The important properties of the polymeric films and coatings that should be investigated to determine their protection efficiencies are: permeability of films to water and food additives and degradation and hydrolysis rates under the required conditions. Rate of degradation by hydrolysis under different conditions may be obtained by determining quantitatively the degradation product with time. The film degradation rate can also be determined by GPC and actually observed under a light microscope or SEM. 2.1.4.1 Permeability: Permeability is a steady-state property that describes the extent to which a permeating substance dissolves and diffuses through a film, with a driving force
related to the difference in concentration of the permeate between the two sides of the film [25]. Permeability is thus defined as:
Permeability =
(
)
(
)
film across difference pressure partial or ion concentrat permeant thickness film area film per time amount in film through permeation of rate state - Steadywhere, the concentration or partial pressure difference is between the phases adjacent to the two sides of the film. Permeability of polymer films to water, oxygen, flavor and colorant is of particular interest. Increasing plasticizer amount, temperature, and relative humidity (RH) generally increases permeability. The challenge is to select a polymer, plasticizer, and film-forming conditions that optimize the desired barrier properties, while achieving other desirable properties, such as film flexibility, strength and solubility.
One method to determine permeation of water and food additives in aqueous state is pervaporation. Pervaporation: According to this process, the liquid feed–mixture is circulated in contact with the membrane, and the permeate (or evaporate) is evolved in the vapor state from the opposite side of the barrier, which is kept under low pressure by continuous pumping (vacuum-pervaporation) or swept by a stream of gas (sweeping-gas pervaporation) [47]. Permeate, thus obtained is finally collected in the liquid state after condensation on a cooled wall. The flux of component i through a pervaporation membrane can be expressed in terms of partial vapor pressures on the either side of the membrane, pi0 and pil by
the equation
Ji = PiG (pi0- pil) l
where, Ji is the flux, l is the membrane thickness, and PiG is the gas separation permeability coefficient. In the case of a pure liquid the activity of the upstream side is unity (a =1), and assuming that the interfaces of the membrane are in thermodynamic equilibrium with the upstream and downstream phase, the activity of the component in the membrane changes from a =1 to a =0 going from the upstream side to the downstream side. In the case of a pure liquid and polymer the activity of liquid just inside the membrane is always the same (a =1) and independent of the polymer used. The concentration however is not. The concentration of liquid inside the membrane is strongly dependent on the interaction between that liquid and polymer.
Experimentally flux and diffusion constant through the dense films can be determined by time-lag technique which is essentially a volumetric method originally developed by Barrer [48]. Before starting the experiment, the pressure of the receiving side is measured accurately and recorded as initial pressure p0. In pervaporation experiments p0 is essentially ‘0’. Then the experiment is started and pressure is read as a function of time and (p-p0) is plotted against time. After a steady-state is reached, the slope of the straight line is extrapolated back and the interception with the leakage line is read as the time lag. Permeability coefficient P is calculated by the equation:
P = 1 Ap l V t p ∆ ∆ where, t p ∆ ∆
= slope of the linear steady state portion of the curve also called flux through the membrane.
A = area of the membrane. l = thickness of the membrane.
V = experimentally determined effective volume of the low pressure side. p1= pressure on high pressure side.
The diffusion constant D is calculated by the equation: D =
τ
6
2 l
where, l = thickness of the film.
τ = lag time.
Diffusion constant is given in units of m2/s. The solubility coefficient S is obtained by dividing P by D and is given in units of cubic centimeters of gas at S.T.P per cubic centimeter of polymer per Pa pressure.
2.1.4.2Hydrolysis of Methacrylates
Weathering resistance and hardness are typical properties of polymethacrylates. Hydrolysis reaction of these polymers and copolymers has been used as a controlled release mechanism for drugs and fertilizers [49]. Poly methacrylates are hydrolyzed to Poly (methacrylic acid) and corresponding alcohols. The hydrolysis rate is a function of temperature, pH, molecular weight and the nature of substitute group [50]. The changing molecular structure along the polymer chain, or even breakdown of the polymer to lower- molecular-weight segments, can further complicate the reaction kinetics [49].
It has been shown that the polymeric methacrylic esters are highly resistant to alkaline hydrolysis, whereas the monomers are not [51, 52]. In general, the differences in rates observed between monomers and their polymers are due to steric differences and to the fact that reaction along the polymer chain involves introduction of a new functional group, i.e. conversion of ester group to an acid group in hydrolysis. This may either retard or accelerate the reaction of neighboring groups. Also, conversion of functional groups along the polymer chain can change the microenvironment and chain coiling, thus complicating reaction kinetics. For example, hydrolysis of 2-hydroxy propyl methacrylate [HPMA], as either the monomer or along a copolymer chain follows second order reaction kinetics, being first order with respect to both hydroxide and HPMA concentration. The rate of hydrolysis of HPMA monomer is very fast, with complete hydrolysis occurring within moments of being in moderately alkaline solutions of pH above 9 at normal ambient temperatures. While following the same reaction kinetics, the rate of hydrolysis of the HPMA polymer is orders of magnitude slower and can be considered stable towards alkaline hydrolysis as it will hydrolyze at an appreciable rate only if the reaction conditions are severe [49].
In contrast to polymethyl acrylate, which can be easily transesterified or saponified to a high extent, polymethyl methacrylate [PMMA] is in general substantially less reactive. Temperatures around 200 °C usually are required to achieve complete alkaline saponification [53]. However, there are reports that ester groups in low molecular weight anionically polymerized polymethyl methacrylate show a striking difference in reactivity upon saponification by ethanolic KOH. Only the terminal ester
group reacts at a constant high rate whereas the other ester groups are saponified not at all or at a considerably lower rate.
Samples of atactic Poly (methyl methacrylate) hydrolyzed in concentrated H2SO4 at different temperatures showed a maximum yield (85%) of Poly (methacrylic acid) at 450C [54]. Hydrolysis of Poly (butyl methacrylate) [PBMA] was examined in acidic and alkaline solutions. Also the kinetics of hydrolysis of PBMA and butyl methacrylate- methacrylic acid copolymer was determined in conc. H2SO4. The highest degree of hydrolysis of PBMA was observed in concentrated polymer solutions. The hydrolysis was first order reaction and rate of hydrolysis increased with increasing temperature. A complete dissolution of PBMA and copolymer was observed after 60 and 30 hours of hydrolysis at 200C. An increase in the degree of hydrolysis increased the softening temperature [55].
Di- and monomethacrylates hydrolyze to methacrylic acid and the alcohol at neutral pH, catalyzed by an unspecified esterase (hydrolase) and by enzymes in saliva. Esterase added to aqueous slurries of polymer powders gave rise to release of methacrylic acid, presumably deriving from degradation of those of the dimethacrylates only bonded in the matrix by one end of the molecule. It is proposed that hydrolases in saliva increase the wear rate of the composite resin fillings [56].