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POLYMERS “Plastics” Third Generation CAPT Science Preparation for Strand II: Chemical Structures and Properties What is a Polymer? Any of numerous natural and synthetic compounds of usually high molecular weight consisting of up to millions of repeated linked units (monomers), each a relatively light and simple molecule. Polymerization Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain. Natural Polymers (Biopolymers) Examples: Cellulose Shellac Amber Proteins Nucleic Acids Synthetic Polymers Examples: Nylon Neoprene PVC Polystyrene Silicone Silly Putty ® Some Common Addition Polymers Name(s) Formula Monomer Properties Uses Polyethylene ethylene –(CH2-CH2)n– soft, waxy solid film wrap, plastic bags low density (LDPE) CH2=CH2 Polyethylene ethylene electrical insulation –(CH2-CH2)n– rigid, translucent solid high density (HDPE) CH2=CH2 bottles, toys Polypropylene propylene atactic: soft, elastic solid similar to LDPE –[CH2-CH(CH3)]n– (PP) different grades CH2=CHCH3 isotactic: hard, strong solid carpet, upholstery Poly(vinyl chloride) vinyl chloride –(CH2-CHCl)n– strong rigid solid pipes, siding, flooring (PVC) CH2=CHCl Poly(vinylidene chloride) vinylidene chloride –(CH2-CCl2)n– dense, high-melting solid seat covers, films (Saran A) CH2=CCl2 Polystyrene styrene hard, rigid, clear solid toys, cabinets –[CH2-CH(C6H5)]n– (PS) CH2=CHC6H5 soluble in organic solvents packaging (foamed) Polyacrylonitrile acrylonitrile high-melting solid rugs, blankets –(CH2-CHCN)n– (PAN, Orlon, Acrilan) CH2=CHCN soluble in organic solvents clothing Polytetrafluoroethylene tetrafluoroethylene non-stick surfaces –(CF2-CF2)n– resistant, smooth solid (PTFE, Teflon) CF2=CF2 electrical insulation Poly(methyl methacrylate) methyl methacrylate lighting covers, signs –[CH2-C(CH3)CO2CH3]n– hard, transparent solid (PMMA, Lucite, Plexiglas) CH2=C(CH3)CO2CH3 skylights Poly(vinyl acetate) vinyl acetate –(CH2-CHOCOCH3)n– soft, sticky solid latex paints, adhesives (PVAc) CH2=CHOCOCH3 cis-Polyisoprene isoprene requires vulcanization –[CH2-CH=C(CH3)-CH2]n– soft, sticky solid natural rubber CH2=CH-C(CH3)=CH2 for practical use Polychloroprene (cis + trans) chloroprene synthetic rubber –[CH2-CH=CCl-CH2]n– tough, rubbery solid (Neoprene) CH2=CH-CCl=CH2 oil resistant Resin Identification Code Symbol Abbreviation Polymer Type PET or PETE Polyethylene Terephthalate HDPE High Density Polyethylene PVC Polyvinyl Chloride or Vinyl LDPE Low Density Polyethylene PP Polypropylene PS Polystyrene Other Polymers or blends of polymers that do not fall into the other 6 classifications. Properties of Polymers Polymers can be very resistant to chemicals. Polymers can be both thermal and electrical insulators. Generally, polymers are very light in weight with significant degrees of strength. Properties of Polymers cont. Polymers can be processed in various ways. Polymers are materials with a seemingly limitless range of characteristics and colors. Polymers are usually made of petroleum, but not always. Polymers can be used to make items that have no alternatives from other materials. Three factors that influence the degree of crystallinity (or “stiffness”) are: • Chain length • Chain branching • Interchain bonding The importance of the first two factors is nicely illustrated by the differences between HDPE and LDPE. HDPE vs LDPE HDPE is composed of very long unbranched hydrocarbon chains. These pack together easily in crystalline domains that alternate with amorphous segments, and the resulting material, while relatively strong and stiff, retains a degree of flexibility. In contrast, LDPE is composed of smaller and more highly branched chains which do not easily adopt crystalline structures. This material is therefore softer, weaker, less dense and more easily deformed than HDPE. As a rule, mechanical properties such as ductility, tensile strength, and hardness rise and eventually level off with increasing chain length. Two Plastic Types Based on Processing •Thermoplastic Polymers •Thermosetting Polymers Thermoplastic Polymers A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding, or pressing. Examples include milk jugs and carbonated soft drink bottles. Thermoset Polymers A Thermoset is a polymer that solidifies or “sets” irreversibly when heated or cured. A thermoset polymer can’t be softened once “set”. Thermosets are valued for their durability and strength and are used extensively in automobiles and construction including applications such as adhesives, inks, and coatings. The most common thermoset is the rubber truck and automobile tire. The End Life of Polymers Durables vs. Non-Durables Products with a useful life of three years or more are referred to as durables. They include appliances, furniture, consumer electronics, automobiles, and building and construction materials. Products with a useful life of less than three years are generally referred to as non-durables. Common applications include packaging, trash bags, cups, eating utensils, sporting and recreational equipment, toys, medical devices and disposable diapers. What do we do with Polymers when they are no longer useful? Three Options and their consequences 1. Disposal in a landfill 2. Incinerate 3. Recycle Disposal in a landfill Polymers are fairly resistant to chemicals and therefore would take a very long time to decompose if just buried in a landfill. In the meantime, the waste polymers take up a lot of space and could possibly decompose or react with other materials that might eventually result in compounds that could be potentially harmful to the environment. Incinerate The burning (combustion) of polymers produces harmful gases that are toxic to the environment. Recycle - The best choice! Mechanical Recycling Feedstock Recycling Source Reduction Mechanical Recycling Once collected, reclamation is the next step where the plastics are chopped into flakes, washed to remove contaminants and sold to end users to manufacture new products such as bottles, containers, clothing, carpet, plastic lumber, etc. Feedstock Recycling Pyrolysis (heating without oxygen) and other chemical recycling is a special case where condensation polymers such as PET or nylon are chemically reacted to form starting materials. Source Reduction Use less! Redesign products and packaging that uses less polymer material. Reduce the amount the amount of polymer products that are purchased. Clean and Reuse the polymer products that have been purchased. What is the first question you are asked at the grocery store checkout: paper or plastic? Paper bags can be recycled. Plastic bags are being recycled in many places, with about 50% of supermarkets now having recycling programs for plastic bags. However, a stack of 1,000 paper bags is 46 inches high and weigh 140 pounds. A stack of 1,000 plastic bags is only 4 inches tall and weighs 16 pounds. That is a 124 pound savings. When you translate these weight and volume differences into transportation efficiencies, it takes seven trucks to haul the same number of paper bags as can be hauled by only one truck carrying plastic ones.
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