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,[1] such as glass-reinforced plasticradar 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,[2] as well as engineering grade thermoplastics.[3] 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.[4]
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.
These are made by the reaction of phenols, formaldehyde and primary amines which at elevated temperatures (400°F (200°C)) 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 490°F (250°C).[5]
Benzoxazine resin synthetic pathway, structure and cure mechanism
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.
Formed by the condensation reaction of a diamine with maleic anhydride, and processed basically like epoxy resins (350°F (177°C) cure).[6] After an elevated post-cure (450°F (232°C)), they will exhibit superior properties. These properties are influenced by a 400-450°F (204-232°C) continuous use temperature and a glass transition of 500°F (260°C).
Bismaleimide resin synthetic pathway and structure
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.
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.[7] 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 752°F (400°C) and wet heat stability up to around 400°F (200°C).
Cyanate ester monomer, prepolymer and polycyanurate structures
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 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.[8] 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.[9] 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.[10]
Diglycidyl ether of bisphenol-A epoxy resin structure
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.
There are two types of phenolic resins[11] - 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
Unsaturated polyester resins are an extremely versatile,[12][13] 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.
Unsaturated polyester resin synthetic pathway and structure
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.
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,[14][15] 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, 600°F (316°C) to 650°F (343°C) temperatures and 200psi (1,379kPa) 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.
Thermoset polyimide prepolymer structure
The cut of polyimides is 450°F (232°C), highest of all thermosets, with short term exposure capabilities of 900°F (482°C). Normal operating temperatures range from cryogenic to 500°F (260°C).
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 excellent physical, electrical, and mechanical properties are maintained over a wide temperature range.[16][17][18][19][20]
High-performance polyimide resin is used in electrical, wear resistant and as structural materials when combined with reinforcement for aircraft-aerospace applications, which are replacing heavier more expensive metals. High temperature processing causes some technical problems as well as higher costs compared to other polymers. Hysols [21] PMR series is an example of this polymer.
Thermoset polyurethane prepolymers with carbamate (-NH-CO-O-) links are linear and elastomeric if formed by combining diisocyanates (OCN-R1-NCO) with long chain diols (HO-R2-OH), or crosslinked and rigid if formed from combinations of polyisocyanates and, polyols. They can be solid or have an open cellular structure if foamed, and are widely used for their characteristic[22] high adhesion and resistance to fatigue. Polyurethane foam structural cores combined with glass-reinforced or graphite-reinforced composite laminates are used to make lightweight, strong, sandwich structures.[23][24][25] All forms of the material, inclusive of flexible and rigid foams, foam moldings, solid elastomeric moldings and extrudates, when combined with various reinforcement–fillers have found commercial applications in thermoset polymer matrix composites.[26]
They differ from polyureas which are thermoset elastomeric polymers with carbamide (-NH-CO-NH-) links made by combining diisocyanate monomers or prepolymers (OCN-R-NCO) with blends of long-chain amine-terminated polyether or polyester resins (H2N-RL-NH2) and short-chain diamine extenders (H2N-RS-NH2). Polyureas are characterised by near instantaneous cure, high mechanical strength and resistance to corrosion so are widely used for 1:1 volume mix ratio spray applied, abrasion resistant waterproofing protective coating and lining.[27]
Silicone resins are partly organic in nature with a backbone polymer structure made of alternating silicon and oxygenatoms rather than the familiar carbon-to-carbon backbone characteristics of organic polymers. In addition to having at least one oxygen atom bonded to each silicon atom, silicone resins have direct bonds to carbon and therefore also known as polyorganosiloxanes. They have the general formula (R2SiO)n and the physical form (liquid, gel, elastomer or solid) and use varies with molecular weight, structure (linear, branched, caged) and nature of substituent groups (R = alkyl, aryl, H, OH, alkoxy). Aryl substituted silicone resins have greater thermal stability than alkyl substituted silicone resins when polymerised (condensation cure mechanism) at temperatures between ~300°F (~150°C) and ~400°F (~200°C). Heating above ~600°F (~ 300°C) converts all silicone polymers into ceramics[28] since all organic constituents pyrolytically decompose leaving crystalline silicate polymers with the general formula (-SiO2-)n. In addition to applications as ceramic matrix composite precursors, silicone resins in the form of polysiloxane polymers made from silicone resins with pendant acrylate, vinyl ether or epoxy functionality find application as UV, electron beam and thermoset polymer matrix composites where they are characterised by their resistance to oxidation, heat and ultraviolet degradation.
Assorted other uses in the general area of composites for silicones include sealants, coating materials, and as a reusable bag material for vacuum-bag curing of composite parts.
Vinyl ester resins made by addition reactions between an epoxy resin with acrylic acid derivatives, when diluted/dissolved in a vinyl functional monomer such as styrene, polymerise. The resulting thermosets are notable for their high adhesion, heat resistance and corrosion resistance. They are stronger than polyesters and more resistant to impact than epoxies.[29] Vinyl ester resins are used for wet lay-up laminating, SMC and BMC in the manufacture and repair of corrosion and heat resistant components ranging from pipelines, vessels and buildings to transportation, marine, military and aerospace applications.
