Glass – a versatile material and valuable recyclate

We find glass everywhere in everyday life. And glass types can be categorized in different ways. We provide an overview with a view to shape and application.

We encounter glass every single day. Be it when looking through windows or typing on our smartphones – it is omnipresent. The versatile material is used in a wide variety of areas. The diversity of glass types making this possible is impressive. Due to the targeted selection of raw materials and the adaptation of manufacturing processes, the properties of glass can be tailored to many needs.

Glass types can be categorised in various ways e.g. by their “genesis”, chemistry, application, shape or finish. For the sake of this overview, we have selected shape and application – and therefore distinguish between the following categories.

Hollow glass comprises glass products that can be filled due to their shape such as beverage bottles, preserving glasses, drinking glasses and other containers. It is produced by blowing or pressing the molten glass into moulds.

The two subcategories container glass and commercial glass are distinguished by their intended use. Container glass serves packaging purposes and is therefore functional and rugged, while commercial glass is often more decorative and diverse in design.

Container glass comprises all glass packaging for food, beverages, medicine or also cosmetics. In most cases, container glass is made of soda lime glass processed by blowing. Common colours are white, green or brown – other colours can be customised.

Container glass primarily serves for storing and transporting liquids and solids and includes:

• Beverage bottles: from small soda pop bottles to big wine bottles – one-way or re-usable – beverage bottles are probably the most well-known examples of container glass.
• Preserving jars: for jams, pickled foods, spices and other foodstuffs thick-walled glass receptacles with screw caps are used.
• Medical and packaging glass: this includes little drug bottles or glass containers for cosmetics.

• Raw materials: container glass is, as a rule, made of soda lime glass, which predominantly consists of natural raw materials such as sand, soda, lime and reclaimed cullet.
• Manufacturing: container glass is predominantly blown. To this end, the molten glass is blown into a mould thereby producing its characteristic shape.
• Colours: container glass is produced in many different colours to protect its content or look appealing. The most customary colours are white, green and brown.
• Recycling: a big benefit of glass is its almost unlimited recyclability. Cullet is collected, shredded and admixed to new glass. It has replaced silica sand as the main component and glass network former today. Today, all glass packaging consists of approx. 60% recycled glass, for green packaging glass this percentage can be up to 90%. The big benefit: next to saving raw materials, clearly less energy is needed for melting. This also translates into less CO2 emitted. Using one ton of recycling glass equates to a saving of over 300 kg of CO2 in the production of new glass packaging.

Commercial glass includes glass tableware for daily use such as drinking vessels with/without stems, carafes, glass plates but also decorative glass objects such as vases, candle holders or bowls. These include:

• Drinking vessels: be it water or wine glasses made of simple glass or crystal glass, with stem or without – drinking vessels and jugs fall into this category.
• Kitchen utensils: glass containers for food, serving trays and other glass kitchen gadgets.
• Decorative glass: vases, bowls and other decorative objects.

Special characteristics:

• Raw materials: alongside soda lime glass, commercial glass is often made of special glass types that make for high transparency, brilliance and a beautiful sound. First and foremost: cut crystal glass since it has a high light refraction, thereby affording ultimate brilliance. Furthermore, crystal glass is particularly well suited for cutting and allows objects with a special look to be designed. The well-known lead crystal is hardly manufactured anymore these days.
• Manufacturing: commercial glass is both blown and pressed. In this process the molten glass is pressed into a mould thereby obtaining its shape.
• Design: commercial glass is often typified by appealing design and high quality.

Flat glass comprises all glass types manufactured in flat sheets. It is used primarily in the construction industry, for windows, doors and façades, for example. Flat glass includes window glass, mirrors and special glass for façades and interior design. Flat glass is predominantly based on soda lime glass manufactured in a float process.

It is used for the following applications:

• Buildings: windows, doors, façade glazing, shower cubicles, mirrors
• Furniture: tables, showcases, shelving
• Mobility: windscreens, side and rear windows, windows/doors in trains, aircraft windows
• Automotive industry: windscreens, car side and rear windows, sunroofs

Float Glass:

• Manufacturing: the float process provides the basis for most flat glass products. In the process, molten glass is poured onto a bed of liquid tin. Due to the surface tension the glass uniformly spreads on the tin bath forming a smooth, flat surface. After gradually cooling down, the flat glass ribbon can be cut to the desired size or be processed further – by tempering, laminating or coating, for example.
• Properties: high transparency, good chemical resistance and an even surface make float glass the ideal base material for many applications.

