What makes polyurethane(PE) or polypropylene (PP) or PET more sustianable?

PVC means POLY VINYL CHLORIDE and is said to be most vulnerable polymer, because of the presence of Chlorine atom in polymer back bone.To make less prone to degradation during processing and end use , lot of stabilsers are being mixed .

But polymer back bone of PE, PP and PET is more stable to heat and Sunlight.So these polymers are mixed only with small amount of Antioxidants to arrest degradation.

Regards

Jinson

PP, PE & PET, as also most of the polymers are not environment-friendly. They do not degrade in landfill, unless compounded with starchy substances. This procedure also make them costlier. Unlike PVC, the named polymers are easily recyclable and reusable. In fact, there are a host of applications using recycled PP, PE & PET.

To know more about the same, please write back to recovery_plastics@yahoo.co.in with your name, address and contact number

I wanted to clear up one point in this thread. There is no signficiant degradation of anything in landfills. A properly managed landfill is designed specifically to prevent (or at least minimize) degradation, so that the surface is stable enough to be useable after the landfill is full. This is done by covering up material as soon as it is put in the landfill to prevent the contact with Oxygen that is required for biodegradation. There have been core samples taken from landfills containing newspapers that are decades old and still legible. Thus, we need to minimize the volume of material that is sent to the landfill rather than worrying about the biodegradability of the material. Because the PP, PE, and PET are more easily recyclable and reusable than the PVC, they achieve this goal by not being sent to the landfill as frequently.

If there is no degredation in landfills, why then are there pipies, vents for methane to escape? Many bacteria are anaerobic and will cause degradation w/o oxygen being present. Degredation of materials can occur via many routes besides oxidation.

I did realize that there is some degradation, which is why I wrote that there was "no SIGNIFICANT degradation". I was just trying to correct the general misconception that anything that is biodegradable is better than things that aren't, because it will disappear in a landfill. This is simply not the case, so the best way to be environmentally beneficial by extending the life of landfills is to either recycle and reuse so less stuff goes to the landfills, or make choices that result in less volume in the landfill, which often means using plastic instead of paper.

I assume that by "sustainable", you mean "not readily degraded, hydrolyzed, decayed" etc. This has to do with the strength of covalent bonds that hold the individual monomers together in their polymers, and the individual atoms are also being strongly bonded together in the individual monomers. In some polymers, certain wavelengths of radiation (sunlight) have the proper frequency (energy) to activate certain atomic bonds. In polyurethanes, aromatic PUs based on MDI are a good example of hydrogen abstraction via incident radiation. Once a C - H bond is broken, the usual result is that a C = C double bond forms between two adjacent carbon atoms. This is a weak link and can be readily oxidized or hydrolyzed. This, in fact, is how oil modified paints "cure", when the added metal catalysts, such as cobalt salts, form a 3-membered ring across the C = C bond, resulting in opening the C = C bond and causing crosslinking, which is where the physical properties of the paint oiriginate. If one looks at the several broad types of polyurethanes available commercially, there are aromatic vs. the much more "stable" aliphatic PUs, and each of those types can be broken down into polyester based soft segment molecules, two types of polyether based PUs, and some very stable and biodurable PUs that resist degredation (hydrolysis, metal ion oxidation, etc) inside the human body. Hope that helps a bit.

PE, PP, and PET are all thermoplastic, which means they can be melted and reprocessed. In addition, they are fairly stable to being remelted and reused. The polymer backbone only slowly breaks down with each reuse. PVC is a thermoplastic and can be remelted, but its melt temperature is very close to its breakdown temperature so there is a lot of breakdown each time it is remelted. Indeed, it is a bit difficult to mold the first time and requires careful molding techniques. It has the nasty habit of releasing hydrochloric acid vapor if slightly overheated. This can be very hard on the mold and equipment.

Polyurethanes are generally thermoset, although they can be formulated to be straight molecular chain thermoplastics. There are recycling methods for the thermosets but it is not as easy as just remelting.

dmap

Mostly polyurethanes( isocynate + linear polyol) are thermoplastics , not themosets.

A thermoset polyurethane is formed when a substantial portion of the polyol and/or the isocyanate molecules contain three or more functional groups

How about a f = 2.3 or 2.5? Wouldn't these be thermoset also?

Just a note to remind everyone that there are a great number of TPUs (thermoplastic polyurethanes) which are thermoplastic and re-processable. Most of my PUs are TPUs, both aliphatic and aromatic, designed to be very bio-durable, where the soft segments are neither polyether or polyester. We make CardioPass(TM) synthetic arteries and vessels which mimic the normal movement of blood vessels, plus they are designed such that the body does not recognize these synthetic vessels as "foreign". Also drug-eluting polymers for various implantable devices, and we do a variety of medical application work, etc. But yes, in the general industry, a goodly number of PUs are thermoset, primarily coatings, but the entire range of PUs can be made thermosetting.