Dipped Cord Testing

Dipped cord testing mainly conducts scientific testing and analysis on adhesion strength, breaking strength and elongation at break, elongation at a specified load, dry heat shrinkage, linear density and twist, and dip pickup and appearance quality, 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

Adhesion strength (H-pull test): This is the core indicator for evaluating the adhesion performance between the dipped cord and the rubber. By pulling the cord out of the vulcanized rubber block under specific conditions, the maximum force value is measured, thereby evaluating the ability of the adhesion interface to resist peeling damage. The result directly affects the durability and service life of products such as tyres.

Breaking strength and elongation at break: This item is used to determine the maximum load that a single dipped cord can bear until breakage under tension (breaking strength) and the percentage of the elongation at the moment of breakage relative to the original length (elongation at break). These two indicators are the key parameters for measuring the basic mechanical properties and toughness of the cord.

Elongation at a specified load: Measures the elongation of the cord under the action of a specified load (usually a certain percentage of the rated breaking strength of the cord). This indicator reflects the dimensional stability of the cord under use conditions, and is crucial for ensuring the dimensional accuracy and uniformity of rubber products (such as tyre belt plies).

Dry heat shrinkage: After treating the cord at a specific temperature (such as 177 ℃) and tension conditions for a certain time, the percentage of the length shrinkage at room temperature is measured. This indicator is used to evaluate the dimensional thermal stability of the cord during high-temperature vulcanization and the subsequent use process. An excessively large shrinkage will cause product deformation or stress concentration.

Linear density and twist: The linear density refers to the mass of the cord per unit length, and is the basis for calculating the specification and dosage of the cord. The twist refers to the number of turns of the yarn or monofilament per unit length, including the initial twist and the ply twist. It directly affects the structural compactness, strength, fatigue performance and the wetting and adhesion effect with the rubber.

Dip pickup and appearance quality: The dip pickup refers to the percentage of the mass of the dipping adhesive attached to the surface of the cord to the total mass of the cord, and needs to be controlled within a reasonable range to ensure the adhesion. The appearance inspection includes checking whether the cord has defects such as fluff, stains, adhesion, broken strands and uneven twist, which will seriously affect the subsequent processing and the final product performance.

Testing Scope

Polyester (PET) dipped cords: Widely used in the carcass and belt plies of passenger car and light truck tyres. Testing needs to focus on its adhesion performance with the rubber, dry heat shrinkage and hydrolysis aging resistance, to adapt to the high-temperature vulcanization process and the complex use environment in tyre manufacturing.

Nylon (PA6, PA66) dipped cords: Mainly used in truck tyres, off-road tyres and transmission belts. The testing focus is on its excellent fatigue resistance, high impact strength and the performance retention rate in high-humidity environments, while its thermal shrinkage behaviour after heating needs to be controlled.

Rayon dipped cords: Commonly found in the belt plies of high-performance radial tyres. In testing, special attention needs to be paid to its dimensional stability (low shrinkage), high modulus and performance under dynamic loads, and the influence of its moisture absorption on the adhesion stability also needs to be evaluated.

Aramid dipped cords: As a high-performance material, used in aviation tyres, special conveyor belts and high-performance tyres. In addition to the conventional mechanical properties, the testing items focus more on the extremely high specific strength, specific modulus, high-temperature resistance and the evaluation of the interfacial adhesion technology with special rubber.

Glass fibre dipped cords: Mainly applied in tyre belt plies and rubber hose reinforcement layers. The testing core lies in its extremely high modulus and dimensional stability, but its fatigue resistance, adhesion durability with the rubber and sensitivity to the dipping treatment process need to be strictly evaluated.

Cords of different specifications and structures: The testing scope covers dipped cords of different linear densities (such as 1000 dtex and 1440 dtex), different ply numbers (such as 2-ply and 3-ply) and different twist direction structures. The performance indicator thresholds and testing emphasis of products of different specifications are different.

Testing Methods

H-pull test method: This is the standard method for the adhesion strength testing. A single cord is embedded in a standard formula rubber block according to a specific process and vulcanized to make a standard specimen, and then the cord is vertically pulled out at a specified speed on a tensile testing machine, recording the maximum force value. This method simulates the actual stress state of the cord-rubber interface, and is a testing means recognized by the industry.

