Flux Paints and Pastes – Part 2


Part: 1: Flux Paints and Pastes – Part 1

Flux and Braze Paste Characteristics

The purpose of flux and/or braze pastes is to provide an initial or supplemental volume of flux and/or filler metal in the form of a viscous liquid at or near the interface of two components:

  • Folded tube seams
  • Tube to header joints
  • Connector tube to manifolds
  • End caps
  • Blocks and fittings
  • Flame braze joints


Flux and braze pastes exhibit pseudoplastic viscosity, meaning to have shear thinning behavior. As the shear stress is increased, the viscosity decreases, but the relationship is not linear. When the shear stress decreases, the viscosity increases, again non-linearly. Other factors affecting viscosity are solids content and temperature. Generally speaking, as the temperature increases, the viscosity decreases. Conversely, as the solids content increases, the viscosity also increases. These dependences illustrate the importance of quoting the parameters under which the viscosity is measured.

The graph below shows the effect of temperature on viscosity:


Note that at each temperature, a double curve is shown representing two sets of measurements: one curve shows the decrease in viscosity as the shear rate is increased and the other curve shows the increase in viscosity as the shear rate is decreased. This mirrored behavior as the shear rate is increased or decreased perfectly illustrates the pseudoplastic behavior.

The following curves show both: – the effect of temperature, and – solids content on viscosity:

These properties allow a bead of flux paste to be dispensed on the surface of a folded tube consistently and continuously at line speeds ranging from 30 m/min to 120 m/min.

Influence of Carrier on Volatilization Behavior

Since the carrier is a main ingredient, the carrier burn off temperature must be considered.

Below are 3 TGA curves showing burn-off temperature of 3 different paste formulations:

Glycol based carrier

Glycol based carrier – burn off at 180°C @ 10°C/min


Acrylate based carrier

Acrylate based carrier – burn off at 400°C @ 10°C/min

Polybutene based carrier

Polybutene based carrier – burn off at 425°C @ 10°C/min


Delivery Systems and Recommendations

When designing a flux delivery system, choosing the right delivery pump is just as important. Below is a list of pros and cons for the associated pumps.

Diaphragm Pump

  • Low cost, low maintenance
  • High flow
  • Generally used for low viscosity fluids
  • Pulsation, viscosity sensitive
  • Poor flow metering

Air Over Liquid (Pressure Vessel)

  • Low cost, low maintenance
  • Temperature sensitive
  • Viscosity sensitive
  • Poor flow metering

Rotary/Gear Pump

  • High viscous liquids
  • Metered dosing
  • Pulsation
  • Does not handle solids

Rotor-Stator Pumps

  • High viscous liquids
  • No pulsation
  • Precision metering
  • Not viscosity sensitive
  • Highest cost

Rotor-stator pumps are the pumps of choice because of their ability to deliver a constant volume of material without pulsation over a range of flow rates. The best example of this is in the localized dispensing of a continuous bead/stripe in folded tube mills where speeds can range from 20 m/min to 120 m/min.

In lower demanding applications where precision dispensing and flow control is not so critical, for example in tube to header fluxing, the lower cost options (air over liquid, diaphragm pumps) are more than adequate.

Flux Paints and Pastes – Part 1


Concept and Definitions

In addition to the standard methods of applying flux to heat exchanger components (wet fluxing and dry fluxing), there is an increasing trend to using sophisticated flux formulations for selective pre-fluxing of components and/or localized fluxing of complicated geometries. The driving force behind this trend is multi-faceted: heat exchanger manufacturers are seeking to out-source flux application, to partially or completely eliminate certain process elements (fluxer, degreaser) and the movement away from seam-welded and extruded tubes to folded tube technology.

