GB/T 42694-2023Textiles - Testing and evaluation for surface wetting resistance - Contact angle and rolling angle method (English PDF)

1 Scope

GB/T 42694-2023 measures water repellency on a fabric's surface by the angles a droplet makes with it. The traditional spray test grades repellency by comparing a wetted specimen against photographic standards - a judgement by eye, which discriminates poorly at the top of the scale where modern finishes now sit. Contact angle and rolling angle measure the physics directly: the contact angle says how much the surface resists being wetted at all, and the rolling angle says how readily a droplet leaves rather than clinging, which is the property a wearer actually notices and the one that distinguishes a merely water-repellent finish from a self-cleaning one. Both are numbers rather than grades, which matters now that fluorine-free finishes are replacing the fluorocarbons being phased out and their performance has to be compared honestly. This document describes the determination of surface wetting resistance of fabrics using contact angle and rolling angle tests and provides an evaluation of the result. It applies to all types of fabrics. It is written for finishers developing repellent treatments, for outerwear and technical textile brands specifying them, and for testing laboratories.

This document describes the method for determination of surface wetting resistance of fabrics using contact angle and rolling angle tests; provides an evaluation of wetting resistance. This document applies to all types of fabrics.

2 Normative references

The contents of the following documents, through normative references in this text, constitute indispensable provisions of this document. Among them, for dated references, only the edition corresponding to that date applies to this document. For undated references, the latest edition (including all amendments) applies to this document.

GB/T 6529 Textiles - Standard atmospheres for conditioning and testing

GB/T 6682 Water for analytical laboratory use - Specification and test methods

3 Terms and definitions

The following terms and definitions apply to this document.

3.1 Contact angle The angle containing the droplet BETWEEN the tangent (the tangent to the gas-liquid interface drawn at the intersection of the gas, liquid, and solid phases) AND the solid- liquid boundary, when the liquid forms droplets on the solid surface and reaches equilibrium.

Note: It is as shown in Figure 1.

5.1.4 Optical imaging device It shall be able to acquire clear images of the droplet and specimen. The acquired images can be viewed on the measurement system.

5.1.5 Liquid supply device The liquid supply device with injection head shall be able to quantitatively add (5±1)µL of liquid. For the injection head, it is recommended to use a flat-headed needle with a diameter of

0.21 mm~

0.52 mm. Stainless-steel needles are mostly used; other types such as Teflon-coated needles can also be used.

5.1.6 Measurement system It can control the light source, optical imaging device, liquid supply device and other components to complete the test operation. It can record and analyze the data of different droplet volume, shape and contact angle, which are obtained from the optical imaging system. The calculation is accurate to 0.1°.

Note: Contact angle calculation methods include Young-Laplace method, width-height method, circle method, ellipse method, tangent method, etc.

5.2 Rolling angle meter The rotating device of the rolling angle meter shall be able to drive the specimen stage to rotate axially continuously, to measure the rotating angle. Its rotation axis is on the surface of the specimen stage. The rotating device has a graduated line showing the rotating angle with an accuracy of 0.1°. It is equipped with a micropipette or dropper capable of dropping about 50 µL. The schematic diagram of rolling angle meter is shown in Figure 4.

7 Test procedure

7.1 General The standard atmospheres for conditioning and testing shall be carried out in accordance with the provisions of GB/T 6529. Before the test, the sample shall be conditioned and balanced. After consultation with relevant parties, conditioning and testing can be carried out under other test conditions; but shall be noted in the test report.

Note: The test results under different environmental conditions are not comparable. The evaluation indicators may not be applicable.

8 Expression and evaluation of results

8.1 Expression of results For the contact angle, the average values theta1 and theta2 of 5 measurements in the warp (longitudinal) and weft (transverse) directions respectively are taken as results; round off to 0.1° according to the rounding principle. If the droplet contacts the surface of the specimen and is completely absorbed by the specimen within 30 s, the contact angle of the specimen is directly recorded as 0°. For the rolling angle, the average values alpha1 and alpha2 of 5 measurements in the warp (longitudinal) and weft (transverse) directions respectively are taken as results; round off to 0.1° according to the rounding principle. When the rolling angle is >90°, or when the droplet is completely absorbed by the specimen after addition, it can be recorded as " >90°".

8.2 Evaluation of wetting resistance If necessary, according to Table 1, evaluate the wetting resistance of the sample. The contact angle shall be evaluated based on the smaller values in the warp (longitudinal) and weft (transverse) directions. The rolling angle shall be evaluated based on the larger values in the warp (longitudinal) and weft (transverse) directions. The contact angle and rolling angle shall both comply with the corresponding wetting resistance grade.