Effects of composition and density of nonwoven fabric on a soil-free growing medium

Abstract

The practice of covering the roof of a building with a layer of living vegetation has many benefits. However, soil is heavy and most soil substitutes present special problems such as a tendency to block drains. Previous studies utilized a lightweight, soil-free growing medium that replaced actual soil to lessen the mechanical load on the roof. This study used a lightweight nonwoven fabric as a growing medium for living roofs. Two types of polyester fibers were blended at various ratios and then thermally dried into various densities. A desirable growing medium must resist moisture deformation and encourage root implantation. The results indicated that 0.030 g/cm³ was the optimum density and that the weight of low-melting-point polyester fibers had to be in excess of 20 wt%. It was anticipated that the manufacturer who adjusts factory parameters to benefit from this research will reduce the production cost.

Introduction

Industrialization has severely polluted the natural environment; the environmental movement seeks to undo the damage by measures such as the alteration of the urban landscape to include green roofs.

Environmental activism has swayed public opinion to the extent that many countries have taken legal action to prevent further environmental destruction [1]. Unquestionably, industrial civilization produces massive quantities of pollutants. Conventional paved surfaces are impervious to water; rain on conventional pavement picks up pollutants and becomes urban runoff [2]. Skyscrapers made of concrete or reflective glass retain heat and contribute to rising temperatures by the thermal island effect [3]. Green roofs can significantly reduce urban runoff, heat islands, vehicle noise, and waste gas produced by factories and transportation [4–7].

Usually, plants are located in soil. Roofs have limited load-bearing capacity and replacement of soil by a lighter medium has many advantages. Many substitutes, such as pellets or sheets, require a nonwoven filter that can remove unwanted particulates that might clog drains. The previous studies have further justified the successful uses of nonwoven layer as a good soil-free growing medium. Featuring the fibrous structure, the growing medium is also lightweight and has great abilities to absorb and retain the water, all of which are beneficial to plant growth [8,9]. Figure 1 depicts a cross-sectional view of a green roof.

Figure 1.

Cross-sectional view of a living roof [10].

Polyester (PET) is known for its workability, its resistance to acids and bases, and acceptable moisture retention. Low-melting-point polyester (LM PET), the most common ingredient in nonwoven materials, also offers a sound bonding effect [11]. PET is a feasible choice for a green system, hence light-weight PET was utilized in this study to replace pellets and sheets.

Soil is a three-dimensional (3D) substance that includes solids, liquids, and gases. As illustrated in Figure 2, plants usually thrive in soils that are 45% minerals, 5% organic solids, and 50% gases and liquids [12]. Optimal porosity is 50%, with water and air in approximately equal amounts. Our nonwoven fabric does not include organic substances but is intended to replace soil for rooftop applications. Thus, in addition to the composition of the growing medium, we examined how vegetables implanted their roots in the growing medium. Further, we examined the drainage of moisture from the medium, because excessive water leads to rotten roots and unhealthy plants.

Figure 2.

Optimal soil composition [12].

Experimental

Materials and process

Three-dimensionally crimped polyester (3D PET) fibers and low-melting-point polyester (LM PET) fibers were prepared. The hollow 3D PET fibers had a melting point of 260℃; their fineness was 12 denier; they were 66 ± 5 mm in length, and had a specific gravity of 1.37–1.39 g/cm³. Low-melting-point polyester fiber (LM PET) with a fineness of 4 denier and a length of 51 mm was a bi-component fiber; the sheath is PET fiber with a melting-point of 110℃ and the core was PET fiber with a melting-point of 260℃. All of these materials were purchased from Far Eastern New Century Corporation.

The aforementioned materials were fed into a carding machine, forming a web.

This procedure was conducted by blending PET with different quantities of LM PET in proportions of 10, 20, 30, and 40 wt%. The samples were produced with densities of 0.03, 0.045, 0.06, 0.075, and 0.09 g/cm³. The maximum and optimal basis weight of each nonwoven layer was 300 g/cm². The web was layered in a module whose total volume was 400 cm³ and height was 1 cm. The module was dried at 110℃ for 30 minutes until the mixture was thoroughly thermally bonded into a nonwoven growing medium.

Tests

Each sample was immersed in the water for 48 hours until it was thoroughly soaked. Then, we measured the sample’s height. The moisture deformation resistance result was calculated by the following formulae (1–7).

