Formaldehyde and Emission Regulations

Turkchem 16 Jul 2020 47 12 dk okuma

TURKCHEM

Formaldehyde

Formaldehyde is one of the most important raw materials in modern chemical industry due to its susceptibility to various chemical reactions and low cost. A colorless gas with a pungent odor at room temperature, formaldehyde is found on the market as aqueous solutions in various concentrations known as formalin. As the shortest-chain member of aliphatic aldehydes, formaldehyde was discovered by Butlerov in 1859 and began to be produced commercially in the early 20th century and gain a foothold in industry. Formaldehyde, containing methanol as a stabilizer and stored commercially at concentrations between 37% and 50%, is obtained through silver and metal oxide processes where the oxidation of methanol in the presence of metal catalysts differs in raw materials, natural gas, electricity and other operational expenses and efficiencies [1].

A Brief Look at Formaldehyde History

1859 – A.B. Butlerov becomes the first person to publish formaldehyde in the literature during the synthesis of methylene glycol in his research laboratory. However, he is unable to clarify its chemical structure. 1868 – Prof. Dr. A.W. von Hofmann, working at Berlin University, becomes the first scientist to determine the chemical structure and identity of formaldehyde. The method von Hofmann employed in formaldehyde production forms the foundation of modern formaldehyde production processes. 1880 – Commercial large-scale production of formaldehyde begins in Germany and spreads throughout Western Europe and America at the beginning of the 1900s. It is initially used as a medical preservative for embalming purposes. This market today accounts for only 1% of total formaldehyde sales. 1907 – Demand for formaldehyde increases continuously in the late 19th century as technology develops and formaldehyde gains new applications. Phenol-formaldehyde resin synthesized by Belgian chemist Dr. Leo Baekeland takes its place in history as the first synthetic polymeric material. 1940 – The first formaldehyde-based particleboard produced in Bremen, Germany pioneered the wood-based panels that became indispensable actors in the construction and furniture sectors. Today – Commercial applications of formaldehyde continue to grow. In Europe alone, 3.6 million tons of formaldehyde (C. 100%) were produced in 2010 [2]. [caption id="attachment_102947" align="aligncenter"] Visual 1: Market shares of formaldehyde derivative products [4].[/caption] Formaldehyde is today a versatile chemical produced on an industrial scale and has become an important raw material in various sectors. Most of the formaldehyde produced through methanol oxidation—particularly formaldehyde-based resins—is consumed after being reacted with other chemicals. Formaldehyde-based resins have become a significant input to the construction and furniture sectors due to their superior adhesive properties and low cost. Subgroups of formaldehyde-based thermoset resins with superior heat and chemical resistance have now taken their place in the automobile and aircraft industries. Formaldehyde consumption in developed countries today is closely related to the construction sector. From the construction of a new living space to its daily use, it is inevitable to encounter materials produced using formaldehyde in every area of our lives. From the plywood used during construction of the structure to the furniture preferred in the home interior, decorative surfaces, television units, various sports equipment, isolation systems, the motor filters of the vehicles we use, and brake systems—it is possible to find traces of formaldehyde. [caption id="attachment_102948" align="aligncenter"] Visual 2: Home applications of formaldehyde and its derivatives [5][/caption]

Formaldehyde and Health Effects

Research conducted by the World Health Organization (WHO) has proven that formaldehyde causes leukemia and adversely affects human health [6]. In 2004, the International Agency for Research on Cancer (IARC) reclassified formaldehyde from Group 2A (possible carcinogen to humans) to Group 1 (carcinogen to humans) [7]. This non-binding advisory decision from this agency affiliated with the World Health Organization (WHO) has prompted environmental organizations, regulatory decisions, relevant committees, and formaldehyde producers/users to take action. Following this IARC report, the U.S. Environmental Protection Agency (EPA) updated its formaldehyde classification in 2010 to B1 (probable carcinogen), while in 2015 the European Chemicals Agency (ECHA) moved formaldehyde classification from Cat 2 (possible carcinogen) to Cat 1B (carcinogen to humans). Following reports from health and environmental authorities, significant restrictions have been placed on formaldehyde emissions both in production facilities where formaldehyde is used and in finished products manufactured using formaldehyde derivatives [8].

