Key role for modern polymer binders
Dr HermanN Lutz of Germany's Wacker Specialties explains the advantages of modern polymer binders in dry-mix mortars in the construction industry.
01 September 2002
For thousands of years, mineral mortars have been used in the construction of buildings.
From antiquity into the Middle Ages, additives and admixes such as soaps, resins, proteins (milk, blood) and ash were mixed on the job site together with the mineral binders (nowadays mainly cement) and aggregate (sand, fillers) in order to improve the technical performance and durability of the resulting mortars.
Today, this so-called job-site mix technology is not able to meet adequately the needs for mortars and their application in the construction industry.
As a practical consequence, the development of the modern construction industry in Western countries from the 1960s onwards was influenced mainly by three important trends, which can today be seen on a global scale:
Dry-mix mortars
Polymer modification techniques were launched in the 1930s , when liquid polymer dispersions were added to job-site mixed mortars and the process was further developed to the use of simple prepacked dry-mix mortars.
However, by using these so-called two-pack or two-component systems, it is not always possible to produce a homogeneous mixture with a defined and constant polymer-cement ratio. Dosing errors may occur accidentally due to insufficient knowledge, experience and education on the part of the workers or even intentionally in order to save money in the short term.
Both result in inadequate product characteristics with possible damage to the applied mortars due to insufficient adhesion bond strength or insufficient flexibility and thus durability.
Other reasons for not using two-pack systems, apart from difficult and risky handling, are the higher expenses and logistics difficulties (such as the need for additional containers with the problem of their disposal; critical storage and transport of liquid dispersions which can deteriorate by microbial attack).
Redispersible powders
A new era in dry-mix mortar technology began with the use of dry polymer-modified prefabricated and pre-packed dry-mix mortars. The key factor in the production of pre-packed polymer-modified mortars was the invention of the redispersible powders by Wacker Chemie in 1953 (under the brand name of Vinnapas redispersible powders).
Redispersible powders are dry polymer binding agents, produced by spray-drying liquid polymer dispersions.
The simple mixing of redispersible powder containing dry-mix mortar with water on the job-site, provides a mortar with the same characteristics and performance compared to a non-polymer-modified mortar to which a liquid polymer dispersion is added.
In the dry-mix mortar plant, redispersible powder is precisely and automatically dosed and homogeneously mixed together with cement, fillers and additives needed for specific types of dry-mix mortars.
Thus the composition, the polymer content and, consequently, the quality and the performance of these polymer-modified dry-mix mortars can be guaranteed.
This is not true for all kinds of two-component systems, where a liquid or a paste-form system is mixed with a dry-mix mortar or cement, which is a permanent source and allows for dosing, mixing and application errors. In contrast to two-component systems, the use of polymer-modified dry-mix mortars allows a safe, quality-controlled and highly efficient mortar application, including silo transport systems, automatic mixing with water and machine spray application.
As a logical consequence, the polymer-modified dry-mix mortars are now used almost exclusively for all kinds of mortar applications.
Applications & advantages
The main applications with dry-mix mortars are: different kinds of ceramic tile and building adhesives, brick-laying mortars and adhesives, jointing and grouting mortars, thermal insulation adhesives and embedding mortars, self-levelling mortars and screeds, concrete repair and concrete rehabilitation systems, all kind of renders and decorative plasters, cement paints, cementitious sealing slurries, joint-fillers and smoothing compounds.
The polymer modification of dry-mix mortars with Vinnapas redispersible powders improves, depending on their dosage, the adhesion bond strength on all kinds of substrates, the flexibility and deformability, the flexural strength and the abrasion resistance, the toughness and impact resistance, the cohesion and the density (impermeability) of the mortar as well as the water retentivity and the workability.
Special Vinnapas redispersible powders can provide a strong hydrophobic (water repellent) effect to the cured mortars. In the graphs 1, 2 and 3, some of the technical improvements of dry-mix mortars, regarding tensile adhesive strength, shear strength and flexibility by the modification with Vinnapas redispersible powders are shown in detail.
