The Relationship between Solid Content and Particle Size Ratio of Waterborne Polyurethane

Abstract

A series of high solid content carboxylic acid/sulfonic acid waterborne polyurethanes was prepared by the emulsion dispersion method. The particle size and solid content were measured. By changing the particle size of the large particles to achieve different particle size ratios, high solid content waterborne polyurethanes were obtained at specific particle size ratios. When the particle size ratio was >7, 4–5 or 2–3, the aqueous polyurethane could reach a higher solid content (more than 56%). This indicated that solid content is related to particle size distribution in high solid content waterborne polyurethane. Moreover, the corresponding three-dimensional stacked models (simple cubic accumulation, face-centered cubic accumulation, cubic close packing and hexagonal closest packing) were established.

1. Introduction

With the improvement of environmental protection requirements and the successive introduction of environmental protection policies in various countries, water-based polyurethanes (WPU) have been widely used because they use water as a solvent, are non-polluting, safe and reliable, and have good mechanical properties and good compatibility [1,2,3,4,5,6]. However, almost all waterborne polyurethane products must address the high cost of drying due to prolonged drying because of the high heat of evaporation of water, high water content, and limited space-time yield [7,8]. It is therefore very important to prepare a high solid waterborne polyurethane.

Until now, studies on the solid content of waterborne polyurethane have been mainly divided into two aspects: formula and particle size. In the formulation aspect, various factors have been researched such as new hydrophilic monomers, different isocyanates, special chain extenders, and polyol [9,10,11,12,13]. Vilas [14] chose a new type of small molecule sulfonate, acrylamide-tert-butane sulfonate, as a chain extender, which greatly improved sticking time, peel strength, and hardness. Li [15] synthesized a waterborne polyurethane with a solid content of 50% using a mixed diisocyanate of HDI (hexamethylene diisocyanate):IPDI (isophorone diisocyanate) with a mass ratio of 30:70. Cao [16] prepared aqueous polyurethane emulsions with a solid content of more than 50% and a viscosity of less than 200 mPa·s using different macromolecular diols as raw materials. However, this cannot be widely used as the application performance is limited by synthetic formulation in many cases [17]. On the other hand, there have been many studies on the improvement of the solid content of waterborne polyurethane by adjusting particle size in recent years. Peng [18] prepared WPU with bimodal distribution by using the emulsion of large particles to disperse another prepolymer. According to the electric double layer theory [19], the charge of the emulsion particles has a significant effect on the interaction between the particles. It is also meaningful to study the effect of charge on the particle size ratio (the ratio of the equivalent particle diameters of two different emulsions’ particle sizes) by the carboxylic acid/sulfonic acid type aqueous polyurethane. The emulsion particle adhesion phenomenon occurs in high solid content carboxylic acid type waterborne polyurethane, but not in sulfonic acid type waterborne polyurethane. In our previous research [17,20], the relationship between particle size ratio and solid content was preliminarily studied. A high solid content waterborne polyurethane with particle bimodal distribution was prepared by a special physical blending method. Therefore, the relationship between particle size ratio and solid content needs further research.

In this paper, a series of high solid carboxylic acid/sulfonic acid waterborne polyurethanes was prepared by the emulsion dispersion method. The particle size of the small particles was maintained constantly while the particle size of the large particles was changed to achieve different particle size ratios of the mixed emulsions. According to the relationship between particle size ratio and solid content, the particle size ratio under high solid content is consistent with that of compact crystal packing. Hence, the corresponding model was established, providing a theoretical basis for the preparation of high solid waterborne polyurethane.

2. Experimental

2.1. Materials

Isophorone diisocyanate (IPDI, analytical grade) was purchased from Shanghai Reagent Factory (Shanghai, China). Dimethylolpropionic acid (DMPA, analytical grade) was from Ling Chemical Co. (Beijing, China). Polyneopentylene-hexamethylene adipate glycol (PHNA, Mw = 2000) was provided by Xuchuan Chemical (Suzhou, China). Butane-1,4-diol (BDO, analytical grade), ethylenediamine (EDA, analytical grade), acetone, and dioctyl sebacate were purchased from Beijing Tongguang Chemical Reagent Corp. (Beijing, China). Sodium ethylenediamine sulfonate (AAS, analytical grade) and sulfonate type polyester diol (s-polyester, analytical grade) were purchased from Transfar Group Co., Ltd. (Hangzhou, China). Dibutyltin dilaurate (T-12, analytical grade) was from the Sinopharm Chemical Reagent Crop. (Shanghai, China).

