Fillers have been in use since the early days of plastics. Today’s enormous growth of the polymer industry is due to the unique properties of fillers they impart to polymers. Glass bubbles (low density hollow glass microspheres) as fillers have been incorporated into thermoset polymers for decades. They are tiny hollow spheres and are virtually inert. These glass bubbles are are compatible with most polymers. Until recently, their use with thermoplastic polymers has been limited because of high rates of bubble breakage from the high shear forces to which they are exposed during such thermoplastic processing operations as extrusion compounding and injection molding. At issue has been the strength of the glass microspheres.
3M have recently developed innovative glass bubbles which offer resistance to extremely high compressive and shear forces. This allows compounders, thermoformers and injection molders to use them to achieve significant weight reductions without restoring to costly equipment modifications. This article will showcase how plastics processors could exploit the advantages of these novel glass bubbles while improving the end-product properties.
1.0 Glass bubbles : Why use them in plastics compounds?
3MTM Glass Bubbles (hollow glass microspheres) (Figure 1) are weight reducing fillers which can be incorporated into various polymers by melt compounding and processed into articles via thermoforming, blow molding, sheet casting, injection molding and etc.
Figure 1. Hollow Glass Microspheres
Compared to other heavy fillers such as talc, calcium carbonate, glass fiber, and clay (from 2.5 to 2.8 g/cc), glass bubbles (GB) for plastics and rubber applications have densities ranging from 0.1 to 0.6 g/cc (Figure 2).
Figure 2. 3MTM Glass Bubbles vs. Other Heavy Fillers
Reducing the weight has been a paramount objective in various industries, such as transportation, aerospace, and hand-held electronics. A few words on the historical trends in transportation industry would be illustrative to set the scene. The trend of reducing the weight which in turn would reduce the fuel consumption in transportation became dominant after the first oil crisis which affected the Western World in mid-1970s and continues to be an active research field. A recent historical analysis of fuel efficiency of US vehicles  before the mid-1970s indicated little change – indeed a net deterioration – in average miles-per-gallon (mpg) of vehicles in US between 1923 (14 mpg) and 1975 (12 mpg). The step change in oil prices during the 1970s (Figure 3) triggered a strong interest in improving the fuel efficiency in transportation which continues to be prevalent today and has resulted in a current fleet fuel efficiency of around 17 mpg . In view of the historical data, it is not surprising that the global rise in oil and energy prices in the early 21st century (Figure 3) triggered a renaissance of light weight material applications. Note that current real (adjusted for inflation) oil prices are higher than what was experienced during the first oil crisis of the 1970s (Figure 3).
Figure 3. Real (adjusted to inflation) US retail gasoline prices. The plotted data were obtained from: www.eia.doe.gov (accessed December 20, 2010)
The importance of reducing the weight and its relationship to fuel consumption is illustrated in the following scenario. One may estimate that the body panels, hoses, bumpers, doors, and the car interior count for about 50% of the total weight of a standard passenger car. On average, about 300 Ibs of filled and unfilled polymers of various densities ranging from 0.9 g/cc to 1.68 g/cc are used in these components . Table 1 summarizes weight reductions that could be achieved in these parts at different GB wt% loadings with two grades of glass bubbles, K42HS and IM30K. The table indicates that a 50 lb to 100 lb weight reduction can readily be achieved in a car having 300 lbs of polymeric parts. Based on a recent study, this would result in an increase of fuel efficiency up to 2 % .
Table 1. % Weight Reductions in the Presence of 3MTM Glass Bubbles
Current interest in new light weight materials covers a broad spectrum from light weight metals, such as aluminum and magnesium, to carbon fiber reinforced plastics. Although the majority of these are technically viable solutions, many of them pose challenges both to the producer and to the customer. New materials, such as carbon fibers, are usually significantly more expensive than the traditional industrial materials and might require additional processing and assembly steps which may increase the total cost. Most crucially, new materials usually require additional time and funds for testing, validation, and design trials which may prolong the product launch and increase cost. 3M glass bubbles offer a ‘plug-and-play’ solution for reducing the weight of plastic parts without requiring significant material and process changes. 3M glass bubbles can be easily compounded with various polymers using the conventional melt processing operations such as extrusion. The resulting compounds are strong enough that the glass bubbles will not break, even in high shear, high pressure processes like injection molding. Figure 4 shows a few examples in different polymer systems.
