Statistical analysis

The statistical model used was a completely randomized block design (DBCA), with a 2 × 2 factorial arrangement (bamboo particle size and bamboo/polypropylene ratio), with seven repetitions for the weight loss, anhydrous density, absorption, swelling, tension and static bending tests, and 20 for impact testing. The design was as follows:

Where:

µ = General mean

αi = The effect due to the i ^th level of particle size factor (40/60 and 60/80)

βj = The effect due to the j ^th level of bamboo/polypropylene ratio factor (50/50 and 70/30)

γk = The block (two types of fungi)

αβij = Represents the interaction effect in the combination ij

εijk = Represents the random error that follows a distribution with zero mean and constant variance σ2 and are independent of each other

An analysis of variance was applied with the Statistical Analysis System software, version 9.2 (^{SAS, 2008}). When differences between treatments were observed (p<0.05), the Tukey mean comparison test was applied.

Biodegradation of bamboo-polypropylene composites

Figure 2 shows the weight loss of bamboo control and bamboo-polypropylene composite exposed to Pycnoporus sanguineus and Rhodonia placenta. The weight loss values of the bamboo-polypropylene composite ranged between 3.3 and 4.9 % for P. sanguineus and from 2.3 to 3.4 % for R. placenta. The highest weight loss was recorded in the composites with the higher proportion of bamboo particles (50/50) and the smaller particle size (-60/+80). Likewise, for the bamboo cane control and the bamboo-polypropylene composite material, a higher weight loss was observed in the presence of P. sanguineus.

Pérdida de peso = Weight loss; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 2 Biodegradation of bamboo-polypropylene composite. 

Regarding the ratio (R), the weight loss caused by both fungal species was observed to increase with higher proportions of bamboo particles in the composite, because there is a greater availability of the organic material for the colonization by and development of the fungi. These results are widely consistent with what has been cited in several studies (^{Pendleton et al., 2002}; ^{Lomelí et al., 2009}; ^{Kartal et al., 2013}; ^{Candelier et al., 2018}).

In regard to the particle size variable (S), there was a possible greater weight loss caused by P. sanguineus in composites with the smaller bamboo particle size; this can be supported by the results documented by ^{Kartal et al. (2013)} and well as by the study by ^{Lomelí et al. (2009)}, who claim that smaller particles have a better degree of distribution and increase the specific surface area susceptible to infestation by fungi.

These results indicate that the bamboo particles are not fully encapsulated by the polymer matrix, which affects the biodegradation of the material; this agrees with the statement of ^{Klyosov (2007)}, namely: that when composite materials contain a significant amount of cellulose, there are enough particles to form extended chains in contact, along which water can permeate most of the material, causing its microbial contamination in the pores and conduits of the matrix, as well as degradation of the cellulosic matter.

Regarding the type of fungus (F), white-rot P. sanguineus was proven to produce more biodegradation than brown-rot R. placenta. This comes from the ability of P. sanguineus to degrade the main components of the cell wall, as pointed out by ^{Encinas and Mora (2003)}. These authors describe the degradation patterns of the white-rot fungi Trametes versicolor (L ex Fr) Pilat. and P. sanguineus in Caribbean pine woods, in whose trees they simultaneously attack the main components of the cell wall: cellulose, hemicellulose and lignin. Likewise, ^{Catto et al. (2016)} point out that white-rot fungi contain such enzymes as lignin peroxidase, laccase and manganese peroxidase, which catalyze the deterioration, through the diffusion of oxidizing agents or specific mediators, which for the most part degrade lignin and in some cases degrade both lignin and cellulose.

Brown-rot fungi, as cited by ^{Mankowski and Morrell (2000)}, ^{Fabiyi et al. (2011)}, ^{Ibach (2013)}, and ^{Candelier et al. (2018)}, contain endoglucanases which tend to degrade cellulose and hemicellulose primarily, so that there is a limited decrease in lignin content, and greater efficiency in the degradation, predominantly of carbohydrates, in conifers (^{Irbe et al., 2006}).

The analysis of statistical variance (ANOVA) showed that the type of fungus (F) and ratio (R) variables were highly significant (with a p-value of 0.0002 and 0.0011, respectively); the particle size variable (S) was also significant (p-value = 0.0176).

Physical properties of bamboo-plastic composites exposed to biodegradation

Density

Figure 3 shows the density of the bamboo-polypropylene composite; in those exposed to the fungi, the lowest value (0.65 g.cm^-3) corresponded to P. sanguineus, with a particle size of 40/60 and a 50/50 bamboo/polypropylene ratio, which is consistent with its higher degradation. Regarding R. placenta, the highest value was 0.89 g.cm^-3, with a 60/80 particle size and a 30/70 bamboo/polypropylene ratio. Likewise, the results for bamboo-polypropylene composites with exposure to fungi were lower than those of the control; which shows that biodegradation by fungi in the composite materials affected the density of the composite.

