HDPE rod (High Density Polyethylene Rod) is a solid rod material formed by extrusion process with high-density polyethylene (HDPE) resin as the base material. High-density polyethylene is a linear polymer polymerized by ethylene monomer (Ethylene, CH₂=CH₂) under low pressure (0.1 - 1.5 MPa) and chromium or Ziegler-Natta catalyst. It has a regular molecular chain structure, high crystallinity (60% - 80%), and a density of 0.941 - 0.965 g/cm³. It is a thermoplastic engineering plastic with high toughness, corrosion resistance and low friction coefficient.
Scientific characteristics
Molecular structure and basic properties
The molecular chain of HDPE is composed of repeated —[CH₂—CH₂]— units and has the following molecular characteristics:
Highly regular linear structure: The molecular chains are closely arranged, with a crystallinity of up to 60% - 80% (the crystallinity of ordinary LDPE is only 40% - 50%), a density of 0.941 - 0.965 g/cm³ (higher than LDPE's 0.910 - 0.925 g/cm³), giving the material high rigidity (tensile modulus of 0.8 - 1.2 GPa);
Non-polar molecular chain: The molecular chain does not contain polar groups (such as hydroxyl groups and carbonyl groups), has low surface energy (about 31 mN/m), and exhibits excellent chemical inertness and low friction coefficient (friction coefficient is only 0.2 - 0.3);
Very few long chain branches: The molecular chain has almost no long chain branches (ordinary LDPE contains a large number of long chain branches), the molecular chains are tightly stacked, and the environmental stress cracking resistance (ESCR) is significantly improved (≥1000 h, ordinary LDPE is only 50-100 h).
Mechanical properties
High toughness and impact resistance
Impact strength: Izod impact strength ≥80 kJ/m² (ISO 180 standard, 23℃), the impact strength of ordinary PVC rods is only 10-20 kJ/m², and it can maintain high impact resistance in low temperature (-40℃) environment (suitable for polar equipment shell);
Elongation at break: ≥500% (ISO 527 standard), the elongation at break of ordinary PP rods is about 200%-300%, which can absorb impact energy and is not easy to break (such as logistics pallets, HDPE rod products have better anti-fall performance than PP rods).
High rigidity and light weight
Tensile strength: ≥25 MPa (ISO 527 standard), close to ordinary PVC rods (20 - 30 MPa), but lower density (HDPE is 0.94 - 0.97 g/cm³, PVC is 1.3 - 1.5 g/cm³), and the specific strength (strength/density) is 1.5 - 2 times that of PVC;
Flexural strength: ≥35 MPa (ISO 178 standard), flexural modulus is 0.8 - 1.2 GPa, suitable for high-load support structures (such as supermarket shopping cart frames, logistics turnover boxes).
Chemical corrosion resistance
Acid and alkali resistance: Stable to dilute acid (such as hydrochloric acid, sulfuric acid, concentration ≤30%) and dilute alkali (such as sodium hydroxide solution, concentration ≤20%), and better chemical corrosion resistance than ordinary PP rods (PP is easily degraded in concentrated sulfuric acid);
Organic solvent resistance: Stable to non-polar solvents such as gasoline, diesel, mineral oil (no swelling after short-term immersion), but sensitive to strong polar solvents (such as acetone, chloroform) (need to avoid contact);
Environmental stress cracking resistance: In aqueous solutions containing surfactants (such as detergents), the environmental stress cracking resistance time is ≥1000 h (ordinary LDPE is only 50-100 h), suitable for chemical pipeline linings and storage tank linings.
Wide temperature range adaptability
Low temperature performance: The brittle temperature is as low as -70℃ (the brittle temperature of ordinary PP rods is -20℃ - -30℃), and it can maintain high toughness in extremely cold environments (such as Arctic scientific research equipment);
High temperature performance: The long-term use temperature is -60℃ - 80℃ (short-term temperature resistance can reach 100℃), and the heat deformation temperature HDT (1.82 MPa) is 70 - 90℃, which is suitable for hot water pipes and food processing equipment components.
Dimensional stability
Water absorption: The saturated water absorption is only 0.01% - 0.03% (0.1% - 0.3% for ordinary PVC rods), and the dimensional change rate is ≤0.02% (0.2% - 0.5% for ordinary PVC rods), which is suitable for precision parts (such as laboratory instrument brackets);
Linear thermal expansion coefficient: 1.2×10⁻⁴/℃ (1.5×10⁻⁴/℃ for ordinary PVC rods), which is less affected by temperature changes (tolerance control ±0.03 mm).
Processing and economy
Easy processing: Good melt fluidity (melt index MI = 0.1 - 10 g/10min, ISO 1133 standard), suitable for extrusion, injection molding, turning and other processes (such as thin-walled parts molding);
Cost-effectiveness: The price is only 1/3 of POM rods and 1/2 of nylon (PA6) rods, which significantly reduces material costs while meeting high toughness requirements (for example, HDPE rods for logistics pallets cost 50% less than PA6 rods).
