Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Plastics Properties and Applications

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Contents

Introduction, benefits and disadvantages of plastics General properties and uses Plastics family tree Properties of thermosets and thermoplastics 35 elected modern plastics, with properties and applications 20 historical plastics, with properties and applications

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

The Plastics

While natural plastics such as Chinese lacquer, bitumen and amber have been known since ancient times, the comprehensive use of plastics in engineered products is a relatively recent phenomenon. The earliest plastics were derived directly from natural materials. From trees and plants came amber, celluloid, ebonite, and gutta-percha. From insects and animals came shellac, horn, casein, etc. Many of these early plastics are out-dated in terms of volume product design, yet some remain in use for the production of jewellery and niche decorative items. Modern plastics were born around the mid 1800’s, since which time the range has expanded and evolved to give the family of plastics available today.

For specific plastics, see the tables for thermosets and thermoplastics, and the Familiar Names table after this section. See the Elastomers section for more on rubber and synthetic elastomers. See the Composite Plastics section for fillers, reinforced plastics, and fabricated components.

Benefits of using plastics: • Vast range of colors and surface finishes available. • Low weight, and high strength to weight ratio, especially when reinforced

or foamed. • Controllable stiffness and resilience. • High electrical and thermal resistance, especially when foamed. • Complicated shapes achievable, fewer components. • Suitable for mass production = low unit labor cost (hence part cost) and

repeatable dimensions. • Wide range of forming techniques available – fewer constraints than metal.

Disadvantages: • Not suitable for components requiring very high absolute strength. • Melting points are generally lower than metals. • Poor thermal and electrical conductivity ( = excellent insulators!).

The Plastics Family Tree

The plastics family tree has two main branches: Thermoplastics and Thermosets. While most plastics belong uniquely to just one of these branches, a small number of plastics, notably the polyesters, imides and urethanes, have formulations in both branches.

Thermoplastics are created by the action of heat and pressure on monomers, usually in the presence of a catalyst, to create very long ‘daisy-chain’ molecules in a process known as polymerization. For example, the polymerization of ethylene gas yields poly-ethylene (polythene). The long chain molecules remain unlinked, like spaghetti. When heated, thermoplastics become soft and may liquefy, but will return to a stiff solid condition on cooling. Polyolefines, Styrenes and Vinyls are all examples of thermoplastics. The cycle of melting and solidification can be repeated many times, although a slight degradation occurs with each cycle. In the case of polythene, for

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

example, repeated melting and solidification will eventually break up the long chains, and the material will revert to its familiar short-molecule form – candle wax. The shape of molecules determines their packing density; low densities generally producing an amorphous material, and high densities producing a semi-crystalline material.

Amorphous thermoplastics such as ABS, acrylic, PVC, polystyrene and polycarbonate soften gradually with heat. They are not generally suited to mechanically demanding situations, but are ideal where low mould shrinkage, high dimensional accuracy and attractive appearance (especially transparency) are required. Their elasticity at softening temperatures makes them suitable for extrusion and vacuum forming. They are often used for consumer product enclosures, although chemical resistance tends to be poor. Fiber reinforcement does not increase strength at elevated temperatures.

Polystyrene (PS) CD jewel cases HIPS disposable razors

ABS consumer electronic products Polycarbonate (PC) lighter and shaver

Semi-crystalline thermoplastics including polythene, polypropylene and nylon tend to have higher melting points but melt abruptly, and carry a higher risk of distortion after molding. They are best suited to applications involving mechanical abuse or abrasion, and are often reinforced to improve otherwise poor strength, dimensional stability and accuracy. Chemical resistance is generally good. Extrusion, vacuum forming and adhesive bonding are often difficult.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Food storage box in polyethylene (PE)

Amorphous and semi-crystalline polymers can be further categorized according to whether they are general purpose (commodity) plastics or engineering (high performance) plastics. The engineering plastics invariably have a higher cost.

