A winter screenwash concentrate formulated without methanol or ethylene glycol can be built around a ternary solvent system of deionized water, 35.0–45.0 wt% propylene glycol, and 25.0–30.0 wt% ethanol according to the composition in Table 1. The exclusion of methanol is driven by REACH Annex XVII Entry 69, which restricts methanol in windscreen washing or defrosting fluids placed on the market to the general public at concentrations ≥0.6 wt%; ethylene glycol is excluded because of its acute oral toxicity and the practical availability of propylene glycol as a lower-risk diol. The concentrate is intended for dilution with water at weight ratios between 1:0.5 and 1:2. At 1:1 dilution the design freeze point is ≤−18 °C; at 1:0.5 dilution the design freeze point is ≤−25 °C, both determined by ASTM D1177-17. Because the mixture is strongly non-ideal, the freeze point cannot be derived reliably from ideal colligative equations and must be measured directly on each batch. The formulation must remain single-phase after 24 h at −20 °C; phase separation in a windscreen washer reservoir can cause pump cavitation and spray nozzle blockage. The pH is buffered to 7.5–8.5, and the corrosion package is silicate-free to avoid cold-temperature gelation and nozzle fouling.

At the production scale, the water balance is not fixed; it absorbs lot-to-lot variation in the ethanol water content and the citric acid addition required to reach the pH target. Sodium benzoate is not required because the ethanol-propylene glycol continuous phase provides adequate short-term preservation of the concentrate. If the diluted product is stored beyond 30 days in warm weather, an isothiazolinone biocide may be required at 0.005–0.015 wt% of the diluted fluid, subject to local labelling. The denatonium benzoate concentration is sufficient to provide a bitterness barrier while remaining below classification thresholds for biocidal activity. The use of denatured ethanol is necessary in consumer product streams; the denaturant package must be confirmed free of methanol because some industrial denaturants contain incidental methanol above the 0.01 wt% raw material specification.

What Limits the Propylene Glycol–Ethanol Ratio in Bulk Storage?

Because ethanol lowers the closed-cup flash point of the concentrate, the propylene glycol–ethanol ratio is bounded at the ethanol-rich end by flammability classification and at the propylene glycol-rich end by low-temperature viscosity and cost. Anhydrous ethanol has a closed-cup flash point of 12–13 °C under ASTM D93-20. The concentrate containing 25.0–30.0 wt% ethanol is designed to remain above 27 °C closed-cup flash point, but published data for the exact water-propylene glycol-ethanol ternary system is limited; each batch should be tested by ASTM D93-20. If the measured flash point falls below 23 °C, the product becomes a flammable liquid Category 2 under Regulation (EC) No 1272/2008; between 23 °C and 60 °C it is Category 3. Storage vessels should be electrically grounded 316L stainless steel or HDPE, with flame arrestors on vents where local fire codes require. Transfer pumps should use EPDM or PTFE diaphragms and seals; nitrile rubber gaskets are not recommended because ethanol can gradually swell the matrix. At the propylene glycol-rich end, increasing propylene glycol above 45.0 wt% raises the concentrate viscosity at −20 °C and reduces the freeze point benefit per unit cost. Published propylene glycol-water freeze point data indicate that a 40 wt% aqueous propylene glycol solution is near −21 °C; the ethanol addition is required to maintain cold-flow performance after dilution. The selected ratio also keeps the concentrate single-phase after 24 h at −20 °C, but uncontrolled water ingress during humid loading can shift the phase boundary and change the flash point. For this reason, closed-loop transfer under dry air is specified when relative humidity exceeds 60%.

The storage upper temperature is limited to 35 °C because ethanol vapor pressure at 30–35 °C can generate a combustible headspace in closed containers. The lower storage limit is −30 °C; the concentrate remains fluid but viscosity may increase to approximately 80–120 mPa·s at −30 °C depending on water content. Published data for this exact viscosity at −30 °C is limited; viscosity should be confirmed by ISO 3219:2021 using a rotational viscometer with a small-sample adaptor at 20 rpm. The concentrate must not be stored in galvanized steel because zinc ions can dissolve and disrupt the benzotriazole passivation film. It must also not be mixed with chlorinated solvents or strong oxidizers because ethanol and propylene glycol can undergo vigorous exothermic reactions under certain conditions. The storage area should be separated from open flames and ignition sources until batch flash point classification is confirmed by the site dangerous goods assessment.

