Research and development of high brightness leveling additives

Research and development of high brightness leveling additives

Hu Sijun, Zhou Shaoying (Wuhan Yincai Technology Co., LTD.)

Abstract: This paper introduces the current situation and theoretical research on high brightness leveling additives in powder coatings, focusing on the performance development of surfactants in acrylic categories such as leveling agent in powder coatings by Wuhan Yincai Company. It also discusses the developed high brightness leveling agent.

I. Introduction

Leveling agents are essential additives in powder coatings. Currently, the industry predominantly uses acrylic-based leveling agents. With expanding sectors like solar power, machinery, automotive, and new energy industries, there is growing demand for coatings and additives that meet emerging industry requirements such as transparency, leveling performance, and extreme environmental adaptability. There is a pressing need to develop high-color-fluorescent leveling agents with excellent compatibility. These agents should effectively combine with coatings to enhance film formation characteristics including flatness, transparency, superior gloss (DOI), and outstanding inter-layer adhesion with excellent anti-peeling properties.

II. Current situation of leveling agents

Most conventional leveling agents are acrylic homopolymers or copolymers. While effective for leveling, they tend to produce orange peel and haze effects. When manufacturing powder coatings under imprecise equipment conditions or aging machinery that compromises extrusion dispersion, or when interacting with low-surface-tension substances during production and application, orange peel patterns may occur. Furthermore, existing leveling agents predominantly consist of opaque particles, making them unsuitable for modern industrial surface coating applications. Maintaining high light transmittance and exceptional flatness is crucial: the former ensures adequate light penetration while the latter minimizes scattering loss. The current limitations in light transmittance result in significant practical issues – although these leveling agents achieve satisfactory leveling performance, their actual light transmittance and scattering loss levels severely restrict practical applications.

Therefore, developing a high-performance leveling agent specifically tailored for emerging industries' surfaces is crucial. By applying the propylene-bonded polymerization principle, functional groups such as hydroxyl, carboxyl, amine, epoxy, silicon, and fluorine are grafted onto the molecular structure of butyl acrylate copolymer-based leveling agents. The incorporation of polar groups enhances pigment and filler dispersion while improving coating film quality. These polar modifications endow the leveling agent with comprehensive and superior performance. Research has documented applications of amino, hydroxyl, carboxyl, and epoxy groups, with corresponding products including BYK360P and 3900pv99, etc. To improve the compatibility of leveling agents, improve the high fresh brightness of leveling agents and reduce fog shadow are the goals that Wuhan Yincai company has been pursuing in terms of leveling additives.

III. Structural studies

The development of high-viscosity leveling agents is based on the composite structure of acrylic long chains and hyperbranched resins. The high compatibility between hyperbranched resin's terminal hydroxyl groups and polyester resin, combined with the surface tension, high gloss, and smooth characteristics of long-chain acrylic resins, constitute key factors in coating technology research. Until today, the most effective results have been achieved through the application of acrylic ester flow aids. Multiple factors influence the flow and leveling processes, including surface tension, inter-facial tension, melt viscosity, curing characteristics, pigment dispersion, film thickness, bubbles, as well as powder particle size and particle size distribution. The technical mechanisms underlying post-coating film spreading are as follows:

Foreign companies have tried to use organosilicon as a branched chain on the main chain of acrylic acid

Modified leveling agent is based on this, with more branched chains of polyether modified

The polyether modified silicone is transplanted to the structural branch of acrylic acid, forming a new type of polyether organic silicon long chain structure.

Thanks to their novel spatial structures and unique properties, dendritic or spherical polymers have sparked widespread interest in the scientific community. Superbranched polymers, which share similar structures and properties with dendrites, are recognized as a new type of material in the 21st century. These materials have garnered significant attention from both academia and industry, becoming a hot research focus in polymer science in recent years. In response, Yinhua Company has pioneered an innovative approach by modifying traditional superbranched resins to develop a novel long-chain composite structure of superbranched resins.