The moisture deformation resistance

Each sample was immersed in the water until it was thoroughly soaked. Then, we measured the sample’s height. The moisture deformation resistance result was calculated by the following formula.

$\text{Moisturedeformationresistance}=[({\text{X}}_1-{\text{X}}_2)/{\text{X}}_1]\times 100$

(1)

Moisture deformation resistance is calculated from the sample immersed in the water.

X₁: height (cm) before immersion

X₂: height (cm) after immersion

The moisture retention

Next, each sample was weighed and its weight was recorded as W₁; it was immersed in water for 48 hours and then suspended until it stopped dripping water. The water-logged weight was recorded as W₂; formula (2) was used to calculate moisture retention.

$\text{Moistureretention}=[({\text{W}}_2-{\text{W}}_1)/{\text{W}}_1]\times 100$

(2)

W₁: dry weight (g)

W₂: water-logged weight (g)

Porosity

The soil-free growing medium was composed of three phases, namely the solid phase, gas phase, and liquid phase. The solid phase consisted of the fibers; the liquid phase consisted of water and nutrients; gas phase was air. The formulae are listed below.

I. D_T (Bulk Density)

${\text{D}}_{\text{T}}={\text{M}}_{\text{T}}/{\text{V}}_{\text{T}}$

(3)

M_T: fiber weight (g)

V_T: total medium volume (cm³)

II. P_F (Fiber volume)

${\text{P}}_{\text{F}}(\%)={\text{V}}_{\text{F}}/{\text{V}}_{\text{T}}\times 100$

(4)

V_F: fiber volume (cm³)

V_T: total medium volume (cm³)

III. P_P (Porosity)

${\text{P}}_{\text{P}}(\%)=(1-{\text{P}}_{\text{F}})\times 100$

(5)

Liquid phase percentage

P_L (liquid phase percentage): This represents the quantity of water taken up by capillary effects in the growing medium.

${\text{P}}_{\text{L}}(\%)=({\text{V}}_{\text{L}}/{\text{V}}_{\text{T}})\times 100$

(6)

V_L= liquid volume (cm³)

(1) P_G (gas phase percentage)

${\text{P}}_{\text{G}}(\%)={\text{P}}_{\text{P}}-{\text{P}}_{\text{L}}$

(7)

Planting test

Brassice Chinensis lonn is shallowly rooted in soil while Wedelia trilobata is deeply rooted in soil. Therefore, both plants are used in this study to evaluate if the nonwoven growing medium is suitable for plant growth and to determine the influence of various medium densities on plant growth.

Results and discussion

Effects of density and LM PET content on moisture deformation resistance

Figure 3 shows that samples that had high LM PET content were able to resist moisture deformation remarkably when samples were at low densities. Samples with high quantities of LM PET showed good bonds between 3D PET and LM PET and high moisture deformation resistance. However, high LM PET content did not have an obvious impact on the moisture deformation resistance of high-density samples. High-density samples had few pores, which limited moisture deformation, showing a better moisture deformation resistance than low-density samples. A high-density sample’s percentage of LM PET did not have a direct influence on its moisture deformation. Figure 3 shows drastic differences between 10 wt% and 20 wt% samples; 20 wt% LM PET content ensured 80% moisture deformation resistance, as shown in Figure 3.

Figure 3.

Moisture deformation resistance of the growing mediums made with 10 wt%, 20 wt%, 30 wt%, and 40 wt% low-melting-point polyester (LM PET).

Figure 4 shows the influence of LM PET content and different densities on the porosity of the growing medium. LM PET content had little effect on porosity of the growing medium; the overall standard deviation was below 1.20%. Porosity was inversely proportional to the sample’s density.

Figure 4.

Porosity of the growing mediums made with 10 wt%, 20 wt%, 30 wt%, and 40 wt% low-melting-point polyester (LM PET) and at different densities of 0.030, 0.045, 0.06, 0.075, and 0.090 g/cm³.

Effects of density and LM PET content on moisture retention rates

It is generally accepted that large contact areas can absorb large volumes of water. Figure 5 presents the effects of density and LM PET content on moisture retention rate.

Figure 5.

Moisture retention rates of the growing mediums made with 10 wt%, 20 wt%, 30 wt%, and 40 wt% low-melting-point polyester (LM PET) and at different densities of 0.030, 0.045, 0.060, 0.075, and 0.090 g/cm³.

