Resistance and Strength of Bio-Compatible Epoxy-Cellulose Composites, as A Function of Concentration and Dispersity of Cellulose Filler

Dmitro Starokadomsky1*, Valerii Barbash2, Maria Reshetnyk3, Alexandra Starokadomska4, L Yudmila Kokhtych5, Sergey Shulga1 and Olha Yashchenko2 1Chuiko Institute of Surface Chеmistry, National Academy of Sciences (NAS) of Ukraine 2National Technical University of Ukraine, Igor Sikorsky Kyiv Polytechnic Institute, Ukraine 3National Museum of Natural History of NAS Ukraine 4Institute of Fish Industry of NAS of Ukraine 5National Technical University of Ukraine, Igor Sikorsky Kyiv Polytechnic Institute and Institute of Physics NAS of Ukraine

A biocomposite's properties are influenced by a number of variables, including the fiber type, environmental conditions

ARTICLE INFO ABSTRACT
The purpose of the present study was the investigation of structural and mechanical properties of epoxy polymer composites filled with nano-, micro-and mesocellulose. In prapared composites, cellulosics play role of a reinforcement constituent while epoxy resin is the matrix. The effect of composite composition with the application of different amounts of nano-and microcellulose on compression and tensile strenght, microhardness, fire-resistance and swelling was investigated. The method of optical microscopy showed that the microstructure is complemented by a long fibrils to give well-compatible compositions, forming hard and resistant plastics. Bending strengt increases in 1.2-1.3 times (after filling by nanocellulose) and Modulus at bending increases from 1.6 to 2.5 (at 20% of nanocellulose). Fire-and Abrasion-resistance increases in 1.2-1.4 times, and adhesion to steel in 2-2.5 times. Filling with nanocellulose and micromeso-disperce particles of lignincellulose (waste-paper utilisated product) let obtain a composites resistant to destruction in acetone solutions and other aggressive media (35-60% H 2 O 2 , sea-water etc). Due to the numerous advantages, such as low cost of cellulosic raw materials, their high availability and abundance, as well as nontoxicity, it can be regarded as perspective filler of the epoxy composits with high mechsnical properties and chemical resistance which may have a wide practical use.
Cellulose is a widespread natural biopolymer of polysaccharide nature and with fibrous structure and unique properties due to which the scope of its use is increasing every year [27][28][29]. More often, cellulose-based materials are used as fillers for various types of composite materials of organic and inorganic nature [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Epoxy-Cellulose Composites (ECC) are biocompatible, tree-imitating and resistant. Therefore, ECC are well for the biо-, restauration and medical applications in the field-terms (field hospitals, departure laboratories, expeditions). According to Nair et al. [15] the adding of 20±2 wt.% of nanocellulose-fiber increases mechanical parameters (modulus, strength, and deformation) and resistance of the resulting composites. Nanocellulose fibers based on bleached softwood pulp were used to prepare cellulose nanocomposites using conventional vacuum infusion [16]. The obtained composites had a porous structure with a random orientation of the fibers. They obtained an increase in strength (from 89 to 107 MPa) and a modulus of elasticity in bending (from 2.8 to 4.2 MPa) -with 13 vol.% of nanocellulose [16]. Moreover, the here, but also mechanisms of physicochemical interaction. The aim of this paper is to evaluate and to compare the effect of cellulose structure and type on properties of the epoxy composits.

Materials
Epoxy polymer composites were prepared, in which cellulose has various dispersion and form of particles. We used microcrystalline cellulose with particle size of 50-300 μm (Russia). Nanocellulose was obtained in Igor Sikorsky Kyiv Polytechnic Institute (Ukraina) in laboratory conditions from organosolvent cellulose from stalks of Miscanthus x giganteus as a result of acid hydrolysis with ultrasonic treatment of suspension [29,30]. Meso-disperce cellulosic product were obtained from waste-paper at Malin paper mill (Ukraine) and the fraction less than 1 mm were used in this research; the content of organic and inorganic substances is 29,7% and 71,3%, respectively.

ECC Preparation
Compositions of the ECC were prepared by mixing weights of cellulose derivatives from 5 wt % to 20 wt% with Epoxy520 epoxy resin (Czech production), followed by the addition of PEPA hardener (resin: hardener ratio 5:1) and constant mechanical stirring under normal conditions. After 3 days of initial curing, samples of the obtained composites were subjected to heat treatment at a temperature of 65 °C for 5-7 hours for mechanical and thermal tests or at 30 °C for at least 5-7 days for tests on swelling and resistance in aggressive liquids.

ECC Structure and Properties Investigation
The surface structure of the cellulosic materials and ECC was studied using an optical microscope and a Scanning Electron (laboratory portable gas mini-burner) as described in [20].

