Microfibrous cyclodextrin boosts flame retardancy of poly(lactic acid)

2-hydroxypropyl-β-cyclodextrin (HP-β-CD) microfibres with diameters ranging between 3-7 μm were prepared by aqueous solution based high-speed electrospinning (HSES) technique and then used as carbonising agent at 3 wt% loading besides 15 wt% ammonium polyphosphate (APP) to obtain flameretarded poly(lactic acid) (PLA) composites. The high specific surface area of the microfibrous HP-β-CD was found to have a crucial role in its flame retardant efficiency. Compared to the effect of the same amount of conventional HP-β-CD powder additive, microfibrous HP-β-CD resulted in higher char yields both during thermogravimetric analysis and cone calorimeter test accompanied with significantly increased mechanical resistance and consequently with improved flame retarding efficacy. The Limiting Oxygen Index (LOI) of the intumescent flame-retarded PLA composite increased noticeably, from 29.0 % to 32.5 %, while the Flame Retardancy Index (FRI), determined from cone calorimetry data, increased from 1.9 (with powder HP-β-CD) to 2.6 (with microfibrous HP-β-CD) only by changing the physical form i.e. the specific surface area of the used HP-β-CD. The advantage of the special microfibrous structure of the oligosaccharide type charring agent lies in the efficient interaction with APP and the attribution of the intumescent char layer with improved thermal and mechanical resistance without compromising its swelling ability. Besides, the microfibrous structure of the HP-β-CD also contributed to the improvement of the mechanical performance of the flame retarded PLA composites.


