Glycerol (which also called glycerin or glycerine) is an organic compound that is colorless and odorless. It is a viscous liquid (viscosity of 1.2 Pa.s) and very stable under normal storage conditions, compatible with many other chemical materials, non-irritating in its various uses, and does not have negative effects on the environment (Pagliaro and Rossi, 2008). As such, it is widely been used in food, medical, pharmaceutical and personal care formulations.
Basically glycerol has three hydrophilic hydroxyl groups (see Figure 1) that are responsible for its solubility in water and its hygroscopic nature. The glycerol substructure is a central component of many lipids. Glycerol is sweet-tasting and low in toxicity (where the lowest LC50 for fish is a 24-h LC50 of >5000 mg/l for Carassius auratus (Goldfish) and for aquatic invertebrates, a 24h EC50 of >10000 mg/l for Daphnia magna is the lowest EC50. No toxicity towards the microorganism Pseudomonas putida was observed at 10000 mg/l after exposure for 16 hours).
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In term of chemistry, glycerol (1,2,3-propanetriol) forms the backbone of triglycerides, and can be produced by saponification of animal fats, e.g. a byproduct of soap-making besides made synthetically using petroleum as a feedstock. It also is a byproduct of the production of biodiesel via transesterification. However due to the high interest on biodiesel, the market for glycerol is depressed, and the old epichlorohydrin process for glycerol synthesis is no longer economical. The worldwide market for refined glycerol for the year 2005 was estimated about 900,000 metric tonnes. In the year 2007, Malaysia exports about 350,000 tonnes (valued about RM 900 millions). In the future, the glycerol price is expected to decline to a large extent due to oversupply from the increase in biodiesel production.
Figure 1.Chemical structure of glycerol
In order to add value to glycerol, a need to find new uses for glycerol is crucial. In organic synthesis, glycerol which has multi-functional structure is considered as a good starting material as a prochiral building block for the synthesis of many other chemicals and intermediates. One of which was polymerization of glycerol where the pre-polymers synthesized could be further reacted to produce longer chains of linear, branched or hyperbranched polymers such as dendrimers (Vogtle et al., 2009; Lin and Long, 2003).
In most of the strategies, used of making polyglycerols do not use glycerol as the major propagating unit. Our goal in this research is to synthesize pre-polymer composed of glycerol diacid units as a polymer propagating monomer that could be further reacted to produce longer chains of polymers. This type of polymer is expected to render new materials as viscosity enhancer/ modifier additives for the use in lubricant applications. It is also might have great potential uses in cosmetics, food additives and surfactants. The success of this work (by using free glycerol) will be able to replace toxic monomers, such as glycidol (which commonly used in polymer based glycerol synthesis), that are environmental hazardous.
In a research conducted by Wyatt and his co-workers (2006), novel oligomeric prepolymers were synthesized by acid-catalyzed condensation of glycerol with iminodiacetic. The prepolymers were obtained after purification by chromatography in an average yield of 62%. The compounds were characterized by using 13C NMR, 1H NMR, matrix assisted laser desorption ionization-time of flight-mass spectrometry, and gel permeation chromatography. It was discovered that linear products bearing cyclic urethane structures were obtained in the reaction between iminodiacetic acid and glycerol.
Lin and Long (2003) studied the polymerization of A2 with B3 monomers to produce hyperbranched poly(aryl estrer)s. A dilute bisphenol A (A2) solution was added slowly to a dilute 1,3,5-benzene tricarbonyl trichloride (B3) solution at 25°C to prepare hyperbranched poly(aryl ester)s in the absence of gelation. The molar ratio of A2:B3 was maintained at 1:1. The maximum final monomer concentration was ~0.08 M. The phenol functionalities were quantitatively consumed during the polycondensation. This was showed in 1H NMR spectroscopy and derivitization of terminal groups. Two model compounds were synthesized to identify 1H NMR resonances for linear, dentritic, and terminal units. The final degree of branching was determined to be ~50%. The hyperbranched polymers exhibited lower glass transition temperatures compared to their analogues. Whereas Stumbe and Bruchmann (2003) used the A2+B3 approach to prepare hyperbranched polyesters with controlled molecular weights and properties. The process was carried out by reacting glycerol and adipic acid without any solvents. Tin catalysts were used. The products were evaluated by size exclusion chromatography (SEC) analysis and NMR spectroscopy to determine molecular weights and degrees of branching.
