ABSTRACT

In this paper, isophorone diisocyanate (IPDI), polyethylene glycol (PEG), dimethylolpropionic acid (DMPA) and internal crosslinking agent trimethylolpropane (TMP) were used to prepare waterborne polyurethane. And then double-crosslinked polyurethane-acrylic composite aqueous dispersion was prepared in which polyacrylate was adopted to modify waterborne polyurethane and some special external crosslinking agents were added including silicone and trifunctional aziridine. The influence of the amounts of internal and external crosslinking agents, emulsifier, initiator on the particle size, particle size distribution, viscosity, molecular weight, as well as water adsorption ratio were studied.

1. Introduction

Polyurethane-acrylate (PUA) waterborne dispersion, with its advantages of excellent weather resistance, pigment affinity, cost-effectiveness over PU emulsion, has been an important research focus. But, to some extent, the application of PUA is limited by its poor chemical resistance, poor mechanical properties and poor water resistance, etc. [1-4]. In recent years, much relative research has been conducted to study how to enhance their properties [5-9], but little has been reported to discuss the effect of synthesis parameters on the properties of PUA in detail. This study was designed to enhance the water resistance of PUA by adding trifunctional polyols and some special external crosslinking agents such as silicone and trifunctional aziridine so as to improve its water resistance. Then the prepared water-resistance PUA emulsion can be used as the film-forming resin in coatings and adhesive.

2. Experiment

2.1. Materials

1) Polyethylene glycol (PEG) 1000 (provided by Shanghai resin factory);

2) Dimethylolpropionic acid (DMPA), trimethylolpropane (TMP) and trifunctional aziridine (QL-1000-Ga) are all CP-grade;

3) Isophorone diisocyanate (IPDI) (provided by BASF);

4) 2-Hydroxyethyl acrylate (HEA), triethylamine (TEA), sodium dodecyl sulfate (SDS), Tween-60, methyl methacrylate (MMA), butyl acrylate (BA), potassium persulfate (KPS);

5) 1-Methyl-2-pyrrolidinone (NMP), ethanol (EtOH) and external crosslinking agents are all AR-grade.

2.2. Experimental Process

Step1: Preparation of PU prepolymer

The PEG 1000 was introduced into a three-necked vessel with reflux condenser, stirrer and thermometer, then heated to 100˚C and dehydrated under vacuum for 1 h. Then the vessel was cooled to 80˚C and dehydrated for about 0.5 h after the DMPA and TMP being added into the reactor. Subsequently, the reactor was cooled again to about 70˚C for the dropping of IPDI with high-speed stirring. When the dropping of IPDI was finished, the mixture was heated up to 80˚C to react for 3 h (some solvent was added), and then cooled to 60˚C for adding HEA. After reacting for 1.5 - 2 h, the prepolymer was neutralized with TEA for approximately 0.5 h. Finally, the vessel was cooled to 20˚C - 30˚C, followed by dropping ice water with stirring to obtain the PU dispersion.

Step2: Preparation of PUA

The obtained preploymer was introduced into a threenecked vessel, dispersed with deionized water followed by adding SDS and TWEEN, stirred and emulsified at 50˚C. And then, the monomer mixture (MMA/BA) and the initiator (KPS) aqueous solution were added. After reacting at 70˚C for about 3 h, the system was cooled to 50˚C - 60˚C. TEA was added to keep the pH value within 8 - 8.5.

Scheme 1. Preparation process of PU prepolymer.

Scheme 2. Preparation process of PUA.

2.3. Characterization

Solid content (C %)

C % = (M₃ − M₁) × 100/(M₂ − M₁)

in which M₁—the original weight of a small glass cup.

M₂—the gross weight of the cup and sample (taking from the obtained dispersion).

M₃—the gross weight of the cup and sample after being placed in an oven at 60˚C for 24 h.

Particle size and particle size distribution

The particle size and its distribution of the obtained dispersion were measured by a LS 230 laser particle sizer produced by British MALVEN Company and the measuring range was 0.02 μm - 2 mm.

