PACKAGING OF DAIRY PRODUCTS

Module 2. Packaging materials

Lesson 8
SOURCES OF DIFFERENT PLASTIC MATERIALS AND PROCESS OF MANUFACTURE

8.1 Introduction

This lesson covers the topics related to history, classification and different methods of manufacture of different plastic materials.

8.2 Developments in Plastics

• By 1936, American, British, and German companies were producing polymethyl methacrylate (PMMA), better known as acrylic glass. Although acrylics are now well known for their use in paints and synthetic fibers, such as fake furs, in their bulk form they are actually very hard and more transparent than glass, and are sold as glass replacements to build aircraft canopies during the war, and it is also now used as a marble replacement for countertops.

• Another important plastic, polyethylene (PE), commonly known as polythene, was discovered in 1933 by Reginald Gibson and Eric Fawcett.

• PEs are cheap, flexible, durable, and chemically resistant. LDPE is used to make films and packaging materials, while HDPE is used for containers, plumbing, and automotive fittings. While PE has low resistance to chemical attack, it was found later that a PE container could be made much more robust by exposing it to fluorine gas, which modified the surface layer of the container into the much tougher polyfluoroethylene.

• Polypropylene (PP) is similar to polyethylene, and shares polyethylene's low cost, but it is much more robust. It is used in everything from plastic bottles to carpets to plastic furniture, and is very heavily used in automobiles.

• Polyurethane was invented in blown form for mattresses, furniture padding, and thermal insulation.

• Epoxies are a class of thermoset plastics that form cross-links and cure when a catalyzing agent, or hardener, is added. They are widely used for coatings, adhesives, and composite materials
• Two chemists named Rex Whinfield and James Dickson, developed polyethylene terephthalate (PET or PETE) in 1941, and it is used for synthetic fibers with names such as polyester, dacron, and terylene.

• PET is less gas-permeable than other low-cost plastics and so is a popular material for making bottles for carbonated drinks, since carbonation tends to attack other plastics, and for acidic drinks such as fruit or vegetable juices. PET is also strong and abrasion resistant, and is used for making mechanical parts, food trays, and other items. PET films are used as a base for recording tape.

• One of the most impressive plastics is polytetrafluoroethylene (PTFE), better known as Teflon, which could be deposited on metal surfaces as a scratch-proof and corrosion-resistant, low-friction protective coating.

• New manufacturing methods were developed, using various forming, molding, casting, and extrusion processes, to make plastic products in large quantities. Consumers enthusiastically accepted the endless range of colorful, cheap, and durable plastic containers/materials being produced.

• One of the most visible parts of this plastics invasion was a complete line of sealable polyethylene food containers which are highly effective and greatly reduce the spoilage of foods in storage. Thin-film plastic wrap that could be purchased in rolls also helped in keeping food fresh.

• Formica, a plastic laminate is used to surface furniture and cabinetry. Formica was durable and attractive. It was particularly useful in kitchens, as it did not absorb, and could be easily cleaned of stains from food preparation, such as blood or grease.

• Polyurethane foam was used to fill mattresses, and Styrofoam was used to line ice coolers and make float toys.

• Plastics are continuously subject to improvement. General Electric introduced Lexan, a high-impact polycarbonate plastic. Du Pont developed Kevlar®, an extremely strong synthetic fiber that is best known for its use in ballistic rated clothing and combat helmets

In the recent years, there has been a tremendous increase in the use of plastics replacing traditional packaging materials such as glass, metal, and paper. The raw materials for plastics are petroleum, natural gas, and coal. They are formed by a polymerization method that creates linkages between many small repeating chemical units (monomers) to form large molecules or polymers. Monomers are subjected to specific temperature and pressure conditions due to which chemical bonding takes place between them resulting in a chain structure. It is known as polymer and process is known as Polymerization (Poly = many, meros = part). The types of plastics are made by either of the following methods which decide their properties also:

• Chains of polymers are also cross - linked with one another

• Structure : Loosely bound or tightly bound

• Depends on extent of branching in polymer chain

• More branching : Less tighter bonding

• Less compact structure

• Low density: e.g. LDPE 0.910-0.920 gm/cc

• Less Branching: More tight bonding

• More compact structure

• Higher density: e.g. HDPE 0.910-0.970 gm/cc

• Branching depends on temperature and pressure employed during polymerization.

8.3 Classification of Plastics

• Plastics can be classified in many ways, but most commonly by their polymer backbone (polyvinyl chloride, polyethylene, polymethyl methacrylate, and other acrylics, silicones, polyurethanes, etc.)

