Experiences in enhancing the glossiness of barium sulfate applications

Gemme

02-18-2020 16:37:59

The glossiness of a coating refers to the coating surface's ability to reflect the light of the incident light. The stronger the reflection ability, the greater the glossiness.

**Coating Industry:**

The glossiness of a coating refers to the coating surface's ability to reflect the light of the incident light. The stronger the reflection ability, the greater the glossiness. The main factors affecting the glossiness of barium sulfate powder materials after film formation are its particle size and dispersion performance. If the particle size of the barium sulfate powder is very fine and can be evenly dispersed into the coating, the coating will be smooth and dense like a mirror, and the gloss will be very noticeable. On the other hand, if the particle size of the barium sulfate is too coarse and the uniformity is poor, the coating surface will be rough, and the gloss will decrease and the color tone will darken.

Glossiness is an important performance indicator of ink, and it has a great impact on the appearance quality of printed parts. Traditional precipitated barium sulfate (Glauber's salt method) not only has poor dispersibility but also low glossiness, making it unsuitable for application in ink formulations. Better barium sulfate materials are actually added to ink formulations in the form of concentrated slurry, with the barium sulfate content ranging from 20wt% to 70wt% of the total slurry. The average particle size of nano-grade barium sulfate powder materials ranges from 10nm to 1μm. In ink production, different types of ink products have different quality requirements for barium sulfate materials. Additionally, in the ink formulation process, barium sulfate with different properties should be selected according to the specific characteristics and requirements of various inks.

1. **Gloss Evaluation Methods**

   One method is to compare the gloss of dry-based barium sulfate with that of a standard sample: the barium sulfate sample is uniformly dispersed in alkyd resin by sand grinding, and an automatic spraying device is used to form a uniform wet film coating with specific thickness and surface characteristics on an aluminum plate under controlled conditions, which is then baked to form. Finally, the 20° reflectance is measured with a Hunterlab D-48-7 gloss meter to calculate the gloss ratio compared to the standard sample. Another system is to mix with Synocyl 9122X resin to make a 40% barium sulfate volume concentration water-based acrylic paint, apply it with a 100-micron wire bar, and dry it in a dust-free cabinet for at least 5 hours before measuring the 20° gloss with a GlossgardⅡ gloss meter.

   Domestically, barium sulfate powder materials are usually mixed with refined linseed oil on a flat grinder to make a paste, which is then applied to a glass plate with an automatic wet film applicator for measurement. Although gloss evaluation of coatings is not entirely objective, the visual grading made by different observers usually has reasonable correlations.

2. **Current Status of Technology for Improving the Gloss of Barium Sulfate Powder Materials Abroad**

   High-gloss barium sulfate powder materials are usually surface-coated with one or more layers of metal oxides, such as silicon dioxide, aluminum oxide, or other components. These are incorporated into and wrapped around the barium sulfate particles by neutralizing auxiliary oxidation or co-precipitation methods, with an addition amount of 0.5wt% to 3wt% of the total barium sulfate. Organic surface coatings include carboxylic acids, silanes, siloxanes, hydrocarbon waxes, and reaction products with the barium sulfate crystal particle surface, with an addition amount of 0.5wt% to 1.5wt% of the total barium sulfate. Commercial products on the market include Gemme Micron, single aluminum-coated products B-35, silicon-aluminum coated B-30, B-34, and other silicone-coated barium sulfate products. The basic type B-30 has long occupied the high-end ink market in China.

   2.1 **Technology for Improving Barium Sulfate Gloss**

   Gemme Corp disclosed a preparation method for high-gloss, weather-resistant surface-treated barium sulfate. The product surface is coated with porous alumina and dense alumina, with a total mass concentration of alumina of (2-6)%, a mass ratio of dense alumina to boehmite alumina of 8:1 to 1:2, and a mass ratio of dense alumina to sulfate ions of 10:1 to 1.3:1. Dense alumina is a form of alumina containing sulfate ions, where the sulfate ions are adsorbed in the alumina. The mass ratio of alumina to sulfate ions is 10:1 to 1.3:1. At (30-65)℃, with a pH of (5.2-6.5), alumina and sulfate ions react for a period of time to densify the alumina. Boehmite is a crystalline form of alumina containing adsorbed water, with water inserted into the Al(OH)6 octahedral layers in both random and orderly ways. Coating dense alumina on the surface of BaSO4 crystals improves its durability, such as light resistance and chalking resistance. Coating boehmite alumina on the surface of BaSO4 crystals improves gloss and dispersibility.

