Keywords

1 Introduction

There has long been a focus on the development of an alternative for conventional paper and plastic to improve global sustainability [1]. The manufacturing of synthetic oil-based plastics such as polypropylene (PP), polyethylene (PE) and polystyrene (PS) has led to an insurmountable amount of waste and CO2 emissions accumulating. This study aims to introduce and investigate a recently developed paper/plastic alternative called LimeX, a material primarily made from limestone (CaCO3) with a plastic additive added to act as a binder for the material, to determine its sustainable performance in comparison to that of conventional paper and plastic. In recent research [2,3,4,5], the Product Sustainability Index (ProdSI) has been instrumental in successfully evaluating and improving the sustainability of products. The ProdSI centers around three basic parameters of economic prosperity, environmental protection and societal well-being which are referred to as the triple bottom line (TBL) [3]. Fiksel et al. determined a list of product sustainability indicators to assess the TBL aspects of products [6]. Then a new and improved framework for the evaluation of product design for sustainability was presented by Jawahir et al. [7]. Although subjective, sustainability evaluations taking the lifecycle of a product into account could lead to a more effective evaluation of the total lifecycle sustainability performance [7]. Finally, Shuaib et al., [8] used the metrics from [7], together with new metrics for TBL criteria, to develop the ProdSI and applied it to compare the sustainability performance of two different generations of consumer electronics products [7]. The metrics highlighted in [7, 8] include waste, extraction, emissions, material recovery and efficiency and recyclability in terms of the environmental impacts; material, production, injury, warranty, labor and raw material costs in terms of economics; and ergonomics, ethics, functionality, safety and health in terms of societal impacts. In this study, similar metrics are highlighted to evaluate the sustainable performance of LimeX products.

2 Experimental Analysis of LimeX

A series of experiments were performed to investigate the information provided by the company that manufactures LimeX, TBM [9]. Part of the investigation into LimeX involved confirming that its composition was primarily limestone, as indicated by the manufacturer. In addition, an evaluation of the strength of LimeX paper was conducted to determine whether it was more durable than conventional paper. The measurements obtained from the analysis were used to inform the ProdSI-based sustainability analysis. The mechanical properties of the products play a role in how the product functions and the feasibility of multi-life-cycle applications of LimeX.

2.1 Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy Analysis of LimeX Pellets

Initial electron microscopy characterization of the LimeX surface topography was performed with a FEI Helios NanoLab 660 microscope. Subsequently, the first few micrographs were used to determine if the LimeX pellets were porous.

Fig. 1.
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Electron Microscopy Micrographs of LimeX Pellet.

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Because of the non-conductive nature of the LimeX, there were difficulties in imaging caused by the charging of electrons on the surface. However, this charging led to some insights in the composition of LimeX. Charging, instead of random placement, appeared to be localized in specific areas of the sample seen in Fig. 1. This was likely due to inhomogeneity in composition. Additionally, the charging of electrons made it difficult to capture extremely magnified images as the charging increases as the beam is localized. A lower voltage along with a circular backscatter detector was utilized in combating charging. After initial analysis, the pellets were imbedded into a conductive, graphite puck to prevent charging issues by giving the electrons a path to disperse more quickly. Grinding away the surface and making a planar sample also reveals that the interior surface of the pellet is not porous. Moreover, EDS results (presented in Fig. 3) show anisotropy of chemical composition suggesting LimeX is non-homogeneous.

Fig. 2.
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Electron Microscopy Micrograph used for EDS.

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The voltage was increased from the previous 2kV to 10 kV to allow for better penetration of electrons and more surface detail when capturing images of the pellets inside the graphite puck. The compositional maps in Fig. 2 show that the material is in-homogeneous with very little calcium being present on the surface compared to the amount of carbon and oxygen present on the surface. What’s more, the magnesium present suggests the mined limestone contained dolomite (CaMg(CO3)2) and not just the typical calcite (CaCO3) found in limestone. The complete chemical composition can be seen in Table 1 below. Conventional paper is primarily made of cellulose.

Table 1. Chemical composition of LimeX pellets.
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Furthermore, there is a small portion of silica and aluminum found in the LimeX pellet. These are thought to be imbedded during the polishing process.

Fig. 3.
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EDS Micrographs showing anisotropy of LimeX’s chemical composition.

