Experimental Study of Reinforcing Mechanical Properties of Nylon-11 by Selective Laser Sintering ^†

Abstract

The main objective of this project was to study the effect of diatomaceous earth (DE) on nylon-11 reinforcement. A Selective Laser Sintering™ (SLS) (SLS uses a high-powered laser to sinter small particles of powdered material to create solid 3D parts) machine was used to sinter polymers with different percentages of DE to form composite mixtures and then rapidly prototype into the tensile bars to be tested in the tensile testing machine. The outcome of the investigation definitely indicated that DE is a feasible option for use as a reinforcement to strengthen the physical properties of polyamides when used in rapid prototyping.

1. Introduction

Polymer reinforcement through the use of fillers is a commonly accepted practice. Once bonded, the mechanical properties of the filler contribute to the overall physical and mechanical properties of the composite. A wide variety of fillers and their effects on new composites have been studied. To create different plastic or metal composite parts, a method of laser sintering through selective laser sintering (SLS) has been successfully utilized by many researchers. Chung and Das investigated the fabrication of functionally graded nylon-11 filled with different volume fractions of glass beads (0–30%) by selective laser sintering (SLS 3D) and tested their composites in both tension and compression [1,2]. Hon and Gill used silicon carbide (SiC) F240 blended with the commercially available Duraform Polyamide as an option for direct production in a SLS process [3,4]. According to findings by [5], mechanical performance in the SLS process correlates strongly with the volume energy density, which includes the effect of varying layer thickness and can be related directly to the melting characteristics of the polymer material.

SLS is one of the most rapidly developing prototyping techniques, which allows us to create full-scale physical prototypes for testing or to rapidly produce 3-dimentional functional parts. SLS operates by using a leveling roller to apply the mixed composite powder to the powder bed, and then a laser melts the powder in the form of the solid model for that layer. The bed then drops, and the next layer is applied and melted. This process is continued until the full build is completed.

The benefit of laser sintering is that parts are developed uniformly and manufactured in a single solid state, whereas normal manufacturing requires different components of a single part to be manufactured separately and then welded or bonded together. For example, a commercial aircraft company uses SLS to reduce the amount of parts of a single air flow channel from 18 individual components to 1 in their aircraft. The high speed manufacturing process reduces the necessary number of components to machine by combining the milling and laser cladding as discussed in [6].

Although there are many different materials that can be used for rapid prototyping, nylon based polymers are most suitable to process by SLS and to produce prototype models [7]. The commonly applied SLS powder to date is polyamide 12 (PA12). This polyamide is “approximately 90% of complete industrial consumption” and the remaining amount is distributed on polyamide 11 (PA11) and some other “exotic” polymers. Schmid et al. [8] pointed that “Industry is awaiting commodity polymers… to open new market segments”.

With each study, new composites are being developed with different physical and mechanical properties, which exhibit better strength than each individual material. This gives the rapid manufacturing industry an increasing variety of plastics to implement and establish new processes to design fully functional and viable prototypes.

The main objective of this research was to investigate the effect of adding different percentages of diatomaceous earth (DE), also known as diatomite, to nylon-11 and how it affects the mechanical properties of the new composite material when sintered in a SLS system.

The key reasons for nylon-11 and DE to be selected for this work were that both materials can be bio-derived, they do not require as much energy to produce as conventional materials, they are practically inexhaustible renewable resources, and SLS Nylon/DE would find many applications.

In previous scientific studies (i.e., [1,9]), materials like glass bead spheres were commonly used as fillers for polymers and, in particular, for nylon-11. It was determined that these beads require coatings of silane (Silane is monomeric silicon compounds with four substituents, or groups, attached to a silicon atom) for the nylon-11 powder to bond with during the sintering process.

Since diatomaceous earth has not been largely investigated as filler at this point, the aim of this research was to answer some important questions: first, will the polymer bond as expected to the diatomaceous earth through its porous surface; second, will the silane coating used for other materials be needed; and third, how does the bonding affect the polymer’s mechanical properties.

2. Materials and Methods

To obtain well-bonded blends within the composite specimens, rapid prototyping technique by the Selective Laser Sintering machine (SLS 3D Model) was used. The changes of physical properties of blended nylon-11 and diatomaceous earth were investigated through testing of composite polymer specimens in a Satec Tensile Testing machine (T20000, 98 kN force capacity).

2.1. Materials

2.1.1. Nylon-11

Nylon-11 is a polymer from the polyamide family. Its mechanical properties, listed in

Table 1. Physical and mechanics properties of nylon-11.

