Microcellular foams, their method of production, and uses...

Details Microcellular foams, their method of production, and uses... Microcellular foams, their method of production, and uses...

2002-09-05

2003-10-28

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S059000, C521S060000, C521S149000, C521S134000

Reexamination Certificate

active

06638984

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of plastic particles, more specifically to the field of expandable and optionally fusible plastic microspheres, and hollow plastic microballoons, microcellular foam or foamed composite materials produced therefrom.
BACKGROUND OF THE INVENTION
Hollow microballoons or microbubbles find prevalent use throughout industry, most commonly as additives or fillers. The primary benefit of hollow microspheres over more conventional fillers (e.g., silicates, aluminates, clays, talcs, etc.) is weight reduction. Hollow microspheres offer a means of introducing controlled, small voids in a closed-cell configuration. This can be difficult to obtain in both viscous and non-viscous fluids, resins, coatings, and cements using conventional foaming agents due to problems associated with the foaming process such as unequal cell growth, time- and temperature-dependent gas diffusion, cell coalescence, etc. Thus, hollow microspheres provide a means for uniformly and homogeneously increasing product bulk while simultaneously decreasing the overall density, lowering product cost on a volumetric basis without sacrificing (or while enhancing) performance.
In addition to the benefits of weight reduction and cheaper product cost, hollow microspheres offer many other advantages in a wide variety of products. For example, fluid products such as printing inks and paints benefit from the spherical shape of hollow microspheres, resulting in viscosity reduction, enhanced flow and leveling, faster dispersion time, smoother surface finish, and an overall increased volume load capacity. Higher loading capacities in turn lead to increased hiding power, maximum tint strength, better gloss control, VOC reduction, dimensional stability, improved applicability, and further overall weight or density reduction.
A wide range of products, such as thermoplastic resins, epoxies, sealants, pipe insulation, potting compounds, spackling compounds, underbody coatings, papers, fabrics, dielectric laminates, prosthetic devices, synthetic foams, cultured marble, polymer concretes, and synthetic cements also benefit from hollow microsphere additives. The primary improvement is again weight reduction, but additional improvements may include: increased volume load capacity; reduced product warpage, shrinkage, and/or cracking; abrasion resistance or abradability; corrosion resistance; increased impact strength; smoother surface finishes; improved molding of intricate parts; disruption of directional orientation (in polymer systems); decreased dielectric constant and/or increased capacitance; increased bulk and stiffness in paper and paperboard; improved sensitivities in explosives (greater thermal insulation and shock resistance); easier machinability; water resistance; better sound absorption; and increased compressibility.
Production methods and compositions for hollow microspheres made from various glass, metallic, or polymeric materials have been disclosed, patented, or used in the past, e.g. see U.S. Pat. Nos. 3,615,972, 3,838,998, 3,888,957, 3,933,955, 3,945,956, 4,133,854, 4,257,798, 4,303,603, 4,349,456, 4,661,137, 4,767,726, 4,782,097, 4,983,550, 5,069,702, and 5,053,436. The particles or microspheres, hollow or otherwise, and/or their method of fabrication as referenced above or in the literature have one or more disadvantages or limitations that have hindered their commercialization or restricted their field of use.
For example, many hollow microspheres are composed of glass or ceramic oxide shell walls, exhibiting a true particle density in the range of 0.1 to 0.4 g/cc. Microspheres such as these must often be washed in a series of treatment baths to reduce alkali content. The microspheres must further be dried from said bath, an operation that is energy inefficient and which leads to clumping unless special drying agents are used. Due to their poor impact strength, glass microspheres are subject to rupture under conditions of high shear, which may be experienced during such common operations as pumping, injection molding, extruding, calendering, or milling. Ruptured microspheres no longer possess the benefit of low density, and the nonspherical shape of the resulting fragments negates many of the other beneficial properties to be realized from the incorporation of spherically shaped additives.
To compensate for the fragility of certain glass compositions, thick-walled glass or ceramic microspheres have been proposed, e.g. see U.S. Pat. No. 3,838,998 (thick-walled glass), U.S. Pat. No. 4,349,456 (ceramic), U.S. Pat. No. 4,983,550 (strong glass), U.S. Pat. No. 5,077,241 (ceramic bubbles), and U.S. Pat. No. 5,225,123 (sintered particle-walls). Thick-walled microspheres exhibit a significantly higher density, however, since their internal void volume is greatly sacrificed to increase structural integrity. As a result, thick-walled microspheres exhibit a particle density of 0.3 to 0.7 g/cc, diminishing the overall effect of density reduction, and are more costly due to increased material usage. Ceramic microspheres tend to have thicker shell walls as well, exhibiting densities up to 0.7 g/cc, and their production requires significantly more expensive precursor materials. Thus, glass or ceramic microspheres possess many disadvantages that inhibit their full commercial exploitation.
To overcome some of the limitations inherent in the production and properties of glass or ceramic microspheres, plastic microspheres have been developed, e.g. see U.S. Pat. Nos. 3,615,972, 3,945,956, 4,049,604, 4,075,134, 4,303,603, and 5,053,436. These hollow microspheres are typically composed of a thermoplastic shell wall material that sometimes encapsulates a solid or liquid core. For instance, Famand and Puddington (U.S. Pat. No. 3,975,194) disclose a process for hollow microsphere production that utilizes a solid core material which sublimes at room temperature by rapid diffusion through the shell wall, leaving behind a liquid shell which is then dried. More commonly, the materials of construction are chosen such that the polymeric shell walls soften upon heating, and a volatile liquid core expands the shell wall by vaporizing to form a hollow, nominally spherical particle. For example, see U.S. Pat. Nos. 3,821,128, 4,108,806, and 5,536,756. Because thermoplastic polymers are incorporated into such particles, the microsphere shell walls are significantly more fracture-resistant than glass, and are therefore less prone to rupture and breakage during high shear operations.
Liquid-filled thermoplastic microcapsules have another advantage over glass or ceramic microspheres in that the expansion of the liquid-filled plastic microcapsules can be triggered after formulation in the end-use product. Glass and ceramic microspheres soften only at extremely high temperatures, making their expansion process incompatible with most end-use products. Liquid-filled, expandable plastic microcapsules on the other hand can be incorporated into products such as resins, coatings, cements, paints or inks in an unexpanded state. These products may then be mixed, pumped, extruded, or otherwise handled and applied in a manner consistent with their use, followed by a heating step which inflates the microcapsules into their hollow, thin-walled, low-density configuration. The expansion caused by the inflating microspheres aides in molding, shaping, or texturing the end-use product. Furthermore, since mixing, extrusion, and molding operations occur while the plastic microcapsules are in an unexpanded (and therefore less fragile) state, the likelihood of rupturing or breaking the particles is greatly diminished. Since the thermoplastic microsphere materials typically have good fracture strength properties and the microballoons are not subjected to high-shear operations after expansion, the microspheres may also be expanded to a greater extent than glass or ceramic microspheres, leading to thin-walled thermoplastic balloons with densities as low as 0.01 g/cc, much lower than hollow glass or ceramic microspheres.
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