In the polymer science field, epoxy polymers offer great versatility and an ample range of enhanced properties, such as stiffness, specific strength, dimensional stability, chemical resistance, and strong adhesion within the matrix [1
]. However a long-standing challenge of the utilization of the epoxy is how to toughen this brittle polymer without sacrificing other important properties [2
]. The advancement in the use of fillers or reinforcements within the epoxy matrix is a common practice. Some of the enhanced properties are brittleness and modulus [1
Cellulose comes from renewable resources which are abundant and readily available in nature. Cellulose provides a stable backbone of β (1→4) linked d
-glucose units which are capable of forming strong hydrogen bonds leading to the formation of crystal fibers [5
]. Other advantages of cellulose particles over traditional materials, such as glass, talc, and mica, include acceptable specific strength properties, low density, enhanced energy recovery, non-toxicity, and low production costs [6
]. The tensile strength of cellulose is particularly important and can improve composite mechanical properties [9
]. However because of the hydrophilic nature of cellulose particles, there is increased moisture absorption followed by dimensional change [11
]. This property results in poor compatibility between cellulose and hydrophobic polymer matrices, such as epoxy.
The efficiency of cellulose reinforced composites depends on the ability to transfer stress from the matrix to the cellulose fiber [13
]. This stress transfer efficiency plays a critical role in the mechanical properties of the composite [5
]. The hydrophilic nature of cellulose derived from the hydroxyl groups on the surface can form inter-macromolecular hydrogen bonds and external bonds with atmospheric hydroxyl groups [16
One key hurdle with cellulose-based composites is the strong attraction between fibers resulting in unwanted agglomeration. The interface energy and excess of hydroxyl groups result in an increased agglomeration within the composite causing stress concentrations during fiber loading [16
]. Currently, there are various suitable treatments, including chemical and physical methods have been tried with technical success, but implementation has been restricted due to high cost barriers [18
]. A potential solution is the encapsulation of cellulose with wax, which could be cost effective given that wax has already been utilized in many composite processes for moisture resistance. Paraffin wax materials can act as a non-polar shell and have been used extensively in wood products, phase change storage materials, pharmaceuticals, and many polymer-based industries [20
]. Wax was used for microcapsules as a curing agent in thermoset resins [21
]. Wax is a potential commercial candidate due to its high reproducibility, low production cost, improved stability, environmental friendliness, and low initial capital investment [22
]. It can also reduce the rate of water flow in capillaries [24
] and significantly increase the dimensional stability of wet specimens [25
]. In another study, wax additives were successfully incorporated into aqueous wood preservatives to reduce checking and improve the appearance of treated wood exposed to outdoor environments [26
]. Finally, MCC has been used as a filler in UV-light curable methacrylic-siloxane resin. MCC addition increased the dynamic moduli and decreased the thermal expansion coefficient. The photocurable microcomposites could be used as innovative protective coatings of damaged wood [27
Recently, a wax encapsulation technique has been developed specifically for microcrystalline cellulose [28
]. In this study, simple cellulose mixing techniques with paraffin wax was achieved to partially or completely encapsulate cellulose particles. To investigate the effect of the performance of encapsulated cellulose dispersed in the epoxy polymer, Epon 828 and microcrystalline cellulose (MCC) were chosen as the composite materials, with paraffin wax for encapsulation of MCC. The water absorption, mechanical properties, FTIR spectra, and thermal properties of the encapsulated MCC (EMC) reinforced epoxy composites were analyzed. The surface morphology of EMC/epoxy composites was also observed using a scanning electron microscope. The possible applications of these composites may include food preservation and innovative packaging, aero-automotive, electrical, and electronics fields [30
2. Materials and Methods
Paraffin wax was purchased from Gulf Oil Corporation, Houston, TX, USA (Gulf Wax, Household, C211P-S-TO). The matrix epoxy polymer (EPON 828) (Catalog No. NC9610653, Manufacturer: E. V. Roberts and Associates Inc. 174-1 gallon) was purchased from Fisher Scientific (Pittsburgh, PA, USA). All materials were stored in sealed containers and placed in a refrigerated room (5 °C). Diethylenetriamine (99%, DETA, CAS No. 111-40-0) curing agent was purchased from VWR Chemical Co. (produced by Alfa Aesar Company, Tewksbury, MA, USA). MCC with an average particle size of 90 μm (CAS: 9004-34-6, MW: 342.3 g/mol; melting point: 260–270 °C; density: 1.27–1.6 g/cm3 (20 °C)), was purchased from VWR Chemical Co. (West Chester, PA, USA). It was stored in sealed a container before use to avoid moisture uptake from the environment.
2.2. Coating Cellulose Particles
For the coating process [28
], EMC particles were obtained by mixing cellulose particles with paraffin wax at 70 °C for 20 min at a constant stirring of 300 rpm. Paraffin wax was first placed in a beaker heated to a temperature of 70 °C and the temperature was maintained until the paraffin wax was completely molten. The subsequent coating was conducted by adding MCC at different concentrations and stirring for 20 min at 500 rpm. The mixture was then cooled to room temperature under constant stirring, and final EMC particles were collected. Ratios of 1:2, 1:3 and 1:4 (wax to MCC, by weight) were utilized to vary the wax coverage on the surfaces of the cellulose particles.
