{{Short description|Synthetic materials with properties similar to natural resins}} [[File:MethmethacrylateBPA-glyc.png|thumb|[[Vinyl ester resin]] is an example of a synthetic resin.<ref name="UllmannEpoxRes2">{{Ullmann|title=Epoxy Resins|last1=Pham|first1=Ha Q.|last2=Marks|first2=Maurice J.|year=2012|doi=10.1002/14356007.a09_547.pub2}}</ref>]] '''Synthetic resin''' is an industrially produced, typically [[Viscosity|viscous]] substance that converts into rigid [[Polymer|polymers]] by the process of [[Curing (chemistry)|curing]]. They are formed by the reaction of [[Dibasic acid|dibasic]] [[organic acids]] and [[Polyhydric alcohol|polyhydric alcohols]]. In order to undergo curing, resins typically contain reactive groups,<ref>{{cite book |title=IUPAC Compendium of Chemical Terminology |date=2014 |website=iupac.org |publisher=International Union of Pure and Applied Chemistry |doi=10.1351/goldbook.RT07166}}</ref> such as [[Acrylate|acrylates]] or [[Epoxide|epoxides]]. Some synthetic resins have properties similar to natural plant [[Resin|resins]], but many do not.<ref name="UllmannResSyn2">{{Ullmann|title=Resins, Synthetic|last1=Collin|first1=Gerd|display-authors=etal|year=2005|doi=10.1002/14356007.a23_089|isbn=3527306730}}</ref>

Synthetic resins are of several classes. Some are manufactured by [[esterification]] of [[Organic compound|organic compounds]]. Some are [[Thermosetting plastic|thermosetting plastics]] in which the term "resin" is loosely applied to the reactant(s), the product, or both. "Resin" may be applied to one or more monomers in a [[copolymer]], the other being called a "hardener", as in [[Epoxy resin|epoxy resins]]. For thermosetting plastics that require only one monomer, the monomer compound is the "resin". For example, liquid [[methyl methacrylate]] is often called the "resin" or "casting resin" while in the liquid state, before it polymerizes and "sets". After setting, the resulting [[poly(methyl methacrylate)]] (PMMA) is often renamed "acrylic glass" or "acrylic". (Trade names include Plexiglas and Lucite).

== Uses == Synthetic resins are also used extensively in [[cured-in-place pipe]] applications. Departments of Transportation in the USA also specify them for use as overlays on roads and bridges. In this application they are known as Polyester Concrete Overlays (PCO). These are usually based on [[isophthalic acid]] and cut with styrene at high levels{{emdash}}usually up to 50%. Synthetic resins are also used in [[anchor bolt]] adhesives though [[epoxy]] based materials are also used.<ref>{{Cite web |title=2K Polymer Systems Ltd: Bonded Anchors: P - Polyester |url=http://www.2kps.net/p-polyester.html |access-date=2018-04-05 |website=www.2kps.net |language=en-GB}}</ref> Many companies have and continue to introduce [[styrene]] free systems mainly due to odor issues, but also over concerns that styrene is a potential carcinogen. [[Drinking water]] applications also prefer styrene-free.

[[Dental material|Dental restorative materials]] based on [[bis-GMA]]-containing resins<ref name="UllmannDental2">{{Ullmann|title=Dental Materials|author1=Robert G. Craig|author2=Dieter Welker|author3=Josef Rothaut|author4=Klaus Georg Krumbholz|author5=Klaus-Peter Stefan|author6=Klaus Dermann|author7=Hans-Joachim Rehberg|author8=Gertraute Franz|author9=Klaus Martin Lehmann|author10=Matthias Borchert|year=2006|doi=10.1002/14356007.a08_251.pub2}}</ref> can break down into or be contaminated with the related compound [[bisphenol A]], a potential [[endocrine disruptor]]. However, no negative health effects of bis-GMA use in dental resins have been found.<ref name="Soderholm19992">{{cite journal |vauthors=Soderholm KJ, Mariotti A |date=February 1999 |title=Bis-GMA–based resins in dentistry: are they safe? |journal=The Journal of the American Dental Association |volume=130 |issue=2 |pages=201–209 |doi=10.14219/jada.archive.1999.0169 |pmid=10036843}}</ref><ref>{{cite journal |last1=Ahovuo-Saloranta |first1=Anneli |last2=Forss |first2=Helena |last3=Walsh |first3=Tanya |last4=Nordblad |first4=Anne |last5=Mäkelä |first5=Marjukka |last6=Worthington |first6=Helen V. |date=31 July 2017 |title=Pit and fissure sealants for preventing dental decay in permanent teeth |journal=The Cochrane Database of Systematic Reviews |volume=2017 |issue=7 |doi=10.1002/14651858.CD001830.pub5 |issn=1469-493X |pmc=6483295 |pmid=28759120 |article-number=CD001830}}</ref>