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2.2 Material Property Testing

A 5 kN Instron Tensile Testing machine and Bluehill Software were utilized to conduct semi-destructive tensile tests that measured the strength and Young’s modulus of four LimeX paper samples with differing thicknesses of 150, 200, 300, and 400 µm. An extension rate of 7.5 mm/min was applied. The experiment was repeated three times per thickness. Table 2 shows the material properties found for LimeX paper. The experimental tensile strength values match the values provided by TBM, but the experimental values of Young’s modulus differ from the values provided by approximately 20% [9]. This error may be ascribed to a difference in the material properties due to changes made by manufacturing processes. To conclude, the strength and toughness values indicate that LimeX paper is stronger and more durable than conventional paper when comparing the tensile strength test to a bursting test [10]. However, conventional paper is anisotropic and dependent on fiber direction leading to a measurement of tensile strength in units per length rather than area.

Table 2. Experimental tensile test results for LimeX paper.
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3 LimeX Sustainability Evaluation

Product sustainability evaluations for product design as researched by Shuaib et al. and Jawahir et al. [3, 8], and additively manufactured products as researched by Zhang et al. and Happuwatte et al. [4, 11] have been developed and have been used to make substantial progress in the assessment of product sustainability. While product sustainability evaluations provide benefits through a rigorous sustainability analysis framework, they are also very subjective due to the weighing and normalizing of individual metrics [11]. The subjectivity involved in sustainability evaluations can lead to different results depending on the biases. Although this is the case, product sustainability assessment methods have been shown to accurately reflect changes in sustainability performance for different generations of products as shown by [3]. For context, there is a five-level hierarchical structure in the construction of a ProdSI [5]. The five levels, top to bottom, include: ProdSI, sub-indices, clusters, sub-clusters, and individual metrics. The sub-indices represent key performance areas relevant to each TBL aspect. The clusters are divided into sub-clusters that address elements relating to the clusters they fall under [5]. Finally, the sub-clusters are divided into individual, measurable metrics [5]. The measured values are then converted into dimensionless scores. Afterwards the dimensionless scores generated are aggregated to determine the ProdSI score. Once the ProdSI score is obtained, the entire process is reviewed for errors and the sustainability performance is determined. Because this paper focused on the introduction of a new paper/plastics product, performing a product sustainability evaluation is a necessary step in investigating its sustainability performance. Due to the lack of access to data for all different metrics, this paper utilized a simplified ProdSI to investigate the performance of LimeX products.

3.1 Selection of Individual Metrics

Firstly, the individual metrics chosen in this study for sustainability evaluation were determined based on elements related to the manufacturing of paper and plastic products. The ProdSI application was simplified (not concentrating on clusters and sub-cluster) to only identify metrics that will provide a general framework to measure performance of the two products under review towards achieving sustainability goals. To start, the key metrics in determining environmental sustainability were identified as energy use, waste generated, and resource efficiency. Next, the economic impacts sub-index is assessed based primarily on the initial investments, costs, benefits and losses. Finally, the societal impacts sub-index was assessed by primarily focusing on safety and health impacts, functional performance, product end-of-life (EOL) management, and product impact. These individual metrics were chosen based on each focus and had to be a quantifiable value with a unit or have shown a clear advantage/disadvantage in terms of sustainability.

3.2 Weighing and Aggregation

Weights are assigned to metric based on the equal weight method, given by [5]. However, not every individual cluster and sub-cluster has a quantifiable unit due to the lack of information on LimeX, and so a simplified approach was taken in which positive and negative values were assigned based on the information obtained for each individual metric. 11 . For example, if the development of LimeX plastic is safer than the development of conventional plastic (no quantifiable measurement), the score for the LimeX plastic safety metric would be slightly higher than the conventional plastic safety metric based on the conclusions of the study taken. The quantifiable measurements can be scored directly based on the measured values and what’s being measured. This is explained in depth in the next section.

$$ProdSI= \frac{1}{3}({E}_{c}+ {E}_{v}+ {S}_{c})$$
(1)
$${E}_{c}+ {E}_{v}+ {E}_{c}=\frac{1}{3}{\sum }_{i=1}^{3}{w}_{i }{C}_{i}+ \frac{1}{3}{\sum }_{i=4}^{7}{w}_{i }{C}_{i}+ \frac{1}{3}{\sum }_{i=8}^{10}{w}_{i }{C}_{i}$$
(2)

Here, Ec, Ev, Sc, wi, and Cj are the sub-index score for economic impact, sub-index score for environmental impact, sub-index score for societal impact, weight of the ith metric and score of jth metric, respectively.

