Advanced Binders for High-Tech Applications
Since the 1930s, ceramists have attempted to develop novel ceramic plate and film forming processes and techniques that would support the growing trend toward miniaturization within the electronic components sector. The first generation of ceramic slips based on resins such as polyvinyl chloride co-polymers required high levels of solvent to generate the necessary low viscosity. Additionally, they contained low levels of ceramic oxide pigment, and the binders did not burn out very efficiently. Successive generations of ceramic slips evolved to incorporate polymers based on vinyl and acrylic chemistry that were suitable for thin-film ceramic tape processing. The major difference between the two polymer classes is the way each degrades in an oxidative environment. Polyacrylates (acrylics) tend to unzip, while polyvinyls (such as polyvinyl butyral [PVB] and polyvinyl alcohol [PVOH]) tend to degrade through elimination and dehydration reactions during binder burnout.
Over the past several decades, an increasing number of electronic component manufacturers have begun using polyvinyl butyral as the temporary binder. With its efficient pigment wetting capabilities, tightly controlled molecular weight distribution, clean endothermic burnout and high oxide ceramic pigment binding capacity, polyvinyl butyral provides opportunities for further advances in oxide ceramic-based passive fabrication technology.
History of High-Tech BindersThe trend toward miniaturization began when the interruption of the international maritime commercial trade routes, which were used for export trade from mineral rich countries such as India during World War II, led to a shortage of capacitor-grade mica. A private/public sector research consortium was established during the 1940s to develop alternatives to mica-based passives. In 1948, Glenn N. Howatt, while working for the consortium, published a major technological breakthrough that paved the way for the formation of thin ceramic plates via a continuous belt.1
During the 1950s, Warren J. Gyurk teamed with Glen Howatt at Glenco Corp. in Metuchen, N.J., to develop some of the earliest prototype passives based on BaTiO3, ferrite and lead zirconate titanate (PZT).2 Afterwards, Gyurk joined the staff at Radio Corp. of America, Inc. (RCA) to collaborate on a military project called the Army Module Program. While working for RCA, he developed several application patents for laminating BaTiO3 films in multiple layers of 25.4 microns each.3 The slip formulation used to produce the ceramic film used a vinyl chloride-vinyl acetate copolymer as the binder. The new manufacturing process would serve as the foundation for the production of a new class of passives—multilayer ceramic capacitors (MLCCs).
During the 1960s, John L. Park, a researcher from the Chattanooga, Tenn.-based American Lava Corp., advanced the work of Howatt with the proposal to discharge the ceramic slip through a small orifice in a pool onto a flexible supporting tape. The patent suggested that the supporting tape could be based on polytetrafluoroethylene or glycol terephthalic acid polyester. Park was one of the first scientists to publish information about ceramic slip formulations based on polyvinyl butyral for use in tape casting applications.4
Today, thin ceramic films that use polyvinyl butyral as the temporary binder are incorporated into MLCCs, low temperature co-fired ceramics (LTCCs), lithium-ion membranes, metal brazing strips, bioceramics, solid oxide fuel cells, thick-film printable polymer insulator pastes, and many other electronic component fabrication processes. As many product engineers have discovered, polyvinyl butyral provides a host of benefits in these and other advanced ceramic applications.
Polyvinyl Butyral DevelopmentPolyvinyl butyral belongs to a class of chemicals called polyvinyl acetals. Starting in 1922, Dr. Willy O. Hermann published a series of patents that described the invention of polyvinyl acetals while working for Consortium fuer Elektrochemishe Industrie GmbH in Munich, Germany.5,6,7 The advances in the field of acetal chemistry made by Hermann and his team at the Consortium—which is now the corporate research facility of Wacker-Chemie GmbH—are commercially available today as advanced polyvinyl butyral resins.*
Polyvinyl butyral is a thermoplastic resin manufactured through a three-step condensation reaction. First, vinyl acetate is polymerized to poly (vinyl acetate). The poly (vinyl acetate) is then saponified to poly (vinyl alcohol), and the alcohol is reacted with butyraldehyde to form polyvinyl butyral. For this reason, polyvinyl butyral should be considered a ter-polymer with functional pendant hydroxyl groups. When used as the main binder in the formulation of ceramic slips, polyvinyl butyral acts as a secondary dispersing agent for the ceramic particle. The polymer exhibits good solubility in low-cost alcohol and glycol ether solvents due to the presence of pendant hydroxyl groups on the polymer chain backbone. It is also soluble in most aromatic and aliphatic solvents.
Advantages in Ceramic ApplicationsLeading-edge researchers of passive components are actively involved in the development of fabrication processes for ceramic tapes yielding film thickness less than 1 µm. To support the advancement of these technologies, manufacturers of ceramic particles and organic binders must develop products with improved performance. The widespread use of submicron ceramic particles has stimulated a new round of research by polyvinyl butyral manufacturers to develop resins with an improved pigment wetting capability, a higher pigment binding capacity and tighter control over the molecular weight distribution. Compared to acrylic and cellulose-based systems, polyvinyl butyral offers broader compatibility with the additives and solvent blends that are often required to support the higher ceramic density of the film upon firing. Typically, as the solids loading increases, drying-related defects in the ceramic film—such as shrinkage, cracking and film porosity—decrease with the use of polyvinyl butyral.
