玻璃胶低温分解粘结剂.doc

The Benefits of Polyalkylene Carbonate Binders QPAC for Low Temperature Glass Frit or Powdered Glass in Low Temperature Processed Thick Film Applications P Ferraro Empower Materials New Castle DE S Hanggodo Empower Materials New Castle DE Abstract QPAC 40 Poly Propylene Carbonate or PPC is an exceptional binder for use in low temperature firing thick film pastes because it decomposes at low temperatures leaving minimal residue after debind The binder s low decomposition temperature allows for binder thermolysis at temperatures well below the sintering temperature of glass and metal powders that are used in ultra low firing thick film applications The potential for dramatically reduced levels of remnant carbon after thermal processing enables improvement in the mechanical optical and electronic properties of a fired thick film device Additionally it is proposed that the use of clean burnout QPAC binders will enable increased utilization of thermal processing equipment and will result in a reduction in periodic maintenance for removal of binder tars and the like from manufacturing thermal processing equipment both which will reduce manufacturing costs and associated per device costs In this paper glass pastes using QPAC PPC carbonate binders were studied Thermal analyses of the rate of debinding of thick film glass pastes made with ultra low firing 420 oC glass were performed and compared with analogous thick film pastes made with other common thick film binders as well as a commercially available ultra low fire thick film paste Rheological properties of the constituent binder vehicles as well as the respective thick film pastes were also evaluated and compared The resulting fired thick film samples were also visually inspected and the QPAC fired thick films most closely resembled the color of the fired neat glass powder while the fired thick films made from other common thick film binders were much darker indicating high levels of remnant carbon and possibly chemical reduction of the ultra low firing glass These obvious advantages can be applied to numerous low fire thick film applications including glass packaging solder glass sealing glasses phosphors dopants and other solar cell device and display sealing applications Introduction Many different electronic and optical products utilize thick film technology in their respective manufacture including but not limited to plasma displays ceramic packaging hermetic packaging ceramic circuits electronic chip components and solar cells and microelectromechanical systems MEMS In general all thick film processes use liquid thick film materials to enable manufacture of the device Typically these liquids are dispersions of inorganic particles dispersed within a liquid solution of multiple components and are comprised of at least the following materials One or more inorganic powders A solution containing oA solvent liquid in which to disperse the powders oA dispersant to enable suspension of the inorganic powder within in the liquid solution oAn organic binder to modify the rheological properties of the liquid to enable precise deposition of the thick film as well as to impart mechanical robustness to the dried deposited thick film oOne or more modifiers to improve either the properties of the dried film or the stability of the liquid or to provide additional properties required by the application In practice the liquid thick film is precisely deposited using screen printing doctor blading or the like then is dried to a solid then heat treated to remove the organic materials as well as to sinter the inorganic powders into the functioning material and configuration device desired As such it is typically desired that the only materials that remain in the final device are the inorganic materials and not the organic materials The organic materials are almost always a means to an end in that they enable precise deposition of the inorganics as well as impart sufficient mechanical robustness to the green unfired film so that it can be handled during manufacture Since the organic materials are intended to be completely removed during thick film processing they are typically called sacrificial materials in the thick film formulation In many cases carbon based organic materials which are not completely removed from the final device will leave carbon impurities that will adversely affect the materials properties of the finished inorganic thick film material These impurities can become gaseous during processing leading to additional porosity in the fired material thereby reducing mechanical strength and resistivity of the fired thick film The carbon based impurities that remain can reduce conductivity in fired metal thick films and can limit current carrying capacity in thick film resistors Carbon based impurities can dramatically reduce dielectric breakdown strength and insulation resistance as well thermal shock robustness Additionally excessive carbon impurities in a thick film dielectric can adversely affect dielectric constant and dielectric conductivity as well as dissipation factor and other key electronic properties For example residual carbon has been found to adversely affect the densification porosity and mechanical properties of metal powders 1 as well as to cause uncontrolled porosity and reduced dielectric breakdown strength in glass and ceramic thick film electronic packaging materials 2 Residual carbon in processed phosphors utilized in plasma displays was found to be responsible for a reduction in emission intensity of as much as 20 3 and has been attributed to a reduction in the optical transmission of glass 4 Residual carbon impurities may influence magnetic properties of ceramic materials5 and also has been found to reduce resistivity of ceramic insulating materials For example residual carbon in barium titanate based dielectric formulations has been found to dramatically increase conductivity of the insulating material in multilayer