Modified Mortars with the Polymers Addition for Thermal Insulation Systems of Polystyrene

Modified Mortars with the Polymers Addition for Thermal Insulation Systems of Polystyrene

Abstract

At the time of the increasing shortage of fossil fuels and their rising prices, energy saving is the main topic. For example, in Germany in the housing sector 75% of the total energy is consumed for the air conditioning in the household, mainly for heating. Today, all new buildings must meet the standards for energy efficiency. The fastest and most efficient way to save energy for heating and improving the climate inside the building is thermal insulation of external walls of the building. In this paper, polymer-cementitious adhesive for bonding and reinforcing, its composition, characteristics and mechanism of binding to the substrate was processed. The influence of the dispersed polymer on the characteristics of polymer-cement mortar for external thermal insulation has been examined in details.

Keywords: Redispersible Polymers; Polymer-Cement Mortar; Polymer-Cementitious Adhesive for Bonding and Reinforcing

Abbreviations: EPS: Expanded Polystyrene; PC: Portland Cement; PPC: Portland Pozzolanic Cement; HAC: High Alumina Cement; MC: Methyl Cellulose; MHEC: Methyl Hydroxyethyl Cellulose; MHPC: Methyl Hydroxypropyl Cellulose

Introduction

A typical system of external thermal insulation consists a polymer-cement adhesive for bonding of insulation material, insulation plates of expanded polystyrene (EPS) or stone wool, then polymer-cement base layer for mounting and reinforcing of facade mesh, fixing components (insulation wedges), one or more finishing layers of polymer-cement adhesive [1]. Components of external insulation system shown in Figure 1. A parameter that characterizes thermal losses through walls is the coefficient of thermal conductivity, This is the amount of heat the building element loses in one second per m² surface at a temperature difference of 1 K, expressed in W/m²K. Lower coefficient of heat conductivity means better thermal protection of the building. The most desirable insulation material is stone wool because it is a good insulator and has the ability to diffuse vapor, and styrofoam is located on second place [2,3] (Figure 1).

Application of Polymer-Cement Adhesive

The first discovery is the development of a powdery redisperse polymer. The polymers added to cement-based adhesives to improve their elasticity and adhesion to the substrate. With the advent of this technology allows the construction chemical industry to present dry polymer-modified adhesives for a variety of purposes:Adhesives based on ceramic bonding cement,

a) Mortars based on gypsum or cement,

b) Cementitious adhesives for thermal insulation,

c) Grout mass,

d) Self-healing mass,

e) Repair mortars,

f) Mortar for masonry,

g) Cementitious waterproofing

Today’s dry polymer-cement adhesives contain numerous additives to improve applicability (open time, consistency, etc.) as well as bonding to the substrate (water retention, elasticity). As the production process is automated and computer controlled, products are produced of precisely defined and uniform quality as well as precisely defined properties with a high degree of reliability in application [1]. When installing an external thermal insulation system, two types of adhesive are required:

A. Adhesive for bonding insulating material to a concrete wall or brick wall (good adhesion to the wall and insulation material as well as good flexibility to handle different coefficients of thermal expansion of the substrate).

B. Adhesive for reinforcement. It should cover the insulation material and to form the base wear-resistant layer for decorative plaster. The reinforcement adhesive requirements are slightly different from the adhesive bonding requirements. Good adhesion to the insulation material, long open time, easy handling, good impact resistance and low water absorption are also important properties of reinforcement adhesives. It is often a compromise and the same material is used for bonding and for reinforcing.

Connective Tissue

The main tasks of the binder are to form a firm bond with the substrate and the material for thermal insulation (adhesion), as well as to link the aggregate grains (cohesion). Today, there are three main types of cement used in the polymer-cement based adhesives: Portland cement (PC), Portland Pozzolanic Cement (PPC) and aluminate cement (High Alumina Cement, HAC) [1]. Portland cement is a hydraulic binder that is obtained by fine grinding portland cement clinker with a specific addition of gypsum, which binds and cures in air and under water. The raw material for the production of portland cement consists of limestone and clay minerals, whereby this ratio is usually 75-80% limestone and 20- 25% clay minerals. The binding speed is regulated by adding 3-5% of the gypsum or anhydrite [4].

