*Article* **Development and Mechano-Chemical Characterization of Polymer Composite Sheets Filled with Silica Microparticles with Potential in Printing Industry**

**Sidra Siraj , Ali H. Al-Marzouqi \* and Muhammad Z. Iqbal**

Chemical Engineering Department, College of Engineering, United Arab Emirates University, Al Ain 15551, United Arab Emirates

**\*** Correspondence: hassana@uaeu.ac.ae

**Abstract:** Polymer composite sheets using a low-cost filler (local natural sand) and polymer (highdensity polyethylene, HDPE) as a replacement of the traditionally used wood-fiber-based sheets for paper-based applications were developed. The sand/polymer composite sheets were prepared by melt extrusion in a melt blender followed by compression molding. The effects of varying particle size, concentration, and the use of a compatibilizer (polyethylene-grafted maleic anhydride) was studied on the mechano-chemical performance properties of the composite sheets such as morphology, thermal and mechanical properties, and wettability characteristics used in the printing industry. In terms of thermal stability, filler (sand) or compatibilizer addition did not alter the crystallization, melting, or degradation temperatures significantly, thereby promoting good thermal stability of the prepared sheets. Compatibilization improved anti-wetting property with water. Additionally, for the compatibilized sheets prepared from 25 μm sand particles, at 35 wt%, the contact angle with printing ink decreased from 44◦ to 38.30◦, suggesting improved ink-wetting performance. A decrease in the elastic modulus was also observed with the addition of the compatibilizer, with comparable results to commercial stone paper. Results from this study will be considered as a first step towards understanding compatibility of local natural sand and polymers for paper-based application.

**Keywords:** sustainability; silica; sand; polymer; composite; synthetic; sheets

#### **1. Introduction**

Traditionally, paper is derived from wood by pressing together moist fibers of cellulose pulp followed by drying. Global use of papers for various applications, such as printing papers, bags, posters, cutlery, etc., is also adding a toll on the environment as it requires a huge amount of water to wash away the initial pulp as well as requires chemicals to bleach such products, thereby also increasing wastewater and sludge disposal problems.

Additionally, increasing population has increased the global production and use of paper and paper products exponentially. An average global growth rate of paper production was almost 3.6% in the 1980s and approximately 3.3% in the 1990s, currently reaching up to a massive 400 million tons per year requirement [1]. On the other hand, increasing use of plastics has become a major cause of concern as it leads to drastic environmental issues due to its non-eco-friendly disposal [2]. An estimate of about 2500 million metric tons (MMT) of plastics are currently in use. It is also reported that over the past 65 years, the cumulative amount of waste generated from primary and recycled plastic waste adds up to a staggering amount of ~6000 MMT. Of this waste, approximately 12% is incinerated and 9% is recycled. Furthermore, over 60% of all plastics produced are directly discarded in nature (marine or in landfills) and continue to accumulate to this date [3]. Figure 1 illustrates a schematic of the worldwide production and disposal of plastics in MMT for the last 65 years. At the current consumption rate of plastics, the Earth will hold an estimate of 3.3 billion tons of plastic by 2050 [4].

**Citation:** Siraj, S.; Al-Marzouqi, A.H.; Iqbal, M.Z. Development and Mechano-Chemical Characterization of Polymer Composite Sheets Filled with Silica Microparticles with Potential in Printing Industry. *Polymers* **2022**, *14*, 3351. https:// doi.org/10.3390/polym14163351

Academic Editor: Edina Rusen

Received: 22 July 2022 Accepted: 11 August 2022 Published: 17 August 2022

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Accordingly, both issues require more innovative methods to help tackle current environmental problems as well as benefit the society in a more sustainable way. Hence, the development of synthetic "paper" came into existence. Synthetic paper or sheet produced from thermoplastic polymers (virgin or recycled) and fillers and additives form a thin composite film that can be used in a variety of paper-based applications. Since the synthetic composite sheet is a "tree-free" product, it will contribute towards reducing the environmental impact of the paper industry, and its potential can be even increased when using recycled plastics [5].

