- What are 100% solids polyurethane and epoxy liquid coatings?
100% solids polyurethane liquid 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. 100% solids epoxy liquid coatings are also generally tow components, containing an epoxy resin part and a hardener/curing agent part.
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 and epoxy coating system having a relatively lower viscosity without using any solvents.
- 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 polyurethane coatings 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 polyurethane coatings are cheaper but more in general more chemical and corrosion resistance. They are 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.
- What are elastomeric polyurethanes or polyureas? What are rigid/structural 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 rigid/structural polyurethane 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.
In different from polyurethane by reacting polyols with isocyanates, 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. Currently almost all the polyurea coatings available in the market are elastomeric polyurea rather than rigid.
The above image illustrates the highly cross-linked molecular structure of rigid/structural polyurethanes vs. the liner structures of elastomeric polyurethane/polyurea. The three dimensional and highly cross-linked molecular structure of the rigid/structural polyurethanes offers them with the corrosion and chemical resistance as well as excellent adhesion to steel substrates. The linear structure of elastomeric polyurethane or polyurea gives better abrasion and higher flexibility but poor adhesion and chemical/resistance.
Like aliphatic polyurethanes, the formulating work for a good rigid/structural 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.
- What advantages do 100% solids rigid/structural 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
- 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.
- Why most good 100% solids polyurethane protective 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 mis-metering problems (often called as off-ratio) during its application. The higher ratio is, the higher possibility it will occur.