Figure 4. Examples of Polymer + 3MTM Glass Bubble Composite Systems
The isostatic crush strength (psi) versus the corresponding true density (g/cc) of the glass bubbles are shown in Figure 5 along with their average diameters indicated in the circles.
Figure 5. Isostatic Crush Strength vs. the Corresponding True Density of the 3MÔ Glass Bubbles (the number in the colored circles indicates the average diameter)
The recently developed 3M glass bubbles such as XLD6000, K42HS, and iM30K, which fall onto the “High Strength GB Trend” line in Figure 5, are optimized to improve their compressive strengths. They have noticeably smaller average particle sizes and narrower particle size distributions compared to their same density counterparts as shown in Figure 6.
Figure 6. Comparison of the Same Density 3MTM Glass Bubbles (0.6 g/cc) with Different Particle Sizes and Size Distributions
The appropriate glass bubble grade for weight reduction is typically the lowest density with the highest survival rate in a given polymer system. The base polymer resin viscosity and the level of pressure fields experienced in the process influences the survival rate of the glass bubbles. High melt viscosity materials in high pressure polymer processes (e.g. injection molding) require stronger glass bubbles such as 3M Glass Bubble iM30K. Nevertheless, there is no strict rule and several grades of lower density bubbles (e.g. 0.3 g/cc at 6000 psi) can still be injection molded successfully with minimal bubble breakage provided that the conditions are set correctly. We will review process parameter considerations in the next section.
2.0 Things to consider during its processing
2.1 Extrusion compounding
Compounding of 3M glass bubbles in a twin screw extruder is similar to that of glass fibers where the fiber attrition is kept to a minimum by introducing the fibers downstream into the molten polymer. The appropriate twin screw extruder is typically a co-rotating intermeshing or a counter-rotating non-intermeshing parallel twin screw extruder. Glass bubbles are preferably added downstream into the main extruder via side stuffing or top feeding ports. For high loadings of glass bubbles, it is important that the screw elements in the glass bubble feed zone have large free volumes with large channel depths. One of the advantages of glass bubbles over other fillers, such as clay or talc, is that they do not form agglomerates in the melt and readily distribute themselves in the molten polymer without having to resort to special distributive mixing elements. Standard forward conveying elements are sufficient to convey and distribute the bubbles in the molten polymer. Pressure generation via reverse elements or wide kneading blocks, typically used for dispersion, i.e. agglomerate break up, should be avoided. If required, narrow kneading blocks with a small number of disks can be used. Another important factor for glass bubble survival in twin screw extrusion is back pressure applied by the presence of the die and melt screen. Increasing the number of strand holes in the strand die along with the diameter of the openings increases glass bubble survival by reducing the back pressure.
The strongest grade of glass bubble, 3M Glass Bubble, IM30K , can provide an economical alternative for compounders who are unable to implement side stuffing or downstream feeding. Compounding trials in which iM30K 3MÔ glass bubbles were fed through or near the hopper indicated relatively reduced extent of breakage of the fillers. However, it should be emphasized that side feeding downstream is still preferable for 3M Glass Bubble iM30K as well.
A preferred screw configuration with downstream glass bubble feeding, along with downstream glass fiber feeding, is shown in Figure 6. Note that the glass bubbles are introduced after feeding all other components to the process.
Figure 7. Representative Extruder Screw Design for Compounding 3MÔ Glass Bubbles
2.2 Injection Molding
Injection molding involves high pressure fields and hence more attention has to be paid to prevent glass bubble breakage. There are two zones in the process where the glass bubble breakage can occur. The first one is the plasticating zones where the polymer is molten and conveyed to get ready for injection into the mold. Back pressure, which increases shearing of the polymer melt, has to be set low enough to prevent glass bubble breakage but high enough to provide a compact melt and prevent air bubble formation in the plastic melt and eventually in the finished part. Next, the injection rate should be kept low enough to prevent injection pressures exceeding the iso-static crush strength of the glass bubbles. Likewise, holding pressure should be lower than what the glass bubble isostatic crush strength is rated for. Injection pressure is also determined by the intensification ratio, i.e. area of the ram /area of the screw, and should not exceed the pressure ratings of the glass bubble . As for tooling, a general-purpose injection screwbarrier is recommended. Double-vane or vented designs are not recommended. Sharp edges on the nozzle should be avoided and sprue orifices should be at least of 0.25 in diameter. Use of full-round or trapezoidal runners cut into both plates having a minimum radius of 0.125 in are recommended.
