Characterization of flame-retardant performance of polyester/flame-retardant viscose blended yarn

Materials and sample preparation

The PET fiber (from Sinopec Yizheng Chemical fiber Co, Ltd, P.R. China, finess: 145 dtex, tenacity: 2.8 CN/dtex, elongation at break: 56.8%) was blended with FRVF (made by our lab, finess: 1.5 dtex, tenacity: 1.8 CN/dtex, elongation at break: 22.9%) by core-spun method. Ring spinning method was used to spinning. PET fiber filament was used as a core, and the FRVF was used as a sheath. After the FRVF was spun into roving, PET fiber filament was added in spun yarn. The core yarn samples were prepared with different weight ratios by adjusting the amount of FRVF. Final samples labeled as CSY-1 (PET:FRVF=60:40), CSY-2 (PET: FRVF=55:45), CSY-3 (PET:FRVF=50:50), CSY-4 (PET:FRVF=40:60), CSY-5 (PET:FRVF=30:70), respectively. The specific spinning process is shown in Figure 1. OPEN IN VIEWER

Characterization of prepared sample

The LOI tests of the samples were carried out by a M606B digital LOI meter (Qingdao Shanfang, P.R. China) according to the ASTM D2863-17 standard. Flammability of the sample was tested on a GT-C32 45-degree flammability tester (GESTER, China), according to 45-degree slope burning method in Japan industry standard JIS L 1091-1999. The samples were subjected to 30 home laundering cycles using a reference detergent according to the ISO 6330:2012 standard. Properties of the fibers before and after the washing were compared.

The SEM images were taken using a S4800 II cold-cathode field-emission SEM (Hitachi, Japan) at various accelerating voltages of 10–15 kV. It should be noted that the accelerating voltage needs to be adjusted in order to prevent the breakdown of nonconductive VF. The SEM samples were coated with gold.

The TGA and DSC of the samples were carried out using a STA449C thermal analyzer (NETZSCH, Germany) at a heating rate of 10℃/min from room temperature to 900℃ in air. The TG-FTIR of the samples was carried out using a STA449C thermal analyzer (NETZSCH, Germany) interfaced to an E55 FTIR spectrometer (Bruker, Germany) in which the spectral range was 4000–400 cm⁻¹, the resolution can achieve 0.2 cm⁻¹, wavenumber accuracy was better than 0.01 cm⁻¹, transmittance accuracy was better than 0.1%. The heating rate was set as 10℃/min from room temperature to 900℃ in air. The airway was maintained at a constant temperature of 200℃ in the TG-FTIR test to ensure that the gases do not condense. One end of airway was connected to the thermal analyzer directly. The 3D diagrams from the TG-FTIR technique can provide a qualitative picture of the overall information of sample thermal decomposition and the absorbance corresponding to the vibrational modes of the different bonds or functional groups that was represented versus the wavenumber and the temperature [13].

The mechanical properties of the samples were tested on an YG020D-50 yarn electronic tension meter (Changzhou Huafang, P.R. China) at 25℃ and relative humidity 65%.

Physical and flame retardancy of samples

The mechanical properties of the samples are shown in Table 1.

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Table 1.

Mechanical properties of samples.

Sample	Finess (dtex)	Elongation at break (%)	Tenacity (CN/dtex)
FRVF	1.5	22.9	1.8
PET	145	56.8	2.8
CSY-1	300	8.77	2.2
CSY-2	332	8.91	2.1
CSY-3	362	9.12	2.0
CSY-4	375	9.23	1.9
CSY-5	460	9.29	1.9

The LOI values of the samples are shown in Figure 2. The LOI value of PET was only 21% before and after washing, and the test found that PET fibers have molten drop phenomenon, which means PET has no flame-retardant properties. However, the LOI of CSY-3 was still >27% after 30 washing cycles, indicating that phosphazene derivatives impart durable flame retardancy to the fibers. The LOI value of samples CSY-1, CSY-2, and CSY-3 was bigger than that of VF and PET samples, and the higher the FRVF content of yarn, the bigger its LOI value, which present the flame-retardant property of CSY can be greatly improved by adding FRVF. This should be mainly caused by the addition of phosphazene derivatives. It can also be found that the LOI value of yarn decreases slightly after 30 washes, indicating that the wash has no significant effect on the flame retardancy. OPEN IN VIEWER

Table 2 gives the 45-degree ignition test result. We can see that the results from 45-degree ignition test have a similar change as LOI test: the damaged length and weight loss of the samples dropped accordingly with the increase of FRVF content both before and after washes. All CSY samples survived three ignitions with no open flame. After burning, char coat was formed on the surface of FRVF, CSY-1, CSY-2, and CSY-3. These results proved that the introduction of FRVF increased the flame retardancy of core-spun yarn. As previously mentioned, CSY-3 has the best flame-retardant properties, so it was chosen as a representative sample for the study.

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Table 2.

