Characterization of PLA-Talc Films using NIR
Chemical Imaging and Multivariate Image Analysis
Techniques
Mémoire
Seyedshahabaldin Amirabadi
Maîtrise en génie chimique
Maître ès sciences (M. Sc.)
Québec, Canada
iii
Résumé
L’emballage joue un rôle important dans l’industrie alimentaire afin de maintenir la qualité des produits le plus longtemps possible. Les films de polymère sont largement utilisés dans l’emballage alimentaire et sont attrayants pour leurs propriétés exceptionnelles. Puisque les polymères à base de pétrole apportent des préoccupations environnementales, les polymères biodégradables tel que le PLA sont étudiés plus intensivement depuis quelques années considérant leurs propriétés écologiques. L’application de films renforcés permet, comparativement aux films simples, d’atteindre des fonctions spécifiques et d’améliorer leurs propriétés tel que l’étanchéité aux gaz.
Toutefois, puisque la structure de ces films est plus complexe, le contrôle de qualité de ces derniers est plus difficile. Dans l’industrie, des méthodes hors-ligne sont très souvent utilisées pour effectuer le contrôle de qualité des films produits. Le contrôle est nécessaire puisque la variabilité de la matière première ainsi que le changement des conditions opératoires amènent des modifications qui changent considérablement les propriétés du film. Par conséquent, une inspection en temps réel ainsi qu’un contrôle des films de polymère est nécessaire sur la ligne de production afin d’obtenir un contrôle de qualité s’approchant de l’analyse en temps réel.
Un système d’imagerie proche infrarouge (NIR) rapide et non-destructif est proposé pour caractériser les films biodégradables d’acide polylactique contenant du talc produits par extrusion-soufflage et utilisés pour l’emballage. Le but ultime est d’utiliser le système pour faire un contrôle de qualité sur la ligne d’extrusion ainsi qu’après le post-traitement thermique, soit le recuit. Un ensemble d’échantillon de film de PLA contenant différentes concentrations en talc ont été fabriqués. Ces derniers ont ensuite été soumis à différentes conditions de recuit. Des images NIR ont été collectées avant la caractérisation des propriétés physiques et mécaniques ainsi que l’étanchéité aux gaz. Des techniques d’imagerie multivariées ont été appliquées aux images hyperspectrales. Celles-ci ont montré que la quantité de talc peut être déterminée et que l’information du spectre NIR permet de prédire les propriétés du film. Dans tous les cas, la méthode proposée permet de déterminer les variations dans les propriétés du film avec une bonne précision.
iv
Abstract
Food packaging plays a great role in the food industry to maintain food products quality as long as possible. Polymer films are widely used in food packaging and also attract attention because of their outstanding advantages. Since petroleum-based polymers are known to cause environmental concerns, biodegradable polymers like PLA were studied more intensively in recent years due to their environmentally friendly properties. The application of reinforced films exceeds simple ones in achieving specific functions and enhancing their properties such as barrier properties. Since the films structures are more complex, quality control is more challenging. In industry, off-line methods are vastly used for quality control of the produced films while variability in raw materials and processing conditions substantially change the film specifications. Consequently, real-time inspection and monitoring of polymer films is needed on the production line to achieve a real-time quality control of the films.
A fast and non-invasive near-infrared (NIR) imaging system is proposed to characterize biodegradable polylactic acid (PLA) films containing talc, and produced by extrusion film-blowing for packaging applications. The ultimate goal is to use the system for quality control on the extrusion line, and after a post-processing via thermal treatment (annealing). A set of PLA-talc films with varying talc contents were produced and submitted to annealing under different conditions. NIR images of the films were collected after which the samples were characterized for their physical, mechanical, and gas barrier properties. Multivariate imaging techniques were then applied to the hyperspectral images. It is shown that various talc loadings can be distinguished, and the information contained in the NIR spectra allows predicting the film properties. In all cases, the proposed approach was able to track the variation in film properties with good accuracy.
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Table of contents
Résumé ... iii Abstract ... iv List of tables ... ix List of figures ... x Nomenclature ... xi Foreword ... xii Chapter 1. Introduction ... 1 1.1 Food packaging ... 1 1.2 Packaging materials ... 2 1.3 Polymer films ... 4 1.4 Film fabrication ... 8 1.4.1 Process... 8 1.4.2 Processing conditions ... 101.5 Quality control of polymer films ... 11
1.5.1 Scattering methods ... 12
1.5.2 Raman spectroscopy ... 14
1.5.3 Near-Infrared (NIR) spectroscopy ... 16
1.6 Thesis objectives ... 22
Chapter 2. Materials and methods ... 24
2.1 Sample preparation ... 24
2.2 Film characterization ... 25
2.2.1 Crystallinity ... 25
2.2.2 Mechanical properties ... 25
2.2.3 Barrier properties ... 25
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2.3.1 NIR imaging system ... 26
2.3.2 Image sets ... 27
2.4 Image calibration and pre-processing ... 28
2.4.1 Image calibration ... 28
2.4.2 Pre-processing ... 28
2.5 Multivariate latent variable methods ... 29
2.5.1 Multivariate image analysis (MIA) ... 29
2.5.2 Multivariate image regression (MIR) ... 33
Chapter 3. Characterization of PLA-Talc films using NIR chemical imaging and Multivariate Image Analysis techniques ... 36
Résumé ... 36
Abstract ... 37
3.1 Introduction ... 38
3.2 Materials and methods ... 41
3.2.1 Sample preparation ... 41
3.2.2 Film characterization ... 42
3.2.3 Hyperspectral image acquisition ... 43
3.3 Multivariate latent variable methods ... 45
3.3.1 Image calibration and Pre-processing ... 45
3.3.2 Multivariate image regression (MIR) ... 45
3.4 Results and discussion... 47
3.4.1 Prediction of PLA-Talc film properties ... 47
3.5 Annealing effect detection using NIR spectra ... 53
3.6 Conclusion... 58
Chapter 4. Conclusions and recommendations for future work ... 62
4.1 General conclusion ... 62
4.2 Recommendations ... 64
viii
Appendix 1. Moisture absorption analysis for PLA films using Multivariate Image
Analysis (MIA) ... 74
A1.1 Introduction ... 74
A1.2 Complementary material and methods ... 74
A1.3 Results and discussion ... 75
Appendix 2. Supplementary results on the measured and predicted film properties for the samples containing different talc contents ... 78
ix
List of tables
Table 1.1 Common polymers used in flexible packaging applications [11]. ... 6 Table 3.1 PLA film samples ... 42 Table 3.2 Predictive ability of the PLS models on the training and validation sets. A separate PLS model was built for each property. Each model has 6 cross-validated components. ... 51
Table 3.3 Predictive ability of the PLS models on the training and validation sets for the annealed films. ... 56
x
List of figures
Figure 2.1. Graphical interpretation of PCA. A) A 3-D space dataset X which was auto-scaled, B) the direction of maximum variance is captured by the first loading vector (p1), and C) the second direction of greatest variance orthogonal to the first is captured by
the second loading vector (p2) and data projection onto the subspace to obtain the
corresponding t1 and t2 values [81]. ... 31
Figure 3.1. PLS model results based on the samples containing different talc content: (A) t1-t2 score plot, (B) PC1 and PC2 loading weights, (C) NIR spectra for PLA
