Temperature and strain characteristics of ceramic-reinforced polyurethane insulation decorative board under artificial accelerated aging conditions

As a new type of thermal insulation material, ceramic-reinforced polyurethane insulation decorative board (IDB) has been applied to a certain extent, but changes in the temperature and strain between this material and the substrate wall during the aging process remain unclear. In this study, an external insulation composite system with ceramic-reinforced polyurethane IDB was subjected to 60 heat-rain cycles, and the temperature and strain at the interface between the IDB and the substrate wall were measured. The results show that the temperature of the substrate wall is about 35 °C lower than the highest temperature of the environmental box, and the temperature fluctuation of the substrate wall during 60 heat-rain cycles always maintains at about 5 °C, which indicates that the ceramic reinforced polyurethane IDB has good thermal insulation performance and durability. During the heat-rain cycles, the vertical strain varies from −750 με to + 2750 με, while the horizontal strain varies from −250 με to + 500 με, indicating that the horizontal and vertical strains have serious inhomogeneity. This local inhomogeneity may be an important reason for the aging and cracking of the IDB. The characteristics of temperature and strain could provide a reference for analyzing the synergistic effect and disbonding mechanism of the interface between the IDB and the substrate wall, and give some guidance for the design and construction of external insulation composite systems.


Introduction
With the increasing energy crisis, high efficiency and energy savings have become important factors in the development of the current era [1].The construction industry accounts for approximately 40% of global energy consumption, and commercial and residential buildings account for approximately 33% of total energy-related greenhouse gas emissions [2], so building energy conservation and emission reduction play important roles.At present, the energy savings of buildings mainly depend on the building enclosure system, and the energy-saving insulation of the enclosure system mainly depends on the form of the composite insulation wall.According to the different relative positions of insulation materials, the building envelope can be divided into three forms: external insulation [3,4], internal insulation [5,6] and external sandwich insulation [7,8].Internal wall insulation occupies valuable indoor space, and there is a problem with hot passages [9,10].External wall sandwich insulation has problems such as wall joints, reliable connections with the main structure and safety issues during use [11].After many studies, it was found that external insulation composite systems have the advantages of easing the thermal bridge, reducing the risk of condensation on the inner surface of the wall, protecting structural components [12], improving construction efficiency and reducing construction costs, and this type of system is an important means to achieve high efficiency and energy savings of buildings and improve living comfort [13,14].However, external insulation composite systems cause a series of problems [15] due to the climatic environment and load during service, such as the hollow drum of the finishing layer, cracking [16,17], degradation of the thermal insulation performance of the thermal insulation layer [18], and debonding of the thermal insulation layer and the base layer [4], resulting in partial or overall failure of the external insulation system.In serious cases, there are safety problems, resulting in loss of life and property [19].
Most traditional external insulation composite systems fix the insulation material onto a wall through binders or anchoring parts and then establish a protective layer and a decorative layer outside the insulation layer [20].This construction technology increases the construction period, and the construction quality is difficult to guarantee, so there are certain hidden dangers [21].To reduce the influence of human factors on the external insulation composite system, at present, by optimizing the structural form, the protective layer and thermal insulation layer are integrated into a whole to form a so-called insulation decoration integrated board (IDB), which provides product functions such as assembly and information [22].Since the installation of the system can be completed with very little on-site manual construction, thus greatly improving the integrity and stability of the system, IDBs have been rapidly developed and used in the field of external insulation in recent years [23].
At present, commonly used insulation materials include polyurethane [24], glass wool [25], and expanded perlite [26].As a kind of polymer compound, polyurethane is widely used in external insulation composite systems because of its good stability, chemical resistance, thermal insulation and resilience [27].Ceramic has the advantages of high temperature resistance, corrosion resistance, high hardness, low creep, etc, and is widely used in many fields [28].The IDB combines polyurethane and ceramics into a composite structure and is applied to the external insulation system.Using polyurethane to improve the insulation performance, ceramic panels serve as protective and decorative layers for polyurethane.The combination of them can fully leverage the advantages of the two materials, which is very significant for improving the energy-saving level of buildings.However, there are few studies on ceramic-enhanced IDBs in the field of building exterior insulation, especially on the influence of external climate in the process of service.In fact, under the influence of temperature, humidity, rain and other factors in the external environment, the IDB will still produce performance degradation which affects the performance of the entire insulation system [29].The influence of the performance degradation is more manifested on the interface between the IDB and the substrate wall [30], which may lead to the debonding.Therefore, the study of the synergistic effect between the IDB and the substrate wall interface under the influence of external climate is of great significance.In this paper, the temperature and strain of the external insulation system with a ceramic-reinforced polyurethane IDB in the process of heat-rain cycles were measured during an artificial accelerated aging test.Through the analysis of the temperature field and the horizontal and vertical strain evolution characteristics at the interface between the IDB and the substrate wall, the weather resistance of the ceramic-reinforced polyurethane IDB system was determined, providing a basis for testing the design and durability of the external insulation composite system with IDB.

