Fabrication and properties of bio-based layer-by-layer coated ramie

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Fabrication and properties of bio-based layer-by-layer coated ramie fabric-reinforced unsaturated polyester resin composites Xiaojuan Yu, Ying Pan, Dong Wang, Bihe Yuan, Lei Song, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00101 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Industrial & Engineering Chemistry Research

Fabrication and properties of bio-based layer-by-layer coated ramie fabric-reinforced unsaturated polyester resin composites

Xiaojuan Yu1, Ying Pan1, Dong Wang1, Bihe Yuan2, Lei Song1* and Yuan Hu1*

1. State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, P. R. China 2. School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, P. R. China

Corresponding Author *Tel./Fax: +86-551-63600081. E-mail: [email protected]. (Lei Song) *Tel./Fax: +86-551-63601664. E-mail: [email protected]. (Yuan Hu)

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ACS Paragon Plus Environment


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Abstract Ramie fabrics were modified with bio-based electrolytes, i.e. cationic chitosan and anionic phytic acid and flame retardant cationic electrolyte melamine by layer-by-layer (LbL) assembly method. Then, the LbL-modified fabric-reinforced unsaturated polyester resin (UPR) composites were fabricated by a hand lay-up method.

Onset

thermal

degradation

temperature

of

the

LbL-modified

fabric-reinforced UPR composites was lower than that of raw fabric-reinforced UPR sample, while char residues of the composites were greatly improved and the release of combustible organic volatiles were suppressed during the combustion. Moreover, flame retardancy of the composites were enhanced with increasing the LbL assembly number, which were evidenced by the markedly reduced peak heat release rate and total heat release values obtained from cone calorimetric tests. The limiting oxygen index value of the composites was increased from 26.0% to 34.5%.

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1. Introduction Unsaturated polyester resins (UPR) are one of the most widely used popular thermoset polymers in fiber/fabric-reinforced polymer composites because of their low cost, easy processing, relatively high strength and good corrosion resistance properties.1-3 With the growing environmental crisis, the utilization of natural fibers as reinforcement has been an increasingly popular alternative to synthetic fiber in the engineering field. Natural fibers have several advantages, such as biodegradable, abundant resource, non-toxic, recyclable, favorable mechanical properties and high strength-to-weight ratio.4-6 Different kinds of natural fibers possess different mechanical performance. The tensile strength of sisal fiber is as high as 400-700 MPa, and Young’s modulus is 9-38 GPa. The tensile strength and Young’s modulus of hemp fiber are 550-900 MPa and 70 GPa, and the specific modulus (modulus/density) is higher than that of E-glass.7 Besides, the mechanical performances of natural fibers (sisal, coir, hemp, jute and kneaf) reinforced polypropylene (PP) composites have been investigated by Paul et al.8 The mechanical properties of the tested natural fiber/PP composites are comparable with the corresponding properties of glass fabric/PP composites in the most cases. In the work by Supranee et al., the mechanical properties of the sisal fiber/UPR composite are enhanced by sisal fiber modified with polymethyl methacrylate, due to the achievement of strong interfacial adhesion.9 Ramie, as a kind of famous natural cellulosic fiber, is derived from the phloem tissue of the plant, which is known as “china grass”. It has significantly smaller diameter (10-60 µm), a low specific density (1.5 g/cm3) and a reasonably higher 3



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tensile strength at 850-900 MPa than other hemp fibers, making it a potential alternative to manmade fibers.10, 11 Ramie also has been utilized as the reinforcement for various polymers such as benzoxazine resin, poly(lactic acid), epoxy and PP.12-15 However, the poor fire resistant of this natural fiber and UPR composite may hinder its application in polymer materials. Flame retardant modification on ramie fiber/fabric and its polymer composites have been explored recently and the flame-retarded fabric may endow good flame retardancy to the whole composite materials.16, 17 Many efforts have been devoted to investigate the flame retardancy of textile fabric, but health risks may arise during the manufacture process. Spirocyclic pentaerythritol diphosphoryl chloride and its phosphorylation derivatives were synthetized to modify the fabrics using the organic solvents, such as dimethylformamide, DMF and the reaction temperature reached 160 °C.18-20 Moreover, the application of commonly durable flame retardants for fabrics may involve the use of formaldehyde to ensure the flame retardant reacting with cellulose-OH moieties.21,