and PLA-3% Talc samples, (D) VIP values, and (E) predicted talc concentration maps... 49 Figure 3.2. Comparison between the measured and predicted film properties for the training and validation set of samples containing different talc contents: (A) talc composition, (B) crystallinity, (C) strain at break, (D) Young’s modulus, (E) O2 permeability and (F) CO2 permeability. ... 52 Figure 3.3. Loading weights of the PLS model built for the crystallinity of annealed samples. The explained crystallinity variance by PC1, PC2 and PC3 are 5.53%, 20.10% and 28.50%, respectively. A plot of PC4-PC7 loading weights is available as supplementary material. ... 54
Figure 3.4. Comparison between the measured annealed film properties and their predicted values using the PLS model: (A,B) crystallinity and (C,D) O2 permeability. ... 56
Figure 3.5 Prediction maps of: (A) crystallinity and (B) O2 permeability for selected annealed films (numbers above the maps correspond to sample numbers as described in Table 3.1). ... 57
Figure S1. Histograms of predicted talc concentration. ... 59 Figure S2. Prediction histograms of: (A) crystallinity and (B) O2 permeability for selected annealed films (numbers above the maps correspond to sample numbers as described in Table 3.1). ... 60
Figure S3. Loading weights of the PLS model components 4-7 built for the crystallinity of annealed samples. ... 61
Figure A1.1. MIA results for the moisture adsorption kinetics of the neat PLA film: (A) t1-t2 score plot, (B) p1-p2 loading plot and (C) t2 score images. ... 76
Figure A1.2. Comparison between the experimental moisture absorption kinetics and the spatial average of the t2 scores for neat PLA. ... 77
Figure A2.1. Comparison between the measured and predicted film properties for the training and validation set of samples containing different talc contents: (A) CH4 permeability, (B) H2 permeability and (C) N2 permeability. ... 78
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Nomenclature
CCD Charge-coupled device
IR Infrared
MIA Multivariate image analysis Mid-IR Mid-infrared
MIR Multivariate image regression MSC Multiplicative scatter correction
NIPALS Non-linear iterative partial least squares
NIR Near-infrared
PCA Principal component analysis
PLA Polylactic acid
PLS Projection to latent structures/partial least squares RMSE Root mean squared error
RMSEC Root mean square error of calibration RMSEP Root mean square error of prediction SAXS Small angle X-ray scattering
SPC Statistical process control SPE Squared prediction error SVD Singular value decomposition
UV Ultraviolet
VIP Variable importance on the projection
VIS Visible
xii
Foreword
This thesis consists of 4 chapters. In the first chapter, the importance of food packaging and polymer film processing is presented. In addition, the most recent literature related with on-line quality control of polymer films is reviewed. The second chapter focuses on the materials and methods used in this project. The details about raw materials and experimental methods, as well as the background on the chemometrics techniques are presented. Chapter 3 consists of an article submitted to an international scientific journal and reports on the main results of this work with discussion and modeling. Finally, chapter 4 presents the general conclusions and recommendations for future works.
Chapter 3:
A fast and non-invasive on-line near-infrared (NIR) imaging system is proposed in this work for the inspection of polylactic acid (PLA) films containing talc. Multivariate imaging techniques were used to analyze NIR hyperspectral images of the samples, and to develop models to estimate various film properties. The proposed imaging sensor successfully detected talc loading in the PLA films and a multivariate regression model was built to estimate relevant properties such as crystallinity, mechanical and barrier properties using the NIR images as the model input. The effect of annealing as a post-treatment applied on the polymer films could also be detected by the sensor. The effect of different annealing temperature and time on PLA films was detected using NIR imaging, and their effect on the annealed film properties was also predicted by the model. This work was accepted in the Polymer Testing journal and should be cited as follows:
Amirabadi S., Rodrigue D., and Duchesne C., Characterization of PLA-Talc films using NIR chemical imaging and Multivariate Image Analysis techniques, Polymer Testing, https://doi.org/10.1016/j.polymertesting.2018.03.047.
xiii Other results were also presented at the following national conference:
Amirabadi S., Rodrigue D., and Duchesne C., Inspection of PLA Films Using NIR Chemical Imaging and Multivariate Image Analysis, 66th Canadian Chemical Engineering Conference, 00177, Quebec City, Canada (2016).
Appendices
Appendix 1:
This appendix contains information related with some preliminary results obtained on the effect of moisture absorption in PLA films. In this section, the experimental data, results analysis, and the developed MIA model are presented.
Appendix 2:
In Chapter 3, a comparison between the measured and predicted film properties for the training and validation sets of samples containing different talc contents was presented for talc composition, crystallinity, strain at break, Young’s modulus, O2 permeability and CO2 permeability. Because of space limitations of the journal, the results and analysis on other gases (CH4, H2, and N2) permeability are presented in this section.
In all the work, sample production was performed by Amir Ghasemi who was a M.Sc. student supervised by Profs. Denis Rodrigue and Carl Duchesne. He also performed part of the characterization tests. I performed all the experimental work related with hyperspectral imaging, image analysis, and additional characterization tests for the polymer film samples. I also wrote the first draft of the article and the whole thesis under the guidance of my supervisors.
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Chapter 1.
Introduction
1.1 Food packaging
These days, food packaging plays an important role in the food industry to maintain the product quality as long as possible. Food packaging should meet all the food regulations which are mainly imposed by governments to ensure a minimum requirement for food quality and safety [1]. In fact, food packaging has several functions. Its primary role is to preserve the food quality and to safely deliver it to consumers as the food quality could decrease during transportation and distribution [2]. For instance, it was estimated that almost half of the globally produced food is wasted and never reaches the consumers, and packaging plays an important role in reducing these food wastes [3]. Generally, protection, containment, convenience and communication are the main functions of the packaging which are defined as [2]:
Protection: this can be considered as the most important packaging function because it preserves the food from external factors and damages such as microorganisms, oxygen, moisture, poisoning and physical damage which have significant effects on food quality degradation. In other words, the package has to chemically, biologically, and physically preserve the food. For the chemical goal, the packaging should decrease the compositional and chemical structure variations of the food caused by moisture, oxygen, light, etc. Biological changes can occur due to microorganisms contamination such as viruses, bacteria, fungi and insects, while physical and mechanical damages during transportation and distribution are the main reasons for foods wastes [2], [4], [5].
