Composite materials were created for usage as reinforcement and to protect the building envelope based on today’s global conditions such as climate change. Composite materials were manufactured using phenol-formaldehyde resin (case of resol) as a matrix, carbon fiber as reinforcement (7.5%
Considering it holistically, built heritage constitutes a living organism of global importance since it carries both the history of humanity and a landmark function for the future. At the same time, today’s global conditions such as climate change that disrupt past experience-based performance expectations of materials in a specific yet now rapidly changing localities, resulting, for example, in abrupt material degradation [
The insulation and the reinforcement of the building envelope in a way that is compatible with traditional construction are set as top priorities [
Composite materials typically consist of a thermosetting polymer matrix with glass fibers as reinforcement. In this form, they are applied in many sectors [
After the cross-linking of the macromolecules, the thermosetting resins—the cured—are in a solid state, and they are insoluble in solvents. The most common resins in this category are phenolic resins, epoxy resins, and polyester. Phenolic resins are produced by polycondensation reactions of phenol-formaldehyde through gradual polymerization with acid catalysts as is the case with novolac resins or base catalysts as is the case with resol resins. Modern technology exhibits increasing demands for composite materials with excellent mechanical properties; however, other properties are often required as well, such as thermal.
Materials that are used for thermal insulation usually have thermal conductivity coefficient (
The aim of the paper is to present materials that can help in the stability of the buildings and possess good mechanical properties as well as heat resistance properties which can safeguard the structural system of the buildings against high temperatures by providing adequate thermal insulation by protecting the building envelope. To achieve this purpose, composite materials were manufactured with polymeric matrix, which has high mechanical properties combined with good thermal resistance and satisfactory thermal insulation properties. To this objective, heat-resistant components are selected for the composite material, in particular, (a) phenol-formaldehyde resin (resol) as a matrix, (b) carbon fibers as a reinforcing agent, and (c) perlite as a component with a low thermal conductivity coefficient. Composite materials of different compositions and proportions of their components are manufactured, and pictures of the samples are taken through scanning electron microscopy (SEM) including elemental analysis (EDS). Furthermore, the mechanical properties of the specimens are measured in terms of flexural strength and shear resistance, while their heat resistance and their thermal conductivity coefficient are determined.
Τhe features that constitute the composite material help to protect the building envelope. Thermal resistance together with thermal conductivity will protect the building in extreme conditions such as extreme weather events. The typical building materials that are used disrupted in the presence of water or even moisture, and the consistency of materials is lost. By manufacturing a heat-resistant material and with good thermal conductivity, the building is protected by the presence of such phenomena. Also, mechanical properties keep the building safe from any displacements due to contraction and expansion of materials or even to some small seismic vibrations.
The resol resin used as a matrix to construct the composite material is laboratory-synthesized through phenol polycondensation (p.a., MERCK) with formaldehyde (p.a., Riedel-de Haen, solution 36.5%) and barium hydroxide (Fluka) as a catalyst. The carbon fibers used had a monofilament number of 3000 tex and a density of 1740 kg/m3. In addition, the perlite is a mineral silicate (70-75% SiO2, 12-15% Al2O3, 3-4% Na2O, 3-5% K2O, and below 1% for Fe2O3, MgO, and CaO) [
It is noted that the density of the respective materials is required to calculate the ratio of components of the composite material (by weight (
The scanning electron microscope used is the FEI QUANTA 200 model. Initially, all specimens were gold plated and then placed on a special sampler, which was inserted into the instrument at a suitable irradiation site under high vacuum and at a voltage of 15 kV. The analysis of the microstructure of the specimens was carried out in magnifications ranging from 100x to 5000x, while in selected samples, an energy distribution analysis (EDS) was performed for their qualitative and quantitative elemental analyses.
The mechanical properties of the composite materials, specifically bending strength and shear strength, were determined by the three-point method according to ASTM D790-71 and D2344-65T (BS EN ISO 14125: 1998). For the bending and shearing tests, the specimen is placed horizontally on a support span, and the load is applied to the center by the loading nose, a special dynamometer, which, by applying pressure, measures the resulting deflection in proportional indication. The result corresponds to a force in a table given by the instrument manufacturer, and then the bending strength
The heat resistance of each material was determined by weight loss after heating in a furnace. The heat resistance is determined separately in the individual components of the composite material as well as in the composite material. The furnace had been heated to 473 K and not higher since phenolic resins decompose and begin to lose their properties above this temperature. The weighed materials were placed therein in the presence of atmospheric air. The materials were weighed every 1 hour for 5 hours, and the weight loss of the materials was determined.
The coefficient of thermal conductivity of the composite material with 10%
The operating principle of the device for measuring the thermal conductivity coefficient
To manufacture the composite specimen resite resin, one direction carbon fibers and perlite were used as shown in Table
Compounds of resol resin–carbon fiber–perlite.
Composite material | Resol |
Perlite (% |
Carbon fiber (% |
---|---|---|---|
R | 100 | 0 | 0 |
R-CF7.5 | 100 | 0 | 7.5 |
R90-CF7.5-P10 | 90 | 10 | 7.5 |
R90-P10 | 90 | 10 | 0 |
R80-P20 | 80 | 20 | 0 |
R70-P30 | 70 | 30 | 0 |
R60-P40 | 60 | 40 | 0 |
The densities of the above materials were as follows: resite
Figure
SEM image of specimen with carbon fibers and perlite (R-P10-CF7.5) at magnification 400x.