Introduction
The growing concern over the environment and sustainability lead to extensive research in developing bio-based green materials as means of solving the disposal problem and reducing the environmental impact of conventional plastics [1]. Among biodegradable thermoplastics, poly(lactic acid) (PLA) (derived from starches and sugars [2]) has received increasing attention from industry and academia [3,4] because of its biodegradability, excellent mechanical properties, thermoplastic processability, and biological properties.
Because of these properties, PLA is proving to be useful in diverse applications as a viable alternative to petrochemical-based plastics [4]. PLA's applications have extended from short life cycle products to the electronics, automobiles, building materials, and the aerospace industry [5][6][7][8]. Since without flame retardants (FRs) PLA cannot meet the safety standards in most cases, the development of effective flame retardants for the modification of PLA is necessitated [9,10]. Intumescent flame-retardant systems (IFRs) are considered promising halogen-free flame-retardant additives that form a foamed cellular charred layer on the surface of the polymer matrix to block the combustion process resulting in lower smoke emission and toxicity and suppressed molten dripping during a fire [11]. These systems mainly include three components; a carbonization agent, an acid source, and a blowing agent [12]. Ammonium polyphosphate (APP) can act both as the acid source and the blowing agent often combined with a polyalcohol type carbonization agent like pentaerythritol (PER) [13]. The use of bio-based excipients is increasingly encouraged and highly preferred especially in biopolymer composites [14,15]. The substitution of PER with renewable char forming materials (mainly polysaccharides) was found to be an obvious and viable solution to provide greener flame retardant formulations, among others, for PLA [16][17][18][19][20][21].
Cyclodextrins are cyclic oligosaccharides formed by the enzymatic treatment of starch [22].
Cyclodextrins and their derivatives are widely used in many industrial and scientific fields [23] and gain popularity in the plastic industry as well. Their inclusion complexes of fragrances, antimicrobial and antioxidant agents, dyes, insecticides, UV-filters can be incorporated into polymers either to ensure slowrelease or homogeneous distribution of the complexed substances [24] utilised in the packaging industry [25,26] and pharmaceuticals [27,28]. Cyclodextrins have also shown a lot of potential as FR components in multiple polymer systems like epoxies [29,30], elastomers [31][32][33], polyesters [34,35] polypropylenes [36][37][38][39][40], and other polymeric systems [41][42][43]. Also, in PLA, β-cyclodextrin (β-CD) has been found to be an effective carbonization agent due to its charring ability and thermal stability. Feng et al. [44] gained a high amount of charred residue in PLA with a 20 wt% β-CD/APP/MA (ratios: 1/2/1) additive system. Teoh et al. [45] showed that in PLA/PMMA system β-CD additive improves the char formation efficiency, as in PLA/PMMA20/FR/β-CD system at 20 wt% additive content with 1:1 FR/β-CD ratio the cyclodextrin suppressed the melt-dripping behaviour and also retained and resisted the loss of phosphorus FR during the polymer combustion. Vahabi et al. [46] synthesised an organic-inorganic hybrid with β-CD (BSDH); and this novel additive has shown to have an excellent synergistic effect in improving the flame retardancy of PLA/APP/BSDH composite, as demonstrated by the significant reduction of peak heat release rate (pHRR) and total heat release (THR) values. Zhang et al. [47] created phospholipidated β-cyclodextrin (PCD) through interfacial polycondensation and found the optimum mass ratio of APP to PCD to be 5 to 1. At this ratio, with 30 wt% loading, the LOI reached 42.6 % accompanied with UL-94 V-0 rating, and in mass loss cone calorimetry (MLC) test the highest amount of char residue (71.5 wt%) was obtained besides significant reduction of pHRR and THR values compared to the neat polymer. Even when the total loading of APP and PCD was decreased to 20 wt%, still V-0 rating according to the UL-94 standard was reached.
It has been shown recently that the particle size of the flame-retardant additives has a significant impact on both the flammability and mechanical performance of intumescent flame-retarded compounds.
Depending on the (relative) particle size and the dispersion of the components of an IFR system in the polymer matrix, their interaction and reaction pathway can change, and thus carbonaceous chars of different composition and structure can be obtained, significantly differing in flame-retarding performance [48].
Electrospinning (ES) technology has been receiving increasing attention due to its capability to produce ultrafine micro-or nanofibres, or fibrous structures proven to be practical in many fields, even in flame retardancy of materials. Vahabi et al. have recently reviewed the state-of-the-art features of flame-retardant polymer materials developed by using electrospinning [49]. Among others, electrospun nanofiber mats were successfully used as fire protecting coatings [50] or as submicronic additives with simultaneous reinforcing and flame retarding capability [51].
Recently, cyclodextrins (CDs) have been successfully electrospun as CDs are capable of forming polymer-like supramolecular structures via intermolecular interactions [52]. Vass et al. [53,54] manufactured grindable HP-β-CD microfibres with diameters in the range of 2-10 µm with a uniquely high production rate of 270 g/h using aqueous high-speed electrospinning technique. The thus obtainable enhanced specific surface area of the fibrous material provides further benefits and widens the perspectives of cyclodextrin-based formulations.
In this work, microfibrous structures were manufactured from the aqueous solution of 2hydroxypropyl-beta-cyclodextrin (HP-β-CD) by high-speed electrospinning method and then used as a biobased charring agent in intumescent flame-retarded PLA. Our hypothesis was that the specific surface area of HP-β-CD plays a role in its flame-retardant efficiency. Therefore, the flame-retardant performance of microfibrous HP-β-CD was compared to that of commercially available powder form in PLA/APP/HP-β-CD systems. Also, the morphological, thermal, and mechanical properties of the flame-retarded biopolymer composites were comprehensively studied.

Preparation of HP-β-CD microfibres
The microfibrous HP-β-CD type additive was prepared by high-speed electrospinning (HSES) and subsequent milling. Figure 1 shows the schematic representation of the preparation steps.

