Food Packaging PDF

Food Packaging General Definitions Packaging Materials (Plastic ; Glass, Paper ; Metal) Packaging of different Food Categories Modified Atmosphere Packaging (MAP) ; Aseptic Packaging Active & Intelligent Packaging Engineering Design for Food Packages Packaging and Food Safety & Quality ( Migration...

Food Packaging General Definitions Packaging Materials (Plastic ; Glass, Paper ; Metal) Packaging of different Food Categories Modified Atmosphere Packaging (MAP) ; Aseptic Packaging Active & Intelligent Packaging Engineering Design for Food Packages Packaging and Food Safety & Quality ( Migration ; Scalping) Edible Packaging New Trends in Food Packaging Sustainability and Packaging and Legislative Aspects Packaging Materials A number of packaging materials are currently in use for food applications, which are accepted by regulatory bodies like the United States Food and Drug Administration (FDA), European Union (EU), etc. The packaging design and material properties determine the package end use and shelf life of packaged foods. Glass, paper, metal, and plastics are the most important groups of materials used for food packaging. Packaging Materials Plastics Polyolefins Copolymers of ethylene Substituted olefins Polyesters Polyamides Biobased plastics (Bioplastics): Starch, Chitosan, Polylactic acid, Poly(hydroxyalkanoates), Biopolyethylene, Biopolyethylene terephthalate, Regenerated cellulose film Glass Metal Paper PLASTIC PACKAGING MATERIALS Plastics The adjective plastic is derived from the Greek plastikos, meaning easily shaped or deformed. It was first introduced into the English language in the 19th century to describe the behavior of the recently discovered cellulose nitrate that behaves like clay when mixed with solvents. The noun “plastics” is often defined in dictionaries as a group of synthetic resinous or other substances that can be molded into any form. From a technical viewpoint, plastics is a generic term for macromolecular organic compounds obtained from molecules with a lower molecular weight (MW) or by chemical alteration of natural macromolecular compounds. Factors Influencing Polymer Structures and Related Properties * 1. Molecular Structure (Classification of Polymers, Polymerization Process) 2. Molecular Weight 3. Density 4. Crystallinity 5. Physical Transitions in Polymers 6. Chemical Structure (Polyolefins, Copolymers of Ethylene, Substituted Olefins, Polyesters, Polycarbonates, Polyamides, Acrylonitriles) 7. Additives in Plastics (Processing Additives, Plasticizers, Antiaging Additives, Surface Property Modifiers, Optical Property Modifiers, Foaming Agents)  Types of Polymers Classification of Polymers Thermoset: cross-linked polymer that cannot be melted (tires, rubber bands) Thermoplastic: Meltable plastic Elastomers: Polymers that stretch and then return to their original form: often thermoset polymers Thermoplastic elastomers: Elastic polymers that can be melted (soles of tennis shoes) Polymer Families Polyolefins: made from olefin (alkene) monomers Polyesters, Amides, Urethanes, etc.: monomers linked by ester, amide, urethane or other functional groups Natural Polymers: Polysaccharides, DNA, proteins Molecular Structure: Classification of Polymers The first group is the linear polymers and they are thermoplastic; that is, they gradually soften with increasing temperature and finally melt because the molecular chains can move independently. They are characterized by extremely long molecules with saturated carbon-carbon backbones. Such polymers may be readily molded or extruded because of the absence of cross-links. If the temperature is raised, they become very flexible and can be molded into shape, even at temperatures below their melting point. Their mechanical properties are rather temperature sensitive. Thermoplastics are the most important class of plastics material available commercially, and account for more than two-thirds of all polymers used in the world today. Thermoplastic polymers constitute the major packaging material in form of films, bottles, cups, jugs, etc. for food industries. The thermoplastic materials used for packaging purpose are qualified based on the code as 1–7  Molecular Structure: Classification of Polymers The second group is the cross-linked polymers and they are thermosetting. As the name suggests, these polymers become set into a given network when manufactured and cannot be subsequently remolded to a new shape. If the temperature is raised to the point where the cross-links are broken, irreversible chemical processes also occur that destroy the useful properties of the plastic. This is called degradation. At normal temperatures, the cross-links make the solid quite rigid. Thermosetting polymers do not melt on heating but finally blister (due to the release of gases) and char. The importance of thermosetting polymers in food packaging is minimal except for epoxy resins that find use as enamels (lacquers) for metal cans. Examples of thermosetting plastics are epoxy resins and unsaturated polyesters. PLASTIC POLYMERS THERMOPLASTICS THERMOSETTINGS characterized by extremely long characterized by the presence of molecules with saturated carbon- cross-links. carbon backbones. set into a given network when molecular chains can move manufactured. independently. gradually soften with increasing do not melt on heating but finally temperature and finally melt. blister