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troubleshooting cure issues in polyurethane coatings: a focus on catalyst concentration

abstract: polyurethane (pu) coatings are widely utilized across diverse industries due to their exceptional durability, flexibility, and chemical resistance. however, achieving optimal coating performance hinges on proper curing, a process significantly influenced by catalyst concentration. deviations from the recommended catalyst levels can lead to various cure-related defects, impacting the final coating properties and longevity. this article provides a comprehensive guide to troubleshooting cure issues in pu coatings specifically related to catalyst concentration, encompassing an in-depth discussion of pu chemistry, catalyst types, common problems, diagnostic techniques, and corrective actions.

1. introduction

polyurethane coatings are formed through the reaction of a polyol (containing hydroxyl groups, -oh) with an isocyanate (containing -nco groups). this reaction, ideally yielding a high molecular weight polymer network, is often accelerated by the addition of a catalyst. the catalyst facilitates the urethane reaction (formation of -nh-coo- linkage), influencing the rate and extent of crosslinking. an inadequate or excessive catalyst concentration can disrupt this delicate balance, leading to a spectrum of cure-related defects. this article aims to equip formulators, applicators, and quality control personnel with the knowledge necessary to identify, diagnose, and rectify cure problems stemming from improper catalyst levels in pu coating systems.

2. fundamentals of polyurethane chemistry and catalysis

2.1 polyurethane formation:

the fundamental reaction in pu coating formation is the step-growth polymerization between a polyol and an isocyanate. this reaction is exothermic and proceeds as follows:

r-nco + r'-oh  →  r-nh-coo-r'
isocyanate + polyol → urethane

the reaction rate is influenced by several factors, including temperature, reactivity of the polyol and isocyanate, and the presence of a catalyst.

2.2 role of catalysts in polyurethane coatings:

catalysts in pu coatings serve to accelerate the urethane reaction, leading to faster cure times and improved throughput. they achieve this by lowering the activation energy of the reaction. different catalysts exhibit varying degrees of selectivity towards specific reactions within the pu system, influencing the final properties of the coating.

2.3 common catalyst types:

several classes of catalysts are employed in pu coatings, each with its own advantages and disadvantages:

• tertiary amines: highly effective in accelerating the urethane reaction, but can also promote side reactions such as isocyanate trimerization and allophanate formation. often used in flexible foam applications and some coating formulations.
• organometallic compounds (e.g., tin, bismuth, zinc): generally provide better control over the reaction and are less prone to side reactions compared to tertiary amines. dibutyltin dilaurate (dbtdl) is a common example, although environmental concerns are driving the development of alternatives.
• delayed-action catalysts: designed to be activated under specific conditions (e.g., temperature, moisture), providing longer pot life and improved application characteristics. examples include blocked catalysts and latent catalysts.

table 1: comparison of common polyurethane catalysts

catalyst type	advantages	disadvantages	common applications
tertiary amines	high activity, cost-effective	strong odor, potential for side reactions (trimerization, allophanate formation), yellowing potential	flexible foams, some two-component coatings
organometallics (e.g., sn)	high selectivity, good control over reaction rate, improved mechanical properties	potential toxicity (especially tin), environmental concerns, hydrolysis sensitivity	two-component coatings, elastomers, adhesives
delayed-action catalysts	extended pot life, improved application characteristics	can be more expensive, require specific activation conditions, potential for incomplete activation	high-solids coatings, coatings requiring long pot life, powder coatings

3. common cure issues related to catalyst concentration

deviations from the optimal catalyst concentration range can manifest in various cure-related defects, affecting the coating’s performance and appearance.

3.1 insufficient catalyst concentration:

• slow cure: the most obvious consequence is a prolonged cure time, delaying the application of subsequent coats and increasing production time.
• incomplete cure: the coating may remain tacky or soft even after the expected cure time, indicating insufficient crosslinking.
• poor adhesion: incomplete crosslinking can weaken the coating’s adhesion to the substrate, leading to delamination or blistering.
• reduced chemical resistance: an under-cured coating is more susceptible to chemical attack and solvent damage.
• lower hardness and abrasion resistance: the coating’s mechanical properties, such as hardness and abrasion resistance, will be compromised.

3.2 excessive catalyst concentration:

• rapid cure: while seemingly beneficial, an excessively rapid cure can lead to several problems.
• short pot life: the working time of the coating mixture is significantly reduced, making application difficult.
• bubbles and pinholes: rapid evolution of carbon dioxide (a byproduct of the reaction of isocyanate with water) can lead to bubble formation and pinholes in the coating.
• cracking and embrittlement: excessive crosslinking can result in a brittle coating that is prone to cracking.
• yellowing: some catalysts, particularly tertiary amines, can promote yellowing of the coating, especially upon exposure to uv light.
• surface defects: rapid skinning can occur, trapping solvents and leading to surface irregularities.

table 2: cure issues and corresponding catalyst concentration problems

cure issue	likely catalyst problem	possible causes
slow cure	insufficient catalyst	incorrect catalyst dosage, expired catalyst, presence of inhibitors, low ambient temperature, incorrect mixing ratio of components
incomplete cure	insufficient catalyst	same as above, plus high humidity (isocyanate reacting with water), insufficient mixing
poor adhesion	insufficient catalyst	same as above, plus improper surface preparation, contamination on substrate
reduced chemical resistance	insufficient catalyst	same as above
lower hardness	insufficient catalyst	same as above
rapid cure	excessive catalyst	incorrect catalyst dosage, highly active catalyst, high ambient temperature, moisture contamination
short pot life	excessive catalyst	same as above
bubbles/pinholes	excessive catalyst	same as above, plus high humidity, insufficient degassing
cracking/embrittlement	excessive catalyst	same as above, plus use of incompatible components, inadequate flexibility of the polyol
yellowing	excessive catalyst	use of amine-based catalysts, exposure to uv light
surface defects	excessive catalyst	rapid skinning, solvent entrapment, poor leveling

4. diagnostic techniques for identifying catalyst-related cure issues

a systematic approach is crucial for accurately diagnosing cure problems related to catalyst concentration. the following techniques are commonly employed:

4.1 visual inspection:

• surface appearance: observe the coating for signs of tackiness, uneven gloss, bubbles, pinholes, cracking, or wrinkling.
• color: check for discoloration or yellowing.
• adhesion: perform a simple adhesion test, such as a cross-cut test, to assess the coating’s bond to the substrate.

