The changes in surface texture and density of porcelain tiles in recent years have become a challenging factor in the performance of the adhesive to bond these tiles to the substrates. Whereas porcelain tiles used to be smaller tiles, the newer large format style tiles can have different mechanical properties in the sense of hardness, density, porosity and surface texture. This new matrix signature can create adhesion problems for low polymer thin set adhesives and poorly applied high polymer adhesives.
The new large porcelain format began appearing in 2008 and around that time, Ardex noticed that ceramic tile adhesives previously used for fixing porcelain tiles were displaying de-bonding. The temperature and length of the firing regime can significantly alter the mineral crystalline phases and the structure of tiles. The modern version of porcelain is produced by a fast firing process at higher temperatures to reduce energy consumption, improve production efficiency and minimise emissions. Apart from improving production efficiency and reducing fossil fuel usage, these conditions alter the properties of the tiles compared to older porcelain tiles.
These new tiles have some common properties including very hard surfaces, high density, semi-conchoidal fracture on breakage and give a ringing sound when struck. They may also have heavily ribbed backs, high contact coverage of white powdery anti-blocking agent and a shiny slightly glittery and glassy appearance. On examination, it is clear the new closed, smooth and almost glass-like nature of the tile back was preventing some cement-based tile adhesives from obtaining a sound bond.
Medium polymer content adhesives are less able to create the mechanical key and do not have enough polymer to produce the required chemical adhesion. Ardex recommends that the higher polymer content of upper range C & R class adhesives is required.
Modifying the application methodologies and better understanding and selection of adhesive will help minimise the issues with installing these tiles.
With large format tiles, the primary difficulty is obtaining sufficient adhesive contact coverage and spread to achieve a strong bond and an effective load carrying bed. When the adhesive contact coverage is reduced, the overall load capacity is reduced, but conversely the actual strains exerted on the adhesive bed increase. The same forces exist (dead load, wind, thermal & moisture related substrate drying movements) but applied over half to two-thirds the contact area means the adhesive bed sees a higher level of applied stress where it is in contact with the substrate on one side and the tile on the other. Ardex testing has found that when the contact coverage dropped to 80 percent overall, the test load to failure dropped by around 30-40 percent.
It is also vital to ensure the application of the tile is performed within the open time of the adhesive. Where the open time is exceeded, the adhesive surface becomes dead to wetting and bonding. As a result, only sufficient adhesive should be spread to lay a few tiles at a time, larger the size the less overall area should be spread.
Another consideration is the drying performance of adhesive and development of strength. The non-porous barrier created by large porcelain tiles acts to delay the loss of water from the adhesive as it cures. Whilst the adhesive might actually chemically cure, the matrix retains the unused water which, with a smaller and porous tile could be absorbed or diffuse out through the tile joints. Larger tiles have fewer grout joints and these are often very narrow (~2-4mm) thus reduced ability for drying quickly. The result of this retained moisture is that adhesive does not develop full strength and behaves like an adhesive in a saturated environment where the actual strength is 30-50 percent less than the equivalent dry or equilibrium strength. This delay needs to be taken into account where loadings are critical from an early age and is a good reason to use fast curing F rated adhesives.
To combat these issues, Ardex recommends selecting flowable type cement based adhesives such as ARDEX X78 and systems with high polymer modification. High end single part or powder/liquid combination C2 rated adhesives have increased polymer levels and will, therefore, meet the adhesive coverage required on the back of a tile as well as having superior overall bond, strength and resilience.
Another method to combat this problem is to short cut the polymer issue by applying a suitable ARDEX primer or bonding bridge to the back of the tile. Typically the primers are polymeric liquids so will effectively adhere to the back face of the tile and the adhesive they are designed to work with.
Another example of a potential method could be to produce a polymer rich adhesive slurry thin coat brush applied to the back of the tile and then placed wet on wet onto the notched adhesive bed.
The property of tiles will continue to evolve and so to must the installation process and products used to ensure a smooth, professional finish. Ardex draws on the expertise of a local and global Research & Development team that monitors tile manufacturing processes as well as emerging trends in the tile market. A result of this is innovative technology such as Microtec reinforcement fibres found in Ardex X77 and Ardex X78 tile adhesives.
For additional information, visit www.ardexaustralia.com and download the technical paper or to speak to an Ardex professional, contact 1300 780 788.