22

Compared to the

traditional flame retardants on fabrics, layer-by-layer (LbL) assembly technique can minimize the risk of the environment pollution. Meanwhile, LbL assembly technique is easy to be operated under mild conditions (e.g. room temperature, atmospheric pressure, using water as solvent), the concentration of immersion solution is relatively low and can be recycled.23 LbL assembly technology, mostly based on the alternating electrostatic attraction of oppositely charged polyelectrolytes, is a relatively novel process to endow the flame 4


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retardancy of fiber/fabric in recent years.24-27 The properties of simplicity, operability, universality and thickness controlling at the nanoscale level make LbL assembly technique be superior to the other traditional coating methods.28 Moreover, considering the toxicity and environmental concern, the “green” materials as electrolytes for LbL assembly have attracted considerable attentions. Chitosan (CH), an amino polysaccharide derived from chitin, is positively charged at low pH (pKa 6-6.5) which make it a favorable candidate for LbL assembly.29 Pan at al. have found that eight-bilayer CH/lignosulfonate coating on flexible polyurethane foams (FPUF) can significantly improve its fire resistance.30 Carosio et al. have employed DNA and chitosan as LbL assembly polyelectrolytes to obtain efficient flame retardant property for cotton.31 Phytic acid (PA) is a kind of organic phosphate compound extracted from plants seeds. The features of environmental friendly, nontoxic and biocompatible make PA be extensively applied as food additive, antioxidant, drug and antistaling agent.32-36 Furthermore, the six phosphate groups in the structure of PA make it be a phosphorous-containing flame retardant.37, 38 The negative charge performance (pKa 1.9-9.5) enable PA and its salts interact with positively charged molecules.39 The eco-friendly intumescent nanocoatings (cationic CH and anionic PA) were deposited on fabrics by LbL assembly in the study by Laufer et al. and it was observed that the fabrics coated with high PA content multilayers can completely extinguish the flame in horizontal flammability test.40 Due to environmental friendly consensus and innovation of biodegradable or recyclable materials, it is preferable to choose “green” materials to modify the 5



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fiber/fabric. Hence, in this work, ramie fabrics were pretreated with cationic CH/melamine (ME) and anionic PA by LbL assembly technique and then immersed in Fe(NO3)3 solution to react with PA. UPR/fabric composites were fabricated by a hand lay-up method, using LbL-modified ramie fabrics as the reinforcement. The effect of LbL-modified fabric on the thermal, flammability and mechanical properties of the UPR composites were investigated. The aim of this work is to provide a potential strategy for constructing a bio-based high performance UPR/fabric composites to enlarge the application of natural fiber/fabric-reinforced polymer. 2. Experimental section 2.1 Materials All the reagents used were of analytical grade. Plain ramie fabrics (21S × 21S/52 × 58) were purchased from Hunan Huasheng Dongting Co., Ltd. UPR (commercial name 196) with the styrene content of about 30 wt% was supplied by Hefei Chaoyu Chemical Co., Ltd. CH, ME, acetic acid, Fe(NO3)3·9H2O and benzoyl peroxide (BPO) were obtained from Sinopharm Chemical Reagent Co., Ltd. BPO was purified by recrystallization from methanol. PA (70% aqueous solution) was acquired from Aladdin Chemistry Co., Ltd. Deionized (DI) water with a resistance of 18.2 MΩ was used for all experiments. 2.2 Layer-by-layer assembly The fabric was modified by LbL assembly technology referring to the previous literature.40 Firstly, cationic deposition solution was prepared by adjusting the pH of DI water to 3 with acetic acid and then 0.5 wt% CH and 1 wt% ME were added. This 6


