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2023 Volume 3
Article Contents
Abstract
Introduction
Material and methods
Plant materials, plastome assembly, and genome annotation
Codon usage and repeat element patterns
Phylogenetic analysis
Haplotype and genetic diversity analyses
Results
General features of the pea pan-plastome
Codon usage and simple sequence repeats (SSRs) patterns in peas
Phylogenetic analysis
Nucleotide variation in the pea pan-plastome
Discussion
Genomic structure
Evolutionary history
Conclusions
Author contributions
Data availability
Acknowledgments
Conflict of interest
Supplementary information
Rights and permissions
References
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ARTICLE   Open Access    

A bio-based hyperbranched flame retardant towards the fire-safety and smoke-suppression epoxy composite

  • 1.

    School of Chemistry and Materials Engineering, Zhejiang A&F University, Hangzhou 311300, China

  • 2.

    Key Laboratory of Wood Science and Technology of Zhejiang Province, Zhejiang A&F University, Hangzhou 311300, China

  • 3.

    Centre for Future Materials, University of Southern Queensland, Springfield QLD 4300, Australia

  • 4.

    School of Agriculture and Environmental Science, University of Southern Queensland, Toowoomba, QLD 4305, Australia

  • These authors contributed equally: Zhiqian Lin, Wangbin Zhang

More Information
  • The design of fully biological flame retardants is vital for environment-friendly and sustainability development. Herein, a fully biological hyperbranched flame retardant named PA-DAD was synthesized successfully through a simple neutralization reaction between 1,10-Diaminodecane (DAD) and phytic acid (PA). PA-DAD as a kind of reactive flame retardant was subsequently employed to composite with epoxy resin (EP). The obtained EP composite, comprising 25 wt% PA-DAD, exhibited excellent flame resistance with an appreciative limiting oxygen index (LOI) of 28.0%, and a desired V-0 rating of UL-94. The favorable attributes stem from the evident flame-retardant characteristics of PA-DAD, manifesting particularly within the gaseous and condensed phases of combustion. Benefitting from the PA-DAD with the synergetic effect of smoke suppression and flame resistance, the EP/25% PA-DAD composite displayed highlighted reductions both in smoke production and heat release rate. In contrast with neat EP, total smoke production (TSP), peak smoke production release (pSPR), the peak heat release rate (pHRR), and the rate of fire growth (FIGRA) of the EP/25% PA-DAD composite were decreased by 49.5%, 57.0%, 72.2% and 77.8%, respectively. Moreover, the EP/25% PA-DAD composite resulted in well-preserved mechanical properties, especially enhanced toughness, compared with the neat EP. The strategies in our work provided a facile, green, and highly efficient way for fabricating high fire-retardant EP composites.

  • Introduction

    The Fabaceae family, the third largest family in angiosperms, contains about 24,480 species (WFO, https://wfoplantlist.org), and has been a historically important source of food crops[1−4]. Peas (formerly Pisum sativa L. renamed to Lathyrus oleraceus – Pisum spp. will be used hereafter due to historical references to varietal names and subspecies that may not have been fully synonymized), are a member of the Fabaceae family and is among one of the oldest domesticated food crops with ongoing importance in feeding humans and stock. Peas originated in Western Asia and the Mediterranean basin where early finds from Egypt have been dated to ~4500 BCE and further east in Afghanistan from ~2000 BCE[5], and have since been extensively cultivated worldwide[6,7]. Given that peas are rich in protein, dietary fiber, vitamins, and minerals, have become an important part of people's diets globally[8−10].

    Domesticated peas are the result of long-term human selection and cultivation, and in comparison to wild peas, domesticated peas have undergone significant changes in morphology, growth habits, and yield[11−13]. From the long period of domestication starting in and around Mesopotamia many diverse lineages of peas have been cultivated and translocated to other parts of the world[14,15]. The subspecies, Pisum sativum subsp. sativum is the lineage from which most cultivars have been selected and is known for possessing large, round, or oval-shaped seeds[16,17]. In contrast, the subspecies, P. sativum subsp. elatius, is a cultivated pea which more closely resembles wild peas and is mainly found in grasslands and desert areas in Europe, Western Asia, and North Africa. Pisum fulvum is native to the Mediterranean basin and the Balkan Peninsula[15,18], and is resistant to pea rust caused by the fungal pathogen Uromyces pisi. Due to its resistance to pea rust, P. fulvum has been cross-bred with cultivated peas in the development of disease-resistant strains[19]. These examples demonstrate the diverse history of the domesticated pea and why further study of the pea pan-plastome could be employed for crop improvement. While studies based on the nuclear genome have been used to explore the domestication history of pea, these approaches do not account for certain factors, such as maternal inheritance. Maternal lineages, which are inherited through plastomes, play a critical role in understanding the full domestication process. A pan-plastome-based approach will no doubt allow us to investigate the maternal genetic contributions and explore evolutionary patterns that nuclear genome studies may overlook. Besides, pan-plastome analysis enables researchers to systematically compare plastome diversity across wild and cultivated species, identifying specific regions of the plastome that contribute to desirable traits. These plastid traits can then be transferred to cultivated crops through introgression breeding or genetic engineering, leading to varieties with improved resistance to disease, environmental stress, and enhanced agricultural performance.

    Plastids are organelles present in plant cells and are the sites in which several vital biological processes take place, such as photosynthesis in chloroplasts[20−24]. Because the origin of plastids is the result of an ancient endosymbiotic event, extant plastids retain a genome (albeit much reduced) from the free-living ancestor[25]. With the advancement of high-throughput DNA sequencing technology, over 13,000 plastid genomes or plastomes have been published in public databases by the autumn of 2023[24]. Large-scale comparison of plastomic data at multiple taxonomic levels has shown that plastomic data can provide valuable insights into evolution, interspecies relationships, and population genetic structure. The plastome, in most cases, displays a conserved quadripartite circular genomic architecture with two inverted repeat (IR) regions and two single copy (SC) regions, referred to as the large single-copy (LSC) and small single-copy (SSC) regions. However, some species have lost one copy of the inverted repeated regions, such as those in Erodium (Geraniaceae family)[26,27] and Medicago (Fabaceae)[28,29]. Compared to previous plastomic studies based on a limited number of plastomes, the construction of pan-plastomes attempts to describe all nucleotide variants present in a lineage through intensive sampling and comparisons. Such datasets can provide detailed insights into the maternal history of a species and help to better understand applied aspects such as domestication history or asymmetries in maternal inheritance, which can help guide future breeding programs. Such pan-plastomes have recently been constructed for several agriculturally important species. A recent study focuses on the genus Gossypium[20], using plastome data at the population level to construct a robust map of plastome variation. It explored plastome diversity and population structure relationships within the genus while uncovering genetic variations and potential molecular marker loci in the plastome. Besides, 65 samples were combined to build the pan-plastome of Hemerocallis citrina[30] , and 322 samples for the Prunus mume pan-plastome[31]. Before these recent efforts, similar pan-plastomes were also completed for Beta vulgaris[32], and Nelumbo nucifera[33]. However, despite the agricultural importance of peas, no such pan-plastome has been completed.

    In this study, 103 complete pea plastomes were assembled and combined another 42 published plastomes to construct the pan-plastome. Using these data, the following analyses were conducted to better understand the evolution and domestication history of pea: (1) genome structural comparisons, (2) codon usage bias, (3) simple sequence repeat patterns, (4) phylogenetic analysis, and (5) nucleotide variation of plastomes in peas.

    Material and methods

    Plant materials, plastome assembly, and genome annotation

    One hundred and three complete pea plastomes were de novo assembled from public whole-genome sequencing data[34]. For data quality control, FastQC v0.11.5 (www.bioinformatics.babraham.ac.uk/projects/fastqc/) was utilized to assess the quality of the reads and ensure that the data was suitable for assembly. The clean reads were then mapped to a published pea plastome (MW308610) plastome from the GenBank database (www.ncbi.nlm.nih.gov/genbank) as the reference using BWA v0.7.17[35] and SAMtools v1.9[36] to isolate plastome-specific reads from the resequencing data. Finally, these plastome-specific reads were assembled de novo using SPAdes v3.15.2[37]. The genome annotation was conducted by Geseq online program (https://chlorobox.mpimp-golm.mpg.de/geseq.html). Finally, the OGDRAW v1.3.1[38] program was utilized to visualize the circular plastome maps with default settings. To better resolve the pan-plastome for peas, 42 complete published pea plastomes were also downloaded from NCBI and combined them with the de novo data (Supplementary Table S1).

