Effect of latex powder and glass fibers on the performance of glazed hollow bead thermal insulation materials

2.1 Experimental materials

In this experiment, glazed hollow beads, whose surface is vitrified and internal structure is porous, were obtained from Shandong Chuangzhi New Materials Technology Company (Weifang, Shandong, China). The apparent appearance and internal structure of glazed hollow beads are shown in Figures 1 and 2, and its main physical properties are given in Table 1. The cement used is 42.5R fast-hardening sulphoaluminate cement. The performance of fly ash is in line with the request of GB/T 1596–2005 “fly ash used for cement and concrete” standards. Alkali-resistant glass fiber with a density of 80–120 kg/m³ is an important reinforced material, which has excellent dispersibility. The elastic modulus and tensile strength of glass fibers are 74 GPa and 1800 MPa, respectively. Redispersible latex powder with a bulk density of 500–600 kg/m³ was adopted from Shandong Xindadi Industry and Trade Company (Jinan, Shandong, China), and its composition is the copolymerization of ethylene/vinyl acetate/VeoVa. Its specific performance parameters are as follows: pH is 5–8, ash is 12%±2% (by mass), nonvolatile matter is more than 98% (by mass), and minimum film-forming temperature is 0°C. Styrene-acrylic emulsion is the production of a paint company.

Figure 1

External morphology of glazed hollow beads.

Figure 2

Internal structure of glazed hollow beads.

2.2 Alkali-resistant glass fiber surface treatment

For the fiber surface coating treatment, alkali-resistant glass fibers were placed in styrene-acrylic emulsion and soaked for 20 min, then fibers were removed and the residual emulsion was squeezed out. Fibers were dried under natural conditions (temperature, 24°C±4°C; humidity, 50%±10%), then fibers were uniformly dispersed. Due to the presence of moisture in the air, the surface of glass fibers is often firmly adsorbed with a layer of water molecules, which interact with alkali metal composition glass fiber to form silicon hydroxyl on the surface of fibers; its forming process is shown in Equation (1) [9]. After fibers are soaked in styrene-acrylic emulsion, silicon hydroxyl on the surface of fibers combines with emulsion macromolecules by van der Waals force; thus, a layer of organic emulsion coating layer is formed on the surface of the glass fibers. Figure 3 shows scanning electron microscope (SEM) pictures of alkali-resistant glass fibers with surface treated and untreated. As can be seen from Figure 3, the surface of glass fiber without surface treatment was smooth, but the surface of treated fiber was covered by large amounts of styrene-acrylic emulsion, which caused significant roughening of fiber surface and the increase in specific surface area, so that the fibers had good mechanical engagement to combine with cement and other cementitious material firmly.

Figure 3

Micromorphology on the surface of alkali-resistant glass fiber.

(A) Without surface treatment of fiber. (B) Surface treatment of fiber.

In the equation, D represents the alkali metal.

2.3 Experimental method

Early experiments have shown that the density and thermal conductivity of glazed hollow bead thermal insulation material increased with the increase in cement and fly ash, and when the content of cement was 40% and the content of fly ash was 10%, the density and thermal conductivity of the samples were 278 kg/m³ and 0.068 W/m·K, respectively, complying with the relevant requirements (density, ≤300 kg/m³; thermal conductivity, ≤0.07 W/m·K). In the experiment, the dosage of glazed hollow beads was fixed at 100%, water at 80%, cement at 40%, and fly ash at 10%, and the dosages of each raw material were expressed in the quality percentage of glazed hollow beads. Then, groups A, R, and S were designed. Group A was mixed with latex powder; the dosages of samples A0–A5 were 0%, 1%, 2%, 3%, 4%, and 5%, respectively. On the basis of the optimal dosage of latex powder, groups R and S were mixed with untreated and treated fibers, respectively, and the dosages of samples R1–R4 and S1–S4 were all 0.6%, 0.8%, 1.0%, and 1.2%, respectively. A variety of experimental raw materials were accurately weighed and mixed uniformly, and the right amount of emulsion and water were successively put into the raw materials. Then, the material was placed in a 500 mm×300 mm×80 mm mold, in the compression molding (molding pressure of 0.48 MPa), and samples were allowed to stand at room temperature for half an hour to gain a thermal insulation board of glazed hollow beads with a dimension of 500 mm×300 mm×40 mm. After being demolded, samples were cured in a cement standard curing box (temperature, 20°C relative humidity, 95%) for 7 days, and then samples were placed in a 110°C electric thermostat blast oven to dry to a constant weight (rate of samples’ quality changed, with two weighing <0.2%). Finally, samples were moved to a dry place to cool to room temperature.