Stability

The samples were placed in TL-5.0W type centrifuge for the measurement of stability.

Water absorption

Water absorption (%) = (Mn₃ − Mn₂) × 100/(Mn₂ − Mn₁).

in which Mn₁—the original weight of each slide.

Mn₂—the weight of slide (dropped with obtained dispersion) after being placed in an oven at 60 ˚C for 24 h.

Mn₃—the weight of slide after being immersed in water for 24 h (removed using dry filter paper).

Viscosity

The viscosity of the samples was measured with DNJ- 9S.

IR

FTIR spectra were obtained with NEXUS2870 type device of U.S. NICOLET Company.

3 Results and Discussion

3.1. Effect of Internal Crosslinking Agent on the Properties of PU

Figure 1 shows the effect of different amount of internal crosslinking agent (TMP) on the viscosity of PU dispersions and water absorption of PU films. The results indicate that the viscosity of PU dispersion increases significantly with the increase of TMP (2% - 6%). The reason may be that a certain degree of crosslinking structure was formed with the addition of trifunctional TMP; but as the TMP content (6% - 10%) increases further, the viscosity begins to fall instead of rising, which seems contradictory to the crosslinking mechanisms. This was mainly because that the crosslinking points became too dense as the TMP content increased from 6% to 10%, which made the dispersion of PU in water difficult. So the observed viscosity reduction might be due to the uneven dispersion, which was also proved by the corresponding water absorption of the PU film as shown in Figure 1. Simultaneously, the crosslinking increased the difficulty of experimental operation. Gelation was likely to form as the amount of TMP increased, leading to much trouble with the following modification by acrylic monomers. This phenomena and results are not consistant with the primary relative research report [1,5]. So, in this case, excessive crosslinking in the polyurethane network is not preferred.

3.2. Effect of Emulsifiers on the Properties of PUA Dispersions

Table 1 shows the influence of different emulsifiers on the properties of PUA emulsions. From the perspective of the appearance, emulsions changed slightly from the white, milk white with blue light, milk white without blue light to white. This could be due to that, at the beginning, the number of latex particles increased while the particle size reduced; but as the emulsifier increases further, the particle size of aqueous emulsion becomed larger. The stability tests indicated that the use of single anionic emulsifier, SDS or nonionic Tween, could not

Table 1. Effect of emulsifier type and amount on the properties of PUA dispersions.

produce stable aqueous dispersion without delamiation, unless the composite emulsifier was adopted. Generally, the particle size was not strictly proportional to the amount of the composite emulsifier.

Figure 2 shows the effect of the emulsifier amount on the viscosity of PUA dispersions and the water absorption of PUA films. The water absorption increased alongwith the increase of the emulsifier amount. This was because that the anionic emulsifier SDS contains sulfonate, whose existence directly affected the water resistance of the films. Thus the water resistance of film reduced with the increase of the emulsifier amount, especially in the case of the anionic emulsifier. So the amount of emulsifier in the emulsions should be kept in a proper scale. And also from Figure 1, viscosity of PUA emulsions remained unchanged alongwith the increase of emulsifier, showing that the concentration of composite emulsifier nearly had no effect on the viscosity of PUA dispersions.

3.3. Effect of the Emulsifier Amount on the Particle Size and Particle Size Distribution of PUA Dispersions

Figure 3 shows the relationship between the emulsifier amount and the particle size and its distribution. The resulting latex particles became smaller with the increase of emulsifier (0.74% - 1.4%). The reason for this change could be that the composite emulsifier improved the function of three-dimensional spaces resistance or electrostatic repulsion on the surface of latex particle, which prevented the coalescence between the dispersed particles and made the particle size small and stable, as corroborated from the data in Table 1. But with the further increase of the amount of emulsifier (1.4% - 2.8%), the resulting particles size become lager, from this point, the function mechanism of anionic emulsifier was different from that of the ordinary emulsion polymerization. This might be due to that the polymerization in the research