PREPARATION OF POLYESTER RESINS

All of the known esterification methods can lead to the synthesis of polyesters. Some specified reactions that do not have an equivalent in simple ester chemistry can also be used for polyester synthesis.

(i) Direct esterification.

One of the most important routes goes via the direct esterification of diols and diacids. An

By varying the hydroxyl excess, the molecular weight of the polyester can be adjusted. When using equimolar quantities of reactants, a high molecular weight polyester can be obtained.

OH – R – OH + HOOC – R’ – COOH → HO – R – O – CO – R’ – COOH + H2O The imbalance in the molar ratio due to inexact weighing and, above all, due to the loss of volatile components (such as neopentylglycol) leads to polyesters with terminal carboxylic or hydroxyl groups which, therefore, limit the molecular weight. 1f the synthesis is carried out in one step, it is difficult to control the molecular weight and, hence, the properties of the polyester. For example, in the synthesis of high molecular weight polyethyleneterephthalate, a two step technique is used. In the first step (the esterification), a di-hydroxyl-diester is obtained in the presence of a large excess of glycol.

2OH – R – OH + HOOC – R’ – COOH → HO – R – O – CO – R’ – CO – R – OH + 2H2O The second step, alcoholysis, is carried out at high temperatures (280°C) with a high vacuum.

This alcoholysis or esterification process enables one to obtain a high molecular weight polymer with the excess glycol being separated by distillation. One necessary condition for the reaction to proceed to completion is a volatile diol. The other method, which is acidolysis, (corresponding to an excess of diacids) is not applicable here since diacids are insufficiently volatile.

The direct esterification of polyols by polyacids (one or two step process) is a generai method for the synthesis of polyesters, whether they are liquid or in powder form. In this type of synthesis all the diacids which can de-carboxylate are to be avoided as well as diols with tertiary hydroxyls which can dehydrate, diphenols of too low reactivity and some short chain diols which can crystallize (1,4 butanediol + isophthalic acid) or which can generate tetrahydrofuran. The formation of tetrahydrofuran from 1,4 butanediol can readily occur.

HO

OH O

Theoretically, a reaction, between stable diols and diacids, only gives linear polyesters with terminal hydroxyl functions. However, there are secondary reactions and the choice of catalyst can be important in order to minimize these. The formation of a certain proportion of ether linkages depends upon the propensity of the diol to form this type of bond. Another secondary reaction consists of an anhydride formation within the chain leading to its rupture.

Some thermal decomposition is also inevitable and this leads to the formation of aceto- aldehydes in small quantities during the polycondensation reaction. Moreover at the same time cyclic polyesters are also formed (up to 1.5 % in polyethyleneterephthalate). These cyclic structures are also found in powder polyesters that are prepared from terephthalic acid and neopentylglycol. Such structures are formed between these two compounds, even in small quantities, and they are responsible for the undesirable blooming effect observed in powder coatings. In low molecular weight polyesters, the cyclic fraction can be significant, which lowers the hydroxyl value of the polyester and, hence, the solvent resistance of the film.

During the preparation of a Polyester using diols such as trimethylolpropane, and especially neopentylglycol, a significant loss of polyol can occur. This will disrupt the structure of the resin if the amount lost cannot be determined and compensation additions made. It is normal

to use a packed column for maximum. Separation of the diol vapors that have been carried over with the water of reaction to minimize polyol losses, but there are losses all the same.

It is possible to determine the amount of polyol lost by measuring how much is in the water obtained from the esterification reaction. This water is collected in a Dean and Stark receiver.

By measuring the refractive index of the water polyol mixture, the amount of polyol present in the water can be estimated and, since the quantity of water recovered is known, the quantity of polyol lost can also be calculated. This should not exceed 10 % of the initial quantity (which means that the temperature at the top of the cooling column should never exceed 105°C) of polyol in the formula.

Levels above this mean that the plant is not suitable for the manufacture of polyester resins.

