Effect of Heat Seal Parameters on Interface Temperature and Seal Strength of Multilayer Structures with Non-Woven

Effects of Heat-Sealing Parameters on the Thermal Profile and Seal Strength of

Multilayer Films and Non-Woven

Divya Ponnambalam

A Thesis

presented to

The University of Guelph

In partial fulfilment of requirements for the degree of Master of Applied Science in Engineering

Guelph, Ontario, Canada

EFFECTS OF HEAT-SEALING PARAMETERS ON THE THERMAL PROFILE AND SEAL

STRENGTH OF MULTILAYER FILMS AND NON-WOVEN

Divya Ponnambalam Advisors: University of Guelph, 2020 Dr. Manickavasagan Annamalai Dr. Loong-Tak Lim

Heat sealing is a common technique for sealing food packages. Unlike continuous films, the seal behaviours are less understood for packages involving both multilayer and non-woven (NW). This study investigated the effects of sealer parameters and seal contaminants on temperature profile and seal strength of multilayer and non-woven films used in coffee capsules. The interfacial temperature of seals with NW1 and NW2 was 5 and 11°C less, respectively when compared to seals without NW. The core and sheath contributed to the higher seal strength in seals with NW2.

The coffee contaminants were visible under the thermal camera even when they were present under the opaque multilayer film. The temperature of the contaminated area was 30°C less in SC and MP and 38°C in SE sample than the clean area. Also, the seal strength decreased significantly in the presence of multiple particles when compared to the single-particle. iii

ACKNOWLEDGEMENTS

I would like to express my profound gratitude to my advisor Dr. Manickavasagan Annamalai for giving me the opportunity to expand my horizons in various areas of research, and for his valuable advice and support throughout the program. I am extremely fortunate to have worked with my co- advisor Dr. Loong-Tak Lim; I thank him for his constant guidance and support during the entire project. A special thanks to my advisory committee member Dr. Yucheng Fu whose door was always open whenever it was needed. I also convey my gratitude to the staff in the School of

Engineering and Food Science.

It would have never been possible for me to have pursued my dreams without the support of my parents and grand parents. I am always grateful and humbled for the love and support that I receive from them. I wish to extend my heartfelt thanks to Hari and Ashwini who always stood by me and continuously encouraged me throughout the pursuit of my degree. Special thanks to my friends

Sindhu, Rathna and Singam who were a constant companion throughout the course of this Masters.

TABLE OF CONTENTS

Abstract ...... ii

Acknowledgements ...... iii

Table of contents ...... iv

List of Tables ...... vii

List of Figures ...... viii

Abbreviations ...... x

Nomenclature ...... xiii

1 Introduction ...... 1

Research objectives ...... 5

Literature Review...... 6

Food packaging overview ...... 6

Methods of heat sealing...... 7

Heat seal process and molecular dynamics mechanism ...... 8

Heat sealing process parameters ...... 10

2.4.1 Sealer jaw and interfacial temperatures ...... 11

2.4.2 Dwell time ...... 13

2.4.3 Effect of pressure ...... 14

Multilayer structures ...... 15

Non-woven in food packaging ...... 16

Heat sealing of non-woven films ...... 17

Seal integrity and maintenance ...... 18

Seal defects ...... 19

2.9.1 Inclusion/contaminated seal ...... 19

2.9.2 Pinholes defects ...... 20

Conventional methods of seal integrity testing ...... 20

Imaging techniques for seal testing ...... 21

2.11.1 Infrared thermal imaging ...... 25

2.11.2 Ultrasonic imaging ...... 26

2.11.3 Terahertz imaging ...... 27

2.11.4 Other Imaging techniques for seal integrity testing ...... 31

Coffee quality and shelf life ...... 32

Coffee packaging requirements ...... 33

Coffee capsule packaging ...... 34

Effect of heat seal parameters on interface temperature and seal strength of multilayer structures with non-woven ...... 36

Introduction ...... 36

Materials and methods ...... 39

3.2.1 Materials ...... 39

3.2.2 Heat sealing ...... 40

3.2.3 Measurement of interfacial temperature ...... 41

3.2.4 Measurement of seal strength ...... 41

3.2.5 Scanning Electron Microscope Imaging ...... 42

3.2.6 Statistical Analysis ...... 43

Results and discussion ...... 43

3.3.1 Interfacial temperature ...... 43

3.3.2 Seal strength ...... 47

3.3.3 Modes of failure ...... 55

3.3.4 Scanning Electron Microscope imaging: ...... 60

Conclusion ...... 64

Effect of coffee particle contaminants on the surface temperature profile and seal strength of multilayer and non-woven packaging...... 66

Introduction ...... 66

Materials and methods ...... 69

4.2.1 Sample preparation ...... 69

4.2.2 Thermal and optical image acquisition ...... 70

4.2.3 Thermal image processing ...... 70

4.2.4 Seal strength measurement ...... 71

4.2.5 Statistical Analysis ...... 71

Results and discussions ...... 71

4.3.1 Thermogram of the heat-sealed films ...... 71

4.3.2 Comparison of thermal images with the optical reference images ...... 76

4.3.3 Seal strength and failure mechanism ...... 79

Conclusions and future recommendations ...... 83

Conclusion ...... 84

References ...... 86

vii

LIST OF TABLES

Table 2.1 Summary of methods for the detection of seal integrity and leak ...... 22

Table 2.2 Ultrasound imaging for detecting package defects in flexible and semi-rigid packaging ...... 28

Table 3.1 Film composition and properties ...... 40

Table 3.2 Abbreviations for interfacial temperature used in the study...... 44

Table 4.1 Comparison of seal strength of contaminated seals with the control sample (n=5). .... 80

viii

LIST OF FIGURES

Figure 2.1 Molecular mechanism involved in the heat sealing of polymer films. Adapted from Najarzadeh (2014)...... 10

Figure 2.2 Heat seal strength versus interface temperature plots for semi-crystalline polymers. Adapted from Stehling & Meka (1994)...... 13

Figure 2.3 Classification of bicomponent non-woven material based on the cross-sectional structure of their fibres (TAPPI, 2002)...... 17

Figure 2.4 Schematic diagram of single-serve-coffee capsule and the enlarged seal region of the capsule...... 35

Figure 3.1 Schematic of heat sealer jaws, film arrangements and thermocouple positions ...... 41

Figure 3.2 Top view illustration of the film specimen with measurements used for (a) sealing and (b) seal strength testing...... 42

Figure 3.3 Change in the interfacial temperature with the sealer jaw temperature for seals without any non-woven and with non-woven (NW1, NW2) for a dwell time of 1 s and 30 psi...... 45

Figure 3.7 Effect of pressure on seal strength for seals with and without non-woven (NW1, NW2) prepared at 240°C for a dwell time of (a) 1s and (b) 1.5 s...... 54

Figure 3.8 Seal failure modes observed under different sealer conditions with and without non- woven conditions. Adapted from Cheng et al., (2007)...... 57

Figure 3.9 Load-displacement plot as observed under different sealer conditions with and without non-woven conditions...... 58

Figure 3.10 Observed failure modes as a function of platen temperature and dwell time for sealing structure (a) without non-woven, (b) with non-woven NW1 and (c) non-woven NW2...... 60

Figure 3.11 SEM micrographs of the cross-sectional view of the lid and wall multi-layer films before sealing...... 62

Figure 3.12 SEM micrograph of a cross-sectional view of lid-non-woven1 (NW1) sample sealed at 235°C, 1.5 s and 30 psi...... 63

Figure 3.13 SEM micrograph of a cross-sectional view of lid-non-woven2 (NW2) sample sealed at 240°C, 1s and 30 psi...... 63

Figure 3.14 Peeled surface of the lid films sealed with non-woven 2 (NW2)...... 64

Figure 4.1 Thermal images of the lidding layer of the sealed samples captured at 3.5 s after the opening of the sealer jaws...... 74

Figure 4.2 Cooling temperature profile (CTP) of the sound and contaminated regions of the MP sample after sealing the films at 240°C, 1s and 30 psi...... 76

Figure 4.3 Overview of the comparison of optical image and thermogram of seals contaminated with coffee particles of MP sample by image overlapping procedure...... 78

Figure 4.4 Failure mode observed for control and contaminated samples obtained during the peel strength test. Adapted from Cheng et al., (2007)...... 81

Figure 4.5 Load-displacement plots observed for control and contaminated samples during the peel strength test...... 82

ABBREVIATIONS

NW Non-woven

PS Poly(styrene)

EVOH Ethylene vinyl alcohol

PET Poly(ethylene terephthalate)

Al Aluminum

LLDPE Linear-low density poly(ethylene)

SE Single particle contaminant at the Edge of the seal area

SC Single particle contaminant at the centre of the seal area

MP Multiple particle contaminants with 25 g/m2 contamination density in the seal area

HSS Heat seal strength

PE Polyethylene

PP Polypropylene

PVDC Poly(vinylidene chloride)

PAA Poly(acrylic acid)

PET Poly(ethylene terephthalate)

LDPE Low-density polyethylene

HDPE High-density polyethylene

PVC Poly(vinyl chloride)

OPP Oriented poly(propylene)

CPP Cast-poly(propylene)

RGB Red-green-blue

MRI Magnetic resonance imaging

CT Computer tomography

IR Infrared

OPA Oriented poly(amide)

VFFS Vertical form filling sealing

UI Ultrasonic imaging

BAI Backscattered amplitude integral

RFCS Radio Frequency Correlation; RFCS- Calculating correlation coefficient over a short segment of RF signal

BEEI Backscattered echo envelope integral

PLSI Polarized light stress imaging

LSI Laser scatter imaging

LED Light emitting diode

CCD Charged coupled device

CGA Chlorogenic acids

PLA Poly(lactic acid)

SEM Scanning electron microscope

ANOVA Analysis of variance

xii

2 R Coefficient of determination

CTP Cooling temperature profile

MHz Mega hertz

T01 Interface temperature at Upper jaw-lid

T02 Interface temperature at Lid-wall

T03 Interface temperature at Wall- Lower jaw

T11 Interface temperature at Upper jaw-lid

T12 Interface temperature at Lid-NW

T13 Interface temperature at NW-Wall

T14 Interface temperature at Wall- Lower jaw

T21 Interface temperature at Upper jaw-lid

T22 Interface temperature at Lid-NW

T23 Interface temperature at NW-Wall

T24 Interface temperature at Wall- Lower jaw

xiii

NOMENCLATURE

Wa Energy required for separating the layers,

훾1 Surface tension of the film

훾2 Surface tension of porous materials

훾12 Interfacial tension between the films.

Tr Reptation time

N Number of monomers crossing the interface t Time

Li Interpenetration depth

M Molecular weight of the sealant polymer

o G N Shear modulus in the rubbery plateau region of the polymer

ρ Density of the polymer molecule

R Gas constant

T Absolute temperature

Me Average molecular weight between the entanglements

1 Introduction

Coffee preparation methods involve turning coffee beans into a beverage. The process has four main steps, roasting, grinding the coffee beans, followed by brewing and, separation of the brew from the spent ground. Freshly brewed coffee is preferred over the old one as it has the best aroma and taste. In the brewing process, the roasted and ground coffee is mixed with hot water (~96°C) to extract the soluble compounds (Mussatto et al.,2011). The brewing method has evolved over the years and falls into four main categories depending on the method of hot water introduction. The methods include a) boiling b) steeping c) drip brewing/ gravitational feed and d) pressurized percolation. In boiling method, the coffee ground is mixed with water and brought to boil and then filtered, while in steeping method the coffee is mixed with boiling water and allowed to steep for few minutes before filtering the brew. In drip brewing method the ground coffee is placed over a filter on which the boiling water is poured, the brew is allowed to drip through the filter by means of gravity. In pressure brewing method the hot water is pressurized over the packed coffee ground to obtain a concentrated brew, the package has an additional filter or serves as a filter itself to separate the brew (Mudgil & Barak, 2018).

The single-serve coffee brewing method- one of the pressure brewing technique is the second most popular coffee brewing method among Americans after drip brewing. The simplicity, ease of use and consumption of relatively lesser time for brewing has made the single-serve coffee capsule a popular product among the consumers. Statistics show that the percentage of Canadians aged between 18-79 who own a Single-cup brewer system has increased from 15 to 36%, from the year 2012 to 2017(CCA, 2017). Coffee capsules are typically made of plastics or drawn aluminum

(Al). Although the aluminium capsules have superior barrier properties against light, oxygen and water, the plastic capsules are more popular because of their much lower price and lesser energy consumption during production (Schrynmakers, 2009; Cozzolino et al., 2015). The plastic capsule has a multilayer thermoformed shell made of polymers which can withstand high brewing temperature during brewing, as well as a barrier layer which protects the coffee ground against oxygen and moisture. Non-woven is placed inside the cup-shaped shell, which holds the coffee particles in place and allows the filtration of coffee infusion while brewing. The capsule is generally sealed using a multilayer metallized lid which provides further protection against oxygen and moisture. The lid, non-woven, and shell components are bonded together, typically by the heat-sealing process (Brommer et al., 2011).

