Beneficial in-situ incorporation of nanoclay to waterborne PVAc/PVOH dispersion adhesives for wood applications

Abstract

Nanoclay/polyvinyl acetate waterborne adhesives were prepared by using a two-step polymerization process. First, seed polymer particles containing the clay were obtained by batch miniemulsion polymerization. Then, the clay was buried within the particles by the addition of a neat monomer in the second step. The final stable dispersions could have up to 50 wt% of solids content. TEM images clearly showed the presence of clay inside the polymer colloids, although not totally exfoliated. The addition of nanoclay produced adhesives with higher water and heat resistance judged by the water swelling behavior and thermal degradation properties, which was reflected also in their better adhesive performance under wet conditions (D3 tests according to EN204) and high temperatures (WATT91) of the samples containing clay with respect to the pristine dispersions.

Introduction

Wood has an intrinsic potential to fulfill the criteria for being a competitive and sustainable engineering material, i.e. a renewable resource available in vast quantities and formed as a natural composite with an extraordinary high strength-to-weight ratio. However, for outdoor use it is necessary to enhance the performance and long-term durability of wood-based materials and the products related with their production, like coatings and adhesives.

The most commonly used adhesives in wood applications are the thermoset formaldehyde-based adhesives (urea–formaldehyde – UF, melamine–formaldehyde – MF, phenyl-formaldehyde, and resorcinol–formaldehyde) which have achieved low formaldehyde emissions [1]. Waterborne poly(vinyl acetate) (PVAc) dispersions stabilized with poly(vinyl alcohol) (PVA) have been used in wood adhesive for more than 50 years due to their good performance and competitive cost [2], [3]. However, since water is a plasticizer for both polymers, these adhesives have low mechanical performance in warm and humid environments [4]. European Standard EN-204 gives the description and the minimal requirements that the PVAc adhesives must fulfill. D1 adhesives show a good resistance only under dry conditions; D2 adhesives should withstand a rather low water presence; D3 adhesives are suitable in the case of contact with cold water; and D4 adhesives are suitable to be used under extreme conditions (resistance to hot water) [5]. Conventional PVAc polymers can be used to formulate standard D1 or D2 adhesives. In general, the performance of these PVAc adhesives is improved to achieve a D3 or D4 performance either by using specific functional comonomers in the emulsion polymerization, by including polyvalent metal salts in the adhesive formulation, or by post-addition of thermosetting resins such as UF, MF or polyisocyanates for two-component systems, which rises the price of the final adhesive [6], [7], makes the application process more complex for the end user, and may lead to formaldehyde emissions.

Functional engineered nanoparticles can also be introduced into water-based wood adhesives in order to improve the properties of wood–adhesive joints. The cost of the improved adhesives is a very critical parameter; thus the identification of cheap sources of nanoparticles will have priority. Nanoclays would be ideal systems due to their natural availability and low cost. Nanoclays have been used together with polymers in nanocomposites for a long time [8]. It has been found that these nanocomposite materials are superior to conventional unfilled materials with respect to the mechanical properties, thermal stability and barrier properties. In the case of water resistance, it has been proved that the organically modified clays have produced a reduction in the water uptake and water vapor permeation rates of polymer/clay nanocomposites [9], [10], [11]. On the other hand, it has been previously found that nanoclay can improve the mechanical properties of thermoplastic adhesives too (for example improvement of the shear strength) [12], [13], [14], [15]. For these improvements, it is important that the nanoparticles are compatible with the polymer matrix and are well dispersed. However, by conventional emulsion polymerization the exfoliation and dispersion of clay are limited, usually producing armored latex particles at low solids content [16], [17], [18].

Miniemulsion polymerization technology seems to be more appropriate to obtain encapsulated morphologies [19], [20]. Upon mixing the aqueous and organic phases, and by applying energy to reduce the size of the droplets of the coarse emulsion, a stable dispersion of nanodroplets containing clay platelets is obtained. This miniemulsion can be polymerized to produce a latex where ideally the clay platelets would be encapsulated inside the polymer particles. Nevertheless, obtaining polymer particles with exfoliated clay incorporated into polymer particles (encapsulated or engulfed morphologies) is elusive. Although the hydrophilic nanoclay particles are usually modified with an organic molecule for its optimal dispersion in a lipophilic polymer matrix, difficulties arise from the fact that organically modified clays (OMC) cannot show full compatibility with the monomers, leading to phase separation [21]. Additionally, the aspect ratio of some clays (>150) makes their incorporation difficult in the typical range of particle sizes obtained in miniemulsion polymerization [21], [22]. Furthermore, the stability of the latexes at high solids content cannot be guaranteed without using a very high amount of emulsifier concentration that in turn reduces the particle size of the miniemulsion droplet and decreases the properties of the latex films [23].

In a previous work a two-step polymerization approach to encapsulate clay within polymer colloids based on PVAc/PVA with 50 wt% of solids content was presented [24]. In this paper, the synthesis of new nanoclay–PVAc/PVA dispersions with a high solids content is presented in order to verify if the extensive encapsulation of clay is beneficial for the PVAc adhesive applications. A commercial organomodified clay was used, and the final clay content was varied between 0.3 and 1.7 wt% with respect to the solids of the dispersion (50 wt%). The morphology of the particles and films was studied by transmission electron microscopy and X-ray diffraction was used to study the degree of exfoliation of the clay platelets. Thermal, water sorption, mechanical and adhesive properties of films obtained from the dispersions were also studied to demonstrate that the presence of clay is beneficial for adhesive performance of these materials under hot and wet conditions.