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  • Plastic foil with anti-microbial properties and improved barrier performance against oxygen migration obtained by atmospheric plasma activation followed by wet-chemical coating with chitosan

Plastic foil with anti-microbial properties and improved barrier performance against oxygen migration obtained by atmospheric plasma activation followed by wet-chemical coating with chitosan

Plastic foil with antimicrobial properties and improved barrier performance against oxygen transmission was obtained by atmospheric plasma activation followed by wet-chemical coating with chitosan.

The combination of tailored chemical surface activation by cold atmospheric plasma treatment and subsequent wet-chemical coating using a chitosan solution resulted in coated plastic foils with good antimicrobial activity and improved barrier properties against oxygen transmission. These findings have been filed in a patent application.

This new material helps in ensuring the quality retention of food and thus reduce product loss and even food poisonings. It can be exploited in various food packaging applications including antimicrobially active vacuum packages and oxygen barrier films.

Chitosan can be immobilized onto plasma activated BOPP by exploiting either carbodiimide or glutaraldehyde chemistries. Carbodiimide is capable of forming covalent bonds between carboxylic groups of the activated BOPP and amino groups of chitosan. In more detail, EDC carbodiimide activated the carboxyl group of the film to form an active O-acylisourea intermediate, allowing it to be coupled to the amino group of chitosan. A by-product was released as a soluble urea derivative after substituted by chitosan, thus no spacer existed between the molecules being coupled.

The mechanism with glutaraldehyde is based on the formation of imine bonds between aldehyde groups of glutaraldehyde and amino groups of chitosan and amino activated substrate. Nitrogen was chosen as the carrier gas during plasma polymerisation because of its excellent properties with respect to activation of polymers and its low price compared to other commonly used carrier gases like helium and argon. NH3 and CO2 were mixed with the carrier gas in order to graft amino and carboxyl functionalities, respectively, onto the polymer surface. Plasma parameters including treatment time, plasma power, frequency, flowrate and electrode distance were optimised for optimal plasma homogenity and maximum concentration of functional groups at the highest temperature acceptable for treatment of BOPP films. N2-plasma + CO2 and N2-plasma + NH3 treatments produced carboxyl group densities of 0.90 nmol/cm2 and amino group densities of 1.7 nmol/cm2 onto BOPP, respectively. 0.1% glutaraldehyde and 0.8% EDC (carbodiimide) were dissolved with 1% chitosan into 0.1 M HCl and further applied onto activated BOPP surfaces.

In the case of glutaraldehyde, the amount of immobilized chitosan was 1.64 g/m2, whereas with carbodiimide only 0.58 g/m2 of chitosan was attached. The coating solutions without any linking agents as well as the BOPP surfaces without plasma activation treatments were incapable of forming permanent chitosan coatings. Due to the better performance of glutaraldehyde combined with N2-plasma + NH3, these were used in further tests. By using 0.1 M acetic acid as a solvent for 1% chitosan and 0.1% glutaraldehyde, the immobilization yield of 1.75 g/m2 was reached. Thus acetic acid was chosen as most applicable solvent for immobilization experiments.

Based on the SEM analysis, the amino activation slightly smoothed the surface of untreated BOPP. The surface roughness did not increase after plasma treatment because of the homogeneous nature of the dielectric barrier discharge. In contrast to commonly used corona treatments, no streamers are formed in this type of plasma. Said streamers can cause local destruction of the polymer substrate, which gives rise to an increased surface roughness. In each case, the observed surface roughness is below 1 m. The bonding between chitosan and BOPP was relatively strong and the thickness of the chitosan layer was about 2 m.

Oxygen transmission rates fell from 1500 to 27 cm3/(m2"24 h) because of chitosan barrier layer. In addition, chitosan formed an effective barrier against both carbon dioxide and ethylene. Good gas barrier properties of chitosan in dry conditions are due to the high amount of hydrogen bonds and crystallinity. Both gas solubilities and diffusion coefficients are extremely low. N2-plasma + NH3 treatment increased the permeability to carbon dioxide as compared with the untreated BOPP. NH3 plasma introduced basic amino groups, which have capability of interacting with dissolved carbon dioxide resulting in an increased transmission.

As the total migration test results indicated, the cross-linked chitosan was permanently immobilized onto BOPP without any notable leaching. The amounts of dissolved substances in 3% acetic acid, 95% ethanol and iso-octane were below 2 mg/dm2, thus the material met the requirements set for the total migration of substances migrated from the packaging materials into foodstuffs stipulated in Directive 2002/72/EC.

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