The term “renewable” is generally applied to those technologies and energy resources whose common characteristics are that they are non-depletable or naturally replenishable.1 Albeit, a major contribution from renewable resources comes in the form of energy; other uses derived from these resources also have equally significant importance. This article deals with a “tree-extract” called ‘Sweet Sap’ which can be used as an emulsifier for making a water-in-oil emulsion leading to the development of low-VOC solventborne enamels. This emulsion is made using mechanical shear.
Immiscibility arising from the difference in surface tension between the two liquids is reduced by using this emulsifier. The ‘Sweet Sap’, by virtue of having both polar and non-polar ends, adsorbs to the water at one end and non-polar liquids such as oils at the other end and, as a result, significantly reduces the interfacial surface tension. Two conventional emulsifiers available in the market were used to compare the properties of the emulsions and the paints made with it. The water-in-oil emulsion without emulsifier was also studied in order to understand the role of emulsifier alone.
Performances were evaluated both in white and colored shades, including their storage stability. The newly used tree-extract in its natural “green-form” was found to be very promising and effective not only from a qualitative point of view, but also because it has enormous potential to use green technology and gain financial dividends.
As the growing thrust on renewable energy has been intensified by government, scientists are trying to find different renewable resources in different fields as these are one of the focused verticals which will drive our future growth and development. This article deals with a ‘tree extract’ called ’Sweet Sap’ that was used as the emulsifier for ‘water-in-oil’ emulsions, which were then employed as a binder to formulate low-VOC enamels. The performance of Sweet Sap was evaluated against two commercial emulsifiers, and in a system without emulsifier to understand the role of this sap.
Emulsions are basically dispersions of one liquid phase in another immiscible liquid phase made with the application of mechanical shear. Immiscibility arises from the difference in surface tension between the two liquids which requires significant reduction for the emulsion to remain stable. This reduction of surface tension is achieved by using surfactants. Surfactants having both polar and non-polar ends adsorb to the polar liquids such as water at one end and non-polar liquids such as oils at other end and, as a result, significantly reduce the interfacial tension at the interface.
The stability of water-in-oil emulsions was evaluated from their extent of separation and their loss of viscosity on storage. The paints made with the emulsions, however, have been evaluated in detail, particularly where emulsion can influence properties like viscosity and its loss on storage, dispersion on storage and pigment flocculation and syneresis. Although the initial study was carried out on white paint, it has been extended further to formulations of colored enamels, e.g., Blue (Phiroza Blue) and Grey (Smoke Grey) based on a combination of organic and inorganic pigments, wherein the extent of flocculation was studied.
Tree extract, or sap, is collected from Date Palm trees and is normally sweet, milky white and translucent, with nearly neutral pH. Initially the sap is free of microorganisms; however, environmental contamination leads to fermentation converting it into acids and alcohol.2 Increased temperatures help in rapid growth of the microorganisms; however, even storage in lower temperatures do not help once contaminated.
Although the Sweet Sap has been used successfully in making emulsions, and subsequently paints, the major challenge was to stabilize it by preventing the initiation of fermentation. Three different means were tried to stabilize the Sweet Sap to prevent fermentation: (a) doping with amino alcohol and maintaining at higher pH; (b) addition of 5% methylated spirit (Trial 1); addition of 2% fungicides (Trial 2).
The first approach, caused a brown coloration on storage and was discarded after four months of storage. Studies have continued with the other two routes.
Clearly it is different from the standard which is visible from the fingerprint region. The same exercise was also carried out after ten months of sample preservation. The stability of the Sweet Sap after 10 months was also checked by FTIR analysis.
In this system of two immiscible liquids, water molecules are dispersed in the continuous phase of a long oil alkyd. The experimental Sweet Sap is added to the alkyd medium thinned with mineral turpentine oil at 600 to 1000 rpm and run for 15 minutes. The Sweet Sap, which is partially soluble in the oil phase, is preferentially adsorbed to the alkyd–water interface. The water is added slowly at 1000 rpm to the system which causes the water molecules to disperse and become coated with hydrophilic Sweet Sap molecules. As the emulsion is sheared, larger water droplets are stretched and ruptured into smaller droplets. The sizes of the dispersed water droplets were in the range of a few nanometers to a few microns and are not visible to the unaided eye.
White pigment (rutile grade TiO2) was dispersed in a long oil alkyd medium without using any dispersing/wetting additive and the emulsion made was added during letdown. Formulations are as shown in Figure 2.
Results And Discussions
The performance of the sap was evaluated by:
1. FTIR re-evaluation of Sweet Sap after 10 months storage;
2. Emulsion performance evaluation; and
3. Performance evaluation of paint made with respective emulsifiers and paint long term storage stability.
As stated earlier the sap is very much susAs stated earlier, the sap is very much susceptible to undergo fermentation leading to acids and alcohol. It was thus preserved with added amino alcohol, methylated spirit and fungicide. The sample with amino alcohol turned reddish after four months and was removed from this re-evaluation study. FTIR analysis of the other two was done after 10 months of storage along with two available emulsifiers (Figures 3-6).
This emulsion basically has oil as a continuous phase and is called an ‘inverse’ emulsion wherein water droplets remain as a dispersed phase. The stability has been evaluated by keeping the emulsion in a measuring cylinder and measuring the extent of syneresis. Figure 7 shows the level of separation after seven days.
The higher viscosity of the emulsion made with sap can be attributed to better emulsification of water in alkyd compared to Additive 1.3 Poor emulsification leads to the lowest viscosity for the emulsion made without emulsifier. Emulsions prepared without emulsifier and with commercially available emulsifier show a higher extent of separation as compared to emulsions based on Sweet Sap indicating the higher efficiency of Sap as an emulsifier and, hence, better emulsion stability.