Containment: in order to avoid food losses between production and consumption, the packaging has to contain the food products. In addition, it makes the food transportation more convenient especially for liquids [5], [6].
Convenience: the food packaging should not only contain the food products and protect them from external attacks, but can also bring convenience to the customers using them. Easy-opening, suitable-adapted size, pressurized and heatable packaging are examples of conveniences which are brought to this industry [6].
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Communication: the written or graphical information could be easily seen on the food packaging and provides the customers useful details about the ingredients, nutrients, shelf life, required instructions (cooking) and storage conditions [2], [6].
1.2 Packaging materials
As the types and characteristics of the packaging material have an important effect on the quality or the functions of the packaging system, choosing the appropriate materials seems to be critical to achieve the goal of food protection [1]. Glass, metal, paper and polymers are the most popular materials used in the food packaging industry. Glass as an inert, transparent and impermeable substance retains the flavor and quality of food products, especially liquids. Moreover, glass, in addition of being thermally resistant, provides appropriate insulation properties making it suitable for different food packaging systems. However, being heavy and fragile limits its use in various applications because transportation imposes high costs in distribution networks [4], [5].
Metals form the other class of materials which have been widely used in the food packaging industry because of several advantages. They offer high mechanical strength, impermeability and good barrier properties, recyclability and high temperature resistance [5]. Aluminium and steel are the most often used metals. Aluminium is used for creating flexible packaging such as metal films and foils, while steel is used as the main source for tinplate making. Nevertheless, metal packages have two main drawbacks. They are corrosion sensitive, which might require further coating like applying tin as a protective layer. This increases their cost and makes them comparatively more expensive than other material. Aluminum, although not having serious problems for corrosion, is far more expensive compared with other materials.
The other commonly used material in the food packaging industry is paper. Paper has several benefits such as being cheap, lightweight, available, and has suitable mechanical strength. However, its sensitivity to moisture limits its application and keeps it from being used in all areas.
3 Polymers are believed to be the most popular and important packaging materials. They have been vastly used in the food industry in various forms: films, bottles, containers, etc. They offer a wide range of advantages and have been used in different applications. In spite of being lighter compared to the other classes of materials, they have excellent strength, mechanical performance, thermosetting, and thermo-stability properties. Moreover, they are transparent, versatile, diverse, both rigid and flexible, and significantly cheaper than other substances, particularly glass and metals [5], [7]. Consequently, they have drawn more attention and interests for application in the food packaging industry. Polypropylene (PP), polyethylene (PE), polystyrene (PS) and polyethylene terephthalate (PET) are good examples of polymers which have been widely used as packaging materials [5].
Nevertheless, they suffer from two important disadvantages, and recent research aims at modifying them in order to resolve or reduce the corresponding problems. Firstly, it has been confirmed that the overwhelming majority of the polymers are non-biodegradable and this leads to serious environmental issues [8]. This explains why there is a strong interest to investigate biopolymer properties and develop their production as an alternative to synthetic polymers. For example, a high number of research projects have been devoted to enhance polylactic acid (PLA) properties as a promising bioplastic in food packaging because it is recyclable and biodegradable in addition of its good mechanical properties, availability and transparency [8], [9].
Secondly, it is generally claimed that polymers are permeable to gases and vapors. Thus, oxygen and water vapor could transfer through the plastics and deteriorate the food quality by reacting with the foods and changing the product constituents. In spite of these drawbacks, a great deal of attention focused on improving their properties because of their outstanding advantages. As a consequence, new approaches and concepts such as active and intelligent packaging have been introduced to enhance the performance of polymer-based packaging systems, and add multi-functions to achieve specific objectives.
For instance, Ahvenainen et al. defined the term “active packaging” as a type of packaging which changes the condition of the packed food to extend shelf-life or to improve safety or sensory properties while maintaining the quality of the packaged food
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[10]. Oxygen, ethylene and other scavengers, moisture absorbers, and releasing systems like antioxidants are examples of this category. In addition, intelligent packaging was considered for monitoring the condition or the environment of the packaged food. Using polymer films is a method for improving food packaging not only because of the mentioned advantages of polymers, but also because of their light weight, flexibility, high strength-to-weight ratio, and the potential for adding various functions [11]. Therefore, they have been widely used in this industry and a high number of researches are being performed to develop their applications.
1.3 Polymer films
Packaging films are very thin and flexible polymers used to wrap a package [7]. They can also be used to avoid permeation of moisture and gases from the outside (environment) to the food, for example. The film thickness depends on its function and application, but typically varies between 10 and 250 µm [3]. The main polymers used for flexible film packaging are listed in Table 1 [11]. It should be noted that several features and functions must be accounted for to describe the performance requirements of polymer films depending on their final application. Barrier properties, selective permeability, mechanical properties, thickness, sealability and optical properties are some examples of the functions and properties expected for various food packaging applications [12].
Barrier properties in polymer films are related with limiting permeation of gases like oxygen, nitrogen, carbon dioxide, and water vapor. For example, oxygen has an important effect on the deterioration of food quality, particularly in the case of oxygen-sensitive foods [12]. Therefore, using polymer films with good barrier properties reducing the oxygen transmission rate through the package is crucial in the food industry. Carbon dioxide prevents the microbial growth inside the packaging [12]. Therefore, a selective permeable film could lead to transferring the carbon dioxide from the surrounding to the inner side of the package while preventing the permeation of other gases to improve food preservation. Hence, various techniques have been used to improve the film structures to achieve specific properties and functions.
5 Langmuir-Blodgett (LB) polymer monolayers have shown good performance. The LB technique is one of the most useful methods for ultrathin film fabrication with a controlled thickness [13], [14]. In this method, a single layer of molecules is first organized on a liquid surface, usually water, before being transferred onto a solid support to form a thin film with the thickness of a constituent molecule. If the process is repeated, multi-layered films can be prepared [13]. Replacing the traditional LB films which were produced by low molecular weight substrates by polymeric ones and performing some process modifications improved their mechanical and thermal properties [13], [15]. However, this technique still has several limitations such as dealing with temperature and solvent changes. Moreover, multifunctional films are far more preferable than simple monolayer LB films.
Self-assembled monolayers (SAM) are another type of films fabricated for specific goals and functions [13]. Although this technique is applicable to a large number of substances and many efforts and strategies were devoted to improve the method, SAM’s are limited to monolayer fabrication. Therefore, this approach cannot be used to incorporate various functions in the films like intelligent and barrier ones.
