For previous reading, please refer to Plastic Materials (Part 8): In-depth Analysis of PC/ABS Materials.
PEHD (High-Density Polyethylene) and PMMA (Polymethyl Methacrylate/Acrylic).
PEHD (HDPE) is the highest density and most rigid variety of polyethylene, renowned for its excellent chemical resistance, environmental stress cracking resistance, and low cost, making it preferred material for fuel tanks, pipes, and packaging.
PMMA, commonly known as acrylic/plexiglass, is transparent plastic with the highest light transmittance (92%+), renowned for its excellent optical properties, high surface hardness, and superior weather resistance, making it a classic material for optical lenses, automotive exteriors, and architectural lighting.

PEHD = High Density Polyethylene, a semi-crystalline thermoplastic polymerized from ethylene, with a density of 0.941~0.965 g/cm³.

Core characteristics of PEHD: High crystallinity (60~80%) → Good rigidity, high strength, and excellent chemical resistance; Amorphous regions provide toughness → Good balance of rigidity and toughness.
PEHD's performance comes from its unique semi-crystalline structure: High crystallinity → High rigidity, high strength, high heat resistance, and excellent chemical resistance; Linear molecular chains → High density, high crystallinity, and low branching; Amorphous regions → Toughness, impact resistance, and resistance to environmental stress cracking; Molecular weight → High molecular weight → Better ESCR and toughness, but more difficult to process.
Main classifications of PEHD

PE vs PP vs PVC: How to choose a general-purpose plastic?

Selection Logic: For chemical resistance/low-temperature resistance/low cost, choose HDPE; for heat resistance/rigidity, choose PP; for flame retardancy/high strength, choose PVC.

PEHD has unique mechanical properties among general-purpose plastics—low rigidity but excellent toughness, and superior creep and fatigue resistance.
Comparison of Typical Properties of Different Types of PEHD

Note: Above are typical ranges for materials of same grade. Specific values vary depending on molecular weight, crystallinity, comonomers, and additives.
Yield-to-tensile strength ratio analysis: PEHD's yield-to-tensile strength ratio (yield strength/tensile strength) has following characteristics: Blow molding/pipe grade: High yield-to-tensile strength ratio (~0.8~0.9), significant cold drawing and necking after yielding, extremely high elongation at break (>500%); Injection molding grade: High yield-to-tensile strength ratio, lower elongation at break (10~50%); High molecular weight grade: High yield-to-tensile strength ratio, better toughness, better ESCR.
Engineering conclusion: PEHD is a typical tough material—it has a distinct yield point and fractures only after undergoing large plastic deformation after yielding. At room temperature, PEHD typically does not fracture upon notched impact (full toughness), and it maintains good toughness at low temperatures. However, its rigidity and strength are not high, making it unsuitable for high-load-bearing structural components.
Influence of Temperature on Performance: PEHD's properties are highly sensitive to temperature: Impact Strength: Extremely high at room temperature (does not crack), maintains good toughness at low temperatures (still has some toughness at -40℃), very low brittle-ductile transition temperature (below -70℃); Stiffness: Decreases rapidly with increasing temperature, HDT (0.45MPa) is only 70~90℃; Creep: PEHD exhibits significant creep, with large deformation under long-term loads, a key limiting factor in pipe and structural component design; Fatigue: PEHD has good fatigue resistance, suitable for cyclic loading applications.

PEHD is one of the most widely used plastics, covering almost all fields.
Automotive Applications

Automotive fuel tanks are the largest automotive application of HDPE. Combination of HDPE's fuel resistance, ESCR, and low-temperature impact makes it preferred material for fuel tanks. Multilayer co-extruded HDPE fuel tanks (with EVOH barrier layer) can meet stringent fuel permeation regulations.
Packaging Applications

Pipelines & Infrastructure

 

Other Applications

 

PMMA = Poly(methyl methacrylate), commonly known as acrylic/plexiglass, is an amorphous transparent plastic polymerized from methyl methacrylate:

Core characteristics of PMMA: Amorphous structure → High transparency; Polar ester groups → High surface hardness, easy to bond; Rigid main chain → High rigidity but brittleness.
Relationship Between PMMA Molecular Structure and Properties
PMMA's superior properties stem from its unique molecular structure: Amorphous structure → High transparency (92%+ light transmittance), low birefringence; Polar ester groups → High surface hardness, easy printing, easy adhesion; Rigid main chain → High rigidity, high strength, but brittleness; Excellent weather resistance → Does not yellow after long-term outdoor use (much better than PC)
Main Classifications of PMMA

PMMA vs PC vs PETG: How to Choose a Transparent Plastic?

Selection Logic: For transparency + hardness + durability, choose PMMA; for transparency + impact resistance + heat resistance, choose PC; for transparency + toughness + easy molding, choose PETG.

