Function-driven Investigation of Non-renewable Energy Use and Greenhouse Gas Emissions for Material Selection in Food Packaging Applications: Case Study of Yoghurt Packaging

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Article

Reference

Function-driven Investigation of Non-renewable Energy Use and Greenhouse Gas Emissions for Material Selection in Food Packaging

Applications: Case Study of Yoghurt Packaging

AGGARWAL, Ankit, et al .

Abstract

Food packaging based on materials from renewable resources, such as biobased plastics, is a subject of continued interest as it offers the potential to reduce environmental impacts of packaging. So far, many food packaging LCA studies involve comparative evaluations of packaging materials without dealing with packaging functions. Although recent studies highlight the need for such an approach, there is a lack of related studies for specific food packaging applications. This study uses a function-driven approach tailored to a specific example of yoghurt packaging to define the functional unit. Key mechanical and barrier functions of yoghurt packaging are expressed and quantified in terms of function-driven parameters based on strength, bending stiffness, oxygen barrier and water vapor barrier of the packaging material. The approach uses the cradle to gate non-renewable energy use and greenhouse gas emissions of the packaging material to exemplify the application of defined function-driven parameters as a basis of comparison for the early phase of packaging development. Finally, the relevance of the function-driven approach [...]

AGGARWAL, Ankit, et al . Function-driven Investigation of Non-renewable Energy Use and Greenhouse Gas Emissions for Material Selection in Food Packaging Applications: Case Study of Yoghurt Packaging. Procedia CIRP , 2018, vol. 69, p. 728-733

DOI : 10.1016/j.procir.2017.11.132

Available at:

http://archive-ouverte.unige.ch/unige:115262

Disclaimer: layout of this document may differ from the published version.

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2212-8271 © 201 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference doi: 10.1016/j.procir.2017.11.132

Procedia CIRP 69 ( 2018 ) 728 – 733

ScienceDirect

25th CIRP Life Cycle Engineering (LCE) Conference, 30 April – 2 May 2018, Copenhagen, Denmark

Function-driven Investigation of Non-renewable Energy Use and Greenhouse Gas Emissions for Material Selection in Food Packaging

Applications: Case Study of Yoghurt Packaging

Ankit Aggarwal

^a,b

*, Markus Schmid

^a,b

, Martin Kumar Patel

^c

, Horst-Christian Langowski

^a,b

aTechnical University of Munich, TUM School of Life Sciences Weihenstephan, Chair of Food Packaging Technology,Weihenstephaner Steig 22, Freising 85354, Germany

bFraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Str.35, Freising 85354, Germany

cEnergy Efficiency Group, Institute for Environmental Sciences and Forel Institute, University of Geneva, Boulevard Carl-vogt 66, Geneva, Switzerland

* Corresponding author. Tel.: +49-8161491481;E-mail address:ankit.aggarwal@mytum.de

Abstract

Food packaging based on materials from renewable resources, such as biobased plastics, is a subject of continued interest as it offers the potential to reduce environmental impacts of packaging. So far, many food packaging LCA studies involve comparative evaluations of packaging materials without dealing with packaging functions. Although recent studies highlight the need for such an approach, there is a lack of related studies for specific food packaging applications. This study uses a function-driven approach tailored to a specific example of yoghurt packaging to define the functional unit. Key mechanical and barrier functions of yoghurt packaging are expressed and quantified in terms of function-driven parameters based on strength, bending stiffness, oxygen barrier and water vapor barrier of the packaging material. The approach uses the cradle to gate non-renewable energy use and greenhouse gas emissions of the packaging material to exemplify the application of defined function-driven parameters as a basis of comparison for the early phase of packaging development. Finally, the relevance of the function-driven approach for packaging design and development is discussed in context of an early stage material selection framework.

Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference.

Keywords:Food packaging functions; Biobased plastics; Non-renewable energy use; Greenhouse gas emissions

1. Introduction

Environmental impact of food packaging is a subject of continued interest in the context of packaging sustainability [1]. Within this perspective, developing food packaging applications based on materials from renewable resources, such as biobased plastics, is considered as a potential solution to reduce the environmental impacts of packaging. The measure of the environmental impacts of food packaging is usually carried out using the standard Life Cycle Assessment (LCA) method.

LCA is a tool to quantify the environmental impacts of a product or a service based on a defined functional unit [2].

The underlying purpose of the functional unit is a quantitative representation of the function provided by the product or service, and this forms the basis for investigating environmental impacts.

