ISO/DIS 4625-3:2025(en)

ISO TC 35/WG 5

Secretariat: NEN

Date: 2025-05-02

Binders for paints and varnishes — Determination of softening point — Part 3: Cup method (without a ball)

© ISO 2025

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Contents

Foreword iv

Introduction v

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Principle 1

5 Apparatus 1

5.1 Softening point apparatus 1

5.2 Central processor 2

5.3 Measuring cell 2

5.4 Sample cup assembly 2

6 Calibration of apparatus using a primary standard 2

6.1 General 2

6.2 Reagent 2

6.3 Procedure 3

6.3.1 Filling the sample cup 3

6.3.2 Heating 3

6.3.3 Cleaning 3

6.3.4 Interpretation 3

7 Instrument check of apparatus using a secondary standard 3

8 Preparation of sample 4

8.1 Preparation 4

8.2 Filling the sample cup 4

9 Procedure 5

10 Expression of results 5

11 Precision and bias 5

11.1 General 5

11.2 Precision 5

11.3 Bias 5

12 Test report 6

Annex A (informative) Details of the interlaboratory comparison for the estimation of precision 7

A.1 General notes on the interlaboratory comparison 7

A.2 Samples 7

A.3 Determinations 7

A.4 Evaluation 8

Bibliography 1

Foreword

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This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes.

A list of all parts in the ISO 4625 series can be found on the ISO website.

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Introduction

The ISO 4625 standards series describes three widely used procedures for the measurement of the softening point of rosin-based resins, ISO 4625‑1,^[1] the ring-and-ball method which was the accepted standard for many years and ISO 4625‑2,^[2] the Mettler cup-and-ball method. This document adds a new method, the Mettler method without the ball.

ISO 4625‑1 is still the only standard test method accepted in regulatory documents such as 21CFR ^[3].

ISO 4625‑2 and this document are both called Mettler cup-and-ball methods. Although the recommended testing conditions differ the only difference between the equipment required is that this document does not use a ball. Surveys have shown that ISO 4625‑2 is the most widely used in the US and this document the most widely used in Europe. These methods are less time consuming than the ring-and-ball manual method and the equipment is less expensive than the ring-and-ball automated method.

As a consequence of the thermoplastic nature of the test resins, the softening points obtained using the recommended test conditions for all three methods are not the generally the same. Consequently, the test method and the testing conditions used should be noted in the final report.

Binders for paints and varnishes — Determination of softening point — Part 3: Cup method (without a ball)

This document specifies a method for determining the softening point of resins (including rosin) and can, under user-defined conditions, give results comparable to those obtained by ISO 4625-1 and ISO 4625-2.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 4618, Paints and varnishes — Vocabulary

For the purposes of this document, the terms and definitions given in ISO 4618 and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

softening point

temperature at which the sample, suspended in a cylindrical cup with a 6,35 mm hole in the bottom and without a stainless steel ball, flows downward a distance of 19 mm, as the sample is heated at linear rate in air

In general, with materials of these types, softening does not take place at a definite temperature. As the temperature rises, these materials gradually change from brittle or exceedingly thick and slow-flowing materials to softer and less viscous liquids. For this reason, the determination of the softening point shall be made by a fixed, arbitrary, and closely defined method if the results obtained are to be comparable.

The disc of the test sample is placed in a cup and heated at a specified rate. The softening sample is forced by its own weight downwards. The temperature at which the sample has sunken by 19 mm is called "softening point".

This commercially available instrument consists of control unit with a digital temperature indicator, matched measuring cell, cartridge assembly – consisting of sample cup and upper portion – and accessories. The control unit automatically regulates the heating rate of the measuring cell. The softening point is indicated on the readout, and the heating programme is stopped, when the sample flow triggers a detector.

This unit shall provide a continuous, linear increase from 25 °C to 375 °C.

This unit shall be capable of heating a sample cup assembly, as described in 5.4, at a linear rate with an accuracy of ±0,2 °C/min. It shall include a sensing system capable of detecting the softening point with a precision of 0,1 °C.

A cup of chromium-plated brass and an upper portion, conforming to the dimensions shown in Figure 1.

Dimension in millimetres

Figure 1 — Cup

This step, required only occasionally, is designed to establish that the temperature indicated by the instrument is in agreement with a known standard. A special cup with a bottom orifice of 2,8 mm, is used instead of the one specified for testing the resin.

Use analytical reagent of primary standard grade benzoic acid for the calibration. As this material is hygroscopic, it shall be stored in a tightly sealed container and replaced with fresh material from a newly opened supply if the melting point of the material is out of the expected range.

Place the cup on a clean, flat surface and fill it with benzoic acid crystals. Hand press the material into the cup with a 9,2 mm rod of dowel stock or equivalent. Refill and repeat the pressing step until the cup is filled with benzoic acid. Remove any crystals from the exterior of the cup.

Preheat the measuring cell to 121 °C, and maintain it at that temperature. Place the cartridge assembly containing the benzoic acid in position in the measuring cell. With the instrument set in the dropping point mode, start the required method, which includes a waiting period of 30 s to allow temperature equilibration between the measuring cell and sample followed by an automatic temperature ramp of 0,2 °C/min. The temperature will rise steadily at the correct rate until the melt point is reached, then remain steady on the readout.

Immediately remove the cartridge assembly. Check to determine that the sample has passed the detector and no pre-triggering has occurred. If a malfunction is suspected, the entire procedure shall be repeated. Inspect the apparatus carefully to ensure that no residue remains. Wash the cartridge parts with a suitable solvent to remove last traces of residue.