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Table 1.1 Common polymers used in flexible packaging applications [11]. Polymer Name Abbreviation Density (g/cm3)
Ethylene acrylic acid EAA 0.925-0.950
Ethylene carbon monoxide ECO 0.930
Ethylene ethyl acrylate EEA 0.925-0.950 Partially neutralized ethylene (meth)acrylic acid (ionomer) ION 0.940-0.950 Ethylene methacrylic acid EMAA 0.925-0.950 Ethylene methyl acrylate EMA 0.930-0.950 Ethylene vinyl acetate EVA 0.925-0.945 Ethylene vinyl alcohol EVOH 1.14-1.16 Maleic anhydride grafted polyethylene PE-g-MAH 0.91-0.940 High density polyethylene HDPE 0.940-0.965 High molecular weight HDPE HMW-HDPE 0.940-0.962 Linear low density polyethylene LLDPE 0.915-0.940 Low density polyethylene LDPE 0.915-0.925 Medium density polyethylene MDPE 0.925-0.940 Metallocene polyethylene m-LLDPE 0.865-0.960 Polyolefin plastomer/elastomer POP/POE 0.856-0.915
Enhanced polyethylene EPE 0.900-0.925
Polyamide (Nylon) PA 1.12-1.14
Polybutylene PB 0.909
Polycarbonate PC 1.2
Polyethylene terephthalate PET 1.3
Polypropylene PP 0.89-0.902
Polystyrene PS 1.04
Polyvinyl chloride PVC 1.16
Polyvinylidene chloride PVDC 1.7
Ultralow density polyethylene ULDPE 0.90-0.915
Food packaging demands for specific properties such as good barrier and mechanical properties, as well as multifunctional features led to developing multilayer films instead of monolayers [12]. Generally, monolayer films have a high permeability to
7 different gases and cannot be considered as a general food packaging material as they cannot provide all the required properties. Thus, multilayer films which are able to bring good mechanical and barrier properties, and multiple functions to the food packaging, have attracted more attention. These films have several layers in their structures: the sealant layer, the core layer and the outer layer. They can be produced by thermal lamination, coating, metallization or co-extrusion technologies [12].
The objective of the sealant layer is to protect the food products by supplying a hermetic seal. Low density polyethylene (LDPE), ethylene vinyl acetate (EVA) and metallocene polyethylene (mPE) plastomers are the most commonly used polymers for the sealant layer [12]. Ethylene vinyl alcohol (EVOH) and polyvinylidene chloride (PVDC) are typically used as the core layer because of their low gas permeability to reduce gas transfer into the package [12]. Lastly, the role of the outer layer is to protect the products against external attacks and stresses. Thus, polymers such as polypropylene (PP), polyamide (PA), polyethylene terephthalate (PET) and high density polyethylene (HDPE) are used as the “abuse resistant” layer [12].
One of the most popular approaches for constructing multilayer films is layer by layer (LbL) assembly because of its robustness and simplicity [13], [16]. This method provides manufactured films with the desired number of layer and thickness, and coating uniformity making it appropriate for gas barrier applications and multifunctional films. Moreover, a wide range of substrates like gold, plastic, clay and nanoparticles could be used to coat the films making LbL more flexible to cope with various applications. Several other modifications and developments have been proposed to improve the multilayer film structure. For instance, different strategies such as surface modification, crystallization and orientation, as well as nanoparticles addition have been applied to improve the barrier properties of polymer films [13]. However, the structure of multilayer films is getting more and more complex by adding different layers, each of them contributing to a specific function. This leads to serious challenges for the quality control of polymer films, which should cope with many film properties such as surface morphology, thickness, mechanical properties (strength and modulus), barrier properties (oxygen transmission rate or OTR), film composition, and also physical defects. As these properties play an important role in
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the performance of the produced films, they should be monitored and submitted to quality control. It should be emphasized that monitoring and quality control of more complex films will impose more challenges.
1.4 Film fabrication
1.4.1 Process
Two of the most popular methods for film production are cast and blown film processes [17]. In the cast film process, the film is extruded from a flat die and rapidly stretched on a chilled roll for casting the film [17], [18]. The chill roll solidifies the product and creates a film with the desired thickness. The main components in cast film extrusion processing are the extruder equipped with a film die, casting rolls, cooling and polishing rolls, vacuum system, puller roll to maintain constant tension on the film, slitter for film cutting and speedy winder to wind up the film [18]. It is also worth mentioning that film dies in this process provide a uniform flow across the die width. As the film leaves the die, it is drawn down by the casting roll moving at a higher speed than the product leaving the die to adjust the film thickness since the film leaving the die is thicker than desired [18]. Moreover, to stabilize the film on the roll and perform uniform drawing, a vacuum system is added to remove the air behind the film. Lastly, the film edges are slit to achieve the desired product width using a slitter [18].
Another important process widely used in the industry is the blown film process [19]. The main difference between this process and cast film is the die design and cooling system. In addition, a single screw extruder is exclusively used in this process while both single and twin-screw extruders can be used in the cast film process [19]. In film blowing, the extruded polymer enters an annular die to form a bubble or a tube which is pulled away from the die vertically or horizontally as the polymer is being cooled by the air [20], [21]. It should be noted that the created bubble can be blown to various diameter leading to films with different widths and thicknesses with the same die. Moreover, as air is used as a cooling medium to solidify the polymer melt, the air flow rate, the film speed and temperature difference can be used to control the cooling rate [19], [20]. After cooling, two
9 nip rolls collapse the bubble into two flat layers and subsequently puller rolls take the film away from the tower. Finally, the slitter is used to cut the produced films in the form of double lay-flat or two lay-flat width [19], [22].
The advantages of the blown film process are the following. A better property balance is maintained between the machine and transverse directions. Low equipment cost and the option of one die for producing films having various widths. Thus, the blown film process is more widely used in the industry for manufacturing films in comparison with the cast film method. As it can be seen, the extrusion process is one of the most prominent parts of both processes for manufacturing the films [21], [23]. The extruder provides continuous heating and mixing of the feed materials to prepare a homogenous melt at constant temperature and pressure before entering the die [21]. Note that the drive system, feed system, screw/barrel system, head/die system, as well as instrumentation and control systems form the main extruder hardware [21].
Typically, there are six functional zones along the extruder and all of them play a great role in determining the quality of the final products [21], [23]. Solids conveying, melting, melt pumping, mixing, degassing and die forming are the functional zones in most extruders. In the solids conveying section, the solid feed drops down on the back of a screw and moves along until melting begins. The melting function consists of the phase change of the polymer from solid granules to a liquid melt. The required energy for this is supplied by an external barrel heater and mechanical heat generated by the screw movement (viscous shear dissipation) [21]. The molten polymer is subsequently conveyed to the end of the extruder, known as the melt pumping function, and this requires applying pressure to push the molten polymer (high viscosity) behind the die. Mixing elements on the screw or static mixers inside the barrel ensure that material components are well-mixed (the mixing function). In addition, removing undesired gases from the polymer melt (degassing) is performed by a vent located upstream of the die. These gases have to be removed before the die to avoid physical defects such as trapped bubbles and surface roughness on the products [21], [23]. Finally, the die is located at the extruder end to form the polymer melt into its final shape as consistently as possible.
