Determination of the Combustion Heat Value of Silicone Coatings

The combustion heat value (gross heat of combustion) of a silicone coating defines how much energy the material releases when it burns completely, and it is a decisive parameter in the fire performance classification of coated products. Determination is typically performed by oxygen bomb calorimetry in accordance with ISO 1716, GB/T 14402, and equivalent standards, providing objective calorific value data in MJ/kg for quality control, certification, and flame-retardant formulation development. This article explains the test principle, applicable standards, sample preparation, measurement procedure, result calculation, typical value ranges, and common applications of silicone coating combustion heat value testing.

Technical Overview

Silicone coatings are film-forming materials based on polysiloxane chemistry, in which an inorganic silicon–oxygen backbone carries organic substituents such as methyl and phenyl groups. This hybrid molecular structure gives silicone coatings outstanding heat resistance, weatherability, and flexibility, and it also produces an intrinsically different combustion behavior compared with purely organic coating systems.

The combustion heat value, also known as the gross heat of combustion (PCS) or gross calorific value, is the energy released per unit mass when a material burns completely in oxygen under standardized conditions, with all combustion water condensed to the liquid state. It is expressed in megajoules per kilogram (MJ/kg). Because the siloxane backbone converts to silica (SiO2) during combustion, and because many silicone coatings contain mineral fillers such as aluminum hydroxide (ATH) or magnesium hydroxide (MDH), their calorific values are generally well below those of hydrocarbon-based binders.

Typical unfilled silicone binders exhibit gross heat of combustion values of approximately 20–26 MJ/kg, while heavily filled flame-retardant or fire-protection grades may fall below 10 MJ/kg. Conventional epoxy, polyurethane, acrylic, or alkyd systems usually range from about 30 to 42 MJ/kg. These differences make calorific value determination an efficient indicator of both the inorganic character and the fire-safety potential of a silicone coating formulation.

The principal test methods are ISO 1716 (EN ISO 1716) and its Chinese equivalent GB/T 14402, which specify the oxygen bomb calorimeter procedure for building products. Related bomb calorimetry methods include ASTM D240 for liquid fuels and ISO 1928 for solid mineral fuels. Results obtained under ISO 1716 are used directly in fire classification systems such as EN 13501-1 and GB 8624, and in material qualification schemes for rail, marine, cable, and electrical applications.

Testing Samples

Cured Free-Film Silicone Coatings

The most common test object is a fully cured coating film. The liquid coating is applied onto a release substrate such as polyethylene or PTFE sheet, cured according to the manufacturer's schedule, peeled off as a free film, and cut into small pieces. This form reflects the true calorific value of the applied and crosslinked coating layer as it exists in service.

Liquid and Uncured Silicone Coating Materials

Liquid products may be tested directly by weighing the specimen into a combustible gelatin capsule or onto a combustible support of known calorific value. The heat contribution of the capsule is determined separately and subtracted during calculation. Alternatively, the liquid is film-cured first and the cured film is tested, which is usually preferred when the result supports a classification.

Coating Removed from Coated Products

When the object of evaluation is a finished coated product, the coating layer can be carefully scraped or milled from the substrate and homogenized before testing. Care must be taken to avoid contamination with substrate material, since even small amounts of metal, mineral wool, or gypsum would bias the measured value downward.

Filled, Flame-Retardant, and Intumescent Silicone Systems

Highly filled coatings contain a large fraction of non-combustible solids and may be difficult to ignite completely. For such materials, a weighed combustion aid of known calorific value, for example benzoic acid or paraffin oil, is added to promote complete combustion, and its heat contribution is subtracted from the measured total.

Sample Quantity and Conditioning

Each firing typically consumes 0.3–1.0 g of specimen, and a complete test including repeats usually requires 10–50 g of homogenized material. Specimens should be conditioned to constant mass, typically at 23 ± 2 °C and 50 ± 5 % relative humidity, so that residual moisture does not distort the measured heat value.

Testing Items

The core deliverable of the test is the gross heat of combustion of the silicone coating material. A typical test program and report cover the following items:

• Gross heat of combustion at constant volume (PCS), in MJ/kg, for the cured coating film or the liquid material as delivered.
• Net heat of combustion (PCI), if required, corrected for the latent heat of vaporization of the combustion water.
• Calorific values of individual significant components for non-homogeneous products, where classification requires each layer to be assessed separately.
• Individual determinations, their arithmetic mean, and the observed scatter across a minimum of three valid firings.
• Observations of ash or silica residue, evidence of complete combustion, and all corrections applied for capsules, combustion aids, and ignition energy.

Testing Methods

Principle of Oxygen Bomb Calorimetry

A weighed specimen is burned in a sealed stainless-steel bomb charged with oxygen at 3.0 MPa (30 bar). The bomb is immersed in a known mass of water inside a calorimeter vessel, and the temperature rise produced by the combustion is measured with high-resolution sensors. Because the effective heat capacity of the calorimeter system has been established by calibration, the released energy, and hence the heat of combustion per unit mass, is calculated from the corrected temperature rise.

Apparatus and Calibration

The equipment comprises a combustion bomb with ignition electrodes, a sample crucible, a calorimeter vessel with stirrer, an isoperibol or adiabatic jacket, precision temperature measurement, and an ignition circuit. Calibration is performed with certified reference benzoic acid, whose certified heat of combustion is typically 26,434 J/g, to determine the effective heat capacity of the instrument. Calibration is verified at defined intervals and after any maintenance, and the calibration status is documented as part of the quality system of accredited third-party laboratories.

