PPG esters are known as polypropylene glycol esters, and they are (according to this link) "a mixture of propylene glycol mono- and diesters of saturated and unsaturated fatty acids derived from edible oils and fats. The products are produced either by direct esterification of propylene glycol with fatty acids or by transesterification of propylene glycol with oils or fats. When prepared by transesterification, the product may contain residual mono- and diglycerides and glycerol." So what the heck does that mean?
PPG esters are cousins of PEG esters - they are esterified versions of these oils that react with propylene glycol to give them that hydrophilic head and lipophilic tail, to make them surfactants. The difference is they are esterified with polypropylene glycol instead of polyethylene glycol.
They have the same kind of naming conventions - PPG-# is the number of propylene glycol molecules that react with one of the original molecules - but the names are much more complicated. They usually end with something like "myristate", which indicates the original fatty acid (like myristic or palmitic acid). The poly- part indicates it's actually a polymer, which means it is a "large molecule (macromolecule) composed of repeating structural units typically connected by covalent chemical bonds". As the number of chains increase, the melting and boiling points and viscosity will increase. What does this mean for us? Well, we generally won't see something with a name like PPG-150 because that would be incredibly thick!
As a note, you might see PPG associated with ethers as well. Read the data bulletin for your ingredient to see if it's an ether or an ester. For instance, PPG-3 benzyl ether myristate has the word "ether" in it, but it's considered an ester. This is because there is an ether functional group in it, but it's an ester.
PPG esters are generally considered non-greasy ingredients and will reduce the feeling of greasiness in your products. They can feel occlusive without being occlusive, but some form light films on your skin. They have very high spreading abilities - some are even easier to spread than our esters! - and should feel less tacky than our vegetables oils on your skin. Some, like Cromollient SCE, are surfactant and water soluble, while others, like Crodamol STS, are dispersible in alcohol, oils, and some lathery surfactants, but insoluble in water. Most tend to be colourless and odourless liquids. Most will avoid foam and lather depression in lathery surfactants, but some might have an effect.
The problem with PPG esters is that there isn't something you can say "this is what they are all like" the way we can with the PEG esters. We get into a whole area of chemistry I don't think I can explain well, so we'll have to take a look at each ester individually instead of making a generalization.
Join me tomorrow for a look at PPG-3 benzyl ether myristate also known as Crodamol STS.