Vinyl ester
Miscellaneous
Amino resins are another class of thermoset prepolymers formed by copolymerisation of amines or amides with an aldehyde. Urea-formaldehyde and melamine-formaldehyde resins, although not widely used in high performance structural composite applications, are characteristically used as the polymer matrix in molding and extrusion compounds where some use of fillers and reinforcements occurs. Urea-formaldehyde resins are widely used as the matrix binder in construction utility products such as particle board, wafer board, and plywood, which are true particulate and laminar composite structures. Melamine-formaldehyde resins are used for plastic laminating.
Furan resin prepolymers made from furfuryl alcohol, or by modification of furfural with phenol, formaldehyde (methanal), urea or other extenders, are similar to amino and phenolic thermosetting resins in that cure involves polycondensation and release of water as well as heat. While they are generally cured under the influence of heat, catalysts and pressure, furan resins can also be formulated as dual-component no-bake acid-hardened systems which are characterised by high resistance to heat, acids and alkalies. Furan resins are of increasing interest for the manufacture of sustainable composites - biocomposites made from a bio-derived matrix (in this case furan resin), or biofibre reinforcement, or both.[30]
Well established processing and application history
Overall, better economics than thermoplastic polymers
Better high temperature properties
Good wetting and adhesion to reinforcement
Disadvantages
Resins and composite materials must be refrigerated
Moisture absorption and subsequent property degradation[31]
Long process cycles
Reduced impact –toughness
Poor recycling capabilities
More difficult repair ability
Related Research Articles
Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also collectively called epoxy. The IUPAC name for an epoxide group is an oxirane.
In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.
Phenol formaldehyde resins (PF) are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial synthetic resins. They have been widely used for the production of molded products including billiard balls, laboratory countertops, and as coatings and adhesives. They were at one time the primary material used for the production of circuit boards but have been largely replaced with epoxy resins and fiberglass cloth, as with fire-resistant FR-4 circuit board materials.
Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.
Pre-preg is a composite material made from "pre-impregnated" fibers and a partially cured polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure. The main idea behind a pre-preg material is the use of anisotropic mechanical properties along the fibers, while the polymer matrix provides filling properties, keeping the fibers in a single system.
Micarta is a brand name for composites of linen, canvas, paper, fiberglass, carbon fiber, or other fabric in a thermosetting plastic. It was originally used in electrical and decorative applications. Micarta was developed by George Westinghouse at least as early as 1910 using phenolic resins invented by Leo Baekeland. These resins were used to impregnate paper and cotton fabric which were cured under pressure and high temperature to produce laminates. In later years this manufacturing method included the use of fiberglass fabric, and other resin types were also used. Today Micarta high-pressure industrial laminates are produced with a wide variety of resins and fibers. The term has been used generically for most resin impregnated fiber compounds. Common uses of modern high-pressure laminates include electrical insulators, printed circuit board substrates, and knife handles.
Powder coating is a type of coating that is applied as a free-flowing, dry powder. Unlike conventional liquid paint, which is delivered via an evaporating solvent, powder coating is typically applied electrostatically and then cured under heat or with ultraviolet light. The powder may be a thermoplastic or a thermosetting polymer. It is usually used to create a thick, tough finish that is more durable than conventional paint. Powder coating is mainly used for coating of metal objects, particularly those subject to rough use. Advancements in powder coating technology like UV-curable powder coatings allow for other materials such as plastics, composites, carbon fiber, and medium-density fibreboard (MDF) to be powder coated, as little heat or oven dwell time is required to process them.
Filament winding is a fabrication technique mainly used for manufacturing open (cylinders) or closed end structures. This process involves winding filaments under tension over a rotating mandrel. The mandrel rotates around the spindle while a delivery eye on a carriage traverses horizontally in line with the axis of the rotating mandrel, laying down fibers in the desired pattern or angle to the rotational axis. The most common filaments are glass or carbon and are impregnated with resin by passing through a bath as they are wound onto the mandrel. Once the mandrel is completely covered to the desired thickness, the resin is cured. Depending on the resin system and its cure characteristics, often the mandrel is autoclaved or heated in an oven or rotated under radiant heaters until the part is cured. Once the resin has cured, the mandrel is removed or extracted, leaving the hollow final product. For some products such as gas bottles, the 'mandrel' is a permanent part of the finished product forming a liner to prevent gas leakage or as a barrier to protect the composite from the fluid to be stored.
Wood glue is an adhesive used to tightly bond pieces of wood together. Many substances have been used as glues. Traditionally animal proteins like casein from milk or collagen from animal hides and bones were boiled down to make early glues. They worked by solidifying as they dried. Later, glues were made from plant starches like flour or potato starch. When combined with water and heated, the starch gelatinizes and forms a sticky paste as it dries. Plant-based glues were common for books and paper products, though they can break down more easily over time compared to animal-based glues. Examples of modern wood glues include polyvinyl acetate (PVA) and epoxy resins. Some resins used in producing composite wood products may contain formaldehyde. As of 2021, “the wood panel industry uses almost 95% of synthetic petroleum-derived thermosetting adhesives, mainly based on urea, phenol, and melamine, among others”.