3.0 A Case Study on the Properties of 3MÔ Glass Bubbles in Glass Fiber filled Nylon 6,6
Glass bubbles are excellent strength/weight optimizers when they are used in filled polymer systems such as glass fiber, talc, and calcium carbonate filled thermoplastics. Replacing a certain percentage of these high density fillers with glass bubbles results in weight reduction while maintaining the original mechanical properties. As an example this is shown in the following case study with glass fiber filled nylon 66.
Commercially available, high strength (18000 psi isostatic crush strength), low density (0.60g/cc) glass microspheres (3MTM glass bubbles S60HS) were selected for these experiments. A commercially available, injection molding grade of polyamide 66 was obtained from E.I. DuPont de Nemours Company under the trade name Zytel®101LNC010 . Glass fibers, PPGÔ Chopvantage 3540, with a density of 2.65 g/cc were obtained from PPG Industries .
3.2 Compounding and injection molding
All samples were compounded on a Berstroff Ultraglide twin screw extruder (TSE 25 mm screw diameter; length to diameter ratio of 36:1) equipped with top feeders for microspheres and glass fibers (GF), a water bath, and pelletizer . Screw speed ranged from 140 to 160 rpm. Temperature set points ranged from 500ºF to 575ºF. Test specimens were then molded on a 150 ton Engel injection molding machine using an ASTM four cavity mold. The screw diameter was 30mm and the injection pressure was maintained below 18000 psi to prevent glass bubble breakage.
3.3 Testing and Characterization
Flexural strength and flexural moduli were determined according to the ASTM D790. Notched Izod impact properties were determined according to ASTM D252. Tensile mechanical properties were determined according to the ASTM D638. The density of the injection molded parts was determined using a helium gas pycnometer.
3.4 Results and Discussion
The results in Table 2 indicate that the presence of 3M Glass Bubble S60HS significantly reduced the density of the injection molded parts. One can also note the strength/weight optimization via the use of glass bubbles in glass fiber filled nylon 66. Glass bubbles allow the end product properties to be retained to a large extent while decreasing the density. For instance, the formulation with the 10wt% GF and 90wt% Nylon 66 (C-3) has a final part density of 1.20 g/cc with a tensile strength (TS) of 73 MPa, tensile modulus (TM) of 5.47 GPa, flexural strength (FS) of 147 MPa, flexural modulus (FM) of 4495, unnotched Impact (UI) of 2.8 J/cm and notched impact (NI) of 0.6 J/cm. If one desires to double all of the mechanical properties, the GF content has to be increased from 10wt% to 33% (see formulation C-1). This would increase the density from 1.20 to 1.39 g/cc. The doubling of these properties can also be obtained using formulation #4 with a density of 1.19 g/cc which contains glass bubbles.
Table 2. Physical Properties of 3MÔ Glass Bubbles S60HSfilled Nylon 66 Composites
In order to improve the properties of composites further, glass bubbles are surface treated with silanes to improve compatibility with various polymers. Figure 7 shows silane treated 3M Glass Bubble IM30K which shows improved adhesion to the polymer matrix.
Figure 8. Scanning Electron Microscopy Image Showing Improved Adhesion to the Polymer Matrix via Silane Treatment
4.0 Additional Benefits
There are several other benefits of adding glass bubbles into thermoplastics compounding formulations. These can be outlined as reduced cycle time in thick molded parts, decreased mold shrinkage and part warpage, and reduced coefficient of thermal expansion as discovered during numerous laboratory and plant trials. Figure 9 shows the effect of glass bubble loading on the Coefficient of Linear Thermal Expansion (CLTE) of the Nylon 66 at different temperature intervals. 25 to 30% decrease in CLTE can be achieved in Nylon 66 with the addition of glass bubbles up to 30 vol %. Likewise, Linear Mold Shrinkage (LMS) decreases as the amount of glass bubble loading is increased. 50% reduction in LMS is observed in injection molded polypropylene containing 30% by volume IM30K.
Figure 9. Coefficient of Linear Thermal Expansion (CLTE) of Nylon 66 as a Function of Glass Bubble Loading
Figure 10. Effect of Glass Bubbles on Linear Mold Shrinkage
5.0 Conclusions and Future Outlook
Compounding 3M Glass Bubbles with polymers offers a ‘plug-and-play’ weight reduction solution. The method is rapid, cost effective, and does not require any complex material or process modification.
