Preparation and Thermal Properties of Modified Cu2O/Polypropylene (PP) Composite

A uniform, monodispersed superfine cuprous oxide (Cu2O) sphere with a mean diameter of 850 nm has been synthesized by solution reduction. The study reported the synthesis and thermal properties of Cu2O/PP composites for the first time. The surface modification of the superfine Cu2O sphere was carried out by using a silane coupling agent KH-570. Fourier-transform infrared (FTIR) spectroscopy and the thermogravimetric analysis (TGA) curve revealed that the Cu2O had been successfully modified by silane coupling agent KH570. The scanning electron microscope (SEM) shows that the modified Cu2O can be uniformly dispersed in the polypropylene (PP) matrix, because through surface modification, there are some active functional groups on its surface, such as the ester group, which improves its compatibility with the PP matrix. The thermal stability of Cu2O/PP composites was improved by adding a small amount of Cu2O (1 wt % of PP). Therefore, based on the potential bacteriostasis of cuprous oxide, the low cost of PP and the results of this study, it is predicted that Cu2O/PP composites can be used in infant preparation (such as milk bottles) with low cost and good thermal stability in the near future.


Introduction
Polypropylene (PP), as a general-purpose plastic, has the advantages of excellent physical properties and easy molding and processing, plays an increasingly important role in the plastics industry, and is widely used in various fields such as chemical, machinery, automobile, food packaging, infant products, etc. [1][2][3][4]. However, PP is a non-polar polymer with poor toughness, high temperature brittleness and large molding shrinkage, which limits the engineering application of PP. How to make plastics and the special functions of PP engineering and to expand its use is an important direction for PP development. Thus, PP is usually modified through being blended with other resins and filled with the introduction of inorganic fillers, such as calcium carbonate [5,6], metallic powder [7,8] and nano-carbon (carbon tube, graphene) [9][10][11][12]. So polypropylene-based nano-composites provide a new way for polypropylene modification. For example, only a small number of nanoparticles can improve significantly the properties of polymers, such as electrical properties [13,14], mechanical properties [15,16] and thermal properties [12,[16][17][18][19]. Bafana A P et al. [12] reported that graphene/polypropylene nanocomposites were prepared using the solution mixing method, and it was found that only a small amount of graphene can improve the thermal properties of PP. Jisheng Ma et al. [19] synthesized Polypropylene/clay (PP/clay) nanocomposites via intercalative polymerization. The transmission electron microscope (TEM) image showed that the clay was exfoliated into nanometer size and dispersed uniformly in the PP matrix, and the thermal stability of the nanocomposites significantly increased.
Therefore, adding nano/micro materials to the PP matrix to improve their performance has become a hot research topic. However, as we all know, when particle size is decreased to the nanoscale range, its properties will significantly change. Among nanoparticles (NPs), metal oxide NPs (i.e., ZnO, ZrO 2 , SiO 2 , Fe 3 O 4 , CuO and etc.) are attracting considerable interest due to their unique physical and chemical properties [20]. Their unique properties of metal oxide NPs strongly depend upon NPs preparation and dimension [21,22]. However, the main problem in NPs application is the high tendency of metal oxide NPs to adhesion and aggregation. Surface modification of the metal oxide NPs with organic compounds has been employed to overcome the problem and reduce the particle surface energy and decrease their tendency to agglomerate [23,24]. In addition, due to their large specific area, inorganic nanoparticles are easy to agglomerate, and are not easily dispersed in the polymer matrix, which limits their reinforcing effect. Therefore, in order to solve this problem, inorganic nanoparticles are often modified [25][26][27]. Xia et al. [26] have used modified ZrO 2 NPs as an additive. To improve the dispersion and interaction of the ZrO 2 with the epoxy coating, the surface of NPs was modified by a styrene coupling agent by the post modification. Based on the above description, the surface of NPs plays an important role in the properties of NPs, counting the dispersity, solubility, stability, reactivity, melting point and electronic structure. Mallakpour et al review the synthesis, properties, applications and surface modification of metal oxide NPs, the classification of coupling agents, and the interaction between metal oxide NPs and coupling agents [28]. Therefore, in this study we reported the surface modification of metal oxide NPs cuprous oxide with silane coupling.
Cuprous oxide (Cu 2 O) is a rare semiconductor material that can be excited by visible light with a band gap of about 2.0 eV. Cu 2 O has nontoxic and low preparation cost, can directly degrade organic matter by sunlight without secondary pollution, and has high theoretical utilization efficiency. Therefore, it is one of the green catalysts with great development prospects. Up to now, cuprous oxide has been prepared by several different methods, such as electrodeposition, solvothermal/hydrothermal methods, sonar-chemical methods, γ-Irradiation, the liquid-phase reduction, etc. [29][30][31][32]. Based on the superior properties of Cu 2 O, it has many potential applications in solar energy conversion [33], photocatalytic degradation [34,35], marine antifouling coating [36], lithium ion batteries [37], carbon dioxide [38], gas sensors [39], and so on.
The incorporation of inorganic nanoparticles in polymer has been found to improve the thermal [16,18,19], mechanical [15][16][17], electrical [14], corrosion resistance [25,26] and antimicrobial properties of the polymer [21,35,40,41]. The improvement of the inorganic nanocomposite properties depends on filler type, size, shape, the degree of dispersion of the NPs in the polymer matrix and the degree of adhesion of NPs with polymer chains [42,43]. Generally, there are two methods by which inorganic NPs are dispersed/incorporated in polymeric materials, either as additives by direct mixing with polymer [12,[40][41][42], or as reactive materials by in situ polymerization [16,19]. To date, there are few studies on Cu 2 O/PP composites. Yongqian Shi et al. [40] reported that the thermal and smoke suppression properties of CuO/PP and graphene/PP. The thermal and smoke suppression properties of CuO/PP and graphene/PP were efficiently improved, compared with native PP. Palza et al. [41,42] prepared copper polypropylene-based composites by melting blend. After 4 h these composites, with 1 v/v % of copper nanoparticles, kill > 99.9% of Escherichia coli (E. coli). In those above studies, they focused on the thermal property, smoke suppression property and antibacterial activity of copper-polypropylene composites. To our knowledge, the study of Cu 2 O/PP composite was fabricated for the first time. Therefore, the development of Cu 2 O/PP composite material with both antibacterial properties and heat resistance is essential. The synthesized Cu 2 O/PP composites are expected to be used in infant products. In this present work, a superfine Cu 2 O sphere was synthesized by solution reduction, and modified Cu 2 O/PP composite materials were synthesized by melting blend. The modification of cuprous oxide is analyzed by an infrared spectrum. Dispersion of modified Cu 2 O in PP was characterized by SEM. The thermal stability of Cu 2 O/PP and modified Cu 2 O/PP composite materials was investigated by TGA and Differential scanning calorimetry (DSC). (Taizhou, China). De-ionized water (DI) was obtained from our laboratory itself. All reagents were used without further purification.

