Selection of monoethylene glycol (MEG) for carboxyl-terminated unsaturated polyester resins begins with the certificate of analysis because acid value control is stoichiometric before it is kinetic. Polyester-grade MEG with a minimum purity of 99.9 wt%, a diethylene glycol (DEG) content not exceeding 0.05 wt%, and a water concentration below 0.08 wt% reduces the variability in the OH/COOH charge ratio that must otherwise be compensated by reactor-level adjustments. In production reactors of 12,000 L to 18,000 L working volume, an initial molar charge ratio of MEG to total dicarboxylic acid in the range 1.05:1 to 1.10:1 is standard for laminating resin grades, while gelcoat formulations often use a tighter ratio of 1.04:1 to 1.06:1 to preserve low hydroxyl end-group content. The acid value at the end of the first esterification stage is typically reduced to 40–60 mg KOH/g at 180–210°C under atmospheric pressure, with water of reaction removed through a packed column and partial condenser. If the MEG water content rises to 0.3 wt%, the effective hydroxyl group concentration decreases by approximately the molar water fraction, and the first-stage acid value plateaus 10–15 mg KOH/g higher under identical time-temperature conditions; this elevation cannot be corrected by increasing reactor temperature alone because elevated temperature accelerates maleate to fumarate conversion and increases DEG formation. Therefore, MEG selection for acid value control is not merely a purity specification but a process stability parameter that determines whether the vacuum stage can begin at the intended carboxyl concentration with predictable chain extension.

What Process Conditions Govern Maleate to Fumarate Isomerization in MEG-Based UPR?

Because the cis-to-trans conversion is thermodynamically controlled, maleate to fumarate isomerization in unsaturated polyester resins proceeds through thermal rotation of the cis configuration to the trans configuration when the maleate half-ester or diester is held at condensation temperatures above 170°C. The reaction is favoured for fumarate because the trans double bond is less sterically hindered and more reactive toward a later free-radical copolymerization with styrene. In MEG-based resins, the rate of isomerization is strongly temperature-dependent; industrial practice maintains the second-stage condensation between 200°C and 220°C to achieve a fumarate mole fraction of 70–90% of total unsaturation, as quantified by 1H NMR integration of the olefinic protons at δ 6.3 ppm for maleate and δ 6.9 ppm for fumarate. Extending the holding time at 220°C beyond the point at which the acid value reaches specification increases fumarate content but also promotes DEG formation and branching. The processing window for maximizing fumarate while keeping acid value between 15 mg KOH/g and 35 mg KOH/g is often no wider than ±5°C at 210°C for an MEG-maleic anhydride-phthalic anhydride ternary system. Published kinetic data for this specific configuration is limited; however, the temperature sensitivity of the isomerization is sufficiently high that a deviation of 3°C near the end point can alter the fumarate fraction by measurable NMR peak area and shift the final styrene-compatibilized resin viscosity by 50–100 mPa·s.

During split glycol addition, the first portion of MEG is charged with phthalic anhydride and maleic anhydride at a 1.0:1.0 stoichiometric ratio of hydroxyl to total anhydride, while the remaining 5–10% of the glycol is withheld until the atmospheric esterification acid value drops below 80 mg KOH/g. Delaying the second glycol fraction reduces the residence time of maleate half-ester at high temperature during the earliest stage, which limits premature isomerization and allows the later vacuum stage to direct both polycondensation and cis-to-trans conversion under more controlled conditions. The withheld MEG also compensates for glycol losses through the distillation column, which vary with reflux ratio and column pressure drop. On a 15,000 L reactor fitted with a structured packed column of 6 m packing height, the overhead distillate typically contains 0.5–1.5 wt% MEG depending on reflux ratio set between 2:1 and 4:1. Acid value samples drawn at 30-minute intervals during split addition show that a deviation of 0.5 wt% in the withheld glycol fraction from the target produces an end-of-stage acid value spread of 8–12 mg KOH/g when the same vacuum profile is applied. This sensitivity is the reason MEG selection and gravimetric or mass flow-based charging must be tied to the same calibration at ±0.5% of full-scale charge weight.

