Natural polymeric water-based adhesive from cork liquefaction

Highlights

• Formulation of two components water-based adhesive from cork for lignocellulosic surfaces.

• Adhesive with no volatile organic compounds.

• Formulation of an adhesive with a good shear strength.

Abstract

Herein we disclose a novel formulation of two components water-based adhesive from a natural source, which is designed to glue lignocellulosic surfaces, mainly cork and wood. It is based on the aqueous extraction of liquefied cork, which results from the polyhydric alcohol acid-catalyzed liquefaction of cork dust or granules. The vacuum concentration of the referred extract leads to the first adhesive component, which is then mixed and homogenized with the second one, a curing aliphatic compound. The addition of “cork powder” to the adhesive formulation creates a stable adhesive with good shear strength values. Thus, a methodology for the valorization of cork dust, a hazardous waste, is here outlined.

Introduction

The bark of Cork Oak (Quercus suber L.), an endemic tree of the Western Mediterranean region, mainly in Portugal and Spain, is considered to be an environmentally friendly and sustainable raw material. The cork is harvested with the preservation of the tree. Its bark is stripped without cutting it down. The new bark is, afterward, allowed to regrow for subsequent extractions (Sierra-Pérez et al., 2015). Cork-based materials, durable products, are believed to play a role in the contribution to decreasing the amount of carbon dioxide in the atmosphere since they are considered to be “carbon neutral” (Gil, 2015). The high interest of the use of Cork in the most diverse forms and applications is related to its peculiar properties, namely elasticity, acoustic insulation, permeability to liquids, low thermal conductivity, low density, high absorption energy, and hypoallergenic properties (Fernandes et al., 2010, Gil, 1997). Cork stoppers manufacture, the most relevant industrial activity of the cork business sector, reaching the annual global production of more than 10¹⁰ units only uses about 25% of the original raw material (Pereira, 2007, Rosa and Fortes, 2009). The resulting by-products are sent to granulation being then used in a variety applications, mainly cork agglomerates, composites, cork pavement, insulation panels, lightweight structures and agglomerated wine closures among others (Anjos et al., 2010, Anjos et al., 2014, Arteiro et al., 2013, Brás et al., 2013, Cardoso et al., 2008, Castro et al., 2010, Demertzi et al., 2015, Gama et al., 2015, Gil, 2015, Jardin et al., 2015, Lakreb et al., 2015, Pereira, 2011, Silva et al., 2005, Soares et al., 2011, Sousa-Martins et al., 2013, Vilela et al., 2013, Yona et al., 2014). On the other hand, the transforming industry generates a residue which is considered to be problematic, the so-called “cork dust”. This residue is a substantial fraction of the total amount of wastes with almost none commercial value. As the name suggests, cork dust includes small size particles. According to the Portuguese standard (NP-114 and NP-273), it has dimensions below 0.25 mm (Gil, 1997). Cork powder is often used in the production of agglomerates, although its inclusion is only possible in small amounts. Several solutions have been envisaged to recycle this hazard residue (Fernandes et al., 2011, Gil, 1997, Gil, 2015). Currently, it is mostly applied as fuel for burning in furnaces: either in the cork industry or even in the ceramic industry, because it presents a high calorific value, which is between 18.9 and 29.3 MJ/kg (Gil, 1997).

Regarding its composition, cork major components are polysaccharides, lignin, suberin and extractives (20, 22, 40, and 15 wt.%, respectively) (Neto et al., 1995, Pereira, 1988).

Liquefaction of biomass, such as lignocellulosic residues, is a process that has been widely investigated and consists in the depolymerization and solubilization of biomass at high temperatures (Balat, 2008, Briones et al., 2012, Hu and Li, 2014, Hu et al., 2014, Jasiukaitytė-Grojzdek et al., 2012, Jasiukaityte et al., 2009, Kunaver et al., 2012). Lignocellulosic materials can be seen as a source of raw materials for the chemical industry. In fact, throughout the liquefaction of that feedstock, products enriched with hydroxyl groups are obtained. Those products can be then promptly used as starting materials for the formulation of environmental friendly polymeric products (Lee et al., 2002).

Lately, cork by-products, have been investigated as a source of biomass for acid liquefaction using polyhydric alcohols under conventional (Soares et al., 2014), microwave (dos Santos et al., 2015) and ultrasounds-assisted methodology (Mateus et al., 2015).

Presently, wood adhesives are polymers produced mainly by petroleum sources, but on the edge of its exhaustion and due to their negative environmental impact, new solutions are needed. However to compete with the fossil sources based adhesives, the bio-based formulations must match the properties of conventional glues and present new valuable technical features at a competitive price (Norström et al., 2014). Thus, the formulation of environmentally friendly adhesives from a renewable and sustainable feedstock, as an alternative to those derived from petroleum, have become a matter of interest among polymer chemists over the last few years (Chen et al., 2013).

Within this context, we planned to develop an innovative and new formulation of a water-based adhesive, derived from the aqueous extract of liquefied cork, for cork or other lignocellulosic materials, which was never yet reported, as far as we know. With the approach outlined during our studies, a procedure to mitigate the impact an industrial waste, considered to be harmful to the environment, was developed. Our work represents, thus, a contribution in the search for a solution for those residues by upcycling them under the concept of biorefineries. Herein, the work that conducted to a patent of a new adhesive for lignocellulosic materials is disclosed (Bordado et al., 2015).