Evaluation of the use of açaí seed residue as reinforcement in polymeric composite

Abstract

Açaí (Euterpe oleracea Mart.) is an important economic source in the north region of Brazil for agro-industry. However, it generates a huge amount of seed residue as agro-waste. The aim of this study is to evaluate the performance of this residue as reinforcement in particleboards. The medium-density particleboards were manufactured according to the experimental design (2^{2 + 1}), considering the parameters with two factors: “Granulometry”, for different açaí seed sizes (2.38, 1.19, 0.29, 0.15 and 0.07 mm) and “%Resin”, for different resin weight fractions (10%, 12.5% and 15%), both applied in the manufacturing of the composites. Measurements for the Apparent Density Thickness Swelling and Water Absorption (WA) of the particleboards were performed. Mechanical properties of particleboards were analyzed through top and surface Screw Withdrawal and Internal Bonding test. In addition, the morphology of surface from particleboards was evaluated through Scanning Electron Microscopy The results of TS for 24 h, WA for 2 and 24 h, and IB showed that the particle size was a significant factor (p < .05). These results established that the highest particle sizes provided lower percentages of absorption. The results of Screw Withdrawal did not achieve the requirements established by NBR 14810–2:2018 standard. Besides that, the results presented statistical difference, considering the particle size and the interaction between parameters (“%Resin” and “Granulometry”), which resulted in a p-value < .05. For the Internal Bonding test, the composite with highest particle size exhibited the best performance compared to the other ones.

Keywords

açaí seed residue mechanical properties particleboard polyurethane resin polymeric composite thermoforming

Introduction

Lignocellulosic resources from agricultural wastes have been an attractive alternative to be used as reinforcement in polymer composites,^1,2 due to their abundant, renewable nature and low cost.³ The use of this waste represents a great potential for the development of sustainable materials based on vegetal fibers and polymeric resins derived from renewable sources.^4,5

Researches have been carried out using lignocellulosic materials like bamboo,⁶ hemp,⁷ wood,⁸ coconut,^9,10 sisal, jute,¹¹ rice straws and coir fibers¹² as reinforcing fillers. Also, many different agricultural wastes were studied to be applied in composites.¹³ Castor-oil polyurethane resin has been considered a suitable option in replacement of synthetic resins.^9,14 Oprea¹⁴ reported that the castor-oil polyurethane composites produced with different cellulose derivatives preserved some properties and increased its degradability, becoming an eco-friendly composite. Chethana¹⁵ analyzed the influence of ginger on the mechanical behavior of polyurethane composites. It was observed that castor oil-based polyurethane has the potential to replace other resins in the production of composites.^16,17 These new eco-friendly materials minimize global warming and energetic impacts,¹⁸ as well as result in products with applications for the industry.¹⁹

In this context, Euterpe oleracea Mart. is a plant from Aracaceae family, and widely found in the Amazon region of Brazil.²⁰ Its fruit, popularly known as “açaí”, is considered an important economic source in the north region of Brazil.²⁰ Pará state is the main producer of açaí, followed by Amazonas, reaching 222,706 tons in 2019.²¹ However, there is a huge amount of açaí seed residues that have been discarded or used as a source of energy in ovens and boilers.²²

The effect of incorporation of açaí fibers in composites with natural rubber from different clones has been studied.²³ Castro²⁴ reported the influence of açaí fiber as a reinforcement in recycled thermoplastics (polystyrene and polypropylene). Espitia²⁵ developed films based on pectin and fibers of açaí. Wataya²⁶ investigated the mechanical, morphological and thermal properties of biodegradable composites based on aliphatic-aromatic copolyester/polylactic acid blends (PBAT/PLA blends) reinforced by açaí fibers, which were fabricated by melting extrusion process, using a twin-screw extruder machine. Santos²² reported the use of short açaí fibers as reinforcement in a blend based on PLA and pine resin. Mesquita¹³ investigated the influence of incorporation of açaí fibers treated chemically on the physical and mechanical properties of eco-particleboards. Martins²⁷ proposed an alternative use for açaí seed as a filler in castor oil-based polyurethane eco-sorbents. As observed, all studies were carried out using açaí residues in polymeric composites, but no reports have been found in scientific literature considering the use of açaí seed residue applied as reinforcement filler for castor oil-based polyurethane resin in wood-based composites.

