Aluminium flakes in AAC

Carlfors Bruk started producing bronze pigment in 1898 at Huskvarna, Sweden. The bronze pigments were produced by stamping machines and it was used as imitation gold pigments. Ever since the first AAC factory was built Carlfors Bruk has been developing and manufacturing aluminium pigments for the AAC industry, first for the Swedish market and later worldwide.

The AAC was a breakthrough process by Swedish Dr. Johan Axel Eriksson in 1920s and the first factory was built by Yxhult AB, in Hällabrottet, Sweden in 1929 for the production of AAC. The Swedish AAC industry contacted Carlfors Bruk at a very early stage to see if aluminum pigments of suitable quality and quantity could be produced for the AAC industry. At the time aluminium pigments were produced by the same stamping machines as used in bronze pigment processes which meant that the quality of the pigments was low when compared to today’s products.

Production of aluminium powder at Carlfors increased as more AAC factories were built in Sweden and the quality of aluminium was vastly improved as a result of good working relationship with AAC manufactures in Sweden. Carlfors also gained more and more knowledge of the functionalities of aluminium in the production of AAC.

In the 1950s, Carlfors introduced and developed ball milling method in the production of aluminium powder for AAC, the new process replaced the old stamping bronze machines. With this new process, Carlfors could deliver finer and thinner aluminum flakes to meet the market’s needs to produce lower density AAC.

Today Carlfors Bruk is one of the leading manufacturers of aluminium powders and pastes for AAC. A unique combination of knowledge, expertise, and research facilities between aluminum and AAC experts leads to many interesting and specialized projects.

One of these interesting projects is evaluating the influence of particle size d50 and Blaine value of aluminium powder in AAC. Particle size d50 is measured with laser diffraction analysis and based on Fraunhofer diffraction theory. d50 particle size of the flake is often an important factor when AAC producers are asking for samples of aluminium powders or pastes whether it is for a new quality or just to source a counter type for existing product already in use.

Blaine value or “Blaine air permeability test” is a method used to measure the fineness of powders such as cement, milled sand and aluminium powder to name a few. The test is based on the fact that the rate at which air can pass through a porous bed of particles under a given pressure gradient is a function of the surface area of the powder.

It is known that the different particle size analyzers from different manufacturers give different values even when measuring the same sample. Results can also be different when using same instrument type of different production versions within the same model line. Differences in the handling of sample, preparation and dispersing media also influence the results. Samples can also provide different values depending on if they are tested dry or in water, alcohol, white spirit or other solvents.

The growing focus on particle size d50 and its influence on AAC has been discussed between aluminium and AAC experts at Carlfors. The discussion led to this project where four powder samples with the similar d50 particle size have been produced from four separate milling processes.

Sample preparation

Milling process used for the trials was a wet milling process in a ball mill containing steel balls and lifter bars. White spirit (WS) was used as solvent and a lubricant for helping aluminium flake milling process was added. Four aluminium powder samples were obtained using the same raw materials (atomized powder), same ball mill, and manufacturing process procedures.

To be able to vary the specific surfaces, different concentrations of solvent (WS), different speed on the ball mill rotation and different size of steel balls were used. After reaching desired d50 values the ball mill was emptied, each of the four samples were then pressed into filter cakes and dried to powder form. Lastly the agglomerations or the off-specification powders were discarded through a 250μm dry sieve processes.

Milling procedures

Sample 1: This sample has been milled according to one of Carlfors standard original manufacturing procedures. The milling balls are 6 to 8 mm diameter.

Sample 2: The milling conditions are the same as in sample 1 but with less amount of WS solvent, the amount of WS solvent was reduced to 50%. The process conditions provide a rougher milling and earlier breakage of the flake thus producing coarser and much thicker flakes.

Sample 3: The milling conditions are exactly the same as in sample 1. In a standard cornflake milling process the atomized powder is hammered out by steel balls and creating flakes with increasing particle size. At a certain point the flakes starts to break down to smaller particle size decreasing d50 values. The ball milling process continues until the desirable particle size is achieved. In sample 3 the ball mill was stopped on the preferred d50 value before the flakes started to break and the ball mill was emptied. In this way a flake with a very great thickness was created.

Sample 4: The milling conditions are kept the same as in sample 1 but with smaller size of steel balls, the size of the balls is 4mm. As a result, the milling time become much longer and is more gentle with smaller steel balls, the flakes became much thinner than previous 3 samples.

Four different powder samples with similar d50 values were produced, the d50 values in the four samples are not identical but are close enough to continue the project.

How will these different samples perform in hydrogen evolution test, rising curve, compressive strength, and pore structure?

Hydrogen Evolution Test

A specific amount of aluminium powder was reacted with Ca(OH)2 in a glass flask at 25ºC. The amount of hydrogen created was measured versus reacting time in minutes.

Results from the hydrogen evolution curves show four different curves with big variation, even though they have similar d50 values. Blaine values seem to correlate well to hydrogen evolution test data.

Lab Castings

Lab castings were made in Carlfors Bruks own AAC laboratory. All the casting raw materials, mixing procedures, and process temperature (around 42ºC) implementations are the same as per standard operating procedures. Dry density of castings is approximately 500 kg/m³. The rising curve graphs show that Blaine value have a big impact on rising speed. Sample 4 has the fastest rising speed and the highest Blaine value. Sample 3 has the slowest rising speed and the lowest Blaine value.

Compressive Strength

The castings were cured in an autoclave for 10 hours at 10.5 bar and then tested for compressive strength (see table 2).

The data also showed that Blaine value have an impact on compressive strength in these trials.

Pore Structure

The cured casting samples were cut open for pore structure inspections.

Sample 1 and 2 have similar structure and sample 4 has the smallest pores. Sample 3 has the biggest pores and it also has the lowest Blaine value.

The data in table 3 show among all the particle size measuring test methods, the Blaine test values have better accuracy representing the performing and final casting results in this project. The results of hydrogen rising curve, casting heights, compressive strength and pore structure are closely following the trend of Blaine test values.

The project started with discussions between AAC and aluminium expert groups at Carlfors Bruk regarding the growing focus on d50 values. In the authors´ work with customer contacts the importance of right d50 values when sending out samples or matching existing grades has grown the last couple of years. In this project the authors have tried to show that d50 values are not the only parameter to take into concern but also other factors influence the performance of Al powders/pastes in AAC.

After reviewing the data from the four milling samples it becomes clear that d50 value alone does not give a full explanation for how a powder or paste will react in a casting. d50 value is a very useful tool for monitoring production and assuring that particles in a given production type remains the same over time, any disturbance in production is quickly revealed by d50 measurements. When comparing different manufacturers however, particle size measurements may vary due to differences in production methods.

Blaine value alone does unfortunately not give a complete picture either. Simply focusing on the highest possible Blaine value is not the answer since that may cause other issues when the rising speed becomes faster as more surface of aluminium is available. Even though d50 is the first requested value, a combination of different tests is usually required in order to find the best possible particles for each process. Making small castings in a laboratory gives a good indication but full-scale trials in a factory are necessary to see how local conditions influence the rising behavior. Upon request these services can be provided in cooperation with customers.

Carlfors Bruk AB

Carlforsvägen, Västra Fabriken

56138 Huskvarna, Sweden

T +46 3638 9500

cb@carlfors.se, www.carlfors.se