Densidad = Density; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 3 Density of bamboo-polypropylene composite exposed to rot fungi. 

The results showed that bamboo-polypropylene composites with exposure to rot fungi and smaller particle size had the highest density values because smaller particles have a better degree of distribution and encapsulation in the polymeric matrix, which reduce porosity and increase density (^{Verhey and Laks, 2002}; ^{Klyosov, 2007}). Also, as the content of bamboo particles in these composites increases, the porosity increases and the density diminishes (^{Lomelí et al., 2009}; ^{Lázaro et al., 2016a}, and ^{Córdova et al., 2020}).

Based on Figure 3, the densities registered with the 50/50 bamboo/polypropylene ratio (<0.84 g.cm^-3) are lower than those documented by ^{Schirp and Wolcott (2005)}, with 1.05 g.cm^-3; by ^{Cárdenas (2012)}, with 1.06 g.cm^-3, and by ^{Kartal et al. (2013)}, with 1.08 g.cm^-3 in specimens unexposed to fungi. This shows that the composites under study had higher porosity levels, which had an impact on the lower density values, possibly because in the present research they were made by extrusion and compression; while in the cited studies the final process was by injection.

The analysis of statistical variance (ANOVA) showed that the variables particle size (S) and ratio (R) were highly significant (p-value < 0.0001).

Absorption

Figure 4 shows the absorption level attributed to the increase in weight of the bamboo-polypropylene composite. Bamboo-polypropylene composites with a smaller particle size (60/80), a 30/70 bamboo/polypropylene ratio, and exposure to R. placenta had the lowest value (7.4 %), while the highest (29.3 %) was recorded in composites with the larger particle size 40/60, a 50/50 bamboo/polypropylene ratio, and exposure to P. sanguineus. These results support the greater influence of the white-rot fungus on the physical property of absorption because the biodegradation was higher (^{Cárdenas, 2012}). Also, the water absorption in the bamboo - polypropylene composites without exposure (control) was evidently lower compared to the composites exposed to the two species of fungi, since penetration by them increased water absorption through the cracks or the greater empty spaces resulting from the separation of the bamboo from the plastic (^{Steckel et al., 2006}; ^{Klyosov, 2007}).

Absorción = Absorption; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 4 Absorption of bamboo - polypropylene composite exposed to rot fungi. 

In addition, bamboo-polypropylene composites with larger particle size and exposure to rot fungi exhibited the highest values for water absorption, which was favored by the larger hydrophilic area exposed to moisture (Figure 5) (^{Verhey and Laks, 2002}; ^{Bouafif et al., 2009}; ^{Lázaro et al., 2016a}). On the other hand, small particles are more effectively encapsulated in the polymer matrix, which reduces the empty spaces for the circulation of water and, therefore, water absorption (^{Stark, 2001}; ^{Wang and Morrel, 2004}).

60/80 (left) and 40/60 particle size (right)

Figure 5 Samples subjected to absorption with a 30/70 bamboo/polypropylene ratio after exposure to Rhodonia placenta (Fr.) Niemelä K.H. Larss. & Schigel. 

Bamboo - polypropylene composites with exposure to rot fungi with a higher content of bamboo particles showed higher water absorption values, due to the increase of their hydroxyl groups and because of their hydrophilic nature (^{Westman et al., 2010}; ^{Ibach, 2013}); in addition, an incompatibility was generated between the particles and the polymeric matrix that originated empty spaces and cracks through which the water penetrated, as pointed out by ^{Klyosov (2007)} and ^{Bouafif et al. (2009)}. That is, the increment of bamboo particles in bamboo-polypropylene composite materials increased water absorption values (^{Mankowski and Morrell, 2000}; ^{Verhey and Laks, 2002}; ^{Barton et al., 2017}; ^{Candelier et al., 2018}).

The absorption values determined for the 50/50 bamboo/polypropylene ratio (>17.9 %) for the two particle sizes exposed to white and brown rot were higher than those reported by ^{Hosseinihashemi et al. (2011)}, of 9.1 and 10.0 %; by ^{Kartal et al. (2013)}, of 16.6 %, and by ^{Cárdenas (2012)}, of 5.1 %; all these authors worked with similar particle proportions and sizes, but with different manufacturing methods.

The analysis of statistical variance (ANOVA) showed that the particle size (S) and ratio (R) variables were highly significant (p-value < 0.0001 for both).

Swelling

In the bamboo-polypropylene composites with exposure, the lowest value, 0.9 %, corresponded to the smaller particle size (60/80), with a 30/70 bamboo/polypropylene ratio and exposure to R. placenta (Figure 6). The highest value, 2.9 %, was for the same particle size and fungal species, but with a bamboo/polypropylene ratio of 50/50 (Figure 6). Also, the swelling of the bamboo-polypropylene composites in the control was lower, compared to the composites with exposure (Figure 6). This can be explained from the penetration of the rot fungi that caused a greater water absorption, due to the cracks or larger empty spaces produced by the separation of the bamboo from the plastic (^{Steckel et al., 2006}; ^{Klyosov, 2007}).

