Thermosetting powder coatings are composed of resins (film-forming agents), additives, pigments, and fillers. Almost every raw material undergoes chemical synthesis or processing, and is not inherently pure, containing varying amounts of small-molecule volatile components. This often results in smoke and unusual odors during the extrusion and spraying/baking processes of powder coatings, significantly impacting the health of on-site workers and posing hazards to machinery and the working environment.
 

While powder coatings are generally considered the most environmentally friendly coating, actual observation and analysis reveal that although powder coatings emphasize minimal VOC emissions, the amount of small-molecule compounds released during the baking process is not insignificant. Furthermore, powder baking equipment typically has limited waste gas recovery capabilities, and equipment manufacturers and spraying plants often neglect this aspect, leading to poorly maintained spraying workshops frequently filled with smoke, causing severe irritation to the mouth, nose, and eyes.

1. Analysis of the Causes of Smoke Generation

The smoke released during powder baking (often accompanied by an irritating odor) almost entirely originates from the raw materials. Below is a brief analysis of the synthesis, raw materials, and chemical basis of various raw materials.
 

First, there is the resin (film-forming material). Polyester (Polyeser Resin) is divided into two types: terminal carboxyl group and terminal hydroxyl group. It is the most important film-forming material for powder coatings. Commonly used polyols and polyacids are produced through esterification and polycondensation. Commonly used polyols include neodymium glycol (NPG), ethylene glycol (EG), ethylene glycol (DEG), trimethylolpropane (tmp), 1.4-butanediol (BDG), cyclohexanediol (CHDM), propylene glycol, and glycerol (glycerol), etc. Commonly used polyacids include purified terephthalic acid (PTA), isophthalic acid (IPA), adipic acid (ADA), trimellitic anhydride (TMA), etc.
 

The first step of the esterification reaction releases water while gradually generating macromolecules. This esterification reaction uses organotin catalysts (containing highly polar or odorous solvents). Incomplete esterification or incomplete removal of esterified water will lead to incomplete polycondensation, as polycondensation is a further esterification reaction. The presence of even a small amount of water will promote the hydrolysis of some polymers. Furthermore, in the later stages of polycondensation, insufficient nitrogen flow, inadequate vacuum time or vacuum level, and the incomplete removal of byproduct water and unreacted esters or acids will result in residual water, raw material esters or acids, and small-molecule polymers in the final product—the so-called volatile components.
 

Currently, commercially available polyester technical specifications include physical properties such as appearance, specific gravity, softening point, TG, color, and viscosity. Chemical properties include acid value, hydroxyl value, and iodine value. Volatile components are rarely tested or specified.
 

Additionally, it should be noted that many so-called dark-colored resins (black, amber, or yellow) sold in the market are usually made from PTA waste or substandard products, including recycled PET packaging materials and waste PTA fibers. The raw materials are diverse, especially PET packaging materials, which often contain pigments, lubricants, antioxidants, etc., resulting in poor color, strong odor, and a strong volatile, unpleasant smell at high temperatures.
 

Epoxy resin, another film-forming material for powder coatings, is most commonly synthesized from bisphenol A and epichlorohydrin (ECH) under the action of NaOH. Synthesis processes are divided into one-step and two-step methods. One-step methods mainly include three processes: water washing, solvent extraction, and solvent extraction. Two-step methods include bulk polymerization and catalytic polymerization.
 

The most widely sold epoxy resin in China is produced by the one-step water washing method, namely E-12. commonly known as 604 epoxy. Although the process is mature, it is generally difficult to completely remove moisture, cyclic chlorides, and many oligomers or hydrolysates from the resin by shortening the reaction time, reducing the number of water washes, and adding white polyester.
 

The third type of resin is acrylic resin polymers, sometimes called acrylic resin, which is a direct translation of its English name. Similar to this series of resins are some additives such as leveling agents and wetting accelerators, which will be explained later.
 

Acrylic resins as film-forming materials are classified into three types: 1) GMA type; 2) Carboxyl-terminated type; 3) Hydroxyl-terminated type.
 

GMA is short for glycidyl methacrylate. Resins using GMA as the terminal polymer are GMA-type resins, possessing epoxy reactive groups. They are mainly used as outdoor lighting resins, combined with carboxyl-terminated polyesters and small amounts of TGIC and HAA. They are typically copolymerized from the following raw materials: methyl methacrylate, butyl acrylate, isooctyl acrylate, and GMA. A small amount of styrene is often introduced during the synthesis process to improve water resistance. Dicranyl peroxide or azobisisobutyronitrile (AIBN) is chosen as the initiator, as some of their decomposition products remain in the final polymer. Toluene or xylene is usually used as a solvent in the synthesis of GMA resins. After the polymerization reaction, the solvent and unreacted residual monomers and oligomers must be removed by vacuum evacuation.
 

Hydroxyl-terminated acrylic resins are generally polymerized from acrylic acid, methacrylic acid, maleic anhydride, etc. These resins have poor weather resistance and are prone to yellowing under light and heat, thus they are rarely used as film-forming materials. Currently, they are mainly used as so-called physical matting agents.
 