Aggregates

Generally, aggregates mean material loosening structures formed in the form of assemblies of more or less identical particles, i.e. material relatively homogeneous in the sense of substancebuilders. Aggregates contain unbound particles (granules) of a certain size. In the narrow sense, aggregates are all grainy (inert) materials that, together with certain bonding materials (cement, lime, bitumen, polymers), are used to obtain various types of mortar and concrete, therefore, materials that are defined by the general term composite materials. Generally, aggregates are classified as natural or artificial [5,6]. In this paper, two natural, inorganic aggregates, ie quartz sand, particle size up to 0.8 mm, and stone dust (so-called filler) were used, particle size up to 0.1 mm. Thus, in this case, a combination of aggregates of different particle sizes is used to ensure a better packing, where smaller particles fill the vacuums between larger particles.

Additives

Additives are substances usually of organic origin, which are added in small amounts to a dry mix, modifying the properties of fresh or hardened glue, mortar or paste. The content of these supplements is usually below 1%. Only redisperse polymers are added in higher percentages. Additives for the production of dry mortar may be added to the mixture solely in powder form. If several different accessories are used, they must be compatible with each other.

Redispersible Polymers

Redispersible polymers are organic polymeric materials produced from aqueous dispersions by spraying technology, as shown in Figure 2. Water dispersions are two-phase systems in which the dispersant is water. Redispersible polymers provide the following properties in adhesives for thermal insulation:

Figure 2: Simplified process of forming redispersible polymers and forming redispersion [6]. The synthetic polymer is dispersed in water.

a) Better adhesion to the substrate,

b) Impact resistance

c) Less damage during impact,

d) Aging properties (good properties after varying climatic conditions),

e) Hydrophobicity (good water odor).

Dosing of redispersible polymers ranges from 0 to 5%, but it should be noted that the overall product price increases with increasing polymer content so that the content of the polymer needs to be accurate enough to achieve satisfactory results as far as the characteristics of the final product are concerned. The following picture shows the structures of the most commonly used polymer in adhesives for external thermal insulation systems [1] (Figure 3).

Water Retention Additives

The main purpose of the water retention additive is to keep the water in the fresh adhesive for a longer period of time so that the cement has enough available hydration water. Water also acts as a lubricant to improve the freshness of fresh adhesives and is also essential for the hydration process of cement. The loss of water from the adhesive is reduced by the adsorption of waterretaining additives on the substrate and on the bonding particles and by forming a thin layer of low permeability. Other parameters influenced by water retention additives are open time, improved wettability of the particles, and reduced subsidence of the adhesive. High water retention can be achieved by chemically modified methyl cellulose (MC). Methyl cellulose modification is performed with ethylene oxide or propylene oxide. The resulting modification products are methyl hydroxyethyl cellulose (MHEC) having the structure shown in (Figure 4) or methyl hydroxypropyl cellulose (MHPC). The MHEC content in formulations does not exceed values greater than 0.5%. The following illustrations show the solubility of MHEC and MC with increasing temperature and retaining water with the addition of MHEC (Figures 5 & 6).

Influence of Temperature on Bonding of Glue: Thermoplastic polymers are semi-crystalline solids consisting of crystalline and amorphous parts. The ratio of crystalline and amorphous parts depends on the primary structure of the polymer and on the parameters of the production process. The amorphous parts of the solid polymer can be in glass or rubber, depending on the ambient temperature. The temperature at which the transformation from the glass state occurs to the rubber and vice versa is called the glass transition temperature (Tg). Below Tg polymers are rigid and fragile. By heating, they soften and become more elastic, i.e. they go into a rubbery state. Another parameter important for the formation of a polymer film is the minimum temperature of film forming (MTF). MTF is the temperature at which the polymer particles will coalesce and form a continuous unopened film. Therefore, it is the lowest temperature that is necessary to meet the conditions to start the formation of the film. The minimum temperature of the film formation depends on the size of the dispersion particles, the drying time, etc., but mainly depends on the glass transition temperature of the polymer. Polymer dispersions do not form a film at a temperature below the MTF, but only white powder is formed. The polymer dispersions that dry at a temperature slightly below the MTF form a white, opaque low-strength film. Just above the MTF deformation and coalescence of the dispersion particles is strong enough to form a continuous film without cracks. Socalled coalescing additives, plasticizers, solvents can lower the MTF far below the Tg of dry polymer. Thus, the choice of coalescing additives is an interesting field of investigation to ensure that polymer dispersions in adhesives form a film even when used at + 5°C. Examples of coalescing additives are dibutylphthalate (DBF), ethylene glycol ethers and organic solvents (e.g., toluene). The MTFF polymer can be reduced by more than 20 °C [1].

