A mechanism for fire retardancy realized by a combination of biofillers and ammonium polyphosphate in various polymer systems

As a matrix of polymer composites, the homopolymer type of PP (type J108M) was purchased from Prime Polymer Co., Ltd., Tokyo, Japan. This polymer has a melt flow rate of 45 g/10 min (230 °C, 2.16 kg) as typical for an injection molding grade material. To improve the compatibility of PP and cellulose, 3.3 wt% of maleic anhydride grafted polypropylene (MAPP) (UMEX1001, Sanyo Chemical Industries Co., Ltd., Kyoto, Japan) was added to PP composites. The value was preliminary optimized against 50 wt% of CPs in terms of tensile strength. PLA (REVODE110) was supplied from Zhejiang Hisun Biomaterials Co., Ltd., Taizhou, Zhejiang, China. This polymer has a melt index of 2–10 g/10 min (190 °C, 2.16 kg), glass transition temperature of 59–62 °C, and melting temperature of 155–160 °C according to the datasheet provided by the supplier.

As a biofiller, CPs (W100-GK) were purchased from Nippon Paper Industries Co., Ltd., Tokyo, Japan. The CPs had a mean particle size of approximately 37 µm. As a halogen-free fire retardant, APP (Taien K) was purchased from Taihei Chemical Industrial Co., Ltd., Osaka, Japan. The phosphorus and nitrogen content in APP were approximately 30% and 16%, respectively. The amount of APP was held constant at 10wt% for PP composites and PLA composites according to our previous study (Arao et al. 2014).

All the composites were prepared by melt mixing through a co-rotating twin-screw extruder (ZSK 18 MEGAlab, Coperion GmbH, Stuttgart, Germany) on a laboratory scale. The screw diameter was 18 mm and the length to diameter ratio (L/D) was 40. The screw configurations for the twin-screw extruder differed between the PP composites and PLA composites. Each screw configuration is shown in Fig. 1a, b. In the case of the PP composites, all materials (i.e., PP granules, MAPP granules, CP and APP) were fed into a main hopper through a volumetric feeder. The temperature profile of the barrel was set as 170 °C/170 °C/140 °C/140 °C/140 °C/140 °C/175 °C from the inlet zone to die head. The rotation speed was 150 min^− 1, and the throughput was 1.0 kg/h.

In case of the PLA composites, the compounding process was divided into two steps due to the concern of hydrolytic degradation of PLA and carbonization of APP and CP. Initially, PLA granules and CP were fed into the main hopper and compounded at a screw speed of 150 min^− 1 and throughput of 1.0 kg/h. The temperature profile of the barrel was set as 180 °C/175 °C/175 °C/170 °C/170 °C/165 °C/165 °C. The compounds were obtained as masterbatch granules in the first process. Subsequently, the masterbatch granules and APP were compounded at the same screw speed and throughput used for the first process, and then the final compounds were obtained. The temperature profile of the barrel was set as 165 °C/165 °C/140 °C/140 °C/140 °C/140 °C/180 °C. Before compounding, PLA was dried in a vacuum oven for 3 h at 90 °C. PP, CP and APP were dried in an oven for 24 h at 80 °C. All formulations are listed in Table 1.

By using the compounded granules through twin-screw extrusion, dumbbell specimens (type 1A, ISO 527) were molded through use of an injection molding machine (PLASTER ET-40V, TOYO MACHINERY & METAL Co., Ltd., Akashi, Hyogo, Japan) with a clamping force of 40 tons. As for the molding conditions, the cylinder temperature was set at 175 °C and 165 °C for PP and PLA, respectively. The obtained specimens were cut for (a) flammability test for the UL 94 standard and (b) measurement of the thickness of the char layer, as shown in Fig. 2. The dimensions are also presented in the same figure. Additionally, plate specimens (50 mm × 50 mm × 3.3 mm) were also molded for the cone calorimetry test. The compounded pellets were dried in an oven for 24 hours at 80 °C before injection molding.

A flammability test was conducted by using the prepared specimens shown in Fig. 2a according to the UL 94 standard (2013). All samples were dried in an oven for 24 h at 80 °C. For evaluating the flammability of the composites, a horizontal burning test and vertical burning test was conducted in this order. In the horizontal burning test, two lines were marked onto the samples in the direction perpendicular to the longitudinal axis at 20 mm and 80 mm from the end that was to be ignited. The samples were clamped at the opposite side of the ignition position, with the longitudinal axis horizontal and transverse axis inclined at 45°. A blue flame that was 20 mm high was applied to the lowest edge of the samples at a distance of 10 mm and angle of 30° to the vertical for 30 s. The time was recorded for the combustion front to travel from the 20 mm mark up to the 80 mm mark for calculation of the burning rate in mm/min. If the burning rate is less than 40 mm/min in case of a thickness of 4 mm, the flammability rating can be expressed as HB. The tests were conducted for 5 times for each specimen.

