1
Introduction
Stephen E. Amos, and Warren Beck
Abstract
Since the dawn of the mankind, there has been a drive to develop lighter materials to enable transport and ease of use. After the industrial revolution and the subsequent development of plastics, there have been ongoing material substitutions from metal, glass, wood, and stone to plastics and composites of these materials to reduce weight. A logical next step in this material evolution was to reduce the weight of plastics. Various, naturally low in density, fillers were first tried with limited density modification capability. In addition, injection or creation of gas in the polymer during the article forming process was also developed and utilized in nonstructural applications such as packaging.
Keywords
3M; Army Cardboard; Coefficient of thermal expansion; Density; Hollow glass microspheres
Since the dawn of the mankind, there has been a drive to develop lighter materials to enable transport and ease of use. After the industrial revolution and the subsequent development of plastics, there have been ongoing material substitutions from metal, glass, wood, and stone to plastics and composites of these materials to reduce weight. A logical next step in this material evolution was to reduce the weight of plastics. Various, naturally low in density, fillers were first tried with limited density modification capability. In addition, injection or creation of gas in the polymer during the article forming process was also developed and utilized in nonstructural applications such as packaging.
By the late-1930s, 3M Company was manufacturing solid glass beads made of scrap window glass. This product was sold to highway departments for reflective road paint. Various reformulating experiments were carried out to attempt to produce hollow glass microspheres (HGMs) but were limited in their success by low yields. By the 1950s, 3M was selling reflective sheeting to the French army, which was called “Army Cardboard”—2×2 sheets of reflective sheeting with low-refractive index glass beads. The sheets were optically designed to be retro reflective for light perpendicular to the sheet plane. These were mounted on the back of convoy vehicles to help prevent night-time accidents.
When the French government placed a large order for Army Cardboard, 3M made the material but it failed to meet the brightness requirements. A 3M scientist, Warren Beck was manager of the Bead Department and he undertook the task of determining why the feed material had failed. Like many scientific discoveries, what was perceived as failure was really a pathway to success for the development of a new product.
When Beck examined the out-of-spec material, he discovered clouds of microbubbles near the surface of the beads. He determined that storing the crushed glass feed particles, for a long period of time in humid weather, had created the conditions to form hollow bubbles. To correct the problem, he recommended crushing the glass and using it immediately. Case solved.
But Beck also knew of the preceding work within 3M attempting to develop such a hollow glass bead, and of earlier patented work by Standard Oil of Ohio on a one step, melt and expand “microsphere” product and process based on either phenolic resin or sodium silicate glass [
1,
2]. After some experimentation, Beck discovered that it was possible to create hollow beads or “HGMs” as 3M would later call them with a two-stage melting and forming process. In 1963, he filed a patent application for creating these unique structures by carefully formulating glass frit, milling it to a specific particle size and particle size distribution, then reheating the particles to form single-wall hollow glass microsphere
Figure 1.1 [3].
Figure 1.1 Visual microscopic image of hollow glass microspheres made by 3M Company. 3M™ Glass Bubbles—Courtesy of 3M.
The Sohio patents were eventually sold to Philadelphia Quartz—PQ Corporation today. PQ currently makes HGMs of this type of glass. The phenolic based microspheres ended up being produced by Union Carbide Corporation.
There have been several other types of materials, discovered or developed over the years that also provide density modification for resin systems. Fly ash is the by-product of powdered coal-burning power plants. It is similar to impure clays in composition in that it is primarily aluminum silicate contaminated with iron, magnesium, calcium, and alkali metal oxides. As the coal particles burn, the ash, which can make up to 10% or more of the coal, fuses to form hollow microspheres. If composition and forming conditions are right up to several percent of the spheres that are produced may be hollow and low enough in density to float on water. This type of bubbled product was first recovered, floating in power-plant ash ponds, in England around 1970 and marketed as “cenospheres.” The density of cenospheres is generally around 0.7 g/cc and their strength is highly variable but usually around 3000 PSI due to imperfections in the sphere wall.
Figure 1.2 Expanded perlite.
Figure From Ref. [4] with Permission. Perlite has been an item of commerce for a number of decades. When heated above the softening point (about 900°
C), water internal to the perlite structure is liberated as steam and the material forms a porous, low density, multicellular material as shown in
Figure 1.2.
When added to liquids or molten plastics, the pores can absorb resin to a degree, depending on the resin viscosity. Some pores are too small to be filled and remain as voids so the material can provide a small amount of density modification to a composite. Generally, perlite is severely degraded in high shear flow environments so it is typically used as a filler in thermoset systems, not in thermoplastics. But, it is primarily used in nonresin applications such as insulation fills in cryogenic liquid storage tanks. There have been various attempts to make fused single cell HGMs from expanding perlite
[5].
Kanamite was a hollow ceramic particle, made from shale, having a particle diameter of 100–600μm. The density varied from 0.4 to 0.8 g/cc. The material was manufactured by the Kanium Corporation of Chicago in the 1960s. It is no longer manufactured today.
Early Application Examples
Various applications were promoted in the early patents for HGMs including the use in plastics, rubber, and other resinous materials for weight reduction. Other application areas of interest were thermal insulation, concrete, synthetic wood, gas storage and transport, and as a flow aid (the ball bearing effect).
One of the first successful applications for HGMs was in dry wall joint sealer. Normal, dense, plaster- or PVA-based joint sealer would shrink and crack requiring multiple applications. The HGM glass material has a very low coefficient of thermal expansion (CTE), preventing shrinkage. Also the wall joint material was very hard after curing and required a significant amount of work to sand to a smooth surface. Providing microvoid spaces improved the postcure processing properties allowing for quick sanding to a smooth surface. One benefit not immediately realized was that of light weighting. This prevents sagging of the compound on vertical surfaces.
An early, unexpected application was the use of HGMs in explosives. Prior to 1951, little was known about the explosive reaction between ammonium nitrate and fuel oil. But a disastrous explosion in Texas City that year resulted in studies leading to an understanding of the mechanism. The Dow Chemical Company, one of the blasting agent suppliers, discovered that this unreliability could be controlled by the incorporation of tiny air bubbles in the slurry. This was originally done by whipping the slurry, but there were problems in controlling the size and distribution of the voids. When the 3M HGMs became available, they were evaluated and eventually used for this job. Still today, cartridges of slurry blasting agents containing 1–2% of HGMs for stabilization have displaced dynamite in mining and construction applications.
Probably, the most obvious use for HGMs was as a functional filler for plastics to enhance properties and/or reduce costs. In the 1970s, the usage of HGMs was mostly for explosives and some for resin applications. Resin applications grew quickly and today these applications include dry wall joint sealer, autobody filler, grout, caulk, potting compound, plastisol, adhesives, sheet molding compound, bulk molding compound, marine applications, extruded and thermoset insulation, buoyancy modules, and thermoplastic injection molded parts for transportation and other applications. New applications combining light-weighting technologies
[5] are being advocated in the plastics marketplace as many processors and end users grapple with questions of renewability, sustainability, CO2 production and carbon taxes, Corporate Average Fuel Economy (CAFÉ Standards), fuel consumption, and release of greenhouse gases to the environment.
There are very few fillers or even additives that are employed in the plastic industry that are lower in density than a typical base resin. This makes these materials unique, and somewhat problematic to handle and formulate. The...
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