[attachment=13221]
RESINSThermosetting &Thermoplastic resins
Introduction
Resins
The primary functions of the resin are to transfer stress between the reinforcing fibers, act as a glue to hold the fibers together, and protect the fibers from mechanical and environmental damage.
Resins are divided into two major groups known as thermoset and thermoplastic.
Thermoplastic Resins
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.
Uses
Thermoset Resins
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 (two-part epoxy, for example), or irradiation such as electron beam processing.
Some examples of thermosets are:
Polyester fiberglass systems: (SMC Sheet molding compounds and BMC Bulk molding compounds)
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)
Process
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.
Uses
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's).
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.
Pro’s of Thermoplastic Resins
High Impact Strength
Attractive Surface Finish
Recyclable / Scrap is Reusable
No Emissions
Can bond to other thermoplastics
Can be molded or shaped with reheat
Con’s of Thermoplastic Resins
Generally softens with heat
More difficult to prototype
Common types of Thermoplastic Resins
Polyethylene (PE)Polypropylene (PP)
Polyamide (PA or Nylon)Polybutylene terephthalate (PBT)Polyethylene terephthalate (PET)Polycarbonate (PC)
Polyethylene (PE)
Polyethylene is a thermoplastic polymer consisting of long chains produced by combing the ingredient monomer ethylene (IUPAC name ethene), the name comes from the ingredient and not the actual chemical resulting.
Polypropylene (PP)
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles (e.g. ropes, thermal underwear and carpets)
Pro’s of Thermoset Resins
Easy to process and laminate
Does not necessarily need pressure or heat to form
Generally inexpensive
Generally stronger than thermoplastics
Generally better suited to higher temperatures then thermoplastics
Con’s of Thermoset Resins
Often release emissions known as volatile organic compounds (VOCs)
Cannot be recycled or reclaimed easily
Short workable pot life, with some exceptions
Less-than-perfect surface finish
Common Types of Themoset Resins
EpoxyPolyesterVinylesterPolyurethanePhenolic.
Epoxy
A major benefit of epoxy resins over unsaturated polyester resins is their lower shrinkage.
Superior mechanical properties, resistance to corrosive liquids and environments, superior electrical properties, good performance at elevated temperatures, good adhesion to a substrate, or a combination of these benefits.
Polyester
Thermoset polyesters are produced by the condensation polymerization of dicarboxylic acids and difunctional alcohols (glycols).
The principal advantage of these resins is a balance of properties (including mechanical, chemical, electrical) dimensional stability, cost and ease of handling or processing.
CONCLUSION
The resins in thermoset composites are an important source of properties and process characteristics. One of the great design strengths of composites is the multiple choice of resins. In order to make effective use of these choices, designers and product specifiers should be familiar with the properties, advantages and limitations of each of the common composite resins. It is common to use the resources of the resin manufacturers laboratories to determine the best resin or an application.