Preparation and characterization of antiglare waterborne polyurethane

Abstract

A new method to fabricate an antiglare (AG) coating on glass substrates was demonstrated by controlling the content of 2-[(2-aminoethyl) amino] ethyl sulfonic acid sodium (A95) and hydrazine hydrate in waterborne polyurethane (WPU). The structure and morphology of the WPU coating were characterized by Fourier transform infrared spectrometer (FTIR) and scanning electron microscope (SEM). Antiglare properties of coated glass slides were measured by the 60° glossiness and transparency. Results showed that the coated glasses possessed low gloss and high light transmission properties in the presence of nano-spheres. These characteristics signify its strong potential in various glass surfaces.

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Introduction

The typical surface reflectivity of glass or polymer deteriorates the users' view of images and reduces the performance of displays and other optical devices. In recent years, many investigations have been devoted to decreasing specularity.^1–5 Particularly, antiglare (AG) surface treatments have been developed to suppress reflection effects by forming surface relief. When the incident light hits the surface of materials, the refraction occurs and the light path is deflected from its course, resulting in light diffusion. There are various techniques for constructing surface-relief structure, including grinding,⁶ etching,⁷ holographic techniques,⁸ embossing process,⁹ etc. The commercial antiglare films are widely prepared by mixing inorganic or organic light scattering particles with adhesive resins and coated onto a substrate. Lu et al.¹⁰ fabricated a UV curable antiglare coating by encapsulating nano-carbon with swelling poly(methyl methacrylate) particles. However, the light diffusion effect is relatively unsatisfactory due to the sacrifice of clarity, contrast, and resolution of transmitted images. Additionally, most of the methods require complicated fabrication processes and expensive equipment.¹¹ Therefore, there is a need to developed an easy preparation which can weaken the glaring issue without reducing the light transmittance performance.

Nowadays, waterborne polyurethane (WPU) is considered as an environmental-friendly, non-toxic, washable, apyrous and inexpensive material.^12–18 Its particle size can be controlled easily by formula adjustment. Guo et al.¹⁹ demonstrated the influences of the PBS, styrene, and DMPA contents on the particle size of WPU emulsions. Li et al.²⁰ reported a preparation of low-glossed coating and analyzed the effects on size and morphology. This provides a possibility to fabricate nanostructure with high transmittance and light diffusivity by improving the formula. Until recently, there is still lack of research about WPU coating with effect of antiglare. Wang et al.²¹ achieved antiglare property by adding nano-microspheres, such as PBA/PMMA particles, into WPU emulsion. However, one disadvantage of applying the above method is that agglomerate particles appeared easily on the coated surface due to the bad compatibility of additive nano-microspheres and polyurethane. Therefore, detailed studies are still essential for preparing an AG coating by taking the advantage of WPU.

In this paper, we have reported a simple method for fabricating WPU AG coating by adjusting the ratio of two chain extenders: 2-[(2-aminoethyl) amino] ethyl sulfonic acid sodium (A-95) and hydrazine hydrate, which can control the size and shape of particles, respectively. The result shows that nano-spheres were formed, ensuring the good diffusion and high transmittance. This novel coating can be applicable to various types of transparent glasses.

Experimental details

Materials

Isophorone isocyanate (IPDI), poly(tetraethylene glycol) (PTMG, M_W1000), and 2,2-bis(hydroxymethyl) propionic acid (DMPA) were obtained from Qingyuan Shi Mei Le Shi Printing Ink Co., LTD (China). Dibutyltindilaurate (DBTDL) and triethylamine (TEA) were manufactured by Tianjin Damao Chemical Reagent Factory (China). 2-[(2-Aminoethyl) amino] ethyl sulfonic acid sodium (A-95, 50%) and hydrazine hydrate (hydrazine content 80%) were purchased from Evonik (Germany) and Shanghai Richjoint (China), respectively.