Figure 1 Preparation of the microfibrous HP-β-CD additive
The HSES setup used for fibre formation consists of a stainless steel spinneret (d = 34 mm) connected to a high-speed motor [53]. The disk-shaped spinneret is equipped with 36 equidistantly distributed orifices (d = 330 μm) located in the sidewall of the wheel. For the solution, 68 wt% HP-β-CD was added to purified water and the mixture was stirred with a magnetic stirrer (100 rpm) at room temperature until complete dissolution (24 h). The solutions were fed with a SEP-10 S Plus syringe pump with a flow rate of 350 mL/h. The rotational speed of the spinneret was fixed at 40,000 rpm. The applied voltage was 40 kV during the experiments (Unitronik Ltd., Nagykanizsa, Hungary). The conical bottom of the drying chamber was grounded, and it acted as the counter electrode. Air knives were used to remove the dried material from the surface of the chamber and a constant airflow (120 m 3 /h, room temperature) was applied to help the dried fibres to reach the cyclone. The experiments were performed at ambient temperature (25 °C). The produced fibrous material was collected by a cyclone. The high throughput fibre forming technology enabled the production of HP-β-CD microfibres with uniquely high productivity of 270 g/h. The obtained electrospun material was then ground to make it suitable for blending with excipients. For this purpose, a hammer mill (IKA MF10, IKA-WERKE GmbH & Co. KG, Staufen, Germany) was used with a 1.0 mm sieve at 3000 rpm. Including the dissolution, spinning, collecting, and grinding losses, a 76 % yield was obtained.
Based on the SEM images presented in Figure1 with identical magnifications of 100×, the morphology of the inherent powder form of HP-β-CD can be compared with that of the obtained fibrous form. The obvious reduction in particle size is accompanied by an approximately 9-fold increase in the surface area to volume ratio, as estimated by image processing of SEM micrographs (supplement), which was expected to be relevant regarding the performance in the IFR system. batches. Before processing, the raw materials were dried in an oven at 80 °C for 6 hours in each case. The internal mixer operated at 180 °C with a continuous rotor speed of 50 rpm. In each case, the polymer mixture was prepared by first adding the pre-measured amount of poly(lactic acid) granules and after adding the additives (15 wt% APP, 3 wt% HP-β-CD) and then mixed for 10 minutes. In Table 1 the compositions of the prepared flame-retarded PLA composites are shown.
The ratio of APP to HP-β-CD was chosen based on the results of Y. Zhang et al. [47], who found the APP to PCD mass ratio of 5 to 1 to be optimal regarding the flame retardant efficiency of the system. For comparison, also test specimens from neat PLA and PLA containing only APP were also manufactured and examined.

Moulding
The mixed materials were dried overnight at 70 °C and then hot-pressed using a Teach-Line Platen Press 200E heated platen press (Dr. Collin GmbH, Munich, Germany). About 30 g of each mixed sample was heated to 180 °C in a mould of 100 × 100 × 2 mm 3 size, then the plates were compressed for 4 minutes with the gradual addition of pressure up to 100 bars and finally cooled to 50 °C under pressure. The specimens for flammability and mechanical testing were obtained by cutting the plates with a disk saw. Prior to performing the experiments, the test specimens were stored in sealed bags at room temperature.

Scanning electron microscopy
Scanning electron microscopic (SEM) micrographs of the powder and electrospun HP-β-CD and the cryogenic fracture surface of the flame-retarded PLA samples were taken using a 6380LVa (JEOL, Tokyo, Japan) type apparatus in high vacuum at an accelerating voltage of 10 keV. Before the examination, all the samples were fixed by conductive double-sided carbon adhesive tape and sputtered by gold using ion sputter (JEOL 1200, JEOL, Tokyo, Japan) to prevent charge build-up on the surface.

Determination of specific surface area of the HP-β-CD additives
Particle sizes and the specific surface area of the HP-β-CD additives were determined by image processing of SEM micrographs with 100× magnification using MATLAB's (The MathWorks, Inc., Natick, MA, USA) Image Processing Toolbox. In the SEM images ( Figure 2), the outline of the identified particles was selected (at least 100 for each type of particle), from which the size values were calculated. The program calculated the area of the particles, from this area the diameter of a circle of equal projection area (d EC , diameter of a circle with an equivalent area of the measured particles) was determined.
Then, the maximum diameter (d M ) was measured, that is the longest distance between any two points along the selected boundary. For the calculation of the aspect ratio (A r ), the minimal diameter (d m ) was estimated from a rectangle with the same area as the particle and the length of which is d M (Figure 3).