==Types== {{See also|Alkyd|Polyurethane|Epoxy resin}}

There are a number of types. One variety is [[epoxy resin]], manufactured through [[polymerization]]-polyaddition or polycondensation reactions, used as a [[thermoset]] polymer for adhesives and [[Composite material|composites]].<ref>{{cite book |author=Gurit Services AG |url=https://www.gurit.com/-/media/Gurit/Datasheets/guide-to-composites.pdf |title=Guide to Composites |website=gurit.com |page=19 |chapter=5.1.3 Epoxy Resins |access-date=2022-10-21}}</ref><ref>Fekiač, Jozef Jaroslav; Krbata, Michal; Kohutiar, Marcel; Janík, Róbert (2025-01-22). [https://archive.today/20251006071248/https://www.mdpi.com/2073-4360/17/3/271 "Comprehensive Review: Optimization of Epoxy Composites, Mechanical Properties, and Technological Trends"]. ''[[Polymers (journal)|Polymers]]''. '''17''' (3). [https://ultracoatingepoxyflooring.ca/ Ultra Coating Epoxy Flooring]: 271. {{ISSN|2073-4360}}. Archived from [https://www.mdpi.com/2073-4360/17/3/271 the original] on 2025-02-14.</ref> Accordingly, it has been mainly in use for industrial flooring purposes since the 1960s. Since 2000, however, epoxy and [[polyurethane]] resins are used in interiors as well, mainly in Western Europe. [[Polyurethane|Polyurethanes]] are another type of synthetic resin.<ref>{{cite journal |last1=Howarth |first1=GA |year=2003 |title=Polyurethanes, polyurethane dispersions and polyureas: Past, present and future |journal=Surface Coatings International Part B: Coatings Transactions |volume=86 |issue=2 |pages=111–118 |doi=10.1007/BF02699621}}</ref>

Synthetic casting "resin" for embedding display objects in Plexiglas/Lucite ([[Poly(methyl methacrylate)|PMMA]]) is simply [[methyl methacrylate]] liquid, into which a polymerization catalyst is added and mixed, causing it to "set" (polymerize). The polymerization creates a block of PMMA plastic ("acrylic glass") which holds the display object inside a transparent block.

Another synthetic polymer, sometimes called by the same general category, is [[acetal resin]]. By contrast with the other synthetics, however, it has a simple chain structure with the [[repeat unit]] of form −[CH<sub>2</sub>O]−. [[Ion-exchange resin|Ion-exchange resins]] are used in [[water purification]] and [[catalysis]] of [[Organic reaction|organic reactions]]. (See also ''[[AT-10 resin]]'', ''[[melamine resin]]''.) Certain ion-exchange resins are also used pharmaceutically as [[Bile acid sequestrant|bile acid sequestrants]], mainly as [[Hypolipidemic agent|hypolipidemic agents]], although they may be used for purposes other than lowering [[cholesterol]]. [[Solvent impregnated resin|Solvent impregnated resins]] (SIRs) are porous resin particles which contain an additional liquid extractant inside the porous matrix. The contained extractant is supposed to enhance the capacity of the resin particles. The production of [[PVC]] entails the production of "vinyl chloride resins", which differ in the degree of polymerization.<ref name="UllmannPVC2">{{Ullmann|title=Poly(Vinyl Chloride|year=2012|last1=Allsopp|first1=M. W.|last2=Vianello|first2=G.|doi=10.1002/14356007.a21_717}}</ref>