Tempered Safety Glass (TSG):

• Manufacturing: float glass is selectively pre-stressed by means of thermal prestressing giving it the high resistance to mechanical stresses. When breaking TSG disintegrates into small, blunt particles.
• Properties: high final strength against mechanical stress, splinter bonding, high resistance to thermal shocks.

Laminated Security Glass (LSG):

• Manufacturing: LSG is made up of two or more glass sheets connected by a plastic layer (mostly PVB) in between. In case of breakage this film bonds the glass splinters.
• Properties: extremely high final strength, splinter bonding, anti-burglary, noise dampening. It is often used in areas where high security standards must be met.

Insulating Glass Units (IGUs):

• Structure: IGUs consist of two or more glass sheets separated by spacers. The void is filled with an inert gas or a vacuum.
• Properties: high insulation value, noise dampening, often with solar or thermal control coatings.

Special glass is developed for a variety of specific requirements and features correspondingly special properties. It is used for industrial, technical and scientific applications that call for dedicated physical, chemical or optical characteristics. These include laboratory glass, optical glass, heat-resistant glass and medical glass.

Raw materials, manufacturing and finishing processes

The raw materials for special glass are varied and selected as a function of the desired properties. In addition to silica sand, soda and lime, metal oxides, rare earths and other substances can also be added.

Special glass is mostly produced in special furnaces at high temperatures. After moulding the glass products are frequently subjected to further finishing processes such as:

• Tempering: the surface is hardened and made more shatter-proof by thermal or chemical treatments.
• Coating: various coatings improve the glass properties in terms of heat insulation, solar protection or scratch resistance.
• Lamination: several glass sheets are connected with each other to produce security or insulating glass units, for example. This is also how the LTG and IGU products categorised as flat glass are produced using special techniques to fulfil specific requirements.

Optical glass:

• Properties: optical glass is characterised by precisely defined optical properties such as special refraction indices and dispersive power. These parameters make for a targeted manipulation of light beams.
• Applications: in addition to lenses for cameras and corrective glasses, prisms, microscopes and telescopes, optical glass is also used in optical measuring and laser technologies as well as in optical fibres.
• Examples: crown and flint glass, barium crown glass

Heat-resistant glass:

• Properties: the admixture of boron oxide during production reduces the thermal expansion of the glass. It is not sensitive to thermal shocks thereby preventing stresses that could lead to glass breakage.
• Applications: heat-resistant glass is found in cookware, laboratory glass, lamps and oven doors but is also used in the chemical industry and in metallurgy.
• Examples: Borosilicate glass (Pyrex)

Chemically resistant glass:

• Properties: the chemical resistance to acids, alkalis and other chemicals is achieved by an especially dense and inert glass surface.
• Applications: in addition to laboratory glass or as a container for aggressive media this glass is also used in the semi-conductor industry and nuclear technology.
• Examples: quartz glass

Tinted glass:

• Properties: glass is tinted by admixing so-called colourants that selectively absorb or transmit specific wavelengths of the transmitted light.
• Applications: tinted glass is used in architecture, art and as filter glass in optics.
• Examples: ruby, cobalt and uranium glass

Smart glass:

• Properties: smart glass uses electrochemical, thermochromic or photochromic effects to dynamically change its optical properties.
• Applications: used as light and UV-blocking glass, for energy savings and privacy purposes, smart glass features in windows and doors – or in displays. It is also used in the automotive industry and air space.
• Examples: electrochromic glass, thermochromic glass, photochromic glass, liquid crystal glass

• Properties: electrical special glass stands out with high electrical insulation, good thermal conductivity and high mechanical strength.
• Applications: this glass type is used in the electronics industry, in sensors and in energy technology as insulating glass in electrical devices or for printed circuit boards.
• Example: glass-ceramics

Touch-sensitive glass:

• Properties: the touch-sensitive surface is obtained by transparent, conductive layers that can produce and detect electrical fields. This glass is typified by high transparency, high hardness, good electrical conductivity and a smooth surface.
• Applications: touch screens form part of nearly all modern user interfaces such as in smartphones, tablets, smart watches. But they are also used in medical devices, air space and in industrial contexts.
• Examples: Gorilla glass, Asahi Dragontrail

Fibre glass:

• Properties: high tensile strength, low density and good electrical insulation are the hallmarks of fibre glass.
  Applications: fibre glass is used in construction, air space and in the sporting goods industry.
  Examples: glass wool, woven glass