Determination of residual water content
To determine the residual water content of the HP-β-CD samples, thermogravimetric analyses were carried out with the same Q5000 type TGA instrument ( Figure 4).

Figure Schematic representation of the char compression test
Upon compression, the normal force required to exert continuous movement transduced by the charred layer was constantly detected and registered.

Results and Discussion
Two types of HP-β-CD additive, differing in particle size and aspect ratio, i.e. commercially available powder and electrospun microfibres, were used as biobased charring agents at 3 wt% loadings besides 15 wt% APP in PLA matrix thus setting the 5:1 mass ratio of APP to CD also found to be optimal by Zhang et al. [47]. The potential role of the morphology of the used HP-β-CD was comprehensively investigated on the thermal, flammability, and mechanical properties of the flame-retarded PLA composites, as well as on the evolution of the intumescent flame-retardant system.

Determination of specific surface area of the HP-β-CD additives
From the data acquired from the analysis of the SEM images taken of the two types of HP-β-CD particles, the average surface to volume area was estimated for both additives. The average values of d EC and d M , the average size of the marked areas, as well as the calculated surface to volume ratios are reported in Table 2. The specific surface area of the powder particles was estimated using the d EC . In the case of the fibrous material the d M value, the average length and diameter were considered to calculate an average specific surface area. Accordingly, a 9-fold increase in the surface area to volume ratio of the HP-β-CD particles was achieved by fibre formation using the aqueous HSES method.