    Codon usage and repeat element patterns

    To investigate the codon usage in the pan-plastome of pea, we utilized CodonW v.1.4.2 (http://codonw.sourceforge.net) to calculate the Relative Synonymous Codon Usage (RSCU) value of the protein-coding genes (PCGs) longer than 300 bp, excluding stop codons. The RSCU is a calculated metric used to evaluate the relative frequency of usage among synonymous codons encoding the same amino acid. An RSCU value above 1 suggests that the codon is utilized more frequently than the average for a synonymous codon. Conversely, a value below 1 indicates a lower-than-average usage frequency. Besides, the Effective Number of Codons (ENC) and the G + C content at the third position of synonymous codons (GC3s) were also calculated in CodonW v.1.4.2. The ENC value and GC3s value were utilized for generating the ENC-GC3s plot, with the expected ENC values (standard curve), are calculated according to formula: ENC = 2 + GC3s + 29 / [GC3s2 + (1 – GC3s)2][39].

    The MISA program[40] was utilized to detect simple sequence repeats (SSRs), setting the minimum threshold for repeat units at 10 for mono-motifs, 6 for di-motifs, and 5 for tri-, tetra-, penta-, and hexa-motif microsatellites, respectively.

    Phylogenetic analysis

    The 145 complete pea plastomes were aligned using MAFFT v 7.487[41]. Single nucleotide variants (SNVs)-sites were used to derive an SNV only dataset from the entire-plastome alignment[42]. A total of 959 SNVs were analyzed using IQ-TREE v2.1[43] with a TVMe + ASC + R2 substitution model, determined by ModelTest-NG[44] based on BIC, and clade support was assessed with 1,000 bootstrap replicates. Vavilovia formosa (MK604478) was chosen as an outgroup. The principal coordinates analysis (PCA) was conducted in TASSEL 5.0[45].

    Haplotype and genetic diversity analyses

    DnaSP v6[46] was utilized to identify different haplotypes among the plastomes, with gaps and missing data excluded. Haplotype networks were constructed in Popart v1.7[47] using the median-joining algorithm. Haplotype diversity (Hd) for each group was calculated by DnaSP v6[46], and the evolutionary distances based on the Tamura-Nei distance model were computed based on the population differentiation index (FST) between different groups with the plastomic SNVs.

    Results

    General features of the pea pan-plastome

    In this study, the pan-plastome structure of peas was elucidated (Fig. 1). The length of these plastomes ranged from 120,826 to 122,547 bp. And the overall GC content varied from 34.74% to 34.87%. In contrast to typical plastomes characterized by a tetrad structure, the plastomes of peas contained a single IR copy. The average GC content among all pea plastomes was 34.8%, with the highest amount being 34.84% and the lowest 34.74%, with minimal variation among the pea plastomes.

    Figure 1.  Pea pan-plastome annotation map. Indicated by arrows, genes listed inside and outside the circle are transcribed clockwise and counterclockwise, respectively. Genes are color-coded by their functional classification. The GC content is displayed as black bars in the second inner circle. SNVs, InDels, block substitutions and mixed variants are represented with purple, green, red, and black lines, respectively. Single nucleotide variants (SNVs), block substitutions (BS, two or more consecutive nucleotide variants), nucleotide insertions or deletions (InDels), and mixed sites (which comprise two or more of the preceding three variants at a particular site) are the four categories into which variants are divided.
    DownLoad: Full-Size Img PowerPoint

    A total of 110 unique genes were annotated (Supplementary Table S2), of which 76 genes were PCGs, 30 were transfer RNA (tRNA) genes and four were ribosomal RNA (rRNA) genes. Genes containing a single intron, include nine protein-coding genes (rpl16, rpl2, ndhB, ndhA, petB, petD, rpoC1, clpP, atpF) and six tRNA genes (trnK-UUU, trnV-UAC, trnL-UAA, trnA-UGC, trnI-GAU, trnG-UCC). Additionally, two protein-coding genes ycf3 and rps12 were found to contain two introns.

    Codon usage and simple sequence repeats (SSRs) patterns in peas

    The codon usage frequency in pea plastome genes is shown in Fig. 2a. The analysis of codon usage in the pea plastome indicated significant biases for specific codons across various amino acids. Here a nearly average usage in some amino acids was observed, such as Alanine (Ala) and Valine (Val). For most amino acids, the usage of different synonymous codons was not evenly distributed. Regarding stop codons, a nearly even usage was found, with 37.0% for TAA, 32.2% for TAG and 30.8% for TGA.

    Figure 2.  (a) The overall codon usage frequency in 51 CDSs (length > 300 bp) from the pea pan-plastome. (b) The heatmap of RSCU values in 51 CDSs (length > 300 bp) from the pea pan-plastome. The x-axis represents different codons and the y-axis represents different CDSs. The tree at the top was constructed based on a Neighbor-Joining algorithm.
    DownLoad: Full-Size Img PowerPoint

    The RSCU heatmap (Fig. 2b) showed different RSCU values for all codons in plastomic CDSs. In general, a usage bias for A/T in the third position of codons was found among CDSs in the pea pan-plastome. The RSCU values among these CDSs ranged from 0 to 4.8. The highest RSCU value (4.8) was found with the CGT codon in the cemA gene, where six synonymous codons exist for Arg but only CGT (4.8) and AGG (1.2) were used in this gene. This explained in large part the extreme RSCU value for CGT, resulting in an extreme codon usage bias in this amino acid.

    In the ENC-GC3s plot (Fig. 3), 31 PCGs were shown below the standard curve, while 20 PCGs were above. Besides, around 12 PCGs were near the curve, which meant these PCGs were under the average natural selection and mutation pressure. This plot displayed that the codon usage preferences in pea pan-plastomes were mostly influenced by natural selection. Five genes were shown an extreme influence with natural selection for its extreme ΔENC (ENCexpected – ENC) higher than 5, regarding as petB (ΔENC = 5.18), psbA (ΔENC = 8.96), rpl16 (ΔENC = 5.62), rps14 (ΔENC = 14.29), rps18 (ΔENC = 6.46) (Supplementary Table S3).

    Figure 3.  The ENC-GC3s plot for pea pan-plastome, with GC3s as the x-axis and ENC as the y-axis. The expected ENC values (standard curve) are calculated according to formula: ENC = 2 + GC3s + 29 / [GC3s2 + (1 − GC3s)2].
    DownLoad: Full-Size Img PowerPoint

    For SSR detection (Fig. 4), mononucleotide, dinucleotide, and trinucleotide repeats were identified in the pea pan-plastome including A/T, AT/TA, and AAT/ATT. The majority of these SSRs were mononucleotides (A/T), accounting for over 90% of all identified repeats. Additionally, we observed that A/T and AT/TA repeats were present in all pea accessions, whereas only about half of the plastomes contained AAT/ATT repeats. It was also found that the number of A/T repeats exhibited the greatest diversity, while the number of AAT/ATT repeats showed convergence in all plastomes that possessed this repeat.

    Figure 4.  Simple sequence repeats (SSRs) in the pea pan-plastome. The x-axis represents different samples of pea and the y-axis represents the number of repeats in this sample. (a) The number of A/T repeats in the peapan-plastome. (b) The number of AT/TA and AAT/ATT repeats of pea pan-plastomes.
    DownLoad: Full-Size Img PowerPoint

    Phylogenetic analysis

    To better understand the phylogenetic relationships and evolutionary history of peas, a phylogenetic tree was reconstructed using maximum likelihood for 145 pea accessions utilizing the whole plastome sequences (Fig. 5a). The 145 pea accessions were grouped into seven clades with high confidence. These groups were named the 'PF group', 'PSeI-a group', 'PSeI-b group', 'PA group', 'PSeII group', 'PSeIII group', and the 'PS group'. The naming convention for these groups relates to the majority species names for accessions in each group, where P. fulvum makes up the 'PF group', P. sativum subsp. elatius in the 'PSeI-a group', 'PSeI-b group', 'PSeII group', and 'PSeIII group', P. abyssinicum in the 'PA group', and P. sativum in the 'PS group'. From this phylogenetic tree, it was observed that the 'PSeI-a group' and the 'PSeI-b group' had a close phylogenetic relationship and nearly all accessions in these two groups (except DCG0709 accession was P. sativum) were identified as P. sativum subsp. elatius. In addition to the P. sativum subsp. elatius found in PSeI, seven accessions from the PS group were identified as P. sativum subsp. elatius.

