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Henry’s law sorption
In the case of the isotherm following Henry’s law, the volatile compound solubility is independent of the concentration. There is a linear relation between the concentration of a substance (C) in the polymer matrix and its partial pressure (p), under equilibrium conditions:
! = !!.! (Equation 1.2)
where kD is the solubility coefficient, S, at a given temperature. In this sorption mode, the gas is dispersed randomly in the matrix, which means that neither polymer-permeant interactions nor the permeant-permeant interactions are preferred. This sorption mode is mainly observed at low pressures and when permeant-permeant and permeant-polymer interactions are low 10 compared with polymer-polymer interactions. This case is observed for sorption of permanent gases in apolar polymers as polyolefins or even for sorption of condensable organic compounds at low activity. For example, literature showed that limonene at concentration below 3.5 ppm was sorbed in Henry mode in OPP (Apostolopoulos and Winters 1991).
Langmuir mode sorption
In the case of Langmuir–type sorption mode, the permeant-polymer interactions are predominant. The diffusing molecules occupy specific sites in the polymer until they become saturated. The concentration of the permeant in the matrix (CH) .
Where C’H is the hole saturation constant, p is the partial pressure and b is the hole affinity
constant. This mode is common for polymers with specific sites of sorption in particular for cellulosic material with water molecules.
Dual sorption model
In the case of sorption in glassy polymers, the dual sorption model combines Henry’s law and Langmuir modes and assumes that there are two populations of diffusing compounds in the polymer. It is only applicable when no strong interactions as swelling or plasticization between polymer and permeant take place. According to this model, the concentration of gas compounds in the polymer matrix .
Where kD is the solubility coefficient, S, at a given temperature, p is the partial pressure, C’H is the hole saturation constant and b is the hole affinity constant.
Flory-Huggins model
If the interactions between permeants are stronger than permeant-polymer interactions, the solubility coefficient of the permeant increases constantly with pressure, and a non-linearly growing sorption isotherm is observed. This curve form is characteristic of the dissolution of the penetrant vapors in the polymer above its glass transition where clustering of penetrant molecules occurs leading to swelling of the polymer at high penetrant activity.
Where the thermodynamic activity of the permeant a is the ratio of the pressure p and the saturation vapor pressure p0 at the experimental temperature; φ is the volume fraction of the permeant in the polymer and χ the enthalpic interaction parameter between the permeant and the polymer.
An example for polymer swelling and plasticization was shown by Apostolopoulos and Winters (1991) for sorption of limonene in polypropylene when the concentration of limonene exceeds 35 ppm.
Diffusion
Diffusion is the phenomenon that describes the mass transport within a polymer membrane by random molecular motions. The molecules diffuse through the polymer from the higher chemical potential side to the other side until equilibrate the chemical potential on both sides. The diffusion coefficient is the term used to describe kinetics in the polymeric matrix and giving indication on the time required to reach the steady state, i.e. the flux is constant with time and the compound concentration does not vary with time.
The general transport expression was derived by Adolf Fick in 1855 and is commonly known as Fick’s second law.
where x is the thickness of the membrane, C the permeant concentration, and t the time.
The diffusion coefficient of organic species in dense polymer materials is a function of molecular geometry of diffusant component and polymer type. Berens (1981) showed in his pioneering work that the diffusion coefficient D decreases with the increase of the size of the organic compound (Figure 1.3).
Figure 1.3. Influence of Van der Waals diameter (d) and the form of organic compounds on their diffusion coefficient (D) in PVC (Berens 1981).
Another way to approximate the geometric effects is the use of the number of carbon-atoms of a homologous series of flavor compounds on diffusion, permeation and sorption, which was reported by Shimoda et al. (1987) for the example of polyethylene films. Figure 1.4 shows that D decreases with the increase of the number of carbon atoms of the compounds.
Figure 1.4. Diffusion coefficient of organic compounds of three homologous series in LDPE:
○ Hydrocarbons; Δ ethyl esters; □ n-aldehydes; ● n-alcohols (Shimoda et al. 1987).
The geometry effect of the molecules can however in most cases be successfully described by the molecular mass. For example, Reynier et al. (2004) showed the effects of molecular weight of some aroma compounds on the diffusion coefficient in different polymers materials. For a given polymer, they showed that the effect of molecular weight of aroma compounds leads to a D variation of less than 1 order of magnitude, while for a given compound, polymer matrix changes lead to variations of D up to 4 orders of magnitude (Figure 1.5).
Factors affecting aroma transport in packaging materials
Large literature has reported on the factors affecting the mass transfer of small molecules in packaging materials. Comprehensive reviews can, for example, be found in literature (Dury-Brun et al. 2007, Giacin 1995, Mark 2003, Strandburg et al. 1990). The main factors that affect the aroma compounds transport in polymer packaging materials, with an emphasis on aroma sorption, can be grouped in factors related to packaging, factors related to aroma compounds, factors related to food matrix and factors related to environment.
