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3 Description of the contamination of food by packaging
substances
3.1 Molecular description
Thermoplastics are abusively called "plastic materials" and consist in intermingled long polymer
chains (typically with molecular mass above 50 kDa). The interminglement and its low
relaxation process at room temperature are responsible for the high cohesion and the density of
material. The polymer chains can present locally a crystalline morphology (semi-crystalline
polymers) or a privileged orientation (oriented polymers). The amorphous phase present two
mechanical behaviors, successively glassy and rubbery, on both sides of a characteristic
temperature so-called the glass temperature. The reader find further details in reference books on
polymers (Painter and Coleman, 1997 ; Sperling, 2006).
Conventionally additives are assumed to be located in the amorphous regions of the polymer.
Additives such as antioxidants or UV absorbers and residues are present at concentrations below
0.5 % w/w (weight of additive per weight of material). Since the plasticizers aim at increasing the
mobility of polymers, they are typically used at concentrations between few and few tens w/w
percents. While the amount of desorbed plasticizers does not modify the mobility of the polymer
and its microstructure, it is thought that the physical description of mass transport in unplasticized
polymers is also valid in plasticized polymers in their normal conditions of use. It is highlighted
that the proposed description is not valid for substances showing blooming effects:
inhomogeneous dispersion or preferential location at the surface of the plastic material (e.g.
lubricant, antistatic agent).

At molecular scale and at low concentration, the transport of substances such as additives or
residues is similar to the dispersion of non-interacting substances or ghost substances due to
thermal agitation. Since the considered substances are larger than voids present in the polymer
and present shapes that differ significantly from polymer chains, the translational motions of
these substances must be envisioned as a consequence of collective motions involving several
polymer segments (Kovarski, 1997; Vitrac and Hayert, 2006). A classical view is a displacement
according to a series of activated hops as described in the Eyring transient state theory (Glasstone
et al., 1941). At the scale of the diffusant, such translations are performed via successive
reorientations which are assessable by dielectric relaxation (Kaji et al., 2003), nuclear magnetic
resonance (Heuer, 1996), echo spin relaxation (Kovarski, 1997) or Rayleigh spectroscopy (Fytas
et al., 1990).
Since on the long term, the trajectory of each substance consists in a collection of uncorrelated
displacements, the local mass flux obeys to the macroscopic law of diffusion and is controlled by
a macroscopic diffusion coefficient: D with SI units in m
2
s
-1
. By contrast to diffusion
coefficients of these substances in liquids, the corresponding diffusion coefficients are broadly
distributed (up to 10 decades) and vary drastically when the mobility of the polymer is changed
or when the size or shape of the diffusing substance is modified. The high molecular mass (noted
M) dependence of the diffusion coefficient lead to scaling laws D
M
-
with 2
6
for
50
1200
M
g
mol
-1
confirms the predominance of trapping and geometric constraints. The
corresponding activation energies are also broadly distributed typically between 40 and 300
kJ
mol
-1
and confirms the drastic effect of temperature (EC-DG SANCO-D3, 2002). From these
considerations, if the temperature is not accurately controlled or if the diffusion coefficient is