2.3. Differential Interference Contrast (DIC) Microscopy
DIC Microscopy (Olympus BX51, Tokyo, Japan) was used to observe the morphology of un-coated MCC and MCC after wax encapsulation. The samples were put on a transparent glass slide and another glass slide was used to separate the sample particles in order to observe individual particles. Then micrographs of each sample were taken using different magnifications.
2.4. Preparation of EMC Reinforced Epoxy Composites
After encapsulation, the EMC particles were added and mixed with epoxy resin prior to cure. EMC loading rates of 0%, 1%, 3%, and 5% (wt % of epoxy resin) were applied at room temperature and mixed with the polymer until fully homogenized. The samples were then obtained by pouring the mixtures into aluminum dishes (10 cm diameter) after DETA was added as a curing agent (10% by weight) to EPON 828 and fully homogenized [31
]. The mixtures were then stored at room temperature for 20 min, then 2 h at 50 °C in an oven, and then 1 h at 100 °C. The nominal thickness of the composites was 2 mm. In order to perform mechanical tests and water absorption experiments, the specimens were cut from the cured samples into rectangular bars with dimensions of 50 mm in length and 8 mm in width. The edges of the specimens were smoothed using 60 grit coarse sandpaper. The final specimens were stored in desiccators. The processes of coating, composite, and sample preparations are shown in Figure 1
2.5. Water Absorption
Water absorption of the composite specimens were performed according to ASTM D1037 [32
]. First the specimens were dried in an oven at 50 °C for 24 h. Six specimens per treatment were submerged in distilled water at 23 °C for 24 h. Excess water was removed from the specimen surfaces with a dry clean cloth. The specimens were weighed within 1 min after removal from water and then flexure testing was performed (next section). The water absorption (water content, Mt
) was determined using the following equation:
Mt (%) = ((Wt − W0)/W0) × 100
is the weight of the sample after soaking and W0
is the weight of the sample before soaking and after drying in the oven.
2.6. Mechanical Properties
Flexure tests performed for both dry and wet samples (before and after 24 h immersion in water absorption test). Flexure tests were conducted using a three point bending protocol at room temperature and at 40% relative humidity. A Z010 Zwick/Roell testing machine (Zwick Roell, Einsingen, Germany) was used with a support distance of 40 mm and testing speed of 1 mm/min (ASTM D790-10) [33
]. Six specimens per group were tested.
2.7. Scanning Electron Microscopy (SEM)
The cross-section morphology of EMC/epoxy composites after tensile tests were observed using a Zeiss Evo 40XVP scanning electron microscope (SEM, Zeiss, Jena, Germany) to better understand the surface properties of the interaction or bond between Epon 828 and wax or EMC particles. The samples were mounted on aluminum stubs using carbon tape. The samples were then coated with a thin layer of gold to prevent charging before the observation by SEM. The accelerating voltage was 20 kV.
2.8. Fourier Transform Infrared Spectroscopy
Fourier transform infrared spectroscopy (FT-IR) measurements were performed using a Perkin–Elmer Spectrum 400 instrument (Waltham, MA, USA) fitted with a single reflectance ATR diamond probe. The samples (MCC, EMC, wax, neat epoxy, neat cured epoxy resin, and EMC/epoxy composite) were measured immediately after manufacture to avoid samples absorbing moisture from the atmosphere.
2.9. Thermal Analysis
In order to study the thermal stability and changes in degradation patterns of MCC, neat EPON 828 and their composites, thermogravimetric analyses (TGA) was carried out using a TGA Q5000 instrument (TA Instruments, New Castle, DE, USA) under nitrogen (20 mL/min) from room temperature to 700 °C and at a heating rate of 15 °C/min.
2.10. Data Statistical Analysis
The data of water absorption and mechanical tests were analyzed using SAS (version 9.4, SAS Institute Inc., Cary, NC, USA). Analysis of variance (ANOVA, α = 0.05) was used to examine differences of water absorption among the wax to MCC ratios and MCC additions. A paired t-test was used to detect the significant differences on the mechanical properties between the composites and the control (pure epoxy).
Wax encapsulated microcrystalline cellulose (EMC) allowed for the homogenous dispersion of microcrystalline cellulose (MCC) in the epoxy matrix without much agglomeration. The wax coverage was not a complete sheath, resulting in adequate cellulose exposure for crosslinking between cellulose and epoxy matrix. In summary, the present wax encapsulation method is simple, cost effective, and may not require additional capital during scale up to a manufacturing process. The EMC particles exhibited many advantages such as limited agglomeration, improved hydrophobicity, and homogenous dispersion in non-polar matrices. The EMC could be employed in the epoxy matrix and provide successful reinforcement at only 1% loading. The results indicated that 1% of EMC loading with a 1:2 ratio of wax to MCC was good for both dry and wet property reinforcement, which suggested that there could be an optimum crosslink density between the cellulose and epoxy matrix. The thermal resistance of the epoxy polymer was also not lowered after addition of EMC. The epoxy cellulosic composites utilizing biomass materials could be beneficial for the polymer and wood-related industries by incorporating wasted materials into new environmental friendly products. Furthermore, the cellulosic epoxy composites could enlarge the applications of epoxy polymers, such as in packaging, automotive, and electrical fields.