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Novolak
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Segment of novolak, illustrating the predominance of cresol subunits and presence of crosslinking.

Novolaks (sometimes: novolacs) are low molecular weight polymers derived from phenols and formaldehyde. They are related to Bakelite, which is more highly crosslinked. The term comes from Swedish "lack" for lacquer and Latin "novo" for new, since these materials were envisioned to replace natural lacquers such as copal resin.

Typically novolaks are prepared by the condensation of phenol or a mixture of p- and m-cresol with formaldehyde (as formalin). The reaction is acid catalyzed. Oxalic acid is often used because it can be subsequently removed by thermal decomposition. Novolaks have a degree of polymerization of approximately 20-40. The branching density, determined by the processing conditions, m- vs p-cresol ratio, as well as CH2O/cresol ratio is typically around 15%.[1]

Novolaks are especially important in microelectronics where they are used as photoresist materials.[2][3] They are also used as tackifiers in rubber.

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References

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Novolak

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from Grokipedia
Novolac, also known as novolak, is a phenolic resin produced through the acid-catalyzed condensation polymerization of phenol and , typically using a molar ratio of to phenol between 0.75 and 0.85, resulting in a non-methylol-bearing structure linked by methylene bridges. This resin is synthesized in a strongly acidic medium ( 1–4) with catalysts such as oxalic or , yielding brittle, solid s with a low that require a curing agent like (HMTA, 8–15% by weight) to cross-link into a thermoset , releasing during the process. Its , characterized by a highly aromatic backbone, imparts notable properties including high dimensional stability, superior mechanical strength, excellent flame resistance, chemical resistance, and an infinite in its uncured form, though it is inherently brittle and often reinforced with fillers for practical use. Novolac resins find widespread application across industries due to these attributes; in , they serve as materials for fabrication, while in composites and laminates, they impregnate substrates like , fabrics, and to produce durable electrical insulators and structural components. They are also employed in friction materials such as brake linings and clutch pads, foundry resins for , protective coatings in automotive and electrical sectors, and as binders in abrasives and products. Derivatives like novolac resins extend these uses to high-performance and chemically resistant paints, enhancing durability in harsh environments.

Etymology and Overview

Etymology

The term "Novolak" is derived from the Latin "," meaning "new," combined with the Swedish "lack," signifying "." This etymological construction reflects the material's development as a novel synthetic substitute for traditional natural lacquers, exemplified by , which had long been used in varnishes and coatings. Early chemists in the late 19th and early 20th centuries adopted this to underscore the innovative replacement of scarce or inconsistent natural resources with engineered phenolic , marking a pivotal advancement in .

Definition and Characteristics

Novolak resin, also known as novolac, is a phenolic produced through the acid-catalyzed of and , utilizing an excess of phenol relative to formaldehyde in a molar ratio typically ranging from 0.5:1 to 0.8:1. This process yields linear oligomers with molecular weights between 500 and 5000, characterized by a predominance of methylene linkages, such as 50–75% 2,4'-methylene bridges, and minimal branching at higher molecular weights. As a precondensate, novolak consists of phenolic units linked by aldehyde-derived bridges, forming a versatile base material for further processing. The defining characteristics of novolak include its behavior, which allows it to soften at temperatures around 50–75°C with a temperature (Tg) of 45–70°C, enabling easy molding and shaping prior to curing. Unlike fully thermoset materials, novolak remains fusible and until cross-linked, exhibiting in polar organic solvents such as alcohols and ketones, which facilitates its dissolution for applications like coatings and composites. It serves primarily as a pre-polymer in formulations, requiring an external crosslinker—typically 5–15% (HMTA) or —to form a three-dimensional, infusible thermoset network upon heating. A key distinction of novolak from other phenolic resins, such as resols, lies in its two-stage curing process: novolaks lack reactive methylol groups and thus cannot self-, necessitating the addition of a curing agent like HMTA to generate methylene bridges during the second stage, whereas resols are one-stage, thermosetting resins formed under alkaline conditions with excess (molar ratio 1.2:1 to 3.0:1) that independently upon heating. This fundamental difference in synthesis and reactivity—acidic and phenol excess for novolak versus basic and excess for resols—results in novolak's greater processability as a precursor.

