Preliminary Report on Ignitibility of Combined Pro-Ecological Waterproofing and Fire Retardant Coatings for Paperboard in Architectural Application

One of the most crucial difficulties to overcome when designing with paper is fire safety – as with most bio-based building materials, paper is easily flammable. However, the combustibility of the material depends on the type of paper-based products used. While products composed of thin sheets of paper with air cavities in between, such as corrugated cardboard and honeycomb panels, ignite and spread fire quickly, thick layers of paperboard present certain fire protection properties. The outer layers of thick paper-board chars, forming a protective layer on the rest of the material – a similar mechanism can be observed in wood [1].

Despite this mechanism, additional fire retardant protection of paper-based building elements is necessary to ensure users safety. Although the legal regulation varies between countries, in most cases it is required that building elements are non-combustible or fire-resistant.

There are three main approaches to reducing the flammability of cellulosic, including paper-based elements.

Material additives during production, e.g. phosphorus compounds, that provide effective long-term protection. However, the presence of retardants, e.g. inorganic salts, in the material may interfere with elements bonding and have an abrasive effect on cutting tools [11].

In several paper-based designs and already completed buildings, a variety of fire retardant techniques has been used (see Table 1).

The role of fire retardants (FR) is to increase the fire safety of the impregnated material by reducing its contribution to a possible fire. This effect can be achieved by delaying the moment of ignition of the material and reducing the combustion intensity and the spread of the fire. Several mechanisms of disruption of the combustion process at different stages can be distinguished.

Formation of coatings that slow down the transport of heat to the protected material, delaying the moment of ignition;

Besides ensuring fire safety, FR should be harmless for people and the natural environment during normal use of the treated objects, as well as during combustion [16].

A large variety of chemical compounds can be applied as fire retardants, especially inorganic salts of phosphorus, boron, chlorine or sulfur. Furthermore, organic substances, like urea and melamine can also be used [16]. In most commercial FR several compounds are combined to employ various retardancy mechanisms and achieve more effective protection. In the last years, the use of some FR has been either discouraged or prohibited due to their toxicity and harmful effects on people's health and the environment. The widely used halogenated FR (especially brominated FR) have been proven to emit toxic halogenated compounds during ignition, that accumulates e.g. in crop plants, penetrate the human body and cause carcinogenic effects [17]. Therefore, it is crucial to use FR that are not only effective but also safe. Besides experimental studies on the use of organic compounds like tannic acid [18], guanidines [19, 20] or plant extracts as FR, e.g. green coconut shell or banana pseudostem [21, 22], widely used groups of non-toxic FR are borates and phosphates.

Borates are one of the most efficient and well-studied FR for wood and other cellulosic materials, such as bamboo [17, 23] or paper [24]. They are also incorporated as a FR additive in the production of paper-based composite materials or laminates [25, 26, 27] and into commercial cellulose-fibre thermal insulation [28]. Borates can also act on these materials as a protection against insects, bacteria and fungi [29]. The two most widely-used FR compounds are boric acid and borax (sodium borate), both non-toxic and cheap. Both of them act by physical mechanism, forming a glassy protective coating on the impregnated surface, as well as chemical, by promoting a char formation and releasing water vapour at high temperature [17, 30, 31]. However, as described by Yu et al., borax presents better performance in slowing down the heat release while boric acid results in a lower amount of total heat released. Therefore, to achieve the best, synergic results the study suggested the use of both compounds in a ratio of 1:1 [23], which also enables solutions of higher concentration to be produced. The differences between the compounds are also noticeable in the degree of resistance to humidity. While boric acid is resistant to water vapour, changes in humidity may cause efflorescence on surfaces coated with borax [11]. Moreover, none of the borates has the ability to chemically bond with cellulose, which may have a negative effect on the durability of the protection, however, allows easy separation of the impregnating agent during the recycling process.

A wide range of Phosphorus-containing compounds can be used as a FR, including organic compounds (e.g. Trialkyl Phosphate or Phosphoramide) and inorganic ones [32, 33]. The most popular are inorganic salts of Ammonium Phosphate and Diammonium Phosphate. Like boron compounds, phosphates are non-toxic and soluble in water, however, they are more sensitive to changes in air humidity [11]. The main fire retardancy mechanism of phosphates in the condensed phase involves the formation of a non-combustible char layer on the impregnated surface [17]. Moreover, the retardants can also inhibit flame in the gas phase [34]. Like borates, inorganic phosphorus salts do not chemically bind to cellulose.