Stabilnost Prema Vodi: After the polymer diffused between adjacent particles, the formed polymer bonds are not soluble in water. However, the film still contains a water-soluble protective colloid that easily absorbs water so that the polymer bonds can hydrate and swell, which can weaken the adhesion strength of the hardened adhesive. Depending on the nature of the polymer, there is a possibility of reacting (saponification) (Figure 11). Poly (ethylene-vinyl acetate) hydrolyzed (saponified) in the presence of alkali in an alcohol and polyvinyl acetate. Alkalinity (pH>12) is present in cementitious adhesives when they come into contact with water. High alkalinity catalyzes EVA hydrolysis (Figure 12). Water solubility can be increased to the extent that the polymer can be washed out of the cement matrix by rain. Polymers containing nonpolar monomers (ethene, butadiene, styrene ...) are not susceptible to saponification [1].

Experiment

We examined the characteristics of the adhesive depending on the share of the redispersion polymers in it:

a) Testing the consistency of fresh adhesive,

b) Testing the adhesion of adhesive to the surface

c) Testing bending strength of hardened adhesive,

d) Testing compressive strength of hardened adhesive,

e) Testing capillary water absorption of hardened adhesive

f) Testing the volume mass of hardened adhesive.

Test results are presented in tables and diagrams, along with photos of the samples and a short description of the test.

Technical characteristics of the components for making the adhesives used in the practical part of the work

Cement: As a cement binder in our experiments we used Portland cement with the addition of fly ash CEM II / B-W 42.5 N, manufactured by Kakanj cement. Cement meets all the prescribed requirements and it is suitable for the preparation of adhesives for sticking EPS plates and adhesives for reinforcement. Additional testing of cement is not required. Proportion of cement in the formulations was 33.1%. The chemical composition of Portland cement CEM II / B-W 42.5 N is shown in Table 4 [8]. Cement producer Kakanj cement is shown in Table 5.

Conclusion

The results of tests carried out in the context of practical part show that the addition of the redispersed polymer directly affects all investigated properties, except the consistency of fresh adhesive where there have been no significant changes with increase of the share of redispersed polymer. The adhesion of adhesive on the surface is increased with the increase of the share of redispersed polymer, because of the occurrence of formation of polymer bond in the cement matrix that additionally binds adhesive on substrate. Adhesion is also the main characteristic of adhesives for external thermal insulation together with the capillary water absorption, so that the redispersed polymer is added in order to increase adhesion of mortar of the substrate and to reduce the capillary water absorption.

The results show decrease of the flexural and compressive strength of the adhesive with the increasing share of redispersed polymer, because of the type of the redispersed polymer used, ie. redispersible polymer used belongs to the class of redisperse soft polymers because of the low glass transition temperature (-6°C). Increase of the share of dispersed polymer leads to decrease of volume mass, primarily because of reduced share of aggregate which has higher specific weight than the specific weight of the polymer dispersion. In practice, we add 1 to 35 of the redispersion polymer. We add 1 to 2% of redispersible polymer in the adhesive for gluing EPS panels, while the adhesive for the reinforcement contains 2 to 3%. It should be noted that the redispersion polymers are most expensive components in adhesives for external thermal insulation, so that with these proportions satisfactory properties of adhesive are achieved and by this competitive product is provided.

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