These are predominantly blends of an EVA copolymer together with a tackifying resin and a hydrocarbon wax. The EVA is the backbone of the formulation and provides the strength in terms of the mechanical properties, film forming nature, flexibility and cohesion. The tackifying resin (there are numerous types with varying chemical structure) delivers the tack at the application temperature, with ultimate adhesion and improvement in substrate wettability.
The wax is a key element of the blend and is primarily employed in fast packaging and bookinding applications. Its function is to modify a number of critical application and performance characteristics. While there is a broad spectrum of ratios for the formulations, a typical example would be:
| Percent | |
|---|---|
| EVA (28% VA, 400 MI) | 40 |
| Tacifying resin | 30 |
| Wax | 30 |
| Antioxidant | 0.5 |
The Marcus low molecular weight polyethelene homopolymers 200,300,500, and 4040 are ideally tailored in structure for use for hot melts due to their:
- Low molecular weight (Mn = 1100).
- High crystallinity (Approximately 70 – 75%).
- Narrow distribution (Mw / Mn = 1.2 – 1.5).
- Exceptionally low melt viscosity (10-60 mPas at 149°C).
These features are essential for the high performance additive function they possess and are crucial in maintaining good compatibility in a diverse range of formulations. Their attributes may be summarized as:
- Reduction of blend viscosity.
- Shortening of setting time.
- Improved wettability.
- Greatly improved heat resistance.
- Diminished blocking tendency.
It is suitable, in some instances, to incorporate a high performance MARCUS polyethylene as the sole wax component, or to completely supplant Fischer-Tropsch wax. However, a combination of paraffinic and microcrystalline waxes is the norm in order to meet specific application requirements. In this respect it is to be noted that the highly crystalline and high melting MARCUS polyethylenes have a far greater, more dramatic and positive influence than any of the other waxes upon:
- Reduction of setting time.
Improvement of heat resistance.