4 Sustainability Results and Discussion

As stated in the introduction, the aim of this paper was to investigate LimeX products and calculate a ProdSI to evaluate its sustainability. The ProdSI will be simplified due to the lack of life cycle data available for the material. The individual metrics generated by the ProdSI will be utilized to quantitatively evaluate and compare its environmental, societal, and economic impacts with that of conventional paper and plastic. This section will include a breakdown of each metric, sub-index and ProdSI score. The physical quantification of individual metrics cannot be grouped together directly, therefore the metrics must be converted into a single normalized scale [3]. In this study, rather than normalizing measured values for the individual metrics [3], a score from 0–10 where 0, 2, 4, 6, 8, and 10 are the worst case, bad, below average, average good, and best respectively was utilized to place the individual metrics on a dimensionless scale. The scores are based on subjective factors specifically pertaining to the importance of each metric in terms of each sub-index. A standard normalization method for the conversion does not exist [12]. In this context, the score of each metric was determined based on the value of each measurement, based on the advantage and/or disadvantage of each measured value pertaining to the overall sustainability, and how relevant the metric is to the sustainability performance. The individual metrics were weighed equally for simplicity. Equation 1 and 2 were used to compute the sub-index and ProdSI scores (Table 3 and 4).

Table 3. ProdSI Metrics selected for analysis.
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Table 4. ProdSI Computation.
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4.1 Environmental Impacts

The energy consumption for LimeX is less than that of paper because LimeX products use more renewable resources and less non-renewable resources than conventional paper and plastic [13]. Secondly, the waste generation metric in the environmental section of the ProdSI was given a score of 6 and 4 because LimeX products emit less CO2 than conventional paper and plastic [14]. Lastly, there is the material use metric. The score given for material use was a 7 and 5 because to date, a higher percentage of LimeX products are recycled than conventional paper and plastics [14].

4.2 Economic Impacts

The cost metric in the economic impacts section of the ProdSI was given a score of 6 and 5 because the cost of conventional paper was more than the cost of LimeX products and because there are fluctuations in the cost of the oil-based resin used in conventional plastic where this is not the case for LimeX products. Additionally, the benefits/losses metric in the economic impacts section of the ProdSI was given a score of 5 and 7 because there are already many recycling facilities for conventional paper and plastic in the US, but there are no recycling facilities for LimeX products. Although LimeX does not use trees in its lifecycle and uses very little water, it does require the mining of limestone. Finally, the initial investment metric in the economic impacts section of the ProdSI was given a score of 6 and 8 because very little investments are required for conventional paper/plastics products that are already the norm whereas there is no infrastructure for LimeX.

4.3 Societal Impacts

In the final sub-index section, safety and health metric in the societal impacts section of the ProdSI was given a score of 7 and 6.5 because the operator safety/health for LimeX products was reported to be higher and better than for conventional paper. Afterwards, the functional performance metric in the societal impacts section of the ProdSI was given a score of 6 and 6 because LimeX is stronger than conventional paper. Lastly, the product EOL management metric in the societal impacts section of the ProdSI was given a score of 7 and 3 because LimeX products are much more recyclable than conventional paper and plastics. Ultimately, the product social impact metric in the product societal impacts section was given a score of 4 and 7 because LimeX products are not visible globally, being relatively new.

5 Conclusion

The ProdSI method adapted from previous methods in this paper compares the paper and plastic substitute material LimeX to currently used conventional paper/plastic products. Results from the simplified ProdSI indicate the sustainability performance of LimeX, producing a ProdSI of 6.78, was better than conventional paper and plastics, with a ProdSI of 6.22. However, since LimeX products are relatively new there has been little to no evidence of a LimeX lifecycle and discarding of LimeX products in the US. Results from the experimental analysis in Sect. 2 indicate that LimeX paper is stronger than conventional paper and that the material is primarily limestone as claimed by TBM [9]. Further research is required to validate long-term sustainability of LimeX. In future work, a life-cycle analysis of LimeX will be conducted in the US.