**4. Discussion**

This study comparatively investigated the e ffect of Al(OH)3 and BaSO4 on performances of ultrafine powder coatings. The results show that the incorporation of Al(OH)3 or BaSO4 make little di fference to several properties of the powder paints and finished films, such as AORs of powders, the flexibility and impact resistance of coating films, while the other properties are influenced significantly by the fillers' types and contents. It is shown that the BERs of powders filled with Al(OH)3 is much higher than those of BaSO4. The specular gloss of coatings incorporated with BaSO4 is much higher in contrast to those with Al(OH)3. Coatings with Al(OH)3 exhibit outstanding corrosion resistance performances when exposed to salt fog, especially for hybrid-based coatings even at higher filler content.

#### *4.1. The E*ff*ect of Fillers' Physical Properties on Performances of Powder Paints and Coating Films*

The relatively smaller median particle size, higher specific gravity of BaSO4 and higher oil absorption of Al(OH)3 may partially result in the di fference of BER and specular gloss between coating films filled with Al(OH)3 and BaSO4 respectively.

Samples incorporated with Al(OH)3 exhibit higher BERs in contrast to those containing BaSO4 at 0.93 cm/s of air velocity, as shown in Figure 7. As described earlier, BaSO4 has a much higher specific gravity than Al(OH)3. Generally, the higher the specific gravity of particles in the fluidized bed, the lower bed expansion is achieved at the same air velocity.

The specular glosses of finished films decreased significantly with the addition of Al(OH)3 in contrast to those of BaSO4, indicating a stronger matting e ffect from Al(OH)3. A matte surface can scatter the light in all directions and hence the surface appears less glossy. A micro-roughness surface is necessary for creating the di ffuse light scattering responsible for the visual e ffects of reduced gloss. Compared with BaSO4, the better matting e ffect of Al(OH)3 incorporated coatings are partially due to the higher oil absorption of Al(OH)3 particles (28g oil/100 g), which is almost 3 times of that of BaSO4 (10g oil/100 g). During the curing reaction, the non-uniform shrinkage in micro scale which results from the cross-linking of binder materials, together with the presence of many filler particles occurs. In general, the higher the oil absorption value of the filler, the more binder it requires to bind it. Compared with BaSO4, the naturally higher oil absorption of Al(OH)3 particles lead to the increase of the content of Al(OH)3 particles in the top-surface of the films and thus the formation of a micro-rough surface during the curing process. The micro-roughness produced is identified by SEM images and profile of surface roughness. Figure 15 shows the cross-section view of coating films PE-A3 and PE-B3. The polymer matrix appears darker than the aluminum substrate, as indicated by the white line which marks out the interface of substrate and the coating film. The coating film adheres tightly to the substrate, without obvious defects or pores at the interface. The thickness of the coating film is approximately 38 μm. Figure 16 exhibits the surface roughness of coating films PE-A3 and PE-B3. The average roughness Ra is the arithmetic average value of the surface profile throughout the length of the testing surface. The Ra of PE-A3 is more than 1.5 times of that of PE-B3, which is due to the micro non-uniformity produced during the curing process.

In general, powder coatings are matted by the help of matting agents. They are incompatible with the coating and can produce micro-roughness of the surface scattering the incident light in di fferent directions. Generally, matting agents are either polyolefinic waxes or inorganic fillers that can decrease the gloss level to about 40% of the initial ones when used in concentrations about 4%. However excessive use of wax can produce haze, yellowing or oily appearance [11,12]. The addition of Al(OH)3 can enhance the matting e ffect without the incorporation of any other matting agents. The specular gloss is achieved to blow 50 with the sample (PE-A3) at the maximum loading of Al(OH)3. In this case, Al(OH)3 is a good alternative to achieve lower gloss finishes in one shot without the necessity of adding matting agents.

 

(**a**) PE-A3-cross-section view

 

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(**b**) PE-B3-cross-section view

 

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**Figure 15.** SEM micrographs of coating films with the addition of Al(OH)3and BaSO4respectively.

**Figure 16.** Roughness of the surface of coating films with the addition of Al(OH)3and BaSO4respectively.

 

 

#### *4.2. The E*ff*ect of Fillers' Chemical Properties on Performances of Coating Films*

 

The chemical properties of fillers make for a grea<sup>t</sup> influence on the properties of coating films, due to the interaction between polymer and fillers during the curing processes. The differential scanning calorimetry (DSC) analysis can reveal the effect of chemical properties on the coatings.

Figure 17 shows the DSC analysis of coatings H-C, H-B3 and H-A3. Two important events of the curing processes of coatings, the glass transition and the crosslinking are identified. At first, the glass transition curves developed much slower at early stages of curing process when the molecular chains formed. Then the glass transition accelerated at later stages in accordance with the chain network formed [41]. With the incorporation of BaSO4 and Al(OH)3, the *T*g increases from 62.1 ◦C (H-C) to 63.9 ◦C (H-B3) and 64.6 ◦C (H-A3). This is partially due to the interaction of the polymer resin systems and the inorganic fillers, which hinder the molecular mobility during the curing processes [42]. Besides, the amount of the crosslinking heat released (Δ*Hc*rosslinking) during the scan were greatly decreased by the addition of two fillers in this study. During the curing process, the fillers absorbed parts of the energy depending on the filler quantity in the resin systems, which is also confirmed by other investigations [42–44]. The dispersion of inorganic fillers in the resin systems hinders the mass and heat transfer, decreasing the reactivity of the system as evidenced by the reduction in the <sup>Δ</sup>*H*crosslinking of the crosslinking processes, as shown in Figure 17. The fillers' addition significantly decreased the amount of heat released. The <sup>Δ</sup>*H*crosslinking of H-A3 and H-B3 are about 39% and 37% of the heat released by sample H-C, which are much lower than the polymer content of 59.8% in H-A3 and H-B3. Meanwhile, fillers with polar groups such as hydrogen bond can interact with the polymeric network, thus influencing the chain formation and flexibility during the curing process, consequently increasing

the *<sup>T</sup>*g, decreasing the <sup>Δ</sup>*H*crosslinking. The H-A3 coatings filled with Al(OH)3 exhibit higher *T*g and lower <sup>Δ</sup>*H*crosslinking than those of H-B3, which is partially due to the -OH bond interacting with the polymer chain.

**Figure 17.** DSC thermograms of hybrid-based coatings with and without the addition of fillers.

The glass transition temperature (*T*g) is a useful parameter to describe curing propagation and can be linked to mechanical properties [45]. During the curing processes, the molecular network mobility decreases according with the increase of *T*g [44]. This can partially explain the significantly improved pencil scratch hardness of H-A3 and H-B3.

Besides, the corrosion resistance is also greatly influenced by the chemical properties of fillers added. It is a crucial property for the coating's application, which strongly depends on a variety of parameters such as quality of resin systems, chemical properties of bulk materials and conditions the films exposed to. The reason that leads to the superior corrosion resistances of H-A could be interpreted as follows. Firstly, the existence of epoxy in hybrid components, a well-known corrosion resistance resin, helps maintaining a good anti-corrosion property. Secondly, a large amount of OH bonds are exposed to the surface of the Al(OH)3 particles which can interact with the resin chains through hydrogen bond [46], as shown in Scheme 1. As a result, higher corrosion resistances of samples H-A1, H-A2, H-A3 and PE-A1 are obtained in contrast to those added with BaSO4. With the increase of filler contents, the deterioration of corrosion resistance is mainly due to the significantly inadequate of resin and curing agent, which are essential in crosslink inside coatings [47,48].

**Scheme 1.** Hydrogen bonding between hybrid resin and Al(OH)3.