• Other classifications include thermoplastic, thermoset, elastomer, engineering plastic, addition or condensation or polyaddition (depending on polymerization method used), and glass transition temperature or Tg.

• Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the attractive intermolecular forces are overcome) and one or more glass transitions (temperatures above which the extent of localized molecular is substantially increased).

• Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets.

• So-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes

8.3.1 Major groups of plastics

8.3.1.1 Thermoplastics

• A thermoplastic is a plastic that melts to a liquid when heated and freezes to a brittle, very glassy state when cooled sufficiently. They pass through several heating and cooling cycles during synthesis and thereafter to form container and films. Thus the reaction is reversible:

• Most thermoplastics are high molecular weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene).

• Thermoplastic polymers differ from thermosetting polymers as they can, unlike thermosetting polymers, be remelted and remoulded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene.

• The difference between thermoplastics and thermosetting plastics is that thermoplastics become soft, remoldable and weldable when heat is added. Thermosetting plastics however cannot be welded or remolded when heated, simply burning instead. On the other hand, once a thermosetting is cured it tends to be stronger than a thermoplastic.Thermoplastic are mostly used for packaging. Some examples of Thermoplastics are:

• Acrylonitrile butadiene styrene (ABS)

• Acrylic

• Cellulose acetate

• Ethylene-Vinyl Acetate (EVA)

• Ethylene vinyl alcohol (EVAL)

• Fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE)

• Ionomers

• Kydex, a trademarked acrylic/PVC alloy

• Polyacrylates (Acrylic)

• Polyamide (PA or Nylon)

• Polyethylene terephthalate (PET)

• Polycarbonate (PC)

• Polyester

• Polyethylene (PE)

• Polylactic acid (PLA)

• Polypropylene (PP)

• Polystyrene (PS)

• Polyvinyl chloride (PVC)

• Polyvinylidene chloride (PVDC)

8.3.1.2 Thermoset plastics

Thermosetting plastics (thermosets) are polymer materials that irreversibly cure, to a stronger form. The cure may be done through heat (generally above 2000C), through a chemical reaction (e.g. two-part epoxy), or irradiation such as electron beam processing. It is possible to burn but re-melting is not possible in such plastic.

• Thermoset materials are usually liquid or malleable prior to curing, and designed to be molded into their final form, or used as adhesives

• The curing process transforms the resin into a plastic or rubber by a cross-linking process

• Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (e.g. unsaturated or epoxy sites), linking into a rigid, 3-D structure

• The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point

• During the reaction, when the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material

• Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material

• Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature of the material

• Generally it is not used for common packaging application. But used where high dimensional accuracy is required viz. automobile parts, machine design, caps, lids of bottle, transparent bus etc
• Some examples of thermosets are:

• Vulcanized rubber

• Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plastic wear)

• Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)

• Melamine resin (used on worktop surfaces)

• Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic)

• Polyimides (used in printed circuit boards and in body parts of modern airplanes)

8.4 Methods of Manufacture of Plastic Packages

8.4.1 Molding process

Molding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a mold. A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape. A mold is the opposite of a cast. A release agent is typically used to make removal of the hardened/set substance from the mold easier.Types of molding include:

• Powder metallurgy and ceramics

• Compaction plus sintering

• Plastics

• Injection molding

• Compression molding

• Transfer molding

• Extrusion molding

• Blow molding

• Rotational molding

• Thermoforming

• Vacuum forming, a simplified version of thermoforming

• Reaction Injection Molding

• Laminating

• Expandable bead molding

• Foam molding

• Rotomolding

• Vacuum plug assist molding

• Pressure plug assist molding

• Matched mold

8.4.1.1 Flexible mold

A mold is a hollow shape which exactly encloses the shape of a desired object. The object is usually created by pouring a liquid into the mold and allowing it to solidify: typical liquids include molten metal or plastic, plaster of Paris, epoxy resin
Molds generally divide into two classes: solid or flexible
There are five different types of flexible mold compounds in significant use today:

• Hot-Melt

• Latex

• Silicone rubbers

• Polysulfide rubbers

• Polyurethane flexible mold compounds

From the standpoint of general utility and economy, the polyurethanes surpass all other type.

8.4.1.2 Injection molding

Injection molding is a manufacturing technique for making parts from both thermoplastic and thermosetting plastic materials in production. Molten plastic is injected at high pressure into a mold / mould, which is the inverse of the product's shape. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture.

