Semicristaline 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.

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.

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