Mechanism of improving the shear strength of EVA hot melt adhesives by fumed silica and microsilica powder

Modification of EVA hot melt adhesives by using inorganic powder is a common method to improve the mechanical properties of hot melt adhesives. In this paper, porous fumed silica with large specific surface area and solid non-porous microsilica powder with small specific surface area were added to EVA hot melt adhesives respectively. The magnitude of the enhancement of the shear strength of EVA hot melt adhesives was explored, and the mechanisms that the two kinds of powders enhance the shear strength were investigated by XPS, ATR-IR, ¹HNMR, FE-SEM and DSC. The addition of fumed silica increased the shear strength of the composite hot melt adhesives, and reached the maximum value when the powder content was 2 %, which increased by about 14.3 % compared with pure EVA hot melt adhesives. Through XPS, ATR-IR and ¹HNMR it was found that the Si-OH on the surface of SiO₂ particles reacted with the ester carbonyl group in EVA to produce polymer alcohol, and the strong polar hydroxyl group on the surface of SiO₂ was changed to an acyl oxygen group. The acylation of the particles surface is conducive to reducing the agglomeration of SiO₂ particles and improving the compatibility between inorganic particles and organic polymers. The composite hot melt adhesive with 1 %, 2 %, and 3 % fumed silica content shows slight agglomeration of inorganic powder, while the agglomeration is significantly severe when the content is 4 %. DSC analysis showed that the crystallinity of EVA composite hot melt adhesives decreased first and then increased with the increase of powder content, and the decrease of crystallinity increased the shear strength. The improvement of the shear strength can be attributed to the enhancement of the compatibility caused by the reaction between fumed silica and EVA organic, as well as the decrease in crystallinity. The addition of microsilica powder improved the shear strength of the composite hot melt adhesives also, and the maximum value was reached when the powder content is 25 %, which was about 10.6 % higher than that of pure EVA hot melt adhesives. The increase amplitude is smaller than that of fumed silica. The reaction between microsilica powder particles and EVA pure hot melt adhesives was not found by XPS and ATR-IR detection, however, the ¹HNMR spectrum showed that there is a partial reaction between them, the degree of reaction was less than that between fumed silica and EVA hot melt adhesive. The crystallinity also showed a trend of first decreasing and then increasing with the increase of powder addition, with the minimum crystallinity at 10 %. The enhancement of the shear strength of hot melt adhesives by microsilica powder may be mainly attributed to the decrease in crystallinity.