[[Silicone resin|Silicone resins]] are [[silicone]]-based polymers that exhibit various useful properties like weatherability (durability), [[Dielectric|dielectricity]], water repellency, thermal stability, and [[Chemically inert|chemical inertness]].<ref>{{Citation |title=Silicone resins |url=https://www.shinetsusilicone-global.com/products/type/resin/index.shtml |publisher=shinetsusilicone}}</ref>

== Composition == Most synthetic resins are viscous, pale coloured liquids consisting of a solution of a polyester in a reactive diluent which is usually styrene,<ref>{{cite web |title=Polyester Resins |url=https://netcomposites.com/guide/resin-systems/polyester-resins/ |url-status=usurped |archive-url=https://web.archive.org/web/20190819083607/https://netcomposites.com/guide/resin-systems/polyester-resins/ |archive-date=August 19, 2019 |access-date=2019-08-19}}</ref> but can also include [[4-Vinyltoluene|vinyl toluene]] and various [[Acrylate|acrylates]].<ref>{{cite journal |author=Joanna Klein Nagelvoort |date=2009 |title=Resin Composition Suitable for (Re)Lining of Tubes, Tanks, and Vessels |volume=EP 2097369 B1}}</ref><ref>{{Cite patent|number=US20210380744A1|title=Non-leaching styrene-free cured-in-place pipe system suitable for potable water applications|gdate=2021-12-09|invent1=Miller|invent2=Moore|invent3=Kinnin|invent4=Waltman|inventor1-first=Gregory C.|inventor2-first=William|inventor3-first=Lucian A.|inventor4-first=Phillip|url=https://patents.google.com/patent/US20210380744A1/en?oq=US+2021380744}}</ref>

Unsaturated polyesters are [[Condensation polymer|condensation polymers]] formed from [[Polyol|polyols]] (also known as [[polyhydric alcohols]]), which have multiple alcohol or hydroxy functional groups. These are treated with unsaturated and in some cases saturated dibasic acids. Typical polyols used are [[glycols]] including [[ethylene glycol]], [[propylene glycol]], and [[diethylene glycol]]; typical acids are [[phthalic acid]], [[isophthalic acid]], [[terephthalic acid]], and [[maleic anhydride]]. Water, a condensation by-product of [[Esterification#Preparation|esterification]] reactions, is continuously removed by distillation, driving the esterification. Unsaturated polyesters are generally sold to parts manufacturers as a solution of resin in reactive diluent. [[Styrene]] is the most common diluent and the industry standard. The diluent allows control over the viscosity of the resin and a participant in the curing reaction. The initially liquid resin is converted to a solid by [[Cross-link|cross-linking]]. Crosslinking is induced by [[Free radical initiator|free radical initiators]]. Unsaturation is generally in the form of maleate and fumarate species along the polymer chain. Maleate/fumarate generally does not self-polymerize via radical reactions, but readily reacts with styrene. Maleic anhydride and styrene are known to form [[Copolymer#Reactivity ratios|alternating copolymers]] and are in fact the textbook case of this phenomenon. This compatibility is one reason that styrene has been so difficult to displace in the market as the industry standard reactive diluent, despite increasing efforts to displace the material such as [[1986 California Proposition 65|California's Proposition 65]]. The initial free radicals are induced by adding a compound that easily decomposes into free radicals. This compound is known as the [[catalyst]]<ref>{{cite book |author1=Erik Lokensgard |title=Industrial Plastics: Theory and Applications |date=19 January 2016 |publisher=Cengage Learning |isbn=978-1305855687}}</ref> within the industry, but [[Radical initiator|initiator]] is a more appropriate term. Transition metal salts are usually added as a catalyst for the chain-growth crosslinking reaction, and in the industry this type of additive is known as a promoter; the promoter is generally understood to lower the [[Bond-dissociation energy|bond dissociation energy]] of the radical initiator. Cobalt salts are the most common type of promoter used. Common radical initiators used are organic peroxides such as [[benzoyl peroxide]] or [[methyl ethyl ketone peroxide]].<ref>{{Citation |last=Weatherhead |first=R. G. |title=Catalysts, Accelerators and Inhibitors for Unsaturated Polyester Resins |date=1980 |work=FRP Technology: Fibre Reinforced Resin Systems |pages=204–239 |editor-last=Weatherhead |editor-first=R. G. |url=https://doi.org/10.1007/978-94-009-8721-0_10 |access-date=2021-05-15 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-009-8721-0_10 |isbn=978-94-009-8721-0 |url-access=subscription}}</ref>