History

Invention

Novolak resins were invented by Belgian-American Leo Hendrik Baekeland during his research into phenolic resins between 1905 and 1907, as he sought to develop synthetic alternatives to natural materials like for use in plastics and coatings. In his early experiments conducted in a private laboratory in , Baekeland investigated acid-catalyzed condensations of phenol and , which produced soluble, fusible resins that served as key intermediates in his broader quest for durable synthetic materials. These novolak resins, however, achieved only limited initial commercial success compared to the infusible, thermosetting resols he later refined for production, as the acid catalysis halted the reaction at a thermoplastic stage, yielding products that could be dissolved in solvents like alcohol or acetone. Baekeland's work on such acid-catalyzed processes was described in U.S. 942,699, filed on , 1907, and granted on December 7, 1909, which primarily focused on methods to convert fusible intermediates into hard, insoluble thermoset products using heat and pressure, though it included details on forming oily liquids that could yield viscous, shellac-like substances suitable for applications under acidic conditions (such as using or ). In 1909, he proposed the name "novolac" for these fusible resins, reflecting their resemblance to a "new " or substitute, marking their initial recognition as a distinct class of materials.

Development and Commercialization

Following Baekeland's initial of phenolic resins in , which laid the foundation for novolak as fusible, materials, commercialization efforts accelerated in the early through the establishment of dedicated production facilities focused primarily on thermosetting . In May 1910, GmbH opened the first commercial phenolic plant in Erkner, , under license from Baekeland, producing resins for industrial applications such as molded products, with novolak used in varnishes and adhesives. Later that year, in October 1910, General Bakelite Company began operations in , scaling up phenolic for similar uses, including early electrical insulators and coatings. These initiatives marked the transition from laboratory-scale synthesis to industrial output, with novolak's and film-forming properties enabling its adaptation for protective coatings and bonding agents. During the and , key advancements refined novolak formulations for enhanced performance in adhesives and coatings, driven by innovations from researchers and companies like Corporation. In 1910, J.W. Aylsworth contributed significantly by developing methods to cure novolak resins using , improving their thermosetting potential for durable bonds in abrasives and composites without excessive brittleness. By the mid-, Corporation introduced novolak-based resinoid grinding wheels and coated abrasives, which offered superior heat resistance and efficiency compared to natural abrasives, spurring adoption in . L. Behrend's 1910 work on oil-soluble modified novolaks further expanded their utility in solvent-based coatings for metals and wood, addressing limitations in . These refinements, often involving controlled acidification and , positioned novolak as a versatile material for industrial adhesives by the . Widespread commercialization in the solidified novolak's role in industrial sectors, with production volumes surging due to demand for heat-resistant materials amid economic recovery and . , which acquired Corporation in 1939, optimized novolak processes for molding compounds, achieving higher mechanical strength and lower free phenol content through techniques, enabling reliable use in automotive parts and electrical components. By the late , annual global production of phenolic resins, including novolaks, exceeded hundreds of thousands of tons, reflecting broad adoption in adhesives for and laminates. This era's milestones, such as steel-belt flaking for solid novolak forms, facilitated storage and , further boosting market penetration. The 1960s and 1970s witnessed a transformative surge in novolak's commercialization, propelled by its integration into as a key component in formulations. Building on earlier discoveries like Oskar Suss's 1949 identification of novolak-diazonaphthoquinone (DNQ) systems for positive-tone resists, adoption accelerated with the rise of semiconductor manufacturing, where novolak's thermal stability and etch resistance supported sub-micron patterning. Companies such as Sumitomo Bakelite and introduced the PAPS (phenol-acetaldehyde-phenol-synthesis) process in the 1970s, yielding novolaks with narrow molecular weight distributions (1.1–2.0) and low impurities, ideal for high-resolution in integrated circuits and LCDs. contributed to scaling these specialized novolaks, optimizing ratios (e.g., meta-para at 40/60) for improved contrast in g- and i-line exposure, as advanced by researchers like Hanabata. By the late 1970s, novolak-DNQ s dominated over 80% of the market, driving the microelectronic revolution with features below 300 nm.

Chemical Composition and Synthesis

Molecular Structure

Novolac resins feature a linear or slightly branched oligomeric structure composed of phenolic units—benzene rings bearing a hydroxyl group—connected primarily by methylene bridges (-CH₂-) at the ortho and para positions relative to the hydroxyl. These bridges form between the carbon atoms on adjacent phenolic rings, resulting in a chain-like that distinguishes novolacs from their crosslinked counterparts. The general formula for novolac can be represented as [\ce(C6H4(OH)−CH2)n−C6H4OH][ \ce{(C6H4(OH)-CH2)_n - C6H4OH} ], where n denotes the (typically 4–15), and the chain terminates with phenolic hydroxyl end groups. This notation highlights the repeating phenolic-methylene motif, with the hydroxyl groups remaining unreacted in the uncured state, contributing to the resin's nature. As oligomers, novolacs exhibit molecular weights generally ranging from 500 to 2000 g/mol, corresponding to low polydispersity and enabling in organic solvents before curing. The ratio of ortho to para linkages, which can vary from predominantly ortho (e.g., >70% ortho under certain acidic conditions) to a more random distribution, influences chain flexibility, , and subsequent reactivity; higher ortho content often enhances due to reduced crystallinity. In their uncured form, novolacs maintain a predominantly linear configuration, facilitating melt processing or dissolution for applications such as photoresists.