While each specific industry applies LCA customized to its standards and practices, a scientifically established LCA approach tailored for food packaging applications of bio-based plastics is still in its early stages [3,4,5,6,7]. Several food packaging LCA studies have been carried out focusing on packaging materials [6,8], food waste and loss [5,9,10,11,12],

Peer-review under responsibility of the scientiﬁ c committee of the 25th CIRP Life Cycle Engineering (LCE) Conference

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729 Ankit Aggarwal et al. / Procedia CIRP 69 ( 2018 ) 728 – 733

shelf life [10,13]; the latter two groups mostly in form of scenario studies. Recent studies have pointed out the importance of considering packaging functions for packaging LCA [14,15]. A recent study [16] presented a systematic theoretical framework to define key packaging functions and their relevance for sustainable development. However, there is a lack of related studies for specific food packaging applications. The current study uses a function-driven approach tailored to yoghurt packaging to define the functional unit. Three key functions for yoghurt packaging are expressed and quantified, in terms of function related properties of the packaging samples under study, based on application specific material properties. Material properties include: yield strength, young’s modulus, yield strain, oxygen and water vapor permeation coefficients. Cradle to gate non- renewable energy use (NREU) and greenhouse gas emissions (GHG) are normalized to several packaging functions provided by the packaging material in yoghurt packaging.

The function-driven approach used in the current study is presented in section 2, in section 3 the approach is applied to a model case study of yoghurt packaging. Section 4 discusses the applied function-driven approach with respect to a material selection framework for its application in early stage packaging design and development. Finally, conclusions are presented in section 5.

2. Materials and Methods:

The current study applies the function-driven approach to actual samples of yoghurt packaging (cups) made from a conventional plastic (PS: polystyrene) and a bio-based plastic (PLA: polylactic acid). The following steps were performed:

2.1. Non-renewable energy use and greenhouse gas emissions as useful parameters in an early stage material selection

The use of NREU and GHG as indicators for a simplified environmental assessment is especially suitable to overcome inherent constraints of data availability and time during early stage research and development of materials and their potential applications [4,17,18]. This technique is useful and relevant for food packaging applications based on novel materials such as bio-based plastics, where specific environmental data on material is limited, packaging applications are still under development and actual end of life options are still unknown and restricted to scenarios. [39,40]

2.2. Characterization of packaging functions and packaging material to determine product specific material properties The classical acronym for the overall food packaging functionality, PCCC, stands for Protection, Containment, Communication and Convenience. Any of these global functions consists of a multiplicity of more specific functions some of which are quantifiable. Only quantifiable functions that are directly related to material properties help to estimate the amount of the specific material needed to produce the package under study. If packagings of the same geometry are to be compared, this amount is represented by the weighted

average of the material thickness. Table 1 exemplifies this for the main functions Protection and Containment for yoghurt packaging. Among these examples, some, but not all properties linearly depend on the material thickness, whereas others are independent of it. This is especially important for the stackability which depends in a complex way on three different material properties. In practice, this function is to be determined empirically with help of prototypes. To address the barrier properties, a proper knowledge of the degradation mechanisms of packed food product is needed which is not often found at material specialists. Moreover, threshold values, such as the maximum allowed uptake of oxygen, are known just for a limited number of products. Thus, in the absence of extensive product knowledge, the known functionality of the long-proven reference package is often taken as a benchmark. In our case, these are the values for oxygen and water vapour transmission rate of the conventional PS cup, see Table 3.

Table 1. Examples for packaging functions, related package properties, corresponding material properties and relation to physical quantities

Packaging Function

Function- related package properties

Material properties

Relation to physical quantities (h:

material thickness) Stackability (i.e.

absence of strong deformation under max.

permissible load)

Strength, S Yield strength ıy 6aıyh Bending

stiffness, St

Young’s modulus Et

St ~ Eth³ Elasticity Elongation at

yield/yield strain İy

Independent of sample dimensions Protection from

oxygen/oxygen barrier

Reciprocal oxygen transmission rate, OTR^-1

Oxygen permeation coefficient PO2

OTR^-1~ h / P_O2

Avoidance of water losses/water vapor barrier

Reciprocal water vapour transmission rate, WVTR^-1

Water vapour permeation coefficient PH2O

WVTR^-1~ h /P_H2O

The packaging materials are characterized using standard testing methods to determine the key properties: density, yield strength, yield strain, young’s modulus, film thickness, oxygen and water vapor transmission rate. This step is important to solve data gaps since information on product specific material properties for novel materials such as biobased plastics is currently limited.