Study of the structure of ECC
Optical microscopy shows (Figure 1) that meso-disperce cellulose (waste-paper) is distributed in epoxy in large interlacing agglomerates up to 200-300 microns in size. Micro-sized cellulose is distributed in form of microfibers up to 500 microns in length.
Nanocellulose has a glassy morphology, with a particle's particle size of less than 500 microns. Thus, these fillers (taken in sufficient quantities, for example 5-20 wt%) should have different effects on the mechanical and resistance properties of the epoxy composite.  Table 1. From Table1 it can be seen that the strength and modulus in bending after filling increases in the case of nanocellulose and mezocellulose. This means that the loose cellulose structures embedded in the polymer network in some cases form a more bending and elastic frame than fragments of an epoxy polymer. At the same time, filling enhances abrasion resistance (since the weight of the abraded mass decreases) and adhesion to steel. The compressive strength after filling does not change ( Table 1).
The fire resistance as a result of filling naturally increases, which is obviously caused by the higher fire resistance of the filler -cellulose. The composites do not acquire the damping properties: as the unfilled polymer, as filled self-ignite after the start of combustion, even without source of fire. Thus, the filling of cellulose derivatives allows to obtain epoxy masses that are well formed and give polymer-composites with enhanced properties. The results of investigation of microhardness of initial unfilled epoxy and ECC is shown in Table 2. As can be seen from Table 2, microhardness of ECC after filling is preserved or slightly reduced. Moreover, the increase in cellulosic additives content leads to the increase in the brittleness of composites.    NaOH are given in Figure 3. In concentrated hydrogen peroxide (a known oxidizing disinfectant in biomedicine), the durability of ECC composites can be higher, compared to a polymer without filling. This is confirmed by a decrease in the degree of swelling for 5% and 20% of filling (but not for 10%, Figure 4).  Figure 5A). But in a 50-60% H 2 O 2 solution, they quickly (in 1-5 days, Figure 5B) swell with a large gain in mass and volume ( Figure   4) -and then degrade. When exposed to seawater (a common corrosive environment for composite products), an initial washout (loss of weight) in 1 day can be seen for all samples (except for the composite with microcellulose, Figure 6).   Note: * -estimated data   Subsequently, the swelling of the unfilled one stops after half a month and does not exceed 2.5% in a month and a half (which corresponds to the standard data for polyepoxides). After filling, the composite swells less actively in the first half-month of aging.
But then the swelling of filled templates continues (albeit at a lower rate) throughout the entire holding time ( Figure 6). The effect of cellulose as a filler is clearly seen when the composites are exposed to acetone-containing solvents (Table3&4). Table 3 shows well the instability of unfilled polyepoxide in acetone solvents. This severely limits the use of epoxy plastics. The introduction of microcellulose leads to an even more noticeable decrease in resistance (destruction during the day, Table 3), while the degree of swelling decreases.
The introduction of 5 and 20% nano-cellulose imparts the highest stability to the composites -up to 10 days without destruction (Nano20%, Table 3). Interesting data on 20% with Waste-paperthe composite does not degrade and swells slightly. The swelling rate then increases, but after 6 days the swelling for 20% Wastepaper stabilises. In pure acetone, the swelling dynamics of unfilled ones is similar to acetone:ethylacetate. Unfilled polymer templates swell quickly and disintegrate on the 3-rd day of exposure (Table   4). Microcellulose slightly improves the resistance to swelling but does not save it from destruction and (Table 4). Nanocellulose and wastepaper provide even higher resistance to swelling and nondegradability.
The photo of epoxy composites for the evaluation of visual changes before and after impregnation in acetone and acetoneethylacetate mix is given in Fig. 7. We see significant destructive changes almost immediately. Decomposition or lay-separation of the samples begins within a few hours, especially in the acetoneethylacetate mix. Their appearance well displays the digital data in Tables 3&4.

Conclusion
As a result of the study, it was found that the introduction of cellulose fillers allows to obtain viscous masses, which after curing turn into wood-like composites. The morphology of the compositions reflects the fibrous nature of cellulose, which therefore integrates well into the supramolecular structure of epoxy. The effect of fillingtype on the strength is ambiguous and depends on the dispersion and concentration of the filler.
A number of the main properties were studied and thay have not changed much after filling (compressive strength, tensile, microhardness). For other characteristics (abrasion resistance, adhesion, elastic modulus), a unambiguous gain was observed.
After filling with cellulosic materials, the resistance of the epoxide to aggressive liquids such as 35-60% hydrogen peroxide, aceton (or acetone:ethylactate) and sea water, increases. As a rule, Epoxy composites with 5 and 20 wt% of cellulose, do not decompose in acetone in first days (unlike unfilled ones) and swell weaker in hydrogen peroxide or acetone:ethylacetate. Also, after filling, the fire resistance of composites increases by 1.3-2 times. As a result, it can be concluded that it is promising and prospactive to obtain ECC composites for the manufacture of bio-eco-compatible wood-based products and of the multi-purpose.

Funding
This research received no external funding.

Conflicts of Interest
The authors declare no conflict of interest.