For the samples that had a satisfactory HB rating, an additional vertical burning test was conducted for judging the achievement of a V-0 rating. The sample was clamped with the longitudinal axis vertical. A blue flame with a height of 20 mm was supplied to the middle point of the bottom edge at a distance of 10 mm for 10 s, and the afterflame time (t₁) was measured. In case the combustion stopped, the sample was ignited again for 10 s and the afterflame time (t₂) and afterglow time (t₃) was recorded again. The V-0 class should satisfy all the following conditions: (a) t₁ or t₂ is less than 10 s, (b) total time of t₁ plus t₂ for the 5 samples is less than 50 s, (c) total time of t₂ plus t₃ is less than 30 s, (d) afterflame or afterglow does not reach the holding clamp and (e) cotton does not ignite by flaming particles or drippings. This test was also conducted for 5 times for each specimen.

Cone calorimetry was performed using a Cone Calorimeter III C3 (Toyo Seiki Seisaku-sho, Ltd., Tokyo, Japan). Through the cone calorimetry tests, HRR was measured for the molded plate samples. The pHRR and total heat release (THR) was also determined from the results. Samples wrapped in aluminum foil were placed with a horizontal orientation above a conical radiant electric heater at a distance of 25 mm. A heater temperature of 734.5 °C and an external heat flux of 50 kW/m² were applied.

The rectangular samples in Fig. 2b were burned enforcedly for observation of the growing thickness of the char layer against elapsed time. The burning time was varied as 10 s, 20 s and 30 s, and the samples were quenched rapidly in water after each time elapsed. The quenched samples were dried for 24 h at 80 °C. Afterward, the samples were embedded into a thermoset plastic, and the centerline of the samples was cut along a longitudinal direction. The 12 samples were prepared for each formulation. The thickness of the char layer at the cross-section was measured for 10 points against each samples (totally 120 points) by optical microscopy.

The thermal decomposition behavior was measured by a TGA apparatus (DTG-50H, Shimadzu Co., Kyoto, Japan) under air atmosphere. A sample with a weight of 10–11 mg was cut from injected specimens. The temperature was increased from room temperature to 600 °C under a heating rate of 10 °C/min. The weight loss was measured while increasing the temperature.

The results obtained from the horizontal burning test and vertical burning test according to UL 94 against PP composites and PLA composites are shown in Table 2. The term self-extinguish denotes extinguishment of combustion after forced ignition against the horizontal burning test. The HRR curve for the cone calorimetry test is depicted in Fig. 3a for PP composites and Fig. 3b for PLA composites. From these curves, the value of the pHRR, average HRR and THR were calculated and summarized in Table 3.

From the previous section, production of water and char by dehydration of cellulose through incorporation of APP is important to achieve the V-0 rating. Therefore, TGA was conducted due to determination of the dehydration temperature during combustion. The mass loss behavior and derivative thermogravimetry (DTG) curves for pure PP (PP-0), pure PLA (PLA-0) and CP are shown in Fig. 5a, b, respectively. In the CP, the thermal decomposition behavior is divided into three steps. In the beginning, cellulose is slightly decomposed until 100 °C due to evaporation of absorbed moisture. After that, a second decomposition was observed from 220 to 350 °C. Finally, decomposition from 350 to 480 °C was confirmed. The pyrolysis of cellulose can be divided into two steps at a threshold temperature of 300 °C (Kandola et al. 1996). At less than 300 °C, cellulose reacts with atmospheric oxygen and forms oxycellulose, which on further heating decomposes with the evolution of water, carbon monoxide and carbon dioxide. Above 300 °C, a gas, tar and char are produced.

When the thermal decomposition behavior is compared for pure PP, pure PLA and CP, the decomposition of CP is found to start at lower temperature (220 °C) than PP (270 °C) and PLA (300 °C). This finding can also be explained from the temperature at the maximum peak in the DTG curve. Therefore, CP addition into the polymer matrix may exhibit a peak attributed to the dehydration reaction at lower temperature than the decomposition temperature of the polymer matrix. Here, the thermal decomposition behavior of PP composites and PLA composites through TGA is shown in Fig. 6a, b, respectively. However, the addition of only CP (PP-3, PLA-3) led to the exhibition of a two-staged decomposition behavior for both composites. The first decomposition temperature was not affected by CP addition and remained at almost the same decomposition temperature observed for the pure matrices although CP shows a lower decomposition temperature than the matrix. Finally, the carbonized residues did not remain at the end as well as for the neat matrices. Additionally, the addition of only APP (PP-4, PLA-4) also exhibited a two-stage decomposition: decomposition of polymer matrix and decomposition of carbon residues, and, finally, more residues were observed than for the system containing only CP.

Interestingly, in case of the combination of APP and CP (PP-6, PP-7, PLA-6, PLA-7), a three-step decomposition behavior was determined against both composites. The first decomposition occurring between 200 and 300 °C can be attributed to the dehydration of CP by interaction of APP. As discussed before, a nonflammable gas is generated here, and, then, char is formed with further heat above 300 °C. The second decomposition corresponds to the decomposition of the matrix polymer. The third decomposition corresponds to the decomposition of the char, and the highest mass of carbonized residues can be obtained through all compositions. Thus, a combination of APP and CP can reduce the dehydration temperature against both composites. Furthermore, a higher CP amount led to a decrease in the temperature of the dehydration reaction of CP. Therefore, the dehydration at lower temperature against the decomposition temperature of the neat polymer is effective to improve the fire retardancy of composites.

