New glass microspheres control density while letting plastics stay super strong.
The next time you watch your bowling ball roll down the gutter, think about how impressed your teammates will be when you turn around to tell them Brunswick Bowling & Billiards, a div. of Brunswick Corp., has been using glass bubbles to control the weight of its 9 to 16-lb balls as early as the 1980s.
Glass bubbles, otherwise known as glass microspheres, have been used as a weight-reducing additive in plastics for years. They come as a dry, white-looking powder and are made from a water resistant and chemically stable soda-lime-borosilicate glass. These glass bubbles (GBs) are water resistant, noncombustible and nonporous so they do not absorb the resin they are mixed with. Older grades range in sizes from 70 to 35 microns with densities from 0.1 to 0.3 gm/cc and strengths of 250 to 2,000 psi. These collapsible strengths are suitable for zero to low-pressure processes. In the case of bowling balls, Brunswick adds glass bubbles to liquid polyester during an open-pour casting operation to make the ball’s inner core.
Ray Edwards, director of Consumer Products R&D for Brunswick Bowling explains, “Other methods to reduce weight for bowling balls such as foaming or injecting air into the matrix don’t give the kind of precise measuring needed to reach the target density. The use of GBs in bowling balls takes what would be an impossible density control issue and makes it manageable.”
Edwards offers an example of how GBs control density: “By regulation, all bowling balls must be between 8.500 and 8.595-in. diameter to be legal for use in competition. The weight of the balls vary from 8 to 16 lb. To make a 10-lb ball that has a 3.67-lb urethane coverstock (the outermost surface of the ball), the 7.7-in. spherical core has to weigh 6.33 lb. The polyester resin used in the core has a specific gravity of 1.23. When the polyester is mixed with the appropriate volume of glass microspheres, the casting mixture can be formulated to have the desired specific gravity of 0.734,” he says.
Seeing what glass bubbles can do for low-pressure processed plastics, 3M Co., St. Paul, decided to spend the last several decades developing glass bubbles for high-pressure processes. Most recently, a quantum-leap advance has enabled a new species called iM30K.
3M’s iM30K has a collapsible strength around 30,000 psi making it suitable for processes like sheet-molding compound (SMC), bulk-molding compound (BMC), resin-transfer molding (RTM), reaction-injection molding (RIM), and pultrusion. It can replace fillers such as clay, acrylics, hydrocarbon-based resin, carbon fiber, organic fillers, and nanoparticles. The GBs are 16 microns in diameter with a density of 0.6 gm/cc, which is one-fifth the density of the most widely used reinforcing fillers such as talc, glass fibers, and calcium carbonate. With this density and strength, the GBs score high for reduced weight, reduced cycle times, less warpage, and acceptable surface finish with improved strength. The automobile industry quickly adopted this new technology.
Automobile manufacturers have been forced to tighten their belt on weight limitations with evermore-stringent Corporate Average Fuel Economy (CAFE) standards. Automakers estimate that every 10% of weight eliminated from a vehicle’s total weight improves fuel economy by about 7%. Methods to reduce weight like retooling and chemical blowing agents are time consuming and costly.
Glass bubbles can replace resin and fillers without requiring added machinery or new, expensive tooling. Due to their low density, GBs are formulated by volume rather than weight. 3M suggests a maximum practical loading at 50% volume of resins without fillers. An example is 30 volume % substitution of polypropylene to create 13% weight savings. Other resins that can be substituted include nylon, ABS, acetal, rubber, plastisols, and other engineered thermoplastics. The more GBs that replace resin, the lower the density and stiffer the part.
When fillers are used to reinforce a plastic, glass bubbles should not be substituted for all the filler. The GBs are spherical and have an aspect ratio of 1, which does not create the same strength seen with a high-aspect ratio filler such as glass fiber because the fiber does not align in the flow direction. To reach a 5 to 10% weight savings, 3M recommends a 10 to 20% substitution of filler if it’s important to maintain physical properties (impact strength, modulus, elongations and heat-deflection temperature) for structural applications.
To use GBs in a process like extrusion or injection molding, 3M recommends adding the bubbles downstream in a side-stuffing operation when the resin is molten. For injection molding specifically, it is important to lower the back pressures, slow down screw speeds, and use larger gates and runners. The GBs are still susceptible to breaking during high shear processing as in gear pumps or two-roll mills.
Because glass bubbles are lightweight, their use creates a shorter cooling time because they reduce the mass of the part. In addition, glass expands and contracts less than most resins so the plastic’s coefficient of linear thermal expansion (CLTE) improves. Because the GBs have an aspect ratio of 1, they create an isotropic filling with more volume loading capacity. All these variables help improve cycle times by reducing warpage and by helping hit target dimensions for parts that are molded, exposed to vibration or need snap fit.
Older grades of GBs cannot be used in parts that need a Class A surface finish. Currently, the most widely used composite for Class A surfaces is sheet-molding compound (SMC), a compression moldable blend of polyester or vinyl-ester resins, chopped glass fiber, and mineral filler (although carbon fiber is finding some applications in low-volume luxury vehicles). The older bubbles at a size of 40 microns at or near the surface of an SMC part would pull out of the panel or fracture during finish sanding, leaving voids and unacceptable paint surface defects.
Continental Structural Plastics Inc. of Troy, Mich., saw firsthand the surface- finish problem with the older grades. The company is a North American producer of SMC and glass-mat thermoplastic (GMT) composite parts for the automotive, heavy truck, and other industries. “We are able to meet OEM Class A standards in low-density, painted appearance panels with the smaller size and increased crush strength of iM30K glass bubbles,” says Mike Siwajek, manager of Materials Development.
Siwajek can say that the company has scored high with the iM30K as he can vouch for improvements in all categories mentioned. “We find that the flow properties of 3M glass bubbles integrate well with SMC production and actually enhance the process by improving resin flow. We can reduce weight by about 25% for a given part with 3M iM30K glass bubbles compared to calcium carbonate.”
Custom formulations of the material can be created upon a company’s request. Examples of applications include automobile parts like doors, fenders, acoustic covers, sunroof shades, and plastisols for underbody coating and seam sealing.
Plastisols are liquids composed mostly of PVC which are heat cured to form a continuous film or solid mass. With an environmental push to reduce the amount of PVC in their product, auto manufacturers started substituting PVC with costly acrylics. Using GBs instead of acrylics, the GBs halved the weight and reduced cost by 15 to 20% while removing PVC resin. Speaking of cost, pricing of the glass bubbles depends upon the density and the package size. SMC part manufacturers using the GBs found the cost of the parts to stay the same, but the new weight yielded a reduced cost in shipping and transit.
Keeping in mind environmental efforts, “GB/plastic composite can be recycled with minimal increase in specific density/gravity. Depending on the recycling method (grinders or heated blade grinders) there may be more or less bubble breakage during each recycle step. The type of resin is not much of a variable and can be recycled with any resin/GB composite to the same effect as the resin would be recycled by itself,” says Steve Amos, senior product development specialist 3M Energy and Advanced Materials Div.
|Rotational molding |
We did not find any documentation explaining the use of GBs in rotational-molded plastics. We contacted James Christie, director of Technical Services at The Step2 Co. LLC, Streetsboro, Ohio, to see how GBs would hold up at the world’s largest rotational-molding facility. James will be updating MACHINE DESIGN with any challenges and/or improvements he nds when he uses glass bubbles to help reduce the weight of their large hollow parts while satisfying the toughest critics in the world, moms.