Heat sealing is the bonding of the polymer films by applying heat and pressure for a specific period (dwell). The sealer parameters such as the sealer jaw temperature, dwell time and pressure influence the seal performance and therefore, the heat seal strength (HSS) of the films

(Soroka, 1995). Along with this, the material properties such as the thickness of the film, melting temperature (Tm), the molecular weight distribution of the polymers also affect the sealing performance. The complex interaction between these parameters is controlled to achieve the desired seal quality for a package (Cheng et al., 2007). HSS is determined by interdiffusion, molecular entanglement and recrystallization of the polymer molecules which occur during the formation of the seal. This molecular mechanism is initiated by achieving melting temperature at the interface of the polymer films during sealing (Karim et al., 1994). Studying the effect of heat- sealing parameters on the seal performance of coffee capsules will help in the maintenance and further improving the quality of the product.

Maintaining the integrity of the package is crucial for providing quality products to consumers. Like any other food packages, the quality of coffee in the packaging is ultimately dependent on seal integrity and performance. Faulty seals might lead to exposure of coffee to the oxygen and moisture from the environment resulting in lipid oxidation, the formation of volatile compounds and increased release of carbon dioxide. These chemical changes in the coffee lead to deterioration of the product quality (Labuza et al.,2001; Illy and Viani 2005; Anese et al.,2006;

Ross et al. 2006;).

Optimization of sealer parameters is one of the critical ways to ensure seal performance and integrity in a package. Effect of heat sealing parameters on monolayer films have been widely studied (Stehling & Meka, 1994; Cheng et al., 2007; Najarzadeh & Ajji, 2014), and the effect of the presence of contaminants in between the sealing layers has given valuable information on how the interfacial temperature affects the seal strength of the films. The studies have established a strong relationship between the seal strength and the heat sealer parameters, which directly affects the interdiffusion of the molecules(Jabbari & Peppas, 1995). The study on heat sealing of multilayer films, in particular the parameter effect, optimization and failure mechanisms have gained focus in the recent years ( Tetsuya et al., 2005; Cheng et al., 2007; Planes & Flandin, 2011;

Guo & Fan, 2016; Iwasaki et al., 2017; Kanani Aghkand et al., 2018). Whereas information on the heat sealing of packages containing multilayer films and non-woven materials, such as those in the coffee capsules, is much less. The first objective of the thesis provides a clear understanding of the influence of the process parameters on final seal performance/ seal strength of the multilayer films and non-woven used in coffee capsules.

In the next objective, the potential of the thermal imaging technique for the detection of the solid coffee particle contaminant in the seal area of the coffee capsule packaging has been discussed. The contaminants in the sealing layer is another parameter that affects the performance of seals in food packages (Willhoft, 1993). Coffee particles tend to get in the interface of the seal layers and contaminate the seal region during filling and sealing of coffee capsules. The presence of contaminants alters the heat transfer and the interface temperature during heat sealing, thus affecting the seal performance. The test for seal performance in industries are mostly offline and hence cannot provide the feedback on the quality immediately. Imaging techniques are gaining popularity in recent years in many industries for their applicability in quality inspection. The application of the imaging technique for detecting the solid coffee contaminant in the seals could help in limiting the number of faulty seals entering the market.

Research objectives

Objective 1: To determine the effect of heat-sealing parameters such as the sealer temperature, dwell time and pressure on the seal strength and interface temperature of the seals involving multilayer films and non-woven.

Hypothesis 1: The interface temperature of the seal structure with non-woven films is comparatively lesser than that of without non-woven films. The seal strength of the films varies based on the type of non-woven involved in the sealing.

Objective 2: To determine the effect of coffee particle contaminants in the seal area, on the surface temperature profile and seal strength of packaging involving multilayer films and non-woven.

Hypothesis 2: The coffee particles present in the seal area of the multilayer film-non-woven interface can create visible thermal artefacts in the thermograms, which could help in the contaminant detection. The lowering of interfacial temperature could decrease the seal strength of the films in the presence of contaminants.

Literature Review

Food packaging overview

Traditionally food is packed to enclose and to protect it from any physical, chemical, biological, and environmental factors. According to Robertson (2016), The primary role of food packaging is containment, protection, convenience and communication. Packaging industry mainly utilizes polymers because of its desirable characteristics; more than 90% of packages are made of plastics

(Akelah, 2013). Food packaging can be broadly classified into flexible, semi-rigid and rigid packages based on their tendency to change its shape or deform due to the internal and external pressures. When the shape of the package is affected by the enclosed products, the package is classified under flexible packages. The shape of the packages which are not affected by the product enclosed by them under atmospheric temperature and pressure but distorted by the external pressure < 70 KPa is classified under semi-rigid packages. While the rigid packages are those in which the shape is not affected by both the product enclosed and the external pressure up to 70

KPa (Von Bockelmann & Von Bockelmann, 1986). Flexible and semi-rigid food packages commonly utilize polymers based on the packaging requirements of the product. Flexible packages are usually made of thermoplastic films which consist of a polymer or a blend of polymers of polyethylene (PE), polypropylene (PP), poly(vinylidene chloride) (PVDC), poly(acrylic acid) (PAA) or poly(ethylene terephthalate) (PET). The most common plastics being used in semi-rigid food packages are low-density polyethylene (LDPE), high-density polyethylene

(HDPE), poly(vinyl chloride) (PVC), poly(styrene) PS, PP, PET. Various methods like thermoforming, blow moulding, injection moulding and compression moulding are used for converting these films into rigid structures for food products (Coles et al., 2003; Akelah, 2013). 6

The heat sealability is one of the essential properties of the polymer packages, and the integrity of the resulting seal is crucial for the overall quality of the package. Several factors are involved in determining the integrity of the heat seal. For example, (1) the heat sealer parameters such as temperature, time and pressure; (2) the film and resin factors such as the density, molecular weight distribution, thickness of the film are controlled in a complex way to attain the desired sealing quality for the particular product (Manley, 2011).

Methods of heat sealing

Heat is provided to the polymers during the sealing process, which helps the molecules from polymer layers to melt and diffuse across each other forming entanglement, thus providing closure to the package (Selke & Culter, 2016). The polymer material is provided with heat through several processes. In conductance sealer, polymer films are sealed between two electrically heated sealer jaws under uniform pressure, to melt and fuse the polymers films. Impulse sealers also have two metal jaws that are heated by electrical impulse only when the polymer film is placed between the jaws and are closed for sealing (Nelson, 2010). In induction sealing, there is no direct contact between the polymer film and the heating element. Heating of the polymer is achieved by employing eddy currents. In the case of ultrasonic sealers, ultrasonic vibration is delivered to the polymer films through a conical welding head which converts mechanical vibration into frictional heat that welds the polymers. Hotwire/ knife sealing is used for polymers with a thickness less than 0.05 mm.

The method uses a heated knife or wire to transfer the pressure to the polymer, the combination of heat and pressure causes sealing. Finally, dielectric sealers have limited

applications as only the polar materials which can form dipole moment can be sealed using this method. Here, brass electrodes are used to supply high-frequency current (50 to 80 MHz) to the polymers compressed under pressure to facilitate bonding of the polymers. (Jenkins et al., 1992;

Robertson, 2016).

Heat seal process and molecular dynamics mechanism

The basic principle behind the process is to provide heat at the interfaces and bring the films to close contact by providing pressure to achieve adequate heat seal strength (HSS) within an acceptable time period (Yam, 2010). Reptation theory is one of the commonly accepted models which explain the polymer molecule interdiffusion during sealing. The molecular mechanism involved in the heat sealing of polymer films is shown in Figure 2.1. The thermal energy provided during the heat sealing breaks the polymer molecules from its tube-like constraint and helps in initiating molecular movement. According to this theory, the molecules which escape from the constraint continues to move. The reptation time (Tr) is the time when the polymer molecules lose the memory of its original position and initiate snake-like motion (Wool et al., 1989). The freely moving polymer molecules inter-diffuse across each of the film boundaries and entangle with the longer chain polymer molecules. The interdiffusion of the polymer molecules is dependent on temperature, dwell time, molecular weight, polymer structure, its distribution, orientation and composition (Jabbari & Peppas, 1995). The interface of the polymer is assumed to be an amorphous structure based on the “reptation theory”. The number of monomers (N) crossing the interface and interpenetration depth (Li) of the molecules in the films is given by the following equations (Whitlow & Wool, 1991).

When t < Tr:

N(t) ∝ t3/4 M-7/4 (2.1)

1/4 -1/4 Li(t) ∝ t M (2.2)

When t > Tr:

N(t) ∝ t1/2 M-1 (2.3)

1/4 -1 Li(t) ∝ t M (2.4)

Where t is time and M is the molecular weight of the sealant polymer.

The polymer chains recrystallize upon cooling and yield a continuum heat seal within the seal area (Kausch & Tirrell, 1989; Rastogi et al., 2005). These entanglements act a topological barrier by preventing the material from flowing under shearing stress (Busse, 1994). The entanglement density depends on the elastic modulus in the rubbery state of melt and the average molecular weight between the entanglements (Me) (Gedde, 1996; Rastogi et al., 2005).

o G N=ρRT/Me (2.5)

o o where G N is shear modulus in the rubbery plateau region of the polymer (G N) as observed in the graph of log (shear stress) versus log (time); ρ is the density of the polymer molecule; R is the gas constant; T is the absolute temperature.

Figure 2.1 Molecular mechanism involved in the heat sealing of polymer films. Adapted from Najarzadeh (2014).

Heat sealing process parameters

The most common method for sealing the polymers in food packaging is by direct conductance method. The sealer parameters which are affecting the HSS and therefore, the performance of the seal, are heat sealing temperature, dwell time and pressure. Previous studies have reported the effect of these sealer parameters on the monolayer polymer films ( Theller, 1989;

Meka & Stehling, 1994; Mueller et al., 1998; Aithani et al., 2006; Mihindukulasuriya & Lim,

2012) and multilayer composite structures ( Oliveira & Faria, 1996; Yuan et al., 2007; Planes &

Flandin, 2011; Nase et al., 2013; Nase et al., 2014; Kanani Aghkand et al., 2018). However, to my

knowledge, there are no systematic studies involving heat sealing of continuous film to non-woven material.

2.4.1 Sealer jaw and interfacial temperatures

Jaw temperature plays a primary role in controlling the HSS of the films. The polymers are always sealed above the glass transition temperature for amorphous polymers and melting temperature for semi-crystalline polymers. The sealer jaws are set at a temperature to reach the required temperature at the interface of the film, and the temperature of the jaws will vary based on the contact area, time, construction material of the jaws and whether one or both the jaws are heated (Farkas, 1964).

The interfacial temperature between the films is essential in controlling the HSS of the films. The temperature at the interface controls the molecular interdiffusion and entanglement necessary for achieving desired seal strength. Stehling & Meka (1994) showed a stronger dependence of the interface temperature on sealer jaw temperature and dwell time from their study on LDPE films. The melting distribution of the polymer at the film interface is dependent on the interface temperature, which determines the seal strength. Maximal seal strength is achieved when crystalline regions of the polymers are completely melted. The typical heat-sealing curve relating interface temperature and the seal strength is represented in Figure 2.2. The curve shows three different temperature regions and the seal strength, which can be observed in semi-crystalline polymers: (1) Seal initiation temperature - the temperature at which measurable but weak seal strength is achieved; (2) Plateau initiation temperature - the temperature after which the ultimate seal strength value is achieved and remains constant over a range; (3) Final plateau temperature -

the temperature where the plateau region ends and the seal strength begin to decrease due to the degradation of the films (Stehling & Meka, 1994; Najarzadeh, 2014).

The temperature at the interface between two sealing films depends on the thickness of the films involved in heat sealing. Planes & Flandin, (2011) observed in their study on multilayer

Al foil-PET/LDPE film that the LDPE layer, which acted as a sealant, did not form seals below

115°C. The Al-PET/LDPE multilayer films did not allow the melting temperature to be reached at the interface due to the higher thickness of the multilayer film. They also observed a significant reduction in the temperature-time window for heat sealing multilayer films when compared to monolayer polymers.

Higher sealer temperature can lead to a decrease in the seal strength of films. Tetsuya et al. (2005) showed that high sealer temperature disrupts the molecular orientation of the multilayer films leading to a decrease in the tensile and seal strength of the films. The seal strength of the bilayer laminate with oriented-polypropylene (OPP)/ cast-polypropylene (CPP) film reached its maximum at the 120°C and decreased when increased beyond this temperature. These studies indicated that the energy required to achieve optimal seal strength is also dependent on the thermal properties of the films, film thickness, crystallinity and the molecular structure of the polymer.