Surfactant phase behavior in high-alcohol matrices is the primary processing bottleneck. The nonionic C9–C11 alcohol ethoxylate with 5–6 mol ethylene oxide and an HLB between 12 and 13 is used at 0.30–0.60 wt%. Added directly to the ethanol or propylene glycol phase, the ethoxylated chain loses hydrogen-bonded water and forms a sticky gel that is difficult to disperse in a low-shear vessel. The manufacturing sequence therefore premixes the nonionic surfactant with an equal mass of deionized water at 18–22 °C before addition to the mixed water-propylene glycol carrier. A secondary anionic wetting agent, sodium dioctyl sulfosuccinate 70% aqueous, is added at 0.10–0.20 wt% to reduce dynamic surface tension during high-speed wiping and improve wetting on hydrophobic insect detritus and salt spray. Foam control is critical because foam entering the washer pump reduces priming and can produce pump chatter; the diluted product is specified to deliver ≤50 mL initial foam and ≤10 mL foam after 5 min by ASTM D1173-07. Corrosion control uses benzotriazole at 0.05–0.10 wt% to passivate copper and brass heater fittings, and sodium molybdate dihydrate at 0.02–0.05 wt% to reduce ferrous corrosion. The inhibitor package is silicate-free because silicate can polymerize in a high-alcohol continuous phase and form nozzle-blocking aggregates during low-temperature storage. Citric acid monohydrate is added at 0.02–0.10 wt% as a buffer to hold pH between 7.5 and 8.5 by ASTM E70-19. A pH below 6.5 accelerates aluminium corrosion, while a pH above 9.5 can attack aluminium and destabilize the benzotriazole film. Amine-based corrosion inhibitors are avoided because their buffering range extends into the alkaline regime where aluminium passivation declines and molybdate solubility changes. In a 1000 kg batch, the water-inhibitor-buffer premix is agitated at 250–350 rpm with a 4-blade pitched turbine for 5–10 min until clear; propylene glycol is added next, followed by the surfactant premix, ethanol, and denatonium benzoate. The finished batch is recirculated through a 100 µm cartridge filter for 10 min to remove undissolved inhibitor particles. If the batch is discharged too quickly after ethanol addition, local low-water zones can retain unmixed ethanol and produce an incorrect flash point result.

When Hard Water Is Used for On-Site Dilution

Dilution water hardness is a critical boundary condition because the dioctyl sulfosuccinate wetting agent forms insoluble calcium salts when total hardness exceeds approximately 150 mg/L as CaCO3, leading to haze and eventual nozzle fouling. The concentrate is specified for dilution with ISO 3696:1987 grade 3 water or softened water with total hardness <150 mg/L as CaCO3. If hard water must be used in the field, tetrasodium EDTA at 0.01–0.02 wt% of the final diluted fluid can be added as a chelating agent, but this addition may alter the corrosion inhibitor balance and should be validated by ASTM D1384-18 corrosion screening if corrosion protection claims are required. Published data for the exact surfactant-hardness interaction in this ternary matrix is limited; a 24 h cold soak at −20 °C followed by turbidity measurement by ISO 7027:2016 with a target ≤5 NTU is a practical field control. Water containing more than 250 mg/L chloride can reduce passivation of aluminium and should not be used unless the diluted fluid is re-inhibited with 0.05 wt% sodium gluconate. Hardness ions do not materially change the freezing point but can deposit scale on nozzle orifices during wet-dry cycling. The dilution ratio must be confirmed by weight rather than volume because the concentrate density is 0.94–0.99 g/cm³ at 20 °C by ASTM D1122-16; a 1:1 volumetric dilution corresponds to approximately 48.0–51.0 wt% concentrate. The freeze point claims apply to weight-based dilution; volume-based field dilution can produce a 1–2 °C warmer freeze point. At 1:0.5 weight dilution the in-service ethanol concentration is approximately 16.7–20.0 wt%, which remains below the 25.0 wt% threshold above which polycarbonate stress cracking risk increases. At 1:1 dilution the ethanol content is 12.5–15.0 wt%, so compatibility with acrylic and polycarbonate external lenses is improved relative to the concentrate. The diluted product should not be applied at temperatures below the measured freeze point of the specific ratio because slush formation in the reservoir can starve the washer pump and cause pump failure.

For routine quality control, the concentrate is specified against the matrix in Table 2. The residual methanol and ethylene glycol screening uses gas chromatography with flame ionization detection on a polar polyethylene glycol column; the reporting limit is 0.01 wt%. Published collaborative trial data for this exact screenwash matrix is limited, so the in-house method is validated by bracketing calibration standards containing methanol and ethylene glycol in a water-ethanol-propylene glycol background. Each raw material lot should be accompanied by a certificate of analysis that reports methanol and ethylene glycol impurities. Because denatured ethanol may contain incidental methanol depending on the denaturant package, the ethanol specification must explicitly prohibit methanol at or above 0.01 wt%. The propylene glycol supplier should provide a specification for ethylene glycol impurity <0.05 wt% and diethylene glycol <0.10 wt%. The concentrate is not a direct substitute for engine coolant and must not be used in automotive cooling systems; it contains surfactants and ethanol that are incompatible with cooling loop materials and operating temperatures.

The formulation has three operational boundaries. First, it must not be diluted with water above 1:2 for winter service because the freeze point rises above −10 °C and wind chill can produce localized slush in the nozzle lines. Second, it must not be combined with anionic or cationic polymer-thickened screenwash additives because the ethanol content can collapse associative thickener networks and alter spray droplet size distribution. Third, it is not compatible with strong oxidizers, concentrated mineral acids, or galvanized steel storage. The addition of dyes is possible at 0.0005–0.002 wt%, but the dye must be tested for cold-temperature solubility; some water-soluble azo dyes precipitate at −20 °C in the presence of ethanol. The use of fragrance is not recommended for a winter screenwash concentrate because hydrocarbon fragrances can increase foam and reduce the flash point.