The quality control of pipe fusion-bonded epoxy powder coating should be done like this!

Abstract: In view of the key performance indicators of pipeline fusion epoxy powder coatings, such as thermal characteristics, density, particle size distribution, volatile matter, gel time, curing time, adhesion, impact resistance, bending resistance, etc.

Keywords: pipeline; fusion bonded epoxy powder; standard; quality control

0 Preface

Since the early 1970s, fusion-bonded epoxy powder coatings have developed into a recognized 4E type coating product that meets high production efficiency, excellent coating performance, eco-friendly and economical [1] coating online coatingol.

Based on relevant standards at home and abroad, this paper has conducted a comparative study on the key performance indicators of epoxy powder coatings for pipelines, and analyzed various factors affecting these key performance indicators, and proposed improvement measures to further improve the fusion bond epoxy powder coatings.

1 Domestic and foreign standards

1.

CASZ245.

The Canadian standard CASZ245.

1.

SY/T0315-2013 "Technical Specification for Fusion Bonded Epoxy Powder Coatings for Steel Pipelines" stipulates the design, construction and inspection requirements for single-layer and double-layer structure fusion bonded epoxy powder coatings for steel pipelines

SY/T0442-2018 "Technical Standard for Fusion Bonded Epoxy Powder Internal Coatings for Steel Pipelines" stipulates the design, production and inspection requirements of fusion bonded epoxy powder internal anticorrosive coatings

2 Key performance index analysis and quality control

2.

The thermal characteristics index requirements of fusion bonded epoxy powder coatings at home and abroad are shown in Table 1

Regarding thermal characteristics, the foreign standards CSAZ245.

It is worth mentioning that CSAZ245.
20-2018 is consistent with SY/T0315-2013 in terms of heating speed; the requirements in CSAZ245.
20-2018 are more detailed, and the location of the Tg point for powder and coating is different
.
SY/T0315-2013 takes the Tg starting point for the Tg point, while CSAZ245.
20-2018 takes the Tg intermediate point
.
Different versions of CSAZ245.
20 have different regulations on Differential Scanning Calorimetry (DSC), but the requirements are becoming more reasonable and closer to the actual production
.

When the operating temperature is higher than Tg, the adhesion and penetration resistance of the coating will decrease, leading to the failure of the FBE coating, causing problems such as adhesion failure, coating blistering, coating peeling, etc.
, and increasing the risk of corrosion [4]
.

The thermal properties of fusion bonded epoxy powder are also related to the epoxy resin Tg.
The higher the epoxy resin Tg, the higher the powder Tg
.
Certain pigments, fillers and bulking agents can also improve the thermal properties of powder coatings
.
Therefore, manufacturers can improve the thermal properties of powder coatings by selecting varieties with relatively high resin Tg, appropriately increasing the quality percentage of pigments and fillers, and adding bulking agents [5-6]
.

2.
2 Density

The density index requirements of fusion bonded epoxy powder coatings at home and abroad are shown in Table 2
.

Regarding the density, foreign standards generally stipulate that it meets the specifications of the powder manufacturer
.
The domestic standard is generally specified as 1.
3~1.
5g/cm3
.
The epoxy powder density has a greater impact on the thickness of the epoxy coating during the production process
.
If the density of the epoxy powder is low, the amount of powder sprayed by the spray gun per unit time (measured by volume) will increase, and the thickness of the epoxy coating will be too high.
Due to the electrostatic repulsion generated in the electrostatic spraying process, it will cause the surface of the epoxy coating to develop Honeycomb, after baking and curing, the coating will produce many pits or unevenness.
Generally, the coating thickness of electrostatic powder spraying is less than 150μm
.
The high density of epoxy powder will reduce the amount of powder sprayed by the spray gun per unit time, and the thickness of the epoxy coating will be low, reducing the anti-corrosion effect, and it is also easy to cause leakage
.
The thickness of the coating that produces ionization rejection in powder spraying is also affected by the variety of powder coating resin, the composition of the powder coating, the coating voltage, and the material to be coated.
It needs to be determined in conjunction with actual tests and production operations
.
Domestic standards stipulate the density of powder coatings, the main reason is to prevent some manufacturers from shoddy manufacturing, affecting the quality of powder coatings
.
The factor that has a greater impact on the density is the resin content in the epoxy powder [7]
.
Therefore, the manufacturer should improve the density of epoxy powder by adjusting the resin content
.