SEM image focused on the breaking surface of the specimen with carbon fibers and perlite (R-P10-CF7.5) at magnification 1000x.
SEM image of carbon fibers (R-CF7.5) focused on the breaking surface where the carbon fibers are distinguishable.
Table
Elemental analysis of SEM-EDS for specimens of resite with carbon fibers and perlite (R-P10-CF7.5) and resite with carbon fibers (R-CF7.5).
Material | Figure | C | O | Na | Al | Si | Au | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
%wt | %at | %wt | %at | %wt | %at | %wt | %at | %wt | %at | %wt | %at | ||
R-P10-CF7.5 | 2 | 69.60 | 79.33 | 21.29 | 18.21 | 0.38 | 0.22 | 0.55 | 0.28 | 3.33 | 1.62 | 4.87 | 0.34 |
R-P10-CF7.5 | 3 | 73.18 | 84.28 | 17.00 | 14.70 | — | — | — | — | 0.79 | 0.39 | 9.02 | 0.63 |
R-CF7.5 | 4 | 78.25 | 85.00 | 18.10 | 14.76 | — | — | — | — | — | — | 3.65 | 0.24 |
Figures
Bending strength of resite (R) and composite materials of resite-perlite (R-P10), resite-carbon fibers (R-CF7.5), and resite-perlite-carbon fibers (R-P10-CF7.5). Variation of
Shear strength of resite (R) and composite materials of resite-perlite (R-P10), resite-carbon fibers (R-CF7.5), and resite-perlite-carbon fibers (R-P10-CF7.5). Variation of
In the case of composite materials containing carbon fibers, the perlite (R-P10-CF7.5) reduces the flexural strength by 7% relative to that of composites without perlite (R-CF7.5). Accordingly, Figure
Figures
Thermal behavior at 473 K for 5 h of perlite and resite-perlite samples at various percentages of perlite.
Thermal behavior at 473 K for 5 h of materials: resite, carbon fiber, and composite material R-P10-CF7.5.
Based on Figure
Weight loss of the composite material of peroxide with additional perlite at various rates, at 200° C, as a function of the content of the resite.
The experimental points were adapted to the exponential function:
Table
Coefficient of thermal conductivity of composite material of phenolic resin– (resite–) carbon fibers–perlite in comparison with other organic thermal-insulating materials.
Material | Polymeric matrix | Additive | Carbon fibers (% |
Coefficient of thermal conductivity | |
---|---|---|---|---|---|
Resol (R) |
Perlite (% | ||||
R-CF7.5-P10 | 90 | 10 | 10 ( | 0.16 | |
Cured phenolic resin | 100 | 0 | 0 | Typ 31 |
0.31 [ |
PU | 100 | 0 | 0 | Solid |
0.25 [ |
Extruded polystyrene | 100 | 0 | 0 | DOW | 0.035 [ |
The value of the thermal conductivity coefficient of the composite material (R-CF7.5-P10) is 4.5 to 1.5 times lower than the thermosetting polymeric materials, while it is 4.5 to 3.2 times higher than the typical foamed heat-insulating polymeric materials. Therefore, the specific composite material is located approximately in the middle between the two categories of materials on the basis of the value of
Nowadays, one must apply two individual processes with independent and separately procured materials in order to secure the thermal protection and the reinforcement of a building envelope, respectively, adding in this way some additional load to the building. Here, a new material is shown, namely, one which can concurrently have both properties, i.e., both high strength and resistance to thermal loads, and thus, it can serve both purposes at the same time.
The replacement of a quantity in the resite matrix with perlite (10% The SEM images and the SEM-EDS elemental analysis of the specimen R-P10-CF7.5 show the presence of perlite on the surface, while at the rupture surface of the composite material, carbon fibers dominate, and the presence of silicon is limited The addition of 10%wt of perlite (R-P10) increases the strength of the material in bending and shearing. In the case of composite material that is fiber reinforced, it is found that the perlite involvement reduces the bending and shear strength due to the additional interfaces created in the composite material mass. Perlite is enhancing the matrix, while the fibers enhance much more than the granular material (perlite) Regarding the heat treatment of materials at 473 K, carbon fibers have the smallest weight loss, while perlite suffers less weight loss, closely followed by the composite, with a proportion of 60-40 wt, with a value of 1% for 5 hours. The resite matrix exhibits the greatest weight loss of all materials at 8.3% for 5 hours. In fiber-reinforced composite materials, the smallest weight loss can be found in the one containing perlite, which confirms the stabilizing role of the substance. The graph of the weight loss of the composite material with perlite (no carbon fibers), as a function of the content of the resite, follows an exponential function, where for a low percentage %wt (60% From the coefficient of thermal conductivity of the composite material (R-CF7.5-P10), it was found that, on the one hand, it is located approximately at the middle between the two categories of materials (thermoset and typically foamed heat-insulating polymeric materials). Base on the For these composites, modifying the proportions of their components and the technique to create a foam matrix, the thermal conductivity coefficient is expected to be further reduced as a typical heat-insulating material Such materials can help in the stability of the modern cultural buildings due to their mechanical properties; and their heat resistance properties safeguard the structural system of the buildings against high temperatures providing adequate thermal insulation. As such, composite materials can play a vital role in keeping the legacy alive both as a reminder of the past as well as a live and constitutive part of our modern world
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The authors declare that they have no conflicts of interest.
We would like to thank Professor Johannis Simitzis for his contribution in the evaluation of the experimental results.