Figure 2.2 Heat seal strength versus interface temperature plots for semi-crystalline polymers. Adapted from Stehling & Meka (1994).

2.4.2 Dwell time

Dwell time is another crucial parameter in heat sealing. Only when sufficient time is provided during heat sealing, the polymer films could melt completely, and the micro-Brownian diffusion of the polymer molecules into the opposite films could occur to form a strong seal. The dwell time is highly dependent on the sealer temperature. Typically, the temperature is held high to minimize the dwelling time requirement for increasing the sealing efficiency. Cheng et al.,

(2007) showed that the multilayer film with linear low-density polyethylene (LLDPE) sealant could be sealed to achieve adequate seal strength at 115°C, 0.2 s or at even lower dwell time of

0.1 s at 118°C. These observations showed that the dwell time is a secondary parameter in heat sealing, increasing of which after reaching a desired interfacial temperature would not increase

HSS and would reduce the production speed (Yuan et al., 2007). Theller (1989) noted that the seal strength reached its maximum at 0.4 s when jaw temperature was 109°C and at 0.35 s when jaw 13

temperature was 105°C in LLDPE films. The HSS did not increase after these dwell times and reached a plateau. A similar pattern of the result was observed by Meka & Stehling (1994). In their study, HSS reached a constant level at 0.4 s. They observed only 10% increase in the HSS when the dwell time was increased from 0.4 to 1.4 s. Although not relevant from an application point of view, Planes et al., (2011) concluded in their study with Al-PET/LDPE films that the lower temperature may be counterbalanced by a more substantial sealing time. Their study showed that an adequate seal strength was achieved for a longer dwell time of 90 s when the films were sealed between 110 and 180°C.

2.4.3 Effect of pressure

To form a strong seal, the heated films must be brought in contact with each other through compression. Typical optimum pressure ranges between ~15 to 30 psi. The application of lower pressure may not be ideal in removing wrinkles which can decrease film contact, while higher pressure can result in "runny" polymer film due to undesirable flow of the molten polymer

(Hishinuma, 2009).

Theller, (1989) found no dependence of sealer pressure on seal strength of HDPE and

LDPE even when contact pressure was increased by a factor of ten at a constant temperature.

Similar results were observed by Najarzadeh & Ajji (2014) in LLDPE films, the effect of sealer pressure was substantial on the seal strength only until intimate contact was achieved between the films after which the increase of pressure did not improve the seal strength significantly. Meka

& Stehling, (1994) did not consider pressure in heat seal modelling as they did not find any improvement in the seal strength when the pressure increased from ~ 7 to 1450 psi in LDPE films at temperatures between 110 and 170°C. They further suggested a similar optimal range of pressure 14

7 to 21 psi to avoid any defects in the seal area during sealing. Although the effect of sealer pressure on the HSS is small within an optimal heat seal process window, the application of a minimal threshold compression pressure is critical to ensure intimate initial film contact to allow molecular interdiffusion of polymer chains essential for forming a strong seal.

Multilayer structures

Multilayer films used in food packaging is a cost-effective manner to meet specific packaging requirements, including properties such as barrier, mechanical, sealing and shelf appearance. The films are designed with sealant (e.g., PE, ethylene-vinyl alcohol copolymer

(EVOH), or ionomers), adhesive, barrier, and abuse resistance layers (Morris, 2017). Multilayer films could vary from 3 to 12 layers, depending on their application. The films are manufactured by either adhesive lamination or co-extrusion process. Prior to selecting the multilayer film manufacturing process, the potential for thermal degradation of the films during extrusion or the need for adhesives must be considered. Multilayer films are widely used in semi-rigid food packaging industries as thermoformed trays to pack the prepared meals, fresh or frozen products, coffee capsules, and as well as in rigid and flexible packages. The films can take advantage of the mechanical properties of one polymer and the barrier/sealant property of another polymer for developing an ideal package for the specific application (Buntinx et al., 2014).

Some multilayer packaging structures that are being heat sealed has an outer face which is non-thermoplastic materials (e.g., paperboard and aluminum foil), or a thermoplastic with a substantially higher melting point than the inner sealant materials. This provides an advantage that the heated sealer bars can be directly applied to the outer part of the packaging film. By contrast,

in a monolayer structure, the material would melt and tend to stick to the surface of the sealing jaws, thereby destroying the seal area (Wagner, 2016).

Non-woven in food packaging

Non-wovens are initially considered as a cheaper replacement for conventional textiles.

The Association of non-woven fabric industry (INDA) defines non-woven as "sheets or web structures bonded together by entangling fibre or filaments, by various mechanical, thermal/chemical processes. These are made directly from separated fibres or molten polymers".

Non-woven materials are engineered for specific needs with the help of different polymer materials or by utilizing bicomponent/multicomponent fibre technologies.

A bicomponent or multicomponent fibre can be defined as fibres with two or more polymer materials extruded from the same spinneret with all the polymers contained within a filament (Xiao, 2004). These technologies help in exploiting capabilities that are not existing in each of polymers when present individually to bring about multifunctional properties.

Bicomponent fibres are classified based on the cross-sectional structures, as summarized in Figure

2.3. Tea/coffee bags are one of the most significant applications of non-woven fabrics in packaging industries by volume. The non-woven used in tea/coffee packaging is primarily produced by wet- laid technology. These non-woven web structures allow an infusion of tea/coffee while also ensuring the retention of fine particles (Kellie, 2016).

Figure 2.3 Classification of bicomponent non-woven material based on the cross-sectional structure of their fibres (TAPPI, 2002).

Heat sealing of non-woven films

Unlike continuous films heat sealing, which is characterized by molecular interdiffusion between layers, mechanical bonding mechanisms are involved during heat sealing of porous non- woven materials. When the melting temperature of the porous material is higher than the film/sealant used, and the energy supplied is not adequate to initiate molecular movement from the nonwoven, the polymer molecules from the sealant film tend to flow into the pores and penetrate them, creating increased interfacial contact. This increases the adhesion strength, and the strength is dependent on the polymer films involved for seal formation ( Wake, 1973; Dartman

& Shishoo, 1993).

The molten polymer liquid that flows into the porous structure can be viewed as the movement of liquid by capillary intake governed by the wettability of the system, capillary force, 17

pore structure of the non-woven material, and processing conditions (Hodgson & Berg, 1988;

Dartman & Shishoo, 1993;). The kinetics of wetting is determined by liquid rheological properties and surface energy, both of which are critical in influencing interface formation and adhesion strength (Bluestein et al., 1975). When the polymer film and non-woven are heated above its glass transition temperature, the polymer chains have the mobility to penetrate deeper by diffusion if adequate dwell time is used, thereby increasing the adhesion strength (Wicks, 2007). Similar to that in the continuous films, the interfacial diffusion in the porous material is governed by the compatibility of the polymers, molecular mobility of the polymer chains, the microstructure of the non-woven materials, and heat-sealing conditions (Schonhorn, 2016).

Seal integrity and maintenance

Seal integrity can be defined as seal continuum, i.e., a complete fusion of the sealing area with no discontinuities (Mihindukulasuriya & Lim, 2012). Poor seal integrity signifies poor manufacturing practices in the industries and facilitates product deterioration through oxygen and moisture transfer through the defects. National Food Processors Association (NFPA) recommends a thorough inspection to ensure that no containers with defects in hermetic seals are distributed

(Ozguler et al., 2001).

Regardless of careful and regular monitoring of sealing operation, the seal defects occur due to:

(1) Improper maintenance of temperature, dwell time and pressure in the sealer; (2) Misalignment of healing substrates; (3) Surface defects of seal jaws; and (4) Presence of contaminants in seal area (CFIA, 2002).

Seal defects

The seal defects can be classified into major and minor based on whether the defect poses a potential safety concern because of the hermeticity of the container. The severity of the defect increases with the extents of damage in the package and the food product involved.

2.9.1 Inclusion/contaminated seal

Seal defects involving contaminates either from the food product or foreign material during filling can compromise the package integrity. Seal area contaminated with lipid, water, or foodstuffs reduces the reliability of seal strength. Mihindukulasuriya & Lim, (2012, 2013) showed that the presence of water or lipid contamination in the seal area could act as a heat sink interfering with the melting of LLDPE films, thereby reducing the interface temperature and seal strength.

The post-process contamination of food can occur when the seal integrity of flexible and semirigid packages is compromised. Channel leaker and pinholes are the common types of leakage found in food package (Song & Hargraves, 1998), due to incomplete sealing across the width of the seal. The minimum channel leaker size for the penetration of vegetative cells in the presence of different food contaminants inside the package was investigated by a few researchers. The channel threshold was found to be 10 µm in PP aseptic cups contaminated with chocolate milk and in PP/EVOH/PP retort trays contaminated with spaghetti and yeast extract. While, PP/EVOH/PP trays with chicken and beef enchilada required a greater size of 70 µm and 200 µm channels

(Hurme et al., 1997; Ravishankar et al., 2005). The less viscous and less acidic yeast extract facilitated the movement and survival of bacteria from the surrounding bacterial solution into the package even in the presence of smaller channels. In contrast, chicken and beef were more viscous to transverse across smaller micro leaks and facilitate mixing of bacterial solution, like that of 19

yeast extract. Short channel length, small bacteria diameter, and channels filled with liquid can exacerbate the risks of microbial contamination.

2.9.2 Pinholes defects

Pinholes or micron-sized holes in the sealing regions are one of the most prevalent defects next to channel leak defects in packaging. Gilchrist et al., (1989) showed that the bacteria were able to pass from 108 tryptic soy broth into 5 µm sized pinholes in trilaminate pouches made of polyethene, aluminum foil and polypropylene. Many studies have concluded that the probability of product contamination via micro-hole increases with increasing hole diameter and the increasing pressure differential across the packaging walls. Moreover, an increase in motility of the microbes and a decrease in viscosity of the liquid contaminants can increase safety risk (Bankes

& Stringer, 1988; McEldowney & Fletcher, 1990; Ahvenainen et al., 1992). Seal defects also increase the permeation of oxygen and moisture into the package, which might affect the quality of the product. Achieving a high performing seal in a package is critical to extending the product shelf life (Solovyov & Goldman, 2008). Seal defects are also perceived as a quality loss and as a severe health risk by the consumers, which directly impact the marketability of the products.

Therefore, there is a great need for the detection of seal defects of packages in the food industries.

Conventional methods of seal integrity testing

The conventional methods to test the seal integrity are chosen based on package compatibility, cost consideration and sensitivity. Both destructive and non-destructive tests are available to test the structural integrity of packages and leaks. The commonly used methods for package integrity and leak detection tests are summarized in Table 2.1.

Destructive tests are optimal for examining the seal strength of the package. However, they are not good indicators of seal integrity, as they cannot detect micro leaks and seals contaminated with food or foreign material. Tests for defect identification are performed at a regular interval, and, in most food industries, half-hourly sampling is prevalent (Dudbridge, 2016).

Imaging techniques for seal testing

Imaging techniques have been widely used in the food and agriculture sectors for quality inspection and grading (Chen, 2013). The imaging system automates the visual inspection of the objects for measurement and classification through the camera. The images obtained are analyzed for the predefined task, such as measuring size, shape, colour or any defects in the sample. The system then classifies the product based on the conclusions drawn from the data (Du & Sun, 2004).

Various imaging techniques such as RGB, ultrasound, magnetic resonance imaging (MRI), X-Ray, computer tomography (CT), infrared (IR), fluorescence and hyperspectral are widely being investigated to analyze the external and internal quality of food and agricultural products. The potential of different imaging techniques for detecting food package defects are still less being explored except for ultrasound imaging. The necessity that the food industries require economical and reliable method for continuously monitoring the integrity of food packages and seals during manufacturing, calls for integrating imaging techniques in the production line.

Table 2.1 Summary of methods for the detection of seal integrity and leak

Test Techniques Description Reference

Destructive

Leak Detection test Biological test • The package is either sprayed with microorganism culture or (Arndt, 2001) dipped in the culture. The entry and growth of microorganism into the package is evaluated after the incubation period.

Bubble test • The package is immersed in a liquid, and the formation of (ASTM F2096-11 bubbles denotes the presence of leaks. The bubble forms due to (2019)) the pressure difference between the package and the environment. Dye penetration test • Presence of leaks is detected by seeping of dye into it. Most (ASTM f1929-98 commonly used after the electrolytic test. (2004))

Electrolytic test • The package is cut into half and filled with electrolyte solution (Smith & Hui, and placed in a vessel containing the different electrolyte. The 2008) presence of the seal leak is indicated by the voltage difference

due to the electron transfer. Tracer gas test • The tracer-gas filled packages are placed inside a vacuum (ASTM F2391 – 05, chamber. The gas flows out in the presence of a leak and is 2016)

detected with the help of sensors. Tracer gases like O2, CO2, N2 and He are commonly used as tracer gases.

