Vulcanized Fiber Testing

Vulcanized fibre inspection mainly conducts scientific testing and analysis on the thickness and density testing, tensile strength and elongation testing, breakdown voltage strength testing, oil absorption and water absorption testing, hardness testing and interlayer bonding strength testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Thickness and density testing: The thickness of the vulcanized fibre is the basic indicator affecting its mechanical properties and insulating properties, and is usually measured at multiple points with a thickness gauge to ensure the uniformity. The density testing is calculated by measuring the mass and dimensions of the specimen, and its value is directly related to the subsequent performances such as the compactness, hardness and oil absorption of the vulcanized fibre. It is the key for judging whether the raw material proportioning and pressing process meet the requirements.

Tensile strength and elongation testing: This testing is used to evaluate the ability of the vulcanized fibre to resist the tensile damage, and is usually carried out on a universal material testing machine. By measuring the maximum load and elongation length of the specimen at the tensile fracture, the tensile strength and elongation at break are calculated. This is crucial for judging its load-bearing capacity and toughness as a structural material or gasket material.

Breakdown voltage strength testing: This is the core indicator for measuring the electrical insulating performance of the vulcanized fibre. During the testing, the specimen is placed between the electrodes, the AC or DC voltage with the uniform step-up is applied until it is broken down, the breakdown voltage value is recorded and the breakdown voltage strength per unit thickness is calculated. This indicator directly determines the safety grade of the vulcanized fibre used as the insulating material in the electrical equipment.

Oil absorption and water absorption testing: The oil absorption testing mainly evaluates the swelling stability of the vulcanized fibre in the media such as the transformer oil, while the water absorption reflects its ability to resist the humid environments. The testing method is usually to soak the specimen in the oil or water at the specified temperature and time, and then measure the mass change percentage. The excessively high oil/water absorption will cause the dimensional deformation and performance decline.

Hardness testing: The hardness of the vulcanized fibre is usually measured with a Rockwell hardness tester or Shore hardness tester, reflecting the ability of its surface to resist the local indentation deformation. The appropriate hardness is the premise for ensuring that the vulcanized fibre has the good mechanical strength and dimensional stability when used as the wear-resistant parts such as the gears and gaskets. If the hardness is too low, it is easy to wear, and if it is too high, the brittleness may increase.

Interlayer bonding strength testing: The vulcanized fibre is made by laminating the multiple layers of the base paper treated with the zinc chloride. The interlayer bonding strength is an important indicator for evaluating the integrity of its internal structure. During the testing, the specimen is peeled or stretched through the specific fixtures, and the force required for the interlayer separation is measured. The insufficient strength is easy to cause the problems such as the delamination and blistering during use.

Testing Scope

Vulcanized fibre for the electrical insulation: Mainly used to manufacture the insulating gaskets, bushings and other parts in the motors, transformers and electrical switches. Its testing core focuses on the electrical performances, such as the breakdown voltage strength, volume resistivity and dielectric loss factor, and at the same time, there are the strict requirements for the oil absorption, to ensure the long-term stable operation in the oil-immersed environments.

Vulcanized fibre for the mechanical structures: Applied to the wear-resistant parts such as the gears, bearings and washers in the fields such as the textile machinery, automobiles and model aircraft. The testing emphasis is on the mechanical performances, such as the tensile strength, compressive strength, hardness, wear resistance and dimensional stability, requiring the material to have the high strength, high hardness and low deformation rate.

Vulcanized fibre for the tooling and dies: Used to make the backing plates and positioning plates of the stamping dies. In addition to the routine mechanical performances, the uniformity, machinability (such as the blanking and drilling performances) and the dimensional change rate after the moisture absorption and heating are particularly emphasized, to ensure the precision and service life of the tooling.

Vulcanized fibre for the special environments: Referring to the variants of the vulcanized fibre used in the high-temperature, corrosion-resistant or special medium environments. The testing scope needs to be extended to the thermal stability (such as the heat deflection temperature), chemical resistance (acids, alkalis and oils) and the swelling in the specific media, meeting the more severe working condition requirements.

Raw material and semi-finished product testing: The testing scope also covers the base papers, treatment liquids (such as the concentration of the zinc chloride solution) and the vulcanized fibre blanks before the vulcanization in the production process. Through the quantitative, whiteness and absorption testing of the raw materials, as well as the pre-curing degree evaluation of the blanks, the quality of the final products is controlled from the source.

Import and export trade compliance testing: In the cross-border trade, the vulcanized fibre products need to be tested according to the regulations and standards of the target markets, such as the EU RoHS Directive (restriction of hazardous substances), REACH Regulation, as well as all the applicable items in the ISO, ASTM and JIS standards designated by the customers, to ensure the compliance with the safety and environmental protection requirements.

