Food Packaging Technology - Plastics PDF

PLASTICS FST648 FOOD PACKAGING TECHNOLOGY Outline INTRODUCTION PLASTIC PROPERTIES MANUFACTURING OF PLASTICS CONTAINERS TYPES OF PLASTICS ADVANTAGES & DISADVANTAGES OF PLASTICS FST648 FOOD PACKAGING TECHNOLOGY What Able to describe manufacturing process...

PLASTICS FST648 FOOD PACKAGING TECHNOLOGY Outline INTRODUCTION PLASTIC PROPERTIES MANUFACTURING OF PLASTICS CONTAINERS TYPES OF PLASTICS ADVANTAGES & DISADVANTAGES OF PLASTICS FST648 FOOD PACKAGING TECHNOLOGY What Able to describe manufacturing process plastic is a Able to discuss the plastic properties goal? Able to differentiate type of plastic materials & its the applications FST648 FOOD PACKAGING TECHNOLOGY INTRODUCTION FST648 FOOD PACKAGING TECHNOLOGY  Plastics A category of synthetic material made from various organic polymers that are malleable and can be molded into a wide variety of shapes. The term "plastic" comes from the Greek word "plastikos," meaning "capable of being shaped or molded." Plastics are typically made from polymers, which are long chains of molecules.  Plastics Macromolecular organic compounds obtained from molecules with a lower MW or by chemical alteration of natural macromolecular compounds. Macromolecular substance obtained by a polymerisation process such as polyaddition or polycondensation or by any other similar process of monomers and other starting substances or chemical modification of natural or synthetic macromolecules or microbial fermentation.  PLASTICS PROPERTIES FST648 FOOD PACKAGING TECHNOLOGY  Molecular structure of polymer Plastic properties are determined by the chemical and physical nature of the polymers used in their manufacture. Polymer properties are determined by - Molecular structure - Molecular weight (MW) - Degree of crystallinity - Chemical structure These factors affects the polymer density and temperature of physical transitions. Homopolymer or Heteropolymer Homopolymer composed of only one (same) repeating building-block unit throughout their molecules. Heteropolymer composed of two or more (different) building-block unit regular or irregularly distributed throughout their molecules. Copolymer composed of 2 different monomers are polymerised together. Terpolymer 3 different monomers polymerised together.  Linear polymer - straight chain molecules that extend in one dimension Linear copolymer - may exhibit any three combinational forms: - Random copolymer - repeating units arrange randomly. - Block copolymer - long sequences/ blocks of each repeating units. - Alternating copolymer - alternating arrangement of the two repeating units. Branched or cross-linked polymer - polymers that have links between the chains.  POLYMER CLASSIFCATION THERMOPLASTIC THERMOSET Linear polymers. Cross-linked polymers. Gradually soften with increasing Become set into a given network temperature and finally melt - when manufactured and cannot be molecular chains can move subsequently remolded to a new independently. shape. Readily moulded or extruded If temperature is raised to the point because of the absence of cross- where cross-links are broken – links. irreversible chemical processes If temperature is raised – become occur (degradation) – destroy the very flexible and can be molded into useful properties of plastic. new shape even at temperature Do not melt on heating but finally below their melting point. blister and char. Eg. polyethylene, polypropylene, Eg. epoxy resins, unsaturated polyesters, polyamides, PVC, PS, etc. polyester, polyurethanes, phenolic resins etc.  Polymerisation Process Addition Reaction Condensation Reaction Ring-Opening Reactions Addition Reaction Involve rearranging electrons of the double bonds within a monomer to form single bonds with other molecules. Reaction proceeds via chain mechanism – Steps: initiation, propagation and termination. Eg. Polypropylene, polyethylene etc. Condensation Reaction Two molecules combine with the loss of a smaller molecule, usually water, alcohol, acid or ammonia. With an appropriate catalyst, a monomer loses H while the other loses OH and combine to form H2O - remaining electrons form a covalent bond - reaction repeats to form long chain of polymers. Eg. polyamides, polyesters etc. Ring-Opening Reactions Derived from the cleavage of cyclic compound followed by polymerisation in the presence of catalyst. MANUFACTURING OF PLASTIC CONTAINERS FST648 FOOD PACKAGING TECHNOLOGY Stage 1: Production of monomer Stage 2: Polymerisation of monomer into polymer Stage 3: Conversion into products  Stage 1: Production of monomer Derived from the cracking process used in petroleum oil refining and natural gas production. It is a chemical process that produces hydrocarbon monomers such as ethylene, propylene, styrene, vinyl chloride, acrylonitrile etc.  Stage 2: Polymerisation of monomer into polymer This process is conducted through polymerisation reactions such as condensation, addition and ring-opening. Polymer produce in the form of pellets, beads or powder.  