Density (g/cc)	Ultimate Strength (MPa)	Yield Strength (MPa)	Young’s Modulus (GPa)	Percent of Elongation at Break	Percent of Elongation at Yield	Melting Point (°C)
1.0–1.05	41.0–68.9	15.0–44.0	0.18–1.80	30.0–400	5.0–42.0	175–191

, show that this polymer can deform by 30–400% (at break) of its original length in tensile testing and may not be stiff enough (yield strength ranges ~15–44 MPa) for some components. One of the important advantages of this polyamide is that it has an ability to accept high loading of fillers, low water absorption, and good chemical resistance [10]. Therefore, it was assumed that it might be possible to increase its stiffness by blending it with filler, like diatomaceous earth, which has a much lower density (0.152–0.208 g/cc), but a much higher melting point (1715 °C) than the nylon-11 (175–191 °C).

2.1.2. Diatomaceous Earth

Diatomaceous earth (DE) represents a material composed of the fossilized remains of diatoms, a chalk-like sedimentary rock, consisting almost entirely of silica. It is a naturally occurring, soft rock that can be crumbled into a fine white to off-white powder. DE has the advantage of being a natural, abundant, and cheap resource that accumulates in oceans or fresh waters.

As determined in the work of Dragan et al. [11], diatoms are characterized by size in the range from under 5 to over 100 µm and structure with openings as small as 0.1 µm in diameter. A magnified view of the diatomaceous earth under the scanning electronic microscope (SEM) in Figure 1 clearly shows that it is a highly porous (Porosity is a measure of the “empty” spaces in material) material with a complex framework. Diatomite consists of billions of minute silica frameworks. The frustules are strong and support intricate internal frameworks with maximum void space.

The small size of the open pores gives diatoms excellent particulate holding and removal properties for filtration. In addition, diatomite has low density, absorptive capacity and it has an extremely high surface area per unit of volume. The physical properties of DE that are relevant to this research are shown in

Table 2. Physical properties of DE.

Density (g/cc)	Diatom Size (μm)	Melting Point (°C)
0.152–0.208	5–100	1715

.

Diatomaceous earth is also sustainable, constantly regenerating, and “removes” as much carbon dioxide as all the rainforests in the world combined [12]. This powder is commonly applied in different areas: medicine and bioprocessing, food and beverage industry, purification of potable water, contaminated ground and surface water, decontamination of sewage liquids and waste water, pharmacy, etc.

2.2. Method

2.2.1. Preparation of Blend

A large portion of this research focused on creating weight-percentage-based blends of polymer with allocated filler. In this process, the first step was to calculate the tapped density of the diatomaceous earth. In order to do this, a container of a known volume and weight was filled to the brim and patted down to remove all of the air. Then, the container was filled to the brim and patted down again, leveled off, and weighed. The weight of the filler was measured, subtracting the weight of the container. This yielded the volume and weight needed to calculate the tapped density and to ensure the proper proportion of weight and volume of the DE to be added to a known volume and weight of nylon-11. As the percent of DE by weight and volume increases so does the tapped density of the blend. Once the appropriate amount of DE was determined, the first blend (10%) was achieved and then placed in the bin of the SLS machine and laser sintered. Each subsequent blend increased by volume fraction of filler by five percent.

To ensure proper mixing before it is placed in the SLS bin, each blend was put into a drum tumbler and allowed to mix for approximately 15 min. Then, it was checked by a particle analyzer to examine the morphology and confirm the distribution of particles is as uniform as possible.

Table 3. Percent of blends by volume of nylon-11 and diatomaceous earth.

Percent of Blend	Volume (Quart 1)
	Nylon-11	Diatomaceous Earth
10	39.960	3.996
15	34.504	5.176
20	31.320	7.830
25	27.020	9.005
30	23.625	10.125

¹ quart = 0.946 L.

shows the composition of nylon-11 and diatomaceous earth for all tested specimens.

2.2.2. Test Specimen

Five different blends by volume fraction were created, and five separate composite specimens (tensile bars) from each blend were manufactured (for a total of 25 test specimens). The particle size of the nylon-11 was roughly 80 µm, while the particle size of the diatomaceous earth was approximately 60 µm. The difference was selected since the diatomaceous earth had a greater range of size variation. The specimens were fabricated by the SLS machine parallel to the build direction to be tested. Each tensile bar was made according to the ASTM D638 standard for plastics [13], with specimen geometry and raster direction shown in Figure 2.

2.2.3. Experimental Procedure

Information about the mechanical properties of pure nylon-11 and the nylon-11 blended with the diatomaceous earth was obtained through laboratory testing at room temperature, where each specimen underwent a slowly increasing axial force in the tensile testing machine until fracture of the composite specimen occurred. From the data of the tensile tests, the corresponding values of stresses and strains for each blend were calculated and were then averaged to report them as the specific properties of each composite material. To quantify the amount of variation of a set of experimental data, the standard deviation for strengths and strains at break for each fabricated specimen was calculated. Scatter plots, shown in Figure 3, allow us to visualize the distribution of mechanical properties along their deviation values.