Testing Methods

Physical property testing method: The thickness is measured with a thickness gauge in accordance with GB/T 451.3 or ISO 534; the density is calculated by measuring the mass and volume; the tensile strength and elongation follow the standards GB/T 12914 or ASTM D828, and are stretched at a constant rate on a universal testing machine until the fracture; the hardness testing commonly uses the methods specified in GB/T 2411 (Shore hardness) or the relevant standards.

Electrical property testing method: The breakdown voltage strength testing is mainly based on GB/T 1408.1 or IEC 60243-1, using the short-time step-up method or progressive step-up method in the oil medium. The volume resistivity and surface resistivity testing is carried out according to GB/T 1410 or IEC 60093, using a high-resistance meter under the specified temperature and humidity conditions.

Environmental adaptability testing method: The oil absorption and water absorption testing usually refers to GB/T 461.3 or the customer-designated standards. The specimen is immersed in the transformer oil at (105±2) ℃ or the distilled water at (23±1) ℃ for the specified time, taken out, wiped dry and weighed for the calculation. The heat resistance testing may include the heat deflection temperature testing (ASTM D648) or the long-term thermal aging experiments.

Chemical analysis and durability method: Including the residual zinc chloride content analysis, determined by the chemical titration method; the resistance to the liquid chemical reagents testing, in which the specimen is immersed in the acids, alkalis and oils of the specific concentrations, and the changes of its mass, dimensions and hardness are observed; the wear resistance testing can adopt the Taber abrasion tester, evaluating its wear amount according to the standards.

Non-destructive and micro testing method: The ultrasonic testing can be used to detect the defects such as the interlayer delamination and bubbles inside the vulcanized fibre. The scanning electron microscope (SEM) can observe its fibre bonding state and cross-section morphology, analysing the influence of the process on the microstructure. These methods provide the in-depth basis for the process improvement and failure analysis.

Standard compliance judgement method: The testing results need to be compared and judged with the standards claimed by the product, such as the national standard GB/T 20631, the International Electrotechnical Commission standard IEC 60667 and the American Society for Testing and Materials standard ASTM D709. During the judgement, attention needs to be paid to the sampling quantity, test conditions, result rounding and the single-item and comprehensive judgement criteria.

Testing Standards

Domestic Standards:

1. GB/T 451.3, Paper and board — Determination of thickness

2. GB/T 12914, Paper and board — Determination of tensile properties

3. GB/T 1408.1, Insulating materials — Methods of test for electric strength — Part 1: Tests at power frequencies

4. GB/T 1410, Methods of test for volume resistivity and surface resistivity of solid electrical insulating materials

5. GB/T 20631, Vulcanized fibre for electrical purposes

International Standards:

1. ISO 534, Paper and board — Determination of thickness

2. IEC 60243-1, Electric strength of insulating materials — Test methods — Part 1: Tests at power frequencies

3. IEC 60093, Methods of test for volume resistivity and surface resistivity of solid electrical insulating materials

4. IEC 60667, Vulcanized fibre for electrical purposes

5. ASTM D828, Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Universal material testing machine: The core equipment for carrying out the mechanical property tests such as the tensile strength, compressive strength, interlayer bonding strength and flexural strength. Its high-precision sensors and control system can precisely record the load-displacement curves and automatically calculate the results, meeting the requirements of the various standards such as GB, ISO and ASTM for the test speed and fixtures.

High-voltage breakdown tester: The equipment specially used to test the electric strength of the solid insulating materials such as the vulcanized fibre. The instrument includes a high-voltage generator, electrode system, oil bath and safety protection devices, and can execute the breakdown testing under the AC, DC or impulse voltages, automatically recording the breakdown voltage values and test time, ensuring the accurate and reliable data.

High insulation resistance meter: Used to measure the volume resistivity and surface resistivity of the vulcanized fibre. This equipment can precisely measure the weak current passing through the specimen under the applied high test voltage (such as 500 V or 1000 V), thereby calculating the resistance values. The equipment is usually equipped with a shielding box to eliminate the interference of the ambient humidity on the test results.

Precision thickness gauge and density measuring device: The thickness gauge needs to adopt the contact-type measuring head and be equipped with a constant pressure device, to ensure the repeatability of the measurement results. The density measurement needs to use a balance with the precision of 0.001 g and vernier callipers or a volume measuring instrument, calculated through the Archimedes principle or the direct measurement method.

Constant temperature and humidity chamber and immersion equipment: Used for the oil absorption, water absorption and environmental aging testing. The constant temperature and humidity chamber can precisely control the temperature and humidity, simulating the different climatic conditions. The immersion equipment usually includes a constant temperature oil bath, distilled water container and the supporting weighing tools, ensuring that the specimens are processed and measured under the standard conditions.

Hardness tester and micro-analysis instrument: The Rockwell hardness tester (usually with the R scale) or Shore hardness tester is used for the rapid detection of the surface hardness of the vulcanized fibre. The scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) are used for the micro-morphology observation and element composition analysis, and are the important tools for studying the material failure mechanism and process optimization.