4.2 touch test:

• tack: gently touch the coating surface to assess its tackiness. a properly cured coating should be tack-free.
• hardness: press a fingernail or a blunt object against the coating surface to evaluate its hardness.

4.3 solvent rub test:

• method: saturate a cotton swab with a specified solvent (e.g., methyl ethyl ketone (mek), acetone) and rub the coating surface a defined number of times (e.g., 50 double rubs).
• assessment: observe the coating for signs of softening, dissolution, or color transfer to the swab. this test provides an indication of the coating’s crosslinking density and solvent resistance.

4.4 differential scanning calorimetry (dsc):

• principle: dsc measures the heat flow associated with transitions in a material as a function of temperature.
• application: dsc can be used to determine the glass transition temperature (tg) of the coating, which is related to the degree of cure. a higher tg generally indicates a more complete cure. it can also identify the presence of unreacted isocyanate or polyol, suggesting incomplete reaction.

4.5 fourier transform infrared spectroscopy (ftir):

• principle: ftir identifies the chemical bonds present in a material by analyzing its absorption of infrared radiation.
• application: ftir can be used to monitor the disappearance of isocyanate (-nco) and hydroxyl (-oh) peaks during the curing process, providing quantitative information about the extent of reaction. it can also identify the formation of urethane linkages (-nh-coo-).

4.6 gel permeation chromatography (gpc):

• principle: gpc separates molecules based on their size.
• application: gpc can be used to determine the molecular weight distribution of the cured coating. a higher molecular weight indicates a more complete crosslinking process.

4.7 titration methods (for unreacted isocyanate):

• principle: quantitative chemical analysis to determine the concentration of remaining isocyanate groups.
• application: useful for determining the degree of cure by measuring the extent of the isocyanate reaction.

table 3: diagnostic techniques and their applications in identifying catalyst-related cure issues

technique	principle	application	information gained
visual inspection	observation of surface characteristics	initial assessment of surface defects, color changes, and adhesion	presence of bubbles, pinholes, cracking, wrinkling, yellowing, and an estimate of adhesion quality
touch test	tactile assessment of surface properties	quick evaluation of tackiness and hardness	indication of cure state (tacky vs. tack-free), relative hardness
solvent rub test	assessment of solvent resistance	evaluation of crosslinking density and solvent resistance	degree of softening, dissolution, or color transfer, indicating the extent of cure and crosslinking
differential scanning calorimetry (dsc)	measurement of heat flow associated with thermal transitions	determination of glass transition temperature (tg) and identification of unreacted components	tg value (higher tg indicates more complete cure), presence of unreacted isocyanate or polyol
fourier transform infrared spectroscopy (ftir)	identification of chemical bonds through infrared absorption	monitoring the disappearance of isocyanate and hydroxyl peaks and the formation of urethane linkages	quantitative information about the extent of reaction, identification of specific chemical groups present
gel permeation chromatography (gpc)	separation of molecules based on size	determination of molecular weight distribution	average molecular weight and distribution, indicating the degree of crosslinking
titration methods (nco)	quantitative chemical analysis of unreacted isocyanate	determining the degree of cure by measuring the remaining isocyanate content	quantitative measurement of unreacted isocyanate, providing a direct indication of the extent of the polyurethane reaction and the completeness of the cure process.

5. corrective actions for catalyst-related cure issues

once the cause of the cure problem has been identified as being related to catalyst concentration, appropriate corrective actions can be implemented.

5.1 insufficient catalyst concentration:

• verify catalyst dosage: carefully check the formulation and ensure that the correct amount of catalyst is being added. use calibrated measuring devices.
• check catalyst activity: ensure that the catalyst has not expired or been contaminated. consider replacing the catalyst with a fresh batch.
• optimize mixing: thoroughly mix the catalyst with the polyol and isocyanate components. ensure that the mixing equipment is functioning correctly.
• increase temperature: increasing the ambient temperature can accelerate the curing process. however, ensure that the temperature does not exceed the recommended limits for the coating system.
• adjust formulation: consult with the raw material suppliers or a coatings expert to explore the possibility of adjusting the formulation to improve cure speed. consider using a more reactive catalyst or increasing the concentration of reactive groups in the polyol or isocyanate.
• consider a co-catalyst: the addition of a co-catalyst can sometimes improve the overall catalytic activity and address issues with slow or incomplete cure. careful selection is needed to avoid adverse side effects.

5.2 excessive catalyst concentration:

• verify catalyst dosage: ensure that the correct amount of catalyst is being added. double-check calculations and measuring devices.
• reduce catalyst concentration: decrease the catalyst concentration to the recommended level.
• control temperature: lowering the ambient temperature can slow n the curing process and extend the pot life.
• adjust formulation: consider using a less reactive catalyst or decreasing the concentration of reactive groups in the polyol or isocyanate.
• use inhibitors: in some cases, adding a small amount of an inhibitor can slow n the reaction and extend the pot life. however, careful selection and dosage are crucial to avoid negatively impacting the final coating properties.
• improve degassing: ensure proper degassing of the coating mixture to remove dissolved gases and prevent bubble formation.

table 4: corrective actions for catalyst-related cure issues

problem	corrective action	rationale
insufficient catalyst	verify catalyst dosage, check catalyst activity, optimize mixing, increase temperature, adjust formulation	ensures correct catalyst level, active catalyst, proper distribution, accelerated reaction, and optimized formulation for curing
excessive catalyst	verify catalyst dosage, reduce catalyst concentration, control temperature, adjust formulation, use inhibitors	ensures correct catalyst level, slowed reaction, extended pot life, and formulation balanced for proper curing and application

6. preventive measures

implementing preventive measures is essential to minimize the occurrence of catalyst-related cure issues.

