Bio-based polyurethane foams toward applications beyond thermal insulation

Highlights

• Coffee grounds wastes were successfully liquefied yielding a bio-based polyol.

• Coffee grounds derived foams formulations were optimized by tuning reagents’ contents.

• The viscoelastic properties of these foams are promising to expand their applications.

Abstract

In this work the preparation of viscoelastic bio-based polyurethane foams (PUFs) using polyols obtained via acid liquefaction of coffee grounds wastes has been optimized. In a first stage, the effect of different ratios of isocyanate content to hydroxyl number (0.6, 0.7 and 0.8) and of three distinct percentages of catalyst (3%, 5% and 7%) on the extent of the polymerization reaction was studied by infrared spectroscopy. Next, different percentages of surfactant (14%, 16% and 18%) and blowing agent (12%, 14% and 16%) were used to assess their effect on the density, thermal conductivity and mechanical properties of the foams, including their recovery time. The mechanical properties of the ensuing foams proved to be very interesting due to their viscoelastic behavior. PUFs were also characterized by scanning electron microscopy (SEM) revealing a typical cellular structure and by thermogravimetric analysis (TGA) which proved that these materials are thermally stable up to 190 °C. These results suggest other potential applications for these materials beyond heat insulation in areas where damping properties can be an added value.

Introduction

Polyurethane foams (PUFs) are a class of lightweight porous materials with enormous interest because of their specific properties and potential application in several fields [1]. PUFs can be classified into two major categories: flexible and rigid foams. In some classifications PUFs are subdivided in semi-rigid and semi-flexible foams [1]. Moreover, some flexible foams can also be classified as viscoelastic when they show a delayed recovery. In fact, other expressions like, visco-hyperelastic, memory, slow recovery, controlled recovery or low resilience foams are also used [2], [3], [4]. Besides applications in thermal insulation, the range of applications of viscoelastic PUFs includes acoustic absorbing materials for noise and vibration control due to their great potential for the damping of mechanical vibrations [5]. Therefore, they are often used to reduce vibrations and harshness and consequently increase comfort [6]. For these reasons, viscoelastic PUFs are commonly used in seats in automobile and aircraft industries. Furthermore, depending on their viscoelastic properties PUFs may also be used as panels in buildings for heat [7], sound [8] and vibration [9] insulation consisting in a significant contribution to energy management, as well as helping to reduce noise pollution. Yet, the use of these porous materials to hamper vibrations requires specific viscoelastic characteristics when they are bound onto the vibrating structure [10].

Currently, the polyurethane (PU) industry is still heavily petroleum-dependent because its two major feedstocks, polyols and isocyanates derive entirely from it. However, due to the uncertainty about the cost of petroleum in the future, as well as the need to move toward more environmentally friendly feedstocks, many recent efforts have been focused on replacing all or a portion of the conventional petroleum-based polyols by counterparts obtained from renewable resources. In fact, many non-petroleum resources, as well as different processes of production of non-petroleum derived polyols have been used in PUs production. Processes such as oxypropylation [11] or acid liquefaction [12] of several biomass residues, as well as the modification of vegetable oils following different strategies [13] have been used to produce renewable polyols for the PUs industry. The acid liquefaction of biomass resources, like starch [14], soybean [15], alginic acid [16], palm [17], sugar-cane bagasse [18], lignin [19], cork [20] or coffee grounds [21], to obtain products which can be used as the polyol component in the production of PUFs is a particular interesting strategy. This is due to the fact that it does not involve the use of a large amount of harmful solvents or reagents nor pressure thus, it presents a relatively low environment impact. However, the majority of these studies are generally associated with the production of rigid PUFs as a result of the functionality of the ensuing polyols.

Due to their unique taste and flavor, coffee brews are among the most consumed beverages in the world and have grown steadily in commercial importance during the last 150 years. With an annual worldwide production of approximately 120 million tons, large quantities of residues are generated [22]. The spent coffee grounds, the solid residues obtained from the treatment of coffee powder with hot water to prepare instant coffee, are the main industrial residues with a worldwide annual generation of 6 million tons [22]. Considering the huge amount of coffee residues produced all over the world, its reutilization and valorization is of major relevance [23]. The composition of coffee grounds may vary from species to species, but they are typically rich in polysaccharides (34–53%, w/w) [22], [24], which make them suitable for liquefaction into bio-based polyols [21] to be used in PUFs formulations.

In the present study liquefied coffee grounds were used as the polyol component for the production of viscoelastic bio-based PUFs. To the best of our knowledge this is the first report on the production of PUFs with a distinct viscoelastic behavior from that of typical rigid foams produced using bio-based polyols. The formulation used in the production of PUFs was first optimized and the resulting foams were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), density and thermal conductivity measurements, compressive tests, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA).