The Use of Straw, Coal and Foam to Improve Thermal and Mechanical Properties of Polyurethane

Introduction. The global energy agenda has led to active research on thermal insulation materials. The effectiveness of these materials is studied in relation to their characteristics and composition [1]. Polyurethane, hardened foam made of polyol and isocyanate, is considered one of the best insulating materials [2]. Natural fillers can be added to the composition to improve its performance, but only in small amounts, as excessive use can lead to decreased adhesion [3].

The scientific literature provides evidence that the addition of straw and coal can improve thermal, mechanical and acoustic properties of polyurethane [4].

The authors [5] obtained a set of samples by mixing purified soil heated to a temperature of 105°C with polyurethane components. Some samples were shaped, others were taken after free foaming. Compression, flexibility, fluidity, and freezing tests were performed. The results were compared with experimental data on marble, ceramic, and basalt samples. It was found that the mixture with purified soil has the best impact strength. However, it is necessary to take into account the lower specific gravity of polyurethane samples.

The researchers [6] integrated recycled and pure polyurethane in various ratios (maximum — 10%). The samples with increased amount of recycled polyurethane foamed faster and had better tensile and compressive characteristics.

In [7], walnut and hazelnut shells are considered as filler for polyurethane. It has been proven that these additives combine well with polyurethane components. Even their small amount increases the mechanical properties and thermal stability of samples.

The possibility of using silica gel as a reinforcing additive for polyurethane was shown in [8]. Four samples containing 0%, 1%, 3% and 5% silica gel were studied. The tests showed that at 5%, thermal conductivity decreased to 0.0268 (W/m·K) compared to a material without a filler of 0.0314 (W/m·K). Stress and compressive strengths increased by 18% with the addition of 3% silica gel, but decreased if the filler volume exceeded 3%.

The author [9] added grain hulls and rubber from recycled tires to polyurethane. The optimal proportion of additives has been established: 5% of grain hulls and 15% of rubber. This ratio ensures the maximum mechanical strength of the material.

The aim of the scientific research described in this article is to study the possibility of improving the properties of polyurethane using additives: straw, coal and foam. It is assumed that the proposed solution will reduce thermal conductivity while maintaining good mechanical, acoustic and physical characteristics. In this case, it will be possible to talk about the economic feasibility of using modified material to increase the energy efficiency of facilities. Moreover, the composition itself should be more affordable, since polyurethane components are partially replaced by cheap fillers.

Materials and Methods. During thermal, physical, mechanical, and acoustic testing, samples of Daumerk polyurethane (Turkey) with and without additives were examined.

Sample preparation. At the first stage of the experiments, four samples of polyurethane were prepared. The main ingredients were mixed without any fillers and then poured into a mold (Fig. 1).

The dimensions of the wooden mold: 150´150´10 mm. Transparent glue was applied to the bottom and inner surface of the lid to prevent the mixture from sticking to the mold. After pouring the mixture, the lid was attached with four screws.

Table 1 shows samples with different ratios of polyurethane components.

At the second stage, the optimal ratios of isocyanate and polyol were selected and six new samples were made. 5% and 10% of coke coal, straw and foam were added to them (Table 2).

Thermal Testing. We used an HTC device (Highlight Tech Corp, China), which allowed us to determine thermal conductivity using the hot plate method in accordance with the American standard ASTM C177¹ (Fig. 2).

The device took into account the current and the applied voltage, and therefore measured heat flux Q between different sides of the sample, as well as temperature difference ∆T between its upper and lower surfaces. Knowing the dimensions of the sample (area A and thickness l), it was possible to determine coefficient of thermal conductivity λ:

(1)

To ensure accuracy of the results, the device was pre-calibrated using three reference samples made of polystyrene, polyethylene and foam with known thermal conductivity [10]. After that, a calibration graph was constructed (Fig. 3).

The total error in measuring the coefficient was 5–7%.

Compression Test. The compression test was performed in accordance with ISO 844². A device was used (Fig. 4), which allowed applying a compression load of up to 5,000 N to the surface of the sample at a given speed. A load at a rate of 1 mm/min was applied to the samples with sides of 10×10 mm until deformation appeared. The results were recorded by a computer connected to the device with special software.

Density Determination. To determine the samples density in accordance with ASTM C1622³ standard, the weight and volume ratios were calculated. To do this, we used:

• scales with an accuracy of ±0.001 g (Fig. 5а),
• vernier calipers with an accuracy of ±0.01 mm (Fig. 5b).

Thus, the volume was calculated with an accuracy of ±0.01 mm³, and then the sample density was determined using the formula:

(2)

where m — mass of the sample, kg; v — volume of the sample, mm.

