PE01879A.gif (1669 bytes) Common Questions on 100% Solids Polyurethane Coatings

  1. What are 100% solids polyurethane coatings?
  2. What are aliphatic polyurethanes? what are aromatic polyurethanes?
  3. What are elastomeric polyurethanes? what are rigid polyurethanes?
  4. What advantages do 100% solids polyurethane coatings offer?
  5. How do 100% solids polyurethane coatings differ from other 100% solids coatings, such as epoxy and powder coatings?
  6. How to apply 100% solids polyurethane coatings? Any special equipment required?
  7. Why most 100% solids polyurethane coatings are in the 1:1 format?
  8. What are the difference between polyurea and polyurethane coatings?


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1. What are 100% solids polyurethane coatings?

100% solids polyurethane coatings usually consist of two components: one isocyanate-rich solution and one polyol-rich solution. This has been defined as an ASTM D16 Type V polyurethane coating. Such a polyurethane coating film is formed when the two components are combined; a rapid and exothermic chemical polymerization reaction takes place.

By definition, the term "100% solids" means the coating system does not use any solvent to dissolve, carry or reduce any of the coating resins. Further, the resins normally still in a liquid state, will convert, 100%, to a solid film after application. The viscosity of the coating system is determined by the selection and design of the resin components. It is not determined by the addition of a solvent, although solvents have usually been used in the industry for the purpose of reducing viscosity. A good chemist shall therefore be able to design a 100% solids polyurethane coating system having a very low viscosity (or any low viscosity) without using any solvents.

 

 

2. What are aliphatic polyurethanes? what are aromatic polyurethanes?

Aliphatic polyurethanes are polyurethanes based on aliphatic isocyanates (e.g. HDI and IPDI) and mostly polyester and/or acrylic polyols. Aromatic polyurethanes are polyurethanes based on aromatic isocyanates (e.g. MDI and TDI) and mostly polyether polyols.

Aliphatic polyurethanes are more expensive, but provide the best UV resistance and color stability among all types of industrial coatings. They are therefore often used for exterior applications and any other places where color stability is concerned. The design of a 100% solids aliphatic polyurethane system is very difficult and few coatings manufacturers have such a technology.

Aromatic polyurethanes are cheaper and often used for interior, lining or underground applications. Depending upon their formulation design, aromatic polyurethanes will exhibit a certain degree of color change ("yellowing") after a few days/months UV exposure. However, their UV resistance is generally better than that of common epoxies. Most 100% solids polyurethanes available today are aromatic polyurethanes.

 

 

3. What are elastomeric polyurethanes? what are rigid polyurethanes?

The properties of 100% solids polyurethanes vary from very soft, rubbery elastomers (like running shoe soles) to hard, ceramic like systems - a good chemist can formulate the 100% solids polyurethanes to do almost anything. The chemical bonds in the more rigid systems are highly cross-linked to each other to create hard, dense systems that have very good chemical and moisture resistance. The rigid systems usually have excellent adhesion and are the best choice for the corrosion protection of metals. On the other hard, the elastomers have a more linear structure with much less cross linking that allows them to be very stretchy and elastic. These systems normally have great impact strength and flexibility, but relatively poor adhesion and chemical resistance. Elastomers are better suited to protecting substrates that tend to move and flex like concrete but do work as well on metallic substrates. The chemical and corrosion resistance of the elastomers can be improved if the systems are applied relatively thick.

Like aliphatic polyurethanes, the formulating work for a good rigid 100% solids polyurethane coating is more difficult than that for a 100% solids elastomer. The ability of designing such systems indicates the technical advanced level of a coatings manufacturer or a coatings chemist.

 

 

4.What advantages do 100% solids polyurethane coatings offer?