Applications of Formaldehyde in Wood-Based Panels and Other Sectors

Formaldehyde is today widely used in obtaining thermoset polymers. Thermoset polymers are three-dimensional macromolecules that are cured after thermal treatment and irreversibly maintain their rigid structure. Urea formaldehyde (UF) resins, melamine urea formaldehyde resins (MUF), melamine formaldehyde (MF) resins, and phenol formaldehyde (PF) resins are used as adhesives and binders in various sectors, particularly in wood-based panels, as formaldehyde-based resins. Wood-based panels are composite materials obtained by bonding wood pieces of various geometries such as fibers, particles, flakes, and sheets with an adhesive in the presence of appropriate pressure and thermal treatment. Particleboard, medium-density fiberboard (MDF), oriented strand board (OSB), and plywood—which we encounter in every area of everyday life—are produced using formaldehyde resins in the manufacture of wood-based panels, as well as in the production of decorative surface papers and laminates. UF and MUF resins emerge as one of the main building blocks of the construction and building sectors. Of the UF resins produced, 95% are used as binders or adhesives in composite panels in plywood and particleboard production and dominate the formaldehyde market. Due to their low cost, stability, strength, rapid curability, and ease of application, UF resins are indispensable to the wood-based panel sector. MF resins appear in the construction sector just like UF and MUF resins. 95% of MF resins are preferred in the decorative surface and laminate sector. Despite being relatively more expensive than other formaldehyde resins, MF resins are in a highly competitive position compared to their alternatives and are indispensable for high-pressure laminates used in outdoor decorative surfaces. Like other formaldehyde resins, PF resins are also an important component for the construction sector. Approximately 60% of PF resins produced find use in insulation materials, wood-based panels, and laminates. Other areas of intensive use are automobile applications (particularly in brake systems) and the casting industry. Due to their high strength, dimensional stability, and resistance to water and high temperatures, phenol-formaldehyde resins are one of the main inputs of the space and aerospace industries operating under harsh atmospheric conditions. In addition to the formaldehyde resins mentioned above, thermoset polymers produced using formaldehyde along with different chemicals such as furan, resorcinol, lignin, and tannin have market shares in various sectors. Besides formaldehyde-based resins, formaldehyde is used as a key input in the production of methylene diphenyl diisocyanate (MDI), one of the main raw materials of the polyurethane sector, in polyoxymethylene (POM) thermoplastics, and in obtaining pentaerythritol, the raw material of alkyd resins.

Formaldehyde Emission and Formation from Wood-Based Panels

Formaldehyde is one of the main components of aminoplastic and phenoplastic resins used in the production of wood-based panels. Formaldehyde emission from finished panels depends on internal and external factors. External factors are temperature, humidity, air movement over the panel surface, and the size of the area where the panels are located, while internal factors are wood types, moisture content of the wood material, type and chemical composition of the binder used, additives added (e.g., catalysts and formaldehyde scavengers), the arrangement of multi-layer panels, panel density, and panel production conditions [9, 10, 11]. Formaldehyde emissions from wood-based panels fundamentally come from three sources: the formaldehyde content of the wood material, free formaldehyde in the unreacted formaldehyde-based resin, and formaldehyde formed from methylene and ether bonds in the resin that undergo hydrolysis during curing. Viewed from this perspective, the chemical structure of the binder resin has a significant effect on formaldehyde emission from wood-based panels. Therefore, a reduction in the free formaldehyde content remaining in the medium without reacting in the resin can directly affect the formaldehyde emission rate of the finished product.

Cross-linking of the resin during the manufacture of wood-based panel products is achieved through a combination of heat and hardener. Acidic components in the wood material serve a function in the hardening of the binder but the desired degree of cross-linking is not achieved. For this reason, catalysts (hardeners) such as ammonium sulfate or ammonium chloride are used, and as these chemicals undergo hydrolysis in an aqueous medium, the acidic ions they create in the medium accelerate the condensation of the resin and curing is achieved.

In liquid resin content, a small amount of free formaldehyde (usually less than 0.1%) is present that ensures cross-linking of the resin in the presence of heat and acid catalyst. This free formaldehyde exists in various chemical forms in the produced panel. It can react with moisture present in wood fibers to convert into compounds such as methylene glycol, polymethylene glycol, or polyoxymethylene hemiacetal; it can also be unstably bonded to wood fiber or thermoset polymer [10]. Free formaldehyde structures that lack hydrolysis resistance and are unstably added to the structure are released from the structure into the atmosphere at high temperatures or in a well-ventilated environment, causing formaldehyde emission. Formaldehyde emission from wood-based panels is not limited to the emission of the free formaldehyde mentioned. Formaldehyde emission may also come from the thermolysis of lignin in wood and polysaccharides in wood fiber [8]. Hydrolysis of partially or fully cured resin over time due to factors such as temperature, humidity, and pH constantly cause emissions. Depending on type, wood particles can release acids within them under certain temperature and humidity conditions. These cause a decrease in pH and hydrolysis of N-methylol groups, leading to formaldehyde formation [12]. The majority of released formaldehyde originates from free formaldehyde in the material. Although formaldehyde emission from the finished product decreases over time, it eventually remains constant after a certain period.