Hinchamiento = Swelling; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 6 Swelling of bamboo - polypropylene composite exposed to rot fungi. 

Results in Figure 6 show that, after exposure to rot fungi, bamboo-polypropylene composites with the higher content of bamboo particles registered the highest values for swelling, in response to the increase of the hydroxyl groups of bamboo and because of its hydrophilic nature; this is consistent with the indications of several authors (^{Westman et al., 2010}; ^{Mankowski and Morrell, 2000}; ^{Verhey and Laks, 2002}). When the bamboo particles swell, microcracks formed between the particles and the polymer matrix, creating porosity and cracks where the water penetrates (^{Bouafif et al., 2009}).

Both, a larger particle size and a higher ratio were used; this originated a larger hydrophilic surface area exposed to moisture, which increased swelling. The opposite occurred with smaller percentages and particle sizes (Figure 7) (^{Verhey and Laks, 2002}; ^{Wang and Morrel, 2004}; ^{Bouafif et al., 2009}). Thus, the degree of degradation by fungi depends on the bamboo content in the composites; that is, in the composites with more bamboo particles, there was a greater penetration of fungi, which favored the separation of the bamboo from the plastic, and this, in turn, caused greater porosity or cracks (^{Steckel et al., 2006}; ^{Klyosov, 2007}) and the swelling of the composites to increase.

Bamboo/polypropylene ratios of 50/50 (left) and 30/70 (right)

Figure 7 Samples subjected to swelling after exposure to Pycnoporus sanguineus (L.) Murrill., with particle size 60/80. 

The swelling values for the 50/50 bamboo/polypropylene ratio (<2.9 %) and the two particle sizes exposed to rot fungi (Figure 6) were lower than those documented by ^{Kartal et al. (2013)}, of 9.4 % and 9.7 %, and those cited by ^{Cárdenas (2012)}, of 10.0 %. These authors worked with similar proportions and particle sizes, and the exposure to white and brown rot fungi lasted 16 weeks; this difference in values comes from the fact that the immersion time of the test specimens in the present study was 24 hours, whereas in the aforementioned studies it was longer (30 and 200 days, respectively); however, the results documented herein are close to those obtained by ^{Hosseinihashemi et al. (2011)}, of 3.6 % and 4.1 %, with an immersion time of the test specimens also of 24 hours.

The statistical analysis of variance (ANOVA) showed that the ratio (R) variable was highly significant (p-value < 0.0001), and the double interactions between type of fungus and the ratio (F*R), as well as between the particle size and the ratio (S*R) were significant (p-value = 0.0411 and 0.034, respectively).

Mechanical properties of bamboo-plastic composites exposed to biodegradation

Static bending

Figure 8 shows the maximum static bending strength of bamboo/polypropylene composite without exposure (control), with exposure to moisture, and with exposure to two rot fungi. In the bamboo/polypropylene composites with exposure, 23.7 MPa was the highest value and corresponded to the composite with the smaller particle size (60/80), a bamboo/polypropylene ratio of 30/70, and exposure to R. placenta; the lowest value (10.4 MPa) was registered in the composite with the larger particle size (40/60), a bamboo/polypropylene ratio of 50/50, and exposure to P. sanguineus.

Resistencia máxima en flexión = Maximum bending strength; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 8 Maximum bending strength of bamboo-polypropylene composite exposed to two rot fungal species. 

These results revealed that the particles of the bamboo-polypropylene composite exposed to rot fungi are smaller than the control composites and than the 15 to 22.5 MPa recorded by ^{Lázaro et al. (2016b)}, who used the same particle sizes and proportions without exposure. This shows that exposure to rot fungi has an inverse relationship to maximum bending strength; this is supported by the evaluation of mechanical properties after exposure to fungi by ^{Stark (2001)}, ^{Schirp and Wolcott (2005)}, ^{Ibach (2013)}, and ^{Candelier et al. (2018)}. Also, the results of the test specimens exposed to moisture were close to those of the test specimens exposed to the fungi, which indicates that moisture may have a similar influence to that of the presence of such organisms (Figure 9), consistently with the results cited by ^{Cárdenas (2012)}.

Figure 9 Samples with a 60/80 particle size, subjected to the action of moisture (left) and of Pycnoporus sanguineus (L.) Murrill. (right). 

Likewise, the bamboo-polypropylene composites with smaller particle size exhibited the highest values of maximum resistance to bending because, depending on the size of the particles, there is a degree of encapsulation in the polymeric matrix; i.e. the small particles are more effectively encapsulated (^{Verhey and Laks, 2002}).

