*Article* **Mechanical Properties of Polyurethane Adhesive Bonds in a Mineral Wool-Based External Thermal Insulation Composite System for Timber Frame Buildings**

**Ewa Sudoł \* and Ewelina Kozikowska**

Construction Materials Engineering Department, Instytut Techniki Budowlanej, 00-611 Warszawa, Poland; e.kozikowska@itb.pl

**\*** Correspondence: e.sudol@itb.pl; Tel.: +48-22-56-64-286

**Abstract:** This paper aims to provide a preliminary assessment of polyurethane adhesive applicability as an alternative to conventional cement-based adhesives used to fix thermal insulation materials to substrates concerning mineral wool-based external thermal insulation composite systems. Currently, polyurethane adhesives are only used in expanded polystyrene-based ETICS. This study discusses the suitability of polyurethane adhesive for ETICS with lamella mineral-wool for timber frame buildings. Bond strength, shear strength and shear modulus tests were conducted. In addition, microstructure and apparent density were analysed. Mechanical properties were analysed in terms of the influence of substrate type and thermal and moisture conditions, taking into account solutions typical for sheathing on timber frame (oriented strand boards (OSB), fibre-reinforced gypsum boards (FGB) and cement-bonded particleboards (CPB)), as well as limit conditions for adhesive application. It was found that PU adhesive can achieve adhesion, both to MW and OSB, and FGB and CPB at ≥80 kPa, which is considered satisfactory for PU adhesives for EPS-based ETICS. Favourable shear properties were also obtained. There was no significant effect of sheathing type on the properties considered, but the influence of temperature and relative humidity, in which the bonds were made, was spotted. The results obtained can be considered promising in further assessing the usefulness of PU adhesives for MW-based ETICS.

**Keywords:** external thermal insulation systems; mechanical properties of bonds; polyurethane adhesive; timber frame building; bond strength; shear properties

#### **1. Introduction**

External thermal insulation composite systems (ETICS) are among the most popular methods for improving the energy efficiency of buildings in Europe [1]. The first ETICS were installed in the 1960s in Germany and later in Switzerland and Austria [2]. The breakthrough for ETICS came in the early 1970s when, as a consequence of the oil crisis, energy prices rose, resulting in considerations concerning the need to minimise heat loss in buildings [3]. The growth dynamics of the insulation sector in Europe in the 1990s was boosted by the accession to the European Union of new countries, which, while meeting their obligations to comply with EU regulations, soon implemented energy efficiency policies. Currently, Central and Eastern Europe, with countries such as Poland, Germany, Austria, the Czech Republic, Slovakia, Lithuania, Latvia and Estonia, are the undisputed leaders in the ETICS sector. In this region, about 120–130 million m2 of building walls are insulated annually. This is more than half of the forecasted European volume of about 215–230 million m2. Turkey holds the dominant position concerning individual countries, and Poland, with 40 million m<sup>2</sup> per year, is in the second place [1,2].

ETICS includes the thermal insulation material fixed to the substrate and the top finishing layer made directly on site, without an air gap or intermediate layers. For many years, expanded polystyrene (EPS) and mineral wool (MW) have been the most commonly

**Citation:** Sudoł, E.; Kozikowska, E. Mechanical Properties of Polyurethane Adhesive Bonds in a Mineral Wool-Based External Thermal Insulation Composite System for Timber Frame Buildings. *Materials* **2021**, *14*, 2527. https:// doi.org/10.3390/ma14102527

Academic Editor: Francesca Ceroni

Received: 31 March 2021 Accepted: 7 May 2021 Published: 13 May 2021

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used insulation materials in ETICS, followed by other factory-made thermal insulation products such as extruded polystyrene (XPS), polyurethane foam (PUR), phenolic foam (PF) and expanded cork (ICB) [1,4]. The ETICS finishing layer consists of a reinforcement layer made of a base coat with glass fibre mesh embedded and a finishing coat (renders). Some systems also contain key coating, primers and decorative coats [1,5,6].

