Compressive Properties of Polyurethane Fiber Mattress Filling Material

Abstract

There is an inevitable trend toward exploring new, environmentally friendly fibers that can be used as raw material for mattresses with moderate hardness and air-permeable characteristics. Ethylene-propylene side by side (ES), high-shrinkage fibers, and thermoplastic polyester elastomer (TPEE) chips were introduced into polyethylene glycol terephthalate (PET)/polybutylene terephthalate (PBT) chip by melt blending to modify PET/PBT fiber. The modified PET/PBT (hereinafter referred to as PLON) is more suitable for mattress filling material than PET/PBT. To explore the compressive properties of PLON cushion made of PLON fiber and expand the scope of the PLON cushion’s application, a layered hardness test, hardness classification test and variance analysis were used to comprehensively evaluate the surface hardness, core hardness, bottom hardness and hardness classification of the mattress made of PLON cushion. The conclusions are: (1) The materials of the support layer have a significant effect on the hardness grade S. The hardness of the mattress with PLON as the support layer is between the spring and the coir; (2) when PLON is used as the material of the support layer, it possesses higher supporting force than coir and the characteristics of light weight and high resilience, which coir does not have; it is also softer than a spring mattress. As cushion material, it provides higher support for mattresses than foam. Practical applications, densities and structure were clarified through the above research, with implications for broader applications for PLON blocks in mattress products.

1. Introduction

Mattresses are essential and inevitable for people’s sleep quality. Higher requirements for mattresses are put forth with the improvement of living standards. Natural latex is expensive and scarce, and artificial latex is a nonrenewable resource. Spring manufacturing is a high energy-consumption industry, which is against the concept of sustainable development and cannot continuously supply a huge mattress market. Thus, there is a trend toward exploring alternatives to nonrenewable resources and environmentally friendly fibers with good performance for mattress production. Palm silk, brown mountain silk, bamboo silk and hemp fiber have been applied as mattress filling materials, but some of their properties are worse than conventional mattress material [1,2,3]. Environmentally friendly fiber has aroused extensive attention for its recyclability, biodegradability and eco-friendliness. Therefore, promoting the performance of environmentally friendly fiber and exploring the scope of its application as mattress filling material and upholstered furniture are possible methods to solve economic and environmental problems.

Researchers have conducted abundant research on different methods of producing and modifying environmentally friendly fiber that can be applied as mattress products [4,5,6,7,8]. Wu [9] found that cattail possesses not only high tensile strength and specific modulus but also the characteristics of light and stable structure and good performance on oil absorption, storage, insect resistance and lipophilic hydrophobicity. Chen et al. [10,11,12] applied loofah to a mattress-supporting layer and studied its effects. Cheng et al. [12] showed that natural loofah fiber possesses a porous structure, a high elastic modulus and good corrosion resistance and hardness.

Polybutylene terephthalate (PBT) fibers have not only low strength, high elongation at break and low initial modulus but also good elasticity and dyeability and a soft hand felling. It is suitable for the production of textiles with high requirements for elasticity. It is also widely used in the field of fiber. The unique properties of polyethylene glycol terephthalate (PET) fiber, which possesses high shrinkage properties, make it increasingly used in clothing, decoration and industrial use. Composite polyurethane fiber PBT/PET, one kind of polyurethane fiber with low cost that is environmentally friendly and has high resilience, is made of PBT chips and PET chips. Xiang [13] et al. added DOPO-based flame retardant to PET using a melt-blending method that proved to be helpful for enhancing flame-retardant properties. Li et al. [14] conducted tests of PET, and the results showed that it has a significant effect on flame-retardant introducing hexakis [p-(hydroxymethyl) phenoxy] cyclotriphosphazene (PN6)/poly (2-phenylpropyl) methylsiloxane (PPPMS) into PET. Modified PET/PBT (hereinafter referred to as PLON) is one kind of fiber with the characteristics of high resilience, strong support and good thermal insulation performance, which broadly expand its application scope in the textile industry. Liu et al. [15] conducted research on cushions made of modified PET/PBT fiber. The results showed that these modified PET/PBT fiber cushions have not only equivalent supporting ability to latex but also higher resilience. A cushion made of modified PET/PBT fiber has great application potential as mattress filling material.

