Difference between three solvents

At present, the most widely used solvents in the coating industry are aromatic hydrocarbon solvents. Due to its complicated source and classification, combined with the names inherited in history and the names standardized in recent years, it has caused confusion to some extent.

According to different sources, aromatic hydrocarbons can be divided into two categories, coking aromatics and petroleum aromatics. Coking aromatics are obtained by fractional distillation of coal tar, and petroleum aromatics are obtained by rectifying petroleum products through platinum reforming oil, catalytic cracking oil, and toluene disproportionation oil.

Coking aromatics and petroleum aromatics are further divided into light aromatics and heavy aromatics according to the number of carbon atoms. Generally, those below C8 (including C8) are called light aromatics, and those above C8, mainly C9 ~ C10 are called heavy Aromatics. Light aromatic solvents for coking aromatics, including coking benzene, coking toluene, coking xylene, and solvent naphtha. Light aromatics of petroleum aromatics include petroleum benzene, petroleum toluene, and petroleum xylene.

Benzene

Industrial benzene is a colorless and transparent liquid with a unique odor of aromatic hydrocarbons. The impurities contained mainly include aromatic homologues, thiophenes, and saturated hydrocarbons, which need to be purified and removed if necessary. Benzene is hardly soluble in water (zero dipole moment). Except for glycerol, ethylene glycol, diethylene glycol, 1,4-butanediol and other polyols, benzene can be used with ethanol, chloroform, carbon tetrachloride, carbon disulfide, and ice. Most organic solvents such as acetic acid, acetone, toluene, xylene, and aliphatic hydrocarbons are miscible. Benzene can dissolve synthetic resins such as rosin, glycerol rosin, glycerol alkyd resin, vinyl resin, styrene resin, acrylic resin. Polyvinyl acetate resin is partially soluble in benzene; ethyl cellulose and benzyl cellulose are soluble in benzene; cellulose acetate and nitro cellulose are hardly soluble in benzene.

The main use of benzene in coatings is to use it in combination with butyl acetate (or ethyl acetate), acetone, and butanol. As a thinner for nitro paints, because benzene vapor is highly toxic to the human body, it is now replaced by other solvents. , Tend to be eliminated.

Toluene

Industrial toluene is a colorless transparent liquid with a odor similar to benzene, and may sometimes contain a small proportion of aliphatic hydrocarbons of the same boiling range. Similar to benzene, toluene is insoluble in water and is miscible with many organic solvents such as methanol, ethanol, chloroform, acetone, glacial acetic acid and benzene. Toluene can dissolve dry oil and other vinyl resins and alkyd resins other than vinyl chloride. Adding methanol and ethanol to toluene can increase the solubility of cellulose acetate.

Because toluene volatilizes quickly (about 3 times as much as xylene), it is rarely used as a solvent. At present, it is mainly used as one of the components in a mixed solvent of vinyl coatings and chlorinated rubber coatings. It is used as a diluent in nitrocellulose coatings.

Xylene

Xylene is a mixture of three kinds of isomers of ortho, meta, and para-xylene, and any of the three isomers is not suitable for use as a solvent alone in coatings.

Industrial mixed xylene-based colorless and transparent liquid with a peculiar odor of aromatic hydrocarbons and sometimes faint fluorescence. Due to different sources, it is divided into petroleum mixed xylene and coking xylene. The former is divided into 3 ° mixed xylene and 5 ° mixed xylene according to different distillation ranges; the latter can be divided into 3 °, 5 ° and 10 ° xylene, and other parameters are basically the same.

Depending on the source and processing route, the contents of the three isomers in the mixed xylene are also different. The following table lists the composition of petroleum mixed xylene prepared by the three routes. At the same time, mixed xylene often contains ethylbenzene, a small amount of toluene, xylene, aliphatic hydrocarbons and sulfides.

Coating products often require the use of anhydrous xylene. To remove a small amount of water in the mixed xylene, calcium chloride, anhydrous sodium sulfate, phosphorus pentoxide or molecular sieve can be used as a dehydrating agent.

Xylene is insoluble in water and can be miscible with ethanol, ether, aromatic hydrocarbons and aliphatic hydrocarbon solvents. Due to its strong solubility and moderate volatilization rate, it is the main solvent for short oil alkyd resins, vinyl resins, chlorinated rubbers and polyurethane resins. It is also a solvent for asphalt and petroleum asphalt. Agent. Add 20% ~ 30% n-butanol in xylene,

Can improve the solubility of xylene to amino resin paints and epoxy resins. Since xylene can be used for drying paint at room temperature or baking paint, it is currently the most widely used and most widely used solvent in the coating industry.

Toluene is an organic compound, which is an aromatic hydrocarbon, with a simple structure of C6H5CH3. It is liquid at normal temperature, colorless and flammable. It has a boiling point of 110.8 ° C, a freezing point of -95 ° C, and a density of 0.866 g / cm3. The toluene thermometer uses its lower freezing point than water, and can be used in high and cold regions; and its boiling point is higher than the boiling point of water, which can measure temperatures below 110.8 ° C. Therefore, in terms of temperature measurement range, it is superior to mercury thermometers and alcohol thermometers. In addition, toluene is cheaper, so the toluene thermometer is cheaper than the mercury thermometer.

Xylene is a colorless and transparent liquid with the structural formula: C6H4 (CH3) 2, xylene (PX) belongs to aromatic hydrocarbons. If people inhale a high concentration of toluene or xylene in a short period of time, they will experience symptoms of central nervous anesthesia. The lighter are dizzy, nausea, chest tightness, fatigue, severe coma or even death due to respiratory failure, mainly from synthetic fibers, plastics, fuel, rubber, etc., hidden in paint, additives for various coatings and various adhesives, The waterproof material can also come from the combustion of fuel and tobacco leaves. Xylene can be divided into p-xylene, o-xylene and m-xylene according to the different positions of two methyl groups on the six-carbon ring. It is an unsaturated hydrocarbon organic compound.