2.2. Preparation of Small-Particle Waterborne Polyurethane (WPU-S)

BDO and PHNA were firstly added and mixed well in a 500 mL three-necked flask immersed in an oil bath at a temperature of 80 °C. Then IPDI and three drops of T-12 (50 wt %) were added to the flask at 80 °C for 4 h. The mixture was cooled and AAS was added to react for 30 min. Water was added to the prepolymer under high speed stirring and emulsified for 20 min. Finally, WPU-S₁ was obtained after removing acetone by rotary evaporation. The formulation of WPU-S₁ is shown in

Table 1. The formulation of small-particle waterborne polyurethane (WPU-S).

Sample Designation	IPDI/g	DMPA/g	AAS/g	s-polyester/g	PHNA/g	TEA/g	BDO/g	Water/g
WPU-S1	12.45	0	4.52	4.0	36.0	0	1.80	25.19
WPU-S2	18.69	2.58	0	0.9	39.1	1.94	1.20	27.6

.

DMPA, BDO, PHNA, and s-polyester were firstly added and mixed well in a 500 mL three-necked flask immersed in an oil bath at a temperature of 80 °C. Then IPDI and three drops of T-12 (50 wt %) were added to the flask at 80 °C for 4 h. The mixture was cooled to room temperature and acetone was added to reduce the viscosity. Then TEA was added and the neutralization reaction proceeded for 40 min. Water was added to the prepolymer under high speed stirring and emulsified for 20 min. Finally, WPU-S₂ was obtained after removing the acetone by use of a rotary evaporation instrument. The particle size of the emulsion was measured and the amount of s-polyester adjusted compared to the particle size of WPU-S₁ until the difference in particle size was ±1 nm. The formulation of WPU-S₂ is shown in Table 1.

2.3. Preparation of Large-Particle Waterborne Polyurethane (WPU-L) and High Solid Content Waterborne Polyurethane (WPU-H)

DMPA, BDO, PHNA, and s-polyester were firstly added and mixed well in a 500 mL three-necked flask immersed in an oil bath at a temperature of 80 °C. Then IPDI and three drops of T-12 (50 wt %) were added to the flask at 80 °C and the flask was kept at 80 °C for 4 h. The mixture was cooled to room temperature and acetone was added to reduce the viscosity. Then TEA was added and the neutralization reaction proceeded for 40 min. The prepolymer of polyurethane was obtained.

Distilled water was added to the prepolymer drop by drop with stirring over 5 min. The mass of distilled water was equal to the sum of the mass of the raw materials used in the synthesis. Finally, WPU-L was obtained after removing the acetone by rotary evaporation. The formulation of WPU-L is shown in

Table 2. The formulation of large-particle waterborne polyurethane (WPU-L).

Sample Designation	IPDI/g	DMPA/g	PHNA/g	s-Polyester/g	TEA/g	BDO/g
WPU-L1	14.90	1.15	40.0	0	0.86	1.20
WPU-L2	14.90	1.15	38.0	2.0	0.86	1.20
WPU-L3	14.90	1.15	36.0	4.0	0.86	1.20
WPU-L4	14.90	1.15	34.0	6.0	0.86	1.20
WPU-L5	14.90	1.15	32.0	8.0	0.86	1.20
WPU-L6	14.90	1.15	30.0	10.0	0.86	1.20

.

Distilled water was added drop by drop with stirring. After the water droplets were added, the addition of the small particle emulsion appeared to stop the emulsification. Finally, WPU-H was obtained after removing the acetone by rotary evaporation. The emulsion prepared by using the WPU-S₁ emulsion for the small particle component is referred to as WPU-Hap, and the emulsion prepared using the WPU-S₂ emulsion for the small particle component is referred to as WPU-Hdp. In each WPU-H (WPU-Hap and WPU-Hdp), the mass ratio of WPU-L (WPU-L₁, WPU-L₂, WPU-L₃, WPU-L₄, WPU-L₅, and WPU-L₆) to WPU-S (WPU-S₁ and WPU-S₂) was 5:1.

The scheme of the formation of WPU-L is illustrated in Figure 1.

2.4. Preparation of WPU Films

Films were prepared by casting the dispersions onto Teflon plates kept at room temperature for 4 days and then at 70 °C in a vacuum drying oven for 2 h. Finally, the dry films of approximately 1 mm thickness were obtained and stored in a desiccator at room temperature for further study.

3. Characterizations

The particle size and polydispersity index (PDI) of the WPU dispersions were analyzed by a Nano-ZS90 Zeta-sizer (Malvern Instrument, Malvern, UK) at 90° under 25 °C. The sample was first diluted in deionized water to a concentration of 0.1 wt %, followed by ultrasonic wave treatment to homogenize the dispersion.