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solution was magnetically stirred until the CH and ME were completely dissolved. Anionic solution was obtained by adding 2.0 wt% PA to DI water and stirred to form a homogeneous solution. Prior to the deposition, ramie fabrics were washed with DI water, and air-dried at room temperature. Then, ramie fabrics were alternately immersed in cationic solution and anionic solution as shown in Figure 1. The initial alternation of dips were carried out for 5 min and subsequent dips were 1 min. After the 15 bilayers (BL) were deposited, the fabrics were soaked in 5 wt% Fe(NO3)3 solution for 10 min. Then the fabrics were washed by deionized water to remove the unreacted ferric ion. The obtained fabrics were dried in a 70 °C oven overnight and defined as L15/Fe-fabric. For comparison, the fabrics with 5 and 15 BL coating without immersion in Fe(NO3)3 solution were marked as L5-fabric and L15-fabric, respectively. 2.3 Preparation of UPR/ramie composites BPO, as the polymerization initiator, was added in the UPR matrix with the content of 2 wt%. Samples were prepared via a hand lay-up method by impregnating the layers of raw or LbL-modified fabrics and then pressed at 6-8 MPa under a 1.00 MN semi-automatic moulding press machine (HPC-100) for 10 min. The curing process was carried out in an oven at 70 °C for 4 h and post cured at 120 °C for 3 h. The formulations of UPR/ramie fabric composites are listed in Table 1. The gel content of UPRs were tested by Soxhlet extraction to determine the effect of LbL modification on its cross-linking density.41 The samples were weighed and extracted using toluene as the solvent for 24 h. The gel content was calculated by the 7



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formula: gel content% = (W2/W1)×100%; W1 and W2 represent the weight of the sample before and after extraction, respectively. From the Table 1, it is clearly found that the gel content of UP-3 is lowest. Higher gel content corresponds to higher cross-linking density of samples. These result indicates that the excess PA may have a little negative effect on the cross-linking density of the composites, because the PO43in PA can partly react with the ester group in UPR during the thermal curing process. 2.4 Characterization. Scanning electron microscopy (SEM) images of fabric were taken on a JSM 6700F microscope (JEOL Co., Ltd., Japan) at the acceleration voltage of 5 kV. SEM images of char residue and fractured surface were acquired by XL-30 environment scanning electron microscopy (FEI Co., Ltd., U.S.A.) at the acceleration voltage of 20 kV. All samples were sputter coated with gold layer before analysis. Thermogravimetric analysis (TGA) were conducted using a Q5000 thermal analyzer (TA Co., Ltd., U.S.A.) at a heating rate of 20 °C/min. Limiting oxygen index (LOI) values were measured by HC-2 oxygen index apparatus (Jiangning Analysis Instrument Co., Ltd., China), according to ASTM D2863. The combustion test was performed on cone calorimeter (Fire Testing Technology, U.K.) under the procedures in ISO5660. Each specimen with the dimensions of 100 × 100 × 3 mm3 (length × width × thickness) was exposed to 35 kW/m2 heat flux. Thermogravimetric analysis-infrared spectrometry (TG-IR) was carried out by TGA Q5000 thermogravimetric analyzer connected with a Nicolet 6700 FTIR 8


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spectrophotometer (Nicolet Instrument Company, U.S.A). Mechanical performance was measured under a CMT6104 universal testing apparatus (Shenzhen SANS Material Detection Co., Ltd., China). Tensile tests were carried out at a cross-head speed of 1.0 mm/min and a gauge length of 50 mm, according to GB/T 1447-2005. Flexural tests were conducted by three-point bending method, according to GB/T 1449-2005. Five replicates were prepared for each test. 3. Result and discussion 3.1 Characterization of LbL-modified ramie fabric and UPR/ fabric composite Surface morphologies of the raw and LbL-modified ramie fabrics are displayed in SEM images, and the element composition of the surface of fabrics is identified by energy dispersive X-ray spectroscopy (EDX), which are shown in Figure 2. It is obvious that the raw ramie fabric presents the typical morphology of cellulosic fibers and the surface is relatively smooth. After the fabrics are covered with 5 BL coating, a thin and homogeneous coating is observed on the surface of L5-fabric. Some fiber edges in L5-fabric are not distinct and filled with the coating. Increasing the assembly layer number to 15 BL results in an increased coating thickness. Meanwhile, the formation of compact coatings on the surface and interface between the adjacent fibers is also observed in L15/Fe-fabric. From the EDX analysis, N and P elements are obviously detected in both L5-fabric and L15-fabric, indicating the presence of flame retardant on the surface of ramie fabric. Furthermore, Fe element is also found in L15/Fe-fabric, which means PA has successfully chelated with Fe3+.42, 43 3.2 Thermal decomposition behavior of ramie fabric and composites 9