2.
3 Particle size distribution

The particle size distribution index requirements of fusion bonded epoxy powder coatings at home and abroad are shown in Table 3
.

The particle size of epoxy powder coating refers to the average particle size of irregularly shaped particles.
The powder particles produced by air classification mill (ACM mill) have different sizes, showing a distribution, that is, particle size distribution
.
For particle size distribution, domestic and foreign standards generally stipulate that the powder on the 150μm sieve is ≤3%, and the powder on the 250μm sieve is ≤0.
2%
.
For today's epoxy powder production technology, such specifications are a bit too broad
.
The particle size distribution has a great influence on the chargeability, powder loading rate, stability, leveling of the coating, coating thickness and texture size of the epoxy powder.
It is an important technical index that affects the coating performance of the powder coating [8]
.
How to control the particle size distribution of powder coatings, make the maximum particle size as small as possible, and control the content of ultrafine powder is of great significance to the production process
.
Whether the particle size distribution of the powder can meet the requirements is mainly affected by the fine crushing, grading and sieving equipment.
These three are a set of integrated linkage devices
.
The main equipment of this system is air classifier, screw feeder, cyclone separator, powder sifter, bag filter, rotary valve, induced draft fan and pulse oscillator
.
After analyzing and comparing the relationship between the speed of the main mill motor, the rotating speed of the classifying rotor (secondary mill) and the particle size distribution of the powder coating, choose a more appropriate speed of the main mill of the powder machine and the speed of the classifying rotor (secondary mill) to get More reasonable particle size distribution
.

2.
4 Volatile matter

The volatile index requirements of fusion bonded epoxy powder coatings at home and abroad are shown in Table 4
.

For the volatile index, CSAZ245.
20-2018 stipulates two methods for determining the volatile content: (1) Titration, the maximum volatile content is 0.
5%; (2) The mass loss method, the maximum volatile content is 0.
6%
.
Among them, the requirement of volatile content ≤ 0.
6% is consistent with the domestic standard, but the way of expression is different
.
General powder coatings are solid powdery substances that do not contain volatile organic solvents and water, and are environmentally friendly coatings
.
However, it still inevitably contains a small amount of volatile components with low relative molecular weight, which leads to frequent smog during the extrusion production and spray baking process of powder coatings, which brings adverse effects to on-site operators and the environment.
[9]
.
Almost all the fumes with pungent odors released during the baking process of powder coatings are derived from raw materials, including resin film-forming substances, curing agents, additives, pigments and fillers
.
The solution can be started from two aspects: "blocking" and "sparse"
.
For the "blocking" aspect, you can consider choosing special materials with adsorption or decomposition properties and adding them to the powder formulation to adsorb or decompose volatile substances
.
The characteristics of this type of material are porosity, micronization, good heat resistance, and no interference with coating performance
.
The solution to the "sparse" aspect is to install vents and collectors on the powder coating extruder to reduce the impact of volatile components on operators and the environment
.
The paint shop should strengthen convection ventilation, strengthen the collection and treatment of volatile gases, and reduce the concentration of smoke
.

2.
5 Gel time

The index requirements for gelation time of fusion bonded epoxy powder coatings at home and abroad are shown in Table 5
.