    The PCA results (Fig. 5b) also confirmed that domesticated varieties P. abyssinicum were closer to cultivated varieties PSeI and PSeII, while PSeIII was more closely clustered with cultivated varieties of P. sativum. A previous study has indicated that P. sativum subsp. sativum and P. abyssinicum were independently domesticated from different P. sativum subsp. elatius populations[34].

    The complete plastome sequences were utilized for haplotype analysis using TCS and median-joining network methods (Fig. 5c). A total of 76 haplotypes were identified in the analysis. The TCS network resolved a similar pattern as the other analyses in that six genetic clusters were resolved with genetic clusters PS and PSeIII being very closely related. The genetic cluster containing P. fulvum exhibits greater genetic distance from other genetic clusters. The genetic clusters containing P. abyssinicum (PA) and P. sativum (PS) had lower levels of intracluster differentiation. In the TCS network, Hap30 and Hap31 formed distinct clusters from other haplotypes, such as Hap27, which may account for the genetic difference between the 'PSeI-a group' and 'PSeI-b group'. The network analysis results were consistent with the findings of the phylogenetic tree and principal component analyses results in this study.

    Figure 5.  (a) An ML tree resolved from 145 pea plastomes. (b) PCA analysis showing the first two components. (c) Haplotype network of pea plastomes. The size of each circle is proportional to the number of accessions with the same haplotype. (d) Genetic diversity and differentiation of six clades of peas. Pairwise FST between the corresponding genetic clusters is represented by the numbers above the lines joining two bubbles.
    DownLoad: Full-Size Img PowerPoint

    Among the six genetic clusters, the highest haplotype diversity (Hd) was observed in PSeIII (Hd = 0.99, π = 0.22 × 10−3), followed by PSeII (Hd = 0.96, π = 0.43 × 10−3), PSeI (Hd = 0.96, π = 0.94 × 10−3), PF (Hd = 0.94, π = 0.6 × 10−4), PS (Hd = 0.88, π = 0.3 × 10−4), and PA (Hd = 0.70, π = 0.2 × 10−4). Genetic differentiation was evaluated between each genetic cluster by calculating FST values. As shown in Fig. 5d, except for the relatively lower population differentiation between PS and PSeIII (FST = 0.54), and between PSeI and PSeII (FST = 0.59), the FST values between the remaining clades ranged from 0.7 to 0.9. The highest population differentiation was observed between PF and PA (FST = 0.98). The FST values between PSeI and different genetic clusters were relatively low, including PSeI and PF (FST = 0.80), PSeI and PS (FST = 0.77), PSeI and PSeIII (FST = 0.72), PSeI and PSeII (FST = 0.59), and PSeI and PA (FST = 0.72).

    Nucleotide variation in the pea pan-plastome

    To further determine the nucleotide variations in the pea pan-plastome, 145 plastomes were aligned and nucleotide differences analyzed across the dataset. A total of 1,579 variations were identified from the dataset (Table 1), including 965 SNVs, 24 Block Substitutions, 426 InDels, and 160 mixed variations of these three types. Among the SNVs, transitions were more frequent than transversions, with 710 transitions and 247 transversions. In transitions, T to G and A to C had 148 and 139 occurrences, respectively, while in transversions, G to A and C to T had 91 and 77 occurrences, respectively.

    Table 1.  Nucleotide variation in the pan-plastome of peas.
    Variant Total SNV Substitution InDel Mix
    (InDel, SNV)    		 Mix
    (InDel, SUB)
    Total 1,576 965 24 426 156 4
    CDS 734 445 6 176 103 4
    Intron 147 110 8 29 0 0
    tRNA 20 15 1 4 0 0
    rRNA 11 3 0 6 2 0
    IGS 663 392 9 211 51 0
     | Show Table
    DownLoad: CSV

    When analyzing variants by their position to a gene (Fig. 6), there were 731 variations in CDSs, accounting for 46.3% of the total variations, including 443 SNVs (60.6%), six block substitutions (0.83%), 175 InDels (23.94%), and four mixed variations (14.64%). There were 104 variants in introns, accounting for 6.59% of the total variations, including 78 SNVs (75%), seven block substitutions (6.73%), and 19 InDels (18.27%). IGS (Intergenic spacers) contained 660 variations, accounting for 41.8% of the total variations, including 394 SNVs (59.7%), nine block substitutions (1.36%), 207 InDels (31.36%), and 50 mixed variations (7.58%). The tRNA regions contained 63 variants, accounting for 3.99% of the total variations, including 47 SNVs (74.6%) and 14 InDels (22.2%). The highest number of variants were detected in the IGS regions, while the lowest were found in introns. Among CDSs, accD (183) had the highest number of variations. In introns, rpL16 (18) and ndhA (16) had the most variants. In the IGS regions, ndhD-trnI-CAU (73), and trnL-UAA-trnT-UGU (44) possessed the greatest number of variants.

    Figure 6.  Variant locations within the pea pan-plastome categorized by genic position (Introns, CDS, and IGS).
    DownLoad: Full-Size Img PowerPoint

    Finally, examples of some genes with typical variants were provided to better illustrate the sequence differences between clades (Fig. 7). For example, the present analysis revealed that the ycf1 gene exhibited a high number of variant loci, which included unique single nucleotide variants (SNVs) specific to the P. abyssinicum clade. Additionally, a unique InDel variant belonging to P. abyssinicum was identified. Similar unique SNVs and InDels were also found in other genes, such as matK and rpoC2, distinguishing the P. fulvum clade from others. These unique SNVs and InDels could serve as DNA barcodes to distinguish different maternal lineages of peas.

    Figure 7.  Examples of variant sites.
    DownLoad: Full-Size Img PowerPoint

    Discussion

    Genomic structure

    The present research combined 145 pea plastomes to construct a pan-plastome of peas. Compared to single plastomic studies, pan-plastome analyses across a species or genus provide a higher-resolution understanding of phylogenetic relationships and domestication history. Most plastomes in plants possess a quadripartite circular structure with two inverted repeat (IR) regions and two single copy regions (LSC and SSC)[20−24]. However, the complete loss of one of the IR regions in the pea plastome was observed which is well-known among the inverted repeat-lacking clade (IRLC) species in Fabaceae. The loss of IRs has been documented in detail from other genera such as Erodium (Geraniaceae family)[26,27] and Medicago (Fabaceae family)[28,29]. This phenomenon although not commonly observed, constitutes a significant event in the evolutionary trajectories of certain plant lineages[26]. Such large-scale changes in plastome architecture are likely driven in part by a combination of selective pressures and genetic drift[48]. In the pea pan-plastome, it was also found that, compared to some plants with IR regions, the length of the plastomes was much shorter, and the overall GC content was lower. This phenomenon was due to the loss of one IR with high GC content.

    Repetitive sequences are an important part of the evolution of plastomes and can be used to reconstruct genealogical relationships. Mononucleotide SSRs are consistently abundant in plastomes, with many studies identifying them as the most common type of SSR[49−52]. Among these, while C/G-type SSRs may dominate in certain species[53,54], A/T types are more frequently observed in land plants. The present research was consistent with these previous conclusions, showing an A/T proportion exceeding 90% (Fig. 4). Due to their high rates of mutation, SSRs are widely used to study phylogenetic relationships and genetic variation[55,56]. Additionally, like other plants, pea plastome genes have a high frequency of A/Ts in the third codon position. This preference is related to the higher AT content common among most plant plastomes and Fabaceae plastomes in particular with their single IRs[57,58]. The AT-rich regions are often associated with easier unwinding of DNA during transcription and potentially more efficient and accurate translation processes[59]. The preference for A/T in third codon positions may also be influenced by tRNA availability, as the abundance of specific tRNAs that recognize these codons can enhance the efficiency of protein synthesis[60,61]. However, not all organisms exhibit this preference for A/T-ending codons. For instance, many bacteria have GC-rich genomes and thus show a preference for G/C-ending codons[62−64]. This variation in codon usage bias reflects the differences in genomic composition and the evolutionary pressures unique to different lineages.