• strict adherence to formulations: follow the recommended formulation and mixing procedures precisely.
• quality control of raw materials: ensure that all raw materials, including the catalyst, meet the specified quality standards.
• proper storage of catalysts: store catalysts according to the manufacturer’s recommendations to maintain their activity and prevent degradation.
• regular calibration of equipment: calibrate dispensing and mixing equipment regularly to ensure accurate dosage and mixing.
• environmental control: maintain consistent temperature and humidity conditions during application and curing.
• training of personnel: train applicators and quality control personnel on proper mixing, application, and inspection techniques.
• documentation: maintain accurate records of all batches, including raw material lot numbers, mixing procedures, and environmental conditions.

7. case studies (illustrative examples)

while specific case studies would require proprietary information, consider these illustrative examples of how the principles outlined above can be applied:

• case study 1: slow cure in a two-component polyurethane floor coating: a two-component pu floor coating exhibited a significantly longer cure time than specified in the technical data sheet. upon investigation, it was discovered that the catalyst batch had been stored improperly, leading to a loss of activity. replacing the catalyst with a fresh batch resolved the issue.
• case study 2: bubble formation in a high-solids polyurethane coating: a high-solids pu coating exhibited excessive bubble formation during application. the catalyst concentration was found to be slightly above the recommended level. reducing the catalyst concentration and improving degassing procedures eliminated the bubble formation.
• case study 3: cracking in a flexible polyurethane coating: a flexible pu coating developed cracks after a few weeks of service. it was determined that an excessively high concentration of a fast-acting amine catalyst had been used, leading to excessive crosslinking and embrittlement. reformulating with a lower catalyst concentration and a more flexible polyol prevented the cracking issue.

8. regulatory considerations

the use of certain catalysts, particularly organotin compounds, is subject to increasing regulatory scrutiny due to environmental and health concerns. formulators should be aware of and comply with relevant regulations regarding the use, handling, and disposal of these substances. the search for safer and more environmentally friendly alternatives is an ongoing area of research and development.

9. conclusion

catalyst concentration plays a critical role in achieving optimal cure and performance in polyurethane coatings. understanding the fundamentals of pu chemistry, the role of catalysts, and the potential consequences of improper catalyst levels is essential for troubleshooting cure-related issues. by employing a systematic approach to diagnosis and implementing appropriate corrective actions, formulators, applicators, and quality control personnel can minimize defects, improve coating quality, and ensure the long-term durability and performance of pu coatings. furthermore, staying informed about regulatory changes and exploring safer catalyst alternatives are crucial for sustainable coating practices.

literature sources (examples – actual citations would be required in a real publication)

• wicks, d. a., et al. polyurethane coatings: formulation, properties, and applications. wiley-interscience, 2007.
• lambourne, r., and t. a. strivens. paint and surface coatings: theory and practice. 2nd ed., woodhead publishing, 1999.
• ulrich, h. introduction to industrial polymers. 2nd ed., hanser publishers, 1993.
• ashida, k. polyurethane and related foams: chemistry and technology. crc press, 2006.
• european coatings journal, various articles on polyurethane technology and catalysis.
• journal of coatings technology and research, various articles on polyurethane coatings.
• astm standards related to coatings testing and analysis.
• technical data sheets and product information from polyurethane raw material suppliers (e.g., polyol, isocyanate, and catalyst manufacturers).

sales contact：sales@newtopchem.com

troubleshooting cure issues related to polyurethane coating catalyst concentration

abstract: polyurethane (pu) coatings are widely used across diverse industries due to their excellent durability, chemical resistance, and flexibility. however, the curing process, critical to achieving desired coating properties, is highly sensitive to the concentration of the catalyst. this article provides a comprehensive analysis of how deviations from the optimal catalyst concentration can manifest as cure defects, detailing the underlying mechanisms and offering practical troubleshooting strategies. product parameters, literature reviews, and standardized troubleshooting protocols are presented to aid formulators and applicators in achieving consistent and high-quality pu coatings.

1. introduction

polyurethane coatings are formed through the reaction of a polyol (containing hydroxyl groups) and an isocyanate (containing -nco groups). this reaction, while thermodynamically favorable, often requires a catalyst to proceed at a practical rate, particularly at ambient temperatures. catalysts, typically organometallic compounds or tertiary amines, significantly accelerate the curing process by lowering the activation energy of the isocyanate-hydroxyl reaction. the concentration of the catalyst plays a pivotal role in determining the cure speed, final properties, and overall performance of the pu coating. insufficient catalyst leads to slow or incomplete curing, while excessive catalyst can cause rapid, uncontrolled reactions resulting in undesirable side reactions and compromised coating integrity. this article aims to provide a systematic approach to troubleshooting cure issues arising from catalyst concentration imbalances in pu coating formulations.

2. polyurethane chemistry and catalysis

2.1 basic polyurethane reaction:

the fundamental reaction in polyurethane formation is the addition of an isocyanate group (-nco) to a hydroxyl group (-oh) to form a urethane linkage (-nh-coo-). this reaction can be represented as follows:

r-n=c=o + r'-oh  →  r-nh-coo-r'
(isocyanate) + (polyol) → (urethane)

2.2 catalytic mechanisms:

catalysts accelerate the urethane reaction through various mechanisms. the most common types of catalysts used in pu coatings are:

• tertiary amines: tertiary amines act as nucleophilic catalysts. they abstract a proton from the hydroxyl group of the polyol, making it more reactive towards the isocyanate. they also coordinate with the isocyanate, increasing its electrophilicity.

• organometallic compounds: organometallic catalysts, such as dibutyltin dilaurate (dbtdl) or bismuth carboxylates, coordinate with both the hydroxyl and isocyanate groups, forming a ternary complex that facilitates the reaction. these catalysts are generally stronger and more efficient than tertiary amines.