Gelation time refers to the total time required for the powder coating to melt into a liquid state at the curing temperature to cross-link and solidify, and the coating cannot be drawn into filaments [10]
.
The gel time is related to the formula of the epoxy powder and directly reflects the curing speed of the epoxy powder
.
Domestic and foreign standards generally require the gelation time to meet the range given by the powder manufacturer or within ±20% of the value given by the manufacturer.
SY/T0315-2013 also specifically requires the gelation time to be ≤30s and meet the range given by the powder manufacturer.
This is mainly related to actual production
.
For epoxy powder, if the gel time is too short, the epoxy powder will not have time to wet on the surface of the steel pipe substrate, which will lead to a decrease in leveling and gloss, and even pinholes and other defects in severe cases
.
The gelation time is too long, and the coating cannot be fully cured within the time required by the customer, resulting in a decrease in coating adhesion, impact resistance and other properties
.
Therefore, the gel time is a very important indicator in the production process
.
For the coating of 3PE and 3PP anticorrosive coatings, before the epoxy powder coating is completely gelled, use an extruder to squeeze the acrylic adhesive to make the adhesive and the epoxy powder coating fuse together well , In order to ensure a good bonding force between the two coatings
.

Temperature is a key factor affecting the gelation time of powder [11]
.
Generally speaking, the higher the temperature, the faster the curing reaction of the powder coating and the shorter the gel time
.
In the production process of powder coatings, the most important processes that affect the change of the gel time are hot mixing and extrusion processes
.
In addition, the type of curing agent is also directly related to the gel time
.
Therefore, in the production process, the temperature and time can be controlled to ensure the proper gelation time of the powder coating, and curing accelerators can be added when necessary
.

2.
6 Curing time

The curing time index requirements of domestic and foreign fusion bonded epoxy powder coatings are shown in Table 6
.

The curing time of powder coatings has an important impact on the production efficiency of pipe coating manufacturers.
Domestic and foreign standards generally require the curing time to meet the range given by the powder manufacturer or the value given by the manufacturer ±20%.
GB/T23257-2017 requires epoxy Powder curing time ≤3min, SY/T0315-2013 requires epoxy powder curing time ≤2min, and meets the range given by the powder manufacturer.
This is mainly related to the production speed of steel pipes
.
The faster the curing time, the faster the production speed .
Appropriate curing conditions are the key to ensuring various performance indicators of epoxy powder coating
.
Generally, epoxy powder manufacturers provide the curing temperature and time of epoxy powder when they provide products.
Under this temperature and time requirements, the curing rate of epoxy powder can reach more than 95%
.
It can be seen that the curing rate of epoxy powder is related to curing temperature and time
.

In the powder coating outer coating production line, the best curing section is the distance between the powder coating spraying room and the water cooling room [12]
.
In the production process of external anticorrosion of steel pipe, the curing time of powder coating is equal to the ratio of curing distance to speed.
The distance of powder coating curing is determined by the production line and is a fixed value
.
Therefore, the curing time of the powder coating is inversely proportional to the linear travel speed of the steel pipe along the production line.
The faster the speed, the shorter the curing time
.
Under normal circumstances, the faster the production speed, the higher the heating temperature of the steel pipe should be, so as to achieve the best curing effect
.
In addition, a satisfactory curing rate can be achieved by increasing the distance of the epoxy powder curing section, which requires readjustment of the operating line
.

2.
7 Adhesion

The adhesion index requirements of fusion bonded epoxy powder coating at home and abroad are shown in Table 7
.

According to domestic and foreign standards, the measurement cycle of adhesion is generally 24h, 48h, 28d, and the index is generally 1-3
.
SY/T0442-2018 also stipulates the use of the pull method to measure the adhesion of the coating
.
The short-term adhesion is to quickly detect the adhesion of online coatings and epoxy powders, and 28d is used as a type test to control and distinguish the quality of epoxy powder coatings and epoxy powder coatings
.
The boiling temperature in the adhesion test is determined according to the operating temperature of the pipeline
.
The higher the temperature and the longer the time, the higher the requirements for epoxy powder coatings [13]
.

