Development of coatings for external façades in the modern building industry

Roberta Pecchini – IMCD Italia Spa

Coatings for building external façades should have not only aesthetic and architectural properties, but also and above all they should protect the structure showing a long lasting durability. It is a not easy challenge as they are also exposed to various weather conditions (very different surface temperatures not only between the various seasons but also between day and night, sun and rain), to the different conditions of the microenvironment where they are applied (near vegetation or high traffic roads, exposure to the different cardinal points, geographic area and others) and they are coated on systems which are technically different (directly on plaster and skim coat or as a finishing coat for thermal insulation systems).

Tab. 1
The standard establishes 3 classes of the capillary water absorption

The architectural project should be supported by highly functional materials which combine the texture and colour design with technical performance, both in case of new constructions and of restructuring works. Furthermore, in this latter case, the products specificity should also guarantee the compatibility with the types of existing materials, so as the support and the techniques which were previously used are compatible to ascertain the durability of the outcome.
The Coatings & Construction IMCD laboratory has developed a high-build coating formulation for façades using vinyl acetate ethylene emulsions, which shows excellent performance and durability.
The vinyl acetate ethylene based emulsions (VAE) are usually used for water based paints for interior, and especially Vinnapas EP 3360 allows to formulate low VOC (< 1 g/l) matt water based, low odor paints, high hiding power (with possible improvement of the titanium content rate), and good wet abrasion resistance.
In addition, Vinnapas EP 3360 show a high solid content (60%) giving an outstanding formulation freedom, and this grade is suitable not only for formulation for interior, but also for exterior, thus allowing the highest efficiency in the stock and purchase handling, meeting the expectations of construction finishes manufacturers.

Tab. 2 
Also the water vapour permeability is classified into 3 classes

Coatings for façades for exterior should be classified according to the EN 15824 standard, defining the final performance requirements for plasters (interior and exterior) based on organic binders. The water is one of the main causes of degradation of the external finishes because it can be absorbed from the outside (condensation, rain) or it can come from the underlying masonry in the form of humidity, and it is therefore essential to check the behavior of the façades cladding with both liquid water and water vapour, measuring the absorption value of capillary water and the vapour permeability rate. The first parameter, W, is measured according to the test method of the EN 1062-3 standard and the second through the measurement of the water vapour transmission rate according to the EN ISO 7783 standard. Another essential property is the adhesion to the substrate and the durability, established according to the capillary absorption rate and also as a function of the accelerated aging tests (normally used, but not included in the EN 15824 standard). All this is followed by the assessment of the flexibility of the coating, in particular if applied as a finish on thermal insulation systems (ETICS), having to resist the impact of shocks accounting for many Joules.

The IMCD formulation and its properties
The capillary water absorption is evaluated as a function of the chemical nature of the polymer used, of the particle size distribution and of the hydrophobic property of the capillary pores; the vapour permeability depends on both the polymer hydrophobicity and its glass transition temperature, Tg. Vinnapas EP 3360 is an ethylene vinyl acetate copolymer which has medium hydrophobicity and a fairly low glass transition temperature, equal to 10 °C.
The ideal outcome would be a coating both in class W3 of capillary water absorption and class V1 of permeability, showing maximum hydrophobicity and the highest resistance to the transmission of water vapour.
The IMCD formulation based on Vinnapas EP 3360 gives excellent results, showing:

W= 0,06 kg/m²h^0,5→Class W₃ 
Sd= 0,101 m →Class V₁
The addition of hydrophobic silicone additive (Silres BS 1306) does not improve the above said properties, which are already highly performing, but it clearly improves the surface hydrophobicity of the coating and the so-called drop effect, as it can be seen from the contact angle parameters and photos listed below:

W= 0,06 kg/m²h^0,5 →Class W₃
Sd= 0,104 m → Class V₁
It is clear that the formulation of the coating has already been optimized using only the VAE polymer, guaranteeing high performance, but the addition of 1% silicone additive gives the additional benefit of an immediate visual effect of water repellency and greater cleanliness of the façade.

Fig. 3 
Cohesive failure of the coating based on Vinnapas EP 3360

Adhesion to the substrate
The adhesion to the substrate should be measured according to the EN 1542 standard and the minimum value required is 0.3 MPa. The IMCD formulation has an adhesion value of 0,92 MPa, with a cohesive failure in the thickness of the coating, as shown in the photo below, showing the high adhesive property of the VAE polymer.

Durability
According to the EN 15824 Standard, it is necessary to carry out further durability tests only if the absorption of capillary water is high (class W₁), but as it is often done in the laboratories of the IMCD group, the formulation developed, despite having a low capillary absorption (W₃) has been tested with accelerated aging cycles in a QUV chamber, according to EN ISO 4892-3. These tests were carried out both with the A340 UV lamp, which simulates the global solar radiation in the wavelength range between 300 and 340 nm, and with the B 313 UV lamp, which has a higher radiation aging peak at 313 nm, compared to the solar radiation, and therefore it represents an aging condition that is even more critical than in the daily routine.
The results are excellent, and after 1000 hours with a UV B lamp, the delta E is equal to 0,707, while after 1500 hours of testing with a UV A lamp the delta E is equal to 1,55, with no cracks, as it is observed from the graph and from the photos.

Flexibility and impact strength
In case of application of the façades cladding as a decorative finish of a thermal insulation system (ETICS), in addition to the above mentioned tests, the impact resistance of 3 and 10 J is also required on the complete system, consisting of insulating panel, reinforced screed and finish. The IMCD formulation, based on Vinnapas EP 3360, shows high impact resistance, thanks to the flexibility of the polymer, which has a glass transition temperature of 10 °C. It is therefore possible to reach the category 1 of impact resistance, since the system, including the finish, does not show any type of degradation after carrying out the tests.
This test, which is not compulsory for the purposes of CE marking of the coating alone, guarantees further durability, in addition to the aging tests.

Conclusions
The IMCD Laboratory in Milan has developed the formulation of a decorative architectural coating based on vinyl acetate ethylene, Vinnapas EP 3360, an emulsion which is typically used for interior products with sustainable solutions at VOC < 1g/l. The technical characterization shows the high quality of this modern coating, achieving performances which are not common to VAE-based products for exterior.
Briefly, the IMCD solution shows low capillary absorption W3; high vapor permeability V₁; excellent adhesion to the substrate; excellent resistance to aging (∆= 0707 after 1000 hours with a UV B lamp and ∆ E = 1.55 after 1500 hours with a UV A lamp)excellent impact resistance, category 1, in thermal insulation systems.