Materials
The ceramic-reinforced polyurethane IDBs used in this test was were composed of an insulation layer and a bonded finishing layer, as shown in figure 1.The insulating layer consisted of polyurethane foam composed of polyether resin as the main raw material and foamed with isocyanate, water, catalyst, foam stabilizer, etc.The density was 140 kg m −3 , the thermal conductivity was 0.044 W/(m•K), the closed porosity was over 95%, and the tensile bond strength was 0.16 MPa.The water absorption capacity was 2.5 g m −2 , and the combustion level was Grade A. The surface of the polyurethane insulation layer was reinforced with fiber mesh cloth, and the ceramic panel bonded on the outside of the insulation layer was reinforced and decorated.With clay and quartz stone as the main raw materials, the ceramic body is obtained by screening, crushing, mixing, forming and other processes.After coating a layer of glaze on the surface, the ceramic body is put into the kiln.After a series of firing processes, the glaze is fused with the surface of the product to form a solid and dense glaze layer.The ceramic density of the test is 2350 kg m −3 , the porosity is 12%, the thermal conductivity is 0.8 W /(m•K), the water absorption is 0.4%, and the combustion grade is Class A. The ceramic board used in the experiment has a thickness of 5 mm and the polyurethane insulation board of 20 mm.The mass ratio of the two is approximately 4:1.Through the multilayer composite structure, the IDB was tightly combined between the layers and exhibited good heat insulation and impermeability.
The special adhesive was used to bond the IDB and the substrate wall, as shown in figure 2. According to the mass ratio of 1:3, the water and the adhesive were evenly mixed and immediately bonded, and the operation was completed within 2 h.The gap between two adjacent IDBs was filled and sealed with the neutral silicone structural adhesive, as shown in figure 2(b).The organic silicone rubber component adopted a single structure, could be used in an environment of −50-150 °C for a long time, and exhibited excellent flame retardancy, aging resistance and sealing performance.

Specimen design and manufacture
A total of 6 ceramic-reinforced polyurethane IDBs of 300 mm × 450 mm were used in this experiment, as shown in figure 2. The thickness of the IDB was 30 mm, of which the polyurethane insulation layer was 25 mm, and the ceramic finishing layer was 5 mm.The width of the silicone structural adhesive sealing joint between the two adjacent IDBs was 20 mm, and the size of the whole specimen reached 940 mm × 920 mm.The IDB was pasted on the concrete substrate wall through a bonding layer; the thickness of the bonding layer was 10 mm, the thickness of the foundation wall was 200 mm, and the strength of the concrete was C30.To simulate the effect of the monolithic board under the environment in one direction, a bitumen waterproof coil was used to seal the edges around the specimen.