Package structural integrity test

Impact tests • The failure of the package is tested by allowing it to collide the (ASTM D1709- wall at different inclinations and speed. 16a (2016))

Tensile test • Seals are mechanically pulled apart under a controlled rate until (ASTM D882- separation to measure the seal strength 18(2018); ASTM D828-16e1 (2016))

Vibration test • Package performance during distribution chain is tested by (ASTM D999 - simulating the vibrations. 08(2015))

Non-Destructive

Leak Detection test Capacitance test • The potential difference between the plates in the presence of (ASTM D150-18 packages is measured using conducting plates, and it varies in the (2018)) presence of leaks.

Eddy current probe • The packages are exposed to metals to induce electrical current (García-Martín et when the magnetic probes are moved over the package, and the al., 2011) irregularities are detected by an interruption in the eddy current flow.

Package structural integrity test

Calliper test • Irregularities in the seal thickness are measured using callipers (Chichester, 1964) Calliper test • Seal integrity is transparent films are tested by passing a beam of (Newman & light Thayer, 2002)

2.11.1 Infrared thermal imaging

Infrared thermography enables the measurement of the surface temperature of a body with high temporal and spatial temperature resolution (Gowen et al., 2010). The principle behind thermal imaging is that all bodies emit infrared energy above 0 K (- 273°C) which could be captured by thermal sensors in the camera. These sensors convert the infrared radiation into electrical signals and display them as a thermal image (Williams, 2009). The potential of the thermal cameras to sense variation in heat diffusion in different materials enables the quality control monitoring of food and agricultural products, such as in the quality evaluation of fruits, vegetables, meat, detection of foreign materials, temperature mapping in cooked food and grain drying (Vadivambal & Jayas, 2011).

The use of IR imaging for quality control of laser sealing in polymer food trays with transparent lids was investigated by Al-Habaibeh et al. (2004). The study showed that the infrared system could monitor the heat pattern during and after sealing, using a low-resolution thermal camera of 16 × 16-pixel and could differentiate between good, faulty and contaminated seals. A thermal camera-based system could detect channels and thermally insulated inclusions in

PE/Al/PP retort pouch of 108 µm thickness. A laser source was used to induce temperature difference in the samples moving at a speed between 0 to 5 cm/s. The monitoring system could detect a 5 mm defect using a thermopile sensor while a microbolometer sensor could detect 1 mm defect. The resolution of the system was improved by 1) improving the speed as it enhanced the variation in thermal output and decreased the distortion by the consequent lateral diffusion of thermal energy and by 2) using a magnifying lens with the trade-off for the field of view (Morris,

2016). IR thermography detected the ground coffee powder of 0.60 mm in the seals of transparent

OPA/PE films polymer bag sealed at 140°C for 0.5 s using VFFS machine. Six different image processing techniques were compared for its detection performance in this study among which fit of the cooling profile was found to be the best; which was based on the intercept of a first-order fit of the cooling profiles in the logarithmic domain (D’huys et al., 2015) .

2.11.2 Ultrasonic imaging

The feasibility of ultrasonic imaging (UI) to identify seal defects like channels and inclusions in flexible and semi-rigid food packaging materials has been widely investigated. The minimum detectable defect size varied based on the packaging material, the frequency and the mode of the ultrasound used and is summarized in Table 2.2. Ultrasound frequency of 2.25 MHz could detect seal defects such as voids, wrinkles, inclusions in plastic-aluminium bilayer film in transmission mode. The sample is required to be placed in between emitting and receiving transducers, and a very high frequency of about 100 MHz is needed to detect the channels of 10

µm (Ayhan & Zhang, 2003). This limited the real-world applications of ultrasonic imaging technique. To overcome this disadvantage, the potential of UI using pulse-echo in reflection and backscattering mode for seal integrity testing was studied. This mode could examine comparatively samples of greater thickness at a lower frequency than the transmission mode, and the ultrasound signal could be probed and received from one side using a single transducer.

The ultrasonic pulse-echo is represented by three signals: (1) A-scan that provides the amplitude of the received sound wave as the function of time; (2) B-scan displays the object's cross-section and indicates the size of the defect and its relative position; and (3) C-scan shows the same information in the object's plane perspective (Yin et al., 2003). Off-seal defects, non-bonded areas, abrasions, food and foreign material inclusions in semi-rigid and polymeric trays were 26

identified using A- and C-scans. The presence of defects was shown in both A and C scans by a decrease in amplitude. The drop-in amplitude was due to reflection and scattering loss from the non-smooth seal region sheet. The results were further confirmed with optical microscope images.

Ultrasound C-scan images could also detect channels that are ≥ 20 µm in packaging involving multilayer tray and lid (Pascall et al., 2002).

Many other researchers found that ultrasound waves could detect channel defect of diameter varying from 6 to 37 µm (Raum et al., 1998; He et al.,2008; Frazier et al., 2000; Xiangtao

Yin et al., 2001; Qi Tian et al., 2000 ) including air and water-filled defects (Ayhan Ozguler et al.,

1999, 2001). Safvi et al., (1997) reported that although the channel defects of 10 µm size were detectable, the method could not distinguish the diameter difference of the channels because of the resolution limit.

2.11.3 Terahertz imaging

Terahertz (THz) in electromagnetic wave space lies between mid-infrared and microwaves. Like radio waves, terahertz radiation can penetrate various materials such as paper, vinyl, plastics, textiles, ceramics, semiconductors, lipids and powders (Kawase et al., 2010). It is widely used to characterize the electronic, vibrational and compositional properties of solid, liquid, and gas in industries (Yan et al., 2006). The presence of even low-density contaminants like insects and plastics in food can be detected using THz (Lee et al., 2012). The ability of 0.6 THz waves to detect air and water-filled channel defects, in both transparent (Poly(amide) PA/PE - 70 µm) and

Table 2.2 Ultrasound imaging for detecting package defects in flexible and semi-rigid packaging

Ultrasound Pulse echo Defect Reference Packaging film Polymer Thickness (µm) Frequency Scan mode Type of defect Minimum detectable (MHz) size range Poly(ester)/Aluminum/oriented 16X103 2.25 Transmission Void 1.4 cm Poly(propylene) (Song et Inclusions al., 1993) dehydrated 0.05 mg Oriented carrot poly(propylene)/Aluminum/poly(p 18X103 ropylene) cooked & 0.1 mg dehydrated beef moisture 0.01 mg bentonite 0.05 mg Wrinkles 0.15 cm difference in length of the seal Poly(ethylene) 68 100 Transmission Channel (saline, 10 µm (Safvi et air and both) al., 1997) Plastic trilaminate 115 Poly (propylene)/Poly (vinylidene 110 17.3 BAI Inclusions (Ozguler chloride)/ Oriented Poly (amide) mouse tail 22.69 µm A et al., tendons 1997) Poly (propylene) /Aluminum /Poly 5-15 µm, 6 µm rarely (ethylene) 120 Channel (air and water filled)

Oriented Poly (amide)/Poly 110 17.3 BAI Channel (air and 25 µm -(90°incidence) (Raum et (vinylidene chloride) water filled) 10 µm -(6° incidence) al., 1998) /Poly(propylene) Poly (propylene)/Poly (vinylidene 110 17.3 BAI Inclusions 20 µm, (Ayhan chloride)/ Oriented Poly (amide) (mouse tail 15 µm Ozguler et tendons) al., 1999) Poly (propylene) /Aluminum /Poly 120 Channel (air and (ethylene) water filled) Poly (propylene)/Poly (vinylidene 110 17.3 BAI, RFS Channel (air and 6 µm (Frazier et chloride)/ Oriented Poly (amide) and RFCS water filled) al., 2000) & Poly (propylene) /Aluminum /Poly 120 (ester) Plastic and aluminium trilaminate - 17.3 BAI, Channel (Qi Tian et RFCS. Plastic 6 µm al., 2000) trilaminate (air and water filled) except for 38 µm water- Aluminium filled and 6 µm air-filled- trilaminate aluminium trilaminate Poly(amide)/Poly (vinylidene 110 17.3 BAI nonbonding and N/A (Ayhan chloride)/Poly(propylene) delamination, Ozguler et 120 blisters, bubbles, al., 2001) Poly (propylene) /Aluminum /Poly and (ester) wrinkles. Poly(amide)/Poly (vinylidene 110 22.9 BAI Channel (air- 38 (Shah et chloride)/Poly(propylene) filled) al., 2001)

Tray- Poly(propylene)/Ethylene 20 C-scan Channel (air- ≥20 µm (Pascall et vinyl alcohol/Poly (propylene) filled) al., 2002)

Lid- Poly (ethylene terephthalate) / Poly (vinylidene chloride)/Poly (amide) /High density poly (ethylene)/Poly(propylene) 103

Semi-rigid containers - 20 A-Scan and short seal, non– N/A (Ayhan & High impact poly (styrene)/Poly C-Scan bonded areas, Zhang, (vinylidene chloride)/Low density inclusion- wire, 2003) poly (ethylene) Teflon and abrasion polymeric trays Poly(propylene)/recycled Poly (propylene)/Ethylene vinyl alcohol/ recycled Poly(propylene) Lid- Poly(olefin)/Aluminum/Poly(amid e) /Poly(ester) Acrylate based transparent 114 17.3 BAI Point reflectors 127.2 µm (Yin et al., polymer film (toner deposits 2003) acted as defects) Poly(ethylene) 80 22.66 BEEI Channel 50 µm (He et al., 2008)

*BAI- Backscattered Amplitude Integral; BEEI- Backscattered Echo Envelope Integral RFC – Radio Frequency Correlation; RFCS- Calculating correlation coefficient over a short segment of RF signal; A-Scan – Amplitude Scan; C- Scan – top view of the material from A-scan. 30

Opaque (LDPE-80 µm) polymers were confirmed in a study conducted by Morita et al., (2012).

The samples were allowed to move at varying speeds ranging from 0.1 to 80 cm/s to simulate the packaging line movement in the packaging line. The minimum detectable limit for channel size varied with the line speed and the type of defect. The air-filled channel had a lower detection limit at any given line speed, as the water absorption and diffraction of THz had a more substantial effect than refraction and diffraction from the air-filled channel. In comparison with the UI technique, terahertz imaging has the advantage that it could detect 38 µm defect at 40 cm/s line speed while in the UI the same channel size defect could be detected only at 0.1 cm/s.

2.11.4 Other Imaging techniques for seal integrity testing

Harper et al., (1995) proposed the use of light penetration test to inspect the seal thickness based on the principle that the thinner/ heat damaged weak seal allows more light to pass through it as it will be translucent when compared to strong seal. (Barnes et al., 2012) investigated the effectiveness of polarized light stress imaging (PLSI) and laser scatter imaging (LSI) to characterize the channel defects in transparent polymer tray sealed with a polymer film. The type of polymer was not mentioned in the study. The defects were detected with 96% accuracy using

PLSI, 90% accuracy with LSI and 95% by combining both. The PLSI utilized camera and polariscope made from the LED monitor and the LSI utilized ranger camera with two different intensities of the laser line. A machine vision system with CCD camera could detect the defects with 93.6% accuracy in transparent films. The type of polymer film and the size of the seal defects were not mentioned in the literature (Shuangyang, 2010).

Coffee quality and shelf life

Coffee, with its unique sensory and physiological effects, is one of the most commonly consumed beverages worldwide. The flavour in the coffee develops due to the complex chemical reactions such as Maillard, Strecker reactions and degradation of polysaccharides, proteins, trigonelline and chlorogenic acid during the roasting process. Chlorogenic acids (CGA) are responsible for astringency, aroma, and pigmentation in coffee (Ky et al., 2001; Farah et al., 2006).

The degradation products of CGA along with caffeine is responsible for the bitterness of coffee

(Casal et al. 2000; Ky et al. 2001; Farah et al. 2006; Keast 2008; Toci et al. 2013). Among the two varieties of coffee, robusta coffee is generally known for its bitter taste and astringency, whereas

Arabica has excellent acidity, flavour, and an intense overall aroma (Ky et al. 2001; Nebesny and

Budryn 2006).

Roasted coffee is prone to chemical and physical changes that may substantially affect the sensory quality of the coffee. The quality of coffee is greatly affected by moisture, oxygen, light, and extraneous odours. Labuza et al. (2001) showed that oxygen was one of the critical factor regulating coffee shelf life and demonstrated that lowering oxygen to 0.5% in a coffee jar could improve shelf life by 20 times. The oxygen exposure leads to the formation of volatile substances by lipid oxidation resulting in odour and flavour loss in coffee (Ross et al. 2006; Toci et al. 2013).