Coating adhesion is affected by many factors, including substrate surface treatment, coating process and coating formulations [14]
.
When the substrate is pretreated, the grease and dirt on the surface of the steel pipe should be removed first, and the steel pipe should be preheated and then surface treated
.
The surface pretreatment should reach Sa2.
5 level, and the depth of anchor pattern should reach 50~75μm
.
Increasing the spraying temperature is conducive to the improvement of coating adhesion
.
In powder coating formulation design, epoxy powder coatings have the best adhesion, followed by epoxy-polyester powder coatings, polyester powder coatings, and polyurethane and acrylic powder coatings
.
The degree of complete curing of the coating also has a great influence on the adhesion of the coating.
If the coating is not fully cured, the adhesion, impact resistance and bending resistance of the coating are not good.
You can adjust the gelation of the powder coating.
Time or adjust the curing process to ensure that the coating is completely cured
.

2.
8 Anti-bending performance

The requirements for bending resistance of fusion bonded epoxy powder coatings at home and abroad are shown in Table 8
.

According to the environmental conditions and temperature requirements of pipeline laying, domestic and foreign standards stipulate the bending resistance test at three temperatures of 0 ℃, -20 ℃, and -30 ℃
.
The specific temperature difference is mainly caused by the inconsistent construction environment and operating conditions at home and abroad.
The standard user should choose the appropriate standard based on the actual situation
.

The bending resistance of powder coatings is mainly related to the formulation
.
The following aspects can be considered to improve the bending resistance of the coating
.
The first is the choice of resin.
The choice of structure is flexible, relative molecular weight is high, and resins with low epoxy value, hydroxyl value and acid value of reactive groups are beneficial to improve the bending resistance of the coating
.
The second is the choice of curing agent.
Choosing a curing agent with a flexible long-chain structure is beneficial to improve the bending resistance of the coating
.
At the same time, when the curing reaction between the resin and the curing agent is complete, the bending resistance of the coating is good.
Therefore, choosing a system with high reaction activity of the powder coating is conducive to the complete curing of the coating, and the bending resistance of the coating will also be good
.
In addition, when designing powder coating formulations, it is possible to shorten the gelation time of the powder coating, add a curing reaction accelerator or adjust the curing process to ensure that the coating is fully cured to improve the bending resistance of the coating [15]
.

2.
9 Impact resistance

The impact resistance index requirements of fusion bonded epoxy powder coating at home and abroad are shown in Table 9
.

The pipeline coating will inevitably encounter collisions during shipping, processing and installation, and its resistance to mechanical damage is determined by its impact resistance
.
This requires epoxy powder coating to have better impact resistance under external impact
.
The impact resistance of the coating is related to its adhesion and hardness
.
At the same time, the environmental temperature has a great influence on the impact resistance of the coating.
The lower the temperature, the worse the impact resistance of the coating
.
Therefore, domestic and foreign standards generally stipulate that the impact resistance test should be carried out at room temperature or -30°C
.

The impact resistance of the coating has a lot to do with the formulation
.
In order to improve the impact resistance of the coating, a resin with strong reactivity and high epoxy value, acid value and hydroxyl value of reactive groups should be selected
.
The curing reaction between this kind of resin and the curing agent is more complete, the crosslinking density of the film-forming material is higher, and the impact resistance of the corresponding coating is also higher
.
In addition, when the Tg of the coating is high, the hardness of the coating will increase correspondingly, but it is easy to cause the impact resistance of the coating to decrease.
Relatively speaking, when the Tg of the coating is low, it is conducive to the improvement of the impact resistance of the coating.
Cause the powder coating storage performance to decline
.
Therefore, Tg needs to be comprehensively considered when selecting resins.
In addition, the appropriate addition of plasticizers, tougheners and other additives to improve the impact resistance of the coating, or the addition of thermoplastic resins, can also have certain effects [15]
.

3 Conclusion

The key performance indicators of pipe fusion bonded epoxy powder coatings include thermal characteristics, density, particle size distribution, adhesion, impact resistance, bending resistance, etc.
The relevant standards at home and abroad have different requirements for these indicators, and pipe fusion ring The formulation design and processing technology of oxygen powder coating have a great influence on these indexes
.
The powder coating manufacturer should control the quality of the product through reasonable formulation design and improvement of the processing technology, thereby improving the overall quality of the pipeline coating
.