2.3 side reactions:

besides the primary urethane reaction, several side reactions can occur, especially at elevated temperatures or with excessive catalyst concentrations. these include:

• allophanate formation: urethane linkages can react further with isocyanates to form allophanate linkages. this reaction increases crosslinking density and can lead to embrittlement of the coating.

• biuret formation: isocyanates can react with urea linkages (formed from the reaction of isocyanates with water) to form biuret linkages. like allophanates, biurets increase crosslinking density.

• isocyanurate formation: isocyanates can trimerize in the presence of certain catalysts (especially strong bases) to form isocyanurate rings. this reaction also leads to significant crosslinking and can improve thermal stability.

• co2 evolution: isocyanates react with water, yielding an amine and carbon dioxide. this is a significant concern because the evolved co2 can cause foaming and bubbling in the coating.

3. impact of catalyst concentration on cure properties

3.1 insufficient catalyst concentration:

when the catalyst concentration is too low, the curing reaction proceeds slowly, resulting in:

• slow tack-free time: the coating remains tacky for an extended period, increasing the risk of dust contamination and handling issues.
• incomplete cure: the coating may not reach its full hardness, chemical resistance, or abrasion resistance. this can lead to premature failure in service.
• poor adhesion: incomplete crosslinking can weaken the interfacial bond between the coating and the substrate, resulting in poor adhesion.
• low glass transition temperature (tg): incomplete curing translates to a lower tg, diminishing the coating’s high-temperature performance and resistance to deformation.

3.2 excessive catalyst concentration:

excessive catalyst concentration can lead to several detrimental effects:

• rapid gelation: the coating may gel too quickly, resulting in poor flow and leveling, surface imperfections, and air entrapment.
• foaming/bubbling: rapid co2 evolution from the isocyanate-water reaction can cause foaming and bubbling, creating a porous and weakened coating.
• embrittlement: excessive crosslinking due to allophanate, biuret, or isocyanurate formation can lead to a brittle coating with reduced flexibility and impact resistance.
• yellowing: some catalysts, particularly tertiary amines, can contribute to yellowing or discoloration of the coating, especially upon exposure to uv light or heat.
• reduced pot life: the time available to apply the coating after mixing the components is significantly reduced, leading to application difficulties.
• poor sag resistance: if the initial viscosity increase is too rapid, the coating may not develop sufficient sag resistance before gelation, leading to uneven film thickness and runs.

4. troubleshooting cure issues: a systematic approach

the following table outlines a systematic approach to troubleshooting cure issues related to catalyst concentration.

problem	possible cause(s)	troubleshooting steps	preventive measures
slow cure/tackiness	1. insufficient catalyst concentration	1. verify catalyst dosage against manufacturer’s recommendations. 2. increase catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. confirm catalyst activity with titration or reaction rate studies.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation. 3. regularly check the expiration date of catalysts.
	2. catalyst inactivation	1. check for potential inhibitors or contaminants in the formulation (e.g., acids, moisture). 2. ensure proper storage of the catalyst to prevent degradation. 3. switch to a more robust or less sensitive catalyst.	1. use high-quality raw materials with low impurity levels. 2. implement stringent moisture control measures during formulation and application. 3. select catalysts compatible with other additives in the formulation.
	3. incorrect catalyst type	1. verify that the chosen catalyst is appropriate for the specific polyol and isocyanate being used. 2. consider using a blend of catalysts (e.g., a tertiary amine and an organometallic catalyst) to optimize cure properties.	1. consult with the catalyst supplier to select the most suitable catalyst for the specific application. 2. conduct preliminary screening tests with different catalyst types to evaluate their performance.
rapid cure/gelation	1. excessive catalyst concentration	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	2. high temperature	1. monitor and control the temperature of the coating components and the application environment. 2. adjust the catalyst concentration based on temperature. 3. consider using a delayed-action catalyst or a catalyst inhibitor.	1. store coating components in a cool, dry place. 2. avoid applying coatings in direct sunlight or during periods of high ambient temperature. 3. optimize ventilation to minimize heat buildup.
	3. highly reactive components	1. consider using less reactive polyols or isocyanates. 2. adjust the catalyst concentration accordingly.	1. carefully select raw materials with appropriate reactivity profiles. 2. conduct preliminary compatibility tests to ensure that the components are compatible with the chosen catalyst.
foaming/bubbling	1. excessive catalyst concentration (leading to rapid co2 evolution)	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	2. moisture contamination	1. ensure that all components are dry and free of moisture. 2. use moisture scavengers (e.g., molecular sieves) in the formulation. 3. protect the coating from moisture during application.	1. store coating components in sealed containers in a dry environment. 2. use dry solvents and additives. 3. monitor humidity levels during application and take appropriate precautions.
	3. incompatible catalyst	1. select a catalyst that is less prone to promoting the isocyanate-water reaction. 2. consider using a catalyst blend to balance reactivity and co2 evolution.	1. conduct preliminary compatibility tests to ensure that the catalyst does not promote excessive co2 evolution. 2. consult with the catalyst supplier for recommendations on suitable catalysts.
embrittlement	1. excessive catalyst concentration (leading to excessive crosslinking)	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	2. incorrect polyol/isocyanate ratio	1. verify the polyol/isocyanate ratio and adjust as needed. 2. ensure that the nco/oh ratio is within the recommended range for the specific system.	1. use accurate weighing and dispensing equipment for polyol and isocyanate components. 2. regularly check the nco content of the isocyanate component.
	3. use of highly functional polyols/isocyanates	1. consider using polyols or isocyanates with lower functionality to reduce crosslinking density. 2. adjust the catalyst concentration accordingly.	1. carefully select raw materials with appropriate functionality for the desired coating properties. 2. conduct preliminary tests to evaluate the effect of functionality on coating performance.
yellowing/discoloration	1. catalyst type (certain tertiary amines)	1. switch to a non-yellowing catalyst (e.g., an organometallic catalyst or a sterically hindered amine). 2. add uv stabilizers to the formulation.	1. select catalysts that are known to have good color stability. 2. use high-quality raw materials with low color impurities. 3. formulate with uv absorbers and light stabilizers to improve color retention.
	2. excessive catalyst concentration	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	3. exposure to uv light/heat	1. add uv stabilizers and antioxidants to the formulation. 2. protect the coating from direct sunlight or excessive heat.	1. formulate with uv absorbers and light stabilizers to improve color retention. 2. apply coatings in shaded areas or during periods of low sunlight. 3. use reflective coatings to reduce heat absorption.
poor adhesion	1. insufficient catalyst concentration (leading to incomplete cure and weak interfacial bonding)	1. verify catalyst dosage against manufacturer’s recommendations. 2. increase catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. confirm catalyst activity with titration or reaction rate studies.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation. 3. regularly check the expiration date of catalysts.
	2. substrate contamination	1. ensure that the substrate is clean, dry, and free of contaminants (e.g., oil, grease, dust). 2. use appropriate surface preparation techniques (e.g., solvent cleaning, abrasion).	1. implement a rigorous surface preparation procedure before coating application. 2. use compatible cleaning solvents and abrasives.
	3. incompatible coating system	1. select a coating system that is compatible with the substrate. 2. use a primer to improve adhesion.	1. conduct preliminary adhesion tests to ensure compatibility between the coating and the substrate. 2. consult with the coating supplier for recommendations on suitable coating systems.

