The first elastomer There was a time long past when the only rubber we had was natural rubber latex, polyisoprene. Straight out of the tree, natural rubber latex isn't good for much. It gets runny and sticky when it gets warm, and it gets hard and brittle when it's cold. Tires made out of it wouldn't be much good unless one lived in some happy land where the temperature was seventy degrees year round. A long time ago...how long, you ask? It was about a hundred and sixty years ago, 1839 to be exact. This was before there were any cars to need tires, but the idea of a useable rubber was still attractive. One person trying to make rubber more useful was named Charles Goodyear, a tinkerer and inventor, and by no means a successful one at this point. While goofing around in his kitchen with a piece of fabric coated with a mixture of rubber latex, sulfur and a little white lead, he accidentally laid it on a hot stove top. It began sizzling like a mass of really smelly bacon or (strangely enough) burning rubber. Wouldn't you know, when he took a look at this mass of rubber, he found it wouldn't melt and get sticky when it was heated, nor would it get brittle when he left it outside overnight in the cold Massachusetts winter. He called his new rubber vulcanized rubber.
Tying it All Together What had happened here? What did the sulfur do to the rubber? What it did was it formed bridges. Which tied all the polymer chains in the rubber together. These are called crosslinks. You can see this in the picture below. Bridges made by short chains of sulfur atoms tie one chain of polyisoprene to another, until all the chains are joined into one giant supermolecule. Yes, folks, this means exactly what you think it does. An object made of a crosslinked rubber is in fact one single molecule. A molecule big enough to pick up in your hand. These crosslinks tie all the polymer molecules together. Because they are tied together, when the rubber gets hot, they can't flow past each other, nor around each other. This is why it doesn't melt. Also, because all the polymer molecules are tied together, they aren't easily broken apart from each other. This is why the Charles Goodyear's vulcanized rubber doesn't get brittle in when it gets cold. We can look at what's going on conceptually, and take a look at the bigger picture. The drawing below shows the difference between a lot of single uncrosslinked polymer chains, and a crosslinked network.
Other elastomers Other kinds of rubber, which chemists call elastomers that are crosslinked include:elastomers Polybutadiene Polyisobutylene Polychloroprene
Crosslinked polymers - thermosets Plastics are also made stronger by crosslinking. Formica is a crosslinked material. Crosslinked polymers are molded and shaped before they are crosslinked. Once crosslinking has taken place, usually at high temperature, the object can no longer be shaped. Because heat usually causes the crosslinking which makes the shape permanent, we call these materials thermosets. This name distinguishes them from thermoplastics, which aren't crosslinked and can be reshaped once molded.thermoplastics Interestingly, the first thermoset was again polyisoprene. The more sulphur crosslinks you put into the polyisoprene, the stiffer it gets. Lightly crosslinked, it's a flexible rubber. Heavily crosslinked, it's a hard thermoset. Other crosslinked thermosets include: Epoxy resins Polydicyclopentadiene Polycarbonates
Environmental Stress Cracking and Crazing (ESC) Some polymers, when stressed, are affected by contact with certain chemical substances. ESC describes a slow brittle failure in stressed polymers by organic substances. For example PVC exposed to certain hydrocarbon impurities may crack and PS in contact with organic liquids may develop crazes. Crazed materials retain considerable strength but crazing may precede cracking. In both ESC and crazing, damage arises from simultaneous action of a substance and environmental stress. The resistance of a polymer to ESC failure depends on structural factors; for example, PE's resistance varies with molar mass, melt flow index, crystallinity and density.
Common engineering polymers XPS Polystyrene Crystal HIPS High Impact Polystyrene SAN Styrene Acrylonitrile Copolymer ABS Acrylonitrile Butadiene Styrene PMMA Polymethylmethacrylate (Acrylic) MBS Polymethacrylate Butadiene Styrene RPVC Rigid Polyvinyl Chloride CPVC Chlorinated Polyvinyl Chloride PVDC Polyvinylidene Chloride PB Polybutylene LDPE Low Density Polyethylene LLDPE Low Linear Density Polyethylene HDPE High Density Polyethylene HMWHDPE High Molecular Weight HDPE LCP Liquid Crystal Polymer PAS Polyarylsulfone PAEK Polyaryletherketone PC/ABS Polycarbonate/ABS Alloy PEEK Polyetheretherketone PEI Polyetherimide PEKEKK Polyetherketoneetherketoneketone PES Polyethersulfone POM Acetal PPA Polyphtalamide PPE Phenylene Ether Copolymer PPS Polyphenylene Sulfide PSO Polysulfone PUR Polyurethane Plastic Rigid TPI Polyimide PP Polypropylene Homopolymer PP/Co Polypropylene Copolymer PP/Talc Polypropylene 40% Talc Filled PP/Glass f Polypropylene 30% Glass Filled EVA Ethylene Vinyl Acetate In Ionomers (Surlyn) CP Cellulose Acetate Propionate TPU Thermoplastic Polyurethane TPO Thermoplastic Elastomer Polyolefin TP Thermoplastic Elastomer Polyester PA6 Polyamide (Nylon) 6 PA66 Polyamide (Nylon) 66 PA11 Polyamide (Nylon) 11 PA12G Polyamide 12, 30% glass filled PA66M Polyamide 66, 40% mineral filled PBT Polybutylene Terephtalate PET Polyethylene Terephtalate PETG Polyethylene Terephtalate Glycol PC Polycarbonate PVDF Polyvinyldene Fluoride
Resources http://www.pslc.ws/mactest/maindir.htm http://www.pslc.ws/mactest/maindir.htm Macrogalleria - all about polymers! http://www.plasticstechnology.com/dp/materials/http://www.plasticstechnology.com/dp/materials/ polymer database http://www.matweb.com/http://www.matweb.com/ searchable database of materials (includes articles) http://www.azom.com/default.asphttp://www.azom.com/default.asp searchable database of materials http://www.goodfellow.com/csp/active/gfHome.csphttp://www.goodfellow.com/csp/active/gfHome.csp periodic table of elements with links to properties http://www.goodfellow.com/csp/active/gfMaterials.csphttp://www.goodfellow.com/csp/active/gfMaterials.csp alphabetic access by name to properties of materials of all descriptions http://www.bpf.co.uk/bpfindustry/plastics_materials.cfm?printable=yes Good resource about different plastics http://www.theotherpages.org/abbrev.htmlhttp://www.theotherpages.org/abbrev.html abbreviations for plastics http://mysite.freeserve.com/designandtech/Materials_Database.xls
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