Specific heat (or specific heat capacity, indicated as c or cₚ in the case of constant pressure) is the amount of energy required to increase by one kelvin the temperature of one kilogram of material.
This quantity measures the ability of a material to store heat: materials with high specific heat require more energy to change their temperature and, under the same heat flux, heat up and cool down more slowly.
Unlike thermal conductivity (λ), which describes the rate at which heat passes through a material, specific heat describes the amount of energy that the material can store. These two properties, together with density (ρ), determine the thermal diffusivity of the material, which expresses the rate at which a temperature variation propagates within the material.
Specific heat is an intrinsic property of the material: it depends on chemical composition and structure (crystalline or amorphous) and may vary as a function of temperature, particularly near phase transitions or structural transformations.
The relationship between exchanged energy, mass and temperature variation is:
Q = m · cₚ · ΔT
where:
Q = exchanged heat [J] | m = mass of the material [kg] | ΔT = temperature variation [K]
From this relation, specific heat can be obtained as: cₚ = Q / (m · ΔT)
The unit of measurement in the International System is J/(kg·K). In some contexts, it is also expressed in kJ/(kg·K) or in cal/(g·°C), a unit of the CGS system historically used in calorimetry.
The distinction between specific heat at constant pressure (cₚ) and specific heat at constant volume (cᵥ) is relevant for gases, while for solids and liquids - materials of technical and construction interest - the difference is negligible and cₚ is conventionally used.
Specific heat is part of a set of interconnected thermophysical properties that define the thermal behavior of materials. The following table shows the relationships:
The product ρ · cₚ is called volumetric heat capacity [J/(m³·K)] and represents the energy required to increase by one kelvin the temperature of one cubic meter of material. This quantity is a fundamental parameter for the evaluation of thermal inertia and the dynamic behavior of materials, both in the building sector and in industrial applications.
Specific heat varies significantly across different classes of materials. Metals, such as steel and iron, exhibit relatively low values (approximately 450–500 J/(kg·K)), while many polymeric materials - such as polyethylene (PE), polypropylene (PP), and polyurethane (PUR) - show significantly higher values, typically ranging from 1,200 to 2,000 J/(kg·K).
For the same mass, polymeric materials are therefore able to store a greater amount of thermal energy and exhibit slower temperature variations compared to metals.
In building and industrial applications, this difference has relevant implications:
In layered components or insulating elements, polymers may contribute to the overall thermal inertia, despite generally having lower density compared to massive materials.
The experimental determination of specific heat requires calorimetric techniques capable of accurately measuring the thermal energy exchanged by the sample. The reference method, both in research and industrial contexts, is Differential Scanning Calorimetry (DSC), standardized by UNI EN ISO 11357-4 for polymeric materials and extendable to a wide range of solid materials.
The principle of DSC consists in subjecting a sample and a reference to a controlled thermal program (heating, cooling, or isothermal) and measuring the difference in heat flow between them. Analysis of the DSC signal allows determination of specific heat as a function of temperature, identification of phase transitions, detection of exothermic or endothermic phenomena, and analysis of structural changes in the material.
DSC is complementary to Light Flash Analysis (LFA): while LFA directly measures thermal diffusivity, the value of specific heat (cₚ) obtained by DSC allows indirect determination of thermal conductivity:
λ=α⋅ρ⋅cp;
The integration of these techniques - LFA for α, DSC for cₚ, and density (ρ) determination by pycnometry or geometric methods - enables complete thermophysical characterization of the material within an integrated experimental campaign, ensuring consistency and metrological traceability among the measured quantities.
An advanced variant is Modulated DSC (MDSC), in which a sinusoidal temperature modulation is superimposed on the thermal signal. This technique allows separation of the reversible contribution (related to specific heat) from the non-reversible one (associated with kinetic phenomena such as relaxations or reactions), improving resolution in the analysis of complex transitions, particularly in polymeric and composite materials.
In addition to the determination of specific heat, DSC allows identification of thermal phenomena relevant for material characterization, particularly in polymers, such as the glass transition and the corresponding glass transition temperature (Tg). This transition appears as a shift in the baseline of the DSC signal and does not represent a phase change, but rather a transition between a rigid (glassy) state and a more deformable (rubbery) state, associated with a change in specific heat.
The glass transition temperature (Tg) is a fundamental parameter for polymer matrix composites (FRP / FRPC), as it defines the upper operating limit of the matrix. Below Tg, the matrix maintains a rigid behavior and ensures effective stress transfer to the fibers; above Tg, a significant reduction in elastic modulus, an increase in viscoelastic deformation, and a decrease in fiber–matrix bonding effectiveness are observed.
For structural applications, the operating temperature must be maintained with an adequate margin below Tg, typically in the range of 20–40 °C, depending on the type of resin and operating conditions.
Specific heat is a relevant parameter in design and qualification phases in numerous application contexts:
Certimac performs specific heat determination by Differential Scanning Calorimetry (DSC) as part of its thermal and thermophysical analysis services. Tests are carried out in accordance with ISO 11357-4 and ASTM E1269, using calibrated instrumentation and in compliance with ISO 17025 accreditation requirements issued by ACCREDIA.
The measurement of cₚ is inherently integrated with other thermophysical tests carried out by the laboratory: thermal diffusivity measured by Light Flash Analysis (LFA) and sample density allow indirect determination of thermal conductivity (λ), completing the thermophysical characterization of the material within a single, consistent experimental campaign. This integrated approach is particularly suitable for innovative materials, composites, and samples for which direct measurement of λ by steady-state methods is complex or not applicable.
Results are issued in the form of a Test Report, usable for product qualification, R&D validation, failure analysis, and support for CE marking.
For further information on test methods and services available at the Certimac thermal analysis laboratory, please visit the dedicated page, contact our experts by filling in the form at the bottom of the page, or write to materiali@certimac.it.
