(164c) Starch-Based Viscosity Modifying Agents of Mortar

In this work, copolymers of methoxy polyethylene glycol (MPEG) were produced with oxidized corn starch (OCS-g-MPEG), and oxidized potato starch (OPS-g-MPEG). For mortar, hhese products increased the stability and acted as viscosity modifying agents (VMAs). Besides, the early cement hydration and the development of compressive strength remained almost unchanged with the addition of the starch-based admixtures. The performance of the copolymers was compared with commercial VMAs, thus evidencing a high potential as alternatives to conventional synthetic admixtures.

1.Introduction

In some countries, 70-80% of concrete contains one or more admixtures¹. In plasticized mortars with a low water-to-cement ratio VMAs can be added to improve both viscosity and stability and reduce the tendency to segregate. In general, these mortars are water-soluble, inorganic, or organic, high-molecular-weight polymers. Organic VMAs can be classified into biopolymers (such as polysaccharides), semi-synthetic (such as cellulose ether derivatives), and synthetic polymers².

According to our knowledge, previous researches are focused on the effect of starches with negatively charged substituents, and other types of polysaccharides, in the mortar. In this study, we evaluated the effect of starches modified simultaneously with anionic groups and a polar copolymer. This work is centered on the production of copolymers of oxidized corn, and potato starch with methoxy polyethylene glycol, and their assessment as mortar admixtures, specifically as VMAs.

2.Materials and methods

Unmodified corn and potato starches were used as raw materials to produce the derivatives. Corn starch was food grade (Ingredion, Cali, Colombia) and Potato starch analytical grade (Panreac AppliChem, Barcelona, Spain). MPEG was purchased from BASF (Florham Park, USA). All the chemicals were analytical grade and used without modifications. To produce the cement pastes, previously homogenized structural type Portland cement (Cementos Argos, Sogamoso, Colombia) was used.

2.1.Production and characterization. Starch oxides were prepared according to the method developed by Chong et al.³. The methodology of Kuakpetoon and Wang⁴ was used to determine the carboxyl content. Modification of MPEG to produce copolymers was carried out according to the procedure followed by Liu et al.⁵ with some modifications.

All the oxidized starches grafted with MPEG and unmodified starches were analyzed using an ATR-FTIR spectrometer (Perkin-Elmer Paragon 1000 FTIR), equipped with a diamond crystal.

The ¹H NMR method was applied to measure the degree of branching and substitution of starches and their copolymers. The substrate was dissolved in d₆-dimethyl sulfoxide, and a small amount of deuterated trifluoroacetic acid also was added. It was used a BRUKER DPX 400 NMR spectrometer.

SEM (HITACHI model S-4800) was used to determinate the particle size of the copolymers. XRD (XPertPRO-MPD, PANalytical Phillips XRD, Alemania), and DSC (model Q100, TA Instruments, New Castle, DE) in an aqueous medium, also were applied to analyze all the samples.

2.2.Performance tests of pastes. The role of a VMA in a paste mixture is increasing the apparent viscosity, as quoted by Papo and Piani⁶. The preparation of pastes was carried out according to ASTM C305 (2014). The initial flow was evaluated by the mini-slump test (DIN EN-1015, 2007). The setting time was determined based on ASTM C403 (2016), and the seven-day compressive strength was evaluated according to ASTM C39 (2018).

3.Results and discussion

3.1.Production and characterization. Figure 1 shows the general process of obtaining the copolymer of starch oxide with MPEG. A dual modification allowed us to evaluate the combined effect of adding an anionic functional group (through an oxidation reaction) and a hydrophilic neutral homopolymer (through a graft copolymerization) in corn, and potato starch. The copolymer's performance, as viscosity modifier of mortar mixtures, was identified based on its structural characteristics.

According to the results in Table 1, under the same reaction conditions, potato starch had a higher degree of substitution with carboxylate groups (0.67) compared to corn starch (0.14).

In the FTIR spectra of the starches and copolymers in Figure 2, the broad peak at ~3300 cm^-1 is characteristic of the free hydroxyl groups in starch. The signals between 1000-1200 cm^-1 are characteristics of the C-O bond of the polysaccharide skeleton and C-O-C vibrations of the MPEG. The symmetrical vibration peaks for the -C=O bonds of the carboxylic anion groups introduced into the starch appear around 1639 cm^-1 and 1470 cm^-1. The band at 1350 cm^-1 represents the CH-bond deformation. The bands at 2915 and 2885 cm^-1 are attributed to the asymmetric CH stretch and are indicative of MPEG conjugation. Therefore, the spectra state the effectiveness of the derivatization and copolymerization as well as the existence of several characteristic functional groups, including carboxyl, methyl, and ether groups.