Thermogravimetric analysis of the additives
The thermal behaviour of the used flame-retardant additives was investigated by thermogravimetric analyses performed in N 2 atmosphere. The resulting thermograms are shown in Figure 5, while the main thermal characteristics are summarised in Table 3. For comparison, the TGA thermogram and relevant data corresponding to neat PLA are also presented. It can be observed that the decomposition of PLA occurs nearly in the same temperature range (290 to 310 °C) as the first decomposition step of APP. For HP-β-CD samples, the initial weight loss up to 100 °C is due to the loss of absorbed water and water of crystallisation [57] which was measured to be 4.5 wt% for the powder and 7.9 wt% for the fibrous material. The significantly increased amount of absorbed water detected in the case of the microfibrous HP-β-CD is associated with its noticeably increased surface area, obtained as a result of fibre formation. The second weight-loss step of the β-cyclodextrins, occurring at around 335 °C is related to their single step decomposition, resulting in a carbonaceous residue (char). The decomposition of the fibrous HPβ-CD begins at a slightly higher temperature and occurs at a lower rate compared to the powdered form. The last degradation stage (T > 400 °C) corresponds to the slow rate degradation of the char. It can be observed, however, that a noticeably higher amount of residue remains at 800 °C from the fibrous HP-β-CD (5.9 wt%) than that from the powder HP-β-CD (1.2 wt%) indicating increased thermal stability of the charred residue that forms from the cyclodextrin with a fibrous supramolecular structure.  Figure 6. In the image of PLA/APP composite, particles stuck together, and detached boundaries of APP particles between APP and PLA (Figure 6 a). Similarly, due to the strong polarity of HPβ-CD, poor interfacial interaction is expected in the HP-β-CD containing PLA composites as well. On the contrary, in Figure 6 b and c, the better dispersion and less sharp phase boundaries of APP particles can be recognised suggesting improved interfacial interaction between APP and the polymer matrix when HP-β-CD is also present in the system. It is proposed that during melt processing the HP-β-CD particles may allocate around the APP surface and act as a surface modifier. A similar phenomenon was revealed by Yin et al. [58], namely that cellulose nanofibres (CNFs) improve the dispersion of APP within the PLA matrix by acting as interfacial adhesion agent. In this relation, the high-surface-area HP-β-CD microfibres are able to form more physical and also chemical interaction with the APP particles.  The thermal characteristics of the PLA composite samples, as measured by TGA in nitrogen atmosphere, are reported in Figure 7 and Table 4, respectively. It can be seen from the presented results that there is no significant difference between the first decomposition stage of the composites; 5 % weight loss can be observed at around 325-340 °C in all cases. The temperature of the maximum decomposition rate does not show a significant difference either. Nevertheless, the lowest maximum degradation rate corresponds to the fibrous HP-β-CD containing sample, while for the sample containing HP-β-CD powder similar value was measured than for the HP-β-CD free PLA/APP sample. Based on this observation, during thermal degradation, a higher degree of interaction is assumed to occur between APP and the microfibrous cyclodextrin. Upon changing the type (i.e. size and structure) of the HP-β-CD used, there is also a difference in the thermal stability and the amount of the residue obtained at 800 °C. In the enlarged part of the figure, it can be seen that above 500 °C, when the second decomposition step of APP occurs, the weight loss is smaller when fibrous HP-β-CD is also present in the system. Above 600 °C, the decomposition of the powder containing sample also accelerates, resulting in less residual mass than remains from the PLA/APP sample. In contrast, the residual weight of the sample containing 3 wt% HP-β-CD in fibrous form remains above the weight of the sample containing only APP even up to 800 °C, thus showing greater thermal stability of the formed char. Based on the TGA analyses performed on the PLA composites it was concluded that the microfibrous structure of the HP-β-CD type bio-based charring agent advantageously influence its interaction with APP and promotes the char formation accompanied with increased thermal resistance that may be crucial regarding the fire protection performance of the intumescent system. calorimetric data are given in Table 5 Comparing the results of the composites, it can be noticed that during the cooling of the fibrous HP-β-CD containing sample, crystallization of PLA started at a noticeable higher temperature (T c = 116 °C), which indicates increased nucleating effect of the high-surface-area cyclodextrin.

Thermogravimetric analysis of the composites
It can also be seen that for all the three flame-retarded composites, the crystallinity determined from the second heating cycle increased compared to the additive-free PLA, which can be connected to the decrease in molecular weight due to high temperature and shear forces occurring during the production of the composites [59].

Tensile tests
Tensile tests were performed to study the effect of the used FR components on the mechanical performance of the PLA composites. In Table 6, the tensile strength, Young's modulus and elongation at yield values measured for the flame-retarded PLA composites are presented. As expected, both neat APP and HP-β-CD act as a non-reinforcing filler in the PLA matrix and result in decreased tensile strength and elongation. Comparing the effects of powder and fibrous HP-β-CD, it can be concluded that using fibrous HP-β-CD slightly better results can be obtained with respect to all the studied mechanical properties (higher modulus, strength and elongation) than with the conventional powder additive. Based on this result it is presumed that the microfibrous HP-β-CD, due to its small particle size and high aspect ratio, can have a reinforcing effect in the system. Also, the surface modifying the efficacy of the microfibrous HP-β-CD is believed to be higher than that of the conventional powder form that results in increased interfacial compatibility between APP and PLA enabling more efficient transfer of external loads from the polymer matrix to the fillers.
Analysis of variance (ANOVA) was performed to evaluate the effect of the used FR additives on Young's modulus. Results indicated that the type of the additive has no significant influence on this mechanical characteristic (p-value = 0.114, significance level: 5 %). However, by performing planned comparisons with contrast coefficients it was confirmed that there is a significant difference between the modulus values of PLA/APP/HP-β-CD powder and PLA/APP/HP-β-CD fibre samples (p-value = 0.0233, significance level: 5 %). Accordingly, it can be concluded that the particle size and structure of the HP-β-CD used at 3 wt% has an effect on the modulus of the flame-retarded composite.

