DOI:10.30919/esmm5f707

ES Materials & Manufacturing, 2020, 8, 36-45

Published online: 25 May 2020

Received 11 Mar 2020, Accepted 25 May 2020

Investigation of Compatibility of Fluorine-Acrylic Emulsion and Sulphoaluminate Cement in the Design of Composite Coating: Effects of Sorbitol and Its Mechanism

Chen Liang, Piqi Zhao, Pengkun Hou, Shoude Wang, Valeria Strokova, Lingchao Lu and Xin Cheng

1. Shandong Provincial Key Lab. of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China

2. School of Materials Science and Engineering, University of Jinan, Jinan 250022, China

3. Department of Materials Science and Technology, Belgorod State Technological University named after V.G. Shukhov, Belgorod, 308012, Russia

Abstract

Both fluorine-acrylic (FA) emulsion and sulphoaluminate cement (CSA) have been proved as satisfactory marine engineering materials, and a better performance can be expected by their combined use. However, their compatibility has rarely been researched. In this study, the effects of sorbitol and its mechanism are investigated by the cement paste fluidity, rheological property, tensile strength, water absorption and SEM tests. The results showed that, compared with sodium tripolyphosphate, borax and sodium gluconate, sorbitol can significantly improve the double electric layer of FA, and the absolute value of Zeta potential increased by 25% compared with the blank sample without any retarder added. With the increase of sorbitol dosage, the fluidity, rheological behavior, tensile strength and water absorption sharply changed first and then tend to be stable after sorbitol dosage reaches the optimal dosage of 0.25% for the compatibility improvement. The morphology analysis also proved its advantages, but the more significant improvement can be ensured by a larger p/c value. Finally, the mechanism hypothesis is proposed. The film formation rate of the emulsion matches the hydration rate of the cement, due to sorbitol addition.

Table of Content

We found sorbitol as compatibilizer to improve compatibility between fluorine-acrylic emulsion and sulphoaluminate cement and the mechanism hypothesis was proposed.

 

 

Keywords: Polymer-modified cement-based coating; Compatibility; Retarders; Rheological properties


1. Introduction

In recent decades, anti-corrosive coatings have been developed rapidly.[1,2] Traditional organic coatings, having poor weather resistance, are easy aging under ultraviolet radiation and easily cause some pollution to the environment. Cement, as a rigid waterproof material, has good durability and poor flexibility. As a composite material of organic-inorganic, the most important feature of polymer-modified cement-based coatings is its designability of properties and structures.[3]

Organic fluorine-modified acrylate emulsion has become the representative of high weatherability acrylate coatings in recent years.[4,5] Because F-C bond energy is 486 kJ/mol, much higher than that of C-C (351 kJ/mol), its molecular structure is more stable. The fluorine atom is not only strongly bonded with the carbon atoms on the main chain, but also closely arranged in the outer layer of the carbon skeleton, which effectively protects the main chain.[6] The addition of organic fluorine can maintain good film-forming property of acrylate emulsion, and it also has the characteristics of water resistance, heat resistance and ultraviolet resistance.[7] In the meanwhile, compared with ordinary Portland cement, sulphoaluminate cement (CSA) has the characteristics of rapid hydration and hardening, satisfactory anti-corrosive properties and excellent volume stability, which makes it widely used in various ocean engineering fields.[8-10] Thus, the composites between them are expected as a powerful combination. But they are not a simple physical blending, which is restricted by the compatibility of polymer emulsion and cement hydration products.[11] The essence of compatibility is the interaction in the process of polymer film-forming and cement hydration. The influence of polymer on cement hydration is that polymer can be adsorbed on the surface of cement particles, which inhibits cement hydration and affects the construction of early inorganic network.[12,13] Actually, the influence of cement hydration, detailed as the increase of pH value, Ca2+ concentration and water loss, on polymer film formation is more significant.[14,15] Take Ca2+ for example, Ca2+ can destroy the stability of emulsion under the action of high valence electrons, resulting in the agglomeration of particles and destruction of emulsion.[16]

Interestingly, compatibility between polymer emulsion and CSA has received only limited attention in literatures and most results are concluded in the view of polymer modification.[17-19] For example, Yilmaz et al.[20] researched the preparation of a tert-Butyl acrylate/Butyl acrylate/ methacrylic acid ternary copolymer/clay nanocomposite containing 3 wt% sodium montmorillonite via seeded emulsion polymerization, finding that a certain amount of emulsifier was necessary to obtain stable latexes and the usage of a low molecular weight water soluble polymer as steric barrier increased the stability of system. Zhao et al.[21] prepared the Ca2+ ion responsive Pickering emulsions based on the properties of poly sodium salt nanoaggregates and indicated that the adsorption of poly sodium salt nanoaggregates at the interface is the key to the stability of the emulsions. Ishikawa et al.[22] found that the steric interaction produced by the surfactant likely resulted in the stable dispersion of this latex.