Synthesized of Superfine Cu 2 O Sphere
The Cu 2 O sphere was prepared by the following procedure. First, solutions of 1 g gelatin in 100 mL deionized water were prepared, and then sonicated for 30 min, and labeled as A solution. A set of solutions were prepared by adding 0.2 mol/L copper sulfate pentahydrate (Cu(SO 4 ) 2 ·5H 2 O) to 100 mL deionized water, in which a certain amount of polyethylene glycol (PEG) as surfactant, was labeled as the B solution. Secondly, B solutions were added to A solutions drop by drop, and then heated at 50 • C in a water bath and stirred continuously. Third, 100 mL 0.1 mol/L aqueous glucose solution was added drop by drop, and then 100 mL 3 mol/L NaOH solution was also added drop by drop. The mixture was kept at 50 • C for another 30 min. Magnetic stirring was continuously applied throughout the entire process of reduction and particle growth. The Cu 2 O particles were separated from the solution by centrifugation at 2000 rpm for 5 min, and was washed several times by deionized water and ethanol. The resultant product was dried at 60 • C in a vacuum oven for 6 h.

Modification of Cu 2 O
First, 1 g dried cuprous oxide was dispersed into 50 mL of distilled water, and the mixture was magnetically stirred for 30 min. Second, 10 mL isopropanol was added into the stirring solution. Third, a certain amount of KH570 was added to the stirring solution, the mixture was stirred for 1 h and then sonicated for 30 min. The suspension was centrifuged, washed by distilled water and absolute ethanol for several times to remove the remaining KH570, and dried in a vacuum oven at 80 • C for 4 h. The product was labeled as M-Cu 2 O. Additionally, the unmodified Cu 2 O is labeled as 0-Cu 2 O.