Hydroxy-terminated acrylic resins use monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl methacrylate, n-butyl acrylate, and styrene, with toluene or xylene as the solvent. The initiator is benzoyl peroxide. The crosslinking agent is blocked isocyanate, resulting in coatings with good gloss retention and excellent weather resistance.
 

2. Solutions

The above provides a basic analysis of the main raw materials for powder coatings. Volatile substances are always present in various raw and by-product materials. During the actual production and spraying/baking processes of powder coatings, smoke and odor problems are always present. In certain environments and seasons, this can cause serious environmental pollution and harm to operators. Solutions can be approached from two aspects: "dredging" and "blocking."
 

From a "dredging" perspective, consider adding exhaust vents and collectors to extruders to reduce volatile organic compound (VOC) pollution in the workshop and harm to operators. Improve convection ventilation in the baking line's drying tunnels to enhance fume collection and treatment, reducing instantaneous fume concentration. Ordinary fixed ovens have poor exhaust performance; improving exhaust and heat circulation can shorten baking and curing time, save resources, and reduce instantaneous fume and irritating odor concentration, benefiting the environment and employee health.
 

The solution to "blockage" involves selecting materials with special properties and adding them to the powder formulation to absorb or decompose volatile substances harmful to the environment and human health. There are many types of adsorption materials; suitable adsorption materials for powder coatings are selected based on factors such as: micronization, porosity, good heat resistance, non-interference, and minimal impact on gloss and leveling.
 

In practical applications, we prioritize porous alumina and molecular sieves.
 

Porous alumina, also known as activated alumina, is a porous, highly dispersible solid material with a large surface area, ranging from 150 to 450 m² per gram. Its microporous surface possesses the characteristics required for catalysis, such as adsorption capacity, surface activity, and excellent thermal stability, making it widely used as a catalyst and catalyst carrier in chemical reactions. Utilizing its adsorption properties, adding 5-10% to powder coating formulations can partially suppress the generation of smoke and odor. Activated alumina has a very small particle size and poor dispersibility; therefore, when selecting a type, a variety with a suitable particle size and price should be considered, and the premixing time should be appropriately extended. If necessary, 0.5% alkyl wax or polyethylene glycol can be added to improve the uniform dispersion of the alumina.
 

Molecular sieves refer to a type of aluminosilicate microporous crystal with uniform micropores, the pore size of which is comparable to the size of a typical molecule. Also known as synthetic zeolite, it has a wide range of applications, including as a highly efficient desiccant, selective adsorbent, catalyst, and ion exchanger. Molecular sieves are composed of a basic framework structure of silicon-oxygen and aluminum-oxygen tetrahedra. Metal cations (such as Na+, K+, Ca2+, Li+, etc.) exist within the crystal lattice to balance excess negative charges in the crystal. Molecular sieves are mainly classified according to their crystal structure into type A, type X, and type Y. Adsorption function: The adsorption of substances by molecular sieves originates from physical adsorption (van der Waals forces). The internal pores of their crystals possess strong polarity and a Coulomb field, exhibiting a strong adsorption capacity for polar molecules (such as water) and unsaturated molecules.
 

In powder coatings, molecular sieves are preferentially considered for textured formulations because they have a large oil absorption capacity and are difficult to disperse in resin. The dosage for planar powder coatings can be 2-5%, and the molecular sieve specifications are 5A-10X.
 

Some natural zeolites also have good adsorption properties, but they are darker in color and cheaper, making them suitable as ordinary pigments for sand and wrinkle textures. When adding molecular sieves or natural zeolites, premixing should be strengthened, and appropriate dispersants such as alkyl waxes are helpful.
 

Simple porous micropowders have limited effect on smoke and odor control in powder coatings, and increasing the amount added significantly impacts leveling, gloss, and powder application rate. Therefore, we have developed smoke-reducing additives specifically for powder coatings, such as the recently launched NS022G. It basically consists of three parts: molecular sieves (partially activated alumina), odor-neutralizing nanopowders, and highly active compounds.
 

The principle and function of molecular sieves have been introduced previously.
 

Odor-neutralizing nanopowders are a type of synthetic material with a special structure, a novel inorganic odor-neutralizing functional powder with broad odor-neutralizing functions, high odor-neutralizing efficiency, and long-lasting effect. It can decompose formaldehyde and various organic compounds, reducing TVOC; it is easy to use, does not affect the construction process, does not contain heavy metals, and is non-toxic and harmless to the human body, making it a new type of environmentally friendly material.
 

The highly active compounds are applied using a special technology, "coated into the inner pores of the molecular sieve," and can form partial hydrogen bonds or covalent bonds with low-molecular-weight compounds such as ethylene glycol and benzoin. Even at higher temperatures such as 150-250℃, their function is not significantly weakened. There are many types of highly active compounds; for powder coatings, ethyl 2-pentyl-3-oxocyclopentyl acetate (methyl acetate) may be considered.
 

The smoke suppressant MS022G is a white micro-powder with good dispersibility and free-flowing ability, suitable for various powder formulations. Adding 1-3% to matte powders has little impact on gloss and leveling.