    This study also comprehensively examined the variant loci of the pea pan-plastome. Among these variant sites, some could potentially serve as DNA barcode sites for specific lineages of peas, such as ycf1, rpoC2, and matK. Both ycf1 and matK have been widely used as DNA barcodes in many species[65−68], as they are hypervariable. Researchers now have a much deeper understanding of the crucial role plastomes have played in plant evolution[69−71]. By generating a comprehensive map of variant sites, future researchers can now more effectively trace differences in plastotypes to physiological and metabolic traits for use in breeding elite cultivars.

    Evolutionary history

    The development of a pan-plastome for peas provides new insights into the maternal domestication history of this important food crop. Based on the phylogenetic analysis in this study, we observed a clear differentiation between wild and cultivated peas, with P. fulvum being the earliest diverging lineage, and was consistent with former research[34]. The ML tree (Fig. 5a) indicated that cultivated peas had undergone at least two independent domestications, namely from the PA and PS groups, which is consistent with former research[34]. However, as the present study added several accessions over the previous study and plastomic data was utilized, several differences were also found[34], such as the resolution of the two groups, referred to as PSeI-a group and PSeI-b group which branched between the PA group and PF group. Previous research based on nuclear data[34] only and with fewer accessions showed that the PA group and PF group were closely related in phylogeny, with no PSeI group appearing between them. One possible explanation is that the PSeI-a and PSeI-b lineages represents the capture and retention of a plastome from a now-extinct lineage while backcrossing to modern cultivars has obscured this signal in the nuclear genomic datasets. However, procedural explanations such as incorrectly identified accessions might have also resulted in such patterns. In either case, the presence of these plastomes in the cultivated pea gene pool should be explored for possible associations with traits such as disease resistance and hybrid incompatibility. This finding underscores the complexity of the domestication process and highlights the role of hybridization and selection in shaping the genetic landscape of cultivated peas. As such, future studies integrating data from the nuclear genome, mitogenome, and plastome will undoubtedly provide deeper insights into the phylogeny and domestication of peas. This pan-plastome research, encompassing a variety of cultivated taxa, will also support the development of elite varieties in the future.

    Conclusions

    This study newly assembled 103 complete pea plastomes. These plastomes were combined with 42 published pea plastomes to construct the first pan-plastome of peas. The length of pea plastomes ranged from 120,826 to 122,547 bp, with the GC content varying from 34.74% to 34.87%. The codon usage pattern in the pea pan-plastome displayed a strong bias for A/T in the third codon position. Besides, the codon usage of petB, psbA, rpl16, rps14, and rps18 were shown extremely influenced by natural selection. Three types of SSRs were detected in the pea pan-plastome, including A/T, AT/TA, and AAT/ATT. From phylogenetic analysis, seven well-supported clades were resolved from the pea pan-plastome. The genes ycf1, rpoC2, and matK were found to be suitable for DNA barcoding due to their hypervariability. The pea pan-plastome provides a valuable supportive resource in future breeding and selection research considering the central role chloroplasts play in plant metabolism as well as the association of plastotype to important agronomic traits such as disease resistance and interspecific compatibility.

  • The authors confirm contribution to the paper as follows: study conception and design: Wang J; data collection: Kan J; analysis and interpretation of results: Kan J, Wang J; draft manuscript preparation: Kan J, Wang J, Nie L; project organization and supervision: Tiwari R, Wang M, Tembrock L. All authors reviewed the results and approved the final version of the manuscript.

  • The annotation files of newly assembled pea plastomes were uploaded to the Figshare website (https://figshare.com/, doi: 10.6084/m9.figshare.26390824).

    • Acknowledgments

    This study was funded by the Guangdong Pearl River Talent Program (Grant No. 2021QN02N792) and the Shenzhen Fundamental Research Program (Grant No. JCYJ20220818103212025). This work was also funded by the Science Technology and Innovation Commission of Shenzhen Municipality (Grant No. RCYX20200714114538196) and the Innovation Program of Chinese Academy of Agricultural Sciences. We are also particularly grateful for the services of the High-Performance Computing Cluster in the Agricultural Genomics Institute at Shenzhen, Chinese Academy of Agricultural Sciences.

  • The authors declare that they have no conflict of interest.

  • Supplemental Table S1 The Various Proportions of the Epoxy Resin Systems.
    Supplemental Table S2 TGA data of PA-DAD and EP composites.
    Supplemental Table S3 Kinetic parameters of the curing reaction.
    Supplemental Table S4 The fire performances of PA-DAD.
    Supplemental Table S5 Structural assignments for the main fragments observed in the Py-GC/MS of EP/25 %PA-DAD.
    Supplemental Table S6 Mechanical properties of neat EP and EP composites.
    Supplemental Fig. S1 1H NMR spectra of (a) PA, (b) DAD, and (c) PA-DAD.
    Supplemental Fig. S2 The DSC curves of (a) neat EP, (b) EP/10% PA-DAD, (c) EP/20% PA-DAD, (d) EP/25% PA-DAD at different heating rates and the fitting curves based on Kissinger, and Ozawa methods for neat EP and EP composites.
    Supplemental Fig. S3 The UL-94 test for neat EP and EP composites.
    Supplemental Fig. S4 The THR curves vs time for EP composites at a flux of 35 kW/m2.
    Supplemental Fig. S5 (a) FTIR spectrogram of the composition of residual char; (b) Raman spectrogram of the composition of residual char; (c)XPS spectrogram of the composition of residual char.
    Supplemental Fig. S6 (a)The C1s spectra of the composition of residual char; (b) The O1s spectra of the composition of residual char; (c) The N1s spectra of the composition of residual char; (d) The P2p spectra of the composition of residual char.
    Supplemental Fig. S7 (a) The Gram-Schmidt curves of neat EP and EP/25% PA-DAD; (b) The intensity of hydrocarbons versus the temperature curves of neat EP and EP/25% PA-DAD; (c) The intensity of CO2 versus the temperature curves of neat EP and EP/25% PA-DAD; (d) The intensity of CO versus the temperature curves of neat EP and EP/25% PA-DAD; (e) The intensity of carbonyl compounds versus the temperature curves of neat EP and EP/25% PA-DAD; (f) The intensity of aromatic compounds versus the temperature curves of neat EP and EP/25% PA-DAD.
    Supplemental Fig. S8 Py-GC/MS spectra of EP/25% PA-DAD.
    Supporting Information Details of characterizations.
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  • Cite this article

    Lin Z, Zhang W, Lou G, Bai Z, Xu J, et al. 2023. A bio-based hyperbranched flame retardant towards the fire-safety and smoke-suppression epoxy composite. Emergency Management Science and Technology 3:21 doi: 10.48130/EMST-2023-0021
    Lin Z, Zhang W, Lou G, Bai Z, Xu J, et al. 2023. A bio-based hyperbranched flame retardant towards the fire-safety and smoke-suppression epoxy composite. Emergency Management Science and Technology 3:21 doi: 10.48130/EMST-2023-0021
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A bio-based hyperbranched flame retardant towards the fire-safety and smoke-suppression epoxy composite

  • 1.

    School of Chemistry and Materials Engineering, Zhejiang A&F University, Hangzhou 311300, China

  • 2.

    Key Laboratory of Wood Science and Technology of Zhejiang Province, Zhejiang A&F University, Hangzhou 311300, China

  • 3.

    Centre for Future Materials, University of Southern Queensland, Springfield QLD 4300, Australia

  • 4.

