The use of agricultural biomass waste as a reinforcing filler in polymer composites is of considerable importance in waste management and the development of sustainable and environmentally friendly biocomposites. The main objective of the present study is to investigate the effect of almond shell powder (ASP) on the mechanical-physical properties of unsaturated polyester (UPE) biocomposites. In this regard, biocomposites containing different amounts of ASP were manufactured by casting molding method and evaluated by various tests. The FTIR test results indicated a suitable interaction between polymer/filler. The addition of almond shell powder increased both tensile and flexural modulus in the biocomposites. A slight decrease (8.29%) in the tensile strength of the biocomposites was observed compared to the pure polymer, which is negligible. Notably, the highest flexural strength was obtained in the UPE/ASP20 biocomposite (containing 20 wt% ASP) at 1.59 MPa. The microstructure of the biocomposites, the distribution of ASP within the polymer matrix, and the various reinforcement mechanisms were investigated using SEM images. Regarding impact strength, a relatively large decrease occurred with the addition of ASP, which was attributed to the more brittle mechanical behavior of the biocomposites due to the presence of rigid ASP. Although the amount of water absorption increased with the addition of ASP, the resulting biocomposites still had acceptable performance in this regard. It is expected that the findings of the present study can be used for the development of environmentally friendly biocomposites.
EXTENDED ABSTRACT

Introduction

Particulate polymer composites are among the most important composite materials that have diverse applications in transportation, urban furniture, construction materials and equipment, toys, and household appliances. Common mineral particles can pose environmental hazards such as contamination from persistent chemicals and the production of fine particulate dust that can be dangerous if inhaled or spread. Nowadays, due to the increasing emergence of environmental challenges, the demand for the development of sustainable and environmentally friendly materials, especially polymer composites reinforced with natural materials such as agricultural wastes, has increased more than ever. In addition to reducing the amount of agricultural waste, this can provide various benefits such as reducing pollution, biodegradability, and reducing the cost of the final composite. Almond hard shell is a type of renewable lignocellulosic biomass that is one of the most abundant agricultural wastes, and this valuable bio-filler seems to be a suitable candidate for the development of polymer biocomposites. Accordingly, the aim of the present study is to evaluate the effect of almond shell powder as a biomass waste on the mechanical and physical properties of polyester biocomposites.
Materials and methods
General-purpose unsaturated polyester resin of orthophthalic type, under the trade name RESITAN UP 747.7, was purchased from Resitan Co. Hard almond shells were also collected and prepared after consumption, and then ground into powder in several stages using an industrial grinder. First, to manufacture the composites, a certain amount of the accelerating agent was added to the polyester resin and mixed. Then, a certain amount of ASP was added to the above mixture and stirred well. Next, the curing agent was added and the mixture were stirred thoroughly. A vacuum pump was used to remove air bubbles resulting from the mixing process. Finally, the biocomposites were molded using a silicone mold and the casting method. Various mechanical tests such as tensile, bending, impact, as well as SEM, FTIR, and water absorption tests were performed on the samples.
Results and discussion
The FTIR results indicated the existence of suitable interactions between the UPE and the ASP, which were mainly physical. These suitable interactions will manifest themselves in the mechanical properties of the composites. The tensile behavior of the composites generally shifted to brittle behavior with the addition of ASP. In addition, the strain at failure decreased with ASP, which is mainly due to the high rigidity of ASP. It was observed that the maximum tensile strength of the biocomposites generally decreased with increasing ASP content. However, this decrease was very small, especially in the two biocomposites UPE/ASP10 and UPE/ASP20. This could indicate a good polymer/filler interaction. Of note, the tensile modulus of the composites increased with increasing ASP content. In this case, the inherent nature of ASP in terms of higher rigidity compared to UPE was suggested to be the main reason for the increase in the tensile modulus of the biocomposites. In general, the flexural strength and flexural modulus are influenced by the amount of ASP filler in the matrix and follow the same trend, both increasing with increasing ASP content. The UPE/ASP20 biocomposite showed the highest flexural strength and flexural modulus (even compared to pure UPE). ASP particles can improve the flexural strength by changing the direction of crack growth within the matrix, as shown in the SEM images of this position. The impact strength of the composites decreased with increasing ASP content. It was observed that the impact strength of the biocomposites increased from 5% to 10% and then decreased from 10% to 20% with increasing ASP content. This could be attributed to the addition of ASP which played the role of heterogeneous nucleation. The water absorption value increased with the increase of ASP content, which is mainly due to the large number of hydroxyl groups on the ASP surface. However, their water absorption values (all below 2%) are still acceptable for practical applications.
Conclusion
Polyester/almond shell powder biocomposites consisting of different weight fractions of ASP were manufactured. In short, mechanical evaluations showed an improvement in the tensile and flexural modulus as well as the flexural strength of the biocomposites with the gradual addition of almond shell powder. Scanning electron microscopy (SEM) images clearly showed that the ASP had a good distribution within the polymer matrix. The biocomposites also exhibited acceptable performance in terms of water absorption.
Polyester/almond shell powder biocomposites consisting of different weight fractions of ASP were manufactured. In short, mechanical evaluations showed an improvement in the tensile and flexural modulus as well as the flexural strength of the biocomposites with the gradual addition of almond shell powder. Scanning electron microscopy (SEM) images clearly showed that the ASP had a good distribution within the polymer matrix. The biocomposites also exhibited acceptable performance in terms of water absorption.