Parametric study on producing Fused Deposition Modelling filament made of recovered carbon black reinforced Acrylonitrile Butadiene Styrene plastics

Additive manufacturing is a process that makes three-dimensional object layer by layer. There are many different types of 3D printer and the most commonly used is Fused Deposition Modeling (FDM). There is a need for a new material for Acrylonitrile Butadiene Styrene (ABS) FDM filament to improve filament strength and reduce the usage of plastics. From literature, there are limited studies available on making 3D printer filament reinforced by recycled carbon black. The limitation hinders the potential of using this material in new applications. In this study, recycled carbon black powder (rCB) was added as a filler reinforcement to enhance the properties of ABS. Parameters considered in this study were percentage of filler weight loading and filler size. Tests and characterisation used in this study were tensile test, thickness test, surface roughness test, scanning electron microscopy, density test and water absorption test. There were improvements in mechanical properties such as tensile test and elasticity of the filament compared to the pure ABS plastic. The higher filler percentage can improve the elasticity of filament and lower filler percentage can improve the strength of the filament. The findings could help in improving marketability status and commercialisation potential of rCB reinforced ABS filament for FDM applications.


Introduction
Additive manufacturing is the construction of three-dimensional CAD or digital 3D Modeling.3D printing also can be referred to a variety of processes in which material is joined under computer IOP Publishing doi:10.1088/1742-6596/2643/1/012011 2 control to create a three-dimensional object or part, with material being added together typically layer by layer.There are various types of 3D printers in the market that are used nowadays.The most common type that is used nowadays is Fused Deposition Modelling (FDM) process.
The pure plastic filament in FDM process is sometimes not strong enough and require reinforcement to improve mechanical properties [1].There are so many different types of mechanical properties that can be tuned from the reinforcement.Corrosion resistance, non-magnetic characteristics, high tensile strength, lightweight, and simplicity of handling are all advantages of reinforcements.In order to make the reinforcement to be more flexible, different types of filler are used to reinforced pure plastic.The usage of filler can also reduce consumption of plastic material.In addition, the use of filler originates from recycled filler can contribute to production of sustainable and environmentally friendly material.One of the potential materials to be used as the reinforcement is recycled carbon black (rCB).Carbon black has good thermal stability, light weight and low cost.The usage in 3D printing application could contribute to cost saving and better environmental impact.However, studies on the feasibility of using the material is rare.
Carbon black composite is a material that came from incomplete combustion or thermal decomposition of petroleum products under controlled conditions.The material has black physical appearance and comes in powder or pellet.Carbon black usually used as a reinforcing filler in rubber products especially tires and it also can be used as a colour pigment for other than rubber products such as plastic, paints and inks.According to a study by Spahr & Rothon [2], about 70% of carbon black is used as a pigment and reinforcing phase in automobile tires because carbon black can help conduct heat away from the tread, can reduce thermal damage and can increase tire life.Then the balance of application is used for other things such as belts, hoses and as ink pigment.Other advantages of using carbon black as filler reinforcement material is carbon black is rubber compound, so it has higher tensile strength, tear strength, modulus and abrasion resistance [3].
With the advantages of carbon black, there are many applications are use this material as filler to reinforced other polymer.In 3D printing applications, study on the potential or using virgin or recycled carbon black as a reinforcement in fused deposition modelling (FDM) filament is still limited.Most of the studies focus on other type of carbon-based reinforcement such as carbon fibre and recycled carbon fibre [4].The effect of parameters which influences mechanical properties of the carbon black reinforced filament remains unknown.
In this study, recycled carbon black acquired from pyrolysis of tire waste was chosen as the filler for ABS plastic.The filler was in powdered form.Effect of material composition on filament physical and mechanical properties was investigated.The parameters chosen in this study where filler weight loading and filler size.Output of this study could assist in making an informed decision towards potential usage of recycled carbon black in improving properties of 3D printed products.

Materials
The ABS used in this study was in pellets form and supplied by Toray Plastics (Malaysia) Sdn Bhd.This thermoplastic material is resistant to chemicals, stress, and creep.ABS provides an excellent balance of impact, heat, chemical, and abrasion resistance, dimensional stability, tensile strength, surface hardness and rigidity.
Recycled carbon black (rCB) used in this study was supplied by Eco Power Synergy Sdn Bhd located in Selangor, Malaysia.The powdered carbon black was used as a reinforcement filler for the ABS plastic.The rCB is originated from a pyrolysis process of tyre waste.The rCB is in powdered form and sieving is needed for size classification.Sieving process is a separation technique based on the difference in particle size.This process will remove larger particles or unwanted particle form carbon black to ensure proper bonding with the ABS plastic.The sieving process was done using UTS sieve shaker machine.The sieve sizes used were 63 µm and 75 µm.The process was repeated three times and each cycle was carried out for 10 minutes.

