Investigation of mechanical properties of plastic composites produced using wood and eggshell

Abstract

In this study, it is aimed to produce wood plastic composite material with eggshell additives and to investigate the mechanical properties of these materials. Mechanical properties such as tensile strength and modulus of elasticity, elongation at break, flexural strength and modulus of elasticity and impact resistance of new materials produced by adding eggshell to wood plastic composite at different rates were investigated. It was determined that tensile strength values gradually decreased due to the increase in the amount of eggshell in the composites produced. The modulus of elasticity of the composite material in tensile strength increases with the increase in the amount of eggshell in the mixture. With the increase of the eggshell ratio used, the elongation values of the plastic composite at break are reduced. The eggshell material used in wood plastic composite reduces the bending strength of the material. The eggshell reduces the modulus of elasticity of the composite material in bending. The eggshell composite material did not cause a significant change in impact resistance.

Keywords

Wood plastic composite polypropylene eggshell mechanical properties

Introduction

Due to its practicality, lightweight nature, and ease of availability, wood as a material predates others such as concrete and steel.¹ Wood has been preferred throughout history not only for its easy access and presence in nature but also for its versatility. Its various forms of use have evolved from past to present, with its most significant application being as a natural building material to meet people's housing needs.²

Wood materials are widely used today because they are renewable and one of the least harmful materials to the environment. In addition to the quantitative dimension of natural materials, its effect on human psychology is an important feature.^3,4 Wood materials have wide usage areas due to their unique properties. Wood material, used in furniture, forestry enterprises, glue and chemical material production areas, sawmill and carpentry workshops, paper production area, construction area (prefabricated house construction), is very important because of the intensive use in such branches.⁵

Building on the environmental and psychological benefits of wood, the development of Wood Plastic Composite (WPC) represents a significant advancement in material science.⁶ WPC is a material created by combining wood fibers or flour with thermoplastic polymer resins, marrying the aesthetic appeal and biophilic benefits of wood with the durability and resilience of plastics.^7,8 This innovative material is increasingly used in outdoor applications, furniture production, flooring, and various construction materials, capitalizing on the strengths of both wood and plastic. It exemplifies a sustainable approach to material design, utilizing the renewable aspects of wood fibers and the recyclability of plastic resins, thus contributing to a more sustainable construction industry.^9,10

Central to the composition of WPC is polypropylene (PP), a versatile thermoplastic polymer that enhances the composite's structural integrity.^11,12 PP is made from the polymerization of propylene monomers, with a chemical structure comprising repeating units of propylene, a basic hydrocarbon compound. The formula for polypropylene is typically denoted as (C₃H₆)n, where ‘n’ indicates the number of propylene units in the polymer chain. Polypropylene is composed mainly of carbon (C) and hydrogen (H) atoms.¹³ It is a relatively inert material, meaning it does not react readily with other chemicals. This makes it suitable for a wide range of applications, including packaging, textiles, automotive parts, and various household products. The inclusion of PP in WPC not only provides strength and durability but also offers resistance to moisture and decay, making it an ideal partner to wood fibers in creating materials that stand the test of time and weather.¹⁴

Enhancing the compatibility and performance of WPC, Maleic anhydride-grafted polypropylene (MAPP) serves as a crucial component. MAPP is a modified form of polypropylene where maleic anhydride groups have been chemically attached to the polypropylene polymer chain.¹⁵ The chemical structure of MAPP consists of the polypropylene backbone with maleic anhydride (C₄H₂O₃) groups attached. The maleic anhydride moiety contains carbon (C), hydrogen (H), and oxygen (O) atoms. The introduction of maleic anhydride groups to the PP chain enhances its bonding with the wood fibers, ensuring a stronger and more durable composite. This modification allows MAPP to act as a coupling agent, promoting adhesion between the hydrophobic polymer and the hydrophilic wood fibers, which is essential for the mechanical integrity and longevity of WPC.¹⁶ The synergistic effect of PP and MAPP in WPC formulation results in a composite material that exhibits superior properties suitable for a wide array of applications.¹⁷

