Polymer

Copolymer is a polymer formed from two or more chemically different monomers, copolymerized into a single chain. It is the basis of most modern plastics: it combines the properties of each monomer to produce materials with a superior stiffness/impact/chemical-resistance balance. ## Copolymer types - Random: monomers randomly distributed. e.g. EVA, random PP - Alternating: A-B-A-B-A-B... (rare in commercial plastics) - Block: A-A-A-B-B-B-A-A-A... e.g. SBS, block PP (impact) - Graft: main chain of A with B branches. e.g. ABS, HIPS - Statistical: similar to random but with a structural bias ## Key commercial examples - EVA (ethylene-vinyl-acetate): PE + acetate → flexible, transparent, sealable; soles, films - POM copolymer: formaldehyde + ethylene oxide; more hydrolysis-stable than homopolymer POM - Impact PP (PP-B): PP matrix + EPDM domains; low-temperature toughness - ABS: styrene + acrylonitrile + grafted butadiene; stiffness + impact + chemistry - PET-G: PET with CHDM as third monomer; amorphous, transparent, easy thermoforming - PVDF copolymer: with HFP; improved flexibility ## Advantages of copolymerization - Fine tuning of properties (Tg, transparency, impact, flow) - Better compatibility with additives / fillers - Better processability without sacrificing mechanics - Tailor-made design for a specific application ## Vs. homopolymer | | Homopolymer | Copolymer | |---|---|---| | Structural purity | High | Medium | | Crystallinity | Higher | Lower (typically) | | Stiffness | Higher | Lower (depends) | | Impact | Lower | Higher (with rubber domains) | | Clarity | Variable | Often improved |

Monomer is the small chemical molecule, with at least a double bond or a reactive functional group, that serves as the building block of polymers via the polymerization reaction. The plastics industry always starts from monomers, generally derived from petroleum or natural gas. ## High-volume monomers - Ethylene (CH₂=CH₂) → polyethylene (PE) - Propylene (CH₂=CH-CH₃) → polypropylene (PP) - Vinyl chloride (CH₂=CHCl) → PVC - Styrene (C₆H₅-CH=CH₂) → polystyrene (PS), ABS, SAN - Acrylonitrile, butadiene → ABS - Caprolactam → polyamide 6 (nylon 6) - Ethylene terephthalate → PET ## Polymerization mechanisms - Addition: the monomer's double bond opens to form chains (PE, PP, PS, PVC) - Condensation: two monomers react releasing a small molecule (water, ethanol). PA, PET, PC, PBT - Ring-opening: caprolactam → PA 6 - Catalysts: Ziegler-Natta, metallocene (PP), peroxides (PE), Phillips (HDPE) ## Monomer vs. polymer - Monomer: small molecule, e.g. styrene (liquid at room temp, soluble in water) - Polymer: macromolecule with thousands to millions of repeat units, e.g. polystyrene (solid at room temp) ## Industrial importance Monomer purity and quality determine the final polymer's properties. Residual monomer in the finished polymer may cause: - Unpleasant odor (residual styrene in PS) - Migration into food contact (vinyl chloride in PVC) - FDA / EU regulatory limits ## Residual monomer Typical levels in commercial polymers: <50 ppm for food grade, <200 ppm for industrial. Steam-stripping reduces residual.