In this study, particleboards reinforced with açaí seeds (Euterpe oleracea Mart.) residues were fabricated. They were manufactured based on a two-level factorial design experiment (2^{2 + 1}). Two factors were considered: “Granulometry”, for different açaí seed sizes (2.38, 1.19, 0.29, 0.15 and 0.07 mm) and “%Resin”, for different resin weight fractions (10%, 12.5% and 15%), both applied in the manufacturing of the composites. They were produced using castor oil-based polyurethane resin as a polymeric matrix. Physical properties of composites were determined through Apparent Density (AD), Thickness Swelling (TS), and Water Absorption (WA). Mechanical performance of composites was evaluated through Screw Withdraw strength (top and surface) and Internal Bonding test (IB). Also, SEM experiments were performed on the surface of the particleboards, in order to characterize the interaction between matrix and filler. The influence of variation of particle size and resin amount was investigated.

Experimental

Materials

The bicomponent castor oil-based polyurethane (PU - Ricinus communis) resin, in a ratio of 1:1 (50% polyol and 50% isocyanate), was donated by Plural Química, São Carlos. This resin was used as a matrix due to its partially sustainable characteristics, as the castor oil-based polyol is derived from renewable sources. Açaí seed (Euterpe oleracea Mart.) residue was obtained from a local market, at street Pedro Botelho no. 70, in Manaus/AM, Brazil.

The seeds were sieved in order to remove stalks and leaves. These seeds were dried in an oven at 100°C for 24 h, aiming to reduce moisture and germination power.²⁸ Subsequently, the seeds were ground in a knife mill (Marconi) and sieved into five mesh size ranges from Tyler scale: 8 (2.38 mm), 14 (1.19 mm), 48 (0.29 mm), 100 (0.15 mm), and 200 (0.07 mm) mesh and only retained particles were used.

Particleboard manufacturing

Table 1 describes the parameters used for a two-level factorial design experiment (2^{2 + 1}). Five types of particleboards were manufactured: G15, G10, Mi, Md15 and Md10. The particle sizes of açaí seed residues in Tyler mesh scale (“Granulometry”) and resin weight fractions (”%Resin”) were evaluated, in order to analyze its influence on properties of the particleboards. Composites G15 and Md15 used the highest resin weight fractions, while Composites G10 and Md10 used the lowest ones. Composite Mi used central values for both factors (resin weight and particle size). Seven experiments were carried out with a triplicate central point of the design experiment.

Table 1.

Parameters “Granulometry” and “%Resin” used in the two-level factorial design experiment.

Nomenclature	Resin (%)	Granulometry (mm)	Typologies
G15	15	2.38 and 1.19 (8 and 14 Tyler)	Gross (G) particleboard
G10	10	2.38 and 1.19 (8 and 14 Tyler)	Gross (G) particleboard
Mi	12.5	2.38 until 0.07 (8, 14, 48, 100 and 200 Tyler)	Mix (Mi) particleboard – central point
Md15	15	0.29 (48 Tyler)	Mean (Md) particleboard
Md10	10	0.29 (48 Tyler)	Mean (Md) particleboard

Particleboards were fabricated through the thermoforming process.^29–31 Castor oil-based PU resin was manually mixed with the açaí seed residue for 20 min. The mixed material was then inserted in a wood-based mold (400 × 400 × 10 mm) and placed in a thermo-hydraulic press (Model PHH 11007, Hidral-Mac^®), configured with 5 MPa pressure and 100°C, for 10 min of pressing. Finally, the composites were conditioned at room temperature around 72 h for the resin polymerization and composite stabilization.²⁹ Figure 1(a) and (b) show, respectively, the particles of açaí seed residue in raw form and the grounded seed, and Figure 1(c) shows the manufactured particleboard based on açaí seed residue and castor oil-based PU resin.

Figure 1.

(a) açaí seed residue in raw form, (b) the ground seed and (c) the manufactured açaí residue composite.

Experimental design

Results were evaluated by analysis of variance (ANOVA), using the software Statistica, version 7. The influence of factors “Granulometry” and “%Resin” was considered at a 5% level of significance.

Physical properties

The measurements for Apparent Density (AD), Thickness Swelling (TS) and Water Absorption (WA) from the particleboards were carried out following the procedures described in standard^30,31 for agglomerated wood-based panels. All samples were prepared with a nominal size of 50 x 50 x 10 mm, following the standard requirements.

