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In oleochemical industry, glycerol (1,2,3-propanetriol) is always produced as a by-product in the manufacturing of acids, soaps, methyl esters, alcohols or nitrogen-containing derivatives. It can also be made from propene via epichlorohydrin (1-chloro-2,3-epoxypropane). However, the petrochemical supply route is less important due to the increasing supply of glycerol from oleochemical industry, the high price of propene and the demand for epichlorohydrin for other purposes (Gunstone & Henning, 2004).
Figure 1.1 Glycerol
Glycerol possesses a unique combination of physical and chemical properties which are utilized in many commercial products. It is hygroscopic, colourless, odorless, viscous, sweet-tasting, low boiling point, non-toxic, emollient, a good solvent, and water soluble. Besides, it is easily biodegradable (Gunstone & Henning, 2004). Furthermore, it is 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 & Rossi, 2008).
The glycerol market is currently undergoing radical changes, driven by very large supplies of glycerol arising from biodiesel production. The effort to reduce the dependence on foreign oil has increased the production of biodiesel and glycerol is the major co-product from the transesterification process used to produce biodiesel. Hence, there is a need to find new uses for glycerol. Polymerization is one of the methods which large amount of glycerol can be used (Wyatt et al., 2006).
There two types of polymerizations. First, soluble products are obtained regardless of the extent to which the reaction is carried toward completion. The products formed are mainly linear polymers. The second type of polymerization is those that lead to gelled or insoluble products, provided that the reaction is carried far enough. The reactants are capable of producing large three dimensional molecules (Flory, 1941).
According to Flory (1941), gelation occurs only when there is the possibility of unlimited growth in three dimensions. It is a significant characteristic of polymerizing systems to have a sharply defined gel point at a certain critical extent of reaction which is independent of temperature, amount of catalyst and so on.
Through polymerization of glycerol, the pre-polymers synthesized could be further reacted to produce longer chains of hyperbranched polymers. Hyperbranched polymers belong to the family of macromolecules known as dendrimers. Dendrimers are highly branched monodispersed molecules produced by multistep syntheses. Preparation of dendrimers requires a high degree of purity of the starting material and high yields of the individual synthetic step. On the other hand, hyperbranched polymers are randomly branched molecules prepared by a simple one-step reaction (Wyatt et al., 2006) via polyaddition, polycondensation, radical polymerisation, and so forth, of an ABn monomer (Vogtle et al., 2009). Due to their unique combination of low viscosity, excellent solubility, and facile synthesis, hyperbranched polymers have received significant attention (Lin, Q & Long, T.E., 2003).
Reaction of the functional A groups with the functional B (coupling) groups of a second monomer molecule gives rise to randomly branched molecule. Since the C groups are present in excess (n â‰¥ 2), crosslinking are avoided from the outset. Reaction can be brought to a standstill by addtion of stopper components. Since the synthesis of hyperbranched polymers does not involve coupling to core molecule, but only ABn monomers react with one another. Both branched molecules and linear sequences maybe formed (Vogtle et al., 2009).
Hyperbranched polymers produced from diacids (A2) and glycerol (B3) are an example of the AB2 system. AB2 monomers are not readily available and kinetic calculations show that the first condensation reaction, which produces an AB2 species, is faster than the subsequent polymer propagation. Thus, the remainder of the reaction progresses as polycondensation between AB2-type species prior to the gel point. Several methods have been used to avoid gelation in A2+B3 systems, including performing the reactions in dilute solutions or reacting them in the absence of solvents while monitoring.
This glycerol-based polymer is expected to show similar properties and characteristics as polyalkylene glycol (PAG). A polyalkylene glycol having the general formula: HO-[R-O-]n H in which n has a value of at least 2 and R is an alkylene radical containing at least 10 carbon atoms. PAG liquid are used as synthetic lubricants in many diverse applications. Thus, glycerol-based polymers could also have the potential to be use as high performance lubricant, coolant or as a lubricant additive (such as viscosity modifier).
Materials with polymeric structures can be used in lubricant to enhance its properties, such as viscosity, pour point and so on. It can be used as starting material for certain types of additives. These polymeric additives can be viscosity modifier, pour point depressants, emulsifiers and demulsifiers, and foam inhibitor in lubricants (Totten, G.E. et al., 2003).
Oils can be effective lubricants at low temperature. However, at higher temperature, they become less effective. To overcome this problem, viscosity modifiers are useful in minimizing viscosity variations with temperature. Viscosity modifier is a polymer with average molecular weights of 10000 to 150000. At all temperatures, viscosity modifier is able to increase oil's viscosity. The thickening of oil at lower temperature is less than that at higher temperature. At low temperatures, the polymer molecules occupying a small volume have a minimum association with the bulk oil. The situation is reversed at high temperatures as the polymer chains expand due to the increased thermal energy. Besides, at higher temperatures, polymers are more soluble and therefore cause the viscosity to increase(Totten, G.E. et al., 2003).
There are two types of viscosity modifiers available commercially: olefin-based polymers and ester polymers. Polyisobutylenes (PIBs), olefin copolymers (OCPs), and hydrogenated styrene-diene (STDs) polymers. Ester polymers include polymethacrylates (PMAs) and styrene ester polymers (SEs) (Totten, G.E. et al., 2003).
In a research done 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.
Qi Lin and Timothy E. 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.
J.F. Stumbe and Bernd Bruchmann (2003) also 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 was used. The products were evaluated by size exclusion chromatography(SEC) analysis and NMR spectroscopy to determine molecular weights and degrees of branching.