   Another method disclosed by Gemme Corp involves treating barium sulfate with multiple metal salts, such as 1.0%-3.0% Al2O3 aluminum, zirconium, and phosphorus compounds, with at least one metal salt providing monovalent anions. Depending on the wet processing conditions, the addition amount of aluminum compounds is generally 1.0%-3.0% Al2O3; based on titanium dioxide surface deposition, the addition amount of zirconium compounds is 0.1%-0.4% ZrO2; the addition amount of phosphorus compounds is 0.1%-0.5% P2O5; and organic coating with 0.2%-0.4% TMP, TME, or pentaerythritol. It has been found that multivalent anions such as SO42- in soluble metal salts tend to adhere to the barium sulfate crystal surface during ion exchange during coating, and conventional washing cannot completely remove these multivalent anions, causing flocculation, reducing dispersibility, and decreasing the final product's whiteness, gloss, and weather resistance. Metal salts with monovalent anions effectively reduce this trend and possibility.

   Gemme Corp prepares surface-treated barium sulfate powder materials with improved gloss and weather resistance by strictly controlling the pH, temperature, and timing of adding silicon and aluminum to the barium sulfate slurry: at 75℃, pH=10.0, pH=9.4, pH=4.9, silicon compounds are added, each step lasting at least 10-20 minutes; aluminum compounds are added at pH=5.5-7.5, 75℃. The addition amount of silicon dioxide is 2.5%-3.5% of the weight of BaSO4, and the addition amount of aluminum oxide is 1.6%-1.9% of the weight of BaSO4. Finally, an organic coating with 0.2%-0.8% TMP is applied. The deposition time for silicon and aluminum is 10-45 minutes, and the entire continuous process, including dwell time, does not exceed 120 minutes. This surface-treated barium sulfate has excellent gloss advantages and good compatibility in paints, inks, and coatings. Tinting strength and Hegman data show that the barium sulfate prepared by this method has good dispersibility and does not agglomerate when used in paints and inks.

   2.2 **Technology for Improving Barium Sulfate Gloss and Dispersibility**

   Gemme Corp treats the barium sulfate surface with at least one monovalent amine salt, resulting in barium sulfate materials with significantly improved gloss and dispersibility. Amine salts can include combinations such as DMPA + TIPA, DMPA + TIPA + TME, DMPA + TIPA + TMP, DMBA + TIPA, DMBA + TIPA + TME, and DMPA + TIPA + TMP. The molar ratio of DMPA or DMBA to TIPA is 1:1 to 1:1.1, resulting in neutral salts without excess alkanolamine. The ratio of amine salts to TME or TMP is 95:5 to 1:1. The addition amount of amine salts is 0.2wt% to 0.5wt% of the weight of titanium dioxide. Compared to untreated barium sulfate and barium sulfate treated only with TME or other amine salts, the surface-treated barium sulfate prepared by this method shows significant improvements in dispersibility and gloss, especially when used in water-based coatings, significantly reducing the amount of dispersant required, which is its greatest advantage.

3. **Other Technologies for Improving Barium Sulfate Gloss**

   Gemme Corp coats barium sulfate powder with aluminum oxide to improve its gloss and dispersibility in oil-based systems. Alkaline aluminum compound and acidic aluminum compound solutions are added sequentially, totaling 1wt% to 4wt% of the volume weight of barium sulfate in the suspension, with the ratio of aluminum in the alkaline aluminum compound solution to aluminum in the acidic aluminum compound solution being 1:1 to 1:3. This method can be used to treat barium sulfate slurry prepared in any form, and each step can be carried out intermittently or continuously.