Overview
Brittle temperature is an important index to evaluate the significant change in mechanical properties of rubber and plastic materials in low temperature environment, that is, the critical temperature at which the material changes from a ductile state to a brittle state. This test usually follows standard methods (such as GB/T 5470, ISO 974, etc.), after the specimen is treated at a constant temperature at a specific low temperature, a single impact force is applied to observe whether it has fractured. As the core equipment that provides a stable and uniform low-temperature environment, the selection of low-temperature test chamber directly affects the accuracy and repeatability of test results.
Key points of selection
The selection of the model should comprehensively consider the requirements of the test standard, material characteristics and actual operating conditions, and the core is to ensure that the environmental parameters in the box can accurately meet the test procedures.
Temperature performance parameters
Matching the temperature range to the test requirements is fundamental. The minimum temperature of the box should be below a certain margin of the expected brittleness temperature of the material, for example, if the test temperature is -40°C, the minimum temperature capability of the equipment is recommended to be -50°C or lower to meet the needs of pre-cooling and process control. Temperature uniformity is a key indicator that refers to the temperature difference between points in the workspace in a steady state, usually no more than ±2°C. The temperature fluctuation reflects the short-term change of the temperature of the control point, which should be controlled within ±0.5°C. The cooling rate needs to meet the cooling time requirements specified in the standard, and too fast or too slow may affect the thermal history of the sample.
Work structure
The size of the chamber should accommodate a sufficient number of specimen racks and impact devices (if built-in) and ensure unobstructed airflow. The liner material should be resistant to low temperatures and corrosion, such as stainless steel. Sealing properties are essential for maintaining stability at low temperatures and preventing frost buildup. The observation window should be equipped with an anti-frost device to ensure that the specimen can be clearly observed during the test. The lead or test holes should be designed to allow for easy passage through the sensor or mechanical linkage without compromising the enclosure seal.
Refrigeration system
Refrigeration systems typically use overlapping mechanical cooling to meet the low-temperature requirements below -40°C. When selecting a model, attention should be paid to its long-term operation reliability, energy consumption and noise level. The control system should have high-precision program control function, which can set constant temperature, program cooling and other modes, and record the temperature curve in real time. The accuracy and position of the control sensor, such as platinum resistors, have a direct impact on the quality of the control.
Safety aids
The equipment should have safety mechanisms such as over-temperature protection, compressor delay protection, and power failure protection. Considering that testing may involve long-term operation, the stability and maintenance convenience of the equipment, such as defrost cycle design and easy replacement of wearing parts, should also be taken into account.
The selection parameters are compared with the simplified table
| Consider dimensions | Brief description |
| Temperature range | The lower limit should be lower than the test temperature, leaving a margin. |
| Temperature uniformity | The temperature difference in the space in the work area affects the consistency of the test. |
| Studio size | Specimen racks need to be accommodated and airflow is ensured. |
| Cooling rate | The time requirements stipulated by the relevant standards must be met. |
| Refrigeration method | Mechanical refrigeration is common, and a stacking system is required for deep low temperature. |
| Recording function | It should be able to record and output temperature-time curves. |
| Safety | Multiple electrical and mechanical safety protection devices. |
When selecting a model, it must first be ensured that the equipment functions meet all the environmental requirements of the test standard on which it is based. Different standards have specific provisions on sample pretreatment time, cooling rate, temperature tolerance, etc. For example, the standard may require the specimen to be kept at a constant temperature for a certain period of time (e.g., 3 minutes or more), which requires the temperature control stability of the test chamber to meet the temperature tolerance requirements during this time period.
Summary
Selecting a cryogenic test chamber for rubber-plastic brittleness temperature testing is a systematic project that balances temperature performance, mechanical structure, control accuracy, safety standards, and long-term operating costs. It is recommended that users evaluate the technical parameters and measured performance of the equipment in detail on the basis of clarifying the test standards and sample characteristics, and if necessary, request the supplier to provide calibration or verification reports that meet relevant standards to ensure the validity and comparability of the test data.
References
GB/T 5470-2008 Determination of embrittlement temperature by impact method for plastics
ISO 974:2000 Plastics Determination of impact embrittlement temperature
ASTM D746-14 Test Method for Impact Embrittlement Temperature of Plastics and Elastomers