What the Grade Number Actually Means
Polyethylene glycol is a linear polyether with the general formula H(OCH2CH2)nOH, and both grades share the same repeating unit. The number in the grade name is the nominal average molecular weight in g/mol, so polyethylene glycol 1500 averages about 1,500 g/mol and polyethylene glycol 3350 about 3,350 g/mol. Because the chemistry is identical, the behavioural differences come almost entirely from chain length: melting range, viscosity, water-binding capacity, solubility in organic media and skin penetration all shift as molecular weight rises. Both grades are supplied as waxy solids at room temperature and both are fully miscible with water.
Physical and Specification Comparison
| Property | Polyethylene Glycol 1500 | Polyethylene Glycol 3350 |
|---|---|---|
| Nominal molecular weight | 1,500 g/mol | 3,350 g/mol |
| Form at 25 degrees C | Soft waxy solid | Firm waxy solid |
| Melting range | Approximately 44 to 48 degrees C | Approximately 54 to 58 degrees C |
| Viscosity in water | Lower at equal concentration | Higher at equal concentration |
| Hygroscopicity | Moderate | Lower |
| Water solubility | Complete | Complete |
| Pharmacopoeial identity | Macrogol 1500 | Macrogol 3350 |
Pharmaceutical Applications
Polyethylene glycol 1500 is used as an excipient in ointments, suppositories, creams and injectable vehicles. Its relatively short chains disperse easily with other ingredients, act as a solubiliser and stabiliser for poorly water-soluble actives, and are cleared from the body comparatively quickly. It also serves as a plasticiser in film coatings and as the base where a soft, low-melting matrix is required.
Polyethylene glycol 3350 behaves differently. It is used as an osmotic laxative and in bowel-cleansing preparations because the longer chains hold more water in the intestinal lumen through hydrogen bonding and draw fluid into the stool. It is also selected for sustained-release matrix systems, where higher viscosity and slower dissolution of the polymer control the release rate of the active ingredient. Grades intended for medicinal use are controlled against pharmacopoeial monographs that limit molecular weight distribution, hydroxyl value, residual ethylene oxide and 1,4-dioxane.
Cosmetic and Personal Care Use
Polyethylene glycol 1500: humectant, lubricant and skin softener. It is readily absorbed and leaves less greasy residue on the skin surface.
Polyethylene glycol 3350: added to lotions and creams that need higher viscosity and stability, where it contributes body, texture and emulsion stability.
Both grades are non-ionic and compatible with most surfactants, which allows them to act as coupling agents in emulsion systems.
Industrial Applications
In coatings and printing inks, polyethylene glycol 1500 acts as a dispersant and levelling agent that improves flow and surface quality, while in plastics and rubber it serves as a lubricant and mould-release agent. Its relatively low molecular weight means the effect is mild and mainly regulates flow and surface properties. Polyethylene glycol 3350 is preferred wherever thicker rheology is needed: as a thickener and anti-sagging additive in coatings and inks, and as a lubricity and corrosion-protection component in metalworking fluids, where it reduces friction and helps protect freshly machined surfaces.
Handling, Storage and Grade Selection
Both grades are hygroscopic and should be stored in sealed containers away from moisture and oxidising agents. Polyethylene glycol 1500 softens at low temperature and is therefore easier to melt or pump in process lines, whereas polyethylene glycol 3350 stays solid at typical ambient site temperatures and is normally dissolved or melted at 60 to 70 degrees C before use. Select the grade by the property that limits the formulation: where softness, a low melt point or rapid clearance is needed, polyethylene glycol 1500 is the natural choice; where viscosity, water retention, film strength or release control dominates, polyethylene glycol 3350 fits better. A certificate of analysis, heavy-metal limits and residual ethylene oxide data are the routine documents buyers should request for either grade.
FAQ
Q: Can polyethylene glycol 1500 be substituted directly for polyethylene glycol 3350?
Not without re-validation. The lower molecular weight changes melting point, viscosity and water retention, so the finished formulation must be re-tested.
Q: Are both grades fully water soluble?
Yes. Both dissolve completely in water, and dissolution is fastest at moderate temperature; very high concentrations can form a gel.
Q: Why is polyethylene glycol 3350 used as a laxative rather than the 1500 grade?
The longer chains are not absorbed and bind more water in the intestine, which produces a stronger osmotic effect with limited systemic exposure.
Q: What is the common CAS number for these grades?
Polyethylene glycol grades share CAS 25322-68-3, and individual molecular weight grades are distinguished by their pharmacopoeial or supplier specifications.
Q: How should the polymer be dissolved in production?
Add flakes or melted polymer slowly to stirred water at 50 to 60 degrees C, keep agitation low shear, and allow full hydration before adjusting the final concentration.
Q: Does molecular weight affect how the material behaves on skin?
Higher molecular weight grades are poorly absorbed through intact skin, which is one reason the 3350 grade and higher are widely used in topical and oral products.