Mixing and filament extrusion process
Mixing of ABS pellets and rCB powder was done using LabTech twin screw extruder machine.The machine consists of two intermeshing, co-rotating screws mounted on splined shafts in a closed barrel.The mixing ratios used in this study were 2.5 wt% and 5 wt% of rCB.For the filler size, 63 μm and 75 μm sizes were used.The size is based on hold sizes of the sieve pans.This study produced five different samples of product as shown in Table 1.Materials that were fed into the machine were heated and blend together.The parameters that were used for this machine in mixing are 20 rpm speed screw, 9 rpm feeding screw and 210 ℃ temperatures.The rotating screws forced the plastic pellet forward into the barrel where the plastic was heated to a desired melt temperature of the molten plastic.The filament was extruded through a die and collected.

Test and characterisation
2.4.1.Thickness test.Thickness test measures the diameter of 3D printer filament after the extruding process.This test is important because if the filament have different thickness, the mechanical properties will significantly vary.Thickness test was carried out using a Vernier calliper.The measurement expected for all products is 3 mm, same as the size of the die opening.

Tensile test.
The test was done using 50 KN Universal Shimadzu tensile machine according to ASTM D638 standard.The machine was equipped with Trapezium X software to support and view the test results.For each product, three samples were tested and the average value was calculated.Each test used 20 mm/min of pulling speed.This machine displayed the result of tensile stress, tensile strain and tensile modulus.

Surface roughness test.
Surface roughness test is a test that determined the texture on the material surface quickly and accurately.In this study, surface roughness of filament was determined using Mitutoyo SJ-410 roughness tester machine.The machine displays the measurement of mean roughness value (Ra) in microns (μm).the sample was clamped on the stage platform.After that, the needle which functioned as the detector, was slowly located on the sample until the needle touch the surface of the sample.Next, the software was initiated in the control penal to get the reading of the sample surface roughness.During this process, the needle was slowly moving along the sample 2.2.Sieving process surface.Travel distance that this machine take is 4.8mm.The surface roughness reading of the sample was taken for 3 times and the average of the surface roughness is calculated and recorded.This step was repeated with samples of the other products.
2.4.4.Density test.Density test is generally to describe the determination of the specific gravity and density of solid plastics in forms such as sheets, rods, tubes, or moulded items.In this study, density value for each product was found by dividing mass of the sample with its volume.

Water absorptivity test.
Water absorption test was conducted to determine the amount of water absorbed under specified conditions.The process to conduct water absorption test was by cut all five sample into 100 mm of length.The sample were dried in an oven for one hour and around 60℃ of temperature.Then, the specimen was cooled down.After that, the specimen was soaked in water at a temperature 23°C for one week.Then, the samples were removed, patted dry with a lint free cloth, and weighed.The amount of water absorption was calculated in percentage by using Equation 1.

Scanning electron microscopy.
The machine used in this study was TESCAN VEGA Compact.
The sample was the fractured side of the tensile tested sample.Length of each sample was taken as 10 mm.Before the SEM analysis was carried out, each sample was coated with a gold layer to enhance the image quality.The image taken were in magnification of 1000x and 2000x.This test examined the bonds of ABS plastic and rCB.

