1. Introduction
The problems of environmental protection, depletion of fossil raw materials (mainly petroleum), and environmental sustainability are important reasons why many producers and scientists undertook intensive research in the field of material engineering, recycling, and technology for the production of new, polymeric, user-friendly materials for the natural environment [
1].
Examples of such materials may be bio-composites composed of natural fibers and matrices of synthetic non-biodegradable polymers that only partially biodegrade over time, or composites containing both biodegradable components called green composites [
2]. The most popular biodegradable materials include polylactide (PLA), poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and thermoplastic starch. The advantages of biodegradable plastics are their high mechanical properties, but most importantly they are environmentally friendly materials, decomposing into products that occur naturally in nature and they are used in the automotive, construction, and fiber optics industries [
3,
4,
5,
6,
7].
Polymers as carbon compounds are most often produced from petrochemical sources, but increasingly from renewable sources. Non-biodegradable plastics from renewable resources include the plastics where biomass is used for production of plastics without the biodegradation property. They are mostly made from bioethanol produced from sugar fermentation, biofuel such as polyethylene (bio-PE). Some other examples are polyvinyl chloride(bio-PVC), polyethylene terephthalate (bio-PET), or polypropylene (bio-PP) [
8].
The basic monomer of polymer polyethylene is ethylene. Ethanol, which is similar to ethylene, can be produced by fermenting corn or sugar cane, which is why the expression “bio-polyethylene (bio-PE).” It is chemically and physically similar to traditional polyethylene. It is not biodegradable, but can be recycled. Bio-polyethylene is currently produced on an industrial scale from bioethanol from sugar cane. In addition, it is an environmentally friendly material and has similar technical properties and processability of the resin from fossil sources. Therefore, processing green plastic does not require any technical adjustments or new investment in equipment [
9].
The continuous increase of requirements for materials, both for general use and construction materials causes the increasingly widespread introduction to the market of petrochemical composite materials with artificial fibers, bio-composites, and green composites. To meet the requirements, especially those regarding environmental changes, natural fibers and particles that do not have a negative impact on the environment are more often used. Natural additives are obtained from renewable sources and also provide fairly good mechanical properties. The most well-known plant fibers are: oil palm, wood, rice straw, sisal, ramie, hemp, doum fruit, bagasse, pineapple leaf, cotton, flax, date palm, rice husk, wheat straw, curaua, coir, jowar, kenaf, bamboo, rapeseed waste, roselle, mesta, banana, abaca, agave, maize, and jute [
10,
11].
Natural fibers (NF) can be divided into wood fiber and non-wood fiber. Wood is a natural and complex polymer composite that essentially contains cellulose, hemicellulose, lignin, and extracts [
12].
Most non-wooded plants are annual plants that develop the full potential of fibers in one growing season. Plant fibers can be in the form of hair (cotton, kapok), hard fibers (pen, sisal), and fiber discs (flax, hemp, jute). There are six types of plant fibers, namely bast fibers (flax, hemp, jute, kenaf, and ramie), leaf fibers (abaca, pineapple, and sisal), seed fibers (coir, cotton, and kapok), straw fibers (corn, rice, and wheat), grass fibers (bagasse and bamboo), and wood fibers (softwood and hardwood) [
13].
Flax fibers are considered to be the most important member of the bast family used as a reinforcement in composites because of their unique properties. The natural high strength and stiffness of flax, low elongation to break, make them particularly interesting in composite research. Flax fibers are not continuous compared to synthetic ones, but have a similar structure as composites and they are hierarchically organized. Their macroscopic properties result from their micro and nano-structural level [
14,
15]. Flax composites were extensively studied. They were introduced to composite materials based on thermoplastics, thermosets, and bio-materials. Yan et al. in his review of composites with flax fiber, presented in detail the advantages and disadvantages of flax fibers, both in the form of cut fibers and fabrics [
14]. The results showed that flax as a reinforcement causes an increase in strength properties, however the results were strongly dependent on the fiber geometry, surface treatment, and humidity. Assarar et al. studied the effect of water ageing on composites based on epoxy resin with glass fiber and flax fiber. Studies proved that water ageing has a strong effect on flax fiber composites, and greatly reduces the Young’s modulus and strain at break compared to glass fiber materials [
16].
Cheour et al. investigated the effect of water ageing on flax fiber-reinforced epoxy composites and their mechanical and damping properties. The results showed a decrease in flexural modulus and an increase in loss coefficients. It was also confirmed that these changes are virtually reversible for loss factors, but irreversible for flexural modules [
17]. Chilali et al. also studied the effect of water on flax fiber composites. Tensile strength tests showed a reduction of approximately 10% in the rigidity of aged composites compared to non-aged materials and indicated a strong effect of water damage behavior [
18]. Moudood et al. examined effect of humidity on flax epoxy composites. The results showed that the production of composites with highly wet fabrics causes deformation of finished elements after treatment, and also leads to poor microstructural quality. The moisture content of the fibers reduces the tensile and flexural stiffness but also increases fracture toughness [
19]. Dhakal et al. also presented the susceptibility of flax fibers to water. Bending properties tend to decrease as the percentage of moisture absorption increases. Comparison of flexural strength and flexural modulus between dry flax bio-composites and wet flax showed that wet samples lost about 50% of strength and stiffness compared to dry flax samples, hence researches were carried out to improve flax fibers behavior [
20]. Stambolis et al. presented research on improved flax fiber, which was 30% less water-sensitive, and composites with those fiber were characterized by maintaining strength and stiffness despite adverse environmental effects [
21].