Synthesis Process

Novolak resins are synthesized through an acid-catalyzed between and , utilizing a molar ratio of phenol to formaldehyde greater than 1:1 to ensure excess phenol and limit the reaction to the stage. Common acid catalysts include , , , or , which maintain a strongly acidic environment with typically between 1 and 4. The reaction proceeds in an aqueous or mixed solvent medium, where is present as an in equilibrium with methylene glycol. The mechanism involves , beginning with the of methylene glycol by the acid catalyst to form a reactive hydroxymethylene . This attacks the ortho or para position of a phenol molecule, leading to a sigma complex that deprotonates to yield a methylol (hydroxymethyl) intermediate. Subsequently, under acidic conditions, the methylol group undergoes to generate a benzylic , which then reacts with another phenol molecule via another to form a (-CH₂-). This stepwise process repeats, building linear or slightly branched oligomers until is depleted. The overall reaction can be represented as: n\ceC6H5OH+(n−1)\ceCH2O→(\ceC6H4OH−CH2)n+(n−1)\ceH2On \ce{C6H5OH} + (n-1) \ce{CH2O} \rightarrow (\ce{C6H4OH-CH2})_{n} + (n-1) \ce{H2O} Typical reaction conditions involve heating the mixture to 80–100°C under reflux for 2–4 hours, allowing water formation and promoting condensation while controlling molecular weight to 500–1000 g/mol. To halt polymerization at the desired oligomer stage and prevent excessive crosslinking, water and excess phenol are removed by distillation or azeotropic methods, followed by neutralization with a base such as sodium hydroxide. In some processes, higher temperatures up to 160°C are employed during water removal to enhance efficiency, particularly in continuous setups. The choice of catalyst concentration, typically 1–6 wt% relative to phenol, influences the ortho/para substitution ratio and final resin properties.

Physical and Chemical Properties

Physical Properties

Novolak resins exhibit a glass transition temperature (Tg) typically in the range of 40–60°C for uncured materials, reflecting their thermoplastic nature at ambient conditions. The softening point varies between 50–100°C depending on molecular weight and composition, allowing processability in applications requiring flow under moderate heat. These resins demonstrate high thermal stability prior to curing, maintaining integrity up to approximately 300°C without significant decomposition, attributable to their aromatic backbone structure. Mechanically, novolak resins appear as brittle solids at , characterized by rigidity and limited flexibility due to their crosslinked potential in precursor form. They possess good to various substrates, enhancing their utility in composite and formulations. The of novolak resins generally falls between 1.1 and 1.2 g/cm³, contributing to their lightweight yet durable profile in solid form. In terms of solubility, novolak resins dissolve readily in polar organic solvents such as alcohols and ketones (e.g., , acetone), facilitating solution processing and film formation essential for thin-layer applications. They exhibit insolubility in water, which supports their use in non-aqueous environments and underscores their hydrophobic character derived from phenolic components.

Chemical Properties

Novolak resins, being in their uncured state, exhibit limited reactivity without the addition of a cross-linking agent. They require a hardener such as (HMTA) to initiate thermosetting behavior, transforming the linear chains into a three-dimensional network. This cross-linking process occurs at elevated temperatures typically ranging from 150°C to 200°C, where HMTA decomposes to generate and , facilitating the formation of methylene bridges (-CH₂-) between the ortho and para positions of adjacent phenolic rings. The curing reaction can be simplified as:
Novolak + HMTA → crosslinked network + NH₃ + ,
with the byproducts and being released during the process. Post-curing, novolak resins demonstrate enhanced , showing resistance to dilute acids and bases due to the robust aromatic and cross-linked matrix that limits penetration and degradation. This stability extends to low flammability, as the resin promotes char formation upon exposure to , which acts as a barrier to further and oxygen access, contributing to self-extinguishing properties. In contrast, the uncured form maintains good storage stability under ambient conditions, remaining solid and non-reactive for extended periods without significant degradation.