Wood plastic composite material produce and it is important to examine the physical, mechanical, thermal and morphological properties of this material.^18,19 WPCs usage areas are increasing in many fields as floor carpets, flower vases, waste paper baskets, parkbenches, picnic tables and plastic lumber.^20,21 Effects of fiber characteristics on the physical and mechanical properties of WPCs are investigated.^22,23 WPCs from agro-waste materials are producted and its mechanical properties are studied.^24–26 The effects of material compositions on the mechanical properties of WPCs are investigated.^27,28

In this study, WPC were produced to utilize eggshell waste. The effects of these composites on mechanical properties are being investigated in detail. Eggshells, as an as yet underutilized waste type, offer innovative applications in sustainable material technologies.^29,30 With a high content of calcium carbonate, eggshells possess a porous structure.³¹ The study aims to address the question of how eggshell additives affect the mechanical properties of WPC.

Building upon this foundation, the current research explores the production of WPC by combining wood flour, PP, eggshell, and MAPP in specific ratios. This approach aims to investigate the mechanical performance of the composites, utilizing the porous structure and calcium carbonate content of eggshells. The study will meticulously evaluate the effect of the eggshell additive on the tensile strength, bending resistance, and impact resistance of the WPC, in accordance with established standards. The comprehensive testing program aims to shed light on the potential of eggshell-enriched WPC in various applications, thus aiming to contribute to the field of sustainable material science.

Experiment and method

Polypropylene (PP) was used as a polymer matrix by purchasing from PETKİM. Waste red pine shavings were used as raw material of wood flour. The eggshell, which is egg waste, was used as filling material. MAPP was purchased from Clariant International as a non-compatibilizer.

Red pine shavings were ground into wood flour to be used in composite material. The obtained wood flour was passed through a 60 mesh sieve and the wood flour remaining on the 80 mesh sieve was used. The wood flour was then dried in an oven at 103 ± 2°C until it reached a moisture level of 1%–3% for 48 h.

The eggshell used in the composite was washed with pure water to remove the egg from the residue. The eggshell was then dried in the oven until it reached 1%–3% moisture for 12 h and ground using a Wiley mill.

Composite sample production was made according to the production recipe given in Table 1 and work flow chart shown in Figure 1. Polypropylene, wood flour, eggshells and homogenous composite mixture were obtained by using a high speed mixer in the proportions specified in MAPP production recipe. The homogeneous mixtures were passed through a single-screw extruder machine with a laboratory type temperature of 90–175°C. Due to its high temperature, the polymer material melts to form a better mixture with wood flour and eggshell.

Table 1.

Test sample rates.

Groups	Red pine wood flour (%)	PP (%)	Eggshell (%)	MAPP (%)
A	30	70	0	0
B	30	65	5	0
C	30	60	10	0
D	30	55	15	0
E	30	50	20	0
F	30	67	0	3
G	30	62	5	3
H	30	57	10	3
I	30	52	15	3
J	30	47	20	3
K	0	70	30	0

The resulting hot homogeneous mixture was cooled in cold water and the water was filtered off. Then the produced material was turned into pellets with the help of crusher. The pelleted material was dried in the oven for 12 h until it reached 1%–3% moisture. Composite samples were produced from dried pellet material by injection molding machine in the temperature range of 180 °C–200°C according to ASTM standards (Figure 2). In this process, the injection speed was 80 mm/sec, the screw speed was 40 r/min and the injection pressure was 100 bar. More information on the production of WPC samples can be found in the master thesis.³²

Scanning Electron Microscope (SEM) image was obtained for structural analysis of produced eggshells and wood plastic composite samples (Figure 3). As seen from the SEM image, a weak bond appears between the eggshell and the wood fibers and the PP matrix. In addition, it is understood that the eggshell separates from the matrix and creates gaps and the mixture is not homogeneously distributed in the eggshell, wood flour and PP polymer matrix. The reason for the lack of homogeneous distribution in the samples is thought to be that the eggshell particles are not equal in size. This situation was considered as a reason for the change in the the mechanical properties of the composite samples.

Mechanical properties such as tensile strength and modulus of elasticity, elongation at break, flexural strength and modulus of elasticity and impact resistance of the composite samples were investigated by the following methods.

Tensile strength tests were performed on the Zwick/Roell Z010 Universal testing machine at a test speed of 5 mm/min. Five tensile samples were prepared and tested in accordance with ASTM D 638 standard,³³ which was prepared at tensile strength test at 0.01 mm sensitivity, for each group of 13 mm width, 4 mm thickness and 165 mm length. Equation (1) was used to determine the tensile strength of composite samples.