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The representative TGA and derivative thermogravimetric analysis (DTG) profiles of raw fabric, LbL-modified fabric and UPR/fabric composites under N2 atmosphere are shown in Figure 3 and 4, and the detail information is summarized in Table 2. The temperatures at mass loss of 10% and maximum decomposition rate are defined as the definitions of onset degradation temperature (T10%) and maximum degradation temperature (Tmax), respectively. As shown in the Figure 3, TGA curves of LbL-modified fabric are similar to that of the raw sample. The main loss stage is mainly ascribed to the decomposition of cellulose macromolecular chains. It is apparent that both T10% and Tmax of LbL-modified fabric are lower than those of raw sample, as shown in Table 2. This can be ascribed to the lower thermal stability of PA in fabric. The degradation of coating on the fabric is beneficial to catalyze the dehydration of cellulose to form thermally stable char residue. The char residue formed can act as thermal shield for inhibiting mass and heat transfer during the pyrolysis process.40, 44, 45 Indeed, with increasing the number of assembly layer, the deposition of electrolytes on the fabric as well as the char residue are increased. Furthermore, Fe3+ chelate with PA, then the acid group on the fabric reduce, thus the T10% of L15/Fe-fabric exhibits a slight increment, as compared with L15-fabric. For UPR/fabric composites, the T10% of UPR/LbL-modified fabrics composite is lower than that of the UPR/raw fabric composite. As observed in the DTG profile, UPR/fabric composites have two thermal degradation stages. The first degradation stage belongs to the decomposition of cellulosic fibers and the second one attributes to the pyrolysis of unsaturated polymer resin.41 The Tmax1 and Tmax2 represent the 10


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temperatures for the weight loss at the first and the second maximum rates, respectively. The incorporation of LbL-modified fabrics results in the reduction of T10% of UP-1. As increasing the BL number, the T10% of UPR/LbL-modified fabric samples are decreased and the higher char residues are obtained. These indicate that the LbL coating can promote the char formation of the composite during the thermal decomposition.46 Besides, the T10%, Tmax1 and Tmax1 of UP-4 are slightly improved in comparison with UP-3 sample. 3.3 Flame retardancy of the composites Flame retardant property of UPR/ramie fabric composites is evaluated by LOI test, which corresponds to the minimum volume percentage of oxygen required for the combustion of the material in oxygen-nitrogen atmosphere. The LOI of UPR/raw fabric composite is 26.0%, as shown in the Table 3. After LbL modification on the fabrics, the LOI value raises from 26.0 to 34.5% as increasing the assembly layer. These results imply that CH/ME and PA coating can endow excellent flame retardancy to UPR/fabric composites. Cone calorimeter is another method to estimate the fire performance of the composites laminates. Heat release rate (HRR) versus time are plotted in Figure 5 and the important parameters including time to ignition (TTI), peak heat release rate (PHRR), total heat release (THR), time to PHRR, fire performance index (FPI) and fire propagation index (FGI) are summarized in Table 3. As expected, LbL-modified fabric-reinforced UPR composites exhibit the relatively lower PHRR and THR, compared to UP-1. In addition, with the increment content of electrolytes, flame retardancy of UPR/LbL-modified fabrics composites are greatly 11



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enhanced. The PHRR of UP-3 sample is reduced from 562 (UP-1) to 387 kW/m2, while the THR is decreased from 60 to 49 MJ/m2. Furthermore, compared to the UP-3, L15/Fe-modified fabric lead to a further reduction in THR while increment in PHRR, which probably due to the existence of small-molecule iron phytate.37 Furthermore, FPI, the ratio between TTI and PHRR, is an important indicator for characterization of fire hazard. Higher value of FPI indicates lower fire hazard. FGI is calculated by ratio of PHRR to the time to PHRR and it is another parameter to evaluate fire risk. The value of FGI is lower, implying that it takes a longer time to reach PHRR and leads to the reduction of fire hazard. Considering the highest FPI and the lowest FGI value, the UP-3 has the best flame retardancy. TG-IR technique was used to investigate the flame retardant mechanism of UPR composites by analyzing the gas products during the thermal degradation. As exhibited in Figure 6a, there are two peaks in the Gram-Schmidt curves of the UPR composites.