Synthetic resins are industrially produced resins, typically viscous substances that convert into rigid polymers by the process of curing. In order to undergo curing, resins typically contain reactive end groups, such as acrylates or epoxides. Some synthetic resins have properties similar to natural plant resins, but many do not.
Polybenzoxazines, also called benzoxazine resins, are cured polymerization products derived from benzoxazine monomers.
Bulk moulding compound (BMC), bulk moulding composite, or dough moulding compound (DMC), is a ready-to-mold, glass-fiber reinforced thermoset polymer material primarily used in compression moulding, as well as in injection moulding and transfer moulding. Typical applications include demanding electrical applications, corrosion resistant needs, appliance, automotive, and transit.
Sheet moulding compound (SMC) or sheet moulding composite is a ready to mould glass-fibre reinforced polyester material primarily used in compression moulding. The sheet is provided in rolls weighing up to 1000 kg. Alternatively the resin and related materials may be mixed on site when a producer wants greater control over the chemistry and filler.
Polymer engineering is generally an engineering field that designs, analyses, and modifies polymer materials. Polymer engineering covers aspects of the petrochemical industry, polymerization, structure and characterization of polymers, properties of polymers, compounding and processing of polymers and description of major polymers, structure property relations and applications.
Cyanate esters are chemical compounds in which the hydrogen atom of the cyanic acid is replaced by an organyl group. The resulting compound is termed a cyanate ester, with the formula R−O−C≡N, where R is an organyl group. Cyanate esters contain a monovalent cyanate group −O−C≡N.
Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin, which then hardens. It is primarily used for small-scale production like industrial prototypes and dentistry. It can be done by amateur hobbyists with little initial investment, and is used in the production of collectible toys, models and figures, as well as small-scale jewellery production.
RTV silicone is a type of silicone rubber that cures at room temperature. It is available as a one-component product, or mixed from two components. Manufacturers provide it in a range of hardnesses from very soft to medium—usually from 15 to 40 Shore A. RTV silicones can be cured with a catalyst consisting of either platinum or a tin compound such as dibutyltin dilaurate. Applications include low-temperature over-molding, making molds for reproducing, and lens applications for some optically clear grades. It is also used widely in the automotive industry as an adhesive and sealant, for example to create gaskets in place.
Glass-filled polymer, is a mouldable composite material. It comprises short glass fibers in a matrix of a polymer material. It is used to manufacture a wide range of structural components by injection or compression moulding. It is an ideal glass alternative that offers flexibility in the part, chemical resistance, shatter resistance and overall better durability.
Transfer molding is a manufacturing process in which casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed rather than open to the fill plunger resulting in higher dimensional tolerances and less environmental impact. Compared to injection molding, transfer molding uses higher pressures to uniformly fill the mold cavity. This allows thicker reinforcing fiber matrices to be more completely saturated by resin. Furthermore, unlike injection molding, the transfer mold casting material may start the process as a solid. This can reduce equipment costs and time dependency. The transfer process may have a slower fill rate than an equivalent injection molding process.
In materials science, a polymer matrix composite (PMC) is a composite material composed of a variety of short or continuous fibers bound together by a matrix of organic polymers. PMCs are designed to transfer loads between fibers of a matrix. Some of the advantages with PMCs include their light weight, high resistance to abrasion and corrosion, and high stiffness and strength along the direction of their reinforcements.
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↑ Handbook of Thermoplastics, ed. O. Olabisi, K Adewale, CRC Press, USA, 2nd edition, 2015, ISBN978-1-466577220
↑ Industrial Polymer Applications: Essential Chemistry and Technology, Royal Society of Chemistry, UK, 1st edition, 2016, ISBN978-1782628149
↑ Handbook of Benzoxazine Resins, ed. Hatsuo Ishida And Tarek Agag, Elsevier B.V., 2011, ISBN978-0-444-53790-4
↑ Stenzenberger, Horst (1988). "Recent advances in thermosetting polyimides". British Polymer Journal. 20 (5): 383–396. doi:10.1002/pi.4980200503. ISSN0007-1641.
↑ Kessler, Michael R. (2012). "Cyanate Ester Resins adapted from Cyanate Ester Resins". Wiley Encyclopedia of Composites. doi:10.1002/9781118097298.weoc062.
↑ Howarth, GA (2003). "Polyurethanes, polyurethane dispersions and polyureas: Past, present and future". Surface Coatings International Part B: Coatings Transactions. 86 (2): 111–118. doi:10.1007/BF02699621. S2CID93574741.
↑ Concise Encyclopedia of Polymer Science and Engineering, ed. J.I. Kroschwitz, Wiley, New York, 1990, ISBN0-471-5 1253-2
↑ F.A. Cassis and R.C. Talbot in Handbook of Composites, ed. S.T. Peters, Springer US, 1998, ISBN978-0-412-54020-2
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