What is Specific Gravity? Definition, Measurement, and Application in Acrylic Products

One-Sentence Definition

Specific gravity is a dimensionless ratio that compares the density of a material to the density of water, commonly used to assess whether a material is lighter or heavier than water.

Detailed Explanation

Specific gravity (SG), also known as relative density, is calculated by dividing the density of a substance by the density of water (typically at 4°C or 23°C for plastics). Unlike density, which has units (e.g., g/cm³), specific gravity is unitless, making it easy to compare materials regardless of measurement system. If a material’s specific gravity is less than 1, it will float on water; if greater than 1, it will sink. This property is crucial in material selection, quality control, and product design, especially in the plastics and acrylic industry.

Measurement Standards:

• ASTM D792 and ISO 1183 are the primary standards for determining the specific gravity of plastics, including acrylic. The most common method involves weighing a specimen in air and then in water, using the difference to calculate SG. Learn more about ASTM D792

Key Components

• Density of the Material: Mass per unit volume, measured in g/cm³ or kg/m³.
• Reference Density: For solids and liquids, this is the density of water (1.0 g/cm³ at 4°C).
• Temperature Control: Both the sample and water must be at a specified temperature for accurate results.
• Measurement Method: Typically, the water displacement method (ASTM D792) is used for plastics.

Practical Applications in Acrylic Products

Specific gravity directly impacts the weight, handling, and application of materials. For example, acrylic (PMMA) has a specific gravity of about 1.19, making it much lighter than glass (~2.5), and similar to polycarbonate (PC, ~1.20) and polystyrene (PS, ~1.05). This lower weight means acrylic products are easier and safer to transport, install, and use—an important advantage for display racks, storage boxes, and protective covers.

Comparison Table: Specific Gravity of Common Materials

Source: SpecialChem Plastics Guide

Case Example: Acrylic display stands from Toyin Acrylic Products Co., Ltd. (TOYIN) are significantly lighter than glass alternatives, reducing shipping costs and installation risks while maintaining excellent clarity and durability. This makes acrylic the preferred choice for retail, office, and home applications.

Related Concepts

• Density: Mass per unit volume; has units (g/cm³).
• Water Absorption: The amount of water a material can absorb, affecting weight and performance.
• Mechanical Strength: The ability of a material to withstand forces without breaking.
• Transparency: Important for display and protective applications.

For more on these topics, see Density vs. Specific Gravity and MT.com: What is Specific Gravity?.

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