3M Glass Bubblescan be added to high density filled polymer composites (talc, glass fiber, carbon black, calcium carbonate and etc) to optimize density-and mechanical strength.
In addition to weight reduction, laboratory and plant trials also indicated that additional processing and material related improvements can be achieved by the addition of glass bubbles into compounding formulations. These can be summarized as reduced cycle time especially in thick injection molded parts, decreased part warpage due to isotropic nature of the fillers, reduced coefficient of thermal expansion, and decreased mold shrinkage.
- M Sivak, O. Tsimhoni. Fuel efficiency of vehicles on US roads: 1923–2006. Energy Policy 37, 3168-3170, 2009.
- N. G. McCrum, C. P. Buckley, C. B. Bucknall. Principles of Polymer Engineering, 2nd ed. Oxford University Press, New York, 1999. pp. 8-10.
- J. Pflug, B. Vangrimde, I. Verpoest. Material efficiency and cost effectiveness of sandwich materials, SAMPE Conference, Long Beach, CA USA, 2003
- iM30K 3MÔ Glass BubblesProduct Information, issues by 3M Company on August 2006.
- 3M™ Glass Microspheres Compounding and Injection Molding Guidelines, Issued by 3M Company 3/08.
- Zytel® is a registered trademark of E. I. du Pont de Nemours and Company.
- The PPG® name is a trademark of PPG Industries.
13M Energy & Advanced Materials Division, 3M Center, St. Paul, MN, USA
2 3M Safety Security & Protect Bus, 3M Center, St. Paul, MN, USA
33M Corporate Research Process Laboratory, 3M Center, St. Paul, MN, USA
Baris Yalcin, Ph.D.
Application Development Specialist, 3M Energy and Advanced Materials
3M Center, 236-0G-B68, St. Paul, MN, 55144-1000, USA
Office: 651 733 6959 | Mobile: 651 387 1460 | Fax: 651 736 7794
Steve E. Amos, Ph.D.
Sr. Product Development Specialist, 3M Energy and Advanced Materials
3M Center, 236-0G-B68, St. Paul, MN, 55144-1000, USA
Office: 651 737 3045 | Mobile: 612 281 3337 | Fax: 651 736 7794
Baris Yalcin, Ph.D.
Baris is an application development specialist in 3M Energy and Advanced Materials Division. He has a Ph.D. in Polymer Engineering from The University of Akron, Akron, OH, USA. From 2004 to 2010, he has worked as the center manager and researcher for the Center for Multifunctional Polymer Nanodevices and Materials (CMPND) at The University of Akron, a third Frontier Project initiative by the State of Ohio to expand Ohio's high-tech research capabilities and promote innovation and company formation. His expertise is on the manufacture and investigation of advanced functional polymer based materials with innovative process design concepts. He has over 25 technical publications/patents. He is a member of the Society of Plastics Engineers and is an active reviewer for several refereed plastics journals.
Steve E. Amos, Ph.D.
Steve has an MS in Polymer Science from the University of Ferrara, Ferrara, Italy and a BS in Chemistry for the University of Wisconsin-Madison. He has been involved with product development, materials and application research at 3M Company since 1994. Materials that he has worked with at 3M include fluoropolymers, glass bubbles, nucleating and clarifying agents, flame retardants, fluorochemicals and ceramic microspheres. He has numerous technical publications and patents. He is a member of the Society of Plastics Engineers (SPE) and a past board member of the Polymer Modifiers and Additives Division of SPE.
I. Sedat Gunes, Ph.D.
Sedat received his PhD in Polymer Engineering from The University of Akron, Akron, OH, USA under the guidance of Professor Sadhan C. Jana. He serves in the Editorial Board of Journal of Plastic Film & Sheeting, in the Society of Plastics Engineers New Technology Forum Organization Committee, and in ASTM Committees D20: Plastics and E56: Nanotechnology. His professional experiences focus on processing and characterization of polymers and nanocomposites.
Andrew D’Souza, Ph.D.
Andrew is a Product Development Specialist in 3M's Safety and Security Protection Services Lab. Prior to that, he has worked in glass bubble technology and polymer processing in 3M's Energy and Advanced Materials Division. He holds several patents and publications in the area of ceramic processing, glass science and technology and specialty coatings. Dr. D'Souza has a Ph.D. in Material Science and Engineering from The Pennsylvania State University