Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D2732-14 – Standard Test Method Technical Guide
📐 Scope and Specimen Geometry
ASTM D2732-14 (Reapproved 2020) defines the standard procedure for determining the unrestrained linear thermal shrinkage of plastic film and sheeting. This test method is specifically applicable to materials with a thickness of 0.76 mm (0.030 in.) or less. It is critical to note that this standard measures the release of internal stresses locked into the film during manufacturing and does not cover dimensional changes resulting from the loss of volatile components or solvents.
Standard specimens are cut to 100 mm ± 2.5 mm squares. The edges must be clean, smooth, and parallel to the machine and transverse directions. Specimens are conditioned according to Practice D618 prior to testing to ensure uniform initial dimensions.
| 🟦 Parameter | 📏 Specification |
|---|---|
| Specimen Nominal Length & Width | 100 ± 2.5 mm |
| Applicable Film / Sheeting Thickness | ≤ 0.76 mm (0.030 in.) |
| Length Measurement Tolerance | ± 0.25 mm |
| Minimum Number of Specimens | 3 per direction (MD & TD) |
⚙️ Apparatus and Test Procedure
The essential apparatus is a constant-temperature liquid bath capable of maintaining the specified temperature within ±0.5°C. The immersion fluid must be inert and not plasticize or react with the specimen; poly(ethylene glycol), glycerin, and water are standard recommendations. The unrestrained specimen is fully immersed in the preheated bath for exactly 10 minutes ± 10 seconds.
⚠️ Critical Requirement for “Unrestrained”: The specimen must be entirely free to move during the test. Any tension, constraint, or friction will inhibit the true “free shrink” response, which is defined in the standard as an irreversible reduction in dimension under nil or negligible restraint.
After immersion, the specimen is removed, cooled to room temperature, and the final dimensions (Lƒ) are measured. The magnitude of shrinkage is highly dependent on the selected test temperature, which should reflect the thermal history or phase transition temperature of the base resin.
📝 Characterization vs. Quality Control: For a complete characterization, perform the test over a range of temperatures spanning the material’s shrinkage range. For routine process or quality control, a single, representative temperature is usually sufficient following the initial characterization.
📊 Calculation and Key Measured Properties
Results are expressed as a percentage of the original dimension. Due to molecular orientation imparted during manufacturing, shrinkage is typically anisotropic; therefore, values must be reported separately for the Machine Direction (MD) and the Transverse Direction (TD).
Calculation Formula:
% Linear Shrinkage = [(L₀ − Lƒ) / L₀] × 100
Where L₀ = original length, Lƒ = final length after heating.
| 📐 Measured Property | 🎯 Description | ⚡ Unit |
|---|---|---|
| Unrestrained Linear Thermal Shrinkage | Irreversible reduction in a specific dimension upon heating | % |
| Test Temperature | Temperature of the constant-temperature liquid bath | °C |
| Immersion Duration | Standardized time specimen is held at temperature | 10 min ± 10 s |
| Directionality (MD / TD) | Specific reporting of shrinkage relative to manufacturing direction | % |
❓ Frequently Asked Questions
🔍 What is the primary difference between D2732 and D1204?
D2732 covers the unrestrained thermal shrinkage of thin films and sheeting (up to 0.76 mm). D1204 covers linear dimensional changes for nonrigid thermoplastic sheeting, which may involve different specimen constraints and broader applications. D2732 is the definitive standard for evaluating free-shrink behavior in packaging films.
💡 Why must shrinkage be tested in both the Machine and Transverse Directions?
Film manufacturing processes (blown or cast extrusion) create molecular orientation. When heated, these oriented molecules relax, causing significant shrinkage along the orientation axis. Testing both MD and TD provides a complete picture of the film’s thermal stability and overall internal stress state.
⚡ What happens if the specimen is not fully unrestrained?
If even slight tension or friction acts on the film during the 10-minute immersion, the measured shrinkage will be artificially low. The standard defines “free shrink” as occurring under nil or negligible restraint. Ensuring absolutely free movement is the most critical procedural requirement for accurate results.
📌 Does this test method apply to films containing plasticizers or solvents?
No. Section 1.1 of D2732 explicitly states that this test does not cover shrinkage from the loss of solvent. The method is designed solely to measure the relaxation of internal stresses locked into the film during thermal processing, not dimensional changes due to the evaporation of volatile components.