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Growing manufacturing of complex multilayer films with barrier properties has created new fabrication processes, most of them being based on extrusion. There are two major multilayer film production processes known as co-extrusion and lamination [22]. Co-extrusion is defined as the process of feeding a single die with two or more different polymer melt streams. Within the die, the various flow streams are combined to form a single-ply film containing the individual layers [21]. In fact, as many extruders are connected to the die as the number of polymer layers to prepare [21]. It is also worth mentioning that the co-extrusion technique is newer and faster than lamination [22]. The benefits of co-extrusion compared to lamination are not only economical, but also the quality of the produced films is higher, especially with respect to thickness, appearance and adhesion properties [22]. As mentioned before, the blown film process has drawn a lot of attention in the film industry because of its undeniable advantages such as excellent manufacturing flexibility and low operational cost. As a result, co-extrusion of blown film is considered as the favorite process for multilayer film production. Furthermore, the ability of co-extrusion in producing thinner layers makes it appropriate for fabricating multilayer barrier films since a lower amount of high-cost barrier materials is consumed [22]. The basic layout of a blown film co-extrusion line is almost similar to a monolayer blown film system with few added equipment such as additional extruders [22].
1.4.2 Processing conditions
This section reviews the effect of various process conditions on the quality of the polymer film produced by blown film co-extrusion. In the extrusion section, the temperature profiles in the different extruder zones, such as the feed, transition, metering and the die zone, and the screw speed are the most important processing conditions which mainly affect the quality of the polymer melt and subsequently the produced film properties. Also, changes in the melt rheology or viscosity influence the polymer flow properties and the final product properties and quality [24]. Among the other process conditions in the extrusion process affecting the film quality is the applied pressure and pressure changes in the die over time which may cause film thickness variations [24]. It is believed that not only the feed rate changes have an impact on film quality and properties (such as thickness), but also raw material variations can substantially affect the process and
11 as a result, the final product quality. Moreover, in the blown film die section, it has been claimed that the nip speed, cooling rate and bubble volume are the process conditions which have the greatest impact on the film properties, and particularly on the film thickness and bubble diameter [21]. For example, spatial variations in air flow might lead to non-uniform cooling creating thickness variations because some parts of the film may be more stretched [24]. It is worth mentioning that physical defects on the film surfaces also need to be avoided. Contamination, streaks lines, gels and rough surfaces are some typical defects caused by fluctuations in process conditions like melt temperature increase, narrow die gap, or low quality feedstocks and foreign materials like impurities or contaminations in feed [24].
Moreover, the bubble characteristics in the air blown die section can dramatically influence the film properties. For instance, the take-up ratio (TUR), blow-up ratio (BUR) and forming ratio (FR) are typical processing parameters affecting the bubble and subsequently the film properties [21]. TUR, BUR and FR are defined as the ratio of film velocity to melt velocity, the ratio of bubble diameter to die diameter and the ratio of TUR to BUR, respectively. In fact, these process conditions are related to the amount of stretching in the machine and transverse directions. In other words, they have a great influence on the molecular structure which, in turn, affects the physical properties of the films. Mechanical, physical, thermal, optical and barrier properties are the most important to measure to determine the final film quality. Therefore, measuring these properties enables obtaining more information about the performance of the produced films, and subsequently leads to solving the corresponding processing problems by applying the optimum processing conditions [21].
1.5 Quality control of polymer films
Monitoring polymer film properties in the industry is still widely performed using off-line analytical techniques. Small samples collected from the production line are transferred to a centralized laboratory to be characterized using different instruments. The main drawbacks of this approach are: 1) the collected samples may not be representative of
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the whole lot of a product, 2) the long delay between collecting and analyzing the samples limits the application of timely feedback control actions which may subsequently lead to off-specification production and defective products [25], [26]. Furthermore, the more complex structure of multilayer films, combined with the variability of feedstocks and raw materials, which is common in the polymer industry, makes off-line characterization inappropriate for rapid monitoring and quality control. It is also worth mentioning that the process conditions play an important role in the quality of the fabricated products. As a consequence, developing real-time monitoring systems allowing to measure the film properties in a rapid and non-destructive fashion to account for unexpected variations is essential to achieving a reliable quality control system for polymer films. In addition, in spite of off-line measurements, real-time monitoring should not be destructive and time consuming; i.e. it should not interfere with the process.
In the next sections, the various techniques considered in the past for polymer films quality control are reviewed. These methods can be divided into three groups: scattering techniques such as light and X-ray scattering, spectroscopic methods such as Raman spectroscopy, mass spectroscopy, ultraviolet-visible (UV-VIS) and near-infrared (NIR), as well as rheometric techniques [27]-[30]. This literature review focuses on rapid and non-destructive techniques, suitable for real-time film inspection. Many other non-non-destructive techniques are available (e.g., acoustics), but we decided to focus mostly on spectroscopic methods which are widely used for polymer products characterization.
1.5.1 Scattering methods
Scattering methods have been used in several investigations to examine the properties of polymer films such as thickness, molecular structure and surface morphology [31]-[36]. Generally, light scattering and X-ray scattering are the most often used techniques in this category. Wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) are two types of techniques, where WAXS uses a scattered angle greater than 1˚ and SAXS less than 1˚ [34].
It has been claimed that SAXS is a non-destructive method having a good potential to be used for monitoring the inner structure of the polymer films, and to provide very useful information about macromolecular parameters, such as specific surface area, surface
13 fractal dimension and film thickness [33]. For instance, Narsimlu et al. applied SAXS for structural analysis of polythiophene derivative films [33]. They determined the macromolecular parameters of poly(3-octylthiophene) (POTH) and poly(3,3"-dioctyl-2,2",5' 2"-terthiphene) (POTTOT) thin films. Chabinyc et al. studied the microstructure of PBTTT (poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) thin films using X-ray scattering [32]. Porzio et al. studied the structure of thin polymer films by applying grazing incidence X-ray scattering (GIXS). Diffraction patterns from various film layers could be recorded leading to complementary structural information [37]. Moreover, one of the benefits of this approach was its ability to be used in more complex multilayer films analysis and characterization. A strategy was designed for using this technique in industry. A simultaneous small and wide angle X-ray scattering combined with a new extruder configuration was used by Cui et al. to monitor the structure of polymer films in real-time [38]. In this work, more structural information could be obtained by combining SAXS and WAXS. Although this study extended the application of X-ray scattering for in-line monitoring of polymer films, it cannot provide information about the finished product structure. It was applied during extrusion on the molten polymer, while the polymer properties change after extrusion during the cooling process.
