PACKAGING OF DAIRY PRODUCTS

Module 2. Packaging materials

Lesson 11
FORMS OF DIFFERENT PLASTIC MATERIALS - 3

11.1 Introduction

In this lesson the topics related to different plastic materials like PET, Polyurethane, Acrylonitrile Butadiene Styrene, Polycarbonate and Ionomers are discussed in detail.

11.2 Polyethylene Terephthalate (PET)

Fig. 11.1 Polyethylene terephthalate

• Polyethylene terephthalate (PET, PETE or the obsolete PETP or PET-P) is a thermoplastic polymer resin of the polyester family.

• It is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber. It is one of the most important raw materials used in man-made fibers. Depending on its processing and thermal history, it may exist both as an amorphous (transparent) and as a semi-crystalline (opaque and white) material.

• Its monomer can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or the transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. Polymerization is through a polycondensation reaction of the monomers.

• The majority of the world's PET production is for synthetic fibers (>60%) with bottle production accounting for around 30% of global demand. In textile applications, PET is generally referred to as simply "polyester" while "PET" is used most often to refer to packaging applications

11.2.1 Processing

There are two basic molding methods, one-step and two-step:

In two-step molding, two separate machines are used. The first machine injection molds the preform. The preform looks like a test tube. The bottle-cap threads are already molded into place, and the body of the tube is significantly thicker, as it will be inflated into its final shape in the second step using stretch-blow molding.

In the second process, the preforms are heated rapidly and then inflated against a two-part mold to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as containers for candy.

In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for molding non-standard shapes (custom molding), including jars, flat oval, flask shapes etc. Its greatest merit is the reduction in space, product handling and energy, and far higher visual quality than can be achieved by the two-step system.

11.2.2 Copolymers

• In some cases, the modified properties of copolymer are more desirable for a particular application.

• Such copolymers are advantageous for certain moulding applications, such as thermoforming, which is used to make tray or blister packaging from PETG film, or PETG sheet. For PET bottles, the use of small amounts of cyclohexane dimethanol (CHDM) or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding ("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages.

11.2.3 Degradation of PET

• PET is subject to various types of degradations during processing.

• The main degradations that can occur are hydrolytic, thermal and probably most important thermal oxidation.

• When PET degrades, several things happen: discolouration, chain scissions resulting in reduced molecular weight, formation of acetaldehyde and cross-links ("gel" or "fish-eye" formation).

11.2.4 Characteristics of PET

• PET can be semi-rigid to rigid, depending on its thickness.

• It makes a good gas and fair moisture barrier, as well as a good barrier to alcohol and solvents.

• It is strong and impact-resistant.

• It is naturally colorless with high transparency.

• When produced as a thin film (trade name Mylar), PET is often metalized with aluminum to reduce its permeability, and to make it reflective and opaque.

• PET bottles are excellent barrier materials and are widely used for soft drinks.

• PET or Dacron is also used as a thermal insulation layer.

• For certain specialty bottles, PET sandwiches an additional polyvinyl alcohol to further reduce its oxygen permeability.

• When filled with glass particles or fibers, it becomes significantly stiffer and more durable.

• While all thermoplastics are technically recyclable, PET bottle recycling is more practical than many other plastic applications. The primary reason is that plastic carbonated soft drink bottles and water bottles are almost exclusively PET which makes them more easily identifiable in a recycle stream.

• PET is also an excellent candidate for thermal recycling (incineration) as it is composed of carbon, hydrogen and oxygen with only trace amounts of catalyst elements (no sulphur) and has the energy content of soft coal.

• PET can withstand temperature upto 15°C and hence it is heat sterilizable and used for boil-in-bags.

11.3 Polyurethane

Fig. 11.2 Generalized polyurethane reaction

Polyurethane, commonly abbreviated PU, is any polymer consisting of a chain of organic units joined by urethane links. Polyurethane polymers are formed by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two alcohol groups in the presence of a catalyst

The physical and chemical character, structure, and molecular size of these compounds influence the polymerization reaction as well as ease of processing and final physical properties of the finished polyurethane

In addition, additive such as catalysts, surfactants, blowing agents, cross linkers, flame retardants, light stabilizers, and fillers are used to control and modify the reaction.
Polyurethane formulations cover an extremely wide range of stiffness, hardness, and densities. These materials include:

• Low density flexible foam used in upholstery and bedding.

• Low density rigid foam used for thermal insulation and e.g. automobile dashboards.

• Soft solid elastomers used for gel pads and print rollers, and

• Hard solid plastics used as electronic instrument bezels and structural parts
  

Commercial production of flexible polyurethane foam began in 1954, based on toluene diisocyanate (TDI) and polyester polyols. These foams were initially called imitation swiss cheese by the inventors

11.3.1 Health and safety aspects

• Fully reacted polyurethane polymer is chemically inert.

• It is not regulated for carcinogenicity.

• Polyurethane polymer is a combustible solid and will ignite if exposed to an open flame for a sufficient period of time.

• Decomposition products include carbon monoxide, oxides of nitrogen, and hydrogen cyanide.

