What exactly is a polymer?
polymer, chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds (see chemical bond). The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer, and three monomers, a trimer. A structural unit is a group having two or more bonding sites. A bonding site may be created by the loss of an atom or group, such as H or OH, or by the breaking up of a double or triple bond, as when ethylene, H2C[double bond]CH2, is converted into a structural unit for polyethylene, -H2C-CH2-. In a linear polymer, the structural units are connected in a chain arrangement and thus need only be bifunctional, i.e., have two bonding sites. When the structural unit is trifunctional (has three bonding sites), a nonlinear, or branched, polymer results. Ethylene, styrene, and ethylene glycol are examples of bifunctional monomers, while glycerin and divinyl benzene are both polyfunctional. Polymers containing a single repeating unit, such as polyethylene, are called homopolymers. Polymers containing two or more different structural units, such as phenol-formaldehyde, are called copolymers. All polymers can be classified as either addition polymers or condensation polymers. An addition polymer is one in which the molecular formula of the repeating structural unit is identical to that of the monomer, e.g., polyethylene and polystyrene. A condensation polymer is one in which the repeating structural unit contains fewer atoms than that of the monomer or monomers because of the splitting off of water or some other substance, e.g., polyesters and polycarbonates. Many polymers occur in nature, such as silk, cellulose, natural rubber, and proteins. In addition, a large number of polymers have been synthesized in the laboratory, leading to such commercially important products as plastics, synthetic fibers, and synthetic rubber. Polymerization, the chemical process of forming polymers from their component monomers, is often a complex process that may be initiated or sustained by heat, pressure, or the presence of one or more catalysts.
What is the significance of polydispersity index?
Polydispersity Index is nothing but the ratio of Weight average molecular weight to number average molecular weight. The weight average MW is always higher than Number average MW and hence the ratio starts from 1.This dispersity index measures the deviation from the uniformity of dispersion. For example if the weight distribution is uniform then the ratio is 1. 1 to 1.1 it is considered to be narrow distribution, 1–2 is moderate distribution and more than 2 is noted as broad distribution
What is the Melt Flow Index (MFI) of PET ?
The "products" tab at www.eastman.com leads you to very complete technical specifications of all their PET grades. You do have to register on their site but it only takes a couple of minutes.
What is the melt flow index?
https://www.google.com.pk/url?sa...Melt Flow Index (MFI) is a measure of how many grams of a polymer flow through the die in ten minutes. The test is performed at a given temperature depending on the plastic. The force used to push the plastic through the system is supplied by a weight which sits on top of a ram. Gravity then takes over and forces the plastic through a die. The test sample is then collected and weighed. The results are then extracted to determine how many grams of plastic came out over a 10 minute period. Therefore the units we use for the test results are “grams/10 minutes”.
What can I get when I'm comparing materials according to their melt flow index? Just viscosity prediction?
As Saeed Doroudiani said, melt flow index is a measure of the viscosity of a polymer melt.This measurement is often used when determining whether too polymers can be welded together. The rule of thumb is that two materials should have a melt flow index within 10% of each other in order to mix well.I like to visualize this by thinking of honey and water. At room temperature, honey is much thicker (more viscous) than water and the two don’t mix. However, if you heat the water, the honey heats up too. At higher temperatures honey is less viscous (thinner) and you can then mix the two together.[image of honey water, from Google images]This illustrates how, when it comes to welding two materials, you need to ensure that the viscosity of each is similar at the welding temperature.Melt flow index is a measure of how much mass of polymer flows through a certain size hole over a certain period of time at a certain temperature under a certain amount of force. It is typically given in g/10-min. In order for this to be a useful measurement when comparing two materials, you must ensure that the test is done with the same parameters (hole diameter, temperature, force) and that the result is provided in the same units.When comparing two materials for the purpose of checking welding compatibility, I would recommend selecting a testing temperature which is just above the melting temperature of the higher melting material (for semi-crystalline polymers). Although, the melting temperatures should be fairly close together anyway.
Polymer Processing: How do you shape lexan, polycarbonate and/or plexiglass?
For polycarbonate (PC) it is typically handled in the “melt” for making parts. I think injection molding is typically used for PC parts. I say “melt” because PC is technically an amorphous polymer and doesn’t really have a true melting point.Plexiglass is actually poly(methyl methacrylate) (PMMA) and it doesn’t have the high performance properties of PC, but it does have very good clarity. Plexiglass is also injection moldable depending on the grade of polymer.Both polymers are amorphous so they yield clear products unless filled (to make a composite) or used in conjunction with a dye (to impart color). Additionally they don’t necessarily melt in the classic sense, but rather they will soften at high temperatures.PMMA will soften before PC and I think a nice working temperature for either for some low viscosity applications will be > 200 C, but you can bend them when they are in the rubbery state. For PC I think this is typically above 150 and for PMMA I think even as high as 100–120 deg C might be good enough depending on the grade.tl:dr - Use elevated temperatures > 200 C for molten applications and just above glass transition for polycarbonate (150 C) and 120–200 C for PMMA.
What is the difference between polypropylene MFI=45 with polypropylene MFI=15?
MFI is short for "Melt Flow Index". The actual units are grams. More precisely, it is the weight of a polymer in grams that flows through a specific capillary at a specified temperature and back pressure. Without those other data, all we can say about polypropylene MFI=45 is that it has a much lower molecular weight than polypropylene MFI=15; perhaps by a value of 10-100x.
Why is polyvinyl acetate so viscous?
If you are asking about the bulk polymer, all polymers are viscous in the melt state because of their large molecular weights and chain entanglements, which hinders fluid flow. If you are asking about polyvinyl acetate latex, which is what white glues such as Elmer's Glue is made of, it is because the polyvinyl acetate particles in the latex are stabilized on their surfaces by poly(vinyl alcohol). This stabilization system is quite viscous because it is itself a polymer that is soluble in water, increasing the viscosity relative to emulsions that are stabilized by surfactants and detergents.