The elongated flexible main chain structure of acrylic resin provides excellent leveling properties. The hyperbranched chains in superbranched resins deliver superior compatibility and moderate strength to reduce surface tension, outperforming copolyacrylic resins in compatibility with polyester and epoxy resins. These resins demonstrate enhanced ability to lower coating surface tension while improving anti-cavitation performance during powder coating application, along with short-wave leveling regulation. Historical experience shows that silicone-based, polyether, and functionalized leveling agents primarily regulate short-wave leveling, whereas acrylic-based leveling agents favor long-wave leveling. To achieve optimal leveling performance, both short-wave and long-wave effects must be considered, along with compatibility between the leveling agent and polyester. In our R&D applications, we employ a spherical hyperbranched resin-acrylic leveling agent containing hydroxyl groups, achieving ideal high fresh film brightness and low haze leveling effects.

This diagram effectively illustrates the mechanism of action of leveling agents. The hyperbranched modified linear acrylic backbone migrates to the film surface, while the hyperbranched resin and polyester exhibit compatibility with moderate surface tension directed toward the air interface. The acrylic structure alone serves as the leveling adjustment component. The acrylic backbone and hyperbranched-modified component, in a highly compatible and nano-spherical state with extremely low melt resistance, migrate to the coating surface and spread uniformly , so it has the characteristics of high fresh brightness and very low fogging and leveling.

Through extensive experimental validation, Wuhan Yincai Technology Co., Ltd. has successfully developed the super-branch modified L87 high-gloss leveling agent. Its nano-cavity structure within the main chain's acrylic composite forms a spherical configuration that fulfills the requirements for powder coatings' high-gloss systems to achieve high mirror finish, low haze, and superior reflectivity. This leveling agent is specifically designed for emerging industries including solar power coatings, mirror finishes, and high-gloss, low-haze premium powder coatings.

IV. THE EXPERIMENTAL PART

4.1 Main raw materials and testing instruments

Butyl acrylate (BA), isooctyl acrylate (HEA), styrene (ST): BASF; azo

(AIBN): Shanghai Aromatics; Toluene (TOL), polyester, TGIC, carbon black, barium sulfate, benzoin, leveling agent L588, brightening agent L701: commercially available. All the above raw materials are industrial grade.

IQ Flex 20/FLex 60 three-angle fog shadow gloss meter: RHOPOINT (UK);

CAP2000* rotational viscometer: DV-Ⅱ, BROOKFIELD; film thickness meter: German Nix QNIX4500; twin screw extruder: SLJ-32, Lingyu powder machinery.

4.2 Synthesis of hyperbranched resins

To prepare hyperbranched resin H104/C100 using the solution polymerization method, the following components were weighed: 30 g H101,300 g mixed monomers, and 5300g toluene. The mixture was transferred to a round-bottom flask containing a 1L mechanical stirrer, thermometer, separatory flask, and nitrogen inlet. Under N2 protection, the temperature was gradually increased to 110℃. The resin was then slowly stirred while being dripped with the reaction proceeding for 1-3 hours. During this period, acid value and hydroxyl value measurements were taken hourly until minimal changes were observed. Subsequently, the reaction was evacuated under vacuum for 1-3 hours, with periodic (30-minute intervals) measurements taken during this process to ensure stability. The resulting polymer was identified as hyperbranched polyacrylic acid resin.

                               The reaction mechanism is shown in Equation (1).

4.3 Preparation of powder coatings and film

Preparation of Powder Coating: Weigh the polyester resin, TGIC, additives, and other components according to the formula in Table 1. After physical mixing, the materials undergo melt extrusion through a twin-screw extruder, followed by tablet pressing, crushing, and sieving to obtain powder coating. The coating is uniformly sprayed onto degreased and rust-removed cold-rolled steel plates using an electrostatic spray gun. After baking at 200°C for 15 minutes, the coated steel sheets are naturally cooled to room temperature to produce the final coating.

