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ISO 305 Thermal stability test by discoloration method

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ISO 305 Plastics — Determination of thermal stability of poly(vinyl chloride), related chlorine-containing homopolymers and copolymers and their compounds — Discoloration method


ISO 305 to determine the thermal stability of poly(vinyl chloride), related chlorine-containing homopolymers and copolymers, and their compounds (collectively referred to as PVC) by assessing the extent of discoloration when exposed to elevated temperatures.


Test Principle and methods: 

The core principle is straightforward: test specimens in the form of thin sheets are exposed to a controlled, elevated temperature for varying durations. As the PVC degrades, it undergoes a visible color change. By recording the time it takes for the first noticeable discoloration to appear, the material's resistance to heat degradation is determined.

Method A (Oil-bath method): A simple, low-cost method where specimens are heated in a temperature-controlled oil bath. The specimens are sandwiched between an aluminum block and cylinder to ensure good heat transfer and limit air exposure, making it nearly anaerobic .

Method B (Forced-air oven method): Requires a forced-air circulation oven. Specimens are laid on aluminum foil supports. This method introduces oxygen, which can accelerate degradation differently than Method A. It is not suitable for materials that might cross-contaminate each other via fumes in the shared oven air.


Test Equipment required for ISO 305

Method A (Oil-bath)

A thermostatically controlled oil bath capable of maintaining the target temperature .

Glass test tubes (external diameter 18 mm, wall thickness 1.2 mm, length min. 150 mm) .

Aluminum blocks and aluminum cylinders (15 mm diameter, 30 mm height) to sandwich the specimen .

A precise timing device (e.g., stopwatch) .

Method B (Oven)

A forced-air-circulation oven with a calibrated thermometer and adjustable airflow (typically around 0.3 m³/min) to ensure uniform temperature distribution .

Removable oven racks (grilles) lined with new, clean aluminum foil to support the specimens .

A precise timing device .


Test Specimen Information

Shape & dimensions:

Method A: Discs, ⌀14 mm, thickness ~1 mm

Method B: Squares, 15 mm side, thickness ~1 mm

Preparation: Punched from sheet; granules/powder/pellets are sheeted on a roll-mill; plastisols are gelled into fused sheets.

Quantity: Test duration (min) ÷ 5; for high-stability materials, use 10–15 min intervals before discoloration starts.

Treatment: Press-polishing is optional for good contact; warm unplasticized sheets before cutting to avoid shattering.


Key Test Parameters

Default test temperature: 180 °C (if no specification or agreement)

Target test duration: 60–120 min

Temperature control: ±0.5 °C

Sampling interval: Normally 5 min; 10–15 min for highly stable materials

Specimen thickness: ~1 mm (critical for consistent results).


Test Procedures

General Setup: Prepare the required number of specimens from a homogeneous sheet. Set the heating apparatus (oil bath or oven) to the target temperature (e.g., 180°C) .

Method A (Oil-bath) Steps:

Place an aluminum block into a test tube, add the PVC specimen, and top it with an aluminum cylinder .

Immerse the tubes in the oil bath and start the timer .

Every 5 minutes, remove one tube, extract the specimen, let it cool, and label it with the exact heating time .

Method B (Oven) Steps:

Place specimens on aluminum foil-lined racks .

Insert the racks into the preheated oven and start the timer. Keep the door closed as much as possible .

At regular intervals, remove a rack with specimens, label them, and continue until all specimens are collected or blackening occurs .

Post-Test: All specimens are mounted on a card noting the exposure time and temperature. They must be stored in the dark. The time to the first color change is recorded.


Test Applications (Industry Fields)

This test is fundamental to any industry that processes or utilizes PVC. Key fields include:

Building & Construction: Evaluating the thermal stability of PVC window profiles, pipes, and siding during high-temperature extrusion processes.

Automotive: Testing PVC components like dashboards, seals, and cable insulation for heat resistance.

Wire & Cable: Determining the suitability of PVC compounds for electrical insulation that may generate heat during operation.

Medical Devices: Ensuring PVC tubing and blood bags can withstand sterilization temperatures without degrading.

Consumer Goods: Assessing the durability of PVC toys, blister packaging, and credit cards.

Related products and device

ISO 305 Thermal stability test heating oven

Electricity hot blast drying oven is mainly used to measure the dimension change of plastic material after heating, suitable for plastic pipe, plastic sheet etc.

ISO 182-1 PVC compounds thermal stability tester

Thermal stability testing machine, suitable for high-temperature testing of plastic products and cable insulation sheaths in oil. The temperature control system adopts microcomputer control with PID function, which ensures high control accuracy and good stability;

Related Standard

ISO 182-1 PVC compounds thermal stability test Congo red method

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 PVC compounds  thermal stability test Congo red method

IS: 4669 Clause 12 thermal stability test Congo red method Methods of test for polyvinyl chloride resins 


This test method is intended to determine the thermal stability of a PVC compound when processed at a high temperature. The method is applicable to all PVC compounds, copolymers and products based on them. A test specimen of the PVC material is heated at a defined temperature, normally 180°C.

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