5. product parameters and specifications

to effectively troubleshoot cure issues, it’s crucial to understand the key product parameters and specifications related to the catalyst and the overall pu coating system. these parameters should be clearly defined and monitored throughout the formulation and application process.

5.1 catalyst parameters:

• chemical composition: the specific chemical identity of the catalyst (e.g., dbtdl, triethylamine, bismuth carboxylate).
• concentration: the recommended concentration range of the catalyst in the coating formulation (typically expressed as a percentage by weight or volume).
• activity: a measure of the catalyst’s ability to accelerate the urethane reaction (can be determined through titration or reaction rate studies).
• viscosity: the viscosity of the catalyst (relevant for liquid catalysts).
• density: the density of the catalyst (relevant for accurate volumetric dispensing).
• solubility: the solubility of the catalyst in the coating solvents and resins.
• storage stability: the shelf life and storage conditions recommended by the manufacturer.
• safety data sheet (sds): provides information on the hazards associated with the catalyst and the necessary safety precautions.

5.2 coating system parameters:

• polyol type and hydroxyl number: the type of polyol used (e.g., polyester polyol, polyether polyol, acrylic polyol) and its hydroxyl number (a measure of the number of hydroxyl groups per unit weight).
• isocyanate type and nco content: the type of isocyanate used (e.g., aliphatic isocyanate, aromatic isocyanate) and its nco content (a measure of the number of isocyanate groups per unit weight).
• nco/oh ratio: the ratio of isocyanate groups to hydroxyl groups in the coating formulation. this ratio is critical for achieving the desired crosslinking density and coating properties.
• solvent type and content: the type and amount of solvents used in the coating formulation. solvents affect viscosity, flow, leveling, and drying time.
• additives: the type and concentration of other additives used in the formulation (e.g., pigments, fillers, uv stabilizers, flow agents).
• viscosity: the viscosity of the mixed coating components.
• pot life: the time available to apply the coating after mixing the components before it becomes too viscous or gels.
• tack-free time: the time required for the coating to become tack-free.
• dry time: the time required for the coating to reach a specified level of hardness.
• hardness: a measure of the coating’s resistance to indentation (typically measured using a pencil hardness test or a durometer).
• adhesion: a measure of the coating’s ability to adhere to the substrate (typically measured using a cross-cut adhesion test).
• chemical resistance: a measure of the coating’s resistance to various chemicals.
• abrasion resistance: a measure of the coating’s resistance to abrasion.
• gloss: the specular reflectance of the coating surface.
• color: the color of the coating.

6. instrumentation and testing methods

several instruments and testing methods are used to assess the cure properties of pu coatings and to troubleshoot cure-related issues.

• viscometers: used to measure the viscosity of the coating components and the mixed coating system. viscosity measurements can be used to monitor the progress of the curing reaction and to assess the pot life of the coating.
• differential scanning calorimetry (dsc): used to measure the heat flow associated with the curing reaction. dsc can be used to determine the glass transition temperature (tg) of the cured coating and to assess the degree of cure.
• dynamic mechanical analysis (dma): used to measure the viscoelastic properties of the cured coating. dma can be used to determine the tg, storage modulus, and loss modulus of the coating, which are related to its stiffness, damping characteristics, and temperature dependence.
• fourier transform infrared spectroscopy (ftir): used to identify the chemical bonds present in the coating. ftir can be used to monitor the disappearance of isocyanate groups and the formation of urethane linkages during the curing reaction.
• pencil hardness test: a simple test used to assess the hardness of the coating. a series of pencils with increasing hardness values are used to scratch the coating surface. the hardness of the pencil that just scratches the coating is recorded.
• cross-cut adhesion test: a test used to assess the adhesion of the coating to the substrate. a series of cuts are made in the coating, forming a grid pattern. adhesive tape is then applied to the grid and pulled off. the amount of coating that is removed with the tape is used to assess the adhesion.
• chemical resistance tests: tests used to assess the resistance of the coating to various chemicals. the coating is exposed to different chemicals for a specified period of time, and the changes in appearance, hardness, and adhesion are evaluated.
• abrasion resistance tests: tests used to assess the resistance of the coating to abrasion. the coating is subjected to abrasion using a specified abrasive material, and the amount of material removed is measured.

7. case studies

(due to space constraints, detailed case studies are omitted. however, examples include:

• a case study on amine blush caused by high humidity and tertiary amine catalyst.
• a case study on blistering caused by excessive dbtdl catalyst.
• a case study on poor scratch resistance caused by insufficient catalyst loading.)