The

absorbance

intensities

of

total

pyrolysis

products

from

UPR/LbL-modified fabrics composites are reduced, as compared with UP-1. Comparing with that of UP-3, the absorbance intensities for the first maximum evolution is increased in UP-4, but the second one is decreased, corresponding to the DTG curve in the Figure 4. The difference may be ascribed to the catalytic effect of ferric phytate: the increasing absorbance intensity for the first stage in UP-4 is probably attributed to the decomposition of ferric phytate and then Fe3+ in the electrolytes on the fabric can catalyze the formation of char residue which serve as protective shield to cut down the heat and oxygen transfer.37, 47 12


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Some low-molecular volatile pyrolysis products of the UPR composites can be identified unambiguously by the characteristic FTIR peaks: the peaks at 3600-3650 cm−1 are stemmed from stretching vibration of -OH; the peaks at 2930-2982 cm−1 are attributed to the aliphatic C-H bonding derived from various alkanes; the band at 2360 cm−1 is ascribed to stretching vibration of CO2 originated from decarboxylation of unsaturated polymer; the peaks at 1762 cm−1 is assigned to the absorbance of stretching vibration of C=O groups from carbonyl compounds; the peaks at 1605, 1416 and 912 cm−1 are associated to aromatic compounds.48, 49 From the Figure 6b-d, the organic volatiles including alkane, aromatic and carbonyl compounds are greatly reduced by the incorporation of LbL-modified fabrics, which result in the decrement fire hazard of polymer in the real fire scene. Furthermore, the intensity of characteristic peak for CO2 is increased for the UPR/LbL-modified fabric composites. The flammable gases during the combustion may be diluted by the nonflammable CO2 in the gas phase. In order to more accurately investigate the whole pyrolysis process, the FTIR absorbance of gas products versus temperature are depicted in the Figure 7. As shown in FTIR spectra of gas products for UPR/fabric composites at 310 °C (Figure 7a), there appears a new absorption peak around 1511 cm-1 in the UP-3 and UP-4, which is stemmed from bending vibration of N-H from the pyrolysis of CH and ME.30, 50 As for the FTIR spectrum of UP-1 (Figure 7b), the peak of C=O at 1762 cm-1 from carbonyl compound of unsaturated polymer emerges at about 270 °C, while the peak of CO2 at 2360 cm-1 derived from the decomposition of terminal and ester group of 13



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unsaturated polymer does not appear until the temperature reaches 335 °C. The peak at 2980 cm-1 derived from alkanes and the peak at 3600 cm−1 for -OH group stemmed from cellulose and unsaturated polymer are observed as the temperature reaches 370 °C. However, the characteristic peak of CO2 appears ahead of time in UP-3 and UP-4, as observed in the Figure 7c and d. This phenomenon can be ascribed to the catalytic effect of PA on fabric and UPR, which is consistent with the results of TGA. Meanwhile, the new characteristic peak at 1511 cm-1 in the UP-3 and UP-4 is assigned to the N-H group, indicating the releases ammonia from the thermal decomposition of CH and ME. This the nonflammable gas NH3 is helpful for diluting the oxygen and flammable decomposition gases to retard combustion.51 SEM images of char residue after cone calorimeter test are presented to deeply investigate the flame retardant performance of UPR/fabric composites during combustion. As shown in Figure 8, it is clear that the skeleton of ramie fabric is retained after combustion. Compared with the relatively fragmentary fibers in char residue of UP-1, it can be concluded that LbL coating on ramie fabric can effectively form a compact protective cover on the fibers. The early decomposition of CH/ME and PA before the pyrolysis of fabric and UPR can preemptively provide a protective layer for underlying composites. Furthermore, the char residue of UP-4 is comparatively loose and fluffy because of the existence of iron phytate.37 Therefore, the LbL-modified fabrics as reinforcement can significantly improve the char formation of UPR/fabric composites. The compact char layer can effectively inhibit the heat and mass transfer, and thus the pyrolysis of underlying materials can be 14


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retarded and ultimately the flame retardant properties of the composites are greatly enhanced. 52, 53 3.4 Mechanical properties of the composites Mechanical performance of the UPR/ramie fabric composites are summarized in Figure 9. The results show that the addition of the L15-fabric reduces the flexural strength of UPR composite by 18.02%. According to the previous literature, the phosphoric acid may have a considerable effect on the thermal treatment of natural fiber. The acid can hydrolyze glycosidic linkages between the hemicellulose and cellulose in natural fiber at lower temperature.54, 55 This degradation may affect the strength

and

stiffness

of

cellulose-hemicellulose-cellulose

the

fiber.

