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Moisture Curing One Component Coatings

The Chemistry of Moisture-Cure 1K Coatings

 

Moisture curing polyurethane coatings are an effective and economical coating solution in many new construction and refurbishment applications in which steel is the substrate material.

Moisture cure polyurethane coatings can be applied independent of the weather (temperature, humidity and dew point), dry quickly and are surface tolerant. In addition, the coatings remain elastic and resistant to UV radiation to provide longterm durability.

Adducts of diisocyanates or polyisocyanate and polyol combinations with an excess of isocyanate groups (prepolymers) can crosslink with atmospheric moisture to give insoluble higher molecular weight polyurethane/polyureas. This reaction describes the curing principle for moisture-curing polyurethane coatings.

Formulating One-Component Moisture-Cure Coatings

There are two ways of formulating one-component coatings (1K coatings) of this kind:

  1. Preparation of prepolymers from suitable polyisocyanates and polyols
  2. Use of Desmodur® E polyisocyanates

 

You can find this information and accompanying illustrations in the Bayer publication, “The Chemistry of Polyurethane Coatings” by clicking here.

Additionally, learn more about waterborne coatings by browsing and downloading our collection of technical papers.

 

Prepolymers from Polyisocyanates and Polyols

Desmodur® N (HDI), Desmodur® L (TDI), and Mondur® MR and MRS (MDI) polyisocyanates are suitable for preparing prepolymers used in formulating one-component moisture-curing coatings. The prepolymers are usually prepared from polyols of low functionality and a corresponding stoichiometric excess of polyisocyanate.

A moisture-curing clear varnish can be made as follows: The polyisocyanate and solvent are mixed for a short time in a dissolver, being sure that moisture is excluded. If a moisture scavenger is required, paratoluene sulfonyl isocyanate (pTSI) can be added. After this, the polyol, other additives, and accelerators are added and the mixture is packed in a dry, airtight container. The coating reaches its final viscosity in approximately seven days. The storage stability of the final product is dependent on the polyol used and should be closely examined.

 

Desmodur® E Polyisocyanates

The trademark Desmodur® E covers a range of ready-to-use polyisocyanates for one-component coatings which may require no other compounding when used in clear coatings except for the addition of solvent where necessary.

 

Properties of Moisture-Curing One-Component Coatings

 

The properties of moisture-curing one-component coatings are principally determined by the nature of the particular base isocyanates used. For example, one-component coatings based on aliphatic isocyanates (HDI, IPDI, Desmodur® W diisocyanate) generally need longer drying times than those based on aromatic isocyanates (TDI, MDI). The drying times depend not only on the temperature, but also on the amount of atmospheric moisture present. With very low absolute moisture content, the drying times may be increased. The aliphatic polyisocyanate based moisture-cured coatings often require metal catalysts (such as dibutyltin dilaurate) to provide reasonable cure times.

The weathering properties of one-component coatings also largely depend on the type of isocyanate used. Coatings based on TDI or MDI have a tendency to yellow in the light and show a relatively rapid loss of gloss on weathering. Those based on HDI, IPDI, and Desmodur W diisocyanate are light stable. Depending on the composition, these one-component coatings may be equivalent in gloss retention and chalk resistance to two-component polyurethane coatings based on Desmodur N polyisocyanates. One-component polyurethane coatings have very good mechanical properties.

The films can range from hard to very flexible. Surfaces are particularly mar resistant and abrasion resistance is exceptionally high. The films also have good resistance to chemicals, including the stronger organic acids, as well as alkalis, alcohols, solvents, and water.

Aliphatic polyisocyanate based moisture curing resins are available which can be formulated into coatings which meet the requirements of the SSPC - Paint 38 Specification “Single-Component Moisture-Cure, Weatherable Aliphatic Polyurethane Topcoat, Performance-Based.”

Pigmented One-Component Coatings

 

Due to the sensitivity of one-component coatings to moisture, a special technique has to be followed when formulating pigmented coatings. This involves the use of pTSI, Additive OF, and splitting the grind procedure into several steps. This process may be accomplished in the same amount of time as the milling of a conventional coating and can be done with the same equipment.

The formulation is divided into four operations:

  1. Weighing
  2. Predispersing
  3. Dispersing in the sand mill
  4. Filling

The individual components are weighed in the given sequence and predispersed with a dissolver. In so doing, two processes occur simultaneously:

  1. Homogenization of the weighted materials
  2. Dehydration of the pigments, extenders, and solvents by pTSI

Carbon dioxide is released through the chemical reaction between water and pTSI, acting as a buffer gas preventing contact with air and humidity. While the dissolver charge is warm, it can be dispersed in a sand mill.

After cooling, the catalyst, and if necessary Additive OF, can be added and the batch adjusted to the desired viscosity with anhydrous solvents. The material should then be packed in dry airtight containers.

Another method of formulating pigmented one-component coatings is to add the pigment in the form of a pre-dispersed paste colorant. In this case, the pigment mill base can be prepared in a binder which reacts as little as possible with the NCO group. The incorporation of pTSI ensures that the pigment paste is free of water. The paste may then be mixed with Desmodur E polyisocyanate binders such as Desmodur E-28.

 

Curing

 

The drying rate of one-component coatings is dependent on the relative atmospheric humidity and the temperature. Low temperature and low atmospheric humidity may slow down the drying considerably.

As with two-component systems, one-component coatings based on aliphatic polyisocyanates often require the incorporation of reaction accelerators. Metal compounds such as dibutyl tin dilaurate are especially suitable for this purpose.

 

Application

 

Moisture-curing coatings based on the Desmodur E polyisocyanates are normally applied by brushing or spraying. Dip coatings and curtain coating cannot generally be used because of the extended contact between the liquid coating and atmospheric moisture.

Unpigmented one-component coatings are used primarily for wood substrates such as parquet flooring and other indoor wood flooring applications. Another area of application is for the sealing or coating of concrete floors, and decorative seamless floors.

Pigmented one-component coatings can be used for anticorrosion coatings for metal, for the coating of concrete substrates, and for various other decorative and protective coatings.

 

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