Limiting oxygen index and UL-94
The results of the LOI and UL-94 measurements performed on the flame-retarded PLA composites are summarized in Table 7. In the case of PLA without flame retardants, during performing the horizontal test the flame spread throughout the test specimen, the average flame spreading rate was calculated to be 34 mm/min.  In the case of the sample with APP alone, a minimal level of charring was observed, while in the case of the specimens combined with HP-β-CD, the phenomenon of charring was more visible. Comparing the powder and fibrous HP-β-CD containing samples, it was observed that on the surface of the out flamed specimens with fibrous HP-β-CD somewhat more carbonaceous char formed. However, apart from this, for each sample, the falling polymer droplets ignited the piece of cotton wool placed underneath, thus achieving a V-2 rating according to the UL 94 standard.
The measured LOI values show that both types of HP-β-CD, when applied at 3 % besides 15 % APP, effectively increase the LOI of the PLA composites. Nevertheless, it was found, that the LOI of the flameretarded PLA composite containing fibrous HP-β-CD is significantly higher than that of the sample containing the same amount of HP-β-CD powder. Consequently, the smaller particle size and increased surface area to volume ratio of the fibrous cyclodextrin is believed to be of key importance regarding its flame retardant efficiency.

Mass loss calorimetry
The heat emission curves gained from the Mass Loss calorimeter measurements of the PLA composites are shown in Figure 8 while the recorded mass loss during the combustion is presented in Figure   9. Main flammability characteristics are summarised in Table 8. Based on the heat emission curves it can be seen that the HP-β-CD type charring agents greatly reduced the amount of heat emitted during the combustion of the PLA composites. Compared to the PLA/APP sample, the heat release rate curves are flatter and more elongated in time when HP-β-CD is also present in the system. This slower decomposition phenomenon can also be read when comparing the slope of the weight loss curves in Figure 9.  The samples containing FR additives show a 20-25 % lower total heat release (THR) compared to neat PLA. The peak heat release rate (pHRR) values show a similar decrease. For both measured quantities, the sample containing fibrous HP-β-CD showed the best results with a 25 % reduction in total heat emission and a 40 % reduction in the peak heat release rate compared to those of neat PLA. The maximum rate of heat emission (pHRR) corresponding to the sample containing fibrous HP-β-CD was also significantly (by 33 %) lower compared to the PLA/APP system, and also the highest amount of carbonaceous residue (m residue ) was formed from this sample. Based on the Flame Retardancy Indices (FRIs) of the composites, both HP-β-CD containing IFR systems provide -good‖ flame retardant performance (1 < FRI < 10) [55]. Nevertheless, FRI increased from 1.9 (with powder HP-β-CD) to 2.6 (with microfibrous HP-β-CD) only by changing the physical form i.e. the specific surface area of the used HP-β-CD. It was concluded that the microfibrous structure of the charring agent is beneficial regarding the flame retardancy performance of the IFR system.
The charred residues obtained after cone calorimeter tests show slightly differing morphology, as can be seen in Figure 10. It is proposed that the well-dispersed microfibrous HP-β-CD improves the integrity of the intumescent char layer, thereby improving its thermal and mechanical stability and thus contributes to better flame retardation. To further investigate this phenomenon, the mechanical resistance of the carbonaceous residues was analysed with compression tests. To characterize the mechanical resistance of the foam structures, the char strength and modulus values were calculated for each tests sample, which can be seen in Table 9 together with the determined initial heights of the examined intumescent char layers. The presented mechanical characteristics can be interpreted as the resistance of the formed carbonaceous residue to deformation. The combustion residues corresponding to the fibrous HP-β-CD containing sample showed improved resistance to deformation even though these char layers had a larger extent (thickness reached nearly 2 cm). The compression strength of the carbonaceous residue corresponding to the PLA/APP/HP-β-CD fibre sample (1107 ± 88 Pa) was measured to be more than two-fold greater than that of the sample containing identical amount of conventional powdered HP-β-CD (542 ±142 Pa). The fibrous HP-β-CD was found to similarly increase the stiffness (modulus) of the charred layers. The thermal and mechanical resistance of an intumescent char fundamentally determines the flame retardant effectiveness of the protective layer. Our results show that the microfibre-structured HP-β-CD effectively strengthens the carbonaceous layer, thereby improving its heat and material transport inhibitory effect. The key importance of compact char formation is more obvious when the ignition source comes from the edges of samples (e.g. LOI test), where the efficient barrier against heat, oxygen and fuel transport can result in immediate fire extinction. Figure 11 Normal force vs. deformation curves measured for the chars formed from the two types of HP-β-CD containing intumescent flameretarded systems The microstructure of the charred residues was also examined by SEM. In Figure 12, micrographs with a magnification of 100× are presented, showing the characteristic pore structure of a relatively large area. Accordingly, the multicellular honeycomb structure is characteristic for both chars, however, they differ in pore size; the charred residue corresponding to the fibrous HP-β-CD containing composite is composed of noticeably smaller cells. It is proposed that the high-surface-area HP-β-CD microfibres may initiate cell nucleation during the simultaneous charring and expansion process, resulting in higher cell density. The increased APP/HP-β-CD interaction proposed based on Figure 6

