Chemical and Physical Modifications

Polymer materials offer low cost and light weight, with good impact and abrasion resis-tance, and are easy to mold and seal. However, most native polymer surfaces have a very low surface energy (Table 2.4) and are chemically inert, which can cause difficulties with adhesion, sealing, and printing. Several physical methods have been developed to change the surface characteristics of polymers, while retaining the bulk properties which make them desirable in food processing and packaging applications [48–50]. These processes can be divided into two major categories: oxidation and grafting.

Oxidation of a polymer surface generally creates a variety of chemical species, includ-ing hydroxyl (-OH), carboxyl (-COOH), carbonyl (C=O), and other polar groups. Most of these species can participate in hydrogen-bonding interactions, thus improving adhe-sion, wettability, and printability of polar liquids, compared to the unmodified surface.

They can also form covalent bonds with neighboring molecules, thus serving as chemical anchors for further modifications. While wet-chemical etching methods can be used to create oxidized chemical groups on the surface of a polymer, these processes tend to be difficult to control and create a liquid waste disposal problem. In contrast, physical meth-ods produce little waste and are relatively efficient.

One simple and rapid method to oxidize a polymer is to simply pass it through an oxygen-rich flame. The process creates a variety of oxidized species by reaction of the surface polymer chains with the abundant free radicals found in the flame. By controlling the contact time, fuel-oxygen mixture, and temperature of the flame, the extent of surface modification can be controlled. However, precise control of modification is difficult, and the process can result in contamination by combustion products, mechanical weakening, or even burning of the polymers [48].

Corona discharge methods use a high-frequency, high-voltage AC electric field to ion-ize the air or other gas molecules near the polymer surface. These ionion-ized species are highly reactive and form a variety of polar, oxidized groups on the polymer surface. The extent of modification must be carefully controlled by changing the voltage and frequency, residence time, gas composition, and other variables. Oxidized surfaces produced by flame or corona methods are highly wettable and exhibit good printing and sealing behavior at first, but these properties typically diminish over time as the oxidized polymer surface relaxes and the hydrophobic chains reorient toward the air to lower the overall surface energy, as previously described [48,50].

Plasma treatments offer various advantages over flame or corona methods, includ-ing rapid, uniform functionalization of the surface at low temperatures, and the ability to process arbitrarily shaped materials. Because surface penetration is very limited, plasma treatment produces very thin (tens of nm), dense layers of surface functional groups. Air or oxygen plasmas produce highly oxidized and hydrophilic surfaces, by the reaction of surface polymer chains with ionized and radical species in the plasma. The treated sur-faces exhibit excellent adhesion and peel strength, and improved wettability and printing properties. In contrast, the use of fluorinated gases (e.g., CF₄ or SF₆) produces hydrophobic and oleophobic (“non-stick”) surfaces with excellent barrier and release properties. Other gases (CO₂, N₂, Ar, NH₃, CH₄, etc.) are also widely used to impart various desirable char-acteristics to surfaces [50]. Plasma polymerization processes can also be used to deposit

suRFACe PRoPeRties

uniform, thin, and highly cross-linked polymer films with a variety of useful properties.

One potential disadvantage of plasma processes is the requirement for the surface to be under vacuum during processing [49,50].

Ultraviolet (UV) radiation uses high-energy photons to initiate various radical reac-tions. Irradiation by shorter wavelengths (e.g., 185 nm from Hg lamps) in the presence of oxygen produces ozone, which promotes the rapid oxidation of polymers and renders their surface strongly hydrophilic. Similar UV methods are also commonly used to cross-link or graft acrylate-based polymers on surfaces, enhancing adhesion, printability, mechanical strength, and protein repulsion [44,51].

Regulatory considerations limit the application of grafted polymers on food-contact-ing surfaces, and the technique is most often used for enhanced printfood-contact-ing, labelfood-contact-ing, and adhesion on packaging. Various other techniques (ion-beam, electron beam, gamma irra-diation, laser, etc.) can also be used to produce controlled oxidation, chemical functional-ization, or polymerization on polymer surfaces [50,51].

Another exciting and rapidly growing area is that of active food packaging. Various antimicrobials, antioxidants, chelators, enzymes, and other bioactive molecules can be incorporated into or immobilized on the surface of food packaging. In particular, nonmi-gratory strategies such as covalent attachment of bioactive agents to a surface avoids the loss of activity caused by migration into the bulk, and can mitigate regulatory concerns [52]. Plasma and UV/ozone oxidation is routinely used to activate substrates for covalent immobilization of enzymes and proteins, antimicrobial peptides, and other bioactive mol-ecules [51,53–55]. Active packaging shows promise for improved food safety, extended shelf-life and freshness, and even “in-package” processing by immobilized enzymes.

2.6 SUMMARY

Properties and predictive relationships relevant to the interfacial energy of solid–fluid and liquid–fluid interfaces have been discussed in this chapter, with reference to the relation-ship between adsorption and interfacial energy in a system. For solid materials, contact angle methods and their interpretation were highlighted in terms of the following:

1. An empirical “critical surface tension,” γc

2. An indication of surface hydrophilic–hydrophobic balance, Wa waterab

3. An equation of state relationship for γs ,

Values of Wa waterab

, were presented for a variety of “low energy” (e.g., hydrophobic poly-mers) and “high energy” (e.g., stainless steel, glass) materials used in food processing and packaging. Measurement of the interfacial energy of liquids and liquid mixtures was also described and some values presented for several pure organic compounds and aqueous ethanol solutions. However, the need for experimentation was presented as necessary for accurate accounting of interfacial energetics in systems containing soluble surface-active components.

While surface properties are initially defined by the bulk properties of the materi-als, they can be modified and controlled by careful choice of surface topography, and chemical or physical treatments. Processes of importance in food applications include

oxidation (to increase adhesion, wettability, or printability), polymerization (to enhance mechanical strength or modify barrier properties), and functionalization to produce bio-active surfaces.

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