The solid content of the WPU was determined by drying an amount of emulsion (about 1.00 g) at 105 °C for more than 3 h, following which the weight ratio of residue to the whole mass was calculated.

The FTIR spectra of films were recorded with a Thermo Scientific Nicolet 6700 FTIR spectrometer (Waltham, MA, USA). All the spectra were scanned within the range of 400–4000 cm⁻¹, and 48 scans were averaged to every single IR spectrum.

4. Results and Discussion

4.1. FTIR Spectra of WPU

The FTIR spectra of WPU-L and WPU-S were similar. The FTIR spectra of the WPU-L₁ film are shown in Figure 2. A peak at 1731 cm⁻¹ was assigned to the stretching vibrations of the carbonyl group. The peaks at 2951 and 2865 cm⁻¹ represent methyl and methylene groups of WPU, respectively. The appearance of peaks at 3322 and 1532 cm⁻¹ was mainly owing to the telescopic vibration and bending vibration of the N–H group, respectively. The appearance of peaks at 801 cm⁻¹ indicated that the sulfonic acid group had been sufficiently introduced into the WPU. The disappearance of peaks at 2250 cm⁻¹ indicated that all of the isocyanate groups had sufficiently reacted with the hydroxyl groups or amino groups.

4.2. Particle Size of Emulsions

The particle size of WPU-S₁ was 54.9 nm, and the particle size of WPU-S₂ was 53.8 nm. The difference in particle size between WPU-S₁ and WPU-S₂ was less than 1 nm; they are the same particle size emulsion. The PDI of WPU-S₁ and WPU-S₂ was less than 0.2. The emulsion particle size distribution was concentrated.

The particle size and PDI of WPU-L are shown in Figure 3 and Figure 4. After adding different amounts of s-polyester, the particle size of the WPU-L emulsion decreased monotonously with the increase of the s-polyester dosage. When the amount of s-polyester was less than 10%, the emulsion particles were significantly reduced in particle size due to hydrophilicity. When the amount was greater than 10%, there was no significant change in the particle size of the WPU-L emulsion, and the hydrophilic chain extender had little effect on the particle size of the emulsion. When the s-polyester content was 0, the emulsion PDI was larger than 0.1. The PDI of the emulsion containing s-polyester was below 0.1, indicating that the particle size distribution was narrow. Moreover, the particle size of the emulsion was about 200 nm or more; the particle size was large, which meets the requirements of the emulsion dispersion method for the large particle size emulsion.

The particle size and PDI of WPU-Hap and WPU-Hdp are shown in Figure 3 and Figure 4. The particle size of WPU-Hap and WPU-Hdp emulsions and the particle size of WPU-L emulsions were similar regardless of the change of the amount of s-polyester. WPU-Hap was composed of WPU-L emulsion and WPU-S₁ emulsion (AAS + s-polyester). WPU-Hdp was composed of WPU-L emulsion and WPU-S₂ emulsion (DMPA + s-polyester). The particle size of WPU-Hap was smaller than that of the WPU-L emulsion, while the particle size of WPU-Hdp was larger; the PDI values of WPU-Hap and WPU-Hdp were substantially larger than that of the WPU-L emulsion. The particle size of the emulsion of WPU-Hdp was larger. Apparently, adhesion of the emulsion particles occurred. This is because the type of hydrophilic group causes the particle charge to be different, and the particles with weak charge are prone to blocking. Another reason for this is the segment is flexible in the emulsion particles, and the particles with flexible segments are prone to adhesion effect. Due to the difference in hydrophilic monomers between the two small particles, WPU-S₁ had more sulfonic acid groups, its charge effect was strong, and its segment flexibility was poor. Thus, WPU-Hap had no obvious adhesion. For WPU-S₂, there were few sulfonic acid groups, the charge was weak, the segments were flexible, and the blocking phenomenon was obvious. Compared to the PDI of WPU-L, the PDI of WPU-H containing s-polyester was larger, which is advantageous for the formation of a high solid content emulsion [20].