This research aims to add ethylene-propylene side by Side (ES) chip, 3D hollow PET fiber and thermoplastic polyester elastomer (TPEE)/PET (hereinafter referred to as EJQ) chips into PBT/PET fiber by melt blending to modify PBT/PET fiber. The study also aims to explore the compressive properties and reasonable application of cushion made of modified PBT/PET fiber (hereinafter referred to as PLON). PLON is more suitable to apply as mattress filling material than PET/PBT for its high resilience, elasticity and tightness. PLON mattresses with different densities and structures were used as the research object, combined with comparative samples such as flexible polyurethane foam (FPF), coir mattresses and spring mattresses. The hardness classification and delamination hardness level were used to characterize the compressive properties of a mattress as a reference for later research: (1) The delamination hardness of different layers was obtained with a delamination hardness test. The surface hardness, core hardness and bottom hardness were analyzed to clarify the compressive properties of different mattress layers; (2) the hardness classification was obtained through the hardness classification test and used to evaluate the total hardness level of the mattress.

2. Materials and Methods

2.1. Materials

The specific fabrication process for the PLON fiber was (Figure 1): (1) First, PET/PBT chip, ES chip, 3D hollow PET fiber and EJQ chips were put into a roller to dry for 10 h.; the specific ratio is 35% EJQ, 40% PET/PBT, 10% 3D hollow PET fiber and 15% ES. The mixing ratio of PET and PBT is 5:5. Then, the dried material was heated by a single screw extruder. (2) The dried and filtered materials were put into a spinneret. The fiber filaments ejected from the spinneret were bundled and drawn through the bundling frame, forming multiple strands from one strand. Then, after slippery in the oil tank, 100-degree steam heating, crimping and setting, a certain degree of curl can be formed according to need. Lastly, the PLON fiber was heated in the dry room at 120 °C for 30 min [16,17,18]. A PLON cushion was made by the Airlaid process. ES fiber is a two-component skin-core structure whose melting point of the skin fiber is low. It can melt at a lower temperature to play a bonding role. EJQ fiber can provide elasticity for the composite fiber and play a bonding role. For component fibers, the best curl performance can be obtained when the mass ratio of the two components is controlled at around 50:50. Either higher or lower than 5:5, the curl performance of composite fibers will be reduced. Thus, the mixing ratio of PET and PBT is 5:5. The Airlaid process can further loosen the fiber collection and improve the separation of fibers with as little fiber damage as possible, dividing the fiber into a single fiber state, and mixing the fibers more uniformly.

PLON mattresses with different densities and structures were selected for the mattress hardness classification test and mattress delamination hardness test as the research objects, which were provided by Jinquan New Materials Co., Ltd. (Suzhou, China). Coir mattresses, spring mattresses and FPF were used as the comparative samples provided by Jinquan New Materials Co., Ltd. (Suzhou, China). Two levels of PLON density were selected: 30 kg/m³ (hereinafter referred to as P30) and 40 kg/m³ (hereinafter referred to as P40). One level of FPF density was selected: 30 kg/m³ (hereinafter referred to as F30). The sizes of the PLON block and FPF block were 380 × 380 × 20 mm, the PLON mattress and coir mattress were 380 × 380 × 95 mm and the spring mattress was 380 × 380 × 215 mm. The repetition amount of each sample was three pieces. All samples were tested in an environment with a temperature of 23 ± 2 °C and a humidity of 45–55%.

Table 1. Mattress structure configurations of different mattress groups.

		G1	G2	G3	G4	G5	G6	G7	G8	G9
Structure		F30+ Spring	P30+ Spring	P40+ Spring	F30+ Coir	P30+ Coir	P40+ Coir	F30+ P50	P30+ P50	P40+ P50
Fabric layer	First layer	5 mm Anti-mite FPF, density 18 kg/m3
	Second layer	10 mm FPF, density 18 kg/m3
	Third layer	non-woven fabric, density 6 kg/m3
Cushion layer	Fourth layer	2 cm FPF, 30 kg/m3	2 cm PLON, 30 kg/m3	2 cm PLON, 40 kg/m3	2 cm PLON, 30 kg/m3	2 cm PLON, 30 kg/m3	2 cm PLON, 40 kg/m3	2 cm FPF, 30 kg/m3	2 cm PLON, kg/m3	2 cm PLON, 40 kg/m3
Supporting layer	Fifth layer	spring	spring	spring	coir	coir	coir	P50	P50	P50
Fabric layer	First layer	nonwoven fabric, density 6 kg/m3
	Second layer	10 mm FPF, density 18 kg/m3
	Third layer	5 mm Anti-mite FPF, density 18 kg/m3

gives the mattress structure configuration of different mattress groups. Nonwoven fabric made of PET fiber and PP fiber is commonly applied on mattresses to separate spring and filling materials. Figure 2 is the scanning electron microscope (SEM) figure of the PLON blocks. The PLON fibers inside the PLON cushion are arranged in a disorderly manner. The fiber network structure is fluffy with a large void structure. There are not many bonding points. After pressing, some fibers will break. ES fiber is made by two components with skin-core structure. The melting point of the skin fiber is low, and it can be melted at a lower temperature to play a bonding role. EJQ fiber can provide elasticity for composite fibers and play a bonding role effect.