Synthesis of Cu 2 O/PP Composite
Cu 2 O/PP composite was synthesized using melt blending [39][40][41]. 10 g polypropylene (PP) and an appropriate amount of xylene were added to the three-necked flask, heated in an oil bath and stirred until completely dissolved. At the same time, a certain amount of Cu 2 O (1 wt % of PP) was added to an appropriate amount of xylene solution, and dispersed ultrasonically for 30 min. After the PP is completely dissolved, the ultrasonicated Cu 2 O solution was added to the PP xylene solution, and stirred 15 min, and then the initiator benzoyl peroxide xylene solution was added to the mixture. During the above reaction, the mixture was heated at 130 • C for 3 h. Then stopping heating, the system was cooled to 70 • C, the reactant was taken out, soaked in absolute ethanol for several times, and dried to constant weight in a vacuum oven at 80 • C. The product was labeled as 0-Cu 2 O/PP. In addition, for comparison, the M-Cu 2 O added to the PP matrix was prepared with the same parameters, and the product was labeled as M-Cu 2 O/PP.
Various functional groups present within Cu 2 O, KH570-Cu 2 O and KH-570 were identified by FTIR spectra by using an Interspec 2020-Spectrolab FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) and KBr as reference. SEM coupled with EDS was used to study the surface morphology and chemical composition of the samples by using scanning electron microscope (Hirachi, Hitachi-S4800, Tokyo, Japan). EDS is an analytical technique used for the elemental analysis or chemical characterization of the materials. The thermal stability of Cu 2 O, KH570-Cu 2 O and Cu 2 O/PP composites was investigated by thermogravimetric analysis (TGA, Netzsch, TG209F3, Germany). The heating rate was 10 • C min −1 , and the experiments were performed in a continuous air flow rate of 20 cm 3 min −1 . The thermal properties of the Cu 2 O/PP composites were determined by using differential scanning calorimetry (DSC, Netzsch, DSC214, Germany). For each test, the sample was first heated from room temperature to 230 • C with a heating rate of 10 • C/min −1 and annealed for 5 min to remove thermal history, followed by cooling down to room temperature at a rate of 10 • C/min −1 for data collection. Experiments were run on samples of about 6-8 mg. In addition, the XRD pattern of the as-synthesized powders in Figure 1 shows the sharp peak shape, indicating that the powders obtained by the current one-pot synthetic approach were well-crystallized. No characteristic peaks of CuO and Cu are observed in XRD patterns, indicating that high-purity Cu 2 O crystalline is successfully synthesized by this method. The pure crystal phase is attributed to the existence of glucose in the process of preparation. Because glucose is a weak reducing agent, only in a strongly basic solution can the Cu 2+ be reduced to Cu + .  It can be clearly seen that the powders have uniform, monodispersed and spherical particles with a mean diameter of 850 nm, because of having polyethylene glycol (PEG) as the surfactant. The steric hindrance effect of PEG can effectively prevent the agglomeration of nano Cu 2 O particles, thereby improving the uniformity of Cu 2 O particles. When the micrograph of the microsphere is further enlarged to high magnification (Figure 2b), it is found that the surface of the microsphere is very smooth, and this is different from other literature [44,45], in which the surface of the reported Cu 2 O particles is very rough, with small flakes and large particles of irregular morphology, or composed of abundant nanoparticles. Figure 2c-f give the SEM/EDS of M-Cu 2 O, and it can be seen that the surface of the M-Cu 2 O with rough surface is obviously uniformly covered by some substances. These substances are attached to the surface of the Cu 2 O sphere in granular form, indicating that the coupling process can be accomplished via the chemical reaction between the alkoxy group of the silane coupling agent and the hydroxyl groups (-OH) on the Cu 2 O surface (due to the hydroxyl groups (-OH) commonly existing on the surface of metal oxide that is used as a reaction site [46]), and a coupling agent layer is formed on the surface of Cu 2 O by a chemical bond. With respect to the above as-synthesied Cu 2 O sphere, gelatin play important roles in the formation of this Cu 2 O sphere. The mechanism of formation of Cu 2 O the sphere was analyzed as follows. When the temperature is relatively high (50 • C), the gelatin sol can exist in the form of droplets in the solution. These droplets should serve as templates for the development of spherical structures. The gelatins contain large amounts of carboxyl and ammonia and the positive Cu 2+ is easily absorbed on the surface of the gelatin droplets, due to electrostatic interaction. The reduction of Cu 2+ by glucose makes Cu + to nucleate and grow at the appropriate position.

