For many years composite material solutions based on SMC have greatly contributed to creating light weight components and assemblies for automotive markets. A major improvement in weight reduction is now possible by using a combination of DSM’s (Schaffhausen, Switzerland) high-performance low profile trademarked additive Palapreg H2700-01 and surface modified hollow glass spheres, says a new white paper written by DSM's Asal Hamarneh, Blazej Gorzolnik, and Andreas Horback.
DSM was able to reduce the density of sheet molding compound (SMC) moldings to ~1.3 g/cm3 compared to the conventional "Class A" formulation of 1.85 g/cm3 (i.e. a 30 percent reduction), while maintaining surface quality and mechanical performance. This combination allows the production of even lighter parts, supporting automotive OEM’s targets of reducing carbon emissions.
Fuel savings and the reduction of carbon emissions are high on the agenda of automotive OEMs, driven by strict legislation and the increasingly greener purchasing behavior of car users. At the same time, drivers want more car functionality and performance for less money, causing OEMs to keep a close eye on manufacturing costs. For that reason car makers have been looking for effective ways to reduce weight and make their manufacturing more consistent at the same time.
DSM is one of the leading suppliers of synthetic resins used in SMC systems. Besides working on new SMC systems that bring better performance, DSM has a track record of introducing novel solutions for improved sustainability. Recent examples include the commercial introduction of styrene-free, cobalt-free, and bio-based material systems.
To obtain lighter weight SMC formulations, high-density inorganic fillers such as calcium carbonate (CaCO3) can be partially replaced by hollow glass spheres (HGS). However, only a fraction of the fillers can be exchanged by these spheres. When higher HGS quantities are added the processability of the SMC paste is normally troublesome due to much lower paste viscosity.
Another challenge is the selection of the right HGS type. Choosing a wrong HGS type will result in crushing of the HGS during the SMC molding process, leading to lower mechanical performance and bad surface quality. In addition to these issues, HGS are known to have poor adhesion to the matrix and to the paint. This is a challenge for parts with high demands on surface quality and for applications where painting is required.
In order to tackle the low weight challenge DSM has used a combination of polyester resin with the newly developed high-performance, low profile additive (LPA) Palapreg H2700-01. This new LPA improves mechanical properties of SMC parts, in addition to providing easier de-molding and lower emissions from cured parts. Palapreg H2700-01 is commercially used in SMC parts made for leading Automotive OEMs. Beside these advantages, we have found that when used together with surface modified HGS this LPA can also promote the adhesion between the hollow glass spheres and the matrix.
A starting formulation for a low density "Class A" SMC paste is shown in Table 1. A formulation with the high performance LPA Palapreg H 2700-01 was compared to the conventional LPA Palapreg H 2681-01. The optimum filler to HGS ratio was found to be 60:28 (on a volume basis) in order to have the right balance of SMC paste viscosity and processability, mechanical performance and molded part surface quality.
In order to optimize density, crush resistance, mechanical strength, and surface quality, we started to evaluate several HGS types. Based on experience it is known that lower particle sizes will typically result in better surface quality, so we selected types with small diameters (see details in table 2). Experimentally no huge differences in mechanical properties were observed when using different HGS types, therefore we selected the VS5500 type based on commercial availability and cost. Next we evaluated the effect of silane types (Table 3) on promoting the adhesion of the HGS to the matrix. The procedure for treating the surface of the hollow glass spheres was adapted from the procedure provided by Momentive Performance Materials, the supplier of the silanes.
Employing these silanes several SMC formulations were prepared, all having a density around ~1.3 g/cm3 and all these formulations could be processed into parts with "Class A" quality. With regard to the flexural strength using the high performance LPA Palapreg H 2700-01 clearly shows a significant increase (~30 %, compared to using the conventional LPA Palapreg H 2681-01). Further details can be found in Figure 1 (bar labeled "no coating"). Employing Palapreg H 2700-01 in combination with sized HGS, the flexural strength on average marginally increased and was relatively insensitive to the type of sizing applied (Figure 1, bars labeled "a" through "d").
Regarding the flexural modulus, employing only Palapreg H 2700-01 again resulted in a significant improvement compared to using the conventional LPA Palapreg H 2681-01 (Figure 2, bar labeled "no coating"). Employing the combination of sized HGS and Palapreg H 2700-01 significantly improved the flexural modulus. Again, with sized HGS tested, the flexural modulus appears to be relatively insensitive to the type of silanes. In this respect the combination of Palapreg H 2700-01 with HGS is quite robust (Figure 2, bars "a" through "d").
When molding parts with a formulation based on the high performance LPA Palapreg H 2700-01 also a much easier de-molding was observed (significantly less stearate required) compared to the conventional LPA, helping to reduce production times for SMC components.
By using a combination of DSM’s high performance low profile additive Palapreg H2700-01 and surface modified hollow glass spheres, it is possible to reduce the density of SMC moldings to ~1.3/cm3 compared to the conventional "Class A" formulation of 1.85 g/cm3 (i.e. a 30 % reduction), while maintaining surface quality and even improving mechanical performance. This combination allows the production of even thinner and lighter parts supporting automotive OEM’s targets of reducing their carbon foot print and fuel emissions.
For a link to the original paper, which includes all tables and graphics, visit the DSM Web site: http://www.dsm.com/corporate/generic/informationcenter.html and click on the paper, entitled "DSM proposes new roads to weight reduction," dated November 21.
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