Abstract
Phenol formaldehyde (PF) is found across a wide variety of applications, from wood adhesives in plywood and particleboard, to friction materials in automotive break parts and as a coating material in several markets. However, it is a petrochemical product and formaldehyde has also been classified as a carcinogen by the Environmental Protection Agency (EPA). Lignin-furfural resins are proposed as sustainable and safe formaldehyde-free alternatives. Lignins recovered from hydrolysates of the hot water extraction (HWE) process of select angiosperms; sugar maple (SM), willow (W), mixture of northern hardwoods (NHW), miscanthus, (MS) and wheat straw (WS) were used as raw materials. In addition, crude lignin recovered after acid hydrolysis of sugar maple hot water extract (SMAH) and SMAH after electron-beam irradiation (EBI) at 50 kGy (SMAH_IRR50) and at 100 kGy (SMAH_IRR100), were also used. The biorefinery lignins were characterized for their chemical composition and inherent properties via wet chemistry and instrumental techniques (HSQC, SEC, TGA and DSC). Two technical lignins, kraft (KL) and a lignin from dilute acid treatment (PL) were also selected for control purposes. The effect of reaction pH, furfural content, curing conditions (180°C and 1.9 MPa) and curing agents (HMTA and furfural) on the mechanical properties of the lignin based resins is determined. Testing of mechanical properties was conducted on resin reinforced glass fiber filters. Glass fiber filters were chosen for their inert nature and any improvement in strength was assumed to be of the resin itself. Based on the results obtained from the tests performed on glass fiber filters, select resins were tested for their mechanical properties on friction paper, and softwood and hardwood kraft papers for their reinforcement ability. Commercially available novolac-type PF resin was used as a control for all mechanical testing. Resins prepared from MS and SMAH lignin show better mechanical properties compared to other biorefinery lignins and are also comparable with PF resin. The good mechanical strength of resins prepared from MS and SMAH is attributed to higher S/G ratio and free phenolic hydroxyl group content – features that determine reactivity of the lignin.