and char. may be readily molded or cannot be subsequently remolded extruded. to a new shape. Polymerization Process Thermoplastics can be made by joining together a sequence of monomers. Under suitable conditions of temperature and pressure, and in the presence of a catalyst called an initiator, the molecular chains grow by the addition of monomer molecules one by one to the ends of the chains. Branching can occur but cross-links are nearly absent. The formation of thermoplastics by a process that involves the joining together of monomers to form polymers that have the same atoms as the monomers in the repeating units is called addition polymerization. Polymerization Process A simple, low MW molecule (which must possess a double bond) is induced to break the double bond and the resulting free valances are able to join up to other similar molecules. This reaction occurs in the form of a chain edition process with initiation, propagation and termination steps. Under normal conditions with the usual catalyst, the spatial arrangements of the branches of the polymers are random; such polymers are called atactic. Some processes give products in which the branches are arranged in an orderly manner; these are called isotactic polymers. In the case of PE, this form of polymerization has advantage of reducing the number of branches that are formed. Thus, the molecules in linear isotactic PE can line up with one another very easily, yielding a tough, high density compound. Atactic PE is less dense, more flexible than, and not nearly as tough as the linear polymer, because the molecules are further apart. Polymerization Process Plastic polymers are also prepared by the process of condensation polymerization that involves two active sites joining together to form a chemical bond, a small molecule being ejected in the process. Addition Polymerization *Condensation Polymerization* Condensation Polymerization The starting monomers are not identical to those of which the chains are to be composed; the superfluous groups of atoms must be ejected when the monomer is added to the end of the chain. If there are enough groups of such superfluous atoms on each monomer molecule, some of them may be temporarily retained on the side of the chain as it grows. This promotes easy branching and leads rather rapidly to a highly cross-linked structure.  Molecular Structure Summary To summarize; thermoplastic polymers can be formed either by addition or condensation polymerization, whereas thermosetting plastics are formed only by condensation polymerization. The degree of cross-linking in a polymer may vary over a very wide range, thus blurring the boundary between thermosetting and thermoplastic materials. MOLECULAR WEIGHT The average number of repeating units in a single molecule of a polymer is known as the degree of polymerization (DP). At DP values of about 10-20, the substance formed is a light oil (paraffin if formed from ethylene). As the DP increases, the substance becomes greasy, then waxy and, finally at a DP of about 1000 it becomes a solid and is then a true polymer. The DP is almost unlimited and may increase to around 100,000 or so. Molecular Weight of Polymers Unlike small molecules, polymers are typically a mixture of differently sized molecules. Only an average molecular weight can be defined. Measuring molecular weight Size exclusion chromatography Viscosity Measurements of average molecular weight (M.W.) Number average M.W. (Mn): Total weight of all chains divided by # of chains Weight average M.W. (Mw): Weighted average. Always larger than Mn Viscosity average M.W. (Mv): Average determined by viscosity measurements. Closer to Mw than Mn DENSITY Density is a function of chemical composition, being dependent on the weight of individual molecules and the way they pack together. The hydrocarbon polymers do not possess heavy atoms, and, therefore, the mass of the molecule per unit volume is rather low. Oxygen, chlorine, fluorine and bromine increase the density of polymers. For example, amorphous hydrocarbon polymers generally have densities of 0.86-1.05 g cm-3, while polymers containing chlorine have densities of 1.4 in the case of PVC and 1.7 for PVdC (poly(vinylidene chloride)).  CRYSTALLINITY When a low MW material such as a metal crystallizes from the molten state, nucleation occurs at various points, from each of which a crystal or grain grows. Likewise, when a molten crystallizable polymer is cooled, crystallization spreads out from individual nuclei. However, instead of individual grains, a considerably more complex structure develops from each nucleus. The degree of crystallinity is an important factor affecting polymer properties. The great length of polymer chains means that a certain amount of entanglement normally occurs and this prevents complete crystallization on cooling as in the case of metals. This phenomenon is due to the difficulty of aligning every portion of each chain of the polymer. Thus, crystallinity in plastics consists of thousands of small “islands” of crystalline regions surrounded by unordered or amorphous (without structure) material CRYSTALLINITY The crystalline areas are known as crystallites. Unlike crystals of small molecules, crystallites are not composed of whole molecules or molecules of uniform size. High MW, narrow MWD and linearity in the polymer backbone can yield high crystallinity. The crystallization rate