100% solids polyurethane coatings offer very unique handling, performance and environmental advantages. To the end users, this means a greater assurance of achieving the desired result on time and on budget. To the applicator, it means high throughput and fewer callbacks. Imagine, for example, coatings that:

are 100% solids, having zero or near zero VOC's
set at virtually any temperature - winter and summer
are non flammable, solvent free and as safe as house paint
are self-priming
can be applied in a single multi-pass coat to any dry film thickness
set in minutes
can be put into service within hours
are easily touched up if damaged
are non-dangerous goods for transportation
require no post curing, ever.

Accompanying these handling advantages are a number of important performance advantages. These include:

superior adhesion without using any primer
resistance to chemical attack
resistance to gouging and abrasion
flexibility and resilience
inertness, freedom from embrittlement
impermeable
resistance to disbondment forces
a number of national and international approvals (UL, NSF, British Gas, etc.)
a totally monolithic, seamless polymer sheet virtually fused to the substrate.

 

 

5. How do 100% solids polyurethane coatings differ from other 100% solids coatings, such as epoxy and power coatings?

100% solids epoxy and 100% solids polyurethane are currently two main sprayable coatings systems that offer the 100% solids advantage to the corrosion industry. They both do, together with some industrial powder coatings, provide excellent corrosion resistance and unique physical and mechanical properties.

Some advantages of 100% solids polyurethanes over 100% solids epoxies are:

polyurethanes could be designed to be from elastomeric to rigid, while epoxies are normally rigid or even brittle;

polyurethanes could be cured within a hour at as low temperature as -40oC, while epoxies cure slowly (weeks) or do not cure at all at such cold temperatures;

polyurethanes generally have the best abrasion resistance among all industrial coatings systems;

aliphatic polyurethanes are well known for their excellent UV resistance and color stability, while epoxies and aromatic polyurethanes are normally not being used for exterior applications; (Is there any 100% solids aliphatic polyurethane coating system? yes, please see my paper "100% Solids Aliphatic Polyurethane Coatings - From Dream to Reality")

Compared to powder coatings, the sprayable 100% solids epoxies and 100% solids polyurethanes will not require application set up or equipment as expensive and complicated as those for powder coatings.

 

 

6. How to apply 100% solids polyurethane coatings? Any special equipment required?

100% solids polyurethane technology is available in a variety of setting times to allow application techniques including "brush and roller", "hand spray", "automatic line travel" and "casting". Typically, the setting (or cured-to-touch) time for these polyurethane systems ranges from less than one minute to several hours.

Fast setting 100% solids polyurethane coatings (with setting time of under 20 minutes) are usually applied with plural component equipment. For further information, please refer to the JPCL December 1995 Feature Article written by me.

 

 

7. Why most 100% solids polyurethane coatings are in the 1:1 format?

Isocyanates and polyols in a 100% solids polyurethane system are reacted normally by around the ratio range of 1:1;

To apply fast setting 100% solids polyurethane coatings with plural component equipment, the 1:1 format is the easiest way to configure the set up. However, it requires that the both components have the same or close viscosity. This is the most difficult challenge for a coatings chemist in designing a 100% solids polyurethane coating system;

A 100% solids polyurethane coating system with a mixing ratio higher than 1:1 will have better chance to cause mismetering problems (often called as off-ratio) during its application. The higher ratio is, the higher possibility it will occur.

 

 

8. What are the difference between polyurea and polyurethane coatings?

In many cases, people tend to mix up polyurea coatings and polyurethane coatings together. And thus polyurethane coatings have become a generic term for coating systems based on polyisocyanate reaction. Polyurea coatings use normally amines as coreactants to react with isocyanates. This reaction is extermely fast (within a few seconds or minutes). As a result, polyurea coatings tend to have a very limited pot life and their recoat time becomes a problem in cases when multiple coats occur. A polyurea linkage, however, will have better heat and high temperature resistance than a polyurethane system with polyols as coreactants.

 

Copyright © Dr. Shiwei Guan, all rights reserved.
Have more questions?
E-mail: sguan@canada.com

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