What is Polyethylene Glycol (PEG)? Grades, Uses & Properties

Polyethylene glycol (PEG) is a family of water-soluble polyether compounds made by polymerizing ethylene oxide with ethylene glycol. The number after "PEG" is the approximate molecular weight, so PEG-200, PEG-400, PEG-600, PEG-1000, PEG-2000, PEG-4000 and PEG-6000 are the same chemistry at different chain lengths. This note explains what PEG is, how the grades differ, where formulators use it, including how the grade number is produced, which grade fits which industry, and how to store and check a delivered lot.

What PEG actually is

PEG is a polyether - a chain of repeating ethylene glycol units capped at both ends. Lower grades (PEG-200, PEG-400) are viscous liquids; higher grades (PEG-4000, PEG-6000) are waxy solids. All are odorless, non-volatile and miscible with water, which is why they appear across pharmaceuticals, cosmetics, textiles and industrial formulations. Our polyether polyol range shares the same polyether chemistry family.

How the grades differ (PEG-200 to PEG-6000)

Common uses by industry

• Pharmaceuticals & personal care: binder, solubilizer, humectant, ointment base
• Textiles: lubricant, antistatic and softening assistant in dyeing/finishing
• Industrial: plasticizer, processing aid, polyurethane chain extender
• Household & cosmetics: solubilizer and viscosity modifier

PEG vs PPG (polypropylene glycol)

PEG is ethylene-oxide based and water-soluble; polypropylene glycol (PPG) is propylene-oxide based and more hydrophobic. PPG shows up where you want water resistance or lower surface activity, while PEG is chosen for solubility and humectancy. Random polyethers blend both behaviors for tuned viscosity and compatibility.

Specifying PEG for your formula

When you shortlist a grade, the working variables are molecular weight, viscosity and any grade-specific monograph (e.g. USP). Exact specifications, SDS and HS code are issued per grade on request - tell us the target market and we will supply the documentation. See our PEG-4000 product page for a representative grade.

Where the name comes from: PEG, polyethylene glycol, PEG glycol

The three names in circulation describe the same polymer. 'Polyethylene glycol' is the systematic name, 'PEG' is the standard abbreviation used in grade names, and 'PEG glycol' or 'polyglycol' are older trade shorthand for the same ethylene-oxide chain. What the name does not tell you is the chain length, which is why every PEG grade carries a number after the letters. A supplier asking for your specification should be able to state that number as a range, and should be able to support it with an identity test - hydroxyl value is the usual one, because it falls predictably as the chain lengthens.

How to read a PEG grade name at a glance

• The letters tell you the backbone: PEG means ethylene oxide, so the chain is water-soluble.
• The number tells you the approximate average molecular weight, and with it the physical form: liquid at the low end, paste in the middle, waxy solid at the high end.
• The certificate tells you whether the batch is what the name promises: molecular weight range, hydroxyl value, water content and colour are the four figures to check first.
• The standard tells you which specification document the grade was made to, which is what makes two suppliers' quotations comparable.

What polyethylene glycol is actually made from

Polyethylene glycol is built rather than extracted. Ethylene oxide is added, unit by unit, onto a short starter molecule - water, ethylene glycol or a low glycol - until the chain reaches the length the grade calls for. Because the addition runs through many repeated steps instead of stopping at one, the batch that comes out is a mixture of chains of slightly different lengths, and the number in the grade name is the average of that mixture. That single fact explains most of what surprises buyers: why a grade is quoted as a range on a specification, why two suppliers' material of the same name can differ slightly in viscosity, and why the hydroxyl value on a certificate says more about the batch you received than the round number on the label does.

PEG and PEO: two names for one family

The same ethylene-oxide chain also travels under a second name. At the low end of the molecular-weight scale the trade convention is polyethylene glycol; at the high end the same polymer is normally called polyethylene oxide (PEO). The boundary between the two names is habit rather than a rule enforced by any standard, and different catalogues place it in different places. That is why the useful question is never which name is on the drum, but what molecular weight range the certificate states. A grade sold as PEG with a very high number, or as PEO with a modest one, is not a contradiction - it is the same chemistry described twice.

What polyethylene glycol is used for, industry by industry

Polyethylene glycol is one of the few industrial polymers that can fairly be called chemically quiet. Its chain carries no charge, its ends are simple hydroxyl groups, it does not ionise in water and it does not precipitate in hard water. That quietness is the reason one family of molecules turns up in so many unrelated industries: the polymer is doing a physical job rather than taking part in a reaction.