It should be noted that other properties like film thickness have also been analyzed by this method. Kim et al. determined the film thickness using wavelength dispersive X-ray fluorescence (WDXRF), which was suited for real-time monitoring of industrial processes while many parts of the used instrumentation are not essential for this measurement [31]. X-ray scattering coupled with confocal micro-X-ray fluorescence (confocal MXRF) and nano-scale X-ray computed tomography (nano-CT) was used as a non-destructive tool to measure the film thickness. In this study, the quantitative analysis of the germanium (Ge) content of metal doped polymers was performed which added new complementary measurements to thickness compared to similar works [39]. This technique was also shown to have good potential as a non-destructive sensor for on-line monitoring of the morphological properties of polymer films like structural and physical (e.g., thickness). However, any reports on the monitoring of barrier properties as a key property using this method have not been published. In addition, this technique was mainly applied using a single point probe because it depends on the incident and scattering angles, as well as the
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polarization and wavelengths of the beams. Hence, it cannot be easily adapted to 2-D surface scans. Moreover, this technique is very expensive. Consequently, it cannot be considered as a comprehensive technique to measure various film properties in industrial processes. Furthermore, the X-ray probes are mostly used to scan a point on the film samples (point-scan mode) instead of line-scan or area-scan mode. Thus, these drawbacks limit the application of this method for non-destructive inspection of polymer films.
1.5.2 Raman spectroscopy
One of the most effective approaches for on-line monitoring of polymer films molecular structure is Raman spectroscopy. It works by radiating a monochromatic light source, such as laser, on a film sample to detect the Raman scattered light. One of the most outstanding features of Raman spectroscopy is that it does not require sample preparation, just like near-infrared (NIR) spectroscopy. Also, it provides real-time monitoring capability and it is not influenced by environmental moisture. Hence, it was used for online monitoring of polymer films in several investigations. Rabolt et al. studied the optical field intensity distribution of multilayer films to analyze their morphological structure using Raman spectroscopy probe [40]. Polymer film orientation changing with preparation conditions was studied by Schlotter et al. using waveguide Raman spectroscopy (WRS) probe as another version of Raman spectroscopy [28]. However, it was observed that when visible wavelength excitation is used in the experiments, high levels of fluorescence limit the WRS. Therefore, Fourier Transform Raman Spectroscopy (FT-Raman) as an improved technique was used by Zimba et al. to study thin polymer films structure in single point (probe) mode [41]. In another investigation, Raman spectroscopy combined with optical spectroscopy was used by Bernard et al. to characterize poly-o-methoxyaniline (POMA) films and perform full structural and chemical identification by the captured intensities (images) [42]. They also compared Raman features of POMA and polyaniline (PANI) films to distinguish them. Improvement of Raman use for polymer film characterization was studied by Musto et al. by combining FT-Raman and confocal-Raman spectroscopy [43]. It should be noted that the characterization was done on syndiotactic polystyrene (sPS) films and FT-Raman probe was used in particular to analyze the sulfonation degree, the
15 crystalline structure, and the local conformational ordering on the samples whereas the sulfonation profile across the sample thickness was monitored by confocal Raman. Meyer et al. investigated the morphological properties of polymer films by applying scanning angle (SA) Raman imaging to analyze the morphology of polymer thin films like poly(3-hexylthiophene) on sapphire, gold and indium thin oxide [44].
In addition to morphological and structural characterization of polymer films by Raman spectroscopy, physical properties like thickness were also determined. Kivioja et al. used total internal reflection (TIR) Raman imaging for the first time to measure the thickness of polystyrene (PS) films on a polypropylene (PP) substrate [45]. It was revealed that this fast and non-destructive technique could be adapted for measuring the thickness of the multilayer polymer films which is one of the most highlighted parts of this research. It is also worth mentioning that the obtained results by this method were validated by attenuated total reflection infrared (ATR-IR) spectroscopy and was compared to other techniques like ellipsometry and spin-coating theory. However, the proposed method was appropriate for a limited range of thicknesses between 40 to 250 nm. Meyer et al. extended the characterization of polystyrene films by measuring their thickness and chemical composition using scanning angle (SA) Raman imaging spectroscopy [46]. The thickness of the samples studied was between 400 nm and 1.8 µm, which is a wider range compared to Kivioja et al. [45].
Raman spectroscopy has shown its potential to be used as a convenient on-line method for monitoring of various polymer thin film properties such as molecular structure, morphology, thickness, and chemical composition. It was also used for characterizing multilayer films doped with metals and other particles. Nevertheless, Raman spectroscopy has several drawbacks. One of the most prominent issues of Raman spectroscopy is the fluorescence phenomenon when dealing with polymer films containing additives or complex structured films like multilayer. In fact, real-time monitoring of polymer films requires fast and non-destructive approaches to guarantee high and accurate monitoring and inspection performance. However, sample heating might occur during exposition to laser, which not only would change the emitted Raman spectrum, but could also affect the sample quality or destroy it in more severe cases. Moreover, the barrier and mechanical properties
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or film defects have never been investigated by Raman spectroscopy. The majority of the studies focused on the structural and morphological properties. Lastly, there is no doubt that Raman spectroscopy instrumentation is more expensive than similar methods like NIR, mainly because it needs a high stability laser source. As a consequence, the application of Raman is limited to special applications and has not be considered as a general technique for on-line monitoring of polymer films to cover all the important features.
1.5.3 Near-Infrared (NIR) spectroscopy
Near-infrared (NIR) spectroscopy is one of the most extensively and efficiently used spectroscopic techniques for on-line quality control of polymers [47]. In fact, the NIR wavelength ranges (800-2500 nm) is located between the visible and mid-infrared regions of the electromagnetic spectrum [48]. Chemical bonds like C-H, N-H and O-H absorb light in the NIR range and therefore can be detected by NIR spectroscopy, enabling to discriminate the chemical composition of the compounds containing these bonds [47]. Several benefits for using NIR such as high speed, non-destructive, low-cost, no sample preparation needed, and remote data collection are possible. Because of these advantages, especially compared with other spectroscopic techniques, there is a growing interest toward using NIR as a reliable on-line monitoring tool in various applications and especially in polymer processing [47]. For instance, mid-IR spectroscopy has some advantages over NIR like the ability to distinguish very similar structures and appropriate calibration transfer between instruments. However, mid-IR instruments are considerably more expensive than NIR instruments while both of them are highly affected by moisture [48]. It should be mentioned that one of the most challenging issues in all spectroscopic techniques is dealing with a large amount of data to perform qualitative and quantitative analysis and to extract useful and relevant information. This is why the chemometrics field was developed to provide multivariate data analysis methods as well as reliable tools to achieve qualitative and quantitative analysis. Chemometrics is defined as the development and application of mathematical and statistical techniques to facilitate chemical data analysis [49]-[51]. Chemometrics methods can be classified into three groups depending on the application: 1) analysis of a single data matrix to extract clustering patterns using principle component
17 analysis (PCA), 2) supervised clustering by partial least squares - discriminant analysis (PLS-DA), and 3) regression between two data matrices using partial least squares (PLS) regression which can also be used for properties prediction [48], [50].