CAUTION It is not possible to tell by visual inspection whether a polymer is amorphous or semicrystalline. Design decisions should be based on specific polymer properties, taking into account the grade of polymer, the degree of chemical modification or alloying, the potential forming and finishing process, and any physical modifications such as filling, foaming or reinforcement.

Some monomer molecules exist in a variety of shapes, and the particular shape of molecule determines the physical properties of the polymer. Where a polymer has been produced from only one shape of monomer molecule, it is known as a homopolymer. Where two varieties are used, the polymer is described as a copolymer, and where three varieties are used, a terpolymer. For example, Acetal is available in homopolymer and copolymer versions.

Note that two completely different plastics can be blended to produce an alloy having a mixture of properties from the two original plastics. An example is polycarbonate/ABS, where the strength and heat resistance of PC is combined with the benign molding characteristics and lower cost of ABS to produce an attractive, strong, relatively low cost plastic alloy.

In addition to the pellet form for molding, thermoplastics can be supplied as thin film, or pre-form blocks for engineering purposes. The properties of thermoplastic moldings are frequently enhanced by the use of fillers or reinforcement, or by foaming.

See composites section for fillers and reinforcements. See thermoplastics tables to identify plastics available as thin films.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

THERMOSETS

Thermosets (thermosetting plastics) are polymerized by curing, an irreversible chemical process that creates a heavily cross-linked polymer. Curing is often described as ‘polymer cookery’. It is not possible to re-melt a cured thermoset. Extreme heating will result in charring and eventual decomposition. For this reason, thermosets are generally more heat-resistant than thermoplastics. Thermosets generally offer high resistance to scratching, creep, solvents and UV radiation, and are frequently filled or reinforced to improve toughness and reduce molding shrinkage. Phenolics, urea formaldehyde, melamine formaldehyde, and epoxides are examples of thermosets. Two-part liquid systems (polyester resin for GRP car bodies, or epoxy adhesives for example) are common, the curing mechanism being addition polymerization. To be useful to the plastic part molder, suitable thermosets are normally supplied partially cured, in the form of powders or dough, the curing process being described as condensation polymerization.

General Purpose

Engineering

General Purpose

Engineering

THERMOPLASTICS

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

In both cases, complete curing is achieved in situ, triggered by heat, UV light, or chemical catalyst. Color ranges can be rather limited.

Phenol formaldehyde (Bakelite) radio Urea formaldehyde power plug and socket (UK, 1947) (UK, 2007)

See the tables Selected Thermosets, Selected Thermoplastics, and Selected Elastomers for more on specific plastics. Also see the foams section, and fillers and reinforcements in the composites section.

CAUTION Polymer chemists and plastics manufacturers often identify families of plastics according to chemical composition. For example the term ‘amides’ is used to mean all members of the nylon family, and ‘olefins’ refers to the chemical family containing polythene and polypropylene. There is a risk of confusion between similar sounding family names, particularly in the case of acetates, acrylics, alkyds, allyls, aramids, amides and imides. Although chemical family names are occasionally useful, it is often better to identify specific polymers by name.

Note also that a pure polymer usually requires the addition of plasticizers, flow modifiers, lubricants and fillers before it may be used as a practical industrial plastic.

General properties of plastics

Appearance The appearance can be transparent or opaque. In general, amorphous thermoplastics offer the best transparency, but when selecting suitable plastics for viewing windows and lenses, optical clarity must be balanced against resistance to impact, UV radiation and solvents. Light transmittance must be over 85% for the plastic to be useful as a lens or window. Domestic window glass generally offers a transmittance of around 90%. Acrylic (92%) and polystyrene (88%) are common, but impact resistance can be poor in both cases. Polycarbonate (89%) is much tougher and is the natural choice for consumer products. Other possibilities include MBS, transparent nylon, polyarylate, PSO, PVC, SAN and SMA. Plastics can be pigmented for any desired color transparent or opaque -although some plastics, notably phenolics, are naturally dark in color and offer less scope for coloring. Plastics can be painted and even metal plated in some cases. Surface textures can be incorporated into the mold, eliminating the need for finishing operations.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Density Relative densities are generally between 0.9 and 1.5 (aluminum = 2.7, and water = 1). Expanded plastics such as polystyrene foam can have relative densities down to 0.01. Structural foams (for example modified PPO) are generally around 0.5.