Synthetic resins are [[Thermoset|thermosetting]] and, as with other resins, [[Curing (chemistry)|cure]] exothermically. The use of excessive initiator especially with a catalyst present can, therefore, cause charring or even ignition during the curing process. Excess catalyst may also cause the product to fracture or form a rubbery material.

Unsaturated polyesters (UPR) are utilized in many different industrially relevant markets, but in general are used as the matrix material for various types of [[Composite material|composites]]. Glass fiber-reinforced composites comprise the largest segment into which UPRs are used and can be processed via [[Sheet moulding compound|SMC]], [[Bulk moulding compound|BMC]], [[pultrusion]], [[cured-in-place pipe]] (known as relining in Europe), [[filament winding]], [[Vacuum bag moulding|vacuum molding]], [[Fiberglass spray lay-up process|spray-up molding]], [[Out of autoclave composite manufacturing#Resin transfer molding - RTM|resin transfer molding (RTM)]]. Wind turbine blades also use them<ref>{{Cite journal |last1=Brøndsted |first1=Povl |last2=Lilholt |first2=Hans |last3=Lystrup |first3=Aage |date=2005-08-04 |title=Composite Materials for Wind Power Turbine Blades |url=https://www.annualreviews.org/doi/full/10.1146/annurev.matsci.35.100303.110641 |journal=Annual Review of Materials Research |volume=35 |issue=1 |pages=505–538 |bibcode=2005AnRMS..35..505B |doi=10.1146/annurev.matsci.35.100303.110641 |issn=1531-7331 |s2cid=15095678 |url-access=subscription}}</ref> as well as many more processes. UPRs are also used in non-reinforced applications with common examples being [[Gelcoat|gel coats]], shirt buttons, [[Underground mining (hard rock)#Ground support|mine-bolts]], [[:File:20181226 Concepts of bowling ball cores.png| bowling ball cores]], [[polymer concrete]], and [[Engineered stone| engineered stone/cultured marble]].<ref>{{Cite web |title=Trusted Solutions &#124; AOC |url=https://aocresins.com/en-amr/home/}}</ref>

== Chemistry == [[File:DMAA_isomerization_mechanism.png|thumb|Mechanism for the DMAA catalyzed isomerization of maleate to fumarate]] [[File:DCPD_Resin_Example.gif|thumb|Example of a DCPD resin]] [[File:Nadic.png|thumb|Example of a Nadic]] In organic chemistry, an [[ester]] is formed as the condensation product of a [[carboxylic acid]] and an [[Alcohol (chemistry)|alcohol]], with water formed as the condensate by-product. An ester can also be produced with an [[acyl halide]] and an alcohol, in which case the condensate by-product is a [[hydrogen halide]].

[[Polyester|Polyesters]] are a category of polymers in which ester functionality repeats within the main chain. Polyesters are a classic example of [[Step-growth polymerization|step-growth polymer]], in which a difunctional (or higher order) acid or acyl halide is reacted with a difunctional (or higher order) alcohol. Polyesters are produced commercially both as saturated and unsaturated resins. The most common and highest volume produced polyester is [[Polyethylene terephthalate| Polyethylene terephthalate (PET)]], which is an example of a saturated polyester and finds utilization in such applications as fibers for clothing and carpet, food and liquid containers (such as a water/soda bottles), as well as films.<ref>{{cite book |author1=Fred W. Billmeyer Jr. |title=Textbook of Polymer Science |date=1962}}</ref>