2.2. Methods

An SLFL-100KN universal testing machine provided by Shimadzu Co., Ltd., (Tokyo, Japan) was used to conduct the mattress hardness classification test and the mattress delamination hardness test. The diameter of the circular loading head of the mattress delamination hardness test is 100 mm, and the test speed is 100 mm/min. The diameter of the circular indenter of the mattress hardness classification test is 355 mm, the radius of the spherical indenter on the bottom is 800 mm and the test speed is 90 mm/min. The center of the indenter was placed directly above the center of the mattress sample. All samples were loaded vertically and set to unload when the load reaches the specified value and download the loading curves. Both tests were referring to the ergonomic evaluation of mattress–Part 1: Test and evaluation method of mattress hardness classification and distribution (T/SZFA 2003.1–2009). D_surface (mm), D_core (mm), D_bottom (mm) and D_total can be obtained by Equations (1)–(4). K (mm²) can be obtained by Equation (6), and the mattress hardness classification S can be obtained by Equation (7).

$$D_{surface}=D_{f40N}-D_{f4N}$$

(1)

$$D_{core}=D_{f200N}-D_{f40N}$$

(2)

$$D_{bottom}=D_{f250N}-D_{f200N}$$

(3)

$$D_{total}=D_{surface}+D_{bottom}+D_{core}$$

(4)

where D_surface (mm) is the surface hardness of the tested mattresses, D_f40N (mm) is the displacement of the loading curves at 40 N, D_f4N (mm) is the displacement of the loading curves at 4 N, D_core (mm) is the core hardness of the tested mattress, D_f200N (mm) is the displacement of the loading curves at 200 N, D_bottom (mm) is the bottom hardness of the tested mattress, D_f250N (mm) is the displacement of the loading curves at 250 N and D_total (mm) is the total hardness of tested mattress,

$$\mathrm{T}=\frac{{\mathrm{T}}_{210}+{\mathrm{T}}_{275}+{\mathrm{T}}_{340}}{3}\left(\mathrm{N}/\mathrm{MM}\right)$$

(5)

where T (N/mm) is the average slope of the loading curves, T₂₁₀ (N/mm) is the slope of the loading curves at 210 N, T₂₇₅ (N/mm) is the slope of the loading curves at 275 N and T₃₄₀ (N/mm) is the slope of the loading curves at 340 N.

$$\mathrm{K}=\frac{A_{45}}{T}$$

(6)

where A₄₅ (N·mm) is the area enclosed by the loading curve and the X axis in the interval 0–450 N and T is the average slope of the loading curves at 210 N, 275 N, 340 N. K < 900 mm² indicates that the mattress is hard, 900 mm² ≥ K ≤ 1800 mm² indicates that the mattress is moderately and K > 1800 mm² indicates that the mattress is soft.

$$S=10 {(1-e^{-\left(K\mathsf{\alpha }+b\right)})}^2$$

(7)

where $\mathsf{\alpha }$ is 7.737 × 10⁻⁴, $b$ is 9.706 × 10⁻² and $e$ is the natural constant. The mattress is hard when 1 ≥ S < 3, moderate when 3 ≥ S ≤ 6 and soft when 6 > S ≤ 10.

The vertical loading method is shown in Figure 3.

3. Results

3.1. Delamination Hardness

Table 2. The ANOVA results for the mattresses’ hardness.

Source	Dsurface		Dcore		Dbottom		Dtotal
	F Value	p Value	F Value	p Value	F Value	p Value	F Value	p Value
Cushion layer material	170.30	<0.0001	394.14	<0.0001	0.27	0.7693	125.03	<0.0001
Supporting layer material	371.72	<0.0001	8562.48	<0.0001	5634.12	<0.0001	6649.70	<0.0001
Cushion layer material × Supporting layer material	27.10	<0.0001	22.89	<0.0001	11.69	<0.0001	13.87	<0.0001

summarizes the ANOVA results obtained from the GLM procedure performed for each mattress layer’s hardness.

Table 3. Mean layered hardness of the mattresses evaluated in this study.

