Discussion about the Influence of Ionomers Neutralization Degree for Hot Tack Property Using New Evaluation Method
Authors: Yoshiura Takuto , Miyata Ken , Koda Tomonori , Nishio Taichi , Nishioka Akihiro , Nakata Kazuyuki 
Published: February 2020

Ionomers are polymer materials with a small amount of ionic functional group, e.g. carboxylic acid, sulfuric acid, or their metal salts. Above all, poly(ethylene-co-methacrylic acid)(EMAA) neutralized with metal cations is the most widely used ionomers for industrial applications. When neutralization degree by metal ions increases, metal ions aggregate to form ion aggregations. In ionomers containing ion aggregations, ion hopping phenomena occurs in which metal ions repeat bonding and dissociating with unneutralized carboxylic acid. As a result, ionomer forms a pseudo-crosslinked structure. With this structure, ionomer realizes excellent mechanical properties for elasticity, processability, toughness, hot tackiness, and so on. These properties extend application of ionomers to sports goods, food packaging materials, and adhesive materials.

In the high-speed automatic filling process, packaging films are heat-sealed almost simultaneously after feeding contents.Hot tack property is important for the high-speed filling process. By hot tack property we mean that adhesive force is stabilized when films in semi-molten state are put together. Development of hot tack property contributes to saving of process time, and simplification of sealing process improving productivity in food packaging.

At present, ionomer is generally recognized to be superior material for hot tack property. The mechanism of the excellence of ionomer for the hot tack property has not been elucidated, yet. It is considered that systematic research of hot tack properties for ionomers should be conducted. In this research, at the beginning, we proposed a new evaluation method of the hot tack property. More reliable evaluation method than conventional one was proposed. Furthermore, by using the new evaluation method, the influence of the neutralization degree of the ionomer on hot tack property was examined.

We examined new evaluation method for hot tack test data. Evaluation of hot tackiness is considered to be essential for the initial stage of peeling. In the conventional evaluation method, force at a specific time, i.e. delay time, is adopted for evaluation. The hot tack strength at the delay time is evaluated. For a new evaluation method, the average of hot tack strength in an appropriate peeling time range was determined. The new evaluation method using average hot tack strength leads to understand peeling behavior precisely in wider time range than conventional method. In this study, the average of hot tack strength in the time range from 0.2 to 0.5 s was employed.

Unneutralized EMAA showed the highest hot tack strength at seal temperature 80℃. However, the hot tack strength drastically decreased at seal temperature higher than 80℃.

Though hot tack strength of ionomer was lower than unneutralized EMAA, it decreased slowly with temperature increase in comparison with unneutralized EMAA samples. Ionomer samples showed higher hot tack strength than unneutralized EMAA finally in the high temperature range (80-120℃). From these results, it was found that the ionomer samples keep hot tackiness in wider temperature range than unneutralized EMAA samples. The neutralization degree of 20% showed the highest hot tack strength. It was found that the ionomer consisting of a neutralization degree of 20% showed the highest hot tack property in the peel time range of 0.2 to 0.5 s in this study

From these results, it was indicated that the presence of sodium ion improves the hot tack property. Neutralization degree of 20% improved the hot tack property remarkably. We expect ion hopping is related for this hot tack mechanism. For the ionomer of low neutralization degree, a large number of unneutralized carboxyl acid groups exit. Ion hopping due to acid cation exchange frequently occur because of existence of unneutralized carboxyl acid groups causing reconstruction to steady structure. In the case of high degree of neutralization, unneutralized carboxyl acid groups hardly exist. Ion hopping is not easy to occur because of lack of unneutralized carboxyl acid groups. Molecular chain diffusion is caused by ion hopping affecting structure of ion bondings. It is considered that lack of molecular chain diffusion caused decrease of hot tack property for ionomer samples of high degree of neutralization. Therefore, the hot tack strength of the ionomer is decreased at high degree of neutralization.