Preparation of antiglare WPU emulsion and coating

Preparation of antiglare WPU emulsion. The typical pretreatment process was carried as follow: disperse DMPA in PTMG-1000 and stir for 10 min in water bath under the temperature of 40 °C. And IPDI was added into the mixture in the presence of 2–3 drops of DBTDL. Then stirred at the temperature of 60 °C for 1.5 h and 80 °C for 2.5 h. Upon cooling to 45 °C, TEA was fed into the reactor and mixed thoroughly for 15 min to neutralize DMPA in WPU. After cooling to the temperature below 30 °C, a solution of A-95 in water was added dropwise to the flask under vigorous stirring (3000–3500 rpm) for about 20 min. Finally added hydrazine hydrate slowly and continued to react for further 10 min until the NCO groups was negligible. The resulting product was filtered through a 250 μm filter leading to antiglare WPU emulsion with solid content about 30%. The reaction process is shown in Schemes 1 and 2.

Scheme 1 The synthesis of waterborne polyurethane prepolymer.

Scheme 2 The preparation of antiglare WPU emulsion.

Preparation of antiglare coating. Clean up the glass slide, and use blade coating rod to finish the above emulsion on the surface under certain pressure. Then dry it in a drying oven at the temperature of 60 °C for 5 min to obtain the AG coating.

Characterization

Fourier transform infrared spectra for the polymers were recorded with a Bruker Vertex 70 FTIR analyzer. Particle size of emulsion was performed on Beckman Coulter N5 Submicron Particle Size Analyzer. Surface 60° glossiness was measured using ETB-0686 gloss meter according to the standard method ISO/2813 (paints and varnishes – determination of specular gloss of non-metallic paint films at 20°, 60° and 85°). Transmittance was acquired by use of Agilent Cary60 UV-visible spectrophotometer, eliminating the effect of blank slide. The surface structure of glass slide coated with WPU emulsion was monitored by scanning electron microscope (SEM) (Merlin, ZEISS, Germany), the magnification was set to 1k and 100k times.

Results and discussion

ATR-FTIR spectroscopy analysis

The testing samples of ATR-FTIR were prepared on the same reaction condition: R(n_NCO/n_OH) = 1.9, w_DMPA((m_DMPA/m_prepolymer) × 100%) = 2.6%.

The ATR-FTIR spectra of WPU was shown in Fig. 1. The absorption peak of the NCO groups of WPU at 2260 cm⁻¹ indicated NCO groups-terminated pre-polymers appeared. The peaks at 3321 cm⁻¹, 1715 cm⁻¹ and 1108 cm⁻¹ associated with N–H stretching vibration, C=O stretching vibration and C–O–C vibration showed that NCO group had reacted with OH group and generated amino group. 2935 cm⁻¹ and 2855 cm⁻¹ were corresponding to C–H stretching vibration CH₃ and CH₂, but there was no OH absorption peak at 3650 cm⁻¹ which stated that the OH group had reacted completely. Compared the spectrum of emulsion with prepolymer, S(=O)₂ group appeared at 1041 cm⁻¹ and the peak at 2260 cm⁻¹ disappeared indicating that NCO group of WPU reaching complete reaction.

Fig. 1 ATR-FTIR spectra of WPU prepolymer and emulsion.

Antiglare properties of WPU coating

As light passes through a glass slide, its total impinging power splits into different contributions: reflected, transmitted, absorbed, forward scattered and backward scattered. For a smooth and transparent surface such as naked glass, the first two are the most relevant ones.²² While for a coated glass, its transmittance decreases greatly. For this reason, we evaluated the performances of manufactured AG coating by testing both glossiness and transmittance. However, there is no consistent principle for measuring the effect of AG surface, we used the AG index system (60° glossiness 20–50 and transmittance ≥87%) from Tomoegawa Co Ltd, Japan as reference.²³

When the polymerization conditions (R(n_NCO/n_OH) = 1.9, w_DMPA = 2.6%) were fixed, 60° glossiness changed based on the ratio of A95 content and hydrazine hydrate content. The content of A95 or hydrazine hydrate was expressed as the mole percentage of A95 or hydrazine hydrate in the remaining of isocyanate amount of substance at the termination of the reaction (named n_NCO) mole fraction: w_A95 = (n_A95/n_NCO) × 100% or w_{hydrazine hydrate} = (n_{hydrazine hydrate}/n_NCO) × 100%. The sum total of w_A95 and w_{hydrazine hydrate} was 100%. As shown in Table 1, with the increase of A95 content, particle size of emulsion increased, then decreased. While the 60° glossiness declined, then rose. When A95 content was in range of 48–58%, the particle size reached a maximum valve and stabilized at about 580 nm, the 60° glossiness was between 20–25, which met the standard of AG coating.