After the samples were prepared, based on GB/T 5486-2008 “Test methods for inorganic rigid thermal insulation”, the flexural, compressive, and water absorption test samples were respectively processed to sizes of 250 mm×100 mm×40 mm, 100 mm×100 mm×40 mm, and 400 mm×300 mm×40 mm, then the samples’ density, flexural strength, compressive strength, and water absorption were tested respectively, where each test was repeated three times, four times, four times, and three times, respectively. In accordance with the National Standard GB/T 10294-2008 “Thermal insulation – Determination of steady-state thermal resistance and related properties – Guarded hot plate apparatus”, test samples were processed into a size of 300 mm×300 mm×20 mm to measure the thermal conductivity of the samples, and each test was run only once. Finally, a QUANTA FEG 250 SEM (FEI Company, OR, USA) was used to observe the fracture morphology of the insulation material.

3.1 Determination of the optimal content of latex powder

The effects of latex powder on the thermal conductivity, strength, and water absorption of the thermal insulation material are illustrated in Table 2 and Figure 4, respectively. According to the analysis of Table 2, the thermal conductivity of thermal materials did not change significantly with the increase in the dosages of latex powders. Figure 4A shows that the different dosages of latex powder improved the compressive and flexural strengths of insulation materials to varying degrees. As the content of latex powder increased, the samples’ compressive strength first increased and then decreased. While the dosage of latex powder was more than 4%, the compressive strength of the samples decreased; this is because the elastic modulus of polymer film is less than the hydration product, so that excessive polymer film, to a certain extent, can be regarded as the pores in the material, which weakens the ability of the samples to withstand compressive stress [9–12]. With the dosage of latex powder increasing, the samples’ flexural strength gradually increased, and when the dosage of latex powder was 4%, the flexural strength tended to be stable. As shown in Figure 4B, the water absorption of the samples first decreased and then tended to stabilize with the increase in the dosage of latex powder. When the dosage of latex powder reached 4%, the increase in the dosage hardly had an obvious impact on the water absorption of the samples. Therefore, the content of latex powder was selected as 4%. At this time, the compressive and flexural strengths of the samples were 0.58 MPa and 0.37 MPa, respectively, which increased by 20.83% and 48%, respectively, compared with blank samples. The samples’ 2-h and 24-h water absorptions were 16.2% and 39.8%, respectively, decreasing by 71.37% and 66.94%, respectively, compared with blank samples. Moreover, the density and thermal conductivity of the samples were 282 kg/m³ and 0.067 W/m·K, respectively, which were in line with the relevant standards (density, ≤300 kg/m³; compressive strength, ≥0.5 MPa; flexural strength, ≥0.3 MPa; thermal conductivity, ≤0.07 W/m·K).

Figure 4

Effect of different contents of latex powder on the strength and water absorption of the samples.

(A) Effect on the strength of the samples. (B) Effect on the water absorption of the samples.

3.2 Determination of the optimal content of glass fibers

The test results of the performance of insulation material affected by glass fibers without surface treatment are presented in Table 3. From the table, the influence of fibers on the density and thermal conductivity of materials is not very obvious, and the addition of untreated fibers did not improve the flexural and compressive strength of thermal materials significantly. Therefore, styrene-acrylic emulsion was used to treat the surface of fibers. Then, the effect of different dosages of treated fibers on the density and strength of glazed hollow bead insulation material was tested.

