Industrial formaldehyde is mainly used in plastic manufacturing through condensation reactions with monomers such as phenol, urea, and melamine to synthesize resin substrates (such as phenolic resin, amino resin, and polyformaldehyde, etc.). The types of by-products are closely related to the reaction system and process conditions (temperature, pH value, raw material ratio), and mainly include volatile organic compounds, small molecule inorganic substances, toxic gases, and reaction residues. Some of these by-products will be released or retained during process steps (such as curing, pyrolysis, and exhaust gas emission).
1. By-products directly generated during resin synthesis reactions 
These by-products result from the condensation and polymerization reactions of formaldehyde and monomers. They are the core by-products generated during the process along with the target resin, and the differences in by-products among different resin systems are significant. 
Synthetic by-products of phenolic resin (PF) 
Phenolic resin is formed by the condensation polymerization of formaldehyde and phenol under the catalysis of acids or bases. During the reaction process, due to the dehydration of the hydroxymethyl intermediate and the cross-linking reaction, two main by-products are produced: one is water, which is the most significant by-product. The hydroxymethyl phenol molecules connect and grow the molecular chain through dehydration, and each cross-linking bond releases one molecule of water. The amount of water is positively correlated with the reaction degree and will affect the viscosity of the resin and the subsequent molding process. The other is free phenol and free formaldehyde. Due to the raw material ratio (such as in thermoplastic phenolic resins, phenol is in excess) and incomplete reaction, some unreacted phenol and formaldehyde will remain in the resin. Free phenol is toxic and irritating, while free formaldehyde will slowly release during the product's usage process. In addition, under an acidic catalytic system, a small amount of bisphenol methane isomers may be accompanied, and these small aromatic compounds will affect the curing rate and mechanical strength of the resin. 
2. By-products from the synthesis of amino resins (urea-formaldehyde, melamine-formaldehyde resins) 
The by-products of amino resins are mainly small molecule compounds, and the nitrogen-containing by-products require particular attention: 
Urea-formaldehyde resin (UF): During the condensation reaction of formaldehyde and urea, in addition to a large amount of water, there will also be free formaldehyde, free urea, and a small amount of formic acid - formaldehyde is prone to undergo slight oxidation during the reaction to form formic acid, causing the pH value of the reaction system to decrease. Extra alkaline solution needs to be added to adjust to maintain the stability of the reaction. The residual free urea is prone to absorb moisture and deliquesce, affecting the water resistance of plastic products. 
Trisodium cyanurate formaldehyde resin (MF): The main by-products are water and free formaldehyde. Due to the high nitrogen content in trisodium cyanurate, during high-temperature curing or subsequent thermal decomposition processes, it will decompose and produce hydrogen cyanide (HCN), a highly toxic gas. Especially in scenarios of insufficient air supply for combustion or thermal degradation, the release of hydrogen cyanide increases significantly, posing extremely great harm to both human beings and the environment. At the same time, a small amount of ammonia may be present, generated by the hydrolysis of the amino group of trisodium cyanurate, and has a pungent odor. 
3. By-products of Polyoxymethylene (POM) Synthesis 
Polycarbonate is produced by the condensation of formaldehyde to form trioxymethylene as the intermediate. During the ring-opening polymerization process, there are relatively few by-products, mainly consisting of unreacted trioxymethylene monomers and a small amount of low-molecular-weight polyoxymethylene oligomers. These by-products have relatively low boiling points and can be separated and recovered through distillation and extraction processes. If they remain in the finished product, they will reduce the heat resistance and dimensional stability of polycarbonate. In addition, during the process of preparing trioxymethylene from formaldehyde, there may be a small amount of impurities such as methanol and methyl formate, which enter the polymerization process along with the intermediate. 
4. By-products resulting from the synthesis of plastic monomers through the condensation of formaldehyde and hydroxyl compounds 
In the process of preparing methyl methacrylate (MMA, a key monomer for plastics) using formaldehyde as the raw material, the by-products of the aldol condensation reaction are complex, including methyl acrylate, methyl propionate, acetone, acrolein, methyl formate, and other esters, ketones, and aldehydes. At the same time, small molecule inorganic substances such as methane and carbon dioxide are also produced. These by-products are mostly volatile organic compounds (VOCs), and some of them are irritating. They need to be removed through separation and purification processes to ensure the purity of the monomer.