ETICS is used as external thermal insulation to the walls of buildings. It can be used on new or existing (retrofit) vertical walls [5]. They can also be used on horizontal or inclined surfaces that are not exposed to precipitation. In Europe, most commonly on masonry walls constructed from units of clay, concrete, calcium silicate, autoclaved aerated concrete laid using mortar and concrete walls made of concrete are either cast on site or as prefabricated panels (Figure 1a) [2]. More and more frequently, ETICS is also being used to insulate walls in timber frame buildings [7]. Then, the thermal insulation material is fixed to the external wall sheathing (Figure 1b), which is usually made of oriented strand boards (OSB), fibre-reinforced gypsum boards (FGB) or cement-bonded particleboards (CPB), less often of fibre-reinforced cement boards or gypsum plasterboards [6–8].

**Figure 1.** ETICS on (**a**) masonry or concrete wall, (**b**) on wood frame wall. Example of purely bonded ETICS.

Based on the method of fixing the thermal insulation material to the substrate, purely bonded ETICS, bonded ETICS with supplementary mechanical fixings, mechanically fixed ETICS with supplementary adhesive and purely mechanically fixed ETICS are distinguished [5,6]. Fixing the thermal insulation material to the substrate is typically carried out using cement-based adhesive. This applies both to ETICS for use on masonry or concrete walls and timber frame buildings. The exception is some EPS-based ETICS [5,6,9,10]. The construction industry is a dynamic field that constantly has new needs. Therefore, academics and manufacturers' continually seek to develop new materials and technologies that can be used as effective alternatives to conventional solutions [11]. As far as ETICS is concerned, attempts are being made to use polyurethane adhesives as an alternative to conventional cement-based adhesives for fixing thermal insulation materials other than EPS to substrates. This is in line with the trend observed for many years of wide, on site use of polyurethane foam in construction [12,13]. Apart from standard applications such as roof insulation [14] or window and door fitting [15], on-site foaming polyurethane adhesives are used to bond masonry elements in wall construction [16,17]. Polyurethane adhesives have numerous advantages, such as effective wetting of most substrates; interaction with substrates through polar interactions (hydrogen bonding); and relatively low molecular

weight/small molecular size, which allows them to permeate porous substrates and to form covalent bonds with substrates that have active hydrogen atoms [18]. They can be applied at low temperatures [12,13,19] and require no preparation activities on site. They are applied directly from a pressurised container to the material to be fixed. Foaming of polyurethane occurs spontaneously in contact with water present in the materials to be fixed. The foaming agent is carbon dioxide formed by the reaction of water with isocyanate groups [12,20].

As mentioned, polyurethane adhesives for fixing insulation material in ETICS are limited to EPS systems. This is due both to formal considerations and to the lack of a sufficient research basis. Placing ETICS on the market, like several other construction products, is regulated by the construction products regulation (CPR) [21], which establishes harmonised conditions for the marketing of construction products. The assessment of the suitability of use of ETICS is carried out through European or national technical assessment. It is subject to testing and assessing properties that affect the object's compliance with the basic requirements. Methods and criteria for assessing the essential characteristics of ETICS are fixed on masonry, and concrete walls are defined in EAD 040083-00-0404 [5] amended ETAG 004 [22] in October 2020. Concerning ETICS for use in timber frame buildings, EAD 040089-00-0404 applies [6]. Both of these documents consider using polyurethane adhesive in ETICS only for bonding EPS to the substrate. For such an application, the issue of testing methodology, taking into account the specifics of an on-site foaming polyurethane adhesive, has been systematised [5,23]. This makes it impossible to obtain, by means of a standard procedure, a European Technical Assessment (ETA) for an ETICS where polyurethane adhesive is used for fixing thermal insulation materials other than EPS and thus makes it challenging to place this system on the EU market.