In unsaturated polyester (UPR) chemistry, unsaturation sites are present along the chain, usually by incorporation of maleic anhydride, but maleic acid and fumaric acid are also used. Maleic acid and fumaric acid are isomers where maleic is the cis-isomer and fumaric is the trans-isomer. The ester forms of these two molecules are maleate and fumarate, respectively. When curing a UPR, the fumarate form is known to react more rapidly with the styrene radical, so [[isomerization]] catalysts, such as N,N-dimethylacetoacetamide (DMAA), are often employed in the synthesis process which converts the maleates into fumarates; the isomerization can also be encouraged with increased reaction time and temperature. Within the UPR industry, the classification of the resins is generally based on the primary saturated acid. For example, a resin containing primarily terephthalic acid is known as a Tere resin, a resin containing primarily phthalic anhydride is known as an Ortho resin, and a resin containing primarily isophthalic acid is known as an Iso resin. [[Dicyclopentadiene| Dicyclopentadiene (DCPD)]] is also a common UPR raw material, and can be incorporated two different ways. In one process, the DCPD is cracked in situ to form cyclopentadiene which can then be reacted with maleate/fumarate groups along the polymer chain via a [[Diels–Alder reaction|Diels-alder reaction]]. This type of resin is known as a Nadic resin and is referred to as a poor man's Ortho, due to sharing many similar properties of an Ortho resin along with the extremely low cost of DCPD raw material. In another process, maleic anhydride is first opened with water or another alcohol to form maleic acid and is then reacted with DCPD where an alcohol from the maleic acid reacts across one of the double bonds of the DCPD. This product is then used to end-cap the UPR resin which yields a product with unsaturation on the end-groups. This type of resin is referred to as a DCPD resin.

Ortho resins comprise the most common type of UPR, and many are known as general purpose resins. FRP composites utilizing ortho resins are found in such application as boat hulls, bath ware, and bowling ball cores.

Iso resins are generally on the higher end of UPR products, both because of the relatively higher cost of the isophthalic acid as well as the superior properties they possess. Iso resins are the primary type of resin used in gel coat applications, which is similar to a paint, but is sprayed into a mold before the FRP is molded leaving a coating on the part. Gel coat resins must have lower color (almost clear) so as to not impart additional color to the part or so that they can be dyed properly. Gel coats must also have strong resistance to UV-weathering and water blistering.

Tere resins are often used when high modulus and strength are desired, but the low color properties of an Iso resin is not necessary. Terephthalic acid is generally lower cost than isophthalic acid, but both give similar strength characteristics to a UPR product. There exists a special sub-set of Tere resins, known as PET UPR resins, which are produced by catalytically cracking PET resin in the reactor to yield a mixture of terephthalic acid and ethylene glycol. Additional acids and glycols are then added along with maleic anhydride and a new polymer is produced. The end product is functionally the same as a Tere resin, but can often be lower cost to manufacture as scrap PET can be sourced cheaply. If a glycol-modified PET (PET-G) is used, exceptional properties can be imparted to the resin due to some of the exotic materials used in PET-G production. Tere and PET-UPR resins are used in many applications including cured-in-place pipe.<ref>{{cite book |author1=Johan Bjorksten |title=Polyesters and Their Applications |author2=Henry Tovey |author3=Betty Harker |author4=James Henning |date=1956}}</ref>

== Advantages == Synthetic resin offers the following advantages:

# Adequate resistance to water and variety of chemicals. # Adequate resistance to weathering and ageing. # Low cost. # Polyesters can withstand a temperature up to 80&nbsp;°C. # Polyesters have good wetting to glass fibres. # Relatively low shrinkage at between 4–8% during curing. # Linear thermal expansion ranges from 100–200 × 10<sup>−6</sup> K<sup>−1</sup>.

== Disadvantages == Synthetic resin has the following disadvantages:

# Strong [[styrene]] odour # More difficult to mix than other resins, such as a two-part epoxy # The toxic nature of its fumes, and especially of its catalyst, [[MEKP]], pose a safety risk if proper protection isn't used # Not appropriate for bonding many substrates # The finished cure is most likely weaker than an equal amount of an epoxy resin

==See also==

* [[Polyester]] * [[Styrene]] * [[Thermoset polymer matrix]] * [[Thermosetting polymer]] * [[Vinyl ester]] * [[Prepolymer]] * [[Resin casting]] * [[Waterborne resins]]

==References== {{Reflist}}

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[[Category:Synthetic resins| ]] [[Category:Plastics]] [[Category:Polymers]]