Mechanical properties

In order to evaluate the influence of the factors “Granulometry” and “%Resin”, the mechanical performance of the particleboards were evaluated through surface and top Screw Withdraw strength test (with a speed of 15 mm/min) and Internal Bonding test (IB). Both tests were performed on samples extracted from the composites, using the equipment MTM – AMSLER, following the recommendations of the NBR 14810–2:2018.³⁰

Scanning Electron Microscopy

SEM experiments were performed for particleboards G10 and Md15 on a Hitachi TM3000 equipment, using 15 kV at 25°C, in order to compare the morphology of surface from particleboards with highest and lowest particle sizes, respectively. Samples of the composites were placed on carbon tape and composites surface-metalized by a sputter coating with a thin gold thickness (10 nm) and analyzed by SEM, aiming to improve the contrast and maximum resolution.³²

Results and discussion

Physical Properties

Figure 2 shows the response surface of the AD effect, as a function of the parameters “%Resin” and “Granulometry”. All results were above 0.8 g/cm³, and according to the standard NBR 14810–2:2018,³⁰ the particleboards were classified as high density. There was no significant difference between factors “Granulometry” and “%Resin” under the studied conditions. However, there is a tendency of higher AD with higher resin weight fraction and smaller particle size, suggesting lower dimensional variation under this condition (composite Md15). According to Mosiewicki,³³ which reported results related to wood-reinforced polyurethane foams, the higher density of composite could be related to more compact internal structure.

Figure 2.

Surface response of the AD effect as a function of the parameters “Granulometry” and “% Resin”.

Also, Mesquita¹³ produced particleboards reinforced with açaí fiber and the same resin used in this work (15% of castor oil-based polyurethane) and its panels presented apparent density around 0.7 g/cm³.

Figure 3(a) and (b) show the response surface of the TS (%) effect for 2 and 24 h, respectively, as a function of the parameters “Granulometry” and “%Resin”. The standard NBR 14810–2:2018³⁰ required the maximum value of 8% for 2 h³⁰ and 17% for 24 h.³¹ The highest swelling value for 2 h in this present work was observed in the particleboards Md15 and Md10, around 2.7%. The lowest swelling value was observed on particleboard G15, achieving less than 1% of swelling after 2 h and approximately 2% in 24 h.

Figure 3.

Surface response of the TS effect as a function of the parameters “Granulometry” and “%Resin” (a) for 2 h and (b) for 24 h.

There was no significant difference (p-value < .05) in TS effect for 2 h. However, in TS for 24 h, it was observed statistic difference for “Granulometry” (p-valor = 0.02). Also, for both ones (2 and 24 h), the TS values increased when the particle size decreased. According to previous work,³³ it was observed that, after the sieving process of açaí seed residue, it was also found residues with particle size on 48 Tyler mesh scale. In this context, they could probably have increased the TS values of composites Md15 and Md10, due to the hydrophilic nature of the residues.^5,34

Composites produced with the açaí seed residue in this work presented lower TS results (8.01%) compared to Mesquita¹³ ones, which composites were produced with castor oil-based polyurethane resin (15%) and açaí fibers (21% and 35% for, respectively, chemically treated and in natura fibers).

Also, composites produced with Pinus³⁵ residues and urban trees prunings from Hymenaea stilbocarpa and Nectandra lanceolata³⁶ presented around 5% of TS after 2 h, and higher dimensional expansion when compared with the particleboards fabricated for this work.

Table 2 exhibits the WA results. The highest value for absorption in 2 and 24 h was, respectively, 16.19 and 56.65%. These results belong to the composite Md10, which has a predominance of açaí fiber residues, according to previous studies,³³ and the lowest resin weight fraction applied for this work (10%). The lowest absorption value after 24 h was observed in composite G15, which resin concentration is higher, as well as the size of the açaí seed particles. Statistic results show that “Granulometry” was a significant factor for the WA effect, for 2 and 24 h, indicating that the highest particle sizes carried out lower percentages of absorption, with a p-value of .02 and .04, respectively.

Table 2.

Water absorption (WA) for 2 h and 24 h.

Nomenclature	A_2h (%)	A_24h (%)
G15	4.89 ± 0.91	21.11 ± 3.68
G10	10.91 ± 1.67	29.67 ± 1.01
Mi 1	10.19 ± 0.37	27.17 ± 1.05
Mi 2	10.43 ± 0.32	27.70 ± 4.80
Mi 3	9.09 ± 1.27	29.43 ± 9.19
Md15	16.20 ± 2.11	47.92 ± 6.04
Md10	16.19 ± 2.20	56.65 ± 11.73

SEM analysis

Figure 4(a) and (b) show the SEM images of the surface from composite G10 and Figure 4(c) shows the image of the surface from composite Md15.

Figure 4.