Apart from expanded polystyrene, the second most common thermal insulation material used in ETICS is mineral wool. In Central Europe, the share of ETICS with EPS is about 84% and with MW about 12%, while in the rest of Europe, it ranges from 60 to 88% and from 9 to 25%, respectively [1,2]. Standard ETICS applications are rock wool in the form of factory-made boards as defined in EN 13162 [24], in which the fibres are either dispersed (standard boards) or oriented parallel to each other and perpendicular to the slab surface (lamella boards). Properties of MW boards used in ETICS are presented in Table 1.

For bonded systems and bonded systems with supplementary mechanical fixing, the adhesion of the individual ETICS layers, including the adhesive bonding to both the substrate and the insulation material, as well the shear properties of bonds are crucial in terms of fulfilment of the fourth basic requirement 'safety in use' [5,6,25,26]. Although several papers have been devoted to the properties of ETICS, looking at a wide spectrum of properties of individual components and their influence on the essential characteristics of the system [1,3,25,27–30], including adhesive bonding [31–34], the authors' attention has been directed towards cement-based adhesive systems. It has been found that bond strength between cement-based adhesive and the concrete, after 28 days under laboratory conditions, can achieve values from 250 kPa to 1000 kPa [31,33], while after 28 days under laboratory conditions and 2 days in water bond strength decreases to 80 kPa [5]. The bond strength between cement-based adhesive and the thermal insulation material strongly depends on the type of insulation material and the model of damage [31,32]. A review of the literature has shown that bond strength between cement-based adhesive and EPS ranges from 80 kPa to 270 kPa [31,33,34] and bond strength between cementbased adhesive and MW achieved values ranges from 30 kPa to 80 kPa [31,32]. Cohesive damage in the insulation material was usually observed [31–34]. The bond strength of polyurethane adhesives in EPS-based ETICS achieved at least 80 kPa [5,6,9,10]. In general, the physical and mechanical properties of polyurethane foam, including polyurethane adhesives, are closely related not only to the rigidity of the polymer matrix but also to the size of the cells and their structure [10,35–38]. Structures with larger cells are characterised by lower apparent density and lower mechanical properties [37,38]. An increase of the water content intensifies the foaming process, leading to an increased cell size [20,39,40].

**Symbol Present in Item Code Property Type of Boards Lamella Standard** - Reaction-to-fire performance Class A1 or A2 - Thermal resistance value declared by the manufacturer according to EN 13162 T5 Thickness tolerance <sup>−</sup>1% or <sup>−</sup>1 mm<sup>1</sup> +3 mm at least −3% or −3 mm<sup>1</sup> +5% or +5 mm<sup>2</sup> DS(70, -) Dimensional stability (48 h, T70 ◦C) thickness change Δε<sup>d</sup> ≤1% length and width change Δε<sup>b</sup> ≤1% DS(70,90) Dimensional stability (48 h, T70 ◦C) thickness change Δε<sup>d</sup> ≤1% length and width change Δε<sup>b</sup> ≤1% WS Water absorption after 24 h of partial immersion ≤1 kg/m2 WL(P) Water absorption after 28 days of partial immersion ≤3 kg/m2 MU1 Water vapour diffusion resistance factor 1 TR Perpendicular tensile strength ≥80 kPa ≥7.5 kPa

In wide bonds, carbon dioxide has the ability to form larger bubbles, resulting in a more porous structure [13,35].


This study aims to assess the applicability of polyurethane adhesive as an alternative to conventional cement-based adhesives for lamella mineral wool boards in ETICS for timber frame building. Bond strength, shear strength and shear modulus tests were conducted. In addition, SEM analysis of the adhesive structure in the bond, as well as apparent density, was performed. These properties were analysed in terms of the influence of substrate type and thermal and moisture conditions, taking into account solutions typical for sheathing on timber frame: oriented strand boards (OSB), fibre-reinforced gypsum boards (FGB) and cement-bonded particleboards (CPB), as well as limit conditions for adhesive application.
