A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications,Polymer Matrix Composites: Materials Usage, Design, and Analysis, SAE International, 2012, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.
They were first used after World War II, and continuing research has led to an increased range of thermoset resins, polymers or plastics,Handbook of Thermoset Plastics, ed. S.H. Goodman, H. Dodiuk-Kenig, William Andrew Inc., USA, 3rd edition, 2013, as well as engineering grade thermoplastics.Handbook of Thermoplastics, ed. O. Olabisi, K Adewale, CRC Press, USA, 2nd edition, 2015, They were all developed for use in the manufacture of polymer composites with enhanced and longer-term service capabilities. Thermoset polymer matrix technologies also find use in a wide diversity of non-structural industrial applications.Industrial Polymer Applications: Essential Chemistry and Technology, Royal Society of Chemistry, UK, 1st edition, 2016,
The foremost types of thermosetting polymers used in structural composites are benzoxazine resins, bis-maleimide resins (BMI), cyanate ester resins, epoxy (epoxide) resins, phenolic (PF) resins, unsaturated polyester (UP) resins, polyimides, polyurethane (PUR) resins, silicones, and vinyl esters.
Benzoxazine resins
These are made by the reaction of phenols, formaldehyde and primary amines which at elevated temperatures () undergo ring–opening polymerisation forming polybenzoxazine thermoset networks; when hybridised with epoxy and phenolic resins the resulting ternary systems have glass transition temperatures in excess of .Handbook of Benzoxazine Resins, ed. Hatsuo Ishida And Tarek Agag, Elsevier B.V., 2011,
Cure is characterised by expansion rather than shrinkage and uses include structural prepregs, liquid molding and film adhesives for composite construction, bonding and repair. The high aromatic content of the high molecular weight polymers provides enhanced mechanical and flammability performance compared to epoxy and phenolic resins.
Bis-maleimides (BMI)
Formed by the condensation reaction of a diamine with maleic anhydride, and processed basically like epoxy resins ( cure). After an elevated post-cure (), they will exhibit superior properties. These properties are influenced by a continuous use temperature and a glass transition of .
This thermoset polymer type is merged into composites as a prepreg matrix used in electrical printed circuit boards, and for large scale structural aircraft – aerospace composite structures, etc. It is also used as a coating material and as the matrix of glass reinforced pipes, particularly in high temperature and chemical environments.
Cyanate ester resins
The reaction of bisphenols or multifunctional phenol novolac resins with cyanogen bromide or chloride leads to cyanate functional monomers which can be converted in a controlled manner into cyanate ester functional prepolymer resins by chain extension or copolymerization. When postcured, all residual cyanate ester functionality polymerises by cyclotrimerisation leading to tightly crosslinked polycyanurate networks with high thermal stability and glass transition temperatures up to and wet heat stability up to around .
Cyanate ester resin prepregs combine the high temperature stability of polyimides with the flame and fire resistance of phenolics and are used in the manufacture of aerospace structural composite components which meet fire protection regulations concerning flammability, smoke density and toxicity. Other uses include film adhesives, surfacing films and 3D printing.
Epoxy (epoxide) resins
Epoxy resins are thermosetting prepolymers made either by the reaction of epichlorohydrin with hydroxyl functional aromatics, cycloaliphatics and aliphatics or amine functional aromatics, or by the oxidation of unsaturated cycloaliphatics. The diglycidyl ethers of bisphenol-A (DGEBA) and bisphenol-F (DGEBF) are the most widely used due to their characteristic high adhesion, mechanical strength, heat and corrosion resistance. Epoxide functional resins and prepolymers cure by polyaddition/copolymerisation or homopolymerisation depending on the selection of crosslinker, hardener, curing agent or catalyst as well as by the temperature.Chemistry and Technology of Epoxy Resins, ed. B. Ellis, Springer Netherlands, 1993,
Epoxy resin is used widely in numerous formulations and forms in the aircraft-aerospace industry. It is regarded as "the work-horse of modern day composites". In recent years, the epoxy formulations used in composite prepregs have been fine-tuned to improve their toughness, impact strength and moisture absorption resistance. Maximum properties have been realized for this polymer.
This is not only used in aircraft-aerospace demand. It is used in military and commercial applications and is also used in construction. Epoxy-reinforced concrete and glass-reinforced and carbon-reinforced epoxy structures are used in building and bridge structures.
Epoxy composites have the following properties:
High-Strength Glass Fiber Reinforced Relative Density 1.6-2.0 Melting temperature (°C) Thermoset Processing Range (°F) C:300-330, I=280-380 Molding pressure 1–5 Shrinkage 0.001-0.008 Tensile strength (p.s.i.) 5,000-20,000 Compressive strength (p.s.i.) 18,000-40,000 Flexural Strength (p.s.i.) 8000–30,000 Izod impact strength (ft·lb/in) 0.3-10.0 Linear expansion (10−6 in./in./°C) 11–50 Hardness Rockwell M100-112 Flammability V-0 Water absorption 24h (%) 0.04-0.20
Epoxy Phenol Novolac (EPN) and Epoxy Cresol Novolac (ECN) resins made by reacting epichlorohydrin with multifunctional phenol novolac or cresol novolac resins have more reactive sites compared to DGEBF epoxy resins and on cure result in higher crosslink density thermosets. They are used in printed wire/circuit board laminating and also for electrical encapsulation, adhesive and coatings for metal where there is a need to provide protection from corrosion, erosion or chemical attack at high continuous operating temperatures.