ceramic capacitor applications 6 7 In general more than 100 ppm residual carbon is thought to be deleterious to at least one electrical property of a thick film ceramic insulating material It is the intent of this study to introduce the QPAC 40 PPC binder system to the low fire and ultra low fire thick film industry in order to address issues of residual carbon and contaminants introduced by current thick film binders The most commonly used binders for making low fire thick film glass pastes and the resulting glass containing thick film devices are binder chemistries based on either ethyl cellulose EC or acrylic AC chemistries The materials properties of the EC AC and QPAC 40 evaluated in this study are illustrated in Table 1 below Table 1 Binder properties BinderQPAC 40 PPCECAC MW300 000NA142 000 Density1 281 121 06 n butyl methacrylate Polyalkylene Carbonate CH CH 3CH2OCO2 n is synthesized through the polymerization of carbon dioxide and epoxides The synthesis process for PPC is environmentally friendly green QPAC 40 PPC is a colorless thermoplastic polymer with low glass transition temperature QPAC 40 PPC binders are available in a broad range of molecular weights As shown in Figure 1 below the decomposition of PPC polymers takes place at a lower temperature than either ethyl cellulose EC or acrylic binders AC that are commonly used binders in thick film pastes EC AC QPAC Figure 1 Thermal decomposition of QPAC 40 PPC EC and AC binders in air and ramping at 2 oC minute The products of the combustion of PPC are relatively environmentally friendly including only carbon dioxide and water vapor Decomposition is complete through three phases solid liquid and vapor so there is no residual carbon or smell an issue at many thick film manufacturing operations Figure 2 Decomposition mechanism of PPC As the binder in a thick film ink is intended to be completely removed during thermal processing of the thick film an important property of the binder system utilized is that it be pure leaving very little residuals ash and that any remaining residual material ash also contain minimal amounts of inorganic impurities that may diminish the performance of the fired thick film material in its intended application An analysis of the residual ash and the ash chemistry of QPAC 40 and a representative EC and a representative AC are indicated in below Table 2 Residuals ash after burn out for QPAC 40 EC and AC Constituent ppm QPAC 40 ACEC Total Ash 10 102300 Chromium 1 1 1 Iron 1 125 Nickel 1 15 Zinc32NA Calcium 1NA4 SodiumNANA1000 Total Metals 103000 It is not uncommon for devices produced using EC AC or other traditional binders to retain several thousand parts per million ppm of carbon after the binder bake out step of thermal processing and up to several hundred ppm of carbon after the firing process A binder system that can reduce or eliminate residual carbon when processed utilizing existing production thermal processing equipment and that can enable a high material throughput rate is highly desirable to the thick film industry QPAC 40 Polypropylene carbonate PPC binders decompose completely at temperatures low enough to ensure almost complete binder removal from all thick film materials that fire at or above 400 oC QPAC 40 PPC binders are removed completely during thermal processing either in oxygen containing atmospheres e g air or in inert atmosphere e g N2 or the like and leave extremely low amounts of residual carbon In the case of low firing and ultra low firing glass thick films complete removal of AC and EC binders before glass sintering is very difficult or impossible to achieve as the relatively high temperatures required to remove these binders encroach on the sintering temperature of the inorganic thick film material This results in entrapment of significant amounts of binder residuals in the fired thick film The entrapped residual materials will likely degrade the properties of the fired material as residual carbon or inorganic contamination can be highly detrimental to materials performance The benefits of using QPAC 40 PPC binders for low firing thick film applications have yet to be fully recognized by the thick film industry It is the intent of this work to introduce QPAC binders to the low fire thick film industry to enable the development of thick film materials exhibiting improved performance Use of QPAC 40 PPC binders allows for binder removal in large quantities more completely and at much lower temperatures than the binders currently used for low temperature thick film applications Use of QPAC 40 PPC binders in low temperature glass thick film formulations shifts the paradigm of compromise between binder content and thermal processing temperature to a level that the industry has not before experienced enabling an additional degree of freedom in the formulation of low temperature fired thick film pastes as well as higher work throughput in thermal processing equipment and improved fired properties of the inorganic thick film such as resistivity of insulators improved mechanical strength toughness and reliability of packaging and sealing materials increased emissivity of phosphors and reduced gas permeability of sealing glasses etc In the present work we have prepared glass pastes made from a commercial low melt glass powder and a commercial polypropylene carbonate PPC binder QPAC 40 in comparison to glass pastes made with a AC acrylic binder as well as a paste made with an EC ethyl cellulose binder and a commercially available ultra low fire thick film glass paste Thermal gravimetric analysis TGA and visual analysis were then compared and contrasted for each of the thick film pastes evaluated The amount of residual carbon of the pastes fabricated using QPAC 40 PPC binder was found to be significantly reduced after thermal processing when compared to the EC and acrylic binder systems Experimental Binder solutions of EC Texanol n butyl methacrylate Texanol and QPAC 40 PPC Triethylene