Publicado en línea: 31 mar 2020
Páginas: 14 - 20
DOI: https://doi.org/10.2478/oszn-2020-0003
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© 2020 Joanna Masiewicz et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.
Epoxy resins (EP) are low molecular weight chemical compounds that contain two or more epoxy groups in their molecules. They have a huge, but still untapped potential in the manufacturing, aviation or automotive industries. Cured sticky liquids of EP become infusible and insoluble plastics with excellent dielectric properties and significant resistance to atmospheric and chemical factors. However, cured epoxy resins gain low impact strength, poor resistance to crack propagation and low elongation at break. Over the past several decades, many works have been devoted to the improvement of mechanics and thermal properties of epoxy resins. Benefits of using fillers or reactive liquid and thermoplastic polymers include both improvement of composites’ strength and their processing properties. Using cross-linkable polymers that create full- or semi-interpenetrating polymer networks also significantly improves epoxy resin's properties. At the time when care for the environment has become a priority, scientists have begun to search for new natural ways to modify the epoxy resin with improvement in properties (Saba, Tahir, 2014). Table 1 shows a comparison of the general characteristics of natural fibres and glass fibres.
Characteristics of natural fibres and glass fibres
| Density | low | 3 x higher |
| Renewability | yes | No |
| Recyclability | yes | No |
| Health risks during inhalation | low | High |
| Biodegradability | yes | No |
Plant fibres are obtained from various parts of plants – seeds, leaves, stems and also fruits. In recent years, they have become a great alternative to replace fillers such as glass fibres or synthetic fibres due to their good mechanical properties, good biodegradability, low density and accessibility. Moreover, it is characterised by low price and is significantly more environmentally friendly than other fillers (Oksman, 2009).
The relatively low prices of natural fibres compared to the prices of glass fibres provide, in a crisis situation, an additional incentive to use them as reinforcement in polymer composites, taking into account the strong dependence of the properties of natural fibres on the geographical location of plantations. The so-called green products are made of materials with a high content of natural resources and they are competitive on global markets, especially in Europe and the USA, where the expectation for eco-friendly products is the highest (Błędzki, Urbaniak, 2014).
The chemical composition of natural fibres is diverse and depends on the type of fibre. Most of all – natural fibres contain cellulose, hemicellulose, pectin and lignin. Hemicellulose is responsible for biodegradation, moisture absorption and thermal degradation of the fibre because it is the least resistant, while lignin is thermally stable and is responsible for UV degradation. Most often, the fibres contain 60–80% cellulose, 5–20% lignin and up to 20% moisture (Sgriccia, Hawley, 2008).
To enhance the mechanical properties of an epoxy resin, Srinivasa's team used Areca fruit hulls. They were initially soaked in distilled water, extracted, washed again and dried to loosen the fibres. Part of the fibres was also subjected to the action of potassium hydroxide, and acetic acid to finally obtain a solution having a neutral pH. Preparation of the fibres allows for easier mixing with the polymer matrix, whose properties have significantly improved compared to the unmodified polymer matrix (Srinivasa, Bharath, 2011).
The effect of the addition of basalt fibres on the properties of epoxy laminates was analysed. The use of basalt fibre reinforcement in epoxy laminates resulted in better mechanical properties compared to epoxy-glass laminates. The difference between the glass transition temperature of epoxy-basalt laminates (113°C) and epoxy-glass laminates (100°C) was 13°C. In addition, epoxy-basalt laminates were also characterized by greater impact strength by 42 kJ/m2. Therefore, the use of basalt fibres as reinforcement in epoxy laminates had a positive effect on the mechanical properties of the composite compared to fiberglass reinforced laminates (Matykiewicz, Barczewski, 2015).
Urbaniak's team undertook to examine the thermal and mechanical properties of biocomposites made on the basis of a standard epoxy system and epoxidized vegetable oil with the addition of 30 parts. wt. microfibers from wheat husk, rice and spelt. The results were compared with the addition of wood microfibers. Studies have shown that the addition of cereal microfibers in enzymatic modification causes an increase in the glass transition temperature of the composite. The results of bending tests of composites from flour scales of flour products, especially spelt, showed the biggest pronounced increase in strength (up to approx. 14%) and bending module (up to approx. 7%) and bending deformation at fracture (up to approx. 15%) in relation for composite with wood microfibers. The Charpy impact toughness also showed an increase of up to 30% in grain biocomposites compared to a composite with wood microfibers. Thus, the fibres studied in the work can serve as one of the alternatives replacing wood microfibers in composites (Urbaniak M., Błędzki, 2013).
It is worth mentioning that not only introducing natural fibres into an epoxy resin (or other polymer matrix) brings beneficial effects to improve the properties of composites, but also chemical and physical modification of the fibres themselves such as fractionation, soaking, heating, digestion in alkaline solutions, significantly improves the final results during the introduction of the fibres to the resin (Błedzki, Reihmane, 1996).
The subject of research was the modification of epoxy resin with various types of natural fillers and demonstrating that they significantly improve the properties of epoxy resin.
The purpose of the work was to receive the epoxy resin modified with collagen, hemp fibres, pepper powder and then identifying and comparing the strength properties of the resulting composites. Epoxy resin Epidian 5 (epoxy resin–diglycidyl ether of bisphenol A, Organika Sarzyna, Nowa Sarzyna, Poland) with a molecular weight of 400 g/mol, viscosity at 25°C around 30 Pa.s, and epoxy number of 0.49–0.52 mol/100 g was used as polymer matrix, for which was dedicated a triethylenetetramine hardener (generally called Z1). Collagen hydrolysate (from Department of Tanning and Fur, UTH Radom), hemp fibres and pepper powder (post-extraction waste from INS Puławy) used as fillers. Ethylene glycol was used as a solvent (only for collagen, Alchem Grupa Sp. z o.o.).