Phenolic (PF) resins
There are two types of phenolic resinsPhenolic Resins Technology Handbook, NPCS Board of Consultants & Engineers, 2007, - novolacs and resoles. Novolacs are made with acid catalysts and a molar ratio of formaldehyde to phenol of less than one to give methylene linked phenolic oligomers; resoles are made with alkali catalysts and a molar ratio of formaldehyde to phenol of greater than one to give phenolic oligomers with methylene and benzylic ether-linked phenol units.
Phenolic resins, originally developed in the late 19th century and, regarded as the first truly synthetic polymer types, are often referred to as the "work-horse of thermosetting resins". They are characterised by high bonding strength, dimensional stability and creep resistance at elevated temperatures, and frequently combined with co-curing resins such as epoxies.
General purpose molding compounds, engineering molding compounds and sheet molding compounds are the primary forms of phenolic composites. Phenolics are also used as the matrix binder with Honeycomb core. Phenolics find use in many electrical applications such as breaker boxes, brake lining materials and most recently in combination with various reinforcements in the molding of an engine block-head assembly, called the polimotor. Phenolics may be processed by the various common techniques, including compression, transfer and injection molding.
Properties of phenolic composites have the following properties:
High-Strength Glass Fiber Reinforced Relative Density 1.69-2.0 Water Absorption 24h(%) 0.03-1.2 Melting Temperature (◦c) Thermo set Processing Range (◦F) C:300-380 I:330-390 Molding pressure I-20 Shrinkage 0.001-0.004 Tensile Strength (p.s.i.) 7000–18000 Compressive Strength (p.s.i.) 16,000-70,000 Flexural Strength (p.s.i.)12,000-60,000 Izod Impact strength (ft-lb/in) 0.5-18.0 Linear expansion (10−6 in./in./°C) 8–21 Hardness Rockwell E54-101 Flammability V-0
Polyester resins
Unsaturated polyester resins are an extremely versatile,Unsaturated Polyester Technology, ed. P.F. Bruins, Gordon and Breach, New York, 1976Composites Handbook, Scott Bader Company Ltd, 2005 and fairly inexpensive class of thermosetting polymer formed by the polycondensation of glycol mixtures often containing propylene glycol, with a dibasic acid and anhydrides usually maleic anhydride to provide backbone unsaturation needed for crosslinking, and phthalic anhydride, isophthalic acid or terephthalic acid where superior structural and corrosion resistance properties are required. Polyester resins are routinely diluted/dissolved in a vinyl functional monomer such as styrene and include an inhibitor to stabilize the resin for storage purposes. Polymerisation in service is initiated by free radicals generated from ionizing radiation or by the photolytic or thermal decomposition of a radical initiator. Organic peroxides, such as methyl ethyl ketone peroxide and auxiliary accelerators which promote decomposition to form radicals are combined with the resin to initiate a room temperature cure.
In the liquid state, unsaturated polyester resins may be processed by numerous methods, including Hand Layup, vacuum bag molding, and spray-up and compression molded Sheet Molding Compound (SMC). They can also be B-staged after application to chopped reinforcement and continuous reinforcement, to form pre-pregs. Solid molding compounds in the form of pellets or granules are also used in processes such as compression and transfer molding.
Polyimides
There are two types of commercial polyimides: thermosetting cross-linkable polyimides made by the condensation of aromatic diamines with aromatic dianhydride derivatives and anhydrides with unsaturated sites that facilitate addition polymerisation between preformed imide monomers and oligomers,D.A. Scola and J.H. Vontel, Polym. Compos., 1988, 9(6), 443-452Polyimides, eds. D Wilson et al., Springer, Netherlands, 1990, and thermoplastic polyimides formed by the condensation reaction between aromatic diamines and aromatic dianhydrides. Thermoset polyimides are the most advanced of all thermoset polymer matrices with characteristics of high temperature physical and mechanical properties and are available commercially as resin, prepreg, stock shapes, thin sheets/films, laminates, and machined parts. Along with the high temperature properties, this thermoset polymer type must be processed at very high temperatures and relative pressure to produce optimum characteristics. With prepreg materials, to temperatures and pressures are required. The entire cure profiles are inherently long as there are a number of intermediate temperatures dwells, duration of which are dependent on part size and thickness.
The cut of polyimides is , highest of all thermosets, with short term exposure capabilities of . Normal operating temperatures range from cryogenic to .
Polyimide composites have the following properties: Good mechanical properties and retention at high temperatures Good electrical properties High wear resistance Low creep at high temperatures Good compression with glass or graphite fiber reinforcement Good chemical resistance Inherently flame resistant Unaffected by most solvents and oils
Polyimide film possesses a unique combination of properties that make it ideal for a variety of applications in many different industries, especially as physical, electrical, and mechanical properties are maintained over a wide temperature range.