Monofilament/cord tensile test method: Carried out on an electronic tensile testing machine in accordance with standards (such as GB/T 32108 and ASTM D885). Dedicated grips are used to fix the two ends of the cord, and it is stretched at a constant rate until breakage, automatically recording the force-displacement curve, thereby calculating a series of parameters such as the breaking strength, elongation at break, elongation at a specified load and initial modulus.

Dry heat shrinkage test method: A specified pretension is applied to a cord specimen of a certain length, and it is treated in a specific high-temperature oven (such as 177±1 ℃) for a specified time (such as 2 minutes). After taking it out and cooling to room temperature, the length change is measured and the shrinkage is calculated. During the test process, the temperature, time and tension need to be strictly controlled to ensure the comparability of the results.

Dip pickup determination method: The solvent extraction method is usually adopted. A certain mass (M1) of the dipped cord specimen is weighed, repeatedly extracted in a Soxhlet extractor with a specific solvent (such as toluene or tetrahydrofuran) to remove the surface dipping adhesive layer, then dried to constant weight, and the mass of the cord framework (M2) is weighed. The dip pickup is calculated by the formula (M1-M2)/M2 ×100%.

Twist determination method: A twist testing machine is used, adopting the untwist-retwist method or the direct counting method. The former untwists in the reverse direction until the monofilaments are parallel and then retwists in the reverse direction to the original length, recording the total number of turns; the latter directly untwists and counts under a specified tension. The twist directly affects the structural uniformity and comprehensive performance of the cord.

Appearance inspection method: Mainly relying on manual visual inspection or combined with automatic image detection equipment under standard lighting conditions. The inspectors compare with the standard sample cards or defect atlases, check whether there are appearance defects such as fluff, knots, stains and poor formation on the surface and inside of the cord package, and carry out grading and judgement.

Testing Standards

Domestic Standards:

1. GB/T 32108, Determination of tensile properties of dipped yarns, cords and tyre cord fabrics

2. GB 18401-2010, National general safety technical code for textile products

3. GB/T 3920-2024, Textiles — Tests for colour fastness — Colour fastness to rubbing

4. GB/T 2910, Textiles — Quantitative chemical analysis

International Standards:

1. ASTM D885, Standard Test Methods for Tire Cords, Tire Cord Fabrics, and Industrial Filament Yarns Made from Manufactured Base Fibers

2. ISO 105, Textiles — Tests for colour fastness

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

Electronic universal material testing machine: This is the core equipment for mechanical property tests such as the H-pull, breaking strength and elongation at a specified load. It is equipped with a high-precision load sensor and an elongation measuring device (such as a non-contact video extensometer), can precisely control the test speed, automatically collect and process the data, and output a complete test report.

Dry heat shrinkage tester: Dedicated equipment, usually composed of a precision oven, specimen holder, loading weights (providing constant tension) and a length measuring system (such as an optical scale or laser rangefinder). It is required that the temperature field inside the oven is uniform and the temperature control precision is high, to ensure the reproducibility of the test conditions and the accuracy of the results.

Soxhlet extractor and analytical balance: The key equipment combination for the dip pickup determination. The Soxhlet extractor realizes the continuous circulation extraction of the solvent and efficiently removes the adhesive. The precision of the analytical balance needs to reach 0.1 mg, and it is used to precisely weigh the specimen mass before and after extraction, which is the basis for calculating the dip pickup.

Twist tester: A dedicated instrument, equipped with a precision gripper, rotary counter and pretension applying device. It can automatically or semi-automatically complete the untwisting and retwisting actions according to the standard method, and precisely record the number of turns. Some advanced models can simultaneously test the twist irregularity.

Constant temperature and humidity chamber: Used for the conditioning of the dipped cord specimens or specific environmental aging tests. Before the test, the specimens need to be placed in a standard temperature and humidity environment (such as 20±2 ℃ and 65±4% RH) for a sufficient time to stabilize their performance, ensuring that all mechanical test results are obtained under a unified environmental benchmark and are comparable.

Appearance inspection bench and image detection system: The appearance inspection bench provides standard D65 light source lighting and a black background, facilitating the inspectors to find subtle defects. The automatic image detection system uses high-resolution cameras and image processing software to automatically identify defects such as fuzz, stains and uneven thickness on the surface of the cord, improving the inspection efficiency and objectivity.