Production of coating powders and particle size of the powder

Introduction

Many aluminum producers, for example, aluminum profiles, or have their own equipment for powder coating, or color them "on the side". В обоих случаях у них возникают вопросы:

  • Что такое «хорошая порошковая краска»?
  • Что такое «хороший производитель порошковой краски»?
  • How to distinguish coating defects, caused by poor paint quality, from those defects, причиной которых является нарушение технология окрашивания?

Below is an overview of a typical powder paint technology.. Critical production stages highlighted, which especially affect the quality of powder paint produced. The role of the distribution of particle size of the powder on the quality of the powder coating is shown..

Exact composition, rigorous technology and close monitoring

General technology scheme

Технология изготовления промышленных порошковых красок состоит из четырех четких этапов [1, 2] (рисунок 1):

  • Weighing the original components of the paint, their pre-mixing and grinding.
  • Melting and extruding the resulting mass, extrudate cooling and coarse crushing.
  • Grinding the crushed extrudate into powder.
  • Добавление компонентов в готовый в порошок (при необходимости), packaging and storage.


Figure 1 – Схема типичной технологии производства порошковой краски [1]

At each stage, the quality of the intermediate product is subject to strict control., because, when powder paint is completely made, it cannot be corrected or corrected in any way. Quality of raw materials, their accuracy and strict adherence to technology are very important for obtaining high-quality powder paint.

Weighing, mixing and grinding

Process

Raw materials typically include resins., hardeners, pigments, fillers and additives, for example, to improve fluidity and facilitate degassing. Each source material undergoes its individual input quality control..

The proportion of each component is carefully weighed with the required degree of accuracy.. For some components, this accuracy may be 0,0001 г [2]. All components, measured in quantity according to the recipe for this brand of paint, loaded into pre-mix container – миксер (рисунок 2).


Figure 2 – Weigh and load
исходных материалов [2]

Then this container is installed on the mixer.. Here, all the starting materials are thoroughly mixed using special cutting blades for a given amount of time.. In this case, all source materials are crushed to the maximum, чтобы на следующих этапах облегчить перемешивание расплава (рисунок 3) [2].

Figure 3 – Предварительное смешивание исходных материалов [2]

Quality control

A sample of this “batch” of raw materials is checked for uniformity and passed through small laboratory extruders and millstones.. The resulting powder is then applied to the control panel., отверждают полученное покрытие в печи и подвергают его следующим испытаниям:

  • Colour, surface spreading and gloss
  • Mechanical properties
  • Длительность отверждения (полимеризации)

If any corrections are required based on the results of these tests, then the mixing process and quality control are repeated until, until the powder obtained meets the established technical requirements.

After this stage of production, no further powder composition modifications are already possible..

Extruding, cooling and crushing

Process

Next, the mixture of raw materials is fed using a dosing system to the extruder. В экструдере поддерживается заданная температура (от 70 by 120 ºС в зависимости от типа краски). This is to ensure, чтобы смола в достаточной мере расплавилась хорошо перемешалась с другими компонентами при прохождении червячного винта экструдера (рисунок 4).

Figure 4 – Extruding, охлаждение и дробление [2]

As a result, at the exit from the extruder, all components are crushed and wetted with resin to form a homogeneous composite. The dispenser feed rate and the screw speed are balanced in this way., to ensure full loading of the extruder. This ensures good mixing of all paint components..

After the extruder, the resulting molten mass is fed to a conveyor cooler for cooling.. Затем материал в виде плоских кусков поступает на дробилку (рисунок 4), where it is broken and crushed into “chips” of size convenient for grinding them at the next production stage 5-10 mm (figure 5).


Figure 4 – Полуфабрикат порошковой краски после экструдера [3]


Figure 5 – Грубо дробленый экструдат порошковой краски [3]

Quality control

At this stage, test the samples obtained "chips". On a laboratory millstone they are crushed into powder. This powder is then applied to the control panel.. Полученное покрытие проверяют на качество по следующим критериям:

  • Colour, gloss, appearance and fluidity
  • Mechanical and chemical properties
  • Electrostatic ability

Too high temperature in the extruder will not only result in low melt viscosity, low shear forces and insufficient pigment dispersion, but will also cause low gloss powder coating. The resin and hardener in the mixture may also begin to react in the extruder, что окажет вредное влияние на характеристики порошковой краски [2].

At this stage of production it is no longer possible to make changes in the composition of the paint. Cheaper to send off-the-shelf extruded chips for recycling now, than later – уже готовый порошок после перемалывания этих чипсов [2].

Grinding the extrudate into finished powder

Process

Дробленный экструдат («чипсы») загружают в агрегат, which is called a mill. In it, the extrudate is ground into powder. Чтобы получить оптимальное распределение размера частиц (РРЧ) порошок проходит несколько этапов обработки: циклонирование, separation, filtering and screening. In modern plants, sieved particles that are too large are automatically sent back to the feed in the grinding unit.