Factors related to packaging
The composition and structure of a polymer affect directly the transport of small, little interacting molecules in polymer matrix. The parameters related to packaging that have a major role on mass transfers through polymeric matrices are free volume, glass transition temperature and crystallinity of the polymer. The free volume is an intrinsic property of the polymer defined as the molecular void volume that is trapped in the matter. For transport, the permeating molecule needs to jump from one void in the amorphous phase to the other. In general, sorption of molecules increases with the increase of free volume.
In a semi-crystalline polymer the permeant molecules cannot be sorbed or diffuse in crystalline phase, transport takes place uniquely in the amorphous phase. In the presence of crystallites the penetrant molecules need to compass the impermeable structures, which lengthen their pathway by making it more tortuous. The macroscopic consequence is a decrease in the diffusion coefficient. This hypothesis was put by Michaels and Bixler (1961) in their pioneering work on the gas diffusion through polyethylene.
Recently, Kanehashi et al. (2010) showed an extensive literature overview on the relationship between crystallinity degree of semicrystalline polymers and barrier properties. Figure 1.6b 15 shows that the oxygen permeability of PE decreases in 4 orders of magnitude with the increase of crystallinity degree. However, this relation does not hold for all polymers (Figure 1.6a). Interestingly, PLA is one of the polymers where the influence of crystallinity on the barrier properties gives contradictory results. Courgneau et al. (2012) reported different types of behaviour of PLA according to the studied molecule, as helium and oxygen. They showed that the helium permeability coefficient decreases with crystallization. In the oxygen case, the permeability coefficient slightly decreases whereas the diffusion coefficient increases with the crystallinity. A similar result was observed by Guinault et al. (2012) for PLA recrystallized at different crystallinity degrees and in different crystalline forms. The authors showed that the diffusion coefficient of PLA increased with crystallinity degree while the sorption coefficient remained constant. They concluded that under specific crystallization conditions a continuous pathway of increased diffusivity was created around PLA crystals (Guinault et al. 2012).
Figure 1.6. Oxygen permeability of semi-crystalline polymers in function of their degree of crystallinity (Kanehashi et al. 2010).
Factors related to aroma compounds
Aroma compounds are generally much more interacting molecules than permanent gases. In that case transfer in polymers can be highly affected by factors linked to the physico-chemical properties of aroma molecules themselves, such as molecular size, structure, hydrophobicity and polarity. Table 1.1. summarizes the effects of these factors on the aroma compound solubility in packaging materials.
One of the main factors is the low condensation temperature of organic vapors, compared to permanent gases. For example, Shimoda et al. (1987) reported the effects of the number of carbon-atoms of three homologous series of aroma compounds on the sorption in polyethylene films. They concluded that the solubility coefficient S increases with the increase of the number of carbon atoms of the compounds, as shown in Figure 1.7.
Figure 1.7. Solubility coefficient of organic compounds of three homologous series in LDPE:
○ Hydrocarbons; Δ ethyl esters; □ n-aldehydes; ● n-alcohols (Shimoda et al. 1987).
Regarding the hydrophobicity factor and polylactide, Auras et al. (2006) studied the sorption of different volatile compounds and showed that PLA had more affinity to more hydrophilic molecules, such as ethyl acetate, compared to hydrophobic one such as limonene. Colomines et al. (2010) reported also a high affinity of PLA for ethyl acetate, the most hydrophilic compound in the homologous series of ethyl esters. So, since the majority of the aroma compounds are more hydrophobic than ethyl acetate, good performance can be expected for the aroma barrier properties of PLA.
Furthermore, aroma compounds are generally present in mixtures and molecules can have antagonistic or synergistic effects. Antagonistic effects can be expected in the low concentration range where polymer morphology remains unchanged while synergy of sorption between aroma compounds is expected in the higher concentration range (approx. > 1 ppm) where sorption causes swelling of the polymer. In the first case the permeation is predicted to follow the laws of permanent gases, which are based on competitive sorption and diffusion (Story & Koros, 1989).
Factors related to food matrix
The food matrix composition has an important role in the sorption of aroma compounds by polymer materials. Oil, polysaccharides, proteins and ethanol can interact with aroma compounds, retaining the aroma compounds in the food matrix and therefore changing the sorption behavior in the packaging materials.
Figure 1.8. Partition coefficients (K) of aroma compounds present in food matrix and in contact with packaging material.
The partition coefficient is defined as the mass concentration relation of a molecule between two phases at equilibrium. Figure 1.8 shows the different partition coefficients involved in the transfer of aroma compound between food matrix and the packaging material in contact.
The retention of aroma compounds can be studied by comparing the headspace – liquid partition coefficients (Khs/liq) in different food matrices and in water. A large literature has been reported about Khs/liq (Ettre et al. 1993, Jouquand et al. 2004, Landy et al. 1996, Savary et al. 2006, Seuvre et al. 2006) and some data regarding an ethyl ester series are presented in Table 1.2. Table 1.2 shows that Khs/liq of ethyl acetate, the most hydrophilic compound, is almost three times higher in aqueous medium than in lipophilic medium.