Acid Value Control Limits Under Reduced Pressure Polycondensation

After atmospheric esterification reaches an acid value between 40 mg KOH/g and 60 mg KOH/g, the pressure is reduced stepwise from 100 kPa to 10–50 kPa initially, then to 10–20 mbar absolute for final polycondensation at 200–220°C. The acid value trajectory under vacuum is controlled by the rate of glycol removal versus the rate of chain extension; if the vacuum level is increased too rapidly when the acid value is above 50 mg KOH/g, MEG and low-molecular-weight oligomers are stripped at the expense of the stoichiometric balance, and the acid value may stall above 25 mg KOH/g even after extended hold. In such instances, the batch is said to have lost its hydroxyl reserve, and attempts to correct the end point by re-adding MEG require re-equilibration times of 45–90 minutes at 210°C to allow redistribution of the newly added glycol. Acid value is determined according to ISO 2114:2000, method A, using a potassium hydroxide volumetric standard and a mixed toluene-ethanol solvent system; the repeatability limit under production laboratory conditions is approximately 0.8 mg KOH/g. The required end-point acid value for a standard laminating resin is typically 20–35 mg KOH/g, for gelcoat resins 15–25 mg KOH/g, and for highly reactive resins 12–20 mg KOH/g; these ranges are stated on the specification sheet and are verified against ISO 2114:2000 before styrene dilution. The vacuum stage also drives maleate isomerization, but the rate of isomerization is slower than the rate of acid value reduction at 200°C; therefore, a resin that reaches 20 mg KOH/g too quickly may still contain less than 70% fumarate, requiring a hold period of 30–45 minutes to complete the isomerization under constant agitation at 210°C.

When relative humidity exceeds 60% RH during storage, polyester-grade MEG in unheated carbon steel tanks under a dry nitrogen pad does not automatically retain the purity shown on the supplier certificate. MEG is hygroscopic, and a 50 m³ storage tank with a fixed-roof design can accumulate water at the liquid surface through breathing cycles; water concentrations in stored MEG can increase from 0.05 wt% to 0.15 wt% over a 30-day period if the vent is not fitted with a desiccant dryer or a nitrogen blanket at 0.2–0.5 bar positive pressure. That increase in water is sufficient to shift the effective OH/COOH ratio by approximately 0.8–1.5%, which manifests as a first-stage acid value that is 5–10 mg KOH/g above the reference curve at the same time point. The acid value deviation is then propagated through the vacuum stage, where the operator must either extend the vacuum hold or accept a resin with a higher carboxyl end-group concentration. Higher residual acid value in a styrene-containing laminating resin, for example above 35 mg KOH/g, alters wet-out on glass fibre and can reduce Barcol hardness development after cobalt-promoted, methyl ethyl ketone peroxide-cured lamination. Therefore, MEG selection must include an assessment of the storage infrastructure at the resin plant, not only the certificate of analysis at the supplier gate.

When Diethylene Glycol Accumulation Exceeds 1.2 wt% in the Finished Resin

Diethylene glycol in MEG-based unsaturated polyester resins is both an incoming impurity and a reaction product of the etherification of two MEG molecules at condensation temperatures above 190°C. The formation of DEG consumes two hydroxyl end-groups and releases water, thereby increasing the apparent acid value and reducing the available hydroxyl reserve. When the total DEG concentration in the finished resin, including DEG derived from MEG and DEG formed during synthesis, exceeds 1.2 wt% of the resin solids, the cured neat resin often shows a measurable reduction in heat deflection temperature under load according to ISO 75-2:2013, typically of 3–5°C per additional weight percent DEG, because the ether linkage increases chain flexibility. The same DEG increase can partially offset the styrene compatibility of the alkyd because the more polar ether segments alter solvency parameters; in severe cases, the resin may require a higher styrene concentration, typically an additional 2–5 wt%, to achieve the target viscosity of 300–500 mPa·s at 23°C when measured by rotational viscometer according to ASTM D2196-20. Control of DEG accumulation is therefore not a minor impurity issue; it is a thermodynamic and kinetic constraint on the upper temperature limit of maleate isomerization. To hold DEG below 1.2 wt%, the second-stage temperature should not exceed 215°C for prolonged periods, and MEG should be selected with an incoming DEG level no higher than 0.05 wt%.