Water Absorption Determination. To determine water absorption of the samples, they were immersed in water for a certain period of time. It was done in accordance with ASTM D2842⁴ standard. The percentage of water absorption was calculated by taking the difference in weight between the sample before and after being immersed, and dividing it by the initial weight of the sample.

Sound Absorption Determination. To measure the sound absorption coefficient, a device was used that operated on the “transmitter —receiver” principle in accordance with ISO 10534-1⁵ (Fig. 6).

The sample was placed between a sound transmitter and a receiver, and a sound wave with certain energy was sent. The wave’s energy was then measured, the frequency range was scanned and analyzed, and the sound absorption coefficient was determined.

Equipment set:

• dual electronic oscilloscope;
• vibrator with a frequency range (1–10 kHz);
• carbon amplifier for transmitting sound waves;
• microphone for receiving sound waves;
• metal bases for mounting the amplifier and microphone;
• bridge on which the amplifier and microphone slide;
• energy meter.

Knowing the wave energy before and after sample installation, it was possible to calculate the sound absorption coefficient:

(3)

where α — sound absorption coefficient; E — wave energy after sample installation (volts); E₀ — wave energy before sample installation (volts).

Results

Density. Figure 7 shows the results of determining the density of the tested samples.

The lowest density of sample 3 (33 kg/m³) was explained by the absence of fillers and the uniformity of its structure. The addition of fillers, especially in a volume of 5%, did not significantly affect the density. This was due, firstly, to the insignificant proportion of the additive. Secondly, the density of additives (especially straw and foam) was close to the density of polyurethane without additives.

Water Absorption. The results of water absorption tests are shown in Figure 8.

A low level of water absorption was recorded in samples without additives (50% isocyanate and 50% polyol) and with the addition of 5% foam (62% isocyanate and 33% polyol). With an increase in the proportion of isocyanate, the water absorption level increased, especially in samples 8 and 10 with 10% straw and coal. This was consistent with the results obtained by other researchers [11]. Sample 2 deviated from the general rule, which may be related to the specifics of its preparation. It is known that straw actively absorbs water. Accordingly, the water absorption is higher for samples with straw. When foam granules were added, on the contrary, water absorption decreased due to the high resistance of this material to water. For this reason, the minimum water absorption was observed for sample 6.

Thermal Properties. Figure 9 provides the results of thermal tests of the samples.

The coefficient of thermal conductivity of samples without additives increased with an increase in the proportion of polyol. With high polyol content, the contact between the components improved, the cell sizes decreased, and the density increased. Dense materials provided more efficient heat transfer, i.e. they had higher thermal conductivity [12]. The addition of foam and coal in high concentrations also increased density and significantly increased thermal conductivity [13]. Adding fillers in small quantities, on the contrary, could improve thermal conductivity. For example, when adding foam and straw in the amount of 5%, it was possible to reduce the values of thermal conductivity to 0.023 W/m·K. The author of [14] obtained almost the same result of 0.024 W/m·K.

Compressive Strength. Figure 10 shows the results of determining the compressive strength.

Compressive strength was proportional to density of the material; therefore, in samples without additives, it increased with increasing polyol content. With the addition of 5% straw and coal, the compressive strength was high due to the uniformity of the cellular structure. With the addition of 10% filler, the mechanical strength decreased due to a decrease in the internal binding force of the sample.

Acoustic Characteristics. Figure 11 shows the results of sound absorption tests.

Among the samples without fillers, the densest one had the best score — 1. With a low additive content (5%), sound absorption increased due to the homogeneous cellular structure. With a higher level of additives (10%), it decreased due to insufficiently strong bonds in the sample material.

Discussion and Conclusion. The results of the research allow us to draw the following conclusions.

1. The addition of fillers in a certain amount can improve thermal, mechanical and acoustic properties of polyurethane.
2. An increase in the proportion of additives generally worsens the studied properties and may increase water absorption. This happens when adding straw.
3. To achieve optimal results, it is recommended to add 5% straw to polyurethane, consisting of 62% isocyanate and 33% polyol. Thermal conductivity of this sample (number 7) was 0.023 W/m·K with a density of 37 kg/m³ and a compressive strength of 358 kN/m².
4. The values of thermal conductivity and mechanical strength of sample 7 are better by 2% and 4%, respectively, compared with the sample without additives (number 3).
5. The advantages of sample 7 should also include its lower cost. Straw is cheaper than the main components of polyurethane.

In the future, it is planned to study other fillers in terms of their effect on the properties of polyurethane.