If the result is not (123,5 ± 0,5) °C, repeat the test. If the second value remains outside of the acceptable range, repeat the test using a fresh sample of benzoic acid. If the results remain outside of the acceptable range, the instrument requires recalibration or repair. Consult the manufacturer's instruction manual.

The primary calibration standard, benzoic acid, might not have a melting point similar to the softening point of the resin being tested. In such cases, it is desirable to calibrate the instrument using a secondary standard having a melting point similar to softening point of the test sample. Materials suitable for use as secondary standards are listed, but are not limited to those, in Table 1. Demonstration of conformance to the melting point of benzoic acid should be completed before a secondary standard is used.

Table 1 — Materials suitable as secondary standards

Follow the same procedure as in 6.3.2 and 6.3.3 but start about 3 °C below the actual melting point of the secondary standard.

Fold the tab on an oil-free disposable aluminium weighing dish to a horizontal position and pinch a pour spout into the dish directly opposite the tab. Weigh 5 g to 10 g of resin into the dish.

Place the dish and sample on a preheated hot plate, and cover with a 600 ml or larger beaker. Heat the sample until it is almost melted.

Remove the beaker from the hot plate, grasp the tab with forceps or needle-nose pliers, and stir the sample with a probe-type thermometer until it is completely melted and homogeneous. Avoid entraining air in the sample melt. The temperature of the sample should be no more than 60 °C above the expected softening point. Remove the thermometer and wipe clean. If the sample smokes or appears to boil, repeat the steps 8.1 to 8.3, setting the maximum temperature 10 °C less than the previous trial.

Arrange two sample cups, small outlet down, on a piece of aluminium foil placed on a smooth, flat surface.

Grasping the dish tab with forceps or pliers, pour the molten resin into both of the sample cups. Fill the cups to the brim.

Allow the resin to cool for at least 10 min. Cleanly trim off the excess resin on the sample cup with a slightly heated knife or spatula, or grasp the cup in a pair of tongs and draw the top surface quickly and firmly over the surface of a heated metal plate. Do not make a determination on a sample which contains air bubbles.

Assemble the cartridge by placing the cup on the collector, then the upper portion on the cup.

Make sure that there is no sample on the outside of the cup, the inside of the jacket, or the collecting sleeve. These components shall be clean during analysis. The assembly should be wiped clean before inserting it into the measuring cell.

The procedure described is for measuring the cup (without a ball) softening point of resins. The heating rate is usually determined by the user. A heating rate of 1,0 °C/min is typical for resins.

If a correlation is desired between cup (without a ball) and ring-and-ball softening point as described in ISO 4625-1 or cup-and-ball softening point as described in ISO 4625-2, a specific heating rate for that correlation shall be determined for each type of resin.

Preset the starting temperature of the central processor from 20 °C to 25 °C below the expected softening point of the sample and maintain this temperature for a few minutes. Place the cartridge assembly in the measuring cell. With the instrument in the softening point mode, start the determination, which includes a waiting period of 30 s to allow temperature equilibration between the measuring cell and sample followed by an automatic temperature ramp at the specified rate in °C/min. Heating will be maintained at this rate until the softening point is reached, after which a steady temperature will be displayed on the readout. Record this temperature as the softening point of the sample.

Remove the cartridge assembly upon completion of the test. Check to ensure that the sample has passed the detector and no pretriggering has occurred. If a malfunction is suspected, reject the result and repeat the test. Inspect the apparatus closely to ensure that no particles of residue remain.

Clean the sample cup by soaking in a suitable solvent.

Care shall be taken in cleaning the sample cup in order not to scratch or alter the size of the cup orifice.

Clean the interior of the cell immediately after removing the sample assembly. Use soft cotton swabs moistened in an appropriate solvent.

Calculate the mean of two determinations and round it to the nearest 0,5 °C.

An interlaboratory comparison of the cup-and-ball softening point of seven resins with low and high softening points was run by 11 laboratories. Details of the round robin test are given in Annex A. The precision information given below is for the comparison of two test results.

Since there is no accepted reference material, method or laboratory suitable for determining the bias for the procedure in this test method for measuring the cup (without a ball) softening point, no statement on bias is being made.

The test report shall include at least the following information

a) the type and identification of the product tested;

b) a reference to this document, i.e. ISO 4625‑3:—;

c) the heating rate used;

d) the result of the test, as indicated in Clause 10;

e) any deviation from the test procedure specified;

f) any unusual features (anomalies) observed during the test;

g) the date of the test.

1. 
   (informative)
   
   Details of the interlaboratory comparison for the estimation of precision
   1. General notes on the interlaboratory comparison

An interlaboratory comparison (ILC) has been carried out in 2014 and 2015 for the determination of precision using the cup-and-ball softening point.

11 laboratories participated in the comparison.

• 1. Samples

For the interlaboratory comparison, seven resins with low and high softening points were tested – see Table A.1.

Table A.1 — Samples

• 1. Determinations

Five different determinations were carried out – see Table A.2.

Table A.2 — Determinations

• 1. Evaluation

The calculation of the precision could be done based on a sufficient number of measurement results obtained. Detailed results and calculated values see Table A.3.

Table A.3 — Results and calculated values<Tbl_--></Tbl_-->

Bibliography

[1] ISO 4625‑1, Binders for paints and varnishes — Determination of softening point — Part 1: Ring-and-ball method

[2] ISO 4625‑2, Binders for paints and varnishes — Determination of softening point — Part 2: Cup-and-ball method

[3] 21CFR – Code of Federal Regulations Title 21, Food and Drugs, U.S. Food and Drug Administration, 10903 New Hampshire Avenue, Silver Spring, MD 20993