Amorphous Materials

Amorphous describes a thermoplastic polymer whose molecular chains lack crystalline ordering. Molecules arrange themselves randomly, without a regular periodic structure, giving the material a generally transparent appearance and reduced shrinkage. ## Behaviour in injection molding Amorphous resins do not show a defined melting point but rather a glass transition temperature (Tg) above which the melt softens gradually. This makes them easy to process and gives them a wider processing window than semi-crystalline materials. ## Typical properties - Mold shrinkage: 0.4 – 0.7 % (very low) - High optical transparency (ABS, PC, PMMA, PS) - Good dimensional stability - Lower chemical resistance than semi-crystalline polymers ## Examples of amorphous polymers ABS, polycarbonate (PC), polystyrene (PS), PMMA (acrylic), rigid PVC and PEI. ## Common issues Environmental stress cracking (ESC), notch sensitivity and surface scratching. UV or impact additives are often required for outdoor applications.

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.

Polymer is a macromolecule formed by the repeated covalent bonding of many small units called monomers. It is the molecular foundation of all plastics, rubbers, fibers and many biological materials (proteins, cellulose, DNA). ## Classification by origin - Natural: cellulose, starch, proteins, natural rubber, lignin - Synthetic: PE, PP, PVC, PS, PET, PA, PC, ABS… (most of the market) - Semi-synthetic: rayon, cellulose acetate, modified natural rubber derivatives ## Classification by architecture - Linear: straight chains (HDPE, PA 66, PS) - Branched: chains with branches (LDPE, ABS) - Crosslinked: PE-X, vulcanized rubber, thermosets - Dendritic: tree-like structures (specialty) ## Classification by thermal response - Thermoplastics: melt and reshape reversibly (PP, PE, PA, PC) - Thermosets: chemically cured, do not remelt (epoxy, phenolic) - Elastomers: flexible, recover shape after deformation (rubber, TPE) ## Classification by composition - Homopolymers: single monomer type (PE, PP-H) - Copolymers: two or more monomers (ABS = acrylonitrile + butadiene + styrene) - Blends: two polymers physically mixed (PC/ABS, PA/PPS) ## Key properties governed by structure - Molecular weight: stiffness and processability - Molecular-weight distribution: process window and toughness - Crystallinity: stiffness, opacity, shrinkage - Chain polarity: chemical resistance, adhesion, transparency

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