Before describing flux pastes and paints in more detail, a few definitions are noteworthy:

  • Flux Paint: Mixture of various powders mixed with a binder which is applied to as substrate in a thin layer. The coating is then converted to a solid film during a subsequent drying (curing) operation thereby adhering to the substrate.
  • Paste: Any mixture of various powders mixed with a carrier. Generally used for application where flux and/or alloy is required for a target location on a heat exchanger assembly or component. The viscosity is adapted to fit the application.
  • Binder: Complex organic compounds that upon curing, reacts to provide adhesion of flux particles to the coated surface.
  • Additives: Organic or inorganic substances used to modify the rheological properties of a fluid.
  • Curing: Drying of the flux painted parts usually with hot air (150°C). Liquid carrier, i.e., water and/or organic solvent(s) evaporate and the binder reacts to provide adhesion.
  • Adhesion: Qualitative or quantitative measure by which bonding strength of the flux particles to the coated surface is determined.
  • De-binding: Process of binder removal from the painted surface either in air or in the furnace atmosphere by the treatment with high temperature.
  • Viscosity: a measure of resistance to gradual deformation by shear stress, corresponding to the informal concept of thickness.
  • Thermogravimetric Analysis (TGA): A technique in which the mass of a substance is monitored as a function of temperature or time as the sample specimen is subjected to a controlled temperature program in a controlled atmosphere.

Paint Flux Characteristics – Viscosity

Viscosity is an essential parameter for the paint flux and is used to determine a suitable application process. A change in viscosity usually requires a change in the design of the application technique or equipment. All paste and paints used in fluxing are non-Newtonian fluids, meaning that the correlation between the applied shear stress and the shear rate is not linear. Many liquids, including paints are typical shear thinning fluids whose viscosities decreases non-linearly with shear stress. When providing a viscosity value, the methodology, specific shear rate and temperature must be provided.


Settling Behavior

Flux powder has very low solubility in water and organic solvent based paint mixtures. During storage, the solid flux particles will eventually settle out, causing a separation of solids and liquid/carrier. The rate of settling and the ease for remixing is therefore an important practical characteristic. The photo below shows an example of different settling behaviors:

Note that the higher value of the settled volume at left means a slower process of settling.

Note that the higher value of the settled volume at left means a slower process of settling.

The settling rate can be affected by several parameters such as binder concentration, flux solids content, storage time and temperature. To ensure complete homogeneity prior to use, a thorough remixing is necessary. Just shaking the container is usually not sufficient. It is recommended to use of a gyroscopic mixer which rotates the container around two perpendicular axes resulting in intensive material flow. These shear forces ensure optimal mixing without affecting the structure of the material.


The degree of paint flux adhesion varies depending on the heat exchanger manufacturer’s requirements. If paint fluxing is performed off-site and the material needs to be transported over long distances, a higher degree of adhesion is required than if the material is coated in house and simply needs to be transported from one station to the next. The degree of adhesion is typically controlled by the binder concentration in the formulation.

While there are standard methods for measuring adhesion (ASTM D 3359 Standard Methods for Measuring Adhesion by Tape Test), some paint flux users have developed their own in-house methods. The advantage of employing standard methods for measuring adhesion allows for a higher degree of inter-laboratory precision and comparison.

Binder Removal

For successful brazing of paint fluxed aluminum components, the binder must be removed before reaching brazing temperature. In the production process, the paint flux carrier is removed immediately after coating in the dry-off / curing operation. When paint fluxed components are put into the brazing line, the increasing temperature is then responsible for decomposing and removing the binder by evaporation. The temperature range at which the binder is removed is determined by Thermogravimetric Analysis (TGA). With this technique, a simulated braze cycle is used to determine at which temperature the binder is removed. The TGA curves below shows the de-binding temperature for a typical paint flux formulation.

gf TGA air nitrogen
Note that whether in air or nitrogen, the binder removal temperature is in the range of 250°C to 450°C. This means that in this case, at least part of the binder will be removed in the brazing furnace. In continuous tunnel furnaces, this is not an issue since the binder evaporation products will be swept away by the counter flow of nitrogen. In semi-continuous or batch type furnaces, the potential influence of binder removal on equipment must be individually considered depending on each brazing line design. In most semi-continuous or batch type furnaces, binder removal takes place during the drying or preheating step in the presence of air – at temperatures below 400°C (to avoid formation of high-temperature oxides).

Special consideration must be given to paint fluxed components which are not boldly exposed to the furnace atmosphere. These areas are usually enclosed spaces such as inside manifolds, sandwiched evaporator plates and turbulators for charge air coolers. In these cases, de-binding products may remain trapped in the enclosed spaces and result in discoloration and black carbon residue deposits. In these cases, it may better to sacrifice some adhesion (lower binder concentration) in order to ensure adequate binder removal.