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.
T.J. Mulkern and N.C. Beck Tan (2000) studied a series of blends of hyperbranched polyester with high molecular weight polystyrenes. 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 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 have good compatibility with a large number of refined and synthetic basestocks.
In a study 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 (Duncan et al., 1997).
Hunt et al. (1993) carried out supercritical fluid extraction to analyse liquid poly(alklene glycol)(PAG) lubricants and sorbitan ester formulations. The PAG matrix was adsorbed onto silica and the selectivity obtained by this method was compared with that obtained by the direct extraction of adsorbed and unadsorbed PAG. Extraction was also done for unadsorbed PAG through the in-line column and it was successful in separating additives from all but the lowest molecular mass PAG oligomers. This extraction procedure enabled fractionation of the product and could be used as a sample preparation technique for further spectroscopic analysis.
It is difficult to produce polymers with narrow molecular weight distributions by traditional methods. Supercritical fluid technology is applied to overcome the conventional methods. The solubilty parameter of supercritical fluid can be tailored. Selective extraction and fractionation are possible from multi-component mixtures. The key to making high quality polymers is to ensure precise control of molecular weight and polydispersity at high yield while keeping residual contaminants below acceptable tolerance levels.
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 (Î»<1) where in rolling contacts the surface mode of failures prevails.
In oils of the same family, the pressure-viscosity coefficient is relatively constant. An increase in viscosity improved the minimum film thickness with the consequent increase of the Î» ratio. Fatigue life is largely a function of the ratio of lubricating film thickness to composite surface roughness (Î» ratio). Differences in Î» ratio for the three polyglycols resulted in different asperity interactions and rolling contact fatigue lives. With regard to rolling contact fatigue, the choice of viscosity class should avoid asperity interaction, so that the only mode of failure will be subsurface failure. Although average pressure in the contact was the same, increase in viscosity from PAG-9 to BREOX-B135X improved the Î» ratio from 0.18 to 0.34. At less Î»<1, operation is in the boundary lubrication regime where rolling contact fatigue life is short, but an increase of Î» ratio reduce the asperity interactions and therefore rolling contact fatigue improved.
An investigation was then carried put by Garcia and co-workers on PC-SAFT volumetric and phase behavior of carbon dioxide + PAG or POE lubricant systems. The densities of synthetic PAG oil was measured from 283.15 K to 333.15 K while the solubilities of CO2 in this oil was measured from 253 K to 333.15 K. Molecular weight of the lubricant was estimated using fast atom bombardment (FAB). Molecular weight and experimental densities were used to calculate characteristic parameters of PC-SAFT model for several commercial PAG oils. Transferable characteristic parameters were used for POEs.
The thermophysical properties and phase behaviour of CO2-lubricant oil mixtures is important for the design of refrigeration and air-conditioning. The circulating fluid comes into contact with the lubricant used in compressors and a portion of the oil is transported into the refrigeration circuit with various effects in terms of performance. If the oil is immiscible with the refrigerant, the compressor may be damaged due to poor oil return to the compressor. Oil may accumulate inside the heat exchanger tubes reducing heat transfer capabilities, enthalpy change and resulting in an overall decrement of the refrigeration capacity and cycle performance. In addition, high solubility of the refrigerant in the lubricant may reduce the viscosity of the oil-rich phase and results in lower lubrication properties which gives rise to breakdown of the compressor mechanical parts.
Hauk & Weidner (2000) studied the thermodynamic and fluid-dynamic properties of carbon dioxide with different lubricants in cooling circuits for automobile application. The datas of the binary mixture were measured at temperatures between 5 and 100 °C under pressure of up to 150 bar. The phase behavior was observed qualitatively in a hugh-pressure view cell and was determined in an autoclave based on a static-analytical method. The viscosity of the lubricant saturated with carbon dioxide was measured with an integrated quartz viscosimeter.
The applicability of lubricants in car-climatization systems can be evaluated with the knowledge of phase behavior and the resulting viscosity of gas-saturated lubricantsThe phase behavior of oils with carbon dioxide can be divided into three different types which are binary systems with closed miscibility gaps, systems with open miscibility gaps, and systems that show barotropic phenomena. Oils that show barotropic behavior in contact with compressed carbon dioxide are not recommended as lubricants. Oils with complete or limited miscibility with carbon dioxide may be used.
Firdovsi & 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 2 stages as listed below. Poly(glycerol-diacid) polymer will be prepared by using different hydrocarbon chain length of diacids (such as azelaic, succinic and adipic acid). The products will be analysed in order to study their chemical and physical properties.
Phase 1 :
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 maintain the reaction progress. Optimization of the reaction parameters will be studied 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.
Phase 2 :
The products obtained will be analysed by using both High Performance Liquid Chromatography (HPLC) 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. Physical properties of the products obtained such as viscosity, solubility, flash point, fire point, density, specific gravity, biodegradability, and oxidative stability will be performed.
Polymers resulting from the copolymerisation of glycerol with diacids of varying carbon chain length, molecular structure, and composition will be obtained. Structures having more than two free acid functionalities at the end-terminals can occur only after branching. As the time of reaction proceeds, the viscosity increases which limits the interaction between the reactants and the growing polymers. The water solubilty of the oligomers decreases with increasing chain length of the diacid monomers of the diacid monomers used in preparing the oligomers. This glycerol based polymers are expected to possess wide range of applications such as cosmetics and lubricants.