the increase in interfacial bonding between DPF

 the increase in interfacial bonding between DPF and the polymer matrix after treating the DPF with NaOH which removes the impurities from the surface and exposes a larger area of the fibre to be bonded with the polymer matrix. The incorporation of DPF lowered the thermal stability, but increased the overall degree of crystallinity of the composites (Dehghani et al. 2013). AlMaadeed and her fellow researchers (2014a; b) also investigated the mechanical properties of date palm wood powder (DPWP) as a filler for recycled LLDPE composite with the fillers concentration ranging between 10 and 70% at 10% increment. The Young’s modulus of the composites increased significantly with increasing DPWP concentration, reaching 466% higher at 70% filler than that for the neat RLLDPE. The stress at break of the composites decreased sharply with increasing DPWP concentration and the composite became brittle when filled with more than 10 wt % DPWP (AlMaadeed et al. 2014b). Research done on TPS as a source of biodegradable matrices to develop hybrid composites with DPF reinforcement showed that the thermal stability increased as the DPF content increases. The tensile strength and modulus also increased by 760% and 600% respectively for 50 wt% DPF reinforcement. Flexural strength and modulus showed similar behavior. 

 At 60 wt% fiber content and above, the mechanical prop- erties started to deteriorate as a result of the increase in the composites porosity (Ibrahim et al. 2014). Afterwards, Saleh et al. (2017) reported similar results as Ibrahim et al. (2014) that DPF reinforced TPS matrices had the highest fatigue and flexural strengths at 50 wt% DPF content. Furthermore, using a mixture of recycled HDPE, LDPE, and PP with 1% maleic anhydride as a compatibilizing agent was effective for chemical modification of the DPFRC, which promoted dispersion and better interfacial adhesion between the polymer matrices and DPF. Thus, the mechan- ical properties of the composites were improved where results showed a noticeable increase in both tensile strain and modulus but a large decrease in the percentage of elongation at break. The physical properties of the composites showed good resis- tance to alkalies and acids. The polymer melting and crystallization temperature were Polymer Matrix Systems Used for Date Palm Composite Reinforcement 137 not affected by the addition of DPF or maleic anhydride body puff ball, indicating that the additives did not affect the thermal behavior of the polymer matrices. A latest study reported (2019) that the biodegradation of linear low density PE (LLDPE) reinforced with powder DPF composites is affected by the fibre treat- ment, UV stabilizers, antiblock additives and polymer additives bath sponge pills. Results showed that the composite containing UV stabilizer degraded faster than the composite containing antiblock additive, which is due to the difference in the chemical additives within them. Finally, a summary on reported studies of DPF reinforced thermoplastic composites are shown in Table 5. 

3 Thermoset Matrix Systems for DPF Composites 3.1 Synthetic Resins for DPF Composites Phenolic resins Phenolic resins are one of the first polymeric products discovered in early 1900s to be composed from simple low molecular weight compounds, being the first legitimately synthetic resins to be used. They are produced from phenol, or phenol derivatives, and formaldehyde by step-growth polymerisation using base or acid catalyst. In the presence of an acid catalyst, the reaction of phenol with less than equimolar proportions of formaldehyde produce novolac resins that contain aromatic phenol units combined mostly by methylene bridges. Additionally, in the presence of a base catalyst unicorn bath pouf, the reaction of phenol or phenol derivative with an excess amount of formaldehyde produces resole resins. Novolac resins are thermally stable and cured by cross-linking with formaldehyde donors, such as hexamethylenetetramine. Moreover, when developing composites using phenolic resins, resoles are the most extensively used phenolic resins for composite development as they are considered less viscous and easier to process than novolac resins. 

Furthermore large body powder puff and container, phenolic resins are mostly used in structural applications, due to their built-in fire-resistant properties. However, they are not tough enough and their curing reaction generates water, which remains trapped in the composite and transforms into steam in the case of a fire which could damage the structure of the material (Ratna 2009). Unsaturated polyesters (UP) resins Unsaturated polyester (UP) resin is the second thermoset resin discovered after phenolic resins, in early 1940s. UP resins consist of an unsaturated polyester, a monomer and an inhibitor. With the increasing environmental concerns, low styrene co-reagent additives are being used in UP resins, usually 50 wt% added to obtain a satisfactory flow behaviour. In some cases, these additives compromise the adhesion by dividing and sealing the surface in the final stages of cure. When cured, UP resins HDT can range from 60 to 230 °C having the lowest glass transition temperature and thermal coefficient expansion of 60 °C and 3.0 °C respectively. Furthermore, UP 138 S. Awad et al. 

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