Inhibitors for Unsaturated Polyester Resins

Inhibitors are chemical compounds that prevent the polymerization process of monomers or other reactive compounds. For unsaturated polyester resins, suitable inhibitor compounds are Quinones and Phenolic compounds that can release weak hydrogen atoms. These hydrogen atoms interact with free radicals released from peroxide decomposition or with a growing polymer chain, preventing further reaction formation. This reaction, which we can call an inhibition reaction, ends after all inhibitor molecules are consumed. For this reason, inhibitors extend pot life as well as storage life in pre-accelerated resins. Selection of inhibitors is made according to the following performance characteristics: • Prevention of gelation during the esterification reaction of unsaturated polyester resin, • Prevention of gelation during the mixture of polyester with vinyl monomer at high temperature, • Providing long shelf life to the resin, • Adjustment of gel time, • Minimum effect on the cure stage following resin gelation. Many commercial polyester resins contain a mixture of two or more inhibitors to achieve optimum stability without interfering with the final cross-linking polymerization process. Regarding inhibitors in the Univar Solutions product portfolio:

4-Tert. Butyl Catechol

An inhibitor that extends pot life and gel time of activated or pre-accelerated unsaturated polyester resins at ambient temperature. The commercial code of the product in technical purity solid form is PERGASLOW BK 100. Its performance in different systems at ambient temperature is as follows: 1) Extends gel time in unsaturated polyester/peroxide/accelerator mixture, 2) Extends pot life in unsaturated polyester/peroxide mixture, 3) Extends pot life in unsaturated polyester/amine accelerator mixture. The product is also available in 10% and 1% solutions under the codes PERGASLOW BK-10 and PERGASLOW BK-1.

2,6-Di-Tert. Butyl-p-Cresol

Extends the gel time of unsaturated polyester resin at ambient temperature. Does not affect cure time. The commercial code of the product in technical purity solid form is PERGASLOW PK 100. Its performance in different systems at ambient temperature is as follows: 1) Extends shelf life of SMC-BMC at ambient temperature, 2) Extends pot life of unsaturated polyester/peroxide mixture at ambient temperature. The product is also available in 40% solution under the code PERGASLOW PK-40.

p-Benzoquinone (PBQ)

It is used for lengthening the pot life of SMC and BMC at ambient temperature.

Hydroquinone (HQ)

A widely used inhibitor. Very effective in protecting resin at ambient temperature and extending gel time. It also increases ambient temperature storage stability and pot life of SMC-BMC.

Tolylhydroquinone (THQ)

Similar in properties to Hydroquinone; however, the presence of a methyl group at the ortho position in the tolylhydroquinone molecule provides some protective effect. It also has better solubility in styrene than HQ. Generally used as a cooking stabilizer for high-reactivity unsaturated polyester. Its effect on cure is minimal.

1,4-Naphthoquinone (NQ)

A storage stabilizer in unsaturated polyester resin.

Mono-Tertiary Butylhydroquinone (MTBHQ):

An effective storage inhibitor for unsaturated polyesters and also useful as a cooking stabilizer for high-reactivity unsaturated polyesters. Provides excellent performance over a wide temperature range and causes only moderate extension in high-temperature resin cure. It has the best solubility among hydroquinone derivatives. Besides its solubility, its good color makes it suitable for use in color-sensitive applications. It is also highly useful for resin systems requiring good thermal stability during synthesis, some of which contain high levels of maleic anhydride.

2,5-Ditertiarybutyl Hydroquinone (DTBHQ)

Increases storage stability of unsaturated polyester resins while having minimal effect on gel time. Used in combination with other inhibitors.

Hydroquinone Monomethyl Ether (HQMME)

A radical polymerization inhibitor for monomers such as acrylates, methacrylates, styrene, etc. A good stabilizer and antioxidant during synthesis. Excellent solubility in organic solvents. For inquiries regarding inhibitors, you can contact Univar Solutions. Nihan Yaman - Composite Sales Manager - Turkey - Univar Solutions

Sources / References: • https://www.pergan.com/files/downloads/Curing_unsaturated_ polyester_vinylester_and_acrylate_resins.pdf • https://www.pergan.com/files/downloads/Curing_UP_EN.pdf, Pergan product brochure Rev. UP/6'e/09/2016 • https://www.eastman.com/Literature_Center/N/N315.pdf