4.3. The Relationship between Solid Content and Particle Size Ratio

In this study, 12 groups of high solid emulsions were prepared by emulsion dispersion using different WPU-L (WPU-L₁, WPU-L₂, WPU-L₃, WPU-L₄, WPU-L₅, and WPU-L₆) emulsions and WPU-S (WPU-S₁ and WPU-S₂) emulsions. The scatter plot of their solid contents is shown in Figure 5, which shows the particle size ratio of each group of emulsions. Since particle adhesion occurs in the emulsion dispersion method, the particle size ratio is the ratio of the particle size of the blended emulsion to the particle size of the small particle emulsion. It can be seen that when the small particle emulsion was the WPU-S₁ emulsion, the solid content was higher when the particle size ratios were 7.00, 4.75, and 2.87, and the solid content was lower when the particle size ratios were 3.48, 4.32, and 5.30. When the small particle emulsion was the WPU-S₂ emulsion, the solid content was higher when the particle size ratios were 7.01, 4.04, and 2.46, and the solid content was lower when the particle size ratios were 6.42, 4.62, and 2.70. This indicates that there are multiple values of particle size ratio that can reach a high solid content.

A three-dimensional physical model can be built to describe and explain this phenomenon. The large particle diameter is a, and the small particle diameter is b. The large particles are arranged in different ways according to Figure 6, while the small particles fill the gaps between the large particles. The emulsion solid content (W) is simulated by the percentage of the total volume of the particles. N_L and N_S are the numbers of large and small particles in the model, respectively. ω₀ is the percentage of the area occupied by only large particles in the model. Particle size ratio (r_p), particle volume ratio (r_v), and solid content (W) are calculated as shown in Equation (1). The simulation curve of the particle size ratio and solid content is shown in Figure 7.

$$\begin{gathered} r_{\mathrm{p}}=\raisebox{1ex}{$a$}\!\left/ \!\raisebox{-1ex}{$b$}\right. \\ {r}_{\mathrm{V}}=\frac{a^3\times N_{\mathrm{L}}}{b^3\times N_{\mathrm{S}}} \\ W=\frac{N_{\mathrm{L}}\times \frac{\mathsf{\pi }}{8}\times a^3+N_{\mathrm{s}}\times \frac{\mathsf{\pi }}{8}\times b^3}{N_{\mathrm{L}}\times \frac{\mathsf{\pi }}{8}\times a^3}\times {\mathsf{\omega }}_0\times 100\% \end{gathered}$$

(1)

When the diameter of small particles is large, small particles cannot fill the voids between large particles, so the solid content is the volume percentage of large particles. When the large particles are arranged in a simple cubic accumulation, body-centered cubic accumulation, body-tightest packing, and hexagonal closest packing (Figure 6), the space utilization ratios are 52% (Figure 6a), 68% (Figure 6b), and 74% (Figure 6c,d), respectively. The first two minimum points on the solid content–particle size ratio curve correspond to the simple cubic accumulation and body-centered cubic accumulation. When the diameter of small particles is gradually reduced so that the particles can enter the gaps between large particles, the solid content increases sharply, corresponding to the first three maximum points on the solid content–particle size ratio curve. As the diameter of the small particles continues to decrease gradually, before the particle size ratio can satisfy the next arrangement, the solid content is obviously reduced. As the diameter of the small particles continues to decrease further, the solid content is gradually reduced until the large particles can accommodate exactly two small particles in the gaps arranged in the closest packing of the hexagons. Then, when two small particles enter the gap, the solid content transitions to a larger level. Thus, there are multiple radius ratios that enable the solid content to reach a large value—the first three are due to the different spatial arrangements of the large particles, and the fourth arises because a plurality of small particles fill the voids between the large particles.

Comparing the particle size ratios of WPU-Hap and WPU-Hdp, it can be found that the particle size corresponding to WPU-Hap was large, due to the influence of the charge effect of small particles. Since the WPU-S₁ emulsion particles had more sulfonic acid groups, the charge effect was strong, so the repulsion between the particles was strong. To achieve the same particle arrangement, the small particle volume can only be smaller, showing a larger particle size ratio. This means that for different formulations of emulsion particles, the appropriate particle size ratio will shift.

5. Conclusions

In this paper, a series of high solid content carboxylic acid/sulfonic acid waterborne polyurethanes was prepared by the emulsion dispersion method and the relationship between the solid content and particle size ratio was studied. We found that the solid content varied with the particle size ratio. When the particle size ratio was >7, 4–5, or 2–3, the aqueous polyurethane could reach higher solid content (more than 56%), and we established a physical model to study the relationship between the particle size ratio and solid content. The plurality of maxima in the particle size ratio–solid content relationship corresponded to different arrangements of the emulsion particles in space. At the same time, the comparison of WPU-Hap and WPU-Hdp showed that the suitable particle size ratio was not a constant but depended on the emulsion particles. The stronger the charge, the smaller the corresponding particle size ratio. Moreover, the difference in chemical composition between small particles and large particles could cause a difference in segment activity, which in turn affected the compatibility of the two particles.