    School of Agriculture and Environmental Science, University of Southern Queensland, Toowoomba, QLD 4305, Australia

  • Received: 17 October 2023
  • Accepted: 12 December 2023
  • Published online: 26 December 2023
Emergency Management Science and Technology  3,  Article number: 21  (2023)  |  Cite this article

Abstract: The design of fully biological flame retardants is vital for environment-friendly and sustainability development. Herein, a fully biological hyperbranched flame retardant named PA-DAD was synthesized successfully through a simple neutralization reaction between 1,10-Diaminodecane (DAD) and phytic acid (PA). PA-DAD as a kind of reactive flame retardant was subsequently employed to composite with epoxy resin (EP). The obtained EP composite, comprising 25 wt% PA-DAD, exhibited excellent flame resistance with an appreciative limiting oxygen index (LOI) of 28.0%, and a desired V-0 rating of UL-94. The favorable attributes stem from the evident flame-retardant characteristics of PA-DAD, manifesting particularly within the gaseous and condensed phases of combustion. Benefitting from the PA-DAD with the synergetic effect of smoke suppression and flame resistance, the EP/25% PA-DAD composite displayed highlighted reductions both in smoke production and heat release rate. In contrast with neat EP, total smoke production (TSP), peak smoke production release (pSPR), the peak heat release rate (pHRR), and the rate of fire growth (FIGRA) of the EP/25% PA-DAD composite were decreased by 49.5%, 57.0%, 72.2% and 77.8%, respectively. Moreover, the EP/25% PA-DAD composite resulted in well-preserved mechanical properties, especially enhanced toughness, compared with the neat EP. The strategies in our work provided a facile, green, and highly efficient way for fabricating high fire-retardant EP composites.

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    Introduction

    • Epoxy resin (EP) plays a vital role in human daily life, which has been employed in different areas of adhesives, coatings, insulation materials, and structural composites for its outstanding chemical resistance, adhesion, high mechanical properties, and electrical insulation[1−6]. However, the highly intrinsic flammability and dense smoke of EP during combustion poses some threats to human safety and property[7−11]. The modification of flame retardancy for EP is imperative and indispensable[12−14]. In this regard, many strategies have been developed to improve flame retardancy of EP composites. Incorporation of flame retardants (FRs) into polymeric matrix is generally one of the most efficient and economical ways to pursue excellent flame retardancy for EP[15,16]. Thus, numerous FRs have been designed to hinder fire hazards of polymers in the past decades[17−21]. Nevertheless, a significant proportion of FRs for the EP matrix are originated from non-renewable petrochemical sources, which goes against the principles of global sustainability and environmental biodegradability. On this account, biological flame retardants have been attracting increasing attention for their inherent renewability, abundance, and biodegradability[22−28].

      So far, numerous bio-based FRs have been prepared and investigated for imparting flame retardancy to EP, such as lignin[29], cardanol[30,31], eugenol[32], phytic acid[33−35], itaconic acid[36], resveratrol[37] and cyclodextrin[38,39], and so on. Among them, phytic acid (PA) possesses high flame-retardant efficiency because of its high phosphorus content of 28 wt%. PA is a phosphorus-rich organic acid, derived from the seeds and roots of the plant, which could provide an acid source for intumescent FRs[40,41]. Wang et al.[35] prepared a nano-layered hybrid as a kind of intumescent flame retardant for EP by a neutralization reaction of melamine and PA, and the obtained composites showed both good smoke suppression and flame resistance. Fang et al.[33] improved the flame resistance of EP via synthesizing one unique phosphorus compound by the neutralization of phenyl phosphonate and PA, affecting condensed and gas phases. Zhu et al.[34] synthesized a macromolecule ammonium phytic acid via a neutralization reaction of PA and N-aminoethyl piperazine for reducing smoke emissions and enhancing flame retardancy as a curing agent for EP. However, it is noted that the amines chelated with PA reported in the literature were petroleum-based products, which did not achieve all bio-based FRs.

      In this work, a fully biological flame retardant (PA-DAD) was designed via a neutralization reaction between 1,10-diaminodecane (DAD) and PA that from renewable castor oil. The resulting PA-DAD exhibited both favourable smoke suppression and flame retardancy due to the generated bi-phase effect from PA-DAD in EP composites. The existing mechanism of flame retardancy for PA-DAD was explored using SEM-EDS, TG-IR, and Py-GC/MS methods. The transferring for heat and oxygen can be declined by the formed compact char, protecting the residual matrix from additional thermal degradation and decreasing emissions of toxic gases. The mechanical performances of EP/PA-DAD composites with a satisfactory toughness were methodically investigated. PA-DAD was demonstrated as a high-efficient green smoke suppressant as well as flame retardant for EP, expecting to contribute to low-carbon programs.

    Experiments and methods

      Materials

    • Dicyandiamide (DCD, 98%), Anhydrous ethanol (99.5%), and 2-methylimidazole (2-MI, 98%) all originated from Aladdin Chemistry Co., Ltd (Shanghai, China). Phytic acid aqueous solution (PA, 70wt%) was purchased by Shanghai Yuanye Bio-Technology Co., Ltd (Shanghai, China). Shanghai Macklin Biochemical Co., Ltd (Shanghai, China) supplied the 1,10-Diaminodecane (DAD, 97%). Anhui Shanfu New Material Technology Co., Ltd (Huangshan, China) supplied solid epoxy resin E12. All reagents were applied with no additional dealing.

      Prepared method of PA-DAD

    • As displayed in Fig. 1a, the PA-DAD was prepared in ethanol-water solution by neutralization reaction. In brief, 18.86 g of PA solution (0.02 mol) and 42.64 g of DAD (0.12 mol) were dissolved into 50 ml of deionized water and 400 ml of anhydrous ethanol, respectively. Then, the PA solution was dropwise added into the stirred DAD solution, yielding in the formation of white precipitate. Subsequently, the final product was acquired by filtration, being washed multiple times by anhydrous ethanol, and dried in a 60 °C vacuum oven for 8 h.

      Figure 1. 

      (a) Schematic diagram for the prepared processing of PA-DAD, (b) FTIR spectra of DAD, PA and PA-DAD, (c) 31P NMR curve of PA-DAD and PA, (d) TG curves of PA-DAD.

      Fabricated method of EP/PA-DAD composites

    • The different EP/PA-DAD composites were prepared as depicted in the following. The solid epoxy resin E12, along with the prepared PA-DAD, promoter 2-MI and curing agent DCD, were thoroughly mixed using a disintegrator to keep the distribution of mixture uniform. The neutralization reaction occurred at a temperature of 25 °C. The different mixing conditions are displayed in Supplemental Table S1. The above mixture was put in a mold made by stainless steel, and then cured with a press vulcanizer by hot pressing under a pressure of 10 MPa for 25 min at 145 °C. When the temperature was cooled to room temperature, the EP/PA-DAD composites to be used for different tests were obtained.

      Characterizations

    • The characterizations employed in this work mainly include: a nuclear magnetic resonance spectrometer (NMR), Fourier Transform infrared spectroscopy (FTIR), Energy-dispersive X-ray spectroscopy (EDS), Scanning Electron Microscope (SEM), Raman spectra, X-ray photoelectron spectroscopy (XPS), Differential scanning calorimetry (DSC), Thermogravimetry Analysis (TG), Limiting Oxygen Index (LOI), Pyrolysis gas chromatography-mass spectrometry (Py-GC/MS), a cone calorimeter, the tensile strength, the impact strength, and the flexural strength and so on. The corresponding details of characterizations are supplied in the Supporting Information.

    Results and discussion

      Characterizations of PA-DAD

    • The FTIR curves of DAD, PA and PA-DAD were carried out as depicted in Fig. 1b. Regarding PA, the 2,400 cm−1 peak was identified as arising from the (P=O)-OH chemical bond[34]. In the curve of DAD, the 3,329 cm−1 absorption peak corresponded to the stretching vibration of N-H, while the scope for 2,800−3,000 cm−1 was related to C-H bonds[42]. By contrast, the peak assigned to N-H and (P=O)-OH of PA-DAD disappeared. Instead, the new appeared peak of 1,554 cm−1 and 1,075 cm−1 were ascribed to NH3+ and PO32−, respectively[35,43]. The above change suggests that the bio-based ionic complex of PA-DAD was successfully prepared.