Figure 5 shows the relationship between the dosage of glass fiber with surface treatment and the density and strength of glazed hollow bead insulation material. Figure 5 shows that the density of insulation material was less affected by the different dosage of treated fiber and that the material strength first increased and then decreased with the increase in the dosage of treated fiber. The mixing of fibers enhanced the samples’ compressive strength, and when the fiber content was 1.0%, the maximum compressive strength was 0.65 MPa, which increased by 8.34% compared with samples R3 mixed with untreated fiber. Meanwhile, fibers significantly improved the samples’ flexural strength, and when the fiber content was 1.0%, the peak of flexural strength was 0.57 MPa, increasing by 35.71% compared with samples R3 mixed with untreated fiber. When the fibers were excessively added, fibers were easily entangled with each other, leading to an uneven distribution of fibers in the material. Because of the uneven distribution, stress concentration occurred easily so that the samples’ flexural strength decreased. For compressive strength, the incorporation of excessive fiber was equivalent to the introduction of porosity defects, causing the material to rupture easily, which adversely affected the material’s compressive strength. According to the above analysis, the optimal content of fiber with surface treatment was fixed at 1.0%, and the density and thermal conductivity of the material were 283 kg/m³ and 0.068 W/m·K, respectively, which were in line with the relevant standards (density, ≤300 kg/m³; compressive strength, ≥0.5 MPa; flexural strength, ≥0.3 MPa; thermal conductivity, ≤0.07 W/m·K).

Figure 5

Effect of different contents of treated glass fiber on the density and strength of the samples.

(A) Effect on dry density of samples. (B) Effect on strength of samples.

3.3 The action mechanism of latex powder and fiber on insulation material

3.3.1 The action mechanism of redispersible latex powder on thermal insulation material

The section morphology of samples A0 and S3 is shown in Figure 6. From Figure 6A, the bonding interface of the aggregate-matrix of sample A0 without mixing latex powder was combined loosely, in which a large number of pores existed and hydration products sporadically covered the aggregate surface. As can be seen from Figure 6B, the interface of the aggregate-matrix of sample S3 mixed with latex powder was bonded tightly and no apparent porosity and cracks existed at the interface. The reason is that polymeric films filled the gap at the interface to reduce the porosity of the interface, which improved the compaction degree of the bonding interface and significantly increased the combination degree of the interface between the aggregates and the matrix. Moreover, polymeric films can hinder the emergence and development of microcracks at the interface, and the macro performance for the material is of high strength.

Figure 6

Micromorphology on the section of samples A0 and S3.

(A) Sample A0. (B) Sample S3.

After water was added to the material, the latex powder rapidly interacted with water to form a polymer emulsion, which uniformly distributed in the matrix. As the hydration reaction proceeded, the emulsion particles gradually dehydrated and formed polymer films, covering the surface of the aggregate, cement particles, fly ash particles, and fibers or filling their pores. Meanwhile, polymer films cross-linked with hydration products of cement and fly ash, which improved the waterproof and mechanical properties of the insulation materials. To further illustrate the action mechanism of emulsion on the performance of insulation material, the structure formation model of insulation material was constructed, and its formation process is shown in Figure 7. As is shown in Figure 7, the process is divided into four steps. (a) Emulsion particles in the matrix are distributed to the surface or surrounding of aggregate, cement particles, fly ash particles, hydration products, and fibers. (b) As hydration reaction proceeds, hydration products increase, at the same time, emulsion particles dehydrate and begin to fuse to form a small area of polymer films, which are dispersed uniformly. Then, the composite cementitious system composed of hydration products and polymer films is initially formed. (c) Due to the ongoing hydration process and the constant loss of moisture, emulsion particles further dehydrate to generate more polymer films, and polymer films combine with each other to grow into films having a larger area, covering or wrapping aggregate, cement particles, fly ash particles, fibers, and hydration products. Moreover, polymer films fill the pores between the aggregate, cement particles, and fibers so that the bonding interfaces of a variety of materials are strengthened. (d) Under the action of cement hydration and surface evaporation, polymer films formed by the emulsion dehydration and hydration products develop coordinately, and both interpenetrate and cross-link with each other, combining aggregate and fibers into a more solid structure. Therefore, a three-dimensional spatial network through the material structure is formed, which prevents the formation and expansion of microcracks in the material, and the polymer films embedded inside the cement hydration products connect with each other to become an irregular polymeric film network, having an enhanced role of “class fiber”, which improves the flexural strength, compressive strength, and water resistance of insulation material.