δ_{t s} = \frac{F_{\max}}{A} (N / {m m}^{2})

(1)where, δ_ts is the tensile strength (N/mm²), F_max is the maximum force at the time of fracture (N) and A is the cross-sectional area of the sample (mm²).

The tensile modulus of elasticity of the composite specimens was carried out on samples conforming to ASTM D 638 standard.¹⁸ Equation (2) is used for elastic modulus in tensile.

E = \frac{δ_{t s}}{ε} (N / {m m}^{2})

(2)where E is the modulus of elasticity at tensile (N/mm²), δ_ts tensile strength (N/mm²) and ε is the unit elongation during the tensile test.

Flexural strength tests of composite specimens were carried out on Zwick/Roell Z010 Universal test machine at a test speed of 2 mm/min and spacing between supports at 80 mm. In bending resistance test, the specimen dimensions according to ASTM D 790 standards,¹⁸ which are prepared at 0.01 mm sensitivity, are 13 mm wide, 4 mm thick and 165 mm long and bending resistance measurements were made by using five samples for each group. Equation (3) was used to determine the tensile strength $σ_{e}$ .

σ_{e} = \frac{3 F_{\max} L}{2 b d^{2}} (N / {m m}^{2})

(3)where L is the clearance between the abutment (mm), F_max is the maximum force at the time of fracture (N), b is the sample width (mm) and d is the sample thickness (mm).

The modulus of elasticity of bending of composite samples was determined in accordance with ASTM D 790 standard.¹⁸ The flexural modulus E was calculated using equation (4).

E = \frac{F l^{3}}{4 b d^{3} ∆} (N / {m m}^{2})

(4)where F is the force (N) that provides the deformation, Ɩ is the distance between the fulcrum (mm), d is the sample thickness, b is the sample width (mm) and Δ is the amount of bending (mm).

The impact resistance of the composite samples was made in accordance with ASTM D 256 standard,³⁴ which was prepared with a sensitivity of 0.01 mm. Five samples were produced for each group, 13 mm wide, 4 mm thick and 64 mm long. A notch of 0.25 mm radius was made on composite samples using the notching tool produced by Polytest RayRan. Impact resistance was measured using Zwick Roell brand HIT5.5P model pendulum shock tester. The impact resistance (fracture energy) $G_{f}$ is calculated using equation (5).

G_{f} = W (h - h_{1}) = W l (\cos β - \cos α)

(5)where W is the weight of the pendulum (N), l is distance of the pendulum center of gravity to the oscillation center (m), h₁ is output height of center of gravity of pendulum (m), h is fall height of the center of gravity of the pendulum (m), β rise angle and α is the angle of fall.

Results and discussion

In this study, wood flour, polypropylene (PP), eggshell and MAPP were mixed in certain proportions to produce wood plastic composites. The effect of the eggshell on the mechanical properties of the produced WPCs was investigated. Tensile strength (ASTM D 638), bending strength (ASTM D 790) and impact resistance (ASTM D 256) tests were performed to determine the mechanical properties of composite samples.

The tensile strength, tensile elasticity modulus and tensile elongation measurements of wood plastic composite samples are obtained. The bending resistance, bending elasticity modulus, impact resistance and hardness measurement values of WPC samples are determined.

Tensile strength

The tensile strength measurement values and standard deviations of composite materials produced by mixing pine wood flour, eggshell, PP and MAPP in certain proportions are shown in Table 2. Tensile strength varies with decreasing of 23.5–19.9 N/mm² in composite samples A, B, C, D and E without MAPP. In the F, G, H, I and J groups produced using MAPP, tensile strength decreases in the range of 30.8–25.5 N/mm². The tensile strength of groups A and K is 23.5 and 22.5 N/mm², respectively.

Table 2.

Tensile strength, tensile elasticity modulus and elongation at break of wood-plastic composite materials.