• Pharmaceutical and nutraceutical processing: binder, carrier and solubiliser, and the basis of solid-dosage and suppository vehicles at the higher grades.
• Personal care and cosmetics: thickener, lubricant, humectant and solubiliser - the same properties that make a grade useful in a cream make it useful in a toothpaste.
• Cleaning and detergents: solubiliser, viscosity modifier and carrier for actives in liquid systems.
• Textiles: lubricant, antistatic and softening assistant through dyeing and finishing, where a water-soluble polymer that leaves no residue on the fibre is worth having.
• Polyurethane and polymer synthesis: polyether diol and chain extender - the PEG and PPG backbone chemistry behind flexible foam, elastomers and sealants.
• Concrete admixtures: the allyl-terminated grade copolymerises into polycarboxylate superplasticiser chains, where the polyethylene-oxide side chain is what keeps the admixture working.
• Coatings, inks and industrial processing: defoaming, levelling and wetting duties, plus plasticiser and processing-aid roles.

The pattern worth noticing is that the grade follows the job. Low grades are chosen where the polymer has to be a liquid that dissolves something or carries it. High grades are chosen where it has to be a solid that binds, coats or holds a shape. Molecular weight is the one dial that moves a single chemistry from one duty to the next.

Four ways the same polymer is asked to work

How to store and handle polyethylene glycol

Polyethylene glycol is hygroscopic. Left open, the low grades pull water out of the air and the higher grades take up enough moisture to cake into blocks that no longer flow. Water content is a specification item rather than a cosmetic detail, so what happens in the warehouse reappears later as an out-of-specification batch on the bench.

• Keep containers closed and store dry. The headspace matters as much as the seal.
• Do not leave flake grades open between shifts. Moisture cycling - damp air overnight, dry air by day - is what turns flake into a solid lump.
• Melt flake grades with a controlled jacket rather than an open steam lance. Local overheating discolours the product, and colour is one of the figures printed on the certificate.
• Liquid grades that have clouded or begun to separate on standing normally come back on gentle warming with mixing; check water content before releasing the lot.
• Treat any container that has stood open as a lot that has to be re-tested, not as the lot you bought.
• Where a water limit is published, hold to it. PPG-400 is specified at water content <=0.5%, which is a figure a formulator can plan around; the current PEG-4000 specification does not state a water limit, and that gap is worth closing in writing before a moisture-sensitive process is built on the grade.

What to check on the batch you receive

Whether the grade is a binder in a tablet or a diol in a polyurethane, the same five figures carry most of the information.

• Molecular weight range. PEG-4000 is quoted as 3600-4400 rather than 4000, and the range is what a certificate can be checked against.
• Hydroxyl value. It counts chain ends per unit mass, so it falls predictably as the chain lengthens and it cross-checks the molecular weight figure.
• Water content. The figure that drifts in storage and moves most after a container has been open.
• Colour, reported as Pt-Co or Hazen. A reading outside the limit usually means oxidation in storage or overheating during melting.
• pH of a 1% solution, plus the standard the grade was made to. The standard is what makes one supplier's quotation comparable with another's.

Representative published values make the set concrete. PEG-4000 (CAS 25322-68-3, made to Q/320621HA024) is a white flake with hydroxyl value 26-32 mgKOH/g, pH 5.0-7.0 at 1% and colour Pt-Co <=50. PPG-400 (CAS 25322-69-4, Q/320621HA046) is a colourless viscous liquid at hydroxyl value 255-312 mgKOH/g, acid value <=0.5 and water content <=0.5% - the propylene-oxide backbone, and the reason its hydroxyl value sits an order of magnitude above the PEG grade at roughly a tenth of the chain length. APEG-1000 (CAS 27274-31-3) carries hydroxyl value 51-62 mgKOH/g alongside an unsaturation figure of >=0.75 mmol/g that the PEG grades do not report at all, because its chain ends in an allyl group instead of two hydroxyls. Random Polyether BPE-1600 (CAS 9038-95-3) shows what a mixed ethylene-oxide / propylene-oxide chain looks like on paper: content >=99.0%, acid value <=0.5 mgKOH/g, colour Pt-Co <=20 and pH 5.0-7.0 at 1%.

What the PEG chemical formula actually says

Written out in full, a polyethylene glycol chain is a string of oxyethylene units with a hydroxyl group at each end, and the shorthand for that is HO-(CH2CH2O)n-H. Two things in that formula do all the work. The bracket holds the repeating unit - one ethylene oxide molecule after it has opened and joined the chain - and the subscript n counts how many of those units the chain contains. The two terminal hydroxyls are why the polymer is a diol rather than an inert plastic: they are the handles that later chemistry, and water itself, can attach to.

The subscript n is not the same thing as the grade number, and reading the two as equivalent is the most common mistake made with PEG nomenclature. A grade name such as PEG-400 states a nominal average molecular weight; the formula states a construction. The link between them is arithmetic rather than definition: the grade number is what the chain averages on a balance, and the repeat-unit count is what the structure contains.

The arithmetic is worth doing once, because it explains why grade figures are always quoted as ranges. Each oxyethylene unit contributes about 44 to the chain's molar mass. Polyethylene Glycol 4000 (PEG-4000, CAS 25322-68-3) is published on this site with a molecular-weight range of 3600-4400 rather than a single figure, and that range implies an average of roughly 82 to 100 repeat units per chain. The implication is approximate by construction: the grade is an average over a distribution of chain lengths, so no single chain in the drum has to match it.