Figure 7

Models of structure formation of insulation material.

(A) Shortly after the mixing. (B) Start of formation of polymer films. (C) Formation of a large area of polymer films. (D) Mutual cross-linking of hydration products and polymer films.

3.3.2 The action mechanism of treated fibers on thermal insulation material

Glass fibers have the characteristics of high elastic modulus and tensile strength, and an appropriate number of surface-treated glass fibers are uniformly distributed in thermal insulation material, which can form a supporting structure of the three-dimensional spatial network. When the material is subjected to flexural destruction, external force generates microcracks on the surface of the material, and the crack tip produces stress concentration, resulting in crack propagation. Fibers can obstruct and intercept the propagation of cracks to weaken the role of external damage, and they can also ease the stress concentration of crack tip. In addition, as glass fibers possess a micrograin role, which have a strong ability to resist flexural and tensile destruction, fibers consume lots of energy when they are drawn or even pulled off. Especially when fibers are treated, they have higher surface roughness to firmly combine with polymer films and hydration products. Thus, when they are pulled out of the material, the need to overcome greater interfacial adhesion and friction and the flexural strength are increased at the macro level. But when the material is subjected to compressive failure, thermal materials rely mainly on the strength of matrix material to resist the pressure damage, and fibers can assume only limited pressure load; thus, the enhancing effect of fibers on the compressive strength of the material is significantly weaker than the enhanced role against flexural strength.

Figure 8 shows the morphology of glass fibers at the fracture of sample S3. As seen in the figure, the surface of the fiber pulled out from the sample was covered by large amounts of gelling substance, and the cross-section from which fibers were pulled out occurred inside the matrix. From the above, we can infer that a relatively stable interface layer existed between fibers and the gelling material, which made the fibers and gelling material adhere together closely, so that the dense structure of “fiber-gelling material-aggregate” was formed inside the samples. From Figure 8B, the element contents of C and O in one point were high, which indicated that the interface layer of the fiber-matrix contained a large amount of organic polymer, and one point also contained the elements Al and Ca, manifesting that cement hydration products existed at the interface. Thus, the interface layer between the fibers and the matrix was constituted by the interleave of hydration products and polymer films. Owing to the high adhesive strength increased by the good bonding interface layer between the fibers and the matrix, fibers played a more important role in the reinforcement effect of the thermal material. On the one hand, fibers can connect across cracks, bridging separated portions caused by cracks together through a good interfacial layer. When the material suffered external stress, the external force passed along the fibers, which were distributed in the three-dimensional space, and continued to pass to the base body by the interface layer between the fiber and the matrix, which can effectively disperse the stress to control the further expansion of cracks. Meanwhile, external force energy was consumed in this transferring process, which led to a substantial increase in the strength of the thermal insulation material. On the other hand, there are simultaneously phenylpropanoid polymer films and starch polymer films, which have good toughness on the interface layer, which can buffer the external force and passivate the stress concentration of crack tips of insulation materials. In addition, polymer films effectively inhibited the propagation of cracks at the interface; thereby, the strength of the material was increased.

Figure 8

Micromorphology of glass fiber on the fracture of sample S3.

(A) Smaller magnification. (B) Energy spectrum of point 1 in panel A.