Patent Application
20210387256
Not Applicable
Not Applicable
The present invention relates to methods and equipment for removing organic binders from green bodies.
Green bodies can be defined as three-dimensional shapes produced from intimate mixtures of a discrete phase comprising particulate materials which, upon sintering, are to yield the desired material composition of the end product, and a continuous phase consisting of a mixture of organic materials the sole purpose of which is to confer the transient property of thermoplasticity to the green mixture so that it can be shaped under the effect of heat and pressure.
The prior art uses various methods to form green bodies, the main ones being:
1. Injection Molding
2. Casting
3. Machining, Also Called Green Machining
4. Additive Manufacturing (AM) Also Called 3D-Printing
It is highly desirable to remove any organic binders from green bodies prior to sintering to avoid carbon inclusion in the end products and contamination of the sintering equipment by condensed binder degradation products as this will shorten the equipment's useful economic lifetime and prevent the attainment of high vacuum levels.
The prior art uses various methods to extract organic binders from green bodies depending on the latter's chemical composition. The most common of these are briefly reviewed.
Wei in U.S. Pat. No. 5,028,367 cites: “[ . . . ] it requires several days to completely remove the binder from the compact.”
C. L. Quackenbush (cf. Non-Patent Literature) reports binder extraction times of 150 hours (6.25 days) for a 3.5 mm thick slab of green silicon nitride.
Another disadvantage of Solvent Extraction (SX) is the recycling or disposal of spent solvent. An environmental concern is that many of today's solvents contain chlorine and are being phased out or banned following the 1978 Montreal Protocol because of concerns over the ozone layer.
Yet another problem with Solvent Extraction (SX) is to determine the time for completion of the debinding step. As it is based on part geometry (part wall thickness or cross-section), it is usually determined empirically or based on engineering studies of specific parts. Part wall thickness can be obtained from CAD drawings. Verification of extraction efficiency implies interrupting the extraction process, drying the parts to remove any solvent locked up in the porosity, and checking the weight loss. If the weight loss is deemed insufficient, the parts must be returned to the solvent bath for additional processing, clearly a costly and counterproductive method. Also, it should be noted that binder formulations are not always constant and may have to be altered to accommodate molding rheology.
Consequently, there is a need for an improved technique that obviates the problems of the prior art.
According to the present invention, there is provided a method and equipment to safely, efficiently and economically remove organic binders from green bodies.
The principle of the instant invention is based on maximizing the solvent diffusion coefficient throughout the debinding process. This is achieved in practice through controlled laminar inundation of the workload. Contrary to what is happening in the prior art where the green parts are invariably immersed in a solvent bath, in the instant invention, the green parts are flooded or inundated in a Reactor Tank by a steady laminar upward stream of freshly condensed solvent while the binder extraction rate is monitored by spectrophotometry of the spent solvent in the Reactor Tank overflow.
The process will be explained in detail below.
It is an object of the present invention to provide an efficient, economical and safe way to remove organic binders from green bodies.
The main advantages of the binder removal system used in the instant invention are:
easy and efficient process control. The end point of the Solvent Extraction (SX) step is reached when the solvent coining out of the System (the spent solvent) is as clean the freshly condensed solvent going in. This is verified by inline spectrophotometry or other suitable trace organic materials analysis. No need to interrupt the process to check the weight loss of the parts,
DWG #1 is a Piping & Instrumentation Diagram (P&ID) showing the main components of the System used in the application of the instant invention, namely:
DWG #2 shows the System in following condition:
DWG #3 shows the System in following condition:
DWG #4 shows the System in following condition:
DWG #5 shows the System in following condition:
The Quartz Reactor Assemblies must be mounted near the Boiler Sumps and at a level such that liquid solvent can flow back from the Quartz Reactors to the Boiler Sumps by gravity.
The Condenser(s) must be mounted at a height such that their bottom outlet is at a level above the overflow weir of the Quartz Reactor Tanks to allow gravity flow of distillate from the Condenser(s) to the Reactor Tanks.
(iii) Loading the Green Parts
The green parts are loaded in stackable carrier baskets or on trays. It is important to allow for the maximum of green part surface to be exposed to the solvent flow. The ideal carriers are stainless steel test sieves used for particle size analysis (PSA). The sieve diameter should be 8″ (203 mm) to fit perfectly into the Quartz Reactor Tanks. The sieves should be of welded construction to withstand exposure to high temperature (max. 600° C.) during LTB.
After loading the green parts into the Quartz Reactor Tank, the Bell Jar is placed over it. A temperature and solvent resistant gasket is used between the Tank and the Bell Jar. The Bell Jar is clamped onto the Reactor Tank.
The SX operational procedure has been explained in foregoing description.
Upon completion of the SX step, the Reactor is drained to Boiler Sump II.
(vii) Reactor Tank Evacuation
After drainage, the Reactor Tank is evacuated to a moderate vacuum (>25″ Hg) to extract any remaining solvent trapped inside the porous green parts. This step is important as the amount of residual solvent can be as high as 50% of part volume. The vacuum pump discharges to the Condensers to recuperate the trapped solvent which flows back to Boiler Sump I.
(viii) Low Temperature Burnout (LTB) Step
Following evacuation of the Reactor Tank and drying of the green parts, the LTB step can be initiated, using hot air or nitrogen gas or a combination of both.