Production and Manufacturing

Industrial Production

The industrial production of novolak resins involves an acid-catalyzed of phenol and , typically using a phenol-to-formaldehyde molar ratio exceeding 1:1 to yield resins. This reaction is predominantly conducted in batch stirred tank reactors for precise control over reaction conditions, though continuous tubular reactors are increasingly utilized for high-volume to improve efficiency and consistency. Following the initial addition of to form hydroxymethyl groups on the phenol ring and subsequent to create methylene bridges, the process includes of the molten to recover excess unreacted phenol, which can constitute up to 10% of the initial charge. is then performed under to eliminate water generated during the reaction, resulting in a high-solids product with yields typically ranging from 85% to 95%. Common acid catalysts employed include , , , or , selected based on the target resin's end-use properties such as and curing behavior. Global production of novolak resins reaches approximately 1.2 million metric tons annually (as of 2024), with the largest share—approximately 36%—concentrated in Asia Pacific, driven by major manufacturing hubs in China and India that leverage cost-effective raw materials and expanding demand in electronics and coatings sectors. To maintain product quality and uniformity, manufacturers monitor the molecular weight distribution of the resins using (GPC), ensuring polydispersity indices suitable for applications like s where consistent behavior is critical.

Variations and Modifications

Novolak resins can be tailored through the substitution of phenolic monomers to enhance specific properties such as and thermal stability. Cresol-novolaks, derived from isomers like and p-cresol, exhibit improved in organic solvents commonly used in formulations, facilitating better dissolution and film formation during processing. These variants maintain the core phenolic structure while adjusting the resin's polarity and molecular weight distribution for optimized performance in lithographic applications. Bisphenol A-based novolak variants incorporate as a co-monomer, resulting in resins with elevated temperatures (Tg) often exceeding 120°C, which provides superior thermal resistance compared to standard phenol-formaldehyde novolaks. This modification increases the rigidity of the backbone, making these resins suitable for high-temperature environments without compromising mechanical integrity. Sustainability-driven modifications involve integrating bio-based phenols, such as those derived from , into the novolak synthesis to reduce reliance on petroleum-derived feedstocks. Lignin-copolymerized novolaks, where depolymerized replaces a portion of phenol, yield resins with comparable mechanical properties and curing behavior while promoting through renewable sourcing. These bio-based variants have been commercialized as solid novolac-type phenolic resins, marking advancements in for industrial adhesives and composites. Epoxy-novolak hybrids combine novolak structures with functionalities, often through glycidyl etherification, to produce resins with enhanced and electrical insulation for . These hybrids offer high functionality and thermomechanical stability, enabling their use in encapsulation where low and rapid curing are critical. Specialized grades of novolak resins feature reduced unreacted phenol content, typically below 1%, achieved through or advanced purification, making them compliant for contact applications by minimizing migration risks. UV-sensitive modifications involve esterification of novolak hydroxyl groups with diazonaphthoquinone (DNQ) moieties, imparting for positive-tone photoresists that undergo solubility changes upon UV exposure. These alterations enable precise patterning in while preserving the resin's etch resistance.

Applications

In Microelectronics and Photoresists

Novolak resins serve as the foundational matrix in diazonaphthoquinone (DNQ)-novolak positive , which are the cornerstone materials for g-line (436 nm) and i-line (365 nm) in manufacturing. These enable the patterning of features with resolutions typically ranging from 0.3 to 1 μm, with i-line enabling finer features down to 0.3 μm and g-line suited for features above 0.5 μm, supporting the fabrication of integrated circuits with critical dimensions suitable for mid-range technology nodes. The combination of novolak's structural integrity and DNQ's photosensitivity allows for precise control over pattern definition in processes like wafer stepping and scanning . In DNQ-novolak formulations, novolak imparts critical properties including high etch resistance during plasma processing, strong to and metal substrates, and overall film mechanical stability to withstand subsequent fabrication steps. The unexposed resist remains insoluble in aqueous alkaline developers due to hydrogen bonding between the phenolic hydroxyl groups of novolak and the DNQ , which inhibits dissolution by a factor of 10 to 100. Upon targeted UV exposure, the DNQ undergoes to generate a base-soluble indene , dramatically increasing the of exposed regions and enabling clean development with sharp contrast and minimal undercutting. This switch mechanism ensures reliable image transfer for device features. The introduction of DNQ-novolak photoresists in the 1970s marked a pivotal advancement in microelectronics, supplanting earlier rubber-based systems and enabling the scaling of integrated circuits from 16 KB to 16 MB densities by providing superior resolution, focus depth, and process latitude. This technology dominated over 90% of the global photoresist market through the 1980s and 1990s, driving the microelectronic revolution and supporting exponential increases in transistor counts per chip in accordance with Moore's Law. Even today, novolak-based systems remain relevant for legacy and cost-sensitive production lines, underscoring their enduring impact on semiconductor fabrication.

