Plastics lose weight with glass bubblesELECTRONICALLY REPRINTED FROM AUGUST 23, 2012

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 dur-ing 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 com-pound (BMC), resin-transfer molding (RTM), reaction-injection molding (RIM), and pultru-sion. 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 cal-cium 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, ex-pensive 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

Authored by:

Lindsey FrickAssociate Editor lindsey.frick@penton.com

Key points:• New glass bubbles can replace resin and fillers

without requiring added machinery or new, expensive tooling.

• The bubbles reduce weight and improve dimensional stability.

• New sizes of glass bubbles help automotive manufacturers meet OEM Class A surface-finish standards.

Resources:3M, www.3m.com

Brunswick Bowling & Billiards, www.brunswickbowling.com

Continental Structural Plastics, www.cspplastics.com

National Highway Traffic Safety Administration, www.nhtsa.gov/fuel-economy

New glass microspheres control density while letting plastics stay super strong.

Brunswick Bowling & Billiards uses glass bubbles to precisely control the weight of its bowling balls.

Compared to irregularly shaped fillers, the spherical shape and low density of 3M glass bubbles create a higher volume loading capacity.

Glass fibers versus glass bubbles

as he can vouch for improvements in all categories men-tioned. “We find that the flow properties of 3M glass

bubbles integrate well with SMC production and actu-ally 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 com-pany’s request. Examples of applications include automobile parts like doors, fenders, acoustic covers, sunroof shades, and plastisols for un-derbody 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 manufactur-ers 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 de-pends 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. Depend-ing 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. MD

bubbles downstream in a side-stuffing operation when the resin is mol-ten. For injection molding specifically, it is important to lower the back pressures, slow down screw speeds, and use larger gates and run-ners. 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 cre-ates 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 ther-mal 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 im-prove 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 first-hand 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 in-dustries. “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 Mate-rials Development.

Siwajek can say that the company has scored high with the iM30K

This Brunswick Bowling ball is cut in half to show the assembly: inner core, outer core, and coverstock. In

bowling balls heavier than 12 lb, the inner and outer cores use multiple shapes and densities to affect

the dynamics and rolling characteristics. Glass bubbles are used in every design.

Rotational moldingWe 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 im-provements he finds when he uses glass bubbles to help reduce the weight of their large hollow parts while satisfy-ing the toughest critics in the world, moms.

Posted with permission from August 23, 2012. Machine Design, Penton Media, Inc. Copyright 2012. All rights reserved.For more information on the use of this content, contact Wright’s Media at 877-652-5295

92625

Headlamp-reflector assemblies

Seam sealers

Structural foam

Grill-opening reinforcement

Trunk-lid inners

Steering wheels

Underbody coating

Body panels

Gas-tank rollover valves and floats

Air intake

Front-end modules

Engine covers

Fan blades and shrouds

Cockpit partsSunroof shadeSunroof frame

Side-body molding

Tires

Manifolds

Automotive application examplesExamples of weight-saving applications for various grades of 3M glass bubbles.