is diminished by the presence of impurities such as catalysts, fillers and pigments. CRYSTALLINITY Stretching a film orients the crystallites and realigns other molecules or segments of molecules, causing the total crystallinity of the film to increase. Such oriented films are generally tougher than either amorphous or unoriented crystalline materials. Orientation and crystallinity are related in that only polymers that are capable of crystallization can be oriented. Non-crystalline, amorphous polymers have no melting point. They simply soften when heated, in much the same way as glass. To summarize, high shear during processing and rapid cooling inhibit crystallinity; annealing and orientation enhance it. CRYSTALLINITY Transparency of unfilled plastics is a function of crystallinity, with non-crystalline polymers such as PS and PC (polycarbonate) having excellent transparency. Other polymers range from cloudy to opaque, depending on the degree of crystallinity PHYSICAL TRANSITIONS IN POLYMERS Noncrystalline (amorphous) polymers are characterized by a glass transition at a temperature called the glass transition temperature (Tg). Crystalline polymers are characterized by a melting transition at a temperature called the crystalline melting temperature (Tm). Tm and Tg are important parameters that define the upper and lower temperature limits for numerous applications, especially for semicrystalline polymers. At a sufficiently high temperature, a thermoplastic polymer is a liquid. In this state, it consists of an amorphous mass of wriggling molecular chains. As it is cooled, the thermal agitation decreases, and, at the Tm, the polymer may crystallize.  The Glass Transition Temperature Tg is the main transition temperature found in amorphous polymers. The underlying molecular process is that frozen backbone sequences begin to move at the Tg. Therefore, Tg is determined not only by the main chain architecture but also by its immediate surroundings. Chain ends and low-MW plasticizers lower the Tg of a polymer; a sufficiently large number of cross-links will increase Tg. Above Tg, a few of the carbon atoms in each chain can still move with relative freedom, but below Tg, nearly all the carbon atoms become fixed, and only side groups or very short chain sections can change position. PHYSICAL TRANSITIONS IN POLYMERS For crystalline polymers, an approximate relationship between Tg and Tm is 2 Tg ≈ Tm (for unsymmetrical chains) 3 and 1 Tg ≈ T (for symmetrical chains) 2 m when both temperatures are expressed in Kelvin. PHYSICAL TRANSITIONS IN POLYMERS An important exception to this occurs with copolymers. Copolymerization also tends to broaden the temperature range over which Tg occurs, due to differences in chemical composition among the copolymer chains in the same sample. Generally, the crystalline copolymer is expected to have a lower melting temperature than the corresponding homopolymer. Ordered copolymers exhibit different transition behavior from random copolymers, generally showing a single, characteristic Tg and, if crystallizable, a single, sharp melting temperature. PHYSICAL TRANSITIONS IN POLYMERS The physical properties of a thermoplastic polymer depend on the values of Tm and Tg relative to room temperature. If both Tm and Tg lie below room temperature, the polymer is a liquid. If room temperature lies between Tm and Tg, the polymer is either a very viscous supercooled liquid or s crystalline solid. If both Tm and Tg are above room temperature, an amorphous polymer is glassy in nature, tending to be brittle. Other properties such as stiffness (modulus), refractive index, dielectric properties, gas permeability and heat capacity all change at Tg  PHYSICAL TRANSITIONS IN POLYMERS The glass transition temperatures of the majority of the commercially important crystallizable polymers lie below 25 °C. The Tg values are low for flexible, linear polymer such as the PEs, and relatively high for stiff chain polymers such as PET (poly(ethylene terephthalate)) and PC, which require higher temperatures for the onset of molecular motions necessary for the glass transitions. PHYSICAL TRANSITIONS IN POLYMERS Bulky side groups decrease the mobility of the chain and, thus, raise Tg. For instance, substitution of alternate hydrogens in the PE with methyl groups to give PP, or with phenyl groups to give PS, increases Tg from -110°C to -18°C and +100°C, respectively. Molecular symmetry tends to lower Tg. For instance, PVC has a Tg of 87 °C, whereas for PVdC the Tg is -17°C. Because bulky side groups tend to restrict molecular rotation, they also raise Tm. For example, the CH3 side groups on PP are larger than the H atoms found on LDPE, and, therefore, the Tms are 176°C and 115°C, respectively. The presence of polar side groups such as Cl, OH and CN even though not excessively large, leads to significant intermolecular bonding forces and relatively high Tms. For instance, the Tm and Tg for PVC is 212°C and for PAN (poly(acrylonitrile)) 317°C. RESIN Identification CODES * The ASTM International Resin Identification Coding System, often abbreviated as the RIC, is a set of symbols appearing on plastic products that identify the plastic resin out of which the product is made It was developed originally by the Society of the Plastics Industry (now SPI: The Plastics Industry Trade Association) in 1988, but has been administered by ASTM International since 2008