Here, we focus on the modification of cement and expect to find an effective retarder for CSA,[23-25] which can match the FA film formation process through the hydration rate control, and has no significant harm to the performance of the polymer. In addition, the effect of the above suitable retarder on the composite coating is investigated by the analysis of cement paste fluidity, rheological property, tensile property, water absorption and microstructures. Finally, the mechanism is also proposed.

 

2. Experimental

2.1 Materials

One of the main materials used in this study is fluorine- containing acrylic ester copolymer emulsion (FA). Originally, three commercially available FAs with over 10% of fluorine contents are selected and labeled as E1, E2 and E3, respectively. The basic properties of FAs are shown in Table 1 (MFT refers to the minimum film forming temperature). Their Fourier-transform infrared (FTIR) spectra are given in Fig. 1, showing a similar group composition. The effective grafting of fluorine monomers is characterized by C-F stretching vibration at 1100 and 670 cm-1, respectively. The stability of Ca2+ in emulsion is an important index for reflecting the initial compatibility between polymer emulsion and cement. Fig. 2 presents the values of critical coagulation concentration (CCC) and the residual ratio on sieve (RRS). E1 shows the highest CCC value and lowest RRS value, indicating the most satisfactory stability for Ca2+ among the above FAs. In the following work, E1 was selected as the target FA raw materials. The sulphoaluminate cement (type 42.5), abbreviated as CSA, used in this research was from China United Cement Corporation, China. The chemical composition of CSA, obtained from XRF analysis, is shown in Table 2.

Table 1. Properties of the FAs.

No.

pH

Solid content/%

Viscosity/
mPa.S

Particle size/um

MFT/

E1

7.6

48

560

0.1-2

30

E2

8.6

45

630

0.1-5

30

E3

7.9

45

470

0.1-2

28

Fig. 1 The FTIR spectrum of FAs

Table 2. Chemical composition of CSA (%)

Composition

CaO

Al2O3

SO3

SiO2

MgO

Fe2O3

TiO2

K2O

Others

LOI

Content (%)

45.26

21.39

15.38

9.52

2.39

2.51

0.70

0.41

0.83

1.61

Fig. 2 The Ca2+ stability of FAs.

Borax (Na2B4O7∙10H2O, CHN), sodium gluconate (SG, C6H11NaO7, CHN), sorbitol (C6H14O6, CHN) and sodium tripolyphosphate (STPP, Na5P3O10, CHN) are compared to obtain the best retarder for improving compatibility. Some admixtures such as the dispersant (sodium hexametaphosphate), film-forming additive (Texanol) and antifoaming agent (Foamastar MO 2190AC) were also used in this study.

 

2.2 Sample preparation

The process flow diagram of coating preparation is shown in Fig. 3. Firstly, the emulsion was stirred at 300 rpm for 1 min to disperse the emulsion particles evenly. Retarder and dispersant (0.3% by mass of FA) were slowly added and continuously stirred at the speed of 600 rpm for 3 min, and then film-forming additive and antifoaming agent were added in amounts about 3% and 0.5% based on the weight of FA to complete the basic preparation of liquid materials in coatings. Secondly, pre-weighted cement was added to the liquid materials and rapidly mixed at the speed of 600 rpm for 5 min to ensure uniform distribution. Finally, the obtained slurry was rested for 2 min for defoaming and then poured into the mould (350 × 320 × 1.5 mm3) for coating preparation. In this research, the curing regime of coatings was adopted, which was cured for 96 h under 25±2 and a relative humidity of 70±10%. Subsequently, after demoulding, the coating was dried in the oven at 45±2 for 48 h. The relevant experiments were carried out at room temperature for at least 24 h.

 

Fig. 3 The flow chart of the process of the polymer cement-based coating.

 

2.3 Methods

2.3.1 Critical coagulation concentration (CCC)

The CCC value test was performed according to the Chinese standard ‘GB/T 20623-2006’. It started with 5wt% of CaCl2 solution and the weight percentage was gradually increased in steps of 0.5wt%. The CCC value was recorded when the emulsion started appearing flocculation.

 

2.3.2 Residual ratio on sieve (RRS)

The residual ratio on sieve (RRS) was determined by the following steps. Firstly, the FA and CSA were mixed for a uniform state with the polymer-cement ratio of 0.1 (the solid mass of the emulsion to the cement quality) and water to cement ratio of 1.0. Then the 120-mesh sieve was used to separate the coarse particles. Following that separation, RRS value was calculated. Notably, the sieve used before coarse particle separation and weight measurement had to be dried to constant.

