The current composite materials must meet a double challenge, one that involves obtaining mechanical properties suitable to the field of activity in which they are used and another one, equally important, that requires that they be renewable. In this paper, we have obtained a category of composite materials that have natural reinforcements (fabrics of flax, cotton, hemp, cattail leaves, and wheat straw). As a matrix, we have used three types of hybrid resin, in the composition of which we used the natural resin dammar, in different majority volume proportions. The differences, up to 100%, were represented by epoxy resin and its associated reinforcement, to generate a quick process of polymerization. We have measured certain mechanical properties and the damping properties of the three types of hybrid resin and of the composite materials under study. Based on these properties, we point out a few fields of activity where these composite materials can be used.

When obtaining environment-friendly hybrid resins made of a blend of Dammar natural resin, in a prevailing volume ratio, with epoxy resin, it is challenging to find alternatives for synthetic resins. Composite materials reinforced with waste paper and matrix made of epoxy resin or hybrid resin with a volume ratio of 60%, 70% and 80% Dammar were studied. All samples obtained have been submitted to tensile tests and Scanning Electron Microscopy (SEM) analysis. The tensile response, tensile strength, modulus of elasticity, elongation at break and the analysis of the fracture surface were determined. The damping properties of vibrations of bars in hybrid resins and in the composite materials under study were also examined. The mechanical properties of the four types of resins and of the composite materials were compared. The chemical composition for a hybrid resin specimen were obtained using the Fourier Transformed Infrared Spectroscopy (FTIR) and Energy, Dispersive X-ray Spectrometry (EDS) analyzes.

As a result of manufacture, composite materials can appear to have variations to their properties due to the existence of structural changes. In this paper, we studied the influence of material irregularity on the mechanical behavior of two categories of bars for which we have used hemp fabric as a reinforcing material. The common matrix is a hybrid resin based on Dammar and epoxy resin. We molded two types of bars within each of the previously mentioned categories. The first type, also called “ideal bar”, was made of layers in which the volume proportion and the orientation of the reinforcing material was the same in each section. The ideal bar does not show variations of mechanical properties along it. The second type of bar was molded to have one or two layers where, between certain sections, the reinforcing material was interrupted in several segments. We have determined some mechanical properties, the characteristic curves (strain-stress), the tensile strength, and elongation at break for all the sample sets on trial. Moreover, we have studied the influence of the non-uniformities on the mechanical behavior of the composites by entering certain quality factors that have been calculated after experimental determinations.

More and more importance has been given lately to the use of environment-friendly materials. Natural resins of vegetal, animal or mineral origin are increasingly employed. This article studies the properties of hybrid composites, obtained by formulating the natural resin Dammar with epoxy resin. The Dammar volume proportion was of 60%, 70% and 80%, respectively. The article is divided in two parts. In the first part, the chemical composition of the natural resin Dammar is analysed. Therefore, the substance was purified by chromatography. The structure and purity of the isolated substances were investigated by 1H-NMR, 13C-NMR experiments and mass analysis. The second part concentrates on structural characterization and on some mechanical properties of the three different hybrid composites. For this purpose, the characteristic curves, modulus of elasticity, tensile strength and elongation at break were measured. Graphic abstract

In the paper, a new Euler-Bernoulli theory is presented, in which the bars eigenpulsations with rectangular section increase with the ratio between the bar width and thickness. This theory is experimentally verified for composite bars made of Dammar reinforced with cotton and flax plus one or two layers of fiber glass fabric. For the considered bars, we have experimentally determined the eigenfrequencies and the damping factor values. We have experimentally determined the Young modulus and breaking strength for the used resin and the obtained composites.