8. conclusion

achieving optimal cure in polyurethane coatings is paramount for realizing their full potential in terms of durability, protection, and aesthetics. catalyst concentration plays a critical role in controlling the curing process, and deviations from the recommended levels can lead to a range of defects. by understanding the underlying chemistry, recognizing the symptoms of catalyst-related cure issues, and implementing a systematic troubleshooting approach, formulators and applicators can effectively address these challenges and produce high-quality pu coatings. regular monitoring of product parameters, adherence to established quality control procedures, and continuous learning from practical experience are essential for ensuring consistent and reliable coating performance. careful selection, handling, and dosage of catalysts are vital components of a successful polyurethane coating application.

9. literature cited

• wicks, z. w., jones, f. n., & rosthauser, j. w. (1999). organic coatings: science and technology (2nd ed.). wiley-interscience.
• lambourne, r., & strivens, t. a. (1999). paint and surface coatings: theory and practice (2nd ed.). woodhead publishing.
• ulrich, h. (1996). introduction to industrial polymers (2nd ed.). hanser gardner publications.
• ashida, k. (2006). polyurethane and related foams: chemistry and technology (2nd ed.). crc press.
• oertel, g. (ed.). (1985). polyurethane handbook: chemistry – raw materials – processing – application – properties. hanser gardner publications.
• randall, d., & lee, s. (2003). the polyurethanes book. john wiley & sons.
• hepburn, c. (1991). polyurethane elastomers. elsevier science publishers.
• woods, g. (1990). the ici polyurethanes book (2nd ed.). john wiley & sons.
• kresta, j. e. (ed.). (1993). polyurethane dispersions. american chemical society.
• prime, r. b. (2014). thermal analysis of polymers: fundamentals and applications. john wiley & sons.

sales contact：sales@newtopchem.com

troubleshooting cure issues related to polyurethane coating catalyst concentration

abstract: polyurethane (pu) coatings are widely used across diverse industries due to their excellent durability, chemical resistance, and flexibility. however, the curing process, critical to achieving desired coating properties, is highly sensitive to the concentration of the catalyst. this article provides a comprehensive analysis of how deviations from the optimal catalyst concentration can manifest as cure defects, detailing the underlying mechanisms and offering practical troubleshooting strategies. product parameters, literature reviews, and standardized troubleshooting protocols are presented to aid formulators and applicators in achieving consistent and high-quality pu coatings.

1. introduction

polyurethane coatings are formed through the reaction of a polyol (containing hydroxyl groups) and an isocyanate (containing -nco groups). this reaction, while thermodynamically favorable, often requires a catalyst to proceed at a practical rate, particularly at ambient temperatures. catalysts, typically organometallic compounds or tertiary amines, significantly accelerate the curing process by lowering the activation energy of the isocyanate-hydroxyl reaction. the concentration of the catalyst plays a pivotal role in determining the cure speed, final properties, and overall performance of the pu coating. insufficient catalyst leads to slow or incomplete curing, while excessive catalyst can cause rapid, uncontrolled reactions resulting in undesirable side reactions and compromised coating integrity. this article aims to provide a systematic approach to troubleshooting cure issues arising from catalyst concentration imbalances in pu coating formulations.

2. polyurethane chemistry and catalysis

2.1 basic polyurethane reaction:

the fundamental reaction in polyurethane formation is the addition of an isocyanate group (-nco) to a hydroxyl group (-oh) to form a urethane linkage (-nh-coo-). this reaction can be represented as follows:

r-n=c=o + r'-oh  →  r-nh-coo-r'
(isocyanate) + (polyol) → (urethane)

2.2 catalytic mechanisms:

catalysts accelerate the urethane reaction through various mechanisms. the most common types of catalysts used in pu coatings are:

• tertiary amines: tertiary amines act as nucleophilic catalysts. they abstract a proton from the hydroxyl group of the polyol, making it more reactive towards the isocyanate. they also coordinate with the isocyanate, increasing its electrophilicity.

• organometallic compounds: organometallic catalysts, such as dibutyltin dilaurate (dbtdl) or bismuth carboxylates, coordinate with both the hydroxyl and isocyanate groups, forming a ternary complex that facilitates the reaction. these catalysts are generally stronger and more efficient than tertiary amines.

2.3 side reactions:

besides the primary urethane reaction, several side reactions can occur, especially at elevated temperatures or with excessive catalyst concentrations. these include:

• allophanate formation: urethane linkages can react further with isocyanates to form allophanate linkages. this reaction increases crosslinking density and can lead to embrittlement of the coating.

• biuret formation: isocyanates can react with urea linkages (formed from the reaction of isocyanates with water) to form biuret linkages. like allophanates, biurets increase crosslinking density.

• isocyanurate formation: isocyanates can trimerize in the presence of certain catalysts (especially strong bases) to form isocyanurate rings. this reaction also leads to significant crosslinking and can improve thermal stability.

• co2 evolution: isocyanates react with water, yielding an amine and carbon dioxide. this is a significant concern because the evolved co2 can cause foaming and bubbling in the coating.

3. impact of catalyst concentration on cure properties

3.1 insufficient catalyst concentration:

when the catalyst concentration is too low, the curing reaction proceeds slowly, resulting in:

• slow tack-free time: the coating remains tacky for an extended period, increasing the risk of dust contamination and handling issues.
• incomplete cure: the coating may not reach its full hardness, chemical resistance, or abrasion resistance. this can lead to premature failure in service.
• poor adhesion: incomplete crosslinking can weaken the interfacial bond between the coating and the substrate, resulting in poor adhesion.
• low glass transition temperature (tg): incomplete curing translates to a lower tg, diminishing the coating’s high-temperature performance and resistance to deformation.

3.2 excessive catalyst concentration:

excessive catalyst concentration can lead to several detrimental effects:

• rapid gelation: the coating may gel too quickly, resulting in poor flow and leveling, surface imperfections, and air entrapment.
• foaming/bubbling: rapid co2 evolution from the isocyanate-water reaction can cause foaming and bubbling, creating a porous and weakened coating.
• embrittlement: excessive crosslinking due to allophanate, biuret, or isocyanurate formation can lead to a brittle coating with reduced flexibility and impact resistance.
• yellowing: some catalysts, particularly tertiary amines, can contribute to yellowing or discoloration of the coating, especially upon exposure to uv light or heat.
• reduced pot life: the time available to apply the coating after mixing the components is significantly reduced, leading to application difficulties.
• poor sag resistance: if the initial viscosity increase is too rapid, the coating may not develop sufficient sag resistance before gelation, leading to uneven film thickness and runs.

