Influence of organic fillers on rheological behavior in phenol-formaldehyde adhesives

Abstract

Phenol-formaldehyde resoles (PF) formulated with wheat flour and organic fillers have long been established for the manufacture of veneer-based wood-composites, and yet much remains unknown about these complex fluids. The rheology of PF/filler formulations was studied as a function of filler type and particle size. Corn cob (Zea mays) residue fillers behaved differently from those made from alder bark (Alnus rubra) and walnut shell (Juglans regia). It was shown that viscoelastic network structures formed within the liquid formulations as a function of shear history, filler type, and filler particle size. The precise nature and origin of these effects is unknown but could involve disintegration of filler particle aggregates on a non-colloidal scale, and/or colloidal effects within the liquid PF medium. In the latter case colloidal structures could form among associated PF chains and also from proteins, polysaccharides, and lignins that leach from wheat flour and filler particles. Relative to alder bark and walnut shell, the unique behavior of corn cob residue was discussed with respect to chemical composition. Many implications for impact on industrial practice are feasible and should be the subject of future research.

Introduction

Phenol formaldehyde (PF) resoles are outstanding wood adhesives that remain as the preferred resin used to manufacture structural wood-based composites from veneer, as in plywood and laminated veneer lumber. In such veneer applications PF resoles are commonly formulated with fillers and extenders [1]. Definition of the terms “filler” and “extender” vary [2], but in the wood products industry fillers are considered to be generally inert, while extenders are proteinaceous and amylaceous materials that exhibit some intrinsic adhesive properties [3]. Contemporary PF/filler/extender formulations are designed to meet a variety of performance criteria including bulk flow, prepress tack, precure moisture retention, gap-filling properties, not to mention post-cure strength and durability [1], [4].

For the applications described here, the fillers are typically organic flours derived from lignocellulosic biomass waste streams such as walnut nutshell (Juglans regia), red alder (Alnus rubra) bark, and furfural production residues, i.e. corn cob residue [1]. While these fillers have held commercial significance for several decades, they have been the subject of little or no detailed analysis resulting in scientific publication. Only Ebewele et al. [5] reported the impact that walnut shell fillers had on adhesive performance. This publication represents an industry/university cooperative research effort intended to broaden the scientific base underlying current and future technologies. The organic fillers studied here, flours of walnut shell, alder bark, and corn cob (furfural production) residue impact the flow properties of PF adhesives. Accordingly, this is a report of how liquid PF adhesive rheology is impacted by the type and particle size of organic filler.