Forty-five-degree ignition test result before and after washing.

Sample	Washing cycles	After glow time (s)	Char length (mm)	Damaged length (mm)	LOI value (%)	Weight loss (%)	Burning phenomenon
FRVF	0	0.1	48	52	28.8	81.8	No open flame, char coat on surface
	30	0.1	46	48	28.6	82.2
PET	0	16.1	0	100	21	99.7	Completely burned, dripping
	30	16.5	0	100	21	99.7
CSY-1	0	2.1	38	54	25.8	87.3	No open flame, uneven char coat on surface
	30	2.0	37	53	25.4	86.8
CSY-2	0	2.1	42	49	27.6	86.7
	30	2.0	40	52	27.1	86.5
CSY-3	0	1.8	45	47	28.6	85.4
	30	2.0	44	45	28.1	85.3
CSY-4	0	1.8	46	47	28.6	84.2
	30	2.0	45	46	28.2	84.2
CSY-5	0	1.8	47	47	28.7	84.1
	30	2.0	46	46	28.2	84.1

Microstructure of the fiber sample

The SEM images of the FRVF (before and after burning), PET (before and after burning), and residues of CSY-3 (after burning) are shown in Figure 2. Before burning, FRVF (a) and PET (c) both have a smooth structure. After burning, in the residues of FRVF the fiber structure was maintained, and the surface of fibers was carbonized as shown in Figure 3(b). And in the residues of PET the fiber structure was lost completely, and the surface of PET was melting into a mass as shown in Figure 3(d). However, CSY-3 has shown a totally different phenomenon to FRVF and PET, as shown in Figure 3(e) and (f). Most of the fibers of CSY-3 still maintain the tight yarn structure of the core yarn after burning. As shown in Figure 3(e) most of the residual fibers have the characteristics of FRVF from the fiber’s shape, which presents a flat fibrous structure instead of the cylindrical structure. And we can see a large number of irregular bulges appeared on the surface of the CSY-3 combustion product in Figure 3(f) also, which is clearly different from Figure 3(b). OPEN IN VIEWER

In the heating process, PET melted at about 280℃ and flowed to FRVF due to the guiding effect of heat flow just like a wick effect. Before PET was decomposed, it was adsorbed by FRVF. Phosphazene derivative which was used as a flame retardant in FRVF (∼10%) was heated during the fire. The decomposition of phosphazene derivative occurred in the range of 177–350℃ [14]. The phosphorus present formed the liquid-state membranes of phosphoric acid when heated and then dehydrated to form polymetaphosphate. A covering layer was formed on the surface of the fiber by polymetaphosphate to prevent further thermal decomposition of FRVF. In the thermal decomposition of phosphazene derivative, nitrogen, nitrogen oxides, and other nonflammable gases were continuously generated. These gases and the liquid film formed a swelling bubble-like structure as shown in Figure 3(b); this air-free coat is more conducive to the heat insulation of the interior of the fiber. It can also be seen that as the inside of the fiber still continues to degrade slowly after carbonization and gas was generated, the carbonized layer is continuously stressed, therefore, cracks appeared on the surface of the carbonized layer.

Since PET was adsorbed by FRVF before being decomposed, the polymetaphosphate not only covered the surface of the FRVF, but also covered PET, and formed a cover layer. PET’s dehydration was accelerated by polymetaphosphate and it formed a char layer before being decomposed. With the release of nonflammable gases, the surface of the sample formed an inflated air-free coat. The rugged surface structure was formed as we can see in Figure 3(e) and (f). This showed a typical intumescent flame-retardant mechanism.

Thermal behavior of the fiber compositions

Thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of PET, FRVF, and CSY-3 are shown in Figure 4. Differential scanning calorimetry (DSC) curves are shown in Figure 5. The thermal degradation process of PET can be divided into two major stages: FRVF can be divided into three major stages, and the CSY-3 can be divided into four major stages. The rates of weight loss were reflected on DTG curves. OPEN IN VIEWER

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Figure 5.

DSC curves of samples. DSC: differential scanning calorimetry; FRVF: flame-retardant viscose fiber; PET: polyethylene terephthalate.

For CSY-3, stage 1 is the dehydration process in the range 50–140℃ with a weight loss of ∼3.7%. As shown in Figure 4, the moisture content of CSY-3 was higher than PET (∼0.4%) and less than half of the FRVF (∼10%). This is caused by the twist of reduced specific surface area.

Stage 2 began at about 150℃ (same as FRVF) and ended at ∼370℃, and the weight loss was 35.6%. As shown in Figure 4, PET has no weight loss before 360℃, and the weight loss of FRVF was 47.4% at this range. Theoretically, CSY-3 should lose about 23.7% of weight loss in this phase if no interaction occurs. The actual test result was 35.6%, which is much larger than the theoretical value. This means that, when the temperature up to ∼280℃ (melting point of PET), PET melted and flowed to FRVF due to the guiding effect of heat flow and accelerated dehydration by polymetaphosphate (produced by the decomposition of FRVF). It is clear that blending with FRVF caused the lower decomposition temperature and more weight loss of primary decomposition for PET.