Artificial accelerated aging test
The artificial accelerated aging test was carried out after the specimen was prepared and cured.In this experiment, a self-developed artificial accelerated aging chamber was used to accelerate the aging of the external insulation composite system with polyurethane IDB.As shown in figure 3, the test device was composed of air conditioning and water spraying systems.The air conditioning system provided a stable and controllable temperature change process for the test piece according to the design conditions of the test, and the water spraying system sprayed water in the direction of the specified flow perpendicular to the surface of the test specimen to simulate the influence of the rainfall process on the test piece.During the test, the specimen was pushed into the aging chamber, and the bolts were tightened to make the two fit tightly to form a closed temperature and humidity environment space.
The EAD 040083-00-0404 standard is revised on the basis of ETAG 004, and it is suitable for various types of insulation materials, especially the composite insulation materials [31].According to the requirements of EAD 040083-00-0404, the hygrothermal test consists of heat-rain cycle and heat-cold cycle.This experiment mainly studies the performance changes of insulation materials after being exposed to high temperature and rain in summer, so that a modified heat-rain test referring to EAD 040083-00-0404 is adopted.In this experiment, the time of each heat-rain cycle was set to 6 h.The first 3 h corresponded to the heating and maintenance phase, in which the heating time was set to increase evenly to 70 °C within 1 h, and then this temperature was maintained for 2 h.After 3 h, the spray system started to work; 15 ± 5 °C water was sprayed into the specimen at a flow rate of 1.0∼1.5 l/(m 2 •min), and this spray of water continued for 1 h to simulate the influence of rainfall.At the end of the spray, the entire system remained in a resting phase for 2 h and gradually returned to room temperature within 2 h.The designed temperature variation in the test chamber is shown in figure 4.
In this experiment, a total of 60 heat-rain cycles were carried out on the external insulation composite system with ceramic-reinforced polyurethane IDBs.During the test, the system automatically measured the temperature and strain every 10 min to analyze the changes in the accelerated aging of the specimens.Figure 4 shows the changes in the actual temperature and the set temperature in the chamber during the first heat-rain cycle.According to the data measured by the temperature sensor installed in the specimen and the chamber, due to the constant changes in the external environment during the test, there was a certain deviation between the actual temperature and the temperature set by the system, but the deviation was small.

Collection of temperature and strain
To gain insight into the real-time temperature and strain of the polyurethane IDBs during the aging process, temperature sensors and resistance strain gauges were installed in the specimen.As shown in figure 2(a), strain gauges were arranged at the four corners and the central position of one side of the insulation decorative integrated board near the bonding layer, as well as on the corresponding substrate wall surface.The temperature sensors were buried in the adhesive layer and arranged at the four corners and the center position corresponding to each IDB.In addition, two temperature sensors were placed on the outer surface of the specimen for collecting the temperature of aging chamber.As shown in figure 2(b), one temperature sensor located in the upper area of the specimen, and the other located in the lower area.The average of the two sensors is taken as the temperature of the chamber.
The temperature sensor used in this test was a high-precision temperature and humidity sensor, and its related test performance parameters are shown in table 1.The sensor was cylindrical, with a diameter of D = 8 mm and a length of H = 35 mm; to facilitate the installation and fixation, the metal round head was removed and replaced by a waterproof breathable protective cover, as shown in figure 5(a).The resistance strain gauge was a biaxial strain gauge, as shown in figure 5(b).Because the strain gauge was a two-axis type, it could be used to collect the horizontal and vertical strains at each layout point.The relevant test performance parameters are shown in table 2. At the same time, the temperature compensation strain gauge was placed outdoors to eliminate the influence of outdoor temperature on the strain.