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A study was also carried out on the glycerol esters from reaction of glycerol with dicarboxylic esters. The glycerol esters were synthesized by the base catalyzed reaction of glycerol with aliphatic dicarboxylic acid esters (such as dimethyl oxalate, dimethyl glutarate, dimethyl adipate, etc). Various parameters that may affect the transesterification were studied in order to optimize the yield of products. The reactions were carried out by varying the glycerol/ester molar ratios. The optimum ratio was 4:1, whereby the quantity of the monoester was 60% after 8 h. The conversion decreased slightly when the molar ratio exceeded 4:1. At higher temperatures, the amount of monoester in the reaction mixtures increased and it reached a maximum level after 6 h when the reaction was carried out at 100 °C to 120 °C. It took 8 h at a lower temperature. However, the overall yield at the end of the reaction was not affected by the temperature. The formation of both monoester and diester were produced in an overall yield of 80% after 15 h of reaction time (Cho et al., 2006).
Sunder et. al. (1999) carried out a controlled synthesis of hyperbranched polyglycerols by ring opening multibranching polymerization. Hyperbranched aliphatic polyethers with controlled molecular weights and narrow molecular weight distribution were prepared via anionic polymerization of glycidol with rapid cation-exchange equilibrium. Glycidol which represents a cyclic AB2 monomer was polymerized in a ring-opening multibranching (ROMBP). The anionic polymerization was carried out under slow addition conditions with partially deprotonated (10%) 1,1,1-tris(hydroxymethyl)propane (TMP) as the initiator. 13C NMR, MALDI-TOF spectrometry, vapor pressure osmometry (VPO), and GPC were used to characterize molecular weights and polydispersities of the polyols formed. The 13C NMR spectra used to assess the degree of branching (DB) ranged from 0.53-0.59. A complete attachment of hyperbranched polymers to TMP initiator and the absence of macrocyclics were showed in MALDI-TOF spectra. There was no macrocyclics or hyperbranched macromolecule obtained, due to slow addtion.
A series of blends of hyperbranched polyester with high molecular weight polystyrenes was studied by Mulkern and his co-worker (2000). The processability and compatibility in the blends were investigated as a function of volume fraction of hyperbranched polyols (HBP) added and reactivity of the matrix phase. Due to its low viscosity and high reactivity, HBP polymers are suitable for reactive polymer blending. Through processing and rheological studies, it was found that HBPs are effective processing aids. A significant drop in the blend viscosity occurs immediately on addition of HBP, even at levels as low as 2 vol. %.
In most cases, materials of polymeric structure such as olefin-based polymer and ester polymer, are been used in lubricant to enhance its properties such as viscosity, pour point and so on (Totten, et al., 2003). Poly(glycerol-diacid) oligomer which exhibits wide viscosity range could also be an excellent candidate materials as viscosity enhancer/ modifier additives for the use in lubricant applications.
In 1934, Herman Bruson discovered a synthetic oil additive when he was exploring the synthesis and possible applications of longer alkyl side chain methacrylates. Bruson's invention, polymethacrylates (PMAs) was found to have the potential to function as thickener or viscosity index improver for mineral oils. It increases viscosity at higher temperature more than at lower temperatures (Kinker, B.G., 2009). The alkly group in the ester portion of the polymer can be altered to obtain products with better oil solubility and viscosity-improving properties. It also has good compatibility with a large number of refined and synthetic basestocks.