The DSC results suggest that the process is much complicated with the blend of FRVF and PET. In this stage, the DSC curve of FRVF has two endothermic peaks at ∼310.3 and ∼375℃, and PET gives no exothermic peak. The DSC curve of CSY-3 gives only one exothermic peak at 337.7℃. It is hard to identify each reaction in this process. However, it is clear that the major decomposition reaction of PET was accelerated by the blend of FRVF.

Stage 3 began at about 370℃, a little lower to FRVF and PET, ended at ∼470℃, and the weight loss was 23.6%. In this stage, the DSC curves of CSY-3 and FRVF give no endothermic peak. The DSC curve of PET gives the only one endothermic peak at 464.4℃. This stage mainly corresponds to the decomposition of PET that was not carbonized in the previous stage. Compared to the weight loss temperature of PET in this range, the weight loss temperature of CSY-3 at this stage is advanced by ∼10℃. However, the weight loss is far less than half of the weight loss of PET in this temperature range (81.5%). This is still due to the polyphosphoric acid produced by the decomposition of FRVF carbonizing PET, causing a portion of the melted PET to be carbonized at a lower temperature. The carbonized products were first formed on the outer layer of the yarn due to the core yarn spinning method. The formed carbonized layer tightly covers the fibers inside the yarn isolating heat and oxygen; thus, the fibers inside the yarn were protected and the structure of the yarn was preserved.

The final stage begins at 550℃ until the end. At this stage, the weight loss of CSY-3 was 22.0%, and the final residual weight was 15.1%. In this stage, the DSC curve of CSY-3 gives two exothermic peaks at 501 and 529℃. The DSC curve of FRVF peaked at 530℃ and the curve of PET peaked at 568.9℃. The residual weight of CSY-3 at this stage is far greater than the average of PET and FRVF residuals (18.7 + 1.3)/2 = 10%. In this range, the carbonized product of FRVF and PET was heated together; all these carbonized products were further oxidized to produce small molecular gases such as CO₂ and H₂O. In the previous temperature range, PET was carbonized by the poly(meta-phosphoric acid) decomposed by FRVF. The carbonized layer caused a lot of PET fail to completely decompose at previous temperature range, but decomposition and carbonization after further heating, which caused a high weight loss rate for CSY-3 at this stage.

Real-time qualitative analysis of volatile products generated during thermal oxidation

The 3D IR spectroscopy of thermal decomposition products of PET (a), FRVF (b), and CSY-3(c) is shown in Figure 6. From the figure, we can find many discontinuous peaks, which is due to the fact that some small particles partially block the airway during the TG-FTIR test. After comparing with the standard pattern (EPA Vapor Phase No. 2911), it was confirmed that 2630 cm⁻¹ was the main characteristic peak of CO₂ and 3725, 3694, 3624, 3602 cm⁻¹ were also the characteristic peaks of CO₂. The main product in the thermal decomposition process of all these three samples is CO₂. A series of small absorption peaks appearing at 3500–3900 and 670 cm⁻¹ are characteristic absorption peaks of gaseous H₂O. OPEN IN VIEWER

For PET, the C=O stretching peak of phthalic anhydride appeared at 1762 cm⁻¹ at 468℃, which means PET produced a large amount of phthalic anhydride when decomposed by heat. The peak gradually disappears at 611℃, which shows that phthalic anhydride was further oxidized and decomposed at high temperatures. The COO antisymmetrical vibration peak appeared at 1540 cm⁻¹ at 380℃ and gradually disappears after 380℃, which means decomposition products of PET at lower temperature have carboxylate COO appearing.

The thermal decomposition products of CSY had only a small amount of CO₂ and H₂O before 300℃, the CO₂ concentration gradually increased after 300℃. Simultaneously, the peaks of the stretching peaks of ester C=O at 1740 cm⁻¹ and the characteristic absorption peaks of small molecules of alkanes at 1080 cm⁻¹ appeared, which indicates that at this temperature CSY-3 decomposes violently and produces a large number of small molecular gases. These gases are partially oxidized to CO₂ and H₂O, and some of them are not oxidized and enter the detection chamber. This corresponds to the second and third stage weight loss in the TGA. When the temperature reached 350℃, the characteristic absorption peak of the small molecule gas disappeared, leaving only the characteristic absorption peaks of CO₂ and H₂O. When the temperature reaches 600℃, the absorption peak of CO₂ also becomes insignificant. This is due to the char layer coated on the surface of the yarn preventing the oxidation and decomposition inside CSY-3; only a small amount of H₂O absorption peak indicates that the dehydration carbonization reaction of the polymetaphosphoric acid inside the yarn is still in progress. The main products in the thermal decomposition of CSY-3 were H₂O and CO₂.