Resin produced from MEG containing 0.2 wt% water and 0.1 wt% DEG often fails to meet the specification for high-gloss gelcoat because the elevated acid value reduces thickening response with magnesium oxide. In a typical gelcoat formulation, the acid value of the base resin is held below 20 mg KOH/g to achieve a thickening index of 2.0–3.0 at 24 hours when measured by rotational viscometer after addition of 1.0–1.5 wt% magnesium oxide paste. If the acid value remains above 25 mg KOH/g, the thickening rate accelerates prematurely and the sprayable pot life may drop below 10 minutes at 25°C, which is unacceptable for boatbuilding. The tensile strength of cured clear castings according to ISO 527-2:2012 is less sensitive to acid value within the 15–30 mg KOH/g range than to DEG content, but the elongation at break may increase from 2.5% to 4.0% when DEG in the alkyd backbone increases from 0.5 wt% to 1.5 wt%, reflecting plasticization by the ether segment. These relationships anchor the MEG acceptance criteria to final application performance rather than to monomer specifications alone.

Catalyst Selection Boundaries for Maleate to Fumarate Conversion

For high-reactivity grades, the uncatalyzed thermal isomerization of maleate to fumarate in MEG-based unsaturated polyester resins is sufficiently fast at 210°C for most laminating and gelcoat grades, but certain formulations require acceleration to avoid prolonged high-temperature exposure that would raise DEG and colour. Organotin compounds, particularly monobutyltin hydroxide and dibutyltin oxide, are added at 0.02–0.1 wt% based on total charge to increase the rate of esterification and isomerization without excessively promoting ether formation. The use of dibutyltin oxide above 0.1 wt% can lower the acid value too quickly during the atmospheric stage, producing a viscosity overshoot before the vacuum stage and shifting the maleate-to-fumarate ratio in an uncontrolled manner. Titanium tetraalkoxides, such as tetrabutyl titanate, are effective esterification catalysts but are generally avoided in unsaturated polyester resin synthesis for laminating applications because titanium residues can cause yellowing and can interact with peroxide cure systems. Zinc acetate at 0.05–0.2 wt% has been used as a co-catalyst for esterification but may suppress maleate isomerization under certain conditions; published data for this specific configuration is limited, and the use of zinc-bearing catalysts requires resin-by-resin validation of fumarate content by 1H NMR or FTIR before scale-up. The practical boundary for catalyst selection is therefore defined by the need to keep the reaction mass below 220°C while reaching a fumarate mole fraction of at least 75% and an acid value between 15 mg KOH/g and 30 mg KOH/g within a vacuum hold no longer than 120 minutes; outside these boundaries, the probability of DEG accumulation above 1.2 wt% and colour formation above 3 Gardner increases sharply.

If the incoming MEG water content exceeds 0.15 wt%, the resin batch is not automatically out of specification provided the reactor control system includes an adaptive stoichiometric correction based on the incoming water analysis. However, the correction must be made before the first heating ramp, because the water competes with MEG for the anhydride ring-opening and forms free carboxylic acid without contributing to chain growth. The first-stage acid value at 90 minutes may still be 70–85 mg KOH/g when the water content is high, which delays the vacuum transition and exposes the maleate half-ester to an extended atmospheric hold. That extended hold can raise the fumarate fraction before the vacuum stage to 50–60%, which is not necessarily detrimental but changes the viscosity profile and requires a reduction in the later vacuum-hold time. Testing of the resin after styrene dilution should include acid value on solids according to ISO 2114:2000, viscosity of the styrene solution according to ASTM D2196-20, and if the application is gelcoat or marine-grade, a gel time test using a cobalt promoter and methyl ethyl ketone peroxide at 25°C. The fumarate content should be checked by 1H NMR for qualification batches but is not routinely required for production. When the resin is intended for food-contact coatings, the selected MEG must comply with the relevant national and regional monomer regulations, and resin migration testing is required under the applicable food-contact legislation; the specification alone does not constitute compliance.