Paint Flux Machines

Paint Flux MachinesAs the trend towards paint fluxing has increased, so has the sophistication of the paint flux machines. The industry has seen the evolution of paint fluxing from simple hand held paint sprayers, to semi-automatic machines incorporating a degreasing chamber, a paint flux spray chamber and drying/curing chamber. Today, the most sophisticated flux paint spray machines can be fully automated and fully integrated from the stamping operation through to core assembly. Machines with conveyor widths of 1500 mm, conveyor speeds of greater than 3.5 meters/minute which can spray top and bottom and be fully integrated with stamping and assembly are not uncommon. An example of such a machine is shown below:


To be continued…

Flux Paints and Pastes – Part 2

NOCOLOK® Binder, Thickener, System Binder and NOCOLOK® Flux plus Binder Mixture

1. Preparation and Application

This article provides information about the application of binder systems for NOCOLOK Flux.

Solvay offers three concepts for flux binder application:

  • NOCOLOK Binder (water-soluble) / NOCOLOK Thickener (water-soluble)
  • NOCOLOK System Binder (water-based)
  • NOCOLOK Flux plus Binder Mixture (water-based)

These products can be used in water-based NOCOLOK Flux slurries to improve flux particle adhesion. This is of particular interest for fluxing of pre-formed components prior to assembly in order to reduce flux fall-off and dust formation. Binders are also helpful to pre-coat certain areas with specific flux loads. All binder mixtures can be applied on external and internal surfaces.

During the brazing cycle, these binders will completely evaporate (mostly between 350 and 400°C). When used as described below, there will neither be detrimental interactions between the binder and the flux, nor between the binder and the aluminum surfaces. Trials have shown that even at four-times the standard flux load with a binder mixture there was still no surface discoloration after brazing.

2. General Comments

The surface areas to be coated with binder mixtures must be free of lubricants, oils, dirt, and dust. Means of application include spraying, dipping, and brushing.

All NOCOLOK Flux binder mixtures can be applied by spraying with a suitable spray gun (1.4 mm – 1.6 mm) at approximately 3 – 5 bar pressure.

The surface temperature should be at least 10°C.

When binders are used for flux application, the recommended flux load for good brazing results is the same as it is for the standard process (i.e. between 3 – 5 g/m2). The thickness of the binder coating is usually between 10 – 30 μm.

Drying can be done in air – requiring approximately 15 – 20 minutes at room temperature for the coating surface – and 50 – 60 minutes before the parts can be handled. Oven and forced convection drying is feasible too: at 50 – 80 °C, parts will dry within 5 – 20 minutes.

Please refer to the MSDS for detailed information regarding the safe handling of the product.

3. Preparation of Binder Mixtures

For all binder concepts and preparations, the mixtures should be prepared or opened immediately before consumption.

To prepare a mixture free of agglomerates and to achieve best coating results, the following procedures must be observed for either binder concepts:

  • NOCOLOK Binder / NOCOLOK Thickener
    • 45 parts (wt%) de-ionized water (as used for preparing standard flux slurries) is mixed thoroughly with
    • 15 parts (wt%) NOCOLOK Binder (water-soluble) and
    • 5 parts (wt%) NOCOLOK Thickener (water-soluble).
    • Once the first three components are completely homogenized,
    • 35 parts (wt%) NOCOLOK Flux powder are added successively under continuous agitation.
  • NOCOLOK System Binder
    • NOCOLOK System Binder (water-based) already contains the binder and thickener component as well as water. Consequently, only NOCOLOK Flux powder must be added.
    • 65 parts (wt%) NOCOLOK System Binder (water-based) plus
    • 35 parts (wt%) NOCOLOK Flux.
  • NOCOLOK Flux plus Binder Mixture
    • NOCOLOK Flux plus Binder Mixture (water-based) is a ready-for-use preparation containing NOCOLOK Binder, NOCOLOK Thickener and NOCOLOK Flux powder.

If necessary, the mixtures can be passed through a sieve prior to use. This will remove any potential agglomerates.

Prior to use the flux powder in the mixture must be suspended. The thickener will prevent the flux powder from settling too fast, however, when stored for some time or diluted, the mixture must be well shaken before spraying.