In a study conducted by Duncan and Turner (1997), blends of lubricant basestocks with high viscosity complex alcohol esters were produced. The blend comprises of a polyhydroxyl compound R(OH)n, a polybasic acid and a monohydric alcohol. The complex alcohol ester showed a pour point of less than or equal to -20°C and a viscosity in the range about 100-700 cSt at 40°C. The lubricating oil according to Duncan and Turner's invention has excellent lubricity as determined by engine performance, vane pump test, Yamaha Tightening Test, and reduced valve sticking. Besides, it has good stability as evidenced by the results of RBOT and Cincinnati Milacron tests. The lubricant has also unexpected biodegradability as measured by Sturm test.
Hernandez et. al. (2005) tested the rolling fatigue of three polyglycols (PAG-9, PAG-12 and BREOX-B-135X). Polyglycols (also called PAG or polyalkylene glycols) are widely used in the lubrication industry. These compounds have very high viscosity indexes, very low pour points, a high thermal conductivity with respect to mineral oils, hydrolytic stability, etc. Rolling fatigue tests were carried out using IP-300 standard in order to obtain the characterization of the fluids. A four ball test machine was used and 10% life (L10) and 50% life (L50) were obtained. The stress-time curves for L10 and L50 were also determined. All polyglycols were tested under boundary lubrication regime (λ<1) where in rolling contacts the surface mode of failures prevails.
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Firdovsi and Yagoub (2006) investigated the synthetic heat carrier oil compositions based on polyalklene glycols. Thermal stability, mass loss on vaporisation at 250 °C, 350 °C and changing the specifications after heating at 300 °C for 10 h were also studied. The prepared PAGs have been taken as basic components for heat carrier oil compositions. It was discovered that the specifications of PAGs such as viscosity indices, pour points, acid number and flash points changed dramatically upon heat treating. In order to improve the thermal stability and viscosity indices, anti-oxidant and anti-foaming additives were added to the base material to reach optimum compositions. The obtained heat carrier oils showed comparable improved properties in comparison with commercially available heat carriers.
This project will be divided into 3 main phases as listed below. Poly(glycerol-diacid) will be synthesized by using different hydrocarbon chain length of diacids (such as azelaic, succinic and adipic acid). The polymer products will be analysed in order to study their chemical and physical properties.
Chemical reactions of glycerol with different hydrocarbon chain length of diacid compounds (e.g. azelaic, succinic or adipic acid) at different mole ratios, are carried out under N2. The mixtures were charged to a reaction vessel equipped with distillation apparatus. The reaction product is allowed to react at the desired temperature and time. Acid value (AV), hydroxyl value (OHV) and glycerol content will be measured to monitor the reaction progress. Optimization of the reaction parameters will be carried out by varying different reaction parameters such as type and amount of diacid, reaction time, temperature and pressure. The final product will be washed, dried and characterised.
The products obtained will be analysed by using both High Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS) and Gas Chromatography-Mass Spectrometry (GC-MS). Other instrumentation such as Fourier Transform Infrared Spectroscopy (FT-IR), Nuclear Magnetic Resonance (NMR) and Gel Permeation Chromatography (GPC) will also be utilised to further confirm their molecular structure.
In this stage, physical properties of the products obtained such as viscosity, solubility, flash point, fire point, density, specific gravity, biodegradability, and oxidative stability will be performed in order to determine their suitability as viscosity enhancer/ modifier for lubricant. Finally several publications and a dissertation will be produced.
Optimum parameters to produce glycerol-based oligomer from copolymerisation of glycerol with different hydrocarbon chain length of diacids will be identified. Branched oligomer is expected to form from this reaction conditions. As the time of reaction proceeds, viscosity of the polymer increases. Solubility of the oligomers in water will be also greatly reduced with increasing hydrocarbon chain length of diacid monomers. The glycerol based polymers are expected to possess wide range of kinematic viscosity which can be use as additive in lubricant applications.