One formula, many chains: why PEG compounds are plural

Polymerisation does not produce one molecule. It produces an ensemble: chains of different lengths, formed in different proportions, all sharing the same formula and the same chemistry. That is why polyethylene glycol is described in the plural - PEG compounds, polyethylene glycols - and why a specification range is more useful than a single number. The published molecular-weight range is the width of that ensemble, not a tolerance around a target that every chain hits.

• Two lots of the same nominal grade can differ slightly in distribution while both meet the same published range. If a process is sensitive to that difference, the range is the figure to tighten, not the grade name.
• Hydroxyl value moves inversely with chain length. Longer chains carry fewer hydroxyls per unit of mass, which is why PEG-4000 is published here at 26-32 mgKOH/g while the much shorter PPG-400 is published at 255-312 mgKOH/g.
• A grade number describes the average, so two suppliers can both be compliant and still not be interchangeable in a sensitivity-critical step. That is a bench question, not a paperwork question.
• The physical form follows the average length as well: the short grades are liquids, the middle grades are pastes and waxes, and the long grades are solids - which is what the grade ladder on this page is showing.

How the grade number is actually produced, and how it is confirmed on a certificate, is the subject of the note on PEG molecular weight grades. For the propylene-oxide counterpart of the same backbone, and for the way chain length flips the hydroxyl figure, compare Polypropylene Glycol 400 (PPG-400, CAS 25322-69-4).

What a PEG derivative is, and which families exist

A PEG derivative is the same oxyethylene backbone with different chemistry at the ends or along the chain. The backbone keeps the properties that make PEG useful - water solubility, no charge, chemical quietness - while the new end groups give the molecule something to do: react into a polymer, hold an oil phase, or sit at an interface. Most commercial PEG products outside the raw polyol range are derivatives of this kind, which is another reason the family is described in the plural.

• PEG esters. A fatty acid is attached to the hydroxyl ends, turning a water-soluble diol into a surfactant with an oil-loving tail. A documented example in this range is the phosphate-ester version used as a combined emulsifier and rust preventative.
• Allyl PEG. One hydroxyl end is capped with an allyl group, which gives the chain a double bond that can be copolymerised. The grade documented here is published as a molecular weight of about 1000 with a hydroxyl value of 51-62 mgKOH/g and an unsaturation of at least 0.75 mmol/g.
• PEG ethers. The chain is grown onto a fatty alcohol instead of water, which is how alcohol ethoxylates are made. The balance of the resulting molecule is set by the number of oxyethylene units added, exactly as it is anywhere else on this page.
• PEG-PPG copolyethers. Ethylene oxide and propylene oxide are polymerised together so that the chain carries both water-loving and oil-loving segments. The balance is then a property of the ratio rather than of the chain length alone.
• PEG sorbitan esters. The chain is attached to a sorbitan ring carrying a fatty acid, which is the polysorbate family. These sit at the water-favouring end of the emulsifier range and pair with the non-ethoxylated sorbitan esters.

Concrete points of reference from the range: Allyl Polyethylene Glycol APEG-1000 (CAS 27274-31-3) for the allyl-terminated family, AEO-9 (C12-14 Alcohol Ethoxylate) for the PEG ether family, and Tween-80 (Polysorbate 80) for the sorbitan-ester family, which is the same one discussed in the comparison of nonionic and anionic surfactants.

Where the chemistry shows up in a real grade

The figures worth reading together on a PEG or PEG-related specification are the molecular weight range, the hydroxyl value, the physical form and the colour, because between them they describe both the chain length and how clean the chain is. Published values from this range make the relationship visible:

One transparency note on that table: the molecular weight shown for BPE-1600 is a series-level reference value, not a figure specific to the grade - the source record lists other members of the series, so the designation should be confirmed with the supplier before it is used in a calculation. Where a figure is not in the documented record, this page leaves it out rather than filling the gap. The same discipline applies to the low-molecular-weight PEG grades: their customer specification values are not part of the record held here, so no numbers are shown for them. For the interface chemistry behind these grades, continue with what an emulsifier is and how the types differ, or send the grade and application through the contact page for a specification pack.

Related reading

For how the grade number is produced and how it behaves on a certificate, continue with what the number after "PEG" tells you. For the same polyether family in foam control and polyurethane work, see polyether polyols and block polyethers. For an application where a water-soluble polymer has to leave nothing behind on the fibre, see levelling, penetration and scouring in textile dyeing. And for the interface chemistry that begins once a polyether is ethoxylated further into a surfactant, see what an emulsifier is and how the ionic types compare.

Next step

Choosing the right PEG grade depends on your formulation and regulatory path. Send us your application and we will shortlist grades and prepare samples.