bond

is

The replaced

comparatively by

the

more

flexible rigid

cellulose-cellulose bond, resulting in restraining the stress redistribution and lowering strength properties.56, 57 As PA is a strong acid that has six acid groups, it can catalyze the hydrolysis of the glycosidic linkages of hemicellulose and cellulose during the thermal-curing process. Furthermore, the lower cross-linking density of UP-3 was determined by Soxhlet extraction test, which also partly affect the strength of composite. Based on the above consideration, the flexural strength of the UP-3 decreases due to the negative effect of the PA. After PA was reacted with Fe3+ to obtain the iron phytate, the flexural strength of UP-4 is improved by 11.28% in comparison with that of UP-3, even though it is slightly lower than that of UP-1. This phenomenon demonstrates that Fe3+ treatment can mitigate the catalytic degradation of PA. 15



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Moreover, the tensile strength of UP-3 is also decreased by 18.93%, compared with that of UP-1. However, the UP-4 has comparable tensile strength value to the UP-1. The LbL-modified fabric leads to the decrement of elongation at break. These results can be explained by the fact that tensile performance is primarily dependent on the fiber reinforcement and the bonding strength between matrix and fibers.58 To characterize the adhesion interface between ramie fabric and resin, SEM was carried out to observe morphologies of fractured tensile specimens (Figure 10). The fractured surface of UP-1 is rough and uneven. The cracks between the fibers and resin can be clearly observed indicating the relatively weak interface compatibility. The cracks also can be found in the fractured surface of UP-3. Whereas, the fractured surface of UP-4 become ordered and the cracks decrease, compared to the UP-1. In addition, the fibers in the UP-4 are enwrapped by the resin which signifies the improved wettability of this modified UPR and ramie fibers. Thus, the presence of PA has an adverse effect on the mechanical properties of UPR/fabric composite. The formation of iron phytate by Fe3+ reacting with PA can reduce its acidity and preserve the mechanical properties in a certain extent.

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4. Conclusions In this work, flame retardant coating was deposited on ramie fabric using bio-based electrolyte solutions (CH/ME and PA) by LbL assembly method. The UPR/fabric composites were prepared by a hand lay-up technique. The incorporation of LbL-modified fabric results in lower decomposition temperature and higher char residues. This phenomenon was ascribed to the early decomposition of LbL coating on the fabric and then the char formation of UPR/fabric composite was promoted. From cone calorimeter results, the HRR and THR of UPR/LbL-modified fabric composites were significantly reduced and the LOI value was raised from 26.0% to 34.5%. This significant enhancement in flame retardancy was attributed to the protective effect of CH/ME and PA char layer. However, the mechanical behavior of the UPR/LbL-modified fabric composites were reduced which were attributed to the hydrolysis of linkage between hemicellulose and cellulose and the slightly decreased cross-linking density of UPR by PA. The chelating of Fe3+ with PA can recover mechanical performance to a certain extent.

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Acknowledgements The work was financially supported by the National Basic Research Program of China (973 Program) (2014CB931804), the National Natural Science Foundation of China (51473154) and the Fundamental Research Funds for the Central Universities (WK2320000032).

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(56). Spiegelberg, H. L., The effect of hemicelluloses on the mechanical properties of individual pulp fibers. Tappi. 1966, 49, 388. (57). Curling, S.; Clausen, C. A.; Winandy, J. E., The effect of hemicellulose degradation on the mechanical properties of wood during brown rot decay. Int Res Group Wood Pres IRG/WP 2001, 01-20219. (58). Gowda, T. M.; Naidu, A. C. B.; Chhaya, R., Some mechanical properties of untreated jute fabric-reinforced polyester composites. Compos. Part A: Appl. Sci. Manuf. 1999, 30, 277.

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Table captions Table 1. The formulations of UPR/fabrics composites. Table 2. TGA curves of fabrics and UPR/fabric composites under N2 atmosphere. Table 3. Cone calorimeter and LOI results of UPR/fabric composites.

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Table 1. The formulations of UPR/fabrics composites.