Results and discussions
Results of this study are shown in Figure 1 and Figure 2. In general, the results shows that the addition of rcB at different weight loadings and sizes have significant effects on mechanical and physical properties of the filament.provide the highest tensile strength.Amin et al [5] reported that, adding filler material such as rubber or carbon with right composition can enhance efficient load transfer and create strong network structures which can increased mechanical properties of the new composite.Thermoplastics usually gives high elongation at breaks, but with the addition of filler content it can reduce or increase the specific properties of this material.In this study, adding the recycled carbon black as filler can improve the elasticity.Figure 1 b) shows that pure ABS plastic has lower elastic modulus at 1143.97 MPa compared to the sample which reinforced with the rCB.The highest tensile modulus is the ABS with 63 µm and 5 wt.% of the filler at 1314.95 MPa.It can be said that by adding the rCB for filler, the elastic properties can be improved and the strain percentage was reduce.This conclusion can be related to the fact that the rCB has rubberise properties compared to the pure ABS plastic sample, which was more rigid and impact resistance.However, the result in Figure 1 is not conclusive due to large error bars which could be due to non-uniform of rCB in the filament.
In Figure 2 a), the differences in term of thicknesses may due to the forced that applied to push the filament.The 3D printer filament as created using a process of heating, extruding and cooling plastic to transform the raw material into the finished product.The process of extruding filament is not same as the process in a FDM 3D printer application where the filament is pulled rather than pushed through the nozzle to create the filament.Differences in the force applied will change the diameter of the filament.Higher force leads to a filament with smaller thickness.Degree of the forces depends on the material composition.Based on Figure 2 a), it can be seen that the sample with 75 µm and 2.5 wt% has the greatest force.The surface roughness results in Figure 2 b) indicates that the ABS with 63 µm and 5 wt.% of the filler has the highest value of the surface roughness which is 1.096 μm followed by ABS with 63 µm and 2.5 wt.% with a value of 0.839 μm.Higher value of surface roughness means that the surface of the sample is rough and not smooth.Based on Figure 2 b) shows that the 2.5 wt.% of filler percentage can produce smooth surface finish compare to the 5 wt.% of filler percentage.The higher percentage of the filler can produce the rougher surface for the filament.The larger particle size of the filler component can have a progressively more dominant effect on the surface.
From the density test result in Figure 2 c), it can be seen that the pure ABS plastic has the highest density value, which is 1.191 g/cm3 compare to another sample.According to data sheet, density of the rCB used in this study is around 0.548 g/cm3, which is lower than the density of the ABS plastic.The density of the reinforced sample are also low because of pores in the sample which can allowed air traps into the filament.
For the water absorptivity, the pure ABS plastic has lower percentage of water absorptivity at 0.019 % compared to other products with rCB filler.The highest product that can absorb water is 0.792 % which is the ABS with 75 µm and 5 wt.% of the filler.This shows that the addition of rCB increased the water to absorb in the filament.According to a study by Al-Ajaj [6], mixing composites with rCB can enhance the moisture abortion into the filament due to chemical reaction between two different types of material.Other factor that influences the higher water absorption sample is pores in the sample which can allowed the water to enter the sample through capillary action.
Figure 3 shows images of scanning electron microscopy of the tensile fractured surfaces, comparing between different rCB sizes used.It is clear that morphology of the sample which have filler sizes of 75 µm and 63 µm are significantly difference.Figure 3 a) show a fewer pores and cracks compared to Figure 3 b).This happens due to the rCB has not mixed well with the ABS and caused air to be trapped.This can cause the reduction in the tensile strength of the sample.From the tensile strength result in Figure 1 a), it shows that ABS with 63 µm of filler can produce higher tensile stress compare to the product that has 75 µm of filler.The SEM images show that bonding between the surface of the ABS plastic matrix and the recycled carbon black filler is good enough.Based on the results in a study by Akonda et al [7], the thermoplastic surface is a non-polar chemical bond and recycled carbon black will melt after being heated.This causes the reaction between the two materials and fused together.Some of the pores are also caused by air trapped during the filament extruding process.

Conclusions
In overall, the introduction of rCB affects mechanical and physical properties of ABS products.Specific conclusions are as follows: • Filler weight and filler size are the important keys to improve the mechanical properties.However, if the filler weight is higher, the maximum tensile strength may be dropped.Higher filler weight can also improve the elastic properties of the filament.• More filler introduction can lead to air trapped and internal crack of the product, which leads to low density and high water absorptivity.• Higher filler size could cause a rougher surface, which may not be ideal for FDM filament as the filament needs to travel inside a tube in FDM machine.Rough surface could cause the tube to be blocked or filament breakage.

Figure 1 .
Figure 1.Tensile properties of the products a) tensile strength, b) tensile modulus

Figure 2 .
Figure 2. Physical properties of the products a) thickness, b) surface roughness, c) density, d) water absorptivity

Figure 3 .
Figure 3.The SEM interface of ABS with different particle size of rCB after tensile testing a) 63 µm filler size, b) 73 µm filler size

Table 1 .
List of product and composition.