Information on the most widely used ASTM standards within the materials testing industry
ISO 182-1, IEC 60811-3-2, IS: 4669, PVC compounds thermal stability test Congo red method electric and optical cable. The determination is carried out on a sample of the PVC compound which is maintained at an agreed temperature in still air until the colour of a Congo red paper held above it changes from red to blue. If universal indicator paper is used, the colour change is that corresponding to pH 3.
ISO 182-1 specifying the Congo Red method for determining the thermal stability of PVC (polyvinyl chloride) compounds and products at elevated temperatures by measuring their tendency to evolve hydrogen chloride (HCl) gas through dehydrochlorination.
Test Methods and Principle
Method: A simple, rapid, qualitative-to-semi-quantitative test that monitors the evolution of HCl gas from heated PVC samples using Congo red indicator paper (or universal indicator paper as an alternative).
The principle relies on acid-induced color change of indicator paper:
A test portion of PVC is heated in still air at an agreed temperature. A strip of Congo red paper (or universal indicator paper) is suspended 25 mm above the sample. As PVC undergoes dehydrochlorination (releases HCl), the paper’s color changes -- from red to blue (Congo red) or to the pH 3 color (universal indicator). The time to this change is defined as the stability time(ts), which reflects the material’s thermal stability.

ISO 182-1 test specimen information:
Sample types: PVC compounds, pellets, extrudates, moldings, sheets, films, and other products based on vinyl chloride homopolymers and copolymers.
PVC plastisols: Gel into 0.5 mm-thick sheets (oven), then cut into ~2 mm squares.
Pellets/extrudates/mouldings/thick sheet: Grind/cut to pass 2.0 mm sieve (ISO 565, R 20/3 series) with >80% material, retaining some on 1.4 mm sieve.
Film/sheet: Cut into squares/cubes with sides ≤2 mm.
Coatings: Separate from substrate, then process as above.
Cable insulation/sheathing: Cut into thin slivers.
ISO 182-1 Congo Red method testing equipment and device:
A PVC compounds thermal stability tester is required:
| Equipment Component | Specifications | Purpose |
|---|---|---|
| Heating apparatus | Thermostatically controlled oil bath or metal block, capable of maintaining 170°C or 210°C (depending on PVC type) with ±0.5°C accuracy; Capacity ≥10 L. | Uniform heating of test specimens |
| Test tubes | Borosilicate glass, External diameter ~17 mm, wall thickness ~0.4 mm, length ≥150 mm; fitted with stoppers containing holes for glass tubes.
| Contain the PVC sample |
| Glass tubes | Internal diameter 2–3 mm, length ~100 mm (holds indicator paper). | Position indicator paper above the sample and prevent contamination |
| Heat-resistant insulating shield | Drilled to hold test tubes, positioning them 50 mm deep in the oil bath. | |
| Congo red paper | pH-sensitive indicator (red to blue at pH < 3); paper width 10mm, length 30mm.
| Detect HCl gas evolution |
| Analytical balance | Precision of ±0.001 g | Weigh test portions accurately |
| Sieves | 1.4 mm and 2.0 mm mesh sizes | Prepare homogeneous powder samples |
Test Procedure:
Sample loading: Fill two test tubes (5.1) with prepared specimen to ~50 mm depth; shake gently to avoid compaction.
Oil bath preheating: Heat to near test temperature, adjust to exact value with thermometer (5.5).
Indicator setup: Cut 30×10 mm indicator paper (4.1/4.2), fold one end, insert into glass tube (5.2). Wet with ISO 3696 Grade 2 water. Insert tube into stopper, then into test tube—position paper 25 mm above sample.
Immersion & timing: Submerge test tubes in oil bath (to sample top), start stopclocks.
Endpoint detection: Stop clock at first clear red→blue (Congo red) or pH 3 color (universal indicator).
Slow change protocol: For vague color shifts, record two times (red→violet, violet→blue) using 4 stopclocks.
ISO 182-1 Test Application (Industry Fields):
PVC manufacturing: Quality control during production of compounds and resins
Plastics processing: Extrusion, injection molding, calendering operations to verify material stability before processing
Product sectors:
Construction (PVC pipes, profiles, fittings)
Electrical (cable insulation and sheathing)
Automotive (interior trim, seals)
Medical devices (non-critical applications)
Packaging (rigid and flexible films)
R&D: Evaluation of thermal stabilizer effectiveness and formulation optimization
Related test standard:
| IEC 60811-3-2 | Insulating and sheathing materials of electric and optical cables - Common test methods - Part 3-2: Methods specific to PVC compounds - Loss of mass test - Thermal stability test; |
| IS 4669 | Methods of test for polyvinyl chloride resins; Clause 12 Congo red Thermal stability test |
| SATRA TM324 | Thermal stability of PVC - Congo red test |
| DIN 53381-1 | Testing of plastics; determination of thermostability of polyvinyl chloride (PVC); dehydrochlorination methods |
| GB/T 2917.1 | Determination of compounds and products based on vingl chloride homopolymers and copolymers to evolve hydrogen chloride and any other acidic products at elevoted temperatures--Congo red method |
Related products and device
FAQs about ISO 182-1 (Congo Red Method for PVC Thermal Stability)
Q1. Why is the ISO 182-1 test important?
The test is primarily designed for simple and rapid quality control during the manufacture and conversion of PVC compounds. It is also used for characterizing PVC compounds and products. A key advantage is its suitability for coloured PVC materials—where discoloration tests (e.g., visual inspection) may be unreliable. Additionally, it focuses on compounded PVC materials/products (not dry blends, which lack homogeneity).
Q2. What reagents and apparatus are required?
Reagents:
Congo red indicator paper (commercial or homemade via 0.15% Congo red in methanol, dried);
Universal indicator paper (pH 1–10, with a color scale).
Apparatus:
Test tubes (Ø≈17 mm, wall≈0.4 mm, length≥150 mm) with stoppers;
Small glass tubes (ID 2–3 mm, length≈100 mm);
Oil bath (capacity≥10 L, operating range 170–210°C, accuracy±0.5°C);
Heat-resistant insulating shield (drilled for test tubes);
Thermometer (range 170–210°C, division 0.1°C);
Stopclocks (2 for standard tests; 4 for slow color changes).
Q3: Why do my test results differ greatly between repeated tests?
A: Common causes include uneven sample particle size, unstable heating bath temperature, indicator paper touching the sample, inconsistent test tube immersion depth, or poor sample homogeneity.
Q4: What is the difference between ISO 182-1, ISO 182-2, and ISO 182-3?
A:ISO 182-1: Congo red indicator method (qualitative/semi-quantitative, fast)
ISO 182-2: pH indicator method (similar principle, different detection)
ISO 182-3: Conductometric method (more accurate, quantitative HCl measurement)
Q5: What happens if a PVC material fails this test (short stability time)?
A: It will degrade rapidly during processing, causing discoloration, loss of mechanical properties, corrosion of equipment, and release of harmful HCl gas, leading to scrap products and safety risks.
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