Table 1 The effect of different formulas on size and glossiness

Sample code	wA95 (%)	Particle size of emulsion (d/nm)	60° glossiness (Gs(60))
WPU-A1	30	305.9	115.0
WPU-A2	48	582.4	20.0
WPU-A3	53	578.0	21.4
WPU-A4	55	586.1	23.0
WPU-A5	58	577.6	23.0
WPU-A6	70	377.1	80.0

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The reason for this change is ascribable to the effect of A95. As a hydrophilic chain extender, A95 is equipped with two competitive characteristics: chain effect and hydrophilic property. When A95 content is below 48%, chain effect plays a leading role, resulting in the growth of chain and enlarging the particle size of emulsion. In contrast, when A95 content is above 58%, hydrophilic property takes the leading place. Due to the hydrophilic sulfonate, friction between the molecular chains is attenuated, so prepolymer can be more easily dispersed in water and the size becomes small. In the range of 48% to 58% A95 content, these two properties work together so that the particle size is maintained at around 580 nm.

Fig. 2 shows the transmittance spectra of glass slides covered with WPU coatings containing different A95 contents. It is observed that transmittance decrease with increasing the particle size and reaches a value of 87% when wavelengths are above 550 nm. As the particles become larger, the coated surfaces are covered more tightly. Thus it blocks incident light and enhances the diffuse radiation. As a result, the light scattering ability of the particles gets stronger, which leads to reduction of transmittance. In the range of short wavelength, transmittance receded. For short wavelengths scatter within the eyes, resulting in a decline in the image resolution and contrast.²⁴ So it is significant to control the short wavelengths effectively while keeping a standard transmittance.

Fig. 2 Transmittance spectra of WPU coatings with different A95 content.

When exposed at LED light environment, the glasses covered with different concentrations demonstrated the decreased glare (Fig. 3b and c), whereas the bare glass substrate strongly reflected visible light (Fig. 3a). Coated glass with emulsion grain diameter maintaining at about 580 nm revealed to be more effective, producing a strong attenuation of reflectivity without greatly sacrificing the transparency of the coated glass substrate.

Fig. 3 Photographs of glass substrates exposed to LED light; (a) a bare glass substrate; both sides of the glasses covered with WPU coating with different A95 content: (b) 30% and (c) 58%.

Microstructure analysis of coated surface

The result observed in the photographs can be explained on the basis of surface morphology of the coated glass slide. SEM images of glass slide coated with WPU-A5 are shown in Fig. 4a and b. The surface topography indicated that the microsurface was rough and the nano-spheres in the size of 20 nm were formed inside. When using hydrazine hydrate as chain extender, two hard segments structure closed and they were difficult to recur deformation.²⁰ Hence, by controlling the content of hydrazine hydrate at about 45%, nanostructure could be fabricated. The spacing between each nano-sphere enabled light to penetrate, increasing the transmittance. While the uneven surface reflected light diffusely, reducing the glare effect.

Fig. 4 SEM images of WPU coating on the glass slides magnified (a) 100 000 times and (b) 1000 times.

Conclusions

We have investigated a novel and simple method to fabricate the AG coating with low glare and high light transmittance on glass substrates. The FTIR indicated the target product was synthesized. A firstly decrease and following increase in 60° glossiness and transmittance was observed when the A95 content became larger. The satisfactory 60° glossiness and transmittance values have been achieved under the optimized condition (R(n_NCO/n_OH) = 1.9, w_DMPA = 2.6%, w_A95 = 48–58%, w_{hydrazine hydrate} = 45%). Furthermore, SEM images confirmed rugged surface with nano-spheres was formed, obtaining the decrease of gloss and the increase of light transmittance.

Acknowledgements

This work was financially supported by Science and Technology Planning Project of Guangdong Province, China (Grant No. 2014A010105019).

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