Paint release control of brush Open Access

Coating materials play a very significant role in industries.^1,2 Equipped with different properties, they are used in various research fields for special multi-surficial functions.^3,4 They prevent oxidation and increase safety and make a surface water-repellent, perfect, and more beautiful.^5,6 Before coating, the paint materials are in the solid state or liquid state. Due to the special wettability and surface tension of paints, they are always brushed on a solid surface. Chinese brush could absorb the ink and write on paper. The capillary force plays a significant role in capturing much more ink before writing.^7–9 Pig’s bristles were the main raw materials for brushes due to their excellent wettability with the paints.^10,11 With the increase in market demand and the rapid development of modern chemistry, polyethylene terephthalate and polybutylene terephthalate (PET and PBT) are becoming the main raw materials for fabricating the brush fiber. Researchers have paid much attention to the absorption and release of paint by designing the topography of fibers. The absorption and release rate of paint on the brush has not been studied in detail.

Preparation of brush: Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) with a weight ratio of 75:25 were mixed together. The mixture was heated at 260 °C for 15 min and then was extruded into an extrusion die with a diameter ranging from 60 µm to 120 µm. The extruded fiber was washed with cold water (temperature of 15 °C) for rapid temperature falling. For getting fibers with different elastic moduli, the glass fiber was doped into the raw materials at the initial section. The fibers were bound into a compact bundle and formed a brush.

Painting process: The brush was dipped into the solutions such as ink (surface tension of 0.692 × 10⁻¹ N/m), paint (surface tension of 1.57 × 10⁻¹ N/m), and liquid metal (GaIn₁₀, surface tension of 7.1 × 10⁻¹ N/m) with a depth of 1 cm for absorbing solutions. After that, the brush was pressed on the solid surface with a tilted angle of 45° and deformation of 30% for the release of solution.

Mechanical property characterization: The mechanical properties of the fiber were tested by an improved force balance meter system (DCA21, Dataphysics, Germany). The fiber was fixed by the clamp. In the bending process, the deformation and stress data were recorded by software.

Scanning electron microscopy: The SEM images of the fiber were observed by Environmental Scanning Electron Microscopy (ESEM, Quanta FEG 250, FEI) under the voltage of 10 kV.

In the absorption process, the brush is dipped into the paint. The brush fibers generate capillary force and absorb the paint.^12–14 The brush fibers are bent by the surface tension of stored paint. In the painting (releasing) process, the brush is bent with larger deformation by external force, resulting in squeezing out of paint, which realizes the coating function on solid surfaces.^15,16 In this research, we find significant roles for controlling the release rate of paint. Figure 1 shows the brushes with different lengths and the paint effect. The brushes with different lengths effectively control the release rate. Figures 1(a) and 1(b) show two brushes with different lengths of fibers and the paint effect. We find that the brush with longer fibers could control the release of paint and paint a longer line. The contribution of the fiber length to the release rate of paint is shown in Fig. 1(c), which illustrates that the longer fibers could extend the painting distance. The shorter fibers play a role in the fast release of paint. The topography details of two brushes are shown in Figs. 1(d) and 1(e). The diameter of the fiber is 60 µm.

Before coating, the brush is dipped into the paint for absorbing paint [Figs. 2(a) and 2(b)]. The coating material is locked by a cluster of fibers. This process is simplified as liquid staying in two neighbor fibers. This state is shown in Fig. 2(c). In this system, r is the radius of the fiber, and the length is defined as l. In the dipping process, the top region of two fibers is closed under external force. The deformation of the fiber is associated with the elastic modulus (E), the length of the fiber, and the state of external force F. In that case, the fiber is simplified as a model of beam under stress. The absorbed liquid reaches a maximum when the hydrostatic pressure (P) generated by the liquid on the fiber achieves the balance with the deformed fiber. The force per unit length of fiber provided by liquid (paint) is shown as [Fig. 2(d)]

where θ is the geometric angle in Fig. 2(e), θ_c is the contact angle of paint on the fiber, P is the hydrostatic pressure, and γ is the surface tension of paint. In this model, the deformation in the y direction (deflection) of the fiber is shown as

According to Eqs. (1) and (2), with the increase in elastic modulus (E), the deformation of the fiber under the same operating condition increases. The increase in the fiber length could increase the maximum deflection between two neighbor fibers and is conductive to the absorption of paint.

In the printing process, characteristics of elastic modulus have the opposite effect. Figure 3(a) shows the printing process with brush. The paint is released with the decreasing rate. The coating thickness is also decreased with the painting process. The state of liquid paint in two neighbor fibers is shown in Fig. 3(b). The external forces acting on the fiber are the surface tension and the liquid pressure generated by paint. In this model, the deformation in the y direction (deflection) of the fiber is shown as

According to Eq. (3), with the increase in elastic modulus (E), the deformation of the fiber under the same operating condition decreases. Decreasing the fiber length could decrease the maximum deflection between two neighbor fibers and is conductive to the release rate of paint. This theory can satisfy the requirements of different construction sites by adjusting the size (radius and length) and elastic modulus of the fiber.

The elastic property of the fiber is determined by the elastic modulus and the size design. Figure 4 shows the relationship between the fiber length and flexibility. Brush fibers with lengths of 4 cm, 3 cm, and 2 cm are tested by an improved force balance meter system. Figure 4(a) shows that the shorter the fiber length is, the harder the larger elastic modulus should be. The test process is shown in Fig. 4(b). The result illustrates that the length of the fiber is one of the significant factors for controlling the release of paint.

According to Eqs. (1) and (2), elastic modulus is one of the parameters to control the paint release rate. For regulating the elastic property, glass fibers are used to enhance the mechanical performance. In the fabrication process of fibers, the glass fibers are mixed with the raw materials. Figure 5(a) shows the elastic modulus of fibers with different content of glass fibers. With the increase in mixed glass fibers, the elastic modulus increases [Fig. 5(b)]. Liquid materials with different surface tension could realize the same rules with the designed brushes. Figures 5(c)–5(e) show that the liquids (ink, paint, GaIn₁₀ liquid metal) with a surface tension of 0.692 × 10⁻¹ N/m, 1.57 × 10⁻¹ N/m, and 7.1 × 10⁻¹ N/m are coated on the paper surface with brushes [Fig. 5(a), E = 1.583 GPa, 4.621 GPa, and 6.936 GPa]. In the same system, the brush (fiber) with lower elastic modulus absorbs much more paint but has a smaller release rate. Instead, the brush (fiber) with larger elastic modulus absorbs less paint but has high release rate.

In this study, we find that the size and flexibility of brush fibers play a significant role in controlling the release of paint. The brush (fiber) with lower elastic modulus absorbs much more paint and shows a smaller release rate, which realizes uniform paint performance. On the contrary, the brush with larger elastic modulus shows a larger release rate of paint, resulting in fast coating. This study is highly significant to design optimal brushes for different coating materials and work conditions.

L.W. supervised the research. L.W. and R.W. designed the experiment and wrote the paper. F.Z., S.T., and J.L. discussed the results. P.L. simulated the experiments.

This work was supported by the National Natural Science Foundation of China (Grant No. 21805294).

There are no conflicts to declare.

The data that support the findings of this study are available from the corresponding author upon reasonable request.