Huynh-Ba et al. (2001) observed that after roasting and processing of coffee, oxidation of lipids to volatiles occurred within the first 24 h.

Studies have shown that the water activity of coffee increases on its exposure to high moisture environment. This accelerates the loss of volatile compounds and therefore shortens the shelf life

(Anese et al., 2006). The coffee absorbs water when stored in humid conditions after being roasted

(Apostolopoulos and Gilbert 1988; Illy and Viani 2005).

Temperature is another factor which affects the shelf life of coffee as it accelerates the deteriorative reactions (Nicoli et al. 2009). Moreover, increased temperature is positively correlated with the release of carbon dioxide and volatiles from coffee (Nicoli et al., 1993), thereby causing staling. Labuza et al. (2001) showed for every 10°C increase in temperature, degassing of roasted whole coffee bean increased by 1.5 times, and for ground coffee, it increased by three times.

Coffee packaging requirements

Coffee packages are designed to maintain the product freshness and to preserve the sensory attributes of the coffee throughout its shelf life. The packages should allow the vent of gases produced by coffee during storage while keeping it away from oxygen and moisture. The

CO2 released by the coffee can cause the package to swell up or even explode if the package cannot vent the gas. The release of CO2 depends on the degree of roasting and grinding, as darker roasts releases more CO2 in comparison with light roast coffees, and the whole coffee beans have the higher need for venting than ground coffee (Yam, 2009; Ferranti et al., 2019).

The material for coffee packaging is carefully chosen to prevent oxygen and moisture ingress based on the oxygen and water vapour transmission rate of the materials. The shelf life of

the coffee decreased by 10% for every 24 h when the coffee was exposed to room temperature

(Anderson et al., 2003). Typical coffee packaging polymer films include Al, LDPE, PET, PP and paper. Hermetic sealing of the packages is the most crucial part of coffee packaging without which the function of the packaging film becomes futile (Kiyoi, 2010). These are some of the challenges the packagers need to be aware of when manufacturing coffee packaging.

Coffee capsule packaging

Coffee capsules are single-serve packages made of either plastic or aluminum in which ground coffee is packed and held back into the packaging system by the filter for brewing. With the evolution in methods of coffee preparation, the population is increasingly switching from the conventional brewing method to single-serve coffee brewers who brew their coffee in seconds.

Coffee is the most popular beverage in North America; Statistics show that the percentage of

Canadians aged between 18-79 who own a Single-cup brewer system has increased from 15 to

36%, from the year 2012 to 2017(CCA, 2017). Unlike other food packaging systems, the coffee capsule is unique in that it commonly involves a non-woven fabric-like material. Plastic capsules with non-woven filter and metallized polymer film are preferred over aluminum as they are cheap and consume lesser energy for production (Li, 2017). The skeleton diagram of the coffee capsule is shown in Figure 2.4. The plastic capsules contain a multilayer wall/shell made up of polymers like PS or PP along with high barrier polymers like EVOH. PP/PS provides superior mechanical strength and heat resistance suitable for brewing coffee at higher temperatures. EVOH provides required gas/aroma barrier properties for preserving the characteristics of coffee throughout its shelf life. Non-woven fibres are highly engineered for a specific purpose, and in this case, it withstands the brewing temperature while allowing coffee infusion to pass through it. The 34

metalized film closures (lids) provide the required barrier and has a sealant layer for helping in achieving proper seal integrity. The aluminum layer in metallized films has the most effective water and gas/aroma barrier property and reduces the transmission rate (Brommer et al., 2011;

Cozzolino et al., 2015).

Figure 2.4 Schematic diagram of single-serve-coffee capsule and the enlarged seal region of the capsule

Effect of heat seal parameters on interface temperature and seal strength of multilayer structures with non-woven

Introduction

Methods of coffee brewing and packaging have evolved over the years to fulfil the needs of increasing customers' convenience. Single-serve coffee capsule systems in coffee packaging have been gaining popularity in the market recently (Parenti et al., 2014). The single-serve coffee- capsules are advantageous over the bulk packaging in that the portion-sized pack limit the product exposure to moisture and oxygen, thereby increasing secondary shelf-life of the coffee. There are three types of single-serve coffee capsules in the market: a) polymer capsules made up of multilayer shell and lid with an integral non-woven filter; b) aluminum coffee capsules made up of aluminum shell and lid; and c) compostable coffee capsules made up of biodegradable polymers

(Li, 2018). Although the aluminum capsules have superior barrier properties against light, oxygen and water, the plastic capsules are more popular because of their much lower price and lesser energy consumption during production (de Schrynmakers, 2009; Cozzolino et al., 2015).

The polymers used for coffee capsules are usually multilayer materials. They are either made of PS or PP film which sandwiches EVOH to enhance oxygen/gas barrier properties. The oxygen barrier property is improved further with the usage of aluminum foil/PE bi-layer lids (Yam,

2009). Additionally, a non-woven is integrated in the capsule to hold the ground coffee particles, as well as acting as a filter to retain the spent coffee ground during the brewing process. Non- woven and wall structures in single-serve capsules must be able to withstand the elevated temperature and pressure conditions in the brewing machines (Kellie, 2016).

The multilayer and non-woven films used as packaging films are usually heat-sealed.

Heat sealing is a critical part of food packaging which helps in preventing the processed food from contamination, leaking, and preserving the characteristics of the food product throughout its distribution chain (Ayhan & Zhang, 2003). The basic principle behind the heat-sealing process is to provide thermal energy to increase the film-film interface temperature and brings the films to intimate contact by compression. The temperature gradient due to the applied thermal energy facilitates heat transfer and thereby increasing the molecular movement in the polymer until it reaches the melting temperature (Tm). With an increase in dwell time, the polymer chains inter- diffuse across the film surface and form molecular entanglements (Rastogi et al., 2005).

Thermoplastics of higher molecular weight have higher entanglement density in the sealing area, giving higher seal strength than low molecular weight polymers (Buckley et al., 2006). The intermolecular interactions are reformed during cooling of the seal area, bridging the gap between the interfaces, forming an intact seal (Mueller et al., 1998).

The most widely used method of heat sealing is the heated tooling sealing/hot bar sealing to seal pouches, lidding to blisters, cups and trays in form fill and seal packaging. The factors such as temperature, dwell time, and pressure plays a vital role in achieving an excellent seal performance (Hishinuma, 2009). The HSS was maximum when the sealing was performed at a temperature closer to the Tm of the polymer – an essential parameter for characterization of the strength and failure mechanism in heat seals (Aithani et al., 2006). The dwell time is important in providing adequate time for the molecular interdiffusion to occur. Dwell time is often minimized to shorten the seal cycle to improve the production rate. The application of a minimum threshold pressure is required to bring the films to close contact to achieve the desired HSS in the films.

Long dwell time and high pressure do not enhance the seal properties; instead, this condition tends to be counterproductive and can induce seal defect due to displacement of molten materials from the seal area (Theller, 1989; Meka & Stehling, 1994; Cheng et al., 2007).

During heat sealing of multilayer films and non-woven film, when the melting temperature of the porous material is higher than the film/sealant, the molten polymer from the sealant layer tend to flow into the pores and penetrate them, creating increased interfacial contact.

Adhesion strength is created when the polymer cools down and solidifies to form mechanical interlocks (Dartman & Shishoo, 1993; Jones & Stylios, 2013; Wake, 1973). When both the films are heated above Tm, the interdiffusion of the molecules occurs at the interface, further increasing the adhesion force with time. The adhesion strength is also dependent on the affinity between the films and the polymer penetration between the pores. The energy required to peel the film from the non-woven film is given by equation (3.1) (Wicks, 2007).

푊푎 = 훾1 + 훾2 – 훾12 (3.1)

Where 푊푎 is the energy required for separating the layers; 훾1 and 훾2 are the surface tension of the film and porous materials, respectively; 훾12 is the interfacial tension between the films.

Heat sealing phenomena of monolayer films have been reported by researchers, including studies looking at the effect of the presence of contaminants in between the sealing layers on interfacial temperature and seal strength of the films (Cheng et al., 2007; S. D. Mihindukulasuriya

& Lim, 2013; S. Mihindukulasuriya & Lim, 2012; Najarzadeh & Ajji, 2014; Stehling & Meka,

1994). Overall, a strong relationship exists between the seal strength and the heat sealer parameters, which directly affects the interdiffusion of the molecules (Jabbari & Peppas, 1995). 38

Most of the studies in the literature are on heat sealing of multilayer films are focused on seal parameter optimization and failure mechanisms (Tetsuya et al., 2005; Cheng et al., 2007; Planes

& Flandin, 2011; Guo & Fan, 2016; Iwasaki et al., 2017; Aghkand et al., 2018). However, information related to heat sealing involving non-woven structures is limited.

This study aims at investigating the effect of sealer parameters such as jaw temperature, dwell time and sealer pressure on the seal strength of the seal structures, which includes multilayer and non- woven. The effect of the non-woven was estimated both quantitatively and qualitatively by analyzing the seal strength and failure mechanisms. Additionally, the film-film interface temperature was measured using thermocouples to study the effect of the non-woven on heat transfer during the sealing process.

Materials and methods

3.2.1 Materials

The multilayer films and non-woven materials used in for coffee capsule manufacturing were provided by a coffee roaster. The characteristics of the film materials are summarized in

Table 4.1. The melting temperature (Tm) of the materials was obtained from the literature and the specifications provided by the company.

Table 3.1 Film composition and properties

Material Composition Thickness (mm) Melting temperature (°C) Lid PET/Al foil/LLDPE 0.07 n/a/660/140 Non-Woven Non-woven1(NW1)- 0.17 140-250 material Poly(olefin)

Non-woven 2 (NW2)- 0.43 Core >250 • Core- Poly(ester) Sheath 140-250

• Sheath-poly(olefin) Wall PS/EVOH/PS 1.26 220/183/220

*PET: poly(ethylene terephthalate); Al: aluminum; LLDPE: linear low-density poly(ethylene); NW: non-woven; PS: polystyrene; EVOH: ethylene vinyl alcohol copolymer.

3.2.2 Heat sealing

The films were sealed using Sencorp Heat Sealer (model 12-ASL/1, Hyannis, MA, USA).

Figure 3.1 shows the relative position of the sealer jaws, films and thermocouples. The film samples were arranged in an order like that in the capsule (lid-NW-wall). Only the upper jaw of the sealer was heated, while the lower one was insulated with the silicone rubber. Thermocouples were placed between the film interfaces for measuring the interface temperature. The sealer jaw dimensions were 30.5 × 2.5 × 2.5 cm. To determine the effect of the non-woven, two cases of seal structures were considered: (a) seal without non-woven (lid-wall); and (b) seal with non-woven

(lid-NW-wall). The films were cut into 15 × 10 cm dimension, arranged as shown in Figure 3.1 and sealed at six different jaw temperatures 225, 230, 235, 240, 245, 250°C and for dwell times

0.5, 1.0, 1.5 and 2.0 s. The sealer pressure was maintained at 30 psi except for the experiment in which the effect of pressure on seal strength was studied; the films were sealed at 10, 30 and 50 psi. 40

3.2.3 Measurement of interfacial temperature

The interface temperature between the films was measured using bare wire k-type thermocouples (0.007 mm diameter, Omega Engineering, Stanford, CT), and temperatures were recorded with a high-speed NI SCXI-1600 data acquisition device (National Instruments, Austin,

TX). NI Labview Signal Express 2.0 software (National Instruments, Austin, TX), was used to record the data at 1 kHz speed. The temperature was recorded for five replicates at each sealer conditions.

Figure 3.1 Schematic of heat sealer jaws, film arrangements and thermocouple positions

3.2.4 Measurement of seal strength

Film specimens of 15 × 10 cm size were sealed as mentioned earlier but in the absence of thermocouples. Samples of dimension 9 × 2.5 cm (length × width) were cut from the specimens and used for seal strength measurement, as shown in Figure 3.2. The samples were conditioned at room temperature for 48 h before testing. The seal strength was measured using Instron Universal

Testing Machine (Model 1122; Instron, Norwood, MA, USA) at a 300 mm/min crosshead speed according to ASTM F88-00. The seal strength values were reported as an average of five measurements. In the case of seal structure which includes non-woven film, the seal strength was measured between the lid and the non-woven film. While in the case of seal structure without non- woven film, the seal strength measured was between the lid and the wall films.

Figure 3.2 Top view illustration of the film specimen with measurements used for (a) sealing and (b) seal strength testing.