Thermal properties Polythene, PVC and polystyrene quickly lose strength at temperatures above about 60°C. At 80°C, UF begins to degrade with time, and very few plastics survive beyond 100°C. GRN can survive for about a year at 150°C, as can most phenolics. Alkyds can survive at 170°C, and PTFE at 250°C, although PTFE is pliable at this temperature. Polypropylene will soften in contact with boiling water, but talc-filled polypropylene is commonly used for jug kettles. In general, thermosets are more heat and flame resistant than thermoplastics. Plastics are excellent thermal insulators (typically 1,000 times better than metal; 10,000 times better if foamed). The coefficient of linear expansion of plastic is up to 10 times that of metal, but can be tamed by the addition of fillers such as mica or glass strand, so that GRN and GRP can have a dimensional stability to match that of die cast aluminum. Flammability varies across plastics, and depends upon the specific formulation and physical orientation. Flame inhibitors can be added to some materials.

Water/Chemical resistance Most plastics are resistant to water-based solvents, although some, notably nylon and cellulose-based polymers, will absorb water from the environment, swelling by around 1%. Most plastics are attacked to some degree by organic solvents and oils, which are absorbed, causing swelling, loss of rigidity, or cracking. Environmental stress cracking occurs in susceptible plastics where a stress cracking agent such as a detergent, lubricant or solvent comes into contact with a plastic component under tensile stress.

Weather and Ultra-violet resistance Acrylics are among the most weather and UV resistant plastics, having a typical outdoor life in excess of 30 years. PVC and polyesters are moderately good. Many plastics are degraded. Polythene will flake.

Strength and Impact resistance Absolute strength is generally lower than for metals, but can be raised by incorporating fillers or reinforcement. For example, chopped glass fibers are used to increase the strength of nylon power tool bodies. The disadvantage is the shortened mold tool life resulting from the abrasive effects of the filler. Best low-cost plastics for impact resistance are polycarbonate, ABS, polyethylene and nylon. Polystyrene is poor (brittle). Impact resistance in the engineering plastics is usually better still. Most thermosets are brittle although addition of reinforcement and fillers can improve them. Many plastics (particularly thermoplastics) experience cold flow (creep) under long-term steady load, making them unsuitable for permanently stressed springs, although some plastics (e.g. acetal) make excellent springs for intermittent use, and the same property can be exploited in the design of snap fit features. Some plastics exhibit ‘plastic memory’, such that a heated and deformed component will, if re-heated, attempt to revert to its original shape. Rigidity can be controlled by the addition of fillers or plasticizers. For example, rigid (unplasticized) PVC is used for window frames and rainwater goods, and plasticized PVC is used for hosepipes and cable insulation.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Friction Friction can be used to weld plastics that have moderate coefficients of friction (e.g. HDPE for gas and water pipes). For a low coefficient of friction, for use in a bearing for example, choose PTFE, or add a lubricant filler to nylon, acetal or phenolic.

Electrical Thermosets are frequently used as structural insulation in mains connectors and lamp-holders, because of their resistance to heat. Where high temperatures are absent, thermoplastics may be used. Mains wiring is insulated with plasticized PVC. Television (VHF/UHF) aerial cable conductors are insulted with polythene, which is an excellent dielectric offering low loss at high signal frequencies. Most plastics have a tendency to accumulate surface electrostatic charge, which attracts dust and can produce sparks, although anti-static treatments are available.

