Resol and novolac are two types of phenolic resins; they are widely used in industry because of their chemical resistance, electrical insulation, and dimensional stability. Phenolic resin-bonded textile felts can be considered to be fiber-reinforced plastic with high levels of fibers; these fibers are derived from textile scraps recycled from the textile industry. Due to the increase of phenol cost, researchers have been working to partially substitute this monomer by natural polymers that present similiar structure close to that of phenolic resin.
The best possible substitutes of phenol based on cost and availabilty are lignosulfonates, which are obtained from the sulphite process. The ammonium lignosulfonate is the best canidate to substitute phenol, because final properties of phenolic resins are better. Lignosulfonates are not very reactive with phenol and formaldehyde, so it is usual to modify their structure by methyloletion and phenolation. Although phenolation is more expensive.
The reaction mechnism associated with the formulation of lignin-novolac is explained in the following text. The alpha-carbon of the lateral chain of the phenyl-propane units is occupied by the sulfonate group. Lignosulfonates may be reacting by itself or with the phenol adding to the Beta-carbon of the lateral chain. The first step consists of the condensation between lignin fragments and the phenol present with formaldehyde. In the second step, vacuum distillation is employed to adjust the content of free phenol in accordance with the required specifications for textile felt applications. The addition of a curing agent (HMTA) with methylene groups is also necessary for the crosslinking. (1)
On page 3/14, of the following link is Figure 2: Mechanism of lignin-novolac resins synthesis substituted with lignosulfonates. Since this peer-reviewed journal is protected, I could not copy the reaction scheme onto my blog. My apologies.
http://www.ncsu.edu/bioresources/BioRes_02/BioRes_02_2_270_283_Perez_RAOE_Novolac_Resin_Lignosulfonate.pdf
References:
1. Characterization of a Novolac; Jaun Manuel Perez; Bioresources; http://www.ncsu.edu/bioresources/BioRes_02/BioRes_02_2_270_283_Perez_RAOE_Novolac_Resin_Lignosulfonate.pdf; Web; (2007)
Nice find Laura...another good example of chemistry at work.
ReplyDeleteGood Job Laura. This was a good article, i had trouble finding one myself. The reactions were clear and well detailed amd you did a nice job explaining them in your post.
ReplyDelete