Test Procedure

• Homogenize and condition the specimen, determine its moisture state, and record whether results are reported on an as-received or dry basis.
• Weigh approximately 0.5 g of specimen to the nearest 0.1 mg into the crucible, adding the capsule or combustion aid if required.
• Attach the ignition wire, for example platinum or nickel-chromium, in contact with the specimen, together with a cotton ignition thread where specified.
• Assemble the bomb, flush it, charge it with oxygen to 3.0 MPa, and verify leak tightness.
• Place the bomb in the calorimeter vessel, allow the system to equilibrate, fire the charge, and record the complete temperature-time curve.
• After each run, inspect the bomb interior for unburned residue or soot; any run showing incomplete combustion is rejected and repeated.
• Perform at least three valid determinations and calculate the mean gross heat of combustion.

Calculation and Corrections

The gross heat of combustion at constant volume is calculated as the calorimeter heat capacity multiplied by the corrected temperature rise, minus the heat contributions of the ignition energy, the capsule or combustion aid, and the nitric acid formed where required, divided by the specimen mass. If the result is reported on a net basis, the energy required to vaporize the water formed in combustion is subtracted. Final results are reported to the nearest 0.05 MJ/kg or 0.1 MJ/kg, as required by the product specification or classification scheme.

Precision and Quality Control

For materials in the range relevant to silicone coatings, repeatability within a single laboratory is typically about 0.1 MJ/kg or better, while reproducibility between laboratories is of the order of 0.2–0.3 MJ/kg. Control charts on reference materials, duplicate testing, and complete records of calibration and corrections are essential elements of a technically defensible result.

Applications

Combustion heat value data for silicone coatings are used in several important technical and regulatory contexts:

• Fire performance classification of construction products under EN 13501-1 and GB 8624, where gross heat of combustion limits of 2.0 MJ/kg (class A1), 3.0 MJ/kg (class A2), and 4.0 MJ/kg (classes B, C, D) apply to homogeneous products and significant components.
• Material qualification for rail vehicles under EN 45545-2 and for marine and offshore installations, where the calorific content of interior surface materials and components feeds into fire hazard assessment.
• Research and development of flame-retardant silicone coatings, where the measured heat value quantifies the effectiveness of mineral fillers such as ATH and MDH and of intumescent additive packages.
• Quality control of incoming silicone binders and finished coating batches, and equivalence evaluation when changing binder grades, filler systems, or suppliers.
• Certification and regulatory documentation, with testing performed by accredited third-party laboratories against the standard required by the target market.

Because silicone coatings combine moderate combustibility with strong char and silica residue formation, a low calorific value is frequently one of the key technical arguments used to position them as fire-resilient protective coatings for steel structures, tunnel linings, cable trays, and electrical insulation systems.

FAQ

Which standards apply to the combustion heat value determination of silicone coatings?

ISO 1716 and its harmonized versions such as EN ISO 1716 and GB/T 14402 are the primary standards for building products and are the methods cited by EN 13501-1 and GB 8624. ASTM D240 describes an equivalent bomb calorimetry procedure used for liquid hydrocarbon materials, while ISO 1928 covers solid mineral fuels. The standard selected should always match the specification or regulation that the result must satisfy.

How is a liquid silicone coating sampled and tested?

The liquid material is either weighed into a combustible gelatin capsule whose calorific value is subtracted afterwards, or it is first applied and cured into a free film and tested as a solid. For the classification of a coated product, testing the cured film is generally preferred because it represents the material exactly as it exists in the end-use configuration.

What is the difference between gross and net heat of combustion?

The gross value (PCS, higher heating value) assumes that all water formed during combustion condenses and releases its latent heat, whereas the net value (PCI, lower heating value) assumes the water leaves the system as vapor. Fire classification systems such as EN 13501-1 use the gross heat of combustion.

How do mineral fillers influence the result?

Mineral fillers such as aluminum trihydrate, magnesium hydroxide, silica, and glass microspheres are essentially non-combustible. They dilute the combustible polymer fraction, and reactive fillers such as ATH and MDH additionally absorb heat through endothermic dehydration. Consequently, filler loading directly lowers the measured calorific value, and highly filled fire-protection silicones can reach single-digit MJ/kg values.

How much sample is needed and how accurate is the test?

A single firing uses roughly 0.5 g of specimen, and a full test with repeats is comfortable with 10–50 g of material. With a properly calibrated bomb calorimeter and verified complete combustion, repeatability of about 0.1 MJ/kg is achievable, which is adequate for the 0.5 and 1.0 MJ/kg steps used in classification thresholds.

Can the test alone prove that a coating is flame retardant?

No. A low heat of combustion demonstrates limited fuel content, but a full fire performance assessment also requires reaction-to-fire testing such as the single burning item test (EN 13823), the small flame ignitability test (EN ISO 11925-2), and cone calorimetry (ISO 5660) where applicable. Calorific value is one component within this wider testing framework.

What should a professional test report contain?

A report should identify the sample and its curing condition, the test standard and apparatus used, the calibration status, the individual and mean PCS values in MJ/kg, all corrections applied, a statement on combustion completeness, and the basis (as-received or dry) of the result. Reports from accredited third-party laboratories additionally carry the accreditation mark and full traceability information for the measurement chain.