NIR spectroscopy has been widely used in polymer processing to analyze and monitor polymer properties such as crystallinity, composition, as well as mechanical and physical [52]-[61]. Hansen et al. studied the in-line rheological properties of polymers using fiber-optic NIR spectroscopy probe [55]. In this work, the polymer melt flow index (MI) and co-monomer concentration of poly(ethylene vinyl acetate) (EVA) were monitored in an extrusion process and the spectral data was modelled using multivariate statistical regression. The robustness of the calibration models was also verified by real-time predictions. In another study, NIR probe was used to quantify polymer blends composition based on polypropylene/ethylene vinyl acetate copolymer and filler contents in polypropylene/pulverized chalk [54]. An in-line NIR system was used to continuously monitor the extrusion process and chemometrics methods like Principal Component Regression (PCR) and Partial Least Squares (PLS) were used to perform quantitative analysis and model development. The results of the PLS and PCR applied on the NIR spectra showed good agreement between the developed model and the values for the filler concentration (EVA).
In one of the most recent studies, in-line NIR was applied to quantify poly(lactic acid) (PLA) as a contaminant in poly(ethylene terephthalate) (PET) during PET processing. In this case, the NIR probes were attached to the injection-molding machine [62]. PLS was used to develop a model to predict the PLA concentration based on the acquired NIR spectrum in a more organized way by dividing the spectra into sub-regions based on different chemical bonds. Finally, a specific wavelength range was selected for building the model as it contained more valuable information. In fact, very few studies were carried out on discriminating PET and PLA in spite of the importance of PET recycling, and the mentioned research could effectively perform this using NIR as a real-time monitoring tool. Although the mentioned investigations made important attempts to develop the real-time monitoring of polymer properties by NIR spectroscopy, most of them used in-line probes for sensing the polymer melt with little time delay. In-line probes may interfere with
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the process (during extrusion for example), and consequently might affect the operation. In addition, this approach cannot provide information about the finished products and guarantee the final product quality. Therefore, on-line sensors are required to complement the information provided by in-line probes to avoid the mentioned drawbacks and achieve a more reliable real-time remote sensing system.
Nagata et al. used NIR probe installed at the foaming extruder die to monitor CO2 concentration in molten polypropylene [60]. A simple linear regression and PLS models were able to correlate the NIR spectra with the dissolved CO2 concentration even though the limitations corresponding to the dispersed material may lead to weak absorbance. The PLS model could predict the CO2 concentration very well. Furthermore, a non-contact NIR imaging system was used by Jiang et al. for on-line monitoring of laid fabric carbon/epoxy resin prepreg [56]. This method coupled with PLS could effectively predict the resin and the volatile content, and can be considered as an effort toward a non-destructive real-time quality control of polymer fabric cloth. Shinzawa et al. investigated the mechanical properties and crystalline structure of poly(lactic acid) (PLA) nanocomposite by thermomechanical analysis (TMA) and NIR spectroscopy, respectively [63]. This work was done by preparing PLA samples with different clay concentration and subsequently collecting hyperspectral NIR images of the samples to identify the relationship between the images and the crystalline structure. The band position shift analysis of the NIR spectra showed the possibility of extracting relevant information about the variation of the crystalline structure closely related with the nanocomposite system. However, this work was limited to qualitative analysis and no quantitative analysis was done to produce a mathematical correlation between nanocomposite concentration using hyperspectral NIR images and crystalline structure.
In summary, these studies have shown that NIR spectroscopy is an effective technique for real-time monitoring of polymer properties in order to perform real-time quality control of polymers. Chemometrics techniques facilitate the analysis of the NIR spectra from probes or imaging systems, which mainly results in model development for predicting these properties. However, most of these past investigations did not focus on thin polymer films. Very few works were found on real-time monitoring of polymer films,
19 which are important in the food packaging industry. Furthermore, the increasing demand for developing and producing more complex films will increase the need for real-time monitoring of polymer films.
Heymann et al. developed a real-time approach for the determination of thin polymer film thickness using a NIR probe [64]. In this work, the thickness of UV-cured acrylic coating on polymer films was monitored in-line on a pilot-scale roll coating machine. It is worth mentioning that the thickness of UV-cured coatings has an important effect on the properties and functions of the coatings and the films. Therefore, the film thickness requires to be monitored in real-time. For performing quantitative analysis, PLS was used to predict the film thickness. However, the obtained correlation was only applicable for the thickness range of 5-35 μm. In addition, one of the other issues concerning this study was the probe which might influence the process.
Barbin et al. analyzed the film properties by using visible and near-infrared (VIS-NIR) imaging spectroscopy [65]. In this work, composition characterization of biofilms as biodegradable products was investigated and correlated with their mechanical properties such as tensile strength, elongation at break and Young’s modulus. In other words, the produced films with different compositions have been inspected by VIS-NIR spectroscopy to make a correlation between spectral information obtained from hyperspectral images and chemical composition and mechanical properties. Therefore, this work took a step further to perform real-time monitoring of polymer films by considering other properties like chemical composition in addition to mechanical properties. The PLS results showed the ability of this approach to predict these properties using hyperspectral images. Although this research demonstrated the potential of NIR as an on-line monitoring technique for composition and mechanical properties of polymer films, other properties like crystallinity, film defects and barrier properties were not considered.
Gosselin et al. developed a non-destructive on-line NIR imaging system to monitor the crystallinity of polymer sheets [66]. They constructed models for the prediction of crystallinity variations across the sheet surfaces by extracting spectral and spatial information from NIR images. Low density polyethylene (LDPE), high density polyethylene (HDPE) and polypropylene (PP) thin sheets were inspected and the images
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were subsequently analyzed by multivariate image analysis and multivariate image regression (MIA/MIR) approaches to predict the crystallinity distribution. This work confirmed the potential and ability of NIR imaging for monitoring sheets crystallinity. Simple, neat and monolayer polymer sheets were considered in this study.