Sample

Fabric

UP-1 UP-2 UP-3 UP-4

pristine L5-fabric L15-fabric L15/Fe-fabric

Weight gain for fabric after LbL self-assembly (wt%) 0 7.7±1.56 20.2±1.71 20.0±2.32

Content of resin (wt%)

Gel content (wt%)

43.3±4.36 45.8±3.28 45.3±3.17 45.6±4.16

93.03±1.77 93.19±0.26 89.87±0.09 93.76±0.18

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Table 2. TGA curves of fabrics and UPR/fabric composites under N2 atmosphere. Sample

T10% (°C)

Tmax-1 (°C)

Tmax-2 (°C)

Char residue (wt%)

Raw fabric L5-fabric L15-fabric L15/Fe-fabric UP-1 UP-2 UP-3 UP-4

316 277 264 281 317 282 260 278

335 298 286 302 345 299 275 303

410 405 396 404

8.10 29.52 39.25 36.00 6.84 8.51 22.91 20.11

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Table 3. Cone calorimeter and LOI results of UPR/fabric composites. Sample

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

UP-1 UP-2 UP-3 UP-4

52 34 60 44

562 408 387 420

60 57 49 45

Time to PHRR (s) 156 148 150 132

FPI (s/(kW/ m2))

FGI (kW/( m2s))

LOI (%)

0.093 0.083 0.155 0.105

3.60 2.76 2.58 3.18

26.0 31.0 34.5 34.5

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Figure captions Figure 1. Schematic of deposition of CH/ME and PA coating on the surface of ramie fabric by LbL assembly method. Figure 2. SEM images of raw fabric, L5-fabric, L15-fabric and L15/Fe-fabric at two different magnifications and EDX spectra for the corresponding fabrics. Figure 3. (a) TGA and (b) DTG curves of untreated and LbL-modified fabric under nitrogen atmosphere. Figure 4. (a) TGA and (b) DTG curves of UPR/fabric composites under nitrogen atmosphere. Figure 5. Heat release rate curves of UPR/fabric composites. Figure 6. TG-IR results of UPR/fabric composites: (a) Gram-Schmidt curves, (b) hydrocarbons, (c) carbonyl compounds, (d) aromatic compounds and (e) CO2. Figure 7. (a) FTIR spectra of pyrolysis products for UPR/fabric composite at 310 °C, FTIR spectra of pyrolysis products at different decomposition stages of (b) UP-1, (c) UP-3 and (d) UP-4. Figure 8. SEM images of char residues of UPR/fabric composites: (a, b) UP-1, (c, d) UP-2, (e, f) UP-3 and (g, h) UP-4. Figure 9. (a) Mechanical properties and (b) stress-strain curves of UPR/fabric composites, respectively. Figure 10. SEM images of fractured surfaces of tensile specimens for UPR/fabric composites: (a, b) UP-1, (c, d) UP-3 and (e, f) UP-4.

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Figure 1. Schematic of deposition of CH/ME and PA coating on the surface of ramie fabric by LbL assembly method.

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Figure 2. SEM images of raw fabric, L5-fabric, L15-fabric and L15/Fe-fabric at two different magnifications and EDX spectra for the corresponding fabrics.

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Figure 3. (a) TGA and (b) DTG curves of untreated and LbL-modified fabric under nitrogen atmosphere.

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Figure 4. (a) TGA and (b) DTG curves of UPR/fabric composites under nitrogen atmosphere.

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Figure 5. Heat release rate curves of UPR/fabric composites.

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Figure 6. TG-IR results of UPR/fabric composites: (a) Gram-Schmidt curves, (b) hydrocarbons, (c) carbonyl compounds, (d) aromatic compounds and (e) CO2.

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Figure 7. (a) FTIR spectra of pyrolysis products for UPR/fabric composite at 310 °C, FTIR spectra of pyrolysis products at different stages of decomposition for (b) UP-1, (c) UP-3 and (d) UP-4, respectively.

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Figure 8. SEM images of char residues of UPR/fabric composites: (a, b) UP-1, (c, d) UP-2, (e, f) UP-3 and (g, h) UP-4.

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Figure 9. (a) Mechanical properties and (b) stress-strain curves of UPR/fabric composites, respectively.

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Figure 10. SEM images of fractured surfaces of tensile specimens for UPR/fabric composites: (a, b) UP-1, (c, d) UP-3 and (e, f) UP-4.

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For Table of Contents Only

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Fabrication and properties of bio-based layer-by-layer coated ramie

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