The binder component is activated by oxygen from air. Once sprayed and dried, the product cannot be recycled or reused.

Any remaining flux / binder mixture should be stored in an airtight and sealed container. We recommend consuming the mixtures within one week after mixing.

4. Additional Information:

  • NOCOLOK Binder, -Thickener, and –System Binder are compatible for standard NOCOLOK Flux, -LM Flux, -Cs Flux, and -Sil Flux. They are not suitable for NOCOLOK CB Flux and -Zn Flux due to chemical reactions between these fluxes and the ingredients.
  • The formulations (mixing ratios) provided in Solvay’s technical information sheets and brochures are intended as general recommendations – They provide the best basis for automated spray application and have been tested with good brazing results.
  • The recipes can be adjusted to specific application needs by changing the mixing ratios within certain ranges.
  • A well balanced ratio of binder and thickener to flux in the mixtures is important for good brazing performance:
  • Higher binder ratios result in a harder coating layer and stronger flux adhesion. But they require more care for the binder removal step.
  • Very high binder and/ or thickener ratios increase the organic content in the mixture – which may result in carbon residues (discoloration) after brazing.
  • It is possible to reduce and/ or to increase the water content of the mixtures – resulting in higher respectively lower viscosity.
  • Water dilution will cause less wetting action and reduced adhesion.
  • A surfactant (wetting agent) is part of the binder formulation – providing uniform coating, and – compensating (to some extent) for surfaces not cleaned prior to application.
  • Thickener is used for adjusting the viscosity and to keep the flux powder longer in suspension – This provides better performance in spray application. Nevertheless, formulations can be prepared and used without the addition of thickener.
  • Cleaning before binder-based flux application is recommended – but not mandatory.
  • A clean surface can be coated more easily and the flux adhesion will be better.
  • Residual oils and lubricants are reducing binder activity and require higher flux load.
  • Higher surfactant levels can compensate for some contamination – but result in more foaming.

5. Binder Flux Mixing Ratios

  • The standard composition is 35% NOCOLOK Flux, 15% NOCOLOK Binder, 5% NOCOLOK Thickener and 45% water. If a product with lower flux ratio is wanted (i.e., with only 10% flux), the composition must be modified. Right now, we are proposing 10% flux, 8% binder, 2% thickener and 80% water. There is only limited experience with this composition, and we are a little concerned. The reasons for our concerns are as follows:
    • With 35% flux, the ratio of flux to binder/ thickener on the surface of the headers is sufficient to combat the effects of the high organic content. Also, 15% binder has reasonable adhesion characteristics.
    • At 10% flux, the ratio of flux to binder/ thickener must be modified; otherwise there may not be sufficient flux to combat the high organic content. This is why we propose to reduce the binder and thickener to 8% and 2%, respectively. In other words, too much binder/ thickener and not enough flux may lead to black deposits on the headers after brazing and/ or difficulties in brazing.

6. Warehousing Considerations and Shelf Life

  • Under standard storage conditions, the shelf life is up to 12 months and probably longer. Standard storage conditions means that the product was stored at less than 30°C, as suggested in the MSDS.
  • The binder product can be stored at a temperature higher than 30°C, but the shelf life will shorten due to premature aging. Therefore, we recommend that the binder products be consumed within six months, if the storage temperature is a constant 40°C. This is not based on experimental data, but on general knowledge of water based polymer systems and adhesives. Any product stored at a temperature higher than 40°C should be consumed more quickly.
  • Under no circumstances should the binder products, in their original packaging, be exposed to a temperature of 60°C or above. We suspect that polymerization will occur, agglomerates will form and the performance will drop.

7. Thermo-Gravimetric Analysis (TGA) for Binder Flux

Please refer to the flyer “NOCOLOK Flux Application with Binders”.

8. Recommendations for Reducing Costs

  • Is not possible to only mix the binder, thickener and flux and just add the water on site. Without the water, the flux/ thickener/ binder mixture forms a rubbery-like substance that is very difficult to work with.
  • The least expensive option is to purchase the binder and thickener separately and do mixing of all ingredients on site. The most convenient option is to have a ready-mix, ready-to-use product supplied.
  • Please see above for additional recommendations for mixing.