	Tensile strength (N/mm²)		Tensile elasticity module (N/mm²)		Elongation at break
Groups	Average	SD	Average	SD	Average	SD
A	23.5	0.44	730	11.7	5.87	0.29
B	21.9	0.37	841.4	17.9	5.26	0.20
C	21.2	0.33	848.7	5.5	4.60	0.32
D	20.5	0.18	788.3	17.5	4.42	0.17
E	19.9	0.62	765.5	18.4	4.81	0.17
F	30.8	0.12	708	9.1	5.64	0.11
G	28.2	0.50	764.3	9.4	4.96	0.20
H	27.8	0.43	730.4	18.2	4.58	0.10
I	26.4	0.40	824.1	20.81	3.93	0.18
J	25.5	0.88	878.5	16.6	3.84	0.10
K	22.5	0.70	553.6	15.6	12.08	0.11

F, G, H, I, and J group composites produced by using MAPP have higher tensile strength than A, B, C, D and E group composites produced without MAPP that is shown in Figure 1. It is understood that MAPP additive used in composite increases the tensile strength. It was found that the tensile strength values decreased gradually due to the increase of the amount of eggshell in the mixture in both MAPP doped and non-MAPP sample groups. When the tensile strength of the K and A control group composites is compared, the tensile strength of the A group composites is higher than the K group, although the plastic matrix material is the same. In this comparison, eggshell has a lower tensile strength compared to wood flour. Figure 4 shows that the tensile strength of PP, which is used as a matrix, is higher than the eggshell.

Tensile elasticity module

Table 2 shows the elasticity measurement values and standard deviations of composite materials produced by pine wood flour, eggshell, PP and MAPP mixture. If tensile modulus of elasticity is evaluated on the basis of samples, the groups A, B, C, D and E produced without MAPP vary in the range of 848.7–730 N/mm². In the F, G, H, I and J groups produced using MAPP, tensile modulus of elasticity decreases in the range of 878.5–708 N/mm². The tensile modulus of elasticity of groups A and K is 730 and 553.6 N/mm², respectively.

The tensile elasticity module value graph of the composite groups is given in Figure 5. As shown in Figure 5, the F, G, H, I, and J group composites produced using MAPP and the A, B, C, D and E groups produced without MAPP additive exhibit similar behavior in tensile modulus of elasticity. While the amount of eggshell increases, the modulus of elasticity increases in tensile and peaks in the values of groups C and I. With the greater increase in the amount of eggshell in the mixture, the tensile elasticity modulus decreases after peaking. When tensile elastic modulus values of K and A control group composites are compared, the tensile elastic modulus values of group A composites are higher than those of group K, although the plastic matrix material is the same. In this comparison, it is seen that the elasticity modulus of the eggshell is low in shrinkage compared to the wood flour.

In previous studies, it has been noted that chemical powders and wood flour impart an effective stress transfer from the polymer matrix to the reinforcing filler, and that organic and inorganic fillers harden the material, altering mechanical behavior, and significantly reducing deterioration tension. The use of eggshell as an additive in WPCs changes the mechanical properties of the material.^35–37

Elongation at break

Figure 6 shows the elongation at tensile breakage of composite sheets produced by mixing wood flour, eggshell, PP and MAPP in certain proportions. The elongation values of the rupture of the wood plastic composite with eggshell were investigated. It was observed that the elongation values of the rupture values decreased with the increase of the eggshell ratio used in the wood plastic composite content produced. In the tests, elongation at break is measured between 3.84%–12%. Sample groups A, B, C, D and E were produced without the use of MAPP. The rate of eggshells in group A produced as control group is 0%. The elongation at break decreases of B, C, D and E composite sample groups whit eggshell additive ratio is increasing by 5% respectively. F, G, H, I and J groups were produced by adding 3% MAPP to wood plastic composite and increasing eggshell additive ratio by 5% respectively. The increase in eggshell appears to decrease the elongation at break of composite material produced using MAPP. Another striking result is the elongation at break rates between the A, F and K groups. These three groups are the same plastic matrix composites. Group A 30% wood flour and 70% PP; F group 30% wood flour, 3% MAPP and 67% PP; and K group consists of 30% eggshell and 70% PP. Although the A, F and K groups have the same matrix, the eggshell increases the elongation at break in comparison to the wood flour. Due to the 3% MAPP used in the F group, there is a slight decrease in elongation at breaks compared to the A group. With the increase of the amount of wood flour and eggshell in the mixture of samples, elongation at break is decreased.