      Additionally, the chemical structure of PA-DAD was demonstrated via 31P NMR,1H NMR, and FTIR. As presented in Supplemental Fig. S1a−S1c and Fig. 1c, the signal peak at 4.8 ppm in all spectra were ascribed to the deuterated water. In the context of PA, two distinct peaks appeared at approximately 4 ppm in the spectrum. Concerning DAD, peaks at 2.5, 1.4, and 1.2 ppm were related to the -CH2- protons in various chemical environments. When it comes to PA-DAD, the two peaks at 2.5 and 1.4 ppm exhibited a noticeable shift towards the left, indicating that a neutralization reaction occurred between DAD and PA. Moreover, in the spectra of 31P NMR, as depicted in Fig. 1b, the signal of phosphorus shifted from −0.32 ppm of PA to 1.47 ppm of PA-DAD.

      The thermal-degradation behaviors of PA-DAD are also shown in Fig. 1d and Supplemental Table 2. The process of thermal degradation for PA-DAD involved two primary stages, with the temperatures of maximum weight loss rates recorded at 227 and 399 °C. The initial degradation stage can be attributed to and the dehydration reaction from phytic acid, and the destruction of ionic bonds within PA-DAD[44]. The second degradation stage was associated with the releasing of NH3, H2O, and also the generated poly-/pyro-/ultra-phosphoric acids as PA-DAD further decomposed[40,45].

      Curing and thermal properties

    • The curing behaviour from effects of PA-DAD on EP for was explored via non isothermal DSC testing. Supplemental Fig. S2a−S2d displayed the DSC curves of EP/PA-DAD composites with different heating rates of 5, 10, 15, 20, 25 °C min−1 separately. As the heating rate increased, the temperatures of the initial peaks were elevated, and the range of curing temperature became broader because of the intensified thermal effects per unit time[46]. The reaction activation energy (Ea) had been calculated from Kissinger and Ozawa equation, as shown in Fig. 2a &b and as well summarized in Supplemental Table S3. The Ea increased significantly when the PA-DAD was incorporated into the EP. The increased presence of active sites, stemming from the amino group of PA-DAD, needed higher energy to be input for chemical reactions during the initial stages of curing. Consequently, compared to pure EP, the EP/PA-DAD systems required higher curing temperatures.

      Figure 2. 

      The calculated results of (a) Kissinger, and (b) Ozawa methods for neat EP and its different composites, (c) TG and (d) DTG curves for EP composites.

      The behaviors of thermal degradation from EP/PA-DAD composites were inspected via TG with a nitrogen atmosphere in Fig. 2c & d, and related data were summarized in Supplemental Table S2. The residual mass for EP composites was improved after imparting the PA-DAD. The yields of char for neat EP, EP/10% PA-DAD, EP/20% PA-DAD, and EP/25% PA-DAD composites increased from 11.1% (for neat EP) to 13.6%, 17.1%, and 17.2% at 800 °C, respectively. A higher residual mass indicated an optimistic impact on high flame resistance of composites. Compared with neat EP, the Tonset (the temperature at 5 wt% mass loss) and Tmax (the maximum degradation temperature) of EP/PA-DAD decreased continuously as the increased addition amount of PA-DAD. The initial thermal decomposition of EP/PA-DAD primarily resulted from the premature breakdown of PA-DAD. Consequently, PA-DAD can be decomposed as poly-/pyro-/ultra-phosphoric acids at lower temperatures, accelerating the decomposition of EP matrix. The generated poly-/pyro-/ultra-phosphoric acids facilitated crosslinking reactions and catalyzed the carbonization of EP molecules. The process could lead to the formatting for a protective and stable char layer during high-temperature stages. The protective layer significantly enhanced the fire retardancy for EP composites during combustion, providing a safeguard for the underlying matrix[47,48].

      Flame retardancy for EP composites

    • Both LOI values as well as the UL-94 rating were initially employed to assess the flame retardancy for EP composites, and corresponding data and results are summarized in Table 1. Neat EP, which is inherently flammable, exhibited a quite low LOI value of 20.4% and did not meet V-0 rating in the test of UL-94. Upon adding PA-DAD into EP, there were significant increases in LOI values of EP composites. When the content of PA-DAD reached 25 wt%, a notably high LOI value with 28.0% for the EP/25% PA-DAD composite was achieved and successfully satisfied V-0 rating in the test of UL-94 (as indicated in Supplemental Fig. S3). The above findings demonstrated that PA-DAD served as an efficient biological flame retardant for EP, enhancing fire resistance of composites.

      Table 1.  Detailed results corresponding to UL-94 and LOI for EP composites.

      SampleUL-94LOI (%)
      RatingDrippingt1/t2(s)
      Neat EPNRYES> 3020.4
      EP/10%PA-DADNRNO> 3023.6
      EP/20%PA-DADV-1NO8.0/2.425.3
      EP/25%PA-DADV-0NO1.1/0.928.0

      Cone calorimetry tests were performed to assess the actual fire performance for EP composites, and the associated results are reviewed in Table 2. EP/PA-DAD composites' time to ignition (TTI) was found to be shorter than that for pure EP. Because an earlier decomposition for PA-DAD would form a stable, and protective char layer, keeping the EP matrix from further thermal degradation. As depicted in Fig. 3a, the highly flammable EP would burn out rapidly after ignition, and its peak heat release rate (pHRR) was 1,007 kW/m2. Reversely, after introducing 25 wt% PA-DAD to EP, the pHRR of the composites has been reduced to 280 kW/m2, decreasing by 72.2% than that of neat EP. Moreover, as shown in Supplemental Fig. S4, the total heat release (THR) reduced significantly from 99.2 MJ/m2 of pure EP to 71.2 MJ/m2 of EP/25% PA-DAD, which had decreased by 28.3%. Besides, the rate of fire growth (FIGRA) was the ratio between pHRR and TpHRR (time to pHRR) according to the HRR curves to evaluate the actual fire hazard for material qualitatively[49,50]. The FIGRA value (1.38 kW/m2·s) of EP/25% PA-DAD was 77.8% lower than that of neat EP with 6.21 kW/m2·s, underscoring the superior fire safety enhancement achieved by PA-DAD in the EP composites. Additionally, the smoke released during combustion was generally recognized as another critical factor in evaluating the fire retardancy of the material. In Fig. 3b & c, the total smoke product (TSP) and peak of smoke product rate (pSPR) values of EP/PA-DAD composites were inhibited significantly. The values of THR and pHRR for the EP/25% PA-DAD composite experienced noteworthy reductions of 49.5% and 57.0%, respectively, in contrast with neat EP. In detail, as presented in Fig. 3d & e, the peak of CO production named pCOP, and the peak of CO2 production named pCO2P for EP/25% PA-DAD decreased by 66.5%, and 78.9%, respectively. In the event of a fire, fatalities can occur due to suffocation when the concentrations of carbon dioxide (CO2), and carbon monoxide (CO) in the surrounding air reach critical levels. Thus, the introduced PA-DAD to EP dramatically improved the fire safety and provided more escaped time for people. Moreover, lower production of CO and CO2 disclosed the more carbon fixed char residue during combustion, contributing to exert condensed phase effect for flame-retardant mechanism. To underscore the superior attributes of the newly formulated PA-DAD, the TSP reduction, pCOP reduction of PA-DAD and previously reported flame retardant for EP are displayed in Fig. 3f. The detailed fire performances are also summarized as in Supplemental Table S4. The biological PA-DAD in this work showed a significant reduction of TSP and pCOP than those of previous reported FRs.

      Table 2.  Data from Cone calorimeter for EP and its composites.

      SamplesTTI
      (s)      		TPHRR
      (s)      		pHRR
      (kW/m2)      		THR
      (MJ/m2)      		pSPR
      (m2/s)      		TSP
      (m2)      		pCOP
      (g/s)      		pCO2P
      (g/s)      		FIGRA
      (kW/m2·s)
      Neat EP86 ± 3162 ± 51007 ± 3299.2 ± 1.90.26 ± 0.01027.7 ± 2.10.0258 ± 0.00070.586 ± 0.0136.21
      EP/10% PA-DAD71 ± 2121 ± 4740 ± 2696.1 ± 2.10.28 ± 0.01225.1 ± 1.70.0274 ± 0.00060.343 ± 0.0096.12
      EP/20% PA-DAD63 ± 2184 ± 4403 ± 2471.9 ± 0.70.12 ± 0.00616.6 ± 0.90.0125 ± 0.00050.176 ± 0.0072.19
      EP/25% PA-DAD34 ± 1203 ± 3280 ± 1971.2 ± 0.90.11 ± 0.00714.0 ± 1.10.0086 ± 0.00030.124 ± 0.0081.38

      Figure 3. 