Fourier-Transform Infrared Spectroscopy (FTIR)
NPs have a strong tendency to undergo agglomeration. Surface modification of the metal oxide NPs with physically (by physisorption) or chemically (through covalent bonding) routes have been employed to overcome this problem. Surface modification refers to the use of surface additives to change the surface state of particles, such as the surface atomic layer structure and functional groups, surface hydrophobicity, electrical properties, chemical adsorption and reaction characteristics. Through surface modification, it can improve the dispersibility, durability and weatherability of the powders, and improve also the surface activity. Thus, new physical and chemical properties can be produced on the surface of the particles. Surface modification of NPs is that a chemical reaction occurs on their surface, which introduces some new interaction forces between the two phases, such as van der Waals forces, hydrogen bonds or coordination bond interactions, and the combinations of ionic or covalent bonds [28]. The infrared spectrum of cuprous oxide before and after KH570 modification and KH570 is shown in Figure 3. The Figure 3a

Thermogravimetric Analysis (TGA) of Inorganic Particles Cu 2 O
Based on infrared spectroscopy, the thermogravimetric analysis (TGA) of the inorganic particles Cu 2 O before and after the modification was carried out. The results are shown in Figure 4. As shown in Figure 4, before 350 • C, the mass loss of the Cu 2 O and Cu 2 O/KH-570 is mainly due to the evaporation of water. After 350 • C, the mass of 0-Cu 2 O in the curve a has only a small amount of change, and the mass loss is due to the binding water between the molecules (Cu (OH) 2 → -Cu-O-Cu + H 2 O). The quality of the M-Cu 2 O is greatly reduced after 350 • C, as shown in the Figure 4b curve. Combined with the infrared spectrum (Figure 3), a chemical bond forms between the silane coupling agent KH-570 and Cu 2 O, which is decomposed by heating, resulting in a rapid decrease in mass.  Figures 5 and 6, the agglomerates of unmodified Cu 2 O in the PP matrix were found, but the KH570-modified Cu 2 O was finely dispersed in the PP matrix, without agglomeration. These results illustrate that unmodified inorganic superfine cuprous oxide has poor surface activity, resulting in aggregation, but the surface of superfine Cu 2 O were modified with KH570 to introduce organic functional groups (such as the ester group) on the surface of Cu 2 O (Figure 3), and improve the compatibility of the Cu 2 O surface (hydrophilic) with a hydrophobic polymer surface. This also increases the dispersion stability in organic media [47]. These results also reveal that the surface of Cu 2 O was successfully modified by the silane coupling agent KH-570.

Thermal Properties of 0-Cu 2 O/PP and M-Cu 2 /PP Composites
Polypropylene (PP) is one of the five general-purpose plastics. It is cheap, non-toxic and tasteless, and has excellent mechanical properties. It is widely used in electrical appliances, automobiles, daily necessities, etc. [1][2][3][4], but its impact toughness is poor, its thermal stability is also poor, and it is easy to burn. Disadvantages greatly limit the application of PP in certain engineering fields. In order to broaden the application range of PP, the modification of PP has become a research hotspot. Making composite materials is a good way. Adding Cu 2 O to the PP matrix can improve the heat resistance of the composite. The TGA curve of Cu 2 O/PP at the rate of 10 • C/min was shown in Figure 7. As can be seen from Figure 7 Figures 8 and 9 show the DSC curves for pure PP and Cu 2 O/PP composites. The crystallizing temperature of PP is increased by approximately 3 • C, when Cu 2 O is added to the PP. However, our results show that an increase of 3 • C in the crystallization temperature is achieved, implying that the Cu 2 O is a nucleating agent with a rather low efficiency, comparable with other widely published sources [48][49][50][51]. It was also found that the melting temperature of PP is higher than that observed for the Cu 2 O/PP composites.

Conclusions
Superfine cuprous oxide (Cu 2 O) spheres with a mean diameter of 850 nm have been synthesized by solution reduction. The surface of M-Cu 2 O is rougher than that of 0-Cu 2 O, and the Cu 2 O/PP composite was prepared by using melting blend. Fourier-transform infrared spectroscopy and the TGA curve revealed that the Cu 2 O had been successfully modified by silane coupling agent KH570. The KH570-modifed Cu 2 O was finely dispersed in the PP matrix from the SEM images, indicating that the modified cuprous oxide with active functional group, such as an ester group, can improve its compatibility with PP. The thermal stability of Cu 2 O/PP composites is better than that of pure PP, especially modified Cu 2 O/PP composites. Therefore, based on the potential bacteriostasis of cuprous oxide, the low cost of PP, and the results of this study, it is predicted that Cu 2 O/PP composites can be used in infant preparation (such as milk bottles) with low cost and good thermal stability in the near future.