Amorphous

Amorphous materials are thermoplastic polymers whose chains lack regular crystalline ordering. Molecules arrange randomly, giving them transparent appearance, low shrinkage and isotropic properties. They are the preferred choice for technical parts with tight tolerances or high cosmetic finish. ## Key characteristics - No defined melting point: only a glass transition temperature (Tg) - Transparency: many are optically transparent (PC, PMMA, PS) - Low shrinkage: 0.3 – 0.7 % vs. 1.5 – 3 % in semi-crystalline - High dimensional stability: little post-shrinkage - Lower chemical resistance than semi-crystallines ## Commercial amorphous polymers - PS (polystyrene): cosmetics, packaging, electronics - ABS (acrylonitrile-butadiene-styrene): housings, automotive, toys - PMMA (acrylic): optics, signage, sanitaryware - PC (polycarbonate): lenses, safety equipment, electronics - SAN, ASA, rigid PVC, PEI, PSU, PES ## Advantages in injection molding - Wide processing window (no risk of poorly controlled crystallization) - Tight tolerances achievable due to low shrinkage - Excellent shot-to-shot repeatability - High surface finish (mirror or fine texture) ## Limitations - Limited chemical resistance vs. semi-crystallines (especially to hydrocarbons) - Prone to Environmental Stress Cracking (ESC) with detergents, oils - Surface scratching tendency (except PC with hardcoat) - Brittle at low temperatures (PS, PMMA) ## Difference vs. semi-crystalline | Property | Amorphous | Semi-crystalline | |---|---|---| | Transparency | High | Low/opaque | | Shrinkage | 0.3-0.7% | 1.5-3% | | Stiffness | Medium | High | | Chemical resistance | Medium | High | | Process window | Wide | Narrow |

Crystalline (Semi-crystalline) describes the microstructure of a thermoplastic polymer in which part of the chains order themselves into regular crystalline regions (spherulites, lamellae) embedded in an amorphous matrix. In commercial polymers there is never 100 % crystallinity — both phases always coexist. ## How crystallinity is measured - DSC (Differential Scanning Calorimetry): integrates melting enthalpy and compares it to a theoretical 100 %-crystalline reference - WAXD (Wide-angle X-ray diffraction): crystalline peak vs. amorphous halo - Density: higher crystallinity → higher density (PE: 0.91 amorphous → 0.97 high crystallinity) ## Factors that increase crystallinity - Higher mold temperature: chains have time to organize - Slower cooling - Annealing post-mold - Nucleating agents added to the compound - Shear during fill (flow-induced crystallization) ## Examples ranked by typical crystallinity 1. POM (acetal): 70 – 80 % 2. HDPE: 50 – 70 % 3. Isotactic PP: 30 – 50 % 4. PA 6, PA 66: 25 – 50 % 5. PET (crystalline parts): 30 – 40 % ## Effect on properties Higher crystallinity → stiffer, better chemical resistance, lower permeability, more opaque, higher shrinkage, worse impact resistance. ## Colloquial vs. scientific "crystalline" In the plastics industry "crystalline" usually means "semi-crystalline with a high crystalline fraction" (HDPE, POM). In polymer chemistry, no commercial thermoplastic is 100 % crystalline.

Semicrystalline materials are thermoplastic polymers with ordered regions (crystals) embedded in an amorphous matrix. The crystalline fraction (typically 20 – 80 %) drives key properties: stiffness, opacity, chemical resistance and shrinkage. ## Thermal behavior Unlike amorphous polymers, semicrystallines have a defined melting point (Tm) as well as a glass transition (Tg): - Below Tg: stiff and brittle - Between Tg and Tm: ductile, properties depend on crystallinity - Above Tm: fluid for processing ## Properties vs. amorphous - Higher crystallinity: stiffness +, chemical resistance +, opacity +, shrinkage + - Lower crystallinity: transparency +, ductility +, shrinkage − ## Typical examples - PP (polypropylene): 30 – 50 % crystallinity - HDPE: 50 – 70 % (high) - LDPE: 40 – 60 % - PA 6 / PA 66 (nylon): 25 – 50 % - POM (acetal): 70 – 80 % (very high) - PEEK: 30 – 40 % - PET: variable with thermal history (bottles vs. technical parts) ## Processing - Mold temperature is critical: hotter → more crystallinity → more shrinkage - POM and PA in molds at 80 – 120 °C for optimal crystallinity - PP / PE in molds at 20 – 60 °C - Slower cooling than amorphous due to latent heat of crystallization ## Key differences vs. amorphous in molding - Shrinkage: 1.5 – 3 % vs. 0.3 – 0.7 % in amorphous - Post-shrinkage: continues days or weeks after molding - Processing window: narrower; too cold creates brittle parts - Appearance: opaque or translucent by default; need nucleating agents for clarity