A new type of glass microsphere additive has been developed for use in plastic extrusion and injection molding processes. The high-strength, low-density glass microspheres, 3M™ Performance Additives iM30K, can survive extremely high compressive forces, providing compounders and processors with new application opportunities. The potential benefits from using these additives include low microsphere breakage, lower part weight, improved thermal expansion properties, improved processing, improved dimensional stability, lower fuel load and reduced injection cycle times in many applications. Addition of these materials can also result in the maintenance of important thermoplastic physical properties.

Low-density glass microspheres have been available as plastics additives for years. Graph 1 shows the product range available for 3M brand glass microspheres. Application of these microspheres has been limited to those where little or no shear or compression forces are used to form or apply the materials, such as applications relating to plastisols, potting compounds and SMC/BMC1. Processors have had limited success incorporating these products into extruded compounds and injection-molded thermoplastics.

At issue has been the strength of the microsphere and its ability to survive the forces present during processing. The optimum balance of strength with low density has been difficult to achieve due to trade-offs in acceptable manufacturing efficiencies to produce a microsphere with a consistent, controlled wall thickness. Through innovative process and composition developments, 3M Company has produced the first 28,000 psi (~200 MPa) isostatic compressive strength glass microsphere, with a density of 0.6 g/cm3.

Enlarge this picture Fig. 1. Particle size distribution, density, and hydrostatic strength of 3M’s extrusion-grade microspheres.

The spherical shape of these microspheres allows for higher filler loading and reduced resin demand, while maintaining a low melt viscosity compared with other highly filled polymeric systems. Data show how the new product, 3M Performance Additives iM30K, can be extruded and injection-molded to provide significant weight reduction. Also shown are improvements in coefficient of linear thermal expansion and dimensional stability while maintaining important physical properties.

The ability to reduce the weight of thermoplastic materials has been pursued by engineers and designers for various reasons such as improved fuel economy in transportation applications, improved creep resistance and ease of handling.

To demonstrate the differences of the new material and previous generations, experiments were conducted that included two grades of 3M glass microspheres, as detailed in Table 1 and Figure 1. The grades were selected to illuminate processing, handling and property differences. They ranged from high strength bubbles currently used in plastic applications (3M™ S60HS Glass Bubbles), to the new product which is higher in strength (3M™ Performance Additives iM30K).

Not only is the iM30K optimized for compressive strength, but it has a noticeably smaller particle size and narrower particle size distribution compared with S60HS. Stronger, smaller microspheres will be able to withstand more stringent processing conditions such as higher rpm screws, higher shear rate mixers, etc. These new bubbles also show improved mechanical properties of the final composite properties.

Enlarge this picture Graph 1. 3M  brand glass microspheres product portfolio.

Extrusion of the iM30K product is more straightforward than for S60HS. Though side-feeding (side-stuffing) of the iM30K product downstream is still desirable, experiments adding the microspheres with resin pellets in the extruder hopper or top-feeding them near the hopper have shown these approaches to be potentially viable. Many processors and compounders are not equipped to do side-stuffing or able to change screw designs. A microsphere that is robust to various processes, such as iM30K additives, can provide an economical solution for many applications.

Table 2 shows examples of formulations and typical physical properties achieved in a commercial grade of Nylon 66 (Zytel®6 101LNC010) compounded with S60HS, iM30K, and S80HP glass microspheres. A Leistritz ZSE-40 twin-screw extruder was used for compounding the glass microspheres into Nylon 66. The glass microspheres were added downstream using a side stuffer. A general-purpose injection-molding machine (BOY 50M) with a three-zone screw (feed, compression and metering) was used to injection mold ASTM test specimens for physical property measurements.

Physical property testing was done in accordance with the following ASTM methodologies:

• Flexular strength and modulus: ASTM D790.
  
• Notched izod impact: ASTM D252
  
• Tensile properties: ASTM D638.
  
• Elongation at break and yield: ASTM D638.

Data in Table 2 show that the part density decreases from 1.14g/cc for regular Nylon 66 to 1.03g/cc at a 20 percent volume loading of iM30K additives and 0.98g/cc at a 30 percent volume loading of iM30K additives. This corresponds to a density reduction of ~10 percent and 14 percent respectively over standard Nylon 66. The formulation containing S60HS shows significant density reductions over Nylon 66.

Enlarge this picture Table 1. Grades of 3M microspheres used in experiments.

All the glass microspheres formulations also provide a significant increase in tensile and flex modulus as well as flex strength as compared to Nylon 66. Although there is a slight decrease in tensile strength with glass microsphere loading levels, this property reduction can be minimized by using the high-strength, small 3M microsphere iM30K.

The biggest drawback of adding hollow glass microspheres to Nylon 66 is reduced notched Izod impact strength of the composite. This is not unexpected as glass microspheres are essentially non-reinforcing fillers dispersed into a plastic material. However, as can be seen in Table 1, over a 50 percent improvement in the Izod impact strength was realized in this experiment by using iM30K microspheres in the formulations instead of S60HS. This is most likely due to the smaller size of these microspheres compared to S60HS.

Another experiment was conducted with a molded 3M™ Filtrete™ Cabin Air Filter (CAF) frame consisting of a standard 20 weight percent talc-filled homopolymer — black, Piolen® PPTV20A157. This has been compared to a 20 weight percent 3M Performance Additives iM30K-filled PP homopolymer — black, Piolen® PPGB20A15. The talc has been replaced with the 3M Performance Additives iM30K and was compounded by the company PIO Kunststoffe GmbH on a conventional twin screw extruder with a standard screw configuration and a side stuffer feeding device. The density of the raw materials were reduced from 1.04 g/cm3 to 0.84 g/cm3, indicating that all 3M Performance Additives iM30K survived the compounding process. (See photo.)

The parts were made on a conventional Klöckner Ferromatic Type FM-250 injection-molding machine under standard process conditions. Results show that shrinkage of the molded CAF part, particularly in the length and width direction, is significantly lower with the 3M Performance Additives iM30K-filled PP homopolymer compared to the 20 percent talc-filled PP homopolymer. Furthermore, as expected, the part weight could be reduced by approximately 17 percent over the standard part weight while maintaining all relevant mechanical properties.

Enlarge this picture Table 2. Mechanical properties of hollow glass microspheres in nylon 66.

Equally important is the improvement in molding cycle time. The cycle time could be reduced from 16.6 sec. down to 13.2 sec. due to a reduced post pressure and quenching phase. This is most likely due to increased isotropic flow behavior and less material to be quenched.

This trial also demonstrates that even very small and thin parts or geometries, such as thin sealing lips or bars, can be molded and filled properly with a 3M™ Performance Additives iM30K-filled PP homopolymer. Furthermore, a reduction in shrinkage marks (sink marks) or dents can be noticed and underline the improvement in dimensional stability. This is most likely due to the small particle size of the new Performance Additives iM30K microspheres.

In summary, the experiments suggest that hollow glass microspheres can be used in thermoplastics to reduce weight, mold shrinkage, warpage and CLTE. Other attributes unique to hollow glass microspheres are their low thermal conductivity and low dielectric constant. The use of new high strength microspheres, such as Performance Additives iM30K can also provide the additional benefits of improved survivability during stringent thermoplastic processing conditions and improved mechanical properties of the final composite.

For more information, email: mmm@dynetechnologies.com