2.3. Calculating function-driven parameters in terms of material substitution factors (MSF’s)

Ashby introduced the concepts of material indices to relate the amount of material required for a specific application to its properties [21]. These material indices can be used to calculate the material substitution factor (MSF) between two different materials; see equation 1 [4,17,18,19]. MSF is an estimate of the amount of material needed to fulfill a defined function in comparison to another reference material. This approach is useful to account for the differences in technical properties between materials for a defined function and has been applied in few recent LCA studies especially for early

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stage design and development of materials and their applications [4,17,18]

The different product specific material properties are used to define a consistent list of function-driven parameters which are then quantified using Material Substitution Factors (MSF) corresponding to the respective parameter. Material substitution factors are defined via equation 1 below.

m

ref

MSF m

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Where:

MSF = material substitution factor of a material compared to the reference (ref), dimensionless

m = mass of a material for a given function

mref = mass of the reference material for the same function

A MSF greater than 1 represents that for providing the same function, greater amount of material is required as compared to the reference material.

2.4. Estimating function-driven NREU and GHG impacts NREU and GHG emissions obtained from section 2.1, are corrected for the differences in the material properties by multiplying them with the MSF to obtain MSF corrected environmental impacts as suggested in [17]. This serves as a basis of comparing impacts in terms of the defined function- driven parameters.

3. Case study and results

3.1. Non-renewable energy use and Greenhouse gas emissions of PLA and PS

NREU and GHG emission values, as shown in Table 2, were obtained using existing generic LCA data of the packaging materials (PS and PLA in this case) from publicly available sources, namely, Plastics Europe [22], earlier scientific studies [34,35,36] and ecoinvent database [23,24].

Lowest as well as highest ranges of values were used to consider uncertainties in data.

NREU and GHG emissions represent environmental impacts related to energy and climate change impacts which are considered representative and useful proxy indicators for simplified environmental impacts of products or services [25,26]. When used in the form of cradle to gate data, they represent impacts related to the manufacturing phase only.

However; they are still useful as best available estimates for early stage technical development and have been used in recent studies [17,19,20]. Several authors [20,37,38] have highlighted the importance of manufacturing phase for environmental assessments. For impact categories; such as land use, eutrophication and acidification; biobased polymers generally have higher impacts than fossil based polymers.

However, there are large differences in impacts both in the family of biobased plastics and fossil based plastics and no

absolute information is available [27]. Acknowledging these data limitations, the current case study is limited to NREU and GWP.

Table 2. NREU and GHG emissions of PS and PLA (i.e. cradle to gate environmental impacts for per kg of material)

Material NREU

(MJ/kg) (low - high)¹

GHG emissions (kg CO2eq./kg) (low - high)¹

PS (HIPS; GPPS) 78.26 – 90.43 2.25–3.46

PLA 37.9 – 42.2 0.9 – 1.3

1. Low – high: lower as well as higher range of values

3.2. Material properties data for a yoghurt cup

Key material properties namely yield strength, E-modulus, thickness of the yoghurt cup, oxygen transmission rate (OTR) and water vapor transmission rate (WVTR), were determined experimentally on samples from the market using standard material characterization methods and tests as shown in Table 3. The thickness of the cups represents an average of measurements on the side wall. As similar thermoforming molds and techniques have been used, it can be assumed that the thickness distribution over the whole area of the cups follows the same ratio for the cups from both materials. It is also assumed that OTR and WVTR vary inversely with the film thickness as measured at the specific location in the side wall, although this is only exact in case of a uniform wall thickness. These assumptions allow to re-calculate the functional properties in case of changes in wall thickness.

Table 3. Material properties data.

Materials PS PLA

Yield Strength¹ıy[Mpa] 81.3 105

Young’s modulus¹Et[Mpa] 1540 2050

<LHOG6WUDLQİy[%] 5.4 5.5

Oxygen Transmission Rate²OTR [cm³(STP)/Package.d.bar] and oxygen permeation coefficient PO2[cm³(STP) 100 μm /( m² d bar) ]

OTR: 4.56 PO2: 580

OTR: 1.49 PO2: 60

Water Vapor Transmission Rate ³WVTR [g/Package d] and water vapour permeation coefficient PH2O[g 100 μm / m² d]

WVTR: 0.07 PH2O: 12

WVTR:

0.18 PH2O: 26 Side wall thickness¹h [μm] 131.8 110.8

Density4ȡ[kg/m³] 1.05 1.25

1. Tensile properties measured as per DIN EN ISO 527 [28]

2. OTR measured at 23 °C, 50% r.h. as per DIN 53380-3 [ 29]

3. WVTR measured using Gravimetric method at 23°C, 85% -> 0 %, as per DIN 53 122-1 [30]

4. Density values were taken from [31] and [32]

3.3. Function-driven parameters in terms of material substitution factors (MSF’s)

Key function-driven parameters used in the current study comprises of strength, bending stiffness, oxygen barrier, water vapor barrier. Table 4 lists the function-driven parameters and material substitution factors for PLA in reference to PS calculated using equation 1. MSF for PLA on the basis of stiffness and especially water vapor barrier is greater than 1.