The detrimental effects of manufacturing paper and paper-based products from wood on the environment have led to the commercialization of synthetic papers on a global scale. Taiwan Lung Meng industries was the first to commercialize a synthetic paper named "stone paper", which is manufactured using minerals dispersed in polyolefins [6]. The stone paper, which is used to produce a variety of paper-based products such as bags, calendars, labels, etc., has been given several other commercial names, such as Rockstock [7] and mineral paper, and is eco-friendly with the main composition being a varying ratio of a polymer and suitable filler.

Synthetic "paper" or filler-polymer composite sheet has properties similar to the traditional cellulose-based paper [8]. It is developed by extruding polyolefin films which may be multilayered and are filled with inorganic particles, more commonly known as fillers. A large number of inexpensive inorganic particulate materials have been widely used as fillers to enhance the thermal and mechanical properties of polymers such as calcium carbonate (CaCO3), glass, titanium dioxide (TiO2), and silica (SiO2) in paper-based applications [9–11]. Out of the mentioned fillers, CaCO3 has been extensively studied and used for paper-based applications as it can provide the properties required such as opaqueness and printability, as well as increase the mechanical properties [2,12]. However, an increase in such a filler leads to an increase in its brittle nature [10,11].

**Figure 1.** Simplified estimation of worldwide production and disposal of plastics in MMT for the last 65 years [9].

The Middle East, including the UAE, similar to any other onshore country, has naturally abundant sand (silica). The sand available in the UAE varies with geological position in composition, color, size, and surface texture. Due to its high thermal stability, high durability, and unreactive nature, silica is used in various fields such as in 3D printing, construction, textile, electronic applications, etc. [13–20]. Using UAE's sand in writing paper applications can be cost-effective and produce more eco-friendly sheets compared to wood-based paper.

In general, the existing literature indicates the extensive development and commercialization in the production of synthetic composite sheets and its potential replacement for wood-based sheets. The literature represents increased mechanical and durable properties as well as increased thermal stability and degradation, signifying its good potential for such a product [13,14,20–23]. The literature also shows the use of sand in polymers to

improve properties in applications such as construction, waste management, and recycling applications [24–28]; however, not much study has been conducted on using natural sand (silica) as a filler to develop sheets for printing applications.

Therefore, the main objective of this research is to utilize the UAE's sand in manufacturing synthetic composite sheets for writing/printing applications using melt extrusion technique followed by compression. The physiochemical properties of developed composite sheets are assessed and compared with commercial stone paper and regular A4 paper for writing applications. The goal is to develop paper-like composite sheets using a natural resource to contribute towards a more sustainable approach of paper production. The presented solution contributes to using sand as a raw material in a useful way and thereby widens its potential for industrial use [29]. Additionally, the novelty of this work lies in attempting to utilize the raw local sand as a potential filler in a composite material that can have a commercial-scale application, such as in the printing industry, which can potentially grow as an alternative to the traditional wood-based fiber sheets which are extensively used.in numerous applications.

### **2. Materials and Methods**

Sand sample was collected from Ras Al-Khaimah, UAE (25.6741◦ N, 55.9804◦ E). The local sand was classified into 3 types (carbonates, silicates, and free silica, which is in the form of detrital quartz). The actual relative abundance in terms of the mineral content was analyzed to be about 47 wt% silicates, 26 wt% carbonates, and 14 wt% quartz. The mineral composition of SiO2 was analyzed to be 36.93 wt% [30]. All chemicals used, unless stated otherwise, were supplied by Sigma Aldrich, Germany and were of appropriate purity for application in this study. High-density polyethylene (HDPE) pellets (ρ = 0.98 g/cm3) were purchased from Sigma-Aldrich. PE-g-MA (ρ = 1.8 g/cm3) pellets were also purchased from Sigma-Aldrich. Sand from the UAE was collected and ground to finer particles using heavy-duty grinders. A sieve (200 mm diameter, 25 microns aperture, stainless steel mesh) was further used to refine the fine powder to a particle size of 25 μm. Grinding sand to 5 μm was performed by Retsch, Haan, Germany.