      (a) HRR, (b) SPR, (c) TSP, (d) COP, (e) CO2P curves vs time for EP composites under a flux with 35 kW/m2, and (f) the TSP reduction vs pCOP reduction of PA-DAD compared with other works.

      Flame-retardant mechanism

    • To disclose the mechanisms from flame retardants within EP matrix, the char residues of EP composites from cone calorimetry were further studied. As displayed in Fig. 4, the digital photos indicated that EP/PA-DAD composites had a dense and intumescent carbon layer. With an increasing loading of PA-DAD, the volumes of residues increased as well expanded gradually. The dense and intumescent carbon layer provided a desired shield to reduce the heat and mass transferring between condensed and gas phases. SEM-EDS was applied to evaluate the quality of char residues. As depicted in Fig. 4a1−d1 & 4e, there were four elements including C, O, N and P in the char residues of EP/PA-DAD composites. And P content of the char layer was increased gradually with the increased loading of PA-DAD. The element P was introduced by PA-DAD, accelerating the catalysis and carbonization for EP to produce a good char layer in dense, continuous, and phosphorous-containing. Thus, the char layers of EP/PA-DAD composites in Fig. 4a2−d2 were relatively compact compared with neat EP, enhancing the flame retardancy via condensed phase.

      Figure 4. 

      Digital photos of residual chars from cone calorimetry for (a) neat EP, (b) EP/10% PA-DAD, (c) EP/20% PA-DAD, (d) EP/25% PA-DAD. (a1)−(d1) EDS results and (a2)−(d2) SEM photos for corresponding residue char. (e) The images of elemental mapping for residue chars of EP/25% PA-DAD.

      The compositions of char residues were also analyzed by FTIR methods. In Supplemental Fig. S5a, the absorption peaks at around 1,618, and 2,852−2,958 cm−1 were related to C=C, and C-H' stretching vibration from CH2 group, respectively[33,43]. For EP/25% PA-DAD, a new 1,160 cm−1 absorption peak indicated the presence of P-O bonds in compound. Two typical peaks of G band at 1,580 cm−1 and D band at 1,350 cm−1 were presented in Raman spectra of char residues for EP and EP/25% PA-DAD composite, which corresponded to graphite char and the disordered char, respectively (see Supplemental Fig. S5b)[51]. The value of ID/IG for EP/25% PA-DAD char was 1.11, higher than that of char from neat EP (0.89), suggesting a smaller microstructure size that enhances fire resistance[52]. As displayed in Supplemental Fig. S5c, the composition of elements from the surface of EP/25% PA-DAD char was analyzed by XPS. The elements including C, N, O, and P existed in the chemical structure of EP/25% PA-DAD char, which was consistent with the results of EDS characterization. The high-solution of elements was shown in Supplemental Fig. S6a−S6d. Three peaks of C-C in aliphatic and aromatic species at 284.5 eV, C=O at 288.5 eV, and C-O in ether and/or hydroxyl group at 286.0 eV were deconvoluted from C1s spectra. In the high-solution O1s, three kinds of oxygen configurations appear at 535.1, 532.1 and 530.4 eV, corresponding to -COOH, -O- of C-O-C or C-O-P and =O from phosphate or carbonyl groups, respectively. The N1s spectra could be divided into two peaks of 400.2 and 399.2 eV, assigning to nitrogen functionality in the pyrrolic group and oxidized nitrogen compounds[53,54]. The only 133.5 eV peak from P2p spectra was ascribed to pyrophosphate and polyphosphate. The results of XPS demonstrated that poly-/pyro-/ultra-phosphoric acids were generated while the combustion occurred, forming a strong char layer via dehydration and esterification of EP.

      The TG-IR was carried out to explore the pyrolysis products of PA-DAD, pure EP and EP/25%PA-DAD composite under a heating rate with 20 °C/min and N2 atmosphere. Figure 5a showed the FTIR 3D and 2D for decomposed fragments from PA-DAD at 200−500 °C when thermal degradation. The primary gaseous products of PA-DAD were hydrocarbons at 2970 cm−1, CO2 at 2360 cm−1, PO· at 1258 cm−1, P-O-C at 1097 cm−1 and NH3 at both 965 and 930 cm−1. The fragments with P-O-C and PO· were demonstrated by a strong signal at 450−500 °C, being cable of quenching active radicals with OH· and H· during burning, thereby suppressing the combustion. The absorption peak for NH3 was displayed between 400−500 °C, which contributed to the dilution of flue gases and a reduction in the rate of burning. In Fig. 5b, both neat EP and EP/25% PA-DAD displayed the analogous characteristic peaks from gaseous products when degradation, such as aromatic compounds at 1,605, 1,510, 834 and 689 cm−1, hydrocarbons at 2,800−3,000 cm−1, CO2 at 2,360 cm−1, H2O at 3,600−4,000 cm−1.

      Figure 5. 

      (a) FTIR 3D and 2D spectra from pyrolysis products for PA-DAD. (b) FTIR 3D and 2D spectra from the pyrolysis products for pure EP and EP/25% PA-DAD with the maximum rate of decomposition.

      Nevertheless, the EP/25% PA-DAD showed the weaker peak intensities compared with pure EP, indicating the incorporation for PA-DAD could obstacle some productions of volatiles effectively during burning. In addition, the curves of total volatile gas intensity versus temperatures, and the Gram-Schmidt (GS) curves for pure EP and EP/25% PA-DAD composite are depicted in Supplemental Fig. S7. The EP/25% PA-DAD composite' intensities of pyrolytic products were lower than that of neat EP consistently. Some reasons for the dramatical reduction of volatile pyrolysis products from EP/25% PA-DAD are as follows. At the beginning of combustion, the earlier thermal decomposition for PA-DAD could lead to the carbonization layers with nano-scale, acting as a block to inhibit the transferring of heat and mass. When combustion continues, the PA-DAD generated poly-/pyro-/ultra-phosphoric acids for catalyzing the degradation of EP, forming intumescent and compact char for protecting the matrix from further thermal degradation.

      Pyrolysis behaviors

    • The pyrolysis products obtained from EP/PA-DAD composites were investigated by the method of Py-GC/MS further. The main pyrolysis products from the pyrogram (in Supplemental Fig. S8) for EP/25% PA-DAD, were also listed in Supplemental Table S5. The 2, 3, 5, 6, and 10 peaks in the pyrogram were attributed to phenol, p-Cresol, 4-isopropyl phenol, 4-isopropenylphenol, and 2,2-bis(4-hydroxyphenyl) propane. The aforementioned substances stemmed from the reactions involving the opening of aromatic rings and the subsequent rearrangement reactions of the bisphenol A chains. The nitrogen-containing compounds of (E)-Alpha-cyanocinnamamide at peak 8, have the capacity to decompose into non-flammable, small-molecule gas NH3. Above decomposition process effectively reduced the concentration of the flammable gases[55]. The detailed pyrolysis routes and speculative flame-retardant mechanism for EP/25% PA-DAD are shown in Fig. 6. During the burning process, the decomposition of PA-DAD can yield products with P-O-C, PO·, and NH3, thereby capturing the active radicals of H·, OH· and decreasing the flammable gases. When combustion continues, the residual phosphorus from condensed generated the poly-/pyro-/ultra-phosphoric acids, catalyzing the degradation of EP for promoting the producing of the intumescent and compact chars. Herein, the formation of intumescent and compact chars reduced the oxygen, and toxic gas emissions to prevent the matrix from continuously degrading. Thus, the above results about incorporated PA-DAD further demonstrate the excellent flame retardancy and enhanced residual char of composites.

      Figure 6. 

      Speculated pyrolysis and flame-retardant mechanisms for composites with PA-DAD.