Below this value, one can compensate during production by the addition of polyols. Tables and graphs of refractive indices for different polyol water mixtures are available. Figures 3-1 to 3-4 reproduced from the Amoco technical literature shows different refractive indices in water for different polyols. The common polyols that present most glycol loss problems include ethylene, propylene and neopentylglycol. Consider as an example neopentylglycol. If the refractive index of the water, glycol mixture in the receiver is 1.346, then it means that the water contains 6% NPG. If the total weight of water collected is known then the total amount of NPG lost is also known. For example, 100 kg of water containing NPG with a refractive index 1.346, means that 6 Kg of NPG has been lost. Depending upon the initial glycol excess used, it may be necessary to add a further 6Kg of NPG to compensate for these losses. It is much more difficult to estimate losses when a mixture of volatile polyols is used.

Figures 3-1 to 3-4, Refractive index of various polyols in water, in order to estimate glycol losses

(ii) Transesterification.

The second equally important method for the preparation of polyesters is based on a transesterification reaction of diols with carboxylic acid esters. Generally methyl esters are used. In the case of powder coating resins, terephthalic or isophthalic acid are directly used in their liquid form. The polycondensation reactions take place at around 280°C. As is the case for direct esterification, the one step reaction leads to low molecular weight polyesters. The two step process is exploited on a large scale for the production of polyethyleneterephthalate for fibres and sheets. The first step forms the dihydroxyl ester, whilst the second step is the same as for the direct esterification.

This method has some advantages compared with the first one such as:

ƒ The reaction between diols and dialkyl esters takes place at a lower temperature (starting at 140°C-150°C).

ƒ The purity of the methyl esters is higher than for their corresponding diacids.

ƒ The melting point of the esters is lower than that of their diacids and it is easier to obtain homogeneous n-mixtures.

ƒ There are fewer secondary reactions such as the etherification of polyols.

ƒ Diols of low reactivity and of low stability can be used (such as phenols).

ƒ Diols that are able to form cyclic structures can be used, such as 1,4 butanediol.

The major drawback with this method remains the high cost of the starting materials.

(iii) Diols and acid chiorides.

A third method for preparing polyesters is based on the reaction of acid chlorides with diols.

OH – R – OH + ClOC – R’ – COCl → HO – R – O – CO – R’ – COCl + HCl

This method is, above all, advantageous when long chain aliphatic diols are used with low reactivity biphenols. This method also leads to sequenced polyesters starting from low molecular weight hydroxylated polyesters or even alternate polyesters.

Various other techniques can be utilised for preparing polyesters from diols and acid chlorides, depending on the requirements of the resin. Some examples follow:

(a) Reaction in an inert medium with a high boiling point (high boiling point hydrocarbons) and under dry nitrogen to eliminate HCI.

(b) Reaction in an organic solvent in the presence of tertiary amines (pyridine or dimethylaniline) which absorb any acid generated.

(c) In some cases, an interfacial polycondensation can be used, for example diphenols in aqueous alkaline solution and diacid chlorides in solvents such as methylene chloride or toluene react rapidly at ambient temperature with vigorous agitation to convert the dispersion from the water immiscible phase into fine droplets.

Interfacial polycondensation also leads to polyamides (nylons). However, this technique is used for the preparation of special linear polyesters. The polycarbonates possess within the structure the carbonate group.

(iv) Self-esterification.

The self-esterification of hydroxyacids is a fourth method for the preparation of polyesters.

This reaction requires a high temperature, up to 250'C, and is often carried out at reduced pressure. In many cases, this method cannot be used since dehydration or cyclisation problems interfere with the esterification reaction. Lactic acid, for example, can be transformed into acrylic acid.

OH

O OH H

O

OH + ^H2O

Short chain hydroxyacids can easily cyclize forming lactones.

(v) Polyesters from lactones.

Lactones constitute the basis of a fifth method for the preparation of polyesters by a ring opening reaction of a lactone. These are reactions that are readily carried out in the presence of catalysts such as inorganic acids or Friedel Crafts complexes. The possibility of the lactone polymerising depends upon the number of carbons forming the ring.

Resins for powder coatings are manufactured at high temperatures in bulk without solvent. At the end of the synthesis, they are cooled and ground into flakes. Very often, terephthalic acid can be found as a major diacid due to its excellent mechanical properties. As a generalisation, formulations are similar to products in solution except that the final characteristics and the molecular weights are adapted for the relevant crosslinked systems mentioned above (high acid value, low OH value etc).