polymers Article Synthesis of an Aqueous Self-Matting Acrylic Resin with Low Gloss and High Transparency via Controlling Surface Morphology Qiwen Yong 1,2,* and Caizhen Liang 3 1 Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637002, China 2 Institute of Applied Chemistry, China West Normal University, Nanchong 637002, China 3 State Key Laboratory of Applied Microbiology Southern China, Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, Guangdong Institute of Microbiology, Guangzhou 510070, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0817-2568637 Received: 25 January 2019; Accepted: 11 February 2019; Published: 13 February 2019 Abstract: This paper reports on a novel, film-forming acrylic polymer resin that exhibits low-gloss surface and high transparency via controlling film morphology at sub-micron roughness levels. Such microstructure is controlled by means of the copolymerization process increasing the allyl methacrylate (AMA) crosslinker content from 0 to 0.4 wt %. This acrylic resin makes it possible to avoid high loadings of matting agents, while also having good abrasion resistance and soft-touch feeling. Gloss levels of as low as 4 units at 60◦ incident angle and light transmittance of up to 85% have been achieved. The chemical structure of the aqueous acrylic resin was characterized by ATR-FTIR and NMR spectroscopy. The film morphology and surface roughness were measured by SEM and AFM analysis. The emulsion particle morphology and glass transition temperature were obtained by TEM and DSC, respectively. The effects of the crosslinker content on the light transmittance, glass transition temperature, and thermal degradation stability were also discussed in detail. The characterization results conclude that an acrylic polymer with interesting optical properties and high thermal stability can be obtained, which is desirable for leather applications. Keywords: aqueous acrylic resin; cross-linking monomer; low gloss; self-matting effect; glass transition temperature; high transparency 1. Introduction Coating systems and processes which give a substrate a uniform and matt surface are of considerable interest. The reasons for this are predominantly practical in nature. In comparison with a highly glossy surface, a matt surface is superior to hide surface imperfections and does not require to be cleaned very frequently [1–3]. The matt surface may also be desirable on safety grounds to avoid the strong reflection of light [4,5]. The reduced glare of surfaces in schools, hospitals, and workplaces helps to minimize the visual distraction, so that people’s concentration is more focused in those environments [6]. It is universally known that aqueous acrylic polymers such as those of lower alkyl acrylates and lower alkyl methacrylates, as well as their copolymers are widely used in the coating industry [7–9]. However, most of the acrylic polymers dry to produce generally highly glossy surfaces [10,11]. To reduce the gloss of aqueous acrylic coatings, varieties of non-soluble particles, known as matting agents or dulling agents, are added into aqueous acrylic resins. During the drying process, the loss of solvents and condensation products result in a fairly high shrinkage of the acrylic coating film. Polymers 2019, 11, 322; doi:10.3390/polym11020322 www.mdpi.com/journal/polymers Polymers 2019, 11, 322 2 of 14 Consequently, a diffusely reflecting micro-textured surface is formed [12–14]. The most common dulling agents are silica or waxes, or both of them in combination [15–17]. Clays, metal soaps, ground-ups, or sometimes hard organics are also used [18,19]. However, the common characteristic of these matting agents is that they are hard, inflexible materials that must be mixed with a liquid-based coating. In addition, the amount of liquid-based resin must be sufficient so that it completely envelops the non-soluble particles and fills the interspace between them. When extremely low gloss levels are demanded, use of such high levels of hard dulling agents would have the problem that they could not always be adequately wetted and surrounded by liquid-based resin, or in other words, they could not be restrained into a uniform and continuous film. In such cases, the mechanical properties of the liquid-based coating would be impaired [20]. Since the matting agents are also individual hard particles, compared to the dense structure of liquid-based resins, it is difficult to obtain a soft-touch surface [21]. Therefore, the development of an alternative method of fabricating a low gloss acrylic polymer without these drawbacks would be highly desirable. This paper reports a novel aqueous emulsion of acrylic polymer that is inherently of low gloss. The polymer meets the main objective we established based on marketplace demand for very low gloss and high transparency surfaces when utilized as a leather finish. The key strategy of preparing