Glycol Dimethyl Ether Triglyme were prepared The viscosity of these solutions are shown below Table 3 Binder solution viscosities at 0 1 rpm and 25 C BinderSolvent solids Viscosity cps 20 15 300 QPAC 40 PPC 300 000 MW Triglyme 10 4 300 Ethyl Cellulose 10 000 MW Texanol 10 1 400 Acrylic Resin 142 000 MW Texanol Figure 3 Viscosity vs shear rate for different binder solutions Using the information gathered from the binder solution study thick film pastes were made using binder solutions made with reagent grade solvents The paste viscosity targets were similar to commercially available glass paste The thick film formulations were targeted to achieve a viscosity of 100Kcps at 10 s 1 for each such binder family The formulation for each is given in Table 4 below Table 4 Thick film paste formulations with different binders PASTE SOLVENTBINDER SOLUTION SOLID BINDER GLASS QPAC 40 PPC Triglyme 20 4 80 ECTexanol 10 2 80 ACTexanol 20 4 80 The thick film pastes were made by mixing the glass powder into the pre dissolved binder solutions The solutions containing the glass were mixed until uniform The paste was then run through a three roll mill The viscosity vs shear rate data at 25 oC for each of these pastes as well as a commercially available ultra low firing thick film glass paste are illustrated in figure 4 below Figure 4 Viscosity vs shear rate for ultra low fire thick film pastes Additionally samples of each thick film paste were analyzed via thermogravimetric analysis TGA 8 from RT to 400 oC in air atmosphere flowing at 20 ml min with temperature ramping rates of 2 oC min Included in the study with the three prepared pastes is a sample of a commercially available glass paste for comparison The data is illustrated in Figure 5 below QPAC EC AC Commercial Rate of Weight Loss Figure 5 TGA data for each of the thick film pastes studied including Normalized weight loss comparison The ingots from the TGA analysis are shown below in Figure 6 Visually the samples vary The PPC based paste shows a color slightly darker than the neat glass while the AC and EC pastes are clearly darkest This most likely indicates the carbon content remaining post sintering Figure 6 Appearance of thermal treated thick film pastes post TGA Results and Discussion The results of the glass paste analysis shows the benefits of using the PPC binder instead of the traditionally used ethyl cellulose or acrylic binders The results are summarized below Paste Stability and Viscosity There are several available solvents to use with the PPC For our study it was determined that Triglyme would be used because of its surface tension and relative evaporation rate are similar to Texanol The viscosity analysis shows that the QPAC has thixotropic properties similar to the EC The 80 solid glass pastes prepared from the three binders all exhibited a creamy appearance All three systems appeared to be compatible with the low temperature melt glass used However the acrylic based paste showed some solvent separation after several days This paste also had a lower viscosity than the PPC and EC pastes Overall the PPC based paste exhibited the best suspension stability with time The rheology curves of the three pastes show that the PPC and EC pastes have similar viscosities at low shear while the AC paste had a lower viscosity note equal amounts of AC and PPC binder were used while 50 less EC binder was used At higher shear rates the PPC paste viscosity decreased more rapidly than the EC paste thereby indicating a higher degree of thixotropy This may have some benefit in different forms of paste applications Overall all three pastes showed thixotropic behavior The spinnability or stringiness of all three pastes was also evaluated The ability to draw a fiber from each paste was evaluated using a small sharp instrument slowly and steadily drawn from the paste and the length of the draw observed The PPC paste showed a fiber draw of 1 5 mm as compared to 15 20 mm for the EC and AC pastes All other items being equal this lower propensity of the QPAC 40 PPC binder to result in long fibers should correspond to improved paste printing e g edge definition when deposited via screen or stencil printing techniques Decomposition Results Temperature The TGA analysis of the three binders showed that the PPC decomposed at a much lower temperature than EC a difference of greater than 100 oC and a lower temperature than the acrylic approximately 60 C The difference in decomposition temperature was even more apparent for the pastes prepared with each of the binders The glass powder likely acts as a catalyst for the decomposition in all three cases lowering the temperature at which the binder is removed as compared to the case of the pure binder The TGA analysis shows a shift of up to 100 oC lower This is reflected in all three of the pastes Again the onset of decomposition of PPC in the paste is removed at temperatures that are at least 100 oC lower than for the EC with an even greater difference for the acrylic The normalized weight loss curves show that virtually all the PPC is removed at temperatures at least 100 oC lower than both the other binders The TGA curve shows that most of the PPC binder is removed by 200 oC and the remainder by approximately 300 oC This can be a great benefit when as in this formulation the glass frit or powder melts at a low temperature If the binder can be removed before the glass melts the resultant final part should have improved porosity and densification because the gases are allowed to more easily escape Additionally if the binder is removed before the glass sinters then the chance to entrap binder is minimized The ability to debind at a lower temperature also allows for more optimized usage of the furnace The lower debind temperature may also be effective in minimizing the effect of reducing the sensitive oxides because the environment is less conducive to reduction Decomposition Results Residual Carbon The PPC binders are known to have lower