Table of contents :
Introduction
1 State of the art
1.1. Food packaging interactions
1.1.1. Mass Transfer of small organic molecules through polymers
1.1.2. Sorption
1.1.2.1. Henry’s law sorption
1.1.2.2. Langmuir mode sorption
1.1.2.3. Dual sorption model
1.1.2.4. Flory-Huggins model
1.1.3. Diffusion
1.1.4. Factors affecting aroma transport in packaging materials
1.1.4.1. Factors related to packaging
1.1.4.2. Factors related to aroma compounds
1.1.4.3. Factors related to food matrix
1.1.4.4. Factors related to environment
1.2. Methodology
1.2.1. Quantification of the VOCs and aroma compounds into polymer
1.2.1.1. Methods of extraction using solvents
1.2.1.2. Methods using volatility of molecules
1.2.1.3. Determination of transport coefficients
1.3. PLA
1.3.1. Structure
1.3.2. PLA Properties
1.3.3. Solubility
1.3.4. Barrier properties
1.3.5. Microstructure
1.3.6. PLA thermal degradation
1.3.7. Additives in PLA
1.3.8. Food packaging applications of PLA
2 Materials and Methods
2.1 Materials
2.1.1 Polymers
2.1.2 Aroma compounds
2.1.3 Reagents
2.1.4 Solvents
2.1.5 Sponge cakes
2.1.6 Food emulsions
2.2 Methods
2.2.1 Procedures of PLA conditioning
2.2.1.1 Conditioning of PLA samples in aroma atmosphere
2.2.1.1.1 Determination of the aroma partial pressure by gas chromatography
2.2.1.2 Conditioning of sponge cakes in PLA pouches
2.2.1.3 Conditioning of PLA samples in rapeseed oil
2.2.1.4 Conditioning of emulsion in PLA trays
2.2.1.4.1 Characterization of emulsions
2.2.2 Moisture loss and water vapor transfer rate (WVTR)
2.2.3 Partition coefficients
2.2.4 Solubility coefficient
2.2.5 Extraction methods
2.2.5.1 Headspace extraction methods
2.2.5.1.1 MHE: Quantification of aroma compounds
2.2.5.1.2 Multiple headspacesolid-phase micro extraction (MHS-SPME)
2.2.5.1.2.1 Quantification of aroma compounds
2.2.5.1.2.2 Sampling procedure for VOCs quantification
2.2.5.1.2.3 Sampling procedure for lactide quantification
2.2.5.1.2.4 Identification by GC-MS
2.2.5.1.2.5 Limit of detection and quantification
2.2.5.2 Liquid extraction methods
2.2.5.2.1 Quantification of oil sorbed in PLA
2.2.5.2.2 Quantification of residual lactide
2.2.5.2.3 Solvent extraction of additives in PLA
2.2.5.2.3.1 Dissolution by reflux – Precipitation
2.2.5.2.3.2 Dissolution by ultrasound– Precipitation
2.2.5.2.3.3 Ethanol extraction by ultrasounds
2.2.5.2.3.4 Analysis by GC-MS
2.2.6 PLA characterization techniques
2.2.6.1 Size Exclusion Chromatography (SEC)
2.2.6.2 Differential scanning calorimetry (DSC)
2.2.7 Statistical analysis
3. Plasticization of poly(lactide) by sorption of volatile organic compounds at low concentration (Publication Nº 1).
3.1. Abstract
3.2. Introduction
3.3. Materials and methods
3.4. Results and Discussion
3.5. Conclusion
4. Sorption of oil and aroma compounds from model foods in poly(lactide)
4.1 Sorption of aroma compounds from flavored sponge cakes in poly(lactide)
4.1.1 Moisture loss and water vapour transfer rate (WVTR)
4.1.2 Quantification of sorbed aroma compounds in pouches and films of PLA by MHS-SPME.
4.2 Sorption of oil and aroma compounds from model food emulsions in poly(lactide) (Publication Nº 2).
4.2.1 Abstract
4.2.2 Introduction
4.2.3 Materials and methods
4.2.4 Results
4.2.5 Conclusion
5 Identification and quantification of additives, lactides and volatile organic compounds (VOCs) in PLA.
5.1 Deformulation of PLA (Publication Nº 3).
5.2 Quantification of lactides in PLA
5.2.1 Quantification of lactides in PLA by the method of NatureWorks
5.2.2 Quantification of lactides by MHS-SPME
5.2.2.1 Optimization of MHS-SPME
5.2.2.2 Lactide quantification
5.3 Identification and quantification of VOCs in PLA by HS-SPME
5.3.1 Identification of VOCs
5.3.2 Optimization of MHS-SPME
5.3.2.1 SPME fibre
5.3.2.2 PLA amount and incubation temperature
5.3.3 Quantification of VOCs in PLA
Conclusions and perspectives
Annexes
References