	Dsurface	Dcore	Dbottom	Dtotal
F30 + spring	19.14 (2) B	58.93 (3) A	11.2 (3) A	89.28 (3) A
P30 + spring	19.79 (1) A	54.68 (1) B	10.63 (2) B	85.1 (1) B
P40 + spring	18.78 (1) B	53.09 (1) C	10.59 (1) B	82.46 (5) C
F30 + coir	13.29 (4) F	25.61 (2) G	2.54 (2) E	41.44 (1) F
P30 + coir	16.67 (5) C	15.99 (2) H	2.49 (2) E	35.15 (5) G
P40 + coir	14.56 (2) E	15.98 (2) H	2.73 (3) E	33.28 (1) H
F30 + P50	14.51 (2) E	37.57 (1) D	4.7 (2) D	56.87 (1) D
P30 + P50	19.09 (1) B	33.63 (1) E	5.32 (3) C	58.03 (1) D
P40 + P50	15.43 (5) D	30.81 (1) F	5.29 (3) C	51.53 (2) E
LSD values	0.5965	1.0172	0.2885	1.5595

Note: Values in parentheses are coefficients of variation. Means without a common letter in the same column are significantly different from each other at the 5% level of significance.

summarized the means of each mattress layer’s hardness, Figure 4 summarized the means for each cushion layer’s hardness,

Table 4. ANOVA results for K and S.

Source	K		S
	F Value	p Value	F Value	p Value
Cushion layer material	1.46	0.2576	3.34	0.0585
Supporting layer material	314.46	<0.0001	642.44	<0.0001
Cushion layer material × Supporting layer material	0.37	0.8249	0.87	0.4991

summarized the means of the supporting layer’s hardness, and their comparisons were performed using the LSD multiple comparisons procedure. The results in Table 3 and Table 4 and Figure 4 indicate that types of material have a significant effect on surface hardness, core hardness and total hardness while having no significant effect on bottom hardness. The mattress’s surface hardness decreases as the PLON hardness of the cushion layer increases. The core hardness and total hardness of the mattress with 30 kg/m³ FPF as cushion layer material are greater, while those of the mattress with 40 kg/m³ PLON as the cushion layer material are smaller. This demonstrates that the mattress’s core hardness and total hardness decrease with the cushion layer material’s hardness increasing in that 40 kg/m³ PLON is harder than 30 kg/m³ FPF. As the indentation hardness of the cushion layer material increases, its ability to resist bending deformation also increases. The deformation of the bedding layer is limited, with the tensile stress of the surrounding material to cushion layer material increasing. In order to overcome the tensile stress, the force acting on the support layer by the cushioning layer material is reduced [19]. Thus, the indentation hardness of the supporting layer material decreases as that of cushion layer material increases.

The results in Table 3 and Table 4 and Figure 4 and Figure 5 demonstrate that the type of supporting layer materials has a significant effect on surface hardness, core hardness, bottom hardness and total hardness. This is consistent with the findings of Yu et al. [20]. The hardness characteristics of mattresses are different between mattresses with the same cushion material but different supporting materials. The indention hardness of each layer decreases first and then increases with the hardness of the supporting layer material increasing. The surface hardness of the mattress with PLON as the cushion layer material is greater than that of the mattress with FPF as the cushion layer material. This may be due to the high fiber content in the PLON cushion; the porosity is smaller than that of the FPF, and the bonding strength between the PLON fibers is high. It is not easy for PLON fiber to be separated, so the hardness of the PLON cushion is higher than that of the FPF cushion. Mattresses that use PLON as support layer materials have a firmness characteristic between spring mattresses and coir mattresses.

3.2. Hardness Grade

Analysis of variance (ANOVA) is a widely used technique for investigating the significance of input parameters and evaluating a model’s adequacy. The ANOVA results of $P_c$ and $\eta$ are shown in Table 2 and Table 3. The p-values of the quadratic models were less than 0.0001, indicating that these two models are extremely significant. For the same reason, all of the input parameters exhibited significant influence on $P_c$ and $\eta$, and many of the interaction terms also had significant influence. Therefore, a reasonable combination of milling parameters was essential to optimizing energy consumption and increasing power efficiency.

Table 4 summarized the ANOVA results obtained from the GLM procedure performed for each layer’s hardness of the mattress.

Table 5. Mean K and S Values of the mattresses evaluated in this study.

