes pre-steamed laminas (S3/D) underwent the longest pre-steaming duration, which allowed higher amount of moisture to be diffused inside it. This interaction enhanced the compressibility of the laminas,thus clarified the reason of 30 minutes pre-steamed laminas (S3/D) to had the lowest mean value of cell lumen area (39.962 μm 2 ); means that most of the cell lumen were completely collapsed, therefore increasing the hydrophobicity of the laminas and affected the contact angle value.

Conclusion
In conclusion, viscoelastic-thermal compression (VTC) enhanced the densities and reduced the wettability of Paraserianthes falcataria (L.), whereby higher pre-steaming duration influenced the compressibility as well as hydrophobicity of the laminas. However, poor wettability is an indicator of inefficient adhesion properties, by which will affect the bonding strength for further product manufacturing process.