Most of the abovementioned FR are water-soluble, hygroscopic and sensitive to air humidity. Paper elements are also particularly sensitive to moisture, therefore it is especially important to protect impregnated paper from contact with water. This can be achieved by cladding the element with other materials, such as membranes and metal sheets or providing an additional layer of waterproof impregnation.

The presented article investigates the use of environmentally-friendly inorganic boron- and phosphorus-based FR on paper-based building elements, as well as the combinations of these FR with biodegradable water impregnates. Although the mechanism of fire retardancy on cellulosic materials is already well known, there is no research available regarding additional water protection of the impregnated elements and their architecture-related applications. As water impregnates tend to increase the combustibility of the treated materials, while fire retardants usually increase their water absorbency, the combination of both types of protection is essential for its use in building elements. Therefore, the main objective of the research is (i) to verify the possibility of layering FR and oil-based waterproofing impregnantes on paperboard, (ii) to test their ignitability when exposed to a single flame, (iii) to compare the effects obtained using FR, water retardant and both, (iv) to observe changes in the impregnated surface over time, and (v) to assess the environmental impact of the FR. The results indicate potential new ways to protect paper-based building components in an environmentally-friendly and safe way.

The tests were carried out on rectangular 90 × 250 mm pieces of three-layer glued paperboard made of 100% recycled paper, with an area density of 1845 gsm (Zing, EskaBoard paperboard, 3 mm thick).

The raw materials for the preparation of the FR solutions were obtained from Warchem Company, a local manufacturer of laboratory reagents from Poland. Three types of chemical compounds were used:

borax (sodium borate) – Na₂B₄O₇ x 10H₂O;

Water impregnation techniques were selected based on previous research regarding biodegradable paper impregnation [31]. The two types of coatings, that had presented the best performance in water-resistant tests were used in the presented research:

a composite coating of linseed oil varnish (Dragon, Linseed oil varnish) and wood wax – a mixture of beeswax, plant-based and synthetic waxes (ICA Poland, Colorit, Paste wood wax);

Four groups of paperboard specimens were tested:

with only water impregnation (Group 1),

For each type, three identical specimens were prepared. Two FR retardant solutions were prepared by dissolving reagents in deionized water with a temperature of 50°C. The borates solution (BA-BX) was prepared from boric acid, borax and water in the ratio of 1:1:8, and phosphorates solution (DP) from diammonium phosphorate and water in the ratio of 3:7. The chosen concentrations result from the solubility of the used raw materials.

The impregnation was done manually, using a brush. Firstly, fire-impregnates were applied in three layers and the specimens were air-dried for four days. The applied amounts allowed to achieve a saturation of approximately 200g/m² for DP and 167g/m² for BA-BX. Secondly, the water impregnants were applied in two layers, and the specimens were conditioned for another 14 days in normal conditions. Types of specimens are described in Table 2 and the impregnation process is illustrated in Figure 1.

Figure 1

The testing methodology was developed based on a single-flame source test described in ISO 11925-2 standard [35]. The tests were carried out in normal condition (approx.. 20°C, 50%RH) with a standardized flame of the propane-butane burner, corresponding to the size of the match flame. The specimens were mounted in a frame, and the flame was applied to their surface at an angle of 45 degrees, 40 mm above the bottom edge of the specimen (see Figure 2). After 30 s of exposition, the flame was extinguished, and the specimen was observed for another 30 s. After 60 seconds from the start the test was finished and the fire was extinguished if necessary. During the tests, specimens were observed in terms of ignition, fire maintenance and flames reaching 150 mm above the fire application point.

Figure 2

After the test, specimens were photographed and the ImageJ software was used to analyze the shapes and calculate the areas of the charred surfaces. Lastly, the specimens were left for a year-long observation for changes in appearance, such as colour change, unintentional interaction between impregnates or possible crystallization of flame retardants on the surface of the specimen.

The Life Cycle Assessment analysis was performed in accordance with ISO 14040 and ISO 14044 standards [36, 37], using OpenLCA software with ecoinvent 3.8 database and ReCiPe LCIA method. The analysis was conducted for paperboards coated with BA-BX mixture, DP and uncoated, for a functional unite of 1 m². The analysis covers the production phase (cradle to gate approach) and the end-of-life phase, with a 90% recycling rate assumed for paperboard. The use phase was excluded as the application of FR paperboard in building elements is not fully addressed yet, therefore there is a lack of reliable data in this area.