From all the results, we proposed the hypothesis that the dehydration temperature of cellulose via the assistance of APP at lower temperature is important for achieving a V-0 rating. The TGA weight loss curves in Fig. 6 were converted to DTG curves, as shown in Fig. 7. From the explanation for the observed thermal decomposition behavior in the previous section, the combination of APP and CP shows three peaks. The first peak between 200 and 300 °C corresponds to the dehydration of cellulose, the second peak between 300 and 400 °C corresponds to the thermal oxidation of the polymer, and the third small peak between 400 and 500 °C corresponds to thermal decomposition of char residues. Here, the temperature at the peak between 200 and 300 °C (dehydration temperature) in the DTG curve is denoted as T_maxC for the composites incorporating CP and APP, the peak temperature for the neat polymer is denoted as T_maxP, and the second peak temperature for CP (dehydration temperature) is denoted as T_maxB. The T_maxC, T_maxP and T_maxB are represented in Figs. 5b and 7. Then, the temperature difference (ΔT_max) between the degradation temperature of the polymer (T_maxP) and dehydration temperature (T_maxC) of the composites can be given by Eq. 1:The relationship between the thickness of the char layer at an elapsed time of 30 s and ΔT_max is plotted in Fig. 8. Furthermore, the relationship between the average HRR and ΔT_max is plotted in Fig. 9. From these figures, the linear correlation between the thickness of the char layer or average heat release rate and ΔT_max was confirmed. Furthermore, the thickness of the char layer increased, and the average HRR decreased with increasing ΔT_max. Here, ΔT_max for PP-7, PLA-6 and PLA-7, which all exhibited a V-0 rating in Table 2, was determined to be 116 °C, 119 °C and 126 °C, respectively. In case of the HB rating, ΔT_max was less than 95 °C. Therefore, it is necessary for ΔT_max to be at least above approximately 95 °C for achieving a V-0 rating. Here, the condition of ΔT_max for achieving a V-0 rating is given by Eq. 2:In addition, the decomposition temperature of the polymer should be higher than the dehydration temperature of the cellulose itself. Therefore, the precondition should satisfy Eq. 3 as follows:From the correlation in Figs. 8 and 9, the minimum thickness of the char layer and the average HRR for achieving a V-0 rating were determined to be above approximately 0.7 mm (for 30 s) and below approximately 80 kW/m², respectively.

Here, the suggested conditions for achieving a V-0 rating were validated by using a different polymer, polyoxymethylene (POM). The flammability of POM is quite high compared PP and PLA due to the very low LOI value of 15–16 (Harashina et al. 2006). Therefore, improvement of the fire retardancy for POM-based composites is a very challenging task. POM (Eupital, F20) was provided by the Mitsubishi Engineering-Plastics Corporation, Tokyo, Japan. Due to material limitation, wood flour was used as biofiller in this validation experiment. The wood flour (BFα) was provided by K.K. Plaisir, Osaka, Japan. The particle size ranged from 50 to 150 µm. Here, we investigated the improvement in the fire retardancy realized by using a combination of APP and this BFα. The flammability test and TGA were conducted against POM composites. The formulations for the POM composites are listed in Table 4. The classification of the flammability for the UL 94 standard is also listed in the same table. Not only did the addition of only BFα not show a V-0 rating in the flammability test but neither did the combination of BFα and APP.

Here, we examined the reason for this from the TG and DTG curves obtained for POM composites according to the suggested condition for achieving a V-0 rating. The TG and DTG curves for pure POM (POM-0), BFα, POM composites with only BFα (POM-1, POM-2) and POM composites with BFα and APP (POM-3, POM-4) are shown in Fig. 10. The combination of APP and BFα showed a peak that was attributed to the dehydration reaction of cellulose at temperature between 150 and 250 °C, although the addition of only BFα did not show a peak. This is the same phenomena observed for the PP composites and PLA composites. The decomposition temperature (T_maxP) for POM was found be approximately 290 °C and the dehydration temperature (T_maxB) for BFα was approximately 300 °C. Each peak temperature is shown in Fig. 10. The decomposition temperature for POM was smaller than the dehydration temperature of BFα, and hence, Eq. 3 was not satisfied in this composition. From the thermal decomposition behavior observed in the DTG curves, ΔT_max was found to be approximately 73 °C against POM-3 and POM-4, and Eq. 2 was not satisfied. Hence, in case of POM-based composites, a V-0 rating could not be achieved. We proved that the precondition of Eq. 3 is important for obtaining the synergetic effect of cellulose and APP through this validation experiment. Through these tests, we discovered that both conditions of Eqs. 2 and 3 are necessary to providing excellent fire retardancy against composites incorporating biofiller and APP.