Bending resistance

The bending resistance measurement values and standard deviations of composite materials produced by mixing pine wood flour, eggshell, PP and MAPP in certain proportions are shown in Table 3. The bending resistance of the A, B, C, D and E groups produced without the use of MAPP varies with decreasing range of 42.9–39.9 N/mm². In the F, G, H, I and J groups produced using MAPP, bending resistance decreases in the range of 53–47.6 N/mm². The bending resistance of the A and K groups were 42.9 N/mm² and 40.1 N/mm², respectively. The bending resistance graph of the composite sample groups is given in Figure 7.

Table 3.

Bending resistance, bending elasticity modulus, impact resistance values of wood-plastic composite materials produced.

	Bending resistance (N/mm²)		Bending elasticity module (N/mm²)		Impact resistance (N/mm²)
Groups	Average	SD	Average	SD	Average	SD
A	42.9	0.51	2210.2	23.7	3.33	0.08
B	41.9	0.75	2318.6	44.9	2.74	0.06
C	42	0.28	2470.6	18.6	3.03	0.07
D	41.1	0.50	2458.5	37.1	2.97	0.08
E	39.9	0.72	2281.3	27.6	3.00	0.10
F	53	0.72	2377.9	40.3	3.07	0.05
G	51.8	0.31	2492.8	61.1	3.31	0.04
H	50.7	0.73	2613.6	36.1	3.00	0.05
I	48.6	0.52	2712.9	9.24	2.97	0.07
J	47.6	1.1	2912.5	11.7	2.94	0.05
K	40.1	0.37	1529.7	43.5	2.97	0.03

As shown in the graph in Figure 7, it was found that the F, G, H, I and J group composites produced using MAPP were higher than the composites A, B, C, D and E produced without MAPP. It is understood that MAPP additive used in composite increases bending resistance. It was found that the bending resistance values decreased gradually due to the increase of the amount of eggshell in the mixture in both MAPP doped and non-MAPP sample groups. When the bending resistance of the K and A control group composites was compared, the bending resistance of the A group composites was higher than the K group, although the plastic matrix material was the same. Eggshell, compared to wood flour is a result of low bending resistance. The bending resistance of PP, which is used as a matrix, is also higher than that of eggshell.

Bending elasticity module

The bending elasticity modulus measurement values and standard deviations of composite materials produced by mixing pine wood flour, eggshell, PP and MAPP in specific ratios are shown in Table 3. Bending elasticity modulus of A, B, C, D and E groups produced without MAPP are increasing in the range of 2210.1–2281.3 N/mm². In the F, G, H, I and J groups produced using MAPP, the modulus of bending elasticity increases in the range of 2377.9–2912.5 N/mm². The bending elasticity modulus of A and K control groups is 2210.1 N/mm² and 1529.7 N/mm², respectively. The bending elasticity modulus of composite sample groups is given in Figure 8.

The bending elasticity modulus increases due to the increase of eggshell in F, G, H, I, and J group composites produced using MAPP. While the amount of eggshell increased in the A, B, C, D and E groups produced without MAPP additive, the bending elasticity modulus values increased and tended to fall after peaking in the C group. It is seen that the elastic modulus value of the bending composite groups with MAPP is higher than the groups produced without using MAPP. When the elastic modulus values of bending of K and A control group composites are compared, the elastic modulus values of bending of A group composites are higher than the K group, although the plastic matrix material is the same. Eggshell, compared to wood flour is a result of low modulus of bending elasticity. Polypropylene based wood composites have higher bending elasticity modulus than Low-density polyethylene and High Density Polyethylene composites.³⁸

Impact resistance

The impact resistance values and standard deviations of composite materials produced by mixing pine wood flour, eggshell, PP and MAPP in certain proportions is shown in Table 2. In the impact resistance of composite samples, physical mixture, particle size, microstructure and temperature factors play an important role. In this study, impact resistance measurements were performed at room temperature. Impact resistance values show the ductility and brittleness properties of the produced material. İmpact resistance A, B, C, D and E groups produced without using MAPP vary between 3.32 and 2.74 kJ/m². The impact resistance of F, G, H, I and J groups produced using MAPP is in the range of 2.94–3.31 kJ/m². The impact resistance of the control groups A and K is 3.33 kJ/m² and 2.97 kJ/m² respectively. The impact resistance graph of the composite groups produced is given in Figure 9.