Coconut fibers are obtained from the fibrous husk of the coconut from the coconut palm. Coconut fibers have a high lignin content and therefore a high cellulose content, making them elastic, strong, and very durable. The properties of composites with coconut fibers as well as with flax fibers and other natural fibers strongly depend on the volume, diameter, and length of the fibers, as well as moisture content [
22]. Singh et al. evaluated the effect of coconut shell powder with different particle sizes and volumes on the tensile and bending properties of composites based on the epoxy resin matrix. Coconut improves the mechanical properties of epoxy resins but changes of properties are influenced by the volume of fillers [
23]. However, in studies provided by Reddy T. on thermoplastic coconut fiber composites, where high density polyethylene (HDPE) was used as a matrix, the mechanical properties were more depended on the fiber length [
24]. Siva et al. studied the optimal volume content of fibers. The study examined four volumetric parts, the results showed that with increasing fiber content, the strength properties increase and the resistance to dynamic impact decreases. However, because of its higher density, thickness, and more de-laminations, high fiber volume fraction may not always be good for reinforced polyester composites [
25]. Onuegbu et al. presented work on various surface treatments of coconut fibers, studies showed that alkali treatment of coconut fibers significantly improves their tensile properties [
26].
Wood flour is obtained from wooden raw material and is characterized by a loose, very fine, and even consistency. In the chemical composition of wood flour we find cellulose in about 50%, lignin in about 20–25%, hemicellulose in 20–25%, and also, among others: resins, waxes, rubber, proteins, and mineral salts. The exact concentration of individual substances depends on the species of trees, soil, and weather conditions in the area where the trees grow. The influence of wood flour size on mechanical properties and density of HDPE composites was investigated. The results showed that the wood particle size had a significant effect on wood plastic composite (WPC) properties. The change in the number of meshes has a great effect on the flexural modulus, tensile modulus, and impact strength, however, it has little effect on flexural and tensile strength [
27].
Thermoplastics commonly modified with wood flour include work on polypropylene composites with wood flour provided by Sheshmani et al. and Ichazo et al. [
28,
29], high density polyethylene, and recycled high density polyethylene presented by Hamzeh et al. and Nourbakhsh et al. [
30,
31]. Despite the advantages, the use of wood in thermoplastic materials has some problems; limitation of wood thermal stability, difficulties in obtaining good dispersion of filler, and weak interphase adhesion. This is due to the natural incompatibility between hydrophilic polar wood fibers and hydrophobic, non-polar thermoplastics. This phase mismatch causes a poor interface between the wood filler and the polymer matrix. In addition, the strong interaction of wood-wood resulting from hydrogen binding and physical entanglement reduces the dispersion of fillers in the viscous matrix.
Recently, basalt fibers (BF) has appeared as a new type of NF. They are widely used in the area of reinforced polymer materials. BF, which are similar to chemical composition of glass fibers, have interesting properties, including high melting points (in the range from 1350 to 1700 °C), high stiffness, excellent heat resistance, and excellent vibration insulators. In addition to these promising properties, BF are relatively cheaper than carbon reinforcement, non-toxic, and natural, which undoubtedly makes them a good alternative to artificial reinforcement polymer materials [
32,
33,
34].
The growing interest in biobased polymer forces researches and manufacturers to work on new composites containing bio-matrix and natural fibers. Serra-Parareda et al. presented research on polyethylene composites with barley straw as an alternative to using agricultural waste. The barley straw content ranged from 15–40% by weight. Strength results showed the fiber has a high ability to provide strength and rigidity for such composites [
35]. Castro et al. used curaua fibers as a reinforcement for high density bio-polyethylene. The presence of curaua fibers improved some properties such as bending strength and storage modulus, but they also pointed to the high importance of manufacturing methods and introduction of other modifiers to obtain high-grade composites [
36]. In another work by Castro et al. it was confirmed that the introduction of castor and canola oils as a potential compatibilizer for bio-Pe composites with curaua fiber additionally improves the strengthening effect [
37]. Kuciel et al. introduced natural additives into bio-polyethylene (HDPE and LDPE) as an additive, such as wood flour, kenaf fibers, cellulose power, and tuff particles. The introduction of natural particles does not affect the results of tensile strength, however, the use of wood flour and kenaf fibers increases the stiffness several times. The introduction of natural particles causes an increase in water absorption that affects the surface quality of the tested materials [
38]. Tarres et al. presented the bio-based polyethylene matrix with thermomechanical pulp (TMP) fibers. In addition, the effect of melamine anhydride on mechanical properties was investigated. Studies pointed an increase in the mechanical properties of the tested composites, especially with the addition of a compatibilizer. Researchers proved that thermomechanical pulp fibers facilitate 3D printing and provide products with high strength properties [
39].
This work presents new composites based on bio-polyethylene with natural fibers as the first stage of research on hybrid connections of natural fibers. Knowledge of the mechanisms of strengthening natural fibers such as flax fiber, coconut, wood flour, and basalt fibers posed at a further stage to use the full potential of hybrid reinforcement for the same fibers.