How to evaluate/test critical adhesive properties fast and without the effect of wood material used?

Abstract

Polyvinyl acetate (PVAc) is one of the most prevalent synthetic polymer types used in adhesives in wood bonding due to many benefits it offers. Examples of these benefits are low cost, good tack properties and compatibility with wood. However, PVAc adhesives are limited by poor resistance to moisture and heat, restricting their performance in demanding conditions. These limitations can be addressed through crosslinking, which improves strength, durability, and environmental resistance.

Durability testing of PVAc in wood bonding, such as EN204 and EN205 standard tests, is time-consuming, resource-intensive, and results are affected by the variability of wood as a substrate. This raised the question of whether PVAc adhesive performance could instead be predicted through film testing, allowing for faster and more cost-effective evaluation. The goal of this work was to identify quick and reliable test methods for PVAc films that could be implemented in product development to efficiently evaluate adhesive performance in wood bonding.

In this thesis, PVAc adhesives with varying crosslinking densities were prepared and analyzed both through standardized wood bonding tests that acted as a reference and alternative analytical techniques for testing adhesive films and different properties of those films. Standardized tests confirmed that increased crosslinking generally enhanced performance, with 5% catalyst content providing the best balance of strength and water resistance. Among the alternative methods, tensile testing of films showed strong correlation with the results of the standardized tests, indicating its potential as a predictive pre-screening tool. Curing time studies showed that PVAc films reach near full strength within couple of days instead of the standard 7-day curing time, which indicates that pre-screening tests could be conducted in a shorter time frame. Dynamic Mechanical Analysis (DMA) also showed promise in detecting crosslinking-related differences, particularly in storage modulus at elevated temperatures. Measurement inconsistencies, however, affected these results, which highlighted the need for further refinement.

Other techniques were also tested but proved less reliable for predicting adhesive performance. Contact angle measurements showed significant scattering and droplet spreading without consistent trends related to crosslinking density, whereas Fourier-transform infrared spectroscopy (FTIR) only revealed minor intensity differences without clear evidence of crosslinking. Swelling tests confirmed reduced water uptake but the method lacked sensitivity to distinguish between moderate and high crosslinking levels. Both Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA) produced small, inconsistent trends that could not be confidently linked to crosslinking.

Overall, this work demonstrates that while not all techniques are suitable for predicting PVAc performance, tensile testing and potentially DMA show strong potential. These methods could be integrated into the product development process and used as a pre-screening tool to identify promising adhesive formulations and eliminate poor ones early. This approach would save time and costs while reducing wasted effort on testing weak formulations in actual wood bonding.