Other Industrial Applications

Novolak resins, valued for their thermal stability, mechanical strength, and ability to form durable bonds, are widely used as binders in abrasives, where they coat and adhere grains such as aluminum oxide or to create grinding wheels and pads that maintain integrity under high-speed and high-heat conditions. In friction materials like linings and facings, these resins bind reinforcing fibers, fillers, and metals, providing consistent al properties, wear resistance, and heat dissipation critical for automotive and industrial braking systems. For foundry operations, novolak serves as a binder in , where it is mixed with sand to produce thin-walled, precise molds and cores that endure molten metal temperatures up to 1,500°C without deformation. Beyond binders, novolak resins find application in coatings and adhesives, forming protective varnishes that offer resistance and to metals and composites in industrial environments. In adhesive formulations, they enable the production of laminates for electrical insulation and structural panels, as well as bonding agents for composites requiring high and chemical durability. Modified novolak variants, often cured with , have been adapted for wood bonding in particleboard and engineered lumber, providing moisture-resistant joints suitable for . In emerging sectors, novolak resins contribute to carbon-carbon composites by acting as a precursor matrix that carbonizes during high-temperature processing, yielding lightweight, high-modulus materials for brakes and heat shields. They are also integrated into flame-retardant formulations for furnace linings, insulation blocks, and molds, where their char-forming during enhances resistance and .

Safety and Environmental Considerations

Health and Safety

Uncured novolak resin acts as an irritant to the skin and eyes upon contact, potentially causing redness, itching, or serious damage depending on exposure duration. Novolak contains low levels of residual and unreacted phenol, with posing a carcinogenic through or absorption, classified as a by the International Agency for Research on Cancer. During curing, often with , novolak releases gas, which can irritate the and eyes at elevated concentrations. To mitigate these hazards, workers should use including chemical-resistant gloves, protective clothing, eye protection, and face shields, along with adequate ventilation to control airborne contaminants. The (OSHA) sets a (PEL) for at 0.75 parts per million (ppm) as an 8-hour time-weighted average, with a of 2 ppm over 15 minutes. Novolak exhibits low , with oral LD50 values exceeding 2,000 mg/kg in rats, indicating minimal risk from single high-dose exposures. However, chronic exposure to impurities such as in certain variants, like novolak resins, may lead to endocrine disruption and reproductive health risks due to BPA's estrogenic activity.

Environmental Impact

The production of novolak resins, which involves the acid-catalyzed condensation of and followed by , generates environmental emissions, including volatile organic compounds (VOCs) such as unreacted and , primarily through air emissions during the reaction and stages. Additionally, the process produces containing residual that require treatment to prevent aquatic contamination. The step is particularly energy-intensive, contributing significantly to the process's environmental footprint in terms of energy-related emissions. Phenol recovery systems, which recycle unreacted phenol from streams, help mitigate these impacts, though they still necessitate advanced and neutralization to handle phenolic effluents. To address these challenges, efforts have focused on bio-based alternatives that reduce reliance on fossil-derived phenol. For instance, from can substitute 25–55% of phenol in novolak formulations, while cardanol from nut shell liquid enables up to 100% replacement, both derived from renewable sources to lower the and decrease emissions in related phenolic resins. from wood extracts can replace up to 40% of phenol, utilizing to promote a and minimize environmental toxicity. Uncured novolak scrap is also recyclable through mechanical or chemical methods, allowing recovery of phenolic monomers for and reducing from production overruns. Regulatory frameworks impose restrictions on formaldehyde in novolak production and use due to its classification as a hazardous air pollutant and probable carcinogen. Under the U.S. EPA's Toxic Substances Control Act (TSCA), formaldehyde emissions from resin manufacturing are regulated; as of January 2025, the EPA's final risk evaluation confirmed unreasonable risks to human health and the environment from industrial releases, including in phenolic resin production, mandating emission controls and monitoring. In the European Union, REACH Annex XVII limits formaldehyde emissions from articles like phenolic resins to 0.124 mg/m³ in indoor air, targeting free formaldehyde content in products to curb ecological release into water and soil. At end-of-life, novolak's high char yield—typically around 60% upon thermal decomposition—facilitates safer incineration by promoting complete combustion and minimizing incomplete burning residues.

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