Essential knowledge for wire and cable inspection - Weight loss test for insulation and sheath

I. Inspection Purpose

The weight loss test of insulation and sheath is a method to assess the thermal aging performance of insulation and sheath materials. Under the conditions of oxygen and heat that promote aging, polyvinyl chloride polymer materials undergo cracking or polymerization, as well as the migration and volatilization of individual components, resulting in the loss of some weight. Over time, they lose their mechanical and electrical properties until they become unusable. Therefore, all products with polyvinyl chloride insulation and sheaths must undergo weight loss tests.

Ii. Scope of Application

It is applicable to the weight loss test of the sheath of polyvinyl chloride insulated cables and wires and cross-linked polyethylene insulated polyvinyl chloride sheaths with the requirements of this test project.

Iii. Inspection Basis

GB/T 2951.32-2008 /IEC 60811-3-2:2003 and related product standards.

Iv. Test Equipment and Apparatus

1. Aging chamber: Natural ventilation or pressure ventilation. In case of dispute, the natural style shall be adopted.

2. Analytical balance: Sensitivity 0.1mg.

3. Desiccant: Use silica gel or a desiccant made of similar materials.

V. Sampling and Testing Equipment

1. Sampling: Take three insulation and sheath samples from the cable, each 100mm long. If the weight loss test and the thermal aging test are conducted in combination, three samples can be taken from the thermal aging test samples, and one set of specimens can be taken from each insulation core and sheath.

2. Sample Preparation: In addition to preparing specimens in accordance with the provisions of "Preparation of Dumbbell Specimens and Tubular Specimens", the following regulations must also be met:

1) The semiconductive layer on the insulation (if any) should be removed by mechanical means instead of solvents.

2) Try to make as large dumbbell specimens as possible. If the size of the insulated wire core is too small to make large dumbbell specimens, then small dumbbell specimens can be made. The two surfaces of the dumbbell specimen should be parallel, with a thickness of 1.0±0.2mm.

3) For specimens with an inner diameter not exceeding 12.5mm, as long as no semiconductive layer adheres to the insulation inside, tubular specimens can be used instead of dumbbell specimens.

4) The two ends of the tubular specimens should not be closed, and the total surface area of each specimen should be no less than 5cm².

5) Tubular specimens and dumbbell specimens are not required to be marked with lines.

6) There are grooves on both sides between the cores of the double-core flat flexible wire. During the test, the insulated cores should not be separated.

Vi. Test Procedures

Number the prepared specimens, measure their thickness and outer diameter, and calculate the volatile surface area of the specimens.

1) Tubular specimens: Surface area A= outer surface area + inner surface area + cross-sectional area

A = 2 ∏ (D - delta) x (L + delta) / 100

In the formula: A - the surface area of the tubular specimen, expressed in cm².

δ - Average thickness of the specimen, mm. If δ≤ 0.4mm, round to two decimal places; if δ > 0.4mm, round to one decimal place.

D - Average outer diameter of the specimen, mm. If D≤2 mm, it is to two decimal places; if D>2 mm, it is to one decimal place.

L - Specimen length, mm. To one decimal place.

a.δ and D were measured on the thin slices cut from the ends of each tubular specimen in accordance with the provisions of "Measurement of Insulation Thickness" and "Measurement of External Dimensions".

b. This formula is also applicable to tubular specimens with serrated inner surfaces.

C. The calculation of the volatile surface area of double-core flat flexible wire can be regarded as two separate tubular specimens.

2) Dumbbell specimens:

Small dumbbell specimen A=624+(118 δ) / 100

The large dumbbell specimen A=1256+(180 δ) / 100

In the formula: A - the surface area of the dumbbell specimen, expressed in cm².

δ - Average thickness of the specimen, measured in accordance with the measurement method for dumbbell thickness in the "Tensile Test of Insulation and Sheath Before and after Aging", expressed in mm, and rounded to two decimal places.

2. Store the specimens in a desiccator at ambient temperature for at least 20 hours. Each specimen was weighed precisely immediately after being taken out of the desiccator, in mg, accurate to one decimal place.

3. The three specimens of the following products shall be stored at atmospheric pressure under the following conditions, at the temperature and for the time specified in the standard.

Materials with significantly different components should not be tested simultaneously in the same oven.

b. The specimens should be vertically suspended in the middle of the oven, with a distance of at least 20mm between each specimen.

c. The volume occupied by the specimen should not exceed 0.5% of the oven volume.

After heat treatment is completed, the specimens should be re-placed in a desiccator at ambient temperature for 24 hours. Then, each specimen should be accurately weighed again with an analytical balance, in mg, accurate to one decimal place.

5. Calculation of weightlessness:

The weight loss of each specimen = the original weight of the specimen - the weight of the specimen after heat treatment/the volatile surface area

The weight loss unit of each specimen is mg/cm².

- The original weight of the specimen is expressed in mg;

The weight of the specimen after heat treatment is expressed in mg.

Vii. Evaluation of Test Results

The median value of the measurement results taken from three specimens shall be taken as the weight loss of the insulation or sheath, in mg/cm². The maximum weight loss of all products shall not exceed 2.0mg/cm².