the low gloss acrylic polymer resin is controlling the microstructure for yielding a tailor-made roughness. Such microstructure is achieved by controlling the amount and addition order of each monomer, as well as the introduction of a cross-linking monomer allyl methacrylate (AMA), which can cause the incompatibility within the resultant resin. This causes the formation of a coating that has a micro-rough surface rather than one that is smooth. The majority of this paper is concerned with what the chemical structure of this acrylic polymer resin is, through the FTIR-ATR and NMR spectroscopies, and what this polymer surface morphology is demonstrating via SEM observation, as well as how much the polymer surface roughness is through the AFM calculations. This paper also gives a reasonable explanation for the formation of the micro-rough surface of acrylic polymer resin with the aid of the TEM observation and DSC analysis. Finally, the effects of the crosslinker AMA content on the glass transition temperature, film transmittance, and thermal degradation stability are discussed in detail. 2. Experimental 2.1. Materials Butyl acrylate (BA), allyl methacrylate (AMA), and tert-butyl methacrylate (t-BMA) were purchased from Shanghai Aladdin Reagent (Shanghai, China). Potassium persulfate (KPS) and sodium lauryl sulfonate (SLS) were purchased from Sinopharm Chemical Reagent (Shanghai, China). All of the reagents were of analytical grades and used as received. Mesh screen (stainless steel, 200 screening size) was purchased by AS-ONE Corporation (Tokyo, Japan). Deionized water (DI) was used for the polymerization and treatment process. 2.2. Preparation of Low Gloss Aqueous Acrylic Resins The composition and amount of raw materials for the preparation of aqueous acrylic resin emulsions are summarized in Table1. First of all, deionized water and a certain amount of emulsifier SLS were charged into a 500-mL round-bottom flask under nitrogen atmosphere. It was stirred at 250 r min−1 with a mechanical stirrer for dissolving and dispersing. When the system temperature was heated to 70 ◦C, the first group of 20% BA/AMA mixed monomers (total weight percentage is fixed at 20 wt %) were dropwise added to the system for emulsifying about 40 min. Subsequently, a certain amount of initiator KPS was pumped into the system to prepare the seed emulsion within 30 min. The remaining 80% of BA/AMA mixed monomers of the first group were dropwise added to the system, followed by reacting at 70 ◦C for 1 h. Then, the second group of t-BMA/BA mixed monomers and the remaining KPS solution were pumped into the flask through two separate tubes at a constant rate within 30 min. Finally, the polymerization reaction continued at 70 ◦C for 2 h. The aqueous resins Polymers 2019, 11, 322 3 of 14 of acrylic polymer were obtained after being filtered through a 200-mesh screen. The pathway for the preparation of the acrylic polymer resins could be seen in Scheme1. Table 1. The composition and amount of raw materials for preparing the low gloss aqueous acrylic resins. Component Abbr. Composition (wt %) BA 19.6–20 AMA 0–0.4 Monomer Polymers 2018, 10, x FOR PEER REVIEW t-BMA 4.8 3 of 15 BA 0.2 rate within 30 min. Finally,Emulsifier the polymerization SD reaction continued 0.075 at 70 °C for 2 h. The aqueous resins of acrylic polymer were obtainedInitiator after being KPS filtered through a 0.15200-mesh screen. The pathway for the preparation of the acrylic polymerMedium resins could DI be seen in Scheme 75 1. CH 2 CH 2 o CH 2 p CH 2 CH q Scheme 1. Pathway for the preparation of the acrylic polymer resins. 2.3. Preparation of of Low Low Gloss Gloss Aqueous Aqueous Acrylic Acrylic Films Films The low gloss polymer filmsfilms were prepared by coatingcoating the aqueous acrylic resins onto a glass µ sheet with a bar coater.coater. The thickness of the wetting filmsfilms was fixedfixed atat 100100 μm.m. TheThe coatedcoated wetting ◦ filmsfilms werewere drieddried atatambient ambient temperature temperature for for 24 24 h h and and then then further further dried dried in in a convection a convection oven oven at 90at 90C for°C for 1 h 1 to h remove to remove the residualthe residual water. water. Finally, Finally, the polymer the polymer films films were were peeled peeled off from off thefrom glass the sheetglass andsheet kept and in kept a vacuum in a vacuum desiccator desiccator prior toprior testing. to testing. 2.4. Characterization Table 1. The composition and amount of raw materials for preparing the low gloss aqueous acrylic 2.4.1.resins. Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) The infrared spectrumComponent of the acrylic polymerAbbr. resinComposition was obtained (wt %) on a Bruker VERTEX70 FTIR BA 19.6–20 spectrometer (Bremen, Germany) under N2 protection. The sample was scanned 16 times at a resolution −1 AMA −1 0–0.4 of 1 cm over the frequencyMonomer range of 4000–600 cm .