 

2.3.3 Zeta potential

Zeta potential values of emulsion were determined using the Zetasizer Nano ZSP of Malvern Panalytical. Based on this test, the polymer-cement ratio and the dosage of retarder were set as 0.1 and 0.45%, respectively.

 

2.3.4 Cement paste fluidity

The fluidity of CSA paste (w/c=0.35) was evaluated according to Chinese standard ‘GB/T 8077-2012’. The dosage of retarder varied from 0.05 to 0.45 wt% with a gradient of 0.1 wt%. The test time was fixed at 5 and 15 min, respectively.

 

2.3.5 Rheological property

The rheology test program is shown in Fig. 4. Firstly, the coating slurry was stirred at 100 s-1 for 30 s in order to make the coating slurry disperse evenly. Thereafter, the flow field was stabilized by stationary 20 s. Finally, the shear rate increased from 0 to 150 s-1 in 1 min and decreased from 150 to 0 s-1, likewise in 1 min. The yield stress and plastic viscosity were fitted by the data of the falling stage.

 

2.3.6 Tensile property test

The dumbbell-shaped samples were used to determine tensile strength according to Chinese standard ‘GB/T 16777-2008’ (tensile speed is 200 mm/min). The samples were measured using the universal testing machine (CMT5504).

 

2.3.7 Water absorption

The samples were cut into a certain shape (40 × 40 × 1.5 mm3) after curing for 7 d. Then the samples were completely immersed in water for 72 h. The weight of the sample before and after immersion was used to calculate the water absorption.

Fig. 4 The test program of rheology

2.3.8 FTIR

The FTIR spectrum of emulsion was determined using Thermo Nicolet (Nexus 870). The emulsion was put into a drying oven at 105 °C for 2 d until constant weight to build a film for FTIR analysis.

 

2.3.9 Scanning electron microscopy (SEM)

The field emission scanning electron microscope (QUANTA 250 FEG) was used to observe the microstructure of the fresh section of coatings (spraying time is 30 s).

 

3. Results and discussions

3.1 The selection of retarder

Previous studies have ever reported that some cement retarders can play a role in improving the compatibility between polymer and cementitious materials.[11,18] Generally, the retarders used for CSA are hydroxyl carboxylate, inorganic phosphate, polyol and borate. But it is still unclear whether they all show the positive effect on the stability of emulsion, which is an important factor for the compatibility.[26] The thickness of the electric double layer of the emulsion determines its ability to resist the damage of Ca2+, thus affecting the compatibility with cement. Hence, the above retarders are firstly compared and investigated by the Zeta potential test. As shown in Fig. 5, the Zeta potential value of blank sample was reported as -2.52 mV. Most retarders, such as sodium tripolyphosphate, borax and sodium gluconate, do not show a satisfactory compatibility and reduce the Zeta potential values instead. However, the sorbitol achieves a better compatibility and increases the Zeta potential value by 25% compared with the blank sample. The added sorbitol is easy to form hydrogen bond with water molecules through hydroxyl groups, and the hydrogen bonds between water molecules form a stable water film on the surface of the emulsion particles to prevent particle contact.[27] It is equivalent to increasing the thickness of the electric double layer.

Fig. 5 The effect of different retarders on Zeta potential of FA.

 

3.2 Fluidity and rheological behavior

Fig. 6 depicts the fluidity of CSA in 5 and 15 min with various dosages of sorbitol, respectively. Both curves show a similar gradually growing tendency. However, the loss of fluidity between 5 and 15 min tends to be stable after the sorbitol is added more than 0.25%. It means that 0.25% of sorbitol is large enough for improving the fluidity of CSa. It is easy for understanding that the improvement is attributed to the retarder effect of sorbitol. Therefore, the hydration process can be retarded and the steric hindrance between cement particles can be reduced so as to improve the mutual fluidity of cement particles.

 

Fig. 6 The effect of sorbitol dosage on the fluidity of CSA

Although the sorbitol can increase the Zeta potential value of FA and reduce the fluidity loss of CSA, its effect on the FA-CSA composite system is worth further investigation. Rheology is the science of micro-fluidity of matter,[28-30] which can reflect particle flow state and microstructure development of polymer-modified cement- based coating. Based on the analysis of stress-rate curves (Fig. 7). The shear stress shows a similar trend between up curves and down curves in both samples with a polymer to cement ratio (p/c) of 0.5 and 1. The shear stress has an obvious decrease with the increase of sorbitol. But the decreased degree in all samples tends to be stable when the dosage of sorbitol is added to 0.25%. In comparison with p/c values, the global shear stress is significantly decreased when the p/c is changed from 0.5 to 1. It is well understood that the larger p/c value will cause the relative motion of interior particles more readily.