LDPE is a low-density polyethylene, which is prepared by the polymerization of ethylene monomer catalyzed by a free radical initiator and does not contain any other copolymer. Its molecular characteristics are quite high branching degree, with a large number of long branched chains, due to the mutual entanglement of molecular chains, so that its toughness is poor, can not be large proportion of stretch, low impact ability.
At the same time, because of its high branching degree, it has a high melt strength, which plays a considerable role in stabilizing the membrane bubble. Due to the unwinding of the molecule in the process of shear, it has obvious shear thinning characteristics, and the melt viscosity is greatly reduced at high shear, which brings good extrusion processing performance, manifested as lower melt pressure, lower melt temperature and motor load.
Because of the above characteristics, LDPE can be used flexibly in the formulation design to achieve the desired performance. There are mainly the following aspects:
1. Improve machining performance
With the increasing market requirements for packaging, the use of metallocene is also more and more large, although the performance of metallocene is very good, but the processing is often its soft ribs, usually in the extrusion process to produce excessive shear heat, the pressure increases, the temperature rises, the membrane bubble is unstable. This can be improved by blending LDPE, the addition ratio can be 15-30%, if the addition ratio is too high, it will directly affect the final physical properties of the film, which needs to be balanced.
2. Improve optical performance
Some films have certain requirements for optical properties. Linear or metallocene LLDPE has general optical properties, mainly because its internal crystal growth is too large. If 5-15% LDPE is added to it, it will help reduce the internal crystal size, so as to improve the fog and transparency.
3. Improve the heat seal performance
The thermal sealing performance of linear or metallocene LLDPE is significantly better than that of LDPE. However, due to the structure of high branched degree and high melt viscosity at low shear, LDPE can prevent the heat sealing defects caused by excessive extrusion of the heat sealing film during heat sealing. At the same time, an appropriate amount of LDPE can improve the thermal bonding strength, but the amount should not be too much. Otherwise, it will make the heat seal worse.
4. Other functional improvements
For example, in the shrinkage film to improve the thermal shrinkage and shrinkage rate; The phenomenon of tiger markings can be improved by winding film. To improve the necking phenomenon in the casting film; In the greenhouse film to improve the stability of the membrane bubble to achieve large scale membrane production, and so on.
It can be seen that LDPE plays an indispensable role in the formulation design of thin films due to its special molecular structure, and reasonable collocation with other polymer materials can achieve the optimization of formulation and the improvement of performance.
Post time: Aug-15-2022