The variances in deviations of these blends may only be explained by the fact that there was more variability in test batches as the percentage of filler increases. This could have been caused by non-uniform distribution of filler particles during the preparation process in a drum tumbler. The hand calculation of the cross-sectional area of specimens, which is subject to human error, may also affect the values of calculated mechanical properties.

3. Results and Discussion

3.1. Analysis of Mechanical Properties of the Reinforced Nylon-11

Table 4. Experimental mechanical properties of pure nylon-11.

% of DE	Area (m2) (10−5)	Yield Strength (MPa)	Ultimate Strength (MPa)	Young’s Modulus (GPa)	% of Strain at Break
0	6.90	10.47	26.86	0.97	46.55
10	3.98	10.41	41.18	1.90	18.12
15	4.74	17.14	40.24	1.88	23.87
20	3.99	19.47	37.93	2.02	11.89
25	4.21	19.38	37.11	2.21	8.51
30	4.25	20.55	38.85	2.39	4.87

shows the averaged experimental results of tensile properties for pure nylon-11 and 10%, 15%, 20%, 25%, and 30% fractions of diatomaceous earth in nylon-11 to be used as the specific mechanical properties of the reinforced nylon-11.

The tabulated data clearly show that the addition of the filler to the polymer will affect its mechanical properties. As the percentage of filler increases, the properties of the blend change noticeably with respect to the native properties of the pure nylon-11. It becomes less elastic as the filler is added, and it may be expected that the blend with the higher percentage of filler, perhaps 30% or greater, will get the properties of the brittle material. However, more testing needs to be done in order to determine an exact percentage of the filler that will make the blend brittle.

The results of experimental testing are used to graphically show the relationship between the specific mechanical properties of the reinforced nylon-11 and predict its behavior as the percentage of filler varies. The plots in Figure 4 demonstrate the trends in changes of strengths and elongations of composite polymers with increasing percentage of filler.

It can be observed that the values of the yield strength and modulus of elasticity have the tendency to increase and the ultimate strength (almost up to 30%) and elongation tend to decrease as the fraction of diatomaceous earth in blend increases. These patterns do comply with each other. If the material yields at a higher strength, it will be stiffer and will be more resistant to elongation. The increase in ultimate strength can be seen as a result of the material becoming more brittle as it stiffens and the increase in modulus of elasticity can be due to the material becoming less elastic, which coincides with the other property trends. Based on these results, it can be predicted that if at some point the ratio of mixture becomes too great in favor of diatomaceous earth, it may eventually cause the composite to become too brittle to use. This will allow the laboratory to select the appropriate blend to have a material available with necessary mechanical properties.

3.2. Crack Inspection

The crack edges of the pure nylon-11, the 10% and the 30% blends, shown in Figure 5, Figure 6 and Figure 7, were examined under a scanning electron microscope (SEM, 3XME) with a 2 nanometer (nm) resolution to view the bonding effects of the material, and determine if this bonding had any effect on the cracks.

The 10% blend tensile bar in Figure 6a breaks more linearly along the edge. The fibers visible in the break of pure nylon-11 are no longer visible here. The composite material appears slightly more porous in the image shown in Figure 6b, where it has clearly seen patches of darker sections that are comprised of the diatomaceous earth. Although it is still the high content of nylon-11, the overall composition remains smooth.

The edge break of the 30% blend in Figure 7a seems to be cleaner compared to the fiber shown in Figure 5. This composite material appears much rougher along the surface seen in Figure 7b due to the larger amount of DE in the mixture and more porous than either of the previous images. This may be confirmed with the graph in Figure 4b indicating a sign of brittleness of the tensile bar with little elongation comparing to the pure nylon-11.

It appears from the images that as the diatomaceous earth increased in the blend, the material bonded more porously and the material broke more cleanly along the edge. The lack of fiber along the edge in the blends would mean the break occurred more cleanly because nylon-11 bonded well to the uncoated diatomaceous earth. This answers questions about the ability of the nylon-11 to bond to the porous diatomaceous earth without the silane coating used on other fibers. If the bonds were poor, future research should add the silane coating to improve bonds.

4. Conclusions

This study indicates that diatomaceous earth (DE) is a feasible option for use as a reinforcing filler in nylon-11 powders during the selective laser sintering process. The study produced results that demonstrated the trends in the relationship between the mechanical properties of the reinforced nylon-11 with an increasing percentage of filler. To find the broader spectrum of relations between the proposed filler and polymer, it is recommended to investigate fabrication of functionally graded blends by selective laser sintering (SLS) of nylon-11 filled with higher volume fractions (>30%) of diatomaceous earth.