The high rate of destruction of plastic waste is very alarming. In 2015 alone, it is estimated that a total of 80% of plastic wastes were destroyed while only 20% were recycled. The destroyed plastic waste constituted pollution to the atmosphere and took space that would have been used for agricultural purposes [10]. Hence it can be seen that the 80% of plastic waste destroyed constituted a loss as they could have been recycled and sold there by increasing revenue generation and reducing unemployment. To this end it can be categorically stated that plastic recycling is a gold mine which should be tapped.

Paramount in the recycling process of plastic is to ensure that the resulting material has mechanical properties which can compete favourably with other counterpart materials in the construction industry [11]. To this end, this research aims at determining the mechanical properties of recycled HDPE plastics with the aim of carrying our performance assessment on the recycles.

The main objective of this research is to study HDPE plastic recycles with the aim of evaluating the products by determining their mechanical, morphological as well as water retention properties.

The specific objectives include to:

I. Recycle locally sourced HDPE plastic.

II. Determine the hardness number, impact stress value and tensile stress value of the recycled HDPE plastic. III. Investigate the surface morphology of the recycled and virgin HDPE plastic.

IV. Investigate the water retention capacity of the recycled and virgin HDPE plastic.

V. Determine the suitability for use of recycled HDPE plastic in production.

VI. Compare the mechanical properties, morphological and water retention properties of recycled HDPE plastic determined with that of virgin HDPE plastic.

The mould was first designed in 2D and 3D drawing by using AutoCAD software. After confirming the design and the dimensions, the mould then was machined through woodwork machining operations, such as roughing, drilling and other operations needed. Next, the assembled wooden mould will be put in the compression moulding machine that will be compressed later with the aid of a screw jack in order to obtain recycled material. The mould was designed in such a manner as to aid in accomplishing the scope of the research. The mould was made of wood. Its function is to ensure that the melted plastic occupies a specific shape. Below is a picture of such mould in Figs. 2-4.

In this stage, the entire specimen in the pan was placed in the oven inside the compression moulding machine and the heat temperature increased gradually with time. The oven is operated by a 220 voltage circuit which is regulated with a PID controller. A type K thermocouple is attached and the PID controller displays the temperature inside oven and temperature set limit, the oven can heat up plastics and melt at a range of 170 – 220c but it was observed that the specimen melts within the range of 130c-140c. Fig. 9 shows the compression moulding machine used for the melting process.

This process involved removing the molten material from the mould and mixing to achieve even coloration and fusion of the melted material. This was done with the use of a wooden stick.

After melting, the hot semi molten plastic was transferred into the mould and lifted up to the top of the machine with the aid of a screw jack after which it was left to solidify. After solidification, the solid recycled material took the shape of the wooden mould and it was machined using a lathe machine to produce finished products such as tiles. The resulting moulded recycled material is shown below in Figs. 10.a and 10.b.

The recycled material will be tested in three conditions being:

I. Tension.

II. Hardness.

III. Impact.

These tests will assist in revealing the mechanical properties of recycled HDPE plastic. The properties observed will be placed side by side with properties of substances like wood and metal to see its suitability when applied in real life manufacturing scenarios.

Evaluation was done on the cooled recycled material to determine its mechanical properties.

Here, the specimen was cut to shapes to make easy the experimentation process. Fig. 11 shows the shapes to which the specimen was cut to.

The aim of this test was to determine the Rockwell Hardness Number (RHN) of the given Specimen using “Rockwell Hardness tester”. Fig. 12 shows the test equipment.

An impact test usually used to evaluate the brittleness of a material under high strain rates [12]. While tensile stress is normally performed at low strain rate, which can be define that the specimen is very slowly elongated. On the other hand, impact test is subjected material to a sudden, intense blow, in which the strain rate extremely rapid, hence the material may behave in a much more brittle manner than is observed in tensile stress. Whereas, energy required to break the samples per original cross section area is impact strength. The test equipment is shown in Fig. 15.

The impact test readings exposed that the sample of HDPE recycled plastic has an average value of 284 J. The first sample gave an impact value of 286J, the second 282J and the third 284J.

The strength of materials can be determined using a tensile test, in which a tensile load (force) is applied to a specimen of the material and its extension is measured with respectto this load, i.e. Tensile test is used to measure the force required to break a specimen and the extent to which the specimen stretches or elongates to that breaking point [13].

Applying this test, the stressstrain diagram is produced, and we can investigate the tensile behavior of the m aterial and its properties from this diagram.

A Scanning Electron Micrograph (SEM) comparison of recycled HDP plastic and virgin HDPE plastic reveals several distinct differences in their surface morphology and properties [16]. The SEM analysis provides valuable insights into the effects of recycling on the physical structure and integrity of the plastic materials [17].

Virgin HDPE plastic, also known as virgin HDPE, exhibits a smooth, glossy surface with minimal surface defects (Fig. 22). The surface is characterized by a uniform texture, indicating a well-ordered molecular structure. The SEM image shows a high level of crystallinity, with a compact and dense arrangement of polymer chains [18]. This is due to the controlled manufacturing process, which involves extrusion and molding under precise conditions. The resulting material has a high degree of purity, minimal impurities, and a consistent chemical composition.

In contrast, recycled HDPE plastic shows a rougher, more porous surface with increased surface defects (Fig. 23). The SEM image reveals a higher level of surface roughness, indicating a more disordered molecular structure. The presence of micro-particles, pits, and cracks is evident, suggesting that the recycling process has introduced impurities and defects into the material [19]. These defects can arise from various sources during the recycling process, such as contamination from other plastics, metals, or chemicals. Additionally, the repeated melting and re-molding cycles can lead to molecular degradation, causing the polymer chains to become shorter and more disordered.

Recycled plastics often contain additives such as fillers, pigments, and antioxidants to improve their mechanical properties and aesthetics [20]. These additives can be seen in the SEM image as small particles or agglomerates scattered throughout the surface (Fig. 23). In contrast, virgin HDPE typically does not contain these additives.

The surface roughness and porosity of recycled HDPE can have significant implications for its performance and durability [21]. The increased surface area and defects can provide more sites for chemical reactions to occur, making the material more prone to degradation over time. This can lead to reduced mechanical properties, such as strength and flexibility, as well as increased susceptibility to chemical attacks. In contrast, virgin HDPE maintains its integrity and mechanical properties due to its smoother surface and well-ordered molecular structure [22].

The water absorption test was conducted in concordance with the following steps in concordance with ASTM D570 which is the standard for carrying out water absorption tests.

I. Preparing samples by cutting into desired shape, drying and weighing.

II. Submersing weighed sample in water at room temperature for a duration of 24 hours III. Removal of sample from water, surface drying and weighing after submersion.

IV. Calculation for water absorption using the formula:

Wet Weight − Initial dry weight

Water Absorption Capacity = X 100.

Initial dry weight

From the result, the recycled HDPE specimens absorbed more water compared to virgin HDPE specimens. This could be due to the microstructure of the recycled HDPE, which has more voids and pores that result in water filling the gaps more effectively [20]. Both specimens seem to absorb water, but the percentages of the absorption are very small, 0.015% and 0.27% for recycled HDPE and virgin HDPE respectively after 24 hrs of testing. The water absorption test results are shown Fig. 24 the higher value for recycled HDPE plastic is due to the presence of impurities and additives [20].

Both water absorption values are low in comparison to polymers and this is why HDPE is often used in water-related applications like pipes and containers.