In this blog post we’ll be looking at melt fracture and the problem it poses regarding extrusion processes in the plastics industry.
The impending ban on PFAS family substances includes polytetrafluoroethylene (PTFE), which is used in many applications due to its unique friction properties and chemical resistance. Industries that use PTFE as polymer processing aid in products such as films, seals and bearings must look for alternatives to meet the new market demands for PTFE-free products.
“You have to differentiate when it comes to this topic. Despite all the criticism, many substances in the PFAS class are also genuine miracle products with a right to exist. As everywhere in life, the dose makes the poison. That's why you have to take a close/comprehensive look at the issue and ask yourself in which areas fluoropolymers can be usefully substituted.” [Tobias Dangel]
“In the case of PTFE, it can be said that whenever PTFE is used as an additive in other systems (such as plastics or lubricants) boron nitride can not only be an alternative, but could also offer real added value as polymer processing aid with its other properties and is furthermore environmentally friendly.” [Michaela Schopp]
Melt fracture is a common problem in plastics processing that can lead to undesirable cracks and uneven surface structures. This phenomenon occurs particularly with highly viscous polymers and significantly impairs the quality of the end product. The cause lies in the unsteady flow speed of the melt, which is created by wall friction. This results in a rough surface when the polymer melt emerges (Fig 4).
PTFE is often used to reduce melt fracture. It serves as a lubricant and also reduces friction in the processing machines. Due to its metal affinity, the PTFE migrates to the metal nozzle and forms a very thin “sliding layer”. This influences the flow speed of the melt whereby the speed at the wall friction is reduced and the flow speed at the edge and in the middle of the melt are equalized.
Due to its outstanding coefficient of friction (even at high temperatures) boron nitride is the first choice for a PTFE substitution. The way in which melt fracture is prevented is different from that of PTFE. While PTFE influences the friction at the melting front due to its affinity to the metal nozzle, boron nitride reduces the elongation viscosity of the polymer. The hexagonal BN particles disrupt the cross-linking density of the polymer chains, which reduces the number of interlocks/interactions between the polymer chains.
The interactions between hBN and the polymer chains reduce the viscosity of the polymer. This in turn reduces the tensile stresses that can lead to melt fractures and at the same time improves the processability of the polymer. Just 100 to max.1000ppm of hexagonal boron nitride powder is sufficient to produce the effects mentioned. Our HeBoFill® BL-SP 020 powder is a suitable alternative here.
In contrast to PTFE, further positive effects can be achieved with hBN:
- Less beard formation
- Reduced need to clean the nozzles
- Increased machine running times
- Lower processing temperatures possible
- Higher shear rates can be maintained
- Increased service life of nozzles and other processing components in the extrusion process
- Reduction of abrasion
For the production of packaging, especially in the food sector, an additive must have food certification. Our HeBoFill powders are approved in accordance with EU Directive (EU) No. 10/2011 for use in plastic materials intended to come into contact with food. We also have suitable powders in our portfolio for FDA certification requirements.
Ask our sales team about this.
Link to technical consulting: https://www.henze-bnp.com/en/service/technical-sales.php
With the food approval of boron nitride in plastics, the issue of melt fracture can be countered despite the impending ban on PTFE. So, regardless of what the EU Commission decides, an initial contribution can already be made in certain areas for people, the environment and technology.
Sources:
Martin Sentmanat, Savvas G. Hatzikiriakos, Mechanism of gross melt fracture elimination in the extrusion of polyethylenes in the presence of boron nitride, Springer, 2004
Igor B. Kazatchkov, Franky Yip, Savvas G. Hatzikiriakos, the effect of boron nitride on the rheology and processing of polyolefins, Springer, 2000
https://www.bn.saint-gobain.com/applications/polymer-engineering/polymer-processing-aid, aufgerufen am 21.08.2024