This means that to provide the same stiffness or water vapor

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barrier to the yoghurt cup, as provided by PS, more amount of PLA will be required.

Table 4. Material Substitution Factors for PLA vs. PS (reference) Function-driven

parameters

Relation to physical quantities

PLA thickness / μm as required for function- based substitution of 131.8 μm PS

Material Substitution Factor (MSF), mass- based, for PLA in reference to PS

Strength ıy 102.1 0.92

Stiffness S ~ Eth³ 119.8 1.08

Oxygen barrier OTR^-1~ h / PO2 36.2 0.33 Water vapor

barrier

WVTR^-1~ h / P_H2O

284.9 2.57

Mass and actual configuration

110,8 1.0

3.4. Function-driven NREU and GHG impacts of PLA and PS for a yoghurt cup

Table 5 and table 6 shows the MSF corrected values of NREU and GHG emissions of using PLA for a yoghurt cup (vs PS as a reference) for each function-driven parameter i.e.

mass, strength, bending stiffness, oxygen barrier and water vapor barrier. The corresponding ratio of NREU and GHG emissions for PLA vs PS for a yoghurt cup are shown in fig.1 and fig. 2.

Table 5. Environmental impacts in terms of NREU (in MJ) Function-driven

parameter

Material Substitution Factor (MSF)

NREU

PS

(in MJ) (low - high)¹ (reference)

MSF corrected NREU _PLA (in MJ) (low - high)¹ Mass and actual

configuration

1

78.26 - 90.43

37.9 - 42.2

Strength 0.92 34.87 - 38.82

Stiffness 1.08 40.93 - 45.58

Oxygen barrier 0.33 12.51 - 13.93

Water vapor barrier 2.57 97.40 - 108.45

4. Discussion

An appropriate sequence for a sustainability assessment in product design has been suggested in a recent study in terms of an early stage material selection framework [17]. When dealing with the selection of an alternative material for a packaging of food, however, this has to be modified due to specific issues. First, we may use an existing product- packaging combination as a reference because it hardly ever happens that a completely new food product and a new type of packaging are being developed at the same time [33].

Fig.1: Ratio of NREU for PLA vs PS for a yoghurt cup

Table 6. Environmental impacts in terms of GHG emissions (in kg CO2

eq./kg) Function-driven parameter

Material Substitution Factor (MSF)

GHG_PS (in kg CO2

eq./kg) (low - high)¹ (reference)

MSF corrected GHGPLA

(in kg CO2

eq./kg) (low - high)¹ Mass and actual

configuration

1

2.25–3.46

0.9 - 1.3

Strength 0.92 0.83 - 1.2

Stiffness 1.08 0.97 - 1.40

Oxygen barrier 0.33 0.3 - 0.43

Water vapor barrier 2.57 2.31 – 3.34

Fig.2: Ratio of GHG emissions for PLA vs PS for a yoghurt cup

Secondly, the true product-specific threshold values for some properties of the packaging are often not known, which asks for a prototype phase in the whole sequence. The following discussion, will illustrate the sequence in terms of our model case study of yoghurt packaging from the German market.

Here, a bio-based alternative to the conventional polystyrene (PS) cup has been developed.

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4.1. Step 1: Defining goals and constraints

In our specific example, the goal is to estimate probable environmental impacts of a PLA cup for yoghurt versus the traditional PS cup while maintaining the required functional properties as shown in Table 1. At this point, the constraints are given by the fact that the material properties required for stackability and barrier against oxygen and water vapor cannot be defined with sufficient precision.

4.2. Step 2: Screening alternative materials using the constraints

Such a screening also includes aspects of availability and cost. In the current example, only Polylactic acid (PLA) remains as a candidate. It is a commercially thermoplastic polymer, available in grades with mechanical properties similar to PS and an even better oxygen barrier. Its water vapour barrier properties, however, are inferior, an issue for further consideration in the design process.

4.3. Step3: Estimate material substitution factors (MSF’s) For our actual example, the required average thickness values of the PLA package have been calculated, specific for every function. Their ratio to the corresponding values of the standard PS cup, multiplied by the ratio of the density of both materials gives the MSF’s for the substitution of PS by PLA in this case which are shown in Table 4.