Temperature field analysis
The temperature field change at the interface between the IDB and the substrate wall during the aging process can reflect not only the deterioration of the insulation performance of the IDB but also the change in the synergistic effect between the IDB and the substrate wall from the side.Due to the long-term effect of the heatrain cycles, only a part of sensors worked well.According to previous studies [16,30], the temperature and deformation at the joints and corners of the IDBs varied the most dramatically, and were the most likely to reflect the changing patterns of the interaction between the IDBs and the substrate wall.Therefore, this article selects the data at the center, corners of the first row and center of the second row of IDBs as representatives for analysis.In this experiment, the temperature at measuring points 1, 2, 7, 12 and 16 under the 1st, 20th, 40th and 60th heat-rain cycles was selected for analysis, and the temperature change curve is shown in figure 6(a)-(d).
According to the temperature change measured at each point under each cycle, the temperature change curve under each cycle exhibited a similar trend across the whole heat-rain aging stage.During the heating phase, the temperature at each measuring point at the interface gradually increased, but the temperature increase rate at each point was different.This temperature increase continued until the end of the maintenance phase.Once the rain started, the temperature at the interface started to decrease almost simultaneously with the temperature in the chamber.It can be seen from the collected temperature that the temperature at the interface and the temperature in the chamber differed by approximately 35 °C, while the temperature change at the interface itself was only approximately 5 °C, indicating that the ceramic-reinforced polyurethane IDB had a good thermal insulation effect.
Notably, the temperature change at measuring point 2 was quite different from that at other measuring points.In each cycle, the temperature at point 2 was approximately 5 °C higher than that at other measuring  points during the heating and maintenance phase and approximately 3 °C lower than that at other points during the spraying and resting phases.This difference occurred because the No. 2 measuring point was located at the corner of the integrated board.Although the integrated board was sealed and insulated, there may have still been a certain amount of heat flowing into or out of the board, resulting in a higher temperature at point 2 and a lower temperature after rain.Due to the relatively large temperature difference, the IDB attached to point 2 was prone to cracking.When subjected to the same heat-rain cycles as in this study, the temperature monitoring results of the foamed ceramic IDBs showed that the temperature change at the corner of the windows were more severe [30], which was consistent with the temperature change pattern at point 2. Intense temperature changes can cause significant strain, making the corners prone to cracking.Under the same ambient temperature, the average temperature change at each measuring point under different cycles can be used to evaluate the aging condition of the IDB.As shown in figure 7, for the average temperature at the five points under different cycles, the change trend at each point was similar.From the beginning of the test to the 20 heat-rain cycles, the average temperature increased, and after the 20 heat-rain cycles, the average temperature at each point decreased, which may be due to the aging phenomenon of the polyurethane insulation material under the action of the whole heat-rain cycle, resulting in a decrease in its insulation performance.The average temperature at the interface between the foamed ceramic IDBs and the substrate wall showed a similar result to this article [30], that is, the average temperature increment at the interface tended to decrease after 20 cycles, but the mechanism was different with this study.In fact, the performance degradation of foamed ceramic under heat-rain cycles was very small, and the change of the interface temperature was mainly caused by the imbalance between the heat flowing in during the heating phase and the heat flowing out during the raining phase.The interface temperature change caused by the insulation performance degradation of the IDBs itself can be almost ignored [30].

Strain field analysis
In this test, during the strain measurements, the strain was positive when the measuring point was expanded, and the strain was negative when the measuring point was contracted.As for the temperature measurements, strains after 1, 20, 40 and 60 cycles were selected for analysis, as shown in figure 8.
As shown in figure 8, the horizontal and vertical strain changes after each cycle differed substantially.During 1-60 heat-rain cycles, corresponding to the four stages of each heat-rain cycle, the strain could be roughly divided into four different change processes.The vertical strain changed substantially throughout the heat-rain cycle.During the heating phase, the strain increased gradually with increasing temperature in the chamber.When the chamber was in the maintenance phase, the strain did not continue to increase but decreased.At the beginning of the raining phase, the strain decreased sharply and then continued to increase.During the resting phase, the strain still gradually increased and then tended to be stable.
It can be seen from the vertical strain change curve under each cycle that the strain could be roughly restored to the initial level of the test after the end of the heat-rain cycle except after the first cycle.The approximate range of vertical strain was between −750 με and + 2750 με, and the strain increased with an increase in the number of heat-rain cycles, indicating that the plastic deformation of the IDB increased.The horizontal strain, from the first to the 60th heat-rain cycle, exhibited similar variation, and the strain value varied between −250 με and + 500 με.During the heat-rain cycle, the variation range of the horizontal strain was small, showing a trend of horizontal change.In particular, the strain change amplitude at Point 6 was slightly larger, which may have been caused by the poor sealing of the IDB at position No 6 due to the wiring embedded in the sensor, and the thermal expansion and cold contraction effect caused by the moisture and heat transfer were more obvious.

