Understanding Stress Cracking in Packaging Materials: Essential Testing Methods

Stress cracking is a significant risk in packaging materials, particularly in plastic closures, containers, and composite packaging. To ensure product safety and long-term performance, standardized stress cracking testing is essential. This article introduces stress cracking test methods applicable to various types of packaging materials.

Purpose of Stress Cracking Tests

The primary goal of stress cracking tests is to standardize the testing method for evaluating stress cracking resistance in packaging materials. This ensures that manufacturers can reliably assess the durability of their products.

Scope of Testing

This testing method applies to all types of packaging materials. This includes plastic, heat-sealed, and composite packaging components. By covering a wide range of materials, manufacturers can ensure comprehensive assessments.

Equipment and Instruments Required

To conduct stress cracking tests, the following equipment is necessary:

• Constant temperature oven (set at 60°C)

• Torque tester

  
  

Test Procedure

1. Sampling

Development Stage: Select 10 representative samples for testing.

Incoming Inspection Stage: Follow GB/T 2828-2012 for sampling (Normal inspection, single sampling plan for attributes).

2. Conditioning

Condition the products by placing them in an environment of 23°C and 50% relative humidity (RH) for 24 hours. Clearly mark the test samples for identification.

3. Application to Stress-Prone Areas

Apply the test product to areas where stress concentration may occur. This includes the thread neck opening down to the base of the threaded area. For threaded products, tighten the closure according to the torque values specified below.

| Thread Diameter (Approx. Major Diameter T, mm) | Applied Torque (N·m) |

|-------------------------------------------------|----------------------|

| 8 | 0.8 |

| 10 | 0.8 |

| 13 | 0.9 |

| 14 | 0.9 |

| 15 | 1.0 |

| 18 | 1.2 |

| 20 | 1.4 |

| 22 | 1.5 |

| 24 | 1.7 |

| 28 | 2.0 |

| 30 | 2.1 |

| 33 | 2.4 |

| 38 | 2.8 |

| 40 | 2.8 |

| 50 | 5.2 |

| 60 | 6.7 |

| 70 | 7.2 |

| 80 | 8.0 |

| 100 | 9.7 |

4. Product Filling and Positioning

Invert all containers and apply the test product to the inner wall of each container. This ensures that stress is applied uniformly during testing.

5. High-Temperature Static Test

This test is applicable to plastic, heat-sealed, and composite products. Record all observations during this phase.

• Place 10 capped containers in a 60°C oven: 5 upright and 5 inverted.

• Observe samples after 24 hours, 48 hours, 72 hours, and 7 days.

• Evaluate samples immediately after removal from the oven (no need to return to room temperature).

• Apply manual pressure to the container to assess forced environmental cracking.

• Resistance during pressure application indicates no micro-cracking.

  
  

⚠️ Note: At 60°C, caps may loosen. After each inspection, manually retighten the caps before returning samples to the oven. After 15 days, remove samples and allow them to equilibrate at room temperature for 24 hours before final evaluation.

6. Accelerated Stress Cracking Test Using Chemical Solution

This test is applicable to polyolefin products, such as plastic stoppers.

Solution Preparation

Prepare a stress cracking solution using TRIDECETH-12 (CAS No. 78330-21-9) at a concentration of 0.5% ± 0.05% (by weight) in distilled or demineralized water. To accelerate dissolution, the solution may be prepared at 35°C.

⚠️ Note: TRIDECETH-12 is a mixed C11–C14 iso-alkyl ether. Due to varying molecular melting points, the solution may partially solidify at room temperature. If this occurs, reheat and mix thoroughly before testing.

Test Method

• Fully immerse the test samples in a glass container filled with the cracking solution (both internal and external surfaces must contact the solution).

• Seal the container to prevent evaporation.

• Place the container in a 55°C oven for 24 hours.

• Remove samples, wipe dry with absorbent paper, and inspect stress-concentrated areas for cracks.

• Record all findings.

  
  

⚠️ Safety Notice: The cracking solution is corrosive. Always wear rubber gloves and protective goggles when handling.

Calculation

Not applicable for this testing method.

Defect Classification and Acceptance Criteria

When requirements differ from specific packaging material standards, the packaging material standard shall prevail.

Defect Classification and Acceptance Criteria

| Description | Defect |

|----------------------|---------------------|

| 0 defect | √ |

| Serious | AQL 0.15% |

| Main | AQL 0.65% |

| Minor | AQL 1.5% |

| Very Minor | AQL 4.0% |

| Bottle crack | √ |

| Minor crack | √ |

| Function defect | √ |

⚠️ Note: When requirements differ from the applicable packaging material standards, the packaging material standards shall prevail.

Sample Retention

All tested samples and original reference samples must be retained for 6 months after testing.

By following these standardized testing methods, brands can ensure that their packaging materials meet the necessary quality and durability standards. This is crucial for maintaining product integrity and consumer trust in the global beverage, spirits, and cosmetic markets.