Polyurethane has an extremely wide range of densities (from soft foam with <0.02 g/cm³ to rigid materials with >1.2 g/cm³), so the determination methods vary depending on the product form and application scenario.

The following are several main methods to determine the density of polyurethane, sorted from highest to lowest precision:

1. Laboratory Precise Measurement Method (Highest Precision)

These methods require professional instruments and are suitable for scenarios such as R&D and quality inspection where data accuracy is strictly required. Displacement Method (Based on Archimedes’ Principle)

Principle: By measuring the mass of the sample in air and the buoyant force difference when it is completely immersed in water, the actual volume of the sample is calculated, and then the density is derived.

Equipment: Electronic balance (precision ≥0.001g), density testing kit (including stand, beaker, basket, anti-floating net).

Steps:

a. Weigh the mass of the dry sample in air (recorded as m_air), precise to 0.001g;

b. Immerse the sample completely in distilled water at room temperature (if the sample absorbs water, pre-treatment to prevent penetration is required; if the density is less than water, an auxiliary submergence device is needed to fix it), and weigh its apparent mass in the immersed state (recorded as m_water);

c. Calculate the density according to the formula: ρ = m_air / (m_air – m_water) × ρ_water (ρ_water is the density of water at room temperature, taken as 1.000 g/cm³).

Advantages: High measurement accuracy, it is the standard test method for solid material density (such as ASTM D792, ISO 1183), and is applicable to various polyurethane products. Dimension Measurement Method Principle: For samples with regular shapes (such as cubes, cylinders, rectangular prisms), precise measurement tools are used to obtain dimensional parameters, calculate the sample volume (V), combine with the mass (m) measured by a precision balance, and calculate the density according to the formula ρ = m / V.

Equipment: Vernier caliper (precision ≥0.01mm) or micrometer, electronic balance (precision ≥0.01g).

Advantages: Simple and intuitive operation, no consumable consumption, especially suitable for rigid polyurethane with regular shapes (such as insulation boards, solid elastomers).

Note: When measuring dimensions, the average value of 3 different positions of the sample should be taken to reduce errors caused by shape deviation.

II. Industrial/Field Rapid Assessment Methods
These methods are suitable for production sites, procurement acceptance, or situations where high precision is not required.
1. Check Technical Parameter Sheets (TPDS)
◦ The most direct method. Reputable manufacturers provide product technical data sheets, where density is a core parameter and is usually clearly marked, such as ‘Density: 35 kg/m³’ or ‘Density: 450 kg/m³’.
◦ Note the units: The common unit for polyurethane is kg/m³. Conversion: 1000 kg/m³ = 1 g/cm³. For example, the density of common rigid foam insulation boards is approximately 30-60 kg/m³ (i.e., 0.03-0.06 g/cm³).

2. Hand-feel and Physical Property Experience Method (Requires Experience)
◦ Soft Foam (Sponge, Slow-Rebound Memory Foam):
■ Low density (<30 kg/m³): Feels soft, poor support, easy to tear, slow rebound, and has large visible pores.
■ High density (>50 kg/m³): Feels firm, good support, excellent elasticity, good toughness, and has fine surface pores.
■ Common judgment: After pressing with hand, if it rebounds quickly and fully, the density is usually higher; if it collapses and recovers slowly, the density is lower.

◦ Rigid Foam (Insulation Boards, Structural Fillers):
■ Low density (<100 kg/m³): Softer texture, easy to indent with fingernails, lower surface strength, prone to powdering, and good thermal insulation performance.
■ High density (>150 kg/m³): Hard texture, difficult to indent with fingernails, smooth and tough surface, strong load-bearing capacity, and often used as structural material.

◦ Elastomers (Wheels, Seals, Shoe Soles):
■ Density is usually very high (500-1200 kg/m³), close to or exceeding that of water. The simplest preliminary judgment is the ‘sink test’: Cut a small piece and put it in water.
■ Sinks: Density > 1 g/cm³ (1000 kg/m³), typically rigid or filled elastomers.
■ Floats or sinks slowly: Density close to 1 g/cm³.
■ Floats: Density < 1 g/cm³, may be micro-cellular foamed elastomers or soft foam.

3. Reverse Inference Based on Usage
◦ Thermal Insulation Materials: Density typically ranges from 30-60 kg/m³, aiming for the best balance between insulation performance and cost.
◦ Furniture/Mattress Sponge:
■ Ordinary seat cushions: Density around 25-40 kg/m³.
■ High-quality mattresses/sofas: Density often 40-60 kg/m³ or higher.
■ Memory foam: Density generally 45-80 kg/m³, with some exceeding 100 kg/m³.

◦ Building Structural Filler/Imitation Wood: Density typically 300-600 kg/m³.
◦ Industrial Casters, Solid Tires: Density can reach 600-900 kg/m³.
◦ CPU/GPU Thermal Pads: Highly filled polyurethane, density can exceed 1.5 g/cm³ (1500 kg/m³).

III. Special Notes

• Open-cell and Closed-cell: Open-cell foam typically has a lower density, is soft and breathable; closed-cell foam has a wide range of densities but offers better hardness, strength, and thermal insulation and water resistance.

Determination method: Cut a small piece and squeeze it with your hand to see if air can be sucked in or expelled, or observe whether the bubbles on the cross-section are connected.

• Additives: Fillers added to increase weight, hardness, or thermal conductivity (such as calcium carbonate, metal powders, glass fiber) will significantly increase density, far exceeding that of pure polyurethane resin (~1.1 g/cm³).

• Anisotropy: Especially for foamed materials, the density gradient in the upward direction may differ from other directions.

Summary and Recommended Judgment Process

1. Preferred: Look up the official technical data sheet, which is the most reliable information source.

2. If no data is available:

◦ For regular solid samples, use the dimension measurement method and calculate it yourself.

◦ For irregular or water-impermeable solids, use the displacement method.

◦ For on-site quick judgment:

■ First, check the application to infer the approximate range.

■ Then, perform preliminary high/low density classification using tactile sensation (pressure rebound, fingernail hardness) and water immersion test.

■ Finally, compare with samples of known density to obtain a relative judgment.

Hope this comprehensive explanation from theory to practice helps you accurately determine the density of polyurethane!