technology | Polymer foam
methacrylate salt that enables reversible grafting to be achieved without use of peroxides. Tests show that even at addition rates of 0.5% there is a notable improvement in melt elongation (Figure 2). Also at Polymer Foam 2016, Professor Chul Park
Figure 1: Graph comparing post-foaming density change of LDPE sheet produced with and without glycol monostearate (GMS) against SABIC LDPE 2102FC
Source: SABIC
from the University of Toronto discussed an innovative micro/nano-fibril technology for improving mechanical properties and foaming ability in injection moulding applications. He opened his discussion by highlighting what he described as an important problem: semi- crystalline polymers exhibit a steep elongational viscosity drop around their processing temperatures and that leaves only a narrow processing window where the melt strength is optimal for foaming. Current solutions to this problem include: long-chain
Figure 2: Tests show that as little as 0.5% Dymalink 9200 is needed in a polypropylene homopolymer to have a significant effect on melt strength Source: T
otal Cray Valley
2016 conference, which was organised by Compounding World publisher Applied Market Information and held in Cologne in Germany last November. He explained how foams made with the new grade show significantly less collapse in the hours immediately after production than foams made with competing grades because the rate at which the blowing agent diffuses out of the foam cells is better matched to the rate at which air diffuses into the cells (Figure 1). At the same conference, Philippe Lodefier, European R&D Manager at speciality resin supplier Total Cray Valley, discussed new ways to modify polypropylene and improve its melt strength for foam applications. He explained that high melt strength is still difficult to obtain in PP as there are some significant trade-offs. He said higher melt strength can be achieved by increasing molecular weight but this has a negative effect on processability; the alternative route of crosslinking is likely to cause more gels. As an alternative, Total Cray Valley has developed its Dymalink additives for ionic crosslinking of polypropylene. Dymalink 9200 is a zinc
64 COMPOUNDING WORLD | January 2017
branching—but the resin is at least twice as expensive as the linear counterpart for PP; crosslinking, which renders the polymer non-recyclable; and the use of nano fillers such as nanoclays, but these require modification of their surface chemistry for uniform dispersion. Specially chemically modified types of PTFE have also been developed as foam enhancing modifiers. He said these have the advantage that PTFE readily undergoes plastic deformation; it exhibits high ultimate strain and entangled fibrils enhance melt strength of the matrix. Using 3% of this PTFE in PP enables a tenfold increase in expansion ratio, an increase of up to two orders of magnitude in cell density, and an open-cell content of almost 98%. The problem is that such a chemically-modified PTFE is very expensive, he said. A possible solution proposed by Park is to use
fibrillated polymer fillers such as PET, which can be produced on fibre-spinning lines. Tests carried out using PET fibrils at an addition rate of 0.1% showed it is possible to increase expansion ratio as much as 15-times and cell density can be up to three orders of magnitude higher. Fibrils have also been produced in other polymers.
Tests have been performed on metallocene PE contain- ing fibrillated PP, for example, as well as PLA contain- ing fibrillated PA6. Most recently, work has been done using cellulose nanofibres. Park explained that the incorporation of 150-200nm fibres with 100-200 aspect ratios dramatically improve the foaming ability of resins that are not normally considered foamable, while the stiffness and strength properties of normally mechani- cally weak foams are dramatically increased.
New nucleators Jérôme Crépin-Leblond, Plastic Application Develop- ment Manager at Imerys, focues on lightweighting and reinforcement in chemical foaming with mineral
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