Fig. 8.1 Standard two plates tooling - core and cavity are inserts in a mold base

Materials: The most commonly used thermoplastic materials are polystyrene (low cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a ter-polymer or mixture of compounds used for everything from Lego parts to electronics housings), polyamide (chemically resistant, heat resistant, tough and flexible - used for combs), polypropylene (tough and flexible - used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).

Injection Process

Fig. 8.2 Small injection molder showing hopper, nozzle and die area

8.4.1.2.1 Injection molding process

• The resin, or raw material for injection molding, is usually in pellet or granule form, and is melted by heat and shearing forces shortly before being injected into the mold

• Resin pellets are poured into the feed hopper, a large open bottomed container, which feeds the granules down to the screw

• The screw is rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flights decreases towards the end of the screw nearest the mold, compressing the heated plastic

• As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets

• Heaters on either side of the screw assist in the heating and temperature control during the melting process

• The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame

• This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection

8.4.2 Plastics extrusion

Fig. 8.3 Cross section view of plastic extruder

Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weather stripping, window frames, plastic sheeting, adhesive tape and wire insulation.

8.4.2.1 Process of plastic extrusion

• In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) which are often used can be mixed into the resin prior to arriving at the hopper.

• The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (~ 200 °C). In most processes, a heating profile is set for the barrel in which three or more independently controlled heaters gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer. Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated.

• At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa). The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer. This breaker plate also does the function of converting "rotational memory" of the molten plastic into "longitudinal memory"

• After passing through the breaker plate, the molten plastic enters the die. The die is the component that gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling.

• The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.

• Sometimes on the same line a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.

8.4.2.1.1 Sheet/Film extrusion

• For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender rolls), mostly 3-4 in number. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface.

• Often coextrusion is used to apply one or more layers to obtain specific properties such as UV-absorption, soft touch, matt surface, energy reflection, etc.

• A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated till soft (plastic), and formed on a mold into a new shape. When vacuum is used, this is often described as vacuum forming. Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer housings), which often look like being injection moulded, because of the various possibilities in thermoforming, such as inserts, undercuts, divided moulds

8.4.2.1.2 Blown film extrusion

The manufacture of plastic film for products such as shopping bags is achieved using a blown film line

• This process is the same as a regular extrusion process up until the die

• The die is an upright cylinder with a circular opening similar to a pipe die

• The diameter can be a few centimeters to more than three metres across

• The molten plastic is pulled upwards from the die by a pair of nip rolls high above the die (4 metres to 20 metres or more depending on the amount of cooling required)

• Changing the speed of these nip rollers will change the gauge (thickness) of the film

• Around the die sits an air-ring. The air-ring cools the film as it travels upwards. In the centre of the die is an air outlet from which compressed air can be forced into the centre of the extruded circular profile, creating a bubble

• This expands the extruded circular cross section by some ratio (a multiple of the die diameter). This ratio, called the “blow-up ratio” can be just a few percent to more than 200 percent of the original diameter

• The nip rolls flatten the bubble into a double layer of film whose width (called the “layflat”) is equal to ½ the circumference of the bubble

• This film can then be spooled or printed on, cut into shapes, and heat sealed into bags or other items

8.4.2.1.3 Overjacketing extrusion

In a wire coating process, bare wire (or bundles of jacketed wires, filaments, etc) is pulled through the center of a die similar to a tubing die. Many different materials are used for this purpose depending on the application. Essentially, an insulated wire is a thin walled tube which has been formed around a bare wire.

8.4.2.1.4 Tubing extrusion

Plastic tubing, such as drinking straws and medical tubing, is manufactured by extruding molten polymer through a die of the desired profile shape (square, round, triangular). Hollow sections are usually extruded by placing a pin or mandrel inside of the die and in most cases positive pressure is applied to the internal cavities through the pin.

8.4.2.1.5 Coextrusion

Coextrusion refers to the extrusion of multiple layers of material simultaneously. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different molten plastics to a single extrusion head which combines the materials in the desired shape. This technology is used on any of the processes described above (Blown Film, Overjacketing, Tubing). The layer thicknesses are controlled by the relative speeds and sizes of the individual extruders delivering the materials.

8.4.2.1.6 Extrusion coating

Extrusion coating is using a blown or cast film process to coat an additional layer onto an existing rollstock of paper, foil or film. This process can be used to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water. The extruded layer can also be used as an adhesive to bring two other materials together. A famous product that uses this technology is tetrapak (used for packing UHT milk).