In a submicron particle size environment, it is important that the binder adequately coat the large surface area of the particles to ensure that they are adequately separated within the green ceramic body. The thermolysis of the ceramic green body within a temperature range above the thermal degradation temperature of polyvinyl butyral (T = 170 degrees C), but well below the classical sintering temperature of 1000 degrees C, reveals that the concentration of the degradation products from polyvinyl butyral rises rapidly and then diminishes as the binders diffuse to the surface of the green body. The propensity of a binder to exhibit clean burnout is extremely important to prevent the development of flaws and to minimize carbon residues that may inhibit sintering. The minimization of carbonaceous residue within the ceramic body is especially important when the ceramic is co-fired with an oxidizing metal such as copper, since the reducing conditions may aggravate carbon formation. Systems based on polyvinyl butyral are known to exhibit clean burnout characteristics in low temperature co-firing systems with metallization.
In thin-film tape casting applications, both degradation reactions and volatile transport occur simultaneously during thermolysis; therefore, it is important to control the molecular weight of the polymer to improve the probability that the degradation byproducts will diffuse uniformly to the surface of the green body. During the manufacture of polyvinyl butyral, care is taken to ensure that the acetalization process of polyvinyl alcohol and butryaldehyde occur within a defined temperature range to generate a targeted molecular weight and distribution. Such control over the molecular weight distribution is necessary to ensure excellent solvent solubility at the individual polymer particle level and uniform burnout during pyrolosis.
Polyvinyl butyral can decompose by a cyclic-elimination mechanism to release butyraldehyde, with associated chain breakage, leaving aldehyde and unsaturated end-groups.8 In the late stages of this process, various products, including crotonaldehyde and unsaturated hydrocarbons, form by breakage at adjacent sites. Masia et. al. studied the effect of oxides on the burnout of binders.9 They concluded that the surface chemistry of oxides in a polyvinyl butyral ceramic oxide film has a strong catalytic effect on the activation energy and, hence, the decomposition rate of the polyvinyl butyral resin. In LTCC applications, a shift to the left of the polymer’s decomposition temperature profile can provide a ceramist more formulation latitude to incorporate more exotic metal compositions during the metallization process.
In addition, Masia found that selected polyvinyl butyral-containing ceramic green bodies undergoing thermolysis in an oxidative environment yield three decomposition phases, and that the degradation temperature of polyvinyl butyral shifts to lower temperatures depending on the type of ceramic used. Therefore, upon fabrication of the ceramic tape to a component, the burnout schedule should match the polymer/oxide ceramic combination to ensure a low probability for sintered part defects due to bubbling and warpage.
Expanding OpportunitiesThe rapid increase in the demand of passive components used in wireless communications, automobiles and computer products has, in turn, generated a significant level of interest in the use of polyvinyl butyral for the fabrication of low dielectric constant LTCC packages. These dielectrics are designed to address the need for higher signal propagation speed, good reliability and easy fabrication. Recent advances in glass-ceramic composites10 will only further reinforce the popularity of polyvinyl butyral in this field, due to its excellent adhesive and wetting properties to glass surfaces.
Polyvinyl butyral resin suppliers are devoting an increasing proportion of their overall research and development expenditures to developing the next generation of polyvinyl butyral resins that meet the higher mechanical and electrical requirements for use in specialized passive component applications. The push toward miniaturization of passive components through the use of submicron ceramic particles and the incorporation of crystallizing and filled glass particles into the ceramic matrix has re-established polyvinyl butyral as a leading binder of choice in critical, high-performance thin-film ceramic tape applications.
For more information:For more information about polyvinyl butyral resin developments, contact Wacker Polymer Systems, L.P., 3301 Sutton Rd., Adrian, MI 49221; (517) 264-8160; fax (517) 264-8137; or e-mail firstname.lastname@example.org (U.S.) or email@example.com (Germany).
References:1. G. N. Howatt, “Method of Producing High Dielectric Constant High Insulation Ceramic Plates,” U.S. Pat. No. 2,582,993, 1948.
2. R.E. Mistler, “Tape Casting: Past, Present, Potential,” October 1998, www.ceramicbulletin.org/months/Oct98/cast2.html
3. W. J. Gyurk, “Methods for Manufacturing Monolithic Ceramic Bodies,” U.S. Pat. No. 3,192,086, 1965.
4. J. L. Park Jr., “Manufacture of Ceramics,” U.S. Pat. No. 2,966,719, 1961.
5. W.O. Hermann, GB Pat. No. 182,459, 1922.
6. W.O. Hermann, GB Pat. No. 184,442, 1922.
7. W.O. Hermann, GB Pat. No. 185,107, 1922.
8. F. Bakt, Pakistan J. Sci. Ind. Res. 26 (1), 1983, p. 35.
9. S.Masia, P.D. Calvert, W.E. Rhine, and H.K. Bowen, “Binder Burnout: The Effect of Surface Chemistry on Carbon Residue Elimination,” J. Mater. Sci., 24, 1907 (1989).
10. K. Niwa, N. Kamehera, H. Yokoyama and Kurihara, “Multilayer Ceramic Circuit Board with Copper Conductor,” Westerville, OH, 1986, p. 4.
*These resins are commercially available as PIOLOFORM, a registered trademark of Wacker Polymer Systems, L.P., Adrian, Mich.