Significant differences in the ignition process of the four tested groups of specimens were noticed (see Table 3). The unimpregnated paperboards in Group 0 showed ignition and incandescence requiring extinguishing at the end of the test. These were the only specimens in which burning occurred through the entire thickness of the material. Specimens from Group 1 (only water impregnation) ignited and maintained the fire, which spread across the surface of the paperboard, resulting in the combustion of a large area. Specimens impregnated with a combination of linseed oil varnish and wax (1-X-LW) were the only ones with flames reaching the height of 15 cm, and the ones with the largest charred area. As expected, specimens from Group 2 (only fire impregnation) presented full fire resistance with no ignition and an uniformed, oval-shaped charred area.

In Group 3 specimens, a significant reduction in flammability was observed compared to Group 1, although the results were slightly worse than in Group 2. None of the specimens maintained fire when the flame was removed, although a small ignition (not reaching a height of 15 cm) did occur for the phosphate-coated specimens (3-DP-O). The spread of the flames was noticeably smaller than in Group 1, which was also reflected in a limited charred area (see Figure 3).

Figure 3

Figure 4

No problems during the impregnation process were observed – precoating with fire retardants did not hinder water impregnation – and no changes in the appearance of the specimens occurred shortly after the impregnation. However, after the year-long exposure to light and changing air humidity, significant changes were noticed in some of the specimens (see Figure 5). On all of the paperboards impregnated with DP efflorescences are visible, while BA-BX specimens remain free from such changes. Moreover, all the specimens with water impregnation have developed a yellow tinge, thaw was especially visible on paperboards with wood oil treatment and DP pre-coating.

Figure 5

The conducted LCA analysis showed a limited impact of both discussed FR techniques on the paperboard component. The BA-BX coating increased the overall environmental impact of the element by 4% in relation to uncoated specimen, and the DP coating – by 7% (see Figure 5). In each case, the highest impact (approx. 50%) was associated with agricultural land occupation, due to the use of land for the cultivation of cellulosic plants. The most significant changes between LCA scenarios were observed in the areas of “climate change, human health”, “particulate matter formation” and “fossil depletion” (see Table 4).

The results obtained, confirmed the validity of using inorganic FR while coating the cardboard with oil- and wax-based waterproofing agents. Coating of borates mixture was more effective in inhibiting ignition than diammonium phosphorate solution, and also more resistant to coating degradation over time. Moreover, it presented a slightly lower environmental impact in the LCA analysis. In the case of water impregnates, wood oil was less combustible than a mixture of linseed varnish and wax. The former, according to previous studies, is also more effective in protecting against water [38]. Therefore, on the basis of the data obtained, the best effect in terms of both water and fire protection and durability of the coating can be achieved by coating paperboard with a mixture of boric acid-borax and wood oil (testing specimen 3-B-O).

The presented technique can be applied to building elements that require moderate water and fire protection. It should be noted that coated paperboard cannot form a building element on its own, but it needs to be combined in sandwich elements with different materials that provide structural stability, e.g. paper honeycomb panels, and corrugated cardboard. In such composition coated paperboard can significantly increase fire, microbes and mechanical resistance of the elements, as well as its acoustic properties if needed. Depending on the type of composite, it can be used as building envelope, building partition, or structural element of small buildings.

The most common application of the proposed technology is in temporary and emergency structures. Paper-based elements can be sustainable alternatives for conventional materials, as their limited life span is more suited for temporary architecture. Still, regarding their life span, temporary architecture has to present a certain level of fire resistance, to ensure users' safety. The proposed impregnation is a pro-ecological alternative to traditional varnishes and foils used in temporary buildings, in which it may act as a single layer of protection. Moreover, the proposed coating can be a part of a double protection system for the envelopes of permanent buildings, in combination with non-combustible finishing materials such as metal sheets or mineral boards. The presented technique can also be used as additional protection between internal layers of the paper-based building envelope, hindering fire penetration into the material in case of fire [39]. Furthermore, paper-based elements are already used in lightweight building partition systems, where incorporation of coating may reduce the need of using finishing materials with a higher environmental burden (e.g. fire retardant plastic).

Architectural boards that are currently used in applications similar to those proposed for paper-based ones are various types of wood-based products (e.g. soft fibreboard), thin metal sheets, plastic panels (e.g. low-density polyethene or polylactic acid) or laminates, such as HPL (that also includes paper) or glass fibre-resin laminate. Most of these existing products present higher mechanical strength but, on the other hand, significantly higher environmental impact. Therefore, product selection should be tailored to the requirements of the specific application.