3.2.5 Scanning Electron Microscope Imaging

Morphology of the cross-section of the films and sealed samples were visualized using a scanning electron microscope (SEM) (Quanta FEG 250, FEI Company, Hillsboro, OR, USA). For a cross-section of the films, the specimens were cut with sharp razors and sputter coated with tungsten before visualization to make the samples conductive.

3.2.6 Statistical Analysis

Seal strength and interface temperature data were subjected to analysis of variance (ANOVA) to determine the effect of sealer temperature, dwell time and pressure and mean comparisons were carried out by Tukey's test (p< 0.05). The correlation between the seal strength and the sealer parameters were analyzed. The statistical analysis was performed using the SPSS package (SPSS for Windows, SPSS Inc., Chicago, IL, USA).

Results and discussion

Interfacial temperature

Interfacial temperature is one of the essential variables in the heat-sealing process. The melting and interdiffusion of the polymer molecules take place when Tm of lid’s sealant layer, non- woven and the PS layer of the wall is reached between their interfaces. Therefore, the interfacial temperature can be a useful measure of seal strength and mode of seal failure. The interface temperature was measured between the films in seals with and without a non-woven filter. The interface temperatures are denoted using the symbol Tab, where ‘a’ represents the film involved in seal structure and ranges between 0 to 2; a = 0 represents seals without non-woven filter, a = 1 and a = 2 represents seals with non-woven filter 1 and non-woven filter 2, respectively. The subscript

‘b’ represents the position of the thermocouple ranging from 1 to 4. The interfacial temperature measured in the study and the abbreviations are summarized in Table 4.2.

Figure 3.3 describes the change in interfacial temperature with the temperature of the jaws. The seals were prepared at a temperature ranging between 225 and 250°C and at dwell time

and pressure, 1 s and 30 psi respectively. The statistical analysis of the temperature data showed that there is a significant difference in the interfacial temperature between all three samples (seals without NW, with NW1 and NW2) and for seals which were heat-sealed at different temperatures.

The interfacial temperature in the presence of non-woven, i.e., T12 and T22 were lower in comparison to T02 by 5°C and 11°C, respectively. The significant difference in the interfacial temperature might be due to the thermal resistance offered by the air trapped in-between the pores of non-woven (Gibson et al., 2007; Kopitar, 2014). The resistance increases with the increase in the thickness of the film; this explains the much lower interfacial temperature in the presence of

NW2 when compared to NW1 (Kucukali Ozturk et al., 2018).

Table 3.2 Abbreviations for interfacial temperature used in the study

Interfacial Seals without non- Seals with NW1 Seals with NW2 temperature woven

Upper jaw-lid T01 T11 T21

Lid-wall T02 - -

Lid-NW - T12 T22

NW-Wall - T13 T23

Wall- Lower jaw T03 T14 T24

The interface temperatures at T13 and T23 were on an average 5 and 8°C lower than T12 and T22. The difference could be attributed to the variation in thickness and conductivity of the non-woven layer.

Figure 3.4 shows the effect of dwell time on the interfacial temperature. The interfacial temperature was measured when the films were sealed at jaw temperatures 225, 235 and 245°Cfor a dwell time ranging from 0.5 to 2.0 s and at sealer pressure of 30 psi. The temperature increased 44

linearly up to dwell time of 1 s for all jaw temperatures, beyond which, the dependence decreased substantially. The increase in interface temperature for 1.5 and 2 s dwell time was slower, indicating the interfacial temperature had approached the steady state. The time at which the steady-state is reached is a useful measure of the time needed to maximize sealing rates in the production line (Stehling & Meka, 1994). At higher seal temperature (245°C) and dwell time, the differences in interface temperatures between T12, T22 and T13, T23 were significantly decreasing, which might be due to the reduction in the thermal resistance of the seal substrates as the polymers were melted. In all the cases, the interface temperatures at T01, T11, T21 were approaching the set sealer jaw temperature. On the other hand, temperature at T03, T14, T24 did not increase beyond

60°C.

Figure 3.3 Change in the interfacial temperature with the sealer jaw temperature for seals without any non-woven and with non-woven (NW1, NW2) for a dwell time of 1 s and 30 psi. Abbreviations: interfacial temperatures: T02- between lid and wall; T12 – between lid and NW1; T22- between lid-NW2; T13- between NW1-wall; T23- between NW2-wall. 45

Figure 3.4 Change in the interfacial temperature with dwell time (0.5 – 2 s) for seals without any non-woven and with non-woven (NW1, NW2) at different sealer jaw temperatures (a)225, (b)235 and (c) 245°C and constant sealer pressure (30 psi). Abbreviations: interfacial temperature between lid-wall(T02); lid-NW1 (T12); lid-NW2(T22); NW1-wall (T13); NW2-wall (T23).

3.2.7 Seal strength

3.2.7.1 Effect of jaw temperature

The impact of jaw temperature on the seal strength of structures which includes non- woven and without non-woven can be seen in Figure 3.5. The seals were prepared over a range of temperatures (225 to 250°C) while maintaining constant dwell time and sealer pressure of 1 s and

30 psi, respectively. The maximum HSS for a seal structure without non-woven film was achieved

at 235°C and was about 632.10 N/mm. The presence of the non-woven in between the lid and wall structures contributed to significantly higher HSS values (p < 0.05) when compared to the

HSS achieved in seal samples without non-woven. The maximum HSS was observed in the presence of NW2 (968.95 N/mm) at 240°C. The HSS of the seal structure with non-woven was higher than the seal structure involving only the continuous films (lid and wall). The higher surface roughness of the non-woven would have facilitated mechanical interlocking or increase the contact area between the lid’s sealant layer and the nonwoven, thereby contributing to the increased HSS.

(Su et al., 2015). The difference in molecular diffusion and formation of molecular entanglement across the interface also would have been responsible for the origin of the difference in the HSS of seal structures (without NW, with NW1 and NW2) (Iwasaki et al., 2017). The temperature was enough to cause complete melting and interdiffusion of the polymer molecules from LLDPE of the lid and the polymer from the non-woven, resulting in more chain entanglements and increased seal strength (Theller, 1989). Iwasaki (2017) suggested that chain mobility could decrease when a large number of long-chain branches are present in a polymer. This low mobility could limit the interdiffusion of the molecules and lower the adhesion between LLDPE and PS. This observation shows that the seal strength was significantly dependent on the type of film used for sealing. The

HSS increased for all seal structures with the increase in jaw temperature, the maximum seal strength was reached at 235°C for seals without non-woven and at 240°C for seal structure with non-woven. The increase in the interface temperature during sealing has led to thermal motion of the polymer chain segments and deepening of the zone of diffusion, thereby increasing the peeling force required for peeling the seals apart (Cheng et al., 2007). The HSS decreased after reaching its maximum value in all the cases tested. A similar trend was observed by Planes and Flandin,

(2011) in multilayer film made up of 3 layers of PET coated with aluminum and a layer of LLDPE sealed at a range of 110 to 180°C and dwell time of 1 to 90 s. The weakening of seal strength at a temperature above 240°C for seal structure with non-woven while above 235°C for a structure without non-woven might be due to the thermal degradation of the polymers in the multilayer film and non-woven. Moreover, elevated temperatures also led to the squeezing out of the molten polymers in the sealed area resulting in poor seal performance (Butler & Morris, 2016).

Figure 3.5 Effect of jaw temperature on seal strength of seals without non-woven (W/o NW) and with non-woven (NW1, NW2). Seal strength data are for seals prepared at a dwell time of 1 s and pressure of 30 psi. “*” indicates no significant difference between the seal strengths, within the films; the color of “*” indicates that it corresponds to the film indicated with the same color in the line graph. 49

3.2.7.2 Effect of dwell time

The HSS values of seals prepared at different dwell times are summarized in Figure 3.6.

The dwell time of 0.5 s did not allow complete heat transfer from the jaw, which could have resulted in poor interdiffusion of molecules and thereby resulting in low HSS. The significant increase (p < 0.05) in the seal strength above 0.5 s dwell time shows that a minimum threshold is required for heat conduction into the substrate layers (Nase et al., 2014). The continuous heat transfer has led to an increase in HSS over time by providing enough time for the polymers to melt and flow across the interface. The polymer molecules diffusion across the interfaces is time dependent. The chain penetrates at a faster rate initially and slows down to attain equilibrium depth after a particular dwell time (Theller, 1989b). This might be the reason for the unaltered HSS observed after 1 s dwell time at 235 °C. The results indicate that in the presence of non-woven, dwell time of 1 s was long enough for achieving a maximum seal strength. While for seals without non-woven, 1.5 s is needed. The results are comparable to that of the previous study where Al- ethylene vinyl acetate (EVAc)/PE multilayer’s seal strength did not increase significantly after a dwell time of 1.5 s when sealed at 150°C and dwell time of 0.5 – 8.0 s (Nase et al., 2014). The exposure of films to higher jaw temperature (245°C) for an extended dwell time can lead to a significant decrease in the seal strength of the films due to thermal degradation and/or squeeze out of the molten polymer (Poisson et al., 2006). These results imply that low jaw temperature can be counterbalanced with a longer dwell time, and vice versa.

Figure 3.6 Effect of dwell time on seal strength of seals with without non-woven and with non- woven (NW1, NW2). Seal strength data are for seals prepared at a pressure of 30 psi and jaw temperature a) 225°C, b) 235°C and c) 245°C. “*” indicates no significant difference between the seal strengths, within the films; the color of “*” indicates that it corresponds to the films indicated with the same colour in the line graph.

3.2.7.3 Effect of sealer pressure

The effect of sealer pressure on the seal strength was tested by varying the pressure from

10 to 50 psi at 240°C, at two dwell times. Figure 3.7 shows that HSS was significantly low (p <

0.05) at 10 psi for structures with and without non-woven in comparison to the seal strengths attained at other sealer pressures. Low sealer pressures do not ensure close contact of the polymer films, and this would have hindered optimal molecular interdiffusion between layers. While the excess jaw pressure (50 psi) caused squeezing out of the polymer and thinning of the seal area, resulting in lower seal strength (Barbarisi, 1967). As the seal strength was calculated with the

assumption that the thickness of the film does not change during sealing, the seal strength was lesser for films sealed at 50 psi when compared to films sealed < 50 psi. It is noteworthy that in calculating HSS, the original thickness of the seal was used in the calculation, which could under- estimate the HSS value. The minimal pressure across a significant fraction of the film surface is required for achieving good seal strength, and a useful quantity of HSS was achieved at 30 psi for both dwell time of 1 and 1.5 s. Studies on heat sealing of monolayer and multilayer films have shown that increasing the pressure beyond optimum pressure which ranged approximately between

7 to 21 psi, did not enhance the seal strength (Yuan et al., 2007; Najarzadeh & Ajji, 2014; Kanani

Aghkand et al., 2018).

The results of pPearson correlation analyses at 0.05 p-value indicated thatfor both structures (i.e., with and without nonwoven) showed that there was were a significant (p<0.05) positive correlations association between the seal strength (with and without nonwoven) and all the three sealer parameters (i.e., sealer jaw temperature, dwell time and sealer pressure). As revealed by tThe correlation coefficients, overall, the seal strength had the highest correlation with the sealer jaw temperature (0.78 and 0.81 for seal structures with and without non-woven, respectively). The dwell time had a lower correlation than the jaw temperature (and without non- woven was around 0.78 and 0.811 for sealer jaw temperature, 0.63 and 0.76 with and without non- woven, respectively), while the sealer pressure has the lowest correlation (for dwell time and 0.62 and, 0.68 with and without non-woven, respectively)for sealer pressure respectively.

Figure 3.7 Effect of pressure on seal strength for seals with and without non-woven (NW1, NW2) prepared at 240°C for a dwell time of (a) 1s and (b) 1.5 s.

3.2.8 Modes of failure

To understand the effect of sealer parameters, the failure behaviour of the heat-sealed samples was studied. The failure modes were classified into four types based on visual observation.

Different failure modes, corresponding load-displacement plots and its occurrence are summarized in Figures 3.8, 3.9 and 3.10, and discussed as follows:

Type A - complete peeling/adhesive failure was observed in heat seal structure with and without non-woven at low temperatures and shorter dwell time (225°C and 230°C, 0.5 s). The temperature and dwell time were insufficient to cause adequate molecular interdiffusion for creating a good seal. Inadequate molecular entanglement led to lower cohesive strength in the seal area when compared to the adhesive strength acting on them (Selke & Culter, 2016). This also resulted in low seal strength and complete peeling of the seal without any deformation.

Type B - the failure was characterized by complete peeling of the seal with elongation of the material. This occurred in heat sealing structure with and without non-woven when the temperature and dwell time are moderate. The seal adhesion was strong to cause the material to yield during the peel test. The temperature and dwell time would have been enough to create molecular interdiffusion and entanglement at the film interfaces, enhancing the seal strength of the material.

