Unsaturated polyester resin Inhibitor

Uses. About 35% of the isophthahc acid is used to prepare unsaturated polyester resins. These are condensation products of isophthahc acid, an unsaturated dibasic acid, most likely maleic anhydride, and a glycol such as propylene glycol. The polymer is dissolved in an inhibited vinyl monomer, usually styrene with a quinone inhibitor. When this viscous hquid is treated with a catalyst, heat or free-radical initiation causes cross-linking and sohdification. A range of properties is possible depending on the reactants used and their ratios (97). 

Diethylene and triethylene glycol (DEG and TEG) are produced as byproducts of ethylene glycol. DEG and TEG are used in polyurethane and unsaturated polyester resins and in the drying of natural gas. DEG is also used in antifreeze and in the synthesis of morpholine, a solvent, corrosion inhibitor, antioxidant, and pharmaceutical intermediate. 

Unsaturated polyester resins are mainly made by condensing a dibasic acid (1,2-propanediol) with an anhydride (maleic or phthalic anhydrides), by forming ester linkages between the dibasic acid (or their anhydrides) and glycols. Then a reactive monomer (mostly styrene or vinyl toluene, MMA or diallyl phthalate) is used to crosslink the system when needed. Unsaturated denotes the uncompleted chemical activity (double bond) in the original structure, which are used for crosslinking afterwards. In this context, an excess of styrene as the crosslinker (10 to 50 %) is usually added to have it ready in the system, as well as to reduce the viscosity. There are also certain accelerators used (such as, cobalt naphthenate or tertiary amines like dimethyl aniline) to facilitate the cure at ambient temperatures. In addition, there may be pigments, fillers, various inhibitors, accelerators, stabilisers and flame retardants, added to the system. Polymerisation is activated whenever a catalyst (i.e., benzoyl or methyl-ethyl-ketone peroxide) is added. 

Formulation of SMC/BMC compounds is a very sophisticated balance of many ingredients to enhance specific properties and/or act synergistically with other components. Most SMC/BMC formulations have three main elements binder, filler and fiber reinforcement, from a choice of ingredients such as unsaturated polyester resin, monomer, catalyst, inhibitor, fillers, TP anti-shrinkage additives, flame-retardant, thickener, release agent, and glass fiber reinforcement. 

For the free radical copolymerisation of unsaturated polyester resins containing an inhibitor, the following simplified mechanism could be used. 

Hydroquinone -kwi- non [ISV] (ca. 1872) (p-dihydroxybenzene, hydroquinol, p-hydroxyphenol, qui-nol) n. C6H4(OH)2. A white crystalline material derived from aniline, used, as are many of its derivatives, as an inhibitor of free-radical polymerization in unsaturated polyester resins and in monomers such as vinyl acetate. 

Vinylester (VE) resins are prepared by an addition reaction between epoxy resin (difunctional or multifunctional) with an unsaturated carboxylic acid such as acrylic acid or methacrylic acid [77]. The simplest form of VE is the product of the reaction between one mole of diglycidyl ether of BPA and 2 moles of methacylic acid (Figure 2.21). The reaction is carried out at about 100 °C using a catalyst such as triphenyl phosphine. To stop the polymerisation of methacylic acid, an inhibitor such as hydroquinone is used. By changing the nature of base epoxy resin, various types of VE resins can be produced. The vinyl-ended prepolymer is then dissolved in styrene to produce a polymerisable resin similar to an unsaturated polyester resin. Styrene content is adjusted to provide a wide range of viscosities, typically 0.1-4 Pa-s. 

Unsaturated Polyester Resins. Unsaturated polyester resins based on DCPD are produced by reacting glycols, maleic anhydride, and DCPD (12). There are at least four different methods (12,53) in making the DCPD-based polyesters. They are usually two-step batch processes. In one commonly used method, the glycol and maleic anhydride are first allowed to react at a temperature below 150°C, typically aroimd 130°C. Then DCPD is added to the mixture and the reactor temperature is gradually raised to 200°C. After an acid number between 30 and 40 is obtained, the polyester is allowed to cool and is diluted with a mixture of styrene and inhibitors. 

Accelerators are also used to reduce the processing time, while inhibitors are used to decelerate the cure cycle. Inhibitors become useful in lengthening the gel time of resins (usually in conjunction with unsaturated polyester resins), to accommodate slow cure under high-tempeiature circumstances. For example, pentanedione, which is used to retard resin gel-time, is customarily used when large pipes are manufactured. 

Known also as Diphenyl Ketone. p-Benzoquinone -kwi- non (1,4-benzoquinone, chinone) A yellow crystalline compound used, along with many of its derivatives, as an inhibitor in unsaturated polyester resins to prevent premature gelation during storage. 

Unsaturated polyesters are based on macromolecules with a polyester backbone in which an unsaturated acid or combination of a saturated with an unsaturated acid are condensed with a glycol. A three-dimensional structure is produced when the macromolecule is crosslinked through the unsaturation. Commercial unsaturated polyester resin formulations, neglecting consideration of additives, initiators, extenders, and fibrous reinforcing materials, consist essentially of a linear resin, a crosslinking (reactive diluent) monomer (ca. 18-40 wt. %), and inhibitors to retard crosslinking until the resin is used by the fabricator. The simplest member of the polyester series, ethylene maleate (or ethylene fumarate), is prepared as follows ... 

