Читать книгу Packaging Technology and Engineering - Dipak Kumar Sarker - Страница 37

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 expensive than simple flint glass, which costs between £0.10 and £0.42 for different sizes and an array of moulded shapes for volumes ranging from 190 to 500 ml. A higher specification for exact hues in pharmaceutical‐grade tinted glass, which is not essential in food packs, lies behind the higher cost of flint over coloured glass; this is also different from food product glasses, in which recycled glass use of differing tints is commonplace.

Type I glass is suitable as a packaging material for most parenteral or non‐parenteral pharmaceutical products and is the pharmacy primary packaging standard because of its inertness and thermal stability. This type of glass possesses the highest T_m values and so is much harder to work and shape to the desired form. The chemical robustness of borosilicate means that the glass is also ideally suited to the containment of strong acids and alkalis. Type II glass containers based on soda lime/silica glass (type III) but treated via a surface‐inactivation process to provide a contact surface that has remnant alkali ions removed is suitable for most acidic or neutral aqueous medicinal preparations, whether for parenteral or non‐parenteral use. The modification of the regular soda lime glass surface with sulfur creates a material with excellent resistance to surface hydrolytic reactions that typically occur with the ageing and weathering of glass. Modification of type III glass in this way to produce type II glass removes the sodium and calcium oxides that can be leached from water in contact with the glass surface, thereby preventing weathering and blooming from bottles. Weatherisation and ‘bloom’ formation refer to haze or visual crystalline carbonate (Na₂CO₃ is the most abundant but it may also contain also CaCO₃) found on the inside of plain soda lime glass. Its appearance can alarm consumers, who mistake the clouding for possible microbial contamination and growth. The effect of weatherisation is actually minimal upon the overall quality of the glass but sodium carbonate can influence the pH of the contacting solution according to the glass formulation chemistry and solvent contact time. The hygroscopic nature of soda lime glass means that water films can easily form and accumulate on the glass surface; this happens particularly on the inside surface, where there may be water‐containing product and therefore intrinsic water vapour. Moist conditions and changes in relative humidity driven by variations in temperature, during, for example, sea freight shipping, can affect the amount of atmospheric moisture that the glass is exposed to during storage and shipment, leading to an alternating process of condensation and evaporation. Such surface adsorption can induce the dissolution of glass‐borne Na⁺ and Ca²⁺ ions, which then reform as water‐dispersible carbonate crystals on the surface of the glass in the presence of carbon dioxide and as the glass surface dries. Such carbonate frosting can disappear or dissolve. Treating the surface of the glass with fluorine gas can make the surface of the glass 10 times more chemically inert and therefore less susceptible to bloom formation.

The lower T_m of type II glass compared with borosilicate glass means that bottles require a lower temperature to be blown and injection‐moulded. Regular soda lime glass (type III) is an untreated version of soda lime glass that is routinely used for foods, with average chemical resistance based around an average chemical composition of 75% silica, 15% sodium oxide (soda), and 10% calcium oxide (lime) but with small additional amounts of aluminium oxide (alumina), magnesium oxide (magnesia), and potassium oxide (potash). Alumina is incorporated to positively influence the chemical durability of the glass and magnesia is used as an agent to lower the overall T_m on the addition of alumina (T_m ∼ 2070 °C), which serves to reduce the temperature required for moulding operations. Type III glass containers are occasionally used as packaging material for parenteral products or powders for parenteral use but only in instances where stability testing data indicate their suitability. This type of glass is, however, used routinely for food products and as the packaging for non‐aqueous and non‐parenteral pharmaceutical preparations. This type of glass is preferred over type IV material and is used for products that need to be autoclaved as it demonstrates a resistance to the erosion reactions and the increased rate instigated by high‐temperature treatment of the glass container. Type IV glass containers (general purpose soda lime glass) or non‐pharmaceutical and non‐parenteral lower grade general application soda lime glass are used universally as the packaging for foods and beverages, where their low hydrolytic resistance is not an issue. Other than food uses, type IV glass containers have applications in the containment of skin creams and topical products or for oral dosage forms, such as food supplements. With lower grades of glass, delamination of the material in pharmaceutical vials is important, as in other sterile products, and negates its use because it can provide a route for microbe entry into the product by permitting the ingress of air [13].

The standard composition of plain, transparent, ‘everyday use’ or general purpose soda glass is shown in Figure 2.4. This is the glass used for conserve jars, clip‐down Kilner jars, beer and wine bottles, and pâté‐like spread jars all blow‐moulded at pressures of 0.28–0.32 MPa (atmospheric pressure). This type of glass is primarily made of fine ground silica (SiO₂) at about 70%. The glass also contains soda (Na₂O) or sodium sulfate as the next biggest component at about 14%, or a blend of the two. Lime (CaO) is present at about 10% as a modifier. Magnesia (MgO) and alumina (Al₂O₃) are present at approximately 3% and 2%, respectively. Other ingredients such as iron oxides and potash (K₂O) are present at 0.7% and 0.3%, respectively. Glass is an amorphous structure made of a complex entwinement and association in lamellae of a basic monomeric unit, which is the sodium, potassium, and calcium silicate lattice formed at temperatures in excess of 1300 °C (normal glass‐making temperature is 1580 °C). The molecular structure of glass is still somewhat of a complex and not yet fully understood subject, but the basic unit of the component and process variable and large ‘sprawling’ molecule might be (−(xR₂O.SiO₂)n(yR′O.SiO₂)n(zR′′₂O₃.SiO₂)n‐)n, where R, R′, and R′′ represent monovalent (Na, K, Li), divalent (Ca, Co, Fe, Pb), and trivalent (Al, Fe) ion species, respectively, which may or may not be present. In this molecular formula n is a variable number, and x, y, and z represent stoichiometric ratio values sufficient to combine in exact molar proportions with oxygen (O). The molecules are bonded together on cooling in a ‘loose’ form but with an immense viscosity (customarily 0.8–1.5 × 10¹⁸ Pas; with extremes of 10¹⁸–10²¹ Pas for speciality glasses). Molten glass has a viscosity of 1000 kPas to 0.1 kPas between 800 °C and 1580 °C, respectively, which is somewhere between that of room temperature butter and glycerine syrup. The glass transition temperature (T_g), which is analogous to ‘melting’, of this amorphous solid is typically in the approximately 580–1125 °C range, depending on the type of glass, and it has a viscosity of approximately 10¹⁴ Pas at 525 °C, which is below the T_g. Any glass can be considered to be a ‘frozen liquid’ in the supercooled state, in which the components do not return to their original form, which prevents extensive ordering and significant crystallinity, and, therefore, means that glasses do not possess a clear T_m. Combinations of starch or water‐dispersible colloidal polymers and water also form glassy materials at room temperature (see Sections 3.2 and 8.8.1). There are many naturally occurring mineral glasses and all are formed as a result of subterranean igneous processes. In fact, it is thought that the original use of glass stems from the naturally occurring material. Conventional glass‐making uses a combination of virgin ingredients and recycled broken or shredded glass, called cullet, incorporated at approximately 25% w/w on average but this can range from 17% to 93%. This reuse has the advantage of requiring lower process temperatures and, therefore, it has a cheaper and less polluting manufacturing cycle [14]. Making glass is thought to generate between 500% and 600% in carbon dioxide of the weight of silicates and additives used to make the final glass. However, the process is still marginally less polluting than steel or aluminium refining.