A plastic is a synthetic or semi-synthetic material whose backbone is a polymer — long chains of repeating molecular units — usually combined with additives that tune color, stability, flow and strength. The word "plastic" refers to its plasticity: when heated it can be shaped and then set into a solid part, which is exactly what injection molding exploits. ## Plastic vs polymer vs resin - polymer: the pure long-chain molecule (e.g. polyethylene). - Plastic: the usable material = polymer + additives, the everyday term for the finished compound. - resin: in a molding shop, the pelletized plastic fed into the machine. In practice molders use "resin," "plastic" and "material" almost interchangeably. ## The two families that matter for molding - thermoplastic: softens when heated and re-solidifies when cooled, reversibly — so it can be melted, molded, reground and re-melted. Virtually all injection molding uses thermoplastics. - thermoset: cures into a permanent crosslinked network and cannot be re-melted (e.g. epoxy, phenolic). Molded by different processes. ## Structure that drives behavior Thermoplastics are either amorphous (random chains — PC, ABS, PS: gradual softening, lower shrink, often clear) or semi-crystalline (ordered regions — PP, PA, POM: sharp melting, higher shrink, chemical resistance). This structure sets melt behavior, viscosity, shrinkage and where the molding process window lives. ## Related terms - See also: polymer, thermoplastic, thermoset, resin, additive ## What is plastic made of? A base polymer — long repeating molecular chains, usually from petrochemicals or increasingly bio/recycled sources — blended with additives such as stabilizers, colorants, lubricants and reinforcements to reach usable properties. ## What is the difference between a plastic and a polymer? A polymer is the pure long-chain molecule; a plastic is the usable material made from that polymer plus additives. In molding, the pelletized plastic fed to the machine is usually called resin. ## What kinds of plastic are used in injection molding? Almost always thermoplastics, which melt and re-solidify reversibly — split into amorphous grades (PC, ABS, PS) and semi-crystalline grades (PP, PA, POM); thermosets cure permanently and use other processes.

Thermoplastic is a polymer that softens and re-melts when heated above its melting or glass-transition temperature and solidifies again on cooling — with no permanent chemical reaction. This reversibility is what enables injection molding, extrusion and mechanical recycling of most plastics. ## Thermoplastic vs. thermoset - Thermoplastic: linear or branched chains without chemical crosslinks. Melts and can be remolded (PP, PE, ABS, PC, PA, PET, POM). - Thermoset: crosslinks chemically during cure (phenolic, epoxy, melamine resins). Cannot be remelted; reheating only degrades it. ## Classification of thermoplastics - Commodity: PP, PE-HD/LD, PS, PVC, PET → high volume, low cost - Engineering: ABS, PA (nylon), PC, POM, PMMA, PBT → improved mechanical properties - High-performance: PEEK, PPS, PSU, PEI, LCP → high service temperature, high cost - By structure: amorphous (PC, PS, ABS) vs. semi-crystalline (PP, PE, PA, POM) ## Processability Almost any thermoplastic can be injection-molded, extruded, thermoformed, blow-molded and roto-molded. Semi-crystalline grades require precise mold temperature to control crystallinity; amorphous grades tolerate wider windows. ## Recyclability and reuse Thermal reversibility allows scrap (regrind) to be ground and reprocessed up to 20 – 30 % mixed with virgin resin with no major property loss, depending on the polymer. Additives, cross-contamination with other resins, and accumulated thermal degradation limit the number of cycles.

Thermoset is the polymer that, during processing, undergoes a chemical crosslinking reaction (cure) that creates permanent covalent bonds between chains. Once cured it cannot be remelted; reheating only degrades it. ## Fundamental difference vs. thermoplastic | | Thermoset | Thermoplastic | |---|---|---| | Processing | One-time (chemical cure) | Multiple thermal cycles | | Recyclability | Difficult (grind as filler only) | Easy (regrind) | | Structure | 3D crosslinked network | Independent chains | | Scrap reuse | Not reprocessable | Reprocessable | | Thermal resistance | Up to degradation | Up to Tm or Tg | ## Commercial thermoset resins - Phenolic (PF, Bakelite): the first synthetic resin, still in use - Epoxy: adhesives, coatings, structural composites - Unsaturated polyester (UP): glass-fiber, gel coat - Vinyl ester: improved polyester, chemically and mechanically - Melamine (MF): tableware, laminates - Urea-formaldehyde (UF): wood particle board - Polyurethane (PU): foams, RIM - Cured silicone elastomer: seals, vulcanizates ## Forming processes - Compression molding: classic, simple, slow - Transfer molding: more complex, better quality - Thermoset injection molding: special machines with cold barrel - RIM (Reaction Injection Molding): two liquid components react in the mold - Pultrusion: continuous profiles with fiber - Hand layup / lamination: large parts by hand ## Advantages - Very high thermal resistance (epoxy: 200 °C; phenolics: 300 °C) - Excellent dimensional stability - Superior chemical resistance - No creep under load (unlike thermoplastics) - Good electrical insulation ## Limitations - Not recyclable at end of life - Long cure time in some processes - Brittle without fiber reinforcement - Risk of residual monomers (formaldehyde, styrene) during cure