Conclusion
It is demonstrated that the efficiency of an intumescent flame-retardant system can be noticeably improved just by reducing the particle size and aspect ratio of the used charring agent. In this study, the specific surface area of HP-β-CD was increased nine-fold by the high throughput aqueous HSES fibre forming technology. The thus obtained high-surface-area HP-β-CD microfibres were found to have better char promoting performance in intumescent flame-retarded PLA system than the conventional HP-β-CD powder. By increasing the specific surface area of the used charring agent, a remarkable increment of the LOI value (from 29.0 % to 32.5 %) and a noticeable improvement in all the combustion characteristics, measured by cone calorimeter tests, were achieved. Furthermore, the investigated mechanical characteristics of the flame-retarded PLA composites also improved when the HP-β-CD additive was used in a microfibrous form. The improved flame retardant performance of the microfibrous charring agent is proposed to rely on multiple effects. On one hand, it was evinced by TGA analyses that the associated supramolecular structure of the electrospun HP-β-CD fibres is thermally more stable than the powdered form, so is the char that remains after decomposition of the fibrous material. On other hand, based on SEM imaging and mechanical characterisation of the composites, HP-β-CD particles tend to allocate around the APP particles during melt processing when the high-surface-area microfibres are able to form more interactions with the acid source.
As a result, the char formation will be more effective, as indicated by the increased amount of residue obtained both after TGA analysis and cone calorimeter test. Besides these effects, the microfibres are believed to have an increased cell nucleating effect during the swelling process, resulting in a honeycomb structured char that is composed of smaller cells accompanied by lower thermal conductivity. Also relating to this effect, the char layer formed from the fibrous HP-β-CD containing PLA composite was found to have about two times higher compression strength and modulus than those of the powdered HP-β-CD containing composite. By this means, an advantage of various microfibrous fillers, namely that they can reinforce the intumescent char layer without compromising its expandability, has been successfully utilised with the cyclodextrin type charring agent. As a consequence of all these, by using HP-β-CD with microfibrous structure, an increased amount of intumescent char forms that is thermally and mechanically more resistant and thus provides improved fire protection to the polymer. The proposed flame retardancy mechanism is also presented schematically in Figure 13.
The new recognition, namely that the particle size reduction of the HP-β-CD type charring agent alone can provide a significant improvement in the flame retardant efficiency and even in the mechanical properties of the composite, is assumed to be successfully effectuated in other polymeric systems.

Acknowledgements
The

Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.