Glass and Plastics: Properties, Types and Commercial Forms

Glass is a material used for many years. It is the first inorganic substance, amorphous and isotropic (its molecular structure is totally disorganized). When cooled from its fluid state at high temperature, it solidifies without crystallizing and forms a rigid, viscous substance. It is transparent or translucent, hard, brittle and impermeable, with high chemical resistance, except against hydrofluoric acid.

Glass properties

Physical properties

• Density: common glass usually has a density of 2.5 kg/dm³; leaded glass has a density of 4.5 kg/dm³.

Thermal properties

• Thermal dilation: glass dilates very little, superior to wood.
• Thermal conductivity: the flow of heat through glass is very low, about 1 kcal/h·m·°C. For glass with a thickness of 1 m, a temperature difference of 1 °C between the interior and exterior produces a heat flow of 1 kcal per hour per square metre.
• Thermal transmission: the amount of heat that passes through windows depends on the thickness and the thermal changes of the surface.
• Fire behaviour: non-combustible.

Optical properties

• Luminous permeability: the ability of glass to let light pass through, depending on the type of glass and the incident luminous flux.
• Solar factor: the ratio between the total energy entering through a glass and the solar energy incident on it.
• Greenhouse effect: energy entering through the glass is absorbed locally by objects or people and re-radiated as long-wave heat. Opaque or impermeable materials, unlike glass (low transmission), accumulate this energy inside, since radiation of wavelength around 5 µm cannot leave.

Mechanical properties

• Hardness: 6.5-7 on the Mohs scale, which is high.
• Fragility: very fragile; it cannot be deformed. Fragility can be reduced by annealing.
• Compressive strength: very high, about 10,000 N/m².
• Tensile strength: about 400 kg/cm². After annealing, 400-500 kg/cm²; tempered glass, 1200-2000 kg/cm².

Chemical properties

• Durability: very high. It is very resistant to most chemical attack, except hydrofluoric acid.

Commercial forms of glass

• Monolithic glass: Float glass is a flat sheet of variable dimensions (3-6 m), 2-19 mm thick. Tempered glass is treated to be used in doors and similar applications. It softens at about 700 °C and, when broken, shatters into small pieces, so it is very resistant to internal stresses.
• Cast glass: Reinforced glass can be corrugated or polished. Cast glass is shaped during manufacture, or can be drawn or engraved afterwards. Some types of glass can also be cast.
• Enamelled glass and glass with coatings: has a sprayed layer applied to it.
• Reflective glass: low emissivity, which reduces thermal emission.
• Special glass: eliminates reflections and offers good acoustic insulation.

Laminated and insulating glass

• Laminated glass: made of alternating flat sheets of glass and plastic (polyvinyl butyral). It may be coloured or transparent, and offers acoustic insulation.
• Double glazing: two sheets of glass separated by an air chamber.
• Sun protection: consists of a metallic or coloured layer applied to the glass.

Glass fibre

Glass fibre is made of filaments and binders, formed by centrifugation, and is supplied as thread or as a woven fabric. It is used in building as acoustic and thermal insulation, usually in the form of wavy panels bonded with polystyrene. It is non-combustible and resistant to external agents.

Other glass types

Electrochromic, electro-optical, structural, extra-clear and polycarbonate-based glasses are resistant to fire and impact.

Plastics

Plastics are synthetic organic materials made of macromolecules, obtained through a polymerization process. They are produced by transforming natural rubber and synthetic resins (mostly derived from petroleum).

Properties of plastics

• Density: very light, similar to some woods, at 1-2 kg/dm³.
• Compressive strength: varies between 500 and 3000 kg/cm², depending on temperature.
• Tensile strength: varies between 300 and 600 kg/cm², depending on the manufacturing process.
• Flexural strength: varies between 5000 and 20000 kg/cm², depending on temperature and loading time.
• Thermal insulation: generally good thermal and acoustic insulators, with very low thermal conductivity.
• Acoustic insulation: depends on the shape of the plastic.
• Electrical insulation: good electrical insulators.
• Water absorption: generally very low, which is why they are used to make pipes.
• Chemical resistance: invariant to acid concentration and dependent on temperature. Generally very resistant, superior to metals.
• Combustibility: most are combustible. When burning, many release toxic gases. Plastics made with polyethylene, polystyrene and nitrocellulose are especially flammable.
• Appearance: can be very varied and can have strong colours. Maintenance is nil, and a single plastic can be produced in different colours.