4. troubleshooting cure issues: a systematic approach

the following table outlines a systematic approach to troubleshooting cure issues related to catalyst concentration.

problem	possible cause(s)	troubleshooting steps	preventive measures
slow cure/tackiness	1. insufficient catalyst concentration	1. verify catalyst dosage against manufacturer’s recommendations. 2. increase catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. confirm catalyst activity with titration or reaction rate studies.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation. 3. regularly check the expiration date of catalysts.
	2. catalyst inactivation	1. check for potential inhibitors or contaminants in the formulation (e.g., acids, moisture). 2. ensure proper storage of the catalyst to prevent degradation. 3. switch to a more robust or less sensitive catalyst.	1. use high-quality raw materials with low impurity levels. 2. implement stringent moisture control measures during formulation and application. 3. select catalysts compatible with other additives in the formulation.
	3. incorrect catalyst type	1. verify that the chosen catalyst is appropriate for the specific polyol and isocyanate being used. 2. consider using a blend of catalysts (e.g., a tertiary amine and an organometallic catalyst) to optimize cure properties.	1. consult with the catalyst supplier to select the most suitable catalyst for the specific application. 2. conduct preliminary screening tests with different catalyst types to evaluate their performance.
rapid cure/gelation	1. excessive catalyst concentration	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	2. high temperature	1. monitor and control the temperature of the coating components and the application environment. 2. adjust the catalyst concentration based on temperature. 3. consider using a delayed-action catalyst or a catalyst inhibitor.	1. store coating components in a cool, dry place. 2. avoid applying coatings in direct sunlight or during periods of high ambient temperature. 3. optimize ventilation to minimize heat buildup.
	3. highly reactive components	1. consider using less reactive polyols or isocyanates. 2. adjust the catalyst concentration accordingly.	1. carefully select raw materials with appropriate reactivity profiles. 2. conduct preliminary compatibility tests to ensure that the components are compatible with the chosen catalyst.
foaming/bubbling	1. excessive catalyst concentration (leading to rapid co2 evolution)	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	2. moisture contamination	1. ensure that all components are dry and free of moisture. 2. use moisture scavengers (e.g., molecular sieves) in the formulation. 3. protect the coating from moisture during application.	1. store coating components in sealed containers in a dry environment. 2. use dry solvents and additives. 3. monitor humidity levels during application and take appropriate precautions.
	3. incompatible catalyst	1. select a catalyst that is less prone to promoting the isocyanate-water reaction. 2. consider using a catalyst blend to balance reactivity and co2 evolution.	1. conduct preliminary compatibility tests to ensure that the catalyst does not promote excessive co2 evolution. 2. consult with the catalyst supplier for recommendations on suitable catalysts.
embrittlement	1. excessive catalyst concentration (leading to excessive crosslinking)	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	2. incorrect polyol/isocyanate ratio	1. verify the polyol/isocyanate ratio and adjust as needed. 2. ensure that the nco/oh ratio is within the recommended range for the specific system.	1. use accurate weighing and dispensing equipment for polyol and isocyanate components. 2. regularly check the nco content of the isocyanate component.
	3. use of highly functional polyols/isocyanates	1. consider using polyols or isocyanates with lower functionality to reduce crosslinking density. 2. adjust the catalyst concentration accordingly.	1. carefully select raw materials with appropriate functionality for the desired coating properties. 2. conduct preliminary tests to evaluate the effect of functionality on coating performance.
yellowing/discoloration	1. catalyst type (certain tertiary amines)	1. switch to a non-yellowing catalyst (e.g., an organometallic catalyst or a sterically hindered amine). 2. add uv stabilizers to the formulation.	1. select catalysts that are known to have good color stability. 2. use high-quality raw materials with low color impurities. 3. formulate with uv absorbers and light stabilizers to improve color retention.
	2. excessive catalyst concentration	1. verify catalyst dosage against manufacturer’s recommendations. 2. reduce catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. calibrate dispensing equipment.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation.
	3. exposure to uv light/heat	1. add uv stabilizers and antioxidants to the formulation. 2. protect the coating from direct sunlight or excessive heat.	1. formulate with uv absorbers and light stabilizers to improve color retention. 2. apply coatings in shaded areas or during periods of low sunlight. 3. use reflective coatings to reduce heat absorption.
poor adhesion	1. insufficient catalyst concentration (leading to incomplete cure and weak interfacial bonding)	1. verify catalyst dosage against manufacturer’s recommendations. 2. increase catalyst concentration incrementally (e.g., 10% increments), observing the effect on cure time and properties. 3. confirm catalyst activity with titration or reaction rate studies.	1. use calibrated dispensing equipment for accurate catalyst addition. 2. implement a quality control procedure to verify catalyst concentration in the coating formulation. 3. regularly check the expiration date of catalysts.
	2. substrate contamination	1. ensure that the substrate is clean, dry, and free of contaminants (e.g., oil, grease, dust). 2. use appropriate surface preparation techniques (e.g., solvent cleaning, abrasion).	1. implement a rigorous surface preparation procedure before coating application. 2. use compatible cleaning solvents and abrasives.
	3. incompatible coating system	1. select a coating system that is compatible with the substrate. 2. use a primer to improve adhesion.	1. conduct preliminary adhesion tests to ensure compatibility between the coating and the substrate. 2. consult with the coating supplier for recommendations on suitable coating systems.

5. product parameters and specifications

to effectively troubleshoot cure issues, it’s crucial to understand the key product parameters and specifications related to the catalyst and the overall pu coating system. these parameters should be clearly defined and monitored throughout the formulation and application process.