Source	K	S
F30 + spring	2208.30 (6) A	6.97 (4) A
P30 + spring	2290.16 (11) A	7.13 (7) A
P40 + spring	2240.70 (18) A	6.98 (12) C
F30 + coir	332.48 (3) C	0.89 (4) C
P30 + coir	414.66 (1) BC	1.17 (17) C
P40 + coir	310.2 (7) C	0.82 (82) C
F30 + P50	533.50 (1) BC	1.6 (60) B
P30 + P50	641.27 (2) B	2 (2.00) B
P40 + P50	371.09 (53) BC	1.06 (1.06) C
LSD values	304.60	0.68

Note: Values in parentheses are coefficients of variation. Means without a common letter in the same column are significantly different from each other at the 5% level of significance.

summarized the means of K and S values of the mattress,

Table 6. The mean hardnesses of the mattresses evaluated in this study (cushion layer).

Cushion Layer Material	K	S
F30	1024.76 (1.01) A	4.15 (1.06) A
P30	1115.36 (0.92) B	3.43 (0.94) C
P40	974.00 (1.13) B	2.95 (1.18) B
LSD values	175.86	0.3922

Note: Values in parentheses are coefficients of variation. Means without a common letter in the same column are significantly different from each other at the 5% level of significance.

summarized the means of K and S of the mattresses with different cushion layer materials, and

Table 7. The mean hardnesses of the mattresses evaluated in this study (supporting layer).

Supporting Layer Material	K	S
spring	2246.38 (1.84) A	7.03 (1.20) AB
coir	352.45 (16.61) A	0.96 (19.14) A
P50	515.29 (26.39) A	1.55 (30.38) C
LSD values	175.86	0.3922

Note: Values in parentheses are coefficients of variation. Means without a common letter in the same column are significantly different from each other at the 5% level of significance.

summarized the means of K and S of the mattresses with different supporting layer materials, and their comparisons were performed using the LSD multiple comparisons procedure. K < 900 mm² means the mattress is relatively hard, 900 mm² ≤ K ≤ 1800 mm² means the mattress is moderately firm, and K > 1800 mm² means the mattress is soft. In the equation 1 ≤ S ≤ 10, the smaller the S, the harder it is; S < 3 means hard, 3 ≤ S ≤ 6 means moderately soft and hard, and 6 < S ≤ 10 means soft.

The results in Table 5 and Table 6 show that the type of cushion layer material has a significant effect on the hardness grade S. The mattress grade S with F30 and P30 as the cushion layer material is greater than 3, which represents that the mattress is moderately soft. Mattress grade S with P40 as the cushion layer material is less than 3, which indicates that the mattress is too firm. The hardness index value S of the mattress used by women is preferably 3~7, so the mattress with F30 and P30 as the bedding material is more suitable for women. According to Table 6 and Table 7, the supporting layer material has a significant influence on the hardness grade S. The degree of influence is greater than that of the cushion layer material. The firmness of the mattress with PLON material as the support layer is between the spring mattress and coir mattress, which is consistent with the layered firmness results. This may be due to high-density PLON possessing high internal fiber content and tiny pores, and the bonding strength between PLON fibers is higher than that of coconut palm fibers. Therefore, the PLON fibers are not easily washed away when stressed [21].

According to the above results, when PLON is used as the material of the supporting layer, it not only has higher supporting force than a coir mattress and the characteristics of light weight and high resilience that a coir mattress does not have, but it is also softer than a spring mattress. As a cushion layer material, it provides higher support for mattresses than foam.

4. Conclusions

A layered hardness test, hardness classification test and variance analysis were carried out to explore the layered hardness and hardness classification of a PLON mattress to explore its application potential and expand the scope of its application. The main conclusions can be drawn as follows:

(1) The type of cushion layer material has a significant effect on the surface hardness, core hardness and total hardness but has no significant effect on bottom hardness. The surface hardness of the mattress with PLON as the cushion material is greater than that of the mattress with FPF as the cushion material due to the high fiber content, the smaller porosity compared with FPF and the high bonding strength between the PLON fibers.

(2) The material of the supporting layer has a significant effect on the surface hardness, core hardness, bottom hardness and total hardness. A mattress with PLON as a support layer material has a firmness characteristic between that of a spring mattress and a coir mattress.

(3) The type of cushion layer material has a significant impact on the hardness grade S. The mattress with P30 as the cushion layer material is moderately soft, and the mattress with P40 as the cushion layer material is hard. The material of the support layer has a significant effect on the hardness grade S. The hardness of the mattress with PLON material as the support layer is between the spring and the coir due to higher internal fiber content, smaller pores and high bonding strength between PLON fibers compared with coconut palm fibers.

(4) When PLON is used as the material of the support layer, it not only has higher supporting force than coir and the characteristics of lightweight and high resilience that coir does not have, but it is also softer than a spring mattress. As cushion material, it provides higher support for mattresses than foam.