Packaging Technology and Engineering. Dipak Kumar Sarker

quality is usually estimated in terms of tear and crease resistance.

2.4 Making Glass

Glass‐blowing is a craft that was demonstrated in the first glass manufactured in 3500 BCE, which comes from either Syria, Mesopotamia, or Egypt. During the late Bronze age, c. 1550–1200 BCE, there was significant growth in the semi‐industrial production of blown glass in Egypt. In the fifteenth century BC in Crete, Egypt, and the western parts of Asia (Near East) improvement in fabrication and technique followed with the very first formal glass‐making manual, which was created in 650 BC. Expertise in glazing, surface texturisation, and material mixing was honed in the third century BC in India, and by the first century BC Roman glass had started to become famed for its clarity and purity by virtue of using high‐grade white sand. In the seventh to eighth centuries patterned Venetian glass became known for its excellence and was highly sought after across the globe. The masters of glass‐making in the past were considered to be the Phoenicians and Romans and significant aspects of their accrued expertise are still used in modern glass manufacture. Simultaneously, in the eighth century, coloured glass innovations and mould‐blowing or mould‐press‐forming mastery developed in the Sasanian Empire, based in Persia (Iran, Iraq, Syria, Egypt, Yemen, Palestine, and Pakistan), led to the invention of fabrication instruments still currently in use. Three centuries later mirrored glass was first fabricated in Moorish Spain. Stained glass, fabricated using thermally stable particulate metals and metal oxide salts, was first used in a civil engineering context in the twelfth century. In 1674 George Ravenscroft created clear lead crystal known as ‘flint’ glass in England, and by 1830–1850 the so‐called Stoddard utility bottle could be found routinely across the USA. This was driven by an increased demand for glass containers and a means of high‐output manufacture blow‐mould technology that was first developed in 1910 and is still used today. Glass is made from sand (∼59%), soda ash–sodium carbonate (∼18.5%), lime–limestone (∼17%), and 4–6% other substances (Fe, Cr, Co, Se, Na₂SO₄, calumite, nepheline syenite). Some glass makes use of cullet (as much as 10%) or ground recycled glass. Many speciality varieties of packaging glass exist in the form of clear, amber, green, blue, white, opalescent, or metallised forms. Preparation of glass materials involves sourcing appropriate materials of the correct grade, melting and conditioning at 1400–1600 °C, fashioning into appropriate forms (Figure 2.4), annealing, inspection for defects, and ultimately dispatch.

In learning from the methods and sourcing of pigments from antiquity, current manufacturers of flint, amber, and green glasses and speciality coloured glasses continue to use identical materials. Metals and metallic compounds used to provide coloration and impart modifiable surface finishes include a mixture of inert and toxic substances. The overall toxicity of the glass surface is very limited, even when made with metals such as lead or cadmium, as the compounds and elements are permanently sequestered within the silicate lattice constituting the glass. Many colours of glass are so well known that they are referred to by their place of origin or the metal that they contain. The use of cobalt oxide in the glass melt is used to provide ‘cobalt blue’, which is produced by adding cobalt oxide to the glass melt. Other famous glass types include ‘jadeite or Vaseline glass’, which is a fluorescent yellow‐green glass that contains small amounts of diuranate; ‘ruby glass’ and ‘cranberry glass’, which are red glasses produced by the addition of gold or gold chloride; ‘selenium ruby glass’, which has a red tint caused by the addition of selenium oxide; and ‘Egyptian blue’, which is produced by the addition of copper. Incorporation of other colours is achieved by use of cadmium sulfide, which gives a yellow colour; cobalt oxide, which gives a violet coloration; manganese dioxide, which yields purple; and nickel oxide, which gives the melt a violet colour. More familiar colours arise from elemental sulfur, which gives yellow and browns; chromium oxide produces green; iron oxide gives green or brown; carbon gives amber or black; antimony oxide gives white; copper compounds give all the primary colours; tin compounds give white; and lead compounds give yellow coloration. In addition, manganese dioxide and sodium nitrate can be used to remove colour and therefore act as bleaching agents.

Figure 2.4 Types of glasses used in packaging applications.

      Glass production is a highly energy‐consuming process in terms of the energy consumption and initial environmental impact of manufacture. Glass formed into 350 g bottles in general produces 1.06 kg of ‘greenhouse’ gases (mainly and conventionally quoted as CO₂ equivalents) per kilogram of bottle material with an energy consumption of 10.5 MJ/kg, whereas polyethylene terephthalate (PET) – often considered a universal substitute for glass for both food and over‐the‐counter pharmaceutical products – produces 0.49 kg of greenhouse gases per kilogram of bottle material with an energy consumption of 0.6 MJ/kg. The implications for food and beverage containment were described as ‘one of the most intense rivalries in packaging’ by Pan Demetrakakes in the online journal Packaging Digest (USA) in September 2013 (https://www.packagingdigest.com/beverage-packaging/material-or). In addition, the recycling of PET is much less energy consuming than that of glass. However, both materials require less energy for recycling than for original fabrication – of the order of only 74%. In general a recycled price of as little as 56% of the virgin material price is seen, with recycled glass being sold as a commodity at £20–23/tonne. Additionally, for example in the USA, approximately 3.8 times more drink bottle glass is sent for landfill disposal than PET.

There are four conventional categories of glass, which are labelled I, II, III, and IV. The classification of glass is fundamentally based around the degree of chemical attack and resistance to water‐driven hydrolysis. The extent of erosion is highly dependent on the degree of hydroxide ion release under the influence of the contact solvent but can be counteracted by inclusion of oxide compounds, such as borate, in the glassy matrix. These glass varieties range from hardened and shock‐proof borosilicate or neutral glass (type I) to general purpose soda glass (type IV), with the former used for analytical instruments and medical vials or ampoules and the latter used typically for food jars. The composition of general purpose type IV glass (Figure 2.4) can be modified by chemical treatment. Types I–III are considered suitable in most cases for injectable or parenteral pharmaceuticals (single‐dose vials, ampoules, bottles, multi‐dose bottles) and therapeutics [12], with type I being the main variety of high‐resistivity glass used for this purpose. Type I glass vessels contain approximately 80% silica, 10% boric oxide (borate), and a small amount of sodium oxide (soda) and aluminium oxide (alumina). This gives the glass a chemical inertness and high hydrolytic resistance owing to the presence of the boric oxide. This form of glass also has the lowest coefficient of expansion and so has high thermal shock properties; however, it has a high cost per unit volume of 20–30 times that of the cheapest commercial glasses. The cost of glass varies between types with pharmaceutical‐ or pharmacopoeial‐grade type I glass being the most expensive basic form of glass. Surface treatments and lithography or printing obviously increase the commodity cost and, therefore, purchase price commensurate with the extent of mark‐up. Plain untinged or flint glass of liquid parenteral product grade is the most expensive, with a cost of approximately £0.018/g in bulk purchasing, followed by amber pharmaceutical type I glass at £0.014/g. The cheaper types of glass, generally used for food and dry medical products, are typically grade IV or superior grade III glass and scale in cost between £0.0007/g and £0.002/g, respectively. The cost is related to the required glass quality, the required transparency, the difficulty in forming the vessel (bottle, jar, ampoule, or vial), the ability to incorporate recycled cullet, and the inclusion of ‘stable’ pigments. For food‐use jars and bottles, amber and green glass are approximately 1.2–1.8 times more