Polyester Resins. Reinforced polyester resins are thermosets based on unsaturated polyesters from glycols and dibasic acids, either or both of which contain reactive double bonds. The ratio of saturated to unsaturated components controls the degree of cross-linking and thus the rigidity of the product (see Polyesters, unsaturated). Typically, the glycols and acids are esterified until a viscous Hquid results, to which an inhibitor is added to prevent premature gelation. Addition of the monomer, usually styrene, reduces the viscosity to an easily workable level. 

Because of all these minor components (e.g., catalysts and inhibitors, added to major ones) the cure of vinyl ester resins is very complex, involving many competitive reactions. There are some new variables to account for, such as the inhibitor and initiator concentrations and induction time. Several papers [81,96,200,201] use the mechanistic approach, claiming that the phenomenological models do not explicitly include these facts, resulting in a new parameter characterization after each change in resin formulation [96]. Despite these arguments, the phenomenological approach is the most widely used and is based on an autocatalytic model which has been successfully applied to epoxy resins. Many authors [30,34,74,199,202,203] proposed the Equation 2.30 to describe the cure kinetic of unsaturated polyesters ... 

Among the emerging pollutants of industrial origin, Bisphenol A [2,2 bis(4-hydroxydiphenyl)pro-pane] (BPA) has special relevance since it was one of the first chemicals discovered to mimic estrogens as endocrine disrupters.147 This compound was first reported by Dianin in 1891.1411 BPA is produced in large quantities worldwide, mainly for the preparation of polycarbonates, epoxy resins, and unsaturated polyester-styrene resins.149 The final products are used in many ways, such as coatings on cans, powder paints, additives in thermal paper, in dental composite fillings, and even as antioxidants in plasticizers or polymerization inhibitors in polyvinyl chloride (PVC). To a minor extent, BPA is also used as precursor for flame retardants such as tetrabromobisphenol A or tetrabromobisphenol-S-bis(2,3-dibromopropyl) ether.150 This substance can enter the environment... 

Polyester resin coatings are synthesized with components that introduce unsaturation into the polymer chain (—C=C—). The paint is manufactured by mixing a dissolved polyester resin in styrene monomer with pigment and reaction inhibitor. Additional styrene and peroxide are packaged in a separate container and are mixed with the paint when applied using a dual-headed spray gun. Peroxide serves as a radical polymerization initiator for the polyester resin with monomeric styrene and cross-linking. Figure 13.5 shows the chemical structure of an isophthahc polyester resin. 

The second synthetic thermoset resin discovered in early 1940 (after phenolic resin) was unsaturated polyester (UPE) resin. UPE consists of an unsaturated polyester, a monomer, and an inhibitor. UPE gained wide industrial applications due to their low viscosity, which offers easy processability, low cost and rapid cure schedules. 

As discussed in earlier sections, UPE resins are a mixture of an unsaturated polyester, styrene and an inhibitor. When the resin is mixed with a peroxide initiator and activator (cobalt octoate/napthenate), free radicals are formed. At the initial stage, all or most of the free radicals generated are consumed by the inhibitor. The driving force for the preferable reaction of free radicals with the inhibitor is the higher stability of inhibitor radicals. Once the inhibitor molecules are depleted, free radicals, produced from the initiator, initiate polymerisation of the polyesters. Styrene serves as an agent to link the adjacent polyester molecules. The curing of UPE resin (polyester + styrene) involves different types of reactions ... 

Unsaturated, thermosetting polyesters are produced by addition polymerization reactions. Polyester resins can be formulated to have a range of physical properties from brittle and hard to tough and resistant to soft and flexible. Viscosities at room temperature may range from 50 to more than 25,000 cP. The polyester resin formulation usually consists of resin, monomer, inhibitors, catalyst or promoters, and appropriate fillers, additives, etc. 

In the coating industry, polyesters are characterized by resins based on components that introduce unsaturation (—C=C—) directly into the polymer backbone. The most common polyester resins are polymerization products of maleic or isophthalic anhydride or their acids. In producing paint the polyester resin is dissolved in styrene monomer, together with pigment and small amounts of inhibitor. A free radical initiator, commonly a peroxide, and additional styrene are packaged in another container. When applied, the containers are mixed. Sometimes, because of the fast initiating reaction (short pot life) they are mixed in an... 

Once unsaturated polyesters are synthesized, they are diluted in reactive diluents that are capable of polymerizing thermally. Therefore, care needs to be taken at this stage to keep the temperature as low as possible. As an additional precaution during thinning, as well as to have better stability on storage, a polymerization inhibitor such as p-tert-butylcatechol or hydroquinone are necessarily added to the monomers in the thinning tank before resin dissolution. 

Uses Cross-linker for polyesters used in surface coatings, flexible unsaturated polyesters, ink resins, elastomers, lubricants, greases, plasticizers, epoxy curing agents, corrosion inhibitors, soaps... 

The free radicals first react with the chemical inhibitor which has previously been added to the resin, since the inhibitor material must be chemically dissipated before any reaction between free radicals and the C=C double bonds can proceed [5]. Apparently, the free radicals serve to open the double bonds in the polyester linear chain to set in motion that portion of the polymerization process designated as initiation. Either the opened double bonds react with the vinyl groups of the monomer, or the free radicals serve to also open (add to) these latter unsaturated C=C bonds, permitting them to perform their cross-linking function, uniting the polyester chains into a three-dimensional network. There is further evidence that free radicals may also, to some degree, react with the unsaturated monomer to form various products of decomposition [5]. 

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