4.4. Step 4: Performing a first screening of environmental and economic performance

By using the material substitution factors from Table 4 and the data from Table 2, specific product-related cradle-to-gate figures can be calculated, in our example shown for non- renewable energy use (NREU) and Global warming potential (GWP) in Figures 1 and 2. The results allow to gain a first impression on the environmental performance of a possible PLA alternative to the PS cup and to identify critical technical issues. In our case, the bending stiffness and especially the water vapour barrier properties appear to be critical properties of the PLA material, but the difference to the PS reference is still in a range to justify prototype production and practical tests, as to be performed in step 5.

4.5. Step 5: Testing of packaging properties relevant for packaging functions

The functional properties of a packaging like a thermoformed cup can only roughly be estimated just by using known data on material properties. To acquire exact figures, a range of prototypes of different wall thicknesses has to be prepared and the relevant properties of complete packages have to be measured. In our example, the actual configuration already shown in Table 4 showed a functional equivalence, with an effective wall thickness of 110.8 μm for the PLA cup versus 131.8 μm for the PS reference.

The stackability showed to be a combined property resulting from strength, bending stiffness and elasticity, as shown in Table 1. In the end, the deficiencies in stiffness of the final PLA package (resulting from a lower wall thickness) were compensated by higher strength and elasticity (as represented by material properties data in Table 3). More surprisingly, the higher water vapour transmission rate of the PLA cup did not compromise the shelf life of the packed product. Therefore, the reference PS cup can be regarded as being overengineered in terms of water vapour barrier whereas the PLA cup shows the same for oxygen barrier. In total, both functions, initially regarded as possibly being critical for a material substitution show to be not relevant for the actual package – product combination.

4.6. Step 6: Final determination of material substitution factors

Regarding the test results as sketched above, an MSF of exactly one resulted (which is sheer coincidence). Thus, the comparative results for “mass” from Figure 1 and 2 have to be taken for the start of step 7.

4.7. Step 7: Analyzing environmental performance

Considering strength, bending stiffness and oxygen barrier, PLA performs better than PS both for NREU as well as GHG emissions (refer figure 1 and 2), whereas PS performs better considering water vapor barrier.

Comparing the resulting cradle-to-gate figures for non- renewable energy use (NREU) and Global warming potential (GWP), PLA generates environmental impacts that are less than half than those for PS in this specific example.

Furthermore, transports and storage should be regarded in a full LCA. In our specific example, however, this stage of the life cycle can be neglected: As the mass and the geometry of the PLA and PS cups are identical and as their functionalities do not require different transport and storage conditions, the environmental impacts generated in this stage are the same for both alternatives. It would have been different if the results of step 4.5 would have shown significant functional differences between the final configurations of the packages. In such a case, even different rates of product losses could be expected.

This would ask for the inclusion of the relevant LCA data of the product into the analysis. Such a study has been performed by [9,10,11], but just on the basis of scenarios. To our knowledge, correlations between packaging functions and product losses have so far not been established in any study and no empirical values are available. End-of-life options are another important stage of a product life cycle. For the product in our example and for the actual German situation, viable alternatives are energy recovery and material recycling.

While the first alternative is most probable to happen today, the second one will be more important in the future. Analysis of both these options is in preparation and outside the scope of the current work. Information on the comparison of environmental impacts of production and end of life options for fossil as well as biobased plastics has been presented in recent scientific studies [39,40]. However, there are variations

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in environmental impacts across different indicators depending upon waste management scenarios; such as incineration, landfilling, compostability; which must be modelled separately for any full scale LCA study.

4.8. Step 8: Ranking and final decision

This is a relatively short task in our example. The only decision is whether to remain with the conventional PS cup or to substitute it by one from PLA. From the cradle-to-gate figures shown, the launch of a yoghurt product in a PLA cup can be justified. Unfortunately, the cost calculation versus the option of a positive impact at the consumer remains confidential, but the fact that the product in the PLA cup is on the market shows that the evaluation ended in favour for the material from renewable sources.

5. Conclusions

The current study successfully exemplifies the application of a function-driven approach to integrate packaging functions in the definition of the functional unit for an early phase of packaging development. – here for the specific example of yoghurt packaging. The use of function-driven parameters and multiple substitution factors to relate NREU and GHG emissions with the mechanical and barrier functions of the packaging can serve as a useful tool for early stage research and development of food packaging especially for identification, evaluation and selection of packaging materials.

Additionally, the application the function-driven approach to early stage material selection framework provided useful inputs that are relevant for further technical development References

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