Formaldehyde Emission Measurements in Wood-Based Panels

Due to the negative health effects of formaldehyde, various regulations have imposed restrictions on the emission levels of wood-based composite structures. As a result of these restrictions, the upper limits of indoor air pollutants have been re-regulated and the emission rates of formaldehyde listed in the indoor air pollutant list have been reduced by 80% [13]. With the increase in market demand for low-formaldehyde emission products, research and development activities have intensified in this direction and formaldehyde tests have increased in importance in quality control procedures.

Different standards and related methods used for formaldehyde emission measurements are listed in Table 1. These are the Chamber, Gas Analysis, Perforator, Desiccator, and Flask Methods. These methods can be classified in two ways: "measurable emission" (amount of formaldehyde released under test conditions) and the "releasable potential" of formaldehyde in the panel (maximum releasable formaldehyde under conditioning) [14].

Test conditions, sample sizes, and equipment to be used vary depending on the method to be used. Formaldehyde is commonly held in water and analyzed using the acetyl-acetone method. Formaldehyde determination is based on the Hantzsch reaction. The Hantzsch reaction converts formaldehyde to diacetyl-dihydrolutidine (DDL), which reacts with ammonium ions and acetylacetone and shows maximum absorbance at 412 nm; based on this absorbance value, the amount of formaldehyde can be measured. Test methods can be divided into three categories: a. "Reference methods" that simulate a standard indoor environment, the most well-known being the Chamber Method (according to EN 717-1) b. "Certification methods" used especially for product certification, such as Perforator and Desiccator Methods. c. "Quality control methods" for rapid and routine production control. Dynamic Micro Chamber (DMC) and Field and Laboratory Emission Cell (FLEC) requiring special equipment use and another method is the flask method [6]. Among the methods, there are differences not only in test conditions and methodology but also in formaldehyde measurement units, and various studies have been conducted to establish correlations between formaldehyde test methods. However, gas analysis and desiccator methods are used in many industries to determine content for rapid quality control. The chamber test method, which best simulates the human living environment, is used as the reference method.

Formaldehyde Emission Standards and Limits in Wood-Based Panels

Recent restrictions on the sector have not been limited only to the emission levels of finished products, but work has also been carried out covering wood-based panel production facilities. The European Union has developed a strategic policy establishing occupational exposure limits (OEL) to protect workers against risks from dangerous substances in the workplace. The European Union Scientific Committee established the occupational exposure limit for formaldehyde in 2008 as 0.2 ppm for an 8-hour exposure period and 0.4 ppm for short-term exposure limits (STEL). In 1980, formaldehyde emissions began to be regulated by some European countries. A mandatory E1 class (0.1 ppm formaldehyde) was created for wood-based panels. In 2004, Europe created E1 and E2 (European Standard EN 13986) emission classes for wood products used in construction. In 2006, the E1 emission class became mandatory for wood-based panel production. Both in Europe and the United States, the requirement for testing products to determine formaldehyde emission has been increased as part of product development and quality control procedures. European formaldehyde limits for wood-based panels are summarized in EN 13986 Standard for E1 and E2 classes. Table 2 presents formaldehyde emission limits for E1 and E2 classes according to European standards. [caption id="attachment_102950" align="aligncenter"] Table 2: Formaldehyde emission levels in various countries for wood-based panels (PB: Particleboard, MDF: Medium Density Fiberboard, PLW: Plywood) [15][/caption] As shown in the tables, formaldehyde emission is limited by region and countries with different standards and emission levels according to the type of finished product. While E1 and E0 products are increasing their market share within European Union limits, products with emission levels complying with CARB Regulation are popular in the United States [16]. [caption id="attachment_102951" align="aligncenter"] Table 3: Comparison of international emission limits [17][/caption]

From the report on the effects of formaldehyde on human and environmental health published by the World Health Organization in the early 2000s to the present, low-emission target finished product research has begun in all sectors where formaldehyde and its derivatives are used, particularly in the wood-based panel sector, and low-emission products have found use in every area of our daily lives. Thanks to restrictions placed on formaldehyde exposure by the European Union, the United States, and Japan, among much of the world, industry and end consumers have been made aware of the negative effects of formaldehyde. As a result of reports from health and environmental authorities and regulations from committees, public awareness has increased and, as seen in Graph 1, formaldehyde emissions from wood-based panels have decreased over time.

[caption id="attachment_102952" align="aligncenter"] Graph 1: Change in formaldehyde emissions from wood-based panels over the years [18][/caption]