SEM images of the surface from (a) and (b) Composite G10, and (c) composite Md15.

Adhesion failures between matrix and particles were observed in the composites (Figure 4(c)). It was also noted the presence of black dots on resin areas in SEM images, which may be related to humidity excess, due to hydrophilic nature of the açaí residues.^5,34 These issues may be corrected with adaptations in the variables of manufacturing process (such as pressure, temperature or pressing time) or treatments on the residue, aiming to remove water excess.⁵

On the other hand, comparing Figure 4(a) and (c), it is possible to observe that composite with highest resin weight fraction and lowest particle size (Md15) presented a more compacted surface, which corroborates with AD results, that indicated that the density may be influenced by the resin content and particle size.

Top and surface screw withdrawal

Figure 5(a) and (b) show the surface response to the Screw Withdrawal test from top and surface, respectively. The results (both top and surface) did not achieve the requirements established by NBR 14810–2:2018,³⁰ which recommends a minimum value of 800 N and 1020 N to top and surface, respectively. Besides that, the factor “Granulometry” and the interaction between parameters (“%Resin” and “Granulometry”) presented a significant difference, with p-value < .05, for both factors. The adhesion failures between matrix and filler observed in SEM images may explain this lower mechanical performance. On the other hand, the composite with higher density (Md15) exhibited the most suitable result for the test, achieving around 400N and 500N to top and surface, respectively. According to Lopez,³⁷ mechanical properties can be influenced by the density profile of composites, indicating that particleboards with higher density may achieve better mechanical performance.

Figure 5.

Surface response of (a) Top screw withdrawal and (b) Surface screw withdrawal.

Internal Bonding test

The values for the IB test from composites achieved the requirements of the standard NBR 14810–2:2018,³⁰ except the composite Md10. The composite G10 (gross particleboard) exhibited the best performance compared to other ones.

Statistic results show that the particle size was a significant factor for Internal Bonding effect, with p-value = .03, indicating that lowest resin weight fractions and highest particle sizes carried out higher bonding, which may explain the higher performance of composite G10. In addition, the predominance of fibers in seed residues with smaller particle sizes (which were not reduced after sieving process) may have reduced their bonding.³⁸

Results for Internal Bonding in this work were superior to the ones presented by Mesquita,¹³ which obtained a result of 0.53 MPa for composites with chemically treated açaí fibers.

Oliveira³⁵ performed IB test in particleboards with sugarcane bagasse, Pinus and Eucalyptus, which achieved similar results compared to composite Md10. The lowest average value found in his work was the one with sugarcane bagasse, which may be related to the lower amount of adhesive available per particle.³⁵

Conclusion

Based on the results obtained in this present work, it is possible to conclude that açaí seed residue can be used as reinforcement for particleboard manufacturing.

The factor “Granulometry” was significant for Water Absorption and Thickness Swelling in 24 h. These results established that the highest particle sizes provided lower percentages of absorption.

SEM images and AD results indicated that there is a tendency of higher density with highest resin weight fractions and smaller particle sizes. The results of the Screw Withdrawal test did not achieve the requirements of the standard NBR 14810–2:2018, which may be related to adhesion failures between matrix and particles observed in SEM images. However, the best results were reported in the composite with smaller particle size (composite Md15), which may be explained by its better compaction.

On the other hand, results for the Internal Bonding test indicated that the particleboards achieved the requirements of the standard, except the composite Md10 (the one with smaller particle size). Also, the composite G10 (gross particleboard) exhibited the best performance compared to other ones, suggesting that the highest particle size carried out higher bonding.

In conclusion, the composite G10 is the most promising one, considering it uses the least resin ratio (which is most ecologically suitable) and achieved the requirements of the standard for the Thickness Swelling, Water Absorption and Internal Bonding, being adequate for non-structural and indoor applications, such as partitions and ceilings.

Footnotes

Acknowledgements

The authors gratefully acknowledge the Coordination for the Improvement of Higher Education Personnel – CAPES (Grant number: 8888.452653/2019-01) for the financial support. Also, the authors are thankful to Plural Química for the donation of bicomponent resin from castor-oil, Pro-rectory of research and graduate course (PROPESP), the University of São Paulo in São Carlos and Pirassununga and the Federal University of Amazonas for providing laboratory facilities to carry out the experiments.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors gratefully acknowledge the Coordination for the Improvement of Higher Education Personnel (CAPES), Grant number: 8888.452653/2019-01, for the financial support.

ORCID iD

Gabriel Messias Medeiros de Melo

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