To study the on-line monitoring of more complex films like multilayer, NIR spectroscopy coupled with multivariate image analysis (MIA) was applied by Ghasemzadeh-Barvarz et al. [67]. They inspected functional multilayer films to detect the absence or presence of specific barrier layers. The effect of printing on the film samples was also considered by inspecting printed and unprinted films each with and without the barrier layer. The film structure was in the following order: polypropylene (PP), primer (PR), top coat (T), and barrier layer (B). The results showed the ability of NIR imaging coupled with PCA as one of the MIA methods to discriminate between the multilayer films with and without barrier layer. This can be potentially used as an approach for defect diagnostic of multilayer films even by considering the effect of printing. The effectiveness of the proposed technique in discriminating the barrier and non-barrier films using PCA was also shown. This work is one of the few investigations on real-time inspection of barrier multilayer films using a fast and non-destructive strategy.
Although there is a strong necessity to inspect film properties (mostly barrier), and to relate film properties to the spectral and spatial information, the most recent studies only discriminated multilayer films based on the presence or absence of a barrier layer and no quantitative model was developed for the prediction of film properties. Consequently, building models correlating NIR images to various film properties such as barrier, mechanical and chemical composition would be very important for on-line monitoring of plastic films.
Palma et al. recently analyzed the mechanical properties of more complex films consisting of gelatin, chitosan and a plasticizer using NIR spectroscopy imaging [68]. They showed that changing the composition of these edible films had a significant effect on the mechanical properties such as tensile strength (TS), elongation at break (%E) and elastic modulus (EM). Multiple linear regression (MLR) was used to correlate the film composition and the mechanical properties of the films. The results showed that there is a
21 strong correlation between variability of gelatin and chitosan (compositional variation) and tensile strength. It is worth mentioning that although a fast and non-destructive technique was proposed for real-time monitoring of edible polymer films, MLR would not be a reliable technique for extracting the most relevant information when dealing with highly correlated data, particularly hyperspectral images. Also, as mentioned before, barrier properties is one of the main properties which should be monitored to avoid the permeation of specific gases like oxygen degrading the majority of the foods quality. But, barrier properties were not considered in this study.
Polymer thin films loaded with paracetamol drug which are applicable for pharmaceutical purposes were also inspected by NIR hyperspectral images [69]. The chemical composition of the films consisting of hydroxyl propyl methyl cellulose (HPMC), poly(vinyl pyrrolidone) (PVP), poly(ethylene glycol) (PEG) and paracetamol (PAR) was monitored and analyzed by extracting spatial-spectral information from all pixels in the images. The models developed by PLS and multivariate curve resolution-alternating least square (MCR-ALS) relating the images to compounds distribution were used to predict the constituents concentration in the pixels. The main contribution of this work consists of showing the ability of NIR chemical imaging to determine the concentration of constituents of the polymer films as a real-time tool for quality control. In this study, besides pixel-to-pixel chemical distribution analysis, the films could be inspected for defects as well.
Gosselin et al. extended on-line monitoring of polymer thin films by considering both spectral and spatial analysis of blended films [70]. Visible-infrared (VIS-NIR) imaging, as a non-invasive on-line technique, was used to measure the composition distribution and mechanical properties of low density polyethylene/polystyrene (LDPE/PS) sheet blends. Extracting spatial information (textural analysis) gave a better chance to diagnose the defects on the polymer surface. As a consequence, this study provided a good potential for defects monitoring of polymer sheets. Determination of local composition distribution and relating them to mechanical properties like tensile strength, Young’s modulus, stress at break, strain at max stress, strain at break and toughness was carried out by applying PLS regression based on multi-resolution multivariate image analysis (MR-MIA) combined with wavelet texture analysis (WTA). One of the most outstanding benefits
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of this work would be the ability of discriminating defects on the polymer products. However, crystallinity and barrier properties were not considered for on-line monitoring purposes.
To enhance crystalline structure of polymer films and consequently to tune other properties, polymer films would occasionally be exposed to thermal annealing as a post-process treatment. Annealing (thermal energy supplied under specific temperature and time) increases the crystallinity and improves other film properties like barrier ones [71]. Detecting defective films prior to applying annealing would help saving the energy required for treating off-specification materials. Meeting specific end-use properties makes inspection of annealed films vital to detect improperly annealed films. The main reason is that quality control of the films before annealing cannot guarantee the final product quality as the samples might be defective or damaged during exposure to annealing. Hence, polymer films must be consistently inspected before and after annealing. Nevertheless, no reports on rapid and non-destructive techniques for quality control of polymer films submitted to annealing were found in the literature.
1.6 Thesis objectives
Polymers manufactured using raw materials obtained from fossil sources cause serious ecological issues, therefore biodegradable polymers attracted more attention in recent years due to their environmentally friendly properties [8], [72]. One of the most promising sustainable alternatives is polylactic acid (PLA) [73]. However, PLA is a biopolymer and suffers from some deficiencies that need to be corrected by reinforcing PLA films with functional fillers to enhance their properties (mechanical) [72]. However, these fillers increase the complexity of the polymer films structure and variability in process conditions and raw materials also influence the quality of the films. Hence, real-time inspection and quality control of the finished film is very important.
However, polymer film fabrication plants are mostly using off-line techniques, which cannot guarantee the quality of the final products. Therefore, fast and non-destructive real-time inspection techniques are required. Several inspection methods have
23 already been proposed in the literature for real-time monitoring of polymer films. According to the literature, there are limitations in using some techniques. For example, Raman spectroscopy sometimes may cause sample heating and is comparatively more expensive than other techniques. Rheometric technique can be used to measure rheological properties of polymer melts. NIR imaging was found to be an excellent tool able of real-time monitoring of polymer film properties. Although there is a limited number of studies on on-line quality control of polymer films using NIR, most of the research focused on discrimination purposes and very few articles were found on developing models to correlate film properties with the NIR spectra enabling film properties prediction. Also, predicting important film properties like barrier properties has not been considered in these studies while it is very critical especially for the films being used in the packaging industries. Moreover, in spite of the importance of film inspection prior and after annealing, quality control of the films submitted to thermal annealing is yet to be studied. It is also worth mentioning that the NIR imaging system used in this study covers the 900-1700 nm wavelength range, and can detect C-H bonds, the most important chemical bond in PLA structure. Indeed, it is expected that the wavelength ranges below 1100 and above 1600 nm are assigned to the second and first overtone of the C-H stretching vibration, respectively.
Therefore, the objective of this study is to extend the application of NIR chemical imaging to characterize PLA films reinforced with talc at different concentrations and submitted to an annealing treatment. PLA-talc films are produced by extrusion film-blowing, and are inspected prior and after annealing performed under different conditions (temperature and cycle time). Multivariate regression techniques are used to predict crystallinity, gas permeability and mechanical properties of the films based on the NIR images of the film surfaces. To the author’s knowledge, this work seems to be the first NIR chemical imaging application for detecting the effect of annealing on polymer films for quality control purposes. Thus, the proposed methodology can be considered as a type of statistical process control (SPC) technique to perform on-line quality control of PLA films.