Also see the section Braving the Elements, for fire rating and weather resistance.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Selected Thermosets

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Selected Thermoplastics

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Selected Thermoplastics cont.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Selected Thermoplastics cont.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Selected Thermplastics cont.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Selected Thermoplastics cont.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Familiar Names (Historical, in Chronological Order)

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

Familiar Names cont.

Author: Andrew Taylor BSc MA FRSA – Design‐Bites

For General Design Tips and Resources, visit:

http://www.design1st.com/Design-Resource-Library/design-resource-center.html

LINKS and RESOURCES

Early Plastics, Sylvia Katz, Shire Publications, 1986. The Engineering Properties of Plastics (Eng Des 17), RM Ogorkiewicz, Design Council, 1977. Bakelite Radios (Collectors’ Corner), Quantum, 1999.

(UK) The Plastics Historical Society: www.plastiquarian.com/museum/index.htm (UK) RAPRA Technology (ex Rubber and Plastics Research Association): www.rapra.net (UK) British Plastics Federation: www.bpf.co.uk (UK) British Plastics & Rubber (magazine for manufacturers): www.polymer-age.co.uk (Eur) Plastics Europe (Association of Plastics Manufacturers in Europe): www.apme.org (US) Society of Plastics Engineers: www.4spe.org (US) The Plastics Mall (directory): www.plasticsmall.com (US) Alliance for the Polyurethane Industry: www.polyurethane.org (US) Plastics Technology (manufacturing): www.plasticstechnology.com (US) The American Plastics Council: www.plasticsresource.com/ (US) Society of the Plastics Industry (SPI): www.plasticsindustry.org (US) Modern Plastics Worldwide (magazine): www.modplas.com/ (US) Plastics Net (manufacturing): www.plasticsnet.com (UK) Plastics links, forums: www.plastics.com (US) Plastics Technology – Materials selector: www.ptonline.com/mat_zone/

(W) Bayer Plastics: http://plastics.bayer.com/plastics/emea/en/home/index.jsp (W) Bayer Material Science: www.bayerplastics.com (UK) BXL (Bakelite Xylonite Limited): now Zotefoams plc. (UK) BIP Oldbury (thermosets): www.bip.co.uk/ (W) Ciba-Geigy: (ceased plastics production). (US) American Cyanamid: (ceased plastics production). (US) Ticona (Hoechst-Celanese, engineering polymers): www.ticona.com (W) DuPont (engineering polymers): http://plastics.dupont.com (W) Dow (engineering polymers): http://plastics.dow.com/ (W) Dow plastics (by category): www.dow.com/products_services/category/plastics.htm (W) GE Plastics (engineering polymers, including ex Borg-Warner): www.geplastics.com (W) GE Polymershapes: www.gepolymershapes.com (UK) Victrex (PEEK): www.victrex.com (US) Voridian (Eastman Chemical, polymers): www.eastman.com or www.voridian.com (W) BASF (engineering polymers): www.basf.com/businesses/plasticportal/pp_home_en.html (W) BASF (by product) http://corporate.basf.com/en/produkte/segmente.htm (US) Ferro (engineering polymers): www.ferro.com (US) 3M (plastic films): www.3m.com (W) Lucite International (transparent plastics, including Perspex): www.lucite.com/ (US) Union Carbide (owned by Dow, polymers): www.unioncarbide.com/index.htm

(W) Albis Plastics (engineering polymers): www.albisna.com/ (US) Boedeker Plastic pre-forms (sheet, rod etc): www.boedeker.com (US) Endura Plastics Inc: www.endura.com

(W) ICI/National Starch: www.ici.com (US) General Plastics (urethane): www.generalplastics.com (US) Durez (phenolics): www.durez.com (US) Quadrant (plastic stock shapes for machining): www.quadrantepp.com (US) San Diego Plastics (plastics stockholders): www.sdplastics.com (UK) Molded grips in rubber and plastic: www.component-force.co.uk