Preparation and Characteristic of Polyurethane Powder Adhesives with Heating Resistance Modified by Nanosilica

Polyurethane powder adhesives (PPAs) have a great application prospect in various areas. However, the weakly adhesion property at high temperature greatly hindered its application. In this study, nanosilica as reinforcing agent and dispersing agent has been introduced during the preparation of PPAs. A series of the polyurethane powder adhesives modified by nanosilica were prepared and the influence of adding order of nanosilica on the heat resistance and adhesive strength properties of PPAs has been investigated. The results showed that the T-peel strength of most samples after second heat (heated at 70°C for 24 hours) decreased about 60-80% comparing with the original T-peel strength. However, PPSY only decreased 17.5%, which indicated the nanosilica added after the chain extension reaction, in the prepolymer synthesis could increase the heat resistance performance of polyurethane powder adhesives. It also means the nanosilica was doped between the PU chains keeping the original form as enhancement reagent adding the nanosilica after the chain extension reaction. The excellent heat resistance performance of PPSY could meet the requirements of the industrial gluing in the fields of footwear.

1. Introduction
Polyurethane (PU) adhesives were a special group of polymeric materials which were in many ways different from most of the other plastic types [1–6]. Today, with continuous improvements in processing techniques, they were widely used in paints, liquid coatings, elastomers and insulators, foams, medical materials and so on [7–10]. The traditional PU adhesives were very popular because of the efficiency and convenience. But it caused a great impact on the environment. Especially the application of the solvent-borne PU commonly involves the evaporation of the volatile organic compounds (VOCs), such as acetone and dimethyl formamide (DMF) [11]. Polyurethane powder adhesives (PPAs) could avoid the environment pollution of the VOCs in the process of usage [12, 13]. The weakly adhesion property in the high temperature greatly limited its application yet. The addition of nanosilica could improve the thermal stability of the polymer [14–18]. It was found that the dispersion of nanosilica particles in coats prepared by in situ polymerization method was slightly better than that by blending method [19]. It also indicated that more polyester segments had reacted with silica particles during in situ polymerization than during the blending method [20]. The effect of different sizes of the nanosilica on the property of PU has been discussed [21–23]. However, the impact of nanosilica on the PPAs was barely reported.

To our knowledge, a vast amount of research has been done about the preparation of polyurethane powder for coating [24–26]. Farshchi and Gedan-Smolka [27] reviewed the most recent advances in the field of powder coating by focusing on polyurethane powder coating as a group of thermoset powder coatings. Shirkavand Hadavand et al. [28] used the electrostatic method on aluminum plates to get the powder coating. Yang et al. [29] found that there were two main methods for the production of powder coatings, dry and wet process production.

A simple, easy to operate, and low-cost granulation method for polyurethane powder adhesives has been given by our group [12]. In order to further improve the heat resistance of PPAs, a series of PPAs modified by nanosilica were successfully synthesized on the basis of the solution polymerization. The prepared PPAs were investigated using different characterization techniques. The influence of adding order of nanosilica on the heat resistance and adhesive strength of PPAs was carried out. The optimal sequence of adding was after the chain extension reaction, during that the nanosilica was doped between the PU chains and kept the original form as enhancement reagent.