5.1 catalyst parameters:

• chemical composition: the specific chemical identity of the catalyst (e.g., dbtdl, triethylamine, bismuth carboxylate).
• concentration: the recommended concentration range of the catalyst in the coating formulation (typically expressed as a percentage by weight or volume).
• activity: a measure of the catalyst’s ability to accelerate the urethane reaction (can be determined through titration or reaction rate studies).
• viscosity: the viscosity of the catalyst (relevant for liquid catalysts).
• density: the density of the catalyst (relevant for accurate volumetric dispensing).
• solubility: the solubility of the catalyst in the coating solvents and resins.
• storage stability: the shelf life and storage conditions recommended by the manufacturer.
• safety data sheet (sds): provides information on the hazards associated with the catalyst and the necessary safety precautions.

5.2 coating system parameters:

• polyol type and hydroxyl number: the type of polyol used (e.g., polyester polyol, polyether polyol, acrylic polyol) and its hydroxyl number (a measure of the number of hydroxyl groups per unit weight).
• isocyanate type and nco content: the type of isocyanate used (e.g., aliphatic isocyanate, aromatic isocyanate) and its nco content (a measure of the number of isocyanate groups per unit weight).
• nco/oh ratio: the ratio of isocyanate groups to hydroxyl groups in the coating formulation. this ratio is critical for achieving the desired crosslinking density and coating properties.
• solvent type and content: the type and amount of solvents used in the coating formulation. solvents affect viscosity, flow, leveling, and drying time.
• additives: the type and concentration of other additives used in the formulation (e.g., pigments, fillers, uv stabilizers, flow agents).
• viscosity: the viscosity of the mixed coating components.
• pot life: the time available to apply the coating after mixing the components before it becomes too viscous or gels.
• tack-free time: the time required for the coating to become tack-free.
• dry time: the time required for the coating to reach a specified level of hardness.
• hardness: a measure of the coating’s resistance to indentation (typically measured using a pencil hardness test or a durometer).
• adhesion: a measure of the coating’s ability to adhere to the substrate (typically measured using a cross-cut adhesion test).
• chemical resistance: a measure of the coating’s resistance to various chemicals.
• abrasion resistance: a measure of the coating’s resistance to abrasion.
• gloss: the specular reflectance of the coating surface.
• color: the color of the coating.

6. instrumentation and testing methods

several instruments and testing methods are used to assess the cure properties of pu coatings and to troubleshoot cure-related issues.

• viscometers: used to measure the viscosity of the coating components and the mixed coating system. viscosity measurements can be used to monitor the progress of the curing reaction and to assess the pot life of the coating.
• differential scanning calorimetry (dsc): used to measure the heat flow associated with the curing reaction. dsc can be used to determine the glass transition temperature (tg) of the cured coating and to assess the degree of cure.
• dynamic mechanical analysis (dma): used to measure the viscoelastic properties of the cured coating. dma can be used to determine the tg, storage modulus, and loss modulus of the coating, which are related to its stiffness, damping characteristics, and temperature dependence.
• fourier transform infrared spectroscopy (ftir): used to identify the chemical bonds present in the coating. ftir can be used to monitor the disappearance of isocyanate groups and the formation of urethane linkages during the curing reaction.
• pencil hardness test: a simple test used to assess the hardness of the coating. a series of pencils with increasing hardness values are used to scratch the coating surface. the hardness of the pencil that just scratches the coating is recorded.
• cross-cut adhesion test: a test used to assess the adhesion of the coating to the substrate. a series of cuts are made in the coating, forming a grid pattern. adhesive tape is then applied to the grid and pulled off. the amount of coating that is removed with the tape is used to assess the adhesion.
• chemical resistance tests: tests used to assess the resistance of the coating to various chemicals. the coating is exposed to different chemicals for a specified period of time, and the changes in appearance, hardness, and adhesion are evaluated.
• abrasion resistance tests: tests used to assess the resistance of the coating to abrasion. the coating is subjected to abrasion using a specified abrasive material, and the amount of material removed is measured.

7. case studies

(due to space constraints, detailed case studies are omitted. however, examples include:

• a case study on amine blush caused by high humidity and tertiary amine catalyst.
• a case study on blistering caused by excessive dbtdl catalyst.
• a case study on poor scratch resistance caused by insufficient catalyst loading.)

8. conclusion

achieving optimal cure in polyurethane coatings is paramount for realizing their full potential in terms of durability, protection, and aesthetics. catalyst concentration plays a critical role in controlling the curing process, and deviations from the recommended levels can lead to a range of defects. by understanding the underlying chemistry, recognizing the symptoms of catalyst-related cure issues, and implementing a systematic troubleshooting approach, formulators and applicators can effectively address these challenges and produce high-quality pu coatings. regular monitoring of product parameters, adherence to established quality control procedures, and continuous learning from practical experience are essential for ensuring consistent and reliable coating performance. careful selection, handling, and dosage of catalysts are vital components of a successful polyurethane coating application.

9. literature cited

• wicks, z. w., jones, f. n., & rosthauser, j. w. (1999). organic coatings: science and technology (2nd ed.). wiley-interscience.
• lambourne, r., & strivens, t. a. (1999). paint and surface coatings: theory and practice (2nd ed.). woodhead publishing.
• ulrich, h. (1996). introduction to industrial polymers (2nd ed.). hanser gardner publications.
• ashida, k. (2006). polyurethane and related foams: chemistry and technology (2nd ed.). crc press.
• oertel, g. (ed.). (1985). polyurethane handbook: chemistry – raw materials – processing – application – properties. hanser gardner publications.
• randall, d., & lee, s. (2003). the polyurethanes book. john wiley & sons.
• hepburn, c. (1991). polyurethane elastomers. elsevier science publishers.
• woods, g. (1990). the ici polyurethanes book (2nd ed.). john wiley & sons.
• kresta, j. e. (ed.). (1993). polyurethane dispersions. american chemical society.
• prime, r. b. (2014). thermal analysis of polymers: fundamentals and applications. john wiley & sons.

sales contact：sales@newtopchem.com