**3. Behavior of viscoelastic biocomposites**

The biocomposites inherit the behavior of the matrix with which they were manufactured, making their mechanical properties strongly dependent on the ratio of applied strain; therefore the mechanical behavior and viscoelastic structural products that are designed as sustainable products, which can be applied to the construction industry and automotive, mainly be affected by the dependence of applied stress and temperature conditions at the time. When biocomposites are processed and take the desired shape, e.g., extruded beams, molded housings, or any product which may be subjected to a constant load, will be generated on these products efforts, bending or tension or combinations thereof, constants, the effect of a constant effort on a product manufactured in biocomposites can be seen reflected in unwanted time warps, inclusive could produce the product failure. In this context, the durability of biocomposites can limit their applications, and implement risk of all the efforts of previous research, in order to develop sustainable materials for sustainable products. Studying the behavior of NFPCs under constant load conditions, where the deformation increases in time, that is, the material flows under the load (effect of creep), can understand that in a system biocomposite load between redistributes the matrix and natural fibers during deformation, when these materials are subjected to constant loads, can be affected by various effects of creep, the matrix, fiber, and the interface. There are several applications that have achieved biocomposites for extrusion with an addition 40–60%, mixed with thermoplastics, such as HDPE, PP, PVC, and materials [6, 49]. Compounds where manufacturers use natural fibers from different sources, as one of the fillers or reinforcements. These thermoplastic biocomposites can be used as tables for decks, fences, railway sleepers, etc. When used under these requirements, the CREEP or viscoelastic deformation becomes a problem, because the application of the effort takes the material to work under load long periods of time (months and years). This has been studied extensively in the case of advanced thermoset composites, and nowadays the investigations on biocomposites observed that the viscoelastoplastic behavior can lead to failure sustainable product, when subjected to large deformations and long periods of time, under conditions of dynamic or static load and temperature variations. These materials progressively accumulate deformation, causing internal damage occurs due to creep and/or fatigue, both cause cumulative damage [7, 50, 51]. There have also been efforts to correlate effects at smaller scales, relating effort plastic flow [52–54], according to the nonlinear response which it is due to permanent deformation. Investigations of some thermoplastic compounds have focused on deformation patterns, and have shown the strain-fluence compounds with particulate wood plastic, with alteration of the compositions and components of compound [49, 55, 56]. It has also been observed that with increased fiber content, the effect of creep decreases. Agro compounds used to develop products for structural construction, often requiring improved mechanical properties, particularly creep performance. It has been shown that the fluence of biocomposites varies with the type of filler and content, coupling treatment, and types of polymer matrices [6, 10]. Several molding techniques have also been applied to analyze the behavior CREEP [7, 10–12, 57].

At present, it is of interest to develop new thermoplastic biocomposites for sustainable products, and it is about the future course of implementation and sustainability over time of biocomposites. There are estimates based on theoretical predictions, especially validated parameters obtained from accelerated tests, using the technique of dynamic mechanical analysis, DMA; this is the case study of creep behavior of composite materials based on different fibers such as bagasse, bamboo, and wood flour as matrix polyvinyl virgin and recycled vinyl and high density polyethylene. They tried to develop and adjust different theoretical models during all stages of CREEP to help predict long-term behavior. And observing different treatments and source matrices, they observed that models fit well in the linear zone CREEP, difficulties in predicting the primary and tertiary CREEP, referring difficulties in predicting the adjustment parameters, on lacking. Experimental long-term break in the tertiary CREEP, for the limited number of experiments that can be done using only accelerated techniques with DMA, especially those of CREEP long term [6, 10]. At constant load level a biocomposite has better creep resistance than ordinary polymer systems at low temperatures. However, biocomposites usually show higher temperature dependence. Various models of creep (Burgers model, model Findley power law, and a model of simple power law two parameters) have been used to adjust the data flow. The principle of timetemperature superposition (TTS) is typically used for predicting long-term creep, where it is important to understand that this method is valid mainly in the linear viscoelastic region of the biocomposites, however this method suffers from a prediction of the aging of natural fibers, including an error in time, which has now become complex including models for developing sustainable biocomposites. It has been shown that the four elements of the model Burgers and the power law with two parameters, adjust flow curves of biocomposite [6, 7]. Other authors have shown that PP-agglomerate compounds show different behaviors yield according to the processing conditions, i.e., with increasing fiber content, fluence compounds for example with wood fibers decreases [11, 56], studies are not derived from expressions that clearly include the flow properties of the matrix and fiber in their models, nor aging, or other factors associated with the nature of natural fibers. Therefore, the constant creep of these mathematical models is fully specified biocomposites, and are only valid for those compounds in particular and the conditions imposed nowadays. There is no complete method that can predict with high accuracy the viscoelastic performance of biocomposites, however, estimates that they are possible to perform with the use of the DMA, achieving reasonably guide the industry to seek applications for the development of new sustainable products, using biocomposite.