Degradation of the IDB insulation performance
Figure 7 shows that with the aging of the heat-rain cycle, the average temperature at each point after each cycle was lower than the temperature at the beginning of the test, showing a downward trend, which indicated that the insulation performance of the IDB had deteriorated to a certain extent.There are various indicators which reflect the degradation of insulation performance.The research of durability on expanded polystyrene (EPS), insulation cork board (ICB), and mineral wool (MW) exterior insulation systems ,which shows that no matter what insulation material is used, there will be an increase in water absorption and thermal conductivity, indicating a degradation of insulation performance [32].In the study of foamed ceramic IDBs, the average temperature increment at interface was used to reflect the insulation performance change of the external thermal insulation system.However, the foamed ceramic IDBs was made by high-temperature firing, with a high closure rate and mechanical strength.Its insulation performance degradation can be almost ignored when subjected to heat-rain cycles [30].As mentioned earlier, the average temperature increment at the interface was only affected by the difference between the heat flowing in during the heating phase and the heat flowing out during the raining phase, which was not related to the aging and performance degradation of foamed ceramics.Obviously, there is a significant difference between the foamed ceramic IDBs and the IDBs used in this study.It is inappropriate to use the average temperature increment at interface to characterize the insulation performance of the ceramic-reinforced polyurethane IDB.In fact, When the performance of thermal insulation materials decreases, it usually means that their heat transfer becomes faster, so in this paper, the heating rate of the IDB in the heating phase and the cooling rate in the raining phase are used to indirectly reflect the degradation of material properties.Figures 6 (a)-(d) shows that the temperature at the interface of the IDB and the substrate wall first increased and then decreased during a heat-rain cycle.Therefore, the temperature heating rate f t1 was defined as the difference between the highest temperature and the lowest temperature in the temperature rise period divided by the duration of the process.The corresponding cooling rate f t2 was defined by dividing the difference between the highest and lowest temperatures by the duration of the process (as shown in figure 9).For insulation materials, the larger f t1 and f t2 are, the more severe their performance decline.
The heating rate and cooling rate at each measuring point determined by the calculation are shown in figure 10.The variation amplitude at all the measuring points except point 2 was not very large and basically fluctuated around 0.02 °C min −1 .Both f t1 and f t2 at test point 2 increased with the increase in the number of heat-rain cycles, indicating that the thermal insulation properties of the materials at test point 2 were aging more rapidly, and the decrease in the performance of the whole board was uneven.The reason for the rapid aging of the integrated plate at test point 2, as mentioned above, was the loose seal on the edge of the specimen, which caused heat and water to enter and exit and thus accelerated the performance decline of the IDB at that site.The study by Abdou and Budaiwi [33]on the thermal conductivity of glass fiber insulation materials at different moisture contents and temperatures showed that the thermal conductivity increases with the increase of moisture content, and also increases with the increase of initial temperature, reflecting the degradation of insulation performance.This is consistent with the results observed in this article.