      Mechanical properties

    • Mechanical performances including the tensile, flexural and impact strength for neat EP as well EP/PA-DAD composites are displayed in Figs. 7 & 8. The above relatively detailed data are reviewed in Supplemental Table S6. In Fig. 7a & d, the typical strain-stress for neat EP and its corresponding composites are presented. As demonstrated in Fig. 7c & f, the toughness values of pure EP and its corresponding composites were calculated by the integration values from stress-strain curves[56,57]. Obviously, the tensile, flexural strength and toughness were firstly increased and then decreased with the increasing contents of PA-DAD. Partial increased modulus can be attributed to the capacity of PA-DAD to enhance crosslink density. The increased toughness was assigned to the ion bonds formed by PA-DAD, which have the effect on dissipating energy[12,58]. The partial agglomeration of PA-DAD will be caused after the added amount reaches a certain level, leading to a noticeable decline in mechanical properties of EP composites[44].

      Figure 7. 

      (a) Tensile stress-strain curves, (b) tensile strength and modulus, (c) tensile toughness, (d) flexural stress-strain curves (e) flexural strength, (f) flexural toughness for EP and its corresponding composites.

      Figure 8. 

      SEM images from the fractured surfaces of tensile samples for (a) neat EP, (b) EP/10% PA-DAD, (c) EP/20% PA-DAD, and (d) EP/25% PA-DAD. (e) EDS mapping photograph of tensile fractured surfaces for EP/25% PA-DAD.

      The PA-DAD were agglomerated in the EP matrix as seen in the tensile fracture when the addition reached 25 wt% (see Fig. 8). The mapping photograph also proved the phosphorus agglomeration in the fracture. When the PA-DAD loading was 10 wt%, the tensile, and flexural strength of composites owned higher values compared with neat EP, which were 17.8% and 26.0% respectively. In particular, the tensile, and flexural toughness were greatly enhanced by 466% and 178% compared with neat EP. When PA-DAD loading reached 20 wt% and 25 wt%, although the tensile, and flexural strength of EP/PA-DAD composites exhibited lower values, the tensile, and flexural toughness still higher than those of neat EP. Some agglomerations caused by the relatively high filler contents led to the tensile and flexural strength decreased, but the ion bonds formed by PA-DAD have the function of dissipating energy for increasing tensile and flexural toughness. In comparison with neat EP, the impact strength (Supplemental Table S6) of EP/10% PA-DAD, EP/20% PA-DAD, and EP/25% PA-DAD were enhanced to 14.61, 13.00 and 8.53 KJ·m−2, and were also enhanced by 99.0%, 77.1%, and 16.2%, respectively.

    Conclusions

    • In conclusion, a fully biological flame retardant named PA-DAD has been successfully prepared through a straightforward neutralization reaction of DAD and PA. When the content of PA-DAD achieved 25 wt%, LOI value for EP/25% PA-DAD composite reached up to 28.0%, and also satisfied a V-0 rating of the UL-94 test. In comparison with neat EP, the EP/25% PA-DAD displayed dramatic reductions in pHRR (72.2%), pSPR (57.0%), THR (28.3%), TSP (49.5%) and FIGRA (77.8%). The imparting of PA-DAD can also inhibit the production of volatiles sufficiently. The synergy between condensed and gas phase contributed to the outstanding flame retardancy from EP/ PA-DAD composites, beneficial to proposed flame retardant mechanisms. For the condensed phases, PA-DAD generated poly-/pyro-/ultra-phosphoric acids and facilitated the production of a protective char shield to insulate the transferring of heat and mass. While in the gas phases, the PA-DAD released the PO·, P-O-C, and NH3 and then quenched the active radicals of H· and OH·. The mechanical performances for EP composites have been enhanced to certain degrees after adding the flame retardant of PA-DAD. Our work supplied a friendly and facile approach for preparing biological flame retardant and smoke suppressant materials for high fire-safety required EP composites.

    Author contributions

    • The authors confirm contribution to the paper as follows: study conception and design: Lin Z, Zhang W, Dai J; data collection: Lin Z, Zhang W, Lou G, Bai Z; analysis and interpretation of results: Lou G, Bai Z, Xu J, Li H, Zong Y, Chen F, Song P, Liu L, Dai J; draft manuscript preparation: Lin Z, Lou G, Chen F, Dai J. All authors reviewed the results and approved the final version of the manuscript.

    Data availability

    • The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

      Acknowledgments

      • This work was financially supported by the National Natural Science Foundation of Zhejiang Province (No. LY21E030001), National Natural Science Foundation of China (No. 51903222), National College Students Innovation and Entrepreneurship Training Program of China (No. 202210341055), the 'Leading geese' projects of Zhejiang Provincial Department of Science and Technology (No. 2022C03128), and the College Student Science and Technology Innovation Activity Plan of Zhejiang Province (No. 2023R412019).

      Conflict of interest

      • The authors declare that they have no conflict of interest.

      • These authors contributed equally: Zhiqian Lin, Wangbin Zhang

      Supplementary information
      • Supplemental Table S1 The Various Proportions of the Epoxy Resin Systems.
      • Supplemental Table S2 TGA data of PA-DAD and EP composites.
      • Supplemental Table S3 Kinetic parameters of the curing reaction.
      • Supplemental Table S4 The fire performances of PA-DAD.
      • Supplemental Table S5 Structural assignments for the main fragments observed in the Py-GC/MS of EP/25 %PA-DAD.
      • Supplemental Table S6 Mechanical properties of neat EP and EP composites.
      • Supplemental Fig. S1 1H NMR spectra of (a) PA, (b) DAD, and (c) PA-DAD.
      • Supplemental Fig. S2 The DSC curves of (a) neat EP, (b) EP/10% PA-DAD, (c) EP/20% PA-DAD, (d) EP/25% PA-DAD at different heating rates and the fitting curves based on Kissinger, and Ozawa methods for neat EP and EP composites.
      • Supplemental Fig. S3 The UL-94 test for neat EP and EP composites.
      • Supplemental Fig. S4 The THR curves vs time for EP composites at a flux of 35 kW/m2.
      • Supplemental Fig. S5 (a) FTIR spectrogram of the composition of residual char; (b) Raman spectrogram of the composition of residual char; (c)XPS spectrogram of the composition of residual char.
      • Supplemental Fig. S6 (a)The C1s spectra of the composition of residual char; (b) The O1s spectra of the composition of residual char; (c) The N1s spectra of the composition of residual char; (d) The P2p spectra of the composition of residual char.
      • Supplemental Fig. S7 (a) The Gram-Schmidt curves of neat EP and EP/25% PA-DAD; (b) The intensity of hydrocarbons versus the temperature curves of neat EP and EP/25% PA-DAD; (c) The intensity of CO2 versus the temperature curves of neat EP and EP/25% PA-DAD; (d) The intensity of CO versus the temperature curves of neat EP and EP/25% PA-DAD; (e) The intensity of carbonyl compounds versus the temperature curves of neat EP and EP/25% PA-DAD; (f) The intensity of aromatic compounds versus the temperature curves of neat EP and EP/25% PA-DAD.
      • Supplemental Fig. S8 Py-GC/MS spectra of EP/25% PA-DAD.
      • Supporting Information Details of characterizations.
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      • Copyright: © 2023 by the author(s). Published by Maximum Academic Press on behalf of Nanjing Tech University. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (8)  Table (2) References (58)
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    Lin Z, Zhang W, Lou G, Bai Z, Xu J, et al. 2023. A bio-based hyperbranched flame retardant towards the fire-safety and smoke-suppression epoxy composite. Emergency Management Science and Technology 3:21 doi: 10.48130/EMST-2023-0021
    Lin Z, Zhang W, Lou G, Bai Z, Xu J, et al. 2023. A bio-based hyperbranched flame retardant towards the fire-safety and smoke-suppression epoxy composite. Emergency Management Science and Technology 3:21 doi: 10.48130/EMST-2023-0021
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  • Introduction
  • Experiments and methods

  • Materials

  • Prepared method of PA-DAD

  • Fabricated method of EP/PA-DAD composites

  • Characterizations

  • Results and discussion

  • Characterizations of PA-DAD

  • Curing and thermal properties

  • Flame retardancy for EP composites

  • Flame-retardant mechanism

  • Pyrolysis behaviors

  • Mechanical properties

  • Conclusions
  • Author contributions
  • Data availability
  • Acknowledgments
  • Conflict of interest
  • Supplementary information
  • Rights and permissions
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