Tham khảo tài liệu 'lubricants and lubrication part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 78 I 5 Synthetic Base Oils presence of Friedel-Crafts catalysts. They are colorless oily or waxy substances of very low toxicity. Polyalkylene glycols have at least one hydroxyl group at one end of the molecule and can therefore be regarded as alcohols. The number of hydroxyl groups is increased by use of water or multifunctional starters. The reaction of the alcohols with acids leads to esters reaction with for example strong acids and olefins leads to ethers. Because the carbon-oxygen bond is stronger than the carbon-carbon bond polyalkylene glycols have solvent properties somewhat different from those of hydrocarbons. Miscibility with water increases with the number of ethylene oxide units. The solubility is a result of hydrogen-bonding of water with the free electron pairs of the oxygen. In solution the water-soluble grades are practically non-flammable. The hygroscopic character of the polyalkylene glycols is dependent on their hydroxyl group content-it decreases with increasing molecular weight and the number of ether bonds. The solubility in water decreases in the same way. That it also decreases with increasing temperature can be explained by a loss of hydrogen bonding. Solubility in hydrocarbons increases with the molecular weight. Polyalkylene glycols in general are soluble in aromatic hydrocarbons. The molecular weight and viscosity of polyalkylene glycols can be significantly influenced during production and be adjusted within narrow limits. The possibility of engineering products in this way distinguishes them from many other lubricants. Low molecular weight polyalkylene glycols containing more than 50 propylene oxide have pour points that go down to -65 C. The lateral methyl groups are responsible for the disruption of crystallization. On the other hand pure high-molecular-weight polyethylene glycols are wax-like solids with pour points near 4 C. Kinematic viscosities range from 8 to 100 000 mm2 s-1 at 40 C. With the change from diols to monoethers .
