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2-Pack mixing techniques

2-Pack mixing techniques

Correct mixing of 2-pack coatings prior to application is an essential step in the whole painting process. Many coating failures can be traced back to inadequate/incorrect mixing.

Mixing generally

All coatings are made of resins, pigments, additives and solvent (which may be organis or water). The heavier pigments often sink to the bottom of the container and must be re-dispersed by good mixing prior to use. Sometimes the resin or solvent can float to the top (a phenomenon called syneresis). This clear liquid is an essential part of the paint formula and must never be poured off - it must be fully re-incorporated before use.

Mixing of 2-packs

There is an additional reason for correct mixing when it comes to 2-pack coatings. The standard 2-pack kit comprises a Part A and a Part B; one part contains the reactive resin, whilst the other contains the curing agent or 'hardener'.

[Some kits in fact comprise multiple packs; reactive colour packs and accelerators are sometimes supplied in addition to the resin and hardener packs.] We ensure that the ratio of resin to hardener is in the correct proportion to achieve complete reaction between the two when properly mixed.

Think of it at a microscopic level; each molecule requires intimate mixing with its reactive counterpart to react. The speed and efficiency of mixing using a power mixer ensures that, on a molecular level, the Part A and the Part B are intimately mixed, and that the reaction can occur correctly and uniformly throughout the mix.

If mixing is not thorough enough, there may be areas that are richer in either Part A or Part B, resulting in incorrect mixing ratios in these areas. This is sometimes called 'off-ration' mixing. Off-ratio mixing may result in any or all of the following:

•  Patchy, inconsistent gloss, and generally lower then specified on the data sheet
•  Inconsistent hardness, areas that are too soft or too brittle (depending on the type of product)
•  Significantly lower hardness than specified on data sheet
•  Coating failure, such as delamination
•  Film defects such as blooming
•  Stickiness or uncured, un-reacted paint.

What is induction time?

Some coatings also require an induction time to allow the curing reaction to proceed efficiently and correctly. The induction time is a period of time in which the freshly mixed material is allowed to stand before application.

And herein lies and additional problem with using inadequate mixing tools (such as paddle stirrers, potato mashers or shakers); the theoretical lenght of time required to mix the materials thoroughly would certainly exceed the induction time of the product, which means that the curing reaction would commence well before mixing is complete!

This is particularly true of solvent-free coatings.

Procedure for mixing of 2-pack coatings

The mixing of single-pack coatings can ususally be adequately achieved by the use of flat, paddle-like stirrers, potato hashers or paint shakers. The mixing of 2-pack coatings (especially those with high volume solids) is somewhat more involved, and cannot be achieved with anything less than a power mixer. The amount of shear introduced by power mixing is significantly greater than hand-mixing and is critical to chemical reaction initiation.

Recommended instruction sequence below for mixing 2-packs:

  1. Ensure the clean-up solvent is available before commencing application.
  2. Separately mix contents of each container thoroughly with a power mixer. Scrape the sides and the bottom of the can as you go. A flat-blade knife may be effective in achieving this.
  3. Ensure bases have been tinted to the correct colour.
  4. Box all containers before use to ensure colour consistency.
  5. Add the components in the recommended sequence under constant stirring. This usually involves two people, one to pour and one to mix. Mix the contents of both packs together thoroughly using a power mixer as per manufacture's specification. Scrape the sides and the bottom of the container as you go to ensure that all of 'Part A' and 'Part B' are mixed.
  6. Perfectionists usually transfer to a separate, clean container to avoid contamination of un-reacted material. While this is to be admired, if the sides and base of both containers are scraped and effectively mixed, then this is usually unnecessary.
  7. Allow mixture to stand for the recommended induction time, as stated on the product data sheet.

2-Pack mixing technques - Questions and answers

Click on a heading to reveal the answer.

I don't have a high speed mixer. Can I use a flat paddle to mix 2-packs?

No! The energy necessary to completely mix 2-packs simply cannot be achieved with a flat paddle stirrer. The amount of energy a power mixer delivers in three minutes would equate to around thirty minutes of energetic stirring with a paddle, even if you had the strength and stamina to do so. This period of time lapsed may also exceed the induction time and/or reaction time, during which the paddle-mixed material would begin to react randomly in localised areas within the container. If you have a powerful drill motor, a drill bit attachment may suffice.

This would ensure thorough mixing within a reasonable period of time, and before the induction time has finished.

The colour in the can seems to change as it being stirred. Why?

When paint is tinted, the coloured pigments in the tinter slowly become dispersed into the paint. The colour of the paint therefore can change from its original colour to that of the formulated colour. The longer the paint is mixed, the more thorough the pigment is dispersed, and the closer the final colour of the paint will match that of the colour card or sample for which the formulation was created.

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Can I mix 2-pack coatings from different suppliers?

No! Each product varies with regard to amount of resin in the base, and amount of curing agent in the hardener. Even if 2-packs from different suppliers have the same mixing ratio (for example, a 4:1 ratio) the level of hardener to resin can be completely different. The 'Part A' in each 2-pack kit specifically requires the exact amount of hardener present in its own 'Part B' in order to properly cure. In fact, the same principle applies to different products with the same mixing ratio from the same supplier!

Therefore, if you attempt to mix apparently similar products from different suppliers, you run the very high risk of application or coating failure due to incorrect resin/curing agent ratios. Manufacturers ensure that for a given 2-pack kit, when the entire contents of 'Part A' is mixed with the entire contents of 'Part B', you have the correct ratio of resin to curing agent.

 

Can I mix single-lack coatings from different suppliers?

No! Even when it comes to single-pack coatings, there are many types of resin or acrylic latex and widely different methods of stabilisation. As additive present in one paint can readily de-stabilise another, so if they are mixed, the result could be immediate flocculation of the latex particles. Flocculated paint resembles cottage cheese or very lumpy custard, and once this happens, even vigorous mixing cannot reverse it.

It is false economy to attempt to mix left-over paint, unless it is the same paint product.

Can I paddle mix with my 110v/240v drill and attached correct paddle for mixing?

No! The compressed air mixer with adaptor is to be used, and ensure the compressed air line is earthed to discharge any static charge that may build-up.

 

 

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