• Polyurethane polymer dust can cause mechanical irritation to the eyes and lungs.

• Liquid resin blends and isocyanates may contain hazardous or regulated components. Isocyanates are known skin and respiratory sensitizers.

11.3.2 Uses of PU

Table 11.1 Characteristics and uses of polyurethane materials

11.4 Acrylonitrile Butadiene Styrene (ABS)

11.4.1 Characteristics of ABS

• Acrylonitrile butadiene styrene, or ABS (C8H8• C4H6•C3H3N)n is a common thermoplastic.

• ABS is derived from acrylonitrile, butadiene, and styrene.

• Acrylonitrile is a synthetic monomer produced from propylene and ammonia; butadiene is a petroleum hydrocarbon obtained from butane and styrene monomers, derived from coal, are commercially obtained from benzene and ethylene from coal.

• ABS polymers are resistant to aqueous acids, alkalis, concentrated hydrochloric and phosphoric acids, alcohols and animal, vegetable and mineral oils, but they are swollen by glacial acetic acid, carbon tetrachloride and aromatic hydrocarbons and are attacked by concentrated sulfuric and nitric acids. They are soluble in esters, ketones and ethylene dichloride.

• The aging characteristics of the polymers are largely influenced by the polybutadiene content, and it is normal to include antioxidants in the composition.

• Though the cost of producing ABS is roughly twice the cost of producing polystyrene, ABS is considered superior for its hardness, gloss, toughness, and electrical insulation properties.

• ABS will be degraded (dissolve) when exposed to acetone.

• ABS is flammable when it is exposed to high temperatures, such as a wood fire. It will "boil", then burst spectacularly into intense, hot flames

11.4.2 Uses of ABS

ABS can be used between −25 and 60°C. It is used to make light, rigid, molded products such as piping, musical instruments, golf club heads (used for its good shock absorbance), automotive body parts, wheel covers, enclosures, protective head gear and toys.It is used for manufacture of inside liner of refrigerators.

11.5 Polycarbonate

Polycarbonates are a particular group of thermoplastic polymers. They are easily worked, moulded, and thermoformed; as such, these plastics are very widely used in the modern chemical industry. Their interesting features (temperature resistance, impact resistance and optical properties) position them between commodity plastics and engineering plastics

11.5.1 Moulding / Extrusion techniques

• Injection moulding into ready articles: lighting lenses, sunglass/eyeglass lenses, safety glasses, automotive headlamp lenses, compact discs, DVDs, lab equipment, research animal enclosures, drinking bottles, iPod/Mp3 player cases.

• Extrusion into tubes, rods and other profiles.

• Extrusion into sheets (0.5-15 mm) and films (<1 mm), which can be used directly or manufactured into other shapes using thermoforming or secondary fabrication techniques, such as bending, drilling, routing, laser cutting etc.

11.5.2 Characteristics of polycarbonate

• Polycarbonates got their name because they are polymers having functional groups linked together by carbonate groups (-O-(C=O)-O-) in a long molecular chain.

• One type of polycarbonate plastic is made from bisphenol A. This polycarbonate is a very durable material, and can be laminated to make “bullet-resistant glass".

• Although polycarbonate has high impact-resistance, it has low scratch-resistance and so a hard coating is applied to polycarbonate eyewear lenses.

• The characteristics of polycarbonate are quite like those of polymethyl methacrylate (PMMA; acrylic), but polycarbonate is stronger and more expensive. This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass.

11.5.3 Uses of polycarbonate

• Polycarbonate is becoming more common in housewares as well as laboratories and in industry, especially in applications where any of its main features—high impact resistance, temperature resistance, optical properties—are required.

• Polycarbonate may be appealing to manufacturers and purchasers of food storage containers due to its clarity and toughness, being described as lightweight and highly break resistant particularly when compared to silica glass.

• Polycarbonate may be seen in the form of single use and refillable plastic water bottles.

• Polycarbonate is now used in returnable milk bottles also.

11.5.4 Health effects

• Bisphenol A appeared to be released from polycarbonate animal cages into water at room temperature and that it may have been responsible for enlargement of the reproductive organs of female mice.

• There is "some concern that exposure to the chemical bisphenol A in utero causes neural and behavioral effects.”

• Using sodium hypochlorite bleach and other alkali cleaners to clean polycarbonate is not recommended, as they catalyze the release of the bisphenol-A, a known endocrine disrupter

11.6 Ionomer

Polymer compound derived from monomeric units where in metal ions is one of the prosthetic group in ethylene. Thus, Ionomers are copolymers of methacrylic acid and ethylene and have some of the hydrogen atoms of the carboxyl groups replaced by either zinc or sodium atoms. Commercially it is known as surlyn.

11.6.1 Characteristics of ionomers

1. Ionomers have exceptional toughness and clarity and heat-seal characteristics.

2. They are commonly used to give a strong seal in laminated films used for packaging products with a high fat content, e.g., meat products.

3. They are also used in skin packaging applications where clarity and toughness are required in the package. Ionomers also form strong bonds with aluminum foil and are acid resistant.