According to Figure 5, when the polyester nonwoven soil-free growing medium came to a certain density, its water retention started declining. This was because when the density of the growing medium was in excess, it absorbed excessive water, leading to an increase in weight. Thus, the gravitational water drained out, decreasing the water retention. There was no regular change in the relationship of LM PET content and water retention due to the fact that LM PET and 3D PET had the same water absorption ability; hence, water retention of the growing medium was free from being influenced by LM PET content.

Effects of density and LM PET content on liquid phase and gas phase arrangements

Figures 6 and 7 display the effects of density and LM PET content on liquid phase and gas phase. There is an inevitable tradeoff between liquid and gas phases in any growing medium; one can only increase at the expense of the other. Figure 6 shows that densities of 0.030–0.075 g/cm³ retain liquid proportional to density, but densities of 0.090 g/cm³ retain relatively little liquid. Samples with a density of 0.075 g/cm³ retain the highest amount of liquid and lesser amount of gas. The high-density samples had many small pores, forming more water membranes that locked moisture inside these pores, and excluded gas. Samples with a density of 0.090 g/cm³ allowed liquids to drain out by the force of gravity; this resulted in higher gas phase content.

Figure 6.

Effects of density and low-melting-point polyester (LM PET) content on liquid phase.

Figure 7.

Effects of density and low-melting-point polyester (LM PET) content on gas phase.

Planting test

3D PET and LM PET were mixed in an 8:2 ratio, carded, laminated, and made into sheets of two densities, 0.030 and 0.090 g/cm³. These sheets were seeded with Brassica chinensis Linn and Wedelia trilobata, exposed to sunshine, watered, and nurtured for 2 weeks. Figures 8 and 9 show that Brassica chinensis Linn produced small crops in a high-density medium and larger crops in a low-density medium. This confirmed that the saturated liquid phase obstructed plant respiration and impaired plant health. Brassica chinensis Linn grew twice as fast in the low-density medium as it did in the high-density medium. The low-density medium was more hospitable to the tested plants, but low density was not an absolute key of success; excessively low density caused poor moisture retention and greater gas phase volume, which ultimately lead to thirsty and withered plants.

Figure 8.

A 4.5 cm tall Brassica chinensis Linn after 2 weeks of growth in the high-density (0.090 g/cm³) medium.

Figure 9.

Brassica chinensis Linn with a height of 9 cm after 2 weeks of growth in the low-density (0.030 g/cm³) medium.

Roots seek air as well as water; a root in a low-density growing medium can access both. Figures 10 and 11 show Wedelia trilobata in high-density and low-density sheets. In Figures 10(a) and 11(a), the Wedelia trilobata growing in the high-density and low-density sheets were 14 and 17 mm, respectively. Moreover, the heights and shapes of the Wedelia trilobata varied. The roots in the high-density medium were straight and deep-rooted, but the roots in the low-density medium had many bifurcations.

Figure 10.

The Wedelia trilobata roots in a high-density (0.090 g/cm³) medium: (a) 14 mm roots after 1 week and (b) after 4 weeks.

Figure 11.

The Wedelia trilobata roots in a low-density medium (0.030 g/cm³): (a) 17 mm roots after 1 week and (b) after 4 weeks.

Figure 12 shows the roots at the bottom of the medium after 4 weeks of growth. The low-density medium allowed nutrients to flow. Many roots and some Penicillium chrysogenum bacteria flourished in the low-density medium, but few roots were present in the high-density medium.

Figure 12.

The roots of Brassica chinensis Linn at the bottom of the sheets after 4 weeks (a) in high-density (0.090 g/cm³) and (b) in low-density (0.030 g/cm³) sheets.

Conclusion

PET and LM PET were blended and processed into nonwoven sheets of soil-free growing medium, which successfully grew Wedelia trilobata and Brassica chinensis Linn. The experimental results showed that liquid and gas phases existed in a ratio of 47:53 for the medium whose density was 0.030 cm³. These parameters allowed optimal respiration and moisture for the plant species in this study. LM PET content was crucial to the medium’s resistance to moisture deformation, and a 20 wt% of LM PET resulted in optimal resistance to moisture deformation of over 80%. When the LM PET content was 20 wt% and the density was 0.075 g/cm³, the growing medium exhibited the greatest liquid phase. This research successfully designed the processing technique of the feasible growing medium which could be manufactured fast and successively; this could not only decrease the production cost of the growing medium but also reduce the amount of metal racks and pots for plants.