Rule of strain change
It can be seen from the strain data after each cycle that the strain gradually increased during the heating phase, then decreased substantially during the maintaining phase, and then increased after a short period of sharply decreasing due to the influence of rain.This increasing trend continued until the resting phase and gradually stabilized.It seems to be contrary to common sense that the strain decreased during the maintenance phase and then increased again after the rain until the resting phase.Sha et al [29] also found that the strain is increasing with the increasing temperature in the weathering chamber, and tends to be stable in the resting phase when studying the strain of the foam glass IDB during the heat-rain cycles.However, the author's previous research on foamed ceramic IDBs had shown that the strain at the interface will continue to increase until reaching its peak during the heating and maintaining phase, and then begin to decrease during the raining phase [30].In fact, the occurrence of this phenomenon was related not only to the temperature change but also to the water absorption and moisture adsorption of the insulation material and the synergistic effect of the substrate wall-insulation layer.
From the first heat-rain cycle until the 60th heat-rain cycle, the strain changes at each point were roughly the same.The increase and decrease in the external temperature caused the temperature at the interface to increase or decrease, which, on the one hand, caused thermal expansion and contraction at the measurement point and, on the other hand, led to the evaporation of high-temperature water vapor.The pores in the IDB provided a channel for water to escape, resulting in a certain degree of water-loss shrinkage.In contrast, the intrusion of rainwater along the pores caused the measuring point to absorb water and expand.Xiong et al [18] conducted heat-rain cyclic tests on the expanded perlite insulation mortar, indicating that the water absorption is related to pore structure during aging.Pores provide channels for the transmission of water, and changes in pore structure are important factors that cause changes in water absorption.The coupling of heating expansion, water-loss  shrinkage and water-absorption expansion is the main reason for the mentioned strain changes that seem inconsistent with common sense.When the temperature continued to increase until it reaches the hightemperature stage, the pore water in the IDB continued to evaporate, resulting in gradual water-loss shrinkage.The negative strain caused by water-loss shrinkage was greater than the positive expansion strain caused by temperature increase, thus resulting in a continuous reduction in strain in the late maintenance phase (as shown in figure 9).The results of cyclic hygrothermal impact on the EPS exterior insulation system also indicate that as the hygrothermal cycle progresses, the strain accumulation increases and the water absorption rate increases to a certain extent [34].The occurrence of spraying caused the temperature to decrease sharply, which caused contraction of the IDB.During the early stage of spraying, less rainwater invaded the IDB, so the negative strain caused by shrinkage was greater than the positive strain caused by water absorption and expansion, and the strain continued to decrease during this period.From the late spraying phase until the resting phase, the expansion caused by water absorption became increasingly obvious, the corresponding positive strain became increasingly larger, and the strain of each measuring point also gradually increased.After standing for a certain time, the temperature became relatively stable, and the water absorption also reached equilibrium.At this time, the strain was basically stable near a certain value, and there was no large change.For thermal insulation materials such as foam glass and foamed ceramics that did not absorb water or had very low water absorption, their interior is always in a dry state no matter at heating phase or the raining phase, and there was basically no deformation caused by water absorption.The change of strain was mainly affected by the change of temperature.The strain variation characteristics of the foamed ceramic IDBs during the heat-rain cycles belong to this situation [30].It can be seen that the physical and mechanical properties of the insulation layer material of the IDBs have an important impact on the temperature and deformation of the external thermal insulation system.

Conclusion
Sixty heat-rain cycles were carried out on the external insulation composite system of ceramic-reinforced polyurethane IDBs, and the temperature and strain variations between the IDB and substrate wall were tested.Through the analysis of the test results, the following conclusions were drawn: (1) The ceramic-reinforced polyurethane IDB had good thermal insulation.In each heat-rain cycle, the temperature difference between the external environment and the concrete substrate wall reached 35 °C, while the temperature fluctuation of the concrete base itself was only approximately 5 °C.
(2) The vertical strain of the IDB varied from −750 με to + 2750 με and increased with the increase in the number of heat-rain cycles.The horizontal strain varied from −250 με to + 500 με, and the variation amplitude was very small during the heat-rain cycle.
(3) The aging of the IDB exhibited local nonuniformity, and some parts may have rapidly aged due to weak points formed by construction and other reasons; this aging became increasingly serious with the progress of the heat-rain cycle.

Figure 2 .
Figure 2. Specimen of external insulation composite system with ceramic-reinforced polyurethane IDBs.

Figure 4 .
Figure 4. Temperature-time curve of the first heat-rain cycle in the chamber.

Figure 5 .
Figure 5. Temperature-humidity sensor and strain gauge used in this experiment.

Figure 7 .
Figure 7. Average temperature during each cycle.

Figure 9 .
Figure 9. Schematic diagram of heating and cooling rates.

Table 1 .
Performance specifications of the temperature and humidity sensors.

Table 2 .
Performance specifications of biaxial strain gauge.