Polyethylene terephthalate is used in the manufacture of Dacron polyester fiber, Mylar polyester film, and Cronar photographic film base or Terylene or Hostaphan.

Stephanus Sulaeman@upv.pertamina

Polyethylene terephthalate is used in the manufacture of Dacron polyester fiber, Mylar polyester film, and Cronar photographic film base or Terylene or Hostaphan.

Polyethylene terephthalate is prepared by a transesterification reaction between dimethyl terephthalate and the dihydric alcohol, ethylene glycol.

2nHOCH2CH2OH + nCH3OOC-C6H4-COOCH3 heat, catalyst→

nHOCH2CH2OOC-C6H4-COOCH2CH2OH + 2nCH3OH

nHOCH2CH2OOC-C6H4-COOCH2CH2OH heat/vacuum, catalyst→

HOCH2CH2-O-[OC-C6H4-COOCH2CH2-O-]nH + (n-1)HOCH2CH2OH

Dimethyl terephthalate is obtained by the esterification of terephthalic acid by methanol. Terephthalic acid is a high melting (above 300oC), insoluble material and requires special conditions for esterification. Two parts of methyl alcohol, 1 part of terephthalic acid and 0.01 part of sulfuric acid are placed in a closed, agitated pressure vessel and heated to 150oC for 2-3 hr. during the last hour, 5-6 parts of additional methyl alcohol is added slowly to the liquid reactants and distilled to remove the water of reaction. By cooling, the dimtehyl terephthalate is completely separated from the solution. Yields run as high as 95%. The dimethyl terephthalate can be purified by crystallization from high-boiling solvents, such as xylene, or it may be distilled.

In the Imhausen process, p-xylene is oxidized by air at elevated temperature to p-toluic acid. p-Toluic acid is soluble and easily esterified. It is converted to methyl p-toluate in the usual way. The methyl p-toluate is then oxidized by air to monomethyl terephthalate. This product is soluble in organic solvents and is esterified by methanol to give dimethyl terephthalate. The purified dimethyl terephthalate from either of these processes is suitable for use in the manufacture of polyethylene terephthalate.

In the first stage of the manufacture of polyethylene terephthalate, dimethyl terephthalate (1 mole) is caused to react with ethylene glycol (2 moles) in the presence of a catalyst. Suitable catalysts are litharge, zinc salts, calcium salts, magnesium salts, alkali metals or their alkoxides, etc. The catalyst concentration may vary from 0.005-0.1%. the reaction begins at 150-160oC, and the methyl alcohol is distilled out through a good fractionating column until the reaction is complete. In the end, the reaction temperature will have risen to about 230oC. the reaction product is statistically bis(β-hydroxyethyl) terephthalate, but actually it is a mixture of free glycol, bis(β-hydroxyethyl) terephthalate and low polymer. Pure bis(β-hydroxyethyl) terephthalate melts at 109oC, but the usual commercial product melts somewhat higher because of the presence of low-molecular weight polymers.

In the second stage of the manufacture of polymer, the temperature is raised further, and the reaction takes place between the hydroxyethyl end groups to produce polymer and glycol. Vacuum is applied slowly and the temperature raised to remove the glycol and to continue the reaction. The final polymerization is usually accomplished at 260-300oC under a vacuum of 0.1-10 mm of mercury.

If an attempt is made to prepare the polymeric ester from 1 mole of glycol and 1 mole of dimethyl terephthalate, low molecular weight polymers are obtained because of the occurrence of competing reactions. As soon as an ester linkage is formed to fix one end of a glycol molecule, the ester group can enter into a transesterification with the hydroxyl group at the end of a previously formed polymeric molecule. The ethylene glycol which is set free can escape with water or alcohol and is lost at the high temperatures which are required for the reaction. However, when the bis(β-hydroxyethyl) terephthalate is heated and vacuum is applied, excess glycol is removed as transesterification occurs, and an exact balance of glycol and terephthalate constituents is approached.

The duration of the polymerization depends on the catalyst concentration, reaction temperature, size of the batch of polymer being produced, and the amount of surface area generated in the polymerization autoclave. In commercial operation, it is desirable not to have too long a polymerization cycle for economic reasons. In a cycle that is too long, the competing and irreversible thermal degradation reaction will have sufficient time to lower the viscosity and to cause discoloration.

The removal of ethylene glycol from the polymerization reaction mixture should be as continuous and as rapid as possible to assure rapid polymerization. To provide for efficient removal of ethylene glycol, it is desirable to conduct the polymerizationin a vessel which provides for maximum surface-area generation. Agitation of the polymerizing mass should provide for the maximum exposure of the batch to the effects of the vacuum. Efficient operation may be carried out in rapidly agitated vessels having relatively large exposed surface areas or in continuous units. The reaction is stopped when the product has attained the desired viscosity. The molecular weight of the polymer is not known accurately, but reproducibility is obtained by control of the viscosity of the polymer. This is usually done by determining the viscosity of a series of dilute solutions of the polymer in a solvent, such as 60:40 phenol : tetrachloroethane, compared with that of the pure solvent. By plotting lnηr/C vs C in which ηr is the viscosity of the dilute solution and C is the concentration in grams of polyester per 100 ml of solution, and extrapolating to zero concentration, the intrinsic viscosity [η0] is determined. Commercially desirable polymer should have an intrinsic viscosity above 0.45. Polyethylene terephthalate can be water white. The pure polymer has a melting point of 265oC, although most commercial varieties melt somewhat lower.

Other glycols, such as tetramethylene- (mppol 226oC), hexamethylene- (mppol 152oC), octamethylene-glycol (mppol 132oC), etc. can be used to make the polymer but have a more lower melting point than ethylene glycol.

Diethylene glycol gives an amorphous, rubbery polymer. Pentaglycol (neopentyl glycol) gives an amorphous, glassy polymer.