Figure 6 – Размалывание дробленого экструдата в готовый порошок [2]

Quality control

Finished powder paint is subject to strict quality control., to make sure, that it meets all established requirements, including, specified requirements for RFC.

Packaging and storage

Process

In order to meet the special requirements of the customer or the special conditions for the use of powder coatings, additives may be introduced at this stage, which are mixed in an already prepared powder.

Порошок может нормально сохраняться в нераспакованной промышленной упаковке (картонной коробке, мешке или контейнере) в сухом, прохладном месте (30 ºС) до 12 months. Higher temperatures and longer storage may result in moisture absorption.. Storage conditions may vary for some paints., therefore, always follow the directions on the paint packaging, as well as in a passport or certificate.

Quality control

It is recommended to check the paint after 6 months of storage, to make sure, that she does not have any quality problems.

Standard particles, small and large

Standard particles

These are particles, which the paint manufacturer wants to pack in boxes and send to the customer. The size range of these particles is very strictly controlled.. Standard particles provide the formation of a powder coating with the highest possible quality and confident fulfillment of all requirements for mechanical and other properties.

Fine particles

These are very small particles., which may cause staining problems, such as a defect "frame", when a thicker coating forms at the edge of the product. Fine powder particles settle on the product at a different speed, than standard particles, especially on the side and back surfaces of the product, resulting in excessive variation in coating thickness. Excessive amounts of fine particles in powder paint can also lead to clogging of guns and hoses., as small particles liquefy more easily, чем стандартные частицы [2].

Large particles

These particles are too large., to get through the last separation screen. Some manufacturers are re-grinding these large particles., to get good powder out of them, some of them just throw them away. it happens, that these large particles are mistaken or due to malfunctioning separation equipment in the box to be sent to the customer. With electrostatic spraying, they will not be able to get the necessary electric charge., as standard particles usually do, and most likely just fall to the bottom of the spray booth. This leads to a decrease in the efficiency of the primary spraying of the powder on the product. Moreover, seen, что чрезмерное количество крупных частиц в порошковой краске может приводить к осыпанию порошка на некоторых участках поверхности изделия в ходе его перемещений по окрасочной линии с образованием пятен с пониженной толщиной покрытия [2].

Powder paint in box

Некоторое количество мелких и крупных частиц порошка могут попадать в коробку с краской независимо от применяемой технологии изготовления порошковой краски (рисунок 7). Каждый производитель порошковой краски имеет специальную машину для анализа распределения размеров частиц (РРЧ). This machine tests paint samples and shows how large the difference is between powder particle sizes.. There should not be an excessive amount of large or small particles in a normal powder box, which can lead to problems for powder coating.

Figure 7 - Typical type of powder particles
в различных порошковых красках [4]

На что влияет размер частиц?

Следующие этапы процесса нанесения являются особенно чувствительными к распределению размеров частиц порошка [2]:

  • Powder transport by coating system
  • Electrostatic charging of powder particles and their penetration into "Faraday cages"
  • The formation of a uniform cloud of powder in the spray booth
  • Deposition and growth rate of coating thickness on the surface of the product
  • Primary Powder Efficiency

Thermosetting powders are applied electrostatically to products.. Therefore, powder particles must be able to charge in an electrostatic field or as a result of friction with a gun. This charge should be sufficient, to attract particles to the surface and edges of the product. However, sedimentation of the first powder particles should not lead to electrical insulation of the surface, так это будет препятствовать формированию покрытия заданной толщины [2].

The fluidity of the powder must comply with all internal and external requirements for its transportation. Particle size to a large extent determines the ability of the powder to flow freely.. This fluidity depends not only on the powder material, но также от формы и размера частиц [2].

Most industrial powder paints have a particle size of 10 by 100 m. For the electrostatic method of powder spraying, powder particles with sizes ranging from 15 by 75 m, and ideal particles - from 25 by 45 мкм [6].

In the drawing graph 8 typical RFC for industrial pigmented epoxy powder paint presented. The graph shows, that some particles are slightly larger, and some are slightly smaller than the average particle size. The sharper the peak of this curve, the more stable the staining process and the more uniform the powder coating itself.
Figure 8 – Типичное распределение размеров частиц порошковой краски [7]

conclusion

Powder paint quality, including, particle size distribution of the finished powder, largely determines the quality of the powder coating. На качество порошковой краски влияет большое количество производственных факторов: качество исходных материалов, precision technology, state of technological equipment, effective quality control.

See also The quality control of coating powders: indicators, methods, standards - Guide

Sources:

  1. The Review of Powder Coatings //Journal of Materials Science and Chemical Engineering, 2016, 4, 54-59
  2. Complete Guide to Powder Coatings – Akzo Nobel – 1999
  3. TCI Powder Coatings Manufacturing – 2020
  4. What Coaters Need To Know About Powder Coating Particle Size – 2017
  5. TCI Powder Coating Troubleshooting Guide – 2020
  6. Pатент №US8377346B2 – Akzo Nobel – 2013.
  7. Powder Coatings: Chemistry and Technology / Emmanouil Spyrou - 2012.