Stage 3: Conversion into products Further processing of polymer with the addition of additives such as plasticisers and other types of modifiers is conducted to convert polymer into food containers such as bottles, trays and films. The methods used either by extrusion moulding, injection moulding, blow moulding or thermoforming. EXTRUSION MOULDING Blown tubular extrusion Commonly use in the production of plastic film BLOW MOULDING Step 1: Formation of a hollow tube called parison from molten polymer (extrusion process) Step 2 and 3: Parison is entrapped between two halves of a mould of designated shape Step 4 and 5: Blowing air or nitrogen gas (~100 psi) into the soft parison, expanding it against the contours of cold mould cavity Step 6: Final shape solidifies upon cooling and then ejected from the mould Step 7: The finished bottle. INJECTION MOULDING Consists of two main components: an injection unit and a clamp unit. The operation involves heating of polymer pellets or powder until it melts and able to flow under pressure, and then injecting the molten material into the mould followed by a holding period in which the plastic melt filled up space within the cold mould until it solidifies, finally the solidified plastic is ejected from the machine. THERMOFORMING Forming moderately complex shaped containers that cannot be injection moulded due to either the container is very large or it has very thin walls. Applied to almost all thermoplastics. Eg. ABS, PP, PS, PVC, polyesters, acrylics, PC, cellulosic and nitrile resins. Examples of plastic packaging products: egg cartons, fast-food disposables, boxes, cups, meat packaging trays etc. Consists of two stages, i) Temperature elevation of a thermoplastics sheet material until it is soft and pliable ii) Moulding or forming the material into the desired container shape. 3 techniques: vacuum, mechanical and air-blowing process. THERMOFORMING TECHNIQUES Vacuum Mechanical Air-blowing TYPE OF PLASTICS & APPLICATIONS FST648 FOOD PACKAGING TECHNOLOGY  Ethylene Basic polymer Propylene  One of the most inert polymers. P Constitutes no hazard in normal handling. O Polymerisation of ethylene: use high pressure process between 1000 to 3000 atm and temperature between L 100 to 350°C. Y Simplest structure of PE – completely unbranched structure of –CH2 - units. E With high pressure process – produced short and long H chains branching – LDPE and LLDPE(Linear Low) T Y L E N E  Low Density PE Named LDPE because it float in a mixture of alcohol and water. Largest volume single polymer used in food packaging in both film and blow-moulded form. L Polymer strands are entangled and loosely organised D with branched structure. P Applications: bread wrappers, bags, milk containers, margarine tubs, frozen food containers, collapsible E tubes, spouts and dispensers. L Properties of LDPE Low MW and density (50-70% crystallinity). D Soft and flexible - softening point ±100oC therefore cannot be P used in steam for sterilisation of food. Tough, slightly translucent material. E Can be blow extruded into tubular film or extruded through slit die and chill-roll cast (clearer film). Good tensile strength, burst strength, impact resistance and tear strength. Good transparency in thin sections; translucent and waxy in thick-walled container. Excellent moisture barrier but not a good barrier to gases. Excellent chemical resistance particularly to acids, alkalis and inorganic solutions. Sensitive to hydrocarbons and halogenated hydrocarbons, oils and greases - LDPE absorbed these compounds and then swells - Environmental Stress Cracking (ESC). Excellent fusibility – able to be fusion welded to gives good, tough, liquid-tight seals.  High Density PE Nonpolar, linear thermoplastic with much more linear structure than LDPE. Has up to 90% crystallinity thus it is usually opaque. Usually blow-moulded into bottles for a variety of H food packaging applications such as milk and juice D bottles. P E  Properties of HDPE H Harder and stiffer than LDPE D Densities range from 941 to 965 kg/m3 - sinks in an alcohol- water mixture. P Higher melting point than LDPE (HDPE softening point is about 121°C). E Low temperature resistance similar with LDPE. Tensile and bursting strength are higher than LDPE but impact and tear strength are lower than LDPE. Have superior chemical resistance than LDPE and better resistance to oils and greases. Have stress crack in the presence of some products such as detergents Excellent moisture protection but slightly worse gas permeability compared to LDPE film.  Linear Low Density PE Similar molecular structure to HDPE but virtually free of long chain branches, contain numerous short side chains. L Short branching interferes with the ability of the polymer L to crystallize, therefore LLDPE having similar density to LDPE. D More crystalline, stiffer but less transparent than LDPE. Linearity provide strength, branching provides toughness. P Combine the main features of both LDPE and HDPE. E LLDPE has been replacing LDPE in many food packaging applications. L L Properties of LLDPE D Improved chemical resistance. P Improved performance at low and high temperature. E Higher surface gloss, higher strength at a given density and a greater resistance to ESC. In film form, LLDPE has improved puncture resistance and tear strength. Has similar heat sealabilty to LDPE. Similar strength & toughness to HDPE. P Consist of HC chains with methyl groups randomly distributed on either side of the chain. O L Linear polymer containing little or no unsaturation. Y Non-crystalline polymer and has a density of about 850 P kg/m3 R O Type of catalyst and polymerisation conditions – P produce three different types of stereo configurations (isotactic, atactic and syndiotactic). Y L E N E P Most common commercial form of PP - isotactic form. O All methyl groups are on the same side of the chain. L Y Stiff, highly crystalline and has high melting point. P Good chemical and heat resistance. R O Greater tensile strength and hardness. P Poor transparency. Y L E N E P Atactic – methyl groups are placed randomly on both sides of the chain. O Produced when polymerisation occurs in absence L of stereospecific catalysts. Y Non-crystalline/amorphous with density ~ 850 P kg/m3 R Soft, tacky (rubbery) and soluble in many O solvents Lower value product, mainly used as hot-melt P adhesives. Y L E N E Syndiotactic – methyl groups alternate above and below the sides of the chain  Properties of PP Low density (900 kg/m3) Higher softening point than PE, high temperature stability. Low water vapour transmission, medium gas permeability. Good resistance to greases and chemicals – free from ESC problems. Good abrasion resistance. P Good gloss and high clarity – ideal for reverse printing. P Can be blow-moulded and injection moulded. Brittle at subzero temperature therefore not recommended for frozen foods. High temperature resistance (160 to 178°C) – enable to be sterilised/reheated in microwave ovens. Also permits use as seal layer in retortable pouches, hot-filled bottles and microwavable packaging. Susceptible to oxidative degradation at elevated temperatures – inclusion of antioxidants necessary in all commercial PP compounds. Not recommended for use with heavy, sharp or dense products unless laminated into stronger, puncture resistance materials. P O Made by addition polymerisation of styrene – double bond between CH2 and CH rearranges to L form a bond with adjacent styrene molecules. Y S Normally atactic and completely amorphous. T Y Isotactic PS can be formed by using special R catalyst E First mouldable clear rigid plastics to be N commercialised in large volumes in late 1940s. E P Available in: O As a homopolymer refered to as crystal grade PS (general purpose polystyrene - GPPS) because of its L clarity and stiffness. Y S T As a blend with either grafted or blended rubber or Y other elastomers known as high impact PS (HIPS). R E N E As expandable PS sheet (EPS) having foam structure.  POLYSTYRENE (PS) ADVANTAGES DISADVANTAGES Low cost material. Poor solvent resistance. Excellent clarity. Brittle unless impact modified. Excellent gloss. Subject to ESC. Excellent stiffness. Not suitable for microwave applications. Low shrinkage rates during thermoforming. Not suitable for freezer applications. Easily processed due to wide Poor barrier properties. melting point. Poor taste and odour migration resistance.  A thermoplastic that is formed by addition reaction of P O chloride with ethylene to form vinyl chloride monomer L (VCM) – another addition polymerisation to produce PVC. Y Generally polymerises in atactic form – amorphous V polymer. I N Can be compounded to produce wide spectrum of Y physical properties. L Brittle - add a plasticiser liquid to make it soft and C mouldable. H L O R I D E P Unplasticised PVC: O Tends to degrade and discolour due to heat processing – suitable L stabilisers have to be included in formulation (eg. salts of tin, Y lead, Cd, Ba, Ca, Zn, along with epoxides and organic V phosphites). I High water vapour permeability but low gas permeability than N polyolefins (PE & PP). Y Excellent resistance to oils, fats and greases, also resistance to L acids and alkalis. PVC film C Excellent gloss and transparency can be obtained with the H addition of correct stabiliser and plasticisers. L Thin, plasticised PVC film O Widely used for stretch wrapping of trays containing fresh red R meat and produce. I High water vapour transmission rate prevents condensation D inside the film. E  Produce by condensation reaction between ethylene P O glycol with terephthalic acid. L Y E Linear and transparent thermoplastic. T H Y Has capacity to crystallise under certain controlled L E conditions – strong, stiff, ductile and tough (glassy state). N E PET bottles and films are largely amorphous with small T E crystallites and excellent transparency. R E P However, crystallised PET containers have higher degree H of crystallinity and opaque white. T H A L A T E  Can be drawn into fibers (eg. Dacron), films (eg. Mylar), P O food storage bags (eg. ziplock). L Y E PET film is an outstanding food packaging material – T great tensile strength, excellent chemical resistance, light H Y weight, elasticity and stability over wide range of T (-60 to L E 220oC) – suitable for ‘boil-in-the-bag’ products. N E PET also used to make ovenable trays for frozen food T and prepared meals. E R E P H T H A L A T E  Characterised by carbonate inter-unit linkage. P An amorphous polymer. O L Prepared by interfacial polycondensation of Y bisphenol A and phosgene in methylene chloride- C A water mixture. R B O N A T E P O L Y C A R B O N A T E  Properties of PC High temperature resistance – retain properties well P with increasing temperature. O L High impact strength - even at low temperature - Y use as impact-resistance substitute for window C glass (shatterproof windows). A R Resistance to dilute acids but strongly attacked by B alkalis such as amines. O N High permeability to water vapour and gases – must A be coated if barrier properties required. T E Summarised