Type C - the failure at film body was observed in structure with non-woven material. The failure far from the seal area suggested a stronger seal area than the body of the film and indicates an excellent quality of heat seal. The HSS exceeded the tensile strength of the film leading to concomitant necking of the material at the remote (Guo & Fan, 2016). The failure was associated

with the strongest HSS and high elongation values than other modes of failure, indicating the optimum sealer conditions for achieving excellent seal performance.

Type D - the failure was observed in structure with, and without non-woven film, in which the material break at the seal edge could be attributed to the weakness of the seal edge due to fusing, laminate structures and sometimes the geometry of the peel itself. The extended dwell time and elevated temperature conditions led to thinning of bonding layer or sometimes thermal degradation of the material, resulting in fracture at the seal edge (Mihindukulasuriya & Lim, 2012). The weakened tensile strength of the film due to degradation could be associated with lower seal strength (Yuan et al., 2007) compared to the films which experienced the type C failure.

Different failure modes were observed under different seal conditions. At 10 psi, the films did not form a strong seal as the pressure was insufficient for bringing the films to intimate contact.

While for seals at 30 psi, the type C failure was observed indicating the formation of a strong seal.

While for the seals prepared at 50 psi, both type C and type D failures were observed. The type D failure was due to weakening of the seals at the edges under high-pressure conditions.

Figure 3.8 Seal failure modes observed under different sealer conditions with and without non- woven conditions. Adapted from Cheng et al., (2007).

Figure 3.9 Load-displacement plot as observed under different sealer conditions with and without non-woven conditions.

Figure 3.10 Observed failure modes as a function of platen temperature and dwell time for sealing structure (a) without non-woven, (b) with non-woven NW1 and (c) non-woven NW2.

3.2.9 Scanning Electron Microscope imaging:

Figure 3.11 reveals the three-layer structure of lid and wall film; The lid was composed of Al, PET, and LLDPE. The wall film was composed of two PS layers with a layer of EVOH in the middle. The irregularities on the cross-sectional surface of the wall film was caused during the sample preparation. The composite films are tailored to meet the critical performance requirement for the coffee capsules: The outer PET layer provides resistance against mechanical abrasions and chemicals (Badia et al., 2012; Erokhin et al., 2019).

The Al foil layer provides superior gas and moisture barrier properties to the package; while also allowing spontaneous conduction of thermal energy making the multilayer film suitable for heat sealing (Marsh & Bugusu, 2007). The LLDPE layer has excellent sealing properties such 60

as broad hot tack curve and low temperature sealability. Thus the LLDPE sealant layer could facilitate faster and robust packaging operations in the packaging lines (Iwu & Egbuhuzor, 2004).

The wall film also showed the three-layer structure with EVOH in the middle and PS layer on both sides of the film. The EVOH layer provides the required gas and aroma barrier properties to extend the shelf life of coffee (Carballo et al., 2005; Maes et al., 2018). The PS layer adds to the gas barrier and thermoformability suitable for forming coffee capsules (Marsh & Bugusu, 2007; Mieth et al., 2016).

The SEM micrography of the lid-NW1 seal (Figure 3.12) shows complete melting and fusion of the LLDPE layer and NW1 non-woven film. This reveals that the materials have melted and entirely merged by inter-diffusing across the sealed interface. This could have attributed to the strong seal strength and the film fractured at the remote indicating stronger seal area.

Figure 3.13 shows that the LLDPE layer of lid film has melted and fused with the poly(olefin) sheath of the non-woven while leaving the core structure intact. The complete melting and interdiffusion of the polyolefin from the non-woven and LLDPE from the lid film have led to higher seal strength. The intact core structure of NW2 could have increased the seal strength further when compared to the maximum seal strength achieved with NW1.

The SEM image in Figure 3.14 (a) shows the peeled surface of the seal made at lower temperature and time (235°C, 0.5 s). The lid surface sealed at 235°C, 0.5 s had fewer non-woven fibres attached to it as compared to the surface sealed at 235°C, 1 s in Figure 3.14 b). Wisps of non-woven fibres attached to the lid film indicate the initiation of cohesive peeling even at lower temperatures. The lower melting point of the non-woven led to the initiation of the molecular inter-

diffusion between lid and non-woven at 0.5 s, which increased with time as a result of which the cohesive strength has increased. The seals do not fail in the adhesive failure mode, although it appears to be by the visual observation.

EVOH

PS Deformed EVOH layer due to sample preparation

Figure 3.11 SEM micrographs of the cross-sectional view of the lid and wall multi-layer films before sealing. It is noteworthy that a sample preparation artefact existed for the wall structure. Because the sample was cut by using a scissor, the EVOH middle layer was distorted considerably, causing it to stretch and drape over the bottom PS wall layer.

Figure 3.12 SEM micrograph of a cross-sectional view of lid-non-woven1 (NW1) sample sealed at 235°C, 1.5 s and 30 psi.

Figure 3.13 SEM micrograph of a cross-sectional view of lid-non-woven2 (NW2) sample sealed at 240°C, 1s and 30 psi.

Figure 3.14 Peeled surface of the lid films sealed with non-woven 2 (NW2).

Conclusion

Heat sealing parameters had a significant effect on the seal strength of the multilayer and non-woven films. The study highlights the importance of sealer parameter, especially jaw temperature and the dwell time in controlling the interfacial temperature and seal strength of the multilayer and non-woven films. The interface temperature, which is an essential indicator of the seal performance decreased significantly in the presence of NW1 and NW2 by 5 and 11°C,

respectively. The reduction in the interfacial temperature was due to the thermal contact resistance in the presence of non-woven. For the dwell time of 1 s, the HSS increased with an increase in jaw temperature up to 240°C in seals with non-woven and up to 235°C without non-woven.

A minimum dwell time of at least 1 s in the presence of non-woven and 1.5 s for the seals without non-woven are required for achieving the desired seal strength in the films. Longer dwell times could be beneficial when jaws are at a low temperature; however, at elevated temperatures, the condition could be detrimental to the seal bonding. The minimum pressure of 30 psi was required in all the cases for creating enough contact between the films to achieve proper sealing.

Higher pressure caused compression and deformation of the seal area, making the seals undesirable. The maximum seal strength (968.95 N/mm) for seals with the non-woven was reached at 240°C for 1 s whereas, at 235°C and 1.5 s, it was 738.69 N/mm, for seal structure without non- woven. The much higher seal strengths in the seals with non-woven caused material necking, a different type of failure to that of observed in seals without non-woven. The microstructure observation of samples having maximum seal strength showed complete melting and fusion of the films. The SEM images of peeled surfaces indicated cohesive peeling even at low dwell time conditions in the presence of non-woven, the reason for higher seal strength. The core and sheath structure of the NW2 enhanced the seal strength as the core remained intact during the sealing, increasing the material strength. Thus, the composition of non-woven, thickness and its microstructure also had an impact on the seal strength apart from sealer parameters. Understanding the process of heat transfer during sealing will help in looking forward to reducing the temperature gradients between the films to achieve excellent seal performance.

Effect of coffee particle contaminants on the surface temperature profile and seal strength of multilayer and non-woven packaging.

Introduction

Seal integrity is essential in food packaging for maintaining the quality attributes of the food products throughout its shelf life. Most of the food packages made of polymers are heat- sealed, during which polymer melting, molecular diffusion and polymer chain entanglement play an essential role to achieve the desired performance of the seal. Any disruption in this process would directly affect the strength and integrity of the seal. The sealer process variables such as jaw temperature, dwell time can be utilized for controlling the film interface temperature responsible for the seal performance.

Seal area contamination is an essential factor which brings down the performance of the seal in food industries. Contaminants (both solid or liquid) in the seal area affect the seal strength, integrity and performance of the seals as they interfere with the molecular mechanisms taking place during the formation of the seals. The effect of a liquid contaminant in the packaging films has been studied (Mihindukulasuriya & Lim, 2012; Delle Cese et al., 2017). Mihindukulasuriya

& Lim, (2012) reported that the presence of vegetable oil in the seal region of an LLDPE packaging film lowered the seal strength compared to the water and the control. The diffusion of vegetable oil into the polymer during heat sealing resulted in weaker boundary layer when compared to water. However, Delle Cese et al. (2017) did not observe any changes in the seal strength in

PET/LDPE/LLDPE polymer bags when the saltwater or vegetable oil was present as a contaminant in the T-Point. The sealing conditions were different in both the studies, the monolayer LLDPE

was sealed in the range 150- 180°C and 0.5 -1.5 s, while the multi-layer packaging was sealed in the range 120 -160°C and 0.2 - 0.4 s. These studies show that a similar type of contaminants could affect the seal performance differently based on the packaging film and the sealing conditions.

Due to its simplified process and consistent taste in every brew, single-serve coffee capsules have become popular among consumers. Maintenance of seal performance and integrity is essential for ensuring that the package can resist mechanical stresses because of handling. Strong seals in coffee packaging prevent the oxidation, flavour and aroma loss of the product. Seal area contamination with the coffee particles could occur during the filling and packaging of the coffee capsules and is one of the major seal defects.

The most common method of monitoring the seal integrity in food industries is by squeezing the packs manually. The squeezing test has limitations because of its subjective nature and the risk of cross-contamination due to manual handling (CFIA, 2002). Tracer-gas detection

(ASTM F2391) method involves infusing tracer gases such as O2, CO2, N2 or He into the package and placing it in a test chamber containing inert carrier gas. The leakage of tracer-gas into the carrier gas is detected using sensors. Bubble tests (ASTM F2096) and pressure decay test (ASTM

F2095) are the other tests preferred for testing the seal integrity. In the bubble test, the package is immersed in a liquid, and the presence of leaks is denoted by the formation of bubbles due to the pressure difference between the package and the environment. The major disadvantages of bubble tests are the long response time, reduced sensitivity and tendency of the leaks to get clogged with the contaminant making the test less reliable. While in the pressure/ vacuum decay test, the package is placed inside a chamber and is exerted a pressure/ vacuum. The rate of pressure/ vacuum change in the chamber is measured over time and compared with control. The tracer gas 67

and pressure decay tests have high precision; the tests require a longer time to show the changes required for defect detection(Akers et al., 2002). Although the tests are non-destructive, Integration of the test systems into the production line remains a great challenge, as a result of which most of the tests are offline. This results in delayed feedback on the quality of the package, which makes the test system slow for continuous implementation. In most of the industries, the offline tests are used to inspect the samples at an interval of 30 min, thus making the tests applicable for only testing the representative samples in a batch. Most of these disadvantages could be overcome using machine vision systems which enables cost-effective and reliable inspection of 100% of the samples with higher sensitivity and accuracy (Smith,1996; Dudbridge, 2016).

The working principle of the thermal camera is based on the sensors which capture the infrared radiations from the body. In the case of active thermography, the thermal response of a body's surface to a heat stimulus is recorded and analyzed. Conversely, in passive thermography, there is no heat stimulus applied to the body.

The main aim of the study was to determine the ability of the thermal imaging technique for identifying the solid contaminants under the surface of the opaque seal layer typically encountered in coffee capsule packaging. The objectives of this study are: (1) distinguishing contaminated and sound areas on the sealing surface based on thermal profile and elaborating an effective segmentation procedure; (2) studying the effect of the contaminant on the seal strength of the package.

Materials and methods

4.2.1 Sample preparation

The packaging materials was contaminated in the seal region and examined under the thermal camera; to investigate the potential of thermal cameras for the detection of solid contaminants.

Lidding film (GPET/Al/LLDPE, 0.07 mm), non-woven filter with a polyester core and polyolefin sheath (0.43 mm thickness), and the wall films (PS/EVOH/PS, 1.26 mm thickness) used in the study were supplied by a coffee roaster. The test specimens were cut into 15 ×10 cm size films.

The films were arranged in an order like that in the coffee capsule, i.e., the lid, filter, and wall materials as a top, middle and bottom layers, respectively. The roasted coffee bean was ground using a coffee grinder and sieved to separate coffee particles of size 0.70 to 0.85 mm.

Contaminated samples were prepared with coffee particles of size ranging between 0.70 to 0.85 mm by placing the particles between the lid and filter layers to simulate contamination during the filling process in capsules.

Three different contaminant scenarios were considered in this study: (i) Single-particle at the center of the seal (SC); (ii) Single-particle near the edge of the seal (SE); and (iii) Multiple particles- 0.02 g of contaminant (25 g/m2 contamination density) across a 20 × 40 mm seal area

(MP). Plastic tapes were used to fix the film samples in place before heat sealing using a heat sealer (model 12-ASL/1, Hyannis, MA, USA). The test sample was carefully placed in between the upper jaw which was at 240°C, and the lower jaw was insulated using silicone rubber. The dwell time and sealing pressures were 1 s and 30 psi, respectively. The sealer conditions were

selected based on the optimization study from the previous chapter. Optical reference images of the contaminant spread in MP samples were compared with their corresponding thermal images.