In Figure 3, five individual signals can be recognized between the 6 and 7 ppm chemical shifts in the ¹H starch-g-MPEG NMR spectra. These are the resonance signals showing that the MPEG segments were successfully grafted onto the starch particles. Additionally, signals from the anomeric hydrogens 1 and 1' appear at 5.11 and 4.75 ppm, respectively.

Figure 4 includes SEM images of corn, and potato starch and their copolymers. In most of the cases, the structure of the granule remains almost intact after chemical modification, where the hydroxyl groups are partially replaced.

Particularly, Figure 4 shows the SEM images of potato starch and its copolymer. Both of them are oval, and their sizes are in the range of 10-71 µm. The chemical modification of starch increases the granular surface area and porosity.

The surface of starch particles is quite smooth and compact with small natural holes scattered over it. Grafting of MPEG molecules brought about significant changes in the surface morphology of the starch granules. A micro-pubescence structure was observed on the surface of the starch-g-MPEG samples. The outer layer of the granules became loose and porous, and this structure is commonly observed on the surface of polymeric materials after graft modification.

The results in Table 2 are based on ¹H NMR spectra of corn, and potato starch and their copolymers. It includes the degree of branching (DB) of the starch molecules. The DB of potato starch (5.7) decreased with chemical modification (3.8), while corn starch (3.8) remained constant on the copolymer.

Table 2 also presents the degree of crystallinity (DC), in terms of the ratio of crystalline phase to the amorphous phase, for corn, and potato starch and their copolymers calculated from XRD. Corn starch has an α crystalline structure (DC=26.4 and 9.0), and potato starch has a β crystalline structure (DC=27.0). Both types of structures were preserved after oxidation and copolymerization, although the degree of crystallinity decreased in each case.

To complement the information of Figure 4, data on particle size and distribution of corn and potato starch and copolymers are included in Table 2, calculated based on the results of SEM analysis. According to it, the average diameter of corn starch granules (12 μm) remained almost unchanged in the copolymers. Opposite, potato starch granules (26 μm) increased (to 34 μm).

Figure 5 shows the temperature of gelatinization in aqueous media of corn starch (69.4 °C) and potato starch (58.9 °C) with their copolymers.

3.2.Performance tests of pastes. Figure 6 includes the initial flow, Figure 7 setting time, and Figure 8 the 7-days compressive strength of pastes with water to cement ratio of 0.45 with starch-based admixtures (0.5% concentration per cement mass) and without them. Figure 6 shows a decrease in the flow of the pastes, mainly those containing OCS-g-MPEG (124 mm) and OPS-g-MPEG (136 mm) compared to the reference (147 mm). In Figure 7, a low increase in paste setting time with the addition of corn (10.7 h) and potato starch (10.38 h) compared to the reference without admixtures (9.5 h) can be observed. The other starch derivatives did not significantly change the initial setting time. Finally, Figure 8 shows a decrease in the 7-day strength of the paste containing corn starch (30 MPa) and OCS (30 MPa). In the potato starch derivatives, only the oxide lowered resistance (31 MPa) in comparison with the reference (36 MPa). On the contrary, OCS-g-MPEG increased it (41 MPa).

4.Conclusion

OPS-g-MPEG, and OCS-g-MPEG were successfully produced through simple reactions maintaining the structure of the starch granule with low degradation. The copolymers presented low loss of the degree of branching and crystallinity. They were incorporated in mortar at concentrations ranging from 0.5-2.5% per cement mass, and their performances as VMAs were evaluated based on flow properties and adsorption measurements. OCS-g-MPEG and OPS-g-MPEG were VMAs, increasing the stability of mortar. Besides, the early cement hydration and the development of compressive strength at 7-day remained almost constant with the addition of the starch-based admixtures.

Acknowledgements. The authors are grateful to Sika Colombia S. A., Colciencias, and the Universidad Nacional de Colombia by funding this research through the contract FP44842-351-2017.

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