In what follows, the invention will be described in more detail by way of a non-binding practical example. The feedstock formulation (based on 100 g. feedstock) used in the example is:
Binder extraction by Solvent Extraction (SX) relies on three simultaneous mechanisms, i.e.:
(i) Dissolution, i.e. the solubility of the wax component in the chosen solvent,
(ii) Diffusion, as a result of the random thermal motion of solute wax molecules,
(iii) Convection, i.e. the transport of solute wax molecules by solvent flow.
The effects of each of these mechanisms on the instant invention will now be reviewed in detail.
Dissolution depends on the solvent's Hildebrand solubility parameter as well as on environmental and economic considerations, e.g. temperature, flammability, pressure, ozone depletion potential (ODP) and cost.
Until the mid-1980s, CFCs, e.g. Freon 112, were in widespread use but in 1987, the Montreal Protocol banned or severely restricted their use. Consequently, chemical companies like DuPont, Wilmington, Del. and others, developed zero ODP solvents. DuPont's Vertrel MCA™, a non-flammable, proprietary azeotrope of 2,3-dihydrodecafluoropentane and trans-1,2-dichloroethylene (1,2 dichloroethene) commonly used as a solvent for waxes, resins, polymers, fats and lacquers has a Hildebrand solubility parameter of 15.2 MPa1/2 that is higher than that of the commonly used hexane (14.1 MPa1/2. This solvent has been used for the design of the equipment of the instant invention.
Fick's First Law of Diffusion states that the diffusive flux goes from regions of high concentration to regions of low concentration with a magnitude proportional to the concentration gradient.
In one spatial dimension:
J=−D*(δΦ/δx)
where
J is the diffusive flux in dimensions [MIL−2T−1], (e.g. mol/m2s)
D is the diffusion coefficient in dimensions [L2T−1], (e.g. m2/s)
Φ is the concentration in dimensions [ML−3], (e.g. mol/m3)
x is the position in dimensions [L], (e.g. in)
In a paper presented by Fan J. L. et al. of the State Key Laboratory for PM, Central South University, Hunan, Changsha, PRC (cf. Non-Patent Literature) the researchers state:
“At the start of debinding, the concentration difference between the specimens and the solvent is large, it is easy for the soluble component to diffuse and dissolve into the solvent from the specimens, so the debinding rate is high. With increasing time, the concentration difference between the specimens and solvent decreases, the solvent debinding enters into the dissolution control period and the concentration difference becomes the main factor to affect the debinding rate. With the decrease of concentration difference, the diffusion and dissolution rate decrease in spite of increase in the total binder weight loss.”
This research merely confirms Fick's Law of Diffusion and that the binder extraction rate will be maximized if and only if the concentration difference is maintained at a maximum which is the fundamental principle on which the instant invention is based.
Convective transport occurs when Organic Soluble (OS), i.e. solvated wax molecules are carried away by the solvent flow.
If θ is the volume concentration of OS molecules in the feedstock (as per feedstock formulation), we have,
dn/dx=dn/dy=dn/dz=θ
or, in one spatial dimension,
dy/dt=(1/θ)*dn/dt
where
dn/dt is the volume fraction of OS molecules being solvated per unit time, i.e. the rate at which OS molecules are being solvated and
dy/dt is the upward velocity.
The number of OS molecules being solvated is equal to the number of available OS molecule sites exposed to the solvent. This number is θ, the volume concentration of OS molecules at the green body/solvent interface.
The volume fraction of soluble matter in the feedstock (θ) is:
4.010 cm3/19.574 cm3=2.049*10−1
The soluble matter in the feedstock is stearin with properties:
molar mass, in: 891.48 g·mol−1
molar volume: 891.48 g·mol−1/0.84 g·cm−3=1,061.29 cm3·mol−1
molecular volume: 1,061.29 cm3·mol−1/6.022*1023 mol−1 or 1.762*10−21 cm3
molecular diameter, a (based on the hard sphere model):
a=6*1.762*10−21 cm3/π)1/3=1.5*10−7 cm
The Diffusion Coefficient is given by:
D=SQRT(k3/π3m)*(T3/2/Pa2)
where k is Boltzmann's constant
yielding D=2.15*10−15 cm2 s−1
Consequently, a 1.5*0−7 cm thick film of solvent covering a 1 cm×1 cm surface of green body (i.e. 1.5*10−7 cm3 of solvent) will generate 2.049*10−1×1.5*10−7 cm3=3.07*10−8 cm3 of solvated matter per cm2 of green body surface.
This solvated matter must be carried away by the solvent stream as fast as practical in order to maintain the maximum concentration gradient in the spent solvent and thereby the highest dissolution rate.
The solvent upward velocity or upflow (mm/s) is the variable controlling the rate at which solute molecules are being carried away. Empirically it has been determined that an upward velocity of about 10 mm/min (1.67*10−1 mm/s) is adequate.
In the example used to illustrate the invention, the green parts are processed in a Ø220 mm×400 mm (15 lit) Quartz Reactor Tank. Thus at an upward velocity of 10 mm/min, it will take 40 min (to fill an empty Reactor Tank, substantially less for a loaded one. This corresponds to a solvent flowrate of 15 lit/0.66 h or 22.52 lph which defines the necessary condensation capacity of the solvent condenser(s).
In conclusion, the major advantage of this invention resides in the ability to safely, economically and efficiently remove organic binders from green bodies.
Although the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications
This application claims priority to U.S. Provisional Patent Application Ser. No. 62/985,330 filed on Mar. 5, 2020.