When the impact resistance values of the K and A control group composites were compared, the impact resistance values of the A group composites were higher than the K group, although the plastic matrix material was the same. In addition, there are small fluctuations in the impact resistance of composite materials produced using eggshell.

Conclusions

In the scope of this research, a novel WPC material, augmented with eggshell as a bio-filler, was synthesized to evaluate its mechanical performance. The utilization of various fillers in the enhancement of WPCs is a well-established practice aimed at improving material properties while also achieving cost-effectiveness in production. Specifically, this study has integrated eggshell waste into the WPC matrix, thereby investigating the resultant alterations in the composite's mechanical characteristics. The incorporation of eggshell as a filler was meticulously examined to discern its impact on the overall structural integrity and performance of the WPC material.

Red pine wood flour, polypropylene, MAPP and eggshells were mixed at certain percentages. As described in the method section, 11 groups of eggshell doped in different proportions and at least five samples were produced in each group experiment. The mechanical properties of the composite materials such as tensile strength, tensile elastic modulus, elongation at break, bending resistance, modulus of elasticity at bending, impact resistance were investigated. The effect of eggshell on the mechanical properties of wood plastic composite was investigated.

1. It is understood that MAPP additive used in wood plastic composite material production increases tensile strength. It was found that the tensile strength values decreased gradually due to the increase in the amount of eggshell in the mixture in both MAPP doped and non-MAPP sample groups. The eggshell is the result of low tensile strength compared to wood flour.

2. With the increase in the amount of eggshell used in the production of wood-plastic composite material, a decrease in the tensile modulus of elasticity was observed. Comparisons made indicated that eggshell has a lower tensile modulus of elasticity compared to wood flour.

3. The elongation at break is reduced as the eggshell ratio used in the wood-plastic composite content increases. While the amount of eggshell increases in the wood-plastic composite content, it decreases the elongation at break with the decrease in the amount of PP in the mixture. The elongation at break is higher compared to the eggshell wood flour.

4. The MAPP additive used in the composite increases the bending strength. Eggshell used for filling purposes in wood plastic composite has a low bending resistance compared to wood flour. The bending resistance of the PP used as the matrix also appears to be higher than the eggshell value.

5. The MAPP additive in wood plastic composites increases the modulus of elasticity of the composites in bending. The eggshell has a low modulus of elasticity in bending compared to wood flour.

6. The use of eggshells in wood-plastic composite materials did not cause a significant change in the impact resistance of the material, but caused small fluctuations.

Footnotes

Acknowledgements

We would like to thank Kahramanmaras Sutcu Imam University (KSU) Scientific Research Projects Coordination Department for financially supported.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Kahramanmaraş Sütçü Imam Üniversitesi (KSU) Scientific Research Projects Coordination Department, under Project No. 2018/1-7 YLS.

ORCID iD

Ali O Özğan

Data availability statement

The data given in this article are provided for informative purposes and serve as a comprehensive representation of the experimental findings.

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Groups	Red pine wood flour (%)	PP (%)	Eggshell (%)	MAPP (%)
A	30	70	0	0
B	30	65	5	0
C	30	60	10	0
D	30	55	15	0
E	30	50	20	0
F	30	67	0	3
G	30	62	5	3
H	30	57	10	3
I	30	52	15	3
J	30	47	20	3
K	0	70	30	0

Groups	Red pine wood flour (%)	PP (%)	Eggshell (%)	MAPP (%)
A	30	70	0	0
B	30	65	5	0
C	30	60	10	0
D	30	55	15	0
E	30	50	20	0
F	30	67	0	3
G	30	62	5	3
H	30	57	10	3
I	30	52	15	3
J	30	47	20	3
K	0	70	30	0

Groups	Red pine wood flour (%)	PP (%)	Eggshell (%)	MAPP (%)
A	30	70	0	0
B	30	65	5	0
C	30	60	10	0
D	30	55	15	0
E	30	50	20	0
F	30	67	0	3
G	30	62	5	3
H	30	57	10	3
I	30	52	15	3
J	30	47	20	3
K	0	70	30	0