Introduction: Development new adhesives and modification of common adhesives are two main fields for wood-based composite studies. Usually, in order to evaluate adhesive performance, some products are made according to certain treatments that increase the research costs and time. The alternative method is the evaluation of strength and rheology properties on a smaller scale but at higher speed and precision in which the adhesive is applied in tiny amounts on smaller samples and hot pressing is done with high accuracy. This technique is performed by an Automated Bonding Evaluation System (ABES) according to a standard method.
Method: In this study it was required to modify UF resin by adding nano-magnetic particles to use in plywood production in high frequency heating process. To prevent nano particle aggregation, encapsulation technique for the particles was applied. For this purpose, three types of material, including starch, PEG and Acacia gum were used under various mixing conditions. Then, the performance of the mixing method and material type was controlled qualitatively via visual inspection. After that, 20% of different suspensions were added to UF and the gel time of the modified adhesives were obtained with the Adhesive Bonding and Rheology Evaluating System and compared to each other. After determination of appropriate composition, another investigation was done to obtain shear strength development isothermal graphs as a function of press time. For this purpose lap-joint samples of Beech strands with 0.6 mm thickness and 20Î5 mm² area, were made under compositions of press temperature and time as variables in three levels of 80, 100 and 120 °C and five levels of 40, 60, 80, 100 and 120 s respectively. Then, the shear strengh of them was tested with tension loading system. By investigating the obtained graphs, the trend of shear strength development due to press time and press temperature were determined in different adhesive compositions and the optimum one was selected. In the final stage, to investigate the effect of the encapsulated nano particles load in the adhesive matrix, different levels (0%, 4%, 8%, 12%, 16% ,20% and 24%) of selected compound were added to UF resin. Lap-joints were made similar to those in previous stage but with the optimum temperature and time conditions (100 °C press temperature and 90 s press time), and the shear strength was measured on the specimens.
Results: The results showed that the type of encapsulating material has certain effect on the unfiorm distribution of particles as well as gelation time, and among the three applied encapsulation materials, starch had the best performance. With the presence of encapsulated nano-magnetic particles in UF, the bondline shear strength improved. The amount of this increase compare to pure UF was obtained 2.2%, 5.96%, 8.2%, 7.3%, 9.4% and 8.92% respectively for treatments with 4%, 8%, 12%, 16%, 20% and 24% encapsulated nano-magnetic particles. The isothermal graphs of shear strength development of the modified adhesive showed that the relation between press time and strength was linear until 80 s and then became nonlinear.
Conclusion: This study found that the encapsulation of nano-magnetic particles with starch could lead to uniform suspension without accumulation and their presence in UF matrix up to 24% based on adhesive solid mass, didn't have a negative influence on the shear strength of the bondline. Application of this suspension not only caused no disruption to UF curing but also reduced the time of this process compared to pure resin.