Engineering Chemistry 3(2+1)

Lesson 30. Thermoplastic and Thermosetting polymers

There are multiple thermoplastic resins that offer various performance benefits, but most materials commonly offer high strength, shrink-resistance and easy bend ability. Depending on the resin, thermoplastics can serve low-stress applications such as plastic bags or high-stress mechanical parts.

Above its glass transition temperature, T_g, and below its melting point, Tm, the physical properties of a thermoplastic change drastically without an associated phase change. Within this temperature range, most thermoplastics are rubbery due to alternating rigid crystalline and elastic amorphous regions, approximating random coils.

Some thermoplastics do not fully crystallize above glass transition temperature Tg, retaining some, or all of their amorphous characteristics. Amorphous and semi-amorphous plastics are used when high optical clarity is necessary, as a light wave cannot pass through smaller crystallites than its wavelength. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure.

Brittleness can be lowered with the addition of plasticizers, which interfere with crystallization to effectively lower T_g. Modification of the polymer through copolymerization or through the addition of non-reactive side chains to monomers before polymerization can also lower T_g. Before these techniques were employed, plastic automobile parts would often crack when exposed to cold temperatures. Recently, thermoplastic elastomers have become available

Thermoplastic Resins

Thermoplastic polymer resins are extremely common, and we come in contact with thermoplastic resins constantly. Thermoplastic resins are most commonly unreinforced, meaning, the resin is formed into shapes and have no reinforcement providing strength.

Examples of common thermoplastic resins used today, and products manufactured with them include:

1)      PET (Polyethylene terphalate)  - Water and soda bottles

2)      Polyproplyene - Packaging containers

3)      Polycarbonate - Safety glass lenses

4)      PBT (Polybutylene terphalate) - Children's Toys

5)      Vinyl - Window frames

6)      Polyethlene - Grocery bags

7)      PVC (Polyvinyl chloride) - Piping

8)      PEI (Polyetherimide)  - Airplane armrests

9)      Nylon - Footwear

 Many thermoplastic products use short discontinuous fibers as reinforcement. Most commonly fiberglass, but carbon fiber too. This increases the mechanical properties and is technically considered a fiber reinforced composite, however, the strength is not nearly as comparable to continuous fiber reinforced composites.

Advantages of thermoplastic Composites

There are two major advantages of thermoplastic composites. The first is that many thermoplastic resins have an increased impact resistance to comparable thermoset composites. In some instances, the difference is as high as 10 times the impact resistance.

The other major advantage of thermoplastic composites is the ability reform. See, raw thermoplastic composites, at room temperature, are in a solid state. When heat and pressure impregnate a reinforcing fiber, a physical change occurs; not a chemical reaction as with a thermoset.

            This allows thermoplastic composites to be reformed and reshaped. For example, a pultruded thermpostic composite rod could be heated and remoulded to have a curvature. This is not possible with thermosetting resins. This also allows for the recycling of the thermoplastic composite at end of life. (In theory, not yet commercial)

Some common advantage of thermoplastic composites

1)      Aesthetically-superior finishes

2)      Chemical resistant

3)      Hard crystalline or rubbery surface options

4)      Eco-friendly manufacturing

Disadvantages of Thermoplastics:

Because thermoplastic resin is naturally in a solid state, it is much more difficult to impregnate reinforcing fiber. The resin must be heated to the melting point, and pressute is required to impregnate fibers, and the composite must then be cooled under this pressure. This is complex and far different from traditional thermoset composite manufacturing. Special tooling, technique, and equipment must be used, many of which is expensive. This is the major disadvantage of thermoplastic composites.

Advances in thermoset and thermoplastic technology are happening constantly. There is a place and a use for both, and the future of composites does not favour one over the other.

1)      Generally more expensive than thermoset

2)      Can melt if heated

Thermosetting

A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat (generally above 200 °C (392 °F)), through a chemical reaction or irradiation such as electron beam processing. These are formed by condensation polymerization.

Thermoset materials are usually liquid or malleable prior to curing and designed to be moulded into their final form, or used as adhesives. Others are solids like that of the moulding compound used in semiconductors and integrated circuits (IC). Once hardened a thermoset resin cannot be reheated and melted back to a liquid form.

According to IUPAC recommendation: A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation, or both. A cured thermosetting polymer is called a thermoset.

The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.

Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material

Thermosetting plastics are generally strong and resistant to heat, but they melt the first time they are heated to a high enough temperature and harden (set) permanently when cooled. They are used in situations where resistance to heat is important, e.g. on kitchen work surfaces, good-quality plastic cups, saucepan handles and plug casings.

Thermoset plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermosets ideal for high-heat applications such as electronics and appliances.

Thermoset plastics significantly improve the material’s mechanical properties, providing enhances chemical resistance, heat resistance and structural integrity. Thermoset plastics are often used for sealed products due to their resistance to deformation.

Traditional Fiber Reinforced Polymer Composites, or FRP Composites for short, use a thermosetting resin as the matrix, which holds the structural fiber firmly in place. Common thermosetting resin include:

1)      Polyester Resin

2)      Vinyl Ester Resin

3)      Epoxy

4)      Phenolic

5)      Urethane

The most common thermosetting resin used today is polyester resin, followed by vinyl ester and epoxy. Thermosetting resins are popular because uncured, at room temperature; they are in a liquid state. This allows for convenient impregnation of reinforcing fibers such as fiberglass, carbon fiber, or Kevlar.

Properties of Thermoset Resins

As mentioned, a room temperature liquid resin is easy to work with. Laminators can easily remove all air during manufacturing, and it also allows the ability to rapidly manufacture products using a vacuum or positive pressure pump. (Closed Molds Manufacturing) Beyond ease of manufacturing, thermosetting resins can exhibit excellent properties at a low raw material cost.