Emulsion stability can be attributed to the homogeniety of the distribution and sizes of the water droplets. The high shear applied during emulsification causes water to convert into smaller droplets; however, sustainability of the size of the droplets depends on the presence and efficiency of the emulsifier used. As reported4, distribution of finer water droplets can be depicted as shown in Figure 8.
In the absence of emulsifier, during the emulsification stage water gets broken into small droplets of size and distribution similar to that of emulsion prepared in the presence of emulsifier. However, with time, some of the smaller droplets will merge into larger droplets due to the absence of the stabilizing effect of the emulsifier (Figure 8). In fact, two primary mechanisms can destabilize the emulsion and cause the change in size-distribution of water droplets. The first mechanism is coalescence where fusion of two or more droplets forms a single larger droplet by rupturing the films of the continuous oil phase. The other way destabilization can occur is if molecules of the dispersed phases have a relatively higher solubility in the continuous phase. The smaller the sizes of the dispersed phase, the less will be the tendency to sediment due to lower gravitational pull.
Performance of preserved Sweet Sap (Trials 1 and 2) was evaluated after 10 months against Additive 1 and Additive 2 (similar commercially available emulsifiers). Both the standard additives and the trial additive show better stability compared to the emulsion without additive (Figure 9). Trial 2 showed slightly better stability in terms of syneresis possibly because of higher viscosity compared to the other samples.
The graphical representation in Figure 10 shows the extent of syneresis of different emulsions for a period of 18 days kept in a measuring cylinder. Initial viscosity measured before testing is also indicative of the trend to separate out as intial low vicosity has greater tendency towards syneresis.
The stability of the emulsion based on the extent of syneresis, as is evident from Figure 10, is also reflected in the final paint. The paint sample made with trial Sweet Sap kept in an incubator for 96 hours at 60 0C shows more resilience than standard product and product made without emulsifier as is shown in Figure 11.
As stated earlier, the experiments have also been extended for co-grind colored systems like Blue (Phiroza Blue) and Grey (Smoke Grey). The samples of Phiroza Blue made without emulsifier, with commercially available additives and with experimental additive were tested for pigment flocculation, reversal of grinding and drop in viscosity. These tests were conducted after a period of ten months with the samples which were kept at room temperature and evaluated against a sample initially kept in an incubator for 96 hours at 60 0C. Samples without emulsifier show significant pigment flocculation which is visible from the drawdown. The samples with commercially available and Sweet Sap as emulsifier, however, do not show any visible flocculation.
In terms of stability with respect to dispersion, it is observed that both the sample kept at room temperature made without emulsifier and Additive 1 have full reversal of dispersion. The product with Sweet Sap, however, shows very insignificant reversal. These are shown in Figure 12.
Initiation of reversal happens after two months for commercially available additives and without additive-based products. Both the products show similar trends of reversal pattern. The product with Sweet Sap, however, shows significantly better stability. The chronological change of reversal is depicted in Figure 13.
The reversal of grinding is also accompanied by a fall of viscosity which is shown in Figure 14.
Evaluation In Alkyd Color Enamel – Grey (Smoke Grey)
PSince pigment flocculation is a common phenomenon in shades with multiple combinations of organic and inorganic pigments, the performance was evaluated in Grey (Smoke Grey). The trial Sweet Sap in its two preserved states (referred to as Trial 1 & Trial 2) was used to make two samples of smoke grey and was compared against commercially available additive based smoke grey shade. After ten months of storage it was observed (against initially stored color reference) that the standard product has undergone a complete change of color, whereas the other two samples made with trial Sweet Sap keep their color intact!
• Newly found product has been found to be better or comparable in some aspects both in emulsion and paint system.
• The term ‘new’ has been justified from the FTIR analysis against market-available standards.
• The stability of the new renewable “Sweet Sap” after ten months of preservation as shown in FTIR analysis justifies the claim of usability.
• The stability of the products made with new renewable “Sweet Sap” has also been studied and found to be better or comparable against the available standards.
• The new renewable “Sweet Sap” can be commercially explored as an effective emulsifier for water in oil system.
The authors would like to express their gratitude to Mr. B. Bera, Sr. Vice President, R&D for technical guidance and support for this work. We also wish to thank Mr. Tapan Dhar, and Mr. Soumitra Bakshi for their input and continuous encouragement for completion of the project.
Special thanks are warranted to little-known Mr. Sujit Pakhira who helped initiate this project and make it successful by procuring the Sweet Sap in the early morning in the winter season before catching the first train for the office! Lastly, we want to thank Mr. Suhas Chakraborty, Mr. Somnath Roy, Mr. Suman Bhattacharya and Mr. Tapas Koley for their help at different points in time for completing the project.
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This paper was presented at The Waterborne Symposium, 2016, New Orleans, LA.References
1. A. John Armstrong, Esq. & Dr. Jan Hamrin, What are “Renewable Resources”?, Chapter 1, The Renewable Energy Policy Manual, Organization of American States, undated. Retrieved 2013-01-05.
2. Naknean,P., Meenune, M. and Roudaut, G., Characterization of palm sap harvested in Songkhla province, Southern Thailand,
3. T.G.Mason, J N Wilking, K Melison, C B Chang and S M Graves, Nanoemulsions: Formation, structure and physical properties.
4. Lorama Inc, January,2011, Revision 3, Polysaccharide Resin Technology.