Collagen has previously been dissolved in ethylene glycol heated to 70°C. Hemp fibre granules were ground to a complete powder. Peppers after extraction were already in the form of powder. Each epoxy composition contained from 5–20% wt. the filler. All the mixtures with fillers were mixed by mechanical stirrer for 10 minutes and then were homogenized in an ultrasonic mixer Hielscher UP 200H (parameters: 10 minutes, amplitude - 100%, cycle - 1), followed by the addition of a stoichiometric amount of Z1 to all of them. Reference samples contained only the epoxy resin and a suitable amount by weight of hardener Z1. All compositions were placed in 10-sided steel moulds with release agent (20 samples each). Samples were allowed to cure and vent for 24 hours at room temperature. After this time, post-curing process was carried out at 80°C for 3 hours.
Measurement of gel time begins when Z1 is added to the composition. During the test, the contents of the beaker should be mixed, checking the consistency of the sample. Completion of measurement occurs when breaking threads are formed, and the physical state of the composition renders stirring impossible (Kimura, Matsumoto, 1997).
The impact strength of the composition was tested using a Zwick 5012 according to ISO 179. Dimensions of samples were 80 mm x 10 mm x 4 mm (1 mm notched and without notched). Five samples from each composition were tested (PN-EN ISO 179-1:2010).
The test consisted of determination of the critical stress intensity factor (Kc). For measurement, the apparatus used were used Zwick / Roell 010 at room temperature, to measure the value by three-point method of bending the samples. The critical stress intensity factor KC was evaluated using the formula given below:
Three-point bending resistance was tested by Zwick/Roell Z010 according to ISO 178 at room temperature. Five samples of each composition were used for the test. Samples had size 80 mm × 10 mm × 4 mm (PN-EN ISO 178:2006).
Initially, the change in gel time of the composition was determined. Table 2 shows the results of this study.
Gel time of composites containing 5–20% natural filler
| % Natural filler | 0 | 5 | 10 | 15 | 20 | 5 | 10 | 15 | 20 | 5 | 10 | 15 | 20 |
| Gel time (min) | 65 | 67 | 68 | 70 | 74 | 104 | 95 | 76 | 58 | 85 | 94 | 104 | 107 |
All compositions have a longer gel time compared to pure epoxy resin. For collagen-modified resin, the gelation time increases as the filler content increases. It looks different for samples modified with pepper powder, where the gel time decreases as the filler content increases. In the case of compositions with hemp fibres, the filler content does not affect the gel time, which is similar in all samples.
In the next step, we determined the impact of the applied composition modification on crack resistance. For this purpose, impact strength and critical stress intensity factor and the results are shown in Figures 2 and 3.
Figure 1
Cost per weight comparison between glass and natural fibres (Błędzki, Urbaniak, 2014)
Figure 2
Impact strength of composites containing 5–20% natural filler
Figure 3
Critical stress intensity factor composites containing 5–20% natural filler
Figure 2 represents the effect of modifiers on the impact strength (IS). It can be seen, that IS increases and then decreases as the amount of modifiers is increased. These results show that the addition of natural filler improves the impact strength of epoxy resin. All modified compositions had higher impact strength than the unmodified epoxy resin. The maximum impact strength was achieved at 5% natural filler concentration. The optimum impact strength is about three times higher than the value observed for unmodified epoxy resin. However, the best results were obtained with epoxy composite containing 5 wt. % pepper powder representing 270% improvement, respectively, in relation to the virgin epoxy resin.
As it is presented in Figure 2, the maximum Kc value was in compositions containing 5 wt. % of natural filler. Similar as in case of impact strength results. Moreover, the critical stress intensity factor for these samples increased from 1.6 to 4.7 MPa/m. Maximum KC value, representing approximately 330% improvement, was obtained for the material based on 5 wt. % pepper powder. However, the Kc value decreased with increasing amount of natural filler. Figure 4 summarizes the results of flexural tests of epoxy compositions containing different amounts of natural filler All compositions containing a natural filler have higher flexural strain values at break than pure epoxy resin. In the case of compositions with pepper powder, the results were similar to unmodified epoxy resin. As it can be seen, the compositions modified with 5% collagen had the highest values of stress at break.
Figure 4
Flexural strength of composites
The effect of modifier content on the stress at break is shown in Figure 4. The best results of flexural stress at break were obtained for compositions containing 5% natural fillers. As the amount of filler increases, the stress decreases. Maximum value was reached with 5% collagen, which represents about 10% increase in comparison with the neat resin.
The addition of natural filler doesn’t improve the flexural modulus at break. It can be noted that modulus values decreased with increasing natural filler content. All tested compositions containing natural filler exhibited a lower modulus in comparison to the unmodified resin, indicating that the modified compositions became more flexible than the matrix.
On the basis of the results obtained, we can make the following conclusions: The IS, the critical stress intensity factor (Kc), as well as the flexural strength of epoxy resin improved with natural filler addition. In all composites, the best results were obtained at 5% concentration. It should be noted that the addition of 15 or 20 wt. % natural filler significantly reduced the composition properties.
The highest values of impact strength and critical stress intensity factor of composites obtained 5% pepper powder. Maximum IS and Kc in comparison with neat epoxy resin increased respectively 270% and 330%.
The addition of 5% collagen resulted in 10% increase of the stress at break and 1.7% value in the strain at break. The addition of natural filler doesn’t improve the flexural modulus at break.
All compositions with natural filler have a longer gel time than pure resin, which can be beneficial when forming products.
The use of natural fillers is an excellent alternative to synthetic ones. In their favour is evidenced by the fact that the natural filler is biodegradable and doesn’t generate harmful substances for humans, as well as can be obtained at a lower price as compared to synthetic.