Table 1 Powder coating formula

4.4 Performance testing

The acid value of leveling agents was determined according to GB/T 6743-2008; the hydroxyl value was measured as per GB/T 12008.3-2009; melt viscosity was assessed using a CAP2000 viscometer at 50℃/min (750 rpm); glass transition temperature (Tg) was tested in accordance with GB/T 19466.2-2004 under an N2 atmosphere with a heating rate of 10℃/min. Coating impact resistance was evaluated per GB/T 1732-2020; coating hardness was measured as per GB/T 6739-2006; curing time was determined according to HG/T 2006-2022. Tapered flow properties were evaluated based on GB/T 28861—2012 for testing; gloss according to GB/T 9754—2007 for testing.

V. Results and discussion

5.1 Influence of synthesis process on properties of super branched modified acrylic resin

Through the formulation design, the superbranched modified acrylic resin (HL-1~3) was prepared by adding superbranched tree H104/100 into acrylic polymerization monomer at different ratios during the stage of acrylic solution polymerization resin synthesis.

The corresponding powder coatings (HLPC-1~3) were obtained, and their properties are shown in Table 2 and Table 3. As can be seen from Table 2 and Table 3, with the increase of the amount of superbranched resin added, the viscosity and glass transition temperature (Tg) of acrylic resin gradually decreased. As the powder coating's flowability gradually improves with decreasing concentration, the coating's DOI value progressively increases. This phenomenon stems from the fact that hyperbranched resins, being low-functionality aliphatic macromolecules, exhibit lower rigidity and better compatibility compared to traditional acrylic esters. These characteristics result in higher gloss, reduced haze, and minimized orange tint in the coating. The elevated DOI value originates from the structural differences between hyperbranched and linear acrylic ester structures when forming three-dimensional spherical configurations. By incorporating hyperbranched resin H104/100 at high temperatures before material discharge, and combining it with antioxidants and additives according to Table 2 ratios, the hyperbranched modified acrylic resin (HL-4~5) was synthesized. The resulting powder coating (HLPC-4~5) demonstrates improved performance as shown in Table 3. However, Table 3 and Table 2 reveal that pre-discharge addition of hyperbranched resin causes poor compatibility in the modified resin, leading to solid precipitation and turbidity. This indicates inadequate reaction efficiency of subsequent additives. Through comparative analysis of two different processes, the addition of hyperbranched resin before polymerization has a greater impact on the viscosity and Tg of acrylic resin, resulting in coatings with higher gloss and brightness. This may be attributed to the fact that the hyperbranched resin added after polymerization physically combines with the acrylic resin, allowing its characteristics to be more prominently expressed in the mixture. In contrast, the hyperbranched resin added before the polymerization stage combines with the acrylic resin system through physical dispersion followed by chemical reactions, enhancing compatibility between the two components and better manifesting the coating's gloss and brightness. Comprehensive comparison of the performance of hyperbranched modified acrylic resin and powder coatings reveals that the acrylic resin modified with hyperbranched resin added before polymerization exhibits superior leveling properties.

Physical and chemical properties of over branched modified acrylic resin HL 1-5 in Table 2

Table 3 Performance of over branched modified acrylic resin powder coating HLPC 1-5

Table 4 Comparison of coating performance between modified leveling agent HL-2 and general leveling agent

As can be seen from Table 1 to Table 4, the overbranched modified acrylic leveling agent has better fresh brightness, very low fog shadow and excellent surface texture than the ordinary liquid leveling agent under the same conditions.

5.2 Conclusions

Through the reaction of hyperbranched resin with acrylic ester before polymerization, the HL series hyperbranched resin-acrylic leveling agent was developed to improve powder coating leveling. The introduction of hyperbranched resin effectively enhances the color brightness of powder coatings. When 1% hyperbranched resin is added before polymerization, the polyacrylate leveling agent exhibits optimal comprehensive performance. Therefore, during production, when structurally optimizing and modifying polyacrylate leveling agents using hyperbranched resin, it is crucial to perform appropriate physicochemical modifications based on the resin's performance indicators to better adapt to the polyacrylate leveling agent resin synthesis system. Additionally, attention should be paid to the compatibility between hyperbranched resin and polyacrylate, meanwhile need to make sure the modified polyacrylate is matched with the appropriate monomer, in order to give the coating excellent leveling performance, excellent film transparency, excellent gloss and DOI (vividness), as well as excellent interlayer adhesion and excellent anti-peel properties.

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