PMMA's mechanical properties are characterized by high rigidity, high strength, but high brittleness.
Comparison of Typical Properties of Different Types of PMMA

Note: Above are typical ranges for materials of same grade. Specific values vary depending on molecular weight, toughening agent content, and additives.
Yield-to-tensile strength ratio analysis: PMMA's yield-to-tensile strength ratio (yield strength/tensile strength) has following characteristics: General purpose/optical grade: Very high yield-to-tensile strength ratio (~0.95+), almost no yield point, direct brittle fracture, very low elongation at break (2~5%); High impact grade: Slightly lower yield-to-tensile strength ratio, some plastic deformation, slightly higher elongation at break (3~8%); Toughening modification: Rubber toughening can improve impact strength, but it reduces stiffness and transparency; Engineering conclusion: PMMA is a typical brittle material—almost no obvious yield, direct fracture. Its notched impact strength at room temperature is very low (1~2 kJ/m²), making it one of the most brittle engineering plastics. High impact modification can improve impact performance, but brittle fracture remains dominant characteristic.
Effect of Temperature on Performance: PMMA's performance is sensitive to temperature: Impact Strength: Very brittle at room temperature; improves slightly with increasing temperature, becomes even more brittle with decreasing temperature. Stiffness: Decreases with increasing temperature; Tg is approximately 105℃; modulus decreases significantly above 80℃. Creep: PMMA has relatively good creep resistance among transparent plastics, but creep still occurs under long-term loads. Dimensional Stability: Has a relatively high coefficient of thermal expansion (~70~80×10⁻⁶/K), resulting in lower dimensional stability compared to PC.

PMMA has a wide range of applications, from optics and architecture to automobiles and everyday consumer goods.
Automotive Applications

 

 

Automotive exteriors are an important application for high-gloss PMMA. Piano black effect of Plexiglas Hi-Gloss series (such as shade 9V022) can achieve Class A surface quality, replacing paint and reducing costs while being environmentally friendly. Pillar trims, spoilers, and roof trims are typical applications.
Optical Applications

Architectural and Advertising Applications

Other Applications

CAE Simulation
Simulation modeling of two materials has its own characteristics.
PEHD Simulation Key Points: PEHD simulation modeling requires special attention to large deformation, viscoelasticity, and creep.

Four key points of PEHD simulation: Large Deformation Behavior: PEHD has a very high elongation at break (>500%), requiring large strain constitutive models for large deformation simulation. Rate Correlation: Yield strength of PEHD increases with increasing strain rate, requiring rate-dependent models for dynamic simulation. Significant Creep: Creep is significant under long-term loads, creep must be considered in design of pipes and structural components. Strong Temperature Sensitivity: Performance changes greatly with temperature, requiring temperature effects to be considered in simulations under temperature environments.
PMMA Simulation Key Points
PMMA simulation modeling requires special attention to brittle fracture and internal stress.

Four Key Points of PMMA Simulation: Brittle Fracture: PMMA is a brittle material, and fracture simulation requires a brittle fracture model; Stress Cracking: Stress cracking under combined action of internal stress and chemical media is a key issue. Rate Dependence: Strength of PMMA increases with increasing strain rate, and dynamic simulation requires a rate dependence model; Temperature Sensitivity: Tg is approximately 105℃; performance degrades significantly above 80℃.
Typical Simulation Scenarios

Failure Modes

 

Six Major Failure Modes of PEHD
1. Environmental Stress Cracking (ESC): Under combined action of stress and surface-active media (detergents, oils, surfactants), PEHD develops cracks that slowly propagate. This is the most typical and important failure mode for HDPE. Common Scenarios: Cracking at pipe joints, cracking of oil tank welds, cracking of chemical containers.
2. Creep Deformation: Under long-term constant load, PEHD slowly deforms, eventually leading to dimensional deviations or failure. Creep in HDPE is very significant. Common failure scenarios: Long-term pressure deformation of pipelines, long-term load deformation of racks, and long-term load sagging of structural components.
3. Slow Crack Propagation (SCG) Under long-term static load, cracks slowly propagate from defect, eventually leading to fracture. This is main failure mode of HDPE pipes. Common failure scenarios: Gas pipeline rupture after long-term use, cracking at water pipe joints.
4. Aging/Oxidation Long-term thermo-oxidative aging causes PEHD to become brittle and lose strength. Outdoor use also involves UV aging. Common failure scenarios: Aging of outdoor pipelines, aging of geomembranes, and aging of products in high-temperature environments.
5. Low-Temperature Brittle Fracture At extremely low temperatures, toughness of PEHD decreases, potentially leading to brittle fracture. Common failure scenarios: Pipeline rupture in extremely cold regions, and drops in low-temperature environments.
Six Failure Modes of PMMA
1. Stress Cracking (ESC) Under combined action of stress and chemical media (alcohol, acetone, oil), PMMA develops streaks and cracks. This is one of the most common failure modes of PMMA. Common scenarios: Cracking after contact with alcohol, long-term cracking due to assembly stress, spontaneous cracking due to internal stress.
2. Impact Fracture/Brittle Fracture: PMMA is brittle and easily fractures upon impact. Its notched impact strength is very low (1~2 kJ/m²). Common scenarios: Drop fracture, impact fracture, fracture at stress concentration points.
3. Abrasion/Scratches: Although PMMA has a higher surface hardness than PC, it is still easily scratched, affecting its appearance and optical performance. Common scenarios: Lens scratches, scratches on decorative parts, surface abrasion on transparent parts.
4. Aging/Yellowing: PMMA has good weather resistance among transparent plastics, but it will still yellow and degrade in performance under long-term UV exposure. Much better than PC, but not as good as glass. Common scenarios: Yellowing of outdoor skylights after long-term use, aging of outdoor signs.
5. Silver Stains/Bubble/Poor Transparency: Silver streaks appear on the surface of product, there are bubbles inside, and transparency does not meet standards. Common scenarios: Appearance defects in transparent and optical components.
6. Heat Deformation/High-Temperature Failure: PMMA has generally low heat resistance (HDT 85~100℃), and deforms and its strength decreases at high temperatures. Common scenarios: Deformation of parts near heat sources, use in high-temperature environments.