Properties of thermoset resins include:

1)      Excellent resistance to solvents and corrosives

2)      Resistance to heat and high temperature

3)      Fatigue strength

4)      Tailored elasticity

5)      Excellent adhesion

6)      Excellent finishing (polishing, painting, etc.)

7)      Highly flexible design

8)      Thick to thin wall capabilities

9)      Excellent aesthetic appearance

10)  High levels of dimensional stability

11)  Cost-effective

Disadvantage:

1)      Cannot be recycled

2)      More difficult to surface finish

3)      Cannot be remoulded or reshaped

In a thermoset resin, the raw uncured resin molecules are crossed linked through a catalytic chemical reaction. Through this chemical reaction, most often exothermic, the resin creates extremely strong bonds to one another, and the resin changes state from a liquid to a solid.

Examples

Some examples of thermosets are:

1)      Polyester fibreglass systems: sheet molding compounds and bulk molding compounds).

2)      Polyurethanes: insulating foams, mattresses, coatings, adhesives, car parts, print rollers, shoe soles, flooring, synthetic fibers, etc. Polyurethane polymers are formed by combining two bi- or higher functional monomers/oligomers.

3)      Vulcanized rubber.

4)      Bakelite, a phenol-formaldehyde resin used in electrical insulators and plasticware

5)      Duroplast, light but strong material, similar to bakelite used for making car parts

6)      Urea-formaldehyde foam used in plywood, particleboard and medium-density fibreboard.

7)      Melamine resin used on worktop surfaces.

8)      Epoxy resin used as the matrix component in many fiber reinforced plastics such as glass-reinforced plastic and graphite-reinforced plastic).

9)      Polyimides used in printed circuit boards and in body parts of modern aircraft.

10)  Cyanate esters or polycyanurates for electronics applications with need for dielectric properties and high glass temperature requirements in composites.

Some methods of molding thermosets:

1)      Reactive injection molding (used for objects such as milk bottle crates)

2)      Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)

3)      Compression molding (used to shape most thermosetting plastics)

4)      Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)

•  Processing Polymers

Once a polymer with the right properties is produced, it must be manipulated into some useful shape or object. Various methods are used in industry to do this. Injection molding and extrusion are widely used to process plastics while spinning is the process used to produce fibers.

• Injection Molding

One of the most widely used forms of plastic processing is injection molding. Basically, a plastic is heated above its glass transition temperature (enough so that it will flow) and then is forced under high pressure to fill the contents of a mold. The molten plastic in usually "squeezed" into the mold by a ram or a reciprocating screw. The plastic is allowed to cool and is then removed from the mold in its final form. The advantage of injection molding is speed; this process can be performed many times each second.

• Extrusion

Extrusion is similar to injection molding except that the plastic is forced through a die rather than into a mold. However, the disadvantage of extrusion is that the objects made must have the same cross-sectional shape. Plastic tubing and hose is produced in this manner.

• FA thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat (generally above 200 °C (°F)), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.

Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC). Once hardened a thermoset resin cannot be reheated and melted back to a liquid form. According to IUPAC recommendation: A thermosetting polymer is a pre-polymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation, or both. A cured thermosetting polymer is called a thermoset.

Process

The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material. Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.

Properties

Thermoset materials are generally stronger than thermoplastic materials due to this three dimensional network of bonds (cross-linking), and are also better suited to high-temperature applications up to the decomposition temperature. However, they are more brittle. Since they are "set" (non-reformable), they tend not to be recyclable.

Examples

Some examples of thermosets are:

• Polyester fibreglass systems: sheet molding compounds and bulk molding compounds)

• Polyurethanes: insulating foams, mattresses, coatings, adhesives, car parts, print rollers, shoe soles, flooring, synthetic fibers, etc. Polyurethane polymers are formed by combining two bi- or higher functional

monomers/oligomers.

• Vulcanized rubber

• Bakelite, a phenol-formaldehyde resin used in electrical insulators and plasticware

• Duroplast, light but strong material, similar to bakelite used for making car parts

• Urea-formaldehyde foam used in plywood, particleboard and medium-density fiberboard

• Melamine resin used on worktop surfaces.

• Epoxy resin used as the matrix component in many fiber reinforced plastics such as glass-reinforced plastic and graphite-reinforced plastic)

• Polyimides used in printed circuit boards and in body parts of modern aircraft

• Cyanate esters or polycyanurates for electronics applications with need for dielectric properties and high glass temperature requirements in composites

• Mold or mold runners (the black plastic part in integrated circuits or semiconductors)

Thermosetting polymer  2?????????

Some methods of molding thermosets are:

• Reactive injection molding (used for objects such as milk bottle crates)

• Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)

• Compression molding (used to shape most thermosetting plastics)

• Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)

 

References

[1] http:/ / old. iupac. org/ goldbook/ TT07168. pdf

[2] The Open University (UK), 2000. T838 Design and Manufacture with Polymers: Introduction to Polymers, page 9. Milton Keynes: The Open

University

[3] Roberto C. Dante, Diego A. Santamaría and Jesús Martín Gil (2009). "Crosslinking and thermal stability of thermosets based on novolak and

melamine". Journal of Applied Polymer Science 114 (6): 4059–4065. doi:10.1002/app.31114.ood Chemistry is a major aspect of food science.

Food chemistry deals with composition and properties of food and chemical changes it undergoes during handling, processing and storage.

Chemically food consists of carbohydrates, proteins, lipids, vitamins, minerals, preservators, colouring and flavouring reagents of food. In this module we will learn more about them in details.