4.2.2 Thermal and optical image acquisition

An infrared (IR) thermal camera (Model: FLIR E53, Flir systems, Burlington, ON,

Canada) was used to capture the temperature of the seal specimens immediately after the sealing process. The IR camera had an image resolution of 240 × 180 pixels and a temperature range of

0°C to 650°C, with an accuracy of ±2°C. The images were recorded for 25 s after sealing at 30 frames per second to obtain a total of 750 frames after sealing. The seal was kept still in a stable position when the jaws were opened after sealing. Optical images of the contaminants were captured using RGB camera of resolution 3072 × 2048 (Model: DFK 33UX178, Imaging source,

Charlotte, NC 28226, United States).

4.2.3 Thermal image processing

The thermal camera recorded a sequence of images of sealed packaging films immediately after the sealing process for 25 s. The thermal images were corrected for brightness and contrast. The preprocessed images with optimum brightness and contrast were cleared of noise by applying the median filter. The edges of the contaminated regions were sharpened by histogram equalization (Baranowski et al., 2012). This enabled the thresholding procedure to select the areas of contamination accurately.

4.2.4 Seal strength measurement

For seal strength measurement 2.5 × 9 cm strips were cut from 15 × 10 cm sound and contaminated samples. In the case of SC, SE contaminated samples, the single particle contaminant was placed at the center and at the edge of the seal region in the samples. For MP samples the contaminant weight was reduced to account for 25 g/m2 contamination density in 20 × 20 mm seal area. The samples were conditioned at room temperature for 48 h after sealing. The seal strength was measured using Instron Universal Testing Machine (Model 1122; Instron, Norwood, MA,

USA) at a 300 mm/min crosshead speed according to ASTM F88-0034. Seal strength values were reported as an average of five measurements. The seal strength values are derived from tests between the lid and nonwoven layers.

4.2.5 Statistical Analysis

Seal strength data and the temperature data were subjected to analysis of variance

(ANOVA) followed by tukey’s multiple comparison test for significance level of 0.05 (p- value).

The statistical analysis was performed using the SPSS package (SPSS for Windows, SPSS Inc.,

Chicago, IL, USA).

Results and discussions

4.3.1 Thermogram of the heat-sealed films

Thermal images of sealed films captured after sealer jaw opening were analyzed to discriminate the contaminated regions within the seal. The threshold segmentation is presented as a discrimination procedure for the contaminated region. Figure 5.1 shows the thermal images of 71

the samples and their respective threshold segmented images. The cooling temperature profiles

(CTP) (Figure: 5.2) of the sound and contaminated seal regions of the samples were constructed after evaluating the selected regions. The temperature evaluation was done by deriving average temperature values from the pixels covering the sound and contaminated seal region for a period of 25 s. From the CTP of all the samples, it can be observed that the temperature difference between the sound and the contaminated region was near-zero immediately after opening the sealer jaws.

The temperature difference in the contaminated region and the region surrounding the contaminant

(ΔT) became prominent with time during cooling.

In the single-particle contaminant samples (SC and SE), an average maximum temperature difference of 0.9°C to 3.1°C was found between the sound region and the contaminated region of the seal. The particle contaminant was distinguishable from the rest of the heat seal region due to the occurrence of low-temperature profile region surrounding the particle.

The maximum temperature difference between the region surrounding the contaminant and the sound region (ΔT) was observed at 3.5 s of opening of the sealer jaws. The ΔT was ~ 30°C and

38°C in SC and SE samples respectively at 3.5 s of CTP. The low temperature profile observed near the edge of the seal indicates that the heat transfer at the edges is lower than the rest of the seal region. The reduced heat transfer at the seal edges would have attributed to a comparatively lower temperature at the contaminated region and the higher ΔT in the SE sample.

In the MP sample, the thermal image displayed patches of lower temperature in the regions of contamination. The individual particles were not visible in the thermogram; this could be due to the overlapping of lower temperature regions surrounding each contaminant. The ΔT in

MP sample was ~ 30°C at 3.5 s after the opening of the sealer jaws, like that of the temperature 72

difference observed in SC seals. However, the number of pixels from which the ΔT was extracted for MP sample is 103 times more than that of the pixels involved in single-particle sample. The difference in the temperature between the contaminated and the sound seal region could be due to the local increase in thermal resistance in the region surrounding the contaminant. The presence of coffee particle contaminant with different thermal diffusivity than that of the film would have been responsible for the heat sink effect of the contaminant and creating thermal contrasts in its presence (Mihindukulasuriya & Lim, 2012).

Previous studies have shown that the defects such as poor welding, gap and insulated material in the seals could be detected using a thermal camera. Al-Habaibeh et al., (2004) employed the thermal cameras for investigating the quality of the seals in food trays with transparent lid films. The seals were made using the 50 W RF excited carbo dioxide laser, a non- contact laser sealing process. The cooling profile of the sealed region after 5 s of sealing, showed that a low-resolution thermal image is sufficient for measuring the changes in the heat distribution in the case of poor welding. (Morris, 2016) showed that the infrared thermography with microbolometer sensor could identify 1 to 10 mm gap defects and the presence of card-paper (0.24 mm thick), in seals of PE/Al/PP retort pouches. The transient thermal artefacts were created by

808 nm diode infrared laser heating source, which was aimed at the moving samples using a webcam.

Figure 4.1 Thermal images of the lidding layer of the sealed samples captured at 3.5 s after the opening of the sealer jaws. SC- seals contaminated with a single particle in the center; SE- single-particle contaminated at the edge SE; MP- 25 g/m2 contamination density.

Figure 4.2 Cooling temperature profile (CTP) of the sound and contaminated regions of the MP sample after sealing the films at 240°C, 1s and 30 psi. The mean temperature of the pixels covering the sound and contaminated regions were used for plotting the CTP.

4.3.2 Comparison of thermal images with the optical reference images

The optical reference image was overlapped with the thermograms to investigate whether all the particles in the optical images are visible in the thermogram. An example of the comparison procedure is presented in Figure 5.3. The optical images were binarized and overlapped with the thermograms, i.e., regions colored red to show the coffee particles and the white lines indicate the edges of the particles as shown in the thermogram. It is evident from the overlap images that most

of the particles seen in the optical image is accounted for by the thermogram. The mean area of the seal with contaminants measured from the thermogram occupied 52% more pixels than those measured in the optical image. This indicated that although the contaminants and its location are precisely visible in the thermogram, the size of the particle in thermogram is larger than the original particle size. This finding shows that external heat source applied during sealing can result in distinctive temperature profile to reveal the presence of contaminants, thereby facilitating their detection, even when they are buried under optically opaque films. However, since non-uniform heating, surface emissivity variations and surface geometry all have an impact on temperature profiles, quantification of the particle size of the contaminant using thermal data could be challenging.

Figure 4.3 Overview of the comparison of optical image and thermogram of seals contaminated with coffee particles of MP sample by image overlapping procedure.

4.3.3 Seal strength and failure mechanism

The seal strength and the failure mode of the sound and contaminated samples are summarized in Table 5.1 and Figure 5.4, respectively. The sound seals had maximum seal strength; however, it did not significantly vary from the seal strength of films contaminated with single particle in the seal area. The significant reduction in the seal strength of the MP sample could be due to the reduction in the number of binding spots between the films because of the presence of more amount of coffee particles.

The failure mode observed for the control followed elongation with a material break at the remote (Type C). By contrast, SC and SE samples showed a combination of peeling with elongation (Type B) and Type C failures. Although the SC and SE seals sometimes showed peeling, it was coupled with material yield (Type B). This indicates that the single-particle contaminants would have been small enough to be caulked by the sealant layer causing strong adhesion resulting in the material to yield before peeling. Michael Li (2016) showed in their study involving 11 different multilayer films with differents sealant that the seal strength of the films in the presence of solid contaminants depends on “caulkability,” i.e., the ability of the sealant to encapsulate the contaminant depends on the contaminant size other than the sealing and films involved. This was also confirmed in a study by Bamps et al. (2019). The study showed that the presence of ground coffee or blood powder contaminant in the seal area decreased the seal strength and narrowed the process window in three different multilayer films: a) PET and three layers of

LDPE and LLDPE blend; b) PET, two layers of LDPE-LLDPE blend and LDPE-plastomer blend

and PET/LDPE-LLDPE/acid copolymer resin and sodium ionomer. The films were sealed in the

2 range 120 to 180 °C, 0.4 to 1.0 s dwell time and 1.0 to 4.0 N/mm pressure. The seal strength of multilayer polymer with plastomer sealant was less affected by the contaminant when compared with LLDPE and sodium ionomer. The response to the single-particle contaminant in the seal strength of a real coffee capsule might be different as it has relatively smaller sealing region compared to the one in this study; leading to increase in the contamination density.

For MP samples, failures mainly showed Type A (complete peeling) phenomenon, indicating poor fusion and chain entanglement of the polymers, thereby weakening the adhesion strength due to the interference from the contaminants.

Table 4.1Comparison of seal strength of contaminated seals with the control sample (n=5). Sample Control SC SE MP

Seal strength 968 .95 ± 58.45a 815.24 ± 52.23a 639.95 ± 78.34a 556 ± 136.23 b

(N/mm)

*same letters indicate that the seal strengths are not significantly different from each other

(p<0.05).

Figure 4.4 Failure mode observed for control and contaminated samples obtained during the peel strength test. Adapted from Cheng et al., (2007).

Figure 4.5 Load-displacement plots observed for control and contaminated samples during the peel strength test.

Conclusions and future recommendations

The study shows that thermography is promising for the detection of coffee contaminants in heat seals, involving multilayer thermoplastic and non-woven structures. The increase in the local thermal resistance in the presence of contaminants are responsible for obtaining thermal contrasts between the sound regions and contaminated regions of the seal. The thresholding segmentation technique shown in the study can be used to automatically detect the seals contaminated with coffee particles of size ranging from 0.70 to 0.85 mm from the thermograms. The presence of multiple particles in the seal region with the contamination density (25 g/m2) decreased the seal strength. In future, investigation on thermograms and various image processing techniques like region growing, clustering, for the different particle size of the contaminants will help in determining the detection limit of the thermal imaging technique. The technology could be applied to other food processing systems to detect the contaminants in the seal area, provided that the contaminant causes detectable temperature contrasts during heat sealing of the packages.

Conclusion

In the first objective of the study, the effect of sealer jaw temperature, dwell time and pressure on seal strength and interface temperature have been studied in the heat seal structures involving multilayer films and non-woven. The interface temperature data showed a decrease in temperature at the interface of lid and non-woven by 5 and 11°C in the presence of NW1 and NW2, respectively when compared to the seal structures without non-woven. This was attributed to the thermal resistance offered by the non-woven film. The thickness of the non-woven was another factor which contributed to the difference in the interface temperatures between the non-woven.

The jaw temperature and dwell time affected the interfacial temperature and therefore the seal strength of all the seal structures. While minimum sealer pressure of 30 psi was required for achieving adequate seal quality. The maximum seal strength for seals with non-woven was achieved at 240°C for 1s (968.95N/mm) whereas at 235°C 1.5 s (738.69 N/mm), for seal structure without non-woven. The microstructure observation of the peeled and cross-sectional surface of seals showed that the bicomponent fibre structure of NW2 has contributed to the increased seal strength in the films. This shows the dependence of the performance of the seal on the composition, thickness and microstructure of the non-woven involved in heat sealing. It can be concluded from the study that the heat-sealing parameters had a significant effect on the seal strength of the multilayer and non-woven films. The study highlights the importance of sealer parameter, especially jaw temperature and the dwell time in controlling the interfacial temperature and seal strength of the multilayer and non-woven films.

In the second objective of the thesis, the potential of a thermal camera to detect the solid coffee particle contaminants in the seal area was studied. The results showed that the thermal camera could detect the coffee particles under the opaque layer of lid film. The local thermal resistance in the films increased in the areas of contamination, creating thermal contrasts after heat sealing of the films. An average temperature difference of 30°C in the case of SC and MP and

38°C in the case of SE sample was observed at 3.5 s of CTP in the areas of contamination. In MP samples, the thermograms accounted for all the particles that are visible in the optical image. The measurement of the contaminant size was challenging as the contaminants occupied 52% more pixels than that of the actual size. The seal strength of the films in the presence of contaminant varied based on the contamination density in the seal area. The single-particle contaminant did not

2 significantly affect the seal strength while the presence of multiple particles (25 g/m contamination density) decreased the seal strength of the films.

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