Figure 8 shows that, in bamboo-polypropylene composites, the increase in bamboo particle content decreased the maximum static bending strength, which is in line with the findings of ^{Schirp and Wolcott (2005)}, ^{Kartal et al. (2013)} and ^{Córdova et al. (2020)}.

The analysis of variance (ANOVA) showed that the particle size (S) and bamboo/polypropylene ratio (R) variables were highly significant (p-value = 0.0001 and < 0.0001, respectively), and that the fungus type variable (F) was significant (p-value of 0.0137).

Tension

In bamboo-polypropylene composites with exposure, 15.0 MPa was observed to be the highest value for composites containing the smaller particles (60/80), with a 30/70 bamboo/polypropylene ratio and exposure to R. placenta (Figure 10); the lowest value (7.7 MPa) was obtained for composites with the larger particle size (40/60), a 50/50 bamboo/polypropylene ratio, and the presence of R. placenta (Figure 10).

Resistencia máxima en tensión = Maximum tensile strength; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 10 Maximum tensile strength of bamboo- polypropylene composite exposed to rot fungi. 

Bamboo-polypropylene composites with exposure to rot fungi had lower values than the unexposed composites (control); the same happens when compared with the results obtained by ^{Lázaro et al. (2016b)} , who reported values between 16 and 17.8 MPa for the same particle sizes and ratios without exposure. This would prove that exposure to rot fungi inversely influences the maximum resistance to tension, which is supported by several studies in which losses of mechanical properties are registered after exposure to biodegrading fungi (^{Stark, 2001}; ^{Cárdenas, 2012}; ^{Ibach, 2013}; ^{Candelier et al., 2018}; ^{Teixeira et al., 2018}). However, the recorded values are close to those of the samples exposed to moisture only, demonstrating the inverse influence of moisture on the tension property.

The bamboo-polypropylene composites with the smaller particle size exhibited a higher maximum resistance to tension for both rot fungi, since the small particles are more effectively encapsulated by the polymeric matrix (^{Verhey and Laks, 2002}), increasing this mechanical property. The decrease of bamboo particle content augments the maximum tensile strength values (^{Lu et al., 2005}). Therefore, the bamboo - polypropylene composites elaborated with lower proportions of bamboo (30 %) had higher values for maximum resistance to traction.

The analysis of variance (ANOVA) showed that the particle size (S) and ratio (R) variables were highly significant (p-value < 0.0001 for both).

Impact

In the bamboo-polypropylene composites exposed to the fungal species, the highest value (0.53 J) was observed to correspond to the 30/70 bamboo/polypropylene ratio for both particle sizes and both rot fungi (Figure 11). The lowest values (0.33 J) were recorded for the 50/50 bamboo/polypropylene ratio, with the largest particle size (40/60) and exposure to P. sanguineus. This is consistent with the highest degradation of the bamboo/polypropylene composite material. Likewise, these results were superior to those of the control composites, which proves that this mechanical property was not affected by the biodegradation of the rot fungi in the composite materials.

Energía = Energy; Testigo = Control; Tamaño de partícula = Particle size; Proporción bambú/polipropileno = Bamboo-polypropylene ratio.

Figure 11 Impact of bamboo-polypropylene composite exposed to rot fungi 

The values of bamboo-polypropylene composites exposed to rot fungi were lower than those quoted by ^{Cárdenas (2012)}, who estimated an average value of 0.6 J for samples made by extrusion and injection and exposed to rot fungi. Likewise, ^{Lázaro et al. (2016b)} documented values between 0.39 and 0.44 J with similar particle sizes and ratios, but without exposure to fungi and using the same manufacturing method. This provides evidence of a greater influence of the manufacturing method of the composite material on the impact mechanical property. However, the mechanical property of impact is influenced by prolonged exposure such as weathering (^{Fuentes et al. 2014}).

In the bamboo-polypropylene composites, higher bamboo particle contents reduced the values for the impact property, indicating that polypropylene is the main responsible for resistance to impact. Particle size did not have a significant influence.

The analysis of variance (ANOVA) showed that the proportion variable (P) was highly significant (p-value < 0.0001).

Statistical analysis

Table 4 shows the values obtained and the standard deviation of the weight loss variables, as well as the physical properties of anhydrous density, absorption and swelling.

Table 5 shows the values and standard deviation of the variables corresponding to the mechanical properties.

Table 6 summarizes the analysis of variance (ANOVA) values for durability, weight loss, density, absorption, and swelling; and Table 7 shows the values for the mechanical properties of maximum strength in static bending, modulus of elasticity in static bending, maximum stress and impact load of bamboo-plastic composites exposed to biodegradation. Finally, Table 8 shows the values of the multiple mean comparison for the same variables using Tukey’s test.