General classification of plastics

Thermoplastics

Thermoplastics can be moulded or shaped by applying heat, and can be reprocessed. They are formed of chains that soften when heated.

• PVC (polyvinyl chloride): used for pipes, sanitation, electrical insulation, bricks, timber substitutes, coatings, netting and paint.
• PVA (polyvinyl acetate): used in plastic paint and glue.
• PS (polystyrene), expanded or extruded (XPS): used as thermal insulation, in the form of balls or boards. Expanded polystyrene is used in facades, ground cover, and for ceilings and vaults.
• ABS (acrylonitrile butadiene styrene): a hard, amorphous, opaque and very resistant plastic.
• PMMA (polymethyl methacrylate, acrylic): transparent, resistant to yellowing from sunlight. Used for wavy and smooth synthetic sheets, and easily worked.
• PA (polyamides, nylon): high mechanical and thermal resistance, and good chemical resistance. Used in protective nets.
• PE (polyethylene): used for bags, packaging sheets, pipes and film.
• PP (polypropylene): used for pipes, bags and tubes.
• PC (polycarbonate): used in paints and varnishes, and as plate material.

Thermosetting plastics

Thermosetting plastics become solid during primary processing and cannot be recovered or reshaped by heat once formed.

• Phenolic resins (PF): used in sheets, paints and varnishes.
• Urea resins (UF): used in adhesives, varnishes, paints and sheets.
• Melamine (MP): used in varnishes, paints and sheets.
• Polyester (UP): used in reinforcements, varnishes, furniture, electrical insulation, paints, sheets and coatings.
• Polyurethane (PUR): used as foam and acoustic insulation between panels.
• Epoxy resins: used in garage floors, repair mortars and continuous pavements.
• Silicone (where present): used in paints, varnishes and as a binding material.

Commercial steel forms

Steel sheets and plates

• Thin plate: flat steel profile 1 to 4 mm thick, with a maximum width of 20 cm.
• Plate: flat rectangular profile 4-10 mm thick and 20 cm wide.
• Flat bar: rectangular steel bar with a width at least twice its thickness.
• Sheet: flat profile with a very thin thickness of 0.6-20 mm, up to 60 cm wide and 12 m long. Types include smooth, ribbed (fluted, wavy), stamped, perforated and drawn.

Bars and sections

• Handrail bar: rectangular section, with its width at least twice its thickness.
• Square bar: square section, for example 20x20.
• Round bar: round or cylindrical bar (over 5 mm) and wire (under 5 mm), always solid.
• Layered profiles: T-section, I-beams (IPE, HEB, IPN), UPN channels and L-shaped profiles.
• Shaped profiles: omega, hat and L and U profiles.

Fasteners and cables

• Key: forged steel piece with a head and a stem.
• Pin: does not distinguish between key and pin; a pin has a simple cylindrical stem.
• Screw: consists of a cylindrical projection with a continuous helical thread.
• Rivet: a steel piece with a cylindrical stem and a head, inserted into the structure and deformed to fix the ends.
• Cables: braided from multiple wires, usually 3 mm in diameter. Quality fabrics are composed of flat or round wires larger than 3 mm.

Meshes and grids

• Mesh: made of wire less than 3 mm in diameter.
• Grids: framing of bars or profiles forming a mesh.
• Electro-welded mesh: designation example: 15x30 (bar spacing in cm), 10-6.5 (corrugated bar diameter in mm), B500T (type ACR), 5x2 (mesh dimensions in m), UNE 36092-96.